Primary Control Loop Research Articles

Coordination in islanded microgrids: Integration of distributed generation, energy storage system, and load shedding using a new decentralized control architecture

For an islanded microgrid (MG) to work reliably, it is essential to manage the control of distributed energy resources, including generation and storage units, as well as loads, in a coordinated manner. In islanded microgrids, the safe energy storage limits must be accounted for coordination to avoid rapid damage or degradation to the storage units. In this paper, a novel control method is introduced to coordinate distributed generation (DG) and energy storage systems (ESS) in an islanded MG to enhance penetration and complete exploitation of DGs and ESSs and maintain balanced state of charge (SoC). A decentralized modified droop function (MDF) is suggested to share power and to control voltage and frequency without communication link. This controller uses virtual impedance to separate active and reactive power in the primary level control loop. Moreover, based on the ESS's charge state and its control as a frequency control support system, the proposed controller enables DGs to operate in both voltage and frequency control mode (VFCM) and active power control mode (APCM) to avoid overcharging. Likewise, to prevent from over discharging and maintain the frequency within permissible range, a new load shedding strategy is implemented to minimize the cycles of load connection and disconnection. The validation and effectiveness of the proposed control system are demonstrated by simulation results.

A new adaptive droop control strategy for improved power sharing accuracy and voltage restoration in a DC microgrid

In a DC Microgrid, accurate power sharing and voltage restoration are two primary control goals to guarantee power quality and reliable operation. Inaccurate power sharing is a significant concern due to discrepancies in feeder line resistance, faulty equipment, lack of monitoring and control, and nonlinear load. Moreover, inaccurate power sharing may lead to overloading of converters and cause a cascade of failures throughout the entire system. This study proposes a new adaptive droop control strategy to address these challenges. To enhance accurate power sharing, error current sharing is formulated by considering bus current and total rated current. This is regulated by the adaptive controller to adjust droop resistance. Additionally, the impact of droop voltage because of feeder line resistance is considered in the proposed strategy. The primary control loop regulates the output voltage of converter utilizing the proposed observer-based optimum sliding mode control (OOSMC). The controller gain of the OOSMC is optimized using a gradient-based method to enhance transient and steady state response of the converter output. In the secondary loop, the twin-delayed deep deterministic policy gradient (TD3) for voltage restoration is employed with a new reward function to optimally tune the proportional-integral (PI) controller. Finally, the superiority of the proposed strategy is evaluated through rigorous testing scenarios, including the use of constant power load (CPL) and sudden failure in the converters. Moreover, the proposed strategy is validated by simulations and laboratory-based experiments. The results show that the OOSMC outperforms GA and PSO while the TD3 has better performance than traditional PI. Furthermore, when the equivalent power-rated converters are implemented in the system, the proposed strategy can transfer identical power sharing of 4 W to the load and provide accurate current sharing of 0.167 A compared to conventional droop control while the DC bus voltage is maintained at the 24 V reference value. In the case of different power-rated converters, the proposed strategy can achieve power sharing accuracy. The first, second and third converters supply 2 W, 4 W, and 6 W, respectively, to the load and provide accurate current sharing. Meanwhile the DC bus voltage can be kept at the reference voltage of 24 V. In the scenario of various kinds of disturbances, the proposed strategy can achieve accurate power and current sharing when the system is tested under load and input voltage variations. Meanwhile the DC bus voltage always returns to the voltage reference. Moreover, the proposed strategy can share power and current accurately when the converter occurs a sudden failure.

Wastewater Treatment System Optimization for Sustainable Operation of the SHARON–Anammox Process under Varying Carbon/Nitrogen Loadings

Partial nitritation (PN) coupled with the anaerobic ammonium oxidation (Anammox) process has improved ammonium removal in wastewater treatment plants (WWTPs). The operation conditions of this process, i.e., the dissolved oxygen (DO) and the influent ammonium and nitrite concentrations, drive the process to an equilibrium to suppress nitrite-oxidizing bacteria and achieve a proper nitrite over ammonium (NO2/NH4) ratio. This study aimed to implement a set of control strategies in a WWTP model BSM2-SHAMX, combining PN in a single reactor system for high-activity ammonia removal over nitrite (SHARON) to an Anammox reactor, using proportional–integrative–derivative (PID) control and model predictive control (MPC) in a cascade. For correct coupling, the PN should maintain an output NO2/NH4 ratio between 1 and 1.3, suitable for the Anammox process. In the cascade controller feedback loop, the primary control loop controls the NO2/NH4 ratio through the DO concentration from the secondary control loop, which guarantees better effluent nitrogen removal. The performance of the plant was assessed by evaluating the control strategies with different influent carbon/nitrogen (C/N) loadings. The study results showed that the MPC controllers provided better results, with an improvement of 36% in the operational cost compared to the base case with a cost around 26,000 EUR/d, and better nitrogen removal surpassing 90% removal, 10% more than the base case.

