OPAL-RT Real-time Digital Simulator Research Articles

A Decentralized Power Management and Sliding Mode Control Strategy for Hybrid AC/DC Microgrids including Renewable Energy Resources

This paper presents a new decentralized robust strategy to improve small and large-signal stability and power-sharing of hybrid AC/DC microgrids and improve its performance for nonlinear and unbalanced loads. In addition to the sliding mode controller for DC/DC converters, for the sake of improving power sharing and regulating active and reactive powers injected by distributed energy resources, and moreover, controlling harmonic and negative-sequence current in the presence of nonlinear and unbalanced loads, two separate controllers for positive sequence power control and negative sequence current control are designed based on the sliding mode control and Lyapunov function theory, respectively. The theoretical concept of the proposed robust control strategy, including mathematical modeling of microgrid components, basic theorems, controller design procedure, and robustness and closed loop stability analysis are outlined. Also, this direct power/current/voltage control scheme is governed by a new hybrid AC/DC hierarchical control scheme that exploits a harmonic virtual impedance loop and voltage compensation scheme. To show the effectiveness of the proposed robust control scheme, offline time-domain simulations are done on a hybrid AC/DC wind/photovoltaic/fuel-cell microgrid with nonlinear and unbalanced loads in MATLAB/Simulink environment, and the results are experimentally verified by OPAL-RT real-time digital simulator.

Intelligent Power Control of Inverter Air Conditioners in Power Systems: A Brain Emotional Learning-Based Approach

Inverter air-conditioning (IAC) units have been proved to be effective in frequency regulation by providing flexible capacities. This paper proposes a brain emotional learning (BEL)-based controller to provide the IACs with control signals to be efficiently involved in the frequency regulation in power systems. The BEL-based controller can learn quick-auto, making it appropriate in systems facing uncertainty. To assess the BEL controller performance in realistic conditions, the uncertainties as a consequence of variations in system parameters and load level are considered. The goal is to use the BEL controller to increase the IAC units' ability to track regulation signals accurately in uncertain circumstances. The controller is compared to a fuzzy-PI control, a proportional control scheme, a model predictive control and a linear quadratic regulator control. A delay-dependent stability criterion is used to calculate the highest time delay in the IACs response under which the system maintains stability. In addition, this paper presents an BEL-based coordinator to coordinate the IACs and traditional generation units for compensating considerable frequency variations caused by the time delays. Case studies are accomplished on a multi-area power system in MATLAB/Simulink environment. Eventually, real-time verifications by OPAL-RT real-time digital simulator on the simulated power system are executed to assess the control method.

SMES-GCSC Coordination for Frequency and Voltage Regulation in a Multi-Area and Multi-Source Power System with Penetration of Electric Vehicles and Renewable Energy Sources

Frequency, tie-line power, and the terminal voltages of synchronized generators must all be kept within prescribed limits to ensure the stability of an interconnected power grid through combined automatic generation control (AGC) and automatic voltage regulator (AVR) loops. Thermal power plants, electric vehicles, and renewable energy sources—including solar and wind, geothermal, and solar thermal power plants—form the two-area integrated power system in present research. A new cascade controller named the cascaded proportional integral derivative (PID) and fractional-order PID (CPID-FOPID) controller is proposed for the first time, whose performance is compared with the PID and FOPID controller. The results show that the proposed cascade controller outperforms PID and FOPID in delivering superior dynamic characteristics, including short settling times and low oscillation amplitudes. A new metaheuristic algorithm named the coot algorithm was applied to optimize the parameters of these controllers. The suggested controller outperforms FOPID in the combined AGC and AVR problem under uncertain conditions (random load disturbance, variable input of solar irradiation, and wind power). Robustness of the controller is tested with significant variation in the turbine time constant of the thermal and geothermal power plant. In this study, authors also investigated the best possible coordination between the superconducting magnetic energy storage (SMES) and gate-controlled series capacitor (GCSC) devices to control both voltage and frequency simultaneously. The effect of communication time to the power system is analyzed in this study. Additionally, the obtained results are satisfactorily validated using OPAL-RT real-time digital simulator.

