Types of Feeder Protection Relay:
A composite transmission system may use one or more of the following types of Feeder Protection Relay.
- Overcurrent Protection: This is of two types: (i) nondirectional time and current graded schemes; (ii) directional time and current graded schemes.
- Distance Protection: This uses high speed distance relays.
- Pilot Protection: This is of three types: (i) wire-pilot protection; (ii) carrier-pilot protection; (iii) microwave-pilot protection;
The factors governing the selection of a particular scheme of protection can be enumerated as follows:
- Economic justifiability of the scheme to ensure 100% continuity of
- Types of Feeder Protection Relay whether radial or ring mains.
- Number of switching stations in series between supply point and the far end of the system.
- Availability of pilot wires.
- System earthing—whether the neutral is earthed or insulated.
Time graded overcurrent relays are normally used for backup protection in large transmission systems or where a time lag can be permitted and instantaneous operation is not necessary. In distribution feeders, they play a more important role and may be coordinated with fuses.
Overcurrent relays are also used for ground faults. Distance protection is applied where time lag cannot be permitted from stability considerations. Again there are many forms of distance protection each form having its own field of applications, e.g. for very short lines reactance type is preferred; for medium length lines impedance relay is suitable but is likely to operate erroneously on severe reactive power surges; mho relays are suitable for phase faults of longer lines. Distance relaying is a high speed form of protection.
The pilot type of protection is a unit form of protection which would operate only for faults occurring within the protected section giving no backup protection. The advantage of this form of protection is that it gives the fastest discriminative clearance of all faults occurring anywhere within the protected feeder. Pilot wire protection is used for short lines where cost of pilot wires is not prohibitive. Carrier and microwave-pilot protection is used for long lines and interconnected lines.
Main transmission lines working on 220 KV or above are equipped with the fastest and the most reliable protection. Carrier-pilot or high speed distance protections are necessarily used on main transmission lines, whereas the 33 KV circuits are provided with directional time lag over current relays or high-speed distance relays.
Radial feeders working on 11 KV or lower are usually equipped with time-lag overcurrent relays supplemented by instantaneous relays. Sometimes only HRC fuses are used on these lines.