Types of Faults
– Transient fault: No longer present if power is disconnected and restored
– Examples: momentary tree contact, bird or animal contact, lightning strike, conductor clashing
– Automatic re-close function used to restore power in overhead lines
– Transient faults may cause damage at the original fault site or elsewhere in the network
– Persistent fault: Present regardless of power being applied
– Common in underground power cables due to mechanical damage or lightning
– Asymmetric fault: Does not affect each phase equally
– Types: line-to-line fault, line-to-ground fault, double line-to-ground fault
– Line-to-ground fault is the most common type in transmission line faults
– Symmetric fault: Affects each phase equally
– Rare compared to asymmetric faults
– Types: line to line to line (L-L-L), line to line to line to ground (L-L-L-G)
– Symmetric faults account for a small percentage of all system faults but can cause severe damage
– Bolted fault: Fault with zero impedance, resulting in maximum prospective short-circuit current
– Unusual in well-designed power systems but can occur by mischance
– Deliberately introduced in some transmission line protection systems to speed up device operation
Ground Fault (Earth Fault)
– Failure that connects power circuit conductors with the earth
– Can cause objectionable circulating currents or energize equipment at dangerous voltage
– Some power distribution systems tolerate a single ground fault and continue operation
– Wiring codes may require insulation monitoring device for alarm and identification of ground fault cause
– Second ground fault can result in overcurrent or component failure, requiring Ground Fault Interrupter
Realistic Faults
– Fault resistance can vary from close to zero to fairly high relative to load resistance
– Power consumed in fault can be significant compared to zero-impedance case
– Arcs are highly non-linear, so simple resistance model is inadequate
– All possible cases need to be considered for accurate analysis
Arcing Fault
– Electric arc may form between power system conductors and ground
– Arc can have high impedance and be difficult to detect with simple overcurrent protection
– Arcs can cause damage before becoming a complete short circuit
– Additional protection devices used in utility, industrial, and commercial power systems
– Residential wiring regulations may require arc-fault circuit interrupters to detect small arcs
Analysis
– Symmetric faults can be analyzed using power flow study or superposition method
– Simplifying assumptions are made, assuming generators are in phase and operating at nominal voltage
– Fault location is supplied with a negative voltage source while other sources are set to zero
– Separate calculations should be performed for subtransient, transient, and steady-state time ranges Source: https://en.wikipedia.org/wiki/Electrical_fault
In an electric power system, a fault or fault current is any abnormal electric current. For example, a short circuit is a fault in which a live wire touches a neutral or ground wire. An open-circuit fault occurs if a circuit is interrupted by a failure of a current-carrying wire (phase or neutral) or a blown fuse or circuit breaker. In three-phase systems, a fault may involve one or more phases and ground, or may occur only between phases. In a "ground fault" or "earth fault", current flows into the earth. The prospective short-circuit current of a predictable fault can be calculated for most situations. In power systems, protective devices can detect fault conditions and operate circuit breakers and other devices to limit the loss of service due to a failure.
In a polyphase system, a fault may affect all phases equally, which is a "symmetric fault". If only some phases are affected, the resulting "asymmetric fault" becomes more complicated to analyse. The analysis of these types of faults is often simplified by using methods such as symmetrical components.
The design of systems to detect and interrupt power system faults is the main objective of power-system protection.
