Line-commutated Rectifiers Research Articles

Direct Interfacing of Parametric Average-Value Models of AC–DC Converters for Nodal Analysis-Based Solution

AC–DC converters are widely used in many power-electronic-based systems. There is an increasing need to simulate such systems using larger time-steps in offline and/or real-time electromagnetic transient (EMT or EMTP) simulators. The so-called parametric average-value models (PAVMs) have been developed to allow larger time-steps and provide fast simulations. However, the application of PAVMs in nodal-analysis-based EMTP programs typically requires a one-time-step delay between the interfacing sources and the network solution (i.e., indirect interfacing), causing inaccuracy and numerical instability at medium-to-large time-steps. This paper presents a direct interfacing method for PAVMs of line-commutated rectifiers (LCRs). The proposed method linearizes the PAVM interfacing equations and incorporates the respective sub-matrices and history terms into the network nodal equations, which eliminates the need for a time-step delay. Simulation studies verify the effectiveness of the proposed method in EMTP-type solution wherein very good accuracy and numerical stability is achieved at fairly large time-steps, which has not been previously possible with conventional methods.

Hybrid Average-Value/Detailed Modeling of Line-Commutated AC–DC Converters With Internal Faults For Electromagnetic Transient Simulations

Recently, a hybrid average-value/detailed modeling technique has been presented for efficient electromagnetic transient (EMT) simulation of power systems containing ac-dc line-commutated rectifiers (LCRs). In this letter, the hybrid modeling methodology is extended to consider asymmetrical operation of LCRs due to internal faults of switches. The hybrid model permits large time-steps as opposed to conventional switching models that require precise handling of switching events and respectively small time-steps. The extended hybrid model is also demonstrated to possess numerical advantages over the existing state-of-the-art average-value models (AVMs) of faulty LCRs while providing more accurate results.

Hybrid Parametric Average-Value/Detailed Modeling of Line-Commutated Rectifiers

Simulations and analysis of power-electronic-based systems are conventionally done using detailed switching models of power-electronic converters that are available in many electromagnetic transient (EMT) simulation programs. Although being accurate, such detailed models typically require small time-steps for accurate detection and handling of switching events, which makes them computationally expensive. Recently, a parametric average-value modeling (PAVM) approach has been developed for system-level modeling and fast simulations of line-commutated rectifiers (LCRs) including several selected ac harmonics. In this paper, a new hybrid parametric methodology is presented, which has the capability of operating at large time-steps while including the details of the ac- and dc-side variables similar to the detailed switching models (but without the need for locating switching events, i.e., zero crossings), or operating as an average-value model. Extensive simulation studies demonstrate advantageous numerical efficiency and accuracy of the proposed hybrid parametric AVM/detailed model compared to the previous PAVMs as well as the detailed switching models of LCRs when using large time-steps for system-level studies.

Average-Value Modeling of Diode Rectifier Systems Under Asymmetrical Operation and Internal Faults

Line-commutated rectifiers (LCRs) are extensively utilized in exciters of large electrical machines, ac–dc power systems of ships, vehicles, aircraft, and many other industrial applications. Efficient and accurate computer simulations are necessary to analyze various aspects of such systems under both normal and unbalanced/faulty operating conditions. The so-called parametric average-value modeling (PAVM) technique has been recently developed and shown to provide accurate and computationally efficient models of power-electronic converters for system-level simulations. In this paper, the PAVM methodology is extended to LCRs with internal faults. The new formulation considers the asymmetrical operation of rectifiers by including the ac-side characteristic (i.e., 5th, 7th, etc.) and non-characteristic (i.e., 2nd, 3rd, 4th, etc.) harmonics in both positive and negative sequences as well as dc components that may be present on ac variables. The proposed PAVM is verified by experimental measurements and detailed model and is demonstrated to have excellent accuracy under various operating modes and fault configurations.

Generalized Parametric Average-Value Model of Line-Commutated Rectifiers Considering AC Harmonics With Variable Frequency Operation

Line-commutated rectifiers are often utilized in machine-converter systems and many energy conversion applications. Simulation of such power systems using detailed switching models of rectifiers is computationally expensive, and as an alternative for system-level studies, the so-called average-value modeling (AVM) techniques have become indispensable. The parametric AVM (PAVM) uses a computerized approach for establishing the key relationships between the averaged ac and dc variables. In this paper, a generalized PAVM (GPAVM) is proposed, which extends several previously proposed models. The new GPAVM includes the ac harmonics in thyristor-controlled rectifier models considering their nonlinear dependency on the line frequency. The new model is verified using detailed simulations and experimental results and is demonstrated to have better accuracy in a wider range of operating conditions and speeds/frequencies.

