CC BY
11The scientific heritage No 51 (2020) 63
ОСНОВЫ СТРАТЕГИИ УПРАВЛЕНИЯ ШИМ-ИНВЕРТОРОМ ЛИНЕЙНОЙ МЕТАЛЛУРГИЧЕСКОЙ МГД-МАШИНЫ
Тяпин А.А.
аспирант, ФГОУ ВО Сибирский федеральный университет, г. Красноярск, Россия
Кинев Е. С.
к.т.н., директор, ООО Тепловые электрические системы, г. Красноярск, Россия
FUNDAMENTALS OF PWM INVERTER CONTROL STRATEGY OF LINEAR METALLURGICAL
MHD MACHINE
Tyapin A.
Postgraduate student, Siberian Federal University
Kinev E.
Ph.D., director of Thermal Electrical Systems LLC
Аннотация
В статье рассмотрены требования к формированию стратегии управления IGBT-ключами трехфазного двухуровневого транзисторного инвертора при работе на укороченную линейную металлургическую МГД-машину. Показано, что алгоритм работы контроллера в условиях комбинированной несимметричной нагрузки может отличаться спецификой, ввиду резкого различия токов в фазах инвертора и обмотках индуктора в установившемся режиме. По условиям эксплуатации соединение обмоток треугольником оказывается предпочтительнее, нежели соединение в звезду.
Abstract
The article discusses the requirements for the formation of a control strategy for IGBT switches of a three-phase two-level transistor inverter when operating on a shortened linear metallurgical MHD machine. It is shown that the algorithm of operation of a PWM controller under conditions of a combined unbalanced load may differ in specificity, due to the sharp difference in currents in the phases of the inverter and the inductor windings in the steady state. According to the operating conditions, the connection of the windings with a delta is preferable to the connection into a star.
Ключевые слова: Транзисторный IGBT-инвертор, линейная МГД-машина, моделирование силовых цепей, трехфазная обмотка индуктора.
Keywords: Transistor IGBT inverter, linear MHD machine, power circuit simulation, multiphase inductor winding.
Introduction. Linear induction machines (LIM) for metallurgical purposes, usually have concentric copper windings and an open magnetic circuit [1]. Inverse switching on of the windings of individual phases, with a large linear current load, determines the asymmetry of currents in the phases of the inductor of the complex for electromagnetic mixing of aluminum melt in furnaces [2]. For MHD stirrers, the value of the natural power factor at a frequency of about 1 Hz is 0.05 - 0.1. In the power supply system of the inductor, transistor IGBT inverters are used, with a capacity of up to 600 kVA at currents of 250-300 amperes and voltages up to 400 volts [3]. The design and overall dimensions of the induction machine depend on the overall capacity of the furnace and the thickness of the lining, which must be overcome to create a traveling magnetic field in the melt [4].
The modes of the inverter when operating for an inductive load are distinguished by phase asymmetry, as well as the probability of overvoltages, especially in case of emergency load shedding due to the lack of
coasting. Therefore, the vector key management strategy should include all possible states of the power supply system. For circuitry of three, four and six-zone windings of induction machines, as a rule, delta connection options are used, since when a star is connected, an excessive (emergency) current occurs in the neutral wire, or an unacceptable voltage unbalance in phases, which sharply reduces the efficiency of the device [2, 5].
An example of the circuitry of the MHD-mixing complex for aluminum melt when using a standard frequency converter for powering a linear induction machine is shown in Fig. 1. This solution can be used for the simplest LIM designs that do not have high efficiency. Moreover, even in a relatively simple configuration of the windings of a linear machine, the traditional strategy turns out to be of little use, and most of the standard protection algorithms have to be turned off due to the numerous conflicts that arise in the nominal mode of the shortened inductor.
Figure: 1. Circuitry of inverter power supply for three-phase LIM
The three-zone version of the inductor in the three-phase design corresponds to the delta pattern most common in practice. The variant of the four-zone three-phase inductor has a split winding of the first phase and a star connection.
Adopted in Fig. 1 designations determine the purpose of the main elements of the circuit: 1 - control panel, 2 - control panel board, 3 - PWM controller, 4 -power module driver, 5 - inverter power switch fuses, 6 - voltage conversion module, 7 - control system board, 8 - cooling system module, 9 - secondary power supply module, 10 - controlled rectifier.
In addition to three-phase inductors, complexes with an increased number of inductive windings, characterized by greater traction efficiency and a complicated equivalent circuit have found application in melt mixing devices [6, 7]. With a six-zone design, LIM has six concentrated windings, 10 - 12 double-layer sections in each, localized between seven teeth. In the diagram shown in Fig. 1 configuration provides for the inclusion of a six-phase winding with the following parameters: the number of phases m = 6, the number of pole pairs 2p = 6/4, the number of teeth Z = 7, the phase zone of the MDS a = 30. In fact, the six-phase inverter provides dual three-phase windings with inverted center phases in the AZBXCY configuration. The presented circuitry for connecting the inductor to the inverter is typical for machines with ring windings [14].
