Dual Voltage Source Inverter

A Grid-Connected Dual Voltage Source Inverter With Power Quality Improvement Features Abstract: This paper presents a dual voltage source inverter (DVSI) scheme to enhance the power quality and reliability of the microgrid system. The proposed scheme is comprised of two inverters, which enables the microgrid to exchange power generated by the distributed energy resources (DERs) and also to compensate the local unbalanced and nonlinear load. The control algorithms are developed based on instantaneous symmetrical component theory (ISCT) to operate DVSI in grid sharing and grid injecting modes. The proposed scheme has increased reliability, lower bandwidth requirement of the main inverter, lower cost due to reduction in filter size, and better utilization of microgrid power while using reduced dc-link voltage rating for the main inverter. These features make the DVSI scheme a promising option for microgrid supplying sensitive loads. The topology and control algorithm are validated through extensive simulation and experimental results. Index Terms Grid-connected inverter, instantaneous symmetrical component theory (ISCT), microgrid, power quality.

I. INTRODUCTION TECHNOLOGICAL progress and environmental concerns drive the power system to a paradigm shift with more renewable energy sources integrated to the network by means of distributed generation (DG). These DG units with coordinated control of local generation and storage facilities form a microgrid [1]. In a microgrid, power from different renewable energy sources such as fuel cells, photovoltaic (PV) systems, and wind energy systems are interfaced to grid and loads using power electronic converters. A grid interactive inverter plays an important role in exchanging power from the microgrid to the grid and the connected load [2], [3]. This microgrid inverter can either work in a grid sharing mode while supplying a part of local load or in grid injecting mode, by injecting power to the main grid. Maintaining power quality is another important aspect which has to be addressed while the microgrid system is connected to the main grid. The proliferation of power electronics devices and electrical loads with unbalanced nonlinear currents has degraded the power quality in the power distribution network. Moreover, if there is a considerable amount of feederimpedance in the distribution systems, the propagation of these harmonic currents distorts the voltage at the point of common coupling (PCC). At the same instant, industry automation has reached to a very high level of sophistication, where plants like automobile manufacturing units, chemical factories, and semiconductor industries require clean power. For these applications, it is essential to compensate nonlinear and unbalanced load currents [4]. Load compensation and power injection using grid interactive inverters in microgrid have been presented in the literature [5], [6]. A single inverter system with power quality enhancement is discussed in [7]. The main focus of this work is to realize dual functionalities in an inverter that would provide the active power injection from a solar PV system and also works as an active power filter, compensating unbalances and the reactive power required by other loads connected to the system. In [8], a voltage regulation and power flow control scheme for a wind energy system (WES) is proposed. A distribution static compensator (DSTATCOM) is utilized for voltage regulation and also for active power injection. The control scheme maintains the power balance at the grid terminal during the wind variations using sliding mode control. A multifunctional power electronic converter for the DG power system is described in [9]. This scheme has the capability to inject power generated by WES and also to perform as a harmonic compensator. Most of the reported literature in this area discuss the topologies and control algorithms to provide load compensation capability in the same inverter in addition to their active power injection. When a grid-connected inverter is used for active power injection as well as for load compensation, the inverter capacity that can be utilized for achieving the second objective is decided by the available instantaneous microgrid real power [10]. Considering the case of a grid-connected PV inverter, the available capacity of the inverter to supply the reactive power becomes less during the maximum solar insolation periods [11]. At the same instant, the reactive power to regulate the PCC voltage is very much needed during this period [12]. It indicates that providing multifunctionalities in a single inverter degrades either the real power injection or the load compensation capabilities.

This paper demonstrates a dual voltage source inverter (DVSI) scheme, in which the power generated by the microgrid is injected as real power by the main voltage source inverter (MVSI) and the reactive, harmonic, and unbalanced load compensation is performed by auxiliary voltage source inverter (AVSI). This has an advantage that the rated capacity of MVSI can always be used to inject real power to the grid, if sufficientrenewable power is available at the dc link. In the DVSI scheme, as total load power is supplied by two inverters, power losses across the semiconductor switches of each inverter are reduced. This increases its reliability as compared to a single inverter with multifunctional capabilities [13]. Also, smaller size modular inverters can operate at high switching frequencies with a reduced size of interfacing inductor, the filter cost gets reduced [14]. Moreover, as the main inverter is supplying real power, the inverter has to track the fundamental positive sequence of current. This reduces the bandwidth requirement of the main inverter. The inverters in the proposed scheme use two separate dc links. Since the auxiliary inverter is supplying zero sequence of load current, a three-phase three-leg inverter topology with a single dc storage capacitor can be used for the main inverter. This in turn reduces the dc-link voltage requirement of the main inverter. Thus, the use of two separate inverters in the proposed DVSI scheme provides increased reliability, better utilization of microgrid power, reduced dc grid voltage rating, less bandwidth requirement of the main inverter, and reduced filter size [13]. Control algorithms are developed by instantaneous symmetrical component theory (ISCT) to operate DVSI in grid-connected mode, while considering nonstiff grid voltage [15], [16]. The extraction of fundamental positive sequence of PCC voltage is done by dq0 transformation [17]. The control strategy is tested with two parallel inverters connected to a three-phase four-wire distribution system. Effectiveness of the proposed control algorithm is validated through detailed simulation and experimental results.

CHAPTER-2 2.1 POWER QUALITY

The contemporary container crane industry, like many other industry segments, is often enamored by the bells and whistles, colorful diagnostic displays, high speed performance, and levels of automation that can be achieved. Although these features and their indirectly related

computer based enhancements are key issues to an efficient terminal operation, we must not forget the foundation upon which we are building. Power quality is the mortar which bonds the foundation blocks. Power quality also affects terminal operating economics, crane reliability, our environment, and initial investment in power distribution systems to support new crane installations.

To quote the utility company newsletter which accompanied the last monthly issue of my home utility billing: ‘Using electricity wisely is a good environmental and business practice which saves you money, reduces emissions from generating plants, and conserves our Natural resources.’ As we are all aware, container crane performance requirements continue to increase at an astounding rate. Next generation container cranes, already in the bidding process, will require average power demands of 1500 to 2000 kW – almost double the total average demand three years ago. The rapid increase in power demand levels, an increase in container crane population, SCR converter crane drive retrofits and the large AC and DC drives needed to power and control these cranes will increase awareness of the power quality issue in the very near future.

2.2 POWER QUALITY PROBLEMS

Any power problem that results in failure or disoperation of customer equipment, manifests itself as an economic burden to the user, or produces negative impacts on the environment.

When applied to the container crane industry, the power issues which degrade power quality include:  Power Factor.  Harmonic Distortion.  Voltage Transients.  Voltage Sags or Dips.  Voltage Swells.

The AC and DC variable speed drives utilized on board container cranes are significant contributors to total harmonic current and voltage distortion. Whereas SCR phase control creates the desirable average power factor, DC SCR drives operate at less than this. In addition, line notching occurs when SCR’s commutate, creating transient peak recovery voltages that can be 3 to 4 times the nominal line voltage depending upon the system impedance and the size of the drives. The frequency and severity of these power system disturbances varies with the speed of the drive. Harmonic current injection by AC and DC drives will be highest when the drives are operating at slow speeds. Power factor will be lowest when DC drives are operating at slow speeds or during initial acceleration and deceleration periods, increasing to its maximum value when the SCR’s are fazed on to produce rated or base speed.

