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The stability of dual active bridge converter (DAB) is threatened when feeding the constant power loads (CPLs). This article proposes a deep reinforcement learning-based backstepping control strategy to solve this problem. First, a nonlinear disturbance observer is adopted to estimate the large-signal nonlinear disturbance. Then, a backstepping controller is used to stabilize the voltage response of the DAB under the large-signal disturbance. Finally, a compensation method based on deep reinforcement learning is developed to intelligently minimize output voltage tracking error and improve the operating efficiency of the system. The proposed controller can guarantee system stability under the large-signal disturbance of the CPL and achieve a fast dynamic response with accurate voltage tracking; it is more adaptive by using the deep reinforcement learning technique through the learning of its neural networks. The effectiveness of the proposed controller is verified by experiments. [ABSTRACT FROM AUTHOR]

Nowadays, with the rapid development of hydrogen powered vehicles, the demand for hydrogen refueling stations (HRSs) in the transportation field is growing. The renewable energy, e.g., wind energy, is used to produce and store hydrogen on site for HRSs, which may be a relatively cheap and clean solution. Considering the wind farm (WF) and HRSs belong to different entities, a new cooperative operation model of the WF-HRSs combined system is proposed. Specifically, the robust optimization methods are implemented to characterize the uncertainties of wind power and market electricity price, respectively, to alleviate risk. To ensure the fairness of profit allocation, the bargaining power is measured by various contribution levels of each stakeholder based on the Nash–Harsanyi bargaining game theory. Furthermore, the model is transformed into two sequential subproblems: energy trading problem (SP1) and the payment bargaining problem (SP2). Accordingly, a solving technique adopting column and constraint generation algorithm is provided to solve the SP1. In addition, for the sake of privacy protection, a distributed approach based on data-centric mode is developed to solve SP2. Finally, the numerical results can validate the effectiveness and scalability for the proposed scheme and algorithms. [ABSTRACT FROM AUTHOR]

Distributed renewable energy source is an advisable solution for dc microgrids to reduce fuel consumption and CO2 emission. In such microgrids, the installation of two or more battery energy storage (BES) units is utilized to compensate the power imbalance between the sources and loads. Nevertheless, energy management with numerous BES units does not simultaneously consider the impacts of distributed generators (DGs) and constant power loads (CPLs). Since the inaccurate current sharing will shorten the lifetime of the batteries and cause instability problem, this article proposes a distributed secondary $H_{\infty }$ consensus approach based on the dynamic event-triggered communication method to realize accurate current sharing and efficient operation in the presence of numerous DGs and CPLs. First, the whole state-space function model of the dc microgrid consisting of DGs, batteries, resistive loads, and CPLs, is first built in detail. This model is further transformed into standard linear heterogeneous multiagent systems, which provides an indispensable preprocessing for advanced control strategy application. Then, the distributed secondary $H_{\infty }$ consensus approach based on the foresaid systems is designed to achieve accurate current sharing. For reducing the communication among batteries and the controller updating frequency, the dynamic event-triggered communication method is proposed. Compared with existing event-triggered methods, the communication and controller updating frequency of the proposed dynamic event-triggered method have been reduced a lot. Additionally, the proposed method can not only avoid the Zeno behavior, but also obtain the lowest bound of the sampled time interval. Finally, the numerical simulation results and experimental results verify the effectiveness of the proposed control strategy. [ABSTRACT FROM AUTHOR]

State-of-charge (SoC) imbalance and bus voltage deviation are two of the main problems in autonomous dc microgrids. Based on this concern, this paper presents an improved dual-quadrant SoC weighted control strategy and a distributed optimization control method to achieve SoC balance, ensuring accurate power-sharing and bus voltage recovery. Firstly, this paper couples the injected/released power with the current SoC and observed average SoC value to weight the droop coefficient, which is based on the charge/discharge mode for the energy storage system. Then a secondary controller is designed based on distributed optimal control to eliminate the bus voltage deviation caused by the line impedance difference. The proposed optimal control method optimizes the average bus voltage to the nominal value and achieve accurate power-sharing by constructing the correlated variables and voltage independent intermediate variables exchanged among bulk energy storage units (ESUs). Since the voltage observer cannot accurately observe the true average bus voltage under the communication delay, the proposed distributed optimal control method without the voltage observer can ensure that the average bus voltage is optimized to the nominal value, thus improving the robustness of the control system. Finally, the correctness and effectiveness of the proposed method are verified in Simulink/MATLAB. [ABSTRACT FROM AUTHOR]

