Grid Restrictions Research Articles

Low voltage grid resilience: Evaluating electric vehicle charging strategies in the context of the grid development plan Germany

The ongoing transition to decentralized renewable energy sources and sector-coupled consumers is reshaping the energy system. Changes at lower grid levels can stress lines and transformers. Crucial for a successful local energy transition are grid relief measures. Battery electric vehicles contribute to higher loads on grid equipment but also offer flexibility. This paper assesses the influence of four different charging strategies for battery electric vehicles across five representative low-voltage grids based on the grid development plan in Germany for the years 2021, 2037, and 2045. Results indicate that grid stress, specifically capacity stress, will emerge by 2037 and 2045. Decentralized photovoltaic systems are the primary contributors to this stress due to high simultaneous generation. Up to nearly 20% of photovoltaic power may need to be curtailed in 2045, especially in rural grids during the summer, to prevent overloads.Charging strategies linked to wholesale power market prices can inadvertently lead to higher consumption-induced grid overloads, necessitating the consideration of local grid restrictions. Implementing grid-friendly charging strategies, such as reduced charging power or alignment with local photovoltaic production, can mitigate those grid overloads from almost 8% down to 0.11%. However, these charging strategies have limited impact on photovoltaic-induced overloads due to the low number of connected battery electric vehicles during the day.In summary, appropriate charging strategies can ease low-voltage grid stress and are suitable measures to manage the challenges of decentralized energy transition and battery-electric vehicle adoption.

Reduced desalination carbon footprint on islands with weak electricity grids. The case of Gran Canaria

The aim of this paper is to present options to make low-carbon footprint large-scale desalination a reality on arid islands with weak electrical grids. Through these options, the goal is to reconfigure on-grid wind energy/desalination systems for large- and medium-scale water production. In this context, it is proposed to use lithium-ion batteries for stationary energy storage together with management strategies aimed at avoiding the wind energy/desalination systems having to consume energy from the conventional grid they are connected to. The control strategy is based on ensuring that the power provided by the wind farm and batteries remains in synchrony with the power demand of the desalination plant throughout the system's useful life. The interannual variation of wind energy is considered when sizing the renewable energy system and processes for its estimation are proposed. The case study is centred on the Canary Archipelago, a region that is especially vulnerable to the impacts of climate change, but which enjoys exceptional characteristics for the exploitation of wind energy. The results obtained show the optimal wind farm and energy storage system capacities of the analysed configurations. The approach presented allows a low-carbon operational footprint. If the control strategy were to be put into practice today, the current grid restrictions and a life cycle assessment of the system carried out in a societal context that continues to be fossil fuel dependent indicate a potential reduction of 77.4% of the footprint. However, the remaining 22.6% could be eliminated in the future when the manufacturing processes of wind turbines, batteries and desalination plants receive the benefits of carbon-neutral societies.

Numerical Simulation of Wave Overtopping of an Ecologically Honeycomb-Type Revetment with Rigid Vegetation

Traditional concrete revetments can destroy the ecological environment and the water landscape. An increasing number of ecological revetment structures have been applied in coastal, lake, and river regulation projects. It has been found that honeycomb-type revetments display a better performance in the attenuation of wave overtopping when compared to experimental data collected using the Eurotop and Muttray’s formula; recording a 40% decrease in the wave run-up in comparison to the latter. To further investigate the wave run-up and overtopping of the ecologically vegetated honeycomb-type revetment, based on OpenFOAM, an open source computational fluid dynamics software, a three-dimensional numerical wave tank was established. The Discrete Particle Method (DPM) was used to simulate gravel movement, and the flexible plant move boundary model was developed to simulate vegetation. The results of wave run-up calculated by the numerical model and those obtained by the experiments were in good agreement, with errors less than 20%. The modeled results of wave overtopping were within the same order of magnitude as those from the experiments; however, critical limitations were noticed due to effects of plant generalization and grid restrictions imposed by DPM methods. The results showed that wave overtopping increased with increasing wave period and wave height. However, with an increase in the wave overtopping, the influence of the wave period on wave overtopping decreased. The increase in vegetation density effectively reduced wave overtopping. Furthermore, an empirical formula for wave overtopping, considering the effects of vegetation density, was proposed.

