Distribution Transformers -Impact of Harmonics, Estimation of Losses and Life expectancy & Mitigation of ill effects

Knowledge Sharingsession onTRANSFORMERSM. Nageswar Rao, Sr.Mgr.(EMD)

Date: 24.03.2015Venue: EDC, Simhadri

ContentsIntroductionTypes of TransformersTransformers in SMPPSpecificationsComponents Loading & Overloading capacityCondition MonitoringTesting

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Types of transformersPower transformationPower transformer , Distribution transformer, Auto transformersVoltage transformation (Step up, Step down)Voltage class (LT, MV, EHV)CoolingOil filled (ONAN, ONAF, OFAF, ODAF)Dry typeCore material (CRGO, AMT)Construction (Core type, Shell type)Phase (1, 3, Multi-phase)3

Transformers in SMPPSt#1 TransformersSt#2 Transformers

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Rating/ specificationsVoltage HV/LVPower rating, MVACooling method% impedanceVector group & clockTaps, tap changerLosses & Loss capitalization chargesTerminal arrangement

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Rating/ specifications% Impedance 6

Vector groupCounter-clockwise phase rotationClock angle 1: LV lags HV by 30 deg11: LV leads HV by 30 degEx.: Dd0: No phase shiftDyn11: LV leads HV by 30 degYNd5 : LV lags HV by 150 deg

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The angle is called phase angle displacement.Phase angle displacement must not be confused with phase rotation. Phase rotation is the order in which the phase voltages reach their peak values. In three-phase systems, the phase rotation can be either A-B-C or C-B-A.The phase rotation can be reversed by interchanging any two phases of the primary circuit or any two phases of the secondary circuit. Interchanging two phases on both the primary circuit and the secondary circuit will revert back to the original phase rotation.

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Vector groupThe standard connection for a -Y transformer is to have the HV side voltages lead the LV side voltages by 30.

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The angle is called phase angle displacement.Phase angle displacement must not be confused with phase rotation. Phase rotation is the order in which the phase voltages reach their peak values. In three-phase systems, the phase rotation can be either A-B-C or C-B-A.The phase rotation can be reversed by interchanging any two phases of the primary circuit or any two phases of the secondary circuit. Interchanging two phases on both the primary circuit and the secondary circuit will revert back to the original phase rotation.

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Vector groupA-B-C, 12-3 phase rotation.A-B-C, 32-1 phase rotation9

Interchanging the phase connections in a -Y transformer not only reverses the phase rotation but changes the phase angle displacement from a standard --30 to +30 displacement.

SLD St#2the 1-3 winding is still magnetically linked to the 1-N winding, the 2-1 winding is still magnetically linked to the 2-N winding, and the 3-2 winding is still magnetically linked to the 3-N winding in exactly the same way.

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Cooling ArrangementONANFor smaller T/f ratingsOil is kept in circulation by the gravitational buoyancy in the closed-loop cooling systemONAFFans are mounted just below the radiators, to blow air from bottom. more fans should be mounted at the top of radiator height.

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Cooling ArrangementOFAFODAF11

Heat dissipation of winding is less

Oil ducting system is used to direct the oil over the windings, hence more heat dissipation.

When the oil is forced into the transformer, its flow is governed by the least resistance path as well as the buoyancy. Hence, part of the oil may not enter either windings or core, and may form a parallel path outside these two.

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Cooling ArrangementCooling pumpUsed for circulating oil against low and high frictional head losses respectively. Two types of pump designs: axial flow in-line type and radial flow typeThe axial flow type offers less resistance when switched-off, and is used with mixed cooling (ONAN/ ONAF/OFAF) since itThe radial flow type pumps, which offer very high resistance to oil flow under the switched-off condition, are used with oil-to-air heat exchangers (unit cooler arrangement) or oil-to-water heat exchangers in which no natural cooling is provided

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Name plate detailsSt#2 GTSt#2 STSt#2 UTSt#2 UAT

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Components of a TransformerWindingsCoreInsulation (paper)Cooling medium (oil)BushingsProtective devicesPRD, Buch.RelayMonitoring devicesMOG, Oil level indicators, RTD & Temperature gauges (WTI, OTI)AccessoriesTap changer, Radiators, Cooling fans & pumps, Oil flow indicators.Conservator, Breather, AircellAux. power supply (Marshalling box) Lightning protectionEarthing arrangement14

Oil filled Transformer15

Dry type transformer16

Core ThicknessGradeCore losses(w/Kg) 1.7T/ 50 Hz0.23 MMM30.900.27 MMM41.120.30 MMM51.300.35 MMM61.450.23 MM23ZDKH850.850.27 MM27ZDKH900.900.23 MM23M-0H1.000.23 MMTCH-00.900.27 MMTCH-11.00

To provide a low-reluctance path for the magnetic flux linking primary and secondary windings.Types of coreCRGOAMT17

The Amorphous Metal, an alloy of Fe78-B13-Si9, has non crystalline structure which is formed by cooling molten metal rapidly at cooling rates of 106 C/sec.The atoms do not get arranged as proper grain structure, but are arranged randomly. After annealing under a magnetic field, this alloy exhibits low losses as it gains excellent Magnetic and Chemical properties as compared to the conventional material.Amorphous Metal Distribution Transformers are the proven energy savers and this is because the Amorphous Metal core reduces the No load losses by about 75% when compared to the conventional CRGO grade.

