Faults in Power System, Overview:
Faults in Power System and protection devices against faults– A fault indicates abnormal conditions in the circuit in which energy is being dissipated in an unwanted manner it may be due to an abnormal condition. Fault is undesirable change which disturbs the balance condition of the system. Most of the power systems leads to a short circuit condition it is the failure of insulation which is the material between high voltage and low voltage to avoid un-expected flow of current.
Three phase balanced system:
The three phase voltage are said to be balanced if their magnitudes and phase difference are equal with respect to each other.
Let us consider star connected three phase systems the voltage phasors are all equal in magnitude and are displace from each other by 120 degrees with each other. In this the angle between VR and VY is 120 degrees. Similarly VY and VB also have 120 degree.
If we look at the phasor diagram the magnitude of VR , VB and VY are equal in magnitude. The self-impedance of all three phases are equal and the mutual impedance if any between the three phase are same.
Symmetrical faults are also known as three phase faults. In this this type of the three wires red, blue and yellow are short circuited. There is another condition in which all three wires are short circuit and attach with the ground due to which huge amount of current flow. This can cause damage to our equipment or system. Symmetrical faults are the most dangerous faults because highest amount of current flow in these types of faults. In symmetrical faults we have no negative or zero component of the sequence. These are balance faults and have only positive sequence.
Types of symmetrical faults:
- Three phase line to ground fault
- Three phase line to line fault
Three phase line to ground fault:
In this type of fault all the three phase wires are shorted and grounded. This fault produce balance fault. The probability of this type of fault occurrence is about 2 to 3%.
Three phase line to line fault:
This type of fault is similar to the three phase line to ground fault but in this fault the phase wires are short circuited but not grounded. This fault also produce balance fault and the probability of this fault is less than 1%.
Unsymmetrical faults are those faults which make the power system unbalanced. These faults have positive, negative and zero sequence components are present in it. There are various types of unsymmetrical faults:
- Single phase to ground fault
- Two phase to ground fault
- Phase to phase fault
- Open circuit fault
Single phase to ground fault:
In this type of fault phase or ground wire is short circuit. This type of fault occurs due to the damage of the insulation or due to the breaking of the phase conductor and short circuit with the ground. The probability of this fault occurrence is about 70 to 80 %. This type of fault is mostly common in power systems because we have three phases on the pole that stretches for a mile and on both side of the pole we have trees or even on one side of that pole we have trees, animals that are around it so if tree branch brushes up against that line that will produce a line to ground fault or if bird or animal gets on the top of the line and it accidently touches the line and ground at the same time. It will produce line to ground fault.
We assume that our system is unloaded at the time of fault which means that load current is zero. We will draw the three-phase diagram in which neutral is grounded. The line current that will flow in the system are IR, IY and IB. While the EMF are ER, EY and EB and the impedance are Z. As our system is unloaded no current will flow through it. All the three phases current will be zero. So let suppose fault has occur we will take R as the reference phase, this fault will be line to ground fault and current IF will flow so this time the IR current will not be zero. Now the IR, IY and IB currents are no longer balance.
So this equation represents the magnitude of the fault current.
Two phase to ground fault:
In this type of fault any two phase got short circuit with the ground wire. The probability of this type of fault occurrence is less than 10 %. Now let us consider that system is unloaded before the occurrence of the current means no current flow in the circuit. So when any two terminals of the system are shorted with the ground. Then this fault will be known as two phase ground fault.
Let us consider that the Y and B two phases are shorted with the ground so in that case the fault current will flow in the system. The IF current will be equal to:
The sequence current will be represented as:
Now the terminal voltage drops of the sequence:
These networks will be connected in parallel. The current will be:
Phase to Phase fault:
In this type of fault the phase wire got short circuit. This type of fault involves two phases and produce unbalanced power characteristics in the power system. The probability of this type fault occurrence is about 15 to 20 %.
Now let us consider that the time of occurrence of the fault the system is unloaded that is the load current are zero. As the fault is not occur so the current in all phases are zero. When fault has occurs let us consider that Y and B phases are short circuited. In this case the IY and IB current are non-zero. As the two terminal are shorted so the potential of the two terminals will be same and VYB will be zero. There will be no potential drop between Y and B phases.
The IR current will be zero.
There will be no current pass through Zin because this path belongs to zero sequence current only. Neither positive nor negative sequence current can pass through this path. For this sequence zero current is absent Zin will not affect the fault current and neutral will be absent which will also not affect the ground current.
Open circuit fault:
In this type of fault the conducting path break due to which the conduction of the current stop. This occur when one or more phase wire break. Due to this type of fault unbalance current will flow due to which the equipment will heat up. This fault can happen due to the blow off of fuses or mal operation of the circuit breaker in or two phases.
