Midway Report: Project Semester: Study of 220KV Substation and SLDC
Midway Report: Project Semester: Study of 220KV Substation and SLDC
Midway Report: Project Semester: Study of 220KV Substation and SLDC
on
Study of 220KV Substation and SLDC
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Contents
1. INTRODUCTION.................................................................................................................................3
Key diagram.......................................................................................................................................5
2. SCOPE OF TRAINING..........................................................................................................................6
3. GOALS and OBJECTIVES.....................................................................................................................7
4. PROJECTS...........................................................................................................................................7
i. Familiarization and Maintenance of Substation components.........................................................7
Transformer and its parts..............................................................................................................7
DC battery system........................................................................................................................10
Circuit breaker.............................................................................................................................10
Isolators & Earth switches...........................................................................................................12
BUS BARS.....................................................................................................................................13
Capacitor bank.............................................................................................................................14
Relays..........................................................................................................................................14
ii. Tan delta testing of insulation......................................................................................................15
Overview......................................................................................................................................15
Modes of Tan delta test...............................................................................................................16
Precautions to be taken before test............................................................................................16
Test Procedure.............................................................................................................................17
5. Remaining Work..............................................................................................................................17
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1. INTRODUCTION
Electrical power is being generated and transmitted to various cities and towns. This
electrical power is transmitted to the consumer through Electrical substations. An Electrical
Substation is a subsidiary substation of an electrical Transmission, Generation and
Distribution system where voltages is transformed from high voltage to low voltages and vice
versa .The transmission system is inevitable long and high voltage transmission lines are
necessary to transmit huge blocks of power from the sources of generation to the load
centres.Substations generally have switching, protection and control equipments and
transformers. In large substations circuit breakers are used to interrupt any short-circuits or
over loaded current while in the small substations fuses are used for the protection purposes.
Sub-stations may be on the surface with enclosures, underground or indoor.
2. SCOPE OF TRAINING
Substations may be owned and operated by an electrical utility, or may be owned by a large
industrial or commercial customer.
Substations generally have switching, protection and control equipment, and transformers. In
a large substation, circuit breakers are used to interrupt any short circuits or overload currents
that may occur on the network. Smaller distribution stations may use recloser circuit breakers
or fuses for protection of distribution circuits. Substations themselves do not usually have
generators, although a power plant may have a substation nearby. Other devices such as
capacitors, voltage regulators, and reactors may also be located at a substation.
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vi. Pulling of circuit breaker switch or pushing buttons to interrupt flow of current in
disturbed line preparatory to repair, and to connect alternate circuit to carry load of
deenergized lines.
vii. Recording of temperature of transformers at specified intervals.
viii. Calculating average and peak load conditions from electric recording instrument data
and compile periodic report of load variations for system planning purposes.
4. PROJECTS
i. Familiarization and Maintenance of Substation components
Substations generally have switching, protection and control equipment, and transformers. In
substation, transformer serve the purpose of converting high voltage to low voltage via
electromagnetic induction at the same frequency. Being a static device, the efficiency of
transformer is on the high side. In a large substation, circuit breakers are used to interrupt any
short circuits or overload currents that may occur on the network. Smaller distribution
stations may use recloser circuit breakers or fuses for protection of distribution circuits.
Substations themselves do not usually have generators, although a power plant may have a
substation nearby. Other devices such as capacitors, voltage regulators, and reactors may also
be located at a substation. Detailed description of components is given below:
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of wear does not arise. Copper and Iron Core can last indefinitely. It is, however,
subjected to many strains and stresses under operation such as occurrences like
lightning surges, short circuits and over voltages; each of which leaves its effect on
the winding structure. Yet being a sturdy equipment, a transformer does survive all
these and gives a long life, if it is installed, operated and maintained intelligently.
Specification
Insulation
The conductor material must have insulation to ensure the current travels around the
core and not through a turn-to-turn short-circuit. In power transformers, the voltage
difference between parts of the primary and secondary windings can be quite large.
Insulation is inserted between layers of windings to prevent arcing, and the
transformer may also be immersed in transformer oil that provides further insulation.
To ensure that the insulating capability of the transformer oil does not deteriorate, the
transformer casing is completely sealed against moisture ingress. The oil serves as
both cooling medium to remove heat from the core and coil and as part of the
insulation system.
