Testing and Condition Monitoring of Substation Equipments

250MVA, 13.8/420KV, 3PHASE, 50Hz, ONAN/ONAF/OFAF COOLED GENERATOR TRANSFORMER (Make: BHEL, Bhopal)Technical Specification Of Generator Transformer:Type of Cooling : ONAN / ONAF / ODAFRating HV (MVA) :150(60%) / 200(80%) / 250(100%)No Load Voltage : HV(KV) 420No Load Voltage : LV (KV) 13.8Line Current HV (Amps) : 344.1Line Current LV (Amps) : 10471.6No of Phases : 3Frequency (Hz) : 50 Vector Group : Ynd11Temp. Rise Oil (C) : 45C Over Ambient of 45CTemp. Rise Winding (C) : 55C Over Ambient of 45C•Impedance Volt At 250 MVA Base HV Position 1/LV 13.5 % Approx. (At Maximum Tap) HV Position 5/LV 14.5 % 10 % Tol. (At Normal Tap) HV Position 13/LV 19.0 % Approx. (At Minimum Tap)•Insulation Level H.V. S1 1050 L1 1300 – AC 364 H.V.N. AC 38 L.V. L1 95 AC 38

Core & Winding (Kg) 135225Weight of Oil (Kg) 53965Total Weight (Kg) 248755Oil Quantity (Liter) 62030Transport Weight (Kg) 159730Untanking Weight (Kg) 13845Tank & Fittings (Including Turrets) Kg 29730Bushings (Kg) 1265Marshalling Box & Wiring (Kg) 1025Cooling Bank (Including Radiators, Pumps, Fans/W, Supports, Header, Conservator etc.) 27545

Oil In Transformer Tank, Turret etc. 49885Oil In Radiators, Conservator, Header & P/W 12120

•Weight

Cooling Equipment :No of Radiator Bank : 2 50 %No of Radiators Per Bank : 12No of Oil Pumps Per Bank : 1 Running 1 Stand byRate of Oil Flow Per Pump : 2700 LPMNo of Fans Per Bank : 6 Running 1 Stand by (36” Size)Air Delivery / Fan : 467 Cum / Minute

OTI Alarm : 95CTrip : 100C

FAN ON : 60COFF: 55C

WTI Alarm : 100CTrip : 110C

PUMP ON : 75COFF: 70C

•OTI & WTI Auxiliary Contacts Settings :

•OFF – CIRCUIT TAP SWITCH : Tapping in 6 (i.e. total 12) equal steps of 1.25 % each, provided at neutral end of HV winding to give 5 % to 10 % variation of high voltage

315 MVA, 415/220/33KV, 3 PH, 50Hz Auto Transformer (Make: BHEL, Bhopal)

As per India Standard: 2026

Type of Cooling: ONAN/ODAF Rating HV & IV (MVA : 189(60%)/315(100%)

Rating LV (MVA) : Stabilising No load Voltage HV (KV) : 415No load Voltage IV (KV) : 220No load Voltage LV (KV) : 33Line Current HV (Amps) : 262.9/438.2Line Current IV (Amps) : 496.0/826.6Line Current LV (Amps) : StabilisingTemp Rise Oil (oC) : 45/45Temp Rise Winding(oC) : 50/55Phase : 3Frequency : 50HzConnection Symbol(Vector group) :Y Naod 11•Impedance Volt at 315 MVA BaseHV - IV : 12.04%HV - LV : 43.68%IV - LV : 87.05%•Insulation Level HV - S1 : 1050 L1 1300 – AC 38HV - L1 : L1 250 – AC 95IV - LI : 950 –

AC 38•Weight in Kg: Core and Winding (Kg) : 111100Weight of Oil (Kg) : 48700Total Weight (Kg) : 209000Oil Quantity (Liter) : 56000Transport Weight (Kg) : 132400Untanking Weight (Kg) : 13500

OTI Alarm: 90CTrip : 100C

FAN ON : 62COFF: 60C

WTI Alarm: 100CTrip : 110C

PUMP ON : 72COFF: 65C

•Cooling Equipment No. Of Radiator Bank : 2-50%No. Of Radiator per Bank : 12No. Of oil pumps per Bank : 1 Running + 1 Stand by Rate of oil flow per pump : 2700 LPMNo. Of fans per Bank : 3 Running+ 1 Stand by Air delivery/Fan : 467 Cu.m per min of air.•OTI & WTI Auxiliary Contacts Setting •Tappings: No tappings provided on the Transformer.

INSULATION RESISTANCE PURPOSEIt is common routine test performed to check healthiness of insulation.

EQUIPMENTS-Megger

POLARIZATION INDEXPI= 10min reading/1 minute reading

CONCEPT-DC voltage injected between terminals containing dielectric .

