Virtual Loop Research Articles

Robust Virtual Inertia Control of a Low Inertia Microgrid Considering Frequency Measurement Effects

Virtual inertia emulation could be regarded as an inevitable component of microgrids with renewable energy, enhancing microgrid inertia and damping properties. In applying this control technique, a phase-locked loop (PLL) is necessary to obtain the estimation of the system frequency data. However, the employment of PLL could cause larger frequency oscillation to the microgrid due to its dynamics. This issue would be exacerbated in a low-inertia microgrid driven by high renewable penetration, severely deteriorating the frequency stability. Thus, the effect of PLL with measurement delay is a critical issue in utilizing the virtual inertia control. To overcome such problem, this paper proposes a robust virtual inertia control for a low-inertia microgrid to minimize the undesirable frequency measurement effects, improving the microgrid frequency stability. The robust H ∞ control design using a linear fractional transformation (LFT) technique is used to develop the virtual inertia control loop, considering the dynamics of PLL with measurement delay and the uncertainties of system inertia and damping. The efficacy of the proposed H ∞ control method is compared to the conventional and optimum proportional-integral (PI)based inertia control. The results show that the H ∞ -based robust virtual inertia control is superior to both conventional virtual inertia control and optimum PI-based virtual inertia control against a wide range of microgrid operating conditions, disturbances, and parametric uncertainties.

Design of a Cooperative Voltage Harmonic Compensation Strategy for Islanded Microgrids Combining Virtual Admittance and Repetitive Controller

Nonlinear loads (NLLs) in three-phase systems are known to produce current harmonics at −5, 7, −11, 13,… times the fundamental frequency. However, harmonics of the same frequencies are induced in microgrid voltage thereby reducing the power quality. Dedicated equipment, such as active power filters, can be used to compensate the microgrid harmonics. Alternatively, each distributed generation (DG) unit present in the microgrid can also be used for the same. The use of the virtual admittance concept combined with a proportional integral with resonant controller (PI-RES) control structure has previously been proposed as a harmonic compensation sharing strategy when multiple DGs operate in parallel. The drawback of this methodology is that a large number of RES controllers might be required to compensate for all the harmonic components induced by NLLs, increasing the tuning complexity as well as the execution time. This paper proposes the combined use of virtual admittance control loop and repetitive controller (RC) for harmonic compensation. The main advantage of the proposed method is that only one RC is required to compensate for all the harmonic components, significantly reducing the computational burden and the design complexity. The dynamic performance of the whole system is tested under variable NLL.

Virtual Impedance Sliding Mode Control-Based MMC Circulating Current Suppressing Strategy

The device rated capacity, system loss, and cost of a modular multilevel converter are proportional to the circulating current value. In order to enhance suppressing the second order harmonic component of circulating current and improve the dynamic response of the system, a novel circulating current suppressing strategy based on virtual impedance sliding mode control (VI-SMC-CCS) is proposed. This paper first analyzes the essential CCS mechanism, then presents a circulating current decoupling model using a state feedback linearization method, improving the system accuracy. Moreover, the sliding mode control and virtual impedance loop are introduced to strengthen system robustness. Different circulating current suppressing strategies are compared by simulation, the results demonstrate the correctness of the VI-SMC-CCS, which not only effectively reduces the second order harmonic component of the circulating current, but also improves the stability, reliability, and rapidity of the system.

Using virtual inductive loops to adapt the signal plans of junctions

Using virtual inductive loops to adapt the signal plans of junctions

Dead Center Identification of Single-DOF Multi-Loop Planar Manipulator and Linkage Based on Graph Theory and Transmission Angle

