DISTANCE RELAY MINIMUM SETTINGS BASIC INFORMATION

The problem of low settings of line distance relays is that for lines that are very short or have a high SIR, available fault current and voltage at the relay location may not provide adequate operating margins. All distance relays have minimum settings criteria that must be met for proper operation.

The following concerns should be addressed:

a) Impedance characteristic.
Minimum fault operating currents must be know; usually, the shorter the reach, the greater the minimum current required to function. The apparent reach setting of the relay decreases with lower relay terminal voltages (Figure below), causing the relay to under reach.

Apparent relay reach vs. relay voltage

b) Directional action.
Minimum polarizing voltages must be known. Sensitivities in the range of 1% of rated voltage may be required; however, at this sensitivity, misoperations may occur for reverse faults due to the effect of arc drop.

c) Memory action.
The memory circuitry of the relay is used for low-voltage conditions by supplying a prefault voltage for polarizing. This circuit may have memory action that lasts only a few cycles.

d) Operating time.
Tripping time (Figure below) may vary with the distance to the fault, the basic minimum reach setting, the fault current magnitude, and the magnitude of relay voltage prior to the fault.

Location of Fault in Percent of Relay Setting

Variation of operating time with distance to the fault

Usually, the lower the ratio of Zapparent to Zsetting, the faster the relay operates except for low current conditions. Under conditions of low current, the relay operate time may actually increase.

e) Maximum torque angle.
Cable circuits may have a very small line impedance angle, especially for pipe-type cables. This may require a maximum torque angle that is not available on the relay and, thus, necessitate using a different range or impedance characteristic (i.e., quadrilateral) relay.

f) Continuous ampere rating.
The engineer must be aware of the continuous operating characteristics for a particular setting. It is possible for a setting to violate the continuous ampere rating, especially if the relay is an electromechanical style.

g) CT and VT errors.
Due to errors in the CTs and VTs, it is possible for a marginal setting to be unusable. On systems where the available voltages and currents are low, CT and VT errors may further reduce the available quantities to the relay.

h) Relay settings.
When the protected line is short, arc impedance must be incorporated into the settings for Zones 2 and 3. It may not be possible to increase the Zone 1 reach setting because of possible overreach of remote terminals, although some relaying principles automatically adapt to this need.

As each distance relay has its own characteristics, it may be necessary to perform fault studies under minimum conditions to confirm that the relay functions properly. If the relay does not meet the minimum requirements, then alternative relay schemes, such as current differential, phase comparison, or pilot-wire, should be considered.