A methodology for developing ‘Chainlink’ converters

ldquoChain circuits or ldquoChainlink convertersrdquo include the Modular Multi-Level Converter (M²LC) and the multilevel Static Compensator (StatCom). They use large numbers of identical series connected modules, each of which contains a capacitor that can be switched in or out of circuit to present a continually variable voltage at the converter terminals. The technique permits multilevel converters to be constructed without the disadvantages associated with other multilevel topologies and have created major interest for high voltage power transmission applications since complex waveforms can be constructed without the filtering associated with converters utilising PWM. The approach proposed is to operate converter topologies built up from combinations of chain circuits as sets of individual current sources. These are arranged as a matrix form so the current from the input and output terminals are able to circulate through all the chain elements in a subtractive manner. To maintain the capacitor charge level in each chain circuit the net power flow must be zero which can commonly be achieved only for AC to AC or AC to DC power conversion. Additional control is required to trim the operation of the chains to ensure average capacitor voltage in a chain is constrained to a defined level, although the simulation results presented do not include this. An example of a topology that can be derived by this approach is the Matrix Chainlink converter that can directly convert three-phase AC to three-phase AC. A second topology is proposed that will give a AC to 4 quadrant DC converter form that will yield a galvanically isolated DC to DC converter.