Admittance Model Research Articles

Selection of intermediate routes for secure data communication systems using graph theory application and grey wolf optimisation algorithm in MANETs

A mobile ad-hoc network (MANET) consists of set of mobile nodes and communication devices to interact without any support of the infrastructure. Unlike wired networks, the challenges faced in MANETs possessed such as security design, network infrastructure, stringent energy resources and network security issues. The need for these security issues is much focused in overcoming the challenges in MANET. The proposed work focuses on the secure communication using a novel approach addressed on radio station problem for route optimisation, which is deployed by grey wolf optimisation algorithm and supportively task based admittance control model is proposed to monitor the network flow and to identify the malicious nodes. It is also called as role-based control model. The communication is performed based on the cluster of vast sizes these trusted in node(s) will probably be conveying together, at the same time non-entry correspondence of the non-authorised node(s) to keep on maintaining a stable, anchored and reliable gathering of mobile nodes. Simulation is performed using network simulator where the network parameters such as throughput, packet delivery ratio and delay and packet loss are analysed to identify the malicious nodes.

Complex-Valued Multifrequency Admittance Model of Three-Phase VSCs in Unbalanced Grids

This article proposes a multifrequency admittance model for voltage-source converters with three-phase unbalanced grid voltages. The model is derived with multiple complex vectors and harmonic transfer functions, which is merely dependent on its own input voltage trajectory, and can accurately capture the frequency-coupling dynamics. The dynamic effects of both the basic synchronous-reference-frame phase-locked loop (PLL) and its alternative with a notch filter of the negative-sequence voltage component are compared. It is revealed that the notch-filtered PLL significantly weakens the frequency-coupling effects, which leads to a reduced order of the admittance model. The developed model is validated by a frequency scan, and the frequency-coupling effects impacted by different PLLs and voltage unbalance factors are verified by the experimental tests. Finally, a case study on stability analysis in unbalanced grids proves the significance of the model.

Impedance and Stability Analysis of Voltage-Source Converter Interconnection Based on a Universal Admittance Model

A universal admittance model of voltage-source converter applicable to various DQ-frame-based control architectures is developed from the sequence-domain complex space vector analysis. The lumped sequence admittances of converter interconnection considering multicoupling effects are derived based on the converter admittance model, in which two coefficients are defined to quantify the cross-coupling effect between the ac and dc networks. From the universal model, the converter admittances with three typical control architectures and the lumped admittances of converter interconnection are calculated, compared, and verified with time-domain point-by-point simulations. Experiments are also conducted for further verification. Stabilities of the converter interconnection with the three typical control architectures are analyzed and compared. The obtained results indicate that introducing the outer dc voltage control can not only reinforce the coupling between ac and dc circuits but also degrade the stability performance of ac and dc circuits both.

A Nearly Decoupled Admittance Model for Grid-Tied VSCs Under Variable Operating Conditions

Impedance/admittance models (IM/AMs) have been effectively utilized for the stability analysis of converter participated emerging oscillation issues. The IM/AMs are either derived from existing theoretical knowledge or measured by experimental approaches. However, it is quite challenging to get the IM/AMs of black/gray-box devices [such as converter based wind turbine generators (WTGs)] over a wide range of operating conditions. This article proposes a method to identify the AM of voltage-sourced converters by exploiting the relationship between measured AMs and the bus voltage and line current at the point of common coupling. First, an AM is analytically derived, which is almost decoupled with the operating points, and is named as nearly decoupled AM (NDAM). Then, a measurement-based method is further proposed to identify the NDAM. Furthermore, time-domain simulations are used to verify the accuracy of the method. Finally, control-hardware-in-the-loop tests are carried out to demonstrate the effectiveness of the developed method to identify the AM of a direct-drive WTG under variable operating conditions.

