Bo Fan
@kth.se
Electric Power and Energy Systems
KTH Royal Institute of Technology
Scopus Publications
Scholar Citations
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Scopus Publications
Zhenghong Tu, Jiangkai Peng, Bo Fan, Liming Liu, and Wenxin Liu
Institution of Engineering and Technology (IET)
AbstractFor pulsed power load (PPL) accommodation in a medium‐voltage DC (MVDC) shipboard power system (SPS), the charging control of energy storage systems (ESSs) and the generation control of distributed generators (DGs) need to be properly coordinated. Targeting the important but not well‐studied problem, an optimal output‐constrained control algorithm for the offline PPL accommodation strategy is presented. Three control objectives including realising the generation and charging control references, maintaining the DC bus and supercapacitor voltages within the safe operating ranges, and minimising the total generation cost of DGs, are fulfilled concurrently. First, an SPS model with multiple DGs, a supercapacitor ESS, and regular loads is developed. By restricting the DC bus and supercapacitor voltages within pre‐defined constraints, both the transient‐ and steady‐state performances of the SPS are guaranteed. Furthermore, by incorporating the cost minimisation objective into designed virtual control signals, the third control objective on energy efficiency is realised. The stability of the presented algorithm is rigorously proven based on the Lyapunov method. Finally, detailed case studies are conducted to validate the performance of the designed algorithm.
Fangzhou Zhao, Tianhua Zhu, Lennart Harnefors, Bo Fan, Heng Wu, Zichao Zhou, Yin Sun, and Xiongfei Wang
Institute of Electrical and Electronics Engineers (IEEE)
Bo Fan and Xiongfei Wang
Institute of Electrical and Electronics Engineers (IEEE)
Grid-forming (GFM) inverters are required to operate robustly against grid faults. However, due to the limited over-current capability of inverters, current-limiting controls are usually applied to protect these semiconductor devices, which may prevent GFM inverters from a successful fault recovery. To understand this phenomenon, this study analyzes the fault recovery process of a GFM inverter with a priority-based current limiter. According to whether the GFM inverter can ensure transient stability and exit the current-limiting mode after fault clearance, three post-fault scenarios are identified, including normal operation, current limitation, and oscillations. Further, the impacts of the short-circuit ratio and control parameters on the post-fault behavior of GFM inverters are demonstrated. To illustrate the implications of these theoretical results, typical numerical examples are presented. Finally, the theoretical findings are validated through experimental tests.
Yunpeng Li, Wenchao Meng, Bo Fan, Shiyi Zhao, and Qinmin Yang
Institute of Electrical and Electronics Engineers (IEEE)
In this article, we present a distributed aperiodic control algorithm for multibus DC microgrids to realize proper current sharing and voltage regulation under denial-of-service (DoS) attacks. To deal with the DoS attacks, an estimation mechanism using only local information is designed when the communication channels are jammed, which avoids persistent load fluctuations or instability of bus voltages caused by malicious attacks. Moreover, an aperiodic communication mechanism is employed using the local and neighbors’ current states stored in the zero-order holder to determine the communication instants. The proposed resilient aperiodic control achieves proportional load current sharing and maintains the weighted average bus voltage invariant simultaneously. Voltage drift under DoS attacks can be avoided. Furthermore, sufficient stability conditions are established for control gains concerning the attack parameters. The Lyapunov synthesis shows that the current sharing error can exponentially converge towards an adjustable set, and the weighted average bus voltage can be maintained at its nominal value. The advantages of the proposed control are illustrated by switch-level simulations and a hardware-in-the-loop experiments.
Zhenghong Tu, Bo Fan, Javad Khazaei, Wei Zhang, and Wenxin Liu
Institute of Electrical and Electronics Engineers (IEEE)
In isolated ac microgrids, multiple controllable distributed energy resources (DERs) may simultaneously participate in load frequency control (LFC). To improve system frequency dynamics and reduce the frequency deviation for such a multiple-DER microgrid,this paper presents a novel LFC method based on an optimal reset control (ORC) scheme. The proposed ORC comprises a baseline linear controller and resetting elements, i.e., the integrator outputs, determined by a linear quadratic regulation problem. The overshoot in frequency regulation induced by integral control is reduced, and the settling time is shortened. Besides, the simple structure facilitates its real-time application, making ORC a simple yet effective solution for the LFC of microgrids. By coordinating the reset controllers of all controllable DERs, the ORC further improves the overall frequency control performance. The asymptotic stability of the closed-loop system is theoretically proved. Finally, the effectiveness of the presented LFC method is validated by switch-level simulation studies.
