Tettex MIDAS 2881, Operating Instructions Manual

Page: 137 / 164

Applications Guide 127
14.2.4 Voltage Transformers
A huge variety of different kinds of voltage (or potential) transformers makes a complete disquisition in
this manual impossible. Therefore only one of the most famous and widespread voltage transformer is
presented here. It is the capacitor voltage transformer (CVT) as available for example by ABB (type
CPA) or by Trench (type WE).
A capacitor voltage transformer consists basically on a capacitor voltage divider and an
inductive/electromagnetic unit. The electromagnetic unit includes a transformer and a reactor whose
inductance is adjusted in resonance to the equivalent capacitance of the voltage divider. The
secondary voltage of the electromagnetic unit is proportional to the primary voltage and differs in
phase from it by an angle which is approximately zero.
The appropriate standard for testing capacitor voltage transformers are IEC 600186 and IEC 600358.
Test Procedure
Before any attempt is made to measure a voltage transformer, the unit should be isolated,
deenergized and grounded effectively.
For test purpose the inductive unit of a capacitor voltage transformer can be disconnected from the
capacitor voltage divider. This allows beside the overall test (voltage ratio, phase displacement)
separate measurements about the condition of the voltage divider and the electromagnetic unit.
A test procedure with the corresponding test modes is shown in the figure below. The connection
between the intermediate voltage of the voltage divider and the tuning reactor must be opened. Then
the capacitance and the loss factor of the capacitor voltage divider can be measured as outlined in the
table below.
Since the high voltage winding of the transformer is not capacitive graded, a measurement of the loss
angle (tan
δ ) will give no significant results. More meaningful tests would be secondary/ adjustment
winding resistance measurements and oil sample analysis.
The applied test voltage for the capacitor voltage divider should be chosen between 90 – 110% of the
rated voltage. In order to reveal any change in capacitance due to the puncture of one or more
elements, a preliminary capacitance measurement can be made at a sufficiently low voltage (less
than 15% of rated voltage). If the rated voltage exceeds the maximum available test voltage,
measurements should be performed at the maximum test voltage.