Medium Voltage Distribution Research Articles

Online Coordinated Voltage Control in Distribution Systems Subjected to Structural Changes and DG Availability

The responses of multiple distributed generation (DG) units and voltage regulating devices, such as tap changers and capacitor banks, for correcting the voltage may lead to operational conflicts and oscillatory transients, where distribution systems are subjected to network reconfiguration and availability of the DG units. Therefore, coordinated voltage control is required to minimize control interactions, while accounting for the impact of structural changes associated with the network. This paper proposes a strategy for coordinating the operation of multiple voltage regulating devices and DG units in medium voltage (MV) distribution systems, under structural changes and DG availability, for effective voltage control. The proposed strategy aids to minimize the operational conflicts by allowing voltage regulating devices to operate in accordance with the priority scheme designed based on the electrical-distance between voltage regulating devices and DG units, while maximizing the voltage support by the DG units. The proposed coordination scheme is designed to enact with an aid of a substation centered distribution management system for online voltage control. The control actions of proposed coordination strategy are tested on a MV distribution system, derived from the state of New South Wales, Australia, through simulations, and results are reported.

Design of 1.15MWp Distributed Photovoltaic Power Generation Scheme

The design scheme for PV system connecting the grid is provided, considering the reactive power compensation, harmonic and lightning effects. Solar power generated connects into the nearby medium and low voltage distribution networks by the inverter. The capacity of solar power total is 1.15MWp. The electricity is generated for personal use; the remaining is connected to the grid system. The scheme is reasonable in design; the system is stable and reliable by the practical testing. It can be used in large range popularization and application.

Optymalizacja konfiguracji dla sieci rozdzielczych SN i nN

Reducing power losses in energy distribution is forced by mandatory law. Optimization of network reconfiguration can lead to reducing power losses by even a dozen or so percent. Two methods of network reconfiguration optimization are presented in the paper: the heuristic method and a method based on genetic algorithms. The presented solutions offer the opportunity to optimize medium voltage and low voltage distribution networks at the same time. Calculation results for real distribution network are presented. The presented results indicate a high efficiency of network reconfiguration optimization. DOI: 10.12736/issn.2300-3022.2014407

Impact of RDG Location on IDMT Overcurrent Relay Operation and Coordination in MV Distribution System

In recent years there has been an intensive effort to increase the participation of renewable sources of electricity in the fuel and energy balance of many countries. In particular, this relates to the power of wind farms attached to the power system at both the distribution network. However, in the presence of Renewable Dispersed Generation, (RDG) some problems in coordination of protection devices will occur, due to changes in fault current levels at different points. By installing RDG in power distribution networks, the fault current levels are changed and may lead to some miscoordination in IDMT Directional Over-Current Relay (DOCR). In this paper, a novel approach is presented to study the impact of RDG location (d RDG ) on IDMT characteristic curve of relay, fault current (I F ), operation time (T) for DOCR, and coordination time interval (CTI) between backup and primary relays and short circuit level index (ISC) in the presence there phase fault on medium voltage (MV) distribution network. This new approach has been implemented on the Algerian 10 kV distribution power system in Constantine.

Research and application of GIS-based medium-voltage distribution network comprehensive technical evaluation system

SUMMARY Comprehensive technical evaluation plays an important role during the course of plan, design, construction, and operation of the medium-voltage (MV) distribution system. Despite of being extensively researched, the evaluation methodology in the developing countries such as China is still not mature. First, this paper presents the comprehensive technical evaluation theory and methods, and then the hierarchy of evaluation indices is proposed for the MV distribution network. Second, the GIS-based comprehensive technical evaluation system was constructed in accordance with the evaluation methodology. Finally, this paper exemplifies how the distribution network evaluation is implemented systematically for a city district MV distribution network in China. Applications of the evaluation system in many cities of China have convinced the feasibility and practicability to assess MV distribution networks technically and comprehensively. Copyright © 2014 John Wiley & Sons, Ltd.

Challenges in Detection of High Impedance Faults on Broken Conductors in MV Lines

The weather conditions in Saudi Arabia are harsh and the inland areas are very hot, dry and sandy. Thousands of kilometers of medium voltage overhead lines (13.8 kV) are located in deep desert areas feeding loads that are located in remote desert areas. Such medium and low voltage distribution networks face difficulties regarding fault detection and localization. High Impedance Fault (HIF) resulting from a broken conductor in MV overhead lines represent the most challenging problem in such environments. Such condition may cause damage, fire or electric shock hazards resulting in life threatening situations. This study reports on a study of electrical characteristics of inland desert sand and typical characteristics of HIF fault when a MV line conductor breaks and touches such sand. It is shown that the arid desert sand has extremely high resistivity and it is difficult to detect the fault by using conventional protective devices. The study presents results of measurements and simulations and suggests that innovative approaches have to be employed to achieve the desired protection in such environments.

