POWER TRANSFORMER RELAYING PHILOSOPHY AND ECONOMIC CONSIDERATIONS


Protective relaying is applied to components of a power system for the following reasons:

a)Separate the faulted equipment from the remainder of the system so that the system can continue to function
b)Limit damage to the faulted equipment
c)Minimize the possibility of fire
d)Minimize hazards to personnel
e)Minimize the risk of damage to adjacent high voltage apparatus

In protecting some components, particularly high-voltage transmission lines, the limiting of damage becomes a by-product of the system protection function of the relay. However, since the cost of repairing faulty transformers may be great and since high-speed, highly sensitive protective devices can reduce damage and therefore repair cost, relays should be considered for protecting transformers also, particularly in the larger sizes.

Faults internal to a transformer quite often involve a magnitude of fault current that is low relative to the transformer base rating. This indicates a need for high sensitivity and high speed to ensure good protection. There is no one standard way to protect all transformers, or even identical transformers that are applied differently.

Most installations require individual engineering analysis to determine the best and most cost-effective scheme. Usually more than one scheme is technically feasible, and the alternatives offer varying degrees of sensitivity, speed, and selectivity.

The plan selected should balance the best combination of these factors against the overall economics of the situation while holding to a minimum

a)Cost of repairing damage
b)Cost of lost production
c)Adverse effects on the balance of the system
d)The spread of damage to adjacent equipment
e)The period of vulnerability of the damaged equipment

In protecting transformers, backup protection needs to be considered. The failure of a relay or breaker during a transformer fault may cause such extensive damage to the transformer that its repair would not be practical.

When the fault is not cleared by the transformer protection, remote line relays or other protective relays may operate. Part of the evaluation of the type of protection applied to a transformer should include how the system integrity may be affected by such a failure.

In this determination, since rare but costly failures are involved, a diversity of opinion on the degree of protection required by transformers might be expected among those familiar with power system relay engineering.

The major economic consideration is not ordinarily the fault detection equipment but the isolation devices. Circuit breakers often cannot be justified on the basis of transformer protection alone.

At least as much weight should be given to the service requirements, the operating philosophy, and system design philosophy as to the protection of the transformer. Evaluations of the risks involved and the cost-effectiveness of the protection are necessary to avoid going to extremes. Such considerations involve the art rather than the science of protective relaying. 

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