INSTALLATION COMMISSIONING OPERATION AND MAINTENANCE OF TRANSFORMERSby C.P.DESHMUKHA.G.M BHEL BHOPAL
ERECTION AND COMMISSIONING OF TRANSFORMERS Site establishment. Infrastructures and T/P mobilization. Receipt storage and inspection. Placement on plinth and installation. Erection of fittings and accessories. Dry out & pre-commissioning checks. Commissioning. Monitoring performance.
AVAILABILITY OF SITE FRONT Identification and allotment of land for Site establishment & construction of stores. Documents like shipping list, drawings and leaflets for site reference. Infrastructures and T/P mobilization. Extensive site co-ordination.
RECEIPT HANDLING STORAGE & INSPECTION Receipt of material, unloading and proper storage. Physical inspection of material and tank for shortages / damages if any. Shortages identification for re-plenishment Indoor Storage of sensitive instrument accessories in well protected store. Storage of accessories outdoor on compacted land to prevent against rusting.
INSPECTION OF TRANSFORMER ON RECEIPT AT SITE • Check N2 pressure. • Identify external damages/ dents on various locations. • Internal inspection to check movement of core and winding during handling / shunting / unloading en route transportation. • Discrepancy if any to be reported to manufacturer. • Healthiness of core and winding to be ascertained by testing if necessary. • After inspection transformer tank to be pressurized to 2.0 psi and maintained till erection is taken up.
INFRASTRUCTURE TOOL & PLANTS Long boom crane for bushing mounting. Vinch and pulley for dragging. High vac filter machine (4000lph min. cap.) Oil storage tank (20kL cap.) Current injection kit for CT testing. Testing instruments for LV test on the trfr. Vacuum pump suitable for 1mm vacuum. Dew point meter if required. Oil BDV and PPM testing kit. Tan delta / Cap. Testing kit.
ERECTION/ INSTALLATION Ensure healthiness of transformer fittings and accessories. Clean oil storage tank and filter machine to remove old / contaminated oil / dirt in it. Availability of site front near / around transformer plinth. Requisite power supplier near foundation.
LONGER STORAGE OF TRANSFORMER If storage without oil more that 6 months dryness of winding insulation must be assessed. Dry nitrogen (-55˚C) to be used to determine dew point of insulation. Higher dew point indicates moisture ingress in solid insulation. Nitrogen purging to remove moisture till desire dew point achieved.
PLACEMENT OF TRANSFORMER ON PLINTH Inspection of plinth as per center line of main tank and cooling bank/ pockets. Placement of main tank on plinth. Start cooling bank erection after installation of A frame / headers after correct marking as per drawing. All the pipe works and fittings to ensure in healthy condition. Welding rectification /painting of pipe work.
TURRET ERECTION Turret CTs to be cleaned and dried with heater to remove moisture if any. Turret CT testing for IR value, mag. Current, ratio and polarity. Results to be compared with works values. Variations in test results to be reported and concurred by manufacturer. Turrets erection to be carried out as per reference mark to ensure correct installation.
BUSHING HANDLING Make bushings vertical on a temporary stand. Observe leakages if any. Clean porcelain with dry cloth. Tighten oil filling plugs and test tap assembly for adequacy. Check oil loss if any from MOG & re- tighten. If oil loss is from porcelain/ flanges joint manufacturer to be consulted. Bushings mounting after testing for tan delta / cap. Values.
BUSHING TESTING AND ERECTION Capacitance and Tan delta testing at 10kV. Tan delta value < 0.007. Capacitance to be comparable with factory results. Adequacy of oil in the bushing. Minor damages on the bushing despite good test results to be shared with manufacturer. Bushing to be erected with immense care. Lead orientation inside the bushing tube and terminal connection must be well tighten.
TRANSFORMER TESTING BEFORE OIL FILLING Polarity, vector group and ratio measurement on all tap positions. winding resistance measurement on normal, minimum and maximum tap positions. Isolation test between core, end frame and tank. Continuity during tap change operation. magnetization current. Note: Consult manufacturer if results do not match with works values for corrective action.
VACUUM PULLING • Erection of all fittings and accessories. • Internal inspection to remove all the dirt and dust from the transformer tank. • Retightening of cleat nuts and bolts. • Re-tightening of on load tap changer leads connected to the selector. • Blanking all the openings and tightening. • Vacuuming upto 1mm of HG. • Checking of leakages and tightening. • Final vacuuming for the period specified (48hrs min).
OIL PREPARATION & FILLING Oil supplied to confirm IS:335. Clean oil storage tank using good oil to remove residual. Recondition & clean filter machine. Preparation of oil in separate tank to achieve BDV more than 60kV. Evacuation of main tank and maintaining vacuum up to 1 torr. For min 48 hrs. Filling of oil under vacuum.