FACTORS AFFECTING- Surface condition of the terminal bushing, quality of oil, quality of winding insulation , temperature of oil(suitable correction factor applied), duration of application and value of test voltage

TEST CONNECTIONS

INTERPRETATION

FACTORS AFFECTING- Surface condition of the terminal bushing, quality of oil, quality of winding insulation , temperature of oil(suitable correction factor applied), duration of application and value of test voltage

DIELECTRIC ABSORPTION

PURPOSE-The Dielectric Absorption (or Time-Resistance) is an extension of IR.

EQUIPMENT USED-Digital megger with timer

CONCEPT-When voltage is applied to an insulation system, three types of current - Leakage current- is the resistive current . Capacitive charging currentDielectric absorption current -polarising current to align the dipoles.

PROCEDURE In this test, a test voltage is applied for, usually thirty minutes. The megger readings

are taken every 10 seconds for the first minute and thereafter every minute – up to 30 minutes. A curve is drawn showing the variation in the value of IR.

INTERPRETATION-Graph

RATIO

PURPOSE -Check rated LV voltageat rated HV voltage and tap changer Operation.

EQUIPMENTS-Multimeter

CONCEPT-HV Volts/LV volts=HV turns/LV turns

.

MEASUREMENTS(230V AC at HV)1R1Y,1Y1B,1B1R,2R2Y,2Y2B,2B2R.

INTERPRETATION-From name plate details

SHORT CIRCUIT CURRENT

PURPOSE-Check for any loose contact in the tap changer, lead connections etc.

EQUIPMENT USED-LV 3 phase supply & amperemeter.

CONCEPT-A 3 phase balanced low voltage a.c supply is applied to the HV winding at rated tap and the current measured. The current measured at rated tap should tally with the calculated value.Secondary should be S.C.

INTERPRETATION-Value of current obtainedShould tally with calculated value. Difference isIndication of lose contact of tap changer.

MAGNETISING CURRENT

PURPOSETest any fault in the magnetic circuit and winding.

EQUIPMENT USED-LV 3 phase supply & clamp meter.

CONCEPTSingle phase from 440 volts source applied to each phase and current

is measured.

INTERPRETATION Current should be in milliamp and comparable.

MAGNETIC BALANCE

PURPOSE-Find out the condition of stacking of core laminations, tightness of core bolts and perfection of magnetic circuit..

EQUIPMENT USED-Multimeter.

CONCEPTHV each phase from source of 440V.Phase and neutral connection for star-1u-1n,1v-1n,1w-1nPhase and neutral connection for delta.1u-1v, 1v-1w,1w-1u

INTERPRETATIONEXAMPLE-Excited R phase-R phase volts=Y Phase volts+B phase volts

VECTOR GROUP

PURPOSE-Polarity and Phasor relationship between HV and LV voltages.

EQUIPMENT USED-Multi-meter.

CONCEPTVector group shows phase relationship between HV &LV windings

INTERPRETATION-Specific set of equations for particular vector group.

WTI

Tripping & alarm check.

OTI

Tripping & alarm check

PRV

Tripping & alarm check

MOG

Tripping & alarm check

BUCCHOLZ RELAY

Tripping and alarm check.Floats check.

BREATHER

Check the colour of the silica gel in the breather.

RADIATOR

Check the radiator for release of air and position of valves.

BUSHINGS

Check the oil level in the bushings if sealed bushings are used. Release air from bushings if air release plugs are provided.

WINDING RESISTANCE

PURPOSE-Resistance of individual winding.

EQUIPMENT USED-Winding resistance kit

CONCEPT-Inter turn fault and winding damage.

INTERPRETATION-Manufacturers recommendations and inter winding comparision.

PURPOSE-To change tap when transformer is loaded.

CONDITION MONITERING-Continuity of taps.Transient resistance. Condition of rolling and fixed taps.

MAINTENENCE-Servicing.

PURPOSE- IS-10028Life increase of transformer.Improvement of IR value. Improvement of quality of oil used.Minor rectifications . Sludge Removal.

PURPOSE- Determine the degradation of insulation.Cracks or voids left of created during manufacturing or ageing.

EQUIPMENT USED-OMNICRON,DOBLE AND MEGGER are standard equipment for high voltage equipment.

SCOPE-Transformer winding, Bushings, Bushing CT, Bushing PT, LA, motors, Generators, Cables .

MODES- UST, GST & GSTg-A

CONCEPTTan delta= Ir/Ic

INTERPRETATION- Value of tan delta should be less than .007 at 20 degree temperature and capacitance of within 5% variation or as per name plate.