Above base speed, the power factor essentially remains constant. Unfortunately, container cranes can spend considerable time at low speeds as the operator attempts to spot and land containers. Poor power factor places a greater kVA demand burden on the utility or enginealternator power source. Low power factor loads can also affect the voltage stability which can ultimately result in detrimental effects on the life of sensitive electronic equipment or even intermittent malfunction. Voltage transients created by DC drive SCR line notching, AC drive voltage chopping, and high frequency harmonic voltages and currents are all significant sources of noise and disturbance to sensitive electronic equipment

It has been our experience that end users often do not associate power quality problems with Container cranes, either because they are totally unaware of such issues or there was no economic Consequence if power quality was not addressed. Before the advent of solid-state power supplies, Power factor was reasonable, and harmonic current injection was minimal. Not until the crane Population multiplied, power demands per crane increased, and static power conversion became the way of life, did power quality issues begin to emerge.

Even as harmonic distortion and power Factor issues surfaced, no one was really prepared. Even today, crane builders and electrical drive System vendors avoid the issue during competitive bidding for new cranes. Rather than focus on Awareness and understanding of the potential issues, the power quality issue is intentionally or Unintentionally ignored. Power

quality problem solutions are available. Although the solutions are not free, in most cases, they do represent a good return on investment. However, if power quality is not specified, it most likely will not be delivered.

Power quality can be improved through:  Power factor correction,  Harmonic filtering,  Special line notch filtering,  Transient voltage surge suppression,  Proper earthing systems.

In most cases, the person specifying and/or buying a container crane may not be fully aware of the potential power quality issues. If this article accomplishes nothing else, we would hope to provide that awareness.

In many cases, those involved with specification and procurement of container cranes may not be cognizant of such issues, do not pay the utility billings, or consider it someone else’s concern. As a result, container crane specifications may not include definitive power quality criteria such as power factor correction and/or harmonic filtering. Also, many of those specifications which do require power quality equipment do not properly define the criteria. Early in the process of preparing the crane specification:  Consult with the utility company to determine regulatory or contract requirements that must be satisfied, if any.  Consult with the electrical drive suppliers and determine the power quality profiles that cable expected based on the drive sizes and technologies proposed for the specific project.  Evaluate the economics of power quality correction not only on the present situation, but consider the impact of future utility deregulation and the future development plans for the terminal

2.3 THE BENEFITS OF POWER QUALITY Power quality in the container terminal environment impacts the economics of the terminal operation, affects reliability of the terminal equipment, and affects other consumers served by the same utility service. Each of these concerns is explored in the following paragraphs. Economic Impact The economic impact of power quality is the foremost incentive to container terminal operators. Economic impact can be significant and manifest itself in several ways:

Power Factor Penalties

Many utility companies invoke penalties for low power factor on monthly billings. There is no industry standard followed by utility companies. Methods of metering and calculating power factor penalties vary from one utility company to the next. Some utility companies actually meter kVAR usage and establish a fixed rate times the number of kVAR-hours consumed. Other utility companies monitor kVAR demands and calculate power factor. If the power factor falls below a fixed limit value over a demand period, a penalty is billed in the form of an adjustment to the peak demand charges. A number of utility companies servicing container terminal equipment do not yet invoke power factor penalties. However, their service contract with the Port may still require that a minimum power factor over a defined demand period be met. The utility company may not continuously monitor power factor or kVAR usage and reflect them in the monthly utility billings; however, they do reserve the right to monitor the Port service at any time. If the power factor criteria set forth in the service contract are not met, the user may be penalized, or required to take corrective actions at the user’s expense. One utility company, which supplies power service to several east coast container terminals in the USA, does not reflect power factor penalties in their monthly billings, however, their service contract with the terminal reads as follows: ‘The average power factor under operating conditions of customer’s load at the point where service is metered shall be not less than 85%. If below 85%, the customer may be required to furnish, install and maintain at its expense corrective apparatus which will increase the power

factor of the entire installation to not less than 85%. The customer shall ensure that no excessive harmonics or transients are introduced on to the [utility] system. This may require special power conditioning equipment or filters. The IEEE Std. 519-1992 is used as a guide in determining appropriate design requirements.’

The Port or terminal operations personnel, who are responsible for maintaining container cranes, or specifying new container crane equipment, should be aware of these requirements. Utility deregulation will most likely force utilities to enforce requirements such as the example above. Terminal operators who do not deal with penalty issues today may be faced with some rather severe penalties in the future. A sound, future terminal growth plan should include contingencies for addressing the possible economic impact of utility deregulation. System Losses Harmonic currents and low power factor created by nonlinear loads, not only result in possible power factor penalties, but also increase the power losses in the distribution system. These losses are not visible as a separate item on your monthly utility billing, but you pay for them each month. Container cranes are significant contributors to harmonic currents and low power factor. Based on the typical demands of today’s high speed container cranes, correction of power factor alone on a typical state of the art quay crane can result in a reduction of system losses that converts to a 6 to 10% reduction in the monthly utility billing. For most of the larger terminals, this is a significant annual saving in the cost of operation.

2.4 Power Service Initial Capital Investments

The power distribution system design and installation for new terminals, as well as modification of systems for terminal capacity upgrades, involves high cost, specialized, high and medium voltage equipment. Transformers, switchgear, feeder cables, cable reel trailing cables, collector bars, etc. must be sized based on the kVA demand. Thus cost of the equipment is directly related to the total kVA demand. As the relationship above indicates, kVA demand is inversely proportional to the overall power factor, i.e. a lower power factor demands higher kVA for the same kW load. Container cranes are one of the most significant users of power in the

terminal. Since container cranes with DC, 6 pulse, SCR drives operate at relatively low power factor, the total kVA demand is significantly larger than would be the case if power factor correction equipment were supplied on board each crane or at some common bus location in the terminal. In the absence of power quality corrective equipment, transformers are larger, switchgear current ratings must be higher, feeder cable copper sizes are larger, collector system and cable reel cables must be larger, etc. Consequently, the cost of the initial power distribution system equipment for a system which does not address power quality will most likely be higher than the same system which includes power quality equipment.

2.5 Equipment Reliability

Poor power quality can affect machine or equipment reliability and reduce the life of components. Harmonics, voltage transients, and voltage system sags and swells are all power quality problems and are all interdependent. Harmonics affect power factor, voltage transients can induce harmonics, the same phenomena which create harmonic current injection in DC SCR variable speed drives are responsible for poor power factor, and dynamically varying power factor of the same drives can create voltage sags and swells. The effects of harmonic distortion, harmonic currents, and line notch ringing can be mitigated using specially designed filters.

Power System Adequacy

When considering the installation of additional cranes to an existing power distribution system, a power system analysis should be completed to determine the adequacy of the system to support additional crane loads. Power quality corrective actions may be dictated due to inadequacy of existing power distribution systems to which new or relocated cranes are to be connected. In other words, addition of power quality equipment may render a workable scenario on an existing power distribution system, which would otherwise be inadequate to support additional cranes without high risk of problems.

Environment

No issue might be as important as the effect of power quality on our environment. Reduction in system losses and lower demands equate to a reduction in the consumption of our natural nm resources and reduction in power plant emissions. It is our responsibility as occupants of this planet to encourage conservation of our natural resources and support measures which improve our air quality

CHAPTER-3 FACTS Flexible AC Transmission Systems, called FACTS, got in the recent years a well known term for higher controllability in power systems by means of power electronic devices. Several FACTS-devices have been introduced for various applications worldwide. A number of new types of devices are in the stage of being introduced in practice. In most of the applications the controllability is used to avoid cost intensive or landscape requiring extensions of power systems, for instance like upgrades or additions of substations and power lines. FACTS-devices provide a better adaptation to varying operational conditions and improve the usage of existing installations. The basic applications of FACTS-devices are:  Power flow control,  Increase of transmission capability,  Voltage control,  Reactive power compensation,  Stability improvement,

 Power quality improvement,  Power conditioning,  Flicker mitigation,  Interconnection of renewable and distributed generation and storages. The usage of lines for active power transmission should be ideally up to the thermal limits. Voltage and stability limits shall be shifted with the means of the several different FACTS devices. It can be seen that with growing line length, the opportunity for FACTS devices gets more and more important. The influence of FACTS-devices is achieved through switched or controlled shunt compensation, series compensation or phase shift control. The devices work electrically as fast current, voltage or impedance controllers. The power electronic allows very short reaction times down to far below one second.

