| Grid-connected inverter, as the interface between the distributed generation power and the grid, is widely used in photovoltaic, wind and other renewable energy power generation systems. Both the harmonic suppression capability and the quality of grid-connected current waveform are important performance indicators of the grid-connected inverter. The paper mainly studied the current control strategy of the single-phase grid-connected inverter with LCL filter. The3kW string inverter based on DSP and PV-IPM was designed, and the relevant verification by simulation and experiment were completed.In this paper, the two-stage system structure of the string inverter was chosen. So the DC/DC stage was mainly used to meet requirements of the maximum power point tracking and the DC voltage input range, and the DC/AC stage was used to control the inverter and grid-connected process. Firstly, the common circuit topology of the DC/DC converter and the DC/AC converter were studied. The applications, advantages and disadvantages of each topology were analyzed. Further, the modulation principle of the MPPT realization by Boost circuit was studied. What’s more, the characteristics of modulation and harmonic through the bipolar, unipolar, unipolar double-frequency sinusoidal pulse-width modulation were research priorities.Since the velocity of amplitude attenuation over the high-frequency range is-60dB/decade, the LCL filter has an advantage of better attenuation characteristics for higher-frequency harmonics over the pure-inductance filter. Nevertheless, for third-order control model, there is one high resonance peak at the resonance frequency, which will give rise to the distortion of current waveform. Therefore, the appropriate control strategy should be taken advantage of to improve the system damping so as to enhance the stability of the system.The traditional passive damping scheme inserting a resistor in serials or parallel into the filter, result in the corresponding damping loss together with weakening the ability to attenuate the high-frequency harmonics, which is unsatisfactory. While the typical active damping technology includes the split capacitor method, the inner-loop feedback of capacitor current method, the deadbeat control method, and so forth. This paper focused on the parameter design methods for the LCL filter. What’s more, in terms of the control characteristics, filter characteristics, the damping characteristics, the control strategies for the passive damping and the active damping were analyzed.The high-frequency harmonics resulted from the modulation would be easy to be eliminated by the filter. While the low-frequency harmonics that are difficult to be filtered, as a result of non-ideal situations and external disturbances, will seriously influence output voltage and grid-tied current. The current controller, thus, needs further optimization to offset the low-order harmonic pollution. In this paper, the model of a single-phase grid-connected inverter with LCL filter is established. Then, it’s quantitatively analyzed that the low-frequency harmonics injected by the inherent non-linear characteristics and dead-time effects of switching tubes along with the DC voltage secondary pulsation and the grid disturbance. Furthermore, a combined control scheme of embedded repetitive control and state feedback is brought forward, in which DC voltage feedforward decoupling and the grid voltage feedforward compensation are added.The3kW single-phase grid-connected string inverter based on DSP and PV-IPM was developed in this project. The design of the main power circuit, the control circuit and the control partial software were completed. Finally, the rationality of the harmonic analysis and the validity of control strategy presented in this article are verified through MATLAB simulation and experiment results of the3kW inverter system. |
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