Detailed Switching Model Research Articles

Bidirectional Power Flow Control in a DC Microgrid Through a Switched-Capacitor Cell Hybrid DC–DC Converter

This paper focuses on a bidirectional hybrid dc–dc converter suitable as an interface between two dc voltage buses in various applications including microgrids. The switched-capacitor cell, incorporated in the converter topology, gives the advantage of high voltage conversion ratio without using a transformer. This paper analyses the converter operation and the stability in the step-up and step-down operating modes through the state-space averaging method and through the pulse width modulation switch model method applied on an equivalent circuit model. The converter employs a current controller designed in frequency domain based on the Bode plot, using the K factor method. The simulation results obtained by means of an average and a detailed switching model prove, in good correspondence, that the controller is able to track the reference current waveform with good dynamic performance . Experimental results obtained from a 2 kW converter prototype confirm the theoretical considerations and the simulation results.

Filament Shape Dependent Reset Behavior Governed by the Interplay between the Electric Field and Thermal Effects in the Pt/TiO2/Cu Electrochemical Metallization Device

The electrochemical metallization (ECM) cell is a feasible contender for high density resistance switching random access memory or neuromorphic devices. This work elucidates the detailed switching model based on the Cu conducting filament (CF) configuration and the interplay between the Joule heating and electric field effects in the Pt/TiO2/Cu ECM cell, which can explain the switching behaviors both in accordance and discordance with the conventional ECM theory. The Cu CF configuration is varied from the conventional conical one for a small compliance current (Icc, ≈1 mA) to the hourglass or even cylindrical one by adopting a high Icc value (≈30 mA). The rupture process of the Cu CF can be precisely modeled by considering the mutual constructive interference or the competition between the Joule heating and electric field effects for the rupture and rejuvenation of the Cu CF. These models are supported by an extensive thermal‐field simulation in 2D, and an hourglass‐shaped CF is identified by transmission electron microscopy, which is in accordance with the suggested model with a moderate Icc. The detailed geometry of the formed CF and relative bias polarity during the reset step play a critical role in determining the bipolar or unipolar switching mode.

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.

Load Model for Medium Voltage Cascaded H-Bridge Multi-Level Inverter Drive Systems

Medium voltage drives (MVDs) are commonly used in high-power applications and show significant impact on the overall system dynamics due to their large size and high power demand. Although detailed switching models for MVDs can be built using MATLAB/Simulink, such models cannot be used in large-scale simulation software for power system dynamic studies. To solve this problem, the dynamic load model for the medium voltage cascaded H-bridge multi-level inverter drive and induction motor systems, which is suitable for power system dynamic studies, is proposed in this paper. Analytical formula of the model is presented. The model includes the aggregated effect of an MVD, an induction motor, and their control system, and thus, it can accurately represent the dynamic responses of the motor drive system under disturbances. Both voltage and frequency dependence are considered in the model. The accuracy of the model is verified by a case study. A sensitivity study is conducted to evaluate the impact of the model parameter variation on dynamic response characteristics. The developed load model can be readily inserted in the large-scale power system simulation software for power system dynamic studies.

Dynamic Phasor Modeling of Multi-Generator Variable Frequency Electrical Power Systems

The dynamic phasor (DP) concept has been widely used in modeling electrical power systems. So far, the DP concept has been restricted to modeling systems with one single electrical source at a fixed fundamental frequency, either one generator or an ideal three-phase AC source. This paper aims to extend the DP modeling methodology to a wider application area. Two major achievements have been introduced: 1) application of DPs for multi-source, multi-frequency systems; 2) modeling of systems with time-varying frequencies. These two techniques enable the use of DPs to study nearly all types of electrical power systems (EPS). The developed theory is validated using a twin-generator system from the More Open Electrical Technologies (MOET) project. The accuracy and effectiveness of the developed models is confirmed by comparing the simulation results of detailed switching models and DP models under both balanced and unbalanced conditions.

Functional Modeling of Symmetrical Multipulse Autotransformer Rectifier Units for Aerospace Applications

This paper aims to develop a functional model of symmetrical multipulse autotransformer rectifier units (ATRUs) for more-electric aircraft (MEA) applications. The ATRU is seen as the most reliable way readily to be applied in the MEA. Interestingly, there is no model of ATRUs suitable for unbalanced or faulty conditions at the moment. This paper is aimed to fill this gap and develop functional models suitable for both balanced and unbalanced conditions. Using the fact that the dc voltage and current are strongly related to the voltage and current vectors at the ac terminals of ATRUs, a functional model has been developed for the asymmetric ATRUs. The developed functional models are validated through simulation and experiment. The efficiency of the developed model is also demonstrated by comparing with corresponding detailed switching models. The developed functional model shows significant improvement of simulation efficiency, especially under balanced conditions.