An Enhanced Control Strategy to Mitigate Grid Current Harmonics and Power Ripples of Grid-Tied PV System Without PLL Under Distorted Grid Voltages

This article proposes an enhanced integrated control strategy to mitigate grid current harmonics and power ripples for a three-phase grid-tied photovoltaic (GTPV) system without a phase-locked loop (PLL) unit. The proposed control strategy consists of an enhanced proportional resonant (EPR) current controller, which suppresses the harmonics of grid injected current under distorted grid voltage conditions. A phase compensated reference current generator (PCRCG) is integrated with the proposed EPR controller and used to eliminate active and reactive power oscillations under grid voltage sags and swells. The proposed integrated control strategy is implemented in the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\alpha \beta $ </tex-math></inline-formula> stationary reference frame. The synchronization task is included in the PCRCG and making the system PLL-less. Thus, the associated computational burden and implementation complexity are reduced. Furthermore, the frequency adaptability of the GTPV system is enhanced as compared with the PLL-based control strategy in the presence of grid frequency variations using the proposed control strategy. To ensure the fast maximum power point (MPP) tracking, a modified incremental conductance MPP tracking algorithm is implemented in integration with the proposed control strategy. The effectiveness of the proposed control strategy is validated using OPAL-RT real-time digital simulator and compared with the conventional PLL-based control strategy.

Parallel inverter control using different conventional control methods and an improved virtual oscillator control method in a standalone microgrid

Partly because of advances in power electronic converters, the share of renewable energy in power generation is steadily increasing. The main medium of interface for integrating renewable energy sources to the utility grid is the power electronic inverter. Virtual oscillator control (VOC) is a time-domain approach for controlling parallel inverters in a standalone microgrid (MG). The concept is to simulate nonlinear deadzone oscillator dynamics in a system of inverters to ensure a stable AC MG in the absence of communication. VOC is a time-domain and self-synchronizing controller that simply requires the measurement of filter current, whereas traditional droop control and the virtual synchronous machine (VSM) require low pass filters for active and reactive power calculations. In this work, a particle swarm optimization (PSO)-based VOC method (VOC-PSO) is proposed, in which the parameters of the VOC are designed using the PSO algorithm. The system performance using droop, VSM, VOC, and VOC-PSO controllers are investigated using MATLAB and Opal-RT real-time digital simulator platforms. The results show that the proposed VOC-PSO gives improved performance over other control strategies. The efficacy of the proposed VOC-PSO control method is also demonstrated by the experimental results.

Coordinated power sharing among multiple subgrids in a photovoltaic-battery based DC microgrid

ABSTRACT In this paper, a coordinated power sharing strategy for multiple subgrid-based dc microgrid (DCMG) is proposed. The DCMG has an SPV (storage-PV) subgrid and two PV subgrids connected through interlinking bidirectional converters (IBCs). In the SPV subgrid, a charging/discharging power and state-of-charge (SoC) of battery-based droop strategy have been implemented for the battery controller for dc bus voltage regulation. In PV subgrid, IBC regulates its dc bus voltage. Further, for coordinated power-sharing, a correction factor-based coordination control strategy for IBC has been proposed. With the proposed coordination strategy, a small change in load demand has been taken care of by the PV subgrid without affecting the state of the battery. Moreover, the battery has been accorded the last priority to respond to the change in load demands in the DCMG. With the proposed strategy, a low SoC battery charges 1.28%/min faster than the conventional strategy. Similarly, a high SoC battery charges 1.07%/min slower than the conventional strategy. Hence, deep discharge and overcharge of the battery are avoided. With the coordination strategy, a higher capacity subgrid takes more load demand than a weak subgrid. The effectiveness of the proposed strategy is verified with MATLAB simulation and OPAL-RT real-time digital simulator.