Buchholz Relay
Buchholz relay is a safety device which is generally used in large oil immersed
transformers (rated more than 500 kVA). It is a type of oil and gas actuated protection
relay. It is used for the protection of a transformer from the faults occurring inside the
transformer, such as impulse breakdown of the insulating oil, insulation failure of
turns etc. Whenever a fault occurs inside the transformer, such as insulation failure of
turns, breakdown of core or excess core heating, the fault is accompanied by
production of excess heat. This excess heat decomposes the transformer insulating oil
which results in production of gas. The generation of gases depend on intensity the of
fault. Gas bubbles tend to flow in upward direction towards conservator and hence
they are collected in the buchholz relay which is placed on the pipe connecting the
transformer tank and conservator.
Breather
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causes the volume of the oil to be decreased which again causes air to enter into
conservator tank.
The natural air always consists of more or less moisture in it and this moisture can be
mixed up with oil if it is allowed to enter into the transformer. The air moisture should
be resisted during entering of the air into the transformer, because moisture is very
harmful for transformer insulation. A silica gel breather is the most commonly used
way of filtering air from moisture.Silica gel breather for transformer is connected with
conservator tank by means of breathing pipe.
Maintenance of Transformer
The transformer shall not run at a load more than the rated load, except for an
unavoidable shorter period.
Check that the fans are running, if the winding temperature of transformer is
60º C or above. In case the fans are not running then they shall be switched on
manually and non-operation of auto system shall be reported to the In charge.
If the winding temperature is below 60º C, check the operation of fans by
switching them on manually and after checking, the Auto /Manual switch be
returned to Auto position. Intimation of defective fans, if any, be reported to In
charge for rectification.
The winding / oil temperature of the transformer shall not be allowed to be
abnormal looking to the load and ambient temperature. Difference in the oil
and winding temperature meters of the transformer shall not be more than 15º
C. If it is more, this may be either due to the defective temperature meters or
due to some abnormality in the transformer.
The colour of silica gel should be blue.
The oil level in the breather oil cup (at bottom) shall be up to required level. If
not, get transformer oil filled up to the marked level.
In case the over-flux alarm is received, the tap position of the transformer
should immediately be lowered to a tap corresponding to the voltage of High
Voltage side. If the voltage is above the rated voltage of tap No.2, then
message should be given to the feeding 400 kV/220 kV/132 kV GSS and Load
Dispatch Center to reduce the system / supply voltage. If the voltage is more
than 6% higher then the rated voltage at tap No.2, the transformer should be
switched off and Load Dispatch Center intimated.
In the event of the operation of buchholz alarm, buchholz trip, OLTC
buchholz surge trip etc., the transformer is to be isolated from the system
immediately. Where the buchholz alarm is not connected to trip the HV/LV
breaker, both the HV/LV breakers shall be tripped manually. Transformer
shall be isolated from system by opening the required HV/LV isolators and
intimation be given to the Incharge. The transformer is not to be energized till
it is thoroughly tested and the possibility of internal damage to the transformer
is ruled out.
In the event of operation of differential protection of the transformer, the
transformer is to be isolated from the system immediately. The Shift-in-charge
shall carry out the physical inspection of all the equipments (such as LAs of
400 kV/ 220 kV/ 132 kV/33 kV/ 11 kV, Cable Boxes and transformer)
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installed between 400 kV, 220 kV, 132 kV and 33 kV or 11 kV transformer
CTs and also 400kV,220kV,132 kV, 33 kV or 11 kV CTs themselves. The
buchholz relay of the transformer shall be checked for any collection of gas.
The relief vent diaphragm is to be checked. The Incharge is to be informed
immediately. The transformer is not to be energized till reason of operation is
diagnosed.
In the event of the transformer tripping on HV side O/C and E/F only, the
following is to be checked. If tripping is due to over current as a result of over
loading, the transformer can be charged after shedding the excess load. Before
energization of the transformer, all the feeders / incoming breakers are to be
tripped manually. In the event of the transformer tripping again on charging, it
is to be isolated from the system and intimation given to the Incharge. The
transformer is not to be re-energized till the same is thoroughly tested and the
possibility of internal damage to transformer is ruled out.
