Specialization: Energy Systems Degree: Master of Science Abstract: Facing the trend of rapid-growing solar PV capacity, there is an increasing demand for achieving efficient and reliable energy conversion between distributed PV plants and the utility power grid. Varieties of the three-phase transformerless solar PV inverters have been developed to enhance the system’s efficiency, reliability, and power density. In such systems, common-mode (CM) leakage current can be produced due to varying common-mode voltage (CMV), which usually requires a CM filter to suppress. Extensive studies have been conducted on reducing the size of CM filters. In particular, the zero CMV (ZCMV) PWM and interleaved PWM are two popular approaches. The ZCMV PWM is proposed to eliminate the CMV in a multilevel inverter. Despite the reduced linear modulation range, this method has been a popular approach in the literature for PV inverters in both 1000V and 1500V systems. The AC voltage can be low. As such, the reduced linear modulation range is not a big limiting factor for the ZCMV PWM in PV inverter application. However, the CMV cannot be completely eliminated due to the inevitable dead-time. As a result, passive CM filter is still required in a PV inverter. Moreover, the ZCMV PWM results in larger harmonics in the output current. As a result, the practical value of the ZCMV PWM for PV inverter application remains unknown. On the other hand, modular parallel PV inverter system has been widely employed in practical applications to enhance system lifetime and efficiency. With interleaved PWM, the overall CMV of the system can be further reduced, thus results in smaller CM filters. The key challenge to implement the parallel interleaved inverters is the control of low-frequency (LF) zero-sequence circulating current (ZSCC) which can be induced by unavoidable hardware parameters mismatch and output power mismatch. This thesis focuses on addressing the key issues of employing ZCMV PWM and interleaved PWM to reduce the CMV effects in different types of the grid-connected PV inverters. A comprehensive evaluation of the ZCMV PWM is conducted on the transformerless T-type 3L-NPC string PV inverters. In this study, two aspects are explored including dead-time effect and resultant filter volume. Impact of dead-time is thoroughly investigated first, which indicates CMV spikes caused by dead-time are inevitable to ZCMV PWMs. To mitigate the dead-time effects, a cost-effective dead-time mitigation strategy has been proposed by using NP-LCL filters. The analysis indicates NP-LCL has better CM performance than conventional LCL filter, which can be applied to mitigate the dead-time effect without introducing extra passive filtering elements. In order to investigate the actual impact of ZCMV PWM on the total output filter volume, a comparative study has been conducted between the conventional PD PWM and ZCMV PWM methods. The study indicates size of the CM filter can be reduced with ZCMV PWM, but at the cost of two times larger of the DM filter and 1.5 times larger of the total filter volume. Considering the dead-time effect and larger DM filter requirements, ZCMV is less applicable than PD in such applications. Interleaved PWM, on the other hand, can be applied to reduce the CMV effects in central or multistring PV inverters where modular parallel VSIs are employed. This thesis proposed an MPC-based centralized control scheme to achieve interleaved PWM and elimination of LF-ZSCC. Carriers are adopted to realize interleaving, where the benefits of MPC and interleaved PWM are combined to further enhance the system performances. A generalized ZSCC model is derived, which allows the proposed method to be applied to any number of modular parallel VSIs. With LF-ZSCC eliminated, the proposed method enables interleaved PWM to be implemented in N paralleled inverter modules for CMV reduction. Compared to the existing MPC-based ZS control schemes, proposed method is more applicable and more flexible, which can be implemented either in a dedicate central controller or in each converter’s local controllers. Effectiveness of the proposed method has been verified by MATLAB simulations and lab-scale experiments using paralleled two-level VSI as an example.