Asymmetrical Short Circuit Research Articles

ЗАСОБИ РОЗРАХУНКУ ТА ВИБОРУ УСТАВОК ДИФЕРЕНЦІЙНО-ФАЗНОГО ЗАХИСТУ ПОВІТРЯНИХ ЛІНІЙ МІКРОПРОЦЕСОРНИХ ЗАХИСТІВ L60 В СКЛАДНИХ ЕЛЕКТРИЧНИХ МЕРЕЖАХ ДО 10 000 ВУЗЛІВ

The means for calculation and selection of phase-differential protection settings of overhead lines of L60 microprocessor protections of the General Electric company in complex electrical networks up to 10,000 nodes was developed. This software allows to select settings according to the possible types of mixed signals: forward-sequenced and reverse-sequenced, as well as additional criteria. The program makes it possible to check the sensitivity of the starting and disconnecting devices in the case of symmetrical and asymmetrical short circuits, in the case of maximum reverse power flows in various mixed starts. Ref. 3 fig. 5.

Development of intelligent protection and automation control systems using fuzzy logic elements

In this article, the causes of technological disturbances in electrical systems are considered, and several characteristic disadvantages of the protection and automation of elements of electrical systems are highlighted. The tendency to decrease the reliability of relay protection associated with the transition from analog to digital types of protection is substantiated. Based on the studied examples, the use of fuzzy logic in protections, the expediency of using fuzzy logic elements in protection devices, and the automation of electrical systems to identify types of short circuits are justified. This article analyzes the most common damages and presents the results of modeling an electrical system with transformer coupling, where all types of asymmetric short circuits were initiated. The dynamics of changes in the symmetrical components of short-circuit currents of the forward, reverse, and zero sequences are determined. Rules have been created for the identification of asymmetric types of short circuits. An algorithm of protection and automation operation using fuzzy logic elements has been developed. The proposed algorithm of protection and automation will reduce the time to determine the type of damage and trigger protections.

A new fault location scheme based on local measurements for transmission lines connected to inverter-based resources

This paper aims to reveal conventional impedance-based fault location methods’ shortcomings in the presence of inverter-based resources (IBRs) through an analytical framework established for the analysis of the IBR sequence impedances. Thereafter, a single-ended method is put forward in order to accurately specify the fault location in the lines emanating from the IBRs. The proposed fault location formulation utilizes the negative and zero sequence currents as alternative polarizing currents, therefore it is independent of the pre-fault data, and it can be applied for all kinds of asymmetrical short circuit fault. Furthermore, the established formulation is further amended to make it applicable to long transmission lines. The suggested method also provides acceptable accuracy in various conditions, such as different fault resistances, different host grid codes, and dynamic behavior of the fault resistance. The performance of the proposed scheme is scrutinized using the PSCAD/EMTDC software with an extensive number of simulations. The obtained results reveal the effectiveness and efficacy of the proposed fault location method in the presence of IBRs.

Current-Limiting Strategy for Asymmetric Short-Circuit of Three-Phase Inverter for Continuous Power Supply of Nonfaulty Loads

Current-Limiting Strategy for Asymmetric Short-Circuit of Three-Phase Inverter for Continuous Power Supply of Nonfaulty Loads

Equivalent Transformations of Complex Circuits in Calculations of Power Supply Systems of Agrocomplex

