Output Impedance Of Converter Research Articles

Inertia and Grid Impedance Emulation of Power Grid for Stability Test of Grid-Forming Converter

The virtual synchronous generator (VSG) control is a promising solution for the grid-forming converters to enhance the inertia of the converter-based power grid. The virtual inertia and output impedance of the VSG are the important behaviors to be focused. However, the virtual inertia of grid-forming converters may cause active power oscillation due to the interaction with the power inertia of grid through the low-frequency band grid impedance. Besides, the resonance can be also triggered by the interaction between middle-frequency impedance of power grid and output impedance of converter. In order to test the stability of grid-forming converter, accurate emulation of grid behaviors from more aspects, including inertia and impedance under wider frequency ranges, is becoming critical. This article proposes a grid emulation method to mimic the inertia and impedance characteristics of power grid, targeting for the stability test of grid-forming converter under multifrequency bands. A high-switching-frequency converter and a low-switching-frequency converter are adopted in the proposed emulator structure to achieve both high capability in respect to control bandwidth and power level. Moreover, the virtual impedance control is integrated with the VSG control to emulate the line impedance and inertia within relatively wider frequency ranges. The realization of virtual impedance is achieved without derivation terms nor low pass filter. Finally, the performance of the proposed emulator is verified by various simulations and experimental measurements.

The Effect of Grid-Connected Converter Control Topology on the Diagonal Dominance of Converter Output Impedance

The Effect of Grid-Connected Converter Control Topology on the Diagonal Dominance of Converter Output Impedance

Analysis and Design of PLL Less Current Control for Weak Grid-Tied LCL-Type Voltage Source Converter

For a weak grid-tied <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$LCL$ </tex-math></inline-formula> -type converter, the conventional current control interacts with a phase-locked loop (PLL) through the voltage at the point of common coupling (PCC). Thus, the dynamics of PLL deteriorate converter stability, hence leads to system instability issues. To avoid the instability issues due to PLL, a PLL less current control is proposed for <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$LCL$ </tex-math></inline-formula> -type grid-connected converter with capacitor-current-feedback active damping. The analysis and design of the proposed PLL less current control is presented in this article, and the performance of the proposed control is compared with conventional current control (i.e., PLL without bandpass (BP) filter) and improved conventional control (i.e., PLL + BP filter). A small-signal model is developed by considering the dynamics of the PLL and BP filter to show the impact of the current control parameters on converter output impedance. An impedance-based stability analysis is performed to show the system stability against short circuit ratio (SCR) variation for three different current control scenarios, such as proposed PLL less, PLL with a BP filter, and PLL without BP filter. The proposed current control works in both stiff grid and weak grid environments. It has strong robustness against a wide range of variation in grid impedance and more adaptable to grid frequency changes. The performance of the proposed control is verified during grid voltage harmonics, unbalanced voltage, and grid voltage variations and further validated by comparing with the conventional control. In addition to the features mentioned above, the proposed current control is able to operate stably at SCR = 1. Finally, the experiment shows the superiority of the proposed control over conventional control.

Analysis of multi-converter network impedance using MIMO stability criterion for multi-loop systems

This work presents a novel method of simplifying multi-converter systems containing complex grid-forming and grid-following controllers for stability analyses. Analysis is conducted via the equivalent converter output impedance. Important points for impedance-based stability analysis of larger networks are considered, including impedance rotations, proper combination of sources and correctly obtaining a minimum system realisation with appropriate tolerances. The proposed method is used to conduct a sensitivity analysis of different grid-forming and grid-following converter control parameters to determine tuning recommendations for the modern electricity network. To achieve this, complex control structures including power/voltage control, negative sequence regulation and synchronous machine emulation are included. Network stability is found to be sensitive to current control, active power control and frequency-based components such as PLLs. However, voltage control exhibits a smaller effect on stability for a range of short circuit ratios. Some recommendations are provided on when the simplified method can be utilised and how to ensure correct application.

Analysis of small signal model of DC/DC converter without source and load effects

The small signal model of DC/DC converter without source and load effects is established and analyzed in detail. Characteristics of input and output impedance of converter in frequency domain in the model are applicable to the stability analysis and design in DC multi-converter power system. Investigation shows that the resonant characteristics of input and output impedance under closed-loop control are closely related to the crossover frequency and phase margin of control loop. The input impedance resonant valley and output impedance resonant peak locate at the same frequency as the loop crossover frequency. Furthermore, when the phase margin decreases, the resonance valley of the input impedance becomes lower and the resonance peak of the output impedance becomes higher. Finally, the accuracy of the model is verified by MATLAB Simulink time domain simulation.

