Practical Distribution Network Research Articles

Improvement methods for solving the distribution network reconfiguration problem

The paper presents methods for solving the distribution network reconfiguration problem based on heuristic algorithms. In particular, the study solved the distribution network reconfiguration problem for active power losses reduction. The application of the method to a sample network proved effective compared to other algorithms, especially in case of large-scale and complex systems. In addition, the paper considers the influence of the location and capacity of distributed generations on the distribution network reconfiguration problem in different cases. The results show that the distribution network reconfiguration problem combined with optimization location and size of distributed generations is the most efficient solution for minimizing power loss and enhancing voltage profile. The proposed methods have been also successfully applied in the small- and medium-scale practical radial distribution networks. The simulation results show that the proposed methods can be used as reference materials for network reconfiguration problems with single and multi-objectives.

Local penetration‐free control approach against numerous changes in PV generation level in MAS‐based protection schemes

Despite the increasing attention paid to multi-agent systems (MASs), their applications in protection schemes of distribution networks would face serious challenges when the generation level of photovoltaic (PV) systems is significantly increased, i.e. the inherent variable generation of PV systems would not only increase the communication burden, but it can also cause protection miscoordination. To solve this problem, this study first classifies the protection tasks into two hierarchical categories as the ‘first-control-level’ and ‘second-control-level’ functions, where the first group is responsible for the urgent task of fault clearing, and the second group updates protection settings in the event of network/generation changes. Given clearing the fault should be accomplished as soon as possible, the first-control-level functions are designed to require the least possible data communication. Presenting a penetration-free approach, the study next describes the mechanism of managing the various generation-change events through a first-control-level function. Therefore, communication failure, as well as protection miscoordination risks, would be mitigated. Finally, the effectiveness of the proposed method is demonstrated using a practical PV-integrated distribution network. This study tackles an important challenge in the protection of distribution systems with a high level of distributed generation penetration where MAS-based schemes are applied.

Composite Reliability Index of Practical Distribution Network with PVDG Integration

The benefits of the photovoltaic distributed generation (pvdg) installation in the distribution network include the total system loss reduction, voltage profile improvement and the reliability enhancement of the distribution network by minimizing the interruption and duration time to customers due to the loss of utility or due to the faults in transmission lines/equipments. in this research, the optimal size of pvdg is connected at the optimal location of practical distribution network (town pyone feeder). The multi-state model is used to calculate the probabilities of the solar irradiance data and the output power of pvdg. These probabilities of solar irradiance are applied to assess the reliability of the practical distribution network with pvdg integration. The reliability within the island operation mode of the distribution system with pvdg is presented. Analytical approach is described to assess the impact of pvdg on the system reliability due to the stochastic behavior of pv output power. a composite index organizing the impact of pvdg on the overall reliability of distribution network including interruption frequency, interruption duration and expected energy not supplied is presented. The results show that the integration of pvdg placement in the practical distribution network can improve the reliability and the applying proposed methodology can enable determining pvdg configuration concerning the overall system reliability improvement.

A novel ANN-based distribution network state estimator

State estimation is fundamental to energy management systems (EMS) employed in control centers for transmission system networks. However, the automation of power grid requires extension of state estimation to distribution networks, which are yet not observable through the available set of measurement units. In this paper, a novel state estimator based on artificial neural network (ANN) is proposed for obtaining the state of the distribution network despite of the low measurement availability. The proposed estimator was tested for IEEE test distribution networks (33-bus and 69-bus), and for a practical 48-bus Danish network under diverse network conditions such as normal system operation, distributed-generator (DG) connection/disconnection (topology variation), load variation and bad data. Results obtained are promising, which shows the viability of the developed estimator for practical distribution networks.

Distribution Network Expansion Based on the Optimized Protective Distance of Surge Arresters

Expansion of the distribution network and close distance of distribution transformers lead to allocation of excessive surge arresters, while the number of arresters failures in the traditional routine may cause unwanted power outages. Therefore, this paper aims to propose a method to decrease the number of surge arresters as well as keep overvoltage protection of the network in the event of direct lightning strokes. According to different bus structures of the distribution network, elimination of allocated arresters is carried out by investigation of their protective distances at different stroke conditions. A guideline is presented for the IEEE 34-bus distribution system and the outcome is compared with the standard. The analysis is also carried out for a practical distribution network at West Mazandaran Power Distribution Company, Mazandaran, Iran. Possibilities for the elimination of surge arresters at the transformer station and installation of new transformers without allocation of new arresters are investigated in the analysis. Simulations are carried out based on the standard 8/20 lightning current wave at a transient analysis of DIgSILENT.

Quantitative Evaluation Of Actual Loss Reduction Benefits of a Renewable Heavy DG Distribution Network

Modern distribution network architectures have greatly changed due to the increasing proliferation of the network with power converters of renewable distributed generators (RDGs) and switching electronic loads. While these RDGs may offer benefits to the distribution network operator in terms of network loss reduction, the quantification of such benefits are usually investigated on the basis that harmonics from the power converters are negligible. In this paper, the electrical power losses in a network with significant penetration of photovoltaics (PVs) is assessed. The harmonics from the power conditioners of the PVs and network background distortions have been taken into account in the analysis. Technical insights from study of practical distribution networks are presented.

A fault-clearing algorithm supporting the MAS-based protection schemes

Despite the fast, reliable and multi-purpose operation of MAS-based protection schemes, the inherent delay or failure of communication system must not affect the vital role of fault clearing, i.e., the ever-increasing renewables in distribution networks as well as network size increase has resulted in a heavy communication burden yielding to delay or even failure in communication. This fact will necessitate having an auxiliary algorithm to save protection system against any load curtailment. To affordably achieve this goal, this paper proposes a self-healing approach having the ability to temporarily replace the common MAS algorithms during emergency conditions. Accordingly, this paper, first, classifies the protection tasks into two hierarchical control levels; the first is responsible for clearing the fault while the second pursues some technical or economical goals. Next, using some less-considered capabilities of the common relays, a penetration-free algorithm is proposed to perform the first-control-level duties without MAS aid. Finally, the effectiveness of the proposed is proved on a practical distribution network. In general, this method can be a complement for the common MAS-based protection schemes to avoid any electricity interruption due to protection failure.

An analysis of harmonic heating in smart buildings and distribution network implications with increasing non-linear (domestic) load and embedded generation

Harmonic distortion is generally not taken into account within domestic installations and the associated wiring systems, as its potential is considered sufficiently small to be neglected. Standards to limit harmonic manifestations in the low voltage (LV) network are available, but these can be breached as a consequence of advancements in power electronics in some modern household devices contributing higher levels of harmonic distortion than permitted. While these devices individually might not be considered serious in terms of system level harmonic distortion manifestations, electrical equipment failures and insulation failures - increasingly being derived from harmonic cable heating - suggest a different story. A 10% increase in THD in a circuit will result in an increase of 10% in cable heat. Recently, attempts have been made to offer harmonic derating factors for building electrical circuit design in BS7671, but this approach currently prioritizes large power devices. This article explores the need for harmonic considerations during the design stage of electrical services engineering projects. Best practice suggestions, in the context of the dissemination of heat caused by harmonics related to household load deployments/configurations, are also provided based on the analysis conducted with real household data. This is further extended to a practical distribution network where the effect of harmonic heating at the network level is explored. The results suggest that the harmonics in the distribution network can amass to cause a cumulative effect on the network. Furthermore, it can be observed from the results that in a distribution network containing (domestic) solar photo voltaic (PV) systems, the harmonic heating issue can be reduced. This benefit is not without consequence however, as increasing PV penetration does not reduce the harmonic content of the overall system and therefore presents a further concern that may need to be addressed in due time.

