Power systems are effectively earthed by the neutral points being connected directly to earth. The only resistance being the inherent resistance of the connections and the earth itself. A system fault comprises either one or more phases being accidentally connected to earth (earth fault) or two or more phases being accidentally connected together clear of earth (phase to phase fault).
During a fault, all generators on the system deliver power into the fault and any unbalanced currents flow back to the neutral points through the earth. The amount of current flowing into the fault is determined by the impedance of the lines and the transformers between the various generators and the point of fault. The more of the system there is between the fault and the generators the lower the current.
The worst situations arise on the transmission network where parallel lines and transformers reduce the effective impedance to the generators. Transformers constitute the major impedance to fault current and, in this respect, single transformers are better than two or more transformers in parallel.
It is therefore common practice at zone and distribution substations to have only one transformer energised at any time even though there may be a second transformer available on standby. The fault level on the LV side of the transformers is therefore reduced to nearly half the fault level that would apply if both transformers were in service. In a typical kV substation the fault level can be 0 MVA or ,000 amps.
Consequences of fault current
Short circuit fault current depends on the power circuit voltage and configuration, method of neutral connections, presence of protective, and the speed of disconnection of the faulted circuit section.
The effects of fault current on an electrical installation can include:
- Large mechanical forces: Where two wires (conductors) in parallel have current flowing in the same direction, the magnetic fields attract each other. With currents flowing in opposite directions the magnetic fields repel each other. When a fault occurs a normal load current of say 00 amps could rise to 0,000 amps almost instantaneously, and similarly, the mechanical forces applied.
- System voltage being reduced to almost zero (at point of fault): This tends to bring the voltage of the system down. It is therefore essential to remove the fault of the system as quickly as possible. As the system voltage falls, total load and frequency also starts to fall.
- Total load and frequency reduced (as system voltage falls): Losing a generator means the remaining generators could have some difficulty coping with the system load until their steam plant has geared up for full output. The frequency will continue to fall as the generators struggle and there is a real danger of generator damage if the frequency falls below Hz. To cover this sort of situation automatic under frequency load shedding is installed in the zone substations. Load shedding is arranged in five stages corresponding to frequencies of 9.00 – .0 hertz in 0. or 0.0 hertz steps and at the fifth step approximately 0% of the load in that zone will have been shed.
- Overheating: A three-second short-time current rating on equipment allows for extended clearance times. This rating applies to all current-carrying equipment and frequently constitutes the most onerous thermal conditions. Care must be taken in the design of conductors to ensure heating damage cannot occur. Particular care is needed with cables as heat cannot be readily dissipated and damage to the insulation may occur.
- Explosions: Refer to the extract from EnergySafety (formerly Office of Energy) below for more information¹: