Medium Voltage Power Networks Research Articles

Ground Fault in Medium-Voltage Power Networks with an Isolated Neutral Point: Spectral and Wavelet Analysis of Selected Cases in an Example Industrial Network Modeled in the ATP-EMTP Package

The paper presents some case spectral analysis of zero-sequence voltages and currents in an example industrial power distribution network. The network layout is based on typical power delivery networks in underground coal mines. Ground fault simulations have been made using an ATP/EMTP program. Due to the high environmental risks, the reliability of the protection relay operation related to their selectivity plays an important role. This paper tries to find the reasons for nonselective operation and unnecessary tripping in extensive mine cable networks, particularly with large power sources of higher-order harmonics. It was found that in transient states—due to the decaying oscillations occurring in complex RLC circuits—the results of short time measurements of the criterion values for ground fault protective relays can be overestimated (particularly for small values of ground resistance) and lead to nonselective tripping of a healthy cable line. Therefore, it might be advisable to increase the integration time used for measuring rms values. Also, if there is a significant level of higher harmonics in the industrial network generated by high-power converters, it should be noted that the higher harmonics of the ground fault current and currents measured by ground fault protection relays assume much higher values, which may also cause nonselective tripping. In this case, it may be advisable to use higher harmonic filters in the measuring circuits and to select a sufficiently high sampling frequency in the digital protective relays.

Medium voltage outdoor compact substations’ earthing system evaluation based on quantified risk analysis

This paper investigates the ISO 31000 Risk Assessment process in evaluating the performance of existing Earthing Systems in Power Distribution Grids and designing new ones, while considering techno-economic criteria. The primary goal of this paper is to investigate Medium Voltage (MV) earthing systems under various scenarios in terms of soil structure, fault clearing time and earthing mat design, using appropriate software tools such as SES CDEGS and DIgSILENT PowerFactory. Furthermore, purpose of this paper is to emphasize the importance of Risk Assessment as a complementary method to the conventional one suggested in IEEE Std. 80 in assessing the likelihood of a fatal accident occurring in the vicinity of an MV electrical equipment in the event of a Short Circuit (SC) such as Single Line to Ground Fault (SL). Since there is no international standard for designing an effective earthing system in MV power networks, this paper serves as a valuable guide, identifying worst-case scenarios and highlighting critical safety parameters for earthing system design.

Analysis of the Influence of Load on the Value of Zero-Voltage Asymmetry in Medium-Voltage Networks Operating with Renewable Energy Sources

The phenomenon of voltage asymmetry is common in electricity networks and is the cause of many unfavourable phenomena occurring, such as an increase in power and energy losses, as well as the abnormal operation of certain loads. Voltage asymmetry can be caused by several different factors, which include the design of the transmission line (e.g., routing of conductors in relation to each other and the earth), the parameters of consumer circuits (e.g., unequal loading of individual phases), or the design of network equipment (e.g., generators or transformers). An asymmetrical condition also occurs during system disturbances such as single-phase short circuits or lightning. One of the many factors impinging on voltage asymmetry in the electrical network is also the effect of load, but in the case of medium voltage power networks, this effect is neglected in the literature. This paper presents and evaluates the influence of the power consumed by consumers on the values of zero-voltage asymmetry factor indices in medium voltage compensated power networks with the neutral point earthed by an inductance (Petersen coil) operating with wind and photovoltaic power plants.

On-Line Partial Discharge Localization in Power Cables Based on Electromagnetic Time Reversal Theory - Numerical Validation

This paper presents the numerical validation of a new on-line location method of partial discharge (PD) in cables of Medium Voltage (MV) power networks based on the electromagnetic time reversal (EMTR) theory. Because PD events are a symptom of cable insulation degradation, PDs localization is a key topic for fault prevention on power networks, increasing their reliability and resilience and to guaranteeing electricity security. A description of the proposed EMTR PD location method is given and its effectiveness and accuracy are analyzed in lines with both homogeneous and inhomogeneous power cables. The accuracy of PD location methods is strongly affected by the distortion of the PD pulses during their propagation on power lines, caused mainly by the skin effect. For this reason, the accuracy of the proposed location method is analyzed using a lossy model of power networks that is able to reproduce PD signals distortion. The model used, realized using the transmission line matrix (TLM) method, is also presented here and theoretically validated.

