Statcom Full Report

CHAPTER

1

INTRODUCTION

1.0 Introduction With increase in the demand for Electricity due to increase in population and industrialization, the Generation of power was really a challenge now a day. If we want to increase the power generated in the conventional way i.e., by means of nonrenewable energy sources like coal, diesel, natural gases and similar fossil fuels, the pollution increases which degrades the Environment and human life style. Disadvantage of using Non-Renewable energy sources are: Non-renewable sources will expire some day and we have to us our endangered resources to create more non-renewable sources of energy. The speed at which such resources are being utilized can have serious environmental changes. Non-renewable sources release toxic gases in the air when burnt which are the major cause for global warming. Since these sources are going to expire soon, prices of these sources are soaring day by day. Thus there is a great need for electric power which has to be produced in a clean way that is through the Renewable energy sources like solar, wind, tidal, geothermal, biomass energy sources. These resources are very cheap and are abundant in nature. We can completely depend on these sources if we got the technology to do so. Compared to the non-renewable energy sources these have the advantages of the following: The sun, wind, geothermal, ocean energy are available in the abundant quantity and free to use. The non-renewable sources of energy that we are using are limited and are bound to expire one day.

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Renewable sources have low carbon emissions, therefore they are considered as green and environment friendly. Renewable helps in stimulating the economy and creating job opportunities. The money that is used to build these plants can provide jobs to thousands to lakhs of people. You don't have to rely on any third country for the supply of renewable sources as in case of non-renewable sources. Renewable sources can cost less than consuming the local electrical supply. In the long run, the prices of electricity are expected to soar since they are based on the prices of crude oil, so renewable sources can cut your electricity bills. Various tax incentives in the form of tax waivers, credit deductions are available for individuals and businesses who want to go green. But even though they have their advantages, they are not preferred due to economical criteria of investing huge funds. Also the problems that we face when we integrate these energy sources to the grid are quite many like power quality maintenance. In this paper we consider Wind power that can be utilised for generation of electrical power using Wind farms with FACTS device P-STATCOM to compensate the disturbances that occur due to the fluctuating nature of the wind. This nature of wind also effects the current and voltage in the grid to which wind turbine is connected.

1.1 OBJECTIVE OF THE PROJECT The causes of power quality problems are generally complex and difficult to detect when we integrate a wind turbine to the grid. Technically speaking, the ideal AC line supply by the utility system should be a pure sine wave of fundamental frequency (50/60Hz). We can therefore conclude that the lack of quality power can cause loss of production, damage of equipment or appliances or can even be detrimental to human health. It is therefore imperative that a high standard of power quality is maintained. This project demonstrates that the power electronic based power conditioning using custom power devices like P-STATCOM can be effectively utilized to improve the quality of power supplied to the customers. The aim of the project is to implement Wind turbine connected to a Grid consisting of Distribution generation and P-STATCOM with Back Up energy storage 3

system (BESS) in the MATLAB, simulink using Simpower systems tool box and to verify the results through various case studies applying Non-linear loads and study them in detail.

1.2 OVERVIEW OF THE PROJECT The Renewable energy sources, which have been expected to be a promising alternative energy source, can bring new challenges when it is connected to the power grid. However, the generated power from renewable energy source is always fluctuating due to environmental condition. In the same way Wind power injection into an electric grid affects the power quality due to the fluctuation nature of the wind and the comparatively new types of its generators. On the basis of measurements and norms followed according to the guidelines specified in IEC-61400 (International Electro-technical Commission) standard, the performance of the wind turbine and thereby power quality are determined. The power arising out of the wind turbine when it connected to grid system concerning the power quality measurements are-the active power, reactive power, voltage sag, voltage swell, flicker, harmonics, and electrical behaviour of switching operation and these are measured according to national/international guidelines. The paper clearly shows the existence of power quality problem due to installation of wind turbine with the grid. In this proposed scheme a FACTS device {STATIC COMPENSATOR (STATCOM)} is connected at a point of common coupling with a battery energy storage system (BESS) to reduce the power quality problems. The battery energy storage system is integrated to support the real power source under fluctuating wind power. The FACTS Device (STATCOM) control scheme for the grid connected wind energy generation system to improve the power quality is simulated using MATLAB/SIMULINK in power system block set. The intended result of the proposed scheme relives the main supply source from the reactive power demand of the load and the induction generator. From the obtained results, we have consolidated the feasibility and practicability of the approach for the applications considered. The STATCOM is a compensating device which is used to control the flow of active and reactive power required to the Induction Generator of the wind turbine. It is a custom power device which is gaining a fast publicity during these days due to its 4

exceptional features like it provides fast response, suitable for dynamic load response or voltage regulation and automation needs, Both leading and lagging VARS can be provided, to correct voltage surges or sags caused by reactive power demands pulse STATCOM can be applied on wide range of distribution and transmission voltage, overload capability of this provides reserve energy for transients from the BESS. The pulse STATCOM is controlled using the PI controller. The complete background of the compensating devices and power electronic application in compensating devices is discussed and also the compensation using the STATCOM modeling is also discussed. Theoretical analyses of the Different types of control strategies use for the control of STATCOM are discussed and the necessary block diagrams and the transformations required are discussed. Conclusions are drawn basing on the simulated results obtained and also the future scope of the project is also included.

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CHAPTER

2

FLEXIBLE AC TRANSMISSION SYSTEMS AND THEORETICAL ANALYSIS

2.1.0 POWER QUALITY AND RELIABILITY: Power quality and reliability cost the industry large amounts due to mainly sags and short-term interruptions. Distorted and unwanted voltage wave forms, too. And the main concern for the consumers of electricity was the reliability of supply. Here we define the reliability as the continuity of supply. As shown in fig.2.1, the problem of distribution lines is divided into two major categories. First group is power quality, second is power reliability. First group consists of harmonic distortions, impulses and swells. Second group consists of voltage sags and outages. Voltage sags is much more serious and can cause a large amount of damage. If exceeds a few cycle, motors, robots, servo drives and machine tools cannot maintain control of process.

Fig.2.1.1 power quality and reliability Both the reliability and quality of supply are equally important. For example, a consumer that is connected to the same bus that supplies a large motor load may have to face a severe dip in his supply voltage every time the motor load is switched on. In some extreme cases even we have to bear the black outs which is not acceptable to the consumers. There are also sensitive loads such as hospitals (life support, operation theatre, and patient database system), processing plants, air traffic control, financial 6

institutions and numerous other data processing and service providers that require clean and uninterrupted power. In processing plants, a batch of product can be ruined by voltage dip of very short duration. Such customers are very wary of such dips since each dip can cost them a substantial amount of money. Even short dips are sufficient to cause contactors on motor drives to drop out. Stoppage in a portion of process can destroy the conditions for quality control of product and require restarting of production. Thus in this scenario in which consumers increasingly demand the quality power, the term power quality (PQ) attains increased significance. Transmission lines are exposed to the forces of nature. Furthermore, each transmission line has its load ability limit that is often determined by either stability constraints or by thermal limits or by the dielectric limits. Even though the power quality problem is distribution side problem, transmission lines are often having an impact on the quality of the power supplied. It is however to be noted that while most problems associated with the transmission systems arise due to the forces of nature or due to the interconnection of power systems, individual customers are responsible for more substantial fraction of the problems of power distribution systems. 2.1.1 Types of Power Quality Problem Some of the power quality disturbance wave forms are shown in fig 2.1.2 2.1.2 Transients These are sub cycle disturbances with a very fast voltage change. They typically have frequencies often to hundreds of kilohertz and sometimes megahertz. The voltage excursions range from hundreds to thousands of volts. Transients are also called spikes, impulses and surges. Two categories of transients are described, impulsive transient and oscillatory transient. Examples of transients include lightning, electro-static discharge; load switching, line/ cable switching, capacitor bank or transformer energizing and Ferro-resonance.

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Fig.2.1.2. Some PQ disturbances 2.1.3 Long- Duration Voltage Variations Long-duration variations encompass root-mean-square (rms) deviations at power frequencies for longer than 1 min. A voltage variation is considered to be longduration when the limits are exceeded for greater than 1 min. These variations are categorized below: Over voltage: An over voltage is an increase in the rms voltage greater than 110 percent at power frequency for duration longer than 1 min. Examples include load switching, incorrect tap settings on transformers, etc. Under voltage: An under voltage is a decrease in the rms ac voltage to less than 90 percent at power frequency for duration longer than 1 min. Examples include load switching, capacitor bank switching off, overloaded circuits, etc. Sustained interruptions: These come about when the supply voltage stays at zero longer than 1 min. They are often permanent and require human intervention to repair the system restoration. Examples include system faults, protection maltrip, operator intervention, etc.

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2.1.4 Short- Duration Voltage Variations Short-duration variations encompass the voltage dips and short interruptions. Each type of variations can be designated as instantaneous, momentary, or temporary, depending on its duration these variations can be categorized as: Interruptions: This occurs when the supply voltage or load current decreases to less than 0.1 pu for a time not exceeding 1 min. The voltage magnitude is always less than 10 percent of nominal. Examples include system faults, equipment failures, control malfunctions, etc. Sags (dips): Sag is a decrease to between 0.1 and 0.9 Pu in rms voltage or current at power frequency for durations from 0.5 cycle to 1 min. Examples include system faults, energization of heavy loads, starting of large motors, etc. Swells: A swell is an increase to between 1.1 and 1.8 Pu in rms voltage or current at power frequency for durations from 0.5 cycle to 1 min. Swells are not as common as sags. Sometimes the term momentary over voltage is used as a synonym for the term swell. Examples include system faults, switching off heavy loads, energizing a large capacitor bank, etc. 2.1.5 Voltage and Current Imbalance Unbalance, or three-phase unbalance, is the phenomenon in a three-phase system, in which the rms values of the voltages or the phase angles between consecutive phases are not equal. Examples include unbalanced load, large singlephase load, blown fuse in one phase of a three-phase capacitor bank, etc. 2.1.6 Voltage Fluctuation The fast variation in voltage magnitude is . Sometimes the term

light

is also used. This voltage magnitude

ranges from 0.9 to 1.1 pu of nominal. One example is an arc furnace.