Periodical validation and calibration to microgrid models are necessary for investigating the behaviors of microgrids and evaluating the stability of connected power grids. However, instead of calibrating the problematic parameters that vary from their actual values, the commonly used methods normally adjust the selected sensitive parameters. The performance of the calibrated models is still in doubt. Therefore, this article proposes a generic approach for microgrid model calibration, which aims to accurately locate the indeed “ill-conditioned” problematic parameters, as well as address the multiple solutions issue. With the application of sensitivity and correlation analysis, the potentially problematic parameters (PPPs), which are sensitive and independent, are selected to ensure the accuracy and uniqueness of parameter calibration results. Then, the problematic parameters are further screened out by comparing the energy distribution of PPPs and model errors in the frequency domain. Also, hybrid dynamic simulation based parameter estimation is employed to adjust the problematic parameters, by which each device in microgrids can be calibrated independently. Finally, the simulation results demonstrate the outstanding performance in parameter calibration of a real microgrid. [ABSTRACT FROM AUTHOR]

A compact interlinking converter modular (ICM) for hybrid ac/dc microgrids with a distributed storage (DS) bus for storage integration is proposed in this article. The advanced ICM for hybrid ac/dc/ds microgrids involves the nine-switch topology with compact configuration and decreased number of active operating switches, which structurally equivalents to afford a dc interconnection port for dc subgrid, an ac interconnection port for ac subgrid, a ds interconnection port for ds subgrid, and two dc–dc converters for the interconnection ports for parallel DSs. In addition to the popular advantage of component elements saving, the suggested ICM, aggregating various interlinking converter cells (ICCs) into an ICM, is also proficient of participating computations from all ICC sensors to enhance the overall modular performance without building extra communication connection links between ICCs. With this in mind, a decentralized power management control strategy is realized by employing a coordination power control methodology to systematically handle various distributed generations (DGs), DSs, and ICM in a decentralized manner with improved communication fault ride-through capability. The controller hardware-in-the-loop experiments on a real-time digital simulator platform validates the achievement of the advanced ICM application in hybrid ac/dc/ds microgrids and the effectiveness of the associated decentralized power management scheme. [ABSTRACT FROM AUTHOR]

This article proposes a circulating current and second harmonic current (SHC) suppression method by introducing virtual impedance into the circulating current and output current feedback control loops of the dc solid-state transformer (DCSST). The virtual impedance is employed to adjust impedance in the circulating current feedback control loop to decrease the circulating current when the transmission efficiency of each dual active bridge is different. And a SHC suppression method, which introduces the virtual impedance containing a bandpass filter, is adopted to reduce the SHC and improve the dynamic performance of the DCSST. Moreover, a relatively independent modular control method, which is conducive to the modular expansion of the DCSST, is proposed. Compared with traditional DCSST control methods, the proposed method can simultaneously realize the output current sharing and effectively reduce the SHC with modular control. Finally, a DCSST prototype is built and the results of the experiment verify the validity and effectiveness of the proposed control strategy and solutions. [ABSTRACT FROM AUTHOR]

In practical microgrids, the inhomogeneous initial values are widely appeared due to soft-starting operation. If traditional model order reduction approaches are applied, the input-output maps error between the original system and reduced-order system is large. To address this problem, this paper proposes a reduced-order aggregate model based on balanced truncation approach to provide the preprocessing approach for the real-time simulation of large-scale converters with inhomogeneous initial conditions in DC microgrid. Firstly, the standard linear time-invariant model with inhomogeneous initial conditions is established through non-leader multiagents concept. To end this, it is convenient for scholars to build complex system modeling with switched topology. Furthermore, the full system is divided into two components, i.e., the unforced component with nontrivial initial conditions and forced component with null initial conditions. Moreover, this paper presents an aggregated approach that involves independent reducing component responses and combining reducing component responses. Based on this, the input-output maps error is reduced. Then, the approximated error estimate of the reduced-order aggregate model regarding large-scale converters in DC microgrid is first provided, which provides prior knowledge and theoretical basis for DC microgrid designers. Finally, the simulation results illustrate the accuracy of the proposed approach. [ABSTRACT FROM AUTHOR]

A decentralized improved I-V droop control strategy for battery-supercapacitor (SC) hybrid energy storage system (HESS) is proposed in this paper. The dynamic power sharing between battery and SC is realized by replacing the constant droop coefficient in I-V droop control with virtual impedance, i.e. virtual inductance for battery side converter and virtual resistance for SC side converter. Besides, by injecting the virtual inductance in the battery side converter, negligible DC bus voltage deviation can be achieved without extra voltage compensator. Moreover, the state-of-charge (SoC) recovery is also considered to extend the service life of the HESS. Furthermore, in the proposed regulated power system, since the power allocation, DC bus stability and SoC recovery are decoupled from each other, the design of control parameters is simple. The corresponding design guideline is demonstrated in this paper. Finally, to verify the accuracy and feasibility of the theoretical analyses, hardware in the loop simulations have been conducted. [ABSTRACT FROM AUTHOR]

