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This paper presents a control strategy for input-series-output-series (ISOS) modular inverters. Each module is a bidirectional high-frequency ac-link (HFACL) inverter composed of an HF inverter, an HF transformer followed by a cycloconverter. The relationship between input voltage sharing (IVS) and output voltage sharing (OVS) is revealed with a general load based on bidirectional ISOS inverter systems. To achieve IVS and OVS, a stable power sharing control strategy eliminating mandatory IVS loops is proposed. For individual modules, the common output voltage regulation loop output multiplies its magnitude compensator loop output to work as its pulse width modulation signal. The loop gain design is also presented. With the proposed control strategy, excellent IVS and OVS can be obtained not only during steady state but also during transients. The effectiveness of the proposed control strategy is verified by simulation and experimental results of a 2420-VA ISOS two HFACL inverters.