Medium Voltage Research Articles

Overvoltages caused by direct lightning strokes to a hybrid overhead line

This paper presents an analysis of the overvoltages caused by direct lightning strokes to a hybrid overhead line with 138 kV, 13.8 kV, and 220 V circuits. The primary focus is on medium voltage line overvoltages and their dependence on key parameters, including ground impedance, soil resistivity, stroke current front time, conductor characteristics, lightning channel impedance, insulator capacitance, conductors' sags, and tower surge impedance. Using specific models for the transmission tower and high-voltage insulators, validated through laboratory tests and EMTP simulations, critical currents are determined for varying soil resistivity and impulse impedances. The analysis confirms the line robust performance against stroke-induced backflashovers.

Voltage/load Control Strategy for Smart Distribution Grids with a High Penetration of Renewable Energy Sources

The exponential increase of renewable energy sources (RESs) penetration in the power grid introduces the of voltage instability when RESs are integrated into medium and low voltage networks. This paper proposes a state-machine based voltage/load control strategy for different operating scenarios in a system comprised of a large RES power plant connected to a medium voltage bus at 11kV, and small RES systems scattered across low voltage networks at 415V. Three modes of operation are summarized, in which the appropriate action is taken according to the availability of RES power generation in the network and the loading condition of this network. Efficient power sharing is achieved via the utilization of a communication network deployed between the RES systems. The communication protocol of the network is represented in four messages adopted by the RES systems integrated into the grid. The effectiveness of the proposed voltage/load control strategy to surpass overvoltage cases caused by over-generation of RES power is illustrated through simulations. The proposed strategy can be adopted by either agreement between a utility and RES owners, or by making it the basis for a standard for RES installation.

Characterising ecological resource utilization by the endangered Egyptian vulture in Oman to better manage and mitigate electrocution risk

Electrocution is a threat to birds and can undermine their conservation status. We tracked 15 globally endangered Egyptian vultures (Neophron percnopterus) in Oman and used Resource Selection Functions (RSFs) to identify habitats important to vultures and locate places where electrocution risk is high. During daytime vultures selected rugged areas with low vegetative cover, and habitats near roads, waste disposal sites, and both high (>50 kV, large pylons) and medium voltage electricity infrastructure. At night vultures also selected rugged habitats with scarce vegetation. At night high voltage infrastructure was selected, especially in areas of relatively low ruggedness at higher elevations; medium voltage infrastructure was avoided at night. The pattern of infrastructure use suggested that vultures were using large pylons for night-time roosting, but used both large pylons and smaller poles for perching during the day. 54.2 % of 11–33 kV powerlines in the study area were within the top six habitat suitability categories, which also contained 87.3 % of 34,778 vulture daytime test locations. Retrofitting powerlines overlapping the highest suitability zones would have the largest immediate impact. Areas surrounding landfills were also important: 10.34 % of all test locations were located within 2 km of landfills, which contained only 14.5 km of 11–33 kV lines. Targeting these areas would provide high impact at low cost. These maps are particularly useful in Oman because it is a stronghold for resident endangered large birds (i.e., Egyptian vulture, lappet-faced vulture Torgos tracheliotos) and an important winter destination for endangered steppe eagles (Aquila nipalensis).

Losses allocation due to penetration of DG and self-consumption operation in distribution systems. Case: PV Solar Energy

This paper deals the losses allocation in distribution systems with distributed generation (DG). From an analysis based on marginalist theory for costs, losses are assigned simultaneously to generators and consumers of electricity. Taking into account, measurement samplings each 15 minutes during a typical day, it is presented an allocation proceeding of losses along the time, managing profiles of demand and generation by voltage levels. The allocation coefficients are calculated from series of power balance with shared self-consumption and power surpluses towards neighboring nodes. It was simulated through a quasi-static modeling for a network of Medium Voltage and Low Voltage, for different levels of penetration PV Solar. It is presented as an alternative for efficient allocation of losses using the Colombian case as study reference.