The dead center position (or singular position) is an important kinematic characteristic in mechanical design. However, its identification is a challenging task and becomes even more complex in multi-loop planar linkages (or manipulators). According to graph theory and transmission angle, this paper proposes a method to identify the dead center positions of single-degree-of-freedom (DOF) planar linkages. The planar linkages can be denoted by a set of independent loops based on topological structural analysis of the kinematic chains and a structural decomposition. According to the relationship between the mobility factor w i and loop factor f i in the same independent loop, the dead center positions can be located when the new planar linkages containing the corresponding instantaneous virtual loop are formed by adjusting f i to equal to w i or (w i -1), in which the corresponding transmission angle is equal to 0° and 180°. The Jacobian method for identifying the dead center position is then used to verify the proposed method. Furthermore, the dead center positions of the single-DOF 10-bar and 12-bar planar linkages are solved. The proposed method is systematic and programmable and can be applied to any single-DOF multi-loop linkage regardless of the number of independent loops or types of joints.

Decentralized Sliding Mode Control of WG/PV/FC Microgrids Under Unbalanced and Nonlinear Load Conditions for On- and Off-Grid Modes

This paper presents a new decentralized robust power/current/voltage/frequency control strategy for islanded and grid-connected modes to enhance power-sharing and small- and large-signal stability of a wind/photovoltaic/fuel cell microgrid and improve its performance for nonlinear and unbalanced loads. First, the distributed energy resources are modeled for unbalanced voltage condition. Then, in order to improve power sharing, regulate voltage and active/reactive powers injected by these resources, and moreover, harmonic and negative-sequence current control in the presence of nonlinear and unbalanced loads, three separate controllers for positive-sequence voltage and power control and negative sequence current control are designed based on the sliding mode control, Lyapunov function theory, and fractional-order sliding mode control, respectively. The theoretical concept of the proposed control strategy, including the mathematical modeling of microgrid components, basic theorems, controller design procedure, and robustness/closed-loop stability analysis, is outlined. Also, this direct power/current/voltage/frequency control scheme is governed by a hierarchical control scheme that exploits a voltage compensation scheme and harmonic virtual impedance loop. To show the effectiveness of the proposed control scheme, offline time-domain simulation studies are performed on a wind/photovoltaic/fuel cell microgrid with nonlinear and unbalanced loads in MATLAB/Simulink environment and the results are verified by OPAL-RT real-time digital simulator.

Use of a Mechanical Circulatory Support Simulation to Study Pump Interactions With the Variable Hemodynamic Environment.

The Virtual Mock Loop, a versatile virtual mock circulation loop, was developed using a lumped-parameter model of the mechanically assisted human circulatory system. Inputs allow specification of a variety of continuous-flow pumps (left, right, or biventricular assist devices) and a total artificial heart that can self-regulate between left and right pump outputs. Hemodynamic inputs were simplified using a disease-based input panel, allowing selection of a combination of cardiovascular disease states, including systolic and diastolic heart failure, stenosis, and/or regurgitation in each of the four valves, and high to low systemic and pulmonary vascular resistance values. The menu-driven output includes a summary of hemodynamic parameters and graphical output of selected flows, pressures, and volumes in the heart's four chambers as well as in the pulmonary artery and aorta. New tools to augment experimental research on implantable heart-assist devices and to increase our understanding of patient-specific pump interactions are in high demand. The purpose of this ongoing study is to demonstrate the use of a system analysis computer simulation to explore and better comprehend the interactions of mechanical circulatory support pumps with a more extensive combination of patient-specific or simulation conditions than can be established by practical experimentation. Usability is an important factor in constructing computer models for research purposes, and among our primary objectives in creating this simulation model were to make it as portable and useful as possible outside the lab environment, by people not involved in the creation of its operational software.

Fast single shot multibox detector and its application on vehicle counting system

Real-time vehicle detection and counting of multiple types is a difficult problem. To solve this problem, this study presents an efficient method based on single shot detection (SSD) to construct a vehicle detection and counting system. The proposed method named Fast-SSD first combines the Slim ResNet-34 with Single Shot MultiBox Detector. Then the authors limit the location prediction at each cell in the feature map and modify the detection network. When the input size of the picture is 300 × 300, Fast-SSD achieves the accuracy of 76.7 mAP on the PASCAL visual object classes 2007 test set. The network can be implemented at the speed of 20.8 FPS based on the GTX650Ti. Furthermore, they obtain the centre point of each type of vehicle which is detected by the Fast-SSD model in the image and set the virtual loop detectors to specify the detection range. The number of vehicles is calculated when the centre of the vehicle passes the virtual loop detector. Results show that the vehicle detection accuracy achieves 99.3% and the classification accuracy is 98.9%.

