Substation Equipment Maintenance

MAINTENANCE OF SUBSTATION EQUIPMENTS INTRODUCTION : Substation equipments are supposed to work round the clock uninterruptedly. To perform in this way requires some maintenance without which a failure is likely to take place. NO PLANTS OR EQUIPMENTS FAIL WITHOUT GIVING ANY DISTRESS SIGNAL. The duty of a maintenance engineer is to identify a signals well before any damage is going to take place and thus to prevent breakdown, i.e. that is what is a preventive maintenance. We know that maintenance requirement of static equipments (like transformers, Capacitors etc. are much less compared to other dynamic equipments (like circuit breakers, motors etc.). we will concentrate here on maintenance of the following components : (i) Various circuit breakers, (ii) Transformers, (iii) Earthing and lightning arrestors, and (iv) Relays. The maintenance schedule is usually in the form of log sheets on which weeks, months of the year are tabulated. Each equipment in the substation or the plant is provided with a column. The maintenance period is indicated against each equipment. Further each major equipment is provided with a history card. The details about inspection, operation and remarks are written in these cards. INSPECTION, SERVICING & OVERHAUL : Maintenance covers a wide range of activities aimed at keeping that equipment in perfect working condition for performing its function as per assigned duties. 1. INSPECTION: It comprises careful observation/scrutiny of the equipment without dismantling it. It usually includes visual and operational checks. 2. SERVICING: This refers to cleaning, adjustment, lubrication and other maintenance functions without dismantling equipment. 3. EXAMINATION: This refers to inspection with necessary dismantling, measurements and non-destructive tests to obtain data regarding the condition of components/sub-assemblies. 4. OVERHAUL: This refers to the work done with the objective of repairing/replacing worn-out parts and defective parts. The equipment, sub-assemblies are dismantled partly or completely. The condition of components is inspected. Dimensions of worn-out components are measured. The components worn-out beyond acceptable limit are replaced. The assembly is followed by functional checks and measurements to ensure satisfactory operation.

R. Sharma (BSNL Electrical Zone Patna)

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MAINTENANCE OF SWITCHGEARS: The requirement of inspection, servicing, examination and overhaul vary with :- - environmental aspects : such as dust, chemical fumes, moisture/humidity, ambient temperature etc, - operating duty: frequency of operation, rated current etc. - switching duty severity: e.g. repeated operations. Thus maintenance schedule has to meet all the above requirements. The fixed parts need regular inspection and servicing for removing dust, damp and corrosion etc. The moving parts need regular inspection and periodic replacement of worn-out parts. The functional readiness of switching devices should also be ensured. The maintenance of switching devices is related mainly with the wearing out of contacts, deterioration of quenching medium and mechanism components. Table -1 below gives recommendation regarding the period of maintenance of contacts and quenching medium in terms of number of load operations and number of short circuit operations on rated short circuit breaking current.

TABLE-1 MAINTENANCE OF QUENCHING MEDIUM AND CONTACTS.

Maintenance of Quenching

Medium Replacement of contacts K=ΣI2n !

Type of C.B. Load * Operation

Fault operation

Load ** Operation

Fault Operaion

Air C.B. 3000 10 to 15 - Bulk oil C.B. 2000 6 2000 6 2000 MOCB 1000 3 1000 6 1000 Life of contacts becomes supposedly bad after certain operations on load and /or after clearing faults on rated short circuit current an indication of which is shown in Table -2.

* Shelf life 20 Years. ** Technical endurance test should be performed with specified number of operations on no load to confirm suitability of mechanism. ! K is the cumulative sum of the product of the number of breaking operation and square of the breaking current in KA and is taken as a guide to the inspection of contacts internal insulation and the quenching medium.

