Power Transformers

edited July 2020 in Power System

Güç trafolarında ki koruma röleleri nelerdir ve bu röleler nasıl çalışır?

"What are the protection relays in power transformers and how do these relays work?"

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    What is a Protection Relay?

    When a fault occurs in a transformer, the resulting damage is usually proportional to the fault occurring time period. Therefore, the transformer needs to be isolated as fast as possible to prevent more damage. For such purposes, fast protective relays are used for detection of faults.

    Basically, a protective relay is a device that senses / detects faults and gives the tripping command to the circuit breaker. Thus, when a circuit breaker receives command from protective relay, it disconnects the transformer from the network.

    How does a protective relay work?

    There are various types of protective relays in transformers, and their operating principle depends on their type. viz;

    • Gas detector relay (Buchholz relay): This relay is basically used in transformers rated more than 500 kVA. It is a type of gas and oil actuated protection relay. Whenever a fault occurs inside the transformer, such as insulation failure of turns, breakdown of core or excess core heating, the fault is accompanied by production of excess heat. This excess heat decomposes the transformer insulating oil which results in production of gas. The generation of gases depend on intensity the of fault. Gas bubbles tend to flow in upward direction towards conservator and hence they are collected in the buchholz relay which is placed on the pipe connecting the transformer tank and conservator.
    • Overload protection relays (Thermal relays): These type of protective relays generally use a thermal sensor circuit. This circuit when senses a temperature rise over a set value, it triggers a signal to the circuit breaker which then disconnects the transformer from the circuit.
    • Overcurrent protection relay: In an over current relay, there would be essentially a current coil. When normal current flows through this coil, the magnetic effect generated by the coil is not sufficient to move the moving element of the relay, as in this condition the restraining force is greater than deflecting force. But when the current through the coil increases, the magnetic effect increases, and after a certain level of current, the deflecting force generated by the magnetic effect of the coil, crosses the restraining force. As a result, the moving element starts moving to change the contact position in the relay. Although there are different types of overcurrent relays but basic working principle of overcurrent relay is more or less same for all.
    • Ground fault protection relay: A current-based ground-fault relay may look for ground-fault current in one of two ways: 1.) Zero sequence. Here, the relay looks at the phase conductors to ensure that all current coming from the source returns on those same conductors. If some of the current is returning to the source through a different path (usually ground), the ground-fault relay will detect this difference and, if it exceeds a pre-determined amount for a pre-determined amount of time, the ground-fault relay will operate. 2.) Direct measurement. A ground-fault relay can also read the current in the connection between the transformer neutral and ground (even with a neutral grounding resistor). A ground fault anywhere in the system will return current through this path.
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