Over Current Protection

9.Over current protection

As the name indicates , this protection is based on comparing the set current in the relay with the fault current and if the fault current is more than the set value in the relay , then relay will operate . There are basically two main settings to be applied in this type of protection ie , the current setting and the Time dial setting . If the relays are static or numerical relays , then we have the facility to select the characteristics in the relay like IEC standard inverse, very inverse , extremely inverse . definite time etc .

The time of operation of the relay is generally depend on the Time dial setting , current setting , and the characteristics selected or various combination of the above - For example if the definite time is the characteristics selected in the relay , then the relay will operate in definite pre set value of time if the current exceeds the set value of the relay . Or in other words the amount of fault current is not a criteria on the time of operation . As long as the fault current is more than the set value , the relay will operate in the preset value of time . On the other hand , if the charecteristics is selected as standard inverse , then the time of operation of the relay will be reduced as the fault current is increased .

The current settings are usually in Amps and is based on the relay setting calculation . Ideally it can be any current above the capacity of the equipment to be protected , however this setting is not straight forward .It depends on the coordination that must be achieved among the other Relays connected to the same equipment and the system as whole .
There are also directional over current relays which is designed to operate only for a particular direction of the current flow . To understand the direction of the current a reference quantity is required. Normally system voltage is taken as the reference quantity or we call by term polarising voltage .The idea of taking the system voltage as the reference quantity is based on the fact that , it does not change its phase angle appreciably during a fault . The direction of the current based on the selected voltage is found by the relay and the operation or non opertaion is decided based on this. Obviously such relays are supplied with CT and VT for its operation .
Cross polarising is the method normally used for the directional relays in which the reference voltage for the faulted phase is the other two phases . ie if there is a fault in phase A , the reference voltage used in this fault will be from the B and C phases. Its advantage to use such a cross polarising so that any change in the magnitude of the faulted phase voltage is not effecting the directional measurement of the faulted current

9.1Instantanious over current protection

Whenever the fault current is very high warrenting a fast operation to clear the fault , there is instantanious over current protection which is normally available as a part of over current relay . It is always a intresting point how to arrive at a proper value of instatntanious setting .The setting of this protection can be theoritically a value above the time delayed over current protection . However proper coordination between the downstream relays shall be done before selection a value for such a setting . For example if there is an over current relay in the primary of the power transformer , it shall be properly coordinated with a down stream setting in such a way that , for a through fault it shall not trip . To arrive a suitable setting for this protection , the power transformer through fault current shall be found out . As the instantanious protection is called upon to operate during the first few cycles of the fault current , the setting can be found out only by finding the both the symmetrical and the asymmetrical current and selected setting shall be above this value so that there is no false or non selective tripping of this protection with respect to the down stream protections.

Directional Over current Protection


As the name suggests , the directional over current protection detects the dirction of current before it take a decision to operate . It can be set to operate in one direction - normally called Forwrd direction and restrains its operation in the other direction - normally reverse called reverse direction

Hence , it requires two element to operate before a directional over current relay can operate . ie the fault current shall be above the set current and the direction shall be Forward . In order to identify the direction of fault one should have a reference quantity .The direction of the fault current with respect to this reference quantity determines whether the flow in forward direction or reverse
The reference quantity is called the polarising quantity and it shall be reasonably constant . Normally the voltages in the power system are taken as reference ( current also can be taken )

Voltage polarisation

There are two types of voltage polarisation
1, Self polarisation - In this case the faulted phase voltage it self is taken as polarising quantity . The disadvantage is that , for close in type of fault , the polarising quantity ( voltage ) may be too less so that the relay may fail to operate

2. Cross polarisation . In this case the opposite side phase voltages are taken as polarising quantity Ie For Phase A fault - BC voltage will be taken as reference which will be reasonably constant .

The Torque equation of the directional relay

The very important concept which shall be kept in mind to understand the concept of the direction relay requires the basic torque equation of the electro mechanical relay which may be written as

Torque = The product of the Flux developed by the Voltage coil ( polarising voltage ) multiplied by the Flux developed by the current coil and the Sin of the angle between them . ie

Phi( V) * PHI (I) Sin angle between the flux developed by the Voltage and the Flux devoloped by the Current
It may be noted that , the flux developed by the Voltage will be lagging the voltage by the 90 degree ( Inductive property) and the flux developed by the current will be in phase with the current so that , the maximum torque occures when the voltage and currents are in phase ( in this case the angle between the flux due to voltage and the flux due to current will be 90 degree and the Sin 90 become one and the torque become maximum

Maximum Torque angle ( MTA)

Now we understood from the forgoing discussion that the maximum torque occurs when the Voltage and current are in phase . But in actual case the voltage of the polarizing voltage and the current will not be in phase because it depends on the type of fault etc . In order to get the maximum torque during a fault , the voltage is shifted towards the expected fault current so that they are approximately in phase . This angle is called Maximum torque angle. it may be noted that , the fault current angle from the faulted phase is an assumed value .

The Relay charecteristics angle (RCA)

The polarizing voltage angle is normally shifted to the expected fault current direction to achieve the maximum torque during the fault . It may be shifted in clockwise direction or anticlockwise direction depending on the type polarizing used . A measure of this angle is called relay characteristics angle .

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The positive and negative torque and the operating region

We know that , the torque equation is explained above is positive for a angle between flux produced by the Voltage and the flux produced by the current between 0 and 180 degree and it is Negative for a phase angle between 180 degree and the 360 degree . The positive torque is taken as the direction of forward fault ( operating direction) . The negative torque is taken non operating region . It may be noted that , the positive torque area need not completely cover the positive power flow area ! Hence we can not use a directional over current relay to find out the direction of the power flow as relay torque characteristics does not completely cover the positive power flow direction





Analysis of the fault based on the types of load


As explained , the torque is positive for any angle between 0 and 180 degree and it is negative between the 180 degree to 360 degree Now we know to get the maximum torque , the angle between the two fluxes shall be 90 degree ( sin 90 =1 ) .

case 1. The load fault path is fully inductive
For a single phase to earth fault ( with self polarizing ) , we know that if the fault impedance is fully inductive , the current lags the voltage by 90 degree . Assume that , the characteristics angle is zero ( meaning the voltage is not shifted to get max torque during the fault )
In this case The flux produced by the voltage will be lagging the voltage by 90 degree ( voltage coil , current lags by 90 degree) and the flux produced by the fault current is in phase with the fault current . Hence the phase angle between the two fluxes is 0 degree and the torque become zero and the relay will not operate

Now suppose the charectristics angle is set to 90 degree , then with the same fault , the torque is maximum . ( as the phase angle between the two fluxe is 90 degree) . Hence depending upon the h polarising quantity is used ,the network condition etc , the charetericts angle is set . If the line cherctristics is known , then , with self polarising method , the charecteristics angle can be set at line angle

Fault current in four quadrants

If the voltage phasor is taken as dircted towards up ,
1, then all lagging current falls in the first quadrant
2. All leading current will be falling in the second qudrant ( counter clock wise direction from first quadrant )
3. All lagging current in the opposit dirction will be in the third quadrant
4. All leading current in the opposit direction will be in the fourth quadrant

Here , for a forward current , the current phasor shall lie in the first and the second qudrant
similarly for a reverse currrent , the current shall lie in the third and fourth quadrant

Normally for a fault , the current will be lagging in nature due to the line inductance and we are interested in checking whether , the current phasor is in first quadrant ( forward ) or in the third quadrant ( reverse)
Also we will try to ensure that , we get the maximum torque when the fault occures











Relay coordination