CHAPTER II
SUB-STATIONS AND SWITCHING STATIONS
20200 Introduction
1. This chapter is divided into 4 sections as under -
Section I- Organization: A broad set up of the organization and duties of Chief Traction Foreman (Power Supply Installations) are covered.
Section II- Operation of Sub-stations: The important points relating to operation of transformers and protective devices are covered.
Section III- Guiding Notes on Maintenance: The important points to be borne in mind in the maintenance of chief power supply equipments are covered.
Section IV- A recommended schedule of maintenance for power supply equipments is given.
2. The following documents have been incorporated as Appendices to this Volume:
2.1 "Code of Practice for Earthing of Power Supply Installations for 25kV. ac, 50 Hz Single Phase Traction System" issued by RDSO (Appendix III).
2.2 "Guidelines for Relay Setting at Traction Substations and Switching Posts" issued by RDSO (Appendix V)
2.3 "Guidelines for Provision of Maintenance Depots, Tools and Plants and Transport Facilities (Appendix VI).
2.4 "List of Specifications and Drawings for Equipments and Materials for Railway Electric Traction" issued by RDSO (Appendix IX).
I. ORGANIZATION
20201 Organizational Set up
The Divisional set up of senior subordinates working under Sr. DEE/DEE TrD has been arranged on two types of patterns:
(a) Territorial basis
(b) Functional basis
In the territorial set up one Sr. Subordinate is responsible for all the activities of maintenance and operation over a predetermined section of electrified territory or a sub-division. The functional set up envisages separate Sr. Subordinate to be incharge of each activity viz. sub-station, OHE, Workshop. PSI etc. in a division or sub-division. Duties, however, have been specified here in relation to particular function. For territorial set up, the CTFO incharge will perform his duties keeping all functions in view, the next in command viz. TFO being the functional incharge of the specific activity.
Remote control system or protective relay testing being a specialized activity, CTFO (RC) and CTFO/ Test Room usually have a functional jurisdiction over the entire division, with Head Quarters at the Remote Control Centre and Divisional Repair Shop respectively. The CTFOs in territorial charge, should keep a constant liaison between themselves since these aspects will have an element of dual control.
20202 Duties of Chief Traction Foreman, Power Supply Installations
He is the senior supervisor working under the control of DEE/AEE (TrD) and directly responsible for the safe and efficient operation and maintenance of traction power supply installations including sub-stations (when owned by the railway), switching stations, booster transformers and auxiliary transformers in his jurisdiction. He shall be thoroughly conversant with all technical details of the equipment under his charge including their rating, trend of power demand as also correct methods of their operation and maintenance. In particular, he shall-
1. supervise the maintenance of installations under his charge in accordance with the prescribed schedules to keep them fully serviceable at all times and in a state of good repair;
2. maintain proper co-ordination with the Traction Power Controller. Chief Traction Foreman (OHE). Supply Authorities and render assistance when required to ensure reliability of power supply;
3. keep his organisation in constant readiness to deal promptly with any break-downs and failures of equipment;
4. ensure that the programme of testing and maintenance of protective relays is adhered to and ensure that other safety equipment including bonding and earthing are functioning effectively;
5. instruct, train and supervise staff under his control and ensure that they do operate and maintain the equipment properly and in particular do actually observe all rules and regulations and safety precautions laid down;
6. depute staff for refresher courses as prescribed, particularly for such staff as are found deficient in their working;
7. ensure that special instruments and tools provided for maintenance operation and testing of all installations are properly cared for;
8. keep a close watch on availability of spare parts and other stores required for maintenance and operation of the installations and initiate timely action to recoup stocks;
9. ensure proper accountal and periodical verification of stores and tools in his charge.
10. depute staff when required to man sub-stations and switching stations in the event of failure of remote control equipment;
11. inspect all installations under his charge at least once a month, with particular attention to safety aspects;
12. submit prescribed periodical returns after careful scrutiny to AEE (TrD) and Sr. DEE/DEE (TrD);
13. keep his superior officers fully informed of all important development and seek their guidance when required;
14. carry out such other duties as may be allotted by superior officers from time to time.
15. carry out inspections as indicated at Annexure 2.01.
II. OPERATION OF SUB-STATIONS
20203 Introduction
Since the electric traction system depends upon continuous availability of power supply, sub-stations and switching stations have to be kept in proper working condition at all the time. To ensure this, the transmission lines, the 25 kV feeder lines and traction transformers with associated switch gear and control and relay panels are duplicated so that if one unit fails, the standby unit can be brought into service to continue power supply. All switching operations are also centralized and controlled by remote operation by a single authority, namely Traction Power Controller.
20204 Inspection Book and Log Book at Sub-stations
An "Inspection Book" shall be maintained at every sub-station in which observations made by supervisory officials visiting the sub-station for periodical inspections shall be recorded. In addition a log book should also be maintained to keep a record of the traction transformer oil temperature, ambient temperature as well as the currents and voltages as indicated on the control panel at a fixed time every morning. If there is any thing abnormal or unusual, CTFO(PSI) will investigate the cause thoroughly and take necessary remedial action.
20205 Overload Capacity of Traction Transformers
Traction transformers usually have the following overload capacity :
1. Overload rating: (a) 50% overload for 15 min., and (b) 100% overload for a period of 5 min.; after the transformer has attained steady temperature on continuous operation at full load.
2. Over an ambient temperature of 45" C the maximum permissible temperature rise shall be as under:
(a) Winding = 50° C (by resistance method)
(b) Oil = 40° C (by thermometer)
(c) Current carrying = 35° C (by thermometer) parts
3. The hot-spot temperature after 50% overload for 15 min. or 100% overload for 5 min. shall not exceed 100° C for an ambient temperature of 45° C.
4. Interval of time permissible between two successive overloads (after continuous working at maximum ambient temperature of 45°C) is 3 hours for both 50% overload for 15 min. and 100% overload for 5 min.
20206 Tap Setting on Traction Transformers
Traction Transformers are usually provided with off-load tap changers (operated locally or by remote control) with taps from + 10% to (-) 15% in steps of 5%. To decide the correct tap setting a recording voltmeter should be connected at the traction sub-station to the secondary side of a potential transformer to ascertain the pattern of voltage variation throughout the 24 hours for at least 3 typical days. Based on the readings from the recording the tap position should be fixed so that the daily OHE voltage peaks at the traction sub-station lie just below 27.5 kV but does not touch 27.5 kV. This will ensure that the OHE voltage is well above the minimum of 19 kV at the farthest point on the system even when heavily loaded. Once a year a 24-hour record of voltages available on the two sides of every neutral section should be taken to make sure that the voltage does not fall below 19 kV at any time.
Since any change in the inter-connections of the grid system would have repercussions on the voltage at the traction sub-station, the CTFO (PSI) should keep in touch with the supply authorities in regard to system changes
so that he may arrange to take another set of 24-hour voltage readings if any change has taken place and to change the tap setting if required.
20207 Tests on Transformer Oil
In order to improve the performance and to prolong the life of the transformers, EHV grade oil is used. The following two specifications - the first one for new oil and the second for oil in service, are adopted:
(a) IS-335 Specification for New Insulating Oils.
(b) IS-1866 Code of Practice for Maintenance and Supervision of Insulating Oil in Service.
A summary of tests for various characteristics, the requirements to be complied with and methods of tests as contained in the two specifications is at Annexure 2.03(A&B). The tests are wide ranging and should be done once a year. However, some of the tests like Break-down voltage (BDV) test, acidity tests, crackle test for moisture, may be carried out in PSI depots or sub-stations once in six months when samples are drawn for condition monitoring as per Para 20216. Procedures for these tests are indicated in IS 1866.
20208 Purification of Transformer Oil
The object of oil purification is to remove all contaminants such as water, carbon deposits, dirt, sludge, dissolved moisture and gases. The most important quality to be preserved is the di-electric strength, which is affected by the presence of moisture.
The insulating materials used in the winding are hygroscopic by nature and therefore moisture is absorbed through defective breathers, gaskets and addition of untreated make-up oil. It is essential to remove these impurities by purifying the oil when the di-electric strength goes below the permissible limits.
