Medium Voltage Electrical Networks Research Articles

OPTIMIZATION OF THE USE OF DISTRIBUTED ENERGY RESOURCES IN LOCAL ELECTRICAL POWER SYSTEMS ACCORDING TO THE CRITERION OF MINIMUM LOSS OF ELECTRICITY

The issue of optimizing the use of distributed energy resources in local electric power systems (LES) based on the criterion of minimum power loss is considered. It was determined that an important step in optimizing the use of distributed energy resources in the LES is the development of an energy router that allows devices that generate, store, and consume electricity to be combined into a single system at low voltage (without integration into medium voltage electrical networks). The energy router itself can be positioned as a basic device that ensures the operation of LES, the interaction of neighboring LES at the level of energy and information exchange, and the integration of specific LES to medium voltage distribution networks.
 It is shown that the basic component of the evaluation of the effective functioning of energy routers and the platform approach in the LES, in accordance with the requirements of the Energy Internet, is the analysis of the components of electricity losses, the influence on their level of various factors, as well as the formation of appropriate energy efficiency criteria and the assessment of the partial impact of LES elements. It is proposed to carry out a description of energy processes during the decomposition of Frize reactive power with the allocation of reactive and active current in the controlled intersections of the system. The decomposition of the QF power into components under the action of various causes (factors) of additional electricity losses in the LES, in particular, due to different modes of operation of generators and LES loads (voltage and current spectra), both during the period of system operation and for an arbitrary period of time, was considered. which is determined by technological factors of system operation.
 The use of Frize power made it possible to take into account the energy supply processes of LES loads on alternating and direct current from a single point of view, in particular, to assess the impact of a combination of various factors of the appearance of additional electricity losses; comparison of electricity losses for different time intervals through a selected controlled intersection, when the direction of the flow of electricity is constant or the direction of the flow of electricity changes during separate time intervals; analysis of electricity losses in direct and alternating current systems of LES or hybrid systems, where modified capacities are introduced, as integral estimates of the discrepancy, which determines the influence of distorting factors; assessment of the impact of changes in voltage and current in the intersection of LES of limited power on the components of additional losses; analysis of changes in load consumption of active power according to the first harmonic and higher harmonics as integral indicators.

OPPORTUNITY OF USING A MIXED NEUTRAL TREATMENT SOLUTION IN THE DISTRIBUTION ELECTRICAL NETWORKS OF THE REPUBLIC OF MOLDOVA

The optimal solution selection for the treatment of the neutral point in distribution networks (6-35 kV) holds significant practical importance as it directly or indirectly affects the continuity and reliability of supplying electrical energy to consumers, the behavior of medium-voltage electrical networks under single-phase fault conditions, and its impact on the quality of the power distribution service (duration and frequency of power interruptions). It also affects resulting overvoltages, adopted technical solutions, level of electrical security assurance, investments, and so forth. Developing a mathematical model for various methods of treating the neutral point in distribution networks allows for the calculation and analysis of these operating conditions, identifying the optimal one, thus avoiding experimental attempts that are limited by the advanced wear of equipment within the electrical networks of the Republic of Moldova. Based on the developed mathematical model, comparative calculations are performed regarding the feasibility of implementing the combined neutral regime.

МЕТОДИКА ВЫБОРА ПАРАМЕТРОВ ПАССИВНОГО ФИЛЬТРА ДЛЯ ПОВЫШЕНИЯ КАЧЕСТВА ЭЛЕКТРОЭНЕРГИИ В ЭЛЕКТРИЧЕСКИХ СЕТЯХ С АКТИВНЫМИ ВЫПРЯМИТЕЛЯМИ

