LIGHTNING IMPULSE TEST In -precisePresented byS.M.G.MUTHAR HUSSAINTransformer Engineer

CONTENTS• WHAT IS LIGHTNING? • WHAT IS IMPULSE? Typical impulse wave shape and parameters• WHAT IS LIGHTNING IMPULSE TEST?• WHY IS LI TEST DONE ON DISTRIBUTION

TRANSFORMERS?• WHY IS SUCH WAVE SHAPE USED?• HOW IMPULSE VOLTAGE IS GENERATED? Generation of Full Wave Impulse Wave shape adjustment with HV winding Wave shape adjustment with LV winding Generation of Chopped wave impulse• TEST CIRCUIT• TERMINAL CONNECTIONS• EARTHING PRACTICES

CONTENTS continued…• FAILURE DETECTION METHODS• TEST PROCEDURES Test sequence Test setup• EQUIPMENTS REQUIRED FOR TEST• METHOD OF OPERATION• MEASURING TECHNIQUES• INTERPRETATION OF RESULTS• DIAGNOSIS General recordings Voltage recordings-Full wave tests Current recordings-Full wave tests Voltage and Current recordings-Chopped wave tests• FOOT NOTES• REFERENCES

What is Lightning?

• Lightning is a natural phenomenon that occurs in the atmosphere when two neighboring clouds of opposite charges strike each other or between a negative cloud and a grounded object on earth. It is associated with an electrical discharge and huge sound during a thunderstorm.

• The phenomenon of Lightning introduces lightning surges in transmission network. The Lightning strokes on transmission lines are of two type direct strokes and induced strokes. When a thunder cloud directly discharges on a transmission line tower or line wire it is called a direct stroke. It is the most severe form of lightning stroke and is rare on transmission systems and only induced strokes occurs.

• When a thunderstorm generates negative charges at its ground end, the earthed objects develop induced positive charge. The earthed objects of interest are transmission lines and towers. Normally, it is expected that the lines are unaffected because they are insulated by strings of insulators.

What is Lightning? Continued…

• However because of high field gradients involved the positive charges leak from the tower along the insulator surfaces to the line conductors.

• This process may take considerable time in the order of some hundreds of seconds. When the cloud discharges to some earthed object other than the line conductor, the transmission line conductor is left with a high concentration of positive charge which cannot leak suddenly. The transmission line and the ground will act as a huge capacitor carrying a positive charge and hence over voltages occur due to these induced charges. This would result in a stroke which is commonly known as an “induced” lightning stroke.

• As a result of a direct stroke or an induced stroke, a large current impulse is injected into the line conductor which produces a voltage surge in the line as high as 5,000kV. However on an average most of the lightning strokes give rise to voltage surges of less than 1000kV on over head lines.

What is Impulse?

•An impulse is defined as a unidirectional voltage (or current) rising quickly to its peak value and then decaying slowly to zero. An impulse voltage is characterized by its peak value Vp, its front time t f and its tail time or time to half t t.

• Thus an impulse has 2 parts i.e. a rising part which is usually realized by charging a capacitor and a decaying part which is realized by discharging a capacitor.

•Impulse voltages are used to simulate the stresses imposed on high voltage apparatus due to lightning or switching surges.

Typical Impulse wave shape and parameters

t f – Front time is the time taken for the wave to reach its Peak value. t t – Tail time or time to half of the peak value.The front time for a standard lightning impulse is 1.2µs while its tail time is 50µs.Tolerance allowed in peak value is ±3%The tolerance allowed for front time is ±30% and that for tail time is ±20%

What is Lightning Impulse test?• It is a dielectric test conducted on an electrical

equipment to demonstrate that the insulation will withstand impulses that strike it externally due to lightning during its service period. High voltage surges resembling lightning that are simulated in laboratories are commonly known as high voltage impulses. These lightning Impulses are used to test the equipment.

• It is important to note that the distribution of impulse-voltage stress through the transformer winding may be very different from the low frequency voltage distribution.

• Low-frequency voltage distributes itself throughout the winding on uniform volts-per turn basis. Impulse voltages are initially distributed on the basis of winding capacitances.