20209 Oil Purification Plant
For purifying the transformer oil, machines conforming to RDSO's Specification No. ETI/PSI/103 may be used. These are normally operated from 240 V single phase supply taken from the 100 kVA Station transformer provided at the sub-station. Supervisory officials in charge of maintenance of transformers should make themselves familiar with the supplier's instructions in regard to the operation and maintenance of the oil purifying equipment.
20210 Insulation Resistance During Drying Out
Readings of temperature and insulation resistance should be recorded every two hours, from commencement until the full operation is completed. If the readings are plotted on a graph, the appearance will be as 3hcwn in Fig, 2.01.
It will be observed that there are four distinct stages:
A. Initially the insulation resistance drops down to a low value because of rise in temperature of the oil upto about 75° C.
B. Insulation resistance will continue to remain at a low level despite temperature being maintained at a high level until most of the moisture from the windings and oil has been driven out.
C. The insulation resistance will thereafter rise gradually and level off, indicating that all moisture has been driven out and the drying out operation has been completed. At this point oil circulation should be discontinued.
D. As the oil cools off, the insulation resistance will rise much above the levelling off point at the end of stage (C). This is because the insulation resistance value doubles for a fall in temperature of about 10°C to 15°C.
20211 Protective Devices
A number of protective devices are provided to ensure safe operation of traction transformers and other equipment (under normal and extended feed condition with appropriate adjustment of settings). Alarm and trip circuit operations are tele-signalled and indicated at the RCC. The TPC shall in every such case advise CTFO, so that he could arrange for the inspection of the sub-station to investigate the cause and take necessary corrective action and submit a detailed report to Sr. DEE/DEE(TrD).
20212 Operation of Temperature Alarm or Trip
Alarm and trip contacts are provided to operate should the temperature of transformer windings or transformer oil exceed pre-set limits. If alarm or the trip contacts have operated, both of which are indicated at the RCC, CTFO should personally inspect the installation. If the dial settings are correct, the reason for excessive temperature rise should be investigated. Normally instantaneous overloads of over 150% of full load are taken care of by overload relays, while sustained overload below 150% are cleared by thermal protection. It is advisable to connect a recording ammeter and get a 24 hour chart showing the current loading of the transformer in services. The shape of the load curve would give valuable clues as to corrective action to be taken.
If the alarm and trip circuits operate frequently during peak periods, attempt should be made with Operating Department to space out the trains more uniformly throughout the day so as to reduce the peak load. If, on the other hand, it is a suburban section and the peak load cannot obviously be brought down, the second standby
transformer may have to be pressed into service for the duration of the peak load. Such parallel operation of traction transformers may sometimes also incidentally result in reduction of the total losses thereby effecting economy. Secondly, it will also result in higher OHE voltage, since traction transformer impedance is now halved as the transformers are identical.
If a sub-station is persistently overloaded and an adjacent sub-station is appreciably underloaded, the possibility of shifting the neutral section may be considered.
20213 Operation of Differential Protection
Apart from operation on account of internal faults in the transformer, the differential relay could also operate either because of current in-rush on account of magnetization of the core at the time of switching on or because of spill current caused by lack of perfect balance between secondaries of EHV and 25 kV current transformers. The causes for such mal-operation may be defective harmonic restraint filters or wrong CT ratios and should be eliminated.
20214 Buchholz Relay
The Buchholz relay assembly is provided on transformers to detect evolution of gas caused due to internal faults. After first commissioning, the upper assembly of the relay may sometimes be found to operate causing the relay to trip. Analysis of the composition of gas collected will indicate the nature of fault. If it is mere air bubbles the transformer is sound. For details of tests manufacturers write up may be referred to. It is always a wise policy to get the di-electric strength of the oil tested, measure the insulation resistance and carry out ratio test.
III GUIDING NOTES ON MAINTENANCE
20215 Introduction
1. For better utilization of traction assets, outage of any traction equipment from service should be minimum without compromising on safety of the equipment and personnel. Monitoring of condition of the equipment by reliable means is essential for following system of need based maintenance i.e. directed maintenance. However, till such time reliable condition monitoring techniques are introduced, the present system of preventive maintenance has to continue.
2. Recommendations of Original Equipment Manufacturer (OEM) and guidelines issued by RDSO, time to time, shall be kept in view while defining the scope and periodicity of the schedules.
3. The tightening torque for fasteners of various sizes is given in Annexure 2.08.
20216 Transformers
1. Condition Monitoring
In oil filled equipment like transformers, normal deterioration or ageing of insulation is caused by thermo-chemical reaction with participation of heat, moisture and oxygen. This results in formation of soluble and insoluble products which accumulate and deteriorate the properties of oil and cellulosic insulation. Whereas the oil ca" be reconditioned to restore functional properties, no such treatment is possible for the cellulosic insulation, which suffers from reduction of mechanical and di-electric strength. The condition of the insulation, therefore, needs to be checked by suitable method.
The thermal and electrical stresses caused during short circuits, overloads and over voltages in the system result in gas formation in appreciable amount and deterioration of di-electric properties and lowering of flash point of oil from 145° C to somewhere between 50° to 80° C in extreme cases. In the case of incipient faults, the gases being soluble, are absorbed in oil. The Buchholz relay cannot respond during early stages of trouble and by the time these devices operate the damage is done. Dissolved gas analysis (DGA) provides an important means in the art of condition monitoring of power transformers and other oil filled equipment. Of the various methods of gas
analysis Gas Chromatography (GC) is one of the most sensitive, efficient and rapid method, eminently suited for detection of incipient faults and for monitoring of growing faults which are not always revealed by established routine tests etc. In order to timely detect the deterioration of insulation, oil sample shall be drawn annually and subjected to gas chromatography.
Guidelines for condition monitoring of traction transformers by Dissolved Gas Analysis technique are appended at Annexure 2.04.
2. Overhaul Of Transformers
a) Overhaul of a transformer is normally undertaken either if it is faulty or at the end of 7-10 years by way of periodic maintenance. This can be done in the Central Repair Shop which is a covered shop having full facilities including a core lifting bay with a crane. Before commencing the work ensure that spare gaskets of proper quality are available. Drain out the oil, disconnect all leads, remove manhole covers where required. The EHV and 25kV bushings are then carefully removed out and stored well protected in a safe place. Then remove the core by means of the lifting hooks and place on shop floor over a trestle in a large receptacle into which oil can drain out.
b) If the transformer has been opened up because of any internal fault, make a careful note of colour of transformer oil, arc-marks, carbon deposits, charring of insulation, condition of the windings, unusual odour and other abnormalities which would all help in ascertaining the cause of the failure. If a coil has been burnt out, the whole transformer will have to be completely dismantled and then the damaged coil replaced with a new coil. In the case of the traction transformers, the replacement of the damaged coil is best done in the Manufacture's works where necessary facilities and staff with the requisite skill are available.
c) Arrange for the interior of the transformer tank to be thoroughly cleaned of all accumulated debris, sludge, etc. and wash with fresh oil. Remove the drain plug, lightly polish the valve seat and renew the oil-tight gasket round the spindle so that when assembled the plug is fully oil tight; the same remarks apply to the oil sampling valve, if provided. Opportunity should also be taken to plug or weld up any small blow holes through which oil seepage was observed earlier. Finally paint the exterior of the tank if necessary after thoroughly cleaning it up of all paint work, rust and traces of oil and dirt.
d) If the coil assembly is lifted up after 5, 10 or more years of service, considerable amount of sludge formation would have occurred on all parts of the transformer, i.e., at the bottom of the tanks, metal work of the transformers, windings and inter-spaces between windings. All these should be scrapped off carefully with a wooden or fibre wedge without .causing any damage to the windings. Traces of the sludge left over in inaccessible places are best removed by directing a thin jet of transformer oil under pressure using small oil purifier. At the same time the old surface contamination should be brushed and washed down, until the clear surface of the winding is exposed.
e) Care should be taken to protect the windings against ingress of moisture particularly during inclement weather. Care should also be taken by wiping off body sweat with a towel. The windings should also be kept warm by surrounding the open windings by a number of infra-red lamps or by other means.
f) Fully push home the wedges between the coils and take up the slackness of end-plates by tightening up the bolts and locking them. These are provided on traction transformers to hold the windings tightly together to withstand the high mechanical forces generated at the time of short circuits. Shrinkage and settlement usually take place within the first six months of the commissioning of a transformer. The coils are also liable to suffer displacement due to short circuit forces. If the coils are not held tightly in position, it will lead to repeated movement of the coils as well as layers and turns which will in turn cause abrasion and wear of insulation and ultimately failure. It is, therefore, sometimes recommended that the first available opportunity should be taken to have the wedges fully home and tighten up the pressure screws where they are provided.
g) Finally put back the core assembly inside the tank, assemble the bushing check tightness of all internal connections, fit the top, provide new gaskets, fill pure oil and dry out as detailed in para 20208 to 20210. Experience has shown that tools like spanners and foreign objects like washers, pieces of cloth, etc. are sometimes inadvertently left behind in the transformers, which present hazard of short circuits. It is, therefore, important that all tools, etc. used in the overhaul work should be listed out at the beginning and accounted for at the end of the work.
