The controlled transition bridge

The Controlled Transition Bridge (CTB) is a converter topology that combines series connected semiconductor “director switches” with chains of switched capacitor modules, “chainlink circuits”, in such a way that the director switches carry the main current for a significant portion of the AC power frequency period and the chainlink elements provide a controlled traverse of voltage between different director switches conducting. The simplest example of this is where the director switches form a six pulse bridge and the chainlink circuits traverse at a constant voltage rate between the upper director switch conducting and the lower director switch conduction etc., so that the output AC waveform is a trapezoid. A modified form of trapezoid waveform has been used to give both AC voltage magnitude control and reduced harmonic level. The paper presents the method of circulating the current through the bridge to give full control over power and reactive power while minimizing the power loss within the chainlink circuits. Aspects of the control of the converter are discussed including the sizing of the submodule capacitors within the chainlink circuits and the method of energy balance. Finally results are presented from a model of a full converter illustrating the different ways in which the current circulates through the converter bridge during the power ramp up.