Abstract:With the intensifying energy crisis and environmental pollution, the Energy Internet and corresponding patterns of energy use have been attracting more and more attention. In this paper, the basic concept and characteristics of the Energy Internet are summarized, and its basic structural framework is analyzed in detail. On this basis, couplings between the electric power system and other systems such as the cooling and heating system, the natural gas system, and the traffic system are analyzed, and the operation and planning of integrated energy systems in both deterministic and uncertain environments are comprehensively reviewed. Finally, the research prospects and main technical challenges of the Energy Internet are discussed.

Abstract:The heterogeneous nature of smart grid componentsand the desire for smart grids to be scalable, stable and respect customer privacy have led to the need for more distributed control paradigms. In this paper we provide a distributed optimal power flow solution for a smart distribution network with separable global costs, separable non-convex constraints, and inseparable linear constraints, while considering important aspects of network operation such as distributed generation and load mismatch, and nodal voltage constraints. An asynchronous averaging consensus protocol is developed to estimate the values of inseparable global information. The consensus protocol is then combined with a fully distributed primal dual optimization utilizing an augmented Lagrange function to ensure convergence to a feasible solution with respect to power flow and power mismatch constraints. The presented algorithm uses only local and neighbourhood communication to simultaneously find the mismatch between power generation, line loss and loads, to calculate nodal voltages, and to minimize distributed costs, leading to a completely distributed solution of the global problem. An IEEE test feeder system with a reasonable number of nodes is used to illustrate the proposed method and efficiency

Abstract:In recent years, the increasing penetration level of renewable generation and combined heat and power (CHP) technology in power systems is leading to signifi-cant changes in energy production and consumption patterns. As a result, the integrated planning and optimal operation of a multi-carrier energy (MCE) system have aroused widespread concern for reasonable utilization of multiple energy resources and efficient accommodation of renewable energy sources. In this context, an integrated demand response (IDR) scheme is designed to coordinate the operation of power to gas (P2G) devices, heat pumps, diversified storage devices and flexible loads within an extended modeling framework of energy hubs. Subsequently, the optimal dispatch of interconnected electricity, natural gas and heat systems is implemented considering the interactions among multiple energy carriers by utilizing the bi-level optimization method. Finally, the proposed method is demonstrated with a 4-bus multi-energy system and a larger test case comprised of a revised IEEE 118-bus power system and a 20-bus Belgian natural gas system.

Abstract:This paper presents a comprehensive approach to enhance the traditionally deterministic-based power grid planning using system well-being analysis concept. The objective is to identify and characterize the existing system reliability concerns inherited from the adopted deterministic criteria, so that power utilities can accordingly adjust their reliability criteria to cope with real-life system uncertainties and hence to enhance the overall system reliability. A determination of transmission capacity reserve derived from incorporating deterministic criteria into a probabilistic framework has been introduced in the paper. An application of the proposed methodology for justifying a transmission addition project to accommodate the system load growth is illustrated by using an actual island power grid. Both technical and economic aspects, greatly concerned in practice, have been taken into consideration in the project justification.

Abstract:Operating in island mode when failure occurs in the active distribution network(ADN) is an effective measure to maintain an uninterrupted power supply to signifi- cant loads. A two-stage approach that integrates optimal island partition and power dispatch is proposed in this paper, considering photovoltaics (PVs), batteries (BEs) and electric vehicles (EVs) as the power sources. In the first stage, energy indices are defined to describe the energy demand and the maximum energy that these distributed energy resources (DERs) can provide, and islands are partitioned based on an energy constraint. Considering the variability the loads and PVs, the energy constraint is a necessary but not sufficient condition for island operation, so in the second stage, a power dispatch model is proposed as a test for the island partition result. Sequential power flow is also simulated to guarantee a feasible and optimized island status. The situations when the tests are not passed are analyzed and classified, and corresponding modifications for the first stage model are provided. Multiple levels of constraints based on the energy index are established for the island partition model. The proposed approach has been validated through simulation using a modified IEEE 69-bus system which is divided into three districts with different load variability characteristics.

Abstract:In a hybrid AC/DC medium voltage distribution network, distributed generations (DGs), energy storage systems (ESSs), and the voltage source converters (VSCs) between AC and DC lines, have the ability to regulate node voltages in real-time. However, the voltage regulation abilities of above devices are limited by their ratings. And the voltage regulation efficiencies of these devices are also different. Besides, due to high r/x ratio, node voltages are influenced by both real and reactive power. In order to achieve the coordinated voltage regulation in a hybrid AC/ DC distribution network, a priority-based real-time control strategy is proposed based on the voltage control effect of real and reactive power adjustment. The equivalence of real and reactive power adjustment on voltage control is considered in control area partition optimization, in which regulation efficiency and capability are taken as objectives. In order to accommodate more DGs, the coordination of controllable devices is achieved according to voltage sensitivities. Simulations studies are performed to verify the proposed method.

