Four-wire Distribution Network Research Articles

Allocation of power losses and energy in the distribution network

The goal is to determine methods for calculating power losses in a three-phase four-wire low voltage distribution network using measurements of a balance smart meter and consumer smart meters, and to establish the factors influencing the power losses and their allocation among individual network wires, loads, and consumers. The study involved examining three methods for determining power losses for current measurement snapshot. The first method suggests calculating losses as the difference between the power supplied to the network and the total power consumed. The second method calculates power losses using the contribution method. The third method, which in addition to measurement information requires knowledge of the topology and parameters of the network components, determines power losses based on the results of the state estimation method. The research proposes an algorithm for transition from a four-wire distribution network modeling to a three-wire one. The algorithm allocates power losses of the neutral wire among the phase wires. The findings indicate that the negative losses in the network with unbalanced phase loads are caused by the presence at the nodes of the least loaded phase of higher voltage than the voltage at the power supply node. The reason for higher losses in phases with minimal load is the uneven allocation of loads in the phases. In addition, the study reveals that the power loss values obtained by the contribution method, i.e. directly from the measurements of smart meters, are closer to the losses determined from the readings of the balance meter and consumer meters, compared to the losses found from the state estimation results. The considered methods for calculation and allocation of power losses are illustrated by an example of a real-world distribution network equipped with smart meters. The paper demonstrates the examples of allocating total power losses between phase wires and a neutral wire, among phase wires only, and between total loads at phase nodes and individual consumers in phases.

Solar Photovoltaic–Small Hydro-Based Charging: Infrastructure for a Three-Phase, Four-Wire Distribution Network

Solar Photovoltaic–Small Hydro-Based Charging: Infrastructure for a Three-Phase, Four-Wire Distribution Network

Optimal power flow in four-wire distribution networks: Formulation and benchmarking

In recent years, several applications have been proposed in the context of distribution networks. Many of these can be formulated as an optimal power flow problem, a mathematical optimization program which includes a model of the steady-state physics of the electricity network. If the network loading is balanced and the lines are transposed, the network model can be simplified to a single-phase equivalent model. However, these assumptions do not apply to low-voltage distribution networks, so the network model should model the effects of phase unbalance correctly. In many parts of the world, the low-voltage distribution network has four conductors, i.e. three phases and a neutral. This paper develops OPF formulations for such networks, including transformers, shunts and voltage-dependent loads, in two variable spaces, i.e. current–voltage and power–voltage, and compares them for robustness and scalability. A case study across 128 low-voltage networks also quantifies the modelling error introduced by Kron reductions and its impact on the solve time. This work highlights the advantages of formulations in current–voltage variables over power–voltage, for four-wire networks. • We develop two quadratic optimization formulations for unbalanced four-wire grids. • The current–voltage formulation is superior to the power–voltage formulation. • Four-wire optimization models require about 30% more time to solve than three-wire ones. • Numerical results support the proposed relaxation and approximation hierarchies.

Three-phase four-wire shunt hybrid active power filter model with model predictive control in imbalance distribution networks

<span lang="EN-US">This paper presents a harmonic reduction and load imbalance model in a three-phase four-wire distribution network. This model uses a hybrid active power filter, a passive inductor and capacitor filter, and an active power filter in the form of a three-phase, four-leg connected grid inverter. The switching of the voltage source converter on this filter uses finite control set model predictive control (FCS-MPC). Control of this hybrid active power filter uses model predictive control (MPC) with a cost function, comparing the reference current and prediction current with mathematical modelling of the circuit. The reference current is taken from the load current by extracting dq, and the predicted current is obtained from the iteration of the voltage source converter (VSC) switching pattern. Each combination is compared with the reference current in the cost function to get the smallest error used as a power switching signal. Modelling was validated by using MATLAB Simulink. The simulation results prove a decrease in harmonics at a balanced load from 22.16% to 4.2% and at an unbalanced load, reducing the average harmonics to 4.74%. The simulation also decreases the load current imbalance in the distribution network. Reducing the current in the neutral wire from 62.01%-0.42% and 11.29-0.3 A.</span>

DSSE in European-Type Networks Using PLC-Based Advanced Metering Infrastructure

This paper addressesthe problem of real-time state estimation in European-type four-wire low-voltage distribution networks with advanced metering infrastructure based on power line communication (PLC). The main drawback of this type of infrastructure is its high latency and a sequential data sampling process. Both factors make it difficult to obtain instantaneous values from the smart meters in a timely and adequate manner to estimate the real-time state of the system. This problem may be solved in the future with the deployment of metering systems based on other technologies such as 5G. However, PLC technology prevails in many countries, and is not likely to be replaced in the medium term. In the meantime, a solution is needed to provide acceptable real-time state estimates in low voltage networks, not to hinder the active management of these assets in the context of smart grids. In this work, a solution is proposed that allows estimating the state of the low voltage network with a reasonable accuracy considering the low quality of the measurements provided by the deployed infrastructure. The system is validated using a portion of a real pilot network located in the north of Spain fed by a transformer station with seven feeders.