A Practical Directional Third Harmonic Hybrid Active Filter for Medium-Voltage Utility Applications

An increased level of harmonics due to the proliferation of single-phase nonlinear loads is raising serious concerns among utilities. Historically, passive filters have been proposed to reduce harmonics in medium-voltage (MV) utility applications. However, due to their limitations, utilities are turning their attention to alternative solutions. At the same time, active filters are prohibitively expensive and are unlikely to become a realistic solution in the near future. In this paper, a practical directional third harmonic hybrid active filter is proposed. A novel feature that adjusts the level of compensation provided by the filter based on the loading conditions of its passive components is introduced. Simulation and experimental results are presented. Issues related to utilizing existing VAr support capacitors for retrofit applications thereby achieving cost reduction and fail normal feature for increased reliability are addressed. Cost versus performance curves are developed using factual utility harmonic data. Finally, the impact of a distributed filtering solution based on the proposed filter is shown using a simplified MV distribution system.

Medium Voltage Smart Grid: Experimental Analysis of Secondary Substation Narrow Band Power Line Communication

This paper is focused on the possibility of employing narrow band power line communication (PLC) in medium voltage (MV) distribution networks. The topic is investigated by means of both simulations and experimental tests, which were carried out in the distribution network of Ustica Island. In detail, a two-way MV communication was tested between two secondary substations connected by an MV cable power line. Each substation has a by-pass connection at MV bus-bars and an MV/LV oil filled power transformer. In this paper, the MV and the PLC systems under test are described and the experimental tests are presented, which were carried out to evaluate the parameters of the simulation model. The aforesaid model is described and a comparison is shown between simulation and results of experiments, which were carried out in the presence of voltage net, i.e., 24 kV. Finally, the possibility is investigated of a reliable communication and the faster achievable bit-rate; the analysis is performed by means of further experimental results of several communication tests, which were carried out with different modulation techniques.

Application of Narrowband Power-Line Communication in Medium-Voltage Smart Distribution Grids

Narrowband power-line communication (PLC) over distribution networks is gaining the attention of researchers, as a communication medium in future power grids. This paper investigates the application of narrowband PLC in smart distribution grids, starting from a historical overview on the accomplished technological progress and continuing with a comparison of the advantages and drawbacks of PLC technology to other smart-grid communication solutions. The analysis shows that narrowband PLC applications are best suitable for medium-voltage (MV) networks, due to the vast and complex geographical extent. The channel and topological characteristics of MV distribution networks are examined for different operational states and configurations, since they are important for the optimal design and implementation of the PLC infrastructure. The analysis is also extended to an existing overhead MV distribution network with distributed-generation units. The obtained results and the proposed methodology are useful and comprehensive tools for the efficient implementation of PLC technology in future smart grids.

Study on Electricity-Theft Criterion Based on the Unchanged Line Resistance of the Distribution Network

The power, voltage and current will change along with the change of grid’s operation mode; however, the line resistance is unchanged. This paper analyzed the limitation and inadaptability of the existed methods used for anti-electricity-theft. And most of the criterions are based on the variable electrical quantities. In order to establish a criterion that can find out the position of electricity-theft accurately, it chose line resistance as a reference value, and calculated the equivalent resistances of each node and branch combining with the actual measurement system. To verify the validity of this criterion, a medium and low voltage distribution network was structured in the Matlab/Simulink simulation platform. The results, which are basically consistent with the actual situations, show that the criterion based on line resistance can well investigate electricity-theft, and also verify that the introduction of this method will bring great benefits to engineering applications.

Impact of MV Connected Microgrids on MV Distribution Planning

The planning and development of distribution networks with a substantial penetration of microgrids connected to the medium voltage (MV) network form the main themes of this paper. The impact of microgrids is assessed in terms of their effect on optimal network topology, losses, reliability, reserve connections, network upgrade and expansion savings. The earning base of the distribution system operator also comes under scrutiny. A suburban MV cable network is planned using a network planning algorithm developed by the authors, first with optimal routing for demand-only nodes and then with a 33% penetration of randomly located microgrids. The network is then expanded to meet the requirements of a future planning horizon, in order to compare the expanded and upgraded optimum MV network topology with and without microgrids. Apart from visually depicting the topological differences, the savings such microgrids can give to the long term distribution network investment and running costs are quantified in terms of the investment costs, loss costs and interruption costs. When networks are planned with optimal rather than full backup, the introduction of microgrids is shown to have a considerable saving impact on all cost components except the cost per unit power transfer in the distribution network.

