Real-time Simulation Studies Research Articles

Active Slip Frequency Based Islanding Detection Technique for Grid-Tied Inverters

In this article, an islanding detection technique (IDT) is proposed based on applying positive feedback of wide area measurement based slip frequency at the inverter reactive power control loop. The classical Q--f droop characteristic based IDT is affected due to variations in load and active power mismatch (APM) conditions that may exist during islanding condition. To overcome these issues, an adaptive reactive power injection based IDT is proposed. Adaptiveness in the proposed method is achieved by considering the gain of the reactive power control loop as a product of rate of change of susceptance with respect to frequency seen at the inverter terminals, square of per unit point of common coupling voltage and slip frequency, which is the difference between distribution generator frequency and a reference substation bus frequency. The application of slip frequency as a positive feedback parameter injects negligible reactive power during grid side frequency disturbances and also associates with under frequency and over frequency ride through requirements. The method is least effected by load quality factor variations and APM conditions. The IDT is able to detect islanding conditions with a negligible non detection zone. Real time simulation studies are performed using dSPACE DS1104 real time controller and RTDS/RSCAD.

Combining Detailed Equivalent Model With Switching-Function-Based Average Value Model for Fast and Accurate Simulation of MMCs

Modeling and simulation play a vital role in the design and testing of modular multilevel converter (MMC) high voltage direct current (HVDC) systems. Detailed equivalent model (DEM) and switching-function-based average value model (SFB-AVM) are two major types of accurate and efficient models to represent the dynamic response of the MMCs. However, the DEM and the SFB-AVM possess unique benefits depending on the purpose of the simulation studies. The DEM provides a detailed representation of submodule (SM) switching events and individual capacitor ripples. The SFB-AVM provides faster simulation speed by using arm equivalent capacitance. Combining both models in a universal arm equivalent circuit gives the users the choice of selecting the most appropriate modeling method during dynamic simulation. This paper proposes a universal modeling framework combining the DEM with the SFB-AVM which allows the DEM and the SFB-AVM smoothly switch from one to the other during dynamic simulation. The proposed SFB-AVM can accurately represent the MMCs with different SM types. The proposed models are validated in offline and real-time simulation studies which demonstrate the improved simulation speeds of the proposed SFB-AVM over the DEM especially for large numbers of SMs.

Droop-Free Distributed Control for AC Microgrids With Precisely Regulated Voltage Variance and Admissible Voltage Profile Guarantees

In this paper, we propose a droop-free distributed cooperative control with precisely regulated voltage variance and admissible voltage profile guarantees for AC microgrids. First a distributed voltage variance observer is proposed and proven to be able to converge to global voltage variance using only local measurements and information from neighboring distributed generators (DGs). A droop-free distributed control with average voltage regulator, voltage variance regulator, and relaxed reactive power regulator is then designed and shown to be able to achieve the regulation of average voltage and voltage variance and relaxed reactive power sharing in steady-state. For relaxed reactive power sharing, one special DG that could be a community-owned or utility-owned DG will not participate in reactive power sharing and this flexibility is utilized to regulate the voltage profile. The small-signal stability of the system with the proposed control is evaluated by measurement-based Prony analysis, which can also guide the control parameter selection. It is also shown that the steady-state solution of the proposed control can be obtained by solving a set of nonlinear equations, which allows more efficient evaluation of the control performance under different operating conditions and various controller settings. To validate the performance of the proposed control, extensive real-time simulation studies are performed in OPAL-RT on a four-DG test microgrid.

