Transformer Condition Monitoring And Diagnostic: Presented By Prof. Vg Patel

Transformer Condition Monitoring and Diagnostic PRESENTED BY PROF. VG PATEL VG PATEL

TRANSFORMER ENCYCLOPAEDIA CONDITION MONITORING OF TRANSFORMER

Transformer Condition Monitoring 9/5/20

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TRANSFORMER ENCYCLOPAEDIA CONDITION MONITORING OF TRANSFORMER

WHY CONDITION MONITORING ?

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• Maintenance with intelligence • To avoid failure of equipment in service • To enhance life of equipment • System stability and reliability • To avoid laborious work • To achieve Equipment Life Extn.

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TRANSFORMER ENCYCLOPAEDIA CONDITION MONITORING OF TRANSFORMER

Why it is required? With recent trends in maintenance of Electrical equipments condition monitoring is required, as healthiness of all S/S equipment is required to provide continuous, stable, quality power supply. Because, if any one out of these goes weak and creates trouble, power supply gets affected. So, to look out that all these equipment remain in perfect working condition and do not fail during service, its condition is to be monitored continuously, need of condition monitoring arise.... 9/5/20

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TRANSFORMER ENCYCLOPAEDIA CONDITION MONITORING OF TRANSFORMER

Service Life OR useful Life of Equipment is the period up to which equipment works properly and after that it may not work properly. After completion of Service Life of any equipment, it can be kept in service with the help of Rectification and Modification and periodically monitoring condition of equipment. The extension period is known as a Life Extension.

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TRANSFORMER ENCYCLOPAEDIA CONDITION MONITORING OF TRANSFORMER

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TRANSFORMER ENCYCLOPAEDIA CONDITION MONITORING OF TRANSFORMER

Why is Condition Monitoring important? • Reduces maintenance cost • Lengthening of equipment life • Minimising severity of destruction • Minimises downtime • Maximises productivity

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TRANSFORMER ENCYCLOPAEDIA CONDITION MONITORING OF TRANSFORMER

BASIC REQUIREMENTS OF CONDITION MONITORING

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TRANSFORMER ENCYCLOPAEDIA CONDITION MONITORING OF TRANSFORMER

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TRANSFORMER ENCYCLOPAEDIA CONDITION MONITORING OF TRANSFORMER

TYPE OF CONDITION MONITORING

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TRANSFORMER ENCYCLOPAEDIA CONDITION MONITORING OF TRANSFORMER

Monitoring can be broadly classified into two categories: On Line Continuous Periodic

Off Line M/c Assembled (But Disconnected) M/c Disassembled

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TRANSFORMER ENCYCLOPAEDIA CONDITION MONITORING OF TRANSFORMER

TRANSFORMER CONDITION MONITORING

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TRANSFORMER ENCYCLOPAEDIA CONDITION MONITORING OF TRANSFORMER

Present scenario in our country       

Transformers are many years old. Overloaded due to high demand. Short circuit current level of grid has gone up due to substantial increase in MVA rating. Magnitude of short circuit forces as gone up substantially. Old transformers more susceptible to failure triggered by lightning/switching over voltages/short circuit faults. In recent years many failures of transformers reported. Loss of power - several hundred millions of units per annum.

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TRANSFORMER ENCYCLOPAEDIA CONDITION MONITORING OF TRANSFORMER

Transformer is a high efficient static electrical device used for power transfer from one voltage level to the other and plays the vital role in electrical transmission and distribution system. From the day of this equipment in service, different stresses like electrical, mechanical, chemical, and environmental factors affect the condition of the transformer. At the initial stage, degradation of insulation quality occurs slowly. But this deterioration multiplies in due course of time and leads to final failure of the transformer. So, to overcome this situation, continuous monitoring of the condition and preventive measures is required for correct maintenance of the transformer. The faults like partial discharge, electrical arcs, or hot spots generally deteriorate the condition of transformer in quick progression. Hence early detection of these faults is very important for saving transformer from any catastrophic failure. Basic theories of the deterioration of insulation due to these heat-generated faults are described below. 9/5/20

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TRANSFORMER ENCYCLOPAEDIA CONDITION MONITORING OF TRANSFORMER

Power transformer is one of the costliest and vital equipment therefore the loss of a transformer can have an enormous impact on continuity, reliability of supply and results into considerable loss of revenue. Now a days emphasis is given to need based remedial action / maintenance that should be carried out based on periodical condition monitoring of various components of equipment. It enables utilities to take appropriate, timely action and avoid premature failure. Electrical and thermal stresses lead to de-gradation of paper insulation and also oil in the Transformer. 9/5/20

