Method Of Symmetrical Components Research Articles

Fault Location of AT Traction Network Adopting Bilateral Power Supply Mode Based on Generalized Symmetrical Components Method

Fault Location of AT Traction Network Adopting Bilateral Power Supply Mode Based on Generalized Symmetrical Components Method

Characteristics and Parameters of a Three-Phase, Three-Wire Balanced Circuit with Non-Linear Load

The existing non-linear load in a three-phase circuit limits the possibilities of using the symmetrical components method to analyse it. This paper presents a method for analysing such a circuit when only the zero and positive components are present. Nonlinear loading in a circuit limits the applicability of the symmetrical components method in the analysis of a three-phase circuit. When there is only a zero component and a positive component, it is possible to carry out the analysis as presented in this article. The analysed circuit is balanced. An equal nonlinear load with a voltage described by a signum function of the current is considered in three phases. This load, through equal series RL elements, is supplied from a symmetrical three-phase voltage source. For a steady state with unbroken current flows, the equations of the circuit are solved symbolically. In the broader scope of the equation, solutions obtained using MATLAB-Simulink R2021b were used. The matching of the symbolic and simulation solutions was obtained. The current and voltage harmonic content coefficients of the receiver, the equivalent resistance and inductance of the nonlinear load, and the distribution of active and reactive power in the circuit were determined. The reactive power analysis shows that the considered load nonlinearity when generating higher harmonics, increases the reactive power of the circuit and the inductance of the circuit, which is seen from the terminals of the power source.

Large Signal Stability Analysis of Hybrid AC/DC Microgrids When a Single-Phase-to-Ground Fault Occurs

Islanded hybrid AC/DC microgrids lack support for a large grid, and the negative incremental impedance of constant power loads (CPLs) aggravates the poor anti-disturbance capability of the system. When a single-phase ground fault (SPGF) occurs, the amount of fault impulse power that islanded AC/DC hybrid microgrids can stably withstand and when the protection equipment can work are both unknown. In this paper, the method of symmetrical components is utilized, and high-signal stability criteria for islanded hybrid AC/DC microgrids when a SPGF occurs are derived based on the mixed potential theory. The proposed criteria place quantitative constraints on the power of the PV unit, DC/AC converter current inner-loop proportional parameters, inductors, and inductor equivalent resistance, as well as energy storage unit power, CPL power, capacitors, DC bus voltage, line equivalent resistance, line equivalent inductance, equivalent inductance in the faulty branch, equivalent resistance in the faulty branch, positive-sequence equivalent impulse power of the SPGF, and zero-sequence equivalent impulse power. Furthermore, the maximum impulse power of a SPGF that islanded hybrid AC/DC microgrids could stably withstand is also presented, providing guidelines for protection equipment to decide when to work. In addition, the allowable maximum CPL power that islanded hybrid AC/DC microgrids could steadily support as a SPGF occurs is deduced, and the power is usually adopted to determine the states of an energy storage unit and load shedding in advance. Simulation and experimental validations prove the correctness of the derived high-signal stability criteria.

Transient Synchronous Stability Analysis of Grid-Forming Photovoltaic Grid-Connected Inverters during Asymmetrical Grid Faults

Compared with the traditional grid-following photovoltaic grid-connected converter (GFL-PGC), the grid-forming photovoltaic grid-connected converter (GFM-PGC) can provide voltage and frequency support for power systems, which can effectively enhance the stability of power electronic power systems. Consequently, GFM-PGCs have attracted great attention in recent years. When an asymmetrical short-circuit fault occurs in the power grid, GFM-PGC systems may experience transient instability, which has been less studied so far. In this paper, a GFM-PGC system is investigated under asymmetrical short-circuit fault conditions. A novel Q-V droop control structure is proposed by improving the traditional droop control. The proposed control structure enables the system to accurately control the positive- and negative-sequence reactive current without switching the control strategy during the low-voltage ride-through (LVRT) period so that it can meet the requirements of the renewable energy grid code. In addition, a dual-loop control structure model of positive- and negative-sequence voltage and current is established for the GFM-PGC system under asymmetrical short-circuit fault conditions. Based on the symmetrical component method, the composite sequence network of the system is obtained under asymmetrical short-circuit fault conditions, and positive- and negative-sequence power-angle characteristic curves are analyzed. The influence law of system parameters on the transient synchronous stability of positive- and negative-sequence systems is quantitatively analyzed through the equal area criterion. Finally, the correctness of the theoretical analysis is verified by simulation and hardware-in-the-loop experiments.

