Preventive measures for tripping of three-side switch of 110kV main transformer


110kV main transformer

The failure of main transformers is generally rare, but after the accident, it will definitely affect the power supply of some users. Moreover, as the main equipment of the substation, the repair and processing time of the main transformer is relatively long, and the economic loss caused is very serious.

1. Failure Case Analysis

The three-side switch of a 110kv main transformer tripped. The engineer inspected and found that the 121 switch on the high voltage side of the main transformer overcurrent ⅱ section ⅰ time limit protection tripped, the operating current ia=7.18a, ib=6.01a, ic=2.31a. After the accident, the dispatching office immediately ordered the switch to the 2# main transformer, but because the 2# main transformer was in cold standby state, the accident caused the entire station to lose pressure for 40 minutes.

At the same time, the Production Technology Department immediately organized the test and maintenance team to inspect and analyze the cause of the accident. The reasons for the backup protection action of the main transformer may be as follows:

(1) Short circuit on the high voltage side of the transformer;
(2) Short circuit on the medium voltage side of the transformer;
(3) Short circuit on the low-voltage side of the transformer;
(4) A failure occurs in the differential protection range of the main transformer, and the differential protection fails;
(5) Misoperation of backup protection;
(6) The medium and low voltage lines fail and the line protection refuses to operate, causing the main transformer backup protection to trip.

2. Basic situation of main transformer and accident analysis report

Basic situation of main transformer

Basic situation of 110kv central substation: main transformer capacity 2×63000kva, 4 circuits of 110kv, 4 circuits of 35kv, 11 circuits of 10kv. It is responsible for the power supply of the whole city. Once the whole station loses voltage, it will cause a large-scale blackout in the whole city.

Detect and test accidents

Combined with the analysis of the possible causes by the Production Technology Department, the 1# main transformer was tested. The test data are as follows:

Carry out the up-current test on the switches on each side: the switch protection on each side can reliably send a signal to trip; Carry out the insulation resistance, absorption ratio and dielectric loss tests on the 1# main transformer. The test data of the 1# main transformer is as follows:

After checking and testing the insulation resistance, absorption ratio and dielectric loss of each side of the main transformer, and analyzing the data, the influence of items (1) to (5) in the estimation of the cause of the accident may be eliminated from the backup protection action of the main transformer.

Check 35kv line

After inspections and tests on the high-voltage, medium-voltage, and low-voltage sides of the main transformer, no cause was found to cause the backup protection action of the main transformer, so further inspections are required. In the process of checking the protection devices of each 35kv line, it was found that the following 35kv line has protection action information:

(1) First of all, a zero-sequence overvoltage alarm appears on the 35kv Zhongping 325 line, and the action parameters are as follows:
ua=67.8 v, ub=118.5v, uc=111.9v, uab=50.7v, ubc=61.8 v, uca=66.2 v,3u0=177.5 v;

(2) Subsequently, a small current grounding alarm occurred on the 35kv Zhonglu 323 line, and the action information is as follows:
ua=46.2 v, ub=63.82v, uc=61.2v, uab=92.4v, ubc=101.8 v, uca=100.2 v,3u0=16.49 v; ia=0.25a, ​​ib=0.9a, ic=0.83a.

Through the detection test of the 35kv line, when a single-phase grounding short-circuit fault is found in the line, the outgoing line protection only operates and alarms, and the line can continue to run for 1-2 hours. Therefore, it can be seen that when a single-phase grounding alarm occurs on two lines of the power grid at the same time, it reflects the system is a two-phase grounding short-circuit fault, but the low voltage cannot meet the medium voltage side 321 switch compound low voltage blocking overcurrent section ⅰ protection, and high voltage Side 121 switch compound low-voltage lockout over-current ⅰ protection compound low-voltage action requirements.

1# The current on the 110kv side of the main transformer increases to meet the action requirements of the overcurrent ⅱ section ⅰ time limit protection current element. The high-voltage side 121 switch is used as a 10kv and 35kv outgoing remote backup protection action to trip. After inspection, the protection settings of 1# main transformer and 2# main transformer of the center change are shown in the following table:

According to the data in the above table, 1#, 2# 110kv side 121 switch, 122 switch main transformer overcurrent ⅱ section ⅰ time limit protection setting value, no “composite voltage lockout” protection; at the same time, 1#, 2# main transformer 321 Switch, 322 switch over-current protection ⅰ time limit, put into “composite voltage lockout” protection.

