An Improved LMSVM Method for Leakage Current Suppression and Neutral-Point Voltage Control in Transformerless NPC Three-Level Inverters

Transformerless three-level inverters have many advantages over two-level inverters in photovoltaic (PV) systems. However, in transformerless systems, except for the inherent unbalanced neutral voltage of the three-level inverter, the leakage current also needs to be limited strictly. Concerning the defects of the conventional common-mode voltage (CMV) suppression methods, an improved LMSVM (ILMSVM) method is proposed. Unlike the conventional CMV suppression method, the ILMSVM method no longer just abandons the high CMV small vectors to limit the range of CMV to ±1/ 6U_{\mathrm {dc}} , but limits CMV variation to \Delta 1/6U_{\mathrm {dc}} in all switching cycles. So, high CMV small vectors also can be used in some sequence to help controlling neutral point (NP) while keeping leakage current well suppressed. Moreover, medium vector sequences are also used as an independent control sequence to help achieving bidirectional control on NP potential. Compared with LMP/NSVM strategy, the ILMSVM method has advantages in regulating NP potential, and leakage current spikes of the LMP/NSVM method are also eliminated. In comparison with the conventional SVPWM (CSVPWM) method, leakage current in the ILMSVM method is also well restrained. Simulations and experiment results verify that the ILMSVM method can well restrain leakage current and maintain good control of NP in comparing with LMP/space vector modulation (SVM) method and CSVPWM method.