A high performance and cost effective drive based power conditioner for critical applications

In this paper, a high performance and cost effective power conditioner based on off-the-shelf adjustable-speed drives (ASDs) is presented for critical applications such as the CT, MRI type loads. The high performance resides in the load voltage regulation of ±1% in steady state with less than 3% total harmonic distortion, ±0.1 Hz with fixed output frequency (60 Hz), and fast dynamic step load response. The system frequency domain analysis, the PID controller design with a unique derivative compensating term based on the DC bus voltage, and active damping for improved load transient voltage characteristics are explored. The control is implemented in the ASD´s user programmable application software layer and interfaces seamlessly with the existing volts-per-hertz algorithms. The overall system simulation has been developed to demonstrate the steady state and load transient characteristics. The distinguishing control implementation is accomplished by using programming languages to comply with the IEC61131-3 standard. A prototype system consisting of a 150 hp, 460 V, 60 Hz ASD, a three-phase output LC filter, and a 75 kVA step-up isolation transformer has been built and tested to verify the analytical and simulation results. The significant contribution of this paper validates the feasibilities of using low cost off-the-shelf ASDs and their existing volts-per-hertz controls on non-motor driven applications such as the critical CT, MRI type loads in medical patient care and diagnostic areas.