


Vol 88, No 7 (2017)
- Year: 2017
- Articles: 16
- URL: https://bakhtiniada.ru/1068-3712/issue/view/14277
Article
Analysis of characteristics of electric arc furnaces as control objects
Abstract
Small-scale analysis of the operating modes of electric arc furnaces (EAFs) has been performed under conditions of basic control and disturbing effects. The influence of the nonlinearity of the current-carrying conductor on the power-transmission coefficients has been investigated for different melting-process stages. The study of the adjusting characteristics of the furnace has shown that power-transmission coefficients have a sign-alternating character during movements of electrodes with transition through zero at maximum power, which makes such a control action ineffective under the above conditions. The nonlinearity of the current-carrying conductor weakly affects the current value at the maximum power, but it reduces the arc voltage and length. The values of power-transmission coefficients comparable in arc length and current in the working range of the operating conditions show an equivalence of the controllers that stabilize the current or the arc length (arc voltage) in terms of the accuracy of controlling the arc power. It has been shown that the current-transmission coefficient during the melting stage reaches considerable values, which causes pronounced changes in the furnace conditions and occurrence of the flicker. It has been established that, to mitigate the adverse effect of the supply-voltage disturbance, it is reasonable to apply current controllers.



Mathematical simulations of the electromagnetic system of a subminiature magnetoelectric engine
Abstract
This paper is devoted to simulations of a valve electric engine with excitation from high-energy permanent rare-earth magnets. A feature of the design of the engine under consideration is that a stator is manufactured using the polycapillary glass-fiber technique. The conductors of the distributed stator winding are placed in holes pm the tube wall. The cylindrical bipolar rotor is made of iron–neodymium–boron. The assumption accepted in the work during the solving the Laplace equation has made it possible to obtain analytical solutions for the magnetic flux, emf, current, electromagnetic torque, and electromagnetic power.



Magnetic conduction of a magnetic reducer modulator
Abstract
The through magnetic conductivities of a modulator have been considered in the magnetic fluxes produced by the stator winding and rotor magnets at observation points that are located on the outer surface of magnets and bore surface of a stator, respectively. The numerical calculations have shown that these conductivities, which substantially differ from each other, are formed by the fundamental harmonics. The contribution of higher harmonics in these conductivities is negligible. The analytical calculations of the through magnetic conductivity of a modulator (based on the classical approaches to electrical machines for calculations of magnetic fields in air gaps with single-sided serration) lead to a significant error. The effect of magnetic saturation of the modulator cores made of laminated electric steel sheets on the through magnetic conductivities has been estimated.



A technique of structural synthesis of a relay current controller in a transistor-based electric drive
Abstract
From a signal methodological perspective, we developed a technique for structural synthesis of a relay current controller in an automatic current-control system for different laws of switching the keys in a valve inverter. The technique consists of several phases: a phase in which timing diagrams of transistor control impulses in the bridge circuit of the valve inverter are constructed, a phase in which the moments of locking and unlocking of the bridge transistors are determined; a phase in which the static characteristics of the relay elements are determined; and a phase in which the number of relay elements of the current controller are minimized and the structure of the controller is formed. Timing diagrams of the transistor control impulses are constructed on the basis of time diagrams, which describe tracking of the control input by the armature current in the direct-current (dc) motors and the armature voltage at the electric-motor armature in all operation modes of the electric drive, and depend on the used law of switching the keys of the bridge circuit in the valve inverter. The moments of locking and unlocking of the inverter bridge transistors are determined using relay elements with an adjustment characteristic of the discontinuous function of a step type, at a smooth alternating signal at the input of the relay element, which is a signal of desynchronization in the automatic currentcontrol system. The technique allows composing the analytical expressions for static characteristics of the relay elements, which define the moments of locking and unlocking the bridge transistors, and minimizing the number of relay elements in the controller. The synthesized relay current controllers with a minimal number of relay elements provide a high accuracy of the armature current in the dc motor in all operation modes of the electric drive.



