SF6 CB

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SWITCHYARD
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 What is a Switchyard ?
It is a switching station which has the following credits :
(i) Main link between Generating plant and Transmission
system, which has a large influence on the security of the
supply.
(ii) Step-up and/or Step-down the voltage levels
depending upon the Network Node.
(iii) Switching ON/OFF Reactive Power Control devices,
which has effect on Quality of power.
SWITCHYARD EQUIPMENTS
•
Equipments commonly found in switchyard :
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
Lightening arrestor
Current transformer
Voltage transformer
Power transformers / I.C.T.
Bus bar and clamp fittings
Support structure
Isolators
Circuit Breaker
Wave traps
Earthing switch
 Functions of various equipment :
* Transformers :
- Transforms the voltage levels from higher to lower level
or vice versa, keeping the power constant.
* Circuit breakers :
- Makes or automatically breaks the electrical circuits under Loaded
condition.
* Isolators :
- Opens or closes the electrical circuits under No-load conditions.
* Instrument transformers :
- For stepping-down the electrical parameter (Voltage or Current) to a
lower and safe value for Metering and Protection logics.
* Earth switch :
- Used to connect the charged body to ground to discharge the trapped
charge to have a safe maintenance zone.
Functions of various equipment :
* Lightning arrestors :
- Safe guards the equipment by discharging the high currents
due to Lightning.
* Overhead earth wire :
- Protects the O/H transmission line from Lightning strokes.
* Bus bar :
- Conductors to which a number of circuits are connected.
* Wave Traps/Line traps :
- Used in PLCC circuits for Communication and Protection of
Transmission lines
* Reactive Power control devices :
- Controls the reactive power imbalance in the grid by switching
ON/OFF the Shunt Reactors, Shunt Capacitors etc.,
* Current Limiting Reactors :
- Limits the Short circuit currents in case of faulty conditions.
Switchyard One line Diagram
Transfer Bus 400 KV
Main Bus II
Main Bus I
Bus
Isolator
CB
CB
CB
Transfer Bus
Bay
GT
20.5/400KV
Gen Bay
Feeder Bay
 Bus Switching Schemes :
Bus Bar Schemes
* Single Sectionalised bus
* Main & Transfer bus
* Sectionalised Main bus with Transfer bus
* Sectionalised Double Main & Transfer bus
Breaker Schemes
* Ring bus
* One and Half breaker
* Double bus Double breaker
* Single Sectionalized Bus-bar system :
I/C Feeders
CB
Bus-bar
Isolators
O/G Feeders
* Main & Transfer Bus-bar system :
I/C Feeders
CB
Transfer Bus
Bus Coupler
Main Bus
Isolators
* Ring Bus system :
Bus
CB
O/G feeder
I/C Supply
* One and Half Breaker scheme :
Main 1
Main 2
Feeder 2
Tie CB
Feeder 1
 What is a Switchgear ?
“The apparatus used for Switching, Controlling and
Protecting the Electrical Circuits and equipment”.
 Need of Switchgear :
* Switching during normal operating conditions for the
purpose of Operation and Maintenance.
* Switching during Faults and Abnormal conditions and
interrupting the fault currents.
Relays
 Relay :
“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”.
* The relays detect the abnormal conditions in the electrical
circuits by constantly measuring the electrical quantities
which are different under normal and faulty conditions.
 Requirements of Protecting relaying :
Selectivity
- Ability to select the faulty part and isolate that part
without disturbing the rest of the system.
Speed
- Ability to disconnect the faulty part at the earliest
possible time.
Sensitivity
- Ability of the relay to operate with low value of
actuating quantity.
Reliability
- Ability of the system to operate under pre-determined
conditions
Simplicity
- Should be so simple so that it can be easily maintained.
- The simpler the protection scheme, the greater is the
reliability
Economy
- Availability at lower cost.
- Generally, the protective gear should not cost more
than 10% of the total cost. However, when the apparatus to
be protected is of utmost importance (e.g. Generator, GT
etc) economic conditions are subordinated to
reliability.
 Basic classification of Relays based on Function :
* Over current
* Under Voltage
* Impedance
* Under Frequency
* Directional
Circuit Breakers
 Main Parts of a Circuit Breaker :
* Fixed Contact
* Movable Contact
* Operating Mechanism
* Arc extinguishing medium
Basic relay circuit scheme
Equipment
CT
CB
Supply from
source
RELAY
Trip Coil
of
Breaker
Battery
 Fault clearing process :
During any Fault…..
* Fault impedance will be low, so fault current will
increase and relay senses this increase in current.
* Relay contacts closes and sends trip signal to circuit
breaker and the trip coil of the circuit breaker will get
energized.
* Operating mechanism of the circuit breaker will
operate and separate the contacts.
* Arc will be initiated between the contacts and it is
extinguished by suitable methods.
 Arcing phenomenon :
- When a fault occurs, heavy current flows through the contacts
of the circuit breaker before they are opened by the protective system.
- At the instant when the contacts begin to separate, the contact
area decreases rapidly and current density (I/A) increases and hence
rise in temperature.
-The heat produced is sufficient to ionise the medium between the
contacts. This ionised medium acts as conductor and an arc is struck
between the contacts.
- The potential difference between the contacts is very small and
is sufficient to maintain the arc.
