Low Voltage Distribution Feeders Research Articles

Techno-economic assessment of grid-connected residential photovoltaic systems considering overvoltage-induced generation curtailment

Grid-connected residential photovoltaic (PV) systems are continuously installed in worldwide communities, predominantly to reduce electricity bills. However, the rapid growth of distributed PV systems inevitably causes overvoltage in distribution networks. Conventionally, PV curtailment is implemented in a grid-connected inverter to prevent the system from overvoltage. Nevertheless, previous studies have failed to consider PV curtailment caused by overvoltage when conducting techno-economic assessments. Therefore, this study conducts a comprehensive techno-economic assessment of grid-connected residential PV systems, considering overvoltage-induced generation curtailment. A volt-watt curtailment strategy is designed based on a droop characteristic. Simulation studies are conducted using real-world load profiles and electricity tariffs retrieved from the Metropolitan Electricity Authority in Thailand. Furthermore, a typical low-voltage distribution feeder in Bangkok, Thailand, is employed using Thailand’s grid code. Simulation results indicate that the optimal PV size could be reduced by up to 58.33% (depending on the numbers and locations of PV systems) when considering overvoltage-induced generation curtailment, resulting in a profitability reduction of 21.34%. It is concluded that considering overvoltage-induced generation curtailment can accurately determine the PV size, preventing oversizing and overvoltage.

Fault Detection and Localisation in LV Distribution Networks Using a Smart Meter Data-Driven Digital Twin

Modern solutions for precise fault localisation in Low Voltage (LV) Distribution Networks (DNs) often rely on costly tools such as the micro-Phasor Measurement Unit (μPMU), which is potentially impractical for the large number of nodes in LVDNs. This paper introduces a novel fault detection technique using a distribution network digital twin without the use of μPMUs. The Digital Twin (DT) integrates data from Smart Meters (SMs) and network topology to create an accurate replica. In using SM voltage-magnitude readings, the pre-built twin compiles a database of fault scenarios and matches them with their unique voltage fingerprints. However, this SM-based voltage-only approach shows only a 70.7% accuracy in classifying fault type and location. Therefore, this research suggests using the cables’ Currents Symmetrical Component (CSC). Since SMs do not provide direct current data, a Machine Learning (ML)-based regression method is proposed to estimate the cables’ currents in the DT. Validation is performed on a 41-node LV distribution feeder in the Scottish network provided by the industry partner Scottish Power Energy Networks (SPEN). The results show that the current estimation regressor significantly improves fault localisation and identification accuracy to 95.77%. This validates the crucial role of a DT in distribution networks, thus enabling highly accurate fault detection when using SM voltage-only data, with further refinements being conducted through estimations of CSC. The proposed DT offers automated fault detection, thus enhancing customer connectivity and maintenance team dispatch efficiency without the need for additional expensive μPMU on a densely-noded distribution network.

Iterative Distribution System State Estimation for Integrated Primary and Split-Phase Secondary Feeder Monitoring

The observability of split-phase low-voltage (LV) secondary distribution feeders continues to pose a challenge in distribution system state estimation, which has traditionally been modeled for medium-voltage (MV) primary feeders. This work aims to improve the observability of LV feeders using smart meter data to enable a joint primary (MV) and secondary (LV) feeder state estimation. Towards this goal, a two-stage MV-LV state estimation algorithm is proposed that is iteratively executed until convergence at the MV-LV boundary. Specifically, Stage-I estimates primary feeder (MV) states using approximated load demands at the MV buses obtained from Stage-II. Stage-II estimates the secondary feeder (LV) states and improved load demands at the MV level using the MV voltage estimates obtained from Stage-I and smart meter data. black The algorithm terminates once the sequence of estimated variables from both stages at the MV-LV boundary bus, i.e., load demand (from Stage-II) and voltage (from Stage-I) have converged. The proposed algorithm is demonstrated using the IEEE 123 bus distribution test feeder modified to include secondary network extensions at 47 different nodes. The simulation results validate that the proposed approach facilitates the observability of the split-phase LV systems and enables the joint estimation of MV-LV states. Moreover, the accuracy of the primary feeder (MV) state estimation results is improved using the proposed approach to model and estimate split-phase LV feeders.