The development of FACTS-devices has started with the growing capabilities of power electronic components. Devices for high power levels have been made available in converters for high and even highest voltage levels. The overall starting points are network elements influencing the reactive power or the impedance of a part of the power system. Figure 1.2 shows a number of basic devices separated into the conventional ones and the FACTS-devices For the FACTS side the taxonomy in terms of 'dynamic' and 'static' needs some explanation. The term 'dynamic' is used to express the fast controllability of FACTS-devices provided by the power electronics. This is one of the main differentiation factors from the conventional devices. The term 'static' means that the devices have no moving parts like mechanical switches to perform the dynamic controllability. Therefore most of the FACTSdevices can equally be static and dynamic.

The left column in Figure 1.2 contains the conventional devices build out of fixed or mechanically switch able components like resistance, inductance or capacitance together with

transformers. The FACTS-devices contain these elements as well but use additional power electronic valves or converters to switch the elements in smaller steps or with switching patterns within a cycle of the alternating current. The left column of FACTS-devices uses Thyristor valves or converters. These valves or converters are well known since several years. They have low losses because of their low switching frequency of once a cycle in the converters or the usage of the Thyristors to simply bridge impedances in the valves. The right column of FACTS-devices contains more advanced technology of voltage source converters based today mainly on Insulated Gate Bipolar Transistors (IGBT) or Insulated Gate Commutated Thyristors (IGCT). Voltage Source Converters provide a free controllable voltage in magnitude and phase due to a pulse width modulation of the IGBTs or IGCTs. High modulation frequencies allow to get low harmonics in the output signal and even to compensate disturbances coming from the network. The disadvantage is that with an increasing switching frequency, the losses are increasing as well. Therefore special designs of the converters are required to compensate this. 3.1 Configurations of FACTS-Devices: Shunt Devices: The most used FACTS-device is the SVC or the version with Voltage Source Converter called STATCOM. These shunt devices are operating as reactive power compensators. The main applications in transmission, distribution and industrial networks are:  Reduction of unwanted reactive power flows and therefore reduced network losses.  Keeping of contractual power exchanges with balanced reactive power.  Compensation of consumers and improvement of power quality especially with huge demand fluctuations like industrial machines, metal melting plants, railway or underground train systems.  Compensation of Thyristor converters e.g. in conventional HVDC lines.  Improvement of static or transient stability.

Almost half of the SVC and more than half of the STATCOMs are used for industrial applications. Industry as well as commercial and domestic groups of users require power quality. Flickering lamps are no longer accepted, nor are interruptions of industrial processes due to insufficient power quality. Railway or underground systems with huge load variations require SVCs or STATCOMs. SVC: Electrical loads both generate and absorb reactive power. Since the transmitted load varies considerably from one hour to another, the reactive power balance in a grid varies as well. The result can be unacceptable voltage amplitude variations or even a voltage depression, at the extreme a voltage collapse. A rapidly operating Static Var Compensator (SVC) can continuously provide the reactive power required to control dynamic voltage oscillations under various system conditions and thereby improve the power system transmission and distribution stability. Applications of the SVC systems in transmission systems:  To increase active power transfer capacity and transient stability margin  To damp power oscillations  To achieve effective voltage control In addition, SVCs are also used: 1. In transmission systems  To reduce temporary over voltages  To damp sub synchronous resonances  To damp power oscillations in interconnected power systems 2. In traction systems  To balance loads

 To improve power factor  To improve voltage regulation 3. In HVDC systems  To provide reactive power to ac–dc converters. 4. In arc furnaces  To reduce voltage variations and associated light flicker Installing an SVC at one or more suitable points in the network can increase transfer capability and reduce losses while maintaining a smooth voltage profile under different network conditions. In addition an SVC can mitigate active power oscillations through voltage amplitude modulation. SVC installations consist of a number of building blocks. The most important is the Thyristor valve, i.e. stack assemblies of series connected anti-parallel Thyristors to provide controllability. Air core reactors and high voltage AC capacitors are the reactive power elements used together with the Thyristor valves. The step up connection of this equipment to the transmission voltage is achieved through a power transformer.

SVC building blocks and voltage / current characteristic In principle the SVC consists of Thyristor Switched Capacitors (TSC) and Thyristor Switched or Controlled Reactors (TSR / TCR). The coordinated control of a combination of these branches varies the reactive power as shown in Figure. The first commercial SVC was installed in 1972 for an electric arc furnace. On transmission level the first SVC was used in 1979. Since then it is widely used and the most accepted FACTS-device.

Comparison of the loss characteristics of TSC–TCR, TCR–FC compensators and synchronous condenser These SVCs do not have a short-time overload capability because the reactors are usually of the air-core type. In applications requiring overload capability, TCR must be designed for short-time overloading, or separate thyristor-switched overload reactors must be employed. 3.2 SVC USING A TCR AND TSC: This compensator overcomes two major shortcomings of the earlier compensators by reducing losses under operating conditions and better performance under large system disturbances. In view of the smaller rating of each capacitor bank, the rating of the reactor bank will be 1/n times the maximum output of the SVC, thus reducing the harmonics generated by the reactor. In those situations where harmonics have to be reduced further, a small amount of FCs tuned as filters may be connected in parallel with the TCR.

SVC of combined TSC and TCR type. When large disturbances occur in a power system due to load rejection, there is a possibility for large voltage transients because of oscillatory interaction between system and the SVC capacitor bank or the parallel. The LC circuit of the SVC in the FC compensator. In the TSC–TCR scheme, due to the flexibility of rapid switching of capacitor banks without appreciable disturbance to the power system, oscillations can be avoided, and hence the transients in the system can also be avoided. The capital cost of this SVC is higher than that of the earlier one due to the increased number of capacitor switches and increased control complexity. 3.3 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 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.

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.

6 Pulses STATCOM.

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.

3.4 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 1 harmonics. There are a fundamental switching will of course produce the 6 N 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 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 phase shift between the two 6 pulse bridges. This

angle to

produce a 30 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.

Substation with a STATCOM.

3.5 Series Devices:

Series devices have been further developed from fixed or mechanically switched compensations to the Thyristor Controlled Series Compensation (TCSC) or even Voltage Source Converter based devices. The main applications are:  Reduction of series voltage decline in magnitude and angle over a power line,  Reduction of voltage fluctuations within defined limits during changing power transmissions,  Improvement of system damping resp. damping of oscillations,  Limitation of short circuit currents in networks or substations,  Avoidance of loop flows resp. power flow adjustments.

3.6 TCSC: Thyristor Controlled Series Capacitors (TCSC) address specific dynamical problems in transmission systems. Firstly it increases damping when large electrical systems are interconnected. Secondly it can overcome the problem of Sub Synchronous Resonance (SSR), a phenomenon that involves an interaction between large thermal generating units and series compensated transmission systems. The TCSC's high speed switching capability provides a mechanism for controlling line power flow, which permits increased loading of existing transmission lines, and allows for rapid readjustment of line power flow in response to various contingencies. The TCSC also can regulate steady-state power flow within its rating limits. From a principal technology point of view, the TCSC resembles the conventional series capacitor. All the power equipment is located on an isolated steel platform, including the Thyristor valve that is used to control the behavior of the main capacitor bank. Likewise the control and protection is located on ground potential together with other auxiliary systems.

Figure shows the principle setup of a TCSC and its operational diagram. The firing angle and the thermal limits of the Thyristors determine the boundaries of the operational diagram.

