Sub-Station Equipment Maintenance Schedule (Fortnightly to Overhaul)

This chapter covers the complete periodic maintenance schedule for 25kV AC sub-station equipment — from Fortnightly checks through to full Overhaul cycles. Two general principles apply throughout: no work of any kind may begin on or near live equipment unless power supply has been switched off and all prescribed safety measures taken, and unattended sub-stations require vigilance against unauthorized interference — the Traction Power Controller (TPC) checks each grid sub-station once a day by telephone for maximum demand/energy consumption figures, recorded in a register.

Fortnightly Maintenance

General Inspection (by SSE/JE, PSI supervisor)

• Go round the whole substation area — inspect for general cleanliness, proper drainage, road and rail access. Surfaces of roadways/pathways should be free and sufficiently elevated to prevent waterlogging. Remove vegetation growth around the outer periphery; cut tree branches likely to come near live lines.
• If transformer oil is stored, inspect for security and fire risk — ensure no combustible material is in the vicinity.
• Examine all Caution/Danger/Shock Treatment and other boards for cleanliness and secure mounting. Inspect fire extinguishers, fire buckets, and first-aid boxes for intactness and serviceability.
• Inspect structure and plant foundations for sinking or cracking. Go round bolts and nuts on structural work checking tightness.
• Inspect all indication lamps on control panels for correct working.

Battery

1. Check all cells for general cleanliness, electrolyte level, colour of plates, and amount of sediment, then release/report as needed.
2. Take specific gravity and cell voltage of pilot cells and record in register. Top up with distilled water exactly to the correct level for each cell.
3. Check operation of battery charger and note trickle charging rate in register.

Monthly Maintenance

Bonding and Earthing

Inspect all earth connections visually, ensure they are in order and every equipment has duplicate earths. Tighten connecting bolts and nuts as necessary. Check that sub-station structures are properly bonded with the feeding post earth grid and the track by two independent connections, where the sub-station and feeding post are close by.

Oil Level in Transformers, Circuit Breakers, CTs, etc.

Check oil level in sight gauge glass and examine all joints, valves, plugs, etc. for oil leakage in each equipment. Rectify leaky parts if found and restore the oil level.

Insulators

Clean insulators of transformers, circuit breakers, or interrupters with dry cloth and look for flashover marks, cracks, or chippings. Badly chipped insulators should be replaced. Minor chippings can be rendered impervious to moisture by a light coating of Araldite or similar epoxy resin.

132/25kV Traction Transformers

1. Clean externally the tank, conservator, radiator, bushings, oil level indicator, gauges, etc. with dry cloth.
2. Make a note in the register of the maximum temperature of transformer oil on the dial indicator; reset the indicator.
3. Check explosion vent diaphragm for any damage or presence of oil.
4. Check silica-gel breather. If turning pink, replace with dry gel (blue colour) and recondition the old silica-gel. If the silica gel is too wet, check dielectric strength of transformer oil.
5. Check for any gas collection in Buchholz relay.
6. Check for leaky oil on transformer body, conservator tank, oil drain valve and foundations. If leaking, take corrective action by tightening bolts, replace gaskets if necessary.
7. Check if heater in the marshaling box is functioning properly, and if all terminal connections are in order.
8. Check PRD (Pressure Relief Device) function.

Circuit Breakers and Interrupters

1. Open the control box cover. Examine the interior and remove accumulated dust; if any part is badly rusted (indicating moisture entry), find the cause, plug the holes, and repaint rusted parts. Check weather-proof gaskets are in good condition (replace if not) to keep the box water-tight. Examine leading-in pipe connections are properly bushed, sealed, and water-tight. Check all pins and check-nuts are in place; also check tie-rod nuts for tightness.
2. Operate the mechanism at least twice manually. Have it operated on remote control from RCC, keeping the control door open. Observe whether the mechanism functions smoothly without rubbing or obstruction, and if the shock absorber functions properly when the circuit breaker is tripped.
3. Examine the commutator of the motor and clean with clean mull cloth. Examine carbon brushes and replace if necessary.
4. Check breather and breather holes for clogging.
5. Check gear oil level in the mechanism and replenish if required.
6. Check if heater is functioning properly.
7. Check interlocks of the device.
8. Check local position indicator and remote semaphore indicator for operation. Observe correct operation of recording counter. After complete checking, close the cover and test the breaker for operation under remote, local, and manual control.

Isolators

• Operate isolator several times and observe if it operates smoothly and correctly. Check interlocks and lubricate moving parts as necessary with petroleum jelly.
• If isolator is motor-operated, check commutator of motor and clean with dry mull cloth. Check carbon brushes for proper bedding and wear. Check motor smoothness. Clean limit-switch and auxiliary switch contacts and check tightness of wiring connections. Examine contactor box and signal box and clean thoroughly. Lubricate all gears, shafts, bearings, contacts, etc.

