Field Test and Analysis of Partial Discharge on GIS Under Impulse Voltage

This paper presents a set of partial discharge (PD) detection system under field impulse, which is applied in the PD detection on 800 kV breaker under standard lightning impulse voltage test and 750kV substation under GIS on-site oscillating lightning impulse handover test. The anti-interference capability of PD detection system is also discussed, which is of great importance to reduce the rate of fault and improve operation reliability.

1.PD Detection System under Field Available Impulse

As shown in Figure 1, PD discharge detection system is mainly composed of voltage generator, voltage divider, GIS group, Rogowski coil, attenuator, transient voltage suppressors (TVS) and oscilloscope. In order not to affect smooth implementation of routine delivery test, impulse voltage test and partial discharge detection must be conducted on site at the same time. However, the signal of partial discharge is weak. Hence, Rogowski coil is used as signal sensor (which is shown in Figure 2) to avoid bringing harm to insulation performance of electric equipment. The Rogowski coil is wrapped around the grounding line of breaker. One line is connected to oscilloscope through attenuator by means of cables while the other is connected to oscilloscope through TVS. Given that the amplitude of displacement current passing through the grounding line of breaker is great (1-3kA), TVS can shield the signal above threshold to avoid PD impulse being covered by displacement current signal. Compared with the attenuator, TVS can better capture PD impulse without digital filter; while the attenuator can effectively collect the signal of displacement current, which passes through the grounding line, and measure and analyze the signal. Figure 3 shows the untreated signal detected by TVS in the actual measurement. 

Fig.1 Circuit diagram of partial discharge test on site

            Frequency/MHz

• Characteristics of amplitude frequency response

Frequency/MHz

• Characteristics of phase frequency response

Fig.2 Characteristics of dynamic response for Rogowski coil

Fig.3 Untreated signal of partial discharge using TVS under positive polarity lightning impulse voltage

2.PD Detection of 800kV Breaker under Lightning Impulse Voltage Test of Delivery Standards

The double-index lightning impulse voltage test circuit of 800kV breaker is shown in Figure 4. The impulse source is 4800kV Marx impulse generator. The output waveform of power-supply voltage is shown in Figure 5 (U is peak voltage; β is lightning over-shoot; t is front time and t is tail time ). The PD measuring system (Fig 1)is utilized to detect the signal of breaker ground line.

Fig. 4 Experiment system under impulse voltage

Fig. 5 Outputting wave of the generator of impulse voltage

First, 1 min power frequency voltage test is conducted to 3 breakers (peak of test voltage is 960kV) and at the same time, Haefely PD 561 instrument is used to observe the PD condition. The test goes well and any abnormal condition is not found; Secondly, conduct 80% positive polarity and 100% negative polarity standard lightning impulse voltage test for three breakers 2 times under open and closed switch condition; meanwhile, partial discharge is also measured. The 100% lightning impulse voltage of 800kV breaker is 2200kV. The test is conducted in the standard high-voltage test hall. The electromagnetic shielding effectiveness S＞60dB and interference level of power frequency PD is below 3pC, which conforms to the requirement of GB/T 12190-1990. The test found that B-phase breaker detects current impulse under both open and closed switch condition and has high repeatability. The moment and impulse characteristics are consistent with the signal of partial discharge obtained from lab research. Because the level of electromagnetic noise in the test is very low and there is no impulse interference source, we can determine that this impulse signal is PD current signal (as shown in Figure 6). In the test, A and C phase  breakers do not have PD signal.

(a)Open-switch condition 80% (1680kV) PD impulse under positive-polarity lightning impulse

(b) closed-switch condition 100% (2100kV) PD impulse under negative-polarity lightning impulse

Fig. 6 Partial discharge test on B-phase breaker under lightning impulse voltage 

Based on Figure 6, there exits interference impulse with great amplitude at the initial moment because of circuit interference caused by sphere gap discharge. The PD signal mostly appears within 3-10μs shortly after the peak. The source interference does not have an impact on PD measurement. The magnitude of PD impulse can even reach about 20V. In addition, the base line in the Figure 6 is not 0V. The base line in Figure 6(a) reaches around -4V, which illustrates that under the impulse voltage there is obvious capacitive current in the GIS cavity, especially when the breaker is in open-switch condition. However, because of the role of voltage attenuator, the current value remains at the low level.

This example proves that the same defect differently reacts to different kinds of voltage. Even though the defect is not found under power frequency, it may be exposed under the impulse voltage of higher filed strength. Hence, the detection of partial discharge under impulse voltage is conducted as diagnosis test. That can better play a part of diagnosis, detect and avoid the fault in time, and cover the shortage of power frequency voltage test.