Ferroresonance phenomena in power systems

It is a known fact that the world's dependence on electrical energy is increasing day by day. The fact that electrical energy has some advantages over other types of energy, the increasing world population and changing living conditions can be considered among the reasons for this increase. This increase in dependence on electrical energy can be met by expanding existing facilities and establishing new facilities. In these studies, the transmission of electrical energy from the place where it is produced to the place where it is consumed poses important problems as the system expands. Some work is required to eliminate these technical and economic problems and to ensure stable operation of the system. These are problems that were not considered beforehand as the system grew. One of these problems is the ferroresonance phenomenon that occurs at the fundamental frequency due to the nonlinear magnetization characteristics of transformers. Ferroresonance is a resonance phenomenon that occurs in an electrical circuit containing an iron-core self-coil as a nonlinear element. This resonance manifests itself with sudden oscillations in the output size of the system caused by a small change in the amplitude or frequency of a magnitude applied to the input of the system. High voltage transmission lines are double-circuit lines that share the same pole. In special cases such as maintenance or malfunction of such a transmission system, one of the lines can be disconnected from the system and continue energy transmission with the other line. In the high voltage transmission line, the system may oscillate due to the stored energy in the magnetic field of the transformer inductance on the disabled line and the electric field of the line capacity. This oscillation disappears after a certain period of time due to iron losses and other circuit losses. When these losses are covered by the other transmission line, permanent oscillations and over voltages may occur at the transformer terminals. All these events are the negative effects of ferroresonance on transmission systems. In the energy transmission system, ferroresonance phenomenon occurs. Over voltages may occur in case of idle power transformers in star point/isolated networks. In medium voltage networks, ferroresonance occurs due to the melting of a fuse, the opening and closing of voltage transformers with one pole, and the dissymmetry that occurs as a result of a connection process. These negative effects caused by ferroresonance in the energy transmission power system are discussed in detail within the subject of this study.

Review of methods for modeling and control of cyber-physical systems in multi-energy microgrids

The article analyzes the development of methods for modeling and control of multi-energy microgrids through cyber-physical systems. We used the methods of literature review and meta-analysis based on publications from international databases Scopus and Web of Science, Russian database eLibrary, digital platform IEEEXplore et al. According to the analysis, Smart Grid implementation drives the development of cyber-physical systems. As summarized in this study, control interfaces, data transmission channels, and remote debugging ports are vulnerable parts of IoT devices that can possibly be attacked by intruders. A review of the recent publications in this field finds multi-agent technologies to be an effective approach not only for the operational control of multi-energy microgrid modes, but also for the construction of its reliable information network at the level of medium and low voltage systems. In the field of distributed energy systems, literature review of information technology indicates that the more capabilities are added to receive and process various kinds of information (transaction data, mode parameters, status of controllers, etc.) from external sources, the more vulnerable a multi-energy microgrid is to any cyber threats. Modern mathematical methods such as artificial intelligence, dynamic optimization, and multi-agent approaches should be used to effectively solve the problem of load distribution between different energy sources with cost minimization.

Distribution network planning method: Integration of a recurrent neural network model for the prediction of scenarios

The integration of new types of consumers and prosumers into distribution networks presents significant challenges for network planners, especially given the uncertainties of future expansions. This paper introduces a method for distribution network planning that integrates a Long-Short Term Memory (LSTM) model with a confidence interval threshold for long-term scenario predictions. The proposed forecasting model considers datasets collected from monitoring systems which include aggregated demand and self-consumption from photovoltaic sources. The use of the LSTM model enables a more accurate prediction of expansions, as it captures nonlinear patterns in time series data while taking into account the inherent characteristics of non-stationary time series data. The proposed method was tested on an existing radial medium voltage distribution network in Spain, taking into account the operational thermal limits for lines and substation time series measurements. The proposed planning approach also considers asset costs, active planning solutions, and time-series load flow analysis. The results obtained indicate that the use of time-based forecasts from aggregated generation and demand is a more realistic solution for flexible planning solutions when they are compared to traditional strategies.

