Multiple Series Excitation Combinations: How to Achieve 100kV by Stacking Two 50kV Transformers?

The maximum output voltage of a single HZG test transformer is limited by its rated voltage, typically at 50kV or 100kV. For applications requiring higher test voltages (such as power frequency withstand tests for equipment rated at 110kV or above), multiple test transformers are connected in series to achieve voltage stacking.

The basic principle of series excitation

The core design of the series excitation series: Except for the transformer of the highest voltage level, an additional "excitation winding" is wound in the high-voltage winding of the transformers at other levels, with parameters identical to the primary winding of the next-level transformer. The control box supplies power to the first-level transformer, connecting the high-voltage output terminal of the first level to the high-voltage tail end of the second-level transformer. The excitation winding of the first level provides excitation power to the second level. Thus, the high-voltage output of the second level is the superposition of the outputs from the first and second levels. When three series excitation transformers are connected, the same principle applies.

Capacity Ratio Rule

Two series-connected machines: the first and the second have a capacity ratio of 2:1, with the total capacity equal to that of the first machine, and the total output voltage being the sum of both. Example from the manual: 5kVA/50kV and 3kVA/50kV series-connected, total capacity 5kVA, total voltage 100kV, output current 50mA.

Three series-excited machines: The capacities are in the ratio 3:2:1, with the total capacity equal to that of the first machine, and the total voltage being the sum of the three machines.

Key Safety Precautions

During series excitation, the casing potentials of the second and third-level transformers are no longer at ground potential but are U and 2U (U being the single-unit output voltage). Therefore, these transformers must be placed on insulated supports and maintained at a sufficient safety distance from personnel. Wiring inspection is particularly critical—reversing the connections will result in zero output voltage. It is recommended to directly monitor the high-voltage output with a voltage divider to confirm correct stacking before conducting the test.