Grid Impact Research Articles

A synthetic, spatially decorrelating solar irradiance generator and application to a LV grid model with high PV penetration

Residential photovoltaic (PV) technology is expected to have mass global deployment. With widespread PV in the electricity distribution grids, the variable nature of the solar resource must be understood to facilitate reliable operation. This research demonstrates that synthetic, 1-min resolution irradiance time series that vary on a spatial dimension can be generated based on the following inputs: mean hourly meteorological observations of okta, wind speed, cloud height and atmospheric pressure.The synthetic time series temporally validate against observed 1-min irradiance data for four locations—Cambourne, UK; Lerwick, UK; San Diego, CA USA; and Oahu, HI USA—when analysing 4 metrics of variability indices, ramp-rate size, irradiance magnitude frequency and clear-sky index frequency. Each metric is calculated for the modelled and observed data at each location and CDF profile correlation compared as well as applying the Kolmogorov-Smirnov (K–S) test with 99% confidence limits. CDF correlation coefficients of each metric are all above R⩾0.908, and a minimum of 90.96% of daily irradiance time series passed the K–S test. A spatial validation was performed comparing the model outputs to real observation data. The spatial correlation coefficient regression with site separation was successfully recreated with MAPE=0.865%, RMSE=0.01 and R=0.955. The spatial instantaneous correlation was shown to behave anisotropically when using fixed cloud direction, with different correlation in along and cross wind directions. Cloud cover states of 40–60% showed the most spatial decorrelation while 0% and 100% had the least.The model outputs are applied to a distribution grid impact model using the IEEE-8500 node test feeder. PV scenarios of 25%,50%, and 75% uptake were modelled across a 1.5×1.5km grid. The magnitude and frequency of severe tap changing events are found to be significantly higher when using a single irradiance time series for all PV systems versus individually assigning spatially decorrelating time series.

Stochastic Residential Harmonic Source Modeling for Grid Impact Studies

With the introduction of more non-linear loads, e.g., compact fluorescent lamps, electric vehicles, photovoltaics, etc., the need to determine the harmonic impact of the residential load is rising, illustrated by the many studies performed on their harmonic impact. Traditionally, these studies are performed for a single new device and single penetration level, neglecting the harmonic interaction between new types of devices, as well as giving little information at which moment in time possible problems may arise. A composite approach to access the impact of harmonic sources on the distribution network is therefore proposed. This method combines a bottom-up stochastic modeling of the residential load with harmonic measurement data and harmonic load-flows all based on a scenario analysis. The method is validated with measurement data and shows a good prediction of the current level of harmonics in a residential neighborhood for the current situation. To demonstrate the applicability of the proposed method, case studies are performed on the IEEE European Low Voltage Test Feeder. These case studies show a marked difference between applying individual device-based models and a composite modeling approach, demonstrating why the proposed approach is an adequate method for the determination of the impact of new devices on the harmonics.

Heat pump and PV impact on residential low-voltage distribution grids as a function of building and district properties

Heating electrification powered by distributed renewable energy generation is considered among potential solutions towards mitigation of greenhouse gas emissions. Roadmaps propose a wide deployment of heat pumps and photovoltaics in the residential sector. Since current distribution grids are not designed to accommodate these loads, potential benefits of such policies might be compromised. However, in large-scale analyses, often grid constraints are neglected. On the other hand, grid impact of heat pumps and photovoltaics has been investigated without considering the influence of building characteristics. This paper aims to assess and quantify in a probabilistic way the impact of these technologies on the low-voltage distribution grid, as a function of building and district properties. The Monte Carlo approach is used to simulate an assortment of Belgian residential feeders, with varying size, cable type, heat pump and PV penetration rates, and buildings of different geometry and insulation quality. Modelica-based models simulate the dynamic behavior of both buildings and heating systems, as well as three-phase unbalanced loading of the network. Additionally, stochastic occupant behavior is taken into account. Analysis of neighborhood load profiles puts into perspective the importance of demand diversity in terms of building characteristics and load simultaneity, highlighting the crucial role of back-up electrical loads. It is shown that air-source heat pumps have a greater impact on the studied feeders than PV, in terms of loading and voltage magnitude. Furthermore, rural feeders are more prone to overloading and under-voltage problems than urban ones. For large rural feeders, cable overloading can be expected already from 30% heat pump penetration, depending on the cable, while voltage problems start usually at slightly higher percentages. Additionally, building characteristics show high correlations with the examined grid performance indicators, revealing promising potential for statistical modeling of the studied indicators. Further work will be directed to the assessment of meta-modeling techniques for this purpose. The presented models and methodology can easily incorporate other technologies or scenarios and could be used in support of policy making or network design.