he tasks at the stage of planning and operation of power supply systems are related to the calculation of the parameters of their operation modes. With the massive nature of such complex calculations, their execution is possible only with the help of appropriate software and computing tools, that is, using an electronic computer. (Research purpose) The research purpose is to substantiate the algorithm of equivalence of power supply systems or their individual sections as an integral part of the algorithms complex for calculating electrical systems. (Materials and methods) It has been shown that at present there are many agricultural consumers, interruptions in the power supply of which lead to significant damage, in this regard, closed power supply systems are becoming increasingly used, as the requirements for reliability, uninterrupted power supply and the quality of electricity supplied to the consumer are increasing. It was determined that the calculation of closed systems is more complicated than open ones. It was revealed that when solving a wide range of tasks related to the creation and modernization of power supply systems, there is a need to simplify the systems or their individual sections. (Results and discussion) The equivalence rule was formed and methods for calculating equivalent resistances and electromotive force of the converted sections of the replacement circuits were given, which simplifies the circuit and accelerates the process of calculating and analyzing the modes of power supply systems. (Conclusions) It was established that the proposed equivalence algorithm is based on universal formulas for calculating intrinsic and mutual resistances, which were previously obtained by the authors for use in problems of calculating steady-state modes of power supply systems. It was stated that the calculation algorithm is also applicable for the equivalence of forward and reverse sequence circuits in the calculation of currents of asymmetric short circuits and in the analysis of the dynamic stability of electric power systems

Resiliency Enhancement Control of Converter Fed Smart Renewable Micro Grids Against Multiple Risks

Increasing the penetration level of converter-based renewable power plants, along with the advanced grid code requirements, has made the role of the control of grid-connected voltage source converters more important. In this paper, an improved combined control method for the grid-connected converters is proposed to improve the resiliency of the microgrid against multiple risks. Two main objectives of the proposed method are to <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">i)</i> improve the steady state and dynamic performance of grid-connected converter, and <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ii)</i> enhance the resiliency of the system. It copes with high power fluctuations, severe load changes, symmetrical and asymmetrical short circuit faults, and unbalanced grid conditions. As a result, no extra means are needed to tackle each undesirable conditions. The advantages of a current control scheme are integrated with a logical switching table in the proposed control method to improve its steady state and dynamic performance. The virtual flux estimation is eliminated from the control system to reduce the dependency of the system on the parameters. It provides stable operation under normal and abnormal conditions such as unbalanced voltages, voltage sag, faults, etc. The performance of the proposed control method is compared with a virtual-based direct power control and a conventional combined control to confirm its effectiveness superiority through simulation and experimental results.

Application of an Artificial Neural Network for Detecting, Classifying, and Making Decisions about Asymmetric Short Circuits in a Synchronous Generator

Fast and accurate detection of emerging faults in synchronous generators, which have found wide application in power and transport systems, contributes to ensuring reliable operation of the entire system. This article presents a new approach to making accurate decisions on the continuation of the operation of damaged generators in accordance with the requirements of IEEE standards. The necessity of limiting the duration of operation of the generator in conditions of asymmetric short circuits in the stator windings is substantiated. The authors of the article, based on an artificial neural network in the Matlab software environment, have developed a model for detecting, classifying, and making quick and accurate decisions about the operation of the generator in the event of asymmetric short circuits in the stator windings of the generator. This makes it possible to simulate the operation of the generator at various parameters. Prior to training the neural network, the database formed by phase current and voltage signals was analyzed by various features. The neural network was trained using the back-error-propagation algorithm. The output 10 neurons of the network showed the state of the phase windings of the stator. The recorded information of the output neurons was evaluated, in terms of meeting the requirements of the IEEE standard, and decisions were made about continuing or interrupting the generator operation. Tests of the effectiveness of the model showed that it could achieve the desired result at step 49, and the calculated accuracy was 99.5833%. The results obtained can be successfully used in the development of high-speed and highly reliable diagnostic systems and control and decision-making systems for generators for various purposes.