Investigation and Assessment of Stabilization Solutions for DC Microgrid With Dynamic Loads

Several studies have been conducted to investigate the interaction dynamics of direct current (dc) microgrids supplying tightly regulated converters, which behave as constant power loads (CPLs). However, the presence of loads with open-loop control or of small closed-loop bandwidth (dynamic loads) in dc microgrids have not been studied to date. To fill this gap, this paper presents a comprehensive stability assessment of a dc microgrid with a high penetration level of dynamic loads. Unlike CPLs, it has been found that dynamic loads would dramatically affect the overall system stability margin at low-power demand than at rated power condition. Therefore, three solutions are proposed to mitigate the stability problems considering different operating and installation scenarios that a system integrator/designer may encounter. Moreover, the impact of system uncertainties, such as dc feeder length, bus capacitance, and the droop controllers, on system stability with/without the stability enhancement methods, is thoroughly addressed. Time-domain simulation studies based on nonlinear models are conducted to validate the analytical results. Furthermore, hardware-in-loop real-time simulation studies demonstrate the feasibility of the hardware implementation.

An Adaptive Augmented Vision-Based Ellipsoidal SLAM for Indoor Environments.

In this paper, the problem of Simultaneous Localization And Mapping (SLAM) is addressed via a novel augmented landmark vision-based ellipsoidal SLAM. The algorithm is implemented on a NAO humanoid robot and is tested in an indoor environment. The main feature of the system is the implementation of SLAM with a monocular vision system. Distinguished landmarks referred to as NAOmarks are employed to localize the robot via its monocular vision system. We henceforth introduce the notion of robotic augmented reality (RAR) and present a monocular Extended Kalman Filter (EKF)/ellipsoidal SLAM in order to improve the performance and alleviate the computational effort, to provide landmark identification, and to simplify the data association problem. The proposed SLAM algorithm is implemented in real-time to further calibrate the ellipsoidal SLAM parameters, noise bounding, and to improve its overall accuracy. The augmented EKF/ellipsoidal SLAM algorithms are compared with the regular EKF/ellipsoidal SLAM methods and the merits of each algorithm is also discussed in the paper. The real-time experimental and simulation studies suggest that the adaptive augmented ellipsoidal SLAM is more accurate than the conventional EKF/ellipsoidal SLAMs.

Primary frequency control of a microgrid with integrated dynamic sectional droop and fuzzy based pitch angle control

In a microgrid (MG), employing conventional fixed droop gain control of wind turbine for mimicking conventional generators may threaten grid stability. In this paper, an integrated control strategy of inertia emulation and dynamic sectional droop control for generating active power reference of the wind power system is presented to provide primary frequency control (PFC) services. The dynamic sectional droop is implemented with the fuzzy logic controller (FLC) regulated pitch angle control mechanism and compared with the conventional proportional-integral (PI) pitch angle control method in the isolated MG. Two different gain values are selected for developing the proposed sectional droop control which is based on the sensitivity of frequency deviation in comparison to the single fixed gain in conventional droop control. The maximum power margin of wind turbine is considered as 25% and 15% for a wind speed higher than (or equal to) the rated speed and lower than the rated speed, respectively. The effectiveness of the proposed method is investigated through Matlab/Simulink based real-time simulation studies. Simulation results demonstrate that the proposed sectional droop control provides superior performance than conventional fixed-gain method and achieves optimum frequency regulation at the rated wind speed and even for a wind speed lower than the rated wind speed.

Scalable Single-Phase Multi-Functional Inverter for Integration of Rooftop Solar-PV to Low-Voltage Ideal and Weak Utility Grid

Integration of rooftop solar-PV (RTSPV) systems and extensive use of nonlinear loads in the low-voltage distribution system (LVDS) leads to poor power quality (PQ). Therefore, it is necessary to address the issues leading to poor PQ at the point of common coupling of the LVDS. In this article, a multi-band hysteresis current control (MB-HCC) for the multi-functional inverter (MFI) is proposed which improves the efficiency of the MFI and also enhances the PQ of the LVDS. The MB-HCC uses simple switching logic and outperforms in its multi-functional tasks such as active power injection and power conditioning. MB-HCC offers better efficiency over variable double-band HCC (VDB-HCC) as it operates at a lower switching frequency. The performance of the proposed system is simulated by using MATLAB/Simulink and validated by OPAL-RT based real-time simulation studies. During the variation of solar irradiation, the proposed MFI has an average efficiency of 98.5% under the ideal grid and 97.34% under the distorted grid. Moreover, the percentage of Total Harmonic Distortion under ideal and distorted grid conditions is brought down to below 5%, and also, reactive power compensation maintains unity power factor operation complying with the IEEE-519-2014 and 1547 standards. These results substantiate the hypothesis of scalability of the single-phase MB-HCC-based MFI for an LVDS contributing to economy and ecology.

