| For distributed power generation equipments, they should not only keep up with the power reference, but also function well to ensure power supply for critical loads in the islanding mode when there are grid faults. While the Vector Current Control can only operate in grid-connected mode, and has to transform to Droop Control when islanding. It means a switch between current form control and voltage form control. Surely the effect can’t be good. Also Vector Control could trigger stability problem like LC resonance because of its fast response. Comparing of that, the Virtual Synchronous Generator(VSG) control can work both in grid-connected and islanding mode due to voltage form control, as well as switch to islanding frequently but no impact. Different from Droop Control, VSG control adds virtual inertia and frequency damping so as to adjusting response time of power, reducing current impact, improving ability of resisting disturbance in power and frequency and system stability. Besides, it can support the frequency and magnitude of grid voltage in a weak grid.First of all, the circuit model of three-phase PWM grid-connected inverter was built in the three kinds of coordinate system. Then the principle and traditional structure of VSG control is introduced. Based on that, a new control structure without phase locked loop was proposed. Besides, a first order inertia link adds to reactive control loop to increase stability. According to the requirement of steady state that frequency deviation should be less than 1 percent. When connecting to grid, we got the selecting rules for frequency damping. The inertia could be designed by the dynamic adjusting time of simplified first order or second order equations.The VSG control loop is a second order system, so that different damping value cause to different dynamic response, like too big damping value leads to longer adjusting time while over-adjustment comes out when it’s too small. Therefore, the influence of the parameters’ change to the dynamic performance of active power, reactive power, voltage frequency and magnitude in a power step change. The pre-synchronization control is used for connecting to grid with no impact. Considering of the drawbacks in present methods, angle difference filtering is joined in the improved method, subtracting the proportion of angle difference from frequency reference, and the angle difference can be regulated close to zero. So a small start current and active power reverse are achieved. Moreover, the active power reverse can be depressed by adding frequency droop link.Ordinary control methods of PWM converter has the stability problems like high order and sub-synchronous resonance, different from that, VSG control consists of synchronous frequency resonance. The paper builds up open loop transfer function between active power and power angle as well as reactive power and magnitude respectively, further we obtain the overall matrix open loop transfer function. The analysis by bode graph indicates the existence of 50 Hz resonance. By the usage of virtual resistance and feedback of high pass current, the resonance is depressed.In order to validate the practicability of theoretical analysis and proposed methods in real environment, a series of experiments are done in a 15 k W experimental platform of three-phase PWM grid-connected inverter. There are experiments about steady atate of traditional and proposed VSG control structure, dynamic response, switch from grid-connected mode to islanding, pre-synchronization and suppressing synchronous frequency resonance. The results indicate the effectiveness of the above analysis and proposed methods. |
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