Dynamic simulation and techno-economic assessment of hydrogen utilization in dual fuel (Hydrogen/biogas) micro gas turbine systems for a wastewater treatment plant

Due to the commitments of many governments, electrical power production from renewables hopefully continues to become widespread at a rapid rate to revolutionize energy infrastructures. Nevertheless, this green power seems to be wasted due to seasonal power swings, grid restrictions, and curtailment. The present research investigates the practicability of hydrogen injection into biogas from anaerobic digestion to generate electricity for a deployed wastewater treatment plant. A Polymer electrolyte membrane electrolyzer powered by Photovoltaic panels would produce hydrogen. An array of commercialized micro gas turbines has been suggested to create electrical power and thermal demand. Transient modeling was carried out using TRNSYS software to analyze the plant's performance from the thermodynamics and techno-economic perspectives. It is found that when biogas mass flow rate fluctuates, hydrogen could be effectively injected into biogas to fuel micro gas turbines. The results revealed that injecting hydrogen into biogas results in up to a 30% reduction in CO2 emissions, with two micro gas turbines producing more than 265 kW of electrical power at full load. LCOH and payback time would be 8.3 $/kg H2 and 6.5 years, respectively, for a hypothetical scenario in which electrolyzer size is anticipated to supply the maximum necessary hydrogen. Also the results indicate that electrical power and thermal energy of 70kWe-400kWe and 120kWth-500kWth could be reached annually, and the highest efficiency for the power generation, CHP, and the overall system becomes 37.5%, 83%, and 78%, respectively.

A novel output power determination and power distribution of hybrid energy storage system for wind turbine power smoothing

This paper deals with the power smoothing of the wind power plants connected to a microgrid using a hybrid energy storage system (HESS). In a HESS, the power should be distributed between the battery and capacitor such that the capacitor supplies the peaks of power and its high-frequency fluctuations, and the battery compensates for the rest. Besides, due to the relatively low lifetime of the batteries compared to the capacitors, it is preferred to transfer power fluctuations to the capacitor as much as possible. In this paper, methods for calculating the output, battery, and capacitor powers are presented. The output power is determined based on the grid restrictions and the battery SOC. The battery and capacitor powers are decided via an adaptive low-pass filter. The cut-off frequency of the low-pass filter is specified by a fuzzy controller such that not only the power spikes and high-frequency fluctuations are transferred to the capacitor but also the battery SOC variations are reduced. The simulation results show that for the presented wind speed profile, the output power determination reduces 13.7% of the HESS exchanged energy compared to the ramp limiting method. Besides, the proposed power distribution method reduces 92.6% of the unbeneficial charges and discharges of HESS and 8% of the battery exchanged energy compared to the condition that the constant cut-off frequency filter is utilised. The experimental results confirm the effectiveness of the proposed output power determination and HESS power distribution methods. Analysing the methods’ cost proves that although the utilisation of the capacitor bank increases the system initial investment cost, it will return after a while relying on the units’ capacity, power fluctuations, control method etc.

A charging station planning model considering electric bus aggregators

Many governments are pushing for cleaner transportation. In particular, public transportation allows massive transportation of passengers, but it remains highly polluting, especially in high elevation cities. Thus, the progressive introduction of electric buses (EBS) will allow mitigating these environmental concerns. However, some technological problems must be addressed considering the massive penetration of EBs. The lack of flexibility and the time connection of scheduling for public transit make EBS harder than internal combustion ones. This work studies the impact of charging EBs at the bus station and suggests a new way to take EB aggregators into account to reduce energy costs while fulfilling grid restrictions. In addition, to find a different number of charging spots to be installed, a scheduling analysis is conducted.

МОДЕЛЮВАННЯ ПРОЦЕСУ КЕРОВАНОЇ ЗАРЯДКИ ЕЛЕКТРОМОБІЛІВ В УМОВАХ ОБМЕЖЕНОГО ЕЛЕКТРОПОСТАЧАННЯ

Purpose. The purpose of this research is to increase the efficiency of the charging station, which is to meet the demand of energy for charging the batteries of electric vehicles without exceeding the limits of the grid restrictions. Methodology. The model of a controlled charging system was considered to study the efficiency of charging stations with the quantitative connection of electric vehicles to the charger under conditions of power limitation. Results. The load graph during the controlled charging method does not exceed the set limit voltage and power consumption settings, but with the uncontrolled method there is an overload of 16% during peak hours. The main disadvantage of the conventional method of charging with restrictions is that EVs that have been connected to the charging slots later than other EVs, were not charged in conditions of limited power supply. The charging stations require a charging system that does not have such a disadvantage – it is necessary that almost all connected electric vehicles fill their required level of charge and meet the limitations of the network at the same time. Investigating such a system is the task of future research. Originality. Every year the number of electric vehicles in Ukraine is growing rapidly. This also determines the development of a network of charging stations, both commercial and for domestic use. According to the Rules for the use of electricity, the contract for electricity consumption specifies the allowable capacity for the household or enterprise. Violation of this limit may cause the facility to become disconnected from the power supply with additional penalties. Practical value. The solutions to the problem of charging a large number of EVs are mainly based on different types of planning and creating a profile of their charging scenarios with technical limitations. These restrictions depend on the situation of the charging station (the number of connected EVs, their charging levels, the possibility of simultaneous charging for a few EVs, and the declared charging time). The charging period convenient for the EV owner and the network load schedule may conflict. The development of a realtime charging control system should provide each EV guaranteed charging even in conditions of limited power consumption. References 13, tables 4, figures 18.

