An electrical substation is a subsidiary station of an electricity generation, transmission and distribution system where voltage is transformed from high to low or the reverse using transformers. Electric power may flow through several substations between generating plant and consumer, and may be changed in voltage in several steps.
The circuit breaker is used for switching the circuit current and short circuit current interruption. There are two types of circuit breakers used in power station.
1. Live tank type: Enclosure-Live, Breaking parts-Live
2. Dead tank type: Enclosure-Grounded, Breaking parts-Live
For arc Interruption in a circuit breaker Air, Oil, Vacuum, Gas (SF6) are used.
Gas Circuit Breaker (GCB)
Current interruption in a high-voltage circuit-breaker is obtained by separating two contacts in a medium, such as SF6, having excellent dielectric and arc quenching properties. After contact separation, current is carried through an arc and is interrupted when this arc is cooled by a gas blast of sufficient intensity. Gas blast applied on the arc must be able to cool it rapidly so that gas temperature between the contacts is reduced from 20,000 K to less than 2000 K in a few hundred microseconds, so that it is able to withstand the transient recovery voltage that is applied across the contacts after current interruption. SF6 is generally used in present high-voltage circuit-breakers.
Surge arrester is a device used to protect equipments against over voltages. MOS (Metal Oxide Surge Arrester) - Without gaps will be used in UKHP. It consists of ZnO element which has excellent non-linear voltage-current characteristic.
MAIN CAUSES OF OVER VOLTAGES
1. Lightning Surges - 8/20 current impulses.
2. Switching Surges - has a peak value of discharge current having a virtual front time greater than 30 μs but less than 100 μs and a virtual time to half-value on the tail of roughly twice the virtual front time.
3. Temporary over voltages - by line-to-ground faults, load rejection & continued for a few milliseconds to a few seconds.
Nominal discharge current - of an arrester is the peak value of lightning current impulse which is used to classify an arrester.
Current interruption in a high-voltage circuit-breaker is obtained by separating two contacts in a medium, such as SF6, having excellent dielectric and arc quenching properties. After contact separation, current is carried through an arc and is interrupted
when this arc is cooled by a gas blast of sufficient intensity. Gas blast applied on the arc must be able to cool it rapidly so that gas temperature between the contacts is reduced from 20,000 K to less than 2000 K in a few hundred microseconds, so that it is able to withstand the transient recovery voltage that is applied across the contacts after current interruption. SF6 is generally used in present high-voltage circuit-breakers.
SF6 gas in the cylinder is compressed by the downward movement
Gas is forced into the nozzle where the arc is drawn.
In early stages pressure inside the cylinder is raised.
Nozzle concentrates the gas flow to the area of the arc.
The maximum voltage at which a circuit breaker can operate for extended periods without undue degradation or safety hazard.
Rated Short Circuit breaking Current
The highest electric current which can exist in a particular electrical system under short circuit conditions.
Rated Operating Sequence
This denotes the sequence of opening and closing operations which the circuit breaker can perform under specified conditions. The circuit breaker should be able to perform the
operating sequence as follows:
Where, O = opening operation
t = time required to be ready to receive closing order from auto- reclosure relay
CO = close operation followed by open operations
T = time required to insulating media for regeneration and operating mechanism
Rated Lightning Impulse Withstand Voltage
Lightning impulse voltage rating which the gas circuit breaker can withstand.
Rated Short Duration Power-Frequency Withstand Voltage
R.M.S. value of sinusoidal power frequency voltage that the breaker can withstand during tests made under specified conditions and for a specified time.
Disconnect switches are used to isolate a component of an electrical system from the power source. Electrical power distribution systems require switching for many reasons, including fault isolation, transfer loads from one source to another, isolation of line segments for purpose of maintenance, and in some instances for shedding loads.
2. Double break
3. Vertical break
4. Knee type
Earthing Switch is necessary to earth the conducting parts before maintenance and also to provide deliberate short-current while testing.
1. Fault making earth switch
2. Maintenance earth switch
A voltage transformer (VT) is a transformer used in power systems to step-down extra high
voltage and provide low voltage either for measurement or to operate a protective relay.