Simulation of electric buses on a full transit network: Operational feasibility and grid impact analysis

This paper presents a twofold modelling exercise to investigate the implementation of battery electric buses (BEBs) in a full transit network. First, using three BEB concepts: flash, opportunity, and overnight, a real-time simulation model is developed for a full transit BEBs operation in order to (1) quantify the energy demands, (2) design the required infrastructure of the charging station, (3) test the transit operation feasibility, and (4) generate the charging load profile. Simulation results show that flash and opportunity BEBs are more feasible for full transit BEBs operation, however they suffer from high and intermittent power demands. Second, the generated charging load profile for each BEB operation is utilized to study its impact on the utilization and lifetime of the transformers, and the operation of the local distribution grid. Results indicate that the operation of overnight electric buses is more feasible as it relates to their impacts on the substation transformer and distribution feeders overloading, voltage regulation and quality aspects, and operation of voltage control devices. Collectively, findings from this study highlight that the selection of BEBs in a full network transit operation hinges on achieving feasible operation while reducing impacts on utility grid. Insights derived from this work can help optimize the implementation of BEBs in transit context.

Impact of dynamic and static fast inductive charging of electric vehicles on the distribution network

Fast inductive charging technologies allow the exchange of high power quantities (>20kW) between an Electric Vehicle and the electrical grid in contactless way. This demand can significantly modify the load profile of a distribution network and affect its operation and planning. Thus, it is necessary to quantify the grid impact of a network of fast inductive chargers and define the maximum allowable deployment level which does not violate the technical constraints of the network. This paper introduces a methodology for grid impact analysis of fast inductive charging technologies into distribution networks. The proposed methodology is implemented in a realistic model of a Greek MV distribution feeder providing indicative qualitative and quantitative results.

Energy End-use and Grid Interaction Analysis of Solar Assisted Ground Source Heat Pumps in Northern Canada

This paper presents an analysis conducted on a ground source heat pump (GSHP) system coupled with evacuated tube solar thermal collectors to reduce the borefield size in Northern Canada. The annual energy consumption, utility costs, grid impact, greenhouse gas emissions, and economics of a solar assisted GSHP system are compared to several conventional and heat pump (HP) space heating, cooling and domestic hot water systems for mid-rise apartment buildings located in Whitehorse, YK and Yellowknife, NT. In both regions the solar assisted GSHP system demonstrated energy savings over a conventional space heating system, while successfully overcoming an annual ground energy imbalance and reducing the required borefield size. From a grid perspective, although the HP systems shifted the space heating fuel source from fuel oil to electricity, the solar thermal system is able to help minimize the use of inefficient auxiliary heat to meet the space heating loads. The high associated capital costs of a GSHP system coupled with a solar thermal system make the economics difficult to justify; however for regions where energy security is of high importance, the energy savings achieved with these systems can have a substantial impact.

Cost-optimal energy system design in Zero Energy Buildings with resulting grid impact: A case study of a German multi-family house

Zero Energy Buildings (ZEBs) are considered as one of the key elements to meet the Energy Strategy of the European Union. This paper investigates cost-optimal solutions for the energy system design in a ZEB and the subsequent grid impact. We use a Mixed Integer Linear (MILP) optimisation model that simultaneously optimises the building’s energy system design and the hourly operation. As a ZEB have onsite energy generation to compensate for the energy consumption, it is both importing and exporting electricity. The hourly time resolution identifies the factors that influence this import/export situation, also known as the building’s grid impact. An extensive case study of a multi-family house in Germany is performed. The findings show that the energy system design and the grid impact greatly depend on the ZEB definition, the existing policy instruments and on the current energy market conditions. The results indicate that due to the feed-in-tariff for PV, the cost-optimal energy design is fossil fuelled CHP combined with a large PV capacity, which causes large grid impacts. Further, we find that heat pumps are not a cost-optimal choice, even with lower electricity prices or with increased renewables in the electric power system.

