How to Select Waveforms for Impulse Withstand Voltage Test ?

The impulse withstand voltage test can be used to detect the insulation performance of the electrical equipment when subjected to atmospheric overvoltage (lighting stroke), and then it is used to study the insulation performance of the power equipment subjected to overvoltage.

The impulse voltage test results can be divided into two categories according to the different insulation properties of the test specimens. One is the withstand voltage test. The test object is non self restoring insulation , self recovery insulation and non self restoring insulation test. The other is destructive discharge test, the test object is a self - restoring insulation test.

Impulse voltage generator is a kind of high voltage generator which can generate pulse wave. In the simulation equipment in the lightning overvoltage and operation test voltage tolerance characteristics of short-time high voltage under impact, impulse voltage can produce standard full wave (lightning), chopper, operation, steep wave wave, meet the requirements of all kinds of electrical equipment for impulse voltage test.

500kV 25KJ Impulse Voltage Generator

Full wave: The full wave is a certain polarity of non periodic pulse voltage wave, the wave of the front part of the voltage rise quickly. If it reached peak and then slowly dropped to zero.

Chopped wave: lightning impulse voltage in the whole wave after a period of time (a few microseconds), cut off by the outer space of the waveform known as the cut-off wave, according to the length of the truncation delay, the cut is divided into wave front truncation and tail truncation.

Operating wave: wave head time Tcr and the definition of the wave tail time T2 the same as the lightning wave, usually in line with Tcr=250 s and T2=2500 s shock wave as a standard operating wave.

Steep wave: voltage conversion, from high to low, and the basic performance of the waveform is not changed as the steep wave. 

The reasons for the formation of the four kinds of waveforms are different, when the electric equipment is subjected to the impulse withstand voltage test, according to the different samples, the impulse voltage generator should correspond to different waveforms in the test.

Electrical equipment	operation waveform	remarks
Transformer	Full wave, chopping wave, operation wave
Switchgear	Full wave, chopping wave	110kV switchgear, GIS
Transformer	Full wave
Lighting Arrester	Full wave	Impulse current
Insulator	Full wave, steep wave	If it is a ceramic or glass insulator, must be equipped with steep wave device; if it is consistent with the insulator, can also be synthesized by computer

FAQ for partial discharge(PD) Test

There are unavoidable defects in the insulation of high voltage equipment (such as interspaced in solid insulation, bubbles in liquid insulation etc.). Partial discharge will occur when the electric field strength of the surface and the solid insulation of the air bubbles reaches a certain value. This discharge does not form a penetrating channel, but it produces heat and active gas (such as ozone, nitrogen oxide) local corrosion insulation, causing irreversible damage. If gradually expanded, the whole insulation breakdown or flashover. Partial discharge test can identify the characteristics of partial discharge, quantitative measurement of its discharge intensity, in order to find out the hidden dangers of product insulation quality and then take appropriate measures.

In this paper, there are frequently asked questions in the process of partial discharge test.

What is the main requirement for coupling capacitors when we do partial discharge of electrical equipment?

The main requirement of the coupling capacitor is that the coupling capacitor should not be partial discharge at the high voltage, otherwise it is impossible to distinguish whether the partial discharge is from the sample or from the coupling capacitor.

Why do we need to have a pre - applied voltage during partial discharge measurements?

In partial discharge of measuring equipment, including test standard in a short time with a high voltage value pre regulated voltage ratio, which is given to the actual operation of partial discharge is often due to excessive voltage excitation, voltage in advance is to artificially create a condition to simulate the overvoltage the actual operation, to observe the insulation level of partial discharge under specified conditions.

How to determine the initial voltage of partial discharge?

When the voltage rises slowly from a low value to a discharge value that has just begun to exceed a specified value, the voltage is called the initial voltage of partial discharge.

Method for determining extinction voltage of partial discharge

When the voltage applied to the test specimen is observed from the higher value of the partial discharge, the minimum voltage can be observed in the test circuit. In practice, the extinction voltage is the minimum voltage at which the partial discharge amplitude is equal to or less than a specified value.

Why the pulse square wave calibration should be carried out in partial discharge measurement?

Instrument for measuring partial discharge may not directly give the value of discharge depending on pulse numerical parameters. It must to simulate the partial discharge pulse through the known size standard square wave inject to test loop. The injection pulse can be generated by a square wave voltage generator with a magnitude of Uc, which is connected in series with a small known capacitance Cg, and the calibration pulse is equivalent to a discharge power of Qc, which is Qc=UcCg. The measured partial discharge amplitude are compared to pulse square wave in order to quantitatively determine the size of the partial discharge, which referred to square wave calibration.