Fault Location Method for Distribution Network Using an Additional Inductance Strategy

In distribution networks, time asynchrony exists between the phasor measuring unit (PMU) at both ends of a line, and the effective measurement time of the devices is short, leading to insufficient accuracy in phasor measurements. This paper proposes a fault location method for distribution networks that employ an additional inductance strategy to address the limited location accuracy caused by time asynchrony and the inadequate accuracy of phasor measurement devices. The method enhances the stability and accuracy of phase measurement by connecting an additional inductance after the online circuit breaker, thus extending the effective measurement time. It uses the symmetrical component method to obtain the positive-sequence and negative-sequence networks following a fault. Time asynchrony is treated as an equivalent asynchronous phase angle, which is then applied to the positive and negative-sequence voltage components. The impact of time asynchrony is mitigated by compensating for the phase angle difference using the ratio of the positive-sequence voltage component to the negative-sequence voltage component. This approach provides the fault location function and has improved the accuracy of fault location, which is advantageous for rapid fault repair.

A laboratory evaluation of the effectiveness of voltage unbalance factor of opposite temperature analysis of a three-phase induction motor

This paper presents, based on experimental tests, an analysis of the effects of unbalance voltage on temperature rise of three-phase induction motor (TIM) with a view to evaluating effectiveness of the method of measurement employed, namely the method of symmetrical components. To do so, one motor is subjected to conditions of unbalance, strategically chosen, and their elevations of temperatures measured in three-phases are displayed. Among the many results obtained, it was found for voltage combinations where the voltage unbalance factor (VUF) has values very close to 0%, the existence of temperature rises higher than those where the VUF is greater than 2%. From investigations made, it is concluded that the use of VUF as single and sufficient index for the quantification of voltage unbalance is inappropriate.

Dual-Sequence Synchronization Stability Analysis and Control of Multi-Paralleled Wind Farms During Asymmetrical Grid Faults

The synchronization stability and instability risk assessment of multi-paralleled wind farms during asymmetrical grid faults have received little attention to date. In this paper, a simplified equivalent model of multi-paralleled wind farms is established based on the symmetrical components method to deal with the problem of dual-sequence synchronization stability during asymmetrical grid faults. Based on the model, the effects of multiple coupling characteristics such as sequence coupling and mutual coupling of wind farms on the stability of dual-sequence synchronization are studied in detail. Then, an assessment method is proposed to evaluate the dual-sequence synchronization stability of multi-paralleled wind farms under various asymmetrical grid faults. This assessment method can judge whether there are positive-sequence (PS) and negative-sequence (NS) steady-state equilibrium points for each wind farm in the system, and identify the wind farms that are instability due to the lack of a PS/NS equilibrium point. In addition, a current control strategy is derived to maximize the stability margin of dual-sequence synchronization. This strategy can ensure that each wind farm in the system has equilibrium point. Finally, the correctness of the theoretical analysis and the effectiveness of the proposed method are verified by simulation.

Analysis of power grid operation modes considering the influence of tooth harmonics

