The Research Of Inverter For PV Generation

With the global environmental pollution and the increasing serious energy crisis today, photovoltaic (PV) power generation is becoming one of the most promising renewable energy technologies. The research in this paper involves Maximum Power Point Tracking (MPPT) techniques, intelligent charge control strategy, topology and control method of the PV grid-connected power generation and unification control method of PV grid-connected generation and active power filtering, etc.Direct current MPPT control strategy is proposed based on disturbance observer method. By directly detecting output current of PV converter, not only voltage sensors and current sensors in disturbance observer method are reduced, but also simple control algorithm and low cost are realized.A practical lead-acid battery charge control strategy combined with MPPT technique for PV system is studied. The novel method can promote PV cell efficiency, shorten battery charge time and improve charge efficiency. A digital control technique for the inverter is described, which is based on available value voltage and instantaneous voltage dual-loop control. Steady-state accuracy and dynamic performance are ensured at the same time by this control method. A 3kW prototype is designed and tested in the lab, and the experimental results verified the proposed control strategy.Considering small volume and low cost of resident PV grid-connected inverter, the topologies of grid-connected based on immittance converter are studied. Compared with the traditional current source inverter, high-frequency inductor and transformer in the proposed inverter replace the power frequency inductor and transformer. Low power loss, small volume and low cost are realized in the new inverter.As to inherent characteristic of immittance converter, this paper shows that the output current of grid-connected inverter is non-sinusoidal under the conventional SPWM method, which leads to harmonic pollution in public supply networks. An improved modulation method is proposed. A high power factor and low harmonic grid-connect power generation are realized by this way. A 3kW single-phase PV grid-connected inverter prototype is designed based on above theory analysis. The feasibility and validity are proved by experimental results.The topologies and control strategy of three-phase current source grid-connected inverter are studied. An improved modulation strategy is adapted and a satisfied solution is achieved on harmonic suppression. Compared with conventional three-phase grid-connected inverter, the novel inverter has the obvious characteristics as following: any phase of the inverter can work normally even if the other one or both phases are destroyed, which enhances the system redundancy. Under unbalanced grid, the inverter also can provide a stable three-phase current, which strong anti-electricity fluctuation capability. Though three sets of immttance converter and transformer are used, power capacity of every phase is only one third of the inverter total power capacity. Consequently, smaller capacities high-frequency inductor and transformer can be selected.A three-phase current source inverter is presented based on immittance inverter. To utilize of characteristics of immittance converter, DC voltage is converted into sine-wave high frequency current and is isolated by high-frequency transformer, then three-phase sine-wave current source is achieved through cycle-converter modulation. Not only the DC input inductor is reduced but also power is transmitted by high frequency converter. Consequently, the volume of output filter inductor and isolated transformer is declined; small volume and low cost are realized.Due to the low output voltage of PV cell arrays, a novel single-stage three-phase boost-type grid-connected inverter is studied. The simulation and experiment results show that current-source grid connection with a high power factor can be realized by single-stage boost DC/AC conversion. Moreover, the power frequency inductors in traditional current-source inverter are removed and only two switches are communicated at any time. Therefore, the proposed inverter has advantages of flexible control, high stability, low cost and high quality grid current based on the DSP control.The unification control method of PV grid-connected power generation and active power filtering are proposed. PV grid-connected generation and harmonic compensation are realized on the same prototype, which overcomes that PV grid-connected power generation equipment is inoperative at night and improves the system efficiency. Reactive current and harmonic current detection method, active current of PV power generation calculation method, current loop controller and voltage loop controller deign method are analyzed in detail. Furthermore, the unification control method and single active power filtering control method are discussed individually. The feasibility of control strategy and system topology is verified by the simulation and experimental results.