Energy performance evaluation of a net plus-energy residential building with grid-connected photovoltaic system in Brazil

The proposition of Net Plus-energy Buildings (NPEB) leads to the need to carry out studies of load matching in contrast with the grid impacts of distributed generation (DG). This paper performs simulations with the EnergyPlus software tool concerning a NPEB operating in four Brazilian metropolitan areas. The analyses include photovoltaic (PV) performance parameters and Load Matching and Grid Interaction indicators (LMGI). New grid impact indicators are defined in order to study the impacts of DG in the power grid. In the second stage, the work investigates economic aspects under net metering supporting. Results show the annual amount of electrical demand covered by PV varies from 29 to 51% with more potential in situations with higher PV production and higher cooling load, and the annual PV electricity that supplies the loads varies from 24 to 36% according to the seasonal variations of PV-load correlation. The levels of exported electricity into the grid are high in Brazil with annual mean power peaks surrounding 0.7 but can surpass 0.8 in the sunniest periods. The economy demonstrates the building achieves grid parity from 6 to 18% discount rates and the payback time is given for different scenarios of investment costs, discount rates and electricity tariffs.

Probabilistic Agent-Based Model of Electric Vehicle Charging Demand to Analyse the Impact on Distribution Networks

Electric Vehicles (EVs) have seen significant growth in sales recently and it is not clear how power systems will support the charging of a great number of vehicles. This paper proposes a methodology which allows the aggregated EV charging demand to be determined. The methodology applied to obtain the model is based on an agent-based approach to calculate the EV charging demand in a certain area. This model simulates each EV driver to consider its EV model characteristics, mobility needs, and charging processes required to reach its destination. This methodology also permits to consider social and economic variables. Furthermore, the model is stochastic, in order to consider the random pattern of some variables. The model is applied to Barcelona’s (Spain) mobility pattern and uses the 37-node IEEE test feeder adapted to common distribution grid characteristics from Barcelona. The corresponding grid impact is analyzed in terms of voltage drop and four charging strategies are compared. The case study indicates that the variability in scenarios without control is relevant, but not in scenarios with control. Moreover, the voltages do not reach the minimum voltage allowed, but the MV/LV substations could exceed their capacities. Finally, it is determined that all EVs can charge during the valley without any negative effect on the distribution grid. In conclusion, it is determined that the methodology presented allows the EV charging demand to be calculated, considering different variables, to obtain better accuracy in the results.

MV and LV Residential Grid Impact of Combined Slow and Fast Charging of Electric Vehicles

This article investigates the combined low voltage (LV) and medium voltage (MV) residential grid impact for slow and fast electric vehicle (EV) charging, for an increasing local penetration rate and for different residential slow charging strategies. A realistic case study for a Flemish urban distribution grid is used, for which three residential slow charging strategies are modeled: uncoordinated charging, residential off-peak charging, and EV-based peak shaving. For each slow charging strategy, the EV hosting capacity is determined, with and without the possibility of fast charging, while keeping the grid within its operating limits. The results show that the distribution grid impact is much less sensitive to the presence of fast charging compared to the slow charging strategy. EV-based peak shaving results in the lowest grid impact, allowing for the highest EV hosting capacity. Residential off-peak charging has the highest grid impact, due the load synchronization effect that occurs, resulting in the lowest EV hosting capacity. Therefore, the EV users should be incentivized to charge their EVs in a more grid-friendly manner when the local EV penetration rate becomes significant, as this increases the EV hosting capacity much more than the presence of fast charging decreases it.

