Transformer bushing CTs should be tested using the Current Ratio test method before the transformer has been completely assembled. CTs should be tested before they are mounted on the transformer. In some cases, CTs may have to be tested by connecting test leads to both ends of an installed bushing. This can be difficult! If the CTs are already mounted in the transformer, large (high-capacity) current-testing leads can be pulled through the CT centers before bushings have been inserted. Occasionally it is not possible to perform a Current Ratio test. CT tap ratios can be verified by applying a voltage across the full CT winding – a Tap Voltage Ratio test -- then measuring the voltage drop across each individual tap. This is a simple test to perform, and voltage ratios will be directly proportional to the CT turns ratio between taps.
In addition to the Tap Voltage Ratio, a secondary Tap Current Ratio test can be performed. For this test, rated or less current is injected through a tap input and the output current of the full CT winding is measured by transformer action. It is equivalent to the procedure used for performing a Short-Circuit Impedance test on an autotransformer.
It is still necessary to verify CT polarity. One method used to establish CT polarity in power transformers is commonly referred to as "Flashing the CTs." This test can be performed by applying 6-to-12 volts DC to the transformer bushings, using a hot stick to
make and break the test circuit. An automobile battery is often most convenient because work vehicles are usually available at the job site, but a lantern battery will work as well. The transformer winding resistance is usually enough to limit the current flow from a 12-volt car battery, but adding series (current-limiting) resistance (a load box) to the test circuit is advisable in any test circuit with an automotive battery.
Be aware that the DC test circuit will generate a voltage kick when disconnected. Take precautions to prevent electric shock. If performing this test directly on CTs, always include a current-limiting resistance (a load box) in the flash lead connections. Lantern
batteries have high internal resistance and don't need an extra series resistor. Arc flash on a power transformer can be limited if the transformer windings are short-circuited on the side opposite those being flashed through
For CT accuracy and performance, flashing may not be a desirable test to perform because a condition of residual magnetism in the CT core can result. In theory, a possible consequence could be an improper relay operation due to CT saturation upon initial energization. If possible, demagnetizing the core is advisable after a DC flash test using a high current. Residual flux is removed by gradually applying AC test current (excitation current) to the high-current primary, or AC test voltage to the low-current secondary (excitation voltage), and forcing the CT just into saturation with the secondary open circuited. After slowly reducing the AC quantity from saturation point to zero, the residual flux will be removed from the core (it will be demagnetized).