Investigation of Nonlinear Droop Control in DC Power Distribution Systems: Load Sharing, Voltage Regulation, Efficiency, and Stability

Linear droop faces the design tradeoff between voltage regulation and load sharing due to cable resistances and sensing errors. Using a larger droop resistance improves load sharing, but requires a wider droop voltage range. In the nonlinear droop, droop resistance is a function of the converter's output current, and its value increases when the output current increases. As a result, the impacts from sensors and cables are reduced. In this paper, the design of nonlinear droop in dc power distribution systems is studied with special emphasis on load sharing, voltage regulation, system efficiency, and stability. After discussing the piecewise linear and nonlinear droop control, a generic polynomial expression is presented to unify different droop equations. The impact of droop on dc system efficiency is analyzed by evaluating cable and power converter losses. The converter's output impedance using nonlinear droop is modeled to analyze the system stability with constant power loads. The selection and design guidelines of nonlinear droop are summarized, considering both the static performance and interaction with load systems. The analysis is verified in 400-V multi-source dc systems. The nonlinear droop is fully distributed as it only needs local information.

Harmonic Virtual Impedance Design for Parallel-Connected Grid-Tied Synchronverters

In recent years, synchronverters have become a preferred control solution for the integration of renewable energy sources and battery systems to power networks. In this control strategy, the converter acts as a fundamental frequency voltage source and, as such, is unable to deal with harmonic power quality issues. In this paper, the harmonic control is frequently addressed using virtual impedances; however, they have been seldom studied for synchronverters. This paper presents a detailed design procedure for harmonic virtual impedances applied to parallel-connected synchronverters. The proposed method offers a tailor-made design of the converter output impedance and large stability margins. The method is used to decouple the dynamics of the synchronverter control, and the proposed virtual-impedance is explained in detail. The obtained results showed the output impedance of each synchronverter can be designed to achieve a tradeoff in power quality between current and voltage. In addition, the effect of frequency variations was explored and two solutions were provided and compared. All the control system improvements were experimentally validated using two 15-kVA battery-supported synchronverters connected to a distorted weak grid.

An Improved Design of Current Controller for &lt;italic&gt;LCL&lt;/italic&gt;-Type Grid-Connected Converter to Reduce Negative Effect of PLL in Weak Grid

For three-phase LCL -type grid-connected converter, when it is attached to weak grid, current control interacts with phase-locked loop (PLL) via point of common coupling voltage. Consequently, PLL dynamic might deteriorate grid current control and even result in system instability. However, the conventional design method of current controller neglects the impact of PLL, and therefore, it is hard for the current controller to mitigate the negative effect of PLL. In this paper, an improved design of current controller, i.e., proportional–integral controller and capacitor-current-feedback active damping, is proposed to reduce the negative effect of PLL on current control. First, a small-signal impedance model is developed to analyze the impact of PLL on current control. Then, the effect of current controller parameters on the converter output impedance is analyzed, and a design guideline to improve the current controller parameters is presented. With the improved parameters, in precondition of satisfying system stability margin under both stiff and weak grid, the negative effect of PLL on current control can be effectively mitigated without employing additional control strategies. Furthermore, the current control has a strong robustness against wide-range variation of grid impedance. Finally, the experiment demonstrates the effectiveness of the proposed design method.

Stabilization and Performance Preservation of DC–DC Cascaded Systems by Diminishing Output Impedance Magnitude

DC–DC cascaded systems are vulnerable to instability and dynamic performance deterioration. These problems arise due to the nonideality in the source converter output impedance and to the negative input impedance of the load converter. Any overlap in magnitude between these two impedances will destabilize the voltage at the dc bus. Thus, we introduce a method to diminish the magnitude of source-converter output impedance such that the source converter will act as an ideal voltage source. As a consequence, the dynamic performance of the source and load converters will be intact, leading to ensuring the bus voltage stability. In addition, the controller is independent of the number of converters that form the load subsystem. To validate the proposed method, an experimental cascaded system was analyzed, simulated, and experimentally tested. Collectively, all of the outcomes proved the effectiveness of the proposed method.

A Multifunctional Control Strategy for Oscillatory Current Sharing in DC Microgrids

In this paper, a multifunctional control strategy is proposed for an accurate sharing of oscillatory and dc components of load currents among distributed generation (DG) units in a dc microgrid. The proposed controller is composed of a voltage control loop and a multiloop current control unit. The voltage control loop is used to regulate the microgrid voltage as well as to share the dc component of load current through the conventional droop scheme. The current controller includes two main units to simultaneously (1) manage the oscillatory current sharing, (2) improve the stability of control unit, (3) diminish the voltage ripples, and (4) protect the DG units against overloads. In this paper, the idea of virtual conductance is introduced to modify the output impedance of converters for oscillatory current sharing among DG units. To restrict the instantaneous current of DG units under overload conditions, a maximum current limiter unit is introduced to the control system. The compensation signals of these units are eventually applied to the inner current control loop. This scheme guarantees the microgrid voltage to be maintained within the tolerable range under a variety of oscillatory conditions. The stability of the system is studied using small signal analysis. The proposed control scheme is examined in two different microgrid configurations of radial and ring structures. The effectiveness of the proposed method is evaluated through simulation studies.