Modelling Grid Constraints in a Multi-Energy Municipal Energy System Using Cumulative Exergy Consumption Minimisation

Efficiency measures and the integration of renewable energy sources are key to achieving a sustainable society. The cumulative exergy consumption describes the resource consumption of a product from the raw material to the final utilisation. It includes the exergy expenses for energy infrastructure as well as the imported energy. Since consumers and renewable potentials are usually in different locations, grid restrictions and energy flows have a significant impact on the optimal energy system design. In this paper we will use cumulative exergy minimisation together with load flow calculations to determine the optimal system design of a multi-cell municipal energy system. Two different load flow representations are compared. The network flow model uses transmission efficiencies for heat, gas and electricity flows. The power flow representation uses a linear DC approximated load flow for electricity flows and a MILP (mixed integer linear programming) representation for heat and gas flows to account for the nonlinear pressure loss relation. Although both representations provide comparable overall results, the installed capacities in the individual cells differ significantly. The differences are greatest in well meshed cells, while they are small in stub lines.

Challenges to use flexibility in low-voltage networks – results of a pilot project

The SERVING project aims to make flexible loads like night storage heaters and water pumps available to the market in such a way that it is ensured that the operation of these flexibilities will not cause contingencies in the network. Therefore an independent platform was established that provides a communication infrastructure for energy suppliers and flexibilities, estimates the low-voltage network state and schedules the flexible loads while considering grid restrictions. The system was tested in two pilot regions with night storage heaters and one region with water pumps. This study describes the SERVING system and its functionality, some of the challenges faced during the project phase and the ways they were solved. It also presents the results. It could be shown that reductions in energy costs are possible by shifting the loads in time without violating any grid restrictions but maintaining the comfort level for the customers.

An Efficient Reactive Power Dispatch Method for Hybrid Photovoltaic and Superconducting Magnetic Energy Storage Inverters in Utility Grids

The intermittent property and increased grid restrictions have become the most critical elements for increasing penetration levels of clean renewable energy sources (RESs). Smart inverters with combined RESs integration and reactive power support for utility grids have recently found widespread applications due to their techno-economic benefits. In smart inverters, the distribution-static compensator (DSTATCOM) functionality is inherently integrated to the RESs inverters. However, optimized reactive power-sharing between these inverters has become a big issue for the control systems of utility grids. There are numerous existing attempts presented in the literature for addressing these challenges, although they disadvantage low efficiency, uneven sharing, complex implementations, and/or costly added devices. Therefore, this paper proposes an efficient reactive power dispatch method between hybrid renewable energy generation and energy storage systems. The proposed method enhances the energy efficiency of the utility grid by adopting the reactive power share between interfacing inverters according to the estimated power losses. Besides, the proposed method enhances the reliability of smart inverters by relieving their thermal stresses through adopting their reactive power share according to the estimated power losses. The hybrid photovoltaic (PV) generation with superconducting magnetic energy storage (SMES) systems is selected as a case study for validating the new proposed reactive power dispatch method. The results, comprehensive discussions, and performance comparisons have verified the superior performance of the new proposed reactive power dispatch method.

The impact of different strategies for generation shift keys (GSKs) on the flow-based market coupling domain: A model-based analysis of Central Western Europe

The trading of electricity across zones relies on cross-border capacities, provided by transmission system operators. The target design of the European Union for capacity calculations is flow-based market coupling, a method that provides trading domains while taking into account grid restrictions. Flow-based market coupling heavily relies on Generation Shift Keys, an essential predictive parameter, translating zonal balance changes that originate from market coupling into nodal injections and consequent line flow changes. This analysis quantifies the effect of different Generation Shift Key strategies on the market coupling domains and individual network elements. A strategy entails suppositions regarding which generating units partake in market changes and to what extent. For this, a novel method for base case computations is proposed that relies on matching historical reference flows of network elements. The results show that different strategies substantially alter the shape and size of flow-based market coupling domains and have a statistically significant impact on individual network elements. For many lines, the average line flow sensitivity to market changes differs between 1% and 5% across strategies and up to 10% for a few lines. Furthermore, the analysis details how the n-1 security criterion influences the composition of domain constraints and to what extent network elements are affected by it. Particularly with regard to the planned geographical expansion of flow-based market coupling and changing regulatory demands for transmission system operators, this work attests to the importance of developing accurate and transparent flow-based market coupling parameters and model-based representations.