What is Lightning Impulse test? Continued…• Since the transients are impulses of short rise time, the

voltage distribution along the transformer winding is not uniform. The initial part of the winding (i.e. close to the HV line lead) will have higher stress than the winding part close to the ground end and thus it may be the first to breakdown in case of high voltage surges. The test is performed at a voltage ≥ the rated basic lightning impulse insulation level (BIL).

• Impulse testing of an oil filled distribution transformer is usually performed using both the full wave (IEC -60076-2000 Clause 13) and the chopped wave (Clause 14) impulses with chopping time ranging from approximately 2 to 6µs. To prevent large over-voltages being induced in the windings that are not under test, they are short circuited and connected to ground through low impedance paths.

Why is Lightning Impulse test done on Distribution transformers?

• Insulation is recognized as one of the most important constructional elements of a transformer. Its chief function is to confine the current to useful paths, preventing it flow into harmful channels. Any weakness of insulation may result in failure of the transformer. A measure of the effectiveness with which insulation performs is the dielectric strength of transformer.

• The purpose of the impulse test is to determine the ability of the transformer insulation to withstand the transient voltages due to lightning.

• It is well known that power system components are subjected to severe over voltage due to internal switching or external lightning surges. Consequently the integrity of the individual components, devices, and subsystems must be checked through high voltage surge testing.

Why is such wave shape used?

•From the data compiled by the 1937 AIEE-EEI-NEMA committee on Insulation Co-ordination about natural lightning, it was concluded that system disturbances from lightning can be represented by three basic wave shapes- full wave, chopped wave and front-of-waves.

•Later it was recognized that lightning disturbances will not always have these three waves but definitely full wave and chopped wave and hence a standard impulse wave is specified by IEC with front time 1.2µs ± 30% tolerance and tail time (time to half) 50µs ± 20% tolerance

How Impulse Voltage is generated?

•Lightning Impulse is generated using an Impulse Generator.

Generation of Full Wave Impulse

How Impulse Voltage is generated? Continued…

• The above figure shows a simple full wave impulse generating circuit. It consists of 2 capacitors C1 (larger) and C2 (smaller). Capacitor C1 is used for charging and Capacitor C2 for discharging. It has 2 Resistors R1 and R2. R1 is connected in series with C1 through a HV switch (e.g. a sphere spark gap). R2 is connected parallel to C2. The output is taken across the parallel combination of R2 and C2.This is a single stage generator.

• In this circuit, R1 and C2 determine the front time while C1 and (R1+R2) determine the tail time of an impulse. The values of these parameters are selected to generate the desired impulse wave form for the required application.

How Impulse Voltage is generated? Continued…

• When very high impulse voltages are needed, multistage generators are used. The basic idea of a multistage impulse generator circuit is to charge several stage capacitors in parallel and then discharge them in series.

• Where standard impulse shape cannot be obtained because of low winding inductance or high surge capacitance to earth wider tolerances may be accepted by agreement between purchaser and supplier.

• Technical papers written on the subject of “Surge generator characteristics for transformer testing” show that the time to crest (front time t f) of an impulse wave is affected by the series inductance, series resistance and load capacitance. The tail of the wave (tail time t t) is controlled by the generator capacitance, load resistance, load inductance and also load capacitance.

How Impulse Voltage is generated? Continued…• Wave shapes Adjustment with HV winding:• The surge capacitance of the transformer under test being constant, the

series resistance may have to be reduced in an attempt to obtain the correct front time T1 (1.2µs ± 30%).

• Wave shapes Adjustment with LV winding:• With the low voltage windings the virtual time to half value T2 (50µs ±

20%) may not be achievable because of the low inductance making the wave shape oscillatory.

• This problem can be solved to some extent by• Use of large capacitance within the generator• By multiple parallel stage operation of generator• By adjustment of the series resistor• By impedance earthing rather than direct earthing resulting in a

significant increase in the effective inductance i.e. earthing the non tested terminals through resistors not exceeding the surge impedance of the cable (400Ω) such that in all circumstances the voltage to earth appearing at the non tested terminals does not exceed 75% of the rated lightning impulse withstand voltage of that terminal for STAR connected windings and 50% of the rated lightning impulse withstand voltage of that terminal for DELTA connected windings (because of opposite polarity voltages to earth on delta terminals)