A Modular Active Front-End Rectifier With Electronic Phase Shifting for Harmonic Mitigation in Motor Drive Applications

In this paper, an electronic phase-shifting strategy has been optimized for a multiparallel configuration of line-commutated rectifiers with a common dc-bus voltage used in motor drive application. This feature makes the performance of the system independent of the load profile and maximizes its harmonic reduction ability. In order to further reduce the generated low-order harmonics, a dc-link current modulation scheme and its phase-shift values of multidrive systems have been optimized. Analysis, simulations, and experiments have been carried out to verify the proposed method.

Dynamic Phasor Modeling of Line-Commutated Rectifiers With Harmonics Using Analytical and Parametric Approaches

Line-commutated rectifiers (LCRs) are widely used in various industrial applications, wherein they are also known to be a significant source of harmonics. The dynamic phasor (DP) modeling approaches have been well studied in the literature to simulate the dynamics of power systems and their components including harmonics. In this paper, the state-of-the-art analytical DP (ADP) models of LCRs, which relate the ac and dc subsystems through complicated switch functions, are first investigated. Then, a new parametric DP (PDP) model of LCRs is proposed, wherein the DP dynamics of rectifier/dc-link are represented using a set of explicit algebraic functions that are numerically established. Rigorous computer studies demonstrate that the proposed PDP methodology is capable of accurately predicting the steady-state and transient responses of LCR systems under a wide range of loading conditions, while providing significant computational advantages over the conventional detailed model and the established ADP models.

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.

Decomposition of Power Electronic and Conventional Loads of Modern Power Systems by Discrete Wavelet Transform

Load composition data is used in different static and dynamic studies of present power systems. Increasing the number and capacity of power electronic loads in the modern power systems, especially microgrids, makes it necessary to perform various harmonic studies, beside considering this new load type in the conventional studies. For this purpose, new methods are required to decompose power electronic loads from conventional loads of power systems. This paper introduces a novel non-intrusive decomposition method, based on digital wavelet transform. The contribution of the proposed method is its ability to decompose most common power electronic loads, including uncontrolled/semi-controlled rectifiers with both constant power and resistive DC loads, in steady state and especially in transient and asymmetric conditions. Moreover, this method is significantly easier and more accurate than available machine learning-based decomposition methods. Another advantage of this paper is employing excellent denoising ability of digital wavelet transform, compared to conventional digital filters, for noise elimination of actual measured waveforms. The validity of proposed decomposition method is evaluated by simulation of dominating rectifiers in combination with induction motor and inductive load, as conventional load, in MATLAB/Simulink. Laboratory-implementation of the test system is also employed for further validation of the proposed method.Keywords: Denoising, Discrete Wavelet Transform, Line-Commutated Rectifiers, Load Decomposition, Microgrids, Transient Conditions

Direct Interfacing of Dynamic Average Models of Line-Commutated Rectifier Circuits in Nodal Analysis EMTP-Type Solution

Dynamic average-value models (AVMs) for AC-DC rectifier circuits are generally formulated in state-space form and hence are straightforward to implement in state-variable-based simulation languages. In the nodal analysis-based approach used in electromagnetic transient (EMTP-type) simulation packages, the development of AVMs requires additional effort to reformulate and interface the models with the external ac and dc networks. This paper proposes a new averaged-circuit model for three-phase line-commutated rectifiers which is directly interfaced with the ac and dc networks, thereby achieving a simultaneous solution of the respective variables in EMTP-type solution. The proposed model is verified against conventionally interfaced model, as well as detailed switching model of the original rectifier circuit. A significant improvement of numerical accuracy and stability of the solution is demonstrated even at fairly large time steps.

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.