Simulating the transients in a load node with a triple-wound transformer at different voltage levels
Abstract
The transient processes in the load node of a power system, which contains powerful induction motors and triple-wound transformers, are simulated. A technique for transient calculation using direct-current synthetic schemes of electric circuits (Dommel’s algorithm) is presented. A dynamic model of an induction motor in the phase domain and a model of a triple-wound transformer based on inductively coupled branches are used taking into account the transformation of voltage levels. It is shown that the mutual influence of different voltage levels in the circuits can be investigated in the phase domain, within one model, and in the course of one simulation process. As an example, we consider a 110/35/10-kV load node that contains an induction motor, a static load, and a reactive power-compensation device in the form of a capacitor on the low-voltage side, as well as power-transmission lines on the medium-voltage side. Dommel’s algorithm makes it possible to investigate various types of nonlinear elements in the system and does not impose any restrictions on the waveform and harmonic composition of signals in the model. The algorithm also allows one to change the parameters and configuration of the timing constraints, which increases the range of the modes considered. When simulating the load node in this domain, various types of asymmetry and complex faults can be considered. The model of the induction motor is highly flexible and yields good results when calculating the dynamic modes.



Creation of a DC forced armor electromagnet supplied by a circuit with a ballast resistor and minimized with respect to the mass of active materials
Abstract
Weight, dimensional, and power parameters mainly determine the characteristics of electrical equipment. For electromagnetic switching equipment, a substantial improvement in these indices can be achieved thanks to forced and pulse driver control. Therefore, the development of an effective technique to design forced driving electromagnets using refined mathematical models and the determination of electromagnetic and thermal characteristics of these magnets taking into account modern operation requirements for switching equipment are an urgent problem. In this paper, a method to design a forced armor plunger electromagnet in a circuit with a ballast resistor has been proposed. This technique makes it possible to minimize the total mass of electrical steel and winding copper used in the manufacturing of the electromagnet. Calculation expressions for the basic proportionality of the electromagnet that minimize its total weight have been obtained.



A control system of a fan drive based on airflow doppler ultrasonic control
Abstract
The control system of a fan drive based on control of the integral (section-average) velocity of the generated airflow is proposed. For this purpose, the flow velocity (rate) is measured using a Doppler ultrasonic device. The results of laboratory experiments under simultaneous contact and remote control of the flow and rotation speed of the fan in the steady-state and transient modes shows that the ultrasonic device can be integrated into the control system. Corresponding elements and their transfer functions for the automatic-control system (ACS) of the fan unit are identified. The stability is determined, and the results of simulation modeling of operation of a closed-loop system in which the integral velocity of the airflow is assumed to be a regulated variable value are given. The drive of the fan unit should be controlled taking into account the changes of the aerodynamic characteristics of the system: the positions of the curtains, blinds, shutters, etc. The proposed approach made it possible to carry out the noncontact control of an open airflow at the system output independently of the drive type and fan itself and introduce appropriate modifications into the traditional algorithms of automated control of drives of fans, curtains, and shutters.



Diagnostics and an on-load operation algorithm of high-speed voltage regulators
Abstract
Diagnostics methods are considered of single- and three-phase high-speed switching units, such as of the PC and PNOA series, mounted on 35- to 330-kV power transformers. The diagnostics is carried out using a multichannel digital oscilloscope without opening the contactor tank and draining the dielectric liquid from it. Algorithms for automated determination of working capacity of switching devices are presented. It is revealed that the developed algorithms make it possible to shorten the time for preparing and obtaining the oscillograms of currents and automatic processing of measured values, automate the determination of working capacity of switching devices, record the diagnosing results in print, and create a database in digital form.