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2.1.7 Power Frequency Variations Power frequency variations are defined as deviation of the power system fundamental frequency from its specified nominal value (e.g. 50 or 60Hz). This frequency is directly related to the rotational speed of the generators supplying the system. There are slight variations in frequency as the dynamic balance between load and generation changes. The size of the frequency shift and its duration depends on the load characteristics and the response of the generation control system to load changes. Examples include faults on transmission system, disconnection of large load, disconnection of large generator, etc. 2.1.8 Waveform Distortion Waveform distortion is defined as a steady-state deviation from an ideal sine wave of power frequency principally characterized by the spectral content of the deviation. Three types of waveform distortion are listed below: Harmonics: These are steady-state sinusoidal voltages or currents having frequencies that are integer multiples of the fundamental frequency. Harmonic distortion originates in the nonlinear characteristics of devices and loads on the power system. Examples include computers; fax machines, UPS systems, variable frequency drives (VFDs), etc. Inter harmonics: These are voltages and currents having frequency components which are not integer multiples of the fundamental frequency. Examples include static frequency converters, cyclo-converters, induction motors and arcing devices. Noise: This is unwanted electrical signals with broadband spectral content lower than 200 kHz superimposed on system voltage or current in phase conductors, or found on neutral conductors or signal lines. Examples include power electronics applications, control circuits, solid-state rectifiers, switching power supplies, etc. 2.1.9 Causes of Power Quality Variations The main causes of poor power quality come from the customers themselves (internal), generated from one customer that may impact other customers (neighbours), and also from the utility. Neighbours here include those in separate buildings near the

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customer and separate businesses under the same roof such as a small business park. The types and causes of power quality variations are as follows: Table 2.1.1 Internal Causes of Power Quality Variations Types

Causes Small lightning strikes at low voltage levels (e.g.500V) can

Transient

disrupt or damage electronic equipment. Reactive loads turning on and off generate spikes. Poor connections in the wiring system lead to arcing-caused transients. Switching of power electronics devices.

Long-duration

Over- and under-voltages are caused by load variations on the

voltage variations

system. Overloaded circuits results in under voltages. Sustained interruptions are caused by lightning strikes.

Short-duration

Sags and swells occurs whenever there is a sudden change in

voltage variations

the load current or voltage. Sags result when a load turns on suddenly (e.g. starting of large motors). Sags do not directly cause damage but initiate problems indirectly. Swells caused by the sudden turning off of loads can easily damage user equipment.

Current distortion affects the power system and distribution equipment. Overheating and failure in transformer and high neutral currents are some direct problems. Current harmonics may excite resonant frequencies in the system, which can cause Waveform

extremely high harmonic voltages to damage equipment.

distortions

Nonlinear loads (e.g. Variable frequency drives, induction motors, and power electronics components) cause voltage

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distortions, which can cause motor to overheat and vibrate excessively, resulting in damage to the shaft of motors. Components in computers may also be damaged. Electrical noise indirectly causes damage and loss of product Process control equipment and telecommunications are sensitive to such noise. Wiring& grounding

Inappropriate or poor wiring and grounding can affect the operation and reliability of sensitive loads and local area networks.

Table 2.1.2 Neighbouring Causes of Power Quality Variations Types

Causes and effects Transients are generated from the switching of loads. In situations where multiple, separate businesses share wiring or other parts of the power system, arcing-based transients are possible. Reactive loads,

Transient

regardless of light or heavy motors, generate spikes. Changing currents interact with the system impedance. Loads in the Long

/

facility must be large and changing enough to affect the is

Short duration

present, then even simple devices may cause similar concerns.

voltage

Overloading may be the cause as well.

variations neighbours draw large amount of distorted current, Waveform

this current will subsequently distort the utility supply voltage, which

distortion business are subjected to potential problems.

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Table 2.1.3 Utility Causes of Power Quality Variations Types

Causes The most common causes of transients come from lightning surges.

Transient

Other causes include capacitor bank energization, transformer energization, system faults These voltage variations are the result of load switching (e.g.

Long-duration

switching on/off a large load, or on/off a capacitor bank). Incorrect

voltage

tap settings on transformers can also cause system over voltages.

variations

Overloaded circuits can result in under voltages as well. These variations are caused by fault conditions, energization of large

Short-duration

loads that require high starting currents, or intermittent loose

voltage

connections in power wiring. Delayed reclosing of protective devices

variations

may cause momentary or temporary interruptions.

Voltage

and Primary source of voltage unbalance is unbalanced load (thus current

current

unbalance). This is due to an uneven spread of single-phase, low

imbalance

voltage customers over the three phases, but more commonly due to a large single-phase load. Three-phase unbalance can also result because of capacitor bank anomalies, such as a blown fuse in one phase of a three-phase bank.

Power

The frequency of the supply voltage is not constant. This frequency

frequency

variation is due to unbalance between load and generation. Short

variation

circuits also contribute to this variation. The amount of harmonic distortion originating from the power system is normally small. The increasing use of power electronics for control of power flow and voltage (flexible ac transmission systems or FACTS) carries the risk of increasing the amount of harmonic

Waveform

distortion originating in the power system.

distortion

Harmonic current distortion requires over-rating of series components like transformers and cables Inter

harmonics

can

excite

unexpected

resonance

between

transformer inductances and capacitor banks. More dangerous are sub-harmonic currents, which can lead to saturation of transformers and damage to synchronous generators and turbines. 13

2.2.0. Power electronic applications in power transmission system The rapid development of power electronics technology provides exciting opportunities to develop new power system equipment for better utilization of existing systems. Since 1990, a number of devices under the term FACTS (flexible AC transmission systems) technology have been proposed and implemented. FACTS devices can be effectively used for power flow control, load sharing among parallel corridors, voltage regulation, and enhancement of transient stability and mitigation of system oscillations. By giving additional flexibility, FACTS controllers can enable a line to carry power close to its thermal rating. Mechanical switching has to be supplemented by rapid response power electronics. It may be noted that FACTS is enabling technology, and not a one-on-one substitute for mechanical switches. FACTS employ high speed Thyristor for switching in or out transmission line components such as capacitors, reactors or phase shifting transformers for desirable performance of systems. The FACTS technology is not a single high power controller, but rather a collection of controllers, which can be applied individually or in coordination with others to control one or more of system parameters. it started with the high voltage DC current (HVDC) transmission, static VAR compensator (SVC) systems were employed later for the reactive power compensation of power transmission lines . Subsequently, devices like thyristor controlled series compensator (TCSC), static compensator (STATCOM), static synchronous series compensator (SSSC), unified power flow controller (UPFC) were proposed and installed under the generic name of flexible AC transmission systems (FACTS) controllers. 2.2.1 PRINCIPLE AND OPERATION OF CONVERTERS: The switching converter forms the heart of the FACTS controllers. Controllable reactive power can be generated by the DC to AC switching converters which are switched in synchronism with the line voltage with which the reactive power is exchanged. A switching power converter consists of an array of solid state switches which connect the input terminals to the output terminals. It has no internal storage and so the instantaneous input and output power are equal. Further the input and output terminations are complementary, that is, if the input is terminated by a voltage source (charged capacitor or battery), output is a current source (which means a voltage source

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having an inductive impedance) and vice versa. Thus, the converter can be voltage sourced (shunted by a capacitor or battery) or current sourced (shunted by an inductor).

Fig:-2.2.1. Operation of converter Single line diagram of basic voltage sourced converter scheme for reactive power generation is shown in fig.2.2.1 for reactive power flow bus voltage V and converter terminal voltage V0 are in phase. Then on per phase basis I= V- V0 / X The reactive power exchange is Q = VI = V (V- V0 ) / X The switching circuit is capable of adjusting V 0 , the output voltage of the converter. For V0 < V, I lags V and Q drawn from the bus is inductive, while for V 0 >V, I leads V and Q drawn from the bus is leading. Reactive power drawn can be easily and smoothly varied by adjusting V0 by changing the on time of the solid state solid state switches. It is to be noted that the transformer leakage reactance is quite small, which means that a small difference in of voltage (V- V0) causes the required I and Q flow. Thus the converter acts as the static synchronous condenser or VAR generator. As the converter draws only reactive power, the real power drawn from the capacitor is zero. Also at DC voltage level for the converter.

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2.3 FACTS CONTROLLERS: The development of FACTS controllers has followed two different approaches. The first approach employs reactive impedances or a tap changing transformer with thyristor switches as the controlled elements, the second approach employs self commutated static converters as voltage sources. In general these are three categories. in series with the power system (series compensation) in shunt with the power system (shunt compensation) both in series and in shunt with the power system

2.3.1. Series compensation In series compensation, FACTS is connected in series with the power system. It works as a controllable voltage source. In series compensation generally inductors are connected in series with the transmission line that is because in case of long transmission line due to series inductance when a large current flows through it causes a large voltage drop. Now to compensate that large voltage drop due to inductance, series capacitances are connected. All series controllers inject voltage in series with the line. If the voltage is in phase quadrature with the line, series controller only supplies or consumes variable reactive power. Any other phase relationship will involve real power also. Tasks of dynamic series compensation: Reduction of load dependent voltage drops Reduction of system transfer impedance Reduction of transmission angle Increase of system stability Load flow control for specified power paths Damping of active power oscillations Static synchronous series compensation (SSSC): Series compensation can also be built up by the use of STATCOM converter technology. Similar valve configurations are used. Above figure shows the connection principle of an SSSC. A series voltage formed by the DC storage capacitor and the converter configuration will be introduced to the system in quadrature to the line 16

current. Capacitive as well inductive compensation is possible. Such SSSC configurations are also used in the Unified Power Flow Controller (UPFC, described later) as series part of the whole device. Two or more of the SSSC can be installed in a system in parallel lines or at major substations with several lines leaving to different areas. Such arrangement allows power flow control under severe system conditions.