How Impulse Voltage is generated? Continued…• In direct earthing only leakage inductance is involved. • In impedance earthing main inductance becomes more predominant and this

makes the effective inductance 100 to 200 times greater than direct earthing.• Lightning Impulse test can done by either apply Full Wave (LI) or Chopped

wave (LIC). • Generation of chopped Wave Impulse• When a full wave surge occurs on a power network and a flashover takes place

across a bushing or an insulator etc., the voltage instantaneously falls to zero resulting in a chopped impulse wave. The voltage chopping can take place either on the front, at the peak or on the tail of a surge. To simulate such a chopped surge wave, a rod-rod chopping gap is normally placed in parallel with the test object across the impulse generator. The distance of the chopping gaps can be adjusted to control the width of the applied chopped wave during the chopped impulse testing. Triggered chopping gaps are often used to control the chopping time. Chopped impulse testing is required in some applications.

• Switching Impulse (SI) is done on power transformers with Um≥72.5kV and where Lightning Impulse becomes a Routine test. SI is usually required for transformers subjected to ACLD (AC induced over voltage test with long duration) with Partial Discharge Measurement.

Test Circuit• Main Circuit: • Impulse Generator • Wave shaping Components• Test object • Measuring circuits•  • Voltage Measuring Circuit: • Voltage divider (Resistance type, Capacitance type, Compensated type)• Coaxial Cable• Digital Impulse Measuring system (DIMS)• Peak Voltmeter (if available)•  • Chopping Circuit if Applicable: • Rod-Rod Chopping gap•  • Test Calibration: • Before a test an overall check of the test circuit and measuring system may be performed at

a voltage lower than the reduced voltage level (50% of the rated full voltage). In this check voltage may be determined by means of sphere gap or by comparative measurement with another approved device. When using a sphere gap it should be recognized that this is only a check and does not replace the periodic calibration of the approved measuring system. After any check has been made it is essential that neither the measuring nor the test circuit is altered except for the removal of any devices for checking. i.e. The impulse circuit and measuring connections shall remain unchanged during calibration (Reduced wave at 50% of test voltage)

Typical impulse test circuit

Terminal Connections  • Normally the non-tested terminals of the phase winding

under test are earthed and the non-tested phase windings are shorted and earthed. However in order to improve the wave tail T2, resistance earthing of the non-tested windings may be advantageous and in addition the non-tested line terminals of the winding under test may also be resistance earthed.

• If a terminal has been specified to be directly earthed or connected to a low impedance cable in service, then that terminal should be directly earthed during the test or earthed through a resistor with an ohmic value not in excess of the surge impedance of the cable (400Ω).

• Earthing through a low impedance shunt for the purpose of impulse response current measurements may be considered the equivalent of direct earthing.

• No Impulse test on the neutral terminal is recommended. During Impulse test on a line terminal the neutral shall be connected directly to earth.

Lightning Impulse test terminal connections and applicable methods of failure detection

Earthing Practices

• During Impulse testing Zero potential cannot be assumed throughout the earthing system due to the high values and rates of change of impulse currents and voltages and the finite impedances involved. Therefore selection of proper earth is important.

• The Current return path between the test object and the impulse generator should be of low impedance. It is a good practice to firmly connect this current return path to the general earth system of the test room preferably close to the test object. This point of connection should be used as reference earth and to attain good earthing of the test object it should be connected to the reference earth by one or several conductors of low impedance.

• The voltage measuring circuit which is a separate loop of the test object carrying only the measuring current and not any major portion of the impulse current flowing through the windings under test should also be effectively connected to the same reference earth.

Failure Detection Methods

• The fault in winding insulation is detected by general observations of noise, smoke, etc. during the impulse voltage application. Moreover, the inspection of voltage and current oscillograms give more accurate indication of the failure especially for the partial failure. A partial or complete failure of winding appears as a partial or complete collapse of the applied impulse voltage. However, the impulse voltage may not show a small partial failure since the sensitivity of the voltage waveform method is low and this method does not detect faults which occur on less than 5% of the total winding. The failure detection in such cases is enhanced by current oscillogram which usually shows a record of the impulse current flowing through a resistive shunt or a high bandwidth current transformer connected between the neutral and the ground or between the low voltage winding and the ground.