Advantages:  Continuous control of desired compensation level.  Direct smooth control of power flow within the network.  Improved capacitor bank protection.  Local mitigation of sub synchronous resonance (SSR). This permits higher levels of compensation in networks where interactions with turbine-generator torsional vibrations or with other control or measuring systems are of concern.  Damping of electromechanical (0.5-2 Hz) power oscillations which often arise between areas in a large interconnected power network. These oscillations are due to the dynamics of inter area power transfer and often exhibit poor damping when the aggregate power tranfer over a corridor is high relative to the transmission strength. 3.7 Dynamic Power Flow Controller:

A new device in the area of power flow control is the Dynamic Power Flow Controller (DFC). The DFC is a hybrid device between a Phase Shifting Transformer (PST) and switched series compensation. A functional single line diagram of the Dynamic Flow Controller is shown in Figure 1.19. The Dynamic Flow Controller consists of the following components:  A standard phase shifting transformer with tap-changer (PST) series-connected Thyristor Switched Capacitors and Reactors .(TSC / TSR).  A mechanically switched shunt capacitor (MSC). (This is optional depending on the system reactive power requirements).

Based on the system requirements, a DFC might consist of a number of series TSC or TSR. The mechanically switched shunt capacitor (MSC) will provide voltage support in case of overload and other conditions. Normally the reactance of reactors and the capacitors are selected based on a binary basis to result in a desired stepped reactance variation. If a higher power flow resolution is needed, a reactance equivalent to the half of the smallest one can be added. The switching of series reactors occurs at zero current to avoid any harmonics. However, in general, the principle of phase-angle control used in TCSC can be applied for a continuous control as well. The operation of a DFC is based on the following rules:

 TSC / TSR are switched when a fast response is required.  The relieve of overload and work in stressed situations is handled by the TSC / TSR.  The switching of the PST tap-changer should be minimized particularly for the currents higher than normal loading.  The total reactive power consumption of the device can be optimized by the operation of the MSC, tap changer and the switched capacities and reactors. In order to visualize the steady state operating range of the DFC, we assume an inductance in parallel representing parallel transmission paths. The overall control objective in steady state would be to control the distribution of power flow between the branch with the DFC and the parallel path. This control is accomplished by control of the injected series voltage. The PST (assuming a quadrature booster) will inject a voltage in quadrature with the node voltage. The controllable reactance will inject a voltage in quadrature with the throughput current. Assuming that the power flow has a load factor close to one, the two parts of the series voltage will be close to collinear. However, in terms of speed of control, influence on reactive power balance and effectiveness at high/low loading the two parts of the series voltage has quite different characteristics. The steady state control range for loadings up to rated current is illustrated in Figure 1.20, where the x-axis corresponds to the throughput current and the y-axis corresponds to the injected series voltage.

Operational diagram of a DFC: Operation in the first and third quadrants corresponds to reduction of power through the DFC, whereas operation in the second and fourth quadrants corresponds to increasing the power flow through the DFC. The slope of the line passing through the origin (at which the tap is at zero and TSC / TSR are bypassed) depends on the short circuit reactance of the PST. Starting at rated current (2 kA) the short circuit reactance by itself provides an injected voltage (approximately 20 kV in this case). If more inductance is switched in and/or the tap is increased, the series voltage increases and the current through the DFC decreases (and the flow on parallel branches increases). The operating point moves along lines parallel to the arrows in the figure. The slope of these arrows depends on the size of the parallel reactance. The maximum series voltage in the first quadrant is obtained when all inductive steps are switched in and the tap is at its maximum.

Now, assuming maximum tap and inductance, if the throughput current decreases (due e.g. to changing loading of the system) the series voltage will decrease. At zero current, it will

not matter whether the TSC / TSR steps are in or out, they will not contribute to the series voltage. Consequently, the series voltage at zero current corresponds to rated PST series voltage. Next, moving into the second quadrant, the operating range will be limited by the line corresponding to maximum tap and the capacitive step being switched in (and the inductive steps by-passed). In this case, the capacitive step is approximately as large as the short circuit reactance of the PST, giving an almost constant maximum voltage in the second quadrant. 3.8 Unified Power Flow Controller: The UPFC is a combination of a static compensator and static series compensation. It acts as a shunt compensating and a phase shifting device simultaneously.

Principle configuration of an UPFC.

The UPFC consists of a shunt and a series transformer, which are connected via two voltage source converters with a common DC-capacitor. The DC-circuit allows the active power exchange between shunt and series transformer to control the phase shift of the series voltage. This setup, as shown in Figure 1.21, provides the full controllability for voltage and power flow. The series converter needs to be protected with a Thyristor bridge. Due to the high efforts for the Voltage Source Converters and the protection, an UPFC is getting quite expensive, which limits the practical applications where the voltage and power flow control is required simultaneously.

3.8.1 OPERATING PRINCIPLE OF UPFC: The basic components of the UPFC are two voltage source inverters (VSIs) sharing a common dc storage capacitor, and connected to the power system through coupling transformers. One VSI is connected to in shunt to the transmission system via a shunt transformer, while the other one is connected in series through a series transformer.

A basic UPFC functional scheme is shown in fig. The series inverter is controlled to inject a symmetrical three phase voltage system (Vse), of controllable magnitude and phase angle in series with the line to control active and reactive power flows on the transmission line. So, this inverter will exchange active and reactive power with the line. The reactive power is electronically provided by the series inverter, and the active power is transmitted to the dc terminals. The shunt inverter is operated in such a way as to demand this dc terminal power (positive or negative) from the line keeping the voltage across the storage capacitor Vdc constant. So, the net real power absorbed from the line by the UPFC is equal only to the losses of the inverters and their transformers. The remaining capacity of the shunt inverter can be used to exchange reactive power with the line so to provide a voltage regulation at the connection point.

The two VSI’s can work independently of each other by separating the dc side. So in that case, the shunt inverter is operating as a STATCOM that generates or absorbs reactive power to regulate the voltage magnitude at the connection point. Instead, the series inverter is operating as SSSC that generates or absorbs reactive power to regulate the current flow, and hence the power low on the transmission line. The UPFC has many possible operating modes. In particular, the shunt inverter is operating in such a way to inject a controllable current, ish into the transmission line. The shunt inverter can be controlled in two different modes:  VAR Control Mode: The reference input is an inductive or capacitive VAR request. The shunt inverter control translates the var reference into a corresponding shunt current request and adjusts gating of the inverter to establish the desired current. For this mode of control a feedback signal representing the dc bus voltage, Vdc, is also required.  Automatic Voltage Control Mode: The shunt inverter reactive current is automatically regulated to maintain the transmission line voltage at the point of connection to a reference value. For this mode of control, voltage feedback signals are obtained from the sending end bus feeding the shunt coupling transformer. The series inverter controls the magnitude and angle of the voltage injected in series with the line to influence the power flow on the line. The actual value of the injected voltage can be obtained in several ways.  Direct Voltage Injection Mode: The reference inputs are directly the magnitude and phase angle of the series voltage.  Phase Angle Shifter Emulation mode: The reference input is phase displacement between the sending end voltage and the receiving end voltage. Line Impedance Emulation mode: The reference input is an impedance value to insert in series with the line impedance.  Automatic Power Flow Control Mode: The reference inputs are values of P and Q to maintain on the transmission line despite system changes. 3.9 STATIC SYNCHRONOUS COMPENSATOR (STATCOM): Introduction

The STATCOM is a solid-state-based power converter version of the SVC. Operating as a shunt-connected SVC, its capacitive or inductive output currents can be controlled independently from its terminal AC bus voltage. Because of the fast-switching characteristic of power converters, STATCOM provides much faster response as compared to the SVC. In addition, in the event of a rapid change in system voltage, the capacitor voltage does not change instantaneously; therefore, STATCOM effectively reacts for the desired responses. For example, if the system voltage drops for any reason, there is a tendency for STATCOM to inject capacitive power to support the dipped voltages. STATCOM is capable of high dynamic performance and its compensation does not depend on the common coupling voltage. Therefore, STATCOM is very effective during the power system disturbances. Moreover, much research confirms several advantages of STATCOM. These advantages compared to other shunt compensators include:  Size, weight, and cost reduction .  Equality of lagging and leading output.  Precise and continuous reactive power control with fast response.  Possible active harmonic filter capability. This chapter describes the structure, basic operating principle and characteristics of STATCOM. In addition, the concept of voltage source converters and the corresponding control techniques are illustrated. STRUCTURE OF STATCOM Basically, STATCOM is comprised of three main parts (as seen from Figure below): a voltage source converter (VSC), a step-up coupling transformer, and a controller. In a very-high-voltage system, the leakage inductances of the step-up power transformers can function as coupling reactors. The main purpose of the coupling inductors is to filter out the current harmonic components that are generated mainly by the pulsating output voltage of the power converters.

