Principle Of Over Current Relay | Inverse Time Over Current Relay

Over Current / Earth Fault

The overcurrent relay is outlined because the relay, that operates only the worth of this is bigger than the relay setting time. It protects the instrumentation of the ability system from the fault current.

Working Principle of Over Current Relay

In associate over current relay, there would be basically a current coil. once normal current flows through this coil, the magnetic result generated by the coil isn’t sufficient to maneuver the moving part of the relay, as during this condition the restraining force is bigger than deflecting force. however once the present through the coil will increase, the magnetic result will increase, and when an exact level of current, the deflecting force generated by the magnetic result of the coil, crosses the restraining force. As a result, the moving part starts moving to alter the contact position within the relay. though there area unit differing kinds of overcurrent relays however basic regulation of overcurrent relay is a lot of or less same for all.

Earth fault protection

A ground fault is associate unintended contact between associate energized conductor andground or instrumentality frame. The come back path of the fault current is through the grounding system and any personnel or instrumentality that becomes a part of that system. Ground faults area unit oftentimes the results of insulation breakdown

Types of Over Current Relay

Depending upon time of operation, there square measure numerous styles of Over Current relays, such as,

    1.Inverse time over current relay.

   2.Definite time over current relay.

Definite time over current relay

Instantaneous Overcurrent relay (Define Current) Definite current relay operate instantly once this reaches a preset worth. Operates during a definite time once current exceeds its Pick-up


Pick-up  worth. Its  operation criterion  is just

current magnitude (without time delay).

The relay has no intentional time delay for operation. The

contacts of the relay area unit closed instantly once this within the relay rises on the far side the operational price. The quantity between the moment pick-up price and also the closing contacts of the relay is incredibly less.

Inverse time over current relay.

The relay operates only if the magnitude of their operational current is reciprocally proportional to the magnitude of the energize quantities. The operational time of relay decreases with the will 

increase within the current. The operation of the relay depends on the magnitude of this.

The characterisic function for the relay is shown within the figure below. The relay won’t operate once price|the worth} of current is a smaller amount than the decide value. The relay is employed for the protection of the distribution lines. The inverse time relay is of varieties.

1.Normal inverse

2.very inverse

3.Extermely inverse

4.Long time inverse


When fault current will flow in each directions through the relay location, it’s going to be necessary to create the response of the relay directional by the introduction of a directional man

agement facility. the power is provided by use of extra voltage inputs to the relay Relay Connections

There area unit several prospects for an appropriate association of voltage and current inputs. the varied connections area unit keen about the point, at unity system power issue, by that this and voltage applied to the relay area unit displaced.

In a digital or numerical relay, the part displacements are obtained by computer code, whereas mechanical device and static relays typically get the specified part displacements by connecting the input quantities to the relay. The history of the subject ends up in the relay associations being outlined as if they were obtained by appropriate connection of the input quantities, no matter the particular methodology used.


Directional Earth Fault Protection

Generally earth faults ar Single Line to Ground (SLG) and Line-Line to Ground (LLG) faults. Earth faults are characterised by the presence of Zero Sequence Curren I0. Since, aside from

unbalance, normal system operation isn’t having Zero Sequence Current I0, way more sensitive pick-up is feasible for earth fault by victimisation zero sequence current element I0 = (Ia + Ib + Ic) / three and declaring a fault if I0 exceeds a threshold.





Related Posts

Leave a comment