Test procedure for measuring the voltage of partial discharge of transformer

The test voltage should be no more than 1/3 . The power supply is switched on, to boost voltage in advance,and keep 10s above, then non-stop to step-down voltage to measuring voltage, maintained above 1min, and then read the discharge quantity. Finally the test voltage drops below 1/3 measuring voltage, and cut off power supply.

This article is posted by Hannah.cao.(https://www.quora.com/profile/Hannah-Cao-4)

Partial Discharge (PD) Tester - Diagrams of Discharge Analysis and Analysis of Interference Causes

In order to ensure the safety of electrical equipment, we often use partial discharge tester to test electrical equipment.

HCL 2010 M Partial Discharge Tester

After the test is finished, the diagram of partial discharge tester will display on the screen based on the measurement results, which is used to show the electrical equipment and the surrounding discharge. How to determine whether it is in a safe range according to the diagram, which requires the analysis of the causes of each diagram.

In this paper, we will list some of the waveform diagram of partial discharge tester, so that the test personnel can correctly judge the results of the test, and record the safety performance of the electrical equipment.

Diagram 1

Typical discharge waveform (1)

This is capacitive discharge waveform, which can occur in insulation or solid insulation bubbles.  It occurs in the oil immersed capacitor , insulation or insulation plastic filler in paper. The discharge amplitude and pulse number increase with the voltage .

Diagram 2 

Typical discharge waveform (2)

The internal discharge of the interlayer medium may also occur in the carbonized discharge of the insulating board and the climbing of the branches.

Diagram 3

Typical discharge waveform (3)

The discharge of an insulating medium that is in contact with each other and the discharge in the oil impregnated paper capacitor.

Diagram 4

Typical discharge waveform (4)

In the same medium, different sizes of bubbles may also form this kind of circular, mainly in the epoxy casting insulation. Discharge volume changes with the voltage, such as the gap between the capacitor layer.

Diagram 5

In the insulation of an electric motor, the discharge in the mica insulation of the motor, and the amount of discharge varies with the time and pressure of the copy.

Diagram 6

The discharge occurs between the metal and the dielectric surface ,which may be an air gap between the metal and the medium, or the surface conductivity is uneven.

Diagram 7

The surface of the metal electrode is discharged, and the exposed metal surface and the medium are discharged. The air bubbles between the metal and the medium or the air bubbles in the lift may contain impurities such as metal or carbon.

Diagram 8

A small part of a metal foil or metallized sheet in a capacitor can move under an electric field.

Diagram 9

Discharge of poor contact or suspended metal , and the test circuit is not reliable connection. The pulse amplitude of the discharge pulse is equal to the positive and negative half cycle of the discharge. The oscilloscope can observe the discharge pulse pairs, figure (a),(b) this is because the oscilloscope effect caused by visual error caused by light.

Diagram 10 

Contact noise discharge, poor contact between metal or semiconductor shielding layer. At the same time, the discharge distribution of the noise is small on the two sides of the test voltage. As the voltage increases, the noise discharge coverage increases, as shown in figure (a). Motor carbon brush to form a spark discharge may occur in this waveform,figure (b).

Diagram 11

The metal electrode discharge concentration field, the amplitude distribution in the isometric peak voltage on both sides, with the corona and air discharge in the same voltage, the discharge AC voltage exists in both positive and negative half weeks, but 2.5 weeks amplitude asymmetry, discharge pulse amplitude larger in the positive half cycle, then discharge at high potential, and vice versa in low potential. Similar in oil and gas.

Diagram 12

Suspended discharge in oil insulation and creepage, pulse number, the critical voltage, discharge is not stable. Sometimes for a few seconds, or stop a few seconds without discharge. The discharge is with large amplitude, but like the external random disturbance which need combined with waveform and judge the disturbance caused by the discharge of fluorescent lamp.

Diagram 13 

Discharge by fluorescent lamp.

Diagram 14

The interference of silicon controlled rectifier is symmetrically distributed, and sometimes it is a single pulse

Diagram 15

In the case of asynchronous induction motor, the response of the motor is independent of the test voltage, which may be caused by the large asynchronous motor, and may be caused by the large load of the transformer. However, when the generator is turned on for the test, the difference between the generator frequency and the power supply of the system instrument will cause the discharge display waveform to rotate.