Reactive power generation by a STATCOM.

CHAPTER-4 HARMONICS The typical definition for a harmonic is “a sinusoidal component of a periodic wave or\ quantity having a frequency that is an integral multiple of the fundamental frequency.” [1]. Some references refer to “clean” or “pure” power as those without any harmonics. But such clean waveforms typically only exist in a laboratory. Harmonics have been around for a long time and will continue to do so. In fact, musicians have been aware of such since the invention of the first string or woodwind instrument. Harmonics (called “overtones” in music) are responsible for what makes a trumpet sound like a trumpet, and a clarinet like a clarinet. Electrical generators try to produce electric power where the voltage waveform has only one frequency associated with it, the fundamental frequency. In the North America, this frequency is 60 Hz, or cycles per second. In European countries and other parts of the world, this frequency is usually 50 Hz. Aircraft often uses 400 Hz as the fundamental frequency. At 60 Hz, this means that sixty times a second, the voltage waveform increases to a maximum positive value, then decreases to zero, further decreasing to a maximum negative value, and then back to zero. The rate at which these changes occur is the trigometric function called a sine wave, as shown in figure 1. This function occurs in many natural phenomena, such as the speed of a pendulum as it swings back and forth, or the way a string on a voilin vibrates when plucked.

Fig. Sine wave

The frequency of the harmonics is different, depending on the fundamental frequency. For example, the 2nd harmonic on a 60 Hz system is 2*60 or 120 Hz. At 50Hz, the second harmonic is 2* 50 or 100Hz. 300Hz is the 5th harmonic in a 60 Hz system, or the 6th harmonic in a 50 Hz system. Figure shows how a signal with two harmonics would appear on an oscilloscope-type display, which some power quality analyzers provide.

Figure. Fundamental with two harmonics In order to be able to analyze complex signals that have many different frequencies present, a number of mathematical methods were developed. One of the more popular is called the Fourier Transform. However, duplicating the mathematical steps required in a microprocessor or computer-based instrument is quite difficult. So more compatible processes, called the FFT for Fast Fourier transform, or DFT for Discrete Fourier Transform, are used. These methods only work properly if the signal is composed of only the fundamental and harmonic frequencies in a certain frequency range (called the Nyquist frequency, which is onehalf of the sampling frequency). The frequency values must not change during the measurement period. Failure of these rules to be maintained can result in mis-information. For example, if a voltage waveform is comprised of 60 Hz and 200 Hz signals, the FFT cannot directly see the 200 Hz. It only knows 60, 120, 180, 240, which are often called “bins”. The result would be that the energy of the 200 Hz signal would appear partially in the 180Hz bin, and partially in the 240 Hz

bin. An FFT-based processer could show a voltage value of 115V at 60 Hz, 18 V at the 3rd harmonic, and 12 V at the 4th harmonic, when it really should have been 30 V at 200 Hz. These in-between frequencies are called “inter harmonics”. There is also a special category of inter harmonics, which are frequency values less than the fundamental frequency value, called sub-harmonics. For example, the process of melting metal in an electric arc furnace can result large currents that are comprised of the fundamental , inter harmonic, and sub harmonic frequencies being drawn from the electric power grid. These levels can be quite high during the melt-down phase, and usually effect the voltage waveform. Some typical types of equipment susceptible to harmonic pollution include: - Excessive neutral current, resulting in overheated neutrals. The odd triple harmonics in three phase wye circuits are actually additive in the neutral. This is because the harmonic number multiplied by the 120 degree phase shift between phases is a integer multiple of 360 degrees. This puts the harmonics from each of the three phase legs “in-phase” with each other in the neutral, as shown in Figure 3.

Figure. Additive Third Harmonics

 Incorrect reading meters, including induction disc W-hr meters and averaging type current meters.  Reduced true PF, where PF= Watts/VA.  Overheated transformers, especially delta windings where triplen harmonics generated on the load side of a delta-wye transformer will circulate in the primary side. Some type of losses go up as the square of harmonic value (such as skin effect and eddy current losses). This is also true for solenoid coils and lighting ballasts.

 Zero, negative sequence voltages on motors and generators. In a balanced system, voltage harmonics can either be positive (fundamental, 4th, 7th,...), negative (2nd, 5th, 8th...) or zero (3rd, 6th, 9th,...) sequencing values. This means that the voltage at that particular frequency tries to rotate the motor forward, backward, or neither (just heats up the motor), respectively. There is also heating from increased losses as in a transformer.

Table. Harmonic Sequencing Values in Balanced Systems  Nuisance operation of protective devices, including false tripping of relays and failure of a UPS to transfer properly, especially if controls incorporate zero-crossing sensing circuits.  Bearing failure from shaft currents through un insulated bearings of electric motors.  Blown-fuses on PF correction caps, due to high voltage and currents from resonance with line impedance.  Mis-operation or failure of electronic equipment.  If there are voltage sub harmonics in the range of 1-30Hz, the effect on lighting is called flicker. This is especially true at 8.8Hz, where the human eye is most sensitive, and just 0.5% variation in the voltage is noticeable with some types of lighting. [2]

4.1 WHERE THEY COME FROM? How this electricity is used by the different type of loads can have an effect on “purity” of the voltage waveform. Some loads cause the voltage and current waveforms to lose this pure sine wave appearance and become distorted. This distortion may consist of predominately harmonics, depending on the type of load and system impedances.

“The main sources of harmonic current are at present the phase angle controlled rectifiers and inverters.” [3] These are often called static power converters. These devices take AC power and convert it to another form, sometimes back to AC power at the same or different frequency, based on the firing scheme. The firing scheme refers to the controlling mechanism that determines how and when current is conducted. One major variation is the phase angle at which conduction begins and ends. A typical such converter is the switching-type power supplies found in most personal computers and peripheral equipment, such as printers. While they offer many benefits in size, weight and cost, the large increase of this type of equipment over the past fifteen years is largely responsible for the increased attention to harmonics. Figure shows below how a switching-type power supply works. The AC voltage is converted into a DC voltage, which is further converted into other voltages that the equipment needs to run. The rectifier consists of semi-conductor devices (such as diodes) that only conduct current in one direction. In order to do so, the voltage on the one end must be greater than the other end. These devices feed current into a capacitor, where the voltage value on the cap at any time depends on how much energy is being taken out by the rest of the power supply. When the input voltage value is higher than voltage on the capacitor, the diode will conduct current through it. This results in a current waveform as shown in Figure 5, and harmonic spectrum in Figure 6. Obviously, this is not a pure sinusoidal waveform with only a 60 Hz frequency component.