Bo Fan, Jiangkai Peng, Qinmin Yang, and Wenxin Liu
Institute of Electrical and Electronics Engineers (IEEE)
This article presents a distributed consensus-based controller for dc microgrids to achieve proportional current sharing and weighted average voltage regulation in the presence of ZIP [constant impedance (Z), constant current (I), and constant power (P)] loads. The proposed algorithm allows the regulation of the global weighted average voltage in a distributed manner. The precondition on initial bus voltages is relaxed. Furthermore, this study investigates the negative conductance introduced by constant power loads. Based on the properties of Laplacian matrices, the positive definiteness requirement on the conductance matrix is relaxed. A sufficient stability condition on ZIP loads is obtained with improved adaptability. By using the Lyapunov method, large-signal stability is analyzed rigorously for a wide range of loading conditions. The current sharing and voltage regulation errors are proved to converge to zero exponentially. Finally, simulations based on a switch-level dc microgrid model illustrate the advantages of the designed control algorithm.
Bo Fan and Xiongfei Wang
Institute of Electrical and Electronics Engineers (IEEE)
This letter develops an equivalent circuit model of a grid-forming (GFM) converter with a circular current limiter and analyzes its transient stability. It is revealed that the inner control loop can be simplified as a voltage source behind an equivalent resistor. Based on the developed model, theoretical analysis and experimental tests demonstrate that the transient stability of a <inline-formula><tex-math notation="LaTeX">$P-f$</tex-math></inline-formula> droop-controlled GFM converter with the circular current limiter can be assured whenever there exist stable equilibrium points.
Jiangkai Peng, Bo Fan, and Wenxin Liu
Institute of Electrical and Electronics Engineers (IEEE)
Steady-state optimization and real-time controls are conventionally achieved by separate controllers acting at different timescales in dc microgrids. The separation between them not only lowers the overall energy efficiency but also can cause the line currents and the output currents of distributed generators (DGs) to exceed their limits. Targeting these issues, an optimal controller for dc microgrids is proposed in this article to simultaneously achieve the real-time minimization of operating losses (converter losses and line losses) and the regulation of line currents and DG output currents. First, the optimization problem is established, then converted to an optimal control problem by a designed penalty function. Thereafter, the necessary and sufficient optimality conditions of the problem are derived, based on which a distributed controller is proposed to dynamically track the optimal operating point. Driven by the proposed optimal controller in real-time, line current and DG output current regulations are ensured with greatly enhanced performance. Finally, the closed-loop system stability is analyzed rigorously through Lyapunov stability synthesis. Simulations based on a switch-level microgrid model validate the performance of the proposed control solution.
Jiangkai Peng, Bo Fan, Zhenghong Tu, Wei Zhang, and Wenxin Liu
Institute of Electrical and Electronics Engineers (IEEE)
This article presents a distributed periodic event-triggered (PET) optimal control scheme to achieve generation cost minimization and average bus voltage regulation in DC microgrids. In order to accommodate the generation constraints of the distributed generators (DGs), a virtual incremental cost is firstly designed, based on which an optimality condition is derived to facilitate the control design. To meet the discrete-time (DT) nature of modern control systems, the optimal controller is directly developed in the DT domain. Afterward, to reduce the communication requirement among the controllers, a distributed event-triggered mechanism is introduced for the DT optimal controller. The event-triggered condition is detected periodically and therefore naturally avoids the Zeno phenomenon. The closed-loop system stability is proved by the Lyapunov synthesis for switched systems. The generation cost minimization and average bus voltage regulation are obtained at the equilibrium point. Finally, switch-level microgrid simulations validate the performance of the proposed optimal controller.
