Showing papers on "Output impedance published in 2022"

Modeling and Advanced Control of Dual-Active-Bridge DC–DC Converters: A Review

Abstract: This article classifies, describes, and critically compares different modeling techniques and control methods for dual-active-bridge (DAB) dc–dc converters and provides explicit guidance about the DAB controller design to practicing engineers and researchers. First, available modeling methods for DAB including reduced-order model, generalized average model, and discrete-time model are classified and quantitatively compared using simulation results. Based on this comparison, recommendations for suitable DAB modeling method are given. Then, we comprehensively review the available control methods including feedback-only control, linearization control, feedforward plus feedback, disturbance-observer-based control, feedforward current control, model predictive current control, sliding mode control, and moving discretized control set model predictive control. Frequency responses of the closed-loop control-to-output and output impedance are selected as the metrics of the ability in voltage tracking and the load disturbance rejection performance. The frequency response plots of the closed-loop control-to-output transfer function and output impedance of each control method are theoretically derived or swept using simulation software PLECS and MATLAB. Based on these plots, remarks on each control method are drawn. Some practical control issues for DAB including dead-time effect, phase drift, and dc magnetic flux bias are also reviewed. This article is accompanied by PLECS simulation files of the reviewed control methods.

Participation Analysis in Impedance Models: The Grey-Box Approach for Power System Stability

Abstract: This paper develops a grey-box approach to small-signal stability analysis of complex power systems that facilitates root-cause tracing without requiring disclosure of the full details of the internal control structure of apparatus connected to the system. The grey-box enables participation analysis in impedance models, which is popular in power electronics and increasingly accepted in power systems for stability analysis. The Impedance participation factor is proposed and defined in terms of the residue of the whole-system admittance matrix. It is proved that, the so defined impedance participation factor equals the sensitivity of the whole-system eigenvalue with respect to apparatus impedance. The classic state participation factor is related to the impedance participation factor via a chain-rule. Based on the chain-rule, a three-layer grey-box approach, with three degrees of transparency, is proposed for root-cause tracing to different depths, i.e. apparatus, states, and parameters, according to the available information. The association of impedance participation factor with eigenvalue sensitivity points to the re-tuning that would stabilize the system. The impedance participation factor can be measured in the field or calculated from the black-box impedance spectra with little prior knowledge required.

Analysis and Mitigation of Low-Frequency Interactions Between the Source and Load Virtual Synchronous Machine in an Islanded Microgrid

Abstract: Source-side virtual synchronous machines (VSGs) and load-side VSMs (LVSM) are gradually utilized together in the microgrid to provide virtual inertia and damping. However, instability occurs in the islanded microgrid system due to the interaction dynamics between the VSG and the LVSM, which has been investigated in this article. At first, the dq -frame impedance models of the VSG and the LVSM are established and compared. It is revealed that the d-d channel impedance of LVSM behaves the negative resistor with a V-type magnitude in the low-frequency range, which easily interacts with the d–d channel impedance of the VSG and leads to instability of the system. Thus, the inductor current feedforward control and the additional voltage feedback control are proposed for the VSG to reshape its impedance. It diminishes the impedance magnitude and generates the active impedance of the VSG. In this way, the low-frequency interaction between the VSG and the LVSM can be mitigated. Besides, the proposed control preserves the dynamic performance of the system. Finally, simulations and experiments verify the effectiveness of stability analyses and the proposed suppression method.

Adaptive Virtual Impedance Controller for Parallel and Radial Microgrids With Varying X/R Ratios

Abstract: In this paper, an adaptive virtual impedance-based universal voltage source converter (VSC) control scheme is proposed to improve the stability, and power-sharing performance of VSC interfaced distributed energy resources in hybrid AC/DC microgrids with different feeder characteristics. The proposed control scheme modulates the VSC output impedance by tracking the active and reactive power transfer difference between the inverter terminal and the point of connection to the microgrid. Firstly, the stability limits of microgrid with resistive and inductive microgrid feeders are investigated by theoretical analysis. Subsequently, influence of the feeder parameters on the stability of microgrid is evaluated. From the theoretical analysis, the proposed control scheme shows resistive inverter output impedance at low frequency, while it shows more inductive behavior at system frequency for both the low and medium voltage microgrid feeders. The effectiveness of the proposed control strategy is demonstrated through a range of scenarios for two different microgrid types. The results show that the proposed controller can autonomously vary the output impedance of the VSC and provides significantly improved damping and power-sharing performance of the microgrid.