A linearized small-signal Thévenin-equivalent model of a voltage-controlled modular multilevel converter

This paper presents a linearized Thévenin-equivalent circuit for a grid-forming Modular Multilevel Converter (MMC) under double closed-loop control, comprehending an outer voltage control-loop together with an inner current control-loop. Both the AC-side Thévenin model and the equivalent admittance of the DC-side were derived in this work. The proposed MMC model takes into account the influence of passive elements and the control-loop effects on the impedances and gains. With this model it was possible to observe how the control system parameters changes resonant frequencies in both DC-side equivalent admittance and AC-side equivalent model. A comparison between the frequency response of the proposed analytical model and the frequency response obtained from the nonlinear time-domain model is presented to validate the proposed MMC model. The proposed model allows analyzing the dynamic response of the MMC as grid-forming converter in the frequency domain. Finally, as an application example, the stability of an ordinary power-electronic-based system (comprised of a grid-forming MMC supplying a current controlled MMC) was assessed and PSCAD/EMTDC simulations are presented to validate the results predicted by the proposed model.

An improved virtual inductance control method considering PLL dynamic based on impedance modeling of DFIG under weak grid

Impedance-based method is an effective method to analyze the system stability. Based on the impedance stability theory, an improved virtual inductance control method considering PLL dynamic is presented for the stability of doubly-fed induction generator (DFIG) under weak grid condition. Firstly, equivalent input admittance model of the DFIG system including all its controller is developed in the synchronous reference (d-q) frame. Secondly, the generalized Nyquist criterion (GNC) proves that the stability of the DFIG system can be improved by increasing the rotor inductance. Then, a virtual inductance term is introduced into the RSC current controller to emulate additional inductance in the rotor. However, the virtual inductance method may be ineffectual due to the influence of the PLL dynamic under weak grid condition. Therefore, an improved virtual inductance control method considering the PLL dynamic is implemented to improve the stability of the DFIG system under weak grid condition, and the simulation is used to verify the effectiveness of the improved virtual inductance control strategy.

Impedance modeling and stability analysis of three-phase three-level NPC inverter connected to grid

The interaction between grid-connected inverters and the grid may cause stability issues, and compromise the reliable operation of the inverters. This study investigates the stability of a three-level neutral point clamped (NPC) inverter connected to the grid using impedance-based methods. Because the impedance model of a three-phase three-level NPC inverter has not yet been reported, this study fills the literature gap by analyzing the influence of three-level DC-side neutral point control on the impedance characteristics. By fully considering the DC bus dynamics and DC voltage control loop, and the current loop and phase-locked loop (PLL), the admittance model of a three-phase three-level NPC inverter is established and verified by simulation. Additionally, in the stability analysis of a three-level NPC inverter grid-connected system, the frequency coupling introduced by the PLL and DC bus dynamics is included with the help of an established admittance model. The stability of the grid-connected system under different grid short circuit ratios (SCR) and operating power levels is analyzed according to the Nyquist stability criterion. The experimental results revealed that the established impedance model of the three-phase three-level NPC inverter can properly represent the stability of this system.

A Robust Adaptive Admittance Control Scheme for Robotic Knee Prosthesis Using Human-Inspired Virtual Constraints

A robust adaptive admittance control scheme using human-inspired virtual constraints was presented for a robotic knee prosthesis. The controller is able to deal with the wearer's motion intention as well as the partial unknown parameter values of the prosthesis dynamics. The desired trajectory of the prosthetic knee joint is parameterized by the amputee-driven thigh phase variable and implemented by human-inspired virtual constraints rather than the preprogrammed time-dependent trajectory or human data replaying. A reference admittance model was set up to make the prosthesis be more compliant in the presence of ground reaction forces impact. A composite reaching law is proposed to cope with the chattering phenomenon and the finite-time convergence problem. The controller is designed by the back-stepping method based on Lyapunov. The tracking performance and the stability of the closed-loop system are proven via Lyapunov stability analysis. The feasibility and effectiveness of the proposed control scheme are proved in simulation results.

