EHT SUB STATIONS
3.7 LIGHTNING SHIELDING
This protection could be achieved by overhead ground wires of 7/4.064 mm hot dip galvanized steel wire, mounted at a h eight above the power conductors maintaining minimum earth clearance. Lightning masts also could be used instead of ground wires.
3.7.2 When overhead ground wires used, all electrical equipment outside the area enclosed by the ground wires shall fall within the area covered by 30 degrees angle to the vertical on the outside of the outer most ground wires. All electrical equipment within the area enclosed by the ground wires shall within the area covered by a 60 degree angle to the vertical on the side of the ground wires.
If lightning masts or rod used all electrical equipment outside the area enclosed by the lightning masts or rods shall fall within the protective cone formed by rotating a line at 30 degrees to the vertical with the lightning mast or rod peak as apex through 360 degrees. All electrical equipment within the area enclosed by the lightning masts or rods shall fall within the protective cone formed by rotating a line at 60 degree to the vertical with the lightning mast or rod peak as apex through 360 degrees.
The ground wire and/or masts/rods shall be connected to the main station earthing system bus solidly (and not through supporting steel structures) for preventing difference of surge potential between the shield and other grounded parts of the station.
3.8 LIGHTING :
IS : 3646 (Part II) Schedule of values of illumination and glare index-recommends values of intensity of illumination. The following uniform illumination levels are being adopted.
i) Switchyard – 25 Lux
ii) Control room and Test room – 300 to 500 Lux
iii) Carrier room MCC room, LT panels, Chargers, Offices, Conference Hall, Rest Room Reception, Workshop, Repair bay etc., - 300 Lux
iv) Battery room, Corridors, PLCC room, Toilets, Store Room, Cloak room, Stairs etc.
– 100 Lux
3.9 SURGE DIVERTOR (Lightning Arrestors) 3.9.1 Guide for selection of LA
3.9.1.1 Before selecting the L.A it should be ascertained whether the system is effectively earthed, non-effectively earthed or having isolated neutral.
3.9.1.2 The system neutrals are considered to be effectively earthed when the co-efficient of earthing does not exceed 80%. In this case, the reactance ratio X0/ X1 (zero sequence reactance/positive sequence reactance) is positive and less than 3 and at the same time the resistance ratio RO/X1 (zero sequence resistance/positive sequence reactance) is less than 1 at any point on the system. For this system the arrestor rating will be 80% of the highest phase to phase system voltage.
3.9.1.3 The L.A voltage rating corresponding to the system voltages normal are indicated below :
3.9.1.4 The L.As are usually procured along with surge counters and leakage current ammeters, if available.
3.9.2 LOCATION OF LIGHTING ARRESTORS : The L.As employed for protecting transformers should be installed as close as possible to the transformer. The electrical circuit length between L.A and the transformer bushing terminal should not exceed the limits given below :
Rated system
Max. distance between L.A and Transformer bushing terminal (inclusive
of lead length) (in metres) Effectively earthed
3.9.4 Station type L.As should be provided on all feeders rated 33KV and below .
3.9.5 Long cables from the station to exposed Overhead lines should be protected by arrestors installed at the junction of the cable to O.H line.
3.10 ISOLATORS
Air break isolators or disconnecting switches are not intended to break load though these are meant for transfer of load from one bus to another and also to isolate equipment for maintenance. These are available mainly in two types vertical break type and horizontal break type. The later type requires larger width. However the space requirement can be reduced in this horizontal break isolators by having double break with a center rotating pillar. Pantograph and semi-pantograph disconnects involve vertical movements of contact arm and therefore require less separation between phases and thereby require less separation between phases and thereby help in reducing the sub-station area to a larger extent. The isolators could be operated mechanically or hydraulically or pneumatically or by electric motor.
Earthing facility shall be provided wherever required.
Rated Insulation Levels (IS 1818 – 1972) Sl.
No.
Item 11 KV 33 KV 66 KV 132 KV 220 KV 400 KV
1. Standard and impulse withstand voltage
2. One minute power frequency with stand
(x) These values apply when the frame of the isolator is connected to the mid point of the voltage source.
Note 1. For earthing devices only, the test voltages to earth and between poles apply Note 2. The table applies at the conditions of temperature, pressure and humidity
which are 20 Degrees Centigrade, 760 mm and humidity which are 20 Degrees Centigrade, 760 mm and 11g of water/cub. Meter.
