Gas protection is the use of the gas relay inside the reaction transformer to protect the transformer. The gas relay is to reflect the internal electrical fault of the transformer tank and the drop of the oil level. It can send an alarm signal or trip action signal for common internal faults such as faults in the turn and the interlayer, insulation drop in the winding, iron core fault and oil level drop. .
The power transformer operation specification [1] stipulates that “the gas relay should have two contacts when the voltage regulator is in operation, and they should be completely isolated from each other. One set is used for light gas alarms and the other set is used for heavy gas tripsâ€. Gas protection is one of the most important primary protections of power transformers and is divided into light gas and heavy gas protection.
The gas relay is the main implementing element of the gas protection and is installed on the connecting oil pipe between the transformer oil tank and the oil pillow. The reasons for the action are the increase of the gas concentration inside the transformer, the impact of the oil flow, and the reduction of the oil level. When a slight internal fault occurs in the transformer, light gas moves and an alarm signal is issued. If a serious fault occurs inside the fuel tank, a large amount of gas will be generated and the oil flow will hit the lower baffle plate. When the oil flow rate reaches the setting value of the gas relay, it is heavy. The gas moves and a trip signal is issued and the circuit breaker trips.
1. Heavy gas accident related situation
At 23:57 on August 11th, 2016, the on-duty personnel of the central control station in the electrical spot inspection area discovered that the 3102 switch tripped at the North 35kV station, the 3# main transformer trip at the 22:59 station at 23:58, and the 2nd and 3rd stage at the 220000 station. Bus outage. After inspection, it was confirmed that there was phase-to-phase short circuit in the cable room of the North Station 35kV 3102 switchgear, and the heavy gas of the 2# station 3# was tripped, and there was no abnormality in other equipment. Subsequently, the relevant personnel quickly organized the 2# main transformer, which was standby for 220,000 stations, into operation and transmission, and informed the lower-level users to resume power transmission.
The power supply of the 3102 switchgear in the North 35kV station is from the 220000 station 35kV 1# bus, and there are 3 master transformers in the 220,000 station, and the second one is prepared. The 1#, 3# main transformer and 2# main transformer are used for normal operation. 220 thousand station 3# main transformer model PZ10-120MVA/220kV, rated capacity 120000KVA, rated voltage 220+8*1.25%/37KV, cooling method OFAN/OFAF. Gas relay model BR80QJ4-25. Pump fan control box model: XKWFP-21, rated voltage 380V, 10 fans, 4 pumps. The transformer oil pump has two working power sources, which are changed from two units.
2. Analysis of the cause of the accident
The accident point was in the switchgear of the 220,000 transformer substation power distribution room, and there was a possibility that the line backup fault protection action [2] was caused. After a fault occurs, the fault phenomena and processing results are analyzed using the correct theoretical methods to comprehensively analyze all test data after the fault, find out the key points, and judge the fault status and results [3].
A phase-to-phase short circuit occurred between the B-phase grounding knife static contact and the C phase arrester in the cable room of the North Station 35kV 3102 switchgear. The principle of discharge is mainly that insulation strength of insulating parts or insulation gap is lower than applied voltage to cause insulation and wear [4]. Substation room environment control and control of temperature, humidity and other factors, did not take effective measures, is the main management cause of this accident.
For transformer gas protection actions, there are many possible reasons [5], and the cause of the accident was investigated step by step.
When the transformer is in normal operation, a heavy gas trip occurs. First, the operation of the heavy gas relay is caused by excluding the misoperation of the operating personnel or the mistaken jump of the main transformer when the maintenance personnel work, and the excitation inrush current and the transformer load increase drastically when the transformer is no-loaded.
Second, check the possibility of transformer gas contact relay misoperation. Through the visual inspection, disassembly inspection and oil flow test of the gas relay, the result was normal. Troubleshoot the possibility of malfunction of the non-electrical protection transformer outlet due to DC insulation problems. The gas protection circuit and gas protection device detection, the results are normal; the main transformer sub-protection device power board factory inspection, the test results are normal. The protection setting value of the relay for the gas relay is calibrated to 1.2m/s. The value is correct and the transmission is normal, eliminating the possibility of malfunction.
Again, the possibility of heavy gas action caused by serious short-circuit faults inside the transformer was investigated. After the accident, there was no apparent failure outside the transformer. Chromatographic analysis of gas content in transformer oil can effectively diagnose potential early-stage transformer failures and is an important method for detecting transformer internal faults.
Transformer oil chromatogram analysis and electrical test are performed to check if there is any internal fault. The 3# main transformer oil chromatogram, body winding insulation, straight resistance, winding deformation were tested and analyzed, and the results were normal. It proves that there is no obvious fault inside the transformer. Chromatographic data is shown in Table 1.
220kV Transformer Heavy Gas Accident Analysis and Prevention
Table 1 Main transformer chromatographic test data after heavy gas operation
First look at the content of the characteristic gas. Regulations for the handover of power equipment and preventive tests [6]: Total hydrocarbons in the running oil <150 μL/L, acetylene <5 μL/L, hydrogen <150 μL/L. C2H2, total hydrocarbons did not exceed the value of attention, and H2 exceeded the standard, H2 exceeded the level of the main change to H2 in the 220KV station (No trace of growth was found in the follow-up test for 1 year since 2013, and the oil sample was periodically taken as the chromatographic test). Deterioration was observed. Data analysis measures the operational status of the transformer.
According to the judgment of the characteristic gases of gas production in "Guidelines for the analysis and determination of dissolved gases in transformer oil GB/T 7252-2001" [7], it is considered that the reason for exceeding H2 in 30000 main transformers at 220,000 stations is that it has experienced three accidents. resulting in.
Considering that the host has become forced-oil circulation air cooling, literature [8] improved the submersible pump and fan control system to reduce the impact of oil flow. The analysis found that the two sections of the main oil pump are a primary and backup mode. Three on-site start/stop tests were performed on the No. 3 main oil pump. During the test, two oil pumps were frequently started, light gas was actuated, and heavy gas was actuated twice. When the four oil pumps were started at the same time, the gas relay was activated by heavy gas. .
After analysis and research by the technical personnel of the parties, it is considered that the oil pump system has two design defects:
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