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TRANSFORMER ENCYCLOPAEDIA CONDITION MONITORING OF TRANSFORMER

For the reasons like abnormal loading, imperfect/faulty design or environmental conditions on the transformer, the temperature of windings, core and insulations rises and results the faults like partial discharge, electric arc and hot spots. Now, because of this temperature rise and presence of oxygen in the insulations (both oil and solid insulations), the oxidation process is developed. The oxidation process is developed. The oxidation of oil results in the formation of oxidation of oil results in the formation of oxidation products (alcohols, aldehydes, acids, esters). The solid matters that formed by oxidation and condensation of polymerization of oil are called sludge. This sludge is usually deposited on the paper insulation and closes the pores on it. 9/5/20

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TRANSFORMER ENCYCLOPAEDIA CONDITION MONITORING OF TRANSFORMER

Hence, the cooling of windings by oil is affected which hampers the performance of the equipment. Similarly the presence of oxygen and acid in the solid insulation (Cellulose) also results the oxidation leads to the formation of carbon dioxide, carbon monoxide, water and the other products. Solid insulation (heat) free glucose molecules, H 2O, CO2, CO cellulose with reduced chain, 2 fur furans. These products being dissolved in oil, deteriorate the condition of insulation in turn the performance of transformer. From the above discussion it concludes that primitive and effective condition are required for transformer. 9/5/20

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TRANSFORMER ENCYCLOPAEDIA CONDITION MONITORING OF TRANSFORMER

The life of a transformer is dependent upon three crucial parameters; temperature, oxygen and moisture. Most power transformers use paper and oil as the main form of insulation and during manufacture stringent efforts are made to ensure that both are as dry as possible when new transformer leaves the factory.

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TRANSFORMER ENCYCLOPAEDIA CONDITION MONITORING OF TRANSFORMER

EFFECT OF MOISTURE Moisture in transformer insulation can affect transformer performance in several ways: Trigger partial discharge Generate bubbles Lower the dielectric strength Age the insulation prematurely

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TRANSFORMER ENCYCLOPAEDIA CONDITION MONITORING OF TRANSFORMER

The first three effects can leads to premature failure of good transformer; the last one could shorten transformer normal life expectancy. Aged transformers have higher moisture content than new transformer, hence are more prone to the above effects.   Moisture reduces the dielectric strength of paper and oil which in turn increases the risk of electrical failure. It also reduces the mechanical strength of the paper. 9/5/20

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TRANSFORMER ENCYCLOPAEDIA CONDITION MONITORING OF TRANSFORMER

Since hot oil is able to support more moisture than cold oil, percentage saturation is temperature dependent. If hot oil is cooled the relative saturation will increase and an emulsion of water and oil may form. Free water may also be produced which will have a serious effect on dielectric strength and can lead to the formation of rust where it collects, typically in the bottom of main tank and conservators. Furthermore, when wet paper gets very hot, for example during a period of sustained over-loading, the possibility of bubble formation arises. When this occurs moisture in the paper is boiled off as water vapour introducing a risk of partial discharge and electrical breakdown. 9/5/20

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TRANSFORMER ENCYCLOPAEDIA CONDITION MONITORING OF TRANSFORMER

For large transformers, we are talking about tonnes of paper, thousands of litres of oil and hundreds of litres of water. The dielectric breakdown strength of the paper is a function of its water content 1. Equilibration between oil and paper based on temperature. 2. Water moves from paper to oil as the temperature increases. 3. As the temperature increases, the water solubility in the oil also increases. Never change oil because it is wet. The new oil will very quickly degrade to the same condition as the old as moisture leaves the paper to re-establish a condition of 9/5/20 24 equilibrium. VG PATEL

TRANSFORMER ENCYCLOPAEDIA CONDITION MONITORING OF TRANSFORMER

Water in oil sample is very small ppm compared to insulation. Comparison of water distribution in oil and paper Sr. No. 1

Temperature Water Water in paper Deg C in oil 20 1 3000 times what is in the oil

2

40

1

1000 times what is in the oil

3

60

1

300 times what is in the oil

It is important that hot oil sample should be taken. (transformer working temperature should be taken.) 9/5/20

