Effect Of Communication Delay Research Articles

Distributed periodic event-triggered secondary control for islanded microgrids with switching topologies and communication delays

With the application of distributed generators (DGs), microgrids (MGs) have gained much attention. Combined with the distributed communication of networked systems, we investigates the secondary restoration problem in islanded microgrids. The periodic event-triggered (PET) controllers are designed for voltage restoration and frequency restoration with accurate active power allocation, which can naturally avoid Zeno behavior. By means of a common Lyapunov function, a PET protocol is designed that requires only the information of neighbors for event detection. Both event-checking sampling period and event trigger parameter are taken into account to reduce computation and communication redundancy by choosing appropriate parameters. Furthermore, considering the effect of communication delay, sufficient conditions are given to accomplish the secondary control objectives with allowable delays. Switching topologies are considered in an extra discussion. Finally, the effectiveness of the proposed PET control strategies is verified by several MATLAB simulations.

Effects of communication delay in the dual cockpit remote robotic surgery system

PurposeTo evaluate the impact of dual cockpit telesurgery on proctors and operators, and acceptable levels of processing delay for video compression and restoration.MethodsEight medical advisors and eight trainee surgeons, one highly skilled per group, performed gastrectomy, rectal resection, cholecystectomy, and bleeding tasks on pigs. Using the Medicaroid surgical robot hinotori™, simulated delay times (0 ms, 50 ms, 100 ms, 150 ms, and 200 ms) were inserted mid-surgery to evaluate the tolerance level. Operative times and dual cockpit switching times were measured subjectively using 5-point scale questionnaires (mSUS [modified System Usability Scale], and Robot Usability Score).ResultsNo significant difference was observed in operative times between proctors and operators (proctor: p = 0.247, operator: p = 0.608) nor in switching times to the dual cockpit mode (p = 0.248). For each survey setting, proctors tended to give lower ratings to delays of ≥ 150 ms. No marked difference was observed in the operator evaluations. On the postoperative questionnaires, there were no marked differences in the mSUS or Robot Usability Score between the proctors and operators (mSUS: p = 0.779, Robot Usability Score: p = 0.261).ConclusionTelesurgery using a dual cockpit with hinotori™ is practical and has little impact on surgical procedures.

Large-Signal Stability Analysis for Inverter-Based Dynamic Microgrids Reconfiguration

Compared to conventional microgrids (MGs) with pre-defined and static topology, dynamic MGs feature varying electric boundaries and enhanced operational resiliency. This paper evaluates the operation stability of inverter-based dynamic MGs in terms of fulfilling the requested MGs topology change and associated network reconfiguration, which falls into the category of large-signal stability analysis. A complete nonlinear state-space model of inverter-based dynamic MGs is derived, where coordinated controls among inverters along with the effect of communication delays have been considered. In response to reconfiguration demands, an evaluation scheme is developed to determine whether the system could perform seamless topology variation without losing synchronism. Specifically, the system’s stability region is quantified by the domain of attraction (DoA) around the desired operating point using Takagi-Sugeno fuzzy modeling (T-S multimodeling). The large-signal stability of a multi-bus inverter-based dynamic MG with both interface inverters and sectionalizing switches have been analyzed. The stability regions estimated under different scenarios of topology variation have been validated using time-domain simulation in MATLAB/Simulink. The developed evaluation scheme is used to quantify the effects of control gain designs and communication delays on system large-signal stability.

A new hybrid control technique for operation of DC microgrid under islanded operating mode

This study proposes a novel combined primary and secondary control approach for direct current microgrids, specifically in islanded mode. In primary control, this approach establishes an appropriate load power sharing between the distributed energy resources based on their rated power. Simultaneously, it considers the load voltage deviation and provides satisfactory voltage regulation in the secondary control loop. The proposed primary control is based on an efficient droop mechanism that only deploys the local variable measurements, so as to overcome the side effects caused by communication delays. In the case of secondary control, two different methods are devised. In the first, low bandwidth communication links are used to establish the minimum required data transfer between the converters. The effect of communication delay is further explored. The second method excludes any communication link and only uses local variables. Accordingly, a self-sufficient control loop is devised without any communication requirement. The proposed control notions are investigated in MATLAB/Simulink platform to highlight system performance. The results demonstrate that both proposed approaches can effectively compensate for the voltage deviation due to the primary control task. Detailed comparisons of the two methods are also provided.

