Large- and Small-Signal Evaluation of Average Models for Multi-Pulse Diode Rectifiers Sebastian Rosado, Rolando Burgos, Fred Wang, and Dushan Boroyevich Center for Power Electronics Systems The Bradley Department of Electrical and Computer Engineering Virginia Polytechnic Institute and State University Blacksburg, VA 24061 USA Abstract—This paper analyses different average models of multi-pulse diode rectifiers. In the analysis a full-order and a simplified average model are compared to a detailed switching model. The main goal is to establish the validity of the simplified model. First the different models are discussed. Then the small and large signal evaluations of the models are presented. The input impedance frequency response of the models is used as basis for the small-signal evaluation. This parameter is important in the small-signal stability of the system. The large- signal evaluation is done by means of time domain simulations carried on with the different models. The results obtained indicate that the simplified average model can provide a good tool for the analysis of circuits using multi-pulse rectifiers. I. INTRODUCTION Multi-pulse diode rectifiers are usually used as front-end converters in aerospace power systems. Compared to other type of converters for the same applications multi-pulse rectifiers are of reduced dimensions, weight, and cost [1], [2]. In addition, increased power rating of the aircraft power system, as well as the variety of loads and their requirements are placing increased requirements on the system and components; this requires careful analysis. Part of this analysis deals with the stability of the system. For simulation based stability studies detailed switching models can be appropriate. On the other side for analytical studies, and when it is necessary to use small-signal linearized models it is useful to count with good average models. Two main approaches have been developed for the average modeling of line commutated converters, namely input-output transfer function [3] and closed-form mathematical expressions [4]. Although the former is very simple and straight forward to implement, its dependence on input-output lookup tables makes it cumbersome to use since any parameter variation normally requires numerous simulations to rebuild these tables [5], [6]. The mathematical approach on the contrary uses explicit equations to relate the input and output variables of the converters, however the complexity of these models is such that especially so for multi-pulse converters numerous simplifying assumptions are required, which end up affecting the accuracy of the model regarding its dynamic and frequency responses. Regardless of the approach This work made use of ERC Shared Facilities supported by the National Science Foundation under Award Number EEC-9731677. taken, the main issue at hand with this type of converter is that its operation depends on the ac system impedance and thus any model developed should capture this into its operation. This is naturally not practical, reason why these average models are developed taking into consideration only line and generator impedances [7]. In this paper an evaluation is realized with previously developed average models based on the mathematical description of the converter—eliminating the need of multiple simulations as in the input-output transfer function modeling approach. The models analyzed range in complexity from: simplified commutation representation to improved AC and DC dynamics. The models are subjected to transient simulations (and compared to a switching model), and small- signal models derived numerically from them are used to calculate the d-q frame input impedance. The results obtained fully characterize the accuracy of the models studied, showing which models are more propitious for time or frequency domain analysis. Insight is given as well into the physical meaning of the impedance calculations both in d-q frame and from the output DC port. All simulations and frequency domain analysis are presented in Matlab/Simulink and Synopsys Saber. II. MULTI-PULSE RECTIFIER MODELS A. Multi-pulse Rectifier Operation Multi-pulse diode rectifiers are composed of an autotransformer and one or more sets of six-pulse diode bridges. The number of bridges is given by the number of phases in the secondary side of the autotransformer, with a minimum of three phases corresponding to one diode bridge. In order to reduce the output voltage ripple the number of phases in the secondary side is increased by an appropriate interconnection of windings. Although the analysis to be presented can be applied to rectifiers of different number of pulses, the discussion concentrates on an eighteen pulse rectifier like the one in Figure 1. The autotransformer in Figure 1 has a three-phase primary and a nine-phase secondary. The nine-phases are equally spaced in the line frequency period being the electrical angle between phases of forty degrees. The output can also be seen as three sets of three-phases equally spaced and each one 0-7803-9724-X/06/$20.00 ©2006 IEEE. 89 2006 IEEE COMPEL Workshop, Rensselaer Polytechnic Institute, Troy, NY, USA, July 16-19, 2006 0-7803-9725-8/06/$20.00 ©2006 IEEE. Authorized licensed use limited to: Dahono Pekik. Downloaded on March 26, 2009 at 00:14 from IEEE Xplore. Restrictions apply.