Causes, Nature and Effect of Faults in Power System

Causes, Nature and Effect of Faults in Power System

A fault occurs due to the failure of the insulation of a conductor at one or more point or a conducting object comes in contact with the live part of the system. The transmission line is most prone to faults. If the conductors of line come in contact with each other or with the ground.

Causes of Faults

There is a number of causes for the occurrence of a fault in the power system. Some of the possible causes of faults are,

  • Overvoltage due to switching surges
  • Severe lightning strokes
  • Aging of conductor
  • Heavy wind, rains, and snowfall
  • Falling trees on the transmission line
  • Excessive internal and external stresses on the conductors
  • High changes in atmospheric temperatures
  • Accident of vehicle with towers or poles of transmission line
  • Perching of birds on the lines
  • Accidental short circuit due to string, snakes
  • Chemical pollution

There is always the possibility of occurrence of a fault in the power system. Therefore, it is necessary to protect the costly equipment.

Most of the area of power system network is covered by the transmission lines and it placed in the open air. Hence, due to atmospheric conditions, the maximum possibility of fault occurrence is on transmission lines.

Most of the faults lead to short circuit condition and because of this, the impedance of the short circuit path becomes very low and very large fault currents flow.

Thus, it is important to evaluate these fault currents under short circuit conditions. And according to this, design and select the protective equipment.

The following table gives us information about the faults distribution in various parts of the system.

Sr. No.

Equipment

% of the total fault

1

Overhead Lines

50

2

Switchgear

15

3

Transformer

12

4

Cables

10

5

Miscellaneous

8

6

Control Equipment

3

7

CT and PT

2

Nature of Faults

The faults are classified as,

  • Symmetric Fault
  • Unsymmetrical Fault

1) Symmetrical Fault

When all three conductors of the three-phase system are brought together into a short circuit, the fault is a symmetrical fault.

It means in symmetrical fault; all three conductors are affected by the fault. This gives rise to heavy fault current in all the lines.

If there is a breakdown in insulation between all conductors, this type of fault will occur. It is also known as LLL fault. The fault current will remain symmetrical.

LLL and LLLG faults are included in the symmetrical fault. These faults are rarely occurring in practice but it is very severe in nature.

2) Unsymmetrical Fault

The fault which gives unsymmetrical current in the three lines and makes unequal displacement of current is known as an unsymmetrical fault.

There are various types of unsymmetrical faults that occur in the power system network. These are categorized below.

Single-Line to Ground Fault

This type of fault occurs when one phase is short-circuited with the ground. This is the most commonly occurring fault.

Double-line to Ground Fault

This type of fault occurs when two phases are short-circuited with the ground. This fault occurs due to insulation breakdown between phases and ground.

Line to Line Fault

In this fault, two conductors are short-circuited with each other.

Open circuit fault

Due to an accident, there is an open circuit in the lines due to the breaking of lines. This fault creates an unbalanced current to flow in the system.

The below table shows the percentage occurrence of different types of faults.

Sr. No.

Type of Fault

% occurrence

1

Line to Ground (L-G)

85

2

Double Line to Ground (L-L-G)

5

3

Line to Line (L-L)

8

4

L-L-L or L-L-L-G

>2

According to nature, the faults are classified as,

  • Transient Fault
  • Permanent Fault.

Transient Fault

Most of the faults are transient in nature. These faults remain for a short duration of time.

For example, if a twig falls across a line and across the arm and burns itself or falls down, then the fault is transient. It will vanish after a few cycles.

In this type of fault, the fault current is high for 3-4 cycles but it decreases rapidly in the next cycles. This zone in which the current is very high but decreases very rapidly is known as ‘sun-transient.’

After that, for a few cycles, the fault current decreases slowly, this region is known as ‘transient’. This state remains for a few cycles.

After the transient state is over, a steady-state is reached. The RMS value of short-circuit current remains constant during the steady-state. The circuit breaker operates during a transient state.

Permanent Fault

In this type of fault, the fault is permanent. For example, the insulation of two conductors are failed and due to this short-circuit occurs in the system.

These faults will not vanish itself. This fault needs to do some maintenance and required more time to solve the fault. This type of fault occurs rarely.

Effect of Faults

The various effect of faults in the power system are,

  • The fault set up very heavy fault current which is very large in magnitude. The fault current can damage the equipment used in the power system network.
  • A high amount of fault current produces heat which causes the overheating and mechanical stress in the conductors.
  • There is always a danger of fire due to arcing caused by heavy currents. If the fault persists for a long time then the fire may spread to other parts of the system.
  • The overheating causes weakening of the insulation of conductors which reduces the life of insulation.
  • The unbalanced current and voltage can cause the heating of rotating machines connected to the system.
  • The generators are interconnected. It must be operated in synchronism. Due to unbalance in current and voltages, generators may lose their synchronism. Which causes the total shutdown of the system and in the worst condition it will lead to the blackout. Thus, the fault affects system stability.
  • There may be an interruption in the supply to the consumers thus reducing the reliability of the system.

Hence, it is necessary to isolate the faulty part as quickly as possible from a healthy system using reliable protecting devices.

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