Introduction to the principle of high-voltage permanent magnet switch

In the failure protection of the main transformer Circuit Breakers in three 220kV substations in a certain region, a typical relay protection design was used in the past. That is, when the high-voltage side (220 kV side) circuit breaker of the main transformer fails, the failure protection is activated, and the failure protection in the busbar differential protection module is activated on the 220 kV busbar differential screen. The busbar differential protection outlet is used to cut off the busbar circuit breaker and all connected circuit breakers on this busbar. However, compared to the main transformer switch, this typical configuration of failure protection cannot effectively isolate the fault point. The incoming line on the other side can reverse power supply to the fault point through other main transformers and 110kV busbars, which will continue to inject short-circuit current into the fault point. Therefore, improving the failure protection configuration of the 220kV main transformer high-voltage side circuit breaker is of great significance. 

Analysis of Typical Failure Protection for Switch 1

This 220 kV main transformer is equipped with two sets of protections, divided into protection screens A and B. The main protections are differential protection and non electricity protection such as gas protection, with zero time limit tripping on the three sides of the main transformer switch. The schematic diagram of the 220kV substation system is shown in Figure 1, which ignores components such as bypass and isolation switches. The CT winding functions of the high-voltage side switch of the main transformer are arranged in the following order: longitudinal difference one and backup, longitudinal difference two and backup, bus difference (failure) one, bus difference (failure) two, and failure protection. 

Both sets of differential protection devices for the main transformer have no delay tripping of the three side circuit breakers of the main transformer. 

Two sets of 220 kV compound voltage overcurrent protection and zero sequence overcurrent protection with the same principle are both activated by delaying the tripping of the three side circuit breakers of the main transformer, activating the zero sequence direction protection, and delaying the tripping of the high-voltage side circuit breaker of the main transformer. Two sets of 110kV compound voltage overcurrent protection with the same principle, the first time limit of section I jumps the 110kV bus tie circuit breaker, and the second time limit of section I jumps the 110kV circuit breaker on the medium voltage side of this transformer; Zero sequence overcurrent causes the main transformer's three side circuit breaker to trip after a delay; Configure zero sequence direction overcurrent sections I and II, divided into two time limits. The first time limit is to trip the 110 kV bus tie circuit breaker, and the second time limit is to trip the 110 kV circuit breaker on the medium voltage side of the transformer. Each relay protector cooperates with each other to protect the main transformer and adjacent lines from faults. As shown in Figure 1, there is a fault within the CT coverage range of the I # main transformer, namely the GZ1 point fault. The differential protection action is triggered and the three side circuit breakers of the I # main transformer are tripped with zero time limit. However, at this time, the high-voltage side circuit breaker I-DL1 fails and refuses to operate. The 220 kV incoming line still supplies power to the I # main transformer, and the differential does not return. The failure protection in the open busbar protection device is activated, and the 220 kV bus tie circuit breaker and all circuit breakers connected to the I bus are tripped, and the fault point is cut off. 

Although the fault point was cut off through the above protective actions, the high-voltage side circuit breaker refused to move, which expanded the fault range and tripped other lines connected to bus I.

Analysis of Protection Actions for Non Typical Malfunctions

When fault c is not within the coverage range of the main transformer protection CT, the failure and refusal of the high-voltage side circuit breaker will bring other effects and expand the fault. 

(1) The fault occurred between the CT windings. When such a fault occurs, due to the fact that the CT windings used in the two sets of differential protection of the main transformer are crossed with each other, this type of fault is within the coverage range of the differential protection, and therefore consistent with the typical failure protection fault action described above. 

(2) The fault occurred between the CT and the high-voltage side circuit breaker.
When the fault point is between the CT and the high-voltage side circuit breaker, as shown in Figure 1, the fault point GZ2 belongs to the range outside the main transformer differential protection and within the range of the busbar differential protection. At this time, the busbar differential protection will activate, tripping the 220 kV bus tie circuit breaker and all outgoing circuit breakers connected to busbar I. But at this time, due to the refusal of the high-voltage side circuit breaker, I-DL1 cannot trip, and the system incoming line injects short-circuit current into the fault point through the II # main transformer, II-DL2, medium voltage side bus tie circuit breaker, and I-DL2 circuit, as indicated in the diagram. If the short-circuit current reaches 220 kV high backup operating current at this time, and regardless of the input direction, after a delay of t1 (delayed by 3.9 seconds according to the new isolated transformer setting), the three side switches of the main transformer can be tripped to cut off the fault. If the short-circuit current does not reach the high backup operating current, the backup protection on the medium voltage side of the II # main transformer will reliably operate due to overvoltage. After a delay of t2 (delayed by 3.3 seconds according to the new isolated transformer setting), the 110kV bus tie circuit breaker will be tripped to cut off the fault. Regardless of the above situation, although the fault can ultimately be removed, the impact of short-circuit current on both main transformers after a certain delay (depending on the size of t1 and t2) has caused significant damage to the main transformers, and after the fault is removed, a portion of the 110 kV load is also lost. 

(3) The fault occurred on Mother I. When the fault occurs on bus I, it is the fault point GZ3. At this time, it belongs to the range of bus differential protection, and its action is basically the same as the second situation mentioned above. 

3. Improvement plan for malfunction protection

In order to improve the failure protection measures, prevent the expansion of faults, and reduce the impact on the main transformer and corresponding lines during faults, the failure protection can be set as: when the high-voltage side circuit breaker of the main transformer fails, quickly trip the three side switches of the main transformer to effectively isolate the fault. 

The main transformer's main body weight gas protection, load-bearing gas protection, and other non electrical protection can be activated by tripping the main transformer's three side switch, connecting the failure protection action node and the bus differential protection action node in series to the non electrical starting relay, as shown in Figure 2. When the failure protection is activated, the corresponding node closes, the relay is activated, and the three side switches of the main transformer are tripped. 

Implementation of Plan 4

According to the above improvement plan, the main transformer failure protection of three 220kV substations has been correspondingly improved, and the renovation situation is shown in Table 1.

Classifying various faults, analyzing the necessity of tripping the three side circuit breakers of the main transformer under various faults, and improving the failure protection of the high-voltage side circuit breakers of the main transformer. The improvement plan is to use non electricity export relays to trip the three side switches of the main transformer, which solves the problem of taking a long time to cut off the fault when the 220kV high-voltage side circuit breaker of the main transformer fails and refuses to operate under various fault conditions, reduces the possibility of transformer being affected by fault current impact, and ensures the safe and stable operation of the power grid.