Optimal Planning of Multiple Distributed Generating Units and Storage in Active Distribution Networks

Traditionally, the energy supply in off-grid communities has relied on diesel generating units based microgrids (MGs) but global environmental concerns and advancements in renewable energy technology are pushing for the transformation of these systems into renewable-based MGs. This transformation is a more complex hybrid system, with dispatchable and non-dispatchable resources, and requires novel planning and design tools to ensure the security of supply in the future. This paper presents a methodology that jointly optimizes the capacities and locations of dispatchable and non-dispatchable distributed generating units and battery energy storage system employed in a stand-alone MG serving conventional residential and electric vehicle charging load. The complex dual optimization problem is solved innovatively in two sequential steps. In the first step, the capacities of solar photovoltaic panels, wind turbines, battery energy storage system, and diesel generator are determined without actually considering the system design itself. While in the second step, a novel and simple methodology is developed which determines the sub-optimal capacities of distributed generation units along with their actual locations in the distribution system. To determine the optimal locations of distributed generation units, we defined a novel factor, losses-voltage-factor (LVF), which ensures reduced losses and better voltage quality. The proposed methodology gives a robust design that not only results in reduced losses and better voltage quality but also have higher reliability. The proposed technique is very practical, which can easily be applied for the planning and design of the practical active distribution networks.

Efficient Reactive Power Compensation Algorithm for Distribution Network

The use of automation and energy efficient equipment with electronic control would greatly improve industrial production. These new devices are more sensitive to supply voltage deviation and the characteristics of the power system that was previously ignored are now very important. Hence the benefits of distribution automation have been widely acknowledged in recent years. This paper proposes an efficient load flow solution technique extended to find optimum location for reactive power compensation and network reconfiguration for planning and day-to-day operation of distribution networks. This is required as a part of the distribution automation system (DAS) for taking various control and operation decisions. The method exploits the radial nature of the network and uses forward and backward propagation technique to calculate branch currents and node voltages. The proposed method has been tested to analyze several practical distribution networks of various voltage levels and also having high R/X ratio.

An offline penetration-free protection scheme for PV-dominated distribution systems

The rapid growth of small-scale photovoltaic (PV) units in distribution grids brings new challenges in the protection coordination of overcurrent relays. So far, various adaptive techniques have been addressed to solve DG units’ mis-coordination problem, however, the un-predictable penetration of installed PV units highlights the importance of a penetration-free methodology. This study presents a novel offline, quick, applicable and cheap solution which guarantees protection coordination for any penetration and location of PV units. To do so, first, under different PV penetration levels and locations, the conventional protection performance is studied to discover the worst mis-coordination cases. Next, according to relays standards, the characteristic curve of the back-up relay is modified such that it can maintain coordination in all worst cases. The proposed methodology is applied to a practical power distribution network equipped with numerous small-scale PV systems. The results successfully prove the effectiveness of the proposed method. Since this technique only applies some changes in characteristic curves of the back-up relays, lack of programmable relay or adaptive protection requirements cannot decrease its performance. This fact makes this method an attractive option for distribution systems with PV units.

Two‐stage heuristic methodology for optimal reconfiguration and Volt/VAr control in the operation of electrical distribution systems

The availability of smart grid technology makes it more feasible to use optimal reconfiguration and Volt/VAr control strategies for minimising electricity losses in distribution systems. This study presents a two-stage heuristic method to calculate the optimal impact of network reconfiguration and switching of already-installed capacitors over electrical distribution losses. The two-stage method searches first for a set of reduced feasible reconfigurations with less losses via a binary particle swarm optimisation and an optimal power flow algorithm, and then performs a Volt/VAr control for each feasible reconfiguration by switching capacitors, until reaching negligible changes in losses. Numerical examples and a test case considering a practical distribution network of the Taiwan Power Company demonstrate the effectiveness of the proposed method.

Optimal Allocation of Measurement Devices for Distribution State Estimation Using Multiobjective Hybrid PSO–Krill Herd Algorithm

This paper proposes a new multiobjective hybrid particle swarm optimization (PSO)–krill herd (KH) Pareto-based optimization algorithm to optimize number and location of the measurement devices for accurate state estimation (SE) in smart distribution networks. Three objectives are considered to be minimized: 1) the total cost; 2) the average relative percentage error (APE) of bus voltage magnitude; and 3) APE of bus voltage angle. As the objective functions are conflicting with respect to each other, a multiobjective Pareto-based nondominated sorting hybrid PSO–KH optimization algorithm is proposed. In this approach, the random variation in loads and the metrological error of the measurement devices are also taken into account. The proposed algorithm minimizes the cost and enhances the accuracy of the distribution state estimator for better monitoring and control of the system. Furthermore, the impacts of distributed generation on SE performance are also investigated. The feasibility of the proposed algorithm is demonstrated on IEEE 69-bus system and practical Indian 85-bus radial distribution network. The results obtained are compared with conventional KH algorithm and PSO, with well-known multiobjective nondominated sorting genetic algorithm and also with an existing technique based on dynamic programming method for validation.

