What is Induction type Over Current Relay?

What is Induction type Over Current Relay?

INTRODUCTION

Relay is a protective device which is used to detects the fault in electrical system. Relay detects the fault and initiates the operation of circuit breaker to isolate the faulty section. The induction type over current relay used to detect the over current fault in electrical power system. The over current relay operates when the value of current in power system increases than certain preset value. The basic working principle of these type of relays is based on electromagnetic induction, this is the reason why these over current relays are called Induction type over current relay. When the over current fault occur in power system, The induction type over current relay detects the fault and closes the trip coil circuit. The trip coil circuit opens the contact of circuit breaker and thus disconnects the faulty section.

Induction-Type-Overcurrent-Relay
Internal View of Induction Relay

WORKING PRINCIPLE

As the name suggests, The working principle of induction type over current relay is based on electromagnetic induction. The induction relay consists of an aluminium disc which is placed between two electromagnets. The aluminium disc is free to rotate and spiral spring is connected on the spindle of disc to provide the controlling torque. When the over current fault occur, current flow through the windings of electromagnets. Due to the interaction of magnetic fields, The torque is exert on aluminium disc and the disc start rotating. The spindle of aluminium disc carries the moving contact which is used to bridges the two fixed contact of trip coil. The aluminium disc is only rotates when the driving torque become greater than controlling torque applied by spiral spring, which is only happens when the value of current become more than normal preset value.  

how-does-a-directional-overcurrent-relay-work
Working

TYPES

Based on the direction, The induction type over current relays are divided into two types:

  • Non-directional Induction Type Over Current Relay
  • Directional Induction Type Over Current Relay

1. Non-directional Induction Type Over Current Relay

It consists of two electromagnets and aluminium disc. The upper magnet has three limbs and look like a letter “E”. The lower magnet has two limbs and look like letter “U”. The primary winding is wound on the central limb of upper magnet. The primary winding has numbers of tapping which is connected with plug setting bridge. Plug setting bridge is used to change the number of active turn of primary winding which further changes the current value, Thus desired current setting is obtained. The primary winding is connected with the current terminals of power line to be protected. The secondary winding is wound on the central limb of upper magnet and on both limbs of lower magnet. The secondary winding is energized by primary winding due to principle of mutual induction and aluminium disc is placed between both electromagnets.

induction-type-overcurrent-relay-non-directional
Non Directional Relay Arrangement

Operation

When current flows in primary winding, flux is produce in it. The flux produced in primary winding linkages with the secondary winding. According to electromagnetic induction, When conductor cut across magnetic field, An EMF is induced in it. Due to this an emf is induced in secondary winding and since the secondary winding is closed, current starts flowing in it. The magnetic field produced in primary winding and the magnetic field produced by secondary winding interact with each other and torque is exert on aluminium disc placed between both windings. The driving torque which is applied on aluminum disc is opposed by the controlling torque which is provided by the spiral spring placed on the spindle of disc.

Under the normal conditions, When there is no fault occur in power system, The controlling torque is greater than driving torque and thus disc remains in stationary position. When the fault occur in power system, The value of current flowing through primary winding is become greater than preset value. The driving torque become more than controlling torque and thus the disc start rotating. The moving contact placed on spindle of aluminium disc rotates with disc and bridges the fixed contacts. Thus it closed the trip coil circuit and initiate the circuit breaker operation which further isolate the faulty section.

2. Directional Induction Type Over Current Relay

Directional induction type over current relay is basically a combination of reverse power relay and non directional induction over current relay. The construction of this type of relay is divided into two parts:

  • Directional Element
  • Non Directional Element

Directional Element

The directional element is basically an reverse power relay which has similar construction like non directional over current relay. The directional element consists of two electromagnets and one aluminium disc which is free to rotate. The tapped winding is wound on the both the limbs of lower magnet. The tapped winding is connected with the current terminals of the line to be protected and known as current coil. The upper magnet has a winding which is wound on the central limb of magnet. This winding is operated by the system voltage and known as potential coil. The driving torque produce on the aluminium disc by directional element is directly proportional to the power. The direction of driving torque depends upon the direction of flow of current.

induction-type-overcurrent-relay-directional
Directional Relay Arrangement

Non Directional Element

Non Directional Element is basically non directional over current relay. The primary winding of non directional relay which is wound on the central limb of upper electromagnet is connected in series with the current coil of directional element. The non directional element contains a plug setting bridge which is used to adjust the current setting. The tapped primary winding is connected with plug setting bridge and by changing the position of plug, The number of active turns can be changed. The operation of non directional element is controlled by directional element because the trip contacts of directional element is connected in series with the lower magnet winding of non directional element.

Operation

Under the normal condition, When there is no fault occur in power system, The power flows in normal direction. The directional element does not operate under this condition. Hence the trip contacts of directional element is connected in series with lower magnet winding of non directional element, The non directional element also remains inoperative.

When the fault occur in power system, The current or power start flowing in reverse direction and the aluminium disc of directional element starts rotating. The moving contact placed on the spindle of disc bridges the trip contacts which closes the circuit of secondary winding of non directional element. The disc of non directional element start rotating which further closed the trip coil circuit and initiate the circuit breaker operation.

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