IEEE C37.91-1985 pdf free.IEEE Guide for Protective Relay Applications to Power Transformers.
IEEE C37.91 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 begreat and since high speed, highly-sensitive protective devices can reduce damage and therefore repair cost, theequipment protective aspects of relays shall be considered when protecting transformers, particularly in the largersizes.
Faults internal to the transformer quite often involve a magnitude of fault current which is low relative to thetransformer rating.This indicates a need for high sensitivity, and high speed to obtain good protection.
Transformers with modern surge protection are generally reliable.However, if a possibility of failure exists, protectionshal be provided. There is not 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 ofsensitivity, speed, and selectivity. The plan selected shall balance the best combination of these factors against theoverall economics of the situation while holding to a minimum:
1)Cost of repairing damage
2)Cost of lost production
3)Adverse effects on the balance of the system
4)The spread of damage to adjacent equipment
5)The period of unavailability of the damaged equipment
In protecting transformers,backup protection needs to be considered.The failure of a relay or breaker during atransformer fault may cause such extensive damage to the transformer that its repair would not be practical. When thefault spreads due to nonclearing by the transformer protection, remote line relays or other protective relaying mayoperate. Part of the evaluation of the type of protection applied to a transformer should include how the systemintegrity may be affected by such a failure. In this determination, since rare but costly failures are involved, a diversityof opinion on the degree of protection required by transformers might be expected among those familiar with powersystem relay engineering.The major economic consideration is not ordinarily the fault detection equipment but theisolation devices. Circuit breakers often cannot be justified on the basis of transformer protection alone. At least asmuch weight should be given to the servicE, requirements, the operating philosophy, and system design philosophy asto the protection of the transformer. Evaluations of the risks involved and the cost effectiveness of the protection arenecessary to avoid going to extremes. Such considerations involve the art rather than the science of protective relaying.See [7],2[12],[16], and [19].IEEE C37.91 pdf download.