A Fuzzy-Based Coordinated Power Management Strategy for Voltage Regulation and State-of-Charge Balancing in Multiple Subgrid-Based DC Microgrid

Multiple DC subgrids in close proximity can be connected to form a DC microgrid. The interconnected DC subgrids enhance reliability and efficient utilization of resources. In this paper, a novel coordinated power management strategy is proposed for interconnected subgrids of a DC microgrid. Each DC subgrid has PV and battery units. The proposed strategy is implemented on interlinking bidirectional converters (IBCs). The strategy considers the state-of-charge (SoC) and charging/discharging current of the batteries for SoC balancing-based power sharing through IBCs. A fuzzy logic controller is implemented to decide the amount of power flow through the IBCs from SoC and the charging/discharging current of battery units. Moreover, with the coordinated strategy, a high SoC battery takes more loads compared to a lower SoC battery. The proposed strategy supports fast charging and fast discharging of batteries depending on the SoC level. In addition, the coordinated strategy can seamlessly transfer the control of IBC from the power regulating mode to the voltage regulating mode without any additional control scheme during an outage of the battery unit. The feasibility of the proposed strategy is validated through simulation in MATLAB/Simulink and OPAL-RT real-time digital simulator.

An efficient SoC-balancing based power management strategy for interconnected subgrids of DC microgrid

In this paper, a coordinated power management strategy is proposed for voltage restoration, SoC balancing, and distributed power sharing among the interconnected subgrids of a dc microgrid (DC-MG). In the proposed strategy, coordinated secondary controllers are implemented on the upper layer of generalized primary controls of distributed generators (DGs) and interlinking bidirectional converter (IBC). The primary controllers can be implemented in a decentralized power management strategy using the dc bus voltage deviation of each subgrid. However, to eliminate the voltage deviations and to restore the dc bus voltage at their nominal value, secondary controllers are implemented to all distributed sources. The secondary controller may hamper the effectiveness of the decentralized power management technique. Hence, to resolve this issue, a coordinated secondary IBC controller based on SoC and charging/discharging power of energy storage system (ESS) is proposed. The proposed secondary IBC controller re-establishes a distributed power sharing strategy among the subgrids with limited information through low bandwidth communication (LBC). Further, a dc bus-signaling-based state transition strategy for PV, battery, and IBC controllers is proposed to optimize the performance of the battery and efficiently utilize the PV generations. The feasibility of the proposed strategy is verified by MATLAB simulation and OPAL-RT real-time digital simulator.

OC/OL Protection of Droop-Controlled and Directly Voltage-Controlled Microgrids Using TMF/ANN-Based Fault Detection and Discrimination

In this article, we propose a generalized overcurrent (OC) and overload (OL) protection scheme for the droop-controlled and directly voltage-controlled (DVC) inverter-interfaced distributed energy resources (IIDERs) to be used for the protection studies of the alternating-current (ac)-islanded (autonomous) microgrids, and provide a management strategy during and after short circuit (SC) and consequent OLs. The fault is detected, and its type is discriminated by a new method that takes the advantages of both artificial neural networks (ANNs) and transient monitoring function (TMF). By using the proposed strategy, the output current magnitudes of the distributed energy resource (DER) units are limited, and after fault clearance, the microgrid is restored to its normal operating conditions. The proposed OC/OL protection scheme limits the output power of the DERs. In this regard, a generalized fault model is presented, which considers the effect of the control system of the IIDERs. This model exploits a current-limiting strategy (CLS) that can be implemented in different reference frames to enhance the microgrid fault ride-through (FRT) capability. Finally, to demonstrate effectiveness, accuracy, authenticity, and feasibility of the proposed OC/OL protection scheme along with the TMF/ANN-based fault detection method and their desirable performance, offline digital time-domain simulations are done in MATLAB/Simulink environment, and the results are experimentally verified by using the OPAL-RT real-time digital simulator (RTDS).