The relevance Due to the wide spread of compact metallurgical plants, which use powerful drives with frequency converters with active rectifiers (FC-AR), an urgent task has become the fight against resonant phenomena in medium voltage electrical networks, since these converters, despite their significant advantages, have a disadvantage — the deterioration of the quality of electricity in the power supply system 6–35 kV. Existing methods of dealing with resonant phenomena in medium voltage electrical networks, in which non-linear elements are present, such as powerful drives of rolling mills with FC-AR used in power supply systems of metallurgical enterprises, do not always allow changing the situation for the better. One way to solve this problem is to use classic traditional filters. Often, the use of traditional filters to eliminate strong voltage distortions in sections of 6–35 kV does not have the desired effect. The installation of traditional filters is accompanied by additional parasitic current resonances, which amplify the intermediate harmonics. These harmonics multiply the oscillations and strongly distort the voltage shape when the frequency response of the distribution network is superimposed with the region of high-frequency harmonics generated by modern FC-AR. Aim of research The article is devoted to the issues of improving the electromagnetic compatibility (EMC) of FC-AR as part of electric drives of rolling mill stands with an internal power supply system for metallurgical plants in the presence of resonant phenomena through the use of specialized passive filters (SPF). Research methods Methods of theoretical and experimental researches are applied in the work. When studying the causes of the occurrence of resonant phenomena, experimental data obtained in the course of research work at the metallurgical plant JSC Metallurgical Plant Balakovo were used. For the processing of experimental data, mathematical modeling methods were used in the Matlab environment with the Simulink application. Results The significance of the results of the work lies in improving the quality of electricity in the distribution electrical networks of mini-plants with FC-AR in the presence of resonant phenomena in the frequency response of the network through the use of SPF. The implementation of the results of the work is an improvement in quality indicators and, as a result, an increase in the reliability of the operation of all power converters in the internal power supply system and a decrease in emergency situations associated with the failure of power elements. The results of the work were implemented at the metallurgical plant of JSC Metallurgical Plant Balakovo. The quantitative indicator of the technical effect of changing the level of the coefficient of the total harmonic components of the voltage in the general sections of the factory GPP КU is: 1) without the use of SPF КU = 10.26 %; 2) when using SPF КU = 1.42 % (relative change δ КU = –72 %). These results can be used at other compact metallurgical enterprises with extended cable lines of medium voltage 6–35 kV and powerful electric drives with FC-AR.

CONSIDERATION OF THE CRITERION OF THE INFLUENCE OF ASYMMETRIC LOAD ON VOLTAGE DISTORTION IN A 10 kV NETWORK FOR RECOMMENDATIONS FOR REDUCING VOLTAGE ASYMMETRY

To date, there is a fairly large number of electrical equipment that is a source of asymmetric modes of operation in medium and low voltage electrical networks. This electrical equipment can create conductive low-frequency electromagnetic interference in electrical networks by the coefficient of voltage asymmetry in the reverse sequence. Studying the methodological basis of research on the theory of conductive low-frequency interference, it was found that when switching from one adjacent electrical network to another (for example, from a 0.4 kV network to a 10 kV network and vice versa) potential for conductive low-frequency interference decreases, i.e. the interference is partially suppressed by a certain amount. This amount is called the criterion determining the changes of the voltage asymmetry coefficient values in the reverse sequence in the transition of conductive low-frequency electromagnetic interference to the adjacent network. The purpose of this article is to develop a mathematical model for the study of network parameters affecting voltage asymmetry in the reverse sequence, which is reduced to a method for determining the value of the criterion for the effect of an asymmetric load on voltage distortion in a 10 kV network for recommendations for reducing voltage asymmetry. The article describes in detail the method of calculating this value and how it can be used to form recommendations for reducing voltage asymmetry. Scientific novelty of the study consists in the fact that an empirical mathematical model has been obtained that determines the criterion for the influence of an asymmetric load in a low-voltage network, the impact on which makes it possible to improve the electromagnetic situation in a 10 kV electrical network. This parameter can be taken into account when designing elements of electrical networks to determine the electromagnetic environment.

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.

