There are a number of problems that commonly occur in industrial and commercial networks, and some of them are covered in the following paragraphs. Coordination malfunctioning is leading to nuisance tripping of circuit breakers and unwanted electrical effects.
- Earth fault protection with residually-connected CTs
- Four-Wire Dual-Fed Substations
For four-wire systems, the residual connection of three phase CTs to an earth fault relay element will offer earth fault protection, but the earth fault relay element must be set above the highest single-phase load current to avoid nuisance tripping. Harmonic currents (which may sum in the neutral conductor) may also result in spurious tripping.
The earth fault relay element will also respond to a phase-neutral fault for the phase that is not covered by an overcurrent element where only two overcurrent elements are applied.
In this case, overcurrent protection must be applied to all three phases to ensure that all phase-neutral faults will be detected by overcurrent protection.
Placing a CT in the neutral earthing connection to drive an earth fault relay provides earth fault protection at the source of supply for a 4-wire system. If the neutral CT is omitted, neutral current is seen by the relay as earth fault current and the relay setting would have to be increased to prevent tripping under normal load conditions.
When an earth fault relay is driven from residually connected CTs, the relay current and time settings must be such that that the protection will be stable during the passage of transient CT spill current through the relay.
Such spill current can flow in the event of transient, asymmetric CT saturation during the passage of offset fault current, inrush current or motor starting current.
Energizing a relay from a core-balance type CT (CBCT) generally enables more sensitive settings to be obtained without the risk of nuisance tripping with residually connected phase CTs. When this method is applied to a four-wire system, it is essential that both the phase and neutral conductors are passed through the core-balance CT aperture.
For a 3-wire system, care must be taken with the arrangement of the cable sheath, otherwise cable faults involving the sheath may not result in relay operation (Figure 1 above).
The coordination of earth fault relays protecting four-wire systems requires special consideration in the case of low voltage, dual-fed installations. Horcher (Overcurrent Relay Coordination for Double Ended Substations. George R Horcher. IEEE. Vol. 1A-14 No.6 1978.) has suggested various methods of achieving optimum coordination.
Problems in achieving optimum protection for common configurations are described below.
When both neutrals are earthed at the transformers and all circuit breakers are of the 3-pole type, the neutral busbar in the switchgear creates a double neutral to earth connection, as shown in Figure 2.
In the event of an uncleared feeder earth fault or busbar earth fault, with both the incoming supply breakers closed and the bus section breaker open, the earth fault current will divide between the two earth connections.
Earth fault relay RE2 may operate, tripping the supply to the healthy section of the switchboard as well as relay RE1 tripping the supply to the faulted section.
If only one incoming supply breaker is closed, the earth fault relay on the energized side will see only a proportion of the fault current flowing in the neutral busbar.
This not only significantly increases the relay operating time but also reduces its sensitivity to low-level earth faults.
A configuration sometimes adopted with four-wire dual-fed substations where only a 3-pole bus section CB is used is to use a single earth electrode connected to the mid-point of the neutral busbar in the switchgear, as shown in Figure 3.
The coordination time between the earth fault relays RF and RE should be established at fault level F2 for a substation with both incoming supply breakers and bus section breaker closed.
When the substation is operated with the bus section switch closed and either one or both of the incoming supply breakers closed, it is possible for unbalanced neutral busbar load current caused by single phase loading to operate relay RS1 and/or RS2 and inadvertently trip the incoming breaker.
Interlocking the trip circuit of each RS relay with normally closed auxiliary contacts on the bus section breaker can prevent this.
However, should an earth fault occur on one side of the busbar when relays RS are already operated, it is possible for a contact race to occur. When the bus section breaker opens, its break contact may close before the RS relay trip contact on the healthy side can open (reset).
If, during a busbar earth fault or uncleared feeder earth fault, the bus section breaker fails to open when required, the interlocking break auxiliary contact will also be inoperative. This will prevent relays RS1 and RS2 from operating and providing back-up protection, with the result that the fault must be cleared eventually by slower phase overcurrent relays.
An alternative method of obtaining back-up protection could be to connect a second relay RE, in series with relay RE, having an operation time set longer than that of relays RS1 and RS2.
But since the additional relay must be arranged to trip both of the incoming supply breakers, back-up protection would be obtained but busbar selectivity would be lost.
An example of protection of a typical dual-fed switchboard is given in this earlier published technical article.
Reference // Network protection & automation guide by Alstom Grid