3-Phase Transformer magnetization current · PDF file3-Phase Transformer magnetization current...

Materiały dydaktyczne dystrybuowane bezpłatnie.

Projekt współfinansowany ze środków Unii Europejskiej w ramach Europejskiego Funduszu Społecznego

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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 E1 is balanced by emf Ei forced by flux exciting current If (which

waveform is different from sinusoidal due to iron core non linear characteristic) and drop of

voltage over winding impedance Z1 = sqrt(X12 + R1

2) – Fig.1.

Fig.1. Equivalent open circuit of the no-load transformer.

Fig.2. Primary voltage U1, emf E1, exciting current i0 i and main flux fi waveforms.

Core exciting current I0 flows in the primary transformer winding independently of the

load. It has two components: If, the magnetizing component which flows 90° lagging behind

induced voltage E1; and Iow, the core-loss current which is in phase with E1. 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



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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.



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The flux waveform has to contain higher third order harmonics and in consequence phase

voltage Uph1 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 U1. 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 Uph1 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.



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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 Uph1 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:



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Voltage measurement

block terminal

Data acquisition device

USB-6251

Current measurement

block terminal

Data acquisition device

USB-6251

U1 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 Uph1N 220V

Voltage Uph2N 220V

Apparent Power SN 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 voltage-

voltage and current-voltage converters that offer galvanic signal separation.

Fig.10. VI interface window.



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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 U1 and secondary side phase

voltage Uph2. Enables comparison of maximum amplitude of both voltages.

Screen 2 – presents voltage U1 spectrum (limited to first 9 odd harmonics). Allows to

control quality of supply voltage.

Screen 3 – presents instantaneous value of three phase voltages Uph1A, Uph1B and

Uph1C. Displays voltage distortions.

Screen 4 – presents phase voltage Uph1A spectrum (limited to first 9 odd harmonics).

Allows to evaluate voltage distortions.

Screen 5 – presents instantaneous value of three phase currents Iph1A, Iph1B and Iph1C.

Displays current distortions.

Screen 6 – presents phase current Iph1A spectrum (limited to first 9 odd harmonics).

Allows to evaluate current distortions.

Screen 7 – presents instantaneous value of zero line current I0, and compensating current

Idelta (if exists).

Screen 8 – presents zero line current I0 spectrum (limited to first 9 odd harmonics).

Screen 9 – presents compensating current Idelta spectrum (limited to first 9 odd

harmonics).

Fig.11. Realization of the VI interface.



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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).



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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 Uph1, phase current Iph1 and if necessary zero line voltage I0 or

compensation current Idelta from VI interface window should be placed in appropriate Table.

Table1

Wye connection without zero line

Uph1=200V Harmonic order

1 3 5 7 9

U1

Uph1

Iph1

I0

Idelta

Table2



1 3 5 7 9

U1

Uph1

Iph1

I0

Idelta



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Table3



1 3 5 7 9

U1

Uph1

Iph1

I0

Idelta

Table4

Wye connection with zero line


1 3 5 7 9

U1

Uph1

Iph1

I0

Idelta

Table5



1 3 5 7 9

U1

Uph1

Iph1

I0

Idelta

Table6



1 3 5 7 9

U1

Uph1

Iph1

I0

Idelta



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Table7

Wye – Delta connection


1 3 5 7 9

U1

Uph1

Iph1

I0

Idelta

Table8



1 3 5 7 9

U1

Uph1

Iph1

I0

Idelta

Table9



1 3 5 7 9

U1

Uph1

Iph1

I0

Idelta

On the base of measurement data on the separate drawings phase voltage Uph1 or phase

current Iph1 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)?



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Literature:

Adam Biernat, Power Transformers – part of the lecture: Electric Machines in the Power

Engineering and Automatics.

Instruction elaborated by Adam Biernat

3-Phase Transformer magnetization current · PDF file3-Phase Transformer magnetization current...

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