Speaker: Eng. Mahmoud Samir Engineer DTS Trainer Measurement of Losses on TransformerIEC-60076-3

There are mainly two types of losses in the transformerNo load lossLoad loss: 1. No load loss Losses are important for an economic operation of the net work. The no load loss is also called as iron loss or core loss. It is the electrical power lost in terms of heat within the core of electrical equipment, when cores are subjected to AC magnetizing force. It is composed of several types of losses - Hysteresis loss, eddy current loss within individual laminations and inter-laminar losses that may arise if laminations are not sufficiently insulated from each other. These losses are occurred in the core with out load on the transformer and hence it is called no load loss.No load Loss on the transformer

Core Materials The core material is made up of Cold Rolled Grain Orient (CRGO)Silicon Steel. The chemical composition of the GO steels has about 3.2% of Silicon as an alloy, thereby increasing the specified volume resistivity of the steel, thereby reducing the eddy currents. GO Steels are also decarbonizes and have no more than 0.06% of carbon in their chemical composition, which prevents ageing of the steel. There is a special carlite insulation coating on the steel, which reduces the inter-laminar eddy current losses within the core.

No load Loss on the transformer

Core material American Iron and Steel Institute which gave for CRGO Silicon Steel materials with M as a prefix and a number following (e.g. M4, M5, M6 etc.) M indicates magnetic material, and the number following approximately indicated the core loss of CRGO material in watts per lb. at 1.5T and 60 cycles. Today however, this number is not relevant, but still denotes the accepted grade and popularly used throughout the world (e.g. M4 denoted magnetic material having core loss of approx.0.4W/lb at 1.5T/60Hz).

No load Loss on the transformer

Improper handling of strip, sheets or long Laminations can introduce stresses that can distort magnetic properties. These stresses are usually plastic stresses. Tests conducted in the laboratory Single Sheet Tester showed a deterioration of 7% in core loss for material that was bent. However after stress relief annealing at 820C, the deterioration was only 2% and most of the original magnetic properties (with respect to core loss) of the material were restored. Processing operations like slitting, shearing, notching, holing etc. all damage the grain structure of the GO material around the area of fabrication and working.

No load Loss on the transformer

No load Loss on the transformer Burrs: The residual steel on the edge of steel sheet where shearing or punching during fabrication has taken place, thereby increasing the thickness on the edge and reducing the stacking factor. Burrs can be reduced by accurate and precise fabrication and having cutting blades and tools well sharpened at all times. They can also be reduced by de burring and stress relief annealing. What is the use of core The core is the main part of a transformer and is for the magnetic path of the flux.

Losses on the transformer Why using laminated core: The core is laminated to reduce no load losses. The eddy current losses due to eddy current circulation in the core is proportional to the square of the thickness of the core. If the thickness is reduce, the eddy current loss and heating of core is also reduced. The surface of the laminated core is providing with cooling of insulating varnish or insulating oxide layer to layer to prevent eddy current. Burrs and sharp edges shall be avoided, the core bolts should be fully insulated to prevent continuous path to circulating current and heating due to hysteresis loss.

No load Loss on the transformer Silicon steel raises the permeability at low flux density and to reduce hysteresis loss and eddy current loss. No load loss having two component Active or working iron loss component, Iw.= Io Cos Magnetizing iron loss component, Im= IoSine Magnetizing Current = Io = Iw2 + Im2 . The no load loss= Vo x Io x Cos

No load Losson the transformerMost of the steel factories have now switched over to the following method of grading Grain Oriented Steels: (Thickness) (Brand Name) (Core loss at 1.7T/50Hz) For e.g.. Nippon Steel grade 23ZH100 means thickness 0.23mm, ZH is the brand name for Hi-B for Nippon Steel and 100 means 1.00W/kg at 1.7T/50Hz. Similarly 23 RGH100 IS Kawasaki Steel the same material name is 23ORSIH100, the Thyssen Krupp Eklectrical Steel (TKES) nomenclature for the same material.

No load Loss on the transformer Hysteresis lossThe power expended in a magnetic material as a result of the lack of correspondence between the changes in induction resulting from the increase or decrease of magnetizing force which is a result of it being cyclic, i.e. alternatingHysteresis loss is proportional to max and f Where max = Flux (maximum), f = frequency, (Permeability of materials : Air=1, Iron=5000, copper= 1, Nickel=600 , vacuum =4 10 -7 hentry/m)Hysteresis loss is depending on the quality of the core materials. High Permeability and low flux density core materials is reducing the hysteresis loss

Eddy Current Loss:This component of core loss is the energy lost by the circulating current induced in the metal by the variation of magnetic fields in the metal. Therefore, more uniform the magnetic field in the metal, lower the eddy current losses. Eddy Current Loss is proportional to = 2 f2 t 2 = Flux , f = frequency, t= thickness of the core materialEddy Current Loss is proportional to the square of the thickness of the core materials.Inter-laminar Loss: The power expended in a stacked or wound core as a result of weak insulation resistance between Laminations resulting in the flow of eddy current within a core, across Lamination sheets.

No load Loss on the transformer

Measurement of No load loss and No load Current This test is a routine test of the transformer during FAT. The no load loss Po and no load current Io are determined at rated voltage and at rated frequency. The test is normally carried out at different voltage levels usually from 90% to 110% at 5% intervals. Some times it is from 85% to 120% according to the requirement of the specification. The voltage shall be applied either from High Voltage (HV) side of the transformer or from the Low Voltage (LV)side of the transformer keeping other side open condition. Suppose, the transformer HV/LV voltage is 132/12 kV and if applied at HV side, it need 132kV +20% source transformer to perform at the above mentioned voltage levels.

