An Improved Control Strategy for the Current Source Back-to-Back Converters

The current source back-to-back converters have been largely neglected because of the slow transient response and low power factor, which is mainly resulted from a fixed PWM pattern and a poor control method. However, current source converters have intrinsic advantages such as the high reliability and the inherent current limiting capability. Due to these advantages, more and more people have been paying attention to the current source converters. In this paper, a current source back-to-back converter based on the front-end PWM current source rectifier, the DC link reactor, and the back-end PWM current inverter is proposed. A combination control of the output voltage control, the modulation index of the inverter, and the dc link current is proposed, which can maintain the dc link current to a value such that the modulation index of the inverter remains high at all times resulting in less energy loss and lower voltage harmonic distortion. In the rectifier side, an advanced power factor control is proposed, which can provide the desired constant dc link current and operate at the maximum achievable input power factor. In the inverter side, an improved voltage control based on the synchronous frame DQ PI is proposed, which can achieve zero steady-state error and have quick dynamic response to the load changes. Through the proposed control strategy, the current source back-to-back converter system can perform as well as the voltage source back-to-back converter system. All these analyses and the effectiveness of the control method are firstly demonstrated by simulation results, and are verified further in a 5kVA laboratory prototype.