Considerations on the Disturbances Occurred in the Operation of Medium Voltage Electrical Networks with Neutral Treated by Suppression Coil

In this paper are analyzed the analytical models of theoretical calculation and analysis of the main consequences of neutral treatment by suppression coil on the operation of medium voltage electrical networks, both in normal regime and especially in single-phase fault regime. The size of stationary and transient overvoltages and fault currents is evaluated for a concrete case study. Also the authors present in this paper the results they obtained after the extension of their research and in the field of harmonic regime produced by the grounding of a power line. This type of fault occurs frequently in the case of medium voltage networks, with neutral treated with suppression coil. Thus, they managed to reconstruct the waveforms of currents and voltages during the fault period (grounding). Then, having these calculation elements available, a comparative analysis of similar incidents (earthing) from a 20 kV network with the neutral treated with classic suppression coil and modern Trench suppression coil, respectively, was made.

Selection of parameters of overhead lines models when calculating transient processes during earth faults in 6–10 kV networks

Single phase-to-ground faults are the most common type of faults in 6–10 kV overhead distribution networks. Arc intermittent single phase-to-ground fault (PSP) are the most dangerous for the network and the damaged element. They are followed by intense transient processes and, as a result, dangerous overvoltage rate and significant transient current surges at the point of insulation damage. PSP transients also have a significant effect on the selectivity and operation stability of protection devices against this type of damage. Therefore, the development of the methods and means to improve the operation efficiency of 6–10 kV overhead networks in case of PSP and technical improvement of protection devices in many cases is due to the need to calculate the transient processes that occur during insulation breakdowns of the network phase to earth. For the systems under consideration, the reliability of transient processes calculations in case of PSP is determined mainly by the accuracy of estimation of the parameters of 6–10 kV overhead lines, first of all, of inductance, which generally depends on the frequency of the transient current components. In the scientific papers devoted to the study of transient processes in case of PSP in medium voltage electrical networks, including 6–10 kV overhead networks, constant (frequency independent) values of inductance are used as a rule in the equivalent circuits and in the models of transmission lines. An urgent task is to estimate errors caused by the application of this approach to determine the parameters of 6–10 kV overhead lines during the calculations and modeling of transient processes during PSP, and cases of its application. Advanced methods of modeling of electric power systems and their elements have been applied with the use of COMSOL Multiphysics and PSCAD software to obtain the frequency dependences of the inductances of a 6–10 kV three-phase overhead line and study of their influence on the calculation accuracy of transient currents and voltages in case of PSP. The parameters of 6–10 kV overhead line models developed in the indicated software packages at a frequency of 50 Hz are set in accordance with the reference data. The authors obtain the errors estimation to determine the parameters of transient currents and voltages during PSP in 6–10 kV overhead networks when using transmission line models. The frequency dependences of inductance, which are up to 40–50 % in amplitude are not considered. The results show that application of frequency-independent models is permissible only in the cases when parameters of the calculated equivalent circuit of the network and position of PSP point remain practically constant, when solving problems that require high accuracy to determine the parameters of transient currents and voltages, for example, to determine remotely the location of a ground fault, it is necessary to use frequency-dependent models of 6–10 kV overhead lines. Introduction of the developed recommendations to determine 6–10 kV three-phase overhead lines parameters allow us to increase the reliability of calculations and to avoid raw errors when solving the problems which are related to the study of transient processes in case of earth faults in the networks of the given voltage class.

On damage rate of overhead power transmission lines in power supply systems

The purpose of the article is to analyze the damage rate of overhead power transmission lines (OPL) in medium-voltage electrical distribution networks of the Irkutsk region. The established International Indices that determine the level of reliability of the functioning of electric networks are considered, and information on the compliance of the condition of electric networks in Russia with these indices is analyzed. Analytical information on the damage rate of elements of these networks and their causes in Europe, America, and Russia is presented. The emphasis is placed on the fact that the most common damage is characteristic of overhead power lines, especially 6–10 kV lines. As an object of research, two branches of the Irkutsk electric grid company (IEC) were taken, one of which provides electricity to rural consumers, the other — mainly to consumers residing in the territory of the city Irkutsk. The characteristics of these electric networks, their territorial location and basic technical data are presented. To conduct analytical monitoring of the level of reliability of overhead power transmission lines, logs of disconnection of the Eastern and Southern electric networks of the IEC over a long-term period were used. On the basis of this information, tables of failures and their consequences in the studied electrical networks were compiled for monthly average data over the period under examination for various causes of damage. To plot time diagrams of the parameters under examination, computer programs were compiled in the Matlab system, the use of which made it possible to obtain visualization of changes in failures for various reasons for the electrical networks under consideration. The information on the time of power supply interruptions in these networks, as well as the amount of electricity underutilized by consumers during these interruptions and its cost are analyzed. It is shown that in the electric networks under consideration, most of the power failures are related to the territorial dispersion of these networks, and the low level of equipment controls and insufficient qualification of operating personnel, as evidenced by the significant number of failures for unknown reasons. Besides, a significant part of the failures is due to damage to the overhead line wires, switching equipment and the effect of wind load. Recommendations for improving the condition of power transmission lines and a number of measures aimed at improving the level of reliability of power supply are presented.

