Research On Discontinue PWM Modulation Based Inverter For Energy Storage

With the deterioration development of global energy crisis, the distributed generation system based on new energy has drawn widely concerns and obtained rapid developments. The randomness characteristics of solar and wind energy lead to the situation that they cannot provide enough electricity during peak time while power resources can be surplus at times of lower demand. The bi-directional flow of energy between energy storage inverter and distributed power grid can achieve the balance and overcome problems presented nowadays with the effective solutions on unbalanced related problems. Thus, energy storage inverter, as one of the greatly significant compositions of the distributed power generation system, has become the research highlight with regard to methods to increase its conversion efficiency. Based on energy storage inverter as the research object, this paper revolves around its modulation strategy, the main circuit parameter design problems, such as intensive research.First of all, the analysis of the mathematical model of three-phase energy storage inverter and control system is conducted in this paper. Based on the main circuit topology and primary theory analysis, we sythesized the mathematical models of the system in ABC, αβ and dq coordinates, which reveals the mathematical relationship between grid-side current and AC voltage. Simultaneously, introductions on the structure of the three-phase energy storage inverter control system are presented in this dissertation, analyzes the PWM modulation role in energy storage inverters.Secondly, we studied the discrete pulse width modulation(DPWM) strategy. Primarily, we introduced the PWM modulation types. Subsequently, the corresponding relationship of four DPWM strategies between the power factor angle and switch loss of DPWM modulation strategy is deduced. Paper obtained the zero sequence voltage expression based on the modulation voltage and output current. And according to zero sequence voltage and its corresponding DPWM modulation strategy, a method to implement the minimum loss modulation signal was proposed here. Finally, through a period of PWM waveform Fourier expansion, the DPWM modulation spectrum distribution of the output phase voltage was presented, which provided the basis for subsequent LCL parameters optimization design.Then, aiming at the DPWM modulation output harmonic enlargement problem, this paper proposed a new method to design LCL filter. With the help of LCL mathematical model, we analyzed the characteristics of LCL filter and the effects of various parameters on the filter. Therefore, a method is given to design LCL parameters based on DPWM modulation strategy. According to the above, with the result of spectrum analysis of the PWM output phase voltage, we recalculated the DPWM modulation and harmonic current value produced by continuous pulse width modulation(CPWM), and a set of experiment platform parameters can be concluded.Finally, simulations and experimental verifications on implementations of DPWM modulation method and the new LCL filter design method were conducted. After the Fourier decomposition of the energy storage inverter output phase voltage, the harmonic calculation method was verified. The experimental results showed that efficiency of the energy storage inverter has been effectively improved, and the output current harmonic complied with the design requirements. This fully prove the validity of the modulation methods used by this paper and the correctness of the LCL filter parameter design.