Fast Voltage-Balancing Control and Fast Numerical Simulation Model for the Modular Multilevel Converter

A methodology of fast voltage-balancing control based on average comparison and a fast numerical simulation model for the modular multilevel converter are proposed. In each control cycle, the average voltage of all the submodules (SMs) in an arm is calculated. The switching state of each SM will be determined by comparing the capacitor voltage of each SM with this average value. Little sorting of the capacitor voltages is required; therefore, the calculation burden on the controller is significantly reduced. The previous switching state of each SM will be mostly retained. Two variables $\Delta U _{{C}\_{\rm sort}}$ and $\Delta U _{{C}\_{\max}}$ are introduced to adjust the equivalent switching frequency of each SM. Simulation results verified the effectiveness of the proposed voltage-balancing control in normal and fault conditions. A fast numerical simulation model for MMC on PSCAD/EMTDC is also proposed. The dynamics of each SM are sufficiently represented in the proposed model. A hybrid simulation model is further developed to incorporate the detailed switching dynamics of specific interested SMs. The simulation results show that the proposed models give almost identical results as the detailed switching models while the simulation speed of the proposed model is approximately 5000 times faster than the detailed switching model.

Average Modelling of Medium Frequency DC–DC Converters in Dynamic Studies

This paper studies the average value modelling (AVM) of medium frequency (200 Hz-2000 Hz) dc-dc converters, which are becoming increasingly important in high-power dc grids and distributing microgrids. The most convenient modelling is based on transfer function and power components which are available in common simulation platforms, such as PSCAD/EMTDC. However, these models are found to require very small simulation steps and the achieved time-domain accuracy is not satisfactory for medium frequency converters. The core issue of accuracy loss is identified and corresponding improvement is proposed. User written state-space models in the abc frame and the dq frame are analyzed in depth. Different solvers, including the Dommel's method, the Heun's method, and the Runge-Kutta method were adopted and compared in solving the state-space models. A high-power 2-kHz LCL dc-dc converter is taken as the test system. It is found that the AVM in the dq rotating frame based on the Runge-Kutta solver is able to generate very accurate time-domain responses compared to the detailed switching model with almost 100 times improvement in simulation speed. It is demonstrated that a dc-dc converter with 2-kHz inner ac circuit frequency can be accurately simulated at a 50- μs time step.

Dynamic average modeling of a bidirectional solid state transformer for feasibility studies and real-time implementation

Detailed switching models of power electronics devices often lead to long computing times, limiting the size of the system to be simulated. This drawback is especially important when the goal is to implement the model in a real-time simulation platform. An alternative is to use dynamic average models (DAM) for analyzing the dynamic behavior of power electronic devices. This paper presents the development of a DAM for a bidirectional solid-state transformer and its implementation in a real-time simulation platform. Several case studies have been carried out in order to evaluate the behavior of the model under different operating conditions, check its feasibility for power quality improvements and explore the implementation in a real-time simulation platform.

Multiconverter Medium Voltage DC Power Systems on Ships: Constant-Power Loads Instability Solution Using Linearization via State Feedback Control

Bus voltage stability is a key issue in future medium-voltage DC (MVDC) power systems on ships. The presence of high-bandwidth controlled load converters (Constant Power Load, CPL) may induce voltage instabilities. A control design procedure is presented which starts at the modeling level and comes to control implementation. A control method based on a Linearization via State Feedback (LSF), is proposed to face the CPL destabilizing effect and to ensure the MVDC bus voltage stability. A multiconverter shipboard DC grid is analyzed by means of a new comprehensive model, which is able to capture the overall behavior in a second-order nonlinear differential equation. Exploiting DC-DC converters that interface power sources to the bus, LSF technique is able to compensate for system nonlinearities, obtaining a linear system. Then, traditional linear control techniques can be applied to obtain a desired pole placement. With reference to system parameters mismatch, LSF control design is verified by means of a sensitivity analysis, evaluating the possibility of an over-linearization strategy. Time-domain numerical simulations are used to validate the proposed control, in presence of relevant perturbations by means of a two-way comparison (average value model and detailed switching model).

Modeling of a PV system with grid code compatibility

Increasing penetration of Renewable Energy Sources (RES) to the power systems leads to the continuous evolution of grid interconnection requirements, which now include Low Voltage Ride Through (LVRT) practically for all types of generating units connected to the MV or HV networks, including photovoltaics (PV). It is essential that the Distributed Generation (DG) models used in power system studies properly represent the enhanced response characteristics of the stations. In this paper, a model is presented for a PV system with grid-code compatibility, which is suitable for power system dynamic and transient analysis studies. For this purpose, an overall control strategy is introduced to achieve LVRT capability, while elements essential for the correct representation of the dynamic performance of the system, such as the Phase Locked Loop (PLL) and the output LCL filters, are properly accounted for. Time domain simulations are provided for balanced and unbalanced voltage dips, in order to demonstrate the performance of the PV system and evaluate the accuracy of the proposed simplified model. The response of the detailed switching model is also included in the simulation results.

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.

