Virtual three-phase laboratory exercise during pandemic situation

FELIX A. HIMMELSTOSS1 & KARL EDELMOSER2

1Power Electronics Department

University of Applied Science Technikum Wien

Hoechstaedtplatz 6, 1200 Wien

2Institute of Energy Systems and Electrical Drives

University of Technology Vienna

Gusshausstraße 24-26, 1040 Wien

AUSTRIA

Abstract: - The Corona pandemic has changed our way to teach, even our laboratory experiments. The

exercises for the three-phase systems had to be transferred to the web. The virtual exercises are described and

the results are shown. The pros and contras of this way to teach are discussed.

Key-Words: - virtual laboratory, three-phase systems, phasor diagram, neutral line interruption, voltage

displacement,

1 Introduction

For many years the first author has taught a course

for Industrial Electronics in the second year of the

Bachelor course for Electronics. Only one week is

assigned for the chapter three-phase systems

(references are e.g. [1, 2]), one lecture for the basics

and one lecture for some calculation examples. A

laboratory exercise rounds off this theme. In this

laboratory exercise the students have to wire a three-

phase load connected in star with a 400 V mains (for

security reasons, a one to one isolation transformer

is used). In the circuits the voltages across the load

and the currents through the wires are measured.

First we use a symmetrical resistive load, then some

asymmetry is included and several errors are

induced (disconnection of wires). The ammeters and

the voltmeters are read for the experiments and

phasor diagrams are constructed. This is very

helpful to get a better understanding of this topic.

During the pandemic situation in the study year

2020/21 all lectures were transferred to Zoom

lectures. Therefore, an online exercise which

simulates the practical laboratory exercise was

made. The useful simulation program LT-Spice [3]

was used, which can be loaded down with no

charge.

2 Basic experiments

2.1 Experiment 0: Power flow

The starting point is the question: why do we use a

three-phase system? It is easy to understand that

with more wires more energy can be transferred and

the other point (which was already discussed in the

lecture) is the fact that the power transfer is nearly

constant compared to a pulsating one in a two wire

system. This was demonstrated at first (Fig. 1). The

voltage sources with the value zero represent the

ammeters A1, A2, A3, A4. The voltages across the

load resistors are equal to the phase voltages (this is

also in reality, because the voltage drop across the

ammeters can be neglected; in reality we use seven

measurement devices three voltmeters and four

ammeters). Fig. 2 shows the constant power flow

and the three pulsating ones for the three one-phase

systems. Also depicted are the currents through all

four wires and the phase voltages.

Fig. 1. Symmetrical load

Contrary to a single phase system which has a

pulsating power flow, the power flow in a

symmetric three-phase system is constant. So no

mechanical stress at the generator is caused. In a

one-phase system the constant mechanical power

from the turbine and the pulsating electrical power

at the electrical output lead to a large mechanical

stress with a frequency of two times the frequency

of the mains .

Received: June 15, 2021. Revised: October 23, 2021. Accepted: December 9, 2021. Published: January 3, 2022.

WSEAS TRANSACTIONS on ADVANCES in ENGINEERING EDUCATION

DOI: 10.37394/232010.2022.19.1

Felix A. Himmelstoss, Karl Edelmoser

E-ISSN: 2224-3410

Volume 19, 2022

Fig. 2. Up to down: power flow of the complete

three-phase system, power flow of the three phases;

currents through the lines; phase voltages

2.2 Experiment 1: Symmetrical load

The model (Fig. 1) is equal to the setup in the

laboratory. The symmetrical load consists of

resistors and the currents are measured in each line.

The voltage sources with the value zero represent

the current measurement devices (ammeters). To get

the rms-value as in reality (keep in mind that in the

laboratory we only get the rms-values with the

measurement devices), one must display one or an

integer manifold of periods and use the key

<control>, and the left key of the mouse when

pointing with the cursor at the symbol of the desired

signal in the probe window. The signals (phase

currents and phase voltages) are shown in Fig. 2 in

the two lower pictures. The rms-values are 1,473 A

in the phases and zero in the neutral line. The

current in the neutral is the sum of the phase

currents. The phasor diagram (Fig. 3) shows that the

current in the neutral is zero.

Fig. 3. Phasor diagram for experiment 1

2.3 Experiment 2: Unsymmetrical load

The load resistor in phase one is reduced to the half

of the original value (Fig. 4). Now the current in

phase one is doubled. The load is unsymmetrical

and the current is equalized by a current through the

neutral line (Fig. 5).

Fig. 4. Experiment 2: unsymmetrical load

Fig. 5. Experiment 2 up to down: currents through

the wires; phase voltages

Reading from the “ammeters” results in I1=2.945 A,

I2=I3=I4=1.473 A. This leads to the phasor diagram

Fig. 6.

