Optimizing distribution protection

Typical distribution feeder protection is based on phase and ground overcurrent relays set to protect the line for three-phase, phase-to-phase or phase-to-ground faults. An instantaneous relay is used to operate for close-in faults and a time overcurrent relay with inverse characteristic provides protection for most faults on the line. The time overcurrent relay has to coordinate with any fuses used to protect distribution transformers connected to the feeder. The coordination requirements for high current faults result in increase in the operating times for faults further down the line, with the longest times for faults at the remote end.

In order to reduce the number of electromechanical or solid state relays, backup protection for bus faults or breaker failure has been traditionally provided by the transformer protection relays. Considering the fact that they also have to coordinate with the feeder relays, it is obvious that the operating times for bus faults or feeder faults with breaker failure will not meet the requirements of sensitive customers.

Modern multifunctional protection relays have many features that allow significant improvements in the performance of the relays under different fault conditions.

Definite Time versus Inverse Time Overcurrent

Modern distribution feeder protection relays have multiple phase and ground overcurrent elements used to reduce the operating time of the relay for different fault conditions.

Multiple independent stages are available for each phase overcurrent element and the two ground fault elements - one based on measured quantities and the second - on derived quantities calculated from the three phase currents.

Each stage may be selected as non-directional or directional (forward/reverse). All stages usually have definite time delayed characteristics and some of the stages may also be independently set to inverse-time IEC or IEEE tripping characteristic (Figure 2). These stages have a programmable reset timer for coordination with other devices.

A voltage controlled overcurrent function can be enabled on phase overcurrent elements. It provides back-up protection for remote phase faults whilst remaining insensitive to load.

The directionality of the ground fault elements is provided by residual voltage or negative sequence voltage polarizing.

A separate multi-stage sensitive ground fault element is provided and is selected as non-directional or directional. Figure 4 shows the inverse time characteristic of a phase overcurrent relay with the operating points for different fault locations on the protected feeder. The characteristic is coordinated with a downstream fuse.

Table 1 shows a comparison of the operating times of the traditional inverse time characteristic, compared with the use of the two additional definite time delayed overcurrent elements with settings of 0.2 s and 0.35 s.

The last column in the table gives the difference in the operating times. The advantages are quite significant, especially for the faults closer to the end of the line.