Symmetrical & Unsymmetrical Faults

Two common types of faults encountered in power systems are symmetrical and unsymmetrical, each with distinct characteristics and implications for system operation and protection. When insulation fails at any point or live wires come into contact, it causes an imbalance in the system, leading to a short circuit or fault in the line. Faults can occur for various reasons, such as natural disturbances like lightning, high-speed winds, earthquakes or insulation breakdown, falling trees, and bird shorting.

Transmission line faults are generally classified into the following categories.

  • Symmetrical faults
  • Unsymmetrical faults

Symmetrical faults

In this type of fault, all the phases are short-circuited to each other and sometimes to the earth. Due to their symmetrical nature, symmetrical faults do not cause any change in the system’s sequence of phase voltages, and as a result, they do not give rise to zero-sequence currents. The system voltage remains symmetrical with respect to its voltage and phase sequence and is displaced by an equal angle of 120°. This fault is very severe and involves the largest current, but it rarely occurs. Therefore, a balanced short-circuit calculation is performed to determine these large currents.


Unsymmetrical faults

Unsymmetrical faults involve a malfunction in only one or two phases of the electrical power system, which causes an imbalance in the three-phase lines. Such faults occur because of short circuits between line-to-ground lines. There are two types of unsymmetrical faults: series and shunt faults. An unsymmetrical series fault happens between phases or between a phase and the ground. On the other hand, an unsymmetrical shunt fault occurs due to an imbalance in line impedance. Shunt faults in a three-phase system can be classified into different types. These are as follows.

  • Single line-to-ground(LG) Fault
  • Line-to-line (LL) fault
  • Double Line-to-ground (LLG) fault
  • Three-phase short circuit (LLG) fault
  • Three-phase-to-ground (LLLG).fault

In the event of a single line-to-ground fault, one conductor makes contact with either the ground or the neutral conductor. A line-to-line fault, on the other hand, occurs when two conductors are short-circuited. If both conductors fall on the ground or come in contact with the neutral conductor, it causes a double line-to-ground fault. LG, LL, and LLG are unsymmetrical faults, whereas LLL and LLLG are symmetrical faults. To determine these large currents, balanced short-circuit calculations are performed.

Effect of faults on transmission line

Faults can damage or disrupt power systems in several ways. When a fault occurs, it can increase the voltage and current at certain points in the system, damaging the insulation and reducing the lifespan of the equipment. Also, faults can cause the system to become unstable, leading to improper operation of the three-phase system equipment. To prevent further damage, the fault section must be disconnected immediately. This will ensure that the normal operation of the rest of the system is not affected.

Related Terms:
1. Symmetrical Components
2. Zero Sequence Current
3. Sequence Impedance

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