Real Time Simulation of Observer Based State Feedback Load Frequency Regulator for Optimally Reduced Power System Model

This paper presents a study of real time simulation of observer based state feedback load frequency regulator for optimally reduced order power system model. The actual system states can be replaced by the observer based states for feedback control design without losing the system stability. State space model of two area interconnected power system model having non-reheat thermal turbine in each area is investigated to develop the proposed control scheme. Moreover, the power system model is optimally reduced with the objective of retaining the dominant characteristics of original system and reducing the complexity and computational burden for the real time simulation of the proposed scheme. Real time simulation is performed on two PC configurations, out of which continuous and discrete time observer based scheme is designed and simulated in non-real time environment on host PC and target PC is used for real time simulation of continuous and discrete time models. It was found that discrete time observer based control scheme for reduced power system model is quite appreciable in terms of dynamic performance and computational burden as compared to continuous time observer based load frequency control for reduced power system model.

Extension of the do-mpc Development Framework to Real-time Simulation Studies

Abstract Nonlinear Model Predictive Control (NMPC) has become a serious option for industrial applications, with advances in software and algorithms making economics optimizing control suitable even for large scale systems. However, the industrial applications are posing a number of engineering challenges, related to the performance of NMPC in real plant environments. In this paper we address the issue of validating NMPC solutions in a real-time simulation environment, which mimics precisely the behavior of the controlled plant. We propose a validation strategy and describe the simulation framework that was developed in order to support the validation process. The software platform used for this purpose is based on the do-mpc framework, benefiting from its modularity and efficient implementation of NMPC algorithms. The focus of this work is the extension of the software environment to a real-application oriented platform where control scenarios can be simulated in real time. The necessity of the proposed validation strategy is demonstrated for a semi-batch polymerization example, where it is shown that feedback delays have a significant impact on the real-time performance. The necessary steps for a transition from simulation studies to the real application are discussed and a method for improving the NMPC performance is proposed based on the real-time simulation studies.

Detailed quantitative comparison of half‐bridge modular multilevel converter modelling methods

This paper presents a detailed comparison of different modelling methods of the half-bridge modular multilevel converter (HB-MMC), namely, switching function, Thevenin equivalent, and averaged, considering both MMC implementations (large and reduced number of cells). The theoretical basis that underpins each modelling method are discussed. Offline PSCAD simulations are used to validate user-defined switching function and averaged MMC models against the Thevenin equivalent model provided in PSCAD library for accuracy, considering steady-state and DC fault conditions. Furthermore, the RTDS based real-time simulation results of the user-defined HB-MMC switching function model are validated against the above mentioned offline models, considering steady-state and DC short circuit fault operations. Simulation speed and efficiency of different offline HB-MMC models being studied here are compared. From comprehensive corroboration of different HB-MMC models presented here, it has been found that the averaged, switching function and Thevenin equivalent models produce practically identical results during steady-state and DC faults. In detailed offline and real-time simulation studies where fundamental and harmonic dynamics are of interest, switching function model is found to be faster and computational efficient compared to the Thevenin equivalent model.

