Electrical Faults are basically abnormalities of parameters either voltage or current or both in a stable system. Faults are generally caused by equipment failure, human error, and environmental conditions. Electrical faults can cause significant economic damage and have the potential to cause fire hazards risking human life.
Table of Contents
CLASSIFICATION OF ELECTRICAL FAULTS
Broadly electrical faults can be classified as i) Shunt Faults, ii) Series Faults
Shunt Electrical Faults (Short Circuits)
When the path of the load current is cut short because of the breakdown of insulation, we say that a ‘short circuit’ has occurred. The insulation can break down for a variety of reasons.
At times the short circuit may be total (sometimes called a dead short circuit), or it may be a partial short circuit. An electrical fault that bypasses the entire load current through itself, is called a metallic fault. A metallic fault presents a very low, practically zero, fault resistance. A partial short circuit can be modeled as a non-zero resistance (or impedance) in parallel with the intended path of the current.
Most of the time, the fault resistance is nothing but the resistance of the arc that is formed because of the flashover. The arc resistance is highly nonlinear in nature. Early researchers have developed models of arc resistance. One such widely used model is due to Warrington, which gives the arc resistance as:
R = 8750 ( S + 3 u t ) / I1.4
Where S is the spacing in feet, u is the velocity of air in mph, t is the time in seconds, and I is the fault current in amperes.
Shunt Faults may be further subdivided into
i) Symmetrical Faults and ii) Unsymmetrical Faults
i) Symmetrical Faults:
These are the faults in which all three phases are involved. It means the system remains balanced during these faults. There are no zero or negative sequence currents
The line-Line-Line fault is a symmetrical fault in which three phases are physically joined with zero impedance.
Line-line-line-ground is another type of symmetrical fault in a power system.
ii) Unsymmetrical Faults:
These types of faults cause an imbalance in the system. It creates a different value of current in different phases of a three-phase system. There are three types:
i) Line-Ground ii) Line-Line iii) Line-Line-Ground
Shunt faults basically occur due to the failure of insulation. The insulation may fail because of its own weakening, or it may fail due to overvoltage. The weakening of insulation may be due to one or more of the following factors:
- Ageing
- Temperature
- Rain, hail, snow
- Chemical pollution
- Foreign objects
- Other causes
Faults, thus, cause heavy currents to flow. If these fault currents persist even for a short time, they will cause extensive damage to the equipment that carries these currents. Over-currents, in general, cause overheating and attendant danger of fire.
Overheating also causes deterioration of the insulation, thus weakening it further. Not so apparent is the mechanical damage due to excessive mechanical forces developed during an over-current. Transformers are known to have suffered mechanical damage to their windings, due to faults. This is due to the fact that any two current-carrying conductors experience a force. This force goes out of bounds during faults, causing mechanical distortion and damage.
Those faults, which involve only one of the phase conductors and ground, are called ground faults. Faults involving two or more phase conductors, with or without ground, are called phase faults.
Power systems have been in operation for over a hundred years now. Accumulated experience shows that all faults are not equally likely. Single line-to-ground faults (L-G) are the most likely electrical faults whereas the fault due to a simultaneous short circuit between all three lines, known as the three-phase fault (L-L-L), is the least likely.
Fault | Probability | Severity |
L-G | 85% | Least severe |
L-L | 8% |
|
L-L-G | 5% |
|
L-L-L, L-L-L-G | 2% | Most severe |
Total | 100% |
|
Further, the probability of faults on different elements of the power system is different. The transmission lines that are exposed to the vagaries of the atmosphere are the most likely to be subjected to faults. Indoor equipment is least likely to be subjected to faults.
Power system element | Probability of faults |
Overhead lines | 50 |
Underground cables | 9 |
Transformers | 10 |
Generators | 7 |
Switchgears | 12 |
CT, PT relays, control equipment, etc. | 12 |
Total | 100% |
Series Electrical Faults
Series faults are a break in the path of current. Normally such faults do not result in catastrophes except when the broken conductor touches another conductor or some grounded part. It is observed in practice that most of the open conductor faults sooner or later develop into some or the other short circuit faults. However, there are some instances where an open circuit can have dangerous consequences For example, the secondary circuit of a current transformer and the field circuit of a DC machine if open-circuited, can have dangerous consequences.
The boundary between the normal and faulty conditions is not crisp. There are certain operating conditions inherent to the operation of the power system that are definitely not normal, but these are not electrical faults either. Some examples are the magnetizing inrush current of a transformer, the starting current of an induction motor, and the conditions during power swing.
BOUNDARY CONDITIONS FOR DIFFERENT TYPES OF ELECTRICAL FAULTS
For three-phase fault
A three-phase fault occurs when all three phases are shorted physically. The boundary conditions for three-phase faults are VA = VB = VC and IA + IB + IC = 0.
Single line to ground fault
A single line to ground fault occurs when a phase is shorted with the earth. Assuming the fault impedance to be ZF the boundary condition for an L-G fault considering the shorted phase as ‘A’, VA= IA ZF, IB = 0, IC = 0, The fault current IF = IA = Ia0+Ia1+Ia2=3Ia1, where Ia0, Ia1, Ia2 are the zero sequence, positive sequence, and negative sequence currents respectively
Line to Line faults
A line-to-line fault occurs when two phases are shorted. Assuming the phases being B and C phase and fault impedance to be ZF the boundary condition for an L-L fault is, VB= VC +IB ZF, IB = 0, IA = 0, IB+IC = 0 or IB = -IC The fault current IF = IB = -j√3Ia1, where Ia0, Ia1, Ia2 are the zero sequence, positive sequence, and negative sequence currents respectively.
Double Line to Ground Faults
Double line to ground faults occurs when two phases are shorted with the earth. Assuming the shorted phases are B and C, the boundary conditions become IA=0, VB=VC= (IB+IC)ZF = 3Ia0ZF. The fault current is given by IB+IC = 3Ia0, where Ia0 is the zero sequence current component.
STEPS FOR REDUCING THE OCCURANCE OF ELECTRICAL FAULTS
Earthing:
Earthing must be proper in an electrical substation and must be checked for any abnormal values. For substations, the earthing resistance must be 1 ohm or below.
Visual inspection:
Visual inspection must be carried out in the switchyard to detect any abnormalities like wear of conductors etc so that preventive measures can be taken before the occurrence of any faults.
Testing:
Testing of electrical equipment must be done periodically or as suggested by the manufacturer of the equipment to ensure proper functioning as per the designed specification. Insulation of equipment must be tested at regular intervals.
Maintenance:
Maintenance includes the tightening of the clamps and connectors which reduces the temperature and resistance if any loose connection persists, cleaning of the insulators, lubricating the gears of the isolator, and other mechanisms. Jungle cutting and corridor clearing, an important tasks of maintenance of the transmission lines as it is often observed that most faults occur due to tree branches touching the live conductors of the lines, it is therefore important to keep the corridor clean.
Occurrence of faults can be minimized by following the above-mentioned steps but it cannot be fully nullified.