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TRANSFORMER ENCYCLOPAEDIA CONDITION MONITORING OF TRANSFORMER

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TRANSFORMER ENCYCLOPAEDIA CONDITION MONITORING OF TRANSFORMER

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TRANSFORMER ENCYCLOPAEDIA CONDITION MONITORING OF TRANSFORMER

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TRANSFORMER ENCYCLOPAEDIA CONDITION MONITORING OF TRANSFORMER

Oxygen causes oxidation (Chemical deterioration) Effects of oxidation:  Results in acids and sludge. Acid attack solid insulation and metal. Sludge causes poor thermal conduction and mechanical hindrance to proper oil circulation. 9/5/20

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TRANSFORMER ENCYCLOPAEDIA CONDITION MONITORING OF TRANSFORMER

TYPICAL CONDITION MONITORING PROGRAM

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TRANSFORMER ENCYCLOPAEDIA CONDITION MONITORING OF TRANSFORMER

TYPICAL CM PROGRAM Historical Data Visual Inspection & examination

Electrical tests

Data Analysis Trend Analysis

Interpretation

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TRANSFORMER ENCYCLOPAEDIA CONDITION MONITORING OF TRANSFORMER

Transformer Diagnostic Techniques

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TRANSFORMER ENCYCLOPAEDIA CONDITION MONITORING OF TRANSFORMER

WHAT TO MONITOR IN TRANSFORMER AND TOOLS FOR MONITORING

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TRANSFORMER ENCYCLOPAEDIA CONDITION MONITORING OF TRANSFORMER

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TRANSFORMER ENCYCLOPAEDIA CONDITION MONITORING OF TRANSFORMER

Tools for Monitoring • • • • • • •

Insulation System Dissolved Gas Analysis Partial Discharge Meas. Hot Spot Temp. Meas. Furan Analysis IR Measurement Dissipation Factor Meas. Recovery Voltage Meas. 9/5/20

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Winding System Reactance Meas. Low Voltage Impulse (LVI) test Frequency Response Analysis Transfer Function Method 36

TRANSFORMER ENCYCLOPAEDIA CONDITION MONITORING OF TRANSFORMER

ELECTRICAL DIAGNOSTIC TOOLS IR Insulation ( DC ) DGA Oil TAN DELTA Insulation ( AC ) SFRA Integrity of Tr. winding DP Insulation ( paper) PD Insulation ( void ) FURAN Oil (indirect for DP) 9/5/20

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TRANSFORMER ENCYCLOPAEDIA CONDITION MONITORING OF TRANSFORMER

Diagnostic Tests for Power Transformer Transformers in service operate under complex environmental conditions and variable thermal and electrical stresses. Hence, its Condition monitoring is complex and consists many tools. • Moisture of oil • IR/AI/PI --- Insulation (DC) • Dielectric Discharge (Tan delta/Capacitance) --- Insulation (AC) • PD (Partial Discharge - inception voltage) --- Insulation (void) • DP --- Degree of Polymerization --- Insulation (paper) • DGA (Online/Offline) --- Oil • SFRA --- Integrity of Tr. Winding • Furan --- Oil (indirect for DP) --- Paper insulation • Hydran --- Dissolved H2 and Hydro Carbons (H2 & HxCy ) • Step Voltage Test --- Insulation • RVM --- Recovery Voltage Measurements --- Paper insulation • Thermography --- Hot spot in conductor joint • Fiber Optic Sensors --- Oil and Winding Temperature • MAP Analysis 9/5/20

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TRANSFORMER ENCYCLOPAEDIA CONDITION MONITORING OF TRANSFORMER

IR, AI AND PI MEASUREMENTS INSULATION (DC)

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RESISTANCE AND POLARISATION INDEX TESTS •INSULATION IR, AI & PI measurements are useful indicators of the presence of contamination and moisture on insulation surfaces of the winding. • They are performed to verify the overall cleanliness, dryness, localized defects and general condition of insulation systems. • In practice, the IR values may vary over a wide range from a few Mega Ohms to several Giga Ohms. • The minimum acceptable lower limit for IR values is given in standard. • When the dc voltage is applied across the insulation, the current flows is the resultant of three currents • Capacitive charging current • Absorption current • 9/5/20 Leakage current 41 VG PATEL

TRANSFORMER ENCYCLOPAEDIA CONDITION MONITORING OF TRANSFORMER

INSULATION RESISTANCE AND POLARISATION INDEX TESTS 

Capacitance leakage current: The current lasts for a few seconds as DC voltage is applied and drops out after the insulation is charged to its full voltage. The time depends on the size and capacitance of the test object. Larger time for larger capacitance objects.