АНАЛІЗ ЕЛЕКТРОМАГНІТНИХ ПЕРЕХІДНИХ ПРОЦЕСІВ В РОЗПОДІЛЬЧІЙ МЕРЕЖІ 10 КВ ПРОМИСЛОВОГО ПІДПРИЄМСТВА

The article is devoted to the issue of short-circuit analysis in the high-voltage network of the power supply system of an industrial enterprise. A short-circuit analysis is based on the methodology recommended by the IEC 60909 International Standard. The method of equivalent voltage source at a shortcircuit point was used to calculate a short-circuit current. The necessity to analyze short-circuit currents at all levels of the power supply system was used as justification of the feasibility of automation of the calculation process. Module "Shortcircuit analysis" of the ETAP software was used as an analysis tool. The nominal parameters of the elements of the power supply system of the industrial enterprise and its topological structure are the initial data for the calculation. A single-line scheme of a 10 kV distribution network of power supply system is designed in ETAP program for implementation of the calculation process. The calculation of the value of the initial and maximum current for symmetric (three-phase) and asymmetrical ("phase-phase" and "phase-land") short circuits at the specified points of the electrical network, as well as the detection of inconsistency of characteristics installed equipment to short-circuit currents and displaying inadequate places are the result of the module "Short-circuit analysis" of the ETAP program. The results of the analysis of short circuit in the investigated distribution network of the 10 kV industrial enterprise did not find inconsistencies of the equipment characteristics of the requirements for the stability of short-circuit currents.

ELECTROMAGNETIC COMPATIBILITY OF CONTROL AND MEASURING CABLES IN NON-STATIONARY MODES

The article deals with questions of researching and analysis of voltages induced in protective shields of three-phase shielded conductor in modes of asymmetric short circuits external to generators of power station. The development of recommendations and measures to ensure electromagnetic compatibility of measuring circuits of microprocessor devices is an urgent research task, since it is generally determined by the electrophysical properties of the interference source and the analog-digital element base of the measuring part of the devices. The scientific novelty of the work consists in the development of measures to eliminate the induced electromagnetic influence on control and measuring cables switched to current and voltage measuring transformers. The purpose of the study is to assess the electromagnetic effect of the industrial frequency field on control and measuring cables laid near a phase-shielded current pipeline, as well as to develop measures to ensure their electromagnetic compatibility. The paper presents a method for calculating the distribution of the intensity modulus H of a magnetic field induced by an industrial frequency current from a generator at an electric power station. For the modes of single-phase, two-phase and two-phase short circuits to the ground, patterns of the distribution of the magnetic field intensity modulus H are constructed, a stiffness class is assigned according to the magnetic field effect on control and measuring cables in accordance with the classification. Recommendations to reduce the level of exposure to the magnetic field intensity H are given. In case of using the screen, a repeated calculation was performed to compare and determine the effectiveness of this event. As a result of numerical experiments, the effectiveness of the use of an additional screen as one of the measures to ensure electromagnetic compatibility is justified. The use of an additional steel shield makes it possible to reduce the magnetic field strength on the surface of control and measuring cables to acceptable values and, as a result, ensure reliable operation of microprocessor control, protection and automation devices.

Features of asymmetric short circuit currents calculation in the elplek software product

In order to simplify the analysis of the electrical networks operation modes , specialized software tools for mathematical modeling of processes in them are used. There are many software products, but most of them are paid. To carry out research in complex power systems and obtain results in certain sections when performing practical and course work, as well as at certain stages of research, the authors actively use the free software product «Elplek», but in a deeper study of the software encountered certain features some parameters, which differ slightly from the generally accepted domestic method, which led to an error in the calculations in some versions of the schemes of power systems. The purpose of the research is to study the peculiarities of calculating the characteristics of asymmetric short circuits in the software product "Elplek" and to check the adequacy of the formed models. At the first stage, the object of research was chosen – this is a segment of the electrical network containing a small number of nodes and a theoretical calculation of the searched parameters of the short circuit mode in this segment of the electrical network was carried out. In the future, an approach was proposed on the task of parameters of elements in the software product "Elplek" and based on this approach, modeling was carried out as a result of which the results were obtained under the condition of the system's task, as sources of infinite power and calculations for the active part differ by 0.013%, and by reactive – 0.004%; subject to the task of the system, as sources of a certain power and calculations on the active part differ by 0.0046%, and on the jet - 0.0044%, which makes it possible to assert the feasibility of using the proposed approach in the analysis of indicators of emergency modes of complex schemes of electrical supply to consumers of certain segments of electric power systems. Features of calculation of characteristics of asymmetric short circuits in the Elplek software product are considered and the adequacy of the generated models is checked. Key words: power system segment modeling, Elplek software product, element parameter setting, single-phase short-circuit currents

