A fault in power system or equipment’s/apparatus is defined as a defect in the electrical circuit due to which current is diverted from the intended path. It is also defined as the abnormal condition which cause a reduction in the basic insulation strength between phase conductors or between phase conductors and earth or any earthed screen surrounding the conductor.
In an electrical power system comprising of generators, switchgear, transformers, power receivers and transmission and distribution circuits, sooner or later some failure will occur in the system. The probability of failure or occurrence of abnormal condition is more on power lines. As the power lines are widely branched, have greater length, operate under variable weather conditions and are subjected to various action of atmospheric disturbances.
Main Kind of Fault in Power System
Short circuit or shunt fault is the most common and dangerous fault occurs in a power system. They occur as a result of breakdowns in the insulation of current carrying phase conductors relative to earth or in the insulation between phases.
The short-circuit faults can be classified as:
|1||Single phase to ground(L-G)||Cause due to the breakdown of insulation between one of the phases and earth.|
|2||Phase to phase(L-L)||Cause due to breakdown of insulation between either of the two phases.|
|3||Two phases to ground(L-L-G)||Cause due to breakdown of insulation between two phases and earth.|
|4||Phase to phase and third phase to ground||Cause due to breakdown of insulation between two phases and simultaneous breakdown of insulation between the third phase and earth.|
|5||All the three phase to ground||Cause due to breakdown of insulation between all the phases as well as to earth.|
|6||All the 3-phases short circuited||Cause due to breakdown of insulation between all the three phases.
Majority of faults occur in the power system are unsymmetrical faults. conditions are not confined to the three phase condition. single phase to ground(L-G), Phase to phase(L-L), Two phases to ground(L-L-G) and Phase to phase and third phase to ground are of unsymmetrical nature and give rise to unsymmetrical currents. There are different current in the three phases.
The line to ground(L-G) faults occurs most commonly in overhead line. A large number of such faults are transitory in nature and vanish within a few cycles. The balanced three-phases fault is very rare in occurrence.
All the three phase to ground(L-L-L-G) and All the 3-phases short circuited are of symmetrical nature and give rise to symmetrical currents. There are currents in the three phases with 120-degree displacement. The symmetrical three-phases faults generally occur due to carelessness of operating personal.
|S.N||Cause||Percentage of Total|
|2||Sleet, wind, mechanical (jumping conductors)||20|
|4||Switching to a fault||20|
|5||(tree falling on line, birdcage, sabotage, accidents)||28|
Frequency of Fault occurrence in Different Links of a power system
|Equipment’s||Percentage of Total|
|Instrument transformers(CTs and PTs)||2|
Relative probability of occurrences of Most Frequent kinds of short-circuit Faults in overhead and cable power Transmission circuits.
|S.N||Type of Short-Circuit fault||Percentage occurrence|
|1||Single phase to ground(L-G)||70|
|2||Phase to phase(L-L)||15|
|3||Two phases to ground(L-L-G)||10|
|4||phase to phase and third phase to ground||2 or 3|
|5||All the three phases to ground(L-L-L-G0||2 or 3|
|6||All the three phases shorted||2 or 3|
Due to a short circuit in a power network causes heavy current to flow in the network. The current attains its maximum value and then decays to some steady-state value.
Sustained current: Sustained current is the fault current that will flow in the circuit if it is allowed to persist after the transient process ends. This value is used in the electrical equipment’s for designing the thermal stability.
First peak short-circuit current: This is the maximum instantaneous value of the current that will reach during the first moment of a short circuit. This value is used in electrical equipment’s for designing the dynamic stability.
Effects of Fault in power system
- The heavy currents due to short circuit cause excessive heating which may result in fire or explosion.
- Stability of the power system may be adversely affected and complete shutdown of system may occur.
- Short circuit takes the form of an arc that may cause considerable damage to the elements of the power system.
- Damage of other apparatus in the system may be caused due to overheating and abnormal mechanical forces.
- Relay having pressure coils fails due to large reduction in voltage.
- Interruption in the power supply to consumer.
- There may be a considerable reduction in voltage on healthy feeders connected to the system having fault. Considerable loss of industrial productions may result as the motor draws the high current.
Fault clearing process
The protective relays are connected in the secondary circuits of current transformers(CTs) and potential transformers(PTs). under the normal operating conditions, the voltage induced in the secondary of the CT is small and therefore the current flowing in the relay operating coil is insufficient in magnitude to close the relay contacts. Hence the trip coil of the circuit breaker de-energised.
The breaker contacts remain closed and it carries the normal load current. On the occurrence of fault, a large current flows through the primary of the CT which increases the voltage induced in the secondary and hence the current flowing through the relay operating coil. The relay contact is closed and the trip coil of the breaker gets energized to open the breaker contacts. The circuit breaker opens its contacts.
Arc is drawn between the contacts as they separate. The arc is extinguished at a natural current Zero. After final arc extinction and final current Zero, a high voltage wave appears across the circuit breaker contacts re-establishing the arc. This transient voltage wave is called transient recovery voltage(TRV).