Partial discharge testing of large transformers often involves grounding issues, such as the grounding of test equipment, grounding of the test circuit of the transformer under test, and grounding to reduce external electric field interference. It can be said that the grounding wire is the "lifeline" of the power industry, and it is crucial for ensuring the correctness of tests and the safety of personnel. Therefore, this article analyzes and discusses several typical grounding problems often encountered in transformer partial discharge testing (hereinafter referred to as PD testing).
1 Test circuit with low-voltage delta connection
Grounding at the middle of the series test transformers, or grounding at the low-voltage side of the transformer under test, or grounding both simultaneously, will not affect the test results. For three-limb core transformers, both should be grounded simultaneously. However, if neither is grounded, a floating potential will be generated after voltage application, increasing the error of the measurement results.
2 Grounding of test circuit with low-voltage star connection
The connection group of No. 2 transformer at Houhu Substation is YN, ao, ynO+dll. It is a 220kV autotransformer with high, medium, and low voltage windings all in star connection. During PD testing, grounding can be done at the middle of two series test transformers, as shown in Figure 1. From the potential diagram, it can be seen that at this time, the x end of phase A winding of the transformer under test is not at ground potential, while the A1 and x2 ends of the two test transformers are at symmetrical potential positions. When the test circuit grounding point is changed to the low-voltage neutral points x, y, z of the transformer under test as shown in Figure 2, since the voltages of the windings of the transformer under test are different (phase a winding is at full voltage, phases B and C are at half voltage), the X1 and A2 ends of the test transformers are no longer at ground potential; the ground potential is located at 2/3 of the winding height from X2. When the test power supply voltage reaches 70kV, the high-voltage winding of the first test transformer will withstand 70×2/3=46.67kV, which exceeds the withstand voltage level of its high-voltage winding (the first test transformer is 35/0.4kV), so this method is not suitable for series connection of two test transformers.
It should be noted that when using this test connection, the X1 and A2 ends of the test transformers and the O end of the transformer under test cannot be grounded simultaneously; otherwise, two loops will be formed in the test circuit, as shown in Figure 3. Due to the voltage difference between the tested phase and the non-tested phases of the transformer under test, a circulating current will be generated between the two grounding points in the test circuit. The appearance of i and i' will increase the burden on the test transformers and may damage the insulation in severe cases.
In PD testing, the symmetrical voltage application method with grounding at the middle of series test transformers is often used. This way, the voltage to ground on the low-voltage side is only half of the test voltage, effectively reducing the PD level on the low-voltage side. However, when using a single test transformer for voltage application, grounding should be done on the low-voltage side of the transformer under test.
3 Grounding of the balancing coil
Transformers generally need to be equipped with a balancing coil to eliminate the third harmonic. The balancing coil has two lead-out terminals, cP and xP. During transformer PD testing, the lead-out terminals of the balancing coil should be short-circuited and grounded, but there are exceptions. Because some main transformers have balancing coils with three lead-out terminals. If all three terminals are grounded during transformer PD testing, it is equivalent to short-circuiting the three phases. From the structure, the balancing coil is wound close to the iron core, with the low, medium, and high voltage windings outside in order. When the balancing coil is short-circuited, the magnetic circuit of the iron core is short-circuited, causing the test voltage to fail to rise. Later, changing to grounding only one lead-out terminal made the test normal.
4 Working grounding and protective grounding of the frequency multiplier power supply vehicle
During the PD test of the No. 1 main transformer of WISCO's self-owned power plant, the "working grounding" and "protective grounding" on the frequency multiplier power supply vehicle were connected together with bare copper wire for grounding. As a result, the interference waveform on the PD instrument display reached full screen, making square wave calibration impossible. Later, the working grounding was connected to the neutral line of the 380V power supply, separated from the protective grounding, and the interference waveform displayed by the PD instrument was almost zero.
5 Short-circuit grounding of bushing CT
CT can convert large currents at high potential into small currents at low potential, with its secondary winding having many more turns than the primary winding. If the secondary winding of the high-voltage bushing CT of the transformer under test is left open-circuited, during voltage application, a dangerous overvoltage much higher than that of the primary winding will be induced in the secondary winding. Therefore, special attention should be paid to the short-circuit grounding of the secondary winding before the test. Since the neutral point is already grounded during the test, the secondary winding of the neutral point bushing CT does not need to be short-circuited and grounded.
6 Shielding grounding of firewalls
During the PD test of three 500kV single-phase transformers, because the three transformers are separated by firewalls, and the firewalls contain steel reinforcement. The transformer bushing is 3.8m away from the firewall, and the distance becomes even smaller after wearing the grading ring, making it easy to produce floating discharge after voltage increase. For this reason, a mesh grounding wire should be installed on the firewall to enhance anti-interference capability.
7 Grounding of test instruments
All test instruments should be protectively grounded. Attention should be paid to the grounding of the test transformer, compensating reactor, PD detector, as well as the detection impedance, shunt, and square wave generator.
During the PD test of the No. 1 main transformer at Dunkou Substation, when calibrating the square wave for phase B, the background interference of the PD instrument was 480PC. After grounding the end screen housing of the high-voltage bushing and recalibrating the square wave, the background interference dropped to 360PC. In addition, the detection impedance should be placed as close as possible to the bushing end screen.
8 Conclusion
Transformer partial discharge testing involves various grounding issues. Due to different on-site test conditions, grounding is sometimes not given enough attention. Forgetting to ground or improper grounding can lead to safety hazards, abnormal tests, and even serious accidents. To ensure the safe operation of the power system and the smooth progress of various tests, grounding issues must be given top priority and implemented in transformer PD testing.