Bus Bars and Connectors

Immediately after switching off supply and earthing the lines, feel by hand all connectors and clamps on busbars and equipment terminals which carry heavy currents to check if they are too hot. If any connection is too hot, it indicates poor contact — open up the contactor, carefully clean the contact surfaces, touch up high spots so mating surfaces bed well together, apply a very light coat of Vaseline, refit, and tighten up.

Control and Relay Panel

1. Make a note of flag indicators, if any, on relays and then reset.
2. Check if all indicating and recording instruments are working properly and pointers are not sticky.
3. Note and record in the register the range of voltage and current variations during a 15-minute period at the time of inspection. Abnormal voltage or current should be noted for corrective action.
4. Clean the panels externally.

Batteries and Battery Chargers

1. Take specific gravity and cell voltage of every individual cell and enter in the register.
2. If the battery is not in a fully charged condition, boost charge should be given as required and trickle charging rate increased to the extent required. This should only be done by a supervisory official after investigating the cause for excessive discharge.
3. Make a general examination of battery charger. Check earth connection to the body.

Half-Yearly Maintenance

• General: SSE(PSI) should visit the grid sub-station and ascertain whether any significant change has occurred in the 132kV grid network during the past six months, or is expected shortly, with particular reference to tap settings of traction transformers.
• Auxiliary Transformers, PTs and CTs: maintained generally on lines similar to 132/25kV traction transformers except items that do not obviously apply. Additionally check fuse holders on the LT side make good wiping contact with fixed contacts and are secured in position.
• 132/25kV Traction Transformers: (i) Test oil sample for BDV — if it drops below 40kV, oil should be filtered. (ii) Check rod gap settings on bushings are in order as per maker's drawings. (iii) Measure and record insulation resistance of all windings to earth and other windings with a 2500V megger, along with temperature of windings and ambient temperature. (iv) Check all alarm/trip devices for proper working.
• Isolators: (i) Observe for overheating signs; check and wipe contact blades. Clean blade tips and fixed contact fingers, lightly Vaseline the contact-making surfaces. (ii) Clean all articulated joints, sliding and bearing surfaces with spirit. (iii) Check all split pins, lock nuts, checknuts for proper condition. (iv) Check correct meshing and alignment of arcing horns. (v) Operate the isolator slowly, check for simultaneous operation of blades on the poles and correct alignment of blade tips in fixed contact jaws; adjust if required so blades are fully home when handle is closed. (vi) Check locking arrangements.
• Control Panel and control cable connections: (i) Check tightness of all connections, remove cobwebs and wipe accumulated dust with dry cloth. (ii) Test relays by manual operation, put back cover, ensure dust-tight. (iii) Check if tap and time settings are in order. (iv) Examine fuses for signs of overheating, aging, springiness, and cleanliness of contact-making parts; clean and Vaseline lightly to ensure proper contact.

Yearly Maintenance

• General: (a) Inspect the fence all-round the sub-station and bonding between metal fencing panels and earth. Oil door hinges; repaint structural parts showing rust signs. (b) Open all trench covers and clean completely; clean all culverts, remove cobwebs, check possibility of lizards/insects entering enclosed control equipment and make insect-proof. (c) Arrange painting of walls and metal-works as necessary. (d) Check all equipment and busbar for thermography to detect hotspots.
• Lightning Arrestors: (i) Check earth terminals and earth strips for proper condition; clean insulators thoroughly. (ii) Where lightning arrestors have discharge counters, record the counter reading. (iii) Check and measure THRC (Total Harmonic Resistive Current) values.
• Bonding and Earthing: (i) Check physically the soundness of bonding and earthing connections to every electrical equipment, structural steel, lighting arrestors, etc., and inter-panel connections. (ii) Record earth resistance to body of electrical equipment and all parts of fencing and structural steel work. (iii) Check terminations of the overhead shield wire covering the whole sub-station are in good mechanical condition and properly bonded electrically to the structures. (iv) Check and record resistance of each group of earth plates after disconnecting from common earth system.
• 132/25kV Traction Transformer: (i) Test oil sample for acidity. (ii) Check oil level in bushings. (iii) Inspect bushing gaskets for leaks and tighten bolts. (iv) Move the tap-settings switch up and down the full range a few times so good contact is assured by self-wiping action; set the tap finally at the correct position ensuring tap-indication corresponds to position of main contacts. (v) Paint transformer tank on such parts as required.
• Isolators: (i) Smoothen burrs, if any, on blade tips and fixed contact fingers with fine emery paper and smear Vaseline. (ii) Measure clearance of blade in open position, record, and adjust crank mechanism if necessary. (iii) Check adjustable stop set-screws for proper condition and correct positioning. (iv) If motor-operated, measure and record insulation resistance of motor windings and contactor coil using a 500V megger. (v) Check interlocking for proper functioning.
• Bus Bars and Connectors: measure with a 'Ductor' or other low-resistance measuring instrument the contact resistances of all connections carrying heavy currents.
• Neutral sections: connect a recording voltmeter and obtain a 24-hour voltage chart separately for each side of every neutral section. If on the high side, take corrective steps.
• Sub-station and Feeding Posts: (a) Insert check meter at grid sub-station to ascertain calibration of MD and energy meters, if such check meter is not permanently installed. If variation exceeds 3%, steps should be taken for calibration. (b) Check and clean up control switches and push-button contacts for burnt or corroded marks, polish surfaces; also check contact springs have correct springiness.
• Batteries: (i) Ensure all batteries are in good condition and fully charged. Give an equalizing charge and correct specific gravity when essential by adding dilute sulfuric acid — this is not ordinarily necessary; only when the cell has been over-watered and electrolyte spilled out or had to be siphoned out. Record cell voltages at the same time. (ii) If a battery is not in a very healthy condition or has excessive sediment accumulation, the whole battery should be taken out for shop attention, replacing it with another good spare battery. In shops, cells should be opened out, cleaned, and re-assembled with fresh electrolyte. After two full cycles of discharge and charge, a capacity test should be taken at the 10-hour rate of discharge. (iii) Open out the covers of the battery charger and blow out all dust. Check tightness of all connections, bolts, nuts, and screws. Measure and record the insulation resistance of the transformer windings with a 500V megger.
• Auxiliary Transformers: (a) Measure insulation resistance of transformer windings and record values along with temperature. (b) Test sample oil for BDV. (c) Check the 25kV fuse-holder drops out freely on raising the spring latch. Measure earth resistance of neutral conductor.
• PTs and CTs: (i) Test oil samples when it can be done. (ii) Check gap setting of the arcing horns. (iii) Measure insulation resistances. (iv) Check conditions of fuses of PTs and terminal connections for CTs.

