Low Voltage Distribution Grids Research Articles

Electromagnetic interferences in smart grid applications: a case study with power line communication smart meters and PV energy generation

Measurements on a low voltage (LV) distribution grid section equipped with smart meters have shown that photovoltaic energy production has an impact on power line communication systems. An interferences analysis method based on simulations with frequency scans of equivalent circuit models of the LV grid section, including active infeed converters, mains communicating systems, and loads, is presented. The proposed method is described and evaluated with simulations and measurements realised on a simple laboratory set-up.

Predictive management of low-voltage grids

This study presents the functional model that provides net-load forecasts for each low-voltage (LV) node (including PV generation and self-consumption), developed for the UPGRID (real proven solutions to enable active demand and distributed generation flexible integration, through a fully controllable low-voltage and medium-voltage distribution grid) framework project. Several tests scenarios were simulated and the results regarding forecast accuracy and computational performance are given. Results demonstrate the applicability of the distribution in memory solution in a practical operational scenario, offering a highly scalable forecasting system for LV networks. Based on forecasts and available real-time information, an architecture for preventive control of LV grids is built upon chronological analysis capabilities of DPlan. An illustration on how such capabilities are used in the context of the foreseen UPGRID preventive control framework is provided.

Connection of the CDER-Algiers photovoltaic system to low-voltage distribution grid

This work deals with the first photovoltaic (PV) system connected to the low voltage electrical grid in Algeria. The system is made of a PV generator and inverters, which convert the generated direct current into to alternating current, and injects it in the low voltage distribution network of Sonelgaz (Algerian electricity provider). The generated current is perfectly compatible with that supplied by the network. The power of the installed PV generator is 9.54 kWp. In order to produce this power, three sub-fields of I-106 modules are used. Each of these sub-fields is connected to an inverter. This work allowed the study of phenomena related to the PV system connection to the network, namely: power quality, reactive power, current THD and voltage, etc. Electrical parameters at the point of common coupling (PCC) were studied, including the impact of this PV installation on the grid.

Validation of a robust neural real-time voltage estimator for active distribution grids on field data

The installation of measurements in distribution grids enables the development of data driven methods for the power system. However, these methods have to be validated in order to understand the limitations and capabilities for their use. This paper presents a systematic validation of a neural network approach for voltage estimation in active distribution grids by means of measured data from two feeders of a real low voltage distribution grid. The approach enables a real-time voltage estimation at locations in the distribution grid, where otherwise only non-real-time measurements are available. The method shows robust behavior in all analyzed aspects, which is vital for real world applications. A methodology to select the most relevant input variables and find the best achievable performance for a particular number of inputs is presented. Moreover, the paper shows that the performance is not sensitive to the number of neurons in the hidden layer of the neural network as long as the model is not underdetermined. The paper examines the quantity of historical data needed to establish an adequately functioning model. To accommodate grid evolution and seasonal effects, the impact of different retraining intervals is investigated. Furthermore, the performance of the model during periods of high PV generation is evaluated. The validation shows that accurate voltage estimation models for distribution grids with high share of dispersed generation can be established with approximately one month of historical data. The model has to be retrained every 10–20 days to retain estimation mean squared errors below 0.35V2. It was also found that the performance does not decline during times of high PV generation.

Fuzzy-Based Reactive Power Control for Smart PV Inverters in LV Distribution Systems

Future society entails new paradigms in how the world will work; new ideas as Smart Grid, Smart Cities, Smart Homes, and their interactions have been established and studied. With the inclusion of Information and Communication Technology (ICT) components into the low-voltage (LV) distribution grid, some measurement signals from smart meters are available for distribution network control with high penetration of photovoltaic systems (PV). This paper takes a local voltage-control strategy for smart LV distribution networks, by using a fuzzy based methodology using local information collected from smart meters as status information source of the grid. Comparative studies are presented for a smart home LV distribution system to validate the method, showing the robustness of proposed method compared with the traditional local approaches. To illustrate the applicability of the proposed method, a real-time digital simulator for smart distribution power grids with a the developed voltage-control system are used. The results show the proposed method can simultaneously mitigate the PV reactive power injection, as well as minimize the voltage rise and power losses.

Voltage Control in Distributed Generation under Measurement Falsification Attacks

Low-voltage distribution grids experience a rising penetration of inverter-based, distributed generation. In order to not only contribute to but also solve voltage problems, these inverters are increasingly asked to participate in intelligent grid controls. Communicating inverters implement distributed voltage droop controls. The impact of cyber-attacks to the stability of such distributed grid controls is poorly researched and therefore addressed in this article. We characterize the potential impact of several attack scenarios by employing the positivity and diagonal dominance properties. In particular, we discuss measurement falsification scenarios where the attacker corrupts voltage measurement data received by the voltage droop controllers. Analytical, control-theoretic methods for assessing the impact on system stability and voltage magnitude are presented and validated via simulation.