Three Time-Scale Singular Perturbation Hybrid Control and Large-Signal Analysis Stability in AC-Microgrids

This work is devoted to propose a distributed control hybrid algorithm for the primary and secondary control loops of AC-bus microgrids in islanded mode and to provide a large-signal stability. A primary control loop based on Hybrid Dynamical System (HDS) theory manages the DC/AC inverters considering structural non-linearities in the model (affine terms and hybrid dynamics of their signals) and implementation constraints (dwell-time) and ensures the robustness of the output voltage with respect to any model variation. Moreover, the secondary control loop is designed based on the droop concept, which ensures that SOCs, reactive and active powers converge to their reference values, and on Multi-Agent System (MAS) theory to get a consensus among the reactive power and the State Of Charge (SOC) of the batteries in discharging mode. Moreover, it guarantees reliability and robustness with respect to any disconnection. The tuned system parameters are chosen in order to fit a three-time scale model (a time scale for each control objective: power converter control, droop control and consensus controls), which allows providing a large-signal stability analysis for the complete hybrid system based on HDS, MAS and singular perturbation theory.

Hybrid Dynamical Control for Discharging Rate Consensus in AC-Bus Microgrids

In this work, a robust distributed hybrid algorithm is proposed for the primary and secondary control loops of an island AC-bus microgrid to provide large-signal stability of the complete system. A secondary control loop is designed from droop control and multi-agent systems theory to ensure that the State Of Charge (SOC) of the batteries in discharging mode converges to a consensus. Furthermore, this distributed strategy ensures robustness with respect to any plug-and-play event or communication failure. The DC-AC power converter of each battery in discharging mode is controlled in the primary loop by using hybrid dynamical system theory, which considers non-trivial issues in the model (switching and affine terms) and in the signals (constraints in the dwell time). A suited selection of gains allows using singular perturbation analysis to provide large-signal stability properties for the complete nonlinear model.

Distributed Mitigation Layers for Voltages and Currents Cyber-Attacks on DC Microgrids Interfacing Converters

The wide use of communication layers in DC microgrids to transmit voltage and current measurements of each distributed generator unit (DGU) increases the possibility of exposure to cyber-attacks. Cyber-attackers can manipulate the measured data to distort the control system of microgrids, which may lead to a shutdown. This paper proposes distributed mitigation layers for the false data injection attacks (FDIA) on voltages and currents of DGUs in meshed DC microgrids. The proposed control strategy is based on integrating two layers for cyber-attack detection and mitigation to immune the primary and the secondary control loops of each DGU. The first layer is assigned to mitigate FDIAs on the voltage measurements needed for the voltage regulation task of the primary control loop. The second layer is devoted to the mitigation of FDIAs on the DGU current measurements, which are crucial for the secondary control level to guarantee the proper current sharing of each DGU. Artificial neural networks (ANNs) are employed to support these layers by estimating the authenticated measurements. Different simulation and experimental case studies are provided to demonstrate the proposed mitigation layers’ effectiveness in detecting and mitigating cyber-attacks on voltage and current measurements. The simulation and experimental results are provided to evaluate the dynamic performance of the suggested control approach and to ensure the accurate operation of DC microgrids despite the existence of cyber-attacks on the measurements employed in the control strategy. Moreover, the control strategy succeeds to keep the maximum voltage error and the maximum error in current sharing within tolerance.

A New Decentralized Robust Secondary Control for Smart Islanded Microgrids.