OIL FILLING IN CONSERVATOR Check functioning of MOG with float arm movement. Inflating rubber bellow of conservator for 0.1psi and check leakages. Topping oil in the conservator from bottom and releasing air from valves . Pressure testing of transformer for 12 hrs at 5 psi & attend leakages if any.
DRY OUT AND TESTING Oil filtration through high VAC filter machine under vacuum and 55 °C temperature of transformer. Continue dry out till BDV > 70kV and PPM < 10 for 400kV class transformers (IS1866). Continuous filtration till desired values by uninterrupted power supply availability. Check PI > 1.25 IR values comparable with factory results.
TRANSFORMER TESTING Measurement of magnetizing current. Resistance measurement of the windings. Ratio test on all tap position. Magnetic balance test. IR value of the winding (60 sec and 600 sec) Isolation of core and end frame. OLTC manual and electrical operation. Tan delta and cap. of bushing and wdg. Dissolve gas analysis of the oil sample. FRA of the transformer.
PRE-COMMISSIONING CHECKS Compare all low voltage test results and short out discrepancy if any. Releasing of air from different pockets after allowing 24 hrs. settling time. Ensure proper earthing of tank, turret, bushings flanges and test tap assembly. Ensure perfect earthing of tank to mother earthing. Ensure perfect bushing top terminal connections to avoid heating. Commissioning the transformer and monitoring performance for 24 hrs.
METHODOLOGY FOR OVERHAULING OF TRANSFORMER IN OPERATION FOR MORE THAN 7 YEARS
Oil surge relay operation • Located between diverter switch and OLTC conservator. • Trips only when oil flow from OLTC exceeds 0.9 m/sec. • Relay does not sense formation of gas in diverter as gas formation due to arcing in OLTC diverter switch is normal feature. • The gas generation during tap change is passed on to oil conservator and will not accumulate in the relay. • If the transition resistors remain connected in the circuit for a longer duration , there will be huge current of resistance power loss depending upon the step voltage, resistance value of power being transmitted. • Heat so generated will cause expansion of oil & flow towards oil conservator thus resulting in tripping ofOSR protective relay
DISSOLVE GASES ANALYSIS INTERPRETATION INCIPIENT FAULT ASSESEMENT ACTION PLAN FOR LIQUIDATION OF DGA PROBLEM
DISSOLVED GAS ANALYSIS (DGA) Tool for condition monitoring of transformers/ reactors . Analysis of gases dissolved in transformer oil. Main gases formed due to chemical and electrical problems inside the transformers.
DISSOLVED GAS ANALYSIS (DGA) H2, CH4, C2H6, C2H4, C2H2, CO & CO2 gases are evolved due to incipient fault. C2H2 is mainly associated with arcing at very high temperature. Periodic data to be analyzed for detection of fault as per IEC 60599 & IS10593.
Conclusion for DGA . • Gases are formed during 1st charging and soaking of electrical circuit. • Allow minimum 24hrs settling time. • Leakage proof erection to prevent ingress of air. • Concentration of gases must be more than 10 times detectable limits for interpretation. • Bench marking DGA before charging, after charging & one hour after loading.
Conclusion for DGA . • No interpretation on single data. • History of transformer for DGA monitoring. • Excess DGA calls for immediate degassing. • Rapid C2H2 generation is serious needs immediate investigation.
CASE STUDY OF 315 MVA ICT 6M6061 PG TRIVANDRUM-1 Observations ( Oil parameters) • BDV reduced from 68kV to 29.2 within 9 months. • PPM level increased to 13 within 9 months • Oil Resistivity reduced from 148 to 4.71 x 10 gig. Ohm cm. • No change in flash point. • Acidity increased to 0.007.
CASE STUDY OF 315 MVA ICT 6M6061 PG TRIVANDRUM DGA monitoring - interpretation • Total gas content increased to 10.39%. • N2 also increased from 3.89 to 7.56%. • O2 increased from 1.11 to 2.24% . • H2 increased to 9ppm. • CH4 increased to 4ppm. • C2H4 increased to 3ppm. • C2H6 increased to 2 ppm. • CO/ CO2 increased to 534 / 1329 ppm.
INTERPRETATION • DGA results indicates healthiness of the transformer as all fault gases are within limits. (H/ Cless than 50ppm). • Even H2 is also less than 10ppm. • Normal ageing. • Oil condition can be improved further by HOC to improve BDV > 70kV and PPM < 10. • DGA result otherwise are absolutely normal. • CO2/ CO ratio indicate normal cellulose ageing. • N2 purging and subsequent dry out improved dielectric conditions of transformer insulation. DGA problem solved.