Online load frequency control in wind integrated power systems using modified Jaya optimization

The Jaya optimization algorithm is recognized as a simple and faster population-based heuristic search algorithm. However, due to the absence of algorithm-specified parameter, its performance may degrade in terms of convergence speed and obtain the optimal value for the real-world complex optimization problems which are usually nonlinear and non-differentiable in nature. To deal with the aforesaid scenarios, a modified Jaya optimization algorithm (MJOA) is proposed by considering a weight parameter in the search process. Two methods are proposed on the basis of the selection of weight parameter, one is a linear weight (LW) and the other is a fuzzy logic based mechanism. It is found that MJOA is quite accurate and faster in convergence for complex problems. The proposed MJOA has used for online tuning the controller parameters of automatic generation control (AGC) of wind integrated power system. Further, a frequency deviation tolerance concept is proposed to reduce the number of executions of MJOA. The proposed methodology helps in recovering the frequency excursions of the power system with a faster convergence performance and requirements of less computational burden. Such features are very important in smart-grid operational necessities for improving its stability performance. The real-time simulation studies are carried out on an embedded platform using xPC target board of the wind farm integrated IEEE-39 bus test system.

Model Reference Adaptive Back-Electromotive-Force Estimators for Sensorless Control of Grid-Connected DFIGs

A back-electromotive force (EMF) model reference adaptive system based sensorless control scheme is proposed for a grid-connected doubly fed induction generator (DFIG). Based on the stator-side and the rotor-side dynamic models of the DFIG, both the reference model and the adaptive model of the proposed back-EMF based DFIG are investigated. The adaptation mechanism of the proposed controller and the stability of the closed-loop system are derived by Popov's criterion. Real-time simulation studies are performed with detailed dynamics including pulse-width modulation synthesis. A 2.2 kW DFIG experimental platform is constructed to validate the proposed method by hardware experiments. Both results indicate that proper speed estimations can be achieved by the proposed method. Experimental results indicate that the proposed method can provide good robustness against rotor speed variations or output power variations.

In-use microbiological assessment of caffeine citrate 10 mg/mL oral solution

ObjectivesThis study was conducted to investigate the microbial contamination of caffeine citrate 10 mg/mL oral solution (CCOS) during a simulated in-use test in a clinical environment.MethodsA real-time in-use simulation study...

Centralized Protection Strategy for Medium Voltage DC Microgrids

This paper presents a centralized protection strategy for medium voltage dc microgrids. The proposed strategy consists of a communication-assisted fault detection method with a centralized protection coordinator and a fault isolation technique that provides an economic, fast, and selective protection by using the minimum number of dc circuit breakers. The proposed method is also supported by a backup protection that is activated if communication fails. This paper also introduces a centralized self-healing strategy that guarantees successful operation of zones that are separated from the main grid after the operation of the protection devices. Furthermore, to provide a more reliable protection, thresholds of the protection devices are adapted according to the operational modes of the microgrid and the status of distributed generators. The effectiveness of the proposed protection strategy is validated through real-time simulation studies based on the hardware in the loop approach.

Multi‐terminal medium voltage DC grids fault location and isolation

Voltage source converters (VSCs) are highly vulnerable to DC fault current; thus, protection is one of the most important concerns associated with the implementation of multi-terminal VSC-based DC networks. This study proposes a protection strategy for medium voltage DC distribution systems. The strategy consists of a communication-assisted fault location method and a fault isolation scheme that provides an economic, fast and selective protection by means of using the minimum number of DC circuit breakers. This study also introduces a backup protection which is activated if communication network fails. The effectiveness of the proposed protection strategy is analysed through real-time simulation studies by use of the hardware-in-the-loop approach. Furthermore, the effects of fault isolation process on the connected loads are also investigated. The results show that the proposed strategy can effectively protect multi-terminal DC distribution networks and VSC stations against different types of faults.

Cooperative Controls for Pulsed Power Load Accommodation in a Shipboard Power System

To isolate the negative impacts of pulsed power loads (PPL) in a shipboard power system (SPS), energy storage systems (ESS) are usually equipped for online ESS charging and offline ESS discharging for PPL deployment. In order to achieve fast and smooth ESS charging, the generation control and ESS charging control need to be well coordinated. In addition, control constraints, such as generation and charging current bounds and ramp rates, and changes of operating conditions during charging, should also be properly considered. To better solve this important but insufficiently investigated problem, two control algorithms are presented in this paper. The first PI-based control algorithm is designed based on analysis of the desired performance and rules for coordination. The second feedback linearization based control algorithm is designed based on a simplified SPS model and ramp rates difference of supply and demand. The algorithms are simple to implement and can effectively reduce the negative impacts during supercapacitor charging. The algorithms are tested with detailed single-generator and multiple-generator SPS models. Power hardware-in-the-loop and real-time digital simulation studies are performed to demonstrate the effectiveness of the proposed algorithms.