Type-2 fuzzy logic based pitch angle controller for fixed speed wind energy system

In this paper, an interval Type-2 fuzzy logic based pitch angle controller is proposed for fixed speed wind energy system (WES) to maintain the aerodynamic power at its rated value. The pitch angle reference is generated by the proposed controller which can compensate the non-linear characteristics of the pitch angle to the wind speed. The presence of third dimension in the Type-2 fuzzy logic controller (FLC) membership function offers an additional degree-of-freedom in the design of the Type-2 FLC controller to improve the system performance. The applicability of the proposed controller is evaluated by considering a nonlinear simulation model of the WES which is developed and implemented using OPAL-RT real time digital simulator. The results show that the proposed controller offers better performance than the conventional PI and fuzzy logic controllers.

Detection of False Data Injection Cyber-Attacks in DC Microgrids Based on Recurrent Neural Networks

Cyber-physical systems (CPSs) are vulnerable to cyber-attacks. Nowadays, the detection of cyber-attacks in microgrids as examples of CPS has become an important topic due to their wide use in various practical applications from renewable energy plants to power distribution and electric transportation. In this article, we propose a new artificial intelligence (AI)-based method for the detection of cyber-attacks in direct current (dc) microgrids and also the identification of the attacked distributed energy resource (DER) unit. The proposed method works based on the time-series analysis and a nonlinear auto-regressive exogenous model (NARX) neural network, which is a special type of recurrent neural network for estimating dc voltages and currents. In the proposed method, we consider the effect of cyber-attacks named false data injection attacks (FDIAs), which try to affect the accurate voltage regulation and current sharing by affecting voltage and current sensors. In the presented strategy, first, a dc microgrid is operated and controlled without any FDIAs to gather enough data during the normal operation required for the training of NARX neural networks. It is worth mentioning that in the data generation process, load changing is also considered to have distinguishing data sets for load changing and cyber-attack scenarios. Trained and fine-tuned NARX neural networks are exploited in an online manner to estimate the output dc voltages and currents of DER units in dc microgrid. Then, based on the error of estimation, the cyber-attack is detected. To show the effectiveness of the proposed method, offline digital time-domain simulation studies are performed on a test dc microgrid system in the MATLAB/Simulink environment, and the results are verified using real-time simulations using the OPAL-RT real-time digital simulator (RTDS).

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.

A Decentralized Robust Mixed $H_{{2}}/ H_{{{\infty }}}$ Voltage Control Scheme to Improve Small/Large-Signal Stability and FRT Capability of Islanded Multi-DER Microgrid Considering Load Disturbances

This paper presents a decentralized robust mixed $H_{{2}}/ H_{{{\infty }}}$ control strategy for autonomous/islanded multidistributed energy resources (multi-DER) voltage-sourced converter based microgrids. The power management system specifies voltage set points for local controllers and the frequency of each DER unit is specified by hierarchical droop-based control structure. A robust mixed H 2/ H ${}_{\infty}$ control system is designed for set point tracking and disturbance rejection, and also improves fault-ride through capability of microgrid and enhances its performance for small- and large-signal disturbances and nonlinear loads. Unlike some of the previous research works, here the load current is modeled as the disturbance. The controller design problem is formulated by a set of linear matrix inequalities and then solved as a multi-objective optimization problem. Fuzzy decision making tool is used to choose the best tradeoff solution for robust mixed $H_{{2}}/ H_{{{\infty }}}$ controller design problem. The theoretical concept of the proposed robust mixed H 2/ H ${}_{\infty}$ control strategy, including the mathematical modeling of microgrid, basic lemma, and controller design procedure, is outlined. To demonstrate the effectiveness of the proposed robust control scheme, offline time-domain simulation studies are performed on a microgrid consisting of three DERs with local loads in MATLAB/Simulink environment and also results are experimentally verified by OPAL-RT real-time digital simulator.