Figure. Current Waveform

Figure. Harmonic Spectrum of Current Waveform Shown

If the rectifier had only been a half wave rectifier, the waveform would only have every other current pulse, and the harmonic spectrum would be different, as shown in Figure. Fluorescent lights can be the source of harmonics, as the ballasts are non-linear inductors. The third harmonic is the predominate harmonic in this case. (See Table 3) As previously mentioned, the third harmonic current from each phase in a four-wire wye or star system will be additive in the neutral, instead of cancelling out Some of the newer electronic ballasts have very significant harmonic problems, as they operate somewhat like a switching power supply, but can result in current harmonic distortion levels over 30%.

Table. Sample of Harmonic Values for Fluorescent lighting [4]. Low power, AC voltage regulators for light dimmers and small induction motors adjust the phase angle or point on the wave where conduction occurs. Medium power converters are used for motor control in manufacturing and railroad applications, and include such equipment as ASDs (adjustable speed drives) and VFDs (variable frequency drives). Metal reduction operations, like electric arc furnaces, and high voltage DC transmission employ large power converters, in the 2-20MVA rating. This type of 3-phase equipment may also cause other types of power quality problems. When the semiconductor device is suppose to turn-off, it does not do so abruptly. This happens under “naturally” commutated conditions, where the voltage that was larger on the anode side compared to the cathode is now the opposite. This occurs each cycle as the voltage waveform goes through the sine waveform. It also happens under “forced” commutation conditions, where the semi-conductor device has a “gate”-type control mechanism built in to it. This commutation period is a time when two semiconductor devices are both conducting current at the same time, effectively shorting one phase to the other and resulting in large current transients. When transformers are first energized, the current drawn is different from the steady state condition. This is caused by the inrush of the magnetizing current. The harmonics during this period varies over time. Some harmonics have zero value for part of the time, and then increase for a while before returning to zero. An unbalanced transformer (where either the output current,

winding impedance or input voltage on each leg are not equal) will cause harmonics, as will overvoltage saturation of a transformer.

4.2 HOW DO YOU FIND THEM? Hand-held harmonic meters can be useful tools for making spot checks for known harmonic problems. However, harmonic values will often change during the day, as different loads are turned on and off within the facility or in other facilities on the same electric utility distribution system. This requires the use of a harmonic monitor or power quality monitor with harmonic capabilities, which can record the harmonic values over a period of time.

Figure. Power Quality Monitor with Harmonic Analysis Typically, monitoring will last for one business cycle. A business cycle is how long it takes for the normal operation of the plant to repeat itself. For example, if a plant runs three identical shifts, seven days a week, then a business cycle would be eight hours. More typically, a business cycle is one week, as different operations take place on a Monday, when the plant equipment is restarted after being off over the weekend, then on a Wednesday, or a Saturday, when only a Skelton crew may be working.

Certain types of loads also generate typical harmonic spectrum signatures that can point the investigator towards the source. This is related to the number of pulses, or paths of conduction. The general equation is h = ( n * p ) +/- 1, where h is the harmonic number, n is any integer (1,2,3,..) and p is the number of pulses in the circuit, and the magnitude decreases as the ration of 1/h (1/3, 1/5, 1/7, 1/9,...). Table 4 shows examples of such.

Table. Typical Harmonics Found for Different Converters. WHEN ARE THEY A PROBLEM? Most electrical loads (except half-wave rectifiers) produce symmetrical current waveforms, which mean that the positive half of the waveform looks like a mirror image of the negative half. This results in only odd harmonic values being present. Even harmonics will disrupt this half-wave symmetry. The presence of these even harmonics should cause the investigator to suspect there is a half-wave rectifier on the circuit. This also results from a full wave rectifier when one side of the rectifier has blown or damaged components. Early detection of this condition in a UPS system can prevent a complete failure when the load is switched onto back-up power. To determine what is normal or acceptable levels, a number of standards have been developed by various organizations. ANSI/IEEE C57.110 Recommended Practice for Establishing Transformer Compatibility When Supplying No sinusoidal Load Currents is a useful document for determining how much a transformer should be derated from its nameplate rating when operating in the presence of harmonics. There are two parameters typically used, called K-factor and TDF (transformer dereading factor). Some power quality harmonic monitors will automatically calculate these values.

IEEE 519-1992 Recommended Practices and Requirements for Harmonic Control in Electrical Power Systems provides guidelines from determining what acceptable limits are. The harmonic limits for current depend on the ratio of Short Circuit Current (SCC) at PCC (or how stiff it is) to average Load Current of maximum demand over 1 year, as illustrated in Table 5. Note how the limit decreases at the higher harmonic values, and increases with larger ratios.

Table. Current Harmonic Limits as per IEEE 519-1992 For voltage harmonics, the voltage level of the system is used to determine the limits, as shown in Table 6. At the higher voltages, more customers will be effective, hence, the lower limits.

Table. Voltage Harmonic Limits as per IEEE 519-1992 The European Community has also developed susceptibility and emission limits for\ harmonics. Formerly known as the 555-2 standard for appliances of less than 16 A, a more encompassing set of standards under IEC 1000-4-7 are now in effect.

43 HOW DO YOU GET RID OF THEM? Care should be undertaken to make sure that the corrective action taken to minimize the harmonic problems don’t actually make the system worse. This can be the result of resonance between harmonic filters, PF correcting capacitors and the system impedance. Isolating harmonic pollution devices on separate circuits with or without the use of harmonic filters are typical ways of mitigating the effects of such. Loads can be relocated to try to balance the system better. Neutral conductors should be properly sized according to the latest NEC-1996 requirements covering such. Whereas the neutral may have been undersized in the past, it may now be necessary to run a second neutral wire that is the same size as the phase conductors. This is particularly important with some modular office partition-type walls, which can exhibit high impedance values. The operating limits of transformers and motors should be derated, in accordance with industry standards from IEEE, ANSI and NEMA on such. Use of higher pulse converters, such as 24-pulse rectifiers, can eliminate lower harmonic values, but at the expense of creating higher harmonic values.

4.4 TOTAL HARMONIC DISTORTION Harmonic problems are almost always introduced by the consumers’ equipment and installation practices. Harmonic distortion is caused by the high use of non-linear load equipment such as computer power supplies, electronic ballasts, compact fluorescent lamps and variable speed drives etc, which create high current flow with harmonic frequency components. The limiting rating for most electrical circuit elements is determined by the amount of heat that can be dissipated to avoid overheating of bus bars, circuit breakers, neutral conductors, transformer windings or generator alternators. DEFINITION THD is defined as the RMS value of the waveform remaining when the fundamental is removed. A perfect sine wave is 100%, the fundamental is the system frequency of 50 or 60Hz. Harmonic distortion is caused by the introduction of waveforms at frequencies in multiplies of

the fundamental ie: 3rd harmonic is 3x the fundamental frequency / 150Hz. Total harmonic distortion is a easurement of the sum value of the waveform that is distorted.

4.5 POWER MEASUREMENT Despite the use of good quality test meter instrumentation, high current flow can often remain undetected or under estimated by as much 40%. This severe underestimation causes overly high running temperatures of equipment and nuisance tripping. This is simply because the average reading test meters commonly used by maintenance technicians, are not designed to accurately measure distorted currents, and can only provide indication of the condition of the supply at the time of checking. Power quality conditions change continuously, and only instruments offering true RMS measurement of distorted waveforms and neutral currents can provide the correct measurements to accurately determine the ratings of cables, bus bars and circuit breakers. 4.6 NEUTRAL CURRENTS High harmonic environments can produce unexpected and dangerous neutral currents. In a balanced system, the fundamental currents will cancel out, but, triple- N’s will add, so harmonic currents at the 3rd, 9th, 15th etc. will flow in the neutral. Traditional 3 phase system meters are only able to calculate the vector of line to neutral current measurements, which may not register the true reading. Integra 1530, 1560 and 1580 offer a 3 phase 4 wire version with a

neutral 4th CT allowing true neutral current measurement and protection in high harmonic environments. 4.7 HARMONIC PROFILES There is much discussion over the practical harmonic range of a measurement instrument, however study of the harmonic profiles of typically installed equipment can guide the system designer to the practical solution. A typical harmonic profile graph will show a logarithmic decay as the harmonic frequency increases. It is necessary to establish the upper level at which the harmonic content is negligible.

