This paper investigates the capacitor-current-feedback active damping for the digitally controlled LCL-type grid-connected inverter. It turns out that proportional feedback of the capacitor current is equivalent to virtual impedance connected in parallel with the filter capacitor due to the computation and pulse width modulation (PWM) delays. The LCL-filter resonance frequency is changed by this virtual impedance. If the actual resonance frequency is higher than one-sixth of the sampling frequency (fs/6), where the virtual impedance contains a negative resistor component, a pair of open-loop unstable poles will be generated. As a result, the LCL-type grid-connected inverter becomes much easier to be unstable if the resonance frequency is moved closer to fs/6 due to the variation of grid impedance. To address this issue, this paper proposes a capacitor-current-feedback active damping with reduced computation delay, which is achieved by shifting the capacitor current sampling instant towards the PWM reference update instant. With this method, the virtual impedance exhibits more like a resistor in a wider frequency range, and the open-loop unstable poles are removed; thus, high robustness against the grid-impedance variation is acquired. Experimental results from a 6-kW prototype confirm the theoretical expectations.

Capacitor-Current-Feedback Active Damping With Reduced Computation Delay for Improving Robustness of LCL-Type Grid-Connected Inverter

A phase-locked loop (PLL) is a nonlinear negative-feedback control system that synchronizes its output in frequency as well as in phase with its input PLLs are now widely used for the synchronization of power-electronics-based converters and also for monitoring and control purposes in different engineering fields In recent years, there have been many attempts to design more advanced PLLs for three-phase applications The aim of this paper is to provide overviews of these attempts, which can be very useful for engineers and academic researchers

/pdf/three-phase-plls-a-review-of-recent-advances-lyks7vfgc3.pdf

Three-Phase PLLs: A Review of Recent Advances

The interconnection stability of a grid-connected voltage-source converter (VSC) can be assessed by the passivity properties of the VSC input admittance. If critical grid resonances fall within regions where the input admittance acts passively, i.e., has nonnegative real part, then their destabilization is generally prevented. This paper presents an overview of passivity-based stability assessment, including techniques for space-vector modeling of VSCs whereby expressions for the input admittance can be derived. Design recommendations for minimizing the negative-real-part region are given as well.

/pdf/passivity-based-stability-assessment-of-grid-connected-vscs-4bfeazafk3.pdf

Passivity-Based Stability Assessment of Grid-Connected VSCs—An Overview

The virtual impedance concept is increasingly used for the control of power electronic systems. Generally, the virtual impedance loop can either be embedded as an additional degree of freedom for active stabilization and disturbance rejection, or be employed as a command reference generator for the converters to provide ancillary services. This paper presents an overview of the virtual-impedance-based control strategies for voltage-source and current-source converters. The control output impedance shaping attained by the virtual impedances is generalized first using the impedance-based models. Different virtual impedances and their implementation issues are then discussed. A number of practical examples are demonstrated to illustrate the feasibility of virtual impedances. Emerging applications and future trends of virtual impedances in power electronic systems conclude this paper.

Virtual-Impedance-Based Control for Voltage-Source and Current-Source Converters

The control of a grid-connected voltage source inverter with an inductive-capacitive-inductive (LCL) filter is a very challenging task, since the LCL network causes a resonance phenomenon near to the control stability boundary. While many active damping methods have been proposed to overcome this issue, the role that pulse width modulation transport delay plays in the effectiveness of these strategies is still not fully resolved. This paper presents a theoretical discrete time-analysis framework that identifies three distinct regions of LCL filter resonance, namely, a high resonant frequency region where active damping is not required, a critical resonant frequency where a controller cannot stabilize the system, and a low resonant frequency region where active damping is essential. Suitable controllers are then proposed for the two stable regions, with gain calculations that allow for the greatest system bandwidth and damping. Simulation and experimental results verify the presented analysis.

Regions of Active Damping Control for LCL Filters

A sectioned-winding permanent-magnet linear synchronous motor (SW-PMLSM) is proposed in this paper, whose iron core is continuous, whereas winding is divided. The model of the SW-PMLSM is derived after the section structure and the winding distribution are introduced. The basic theory and implementation of the current error vector-based predictive control (CEVPC) and space-vector pulse-width modulation deadbeat-predictive control (SVPWM-DBPC) are proposed. The moving stroke of the SW-PMLSM is divided into five zones according to the position of the secondary, and the implementations for both control methods are investigated in the five zones. Simulation results of both control methods validate the excellent dynamic performance and show that the SVPWM-DBPC is better than the CEVPC.

Comparison of the Two Current Predictive-Control Methods for a Segment-Winding Permanent-Magnet Linear Synchronous Motor

This paper presents a design-oriented transient stability analysis of a droop-controlled voltage-source converter (VSC), where the effect of reactive power control is in focus. By looking into the dynamic coupling between the active power loop and the reactive power loop, a large-signal model of the droop controller is built first. Based on this model, the transient responses of the droop-controlled VSC under grid faults are then evaluated using the phase portrait. It reveals that the reactive power control will generate a transient voltage drop to the VSC, which takes a positive feedback effect and deteriorates the transient stability. Fortunately, such a voltage drop can be offset by raising the reactive power reference. Finally, simulations and experiments are performed to verify the theoretical analysis.