Failure Detection Methods continued…• The current oscillogram usually consists of a high

frequency oscillation, a low frequency disturbance and a current rise due to reflections from the ground end of the windings. When a major fault such as breakdown between turns or between one turn and the ground occurs, high frequency pulses are observed in the current oscillogram and the wave shape changes. For local failure such as a partial discharge only high frequency oscillations are observed without a change of wave shape. To detect any failure, voltage and current oscillographs for the full wave impulses are compared with the initial records corresponding to the reduced full wave.

• Similarly chopped impulses are compared as well. The IEC test criteria states that “the absence of significant differences between voltage and current transients recorded at reduced voltage and those recorded at full test voltage constitutes evidence that the insulation has withstood the test.” If there are doubts about the interpretation of possible discrepancies between oscillograms, three subsequent impulses at full test voltage shall be applied or the whole impulse test on the terminal shall be repeated.

Test Procedures

• Lightning Impulse test (LI) is performed on transformers whose winding terminals are brought out for accessibility. For Oil immersed transformers the test voltage is normally of Negative polarity because this reduces the risk of erratic external flash over in the test circuit. Bushing spark gaps may be removed or their spacing increased to prevent their spark over during the test.

• The test impulse shall be a full standard lightning impulse 1.2µs ± 30% / 50µs ± 20% waveform. However there are cases where this standard impulse wave shape cannot be reasonably obtained because of low winding inductance or high capacitance to earth. The resulting impulse shape is then often oscillatory. Therefore in such cases wider tolerances may be permitted by agreement between the parties. The amplitude of opposite polarity of an oscillatory impulse should not exceed 50% of the first amplitude.

• The test is usually performed at a voltage ≥ the rated basic lightning impulse insulation level (BIL).

• Impulse testing of an oil filled transformer is usually performed using both the full wave (Clause 13) and the chopped wave (Clause 14) impulses with chopping time ranging from 2 to 6 µs as per the requirements of IEC-60076-3 (2000 edition), Clauses 13 and 14. To prevent large over-voltages being induced in the windings, those are not under test are short circuited and connected to ground through low impedance paths.

Test Sequence • Following sequence of impulse voltage applications is specified for oil filled

transformers as per IEC 60076-3, if test is required for full and chopped waves i.e. Clause 13 and 14:

• One or more reduced full wave impulse at 50-75% of BIL.• One full wave impulse of 100% BIL.• One or more reduced chopped wave impulse at 50-75% of BIL.• Two chopped impulses at 110% BIL and• Two full wave impulses at 100% BIL.•  • If the chopped wave testing is not required sequences 3 and 4 mentioned

above are ignored. Thus for full wave test according to Clause 13 only, one or more full wave reduced impulse and three full wave impulses at 100% BIL are applied. Since the insulation is of non-self restoring type, the transformer must withstand all the three impulses at 100% of BIL.

• If the LI test is performed as per ANSI C57 test procedures, the test voltage applications should have the following sequence

• One reduced full wave impulse at 50-70% of BIL.• One reduced chopped wave impulse at 50-70% of BIL.• Two chopped wave impulses ≥ 115% BIL and• One full wave impulse at 100% BIL• The other criterion is more or less similar to the IEC requirements given

above.

Test set up

• The schematic diagram showing typical connections for the impulse testing of a three phase DELTA/STAR distribution transformer is given in Fig 11.10. Here winding UW is under test with full impulse voltage application. Moreover, it is ensured that windings UV and VW are subjected only to half of the test voltage with the help of an external voltage divider. In case of STAR/STAR winding connections, each HV winding is tested separately and the other two windings are short circuited and grounded. In transformer testing, it is essential to record the waveforms of the applied voltage and the resulting current transients through the winding under test. Sometimes, the transferred voltages in the secondary winding and/or the neutral currents are also recorded.

Arrangement of a transformer for Impulse voltage test

Equipments required for test

•Transformer to be tested•Triggered sphere spark gaps•Impulse generator •Voltage divider (Resistive or Capacitive)•Dual Channel digital impulse measuring

system (DIMS) •Components ( High Bandwidth CT,

Current shunt )•Coaxial cable & Connection wires

Method Of Operation

• Energize the circuit from the output of the HVAC transformer through a half wave negative polarity rectifier.

• Adjust the sphere gap’s separation so that it can breakdown at the rated full impulse voltage level or slightly higher than that.

• Increase the HVAC and thus the HVDC charging voltage slowly till the breakdown of the spark gap is possible.