Neural Learning Enhanced Variable Admittance Control for Human–Robot Collaboration

In this paper, we propose a novel strategy for human-robot impedance mapping to realize an effective execution of human-robot collaboration. The endpoint stiffness of the human arm impedance is estimated according to the configurations of the human arm and the muscle activation levels of the upper arm. Inspired by the human adaptability in collaboration, a smooth stiffness mapping between the human arm endpoint and the robot arm joint is developed to inherit the human arm characteristics. The estimation of stiffness term is generalized to full impedance by additionally considering the damping and mass terms. Once the human arm impedance estimation is completed, a Linear Quadratic Regulator is employed for the calculation of the corresponding robot arm admittance model to match the estimated impedance parameters of the human arm. Under the variable admittance control, robot arm is governed to be complaint to the human arm impedance and the interaction force exerted by the human arm endpoint, thus the relatively optimal collaboration can be achieved. The radial basis function neural network is employed to compensate for the unknown dynamics to guarantee the performance of the controller. Comparative experiments have been conducted to verify the validity of the proposed technique.

The Swing Control of Knee Exoskeleton Based on Admittance Model and Nonlinear Oscillator

It is important to control exoskeleton to act synchronously with the user for movement assistance. A knee exoskeleton was constructed for gait rehabilitation training, and a control method based on adaptive frequency oscillator (AFO) was presented for swing motion of knee exoskeleton. Firstly, during the learning mode, the user flexed and extended with the exoskeleton leg together, and the exoskeleton leg was not actuated. The phase of the exoskeleton angle extracted, and the envelope curve of the user’s EMG is generated based on AFO. Then, during power-assisted mode, the assistive torque is generated according to the angle phase on-line and the restructuring muscle torque. At last, the assistive torque as a feedback signal was input to the admittance model for the swing control of the knee exoskeleton. The experiments of swing control showed that knee exoskeleton can follow the user’s motion well.

New Perspectives on Stability Analysis of Three-Phase Grid-Connected Inverter Considering System Couplings

This paper considers the three-phase grid-connected inverter (GCI) as a MIMO system, and presents some new perspectives on the stability of GCI. It is found that with traditional Nyquist criterion (NC)-based method which ignores the off-diagonal components of impedance or admittance matrix, the stability results of impedance and admittance models are different. To make up this disadvantage, a simple stability analysis method based on system feedback ratio matrix (SFRM) is proposed. Instead of ignoring the couplings of impedance or admittance, the SFRM in the proposed method can be equivalent to diagonal matrix by adding a perturbation matrix into the system model, so identical results of stability analysis under impedance and admittance models are attained. Furthermore, to ensure the correctness of stability result, the boundary conditions of the proposed method are also provided. Theoretical analysis, simulation and experimental results show that compared with the state of art methods, the proposed SFRM-based method is of low computational complexity and high accuracy, and is more suitable for engineering application.

Admittance Modeling and Stability Analysis for Inverter-Based Microgrid With Voltage-Source-Rectifier Load

Instability and low-frequency oscillation phenomenon of inverter-based microgrid (IBM) with voltage-source-rectifier load have obtained increasing concern. However, the influence of instantaneous angle of operation point on IBM is usually neglected during voltage faults, and it may generate the uncertain impact of voltage faults on the stable and safe operation of IBM. Besides, the influence of instantaneous angle of operation point may cause varying degrees of system oscillation and stability at a different angle of system states' change, including fault recovery. To solve this issue, a systematic approach of small-signal admittance modeling for IBM in the αβ frame is proposed in this article. The small-signal admittance model of each submodule (SM) of IBM is established and formed as a multi-input-multi-output (MIMO) system considering the actual closed-loop controls and decoupled strategies in the dq frame. Based on the proposed admittance models of SMs, the complete model of IBM is integrated and linearized, and the influence of both amplitude and instantaneous angle of operation point on the IBM stability is analyzed in small-signal model comprehensively. Besides, using the generalized Nyquist stability criterion (GNC), the influence of instantaneous angle in large-signal operation point on oscillation amplitude and stability of IBM is investigated. The proposed analysis approach can analyze and determine the responded oscillation and stability of the system more accurately during system states' variation, such as voltage sag and sudden changes in load. It also reduces the uncertainty of IBM behaviors during states' variation to ensure that IBM operates smoothly, flexibly, and stably. Finally, experimental results are presented to validate the correctness of the proposed analysis method.