Mitigation of power quality problems using voltage-controlled based modified DSTATCOM

ABSTRACT This paper proposes a modified distribution static compensator (DSTATCOM) with the voltage controller. The modified DSTATCOM is incorporated in a three-phase four-wire distribution network. The design criteria of the modified DSTATCOM parameters is introduced. The Hysteresis Voltage Controller (HVC) is used to drive the Voltage Source Converter (VSC) and its reference voltage signal is generated based on the Instantaneous Symmetrical Components Theory (ISCT) to provide the proper compensation to stabilise the system voltage, improve the power factor and mitigate the harmonic distortion. Matlab/Simulink software is used to verify the performance of the modified DSTATCOM with the used controller under different load types and supply voltage conditions. The control algorithm ensures that the bus voltage is maintained at its rated value during different connected loads and supply conditions. Also, it compensates the non-linear and unbalanced supply currents and reducing the Total Harmonic Distortion (THD) of both current and voltage. The proposed modified DSTATCOM gives better results in comparison with the conventional DSTATCOM.

Optimal Power Quality Compensation of Energy Storage System in Distribution Networks Based on Unified Multi-Phase OPF Model

Energy Storage System (ESS) is a promising solution to suppress the peak-valley difference of residential distribution networks (RDN) with high penetration of distributed photovoltaic generations. Meanwhile, it can also provide certain power quality compensations to RDN due to the flexible adjusting ability of its converter interface. To make full use of this feature, this paper investigates the optimal power quality compensation of the ESS. This is achieved by formulating and solving an optimal power flow (OPF) problem, the objectives of which are to minimize the power loss, harmonic distortion, and voltage unbalance of the network. Noticeably, the established model includes the neutral line and inter-phase loads; thus, it is unified and can be easily extended to networks with different structures. However, the coupling between different phases and the inclusion of harmonic variables significantly increase the complexity of the OPF problem. To address this issue, the optimization model is further transformed into a convex quadratic constrained quadratic program (QCQP) by replacing power flow equations with current injection equations. As a result, the problem can be solved by the primal-dual interior-point method (PDIPM) with guaranteed convergence and computational efficiency. An iterative solution method is further proposed to improve the accuracy of the solution. Finally, the proposed method is verified using a three-phase three-wire 25-node distribution network and a three-phase four-wire 162-node distribution network in North America.

Three-Phase Four-Wire OPF-Based Collaborative Control of PV Inverter and ESS for Low-Voltage Distribution Networks With High Proportion PVs

The use of photovoltaic reactive power and energy storage active power can solve the problems of voltage violation, network loss, and three-phase unbalance caused by photovoltaic connection to low-voltage distribution networks. However, the three-phase four-wire structure of the low-voltage distribution network brings difficulties to power flow calculation. In order to achieve photovoltaic utilization through optimal power flow, a photovoltaic-energy storage collaborative control method for low-voltage distribution networks based on the optimal power flow of a three-phase four-wire system is proposed. Considering the amplitude and phase angle of voltage and current, a three-phase four-wire node admittance matrix was used to establish the network topology of the low-voltage distribution network. Also, to minimize the network loss, the three-phase unbalance and voltage deviation. a multi-objective optimization model based on three-phase four-wire network topology was established considering the voltage constraints, reverse power flow constraints and neutral line current constraints. Through improving the node admittance matrix and model convexity, the complexity of solving the problem is reduced. The CPLEX algorithm package was used to solve the problem. Based on a 21-bus three-phase four-wire low-voltage distribution network, a 24-h multi-period simulation was undertaken to verify the feasibility and effectiveness of the proposed scheme.

Voltage violation in four‐wire distribution networks integrated with rooftop PV systems

Integration of rooftop photovoltaic (PV) systems in a three-phase four-wire distribution network cause voltage-violations namely voltage-rise and voltage unbalance. This study investigates the factors that affect both the voltage-rise and voltage unbalance in low voltage distribution network integrated with the rooftop PV systems. The concerning factors are classified into active factors such as; loads active powers, PV active powers, and bus reactive powers, and passive factors such as; numbers of feeder buses and neutral-grounded resistances. The study also determines the factors conditions at which the highest values of both voltage-rise and voltage unbalance occurred. Moreover, the most and least significant effects of individual factors on both voltage-rise and voltage unbalance are studied. The studied system is simulated and implemented in MATLAB software environment and the feeder loads are modelled based on Back–Forward Sweep method. The simulation results identify that the conditions of the worst voltage-rise and voltage unbalance cases depend on the collective influence of the studied factors.

