Research On Synchronous And Grid-connected Control Methods Of Three-phase Inverter Under Unbalanced Grid

The increasing popularity of renewable energy and distributed generation systems puts higher demands on grid-connected inverters.For the control method of the grid-connected inverter,synchronous control and grid-connected current control are the key points.With the large number of applications of new energy power systems,the access of a large number of distributed power sources,the access of high-power single-phase loads and the uncertainty of single-phase load power consumption,the system will be unbalanced,which will brings challenges to the synchronous control and the grid-connected control.This paper takes the three-phase inverter under unbalanced grid as the research object,and studies the fast,accurate and simple synchronous control method of the inverter under unbalanced grid to eliminate the adverse effects of unbalanced amplitude,harmonics and DC offset,and the accurate estimation of the phase angles of the three-phase voltages even under phase unbalanced conditions.A discrete proportional resonance(PR)controller which can improve the quality of the grid-connected current and has no resonance frequency shift is designed.First,this paper studies the unitization algorithm of voltage amplitude under unbalanced grids,and eliminates the adverse effects of harmonics and DC offsets through the cascading delayed signal cancellation algorithm.Then,according to the anti-conjugate decomposition principle,the output voltages of the cascade delayed signal cancellation algorithm are orthogonal And the sum of the squares of the two voltages are added to obtain the voltage amplitude of each phase,and then the amplitude of each phase is unitized to obtain a balanced three-phase voltage.Finally,a simulation model is built under the PLECS simulation environment to verify the effectiveness of the method.In order to obtain the frequency information of the grid,the Teager energy operator(TEO)algorithm is used to calculate the grid frequency through three sampling points in an open loop,without any parameter adjustment of the controller.And it is not affected by the phase angles.Finally,simulation results prove that this method can quickly and accurately obtain the grid frequency even under the conditions of frequency fluctuation,amplitude imbalance and phase imbalance.In order to analyze the performance of the discrete proportional resonant controller obtained by different discretization methods,the discretization methods are divided into five groups according to their poles.By anal yzing the pole trajectory of the discrete transfer function,it can be seen that these discretization methods may not achieve zero damping,or have resonance pole shift,or have heavy calculation burden.To this end,this paper proposes a discrete PR contr oller based on the state transition matrix.With this method,the resonance frequency shift can be eliminated and the no static-error tracking performance can be improved.And by feeding forward the grid voltage processed by the CDSC algorithm,the dynamic response performance of the system is improved and the power quality of the grid-connected current can also be enhanced.Finally,the performance of the method is verified by simulation.In order to accurately estimate the phase angles of the unbalanced three-phase voltages,the energy operator can also be used to obtain the phase information by calculating the energy cross product.It can accurately get the phase angles even under phase unbalanced conditions.Finally,combining the aforementioned amplitude unitization algorithm,frequency-locking algorithm and discrete PR control,the overall control block diagram of the three-phase inverter is obtained.And the performance of the synchronous and grid-connected control method proposed in this paper is verified by simulation.It can be seen that the method proposed in this paper can quickly and accurately estimate the grid voltage information even when the grid voltage contains unbalanced amplitude,unbalanced phase angles,harmonic distortions and DC offset.And the output grid-connected current waveform has high quality.