Maintenance Schedule for Circuit Breakers/Interrupters (Yearly)

This schedule applies to all circuit breakers and interrupters with the following provision: 132kV CBs — every 12 months or after 6 trippings on short circuit. 25kV CBs — 30 trippings on short circuit. 25kV interrupters — 250 openings on normal current for minimum-oil type and 500 openings on normal current for bulk-oil type / vacuum / SF6 types, applicable and recommended by manufacturer.

1. In oil type: open the extinction chamber, examine the contact rod arcing tip, upper and lower contact fingers, and fixed arcing contacts for burns or pitting. Check contact springs for loss of temper, breaks or other deterioration; replace where necessary. Remove any beads of fused metal from arcing tips and clean pitted surfaces. Change contacts when wear reaches limits prescribed by the manufacturer. Tighten all bolts and current-carrying parts. Check contact rod for correct alignment and setting.
2. In oil type: test oil sample for BDV. If it falls below 40kV for 132kV circuit breakers and 30kV for 25kV circuit breakers and interrupters, the oil should be purified or replaced.
3. In oil type: clean the explosion chamber with dry and clean cloth. Remove carbon deposit if any; wash out all traces of moisture from all parts with fresh oil having high dielectric strength and refill with good oil.
4. The operating mechanism of circuit breakers and interrupters should additionally be attended to annually as under: (i) Lubricate bearing surfaces of roller bearings and sliding surfaces with good-quality machine oil, used sparingly (oil tends to collect dust/dirt); wipe off surplus with clean cloth. (ii) See all links and levers move freely; operate the mechanism slowly by hand to see all parts move freely and no undue friction is noticeable. (iii) Check all pins, latches, etc. for bending and misalignment; check latch carefully so it is not getting worn so as to cause unlatching from vibration or sticking/failure to trip. (iv) Check the mechanism operates with 80% of the normal operating voltage; check and record insulation resistance. (v) Observe operation of trip coil during electrical tripping and the plunger for fast action and freedom from stickiness — the plunger should have sufficient travel to ensure adequate impact that will positively release the breaker latch; check insulation resistance of the coil. (vi) Check the breaker mechanism operates smoothly and freely without binding; check the contact rod is not binding against its guide. (vii) Wash out bearings/pivots etc. with carbon tetra-chloride if dirty and lubricate very lightly. (viii) Examine the accelerating springs and see adjusting nuts (if any) are locked tight. (ix) Measure the length of the breaker stroke and check/adjust in accordance with manufacturer's instructions. (x) Check opening and closing position of auxiliary contacts with respect to main contacts; adjust where necessary. Check condition of contacts and refinish with fine file if burnt or corroded, ensure good contact without excessive friction. Check operating rods/levers are secure and move freely. Smear auxiliary contact surfaces lightly with Vaseline/petroleum jelly. Drain oil from gear box of the spring-charging motor and refill to correct level with the right grade of oil. Measure the duration for which the motor runs to charge the springs and compare with Maker's instructions.

Overhaul Schedule