Increasing Unbalanced Distribution Network’s Hosting Capacity for Distributed Energy Resources by Voltage Regulators

Renewable energy sources (RESs) are becoming promising nowadays. Deploying distributed energy resources (DERs) is thus increasing accordingly, but the intermittent nature associated with RES remains a significant challenge. Also, the distribution grid faces technological difficulties with the large-scale integration of PV generations and EV charging loads. This paper suggests a strategy for a novel optimal voltage regulator (VR) placement model associated with the optimal placement of electric vehicle chargers (EVCs) and photovoltaic (PV) cells. Also, VRs’ reference voltages are determined optimally to enhance the hosting capacity (HC) of three-phase unbalanced distribution feeders for large-scale residential/public EVCs and PVs. Two scenarios are defined to study the impact of VRs on the network’s HC for DERs, namely with and without VRs in the real three-phase unbalanced feeder. The developed model determines the optimal placement of VRs and their reference voltages linked with EVCs and PVs penetration. Time-Varying Acceleration Coefficients Iteration Particle Swarm Optimization (TVAC-IPSO) algorithm has been implemented to improve the real three-phase unbalanced distribution feeder’s HC for DERs. The efficiency of the proposed model and the solution method is validated on the modified IEEE 37-node medium voltage (MV) radial distribution network connected to 35 low voltage (LV) distribution feeders. The obtained results represent that installing a proper number of VRs will increase the network’s HC for DERs.

Stability Analysis of Low-Voltage Distribution Feeders Operated as Islanded Microgrids

The increasing penetration of behind-the-meter distributed energy resources opens up opportunities to improve distribution grids’ resilience and reliability. In this context, the paper studies islanded operation of radial low-voltage (LV) distribution feeders with a high penetration of prosumers equipped with grid-forming inverters. In contrast to the existing studies that focused on droop control as the main source of instability, we extend the analysis to the microgrid topology, in particular the number of grid-forming inverters. The eigenvalue analysis shows that with the increasing number of grid-forming inverters, the system becomes practically unstable with its critical eigenvalue approaching zero as progressively more inverters are added. Furthermore, we extend the analysis by explicitly modelling the DC side dynamics, including the DC/DC converter with its controller and inherent dynamic response of the DC energy source. Eigenvalue analysis shows that the DC voltage controller impacts the system stability by introducing new dominant eigenvalues. The results suggest that the DC energy source should have a fast response to ensure a stable operation. The small-signal stability results, benchmarked against time-domain simulations, indicate that careful parameter tuning is required to ensure stable islanded operation of LV feeders.

Non-technical loss detection in limited-data low-voltage distribution feeders

Non-technical loss (NTL) is the major source of energy loss in distribution networks. Since the direct calculation of NTL is impossible, it is mostly being obtained by calculating the difference between total loss and technical loss (TL). This is rather not possible without adequate equipment for measuring the TL. However, time-worn distribution networks with high limitations and lack of equipment could identify the regions with a high share of NTL to reduce/eradicate the sources of NTL. This strategy can increase efficiency and reduce costs compared to the one-by-one inspection of customers. This paper presents a novel NTL detection procedure based on load estimation in highly limited distribution networks. First, the low-voltage (LV) distribution network will be divided into a few smaller hypothetical sub-networks through a fuzzy c-means (FCM) clustering technique. Then, a meter placement based on the maximum likelihood criterion is deployed to find the best places for installing the existing meters in each cluster. In the next stage, a linear system of equations is utilized to extract the pattern load profiles (PLPs) related to each class of customers for each hypothetical sub-network separately. These PLPs will be used to estimate the transformers’ load in all LV feeders based on their share in the total consumption of the entire network. Finally, NTL possibility per capita will be achieved for each LV feeder by defining an index that estimates the degree of NTL in those areas.

Phase Identification of Single-Phase Customers and PV Panels via Smart Meter Data

With proliferation of single-phase rooftop photovoltaic (PV) panels, phase balancing in low voltage (LV) distribution feeders becomes the point of concern. In this way, identification of the hosting phase of connected single-phase customers and PV panels is a prerequisite. This paper proposes an optimization model for the phase identification problem. The objective is to minimize the summation of the absolute error between estimated and measured variables. Smart meters (SMs) data including active and reactive power absorptions/injections, nodal voltage magnitudes, and network configuration data form the input of the model. Potential errors in the input data are captured in the model while their adverse effects are tackled by incorporating data associated with numerous consecutive time intervals. The problem is thus inherently large-scale with prohibitively heavy computation. Accordingly, the proposed method takes Benders decomposition in use to make the model tractable. The final model lies in the mixed integer linear programming (MILP). For the sake of performance verification, three different feeders with 34, 457, and 906 nodes along with SM data are studied and discussed thoroughly.