Analysis and Control of Modular Multilevel Converters Under Unbalanced Conditions

This paper investigates the contents of submodule voltage ripples, circulating currents, and internal converter voltage in modular multilevel converters (MMCs) theoretically. The operation of MMCs is studied under asymmetry of the upper and lower arm, as well as the unbalanced ac system. In three-phase MMCs, the analysis shows that the second harmonic circulating currents will be asymmetric under an unbalanced ac system and can be decomposed into positive-, negative- and zero-sequence parts. The positive- and negative-sequence components affect neither the ac side nor the dc side of MMCs while the zero-sequence components will flow into the dc side, aggravating the power fluctuations of the dc side. In order to solve this problem, a new controller based on the instantaneous power theory and proportional-resonant scheme is designed. Simulations with a detailed switching model on the PSCAD/EMTDC platform verify the theoretical analysis, and demonstrate that the proposed controller eliminates the active power fluctuations and suppresses the harmonic circulating currents as well.

Hybrid converter with ac side cascaded H‐bridge cells against H‐bridge alternative arm modular multilevel converter: steady‐state and dynamic performance

This study explorers the operating principles of the hybrid converter with ac side cascaded H-bridge cells and the H-bridge alternative arm modular multilevel converter, where the main objective is to compare their steady-state and transient performance. This comparison focuses on steady-state aspects such as active and reactive power control and semi-conductor losses and the transient performance of these converters during ac and dc network faults. To facilitate this comparison, detailed switch models of both high-voltage direct current (HVDC) converters are developed and simulated in Matlab-Simulink. The major outcomes are discussed and strengths and weakness associated with each converter are highlighted.

Efficient Approaches for Modeling and Simulating Photovoltaic Power Systems

Modeling and simulation of photovoltaic (PV) power systems have become increasingly important with wide acceptance and integration of solar energy in modern electric grids. The transcendental nonlinear equations describing the PV generator, which are coupled with the detailed switching models of the power electronic converters, generally result in slow and inefficient simulations, especially when long-term analyses are required. This paper focuses on simple and efficient modeling approaches that are suitable for long-term and large PV system analyses. This study provides a simplified PV-cell model and its parameterization, guaranteeing that the I-V characteristic curves pass through the typical points given in manufacturers' datasheets. Furthermore, several power interface models are provided for fast simulation purpose. A classical two-stage power processing system with intermediate dc link used as a string inverter, as well as a single-stage conversion unit used in distributed module-dedicated PV applications, are taken as application examples. The generalized modeling approach is thoroughly evaluated by comparing the simulation results with the experimental data of a practical 2.4-kW grid-tied PV solar unit. The proposed methodology is shown to have advantages over conventional modeling approaches to simulate long-term grid-tied operation.

Modeling and Control System Design of a Grid Connected VSC Considering the Effect of the Interface Transformer Type

In this paper, modeling and control system design of a three-phase VSC system are investigated. The linear model and the control design are developed in the stationary-frame. The model shows a highly coupled system along and coordinates when using a -Y transformer vs. two completely decoupled subsystems in the case of Y-Y transformer. This issue was often overlooked in literature. The developed model includes the phase shift caused by the -Y transformer to address its effect on the steady-state and the dynamic behaviors under balanced and unbalanced distorted three phase conditions. Linear state-space model of the closed-loop system is developed to study the dynamics and stability of the two configurations. Modal analysis shows that the phase shift introduced by the -Y configuration can reduce the ultimate gain of the open loop system by 62% as compared to the system interfaced using Y-Y configuration. The developed linear model has been simulated in MATLAB/SIMULINK and validated using a detailed switching model in PSCAD/EMTDC under balanced and unbalanced three-phase conditions. The proposed control method can provide fast active and reactive power control with a minimum harmonics distortion in the line current.

Optimized InfiniBandTM fat‐tree routing for shift all‐to‐all communication patterns

AbstractClustered systems have become a dominant architecture of scalable high‐performance super computers. In these large‐scale computers, the network performance and scalability is as critical as the compute‐nodes speed. InfiniBandTM has become a commodity networking solution supporting the stringent latency, bandwidth and scalability requirements of these clusters. The network performance is also affected by its topology, packet routing and the communication patterns the distributed application exercises. Fat‐trees are the topology structures used for constructing most large clusters as they are scalable, maintain cross‐bisectional‐bandwidth (CBB), and are practical to build using fixed‐arity switches. In this paper, we propose a fat‐tree routing algorithm that provides a congestion‐free, all‐to‐all shift pattern leveraging on the InfiniBandTM static routing capability. The algorithm supports partially populated fat‐trees built with switches of arbitrary number of ports and CBB ratios. To evaluate the proposed algorithm, detailed switch and host simulation models were developed and multiple fabric topologies were run. The results of these simulations as well as measurements on real clusters show an improvement in all‐to‐all delay by avoiding congestion on the fabric. Copyright © 2009 John Wiley & Sons, Ltd.