Small Signal Modeling Based Virtual Synchronous Generator Stability Analysis And Design

In the last decade, environmental pollution is one major bi product of energy generation. Recently, renewable energy has emerged as clean energy source with environmentally friendly features. In comparison with traditional renewable energy distribution mechanism from generations companies by an integrated network,distribution generation based schemes have the potential to improve the overall stability of the power grid. With advantages of higher reliability, renewable energy distribution generation had attracted attention of researchers in recent years.As a special distribution generation, microgrid integrates distribution power supply,load, electronic devices, energy storage into an independent generation and supply system. Microgrid is conducive to large-scale integration of renewable energy, and has gained popularity amongst various research groups. Microgrid based inverters serve as a primary interface to connect renewable energy sources with the grid, therefore the control strategy of these inverters impose a crucial impact on the microgrid performance.Some traditional control strategies encounter complexities to fulfil the requirements of microgrid independent autonomous operation. For the same effect, much of the research activity is focused on virtual synchronous generator (VSG) technology in microgrid. In our dissertation, we have focused on the control strategy of the virtual synchronous generator and parameter design. The main contents of this research are:1. Compared with the droop control and synchronous generator small signal modeling, it proves that the droop control is a special case of virtual synchronous generator. Because the droop control is lack of inertia and it is easily to produce oscillation under disturbances. Referring to synchronous generator, power system stabilizer is introduced. Adding a power differential term in droop equations is an effective way to suppress oscillation. Micro-grid's small-signal models of the conventional droop control and the power differential droop control are established. The small-signal analysis is presented which helps in identifying the relationship between the controller parameters, line parameters and the system stability. The methods for selecting control parameters are also summarized. Then, the sensitivity analysis is presented which helps in contrasting the degree of participation of state variables in the low-frequency modes. The association between the line parameters and the participation factors is studied. The simulation results for various conditions have verified the model and analysis. Therefore ,it was concluded that the system damping increases with the increase of length or impedance ratio of the line. We have also discovered the dominance of eigenvalues on each inverter source is prone to changes.2. A control strategy without current sensor of virtual synchronous generator is designed. We established the fact that for power calculation equations of transmission line, virtual electromagnetic torque from the transmission line power does not need current feedback. Furthermore, the small signal model of the VSGs is established. Also,the methods for selecting control parameters combined with synchronous generator theory are summarized. Simulation results under multiple parameters verify the correctness of summarized parameter design methodology.We came to a conclusion that the synchronizing torque of VSGs was decided by the filter parameters, the transmission line parameters, the voltage gain and the reactive power gain. Moreover, when compared with the voltage gain, the reactive power gain has a larger impact on the synchronizing torque. The damping torque was decided by damping ratio and frequency damping factor. The reactive power output ability of virtual synchronous generators was determined by the voltage gain and the reactive power gain.3. The proposed control strategy was designed in line with the one for synchronous generators. A control strategy is designed which included practical aspects such as the prime mover and the rotor motion equation, the virtual excitation controller, the virtual electromagnetic transient controller and the dual PI controller. The small signal model of the consistent virtual synchronous generators is established.Partial of the VSG control parameters and circuit parameters impact on system small signal stability were identical to the results as follows: (1). the synchronizing torque of VSGs was decided by the filter parameters and the transmission line parameters; (2). the damping torque was decided by damping ratio and frequency damping factor. Moreover, parameter design methods of the virtual excitation controller and the dual PI controller are analyzed. Eventually, simulation results validates the fact that the virtual synchronous generator has the same response with real synchronous generator. It also affirms the small signal modeling accuracy.