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OilOil parametersBDV MOISTURE CONTENTTAN DELTARESISTIVITYACIDITY/SLUDGE CONTENTINTERFACIAL TENSIONIf BDV and Moisture content deteriorate oil filtration is to be done.If Tan Delta & Acidity content deteriorate beyond permissible values, oil needs to be replaced.

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Insulation Minor insulation Like inter turn insulation, is achieved using cellulosic paper.Major insulation like between primary and secondary, phase to phase and inner coil to core is achieved by Bakelite, wooden blocks, cellulosic paper cylinders.

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BushingTypesPlain type bushings (1000 degC)ArcingHigh level energy faultKey gases: Hydrogen and acetylene CO, CO2 (If the cellulose material (paper, insulating board etc.) is involved) 30

Thermal faults:As the fault temperature rises, the formation of the degradation gases change from methane (CH4) to ethane (C2H6) to ethylene (C2H4).

DGAThe distribution of released gases can be related to the type of electrical fault and the rate of gas generation can indicate the severity of the fault.

MethodsRogers Ratio Method, IEC Basic Ratio Method, Duval Triangle method and Key Gas Method

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DGA (Rogers Ratio Method)Three ratiosRatio 1 (R1)=CH4/H2Ratio 2 (R2)=C2H2/C2H4Ratio 5 (R5)=C2H4/C2H6No minimum levelssuggested when normal levels exceeded

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DGA (IEC 60599)Identifies 6 different fault typesPD: Partial DischargeD1: Discharge of low energyD2: Discharge of high energyT1: Thermal fault, t 700 CUses a combination of ratios (based on Rogers Ratios), gas concentrations and rates of gas increase.

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Moisture content in cellulose insulationWater heat run testRelative saturation (RS) of water in oil

Water heat run testTest should be carried outFor 3 daysWith top oil temp at 60 deg.Moisture will migrate from solid insulation to oil35

When the oil is forced into the transformer, its flow is governed by the least resistance path as well as the buoyancy. Hence, part of the oil may not enter either windings or core, and may form a parallel path outside these two.

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Relative saturation (RS) of water in oilAccurate testResults should be corrected to top oil temperature

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Ps = Saturation vapor pressure at temperature Tto.36

Transformer testingVoltage ratio test1 Excitation Current at 415VPolarity check Winding Resistance IR of winding & coreCapacitance & Tan delta of winding & bushing

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Voltage ratio testTo measure the voltage ratio of one winding to another associated with a lower or equal voltage.Accepted criteria% Ratio Error : 0.5 % of declared ratio on all taps Phase Angle error: 0.5% radian % Ratio error (Deviation) = [(measured ratio calculated ratio) / calculated ratio] x 100 Calculated voltage ratio = HV winding voltage / LV winding voltage

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1 Excitation Current at 415V To measure the excitation current at 415v in order to cross check the results at site before commissioning. Procedure:Single phase 415V, 50Hz supply is given to low voltage winding. Other windings are kept open. Excitation current is measured of supplied winding.

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Polarity check Procedure1-ph supply given to 1.1 & 2.21.2 & 2.1 are shorted.Voltage of 1.1 -1.2 & 2.1 -2.2 are measured.Acceptance criteriaSum of voltages 1.1-1.2 & 2.1-2.2 = supply voltage

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Winding ResistancePurpose To measure the winding resistance & Calculation of I2R component of conductor losses. To check faulty joints or breaks. To check loose connection.

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IR of winding & coreTo determine the insulation resistance from individual windings to ground or between individual windings. Insulation Resistance will be measured between (HV+N) / LV (HV+N) / (LV+E) LV / (HV+N+E)

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IR of winding & coreCalculate the Polarization index (P.I) which is equal to ratio of IR after 600 sec.to IR after 60 sec. Acceptance:P.I value should be greater than or equal to 1.3

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Capacitance & Tan delta of winding & bushing PurposeTo check the tan delta and capacitance of the transformer windings. For comparison with field measurements in order to assess the probable condition of the insulationMethod Test voltage of 0.5 to 12 kV @ 50Hz. Test done for windings to ground , between windings.Acceptance Tan Delta of winding @ 20C = 0.5% Max.

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Diagnostic testingFrequency response analysis (SFRA)Degree of polymerization indexFuran analysis (furfural content)

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SFRATo detect movement of windings under short circuit conditionsProcedureLow voltage sinusoidal output to one end of winding from Network Analyser (NA).Voltage transfer function measured from 5hz to 10mhzNA programmed to measure amplitude and phase shift at discreet intervals of frequencies.

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SFRACondition assessment based oncomparison of responses from different phases for each winding and tap positioncomparison of responses from different transformer of the same designFrequency bandwidth5hz to 2khz indicate mag circuit condition2khz to 200 khz changes in leakage impedance1mhz and above indicate capacitance changes

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dP indexLength of cellulose molecule is measured in terms of dp index.Depends on dp index of paperNew paper 1200 to 2000Below 250 a matter of concernBelow 150 little life left and close to failure

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Thank you all49

TRANSFORMER EXPLOSION PREVENTION ANDFIRE EXTINGUISHING SYSTEM50

Air cell is made from Nylon fabric coated with Nitrile rubber

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TransportationWith oil10% of the tank volume is left clear for expansion to limit the excess internal air pressure to 0.35 kg/cm2.Fittings dismantled before transport are packed in packing cases in line with shipping list.With N2Transformer tank is filled with dry Nitrogen at a positive pressure of 0.175 kg/cm2 (2.5 psi)N2 regulator reduces cylinder high pressure of 120 to 140 ksc to required low pressure of 0.175 ksc.

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