Fig. 6. Phasor diagram for experiment 2

I =

NI +

1I +

WSEAS TRANSACTIONS on ADVANCES in ENGINEERING EDUCATION

DOI: 10.37394/232010.2022.19.1

Felix A. Himmelstoss, Karl Edelmoser

E-ISSN: 2224-3410

Volume 19, 2022

2.4 Experiment 3: interruption of phase 1

Now we interrupt phase one (Fig.7). This could be

by the blow of a fuse. Now we have only two

working phases and again a current has to flow

through the neutral line.

Fig. 7. Experiment 3: interruption of phase 1

Fig. 8. Experiment 3 up to down: currents through

the wires; phase voltages

Reading our “ammeters” leads to zero for phase one

and again 1.473 A for all other currents. To equalize

the system, again a current has to flow through the

neutral line. Constructing the phase diagram (Fig. 9)

helps again to understand this.

Fig. 9. Phasor diagram for experiment 3

2.5 Experiment 4: Additional interruption of

the neutral line

Now a second error occurs, the neutral line is also

interrupted (Fig. 10).

Fig. 10. Experiment 4: interruption of phase 1 and

of the neutral

Now two things occur: the star point of the source

which is normally grounded and the star point of the

load are different and a dangerous voltage between

the star point of the load and earth can occur, and

secondly the voltages across the load differ from the

phase voltages. Fig. 11 shows the shift of the two

star points, the voltages across the loads, the phase

currents, and the phase voltages. Reading our

“ammeters” shows the same current 1.276 A in line

2 and line 3. The voltages across R2 and R3 are the

same.

Fig. 11. Experiment 4 up to down: shift of the star

point, voltages across the loads, phase currents,

phase voltages

The phase current is the same in both phases. Across

the series connection of both load resistors lies now

the line-to-line voltage. The voltage across the two

load resistors is now only 200 V (exact 199 V in our

case). The most interesting fact is the voltage

difference between the earthed voltage source and

the star point of the load. There is a voltage shift of

115 V, already a dangerous value! The interruption

of the neutral line can be very dangerous when the

load is unsymmetrical!

The construction of the phasor diagram (Fig. 12)

helps for better understanding. The construction

follows Kirchhoff’s voltage law. The phase voltage

is the sum of the voltages across the load and the

difference between the two star points. One gets

WSEAS TRANSACTIONS on ADVANCES in ENGINEERING EDUCATION

DOI: 10.37394/232010.2022.19.1

Felix A. Himmelstoss, Karl Edelmoser

E-ISSN: 2224-3410

Volume 19, 2022

KTLASTSTERNPStrSpg VVV  22 (1)

KTLASTSTERNPStrSpg VVV  33 . (2)

With the measured values and with the help of a pair

of compasses one can construct the phasor diagram

(Fig. 12).

Fig. 12. Phasor diagram for experiment 4

2.6 Experiment 5: repair of the system

The system is again the system as shown in Fig. 1.

When the load is equal for all three phases, an

interruption of the neutral point does not matter.

(High voltage transmission lines have only a thin

earth line which conducts only the current caused by

some unavoidable asymmetry and are necessary in

case of a system fault; district lines in the tens of kV

range have only three lines.)

There is no voltage difference between earth (the

neutral point or star point of the source) and the star

point of the load.

2.7 Experiment 6: Capacitive load in

phase 1

The system is now repaired, but we change the load

of phase 1 by a capacitor which draws about the

same current as the original resistor (Fig. 13). From

the reactance of a capacitor



. (3)

one gets

4.20

1562

11 







µF. (4)

In our laboratory we have 20 µF capacitors which

can be used in this case. Now the load is asymmetric

and a current is flowing through the neutral line.

The results are shown in Fig. 14 and the phasor

diagram in Fig. 15. The “ammeters” show

I1=1.443 A, I2=1.473 A, I3=1.473 A, I4= 2.063 A

(measured during the fourth and fifth period).

Fig. 13. Experiment 6: capacitive load in phase 1

Fig. 14. Experiment 6 up to down: voltages across

the load, current through the lines, phase voltages

Fig. 15. Phasor diagram for experiment 6

2.8 Experiment 7: capacitive load in

phase 1 and interruption of the neutral line

What happens when the neutral line is now

interrupted (Fig. 16)?

Fig. 16. Experiment 7: interruption of the neutral

line

US3 US2

locus of the arrow of the

displacement voltage

US3 US2

I3I2

WSEAS TRANSACTIONS on ADVANCES in ENGINEERING EDUCATION

DOI: 10.37394/232010.2022.19.1

Felix A. Himmelstoss, Karl Edelmoser

E-ISSN: 2224-3410

Volume 19, 2022

For the measurement one has to check that the

system is in the steady state. Fig. 17 shows the shift

of the load star point, the voltages across the load,

the current through the phases, and the phase

voltages.

