Usually, a power system operates under balanced conditions with all equipment's carrying normal load currents and also the bus voltages inside the prescribed limits. This condition can be disrupted because of fault within the system. If the electrical fault current exceeds the interrupting rating of the protective device, the consequences can be devastating. It can be a serious threat to human life and is capable of causing injury, extensive equipment damage.
Short circuit fault current is many times larger than the normal current. A short circuit is simply a low resistance connection between the two conductors supplying electrical power to any circuit. This results in excessive amount of current flow in the power systems through the path of low resistance and may even cause the power source to be destroyed and causes more heat and fires.
The fault analysis of a power system is needed in order to provide information for the choice of switch-gear, size of conductors, setting of relays, finding the rating requirements of other power equipment and confirming system stability. All the equipment must be chosen to work with the fault current that sometimes flows in great quantity. At AllumiaX, our goal is to interrupt the fault current as early as possible with minimum disruption to upstream devices by achieving good coordination between protective devices and precise trip settings.
The faults in the power system are mainly categorized into two types:
The open circuit fault happens due to the failure of one or two conductors. These faults take place in series with the line so referred as series fault. Such types of faults have a strong impact on the reliability of the system. The open circuit fault is classified as:
The short-circuit fault is commonly divided into symmetrical and asymmetrical types. These faults are further categorized as one of five types. In order of frequency of occurrence, they are:
Asymmetrical faults mandate the calculation of positive negative and zero sequence components separately.
Single Line to Ground Fault: This type of fault occurs when you have one of the phases (A, B or C) is shorted with the ground.
Line to Line Fault: This type of fault occurs when you have one of the phases (A, B or C) is shorted with the ground.
Double Line to Ground Fault: This type of fault occurs when two phases are shorted with the ground together (A-B-G, B-C-G or C-A-G)
Symmetrical faults do not give rise to zero sequence or negative sequence components because they are perfectly balanced, symmetrical faults only have positive sequence values.
Three Phase Line to Ground Fault: The 3-phase to ground faults are faults in where all the phases (A, B and C) are shorted together and they are grounded.
Three Phase Line to Line Fault: The three phase faults occur when you have A, B and C phases are shorted together but ground is not involved.
|Type of Faults
Symmetrical or Unsymmetrical
|Probability of Occurrence
|Three phase line to ground fault
|2 - 3%
|Three phase line to line fault
|Single line to ground fault
|70 - 80%
|Line to line fault
|15 - 20%
|Double line to Ground fault
Hello there! On a related topic, we previously wrote a blog about Symmetrical Components. If this peaks your interest, check it out and let us know what you think
Here, we will outline 7 steps required in order to perform a fault analysis in a power system for a given fault.
1. Converting the System to Base Values: Convert the system to the per-unit system, based on the same base value.
2. Type of Fault: Identify the type of fault that is being analyzed.
3. Constructing Sequence Networks: Draw the sequence networks for each of the positive negative and zero sequence networks from the system that was converted in step 1.
4. Make a Faulted Sequence Network Diagram: Take the un-faulted sequence networks and modify and interconnect them according to the type of fault to make a faulted sequence network diagram.
5. Hand Calculation: Hand calculate the sequence currents and voltage quantities during a faulted condition.
6. Convert per unit value into three phase current and voltage quantities: Convert the per unit values that we calculated in step 5 into three phase current and voltage quantities that can be actually used and understood intuitively.
7. Fault calculation on LV side of the transformer: Calculating the fault that occurs on the low voltage side of the transformer.
The weather factors that typically cause power system faults are: lightning strikes, accumulation of snow on transmission lines, heavy rains, high speed winds, earthquake, salt pollution depositing on overhead lines and conductors, floods and fires adjacent to electrical instrument, etc. These environmental conditions interrupt the power supply and also damage electrical installations.
Electrical equipment's like machines, motors, generators, transformers, cables, reactors, switch devices, etc. causes electrical faults. These faults may be caused due to malfunctioning, ageing and degradation, insulation failure of cables and winding, breakdown due to high switching. These failures lead to high current to flow through the devices or equipment which further damages it.
Electrical faults are also caused due to human errors like choosing improper rating of equipment or devices, forgetting metallic or electrical conducting components once coupling or maintenance, switching the circuit while its below servicing, etc. A classic example is one wherever maintenance staffs unwittingly leave isolated instrument connected through safety earth clamps once maintenance work is completed. A three-phase to earth short-circuit fault occurs when the equipment is re energized to return it to service.
The smoke of fires under overhead lines consists of tiny particles results in spark between the lines or between conductors to insulator. This arc causes insulators to lose their insulting capability because of high voltages. The hot air in the flames of a fire has a much lower insulation strength than air at close temperature.
One of AllumiaX's recent initiatives is a corporate sponsorship for the GeneralPAC platform which provides tutorials for power systems protection, automation and controls. Here, you will find the video series of fault analysis in power system. In this series they will be going over the analysis of various types of faults that occur in power systems and at the same time intuitively understanding the hand calculations involved.