Search

This paper presents the design of an energy management system (EMS) capable of forecasting photovoltaic (PV) power production and optimizing power flows between PV system, grid, and battery electric vehicles (BEVs) at the workplace. The aim is to minimize charging cost while reducing energy demand from the grid by increasing PV self-consumption and consequently increasing sustainability of the BEV fleet. The developed EMS consists of two components: An autoregressive integrated moving average model to predict PV power production and a mixed-integer linear programming framework that optimally allocates power to minimize charging cost. The results show that the developed EMS is able to reduce charging cost significantly, while increasing PV self-consumption and reducing energy consumption from the grid. Furthermore, during a case study analogous to one repeatedly considered in the literature, i.e., dynamic purchase tariff and dynamic feed-in tariff, the EMS reduces charging cost by 118.44 $\%$ and 427.45 $\%$ in case of one and two charging points, respectively, when compared to an uncontrolled charging policy.

Solar-battery systems are still expensive, bulky, and space consuming. To tackle these issues, we propose a novel device that combines all the components of a solar-battery system in one device. This device might help reduce installation cost compared to the current solar-battery systems as well as provide a plug-and-play solution. However, this physical integration means higher temperatures for the components. Therefore, this paper presents a thermal analysis of the physical integration concept to evaluate its feasibility, focusing on the batteries, the most delicate components. The thermal analysis was conducted using a Finite Element Method model and validated with experimental results on a prototype. According to the model, the temperature of the components (battery and converters) reduced drastically by adding an air gap of 5–7 cm between the solar panel and the components. Even under severe conditions, maximum battery temperature never surpassed the highest temperature of operation defined by the manufacturer. Moreover, the maximum battery temperature decreases even further by applying a phase change material as a passive cooling method, reducing it by 5 °C. As a result, the battery pack operates in a safe range when combined with a 265 Wp solar panel, demonstrating the potential of this concept for future solar-battery applications.

Due to an increasing number of power generation units and load devices operating with direct current (DC) at distribution level, there is a potential benefit of leading efforts toward building a DC distribution system. However, the implementation of DC distribution systems faces important challenges, including the market inertia of AC systems and standardization. Many of the benefits that are attributed to DC can only be realized if a complete DC system is developed, and not if only a few components are replaced. This paper presents the concept of a universal DC distribution system, as envisioned by the authors. The universal DC distribution system could be implemented in various use cases, but could also completely replace AC distribution grids. The paper covers the possibilities of having DC nanogrids inside buildings, DC microgrids in neighborhoods, and the connection to AC and DC medium voltage grids. Furthermore, considerations regarding flexibility, electricity market design, control, and protection are presented.

Constant power loads combined with low inertia form a major challenge for future distribution grids. This paper presents a state-space representation to model dc distribution systems. Two methods are discussed to analyze the (small-signal) stability of these dc distribution systems; an algebraic method and a Brayton-Moser method. The system models and the methods for stability analysis were verified using an experimental dc microgrid set-up. Furthermore, it was found that the instability of dc distribution systems can be classified into two categories: equilibrium instability and oscillatory instability.

Flexible dc interlinks can be used for compact and efficient active power redirection by creating a meshed structure in existing radial distribution network. It is shown that with optimal location and sizing of a back-to-back voltage sourced converter based point-to-point dc link connecting two ac buses of the test 33-bus radial distribution system, 10-15% network loss minimization can be achieved. This possibility can be advantageous in efficiently redirecting active power flows in urban and semi -urban areas because intermittent distributed grid resources (such as photovoltaics and wind farms) and energy intensive loads (such as ultra fast charging stations for electric vehicles) are resulting in local pockets of excess generation and demand. The impact of distributed generation and high powered loads on placement of dc interlink is numerically quantified to suggest that both capacity and efficiency enhancement can be achieved in the distribution grid. It is envisioned that this method of integrating dc power transmission can be the first realistic step towards realizing a multi-terminal dc medium voltage distribution evolving out of the existing infrastructure.

DC Microgrid with integrated photo-voltaics (PV) and battery storage system is a promising technology for future smart grid applications. This paper compares three battery storage technologies namely: lithium-ion (Li-ion), lead-acid, and vanadium redox flow battery (VRFB) in the residential low-voltage dc grids, to increase the penetration of PV to improve self-sufficiency and grid independence. A multi-objective optimisation method is proposed to visualise the trade-offs between four objective functions: cost of electricity, energy autonomy, peak power reduction, and lifetime capital cost. The method is applied to a near-future scenario, based on a real residential feeder. Pareto fronts of the dominant designs are used to compare different battery technologies for this PV-battery integrated dc microgrid application. Additionally, the results provide insight into the dimensioning the battery, the PV, and the front-end ac-dc converter.