Fig.2.2.2.Static synchronous series compensator (SSSC) Thyristor-Controlled Series Capacitor (TCSC) The two basic schemes of thyristor-controlled series capacitors, using thyristorswitched capacitors and a fixed capacitor in parallel with a thyristor-controlled Reactor, are shown schematically in Fig- 2.2.3 a and b. In the thyristor-switched capacitor scheme of Figure 2.2.3 a, the degree of series compensation is controlled by Increasing or decreasing the number of capacitor banks in series. To accomplish this, each capacitor bank is inserted or bypassed by a thyristor valve (switch). To minimize valves is coordinated with voltage and current zero crossings. In the fixed-capacitor, thyristor-controlled reactor scheme of Figure 2.2.3 b, the degree of series compensation in the capacitive operating region (the admittance of the TCR is kept below that of the parallel connected capacitor) is increased (or decreased) by increasing (or decreasing) the thyristor conduction period, and thereby the current in the TCR. Minimum series compensation is reached when the TCR is off. The TCR may be designed to have the capability to limit the voltage across the capacitor during faults and other system contingencies of similar effect. The two schemes may be combined by connecting a number of TCRs plus a fixed capacitor in series in order to achieve greater control range and flexibility. 17

(a)

(b) Fig.2.2.3. (a) TCSC with thyristor switched capacitance (b) TCSC with fixed capacitor

2.3.2. Shunt compensation: This may be variable impedance, variable source or combination of these. All shunt controllers inject current into the system at the point of connection. Combined seriesseries controllers can be combination of separate series controllers which are controlled in a coordinated manner. Combined series and shunt controllers either controlled in coordinated manner as in fig. or a unified power flow controller with series and shunt elements as in fig. for a unified controller there can be real power exchange between the series and shunt controllers via dc power link. Tasks of dynamic shunt compensation: Steady state and dynamic voltage control Reactive power control of dynamic loads Damping of active power oscillations Improvement of system stability

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Examples of shunt compensation: STATIC VAR COMPE NSATOR (SVC): SHUNT-connected static var compensators (SVCs) are used extensively to control the AC voltage in transmission networks. Power electronic equipment, such as the thyristor controlled reactor (TCR) and the thyristor switched capacitor (TSC) have gained a significant market, primarily because of well-proven robustness to supply dynamic reactive power with fast response time and with low maintenance. With the advent of high power gate turngeneration of power

electronic equipment, STATCOM, shows great promise for

application in power systems .Installation of a large number of SVCs and experience gained from recent STATCOM projects throughout the world motivates us to clarify certain aspects of these devices.

Fig.2.3.1 Static var compensator Fig.2.3.1 shows a schematic diagram of a static var compensator. The compensator normally includes a thyristor controlled reactor (TCR), thyristor-switched capacitors (TSCs) and harmonic filters. It might also include mechanically switched shunt capacitors (MSCs), and then the term static var system is used. The harmonic filters (for the TCR-produced harmonics) are capacitive at fundamental frequency. The TCR is typically larger than the TSC blocks so that continuous control is realized. Other possibilities are fixed capacitors (FCs), and thyristor switched reactors (TSRs). Usually a dedicated transformer is used, with the compensator equipment at medium voltage.

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The transmission side voltage is controlled, and the Mvar ratings are referred to the transmission side. TYPICAL CONFIGURATIONS IN SVC: The SVC typically consists of a TCR (Thyristor Controlled Reactor), a TSC (Thyristor Switched Capacitor) and fixed capacitors (FC) in a harmonic filter arrangement as shown in Figure 2.3.2. The TCR consists of reactors and thyristor valves. The TCR continuously controls reactive power by varying the current amplitude flowing through the reactors. The TSC consists of capacitors, reactors and thyristor valves. The TSC switches on and off the capacitors. The AC filters provide fixed reactive power and absorb the harmonic current generated by the TCR. The TCR+FC is the most basic configuration of the SVC. The TCR+TSC+FC, the more advanced configuration, can be tuned to minimize the losses at the most frequent operation point.

Fig.2.3.2. Typical configuration of SVC TCR (Thyristor Controlled Reactor) The amplitude of the TCR current can be changed continuously by varying the thyristor firing angle (Figure 2.3.3). The firing angle can be varied from 90 degrees to 180 degrees. The TCR firing angle can be fully changed within one cycle of the fundamental frequency, thus providing smooth and fast control of reactive power supply

to

the 20

system

Fig.2.3.3. TCR curren t and firing angle TSC (Thyristor Switched Capacitor): The TSC is used to switch on and off the capacitor bank. The TSC does not generate any harmonic current components. The capacitor switching operation is completed within one cycle of the fundamental frequency. The TSC provides a faster and more reliable solution to capacitor switching than conventional mechanical switching devices. The TSC can operate in coordination with the TCR so that the sum of the reactive power from the TSC and the TCR becomes linear. Applications with only TSC's are also available, providing stepwise control of capacitive reactive power.

Fig.2.3.4 TCS current and firing angle

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Unified power flow conditioner (UPFC): The Unified Power Flow Controller (UPFC) is the first member of an emerging family of advanced Flexible AC Transmission System (FACTS) controllers that use multiple Synchronous voltage sources (SVS) operated conjunctively to optimize the use of electric power transmission networks. Each such SVS is typically an electronic voltage-sourced inverter that can be shunt-connected (STATCOM) or series-connected (SSSC) to the power network. A STATCOM or an SSSC can operate on its own, using the inherent ability to generate or absorb reactive power at its ac terminals. These devices are, however, unable to negotiate real power from the network unless they are equipped with an additional source or sink of real power at their dc terminals. This leads to the concept of joining multiple STATCOMs and/or SSSCs together at their dc terminals. The joined units are thus free to negotiate real power at their ac terminals, subject only to the constraint that the total average power at the dc bus must be zero.

Fig.2.3.5. Unified power flow controller Dynamic voltage restorer (DVR): The DVR mitigates voltage sags by injecting a compensating voltage into the power system in synchronous real time. The DVR is a high-speed switching power electronic converter that consists of an energy storage system that feeds three independent singlephase pulse width modulated (PWM) inverters. As shown in Fig.2.4.1, the energy storage system for the DVR is a dc capacitor bank, which is interfaced to the PWM inverters by using a boost converter (dc to dc). The boost converter regulates the voltage across the dc link capacitor that serves as a common voltage source for the PWM inverters. The three voltage source single-phase PWM inverters (dc to ac) synthesize the ap control system. This compensating voltage waveform is injected into the power system 22

through three single-phase series injection transformers. The DVR control system compares the input voltage to an adaptive reference signal and injects voltage so that the output voltage remains within specifications (e.g.,1.0 per unit).

Fig.2.3.6 dynamic voltage regulator (DVR) Under normal operating conditions (no sag), the DVR injects only a small voltage to compensate for the series reactance of the injection transformers and device losses. During sag, the DVR control system calculates and synthesizes the voltage required to maintain the output voltage and injects this voltage in synchronous real time. 2.4.1. STATCOM In 1999 the first SVC with Voltage Source Converter called STATCOM (static compensator) went into operation. The STATCOM has a characteristic similar to the synchronous condenser, but as an electronic device it has no inertia and is superior to the synchronous condenser in several ways, such as better dynamics, a lower investment cost and lower operating and maintenance costs. A STATCOM is build with Thyristors with turn-off capability like GTO or today IGCT or with more and more IGBTs. The static line between the current limitations has a certain steepness determining the control characteristic for the voltage. The advantage of a STATCOM is that the reactive power provision is independent from the actual voltage on the connection point. This can be seen in the diagram for the maximum currents being independent of the voltage in comparison to 23

the SVC. This means, that even during most severe contingencies, the STATCOM keeps its full capability. In the distributed energy sector the usage of Voltage Source Converters for grid interconnection is common practice today. The next step in STATCOM development is the combination with energy storages on the DC-side. The performance for power quality and balanced network operation can be improved much more with the combination of active and reactive power.

2.4.1 STATCOM structure and voltage / current characteristic STATCOMs are based on Voltage Sourced Converter (VSC) topology and utilize either Gate-Turn-off Thyristors (GTO) or Isolated Gate Bipolar Transistors (IGBT) devices. The STATCOM is a very fast acting, electronic equivalent of a synchronous condenser. If the STATCOM voltage, Vs, (which is proportional to the dc bus voltage Vc) is larger than bus voltage, Es, then leading or capacitive VARS are produced. If Vs is smaller then Es then lagging or inductive VARS are produced.

Fig 2.4.1.1 6 Pulses STATCOM 24

The three phases STATCOM makes use of the fact that on a three phase, fundamental frequency, steady state basis, and the instantaneous power entering a purely reactive device must be zero. The reactive power in each phase is supplied by circulating the instantaneous real power between the phases. This is achieved by firing the GTO/diode switches in a manner that maintains the phase difference between the ac bus voltage ES and the STATCOM generated voltage VS. Ideally it is possible to construct a device based on circulating instantaneous power which has no energy storage device (ie no dc capacitor). A practical STATCOM requires some amount of energy storage to accommodate harmonic power and ac system unbalances, when the instantaneous real power is non-zero. The maximum energy storage required for the STATCOM is much less than for a TCR/TSC type of SVC compensator of comparable rating.

