UNIT 5 : HIGH VOLTAGE TESTING AND INSULATION COORDINATION

UNIT 5 : HIGH VOLTAGE TESTING AND INSULATION COORDINATION
5.1 NEED FOR HIGH VOLTAGE TESTS Verify the name plate details Confirm the design specifications Dr M A Panneerselvam, Professor, Anna University

5.2 CLASSIFICATION OF HIGH VOLTAGE TESTS
Ascertain the maximum withstand values (capacity) Research and development of insulating materials 5.2 CLASSIFICATION OF HIGH VOLTAGE TESTS High voltage tests can be classified under various heads Dr M A Panneerselvam, Professor, Anna University

Dr M A Panneerselvam, Professor, Anna University
as given under: 5.2.1 Based on the type of tests: Type tests Routine tests Special tests Acceptance tests Dr M A Panneerselvam, Professor, Anna University

Type tests are tests conducted on a randomly selected sample out of a given lot , confirming to the same rating and specification , and manufactured over a given period of time. These tests are conducted by a neutral authority Dr M A Panneerselvam, Professor, Anna University

and witnessed by both manufacturer and customer. Routine tests are conducted on all piece of equipment generally by manufactures in their own premises. Dr M A Panneerselvam, Professor, Anna University

Special tests are certain tests conducted only on the request of customer otherwise these tests are not conducted normally.( Ex. Impulse test including chopped waves on transformers) . Dr M A Panneerselvam, Professor, Anna University

Acceptance tests are tests conducted at the customer’s premises during receipt of consignment (equipment). The customer may specify any test for this purpose. Dr M A Panneerselvam, Professor, Anna University

5.2.2 Based on the nature of voltages: DC voltage test AC voltage test Impulse voltage/Impulse current test Switching surge test Dr M A Panneerselvam, Professor, Anna University

5.2.3 Based on the nature of breakdown: Withstand tests Flashover/Sparkover tests Puncture tests Dr M A Panneerselvam, Professor, Anna University

5.2.4 Based on the environment: Dry tests Wet tests Dr M A Panneerselvam, Professor, Anna University

5.3 ELECTRICAL EQUIPMENT CONSIDERED FOR TESTING
Power transformers ,distribution transformers, instrument transformers,reactors , etc., Protective devices like circuit breakers, lightning arresters , etc., Dr M A Panneerselvam, Professor, Anna University

Porcelain insulators and other types of insulators Cables , capacitors and other dielectric materials and Busducts(isolated and 3 phase busducts) and other sundry equipment. Dr M A Panneerselvam, Professor, Anna University

5.4 STANDARD HIGH VOLTAGE TESTS
Out of all the equipment listed above, Power transformers , Distribution transformers ,Lightning arresters , Circuit breakers, Line insulators and Power cables form the major components of a power system. Dr M A Panneerselvam, Professor, Anna University

The high voltage tests conducted in general on the above equipment are classified as follows: 5.4.1 Direct voltage tests: Tests on cables and other dielectric materials. Dr M A Panneerselvam, Professor, Anna University

5.4.2 Power frequency tests: Induced overvoltage test and separate source voltage withstand test on power transformers, one minute withstand / flashover test under dry and wet conditions , puncture test on insulators Dr M A Panneerselvam, Professor, Anna University

power frequency sparkover test on lightning arresters and dry and wet withstand tests on circuit breakers. 5.4.3 Impulse voltage tests: Withstand test on power transformers, Impulse sparkover test and front of wave impulse Dr M A Panneerselvam, Professor, Anna University

sparkover test(V-T curve) on lightning arresters, withstand/ flashover tests on insulators and isolators and withstand/flashover tests on bus ducts. 5.4.4 Switching surge tests : Withstand tests on power transformers, insulators, lightning arresters , circuit breakers , etc., Dr M A Panneerselvam, Professor, Anna University

All the tests mentioned above can be grouped under two heads ,viz.,(1) Tests conducted on external insulation and (2) Tests conducted on internal insulation of electrical equipment. Dr M A Panneerselvam, Professor, Anna University

