The turn’s ratio test on a power transformer is an AC low voltage test that determines the high voltage winding’s ratio to all other windings while the transformer is not in use. On all taps, every winding is subjected to a turns ratio test.
Since CT ratios are the rated primary current to the rated secondary current at a specific load, the transformer ratio test mainly focuses on current injection in auto variable transformer for metering and protection.
The Raptor System offers many auto variable transformer manufacturer test templates and functions for CT/VT and PT transformer testing; the Raptor’s portability also allows the test set to be put closer to the transformer under test, lowering the length of test leads and power required; The Raptor includes the following transformer ratio test templates:
CT ratio determines the polarity and angle error. It also connects the burden in VA at the test current, in impedance, and the power factor of the load, using the same template. If the nominal ratio is entered, the transformer ratio test also displays the ratio error; it is advisable to test the ratio error and phase angle at different percentages. The auto variable transformer exporter can also show the ratio error.
CT Ratio, Voltage Method
When it is not possible to inject primary current into the CT for whatever reason, this template is used. The transformer ratio test is performed by applying a voltage to the secondary side of the CT while measuring the primary voltage. This method provides no information about the influence of the magnetic core on the total accuracy at its precision load.
By injecting primary current and measuring secondary voltage, the transformer ratio test for Rogowski and low-power CTs determines the CT ratio, polarity, and phase angle of this specific type of current transformers or sensors, with the turn ratio defined as a proportion of the primary current to the low-level secondary voltage.
CT ratio for Rogowski and low power CTs
By applying a voltage to the primary, measuring the secondary voltage, and confirming the VT turn ratio, polarity, and phase angle, this voltage measurement transformer ratio test is now finished.
If the neutral is not accessible, the injection is between two phases, measured at the low side according to the calculation, and the required connection diagram depending on the PT connection group. If the PT is three-phase, a measurement must be carried out in each phase; the injection is between two phases and measured at the low side according to the calculation and required connection diagram depending on the PT connection group.
Any of the SMC product range’s primary and secondary injection test sets can be used as a current or voltage source. As long as the injection range is satisfied, an integrated injection and measurement system, such as the Raptor or the ETP-1, with an appropriate external metre for the auto variable transformer ratio test, is preferred, as it contains all data and results in the same report, among other benefits.