Bo Fan and Xiongfei Wang
Institute of Electrical and Electronics Engineers (IEEE)
For grid-connected voltage-source converters (VSCs), it is commonly required that its power outputs can track the power references given by operators under various grid conditions. To achieve this objective, this article presents a Lyapunov-based nonlinear power control algorithm. The system dynamics is developed in the stationary frame, which facilitates the design of the control algorithm in the absence of phase-locked loops that may cause instability issues in ultraweak grids. A virtual resistance is then introduced to allow fast power tracking performance and relax the requirement on accurate system parameters. Furthermore, to simplify the control design, the quasi-stationary line impedance model is applied, based on which a nonlinear control algorithm that only utilizes the output voltage and current information is developed. Afterward, the stability of the closed-loop system is analyzed via the Lyapunov theory. The theoretical results illustrate that the power regulation goal can be achieved and the VSC can maintain synchronization with the power grid. Finally, experimental results demonstrate the effectiveness of the proposed control algorithm under temporary grid faults and various short-circuit ratios.
Jiangkai Peng, Bo Fan, Qinmin Yang, and Wenxin Liu
Institute of Electrical and Electronics Engineers (IEEE)
In a multibus dc microgrid, both the constant power loads (CPLs) and the discrete-time (DT) sampling of digital controllers tend to destabilize the system. To address these issues, a novel distributed DT control scheme is presented in this article to achieve proportional load current sharing and weighted average bus voltage regulation in dc microgrids with constant impedance loads, constant current loads, and CPLs (ZIP loads). First, a dc microgrid model with ZIP loads is established in the DT domain, based on which the distributed DT controller is developed. A sparse communication network is then exploited to achieve the proportional load current sharing by exchanging sampled current information between neighbors. Specifically, a distributed voltage observer utilizing the neighbors’ current information and local voltage information is designed to estimate and regulate the weighted average bus voltage. To alleviate the destabilizing effect induced by the DT sampling of digital controllers, a constrained condition on control gains is established. Additionally, a sufficient condition on CPLs that guarantees their negative impact on stability being locally counteracted is proposed. Through Lyapunov synthesis, the objectives of current sharing and voltage regulation are proved to be achieved simultaneously. Simulation studies on a detailed switch-level model validate the control scheme and demonstrate its robustness against communication network imperfections and the capability of accommodating plug-and-play operations.
Bo Fan, Teng Liu, Fangzhou Zhao, Heng Wu, and Xiongfei Wang
Institute of Electrical and Electronics Engineers (IEEE)
Grid-forming (GFM) inverters are recognized as a viable solution to increase the penetration of renewable energy in bulk power systems. However, they are physically different from synchronous generators in terms of overcurrent capability. To protect the power semiconductor devices and support the power grid under severe symmetrical disturbances, the GFM control systems should be able to achieve the following requirements: current magnitude limitation, fault current contribution, and fault recovery capability. Various current-limiting control methods are reported in the literature to fulfill these goals, including current limiters, virtual impedance, and voltage limiters. This paper presents an overview of those methods. Emerging challenges that need to be addressed, including temporary overcurrent, unspecified output current vector angle, undesired current saturation, and transient overvoltage, are pointed out. Comparative simulations are conducted to demonstrate the performance of different methods under grid voltage drops and phase jumps. Finally, open issues of current-limiting control methods for GFM inverters, including transient stability assessment, voltage source behavior under overcurrent conditions, and windup of voltage controllers, are shared.
Bo Fan and Xiongfei Wang
IEEE
This study investigates the transient stability of a virtual-admittance-controlled grid-forming (GFM) converter with a circular current limiter. Firstly, to facilitate the transient stability analysis, the inner control loops along with the circular current limiter are represented as a voltage source behind adaptive virtual impedance. Next, a time-rescaling technique is utilized to simplify the closed-loop system dynamics. It is found that transient stability is only affected by the “relative speed” defined as the ratio between the voltage magnitude control gain and the power one. Based on the simplified system model, the phase plain analysis is applied to evaluate the transient stability of the GFM converter. The results reveal that transient stability can be jeopardized when the circular current limiter is applied, while reacquired by increasing the relative speed. Finally, simulation and experimental tests are conducted to verify these findings.