Output-Impedance Shaping of Bidirectional DAB DC–DC Converter Using Double-Proportional-Integral Feedback for Near-Ripple-Free DC Bus Voltage Regulation in Renewable Energy Systems

This paper investigates the design and implementation of virtual-output-impedance shaping on an inverter-loaded, fuel-cell-battery-powered dc–dc converter system for achieving near-ripple-free dc bus voltage regulation. The method is based on the insertion of a second output-voltage feedback loop as can be inferred from the Mason's gain formula. Three basic modes of virtual-output-impedance shaping (proportional, derivative, integral) are discussed and the closed-loop output-impedance characteristics due to each of them are analyzed in detail and with their Thevenin equivalent circuits derived. Despite the suitability of integral feedback for minimizing converter's output impedance, it can give rise to an unwanted resonance peak near the converter's crossover frequency, thus potentially destabilizing the system. The solution to the problem using combination of basic virtual-output-impedance shaping modes and its practical implementation are discussed. When implemented on a fuel-cell-battery-powered dual-active-bridge dc–dc converter, the second-harmonic distortion of dc bus voltage is shown to have been reduced by 85.5% compared to a conventionally PI-compensated system.

Systematic Derivation of a Family of Output-Impedance Shaping Methods for Power Converters—A Case Study Using Fuel Cell-Battery-Powered Single-Phase Inverter System

For power converters used in renewable energy systems, output-impedance design has become an important design consideration for minimizing the impacts of low-frequency harmonic current on the lifetime of ripple-sensitive energy sources such as fuel cells and photovoltaic cells. In the literature, various methods are proposed to tackle this design issue but they are frequently treated in isolation from each other and specific to the systems being discussed. In this paper, a systematic derivation of four basic modes of output-impedance shaping method is presented. These basic modes can be directly inferred from the Mason's gain formula and other methods are in essence derivatives or combinations of these basic modes. By using a fuel-cell-battery-powered single-phase inverter as an implementation example, their characteristics are discussed thoroughly and their performances in shaping converter's output impedance are evaluated experimentally.

Active damping method to reduce the output impedance of the DC–DC converters

In the cascaded power system, instability may occur because of the impedance interaction. The system can be stabilised by passive damping or control methods to reduce the output impedance of the source converter and improve the damping characteristics. This study focuses on the source converter in the cascaded system and presents a new active damping control to reduce the output impedance of the converter by a proper active damping controller. The method to compute the corresponding active damping controller is proposed in this study. By the application of the active damping method, a virtual resistor in parallel with the output of the converter's small signal model can be realised. Therefore the output impedance can be reduced. Because the virtual resistor exists only in the small signal model, the steady-state operating point is not affected and the value of the virtual resistor can be changed. The experiment results of a Buck converter with the active damping controller under different conditions are proposed to verify the effectiveness of the active damping control. Besides, an active damping controller is applied to an unstable cascaded system in simulation to stabilise the cascaded system so the effectiveness of this method is also verified.

A Sizing Methodology for On-Chip Switched-Capacitor DC/DC Converters

This paper proposes a systematic sizing methodology for switched-capacitor DC/DC converters aimed at maximizing the converter efficiency under the die area constraint. To do so, we propose first an analytical solution of the optimum switching frequency to maximize the converter efficiency. When the parasitic capacitances are low, this solution leads to an identical contribution of the switches and transfer capacitors to the converter output impedance. As the parasitic capacitances increase, the optimum switching frequency decreases. Secondly, optimum capacitor and switch sizes for maximum efficiency are provided. We show that the overdrive voltage strongly impacts the optimum switch width through the modification of their conductance. To support the sizing methodology, a model of the efficiency of switched-capacitor DC/DC converters is developed. It is validated against simulation and measurement results in 65 nm and 0.13 μm CMOS, respectively. The proposed sizing methodology shows how the converter efficiency can be traded-off for die area reduction and what is the impact of parasitic capacitances on the converter sizing.

A Droop Control Strategy with Impedance Compensation for Low Voltage Microgrid

With conventional droop control, parallel operation of distributed generations (DG) in microgrid would lead to unbalanced power sharing. In this paper, inherent limitation of conventional droop control is analyzed. Analysis results show that different converter output impedance and line impedance make the power sharing unbalanced. In order to weaken or eliminate impedance difference from point of common coupling (PCC) to DGs, virtual impedance is introduced. By the introduction of designed virtual impedance, a novel droop control strategy with impedance compensation is proposed in this paper. Simulation results are presented from a two converters parallel-connected microgrid, showing the effectiveness of the droop control with impedance compensation. Simulation results show that DGs with proposed approach can allocate the power equally, and work stably in grid-connected mode, island mode and progress of reconnection to grid.