What Are the Protections Used For Transformers

What Are The Protections Used For Transformers

The electrical equipment and circuits in a substation must be protected in order to limit the damages due to abnormal currents and over voltages.

All equipment installed in a power electrical system have standardized ratings for short-time withstand current and short duration power frequency voltage. The role of the protections is to ensure that these withstand limits can never be exceeded, therefore clearing the faults as fast as possible.

In addition to this first requirement a system of protection must be selective. Selectivity means that any fault must be cleared by the device of current interruption (circuit breaker or fuses) being the nearest to the fault, even if the fault is detected by other protections associated with other interruption devices.

As an example for a short circuit occurring on the secondary side of a power transformer, only the circuit breaker installed on the secondary must trip. The circuit breaker installed on the primary side must remain closed. For a transformer protected with MV fuses, the fuses must not blow.

They are typically two main devices able to interrupt fault currents, circuit breakers and fuses :

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

Transformer Protection

Stresses generated by the supply

Two types of over voltages may stress and even destroy a transformer:

  • The lightning over voltages due to lightning stroke falling on or near an overhead line supplying the installation where the transformer is installed
  • The switching over voltages generated by the opening of a circuit breaker or a load break switch for instance.

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

Stresses due to the load

A transformer overload is always due to an increase of the apparent power demand (kVA) of the installation. This increase of the demand can be the consequence of either a progressive adjunction of loads or an extension of the installation itself. The effect of any overload is an increase of the temperature of oil and windings of the transformer with a reduction of its life time.

The protection of a transformer against the overloads is performed by a dedicated protection usually called thermal overload relay. This type of protection simulates the temperature of the transformer’s windings. The simulation is based on the measure of the current and on the thermal time constant of the transformer. Some relays are able to take into account the effect of harmonics of the current due to non-linear loads such as rectifiers, computers, variable speed drives etc. This type of relay is also able to evaluate the remaining time before the emission of the tripping order and the time delay before re-energizing the transformer.

In addition, oil-filled transformers are equipped with thermostats controlling the temperature of the oil.

Dry-type transformers use heat sensors embedded in the hottest part of the windings insulation.

Each of these devices (thermal relay, thermostat, heat sensors) generally provides 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.

Internal faults in oil filled transformers

In oil filled transformers, internal faults may be classified as follow:

  • Faults generating production of gases, mainly:
    • Micro arcs resulting from incipient faults in the winding insulation
    • Slow degradation of insulation materials
    • Inter turns short circuit
  • Faults generating internal over pressures with simultaneously high level of line over currents:
    • Phase to earth short circuit
    • Phase to Phase short circuit.

These faults may be the consequence of external lightning or switching over voltage.

Depending on the type of the transformer, there are two kinds of devices able to detect internal faults affecting an oil filled transformer.

  • The Buchholz dedicated to the transformers equipped with an air breathing conservator .

The buchholz is installed on the pipe connecting the tank of he transformer to the conservator . It traps the slow emissions of gasses and detect the flow back of oil due to the internal over pressures

Breathing transformer protected by buchholz

  • The DGPT (Detection of Gas, Pressure and Temperature, for the integral filled transformers . This type of transformer is manufactured up to around10 MVA. The DGPT as the buchholz detects the emissions of gasses and the internal over pressures. In addition it monitors the temperature of the oil.
 Integral filled transformer

 DGPT (Detection of Gas, Pressure and Temperature) protection relay for integral filled transformers

Concerning the monitoring of gas and temperature the buchholz and the DGPT provide two levels of detection:

  • A low level used to generate an alarm to advise the maintenance staff,
  • A high level to trip the switching device installed on the primary side of the transformer (circuit breaker or load break switch associated with fuses).

In addition, both the buchholz and the DGPT are suitable for oil leakages detection.

Overloads and internal faults in dry type transformers

The dry type transformers are protected against over-heating due to possible downstream overloads by a dedicated relay monitoring thermal sensors embedded in the windings of the transformer .

The internal faults, mainly inter turns and phase to earth short circuits occurring inside a dry type transformers are cleared either by the circuit breaker or the fuses installed on the primary side of the transformer. The tripping of the circuit breakers when used is ordered by the phase to phase and phase to earth over current protections.

Inter turns faults need a dedicated attention:

  • They generally generate moderate line over currents. As an example when 5 % of a HV winding are short circuited the line current of the transformer does not exceed 2 In, for a short circuit affecting 10 % of the winding the line current is limited around 3 In.
  • Fuses are not appropriate to clear properly such currents
  • Dry type transformers are not equipped with additional protection devices such as DGPT dedicated to internal faults detection.

Hence, internal faults generating low level of line over current may not be safely cleared by fuses. Protection by means of over current relay with adequate characteristic and settings is preferred (Schneider Electric VIP relay range for example).

Dry type transformer
Thermal relay for protection of dry type transformer

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