Zhenghong Tu, Bo Fan, Wei Zhang, Jiangkai Peng, and Wenxin Liu
Institute of Electrical and Electronics Engineers (IEEE)
Pulsed power loads (PPLs) consume a huge amount of energy within a short time. An energy storage system (ESS) is usually installed in a shipboard power system (SPS) for PPL accommodation. Online accommodation strategies, where the PPL is connected to the SPS through the ESS all the time, have great advantages in reducing the hardware cost and increasing the PPL deployment frequency. But there is a lack of online accommodation solutions to deal with the operational requirements. This article presents a barrier-Lyapunov-function-based optimal control solution for online PPL accommodation in dc SPSs. By introducing the state-constrained technique, the operational constraints on the supply current for PPL accommodation and the bus voltage are all respected. Three control objectives, including safe and fast ESS charging, high-quality regulation of the dc bus voltage, and generation cost dynamic minimization, are realized simultaneously. The stability and optimality of the control solution are guaranteed rigorously via the Lyapunov synthesis. Simulations based on a switch-level SPS model demonstrate the effectiveness of the proposed control solution.
Zhen Fan, Bo Fan, Jiangkai Peng, and Wenxin Liu
Institute of Electrical and Electronics Engineers (IEEE)
DC microgrids are growing in popularity due to their advantages in terms of simplicity and energy efficiency while connecting dc sources and dc loads. In traditional hierarchical schemes, optimization and control are implemented at different time scales. The loose integration lowers its energy efficiency and makes it hard to achieve real-time optimization. Even a slight disturbance can result in deviations of bus voltages and output currents from their optimal operating points. Additionally, most real-time control schemes cannot guarantee the boundedness of individual bus voltages. Targeting these problems, a distributed optimal control algorithm is presented in this article for dc microgrids to minimize operation loss (converter loss and distribution loss) in real time and maintain all bus voltages within predefined ranges. First, the Karuch–Kuhn–Tucker condition of the original constrained optimization problem is converted to an equivalent optimality condition, which is suitable for control design. Then, a distributed control algorithm is designed to drive the system's operating condition toward the optimal one. Convergence to the optima is guaranteed through rigorous Lyapunov-based stability analyses. Finally, simulation studies with a detailed switch-level model demonstrate the merits of the proposed controller.
Zhen Fan, Bo Fan, and Wenxin Liu
Institute of Electrical and Electronics Engineers (IEEE)
DC microgrids are increasing in popularity due to their simplicity and high energy efficiency, and becoming an appealing solution for the coordination of multiple conventional generators (CGs) and renewable generators (RGs). This article presents a distributed discrete-time control scheme to achieve the optimal coordination of CGs and RGs, where the generation cost of the CGs is minimized and the energy utilization of RGs is maximized. A certain degree of proportional load sharing among the RGs is also achieved to improve the stability margin and dynamic performance of dc microgrids. The designed control algorithm can maintain the bus voltages in their safe operating ranges. Besides, since the proposed control algorithm is developed in the discrete-time domain directly, it can avoid the possible instability impact of the digital implementation of control algorithms. Based on the Lyapunov analysis, the stability and convergence of the closed-loop system are analyzed rigorously. Finally, simulation results based on a detailed switch-level dc microgrid model illustrate the advantages of the proposed optimal control algorithm.
Bo Fan and Xiongfei Wang
Institute of Electrical and Electronics Engineers (IEEE)
To avoid potential privacy threats and associated cyber-security issues in microgrids, this letter presents a distributed active power sharing and frequency regulation method with preserved privacy of local information. In the proposed approach, the transmitted data including the active power outputs and capacities are protected by adding noise to the original ones. Theoretical analysis and verification studies are performed to illustrate the advantages of the proposed method.