Small-Signal Modelling and Stability Analysis of Current-Controlled Virtual Synchronous Generators

Virtual Synchronous Generators (VSG), as an effective mean to improve the frequency and voltage regulation ability of the renewable power, had aroused wide concern. Researchers have proposed several VSG implementations, which can be classified into two categories: voltage-controlled VSG and current-controlled VSG. Voltage-controlled VSG have been extensively studied. However, few researches focused on current-controlled VSG. In this paper, we concentrated on the current-controlled VSG and its stability problem. Firstly, a small-signal model of current-controlled VSG was established to identify the oscillation modes of grid-connected VSG system. Moreover, the developed model was used to investigate damping characteristics of the system under various scenarios and with different control parameters. The results revealed that stability of VSG system was sufficiently impacted by virtual inertia and phase lock loop in VSG. Under some unfavourable conditions, current-controlled VSG would contribute to high-frequency or subsynchronous oscillation.

Research of the high supply voltage quality control for solid‐state transformer

The harmonics flow characteristic of microgrid will be changed when solid-state transformer (SST) replaces solid-state switch or traditional transformer (TT). The supply voltage quality of SST is related to its control strategy at this time. Therefore, a high supply voltage quality control strategy for SST has been proposed in this study. First, the supply voltage characteristics of SST and TT have been compared, and finding that the voltage quality is easy to worsen when SST working without power quality control strategy. Then, the voltage quality regulating mechanism of SST has been analysed. In addition, a novel control strategy which includes the direct voltage control loop and the virtual impedance loop has been proposed for SST. The characteristics of the voltage loop and virtual impedance loop are analysed. The computational burden, harmonic extraction and phase-locked-loop are considered in the voltage control loop. The compensational state can vary with the surplus compensation capacity by adopting the virtual impedance loop. Finally, the experimental results verified the validity and effectiveness of the presented control strategy.

Multiple Fusion Based on the CCD and MEMS Accelerometer for the Low-Cost Multi-Loop Optoelectronic System Control.

In the charge-coupled device (CCD) and micro-electro-mechanical system (MEMS) accelerometer based low-cost multi-loop optoelectronic control system (OCS), due to accelerometers’ drift and noise in low frequency, the disturbance suppression (DS) is insufficient. Previously, based on the acceleration and position dual-loop control (ADLC), researchers combined a disturbance observer (DOB) with a virtual velocity loop to make some medium-frequency DS exchange for low-frequency performance. However, it is not optimal because the classic DOB based on accelerometers’ inaccurate signals cannot observe accurate disturbance in low frequency and the velocity based on a CCD and accelerometer time-domain fusion carried the CCD’s delay, resulting in the drop of medium-frequency DS. In this paper, considering the CCD’s advantage in low frequency and the accelerometer’s strength in high frequency, we propose to fuse their signals twice with a modified complementary filter method to respectively acquire an acceleration and velocity. The new acceleration with no drift and less noise but lower bandwidth creates a new acceleration model and is only used in fusion DOB (FDOB), while the velocity with little delay is to build an additional velocity loop. Compared with the traditional DOB enhanced by the time-domain fusion velocity loop, experiments verify that the proposed multiple fusion would apparently enhance the system’s DS, especially in low and medium frequency.

Image-based approach for detecting vehicles in user-defined virtual inductive loops

A camera-based scheme is proposed for detecting vehicles at user-defined virtual loops, simulating the operation of inductive loops. False vehicular detections are minimized by a combination of efficient edge detection and color information. The experimental results suggest that the proposed scheme potentially can detect and count vehicles at user-defined virtual loops accurately (with more than 98% correct detection rate, in average), besides being more robust to cast shadows and sudden illumination changes than comparable methods that represent the state of the art.

