The relay selection for transmission line protection utilizes one of the two major relaying philosophies. One is distance protection and the other is differential protection. Both of these protection philosophies are discussed in detail in previous posts. The other relays such as an overcurrent and earth fault relay is used as a backup relay for the protection of transmission lines at higher voltage levels and main protection for lines at comparatively lower voltage levels.
The directional earth fault and over-current relays are basically used for the protection of transmission lines at higher voltage levels as they are interconnected and parallel. However, the non-directional earth fault and over-current relays are used for the protection of distribution lines at comparatively lower voltage levels such as 11KV.
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Relay selection based on voltage level
For 33 KV and 11 KV lines, Overcurrent and earth fault relays, 51 and 51N are mostly used as main protection. No main-2 redundant and backup protection relays are used in 11KV voltage-level transmission lines.
For the 66 KV & 132 KV, the relay selection for transmission lines includes a switched distance or numerical distance relay 21, which is used as a main protection along with it, directional overcurrent and earth fault relay with high set 67-67N is used as a backup protection for the lines.
For voltage levels above 132KV, the relay selection for transmission lines includes main protection relays that are duplicated for redundancy as Main 1 and Main 2 relays along with the main protection relays backup relays are also used. The main protection includes numerical distance or line differential relay based on the utility’s choice and the backup protection relay includes the directional overcurrent and earth fault relay. If the main 1 relay is a distance relay the main 2 relay will be a line differential or distance relay but with a different make for redundancy.
Relay selection for transmission lines (SHORT):
The differential protection is used for short transmission lines. This is because for short transmission lines the resistance and reactance of the conductor are very low, therefore, the measured impedance will also be less and hence the impedance relay (distance) tends to malfunction.
Another practical reason is being the distance of fault is not so necessary in short transmission lines as it can be easily detected by the line maintenance personnel.
Relay selection for transmission lines (LONG):
The main protection used for long transmission lines is normally distance relay (impedance). This is taken into use as the distance of fault is given out by the relay and as the line length increases the fault impedance increases which gives in easy fault detection by the impedance relay.
It may be noted that relay manufacturers have distance and differential protection elements embedded into a single unit example is ABB’s RED 670 relay which has both functions.
However, in addition to the main protection relays, the transmission lines normally have backup protection as directional earth fault relay and overcurrent relay.
Conclusion:
As per the Central Electricity Authority, CEA India, the short transmission line length is any length below 30Kms. And above 30Kms length, the line is treated as a long transmission line. This may differ from country to country as per specified standards.
In high voltage transmission lines beyond voltage levels of 132KV, for short transmission lines, line differential relay is used as main 1 protection and distance relay is used as a main 2 protection. The opposite holds true for long transmission lines. However, in some utilities differential relay is used as the main protection, and distance is used as backup protection, for short transmission lines.