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TRANSFORMER ENCYCLOPAEDIA CONDITION MONITORING OF TRANSFORMER

INSULATION RESISTANCE AND POLARISATION INDEX TESTS 

Absorption leakage current:



It is caused by polarization of molecules within dielectric material. In low capacitance equipments the current is high for first few seconds and decrease slowly to nearly zero. In high capacitance equipment or wet and contaminated insulation, there will be no decrease of absorption current for long time.





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TRANSFORMER ENCYCLOPAEDIA CONDITION MONITORING OF TRANSFORMER

INSULATION RESISTANCE AND POLARISATION INDEX TESTS

 Conduction or leakage current: 

 



This is the current that normally flows through the insulation ,between conductors or conductors to ground. It increases quickly and becomes stable. This current increases as insulation deteriorates and becomes predominant after absorption current vanishes. It is steady and time independent. Hence the important current for measuring insulation resistance.

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TRANSFORMER ENCYCLOPAEDIA CONDITION MONITORING OF TRANSFORMER

INSULATION RESISTANCE AND POLARISATION INDEX TESTS

IR MEASUREMENTS

• THE TEST VOLTAGES FOR IR TRANSFORMER ARE AS UNDER. RATING

6.6 kV 11 kV & ABOVE

OF

TEST VOLTAGE

415/440 V 3.3 kV

MEASUREMENT

500 V 1000 V 2500 V 5000 V

THE LOWER LIMITS MAY DIFFER DEPENDING ON EXPERIENCES COLLECTED FOR A PARTICULAR SITE CONDITION. THE MINIMUM ACCEPTABLE LOWER LIMIT FOR IR VALUES IS GIVEN IN STANDARDS AS (V L-L / 1000) + 1 M Ohm. 9/5/20

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TRANSFORMER ENCYCLOPAEDIA CONDITION MONITORING OF TRANSFORMER

HEALTH OF INSULATION – (I.R.VALUE) Basic property of good insulating material is to separate all electrical circuit at different potential from earth. •IR values to be measured with help of megger are (1) PH. to E (2) PH. To PH. (3) complete winding to E (4) HV to LV windings. • It varies with duration and magnitude of test voltage & temp. RISE IN 10 deg C, IR BECOMES HALF. •Minimum IR value required =[(un/1000)+ 1] M. Ohm. IR value is usually indicates overall cleanliness and dryness of insulation. It does not confirm that insulation is free from mechanical damage or physical weakness. 9/5/20

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TRANSFORMER ENCYCLOPAEDIA CONDITION MONITORING OF TRANSFORMER

The purpose of an insulation resistance (IR) reading is to evaluate the condition of the insulation between conductors and ground. This is done by applying a direct voltage between the conductors (windings) and the casing of the transformer and measuring current leakage across the insulation system. The readings are applied to Ohm's law (R=V÷I), which provides a resistance. In the case of insulation system, current may be measured in milli or micro-ohms. The lower the current reading, the higher the insulation resistance reading. These IR readings change over time because of dielectric absorption. Basically, the insulation system consists of polarized atoms that line up (polarize) with the applied DC voltage. As they polarize, the insulation resistance will increase. 9/5/20

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IR MEASUREMENTS

Capacitance charging current is initially high and drops as the insulation becomes charged up to the applied voltage. The actual length of time depend upon the size and capacitance of the test object: larger time for large capacitance objects.

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Absorption current is also initially high but drops at a much slower rate than the charging current. It depends on the nature of the insulation material The conduction or leakage current builds up quickly to a steady value and then remains constant for a particular applied voltage under stable conditions. It is this current that is affected by moisture, dirt etc. and the degree to which it flows bears a direct relation to the quality of insulation. An increase in the leakage current is a pointer to possible future problems.

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ABSORPTION OF MOISTURE BY INSULATING MATERIALS Solid insulation in transformers is very porous and absorbs much water.  Some of the water that is dissolved in the oil is absorbed from the oil by the insulation. As more water is dissolve in the oil, more water is absorbed by the insulation. The most effective method for drying out the insulation in transformers is with heat and vacuum. The transformer insulation must be dried by circulation of hot, dry oil. This oil should then be cooled and dried.  Since the dielectric strength of insulation is reduced by moisture, it is important that the insulation not be allowed to absorb the moisture the first place. 9/5/20