DC battery system
D.C. auxiliary Power supply: Being most reliable, the D.C. auxiliary power supply is
provided for supplying power to protection relays, circuit breakers, indicating circuits,
communication system, etc. D.C. Auxiliary power supply is provided from storage
batteries maintained continuously charged by some source of D.C. supply set or a
charger. The voltage of the D.C. auxiliary supply is maintained at 110/220 volt for all
the time for effective protection & control of substation equipment. DC float voltage
is to be maintained between 118 to 121 volts for 110-volt system while it is to be 236
to 242 volt for 220 volt system. For conventional type of battery sets, measure the
gravity of three battery cells at random which shall be in the range of 1210±5
(corrected to 27° C) for Standard makes battery cells and 1200±5 for EXIDE /EMCO
make battery cells. If the measured gravity is outside this range, matter is to be
reported to for the in -charge for remedial action. Temperature correction is to be
applied to the readings of the specific gravity for converting it to the reference
temperature of 27º C. For cell electrolyte temperature 1ºC above 27º C , 0.7 is to be
added to the reading of the hydrometer for correcting it to 27º C and for below 27º C,
0.7 is to be subtracted from the reading of the hydrometer for correcting it to 27º C .
[SG27 = SGt + 0.7 (t-27)].
Circuit breaker
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to detect a fault condition and interrupt current flow. Unlike a fuse, which operates
once and then must be replaced, a circuit breaker can be reset (either manually or
automatically) to resume normal operation. Circuit breakers are made in varying
sizes, from small devices that protect an individual household appliance up to large
switchgear designed to protect high-voltage circuits feeding an entire city.
They can be classified into:
➢ Oil circuit breaker
➢ Air-blast circuit breaker
➢ Sulphur hexafluoride circuit breaker (SF6)
➢ Vacuum circuit breakers
SF6 and Vacuum circuit breaker are being used in 220KV distribution substation.
Maintenance
PREVENTIVE MAINTENANCE:
Preventive maintenance of electrical equipment primarily amounts to testing the
insulation and contact resistance of the various pieces of equipment.
RESISTANCE TESTING:
Insulation resistance test reveal defects and faults which cannot be detected by
external
inspection. Though not upsetting normal operation of the equipment at the time of
testing, these defects may later lead to a breakdown.
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Insulation defects may be due to variety of causes, such as overheating due to
overloads,
excess moisture, mechanical injury, ageing etc.
For preventive purpose, it is normal practice to test the insulation of support insulators
and circuit breaker. Insulation resistance may be tested with a megger only on de -
energized circuits or equipments.
Moisture absorption is usually evaluated from the ratio of two values of insulation
resistance measured with a megger after 60 seconds and 15 seconds respectively. This
ratio is called the coefficient of moisture absorption and is calculated by the Equation
‘K’ = (R-60) /(R-15)
Where, ‘K’ is Co-efficient of moisture absorption.
R-60 insulation resistance as measured by megger after 60 seconds.
R-15 insulation resistance as measured by megger after 15 seconds.
Damp insulation will have co-efficient value closed to 1. As the insulation dries out,
the coefficient rises and when it becomes 1.3 or above, the insulation is considered to
be dry.
i. External cleaning
The insulators of the Breaker should be cleaned from salt and dirt/dust deposition
together with the cleaning of the other insulators in the substation. The time interval
for this cleaning shall be based on the polluting atmosphere or the periodicity
mentioned in maintenance schedule.
ii. Rust Protection :
Some parts of the mechanism in the operating mechanism are made of steel and are
surface treated against rust. In spite of the good rust protection, minor corrosion may
occur after some years, especially when the breaker is standing in strong corrosive
surroundings. The rust stains shall be cleaned with emery paper and new rust
protection shall be painted or sprayed on. As rust protection, grease G or Tectyl 506 is
recommended.
iii. Tightness check :
The breakers are provided with density monitor switches (temperature compensated
pressure gauges). Every density monitor switch is provided with an alarm contact
which gives an electrical signal if abnormal leakage takes place. With the first
inspection, the bolted joints on the breaker and operating mechanism shall be
tightened up. All the wiring joints in the terminal blocks of the operating mechanism
shall be re-tightened at regular intervals as per maintenance schedule. It is not
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necessary to repeat this tightening, only after bigger overhauls. SF6 gas leakage is to
be detected using suitable gas leak detector.
iv. Air protection:
After maintenance/overhaul of the Circuit Breaker, it should be evacuated by vacuum
pump before filling in the SF6 gas, so that SF6 gas does not mix with ambient air and
also humidity and dust particles are removed from the Breaker. With vacuum pump, a
final vacuum must be reached less than 5 mbar.
