The hydel power development in Kerala began with the commissioning of Pallivasal hydro electric project in 1940. Next few decades saw the progressive developments of various schemes. There are 2 main feeders into this substation .It consists of a upper yard and a lower yard. The 2 feeders are from Kattakada substation. These are 110kV feeders. The substation supplies power in 3 ratings 66kV,22kV and 11kV.The 11kV supply feeds the following : Pappancode, Poojapura, Velleyambalam, Killipalam, Peyad, Neyamam, Karamana
2 COMPONENTS OF
SUB-STATION
The power transformers are used to
step down the voltage. The transformer core is made of high grade non-ageing
cold rolled grain oriented steel laminations. The cooling is provided by means
of ONAN(Oil Natural Air Natural) and ONAF(Oil Natural Air Forced). There are
fans rotating at very high speed near to fins (used to increase the surface
area).
Tap changing mechanism is provided for
maintaining the secondary voltage constant with varying primary voltage. Tap
changing causes change in leakage reactance, core loss, I2R loss etc. The tap
changing means changing the turns ratio. The tapings are provided on the HV
side so that it carries minimum current.
4.2.1
Tank
Tank is welded construction. It is
stiffened by structured steel. It is provided with 2F earth terminals. HV and
LV bushings are transported in crates.
4.2.2
Conservator
Conservator is a cylindrical tank
divided into 2 chambers-one for the main tank and other for OLTC. The function
of the main conservator is to keep the main tank completely filled with oil.
OLTC is fitted with an oil sight window. Explosion vent has a bursting plate
and is used as a safety device. When the internal pressure of transformer
increases by abnormal phenomenon the diaphragm bursts. In latest models this is
replaced by a pressure release valve.
4.2.3
Breather
When transformer becomes warm, the oil
and gas expands. The gases at the top of the oil expel out and air is drawn
into the transformer. This process is called breathing. The air entering is
made moisture free by letting it to pass through the breather. It consists of a
small container with silica gel crystal. The material is blue when dry and
whitish when damp. At the bottom of the container some oil is present along
with aluminium silicate in order to absorb dust particles. There are 2
breathers, one for the main conservator tank and the other for OLTC.
Two gauges keep track of the
temperature of the oil and winding and an alarm goes off when the temperature
rises beyond the limited value. The gauges are fixed on the mounting plate of
the transformer. The 2 types of indicators are –Oil Temperature Indicators
(OTI)-Winding Temperature Indicators (WTI). There is provision for the control
of fan motor by sensing the oil temperature.
4.2.4
Isolators
In order to disconnect a part of
system for maintenance and repair isolators are used. It is a knife switch
designed to open a circuit under no load. If isolators are to be opened, the CB
connected must be opened first. Otherwise there is a possibility of occurrence
of a spark at the isolator contacts. After repair, first isolators are closed
and then CB. There are two types of isolators- line isolators and bus
isolators. For bus isolators, there is no earth switch. During maintenance
works the line isolator contacts are opened, so that the three phases trip
simultaneously. For the ease of earthing, dead weights are provided at the end
of earthing arms.
Ratings
|
||
Ø Current
|
:
|
800A
|
Ø Max design voltage
|
:
|
125kV
|
Ø Impulse withstand voltage
|
:
|
550kV
|
4.2.5 Circuit Breakers
A circuit breaker is an equipment
which can open or close a circuit under normal as well as fault conditions. It
can be operated manually from the yard as well as from control room using a
pulse trigger under normal condition and automatically under fault condition.
For latter relay circuits are used.
It consists of a moving and fixed
contacts enclosed in strong metal tank. Under normal conditions contacts remain
closed and the CT senses full load current. When fault occurs, the over-current
in the CT primary winding increases the secondary emf thus energizing the trip
coil and moving contacts are pulled down, thus opening the contacts and hence
the circuit. The arc produced is quenched by SF6(outdoor CB) and vaccum(indoor
CB).
Its closing is by spring action and
tripping is in air. Each CB has an air tank in which pressure is maintained at
15kg/cm2. If pressure goes below this a rotary compressor is automatically
activated. Pressure of SF6 is continuously monitored. SF6 being costly, is
filled separately in each CB. The gas can be reconditioned after each
operation. Operation mechanism is through air, which is being stored in a
closed tank. Other CBs are interconnected through tubes. Gang operation is
initiated while tripping.