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With the release of IEEE Std. 43-2000, new limitations on the use and evaluation of PI were issued. This test is no longer as easy and straightforward as it once was because most new insulation systems start with test results in the Giga-ohm and Terra-ohm (billions and trillions of ohms) range. As a result, leakage current between the windings below the micro-ohm range require the instrument and test methods used, including how to position the conductors, to be extremely accurate. The readings must also be taken at the transformer itself. Additional concerns with PI testing are outlined within IEEE Std. 432000, including temperature limitations, dew-point limitations, trending tests, surface conditions, types of windings to be tested, and more. For instance, the tests must be performed above the dew point and corrected for temperature to 40°C. This is a change from the 1974 version of the standard, where it was assumed that PI wasn't a temperature-dependent test. 9/5/20

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The main change has to do with the polarization of new insulation systems based upon temperature because unlike conductors where the resistance increases with temperature, insulation resistance is inversely proportional to temperature so insulation resistance decreases with temperature. Therefore, a PI performed on a hot insulation system may be dramatically different than a PI performed on a cold one (i.e., in storage). The only true way to trend the PI is to perform each test with the winding at about the same temperature, over the dew point.

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TRANSFORMER ENCYCLOPAEDIA CONDITION MONITORING OF TRANSFORMER

•If the leakage current component is very much larger than the other two components, then the total current (& hence the IR value) will not change significantly with time. •Hence in order to help determine how dry and clean the winding is, the polarization index is calculated. •Polarization index = ratio of 600 second IR value to 60 second IR value. •The value of PI approaches 1 for bad insulation while PI in the range of 2 to 4 can be considered as an indication of good insulation in case of rotating machines (in case of transformer, range is 1.2 to 1.5 can be considered as an indication of good insulation). •A very high value of PI is also not advisable, since it shows the brittleness of insulation. •Since, PI is the ratio, it is independent of temperature. •It gives quantitative information about the insulation W.R.T. moisture, dust & other contamination. 9/5/20

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Polarization index: Prior to 1974, the industry relied on nonstandardized systems to evaluate the polarization of the insulation system. This led the IEEE to form a standards committee to address this situation. It was discovered that many insulation systems would polarize in anywhere from 10 minutes to several hours, so it was possible to produce a ratio using the insulation readings at the one-minute and 10-minute marks. This ratio could be trended or compared to a simple table that would provide an indication of the condition of the winding. It was then possible to analyze PI by either providing the direct ratio or plotting the readings in steady increments over time. In the past, a PI ratio of less than 2.0:1.0 would indicate a problem with the insulation system; usually the insulation would be carbonized (burned) or would have absorbed contamination. A poor reading would indicate that additional testing was necessary. 9/5/20

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HEALTH OF INSULATION – (P.I. VALUE)

Polarization index: •It is a ratio of IR values. PI = IR at (10 min./1 min.) •I.R. = Vdc/(Ic + Ia + Ilc + Ils) where Ic = cap. Current, Ia = dielectric Current. Ic & Ia reduces after 10 min •Ilc& Ils = leakage current thro’ insulation & surface. •Since PI is the ratio, it is independent of temperature. •It gives quantitative information about the insulation W.R.T. Moisture, dust & other contamination. •High value of PI indicates good condition of insulation. For clean and dry windings PI > 1.3 to 1.5. •A very high value of PI is also not advisable, since it shows the brittleness of insulation. 9/5/20 54 VG PATEL

TRANSFORMER ENCYCLOPAEDIA CONDITION MONITORING OF TRANSFORMER

Interpretation of results

IR measurements 



If IR value shows a decreasing trend it shows gradual deterioration of insulation quality due to humidity, dust accumulation etc.. A very sharp drop in IR value indicates insulation failure.

Polarization index measurements  

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Polarization Index=600sec IR value/60 sec IR value PI<1 , for bad insulation and PI in the range of 1.2 to 2 can be considered as an indication of good insulation. A very high value of PI is also not advisable since it shows the brittleness of insulation. 9/5/20

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CAPACITANCE & TAN δ

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TANDELTA AND CAPACITANCE TEST:oPerfect dielectric is a dielectric in which all of the energy required to established an electric field in the dielectric is recoverable when the voltage is removed. oIn a perfect capacitor, the voltage and current are phase shifted 90 degrees oImperfect dielectric is a dielectric in which a part of the energy is not returned when voltage removed. The energy which is not returned is converted in to heat loss. oIf there are impurities in the insulation, The current and voltage will no longer be shifted 90 degrees. oIf the insulation is free from defects, like water trees, electrical trees, moisture and air pockets, etc. insulation approaches the properties of a perfect capacitor. The presence of any undesirable conditions can be identified such as • Moisture content • Conductive contaminants • Internal partial discharges 9/5/20

