2. Outline.........
• Introduction
• Basic Element of power system
• Switch gear and Protection
• Abnormalities and fault in power system
• Function of a Protective system
• Back up protection
3. Introduction
⚫ Power supply system is wide and complicated.
⚫ There is every possibility of occurring fault in power
System.
⚫ Voltages of 11kv, 132 kv, 220 kv, or 400 kv are used
in power system.
⚫ Protective Relays are employed in power System.
⚫ The three – phase system can be used in two ways.
⚫ The first method the neutral is not ground. So that's
called the Isolated neutral System Or Free neutral
System.
⚫ The Second method the neutral is earthed in directly or
through resistance or reactor.
4. Basic Element of a Power System
⚫ Power system can be divided in to three main parts.
1. Generating Station
2. Load centre
3. Link joining 1 and 2
5. Switchgear and Protection
⚫ The Power System can be divided in to three main parts.
These three parts include generator, Step up transformer,
high voltage and low voltage transmission line, stem down
transformer, distribution lines , feeder, Distribution
transformer etc. All these devices are to be switched on and
off. Device used to switch on and off the circuits are called
switchgears.
⚫ Circuit breaker has to operate in two situation
1. Switch on and off the circuit in healthy
condition
2. It breaks the circuit in abnormal condition such
as short circuit or fault.
6. Abnormalities and fault in power system
⚫When the power system is operating
properly it is called the normal healthy
condition.
⚫But the power system does not work
always in the normal or healthy condition
this is called Abnormal condition.
7. Normal Operation of power system
1. Voltage and current are balanced i.e. The three –
phase voltage are equal and the three-phase currents
are also almost equal.
2. Frequency of the supply is equal to the rated
frequency. There is very little change in the frequency.
3. There is no change in the voltage beyond some limit.
4. Power flow is in the desired direction.
5. Value of current is equal to the rated value or less
than it.
8. Abnormal condition of a power system
1. Overloading of equipment.
2. Unbalanced loading.
3. Failure of prime mover in power station.
4. Failure of exciter in power station.
5. Loose contacts
6. Over voltage surge.
9. Causes of fault
1) Defect in insulation of the winding of electrical
equipment.
a) Ageing
b) Voltage surge
2) Defect in underground cable
3) Defect in overhead line
4) Defect in the bus bars of control panel of the
substation.
10. Consequences of Fault
1. Arc may be produced due to the fault.
2. There is possibility of fire when the arc is
produced in the equipment due to the fault.
3. Large current flows due to short circuit.
4. Motors take increased current due to the
reduced voltage so the motors are over
heated.
5. Pressure coil of some electromagnetic relay
cannot operate due to the reduced voltage.
6. Large electromagnetic force are produced due
to large value of the short circuit current.
11. Statistics of various types of fault
Equipment % of fault
Over head line 50
Underground cable 10
Switch gear 15
Transformer 12
CT and PT 2
Control equipment 3
Miscellaneous 6
12. Types of fault in Overhead lines:-
1. Line to ground fault (LG)
1. Line to line fault (LL)
1. Line to line to ground fault (LLG)
1. Line to line to line fault (LLL)
13. Function of a protective system
⚫Primary Function:- Protective relay detects the
fault and the faulty component is isolated from the
system with the help of the switchgear. This is the
primary function of the protective relay.
⚫Secondary Function:- The primary function of
the protective relay. Some relays also give information
about the type of fault and its location. This is the
secondary function of the protective relay.
14. Basic element of a protective system
1. Sensing element
(CT and PT)
2. Selector
(Summation
transformer)
3. Measuring
element
(Comparator
and relay)
4. Amplifier
5. Actuator
16. Zones of Protection
1. Generator
2. Low voltage Switch gear
3. Power Transformer
4. High voltage switchgear
5. Transmission line
17. Back up Protection
• When the fault occurs in the system the
primary protection system starts working
and the rely operates in the specified
period and trips the circuit breaker.
• The back up protection system also starts
working as soon as the fault occurs.
• But the operating time of the backup
protection is kept more than that of the
primary operating system.
19. Necessity of back up protection
1. The back up protection system operates and clears the
fault when the primary protection system fails.
2. When the primary protection system is under repairs or
in maintenance the back up protection system works as
the primary protection system.
Reasons of failure of the primary protection system are as
follows
1. Due to the failure of the voltage and current supplied to
the relay
2. When the trip coil of the circuit breaker dose not get d.c.
Supply.
3. When the defect is developed in the protective relay.
4. Due to the defect in breaker mechanism.
5. Due to the defect in the circuit breaker.
20. Types of back up Protection
1. Relay Back Up
2. Breaker Back Up
3. Remote Back Up