The use of shield wires for reducing induced voltages from lightning electromagnetic fields

The mechanisms from which lightning overvoltages can be produced on a power line depend on the system voltage. In medium voltage (MV) overhead distribution systems, lightning transients can be originated from either direct or indirect strokes. The main methods that can be adopted to improve the line lightning performance concern the increase of the critical impulse flashover voltage (CFO) of the line structures, the installation of surge arresters, and the use of one or more shield wires. This paper deals with the evaluation of the effectiveness of shield wires in reducing the magnitudes of the surges induced by nearby strokes on MV distribution lines. Such effectiveness depends on the combination of several parameters such as the relative position of the shield wire with respect to the phase conductors, the grounding interval, the ground resistance, the stroke current steepness, and the relative position of the stroke channel with respect to the grounding points. Realistic situations corresponding to typical configurations of a rural distribution line with either an overhead ground wire or a neutral conductor are considered. The analysis is carried out based on both computer simulations and test results obtained from scale model experiments, under controlled conditions.

Power Factor Optimization of Distributed Generations in Distribution Networks Based on Improved Particle Swarm Optimization Method

The gradually extensive penetration of small-scale distributed renewable generators in existing medium-voltage power distribution networks highlights many technical challenges which call for urgent solutions from power utilities. This paper attempts to optimize the power factor of distributed generators (DGs) integrated in distribution networks and presents a novel algorithmic solution. With the aim of minimizing power loss whilst maintaining the node voltage, the problem is formulated with a mathematical model elaborating the DGs and a set of constraints in distribution networks and addressed through adopting an extended particle swarm optimization (PSO) approach. The suggested algorithm is assessed through numerical simulation experiments with the IEEE 33-bus system and the outcome shows that the optimization algorithm can effectively reduce the power loss and promote the node voltages across the overall distribution network.

Reconfiguration of large-scale distribution networks for planning studies

Medium voltage (MV) distribution networks are usually operated radially. However, its structure may be designed meshed, so that in case of a failure of an element, the load downstream can be supplied by adjacent feeders through reconfiguration of the network. Under this assumption, a reconfiguration tool is a key resource for the planning and the operation of MV distribution networks. This paper presents a heuristic reconfiguration algorithm of MV distribution networks. The algorithm determines the configuration that minimizes the Non-Delivered Power not only in case of the failure of a single branch (line, transformer) but also in case of a complete failure of a high voltage to medium voltage substation. The constraints considered are that the radial structure of the network must be maintained and that line thermal limits, transformer capacities and bus voltages must be within their admissible ranges. The results of the algorithm show up weak regions of the network where planning actions should be carried out. The main features of the algorithm are interpretability of the solution and low computation times while handling complex and large-scale MV distribution networks. The algorithm has been tested on several actual distribution networks. Results of the application of the algorithm to real scenarios of Madrid’s distribution network are provided.

Voltage Stability of Medium and Low Voltage Distribution Networks with Wind Generation Based on Hilbert-Huang Transform

Nowadays the voltage instability problems occur in the distribution grid due to the integration of more and more fluctuant distributed generators. This paper focuses on voltage stability of distribution networks with wind generation. A voltage stability index is presented, and calculated by an expanding Newton-Raphson power flow method in which the wind power generation nodes are modified according to the P-Q(V) character. In order to obtain the key components of fluctuant wind power, the Hilbert-Huang transform algorithm is utilized to reveal the inherent characteristics of wind power. The extrema extending method based on the mirror periodic is used in the empirical mode decomposition and can handle the end effects. According to the Hilbert spectrum and instantaneous energy of each intrinsic mode functions, the components with lower instantaneous frequency are selected to rebuild the wind power. The new rebuilt series consist of stationary and monotonic components which are smoother, and the rebuilt series can reflect the main fluctuant characteristic of the initial wind power data by comparing the index.

Maintenance of Three-phase Load Voltage during Single Phase Auto Reclosing in Medium Voltage Radial Distribution Lines

Most faults in medium voltage (MV) distribution lines are temporary line to ground (LG) faults. Three-phase auto reclosing (TPAR) is commonly used to remove this fault with temporary disconnection of all the phases. Multi-shot single-phase auto reclosing (SPAR) may also be used to remove the LG fault. But it produces highly unbalanced and low voltage across the load during the reclosure dead time. It is proposed to connect a zigzag winding grounding transformer at the load bus to maintain the 3-phase load voltage when one phase opens during the SPAR. With low value of grounding resistance the 3-phase voltage during the SPAR dead time becomes approximately balanced. Directional over current relays may be used for the protection. Analysis of a MV radial distribution system having a zigzag transformer connected to the remotest load bus is presented with the computation of voltages during the dead time of SPAR.