When overhauled transformers are to be commissioned the same procedure as detailed in Chapter IX for new transformers should be followed.
Each railway should plan, taking into consideration the resources available with them to carry out the POH and repairs of the transformer and decide the agency to execute the work.
3. Investigation into Causes of Failures of Transformers
In most cases the causes of the fault can be surmised by careful observation of the condition the windings, e.g. displacement of the turns or coils, coil insulation (brittle or healthy), evidence of overheating, carbon deposit or flash marks on the core, supports, the inner surface of the tank or cover. The following notes may be of help in identifying the cause:-
(a) Failure due to lightning discharge or overvoltages — This is characterized by break-down of the end turns close to the line terminal. There may be a break in the turns or end lead, and also flash marks on the end coil and earthed parts close to it, but the rest of the coils will be found to be healthy,
(b) Sustained overloads. — The windings in one or all phases would show signs of overheating and charring; the insulation would be very brittle and would have lost all its elasticity.
(c) Inter-turn short, inter-layer short, or inter-coils short — The same signs as for indicated for sustained over load would be noticed, but only on affected coils, the rest of the coils being intact. This Is likely if the differential relay or the Buchholz relay has operated.
(d) Dead short-circuit.— This can be identified by the unmistakable, lateral or axial displacement of the coils. The coils may be loose on the core, some turns on the outermost layer may have burst outwards and broken as if under tension. If, in addition to these signs, the windings are also completely charred, it is conclusive evidence that the short circuit has continued for an appreciable period, not having been cleared quickly by the protective relays.
(e) If the upper chamber of the Buchholz relay alone has tripped, check the insulation of core bolts, by applying a voltage of 230 V to 1000 V between the core and each bolt. If it fails, renew the insulating bush. Observe also all the joints, and tap-changer contacts, for over-heating and arcing.
(f) If the oil shows a low BDV, it does not necessarily mean that it has caused the breakdown. At high voltage ratings, excessive moisture content in the oil may result an internal flashover between the live parts and earth, which will leave corresponding tell-tale marks.
20217 Circuit Breakers and Interrupters
The following types of circuit breakers and interruptors are now in use: Circuit Breakers:
(a) 220/132/110/66 kV, Double pole:
- Minimum oil type -SF6type
(b) 25 kV, Single Pole
- Minimum oil type
- SF6 type
- Vacuum type
(c) Interruptors
- Bulk oil type
- Minimum oil type
-SF6
- Vacuum type
Oil type circuit breakers/interruptors require considerable attention for maintaining satisfactory condition of the oil. In case of minimum oil type equipments frequent replacement of oil is necessary on account of service conditions . To overcome these limitations , SF6 type and vacuum type circuit breakers and interruptors are now standardized.
Manufacturer's detailed instructions may be referred to for installation, commissioning, operation and maintenance for all types of breakers/interruptors. RDSO's additional instructions on maintenance and modifications to the circuit breakers/ interruptors should also be followed. Some tips for the maintenance of circuit breakers and interruptors, in general, are given in the succeeding paragraphs.
20218 Guidelines for Maintenance of Circuit Breakers and Interruptors
A. Minimum Oil Circuit Breakers: 1. Oil
When a circuit breaker trips the arc is extinguished by the oil and, therefore, the oil gets carbonized and contaminated. Although 25 kV traction circuit breakers are designed for 100 successive trippings or for short circuit currents upto 4000 A, without requiring replacement of oil, experience has shown that because of heavy contamination of oil, insulation level comes down rapidly even with 40 or 50 trippings. A record should, therefore, be maintained by the ATFO, based on TPC's register / Computer print out of the number of trippings of each circuit breaker since last attention. The CTFO (PSI) should keep a watch over these figures and arrange to measure the BDV of oil samples at intervals to be decided in the light of experience. Oil shall be purified or replaced if the di-electric strength falls below the prescribed value.
2 Caskets
During inspections, look for evidence of oil leaking out of bushings, tank body, etc. Slight oil leakage could occur due to (a) the mating surfaces are not perfectly flat (b) the gasket is not being quite free from dirt or extraneous matter (c) the gasket is not being properly compressed (d) gasket has lost its elasticity.
Leaky gaskets shall be attended to or replaced without delay, as they may allow moisture to get in and contaminate the oil.
3. Bushings
The porcelain exterior surfaces should be maintained in a perfectly clean and bright condition. Deposits of salt and industrial dust should be removed periodically so that there may be no possibility of flash-over. Where the
deposits are heavy a wet cloth may be used for wiping it off, failing which carbon tetra-chloride may be used. After removing the contamination the surfaces should be wiped out with a dry cloth so that the gloss on the surface is restored.
During inspection the insulator surface shall be very carefully examined all round for any fine surface cracks so that the damaged insulators may be replaced. If the chipping on bushing sheds is only very slight, replacement may not be necessary.
4. Internal Insulating Parts
Every time an oil circuit breaker is opened and oil has been drained out, all the interior parts including the bushing in the minimum oil type units or tank in the bulk oil units should be thoroughly cleaned to remove all traces of carbon or sludge that may have adhered thereto. Thereafter, the parts should be first cleaned with dry non-fluffy cloth (never with cotton waste), then thoroughly flushed out with good oil and are assembled.
5. Contacts
It is essential for the contact surfaces to be properly aligned with sufficient area of contact and pressure between the contacts to ensure that when carrying full-load currents, the contacts remain quite cool. Badly pitted or burnt-out contacts should be replaced before they cause damage to other parts. If they are only rough, the surface irregularities should be smoothed out by touching up lightly with a clean fine file taking care not to overdo it. Where the contact mechanism consists of main and secondary contacts, the main contacts do not normally require any maintenance, because the current is made or broken only at the secondary contacts. The Main contacts should, however, be inspected for good condition and kept smooth and clean. The adjustment of contacts should be made in accordance with Maker's instructions.
6. Operating Mechanism
Check whether the elastic mounting pads are in good condition and capable of absorbing the mechanical shocks during operation. The length of the breaker stroke should be measured and adjusted if necessary in accordance with the Maker's instruction book. Ensure that the operating mechanism functions smoothly and freely through the entire stroke and without binding. Lubricate all pins and bearing surfaces with light oil and wipe off the excess. Ensure that all cotter pins are open and locking plate and nuts are in place and properly tightened.
The operating springs are quite powerful and when they are in the charged condition considerable energy is stored. Serious damage and injury to personnel may be caused if they are released inadvertently.
Parts which are scored or which show excessive wear should be replaced and adjustment should be made within the tolerances as indicated in the Maker's instruction book.
The circuit breaker or interruptor is guaranteed to work satisfactorily between the specified voltage limits. During annual inspection check performance of interruptors and circuit breakers to ensure that they function properly at the lowest specified voltage for the operating coil. Measure and record the opening and closing times both at normal and minimum voltage.
7. Interlocks
Pay attention to the proper functioning of interlocks, both mechanical and electrical, and particularly to auxiliary contacts, spring tension and screws, bolts, pins, etc. securing the contacts. Check that the wiring connections at terminal boards are properly secured.
B. SF6 Circuit Breakers:
1. Gas System.
The SF6 gas in a pure state is inert, exhibits exceptional thermal stability and has excellent arc quenching properties as well as exceptional high insulating properties. Physical properties of SF6 gas are indicated in the Annexure 2.05. There is very little decomposition of the gas after long periods of arcing. Such decomposition has virtually no effect upon dielectric strength and interrupting capability. The solid arc product formed by arcing is metallic fluoride which appears in the form of a fine grey powder which has high dielectric strength under dry conditions as existing in the breaker. A good quality absorbent is used in the apparatus to remove decomposed gaseous by-product. During the maintenance record gas pressure and temperature. Supply the gas if pressure is less than the prescribed value. Check setting of gas pressure switches.
