Electronics and Electrical Engineering

Power Systems / Electrical Energy Systems Laboratory

Unloaded Three Phase Transformer

1. Objective

The objective of this experiment is to study the effect of various transformer

connections on the losses and harmonic content of the primary current and voltage waveforms in a three-phase transformer with no connected load. Operation of the transformer under overvoltage conditions will also be observed.

2. Introduction

A large power system has a lot of relatively small transformers located close to the

customers. These small transformers spend most of their working life (40–60 years)

with minimal load and only a very small part of their working life operating close to

full load. Consequently the light load losses of these smaller transformers have a huge economic significance to the power system operator. The power system operator must make a trade–off between the initial cost of a transformer and the lifetime financial cost of the losses in the transformer. Clearly the cheapest transformer to buy (using the

least amount of material) is unlikely to have the lowest total lifetime cost of ownership.

In this experiment we will study some of the factors that influence the no-load losses of a typical distribution transformer.

3-phase Power Supply

The 3 phase power supply is an electronic source which can provide up to 42V line (24V phase) RMS.

The “Output Waveform” has three different settings:-

(a) “sine”:                    an ideal, balanced 3 phase sinusoidal supply, with no harmonics.

(b) “unbal sine”:         an unbalanced 3 phase supply with one phase reduced in amplitude (not used during this experiment).

(c) “sine +3rd”:           a balanced 3 phase supply with a significant amount of 3rd harmonic distortion.

This supply can provide up to 1A continuously and up to 2A for a few minutes before it gets too hot. Take care when operating to make sure that the current supplied is not too

great. Leave the green illuminated power switch ON to keep the internal fan running and turn the "Output" switch On and Off as required.

If the power supply overheats you will need to leave it ("Output" set to "Off") for about 15 minutes with the fan running before it cools sufficiently to allow further use.

3-phase Wattmeter

The 3 phase power meter has connections for power supply and load. Keep the neutral connection connected to the transformer star point to minimize measurement errors.

This meter measures and displays RMS quantities. The default display shows line

voltage, average line current, total power consumed and overall power factor. Pressing the red button will cycle the display through different measured values (individual

phase voltage, currents and power are the most useful). Power is on page 1, Line Currents are on page 3. Note carefully the displayed units of measure.

Transformer

The transformer that you will be testing is a scale model of a mass-produced

distribution transformer. The transformer is rated for a full load of approximately

200VA. Each phase has a primary with a tap changer and two independent secondary

windings. The primary tap changer allows operation with -5%, NOM(nominal), or +5% primary turns. Note that because of the core geometry, this transformer is NOT

perfectly balanced; the centre limb has two short flux paths and each outer limb has a long and a short flux path. In a power system each transformer of this type would be connected to the source differently so that the whole power system operates with a

balanced load overall.

Potential Transformers and Current Transformer

In a large power system the voltages and currents used are too dangerous to attempt to measure directly. To reduce the system voltages and currents to a safe level for

observation, potential and current transformers (also known as instrument

transformers) are installed at strategic locations. In this experiment you will use the

instrument transformers to observe the power transformer voltage and current

waveforms on an oscilloscope. Note the scaling factor printed on each transformer box. The current transformer connects with a jack plug to the circuit being observed. The

jack socket contains a switch to break the circuit and put the current transformer primary in series with the circuit.

The no–load current waveform. of an over–excited transformer will contain significant harmonic distortion, mainly third,fifth, and seventh harmonics. This can be explained in terms of magnetic saturation and hysteresis. On the one hand, a sinusoidal induced emf at fundamental frequency requires a similar sinusoidal variation of flux, which in turn requires a non–sinusoidal magnetizing current. On the other hand, a sinusoidal

magnetizing current produces non–sinusoidal flux and a corresponding non–sinusoidal emf. In a balanced, three–phase circuit, third harmonic components are co–phasal.

Accordingly, third harmonic currents do not add up to zero at the star point and a star– neutral connection provides a return path for 3rd harmonic currents.

Preliminary exercise: Sketch the fundamental sine waves of the three-phases, 120

degrees apart, and their third harmonic components on the same axis on graph paper in your laboratory book. This will show that the third harmonic components are co-

phasal.

3. Procedure

Measurements

During the experiment, the transformer will be connected in different ways and under different conditions. For each setup (described in the following pages), record each of the measurements in the list below.

Set  "Output" to "On" before taking measurements, and to "Off" when finished; try to minimise the amount of time that it is "On".

Using the Digital Multimeter:-

Primary Line Voltage "VRY"

Secondary No. 2 Line Voltage ("Vry" for Star-Star ; "Vr1r2" for Star-Delta) Primary Phase Voltage "VRN"

Secondary No. 2 Phase Voltage "Vrn" (not applicable for Star-Delta)

Using the Wattmeter:-

Primary Line Current "IR"

Primary Line Current "IY"

Primary Line Current "IB"

Total Power consumed (i.e. Power Losses)

Using the Potential Transformer and Digital Storage Oscilloscope:-

Primary Line Voltage "VRY"

Secondary No. 2 Line Voltage ("Vry" for Star-Star ; "Vr1r2" for Star-Delta) Phase-shift of Transformer ("VRY" to Sec2 "Vry" for Star-Star  ;

"VRY" to Sec2 "Vr1r2" for Star-Delta)

Primary Phase Voltage "VRN"

Secondary No. 2 Phase Voltage "Vrn" (Not applicable for Star-Delta)

Using the Current Transformer and Digital Storage Oscilloscope:-

Primary Neutral Current "IN"

Current in Delta-Connected Secondary No. 2 (Extended Star-Delta only)

Tabulate the results.

