Power Transformer Protection Relay: Principles, Settings, And Verification

The entire set of equipment responsible for rapid transformer fault removal is commonly referred to internationally as power transformer protection relay, corresponding to what we often call transformer protection relay. If the scope is relaxed a bit and all kinds of alarm monitoring such as current transformers, circuit breaker trip circuits, temperature, pressure, etc. are included, it falls under the broader term power transformer protection devices. The two names have slightly different focuses, but the goal is the same: to remove the transformer from operation within a very short time of the fault and minimize damage to the windings and core.

Several goals to be addressed in protection

The transformer protection target can be split into two main lines. The first is to quickly remove internal faults and limit equipment damage, which requires extremely fast movement speed. The second is to provide alarm or backup protection for overload, overexcitation, external failure and cooling abnormalities. Such problems do not necessarily require immediate tripping, but timely reminders are required. The two main lines target completely different scenarios and cannot be confused, so one cannot expect to cover all the risks with a single device.

Based on this, protection schemes must be designed according to capacity, voltage, system grounding method, importance, number of windings, tap-in method, and available circuit breaker configuration. Using a set of fixed values based on just one kVA value, or covering all risks with a universal relay, is not feasible.

Differential protection and excitation surge

Longitudinal differential protection is the main force in transformer protection. Its principle is to compare the currents on each side of the transformer. Once a phase or ground fault occurs in the protected area, the difference in currents on both sides will increase abnormally, thereby triggering a trip. The real difficulties lie in rectification: the variable ratio, the phase displacement caused by the connection group, and the error of the current transformer itself must be compensated for each item so that the differential flow criterion can stand.

Excitation surge is another unavoidable obstacle. When the transformer is closed without load, the excitation surge may behave like differential flow. If differential protection is not differentiated, normal closing may also be misguided. Therefore, differential protection usually uses harmonic or waveform criteria to distinguish the surge from the real fault and avoid accidental jumps.

Backup protection and supplementary measures

Differential movement alone is not enough. Overcurrent protection is often used as a backup for external short circuits or internal faults, thermal protection is used to deal with overload and hot spot risks, and V/Hz protection focuses on core overexcitation. Liquid-immersed products can also be configured with gas, burst pressure, pressure relief and oil temperature protection; dry-type products rely more on the combination of winding temperature, fan status, overcurrent and differential. When the ground fault is relatively hidden, limiting the ground fault protection can improve the sensitivity, but whether it is applicable depends on the specific configuration of the neutral point and current transformer, and cannot be copied from other items.

Setting value, coordination and power supply

Fixed values are the most important part of the protection plan that requires on-site judgment. The rated value and action curve of the fuse or overcurrent protection must not only avoid the allowable load and excitation surge, but also remove the fault between the equipment tolerance range and the upper and lower level protection. Before the first excitation, the impact of excitation surge on differential and overcurrent protection must be evaluated to confirm that the circuit breaker, protection and system voltage conditions allow for power resupply. After tripping, you cannot automatically reopen the gate. You must first confirm the fault category, evidence of protection actions, equipment status and system risks.

Verify that the complete link is covered

Finally, there is verification. Protection function testing cannot only measure the relay body. It must go from measuring input, logic, tripping loop, circuit breaker, alarm all the way to the monitoring end, and end-to-end general operation counts. Testing one relay alone does not prove that the entire protection chain is reliable. Many hidden dangers lie precisely in the transformer polarity and secondary circuit wiring, and rarely in the relay body.

If we string these links together, transformer protection is never something that ends with selecting a piece of equipment and filling in a fixed value. First, think clearly about the location and risks of the equipment in the system, match it with appropriate protection, and then use complete field tests to verify each link in place. Only then can the configured power transformer protection relay be truly reliable.