Mitigation of Negative Impedance Instabilities in a DC/DC Buck-Boost Converter with Composite Load

A controller to mitigate the destabilizing effect of constant power load (CPL) is proposed for a DC/DC buck-boost converter. The load profile has been considered to be predominantly of CPL type. The negative incremental resistance of the CPL tends to destabilize the feeder system, which may be an input filter or another DC/DC converter. The proposed sliding mode controller aims to ensure system stability under the dominance of CPL. The effectiveness of the controller has been validated through real-time simulation studies and experiments under various operating conditions. The controller has been demonstrated to be robust with respect to variations in supply voltage and load and capable of mitigating instabilities induced by CPL. Furthermore, the controller has been validated using all possible load profiles, which may arise in modern-day DC-distributed power systems.

MRAS speed estimator with fuzzy and PI stator resistance adaptation for sensorless induction motor drives using RT-lab

Summary This paper presents a real-time simulation study of Model Reference Adaptive System based rotor speed estimator with parallel stator resistance adaptation mechanism for speed sensorless induction motor drive. Both, the traditional Proportional Integral and Fuzzy logic based control mechanisms are utilised for stator resistance adaptation, while, the rotor speed is estimated parallely by means of Proportional Integral based mechanism. The estimator's response to dynamic changes in Load perturbation and doubling of the nominal value of the actual stator resistance of the motor is observed. The superiority of the fuzzy based stator resistance adaptation in the Model Reference Adaptive System estimator is proved through results validated in real-time. The purpose of employing a fairly new real-time platform is to reduce the test and prototype time. The model is initially built using Matlab/Simulink blocksets and the results are validated in real time using RT-Lab. The RT-lab blocksets are integrated into the Simulink model and then executed in real-time using the OP-4500 target developed by Opal-RT. The real-time simulation results are observed in the workstation.

Action-level real-time DEVS modeling and simulation

For some classes of systems, it is advantageous to develop real-time models instead of step-wise or logical-time models. Toward this goal, the action-level real-time (ALRT) discrete-event system specification (DEVS) modeling and simulation approach is proposed. Modeling of actions is introduced into the parallel DEVS formalism using time invariants defined for real-time statecharts. Actions are specified in terms of time-windows that are to be executed in real-time. An abstract simulator protocol is devised for executing the ALRT-DEVS models under constrained computational resources. The approach is implemented in the parallel DEVS-Suite simulator. These models can be simulated on unitary computing platforms. A simple example switch model is detailed and tested to show the kinds of real-time modeling and simulation studies that can be supported.

Architecture of Nan'ao multi-terminal VSC-HVDC system and its multi-functional control

This paper presents a framework of a multi-terminal HVDC transmission system and its multi-functional control strategy. The framework possesses the basic characteristics of the DC-grid and is suitable in integrating distributed power sources. The paper proposes the first architecture for a multiterminal HVDC transmission system using the VSC technology. Its control strategy offers various functionalities that include controls for operation mode, start-up and shutdown, DC voltage, and station online re-connecting, which are significantly different from the control of point-to-point VSC-HVDC systems. The framework has not only been evaluated in real-time simulation studies, but has also been implemented onsite for the first time via the China Southern Grid's Nan'ao Multi-terminal VSC-HVDC (VSC-MTDC) project. This paper gives a brief review of the current research and engineering achievements in this field, which includes four aspects: the architecture of the VSC-MTDC system, the structure of the control and protection system, simulation verification tests setting, and the results of real-time hardware in hardware in loop (HIL) simulation studies and onsite tests.