A harmonic analysis of the magnetic field of an asynchronous motor was carried out, taking into account bilateral gearing and magnetic asymmetry of the air gap. The spectrum is obtained in the form of infinite series of spatio-temporal harmonic components. Magnetic asymmetry of an asynchronous motor takes into account static and dynamic eccentricity, magnetic asymmetry of steel. 27 different spatial harmonics of the field in the air gap of the induction motor were obtained. The considered spectrum of harmonics of the magnetic field will be induced in the stator windings by the electromotive force with the corresponding frequencies. The fields with γр ± κ pairs of poles rotate asynchronously with the rotor and are damped by the fields created by the currents of the rotor cage. Thus, the magnetic asymmetry affects the spatial harmonics of the magnetic field, changing the angular velocities of their rotation in the air gap. The appearance of magnetic asymmetry causes a redistribution of the spatio-temporal distribution of the harmonics of the magnetic field of the tooth frequency, which leads to a change in the amplitude and phase relationships between the quantities. This leads to the appearance of an asymmetric system of electromotive force of tooth frequency in the stator windings, and, therefore, to a different effect of the asymmetry of the air gap at the level of symmetrical components. In the general case, at the level of symmetric components, the electromotive force of the gear frequency is influenced by two groups of factors that determine asymmetric or different from the nominal operating modes of an asynchronous motor of the same size. An increase in the magnetic asymmetry of the air gap causes a nonlinear, close to parabolic character of the growth of the existing symmetrical components. Symmetric components of the electromotive force of the toothed frequency cause currents of the corresponding components to flow through the closed circuits of the stator windings of the asynchronous motor and the entire power supply system. The calculation scheme for replacing a three-phase network for toothed harmonics of electromotive force is given. The phase resistances of the windings of rotating electric machines for asymmetric three-phase networks are not the same for different sequences. The main method of calculating the considered network is the method of symmetrical components. The calculation is made for one main phase according to the substitution schemes, which corresponds to the symmetrical components of the electromotive force of the tooth frequency. As a result of theoretical studies, functional dependences of the influence of magnetic asymmetry on the level and distribution of phase currents of the toothed frequency were obtained. The nature of the influence at the level of symmetric components is close to parabolic, and the most sensitive to changes in magnetic asymmetry is the minor symmetric component of the zero sequence. The method of calculating the currents and voltages of the toothed frequencies of asynchronous motors in the conditions of power consumption has been developed. Theoretical propositions are confirmed by experimental studies. It has been established that the levels of toothed harmonics in power supply systems, as a rule, do not exceed 10–15 % of the level of the fundamental frequency. This leads to a decrease in the reliability of the operation of electrical networks and a shortening of the service life of the main energy equipment at the enterprise. However, frequency-amplitude resonance at tooth frequencies causes the greatest threat to the power plants of industrial enterprises. This is due to the specific nature of gear harmonics of asynchronous motors (parity, dependence on load and various technological factors). The biggest threat arises when using the same type of asynchronous motors with similar operating modes. Oscillograms of experimental studies are given.

Simulation of protection against unbalanced high-voltage asynchronous drive of recycle compressor in fuel hydrotreating unit

In the oil and gas industry, continuous processes such as oil and gas refining play a great role and are sensitive to many external factors. Such processes require special procedures for stopping and restarting. In order to maintain a sustainable process, the entire system needs to be cleaned by removing of unreacted components. Rejected raw materials are often dumped into a flare leading to tangible environmental problems and significant economic disadvantages. Electrotechnical systems (ETS) play an important role in ensuring continuous technological processes in oil and gas industry. Electric motors are one of the key elements of ETS. The majority of the electrical machines used in industry today are Asynchronous motors (AM) – no less than 80 %. Ensuring their trouble-free operation is one of the key factors in the design, simulation, and analysis of asynchronous motor relay protection systems, including unbalanced conditions of their operation. These conditions can occur due to unbalanced AM connection circuits, supply voltage unbalance, or as any faults in a machine itself. Operating a motor under these conditions will result in shorter motor life, reduced power, wear and aging of the insulation. The study subject was an Asynchronous motor drive of a recycle compressor of a gasoline hydrotreating unit at the fuel hydrotreating integrated unit at the Astrakhan Gas Refining Plant (AGRP). The authors used Matlab simulations to study the facility and its protection systems/devices operation. The method of symmetrical components was selected as the main theoretical method. The authors developed a model of an asynchronous motor drive of a recycle compressor. This involved establishing a set of relay protections (RP) and developing the models of the following protections: sequence filter (symmetrical component filter) (SF), negative sequence (nps) O/C protection, and overload protection. It was demonstrated that the specified relay protection complex fully protects the motor from unbalanced operation conditions. The authors conducted a study of the protection complex operation under different supply voltage unbalances, with different motor loads. They formed a conclusion about the performance of the developed protection complex, and gave recommendations for its technical implementation in a business environment. The study findings can be used as a basis for the development and testing of relay protection components of the entire electrical system of the fuel hydrotreating unit at the Astrakhan Gas Refining Plant.