The effect of communication delays on the frequency stability of power systems integrated with inverter air conditioners

Massive inverter air conditioners (IACs) are widely integrated into the smart grid, which can be employed to provide frequency support. However, the participation of IACs may inevitably introduce communication delays in the process of data detection and transmission, which easily leads to oscillation and instability of power systems. In this paper, we explore a delay-dependent stability of the power system with the participation of IACs and further reveal the impact of delay in the IACs control loop during frequency regulation. Initially, a linear time-delay system based on the state-space model is established, making it possible to investigate the effect of delays by a time-domain method. Then, applying Lyapunov theory and linear matrix inequality technique, a delay-dependent stability criterion is proposed, considering the tradeoff between accuracy of delay margins and computational efficiency. Finally, the interactive effects among delay margin and PI controller gains are explored in different scenarios and the validity of the proposed criterion is verified, with a less computational burden that can achieve nearly accurate delay margins. The proposed criterion supplies reference for determining the delay requirement and selecting controller parameters of IACs participating in frequency regulation.

Distributed secondary controller to ensure proportional sharing of reactive power in AC microgrid

In an ac microgrid, reactive power sharing accuracy is affected due to unequal values of interconnecting cable impedances. To resolve this issue, secondary controllers are used to compensate the effect of cable impedances. Various types of secondary controllers are suggested in the literature which includes linear proportional plus integral (PI) controllers to minimize the difference in reactive power sharing created by conventional E−Q droop law. The reference value of the PI controller is the average value of reactive powers supplied by the sources. When the PI controller is used to minimize the difference between the actual and reference value of reactive power, the difference between the algebraic sum of reactive powers supplied by the sources and the reactive powers demanded by the loads has a nonzero value. Therefore, the performance of these controllers becomes poor in the case of ac microgrid having sources of unequal ratings. To resolve this issue, a proportional reactive power-sharing (PRPS) controller-based distributed secondary controller is proposed in this paper. The PRPS controller modifies the droop gain of the E−Q droop control loop of each source in such a way that each source supplies reactive power equal to its proportional value of reactive power. The proportional value of reactive power supplied by each source is the power when the equivalent output impedance of all sources as seen by the loads are identical. The key advantage offered by the proposed controller is the accurate reactive power-sharing in the case of ac microgrid having sources of unequal ratings. The proposed controller ensures zero value of the difference between the algebraic sum of reactive powers supplied by the sources and the reactive powers demanded by the loads. This ensures accurate sharing of reactive power among the sources. The validation of the proposed controller is carried out for islanded mode of operation of ac microgrid. Further, the proposed controller requires of low bandwidth communication for its implementation. The effect of the proposed controller on the stability of the system is demonstrated using reduced order small-signal model. The effect of communication delay on the performance of the system is analyzed with the help of roots locus plots. To validate the efficacy of the proposed controller, detailed simulation studies are carried out in Matlab/Simulink.

Design of Improved Nonlinear Active Disturbance Rejection Controller for Hybrid Microgrid With Communication Delay

In this paper, an improved nonlinear active disturbance rejection control (INLADRC) approach is proposed for load frequency control of hybrid microgrid with communication delay. The INLADRC technique includes a nonlinear extended state observer (NESO) to estimate the system states and a nonlinear controller for the cancellation of the generalized disturbance acting on system. The effect of communication delay in the hybrid microgrid system is alleviated via the incorporation of an auxiliary structure that also restores the synchronization between the inputs of NESO. The efficacy of the proposed approach is validated via a comprehensive analysis, wherein step variations as well as random perturbations are considered in solar power, wind power and load demand. Further, the performance of proposed technique is also corroborated via consideration of real time solar and wind data. Moreover, the robustness of INLADRC technique is investigated in presence of parametric uncertainty, model-plant mismatch, delay time mismatch as well as varying communication delay. An extensive comparative analysis is a testimony to the effectiveness of INLADRC approach.

