Modeling and Control of Grid Inverters Part I

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Modeling and Control of Grid Inverters Part I

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Instructor: Mark Dehong Xu, Zhejiang University, China With the increase of renewable energy penetration to the utility, the requirements to the gird inverter become higher and higher. One aspect of these requirements is that the grid inverter has higher power quality. Therefore, LCL filters have been widely used in the grid inverter. However, the grid inverter with LCL filter is sensitive. The damping control of it is critical to its proper operation. To analytically design the controller for the grid inverter, dynamical models for either single-phase or three-phase inverters with LCL filter are needed. With the dynamical model, transfer functions in dq rotating frame are able to be derived. Then the control can be designed analytically with traditional control theory. Since the transfer function of the grid inverter with LCL filter is the 3rd order, the bandwidth of the close loop will become narrower with the conventional PI controller design methodology. In this tutorial, a novel control scheme is introduced, which is kwon as Weighted Average Current Control(WACC). With WACC, the transfer function is transformed into the 1st order. It can widen the bandwidth significantly and realize damping control at the same time. Extensions of Weighted Average Current Control will be also introduced. DC current injection to the grid, which is caused by the grid inverter, is harmful to devices in the distribution systems. Generally the DC current injection occurs due to the asymmetry of the power device characteristics and control or gating drives in the grid inverter. An active DC current rejection control to the Grid is introduced. It is effective for both single phase and three phase grid inverter. The power quality of the grid inverter is deteriorated when the harmonics of the PCC voltage is poor. It is especially serious for high power grid inverters since the PWM switching frequency is deliberately selected to be lower to satisfy the efficiency requirement. Selective harmonics suppressing control loop may be added to reduce the effect of PCC voltage harmonics influence. For three phase gird inverter, there exist different PWM methods such as Continuous PWM, Discontinues PWM, Selective Harmonics Elimination PWM(SHEPWM) etc. The power loss distribution in the components of the grid inverter varies with the changing of the loading. One PWM method, which is good for the heavy load, may be not ideal for the light load. On the other hand, one PWM method, which is good for static state such as SHEPWM, may be not suitable to Low Voltage Ride Through condition. Hybrid PWM for the grid inverter comprises with the different operating conditions. It can increase the performance of the grid inverter with the regards to the entire loading condition.
Instructor: Mark Dehong Xu, Zhejiang University, China With the increase of renewable energy penetration to the utility, the requirements to the gird inverter become higher and higher. One aspect of these requirements is that the grid inverter has higher power quality. Therefore, LCL filters have been widely used in the grid inverter. However, the grid inverter with LCL filter is sensitive. The damping control of it is critical to its proper operation. To analytically design the controller for the grid inverter, dynamical models for either single-phase or three-phase inverters with LCL filter are needed. With the dynamical model, transfer functions in dq rotating frame are able to be derived. Then the control can be designed analytically with traditional control theory. Since the transfer function of the grid inverter with LCL filter is the 3rd order, the bandwidth of the close loop will become narrower with the conventional PI controller design methodology. In this tutorial, a novel control scheme is introduced, which is kwon as Weighted Average Current Control(WACC). With WACC, the transfer function is transformed into the 1st order. It can widen the bandwidth significantly and realize damping control at the same time. Extensions of Weighted Average Current Control will be also introduced. DC current injection to the grid, which is caused by the grid inverter, is harmful to devices in the distribution systems. Generally the DC current injection occurs due to the asymmetry of the power device characteristics and control or gating drives in the grid inverter. An active DC current rejection control to the Grid is introduced. It is effective for both single phase and three phase grid inverter. The power quality of the grid inverter is deteriorated when the harmonics of the PCC voltage is poor. It is especially serious for high power grid inverters since the PWM switching frequency is deliberately selected to be lower to satisfy the efficiency requirement. Selective harmonics suppressing control loop may be added to reduce the effect of PCC voltage harmonics influence. For three phase gird inverter, there exist different PWM methods such as Continuous PWM, Discontinues PWM, Selective Harmonics Elimination PWM(SHEPWM) etc. The power loss distribution in the components of the grid inverter varies with the changing of the loading. One PWM method, which is good for the heavy load, may be not ideal for the light load. On the other hand, one PWM method, which is good for static state such as SHEPWM, may be not suitable to Low Voltage Ride Through condition. Hybrid PWM for the grid inverter comprises with the different operating conditions. It can increase the performance of the grid inverter with the regards to the entire loading condition.