Special Issue on Modeling and Analysis of Interaction Between Grids and Grid-Connected Power Electronics Converters in Distribution Networks

Increasing penetration of grid-connected renewable energy systems and smart loads based on power electronics converters (such as solar inverters, wind turbines, and variable speed motor drive systems) has been increasing in low- and medium-voltage power networks. High penetration of power electronics converters has the potential to degrade the power quality of grids introducing strong resonances in grids, waveform distortion (e.g., harmonics), transients, or flicker, which pose severe risks to the distribution networks and connected equipment. Especially waveform distortion above and below 2 kHz, as well as fast transients, can cause failures in grid communication, degradation of equipment such as distribution transformers, and losses of energy, and can affect system protection. This further faces the community with challenges in the measurement of voltage and current as well as in maintaining the requirements of the electricity codes and contractual obligations between network operators and users. The automobile industry is going through an incredible transformation from internal combustion engines (ICEs) to all-electric vehicles (EVs). Some European countries have decided to ban all petrol-powered cars in the next 25–40 years. The fast-growing penetration of EVs charger will result in new challenges in distribution networks and connected loads. For example, grid impedance and its variation have a significant impact on grid-connected power converter behavior, interaction, stability, and harmonic emission and immunity. Thus, grid impedance modeling and estimation are required for existing and future analysis of distribution networks with a high number of grid-connected inverters. This Special Issue aimed to emphasize and document the latest research that addresses the abovementioned emerging challenges. In response to the call for articles, the total number of manuscripts received was 53, and 15 articles were accepted for publication. The submitted manuscripts came from 19 different countries, covering most of IEEE regions, i.e., Australia, China, Hong Kong, India, Bangladesh, Iran, Turkey, Finland, Sweden, Denmark, Belgium, France, Germany, the U.K., the United States, Canada, South Africa, Mexico, and Chile. From the accepted articles, all 15 articles address issues directly relevant to this Special Topic. Broadly, the articles can be categorized as follows: control and stability, assessment of frequency-dependent impedance, power quality aspects in distribution networks, and grid distortion impacts on power converters.

Statistical Assessment of Electric Shock Hazard in MV Electrical Power Substations Supplied from Networks with Non-Effectively Earthed Neutral Point

This paper focuses on the evaluation of the electric shock hazard accompanying earth faults in a non-effectively earthed medium-voltage (MV) electrical power network. This hazard depends on the duration and value of the fault current. While the fault current depends on several factors, the most important is the neutral point earthing method. The value of the fault current affects the earthing-electrode voltage value, being the basis for the assessment of electric shock hazard in MV/LV substations. The earthing-electrode voltage is also influenced by the resistance of the substation earthing, which in practice is random. Therefore, an original statistical evaluation method for assessing the electric shock hazard has been developed and presented in this paper. It is based on a statistical model of the MV/LV substation earthing resistance, worked out on the basis of experiments and measurements in real electrical power networks. This method can be used for the determination of statistical distributions of earthing-conductor voltages in real electrical power networks, and on this basis, the MV/LV substations with rates of electric shock hazard that are too high can be indicated. This also makes it possible to determine the longest permissible fault current interruption times or the highest permissible earthing resistances for specific substations. This method is also applicable when selecting the neutral point earthing method. The developed method was used in all of the above proposed areas by performing calculations on a model of a real 15 kV network with the neutral point earthed by a resistor. This analysis proves the practicability of the method both at the stage of designing power networks, and when a change in the neutral point earthing method in existing networks is being considered. Particularly valuable is also the statistical model of an earthing electrode resistance, developed on the basis of measurements in 2408 MV/LV substations, which may also be applicable in the future studies.

An Improved Hybrid Particle Swarm Optimization and Tabu Search Algorithm for Expansion Planning of Large Dimension Electric Distribution Network

Optimal expansion of medium-voltage power networks is a common issue in electrical distribution planning. Minimizing the total cost of the objective function with technical constraints make it a combinatorial problem which should be solved by powerful optimization algorithms. In this paper, a new improved hybrid Tabu search/particle swarm optimization algorithm is proposed to optimize the electric expansion planning. The proposed method is analyzed both mathematically and experimentally and it is applied to three different electric distribution networks as case studies. Numerical results and comparisons are presented and show the efficiency of the proposed algorithm. As a result, the proposed algorithm is more powerful than the other algorithms, especially in larger dimension networks.

Analysis and evaluation of dielectric parameters for design verification and calibration of a newly developed diagnostic system for MV power cables

For the purpose of a condition-based asset management, in-depth knowledge of the equipment condition is needed. In medium-voltage power networks, paper-insulated lead-covered (PILC) cables are to see as special critical assets. In order to evaluate the most appropriate diagnostic parameters and their characteristic values, multiple PILC cables have been artificially aged at power frequency during a long-lasting laboratory ageing experiment. Over this process, the relevant parameters like dissipation factor and partial discharge behaviour have been monitored and recorded and a fundamental database was built up. As a next step, field measurements of these values need to follow. Therefore, a mobile measurement system has been furtherly developed and integrated in a cable test van. In order to get improved diagnostic criteria the mobile test system facilitates, that the dielectric properties of the PILC cables are determined at 0.1 and 50 Hz test voltage frequency. Here, the development, the calibration and the adjustment of the measurement system as well as influences on field measurements will be described and discussed.