2. Interrupting Unit
Clean the surface of the porcelain and other parts. Contacts should be inspected and replaced if necessary. Renew the absorbent taking care that exposure of the absorbent to the atmosphere is minimal. The breaker should be evacuated as soon as possible.
3. Operating Mechanism
Check stroke from closed position to completely opened position and over stroke from completely opened position to stopped position. Check prescribed clearances. Relubricate moving parts. Check that pressure gauge is working correctly. Check pneumatic system for leakage.
The housing should be checked for water penetration and rust! Ensure that fasteners are not loosened. Check connections of control circuit wires for tightness.
C. Vacuum Circuit Breaker:
Guidelines as indicated above in case of the other two types of circuit breakers in respect of operating mechanism and its housing and other components are generally applicable in case of vacuum circuit breakers also except the interrupting chamber and pneumatic circuit. As regards interrupting chamber (vacuum bottle) no maintenance as such is required to fee carried out.
20219 Lead Acid Batteries
A battery is considered to be very vital equipment in the power supply installations and therefore its proper maintenance is imperative.
On electrified sections batteries and battery chargers are installed at the following locations.-
1. Traction Sub-stations -
110V, 200Ah.lead acid cells for control, protection and indication circuits.
2. Switching Stations -
110V or 72V, 40Ah lead acid batteries for operation of circuit breakers and interruptors and motor-operated isolators.
3. Remote Control Equipment -
Batteries of suitable voltage and capacity at remote control centre, traction sub-station and switching stations.
To reduce number of batteries at TSS/SS the remote control equipment is now being connected to the battery of TSS/SS.
In all cases, mains operated battery-chargers are provided with facilities for either trickle charge or boost charging. The rating of the battery charger should related to the capacity of the battery.
20220 Guidelines for Maintenance of Batteries
1. As the entire system of protection at a sub-station depends upon a sound battery it should always be in proper condition. It should under no circumstances be disconnected when the sub-station is in operation.
Batteries should be maintained keeping in view instructions of the manufacturer by a trained staff. The points to be observed during the inspections are summarized below -
a. General condition of the battery room and cells
b. Specific gravity of electrolyte in the cells
c. Charging current
d. Cell voltage
e. Condition of the plates and extent of deposits
f. Inter-cell connectors and main battery terminals
A detailed history of every battery should be separately maintained in which all relevant information is periodically entered. Fortnightly specific gravity readings should be taken and recorded in appropriate forms.
Smoking or the use of open flames or tools which may generate sparks is strictly forbidden in the battery room. The battery room should be well ventilated and dust free and should have acid proofing done on the walls and flooring. It should be kept isolated from other electrical equipment. Appropriate fuse protection for short circuit in the wiring between the battery and distribution switch board should be provided.
2. Specific Gravity
The specific gravity of the electrolyte should be maintained at about 1.210 at 27° C and when it drops to 1.150 the cell may be considered discharged. These values vary with the type of battery, temperature, age and working conditions.
Specific gravity is related to electrolyte temperature. For the purpose of test requirements, the fully charged specific gravity shall be l,20± 0.005 corrected to 27° C. Temperature correction hydrometer readings of specific gravity shall be made as follows (Ref. IS 1652):
a) For each 1° C above 27° C, add 0.0007 to the observed reading and
b) For 1° C below 27° C deduct 0.0007 from the observed reading.
When the battery is first commissioned the specific gravity of the all cells would be almost equal. Subsequently during periodical inspections, variations in specific gravity may be observed due to unequal rate of evaporation. This should be corrected by adding distilled water. In no circumstances should concentrated or diluted sulphuric acid be added to any cell except when acid is known to have spilled out. Distilled water alone should be used for topping up the level.
Hydrometer readings taken when a cell is gassing freely gives the specific gravity of a mixture of gas bubbles and
electrolyte and not the true specific gravity of the electrolyte. The readings should therefore be taken after allowing all bubbles to subside. Hydrometers of reputed make should only be used. Hydrometers of 300 mm length are necessary to give required accuracy. Two hydrometers should always be maintained in a station and they should be periodically checked to see that they read alike.
3. Pilot Cells
One of the cells in each row of the battery set should be selected and kept as the pilot cell. Readings should be taken on these cells with sufficient frequency to indicate its state of discharge and charge and serve as a guide to the condition of the other cells. The pilot cell when once selected should not be changed unless the cell has to undergo special treatment or repairs in which case a note should be made immediately on record sheets. The height of the electrolyte in the pilot cell should invariably be kept at a fixed point (say 12 mm) above the top of plates by adding distilled water every fortnight, if necessary.
4. Trickle Charging
Lead acid batteries are very sensitive to overcharging as well as over-discharging. If over charged, the positive plates will shed their active material quickly. If kept in discharged condition for long, the plates will suffer 'sulphation' evidenced by appearance of whitish deposits on the plates. Prolonged charging at a very low rate after emptying the electrolyte and filling the cell with distilled water is sometimes useful if the sulphation is very light. However, there should be no occasion at all for any battery set used in stationery traction installations to be sulphated, as they are continuously on trickle charge. A long life for the battery is achievable if the battery is kept floating on a battery charger so that the terminal voltage of each cell is maintained close to 2.15V.
This can be achieved if the battery is kept on a very low rate of charge, say, 1 milli-ampere per Ah capacity of the battery. The exact rate of charge should be fixed having regard to the normal and intermittent rates of discharge over a period of 24 hours, so that the battery is always kept in fully charged condition and never overcharged or over-discharged.
When a battery is being properly float-charged very small gas bubbles (about the size of a pin head) rise slowly from the plate to the surface of the electrolyte. In batteries, that are being overcharged the bubbles are much larger and reach the surface at a higher rate.
5. Cell Voltage
The voltage of cell at the end of a charge is not a fixed value but will vary depending on the age of the battery, the temperature, specific gravity of the electrolyte and charging rate. The voltage of new cells at the end of a full charge will be about 2.5 to 2.75 V when it is receiving charge at the 10 hour rate. This gradually decreases as the age of the battery increases until it comes down to 2.4 V with normal temperature and charging rate.
No cell should ever be discharged below the point where the cell voltage reaches 1.85 V as measured when the cell is discharging at the normal 10 hour rate.
It should be noted that the voltage of a cell gives an approximate indication of its state of charge (or discharge) only when it is being discharged, say at the 10 hour rate, and not when the cell is an open circuit.
Sulphated plates, lug corrosion, partial short circuit due to cracked separators and other defects of a lead-acid cell cause a noticeable drop in the terminal voltage with current flowing in the cell. This drop varies with the amount of current flowing and in order to get voltages that can be compared from month to month, the voltages should be taken with the same current flowing in the cell. The cell testing voltmeters in use should be periodically checked and recalibrated, if necessary. When not in use they should be kept in a safe place.
6. Condition of Plates and Deposits
The active material in the positive plates in healthy cells in use for more than 12 months (when fully charged) should be chocolate in colour and negative plates light or bluish grey according to age. The chief indications of weak cells are badly coloured plates, irregularity in gassing or entire failure to gas and a fall in voltage and specific gravity below that of other cells.
In new batteries, flakes of brown scale will be seen getting detached from edge of positive plates. This formation of scale is normal. Until all this scale is dispersed, the plate cannot be considered as stabilized. Sometimes pieces of this scale may lodge across adjacent negative plates and cause a partial short circuit. Such flaked pieces should be gently dislodged with a thin piece of wool and allowed to fall to the bottom of the cell. This scaling occurs only on the edges of the plates. The removal of the scales should be done very carefully so that the plates are not damaged.
Examine carefully the physical condition of the plates such as cracks, distortions, accumulation of whitish deposits, etc.