Dealing with the islanded operation of a microgrid (MG), the micro sources must cooperate autonomously to regulate the voltage and frequency of the local power grid. Droop controller-based primary control is a method typically used to self-regulate voltage and frequency. The first problem of the droop method is that in a steady state, the microgrid's frequency and voltage deviate from their nominal values. The second concerns the power-sharing issue related to mismatched power line impedances between Distribution Generators (DGs) and MGs. A Secondary Control Unit (SCU) must be used as a high-level controller for droop-based primary control to address the first problem. This paper proposed a decentralized SCU scheme to deal with this issue using optimized PI controllers based on a Genetic Algorithm (GA) and Artificial Neural Networks (ANNs). The GA provides the appropriate adjustment parameters for all adopted PI controllers in the primary control-based voltage and current control loops and SCU-based voltage and frequency loops. ANNs are additionally activated in SCUs to provide precise online control parameter modification. In the proposed control structure, a virtual impedance method is adopted in the primary control scheme to address the power-sharing problem of parallel DGs. Further, in this paper, one of the main objectives includes electricity transmission over long distances using Low-Voltage DC Transmission (LVDCT) systems to reduce power losses and eradicate reactive power problems. Voltage Source Inverters (VSIs) are adopted to convert the DC electrical energy into AC near the consumer loads. The simulation results illustrated the feasibility of the proposed solutions in restoring voltage and frequency deviations, reducing line losses, as well as achieving active and reactive power sharing among the DGs connected to the MG.

A Coordinated Control Architecture With Inverter-Based Resources and Legacy Controllers of Power Distribution System for Voltage Profile Balance

In this article, a coordinated control architecture is proposed that utilizes inverter-based distributed energy resources (DERs) and legacy controllers such as voltage regulators in a power distribution system and coordinates the reactive power support such that the voltage balance at a particular node of interest can be achieved. The approach is developed based on the alternating direction method of multipliers and the optimal control framework, where a primary control loop is used to provide the necessary reactive power set point to mitigate the voltage deviation and a secondary control loop is used to balance the reactive power from multiple devices such as DERs. The methodology is tested on the IEEE 123-bus distribution feeder with multiple three-phase DERs in coordination with a voltage regulator. It has been observed that the proposed architecture can reduce regulator tap operation, improve the voltage deviation (at least by 20%), and, at the same time, mitigate the negative effect of the reverse operation of the regulators.

An Enhanced Z-Source Switched MLI Capacitor for Integrated Micro-Grid with Advanced Switching Pattern Scheme

In this paper, the Enhanced Z-Source Switched Capacitor Multi-Level Inverter (EZSC-MLI) is presented, which can generate a greater number of levels and magnitude in output. The proposed MLI has greater recognition due to its low harmonic profile, fewer switching components, compact size, low switch stress, isolated DC supply, high efficiency, and low cost. A high voltage boost factor is achieved by using the Z-Source. The Switched Capacitor module is used for charging all the capacitors to equal voltage magnitude based on a self-balanced scheme. The proposed topology for grid integration requires dual control loops, a primary voltage control loop, and a secondary current control loop. The performance of the proposed 7-level topology for grid integrated systems is verified with multi-carrier advanced modulation schemes and by simulations carried out in Matlab/Simulink.

Simulation framework for automatic load frequency control studies of VSC-based AC/DC power grids

This paper introduces an efficient simulation framework for automatic load frequency control (ALFC) studies of VSC-based AC/DC power grids. It is aimed at slow transients caused by large megawatt imbalances occurring either in the AC systems or DC grid. These studies involve the coordinated action of the primary and secondary control loops of power plants, ones requiring long simulation times ranging from various tens of seconds to minutes. To address these overly challenging AC/DC power system simulations, the developed modelling framework resorts to suitable linear models, i.e., VSC dynamic controls are devised to conform to the slow dynamics featuring the ALFC. The most fundamental dynamics of AC systems, DC grid and VSC stations are compiled with reasonable precision. This is demonstrated using a three-terminal, multilevel VSC-based HVDC system implemented in the PSCAD simulation package for comparative purposes; both solutions are shown to concur very well because root mean squared errors inferior to 3% are obtained. The practicality of this effective simulation framework is also demonstrated by studying a five-terminal VSC-based transmission grid facing large power imbalances imposed by load disconnections and VSC outages.

Model-based two-layer control design for optimal power management in wind-battery microgrids

In this paper, a comprehensive model in Hamiltonian form of a Microgrid (MG) composed of heterogeneous components, i.e. wind turbine generator, battery storage and local loads together with their power conversion units, is developed. The proposed model analytically captures the energy conversion capabilities of different sustainable energy sources. Based on this model description, novel primary (nonlinear PI) and secondary controllers (receding horizon) are proposed that ensure boundedness of the currents injected by each energy source and optimal power management operation of the entire MG. Furthermore, closed-loop stability analysis is rigorously proven for both primary and secondary control loops taking into account the accurate Hamiltonian description of the whole MG that includes the energy conversion characteristics. Detailed simulation results of the entire MG connected to a weak grid and operating in islanded mode are provided to validate the proposed model, the control design and the stability analysis under various scenarios.