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TRANSFORMER ENCYCLOPAEDIA CONDITION MONITORING OF TRANSFORMER  THE ABSOLUTE VALUES OF CAPACITANCE & TAN DELTA WILL VARY DEPENDING ON TYPE OF INSULATION AND CONSTRUCTION OF EQUIPMENT.  MEASURED TAN DELTA AT LOW VOLTAGE GIVES AN INDICATION OF THE WINDING CONTAMINATION, WHILE THE TAN DELTA TIP UP GIVES AN INDICATION OF THE VOID CONTENT.  A HIGHER TAN DELTA TIP UP AT A CERTAIN APPLIED VOLTAGE INDICATES PRESENCE OF VOIDS/IMPURITIES IN THE INSULATION AND THE INCEPTION OF PDS AT THIS VOLTAGE.  THE INCREASE IN TAN DELTA WITH INCREASE IN VOLTAGE AND PASSING OF TIME SHOWS THE DETERIORATION OF INSULATION.

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Concept of Tan 

• Theoretically an ideal capacitive current I c leads the voltage in phase by 90°. • If there are impurities in insulation, the resistance of insulation decrease resulting in increase of resistive current. • However, in practice it draws both charging current and a small dissipative or loss current component current. Thus the total current I = Ic + Ir which leads the voltage by a phase angle < 90°. and lags the Ic by an angle . • The tangent of this angle directly indicates the heat dissipation that takes place inside the dielectric material. • The values obtained on new insulation forms the reference value for periodic measurements. • By measuring the latest values of C & tan of the insulation of any equipment with those monitored earlier. 9/5/20

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Tan  - Analysis

• A high tan will result in thermal breakdown of the insulation at a lower voltage. • A lower tan is generally desirable, but a high dielectric loss does not necessarily mean that the insulation is inferior. • If the dielectric loss increases with the increase in voltage, it indicates voids & pd phenomena in the insulation. • If the Tan values increase with respect to time it indicates the deterioration of insulation. • Detailed Tan analysis depends on the following : • Magnitude of Tan at various voltages & max increase in tan with respect to increase in voltage. • Steeper slope in the tan values wrt voltage shows defective insulation. • Tan rise wrt time indicates insulation deterioration. 9/5/20

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Dissipation Factor / Power Factor •With the application of voltage, voids / impurity present in insulation may discharge partially. •Increase in capacitance w.r.t. applied voltage indicates deterioration of insulation. •Variation of Tanδ w.r.t applied volt. is know as Tanδ TIPUP. •Increase in Tanδ tip-up indicates deterioration in insulation. •If two successive result indicates rise< 10% it is considered stable but > 10%, measuring frequency can be increased. •Steeper slop of TANδ w.r.t. Voltage indicates defective insulation. 9/5/20

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C -TAN DELTA

•Frequency can be once in a year, preferably in same period to avoid ambient effect. •All readings should be converted to 20 deg. C and compared. •Reading should never be taken if humidity is > 50%. •Transformer oil may contain moisture, small floating metallic or dust particles, which can change result much. Reading should be taken after filtering. •Test equipment should be isolated, discharged, kept at room temp. for long time so that all components attains room temp. It should be perfectly cleaned. 9/5/20

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HEALTH OF INSULATION: (C – TAN DELTA) • Capacitance is charge it can stored at a given voltage. • Capacitance value depends on the type & characteristic of the dielectric material. • Real dielectric is associated with some loss, like conduction loss due to partial discharge. • It is measure of quality of insulation • δ depends upon the impurities present in the insulating material. PF = COS θ DISSIPATION FACTOR = TAN δ IC LEAD V BY 90° ic LEAD V BY (90 – δ)° 9/5/20

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TAN  MEASUREMENT

C - TAN MEASUREMENT IS ONE OF THE MOST USEFUL, RELIABLE, EFFECTIVE, NON-DESTRUCTIVE DIAGNOSTIC TOOL. • THE CAPACITANCE VALUE IS DEPENDENT ON:  THE CHARACTERISTICS OF THE DIELECTRIC MATERIAL.  THE PHYSICAL CONFIGURATION OF THE ELECTRODES. • HENCE C WILL INCREASE WITH INCREASE IN VOLTAGE, WHICH INDICATES PRESENCE OF PD AND THE DETERIORATION OF INSULATION. •