Development of models of current relays and power direction relays in the MATLAB software

During pandemics and military aggressions, the continuation of education is ensured by the active implementation of distance learning technologies. A significant number of real-life processes can be described using mathematical models. Their further implementation in the program code and the finished software product allows them to be effectively used in virtual studies of relay protection schemes. For the correct functionality of any virtual relay or protection complex, it is necessary to build a model that takes into account the real characteristics of its constituent elements. In case of absence of the latter, they should be determined empirically. To create a virtual laboratory work on the study of the RT-40 current relay, based on the parameters specified in the passport for this device, an algorithm was created that allows the user to interactively determine the main characteristics of the relay: tripping current, release current, and response time depending on the current through the relay using various circuit elements (sliders, regulators, etc.). The algorithm of the program “Research of characteristics of power direction relay RBM-171” uses the angular characteristics and sensitivity characteristics of the real relay RBM-171, which were converted into digital form. For the RBM-178 relay, which has a different connection scheme and maximum sensitivity angle from the previous test stand, a separate test stand “Study of the RBM-178 power direction relay” was created. The logic of its operation is based on the detection of zero-sequence currents and voltages using the method of symmetrical components. All laboratory circuits have visualization and indication of the states of the circuit elements present on the stands. This allows the user to achieve more accurate results and get closer to testing relays in real laboratory conditions.

A Comprehensive Review on Distributed Feeder Protection Problems in the Gaza Strip 22/0.4 kV Overhead Power Distribution System – A Case Study

This paper provides a perspective on protective system at 22kV overhead feeders in Gaza distribution network using symmetrical components method and ETAP software. Electrical disturbances from high rate of outage feeders at the 22 kV overhead power lines frequently cause disruptions on the generating plant and customer loads. One sub-transmission feeder from a parallel set of ten 22 KV feeders (140 MW) in the Gaza west substation is selected as a case study in this research. Because the protection system lacks a data fault recorder (DFR), the paper calculates the fault currents at both the nearest substation bus and the last bus in the distributed feeder using the symmetrical components method and verifies the results by using ETAP software simulation. The results from calculating the minimum fault at the last bus indicated that the current protective parameters are reasonable and satisfied for detecting and interrupting the occurring faults. This author paper proposes increasing the fault clearing time at farthest feeder bus to make protective functions less sensitive by adjusting TMS to 0.08 for time overcurrent and earth fault and setting pickup level to 3.8 for instantaneously overcurrent. The design model is run on the selected feeder, and simulation results guarantee the proposed model's accurate performance for both overcurrent and earth faults after changing the fault clearing time. There will be no miscoordination of protective devices or operational risks while running the design model for the selected feeder with new settings. Gaza's distribution feeder network requires structural improvements to become more reliable and to reduce the high rate of feeder trip, and practical recommendations to improve current installation distribution grid are included in this paper.