Nonlinear Load Sharing and Voltage Compensation of Microgrids Based on Harmonic Power-Flow Calculations Using Radial Basis Function Neural Networks

A new decentralized hierarchical control scheme is presented to improve power sharing of multidistributed energy resources microgrids including nonlinear and sensitive loads. In this systems, electronically coupled distributed energy resources are responsible to perform the compensation to reduce the voltage harmonics at the point-of-common coupling. The proposed control scheme adds a new virtual impedance scheme, power calculation unit, and also a complementary loop to improve small- and large-signal stability margins and includes detailed modeling for all hierarchical control levels (either for grid-connected or islanded modes). Compared to conventional virtual impedance methods that add only line current feedforward terms to the voltage reference, here, the line current and voltage at the point-of-common coupling regulate the virtual impedance at fundamental and harmonic frequencies, respectively. So, mismatches in the feeder and line impedances are compensated. Moreover, a power calculation method based on harmonic power flow is presented, which exploits the nonlinear mapping ability of radial basis function neural networks to solve harmonic power flow and obtain voltage harmonics and active and reactive powers. To show the effectiveness of the proposed control scheme, offline time-domain simulation studies have been done on a test microgrid by MATLAB/SIMULINK software and verified experimentally using OPAL-RT real-time digital simulator.

A generic microgrid controller: Concept, testing, and insights

Microgrids have garnered attention in recent years as a way to increase the reliability of the grid, increase the reliability of electricity service to customers, adapt to an increasing percentage of intermittent renewable generation, and serve both customer critical loads and the needs of adjacent communities in the case of emergencies such as natural disasters. One barrier to microgrids is the historic cost and lack of standardization associated with microgrid controllers. To reduce this cost and address standardization, specifications for a Generic Microgrid Controller (GMC) were developed with the goal to facilitate the design and ease of adaptation of microgrid controllers to various microgrids of different sizes and with different resources. A GMC must address two core functions, Transition and Dispatch, as well as several optional higher level functions such as economic dispatch, and renewable and load forecasting.The GMC specifications were demonstrated and evaluated using a commercial simulation platform for two different microgrids, a 20 MW-Class community microgrid and a 10 MW-Class medical center microgrid. For each microgrid, the GMC was tested in hardware-in-the-loop (HIL) using an OPAL-RT real-time digital simulator, and the two core functions were assessed. The results established that (1) the GMC is successful in seamlessly transitioning the microgrids to and from an islanded mode, (2) a load/generation mismatch at the time of separation depends on the microgrid configuration and must be lower than a specific value determined by simulation testing, and (3) the GMC Dispatch Function response is acceptable in maintaining, in the islanded mode, 60 Hz for a range of load changes.

Unbalanced harmonic power sharing and voltage compensation of microgrids using radial basis function neural network‐based harmonic power‐flow calculations for distributed and decentralised control structures

A new hierarchical control scheme is proposed to improve power sharing of multi-distributed energy resources (DERs) microgrids including non-linear and unbalanced loads. The electronically coupled DERs are responsible to perform the harmonic and unbalance compensation to reduce the voltage harmonics at the point of common coupling (PCC) and improve power quality. The proposed scheme uses a complementary control loop for small/large-signal stability enhancement, and moreover exploits new concept for fundamental and harmonic virtual impedance scheme for positive and negative sequences based on IEEE standards. Compared to conventional virtual impedance methods that add only line current feed-forward terms to the voltage reference, here, the line current and voltage at the PCC regulate the virtual impedance at fundamental and harmonic frequencies, respectively. So, mismatches in the feeder and line impedances are compensated. Also, a harmonic power calculation is presented based on the non-linear mapping capability of radial basis function neural networks to obtain voltage harmonics and active and reactive powers for balanced/unbalanced operation modes. To show the effectiveness of the proposed control scheme, offline time-domain simulation studies have been performed on a sample microgrid using MATLAB/Simulink software and OPAL-RT real-time digital simulator for verification.