As the tests conducted are large in numbers and are on different type of equipment , it is not possible to give the meaning of all the terminology adopted and they can be easily verified from the relevant standards. Dr M A Panneerselvam, Professor, Anna University

Depending upon the country in which the equipment is going to be installed it can be tested as per the standards of that country. (Ex., Indian standard, British standard, NEMA standard etc.,) Dr M A Panneerselvam, Professor, Anna University

5.5 LIST OF IMPORTANT INDIAN STANDARDS USED IN HIGH VOLTAGE TESTING
Method for high voltage measurement by means of sphere gaps (one sphere earthed ) :IS Dr M A Panneerselvam, Professor, Anna University

2)Method of high voltage testing-Part I : General definitions and test requirements : IS 2071(Part I)-1974 3) Method of high voltage testing – Part II:Test procedures :IS 2071(Part II) 1974 Dr M A Panneerselvam, Professor, Anna University

4) Method of high voltage testing-Part III: Measuring devices:IS 2071(Part III)1976 5) Specification for power transformers-Part III :Insulation levels and dielectric tests : IS 2026(Part III) -1981 Dr M A Panneerselvam, Professor, Anna University

6) Specification for lightning arresters for alternating current systems-Part I :Non linear type lightning arrester : IS 3070 (Part I ) 7) Specification for alternating current circuit breakers - Part II : IS 2516 (Part II/ Sec.2) Dr M A Panneerselvam, Professor, Anna University

8) Direct voltage tests on cables: IS -698. (Cables are generally tested with DC voltages to avoid heavy charging currents and loading of generator) Dr M A Panneerselvam, Professor, Anna University

When specific standard is not available for testing of newly developed equipment the tests can be conducted in general agreement with existing standards based on the type of test and the nature of insulation. Dr M A Panneerselvam, Professor, Anna University

5.6 GENERAL REQUIREMENTS OF HIGH VOLTAGE TESTING
Prior to starting of the actual test on a given equipment there are certain important prerequisites and details to be looked into , viz, Dr M A Panneerselvam, Professor, Anna University

(1) Arrangement of test object (2) Conditioning of test object and (3) Correction factors for atmospheric conditions ( air density and humidity). 5.6.1 Arrangement of test object: Dr M A Panneerselvam, Professor, Anna University

Clearance to extraneous structures and general disposition of the object , viz, height above ground level , the arrangement of high voltage lead etc. is of importance. If the clearance to extraneous structures is at least 1.5 times Dr M A Panneerselvam, Professor, Anna University

the flashover distance between the electrodes of the test object it is found to be sufficient. 5.6.2 Conditioning of test object: Depending upon the requirements of testing ,the tests are conducted under dry or wet Dr M A Panneerselvam, Professor, Anna University

conditions. The rain conditions are simulated using spray of water through nozzles falling on the test object at angle of 45 0 to the vertical axis. The characteristics of the spray for tests with alternating voltages are given in the following slide. Dr M A Panneerselvam, Professor, Anna University

ARRANGEMENT FOR WATER SPRAY DURING WET TEST
Dr M A Panneerselvam, Professor, Anna University

Average precipitation rate for measurement of vertical component = 3 ± 10 % mm/min Limit for individual measurement = 3 ± 25 % mm/min Resistivity of collected water = 100 ± 10 % ohm-m Dr M A Panneerselvam, Professor, Anna University

Duration of wet withstand test = 60 sec. The characteristics of water spray for switching surge test for voltages above 500 kV are : Vertical component = 1 to 1.5 mm/min. and Horizontal Dr M A Panneerselvam, Professor, Anna University

component = 1 to 1.5 mm/min. Limit for individual measurements = 0.5 to 2.0 mm/min Resistivity of water=100± 15 % ohm-m. Dr M A Panneerselvam, Professor, Anna University

5.6.3 Atmospheric correction factors: As the high voltage tests are invariably conducted at conditions other than standard temperature, pressure and humidity the voltage values should be multiplied by Dr M A Panneerselvam, Professor, Anna University

atmospheric correction factors, viz, Air density correction factor , kd Humidity correction factor , kh Standard atmospheric conditions (STP) are : Temperature = 20 0 C , Pressure = 760 mm of Hg. (1013 millibars) and absolute humidity Dr M A Panneerselvam, Professor, Anna University