- The current flow depends upon the Arc resistance.
 Few definitions :
Breaking Capacity Max fault current at which a CB
of breaking a circuit.
is capable
Making Capacity Max current a CB can withstand if it closing on
existing Short circuit.
Restriking Voltage –After the arc has been extinguished, the
voltage across the breaker terminals does not normalize
instantaneously but it oscillates The transient voltage which appears
across the breaker contacts at the instant of arc being extinguished.
Recovery Voltage –Power frequency voltage which appears across
the breaker contacts after the arc is finally extinguished and transient
oscillations die out.
 Events/Timings during fault clearing process :
Fault clearing
Time
Relay time
Circuit breaker
Time
=
Relay time
=
Instant
Of
Fault
=
Closure of
Trip
Circuit
+
Circuit breaker
Time
to
Closure of
Trip
Circuit
to
Final arc
Extinction
 Various types of CBs :
(i) Miniature CB
(ii) Air Break CB
(iii) Air Blast CB
(iv) Oil CB
(v) SF6 CB
(vi) Vacuum CB
Bulk Oil CB
Minimum Oil CB
 Air Break CB :
 Air Blast CB :
ABCB- Principle of arc quenching
 Bulk Oil CB :
 Minimum Oil CB :
 SF6 CB :
1. Op mechanism
8. Puffer cylinder
2. Interrupter
9. Nozzle
3. Support
10. Fixed position
4. Op rod
11. Fixed contact
5. Linkage
12. Moving contact
6. Terminals
13. Gas inlet
7. Filters
Advantage of SF6
* Inert gas with high dielectric strength.
* Colour less and odour less.
* Non-toxic and non- inflammable.
* Sf6 is blown axially to the arc, hence it removes the heat by axial
convection and radial dissipation. As result the arc dia reduces
and comes to zero at current zero.
* Gas pressure in the chamber is at 5 ksc.
* SF6 is filled at a pressure of 12 ksc in the tank and maintained by
means of an individual or a common compressor.
* The decomposition products of arcing are not explosive hence no
chance of fire.
Disadvantages
* SF6 gas condensates at low temperature & high pressure
 Vacuum CB :
Advantage of vacuum CB
* Used up to 66 KV.
* Vacuum is highly dielectric, so arc can’t persists.
* Separation of contacts causes the release of metal vapour from
the contacts, the density of vapour depends on the fault current.
* At current zero the vapour emission will tends to zero and the
density will becomes zero and dielectric strength will build up and
restriking will be prevented.
* No emission to atmosphere, hence pollution free.
* Non- explosive and silent operation.
* Compact size.
Disadvantages
* High initial cost.
* Surge suppressors (R or RC combination) are to be connected
at load side for limiting switching over-voltage while
switching low pf loads.
Isolator- functions
 Operates under off-load conditions
 Physically isolates the electric circuit
 Does not have any current making & breaking
capacity
 Used in addition to C.B.
Earth-Switch
 Operated when Isolator is open.
 Connected between Line conductor and Earth
whenever required.
 Discharge the voltage trap in line.
Isolator
Isolator
CB
Source
Earth S/W
* Practically, Pantograph type Earth switches were used.
Load
Current transformers
 Purpose :
- To step-down the high magnitude of current
to a safe value to incorporate Measuring and
Protection logics
• Current transformers are used for the
instrumentation, protection or metering of power
systems and are normally installed on each side
of a circuit breaker.
Voltage transformers
 Purpose :
- To step-down the high magnitude of voltage
to a safe value to incorporate Measuring and
Protection logics.
• Voltage transformers serve a number of functions
in a power system. They are required for the
operation of many types of instrumentation and
relay protective systems. They measure voltage
and in conjunction with CT , they measure
power. They feed synchronizing equipment. They
can be used as coupling capacitors in power
line carrier network.
CAPACITIVE VOLTAGE TRANSFORMER
• - Primary voltage is applied to a series of
capacitors group. The voltage across one of the
capacitor is taken to aux PT. The secondary of
the aux PT is taken for measurement and
protection.
• SECONDARY VOLTAGES(110 VOLTS AC) FOR
METERS AND ENERGY METERS
• VOLTAGES FOR PROTECTIVE RELAYS
• VOLTAGES FOR SYNCHRONIZING
• DISTURBANCE RECORDERS AND EVENT LOGS
• OVERFLUX RELAYS
• PLCC
Lightening Arrestor
 Purpose :
- To discharge the high voltage surges in the power
system due to lightning to the ground.
 Apparatus to be protected :
* Overhead lines………Earth/Ground wires (PA=30 deg)
* HV equipment………LAs
* Substation…………...Lightning Masts, Earth wires
 Types :
Rod gap LA :
* Gap length is such that
the break-down occurs at
80% of the spark voltage
* After the surge, the arc
in the gap is maintained
by the normal supply
voltage. So, only used as
a back-up.
Insulator
Rod gap
Equipment
body
Wave Trap
Transmission Line
S/S-1
Wave
Trap
To control room
of S/S-1
S/S-2
Wave
Trap
To control room
of S/S-2
* Wave trap is used for Protection of the transmission line and
communication between the Substations.
* VHF signal is transmitted from one end to the another through the
same power line.
* Sends inter-trip signal to the other end CBs so that fault can be isolated
at the earliest time.
THANK
YOU
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