Thursday, 27 February 2020

Directional Over Current Relay- Use of directional and non directional relay in over current relay

Directional Over Current Relay

  • Directional Over Current Relay is used to transmission line where the power flow is associated in a special direction.
  • From the word over current, it means that the relay will be operated for only one case if the current value rises from the set value.
  • And directional word is for the current direction that is if relay is set for only forward direction current, it will sense forward direction current, it will not sense reverse direction over current and vice versa.
  • These are suitable in ring main system and radial system.

How does the directional Relay Works?

Directional Over Current Relay
Directional Over Current Relay 

  • Directional over current relay respond to excessive current flow in a particular direction in the power system. The relay typically consists of two elements. One is directional element, which determines the direction of current flow with the respect to the voltage reference.
  • When this current flow is in the predetermined trip direction, this directional element enables (turn on) the other element, which is the standard over current relay, complete with taps and time dial, as found on a normal non- directional over current relay. 
  • Because of these relay is designed to operated on fault current, the directional unit is made so that it operates best on a highly lagging current, which is typical of faults in power systems. So the zone of fault is identified with the help of relay characteristics and maximum angle setting.
  • Directional over current relay are normally used on over coming line circuit breakers on buses which have two or more sources. 
  • They are connected to trip an incoming line breaker for fault current flow back into the source, so that a fault on one source is not fed by the other sources. 
  • In complex distribution or sub-transmission network, these relay may be used to improve coordination of the system.

What is Non Directional Relay?
  • When there is fault in power  system, power flows through fault. Non directional relay operates irrespective of direction of flow of current. 
  • Ex. Breaker at generator end. If there is fault on generator secondary relay has to operate to open circuit breaker.  
  • If there is fault in a winding generator and its drawing power from grid then also circuit breaker has to operate. So we use a non directional relay. It has to operate in fault conditions irrespective of direction of power flow. 

Use of Directional and Non- Directional Relay in over Current Relay

  • In an over current relay we use both directional as well as non directional over current relay.
  • Directional relay is connected towards load side of the line which will trip if over current flows from load towards the source and 
  • The non directional relay is connected at the source side which will trip if fault occurs irrespective of the direction of current. 

  • Now a days we use multiple conductors or transmission line forming a ring structure so as to avoid interruption of power supply to the costumers in case of fault in any line.  
  • Therefore when any of line undergoes a fault. The non Directional relay at the source will automatically trip and separate the source from fault point but load side is still connected to the fault point. 
  • Due to the other transmission lines present in the system the power will still reach to the costumers. As the fault is still present therefore other transmission line will also feed this fault point and thus system might get damage. 
  • Therefore the directional relay we connected at the load side will detect the reverse flow of current and thus trip and avoids other transmission to feed the fault location.


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