Negative Sequence Overcurrent Protection

Overcurrent elements in traditional phase overcurrent protection must be set higher than maximum load current. This limits the sensitivity of the phase relays and at the same time results in increased operating times for line end faults. Since the levels of the phase fault currents for phase-to-phase faults are lower than the levels for three-phase faults, this will result in further increase of the fault-clearing time.

If we consider an example for a phase-to-phase fault at 75 % of the protected feeder, the voltage at the relay location is 0.79 Unom and the phase overcurrent relay with inverse characteristic used for Table 1 and Figure 4 will operate in 0.71 s. For the same three-phase fault the voltage drop seen by the relay is 0.71 Unom with operating time of 0.53 s.

Any unbalanced fault condition will produce negative sequence current of some magnitude. Thus, a negative sequence overcurrent element can operate for both phase-to-phase and phase-to-ground faults. If a definite time negative sequence overcurrent element is used, it may be set to clear the same phase-to-phase fault in less than 0.3 s. This will bring the duration of the fault within the acceptable region of No Interruption in Function. Negative sequence overcurrent elements also give greater sensitivity to resistive phase-to-phase faults or high resistance phase-to-phase-to ground faults, where phase overcurrent elements may not operate at all.

RMS Thermal Overload

Thermal overload protection based on a thermal replica using RMS load current to model heating and cooling of the protected equipment can be used with both alarm and trip stages. The heat generated within a cable or a transformer is directly proportional to current squared. The thermal time characteristic used in the relay is therefore based on current squared, integrated over time. The largest phase current should be used for input to the thermal model. The thermal element may be set with either a single time constant characteristic for the protection of cables or dry transformers, or a dual time constant characteristic to protect oil filled transformers. See the characteristic in Figure 5 bellow.

Distribution Bus Protection

The selection of protection equipment for bus faults until recently was based on the requirements for stability of the power system. Because of that the protection of buses in the case of short circuit faults at the transmission level is usually provided by high or low impedance bus differential relays. The distribution bus protection has been done by the backup time delayed overcurrent protection of the transformers.

The understanding of the effects of longer fault clearing times on sensitive industrial equipment results in a change in the philosophy on distribution bus protection. It is now based on exchange of signals between the feeder relays and the transformer protection relays.

All overcurrent starting signals from the multiple feeder relays are paralleled and used to energize an opto-input of the transformer overcurrent protection relay. For a fault on any of the distribution feeders, the relay protecting the faulted feeder will start and with or without time delay (depending on the fault location) will issue a Trip signal to clear the fault.

Figure 6 shows a simplified block diagram of a distribution bus protection. If the fault is on one of the feeders, the protective relay of that feeder will immediately operate an output that is wired to an input of the transformer relay. This signal will indicate a feeder fault and will block the operation of the bus protection function.

If the fault is on the bus, no feeder relay will operate, thus indicating to the transformer protection relay that it is a bus fault. The overcurrent elements that are used to implement a distribution bus protection scheme have to be set with a certain time delay that allows the receiving of a signal from any of the feeder relays. At the same time each feeder relay should be able to communicate the starting of an overcurrent element that is used to block the bus protection element.

The advantage of this type of scheme therefore is that it allows fast fault clearing of distribution bus faults without the need for installation of a distribution bus differential protection.