Current limiting strategy for grid-connected inverters under asymmetrical short circuit faults

Grid-connected inverter plays an essential role as an interface between energy resources and the power grid. The performance of the inverters is adversely affected by the grid disturbances such as imbalances and asymmetrical short circuit faults. Then, it is necessary to enhance the functionality of the inverter under such conditions. This paper proposes an unbalance current limiting strategy for grid-connected inverters under asymmetrical short circuit fault conditions. In the proposed current limiting strategy, two main features are included: (i) second-order harmonic elimination from instantaneous active power injected into the grid, and (ii) reactive current injection according to the grid code demand. In the proposed current limiting strategy, proper positive and negative sequence current components are calculated, by which the aforementioned goals are achieved. Furthermore, to be compatible with the well-known control structure of the power converters, the current references are obtained in the form that is applicable with the commonly-used double-synchronous-references-frame control structure. To verify the effectiveness of the proposed method, the behavior of the current limiting strategy is examined by simulation of an asymmetrical short circuit fault condition in a test system. In the simulation, both the transient response upon fault occurrence and steady-state of fault condition are considered and analyzed. Furthermore, the simulation results are supported by experimental test results, verifying the effectiveness of the proposed strategy.

Ways of constructing of measuring and starting elements of microprocessor devices of fast acting automatic transfer switch

Microprocessor devices of fast-acting automatic transfer switch (FATS) are widely spread in modern net-works to increase the reliability of power supply of industrial plants in case of short-term violations of normal power supply mode. Presence of motor load in such networks requires minimizing the power interruption time to provide more favorable conditions for self-starting of electric motors. However, currently existing operation algorithms of measuring elements of FATS devices are based on the decomposition of the measured signal by using a Fourier integral, and require observation interval equals to the period of the industrial frequency. The known directional elements that are used in microprocessor-based FATS devices do not ensure reliable operation of the device with asymmetric and nearby fault. Therefore, the purpose of the work is to increase the speed of operation of the algorithms of the measuring elements of microprocessor devices of FATS, as well as to increase the reliability of the starting directional element in case of nearby fault and deep voltage drops. To achieve this goal the author uses the method of computer simulation. The method of a full-scale experiment on a laboratory bench to verify the obtained theoretical results is used. The author improves the algorithm to determine the orthogonal components of the measured current and voltage signals by integral averaging of instantaneous discrete values of the measured signals and their derivatives over the observation interval with a duration of less than a half-period of the industrial frequency. The algorithm of the operation of the starting directional element in case of deep voltage drops caused by nearby fault in both the external and internal power supply network has been improved by using the main power supply voltage from the added memory element of the pre-fault measurement cycle as a reference voltage to determine the power direction. The obtained results show that the proposed algorithm for the operation of the measuring elements will make it possible to obtain the value of the phase and amplitude of the measured signal in a time equals to 0,005 s. and use of an improved starting directional element makes it possible to increase its reliability in case of deep voltage drops and asymmetric short circuits. The proposed algorithms of operation of the measuring elements and the starting directional element can be used in FATS devices.

