Hierarchical Control Structure Research Articles

Experimental framework for laboratory scale microgrids

This paper presents a proposal for a microgrid test rig for laboratory use. It aims at high flexibility using a modular approach with a common hardware for most of the tasks. The proposed framework for laboratory scale microgrid addresses the requirements for teaching and research. This objective is attained with a reconfigurable power electronics stage, used for test and design of new topologies. The experimental framework also allows testing algorithms at different levels in the hierarchical control structure, while giving access to emulation and simulation of elements commonly found in microgrids and to low-level programming of communication protocols for studying the communications channel. The processing unit in each module, called local controller in the paper, uses a high performance digital signal processor (DSP). This processing unit allows reconfiguration of each module, to assume any of the tasks in the microgrid, i.e. controllable loads, storage, wind, photovoltaic generation, etc. The proposed hardware was tested as a simulator/emulator of the different subsystems. The communications with a microgrid central controller (MCC) is provided with standard embedded processors, capable of implementing the communication protocols suitable for micro-grid environments.

Pathway Selection in Wet-Chemical Reaction for Hierarchical High Surface Nano-Ceria

Ceria has been received great attention owing to its unique optical, electrical and magnetic properties that make it valuable material for solid oxide fuel cells, chemical mechanical polishing, gas sensors, UV-adsorbents and automotive three-way catalysts. We study to find the right pathway in low-cost synthesis processing for nano-ceria with tremendous high surface reactivity and redox/thermal stability. Wet chemistry needs to be designed in fastidious details in order to obtain the nano-ceria with anticipated properties by the control of hierarchical surface structure of the nano-ceria. Each cerias we make has been showed poor repeatability resulted from the final architectures of it, albeit, it is synthesized from the same reactant concentration. It would be the thermodynamic differences of the pathways lead to crucial distinction in structure of nano-ceria depending on reaction time, the moment concentration and the temperature. The Hierarchically well-constructed nano-ceria shows high thermal stability in consequence of sufficient space to avoid the agglomerating among the crystals could maintain its structure at high temperature. Thus, to determine the relation between the pathway and result thermodynamically is one of the crucial part of design the right wet-chemical reaction, in order to get the nano-ceria with high surface reactivity and the redox/thermal stability with good repeatability. Furthermore, hierarchical high surface nano-ceria could be used for electrochemical device operating in extreme condition. Figure 1

Amorphous Silk Nanofiber Solutions for Fabricating Silk-Based Functional Materials.

As a functional material, silk has been widely used in tissue engineering, drug release, and tissue regeneration. Increasing subtle control of silk hierarchical structures and thus specific functional performance is required for these applications but remains a challenge. Here, we report a novel silk nanofiber solution achieved through tuning solvent systems used to generate the material. Unlike the β-sheet rich silk nanofibers reported previously, these new silk nanofibers are mainly composed of amorphous structures and maintain a solution state in aqueous environments. The amorphous silk nanofibers are stable enough for storage and also metastable, making them easy to use in the further fabrication of materials through various processes. Silk scaffolds, hydrogels, and films were prepared from these silk nanofiber solutions. These silk materials from amorphous nanofiber solutions show different properties and tunable performance features. Therefore, these amorphous silk nanofibers are suitable units or building blocks for designing silk-based materials.

Energy management of an experimental microgrid coupled to a V2G system

Abstract This paper presents an algorithm for economic optimization of a laboratory microgrid. The microgrid incorporates a hybrid storage system composed of a battery bank and a hydrogen storage and it has a connection with the external electrical network and a charging station for electric vehicles. To study the impact of use of renewable energy power systems, the microgrid has a programmable power supply that can emulate the dynamic behavior of a wind turbine and/or a photovoltaic field. The system modeling was carried out using the Energy Hubs methodology. A hierarchical control structure is proposed based on Model Predictive Control and acting in different time scales, where the first level is responsible for maintaining the microgrid stability and the second level has the task of performing the management of electricity purchase and sale to the power grid, maximize the use of renewable energy sources, manage the use of energy storages and perform the charge of the parked vehicles. Practical experiments were performed with different weather conditions of solar irradiation and wind. The results show a reliable operation of the proposed control system.