ДВУХСТОРОННИЙ МЕТОД ОПРЕДЕЛЕНИЯ МЕСТА ОДНОФАЗНОГО ЗАМЫКАНИЯ НА ЗЕМЛЮ В СЕТЯХ С ИЗОЛИРОВАННОЙ НЕЙТРАЛЬЮ

Relevance Currently, on the part of enterprises operating 6–35 kV distribution networks, there is a request for a deep modernization of networks and the transformation of existing, outdated networks into digital ones. A separate aspect of modernization is the introduction of high-precision measuring devices and new principles of data transmission. At the same time, the probability of damage for various reasons remains. Single-phase earth faults in networks with isolated neutral are still the most common problem in distribution networks that do not have universal solutions. The problem is particularly acute with determining the location of single-phase earth faults. The opportunities provided by the introduction of digital networks can become the basis for the implementation of new algorithms for relay protection and determining the location of damage. Aim of research The article considers, developed by the authors, a two-way method for determining the location of damage in single-phase earth faults according to the parameters of the emergency mode when installing current transformers in all three phases. The two-way method of determining the location of damage is based on the use of zero-sequence currents and voltages, which are determined by calculation for phase current and voltage differences at the beginning and end of the line. The algorithm is focused on digital electrical networks equipped with high-precision measuring transformers and a communication channel. To calculate the parameters of the zero sequence, the differences of the phase emergency parameters at the beginning and at the end of the line are used. Research methods To develop and study a two-way method for determining the location of damage in single-phase earth faults, the principle of superposition in combination with the method of symmetrical components was used. Decomposition of three-phase currents and voltages into symmetrical components is a linear procedure. Therefore, the zero-sequence current in a three-phase line, in accordance with the superposition principle, can be represented as the sum of three zero-sequence currents, each of which is determined by one of the phase currents at zero values of phase currents in the other two phases. This application of the method of symmetric components is valid only for purely emergency values of phase currents. Therefore, in advance, the load current must be excluded from the phase currents of the emergency mode. Results It is shown that the use of the method of symmetric components for single-phase earth faults separately for the damaged phase and undamaged phases makes it possible to determine the zero-sequence current at the site of damage and the intrinsic zero sequence current of the damaged line. It is shown that this separation of the zero-sequence current makes it possible to distinguish two components of the zero sequence in the voltage difference at the ends of the line. Based on the allocation of two components in the current and voltage of the zero sequence, an algorithm for two-way determination of the damage location has been developed, which allows determining the distance to a single-phase earth fault in networks with an isolated neutral. To study the two-way method of determining the location of damage, a network model and a device for determining the location of damage were implemented in the MATLAB Simulink software package. A functional diagram of a two - way damage location detection device is presented. The performed studies on the model of the electrical network have shown the high accuracy of the proposed algorithm for determining the location of damage.

Concept and Definition of Dynamic Symmetrical Components With Time-Varying Amplitude and Frequency

This paper investigates the dynamic symmetrical component method of internal voltage vector with time-varying amplitude and frequency. With the large-scale integration of power electronic devices into the power grid, when there is an asymmetrical fault in the AC network, the voltages and currents present the characteristics of amplitude and frequency time-varying modulation. Hence, the traditional asymmetrical fault analysis method and concept based on phasor models with constant amplitude and frequency are not adaptive enough. In order to supplement the traditional symmetrical component method and concept, the phasor models and vector models with time-varying amplitude and frequency are established. Then, the asymmetrical three-phase phasors with time-varying amplitude and frequency are decomposed into three groups of symmetrical phasors with time-varying amplitude and frequency. Based on the mathematical and physical characteristics of symmetrical components, the applicable concept and definition are reestablished. Lastly, time-domain simulations corroborate the theoretical analysis.

Research on Open Circuit Fault Modeling and Fault Tolerant Control Strategy of Five-Phase Induction Motor

Five-phase induction motors have the advantages of high reliability and strong fault-tolerant performance, so it’s open circuit fault model and fault-tolerant control strategy are widely studied. Based on the normal operation of the five-phase induction motor, the mathematical model of the five-phase induction motor under the conditions of single-phase open circuits, adjacent two-phase open circuits, and non-adjacent two-phase open circuits are established by using the reduced order decoupling transformation. Based on the principle of constant magnetic potential, the relationship between magnetic potential and each phase current is analyzed by using the symmetrical component method (MSC). The fault-tolerant control strategy of a five-phase induction motor with the above three open-circuit faults is designed. Through simulation and prototype experiments, the phase current and speed conversion under three open-circuit faults are analyzed. The results show that after the open-circuit fault of a five-phase motor, the residual phase current is no longer balanced, the motor speed is decreased, and the vibration is increased significantly. After fault-tolerant control, the residual phase current is balanced, the rated speed can be reached, and the vibration of the motor is reduced. Thus, the validity and correctness of the fault-tolerant control strategy for a five-phase induction motor are verified.

Fault interaction analysis approach for hybrid AC/DC power grids

Heretofore, the comprehensive fault analysis approach for AC/DC hybrid grids has not been sufficiently investigated. Inspired by the conventional symmetrical components method, a novel “sequence component network” model based on the mapping relation is proposed to study the characteristics of AC (DC) side after DC-side (AC-side) faults in AC/DC hybrid grids. Subsequently, the AC-side electrical characteristic after DC-side faults was discussed employing the “sequence component network” model. Finally, taking into account the influence of different grounding modes, a DC-type integration system model of PV stations was established in PSCAD/EMTDC to verify the proposed approach.

