Abstract
As to the concrete topology of three-phase LCL type grid-connected inverter with damping resistance, mathematical model was deduced in detail, using method of equivalent transformation to the structure diagram, damping resistance was virtualized, mathematical model under the DQ frame that can realize decoupling control was established, a dual-loop control strategy for grid-connected inverter with LCL filter was proposed, the system stability was analyzed and the design method of controller was given. The proposed method overcame the flaws of loss increase, efficiency reduce and cost increase which were caused by damping resistance in LCL type grid-connected inverter, the system efficiency and power supply quality of the output were improved. Feasibility and effectiveness of the new method were validated by simulation and experimental results.
Highlights
LCL filter compared with the traditional L filter, it needs smaller inductance value and it is more effective for restraining higher harmonic when they achieve the same filtering effect, but resonance problem exists in LCL filter itself, it will cause system instability [1,2,3]
As to the concrete topology of three-phase LCL type grid-connected inverter with damping resistance, mathematical model was deduced in detail, using method of equivalent transformation to the structure diagram, damping resistance was virtualized, mathematical model under the DQ frame that can realize decoupling control was established, a dual-loop control strategy for grid-connected inverter with LCL filter was proposed, the system stability was analyzed and the design method of controller was given
If “virtual damping resistance” that is from the equivalent transformation is directly used in controlling, due to the order of its transfer function numerator is higher than that of the denominator, feedback quantity is the filter capacitor current, the current harmonic is high and the control is more difficult, it is difficult to achieve the effect of inhibiting resonance and system stability and this is verified by simulation result
Summary
LCL filter compared with the traditional L filter, it needs smaller inductance value and it is more effective for restraining higher harmonic when they achieve the same filtering effect, but resonance problem exists in LCL filter itself, it will cause system instability [1,2,3]. There are two common methods to solve it [4, 5]: one is passive damping, that is, a damping resistance is connected with the capacitor branch in series to inhibit resonance [6,7] This method is simple and reliable, but the LCL filter’s ability of restraining higher harmonic was reduced, it will bring extra system loss for the damping resistor; another one is active damping, namely the modified control algorithm was adopted to inhibit resonance and make sure the system stability [8, 9]. According to the principle of inhibiting resonance by using damping resistance, and using equivalent transformation, a dual-loop control strategy for grid-connected inverter with LCL filter was proposed in this paper, this new method was used to inhibit resonance, ensure system stability. The Topology and Mathematical Model of Grid-connected Inverter with LCL Filter
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