Maintenance
Insulator – Insulation
They should be cleaned and inspected; porcelain insulation should be examined for
cracks and other defects. Maintenance as given under bushings in sections 1.5.10 and
2.1.4 under transformers and OCB should be followed:
2. Contacts – fixed and moving
Contacts should be examined, over heating and / or other damages. They should be
reconditioned or renewed as required. Fixed and moving copper contacts may be
dressed by using a fine file or fine glass paper. Evmy or corborundun paper should not
be used Earth connections of the isolator with earth blades should be checked and
tightened. If the isolator is provided with sparking tips (arching horns) , they should
be checked and ensured that sparking tips touch before the main contacts close these
tips and replace if badly damaged. Check auxiliary contacts for correct alignment
lightly spray with petroleum jelly.
3. Mechanism Cleaning;
Examine the isolator control mechanism and renew parts. Lubricate and check for
correct operation. In case motor operated mechanism, check operation of motor,
motor brakes, limit switches etc.
4. Indicating devices and interlocks
Examine ‘ON’ and ‘OFF’ indicators and also interlocks, padlocking devices and test
the same, whenever necessary.
5. Check supporting structure for loose nuts & bolts.
Check earthing connections of supporting structure and metallic parts of the isolator
including operating handle.
6. Caution: Mechanical/electrical interlocks of earth switches with circuit breaker
closing circuit and line isolators must always be kept in proper working condition.
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BUS BARS
Maintenance
Bus bar of a substation is the only part which has to carry maximum load current
during
normal operation as well during fault conditions.
Any poor joint or loose jumper in this part may create heating, voltage drop and
current
unbalance in three phase system during normal operation. Some loose jumper or a
poor joint may open or fail during fault condition when heavy fault current is being fed
by a bus bar section, this may result in ‘bus fault’. And a bus fault is the worst kind of
fault in a power system.
Thus a bus bar itself and all other components located in the ‘bus bar zone ‘ like
jumpers
isolators, CTs, PTs, breakers etc. form a critical part of power system which if fail may
create a bus fault. Thus it is very important to maintain this part of power system in
perfect condition.
Capacitor bank
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above 0.98 lagging or the voltage exceeds the normal voltage even after reducing the
tap position of the transformer, the capacitors bank is to be switched OFF.
Prevention
Whenever any capacitors are put into circuit, the load current on the transformer
incoming panel should be observed. If the load current remains the same or is
increased by switching on the capacitors, the capacitor should be immediately
switched OFF. If the load current is reduced, then only the capacitor shall be kept in
the circuit.
Wherever there are two or more separate capacitor banks of the same voltage rating on
the same bus but controlled by separate breakers, then two or more of them cannot be
switched on unless they are provided with series reactors. At sub stations, where two
or more capacitor banks are installed and one of them is not provided with series
reactor, then the capacitor bank without series reactor is to be switched ON first and
the capacitor bank (s) with series reactor is (are) to be switched ON there after. To
explain, the first capacitor bank to be switched ON can be without series reactor, but
any second or later capacitor bank switched on should have series reactor.
The capacitors should not be switched on within 5 minutes of switching OFF so that
by that time the capacitors are discharged to a safe value.
Relays
Maintenance
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i.Maintenance and testing procedure given by manufacturers of relays and equipments
are generally found to be sufficient and adequate and, therefore, are to be strictly
followed by the utilities. The periodicity of maintenance and testing as recommended
by the manufacturers may, however, be reviewed by utilities depending on the
working environment, site conditions, frequency of faults, behavior and operation of
relays/equipments and type of relays etc. However, it is suggested that periodic testing
of relays should be done once in a year.
ii. All the protective devices including CT, PT, relay panels etc. should be earthed
properly. Earthing of PT primary separately and adequately must be ensured. Earthing
of star point of PT secondary/CT secondary must be done effectively. However,
earthing of CT PT secondary at only one point should be adopted.
iii. Multiple earthed CT/PT secondary winding are a source of trouble and responsible
for mal-operation of protective relays.
iv. Proper earthing of the relay panel should be ensured. This is very important from
the
point of operation of static relays.
