• Title, Summary, Keyword: Virtual impedance

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Effects of a Human Impedance and a First-Order-Hold Method on Stability of a Haptic System with a Virtual Spring Model (인간 모델과 1차 샘플-홀드 방식이 가상 스프링 모델 시스템의 안정성에 미치는 영향 분석)

  • Lee, Kyungno
    • Journal of Institute of Convergence Technology
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    • v.3 no.2
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    • pp.23-29
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    • 2013
  • When a human operator interacts with a virtual wall that is modeled as a virtual spring model, the lager the stiffness of the virtual spring is, the more realistic the operator feels that the virtual wall is. In the previous studies, it is shown that the maximum available stiffness of a virtual spring to guarantee the stability can be increased when the first-order-hold method is applied, however the effects of a human impedance on the stability are not considered. This paper presents the effects of a human impedance on stability of haptic system with a virtual spring and a first-order-hold (FOH) method. The human impedance model is modeled as a linear second-order system model. The relations between the maximum available stiffness of a virtual spring and the human impedance such as a mass, a damping and a stiffness are analyzed through the MATLAB simulation. It is shown that the maximum available stiffness is proportional to the square root of the human mass or damping respectively.

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Improved Reactive Power Sharing and Harmonic Voltage Compensation in Islanded Microgrids Using Resistive-Capacitive Virtual Impedance

  • Pham, Minh-Duc;Lee, Hong-Hee
    • Journal of Power Electronics
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    • v.19 no.6
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    • pp.1575-1581
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    • 2019
  • Due to the mismatched line impedance among distributed generation units (DGs) and uncontrolled harmonic current, the droop controller has a number of problems such as inaccurate reactive power sharing and voltage distortion at the point of common coupling (PCC). To solve these problems, this paper proposes a resistive-capacitive virtual impedance control method. The proposed control method modifies the DG output impedance at the fundamental and harmonic frequencies to compensate the mismatched line impedance among DGs and to regulate the harmonic current. Finally, reactive power sharing is accurately achieved, and the PCC voltage distortion is compensated. In addition, adaptively controlling the virtual impedance guarantees compensation performance in spite of load changes. The effectiveness of the proposed control method was verified by experimental results.

An Islanding Microgrid Power Sharing Approach Using Adaptive Virtual Impedance control scheme

  • Hoang, Van-Tuan;Lee, Hong-Hee
    • Proceedings of the KIPE Conference
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    • pp.315-316
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    • 2016
  • This paper proposes an enhanced distributed generation (DG) unit with an adaptive virtual impedance control approachin order to address the inaccurate reactive power sharing problems. The proposed method can adaptively regulate the DG unit thanks to the equivalent impedance, andthe effect of the mismatch in feeder impedance is compensatedto share the reactive power accurately.The proposed control strategy can be implemented directly without any requirement of pre-knowledge of the feeder impedances. Simulations are performed to validate the effectiveness of the proposed control approach.

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A Droop Method for High Capacity Parallel Inverters Considering Accurate Real Power Sharing

  • Kim, Donghwan;Jung, Kyosun;Lim, Kyungbae;Choi, Jaeho
    • Journal of Power Electronics
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    • v.16 no.1
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    • pp.38-47
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    • 2016
  • This paper presents DG based droop controlled parallel inverter systems with virtual impedance considering the unequal resistive-inductive combined line impedance condition. This causes a reactive power sharing error and dynamic performance degradation. Each of these drawbacks can be solved by adding the feedforward term of each line impedance voltage drop or injecting the virtual inductor. However, if the line impedances are high enough because of the long distance between the DG and the PCC or if the capacity of the system is large so that the output current is very large, this leads to a high virtual inductor voltage drop which causes reductions of the output voltage and power. Therefore, the line impedance voltage drops and the virtual inductor and resistor voltage drop compensation methods have been considered to solve these problems. The proposed method has been verified in comparison with the conventional droop method through PSIM simulation and low-scale experimental results.

Real-Time Haptic Rendering for Tele-operation with Varying Communication Time Delay (가변적인 통신지연시간을 갖는 원격 작업 환경을 위한 실시간 햅틱 렌더링)

  • Lee, K.;Chung, S.Y.
    • Journal of the Korea Society For Power System Engineering
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    • v.13 no.2
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    • pp.71-82
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    • 2009
  • This paper presents a real-time haptic rendering method for a realistic force feedback in a remote environment with varying communication time-delay. The remote environment is assumed as a virtual environment based on a computer graphics, for example, on-line shopping mall, internet game and cyber-education. The properties of a virtual object such as stiffness and viscosity are assumed to be unknown because they are changed according to the contact position and/or a penetrated depth into the object. The DARMAX model based output estimator is proposed to trace the correct impedance of the virtual object in real-time. The output estimator is developed on the input-output relationship. It can trace the varying impedance in real-time by virtue of P-matrix resetting algorithm. And the estimator can trace the correct impedance by using a white noise that prevents the biased input-output information. Realistic output forces are generated in real-time, by using the inputs and the estimated impedance, even though the communication time delay and the impedance of the virtual object are unknown and changed. The generated forces trace the analytical forces computed from the virtual model of the remote environment. Performance is demonstrated by experiments with a 1-dof haptic device and a spring-damper-based virtual model.