This type of fault occur when the transformer or motor winding damage. When the number of turns in the winding got short circuit.
This type of fault is also known as multiple fault. In this type of fault two or more than two faults occur simultaneously.
Difference between Shunt faults and series faults:
Shunt faults are identified when there is increment in the current, if there is any sudden change in current and sudden damping the voltage so how much current will increase; current will increases to a very high value and the voltage decreases to almost zero value in the shunt fault condition.
In series faults reverse condition will be occur when the series fault occur the circuit will be open the current will be zero and the voltage will be maximum. During the series fault condition the current will decreases almost zero and the voltage increases to a high value.
Fundamental characteristics of faults in power system:
The operation of the protection and indeed the detection of the fault condition depend upon very much on the characteristics of the fault itself. Suddenly the nature of the fault is the principal factor in determining:
- Magnitude of fault current
- Change in magnitude of the voltage
- Change in phase angle relationships
A ground fault occurs when a live wire or component touches the metal housing of the appliance. The housing is now energized and the ground wire returns the electricity to the source. Electricity will always takes the path of the least resistance if we touch energized housing without a ground wire we might be electrocuted. It the ground fault happens before the load, the circuit is shorted , the current increases, and the breaker or fuse will blow cutting power to the appliance. Now if the ground fault happens after the load, there is still resistance in the circuit so the breaker might not trip. The appliance might still be functional but this depend upon on the component affected.
To test whether the appliance is shorted to ground, we can plug it into circuit protected by ground fault circuit interrupter (GFCI). GFCI will monitor the current between the live and neutral wire lines. If there is even a small amount of current leaking to ground then the GFCI will trip disconnecting the power until it reset. Components can also be tested for shorts. If the component has continuity between either terminal and the metal frame then it is shorted and will need to be replaced.
Protection devices against faults:
There is limit of current that any device can withstand. If the current exceeds that limit the components of the device heat up and this may even lead to fire. So to prevent device from such condition we will use protection devices.
Electric fuse come in many shapes and sizes which has fuse wire and connected to the two terminals of some metal. This wire is made up of copper, zinc or aluminium. The rest of the switch is basically made up of non-combustible material. We should make sure that the device which is used to prevent fires in other devices must not catch fire. Now we will discuss that how it prevent household electrical appliance from catching fire. This fuse is introduced in every circuit in our household. When the current flows through the circuit, it heats up the fuse wire and when there is heavy current in the circuit, the wire simply melts resulting in the broken circuit. The circuit will break and the current will not reach to the appliance. Once the fuse wire melts it cannot reform itself we have to replace the fuse with the new one manually.
A circuit breaker is device that automatically or manually breaks the circuit in faulty condition. In faulty condition abrupt current will flow in the circuit.
Inside the circuit breaker we find couple of key components.
- The actuator mechanism
- Terminal where we make the wire connection
- Thermal magnetic strips
- Lever which will be used to turn the circuit breaker on or off and resets in case it trips
When the current increase from the rating of the circuit breaker this will be over condition which will heats the bimetallic strip element and changes it shape to the point where it releases the lever trips the circuit breaker and interrupts the circuit.
It is a device that detects fault and initiates the operation of the circuit breaker to isolate the defective part of the system. The relay detects the abnormal condition in the power system by constantly measuring the voltage and current through instrument transformer that are the CTs and PTs which are different under normal and faulted condition. The electrical quantities which may change under faulted condition are voltage, current, frequency and phase angle. For example if there is short circuit a very high current will flow in the line or if there is voltage sag line voltage will drop. Now through the changes of one or more of these quantities the protective relays will be able to detect the abnormality their presence, type and location and other very useful information. When the fault is detected through the instrument transformer the relay operates enable the trip of the circuit breaker. This results in the opening of the breaker and disconnecting the faulty segment of the line.
Lightning rods are devices which that divert the lightning surges to ground but unlike arresters they are simple conductive terminals that are always at ground potential and are never energized. According to most definitions a lightning arrestor is a device used to in power system above 1000 volts to protect other equipment insulation from being damaged by the lightning and switching surges. The SPD (surge protective device) and TVSS (transient voltage surge suppressor) are similar to lightning arrestors in that they are surge diverters but they are generally used for the voltage below 1000 volts.
When a lightning strike the line the current will down the line into the transformer this action will mostly result in damage to the transformer and the lights will go out. The transformer is protected the arrestor and the line is hit by the lightning the arrestor will turn on and divert the surge to the ground. The arrestor protect the transformer from damage. The lightning arrestor does not absorb the lightning and it does not stop the lightning. It does however clamp or limit the voltage produced by the lightning and it does divert the lightning current to ground. It should be noted that the arrestor will only protect the equipment that is electrically parallel with it.