Fig. 17. Experiment 7 up to down: shift of the load

star point, voltages across the load, current through

the phases, phase voltages

Fig. 18. Phasor diagram for experiment 7

For the steady state one gets the values

I1=1.9443 A, I2=2.1905 A, I3=0.5873 A for the

currents, for the load voltages UC1=309,5 V,

UR2=341.8 V, UR3=91.68 V, and for the

displacement of the star point of the load 144.44 V

(the measurement was done in the fifth period). The

voltage across load R2 is much higher than the

normal phase voltage of 230 V. Therefore, this load

can be destroyed (e.g. a TV set or other electronic

equipment)! The load across R1 is much lower than

the normal voltage and this can lead to a

malfunction. The voltage across the capacitor is also

significantly higher than the nominal voltage.

Asymmetric load and interruption of the neutral can

be very dangerous. Again the drawing of the phasor

diagram (Fig. 18) enhances the understanding. The

current in phase one leads the voltage across the

load capacitor by ninety degrees.

2.8 Experiment 7: star point displace-

ment Sternpunktverschleppung

The last of our basic experiments shows that it can

be very dangerous when the neutral is interrupted.

For example, two flats are supplied from the same

source. Therefore, they use the same neutral line. If

this line is interrupted and the load in one flat is

asymmetric, then not only the voltage at the loads

can destroy the loads in this flat, but also in the

neighboring one!

Fig. 19. Experiment 8: Voltage displacement

3 Extensions of these laboratory

experiments

Depending on the time one can expand these

experiments, e.g. the load can be connected in

triangle, the three-phase system can be rectified, or

the load can be an induction machine.

3.1 Experiment 9: Triangle load

Fig. 20 shows the circuit diagram for a triangle load.

The current in all load branches is also measured.

One can extend this experiment by studying the

influence of interruptions of different wires after

first considering it. First it must be done with paper,

pencil, and India rubber!

Fig. 20. Experiment 9: triangle load

WSEAS TRANSACTIONS on ADVANCES in ENGINEERING EDUCATION

DOI: 10.37394/232010.2022.19.1

Felix A. Himmelstoss, Karl Edelmoser

E-ISSN: 2224-3410

Volume 19, 2022

3.2 Experiment 10: three-phase

rectification

Another expansion could be a rectification [4] with

a B6 bridge (Fig. 21). This should, however, also be

studied with paper and pencil before. The results are

shown in Fig. 22, where the current through the

lines and the phase voltages are depicted together.

In the last picture the rectified output voltage and

the input voltages are shown. The currents through

the diodes and the voltages across them can be

studied in another experiment. In a next step the

influence of inductors in the lines and in the load

can be treated.

Fig. 21. Experiment 10: B6 rectifier

Fig. 22. Experiment 10 up to down: voltage and

current phase 3, voltage and current phase 2, voltage

and current phase 1, rectified voltage and phase

voltages

3.3 Experiment 11: induction machine

In this experiment an induction machine modelled

by the T-model can be treated. Fig. 23 shows a

machine in the nominal point.

Fig. 23. Experiment 11: induction machine at the

nominal point

4 Conclusion

The main idea of these exercises in the laboratory is

that the students of the Electronics curricular should

at least work one time in their course at the 400 V

three-phase mains. To reduce the risk of hazards an

isolation transformer separates the circuits from the

distribution network. We also use cables with

connectors which are especially isolated. But

carelessness can always occur, e.g. if the voltages

should be measured and the measurement device is

used as an ammeter or a cable is wrongly connected.

In the virtual lab exercise these errors have no

dangerous consequences and there is no stress for

the responsible lecturer (I was always glad when

these exercises were finished). The second task of

these experiments is to practice measurements and

to construct the phasor diagrams. Measurements in

the virtual exercises are easier, because there are no

fluctuations of the voltages. The instruments have to

be read at the same time (the students now make a

picture with their mobile phones to get the results at

the same moment). The problems with the

construction of the phasor diagrams are the same

both after the real measurement or after the

simulation (when all ammeters and voltmeters are

working correct). The results of the simulation and

their interpretation are the same and help to get a

better understanding of the three-phase system. The

exercises last shorter than in reality and in the same

available time more experiments can be carried out.

Interested students can repeat these experiments at

home and change the tasks and check other

problems. Nevertheless, the practical work with

hardware should be done in our courses.

References:

[1] Olle I. Elgerd, and Patrick D. van der Puije,

Electronic Power Engineering, Chapman &

Hall, 1998.

[2] Mohamed E. El-Hawary, Electric Power

Systems, IEEE Press, 1995.

[3] Analog Devices, LTspice (can be downloaded

for free)

[4] Ned Mohan, Torr M. Undeland, William P.

Robbins, Power Electronics, John Wiley &

Sons, 2003.

Creative Commons Attribution License 4.0

(Attribution 4.0 International, CC BY 4.0)

This article is published under the terms of the Creative

Commons Attribution License 4.0

https://creativecommons.org/licenses/by/4.0/deed.en_US

WSEAS TRANSACTIONS on ADVANCES in ENGINEERING EDUCATION

DOI: 10.37394/232010.2022.19.1

Felix A. Himmelstoss, Karl Edelmoser

E-ISSN: 2224-3410

Volume 19, 2022