Analysis and Resolution of Abnormal Multi-Point Grounding in Power Transformer Cores

The existence of multi-point grounding in transformer cores causes two major problems: firstly, it can lead to local short-circuit overheating in the core, and in severe cases, cause localized burning damage to the core; secondly, circulating currents generated in the normal core grounding wire can cause localized overheating in the transformer and potentially lead to discharge-type faults. Therefore, multi-point grounding faults in power transformer cores directly threaten the daily operation of substations. This paper analyzes an abnormal multi-point grounding issue in a power transformer core, introducing the fault analysis process and on-site resolution measures.

1.Grounding Fault Overview

The No. 1 main transformer at a 220 kV substation is model SFPSZB-150000/220, manufactured on November 11, 1986, and commissioned on August 8, 1988. It originally used forced oil circulation air cooling but was converted to natural circulation air cooling in 2012. On March 5, live testing of the core grounding current for the No. 1 main transformer showed 40 mA, a significant deviation from previous test results. Examination of the core grounding online monitoring and current-limiting device showed a core grounding current of 41 mA. 

Historical records indicated that the device had automatically engaged an 115 Ω current-limiting resistor on February 27. After determining that the No. 1 main transformer might have a core multi-point grounding issue, personnel reviewed the chromatographic online monitoring data but found no abnormalities. Oil testing personnel collected samples from the No. 1 main transformer on the afternoon of March 5 for oil chromatographic analysis, but the test data showed no significant changes, as shown in Table 1 for dissolved gas chromatographic test results. According to the online monitoring device settings, when the grounding current exceeds 100 mA, the device will automatically engage a resistor to limit the grounding current. Based on this, it was determined that the No. 1 main transformer has a core multi-point grounding fault.

2 Equipment Fault Analysis

The core grounding current test data of the main transformer over the past three years is shown in Table 2. Comparing historical test data reveals that the core grounding current measurements for the No. 1 main transformer have consistently remained within normal ranges, with no abnormal trends detected in dissolved gases in oil. However, the grounding current has shown significant growth, and the current-limiting device has automatically engaged the current-limiting resistor.

Based on comprehensive analysis of these conditions, it can be determined that the No. 1 main transformer has a core multi-point grounding fault. However, when the multi-point grounding occurred, the core grounding online monitoring and current-limiting device immediately engaged the resistor at the moment of current increase, effectively limiting the current magnitude. As a result, no abnormalities appeared in the dissolved gas chromatographic analysis of the transformer oil.

On March 28, during a routine power outage test of the No. 1 transformer, core insulation resistance measurements confirmed the multi-point grounding condition. Test personnel measured the core insulation resistance using 1,000V voltage, which displayed an insulation resistance of "0". Using a multimeter to measure the core grounding resistance showed a continuity status of "conductive" with a resistance value of "0". These measurements proved that the No. 1 main transformer core had multi-point grounding, specifically metallic grounding.

3 Resolution Measures

(1) Considering the grounding fault might be caused by a soft metallic contact, the capacitor impulse method was attempted to eliminate the fault: A capacitor (with capacitance of 26.94 μF) was charged to 2,500 V and discharged three times into the No. 1 main transformer. After the impulses, the core insulation resistance was measured to determine if it had recovered. If not recovered, the test voltage was increased to 5,000 V for another three impulses. If the fault still persisted, further attempts would be discontinued.

(2) If the capacitor impulse method failed to eliminate the grounding fault, a hood lifting inspection of the transformer would be performed when conditions permitted, to directly locate the grounding point and fundamentally eliminate the core multi-point grounding fault.

(3) If the main transformer could not be immediately de-energized for hood inspection and maintenance, a temporary measure of connecting a current-limiting resistor in series with the grounding down conductor could be implemented. The No. 1 main transformer was equipped with a JY-BTJZ core grounding online monitoring and current-limiting device containing four resistance settings (115, 275, 600, and 1,500 Ω), which had already automatically engaged the 115 Ω resistor based on the grounding current magnitude. After equipment commissioning, monitoring was intensified with shortened testing cycles for core grounding current measurements and transformer oil chromatographic analysis for tracking purposes.

The specific field implementation process was as follows: First, the external core grounding connection was disconnected, and a DC high voltage generator was used to charge the capacitor. After approximately 3 minutes of charging, the voltage reached 2.5 kV. Then, using an insulated rod, the lead wire was connected to the core down conductor to discharge the capacitor into the transformer core. After a single capacitor discharge to the No. 1 main transformer core, the 60-second core insulation resistance recovered to 9.58 GΩ, with an absorption ratio of 1.54, consistent with previous test results. The grounding point was successfully eliminated.

After the No. 1 main transformer was returned to service, we measured the core grounding current using a core grounding current tester, which showed 2 mA. Simultaneously, the real-time core grounding current monitoring device also displayed 2 mA, confirming that the fault had been eliminated.