Traditional optimization and control techniques are no longer suitable to account for the interactions introduced by new technologies like combined heat and power units (CHP) and renewable sources. In this paper a general optimization framework and a hierarchical control architecture are presented for systems with multiple energy carriers, i.e. electricity, heat, gas, etc. The proposed framework is based on the energy hub approach and is an extension of previous research done in this field. Two-level architectures can be adjusted to fulfill the needs of future power systems, where there will be a higher participation of decentralized generation. Nevertheless, the papers generally focus exclusively on electricity flows, even when they include cogeneration units like fuel cells. For this reason the contribution of this paper is to propose a two-level control strategy that can be applied in systems with multiple energy carriers and to provide an illustrative example in which the results of using the strategy can be observed. The complete framework presented in this paper consists of an optimization algorithm and a real-time control algorithm. The optimization algorithm indicates when to turn on and turn off a generation unit and how much power it should deliver at a certain time period. The optimization is done for a forecasted period of 24 h and the real-time control strategy runs continuously to compensate for the mismatches between the scheduled load and the real load by means of control actions.

Since shipboard power systems are inherently islanded and consist of energy intensive sources and loads, it is an interesting application for dc microgrids. The object of this paper is to explore the merits of dc based shipboard power distribution as compared to conventional ac solutions. A survey of current initiatives, emerging concepts and design challenges is carried out. Based on the review, insight is offered on different aspects like efficiency, safety, and reliability and their relevant tradeoffs. It will be discussed how the interplay of reliability and availability, driven by the computational cost of optimization and restoration goals, forms the heart of reconfigurability.

Due to low inertia and constant power loads, stability is a major issue for dc distribution grids. Previous research focuses mostly on the (mathematical) analysis of the stability, but does not provide any guidelines to ensure stability. This paper presents four guidelines that ensure the existence of an appropriate equilibrium and stability around that equilibrium. From a linearized model of the dc distribution grid the location of the equilibrium and the stability of the system are derived. Subsequently, guidelines for the existence and propriety of the equilibrium, and the static and dynamic stability around that equilibrium are developed. Furthermore, the accuracy of these guidelines is determined. Additionally, possible applications of the guidelines are discussed.

This paper presents a transient analysis of various configurations of DC distribution systems. A combination of theoretical and practical methods were used to investigate the behavior of the system's variables after the occurrence of faults and the protection operation. A number of networks of varying complexity were built with detailed cable models using the EMTP/ATP software package in order to simulate a multitude of faults and assess each network's response at various positions and most crucially, the effectiveness of the protection scheme in reducing the impact of the transients after the fault. The travel speed of the transients was also investigated. At the same time, a set of measurements was conducted on a DC street lighting network to study the transient behavior of a real system. Another model was built to resemble the measured system, in order to compare between the simulation and measurement results. Finally, the evaluation of the measurement and simulation results leads to conclusions that will contribute to the creation of better, safer and more versatile DC distribution grids.

Connecting large dc distribution grids on the same voltage level can allow superior utilization of infrastructure. Selective ground fault protection is necessary and can be done using low impedance grounding. Distributed energy resources would allow for higher resilience of the grid if islanding operation of dc microgrids is allowed. For safety, grounding is essential in these islanded microgrids so individual grounding points are needed. Multiple low impedance grounding points would cause dc ground currents that lead to corrosion. This paper introduces capacitive grounding which is high impedance in steady-state effectively eliminating ground currents but is low impedance for fault transients and thus can allow for selective ground fault protection. The sensitivity of the capacitor sizing for two grounding points is shown. Further an initial filter for ground fault discrimination is analyzed.

This paper provides an empirical study on series arc behavior in low voltage dc microgrids. The response of an R-L-C dc microgrid abstraction towards series arcs is studied experimentally for varying grid inductance, dc voltages, load capacitances and load currents. In order to account for the stochastic nature of arcs, experiments are repeated multiple times under similar conditions to gain statistical significance. Thereby, insight on percentage occurrence and burn time of initiated series arcs is provided. Load side voltage response is studied to gain insight on the expected peak drop and fall time. This empirical evidence was judged to be a necessary requirement in developing a novel series arc extinguishing method from load side power electronic devices.