Virtual Inertia Control Application to Enhance Frequency Stability of Interconnected Power Systems with High Renewable Energy Penetration

Due to a rapid increase in the utilization of power converter-based renewable energy sources (RESs), the overall system inertia in an interconnected power system might be significantly reduced, increasing the vulnerability of the interconnected power system to the system instability. To overcome problems caused by the significant reduction in system inertia, this paper proposes a new application of virtual inertia control to improve frequency stability of the interconnected power system due to high penetration level of RESs. The derivative control technique is introduced for higher-level applications of virtual inertia emulation. Thus, the proposed virtual inertia control loop has a second-order characteristic, which provides a simultaneous enabling of damping and inertia emulations into the interconnected power system, enhancing frequency stability and resiliency. System modeling and simulation results are carried out using MATLAB/Simulink® (R2016b, MathWorks, Natick, MA, USA). Trajectory sensitivities are also performed to analyze the dynamic effects of virtual inertia control’s parameters on the system stability. The effectiveness of the virtual inertia control concept on stability improvement is verified through a multi-area test system with high RESs penetration level for different contingencies.

Enhanced proportional power sharing strategy based on adaptive virtual impedance in low‐voltage networked microgrid

The variation of the electrical distance and the complexity of the electric network lead to the variations of feeder impedances between distributed generation units and load points. It is determined that conventional droop control has drawbacks in achieving accurate power sharing due to the effects of mismatched impedance. Therefore, this study proposes an enhanced proportional power sharing strategy based on adaptive virtual impedance in a low-voltage networked microgrid. The improved R-L type droop control can effectively prevent the coupling between real and reactive powers. Furthermore, an adaptive virtual impedance loop is introduced to counteract the feeder voltage drop. The method utilises real and reactive power mismatching which were fed into integral controllers, and then generates the virtual inductive and resistive components, respectively. This proposed strategy is able to enhance power sharing accuracy without requiring the knowledge of feeder impedance, and it is more adaptive to the complex impedance. The simulation experiments carried out under the environment of MATLAB/Simulink, and results verify the effectiveness of the proposed strategy.

Frequency Response Analysis of a Single-Area Power System with a Modified LFC Model Considering Demand Response and Virtual Inertia

The modern electric power system is foreseen to have increased penetration of controllable loads under demand response programs and renewable energy resources coupled with energy storage systems which can provide virtual inertia. In this paper, the conventional model of an electric power system is appended by considering, individually and collaboratively, the role of demand response and virtual inertia for the purpose of frequency analysis and control. Most existing literature on this topic either considers one of these two roles or lacks in providing a general model of power system with demand response and virtual inertia. The proposed model is presented in general form and can include/exclude demand response and/or virtual inertia. Further, power system operator can opt the power shares from conventional, demand response, and virtual inertia loops for frequency regulation and can also evaluate the impact of other parameters such as time delays and frequency deadbands on system frequency response. The mathematical formulation of steady-state values of frequency deviation and power contribution from all resources is provided and validated by simulation results under various scenarios including a case of wind intermittency.

Application of the Feynman-tree theorem together with BCFW recursion relations

Recently, it has been shown that on-shell scattering amplitudes can be constructed by the Feynman-tree theorem combined with the BCFW recursion relations. Since the BCFW relations are restricted to tree diagrams, the preceding application of the Feynman-tree theorem is essential. In this way, amplitudes can be constructed by on-shell and gauge-invariant tree amplitudes. Here, we want to apply this method to the electron–photon vertex correction. We present all the single, double, and triple phase-space tensor integrals explicitly and show that the sum of amplitudes coincides with the result of the conventional calculation of a virtual loop correction.