Human Trafficking in Malaysia

Recent studies have suggested that migrants are highly vulnerable to being trafficked. Malaysia, with its large number of migrants is vulnerable and is struggling to implement its Anti-Trafficking in Persons and Anti-Smuggling of Migrants Act 2007. There is also little information on implementation experience so far. Through in-depth interviews with government bureaucrats and other stakeholders, this study seeks to gain some insight into Malaysia’s policy implementation experience. It finds inadequate coordination, limited resources, poor information flow, enforcement approach, hierarchical control structure, and need for change as the major factors constraining effective policy implementation.

Multi‐terminal HVDC grids with inertia mimicry capability

The high-voltage multi-terminal dc (MTDC) systems are foreseen to experience an important development in the next years. Currently, they have appeared to be a prevailing technical and economical solution for harvesting offshore wind energy. In this study, inertia mimicry capability is added to a voltage-source converter-HVDC grid-side station in an MTDC grid connected to a weak ac grid, which can have low inertia or even operate as an islanded grid. The presented inertia mimicry control is integrated in the generalised voltage droop strategy implemented at the primary level of a two-layer hierarchical control structure of the MTDC grid to provide higher flexibility, and thus controllability to the network. Besides, complete control framework from the operational point of view is developed to integrate the low-level control of the converter stations in the supervisory control centre of the MTDC grid. A scaled laboratory test results considering the international council on large electric systems (CIGRE) B4 MTDC grid demonstrate the good performance of the converter station when it is connected to a weak islanded ac grid.

Development of a PEM Fuel Cell City Bus with a Hierarchical Control System

The polymer electrolyte membrane (PEM) fuel cell system is considered to be an ideal alternative for the internal combustion engine, especially when used on a city bus. Hybrid buses with fuel cell systems and energy storage systems are now undergoing transit service demonstrations worldwide. A hybrid PEM fuel cell city bus with a hierarchical control system is studied in this paper. Firstly, the powertrain and hierarchical control structure is introduced. Secondly, the vehicle control strategy including start-stop strategy, energy management strategy, and fuel cell control strategy, including the hydrogen system and air system control strategies, are described in detail. Finally, the performance of the fuel cell was analyzed based on road test data. Results showed that the different subsystems were well-coordinated. Each component functioned in concert in order to ensure that both safety and speed requirements were satisfied. The output current of the fuel cell system changed slowly and the output voltage was limited to a certain range, thereby enhancing durability of the fuel cell. Furthermore, the economic performance was optimized by avoiding low load conditions.

An integrated hazard identification method based on the hierarchical Colored Petri Net

The traditional hazard analysis approaches applied to the socio-technical system cannot cover the complex organization structures, the interactions between systems and human behaviors, the interrelated factors among sub-systems and the safety culture of specific societies. This paper presents an integrated hazard identification approach named System-Theoretic Process Analysis Based on Formalization Model (BFM-STPA), which can solve the above issues. Firstly, the hierarchical control structure models of the socio-technical system are established through Colored Petri Nets (CPN) due to its strong description ability and executable ability, which may also be regarded as the graphical system specification. Secondly, the hazards can be identified according to the CPN models following a series of guide conditions. Thirdly, the comprehensive contributing factors to the hazards will be found out by tracing the former states within the reachability graph generated from CPN model. Finally, an integrated hazard log can be derived for further hazard analysis and safety-guided design. In this paper, the above method was applied in the Chinese Train Control System level 3 (CTCS-3). And the process of hazard identification for the scenario of Temporary Speed Restriction issued was elaborated in detail. Compared with the hazard log generated by Hazard and Operability (HAZOP), the hazard log generated by BFM-STPA covered not only the subsystem failures, but also the deviation of interactions among subsystems from design intent, human errors and socio-technical drawbacks related to the TSR Issued scenario of CTCS-3.