=11 gm/m3. For tests conducted on external insulations the voltage at room temp. V(RTP) is related to V(STP) as below: V (RTP) = V (STP) kd/kh In case of sphere gap test, only air Dr M A Panneerselvam, Professor, Anna University

density correction factor is applied and the humidity correction factor is not applied. Air density factor ‘ d ‘ is given by, d = b / 273+ td Here ‘b’ is pressure in millibars and ‘ td ‘ is the dry bulb temp. 0C Dr M A Panneerselvam, Professor, Anna University

When pressure is given in mm-Hg , d = b/ 273+ td The table in next slide, gives the values air density correction factor for the calculated value air density factor ‘d’. Dr M A Panneerselvam, Professor, Anna University

TABLE SHOWING AIR DENSITY CORRECTION FACTOR AND AIR DENSITY FACTOR
Note : FOR CHENNAI WEATHER CONDITIONS AIR DENSITY CORRECTION FACTOR IS EQUAL TO AIR DENSITY FACTOR Dr M A Panneerselvam, Professor, Anna University

Humidity correction factor(kh):
COMPUTATION OF ABSOLUTE HUMIDITY FROM DRY AND WET BULB THERMOMETER TEMPERTURES Dr M A Panneerselvam, Professor, Anna University

HUMIDITY CORRECTION FACTORS Dr M A Panneerselvam, Professor, Anna University

Calibration of Impulse Generator: Calibration means getting the relationship between the input charging voltage and output voltage of the Impulse Generator for a given connected load. Dr M A Panneerselvam, Professor, Anna University

Let us take testing of insulators for use in 132 kV system : The maximum system voltage is132 x 1.1 = 145 kV . Assuming the system is effectively earthed the Power frequency withstand voltage level = 230 kV and the impulse voltage withstand level = 550 kV. Dr M A Panneerselvam, Professor, Anna University

To test the insulator for an impulse voltage of 550 kV we should know the corresponding input charging level of the Impulse Generator and hence we do the calibration of the generator. Dr M A Panneerselvam, Professor, Anna University

The calibration is generally done at a voltage of 50 to 75 % of the test voltage. In this case since the test voltage is 550 kV the calibration is done at a level of 275 kV to 413 kV. Let the Impulse Generator has 6 stages with a charging voltage of Dr M A Panneerselvam, Professor, Anna University

200 kV. When the generator is connected for full series the output voltage of the generator is approximately 200 x 6 = 1200 kV. The test object is connected to the Impulse Generator as shown in the next slide. Dr M A Panneerselvam, Professor, Anna University

MULTISTAGE IMPULSE GENERATOR CONNECTED TO POTENTIAL DIVIDER, MEASURING SPHERES AND LOAD Dr M A Panneerselvam, Professor, Anna University

As the calibration has to be done at about 275 kV and referring to sphere gap standard , for 50 cm dia. spheres , a gap set for 15 cm the disruptive discharge voltage is 374 kV (which is 68 % of the test level) at STP for positive impulse. Dr M A Panneerselvam, Professor, Anna University

Let us assume the following atmospheric conditions during the testing: Dry bulb temp. = 30 deg. C , Wet bulb temp. = 24 deg. C and Pressure = 758 mm of Hg. Dr M A Panneerselvam, Professor, Anna University

The air density factor = 0.386x758 / = 0.965 For Chennai weather conditions Air density correction factor = Air density factor , kd. Dr M A Panneerselvam, Professor, Anna University

Hence, the disruptive discharge voltage for a gap setting of 15 cm for 50 cm dia. spheres = 374 x kd = 374 x = 361 kV ( Note: Humidity correction is not applied for sphere gaps). Dr M A Panneerselvam, Professor, Anna University