Overvoltage Prevention and Curtailment Reduction Using Adaptive Droop-Based Supplementary Control in Smart Inverters

Recent developments in the renewable energy sector have seen an unprecedented growth in residential photovoltaic (PV) installations. However, high PV penetration levels often lead to overvoltage problems in low-voltage (LV) distribution feeders. Smart inverter control such as active power curtailment (APC)-based overvoltage control can be implemented to overcome these challenges. The APC technique utilizes a constant droop-based approach which curtails power rigidly, which can lead to significant energy curtailment in the LV distribution feeders. In this paper, different variations of the APC technique with linear, quadratic, and exponential droops have been analyzed from the point-of-view of energy curtailment for a LV distribution network in North America. Further, a combinatorial approach using various droop-based APC methods in conjunction with adaptive dynamic programming (ADP) as a supplementary control scheme has also been proposed. The proposed approach minimizes energy curtailment in the LV distribution network by adjusting the droop gains. Simulation results depict that ADP in conjunction with exponential droop reduces the energy curtailment to approximately 50% compared to using the standard linear droop.

Design of On Load Tap Changing Transformer (OLTC) to Increase Photovoltaic Penetration Level in Low Voltage Distribution Feeder

This paper investigates a method for regulating the voltage profile and reducing the voltage unbalance at low voltage (LV) residential feeders with single-phase rooftop photovoltaic (PV) installed randomly along with the feeders. The used algorithm considers the distribution transformers that have on-load tap changers (OLTC) and can automatically control the voltage to prevent voltage rise in the feeder. MATLAB-based simulation results demonstrate the effectiveness of the discussed approaches.

Fast-decoupled power flow method for integrated analysis of transmission and distribution systems

The growing participation of distributed generation in the electricity supply along with the advances in smart grid technologies have emphasized the active role of distribution systems and their more complex interaction with the transmission system. As a consequence, the operation of the power grid requires closer coordination between transmission and distribution systems. This paper proposes an integrated analysis that allows a unique power flow method to be effectively applied to any voltage level of the power system, from the high-voltage bulk transmission network to the low-voltage distribution feeders. The proposed methodology combines the decoupled power flow approach and complex per unit normalization technique to efficiently deal with the high dimension of unified network analysis and the different characteristics of transmission and distribution networks, regarding line parameters and topological arrangements. Simulations are conducted in an integrated T&D network, and in a large primary-secondary distribution system as well. The results demonstrate the effectiveness of the proposed approach and attest the relevance of integrated power flow analyzes to adequately determine the interrelationships between the different operating levels of the electrical network.

Harmonic Analysis of Grid-Connected Solar PV Systems with Nonlinear Household Loads in Low-Voltage Distribution Networks

Grid-connected rooftop and ground-mounted solar photovoltaics (PV) systems have gained attraction globally in recent years due to (a) reduced PV module prices, (b) maturing inverter technology, and (c) incentives through feed-in tariff (FiT) or net metering. The large penetration of grid-connected PVs coupled with nonlinear loads and bidirectional power flows impacts grid voltage levels and total harmonic distortion (THD) at the low-voltage (LV) distribution feeder. In this study, LV power quality issues with significant nonlinear loads were evaluated at the point of common coupling (PCC). Various cases of PV penetration (0 to 100%) were evaluated for practical feeder data in a weak grid environment and tested at the radial modified IEEE-34 bus system to evaluate total harmonic distortion in the current (THDi) and voltage (THDv) at PCC along with the seasonal variations. Results showed lower active, reactive, and apparent power losses of 1.9, 2.6, and 3.3%, respectively, with 50% solar PV penetration in the LV network as the voltage profile of the LV network was significantly improved compared to the base case of no solar. Further, with 50% PV penetration, THDi and THDv at PCC were noted as 10.2 and 5.2%, respectively, which is within the IEEE benchmarks at LV.