Design and implementation of partial feedback linearizing controller for grid-connected fuel cell systems

This paper presents an approach to design a nonlinear controller for grid-connected fuel cell (FC) systems in order to provide ancillary services to low or medium voltage power networks. In this paper, these ancillary services mainly refer to the delivery of both active and reactive power from the FC to the grid under different operating conditions. The controller is designed based on the dynamical model of a proton exchange membrane fuel cell (PEMFC) which is developed from the electrical equivalent circuit of a grid-connected PEMFC through a voltage source converter (VSC). Partial feedback linearization technique is then employed to obtain the control laws with an aim of regulating respective currents related to both active and reactive power through the switching action of VSCs. The stability of the internal dynamics of the PEMFC system is also analyzed in this paper as the proposed control scheme cannot be implemented for unstable internal dynamics. A rate limiter is used with the proposed partial feedback linearizing controller to ensure the suitability under the worst-case conditions as well as to prevent the over-modulation in the rate of change of power references. The applicability and performance of the proposed controller is justified on a simple system as well as on a 12-bus balanced test distribution system and the CIGRE low voltage test distribution network under different grid events. The simulation results show the superiority of the proposed controller as compared to the vector controller.

Oil-Filled MV/LV Power-Transformer Behavior in Narrow-Band Power-Line Communication Systems

The aim of this paper is to characterize oil-filled power transformers in the narrow-band power-line communication (PLC) frequency range of 50–150 kHz. A new model is proposed for such type of a transformer, in the aforementioned frequency range of interest, in order to study its influence on the signal transmission and to investigate the possibility that the PLC signal can cross the power transformer itself. The new model is simpler than those previously proposed in literature, and it can be used in the whole frequency range available for NB PLC according to the CENELEC EN 50065-1 A-band. In order to validate the model, simulation and measurement results are compared for four oil-filled power transformers with different rated power values. A complete PLC system in the case of a medium-voltage (MV) power network is also simulated, in order to perform a preliminary evaluation of the influence of power transformers on the communication system. The MV overhead power line, the oil-filled power transformers, the MV signal coupling network, and two ST7540 frequency shift keying (FSK) power-line transceivers are included in the simulated system. The performances of the complete PLC communication system are evaluated in the case of both MV to low voltage (LV) and LV to MV transmissions. The analysis is carried out by evaluating the signal attenuation, which is computed as the ratio between the received and transmitted voltage signals, in the case of different line lengths and carrier frequency values.

Synchronized Phasors Monitoring During the Islanding Maneuver of an Active Distribution Network

The paper describes the performances of a phasor measurement unit (PMU) prototype based on a synchrophasor estimation algorithm conceived for the monitoring of active distribution networks, as well as its experimental application during some intentional islanding and reconnection tests of an urban medium voltage power network. With respect to typical applications in transmission networks, the use of PMUs in distribution networks requires very low values of total vector error (TVE), which involves particular low values of phase errors of the synchrophasor estimates. These requirements are met by a specifically developed PMU, the characteristics and experimental characterization of which are illustrated in the paper. Three of these PMUs have been then used to monitor experimental tests carried out to assess the capability of a urban distribution network to operate autonomously when fed by a local 80 MW combined-cycle power plant. The information provided by the installed PMUs significantly facilitate the operator maneuvers and appear to be useful for the development of an improved control and management system of the active distribution network.

Agent-based coordination techniques for matching supply and demand in energy networks

There is a lot of effort directed toward realizing the power network of the future. The future power network is expected to depend on a large number of renewable energy resources connected directly to the low and medium voltage power network. Demand side management (DSM) is an important element for reaching this goal. Demand side management is concerned with actions that reshape the energy consumption pattern for the end-users during peak time or during times when consumption exceeds generation. To control and coordinate the operation of the network with these new characteristics efficiently and with a certain degree of reliability, intelligent coordination approaches are required. The intermittency and variability of renewable energy resources, the large number of distributed consumption and generation units, and the local operation constraints for these units should be considered when designing the appropriate control and coordination systems. Agent-based systems are proposed and being implemented to control and coordinate the operation of different entities inside the future power network. By using agent technology we can satisfy the local constraints of different entities while simultaneously satisfying global goals by using coordination approaches for coordinating and adapting the agents' actions. Approaches developed rely on two control types for altering demand: price-based control (indirect control) and direct-load control. In this survey article we review previously and currently developed agent-based coordination approaches that depend on direct-load control for matching supply and demand in the power network of the future.