TRANSFORMER FAILURE CASE STUDY-2 SITE - NTPC KAYAMKULAM TRFR RATING - 160 MVA, 11/230 KV GEN. TRFR. 1ST COMMG. - JAN 2000 PROBLEM - DGA analysis show high gas concentration & rising trend of fault gases. TYPE OF FAULT - Internal thermal fault of high temp. 700•C. ACTUAL FAULT - Insulation failure of top yoke shunt. Transformer repaired and re-commissioned successfully in June 2004.
DRY OUT AND TESTING Oil filtration through high vac filter machine under vacuum and 55 °C temperature of transformer. Continue dry out till BDV > 70kV and PPM < 10 for 400kV class transformers (IS1866). Mounting of accessories and cabling to the M box / RTCC panel. Earthing main tank. Check OLTC / off CKT. Tap switch operation of all taps and ensure continuity. Conducting low voltage tests like IR values, Mag. Current, Winding resistance, ratios and impedance measurement. Vector group and polarity testing. Final IR measurement after connecting transformer to the system.
TRANSFORMER FAILURE CASE STUDY-3 SITE - MSEB KHAPERKHEDA TRFR RATING - 250 MVA 220KV GT SL.NO. 6004888 1ST COMMG. - 08.01.1990 HISTORY - GT Overhauled in Aug- 03. Transformer oil replaced by MSEB.. PROBLEM - High DGA reported. Excessive high content of H2 (6383ppm) & evolution of CH4 noticed. INTERPRETATION - Oil decomposition and corona discharges in the vicinity of magnetic circuit. Corona in oil.
TRANSFORMER FAILURE CASE STUDY-3 INSPECTION AFTER - Rusting and black spot on top cover lifting core surface below cross beam plate at 3 places. LV bus bar “U” phase braze joint blackened. CORRECTIVE ACTION- Thorough cleaning with new oil. Removal of complete sludge. laminations cleaned and varnished. Yoke cross beam insulation replaced. LV bus bar joint cleaned and re-insulated. Dry out by N2 purging and hot oil circulation . COMMISSIONING - GT charged on 02.02.07. DGA results are absolutely normal .
Impact-O-Graph for transformer transportation to site. • Transformers transportation • By wagon ( 180 / 220 tons) • By road on trailer ( Axle combination) • By ship for overseas destination • Road / rail / ship ODC sanction.
Impact-O-Graph for transformer transportation to site. • Recording of shock and impact from all directions. • Lateral shock (“X” axis) • Longitudinal shock (“Y” axis) • Vertical Shock (“Z” axis)
Impact-O-Graph analysis. • Max. shock in all the three axis. • Magnitude and duration as per records. • Total transit duration. • Impact / shocks to be less than design.
DO'S & DON'TS AND PREVENTIVE ACTION & MAINTENANCE BY:- C. P. DESHMUKH AGM(TXX) BHEL BHOPAL
DO’S FOR POWER TRANFORMER/REACTOR 1. Check & thoroughly investigate the trfr/reactor whenever any alarm or protection operated. 2. Check air cell in conservator. 3. Attended the leakages on the bushing immediately. 4. Examine the bushings for dirt deposits & coats, & clean them periodically. 5. Check the oil in trfr & OLTC for di-electric strength and moisture content & take suitable action for restoring the quality. 6. Check the oil level in oil cup & ensure air passages are free in the breather. If oil is less, make up the oil. 7. Check the oil for acidity and sludge as per IS:1866. 8. If inspection covers are opened or any gasket joint is to be tightened, then tighten the bolts evenly to avoid uneven pressure.
DO’S FOR POWER TRANSFORMER/REACTOR 9. Check & clean the relay and alarm contact. Check also their operation, & accuracy and if required change the setting. 10. Check the protection ckts. Periodically. 11. Check the pointers of all gauges for their free movement. 12. Clean the oil conservator thoroughly before erecting. 13. Check the buchholz relay & readjust the floates, switches etc. 14. Inspect the painting & if necessary retouching should be done. 15. Check the OTI & WTI pockets & replenish the oil, if required. 16. Remove the air through vent plug of the diverter switch before you energise the trfr. 17. Check the oil level in the diverter switch and if found less, top up with ffresh oil conforming to IS:335 `
DO’S FOR POWER TRANSFORMER/REACTOR 18 Examine & replace the burnt or worn out contacts as per annexure 4.1 of maintenance schedule. 19 Check all bearings and operating mechanism & lubricate them as per schedule. 20 Open the equalizing valve between tank and OLTC, wherever provide at the time of filling the oil in the tank. 21 Fill the oil in the trfr/reactor at the earliest opportunity at site and follow storage instructions. 22 Equalize the diverter compartment ofnthje OLTC by connecting equlising pipe between flange joints provided on the tap changer head.