Flatness-Based Discrete-Time Control of a Battery Emulator Driving a Constant Power Load

In this article, a new method for controlling battery emulators in the shape of bidirectional step-down converters driving constant power loads (CPL) is presented. The CPL introduces nonlinear behavior into the system and causes unstable system dynamics. This property and the discontinuity introduced by the switches make it challenging to develop a suitable control strategy. Usually, switching frequencies are assumed to be fast enough for the controller to neglect the fact that the switches are either ON or OFF. As the desired dynamics of the system approaches the switching frequency of the converter, the discontinuity of the switches can no longer be ignored and the controller has to generate a suitable piecewise constant control input. A novel approach for arriving at a piecewise constant control input is presented and compared to state-of-the-art approaches. The advantages of the new control method are demonstrated with simulations as well as measurements from a hardware-in-the-loop testbed.