23-02-2013, 12:11 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 Protection
There are no strict guidelines as to what protection devices should be used for a particular
transformer. However, typically Transformers below 5000 KVA (Category I & II) are protected using
Fuses. Transformers above 10,000KVA (Category III & IV) have more sensitive internal fault
detection by using a combination of protective devices as shown in Figure 1. For ratings between the
above a protection scheme is designed considering the service criticality, availability of standby
transformers, potential of hazardous damage to adjacent equipment and people etc.
Transformer Protection
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Section 2
Electrical Protection
The electrical protection of the Transformer comprises of the following and each is
elaborated further.
• Fused Protection
• Differential Current Protection
• Over Current Protection
• Over Excitation Protection
• Over Voltage Protection
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 Through Fault Withstand Standards
The philosophy of transformer over current protection is to limit the fault current below the
transformer through fault with stand capability. The fault withstand capability in turn is based on the
possibility of mechanical of the windings due to the fault current, rather than on thermal
characteristics of the transformer.
Transformer Protection
a) It’s classified as Frequent Faults if the number of faults over the transformer life time is more than the
number shown. Else it’s classified as infrequent faults.
For category II & III the frequent fault curve may be used for backup protection in case it’s exposed to
frequent faults, but is protected by high speed primary relays
See Figure 3 – Guide to determine fault frequency
b) I, symmetrical short circuit current in per unit of normal base current based on minimum nameplate
KVA rating; t, time in seconds; f, frequency in HZ.
For Category I frequent &
Category II & III infrequent
For Category II frequent
Transformer Protection
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For Category III Frequent Figure 3 Guide to Fault Frequency
& Category IV infrequent
The procedure to decide on, over current protection rating, as per the transformer fault
withstand ratings is as follows:
1. Determine the transformer category from the above table
2. If in category II or III determine if it will be subject to faults frequently or
infrequently. Use figure 3 – Guide to fault frequency
3. Based up on the above determine the curve applicable
4. Replot the curve determined in step 3 specifically for the transformer under
consideration using the secondary or the primary amperes as the abscissa,
secondary amperes is preferred for coordination with down stream protective
devices.
5. Select the proper fuses or relays – tap, time dial setting etc such that coordination is
maintained and within the within the transformer withstand curve determined
above.
The determination of the transformer fault withstand curve using above procedure is
explained using an example below: