What is a fault?

In an electrical engineering system, a fault is an abnormal electric current flowing through the system, and fault analysis is the study of these faults. For instance, a short-circuit is a fault where the live wire touches the ground or neutral wire. If a circuit is interrupted by a failure of a current-carrying wire then an open circuit fault occurs. The open-circuit faults are also called series faults. One or more phases or ground may be involved in a three-phase fault or may occur only between two phases. Current flows into the earth in case of a ground fault or earth fault. For most situations, the prospective short circuit current of a fault that is predictable can be calculated. To limit the loss of service because of a failure, the protective devices can detect fault conditions and operate circuit breakers and also other devices. All phases equally can be affected in a poly-phase system which is known as symmetric electrical fault. The asymmetric fault becomes more complicated to analyze if only some phases are affected. By using methods such as symmetrical components, the power system analysis of these types of faults is often simplified. The main objective of power system protection is to design, detect, and interrupt power system faults.

Transient fault

If power is disconnected for a short time and then restored and the fault is no longer present, it is called a transient fault. It is also a type of insulation fault that affects the device temporarily and the dielectric properties are restored after a short interval of time. In overhead power lines, many faults are transient. When a fault occurs in the equipment which is used for power system operation, it operates to isolate the area of the fault. The power line can be returned to service after the transient fault gets cleared. Some of the examples of transient faults are momentary tree contact, bird or other animal contacts, lightning strike, or conductor clashing.

Persistent fault

Regardless of power being applied, a persistent fault is present. Due to mechanical damage to the cable, the faults in underground power cables are most often persistent but are sometimes transient due to lightning.

Types of fault

Asymmetric fault

Each of the phases is not affected equally in case of asymmetric or unbalanced fault. The types and causes of asymmetric faults are mentioned below:

• Line-to-line fault: It is a type of fault that occurs when there is a short circuit between lines that is caused by the ionization of air or when the lines come in physical contact. For instance, due to broken insulators in transmission line, roughly 5% - 10% faults are asymmetric line-to-line faults.
• Line-to-ground fault: This type of fault occurs when a short circuit between one line and ground is very often caused by physical contact. For instance, due to lightning or often storm damage. The asymmetric line-to-ground faults in transmission line faults are roughly 65% to 70%.
• Double line-to-ground fault: This type of fault occurs when the two lines come in contact with the ground also commonly occurs due to storm damage. The double line-to-ground faults in the transmission line are roughly 15% to 20%.

Symmetric fault

In fault engineering, each of the phases is affected equally in the case of a symmetric fault. Roughly 5% of faults in transmission lines are symmetric faults. Compared to asymmetric faults, these faults are rare in nature. Two types of symmetric electric faults are line to line to line (L-L-L) and line to line to line to ground (L-L-L-G). Of all system faults, the symmetric faults account only for 2% to 5%. Even though the system remains balanced, it can cause severe damage to equipment.

Fault analysis

Via the same methods, the symmetrical faults can be analyzed as any other phenomenon in power system fault analysis. There are also many softwares that exist to accomplish these types of fault analysis modules automatically. There is another method that is more accurate and is more instructive in fault engineering. Some simplifying assumptions are made at first. It is assumed that all the electrical generators are in phase and are operating at a voltage called nominal voltage of the system. The electric motors can be considered as a generator because when a fault occurs, they supply power instead of drawing power. For this base case, the voltages and currents are then calculated. Then the fault location is considered to be supplied with a negative voltage source which is equal to the voltage at that location while all other sources are set to zero. The principle of superposition is used in this method. These calculations should be done separately for three separate time ranges to obtain accurate results.

1. The sub-transient is the first and this is associated with the largest currents.
2. Transient comes between sub-transient and steady-state.
3. The steady-state occurs after the settle down has happened by all the transients.

The underlying assumptions in the fault analysis study are broken by the asymmetric fault used in three-phase power namely that the load is balanced in all three phases. Where only one phase is considered, there it is impossible to directly use tools such as the one-line diagram. It is usual to consider the resulting voltages and currents as a superposition of symmetrical components due to the linearity of power systems to which three-phase analysis can be done. The power system is seen as a superposition of three types of components in the method of symmetric components which are :

1. A positive sequence component: Here, the phases are in the same order as the original system that is abc.
2. A negative sequence component: Here, the phases are in the opposite order as the original system that is acb.
3. A zero sequence component: Here, all the three phases are in phase with one other.

One must know the per-unit zero-, positive- and negative-sequence impedance’s of the transmission lines, generators, and transformers involved to determine the currents resulting from a symmetric fault. Using these impedances, three separate circuits are obtained. Depending upon the type of fault the circuits are connected in a particular arrangement. The network can be now analyzed using classical circuit analysis techniques once the sequence circuits are properly connected. The solution results in currents and voltages that exist as symmetrical components.

Context and Applications

This topic is significant in the professional exam for graduate and postgraduate courses.

• Bachelors in Electrical Engineering
• Masters in Electrical Engineering

Practice Problems

Q1. What is the other name of open circuit faults?

1. Series faults
2. Shunt fault
3. Parallel fault
4. None of these

Answer: Option a

Explanation: If a circuit is interrupted by a failure of a current-carrying wire then an open-circuit fault occurs. The open-circuit faults are also called series faults.

Q2. How does the transient fault affect the device?

1. Permanently
2. Temporarily
3. For an instance
4. None of these

Answer: Option b

Explanation: If power is disconnected for a short time and then restored and the fault is no longer present is called a transient fault. It is also a type of insulation fault that affects the device temporarily and the dielectric properties are restored after a short interval of time.

Q3. How many types of asymmetric faults are there?

1. 2 types
2. 3 types
3. 4 types
4. 5 types

Answer: Option b

Explanation: There are three types of asymmetric faults which are LL, LG, and LLG.

Q4. How many percent of total system faults are symmetrical faults?

1. 1 %
2. 3%
3. 5%
4. 25%

Answer: Option c

Explanation: Roughly 5% of faults in transmission lines are symmetric faults.

Q5. How many types of sequence components are there in fault study?

1. 2 components
2. 3 components
3. 4 components
4. 5 components

Answer: Option b

Explanation: There are three sequence components which are positive, negative, and zero sequence.

Related Concepts

• Electrical safety
• Fault technology