Modeling, Control, and Stability Analysis of a Converter in Multi-Converter Systems Including Positive Feedforward Control.

In large interconnected systems such as advanced automotive systems, aircrafts, and more-electric ships, multi-converter systems consisting of converters and motor drives are increasingly being used to replace earlier mechanical, pneumatic, and hydraulic systems. All these power electronic systems have their advantages in terms of weight, size, flexible system configuration, high efficiency energy conversion, isolation, and ability to satisfy a variety of control objectives. In addition, the application of a modular system design approach also leads to multi-converter systems due to their advantages in terms of system reconfiguration and flexibility. However, multi-converter systems are prone to potential instability because of interactions between subsystems. The destabilizing effect by the subsystem interactions is known as the so called negative impedance instability phenomenon. That is because tightly regulated power electronic devices behave like constant power loads within their control bandwidth. In other words, the subsystem interactions affect the total system stability and dynamic performance, even though each subsystem is individually well-designed and standalone stable. This study addresses these problems and proposes a converter system modeling technique in a multi-converter system, a generalized stability criterion based on a converter system and a positive feedforward control technique to improve system stability. The small-signal modeling technique of a converter system in a multi-converter system using the g-parameter representation is developed to analyze the subsystem interactions under both feedforward and feedback control. In particular, the small-signal models including the subsystem interactions such as the source, the load, and simultaneous subsystem interaction are derived using Middlebrook's Extra Element Theorem (EET) and Two Extra Element Theorem (2EET). For stability analysis in a multi-converter system, a unified impedance criterion for both source and load subsystem interactions is proposed in the form of a minor loop gain using the Middlebrook's impedance criterion and the two EET. The unified impedance criterion is achieved by applying the Nyquist criterion to the so called minor loop gain, which is derived from the denominator of the correction factor obtained applying the 2EET. The unified impedance criterion can be also applied to a single interaction by just neglecting the other interaction. As an active approach to improve the stability which is degraded by the subsystem interactions, a positive feedforward (PFF) control technique is proposed. Compared with the conventional passive filter damping approach, it has advantages such as no modification of physical hardware, relatively good dynamic performance, and simple controller design using linear control design techniques. The PFF control, which is combined with the conventional negative feedback control, behaves like an active damping filter by creating positive input impedance at high frequencies while allowing the negative feedback controller to maintain tight output voltage regulation at low frequencies. As an illustrative example, a buck converter with an input filter and a constant power load are taken into consideration. The stability improvement by the PFF control is demonstrated through simulation and verified by experimental results. An FPGA based digital control technique is utilized to realize the controllers in the buck converter.