Measurement of currents in systems of power-system protection from single-phase earth faults and in automated control by arc-suppression reactors
Abstract
The behavior of a null-string current transformer as an inertial element was analyzed, and the transfer function was derived in accordance with the equivalent circuit of current transformer. The oscillograms of real null-string currents and their mathematical description in the time and frequency domains were presented. The spectral characteristics of mathematical models of signals depending on the form and duration of the signal observation were shown. The signal passage through the measurement transformer in the transient mode was investigated. Mathematical expressions in the time and frequency domains that describe the output signals and errors of transform of the null-string current transformer for various input actions were obtained. The experimental results are presented, in particular, the oscillograms of real transient processes for various transformers, which confirm the reliability of the used mathematical models. The relations of the parameters influencing the transform error during the transformer operation in the dynamic mode were found. A suitability of the existing null-string current transformers for power protection purposes was estimated. Requirements for new null-string current transformers taking into account the features of modern digital power-system-protection devices were formulated.



A DC voltage converter with a series resonant inverter and asymmetric control of power transistors
Abstract
The interest in dc converters with a series resonant inverter (resonant converters) is due to the fact that they easily realize switching of semiconductor devices at zero current and zero voltage in a device. This makes it possible to significantly reduce power losses during switching of semiconductor devices and to realize the operation of a dc–dc converter at higher frequencies with a high efficiency. In the foreign literature, many circuits of resonant converters are described and many results of experimental studies are presented. However, the theory of such converters has not been sufficiently developed and theoretical studies are conducted using approximate methods, for example, fundamental-harmonic method. This paper describes the operating principle of the power part of a dc–dc converter with a series resonant inverter with asymmetric control in the discontinuous conduction mode (DCM), when the switching frequency is less than the resonance frequency of the LC circuit. The features of symmetric and asymmetric control of the inverter’s power transistors are described, in which bipolar current pulses are formed in half of the switching period. An algorithm for asymmetric control of transistors is proposed, and a scheme for implementing this algorithm on discrete components in the form of a virtual model in the Matlab–Simulink environment is presented. The results of modeling (transient and steady-state conditions, external characteristics, and other dependences), as well as the results of a comparison of the taken and theoretical (constructed from analytical relationships) characteristics, are presented.



The use of statistical methods to analyze electric-energy consumption by budget institutions
Abstract
The possibilities of applying statistical methods to analyze the consumption of electric energy by budget organizations and control energy efficiency are studied. Statistical methods are used to consider the influence of the season and climatic conditions on consumption of electric energy, which allowed a simulation model to be created of electric-energy consumption for illumination and other needs according to data of devices considering only the total consumption of electric energy by all groups of consumers. The change in monthly consumption of electric energy within a year is a stochastic value depending on the duration of light in a day and other factors. The statistical characteristics of the monthly consumption of electric energy by individual buildings of Chuvash State University are determined. Dispersion analysis shows that distribution of the values of the monthly electric-energy consumptions by most buildings corresponds to the normal law. The correlation between the monthly electric-energy consumption and duration of light in a day permitted one to carry out a regression analysis and determine the actual average power of the lighting system and actual average power of the rest of the energy consumption of each facility. The results of the statistical analysis make it possible to compile the electric-energy consumption balance, predict electric-energy consumption by the different group of consumers, estimate the efficiency of energy-saving measures, and obtain the data necessary to fill in energy declarations and energy-performance certificates.



Adjustable magnetic gear
Abstract
A mathematical interpretation of a combined electromechanical device possessing the properties of a synchronous machine connected to a frequency converter and of a magnetic gear with a variable reduction factor is considered. The stator winding of the synchronous machine is concentrated (tooth winding), and its spatial period covers the full number of teeth z. An expression for the frequency response of the magnetomotive force (MMF) of the tooth winding was obtained. In the variant under consideration (z = 36), the fundamental harmonic of the magnetic field, which sets the number of stator pole pairs, is v = p = 17. The MMF of the magnets of the internal rotor is represented as a Fourier series. Formulas are given making it possible to calculate the magnetic fields in two air gaps of the magnetic gear caused by the stator current and magnets of a high-speed rotor (using the method of specific magnetic conductivity). The electromagnetic torques of the gear shafts are calculated by the stretch method. The electromagnetic torque caused by the stator winding is determined on the basis of the instantaneous values of the EMF of rotation and transformation of each winding coil without using the winding coefficients. A differential equation system for calculating the transient processes in three channels of a wind-power plant is obtained: in two mechanical channels and in one electrical channel. A closed system with a PID controller is proposed making it possible to stabilize the rotational speed of the electric generator of a wind-power plant with a variable wind speed.



