DISTANCE PROTECTION

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From the name itself it may lighten that it’s about the transmission line protection strategy.


Ø  When do we need this?

     We need to protect a long transmission line with good sensitivity and selectivity

Ø   Why not over current Protection?

Of course it is possible as it’s a cheaper. But not recommended where we need higher sensitivity and selectivity. That is, if the load current and fault current are almost comparable, overcurrent protection fails to distinguish between the two. Also it is more susceptible in taking action with the change in fault location.

Ø   Advantages over other methods

·         Provides faster operation.

·         Has the directional feature.

·         Greater accuracy as more information is using for taking decision.

·         As a backup protection for cable differential.

·         Independent of source impedance.

·         To detect fault location from the recorded fault impedance.

Ø  How its working?

The Distance protection takes the Current and voltage with the direction as inputs and calculates the impedance (ZF). Then it compares with the line impedance (ZL). If ZF > ZL initiate trip else restrain.

For nearby faults the current will be higher and voltage will be lower. Voltage and current will be moderate for faults at moderate distance from the substation. And the Voltage will be high and current will be low, if the fault is located at almost at the end of the line.

If the Calculated impedance is less than the set impedance, then it is termed as under reach and else over reach. In a simple scheme at the point of ZMeasured = ZSet   relay will be on the verge of operation/pick condition and for ZMeasured > ZSet relay will restrain. Based on this we could grade the protection as different zones.

There are three different types of distance protection methods as per the ratio between V and I as follows

1.Impedance relay

It solely relies on the impedance calculation only with considering magnitude of parameters. So it does not have the directional feature. The output from CT and PT are fed to relay and form torques proportional to the input. So, at normal conditions it balances and while there is a fault the current increases and produces the pickup (or the operating) torque and the restraining torque supplied by voltage will fail to balance it. Which contributes to the relay contact for initiating the trip.

Characteristics of impedance Relay

The relay possesses only one set parameter which is ZSet.

As it’s a simple impedance relay there are some difficulties while handling the fault condition like it cannot discriminate the fault whether it is internal or external, not at all considering the arc that may be formed during fault, nuisance tripping initiation while power swings are present

But these demerits are rectifiable with enhanced circuits. 

2.Reactance Relay

Here the current provides the operating torque while the restraining is introduced by current-voltage element in order to have the directional feature. But the same directional unit cannot contribute to directional feature for operation.


Characteristics of Reactance Relay

 

The relay operates based on the reactance only.so it does not bother about the resistance component in the line impedance.

The set parameter is X. that is, for any value below set, the relay will produce positive torque.

On the basis of characteristics, its technically a non-directional relay and so, it is not capable to discriminate neither faults near to relaying point nor the adjoining section.

3.Mho or Admittance relay

The Mho relay is made directional by adding a polarising coil deduced from voltage. Now the operating torque is provided by the product of  V&I and restraining by voltage element.


Characteristics of Mho Relay

It’s the most versatile distance relay protection strategy for long lines due to its faster operation, immune to most of the power swings.

 

Simply, the performance of distance relay is based upon the measurement of impedance, reactance or admittance of line between the location of line and the fault location.


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