Inside the meshed LVDC distribution grids the power flow is predominantly limited by the line impedances. In order to achieve economical and flexible operation of the meshed LVDC distribution grids, it is required to control the power flow. For that end, the line impedances need to be adjustable. The line impedance can be controlled by a DC-DC transformer, however, it needs to be rated for the full grid power. In order to reduce the installation costs, a partially rated device is desirable. Therefore, a partially rated power flow control converter (PFCC) is proposed. This paper presents the PFCC performance estimate and demonstrates the PFCC functionality. The PFCC is an economical solution for increasing the controllability of the power flow in the meshed LVDC distribution grids. However, due to high step down ratio inside the PFCC achieving efficient performance is challenging. Furthermore, due to unavoidable MOSFET paralleling the part count rises. The proposed PFCC consists of two cascaded converters, therefore the control range and stability depends, besides else, on the impedance interaction between the two stages.

This paper considers a three-phase grid-connected photovoltaic (GCPV) system, which comprises a 120kW PV farm connected to the grid through a step-up transformer. GCPV is considered as a distributed generation (DG) serving a variable local load (LL) connected to the low voltage side of the transformer. A power conditioning unit (PCU) consisting of a two-stage energy conversion device is used to (i) maximize the extracted PV farm output power, and (ii) control power exchange with the grid. The PCU regulates the LL voltage level by means of reactive power compensation (RPC). The proposed system is equipped with an intelligent energy harvesting method based on particle swarm optimization-subgradient algorithm (PSO-SG). It is shown that the system operates efficiently using the proposed PCU. Validation of the model has been carried out by using MATLAB/Simulink.

It is anticipated that with the thrust towards use of clean energy resources such as electric vehicles, future distribution grids will face a steep increase in power demand, forcing the utility operators to invest in enhancing the power delivering capacity of the grid infrastructure. It is identified that the critical 5–20 km medium voltage (MV) underground ac distribution cable link, responsible for bulk power delivery to the inner urban city substation, can benefit the most with capacity and efficiency enhancement, if the existing infrastructure is reused and operated under dc. Quantification of the same is offered in this paper by incorporating all influencing factors like voltage regulation, dc voltage rating enhancement, capacitive leakage currents, skin and magnetic proximity effect, thermal proximity effect and load power factor. Results are presented for three different ac and dc system topologies for varying cable lengths and conductor cross-sections. The computed system efficiency is enhanced with use of modular multilevel converters that have lower losses due to lower switching frequency. A justified expectation of 50–60% capacity gains is proved along with a generalized insight on its variations that can be extrapolated for different network parameters and configurations. Conditions for achieving payback time of 5 years or lower due to energy savings are identified, while the socio-economic benefits of avoiding digging and installing new cable infrastructure are highlighted. The technical implications of refurbishing cables designed for ac to operate under dc conditions is discussed in terms of imposed electric fields, thermal profile and lifetime. A novel opportunity of temperature dependent dynamic dc voltage rating to achieve additional capacity and efficiency gains is presented.

Localization of series arc faults in dc microgrids is an important requirement to guarantee operational safety and uninterrupted power to end users. In the previous EPE-ECCE proceeding, a theoretical proof was provided for the selective series arc detection using a novel algorithm. In the current paper, the concept is validated by showcasing real time localized detection when series arc is initiated between point of common coupling and one of the parallel loads. Experiments are repeated several times to gain statistical significance to account for the stochastic nature of the arcing phenomenon.

This article describes the circulating current phenomenon that takes place in bipolar, meshed DC distribution grids. A difference in resistance between the conductor changes the distribution of the current on the conductors. This provokes a net current to flow, which is in turn measured by the residual current protection scheme, used to detect and isolate ground faults in the system, even though there is no fault in the system. Due to the circulating tendency of this net current, the operation of the protection scheme can be prevented in such a case by the addition of a clever communication system.

This study evaluates the dynamics of DC microgrids (DCMG) with the integration of voltage dependent demand response (VDDR). Previous work evaluates price prediction models for grid operators to estimate local price as a demand side management (DSM) technique. VDDR has been proposed to implement DSM without the need of a dedicated communication link. The dynamic response of a DCMG with VDDR has been evaluated in a situation with limited source power output. This has been done with two different VDDR controller logics: dynamic and fixed hysteresis control. Furthermore, the effect of current rate of change control is presented. The results provide further understanding on the dynamic behaviour of DCMGs, which can be used to create a guideline for the design of source and load control systems.

Refurbishing existing ac distribution cables to operate under dc conditions can offer several advantages in terms of capacity enhancement, efficiency and flexibility in power and voltage control, among others. In this paper, technical aspects such as insulation ageing, capacity and efficiency enhancement are explored. A novel idea of dynamic cable voltage rating based on the temperature dependent electric field is developed. The proposed algorithm can be incorporated in the dc link converters to obtain additional efficiency and capacity gains. Finally, challenges in the presented concept are highlighted.

