Faults in electrical power system are the defect in electrical equipment/apparatus. Due to this current is diverted from the intended path.
The fault is an abnormal condition that results in the reduction of the basic insulation strength between phase conductors, or between the phase conductor and earth. The reduction of insulation strength alone is not considered a fault until it affects the system. The reduction of insulation strength if results either in the excessive flow of current (short circuit current) or in reduction of the impedance between conductors or between conductor and earth to a value below that of the normal load impedance of the circuit.
Electrical power system compromises of the generator, switchgear, transformer, power receivers, transmission and distribution circuits, and various electrical equipment. It is uncertain that they will be working under normal conditions of operating continuously. Sometimes, sooner or later the same failure may be encountered by the system. The probability of occurrence of a fault is more on the power lines. As power lines are widely branched with greater length, they operate under variable weather conditions and are prone to atmospheric disturbances.
The cause of failure of the power lines are mentioned below;
- At normal voltage, breakdown may occur due to deterioration or aging of insulation. Also by damage is caused by unpredictable happenings such as falling of trees across lines, blowing of heavy winds, birds shorting out the lines, vehicles colliding with poles or towers, etc.
- Due to abnormal voltage, breakdown may occur by switching surges or lightning strokes which may be either direct or induced.
Statistics of Faults in Electrical Power System
The table below shows the fault statistics. Table 1 gives the idea of the percentage of various causes of faults and frequency of fault occurrence in the various links of the power system.
|Causes||Percentage of Total|
|2. Sleet, wind, mechanical (jumping conductors)||20|
|3. Apparatus failure||20|
|4. Switching to a fault||20|
|5. Miscellaneous (tree falling on a line, accident, etc.)||28|
Table 1: Percentage of various causes of faults
Table 2 gives the idea of the percentage of frequency of fault occurrence in various links of the power systems.
|Equipment||Percentage of Total|
|Instrument Transformer (CTs and PTs)||2|
Table 2: Frequency of fault occurrence in various links of power system
Types of faults in electrical power system
The most common fault occurring in a power system is the short circuit fault. Short circuit fault is the result of the breakdown of insulation of current-carrying phase conductors relative to earth or breakdown of insulation between phase conductors.
Table 3 shows the classification of fault in a 3-phase ac power circuit along with the relative probability of occurrence of the fault in overhead and cable power transmission circuits.
|S.No.||Types of Short-Circuit Fault||Representation||Remarks||Percentage of Occurrence|
|1||Single-phase to the ground (L-G)||Due to the breakdown of insulation between one of the phases and the earth.||70|
|2||Phase to phase (L-L)||Due to the breakdown of insulation between either of the two phases.||15|
|3||Two phases to the ground (L-L-G)||Due to the breakdown of insulation between two phases and earth.||10|
|4||Phase to phase and third phase to ground||Combination of faults given at 1 and 2.||2 or 3|
|5||All the three phases to the ground (L-L-L-G)||Due to the breakdown of insulation between all the three phases as well as to the earth.||2 or 3|
|6||All three phases shorted (L-L-L)||Due to the breakdown of insulation between all three phases.||2 or 3|
The probability of occurrence of line to ground (L-G) fault is more in overhead lines. A large number of such faults are temporary and may vanish within a few cycles. Such temporary faults can be a small branch of tree falling across a line and cross an arm and burning itself or falls. Balanced three-phase faults are the rarest with the probability of occurrence accounting for about 5% of the total. But it is the severest of all types of fault.
Symmetrical and Unsymmetrical Fault
The first four faults mentioned in table 4 (L-G, L-L, L-L-G, phase to phase, and third phase to ground) are unsymmetrical. Unsymmetrical faults give rise to unsymmetrical currents which mean that different fault currents in phases.
The last two faults mentioned in table 4 (L-L-L-G, L-L-L) are symmetrical. Symmetrical fault gives rise to symmetrical currents which means that equal fault currents in all three phases with a displacement of 1200.
Fault Clearing Process
On the secondary of the current transformer (CT) or potential transformer (PT), protective relays are connected. Under normal operating conditions, the current through the secondary of the CT is insufficient for the relay contacts to close and the trip coil remains de-energized. The protective relay under abnormal fault conditions will complete the trip coil circuit. As the fault will result in the inflow of heavy short–circuit current which in turn induces sufficient current in the secondary of CT for relay contacts to come into action. The relay contacts become closed and the trip coil of the breaker gets energized opening the contact of the circuit breaker.
Arc is drawn between the contacts of the breaker as they separate. This arc is extinguished at the natural zero of the ac wave by suitable technique and medium. When the final arc is extinguished and the final current is zero. A high voltage wave appears across the breaker contacts which may re-establish this arc. This transient high voltage wave is termed transient recovery voltage (TRV).
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