What is a Transformer ? Types and Transformer protection

What is a Transformer device?

The transformer is a device that transfers electrical energy from one circuit to another through the magnetic field without change of frequency.

Types of Transformer

Based on service

  1. Power Tr
  2. Distribution Tr.

What is the use of transformer?

In this age of modern transmission system with optimal loss it is necessary to transmit the power at very high level.it is not economical to produce electrical power at such a high voltage.Here comes the vital role of transformer, which enhances the voltage level from generating voltage to such a high level where the loss is minimal.

It is also used as booster in distribution system to give better voltage regulation.

General Fault In Transformer

General Fault in transformer are

  1. Open circuit Fault
  2. Over heating
  3. Winding short circuit fault
  4. faults due to lightening

Protection For Transformer

  1. Buchholz Relay
  2. Earth Fault Relay
  3. Over current relay
  4. Differential relay
  5. Digital relay

Earth Fault Protection

An earth fault usually involves a partial breakdown of winding insulation to earth Under this circumstance it is profitable to employ an earth fault relay One method of protection is core balance protection

Differential Protection

Merz price circulating current principle or differential protection is commonly employed for power transformer protection from internal fault.

Points to be noted

  1. The different of current in primary and secondary must be equalized in diff relay by using appropriate turns on CT’s connected with both primary and secondary

  2. The phase difference of current still generates a relay current. To neutralize it is case of healthy condition it is required to connect the CT’s in phase opposition with respect to adjoint.

Protection Against Lightening

It is necessary to provide protection of transformer against transient & surges.The most commonly used devices for protection against lightening surges are

a). Earthing screen

b) Overhead Ground wires

c) Lightning arrestor

Transformer faults & protection type

Fault type
Protection used
Primary winding Phase-Phase fault Differential; Overcurrent
Primary winding Phase-Earth fault Differential; Overcurrent
Secondary winding Phase-Phase fault Differential
Secondary winding Phase-Earth fault Differential; Restricted Earth Fault (REF)
Interturn Fault Differential; Buchholz
Core Fault Differential; Buchholz
Oil tank Fault Differential; Buchholz; Tank Earth
Overfluxing Overfluxing
Overheating Thermal

Thank you for your explanation from above. I always like to see there are a guys out there trying to help explaining about the mandatorily need of protection devices. I´d like to add some information regarding protection devices. Generally there needs to be distinguished between protection devices for hermetically sealed transformers and devices for transformers with expansion tank (get in direct touch with its ambient air).

Devices for Transformers with expansion tank

  1. Buchholz relay MBP. This great device has experienced many improvements the last several years. Now its a reliable and durable device which detects oil level and gas accumulation as well internal pressure.
  2. Magnetic oil level indicator MOLC which is placed on the front side of the expansion tank. It meaures oil level and can be equipped with min. and max. switches.
  3. Pressure safety valve MPRV which works with a pre-adjusted pressure value. If this value gets exceeded, the valve opens up and releases internal pressure. After decreasing pressure, the valve closes. It can be equipped with either one or two switches.

Protection devices for hermetically sealed distribution transformers There are also protection devices for hermetically sealed transformers, you can find a short list below.

1.transformer protection device MCHD which detects oil level and temperature, gas accumulation and internal pressure. It´s the mostly used device for hermetically sealed distribution transformers as it is a all-in-one device. 2. Bimetallic thermometer MBT which detects oil temperature. It can be equipped with min. and max. limit switches. 3. Protection device MHPR which detects oil level. It is equipped with a switch to alarm. 4. Pressure safety valve MPRV as mentioned above.

I´m now working in this market for a long time and I hope to have shown you a short overview about the protection devices. As said, depending on the transformer properties, there are different devices needed. If somebody has a question about this products, you can gladly get in touch with me.:grinning:

A few pictures of the devices

Protection device MCHD

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

What is the role of power transformer protection?

You need protection to ensure that the set withstand limits are never exceeded, hence the need for 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 protection 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 , 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 Criteria Protection System Failure Location (Internal/External)
When talking about current differences criteria Differential protection It is Internal/external protection
When talking about high current criteria Overcurrent protection Then External protection
When talking about gas evaluation criteria Buchholz relay Then Internal protection
When talking about high-temperature criteria Thermal overload protection Then Internal protection
When talking about zero-sequence current criteria Ground fault protection Then External protection
When talking about impedance criteria Distance protection Then 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 Conditions Protection System
Going through overloading or overheating Opt forThermal overload protection
Going through an external short circuit in the network Opt for Overcurrent and distance protection
Going through transformer internal short circuit Opt for Differential Relays, overcurrent and Buchholz relay
Going through transformer internal single phase short circuit or ground-fault Opt 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.