This paper derives the mathematical expressions for computation of the load impedance magnitude, such that rated cable current is imposed at the sending end, with sending end voltage as the reference phasor. Thereby, the variation in maximum power transferred to the receiving end of a medium voltage cable link is described for varying link length, conductor cross-sectional area and load power factor. The percentage error in transmitted power computation due to simplification by neglecting the cable capacitance is quantified. The merit of the developed theory for future use is highlighted.

In the future, the number of plug-in electric vehicles (PEVs) that will participate in the primary frequency control (PFC) is likely to increase. In our previous research, the computational complexity of the PFC problem for a large number of PEVs was reduced using aggregate models of PEVs. However, in the literature on the PFC, the distribution network characteristics have not been included in the aggregate models of PEVs for the PFC, despite the fact that PEVs will be dispersedly connected to the distribution network. This paper proposes an aggregate model of PEVs for the PFC that further incorporates distribution network characteristics, i.e., the distribution network power loss (DNPL) and the maximum allowed current (MAC) of the lines and transformers. The DNPL variation is formulated according to the line and transformer impedance, spatial distribution of PEVs and loads, and active power variation of PEVs. Then, DNPL variation together with the MAC of the lines and transformers are incorporated in the proposed model of PEVs. Finally, the simulation results show an excellent agreement of 98% between the detailed model and the proposed aggregate model of PEVs.

Unplugging of energized loads in dc microgrids can cause safety concerns to users in residential areas. Sustained series arcs are more relevant in dc systems than in ac due to absence of current zero crossing. In this paper, the series arcs are detected from the load side, wherein, the proposed algorithm monitors the drop in load capacitor voltage associated with the intial electrode dependent series arc voltage. The original contribution of this work includes the experimental analysis of response of the arc detection algorithm with different discrete bandpass filter configurations and time constants. Experiments are conducted by generating series arcs for a stable 100V supply and noisy 600V supply with 300 Hz noise. The design requirements like robustness to low frequency grid voltage fluctuations, high frequency switching noise, detection time, and the threshold detection voltage are achieved and the most favourable design choices are highlighted.

The paper provides a comparison of four PV-battery architectures with dc and ac backbones, in terms of autarky, energy efficiency, battery size and reduction of annual electricity cost. The comparison is conducted based on the residential load and irradiation data from the Netherlands. The effect of different PV generation is also analyzed by comparing the results with irradiation data from Costa Rica. The results show that the ac coupled architecture gives the best performance.

We present a novel approach to physically integrate a PV module, dc/dc converter, dc/ac microinverter and a battery pack. The main advantages and challenges to realize this integration are presented. Since the thermal management is one main challenge to tackle, a 1D thermal model is developed, and the respective simulation results are obtained.

This paper focuses on the most common PV-storage architectures that are designed for residential applications and that incorporate storage devices like batteries, hydrogen systems, supercapacitors, and flywheels. The main motivations and a comparison of the advantages and disadvantages of the architectures are presented. Moreover, some common approaches to performing intelligent power management are introduced.

EV Charging is expected to be a significant load on the grid in the future. But they can also be a part of the solution to grid violations. As the amount of renewable energy generation increases, there is a large amount of power flowing in the reverse direction in the distribution network, which is undesirable. To reduce this, EVs can charge at strategically located “Grid Management Parking Lots”. Here the EV charging power level varies in accordance with the amount of capacity available in the grid. This is a win-win situation for both parties, the grid operator and the EV owner, as the grid operator reduces the amount of violations in the grid and the EV owner has his/her vehicle charged sooner.

Micro-grid for villages using renewable energy is becoming a trend for developing countries like in Africa. Commonly, ac microgrids are used for the simple reason of ac household equipments and protection abilities. DC microgrid is a better choice in terms of efficiency, control simplicity and converter prices. However, whether it is an AC or DC, the demand-supply management is the most basic problem in developing countries which causes more outage due to overloads and/or equipment failures. This paper proposes, a weakly coupled dc grid, a systematic approach to building electrical grid with the consideration of social behavior, future power system considerations and social participation in decision making.

DC renewable energy sources and dc loads are gaining attraction and they are preferably connected in dc microgrids. To increase the availability of dc microgrids, voltage dependent demand response should be used to switch off low priority loads in case of insufficient supply. This is possible even without communication with the loads. In this paper it is shown that using a voltage hysteresis with fixed thresholds to switch loads on and off, does not provide a general solution and oscillations can occur in specific cases. Dynamic hysteresis thresholds are introduced as a mean for devices to autonomously learn the present behavior of the grid to which they are connected.