In dc microgrid, energy storage system (ESS) plays a crucial role to provide short-or-long term and high-quality electric energy. The different control strategies for bidirectional dc-dc converter (BDC) of ESS in grid-tied and islanded modes pose challenges to the coordination control of the dc microgrid. This paper proposes an autonomous control scheme for the BDC in dc microgrid. The proposed control scheme is based on $V^2-P$ droop control and unifies bus voltage regulation and power regulation in a single control structure. Thus, global smooth transition between various operation modes can be achieved without any control strategy changes, which avoids various mode switch detection mechanisms and improves system stability. Furthermore, the proposed control scheme is fully decentralized which reduces the reliance on communication, and enhances the reliability of the microgrid. On the other hand, $V^2-P$ droop approach eliminates the negative effect of widespread constant power loads (CPLs) in dc microgrid. The stability of the proposed control method is illustrated, and the design guideline of some critical control gains is addressed. Finally, the effectiveness of the proposed control scheme is validated by the real-time hardware-in-loop (HIL) platform. [ABSTRACT FROM AUTHOR]

The high penetration of renewable energy sources always leads to the fluctuation of the droop coefficients which are designed in inverse proportion to their rated capacity, and power electronics devices are prone to static instability. Although the impedance-based approaches have been widely studied to deal with this problem, the stability-oriented droop coefficients region identification due to the fluctuation of renewable energy sources is not provided. Thus, this article proposes an impedance-based approach to assess the droop coefficients stability region in the power system consisting of numerous distributed generators (DGs). First, the modified phase margin and opposing argument (MPMOA) forbidden criterion is constituted to acquire the complementary space of the droop coefficients stability region. The MPMOA forbidden criterion is first transformed into the condition that the generalized return-ratio matrix is Hurwitz through mirror, rotation, and translation mapping. In comparison with the previous simplified stability criteria, the conservatism of the proposed criterion is reduced a lot. Thereafter, the droop coefficients stability region is directly calculated by the generalized return-ratio matrix and guardian map theory. Eventually, the simulation and experimental results are provided to validate the conservatism and effectiveness of the impedance-based stability region identification approach. Therein, the simulation results illustrate that the proposed droop coefficients stability operation criterion has lower conservatism than these of the previous simplified stability criteria. Furthermore, the simulation and experimental results illustrate that the proposed stability-oriented droop coefficients region identification approach can provide an effective parameter stability region. [ABSTRACT FROM AUTHOR]

The hybrid fuel cell/supercapacitor (FC/SC) system is a promising onboard power supply system for more electric aircraft (MEA), where system stability is a critical issue due to the high penetration of constant power loads (CPLs) in MEA. This article proposes a composite finite-time controller for decentralized power sharing and stabilization of the hybrid FC/SC system with CPLs. It consists of an integral droop (ID) + finite-time controller for the SC converter and a proportional droop (PD) + finite-time controller for the FC converter. First, the coordination of PD and ID achieves decentralized power sharing between FC and SC such that SC only compensates fast fluctuations and FC provides smooth power at the steady state. Then, a finite-time observer is designed to provide feedforward compensation for the disturbances and enables accurate tracking with fast dynamics. Finally, a composite finite-time controller is constructed following a nonrecursive synthesis procedure with a rigorous large signal stability analysis. The proposed controller guarantees finite-time convergence even under large signal variations and can be easily implemented with a practical gain tuning procedure. Simulations and experiments are conducted to verify the proposed technique. [ABSTRACT FROM AUTHOR]

This article investigates the current sharing and voltage regulation problem of a DC microgrid by a distributed supervisory control method. In the existing secondary control methods, current sharing feedback control loop and voltage regulation feedback control loop are linearly integrated to shift the droop equation. However, the linear combination of two loops may easily offset the control effect of each other, as the control dimension is less than the objective dimension. This fact motivates us to design a distributed supervisory secondary control (DSSC) scheme to achieve a more flexible configuring of each converter. The DSSC consists of two candidate controllers and a supervisor. The two controllers refer to current sharing configuring and DC bus voltage regulation respectively, and the supervisor is to orchestrate the switching between candidate controllers based on the system environment of DC microgrid. Under the proposed DSSC, each converter selects to configure the current sharing or regulates the DC bus voltage according to its need. Theoretical stability analysis is rigorously conducted. Finally, both simulation and experiment results are presented to demonstrate the effectiveness of DSSC. [ABSTRACT FROM AUTHOR]

The interleave boost converter (IBC) has been used as the interface converter between the low-voltage electrical energy sources, such as lithium-ion battery banks, solar panels, and fuel cells, and the dc bus of dc microgrids. With increasing penetration of tightly regulated power electronic loads, which behave as constant power loads, the stability of microgrid dc-bus voltage that is fed by the IBC is threatened by the loads’ negative incremental impedance feature. To ensure the stability of the bus voltage, this article proposes a nonlinear disturbance observer (NDO)-based sliding-mode control algorithm. The proposed algorithm has excellent robustness, low computational burden, and no extra hardware cost. A generalized average state-space model is proposed to facilitate the control design. In addition, an NDO is employed to estimate the output power of IBC rapidly and accurately. To verify the effectiveness of the proposed algorithm, simulation and experimental results are presented. [ABSTRACT FROM AUTHOR]

