Ying Jiang

In transformerless three-level photovoltaic inverter systems, the modified LC filter, which directly connects the dc-side neutral point to the common point of filter capacitors, is considered to be an ideal scheme to suppress the troublesome leakage current. However, a new common-mode (CM) resonant current is emerged, which leads to inverter-side current distortion and even system instability. Therefore, a second-order passivity-based controller in the time domain featuring excellent system stability is proposed for the CM resonant current mitigation. First, the reason for the CM resonant current generation with the modified LC filter is analyzed in detail. Then, the CM current model is derived, which reveals that it is a second-order system. The passivity of this second-order model is strictly proved. Moreover, based on the passive control theory, zero state detectable theory, and Lyapunov stability theorem, a second-order passivity-based controller is designed step by step to suppress the CM resonant current, and the system global asymptotic stability is also proved. The proposed controller is based on the time domain, which is more intuitive and avoids the error amplification in the conversion from frequency domain to time domain. Finally, the effectiveness and the correctness of the theoretical derivation of the passivity-based control method are validated by experiments. Compared with the passive damping resistor method, the proposed method not only has better performance but also avoids the use of real resistors and reduces the cost by using software instead of hardware.

The five-level active neutral-point-clamped (5L-ANPC) inverter has been applied to increase the power capacity of photovoltaic (PV) systems. However, this type of configuration requires to balance neutral point (NP) voltage and flying capacitor (FC) voltage properly. For the operation of the transformerless five-level inverter, another requirement is to suppress the leakage current associated with the common-mode voltage (CMV) vectors. Due to the mutually coupled issues, the conventional model predictive control (MPC) methods reported so far cannot deal with them properly at the same time. To address these problems, a novel double-vector MPC (DV-MPC) strategy with zero CMV vector is proposed in this article. First, 19 voltage vectors with zero CMV are implemented as candidate vectors. Then, for the purpose to reduce output current ripples, two voltage vectors are penalized by the cost function without weighting factors to track the reference voltage. In addition, the dwell times are optimized to further mitigate current ripples. Finally, to avoid the sharply increasing computational burden of the online iteration caused by the introduction of the second vector and its corresponding redundant switching states, a voltage-balance strategy is presented for the DV-MPC method, which can determine the suitable switching combinations directly and adapt them to the selected vectors to balance the voltages of the NP and FCs together. The effectiveness of the proposed method is verified by the simulation and experiment at the end of this article.

The three-level quasi-Z-source inverter (TLQZSI) is an attractive topology that is suitable for voltage buck and boost operation. This paper further presents a hybrid space vector modulation (HSVM) scheme to reduce the common-mode voltage (CMV) magnitude and frequency simultaneously. At first, the selection of basic vectors is optimized. Six medium vectors, six small vectors that have low CMV magnitudes, one zero vector, and three shoot-through vectors are utilized to produce the ac output voltage. The shoot-through state is properly inserted within the zero vector without affecting the normal ac output voltage. The effective small vector is selected according to the neutral-point voltage (NPV). Duty cycles of the adopted vectors are updated accordingly, and the restriction of the duty cycle of effective small vector is also derived. When comparing with the conventional SVM method, the CMV magnitude for the proposed scheme is suppressed by half. Furthermore, the CMV frequency is reduced by comparison to the conventional CMV reduction method. The feasibility of the proposed scheme has been tested by using Simulink platform and a hardware-based test rig.

In high-power applications, selective harmonic elimination pulsewidth modulation (SHEPWM) method is widely used to eliminate low-order harmonics in three-level inverters with low power losses. However, it faces the challenges that the conventional optimization methods for solving harmonic elimination equations in SHEPWM are easy to be trapped in local optimum. Moreover, the neutral point (NP) voltage of three-level inverter cannot be balanced under low-voltage ride-through (LVRT) condition with conventional SHEPWM method. To address these problems, a SHEPWM scheme is proposed, which is on the basis of an improved particle swarm optimization (IPSO) algorithm to solve the harmonic elimination equations and an enhanced NP balance control method in LVRT operation. First, an IPSO algorithm is proposed to calculate the switching angles of the transcendental equations for harmonic elimination. The IPSO algorithm defines a nonlinear negative exponential inertia weight considering the current and optimal fitness value, which can dynamically adjust the local and global searching speed and step size according to the actual situation, greatly improving the convergence speed and solution accuracy and avoiding falling into the local optimum. Then, by increasing the degree of freedom of the output current direction and combining with the NP voltage deviation, an enhanced NP voltage balance control method of SHEPWM in LVRT operation is proposed. The validity of the proposed method is proved by simulation and experimental results.

Fast dynamic performance of photovoltaic (PV) inverter is one of the most important issues in low-voltage ride-through (LVRT) operation, as the voltage dip period is short. Nevertheless, it is difficult to control the output currents with conventional methods, especially during transients or under low power factor operation. In order to satisfy the LVRT requirements and improve the performances of three-level T-type inverter (3LT2I) in PV system, the hybrid passivity-based control (HPBC) strategy with fast transient response and strong robustness to system parameter changes and external disturbances is proposed in this paper. Firstly, the Euler-Lagrange model of 3LT2I under LVRT condition is derived and analyzed. Then, the 3LT2I system is theoretically proved to be strictly passive by using some ingenious arithmetic and differential calculation techniques. Furthermore, based on error energy function and Lyapunov stability criterion, a passivity-based controller including negative-sequence of grid voltage feedforward control terms is designed to suppress the negative-sequence currents effectively. The balanced grid currents are obtained, which improve the stability of the system in LVRT operation. Compared with conventional methods, the proposed HPBC scheme has the superior characteristics of fast and non-overshoot response in transient state and accurate current tracking performance in steady-state. The theoretical analysis and effectiveness of the proposed strategy have been validated by means of both simulations and experiments.