Protection of TRANSFORMERS

Protection  of transformers

Transformers are static devices, totally enclosed and generally oil-immersed.  Therefore, chances of faults occurring on them are very rare.  However, the consequences of even a rare fault may be very serious unless the transformer is quickly disconnected from the system.  This necessitates to provide adequate automatic protection for transformers against possible faults. 

Small distribution transformers are usually connected to the supply system through series fuses instead of circuit breakers.  Consequently, no automatic protective relay equipment is required.  However, the probability of faults on power transformers is undoubtedly more and hence automatic protection is absolutely necessary. 

Common transformer faults.  

As compared with generators, in which many abnormal conditions may arise, power transformers may suffer only from : 

(i) open circuits 

(ii) overheating 

(iii) winding short-circuits e.g. earth-faults, phase-to-phase faults and inter-turn faults. 

An open circuit in one phase of a 3-phase transformer may cause undesirable heating.  In practice, relay protection is not provided against open circuits because this condition is relatively harmless.  On the occurrence of such a fault, the transformer can be disconnected manually from the system.

Overheating of the transformer is usually caused by sustained overloads or short-circuits and very occasionally by the failure of the cooling system.  The relay protection is also not provided against this contingency and thermal accessories are generally used to sound an alarm or control the banks of fans. 

Winding short-circuits (also called internal faults) on the transformer arise from deterioration of winding insulation due to overheating or mechanical injury.  When an internal fault occurs, the transformer must be disconnected quickly from the system because a prolonged arc in the transformer may cause oil fire.  Therefore, relay protection is absolutely necessary for internal faults.

Protection  Systems  for  Transformers

For protection of generators, Merz-Price circulating-current system is unquestionably the most satisfactory.  Though this is largely true of transformer protection, there are cases where circulating current system offers no particular advantage over other systems or impracticable on account of the troublesome conditions imposed by the wide variety of voltages, currents and earthing conditions invariably associated with power transformers.  Under such circumstances, alternative protective systems are used which in many cases are as effective as the circulating-current system.  

The principal relays and systems used for transformer protection are : 

(i) Buchholz devices providing protection against all kinds of incipient faults i.e. slow-developing faults such as insulation failure of windings, core heating, fall of oil level due to leaky joints etc. 

(ii) Earth-fault relays providing protection against earth-faults only. 

(iii) Overcurrent relays providing protection mainly against phase-to-phase faults and overloading. 

(iv) Differential system (or circulating-current system) providing protection against both earth and phase faults. 

The complete protection of transformer usually requires the combination of these systems.  Choice of a particular combination of systems may depend upon several factors such as 

(a) size of the transformer 

(b) type of cooling 

(c) location of transformer in the network 

(d) nature of load supplied and 

(e) importance of service for which transformer is required.  In the following sections, above systems of protection will be discussed in detail.