In hybrid ac/dc microgrids, a bus converter system involving the use of a high-frequency dc transformer (DCT) plays a vital role to transmit power between dc and ac subgrids. However, the multiple control tasks and power transfer mode switching of bidirectional dc/ac converter create big challenges for the systematic controller design of DCT. To address this concern, a comprehensive coordinated frequency control (CCFC) is put forward in this article for the symmetrical CLLC-DCT to adapt the complicated hybrid ac/dc microgrid operation. By establishing the mathematical model on the voltage conversion gain (VCG) and active power transmission ratio (APTR), four operation modes are proposed based on power transfer conditions. The corresponding operators are derived based on the circuit feature to avoid the redundant manipulations. Frequency threshold is determined to avoid both overvoltage and low-voltage issues based on the known allowable range of VCG. The proposed CCFC scheme is detailedly realized by combining both the open-loop and closed-loop schemes to facilitate the design procedure. The prototype DCT demonstrates the proposed CCFC scheme can achieve a peak efficiency of 98.2% and satisfied performances on both VCG and APTR. [ABSTRACT FROM AUTHOR]

The multilevel boost converter (MBC) has been widely adopted in the dc microgrid systems due to its high voltage gain and simple structure. In recent years, the power electronic loads, which usually behave as constant power loads (CPLs), are penetrating in microgrids. The incremental negative impedance of CPLs degrades the stability of microgrid systems. To ensure effective power flow and guarantee system stability, eliminating the undesired effects of CPLs is a necessity. In this article, a novel assorted nonlinear stabilizer, which is integrated with an extended nonlinear disturbance observer (NDO) and an adaptive backstepping controller, is proposed for stabilizing the MBC-fed microgrid system with CPLs. First, the reduced-order MBC model is transformed into the Brunovsky's canonical form using the exact feedback linearization technique. Second, owing to the NDO, fast system dynamic responses are achieved. Using the estimations of NDO, an adaptive backstepping controller is developed to strictly guarantee the microgrid bus voltage stability in the sense of a large signal. Simulation and experiment results are presented to verify the effectiveness and feasibility of the proposed stabilizer. [ABSTRACT FROM AUTHOR]

The Newton-Raphson (NR) method is a powerful tool in nonlinear equations. However, a well-known disadvantage of this method is that the initial iteration value must be chosen sufficiently close to a true solution in order to guarantee its convergence. The Kantorovich theorem is virtually the only known sufficiency condition for convergence of NR method, and gives very conservative bounds. This letter analyzes the convergence of the NR method in feasible power-flow for direct-current (DC) network, and an explicit convergence condition is obtained. Comparing with the existing results, the proposed convergence condition is more concise and less conservative. Moreover, according to the proposed condition, one can easily set the initial iteration value to guarantee the convergence of the NR method. Case studies verify the correctness of the presented analysis. [ABSTRACT FROM AUTHOR]

The penetration level of photovoltaic (PV) keeps increasing in modern distribution networks, which leads to various severe voltage limits violation problems. This article aims to aggregate and utilize the PV inverters for voltage regulation by a fully distributed two-level Volt/VAr control (VVC) scheme. In the lower-level VVC (real-time scale), the rooftop PV inverters are aggregated via consensus algorithms and then governed by droop controllers in medium-voltage networks. The droop controller adjusts the reactive power output of each PV aggregator in real-time from its dispatched value depending on the bus voltage variations. In the upper-level VVC (15-min timescale), the reactive power of PV aggregators is dispatched for power loss minimization, where control signals are set as base values for PV aggregators. This problem is formulated as second-order cone programming and solved by the alternating direction method of multipliers. The simulation results demonstrate the effectiveness of the proposed method in both short-term and long-term scenarios with different system scales. [ABSTRACT FROM AUTHOR]

Different from single-bus dc microgrids (MGs), a multibus MG normally bridge multiple dc buses via a complex line impedance network. This intricate configuration together with tickling constant power loads may cause severe stability issues. For system stability improvement, conventional ways include designing small-signal stabilizers in a decentralized way and constructing large-signal stabilizers by solving Lyapunov equation in a central controller. Note that centralized methods may be vulnerable to single point of failures, while decentralized patterns are commended to reinforce MG reliability and scalability. To implement large-signal stabilization in decentralized mechanisms, which has rarely been investigated in the existing literature, a novel uniform nonsmooth control scheme (UNCS) is proposed, in this article. UNCS has the following fourfold advantages. First, UNCS helps to express commonly used dc/dc converters, boost, buck, and buck–boost, into a uniform model. The model substantially facilitates a uniform stabilizer design that accommodates the preceding three converters despite their differences in topology. Second, UNCS assigns each converter based distributed generator (DG) with a composite large-signal stabilizer comprising a nonsmooth observer and a nonsmooth controller. The controller neutralizes the disturbances estimated by the observer, and it also rigorously stabilizes the internal states in DG uniform model. Third, the stabilizer also provides an additional control degree of freedom, a finetuning factor, to refine DG dynamic responses to meet high requirements for voltage and power qualities. Fourth, when numerous DGs with the proposed stabilizer congregate to build a multibus MG, the overall system large-signal stabilization is autonomously attained without communication links, which is mathematically justified by Lyapunov stability analyses. As such, DGs are allowed to randomly plug in/out without worrying about impairing system level stability. Simulations help to properly select relevant key parameters. Experiments show that UNCS outperforms the standardly designed PI controls, and it will stabilize the multibus MG in a large operation range. [ABSTRACT FROM AUTHOR]