Estimation of uncertainty in computation of importance factors and probability of undesirable events for safety-critical multi-unit systems, based on Binary Decisions Diagrams

This paper addresses current needs of the Multi-Unit Probabilistic Safety Assessment (MUPSA) methodology, which consists of a series of eight essential steps for conducting Probabilistic Safety Assessment (PSA) in the context of a multi-unit Nuclear Power Plant (NPP), co-located on a single site. The focus of this study revolves around one of the steps of the MUPSA methodology, which includes the examination of the importance factors and uncertainty analysis. Emphasizing the role of this step within MUPSA, the paper highlights the hypothesis and estimations made, along with the determination of system importance factors and failure frequency. To illustrate the practical application of these concepts, the paper presents three illustrative examples. The first example involves a 3-terminal reliability network, while the second example features an illustrative calculation applied to a system designed for potential use in a real NPP, compared to a Monte-Carlo simulation. Moreover, the second example involves the evaluation of a medium voltage distribution system (MVDS) designed to meet the auxiliary supply requirements of a nuclear power plant. This analysis is performed using a binary decision diagram (BDD), and the results derived from the BDD evaluation are then verified with those obtained through a Monte Carlo simulation for a comprehensive comparative analysis. Finally, an uncertainty analysis is achieved using a BDD reliability model, using the results of fault trees developed by National Company SN Nuclearelectrica S.A. (SNN), Department of Nuclear Safety, in the context of the Coordinated Research Project (CRP- I31031).

Hybrid AC/DC architecture in the CE.D.E.R.-CIEMAT microgrid: demonstration of the TIGON project.

This article presents the demonstrative development of the Towards Intelligent DC-based hybrid Grids Optimizing the Network performance (TIGON) project at the Centre for the Development of Renewable Energy - Centre for Energy, Environmental and Technological Research (CE.D.E.R.-CIEMAT), as well as the established objectives to be achieved with the implementation of a microgrid with smart grid architecture based on direct current (DC) and integrated into the current energy system. This type of architecture is proposed as a future solution to reduce energy losses caused by DC-alternating current (AC) conversions, increasing the overall performance and profitability of hybrid grids. All this without forgetting to ensure the supply, stability and reliability of the system with the development of all the necessary equipment and protections to make this approach a reality. The microgrid design and process of implementation start from a transformation centre, from which the medium voltage direct current (MVDC) grid will be created by the Solid State Transformer (SST). In the MVDC grid, we will find a bank of lead-acid batteries and other essential equipment in the microgrid, a DC/DC converter that will create the low voltage direct current (LVDC) grid. On the LVDC side, several branches have been designed to connect the rest of the systems; generation (mini-wind and photovoltaic), storage (LFP batteries) and loads (AC and DC loads). Each of the equipment will have a connection to the DC grid through converters made exclusively for this equipment and connexion to the AC grid, which will allow us to obtain all the necessary data to carry out the required studies to achieve the established objectives of the project.

Modeling and experimental validation of a proposed distance protection system for MVDC transmission lines

Protecting direct current transmission lines (DC-TLs) against faults is challenging and crucial, especially when locating faults. This research aims to develop a novel distance protection approach for Medium voltage direct current transmission lines (MVDC-TLs). Various faults in the computerized experiments and practical models have been applied to the proposed system in the AC network and DC transmission lines to evaluate the performance of the proposed method. The proposed protection system employs the wavelet transforms (WT) and ANFIS as detailed coefficients ‘feature extracting’ tools and classifiers, respectively. The mechanism operation of the proposed method is composed of three stages. Firstly, it extracts the features of the faults by WT. Hence, the ANFIS training process uses such features to identify and classify the faults. The third stage utilizes the ratio value between the fault current and the healthy operation current as an input of another ANFIS model to locate faults on the DC TL. The performance of the proposed protection method has been tested and evaluated for MVDC and MVAC networks. Also, the research executes both practical and simulation experiments utilizing different faults. It examines various fault locations on the entire MVDC line. The mechanism measures the current signals on both sides of the DC-TL. The paper built the MVDC-TLs model and the different cases using the software PSCAD/EMTDC and the proposed method via MATLAB. The study tested faults along the TL in the presence of the AWGN noise. The research utilizes practical experiments to verify the accuracy of the proposer. The accuracy of the proposed fault location is 99.923 %. Based on the results of this research paper and comparison with the recent related research works, the proposed method is more effective and efficient than the previous methods in detecting, classifying, and locating system faults.