System efficiency prediction of a 1kW capacity grid-tied photovoltaic inverter

<p>This article presents the system design and prediction performance of a 1kW capacity grid-tied photovoltaic inverter applicable for low or medium-voltage electrical distri-bution networks. System parameters, for instance, the longitude and latitude of the solar plant location, panel orientation, tilt and azimuth angle calculation, feasibility testing, optimal sizing of installment are analyzed in the model and the utility is sim-ulated precisely to construct an efficient solar power plant for residential applications. In this paper, meteorological data are computed to discuss the impact of environmen-tal variables. As regards ensuring reliability and sustenance, a simulation model of the system of interest is tested in the PVsyst software package. Simulation results yield that the optimum energy injected to the national grid from the solar plant, specific pro-duction, and performance ratio are 1676kWh/year, 1552kWh/kWp/year, and 79.29% respectively. Moreover, the predicted carbon footprint reduction is 23.467 tons during the 30 years lifetime of the system. Therefore, the performance assessments affirm the effectiveness of the proposed research.</p>

Analysis of the Influence of the Insulation Parameters of Medium Voltage Electrical Networks and of the Petersen Coil on the Single-Phase-to-Ground Fault Current

Settings of protection are essential to ensure the sensitivity and selectivity needed to detect defects. Making the correct settings requires the calculation of the fault currents with as little error as possible. Fault currents are influenced by the parameters of the electrical networks, including the state of the insulation and the Petersen coil, which changes during their operation electrical networks. This paper analyzes how the insulation parameters of medium voltage power lines, the parameters of the Petersen coil used to treat the neutral of the medium voltage electrical network and the value of the resistance at the fault location influence the fault current in the case of a single-phase fault. The large number of single-phase faults that occur in medium voltage electrical networks justifies this analysis. The symmetrical components method was used to calculate the fault current. The results obtained by calculation were verified experimentally by causing a single-phase-to-ground fault in a real medium voltage network. The paper presents the situations in which the analytical calculation of the single-phase-to-ground fault current can lead to inadmissibly large errors, even over 50%, but also the situations in which the errors fall below 3%.

Evaluation of Medium Voltage Network for Propagation of Supraharmonics Resonance

Power converters with high switching frequency used to integrate renewable power sources to medium and low voltage networks are sources of emission in the supraharmonic range (2 to 150 kHz). When such converters are connected to a medium voltage (MV) network these supraharmonics propagate through the MV network and can impact network and customer equipment over a wide range. This paper evaluates an existing Swedish MV electrical network and studies the pattern of supraharmonic resonance and the propagation of supraharmonics. The MV network consists of eight feeders including a small wind farm. Simulations reveal that, the bigger the MV network, the more resonant frequencies, but also the lower the amplitude of the resonance peaks in the driving point impedance. It was also identified that for short feeders as length increases, the magnitude of the transfer impedance at supraharmonic frequency decreases. For further increment in feeder length, the magnitude increases or becomes almost constant. For very long feeders, the transfer impedance further starts decreasing. The eight feeders in the network under study are similar but show completely different impedance versus frequency characteristics. Measurements at the MV side of the wind farm show time varying emissions in the supraharmonic range during low power production. The impact of these emissions coupled with system resonance is examined.

