Transformer differential protection [87T] is something we look at during our power system studies to safeguard transformers from faults and abnormal conditions. Transformers play a critical role in power transmission and distribution by stepping up or stepping down voltage levels. To ensure their reliable and safe operation, differential protection is employed. 

Low-impedance transformer differential protection is a variation of transformer differential protection that is designed to provide enhanced sensitivity and reliability in detecting internal faults in power transformers.

Transformer differential protection is a popular choice for protecting critical transformers in power systems, especially when high sensitivity is required to detect internal faults at an early stage. It’s commonly used in substations and industrial facilities to safeguard transformers and maintain power system reliability. The benefits of Differential protection are:

  • It can detect even small fault currents and, therefore, provides better protection against incipient (early-stage) faults within the transformer. This sensitivity is particularly useful for detecting winding faults and incipient problems that might develop into more severe issues if left undetected.
  • Once the relay detects a fault or an abnormal condition, it sends a trip signal to the circuit breaker to disconnect the transformer from the system. This rapid tripping helps prevent further damage to the transformer and minimizes system disturbances.

The image below demonstrates a typical transformer differential protection scheme. The purpose of the transformer differential protection is to determine whether a fault is within the protected zone, or outside of the protected zone. The protected zone is limited by the position of current transformers and in principle can include more objects than just a transformer. If the fault is found to be internal, the faulty power transformer must be quickly disconnected from the system.

When designing the setting in the relay for [87T], it is important to carefully select settings that will only trip as intended and not operate for faults outside of the relay zone. Premium Power has extensive experience in this area and would be glad to assist you.