Вектор Пойнтинга и новая теория трансформатора. Ч. 11. Трехфазные трехстержневые трансформаторы без нейтрального провода

A topological equivalent circuit for a three-phase three-core transformer reflecting the spatial structure of its magnetic system is developed. Owing to this approach, it became possible to represent the magnetic fluxes of the magnetic circuit’s all main sections and the apertures for each of three phases directly in the circuit in the absence of the windings’ neutral wires. The circuit is constructed by stitching together the anatomical circuit models of single-phase transformers obtained in the previous parts with taking into account the relationships between the fluxes at the junctions of the phase zones in iron. Its validity is confirmed by the rigor nature of the physical and mathematical relations for idealized transformers with infinite magnetic permeability of iron and simplified magnetic field patterns, which corresponds to the generally accepted approach with neglecting the magnetization currents. The difference lies in the fact that the developed model takes into account the heterogeneity of magnetization in different parts of the magnetic circuit with allocating more than 30 sections in the iron and apertures. The transition to the model of a real three-core transformer is carried out by adding four nonlinear transverse magnetization branches in each extreme phase zone and eight branches in the central phase zone to the idealized equivalent circuit. It is shown that in cases of winding connections without neutral wires, there is no flux of the Poynting vector in interphase zones in any unbalanced mode. In this case, the problems connected with the occurrence of fluxes exceeding the no-load fluxes under the conditions of symmetric and asymmetric short circuits, as well as the occurrence of buckling fluxes in these modes in the region outside the transformer iron, are solved.

Transient Roebel Bar Force Calculation in Large Salient-Pole Synchronous Machines

The radial and tangential force components acting on each sub-conductor of a Roebel bar in a large generator lead to mechanical vibration of the stator bars in the slot. This double-frequency vibration results in additional mechanical stress on fixation materials in the slot and causes insulation deterioration and looseness of stator bars. Particularly during short circuits, the stator bars are subjected to immense radial forces. Understanding the origin of these vibrations and developing models to anticipate them is essential to increase the reliability and expected life time of large generators. This paper presents an analytical method for the estimation of Roebel bar forces during symmetrical and asymmetrical short circuits. For this purpose, an analytical calculation method is developed to estimate the transient three-phase currents during different short circuits. These currents induce tangential and radial flux density components along the slot width and length, respectively. An analytical approach is introduced to estimate these flux densities as a function of the slot length. Subsequently, an analytical method for calculating the Roebel bar forces considering three-dimensional transpositions is developed and verified with finite element calculations. Beyond the state of the art, the impact of the rotor field on the radial and tangential components of the stray fluxes are analytically estimated. The result is a comprehensive analytical tool for the estimation of Roebel bar forces during different short circuits in large generators.

Researching the Blocking of Current Protection of Electrical Power Units with Transformers

The magnetizing current inrush appears in electrical power units equipped with transformers in case of no-load energizing of the power transformers and in a number of other cases. This phenomenon could cause a false triggering of the current protection. To prevent incorrect actions of the current protection during the magnetizing current inrush a protection blocking is carrying out. The blocking principle operation is based on the fact that in a three-phase system in normal mode and in case of symmetrical short circuit the first harmonics contained in the phase currents of electrical installations form a direct sequence and the second ones – the negative sequences. In case of an asymmetric mode, including an asymmetric short circuit, the negative sequence appears, formed by the first harmonics that are part of the phase currents of the specified system. In magnetizing current inrush modes, second harmonics of significant magnitude are present in phase currents, which form the negative sequence. Based on the analysis of the information parameters of the specified sequences currents, identification of the magnetizing current inrush and short-circuit modes is performed with the implementation of the protection blocking if necessary. The study of the current protection blocking functioning was performed using computational experiment by analyzing the calculated changes of blocking parameter compared with the threshold setpoint. The specified researching is done by using the digital model that is implemented in the dynamic modeling environment MatLab-Simulink. As a result of the performed calculations, the principal operability of the proposed current protection blocking was confirmed that provides a fairly reliable identification of the magnetizing current inrush and short-circuits modes, regardless of the degree of saturation of current transformers. It was found that the proposed principle of the current protection blocking has a higher sensitivity in comparison with the classical one based on the estimation of the ratio of the second and first harmonics of the phase currents. In short-circuit modes in an electrical power units the proposed blocking causes a current protection operation delay that can be reduced by digital filters performance improvement.