Модель распределения электрического напряжения и изменения электрического тока в сети с наличием неучтённого потребления электрической энергии

The aim of the study is control of commercial losses in electrical grids, especially in low voltage grids, which is one of the priority lines of activities conducted by electric network companies. The complexity of solving this problem is stemming from the difficulty of exactly locating the commercial loss occurrence place under the conditions of extensively branched low and medium voltage electrical networks. Various methods are currently used to determine the commercial loss occurrence places. However, no effective methods have been created for determining the fact and place of unaccounted electricity consumption in networks under the conditions of performing remote analysis of networks based on the data from modern electricity meters used in the automated fiscal electricity metering system. These difficulties can be overcome by developing a model of voltage distribution and change of current in distribution networks of the 0.4--35 kV nominal voltage levels. A model of voltage distribution and changes of current for a network containing unaccounted electricity consumption is proposed. The effectiveness of using the proposed model has been theoretically substantiated; its applicability limits are defined, and the accuracy of the obtained results is estimated. Graphical representation of the proposed model, which is one of the electrical network digital imaging forms, can be used to analyze electrical networks for revealing if there is unaccounted electricity consumption in them. By using the proposed model of voltage distribution and change of current in the network, it is possible to represent the electrical network as a set of electrical parameters to analyze electrical networks for the presence of commercial losses.

Compound Admittance Matrix Estimation of Three-Phase Untransposed Power Distribution Grids Using Synchrophasor Measurements

This article considers the problem of estimating the parameters of the compound admittance matrix of three-phase untransposed low- and medium-voltage electrical distribution networks using synchrophasor measurements from phasor measurement units (PMUs). The work proposes and analyzes the performance of a preprocessing strategy on the PMU's raw measurements, which consists in grouping the raw measurements in clusters and, then, using the averaged measurements from each cluster for admittance matrix estimation. This step reduces the noise level and discards similar measurements from each cluster, ultimately improving the estimation quality of regression-based estimation methods such as least squares (LS) and total least squares (TLS). The proposed approach uses a linear estimation model with phasor measurements of nodal voltages, nodal injection currents, and branch currents. We adopt a realistic measurement noise model in polar coordinates, which reflects the accuracy class of measuring instruments and is projected to rectangular coordinates. The proposed approach is validated by performing simulated experiments for different CIGRE and IEEE benchmark grids. Furthermore, the work includes different sensitivity analyses on the preprocessing policy (cluster type and size), availability of branch or injection currents measurements, as well as with different noise levels on the measurement data.

Overvoltage Impact on Internal Insulation Systems of Transformers in Electrical Networks with Vacuum Circuit Breakers

Vacuum circuit breakers are increasingly used as switching apparatus in electric power systems. The vacuum circuit breakers (VCBs) have very good operating properties. VCBs are characterized by specific physical phenomena that affect overvoltage exposure of the insulation systems of other devices. The most important phenomena are the ability to chop the current before the natural zero crossing, the ability to switch off high-frequency currents, and the rapid increase in dielectric strength recovery. One of the devices connected directly to vacuum circuit breakers is the distribution transformer. Overvoltages generated in electrical systems during switching off the transformers are a source of internal overvoltages in the windings. The analysis of the exposure of transformers operating in electrical networks equipped with vacuum circuit breakers is of great importance because of the impact on the insulation systems of switching overvoltages (SO). These types of overvoltages can be characterized by high maximum values and atypical waveforms, depending on the phenomena in the circuit breaker chambers, system configuration, parameters of electrical devices, and overvoltage protection. Overvoltages that stress the internal insulation systems are the result of the windings response to overvoltages at transformer terminals. This article presents an analysis of overvoltages that stress the transformer insulation systems, which occur while switching off transformers in systems with vacuum circuit breakers. The analysis was based on the results of laboratory measurements of switching overvoltages at transformer terminals and inside the winding, in a model medium-voltage electrical network with a vacuum circuit breaker.

