Sina Chiniforoosh

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.

Evaluating misalignment of hall sensors in brushless DC motors

Brushless DC (BLDC) motors are used in a wide variety of applications and have been well investigated in the literature. In a typical low-cost drive the voltage source inverter is controlled by the signals produced by three Hall effect sensors that are used to detect the rotor position. These sensors are supposed to be positioned exactly 120 electrical degrees apart. However, this assumption may not always be true especially in medium- and low-precision motors. This paper demonstrates the effects of misaligned Hall sensors on stator currents and resulting torque, and presents a method for evaluating misplacement of each Hall sensor. Several industrial prototype BLDC motors are used to support the investigation and verify the developed methodology.

Dynamic average modeling of line-commutated converters for power systems applications

Detailed switch-level models of high-pulse-count converters are relatively straightforward to implement using commonly available simulation packages used for digital time-domain simulations and studying of power systems transients. However, such models are computationally intensive due to switching, and could become the bottle-neck for system-level studies with large number of components and controllers. This paper describes approaches for developing dynamic average-value models, i.e., the analytical derivations and parametric modeling. The resulting approximate models do not represent switching but still capture the transient behavior of the original converter circuit. The paper presents the results for 3- and 6-phase rectifiers implemented in PSCAD/EMTDC and Matlab/Simulink and shows that dynamic average models can be very effective. The paper also shows that as the number of pulses/phases increases, so does the complexity of switching pattern that defines the operating modes.

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.

An improved V/F control scheme for symmetric load sharing of multi-machine induction motor drives

The traditional low-cost Volts-per-Hertz (V/F) induction motor (IM) drives typically operate based on speed command, whereas the developed torque is consequently determined according to the torque-speed characteristics of the machine. In multi-machine load-sharing applications, it is preferred to have number of identical IMs; whereas in practice, deviations among motor parameters is probable and will result in disproportionate sharing of the mechanical load and even overloading one or several machines. In this paper, an improved V/F scheme is presented, which compensates for possible variations in the motor parameter (e.g. rotor resistance) and balances the load accordingly. The new method is shown to be effective and easy to implement, and may be readily extended to an arbitrary number of motors driving a common load.

Averaged-circuit modeling of line-commutated rectifiers for transient simulation programs

Dynamic average-value models (AVM) for line-commutated rectifier circuits are generally formulated in a state-space form and hence are straightforward to implement in state-variable-based simulation languages. In nodal-analysis-based languages, however, developing AVMs requires additional effort to reformulate the models and interface them with the external circuit networks. This paper proposes an averaged-circuit model for representing three-phase line-commutated rectifier in nodal-analysis-based simulation languages. The model derivation and its interface with the network are presented. The proposed model is verified against a detailed switch-level implementation of the system.

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.