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THE FOLLOWING ARE THE MAIN EQUIPMENT AT 33/11 KV
SUBSTATIONS
Circuit breakers 33 kv, 11 kv
Power Transformer 33/11 kv
AB Switches 33 kv,11 kv
Station Battery or small size multiple tripping units foreach CB
Lightening Arrestors 33 kv, 11 kv
Instrument transformers CTs & PTs 33 kv,11 kv
Control & Relay panels Control cables and circuitry
Station Earth
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Regular inspections and preventive maintenance to bedone on each of the above equipment are codified as
Preventive Maintenance schedules fixing periodicity for each
item.
The main objective of the maintenance is to maintainthe insulation in good condition and to avoid entry of
moisture and to remove dirt.
The sustained operating temperature of about 8 to 10
Degrees Centigrade more than the operating temperature of
75 Degrees Centigrade will shorten the life of transformer oil,
circuit breaker etc. Hence, over loading should be avoided.
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As far as possible the temperature of oil and
windings shall be maintained at 4
0
C and 45
0
C aboveambient temperature. If the acidity of oil exceeds 1.0
mg. KOH/Gm of Oil, the oil should be replaced with
fresh oil.
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STATION BATTERY
Battery Operation and Maintenance
(A) The Ampere hour capacity of 220V batteries at
smaller Sub- station shall be 80.
(B) The same at EHT Sub-stations shall be 200
(C) Batteries with 300 Amps hour capacity shall be
used only at Power house or Sub- stations where
solenoid closing of circuit breakers is in use.
The trickle charging rate shall be
Amp hour capacity x 2/24x100 plus regular discharge
in amps.
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The boost charge rate shall not exceed Amp. Hour
capacity divided by ten. The individual cell voltage shall not go down below 2.1
volt.
The specific gravity should not differ by more than 30
points between cells in the same battery maximum and
minimum. Where the difference is more: electrolyte
should be diluted by adding distilled water in cells with
higher specific gravity thus narrowing down the
difference and all cells in the battery given a boost
charge. Under no circumstance electrolyte orconcentrated acid should be added to cells with low
specific gravity.
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Usage of alkali cells and acid cells in the same substation
should be avoided to avert inadvertent mix up of
electrolyte or usage of accessories of one with the other.
Leakage indication lamps should be compulsorily connectedon the charges panel for continuous indication of healthiness.
Every D.C. Circuit takes off should be through protective fuses
(H.R.C) or m.c.bs.
(i) Once in a day A.C. Supply to charges should beswitched off and D.C voltage measured and noted.
(ii) In that condition with no A.C. supply to charger, the
duty performance of the battery by closing or
tripping of a relatively un-important breaker is to be
ensured. Mere availability of D.C Voltage is no indexof healthiness of battery.
(iii) A.C Supply to charger is to be restored immediately
after this test.
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Certain charger panels have Switch Off
arrangement whenever A.C supply fails. There
should be switched on after each restoration ofsupply.
Leakages in D.C circuitry should be attended on
top priority first by sectionalisation, then byisolation and finally be rectification.
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Lead Acid Batteries (Common)
Check up the Electricity to level in the cell of all the cells
in morning shift every day. If the electrolyte is low, top upthe cell with distilled water.
Check up the voltage and specific gravity of six cells in
each shift and record as per cell numbers given ( PILOT
CELLS)
Check up the D.C voltage of the combined cell (battery)
after switching off the battery charger every day in the
morning shift. Record the battery voltage and current
with the charger off.
Ensure that the battery is in floating condition normallyby adjusting the rheostat of the charger.
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Never charge a battery at a voltage higher than 2.4 V per
cell i.e., it works out to 264 V for 220 V Battery and limit
the charger ampere capacity also.
If any of the cells is found to be having low voltage and
specific gravity, by pass the cell. The removed cell should
be charged separately and replaced after the voltage and
specific gravity attains normal value. Improvementshould not be attempted by adding acid to electrolyte.
Check the condition of all the cells every month for
voltage and specific gravity. Charge the battery if
required. The voltage and specific gravity readings shall
be taken before and after charging . The charging rate
shall be limited to 10 hours rating to avoid excessive
gassing and evaporation of Electrolyte.
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All the cells shall be physically checked for fracture and
buckling of the container and excessive rise. Check
battery terminals for tightness. Apply Vaseline
( Petroleum jelly) whenever necessary. This check shouldbe attempted after the battery is brought to full charge.
Keep the charger off, observe the voltage with D.C lighting
on continuously for two hours. Observe the voltage
and current. The voltage should not fall by 10% when
the discharge on the battery is at 10Hr i.e., 25 Ampsfor 250Hrs. battery 30 Amps for 300hrs. battery etc.
Every discharge shall be compensated by charging.
As the ampere hour efficiency is always less than unity,
the charging ampere hours shall be more than discharge
ampere hours.
The battery charger panels, in some case have a no volt
trips. Whenever A/C supply fails, the charger should be
switched on after restoration ofAC supply.
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Instrument Transformers
An instrument transformers role is to provide accurate
inputs to protection, control and metering systems includingrevenue metering.
The main tasks of Instrument Transformers are:
To transform currents or voltages from usually a highvalue to a value easy to handle for relays and
instruments.
To insulate the metering circuit from the primary high
voltage system
To provide possibilities of standardizing the instruments
and relays to a few rated currents and voltage.
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Current Transformer:
The Primary winding in a current transformer is
incorporated in the line and carries the current flowing in the
network. The magnitude of the secondary current is dependent
of the Ratio of the transformer.
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Accuracy:
The quality of measurement which characterize the
ability of measuring instrument to give indication equivalent tothe true value of the quantity measured. Accuracy limits
specified in a particular standard for different loads and power
factor differs.
Failure of Its can cause malfunction of system
protections , controls and metering devices and, in some
catastrophic failures, damages to nearby power equipment, or
injury to personnel in the vicinity.
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Dissipation Factor:
The ratio of the power loss in a dielectric material to thetotal power transmitted through the dielectric, the imperfection
of the dielectric. Equal to the tangent of the loss angle.
Most plastics have a low Dissipation Factor , a desirable
property because it minimizes the waste of elec. Energy as heat.
= tan De=1/2
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Where
F= Frequency of applied voltageCp= Equivalent parallel capacity
Rp= Equivalent parallel Resistance
= Loss angle
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Loss Angle
A measure of the power loss in an inductor or acapacitor, equal to the amount by which the angle
between the phasors denoting voltage and current
across the inductor/ capacitor differs from 900.
For an ideal capacitor , it is ZERO.
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