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This paper explores a new configuration for modular inverter systems, namely, input-series-output-series (ISOS) connected inverter systems, which are very suitable for high-input-voltage and high-output-voltage applications. The control objective of such systems is to achieve input voltage sharing (IVS) and output voltage sharing (OVS) among the constituent modules. This paper first reveals the relationship between IVS and OVS and points out that IVS and OVS can be simultaneously achieved only by the application of a compound strategy, which involves controlling IVS as well as controlling the magnitudes or phases of the modules' output voltages. Then, a practical implementation of the compound strategy, which combines a method of IVS with synchronization of the output phase angles, is proposed. An analysis of the decoupling of the two control loops, along with the detailed description of their design, is also presented. Finally, experimental results are presented to verify the theoretical analysis.