Main Transformer Backup Protection Principle and Protection Scope

Main Transformer Backup Protection Principle and Protection Scope

Function: Power transformers should be equipped with overcurrent protection devices for external grounding and phase-to-phase short circuits, as well as neutral point overvoltage protection devices, to serve as backup protection for adjacent components and internal transformer faults. The transformer's backup protection is the standby protection for its main protection. When the main protection fails, the backup protection operates to ensure equipment and personnel safety. Its protection scope includes the transformer, the power supply circuit, and the load equipment on the circuit. Backup protection refers to impedance protection, undervoltage overcurrent protection, composite voltage overcurrent protection, and overcurrent protection. They can all detect the overcurrent state of the transformer, but their sensitivities differ; impedance protection has high sensitivity, while overcurrent protection has low sensitivity.

III. Classification of Backup Protection Remote Backup Protection: Backup protection implemented by the protection of adjacent power equipment or lines when the main protection or circuit breaker fails to operate. Near-backup protection: When the main protection fails to operate, backup protection is provided by another set of protection on this equipment or line; when the circuit breaker fails to operate, near-backup protection is provided by the circuit breaker failure protection. High backup protection and low backup protection are relative to the transformer. Backup protection on the high-voltage side of the transformer is called high backup, and backup protection on the low-voltage side of the transformer is called low backup.

1. Backup protection is used to protect the transformer in the event of a failure of the main protection to operate. It generally includes:

 (1) Overcurrent protection initiated by composite voltage on the high-voltage side;

(2) Overcurrent protection initiated by composite voltage on the low-voltage side;

 (3) Zero-sequence current and zero-sequence voltage protection against external grounding short circuits;

(4) Overload protection to prevent symmetrical overloads;

 (5) Protection connected to the high-voltage side busbar: high-voltage side busbar differential protection, circuit breaker failure protection;

(6) Related protection connected to the low-voltage side busbar: low-voltage side busbar differential protection, etc. The working process of the transformer protection device is shown in Figure 6-1. When the measured parameters of the transformer do not exceed the set values, the protection is in normal operation. When a fault occurs, each protection unit determines whether the fault occurs within its respective protection range based on the measurements. When a fault occurs inside the transformer, the differential protection trips; if the fault point is inside the tank, the gas protection can trip with high sensitivity. Regardless of whether the fault is internal or external, the transformer's phase-to-phase backup protection should be activated. If it is a ground fault, the zero-sequence protection, as a backup protection for ground faults, also activates simultaneously.

If the fault disappears within the backup protection's operation delay, the backup protection returns to normal operation; if the fault still exists, it trips, disconnecting the transformer from the power grid.

Furthermore, when the transformer experiences overload or other abnormal operating conditions, the corresponding protection will activate and issue a signal. IV. Backup Protection for Phase-to-Phase Short Circuits The main protection of a transformer typically uses differential protection and gas protection. In addition to the main protection, the transformer should also be equipped with backup protection for phase-to-phase short circuits and ground faults. Backup protection serves to prevent overcurrent in transformer windings caused by external faults, and acts as a backup for the protection of adjacent components (busbars or lines), and, where possible, as a backup for the main protection in case of internal transformer faults. Transformer phase-to-phase short-circuit backup protection typically employs overcurrent protection, undervoltage-starting overcurrent protection, composite voltage-starting overcurrent protection, and negative-sequence overcurrent protection. Impedance protection is also sometimes used as backup protection.

1. Overcurrent Protection The principle wiring diagram of the overcurrent protection device is shown in Figure 5-18. Its working principle is the same as that of line definite-time overcurrent protection. After the protection operates, the circuit breakers on both sides of the transformer are tripped. The starting current of the protection is set according to the maximum possible load current of the transformer, i.e., where: Krel — reliability coefficient, taken as 1.2-1.3; Kr — return coefficient, taken as 0.8-0.95; IL·max — the maximum possible load current of the transformer. IL·max can be considered under the following circumstances and the maximum value should be taken:

(1)For transformers operating in parallel, the overload that occurs in other transformers when the transformer with the largest capacity is disconnected should be considered. When the capacity of each transformer is the same, the calculation formula is: Where n——the minimum possible number of transformers operating in parallel.