Hamid Atighechi

Dynamic Average Modeling of Front-End Diode Rectifier Loads Considering Discontinuous Conduction Mode and Unbalanced Operation

Electric power distribution systems of many commercial and industrial sites often employ variable frequency drives and other loads that internally utilize dc. Such loads are often based on front-end line-commutated rectifiers. The detailed switch-level models of such rectifier systems can be readily implemented using a number of widely available digital programs and transient simulation tools, including the Electromagnetic Transient (EMT)-based programs and Matlab/Simulink. To improve the simulation efficiency for the system-level transient studies with a large number of such subsystems, the so-called dynamic average models have been utilized. This paper presents the average-value modeling methodologies for the conventional three-phase (six-pulse) front-end rectifier loads. We demonstrate the system operation and the dynamic performance of the developed average models in discontinuous and continuous modes, as well as under balanced and unbalanced operation.

Dynamic Average-Value Modeling of 120° VSI-Commutated Brushless DC Motors With Trapezoidal Back EMF

The 120° voltage source inverter driven brushless dc (BLDC) motors are very common in many applications. This paper extends the previous work and presents an improved dynamic average-value model for such BLDC motor-drive systems. The new model is explicit and uses a proper qd model of the permanent magnet synchronous machine with nonsinusoidal rotor flux. The model utilizes multiple reference frame theory to properly include the back EMF harmonics as well as commutation and conduction intervals into the averaged voltage and torque relationships. The commutation angle is readily obtained from the detailed simulation. The proposed model is demonstrated on a typical industrial BLDC motor with trapezoidal back EMF waveforms. The results of studies are compared with experimental measurements as well as previously established models, whereas the new model is shown to provide appreciable improvement.

Average-Value Modeling of Synchronous-Machine-Fed Thyristor-Controlled-Rectifier Systems

Due to the repeated switching, the detailed switch-level models of electrical machines coupled with power-electronic converters are computationally expensive and hard to linearize for small-signal frequency-domain analysis. Average-value modeling (AVM) has become an effective tool for small-signal analysis of power electronic systems and time-domain transient studies where the details of switching are not important and can be neglected. Recently, a parametric AVM (PAVM) approach has been developed for machine/diode rectifier systems. This paper extends the parametric approach to the machine/thyristor-controlled-rectifier systems, where the thyristor firing may be referenced to either the line voltages or the rotor position. An analytical average model for this system is also developed based on the recently proposed constant-parameter voltage-behind-reactance synchronous machine model. The new PAVM is compared against the original switching system, as well as the analytical AVM. It is shown that the PAVM can accurately predict both small-signal characteristics and large-signal transients of the original switching system in light and heavy modes, which represents an advantage over the analytical models which are typically implicit.

Steady-State and Dynamic Performance of Front-End Diode Rectifier Loads as Predicted by Dynamic Average-Value Models

Summary form only given. The detailed switch-level models of front-end diode rectifier loads can be readily implemented using a number of transient simulation programs, such as PSCAD/EMTDC, and the toolboxes in Matlab/Simulink. To improve the simulation efficiency for the system-level studies, the so-called dynamic average models have been widely used by researchers and engineers. Recently, several average-value modeling methodologies for the conventional three-phase (six-pulse) front-end rectifier loads have been discussed, and the dynamic performance of several developed models has been demonstrated in discontinuous and continuous modes. In this paper, the effects of topological variations of the ac-side filters on the system performance are investigated. Also, the steady-state and dynamic impedances predicted by the average models under balanced and unbalanced operation are compared. The studies and analyses presented here extend and complement those set forth in the preceding companion publication.

Dynamic Average-Value Modeling of CIGRE HVDC Benchmark System

High-voltage direct-current (HVDC) systems play an important role in modern energy grids, whereas efficient and accurate models are often needed for system-level studies. Due to the inherent switching in HVDC converters, the detailed switch-level models are computationally expensive for the simulation of large-signal transients and hard to linearize for small-signal frequency-domain characterization. In this paper, a dynamic average-value model (AVM) of the first CIGRE HVDC benchmark system is developed in a state-variable-based simulator, such as Matlab/Simulink, and nodal-analysis-based electromagnetic transient program (EMTP), such as PSCAD/EMTDC. The 12-pulse converters in the HVDC system are modeled with a set of nonlinear algebraic functions that are extracted numerically. The results from the average-value models are compared with the results of the detailed simulation to verify the accuracy of the AVMs in predicting the large-signal time-domain transients. The developed dynamic average models are shown to have computational advantages.

