Transformer TestingWinding Resistance measurementIEC11.2 Measurement of winding resistance11.2.1 GeneralThe resistance of each winding, the terminals between which it is measured and the temperature of the windings shall be recorded. Direct current shall be used for the measurement.In all resistance measurements, care shall be taken that the effects of self-induction are minimized.11.2.3 Liquid-immersed type transformersAfter the transformer has been under liquid without excitation for at least 3 h, the average liquid temperature shall be determined and the temperature of the winding shall be deemed to be the same as the average liquid temperature. The average liquid temperature is taken as the mean of the top and bottom liquid temperatures.In measuring the cold resistance for the purpose of temperature-rise determination, special efforts shall be made to determine the average winding temperature accurately. Thus, the difference in temperature between the top and bottom liquid shall not exceed 5 K. To obtain this result more rapidly, the liquid may be circulated by a pump.

IEEE

5. Resistance measurements

Resistance measurements are of fundamental importance for the following purposes: a) Calculation of the I^2* R component of conductor losses b) Calculation of winding temperatures at the end of a temperature test c) As a quality control test of the manufacturing process d) As a base for assessing possible damage in the field

5.1 Determination of cold temperatureThe cold temperature of the winding shall be determined as accurately as possible when measuring the cold resistance. The precautions in 5.1.1, 5.1.2, and 5.1.3 shall be observed.

5.1.1 General Cold-resistance measurements shall be made on a transformer only when the liquid or winding temperature is stable. The temperature is considered stable if the top liquid temperature does not vary more than 2 C in a 1 h period.5.1.2 Transformer windings immersed in insulating liquid The temperature of the windings shall be assumed to be the same as the average temperature of the insulating liquid, provided a) The windings have been under insulating liquid with no excitation and with no current in the windings for a minimum of 3 h for a transformer without pumps and for 1 h for transformer with pumps running before the cold resistance is measured. b) The temperature of the insulating liquid has stabilized, and the difference between top and bottom temperature does not exceed 5 C. 5.1.3 Transformer windings out of insulating liquid The temperature of the windings shall be recorded as the average of several thermometers or thermocouples inserted between the coils, with care used to see that their measuring points are as nearly as possible in actual contact with the winding conductors. It should not be assumed that the windings are at the same temperature as the surrounding air. 5.2 Conversion of resistance measurements Cold winding resistance measurements are normally converted to a standard reference temperature (Ts) equal to the rated average winding temperature rise plus 20 C. In addition, it may be necessary to convert the resistance measurements to the temperature at which the impedance loss measurements were made. The conversions are accomplished by Equation (1):

Rs = Rm * (Ts +Tk)/(Tm+Tk) (1)

where Rs is the resistance at desired temperature Ts ()Rm is the measured resistance () Ts is the desired reference temperature (C) Tm is the temperature at which resistance was measured (C) Tk is 234.5 C (copper) or 225 C (aluminum) NOTEThe value of Tk may be as high as 230 C for alloyed aluminum.

5.3 Resistance measurement methods

5.3.1 Voltmeterammeter methodThe voltmeter-ammeter method is the most common method used for transformer winding resistance measurement. Resistance-measuring systems employing computer-controlled digital voltmeters, current measuring shunts, and/or digital ammeters of appropriate accuracy are commonly used for cold-resistance measurements and in connection with temperature-rise determinations.To use this method, the following steps should be taken:a) Measurement is made with direct current, and simultaneous readings of current and voltage are taken using the connections in Figure 1. The required resistance is calculated from the readings in accordance with Ohms Law. Electronic switching power supplies may be used as voltage sources; however, batteries or filtered rectifiers may also be used, especially in instances where less ripple is desired in the measurement. Automatic recording of the resistance data is recommended so that the time to saturation and the variability of the resistance readings after stabilization can be documented.

b) The voltmeter leads shall be independent of the current leads and shall be connected as closely as possible to the terminals of the winding to be measured. This step is to avoid including in the reading the resistances of current-carrying leads and their contacts and of extra lengths of leads.c) When making manual resistance measurements:

1) The measuring instruments shall have ranges that are close to full scale to minimize errors of observation.2) The voltmeter may be disconnected from the circuit before switching the current on or off to protect the voltmeter from injury by off-scale deflections. To protect test personnel from inductive kick, the current may be switched off by a suitably insulated switch with a protective circuit to discharge the energy.3) Due to inaccuracy of deflecting ammeters and voltmeters, current shunts and digital voltmeters or high-accuracy digital ammeters or other high accuracy instrumentation should be used that meets the requirement of IEEE Std C57.12.00.

d) Resistance is recommended to be measured at intervals of 5 s to 10 s, and the readings used shall be after the current and voltage have reached steady-state values.When measuring the cold resistance, preparatory to making a heat run, note the time required for the readings to become constant. That period of time should be allowed to elapse before taking the first reading when final winding hot-resistance measurements are being made. The residual flux in the core should be made the same for both the cold-resistance and hot-resistance measurements by saturating the core with direct current prior to the measurement.In general, the winding will exhibit a long time constant. To reduce the time required for the current to reach its steady-state value, a noninductive external resistor may be added in series with the dc source. It may then be necessary to increase the source voltage to compensate for the voltage drop in the series resistor. The time will also be reduced by passing a direct current through other windings in either the same polarity as the winding being tested for windings on the same phase or opposite polarity for other phases during these tests. For delta-connected windings, the time can also be reduced by opening the delta connection.

e) It is recommended that ten or more readings, but a minimum of four readings, should be used for each cold-resistance measurement, and the average of the resistances calculated from these measurements shall be considered to be the resistance of the circuit. The current used shall not exceed 15% of the rated current of the winding whose resistance is to be measured. Larger values may cause inaccuracy by heating the winding and thereby changing its temperature and resistance.

5.4 Resistance measurement connections and reportingThe individual phase- or terminal-to-terminal resistance readings shall be reported along with the sum total winding resistance.5.4.1 Wye windingsFor wye windings, the reported resistance measurement may be from terminal to terminal or from terminal to neutral. For the reported total winding resistance, the resistance of the lead from the neutral connection to the neutral bushing may be excluded. For terminal-to-terminal measurements, the total resistance reported is the sum of the three measurements divided by two.

5.4.2 Delta windingsFor delta windings, the reported resistance measurement may be from terminal to terminal with the delta closed or from terminal to terminal with the delta open to obtain the individual phase readings. The reported total winding resistance is the sum of the three phase readings if the delta is open. If the delta is closed, the reported total winding resistance is the sum of the three phase-to-phase readings times 1.5.

Electric Power Transformer Engineering

17.6.3 Winding Resistance Measurements

17.6.3.1 Purpose of Winding Resistance MeasurementsMeasurements of dc winding resistance are of fundamental importance because they form the basis for determining the following:Resistance measurements, taken at known temperatures, are used in the calculation of winding conductor I^2*R losses. The I^2*R losses at known temperatures are used to correct the measured load losses to a standard reference temperature. Correction of load losses is discussed in

Resistance measurements, taken at known temperatures, provide the basis to determine the temperature of the same winding at a later time by measuring the resistance again. From the change in resistance, the change in temperature can be deduced. This measurement is employed to determine average winding temperatures at the end of heat run tests. Taking resistance measurements after a heat run test is discussed in Section 17.6.4.Resistance measurements across the transformer terminals provide an assessment of the quality of internal connections made to the transformer windings. Loose or defective connections are indicated by unusually high or unstable resistance readings.

17.6.3.2 Nature of the Quantity Being MeasuredThe dc winding resistance differs from the value of resistance indicated for the resistor shown in Figure 17.16 or the resistors that appear in textbook illustrations of the PI or T equivalent circuits of transformers to represent the resistance of the windings. The resistors in the equivalent circuits include the effects of winding I^2*R loss, eddy loss in the windings, stray losses in structural parts, and circulating currents in parallel conductorsnamely, they represent the resistive components of the load loss. The resistors shown in the equivalent circuits can be thought of as representing an equivalent ac resistance of the windings. The dc resistance of the windings is a different quantity, one that is relevant for calculating I^2*R, for determining average winding temperature, and for evaluating electrical connections.