II. DUAL VOLTAGE SOURCE INVERTER A. System Topology The proposed DVSI topology is shown in Fig. 1. It consists of a neutral point clamped (NPC) inverter to realize AVSI and a three-leg inverter for MVSI [18]. These are connected to grid at the PCC and supplying a nonlinear and unbalanced load. The function of the AVSI is to compensate the reactive, harmonics, and unbalance components in load currents. Here, load currents in three phases are represented by ila, ilb, and ilc, respectively. Also, ig(abc), iμgm(abc), and iμgx(abc) show grid currents, MVSI currents, and AVSI currents in three phases, respectively. The dc link of the AVSI utilizes a split capacitor topology, with two capacitors C1 and C2. The MVSI delivers the available power at distributed energy resource (DER) to grid. The DER can be a dc source or an ac source with rectifier coupled to dc link. Usually, renewable energy sources like fuel cell and PV generate power at variable low dc voltage, while the variable speed wind turbines generate power at variable ac voltage. Therefore, the power generated from these sources use a power conditioning stage before it is connected to the input of MVSI. In this study, DER is being represented as a dc source. An inductor filter is used to eliminate the high-frequency switching components generated due to the switching of power electronic switches in the inverters [19]. The

system considered in this study is assumed to have some amount of feeder resistance Rg and inductance Lg. Due to the presence of this feeder impedance, PCC voltage is affected with harmonics [20]. Section III describes the extraction of fundamental positive sequence of PCC voltages and control

strategy for the reference current generation of two inverters in DVSI scheme.

B. Design of DVSI Parameters 1) AVSI: The important parameters of AVSI like dc-link voltage (Vdc), dc storage capacitors (C1 and C2), interfacing inductance (Lfx), and hysteresis band (±hx) are selected based on the design method of split capacitor DSTATCOM topology [16]. The dc-link voltage across each capacitor is taken as 1.6 times the peak of phase voltage. The total dc-link voltage reference (Vdcref) is found to be 1040 V. Values of dc capacitors of AVSI are chosen based on the change in dc-link voltage during transients. Let total load rating is S kVA. In the worst case, the load power may vary from minimum to maximum, i.e., from 0 to S kVA. AVSI needs to exchange real power during transient to maintain the load power demand. This transfer of real power during the transient will result in deviation of capacitor voltage from its reference value. Assume that the voltage controller takes n cycles, i.e., nT seconds to act, where T is the system time period. Hence, maximum energy exchange by AVSI during transient will be nST. This energy will be equal to change in the capacitor stored energy. Therefore

where Vdcr and Vdc1 are the reference dc voltage and maximum permissible dc voltage across C1 during transient, respectively. Here, S = 5 kVA, Vdcr = 520 V, Vdc1 = 0.8 ∗ Vdcr or 1.2 ∗ Vdcr, n = 1, and T = 0.02 s. Substituting these values in (1), the dclink capacitance (C1) is calculated to be 2000 µF. Same value of capacitance is selected for C2. The interfacing inductance is given by

Assuming a maximum switching frequency (fmax) of 10 kHz and hysteresis band (hx) as 5% of load current (0.5 A), the value of Lfx is calculated to be 26 mH. 2) MVSI: The MVSI uses a three-leg inverter topology. Its dc-link voltage is obtained as 1.15 ∗ Vml, where Vml is the peak value of line voltage. This is calculated to be 648 V. Also, MVSIsupplies a balanced sinusoidal current at unity power factor. So, zero sequence switching harmonics will be absent in the output current of MVSI. This reduces the filter requirement for MVSI as compared to AVSI [21]. In this analysis, a filter inductance (Lfm) of 5 mH is used. C. Advantages of the DVSI Scheme The various advantages of the proposed DVSI scheme over a single inverter scheme with multifunctional capabilities [7]–[9] are discussed here as follows: 1) Increased Reliability: DVSI scheme has increased reliability, due to the reduction in failure rate of components and the decrease in system down time cost [13]. In this scheme, the total load current is shared between AVSI and MVSI and hence reduces the failure rate of inverter switches. Moreover, if one inverter fails, the other can continue its operation. This reduces the lost energy and hence the down time cost. The reduction in system down time cost improves the reliability. 2) Reduction in Filter Size: In DVSI scheme, the current supplied by each inverter is reduced and hence the current rating of individual filter inductor reduces. This reduction in current rating reduces the filter size. Also, in this scheme, hysteresis current control is used to track the inverter reference currents. As given in (2), the filter inductance is decided by the inverter switching frequency. Since the lower current rated semiconductor device can be switched at higher switching frequency, the inductance of the filter can be lowered. This decrease in inductance further reduces the filter size. 3) Improved Flexibility: Both the inverters are fed from separate dc links which allow them to operate independently, thus increasing the flexibility of the system. For instance, if the dc link of the main inverter is disconnected from the system, the load compensation capability of the auxiliary inverter can still be utilized. 4) Better Utilization of Microgrid Power: DVSI scheme helps to utilize full capacity of MVSI to transfer the entire power generated by DG units as real power to ac bus, as there is AVSI for harmonic and reactive power compensation. This increases the active power injection capability of DGs in microgrid [22].

5) Reduced DC-Link Voltage Rating: Since, MVSI is not delivering zero sequence load current components, a single capacitor three-leg VSI topology can be used. Therefore, the dclink voltage rating of MVSI is reduced approximately by 38%, as compared to a single inverter system with split capacitor VSI topology. III. CONTROL STRATEGY FOR DVSI SCHEME A. Fundamental Voltage Extraction The control algorithm for reference current generation using ISCT requires balanced sinusoidal PCC voltages. Because of the presence of feeder impedance, PCC voltages are distorted. Therefore, the fundamental positive sequence components of the PCC voltages are extracted for the reference current generation. To convert the distorted PCC voltages to balanced

sinusoidal voltages, dq0 transformation is used. The PCC voltages in natural reference frame (vta, vtb, and vtc) are first transformed into dq0 reference frame as given by

In order to get θ, a modified synchronous reference frame (SRF) phase locked loop (PLL) [23] is used. The schematic diagram of this PLL is shown in Fig. 2. It mainly consists of a proportional integral (PI) controller and an integrator. In this PLL, the SRF terminal voltage in q-axis (vtq) is compared with 0 V and the error voltage thus obtained is given to the PI controller. The frequency deviation Δω is then added to the reference frequency ω0 and finally given to the integrator to get θ. It can be proved that, when, θ = ω0 t and by using the Park’s transformation matrix (C), q-axis voltage in dq0 frame becomes zero and hence the PLL will be locked to the reference frequency (ω0). As PCC voltages are distorted, the transformed voltages in dq0 frame (vtd and vtq) contain average and oscillating components of voltages. These can be represented as

where vtd and vtq represent the average components of vtd and vtq, respectively. The terms vtd and vtq indicate the oscillating components of vtd and vtq, respectively. Now the fundamental positive sequence of PCC voltages in natural reference frame can be obtained with the help of inverse dq0 transformation as given by

These voltages v+ ta1, v+ tb1, and v+ tc1 are used in the reference current generation algorithms, so as to draw balanced sinusoidal currents from the grid. B. Instantaneous Symmetrical Component Theory ISCT was developed primarily for unbalanced and nonlinear load compensations by active power filters. The system topology shown in Fig. 3 is used for realizing the reference current for the compensator [15]. The ISCT for load compensation is derived based on the following three conditions.