One New Super-Twisting Sliding Mode Direct Thrust Control for Linear Induction Machine Based on Linear Metro

In this article, a new supertwisting sliding mode direct thrust control (NSTSM-DTC) scheme is proposed to improve the dynamic response, robustness ability of the linear induction machine, and overall drive performance. First, a NSTSM algorithm is proposed and implemented in both electromagnetic thrust and primary flux-linkage control loops. The design aspects, implementation process, and stability analysis of the NSTSM based on the DTC method are fully investigated, which can effectively achieve faster dynamic response, smaller steady state speed tracking error, lower electromagnetic thrust ripple, and increase the system robustness against load variation in comparison to those of the conventional control methods. The proposed method is fully confirmed by the comprehensive simulations and experiments based on one prototype of 3 kW arc induction machine.

Conversion and verification of proportional–integral​ controller parameter in Daya Bay distributed control system modification project

The nuclear island analog system in the Daya Bay Nuclear Power Plant is made up of discrete analog boards. Owing to the difficulties in purchasing spare parts and maintaining systems, the overall digital upgrade and modification shall be carried out during the 30-year overhaul period. In view of the difference between analog and digital systems in expressing and processing process signals, this paper proposed the proportional–integral (PI) controller parameter conversion method for digital modification, highlighting the importance of controller input/output range matching in the process of parameter conversion. The Daya Bay process simulation model, minimum digital system, and minimum analog system are built into a system verification platform through a programmable logic controller (PLC). Taking primary single-phase pressure control loop system and downflow pressure control loop system as research objects, the disturbance was introduced through the step of the primary pressure setting value and opening/closing of the second downflow orifice plate respectively, and the adjustment time、 overshoot and other adjustment quality differences in different systems are compared. The correctness of the parameter conversion method is verified through the system verification platform, which not only ensures the correct input of controller parameters of the Daya Bay DCS modification project but also provides invaluable experience for other digital modification projects of analog systems.

MPI-Based System 2 for Determining LPBF Process Control Thresholds and Parameters

Determining thresholds of the primary control loops (System 1) of an additive manufacturing (AM) process is challenging when realizing System 1 with its fast and intuitive capability for adapting to different metal powers, machine configurations, and process parameters. Based on the convolution neural network and long short-term memory models, this letter presents a secondary tuning loop (System 2) to classify the types of melt-pool images (MPIs) from a coaxial camera online, suggest polishing parameters, and determine the control thresholds of System 1 offline. Case studies indicate that the thresholds and parameters of System 1 including smoke discharging, powder coating, and laser polishing of control loops of a laser powder bed fusion (LPBF) machine can be more deliberatively and logically decided by the proposed MPI-based System 2.

An active parallel power decoupling circuit with a dual loop control scheme for micro‐inverters

AbstractAn important challenge in single‐stage single‐phase micro‐inverters is reducing required capacitance by various active power decoupling (APD) techniques in order to use film capacitors with a longer lifetime instead of electrolytic capacitors. In this paper, a photovoltaic (PV)‐side parallel APD circuit based on a bidirectional flyback converter is proposed. This topology is able to increase the average and ripple voltages for the capacitor due to having a transformer and thus can reduce the required capacitance. In the control scheme, dual control loops on the main and decoupling circuits manage the micro‐inverter operation. The primary control loop is based on the instantaneous active and reactive power theory and a proportional resonant (PR) controller that is responsible for delivering the PV panel energy to the grid. The secondary control loop regulates the pulsation power on the dc‐link capacitor through the PR controller. Simulation results demonstrated that the proposed APD circuit and the control method could properly damp the pulsation power in the normal and reference tracking operations. Furthermore, the good robustness of the micro‐inverter was confirmed in shutdown and start‐up. Finally, the experimental results with a 180‐W single‐phase micro‐inverter prototype are presented, which validates the advantages of the proposed configuration.

Development of an Antiwindup Technique for a Cascade Control System.