17.6.3.3 How Winding-Resistance Measurements Are MadeThe measurement of power transformer winding resistance is normally done using the voltmeter ammeter method or using a ratio metric method to display the voltage-current ratio directly. A circuit for the measurement of winding resistance is shown in Figure 17.17. A dc source is used to establish the flow of steady direct current in the transformer winding to be measured. After the R-L transient has subsided, simultaneous readings are taken of the voltage across the winding and the current through the winding. The resistance of the winding is determined from these readings based on Ohms law.

17.6.3.4 Discussion of the Measurement ProcessIf a dc voltage is applied as a step to a series R-L circuit, the current will rise exponentially with a time constant of L/R. This is familiar for the case where both resistance and inductance remain constant during the transient period. For a transformer winding, however, it is possible for the true resistance, the apparent resistance, and the inductance of the winding to change with time. The true resistance may change if the direct current is of high-enough magnitude and is applied long enough to heat the winding substantially, thereby changing its resistance during the measurement. The inductance changes with time because of the nonlinear B-H curve of the core steel and varies in accordance with the slope of the core-steel saturation curve. In addition, there is an apparent resistance, Ra, during the transient period.

Ra =V/I=R+(L/I)*I

Note that the apparent resistance, Ra, is higher than the true resistance, R, during the transient period and that the apparent resistance derived from the voltmeter and ammeter readings equals the true resistance only after the transient has subsided.Resistance measurement error due to heating of the winding conductor is usually not a problem in testing transformers, but the possibility of this effect should be taken into consideration, especially for some low-current distribution transformer windings where the dc current can be significant compared with the rated current. It is more likely that errors will occur because of meter readings taken before core saturation is achieved. The process involved in core saturation is described below.Compared with the exponential current-versus-time relationship for the R-L circuit with constant R and constant L, the current in a transformer winding, when a dc voltage is first applied, rises slowly. The slow rate of rise comes about because of the high initial impedance of the winding. The initially high impedance results from the large effective inductance of the winding with its iron core. As the current slowly increases, the flux density in the core slowly rises until the core begins to saturate. At this point, the winding no longer behaves like an iron-core coil and instead behaves like an air-core coil, with relatively low inductance. The rate of rise of the current increases for a period as the core saturates; then the current levels off at a steady-state value. Typical shapes for the voltage, current, and apparent resistance are shown in Figure 17.18. The magnitude of the dc voltage affects the rate at which flux builds up in the core, since V = N(d/dt). The higher the magnitude of the dc voltage, the shorter is the time to saturation because of a higher value for d/dt. At the same time, though, the coil must be able to provide the required magneto motive force in ampere turns, N I, needed to force the core into saturation, which leads to a minimum value for the dc current. Of course, there is an upper limit to the value for dc current, namely the point at which conductor heating would disturb the resistance measurement.Note the time scale of the graph in Figure 17.18. It is very important that the steady-state dc current be attained before meter readings are taken. If this is not done, errors in excess of 20% are easily realized.

17.6.3.5 Winding Resistance and Average Winding TemperatureTwo of the three purposes listed above for measuring the dc resistance of a transformer winding inherently involve a concomitant measurement of temperature. When measuring resistance for the purpose of calculating I^2*R at a given temperature, the I^2*R value obtained will be used to determine the load-loss value at a different temperature. When the winding resistance is measured before and during a heat run, the determination of average winding temperature at the end of a heat run test requires knowledge of winding resistance at two temperatures.The winding dc resistance at two temperatures, T1 and T2, will have values of R1 and R2, respectively, at the two temperatures. The functional relationship between winding resistance and average temperature is shown in Equation 17.4:

R1/R2 = T1+Tk / T2+Tk (17.4)

WhereR1 is the value of winding resistance, corresponding to average winding temperature of T1R2 is the value of winding resistance, corresponding to average winding temperature of T2Tk is 234.5C for copper, 225C for aluminum

Correction of load loss for temperature is covered in Section 17.6.2. Determination of average winding temperature in a heat run test is covered in Section 17.6.4.

J&P Transformer Handbook

Resistance of windings

The DC resistances of both HV and LV windings can be measured simply by the voltmeter/ammeter method and this information provides the data necessary to permit the separation of I^2*R and eddy-current losses in the windings. This is necessary in order that transformer performances may be calculated at any specified temperature.The voltmeter/ammeter method is not entirely satisfactory and a more accurate method such as measurement with the Wheatstone or Kelvin double bridge