2.4.1.2. STATCOM Equivalent Circuit Several different control techniques can be used for the firing control of the STATCOM. Fundamental switching of the GTO/diode once per cycle can be used. This approach will minimize switching losses, but will generally utilize more complex transformer topologies. As an alternative, Pulse Width Modulated (PWM) techniques, which turn on and off the GTO or IGBT switch more than once per cycle, can be used. This approach allows for simpler transformer topologies at the expense of higher switching losses. The 6 Pulse STATCOM using fundamental switching will of course produce . There are a Variety of methods to decrease the harmonics. These methods include the basic 12 pulse configuration with parallel star / delta transformer connections, a complete elimination of 5th and 7th harmonic current using series

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connection of star/star and star/delta transformers and a quasi 12 pulse method with a single star-star transformer, and two secondary windings, using control of firing angle to produce a 300 phase shift between the two 6 pulse bridges. This method can be extended to produce a 24 pulse and a 48 pulse STATCOM, thus eliminating harmonics even further. Another possible approach for harmonic cancellation is a multi-level configuration which allows for more than one switching element per level and therefore more than one switching in each bridge arm. The ac voltage derived has a staircase effect, dependent on the number of levels. This staircase voltage can be controlled to eliminate harmonics.

2.4.2. REAL AND REACTIVE POWER CONTROL:Basic operating principle of a SATCOM is similar to that of synchronous machine. The synchronous machine will provide lagging current when under excited and leading current when over excited. STATCOM can generate and absorb reactive power similar to that of synchronous machine and it can also exchange real power if provided with an external device DC source. 1)

Exchange of reactive power:- If the output voltage of the voltage source converter

is greater than the system voltage then the SATCOM will act as capacitor and generate reactive power(i.e.. provide lagging current to the system) 2)

Exchange of real power: -

As the switching devices are not loss less there is a

need for the DC capacitor to provide the required real power to the switches. For long duration of real power requirement even after the primary supply failed back up energy storage system (BESS) is used. Hence there is a need for real power exchange with an AC system to make the capacitor voltage constant in case of direct voltage control. There is also a real power exchange with the AC system if STATCOM is provided with an external DC source to regulate the voltage in case of very low voltage in the distribution system or in case of faults. And if the VSC output voltage leads the system voltage then the real power from the capacitor or the DC source will be supplied to the AC system to regulate the system voltage to the =1p.u or to make the capacitor voltage constant.

26

Hence the exchange of real power and reactive power of the voltage source converter with AC system is the major required phenomenon for the regulation in the transmission as well as in the distribution system.

2.4.3 BASIC OPERATING PRINCIPLES OF STATCOM: The STATCOM is connected to the power system at a PCC (point of common coupling), through a step-up coupling transformer, where the voltage-quality problem is a concern. The PCC is also known as the terminal for which the terminal voltage is UT. All required voltages and currents are measured and are fed into the controller to be compared with the commands. The controller then performs feedback control and outputs a set of switching signals (firing angle) to drive the main semiconductor switches of the power converter accordingly to either increase the voltage or to decrease it accordingly. A STATCOM is a controlled reactive-power source. It provides voltage support by generating or absorbing reactive power at the point of common coupling without the need of large external reactors or capacitor banks. Using the controller, the VSC and the coupling transformer, the STATCOM operation is illustrated in Figure below.

Fig 2.5 . STATCOM operation in a power system The charged capacitor Cdc provides a DC voltage, Udc to the converter, which produces a set of controllable three-phase output voltages, U in synchronism with the 27

AC system. The synchronism of the three-phase output voltage with the transmission line voltage has to be performed by an external controller. The amount of desired voltage across STATCOM, which is the voltage reference, Uref, is set manually to the controller. The voltage control is thereby to match U T with Uref which has been elaborated. This matching of voltages is done by Varying the amplitude of the output voltage U, which is done by the firing angle set by the controller. The controller thus sets UT equivalent to the Uref. The reactive power exchange between the converter and the AC system can also be controlled. This reactive power exchange is the reactive current injected by the STATCOM, which is the current from the capacitor produced by absorbing real power from the AC system.

Where, Iq is the reactive current injected by the STATCOM UT is the STATCOM terminal voltage Ueq is the equivalent Thevinen voltage seen by the STATCOM Xeq is the equivalent Thevinen reactance of the power system seen by the STATCOM If the amplitude of the output voltage U is increased above that of the AC system voltage, UT, a leading current is produced, i.e. the STATCOM is seen as a conductor by the AC system and reactive power is generated. Decreasing the amplitude of the output voltage below that of the AC system, a lagging current results and the STATCOM is seen as an inductor. In this case reactive power is absorbed. If the amplitudes are equal no power exchange takes place. A practical converter is not lossless. In the case of the DC capacitor, the energy stored in this capacitor would be consumed by the internal losses of the converter. By making the output voltages of the converter lag the AC system voltages by a small balance the losses in the converter. The diagram in Figure below illustrates the phasor diagrams of the voltage at the terminal, the converter output current and voltage in all four quadrants of the PQ plane.

28

Fig 2.6 Phasor diagrams for STATCOM applications reactive power generation or absorption by increasing or decreasing the capacitor voltage Udc, with reference with the output voltage U. Instead of a capacitor a battery can also be used as DC energy. In this case the converter can control both reactive and active power exchange with the AC system. The capability of controlling active as well as reactive power exchange is a significant feature which can be used effectively in applications requiring power oscillation damping, to level peak power demand, and to provide uninterrupted power for critical load.

2.4.4 CHARACTERISTICS OF STATCOM: The derivation of the formula for the transmitted active power employs considerable calculations. Using the Variables defined in Figure below and applying Kirchoffs laws the following equations can be written;

Fig 2.7 .Two machine system with STATCOM 29

By equaling right-hand terms of the above formulas, a formula for the current I1 is obtained as

Where UR is the STATCOM terminal voltage if the STATCOM is out of operation, i.e. when Iq = 0. The fact that Iq

R can

be used to express Iq

as

Applying the sine law to the diagram in Figure below the following two equations result

The formula for the transmitted active power can be given as

30

To dispose of the term UR the cosine law is applied to the diagram in Figure above Therefore,

Fig 2.8 Transmitted power versus transmission angle characteristic of a STATCOM With these concepts of STATCOM, it is thus important to utilize these principles in accommodating shunt compensation to any system. Since this thesis only reflects on the voltage control and power increase, the requirements of the STATCOM would be further elaborated.

2.4.5 FUNCTIONAL REQUIREMENTS OF STATCOM: The main functional requirements of the STATCOM in this thesis are to provide shunt compensation, operating in capacitive mode only, in terms of the following; transmission. This compensation of voltage has to be in synchronism with the AC system regardless of disturbances or change of load.

reactive power flow under heavy loads and for preventing voltage instability STATCOM into the system The design phase and implementation phase (as presented in the next chapter) would refer to the theoretical background of STATCOM in providing the requirements 31

CHAPTER

3

DISTRIBUTION GENERATION 3.1.1 .DISTRIBUTED GENERATION The centralized and regulated electric utilities have always been the major source of electric power production and supply. However, the increase in demand for electric power has led to the development of distributed generation (DG) which can complement the central power by providing additional capacity to the users. These a r e s m a l l generating units which can be located at the consumer end or anywhere within the distribution system. DG can be beneficial to the consumers as well as the utility. Consumers are interested in DG due to the various benefits associated with it: cost saving during peak demand charges, higher power quality and increased energy efficiency. The utilities can also benefit as it generally eliminates the cost needed for laying new transmission/distribution lines. Distributed generation employs alternate resources such as micro-turbines, solar photovoltaic systems, fuel cells and wind energy systems. This thesis lays emphasis on the fuel cell technology and its integration with the utility grid.

3.1.2 DISTRIBUTED GENERATION SYSTEMS BACKGROUND Today, environmental

new

advances

regulations

in

power

encourage

a

generation

technologies

significant

increase

of

and

new

distributed

generation resources around the world. Distributed generation systems (DGS) have mainly been used as a standby power source for critical businesses. For example, most hospitals and office buildings had stand-by diesel generators as an emergency power source for use only during outages. However, the diesel generators were not inherently cost-effective, and produce noise and exhaust that would be objectionable on anything except for an emergency basis. On the other hand, environmental-friendly distributed generation systems such as fuel cells, micro turbines, biomass, wind turbines, hydro turbines or photovoltaic arrays can be a solution

to

meet

both

the

increasing

demand

of

environmental regulations due to green house gas emission.

32

electric

power

and

As illustrated in these figures, the currently competitive DGS units will be constructed on a conventional distribution network, instead of large central power plants because the DGS can offer improved service reliability, better economics and a reduced dependence on the local utility

Figure 3.1 A large central power plant and distributed generation systems Recently, the use of distributed generation systems under the 500 kW level is rapidly

increasing

due

to

technology

improvements

in

small generators,

power electronics, and energy storage devices. Efficient clean fossil-fuels technologies such as micro-turbines, fuel cells, and environmental-friendly renewable energy technologies such as biomass, solar/photovoltaic arrays, small wind turbines and hydro turbines, are growingly used for new distributed generation systems. These DGS are applied to a standalone , a grid-interconnected, a standby, peak shavings , a cogeneration etc. and have a lot of benefits such as environmental-friendly and modular electric generation, increased reliability/stability, high power quality, load management, fuel flexibility, uninterruptible service, cost savings, on-site generation, expandability, etc.