A model of an intact power facility as an indicator of damage
Abstract
A general approach to the detection of faults in observed power system facilities is considered. An important property of the intact object model, namely, its capability of selecting the modes of the electric system that includes the protected object, is indicated. The entire set of the modes is divided into two subsets designated as the α and β modes. If it is in the α modes, the protected object is damaged, and, if it is in the β modes, it is in operating conditions. The selectivity of the relay protection is ensured by complete separation from the β modes. The model adequate to the intact power facility copes with this task. Upon damage to the object when the adequacy of the model is violated, the necessary prerequisites are created to ensure the acceptable sensitivity and speed of the relay protection. As an example, two power facilities are considered, i.e., a power-transmission line with one-sided monitoring and a three-phase transformer with a DYN winding connection. The β modes are faults outside the protected zone for the power-transmission line and magnetizing- current inrushes for the transformer.



Increasing the efficiency of joint operation of a solar-power plant with an industrial alternating-current network
Abstract
The use of photoelectric-converter modules (PCM) with integrated direct-current regulators in low- and medium-power-supply systems working jointly with an industrial network permits production to be increased of the power generated by photoelectric-converter modules due to operation of all modules in the maximum power-generation mode. With the decrease in the intensity of solar radiation of some of the chain of the series-connected photoelectric-converter modules, the simultaneous work of all modules of the chain in the maximum power selection mode can lead to the appearance of overvoltages on elements of direct-current regulators integrated into the modules. A means is proposed in this article to control the chain of seriesconnected photoelectric-converter modules with integrated direct-current regulators. The control algorithm of the direct-current regulators permitting one to equalize the voltage at the output of the series-connected regulators under very nonuniform illumination of the photoelectric-converter modules. Computer models are studied, and results of simulation in Matlab/Simulink software system are presented.



Construction of combined models of the properties of bulk high-temperature superconducting materials
Abstract
The article contains the results of research that was aimed at improving the models of properties of bulk high-temperature superconducting (HTS) materials, which are necessary for numerical analysis of electromagnetic fields in electrical devices containing elements of high-temperature superconductors. Approximating combined models for a set of transport and bound currents are considered, which are determined, respectively, by the resistive model for currents and the nonlinear hysteresis model for the magnetization. The model of transport currents is based on the known types of the critical-state model. The material-magnetization model is composed for a set of magnetic moments of elementary superconducting cylinders with diameters that are much smaller than their length, which are oriented in the direction of the principal anisotropy axis of the material. The approximation to the actual conditions is realized by the statistical characteristics that specify the possible changes in the critical current density, the critical field strength, and the spatial orientation of the basic axes of the elementary cylinders. The results of studies of a laboratory model, which consists of a cylindrical HTS element and an annular permanent magnet, are presented and compared with the results of electromagnetic-field calculations according to the proposed models of properties.



Small devices for longitudinal compensation in overhead power lines
Abstract
Distributed-series impedance (DSI) technology for overhead power lines is under active development in the United States and Russia. DSI can be used for a wide range of application, for example, for redistribution of power flows in parallel lines and cross sections; phase balancing; increasing the power-transfer capability, stability, and reliability of the functioning of an electric-power network; and reducing power losses. This article describes the principles of operation of the DSI and proposes a generalized analytical model, which illustrates the concept of controlling the reactance introduced by means of a device in a power line. The structure of an inductive-capacitive DSI with two capacitors made by OAO G.M. Krizhizhanovskii Energetics Institute in cooperation with AO NTTs FSK EES is described in detail. This work discusses general characteristics and parameters of the device. The main modes of operation of the developed DSI prototypes are presented. The feasibility of the devices has been confirmed during trial operation.