Mitigating inter-area oscillation of an interconnected power system considering time-varying delay and actuator saturation

A wide area damping controller (WADC) is designed in this paper to mitigate the frequency deviation of interconnected power system. The effect of time delay during wide area measurement system (WAMS) and saturation of the actuator is also considered in this work while designing the WADC. The effect of communication delay along with the saturation of a actuator can make the interconnected power system unstable. This paper focuses on finding a stabilization criterion of the power system connected with a WADC. The WADC is designed taking the effect of time-varying delay in the WAMS and saturation of the actuator into consideration. The saturation of the actuator is modelled with the help of dead zone type non-linearity. In this paper, a Lyapunov–Krasovskii functional is used to design a state feedback control law based on linear matrix inequality (LMI) taking time-varying delay and the actuator saturation into consideration. A thyristor controlled series capacitor (TCSC) is used as an actuator. A supplementary damping controller (SDC) is used as the WADC in this paper to provide sufficient damping. Simulation study on four machine 11 bus power system and New England 39 bus power system is performed to understand the performance of the designed controller. The results are also verified in real-time using OPAL-RT.

Distributed Voltage Regulation for Low-Voltage and High-PV-Penetration Networks With Battery Energy Storage Systems Subject to Communication Delay

The increasing penetration level of photovoltaic (PV) systems in low-voltage networks causes voltage regulation issues. This brief proposes a new voltage regulation strategy utilizing distributed battery energy storage systems (BESSs) while incorporating the inevitable communication delays. The proposed strategy ensures that the voltage regulation burden is appropriately shared among all involved BESSs in the network. The effects of communication delays on the performance of the proposed strategy are analyzed and an admissible delay bound is established. Case studies are conducted to verify the effectiveness of the proposed strategy.

Discontinuous event-trigger scheme for global stabilization of state-dependent switching neural networks with communication delay* *Project supported by the National Natural Science Foundation of China (Grant Nos. 62003194, 61973199, 61573008, and 61973200).

This paper is concerned with the global stabilization of state-dependent switching neural networks (SDSNNs) via discontinuous event-triggered control with network-induced communication delay. Aiming at decreasing triggering times, a discontinuous event-trigger scheme is utilized to determine whether the sampling information is required to be sent out or not. Meanwhile, under the effect of communication delay, the trigger condition and SDSNNs are transformed into two tractable models by designing a fictitious delay function. Then, using the Lyapunov–Krasovskii stability theory, some inequality estimation techniques, and extended reciprocally convex combination method, two sufficient criteria are established for ensuring the global stabilization of the resulting closed-loop SDSNNs, respectively. A unified framework is derived that has the ability to handle the simultaneous existence of the communication delay, the properties of discontinuous event-trigger scheme, as well as feedback controller design. Additionally, the developed results demonstrate a quantitative relationship among the event trigger parameter, communication delay, and triggering times. Finally, two numerical examples are presented to illustrate the usefulness of the developed stabilization scheme.

Centralized Airflow Control to Reduce Output Power Variation in a Complex OWC Ocean Energy Network

A centralized airflow control scheme for a complex ocean energy network (OEN) is proposed in this paper to reduce the output power variation (OPV). The OEN is an integrated network of multiple oscillating water columns (OWCs) that are located at different geographical sites connected to a common electrical grid. The complexity of the OWC-OEN increases manifolds due to the integration of several OWCs and design of controllers become very challenging task. So, the centralized airflow control scheme is designed in two stages. In control stage-1, a proportional-integral- (PI-) type controller is designed to provide a common reference command to control stage-2. In control stage-2, the antiwindup PID controllers are implemented for the airflow control of all the OWCs simultaneously. In order to tune the large number of control parameters of this complex system, a fitness function based on integral squared error (ISE) is minimized using the widely adopted particle swarm optimization (PSO) technique. Next, the simulation results were obtained with random wave profiles created using the Joint North Sea Wave Project (JONSWAP) irregular wave model. The OPV of the proposed OWC-OEN was reduced significantly as compared to the individual OWC. It was further observed that the OPV of the proposed scheme was lower than that achieved with uncontrolled and MPPT controlled OWC-OEN. The effect of communication delay on the OPV of the proposed OWC-OEN scheme was also investigated with the proposed controller, which was found to be robust for a delay up to 100 ms.

Effect of Communication Delay on Consensus-Based Secondary Controllers in DC Microgrid

In a dc microgrid, proportional current sharing among the sources is recommended while maintaining the system voltage within the limits. To achieve this, secondary control is often employed along with the primary droop controllers. As secondary controllers rely on communicated values, a larger delay in the communication channel can make the microgrid system unstable. Some specific studies on stability issues related to communication delay are reported in the literature using simulation studies, polynomial approximation of delay term, or by tracing the eigenvalues. However, these methods are either impractical or will give erroneous results. Considering increasing deployment of dc microgrids globally using different communication technologies and associated delays, a generic analysis is required in this area. In this article, existing secondary controllers are categorized into four classes. A comparison of these four classes based on the maximum delay before becoming unstable, termed as delay margin, is performed. Further, an expression for sensitivity of delay margin to system parameters at steady-state operation is derived. This analysis would help in determining the suitability of different system parameters for dc microgrid applications. The analytically obtained delay margin values are verified with the help of simulation studies as well as experiments.

