Uncertainty of the wind results in a fluctuating power output of wind turbine generators, and consequently, causes adverse impacts on the stability of system frequency and voltage. To smooth such power fluctuation, this study proposes a power quality control strategy based on a three-level hierarchical structure for windbattery energy storage hybrid power system, including grid demand calculating level, energy management level and voltage-source converters control level. The advantage of the control strategy is that the state of charge of a battery energy storage system is regulated within proper range and the voltage at the point of common coupling is kept stable while smoothing wind power. Effectiveness of the proposed method is validated by MATLAB/SIMULINK simulations and experiments.

Transformerless inverter for grid-tied photovoltaic (PV) system has been widely used due to lower cost, higher efficiency and lighter weight. Various transformerless inverter topologies have been proposed to meet the safety requirement of low leakage current and obtain the reactive power capability. To get better performance, a novel transformerless hybrid-H6 inverter with an improved modulation technique is proposed in this study. By adopting the improved modulation technique, two symmetry paths are realised to share the current during the freewheeling mode. Thus, without paralleling any additional capacitors to the switch, the inverter can reduce the influence of junction capacitance on common mode voltage naturally which results in mitigating the leakage current issue. Thanks to the dead time reduction through the improved modulation, the qualities of output waveforms are improved. Moreover, reactive power control is achieved without any modification of the inverter structure. Finally, a 1 kW prototype is simulated and tested to verify the theoretical analysis of this study. Not only the reactive power capability is obtained for the proposed inverter, but also the small common mode voltage fluctuation is achieved at the same time. In addition, the total harmonic distortion of the current is decreased by more than 1.7%.

Due to its low conduction loss, hence high current ratings, as well as low cost, Silicon Insulated Gate Bipolar Transistor (Si IGBT) is widely used in high power applications. However, its switching frequency is generally low because of relatively large switching losses. Silicon carbide Metal-Oxide-Semiconductor Field-Effect Transistor (SiC MOSFET) is much more superior due to their fast switching speed, which is determined by the internal parasitic capacitance instead of the stored charges, like the IGBT. By the combination of SiC MOSFET and Si IGBT, this paper presents a novel series hybrid switching method to achieve IGBT's dynamic switching loss reduction by switching under Zero Voltage Hard Current (ZVHC) turn-on and Zero Current Hard Voltage (ZCHV) turn-off conditions. Both simulation and experimental results of IGBT are carried out, which shows that the soft switching of IGBT has been achieved both in turn-on and turn-off period. Thus 90% turn-on loss and 57% turn-off loss are reduced. Two different IGBTs' test results are also provided to study the modulation parameter's effect on the turn-off switching loss. Furthermore, with the consideration of voltage and current transient states, a new soft switching classification is proposed. At last, another improved modulation and Highly Efficient and Reliable Inverter Concept (HERIC) inverter are given to validate the effectiveness of the device level hybrid soft switching method application.

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To suppress zero-sequence circulating current in parallel operating inverters with common DC power supply, this paper proposes a new strategy based on Dual-carrier SVPWM and proportional resonant (PR) control, which use two opposite active vectors instead of zero vectors to suppress the higher frequency circulating current and reduces the lower frequency circulating current by means of zero-sequence circulating current closed-loop control based on PR controller. It has the advantages of high-efficiency usage of DC voltage, needless carrier synchronization and eliminating the circulating current. Simulation experiments verify the strategy's effectiveness in the two parallel inverters.