Passivity-Based Stabilization of LCL-Type Grid-Connected Inverters via a General Admittance Model

For LCL-type grid-connected inverters, inverter-side current, grid-side current, capacitor current and voltage are four commonly used variables in the control serving as different roles, such as control objective, active damping, synchronization, etc. This article builds a general admittance model, in which the total output admittance is decomposed into four paralleled subadmittance corresponding to the above four variables in the control law, to facilitate the passivity-based stability assessment and controller parameter optimization for different control objectives and schemes. Based on the passivity analysis, an appropriate combination scheme of subadmittance and a simple unified analytical controller parameter design method are proposed to obtain a passive output admittance from 0 Hz up to Nyquist frequency for either inverter-side or grid-side current control. Therefore, it can not only ensure the stable operation of LCL-type inverters regardless of grid impedance (either inductive or capacitive) but also avoid the conventional design constraints of an LCL filter, i.e., f <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">r</sub> <; or > f <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">s</sub> /6 is required for the single-loop inverter-side or the grid-side current control. Finally, experiments verify the theoretical results.

Stability and Transparency of Delayed Bilateral Teleoperation with Haptic Feedback

Abstract Haptic guidance can improve control accuracy in bilateral teleoperation. With haptic sensing, the human operator feels that he grabs the robot on the remote side. There are results on the stability and transparency analysis of teleoperation without haptic guidance, and the analysis of teleoperation with haptic feedback is only for linear and zero time-delay systems. In this paper, we consider more general cases: the bilateral teleoperation systems have time-varying communication delays, the whole systems are nonlinear, and they have force feedback. By using the admittance human operator model, we propose a new control scheme with the interaction passivity of the teleoperator. The stability and transparency of the master-slave system are proven with the Lyapunov–Krasovskii method. Numerical simulations illustrate the efficiency of the proposed control methods.

Amplitude Distribution of Partial Discharge Signals on Tunnel-Installed High-Voltage Cables

For 110 kV and above tunnel-installed high-voltage (HV) cross-linked poly-ethylene (XLPE) cable systems, it is a normal procedure to adopt a cross-bonding scheme. The high-frequency current method is frequently used in the cross-bonded cable systems for on-site or online partial discharge (PD) detection by monitoring the signals on the cross-bonding wires. To further study the amplitude distribution characteristics of the PD signals, a parametric characteristic admittance model of a three-phase cable system in a tunnel is established based on Tylavsky’s formulas. The model is used to calculate the amplitude distribution formula of the PD pulse current on the cross-bonding wires. In addition, the influence of cable laying and tunnel environment on the amplitude distribution is also studied. Finally, the correctness of the model and the conclusion are verified by simulation experiments and on-site tests. The results show that the signal amplitude distribution is determined by the ratio of the characteristic admittances. As the distance between the cables and the distance from the inner wall of the tunnel increase, the amplitude difference gradually decreases.

Admittance control based robotic clinical gait training with physiological cost evaluation

In this paper, we investigate an effective control strategy for a cane-type assistive mobile robot toward clinical gait training. Assistive robots are expected to provide aid in order to reduce the burden of caregivers and physical therapists, e.g., in gait rehabilitation of elderly people. Our group has been developing a series of cane-type walking assistive robots named Intelligent Cane as a mobile hand-holding device based on admittance control to provide safe and efficient gait training. This paper explores a systematic design methodology of the admittance control model in order to provide suitable walking load during gait training. We first conduct a pilot experiment to investigate the relationship between the physiological cost of user’s walking and the coefficients in the admittance control model of the cane robot. Then, we present a clinical gait training study conducted in a hospital to evaluate the feasibility in practical use of the proposed control strategy of our cane robot in gait rehabilitation. These experimental results suggest the effectiveness of the proposed gait rehabilitation strategy with our robot.