FACTORS AFFECTING-Oil condition, Dirt on surface, Temperature, Loose test co

PURPOSE- Operate equipment in safe temperature zone.Loose contacts detection.Local hot spots.

PURPOSESFRA analysis can detect problems in transformers such as:Winding deformation – axial & radial.Displacements between high and low voltage winding.

EQUIPMENTS-SFRA kit.

CONCEPT-Transformers consist of multiple complex networks of capacitances and resistors that can generate a unique signature when tested at discreet frequencies and plotted as a curve. The distance between conductors of the transformer forms a capacitance. Any movement of the conductors or windings will change this capacitance. This capacitance being a part of complex L (inductance), R (Resistance) and C (Capacitance) network, any change in this capacitance will be reflected in the curve or signature.

INTERPRETATIONTime-based – current SFRA results compared to previous results of

the same unit.Type-based–SFRA of one transformer will be compared to equal type

of transformer.Phase comparison – SFRA results of one phase will be compared to

the results of the other phases of the same transformer.

PURPOSEOil analysis of switchgear elements like OCB, Transformers, CT, PT

etc. is important not only for smooth operation but also for faults detection inside these equipments

ABBREVATIONS- O- Power Transformer above 420kv A- Power Transformer 170kv-420kv B- Power Transformer 72.5kv-170kv C-Power Transformer up to 72.5kv D-Instrument Transformer above170Kv E-Instrument Transformer up to 170Kv G-OCB

WATER CONTENT

IS-13567

PURPOSE-Effects insulation quality and decrease BDV.

LIMIT(PPM) OAD-00 ,B-40 , E-30, C-50

BREAK DOWN VOLTAGE

IS-6792

PURPOSE- To avoid sparking.

LIMIT(KV rms) OAD-50 ,BE-40 ,C -30

DIELECTRIC DISSIPATION FACTOR AT & SPECIFIC RESISTANCE OR RESISTIVITY AT 90 DEGREE

IS-6262

PURPOSE- Property of good dielectric.

LIMIT(%) OAD-0.20 ,BE-1,C -.30

IS-6103

PURPOSE Property of good dielectric.

LIMIT(ohm-cm)-0.1*10^12

INTERFACIAL TENSION

IS-6104

PURPOSE-molecular attractive forces Between water and oil

LIMIT(N/m)-0.015

FLASH POINT

IS-1448

PURPOSE-avoid flash as temperature rises.

LIMIT(degree celcius)-125

NEUTRALIZATION VALUE

IS-1448

PURPOSE- Acidity degrades paper insulation

LIMIT(mgKOH/gm) -0.3000

FURON ANALYSIS

IS-9434

PURPOSE-To detect healthy condition of paper insulation.

LIMIT(PPM) -1.001

DISSOLVE GAS ANALYSIS

IS-9435

PURPOSE-To detect faults inside transformer.

LIMIT(PPM) GAS INDICATIVE FAULT LIMITMETHANE LOCAL OVERHEATING 120ETHYLENE THERMAL DEGRADATION

OF OIL50

ETHANE OVERHEATING 65ACETYLENE ARCING & SPARKING 35HYDROGEN ELECTRICAL/THERMAL

FAULT100

CARBON MONOXIDE THERMAL AGEING OF PAPER

350

FOLLOWING PARAMETERS SHOULD BE MONITERED PERIODICALLY-

1.CLOSING TIME-150ms2.TRIP TIME-150ms3.TAN DELTA OF GRADING CAPACITORS-.0074.CONTACT RESISTANCE-150m-ohm5. IR VALUE-1000mega-ohm

FOLLOWING PARAMETERS SHOULD BE MONITERED PERIODICALLY-

1.IR-1000Mega ohm2.TAN DELTA-.0073.CT RATIO ERRORS FOR PROTECTION CORES-+3%4. CT RATIO ERRORS FOR METERING CORES-+3%5.CURRENT IN METERING AND PROTECTION CIRCUITS6.CAPACITANCE-+10%&-5%

FOLLOWING PARAMETERS SHOULD BE MONITERED PERIODICALLY-

1.IR-1000Mega ohm2.TAN DELTA-.0073.CVT RATIO ERRORS FOR PROTECTION CORES-+5%4. CVT RATIO ERRORS FOR METERING CORES-+0.5%5.CURRENT IN METERING AND PROTECTION CIRCUITS.6.CAPACITANCE-+10%&-5%

FOLLOWING PARAMETERS SHOULD BE MONITERED PERIODICALLY-

LEAKAGE CURREAT-500m-ampIR VALUE-1000 M-OHM

FOLLOWING PARAMETERS SHOULD BE MONITERED PERIODICALLY-

TRIPPING TIME CLOSING TIMERESET VALUEPICK UP VALUE