Improved Weak Grids Synchronization Unit for Passivity Enhancement of Grid-Connected Inverter

Abstract: The weak grids may be unstable due to the negative resistance behavior of the grid-connected inverter (GCI) caused by the synchronization unit with the phase-locked loop (PLL). This article proposes a passivity enhancement method to attain the positive output resistance of GCI in the qq channel. Initially, the impedance decomposition is utilized to reveal the reason for the negative output resistance brought by PLL. Secondly, with decomposed impedance, a novel PLL by adding a prefilter and an impedance phase regulator with a normalized amplitude to the input of traditional synchronous reference frame PLL (SRF-PLL) is proposed. Afterward, the effect of the critical system parameters and GCI operating conditions on the GCI’s passivity is discussed based on the established small-signal impedance model. Theoretical analysis manifests that the proposed PLL does not affect the system’s stability under rectifier mode and brings the passivity of GCI’s output impedance in the qq channel in broad system parameters under inverter mode. Lastly, the experiments are implemented, verifying that using the proposed PLL, the instability caused by the intersection between grid and GCI impedances is removed due to the passivity, and the system can operate at 1.0 per unit (pu) active power reference under the very-weak-grid conditions.

A Critical Aspect of Dynamic Stability in Autonomous Microgrids: Interaction of Droop Controllers Through the Power Network

Abstract: It is explored in this article that the interaction of droop controllers through the power network (IDCPN) is the dominant factor affecting the dynamic stability of an autonomous networked microgrid (ANMG) and new models are developed to support the IDCPN. The models are developed to analyze the impacts of three parts on the IDCPN: 1) the X/R ratio of the power network impedance; 2) the droop controllers including low-pass filters, and 3) the impedance characteristics of the grid-forming voltage source inverters (VSIs). The low-frequency oscillations (LFO), excited by IDCPN, is identified and quantified by modeling the first and second parts. The critical impact of the low X/R ratio on amplifying the LFO is clarified. Then, the output impedance of the grid-forming VSI, including the virtual inductance loop (VIL), is developed and rigorously observed that reveals inefficient performance. It is shown that the VIL improves dynamic performance by providing sufficient damping to suppress LFOs and not by effectively boosting the X/R of the VSI output impedance. This, however, is problematic in the current limiting that puts the ANMG at instability risk because of the resistive–capacitive impedance characteristics of the VSIs. A ${{\boldsymbol{H}}_\infty }$ robust controller is proposed to replace VIL for suppressing LFO and stabilizing ANMG. Numerical/simulation results are provided to prove the accuracy of the models.

Sequence Impedance Measurement of Utility-Scale Wind Turbines and Inverters – Reference Frame, Frequency Coupling, and MIMO/SISO Forms

Abstract: Sequence impedance responses are increasingly used for the stability analysis of converter-grid systems; however, many aspects of the sequence impedance measurement process, particularly those resulting from the frequency coupling between the positive- and negative-sequence impedances, are not yet fully explored. Existing methods for measuring sequence impedance with frequency coupling are complicated, not suitable for large wind turbines and inverters, and they can measure the sequence impedance in only one of the MIMO and SISO forms. This paper shows that the sequence impedance has a reference frame similar to the dq impedance and presents a method for measuring the sequence impedance with frequency coupling. The proposed method aligns the sequence impedance reference frame based on the estimation of the grid voltage angle and obtains the impedance response in both MIMO and SISO forms. The paper also demonstrates the impact of the fundamental frequency on the accuracy of the impedance measurements. The proposed method and practical issues associated with the impedance measurement of utility-scale wind turbines and inverters are demonstrated on a 1.9-MW Type III wind turbine and a 2.2-MVA inverter using an impedance measurement system built around a 7-MW/13.8-kV grid simulator and a 5-MW dynamometer.

An Impedance-based Parameter Design Method for Active Damping of Load Converter Station in MTDC Distribution System

Abstract: To achieve the efficient application of impedance analysis in the stability assessment and enhancement of multi-terminal DC distribution systems, this paper proposes the DC-side reduced-order impedance models with power control and AC voltage control, respectively, by taking the load converter station as the object. By using the DC-side current as the feed-forward state, the active compensator applied to the load converter station with two control modes is also derived as well as the corresponding reduced-order impedance models. Combined with the reduced-order impedance models, a method based on damping factor sensitivity is further proposed to design the parameters of the derived active compensators. The verification results in the frequency domain and time domain demonstrate the accuracy of the reduced-order impedance and the effectiveness of the proposed compensator parameter design method.

