Exploring Power Transformer Protection for Power Systems: Failure Types and Differential Protection

A power transformer is the most significant and expensive part of the electrical system. The basic function of electrical equipment like instrument transformers and circuit breakers is to protect the power transformers. If you take into consideration the true value of the transformer and its high cost compared to the other equipment, it is fair to install high-quality systems for better protection against external failures that can be caused due to network or internal transformer failures.

What are transformer protection systems?

External failures that appear somewhere in a network (due to short circuit, overvoltage, overload, atmospheric discharge, etc.) have the potential to create problems for the transformer. For instance, short circuits in the network can lead to significant heating of the transformer busbars and windings.

If you think failure can only occur outside, then you’d be surprised to know that it can appear inside as well. For instance, windings short circuits, short circuits between phases, inter-turns, or faults in the core, they all take place inside and can do a lot of damage. When it comes to failure location, you can divide the transformer protection into two parts: internal and external.

The main duty of the system is to separate the transformer from the energy supply as soon as possible, and it helps prevent unintended consequences and major transformer damages.

The system is designed, keeping in mind, to signal if irregularities have taken place in the electrical system, which could, unfortunately, lead to transformer failure. After the pre-set relay blocking time, what the system does then sends a signal to the circuit breaker when it removes the transformer from the system before it is affected by the failure. 

Now let us talk about the transformer protection systems completely based on operation criteria

In the table below, you can see different transformer protection systems according to operation criteria.

Table 1

Operation Criteria Protection System Failure Location (Internal/External)
In current differences criteria Differential protection Internal/external protection
In the high current criteria Overcurrent protection External protection
In the gas evaluation criteria Buchholz relay Internal protection
In the high-temperature criteria Thermal overload protection Internal protection
In the zero-sequence current criteria Ground fault protection External protection
In the line impedance criteria Distance protection External protection

Then comes the transformer protection types by failure conditions

If you have different protection systems, it will help you detect different faulty conditions in the transformer. In table number 2, it shows which failures can be detected with its corresponding protection system.

Table 2

Transformer Faulty Conditions Protection System
If you have the overloading or overheating Thermal overload protection
If you have an external short circuit in the network Overcurrent and distance protection
If the transformer has an internal short circuit Differential Relays, overcurrent and Buchholz relay
If the transformer has an internal single phase short circuit or ground-fault Single phase overcurrent, ground fault and tank ground-fault protection

Let us talk about Differential Protection (ΔI)

Differential protection is a reliable and safe protection method and a vital and commonly used transformer protection. You can use it for protecting the transformer that has nominal power above 8 MVA. For power up to 4 MVA, this is usually not preferable.

With the help of this system, you can cover short circuits that take place inside the transformer like short circuits between phases, inter-turns, and between phase and ground. In case the transformer neutral is directly grounded, then the protection will also cover insulation breakthrough through all windings. If it is isolated, then it will cover only faults between two phases and not just a single phase failure.


This article intends to give you a basic understanding of power transformers and its functions.

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