Adaptive Higher-Order Sliding Mode Control of Series-Compensated DFIG-Based Wind Farm for Sub-Synchronous Control Interaction Mitigation

Sub-synchronous control interaction (SSCI) is an oscillation phenomenon caused by the interaction of converter control and series-compensated transmission line. This paper proposes a novel adaptive higher-order sliding mode (AHOSM) control strategy for damping the SSCI of a series-compensated DFIG-based wind power system. On the basis of system modeling and oscillation mechanism analysis, SSCI suppression is converted to the current tracking control problem. Firstly, an auxiliary feedback control is employed for the nonlinear series-compensated system, then integral sliding mode functions are defined to design a second-order sliding mode control law for the equivalent system. Adaptive laws for the control gains are then conceived based on the Lyapunov function considering unknown upper bounds of uncertainty derivatives. System stability is also analyzed in detail along with adaptive laws’ design. The effectiveness of the proposed control scheme is verified under different series-compensated level, different wind speed, symmetric and asymmetric short circuit fault, and internal and external disturbances. The PI control and conventional first-order sliding mode control scheme are also executed to compare the damping effect.

Analysis of transient waveforms in a power system at asymmetrical short-circuits

Analysis of transient waveforms in a power system at asymmetrical short-circuits

An Asymmetric Short-Circuit Fault Ride-Through Strategy Providing Current Limiting and Continuous Voltage Supply for Three-Phase Three-Wire Stand-Alone Inverters

The three-phase three-wire stand-alone inverter is required to have short-circuit fault ridethrough ability to achieve continuity of power supply. Generally the inverter works in current-controlled mode in symmetric short-circuit conditions, but voltage limiting and harmonic distortion will appear under this method when asymmetric short-circuit fault occurs. Several existing methods for asymmetric fault ridethrough can achieve current limiting without voltage limiting, however, none of them can guarantee that the healthy phase voltage keeps constant before and after fault. This article proposes a fault ride-through strategy for asymmetric short-circuit conditions, which not only keeps the healthy phase voltage constant with different loads, but also makes the fault phase current limited and controllable. This goal is achieved by a simple control structure combining α-axis voltage control and β-axis current control at the same time. Furthermore, the voltage and current characteristics of this method under different load conditions have been analyzed, the analysis results indicate that the impact of different loads on fault ride-through performance is almost negligible. The correctness of theoretical analysis and the effectiveness of proposed strategy is verified by experimental results.

3D Transient Electromagnetic-Temperature Field Analysis of the Loss and Heat of the Damper Bars of a Large Tubular Hydro-Generator During Short Circuit Faults

This paper studies a real 36-MW large tubular hydro-generator to solve the transient variation problem of the loss and heat of damper bars during short circuit faults in tubular hydro-generators. The transient analysis method of electromagnetic-temperature fields is adopted to study the transient variation problem of the loss and heat of damper bars during three-phase symmetrical short-circuits, single-phase to ground asymmetrical short-circuits, and phase-to-phase asymmetrical short-circuits under rated operation states. The research work provides valuable findings for improving the level of state analysis, the design and manufacturing processes, and the operation and maintenance of large-scale tubular hydro-generators.

Practical calculation of asymmetric short circuit current of DFIG connected to distribution network

In view of the existing complex analytic calculations of DFIG short-circuit current are hardly applied in engineering projects, a practical calculation of asymmetric short-circuit current of DFIG is proposed. According to the complex sequence and their Thevenin equivalent model of DFIG network, the composition of each sequence component of DFIG short-circuit current is analysed. Moreover, considering the low-voltage ride through strategy of DFIG, the negative sequence periodic components of short circuit current are well analysed during the crowbar activation and deactivation, and the formula of the negative sequence periodic components of short circuit current are derived. On the basis of positive and negative sequence open circuit voltage, calculating impedance and rotor current, the judgement of crowbar activation is established. The pre-calculated surfaces of negative sequence periodic components of short circuit current are proposed, and the procedure for calculating the asymmetric short-circuit current of DFIG is designed. Finally, the proposed method is verified by simulation.