Stability Assessment of A Radial Grid With Power Converters

Abstract: The increase proportion of power converters causes stability problems in the radial grid. To address these issues, this paper firstly proposes a unified modeling procedure and applies it to two types of converters. Secondly, the impedance models are used to analyze the stability of a radial grid with converters under four cases. The influence of the short circuit ratio (SCR) and line impedance on system stability are investigated. Then, a comprehensive sensitivity analysis is performed in order to investigate the effects of parameters variations on the closed-loop poles of the system under different conditions. Finally, the accuracy of the model and theoretical analysis are verified by simulations and Hardware-in-the-Loop (HiL) tests.

Sensorless Temperature Estimation of Lithium-Ion Battery Based on Broadband Impedance Measurements

Abstract: Temperature monitoring is of paramount importance for guaranteeing the safety and proper operation of lithium-ion batteries. Traditional temperature sensors suffer from heat transfer delay, where internal battery temperature cannot be measured directly. Motivated by this, this letter proposes a novel sensorless temperature estimation method based on broadband impedance spectroscopy. In this letter, pseudorandom sequence (PRS) with finite signal levels is utilized for impedance measurements. The measured impedance information is merged into an impedance-temperature model, which cooperates with a specially designed least-square method for temperature estimation. The proposed framework is robust against interference, whereas simple enough for online implementation. Experimental results suggest excellent estimation accuracy of the proposed method under different circumstances.

Investigation and comparison of the electrochemical impedance spectroscopy and internal resistance indicators for early-stage internal short circuit detection through battery aging

Abstract: It is crucial to identify the battery's internal short circuit (ISC) for safety. The study aims to explore the effectiveness of ISC detection methods through battery aging. Two types of method are compared in this work: diffusion coefficient calculation based on electrochemical impedance spectroscopy and conventional internal resistance monitoring through algorithms such as recursive least square (RLS). In this work, the frequency-domain P2D model is built to simulate the behavior of the impedance response with different levels of ISC. Then, coin cells are assembled with conductive contaminants to examine the accuracy of the model simulation and the actual influences of the ISC on the impedance. Afterwards, with the proven impedance model and potential influences, the substituted ISC test on commercial 18,650 cells is applied to examine ISC detection based on EIS under different aging statuses. Finally, the conventional internal resistance identification method is used for comparison. The coin cell experimental results in the study indicate that the inception of the ISC mainly alters the low-frequency impedance with much reduced phases, which is also predicted by the model simulation. Similar results with the impacted low-frequency impedance can also be observed in the substituted ISC test for the commercial cells. The diffusion coefficients, the parameter mainly associated with the low-frequency impedance response, increased by 47–143 % as the battery SOH decreased from 100 % to 87 %. In comparison, the internal resistance identification method indicates that the ohmic component is relatively stable after the inception of the ISC. The polarization component of the internal resistance exhibits a significant change as high as 234 %. However, as the battery's SOH decreases, the effectiveness of the polarization resistance fades toward 23 %.

Mixed-Mode Electronically-Tunable First-Order Universal Filter Structure Employing Operational Transconductance Amplifiers

Abstract: In this paper, a new mixed-mode first-order universal filter configuration is presented that employs three operational transconductance amplifiers (OTAs) and one grounded capacitor (eminently suitable for IC chip fabrication). All three first-order generic filter functions, namely low pass filter (LPF), high pass filter (HPF) and all pass filter (APF) in all the four possible modes, namely voltage mode (VM), current mode (CM), transresistance mode (TRM) and transconductance mode (TCM) can be realized. The proposed configuration offers high input impedance and high output impedance. The pole frequency of the filter can be controlled electronically by varying a single transconductance. Nonideal analysis of the proposed filter structure has also been carried out and the results have been compared with those obtained from ideal analysis. The performance of the presented filter configuration has been corroborated through PSPICE simulations as well as experimental results. The various simulation and experimental results validate the practical viability of the proposed configurations.