Symmetric Admittance Modeling for Stability Analysis of Grid-Connected Converters

This paper proposes a novel admittance model for the stability analysis of grid-connected voltage-source converters (VSCs) which features a symmetric structure. On this basis, the grid-connected VSC system can be represented by a reciprocal two-port circuit to explore the oscillation mechanism of the system. Furthermore, the grid-connected VSC system can be modelled as a single-input single-output (SISO) system rather than a multi-input multi-output (MIMO) system. We analytically evaluate the influences of the VSC control parameters and the power factor on the stability of the grid-connected VSC by employing the Nyquist criterion and the stability margin. In addition, we demonstrate that the VSC's current output is in inverse proportion to the system's gain margin. Further, an analytical expression of the current limit constrained by small-signal stability is derived in closed form so as to calculate the admissible current output. All of analysis results are verified by simulations based on Simulink and hardware-in-the-loop (HIL) platform.

Frequency-Coupling Admittance Modeling of Converter-Based Wind Turbine Generators and the Control-Hardware-in-the-Loop Validation

Recently, sub- and super-synchronous oscillation (SSO) associated with converter-based wind turbine generators (WTGs) has been attracting wide attentions. To investigate such SSO incidents, impedance or admittance-based modeling and analysis were widely used. It was discovered that coupling effects exist between impedances or admittances at complementary (i.e., sub and super-synchronous) frequencies. However, there is a lack of experimental validation on the coupling effects of different WTGs. In this paper, the frequency-coupling admittance models (FCAMs) of two types of widely adopted WTGs, i.e., direct-drive and doubly-fed WTGs, are presented at first. Secondly, a perturbation based identification method is proposed to obtain the FCAMs. Next, the method is implemented in a control-hardware-in-the-loop (CHIL) platform based on RTDS. Then with extensive CHIL tests, the FCAMs of both types of WTGs are obtained, and the impacts of frequency-coupling effects on SSO are validated. Finally, the characteristics of frequency-coupling effects and the root causes are further examined. This study is of important reference value to select appropriate models for oscillatory stability analysis.

An admittance controller based on assistive torque estimation for a rehabilitation leg exoskeleton

A rehabilitation exoskeleton leg was constructed for gait training, and a method for exoskeleton leg swing control based on admittance model was studied. The man–machine interaction torque, the inertia compensator torque, and the exoskeleton’s assistive torque act as the inputs to the admittance control system for realizing the effective assistance to the wearer. The exoskeleton’s assistive torque is generated by estimating net torque exerted by the muscle based on adaptive frequency oscillator. A variable swing frequency experiment for simulating physical rehabilitation exercise was implemented. The results show that the inertia compensator adjusting the compensation varies with the swing frequency. The control system provides effective assistance to the wearer. The coordinated control experiment of hip and knee joints was also implemented. The results show that the interaction forces are all controlled in a reasonable and small range, and there is a good coordination between the hip joint and knee joint during swing motion.

Quantifying the Interface Defect for the Stability Origin of Perovskite Solar Cells

AbstractThe stability issue that is obstructing commercialization of the perovskite solar cell is widely recognized, and tremendous effort has been dedicated to solving this issue. However, beyond the apparent thermal and moisture stability, more intrinsic semiconductor mechanisms regarding defect behavior have yet to be explored and understood. Herein, defects are quantified; especially interface defects, within the cell to reveal their impact on device performance and especially stability. Both the bulk and interface defects are distinguished and traced in situ using an expanded admittance model when the cell degrades in its efficiency under illumination or voltage. The electric field‐induced interface, rather than bulk defects, is found to have a direct correlation to stability. Releasing the interface strain using a fullerene derivative is an effective way to suppress interface defect formation and improve stability. Overall, this work provides a quantitative approach to probing the semiconductor mechanism behind the stability issue, and the inherent correlation discovered here among the electric field, interface strain, interface defects, and cell stability has important implications for ongoing device stability engineering.