Towards metamodeling the neighborhood-level grid impact of low-carbon technologies

Heating electrification and distributed renewable generation in the residential sector are among prominent solutions advocated for energy saving and carbon emission reduction. However, research shows these low-carbon technologies may create issues at the low-voltage (LV) distribution grid. High-level policy assessment currently lacks the support to take into account such local grid restrictions. To achieve this, we propose the use of a probabilistic simulation framework in combination with metamodeling, that allows to assess the potential LV grid impact for a wide range of cases. The probabilistic framework is first presented, which is developed for Belgian residential neighborhoods with air-source heat pumps and rooftop PV, based on previous work. Given the complexity and computational requirements of this approach, the paper furthermore proposes metamodeling as a technique to obtain inexpensive evaluation of low-voltage grid impact indicators, suitable for high-level assessments. Although metamodeling is extensively used in various engineering domains, no application in district-level grid-related indicators is available. Consequently, this paper’s focus lies on discussing the various steps and options of the metamodeling procedure, while emphasizing problem-specific challenges. Lastly, the proposed metamodeling methodology is used for training simple metamodels for voltage indicators in neighborhood-level LV grids. Linear regression performed fairly well in predicting the minimum voltage levels, though less accurately close to the lower voltage limit, while logistic regression effectively detected feeders with violations.

Charge scheduling for electric freight vehicles

We consider a fleet of electric freight vehicles (EFVs) that must deliver goods to a set of customers over the course of multiple days. In an urban environment, EFVs are typically charged at a central depot and rarely use public charging stations during delivery routes. Therefore, the charging schedule at the depot must be planned ahead of time so as to allow the vehicles to complete their routes at minimal cost. Vehicle fleet operators are subject to commercial electricity rate plans, which should be accounted for in order to provide an accurate estimation of the energy-related costs and restrictions. In addition, high vehicle utilization rates can accelerate battery aging, thereby requiring degradation mitigation considerations. We develop and solve a comprehensive mathematical model that incorporates a large variety of features associated with the use of EFVs. These include a realistic charging process, time-dependent energy costs, battery degradation, grid restrictions, and facility-related demand charges. Extensive numerical experiments are conducted in order to draw managerial insights regarding the impact of such features on the charging schedules of EFVs.

Playing Pollution Games with Thermal Electricity Generators

This paper discusses the economic and environmental implications of a stylised electricity market with transmission grid constraints and shared temporal pollution standards that restrict the joint strategy space of the agents. These are problematic to enforce if individual monitoring is impossible or very expensive. For such situations, we propose a time-dependent (or “open-loop dynamic”), game-theoretic model capable of analysing coupled constraints equilibria, also known as generalised Nash. We compute these equilibria for thermal generators subjected to annual pollution limits and instantaneous grid restrictions for a three-node dc model with a two-period load duration curve. The model illustrates the possibility that well-meaning environmental regulation might harm consumer surplus. It also highlights the cost to efficiency of regulatory attempts to ease the burden of compliance.

The eGo grid model: An open source approach towards a model of German high and extra-high voltage power grids

There are several power grid modelling approaches suitable for simulations in the field of power grid planning. The restrictive policies of grid operators, regulators and research institutes concerning their original data and models lead to an increased interest in open source approaches of grid models based on open data. By including all voltage levels between 60 kV (high voltage) and 380kV (extra high voltage), we dissolve the common distinction between transmission and distribution grid in energy system models and utilize a single, integrated model instead. An open data set for primarily Germany, which can be used for non-linear, linear and linear-optimal power flow methods, was developed. This data set consists of an electrically parameterised grid topology as well as allocated generation and demand characteristics for present and future scenarios at high spatial and temporal resolution. The usability of the grid model was demonstrated by the performance of exemplary power flow optimizations. Based on a marginal cost driven power plant dispatch, being subject to grid restrictions, congested power lines were identified. Continuous validation of the model is nescessary in order to reliably model storage and grid expansion in progressing research.