Modeling and Simulation of Voltage Source Converter–Medium-voltage DC System for Stability Analysis

This article presents the modeling and simulation of a simple two-terminal and a multi-terminal voltage source converter based medium-voltage DC system, representing a simplified shipboard distribution network, for angular stability analysis. The control modes considered for the voltage source converter include the voltage control mode and the active and reactive power control schemes. The impact of these control modes on maintaining the DC voltage and, subsequently, on the transient as well as small-signal stability of the medium-voltage DC systems has been examined with the help of simulation in MATLAB/Simulink® (The MathWorks, Natick, Massachusetts, USA), and the results have been compared with an alternative medium-voltage AC distribution scheme. The impact of adding a power system stabilizer to enhance the small-signal stability has also been examined in the case of the medium-voltage AC architecture. Results indicate that that generators with the type 0 exciter are unstable with the medium-voltage AC architecture and stable with the medium-voltage DC architecture; using a type 1 exciter in the medium-voltage DC system improved the damping of the system. Use of a power system stabilizer for a medium-voltage AC system improved the damping, but the improvement was more significant with the placement of an static var compensator, in agreement with the results of participation analysis.

On Human Life Risk-Assessment and Sensitive Ground Fault Protection in MV Distribution Networks

The minimum phase-to-ground fault current required to be sensed by protection systems in medium-voltage (MV) networks can be as low as 0.7 A in a few countries, leading to a lot of undesired relay trips and poor service quality to costumers. However, these settings raise the protection threshold above the minimum fault current that concerns network operators regarding human safety, although they cannot be practiced when grounding is distributed. The purpose of this paper is to present a risk assessment foundation to determine the required protection sensitivity to ensure human safety in MV distribution networks. The proposed approach is based on a biophysical model included in IEC standards, the consideration of current paths models for typical faults and Monte Carlo methods to deal with nonlinearity, and the many involved random variables. Downed conductors and line-to-concrete pole faults are investigated and sensitivity analysis performed to highlight some important determinants of the results.

Characterization of Ageing for MV Power Cables Using Low Frequency Tan δ Diagnostic Measurements

This paper describes very low frequency (VLF) tan delta experiments performed on field-aged and non-aged distribution medium voltage (MV) cable samples. The field aged samples constitute a uniform set of Cross-linked Polyethylene (XLPE) of 15 kV unjacketed cables removed from the same service area having experienced similar operating and ageing conditions. The non-aged samples are a diverse set of crosslinked polyethylene (XLPE) and water tree retardant cross-linked polyethylene (WTRXLPE) cables of 15 kV and 25 kV and Ethylene Propylene Rubber (EPR) cable of 25 kV. The experiments are designed to contribute in understanding, time, voltage and discharge time dependence of Tan delta diagnostic measurements at VLF of 0.1 Hz. Results help in clarifying issues that arise when characterizing MV cable insulation by Tan delta diagnostic measurements. The issues include time-on-test, voltage level as a diagnostic tool, diagnostic features, and reproducibility and repeatability of the measurements. The paper shows that higher insulation losses, non-linearity, hyteresis, and variation in voltage and time of Tan delta diagnostic measurements at VLF are indicators that can be used to properly characterize the insulation and enhance the diagnosis.

Critical Radius of Axial Magnetic-Field Contacts in Vacuum Interrupters

Vacuum circuit breakers (VCBs) are widely used to protect medium-voltage (MV) power distribution circuits. Since SF6 gas is specified as a global warming gas, VCBs are stepping into a higher voltage sector to protect power transmission circuits. Axial magnetic field (AMF) contacts are widely used in vacuum interrupters. In this paper, we propose a concept of critical radius of AMF contacts. When coil width and coil height are fixed, the axial magnetic flux density increases first with increasing contact diameter. Then, it reaches a maximum value. Thereafter, the axial magnetic flux density decreases. The contact radius corresponding to the maximum axial magnetic flux density is critical radius. The concept of critical radius is validated by coil-type AMF contacts and slot-type AMF contacts in MV vacuum interrupters with finite-element analysis. Critical radius is only related to contact geometry parameters and the current has no influence on critical radius. Critical radius increases with increasing contact gap, coil width, coil height, and thickness of the contact plate. In high-voltage AMF vacuum interrupters, axial magnetic flux density per kiloampere increases with increasing contact diameter since the critical radius is typically high with a high contact gap and it is in the rising branch. In an MV AMF vacuum interrupter, interrupting capacity could increase with an increase of the contact diameter with a different rising rate before and after the critical radius. This is expected to be experimentally validated.