With more distributed energy resources penetrated into the residential community, nanogrid based peer-to-peer (P2P) energy market has rapidly emerged over recent years. Due to the complexities on the decision-making process of each market participant, an efficient, fair and beneficial oriented bidding strategy is thus necessary. In this article, a two-stage bidding strategy for P2P trading of nanogrid is proposed. To overcome the limitations of traditional methods, in the first stage, a supply-demand relationship considered two-step price predictor, which aims to promote the usage of local renewable energy, is formulated to provide the guidance on transaction adjustment. In the second stage, trading preference based simultaneous game-theoretic approach is fully introduced, which can optimize the market equilibrium and then increase the social welfare of the P2P market. Additionally, to mitigate the possible failure of price matching, value-at-risk is implemented through the trading process as a risk hedging tool. To verify the effectiveness of the proposed strategy, usages of local renewable energy, economic benefits and success rates of transaction is compared against the traditional method for various cases. [ABSTRACT FROM AUTHOR]

This paper proposes a semi-consensus strategy for multi-functional hybrid energy storage systems (HESSs) in DC microgrids. Batteries in a HESS are regulated by conventional V-P droops and supercapacitors (SCs) are with integral droops (ID). Only batteries are assigned with local distributed compensators which exchange information through sparse communication links. Those SCs are exempted from data exchange process, which would save system investment costs. Within the semi-consensus scheme, the most essential function is the cooperation of V-P droop and ID that helps to naturally allocate low frequency components of load power to batteries and high frequency components to SCs, thus prolonging the overall life time of HESS. In addition to the transient power allocation function, there are other three functions endowed by the proposed strategy, which are autonomous DC bus voltage recovery to its nominal level, spontaneous SC state of charge (SOC) restoration, autonomous power sharing and SOC balancing among batteries. It is the simultaneous realization of above four functions with limit communications that makes up the main contributions in this paper. A generic mathematical modeling of HESS with the semi-consensus strategy is established. The model allows for dynamic analyses to theoretically validate the effectiveness of proposed method in both frequency and time domains. In-house experimental results are shown fully consistent with the dynamic analyses and also effectively corroborate the intended HESS multi-functional operations. [ABSTRACT FROM AUTHOR]

A virtual synchronous generator (VSG) control based on adaptive virtual inertia is proposed to improve dynamic frequency regulation of microgrid. When the system frequency deviates from the nominal steady-state value, the adaptive inertia control can exhibit a large inertia to slow the dynamic process and, thus, improve frequency nadir. And when the system frequency starts to return, a small inertia is shaped to accelerate system dynamics with a quick transient process. As a result, this flexible inertia property combines the merits of large inertia and small inertia, which contributes to the improvement of dynamic frequency response. The stability of the proposed algorithm is proved by Lyapunov stability theory, and the guidelines on the key control parameters are provided. Finally, both hardware-in-the-loop and experimental results demonstrate the effectiveness of the proposed control algorithm. [ABSTRACT FROM AUTHOR]

High penetration of renewable energies through fast-response power converters results in a considerable displacement of conventional synchronous generators and losing of system inertia for frequency control. Modular multilevel converters (MMCs) can be employed serving as an interface between the large-scale renewable generation and power grids. In a microgrid with high shares of renewables integrating through MMCs, submodule (SM) capacitors can be used as energy storage to provide a degree of synthetic inertia for system frequency support. This paper quantitatively exploits the MMC synthetic inertia in a microgrid with flexible renewable penetration levels. An MMC inertia coefficient concept is proposed. It is mainly affected by the penetration ratio, the SM capacitance, and the modulation index of an MMC, with the system operation constraints. According to the analysis, a substantial portion of system inertia can be provided by properly designed MMCs. Detailed MMC frequency control loops are presented. The capacitor average voltage is proportionally linked to the frequency deviation, in order to flexibly adjust the capacitor energy during frequency events. The MMC output active power is deliberately and simultaneously regulated according to the capacitor energy change rate. By controlling the MMC capacitor voltage, dc side power, and output active power, an amount of energy can be released or absorbed by SM capacitors to improve the system frequency response during the frequency event transients, while the renewable generation is scarcely influenced. The proposed concept is experimentally verified and the results show that, with proper system parameters and control loops, the line frequency deviation and the rate of change of frequency can be significantly reduced. Good agreements of the system frequency characteristics have been achieved between the theoretical calculation and experimental results. [ABSTRACT FROM AUTHOR]

