NON DIRECTIONAL OVERCURRENT RELAY SCHEMES FOR TRANSMISSION LINES

Overcurrent protection is the simplest and least expensive form of fault protection that can be placed on transmission lines. The operating principles depend only on current magnitude. The ac connections for three-phase and one ground time overcurrent (TOC) relays and instantaneous overcurrent (IOC) relays are shown in below.

Connections for overcurrent phase and ground relay

Phase currents or sequence currents can be used as the operating quantity. Phase overcurrent relays operate for all possible fault types, but require their pickup settings to be higher than the maximum expected normal or emergency load flow condition.


Negative and zero-sequence overcurrent relays do not operate for balanced loads or for three-phase faults, but can have pickup settings well below the expected load. Tripping may be instantaneous, delayed for a fixed time, or delayed for a time inversely proportional to the current magnitude.

Figure below shows some of the various shapes of time/current characteristics that may be used.

TYPICAL TOC CURVES

Time Dial 5

Multiples of pu

Time overcurrent curve shape comparison

Instantaneous tripping can be applied if the pickup point of the instantaneous unit can be set higher than the maximum contribution to faults outside the protected line. The percentage of a line that can be protected by an instantaneous overcurrent relay will vary with line length and source impedance.

To protect an entire nonradial line, time delays are generally required to achieve coordination with downstream protective devices.

Figure below shows how coordination is achieved between a relay with a time and instantaneous element (the primary relay), and an upstream (backup relay) with only a time element.

Coordination of time overcurrent relays

To ensure proper coordination, the pickup point of the instantaneous unit should be set higher than the maximum contribution to faults outside the protected line.

The pickup value of the time element should be set to prevent tripping for the maximum load current that can flow in either direction on the line. The time adjustment (i.e., dial) should generally be set to produce the fastest operating time that will not result in miscoordination with other protection behind or in front of the terminal.

The effect of varying the time adjustment is illustrated in Figure below for a typical time overcurrent relay.

Very Inverse

Effect of Time Dial

Varying time adjustment on overcurrent relay characteristic curves

Nondirectional overcurrent relay schemes find limited application on transmission lines. Because transmission lines usually have at least two sources of fault current, the nondirectional elements have to be coordinated with protective devices both in front of and behind the line terminal.

This makes the coordination of nondirectional relays more difficult, and sometimes impossible. In some applications, these relays may be applied only at the terminal with the higher fault current source.