05-09-2012, 01:43 PM
Transformer Protection
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Introduction
The primary objective of the Transformer Protection is to detect internal faults in the
transformer with a high degree of sensitivity and cause subsequent de-energisation and, at the same
time be immune to faults external to the transformer i.e. through faults. Sensitive detection and deenergisation
enables the fault damage and hence necessary repairs to be limited. However, it should
be able to provide back up protection in case of through faults on the system, as these could lead to
deterioration and accelerated aging, and/or failure of the transformer winding insulation due to over
heating and high impact forces caused in the windings due to high fault currents. In addition to the
internal faults, abnormal system conditions such as over excitation, over voltage and loss of cooling
can lead to deterioration and accelerated aging or internal failure of the transformer. Hence protection
again these failures should be considered in as part of the comprehensive transformer protection
scheme.
Transformer protection can be broadly categorized as electrical protection implemented by
sensing mainly the current through it, but also voltage and frequency and, as mechanical protection
implemented by sensing operational parameters like oil pressure/ level, gas evolved, oil & winding
temperature.
Like in most things in Transformer Protection too, the extent of protective devices applied to
a particular Transformer is dictated by the economics of the protection scheme vis-à-vis the
probability of a particular type of failure and the cost of replacing and repairing the transformer as
well the possibility of the failure leading to damage of adjacent equipment or infrastructure. Failure
costs include all the direct and indirect costs associated with it. The protection scheme cost includes
the cost of the protective device but is mainly the cost of the disconnecting device i.e. the Circuit
Breaker and other auxiliaries like batteries and necessary infrastructure. Further the life cycle cost is
taken into account.
Transformer Over Current Protection
Over current protection is commonly used for protection from phase and ground faults. It’s
used as primary protection where differential protection is not used – typically for category I & II
transformers and as backup protection if differential protection has been used – typically for category
III & IV transformers. The protection zone of over current devices is normally more than the
transformer. Hence they are part of the system protection and need to be coordinated with the other
system protection devices.
Typically, fuses are used as primary protection for transformers below 10MVA. Above
10MVA over current relays are used as back up along with differential relays as primary protection
for transformers. Instantaneous over current relays are also used for back up where differential relays
have been used. Typically they are set to 150% to 200% of the maximum of
1. Magnetising current inrush (If harmonic restraint is not used)
2. Short time load – Cold Pickup
3. Maximum 3 phase short circuit current
Transformer Differential Relay
To account for the above variables less sensitive Percentage Differential Relays with
percentage characteristics in the range of 15 to 60% are applied to transformers. Additionally, in
modern microprocessor and numeric relays harmonic restraints can be applied.
The second harmonic is the dominant harmonic in the magnetic inrush current. Hence a
second harmonic restraint is utilised to prevent the relay from operating during the inrush.
The excitation current contains high magnitudes of the odd harmonic, typically 25% of the
third component and 11% of the fifth component. The fifth component is utilised to sense over
excitation. If an over excitation relay has been applied, the fifth harmonic signal is used to block the
differential trip signal so as to have easy fault discrimination during trip analysis. Otherwise, it is
used to restraint the relay operation.
In addition to the fixed the percentage differential relays, variable percentage relays are also
used. In this case, the percentage restraint increases as the transformer through current increases. This
limits the adverse effect of CT saturation if any.