The electric energy or power is transmitted and distributed from various generating station to load end. There are various fault which may occur in power system during transmission and distribution of electrical energy.
Under Normal Condition, the electrical circuit or device operates at normal voltage and current, once the fault take place the voltage and current deviates (change) from their normal value.
Therefore the state of power system when current and voltage are other than their normal value is known as ‘electric fault ‘.
In simple words, fault is defined as abnormal conditions of power system. Fault is said to be a occur in power system when its voltage and current are different from rated value.
Types of faults
There are various fault which may occur in overhead power system underground cable. Before studies these faults you most familiar with short circuit, open circuit and overload.
When two conductors of different potentials come directly in contact with each other at any point, the voltage at that point reduced to zero and very high current flow in a circuit. This abnormal condition is said to be short circuited.
When the wire of a closed circuit is cut or disconnect by switch so there will be no current flow in the circuit but only voltage is present, the circuit is said to be open circuit.
When load is connected to power system is greater than designed value of system, the current flow in the system is greater than its rated value. This condition is called overload.
Short circuit, open circuit and overload give rise to fault in system respectively known as short circuit fault, open circuit fault and overload fault. Open circuit fault is the most common fault in overhead system.
Types of fault in overhead system
There are mainly two types of faults in 3-phase overhead system. Those are symmetrical faults and unsymmetrical faults.
(1) Symmetrical faults
When a symmetrical fault occur, it effect each of the three phase equally. In other words, when a symmetrical fault occur, the fault current flow through all the three lines are equal in magnitude. These faults are also known as balanced faults.
In overhead system symmetrical faults are
1. All three phase short circuited (L-L-L): This fault occur in over head line when all the three phase get short circuited.
- All three phase to earth (L-L-L-E):This fault occur when all the three phase break and fall down on the ground.
The symmetrical faults are really occur, only 2 to 5% of system fault are symmetrical fault. When these fault occur, the system remain balanced but it may cause serious damage in power system.
(2) Unsymmetrical faults
An unsymmetrical faults does not affect each of 3-phase equally. When these fault occurs, the fault current in all lines are unequal. These faults are also known as unbalanced faults.
In overhead lines unsymmetrical faults are
1. Single phase to earth (L-E): This fault occur when a line conductor break and fall on the ground.
- Phase to phase fault (L-L): This may occur when one of line conductor break and fall on the other line conductor.
- Two places to earth (L-L-E): This fault occurs when two conductor of break and fall on ground.
- Phase to phase and third phase to ground:This fault occurs when one phase break and fall on ground and other two phase gets short circuited.
Single phase to earth (L-E) is the most common fault occur in overhead lines. Unsymmetrical faults may cause heavy damaged in power system.
Types of fault in underground system
The electrical energy is also transmitted by underground cables. The main fault which may occur in underground cables are
- Ground or Earth faults:
This fault occur when the insulation of underground cables get damaged and current start flowing from cable conductor to earth.
- Short circuit faults:
When the insulation of two underground cables get damaged and two cores (conductors) come directly in contact with each other, current start flowing from one core to other instead of flowing through the load. Such faults are known as short circuited faults.
- Open circuit faults:
When the cable connectors break, no current flow through the cables. Such faults are known as open circuit faults.
Cause of electric faults
There are a number of reason which are responsible for electric fault in power system.
- Apparatus Failures: It includes the failure of transmission line, generator, motors, transformer, switching devices due to
– insulation failure
- Weather Condition:
– lighting strikes
– high speed winds
– heavy rains
– snow and ice etc.
All these are responsible for heavy faults.
- Human Error:Human error such as
– improper rating of equipment
– improper maintenance etc
may also give rise to electric faults.
- Miscellaneous:Electric faults may also occurs due to
– tree falling on line
– ionization of air
– birds shorting the lines etc.
These interrupt the power supply and also damage electrical system.
Effect of electric faults
- Large Current: When a fault occurs a large current flow in the system which may damage the equipment.
- Reduce Reliability: When a faults occur, it may interrupt the continuity of supply and reduce the reliability of the system. It causes the system to become unstable.
- Overheat: Faults can cause large current which overheat the system equipment and cause serious damage.
- Unbalanced system: When a fault occur, it may unbalance 3-phase system this and this affects the operation of three phase equipment.
- Fire: Faults can cause fire in power system and domestic which give rise to serious damage.
Purpose of fault analysis
The purpose of fault analysis is to determine the voltage and current in power system during the faults so that we can select the proper rating of protection device like circuit breaker, relay, etc for the protection of power system.