Multi-point grounding of the transformer core is one of the more common faults in transformers. It is difficult to find and handle. The conventional method is a hood inspection. If the fault point cannot be found intuitively, the DC method or the AC method is generally used for searching. Not only the workload is large, the cost is high, and the power outage time is long to affect the user's electricity consumption, but also the large transformer hanging hood. There is a great risk. The following describes a method of handling the multi-point grounding of a transformer core using a capacitor discharge shock method.
When the company's Xingan Substation made a preventive test on June 7, 1999, it found that the insulation resistance of core 2# was seriously reduced (the core was led to the ground outside the shell via a small bushing), and the insulation resistance reading was measured with a megohmmeter. Sometimes it is 0. At this time, using a multimeter to measure the resistance is more than a dozen ohms; sometimes in the 0 ~ 40MΩ swing, while hearing a slight discharge sound inside the transformer. Other test items are normal (no color filter, no analysis of insulating oil color). The preliminary analysis considers that residual debris causes the core to be grounded.
Basic situation of transformer
This transformer was produced by the Harbin Transformer Factory in September 1992 and its model number was SFZ7-40000/110. It was put into operation in November 1992. There was no abnormality in the hood inspection and test before the commissioning. In 1997, the 6KV side short-circuit current impacted for several minutes due to interruption of the protection power supply caused 6KV three-phase bushings to burn out, the transformer oil leaked out of ignition, and 110KVA phase bushings flashed. After the cover inspection, copper beads were found on the bottom of the transformer. There was no abnormality in the measuring coil DC resistance, coil insulation resistance, and insulation resistance of the core to the ground. After replacing the casing, no problems were found in the tests.
Initial treatment
Xing'an Transformer is responsible for the power supply tasks of Xingan and Junde, which produce nearly 3 million tons of high-gas and large-volume coal mines and surrounding areas. This substation was built in the early days of the liberation. After several expansions and capacity expansion, the space around the transformer was very small. When the hood was lifted, it needed to be transported away from its current operating position. This meant that this transformer required a long time power cut and would directly affect the production of coal mines. Safe, this is not allowed. According to the above circumstances, it was decided to open the manhole after oil draining and flush the iron core with high-speed oil flow. No problem was found when the manhole was opened. After the iron core was flushed, the insulation of the core to the ground was measured to be 5000 MΩ, and the normal value was restored. The retest after oil injection became 0MΩ again. Putting the transformer into operation with the load measuring core current to ground was 0.6A, indicating that this treatment had no effect, but it was further confirmed that the residue was caused by the core grounding.
Capacitor discharge shock
According to relevant information (1), grounding of cores caused by debris floating can be handled by capacitor discharge shocks. The huge current generated by the momentary discharge of the capacitor will melt or blow the residual debris, or the electric current generated by the huge instantaneous impact current of the capacitor will cause the residual debris to move away from its original position. However, how to implement this method, such as how to choose the capacitor capacity, how high the impact voltage, and what harm to the transformer, etc., are not described in the data. After careful research and analysis, it was decided to try using two 6.6KV 40Kvar shunt compensation capacitors plus 3000V first:
capacitance:
Discharge impact energy:
Inrush current:
Maximum impact power:
Where: Q - shunt compensation capacitor capacity, Kvar
UN - shunt compensation capacitor rated voltage, KV
U——Capacitor applies DC voltage (impact voltage), V
R——resistance measured in discharge circuit, 2.7Ω
Note: Because of the long inductance of the loop connection wire, it has a certain influence on the amplitude of the discharge current and the discharge time. Therefore, the maximum discharge current impact value and the maximum impact power are for reference only.
The figure is the wiring diagram of the charging and discharging principle of the capacitor. After the line is connected according to the figure, the capacitor is charged. Pay attention to the slower step-up speed. When the voltage reaches 3000V, the connecting rod of the capacitor and the DC voltage generator is disconnected by the insulating rod and contacts with the outer lead of the transformer core, and a clear discharge sound is heard to complete the discharge shock.
After the impact, the core-to-ground insulation resistance was measured to be 5000 MΩ. The measured grounding current was not measured. On the 19th day of operation, the core grounding current suddenly increased to 0.45A, and the retested core-to-ground insulation of the power failure was still 0 MΩ. This indicates that the impact of the first capacitor discharge impact was not obvious. The analysis may be due to the small discharge current. The second impact will be performed the next day, raising the capacitor charging voltage to 6KV.
Discharge impact energy:
Inrush current:
Maximum impact power:
After the impact, the core-to-ground insulation resistance was measured to be 5000 MΩ. The insulation resistance, dielectric loss, and leakage current of the measurement coil were basically the same as those in the pre-test. It has been in operation for more than three years since it was put into operation on that day. After core current monitoring and three-year pre-test, there are no abnormalities, indicating that this method has achieved the desired results.