Approximate dynamic average-value model for controlled line-commuted converters

Dynamic average-value modeling of power electronic-based systems has evolved and is widely used for system-level transient simulations and small-signal analysis. This paper presents an approximate average model for controlled line-commutated converters, which is developed based on the parametric average modeling methodology previously established for the diode bridge rectifiers. The proposed model is compared against a previously-developed analytical average-value model as well as a detailed switch-level implementation of the system. The new model is demonstrated to have good accuracy in both time- and frequency — domain for predicting the system-level behavior.

A Generalized Methodology for Dynamic Average Modeling of High-Pulse-Count Rectifiers in Transient Simulation Programs

High-pulse-count rectifier systems (with more than six pulses) are often used in high-power applications. Dynamic average value modeling (AVM) has proven to be indispensable for simulation and analysis of such power electronic systems. However, developing accurate AVMs for such line-commutated converters is challenging due to the presence of complicated switching patterns (operating modes), configuration of multi-phase transformers and/or rotating machines, inter-phase transformers, filters, etc. This paper presents a generalized methodology for full-order dynamic average modeling of high-pulse-count line-commutated rectifiers, which is based on the recently proposed parametric AVM approach. Extensive simulation studies are carried out considering the 18-pulse rectifier example system. The accuracy and numerical advantages of the developed generalized AVM is verified against the detailed switching model, and a previously established average model in time and frequency domains.

Using Multiple Reference Frame Theory for Considering Harmonics in Average-Value Modeling of Diode Rectifiers

Line-commutated converters (LCCs) are widely used in various high-power applications such as generator-rectifier systems, exciters, front-end rectifier loads, and classic high voltage dc systems. Among various techniques used for modeling LCC systems, the dynamic average value modeling (AVM) wherein the effect of switching is neglected or averaged over a prototypical switching interval has become indispensible since it results in continuous, linearizable, and computationally efficient models. The conventional AVMs can only predict the fundamental component of the ac voltages and currents, and neglect the harmonics injected at the ac side by the switching converter. In this paper, a recently proposed parametric average-value modeling (PAVM) approach is extended using multiple reference frame theory to include the significant harmonics of interest (e.g., fifth and seventh) for diode rectifiers. The new PAVM is verified against the detailed simulation in steady-state and transient studies, and is effective in predicting the transient waveforms, while achieving significant computational advantage (speed up) in time-domain simulation over conventional models.

Verification of Parametric Average-Value Model of Thyristor-Controlled Rectifier Systems for Variable-Frequency Wind Generation Systems

Line-commutated thyristor-controlled rectifiers are often used in many industrial and renewable energy applications where controllable dc voltage is required. The derivation of accurate dynamic average-value models for thyristor-controlled systems is challenging. The recently proposed parametric average value modeling (PAVM) avoids the discrete switching states of the converters and results in computationally efficient models that are suitable for system level studies. This paper extends the PAVM recently developed for synchronous-machine-fed thyristor-controlled-rectifier systems to a permanent magnet synchronous machine wind generation system where the operation in variable speed and frequency is required.

Numerical average value modeling of second order flyback converter in both operational modes

Average-value modeling (AVM) provides an efficient tool for studying power electronic systems, including DC/DC converters. Analytical derivation of average models that are valid in different operating modes is difficult, especially when parasitics and non-idealities are included in converter circuit. Recently, a numerical computer-aided method of obtaining the average-value model of power electronic systems has been proposed based on corrected state space average-value modeling, wherein several parametric functions are obtained using the detailed simulation. This paper considers the second order flyback converter, which has transformer isolation and additional losses that have not been sufficiently addressed in the prior literature. The paper extends the parametric AVM approach and suggests a modified averaged circuit model for this type of converter. The proposed models are compared in predicting both large signal time-domain transients and small-signal frequency domain characteristics.