Efficient Energy Management in Smart Micro-Grids: ZERO Grid Impact Buildings

In a smart micro-grid (MG) each generator or load has to take part in the network management, joining in reactive power supply/voltage control, active power supply/frequency control, fault ride-through capability, and power quality control. This paper includes a new concept for building integration in MGs with zero grid-impact so improving the MG efficiency. These aims are shown to be achievable with an intelligent system, based on a dc/ac converter connected to the building point of coupling with the main grid. This system can provide active and reactive power services also including a dc link where storage, generation, and loads can be installed. The system employed for validation is a prototype available at ENEA Laboratories (Italian National Agency for New Technologies). A complete and versatile model in MATLAB/SIMULINK is also presented. The simulations results and the experimental test validation are included. The trial confirms the model goodness and the system usefulness in MG applications.

Grid Impact Indicators for Active Building Simulations

This paper presents grid impact indicators, developed to evaluate the performance of local control mechanisms, affecting the impact of a net-zero energy building on the electricity grid, without the need to simulate the grid itself. The capacity factor, loss-of-load probability, cover factor for supply and demand, load match index, power exchange variability, one-percent peak power, peaks above limit, dimensioning rate, and kVA credit are proposed and applied to example single-building simulations. These simulations include control mechanisms affecting the power exchange of the building with the grid by adjusting the set-point temperature of the thermal energy storage tank of the heat pump, shifting local consumption to better coincide with local photovoltaic generation, or by directly limiting the power supplied to the grid. The simulations focus on a single net-zero energy building and use one such building as a reference and others for comparison of trends in simulation results. All modeling is done in the equation-based object-oriented modeling language Modelica. Simulations are performed using the Dymola environment.

Method for modelling fluctuating energy generation plants for the assessment of their impact on medium and low voltage grids

This paper describes a method for taking the forecasting uncertainty into account when assessing the impact of volatile generation on power grids. To this end the generation of feed-in scenarios for generation plants that include the uncertainty of the weather forecast is described. With a three-step model that firstly forecasts local weather parameters and, secondly, generates scenarios for these parameters, thirdly, a reduced number of resulting feed-in profile scenarios for each plant is computed. With these scenarios the assessment of the grid-impact of the plants can be calculated taking into account the uncertainty of the prognosis. In a case study, feed-in profiles for the plants in an exemplary region are generated which can be used for an assessment of the grid impact in this region using probabilistic load flow calculation.

Initial stabilization of a statistical sample of forty-four monocrystalline photovoltaic modules

The paper discusses the methodology and presents the results of the stabilization process for a statistic of forty-four monocrystalline silicon photovoltaic (PV) modules. The modules have been exposed to real sunlight in three different sessions and have been measured prior exposure and after each exposure session using a pulsed sun simulator. The first session of exposure shows the highest power loss in the modules, with an average 1.8%. During this initial stabilization step, 17 modules had a loss in the maximum power above 2% (registered losses from 2.1% to a maximum of 2.8%). The overall power losses are in the range 1.7–3.4%, with an average loss of 2.4% for the considered module population. Visual inspection and the analysis of the measured I–V curves revealed some imperfections in the modules. However, those imperfections do not impact on the actual performance at the present time. After the stabilization, the modules have been installed in the Northeast Solar Energy Research Center (NSERC) array, located at Brookhaven National Laboratory. The array will be used to analyze module degradation in the Northeast environmental conditions and to study the grid impact of utility-level PV plants.

Electric Vehicle Charging in an Office Building Microgrid With Distributed Energy Resources

This paper discusses the charging of plug-in hybrid electric vehicles (PHEVs) in an existing office building microgrid equipped with a photovoltaic (PV) system and a combined heat and power (CHP) unit. Different charging strategies and charging power ratings for workplace charging are examined for their grid impact and their impact on the self-consumption of the locally generated electricity. The grid impact can be significantly reduced by using strategies that require limited future knowledge of the EV mobility behavior and limited communication infrastructure. These strategies allow a high number of EVs to be charged at an office building, even with a limited number of charging spots, due to the large standstill times.

