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This paper investigates the operation of the back-to-back neutral point clamped (BTBNPC) converters when the ac voltages on both ends are asynchronous. The zero-sequence injection (ZSI) technique has been used to extend the linear modulation range of the pulsewidth modulation operation. This paper investigates the ZSI of the BTBNPC to maximize the dc-bus voltage utilization while the ac voltages of both ends have different magnitudes and phases. This study further explores the reactive current injection to reduce the risk of overmodulation when grid voltages surge. The combination of these techniques can extend the operation range of the BTBNPC over varying ac voltages of both ends with the available dc-bus voltage. In addition, the issue of the neutral-point balance is also taken into consideration. Laboratory test results are provided to verify the proposed technique. [ABSTRACT FROM AUTHOR]