Analysis of the cause of the accident

It can be preliminarily judged that the reason for the backup protection action of 1# main transformer is the ground fault of line 325 and line 323 at the same time, which caused the current and voltage to fail to meet the protection operating conditions of the medium voltage side of the main transformer during operation, but the operating current has reached the high voltage side of the main transformer. Overcurrent ⅱ section ⅰ time limit protection conditions and tripping.

Therefore, it is necessary to go to the site to detect the specific reasons that cause the 325 and 323 lines to be grounded at the same time, and check whether it is caused by external force or the state of the line itself.

Therefore, according to the actual operation of the power grid, it is recommended to modify the setting value of the main transformer backup protection:
⑴Adjust 1#, 2#110kv side 121 switch, 122 switch main transformer overcurrent ⅱ section ⅰ time limit protection setting value, and put into the “composite voltage lockout” of this protection;
⑵Adjust 1#, 2# main transformer 321 switch, 322 switch overcurrent ⅰ protection ⅰ time limit, and exit the “composite voltage lockout” of this protection.

3. Preventive measures

In order to ensure the reliable and effective operation of the substation, the following measures can be taken to prevent:

(1) Start from the selection of equipment. When selecting major equipment such as transformers, the quality must be strictly controlled to avoid design defects in the process structure of these equipment. Strengthen management from equipment installation, transformation and overhaul project acceptance, etc., especially the part of inspection and acceptance work that may be hidden. It is necessary to always ensure that the equipment can operate safely. After the equipment is installed and operated, the transformer should be maintained and regularly repaired and monitored.

(2) The staff must strengthen the operation management work. When conducting on-site equipment inspections, they must strictly follow the inspection items specified in the equipment operation regulations, and conduct comprehensive and detailed inspections and maintenance work on a regular basis.

(3) Promote the implementation of online testing of large main equipment such as transformers, and install transformer online measurement devices on transformers. Achieve regular and real-time inspections of the operation of main equipment, make reasonable judgments on the early signs of accidents, and take preventive measures in advance to reduce the frequency of equipment accidents. In the tracking and monitoring of the main equipment transformer, oil chromatographic tracking analysis can be used, and the chromatographic tracking period can be determined according to the side general tracking situation.

(4) In view of the staff’s weak sense of responsibility, formulate a reasonable reward and punishment system, clarify responsibilities, and strengthen the sense of responsibility of the staff. Establish an economic responsibility assessment system for staff to supervise whether the work content of staff is in compliance with regulations, to ensure that daily work meets business standards and compliance with regulations, and minimize the probability of accidents.

(5) As the frequency of extreme weather has increased in recent years, equipment inspections and preventive measures must be fully implemented. The main preventive measures for tripping faults caused by natural disasters and external forces are: to avoid tripping accidents caused by lightning strikes, you can use lightning protection measures such as installing lightning rods and installing lightning protection wires, and you can overhaul the grounded module to ensure that the grounding resistance reaches the quality Standard. Tripping accidents caused by bird damage can be equipped with a triangular link, which can effectively prevent birds from staying or nesting

(6) The power supply unit needs to appropriately carry out a variety of learning activities and create a good learning atmosphere to improve the overall technical level of business personnel. Train staff regularly. Carrying out rigorous and intensive training on handling accidents for operating personnel, able to correctly determine the cause and nature of the accident, deal with the accident quickly and efficiently, and reduce the degree of loss caused by the accident.

(7) For accidents caused by the overloaded working conditions of equipment, the power grid can be improved, and the power distribution and other equipment can be tested regularly, and the fuse setting value of the dropped fuse can be recalculated, and its reasonable configuration.

(8) Be good at summarizing accident experience, strengthening technical supervision functions and analyzing and managing test results. Effectively analyze and summarize the accidents that have occurred, analyze the causes and look forward to improvement. Under bad conditions, some equipment can be tracked and inspected for individual items to find hidden defects in the equipment in time and improve work efficiency.

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