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TRANSFORMER ENCYCLOPAEDIA CONDITION MONITORING OF TRANSFORMER

TAN  MEASUREMENT

• A SINGLE C/TAN  MEASUREMENT ON A COMPLETE WINDING BY ITSELF IS OF LIMITED USE. • PERIODIC MEASUREMENTS SHALL BE CONDUCTED FOR TREND ANALYSIS. • INITIAL TEST RESULTS CAN BE CONSIDERED AS BENCHMARK FOR FURTHER CONDITION MONITORING • FOR EVALUATION PURPOSE FOLLOWING DATA ARE ANALYSED: oMEASURED ABSOLUTE VALUES AT STEPS OF VOLTAGE oTIP UP VALUES oCOMPARISON WITH PREVIOUS TEST DATA & MANUFACTURERS’ RESULTS, IF ANY oCOMPARISON WITH DATA OF SIMILAR EQUIPMENT WORKING UNDER SIMILAR ENVIRONMENTS. 9/5/20 66 VG PATEL

TRANSFORMER ENCYCLOPAEDIA CONDITION MONITORING OF TRANSFORMER

TANDELTA (DISSIPATION FACTOR) :oWHAT ARE PERMISSIBLE LIMITS ? oIdeally, Tan —Delta of insulation should be zero oBut in actual it has small loss (Ir) component. oIt is trend test and value of Tan-Delta is to be compared with previous value. Transformer winding :oNEW : 1 % is Max. permissible value oIN SERVICE : 2 % is Max. permissible value

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TRANSFORMER ENCYCLOPAEDIA CONDITION MONITORING OF TRANSFORMER

SWEEP FREQUENCY RESPONSE ANALYSIS [SFRA]

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TRANSFORMER ENCYCLOPAEDIA CONDITION MONITORING OF TRANSFORMER

SFRA SFRA Capable of Detecting

When compared with signature results any deviation is an indicator of Coil deformation (axial or radial) Core Movements Faulty core ground Partial winding collapse, Winding Deformations, Displacement Hoop buckling Broken or loosen clamps Shorted turns & open windings. 9/5/20

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TRANSFORMER ENCYCLOPAEDIA CONDITION MONITORING OF TRANSFORMER

SFRA (Sweep Frequency Response Analysis) Reliable tool for mechanical condition assessment of the windings. Transformers subjected to mechanical stresses during ■ Transportation ■ Short circuit faults near the transformer ■ Transient over voltages such as switching, lightning etc. Mechanical Stresses cause ■ Winding displacement or deformation ■ Winding collapse in extreme cases ■ Such mechanical defects eventually lead dielectric faults in the winding 9/5/20

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TRANSFORMER ENCYCLOPAEDIA CONDITION MONITORING OF TRANSFORMER

FREQUENCY RESPONSE ANALYSIS (FRA) The winding of every transformer has an unique RLC network depending on its geometry, used materials and manufacturing. The frequency response analysis for transformer is a very sensitive technique for detecting winding movement caused by loss of clamping pressure or by short circuit forces, in service. A change of RLC network appears, it means a change of frequency response characteristics that can be identified by a comparative analysis with signature. FRA consists of measuring the impedance of transformer windings over a wide range of frequencies and comparing the results of these measurements with a reference set. 9/5/20

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TRANSFORMER ENCYCLOPAEDIA CONDITION MONITORING OF TRANSFORMER

FUNDAMENTALS OF SFRA We all know three main Element in Electrical network: o ResistanceR

Not vary with frequency

o Inductance L=2πfl

Directly vary with frequency

o Capacitance C=1/2πfc Inversely vary with frequency

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TRANSFORMER ENCYCLOPAEDIA CONDITION MONITORING OF TRANSFORMER

HOOP BUCKLING: Produced within a winding due to excessive compressive forces during a fault. Winding loses shape and gains a 'bump' when seen end on. Results in a bent, (but not broken) winding. Gassing may result. Transformer likely to be able to carry on in service. Transformer integrity is compromised.