To get an output voltage of 361 kV , the charging level to the first stage of the Impulse Generator is approximately = 361 / 6 = 61 kV. Starting from a charging voltage of 55 kV to 65 kV, the 50 % disruptive discharge voltage for Dr M A Panneerselvam, Professor, Anna University

this setting is obtained using average or up and down method. Let the 50 % disruptive discharge voltage so obtained is 63 kV. That is, a charging voltage of 63 kV gives an output voltage of 361 kV at RTP conditions. Dr M A Panneerselvam, Professor, Anna University

Before the calibration procedure starts it is important to check the wave form of the impulse voltage and if it is not as per specification the parameters (R1 and R2)are changed to get the proper wave shape of 1 .2 / 50 μs. Dr M A Panneerselvam, Professor, Anna University

Once the calibration is over , the test voltage levels to be applied at RTP conditions are calculated. i.e., V(RTP) = V(STP) x kd/kh Humidity correction factor is obtained from the dry bulb Dr M A Panneerselvam, Professor, Anna University

and wet bulb temperatures. For td = 30 deg.C and tw= 24 deg.C the humidity is obtained from the graph as gm/m3 . From another graph the humidity correction factor for power frequency, positive and Dr M A Panneerselvam, Professor, Anna University

negative impulses are obtained as 0.910, and respectively. Hence the positive impulse to be applied at (RTP) conditions = 550 x / = 571 kV and for negative impulse =550x0.965 / = 565 kV Dr M A Panneerselvam, Professor, Anna University

For power frequency tests, the cascaded transformer is calibrated and the relation between the input voltage and the output voltage is obtained. The power frequency withstand Dr M A Panneerselvam, Professor, Anna University

level at STP for 145 kV insulator is 230 kV (RMS) The test voltage to be applied at RTP conditions =230 x / = 244 kV (RMS). Dr M A Panneerselvam, Professor, Anna University

5.7 TESTS ON EXTERNAL INSULATIONS(WITHSTAND/ PUNCTURE/FLASHOVER TESTS UNDER DRY/WET CONDITIONS) 5.7.1 DC voltage tests: The voltage should be applied to the test object starting with a low value Dr M A Panneerselvam, Professor, Anna University

to prevent overvoltage due to switching transient but not so slowly as to cause unnecessary stress on the test object. The voltage is maintained for a specific time (as per standards) and then rapidly reduced. The Dr M A Panneerselvam, Professor, Anna University

test object is supposed to have withstood the test if no discharge occurs.To obtain the disruptive discharge voltage , the voltage is raised as described above till the disruptive discharge occurs and the level is noted. Dr M A Panneerselvam, Professor, Anna University

5.7.2 AC voltage tests: The voltage should be applied as specified earlier (under DC test). The requirements during voltage application are generally met if the rate of rise above 75% of the test voltage is about 2 % per second. It should be maintained Dr M A Panneerselvam, Professor, Anna University

for specified time (generally 60 sec.) and then rapidly decreased. The test object is supposed to have withstood the test if there is no disruptive discharge. For wet tests water spray as per specification should be used. Dr M A Panneerselvam, Professor, Anna University

The voltage is increased further to obtain the disruptive discharge level. 5.7.3 Impulse voltage tests: Generally 15 impulses of the rated withstand voltage and of specified wave shape and polarity Dr M A Panneerselvam, Professor, Anna University

are applied. The test object is supposed to have withstood if no disruptive discharge occurs in the non self restoring insulation and not more than one disruptive discharge occur in self restoring insulation. In addition the Dr M A Panneerselvam, Professor, Anna University

oscillograms of the applied voltages are recorded and there should be no difference between the first and the fifteenth impulse and both should be exactly identical and look one and the same. Dr M A Panneerselvam, Professor, Anna University

5.7.4 Switching surge tests: As the system voltage increases beyond 400 kV, switching surge magnitude increases and becomes more severe. The general wave shape for the test is 250/2500 microseconds. The test Dr M A Panneerselvam, Professor, Anna University

procedure for switching surges is the same as that of lightning impulse tests. Depending upon the particular type of equipment tested the wave shapes vary and 200/2000 microseconds or /3000 microseconds waves are used. Dr M A Panneerselvam, Professor, Anna University