Optimal multiple microgrids based forming of greenfield distribution network under uncertainty

Multiple microgrid (MMG) operation concept of community microgrid (MG) provides enhanced resiliency and reliability through self-healing, enabling a high penetration of distributed generations (DGs), power interchange between MGs and real-time communication. In this study, the methodology of optimal design of low-voltage greenfield distribution grids based on multiple MGs in the presence of uncertainties is presented. The electrical and geographical borders of MGs community are determined using some important criteria such as power balance and spatial distribution of load. Within MGs, both of dispatchable and non-dispatchable DGs are considered. The external grid is assumed as a backup for each MG in abnormal conditions. The proposed method determines the optimal borders of MGs, optimal size and site of DGs for each autonomous MG simultaneously. The imperialist competitive algorithm is used to optimise the total cost of optimal MG clustering problem. The method is implemented to a vacant area with residential and commercial customers. The MGs optimal service area, DGs location, size and type within each MG and LV feeder's route are indicated and compared both for deterministic and probabilistic planning cases.

Influence of increasing numbers of RE-inverters on the power quality in the distribution grids: A PQ case study of a representative wind turbine and photovoltaic system

This paper presents the selected power quality (PQ) indicia of a wind generator and a photovoltaic installation considered to be the representative of medium voltage and low voltage distribution grids. The analysis of measured values suggests that the decrease in PQ is a case of specific combination of distributed generation, grid parameters and load behaviour. Modern generators have a limited impact on PQ. On the other hand, fluctuations in power generation are regarded as an emerging PQ indicator. The growing number of distributed renewable installations causes stochastic, variable, and hardly predictable power flows in the distribution grid. The nature of fluctuations in wind and solar generation is different. In both cases, new indexes for the quantification of fluctuations are needed and are yet not standardised. Proper assessment of these fluctuations enables definition of useful fluctuation limits and rules for optimal storage system integration.

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.

Impacts of Low-Voltage Distribution Grid Codes on Ancillary Services and Anti-Islanding Detection of Inverter-Based Generation

The capacity to provide ancillary services has become more and more important for generators in the distribution grid. New grid codes have been implemented which regulate inverter behavior regarding different aspects of system safety and stability. This mainly includes the frequency-dependent power reduction or the allocation of reactive power. These standards affect mains monitoring because the power control directly changes penetration after the generator has islanded. In this paper, equations for active and reactive power balance are set up and developed further. Non-Detection Zones for different control methods are shown. These are then described and interpreted in detail to show how different effects that result from grid codes interfere with one another. Frequency-dependent power reduction was found to change the size and shape of the Non-Detection Zones. Reactive power control results in a shift of the Non-Detection Zones. In fact, even the initial operating point before loss of mains has an impact on inverter behavior after islanding. Islanding operating points are sometimes undefined which leads to oscillation. An interpretation of the results and advice for future revisions of grid codes conclude the paper.

Optimal Placement and Sizing of Active In-Line Voltage Regulators in Flemish LV Distribution Grids

Over the past few years, both the integration of small-scale embedded generators and the electric power consumption have been increased rapidly in the low-voltage (LV) distribution networks. These two trends are likely to result in a number of power quality issues such as voltage rise/drop and voltage unbalance. Regarding the mitigation of the voltage rise/drop effects, several utilities have already implemented the application of on-load tap changer (OLTC) in secondary distribution transformers. However, the voltage control via OLTC transformers might be infeasible in the case of radial feeders with both large number of branches and long distribution lines. This paper evaluates the performance of active in-line voltage regulators (IVRs) in LV grids. The study also investigates the optimal placement and sizing of active IVRs in Flemish LV networks. Concerning the OLTC control of the IVRs, an algorithm from a previous work is used. Finally, the influence on various power quality metrics is tested, when IVRs are incorporated into the system.

