Selection of Distance Relay:
The factors to be considered for the Selection of Distance Relay scheme can be enumerated as follows:
Speed of operation.
Measuring relay characteristics.
1. Speed of Operation:
Operating time is of great importance on systems which are liable to become unstable if the fault persists for more than 5 cycles or so. Zone 1 operation in non-switched schemes where separate measuring units for different phases are used is in the region of 2-3 cycles; but about 40% of the protected section viewed from both ends is covered in zone 2 operating time. Such a scheme in conjunction with the power line carrier is used to extend the zone 1 reach for internal faults, thereby ensuring simultaneous high-speed operation of breakers at both ends, the scheme however works as a plain distance backup for external faults.
All switched schemes which use single measuring unit for different phases switched by proper selectors or starters, are slow speed protections. Thus in general non-switched schemes are applied on all important feeders at sub-transmission and distribution voltage levels, and in combination with power line carrier, on all feeders at bulk transmission voltages. Switched schemes are applied as an economic alternative to time graded forms of protection on radial and ring distribution circuits and on less important transmission lines.
Because of the long clearance times with the switched schemes, it is usual to add out-of-step blocking and tripping relays to the scheme to prevent serious shut-down during power swing condition.
2. Measuring Relay Characteristics:
The field of application of various Selection of Distance Relay overlap to a large extent and as such a hard line cannot be drawn for their respective fields. The length of the line to be protected and the voltage of transmission play a very important role in the choice of these
Reactance relays are preferred for short line sections because of being insensitive to fault resistances which may be large compared with the line impedance. Since power swings affect reactance relays to a large degree than the other factors these relays are unsuited to long line protection.
The impedance relay is better suited for phase-fault relaying for lines of moderate lengths. The effect of arc on impedance relay is more than in a reactance relay but less than a mho relay. Power swings affect an impedance relay less than a reactance relay but more than a mho relay, but schemes using either may require supplementary relays to prevent maloperation. The impedance relay will always require a separate directional unit.
As the mho relay is less affected by power swings and encircles the smallest area on the complex plane it is best suited for long lines. It has an additional advantage of being inherently directional which renders starting relays unnecessary. For very long heavily loaded lines, mho unit blinders or elliptical relays are better suited.
With cables, it is the sensitivity of the relay rather than the fault resistance that determines the minimum length of cables that can be protected. The conductance relay is more tolerant to fault resistance than mho or plain impedance and more economical than the reactance unit because it needs no directional unit. It is applicable to distribution lines both overhead and cable.
3. Fault Coverage:
Where a system is not effectively earthed or earthed through an arc suppression coil it is only necessary to apply phase-fault schemes of protection; while on effectively earthed systems fault coverage for both phase and earth faults is provided.
4. Economic Considerations:
The foregoing discussion makes it amply clear that there are a number of alternatives to achieve the objective of feeder protection. Maybe time graded overcurrent protection is sufficient under particular circumstances. As far as earth-fault protection is concerned overcurrent earth-fault protection works quite satisfactorily and is normally provided with phase protection as one of the Selection of Distance Relay schemes. The occasional slower clearing time with overcurrent relays is unimportant because single phase of earth-faults have negligible effect on system stability.