Fair coordination of distributed energy resources with Volt-Var control and PV curtailment

This paper presents a novel distributed optimal power flow (DOPF) method for fair distributed energy resource (DER) coordination in the context of mandated rooftop PV inverter control modes. In practice, inverter reactive power control is increasingly required by grid connection codes, which often unfairly curtail PV generation of prosumers towards the end of low-voltage feeders. Similarly, optimization-based DER coordination methods that aim solely for technically-efficient DER coordination do not consider the distribution of PV curtailment across customers. To address these concerns, we develop a tractable multi-objective DOPF method for optimal DER coordination that (i) curtails PV generation fairly across prosumers, and (ii) incorporates a standard piecewise-linear Volt-Var control reactive power control function without using integer variables. Three equity principles representing different interpretations of fairness are implemented in our coordination method; namely, egalitarian, proportional and uniform dynamic PV curtailment redistribution. The performance of our approach is demonstrated on low-voltage distribution feeders of different sizes (5, 10, 25, 50 and 100 prosumers) using two network topologies: line topology without lateral spurs and tree topology with lateral spurs. Each network considers three levels of PV penetration, giving 30 test systems in total. The results demonstrate the effectiveness of the proposed DOPF method for fair DER coordination: PV curtailment is equitably distributed among prosumers with a computational burden on par with conventional DOPF approaches. Moreover, different fairness methods result in different patterns of curtailment, which a regulator may choose between.

A UPFC for Voltage Regulation in LV Distribution Feeders With a DC-Link Ripple Voltage Suppression Technique

The large-scale integration of distributed photovoltaic generation causes several power quality issues in low voltage (LV) distribution networks. Network voltage profile variations severely affect the LV distribution networks. The four-leg unified power flow controller (4L-UPFC) has series and shunt converters that can address the power quality issues. However, instantaneous power theory shows that second-order harmonic voltage (2ωHV) appears at the dc-link capacitor of the 4L-UPFC during any unbalanced operations. This article proposes control strategies for series and shunt converters that will simultaneously regulate the load voltages of a distribution feeder while suppressing the 2ωHV term on the dc-link of the UPFC. A controlled negative sequence current from the shunt converter is used to suppress the 2ωHV term on the dc-link. The active suppression of the 2ωHV term allows electrolytic capacitors to be replaced with small long life ceramic or film capacitors, and this does not require additional passive compensation. Stability analysis of the control loops demonstrates the overall stability of the converter system. The proposed control methods have been implemented on a Texas DSP (F28377D). An experimental demonstration on a laboratory scale prototype shows that the proposed control methods can effectively regulate the load voltages at LV distribution feeders and suppress the 2ωHV on the dc-link of UPFC during unbalanced loads and supply conditions.

Unbalance mitigation by optimal placement of static transfer switches in low voltage distribution feeders

With the rapid proliferation of residential rooftop photovoltaic (PV) systems, current and voltage unbalance issues have become a matter of great concern in low voltage (LV) distribution feeders. To overcome the issues, this study proposes a model to optimally rephase customers and PVs among the three phases via static transfer switches (STSs). The optimal STS placement is also considered in the model to achieve a cost effective solution with optimum number and location of STSs. The objective is to minimise total energy losses caused by current unbalance, minimise the number of STSs, and keep voltage unbalance along the feeder within the acceptable range. The model is solved via a non-dominated sorting genetic algorithm-II (NSGA-II) which provides a Pareto front. A fuzzy decision-making approach is then applied to choose the final solution among the Pareto front points. The proposed model is simulated on the IEEE 123-Node Test Feeder. The simulations are conducted in MATLAB where the COM interface capability is used to call OpenDSS to evaluate NSGA-II populations. According to the achieved results, the proposed model can effectively and affordably apply STSs to mitigate unbalance issues in LV feeders hosting high penetration of rooftop PVs.

Photovoltaic Hosting Capacity Sensitivity to Active Distribution Network Management

The increasing integration of rooftop photovoltaic (PV) systems in low voltage (LV) distribution systems may result in technical problems such as over-voltage. Thus, utilities have been employing different active distribution network management schemes to enhance rooftop photovoltaic hosting capacity (PVHC) by mitigating the technical issues. As there are different control schemes to resolve the over-voltage issue and increase PVHC, it is necessary to develop a comprehensive method to compare their effectiveness. The aim of this paper is to investigate the effects of autonomous voltage control strategies on the PVHC in LV distribution feeders. The investigated operation strategies are based on the active and reactive power control capabilities of PV systems as well as the on-load tap changer of transformers. We propose an optimization-based framework to determine the PVHC considering the voltage control capabilities. We do this by utilizing the linear mathematical model of the PVHC, in which the voltage control strategies are modeled as equality and inequality constraints. The proposed methodology is examined on IEEE 123-bus test system as well as 128 LV UK feeders to identify the potential of autonomous voltage control strategies in increasing the PVHC. Finally, the sensitivity of the obtained results to the R/X ratio of the feeders and size of PV systems is assessed.

