Transformer differential protection common incorrect action analysis - Database & Sql Blog Articles

The polarity of the current transformer is reversed, causing the fault outside the differential protection zone to malfunction.

A 220kV outlet on a substation has a fault. After the line trips, the reclosing action occurs and a three-phase short circuit occurs. At this time, the No. 1 main transformer differential protection action, the transformer was removed. 
After the accident, the secondary circuit of the protection device was tested and found that the polarity of the phase A differential TA on the 220kV side of the transformer was reversed.
The reason for the misoperation outside the area is: the polarity of the TA is wrong, and the fault outside the area is equivalent to the fault in the area. Therefore, the relevant regulations should be strictly implemented: when the differential protection is officially put into operation or after the secondary circuit is changed, the load inspection must be carried out to make a hexagonal diagram of the differential TA to ensure that the differential TA wiring is correct. 

Misoperation of differential protection when transformer is dropped

When the substation has five air-drop main transformers, the differential protection is faulty five times. Once the system is checked, there is no abnormality, and there is no problem in protecting the secondary circuit. After the accident, the differential protection test found that the second harmonic braking coefficient of the protection device has a large fixed value error, and the second harmonic braking coefficient setting value is 20%. In fact, the second harmonic braking is braked at 23%. . After the protection device is replaced, the differential protection has an accurate braking characteristic at 20% braking current. The transformer is again idling and the differential protection is not malfunctioning. 
The cause of the accident is: the differential protection device has poor characteristics, the second harmonic braking coefficient is higher under the fixed value, the excitation inrush current is large when the transformer is aired, and the second harmonic braking ability is too small, so it causes misoperation in the airdrop transformer. . 
The newly installed protective device shall be inspected in a comprehensive manner according to the regulations. It shall be found that the characteristics of the protective device are not good. The airborne test of the newly installed or overhauled transformer shall be carried out 3 to 5 times, and the magnetizing inrush shall be analyzed. Size and harmonic content. Adjust the second harmonic braking ratio according to the actual situation.

Differential protection misoperation caused by insulation damage of TA secondary circuit insulation damage

A substation 2# main transformer differential protection operates in the absence of a fault in a system, and the main transformer is removed. After the accident, it was found that the 2# main transformer differential protection high-voltage side C-phase TA to the switch terminal box secondary cable insulation damage, the ground insulation is zero, thus shorting one phase TA. A differential current is generated in the differential relay to cause the protection to malfunction.
The cause of the accident is: the pipe of the C-phase TA is not tightly sealed at the pipe, and the water is poured into the pipe. The temperature in winter is low and the ice is damaged. 
Therefore, it is necessary to strictly control the quality during construction. First, the cable cutter is prevented from damaging the outer insulation of the core wire when the cable head is peeled. Second, the nozzle of the cable through the pipe must be well sealed. Before the protection is put into operation, after the overhaul or every spring inspection, the 1 000V megohmmeter is used to measure the insulation between the TA core wires and the core wires, and the hidden dangers are detected in time. 

TA model and ratio selection error 

Transformer differential protection should choose D-class iron core TA with air gap. Because the transformer used for transformer differential protection is not the same in type and ratio, its characteristics and residual magnetization are not completely the same, so it is normal. There is always an unbalanced current flowing through the differential circuit during operation. The degree of core saturation has a significant effect on the unbalanced current, and it increases significantly with the increase of the primary current, and the short-circuit current is large. In order to reduce the unbalanced current, it is necessary to take measures in the structure of the TA, the core material, etc., so that the iron core is not easily saturated when the primary side passes a large short-circuit current, and the D-class iron core TA with an air gap has the above functions. , is a special TA dedicated to differential protection. 
When selecting the TA ratio, based on the original routine calculation, the 1 to 2 gears can be appropriately increased according to experience, that is, the TA of the large ratio can be appropriately selected, so that the short circuit current multiple can be reduced, and the difference generated in the differential circuit can be reduced. Balance the current, effectively weaken the magnetizing inrush current and improve the sensitivity of the differential protection. This is a more effective way to avoid faults outside the protection zone and prevent the transformer differential protection from malfunctioning. 
The correct choice of the TA model and the ratio is the basis for ensuring the reliability of the differential protection action. If the TA model is selected incorrectly or the selected change is small, the TA core is rapidly saturated when the fault occurs outside the protection zone, and the unbalanced current increases rapidly, which will cause the differential protection to malfunction. Therefore, it is necessary to pay attention to the TA model and the ratio. select. 

Differential protection secondary current loop grounding method error

When the secondary current loop of the differential protection is grounded, the secondary current loop including the TA on each side must be connected to the grounding grid through one point. Because the grounding grid of a substation is not absolutely equipotential, there is a certain potential difference between different points. When a short-circuit fault occurs, a large current flows into the grounding grid, and a large difference will occur between the points. Potential difference. If the secondary current loop of the differential protection is grounded at different points of the grounding grid at the same time, the current generated by the potential difference between the different grounding points in the grounding grid will flow into the protection secondary circuit, and this current will increase the difference in the differential circuit. Unbalanced current causes the differential protection to malfunction. 
The requirement for differential protection secondary circuit grounding is that the secondary current loops of each side TA are connected in parallel to the differential current loop of the protection device, and all secondary current loops must also be grounded at a common point in parallel. In order to avoid the influence of the current generated between different grounding points on the reliability of the differential protection action.

Differential protection caused by staff mistakes 误

A substation 2# main transformer 10kV side 952 switch power failure, 220kV, 110kV side switch normal operation, the metering personnel to test the 2# change 952 switch metering circuit TA, because the staff is not familiar with the loop, mistake the differential circuit as In the metering circuit, when the current is applied, the main transformer differential protection action trips the high- and medium-voltage side main-change switch, causing the main transformer protection power failure.
Due to the staff's mistakes, the differential protection accidents are serious in nature, and they belong to the assessment accidents. Everything should be avoided. Responsible for staff to strengthen safety education and technical training, personnel working in the secondary circuit of equipment should have strict written safety measures, item by item, and should be guarded by special personnel, staff should strengthen the responsibility of the work, work time Can not rely on the impression, according to the drawings, to prevent such accidents. 
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Differential protection is the primary protection of the transformer and should meet the requirements of reliability, selectivity, sensitivity and fastness. However, due to its complicated structure and complicated wiring, it is very likely to leave hidden dangers during the installation and inspection and modification process. Therefore, in the process of design, construction and subsequent inspection and modification, we must strictly follow the requirements of the regulations, carefully analyze, and ensure each model is closed to ensure that the TA model, ratio, secondary wiring and secondary current grounding are correct. To prevent the occurrence of differential protection accidents.