Transformer Protection Scheme

AC system cannot be imagined without Transformer. Starting from power generating station to distribution system, everywhere transformer is used at different levels to step up or step down voltage. Not only this, it is an important and costly element of power system. The cost of a power transformer is in the range of millions of dollar. Also, transformer outage results in plant shutdown and hence loss of production revenue. Therefore the need for transformer protection cannot be ignored.

Transformer protection means protecting the transformer from internal as well as external faults. The protection scheme must automatically isolate it in case of faults within stipulated time. Since small distribution transformer is connected to the supply system through series fuse wire instead of breaker, no automatic protection scheme is required. Thus all the protection discussed in this post is applicable for power transformer unless explicitly specified.

Different Kinds of Transformer Faults

A power transformer may suffer from the following kind of faults:

• Open circuit
• Overheating
• Winding short circuit or inter-turn faults
• Phase to earth Faults
• Phase to phase Faults

An open circuit in transformer results in undesirable overheating. Though for power transformer, this is quite rare and pole discrepancy protection of breaker takes care of this.

Overheating of transformer may be due to prolonged overloading, short circuit or by the failure of cooling system. Overheating will obviously cause heating of winding and oil. Thus the temperature of oil and winding will increase. Oil Temperature Indicator (OTI) and Winding Temperature Indicator (WTI) are mounted on the transformer tank to monitor the oil and winding temperature respectively.

Winding short circuit is also known as internal faults. An internal fault in transformer is basically due to deterioration of winding insulation. The winding insulation may degrade due to variety of reasons. One of them is increase in moisture content in transformer oil. Normally the moisture content in power transformer must not exceed 10 ppm. In case moisture content is more than 10 ppm, hot oil circulation (HOC) is required to be carried out.

Different transformer protection scheme is adopted by using different types of transformer protection relays to protect transformer from these kinds of faults.

Transformer Protection Scheme

There is mainly two transformer protection: physical protection and electrical protection.

The protection elements which sense the physical parameters viz. oil temperature, winding temperature, oil pressure etc. are categorized under physical protection. These protection elements operate if the value of any of the physical parameter exceeds the set value. For example, if oil temperature exceeds 80 degree celcius, OTI annunciates and if it exceeds 85 degree celcius, OTI actuates to isolate the transformer by opening its HV and LV breaker.

Various types of transformer protection relays under this category are listed below:

• Buchholz Relay
• Oil Temperature Indicator (OTI)
• Winding Temperature Indicator (WTI)
• Pressure Relief Device (PRD)
• Oil Surge Relay (OSR)

Electrical transformer protection element senses any abnormal condition in the transformer by sensing the electrical parameters like voltage, current or combination of voltage & current. These protections are provided to protect transformer winding or core from electrical faults. For this purpose different types of transformer protection relays are used which work on different principle.

There are basically two types of electrical transformer protection: Primary Protection and Back-up Protection. Primary protection includes:

• Differential Protection
• Overflux Protection

Back-up protection comprises of following protection:

• Directional Over-current Protection
• Restricted Earth Fault (REF) Protection 

In addition to the above physical and electrical transformer protection, High Velocity Fire Water Spray System (HVWS) is also implemented to protect transformer from fire. In fact without having HVWS system, the competent authority i.e. Central Electricity Authority (CEA) for India, is not going to give approval for transformer charging.

In case of fire, HVWS system actuates to spray water on the transformer and sends tripping command to HV and LV breaker to isolate it. You might surprise that how water spray will extinguish the fire in transformer as it contains oils. Actually, water spray makes a curtain of water vapor around the transformer and hence blocks the supply of oxygen. Thus due to lack of oxygen, the fire gets extinguished.