What is Power transformer protection?

Credit to Author: Kavita Manral| Date: Tue, 04 Aug 2020 16:08:51 +0000

When you think of a plant/industries, one of the things that you want to protect are your electrical equipment and circuits. In order to minimize the damages due to abnormal current and overvoltages, electrical equipment and circuits should be protected in a substation. The equipment that you install in a power electrical system has standardized ratings for short-time withstand current and short duration power frequency voltage.

What is the role of power transformer protection?

 Well to answer this question- you need protection to ensure that the set withstands limits are never exceeded, hence clearing the faults as soon as possible. Furthermore, the requirement of a system must be selective. Now, what does selectivity mean? It means that any fault must be cleared by the device of current interruption being the nearest to the fault, and it doesn’t matter even if the fault is detected by other protections that are associated with other interruption devices.

Let us explain this with an example, for a short circuit to occur on the secondary side of a power transformer, then the circuit breaker that has been installed on the secondary must trip. And, the circuit breaker that was installed on the primary side must remain closed. Thus, for a transformer protected with MV fuses, the fuses must not blow.

There are usually two primary devices that can easily interrupt fault current, fuses, and circuit breakers :

  • You need to make sure that circuit breakers must be associated with a protection relay having three vital functions:
    • Detection of the fault
    • Measurement of the currents
    • Emission of a tripping order to the breaker
  • The fuses blow under certain fault conditions.

Now let us have a look at the transformer protection systems based on their operation criteria

Below you will see different types of transformer protection systems based on their operation

Table 1

Operation CriteriaProtection SystemFailure Location (Internal/External)
When talking about current differences criteriaDifferential protectionIt is Internal/external protection
When talking about high current criteriaOvercurrent protectionThen External protection
When talking about gas evaluation criteriaBuchholz relayThen Internal protection
When talking about high-temperature criteriaThermal overload protectionThen Internal protection
When talking about zero-sequence current criteriaGround fault protectionThen External protection
When talking about  impedance criteriaDistance protectionThen External protection

Let us move on to transformer protection types by failure conditions

If there are several protection systems, then this will help you detect different faulty conditions in the transformer.

Table number 2 talks about which failures can be detected with its corresponding protection system.

 Table 2

Transformer Faulty ConditionsProtection System
Going through overloading or overheatingOpt forThermal overload protection
Going through an external short circuit in the networkOpt for Overcurrent and distance protection
Going through transformer internal short circuitOpt for Differential Relays, overcurrent and Buchholz relay
Going through transformer internal single phase short circuit or ground-faultOpt for Single phase overcurrent, ground fault and tank ground-fault protection


We are aware of the fact that transformers are slightly expensive but they play an important role in power systems and protection schemes should be present in order to detect and solve any unprecedented condition in a jiffy. And, for that we have Schneider Electric India, we  implement the best techniques like multiple winding differential, REF and thermal algorithms, and collect service data to aid the scheduling of preventive maintenance.

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