CVT capacitor voltage divider terminal discharge fault test and improvement measures

Author： GOZ Electric Time：2024-06-23 09:35:53 Read：15

1. Product disassembly analysis and test verification

The capacitor voltage divider and electromagnetic unit of the product were disassembled and inspected. It was found that there was no shorting piece between the terminals N and E in the terminal box of the product. Only the end E of the intermediate transformer was grounded through the grounding wire. The N end discharged to the metal pressure plate of the terminal board, and there were obvious discharge burning marks. In order to verify the specific cause of the product, the electromagnetic unit and the capacitor voltage divider were tested according to the judgment method described above.

1) The no-load test of the intermediate transformer of the electromagnetic unit was carried out. The rated voltage was applied from the secondary side to test the no-load current and no-load loss of the iron core of the intermediate transformer. After measurement, the no-load current and no-load loss changed very little compared with the factory value, indicating that there was no inter-turn short circuit between the secondary windings.

2) The ferromagnetic resonance test of the entire product was carried out by suddenly eliminating the secondary short circuit. The oscillation end time was within the standard range, proving that the damper was not damaged.

3) All other tests of the corresponding electromagnetic unit were also qualified, and the problem of the electromagnetic unit could be basically ruled out. In order to verify whether the capacitor element of the capacitor voltage divider C part is damaged, the product is disassembled and the dielectric loss and capacitance of the capacitor voltage divider are measured separately.

According to the test data, the capacitance values are all within the qualified range, and the initial value difference is small, so it can be judged that the capacitor element of this product is normal and the capacitor voltage divider is not damaged.

4) If the terminal N of the capacitor voltage divider is not effectively grounded, then as long as the capacitance value of C3 (N terminal to ground capacitance, pF) between the N terminal and the ground is measured, the floating voltage of the N terminal can be calculated. Clean the discharge traces between the N terminal and the ground of the capacitor voltage divider of the faulty product, and use a special digital capacitance meter to test the N terminal to ground capacitance. In order to ensure the accuracy of the data, two new products of the same model are selected and the N terminal to ground capacitance is tested in the same way.

Through the above test verification and the description of the on-site fault situation, it can be seen that the N terminal of the capacitor voltage divider is not grounded, and an extremely high floating voltage is generated during operation, which is the main cause of this fault. High-energy discharge causes cracks in the secondary terminal board, causing transformer oil leakage in the electromagnetic unit. Therefore, when the capacitive voltage transformer is in operation, the end N of the capacitor voltage divider must be reliably grounded. If there is no grounding, the ultra-high floating voltage can cause surface flashover breakdown to the ground in a very short time. Its discharge route is clear and the breakdown time is very short. Usually, the secondary monitoring voltage will not find obvious abnormalities.

2. Product design improvement

1) Improve the grounding method of the end of the capacitor voltage divider. The end N of the CVT capacitor voltage divider and the end E of the intermediate transformer are grounded in the secondary terminal box of the product through two independent grounding wires. At one end of the grounding, the two grounding wires are welded to the same OT terminal and then grounded. This can effectively avoid the on-site acceptance test or product installation and commissioning process. Due to negligence, the copper shorting piece is removed and forgotten to be restored, which eventually causes the end of the capacitor voltage divider to be grounded.

2) A set of discharge gaps are connected in parallel between the end N of the capacitor voltage divider and the grounding terminal. The discharge gap action voltage value is set to 3kV. When the grounding of the N end is disconnected, it can be connected to the ground through the discharge gap. The discharge sound after the discharge gap is actuated can prompt the on-site patrol personnel to find abnormalities as soon as possible, effectively preventing the failure of N-end suspended discharge.

3. Conclusion

There are many reasons why the N-end of the capacitor voltage divider is not grounded, including the OT terminal of the product grounding wire is not welded firmly, or the tester forgets to restore the short-circuit after the product on-site acceptance test, etc., resulting in a floating voltage between the N-end and the ground after the product is put into operation. The voltage value has far exceeded the standard withstand voltage value that the N-end can withstand to the ground. It will discharge to the nearest grounding end in a very short time and cause the secondary terminal board to crack. If it is not discovered for a long time, it will threaten the safety of the entire transformer. Through the abnormal analysis and verification of the cause of this fault, the design of the grounding at the end of the capacitor voltage divider has been improved, and similar problems have been prevented from occurring.