01-01-2013, 02:24 PM
Transformer Tests
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Introduction :
The methods used during tests and measurements of the Power Transformers, test and measurement circuits, calculations and evaluation criterias are included in this manual. At the end of this manual, BEST Balıkesir Transformer Factory laboratory hardware and measurement and test equipments are listed.
For insulation levels of transformers, electrical characteristics and evaluation, please refer to national and international standards and customer specifications.
Measurement of winding resistance
Measurement is made to check transformer windings and terminal connections and also both to use as reference for future measurements and to calculate the load loss values at reference (e.g. 75C) temperature. Measuring the winding resistance is done by using DC current and is very much dependent on temperature. Temperature correction is made according to the equations below:
Measuring circuit and performing the measurement
The transformer winding resistances can be measured either by current-voltage method or bridge method. If digital measuring instruments are used, the measurement accuracy will be higher. Measuring by the current-voltage method is shown in figure 1.1
In the current – voltage method, the measuring current passing through the winding also passes through a standard resistor with a known value and the voltage drop values on both resistors (winding resistance and standard resistance) are compared to find the unknown resistance
Determining the Connection Group
Depending on the type of the transformer, the input and output windings of a multi-phase transformer are connected either as star ( Y ) or delta ( D ) or zigzag ( Z ). The phase angle between the high voltage and the low voltage windings varies between 0 and 360
Representing as vectors, the HV winding is represented as 12 (0) hour and the other windings of the connection group are represented by other numbers of the clock in reference to the real or virtual point. For example, in Dyn 11 connection group the HV winding is delta and the LV winding is star and there is a phase difference of 330 (11x30) between two windings. While the HV end shows 12 (0), the LV end shows 11 o’clock (after 330).
Determining the connection group is valid only in three phase transformers. The high voltage winding is shown first (as reference) and the other windings follow it.