Ensure Primary Voltage measurements are made directly at the Transformer Primary  (For the "Extended Star" connections, the voltages should be higher than shown on the Wattmeter).

Use the printer to plot any waveforms which have harmonic content : these will be required for discussion, later.

Annotate the printouts so that you can identify them later.

3.1 Star-Star Connection

Fig. 1   Star-Star Connection

1)  3-Phase Power Supply setup

Set "Power" to "On"

Set "Output Level" to "max" (42V Line Voltage)

Set "Output Waveform" as requested, below.

2)  Transformer connection and setup

Before altering connections on the transformer, always ensure that the "Output" switch on the 3-phase Power Supply is set to "Off".

Connect the three-phase transformer as shown in Fig. 1 (Secondary no. 2 is connected as a star).

At this stage, ensure that the Neutral connection on the Power Supply is not connected to anything (i.e. floating star).

Set Primary Tap Changer to "NOM".

3)  Measurements / Observations

For each case, below, take a full set of measurements (see page 3)

Case 1: (Output Waveform "sine"   ;   Neutral wire NOT connected ; Tap NOM)

Case 2: (Output Waveform "sine"   ;   Neutral wire connected  ; Tap NOM)

Case 3: (Output Waveform "sine +3rd" ; Neutral wire NOT connected ; Tap NOM)

Case 4: (Output Waveform "sine +3rd"   ;   Neutral wire connected  ; Tap NOM)

3.2 Extended Star-Star Connection

Fig. 2   Extended Star-Star Connection

1)  Connect the three-phase transformer as shown in Fig. 2.

Set Primary Tap Changer to +5%.

The primary winding and secondary winding no. 1 are connected in series, to make an extended star. This connects secondary no.1 in antiphase in series with the

primary. The primary voltage will now be considerably higher than design normal and operates the transformer primary under overvoltage conditions. This

connection setup would never be used in a real system; here we are using it to

significantly increase the primary voltage beyond what the power supply is capable of to the point of seriously over–exciting the transformer. Secondary no. 2 is

connected as a star and used for measurements.

DO NOT leave the power supply and the transformer operating (i.e. "Output" set to "On")  for more than a few seconds at a time when operating with this

setup. The power supply is heavily loaded and will easily overheat. Use the

trace storage facility on the oscilloscopes to ensure that the transformer is not being driven in this way for more than a short time.

2) Measurements / Observations

Case 1: (Output Waveform "sine"  ;  Neutral wire NOT connected  ;  Tap +5%)

(a) Take a full set of measurements (see page 3).

(b) Observe the Primary Line Current “IR” waveform. while changing the Primary Tap Changer from +5% to NOM and -5%. Note any effect on

harmonics and losses.

Case 2: (Output Waveform "sine"  ;  Neutral wire connected  ;  Tap +5%)

(a) Take a full set of measurements (see page 3).

(b) Observe the Primary Line Current “IR”, and “Neutral Output Current”

waveforms while changing the Primary Tap Changer from +5% to NOM and

-5%. Note any effect on harmonics and losses.

Case 3: (Output Waveform "sine+3rd" ;  Neutral wire NOT connected  ;  Tap +5%)

(a) Take a full set of measurements (see page 3).

(b) Observe the Primary Line Current “IR”, waveform. while changing the Primary Tap Changer from +5% to NOM and -5%. Note any effect on

harmonics and losses.

Case 4: (Output Waveform "sine +3rd"   ;   Neutral wire connected  ;  Tap +5%)

(a) Take a full set of measurements (see page 3).

(b) Do NOT change the Primary Tap Changer to the other two positions

because you will probably overload the power supply. What do you think would happen to the transformer under test if the Primary Tap Changer was set to the  other two positions?

3.3 Extended Star-Delta Connection

Fig. 3   Extended Star-Delta Connection

1) Connect the three-phase transformer as shown in Fig. 3.

Set Primary Tap Changer to +5%.

The primary winding and secondary winding no. 1 are connected in series—extended star. The secondary winding no. 2 is connected as a delta. Secondary no. 2 is used for measurements.

As before, DO NOT leave the power supply and the transformer operating (i.e.

"Output" set to "On")  for more than a few seconds at a time.

2) Repeat Section 3.2.2.

4. Calculations

Calculate the line-to-line voltage ratio for each of the three transformer connections.

5. Conclusions

Discuss the presence or absence of harmonic components in the waveforms.

Discuss the advantages and disadvantages ofthe various transformer connections.

What connection method has the lowest no-load losses? What effect does the primary neutral connection have on losses?

Standard practice in the UK for connecting distribution transformers is to use a delta

connected primary and a star connected secondary. What are the advantages and

disadvantages for this connection method if the connected load draws a non-sinusoidal current? (Consider triplen and non-triplen harmonics and losses).