CFOA‐based simple mixed‐mode first‐order universal filter configurations

Abstract: This brief communication presents two current feedback op‐amp (CFOA)‐based simple first‐order mixed‐mode universal filter topologies—one of them providing first‐order low‐pass (LP), high pass (HP), and all pass (AP)—all the three filters in voltage‐mode (VM)/trans‐admittance‐mode (TAM) while the other one realizing the quoted three functions in current‐mode (CM)/trans‐impedance‐mode (TIM). Except the all pass filter realization from the first configuration which needs a second CFOA also as gain‐of‐two amplifier, in all the remaining cases of both the configurations, only a single CFOA is needed. The other notable features of the circuits are employment of only three resistors and a grounded capacitor (as preferred for IC implementation) and the availability of independent tunability of the gain (H0) and cut‐off frequency/pole frequency (ω0) of all the three filters, in both the cases. In spite of not offering ideally infinite input impedance in VM/TAM mode and ideally zero input impedance in CM/TIM mode, the circuits still possess easy cascadability due to providing a voltage output from a low‐output impedance terminal W while the current output from the high output impedance terminal Z. Non‐ideal and sensitivity analyses have also been carried out. The workability of the proposed circuits has been demonstrated through PSPICE simulations and experimental results using the commercially available AD844‐type IC CFOA.

Small-Signal Modeling and Controller Parameters Tuning of Grid-Forming VSCs With Adaptive Virtual Impedance-Based Current Limitation

Abstract: The adaptive virtual impedance (VI) control emerges as an attractive solution to limit the current of grid-forming voltage-source converters (GFM-VSCs) during grid faults. Yet, the adaptive VI-based current limitation relies on the detection of the current magnitude, which introduces nonlinear dynamics that challenges the controller parameters tuning of GFM-VSCs. This article develops the small-signal model of GFM-VSCs with adaptive VI-based current limitation, from which, the dynamic impact of the adaptive VI is explicitly revealed. Considering such impact, a holistic controller parameters tuning method for GFM-VSC is proposed to guarantee the small-signal stability of the system. Simulation and experimental tests are performed to verify the effectiveness of the theoretical analysis and the proposed parameters tuning method.

Fast LDO Handles a Wide Range of Load Currents and Load Capacitors, up to 100 mA and Over 1μF

Abstract: This paper proposes a low dropout voltage regulator (LDO) that exhibits both a fast response to load transients and the ability to handle practically any load capacitor. Starting from a typical LDO topology, an error amplifier (EA) that drives a PMOS pass transistor and a passive feedback network, we inserted a novel circuit, with the input AC-coupled to the LDO output and the output connected directly to the pass transistor gate. This circuit creates an inner feedback loop able to react quicker than the main feedback loop to variations in the output voltage, and appropriately inject or sink current to/from the gate node. Moreover, the inner feedback loop helps reduce the equivalent small-signal impedance at the LDO output, which in turn reduces the impact the pole associated with the output node has on the LDO stability. A compact circuit implementation of this topology is presented in this paper: it combines the proposed fast transient & frequency compensation circuit with a high slew-rate EA. The resulting LDO was integrated in a 130 nm standard CMOS technology. The measurement results are in good agreement with simulations and validate the concept and design. The LDO provides a steady 1 V output with the supply voltage varying from 1.2 V to 1.5 V and the load current going up to 100 mA. Its fast response to load transients helps maintain the output voltage overshoot and undershoot below 250 mV for C _L = 0 and under 60 mV for $\text{C}_{\mathrm {L}} =1\,\,\mu \text{F}$ , when the load current varies between 1 $\mu \text{A}$ and 100 mA in 1 $\mu \text{s}$ . The LDO requires only 6.2 $\mu \text{A}$ of quiescent current and occupies 0.018 mm ² of die area.

An Iterative Virtual Impedance Regulation Strategy in Islanded Microgrids for Enhanced Balanced, Unbalanced and Harmonic Current Sharing

Abstract: The balanced, unbalanced and harmonic current sharing inaccuracy caused by the mismatched wire impedances of distributed generation (DG) units influences the normal operation of islanded microgrids. To deal with this issue, an iterative virtual impedance regulation (IVIR) strategy is proposed in this paper, both the resistive and reactive parts in the output impedances of DG units are regulated. The whole process is decomposed into several successive iterations. In each iteration, the fixed virtual impedance and adaptive virtual impedance regulation are combined to generate a virtual resistance increment and uniform virtual reactance increment according to the uniform output current magnitude and phase angle respectively. Based on these increments, the virtual impedance at fundamental-negative and harmonic frequencies also are modified. With the proposed strategy, all the balanced, unbalanced and harmonic current sharing is accurate with a relatively small virtual impedance. Only the local current information is required for the IVIR process, and the communication is merely used to exchange the VIR mode status flags at the beginning and the end of the IVIR process, the dependency on communication is minimized. The feasibility of the proposed strategy is verified with the simulation and experimental results.

