Protection of transformer and circuits

General

The electrical equipment and circuits in a substation must be protected in order to avoid or to control damage due to abnormal currents and/or voltages. All equipment normally used in power system installations have standardized short-time withstand ratings for overcurrent and overvoltage. The role of protective scheme is to ensure that this withstand limits can never be exceeded. In general, this means that fault conditions must be cleared as fast as possible without missing to ensure coordination between protective devices upstream and downstream the equipement to be protected. This means, when there is a fault in a network, generally several protective devices see the fault at the same time but only one must act.
These devices may be:

• Fuses which clear the faulty circuit directly or together with a mechanical tripping attachment, which opens an associated three-phase load-break switch
• Relays which act indirectly on the circuit-breaker coil

Transformer protection

Stresses due to the supply network
Some voltage surges can occur on the network such as :

• Atmospheric voltage surges

Atmospheric voltage surges are caused by a stroke of lightning falling on or near an overhead line.

• Operating voltage surges

A sudden change in the established operating conditions in an electrical network causes transient phenomena to occur. This is generally a high frequency or damped oscillation voltage surge wave.
For both voltage surges, the overvoltage protection device generally used is a varistor (Zinc Oxide).
In most cases, voltage surges protection has no action on switchgear.

Stresses due to the load
Overloading is frequently due to the coincidental demand of a number of small loads, or to an increase in the apparent power (kVA) demand of the installation, due to expansion in a factory, with consequent building extensions, and so on. Load increases raise the temperature of the wirings and of the insulation material. As a result, temperature increases involve a reduction of the equipment working life. Overload protection devices can be located on primary or secondary side of the transformer. The protection against overloading of a transformer is now provided by a digital relay which acts to trip the circuit-breaker on the secondary side of the transformer. Such relay, generally called thermal overload relay, artificially simulates the temperature, taking into account the time constant of the transformer. Some of them are able to take into account the effect of harmonic currents due to non linear loads (rectifiers, computer equipment, variable speed drives…).This type of relay is also able to predict the time before overload tripping and the waiting time after tripping. So, this information is very helpful to control load shedding operation.
In addition, larger oil-immersed transformers frequently have thermostats with two settings, one for alarm purposes and the other for tripping.
Dry-type transformers use heat sensors embedded in the hottest part of the windings insulation for alarm and tripping.

Internal faults
The protection of transformers by transformer-mounted devices, against the effects of internal faults, is provided on transformers which are fitted with airbreathing conservator tanks by the classical Buchholz mechanical relay (see Fig. B4). These relays can detect a slow accumulation of gases which results from the arcing of incipient faults in the winding insulation or from the ingress of air due to an oil leak. This first level of detection generally gives an alarm, but if the condition deteriorates further, a second level of detection will trip the upstream circuit-breaker.

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Fig. B4: Transformer with conservator tank

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An oil-surge detection feature of the Buchholz relay will trip the upstream circuit-breaker “instantaneously” if a surge of oil occurs in the pipe connecting the main tank with the conservator tank.
Such a surge can only occur due to the displacement of oil caused by a rapidly formed bubble of gas, generated by an arc of short-circuit current in the oil.
By specially designing the cooling-oil radiator elements to perform a concerting action, “totally filled” types of transformer as large as 10 MVA are now currently available.
Expansion of the oil is accommodated without an excessive rise in pressure by the “bellows” effect of the radiator elements. A full description of these transformers is given in Sub-clause "Choice of MV/LV transformer" (see Fig. B5).

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Fig. B5: Totally filled transformer

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Evidently the Buchholz devices mentioned above cannot be applied to this design; a modern counterpart has been developed however, which measures:

• The accumulation of gas
• Overpressure
• Overtemperature

The first two conditions trip the upstream circuit-breaker, and the third condition trips the downstream circuit-breaker of the transformer.

Internal phase-to-phase short-circuit
Internal phase-to-phase short-circuit must be detected and cleared by:

• 3 fuses on the primary side of the tranformer or
• An overcurrent relay that trips a circuit-breaker upstream of the transformer

Internal phase-to-earth short-circuit
This is the most common type of internal fault. It must be detected by an earth fault relay. Earth fault current can be calculated with the sum of the 3 primary phase currents (if 3 current transformers are used) or by a specific core current transformer.
If a great sensitivity is needed, specific core current transformer will be prefered. In such a case, a two current transformers set is sufficient (see Fig. B6).

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Fig. B6: Protection against earth fault on the MV winding

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Protection of circuits

The protection of the circuits downstream of the transformer must comply with the IEC 60364 requirements.

Discrimination between the protective devices upstream and downstream of the transformer

The consumer-type substation with LV metering requires discriminative operation between the MV fuses or MV circuit-breaker and the LV circuit-breaker or fuses. The rating of the MV fuses will be chosen according to the characteristics of the transformer.
The tripping characteristics of the LV circuit-breaker must be such that, for an overload or short-circuit condition downstream of its location, the breaker will trip sufficiently quickly to ensure that the MV fuses or the MV circuit-breaker will not be adversely affected by the passage of overcurrent through them.
The tripping performance curves for MV fuses or MV circuit-breaker and LV circuit-breakers are given by graphs of time-to-operate against current passing through them. Both curves have the general inverse-time/current form (with an abrupt discontinuity in the CB curve at the current value above which “instantaneous” tripping occurs).
These curves are shown typically in Figure B7.

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Fig. B7: Discrimination between MV fuse operation and LV circuit-breaker tripping, for transformer protection

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• In order to achieve discrimination (see Fig. B8):

All parts of the fuse or MV circuit-breaker curve must be above and to the right of the CB curve.

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Fig. B8: MV fuse and LV circuit-breaker configuration

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• In order to leave the fuses unaffected (i.e. undamaged):

All parts of the minimum pre-arcing fuse curve must be located to the right of the CB curve by a factor of 1.35 or more (e.g. 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...) and, all parts of the fuse curve must be above the CB curve by a factor of 2 or more (e.g. 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 standard maximum manufacturing tolerances 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.
Where a LV fuse-switch is used, similar separation of the characteristic curves of the MV and LV fuses must be respected.

• In order to leave the MV circuit-breaker protection untripped:

All parts of the minimum MV CB curve must be located to the right of the LV CB curve by a factor of 1.35 or more (e.g. 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...) and, all parts of the MV CB curve must be above the LV CB curve (time of LV CB curve must be less or equal than MV CB curves minus 0.3 s)
The factors 1.35 and 0.3 s are based on standard maximum manufacturing tolerances for MV current transformers, MV protection relay 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.

Choice of protective device on the primary side of the transformer

As explained before, for low reference current, the protection may be by fuses or by circuit-breaker.
When the reference current is high, the protection will be achieved by circuit-breaker.
Protection by circuit-breaker provides a more sensitive transformer protection compared with fuses. The implementation of additional protections (earth fault protection, thermal overload protection) is easier with circuit-breakers.