• Then apply a trigger pulse so that a spark occurs in the spheres and an impulse is generated.

• Apply one or more full wave impulse voltage at reduced magnitude of 50 to 75% of BIL for calibration purpose between the phases UW (φ U) of a DELTA connected winding (HV Winding)

• Measure the impulse (peak value) at reduced voltage using the voltage divider and DIMS and record its waveform parameters through one channel.

• Measure the impulse response current (winding current) passing through the high bandwidth CT or Current shunt connected between the “W” terminal and ground using the second channel of the DIMS.

Method Of Operation continued…

• Do not alter either the test circuit or the measuring circuit after calibration.

• Now apply three full wave impulses of 100% BIL in succession between phases UW (φ U).

• Measure the impulse (peak value) at full rated voltage and impulse current using the two channels of DIMS and record its waveform parameters.

• Repeat the above procedure from 5 to 10 for the other two phases VU (φ V) and WV (φ W) and record its voltage and current waveforms.

• Compare the wave shapes of voltages and currents recorded between reduced and full impulse voltage levels or between successive records at rated test voltage.

• If no significant difference is found between voltage and current transients recorded at reduced voltage and those recorded at full test voltage it is evident that the insulation has withstood the test and passed the LI test.

Measuring Techniques•Impulse response current measurement is

done through low impedance shunt.•Digital Recordings: •Oscillograph / Digital records obtained

during Calibration and tests shall clearly show the applied lightning impulse voltage, impulse shape (front time, time of half value, amplitude) and impulse response current wave with the help of at least 2 independent recording channels.

Interpretation Of Results

•Assessment of test results is primarily based on the comparison of wave shapes of voltages and currents recorded between reduced and full impulse voltage levels or between successive records at rated test voltage.

DIAGNOSISGeneral RecordingsDiscrepancy Possible Cause Check & RemedyVariation in wave shapes ofVoltages and current records

Disturbances due to test circuit, measuring circuit and earthing methods

Check and if so eliminate or minimize their effect

Variation in the amplitude of current records with high frequency initial oscillations

Difference in firing times of the individual stages of a multi stage generator

Check and if so correct the firing timing to be the same

Variation in the amplitude of current records with high frequency initial oscillations

Discharge circuits are not coincident in time

Check and if so make new settings of the discharge gaps on generator

Logical and Progressive change with increasing voltage levels

Core earthing or non-linear elements (surge arrestors) disturbances

Check and if so eliminate by solid core earthing and removing non-linear resistors.

Even after eliminating the above sources of discrepancies, any variations in the wave shape of voltage and current records between the reduced and rated test voltage or between successive records at rated test voltage which cannot be proved to have originated from the test circuit or in non-linear resistors within the test object, it is evident that the insulation has failed during the test.

DIAGNOSISVoltage Recordings-Full wave testsDiscrepancy Possible Cause Check & RemedyDirect earth fault near the terminal under test resulting in rapid and total collapse of the voltage

Defective Insulators. Low Line leads to earth clearance.

Check insulators for any defect such as crack, dust and spots and replace it. Line leads from inside to be well protected from earth.

Total flash over across the winding under test resulting in slower collapse of voltage

Insufficient insulation at the initial part of the winding and line leads not well insulated.

Additional insulation between the first 2 layers and last 2 layers. Start and Finish line leads to be extra insulated.

Part flash over across the winding results in reduced impedance, decrease of time to half & characteristic oscillations in voltage wave at the moment of flashover

Insufficient/ Weak layer insulation of the winding

Check the weak point and increase the layer insulation accordingly

Less extensive faults detected as high frequency oscillations

Breakdown between Coil to Coil or turn to turn insulation

Impulse response current recordings to be done

DIAGNOSISCurrent Recordings-Full wave testsDiscrepancy Possible Cause Check & RemedyMajor change in amplitude and frequency in Current records

Winding breakdown within the tested winding, between windings or to earth

Increased layer insulation, sufficient Coil to Coil Clearance, adequate phase barriers, improved line to earth clearances.