A generalized descriptor-system robust H∞ control of autonomous microgrids to improve small and large signal stability considering communication delays and load nonlinearities

This paper presents a descriptor system H∞ approach to enhance performance of robust H∞ controller and previously-reported decentralized robust servo-mechanism (DRSM) control scheme for autonomous voltage sourced converter (VSC)-based microgrids including multiple distributed energy resources (DERs). The power management system specifies voltage set points for local controllers and the frequency of each DER unit is specified by a hierarchical droop-based control structure. A descriptor system H∞ robust controller is designed based on a closed-loop representation of microgrid either with H∞ or DRSM controller for set point tracking, disturbance rejection and improving performance of microgrid for small/large-signal disturbances and nonlinear loads. Here, unlike some of the previous researches, the load current is modeled as disturbance and also communication time-delay is considered. The theoretical concepts of proposed control strategy, including mathematical modeling of microgrid, basic theorems, and design procedure are outlines. Then, design problem is formulated by a set of linear/bilinear matrix inequalities and then solved using a new iterative algorithm in the form of convex optimization problem. To demonstrate effectiveness of the proposed control scheme, offline time-domain simulation studies are performed on a multi-DER microgrid in MATLAB/Simulink environment and also the results are experimentally verified by OPAL-RT real time digital simulator.

Eigenvalue, Robustness and Time Delay Analysis of Hierarchical Control Scheme in Multi-DER Microgrid to Enhance Small/Large-Signal Stability Using Complementary Loop and Fuzzy Logic Controller

In this paper, the proposed hierarchical control scheme adds new control loop to control the reactive power reference by a fuzzy logic controller to have the benefit of increasing the system stability margins and moreover, eigenvalue, robustness and time delay analysis of proposed control scheme are presented. The reported droop-based control methods of VSI-based microgrids including hierarchical droop-based control scheme are limited to primary and secondary control levels while the proposed control scheme is completely analyzed so that the three hierarchical control levels modeled for both grid-connected mode and islanded mode. This scheme maintains the stability of microgrids not only for the small-signal events, but also for large-signal disturbances such as three phase and single phase to ground faults, heavy motor starting, etc. However, power sharing to loads and network is sufficiently done. To demonstrate the effectiveness of the proposed hierarchical controller, simulation studies have been performed on a microgrid consisting of four units of distributed generation with local loads and in presence of main grid using MATLAB/SIMULINK software and validated using OPAL RT real-time digital simulator.

Real‐time verification of new controller to improve small/large‐signal stability and fault ride‐through capability of multi‐DER microgrids

Decentralised control methods of voltage-source inverters-based multi-distributed energy resource (DER) microgrids and microgrids hierarchical control scheme studied in earlier researches limited to primary and secondary control levels while the proposed control scheme is completely analysed so that the three hierarchical control levels modelled for both grid-connected mode and islanded modes. In this study, the proposed hierarchical control scheme adds new control loop to control the reactive power reference by a fuzzy logic controller to have the benefit of increasing the system stability margins. In addition to the small-signal events, it maintains the stability of microgrids faced with large-signal disturbances such as short circuits, line outage, heavy motor starting etc., and improves fault ride through capability. However, power sharing to loads and network is sufficiently done. To demonstrate the effectiveness of the proposed hierarchical controller, simulation studies have been performed on a microgrid consisting of four units of distributed generation with local loads and in presence of main grid using MATLAB/SIMULINK software (offline simulations) and OPAL-RT real time digital simulator for verification.

Dynamic Reference-based Voltage Droop Control for VSC-MTDC System

The use of voltage source converter multi-terminal direct current (VSC-MTDC) systems is anticipated to increase from the introduction of wind farms and super grids in the near future. Effective control of the DC voltage in VSC-MTDC systems is an important research topic. This paper proposes a new dynamic reference-based voltage droop control to control the DC voltage in VSC-MTDC systems more effectively. The main merit of the dynamic reference-based voltage droop control is that it can reduce the steady-state error in conventional voltage droop control by changing references according to the system operating conditions. The performance of the proposed control was tested in a hardware-in-the-loop simulation (HILS) system based on the OPAL-RT real-time digital simulator and four digital signal processing boards.