Measurement of No load loss and No load Current Hence, normally all transformer manufacturers are doing the no load loss by exciting LV side of the transformer. Rated excitation is required either exciting from HV side or LV side with the respective rated voltage of the winding. Following losses will be occurred at no load condition of the transformer.Iron loss or core lossDielectric loss in the insulation load loss caused by the no load current. The last two losses are very small and hence, generally ignored.

Measurement of No load loss and No load Current During the no load loss measurement, the applied voltage wave from some what differ from the sinusoidal form. This is due to the harmonics in the magnetizing current which caused additional voltage drop in the impedance of the applied voltage. Hence during this loss measurement, an average type volt meter ( Mean Value Volt meter) is referred in the VT secondary for the applied voltage as required the % of excitation and taken as U. At the same time, a volt meter responsive to the r.m.s value of the applied voltage shall be connected in parallel with the mean value voltmeter and taken as U. The reading of the mean value and r.m.s voltmeters are different. The test voltage wave shape is satisfactory if, the reading of U and U are equal or with in the 3% range.

Measurement of No load loss and No load Current A wave form correction factor shall be applied to the measured loss as per the below formula for getting actual loss at sinusoidal wave form of the applied voltage. Po = Pm (1+d) Where d= U-U ( usually negative value ) U Where Po = corrected loss, Pm= Measured loss, U= mean value Voltmeter reading and U= r.m.s Voltmeter reading.

Measurement of No load loss and No load Current The frequency of the applied voltage shall be at the rated frequency of 50Hz. If the frequency is differ from the rated value and is with in the IEC tolerance of 3% (48.5 to 51.5 Hz) a frequency correction factor also required to the applied voltage. Consider the test frequency is 49Hz and 51 HZ. The corresponding correction in the applied voltage will be 49 X V and 51 X V respectively, since the 50 50 core is designed for the rated frequency of 50Hz. If less frequency from rated, the excitation of the core will be more than rated excitation and vice versa.

Measurement of No load loss and No load Current For loss measurement watt meters are using to measure the loss and now a days Power Analyzer is using in place of watt meters. Two connection methods are using in three phase transformer for loss measurement.Two watt meter method Three wattmeter method Generally three wattmeter method are using and more accurate than using two watt meters. Two watt meter method Equipments required:Source Transformer with suitable voltage regulating systemAverage type volt meterR.m.s type Voltmeter

Measurement of No load loss and No load Current For loss measurement watt meters are using to measure the loss and now a days Power Analyzer is using in place of watt meters. Two connection methods are using in three phase transformer for loss measurement.Two watt meter method Three wattmeter method Generally three wattmeter method are using and is more accurate than using two watt meters. Two watt meter method Equipments required:Source Transformer with suitable voltage regulating system

Measurement of No load lossCTs 3 nos (suitable to required rating of voltage and current)VTS - 2 Nos line to line (suitable to required rating of voltage)Frequency meter.Average type volt meterR.m.s type Voltmeter (V1,V2&V3)3 Ammeters with suitable range (A1,A2 &A3 )2 Watt meters with low power factor (W1& W2) or Power analyzer.

Measurement of No load loss Three watt meter method Equipments required:Source Transformer with suitable voltage regulating systemCTs 3 nos (suitable to required rating of voltage and current)VTS - 3 Nos Phase to Earth (suitable to required rating of voltage)

Measurement of No load loss and No load CurrentFrequency meter.Average type volt meterR.m.s type Voltmeter3 Ammeters with suitable range (A1,A2 &A3 )3 Watt meters with low power factor (W1, W2 &W3) or Power analyzer.

Measurement of No load loss and No load Current

Measurement of No load loss and No load Current With the above test conditions, the voltage shall be raised up to the saturation level and slowly reduced to the minimum possible value and repeated two three times for demagnetizing the core. After demagnetization, the voltage shall be applied to the required levels and take all readings in all levels. Calculations Total measured loss =Po= W1+W2 or W1+W2+W3 Corrected loss =Pm= Po X wave form correction factor X frequency correction factor, if required. Magnetizing Current (average)=Io= A1+A2+A3/3 Applied Voltage (average) =Vo = V1+V2+V3/3 or Direct readings in the Power Analyzer.

Measurement of No load loss and No load Current

Wattmeter constant and Multiplication FactorWattmeter constant : Voltage selection in WM x Current selection in WM x power factor (WK) Full scale division Multiplication Factor: Wattmeter constant x CT Ratio x VT Ratio kW (MF) 1000I.e., measured power in kW = whatever readings in the wattmeter x Multiplication factor.Magnetizing Current in Amps=Io= Measured average Current X CT ratioApplied Voltage in kV =Vo= Measured average Voltage X VT ratio

Tolerance of losses The no load loss is always in the core when a transformer is energized at no load condition and hence no load loss of a transformer is a permanent losses of the transformer. The penalty of the excess no load loss than the specified limit is more than that of load loss since load loss is varying according to the load. As per IECLosses has no positive tolerance and is specified the maximum value. That means, the measured value should not be more than the specified value.Total losses has a positive tolerance of 10% is allowedComponent losses has a positive tolerance of 15% is allowed provided that the tolerance for the total losses is not exceeded.

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Penalty of losses Penalty for excess loss as per DEWA specification

For no load loss: 36,000.0 Dhs/kW within 5% in excess of guarantee values and 72,000.0 Dhs/kW above 5% in excess of guarantee values.

For load loss: 7,200.0 Dhs/kW within 5% in excess of guarantee values and 14,400.0 Dhs/kW above 5% in excess of guarantee values.