Microstructure and arc erosion behavior of WC/CuCr30 composites based on nano-Cr precipitation

CuCr alloys with high contact performance for medium and high voltage vacuum circuit breakers are becoming increasingly urgent. In this work, WC/CuCr30 composites were prepared by SPS process, and nanometer-sized precipitated Cr phases were obtained by subsequent heat treatment. The microstructure and arc erosion behavior were investigated. The results show that nano-Cr phase precipitated in copper matrix can effectively improve the interfacial bonding strength between the Cu matrix and WC particles, and part of the precipitated nano-Cr phase is combined with the C element in WC to form nano-Cr23C6. Both nanophases can improve the resistance to dislocation and sub-grain boundary movement in the deformation process of WC/CuCr30 composite, thus improving the hardness of the copper matrix with a slight decrease in electrical conductivity. The results of electrical contact show that the addition of WC particles and nano-Cr precipitates can not only extend contact life of CuCr material, but also help to disperse the arc to avoid concentrated erosion. The presence of Cr23C6 phase around WC particles effectively improves the interfacial bonding between Cu phase and WC phase and reduces the probability of pore existence at the interface, which is beneficial to vacuum breaking performance.

The use of PSS2A system stabilisers to damp electromechanical swings in medium voltage networks with distributed energy sources

The use of PSS2A system stabilisers to damp electromechanical swings in medium voltage networks with distributed energy sources

A new method for estimating fault location in radial medium voltage distribution network only using measurements at feeder beginning

A new method for estimating fault location in radial medium voltage distribution network only using measurements at feeder beginning

Parallel Operation for 11 kV Ring System with Normally Open Point at Distribution Networks in AADC

Currently, Al Ain Distribution Company (AADC) 11 kV distribution circuits are electrically split by Normal Open Points (NOP)s. NOPs can be closed for load restoration in case of planned or unplanned outages. Also, changing the NOP locations helps in load shifting to achieve network optimization between adjacently located 33/11 kV primary substations. This paper discusses a new operation mode in medium voltage distribution power network of AADC at Al Ain area in the United Arab Emirates by introducing closed loops of parallel 11 kV feeders from different busbars or substations. This operation mode is intended to end the continuous debate of optimal NOP locations in addition to expected reduction in the required time for load restoration in case of failures at distribution (11/0.4 kV) substations. A detailed case study is performed to explore the achieved benefits and enhancements on load balancing, voltage profile, and power factor in medium voltage networks. On the other hand, the impact on load flow, protection scheme, and loading violation are evaluated. Furthermore, the proposed model is in line with the regulations of the Department of Energy (DoE), regulatory body in Abu Dhabi Emirate, and should fully comply with Electricity Distribution Code (EDC) of Security of Supply Standards. Accordingly, while the current used operation mode is still valid, it is found that closed loops arrangement can be only allowed for rings from same 33/11 kV substations feeding from same source with considerable protection limitations. Moreover, it was concluded that closed loops from different 33/11 kV Substations will result in load flow problems.

Low Frequency Versus High Frequency PWM in Medium Voltage, High Power, Higher Level Inverters: THD, Harmonic Filtering, and Efficiency Comparison

Low Frequency Versus High Frequency PWM in Medium Voltage, High Power, Higher Level Inverters: THD, Harmonic Filtering, and Efficiency Comparison

A Power Self-Balanced High Step-Down Medium Voltage DC Converter With Expandable Soft-Switching Range

This paper proposed a power self-balanced high step-down medium voltage dc converter with expandable soft-switching range. The proposed one features the input voltage self-balance of each submodule and the current self-balance of each MOSFET, due to the sub-modules transmitting the same power through LC-coupled branches. The LC-coupled branch provides multiple direct AC power paths from medium voltage input to load, eliminating the losses caused by conventional multi-stage transmission. Each submodule of the proposed converter is a soft-switching unit that uses an auxiliary inductor to achieve soft switching, which reduces the resonant current and facilitates the expansion of the number of modules, without redesigning the transformer, instead of the conventional resonant converter that reduces the transformer magnetizing inductance to achieve soft-switching. Moreover, an auxiliary power supply scheme for the proposed converter is provided, which is used to solve the problem of difficult startup of the medium voltage DC converter, where the negative impedance characteristics of the series-connected auxiliary power supply are suppressed without the need for lossy voltage sharing resistors. A prototype with 1kV dc input and 100V/1kW output is built to verify the feasibility of the proposed topology.