Diagram 16

Industrial high frequency equipment interference, industrial high frequency equipment such as ultrasonic generator, induction heater etc.

Diagram 17

The interference of amplitude modulated sine wave signal is mainly caused by the high power high frequency power amplification or the radio emission or radiation interference of the oscillator and the interference of the radio station.

Diagram 18

corona discharge

Corona discharge occurs at the metal tip or the edge of the electric field. The corona discharge occurs only in the half of the test voltage, and symmetrically distributed on both sides of the voltage peak, figure (a). As the voltage increases, the number of pulses (width) increases symmetrically, as shown in figure (b). If the discharge tip electrode is in a high potential, the corona discharge occurs at the negative half of the test voltage. If the tip electrode is in the ground potential, the discharge pulse appears in the positive half cycle. On one electrode, the initial corona appeared in 1.5 weeks, but when the voltage rises above the initial voltage of many, in the other 1.5 weeks will also appear in large amplitude, discharge less pulses, as shown in figure (c).

A Study of Characteristics of Transformer Switching Impulse Waves

Because the equivalent frequency of operating voltage is higher than the power frequency voltage, duration is shorter than a minute frequency. Insulation structure in the operating voltage of the discharge characteristics, the breakdown mechanism and frequency, the impact is not the same. The operation does not cause the effect of wave oscillation, a long time, for the same voltage, the coil will be more severe than the impulse test. Compared to frequency voltage,operation wave is directly caused by the oil gap breakdown. The small oil cylinder gap thin insulation structure of transformer is more necessary.

If insulation level is reduced, the power frequency and frequency test may cause partial damage. This residual weakness in the test is not easy to find. But under the long term working voltage, it will gradually develop to lead to the breakdown of insulation. Generally partial discharge occurs in operation wave , it will not caused residual damage.

Characteristics of inductance operating impulse wave

The operating waveform is close to the voltage in the operation. The authenticity is strong. The equivalent power frequency voltage is more reasonable, and the main longitudinal insulation are all evaluated, which has the advantages of portable equipment, sensitive display, no large power supply and so on.

The frequency or frequency test, ready to spend huge sums of equipment, heavy workload, the scene often give the test of safety equipment will be without clear aims. It is necessary and feasible to test the insulation of the transformer by using the operating wave test.

 Inductance operating impulse waveform

Wave front length; T_f ≥100μs

Wave tail length; T_t ≥100μs

90% amplitude duration: ≥200μs

peak reverse voltage:U2M≤0.5 UM

≥－[100×1000(0)×200(90)]μs

Circuit of inductance operating impulse wave

Operation Method of AC withstand voltage and series resonant test for Generator

With the development of power system , the capacity of the operating equipment is larger, such as thermal power generating units whose capacity is more than 1000MW, and hydraulic power unit which the largest single capacity has reached 800MW; To do AC power frequency withstand voltage test of these devices, if you use the traditional test equipment (frequency test transformer), due to the need of larger capacity , transformer and voltage regulator are very heavy. The power frequency resonance device has great advantages in the aspects of test power supply capacity, equipment weight, test waveform and investment. The device is mainly designed for the AC voltage withstand test of the generator with a capacity of 250MW or less. Specific methods of operation are as follows:

Over current protection setting, 1.2 times the rated current of high voltage side of transformer. For example: the high voltage side rated current of exciting transformer is 50A, the configuration of the transformer is 50/5, so the current relay should be adjusted to 5A.

Over voltage protection settings, according to the test voltage 1.1 times set.

For example: When the test voltage is 39kV, the display should be adjusted to 43kV.

The divider wire is connected to the high voltage current two terminal of the console.  Note: All wire must be connected, firm and reliable.

The high voltage current on the exciting transformer is connected to the high voltage current two terminal of the console.

Pressure time setting: set the test time on the time relay to the required test time.

According to the above connection that test wiring is correct, close circuit breaker of the console . At this time if the regulator is not zero, the regulator will automatically return to zero. Do not turn on the main power source, adjust the reactor core clearance, observe the lifting and clearance limit protection function to make sure whether it is normal.

Close the main power source, push the "boost" button to boost on the reactor gain hundreds of volts, by changing the air gap to tune the output voltage to reach the maximum. At this time it can be tuned, and boost to the value of the test voltage. When the voltage reaches the set value , the equipment will automatically return to zero.

Disconnect main power source.

During the test, the relevant personnel should strengthen the monitoring of the test equipment. Once the abnormal phenomenon occurs, the voltage regulator should be quickly reduced, and the power source will be cut off.

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