Photovoltaic (PV) generators have continuously increased in recent years, whose power is usually controlled through the perturbation and observation (P&O) method. In essence, the P&O method is nonlinear and discontinuous. Hence, the conventional small-signal stability analysis is not suitable anymore when the influence of the P&O-based power control is considered. Focusing on this problem, this paper adopts the nonlinear describing function (DF) method to conduct the accurate stability analysis of PV generators with consideration of P&O-based power control. The detailed procedures about the DF method are introduced, and then the related influence factors like perturbation size, filters, and so on are analyzed quantitatively. Furthermore, the comparison with the conventional stability analysis methods is made, which suggests that the DF method can effectively enhance the accuracy of the stability analysis. All the conclusions are verified by the real-time hardware-in-loop tests. [ABSTRACT FROM AUTHOR]

The rapidly developing data center industry results in a large amount of energy consumption. Considering its unique demand characteristics, it becomes desirable to improve its energy efficiency by reusing its waste heat. In this paper, an integrated energy management scheme is proposed to optimize the operation cost for a data center microgrid. The waste heat recovered from data center operation is optimally scheduled with other resources in the integrated energy management model to minimize the operation cost of data center microgrid. Taking the uncertain nature of renewable supply, power and heat demand into consideration, a two-stage stochastic programming model is formulated and analyzed under different test cases. The numerical study results demonstrate the effectiveness of the proposed integrated energy management model. [ABSTRACT FROM AUTHOR]

CLLC-type dc transformer (CLLC-DCT) is very popular in the hybrid ac–dc microgrid thanks to its high-power density advantage and good bidirectional power transfer capacity. In the hybrid ac/dc microgrid, the open-loop control is always utilized by the CLLC-DCT to cooperate with the bidirectional interlinking converter to realize the power and voltage conversion between the ac and dc bus. This paper first studies the circuit parameters design of the open-loop controlled CLLC-DCT with consideration of such a realistic problem: The real inductors/capacitors values are actually different with their theoretically designed values due to the operation power and temperature variation. To solve this problem, a robust circuit parameters design scheme is proposed for the CLLC-DCT in this paper. With the proposed scheme, the designed CLLC-DCT exhibits good power transmission and voltage regulation ability in the hybrid ac/dc microgrid even when its actual inductors/capacitors values vary with the practical power and temperature. The robust design method is experimentally verified in a hybrid ac/dc microgrid prototype. [ABSTRACT FROM AUTHOR]

Internet data centers are booming in the last few years, leading to gigantic amount of energy consumption and greenhouse gas emission. In this paper, a data center microgrid including conventional units, energy storage, and renewable generation is modeled to optimize its electricity bill and carbon footprint. Considering the randomness of electricity price, renewable output and workload distribution, the volatility risk is incorporated to measure the associated operation risk. Then, a day-ahead emission-aware stochastic resource planning scheme is formulated to decide the strategy on power procurement, energy storage operation, batch workload allocation, and unit commitment of conventional units. Simulation results show the effectiveness of the proposed approach. [ABSTRACT FROM AUTHOR]

This paper presents a uniform control strategy for the bidirectional ac/dc interlinking converter (BIC) in hierarchical controlled hybrid microgrid. The uniform control strategy unifies different control structures for multifunctional BIC in terms of power management, ac and dc voltage support. With the unified control structure, various triggering mechanisms for BIC mode-switch are not compulsory in response to the relevant scenario changes in the power network. Therefore, the negative consequences, such as unsmooth transition and system collapse due to the inaccurate or slower mode switch, can be avoided. Moreover, the uniform control strategy is also applicable for the hierarchical controlled hybrid microgrid, whose control architecture is typically comprised of the centralized and decentralized levels. This enables the communication fault ride-through capability and the impact from the communication failures can thus be mitigated. To sum up the above two features, the proposed control strategy is able to provide a smooth operation for the BIC without control mode switch in the event of unintentional scenario changes in power or communication network. To verify the validity of the proposed uniform control strategy, the controller hardware-in-the-loop experimental results have been provided in this paper. [ABSTRACT FROM PUBLISHER]

Power allocation in hybrid energy storage systems (HESSs) is an important issue for dc microgrids. In this paper, an integral droop (ID), inspired by the electrical characteristics of capacitor charging/discharging process, is proposed and applied to a cluster of energy storages (ESs) with high ramp rates. Through the coordination of the ID and conventional V-P droop, the transient power allocation in HESSs can be intrinsically realized in a decentralized manner. The high-frequency components of power demand can be compensated by the ESs with ID, whereas the ESs with V-P droop respond to the smooth change of load power. Additionally, the ID coefficient can be designed according to the nominal ramp rate of the ESs with slow response, which helps to extend the lifespan of the HESS. On the other hand, to easily assess the stability of the system feeding constant power loads, a minimum relative impedance criterion (MRIC) is developed. Based on MRIC, it is revealed that the proposed ID can shape the output impedance of the HESS and stabilize the entire system. The feasibility and effectiveness of ID are verified by both simulations and experimental results. [ABSTRACT FROM PUBLISHER]

