Analysis of the hottest single-phase grounding fau

2022-08-11
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Analysis of a single-phase grounding fault in a small current grounding system

Yang Shan and Zhuo Chen, researchers of guozhoushan power supply company and Zhejiang zheneng Zhongmei Zhoushan Coal Power Co., Ltd., wrote an article in the 12th issue of electrical technology in 2018, analyzed in detail a tripping accident of a small current grounding system caused by a single-phase grounding fault, and discussed the relay protection action process and the cause of the accident, According to the actual operation characteristics of the substation, the corresponding rectification measures are put forward

small current grounding system refers to the system in which the neutral point is not grounded or the neutral point is grounded through arc suppression coil. When a single-phase metallic grounding fault occurs in a small current grounding system, the phase voltage of the non fault phase rises to the line voltage, and the line voltage and phase remain unchanged and symmetrical. The phase insulation of the system can meet the operation requirements, and the grounding point only flows through the total capacitive current of other outgoing lines. Therefore, when a single-phase grounding fault occurs in a small current grounding system, the system can usually operate for a period of time without tripping

at 0:45 on January 4, 2018, a phase a insulation breakdown at the intermediate joint of a 35KV incoming tower in a substation caused the single line to be grounded, causing the BC phase high-voltage fuse of the bus voltage transformer to fuse, causing multiple bays in the station to trip

1 basic information

1.1 substation operation

Goulu 3521 line, gouwan 3537 line and Shanmen 3535 line on 35kV section I bus of the substation are in operation at 6:07 on 30th (opposite side hot standby), 35kV switch of No. 1 main transformer is in operation, and line 3531 is in line maintenance, as shown in Figure 1. 35kV bus separation hot standby, standby automatic operation. The protection and measurement devices of Goulu 3521 line, gouwan 3537 line and Shanmen 3535 line are the whx-825a/p protection and measurement devices of XJ electric, and the model of 35kV standby automatic switching is wbt-822a/p of XJ electric

Figure 1 35kV section I bus wiring diagram of the substation

1.2 protection action

at 0:00 on January 4, the 35kV Goulu 3521 line of the substation was single-phase grounded, 00:45, 35kV Goulu 3521 line, gouwan 3537 line, Shanmen 3535 line distance section I acted, and the switch tripped. At the same time, the B and C phase high-voltage fuses of 35kV section I bus voltage transformer were blown. 35kV standby automatic switching action, tripping 35kV switch of No. 1 main transformer and closing 35kV bus switch. The reclosing action of Shanmen 3535 line was successful. There is no ranging information on the three lines

field background message and device message are consistent. The action is as follows:

1) 00:45:27.043, the distance protection section I of Shanmen 3535 line acts

2) 00:45:27.372, hook 3537 line distance protection section I action return

3) 00:45:28.151, the reclosing action of Shanmen 3535 line

4) 00:45:21.107, hook up 3521 line distance I action

5) 00:45:21.228, hook 3521 line distance I action return

6) 00:45:21.125, hook 3537 line distance protection section I action

7) 00:45:21.191, hook Wan 3537 line distance protection section I action return

8) 00:45:26.875, standby auto throw jump into the line

9) 00:45:27.633, standby automatic switching on and off bus switch action

there is only the main transformer fault recorder on site, which cannot collect the relevant voltage and current information of Goulu 3521 line. According to the fault analysis theory, when Goulu 3521 line is grounded in phase a, the B and C phases of Goulu 3521 line, gouwan 3537 line and Shanmen 3535 line will provide the capacitive current to phase a of Goulu 3521 line through the equivalent capacitance to the ground, which is the sudden change current of phase a at 35kV side of No. 1 main transformer

the sum of current changes of other two phases of Goulu 3521 line, gouwan 3537 line and Shanmen 3535 line is the sudden change current of other two phases at 35kV side of No. 1 main transformer. Therefore, analyze the voltage and current of No. 1 Main Transformer [3]. The voltage waveform of 35kV section I bus and the 35kV side current waveform of No. 1 main transformer are shown in Figure 1 and Figure 2 respectively

Figure 2 voltage waveform of 35kV section I bus

it can be seen from Figure 2 that the amplitude of phase a of 35kV section I bus voltage decreases significantly and phase BC increases around 00:45:11, that is, at this time, 35kV section I bus has a phase a single-phase grounding fault. It can be seen from Figure 3 that the effective value of three-phase current before the fault is about 0.47a. After the fault, the three-phase current has a certain degree of distortion, of which phase a distortion is the largest, but there is no obvious fault current in the whole process of the accident. Then, what causes the single-phase grounding of Goulu 3521 line? What is the reason for the line protection action

