MEASUREMENT OF TRANSFORMER NO LOAD LOSSES BASIC INFORMATION

Measuring no-load losses of a transformer when subjected to a sinusoidal voltage waveform can be achieved simply by using a wattmeter and a voltmeter; refer to Figure 1. Transformers may be subjected to a distorted sine-wave voltage.

In order to achieve the required measuring accuracy, the instrumentation used should accurately respond to the power frequency harmonics encountered in these measurements. Also, measured values need to be corrected to account for the effect of the voltage harmonics on the magnetic flux in the core and hence on both the hysteresis and eddy current loss components of iron losses.

The hysteresis loss component is a function of the maximum flux density in the core, practically independent of the waveform of the flux. The maximum flux density corresponds to the average value of the voltage (not the rms value), and, therefore, if the test voltage is adjusted to be the same as the average value of the desired sine wave of the voltage the hysteresis loss component will be equal to the desired sine wave value.

The average-voltage voltmeter method as illustrated in Figure 1 utilizes an averagevoltage responding voltmeter based on a full-wave rectification. These instruments are generally scaled to give the same indication as a rms voltmeter on a sine-wave voltage.

The figure shows the necessary equipment and connections when no instrument transformers are needed. As indicated in Figure 1, the voltmeters should be connected across the winding, the ammeter nearest to the supply, and wattmeter between the two; with its voltage coil on the winding side of the current coil.

The average-voltage responding voltmeter should be used to set the voltage.

NOTE

‘F’ is a frequency meter

‘A’ is an ammeter

‘W’ is a wattmeter

‘V’ is a true rms voltmeter

‘AV’ is an average-responding, rmscalibrated voltmeter

The eddy-current loss component of the core loss varies approximately with the square of the rms value of the core flux. When the test voltage is held at rated voltage with the average-voltage voltmeter, the actual rms value of the test voltage is generally not equal to the rated value.

The eddy-current loss in this case will be related to the correct eddy-current loss at rated voltage by a factor k given in Equation 8.2, Clause 8 of the IEEE Std. C57.12.90-1993 and C57.12.91-1979 Standard. This is only correct for a reasonably distorted voltage wave.

ANGULAR DISPLACEMENT OF THREE PHASE TRANSFORMERS BASIC AND TUTORIALS

Angular displacement is defined as the phase angle in degrees between the line-to-neutral voltage of the reference-identified high-voltage terminal and the line-to-neutral voltage of the corresponding identified low-voltage terminal.

The angle is positive when the low-voltage terminal lags the high-voltage terminal. The convention for the direction of rotation of the voltage phasors is taken as counterclockwise.

Since the bulk of the electric power generated and transmitted is three-phase, the grouping of transformers for three-phase transformations is of the greatest interest. Connection of three-phase transformers or three single phase transformers in a three-phase bank can create angular displacement between the primary and secondary terminals.

The standard angular displacement for two-winding transformers is shown in Figure above. The references for the angular displacement are shown as dashed lines.

The angular displacement is the angle between the lines drawn from the neutral to H1 and from the neutral to X1 in a clockwise direction from H1 to X1.

The angular displacement between the primary and secondary terminals can be changed from 0 to 330 degrees in 30deg steps simply by altering the three-phase connections of the transformer.

Therefore, selecting the appropriate three-phase transformer connections will permit connection of systems with different angular displacements.

Figure shows angular displacement for common double-wound three-phase transformers. Multicircuit and autotransformers are similarly connected.