Protection of transformer and circuits

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 or discrimination 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 voltages 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 (see Fig. B16a)

The buchholz is installed on the pipe connecting the tank of he transformer to the conservator (see Fig. B16b). It traps the slow emissions of gasses and detect the flow back of oil due to the internal over pressures

Fig. B16a: Breathing transformer protected by buchholz

Fig. B16b: Transformer with conservator

• The DGPT (Detection of gas, pressure and temperature) for the integral filled transformers (see Fig. B17 and Fig. B18). 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.

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.

Fig. B17: Integral filled transformer

Fig. B18: DGPT for protection of Integral filled transformer

Overloads and internal faults in dry type transformers

(see Fig. B19 and Fig. B20)

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 (see Fig. B20).

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 example when 5 % of a HV winding are short circuited the line current of the transformer does not exceed 2 In, for un 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 (VIP range for example) is preferred.

Fig. B19: Dry type transformer

Fig. B20: Thermal relay for protection of dry type transformer

Discrimination between the protective devices upstream and downstream of the transformer

It is a common practice to ensure the discrimination between the MV circuit breaker or fuses installed on the primary side of a transformer and the LV circuit breaker.

The characteristics of the protection ordering the tripping or the MV circuit breaker or the operating curves of the fuses when used must be such as in case downstream fault the LV circuit breaker only trips. The MV circuit breaker must remain closed or the fuse must not blow.

The tripping curves of MV fuses, MV protection and LV circuit breakers are given by graphs giving the operating time as a function of the current.

The curves are in general inverse-time type. LV circuit breakers have an abrupt discontinuity which defines the limit of the instantaneous action. Typical curves are shown in Fig. B21.

Discrimination between LV circuit breaker and MV fuses

(see Fig. B21 and Fig. B22)

• All parts of the MV fuse curve must be above and to the right of the LV CB curve.
• In order to leave the fuses unaffected (i.e. undamaged), the two following conditions must be satisfied:
  • All parts of the minimum pre-arcing fuse curve must be shifted to the right of the LV CB curve by a factor of 1.35 or more.
    Example: where, at time T, the CB curve passes through a point corresponding to 100 A, the fuse curve at the same time T must pass through a point corresponding to 135 A, or more, and so on.
  • All parts of the fuse curve must be above the CB curve by a factor of 2 or more

Example: where, at a current level I the CB curve passes through a point corresponding to 1.5 seconds, the fuse curve at the same current level I must pass through a point corresponding to 3 seconds, or more, etc.

The factors 1.35 and 2 are based on the maximum manufacturing tolerances given for MV fuses and LV circuit breakers.

In order to compare the two curves, the MV currents must be converted to the equivalent LV currents, or vice-versa.

Fig. B21: Discrimination between MV fuse operation and LV circuit breaker tripping, for transformer protection

Fig. B22: MV fuse and LV circuit breaker configuration

Discrimination between LV circuit breaker and MV circuit breaker

• All parts of the minimum MV circuit breaker curve must be shifted to the right of the LV CB curve by a factor of 1.35 or more:
  • Example: where, at time T, the LV CB curve passes through a point corresponding to 100 A, the MV CB curve at the same time T must pass through a point corresponding to 135 A, or more, and so on.

• All parts of the MV CB curve must be above the LV CB curve. The time difference between the two curves must be 0.3 s at least for any value of the current.

The factors 1.35 and 0.3 s are based on the maximum manufacturing tolerances given for MV current transformers, MV protection relay and LV circuit breakers.