A multiobjective approach for optimal placement and sizing of distributed generators and capacitors in distribution network

Both active and reactive power play important roles in power system transmission and distribution networks. While active power does the useful work, reactive power supports the voltage that necessitates control from system reliability aspect as deviation of voltage from nominal range may lead to inadvertent operation and premature failure of system components. Reactive power flow must also be controlled in the system to maximize the amount of real power that can be transferred across the power transmitting media. This paper proposes an approach to simultaneously minimize the real power loss and the net reactive power flow in the system when reinforced with distributed generators (DGs) and shunt capacitors (SCs). With the suggested method, the system performance, reliability and loading capacity can be increased by reduction of losses. A multiobjective evolutionary algorithm based on decomposition (MOEA/D) is adopted to select optimal sizes and locations of DGs and SCs in large scale distribution networks with objectives being minimizing system real and reactive power losses. MOEA/D is the process of decomposition of a multiobjective optimization problem into a number of scalar optimization subproblems and optimizing those concurrently. Case studies with standard IEEE 33-bus, 69-bus, 119-bus distribution networks and a practical 83-bus distribution network are performed. Output results of MOEA/D method are compared with similar past studies and notable improvement is observed.

Maximizing Available Delivery Capability of Unbalanced Distribution Networks for High Penetration of Distributed Generators

The network reconfiguration technique has been used to reduce network losses, balance transformer loading, and restore the power supply of the outage area in distribution networks. With a large number of uncontrollable distributed generators (DGs) added to distribution networks, the reconfiguration technique can be applied to enhance the available delivery capability (ADC) of distribution networks. This paper presents a two-stage methodology for determining optimal network topologies for improving the ADC of distribution networks for supporting more renewable energies. The first stage is a group-based binary particles swarm optimization (BPSO), which takes advantage of the BPSO's global capability while improving its slowness in finding optimal solutions. The second stage is a hybrid of greedy-based and domain-knowledge-based methods. When applied to multiple scenarios of DGs, the proposed methodology can effectively deal with the uncertainty of the DGs' outputs and time-varying loading conditions. As a byproduct, the optimal network topology found by the proposed methodology also decreases the network power losses due to the increase of the ADC of the distribution networks. The IEEE 123-bus test network and a practical 1001-node distribution network are used to verify the proposed methodology, and the numerical studies demonstrate the effectiveness of the proposed methodology in greatly increasing the ADC via optimal network reconfigurations.

Distributed generation allocation with on-load tap changer on radial distribution networks using adaptive genetic algorithm

This paper presents a distribution generation (DG) allocation strategy along with on-load tap changer (OLTC) for radial distribution networks using adaptive genetic algorithm (AGA). The optimal locations for DG units and OLTC, the optimal generation for the DG units, and the OLTC tap ratio are determined by minimizing a weighted objective function comprising of network power loss and maximum bus voltage deviation. A planning model incorporating DG and OLTC models is devised. Three different types of DG units, i.e., DG operating at lagging, leading, and unity power factor are used in the planning. Two new AGA variants based on adaptively varying crossover and mutation probabilities are proposed and used in this work as solution strategies and their performances are compared with some of the existing similar type of AGA variants using the results of multiple runs. The performances of the proposed AGA variants are found to be better in terms of quality of the solutions and consistency. The results also show that the combined operation of DG and OLTC reduces significant amount of power loss and bus voltage deviation. A direct performance comparison with some of the similar approaches shows that the solutions obtained with the proposed approach provide similar amount of power loss reduction and bus voltage improvement with lesser amount of DG penetration level. The approach is demonstrated on a 69-bus test distribution network and a 52-bus Indian practical distribution network.

