Thyristor-Based High-Power On-Load Tap Changers

The volatile nature of renewable power sources leads to increased voltage fluctuations in the medium-voltage and low-voltage ac grids to which they are connected. Mechanical on-load tap changers are widely used to keep the voltage in these grids within prescribed limits. However, mechanical on-load tap changers are not designed for the high number of switching cycles that are required in grids that have high capacity on volatile sources. Their contacts can only handle a limited number of switching actions during their lifetime.

This thesis investigates fully electronic thyristor-based on-load tap changers that are capable of switching extremely fast and that exhibit virtually no wear due to switching actions. For the control of the commutation process a robust control is developed. The new control scheme uses separate gate trigger signals for each thyristor of a bidirectional thyristor pair. Thereby, safe commutation at highly disturbed load currents is achieved. Undesired short-circuits from tap to tap during commutation are avoided effectively.

A detailed simulation model of transformer and tap changer is synthesized to comprehensively test the developed control under highly disturbed load conditions. The model is verified using a low-power experimental setup of a 9-step on-load tap changer prototype. Basic switching tests are performed on a 27-step on-load tap changer prototype with a nominal regulating range of ±7600 V and a current capability of 950 A.