Ratings
|
||
Ø Rated voltage
|
:
|
145kV
|
Ø Normal current
|
:
|
1250A
|
Ø Frequency
|
:
|
50Hz
|
Ø Lightning impulse withstand voltage:
|
650kV(peak)
|
|
Ø First pole to clear factor
|
:
|
1.5
|
Ø Short time withstand current
|
:
|
31.5kA
|
Ø Duration of SC current
|
:
|
5
sec
|
Ø SC breaking current
|
||
Symmetrical
|
:
|
31.5kA
|
Asymmetrical
|
:
|
32.7kA
|
Ø SC making current
|
:
|
80kA
|
Ø Out of phase breaking current
|
:
|
7.9kA
|
Ø SF6 gas pressure at 300C
|
:
|
6.3bar
|
Ø Total mass of SF6 gas
|
:
|
8.7kg
|
Ø Total mass of circuit breaker
|
:
|
1300kg
|
4.2.6 Insulation Transformers
The lines in the substation operate at
high voltage and current. They will not work satisfactorily if mounted directly
on power lines. The difficulty is overcome by installing instrument
transformers. The function is to transfer voltage or current in the power line
to values
convenient for the operation of measuring instruments and
relays. The two type of instrument transformer in the substation are:
Current
transformer
Capacitive
voltage transformer
4.2.6.1
Current transformer
The current transformer is used to measure the very high current
passing through the bus. It steps down the current and measurements are taken
in control room through cables.
Ratings
Ø 110kV side :
600-300-150-75/1-1-1-1
4.2.6.2 Capacitive voltage transformer
This is a step down transformer, which steps down the high
voltage to a value that can be measured using the measuring instruments in the
control room. This has an additional core for the carrier communication. The
CVT are between phases and ground in parallel to the circuit.
Ratings
Ø 110kV feeder Ratio : 110kV/3/110V
4.2.7
Arcing Horn
Arcing horns are the protection of the
insulators in case of high voltage, which it cannot withstand. They are 2 metal
rods fitted at the top most and bottom most parts of the insulator. During high
voltage, insulators can’t resist this and cracks may be developed. In order to
avoid these arcing horns are provided. They conduct the high voltage to the
ground and protect the insulator.
4.2.8
Lightning Arrester
Whenever an incoming comes to a
substation, initially the line is connected through a lightning arrester. This
is for the protection of the station. Generally a lightning arrester seems like
a set of insulators connected together with a ring in the top.
This ring is called grading ring. The purpose of grading rings is that in case
of heavy voltage surge the charge is distributed uniformly through the ring and
then the discharge occurs. An ammeter is connected with the arrester, which
record the maximum current passed through it. The ammeter is reset. The ammeter
in the arrester carrying the topmost conductor will have maximum current
passing through it.
4.2.9
Earthing Systems
All instruments in the yard are
earthed by means of an earthling mat of mild steel rod, forming crises-cross
squares of 3.5m side. The mat is placed 60cm deep and they are welded at their
joint. As initially the earth resistance was found to be greater than one,
ground rods were inserted to a depth of 3m at the periphery of the mat. Each
instrument is earthed by connecting to a node by GI rod or GI flat. In addition
auxiliary mats are provided to prevent the operating person from getting
electric shocks.
4.2.10
Capacitor Banking
Capacitor bank is provided for power
factor improvement. It decreases the reactive power and increases the active
power. 20 MVAR capacitor is connected in the 110kV side. During offload periods
the capacitor bank is shutdown to reduce current consumption.
4.2.11
Busbar
The lines operating at the sane
voltage is directly connected to a common electrical component called busbar.
Busbar is of Cu or Al and is rectangular in cross-section. Conductor used is
moose. The incoming and outgoing lines in the substation are connected to the
busbar.
4.2.11.1
Single busbar with sectionalisation
In this, busbar is divided
into sections and load is equally distributed on all sections.
The
advantage of this arrangement is as follows:
Ø If fault occurs in any section of the busbar, that section can
be isolated without affecting the supply from other sections.
Ø Repair and maintenance of any section of the busbar can be
carried out by de energizing that section only. Thus eliminating the
possibility of complete shut down
4.2.11.2
Double busbar with bus coupler
Buses are coupled by means
of two isolators and a coupler.