A pure insulator when is connected across line and earth, it behaves as a capacitor. In an ideal
insulator, as the insulating material which acts as dielectric too, is 100 % pure, the electric
current passing through the insulator, only have capacitive component.
There is no resistive component of the current, flowing from line to earth through insulator as
in ideal insulating material, there is zero percent impurity.
In pure capacitor, the capacitive electric current leads the applied voltage by 90°.
In practice, the insulator cannot be made 100% pure. Also due to ageing of insulator the
impurities like, dirt and moisture enter into it.
These impurities provide conductive path to the current. Consequently, leakage electric
current flowing from line earth through insulator has also resistive component.
Hence, it is needless to say that, for good insulator, this resistive component of leakage
electric current is quite low. In other way the healthiness of an electrical insulator can be
determined by ratio of resistive component to capacitive component. For good insulator this
ratio would be quite low. This ratio is commonly known as tanδ or tan delta. Sometimes it is
also referred as dissipation factor.
There are two ways to predict the condition of an insulation system during tan delta or
dissipation factor test. First one is, comparing the results of previous tests to determine, the
deterioration of the condition of insulation due ageing affect.
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Second one is, determining the condition of insulation from the value of tanδ, directly. No
requirement of comparing previous results of tan delta test.
If the insulation is perfect, the loss factor will be approximately same for all range of test
voltages. But if the insulation is not good enough, the value of tan delta increases in higher
range of test voltage.
UST
Test set connected for Ungrounded Specimen Test mode. This is used when specimen is
isolated from earth e.g. Transformer bushing, CTs with test tap, CVTs and CB voltage
grading capacitors. The test mode is often used to reduce the effect of stray capacitance losses
to ground, and to reduce the effect of interference pickup from energized apparatus.
GST
Test set connected for grounded specimen test mode. This is used when specimen do not have
two specific points (isolated from ground) for Tan delta measurement
e.g.Transformer/Reactor winding, CTs without test tap etc.
GSTg:
Where the Guard (g) is used for measuring the Capacitance between Transformer Winding
and Earth.
Objective
The Objective of this Test is to ascertain the general condition of the ground and inter-
winding insulation of transformer
1. The test set is a source of high voltage electrical energy and operator must use all
practical safely precautions to prevent contact with energized parts of the test
equipment and related circuits.
2. Never connect the test set to energized equipment.
3. The ground cable must be connected first and removed last.
4. The ground terminal of the input supply card (green lead) must be connected to the
protective ground (earth) terminal of the line power source.
5. Keep the high voltage plugs free from moisture, dust during installation and
operation.
6. Adequate clearance (Min 1 foot i.e. 30 cm) arc maintained between energized
conductor and ground to prevent any arc over.
7. It should be ensured that test specimen is de-energized and grounded before making
any further connection and no person may come in contact with HV output terminal
or any materiel energized by the output.
8. Porcelain of the bushings shall be clean and dry. Remove any dirt or oil with clean
dry cloth.
9. Test shall not be carried out when there is condensation on the porcelain. Preferably,
tests shall not be carried out when the relative humidity is in excess of 75%
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Test Procedure
1. Isolate Transformer from High Voltage & Low voltage side with physical inspection
of open condition of the concerned isolators/disconnector.
2. Measure and record the ambient temperature and relative humidity for reference.
3. Short the Terminals of the bushings of each winding together using bare braided
copper jumper. These jumpers shall not be allowed to sag.
4. Connect the ground terminal of the test set to a low impedance earth ground (to Earth
mat of the substation).
5. Connect the external interlock cable to the ‘Interlock terminal of the test set.
6. Test kit HV Lead connected to Transformer Hv Winding & Test Kit LV Lead
connected to Transformer LV Winding. Carry out the Test in UST, GST and GSTg
mode.
7. Test kit HV Lead connected to Transformer LV Winding & Test Kit LV Lead
connected to Transformer HV Winding. Carry out the Test in GST and GSTg mode.
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