Testing voltage transformers

Performance tests on current and voltage transformers to verify ratio and polarity. These may include the injection of current into the primary load circuit (primary injection). 

In order to limit size and weight of T, resonant circuits or low frequency (e.g. 0.1 cps) test voltage sources may be used [20] instead of the conventional transformer. 

Test objects acting as a high voltage source, e.g. a voltage transformer, may be used as their own testing voltage source (see Fig. 8.6). 

Rated power of voltage transformer Test current... 

This test can be done using a low-voltage transformer and suitably connected ammeters and voltmeters, but a number of commercial instruments are available, such as the Clare tester, which give a direct reading in ohms. 

J) Source side resistance R (to control the p.f. as per the test requirements) High current step down transformer Test object (j Current shunt (2) R-C voltage dividers (12) Recording instruments ... 

Polarity test to check the connections through the potential transformer to ensure that the connections between the transformer and the meters or relays have a correct relative polarity. Otherwise the meters would show erratic readings, while the relays would transmit wrong signals. This test may also be conducted with a low-voltage source of 10 V by ob.serving the deflection of the instruments. 

Type tests These are conducted on a finished voltage or current transformer, one of each design and type, to verify their compliance with the design data and relevant Standards. 

Routine tests These are conducted on each finished voltage or cuiTent transformer to verify their suitability for the required duty. 

The dieleetrie isolation of the transformer should be at least able to withstand twiee the input voltage. Although this transformer is not tested during the HIPOT test, failure of its insulation would eause a eatastrophie failure in the eontrol eireuitry in the event of a power switeh failure. 

High-voltage withstand tests. The rated withstand voltages applicable to transformers of up to 36 kV are shown in Table 16.5. 

Where V and I refer to full-load current and rated voltage and IZ is the voltage measured at rated current during a short-circuit test transformer. In the case of a transformer... 

The efficiency of transformer oils as dielectrics is measured by electric strength tests. These give an indication of the voltage at which, under the test conditions, the oil will break down. Various national standards exist that all measure the same basic property of the oil. There is an international specification, lEC 296/1982, which may be quoted by equipment manufacturers in their oil recommendations. 

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