The
advantage of this arrangement is as follows:
Ø Load
division is better.
Ø Even if one bus fails the other bus can supply the load.
4.2.12
Insulators
The overhead line conductors are
supported on poles or towers in such a way that currents from conductors do not
flow to earth through these supports, i.e. lands are properly insulated from
supports by insulators. Materials used for constructing the insulators are made
of porcelain. The insulator used is of suspension type, stringing type and
strain type.
4.2.13
Relays
Relays are one of the most important
part of sub-station. The function of a relay is mainly incorporated in the
control panel section of the substation. A protective relay is mainly
incorporated in the control panel section of the substation. A protective relay
is a device that detects the fault and initiates the operation of the CB to
isolate the defective element from the rest of the system. The relay detects
the abnormal condition such as voltage, current, frequency, phase angle and
temperature. Pothencode substation has control panels for its incoming as well
as outgoing feeders. And each control panel has various relays. The different
types of relays which are used here are:
Ø Distance protection relay
Ø Auto reclose relay
Ø Synchronizing relay
Ø Differential relay
Ø Over current relay
Ø Earth fault relay
4.2.13.1
Distance protection relay
It is a special type of relay used to
know at which place the line has failed. The lines are divided into zones. The
relay will indicate the rough distance between the station and the point at
which the breaking has occurred. The connection to the main relay is made
through an auxiliary relay. This relay is very helpful in remote areas. The
control panel has 2 types of distance protection relays.
4.2.13.2
Auto reclose relay
Auto reclose relay is of mechanical
type. It helps in speedier tripping and making up of the connection after fault
rectification.
4.2.13.3
Synchronizing relay
Advantages of bus couplers can be
obtained only if the voltage and frequency of the bus bars to be coupled are
the same. Synchronized relay does the function of constantly comparing the two
voltages and frequency and thereby initiating the tripping mechanism at time of
fault. The synchronoscope aids it.
4.2.13.4
Differential relay
The relay is activated at difference
in current flowing through the relay. In case of equipments like CT the relay
is connected in between the equipments. In normal conditions the current
through the relay is the same as the equipment current but when any fault
occurs in the line enclosed then there is a rise in current through the relay
at the fault side above that which is on the other side. This activates the
relay, tripping occurs.
4.2.13.5
Over current relay
The relay activates when current
exceeds the permissible limits. It will be connected to the circuit breaker in
case of any fault due to over current. The relay acts and activates the circuit
to the breaker hence tripping the breaker. DC supply is always given to the
relay as it should trip even if there is an interruption in the power supply.
4.2.13.6
Earth fault relay
It will be activated when there is any
fault in the earthing of the equipment. It is also connected to circuit breaker
to help tripping the circuit. The main applications of this relay are in
control, alarm, indication and other auxiliary duties in AC or DC systems. CAA
& VAA relays are current and voltage operated respectively. These relays
are attracted in armature units of compact design supplied with reset contacts.
Standard contacts are of silver or copper alloys. When it is necessary to break
heavy or highly inductive DC loads, heavy duty magnetic blow type contacts are
used. They use the magnetic field of a small permanent magnet to force the arc
onto the arcing horns away from the contact tips. The new control panel uses
automatic semaphore (mimic) indicators for better control.
4.2.14
Power Line Communication
Carrier communication is basically the
communication between the substation head offices through power lines. Each
substation has wave trap arrangement, which consists of capacitor and inductor
coupling circuit, which is used to separate the speech from 50Hz frequencies.
There are controls of the communication lines in a separate area. It has an
amplifier, modulator, interface etc. used for amplifying, modulating and making
intermediate connections. It cannot be made for practical domestic distribution
as this will make the signal very weak. Using PLCC it is possible to make
remote controlled connections to the CT, PT, CB etc. which will make the job
easier. In this type of communication, there will be no interference from
outside. Microwave communication can be used to link the dispatched centre
within the substation and also to the head office. 9505 power line carrier
terminals are intended for the transmission of speed, telemetering,
teleprinting, telecontrol, teleindication & teleprotections signals in the
carrier frequency range between 50Hz to 500 kHz over the following
communication media with suitable line equipment.