Symmetrical Components and Sequence Networks Connections for Short-Circuit Faults in Five-Phase Electrical Systems

The method of symmetrical components is an important mathematical tool for electrical engineering, as it simplifies the analysis of unbalanced electrical circuits. The method is used almost exclusively for three-phase networks, but with the advancement of multiphase electrical systems, it could be convenient to utilize it for such systems as well. In this paper, the method of symmetrical components is used to analyze a generic five-phase electrical system for various short-circuit faults and to determine the sequence networks connections for these faults. The analysis performed covers the derivation of the symmetrical components for voltage/current and of fault currents. The analytical results and the inferred sequence networks connections are validated by computer simulations. This paper therefore extends the literature on short-circuit analysis of multiphase electrical systems using the method of symmetrical components.

Short-circuit calculation method for unbalanced distribution networks with doubly fed induction generators

In an unbalanced distribution network, transforming three-phase unbalanced components into components with positive, negative, and zero sequences is difficult. Moreover, the fault currents of doubly fed induction generators (DFIGs) exhibit a complex nonlinear relationship. The fault characteristics of DFIGs with an activated crowbar differ from those with a non-activated crowbar. Moreover, distinguishing between these two crowbar conditions based on calculations is difficult. Therefore, the traditional symmetric component method is unsuitable for unbalanced distribution networks with DFIGs owing to its low calculation accuracy. In this work, the equivalent calculation phase component models of DFIGs are analyzed using low-voltage control strategies. In particular, the activation criterion of crowbar protection in the time domain is proposed. A novel fault analysis method for unbalanced distribution networks with DFIGs is also proposed. The action condition of crowbar protection is assessed using the traversal method, and the coupling relationship among DFIGs is determined using an iterative calculation method. Finally, the effectiveness of the proposed fault analysis method is verified by simulation.

A Novel Microgrid Islanding Detection Algorithm Based on a Multi-Feature Improved LSTM

Islanding detection is one of the conditions necessary for the safe operation of the microgrid. The detection technology should provide the ability to differentiate islanded operations from power grid disturbances effectively. Given that it is difficult to set the fault threshold using the passive detection method, and because the traditional active detection method affects the output power quality, a microgrid islanding detection method based on the Sliding Window Discrete Fourier Transform (SDFT)-Empirical Mode Decomposition (EMD) and Long Short-Term Memory (LSTM) network optimized by an attention mechanism is proposed. In this paper, the inverter output current and voltage at the point of common coupling (PCC) are transformed by the SDFT. The positive sequence, zero sequence, and negative sequence components of voltage and current harmonics are calculated and reconstructed by adopting the symmetrical component method (SCM). Meanwhile, the current and voltage are decomposed into a mono intrinsic mode function (IMF). The symmetric components of voltage, current, and IMFs are used as inputs to the deep learning algorithm. An LSTM with the features extracted to classify islanding and grid disturbance is proposed. By using the attention mechanism to set the weight values of the features of hidden states obtained by the LSTM network, the proportion of important features increases, which improves the classification effect. MATLAB/Simulink simulation results indicate that the proposed method can effectively classify the islanding state under different working conditions with an accuracy level of 98.4% and a loss value of 0.0725 with a maximal detection time of 66.94 ms. It can also reduce the non-detection zone (NDZ) and detection time and has a certain level of noise resistance. Meanwhile, the problem whereby the active method affects the microgrid power quality is avoided without disturbing the current or power of the microgrid.