The colour of the deposits gives a good indication of the state of health of the cells. Whitish deposit indicates undercharging leading to discharged condition. In healthy cells, the deposit is brown in colour but excessive shedding of active material from the positive plates indicates overcharging of the battery. If this is noticed, reduce the rate of charge immediately. If all the cells in a battery show whitish deposits immediate action should be taken to give a boost charge at an appropriate rate and then to increase the trickle charging rate sufficiently to keep the battery in a healthy condition all the time. Weak ceils should be immediately examined for any possible short circuit or metallic contact between positive and negative plates. The short circuit should be removed and the cell should then be given special additional charging by cutting it out and putting it back again when a healthy condition is regained, after it is attended to.
7. Inter-Cell Connectors
The inter-cell connectors of the battery should be examined to ensure that they are clean and tight, making perfect contact with cell lugs and that no corrosion is taking place. Light vaseline should be applied to prevent corrosion.
Inspection of copper inter-row connectors should also be made for any signs of copper sulphate corrosion which should be cleaned up. Acid-proof paint or enamel should be applied to all exposed copper work in the battery room and any flaking of paint work given prompt attention.
20221 Protective Relays
1. Each electrified division shall have specialist staff attached to the Central Repair Shop trained in the maintenance, overhauling, testing adjustment and calibration of protective relays as well as indicating, integrating and recording instruments. Such specialist staff shall hold competency certificate No. TR-7 as explained in Chapter XII.
2. The Central Repair shop should be fully equipped with necessary apparatus, instruments, tools and equipment for overhauling, testing and calibration of relays.
3. Each Supervisor responsible for maintenance and testing of protective relays should maintain a register in which full details regarding each relay should be entered. The details to be recorded are - the type and serial number, PT and CT ratios, range of settings available, characteristic curves (where applicable), location where installed, schematic diagram of connections, normal settings and details of calculations for fixing the normal setting. Details of tests as well as repairs carried out should be entered in this register from time to time. These particulars should also be maintained in the office of Sr. DEE (TrD).
4. No alterations in the settings of protective relays should be carried out without the written authorization of Sr. DEE (TrD), who will submit proposals including detailed calculations for changes required, if any, for prior approval of CEE. Guidelines for setting of relays are given in the Appendix V.
5. The procedure for commissioning of protective relays has been given in Chapter IX.
6. The normal maintenance attention required for relays in service is generally as under -
(a) It is essential to ensure that the cover gaskets are in good condition and the fixing screw quite tight, so that the instrument is dust-tight.
(b) Manual operation to confirm that the relays do operate the trip circuits in the manner prescribed. These tests should be carried out by at least at the level of AEE once in a year for all relays. Simultaneously visual checks on relay connections, condition of the trip battery, trip and alarm circuits, and also the dust-tightness of protective covers should be made. The relay cover should then be sealed. A record should be maintained showing the date and time this is done.
On each occasion when the seal is broken subsequently the reasons should be recorded in the log book.
(c) Distance protection relay may be tested for calibration once in a year with primary injection set.
(d) Secondary injection tests .- These should be done annually preferably before onset of busy season, making use of portable testing equipment and at the settings approved by the competent authority. Apart from testing the operation at the normal setting tests should also be carried out at other settings to make sure that the relay has the required characteristic.
(e) Overhaul, bench tests and calibration: These are necessary once in ten years or when a relay is not found functioning correctly. This work should invariably be carried out only in the Central Repair Shop by highly skilled technicians fully conversant with all details of construction and adjustment.
The bench tests and final calibration should be carried out after overhaul of the moving parts and measurement of coil resistance and other data. Transport of the relays to and from the Central Repair Shop also requires utmost care including locking of the moving parts and careful packing and handling. When laboratory tests are fully satisfactory, the relays should be sealed and date of overhaul painted on the outer cover of the relay.
20222 Guidelines for Maintenance of Switching Stations
The maintenance required for equipment in switching stations is more or less similar to that for traction sub-station equipment, except that traction transformers, circuit breakers and current transformers ate not present and area is much smaller. However, the only additional but important item which requires attention is condition of the return feeder connection to all the rails (at the feeding posts). These return feeder connections are liable to be damaged by Permanent Way gangs in their normal work of packing and maintaining the permanent way. Supervisory officials, therefore, should stress the importance of these from the electrical point of view to the PWIs so that they in turn may warn their maintenance gangs not to damage the connections. In addition, the supervisory officials shall, during their periodical inspections, make it a point to inspect the return feeder rail connections and ensure that they are in excellent condition.
IV MAINTENANCE SCHEDULES
20223 Schedules of Inspection
1. In order to achieve high reliability and ZERO DEFECT, and to ensure effective checks on the maintenance work minimum schedules of inspections to be carried out each month by the TrD officers and Senior Subordinates in charge of operation and maintenance of PSI equipments, are indicated at Annexure 2.01.
The schedule of inspections as indicated is the minimum quota to each official per month and should be independent of other tasks. They will not be of routine nature but shall be carried out in depth to identify:
(i) Deficiencies and short comings,
(ii) Lack of skill amongst staff.
(iii) Inadequacies in maintenance facilities.
(iv) Constraints experienced.
(v) Conditions of environment which lead to poor quality of work if any.
2. The inspecting officials should adjust their inspections in such a manner as to cover all of the installations in their jurisdiction within the stipulated periods and stagger the inspections among themselves to avoid over inspections of the some installations repeatedly in a very short time and neglect of other installations. A check list in brief for various inspections is given in the Annexure 2.02.
3. The items of attention listed hereunder at any particular periodicity are over and above those mentioned in the previous schedule. This should be kept in view while carrying out maintenance work.
4. The periodicity of the items of attention listed in the following paragraphs may be modified to suit local requirements with the approval of CEE.
5- As regards new equipments, if schedules have not been drawn up. tentative schedules may be evolved based on the Original Equipment Manufacturer's guidelines and RDSO's recommendations, keeping in view the local conditions also and followed with the approval of CEE.
6. Schedules for maintenance of SF6 type circuit breakers as recommended by one of the manufacturers are indicated in the Annexure 2.06. Schedules for maintenance of vacuum circuit breaker as recommended by one of the manufacturers are indicated in the Annexure 2.07. Schedules as indicated in the following paragraphs are for minimum oil circuit breakers.
20224 General
1. No work of any kind shall be commenced on or in the vicinity of live equipment unless power supply to the particular part has been switched off and all other prescribed safety measures taken.
2. To guard against the possibility of unauthorized interference and pilferage from unattended sub-stations and switching station, all electrical department staff shall be vigilant and watch for any such activity when they are in the vicinity. Surprise checks coupled with periodical inspections will also act as deterrents.
3. The TPC shall once a day check up communication to each of the grid sub-stations and obtain the maximum demand and energy consumption for the previous 24 hours and enter the figures in a register. Whenever inspecting staff visit the sub-station or switching station, they shall contact the TPC on the telephone.
FORTNIGHTLY MAINTENANCE
20225 General Inspection by a PSI Supervisor
1. Go round the whole area of the sub-station; inspect for general cleanliness, proper drainage, road and rail access. The surface of the roadway and pathways in the sub-station should be firm and sufficiently elevated to prevent water-logging. Remove any undergrowth of vegetation around the outer periphery; cut any tree branches likely to come in the vicinity of live lines.
2. If lubricating or transformer oil is stored, inspect for security and fire risk and see that no combustible material is in the vicinity.
3. Examine all "Caution", "Danger", "Shock Treatment" and other boards, whether they are clean and well secured. Inspect fire extinguishers, fire buckets and First Aid Boxes, if they are intact and serviceable.
4. Inspect structure and plant foundations for any sinking or cracking. Go round the structural work for checking tightness of various bolts and nuts.
5. Inspect all indication lamps on control panels for correct working.
6. Carry out inspections as indicated at Annexure 2.01.
20226 Battery
1. Check all cells generally in accordance with para 20220.
2. Take specific gravity and cell voltage of pilot cell and record in register. If any significant change is noticed specific gravity and voltage for all cells should be taken to identify any weak cells. Then top up with distilled water exactly to the correct level for every cell.
3. Check operation of battery charger and note charging rate in register.
MONTHLY MAINTENANCE
20227 Bonding And Earthing
Visually inspect all earth connections and see that they are in order and that every equipment has duplicate earths. Tighten connecting bolts and nuts as necessary. Where the sub-station and feeding post are close by ensure that sub-station structures are properly bonded with the feeding post and the track by two independent connections.
20228 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.