A control scheme for voltage unbalance mitigation in distribution network with rooftop PV systems based on distributed batteries

Unbalanced loads create unacceptable voltage unbalance (VU) in low voltage (LV) three-phase four-wire distribution networks with unbalanced rooftop photovoltaic (PV) systems. This paper proposes a mitigation strategy for restricting the VU depending on the distributed batteries that included in grid interfaced rooftop PV systems. Moreover, it introduces a dynamic control model based on two PI-controllers for determining the required batteries’ compensating currents and ensuring equality resultant stress current on each bus. It overcomes three existing challenges; the inequality stresses currents on the distributed batteries along the feeder, the insufficient size or non-existing of the distributed batteries, and satisfies the same stress among batteries on the same bus. For investigating the influence of equal phase magnitude batteries’ compensating current at the same bus on the whole distribution network (DN). Particle Swarm Optimization (PSO) and Bat optimization techniques are used to determine the optimum self and mutual sharing co-efficients of each battery. The limits of the distributed batteries' size are taken into consideration in the optimization process. The proposed dynamic model is implemented using MATLAB software. The simulation results show the effectiveness of the proposed control scheme for mitigating the VU on the distribution feeder’s buses at three scenarios; different loads, PVs, and states of charge of batteries. Moreover, the batteries’ compensating currents are presented to examine the implementation feasibility.

State estimation of a three-phase four-wire secondary distribution network

A state estimation algorithm for assessing the state of a power delivery network according to the measurement information performed by smart meters is proposed and its efficient use in a real low-voltage three-phase four-wire secondary distribution network demonstrated. Nonlinear state estimation is performed by a method of simple iterations, at each step of which an overdetermined system of linear equations of measurements is solved by the method of weighted least squares. State estimation is performed independently for each phase and the neutral wire; phase voltage estimates relative to the neutral wire are determined according to estimates of the phase wire voltages and the neutral wire relative to the ground. Testing of the method is carried out on the example of a real 11-node main feeder, on the poles of which MIR S-04, MIR S-05 and MIR S-07 single-phase and three-phase meters of Russian production are installed. For the state estimation, information was used on the hourly average power of loads and voltage modules for 576 measurement sections taken from the protocols of an automated commercial electricity accounting system. The high accuracy of the obtained estimates is confirmed by no more than 1.2 V residues between the measured values of the variables and their estimates. Two methods for determining energy losses by estimating voltages - with respect to earth and a neutral wire, respectively - are presented. The possibility of balancing loads by moving single-phase loads of the most loaded phase to a less loaded phase is demonstrated. The calculations performed for a real network show the proximity of variables measured by smart meters to their estimates, confirming the effectiveness of the presented algorithm for assessing the state of the secondary distribution network with explicit consideration of the neutral wire.

Review on voltage‐violation mitigation techniques of distribution networks with distributed rooftop PV systems

The integration of the various types of distributed generators in low-voltage (LV) distribution networks becomes a great concern, especially the rooftop photovoltaic (PV) systems. The negative impacts of the rooftop PVs on the distribution feeder buses' voltage include voltage rise and voltage unbalance (VU). Such a voltage-violation condition depends mainly on the PVs ratings and the network unbalance percentage. This study presents a review for different techniques used to mitigate the voltage violation resulting from PVs integration in a typical three-phase four-wire LV distribution network case study. The voltage-violation mitigation techniques studied in this study are enhancement of the feeder, on-load tap changer, demand-side management, active power curtailment, a reactive power control, static transfer switch, energy storage systems and hybrid strategies. The LV distribution network case study was modelled based on constant power model method using MATLAB software environment. The simulation results demonstrate both voltage regulation and alleviating VU capabilities of each technique.