A cascade control system comprising the primary and secondary controllers suffers from a cascade-type integral windup problem in which the output saturation of the secondary control loop can considerably increase the integral part of the primary control loop. In this paper, we present a new predictive antiwindup technique that can completely eliminate the possibility of secondary controller output saturation, resulting in no cascade-type integral windup phenomenon. The proposed method does not require any type of process models; thus, its implementation is simple and straightforward, which is a very favorable advantage compared to the model-based antiwindup techniques from the practical viewpoint. Our simulation confirms that the proposed method can completely remove the primary controller’s cascade-type integral windup resulting from the saturation of the secondary controller output. Further, the proposed method exhibited good control performance without needing any type of process model for the various types of processes and controllers. Our experimental study successfully demonstrated that there are no problems in applying the proposed method to real plants.

Control and synchronization of a fixed-frequency control method in microgrid based on global positioning system

Control methods based on global positioning systems (GPS) are reported in multiple literatures recently, which achieve a fixed frequency operation of the microgrid and therefore has the tremendous benefit that any problems related to frequency instability are eliminated. However, the possible interruption of GPS timing signals may cause current circulating and finally leads to instability of the microgrid, yet it is largely neglected in literatures. This paper presents an angle synchronizing mechanism which utilizes the timing signal from GPS satellites and an auxiliary frequency droop loop to ensure synchronization during GPS offline events. Smooth transfer is guaranteed between primary and auxiliary control loops with discrete control architecture. Also, to allow microgrid using GPS-based control to work in tandem with the bulk power system or another frequency droop microgrid, an extra synchronization algorithm is proposed. The viability and performance of the proposed control structure is validated by case studies.

Controlling DC microgrids in communities, buildings and data centers

Microgrid technology is poised to transform the electricity industry. In the context of commercial/domestic buildings and data centers, where most loads are native direct current, DC microgrids are in fact a natural choice. Voltage stability and current/power-sharing between sources within a DC microgrid have been studied extensively in recent years. DC voltage droop control is known to have its drawbacks in that current or power-sharing is relatively poor. To eliminate this drawback, some have proposed to add a communication-based consensus control in addition to the primary voltage droop control loop. The current sharing performance is improved, however, the voltage deviation inherent in droop control requires a further, slower control to achieve voltage quality control. To overcome this complication, and reduction in response time, a low latency communication-based control technique that achieves proportional current sharing without significant voltage deviations is proposed in this work. The stability of the proposed control technique is compared to state-of-the-art using eigenvalue and transient analyses. The negative impact of communication delays on proposed control is discussed in detail.

To study unplanned islanding transient response of microgrid by implementing MSOG and SRF‐PLL based hierarchical control in the presence of non‐linear loads

This study considers the hierarchical control of microgrid and harmonic compensation of non-linear load specially in transition from grid-connected to unplanned islanding. The proposed controllers are based on the stationary reference frame with hierarchical control levels (inner, primary, and secondary). In the inner control loop, proportional and multi-resonant voltage and current controllers for three-phase voltage source inverters are proposed. Moreover, a droop control is used to share active and reactive power in the primary control loop. To achieve decoupled active and reactive power sharing, virtual impedance loop is implemented, too. The mentioned procedure increases the total impedance and consequently reduces the initial current and subsequently facilitates the continuous connection of DGs to the MG. To attain smoother transition, selective harmonic virtual impedance is applied by multiple second-order generalised integrator module to extract the fundamental positive sequence and harmonic components of the output current of the DGs. In secondary control level, synchronisation procedure is considered to make connection and restoration by the advanced three-phase synchronous reference frame PLL with in-loop moving average filter which significantly enhances its disturbance rejection capability. The results are shown smoother transition, robustness, and high performance of the proposed hierarchical control VSIs during variation of system configuration.

Efficiency Improvement of Wireless Power Transfer Based on Multitransmitter System

Multitransmitter wireless power transfer system gets more and more applications as it can maintain high efficiency in a large space range. This article focuses on modeling the multitransmitter system, and proposes an impedance matching method to improve efficiency under lateral-misalignment conditions. The theoretical analysis shows that the maximum transfer efficiency can be achieved as long as the equivalent input impedance of the transmitters is regulated to be the same under misalignment condition. Hence, a double closed-loop control strategy is proposed to realize maximum efficiency point tracking, where the primary control loop adjusts the input impedance of the transmitters through pulsewidth modulation, while the second control loop maintains constant output power by a dc–dc converter. A 500-W prototype is built to validate the feasibility of the proposed method. The efficiency is always higher than 90% with an improvement of up to 20.3% compared to that without such a method when the lateral misalignment increases from 0 to 10 cm (0%–41% of maximum coil size).