20229 Insulators
Clean all insulators with dry cloth and look for any flashover marks, cracks, chippings. Insulators which are badly chipped should be replaced. Minor chippings can be rendered impervious to moisture by a light coating of Araldite or similar epoxy resin.
20230 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 dial indicator; reset indicator.
3. Check explosion vent diaphragm for any damage and presence of oil.
4. Check silica-gel breather. If turning pink in appearance, replace it with dry gel (blue colour) and recondition the old silica-gel. If the silica-gel is too wet, check di-electric strength of transformer oil.
5. Check for gas collection, if any, in Buchholz relay
6. Check for oil leakage on transformer body, conservator tank, oil drain valve and foundations. If leaking, take corrective action by tightening the bolts; replace gaskets, if necessary.
7. Check if heater in the marshalling box is functioning properly, and if all terminal connections are in order.
20231 Operating Mechanism of Circuit Breakers and Interruptors
1. Open the cover of control box. Examine the interior and remove the accumulated dust. If any part of the interior is badly rusted indicating entry of moisture, find out the cause, plug the holes and repaint the rusted parts. Check in particular if the weather-proof gaskets are in good condition; if not, replace them to make the control box water-tight and dust-tight. Examine if the leading in pipe connections are properly bushed, sealed and watertight. Check if all pins and checknuts are in place. Check also 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 any rubbing or obstruction, and also if the shock absorber functions properly when circuit breaker is tripped.
3. Examine the commutator of the motor and clean with muslin 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 it, if required. '
6. Check if heater is functioning properly.
7. Check interlocks of the equipment and associated isolators.
8. Check local position indicator and remote semaphore indicator for operation. Observe for the correct operation of recording counter.
After complete checking, close the cover and test the breaker for operation under remote, local and manual control.
20232 Isolators
1. Manually operate isolator several times and observe if it operates smoothly and correctly. Check interlocks and integral lock, lubricate moving parts as necessary with appropriate lubricant.
2. If isolator is motor-operated, check commutator of motor and clean with dry mull cloth, and check carbon brushes for proper bedding and wear. Check if motor is working smoothly, clean limit-switch and auxiliary switch contacts and check tightness of wiring connections. Examine contactor box and signal box; clean thoroughly and lubricate all gears, shafts, bearings, contacts etc.
20233 Busbars, Clamps and Connectors
Immediately after switching off the power supply and earthing the lines, feel by hand all connectors and clamps on busbars and equipment terminals which carry heavy currents to see if they are too hot. If any connection is too hot, it indicates poor contact. Open up the connector; carefully clean the contact surfaces, touch up the high spots on the contact surfaces so that the mating surfaces bed well together; apply a very light coat of vaseline, refit and tighten up. Wherever applicable, replace bi-metallic strip.
20234 Control and Relay Panels
1. Make a note of flag indications, if any, then reset.
2. Check if all indicating and recording instruments are working normally and the 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 the day when inspection was carried out Abnormal voltage or current should be noted for corrective action.
4. Clean the panels externally.
QUARTERLY MAINTENANCE
20235 Batteries and Battery Chargers
1. Take specific gravity and eel! 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 causes for excessive discharge.
3. Make a general examination of battery charger. Check earth connection to the body.
20236 PTs and CTs
These should be maintained generally on lines similar to that of traction transformers except for items which do not obviously apply. In addition, for PT check the fuse holders on the LV side to see if they are in order.
20237 Booster Transformers
a) Replace or recondition silica-gel breather, if necessary
b) Check earthing connections from bottom of structure to the earth electrodes or to the rails. Check the availability of duplicate earth strip and its proper connection.
c) Check all caution boards, name plates and anti-climbing devices for proper condition.
d) Check foundation for any sinking or cracking : Check all structure bolts and nuts for proper condition.
Annual maintenance and periodical overhaul are to be carried out, generally as indicated for the traction transformers.
20238 Auxiliary Transformers
1. Measure insulation resistance of transformer winding and record values alongwith temperature.
2. Test a sample of oil for BDV.
3. Chek that the 25 kV fuse-holder drops out freely on raising the spring latch. Check rod gap setting. Measure earth resistance of neutral conductor.
Annual maintenance and periodical overhaul are to be carried out, generally as indicated for the traction transformers.
HALF YEARLY MAINTENANCE
20239 General
CTFO (PSI) should visit the grid sub-station and ascertain whether any significant change in the EHV grid network has occurred during the past six months or are expected shortly.
20240 Traction Transformers
1. Test oil sample from tank bottom for crackle test, acidity and BDV. If BDV is below the prescribed value, oil should be dried out.
2. Check whether the rod gap settings on bushings of transformers are in order, as per Maker's drawings.
3. Measure and record insulation resistance of all windings to earth and other windings with a 2500 V megger, along with temperature of windings and ambient temperature.
4, Check all alarm and trip devices for proper functioning.
20241 Isolators
1. Observe for any signs of overheating and check the wipe of contact blades. Clean blade tips and fixed-contact fingers and lightly vaseline the contact making surfaces.
2. Clean all articulated joints, sliding and bearing surfaces thoroughly.
3. Check all split pins, lock nuts and check nuts for proper condition.
4. Check for correct setting and alignment of arcing horns.
5. Operate the isolator slowly, check for simultaneous operation of the blades on the poles and correct alignment of blade tips in the fixed contact jaws of the poles. Adjust if required to ensure that the blades are fully home between the contacts when handle is in closed position.
6. Check locking arrangements.
20242 Control and Relay Panels
1. Check tightness of all connections, remove cobwebs and wipe off accumulated dust with dry cloth.
2. Check if tap and time settings of the relays are in order.
3. Examine fuses for signs of overheating or aging, springiness and cleanliness of contact making parts. Clean up and lightly vaseline to ensure proper contact.
YEARLY MAINTENANCE 20243 General
1. Inspect the fence all-round the sub-station and bonding between metal fencing panels and to earth. Put a drop of oil in the hinges of all doors. Repaint any of the structural parts as necessary.
2. Open all the trench cover and clean them completely, dean all culverts and remove cobwebs; check possibility of lizards or other insects gaining entry into enclosed control equipment, and make them insect-proof.
3. Arrange for painting of walls and metal-works as necessary.
4. Check all explosion vent diaphragms for any damage.
5. Check rod gap setting.
20244 Lightning Arresters
1. Check earthing terminals and earth strips for proper condition. Check connection to the line.
2. Where lightning arrestors are provided with discharge counters, record the counter reading.
20245 Bonding and Earthing
1. Check physically the soundness of bonding and earthing connections to every electrical equipment, structural steel, lightning arrestor etc. and inter-panel connections.
2. Record earth resistance to body of electrical equipment as well as to all parts of the fencing and structural steel work.
3. Check if the terminations of the overhead shield wire covering the whole sub-station are in good physical condition and properly bonded electrically to the structures.
4. Check and record resistance of each group of earth electrodes, after disconnecting it from common earth system. Improve, if necessary.
5. Check condition of connections to the burned rails.
20246 Traction Transformers
1. Send samples to approved laboratory for all tests listed at Annex. 2.03B (IS 1866) including dissolved gas analysis.
2. Check oil level in bushings.
3. Inspect bushing gaskets for leaks and tighten bolts.
4. Move the tap-setting switch up and down the full range a few times so that by self-wiping action good contact is assured. Set the tap finally at the correct position making sure that tap-indication corresponds to position of main contacts.
5. Paint transformer tank on such parts as required.
20247 Isolators
1. Smoothen burrs, if any on the blade tips and fixed contact fingers with fine emery paper and smear vaseline.
2. Measure clearance of blade in open position and record and adjust crank mechanism, if found necessary.
3. Check the adjustable stop set-screws for proper condition and correct positioning.
4. If the isolator is motor-operated, measure and record insulation resistance of motor windings and contactor coil using a 500 V megger.
20248 Bus Bars and Connectors
Measure with a 'Ductor' or other low resistance measuring instrument the contact resistances of all connections which are carrying heavy currents.
20249 Control and Relay Panels
1. Carry out maintenance on relays as detailed in para 20221.
2. Check and clean up control switches and push-button contacts for burnt or corroded marks; polish the surfaces. Check also if the contact springs have the correct springiness.