A new under‐voltage load shedding scheme for islanded distribution system based on voltage stability indices

Under‐voltage load shedding (UVLS) is an important technique to maintain the voltage stability and frequency of a power system network. UVLS has been applied widely in transmission systems to avoid system blackouts. However, with increasing penetration of distributed generation such as photovoltaic (PV) systems, the application of UVLS becomes important for islanded distribution systems. Under this condition, the network does not have a frequency reference as when it is connected to the grid. In this condition, when the load demand exceeds the PV capacity, UVLS is the only option to stabilize the system by shedding the load based on the changes of the voltage magnitude. In this work, a new UVLS scheme based on voltage stability indices is proposed. Four voltage stability indices are used as indicators for load shedding. Based on the stability indices, the loads that have the highest tendency of voltage collapse shall be the first ones to be shed. The proposed scheme is tested on a practical distribution network energized by a grid, a mini hydro generator, and a PV system. The test results on various scenarios prove that the proposed method is able to restore the system stability. © 2017 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.

Approach to setting gateway reactive power control band for distribution networks with wind power

The integration of distributed wind farm leads to additional fluctuation of power flow in a high-voltage distribution network, which has become a major concern in automatic voltage control. This study proposes a slack optimal control method to determine the tolerance band of gateway reactive power (GRP) which takes into account voltage constraints, energy loss and excessive operations of control devices. Analysis of network characteristics shows that the GRP control band should be set according to both load levels and the outputs of distributed wind farm. Based on the analysis, slack optimal active loss difference strategy is proposed to set the slack optimal band of GRP. A slack optimal band matrix is defined and a partitioning intersection strategy is proposed to implement the differential setting of the slack optimal control band parameters of GRP. Simulation studies are conducted on a practical distribution network in China and the results show that the proposed method performs well in energy saving, control device regulating, voltage profile keeping, and reactive power balancing in a simple and practical way.

Effect of placement of droop based generators in distribution network on small signal stability margin and network loss

For a utility-connected system, issues related to small signal stability with Distributed Generators (DGs) are insignificant due to the presence of a very strong grid. Optimally placed sources in utility connected microgrid system may not be optimal/stable in islanded condition. Among others issues, small signal stability margin is on the fore. The present research studied the effect of location of droop-controlled DGs on small signal stability margin and network loss on a modified IEEE 13 bus system, an IEEE 33-bus distribution system and a practical 22-bus radial distribution network. A complete dynamic model of an islanded microgrid was developed. From stability analysis, the study reports that both location of DGs and choice of droop coefficient have a significant effect on small signal stability, transient response of the system and network losses. The trade-off associated with the network loss and stability margin is further investigated by identifying the Pareto fronts for modified IEEE 13 bus, IEEE 33 and practical 22-bus radial distribution network with application of Reference point based Non-dominated Sorting Genetic Algorithm (R-NSGA). Results were validated by time domain simulations using MATLAB.

Evaluating maximum photovoltaic integration in district distribution systems considering optimal inverter dispatch and cloud shading conditions

As photovoltaic (PV) integration increases in distribution systems, to investigate the maximum allowable PV integration capacity for a district distribution system becomes necessary in the planning phase, an optimisation model is thus proposed to evaluate the maximum PV integration capacity while guaranteeing the entire system operating constraints (e.g. network voltage magnitude) within reasonable ranges in this study. Meanwhile, optimal inverter dispatch is employed to further improve the PV integration by ensuring the optimal set-points of both active power and reactive power for the PV inverters. However, the intermittency of solar PV energy (e.g. due to passing clouds) may affect the PV generation in the district distribution network. To address this issue, the voltage magnitude constraints under the cloud shading conditions should be taken into account in the optimisation model, which can be formulated as a mixed integer non-linear non-convex programming. Furthermore, a sequential interior-point method is utilised to solve this problem. Case studies on the IEEE 33-bus, 69-bus distribution networks and two practical distribution networks in China demonstrate the effectiveness of the proposed method.