A fault in a power system or circuit is a failure which interferes with the normal flow of current. The faults are associated with abnormal change in current, voltage and frequency of the power system. In general faults occur in power system networks due to insulation failure of equipment, flashover of lines initiated by a lightning stroke, or due to accidental faulty operation.
Need for fault calculation:
When the fault occur in a part of power system, heavy current flows in that part of circuit which may cause permanent damage to the equipments. The selection of the circuit breaker depends on the current flowing immediately after the fault occurs. The estimation these currents for various types of faults at various locations in the system are called fault calculation. The data obtained from fault calculations are also used to determine the settings of the relay which control the circuit breakers.
Types of faults
Faults can be broadly classified into
- a) Shunt faults (short circuit)
- b) Series faults (open conductors)
The shunt type of faults involves short circuit between conductor and ground or short circuit between two or more conductors. The shunt faults are characterized by increase in current and fall in voltage and frequency.
The series faults may occur with one or two broken conductors which creates open circuits. The series faults are characterized by increase in voltage and frequency and fall in current in the faulty phase.
Short circuit faults: A short circuit fault is an abnormal connection of very low impedance between two points of different potential. These are the most common and severe kind of faults, resulting in the flow of abnormal high currents through the equipment or transmission lines. Short circuit faults are also called as shunt faults.
Symmetrical faults: These are very severe faults and occur infrequently in the power systems. It affects each of the three phases equally. In transmission line faults, roughly 5% are symmetric. Like, Line to line to line to ground (L-L-L-G) , Line to line to line (L-L-L)
Unsymmetrical faults: These are very common and less severe than symmetrical faults. It does not affect each of the three phases equally. In transmission line faults, roughly 95% are unsymmetrical. Like, Line to ground (L-G),Line to line (L-L), Line to line to ground (L-L-G).
Open circuit faults: These faults occur due to the failure of one or more conductors. Open circuit faults are also called as series faults. These are unsymmetrical or unbalanced type of faults except three phase open fault. Fault like, One conductor open, Two conductor open
Transient fault: A transient fault is a fault that is no longer present if power is disconnected for a short time and then restored. Basically, transients are momentary changes in voltage or current that occur over a short period of time i.e. interval is usually described as approximately one sixteenth of a voltage cycle.
Occurrence of faults in the power systems
3-Phase fault – 5%
Double line to ground fault – 10 %
Line to line fault – 15 %
Single line to ground fault – 70 %
Fault analysis
The faults are analysed easily by making use of Thevenin’s theorem. As the readers know that this theorem can be used for determining the changes that take place in currents and voltages of a linear network when an additional impedance is added between two nodes of the network.
The theorem states that: The changes that take place in the network voltages and currents due to the addition of an impedance (a short circuit) between two network nodes are identical with those voltages and currents that would be caused by an e.m.f. placed in series with the impedance and having a magnitude and polarity equal to the pre-fault voltage that existed between the nodes in question and the impedance as seen between the nodes with all active voltage sources short circuited.
To determine the current and voltage distribution in the system, the distribution in each of the sequence networks must first be determined. The Thevenin’s equivalents of positive, negative and zero sequence networks are identical to those of a network of single generator. Consider the system in Fig. shown below for illustration of the application of Thevenin’s theorem for determining the equivalent positive, negative and zero sequence networks.
Thevenin’s equivalent of positive sequence networks is obtained from the positive sequence network. The Thevenin’s equivalent voltage source is the prefault voltage at the fault point and the equivalent impedance Z1eq is the impedance as seen between the fault point and the zero potential bus shorting the voltage sources.
It is to be noted here that positive sequence impedance of the alternator or the synchronous machine depends upon the state of the machine i.e., whether it is sub-transient, transient or steady state.
Similarly, the Thevenin’s equivalent negative and zero sequence networks are obtained from the negative and zero sequence networks respectively. Since the system is balanced, no negative or zero sequence currents are flowing before the fault occurs.
The prefault negative and zero sequence voltages at the fault point are zero. Therefore, no e.m.fs appear in the equivalent circuits. The impedances Z2eq and Z0eq are measured between the fault point and the reference bus in their respective networks.
In the positive network, the currents throughout the system due to the fault can be added to the load currents before the fault to give the total positive sequence current during the fault. The net fault current is the fault current considering the system
Frequently Asked Question (FAQ)
What is a fault in a power system?
A fault in a power system is an abnormal condition that occurs due to a deviation from normal operating conditions, resulting in a short circuit or interruption in the flow of electricity.
What are the main types of faults in power systems?
The main types of faults in power systems are:
- Short Circuit Faults
- Open Circuit Faults
- Ground Faults (Earth Faults)
- Symmetrical Faults
- Asymmetrical Faults (Unsymmetrical Faults)
What is a Short Circuit Fault?
A short circuit fault occurs when two or more conductors come into contact, creating a low-resistance path for current flow. This results in a sudden increase in current, potentially causing equipment damage or system instability.
What is an Open Circuit Fault?
An open circuit fault occurs when there is a break or discontinuity in a conductor, interrupting the flow of current. This can lead to a loss of power in downstream circuits or equipment.
What are Asymmetrical Faults?
Asymmetrical faults, also known as unsymmetrical faults, are faults that result in unbalanced currents flowing in the phases of a power system. These faults can occur due to various factors such as phase-to-phase faults, phase-to-ground faults, or unequal impedance.
How are Faults Detected in Power Systems?
Faults in power systems are detected using protective relays and sensors installed throughout the network. These devices continuously monitor electrical parameters such as current, voltage, and impedance to identify abnormal conditions indicative of a fault.
What are the Consequences of Faults in Power Systems?
Faults in power systems can lead to disruptions in electrical supply, equipment damage, safety hazards, and financial losses for utilities and consumers. Prompt detection, isolation, and restoration of service are essential to minimize the impact of faults on power system operation.
What is a Ground Fault (Earth Fault)?
A ground fault occurs when a conductor unintentionally comes into contact with the ground or a grounded object, creating an alternative path for current to flow to the earth. Ground faults can pose safety risks and may cause equipment damage.
What are Symmetrical Faults?
Symmetrical faults are faults that result in balanced currents flowing in all phases of a power system. These faults typically occur due to uniform impedance imbalances and are often easier to analyze and mitigate compared to asymmetrical faults.
How are Faults Mitigated in Power Systems?
Faults in power systems can be mitigated through various measures, including the installation of protective relays, circuit breakers, fuses, and surge arresters. Additionally, proper maintenance practices, periodic inspections, and system upgrades can help identify and address potential fault conditions before they escalate into major disruptions.
Pingback: Power line carrier communication (PLCC) in Substation