20250 Batteries and Battery Chargers
If the battery is not in a healthy condition or if there is excessive accumulation of sediment, the whole battery should be replaced with a new set.
Battery Charger
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 of the battery charger with 500 V megger.
20251 PTs and CTs
1. Test oil samples if possible.
2. Check rod gap setting, if provided.
3. Measure insulation resistance.
4. Check conditions of fuses of PTs and terminal connections for CTs.
20252 Special Maintenance Schedules for Minimum Oil Circuit Breakers and Interruptors
This schedule will apply to minimum oil circuit breakers and interruptors with the following proviso:
132 kV CBs.............................................. - 6 trippings on fault
25 kVCBs............................................... - 30 trippings on fault
25 kV Interrupters...................................... - 250 openings on normal current for minimum oil type
and 500 openings on rated current for bulk oil type.
a) Open the extinction chamber, examine the contact-rod arcing-tip, upper and lower contact fingers and fixed arcing-contacts for burring or pitting. Check contact springs for loss of temper, breaks or other deterioration; replace wherever necessary. Remove any beads of fused metal from arcing-tips and clean pitted surfaces. Change the contacts when the wear reaches the limits prescribed by the manufacturer. Tighten up all bolts and current carrying parts. Check contact rod for correct alignment and setting.
b) Test oil sample for BDV. If it falls below the prescribed value the oil should be purified or replaced.
c) Clean the explosion-chamber with dry and clean cloth. Remove the carbon deposits if any; wash out all traces of moisture from all parts with fresh oil having high di-electric strength and refill with good oil.
d) In addition, the operating mechanism of circuit breakers and interruptors should be attended to annually as under -
1. Lubricate bearing surfaces of rollers, bearings and sliding surfaces with good quality machine oil. Since oil tends to collect dust and dirt, it should be used sparingly and any surplus should be wiped off with a clean cloth.
2. See that all links and levers move freely. Operate the mechanism slowly by hand to see that all parts move freely and no undue friction is noticeable. Observe the mechanism to see that everything is in working order.
3. Check all pins, latches, etc. for binding and mis-alignment. Check latch carefully to see that it is not getting worn so as to cause unlatching from vibration or sticking and failure to trip.
4. Check that the mechanism operates with 80% of the nominal operating voltage. Check and record the insulation resistance.
5. Observe operation of trip coil during electrical tripping and the plunger for fast action and freedom from any stickiness. The plunger should have sufficient travel to ensure an adequate impact that will positively release the breaker latch. Check insulation resistance of the coil.
6. Check if the breaker mechanism operates smoothly and freely without binding. Check that the contact rod is not binding against its guide.
7. Wash out bearings, pivots, etc. with carbon tetra-chloride if they are dirty, and lubricate very lightly.
8. Examine the accelerating spring and see that adjusting nuts are locked tight.
9. Measure the length of the breaker stroke and check and adjust in accordance with Manufacturer's instructions.
10. Check opening and closing position of the auxiliary contacts with respect to the main contacts. Adjust where necessary. Check the condition of the contacts and refinish with fine file if burnt or corroded. Ensure that good contact is made without excessive friction. Check operating rods and levers to ensure that they are secure and move freely. Smear auxiliary contact surfaces lightly with vaseline or 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 spring and compare with Maker's instructions.
20253 Pre-Monsoon Checks
Before onset of monsoon season, it should be ensured that for every equipment no scheduled maintenance work is overdue. In the scheduled inspection just preceding the monsoon, special attention should be paid to the vulnerable points likely to permit ingress of moisture resulting in reduction in dielectric strength of the equipments and rusting of parts.
20254 Overhaul Schedule for Equipment
1. Transformers.......................... In case of an internal fault or once in 7-10 years.
2. Operating mechanism of ............Once in 10 years or as and when any major part like springs have to Circuit Breaker and Interruptors. to be replaced or the mechanism is sluggish, and needs shop attentand overhaul.
Annexure 2.02
(Para 20223)
CHECK LIST FOR INSPECTIONS
1.0 PSI depots including Subordinate Offices
a) OHE/PSI Depots.
check 1. Staff grievance register.
2. Quarter register.
3. Attendance register.
4. Cleanliness of depot.
5. Upkeep of Stores. 6. Stock position in Stores. 7. Compliance of audit & account inspection notes. 8. Test & Trial report. 9. Availability of latest drawings and specifications. 10. Planning and progress of section works.
b) Subordinate office:
check 1. Attendance register.
2. Compliance of audit & account inspection notes.
3. Compliance of Officer's inspection notes.
4. Test & Trial report.
5. Availability of Drgs. & specification.
6. Progress & planning of section works. 2.0 Inspection of Grid Substation
1. Be on look out for any modifications made / being made in the power supply arrangement.
2. Check up if there is any equipment under breakdown which is likely to increase risk of interruption ' in power supply to traction.
3. Note down meter readings and scrutinize and record important data regarding power supply parameters including daily MD, variation in voltage, frequency and power factor 3.0 Inspection of Traction Sub-station
a) Switch Yard
check 1. For vegetation growth and spreading of pebbles.
2. Painting of fencing and equipments.
3. Condition of cable trenches & trench covers.
4. Condition of approach road. b) Power Transformer
check 1. O.T.I, and W.T.I, temperature - present and maximum readings.
2. Oil level in conservation tank.
3. Tap changer position of standby & service transformer.
4. For abnormal humming.
5. Colour of silica gel.
6. For leakage of oil on transformer body, conservator tank, oil drain valve and radiator.
c) Circuit breaker & interrupters:
check 1, Control box gaskets for water & dust tightness.
2. Operation by local & remote control.
3. Operating mechanism for smooth operation.
4. Oil level & leakages.
5 Closing time of interruptor.
6. Number of trippings since last replacement of oil in case of circuit breaker and counter reading of interruptor.
d) PT, CT, AT
check 1. Leakage of oil.
e) Isolator:
check 1. Locking arrangements.
2. For correct alignment of blade tip in the fixed contact jaws.
3. For correct matching & alignment of arcing horns.
f) Control Panel:
check 1. Fuses for the correct size, overheating or aging signs.
2. For loose connections at terminal Boards.
3. Functioning of Alarms & visual indication on control panel.
4. Functioning of auxiliary relays.
g) Battery Charger & Batteries: check 1. Acid level
2. presence of sedimentation
3. Specific gravity & voltage of pilot cell
4. Presence of sulphation and tightness of inter cell connectors.
5. Size of fuses of Battery charger.
6. Voltmeter and ammeter readings. h) Energy meter
check 1. Recorded maximum demand.
2. Condition of the seal. i) Earthing check 1. Soundness of earth connection to each electrical equipment and structure.
2. Last recorded earth resistance readings.
3. Buried rail connection. j) Remote control equipment
check 1. General function of relays & selectors.
2. Wiring for loose connection if any.
3. For presence of dust & condition of cubicle gaskets. k) General
check 1. Availability of fire buckets, Respiration chart, First Aid Box, Tools & Plants.
2. Working of TPC Phones & emergency sockets. 3. Inspection Register and remarks made therein.
4. History sheets of various equipments. 4.0 Switching Stations
a) Switch Yard:
check 1. For vegetation and spreading of Pebbles.
2. Painting of fencing & equipments.
3. Condition of cable trenches & trench cabins.
b) Interruptors:
check 1. Control box gaskets for water & dust tightness.
2. Operation by local & remote control.
3. Operating mechanism for smooth operation.
4. Oil level & leakages.
5. Interlocking of interruptors & under voltage relay operation at SP.
c) PT&AT
check 1. Leakage of oil.
d) Isolator
check 1. Locking arrangements.
2. For correct alignment of blade tips, in the fixed contact jaws & alignment of arcing horns.
e) Battery charger & Batteries: check 1. Acid level.
2. Presence of sedimentation.
3. Specific gravity & voltage of pilot cells.
4. Presence of sulphation & tightness of inter-cell connectors.
5. Size of fuses of battery charger.
6. Voltmeter & ammeter readings.
f) Earthing:
check 1. Soundness of earth connection to each electrical equipment & structures.
2. Last recorded earth resistance readings.
g) General
check 1. Availability of fire buckets, respiration chart, First Aid box, Tools & Plants.