Abstract:With the global consciousness of climate change, renewable energy systems are prioritized over the conventional energy systems. The deep injection of renewables into the power systems is creating several challenges to the grid due to wide variations in their output power depending on the time of the day, weather etc. Of these challenges, frequency change plays a vital role in maintaining the power quality. This paper presents a novel sliding mode controller with non-linear disturbance observer to effectively mitigate the wide changes in the frequency. A sliding mode surface based on estimated disturbance along with states is designed. A sliding mode control law is proposed to compensate disturbances including variations in renewables, load and parameters under mismatched uncertainties. The proposed observer based controller is tested for three area multi-machine power system in MATLAB/Simulink. The simulated results proved to alleviate the frequency variations effectively compared to the conventional controllers.

Abstract:This paper investigates the use of a virtual synchronous generator (VSG) to improve frequency stability in an autonomous photovoltaic-diesel microgrid with energy storage. VSG control is designed to emulate inertial response and damping power via power injection from/to the energy storage system. The effect of a VSG with constant parameters (CP-VSG) on the system frequency is analyzed. Based on the case study, self-tuning algorithms are used to search for optimal parameters during the operation of the VSG in order to minimize the amplitude and rate of change of the frequency variations. The performances of the proposed self-tuning (ST)-VSG, the frequency droop method, and the CP-VSG are evaluated by comparing their effects on attenuating frequency variations under load variations. For both simulated and experimental cases, the ST-VSG was found to be more efficient than the other two methods in improving frequency stability.

Abstract:As power to gas (P2G) technology gradually matures, the coupling between electricity networks and natural gas networks should ideally evolve synergistically. With the intent of characterizing market behaviors of integrated electric power and natural gas networks (IPGNs) with P2G facilities, this paper establishes a steady-state model of P2G and constructs optimal dispatch models of an electricity network and a natural gas network separately. In addition, a concept of slack energy flow (SEF) is proposed as a tool for coordinated optimal dispatch between the two networks. To study how the market pricing mechanism affects coordinated optimal dispatch in an IPGN, a market equilibrium-solving model for an IPGN is constructed according to game theory, with a solution based on the Nikaido-Isoda function. Case studies are conducted on a joint model that combines the modified IEEE 118-node electricity network and the Belgian 20-node gas network. The results show that if the game between an electric power company and a natural gas company reaches market equilibrium, not only can both companies maximize their profits, but also the coordinated operation of the coupling units, i.e., gas turbines and P2G facilities, will contribute more to renewable energy utilization and carbon emission reduction.

Abstract:The increasing importance of energy efficiency has led to several studies related to the construction of a reliable low voltage DC (LVDC) distribution system. Specifically, studies on a protection scheme that considers the fault characteristics of an LVDC distribution system are essential to improve system reliability. When compared to a conventional distribution system, the most distinct feature of an LVDC distribution system is the existence of a capacitive discharge current from converters under a fault condition that results in the prompt operation of protection devices. Therefore, this study involves proposing a precise and rapid technique to identify the fault section in an LVDC distribution system. The technique involves two stages, namely: an analysis stage to analyze the capacitive discharge current and a decision stage to identify the fault section based on the fault type. A detailed discussion of each step is presented and its feasibility is verified based on the results of simulations with an ElectroMagnetic Transients Program and MATLAB.

Abstract:With the large-scale development of distributed generations (DGs) and the connection into the main grid of active distribution networks (ADNs), traditional centralized dispatch of power system has encountered enormous challenge. In a bilateral electricity market, introducing ADN resources in the day-ahead generation schedule will not only enrich the dispatch patterns to the power system, but also reflect the initiative of ADNs. This paper proposes a coordinated scheduling model of power system with a plurality of ADNs based on multi-agent system where ADN agents are brought in the day-ahead market clearing. The process of market clearing and the dispatch of DGs in ADNs are independent with each other but linked together through the market clearing price (MCP) and bid volume. The optimal operating point of the whole system is achieved through multiple information exchange. In comparison with the dispatch without interaction between ADNs and the market operator (MO), the coordinated scheduling model is applied in a system with four ADNs to verify that the proposed method can improve the overall interests of ADNs. Finally, the effects of storage device and tie-line power limit are analyzed.

Abstract:The impacts of outlying shocks on wind power time series are explored by considering the outlier effect in the volatility of wind power time series. A novel short term wind power forecasting method based on outlier smooth transition autoregressive (OSTAR) structure is advanced, then, combined with the generalized autoregressive conditional heteroskedasticity (GARCH) model, the OSTAR-GARCH model is proposed for wind power forecasting. The proposed model is further generalized to be with fat-tail distribution. Consequently, the mechanisms of regimes against different magnitude of shocks are investigated owing to the outlier effect parameters in the proposed models. Furthermore, the outlier effect is depicted by news impact curve (NIC) and a novel proposed regime switching index (RSI). Case studies based on practical data validate the feasibility of the proposed wind power forecasting method. From the forecast performance comparison of the OSTAR-GARCH models, the OSTAR-GARCH model with fat-tail distribution proves to be promising for wind power forecasting.

