A faultin an electric power system can be defined as, any abnormal condition of the system that involves the electrical failure of the equipment, such as, transformers, generators, busbars, etc.
EFFECT OF FAULT
- Under normal or safe operating conditions, the electric equipments in a power system network operate at normal voltage and current ratings.
- Once the fault takes place in a circuit or device, voltage and current values deviates from their nominal ranges.
- The faults in power system causes over current, under voltage, unbalance of the phases, reversed power and high voltage surges.
- This results in the interruption of the normal operation of the network, failure of equipments, electrical fires, etc.
Usually power system networks are protected with switchgear protection equipments such as circuit breakers and relays in order to limit the loss of service due to the electrical failures.Electrical networks, machines and equipments are often subjected to various types of faults while they are in operation. When a fault occurs, the characteristic values (such as impedance) of the machines may change from existing values to different values till the fault is cleared.
Faults – Types and their Effects
- Active faults
- Passive faults
- Transient faults
- Permanent faults
- Symmetrical Faults
- Asymmetrical Faults
- Open circuit faults
- Short circuit faults
- Active Faults
The “Active” fault is when actual current flows from one phase conductor to another(phase-to-phase) or alternatively from one phase conductor to earth (phase-to-earth).This type of fault can also be further classified into two areas, namely the “solid” fault and the “incipient” fault.The solid fault occurs as a result of an immediate complete breakdown of insulation as would happen if, say, a pick struck an underground cable, bridging conductors etc. or the cable was dug up by a bulldozer. In mining, a rockfall could crush a cable as would a shuttle car. In these circumstances the fault current would be very high, resulting in an electrical explosion.This type of fault must be cleared as quickly as possible, otherwise there will be:
- Greatly increased damage at the fault location. (Fault energy = 1² x Rf x t where t is time).
- Danger to operating personnel (Flash products).
- Danger of igniting combustible gas such as methane in hazardous areas giving rise to a disaster of horrendous proportions.
- Increased probability of earth faults spreading to other phases.
- Higher mechanical and thermal stressing of all items of plant carrying the current fault. (Particularly transformers whose windings suffer progressive and cumulative deterioration because of the enormous electromechanical forces caused by multi-phase faults proportional to the current squared).
- Sustained voltage dips resulting in motor (and generator) instability leading to extensive shut-down at the plant concerned and possibly other nearby plants.
The “incipient” fault, on the other hand, is a fault that starts from very small beginnings, from say some partial discharge (excessive electronic activity often referred to as Corona) in a void in the insulation, increasing and developing over an extended period, until such time as it burns away adjacent insulation, eventually running away and developing into a “solid” fault.
- Passive Faults
Passive faults are not real faults in the true sense of the word but are rather conditions that are stressing the system beyond its design capacity, so that ultimately active faults will occur. Typical examples are:
- Overloading – leading to overheating of insulation (deteriorating quality, reduced life and ultimate failure).
- Overvoltage – stressing the insulation beyond its limits.
- Under frequency – causing plant to behave incorrectly.
- Power swings – generators going out-of-step or synchronism with each other.
It is therefore very necessary to also protect against these conditions.
- Transient & Permanent Faults
Transient faults are faults which do not damage the insulation permanently and allow the circuit to be safely re-energized after a short period of time.
A typical example would be an insulator flashover following a lightning strike, which would be successfully cleared on opening of the circuit breaker, which could then be automatically reclosed.
Transient faults occur mainly on outdoor equipment where air is the main insulating medium.
Permanent faults, as the name implies, are the result of permanent damage to the insulation. In this case, the equipment has to be repaired and reclosing must not be entertained.
- Symmetrical & Asymmetrical Faults
A symmetrical fault is a balanced fault with the sinusoidal waves being equal about their axes, and represents a steady state condition.
An asymmetrical fault displays a d.c. offset, transient in nature and decaying to the steady state of the symmetrical fault after a period of time. The amount of offset depends on the X/R (power factor) of the power system and the first peak can be as high as 2.55 times the steady state level.