Analysis of Internal Overvoltages in Transformer Windings during Transients in Electrical Networks

Due to the increasing requirements for the reliability of electrical power supply and associated apparatus, it is necessary to provide a detailed analysis of the overvoltage risk of power transformer insulation systems and equipment connected to their terminals. Exposure of transformer windings to overvoltages is the result of the propagation condition of electromagnetic waves in electrical networks and transformer windings. An analysis of transformer winding responses to transients in power systems is of particular importance, especially when protection against surges by typical overvoltage protection systems is applied. The analysis of internal overvoltages in transformers during a typical transient related to switching operations and selected failures is of great importance, particularly to assess the overvoltage exposure of insulation systems in operating conditions. The random nature of overvoltage phenomena in electrical networks implies the usage of computer simulations for the analysis of overvoltage exposures of electrical devices in operation. This article presents the analysis of the impact of transient phenomena in a model of a medium-voltage electrical network during switching operations and ground faults on overvoltages in the internal insulation systems of transformer windings. The basis of the analysis is simulations of overvoltages in the windings, made in the Electromagnetic Transients Program/Alternative Transients Program (EMTP/ATP) using a model with lumped parameters of transformer windings. The analysis covers the impact of the cable line length and the ground fault resistance value on internal overvoltage distributions.

Применение класса напряжения 20 кВ в системах электроснабжения современных мегаполисов

Megacities are characterized by a high level of energy consumption, a significant density of electrical loads and increased requirements for the reliability of power supply to consumers. Traditionally in the USSR and further in Russia, voltage classes of 6 and 10 kV are used for urban distribution electric networks. The increasing density of electric load and electricity consumption does not allow in extremely cramped conditions of developing cities to provide electricity through existing cable lines with a voltage of 6-10 kV due to their limited bandwidth and lack of reliability. The technical condition of the equipment and cable lines in the 6-10 kV supply networks has significant physical and moral deterioration. Existing networks have insufficient redundancy and have practically exhausted their ability to connect new consumers. These circumstances required a decision to switch to a voltage class of 20 kV, which is a strategic direction for the development of medium voltage electrical networks in Moscow. According to this decision, since the 2000s, electric networks with a voltage of 20 kV have been actively developing in Russia and in Moscow. This article analyzes the implemented approaches to the use of an electric network of 20 kV in megacities of different countries.

Разработка стенда для демонстрации поражения человека напряжениями прикосновения и шага вблизи воздушной линии 10 кВ

An objective to develop a labor protection engineering stand was set forth as part of the research and development work "Carrying out theoretical and experimental investigations and development of stands for demonstrating the danger of electric shock at a training ground" for visually demonstrating the danger of injuring a person by touch and step voltages in 3…35 kV medium voltage electric networks. The technical solutions for practically implementing the stand were adopted based on an analysis of regulatory documents, conditions under which dangerous touch and step voltages occur, the most typical human injury cases, and calculation results. Specialists of the National Research University Moscow Power Engineering Institute Department of High Voltage Engineering and Electrophysics, working jointly with specialists of PJSC Rosseti Moscow Region have developed, manufactured, tested, and put into operation a stand for demonstrating human injury by the touch and step voltages when a 10 kV wire falls on the ground, on a car, and when a fault of a live wire on the overhead line support occurs. With the stand having been put in use at the PJSC Rosseti Moscow Region Training Center Ground, this will allow the staff to form a clear understanding of the electric shock danger, to focus on the effects caused by voltage and current, and thereby significantly reduce electrical injuries.