33

3.2 Benefits In the last decade, the concept of many small scale energy sources dispersed over the grid gain a considerable interest. Most of all, technological innovations and a changing economic and regulatory environment were that main triggers for this interest. International Energy Agency IEA lists five major factors that contribute to this evolution, such as developments in distributed generation technologies, constraints on the construction of new transmission lines, increased customer demand for highly reliable electricity, the electricity market liberalization and concerns about climate change. Especially the last two points seem to offer the most significant benefits, as it is unlikely that distributed generation would be capable of avoiding the development of new transmission lines. At minimum, the grid has to be available as backup supply. In the liberalized market environment, the distributed generation offers a number of benefits to the market participants. As a rule, customers look for the electricity services best suited for them. Different customers attach different weights to features of electrical energy supply, and distributed generation technologies can help electricity suppliers to supply the type of electricity service they prefer. One of the most interesting features is the flexibility of DG that could allow market participants to respond to changing market conditions, i.e. due to their small sizes and the short construction lead times compared to most types of larger central power plants.

3.3. Flexibility in price response Important aspects of the abovementioned flexibility of distributed generation technologies are operation, size and expandability. Flexible reaction to electrical energy price evolutions can be one of the examples, allowing a DG to serve as a hedge against these price fluctuations. Apparently, using distributed generation for continuous use or for peak shaving is the major driver for the US demand for distributed generation. In Europe, market demand for distributed generation is driven by heat applications, the introduction of renewable and by potential efficiency improvements. 1) classify in view of this definition. 2) The value of their flexibility is probably understated when economic assessments of distributed generation are made. Recent work based on option value theory suggests that flexible power plants. 34

3.4. Flexibility in reliability needs Reliability considerations of the second major driver of US demand for distributed generation is quality of supply or reliability considerations. Reliability problems refer to sustained interruptions in electrical energy supply (outages). The liberalization of energy markets makes customers more aware of the value of reliable electricity supply. In many European countries, the reliability level has been very high, mainly because of high engineering standards. High reliability level implies high investment and maintenance costs for the network and generation infrastructure. Due to the incentives for cost effectiveness that come from the introduction of competition in generation and from the re-regulation of the network companies, it might be that reliability levels will decrease. However, for some industries, such as chemical, petroleum, refining, paper, metal, telecommunication, a reliable power supply is very important. Such companies may find the reliability of the grid supplied electricity too low and thus be willing to invest in distributed generation units in order to increase their overall reliability of supply. The IEA recognizes the provision of reliable power as the most important future market niche for distributed generation. Fuel cells and backup systems combined with an UPS (Uninterruptible Power Supply) are identified as the technologies that could provide protection against power interruptions, though it has to be noted that the fuel cell technology is currently not easily commercially available.

3.5. Flexibility in power quality needs Apart from large voltage drops to near zero (reliability problems), one can also have smaller voltage deviations. The latter deviations are aspects of power quality. Power quality refers to the degree to which power characteristics align with the ideal sinusoidal voltage and current waveform, with current and voltage in balance. Thus, strictly speaking, power quality encompasses reliability. Insufficient power quality can be caused by failures and switching operations in the network (voltage dips and transients) and by network disturbances from loads (flickers, harmonics and phase imbalance).

35

3.6. Environmental friendliness Environmental policies or concerns are probably the major driving force for the demand for distributed generation in Europe. Environmental regulations force players in the electricity market to look for cleaner energy- and cost-efficient solutions. Many of the distributed generation technologies are recognized environmentally friendly. Combined Heat and Power (CHP) technology, allowing for portfolio optimization of companies needing both heat and electrical energy, is one of the examples. Compared to separate fossil-fired generation of heat and electricity, CHP generation may result in a primary energy conservation, varying from 10% to 30%, depending on the size (and efficiency) of the cogeneration units. Furthermore, as renewable energy sources are by nature small-scale and dispersed over the grid. Installing distributed generation allows the exploitation of cheap fuel opportunities. For example, DG units could burn landfill gasses in the proximity of landfills, or other locally available biomass resources. Most government policies that aim to promote the use of renewable will also result in an increased impact of distributed generation.

3.7. Impacts on power quality The installation and connection of distributed generation units can positively affect the power quality. However, a converse effect could also be noted. DG units are likely to affect the system frequency. As they are often not equipped with a load-frequency control, they will free ride on the efforts of the transmission grid operator or the regulatory body to maintain system frequency. Therefore, connecting a large number of DG units to the grid should be carefully evaluated and planned. Moreover, the impact on the local voltage level of distributed generation connected to the distribution grid can be significant. Especially raising voltage levels in radial for Some times the power injections need to be corrected for the transmission losses, meaning that an ARP should inject some 3¸4% more than it withdraws.

36

3.8. Connection issues It can be taken as given that the electric power flows from the higher voltage grid to the lower voltage grid. Increased share of distributed generation units may lead to inducing power flows from the low voltage into the medium-voltage grid. This bidirectional power flows asks for different protection schemes at both voltage levels. Moreover, the added flexibility of DG asks for extra efforts on the grid operation side. As

e during an outage, they

should also meet the requirements for such operation mode. Next to guarantying no power supplied to the grid, they must be able to provide the auxiliary services needed. Moreover, once the distribution grid is back into operation, the DG unit must be able to be re-synchronized.

37

CHAPTER

4

WIND POWER GENERATION 4.1. INTRODUCTION: Wind power is the conversion of wind energy into useful form, such as electricity, using wind turbines. In windmills, wind energy is directly used to crush grain or to pump water. Wind energy is plentiful, renewable, widely distributed, clean, and reduces greenhouse gas emissions when it displaces fossil-fuel-derived electricity. The intermittency of wind seldom creates insurmountable problems when using wind power to supply a low proportion of total demand, but it presents extra costs when wind is to be used for a large fraction of demand. 4.1.1Wind Turbine Types: Modern wind turbines fall into two basic groups; the horizontal-axis variety, like the traditional farm windmills used for pumping water, and the vertical-axis design, like the eggbeater-style Darrieus model, named after its French inventor. Most large modern wind turbines are horizontal-axis turbines. Turbine Components Horizontal turbine components include: 1) Blade or rotor, which converts the energy in the wind to rotational shaft energy; 2) A drive train, usually including a gearbox and a generator; 3) A tower that supports the rotor and drive train; 4) And other equipment, including controls, electrical cables, ground support equipment, and interconnection equipment. Wind turbines are often grouped together into a single wind power plant, also known as a wind farm, and generate bulk electrical power. Electricity from these turbines is fed into a utility grid and distributed to customers, just as with conventional power plants.

38

FIGURE.4.1 Wind turbine 4.1.2. Wind Turbine Systems: Wind turbines can operate with either fixed speed (actually within a speed range about 1 %) or variable speed. For fixed-speed wind turbines, the generator (induction generator) is directly connected to the grid. Since the speed is almost fixed to the grid frequency, and most certainly not controllable, it is not possible to store the turbulence of the wind in form of rotational energy. Therefore, for a fixed-speed system the turbulence of the wind will result in power variations, and thus affect the power quality of the grid. For a variable-speed wind turbine the generator is controlled by power electronic equipment, which makes it possible to control the rotor speed. In this way the power fluctuations caused by wind variations can be more or less absorbed by changing the rotor speed and thus power variations originating from the wind conversion and the drive train can be reduced. Hence, the power quality impact caused by the wind turbine can be improved compared to a fixed-speed turbine. The rotational speed of a wind turbine is fairly low and must therefore be adjusted to the electrical frequency. This can be done in two ways: with a gearbox or with the number of pole pairs of the generator. The number of pole pairs sets the mechanical speed of the generator with respect to the electrical frequency and the gearbox adjusts the rotor speed of the turbine to the mechanical speed of the generator.

39

FIGURE.4.2 In this section the following wind turbine systems will be presented: 1. Fixed-speed wind turbine with an induction generator. 2. Variable-speed wind turbine equipped with a cage-bar induction generator or synchronous generator. 3. Variable-speed wind turbine equipped with multiple-pole synchronous generator or multiple-pole permanent-magnet synchronous generator. 4. Variable-speed wind turbine equipped with a doubly-fed induction generator. 4.2.0. Fixed-Speed Wind Turbine For the fixed-speed wind turbine the induction generator is directly connected to the electrical grid according to Fig.4.3 The rotor speed of the fixed-speed wind turbine is in principle FIGURE 4.3

determined by a gearbox and the pole-pair number of the generator. The fixed-speed wind turbine system has often two fixed speeds. This is accomplished by using two 40

generators with different ratings and pole pairs, or it can be a generator with two windings having different ratings and pole pairs. This leads to increased aerodynamic capture as well as reduced magnetizing losses at low wind speeds. This system (one or two1980s and 1990s. 4.2.1. Variable-Speed Wind Turbine FIGURE 4.4

The system presented in Fig. 4.4. consists of a wind turbine equipped with a converter connected to the stator of the generator. The generator could either be a cage-bar induction generator or a synchronous generator. The gearbox is designed so that maximum rotor speed corresponds to rated speed of the generator. Synchronous generators or permanent-magnet synchronous generators can be designed with multiple poles which imply that there is no need for a gearbox, see Fig. 4.4.

-

advantage with this system is its well-developed and robust control. 4.2.2 Variable-Speed Wind Turbine with Doubly-Fed Induction Generator The system, see Fig. 4.5, consists of a wind turbine with doubly-fed induction generator. This means that the stator is directly connected to the grid while the rotor winding is connected via slip rings to a converter. This system has recently become very popular as generators for variable-speed wind turbines. This is mainly due to the fact that the power electronic converter only has to handle a fraction (20 30%) of the total power .Therefore, the losses in the power electronic converter can be reduced, compared to a system where the converter has to handle the total power. In addition, 41

the cost of the converter becomes lower. There exists a variant of the DFIG method that uses controllable external rotor resistances

FIGURE 4.5

(compare to slip power recovery). Some of the drawbacks of this method are that energy is unnecessary dissipated in the external rotor resistances and that it is not possible to control the reactive power. Of all the above mentioned types we are going to use the first type because of its simplicity that is fixed speed wind turbine method with induction generator. Due to the advantages that we considered in real time application and also in simulink designing we adopted the induction generators.