Impact of a Medium-Size Wave Farm on Grids of Different Strength Levels

Power fluctuations generated by most oscillating wave energy converters may have a negative impact on the power quality of the local grid to which the wave farms will be connected. Hence, assessing their impact is an important step in the selection process of a suitable deployment location. However, site-specific grid impact assessment studies are relatively time-consuming and require a high level of detail on the local network. Both of these constraints mean that grid impact studies are usually not performed in the preliminary stages of the site selection process, despite the extremely negative consequences resulting from poor power quality. This paper details a comprehensive study based on a relatively typical wave farm design connected to networks of different strength levels. The study was performed using experimental electrical power time series of an oscillating water column (OWC) device generated under the framework of the European FP7 project “CORES”. Simulations were performed using DIgSILENT power system simulator “PowerFactory”.

Apartment Building Electricity System Impact of Operational Electric Vehicle Charging Strategies

This paper discusses the charging of multiple plug-in hybrid electric vehicles (PHEVs) in an apartment building, equipped with a photovoltaic (PV) system. Different charging strategies and charging power ratings are examined, which are assessed in terms of their grid impact, the self-consumption of local electricity generation, and the electric driving range. The impact of a residential building, which incorporates electric vehicle (EV) charging, on the distribution grid can be significantly reduced by using simple EV charging strategies. These strategies include complementing night-time with day-time charging, peak shaving at vehicle level, and charging the surplus of local generation. Effective results are obtained using only the knowledge of the present battery state of charge, next departure time, and the instantaneous local generation surplus. The simultaneity of the EV charging and the PV production increases. The increase in electric driving range is negligible for three-phase charging.

A Spatial–Temporal model for grid impact analysis of plug-in electric vehicles

A Spatial–Temporal model (STM) was developed to evaluate the impact of large scale deployment of plug-in electric vehicles (EVs) on urban distribution networks. The STM runs based on the integration of power system analysis and transportation analysis. Origin–Destination (OD) analysis from intelligent transportation research was used to model the EV mobility. Based on the EV technical and market information provided by the EU MERGE project and the output of OD analysis, a Monte Carlo simulation method was developed within the STM to obtain the EV charging load of each load busbar over time. The STM aims to facilitate power system evaluation and planning, and is able to provide both average values and probabilities of nodal bus voltages and branch loadings. The STM is able to identify the critical network components that will require to be upgraded. A high customer density urban network from the United Kingdom Generic Distribution System combined with geographic information was used as a test system. Two EV charging strategies, “dumb” charging and “smart” charging, were simulated and compared under different EV penetration levels (0%, 25% and 50%) to verify the effectiveness of STM.

Research on Control Method of DC Voltage Based on Power Feedforward

DC-bus voltage is affected by fluctuations of load and grid impact, threating the safe operation of the system. According to the instantaneous active power balance, a power feedforward control strategy was proposed to reduce the DC voltage fluctuations. In this strategy, the load active power was directly fed-forward to the grid-side given instantaneous active power node through a constructed load state feedforward channel, which avoided the slower process of indirect power adjustment through outer voltage loop. So, the system response speed was greatly accelerated, the DC-bus voltage fluctuation was inhibited effectively, and the dynamic performance of the system had improved. Test results demonstrate the effectiveness of the proposed strategy.

Grid Impact of Voltage Control and Reactive Power Support by Wind Turbines Equipped With Direct-Drive Synchronous Machines

This paper proposes voltage control and reactive power support with direct-drive synchronous machines in wind turbines during normal operation and transient events. The wind turbines are equipped with voltage and reactive power control for normal operation and undervoltage ride-through modes for voltage dips. The wind turbine contributions are evaluated both for theoretical test cases as well as for two specific locations in the Belgian grid. Simulations reveal that the preferred mode for voltage support during a voltage dip depends on the grid characteristics short-circuit power and <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">X</i> - <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">R</i> ratio. Additionally, the angle stability of both induction motor load and nearby synchronous generators can be improved by adding reactive power support by the wind turbines. From the Belgian case study, it is concluded that voltage control by wind power plants is preferred, especially in remote areas where the additional control becomes necessary to maintain operation in between voltage limits. In case of voltage dips, a coordinated control of wind power plants in the area lowers the voltage reduction.