Investigating Phase Over-Current (OC) Protection in Medium-Voltage networks

Overcurrent protection is protection against excessive currents or current beyond the acceptable current rating of equipment. It generally operates instantly. Short circuit is a type of overcurrent. Magnetic circuit breakers, fuses and overcurrent relays are commonly used to provide overcurrent protection. There is always a need to protect expensive power equipment. Protective relaying is a very important part of any electric power system that comes into play during trouble, fault or abnormal condition. The purpose is to isolate unhealthy part of electrical power system while the rest continue their normal operation. The entire electric power system from source to load centers is exposed and subject to natural hazards. The effects of these hazards are capable of interrupting normal operations of the system. Since these hazards cannot be prevented, precautions are taken to minimize or eliminate their effect on the system. The relay is a basic component of any protection scheme. The information (or signals) received from the power system actuates the relay, when necessary, to perform one or more switching actions. The signals are proportional to the magnitudes and phase angles of power system voltages and currents. When the relay receives these signals, it decides to close (or open) one or more sets of normally open (or closed) contacts, and consequently, the trip coil of a circuit breaker will be energized to open the power circuit. This paper investigates over-current relay protection scheme applied to medium-voltage electrical network. Methods of current and time grading have been applied in the coordination of the overcurrent relays in a radial network. Different time/current characteristics of relays such as the normal inverse (NI), very inverse (VI), and extreme inverse (EI) have been examined in order to obtain optimum discrimination.

Investigating Phase Over-Current (OC) Protection in Medium-Voltage networks

Overcurrent protection is protection against excessive currents or current beyond the acceptable current rating of equipment. It generally operates instantly. Short circuit is a type of overcurrent. Magnetic circuit breakers, fuses and overcurrent relays are commonly used to provide overcurrent protection. There is always a need to protect expensive power equipment. Protective relaying is a very important part of any electric power system that comes into play during trouble, fault or abnormal condition. The purpose is to isolate unhealthy part of electrical power system while the rest continue their normal operation. The entire electric power system from source to load centers is exposed and subject to natural hazards. The effects of these hazards are capable of interrupting normal operations of the system. Since these hazards cannot be prevented, precautions are taken to minimize or eliminate their effect on the system. The relay is a basic component of any protection scheme. The information (or signals) received from the power system actuates the relay, when necessary, to perform one or more switching actions. The signals are proportional to the magnitudes and phase angles of power system voltages and currents. When the relay receives these signals, it decides to close (or open) one or more sets of normally open (or closed) contacts, and consequently, the trip coil of a circuit breaker will be energized to open the power circuit. This paper investigates over-current relay protection scheme applied to medium-voltage electrical network. Methods of current and time grading have been applied in the coordination of the overcurrent relays in a radial network. Different time/current characteristics of relays such as the normal inverse (NI), very inverse (VI), and extreme inverse (EI) have been examined in order to obtain optimum discrimination.

Dynamic stability of the functioning of current earth fault directional protection in networks with isolated neutral

In distribution 6–10 kV networks with an insulated neutral for earth fault protection, zero sequence current directional protection devices are commonly used. According to the operation data, the main disadvantage of such kind of protection is the possibility of their functioning failures in transient conditions with the most dangerous for network intermittent arc earth faults. It is known that most earth faults in 6–10 kV networks, primarily in the initial stage of insulation damage, have an intermittent arc. Operation failures of zero sequence current directional protection in case of arc faults reduce the operational reliability of the protected network and, as a result, the reliability of power supply to consumers. Nowadays, new developments of electrical power systems relay protection devices, including earth fault protection of medium voltage distribution electrical networks, are implemented only on a microprocessor base. Therefore, the selection and justification of the implementation principles of zero sequence current directional protection which can provide high dynamic stability of functioning is a relevant objective. When analyzing the dynamic stability of the functioning of zero sequence directional current protection, regarding the complexity of transients during intermittent arc earth faults in medium voltage electrical networks with an isolated neutral, the simulation in Matlab using SimPowerSystem and Simulink was carried out. This study focuses on transient currents and voltages as the main factor influencing dynamic stability of the functioning of zero sequence current directional protection. The impact of other factors, for example, the inaccuracies of the primary zero sequence current and voltage transducers, the scheme of formation of compared quantities, etc. was not taken into account in simulation models. The study has allowed determining the causes of possible functioning failures of digital current earth fault directional protection in dynamic operation modes. It has been shown that the usage of orthogonal components of fundamental frequency of zero sequence voltage and current in current directional protection devices eliminates the failure of their operation with any kind of arc earth faults. To ensure high dynamic stability of operation under the influence of transients during arc intermittent earth faults, current directional protection for this type of damage should be performed on the basis of monitoring the phase relationships of the fundamental frequency components of 50 Hz of zero sequence voltage and current, but not their full values.