/pdf/transient-stability-impact-of-reactive-power-control-on-grid-2yyv4rkcp6.pdf

Transient Stability Impact of Reactive Power Control on Grid-Connected Converters

The key technology of the air-core monopole linear motor (AMLM), a core component of precision motion systems, is the distribution of the electromagnetic structure and cooling structure. Thus, AMLM can achieve small thrust fluctuation, high thrust density, and low surface temperature rise. Consequently, the multiobjective optimal design of AMLM under the multiphysical fields has become the key technology of AMLM. Consideration of the strict requirements of AMLM model calculational accuracy will cause an excessively long computation time for the finite-element analysis (FEA) software three-dimensional (3-D) model, and it is not convenient for an optimal computation of the multiphysical fields model. Therefore, a surface magnetic charge model and an image method are proposed in the paper to establish an air gap magnetic field model of AMLM. The saturation characteristic of magnetic materials can be reflected through saturation coefficient of the model. In addition, the air gap field distribution status of AMLM can be better predicted. An AMLM thermal field model is established by adopting the thermal network method. In the model, the heat transfer coefficient of cooling water is calculated based on the current curve obtained from the FEA software. Parametric modeling was performed on the AMLM electromagnetic and cooling structure through definition of 7-D proportionality coefficients. Based on the aforementioned electromagnetic field and the thermal field model, genetic algorithm (GA) was adopted for AMLM optimization design by setting cooling capacity and size as constrain conditions as well as by setting thrust density, thrust fluctuation, mover's mass, and motor constants as optimization objects. The AMLM electromagnetic field, the accuracy of thermal field model, and the validity of the AMLM optimization method were verified through simulation and tests. The method proposed has significantly improved the efficiency of optimization calculation.

Modeling and Optimization of Air-Core Monopole Linear Motor Based on Multiphysical Fields

This article characterizes the thermal behavior of a commercialized silicon carbide (SiC) power MOSFET module with special concerns on high-temperature operating conditions as well as particular focuses on SiC MOSFET dies. A temperature-dependent Cauer-type thermal model of the SiC MOSFET is proposed and extracted based on offline finite-element simulations. This Cauer model is able to reveal the temperature-dependent thermal property of each packaging layer, and it is suitable for the high-temperature thermal-profile prediction with sufficient computational efficiency. Due to the temperature-dependent thermal properties of the SiC die and ceramic material, the junction-heatsink thermal resistance can be increased by more than 10% under high-temperature conditions (up to 200 °C), which can considerably worsen thermal estimations of the SiC die and its packaging materials. Furthermore, the experimental measurement of transient thermal impedance was conducted under operating temperature variations (with virtual junction temperature ranging from 60.5 °C to 199.6 °C), and the effectiveness of the proposed temperature-dependent Cauer model was fully validated.

https://vbn.aau.dk/ws/files/337067180/Thermal_Characterization_of_Silicon_Carbide_MOSFET_Module_Suitable_for_High_Temperature_Computationally_Efficient_Thermal_Profile_Prediction.pdf

Thermal Characterization of Silicon Carbide MOSFET Module Suitable for High-Temperature Computationally Efficient Thermal-Profile Prediction

As is known, improving launcher thrust is the target that the mankind is pursuing. There is no saturation phenomenon in the Ironless permanent magnet linear motor(IPMLM), so the thrust of IPMLM has a linear relationship with current through windings to provide larger thrust. The topology of IPMLM determines thrust fluctuation is smaller than Iron-PMLM. In this paper, the model of airgap magnetic field is established by magnetic charge method and image method for global optimization of IPMLM thrust. All dimensions of IPMLM are described by motor thickness and three ratio coefficients(α, β and γ). The cooling power of cooling system under different temperature gradients, which determines current density of winding, is calculated by Comsol software. When α, β and γ are different means each dimension of IPMLM is different, the current density in different winding could be determined by constraint condition, p cu ≤ Q. So we can obtain the surface of current density and dimension, and the surface of thrust and dimension. For obtaining maximum thrust in the same volume, the size ratio among magnetic structure, winding structure and cooling structure is derived by the thrust analytical expression. The distribution of IPMLM thermal field is analyzed by 3-D finite element method, and this optimization method of IPMLM thrust density is proved by experiment.

Donghua Pan

Papers

Comparison of the Two Current Predictive-Control Methods for a Segment-Winding Permanent-Magnet Linear Synchronous Motor

Transient Stability Impact of Reactive Power Control on Grid-Connected Converters

Modeling and Optimization of Air-Core Monopole Linear Motor Based on Multiphysical Fields

Thermal Characterization of Silicon Carbide MOSFET Module Suitable for High-Temperature Computationally Efficient Thermal-Profile Prediction

Analysis and optimization of Ironless permanent magnet linear motor for improving thrust