WARSAW UNIVERSITY OF TECHNOLOGY INSTITUTE OF ELECTRICAL MACHINES ELECTRICAL MACHINES IN THE POWER ENGINEERING AND AUTOMATIC 3-Phase Transformer magnetization current test Introduction Primary voltage E 1 is balanced by emf E i forced by flux exciting current I f (which waveform is different from sinusoidal due to iron core non linear characteristic) and drop of voltage over winding impedance Z 1 = sqrt(x 1 2 + R 1 2 ) Fig.1. Fig.1. Equivalent open circuit of the no-load transformer. Fig.2. Primary voltage U 1, emf E 1, exciting current i 0 i and main flux fi waveforms. Core exciting current I 0 flows in the primary transformer winding independently of the load. It has two components: I f, the magnetizing component which flows 90 lagging behind induced voltage E 1 ; and I ow, the core-loss current which is in phase with E 1. Ordinarily this current is small and produces negligible voltage drop in the winding. Core-loss current has two components: eddy current and hysteresis. Eddy-current loss is caused by current circulating in the core laminations. Hysteresis loss is the power required to magnetize the core 1
first in one direction and then in the other on alternating half-cycles. Hysteresis loss and magnetization are intimately connected, as can be seen from Fig.3. Fig.3. Transformer voltage, flux, and exciting current. Although the applied voltage to a transformer is sinusoidal, the magnetization current related to the flux through the magnetization curve is non-sinusoidal, as shown in Fig.3. The non-sinusoidal current is symmetrical around its peak. Such a waveform is mainly composed of odd harmonics. In particular, the 3rd harmonic, as well as the 5th and the 7th harmonics have a significant contribution. 3-Phase Transformer connections influence on phase current and voltage higher harmonics Wye connection without zero line (Fig.4) cannot provide higher third order harmonics (3-rd and 9-th order triplen harmonics) of current to flow. Fig.4. Wye connection without zero line. 2
The flux waveform has to contain higher third order harmonics and in consequence phase voltage also contains higher third order harmonics as it is explained in Fig.5. Fig.5. Phase voltage in case of absence of third order harmonics in the magnetizing current. Notice that Wye connection without zero line can provide other higher odd current harmonics (for example 5-th and 7-th order harmonics). However higher odd harmonics do not exist in the line voltages. Distorted phase voltage has higher peak value that can be dangerous for load. Wye connection with zero line. Additional zero line allows third order current harmonics to flow (Fig.6). The neutral wire carries three times the third order harmonic current of one transformer as these currents are co-phasal. Fig.6. Wye connection with zero line. In consequence the flux waveforms don t contain third order harmonics and in consequence phase voltage also doesn t contain higher order harmonics (Fig.7). Higher current harmonics exist in the phase current. Fig.7. Phase voltage in case of existence of third order harmonics in the magnetizing current. 3
Wye Delta connection (Fig.8). In this case the higher current harmonics (especillly higher third order harmonics) can flow in delta circuit, the flux waveforms don t contain higher odd harmonics. The third order harmonic currents inside the closed delta winding correct the flux wave to be nearly sinusoidal and in consequence phase voltage also doesn t contain higher order harmonics (Fig.7). However higher current harmonics (other than higher third order harmonics) can exist in the phase current. Fig.8. Wye-Delta connection. Test circuit connections Tested Transformer is 5-column symmetrical 3-pase one with Wye primary side connection. Fig.9. Connection diagram. Voltage and current measurement block terminals should be connected to the data acquisition device USB-6251 in the following order: 4
Voltage measurement block terminal Data acquisition device USB-6251 Current measurement block terminal Data acquisition device USB-6251 AI 0 I1 AI 6 U2 AI 1 I2 AI 7 U3 AI 2 I3 AI 8 U4 AI 3 I4 AI 9 U5 AI 4 I5 AI 10 U6 AI 5 I6 AI 11 Ratings of the transformer to be tested: Ratings Voltage N 220V Voltage Uph2 N 220V Apparent Power S N 3x3 kva Connection diagram is presented in Fig.9.Supply voltage unit consists of Induction Voltage Regulator and additional Wye-Wye connected transformer securing possibility to realize (using W1 switch) zero line connection. Tested transformer secodary side can be Delta connected using W2 switch. Transformer voltages and currents are measured using voltagevoltage and current-voltage converters that offer galvanic signal separation. Fig.10. VI interface window. 5