Distributed algorithms for resource allocation in cyber-physical energy systems with uniform/nonuniform communication delays

As the communication delays in cyber-physical energy systems are considered, the distributed optimal resource allocation problem has not been well studied. In this paper, distributed algorithms are proposed to solve this problem and the effects of communication delays are analyzed. Firstly, as the uniform and nonuniform communication delays in cyber-physical energy system are taken into account, two distributed resource allocation (RA) algorithms are proposed respectively. By using the eigenvalue perturbation theory and the Generalized Nyquist Criterion, the impacts of time delays on the distributed algorithms are strictly analyzed and the maximum allowable delay bounds are derived. Then, as the generation capacity constraints are further considered, the distributed iterative method based on the unconstrained RA solutions is presented. Finally, several simulation studies are carried out to validate the theoretical results.

Leader-Following Diffusion-Based Reactive Power Coordination and Voltage Control of Offshore Wind Farm

This paper proposed a distributed reactive power coordination and voltage control of offshore wind farm based on the leader-following diffusion algorithm. Designating several wind turbine generators (WTGs) as the leaders to receive the information of voltage at point of common coupling (PCC), the reactive power generations required to minimize the voltage deviation could be computed by these leaders. The required reactive power generations are diffused throughout all WTGs by the diffusion algorithm, resulting in the coordinated operation of WTGs to regulate the PCC voltage. The proposed offshore wind farm controller is based on the hierarchical control strategy, which consists of primary and secondary layers. The primary layer is responsible for the inner current, voltage or power regulations whereas the secondary layer is based on the proposed diffusion algorithm to achieve the coordinated operation among WTGs. The proposed strategy could maintain accurate reactive power sharing among WTGs and regulate the PCC voltage. A comparison study with the conventional consensus-based control is presented to show the effectiveness of the proposed diffusion controller. The comparison results show the better performance of the proposed method in terms of dynamic responses of PCC voltage and reactive power coordination. Simulation scenarios of constant wind speed, variable wind speed, and voltage sag in the utility grid are carried out to evaluate the performance of proposed method. The proposed diffusion control is tested either in a small or large offshore wind farm systems. Effect of communication delay on the performance of proposed diffusion control is also described. An experiment of the small-scale wind farm system is conducted to show the feasibility of proposed diffusion strategy.

A robust firefly–swarm hybrid optimization for frequency control in wind/PV/FC based microgrid

This paper presents frequency regulation in a microgrid consisting of wind, photovoltaic (PV), fuel cell (FC) and diesel engine generator (DEG) along with the energy storage system such as flywheel energy storage (FES) and battery energy storage (BES). In this study, firefly and particle swarm (FF–PSO) based hybrid optimization technique is proposed to tune parameters the proportional–integral–derivative (PID) controller to minimize the frequency deviation in microgrid system under different operating conditions such as variation in wind speed, and load demand. Further, a comparative analysis for tuning of the PID controller is performed using particle swarm optimization (PSO), and firefly algorithms (FFA) to minimize the deviation in frequency. The power management as well as frequency control in microgrid is studied with the help of the performance indices; integral absolute errors (IAE), peak overshoots, settling times and standard deviation (SD). In addition, the effect of communication delay with higher non-linearity is tested to assess the performance of the controllers. Further, the frequency control is validated in real-time digital simulation environment using OPAL-RT. The qualitative, quantitative and real-time analysis reflects the improvements in frequency deviation in microgrid using the proposed hybrid FF–PSO optimized PID as comparison to the PSO/FF optimized PID controllers.