SHORT TURN: Produced within transformer by a turn-to-turn fault Adjacent turns lose paper and braze / weld together Results in a solid loop around the core. 9/5/20

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TRANSFORMER ENCYCLOPAEDIA CONDITION MONITORING OF TRANSFORMER

Many organizations use SFRA technique:  In SFRA a steady sinusoidal input applied to a test object and measured output, Sweeping through the frequency range The ratio of Vout / Vinput indicate frequency response  For Analysis, this Ratio converted in to graph DB v/s frequency by using formula DB = 20 logl 0(V out/ V input )  Each 20 dB drop means we are looking at a tenth of the previous Vout / Vin

dB Response: as impedance increases, V out falls

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TRANSFORMER ENCYCLOPAEDIA CONDITION MONITORING OF TRANSFORMER

Test method: ■ Consists of application of a sinusoidal signal to one end of the winding. ■ Output voltage is measured at the other end of the winding, other windings are left open. Transfer function (Vo/Vi) is measured for three frequency ranges: ■

Low frequency range 50 Hz to 2 kHz

■

Medium frequency range 50Hz to 20 kHz

■

High frequency range 5 kHz to 2 MHz 9/5/20

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TRANSFORMER ENCYCLOPAEDIA CONDITION MONITORING OF TRANSFORMER

Interpretation Condition assessment is based on comparison of the present signature with the earlier patterns obtained on the same winding under healthy condition. ■ Comparison of responses of different phases of the same winding at the same tap position. ■ Comparison of responses of different transformers of the same design. ■

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TRANSFORMER ENCYCLOPAEDIA CONDITION MONITORING OF TRANSFORMER

TYPICAL RESPONSES TYPICAL RESPONSE OF A HEALTHY TRANSFORMER  Starting db value is what we would expect.  The shape is consistent with a star configuration.  All phase lineup with each other. TYPICAL RESPONSE OF A TRANSFORMER WITH HUGE PROBLEMS  Starting DB value of the white phase is unexpected.  The shape is not consistent with a star configuration.  Phase do not lineup with each other.  Clear frequency shift with the formation of new resonant point. 9/5/20

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TRANSFORMER ENCYCLOPAEDIA CONDITION MONITORING OF TRANSFORMER

Summary - conclusion  SFRA is an established methodology for detecting electromechanical changes in power transformers  Collecting reference curves on all mission critical transformers is an investment!  Ensure repeatability by selecting good instruments and using standardized measurement practices 9/5/20

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TRANSFORMER ENCYCLOPAEDIA CONDITION MONITORING OF TRANSFORMER

PARTIAL DISCHARGE (PD) (Pl refer topic 10A)

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TRANSFORMER ENCYCLOPAEDIA CONDITION MONITORING OF TRANSFORMER

Partial discharge (summarizing): • It is an electrical discharge that occurs across a portion of the insulation between two conducting electrodes without completely bridging the gap. • This results in localized, nearly instantaneous release of energy. • The most convention unit for quantifying the PD quantity is Pico coulomb. 9/5/20

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TRANSFORMER ENCYCLOPAEDIA CONDITION MONITORING OF TRANSFORMER

DISSOLVED GAS ANALYSIS (DGA)

(Pl refer topic 10C)

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TRANSFORMER ENCYCLOPAEDIA CONDITION MONITORING OF TRANSFORMER

Following gases are generally found dissolved in transformer oil:

OXYGEN - O2 NITROGEN - N2 HYDROGEN - H2 CARBON MONOXID - CO CARBON DIOXID - CO2 METHANE - CH4 ETHANE - C2H6 ETHYLENE - C2H4 ACETYLENE - C2H2 PROPANE - C3H8 PROPYLENE - C3H6 9/5/20

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TRANSFORMER ENCYCLOPAEDIA CONDITION MONITORING OF TRANSFORMER

SUMMARIZING: • By condition monitoring we mean continuous evaluation of the health of plant and equipment throughout its serviceable life. • Condition monitoring and protection are closely related functions. The approach to the implementation of each is, however, quite different. • Condition monitoring can, in many cases, be extended to provide primary protection, but its real function must always be to attempt to recognize the development of faults at an early stage.

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TRANSFORMER ENCYCLOPAEDIA TRANSFORMER TROUBLE SHOOTING

CONCLUSION • Condition monitoring provides more reliable scheduling tool for preventive maintenance. • Accurate fault diagnostic reduces numbers unexpected failures. • Trend, comparison of conditions at different time is important. • One has to filter out electrical, magnetic noise to evaluate correct condition of transformer.

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TRANSFORMER ENCYCLOPAEDIA TRANSFORMER TROUBLE SHOOTING

CONCLUSION • Various techniques in combination can be used to identify the root cause of problem. • One has to adopt many new monitoring techniques now available. • On line monitoring can bridge the gap between two monitoring events. • Humming of transformer is irritating but silence with bang is catastrophic.

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TRANSFORMER ENCYCLOPAEDIA

OPEN FORUM 9/5/20

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TRANSFORMER ENCYCLOPAEDIA

THANQ

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