TABLE-2

MAINTENANCE OF CONTACTS


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Life of contacts Type of C.B Number of load operations on

rated load current Number of fault operations on rated short circuit current

A.C.B. 10,000 50-100 M.O.C.B. 1,000 3-6 MAINTENANCE SCHEDULE CHECK LIST The check list of maintenance of switchgear gives the list of activities and there periodicity. The periodicity of activities vary with: - type of switching devices, - frequency of operating duty and breaking current, and - environment. Table 3, below, gives the periodicity of important checks on C.B. during routine maintenance.

TABLE - 3

Important checks on C.B. during routine maintenance Sl. No. Activity Quarterly Half yearly Annual

1. Check insulation resistance of each pole of

phase of ground across terminals X X X

2. Check the dielectric strength X 3. Check mechanical Operations X 4. Check tightness of bolts X 5. Check oil level X 6. Clean porcelain insulators X 7. Check contact length and simultaneous

contact touch. X

8. Measure contact resistance X 9. clean cross jet pots, recondition contacts and

fill oils. X

10. Measure contact speeds X 11. Measure break time and make time. X 12. Level of oil/gas pressure X The check list and maintenance schedule for various types of circuits breakers are indi-cated below in table 4. The following abbreviations have been used: DI= Daily inspection; WI= Weakly inspection; MI= Monthly inspection; QI= Quarterly inspection; HI= Half yearly inspection; AI= Annual maintenance; N = Normal duty;


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R = Repeated operations on rated load current.

TABLE -4 Check list and maintenance schedule for circuit Breakers

Maintenance Activity

Indoor Metal clad switchgear

Minimum oil C.B.

Vacuum C.B.

N. R. N. R. 1. General Inspection HI QI MI AI QI 2. Cleaning of external insulation. HI QI

QI QI

3. Contacts, arc control devices, contact position external (VCB) contact resistance.

- HI QI AI AI

4. Quenching medium in C.B. and topping up

QI HI QI - -

5. Cleaning of internal insulation of C.B.

- AI HI - -

6. Terminal connections, earthing connections, Cabinets Cable, Structure etc.

QI AI HI AI AI

7. Main mechanism Operating Checks, inter-pole Linkages.

- HI QI AI AI

8. Servicing AI AI HI 5Y AI 9. Control wiring AI AI AI AI AI 10. Operating checks & I.R.

measurement, contact resist. measurement

AI HI AI AI

11. Post fault maintenance. after no. of fault operations.

25 6

50

12. Overhaul after no. of years No. of fault operation

1

10

No. of load operation 1,000 10,000 STEPS IN MAINTENANCE OF CIRCUIT BREAKERS : (A) GENERAL INSPECTION : Observe the C.B. visually. Note the cleanliness of terminals, earth connections, reading of counters, levels of quenching medium (in case of oil C.B.) etc. (B) CLEANING AND DRYING :


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Use CTCL or trichloroethylene or other cleaning agent as recommended by the manufacturer. Use air pressure jet (3 kg/cm2) for cleaning. Use clean cloth which does not leave fibers or particles on the surface. Avoid water, moisture or dampness during the cleaning. Care should be taken to avoid falling of dust, iron particles, nut-bolts, washers etc. inside the breaker. Terminals should be cleaned of dust, oxide coating if any by emery paper without iron particles. (C) INSULATION SURFACES : Inspect visually, carefully for signs of cracks, tracking or any other defects. It should be free from electrical or mechanical damages. Finally, insulation resistance measurement gives indication about the health of the insulation. The internal insulation should be cleaned thoroughly by trichlorethylene, lint free cloth and air jet. Any deposition of sludge and carbon particles, conducting dust particles etc. shall be removed before reassembly. After cleaning and drying insulation resistance is measured by the help of Megger between the terminal and earth. INSULATION RESISTANCE MEASUREMENT : Insulation resistance tests are strictly comparative in that for each item tested a rejection value can only be fixed on the basis of experience by comparison with earlier results. For this reason the test equipment and method used should be the same on each occasion. The test values obtained should be logged on a standard form designed for the purpose, together with the humidity and temperature at the time of the test, and in general a steady fall of insulation resistance over a period of time is a more reliable indication of deterioration than is a relatively low value which remains reasonably constant. During overhaul of the electrical equipment the insulation surface should be cleaned with trichloroethylene, clean cloth and compressed air. For h. v. switchgear 1000 V or 5000 V (d.c.) megger is preferred. The insulation resistance of H.V. C.B. is very high (above 10000 Mega. Ohm). Test voltages which could be applied to primary insulation when making resistance test varies according to the voltage rating of the switchgear. An indication to this is given below in Table 5.