Economic operation is a major concern for microgrids (MGs). System operation cost is optimized when the incremental costs (ICs) of all distributed generators (DGs) reach equality. Conventionally, economic dispatch of DGs is solved by centralized control with optimization algorithms or distributed control with consensus algorithms. To improve the reliability, scalability, and economy of MGs, a fully decentralized economic power sharing strategy is proposed in this paper. As frequency is a global state in ac MG and dc bus voltage serves as a natural indicator in dc MG, a frequency-IC droop scheme is proposed for ac MG, a voltage-IC droop scheme is proposed for dc MG, and a normalization scheme is proposed for hybrid ac/dc MG. By using the proposed technique, ICs of DGs reach equality with the convergence of the system global indicator (frequency or dc bus voltage). Then power sharing of each DG is automatically achieved based on its relevant IC function and the total operating cost can be optimized without any communication or central controllers. The proposed approach is implemented in an ac MG, a dc MG, and a hybrid ac/dc MG in MATLAB/Simulink to verify its effectiveness. [ABSTRACT FROM AUTHOR]

For hybrid energy storage system in dc microgrid, effective power split, bus voltage deviation, and state-of-charge (SoC) violation are significant issues. Conventionally, they are achieved by centralized control or hierarchical control methods with communications. This paper proposes a simple and effective strategy to solve the problem in a decentralized manner. A high-pass filter-based droop (HPFD) controller is proposed to regulate the battery converter, and a virtual capacitance droop (VCD) controller is implemented for a supercapacitor (SC) converter. The cooperation of HPFD and VCD first achieves autonomous power split that high-frequency fluctuation is buffered by SC and low-frequency power is supplied by battery. Meanwhile, the bus voltage deviation induced by the droop-based power sharing is eliminated automatically at steady state. The resulted bus voltage restoration simultaneously enforces SC SoC back to its nominal value, and, thus, ensures continuous operation of SC as a power buffer without the violation of its SoC boundary. A design guideline is developed to ensure expected system dynamics. The effectiveness of the proposed method and analytical results are validated by simulations and experiments. [ABSTRACT FROM PUBLISHER]

Power allocation is a major concern in hybrid energy storage system. This paper proposes an extended droop control (EDC) strategy to achieve dynamic current sharing autonomously during sudden load change and resource variations. The proposed method consists of a virtual resistance droop controller and a virtual capacitance droop controller for energy storages with complementary characteristics, such as battery and supercapacitor (SC). By using this method, battery provides consistent power and SC only compensates high-frequency fluctuations without the involvement of conventionally used centralized controllers. To implement the proposed EDC method, a detailed design procedure is proposed to achieve the control objectives of stable operation, voltage regulation, and dynamic current sharing. System dynamic model and relevant impedances are derived and detailed frequency domain analysis is performed. Moreover, the system level stability analysis is investigated and system expansion with the proposed method is illustrated. Both simulations and experiments are conducted to validate the effectiveness of the proposed control strategy and analytical results. [ABSTRACT FROM PUBLISHER]

Repetitive control strategies have been commonly applied in pulse-width-modulated (PWM) voltage source inverters (VSIs) for many industrial applications. This paper presents a repetitive controller for voltage harmonic mitigation of VSI-based islanded microgrids. The phase delay in the overall control system, e.g., the delay caused by the digital duty cycle calculation, PWM generation, and repetitive controller, has to be compensated by elaborately designed phase lead filters in order to prevent control performance deterioration and system instability. Nevertheless, quantificational analysis and practical design of the time advance unit in such filters are hardly found in existing literature works. In view of this, this paper proposes an explicit analysis of the phase lead filters and a novel design method of the time advance unit in repetitive controllers to ensure system stability. Moreover, with the help of the proposed method, the overall system stability margin is predictable and improved controller performance is achieved as well. The proposed method is implemented experimentally to show the accurate stability margin calculation as well as the excellent steady state and dynamic performances of the repetitive control scheme. [ABSTRACT FROM PUBLISHER]

Multi-level energy management system (EMS) is proposed for dc microgrids operations to ensure system reliability, power quality, speed of response, and control accuracy in this paper. System distributed control is scheduled as the primary control. Battery energy storages (BESs) operate in voltage regulation mode with droop control for power sharing. However, bus voltage deviation and power tracking error due to line impedance are the main drawbacks of distributed control. To enhance the system power quality and control accuracy, bus voltage restoration and power sharing compensation are implemented in secondary control. Economic dispatch based on comparison of system units' marginal operation costs is carried out in tertiary control to minimize the system operation cost. Detailed elaboration has been derived to quantify BES utilization cost due to the cumulated lifetime degradation. In case of communication failure, system operation can still be retained with primary control without operating mode change so that to enhance system reliability. A lab scale dc microgrid is developed for the verification of the proposed multi-level EMS and relevant control techniques. [ABSTRACT FROM PUBLISHER]