The
different types of power lines
Ø High
volume and medium volume power lines
Ø Open wire lines exposed to power lines
Model 9505 PLCC provides single or twin channel voice grade for
the transmission of
speech or audio tones over high volume
transmission lines. The transmitted audio tones can be used for telemetering
supervisory control, protective relaying, data or other purposes. When used
for data only, each channel carries onto typically 2450-based
telegraphed channels or a small no of channels at high base rates.
Features of PLCC are: programming,
efficiency, flexibility, voice grade connections and output power. Thus they
are used for 3 purposes.
Ø Station to station communication
Ø Data transmission
Ø Protection purpose
4.2.14.1
Principle
AF signals are converted into IF signals using IF carriers of
5.12 MHz generated in the system using a crystal oscillator. The required IF
signals are filtered out using IO pole crystal filter to a final mixer stage.
The carrier required for final mixing is derived from a VCO. The section works
on PLL principle and can be programmed to oscillate so as to give HF output in
the range of 50-500 kHz in steps of 0.5 kHz. Programming can be achieved by
simple strapping.
4.2.15
Battery & Battery Charger
The station DC source is facilitated
through battery of 400 Ah capacity and 200 Ah capacity. The 400 Ah battery bank
No 1 is fed through the battery charger from the main control room. This is of
110kV, 50 A capacity. The second 400Ah battery bank has the same capacity. 200
Ah bank is fed through the battery charger located in the old control room.
This is of 110kV, 15 A capacity. This feeds only 11kV cubicles located in the
old control room. 110 volt supply is always provided as a standby as there is
possibility of power failure in station. At this time also the tripping in case
of fault should continue, for this the 80V DC supply is very essential. 55
batteries each of 2 volt are provided giving a total of 110 V. In some area the
required voltage is less, in such cases the batteries used also should be less.
The batteries are lead acid cells and have sulphuric acid as its electrolyte
with lead electrode along with spongy lead in between. They have 400 Ah
capacity i.e. they can supply a current of 400 A for a time of 1 hour. So it
can be used to supply 200 A at intervals of 2 hours. This voltage always
provided in parallel with the AC supply. It can be used in case the AC fails.
The batteries can be charged in 2 modes, float charging and boost charging.
Float charging is used when AC is present and Boost charging is used when the
battery is in the back up mode. Battery is regularly checked in the substation
to check the acidity
of electrolyte. A hydrometer is used
to measure the same. To measure the voltage there is the centre zero voltmeter.
4.3
SPECIFICATIONS & RATINGS OF TRANSFORMERS
4.3.1
63MVA, 110/66kV Autotransformer
Ø Volts at no load
|
HV
|
:
|
11000
|
LV
|
:
|
66000
|
|
Ø Ampere
|
HV
|
:
|
220/331
|
LV
|
:
|
367/551
|
|
Ø Phase
|
HV
|
:
|
3
|
LV
|
:
|
3
|
|
Ø Type of cooling
|
:
|
ONAN/ONAF
|
|
Ø Frequency
|
:
|
50HZ
|
|
Ø Impedance voltage
|
:
|
14.02%
|
|
Ø Weight of Core and windings
|
:
|
26200kg
|
|
Ø Weight of oil
|
:
|
18740kg
|
|
Ø Volume of oil
|
:
|
20829litres
|
|
Ø Guaranteed maximum
|
|||
Temperature
rise in oil
|
:
|
450C
|
|
Ø Number of working taps
|
:
|
9
|
|
4.3.2 10 MVA, 110/11kV Transformer
|
|||
Ø kVA
|
HV
|
:
|
10000
|
LV
|
:
|
10000
|
|
Ø Volts on no load
|
HV
|
:
|
110,000
|
LV
|
:
|
11,000
|
|
Ø Amperes
|
HV
|
:
|
52.48
|
LV
|
:
|
524.86
|
|
Ø Phase
|
HV
|
:
|
3
|
LV
|
:
|
3
|
|
Ø Type of cooling
|
:
|
ONAN
|
|
Ø
Frequency
|
:
|
50Hz
|
|
Ø
Impedance voltage
|
:
|
9.65%
|
|
Ø
Un-tanking mass
|
:
|
11500kg
|
|
Ø
Mass of oil
|
:
|
7920kg
|
|
Ø
Total mass
|
:
|
26200kg
|
|
Ø
Volume of oil
|
:
|
8800litres
|
Ø Guaranteed maximum
Temperature
rise in oil : 450C
Ø Maximum temperature
rise
of winding : 550C
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