A symmetrical fault gives rise to symmetrical fault currents that are displaced with 120˚ each other. Symmetrical fault is also called as balanced fault. This fault occurs when all the three phases are simultaneously short circuited.
These faults rarely occur in practice as compared with unsymmetrical faults. Two kinds of symmetrical faults include line to line to line (L-L-L) and line to line to line to ground (L-L-L-G) as shown in figure below.
The analysis of these faults is required for selecting the rupturing capacity of the circuit breakers, choosing set-phase relays and other protective switchgear. These faults are analyzed on per phase basis using bus impedance matrix or Thevenins’s theorem.
The most common faults that occur in the power system network are unsymmetrical faults. This kind of fault gives rise to unsymmetrical fault currents (having different magnitudes with unequal phase displacement). These faults are also called as unbalanced faults as it causes unbalanced currents in the system.
The figure below shows the three types of symmetrical faults occurred due to the short circuit conditions, namely phase or line to ground (L-G) fault, phase to phase (L-L) fault and double line to ground (L-L-G) fault.
A single line-to-ground (LG) fault is one of the most common faults and experiences show that 70-80 percent of the faults that occur in power system are of this type. This forms a short circuit path between the line and ground. These are very less severe faults compared to other faults.
- Open Circuit Faults
These faults occur due to the failure of one or more conductors.
The figure below illustrates the open circuit faults for single, two and three phases (or conductors) open condition.
The most common causes of these faults include joint failures of cables and overhead lines, and failure of one or more phase of circuit breaker and also due to melting of a fuse or conductor in one or more phases.
Open circuit faults are also called as series faults. These are unsymmetrical or unbalanced type of faults except three phase open fault.
- Abnormal operation of the system
- Danger to the personnel as well as animals
- Exceeding the voltages beyond normal values in certain parts of the network, which further leads to insulation failures and developing of short circuit faults.
Although open circuit faults can be tolerated for longer periods than short circuit faults, these must be removed as early as possible to reduce the greater damage.
- Short Circuit Faults
A short circuitfault can be defined as an abnormal connection of very low impedance between two points of different potential, whether made intentionally or accidentally.
These are the most common and severe kind of faults, resulting in the flow of abnormal high currents through the equipment or transmission lines. If these faults are allowed to persist even for a short period, it leads to the extensive damage to the equipment.
Short circuit faults are also called as shunt faults. These faults are caused due to the insulation failure between phase conductors or between earth and phase conductors or both.
The various possible short circuit fault conditions include three phase to earth, three phase clear of earth, phase to phase, single phase to earth, two phase to earth and phase to phase plus single phase to earth as shown in figure.
The three phase fault clear of earth and three phase fault to earth are balanced or symmetrical short circuit faults while other remaining faults are unsymmetrical faults.
These may be due to internal or external effects
- Internal effects include breakdown of transmission lines or equipment, aging of insulation, deterioration of insulation in generator, transformer and other electrical equipments, improper installations and inadequate design.
- External effects include overloading of equipments, insulation failure due to lighting surges and mechanical damage by public.
- Arcing faults can lead to fire and explosion in equipments such as transformers and circuit breakers.
- Abnormal currents cause the equipments to get overheated, which further leads to reduction of life span of their insulation.
- The operating voltages of the system can go below or above their acceptance values that creates harmful effect to the service rendered by the power system.
- The power flow is severely restricted or even completely blocked as long as the short circuit fault persists.
PROTECTION OF SYSTEM FROM FAULTS
|To Sense the fault and give a trip signal to CB.
|For protective circuits between CT’S , VT’s, relays
|Trip circuit cables
|For trip signal between the protective relays and CB trip coil.
|Opening and clearing the fault
Whenever a fault occurs, the relay identifies the normal and the abnormal conditions. When an abnormal condition occurs, the relay closes its contacts and trips the CB. Therefore, the CB opens and faulty part gets isolated from the supply. This entire process is automatic and fast.