This paper presents two linear power sharing schemes for distributed generators (DGs). The purpose is to reduce the total generation cost (TGC) of the considered autonomous microgrid. The saving in cost is realized by tuning the DG droop gradients in accordance to their respective maximum or mean generation costs. Compared with the existing nonlinear cost-based schemes, the proposed schemes use linear droop functions, which are easier to tune and implement in practice for a greater reduction in TGC. The proposed schemes have been tested experimentally with an example microgrid powered first by dispatchable DGs only, and then by a combination of dispatchable and nondispatchable DGs. Experimental results obtained have demonstrated the anticipated outcomes under different operating conditions. [ABSTRACT FROM AUTHOR]

Multiple-voltage-region control, in which the bus-voltage range is divided into several regions, is usually implemented for dc microgrid operation in distributed manner. Voltage/power droop relationships are imposed for active power sharing among slack terminals. Conventionally, threshold voltages for voltage region partition are determined with fixed percentage of variation around the nominal value, which may result in unevenness of droop coefficients in different regions. If system droop coefficient is too high, significant bus-voltage step change due to load variation will occur. On the other hand, significant power sharing error among slack terminals will be induced if the droop coefficient is too low. In this paper, a compromised solution with power-capacity-based bus-voltage region partition is proposed to equalize the droop coefficients in different regions. However, the droop coefficients are determined based on the rated power capacity of system units. Bus-voltage discontinuity appears when the power capacity reduces in actual implementation. To eliminate the voltage discontinuity, online droop coefficient tuning according to the real-time power capacity is implemented. Algorithms for local power capacity estimation of solar photovoltaic (PV) and battery energy storage have been proposed. A lab-scale dc microgrid has been developed for verification of the proposed methods. [ABSTRACT FROM AUTHOR]

This paper presents a probabilistic approach in the modeling of stand-alone microgrids to predict their operational adequacy performance considering uncertainty of energy storage system (ESS), photovoltaic system (PVS) and conventional generator (CG). Instead of using the daily or hourly time step, operating period a minutely time step is considered to incorporate the effect of fast ramp up/down of system components on microgrid operating adequacy through expected energy not supplied (EENS) and expected energy not used (EENU), due to load and resource variations. A time varying state of charge (SOC) model is proposed to determine power output of an ESS in reliability modeling. The reliability of a PVS is modeled in detail based on the total cross-tied configuration (TCTC) of photovoltaic (PV) cells and arrays. The proposed technique and indices will be useful for system planners to select the type and size of microgrid systems that contain alternative energy sources and storage. [ABSTRACT FROM PUBLISHER]

This paper presents a distributed control scheme for reliable autonomous operation of a hybrid three-port ac/dc/distributed storage (ds) microgrid by means of power sharing in individual network, power exchange between ac and dc networks, and power management among three networks. The proposed distributed control scheme includes: 1) a fully decentralized control, which is achieved by local power sharing (LPS) in individual ac or dc network, global power sharing (GPS) throughout ac/dc networks, and storage power sharing (SPS) among distributed storages. Upon fully decentralized control, each power module can operate independently without communication links. This would benefit for riding through communication malfunction in multilayer supervision control system; 2) a multilevel power exchange control for scheduling LPS, GPS, and SPS has been developed to reduce unnecessary power exchange between ac/dc networks and operations of DS units with the benefit of reducing power exchange losses and prolonging storage lifetime. The proposed distributed control strategy has been verified by the simulation and experimental results. [ABSTRACT FROM PUBLISHER]

Droop schemes have traditionally been applied to the control of parallel synchronous generators in power systems. It has subsequently been brought over to the control of distributed generators (DGs) in microgrids with the same retained objective of proportional power sharing based on ratings. This objective might, however, not suit microgrids well since DGs are usually of different types, unlike synchronous generators. Other factors like cost, efficiency, and emission penalty of each DG at different loading must be considered since they contribute directly to the total generation cost (TGC) of the microgrid. To reduce this TGC without relying on fast communication links, an autonomous droop scheme is proposed here, whose resulting power sharing is decided by the individual DG generation costs. Comparing it with the traditional scheme, the proposed scheme retains its simplicity and it is hence more likely to be accepted by the industry. The reduction in TGC has been verified by experiment. [ABSTRACT FROM PUBLISHER]

DC microgrids are becoming popular in low-voltage distribution systems due to the better compatibility with photovoltaic panels, electric vehicles, and dc loads. This paper presents a practical dc microgrid developed in the Water and Energy Research Laboratory (WERL) in the Nanyang University of Technology, Singapore. The coordination control among multiple dc sources and energy storages is implemented using a novel hierarchical control technique. The bus voltage essentially acts as an indicator of supply–demand balance. A wireless control is implemented for the reliable operation of the grid. A reasonable compromise between the maximum power harvest and effective battery management is further enhanced using the coordination control based on a central energy management system. The feasibility and effectiveness of the proposed control strategies have been tested by a dc microgrid in WERL. [ABSTRACT FROM PUBLISHER]