4.3. INDUCTON MACHINE: Induction machines are often

reflects

the reality of the qualities of these machines. They are cheap to manufacture, rugged and reliable and find their way in most possible applications. Variable speed drives require inexpensive power electronics and computer hardware, and allowed induction machines to become more versatile. In particular, vector or field oriented control allows induction motors to replace DC motors in many applications 4.3.0. DESCRIPTION The stator of an induction machine is a typical three phase one. The rotor can be one of two major types. Either of the following :42

a) It is wound in a fashion similar to that of the stator with the terminals led to slip rings on the shaft, as shown in figure , or

FIGURE 4.6

b) It is made with shorted Fig Wound rotor slip rings and connections bars..The picture of the rotor bars is not easy to obtain, since the bars are formed by casting aluminium in the openings of the rotor laminations. In this case the iron laminations were chemically removed. 4.3.1. CONCEPT OF OPERATION As these rotor windings or bars rotate within the magnetic field created by the stator magnetizing currents, voltages are induced in them. If the rotor were to stand still, then the induced voltages would be very similar to those induced in the stator windings. In the case of squirrel cage rotor, the voltage induced in the bars will be slightly out of phase with the voltage in the next one, since the flux linkages will change in it after a short delay. If the rotor is moving at synchronous speed, together with the field, no voltage will be induced in the bars or the windings. FIGURE 4.7

0,

s

frequency of the induced voltages will be fr

r

s

-

0

becomes here: g----------------------------------------

(1)

where v is the relative velocity of the rotor with respect to the field: s

-

0-----------------------------------------------------------

43

(2)

the

Since a voltage is induced in the bars, and these are short circuited, currents will flow

s

-

0

Bg

s

-

0)

Bg

We define as slip s the

s

-

0

s

FIGURE 4.8

0. Above Synchronous speed s < 0, and when the rotor rotates in a direction opposite of the magnetic field 1< s Three-phase motors: Operation principles

upon the speed difference between the rotating stator field and the rotor. The frequency of the rotor current is 50 Hz when the rotor is stationary. starts to rotate the speed difference are reduced, which results in reduction on the frequency of the induced voltage in the rotor. reduced magnitude of rotor current and induced voltage. FIGURE 4.9

Force generation: tion generates the driving force. The force drives the motor

44

current and torque become zero. Therefore the motor speed must be less than the synchronous speed. and the actual rotor speed. The speed difference is called slip (s) and defined as: s = (ns - nr) / ns where ns =120 f / p r

=sf

4.3.2. Development of equivalent circuit: /generator consists of a two magnetically connected systems, Stator and rotor. primary and secondary windings. -phase voltage that drives a three-phase current through the winding. This current induces a voltage in the rotor. 1)

across phase A is equal to the sum of the

induced voltage (E1). Voltage drop across the stator resistance (I1 R1). Voltage drop across the stator leakage reactance (I1 j X1). V1 = E1+ I1 ( R1+ j X1) 1

induced voltage generates a voltage E2 in the rotor through the magnetic

coupling. If the rotor is at stand still, the induced voltage E 2 is proportional to E1 times the turn ratio. T = Nstat / Nrot = N1 /N2. The value is: E2 = E1 (N2 /N1 ) = E1 / T If the rotor is rotating, the voltage induced in the rotor is multiplied by the slip s, because the induced voltage is proportional to the speed difference between the stator field and rotor. E2 = s E1 / T resistance (I2 R2), and the leakage inductance (I2 X2).

45

I2 j wr L2 = I2

r)

L2 = I2

2

= I2 j s (w L2) = I2 j s X2

E2 = I2 (R2 + j s X2 ) V1 = E1+ I1 ( R1+ j X1) E2 = s E1 / T E2 = I2 (R2 + j s X2 ) I2 = I1 (N1/ N2) = I1 T E1 = E2 T / s = T I2 (R2 + j s X2 ) /s = I1 T2 (R2 /s + j X2 ) = I1 [(R2 T2 /s) + j (T2 X2 )] = I1 (R*2 /s) + j X*2 ) where: R*2 = R2 T2 and X*2 = T2 X2 are rotor resistance & reactance referred to stator. V1 = E1+ I1 ( R1+ j X1) E1 = I1 (R2* / s + j X2* ) for the induction motor: V1 = I1 (R2* / s + j X2* ) + I1 ( R1+ j X1) = I1 [( R1 + R2* / s) + j ( X1+ X2*)] V1 = I1 [( R1 + R2* / s) + j ( X1+ X2*)] V1 = I1 [(R1 + R2* / s) + j ( X1+ X2*)]

connected in parallel. The resistance represents the hysteresis and eddy current losses. The reactance represents the magnetizing current that generates the air-gap magnetizing flux. The induction motor/generator equivalent circuit is: FIGURE 4.10

46

4.4. INDUCTION GENERATOR OPERATION Figure 5 shows the speed torque characteristics of an induction motor operating from a constant frequency power source. Most readers are familiar with this characteristic of the Induction motor operation. The operation of the induction motor occurs in a stable manner in the region of the speed torque curve indicated in Figure 5. The torque output as well as the power delivered by the motor varies as the motor speed changes. At synchronous speed no power is delivered at all. The difference between the synchronous speed and the operating speed is called the slip. The output torque and power vary linearly with the slip. If the induction motor is driven to a speed higher than the synchronous speed, the speed torque curve reverses as shown in Figure 6. In the stable region of this curve, electric power is generated utilizing the mechanical input power from the prime mover. Once again the generated power is a function of the slip, and varies with the slip itself

Figure 4.11: Induction Motor Torque v/s Speed

47

Figure 4.12: Induction Generator Torque v/s Speed In the generator mode, if the slip is controlled in accordance with the load requirements, the induction generator will deliver the necessary power. It must be remembered that the synchronous speed is a function of the electrical frequency applied to the generator terminals. On the other hand, the operating shaft speed is determined by the prime mover. Therefore to generate power, the electrical frequency must be adjusted as the changes in the load and the prime mover speed occur. In addition to the requirement stated above, the excitation current must be provided to the generator stator windings for induction into the rotor. The magnitude of the excitation current will determine the voltage at the bus. Thus the excitation current must be regulated at specific levels to obtain a constant bus voltage. The controller for the induction generator has the dual function as follows: i) Adjust the electrical frequency to produce the slip corresponding to the load requirement. ii) Adjust the magnitude of the excitation current to provide the desirable bus voltage. Figure 7 depicts the region of generator mode operation for a typical induction generator. A number of torque speed characteristic curves in the stable region of operation are shown to explain the operation. As an example, consider the situation when the prime mover is at the nominal or 100% speed. The electrical frequency must be adjusted to cater for load changes from 0 to 100% of the load. If a vertical line is 48

drawn along the speed of 100%, it can be observed that the electrical frequency must be changed from 100% at no load to about 95% at full load if the prime mover speed is held at 100%.

Figure 4.13: Induction Generator Torque v/s Speed in Operating Range 4.5. BENEFITS OF INDUCTION GENERATOR TECHNOLOGY Induction generator has several benefits to offer for the micro, mini power systems under consideration. These benefits relate to the generator design as follows: i) Cost of Materials: Use of electromagnets rather than permanent magnets means lower cost of materials for the induction generator. Rare earth permanent magnets are substantially more expensive than the electrical steel used in electromagnets. They also must be contained using additional supporting rings. ii) Cost of Labour the rotor structure by installation of the containment structure. Handling of permanent magnets that are pre-charged is generally difficult in production shops. These requirements increase the cost of labour for the PM generator. iii) Generator Power Quality: The PM generator produces raw ac power with unregulated voltage. Depending upon the changes in load and speed, the voltage variation can be wide. This is all the more true for generators exceeding about 75 kW power rating. The induction generator produces ac voltage that is reasonably sinusoidal as shown in the example from an actual test in Figure 9. This voltage can be rectified easily to produce a constant dc voltage. Additionally, the ac voltage can be stepped up or down using a transformer to provide multiple levels of voltages if required. 49

Figure 4.14: Induction Generator AC Output Voltage Waveform iv) Fault Conditions: When an internal failure occurs in a PM generator, the failed winding will continue to draw energy until the generator is stopped. For high-speed generators, this may mean a long enough duration during which further damage to electrical and mechanical components would occur. It could also mean a safety hazard for the individuals working in the vicinity. The induction generator on the other hand is safely shut down by de-excitation within a few milliseconds, preventing the hazardous situations.

4.6. MATHEMATICS OF WIND POWER The amount of mechanical power captured from wind by a wind turbine can be formulated as: Pm=(1/2) ACpv3

-------------------

(1)

= Air density (Kg/m3) A = Swept area (m2) CP = Power coefficient of the wind turbine V = Wind speed (m/s) Therefore, if the air density, swept area and wind speed are constant the output power of the turbine will be a function of power coefficient of the turbine. In addition, the wind turbine is normally characterized by its CPgiven by: =( R)/

-----------------(2)

In (2), 50

opt,

which results in

optimum efficiency; therefore, maximum power is captured from wind by the turbine. The output power of a wind turbine versus rotor speed while speed of wind is changed from v1 to v3 (v3>v2>v1). They show that if the speed of wind is v1, then the maximum power could be captured when the rotor speed is 1; in other words, the operating point of the system is point A, which corresponds to the maximum output power. If wind speed changes from v1 to v2 while the rotor speed is fixed at 1, the operating point of system is point B, which does not correspond to maximum power tracking. The rotor speed should be increased from,

1 to

2, which results in the maximum power at

operating point C.

