5

1. THREE-PHASE TRANSFORMER. SHORT CIRCUIT TEST

1.1 INTRODUCTION. DESCRIPTION OF THE EXPERIMENT

The short-circuit test consists of measuring the input quantities of the transformer when its secondary winding is short-circuited and the primary winding is supplied with a suitably decre-ased voltage, so that the currents in both windings are equal to the rated currents.

The input power of the transformer in short-circuit operation is coincident with the copper losses in the transformer. In fact, the supply voltage is completely used to overcome the volta-ge drops of the windings and the only flux generated is the leakage flux, whose path is almost exclusively developed in air. Being affected by a virtually null flux (main flux), the core does not give rise to any loss.

In this experiment, students will measure the value of the short-circuit voltage VSC and of the power factor cosφSC. These values are essential for the calculation of the voltage drops under any load condition. They are useful to define the conditions of load division in case of parallel operation with other transformers.

ObjectivesBy performing this experiment, the students will study the short circuit operation of a three-phase transformer while reaching the following main objectives:

¾ To understand the schematic diagram corresponding to the short circuit test of a three-phase transformer.

¾ To perform the three-phase transformer wiring connections, in order to run the short circuit test.

¾ To obtain the characteristic curve related to the short circuit test (VSC - short-circuit voltage, ISC - short circuit current):

VSC = f(ISC) and cosφSC = f(ISC)

1.2 COMPONENTS LIST

The modules required for this experiment are: ¾ DL 1080 Three-Phase Transformer ¾ DL 10065N Electrical Power Digital Measuring Unit ¾ DL 1013M2 Power Supply Module

1.3 PROCEDURE OUTLINE

Schematic diagramThe values of Psc and Isc are those corresponding to an input current that is normally made equal to the rated current. It is preferable to perform the test under different current values, to draw the graphs of the individual quantities as a function of the short-circuit current Isc. The advantage is obtained that the measurement errors are reduced through the graphical inter-pretation of possible anomalous results. This is rather easy because the curves have a compul-sory and foreseeable behavior. When selecting the test currents, it is worth to stick to a wide range around the rated value

6

of the input winding. This does not have to be exceeded by over the 10 ÷ 15%, in order not to significantly heat the windings. The test results are affected by the temperature of the windings and the latter must be provi-ded to have an accurate meaning. It is, therefore, suggested to perform the test very fast and to start the measurements from the highest current values.

The circuit diagram for testing the three-phase transformer in short circuit is shown in figure 1. Supply the transformer through the high voltage side from a variable power supply, to prevent too high currents from flowing through the measuring circuit.

Figure 1. Circuit diagram for the short ciruit test of a three-phase transformer

The LV side of the transformer is short circuited and the wattmeter (W), the voltmeter (V) and the ammeter (A) are connected to the HV side of the transformer.

The test can be performed by selecting at will the input winding, because neither the Psc va-lue nor the Vsc change. Since in the short-circuit operation the behavior of the transformer is perfectly balanced and no reasons exist of wave deformation, also the connection of the input winding is perfectly free and can be indifferently selected as a function of supply and measu-rement convenience.

Characteristics curvesThe short circuit test of the three-phase transformer refers to the following curves presented in figure 2. The behavior of the test diagrams can be justified by analyzing:

1. The short-circuit voltage is expressed by the formula:VSC = ZS ISC

where ZS represents the equivalent impedance of the transformer, referred to the input side. This impedance is composed of the equivalent resistance and the leakage inductance of the windings.

7

Figure 2. The characteristic curve for the short ciruit test of a three-phase transformer

Both of them have no reason to change when the current Isc is varied, because: ¾ the Re could be modified only as a consequence of a winding temperature variation. The test must prevent significant heating from being generated.

¾ the Xe is only generated by the core flux. Therefore, its value is surely constant.Being Ze constant, the graph Vsc = f(Isc) will have to be a straight line crossing the axes origin.

