Control Strategy For Input-Series-Output-Parallel Inverter

Input-Series-Output-Parallel (ISOP) inverter, which consists of multiple inverter modules connected in series at the input side and parallel at the output sides, is suitable for high input voltage and high output current applications. Input voltage sharing (IVS) and output current sharing (OCS) are necessary requirements for ISOP inverter to work properly. So the dissertation investigates the control strategies to achieve IVS and OCS for ISOP inverter.This dissertation reveals the relationship between IVS and OCS, and shows that if OCS is achieved, IVS will be achieved automatically. However, control aiming to meet OCS will lead to instability. If IVS is achieved, once either magnitudes or the power factor angles of output currents are equal, OCS will be achieved. It will be stable for ISOP inverter. Control method is presented to achieve IVS and OCS for ISOP inverter. By the method, IVS is achieved by controlling magnitudes of the inductor currents according to the input voltage errors. OCS is achieved by keeping the phases of the filter inductor currents constant, which means power angles of the output are invariable.A centralized control strategy for ISOP inverter is proposed based on above analysis. The control strategy is composed of output voltage regulating (OVR) loop, input voltage sharing regulating (IVSR) loop and inductor current regulating (ICR) loop. All modules share the common OVR loop,which provides references for all the ICR loops. IVSR loop regulates the magnitudes of the references of ICR loops to achieve IVS. During regulating process, the output power angles of the modules are kept constant, OCS will be achieved when IVS is achieved. Based on the centralized control strategy, a distributed control strategy is proposed to keep the control system independently. The strategy distributes control circuits into each module, and control circuits of the modules are connected by the output voltage reference line and the IVS line, which makes modules work independently or corporately with other modules. The importance of each modules is same in the system, which improves reliability, redundant and modularity of the system.Input voltages, output voltage and output currents are controlled in ISOP inverter, thus the control system seems to be complex. Analysis shows that OVR loop is decoupled with IVSR loops, so control system could be decoupled into two independent parts: IVSR loops and OVR loop. Each part could be designed independently. IVSR loops are a multi-variables system, so Lyapunov theorem is used to design it. OVR loop is a single-variable system, and frequency domain analysis method could be used. The dissertation provides design method in detail.A 2kVA prototype of two-module ISOP inverter is fabricated and tested in the lab. Experimental results verified the effectiveness of the proposed control strategies.