This paper examines the use of a H-bridge cascade multilevel STATCOM for symmetry compensation. One of the particular problems H-bridge based STATCOMs have when used in these applications is maintaining correct voltages on the H-bridge capacitors for each of the individual phases of the STATCOM. This difficulty is the result of average real power flowing in or out of the individual phase legs. A solution has already been published for the delta connected STATCOM, but has not, until this paper, been solved for the wye connected topology. This paper uses a new approach to solve the phase leg power balance problem using zero sequence current and voltage injection. It allows the individual phase voltages for the both the delta and wye connected cascaded H-bridge STATCOM to be controlled. Furthermore, when implemented for the delta connected STATCOM it leads to a more elegant and parameter independent control system architecture.

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A note on versions:The version presented here may differ from the published version or from the version of record. If you wish to cite this item you are advised to consult the publisher's version. Please see the repository url above for details on accessing the published version and note that access may require a subscription. Abstract-This paper further develops a Model Predictive Control (MPC) scheme which is able to exploit the large number of redundant switching states available in a multi-level H-bridge StatCom (H-StatCom). The new sections of the scheme provide optimised methods to trade off the harmonic performance with converter switching losses and capacitor voltage ripple. Varying the pulse placement within the modulation scheme and modifying the heuristic model of the voltage balancing characteristics allows the MPC scheme to achieve superior performance to that of the industry standard phase shifted carrier modulation technique. The effects of capacitor voltage ripple on the lifetime of the capacitors is also investigated. It is shown that the MPC scheme can reduce capacitor voltage ripple and increase capacitor lifetime. Simulation and experimental results are presented that confirm the correct operation of the control and modulation strategies.

A multi-level H-bridge StatCom inherently contains redundancy in the available switching states. This paper develops a variation on the typical Model Predictive Control (MPC) scheme which is able to exploit this redundancy to simultaneously balance the H-bridge capacitor voltages, provide excellent current reference tracking and minimise converter switching losses. The scheme consists of a dead-beat current controller that has been integrated with heuristic models of the voltage balancing and switching loss characteristics. The integration of a Pulse Width Modulation (PWM) scheme is also described. Simulation and experimental results are presented that confirm the correct operation of the control and modulation strategies. Comparison with traditional control and modulation schemes is provided in terms of the key performance indicators associated with multi-level H-bridge StatComs.

Current control in inverter driven machine systems is arguably the most important part of the control system. If accurate and rapid current control is achieved then, given the correct reference currents, fast and accurate torque control can be achieved. Unfortunately the implementation of current control in power electronic systems is not ideal. Practical effects can have a significant influence on its performance. This paper examines one of these effects, dead-time, and considers the influence it has on the performance of Predictive Current Controllers (PCCs). The paper presents analysis that shows that PCC implicitly compensates for voltage loss due to dead-time. Also a modified PCC is introduced that reduces the zero current clamp problem caused by dead-time. Simulation and experimental results are presented to verify the analysis and confirm the performance of the new algorithm. 1 We are not considering the interactions that dead-time may have on the dynamic performance of a machine.

This paper considers specific issues related to using the cascade (also known as the H-bridge) multilevel STATCOM with unbalanced voltages and currents at the STATCOM-AC line connection terminals. Under this condition the average power into the individual phase legs of the converter, in general, is not zero, although the total three phase power is zero. The resultant phase cluster power unbalance can result in increasing or decreasing capacitor voltages within a phase leg. This paper will present a technique for zeroing the phase cluster average power under unbalanced line conditions when the control strategy is to rebalance unbalanced voltages.