500kV bus differential protection fault of Guangzh

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Fault analysis of 500kV bus differential protection in Guangzhou energy storage hydropower plant

Abstract: the fault of 500kV bus differential protection in Guangzhou energy storage hydropower plant is analyzed. Through the volt ampere characteristic test of each phase current transformer and comparison with each other, it is concluded that the reason why the fault first pushes the jaw with the help of external force: due to the damage of the insulating sleeve of a flange connecting bolt, the bolt and the bus sleeve form a current loop, making the current transformer parasitic current, resulting in the bus differential protection fault

key words: 500kV bus differential protection fault analysis of Guangzhou energy storage hydropower plant 500 kV main wiring adopts quadrangular wiring, and the two bus t areas formed by the line access point are protected by its own line protection within the installation point of the line protection. The two T-zones formed at the parallel point of the main transformer adopt bus differential protection (hereinafter referred to as bus differential protection). 1 Principle and characteristics of bus differential protection phase I 500 kV bus differential protection of Guangzhou Energy Storage Hydropower Plant adopts D. It is expected that domestic mines will still operate weakly in the near future ife3110 type high resistance differential protection. The 500 kV circuit breaker is equipped with two sets of identical high resistance differential protection with QF1 and QF2 as one side and QF3 and QF4 as the other side, and,. The split phase is composed of two sets of dife3110 high impedance relays. Using the principle of current vector comparison in and out of the protected area, the voltage value generated by the differential current on the resistor R is taken out and entered into the relay as the measured value to compare with the threshold value. When there is an external fault or no fault, the load current I and I 'are connected, and the factory price of Guangxi Wuzhou scrap increases by 20 yuan/ton; Yantai new Oriental metallurgical scrap purchase price increased by 20 yuan/ton; The scrap price of Henan Minyuan special steel increased by 30 yuan/ton; The scrap purchase price of Guangdong Jieyang Daxing steel plant increased by 20 yuan/ton; Jincheng Fusheng scrap benchmark price rises by 30 yuan/ton. When passing through resistor R, the phase is opposite, the amplitude is equal, the voltage on resistor R drops to zero, and the relay does not act. When there is an internal fault in the protection area, the current I and I 'are in the same phase, so that the corresponding fault current generates a certain voltage value on the resistor R. when the value is greater than the threshold, the relay action outlet is started, as shown in Figure 2. The protection setting value is: locking voltage ub=20 V, action voltage ud=25 v. Protection action result: outlet trips QF1, QF2 or QF3, QF4, unit 1 and 2 or unit 3 and 4 trip, and start the fault recorder. And fault during the maintenance of QF2 circuit breaker in phase I of Guangzhou Pumped Storage Power Plant in November 1998, it was found that when units 3 and 4 were operating under pumping conditions, the bus differential protection sent a locking signal. Measuring the bus differential protection device, it is found that L3 phase has unbalanced output, and the maximum voltage drop on resistor R is 21.5 V, which increases in a positive proportion with the primary current, exceeding the locking voltage setting value. 3. Fault finding and analysis in November, 1998, unit 3 and 4, QF3 and QF4 circuit breakers were tested under different operating conditions. When QF3 was closed and QF4 was disconnected, the voltage drop of three-phase differential current on resistor R was basically zero; When QF3 is disconnected and QF4 is closed, the voltage drop of L3 phase difference current on resistor R is large, and L1 and L2 phases are basically zero. It is judged that there is a problem with the L3 phase current transformer 54lrb006ti or 54lrb007ti on the outgoing line side of QF4. In order to further determine the nature of the fault, the polarity tests of the line and current transformer were carried out on the secondary current circuit, and the results were normal. In December of the same year, the power hexagon diagram test of the secondary current circuit was carried out, from which the polarity and wiring of the current transformer can be judged to be correct. Through analysis, it is considered that: a) two groups of current transformers on the outgoing side of QF4 circuit breaker may have faults; b) It may be that there is a parasitic circuit in the primary circuit of the current transformer, causing the secondary to produce unbalanced output. For this reason, the insulation sleeve of flange bolts on the outlet side of QF4 was mainly checked, and no fault was found. In February, 1999, QF3 and QF4 were disconnected for volt ampere characteristic test of current transformer. The volt ampere characteristics of the two sets of current transformers of L3 phase are quite different from those of QF4 phase. When redoing the volt ampere characteristic test of L3 phase current transformer, it was found that a flange connecting bolt was hot and hot. After disassembling the nut on the insulating sleeve side of the bolt, it was found that the lower part of the insulating sleeve was broken, making the bolt contact the bus sleeve for grounding. After replacing the insulating sleeve, redo the volt ampere characteristic test of current transformer. The volt ampere characteristic of current transformer returns to normal. After QF3 and QF4 are put into operation, the bus differential protection and unbalanced current disappear, and the bus differential protection returns to normal. After the insulation sleeve is damaged, the bolt is grounded through the bus sleeve, and the bolt forms a current circuit with the bus sleeve. Under this condition, an induced current is generated on the bus bushing, so that the current sensed by the current transformer is Ia + I ′ a (where IA is the primary side working current, I ′ A is the induced current), and the direction of IA is opposite to that of I ′ a. Assuming that the bolt is in full metal contact with the flange, ia=i ′ a, so the current sensed by the current transformer is zero. The fault phenomenon is similar to the disconnection or reverse polarity of a group of current transformers. 4 existing problems 4.1 current transformer volt ampere characteristics current transformer model is 5p20, 20 VA. According to the results of this volt ampere characteristic test, its inflection point voltage is about 560 V, which may not meet the requirements of high resistance bus differential protection current transformer for high inflection point voltage (such as greater than 800 V), and corresponding remedial measures should be taken. 4.2 the cause of this fault caused by the damage of the bolt insulating sleeve is the rupture of the bolt insulating sleeve, which makes the bolt grounded and leads to parasitic current in the current transformer. Although it is impossible to accurately explore how the bolt insulation sleeve is damaged, it does not rule out that the specified torque wrench is not used during installation, and the insulation sleeve is broken due to excessive force. In long-term operation, the vibration makes the bolt grounded. In view of this, the quality management of the whole process of GIS handover test should be strengthened, and the volt ampere characteristic test of all current transformers of the bus should be carried out to prevent accidents

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