A discrete-event and hybrid traffic simulation model based on SimEvents for intelligent transportation system analysis in Mcity

An intelligent transportation systems (ITS) is a typical cyber-physical system (CPS) in which physical components, for example, Connected Automated Vehicles (CAVs), are monitored and controlled through a network of cyber and physical components. Such systems, therefore, contain event-driven dynamics along with time-driven dynamics. The proposed discrete-event and hybrid simulation framework based on SimEvents facilitates testing for safety and performance evaluation of an ITS and has been used to build a traffic simulation model of the Mcity test facility. It is specifically designed for testing CAVs and contains various road/lane configurations and a complete instrumentation system. This enables users to study traffic at the microscopic level, including the effectiveness of new control algorithms for CAVs under different traffic scenarios, the event-driven aspects of transportation systems, and the effects of communication delays. The framework spans multiple toolboxes including MATLAB$^{{\circledR }}$, Simulink$^{{\circledR }}$, and SimEvents$^{{\circledR }}$.

A Novel Communication-Based Average Voltage Regulation Scheme for a Droop Controlled DC Microgrid

In a dc microgrid, enhanced voltage regulation (VR) and load sharing (LS) are necessary for the economic and reliable operation of the system. Traditionally, droop control is used for LS in microgrids. However, because of the nature of the droop control, good VR and accurate LS cannot be achieved simultaneously. To overcome this drawback, a low-bandwidth, communication-based average VR technique along with an algorithm for equal voltage correction factor is proposed in this paper. Here, precise LS is achieved via droop control using a high value of droop gain, while good VR is obtained through low-speed control area network (CAN) communication, which provides fault tolerance and expandability feature to the system. Only the local dc bus voltage information is exchanged over CAN by the grid connected converters, which ensure low communication traffic. Thus, sufficient bandwidth is available on the CAN bus that can be used for control and monitoring purpose—a highly desirable proposition for a smart grid application. Effect of communication delays on system stability has been modeled and analyzed. Simulation and experimental results are presented to validate the feasibility of the proposed scheme. Plug-n-play and fail-safe features of the proposed technique are highlighted.

Distributed Cooperative Optimal Control of DC Microgrids With Communication Delays

One of the fundamental and challenging issues in microgrids is to guarantee fairness of load sharing while realizing voltage regulation of distributed generations. In order to address this issue, a new multiobjective optimization problem with tunable weighting coefficients is first formulated for dc microgrids. Second, a new distributed control scheme, which only requires local communications among neighbors, is proposed to solve the optimization problem. It is theoretically proved that the distributed control scheme can exponentially achieve the global optimal outputs of voltages and currents at distributed generations. Compared with a centralized control scheme, the proposed distributed control scheme provides remarkable advantages in improving reliability and scalability of microgrids. Third, the distributed control scheme is extended to accommodate a constant communication delay. The effects of the communication delay on the stability of microgrids are explicitly characterized. Finally, the performance of the proposed control schemes is evaluated by a modified six-bus microgrid with dc-powered trolleybus systems in terms of their convergence, robustness to load variations, plug-and-play functionality, tradeoff ability, and effects of communication delays.

Ideal current‐based distributed control to compensate line impedance in DC microgrid

To achieve proportional power sharing among similar sources in DC microgrid, droop controllers are used. However, due to interconnecting cable impedances, power shared by the sources deviate from their desired values. To alleviate this problem, techniques utilising shift in droop characteristics and/or modification in droop gain are suggested in literature. Methods which use both of them offer better performance due to higher degree of freedom in control. However, these methods utilise distributed proportional-integral control, which leads to winding-up phenomenon. To address this problem, a distributed secondary controller using full communication is proposed. The current of each source is shared with other sources using low-bandwidth communication. Using this information, each source calculates the ideal value of current to be supplied, which is then used to update its droop gain. To improve the average system voltage, shifting of droop characteristics is incorporated. Detailed small signal model of the proposed controller is developed. Effect of communication delay on the performance of the proposed controller is analysed using the characteristic equation of the system. To evaluate the performance of the proposed scheme, detailed simulation studies are carried out. A scaled down laboratory prototype is developed to determine the viability of the proposed controller.

Multiagent-Based Coordination Consensus Algorithm for State-of-Charge Balance of Energy Storage Unit

A multiagent-based coordination consensus algorithm was designed to simultaneously meet state-of-charge consensus and DC bus voltage stability requirements. A distributed energy storage system model with four batteries was built in Matlab/Simulink, and local droop control was employed to achieve the proposed algorithm. A multiagent system and agent weak communication network were constructed in the JADE platform, and the effectiveness of the proposed algorithm was verified by Simulink and JADE interactive simulation. The effect of communication delay on the system was also analyzed.