Control Schemes For LCL-VSR Under Unbalanced Supply Voltage Conditions

The PWM voltage saurce converter(VSC) has been increasingly employed in recent years owing to its advanced features including sinusoidal input current, small size of input and output filter,bi-directional power flow, high control ability of grid-side active and reactive power, etc. Compear with L-type filter, LCL-filter has higher attenuation ability of current harmonics at higher frequencies, so it has been more and more employed in VSC system.The control schemes of voltage source PWM rectifier with an LCL filter under unbalanced supply voltage conditions is studyed in this paper. The LCL active damping control strategy and unbalanced contol strategy are combined.In the traditional output current balanced control scheme,third order harmonic appears in the output current because of the second order hrmonic consistent in the output current's positive sequence active power command,the control scheme is improved by adding an notch -filter to avoid the second order hrmonic coupling in the current commands, so the third order harmonic disappeared from the output currents; The DC voltage ripple suppression control schemes is improved to solve the problem of its complex output current command algorithm and difficult for implementation, the new control scheme is achieved by simplyfying the current command algorithm. Simulation and experiment works were taken to prove the proposed control schemes.Ths results show that the improved output current control scheme can eliminate the third order harmonic in the output currents; And the new current command algorithm is easy to implementation and the output voltage ripple is suppressed, at the same time the VSR can also been seen as operating in unit power factor under the unbalanced condition.When using LCL-VSC working as an inverter(VSI) in the DG system, it's expected to provide power to local loads after it disconnects from the grid, and achieve seamless transfer between the grid-connected mode and stand-alone mode to reduce the impact on critical local loads and the grid during the transition. A new seamless transfer control scheme based on multi-loop control structure is proposed in this paper. The grid connected current is directly controled in the grid-connected mode and the load voltage(capacity voltage) is directly controled in the stand-alone mode. The load voltage distortion problem caused by the startup of voltage loop during the transfer is solved due to the fact that in the new scheme the capatity voltage loop always keeps working. The unbalanced control algorithm is introduced in the seamless transfer scheme to elimination the negative sequence component of grid connected current in grid-connected mode and negative sequence component of capacity voltage in stand-alone mode cause by unbalanced local loads. So unbalanced loads will not be a problem of the system. Simulation and experiment results show that the new seamless transfer scheme make the load voltage only changed slightly between the transfer of the two modes, so the goal of seamless transfer is received. And loads unbalanced does not affect the system operation.