Using this method to deal with the residual ground due to debris caused by the obvious effect of the earth fault, save time, save manpower and resources, simple and practical. However, iron core grounding caused by moisture or insulation breakdown of core insulation can not be used this method, still need to hang treatment.
When the company's Xingan Substation made a preventive test on June 7, 1999, it found that the insulation resistance of core 2# was seriously reduced (the core was led to the ground outside the shell via a small bushing), and the insulation resistance reading was measured with a megohmmeter. Sometimes it is 0. At this time, using a multimeter to measure the resistance is more than a dozen ohms; sometimes in the 0 ~ 40MΩ swing, while hearing a slight discharge sound inside the transformer. Other test items are normal (no color filter, no analysis of insulating oil color). The preliminary analysis considers that residual debris causes the core to be grounded.
Basic situation of transformer
This transformer was produced by the Harbin Transformer Factory in September 1992 and its model number was SFZ7-40000/110. It was put into operation in November 1992. There was no abnormality in the hood inspection and test before the commissioning. In 1997, the 6KV side short-circuit current impacted for several minutes due to interruption of the protection power supply caused 6KV three-phase bushings to burn out, the transformer oil leaked out of ignition, and 110KVA phase bushings flashed. After the cover inspection, copper beads were found on the bottom of the transformer. There was no abnormality in the measuring coil DC resistance, coil insulation resistance, and insulation resistance of the core to the ground. After replacing the casing, no problems were found in the tests.
Initial treatment
Xing'an Transformer is responsible for the power supply tasks of Xingan and Junde, which produce nearly 3 million tons of high-gas and large-volume coal mines and surrounding areas. This substation was built in the early days of the liberation. After several expansions and capacity expansion, the space around the transformer was very small. When the hood was lifted, it needed to be transported away from its current operating position. This meant that this transformer required a long time power cut and would directly affect the production of coal mines. Safe, this is not allowed. According to the above circumstances, it was decided to open the manhole after oil draining and flush the iron core with high-speed oil flow. No problem was found when the manhole was opened. After the iron core was flushed, the insulation of the core to the ground was measured to be 5000 MΩ, and the normal value was restored. The retest after oil injection became 0MΩ again. Putting the transformer into operation with the load measuring core current to ground was 0.6A, indicating that this treatment had no effect, but it was further confirmed that the residue was caused by the core grounding.
Capacitor discharge shock
According to relevant information (1), grounding of cores caused by debris floating can be handled by capacitor discharge shocks. The huge current generated by the momentary discharge of the capacitor will melt or blow the residual debris, or the electric current generated by the huge instantaneous impact current of the capacitor will cause the residual debris to move away from its original position. However, how to implement this method, such as how to choose the capacitor capacity, how high the impact voltage, and what harm to the transformer, etc., are not described in the data. After careful research and analysis, it was decided to try using two 6.6KV 40Kvar shunt compensation capacitors plus 3000V first:
capacitance:
Discharge impact energy:
Inrush current:
Maximum impact power:
Where: Q - shunt compensation capacitor capacity, Kvar
UN - shunt compensation capacitor rated voltage, KV
U——Capacitor applies DC voltage (impact voltage), V
R——resistance measured in discharge circuit, 2.7Ω
Note: Because of the long inductance of the loop connection wire, it has a certain influence on the amplitude of the discharge current and the discharge time. Therefore, the maximum discharge current impact value and the maximum impact power are for reference only.
The figure is the wiring diagram of the charging and discharging principle of the capacitor. After the line is connected according to the figure, the capacitor is charged. Pay attention to the slower step-up speed. When the voltage reaches 3000V, the connecting rod of the capacitor and the DC voltage generator is disconnected by the insulating rod and contacts with the outer lead of the transformer core, and a clear discharge sound is heard to complete the discharge shock.
After the impact, the core-to-ground insulation resistance was measured to be 5000 MΩ. The measured grounding current was not measured. On the 19th day of operation, the core grounding current suddenly increased to 0.45A, and the retested core-to-ground insulation of the power failure was still 0 MΩ. This indicates that the impact of the first capacitor discharge impact was not obvious. The analysis may be due to the small discharge current. The second impact will be performed the next day, raising the capacitor charging voltage to 6KV.
Discharge impact energy:
Inrush current:
Maximum impact power:
After the impact, the core-to-ground insulation resistance was measured to be 5000 MΩ. The insulation resistance, dielectric loss, and leakage current of the measurement coil were basically the same as those in the pre-test. It has been in operation for more than three years since it was put into operation on that day. After core current monitoring and three-year pre-test, there are no abnormalities, indicating that this method has achieved the desired results.
Using this method to deal with the residual ground due to debris caused by the obvious effect of the earth fault, save time, save manpower and resources, simple and practical. However, iron core grounding caused by moisture or insulation breakdown of core insulation can not be used this method, still need to hang treatment.
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