Dynamic Energy Management of Hybrid Energy Storage Systems with a Hierarchical Structure

In power system control unicontrol with single storage units or centralized control with multiple storage units to meet different level targets is challenging. Considering the charge and discharge characteristics of storage devices, this paper proposes a hierarchical configuration structure of a battery and supercapacitor mixed storage scenario, and develops a convenient control method for accessing various DC loads and can central manage mass batteries in one place. Aiming at the optimal management of large scale battery storage, the paper proposes a three-layer battery hierarchical control structure and the control objects and control circuits are discussed. Simulation studies are used to verify the control effect of the hierarchical storage system and the results show that the strategy can effectively decrease photovoltaic output fluctuation.

An Enhanced Power Sharing Scheme for Voltage Unbalance and Harmonics Compensation in an Islanded AC Microgrid

In this paper, an enhanced hierarchical control structure with multiple current loop damping schemes for voltage unbalance and harmonics compensation (UHC) in ac islanded microgrid is proposed to address unequal power sharing problems. The distributed generation (DG) is properly controlled to autonomously compensate voltage unbalance and harmonics while sharing the compensation effort for the real power, reactive power, and unbalance and harmonic powers. The proposed control system of the microgrid mainly consists of the positive sequence real and reactive power droop controllers, voltage and current controllers, the selective virtual impedance loop, the unbalance and harmonics compensators, the secondary control for voltage amplitude and frequency restoration, and the auxiliary control to achieve a high-voltage quality at the point of common coupling. By using the proposed unbalance and harmonics compensation, the auxiliary control, and the virtual positive/negative-sequence impedance loops at fundamental frequency, and the virtual variable harmonic impedance loop at harmonic frequencies, an accurate power sharing is achieved. Moreover, the low bandwidth communication (LBC) technique is adopted to send the compensation command of the secondary control and auxiliary control from the microgrid control center to the local controllers of DG unit. Finally, the hardware-in-the-loop results using dSPACE 1006 platform are presented to demonstrate the effectiveness of the proposed approach.

An Effective Nonlinear Multivariable HMPC for USC Power Plant Incorporating NFN-Based Modeling

The ultra-supercritical (USC) unit is an advanced power generation technology with high plant efficiency, high coal utilization, and low emission. However, it is difficult to realize a coordinate control for the USC unit to achieve fast and stable dynamic response during load tracking and grid frequency disturbances, because it is complex, nonlinear, and large scale. This paper presents a nonlinear hierarchical model predictive control (HMPC) to incorporate both the plant-wide economic process optimization and the regulatory process control into a hierarchical control structure, in which the model predictive control (MPC) technology is utilized to solve the multilayer optimization problem. While the nonlinear HMPC optimization problems can be nonconvex, the neuro-fuzzy network (NFN) modeling on USC is incorporated to facilitate the convex quadratic program (QP) routine. Detailed analysis on load tracking and grid frequency disturbances via simulations has been addressed to demonstrate the effectiveness of the proposed nonlinear HMPC.

Fully Electrified Regenerative Braking Control for Deep Energy Recovery and Maintaining Safety of Electric Vehicles

Recycling braking energy is significant to improve the total energy efficiency of electric vehicles (EVs). Moreover, braking safety must be maintained under complex conditions. As the actuator, the electric traction motor has much better features than the internal combustion engine, e.g., faster torque response and capability for energy feedback. From the perspective of energy efficiency and safety, highlighting regenerative braking is a meaningful but challenging problem. In this paper, a braking system using only electric motors/generators as the actuators is investigated, in which the energy may be potentially fed back to the onboard energy storage system (ESS) as much as possible. The energy that may be recovered to the ESS is analyzed. To maintain the stability and to improve the performance of the regenerative braking in unknown tire–road conditions, a knowledge-based methodology in a hierarchical control structure is proposed, where the maximum adhesion force and the motor reference torque (MRT) are determined online. The proposed methodology avoids the complex determination of the optimum slip ratio, while acquiring nearly the optimum antiskid braking performance. Simulation and experiment were carried out to validate the effectiveness.

Application of systems and control theory-based hazard analysis to radiation oncology.