Jiangkai Peng, Bo Fan, Qinmin Yang, and Wenxin Liu
Institute of Electrical and Electronics Engineers (IEEE)
A dc microgrid needs to be well controlled to fully unlock its potential. This article presents a distributed event-triggered control algorithm for accurate load current sharing and voltage profile regulation of dc microgrid. According to the control design, a subsystem controller only communicates with its neighboring controller(s) when a certain condition evaluated using local measurements is triggered. Thereafter, control signals are updated when such events are triggered locally or at its neighboring subsystem(s). Since the algorithm can significantly lower the requirements for communication and computation, it is efficient and easy to implement. Lyapunov synthesis is performed to show that the reduced communication does not compromise the control performance. Simulation with a detailed switch-level dc microgrid model further validates the effectiveness of the proposed control algorithm.
Jiangkai Peng, Bo Fan, and Wenxin Liu
Institute of Electrical and Electronics Engineers (IEEE)
In conventional DC microgrid control schemes, optimization and real-time control are usually performed at different time scales. It is hard for such control schemes to achieve real-time optimization. Even a small disturbance can result in deviations of bus voltages and output currents from their optimal operating points. In addition, most real-time controllers cannot guarantee the satisfaction of pre-defined constraints on individual bus voltages due to the disconnection between steady-state optimization and real-time control. In this paper, a distributed optimal control scheme is presented for DC microgrid. The objectives are to simultaneously realize generation cost minimization and individual bus voltage regulation. First, the optimal control problem is formulated, based on which a necessary and sufficient optimality condition is derived that characterizes the optimal operating points. The distributed optimal controller is then proposed to dynamically drive the system to operate at the optimality condition. Convergence to the optimal operating points that minimizes generation cost under constraints is guaranteed through rigorous Lyapunov synthesis. Each individual bus voltage can also be maintained within bounds in both transient- and steady-state. The performance of the proposed controller is validated through simulations based on a switch-level microgrid model.
Jiangkai Peng, Bo Fan, Hao Xu, and Wenxin Liu
Institute of Electrical and Electronics Engineers (IEEE)
In this paper, a novel discrete-time self-triggered (DTST) distributed controller is developed for DC microgrids with data dropouts and communication delays. To meet the discrete-time (DT) nature of digital systems, a DT distributed controller is first proposed to achieve load current sharing and average voltage regulation simultaneously in DC microgrids. To further relax the communication restrictions, a novel self-triggered mechanism is developed and integrated along with the DT controller, where the local controller proactively estimates the next communication instant ahead of time. Since the controller can only be triggered at DT instants, the Zeno phenomenon is avoided. Moreover, an algorithm is further proposed to compensate for the effect of data dropouts and communication delays. Through Lyapunov synthesis, stability and convergence of the closed-loop system with the proposed design is guaranteed. Finally, detailed simulation studies validate the effectiveness of the proposed DTST controller under communication imperfections.
Bo Fan, Shilin Guo, Jiangkai Peng, Qinmin Yang, Wenxin Liu, and Liming Liu
Institute of Electrical and Electronics Engineers (IEEE)
In dc microgrids, load power sharing and bus voltage regulation are two common control objectives. In this article, a consensus-based algorithm is presented to achieve proportional power sharing and regulation of weighted geometric mean of bus voltages in dc microgrids with ZIP (constant impedance, constant current, and constant power) loads simultaneously. By using the virtue of the Laplacian matrices of undirected connected graphs, a lemma is derived to assist the stability analysis of the dc microgrids. Thus, a sufficient condition that stabilizes the system with ZIP loads is established. In addition, with the help of a distributed voltage regulation error estimator, the tuning of the bus voltages can be realized without the requirement on initial voltage conditions. Through the Lyapunov synthesis, the large-signal stability of the closed-loop system is theoretically ensured for a wide range of load conditions. Finally, simulation studies are performed to validate the merits of the proposed consensus-based algorithm.