The impulse , switching surge and power frequency test voltage levels for testing equipment at different system voltages are given in the next slide. Dr M A Panneerselvam, Professor, Anna University

WITHSTAND LEVELS FOR SYSTEM VOLTAGES < 245 kV
POWER FREQUENCY AND IMPULSE TEST VOLTAGE WITHSTAND LEVELS FOR SYSTEM VOLTAGES < 245 kV Dr M A Panneerselvam, Professor, Anna University

INSULATION LEVELS FOR SYSTEM VOLTAGES > 245 kV Dr M A Panneerselvam, Professor, Anna University

SPECIMEN OSCILLOGRAMS TAKEN DURING IMPULSE TEST ON 11 kV INSULATOR

5.8 POWER FREQUENCY / IMPULSE VOLTAGE WITHSTAND/FLASHOVER TESTS ON THREE PHASE CIRCUIT BREAKERS / ISOLATORS Dr M A Panneerselvam, Professor, Anna University

5.8.1 Modes of connections : High voltage tests on 3 phase circuit breakers / isolators are conducted by connecting them in 5 different modes as given below: With breaker/isolator contacts closed: Dr M A Panneerselvam, Professor, Anna University

Mode 1 : Phase ‘RY’ connected together and earthed with frame and high voltage applied to ‘B’pahase. Mode 2: Phase ‘YB’ connected together and earthed with frame and high voltage applied to ‘R’ phase. Dr M A Panneerselvam, Professor, Anna University

Mode 3: Phase ‘BR’ connected together and earthed with frame and high voltage applied to ‘Y’ phase. With breaker / isolator contacts open: Mode 4: One side of each pole connected together and earthed Dr M A Panneerselvam, Professor, Anna University

and the other side of the poles connected together and applied with high voltage.The frame is isolated. Mode 5: Same as ‘Mode 4’ but high voltage lead and earth connections inter changed. Dr M A Panneerselvam, Professor, Anna University

TESTING OF 3 PHASE CIRCUIT BREAKER / ISOLATOR Dr M A Panneerselvam, Professor, Anna University

OSCILLOGRAMS TAKEN DURING IMPULSE TEST ON 11 kV AIR BREAK SWITCH

TESTING OF 3 PHASE 230 kV DISCONNECTING SWITCH

TESTING OF 500 kV DISCONNECT SWITCH

5.9 IMPULSE VOLTAGE WITHSTAND TEST(INCLUDING CHOPPED WAVES) ON POWER TRANSFORMERS / DISTRIBUTION TRANSFORMERS Dr M A Panneerselvam, Professor, Anna University

5.9.1 introduction : In case of power transformers both primary and secondary windings are subjected to impulse test as both the windings are HV windings. They are exposed to open atmosphere and hit by lightning strokes (Ex. 66 kV/ 11 kV). Dr M A Panneerselvam, Professor, Anna University

But, the distribution transformers are subjected to impulse test only on high voltage winding. The low voltage winding is generally connected to distribution network and are not likely to be hit by lightning (Ex.11 kV/ 420 V). Dr M A Panneerselvam, Professor, Anna University

IMPULSE TESTING OF 3 PHASE TRANSFORMER

Unless and otherwise stated , impulse test on transformers are conducted generally with negative polarity impulse. The sequence of voltage applications given below: Dr M A Panneerselvam, Professor, Anna University

One full wave at reduced BIL recorded with lesser time base. (ii) One full wave at reduced BIL(same as (i)) recorded with higher time base. Dr M A Panneerselvam, Professor, Anna University

(iii) One full wave at 100 % BIL recorded with higher time base. (iv) One chopped wave at reduced BIL recorded with lesser time base.. Dr M A Panneerselvam, Professor, Anna University

(v) & (vi) Two chopped waves at 100 % BIL recorded with lesser time base. (vii)&(viii) Two full waves at 100 % BIL recorded with higher time base Dr M A Panneerselvam, Professor, Anna University