Coordinated Control of Active and Reactive Power Compensation for Voltage Regulation with Enhanced Disturbance Rejection Using Repetitive Vector-Control

Voltage profile is one of many aspects that affect power quality in low-voltage distribution feeders. Weak grids have a typically high line impedance which results in remarkable voltage drops. Distribution grids generally have a high R/X ratio, which makes voltage regulation with reactive power compensation less effective than in high-voltage grids. Moreover, these networks are more susceptible to unbalance and harmonic voltage disturbances. This paper proposes an enhanced coordinated control of active and reactive power injected in a distribution grid for voltage regulation. Voltage drop mitigation was evaluated with power injection based on local features, such loads and disturbances of each connection. In order to ensure disturbances rejection like harmonic components in the grid voltages, a repetitive vector-control scheme was used. The injection of coordinated active and reactive power with the proposed control algorithm was verified through simulations and experiments, demonstrating that it is a promising alternative for voltage regulation in weak and low-voltage networks subject to inherent harmonic distortion.

A New Approach to Reliability Assessment and Improvement of Synchrophasor Communications in Smart Grids

Wireless communications can facilitate transfer of synchrophasor data between spatially separated phasor measurement units (PMUs) and phasor data concentrators (PDCs). However, such communication systems may impose random access delay and failure on PMU channels that lead to missing synchrophasor data frames at the output of the PDC. This paper presents a new approach for online reliability assessment and improvement of synchrophasor data communications. The proposed approach involves elaborate estimation and probabilistic prediction algorithms that trigger a prioritized handover mechanism in order to minimize the number of synchrophasor data frames that are missing over successive time stamps. Extensive simulations based on the LTE communications in a low-voltage distribution feeder confirm significant performance improvement and fast fault detection under the communication link failures. This non-intrusive approach can be adopted by network operators to ensure reliable transmission of synchrophasor data to monitoring, control, and protection applications in smart grids.

Distributed load management scheme to increase PV hosting capacity in LV feeders

Proliferation of photovoltaic (PV) panels may cause voltage increases in low voltage (LV) distribution feeders. This issue confines PV hosting capacity of the feeders. To overcome this issue and enable higher PV hosting capacity, a model to manage the operation cycles of the domestic electric water heater and heating, ventilation, and air conditioning loads in order to avoid the voltage rise, thereby increasing PV hosting capacity in LV feeders is presented in this study. The model respects users' thermal comfort by setting the associated temperatures within allowed ranges. To preserve customers' privacy, the presented model is then decomposed into a master problem and several sub-problems using the Dantzig-Wolfe decomposition algorithm. Thereafter, an effective scheme is designed to solve the decomposed problem in a distributed fashion. In the scheme, the master problem is solved by the system-wide module and the sub-problems are solved by house-wide modules. The problems are all in linear format which can be easily solved with affordable computation. Importantly, the proposed model can reach the optimal solution while minimal data is shared between houses and control centre.

The simultaneous mitigation of slow and fast voltage fluctuations caused by rooftop solar PV by controlling the charging/discharging of an integrated battery energy storage system

Abstract Both slow and fast voltage fluctuations in the connected low voltage (LV) distribution feeder are caused by intermittent variations in solar PV power output, in addition to the variations in load demand where rooftop solar photo-voltaic (PV) unit penetration is higher. A single energy storage system integrated with the solar PV unit can mitigate these fluctuations in voltage profile. A novel analytical approach to mitigate both slow and fast voltage fluctuations simultaneously in the connected LV distribution feeder is proposed, which has not explicitly been addressed in the literature. Integrated battery energy storage systems will be dynamically charged during mid-day to alleviate the voltage rise and discharged during the evening peak hours to alleviate the voltage drop, while simultaneously controlling the fast fluctuations of the PV inverter output to a specified value. The proposed control strategy has been validated using a hypothetical distribution feeder system and results have demonstrated that both the slow and the fast voltage fluctuations in the voltage profile can effectively be alleviated, if the proposed strategy is implemented.

Analysis of Voltage Unbalance due to Single-Phase Dispersed Generation Systems in Three-Phase Low-Voltage Distribution Feeders

Analysis of Voltage Unbalance due to Single-Phase Dispersed Generation Systems in Three-Phase Low-Voltage Distribution Feeders