3.10.2 Switches controlling transformers or feeders should be capable of breaking their charging currents.
3.10.3 High pressure contact switches for outdoor service are generally with a contact pressure of 0.227 kgs per Amp. Capacity (rough guide)
3.10.4 Breaking capacity of switches :
Breaking current of ASEA switchs – Empirical formula
I = K L E Where
I = Breaking current in Amps (r.m.s)
K = 0.06 when breaking a capacitor or unloaded cable
0.15 when breaking an unloaded overhead transmission line 0.20 when breaking unloaded transformer
0.40 when breaking load current
L = Centre to Centre phase distance in mm. though not exceeding four Times the flash over distance across an open pole.
E = Breaking voltage, Main voltage Kv-rms
1. MAINTENANCE SCHEDULE OF E.H.V. POWER TRANSFORMERS
Sl.No. Item of Maintenance Periodicity Remarks
1. Checking the colour of silicagel in the breather and replacement or reconditioning if colour changes from blue to pink say about 50% of the total quantity. Checking up the oil level of the oil seal (to be upto the level marked in the cup)
Daily
2. Checking of oil level in a) Main conservator b) OLTC conservator c) bushings and examining for leaks of oil abnormality from oil pumps and cooling fans
Daily 5 Checking up of oil and winding temperatures Hourly 6 Checking for pressure relief explosion vent 8 Ensuring that oil comes out when air release valve
is opened (of the main tank)
Quarterly 9 Measuring insulation resistance of windings with
an appropriate Megger (note down oil temp.)
Quarterly
Ensure oil level in OLTC
Quarterly 12 Main tank oil testing for BDV and moisture
content
Half yearly
13 a) Checking of Bucholtz
relay for any gas collection and testing the gas collected
Quarterly or during fault
Half yearly or
b) Checking of operation of Bucholtz relay by air injection
c) Noting the oil level in
a) Lubricating / greasing all moving parts Quarterly 15 Checking of all connection on the transformer for
tightness such as bushings, tank earth connection,
etc. Quarterly
16 Forced cooling system
a) Megger testing of
motors (pump) lubricating the mechanical parts and cooling fans
b) Cleaning of water jacket
c) Cleaning of water jacket
circuits of oil pumps and cooling fans for earth leakage overload trip alarm, etc.
d) Checking of interlocks for pumps. Cooling of interlocks for pumps.
Cooling Air Blast fans for Auto Start and Stop operation at correct temperature setting and for manual operation
Yearly
Quarterly Quarterly
Quarterly
17 Oil level in oil seal and replacement Quarterly 18 Testing of oil for dissolved gas analysis for 100
MVA and above. If the results show abnormality, frequency of DGA may be increased as per the
19 Pressure testing of oil coolers Half Yearly
20 Testing of motors, pumps and calibrating pressure gauges, etc.
Half Yearly 21 Overhauling of pumps, motors and cooling fans Yearly or as and
when necessary 22 Testing of oil in main tank for acidity, tan delta,
IFT and resistivity
Yearly
23 Bushings testing for tan delta Yearly
24 Calibration of oil and winding temperature indicator
Yearly 25 Measurement of excitation current at low voltage
at normal tap and extreme taps
Yearly 26 Measurement of DC winding resistance Yearly
27 Ratio test at all taps Yearly
28 Checking the bushing CT for WTI for correct ratio Yearly
29 OLTC
a) Inspection of contacts in diverter Depending upon the no. of operations as recommended by
the manufacturer b) Driving mechanism visual check-up,
overhauling if necessary
Yearly
30 a) Tap position indicator Yearly
b) Checking for proper working of remote tap position indicator, remote winding test indicator
Yearly 31 Operatingof Bucholtz relay for alarm and trip by
draining of oil and injection of air with cycle pump
Yearly 32 Checking for leakage in air cell (for transformers
fitted with Air Cell)
Yearly 33 Oil level in Thermometer pocket top up if required Yearly 34 Bushing partial discharge test and capacitance Once in 5 years
35 General Overhaul Once in 10 years
a) Core tightening
b) De-sludging / washing of windings c) Inspection of core and windings d) Replacement of all gaskets e) Check core bolt insulation
36 Filtration of oil Whenever oil test
results are below permissible limits
INTERPRETATION OF THE ANALYSIS OF GASES IN TRANSFORMERS AND