Both humans and software are notoriously challenging to account for in traditional hazard analysis models. The purpose of this work is to investigate and demonstrate the application of a new, extended accident causality model, called systems theoretic accident model and processes (STAMP), to radiation oncology. Specifically, a hazard analysis technique based on STAMP, system-theoretic process analysis (STPA), is used to perform a hazard analysis. The STPA procedure starts with the definition of high-level accidents for radiation oncology at the medical center and the hazards leading to those accidents. From there, the hierarchical safety control structure of the radiation oncology clinic is modeled, i.e., the controls that are used to prevent accidents and provide effective treatment. Using STPA, unsafe control actions (behaviors) are identified that can lead to the hazards as well as causal scenarios that can lead to the identified unsafe control. This information can be used to eliminate or mitigate potential hazards. The STPA procedure is demonstrated on a new online adaptive cranial radiosurgery procedure that omits the CT simulation step and uses CBCT for localization, planning, and surface imaging system during treatment. The STPA procedure generated a comprehensive set of causal scenarios that are traced back to system hazards and accidents. Ten control loops were created for the new SRS procedure, which covered the areas of hospital and department management, treatment design and delivery, and vendor service. Eighty three unsafe control actions were identified as well as 472 causal scenarios that could lead to those unsafe control actions. STPA provides a method for understanding the role of management decisions and hospital operations on system safety and generating process design requirements to prevent hazards and accidents. The interaction of people, hardware, and software is highlighted. The method of STPA produces results that can be used to improve safety and prevent accidents and warrants further investigation.

Distributed Tertiary Control of DC Microgrid Clusters

A distributed control method is proposed to handle power sharing among a cluster of dc microgrids. The hierarchical control structure of microgrids includes primary, secondary, and tertiary levels. While the load sharing among the sources within a dc microgrid is managed through primary and secondary controllers, a tertiary control level is required to provide the higher level load sharing among microgrids within a cluster. Power transfer between microgrids enables maximum utilization of renewable sources and suppresses stress and aging of the components, which improves its reliability and availability, reduces the maintenance costs, and expands the overall lifespan of the network. The proposed control mechanism uses a cooperative approach to adjust voltage set points for individual microgrids and, accordingly, navigate the power flow among them. Loading mismatch among neighbor microgrids is used in an updating policy to adjust voltage set point and mitigate such mismatches. While the voltage adjustment policy handles the load sharing among the microgrids within each cluster, at a lower level, each microgrid carries a communication network that is in contact with the secondary control system. It is this lower level network that propagates voltage set points across all sources within a microgrid. Load sharing and set point propagation are analytically studied for the higher and lower level controllers, respectively. Experimental studies on two cluster setups demonstrate excellent controller performance and validate its resiliency against converter failures and communication losses.

Correlation between Hierarchical Structure and Processing Control of Large-area Spray-coated Polymer Solar Cells toward High Performance.

The formation mechanism of a spray-coated film is different from that of a spin-coated film. This study employs grazing incidence small- and wide-angle X-ray Scattering (GISAXS and GIWAXS, respectively) quantitatively and systematically to investigate the hierarchical structure and phase-separated behavior of a spray-deposited blend film. The formation of PCBM clusters involves mutual interactions with both the P3HT crystal domains and droplet boundary. The processing control and the formed hierarchical structure of the active layer in the spray-coated polymer/fullerene blend film are compared to those in the spin-coated film. How the different post-treatments, such as thermal and solvent vapor annealing, tailor the hierarchical structure of the spray-coated films is quantitatively studied. Finally, the relationship between the processing control and tailored BHJ structures and the performance of polymer solar cell devices is established here, taking into account the evolution of the device area from 1 × 0.3 and 1 × 1 cm2. The formation and control of the special networks formed by the PCBM cluster and P3HT crystallites, respectively, are related to the droplet boundary. These structures are favorable for the transverse transport of electrons and holes.