Integrating multi-domain distributed energy systems with electric vehicle PQ flexibility: Optimal design and operation scheduling for sustainable low-voltage distribution grids

Abstract Electricity and transport sectors have to be decarbonised in order to mitigate climate change effects leading to increased penetration of distributed energy systems (DES) and electric vehicles (EV) which can threaten the security of distribution grid operation. Proper design and operation of such systems are crucial if the adverse effects on the grid are to be avoided. Moreover, EVs represent a high load and should not be considered merely as passive assets since they can provide various flexibility services for maintaining the grid stable. This paper presents a multi-domain optimisation framework for minimising carbon emission in low-voltage distribution grids with high share of distributed energy resources and electric vehicles. The framework determines optimal EV flexibility usage (both active and reactive) while satisfying electric and thermal building demands, and maintaining the distribution grid in the stable operation. The model was applied to a real low-voltage Danish distribution grid where measurement data is available on hourly basis in order to determine EV flexibility impacts on carbon emissions, as well as the benefits of optimal DES design. The influence of EV reactive power control on the grid operation, in addition to coordinated charging, is analysed. Results showed that when the system can be optimally designed, emissions decrease by 64% and additionally 32% with proactive EV integration, whereas EV reactive power control enables integration of larger EV amounts and provides significant voltage support without affecting the user comfort.

Control approach of three‐phase grid connected PV inverters for voltage unbalance mitigation in low‐voltage distribution grids

Low-voltage distribution grids (LV) experience unbalanced voltage condition due to the high penetration levels of rooftop mounted single-phase photovoltaic (PV), the existence of single-phase transformers, unbalanced cable impedances and different load levels on each phase. The voltage unbalance (VU) has a harmful impact on the loads and equipment connected to grid that leads to overheating and fast thermal ageing. There are several solutions proposed to mitigate the VU, that requires the installation of extra hardware components or incorporate complex type of controllers that need special tuning procedures and special arrangements. Therefore, in this study a new and simple control approach of three-phase grid connected PV inverter is proposed to mitigate the unbalanced voltage. The new control approach is able to fully mitigate the unbalanced voltage without the need to extra hardware installation or negative sequence voltage magnitude and phase angle measurements, provided that the ampacity of the converter is high enough. The new system is tested in a LV grid with high PV penetration from Germany. The results show that the VU factor did not exceed 0.8% at the feeder connection point.

Quantifying residential PV feed-in power in low voltage grids based on satellite-derived irradiance data with application to power flow calculations

A scheme using satellite-derived irradiance measurements to model the feed-in power of residential photovoltaic (PV) systems in a low voltage distribution grid is described. It is validated against smart meter measurements from a test site with 12 residential PV systems in the city of Ulm, Germany, during May 2013 to December 2014. The PV feed-in power is simulated in a 15-min time resolution based on irradiance data derived from Meteosat Second Generation satellite images by the physically based retrieval scheme Heliosat-4. The PV simulation is based on the nominal power and location of the PV systems as provided by the distribution system operator. Orientation angles are taken from high resolution aerial laser-scan data. The overall average mean error of PV feed-in power is 4.6% and the average root-mean-squared error is 12.3% for the individual systems. Relative values are given with respect to the total installed power of 152.3kWp. Sensitivity studies discuss the need for knowing the exact orientation angles of each individual PV system or the usefulness of a single ground-based measurement as alternative to satellite observations. As an application of the scheme, the modelling of the effect of the power flow from the residential PV on the load flow of the low voltage distribution grid transformer is described and illustrates the advantage of the discussed approach for distribution system operators.

Phase-wise enhanced voltage support from electric vehicles in a Danish low-voltage distribution grid

High deployment of electric vehicles (EVs) imposes great challenges for the distribution grids, especially in unbalanced systems with notable voltage variations which detrimentally affect security of supply. On the other hand, with development of Vehicle-to-Grid technology, EVs may be able to provide numerous services for grid support, e.g., voltage control. Implemented electronic equipment will allow them to exchange reactive power for autonomous voltage support without communicating with the distribution system operator or influencing the available active power for primary transportation function. This paper proposes a voltage dependent EV reactive power control and quantifies its impact on a real Danish low-voltage grid. The observed network is a heavily unbalanced three-phase four-wire grid modeled in Matlab SimPowerSystems based on real hourly measurement data. Simulations are performed in order to evaluate phase-to-neutral voltage support benefits as well as to address neutral-to-ground values, active power losses and the unbalances at the same time. The analysis shows that reactive power support both raises minimum phase-to-neutral voltage magnitudes and improves voltage dispersion while the energy losses are not notably increased. Further on, since the control is voltage dependent, provided reactive power is unequal among the phases leading to greater support on heavily loaded phases and decreased unbalances caused by residential consumption. Hence, implementation of such a phase-wise enhanced voltage support could defer the need for grid reinforcement in case of large EV penetration rates, especially in highly unbalanced networks.