Application of image processing to calculate the number of fish seeds using raspberry-pi

Many fish cultivator in Indonesia who suffered losses due to the sale and purchase of fish seeds did not match the agreed amount. The loss is due to the calculation of fish seed still using manual method. To overcome these problems, then in this study designed fish counting system automatically and real-time fish using the image processing based on Raspberry Pi. Used image processing because it can calculate moving objects and eliminate noise. Image processing method used to calculate moving object is virtual loop detector or virtual detector method and the approach used is “double difference image”. The “double difference” approach uses information from the previous frame and the next frame to estimate the shape and position of the object. Using these methods and approaches, the results obtained were quite good with an average error of 1.0% for 300 individuals in a test with a virtual detector width of 96 pixels and a slope of 1 degree test plane.

Two-Stage Battery Energy Storage System (BESS) in AC Microgrids with Balanced State-of-Charge and Guaranteed Small-Signal Stability

In this paper, a two-stage battery energy storage system (BESS) is implemented to enhance the operation condition of conventional battery storage systems in a microgrid. Particularly, the designed BESS is composed of two stages, i.e., Stage I: integration of dispersed energy storage units (ESUs) using parallel DC/DC converters, and Stage II: aggregated ESUs in grid-connected operation. Different from a conventional BESS consisting of a battery management system (BMS) and power conditioning system (PCS), the developed two-stage architecture enables additional operation and control flexibility in balancing the state-of-charge (SoC) of each ESU and ensures the guaranteed small-signal stability, especially in extremely weak grid conditions. The above benefits are achieved by separating the control functions between the two stages. In Stage I, a localized power sharing scheme based on the SoC of each particular ESU is developed to manage the SoC and avoid over-charge or over-discharge issues; on the other hand, in Stage II, an additional virtual impedance loop is implemented in the grid-interactive DC/AC inverters to enhance the stability margin with multiple parallel-connected inverters integrating at the point of common coupling (PCC) simultaneously. A simulation model based on MATLAB/Simulink is established, and simulation results verify the effectiveness of the proposed BESS architecture and the corresponding control diagram.

Fully Distributed Hierarchical Control of Parallel Grid-Supporting Inverters in Islanded AC Microgrids

In this paper, a fully distributed hierarchical control strategy is proposed for operating networked grid-supporting inverters (GSIs) in islanded ac microgrids (MGs). The primary control level implements frequency and voltage control of an ac MG through a cascaded structure, consisting of a droop control loop, a virtual impedance control loop, a mixed ${H_2}/{H_\infty }$ -based voltage control loop, and a sliding-mode-control-based current loop. Compared to conventional proportional-plus-integral-based cascaded control, the proposed cascaded control does not require a precise model for the GSI system. The proposed secondary control level implements distributed-consensus-based economic automatic generation control and distributed automatic voltage control, which integrates the conventional secondary control and tertiary control into a single control level by bridging a gap between traditional secondary control and tertiary control. Simulation results demonstrate the effectiveness of the proposed hierarchical control strategy.

Robust Virtual Inertia Control of an Islanded Microgrid Considering High Penetration of Renewable Energy

This paper presents robust virtual inertia control of an islanded microgrid considering high penetration of renewable energy sources (RESs). In such microgrids, the lack of system inertia due to the replacement of traditional generating units with a large amount of RESs causes undesirable influence to microgrid frequency stability, leading to weakening of the microgrid. In order to handle this challenge, the $H_{\mathbf {\infty }}$ robust control method is implemented to the virtual inertial control loop, taking into account the high penetration of RESs, thus enhancing the robust performance and stability of the microgrid during contingencies. The controller’s robustness and performance are determined along with numerous disturbances and parametric uncertainties. The comparative study between $H_{\mathbf {\infty }}$ and optimal proportional-integral (PI)-based virtual inertia controller is also presented. The results show the superior robustness and control effect of the proposed $H_{\mathbf {\infty }}$ controller in terms of precise reference frequency tracking and disturbance attenuation over the optimal PI controller. It is validated that the proposed $H_{\mathbf {\infty }}$ -based virtual inertia controller successfully provides desired robust frequency support to a low-inertia islanded microgrid against high RESs penetration.