Table - 5

Test Voltages 3-phase system rating of primary insulation of switchgear.

Test voltage recommended to insulation resistance test (to earth and between phases) KV (d.c.)

upto 1 KV 1 Above 1 KV to 3.6 KV 2


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Above 3.6 KV to 12 KV 5 Above 12 KV 5 (D) MECHANICAL ASSEMBLY: Check that all the nuts and bolts are in their position and check their tightness. Check circlips, split pins, oil dashpot (if any for current level and its operation. Also air dashpots should be checked for current operation. Clean, inspect and replace worn-out parts during overhauls., In case of spring operated mechanism check the ratchet wheels and prawls for broken or chipped teeth. Values of pneumatic mechanism or hydraulic mechanism should not be disturbed unless the diagnostic tests indicate the need for their readjustment. (E) INTER-POLE LINKAGES : Inter-pole linkages should be checked to ensure simultaneous operation of 3 poles (within pole discrepancy of 5 ms) and deteriorations, found if any, of springs and other components should be checked and set right. Linkage pins, circlips, nut-bolts etc. should be checked. Mal-operation of mechanism. inter-pole linkages dashpots etc. can be revealed by the time/travel record of 3 poles which shall be described in the subsequent discussion. The moving contacts of each phase should meet the fixed contact practically simultaneously which can be easily checked with a simple practical arrangement as shown below:

LR LY LB

Battery Supply

R

Y

B

C.B. Fig. : Method of checking simultaneous

contact meeting during slow closing


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METHOD OF OBTAINING TIME-TRAVEL CHARACTERISTIC ON NO-LOAD : This is extremely important because the breaking capacity is related with the time/travel characteristics of moving contacts. No load characteristics give sufficient indication about the health of mechanism linkages. For satisfactory arc interruption the moving contact should open and travel with optimum characteristic. Slow movement of contact initially, during middle of stroke and during final part of the stroke indicate excessive friction between sliding parts, very high dynamic loading during arc quenching and excessive damping respectively. A straight rod is connected to the moving contact or movable part. This rod is in turn connected to the curvo-roller which is a specially designed motor driven drum mounted on the top hood of the MOCB. The moving contact touches the paper on the drum. The motor is driven electrically. The drum rotates at known speed. The graph sheet fixed on the drum has definite circumferential speed. During the opening stroke and closing strokes, the pencil gives the time/travel characteristic marking on the graph sheet on the curvo-roller. (F) CHECK AND MAINTENANCE OF OTHER IMPORTANT ACCESSORIES : Wherever required check for cleanliness, loose connection, dirt and moistures, tightness of joints, proper contact matching and sequential operation of auxiliary switches and interlocks etc. The following accessories must be properly attended : Main connections, Secondary wiring and fuses, Earth connections ; Heater, Safety shutters (check for proper operation after de-energisation of bus) Bus bars and bus chambers, Auxiliary switches, Indicating devices and interlocks, Isolating contacts, Overload devices and protective relays, Instrument transformers and instruments, Contactors. Particular attention must be paid to interlocks and locking devices especially those associated with earthing and testing facilities . A defective or worn device may result in a dangerous condition. Any incorrect operation must be rectified.


