Control Strategies For Input-Parallel-Output-Parallel And Input-Series-Output- Series Connected Combined Inverter Systems

Series-parallel power conversion systems, in which multiple standardized converter modules are connected in series or parallel at the output and input sides, can be classified into four possible architectures on the basis of the connection forms, namely, input-parallel-output-parallel (IPOP), input-parallel-output-series (IPOS), input-series-output-parallel (ISOP), and input-series-output-series (ISOS). The advantages of systems constructed from connecting multiple converter modules include ease of thermal design, increased overall system reliability, shortened design process and lowered cost for the system; improved system reliability, and ease of expansion of power system capability. The standardized converter modules include four possible kinds, i.e., dc/dc converter, dc/ac inverter, ac/dc rectifier, and ac/ac cycloconverter. This dissertation focuses on the control of IPOP and ISOS dc/ac inverter systems. IPOP inverter system is suitable for the high output current applications such as UPS and aviation static inverter, while ISOS inverter system is suitable for high input voltage and high output voltage applications.A general control strategy is proposed for the series-parallel connected inverters system in this dissertation. In light of the power balance of both the input and the output sides, the general control strategy can be divided into two kinds: the strategy of controlling the output and the compound strategy. For the input-parallel inverters system, the strategy of controlling the output and the compound strategy can both be considered and the former is simpler. However, for the input-series inverters system, the stable operation of the system can not be ensured if the strategy of controlling the output is employed. So the compound method should be adopted. That is, controlling ICS or IVS as well as controlling the magnitude or phase of output current or output voltage to be equal.In the case of the IPOP inverters system, a distributed control strategy based on average current control has been proposed to obtain output current sharing by synchronizing the voltage reference and averaging the current reference. In this method the instantaneous current sharing is acquired with simple circuit, and hot swap and redundancy are easy to achieve. While the inductor-current feedback control is employed in this method, the output characteristics are poor. In this dissertation, the load current feed-forward control is introduced into the above average current control method and it is located before the point of averaging current reference. As a result, the output characteristics of each module and the whole system are improved, and meanwhile, the functions of output current limiting and the effect of current sharing are kept as the original method. Then, the output characteristics and circulating current between the original and improved methods are compared, and what's more, by theoretical and phasor means, the dissertation points out two key factors, namely the difference of KI and the difference of Cf of the inverter modules operating in parallel, determine the circulating current under both the two strategies. The difference of KI causes real and reactive circulating current, and the difference of Cf only results in the reactive circulating current. The output current sharing will be implemented only when KI and Cf of all the modules are both matched. Finally, the simulation and experimental results are presented to verify the effectiveness of the improved method, and the prototype is also achieve hot swap and redundancy.The control objective of the ISOS inverter system is to achieve input voltage sharing (IVS) and output voltage sharing (OVS) of the constituent modules. This dissertation firstly reveals the relationship of IVS and OVS. If OVS is achieved, so is IVS, but the controlling the output voltage of the modules to achieve OVS is not stable. If IVS is achieved, OVS is not ensured, it is because only the output real powers of the modules are balanced, but the output reactive powers of the modules are not always balanced. OVS can be achieved by controlling the magnitude or phase of output voltage to be equal based on controlling IVS. Moreover, this dissertation proposed a practical implementation of the compound strategy, i.e., the method of IVS combined with the same output voltage angle. The compound strategy includes IVS loop, system output voltage loop, and current loop of each module. In the above strategy, the capacitor-current feedback control is introduced to accomplish the same phase of output voltage of all modules. And then combined with controlling IVS, OVS is achieved. Furthermore, this dissertation also proposes another three-loop strategy based on the inductor-current feedback control which can realize IVS but can reach OVS only when the output filter capacitors of all the modules are matched. The defect of this method demonstrates the superiority of aforementioned three-loop strategy based on the capacitor-current feedback control which can obtain OVS unconditionally. Finally, the decoupling relationship between IVS loop and output voltage loop is analyzed and the design of two control loops is also presented.