The increasing amount of distributed energy resources requires significant changes to today's power systems. Most of the distributed resources are dc inherently or have a dc link. Therefore, connecting them with a dc distribution system seems beneficial. This paper presents the opportunities and challenges of dc distribution systems, starting with the requirements of future power systems. It will be looked at how dc can fulfill them with meshed grid architectures, increased system availability and new market models. New protection strategies for large dc distribution systems and their open research questions are discussed.

Meshed dc distribution grids have advantages over radial connected ones. Challenges arise when this is done on the same voltage level and the full potential power might not be usable in certain configurations. This paper shows how the power flow in meshed dc grids can be increased by limiting the currents at congested lines. This can be done in a decentralized way with only local current measurements. Several realization possibilities for current limiting devices are introduced and discussed. Dynamic simulations are performed.

This paper describes the behavior of voltage weak dc microgrids. These are dc microgrids with a relatively small system capacitance. The large amounts of stored energy in the passive component of a network has a considerable effect on the size of the fault currents, control and reliability. The use of a complex control approach to stabilize voltage weak microgrids is a possibility that requires further attention. In this paper, however, a simple way to stabilize such a system is proposed by limiting the rate of change of the power electronic interfaces. The small signal analysis of a three node system is analyzed to see the effect of system capacitance, inductance, resistance and PI and Droop values on the control of the system. The small signal analysis is implemented to estimate the rate of change of loads and the effect of step load changes on the system. In the modeling, a combination two types of loads, Constant power loads, and resistive loads, is used to see the effect of on the system stability. The source and converters are modeled as droop controlled current sources in parallel with capacitors.

Due to the absence of current zero crossing, dc arcs do not extinguish as easily as ac ones. It is essential to detect and eliminate series arc faults in dc microgrids in order to ensure safety, particularly while unplugging loads. Several detection and extinction methods using various means are known in literature. In this paper a novel arc detection method is proposed. It detects the load side input voltage drop due to the initial electrode specific minimum arc voltage. Only the local input voltage has to be measured and selectivity is given. The arc can be extinguished by shutting down the load's power electronics converter. The proposed method is elaborated through simulation of arc behaviour for constant resistor and constant power loads with input capacitors. First experimental results correspond with the theoretical analysis.

The traditional ac power system is challenged by the increased amount of distributed energy resources. Enormous changes and investments are necessary in order to achieve an ac smart grid capable of coping with the challenges introduced. In dc, solutions seem to be more straightforward since most of the distributed energy sources and most of the loads connected to the low voltage grid operate with dc. For this reason, dc distribution systems should be considered as an alternative with significant potential. Nevertheless, nowadays research is mainly focused on local dc nano- and microgrids. This paper introduces relevant aspects related to dc distribution networks. A wide field of opportunities and challenges are briefly exposed.

Plug-in electric vehicles (PEVs) have great potential in the near future to be connected in a large number to the power systems. This will lead to influence the overall dynamic behavior of power systems, specifically when PEVs participate in the primary frequency control (PFC). Modeling a large number of PEVs for PFC can be a complex and time consuming task. In order to reduce computational complexity, in [1], we proposed that a large number of PEVs was represented by an aggregate model which was connected to the transmission system. In [1], in the aggregate model of PEVs for the frequency stability analysis, the distribution network has been neglected, although in reality PEVs will be dispersedly connected to the distribution network. This paper proposes an aggregate model of PEVs that incorporates distribution network characteristics, i.e. the power losses. Thus, the proposed aggregate model represents the active power response of PEVs taking into consideration distribution network power losses (DNPL). To achieve this goal, the DNPL increment, which is caused by the participation of PEVs in the PFC, is mathematically formulated with respect to the active power variation of PEVs following a disturbance. Then, the proposed aggregate model of PEVs is obtained according to the PEV fleet behavior and also the calculated power loss variation. In order to compare the detailed model and the proposed aggregate model, the simulations are carried out in Matlab / Simulink. Finally, the simulation results show high level of correspondence between the detailed and the proposed aggregate model of PEVs following a disturbance in the power system.

This paper presents a control strategy for the operation of an autonomous distributed generation system (DGS) that includes multiple energy carries, storage devices and wind energy. The simulation is performed for the heat and electricity demand of 200 households. The proposed control strategy provides suitable performance results and improved energy efficiency for the system under study.

During the last decade there has been a strong development of energy applications that make use of multiple energy carriers. A new approach is necessary to tackle problems related to planning and operation of energy systems, since conventional techniques are mostly oriented to deal with independent energy carrier infrastructures only. The contribution of this paper is to offer a general unit commitment framework for energy systems that contain multiple energy carriers and to present illustrative examples on the topic. The unit commitment problem is solved using the optimization software AIMMS.