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INTRODUCTION
Protective relaying is an integral part of any electrical power system. The fundamental objective of system protection is to quickly isolate a problem so that the unaffected portions of the system can continue to function. The flip side of this objective is that the protection system should not interrupt power for acceptable operating conditions, including tolerable transients.
• The choice of protection depends upon several aspects such as type, rating of the protected equipment, its important location, probable abnormal conditions, costs etc.
• A fault in electrical equipment is defined as a defect in its electrical circuit due to which the flow of current is diverted the intended.
• Faults can be minimized by improving system design, improving quality of component, better and adequate protective relaying, better operation and maintenance; however the fault can’t be entirely eliminated.
• The protective relay senses the abnormal condition in a part of power system and given an alarm or isolate that part from the healthy system.
• When abnormal conditions occur three basic objectives must always be met:
• All endangered equipment must be protected from damage
• The faulted components must be isolated and if not damaged, reenergized as rapidly as possible.
POWER TRANSFERORMER PROTECTION
A power transformer constitutes an important and expensive component in a power system. It is, therefore essential to provide an efficient protective relay scheme to protect the transformer from any severe damage which might likely to be caused by short-circuited faults with in the equipment itself or any sustained overload or fault conditions in the power systems.
Protective relaying is necessary for every power transformer. The choice of protection depends upon several aspects such as type, rating of transformer, its location, its importance, probable abnormal conditions, cost etc. There are several transformers of various ratings.Each needs certain adequate protection.
The protective relaying senses the abnormal conditions give an alarm or isolate that part from the healthy system. The relaying are compact, self contained devices which respond to abnormal condition. The relay distinguishes the normal and abnormal conditions. When an abnormal condition occurs relay closes its contacts there by trip circuit breaker opens and faulty part is disconnected from the supply. The entire process is automatic and fast.
Circuit breakers are switching devices which can interrupt normal and abnormal currents. Besides relays and circuit breaker there are several other important components in the protective relaying scheme. These include protective current transformer, voltage transformers, protective relays, time delay relays, Auxiliary relays, trip circuits, secondary circuits, auxiliary and accessories etc.
BUCHHOLTZ RELAY:
The incipient faults in the transformer tank below oil level actuate buchholtz relay to given an alarm. The arc produced due to fault causes decomposition of transformer oil. The product of transformer oil decomposition contains more than 70% of hydrogen gas. This hydrogen gas being light rises upwards and tries to go into the conservator. The buchholtz relay is fitted in the pipe leading to the conservator. The gas gets collected in the upper portion of the buchholtz relay.
There by the oil level in the buchholtz relay drops down. The float, floating in the oil tilts down with lowering oil level while doing so the mercury switch attached to the float is closed and the mercury switch closes the alarm circuit. There by the operator know that there is some incipient fault in the transformer. The transformer is disconnected as early as possible and the gas sample is tested. The testing of gas gives clue regarding the type of insulation failure. Buchholtz relay gives an alarm so that the transformer can be disconnected before the incipient fault grows into the serious one.
When a serious short circuit occurs in the transformer, the pressure in the tank increases. The oil rushes towards the conservator and it passes through Buchholtz relay. The plates in the buchholtz relay get presses by rushing oil. There by they close another switch which inturn closes the trip circuit of circuit breaker. There after the transformer is removed.
The decomposition of transformer oil starts at about 3500C. The gas accumulated in the upper portion of the relay can be trapped. The gas is tested for color, combustibility, chemical test etc.