Page 374 - Offshore Electrical Engineering Manual

Voltage Transformers (Refer to BS EN 61869 and BS EN 50482) 361

Table 7.4.1 Recommended Test Voltages for Voltage Transformers (Fully
Insulated Primary) and Current Transformers

Rated Highest Equipment Voltage (kV AC) Test Voltage (kV DC)
1. Primary insulation test
Up to 0.66 1.5
Up to 3.6 7.5
Up to 7.2 15.0
Up to 12.0 25.0
Up to 17.5 32.0
Up to 24.0 45.0
Up to 36.0 66.0
1. The application time should be 15 min applied between the primary and earth, with
secondary windings shorted and earthed.
2. Secondary insulation tests. All secondary insulation tests should be conducted using a
test voltage of 500 V DC to earth for 1 min duration.
AC, alternating current; DC, direct current.

POLARITY CHECK
The VT polarity can be checked using the simple circuit shown in Fig. 7.4.1.
The battery must be connected to the primary winding with the polarity ammeter
connected to the secondary winding. For the circuit shown, the ammeter will flick
positive on making the switch and negative on opening the switch.

VOLTAGE RATIO TEST
The approximate voltage ratio of the transformer can be tested by a simple voltage
injection test on the primary winding by measuring the primary and secondary voltages.
Extreme care must be exercised when carrying out this test to ensure that the test
voltage is applied to the primary and not the secondary winding. Otherwise, very
high voltages may be applied to the test instruments and create a hazard to the tester.

PHASING CHECK
It is essential to check the phasing of the secondary connections on three-phase VTs,
or three single-phase VTs, when these are used for either metering or protection
purposes.
The simplest method of testing the phasing is by the application of the system
voltage on to the primary winding of the transformer, i.e., with the main busbars live.
The secondary voltages can then be checked between phases and neutral, and the
phase relationship is verified with a phase indication meter.
Open-delta secondary windings of three-phase transformers can also be checked
to verify a balanced primary three-phase supply.