TYPICAL OSCILLOGRMS TAKEN DURING IMPULSE TEST ON TRANSFORMER

100kVA, 11kV/433V three phase Distribution Transformer
Oscillograms taken during the impulse voltage withstand test on 100kVA, 11kV/433V three phase Distribution Transformer Plate 1 : Reduced BIL waves (56.25kV at STP) recorded on 10s / division. 100% BIL waves (75kV at STP) recorded on 10s / division Plate 2 : Reduced chopped wave (60kV at STP) recorded on one s./division Plate 3 : First 100% chopped waves (75kV at STP) recorded on one s./division Second 100% chopped waves (75kV at STP) recorded on one s./div. Plate 4 :100% BIL waves (75kV at STP) recorded on 10s / division % BIL waves (75kV at STP) recorded on 10s / division Dr M A Panneerselvam, Professor, Anna University

OSCILLOGRAMS TAKEN DURING IMPULSE VOLTAGE WITHSTAND TEST ON 250kVA, 11kV/433V, THREE PHASE DISTRIBUTION TRANSFORMER PHASE A Dr M A Panneerselvam, Professor, Anna University

PHASE B Dr M A Panneerselvam, Professor, Anna University

PHASE C Dr M A Panneerselvam, Professor, Anna University

5.10 FAILURE REPORTS Case 1 : 100kVA, 11kV/433V Al WOUND TRANSFORMER (shows the progressing failure in the winding insulation) Dr M A Panneerselvam, Professor, Anna University

Case II : 315kVA, 22kV/433 V Cu wound three phase distribution transformer Dr M A Panneerselvam, Professor, Anna University

Case iii : 22kV metering Unit (CT) Dr M A Panneerselvam, Professor, Anna University

Case iv : 12KV, 630A, 20KA, SF6 Load Break Fault Make switch FIRST AND FIFTEENTH POSITIVE AND NEGATIVE IMPULSE VOLTAGES APPLIED AT 75 KV IN MODE I TO MODE IV. Dr M A Panneerselvam, Professor, Anna University

5.10 INSULATION COORDINATION
Insulation Coordination is the correlation of the characteristics of the protective devices and the equipment connected in the power system Dr M A Panneerselvam, Professor, Anna University

such that the equipment and the system are well protected by the protective devices in the event of excessive over voltages. Dr M A Panneerselvam, Professor, Anna University

Fixing of BIL for power system operating at 132 kV: Dr M A Panneerselvam, Professor, Anna University

Let us first consider that the system is not effectively earthed, in which case the coefficient of earthing (COE) is unity. The rating of the lightning arrester is chosen as 132 x 1 = 132 kV Dr M A Panneerselvam, Professor, Anna University

The discharge voltage for this rating of the arrester = 132 x 3 (taken from arrester manual)= 396 kV. Allowing 40 % excess for reflection and refraction at the terminal end, the voltage at the transformer terminal = 396 x 1.4 = 555 kV. Dr M A Panneerselvam, Professor, Anna University

Further allowing a margin of about 60 kV the insulation level becomes = = 615 kV Choosing the next slab in the insulation level, the BIL is selected as 650 kV. Dr M A Panneerselvam, Professor, Anna University

Let us now assume the system to be an effectively earthed system, for which the COE is taken as less than 75 % Working on the same line as earlier, the rating of the lightning arrester is chosen as 132x0.75 = 99 kV. Dr M A Panneerselvam, Professor, Anna University

The residual voltage for this rating from the arrester manual = 99 x 3 = 297 kV . Allowing 40 % excess for reflection and refraction , the voltage at the transformer terminal rises to = 297 x 1.4 = 416 kV. Dr M A Panneerselvam, Professor, Anna University

Allowing a margin of 60 kV , insulation level becomes = = 476 kV Choosing the higher slab of insulation the BIL is selected as kV. This level is known as reduced BIL. Dr M A Panneerselvam, Professor, Anna University

Hence, it is seen that as the system becomes effectively earthed the insulation level is reduced resulting in great saving of insulation and cost. Dr M A Panneerselvam, Professor, Anna University

UNIT 5 : HIGH VOLTAGE TESTING AND INSULATION COORDINATION

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