CCII-Based Voltage-Mode and Current-Mode High-Order Filters with Gains and Grounded Passive Elements Only

Abstract: Dual output plus-type second-generation current conveyor-based high-order voltage-mode (VM) and current-mode (CM) analog filters with gains are proposed in this manuscript. The proposed VM and CM high-order filters can simultaneously provide low-pass and high-pass responses. Also, if an order of the filter is even, the VM one can realize band-pass response. Both filter circuits do not suffer from any passive element matching problems. Additionally, they are composed of only grounded passive components, suitable for integrated circuit processes. The proposed VM filter has high input impedance, and the CM one possesses high output impedances; thus, both filters can be easily cascaded. Nevertheless, if necessary, VM one needs two voltage followers to obtain low output impedances, and CM one requires one current follower to obtain low input impedance. Non-ideality analyses are included for both filter structures. Many simulations based on the SPICE program and several experiments are realized to affirm the theory.

Steady-State Analysis and Optimal Design of an LLC Resonant Converter Considering Internal Loss Resistance

Abstract: In this paper, a steady-state model of an LLC resonant half-bridge converter with internal loss resistance is proposed, in order to maximize power conversion efficiency, and steady-state characteristic equations of DC voltage gain and input impedance are derived for the optimal design of the converter. First, to confirm the validity of the steady-state characteristic equation and the optimal design process, a prototype converter with a maximum output of 2 kW was designed. Through comparison of simulation, calculation, and experimental results obtained from the prototype test, it is shown that the calculation results proposed in this paper were closer to the experimental results than the calculation results obtained under the lossless condition. In addition, the relationship between the switching frequency and the load current of the prototype was compared, in order to determine the operating range of the switching frequency, which is important in the converter design stage. In this case, it was confirmed that the calculated value reflecting the internal loss showed a close result. In conclusion, we confirm the usefulness of the analysis results reflecting the internal loss resistance proposed in this paper and the optimal design process.

Analysis of the discharge plasma impedance of copper vapor laser

Abstract: In the pursuit to analyze the impedance of copper vapor lasers (CVLs) in different conditions, a novel approach has been proposed in this paper. The underdamped behavior of the voltage waveform across the CVL is leveraged to compute the impedance of high-voltage discharge plasma in the laser. This methodology provides an accurate idea of the discharge plasma resistance and inductance as it is calculated on the basis of experimental voltage waveforms obtained from the laser system. The laser head inductance remains almost fixed and equivalent to ∼0.47 µH whereas the laser resistance changes between 34 Ω and 11 Ω depending on the discharge condition and its constituents. A critical evaluation of CVL impedance is done in all experimentally possible conditions, and a methodology has been proposed to maintain the CVL impedance, which results in the power stability of the laser in oscillator–amplifier configuration. The laser impedance variation w.r.t. time, pressure, operating voltage and electrode pin configuration has been investigated. The impact of the localized electric field at the electrode on the laser resistance has also been emphasized in this paper. A good concurrence exists between the calculated laser impedance and its experimental behavior.

Online Broadband Battery Impedance Spectroscopy Using Current-Mode Boost Converter

Abstract: In this article, an online method of broadband battery impedance spectroscopy is proposed, in which the excitation signal is injected through the inner feedback loop of a dc/dc boost converter with average current-mode control (ACMC). By deriving the average state-space model of the conventional boost converter, it is shown that an open-loop converter exhibits a resonant frequency response in regard to the injected signal, whereas the compensated current loop in ACMC produces a controllable, flat-gain closed-loop transfer function that allows broadband current excitation signals to be injected into the battery without the distortion that is seen in the open-loop configuration. In the presented method, the converter is set to discharge the battery at a constant C-rate during impedance measurement, while a broadband signal is superimposed onto the loop reference current. A low-dropout (LDO) regulator is then used to filter out disturbances caused in the converter output. The proposed approach is experimentally implemented, and a discrete-interval binary sequence (DIBS) signal is utilized to measure the impedance of two 18650 lithium-ion batteries in several arbitrary frequencies at once in the 0.1 Hz–1 kHz range. The measured spectra are validated by a commercial impedance analyzer.