Thermal Control of Quasi-2-Level Super-Switch by Power Routing

The requirement of power converters for Medium Voltage (MV) and High Voltage (HV) applications accelerates faster than the development of semiconductor switches capable of handling such voltage levels. For that reason, it is common to use the series connection of semiconductors with auxiliary snubbers. In contrast, an increased number of semiconductors is more likely to cause reliability problems due to increased redundancy, unequal thermal conditions, or different remaining lifetime. On the other hand, the quasi-2-level super-switch (Q2L SS) yields increased flexibility, enabling precise readjustments of the voltage and switching losses of individual devices of the converter due to the utilization of small capacitors. The power routing capability of the Q2L SS to redistribute the switching losses from the overheated semiconductors is presented in this paper. The influence of power routing on performance of Q2L SS is investigated with the Double Fourier analysis and experimental results.

New single‐stage bidirectional three‐phase ac‐dc solid‐state transformer

AbstractHigh‐power applications with low‐voltage (LV) dc loads, for example, fast charging stations for electric vehicles (EVs), are typically supplied from the medium‐voltage (MV) grid. Aiming for low volume, MVac‐LVdc solid‐state transformers (SSTs) that provide galvanic separation with medium‐frequency transformers (MFTs) are thus considered, which are conventionally realized as two‐stage systems that consist of an ac‐dc power‐factor‐correction (PFC) rectifier and an isolated dc‐dc converter. This letter extends a new single‐stage isolated bidirectional PFC rectifier concept to MV levels, resulting in an SST that performs MVac‐LVdc conversion in a single converter stage and, unlike many modular SST concepts, employs only a single MFT. The SST's primary side operates modular‐multilevel‐converter (MMC) bridge legs, whose input voltages contain large ac components, in the quasi‐two‐level mode to minimize the cell capacitors. The secondary side employs a standard LV three‐phase rectifier, and a dual‐active‐bridge‐(DAB)‐like modulation strategy allows power flow regulation and ensures sinusoidal grid currents. The concept is explained and validated using detailed circuit simulations of an exemplary 1‐MW system operating between a 10‐kV three‐phase grid and an 800‐V dc output. The component stresses are evaluated, and an efficiency of 98.1% and a power density of up to 0.6 kW/dm3 are estimated.

Maximum Reactive Power Generation Method Based on Limitation of Output Capacity for Star-Connected Cascaded H-Bridge STATCOM Under Voltage Sag

Star-connected cascaded H-bridge (SCHB) STATCOM is being widely used in medium and high voltage to support grid by injecting positive and negative sequence current. Under balanced conditions, the rated reactive power is usually utilized to calculate the required reactive current reference based on instantaneous power theory. However, under grid voltage sag, the rated reactive power changes due to the introduced negative sequence current and extra zero sequence voltage for cluster voltage balancing, which may increase overcurrent and overmodulation risk. Therefore, how to derive the maximum compensable reactive power is required for SCHB STATCOM under the grid voltage sag. In this paper, the quantitative relationship between the imbalanced degree of grid voltages and reactive power reference is fully explored. On this basis, the maximum reactive power is calculated under voltage sags by considering current and voltage limitation. In this calculation, a new geometrical analysis method is proposed, which makes the calculation speed faster for online calculation. Therefore, a new grid support control strategy is proposed for the SCHB STATCOM under grid voltage sag, which can effectively support the grid with the maximum output reactive power. Finally, the proposed method is verified by experimental results on a 400V/7.5kVar SCHB STATCOM prototype.

PROTECTION COORDINATION OF PHOTOVOLTAIC POWER PLANT IN THE TIME DOMAIN

The safety of workers and equipment in the power grid requires the shutdown of power plants in case of maintenance or malfunction. The shutdown relies on protective devices that must be properly coordinated to isolate only the part of the grid affected by the malfunction. Using the DIgSILENT PowerFactory software, an analysis was conducted on a model of an accurate real grid integrating a photovoltaic power plant with a capacity of 390 kW. Protection coordination testing was carried out for three-phase, two-phase, and single-phase short circuit currents at arbitrarily selected locations at medium and low voltage levels. Protection in the time domain is coordinated on lines and busbars to determine the speed and selectivity of protective devices. The analysis results indicate that adequately adjusting the three-phase short circuit at the main transformer output 110/10 kV with an impedance of 0 Ω and an allowed protection operation time of up to two seconds can be correctly addressed within 36.7 ms.