1) The source neutral current must be zero. Therefore

2) The phase angle between the fundamental positive sequence voltage (v+ ta1) and source current (isa) is φ

3) The average real power of the load (Pl) should be supplied by the source

Solving the above three equations, the reference source currents can be obtained as

where β = tan√ φ 3 . The term φ is the desired phase angle between the fundamental positive sequence of PCC voltage and source current. To achieve unity power factor for source current, substitute β = 0 in (9). Thus, the reference source currents for three phases are given by

where i ∗ sa, i ∗ sb, and i ∗ sc are fundamental positive sequence of load currents drawn from the source, when it is supplying an average load power Pl. The power Pl can be computed using a moving average filter with a window of one-cycle data points as given below

where t1 is any arbitrary time instant. Finally, the reference currents for the compensator can be generated as follows:

Equation (12) can be used to generate the reference filter currents using ISCT, when the entire load active power, Pl is supplied by the source and load compensation is performed by a single inverter. A modification in the control algorithm is required, when it is used for DVSI scheme. The following section discusses the formulation of control algorithm for DVSI scheme. The source currents, is(abc) and filter currents if(abc) will be equivalently represented as grid currents ig(abc) and AVSI currents iμgx(abc), respectively, in further sections. C. Control Strategy of DVSI Control strategy of DVSI is developed in such a way that grid and MVSI together share the active load power, and AVSI supplies rest of the power components demanded by the load. 1) Reference Current Generation for Auxiliary Inverter: The dc-link voltage of the AVSI should be maintained constant for proper operation of the auxiliary inverter. DC-link voltage variation occurs in auxiliary inverter due to

its switching and ohmic losses. These losses termed as Ploss should also be supplied by the grid. An expression for Ploss is derived on the condition that average dc capacitor current is zero to maintain a constant capacitor voltage [15]. The deviation of average capacitor current from zero will reflect as a change in capacitor voltage from a steady state value. A PI controller is used to generate Ploss term as given by

where evdc = Vdcref − vdc, vdc represents the actual voltage sensed and updated once in a cycle. In the above equation, KP v and KIv represent the proportional and integral gains of dc-link PI controller, respectively. The Ploss term thus obtained should be supplied by the grid, and therefore AVSI reference currents can be obtained as given in (14). Here, the dc-link voltage PI controller gains are selected so as to ensure stability and better dynamic response during load change [24]

2) Reference Current Generation for Main Inverter: The MVSI supplies balanced sinusoidal currents based on the available renewable power at DER. If MVSI losses are neglected, the power injected to grid will be equal to that available at DER (Pμg). The following equation, which is derived from ISCT can be used to generate MVSI reference currents for three phases (a, b, and c)

where Pμg is the available power at the dc link of MVSI. The reference currents obtained from (14) to (15) are tracked by using hysteresis band current controller (HBCC). HBCC schemes are based on a feedback loop, usually with a two-level

comparator. This controller has the advantage of peak current limiting capacity, good dynamic response, and simplicity in implementation [14]. A hysteresis controller is a high-gain proportional controller. This controller adds certain phase lag in the operation based on the hysteresis band and will not make the system unstable. Also, the proposed DVSI scheme uses a first-order inductor filter which retains the closed-loop system stability [25]. The entire control strategy is schematically represented in Fig. 4. Applying Kirchoff’s current law (KCL) at the PCC in Fig. 4

By using (14) and (16), an expression for reference grid current in phase-a (i ∗ ga) can be obtained as

It can be observed that, if the quantity (Pl + Ploss) is greater than Pμg, the term [(Pl + Ploss) − Pμg] will be a positive quantity, and i ∗ ga will be in phase with v+ ta1. This operation can be called as the grid supporting or grid sharing mode, as the total load power demand is shared between the main inverter and the grid. The term, Ploss is usually very small compared to Pl. On the other hand, if (Pl + Ploss) is less than Pμg, then [(Pl + Ploss) − Pμg] will be a negative quantity, and hence i ∗ ga will be in phase opposition with v+ ta1. This mode of operation is called the grid injecting mode, as the excess power is injected to grid IV. SIMULATION STUDIES

The simulation model of DVSI scheme shown in Fig. 1 is developed in PSCAD 4.2.1 to evaluate the performance. The simulation parameters of the system are given in Table I. The simulation study demonstrates the grid sharing and grid

injecting modes of operation of DVSI scheme in steady state as well as in transient conditions. The distorted PCC voltages due to the feeder impedance without DVSI scheme are shown in Fig. 5(a). If these distorted voltages are used for the reference current generation of AVSI, the current compensation will not be proper [14]. Therefore, the fundamental positive sequence of voltages is extracted from these distorted voltages using the algorithm explained in Section III-A. These extracted voltages are given in Fig. 5(b). These voltages are further used for the generation of inverter reference currents. Fig. 6(a)–(d) represents active power demanded by load (Pl), active power supplied by grid (Pg), active power supplied by MVSI (Pμg), and active power supplied by AVSI (Px), respectively. It can be observed that, from t = 0.1 to 0.4 s, MVSI is generating 4 kW power and the load demand is 6 kW. Therefore, the remaining load active power (2 kW) is drawn from the grid. During this period, the

microgrid is operating in grid sharing mode. At t = 0.4 s, the microgrid power is increased to 7 kW, which is more than the load demand of 6 kW. This microgrid power change is considered to show the change of operation of MVSI from grid sharing to grid injecting mode. Now, the excess power of 1 kW is injected to the grid and hence, the power drawn from grid is shown as negative. Fig. 7(a)–(c) shows the load reactive power (Ql), reactive power supplied by AVSI (Qx), and reactive power supplied by MVSI (Qμg), respectively. It shows that total load reactive power is supplied by AVSI, as expected. Fig. 8(a)–(d) shows the plots of load currents (il(abc)), currents drawn from grid (ig(abc)), currents drawn from MVSI

(iμg(abc)), and currents drawn from the AVSI (iμx(abc)), respectively. The load currents are unbalanced and distorted. The MVSI supplies a balanced and sinusoidal currents during grid supporting and grid injecting modes. The currents drawn from grid are also perfectly balanced and sinusoidal, as the auxiliary inverter compensates the unbalance and harmonics

in load currents. Fig. 9(a) shows the plot of fundamental positive sequence of PCC voltage (v+ ta1) and grid current in phase-a (iga) during grid sharing and grid injecting modes. During grid sharing mode, this PCC voltage and grid current are in phase and during grid injecting mode, they are out of phase. Fig. 9(b) establishes that MVSI current in phase-a is always in phase with fundamental positive sequence of phase-a PCC voltage. The same is true for other two phases. Thus the compensation capability of AVSI makes the source current and MVSI current at unity power factor operation. The dc-link voltage of A

is shown in Fig. 10(a) and (b). These figures indicate that the voltage is maintained constant at a reference voltage (Vdcref) of 1040 V by the PI controller. All these simulation results presented above demonstrate the feasibility of DVSI for the load compensation as well as power injection from DG units in a microgrid.

VI. CONCLUSION A DVSI scheme is proposed for microgrid systems with enhanced power quality. Control algorithms are developed to generate reference currents for DVSI using ISCT. The proposedscheme has the capability to exchange power from distributed generators (DGs) and also to compensate the local unbalanced and nonlinear load. The performance of the proposed scheme has been validated through simulation and experimental studies. As compared to a single inverter with multifunctional capabilities, a DVSI has many advantages such as, increased reliability, lower cost due to the reduction in filter size, and more utilization of inverter capacity to inject real power from DGs to microgrid. Moreover, the use of three-phase, threewire topology for the main inverter reduces the dc-link voltage requirement. Thus, a DVSI scheme is a suitable interfacing option for microgrid supplying sensitive loads.

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