Multifunctional Distributed MPPT Controller for 3P4W Grid-Connected PV Systems in Distribution Network with Unbalanced Loads

The integration of photovoltaic (PV) systems with three-phase four-wire (3P4W) distribution networks has imposed several challenges related to existing unbalanced loads, reactive power generation and harmonics content. In this paper, a multifunctional distributed maximum power point (MPPT) controller for grid integration of PV systems is proposed. The proposed distributed MPPT controller is developed based on employing a four-leg three-level T-type multilevel inverter. The proposed inverter performs multifunctionalities, including distributed MPPT, neutral current compensation for the unbalanced loads, supplying reactive power into the grid and the grid integration. Moreover, the proposed inverter overcomes the stochastic behavior of both the PV generation with partial shading problems and its operation with unbalanced loads as well. Furthermore, the new proposed controller injects sinusoidal output currents with decreased levels of total harmonic distortion (THD) into the grid. The tested case study is investigated for the various operating scenarios of PV generation and load demands. The results and tabulated performance comparisons have proven the superior performance of the proposed multifunctional PV generation system. The results show the ability of the proposed controller to efficiently extract distributed MPPT for all PV modules at all the tested scenarios. Additional improvement of the energy efficiency is achieved through the elimination of the neutral current due to existing unbalanced loads.

Сomprehensive provision of standard power quality parameters in three-phase networks by the use of hybrid filter-compensating converters

In this work the conditions of the most effective use of the developed perspective means for the solution of the actual problem of complex providing of electromagnetic compatibility in modern distribution public electricity networks are considered. This is achieved by improvement of a number of voltage quality characteristics (harmonics, asymmetry and voltage fluctuations). The simulation model of a three-phase four-wire distribution network is proposed, which allows estimating voltage quality characteristics when connecting non-linear and asymmetric loads. The simulation modeling of the operation of the specified network with certain characteristics, with different values of the power of single- and three-phase nonlinear load (rectifiers) is carried out. The simulation results showed that in modern conditions, with a high concentration of electronic equipment in the distribution network, it is not enough for each separate unit of equipment to have a built-in individual means for reducing the emission of disturbances, which ensures compliance with the requirements of the European Harmonized Standards for Electromagnetic Compatibility. Therefore, in order to solve this problem in the electricity networks it is suggested for consumers to additionally apply group electromagnetic compatibility means, including hybrid filter-compensating converters that combine functional capabilities of a regulated filter balancing device which based on an autotransformer with the zig-zag connection of windings and an active parallel power filter. The analysis of simulation results of a three-phase electric network with standard characteristics of a distribution transformer allowed to determine the ratio of power of one- and three-phase nonlinear load of consumers and linear load asymmetries in network, for which a hybrid filter-compensating converter has an advantage over the known passive group means of improving the quality of voltage and allows you to provide the quality of voltage in accordance with the norms of the international standards.

New method for calculating the susceptances of a balancing capacitive compensator for a three-phase four-wire distribution network

Nowadays, there is an increasing development of equipment type Custom Power Devices (CPDs) designed to ensure a high level of electrical power quality at the Point of Common Coupling (PCC). Most of these devices are built as advanced power electronics applications, such as the Switching Power Converters (SPCs) category. However, for a wide range of applications in consumer installations and electricity distribution operators, such as power factor correction, load balancing, voltage regulation, or flicker mitigation, the reactive power compensators based on passive circuit elements, Reactive Power Compensators (RPCs) are successfully used. The most efficient RPCs are those built so that they may adapt to variable regimes, known as Static Var Compensators (SVCs). Two of the usual functions of a SVC, namely power factor improvement and load balancing in a three-phase distribution network, can be achieved simultaneously by using an unbalanced SVC built in the version of Adaptive Balancing Reactive Compensator (ABRC). The paper briefly–outlines the mathematical model and operation of a Balancing Reactive Compensator (BRC) for a three-phase four-wire network. Then, moves on to develop a direct method of sizing based on equations that directly link the susceptances’ values of the compensator with the real and imaginary components of load sequence currents. The problem is to determine the weight of these components, meaning their degree of compensation, by requiring only capacitive or null susceptances and avoiding capacitive over-compensation of positive sequence currents. Thus, a particular case of BRC, i.e. a Balancing Capacitive Compensator (BCC) is obtained, which keeps the same functions, but has the advantage of reducing costs by eliminating high-power single-phase coils and by simplifying the compensation control system. The paper also presents the numerical results and the conclusions of a case study on the application of the method, performed using two computational and modeling software tools. Experimental laboratory determinations and modeling under nonsinusoidal conditions complete the analysis. The results demonstrate the correctness of the new method of solving the sizing problem and once again validate the unbalanced capacitive compensation as an efficient method of power factor improvement and load balancing at a PCC belonging to a three-phase four-wire distribution network.