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An Enhanced Power Sharing Strategy for Islanded Microgrids Considering Impedance Matching for Both Real and Reactive Power

  • Lin, Liaoyuan;Guo, Qian;Bai, Zhihong;Ma, Hao
    • Journal of Power Electronics
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    • v.17 no.1
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    • pp.282-293
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    • 2017
  • There exists a strong coupling between real and reactive power owing to the complex impedances in droop based islanded microgrids (MGs). The existing virtual impedance methods consider improvements of the impedance matching for sharing of the voltage controlled power (VCP) (reactive power for Q-V droop, and real power for P-V droop), which yields a 1-DOF (degree of freedom) tunable virtual impedance. However, a weak impedance matching for sharing of the frequency controlled power (FCP) (real power for $P-{\omega}$ droop, and reactive power for $Q-{\omega}$ droop) may result in FCP overshoots and even oscillations during load transients. This in turn results in VCP oscillations due to the strong coupling. In this paper, a 2-DOF tunable adaptive virtual impedance method considering impedance matching for both real and reactive power (IM-PQ) is proposed to improve the power sharing performance of MGs. The dynamic response is promoted by suppressing the coupled power oscillations and power overshoots while realizing accurate power sharing. In addition, the proposed power sharing controller has a better parametric adaptability. The stability and dynamic performances are analyzed with a small-signal state-space model. Simulation and experimental results are presented to investigate the validity of the proposed scheme.

Stability Analysis Using G-Parameters of Converters Constituting DC Microgrid and Stability Enhancement Through Virtual Impedance (G-parameter를 이용한 직류 마이크로그리드의 컨버터 상호 안정도 분석 및 가상 임피던스를 이용한 안정도 향상)

  • Lee, Jae-Suk;Lee, Gi-Young;Kim, Rae-Young
    • The Transactions of the Korean Institute of Power Electronics
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    • v.23 no.5
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    • pp.321-327
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    • 2018
  • DC microgrid system composed of multiple converters has a tendency to make the system unstable due to the interaction of converters. To solve this problem, in this paper, the interaction between cascaded converters with LC input filter is analyzed with impedance modeling using g-parameter. The input impedance and the output impedance of the system can be obtained through this technique. The stability of the system can be determined by applying Middlebrook's stability criterion to the impedance. Virtual impedance is added to the controller to enhance stability. The validity of the analysis is verified by the result of several simulations and experiments.

A Study for the Effect of a Virtual Mass with a Low-Pass Filter on a Stability of a Haptic System (가상질량과 저주파통과필터에 의한 햅틱 시스템의 안정성 영역에 관한 연구)

  • Lee, Kyungno
    • Journal of Institute of Convergence Technology
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    • v.7 no.2
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    • pp.25-30
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    • 2017
  • This paper presents the effects of a virtual mass with a low-pass filter on the stability boundary of a virtual spring in the haptic system. In general, a haptic system consists of a haptic device, a sampler, a virtual impedance model and zero-order-hold. The virtual impedance is modeled as a virtual spring and a virtual mass. However the high-frequency noise due to the sampling time and the quantization error of sampled data may be generated when an acceleration is measured to compute the inertia force of the virtual mass. So a low-pass filter is needed to prevent the unstable behavior due to the high-frequency noise. A finite impulse response (FIR) filter is added to the measurement process of the acceleration and the effects on the haptic stability are simulated. According to the virtual mass with the FIR filter and the sampling time, the stability boundary of the virtual spring is analyzed through the simulation. The maximum available stiffness to guarantee the stable behavior is reduced, but simulation results still show that the stability boundary of the haptic system with the virtual mass is larger than that of the haptic system without the virtual mass.

Reproduction of Arm Kinesthetic Sense in Virtual Environment Using Bilateral Control (양방향 제어를 이용한 가상환경에서의 팔운동감 제시)

  • 정웅철;민두기;송재복;김용일
    • Proceedings of the Korean Society for Emotion and Sensibility Conference
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    • pp.97-100
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    • 1999
  • Human feels kinesthetic sense in response to the force acted on him. In order to represent kinesthetic sense, a force is analyzed as mechanical impedance (i.e., stiffness or damping) and implemented by active impedance control. In this research, a 3-dimensional arm motion generator is developed to present various mechanical impedance characteristics to an operator. An introduction of virtual reality provides not only a visual effect in virtual environment but also the change in force synchronized with the visual effect in real time.

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Distributed Adaptive Virtual Impedance Control to Eliminate Reactive Power Sharing Errors in Single-Phase Islanded Microgrids

  • Hoang, Tuan V.;Lee, Hong-Hee
    • Proceedings of the KIPE Conference
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    • pp.120-121
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    • 2017
  • This paper proposes an enhanced distributed generation (DG) unit with an adaptive virtual impedance control approach in order to address the inaccurate reactive power sharing problem. The proposed method can adaptively regulate the DG virtual impedance, and the effect of the mismatch in feeder impedances is compensated to share the reactive power accurately. The proposed control strategy is fully distributed and the need for the microgrid central controller is eliminated. Furthermore, the proposed method can be directly implemented without requirement of pre-knowledge of the feeder impedances. Simulations are performed to validate the effectiveness of the proposed control approach.

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