Enhancing operation of decentralized energy systems by a regional economic optimization model DISTRICT

In this paper, a regional energy system model is introduced. The model targets the cost minimal operation of a regional energy system with the possibility of trading electricity at a spot and reserve market outside of the system borders. The system boundaries are set to be around the regional energy system, with one connection point to the rest of the system and hence the possibility to trade across the system border. Within the system boundaries, the system is divided into model areas where demand and generation are located, using a transport model approach to account for grid restrictions between the model areas. In addition to generation technologies, the model allows for demand side management and storage usage. For the trade at central markets, spot and reserve markets are integrated, using a price-taker approach. The paper shows the mathematical formulation in detail. To test the formulation and methodology of the model presented here, a case study based on real data is conducted. The case study highlights the fact that the model accounts for operational interconnections of technologies at distribution grid level. It also enables looking at effects on the grid and indicates where a more detailed power flow analysis might be required. If the proclaimed flexible energy systems come to reality, the model enables stakeholders to improve their knowledge of their distributed system, the effects of small-scale technologies and the interplay with the central system. Hence, the application demonstrates that the model adds value to closing the knowledge gap between stakeholders’ concerns about efficient operation planning of their system at distribution grid level.

Forecast the grid oriented battery operation to enable a multi-use approach and discussion of the regulatory framework

Battery projects are getting increasingly attractive for various applications due to decreasing prices. One possible use case is also to exploit the flexibility for grid usage to avoid/defer conventional grid reinforcements and to effectively integrate decentralised renewables, active prosumers and new loads in the future into the grid. The profitability of storage assets can be improved substantially if a ‘multi-use-approach’ can be realised. These approaches consider grid usage in time periods with grid restrictions and system- or market usage could be applied in non-critical time periods. Based on real-world projects, approaches for such operations have been derived and implemented. Next to some elaborations on these results, the discussion on possible business models considering the regulatory framework is also a main aspect of this study.

Metamodeling energy indicators in neighborhoods with growing deployment of heat pumps and rooftop photovoltaics

This paper evaluates simple metamodels to predict local electricity demand and grid restrictions, in residential neighborhoods with heat pumps and photovoltaics. The procedure and challenges of developing such models are described. Modeling is based on results obtained from detailed simulation of buildings and the grid. Linear and logistic regression models are developed for electricity demand and minimum voltage respectively, as a function of neighborhood characteristics, related to both building and electrical network properties. The paper shows that linear regression can be used for a first evaluation of electricity demand. For voltage violations, logistic regression gives acceptable results; however, more complex models are needed to approximate voltage levels.

Project ‘power-to-heat in smart grids’– a multi-objective approach for a maximised value of flexibilities in grids

The integration of power-to-heat (PtH) assets as grid-oriented flexibilities can help balancing and stabilising the increasingly strained distribution grid level. As a result of using demand-side response, conventional grid reinforcements could be avoided/deferred in certain cases. Furthermore, decentralised renewables could be integrated more effectively. A multi-objective approach aims to improve the economics of the PtH asset using the flexibility in an optimised way, whereas the grid-oriented usage is chosen in certain times of grid restrictions due to fluctuating wind feed-in, system-/market-oriented flexibility usage can be applied during the remaining time periods. Besides the technical solution of using different optimised flexibility dimensions, also the regulatory discussion for distribution grid operators is part of this study indicating a need for actions to enable a suitable usage of demand-side response potentials.

Electricity storage systems in the future German energy sector: An optimization of the German electricity generation system until 2040 considering grid restrictions

Due to the growing feed-in of electricity based on renewables, electricity storage systems will be essential in the future energy sector. Because of the volatile feed-in, electricity will have to be shifted temporally. Additionally, load centers and regions of potentially high wind-based electricity production are located far away from each other in Germany, resulting in the need to transport electricity from the north to the south. According to the targets defined by the German government, more than 60% of electricity generation in 2040 is to be based on renewables. A strategic allocation of storage systems might help to improve the utilization of grid capacities and integrate renewables at the same time. To analyze this, we implemented the possibility to commission storage systems throughout Germany in the energy system model PERSEUS-NET-ESS. This investment and dispatch model includes a DC approach of the German transmission grid and, thus, calculates not only the installed capacities, but also their optimal allocation. Besides storage systems, gas turbines or load shift potentials can be used for the integration of renewables. In this paper, we use PERSEUS-NET-ESS to evaluate the alternatives taking the grid restrictions into account. Results indicate that it is beneficial to commission about 3.2GW of battery storage systems until 2040, provided that storage investment will drop to about 150€/kWh until then. The main part of the capacity is to be deployed in northern Germany close to the sea, where electricity from off-shore wind parks will be fed into the grid. At the same time, the storage systems will be located mainly close to congested grid lines. For the case of battery storage systems being impossible in the model, gas turbines are commissioned instead. Modeling will also consider the load shift potential due to electric mobility. It can substitute almost all of the commissioned storage systems and at the same time reduce the total generation capacity needed.