Robust Optimal Allocation of Decentralized Reactive Power Compensation in Three-Phase Four-Wire Low-Voltage Distribution Networks Considering the Uncertainty of Photovoltaic Generation

Due to the unbalanced three-phase loads, the single-phase distributed photovoltaic (PV) integration, the long feeders, and the heavy loads in a three-phase four-wire low voltage distribution network (LVDN), the voltage unbalance factor (VUF), the network loss and the voltage deviation are relatively high. Considering the uncertain fluctuation of the PV output and the load power, a robust optimal allocation of decentralized reactive power compensation (RPC) devices model for a three-phase four-wire LVDN is proposed. In this model, the uncertain variables are described as box uncertain sets, the three-phase simultaneous switching capacity and single-phase independent switching capacity of the RPC devices are taken as decision variables, and the objective is to minimize the total power loss of the LVDN under the extreme scenarios of uncertain variables. The bi-level optimization method is used to transform the robust optimization model with uncertain variables into bi-level deterministic optimization models, which could be solved alternately. The nonlinear programming solver IPOPT in the mature commercial software GAMS is adopted to solve the upper and lower deterministic optimization models to obtain a robust optimal allocation scheme of decentralized RPC devices. Finally, the simulation results for an actual LVDN show that the obtained decentralized RPC scheme can ensure that the voltage deviation and the VUF of each bus satisfied the secure operation requirement no matter how the PV output and load power changed within their own uncertain sets, and the network loss could be effectively reduced.

A system optimization method for mitigating three-phase imbalance in distribution network

Three-phase imbalance is a common phenomenon in three-phase four-wire distribution network systems (DNSs), which may cause power quality deterioration, increase power losses, and can even damage appliances as well. The situation is becoming even worse because of the increasing integration of the distributed generations (DGs) and storage-capable loads into DNSs. In this paper, a joint optimization model based on the Y-connected and Δ-connected static reactive power compensation devices are proposed to mitigate the three-phase imbalance and minimize the active power losses in DNSs. The model is formulated to optimize the operation of all the devices in order to mitigate the imbalance of the whole DNS rather than a single feeder or a particular area. Meanwhile, the current imbalance of the transformer and the voltage imbalance of all three-phase nodes are simultaneously considered in the model according to the practical requirements. Several linearization techniques are applied to simplify the computation and accelerate the solving speed. These techniques convert the non-linear and non-convex model into a mixed-integer linear programming model, which can be effectively solved by off-the-shelf solvers. The effectiveness and scalability of the proposed method are verified by the test results obtained from the modified IEEE 34 and IEEE 123 test systems.

Photovoltaic Power Converter Management in Unbalanced Low Voltage Networks with Ancillary Services Support

The proliferation of residential photovoltaic (PV) prosumers leads to detrimental impacts on the low-voltage (LV) distribution network operation such as reverse power flow, voltage fluctuations and voltage imbalances. This is due to the fact that the strategies for the PV inverters are usually designed to obtain the maximum energy from the panels. The most recent approach to these issues involves new inverter-based solutions. This paper proposes a novel comprehensive control strategy for the power electronic converters associated with PV installations to improve the operational performance of a four-wire LV distribution network. The objectives are to try to balance the currents demanded by consumers and to compensate the reactive power demanded by them at the expense of the remaining converters’ capacity. The strategy is implemented in each consumer installation, constituting a decentralized or distributed control and allowing its practical implementation based on local measurements. The algorithms were tested, in a yearly simulation horizon, on a typical Portuguese LV network to verify the impact of the high integration of the renewable energy sources in the network and the effectiveness and applicability of the proposed approach.

Using smart meters for checking the topology and power flow calculation of a secondary distribution network

The paper analyzes options of using smart meters for power flow calculation and for assessing the state of a real three-phase four-wire secondary distribution network based on measurements of average values of active and reactive power and of voltages. The work is based on the authors’ research on allocation of measurements to ensure secondary distribution network observability and on selection of the most efficient method for linear and non-linear state estimation. The paper illustrates solution of a problem on identification composition of load nodes in the phases and reveals challenges related to voltage account in the neutral wire and in its grounding.

Rooftop PV systems with distributed batteries for voltage unbalance mitigation in low voltage radial feeders

This paper discusses the alleviation of voltage unbalance (VU) in low voltage four-wire ungrounded distribution networks. Unbalanced allocation of distributed rooftop Photovoltaic (PV) ratings and loads creates considerable neutral current in the neutral line. Because of the size limitation or non-connection of distributed batteries, this paper proposes three strategies for using the available distributed energy storage to reduce VU and neutral potential under a high penetration of distributed rooftop PVs. VU mitigation algorithms are presented and implemented in the MATLAB/software environment.