3.PD Detection of 750kV Substation under GIS on-site Oscillatory Lightning Impulse Handover Test

The test circuit of 750kV substation GIS on-site oscillatory lightning impulse handover test is shown in Figure 7. The impulse source is 2400kV Marx impulse generator. Output waveform of oscillatory lightning is shown in Figure 8. Please refer to Figure 1 for PD measuring system under oscillatory lightning impulse. Since GIS group has many grounding points on site, the sensor is connected to one of them to detect the signal.

Fig. 7 Equipment and test object of field experiment

(a) Oscillatory lightning wave of positive polarity (U = 1547.53kV, t= 2.18μs, t = 17.12μs )

(b) Oscillatory lightning wave of negative polarity (U = 1523.57kV, t= 2.30μs, t = 17.24μs )

Fig.8 Oscillatory lightning impulse voltage wave created by the generator of impulse voltage

Now conduct 75% positive polarity and 100% negative polarity oscillatory lightning impulse voltage test 3 times under closed-switch condition to A, B and C Phase. The result is good. There is no breakdown found in any phase; measurement results of impulse voltage PD detection system show that there is no partial discharge. Take C-Phase as an example, the detection result is illustrated in Figure 9. There is source interference with great amplitude at the initial moment.

(a) 75% (1680kV) oscillatory lightning wave of negative polarity

(b) 75% (1680kV) oscillatory lightning wave of positive polarity

Fig.9 Partial discharge test on C-phase GIS group under oscillatory lightning impulse voltage=

4.Discussion on PD detection Anti-interference Measures

The basic idea of partial discharge detection under impulse voltage is to detect high-frequency current impulse, which flows through GIS test object insulation structure. In the actual detection, it is likely that the interference generated by test system or the outside environment generates high-frequency current impulse similar to PD signal. The measurement results of Figure 6 and 9 also prove that. Therefore, test personnel should distinguish between outside interference signal and actual PD signal, and prevent incorrect judgment from causing serious consequences. In addition, on-site test environment is complicated, and each interference signal can reduce detection sensitivity and increase smallest measurable level. So, the ability of distinguishing interference waveform must be improved, and meanwhile, anti-interference capability of test system needs to be enhanced. 

Based on some materials at home and abroad, interference sources of PD detection on site are summarized as follows:

• Space electromagnetic interference generated by the operation of external electric equipment and electromagnetic interference by sphere gap double hit and defect model discharge;
• Direct interference generated by each kind of high-frequency signal in the power distribution network;
• Neutral point potential of power source led by the power-source line of measurement instrument and electromagnetic interference;
• Interference caused by transient current flowing through outer shielding layer of cables.

Against the above interference, please refer to the following anti-interference measures:

• One-point grounding of test circuit. Lay the copper belt as grounding wire between voltage divider and measurement instrument; the voltage divider should approach the grounding electrode as close as possible; lay the measurement cable along the copper belt to reduce the circuit area as much as possible; use tin paper to wrap the cable joint.
• The power source for detecting the system should be separated by isolation transformer and adopts optical and photoelectricity to transmit the detected signal.
• Adopt digital filter. The digital filter can utilize the discrete system to change the waveform or spectrum of input digital signal, allowing useful signal frequency to pass and restrain useless signal output. In the test, the filter must ensure that PD signal bear the characteristics of constant amplitude-frequency response and linear phase. Compared with IIR (infinite impulse response) digital filter, FIR (finite impulse response) digital filter not only ensures straight amplitude frequency feature, but also gains strict linear phase feature, avoiding phase distortion. Therefore, the FIR is recommended. The effect drawing of digital filter filters out the displacement currents flowing through GIS (Figure 10). Based on the drawing, displacement current of the oscillating is filtered out.

Fig. 10 Digital filter filters out the displacement currents flowing through GIS 

5.Conclusions

Safe operation of super-high voltage GIS equipment is directly related to the reliability of main power gird. In recent years, many researchers at home and abroad have been focusing on utilizing impulse voltage test and its PD detection to conduct GIS insulation breakdown diagnosis and analysis. We designed a set of PD detection and analysis device under GIS impulse and gained the experience about on-site diagnosis of GIS equipment fault.

• The PD detection device under impulse voltage can effectively extract PD signal without affecting normal delivery test. This new trial can be a supplementary to PD and impulse voltage test and is of practical significance to the evaluation of equipment safety.
• Hidden troubles in the GIS can have different effects on gas insulation under different kinds of voltage. PD test under the impulse can cover the shortage of available field test and can effectively find fault defect; in the actual test of 800kV GIS breaker, it is found that if the quantity of power frequency PD is not exceeding the limits, partial discharge occur at 80% positive polarity lightning impulse under open-switch condition and 100% negative polarity lightning impulse under closed switch condition.

This paper utilizes TVS to detect PD signal in the strong displacement  current and obtains good effect. However, the current condition cannot be detected as a whole during the whole process of applied voltage. Hence, the PD detection system under GIS impulse on site needs to be further improved and practicalized. Various methods are utilized to improve anti-interference ability of measuring system to effectively distinguish and eliminate interference source.