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Auxiliary contacts controlling the trip circuit shall also receive particular attention for its operation within the required timing. (G) FINAL VERIFICATION : After overhaul the C.B. is subjected to operational checks by performing C,O,CO, Operations from local control cabinet and from the control room (if remote control is also there). Simultaneous touching of the contacts of three phases is verified. Insulation resistance is measured between the terminals and between the lower terminal and earth. Insulation resistance of auxiliary winding is also measured. (H) TYPICAL MAINTENANCE RECORD DATA : History card for each is kept. It may be recorded in the following from : Circuit breaker serial number : Name of manufacturer : Year of purchase : Cost of purchase : Current and voltage rating : Making capacity (Symmetrical) : Breaking Capacity (Symmetrical) : Short time rating : Inspection date :

Component Observation Action taken Initials Mechanism Linkages Insulation Quenching medium Main contacts Arcing contacts Terminals Final verification

MAINTENANCE OF A.C.B., FUSE GEAR FOR LOW & MEDIUM VOLTAGES : Maintenance schedule depends upon frequency of load operations, fault operations and environment of operation and as such no uniform guide lines can be prescribed. However, for indoor, dust free installation with infrequent load operations the following schedule may be recommended :

- Inspect as often as possible with maximum interval of 12 months ; - Examine at 5 years interval ; and - Overhaul when examination and diagnostic tests indicate need. Maximum interval of

15 years.

TABLE 6 : MAINTENANCE. OF A.C.B. & SWITCHGEAR


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S. No.

Maintenance Operation

Routine Maintenance Post fault

Inspection Examination and Overhaul

Maintenance

1. Operational checks X X 2. Visual Inspection X X X 3. Cleaning X X 4. Insulation resistance X X 5. Circuit breaker enclosure X X 6. Main/arcing contacts X X 7. Arc control devices and inter-phase

barriers X

8. Mechanisms X 9. Auxiliary switches, indicating

devices and interlocks X

10. Isolating contacts X 11. Overload devices and protective

relays X X X

12. Instruments and instrument transformers

X

13. Contactors X 14. Main connections X 15. Secondary wiring and fuses X 16. Earth connections X 17. Shutters X X 18. Switchgear spouts X 19. Heaters X 20. Buses and bus bar chambers X 21. Maintenance of auxiliary equipment X 22. Diagnostic testing X 23. Final verification X X X MAINTENANCE OF OIL CIRCUIT BREAKERS (B.O.C.B., MOCB) : The following schedule may be followed for the purpose (O.C.B. installed outdoor and in damp/rainy areas) : (i) Under normal conditions for frequency operations, once in 6 months with maximum

period of 9 months. (ii) After fault clearance : If fault level is high; inspect as early as possible. Otherwise after

fault opening operations. (iii) Overhaul : Once in 5 years for normal and once in 3 years for repeated/frequenct fault

duty. (iv) Following checks should be done during periodic check up :

(a) Level and condition of oil (b) Condition of insulators; if required cleaning may be resorted to.


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(c) Contacts (d) Operating mechanism (e) Auxiliary switches (f) Tighten nuts & bolts. (g) Insulation resistance by megger (use 500 V megger for control circuits).

(v) When the breaker operates on faults, the internal and external inspection should be

carried out as soon as the operating schedule permits. (a) Examine the oil; if badly deteriorated, change it. (b) Check arcing contacts and take remedial measures. (c) Inspect the insulation. (d) Check the arc control devices and do the needful. (e) Check the tripping circuit and operating mechanism. (f) Be sure that no tools are left in the tank. (g) Check the Dielectric property of oil. Limiting value for various properties of

Dielectric oil is tabulated below :

TABLE 7 : PROPERTIES OF DIELECTRIC OIL

Characteristic Limiting value Sludge (maximum) 1.200 % Flash point (closed minimum) 1460C Viscosity at 700F 37 CS Electric Strength, 1 minute minimum 40 K.V. r.m.s. Saponification value 1.00 mg KOH/gm Copper discolouration Negative Crackle Shall pass test.

While using centrifugal separators for removing free water and fine solid

impurities, the oil is heated to about 700C by means of electric heater. Hot oil should not be used to fill switchgear. There should be as little aeration of

the oil as possible during filling of the tank. A standing time should be allowed after the tank is filled before commissioning the equipment.