High-Resistance Connection Diagnosis in Five-Phase PMSMs Based on the Method of Magnetic Field Pendulous Oscillation and Symmetrical Components

An online approach for diagnosing high-resistance connection (HRC) faults in five-phase permanent magnet synchronous motor drives is presented in this article. The development of this approach is based on a so-called “magnetic field pendulous oscillation (MFPO)” technique and symmetrical components method. Under HRC fault condition, a “swing-like” MFPO phenomenon is observed compared to the healthy condition. Furthermore, with the extracted current features in symmetrical components domain, different HRC fault types are successfully identified and distinguished. These fault types include single-phase faults, e.g., HRC fault in phase-A; two-phase nonadjacent faults, e.g., HRC fault in phase-A&C; and two-phase adjacent faults, e.g., HRC fault in phase-A&B. Meanwhile, the localization of the faulty phase/phases is also accomplished, and the fault severity is estimated. In this approach, only sensing of the phase currents is needed. Hence, the implementation cost is very low since the sensory data of the currents are typically already available in the closed-loop vector-controlled drives for control purpose and no additional sensors or related signal conditioning circuits are required. The effectiveness of the presented diagnostic approach is verified by simulations and experimental results.

АНАЛІЗ РЕЖИМУ РОБОТИ ЕЛЕКТРИЧНОЇ МЕРЕЖІ ПРИ ОДНОФАЗНИХ КЗ НАПРУГОЮ 6-35 КВ

In three-phase electrical networks during the operation of power supply systems, damage to electrical equipment and difficult modes of operation are possible. Damage associated with insulation failure, rupture of wires and cables of power lines, personnel errors when switching, lead to a short circuit between the phases or on the ground. At a short circuit in a closed circuit there is a big current, voltage drop on elements of the equipment increases that leads to the general decrease in voltage in all points of a network and disturbance of work of consumers. Complex modes of operation of electrical networks occur, as a rule, as a result of accidents or after emergency shutdowns of equipment, with subsequent overloads and voltage deviations from the nominal values. And although these modes have been considered acceptable for some time, they create the preconditions for various types of damage and disorders in the operation of power grids. To ensure normal operation of electrical networks and prevent the development of accidents, it is necessary to respond quickly to changes in the mode of operation of the electrical network, immediately separate damaged equipment from serviceable and, if necessary, turn on a backup power supply. These functions are performed by relay protection and automation devices. Principles of operation of relay protection devices. 1. Current protection. Short circuits are accompanied by a sharp increase in current that exceeds the value predetermined by the calculation, which will perform simple relay protection devices that can control the value of currents. Separate the maximum current protection, which acts to disconnect the damaged network element with a time delay and current cut-offs, which operate without a time delay. The difference between them is in the choice of how to ensure selectivity. The currents controlled by the relay protection device can be measured in phases (through current transformers), or individual components of phase currents can be measured (currents of direct, reverse and zero sequences. This method is based on the method of symmetrical components). Current types of protection are divided with control of power direction (directional) and without control (non-directional). Current protection works on the principle of operation not only in case of damage to one of the network elements, but also to adjacent ones. Therefore, current protection is referred to as protection with relative selectivity. 2. Differential protection. The basis of the principle of differential protection is the comparison of homogeneous, instantaneous values at the ends of the protected element of the network (transformer, busbar section, generator, overhead line). Usually compare currents in magnitude and phase. Differential protection by its principle works only in case of damage to the protected element of the network, so it is performed without time delay and differential protection is called protection with absolute selectivity. 3. Remote protection. In the event of short circuits in the electrical network is not only a sudden increase in current, but also a sharp decrease in voltage, ie a decrease in resistance. The advantage of relay protection devices based on this principle is that the resistance to the fault does not depend on the current and voltage, but only on their ratio. This allows protection at short-circuit currents less than the nominal for electrical equipment. Single-phase earth faults account for about 70% of all damage to electrical equipment. According to the Rules of technical operation in the event of a single-phase short circuit to "ground" in 6-35 kV electrical networks, relay protection devices must act either to turn off the equipment or to "signal". At the enterprises with special working conditions and the increased risk of damage of the equipment and defeat of people by action of an electric current protection against single-phase short circuits on the earth operates on shutdown with the minimum endurance of time. Thus, after the analysis of the modes of operation of power supply systems, the importance of using switching devices of relay protection to ensure normal operating conditions of electrical networks and prevent accidents is confirmed. Also, the use of relay protection devices must be justified for each individual electrical equipment according to the principles of operation and characteristics of the proper functioning of the switching apparatus. In case of emergencies, namely single-phase earth faults, 6-35 kV electrical network operation modes and operation of single-phase earth fault protection are performed according to various technical parameters, depending on the type and connection scheme of the neutral of this power system.