Abstract:Renewable energy based distributed generation (DG) has the potential to reach high penetration levels in the residential region. However, its integration at the demand side will cause rapid power fluctuations of the tieline in the residential region. The traditional generators are generally difficult to manage rapid power fluctuations due to their insufficient efficiency requirements and low responding speed. With an effective control strategy, the demand side resources (DSRs) including DGs, electric vehicles and thermostatically-controlled loads at the demand side, are able to serve as the energy storage system to smooth the load fluctuations. However, it is a challenge to properly model different types of DSRs. To solve this problem, a unified state model is first developed to describe the characteristics of different DSRs. Then a load curve smoothing strategy is proposed to offset the load fluctuations of the tie-line of the residential region, where a control matrix deduced from the unified state model is introduced to manage the power outputs of different DSRs, considering the response order and the comfort levels. Finally, a residential region with households is used to validate the load curve smoothing strategy based on the unified state model, and the results show that the power fluctuation rate of the tie-line is significantly decreased. Meanwhile, comparative study results are shown to demonstrate the advantages of the unified state model based load curve smoothing strategy.

Abstract:This study focuses on the potential role of plugin electric vehicles (PEVs) as a distributed energy storage unit to provide peak demand minimization in power distribution systems. Vehicle-to-grid (V2G) power and currently available information transfer technology enables utility companies to use this stored energy. The V2G process is first formulated as an optimal control problem. Then, a two-stage V2G discharging control scheme is proposed. In the first stage, a desired level for peak shaving and duration for V2G service are determined off-line based on forecasted loading profile and PEV mobility model. In the second stage, the discharging rates of PEVs are dynamically adjusted in real time by considering the actual grid load and the characteristics of PEVs connected to the grid. The optimal and proposed V2G algorithms are tested using a real residential distribution transformer and PEV mobility data collected from field with different battery and charger ratings for heuristic user case scenarios. The peak shaving performance is assessed in terms of peak shaving index and peak load reduction. Proposed solution is shown to be competitive with the optimal solution while avoiding high computational loads. The impact of the V2G management strategy on the system loading at night is also analyzed by implementing an off-line charging scheduling algorithm.

Abstract:This paper aims to accurately identify parameters of the natural charging behavior characteristic (NCBC) for plug-in electric vehicles (PEVs) without measuring any data regarding charging request information of PEVs. To this end, a data-mining method is first proposed to extract the data of natural aggregated charging load (ACL) from the big data of aggregated residential load. Then, a theoretical model of ACL is derived based on the linear convolution theory. The NCBC-parameters are identified by using the mined ACL data and theoretical ACL model via the derived identification model. The proposed methodology is cost-effective and will not expose the privacy of PEVs as it does not need to install sub-metering systems to gather charging request information of each PEV. It is promising in designing unidirectional smart charging schemes which are attractive to power utilities. Case studies verify the feasibility and effectiveness of the proposed methodology.

Abstract:This paper presents a Phillips-Heffron model for the generation unit with current-controlled (CC) voltage source converter (VSC) as the interface. A concept of current angle is put forward for the CC-VSC, and the relationship between the current angle and the power angle is also quantified. Based on the current angle, a PhillipsHeffron model is established for the generation unit with CC-VSC, considering the dynamic of phase-locked-loop (PLL) in the weak grid. The model demonstrates that small-signal dynamics of the generation unit is similar to that of the traditional synchronous generator (SG) which is characterized by the electromechanical swing equations. Then the dynamics can be depicted by the famous inertia, synchronizing and damping coefficients. Small-signal stability of a CC-VSC-based single machine infinite bus system is analyzed by means of the traditional theory of power system. Based on the relationship between the current angle and the power angle, the Phillips-Heffron model of the CC-VSC is also used in stability analysis of multimachine power system, and parameter optimizations of the CC-VSC are also studied for stability improvement.

Abstract:The admittance is a strong tool for stability analysis and assessment of the three-phase voltage source converters (VSCs) especially in grid-connected mode. However, the sequence admittance is hard to calculate when the VSC is operating under unbalanced grid voltage conditions. In this paper, a simple and direct modeling method is proposed for a three-phase VSC taking the unbalanced grid voltage as a new variable for the system. Then coupling in the three-phase system can be calculated by applying the harmonic linearization method. The calculated admittance of three-phase VSCs is verified by detailed circuit simulations.

Abstract:High efficiency and fast dynamic response are two main control objectives for dual active bridge (DAB) DC-DC converters. Traditional extended phase shift (EPS) control can significantly enhance the conversion efficiency of DAB DC-DC converters by reducing current stress; however, it cannot fulfill fast dynamic response requirements. In this paper, a novel hybrid control scheme consisting of EPS control and direct power control (DPC), named as EPS-DPC, is proposed. EPS-DPC control has salient features in both efficiency and dynamic performance. In order to verify the outstanding performance of the proposed EPS-DPC scheme, an experimental comparison was carried out on a scale-down DAB DC-DC converter among several control strategies, including single phase shift control with traditional voltage-loop (SPSTVL), EPS control with traditional voltage-loop (EPSVTL), and EPS-DPC. Experimental results have been high consistent with theoretical analysis, and verified these advantages of the proposed EPS-DPC scheme.