1. Inductive Voltage Transformer (IVT)
2. Capacitor Voltage Transformer (CVT)
Accuracy Class Designation
Measuring Voltage Transformers
The accuracy class is designated by the highest permissible percentage voltage error at
rated voltage and with rated burden, prescribed for the accuracy class concerned.
Standard accuracy classes: 0,1 – 0,2 – 0,5 – 1,0 – 3,0 43
Protective Voltage Transformers
The accuracy class for a protective voltage transformer is designated by the highest
permissible percentage voltage error prescribed for the accuracy class concerned, from 5 %
of rated voltage to a voltage corresponding to the rated voltage factor. This expression is
followed by the letter P.
Standard accuracy classes 3P and 6P
A protective relay is a device that detects the fault and initiates the operation of the circuit
breaker to isolate the defective element from the rest of the system. Most of the relays in
service on power system today operate on principle of electromagnetic attraction or
electromagnetic induction. Regardless of the principle involved, relays are generally
classified according to the function they are called upon to perform in the protection of
Type of Protection
When a fault occurs on any part of electric power system, it must be cleared quickly in
order to avoid damage and /or interference with the rest of the system. It is usual practice
to divide the protection scheme into two classes those are primary protection and back-up
It is the protection scheme which is designed to protect the component parts of the power
system. If a fault occurs on any line, it will be cleared by its relay and circuit breaker. This
forms the primary or main protection and serves \as the first line of defense. The service
record of primary relaying is very high with well over ninety percent of all operations
being correct. However, sometimes faults are not cleared by primary relay system because
of trouble within the relay, wiring system or breaker.
It is designed to operate with sufficient time delay so that primary relaying is given enough
time to function if it is able to. Thus referring to fig…, relay A provides back-up protection
for each of the four lines. If a line fault is not cleared by its relay and breaker, the relay A
on the group breaker operate after a definite time delay and clear and entire group of lines.
It is evident that when back-up relaying functions, a larger part is disconnected than when
primary relaying functions correctly. Therefore, greater emphasis should be placed on the
better maintenance of primary relaying.
This type of relay works on the induction principle and initiates corrective measures when
current in the circuit exceeds the predetermined value. The actuating source is a current in
the circuit supplied to the relay from a current transformer. These relays are used on a.c.
circuits only and can operate for fault current flow in either direction.
Directional Power Relay
This type of relay operates when power in the circuit flows in a specific direction. Unlike a
non-directional over-current relay is so designed that it obtains its operating torque by the
interaction of magnetic fields derived from both voltage and current source of the circuit it
protects. Thus this type of relay is essentially a wattmeter and the direction of the torque
set up in the relay depends upon the direction of the current relative to the voltage with
which it is associated.
Distance or Impedance Relay
The operation of the relay is governed by the ratio applied voltage to current in the
protected circuit. Such relays are called distance or impedance relays. In impedance relay,
the torque produced by ac current element is opposed by the torque produced by a voltage
element. The relay operates when the ratio V/I is less than a predetermined value.
Differential relay one that operates when the phasor difference of two or more similar
electrical quantities exceeds a pre-determined value. It has advantages over other relays. It
is more sensitive than other relays and makes correct distinction between heavy load
conditions and minor fault conditions. Thus a current differential relay is one that
compares the current entering a section of the system with the current leaving the section.
Under normal operating conditions, the two currents are equal but as soon as a fault occurs,
this condition no longer applies. The difference between the incoming and outgoing
currents is arranged to flow through the operating coil of the relay. If this differential
current is equal to or greater than the pickup value, the relay operates and opens the circuit
breaker to isolate the faulty section.
The primary of a current transformer typically has only one turn. This is not really a turn or wrap around the core but just a conductor or bus going through the “window.” The primary never has more than a very few turns, while the secondary may have a great many turns, depending upon how much the current must be stepped down. In most cases, the primary of a current transformer is a single wire or bus bar, and the secondary is wound on a laminated magnetic core, placed around the conductor in which the current needs to be measured.
If primary current exists and the secondary circuit of a CT is closed, the winding builds and maintains a counter or back EMF to the primary magnetizing force. Should the secondary
be opened with current in the primary, the counter EMF is removed; and the primary magnetizing force builds up an extremely high .