Input Impedance Modeling and Analysis of Line-Commutated Rectifiers

This paper presents the modeling and analysis of small-signal input impedance for line-commutated rectifiers. The recently developed impedance mapping method, which has been applied to single-phase rectifiers with sinusoidal line inputs, is generalized to single-phase rectifiers with distorted lines as well as three-phase rectifiers. The modeling method assumes known impedance of the circuit on the dc side of the rectifier bridge (including its output filter and the load), and determines the corresponding ac input impedance by using a current and voltage mapping function that describes the operation of the rectifier bridge in the frequency domain. In this approach, closed-form analytical expressions are derived for the ac input impedance as functions of the dc circuit impedance. The resulting models are intended for stability analysis and design of large ac power systems involving significant number of rectification loads, such as more-electric aircraft power systems and microgrids. Detailed derivation of the models is presented in this paper. Numerical simulation and experimental measurement results are also presented to validate the developed models.

Harmonic improvement in 12-pulse series-connected line-commutated rectifiers

An innovative method for reducing current distortion on the ac side of 12-pulse series-connected line-commutated ac/dc rectifiers is presented. Furthermore, the overlap conduction of thyristors is completely eliminated. The methodology involves an accurate shaping of the dc current by using two self-commutated switches (IGBTs). This dc-current shaping is reflected back into the shaping of the ac input currents, which become pure sine waves. Both rectifying and inverting operations are possible with a simple control circuit, which is able to deal with both rapid load variations and failures in the self-commutated switches. HVdc applications of this innovative method are briefly examined, together with experimental results showing both the steady state and the transient behaviour of the technology by using a 380 V 20 kVA 50 Hz laboratory prototype.

Method and apparatus to reduce current distortion in line-commutated rectifiers

The use of a method and an apparatus to different configurations of line-commutated ac/dc rectifiers to reduce the distortion of currents flowing from the ac supply is described. The load may be either inductive or capacitive. The technology involves an accurate shaping of the dc current using two self-commutated switches (IGBT's). This dc-current shaping is reflected back into the shaping of the ac input currents, which become pure sine waves. Thyristor-based rectifying operation is possible with a simple control circuit, which is able to deal with both rapid load variations and failures in the self-commutated switches. Furthermore, the overlap conduction of bridge thyristors is eliminated completely. Experimental verification is provided from a 400-V, 30-kVA and 50-Hz laboratory prototype.

State variable decoupling and power flow control in PWM current-source rectifiers

Pulsewidth modulated (PWM) current-source rectifiers (CSR), among other alternatives, offer marked improvements over thyristor line-commutated rectifiers as a source of variable DC power. Advantages include reduced line current harmonic distortion and complete displacement power factor control, including unity displacement power factor operation. However, due to nonlinearities of the PWM-CSR model, their control has usually been carried out using direct line current control in a three-phase stationary frame (abc). This paper proposes the application of a nonlinear control technique that introduces more flexibility in the control of the rectifier and results in a more straightforward approach to controller design. The proposed technique is based on a nonlinear state variable feedback approach in the rotating frame (dq). The approach allows the independent control of the two components of the line current (active and reactive) with the same dynamic performance, regardless of the operating point. The control strategy also eliminates the need for input damping resistors and rejects the effect of supply voltage variations. Furthermore, a space vector modulation (SVM) technique is used to maximize the supply voltage utilization. This paper includes a complete formulation of the system equations and a controller design procedure. Experimental results on a 2 kVA digital-signal-processor-controlled prototype confirm the validity of theoretical considerations.

Power electronics in German railway propulsion

The state of the art of power electronics is reviewed. Because of the different requirements, such as suburban or long distance traffic and DC or AC supply, various systems are in use and some peculiar configurations, as resistor braking, have become important. With AC supply special line-commutated rectifiers causing low displacement factor and small harmonic content are used for DC motor propulsion. With DC supply choppers allow nondissipative motor control, often combined with special circuits for field weakening and for change of driving/breaking. Particularly important is three-phase propulsion with robust squirrel-cage motors, made possible by the development of suitable variable-frequency inverters. Both current source and voltage source inverters are used in suburban cars as well as for locomotive drives. The most universal but also most complicated system served by the voltage source inverter is described in more detail.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

PWM Control Techniques for Rectifier Filter Minimization

Minimization of input/output filters is an essential step towards manufacturing compact low-cost static power supplies. Three PWM control techniques that yield substantial filter size reduction for three-phase (self-commutated) rectifiers are presented and analyzed. Filters required by typical line-commutated rectifiers are used as the basis for comparison. Moreover, it is shown that in addition to filter minimization two of the proposed three control techniques improve substantially the rectifier total input power factor.