Improved Stationary Frame AC Current Regulation using Feedforward Compensation of the Load EMF

Current regulation plays a key role in modern power electronic AC conversion systems. The most obvious strategy is to use a simple closed loop PI regulator. However it is well known that this approach cannot achieve zero steady state error because of the instability that inevitably occurs as the PI gains are increased to reduce this error. Linear analysis identifies that an AC load controlled by a PI regulator is a second order system with no theoretical stability limits as the gains are increased. Hence other factors must be responsible for the gain limits that are known to occur in practice. This paper revisits the use of a PI regulator to control AC currents and identifies that it is control loop delays that limit the PI regulator's gains. It is this limit that then constrains the controller's performance, particularly when the system feeds into backemf type loads. Methods are then presented to analytically determine the best possible gains that can be achieved within this constraint, and to improve the performance of a PI regulator by incorporating feedforward compensation of the load backemf into the control loop.