Power Transformer Protection Schemes Selection And Coordination

Whether a power transformer protection scheme can truly protect the equipment often depends not on how advanced the relay itself is, but on whether it is matched according to the transformer’s capacity, structure, system grounding, available fault current, and the consequences of the fault. This is also the easiest place to go astray in power transformer protection. Protection is not about buying a universal part, installing it and then finishing it. Different devices and different systems have completely different risk points.

Protect the target first

The goals of transformer protection can be viewed from two perspectives: quickly remove internal faults and minimize equipment damage; at the same time, provide alarm or backup protection for situations such as overload, overexcitation, external faults and cooling abnormalities that may not trip immediately. Think about these two points clearly. The benchmark for judgment is which plan to choose and what value to set later, and the more expensive the better will not be led by it.

Differential protection is the main

Longitudinal differential protection is the backbone of the entire scheme. It compares the current on each side of the transformer and is used to detect phase-to-phase or ground faults within the protected area. During the adjustment, the ratio change, coupling group phase shift and current transformer error must be compensated. This step is not done carefully enough and may cause misoperation during normal operation. The excitation surge is a classic trap. It will appear in the differential flow at the moment of closing the gate, so differential protection usually uses harmonic or waveform criteria to identify it to avoid normal power supply being misjudged as a fault.

How to fill the gap in backup protection

No matter how strong the differential protection is, overcurrent protection is also needed behind the scenes. Overcurrent protection can be used as a backup for external short circuits or internal faults, but its ratings and curves must avoid both permissible loads and excitation surges, and faults must be cleared within the equipment’s tolerance range and the upper and lower protection ranges. There is no universal specification that can be determined based solely on capacity. Ground fault protection also depends on the configuration of the neutral point and current transformer. Limiting ground fault protection can improve the sensitivity of ground faults in specific windings. Overexcitation is monitored by V/Hz protection, while temperature protection is responsible for overload and hot spot risks.

Dry and liquid immersion types are available in their own way

In addition to the above, liquid-immersed products can also be equipped with gas, burst pressure, pressure relief and oil temperature protection; dry products usually rely on winding temperature, fan status, overcurrent and differential means. The two types of equipment have different structures and different risk points, so the protection configuration should naturally not be copied. The through-fault curve is used to assess the tolerance and protection coordination under external short-circuit current and duration. It refers to how long the equipment can withstand the load and does not mean that the transformer has the normal overload capacity for the same period of time.

The process of setting and restoring power must be explained

The protection plan must eventually be implemented at a fixed value, which comes from the protection and cooperation research, not from moving other projects over and changing a few numbers. When re-energizing after tripping, the fault category, evidence of protection actions, equipment status and system risks must also be confirmed first. Closing the switch cannot be regarded as the default recovery step. The protection and fire protection specifications should also clearly state the action, alarm, lock, jump, backup power and failure status, and finally verify the entire link through on-site functional tests.