Protection of Power Transformers

The electrical equipment and circuits in a substation have to be safeguarded in order to negate the losses incurred as a result of abnormal currents and overvoltages.

In most cases, all the equipment installed in a power electrical system possess standardized ratings for short-time current along with short duration power frequency voltage. The function of the protections is to guarantee that these withstand limits are not exceeded, therefore dealing with the faults as fast as possible.

In addition to this first requirement, a system of protection must be selective. This means that any fault must be dealt with by the device of current interruption (circuit breaker or fuses) being the closest to the fault, even if the fault is identified by other protections associated with other interruption devices. As an example of a short circuit transpiring on the secondary side of a power transformer, only the circuit breaker installed on the secondary must trip. The circuit breaker connected on the primary side should be closed in this situation. For a transformer protected with MV fuses, the fuses must not blow in any situation.

Transformer Protection

Below are the key points for transformer protection

Supply Causes 

Two types of overvoltage stresses that can destroy a transformer:

  • The lightning overvoltages caused by lightning stroke falling on or near an overhead line supplying the system where the transformer is installed
  • The switching overvoltages caused by the opening of a circuit breaker or a load break switch for instance.

Depending on the application, protection against these two types of voltage surges may be necessary and are often ensured by means of surge arresters, that is preferably connected to the MV bushing of the transformer.

Stresses Due To The Load 

A transformer overload takes place as a result of an increase in the apparent power demand (kVA) of the installation. The increase in this demand can be the result of either a progressive adjunction of loads or an extension of the installation itself. The impact of any overload is an increase in the temperature of oil and windings of the transformer which reduces its lifetime.

The protection of a transformer against the overloads is carried by a specific protection usually called thermal overload relay. This type of protection resembles the temperature of the transformer’s windings. The simulation is formed on the measure of the current and on the thermal time constant of the transformer. Some relays are able to account for the effect of harmonics of the current caused by non-linear loads such as rectifiers, computers, variable speed drives etc. This type of relay is also able to estimate the remaining time before the emission of the tripping order and the time delay before re-energizing the transformer.

In addition to this, oil-filled transformers are implemented with thermostats controlling the temperature of the oil. Dry-type transformers make use of heat sensors installed in the hottest part of the windings insulation.

In most cases, these devices (thermal relay, thermostat, heat sensors) generally offer two levels of detection:

  • A low level used to generate an alarm to advise the maintenance staff,
  • A high level to de-energize the transformer.

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