Statistical Analysis and Modeling of Noise on 10-kV Medium-Voltage Power Lines

This paper presents a statistical method to model the noise on 10-kV medium-voltage power networks. The cumulative distribution function of the Nakagami-m distribution is calculated and compared with the corresponding empirical cumulative probability distribution in time domain for both the total noise power and the noise power at certain frequencies. It shows that the Nakagami-m distribution offers an excellent fitting to the measured data. The noise spectrum is also analyzed and the exponential and Gaussian functions are used to fit the spectrum. All of the parameters of these fitting functions are random variables which follow the Nakagami-m distribution.

Research of Impedance Characteristics for Medium-Voltage Power Networks

The general method to investigate the impedance characteristics with a network analyzer is discussed. The common properties of the impedance are studied based on two-months observation of two 10-kV medium-voltage power networks. Meanwhile, the difference of the impedance measured at the headside and tailside of a feeder, the impedance characteristic impacted by the operating mode of the power system, the time variance of the impedance, and the characteristic obtained from a different coupling mode are also studied carefully. The results show that the characteristic impedance is in correlation with the power system structure and its configurations, and its value is about several tenths of ohms. The input impedance varies scarcely with the time. The capacitor group connected to the bus has no influence on the input impedance. And the input impedances in common mode and differential mode are both symmetric. The impedance value in differential-mode coupling is higher than that in common-mode coupling

VOLTAGE REGULATION IN NETWORKED ELECTRICAL POWER SYSTEMS FOR DISTRIBUTED GENERATION: A CONSTRAINED SUPERVISORY APPROACH

In this paper a constrained control strategy based on predictive control ideas is used to regulate the voltage in electrical Medium Voltage (MV) power networks. The aim is to regulate the voltage at certain nodes of the network in the presence of Distributed Generation (DG) units and despite of various changing loads acting as exogenous disturbances. The voltage regulation problem is rephrased as a constrained tracking control problem by imposing that the voltages at certain loads have minimum deviation from their nominal values. Simulations studies show the effectiveness of the proposed approach.

Detection of broken conductor with ground contact faults in medium voltage power networks

A major problem concerning the operation of medium voltage power networks refers to the neutral-point grounding system. Several technical solutions are in use, none of them being completely satisfactory. One criterion to appreciate the efficiency of a grounding system is the ability of the network?s protective system to accurately detect various types of faults such as broken conductor with ground contact faults. This type of fault make a relatively important figure (3 to 10 %) in the total faults statistics for a medium voltage network. In order to detect such faults, the sensed variables are the zero-sequence voltage and the neutral-grounding current. In this paper an analytical and computational analysis is performed to see how various fault and network parameters affect these variables, assuming different grounding systems. Measurements made in two real medium voltage networks show good agreement with the theoretical results.

An improved branch-exchange algorithm for large-scale distribution network planning

Design of optimal layout for medium-voltage power networks is a common issue in electrical distribution planning. Technical constraints (radial structure, voltage drops, and equipment capacity), and reliability limits must be fulfilled. The function for minimizing includes investments, power losses, and quality of supply costs. The authors present in this paper an improved algorithm based on a branch-exchange technique to solve large-scale problems. A heuristic algorithm for solving a Euclidean Steiner problem is used to improve the network by including transshipment nodes.

An Improved Branch Exchange Algorithm for Large Scale Distribution Network Planning

Design of optimal layout for medium-voltage power networks is a common issue in electrical distribution planning. Technical constraints (radial structure, voltage drops, and equipment capacity), and reliability limits must be fulfilled. The function for minimizing includes investments, power losses, and quality of supply costs. The authors present in this paper an improved algorithm based on a branch-exchange technique to solve large-scale problems. A heuristic algorithm for solving a Euclidean Steiner problem is used to improve the network by including transshipment nodes.

A new efficient procedure for the transient analysis of dissipative power networks with nonlinear loads

A new procedure, based on a combined frequency- and time-domain approach, is presented for the transient analysis of dissipative power networks with nonlinear loads described by monotone characteristics. Special care is paid to the solution of the power network transient equations in case the line-sections are closed by protective devices such as varistors. Networks excited by transient internal sources or external electromagnetic fields are analysed. A simple procedure is implemented to include the simulation of coupling phenomena in the Electromagnetic Transient Program (EMTP). Applications are carried out to predict transient voltages and currents in medium-voltage power networks.