Fig 4.15: Power Coefficient vs. Tip-Speed Ratio.

Fig 4.16: power with different TSR Based on (2), the optimum speed of rotor can be estimated as follows:

v=

--------(3)

Unfortunately, measuring the wind speed in the rotor of turbine is very difficult; thus, to avoid using wind speed, (1) needs to be revised. By substituting the wind speed equivalent from (3) into (1), the output power of the turbine is given as:

Finally, the target torque can be written as: Where,

=

51

A

CHAPTER

5

BACK UP ENERGY STORAGE SYSTEM AND NONLINEAR LOAD 5.1 Energy Storage: Electricity is more versatile in use than other types of power, because it is a highly ordered form of energy that can be converted efficiently into other forms. For example, it can be converted into mechanical form with efficiency near 100% or into heat with 100% efficiency. Heat energy, on the other hand, cannot be converted into electricity with such high efficiency, because it is a disordered form of energy in atoms. For this reason, the overall thermal-to-electrical conversion efficiency of a typical fossil thermal power plant is less than 50%. Disadvantage of electricity is that it cannot be easily stored on a large scale. Almost all electric energy used today is consumed as it is generated. This poses no hardship in conventional power plants, in which fuel consumption is continuously intermittent sources of power, cannot meet the load demand at all times, 24 h a day, 365 d a year. The present and future energy storage technologies that may be considered for standalone wind or PV power systems fall into the following broad categories:

5.2 BATTERY: The battery stores energy in an electrochemical form and is the most widely used device for energy storage in a variety of applications. There are two basic types of electrochemical batteries: The primary battery, which converts chemical energy into electric energy. The electrochemical reaction in a primary battery is non-reversible, and the battery is discarded after a full discharge. For this reason, it finds applications where a high energy density for one-time use is required. 52

The secondary battery, which is also known as the rechargeable battery. The electrochemical reaction in the secondary battery is reversible. After a discharge, it can be recharged by injecting a direct current from an external source. This type of battery converts chemical energy into electric energy The internal construction of a typical electrochemical cell is shown in Figure. It has positive and negative electrode plates with insulating separators and a chemical electrolyte in between. The two groups of electrode plates are connected to two external terminals mounted on the casing. The cell stores electrochemical energy at a low electrical potential, typically a few volts. The cell capacity, denoted by C, is measured in ampere-hours (Ah), meaning it can deliver C A for one hour or C/n A for n hours. The battery is made of numerous electrochemical cells connected in a series parallel combination to obtain the desired battery voltage and current. The higher the battery voltage, the higher the number of cells required in series. The battery rating is stated in terms of the average voltage during discharge and the ampere-hour capacity it can deliver before the voltage drops below the specified limit. The product of the voltage and ampere-hour forms the watt-hour (Wh) energy rating the battery can deliver to a load from the fully charged condition. The battery charge and discharge rates are stated in units of its capacity in Ah. For example, charging a 100-Ah battery at C/10 rate means charging at 100/10 = 10 A. Discharging that battery at C/2 rate means drawing 100/2 = 50 A, at which rate the battery will be fully discharged in 2 h. The state of charge (SOC) of the battery at any time is defined as the following: SOC

5.3 TYPES OF BATTERY: There are at least six major rechargeable electro-chemistries available today. They are as follows: -acid (Pb-acid) -cadmium (NiCd) -metal hydride (NiMH) -ion (Li-ion) -polymer (Li-poly) -air

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5.3.1 LEAD-ACID This is the most common type of rechargeable battery used today because of its maturity and high performance-over-cost ratio, even though it has the least energy density by weight and volume. In a Pb-acid battery under discharge, water and lead sulphate are formed, the water dilutes the sulphuric acid electrolyte, and the specific gravity of the electrolyte decreases with the decreasing SOC. Recharging reverses the reaction, in which the lead and lead dioxide are formed at the negative and positive plates, respectively, restoring the battery into its originally charged state. The Pb-acid battery comes in various versions. The shallow-cycle version is used in automobiles, in which a short burst of energy is drawn from the battery to start the engine. The deepcycle version, on the other hand, is suitable for repeated full charge and discharge cycles. Most energy storage applications require deep cycle batteries. The Pb-acid electrolyte into non-spillable gel. The gel-cell battery, therefore, can be mounted sideways or upside down. The high cost, however, limits its use in military avionics. 5.3.2 NICKEL-CADMIUM The NiCd is a matured electrochemistry, in which the positive electrode is made of cadmium and the negative electrode of nickel hydroxide. The two electrodes are separated by Nylon TM separators and placed in potassium hydroxide electrolyte in a stainless steel casing. With a sealed cell and half the weight of the conventional Pbacid, the NiCd battery has been used to power most rechargeable consumer applications. It has a longer deep-cycle life and is more temperature tolerant than the Pb-acid battery. However, this electrochemistry has a memory effect (explained later), which degrades the capacity if not used for a long time. Moreover, cadmium has recently come under environmental regulatory scrutiny. For these reasons, NiCd is being replaced by NiMH and Li-ion batteries in laptop computers and other similar high-priced consumer electronics.

5.3.3 NICKEL-METAL HYDRIDE NiMH is an extension of the NiCd technology and offers an improvement in energy density over that in NiCd. The major construction difference is that the anode is made of a metal hydride. This eliminates the environmental concerns of cadmium. 54

Another performance improvement is that it has a negligible memory effect. NiMH, however, is less capable of delivering high peak power, has a high self-discharge rate, and is susceptible to damage due to overcharging. Compared to NiCd, NiMH is expensive at present, although the price is expected to drop significantly in the future. This expectation is based on current development programs targeted for large-scale application of this technology in electric vehicles.

5.4 EQUIVALENT ELECTRICAL CIRCUIT: For steady-state electrical performance calculations, the battery is represented by an equivalent electrical circuit shown in the figure. In its simplest form, the battery works as a constant voltage source with a small internal resistance. The open-circuit (or electrochemical) voltage Ei of the battery decreases linearly with the Ah discharged , and the internal resistance Ri increases . That is, the battery open-circuit voltage is lower, and the internal resistance is higher in a partially discharged state as compared to the E0 and R0 values in a fully charged state.

Fig 5.1 Equivalent electrical circuit of battery showing internal voltage and resistance. 5.5.0 NON LINEAR LOADS: Applies to those ac loads where the current is not proportional to the voltage. Foremost among loads meeting their definition is gas discharge lighting having saturated ballast coils and thyritor (SCR) controlled loads. The nature of non-linear loads is to generate harmonics in the current waveform. This distortion of the current waveform leads to distortion of the voltage waveform. Under these conditions, the voltage waveform is no longer proportional to the current.

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Non

Linear

Loads

are:

COMPUTER,

LASER

PRINTERS,

SMPS,

RECTIFIER, PLC, ELECTRONIC BALLAST, REFRIGERATOR, TV ETC. 5.5.1 LINEAR LOAD: AC electrical loads where the voltage and current waveforms are sinusoidal. The current at any time is proportional to voltage. Linear Loads are: POWER FACTOR IMPROVEMENT CAPACITORS, INDESCENT LAMPS, HEATERS ETC.

5.5.2 DIFFERENCE BETWEEN LINEAR AND NON LINEAR LOADS

Fig: 5.2 Difference between linear and non linear loads If the system is integrated with non linear loads then the system shall be earthed through conducting material suitable to carry the fault current. The minimum cross section of the earth conductor shall be calculated based on maximum current, which can flow at the time of short circuit/earth fault. though in some cases the wave forms will be distorted in nature.

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5.5.3 DIFFERENCE IN CURRENT WAVEFORMS

Fig : 5.3 Current waveforms of linear and non linear loads

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CHAPTER

6

IMPLEMENTATION IN MATLAB (SIMULINK) 6.1 Introduction to MATLAB The name MATLAB stands for Matrix Laboratory. MATLAB is a software package for high performance numerical computation and visualization. It provides an interactive environment with hundreds of built-in functions for technical computation, graphics and animations. The combination of analysis capabilities, flexibility, reliability and powerful graphics makes MATLAB the premier software package for electrical engineers. best of all, MATLAB provides easy extensibility with its own high level programming language. MATLAB provides an interactive environment with hundreds of reliable and accurate built-in mathematical functions, these built-in functions provide excellent tools for linear algebra computations, data analysis, signal processing, optimization, numerical solution of ODEs, quadrature and man other types of scientific computations. They provide solutions to a broad range of mathematical problems including matrix algebra and complex arithmetic. There are also numerous an external interface to run programs written in FORTAN or C language from MATLAB. TYPICAL USES OF MATLAB 1. Math and computation. 2. Algorithm development. 3. Modelling, simulation and prototyping. 4. Data analysis, exploration and visualization. 5. Scientific and engineering graphics. 6. Application development including graphical user interface. Since the basic data element in MATLAB is an array which does not require dimensioning, this allows us to solve many technical computing problems in a fraction of time it would take to write a program in a scalar non-interactive language such as C or Fortran. THE MATLAB system has five main parts: 1. MATLAB language. 2. MATLAB Working Environment. 58

3. Handle Graphics. 4. MATLAB Mathematical Function Library. 5. MATLAB Application Program Interface (API).

6.2 INTRODUCTION TO SIMPOWER SYSTEMS: SimPowerSystems and other products of the Physical Modeling product family work together with Simulink to model electrical, mechanical, and control systems. SimPowerSystems operates in the Simulink environment. Power systems are combinations of electrical circuits and electromechanical devices like motors and generators. Engineers working in this discipline are constantly improving the performance of the systems. Requirements for drastically increased efficiency have forced power system designers to use power electronic devices and sophisticated control system concepts that tax traditional analysis tools and techniques. Further complicating the analyst's role is the fact that the system is often so nonlinear that the only way to understand it is through simulation. Land-based power generation from hydroelectric, steam, or other devices is not the only use of power systems. A common attribute of these systems is their use of power electronics and control systems to achieve their performance objectives. SimPowerSystems is a modern design tool that allows scientists and engineers to rapidly and easily build models that simulate power systems. SimPowerSystems uses the Simulink environment, allowing you to build a model using simple click and drag procedures. Not only can you draw the circuit topology rapidly, but your analysis of the

circuit can include its FIGURE 6.0 59

interactions with mechanical, thermal, control, and other disciplines. This is possible because all the electrical parts of the simulation interact with the extensive Simulink modeling library. SimPowerSystems and SimMechanics share a special Physical Modeling block and connection line interface.