(h) Silica-jel breather should also be maintained properly it the O.C.B. or transformer

contains it. When the colour of silica-jel changes from the blue to pink, it requires reactivating. This is done by baking it at a temperature of about 2000C until the whole mass has restored its original blue colour.

POST FAULT-MAINTENANCE OF OIL CIRCUIT-BREAKERS :

Depending upon the design and duty of the circuit breaker, inspection may be required

after about six fault operations or the number specified by the manufacturers and based on local fault level. When such an inspection is necessary special attention shall be given to the points tabulated below under table 8 :


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TABLE 8 :

MAINTENANCE FOR OIL-CIRCUIT BREAKER IN METAL ENCLOSED SWITCHGEAR

Routine maintenance Post fault Sl. No. Maintenance operation Inspection Examination

and overhaul maintenance

1. Operational check ; C-O-CO X X X 2. General Inspection X X X 3. Cleaning X 4. Insulation, insulating oil X X X 5. Circuit breaker enclosure X 6. Main arcing contacts X 7. Arc control devices X X 8. Isolating contacts X X 9. Venting and gas seals X X 10. Mechanism X X 11. Auxiliary switches, Indicating devices and

interlocks X

12. Overload devices and protective relays X X 13. Instruments and Instrument Transformers X 14. Control relay or contactor X 15. Insulating oil X X 16. Tank and tank lining X X 17. Tank lifting mechanism X X 18. Main connections X 19. Secondary wiring and fuses X 20. Earth connections X 21. Heaters X 22. Shutters X


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Routine maintenance Post fault Sl. No. Maintenance operation Inspection Examination

and overhaul maintenance

23. Switchgear spouts X 24. Busbars and busbar chambers X 25. Weather shields X 26. Final verification X X X 27. Joints and seals X 28. General Mechanical Inspection X 29. Maintenance of oil, switches etc. X 30. High voltage fuse connection and

associated linkages X

31. Maintenance. of air break isolating devices associated with oil switchgear

X

32. Maintenance. of auxiliary equipment X X MAINTENANCE SCHEDULE OF OUTDOOR UNATTENDED SUBSTATIONS : The maintenance schedule (for transformer) as listed in Table 9 below may be followed for outdoor unattended substations (though very less in our deptt.) :

TABLE 9 Period Items to be checked Quarterly (i) Clean bushes of all dirt and oil

(ii) Examine condition of switches and tighten connections. (iii) Look for any leaks (iv) Check condition of silica-gel. See that the air passages in the

dehydrator are free. (v) Check oil level and top up, if necessary. (vi) Look for broken bushes.

Yearly (i) Check dielectric strength of oil (ii) Check earth resistance (iii) Check lightning arrestors.

Five yearly Open tank for cleaning and reassembly. The following measures may be taken to keep observation on the transformer oil so far as its acidity is concerned : (i) Acidity below 0.5 mg KOH/gm - permissible, if oil is satisfactory otherwise (ii) Acidity between 0.5 & 1.0 mg KOH/gm - Keep oil under observation (iii) Acidity above 1.0 mg KOH/gm - Discard oil. The acidity of oil should be checked every two years for transformers upto 1000 KVA capacity and annually for transformers above this size. MAINTENANCE SCHEDULE OF TRANSFORMERS ATTENDED SUB-STATIONS :


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Transformers will run practically trouble-free if proper maintenance is done. For this purpose a maintenance schedule can be drawn. Though this schedule shall be only indicative in nature and tests may be conducted at more frequent intervals depending upon the severity of service. Such a maintenance schedule is given below in table 10.

TABLE 10 Inspecting Frequency

Items to be inspected Remarks

(I) Hourly Ambient temperature, winding temperature, oil temperature, load and voltage

Adjust load if temperature is high.