2. The power Psc measured during the test represents the total copper losses. It includes the measured Joule effect losses due to the winding resistances and the additional losses due to the eddy currents induced, by the leakage flux, in the mass of both the windings and the sur-rounding conductive materials.

3. The function cosφSC = f(Isc) must be a constant. In fact, being constant the equivalent para-meters RS and XS of the transformer, also will be constant when Isc is varied:

cosφSC = RS / ZS

Setup and connection diagram

Figure 3 shows the schematic diagram of the short circuit test, where the three-phase tran-sformer is supplied from the AC three-phase variable section of the power supply DL 1013M2 (0÷240V/8A). The transformer parameters are measured with the measuring module DL 10065N.

The schematic diagram from figure 3 is close to the student’s theore-tical knowledge (it contains the classical electrical symbols). We invi-te you to use this diagram while performing the experiment, having the wiring diagram from figure 4 as a reference.

8

Figure 3. Schematic diagram of the short circuit test of the three-phase transformer

Follow the diagram below to connect the power cables:

Figure 4. Wiring diagram of the short circuit test for the three-phase transformer

Before starting any wiring activity, check all the power connections: all switches must be OFF.

Do not forget to connect the ground terminal! As shown in the dia-gram with specific symbols, all the equipment is connected to the protective network with a dedicated connector and cable.

Experimental procedure and learning planBefore starting the experiment, connect all the modules to the main power supply using the supply cables.

Perform the circuit configuration shown in the wiring diagram in figure 4. Power ON the DL 10065N measuring device.

9

Follow the next steps to enable and prepare the DL 1013M2 power supply for use:

¾ Raise up all the switches on the power supply.

¾ Turn the key clockwise. from position 0 to 1. ¾ Switch the selector "a0b" to position "b". We will use the AC part (0÷240V/8A) of the power supply. This action is necessary in order to start the power supply

¾ Press the green “start” button on the power supply module.

Before using the power supply, make sure that the safety connector (dongle) K1 is installed on the DL 1013M2 module (see figure 4).

Supply voltage to the three-phase transformer (DL1080) using the DL 1013M2 power supply.

Make sure that the knob of the power sup-ply is turned counterclockwise at “0” posi-tion and the main switch to “b”. Switch the selector of DL 1013M2, corresponding to the variable AC voltage "L1L2L3/ 0÷240V•8A", from off (O) to the on (I) position.

Perform the measurements starting from the high current values, with a certain speed between measurements, to avoid a possible thermal jump due to the nature of the test, so that the temperature remains approximately constant throughout the experiment.

Gradually increase the voltage and, while adjusting the knob, read the voltmeter V, using the DL 10065N module. For each voltage, measure the corresponding input currents and powers through the ammeter A and the wattmeter W (use the arrows from the front panel of the DL 10065N module to switch between voltage, current and power).

10

Table 1. Measured short-circuit values of the three-phase transformer

Calculate the power factor using the following formula and compare the result with the power factor measured using the DL 10065N module:

When the experiment is completed, turn off the power supply and switch all the selectors to “off”, the “a0b” to position zero and turn the knobs fully-counterclockwise to the zero position.

1.4 QUESTIONS

Answer the following questions related to the experiment.

1. Explain about the short circuit test on transformers.2. Why is the transformer rated in kVA and not in kW?3. How are the copper losses determined in a transformer?

11

1.5 CONCLUSIONS

From the no load and the short circuit transformer tests, it can be seen that the copper loss of a transformer depends on the current, while the iron loss depends on the voltage. Thus, the total transformer loss depends on volt-ampere (VA). It does not depend on the phase angle between voltage and current (the transformer loss is independent on the load power factor). This is the reason why transformers are rated in kVA.

The short circuit test is used to determine the values Re and Xe of the series branch of the equi-valent circuit.These impedances are usually very low, but they appear higher in value when referred to the high voltage side. This test is consequently performed on the high voltage side of the transfor-mer in order to keep the current drawn by these impedances at an acceptable level.