6.3 SIMULINK BLOCKS USED AND THEIR FUNCTIONS Synchronous Machine: Model the dynamics of three-phase round-rotor or salient-pole synchronous machine. The synchronous Machine block operates in generator or motor modes. The operating mode is dictated by the sign of the mechanical power (positive for generator mode, negative for motor mode). The electrical part of the machine is represented by a sixth-order state-space model and the mechanical part is the same as in the Simplified Synchronous Machine block. Three-Phase Transformer (Two Windings): Implements three-phase transformer with configurable winding connections. he Three-Phase Transformer (Two Windings) block implements a three-phase transformer using three singlephase transformers. You can simulate the saturable core or not simply by setting the appropriate check box in the parameter menu of the block Asynchronous Machine: Model the dynamics of three-phase asynchronous machine, also known as induction machine. The Asynchronous Machine block operates in either generator or motor mode. The mode of operation is dictated by the sign of the mechanical torque: If Tm is positive, the machine acts as a motor. If Tm is negative, the machine acts as a generator. Universal Bridge: Implements universal power converter with selectable topologies and power electronic devices. The Universal Bridge block implements a universal three-phase power converter that consists of up to six power

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switches connected in a bridge configuration. The type of power switch and converter configuration is selectable from the dialog box. Wind Turbine: Implements model of variable pitch wind turbine. The model is based on the steady-state power characteristics of the turbine. The stiffness of the drive train is infinite and the friction factor and the inertia of the turbine must be combined with those of the generator coupled to the turbine.

6.4 MODELLING THE SUB SYSTEMS: 6.4.1 DISTRIBUTION GENERATOR

FIGURE 6.1 Hydraulic generation as a distribution generation Here from the fig 6.8 we can see the synchronous alternator that is used in the hydraulic power plant. HTG is the hydraulic turbine governor used to give the power input to the Alternator. The poles are excited by the excitation system. Here in the simulation we are virtually regenerating the 3 phase power output by giving the power output as input to the hydraulic turbine governor. Similarly the voltage at the direct and quadrature axis are fed back to generate the required dc voltage by the excitation system. The generation is at13.8 KV at power of 200MVA. A step up transformer is used to step up the voltage to 230 KV. Here the shunt loads are just to avoid the error being induced due to connecting the machines alternator and transformer in series. The output ports A,

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B, C are given to the grid through a step down transformer, as the grid voltage is at 415V. 6.4.2 WIND POWER GENERATION

FIGURE 6.2 wind power generation Wind generation using wind turbine, pitch control, Induction Generator. Here we are using the induction generator as generating machine due to its advantages over other machines for its simplicity and economical factors. The pitch angle controller makes the angle of the turbine blade to adjust in such a way that the speed of rotation at every velocity of the wind is maintained constant. And the parallel capacitive bank is to supply the reactive power to the IM running as the generator. Here we considered the per unit values in the closed loop that can be seen from the fig 6.9. The rms values of the current and voltage generated is taken and the power is being calculated at every sampling time interval and the wave form is being traced in the scope. A timer is used in fig for assigning the wind velocity at 3 different states which will be linearise after some loop operations.

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6.4.3 STATCOM (VSI) WITH BESS AND CONTROLLER

FIGURE 6.3 Voltage source inverter with battery and controller. Here the VSI used comprises of the 6 pulse converter in which the components are the llel diodes. It consists of a capacitor and back up energy storage system for back up under long duration real power outage. Here we are using the PI controller and PWM (pulse width modulation technique) to generate the gate pulses to

FIGURE 6.4 CONTROLLER WITH PWM

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The DC link voltage Vdc is sensed and is given to the controller. And also the grid From fig 6.11 the error from the Vabc and the 1.0 pu value is given to the PI controller the transfer function generates the control voltage. Similarly from the Vdc voltage the phase angle is adjusted accordingly. Here the control method adopted is phase shift control and Regulation of ac bus and dc link voltage. 6.4.4 NON LINEAR LOAD

FIGURE 6.5 NON LINEAR LOAD (RECTIFIER) The nature of non-linear loads is to generate harmonics in the current waveform. This distortion of the current waveform leads to distortion of the voltage waveform. Under these conditions, the voltage waveform is no longer proportional to the current. Non Linear Loads are: COMPUTER, LASER PRINTERS, SMPS, RECTIFIER, PLC, ELECTRONIC BALLAST, REFRIGERATOR, and TV ETC.

6.5 PARAMETERS Grid voltage - 415 V. Operating frequency - 60 HZ. Induction generator - 3.35KVA, 415V, 60 Hz, P=4, Speed=1440rpm,

Ls=Lr=0.06H.

Inverter - DC Link Voltage=800V, DC Switching Frequency=2 kHz. Non linear load

25 KW.

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FIGURE 6.6 over all circuit diagram in simulink with sub systems

6.6 Simulation Result

FIGURE 6.7.1 grid voltage without statcom

65

FIGURE 6.7.2 Voltage and current from DG

FIGURE 6.7.3 Injected current into the grid

66

FIGURE 6.7.4 compensated wind output

FIGURE 6.7.5 compensated grid voltage Here the voltage is in P.U. That is as below 415 V/ 13.8 KV = 0.03007 p.u

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CHAPTER

7

CONCLUSION AND FUTURE SCOPE 7.1 CONCLUSION In this paper we present the FACTS device (STATCOM) -based control scheme for power quality improvement in grid connected wind generating system and with nonlinear load. The power quality issues and its consequences on the consumer and electric utility are presented. The operation of the control system developed for the STATCOM in MATLAB/SIMULINK for maintaining the power quality is to be simulated. It has a capability to cancel out the harmonic parts of the load current. It maintains the source voltage and current in-phase and support the reactive power demand for the wind generator and load at PCC in the grid system, thus it gives an opportunity to enhance the utilization factor of transmission line. Thus the integrated wind generation and FACTS device with BESS have shown the outstanding performance in maintaining the voltage profile as per requirement. Thus the proposed scheme in the grid connected system fulfils the power quality requirements and maintains the grid voltage free from distortion and harmonics.

7.2 FUTURE SCOPE: STATCOM can be replaced with UPQC for better power control. Replacing the Induction Generator with Doubly fed Induction generator is preferred for better results. Now a day the statcom control scheme is based on various methods mentioned in chapter 2 basing on the requirements. In future the off-shore wind turbines will be well implemented due to its advantages of producing high power.

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REFERENCES: [1]

Yuvaraj and Pratheep Raj, Anna University of Technology

Power Quality

Improvement for Grid Connected Wind Energy System using FACTS device

IEEE

Trans. on E. Conv., vol. 23, no. 1, pp. 163 169, 2008. [2]

Energy conversion system models for

adequacy assessment of generating systems incorporating

IEEE Trans.

on E. Conv., vol. 23, no. 1, pp. 163 169, 2008. [3]

grid integration of renewable

energy source: A

IEEE Trans. Ind. Electron., vol. 53, no. 4, pp. 1002 1014,

2006. [4] M. Tsili and S. for wind Proc. IET Renew.power gen., vol. 3, pp. 308 332, 2009. [5] J. J. Gutierrez, J. Ruiz, L. Leturiondo, and A. system for wind turbine certificat

IEEE Trans. Instrum. Meas., vol. 58, no. 2, pp. 375 382,

Feb. 2009. [6] Indian Wind Grid Code Draft report on, Jul. 2009, pp. 15 18, C-NET. [7] C. Han, A. Q. Huang, M. Baran, S. Bhattacharya, and W. Litzenberger, ation of a large wind farm into a weak loop power

IEEE Trans. Energy Conv., vol. 23, no. 1, pp. 226 232, Mar. 2008.

in IEEE PES Gen. Meeting, 2005, vol. 2, pp. 1483 1488.

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APPENDIX Sinusoidal Pulse Width Modulation (SPWM) generator : For each arm of the VSC shown in fig pulses are generated by PWM generator. It compares a triangular carrier waveform to a reference modulating signal as shown in fig .The modulating signals can be generated by the PWM generator itself. Three reference signals are needed to generate the pulses for a three-phase, single or

double bridge. The reference signals used here are three-phase sinusoidal signals. These are generated by controller circuit. The output of PWM generator is given by: When Va0> VT T+ on; T- off; Va0 = ½Vd, and when Va0 < VT T- on; T+ off; Va0 = -½Vd

Fig Sinusoidal Pulse Width Modulation (SPWM) technique The DC link voltage Vdc is sensed and is given to the controller. And also the grid om signal routing. From fig the error from the Vabc and the 1.0 pu value is given to the PI controller the transfer function generates the control voltage. Similarly from the Vdc voltage the phase angle is adjusted accordingly. Here the control method adopted is phase shift control and Regulation of ac bus and dc link voltage. 70