| Virtual Synchronous Generator (VSG) has received more and more attention because of unique technology advantage, such as virtual inertia, virtual damper, droop control, seamless switching between grid-connected and islanding mode. With the support of "The National Twelth Five-Year Research Programme of China " (2014BAA04B02), several topics,such as math model of microgrid inverter, control strategy, key parameters design, lab simulation, have been studied in this paper. The main contents and creative ideas are as follows:(1) The basic control principle of synchronous generator is introduced and the characteristic of VSG control, droop control and the real synchronous generator is analysized and compared. Several topologies applied on high-power microgrid demonstration project are introduced and the advantage and disadvantage are analysized.(2) Positive and negative sequence math model, circling current and output impedance model of three-phase microgrid inverter are established using complex vertor transformation during single-inverter islanding mode, single-inverter grid-connected mode, parallelled-inverters islanding mode, parallelled-inverters grid-connected mode. The dynamic and steady decoupled characteristic is derived accordingly. The effect of sampling error and dead-time error is also explored and the key control paramaters and variables are then derived accordingly.(3) The key control variables are classified according to their control characteristic. Combining math model and stability analysis, the general control equations of power control loop and voltage double loop are established. Power calculation method based on multi-notch filter is given to improve power calculation response and system stability. In order to solve the problem between dynamic voltage response and circling current during parallel operation, output impedance decoupled control is proposed. In order to solve the problem between unbalanced voltage suppression and circling current during parallel operation, balanced control scheme is proposed under unbalanced loads. In order to suppression DC component of transformer, DC component suppression method based quai-resonant controller is given. In order to provide constant power during grid-connected mode according to grid dispatcher, conatant power control strategy is given. In order to suppress subsynchronous resonant, subsynchronous resonant suppression control scheme is given. In order to meet the requirement of reactive power compensation, a reactive power compensation scheme based on active component compensating reactive current is proposed. In order to combine PV generation and storage energy compensation, a general control strategy is establisehed.(4) Three test beches are built with the help of lab mambers to validate the effectiveness of the proposed control scemes. The firstne bench is dynamic simulation machine and Wind-PV-Storage energy physics simulation. The second is PV-Storage energy-Diesel experiment system and the last is smart microgrid demonstration project in Cuoqin, Xi Zang province. The lab simulation scheme is introduced briefly, including grid simulation, PV cell simulation and storage battery simulation. The key paramaters design method is given. According to the effect of digital control time delay, stability region criterion of LC/LCL resonant frequency based on generalized Nyquist stability criterion is proposed and disreste stability region is obtained. Combining this with the maximum ripple current of converter-side inductance, a LC/LCL paramater design method is given. The experiment results of different control scemes mentioned above are given and the control sceme are also verified through field experiment of demonstration project in Cuoqin, Xi Zang province. |
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