Figure 3 35kV side current waveform of 1 Main Transformer

2 cause analysis

2.1 single line grounding cause analysis

the maintenance personnel inspected the whole line of Goulu 3521 line, and found that the insulation breakdown of the A-phase cable joint of the 7 intermediate joint well between tower 13 and tower 14 of Goulu 3521 line made the A-phase of Goulu 3521 line grounded. The cable and the surrounding environment were carefully checked on site, and no one was found to have caused damage or environmental impact, After reading the historical data and maintenance records of Gelu 3521 line, it can be seen that the cable joint was put into operation in December 2016, and the voltage withstand test was not carried out as required before it was put into operation. The long-term operation led to the decline of its insulation, resulting in insulation breakdown and single-phase grounding

2.2 cause analysis of protection action

according to the manufacturer's instructions, the phase current sudden change starting element of wxh-825a type protection device with three line intervals judges whether the protected line has a fault by detecting the change of the instantaneous value of each phase current sampling in real time. The criterion is

if any phase current meets the criterion for two consecutive points, it is judged as protection startup. Once it is started, it will return with a delay of 10s. If it meets the mutation criterion before returning, it will be timed back again. According to the analysis and calculation of the current of No. 1 main transformer, there are four consecutive points in the weekly wave of the 35kV side A-phase current of No. 1 main transformer that meet the sudden variable current criterion, and the sampling frequency of the protection device is larger, that is, the protection device has at least four consecutive points that meet the criterion, and the protection device will judge the protection to start. When the protection device is in the starting state, the PT disconnection monitoring is locked. After the protection device is started, it returns with a delay of 10s. During this period, the BC phase fuse is fused, the capacitive current is large, the ranging impedance falls within the impedance range of distance section I, and the distance protection acts and the outlet trips

to sum up, phase a of Goulu 3521 line is grounded, causing intermittent mutation of three-phase current, which leads to the starting of the protection devices of Goulu 3521 line, gouwan 3537 line and Shanmen 3535 line, while the distance protection will no longer judge Pt disconnection and locking after the device is started. Therefore, after the single-phase grounding, the high-voltage fuses of phase B and C were fused. When the measured voltage of the protection of Goulu 3521 line, gouwan 3537 line and Shanmen 3535 line became zero, the line protection did not return due to the sudden change of current, and the PT line was also in the locked state, so the voltage phase failure could not be identified, resulting in the action of distance protection

3 rectification measures

the maintenance personnel replaced the broken cable connector and the fused voltage fuse, and timely restored the system operation state. However, in order to prevent such accidents from happening again, the following rectification measures can be taken to prevent tripping of small current grounding system when single-phase grounding

1) install arc suppression coil

this substation belongs to a small current grounding system with high degree of cable talk. When a single-phase grounding fault occurs in the system, there will be a large capacitive current. Due to the serious deviation between the price and value of rare earth in China, the substation can be equipped with arc suppression coil to reduce the capacitive current in case of single-phase grounding fault, so as to prevent the false start of the protection device. At present, many methods around intelligent arc suppression coil have been widely used in small current grounding systems

2) install line selection equipment

for the small current grounding system, in order to prevent its single-phase fault tripping, there are currently various line selection and fault location devices based on wavelet analysis, BP neural network and other pre selection methods. You can choose stable, reliable and economical devices to install in the substation to locate the single-phase fault point in advance to prevent tripping accidents

3) ensure the quality of the equipment put into operation

the intermediate joint of the cable in the substation is the accident prone point, and its quality and workmanship seriously affect the operation of the cable, especially affected by salt fog, plum rain and other weather, the cable equipment exposed outdoors should have better quality, which can meet the needs of long-term operation

4) implement the supervision of operation process

before putting into operation, cables and other related equipment should be comprehensively tested in accordance with relevant regulations. Relevant units and departments should strengthen the supervision of operation process, thoroughly implement standardized operation, and prevent the cable from being put into operation without voltage withstand test. 3) the number of experimental curve boxes can be arbitrarily selected from 1 to 3, so as to completely ensure that the re operation equipment and facilities have passed all testing tests, Comply with relevant technical standards, process standards, etc

5) strengthen the operation and maintenance of equipment

strengthen the operation and maintenance of power facilities and equipment after putting into operation. Through various ways and methods such as condition monitoring and manual inspection, and now many inorganic organic composite materials just hide the combustible B3 level of core materials or combustible B2 level problems that are strictly restricted in use. Strengthen the supervision of the real-time state of operating equipment, so as to achieve early prevention, early detection and early treatment, and early prevention of hidden danger equipment Deal with defective equipment early to reduce the accident rate and reduce the scope of power failure

conclusion

small current grounding system, with its high reliability, is often used in power systems of 35kV and below. Although the small current grounding system can continue to operate for several hours in case of single-phase grounding fault, the increased phase voltage and comparative capacitive current will affect the system, and the long-term single-phase grounding fault will also cause two-phase fault or even three-phase fault, expanding the scope of the accident. Therefore, how to prevent single-phase grounding and effectively suppress capacitive current becomes very important. Before the substation is put into operation, effective preventive measures should be taken according to the load conditions after operation and the station conditions to reduce capacitive current and equipment failure rate

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