A Novel Droop Control Strategy of Reactive Power Sharing Based on Adaptive Virtual Impedance in Microgrids

Abstract: Droop control strategy is widely used to control parallel inverters in a microgrid. However, due to the mismatched line impedance, the traditional droop control strategy is difficult to evenly share the reactive power and restrain the circulating current among multiple parallel inverters. In this article, an adaptive virtual impedance-based droop control strategy without communications is proposed to solve the reactive power sharing problem caused by the difference of line impedance. The reactive power output and voltage of each distribution generator are introduced into the adaptive virtual impedance controller to achieve a tradeoff between reactive power sharing accuracy and voltage drop. Small-signal stability analysis of the inverters equipped with the proposed controller is conducted to determine the optimal control parameters. Simulation and experimental results demonstrate the effectiveness of the proposed droop control strategy.

A Linear Wideband CMOS Balun-LNA With Balanced Loads

Abstract: This brief presents a novel structure on balun-LNAs which has a differential output with symmetric loads without any need for current bleeding circuit. The proposed structure is based on the common-gate (CG) common-source (CS) cascode LNA with identical transconductances for the CG and CS stages using a positive feedback for input matching compensation. This brief also introduces a new linearity improvement technique based on post distortion and derivative superposition linearization techniques without affecting the input impedance matching condition or requiring considerable power overhead. By this way, despite other linearity improvement techniques, the voltage gain not only is not decreased but is also improved. The proposed balun-LNA structure is designed in a 65 nm CMOS technology and covers the frequency range of 0.47-3.3 GHz. It has symmetrical loads with the maximum S ₂₁ of 22 dB and a minimum noise figure (NF) of 2.57 dB. The achieved third-order input intercept point (IIP3) and second-order input intercept point (IIP2) are +2.81 dBm and 29.27 dBm, respectively. The circuit consumes 8.33 mA from a nominal supply voltage of 1.5 V, and excluding the pads, it occupies 0.057 mm ² silicon die area.

Resistorless First-Order Universal Filter Structures Employing OTAs with Independent Controllability of Gain and Pole Frequency

Abstract: This communication presents two new first order universal active filter configurations employing operational transconductance amplifiers (OTA) and use grounded capacitor which is prominently suitable for integrated chip (IC) fabrication. One of the proposed topologies provides voltage mode (VM) and transconductance mode (TCM) of operations while the other circuit provides current mode (CM) in addition with transresistance mode (TRM) filter functions. All three first order filter functions namely low pass filter (LPF), high pass filter (HPF) and all pass filter (APF) are configurable with appropriate choice(s) of input voltage/current signals. An equal transconductor condition (which can be easily achieved) is required for the realization of APF functions. The proposed filter configurations possess high input impedance and high output impedance and also offer independent electronic tunability of pole frequency/cut-off frequency. The gain of filters realized in VM, TCM and TRM has independent electronic tunability feature and can be adjusted through transconductor. For the usability of proposed filters, multi-phase sinusoidal oscillator has also been demonstrated. To assure the workability of the proposed filter structures, PSPICE simulations with CMOS OTA implemented in 0.18 µm TSMC technology parameters has been used. Various simulation results including Monte-Carlo simulations, process corner analysis, total harmonic distortions (%THD) and temperature analysis have also been appended. Experimental results employing off-the-shelf IC LM13700 type OTAs have also been included to verify the theoretical propositions of first order universal filters and multiphase sinusoidal oscillators.

A Virtual Impedance Based Control for Power Sharing in a Microgrid with Uncertainty in Line Impedance

Abstract: In a microgrd, power sharing among different power converters with different energy sources is important especially when it works in the island mode. The droop control which is broadly used in the microgrid has the merits of simplicity since it only uses the local signals and does not require communication. However, in the conventional droop control, there is inherent trade-off between the power sharing and the voltage regulation in selecting the droop coefficient. In this paper, a virtual impedance-based control for sharing power is proposed for improving the voltage various while maintaining power sharing among different power converters when there is uncertainty in the line impedance. Experiments are carried out to compare with the conventional droop control method. It shows that the virtual impedance based decentralized control has drastically decreases the voltage variation from 5% to 0.8% while maintaining the same power sharing, which demonstrate its superiority over the conventional droop control when there is impedance uncertainty in the grid.