Significant increase with change in the superimposed frequency in neutral current

Fault within the tested winding Check for the break down point and do insulation co-ordination

Significant decrease with change in the superimposed frequency in neutral current

Fault from the tested winding to an adjacent winding or to earth

Adequate Core to Winding insulation, sufficient Coil to Coil Clearance, adequate phase barriers, improved line to earth clearances

Instantaneous decrease in amplitude with change in polarity and basic frequency of capacitively transferred current

Fault in the tested winding or to earth Check for the break down point and do insulation co-ordination and improve line to earth clearance.

Instantaneous increase in amplitude and basic frequency in same polarity of capacitively transferred current

Fault from the tested winding to an adjacent winding

Adequate Core to Winding insulation, sufficient Coil to Coil Clearance, adequate phase barriers.

Small, local, jagged disturbances spread over 2µs or 3µs

Severe Partial discharge or insulation breakdown between turns, coils or connections

Vacuum Oil filling to be done. Avoid sharp edges in insulation materials, copper bus bars, core clamping and tie rods

DIAGNOSISVoltage and Current Recordings-Chopped wave testsDiscrepancy Possible Cause Check & Remedy Change in frequency of Voltage and Current recordings after chopping

Flash over in the return loop to the laboratory earth or an internal failure in the test object

Increase clearance to earth to avoid flash over. Improved Insulation co-ordination necessary

Failure of the chopping gap to chop or any external part to spark over

Failure either in the test circuit or in the test object

Check the test circuit for any such discrepancy or identify the breakdown point in the test object and do insulation co-ordination

Fault occurring before chopping Condition resembles a full wave test failure due to insulation breakdown

Increased layer insulation, sufficient Coil to Coil Clearance, adequate phase barriers, improved line to earth clearances

FOOT NOTES

• Use of surge diverters/arrestors is to limit the transferred over voltage transients.

• Transients - Impulses of short rise-time.• Effective impedance – It is the total impedance between the impulse terminal

of the transformer and ground.• 400Ω is the maximum Surge impedance of a transmission line.• SI is done on power transformers with Um ≥ 72.5kV and where LI becomes a

Routine test.• SI is usually required for transformers subjected to ACLD with Partial

discharge measurement.• No Impulse test on the Neutral terminal is recommended. During Impulse

test on a line terminal, the neutral shall be connected directly to earth. • For Oil immersed transformers the test voltage is normally of Negative

polarity because this reduces the risk of erratic external flash over in the test circuit.

• In direct earthing only leakage inductance is involved. • In impedance earthing main inductance becomes more predominant and this

makes the effective inductance 100 to 200 times greater than direct earthing.• Voltage to earth appearing at the non tested terminals in a DELTA Winding is

50% of the rated lightning impulse withstanding voltage of that terminal is because of opposite polarity voltages to earth on delta terminals.

FOOT NOTES Continued…

• Impulse response current measurement is done through low impedance shunt.• A triggered spark gap is needed in order to initiate the generation of the impulse.• 1.2µs is for front time (T1) to reach at least 50% of the full impulse voltage level.• 50µs is for the tail time (T2) to sustain without collapsing. • Impulse current is normally the most sensitive parameter in failure detection.

Therefore the recorded current waves are the main criteria of the test result.• When recording the winding current the recording should continue till the

inductive peak reaches which will permit examination of the wave to determine whether there is a change in inductance due to any shorting of turns as a result of insulation failure.

• Apply one impulse of a voltage 50 -75% of the full test voltage for calibration purpose and three subsequent impulses at full voltage on each of the line terminals of the tested winding (Example HV winding) in succession. In a DELTA connected 3φ transformer, the other line terminals of the winding shall be earthed directly or through low impedance not exceeding the surge impedance of the connecting line.

• If the winding has a neutral terminal (Example LV winding STAR connected), the neutral shall be earthed directly or through low impedance such as a current measuring shunt.

• The tank shall be earthed.• In case of a transformer with tertiary winding the terminals of the windings not

under test shall be earthed directly or through low impedance.

 REFERENCES

• Fundamentals of High Voltage Engineering by Abdulrhman Al-Arainy, Mohammad Iqbal Qureshi, Nazar Malik

• Experiments in High Voltage Engineering by Abdulrhman Al-Arainy, Abderrahmane Beroual, Nazar Malik

• International Standard IEC 60076-3 Second edition 2000-03

• International Standard IEC 60076-4 First edition 2002-06

• IEEE Guide for Transformer Impulse Tests IEEE Std C57.98-1993