ELEC-E8407 Electromechanics. Laboratory exercise 1: Transformer

Transcription

1 ELEC-E8407 Electromechanics Laboratory exercise 1: Transformer

2 ELEC-E8407 Electromechanics 2 In the exercise, the properties of a core-type transformer are tested by carrying out the following measurements (Standard IEC ) on the transformer: winding resistances, voltage ratio and phase difference between primary and secondary voltages, short-circuit impedance and shortcircuit losses, and no-load losses and no-load current. The exercise is a familiarisation with making three-phase measurements of power, voltage and current, and, on the basis of measured or given values, with making a single-phase equivalent circuit for a three-phase transformer. You will also learn the effect of temperature on resistances, and the test procedure in accordance with the Standard. Useful reference information includes the study material supplied for the course, and basics of circuit theory (e.g. the material supplied for Circuit Analysis). The single-phase equivalent circuit is formed with the assumption that the primary and the secondary are star-connected (regardless of their actual connections). The voltages on the singlephase equivalent circuit are always phase to neutral. The following symbols are used for the quantities on the single-phase equivalent circuit: U V1 Primary phase to neutral voltage I 1 Primary current R1 Primary resistance X 1 Primary stray reactance U V2 Secondary phase to neutral voltage I 2 Secondary current R 2 Secondary resistance X 2 Secondary stray reactance R Fe Iron loss resistance X m Magnetising (principal) reactance I 1 R 1 X s 1 X s' 2 R 2 ' ' I 2 U V1 RFe X m ' U V2 Fig. 1. Equivalent circuit of a transformer. The primed quantities indicate that they are reduced to the non-primed voltage side (in this case secondary quantities are reduced to primary side).

3 ELEC-E8407 Electromechanics 3 PRELIMINARY REPORT 1. How can the locations and polarities of the windings be tested on the various legs of a core transformer when it has two open (-circuited) three-phase windings (III and iii)? What measurements are required, assuming that the transformer is so enclosed that only the 12 winding terminals are accessible? The available equipment consists of voltmeters, an ohmmeter and a 1-phase voltage supply. 2. What measurements are required for calculating the equivalent circuit of Fig. 1? How can the measured values be used to obtain the component values in the equivalent circuit? Assume that the primary is delta-connected and the secondary is star-connected. 3. The ratings of the three-phase transformer are S N = 1000 kva, U N1/U N2 = 6000V/400V, f N = 50 Hz. The connection is Dy11. The AC resistance of the primary winding is R 1 = 0.51 at a winding temperature of 20 º C (Note the delta connection!). A no-load test was made on the high-voltage side at rated voltage (the low-voltage side is open and the high voltage side is supplied) and a short-circuit test on the low-voltage side (the low voltage side is short-circuited and the high voltage side is supplied) at rated current at a winding temperature of 75 º C. The measured values were: no-load test: I 0 = 0.67 A P 0 = 1550 W short-circuit test: U k = 360 V P k = 8200 W Determine the equivalent circuit parameters as seen from the high-voltage side. Assume that the leakage flux is the same both on the HV and the LV side. (Note the test temperature!). 4. How is the two-wattmeter method (Aaron connection) used to measure the power and power factor of a three-phase system? Draw the connection and derive the equations required. Do not use separate volt- or ammeters. (See the circuit theory course material!).

4 ELEC-E8407 Electromechanics 4 MEASUREMENTS 1. Measure DC-resistances of the primary and secondary coils at room temperature. Use the 4-wires method. 2. Determine the transformer s voltage ratio in the Dy11 connection. Measure the phase displacement between the primary and secondary voltages. 3. Carry out measurements on the transformer as required for calculating the equivalent circuit. 4. Measure the transformer s secondary voltage, efficiency and power factor at resistive, capacitive and inductive loads. 5. Measure DC-resistances of the primary and secondary coils after the loading.

5 ELEC-E8407 Electromechanics 5 FINAL REPORT 1. Use the measured values to determine the phase displacement between the primary and secondary voltages. 2. Calculate the single-phase equivalent circuit of the transformer in the Dy11 connection. Assume similar leakage flux on both the primary and the secondary. In the calculation of the resistances, make use of the ratio of the DC resistances of the windings as reduced to the primary side. Also take into account the temperature dependence of the resistances. The effect of temperature on the iron losses is negligible. 3. From the equivalent circuit, calculate the transformer s efficiency at a resistive load at different values of load current, and compare the result to the measured values. Take into account the no-load and load losses. 4. Use the measured values to make a graphic presentation of the voltage difference ' D U = U1- U2 = f( I2) at resistive, inductive and capacitive loads. From the equivalent circuit, calculate the transformer s voltage difference in rated service at resistive, inductive and capacitive loads, and compare the result to the measured result. + Your opinions on the exercise and on the preliminary and final reports

6 ELEC-E8407 Electromechanics 6 APPENDIX 1: TRANSFORMER The transformer used for the exercise is a three-leg transformer with a symmetrical iron core. The figure shows the windings on a single leg. The current-carrying capacities of the windings are: 380 V: 2.9 A 110 V: 5.0 A The connections are made as series connections of the 380 V windings on the primary (the 220 V tappings are not used) and the 110 V windings on the secondary. In this exercise the rated primary voltage is 380 V. The transformer also has 10 V (0.1 A) windings, which are not used in this work.