2. Inspection Register and remarks made therein.
3. History sheets of various equipments.
 
GUIDE-LINES FOR CONDITION MONITORING OF TRACTION POWER
TRANSFORMER BY DISSOLVED GAS ANALYSIS (DGA) TECHNIQUE
(Reference : RDSO's Circular No. ET1/PSI/M/4 dated 5-2-91)
1.0 Introduction
1.1 Dissolved gas analysis (DGA) is a powerful diagnostic technique for monitoring the internal condition of transformer as it is capable of detecting faults in the incipient stage, before they develop into major faults and result in the outage of the transformer. The conventional BUCHHOLZ RELAY is universally used in transformers to protect against severe damages. However, its limitation is that enough gas must be generated first to saturate the oil fully and then to come out or there should be a gas surge to operate this relay. Moreover, Buchholz Relay is never meant to be a diagnostic device for preventive maintenance of transformers.
1.2 The DGA technique is very sensitive as it detects gas in parts per million (ppm) of the oil by use of the GAS CHROMATOGRAPH. It is possible to check whether a transformer under service is being subjected to a normal aging and heating or whether there are incipient defects such as Hot Spots, Arcing, Overheating or Partial discharges. Such incipient faults otherwise remain undetected until they develop into a major failure.
2.0 Formation of Gases in Oil Filled Transformers
2.1 It is well known that insulating oil in high voltage equipments can break down under the influence of the thermal and electrical stresses to produce hydro-carbon gases, hydrogen and carbon oxides. Gases may be formed in transformers and other high voltage oil filled equipment due to aging and to a greater extent as a result of faults. The accumulation of gases in transformer oil may be sudden due to a severe arcing fault or more gradual as in the case of slow deterioration of insulation. The principle mechanism of gas formation in a transformer tank can be classified as under:
a) Oxidation, b) Vapourisation, c) Insulation decomposition, d) Oil break down, e) Electrolytic action.
2.2 Oxidation
Carbon dioxide is the gas predominantly liberated during the process of oxidation. The process begins when small quantities of oil combine chemically with the dissolved oxygen in the oil resulting in formation of traces of organic acids. These acids react with the metal of the transformer, forming metal based soaps which dissolve in the oil and act as a catalyst to accelerate the process of oxidation.
2.3 Vapourisation
The vapourisation of oil occurs at about 280° C while that for water occurs at about 100 °C. The false alarm of a Buchholz relay may be attributed to the fact that the condensation of water vapour takes place when the excess moisture in the tank is vapourized by a heat source. False alarm can also occur, when hydro-carbons, the constituents of the insulating oil, vapourize.
2.4 Insulation Decomposition
The solid insulants in power transformers are mainly of cellulose or resinous type, viz., paper, press board, cotton, resins and varnishes. These substances contain in their molecular structure substantial amounts of oxygen, carbon and hydrogen. In the temperature range of 150 °C to 400 °C, the insulation breakdown results in liberation of hydrogen, carbon dioxide and carbon monoxide. Above 400 °C, the gases formed are relatively less.
2.5 Oil Break Down
The direct break down of oil by arcing results in cracking of the oil. The aromatic contents breakdown into simple hydro-carbon gases and hydrogen. Acetylene and methane are the major constituents. Other hydrocarbon gases may also be liberated due to cracking, if the necessary temperature is maintained for their stable formation.
2.6 Electrolytic action
Hydrogen and oxygen are liberated during electrolytic action. Presence of minute and small particles of fibres within the oil leads to electrolytic action. Light hydrocarbon gases may also be present, if solid insulation is involved.
3.0 Types of Fault Conditions
There are three main types of fault viz.- overheating of windings, core and joints; partial discharges; and arcing.
3.1 Overheating
Overheating metallic parts heat up the surrounding regions such as paper insulating tapes and oil. This leads to thermal deterioration of these materials. Thermal degradation of paper produces carbon dioxide, carbon monoxide and water. The ratio of carbon dioxide to carbon monoxide is typically five; but if the ratio falls below three, there is indication of severe overheating of the paper. Oil degradation produces a number of hydro-carbon gases such as methane, ethane, ethylene, and acetylene. Methane and ethane are decomposition products that appear above 120°C; ethylene appears above 150°C while acetylene is a high temperature product, appearing at several hundred degrees centigrade. Some hydrogen is also produced alongwith the hydro-carbons gases. The proportion of the various hydrocarbons varies with temperature. This is the basis of the well known Ratio Code introduced several years ago by Dorenberg and R.R. Rogers.
3.2 Partial Discharge
The second type of fault condition is partial discharge which occurs due to ionization of oil in highly stressed areas where gas/vapour filled voids are present or the insulation is containing moisture. The main product during partial discharge is hydrogen, though small amounts of methane and other gases would also be present depending upon thermal degradation. The disintegration of oil and cellulose due to partial discharge is characterized by the removal of the outer hydrogen atoms to form hydrogen gas. The remaining molecular framework polymerizes and long chain products such as waxes are formed. Thermal degradation is a more predictable phenomenon which involves the break-up of chemical bonds. Cellulose decomposes ultimately to CO. CO2 and water; oil break up into lower molecular hydro-carbons.
3.3 Arcing
The third type of fault condition is arcing. Arcing can occur between leads, between lead and coil and between other highly stressed regions weakened by fault conditions. The high temperatures caused by arcing results in the production of acetylene and hydrogen.
3.4 Pattern of generation of gases in transformer is summarized below: FAULT/PATTERN . " KEY GAS Conductor Overheating C0/C02 (Carbon Oxides) Oil Overheating C2 H4(Ethylene)
Partial Discharge H2 (Hydrogen)
Arcing C2H2 (Acetylene)
4.0 Solubility of Gases
4.1 The solubility of gases in oil varies with temperature and pressure. While solubility of H2, N2, CO, O2 in
oil increases with temperature and that of C02, C2 H2, C2 H4 and C2 H6 decreases with temperature, solubility of CH4 remains essentially constant.
All the gases become more soluble in oil with increase in pressure. Solubility of gas is one of the factors contributing to the complexities in formulating permissible levels of gases on the basis of service life of a transformer. Table 1 show solubility of different gases 25°C and at 1 atm. The homogeneity of the gases in the oil is dependent on the rate of gas generation, access of the fault area to flowing oil, rate of oil mixing and presence of gas blanket.
5. Dissolved Gas Analysis (DGA)
5.1 Dissolved gas analysis (DGA) of the oil of a transformer in operation is a specialized technique to assess the internal condition of the transformer. DGA is performed by Gas Chromatography. The gases extracted from the oil by a suitable apparatus are transferred to the Gas Chromatograph system for analysis.
5.2 The knowledge of solubility of Hydro-carbon and fixed gases at different temperatures, in insulating oils helps in interpretation of gas analysis. The permissible concentration of dissolved gases in the oil of healthy transformer is shown in Table II. The combinations of Gas levels for different types of faults are shown in Table III while Table IV shows the gas composition by volume under arcing fault with participation of various components of solid dielectrics in a transformer.
5.3 While the absolute concentration of fault gases gives an indication of status of insulation of transformer, whereas the relative concentration of these gases provides a clue to the type of fault. For fault diagnosis the method based on Rogers' Analysis is adopted.
5.4 Roger's method:
This method holds good for hydro-carbon gases. By evaluating the gas ratios, the type of fault is detected. Four ratios are used viz., Methane/Hydrogen, Ethane/Methane, Ethylene/Ethane and Acetylene/Ethylene. The value of ratios can be greater or smaller than unity. The ratio and type of fault represented by that ratio are given in Table V.
6.0 Data Collection and Analysis
6.1 It is recommended that DGA be performed regularly once a year on every transformer upto 4 years of service and thereafter twice a year upto 10 years and the frequency thereafter may be increased to thrice a year.
Note: Wherever the Buchholz relay operates, the dissolved gas analysis be carried out immediately after operation of the relay to ascertain the cause of fault.
6.2 The results of the DGA for each transformer should be built into a data bank and based on the trend of the gas levels over a period of time as well as the faults, if any, that the transformer had suffered, an analysis may be done to establish the exact nature of the incipient fault that may be developing in the transformer.
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SWITCHGEAR INSPECTION AND TEST PROCEDURE PKR ELECTRICAL ENGINEERING CONFIDENTIAL TABLE OF CONTENTS 1) VISUAL INSPECTION 2) W...
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