(ii) Daily Oil level Relief diaphragm Dehydrating breathers (check for free air passages. Check colour of silica-gel

If low top up with dry oil Look for any sign of leakages Replace if broken Change if colour is pink

(iii) Quarterly Lubricate bearings etc. of cooler fans, circulating pumps etc., if any.

(iv) Half yearly Check oil for dielectric strength and presence of moisture

Filter or replace if necessary

Check bushes, insulators etc. Check cable boxes etc.

(v) Yearly Check oil for acidity, sludge and flash point, if possibly.

Filter or change oil as necessary.

Check relays, alarms, contacts etc.

Check earth resistance Methods of lowering resistance may be followed if required.

Check lighting arrestors (vi) Five yearly Over-all inspection including lifting

of core and coils Wash by hosing down with clean dry oil.

MAINTENANCE OF TRIP CIRCUIT & BATTERY SUPPLY : Maintenance of the trip circuit and battery supply is essential for the satisfactory operation of all protective relays (for shunt trip type). Following care and maintenance of batteries are required to be taken : (i) Electrolyte level must be maintained 10 to 15 mm above the plates for the proper

reaction of the constituents, (ii) Terminal voltage of the cell must not be allowed to fall below 1.8V, (iii) Battery should be charged to its rated capacity to increase its efficiency, (iv) Do not allow battery to remain in semi-discharged condition for long otherwise the life

of the battery will be reduced, (v) Commercial sulphuric acid should not be used otherwise the active components will be

damaged soon, (vi) Use distilled water for topping the level of electrolyte,


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(vii) Avoid excessive charging since it reduces life of the battery, (viii) Bare and insulated lead should be painted with recommended paint to avoid accidental

short circuit, (ix) Battery should be kept clean and dry and battery room should be well ventilated to

avoid suffocation, (x) If acid falls out of battery, wipe over with wet rag and dry thoroughly to save the

components from being damaged, (xi) Terminal posts and connectors should be clean and free from corrosion to increase the

life of the battery, (xii) Nuts and bolts of cell connectors should be kept tightened and smeared with Vaseline to

avoid rusting; and (xiii) it should not be left in discharged condition for long otherwise the defects of sulphation

will occur. Every relay should be tested once in six months, with suitable testing set and the records of such tests should be logged in a maintenance register. During tests a check should be made if any of the overload or time setting on the relay require change due to the increase or decrease in the load condition since the date of last test. Following defects are possible : (a) Improper contact or misalignment of the contact prongs of the trip battery circuits,

between the cubicle and the draw-out truck (b) Discharged battery, (c) Wrong current transformer connections, (d) Wrong relay settings for the load conditions, and (e) Stiffness of circuit breaker operating mechanism due to dust, dirt, rust or lack of

lubrication. CARE AND MAINTENANCE OF EARTHING : (i) Check the earth resistivity during dry season half yearly and maintain and compare the

record with previous reading. (ii) Water should be poured at regular intervals during dry season at the small substations. (iii) When the system is expanded, the earthing system should also be expanded by using

additional earth electrodes and earth wire separately. (iv) Electrodes should be placed in the earth in the up[right vertical position. (v) The electrodes and the earth continuity conductor used in the circuit should be of same

material i.e. copper or G.I. (vi) Check the earth connections and tighten them properly. (vii) Pass the earth continuity conductor through the galvanised pipe from being damaged. (viii) Inspect the broken or rusted earth continuity conductors and replace them with proper

size. (ix) The earth resistance of the following equipments must be measured during the driest

part of the year :

(a) transformer body and other metal parts, (b) neutral of the transformer, and (c) lightning arrestors.


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The value of earth resistance should not be allowed more than 2 ohms. (x) Mixture of salt and water may be added to improve the earth resistance. (xi) In the corrosive earth, the steel electrode should be replaced by copper coated steel

electrode. (xii) The earth continuity conductor should be properly welded with the earth electrode. (xiii) Avoid any jointings from earthing conductor.

Substation Equipment Maintenance

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