Xuguo Jiao, Qinmin Yang, Bo Fan, Qi Chen, Yong Sun, and Lin Wang
ASME International
Abstract As wind energy becomes a larger part of the world's energy portfolio, the control of wind turbines is still confronted with challenges including wind speed randomness and high system uncertainties. In this study, a novel pitch angle controller based on effective wind speed estimation (EWSE) and uncertainty and disturbance estimator (UDE) is proposed for wind turbine systems (WTS) operating in above-rated wind speed region. The controller task is to maintain the WTS's generator power and rotor speed at their prescribed references, without measuring the wind speed information and accurate system model. This attempt also aims to bring a systematic solution to deal with different system characteristics over wide working range, including extreme and dynamic environmental conditions. First, support vector machine (SVR) based EWSE model is developed to estimate the effective wind speed in an online manner. Second, by integrating an UDE and EWSE model into the controller, highly turbulent and unpredictable dynamics introduced by wind speed and internal uncertainties is compensated. Rigid theoretical analysis guarantees the stability of the overall system. Finally, the performance of the novel pitch control scheme is testified via the professional Garrad Hassan (GH) bladed simulation platform with various working scenarios. The results reveal that the proposed approach achieves better performance in contrast to traditional L1 adaptive and proportional-integral (PI) pitch angle controllers.
Bo Fan, Jiangkai Peng, Qinmin Yang, and Wenxin Liu
Institute of Electrical and Electronics Engineers (IEEE)
In modern control systems, most control algorithms, especially system-level ones, are implemented in discrete-time (DT) with digital controllers and digital communications. In this paper, a distributed DT algorithm is developed to achieve proportional load current sharing and average bus voltage regulation in DC microgrids. In order to reduce the communication requirement among the controllers, a periodic event-triggered (PET) DT algorithm is proposed by introducing a novel PET condition. Since the PET condition is detected periodically, the Zeno phenomenon existing in continuous-time (CT) event-triggered algorithms can be avoided. The communication burden for detecting the PET condition is also relieved since only the local and neighbors’ triggered information is required. Through the Lyapunov synthesis, the current sharing error is proved to converge to zero asymptotically. Finally, comparative results among different algorithms, based on a detailed switch-level microgrid model, are given to validate the effectiveness of the proposed PET control design.
Xin Huang, Keyou Wang, Bo Fan, Qinmin Yang, Guojie Li, Da Xie, and Mariesa L. Crow
Institute of Electrical and Electronics Engineers (IEEE)
This paper presents the design of a filtered tracking error based robust current controller for three-phase grid-tied inverters interfacing distributed renewable resources into the grid. An uncertainty and disturbance modeling based control law is developed for achieving the robustness against non-ideal grid conditions, including the grid impedance variations, grid voltage harmonics, and fluctuations in grid voltage magnitude (symmetrical/asymmetrical), frequency, and phase. The proposed controller is shown to have superior current tracking performance to directly control the current injected into the grid being pure sinusoidal and three-phase balanced. In addition, high dynamic and tracking performance can be further ensured since all the phase-locked loops and multi-loop controllers are eliminated, which also delivers the advantage of a simple implementation. Especially, the system stability is proven by using the Lyapunov function. Both simulation and hardware-in-the-loop experimental results of the proposed robust controller, as well as the proportional–integral controller and the parallel proportional-resonant controller, are given and compared, which validates the performance and effectiveness of the proposed control strategy.
Jinglan Li, Qinmin Yang, Bo Fan, and Youxian Sun
Institute of Electrical and Electronics Engineers (IEEE)
The coaxial-rotor micro-aerial vehicles (CRMAVs) have been proven to be a powerful tool in forming small and agile manned–unmanned hybrid applications. However, the operation of them is usually subject to unpredictable time-varying aerodynamic disturbances and model uncertainties. In this paper, an adaptive robust controller based on a neural network (NN) approach is proposed to reject such perturbations and track both the desired position and orientation trajectories. A complete dynamic model of a CRMAV is first constructed. When all system states are assumed to be available, an NN-based state-feedback controller is proposed through feedback linearization and Lyapunov analysis. Furthermore, to overcome the practical challenge that certain states are not measurable, a high-gain observer is introduced to estimate the unavailable states, and then, an output-feedback controller is developed. Rigorous theoretical analysis verifies the stability of the entire closed-loop system. In addition, extensive simulation studies are conducted to validate the feasibility of the proposed scheme.