Energy management system for stand-alone diesel-wind-biomass microgrid with energy storage system

An energy management system for stand-alone microgrid composed of diesel generators, wind turbine generator, biomass generator and an ESS (energy storage system) is proposed in this paper. Different operation objectives are achieved by a hierarchical control structure with different time scales. Firstly, the optimal schedules of the diesel generators, wind turbine generator, biomass generator and ESS are determined fifteen minutes ahead according to the super short-term forecast of load and wind speed in the optimal scheduling layer. Comprehensive analysis which takes the uncertainty of load and wind speed into account is conducted in this layer to minimize the operation cost of the system and ensure a desirable range of the state of charge of the ESS. Secondly, the operation points of each unit are regulated dynamically to guarantee real-time power balance and safety range of diesel generation in the real-time control layer, based on which the response capability when suffering significant forecast deviation and other emergency issues, e.g. sudden load-up can be improved. Finally, the effectiveness of the proposed energy management strategy is verified on an RT_Lab based real-time simulation platform, and the economic performances with different types of ESS are analyzed as well.

Economic Model Predictive Control (EMPC) of an Industrial Diesel Hydroprocessing Plant

Diesel hydroprocessing is a refinery process by which the sulfur impurities are removed by hydrodesulfurization and the main product diesel is obtained by hydrocracking. The industrial Diesel Hydroprocessing Plant considered in this study consists of two hydrodesulfurization reactors and one hydrocracking reactor in series. The feed to the plant is a blend of four different raw material streams which are heavy diesel (HD), light diesel (LD), light vacuum gas oil (LVGO) and imported diesel from another refinery. A two-layer, hierarchical Economic Model Predictive Control (EMPC) structure is proposed to maximize the profit of the plant. The plant-wide profit is maximized by computing the optimal set-points by the upper economic model predictive control layer while these set-points are tracked by the regulatory model predictive controllers in the lower level. Set-point tracking and disturbance rejection performances of the proposed EMPC structure are tested through closed-loop simulations.

A Decentralized Hierarchical Control Structure and Self-Optimizing Control Strategy for F-P Type DGs in Islanded Microgrids

A decentralized and self-optimizing control strategy is proposed for the frequency-power droop type distributed generators (DGs). The main characteristics of the proposed control strategy are the following: 1) it is hierarchical and fully decentralized, i.e., the functions of primary, secondary and tertiary frequency control can be realized locally, with the microgrid centralized controller making communication networks unnecessary and 2) it can make DGs share loads according to the equal incremental principle. Simulation results verify the effectiveness of the proposed control strategy.

Dynamic Management of Electrical Power Distribution Networks

This paper considers the problem of optimal dynamic management of electrical power distribution networks with distributed generation and storage. Initially, analysis is performed of the Optimal Power Flow (OPF) problem—a paramount optimization problem that needs to be solved to ensure optimal steady-state power network operation. In the rest of the paper we present a hierarchical control structure for solving the considered optimal control problem in a dynamical framework. At the upper level a dynamic OPF solver computes the optimal power references for distributed generators and storages at slow rate. These references are then transmitted to the intermediate level, where a faster Model Predictive Control algorithm computes small deviations from power references given by the OPF solver to take into account the variability of load profiles that was neglected at the upper layer. Finally, the power references are forwarded to the primary level where local controllers track these power reference values. A realistic simulation case study of a Croatian power distribution grid is used for testing purposes and to demonstrate the applicability and usefulness of the proposed control strategy.

Occupant feedback based model predictive control for thermal comfort and energy optimization: A chamber experimental evaluation

In current centralized building climate control, occupants do not have much opportunity to intervene the automated control system. This study explores the benefit of using thermal comfort feedback from occupants in the model predictive control (MPC) design based on a novel dynamic thermal sensation (DTS) model. This DTS model based MPC was evaluated in chamber experiments. A hierarchical structure for thermal control was adopted in the chamber experiments. At the high level, an MPC controller calculates the optimal supply air temperature of the chamber heating, ventilation, and air conditioning (HVAC) system, using the feedback of occupants’ votes on thermal sensation. At the low level, the actual supply air temperature is controlled by the chiller/heater using a PI control to achieve the optimal set point. This DTS-based MPC was also compared to an MPC designed based on the Predicted Mean Vote (PMV) model for thermal sensation. The experiment results demonstrated that the DTS-based MPC using occupant feedback allows significant energy saving while maintaining occupant thermal comfort compared to the PMV-based MPC.