Data collection and presentation is executed under LabViev Virtual Instrument (VI) equipped with A/D converter. In Fig.10 the VI interface window is presented. Screen 1 presents instantaneous value of line voltage and secondary side phase voltage Uph2. Enables comparison of maximum amplitude of both voltages. Screen 2 presents voltage spectrum (limited to first 9 odd harmonics). Allows to control quality of supply voltage. Screen 3 presents instantaneous value of three phase voltages A, B and C. Displays voltage distortions. Screen 4 presents phase voltage A spectrum (limited to first 9 odd harmonics). Allows to evaluate voltage distortions. Screen 5 presents instantaneous value of three phase currents A, B and C. Displays current distortions. Screen 6 presents phase current A spectrum (limited to first 9 odd harmonics). Allows to evaluate current distortions. Screen 7 presents instantaneous value of zero line current I 0, and compensating current (if exists). Screen 8 presents zero line current spectrum (limited to first 9 odd harmonics). Screen 9 presents compensating current spectrum (limited to first 9 odd harmonics). Fig.11. Realization of the VI interface. 6
In the area above screens RMS values of appropriate qantities are presented. In Fig.11 realization of the VI interface window is presented. Starting of VI instrument and test circuit arrangement. Transformer test circuit is already connected. However some additional connections should be done in appropriate (described below) sequence: 1- Switch on the computer (wait for operation system up-loading). 2- Switch on the data acquisition device USB-6251. On the computer screen the tool program selection window appears (Fig.12). Fig.12. Tool program selection window. 3- Activate the LabViev 8.21 icon. 4- Chose the file magnesowanie 5k EN that appear in the starting window Open of the LabView program. The VI window as presented in Fig.10 should appear. 5- Switch on the voltage-voltage and current-voltage converters box. 6- Arrange proper connection of tested transformer supply voltage using the Induction Voltage Regulator. Notice that initial voltage setting should be should be set to minimum. 7- Switch on the Induction Voltage Regulator. 8- Set the appropriate supply voltage. 9- Set appropriate transformer connection (Wye without zero line, Wye with zero line or Wye-Delta). 10- Start measurements activating VI instrument START button positioned in the tools rib (Fig.13). 7
Fig.13. VI instrument START button. After finishing measurements firstly: 1- Switch off the Induction Voltage Regulator after setting output voltage to minimum. 2- Disconnect the Induction Voltage Regulator. 3- Switch off the voltage-voltage and current-voltage converters box. 4- Switch off the data acquisition device USB-6251. 5- Close the computer. Measurement data collection and elaboration Measurement should be performed for 3 values of supply voltage for each transformer connection (Wye without zero line, Wye with zero line or Wye-Delta). Results of harmonic analysis of phase voltage, phase current and if necessary zero line voltage or compensation current from VI interface window should be placed in appropriate Table. Table1 =200V Table2 =300V Wye connection without zero line Wye connection without zero line 8
Table3 =350V Wye connection without zero line Table4 =200V Wye connection with zero line Table5 =300V Wye connection with zero line Table6 =350V Wye connection with zero line 9
Table7 =200V Wye Delta connection Table8 =300V Wye Delta connection Table9 =350V Wye Delta connection On the base of measurement data on the separate drawings phase voltage or phase current harmonic content should be presented in two ways. First revealing the development of higher order harmonics with supply voltage change (parameter is the winding connection arrangement). Second revealing the content of higher harmonics that depends on winding connection arrangement (parameter is the supply voltage). Drawings should be commented: 1- What winding connection arrangement guarantees best quality of distributed electrical energy? 2- What is the advantage (if there is any) of Wye connection with zero line against Wye-Delta connection (or opposite)? 10
Literature: Adam Biernat, Power Transformers part of the lecture: Electric Machines in the Power Engineering and Automatics. Instruction elaborated by Adam Biernat 11