Analysis of $\Delta P -\Delta Q$ Area of Uncontrolled Islanding in Low-Voltage Grids With PV Generators

The large penetration of distributed energy resources (DERs), especially photovoltaic (PV), in low-voltage (LV) distribution grids has raised the concern of unintentional islanding operations. The probability of unintentional islanding is increased mainly due to the newly introduced standards with extended frequency and voltage thresholds and the additional grid support functions, such as active and reactive power regulation using P/f and Q/V droop characteristics. Moreover, islanding operation due to faulty events at the medium-voltage (MV) distribution level may be dangerous in the presence of automatic reclosing procedure leading to out-of-phase reconnections. In this paper, an analysis of this phenomena is provided using simplified generator and load models. Simulation results based on MATLAB and DIgSILENT tools on a simplified LV distribution network are reported together with experimental results on a lab-scale prototype.

Impact of residential electricity tariffs with variable energy prices on low voltage grids with photovoltaic generation

This paper presents an approach integrating simulation models for residential electricity demand with price elasticity and electricity generation from photovoltaic systems as well as for load flow analysis using Monte Carlo simulation in low voltage distribution grids. The price elasticity model, using metering data collected within a field test including approximately 1100 households, and the application of the approach for analysing the impact of alternative electricity tariffs on grid utilisation for different use cases constitute the main contribution. We compare a tariff using constant prices with tariffs using variable prices, one of which is directly derived from the central wholesale market, while another one is subject to sales-driven rules and regional renewable electricity availability. The results reveal that generation peaks lead to power flow reversions and an exceedance of the limit for voltage deviation in several hours for high photovoltaic installation rates and thus to more critical situations than demand peaks for the considered low voltage grid. While the considered tariffs can, generally, support grid stability, several drawbacks should be reduced in future tariff design: the considered sales-driven tariff may aggravate critical grid situations during demand peaks while a tariff, solely deriving its variable prices from a central wholesale market, may not be suitable to relieve critical grid situations during distributed generation peaks.

LV distribution network voltage control mechanism: Analysis of large-scale field-test

Abstract This work presents the results of an extensive large-scale field test of a local voltage control mechanism in Low Voltage (LV) distribution grids. The main goal of the voltage control system is to mitigate over- and undervoltages in the feeder, and for that the readily available flexibility of residential smart appliances is used. The advantage of the control system is that there is no need for a communication network between the different households within the LV network. The control system merely requires communication between the smart appliances within one household, and uses locally available measurements, such as the household supply voltage provided by e.g. a smart meter. The control system was rolled out in the LINEAR residential demand response pilot in 85 families, and was tested from December 2013 to September 2014.

Efficient Control of Energy Storage for Increasing the PV Hosting Capacity of LV Grids

Photovoltaic (PV) systems are among the renewable sources that electrical energy systems are adopting with increasing frequency. The majority of already-installed PV systems are decentralized units that are usually connected to low-voltage (LV) distribution grids. The PV hosting capacity of an LV grid is usually limited by overvoltage, and the efficient control of distributed electrical energy storage systems (EESSs) can considerably increase this capacity. In this paper, a new control approach based on the voltage sensitivity analysis is proposed to prevent overvoltage and increase the PV hosting capacity of LV grids by determining dynamic set points for EESS management. The method has the effectiveness of central control methods and can effectively decrease the energy storage required for overvoltage prevention, yet it eliminates the need for a broadband and fast communication. The net power injected into the grid and the amount of reactive power absorbed by PV inverters are estimated using the PV generation forecast and load consumption forecast, and the dynamic operating points for energy storage management are determined for a specific period of time by solving a linear optimization problem. Simulations performed on a realistic LV feeder of the Danish island Bornholm verify the performance of the proposed method.

Electric Power Quality Analysis 6-10/0.4 kV Distribution Networks

The paper reports quality analysis and evaluation at 6 - 10/0.4 kV low-voltage distribution grids in Uzbekistan. Power quality frequently does not correspond to the rated value which is largely due to unbalanced phase loading in grids and which also results in increased power loss. The study of the asymmetrical operating modes of the rural distribution networks of 0.4 kV was conducted in three steps: measurement, calculations and analysis of relevant data; providing practical guidelines and finally, implementing instruments to normalize grid operation. Measuring was conducted using certified instrumentation analyzer “MALIKA” designed by authors. The study and analysis of additional power losses as the function of indicators of asymmetrical features of voltage and current in operating 0.4 kV grids reveals that, quality of electric power at grids under investigation, merely does not meet the requirements of the Interstate Standard.