Taking into account the variety of LIM modes with a complex configuration of windings, it is necessary to formulate a set of requirements to be taken into account when developing a strategy for controlling the keys of a frequency inverter [8-12]. Each of the requirements must be analyzed in the system of mathematical modeling and quantitative characteristics of the dynamics and statics of the operating parameters that need to be taken into account when building the algorithm of the PWM controller operation must be obtained. To simulate the steady-state modes of MHD machines, you can use the built-in circuitry analysis subsystems of the Ansys software environment. In addition, it is possible to use any software simulator with a developed library of models. A very urgent task can be considered the construction and modeling of control systems for inductor modes using the MATLAB & Simulink program components, which make it possible to implement optimization algorithms for the operation of inductors, especially in the presence of a reverse.
An example of the configuration of the power supply system for a flat shortened six-zone inductor with two three-phase windings is shown below. The connection diagram of the power link of a frequency inverter built on the basis of half-bridges (A1, A2, A3 and A4, A5, A6) connected to a six-zone inductor is shown in Fig. 2. The following designations are adopted: power module -1, induction machine - 2.
Figure: 2. Configuration of the power section of the inverter for six-zone LIM
The presented circuit solution is more typical for electromagnetic trays, since it implements a one-sided longitudinal magnetic field inductor with annular windings. However, under certain conditions, a similar solution can be applied to transverse field inductors, which
are more efficient. In addition, with increased working clearances, it is possible to use toothless inductor designs. In this case, the windings should be made monolithic using a special molding technology with baking in special adhesive compounds.
A sketch of the construction of a six-zone three-phase flat MHD machine of a longitudinal magnetic field is shown in Fig. 3.
Figure: 3. Sketch of the construction of a six-zone linear MHD machine
The design of a one-sided linear MHD machine shown in the figure is used in the case when it is not possible to use two-sided inductors. To increase the intensity of the electromagnetic effect on the melt, transverse field inductors with concentric windings are often used [18]. In any case, edge effects and magnetic field distortion are present in shortened inductors. Therefore, for an adequate representation of three-phase and polyphase machines, the creation of an appropriate equivalent circuit is required.
It is possible to use even more complex designs of inductors, which are characterized by increased traction efficiency. Each type of LIM inductors corresponds to its own diagram of the magnetizing forces of the windings, built taking into account the traditional conditions
for inverting magnetically coupled windings of different phases.
Typical vector diagrams of magnetizing forces of concentric windings of a six-zone MHD inductor are shown in Fig. 4. To the phase relationships of the tooth magnetizing forces with index 1 of the three-phase inductor windings (Fig. 1) correspond to the group of modules A4-6. The distribution of magnetizing forces with index 2 is provided by the group of modules A1-3. The diagram shows that an initially symmetrical system can become unbalanced when the middle phase winding is inverted in each coil group [13]. This feature should also be taken into account when analyzing the modes of the inductor.
a 6 e
Figure: 4. Vector diagrams of the magnetizing forces of the windings of the six-zone LIM
An example of a circuit design for a universal power module of a frequency converter is shown in Fig. 5.
The high equipment of the power module with half-bridge racks is largely justified in the implementation of reconstruction projects for enterprises using
workable units of outdated types of transverse field inductors of two-phase or three-phase connection. With this approach, some part of the inductor fleet has to be replaced with modern linear machines. If new power supplies are combined with dissimilar inductors, additional requirements for the control strategy of frequency converters also arise.
Figure: 5. Schematic solution of the universal power module of the inverter
Let us formulate a set of the most important requirements for the synthesis of an algorithm for controlling the power switches of a frequency inverter.
1. The control strategy is created from the conditions for creating disconnected three-phase (n-three-phase, including six-phase) EMF systems.
2. Controlling the phase shifts of the operating parameters of each phase should provide properties approaching the idealized current source.
3. Management - vector. The inverter is no more than three levels.
4. Energy transport in operating mode, as well as in emergency conditions, should provide for automatic recuperation of arbitrary phases.
5. Regularities of phase reversal from the consumption mode to the generation mode are programmed according to the results of the analysis of the set of steady-state modes, of a specific type of machine.
6. Sampling intervals are selected taking into account the condition that the PWM frequency should be three or more orders of magnitude higher than the operating frequency of the induction machine.
7. Operation of the control system in emergency mode should provide for power release at time intervals that are a multiple of the rate of rise of the DC bus voltage.
8. The key management strategy should provide for the possibility of reversing the electromagnetic mode in any time interval of the steady state of the system.
9. Both the acceleration and deceleration of the power supply system of the linear induction machine must have a duration that is one order of magnitude less than the duration of the technological regime.
10. The speed of feedback of the automatic control system must be two or more orders of magnitude greater than the duration of time intervals due to sampling.
11. The stability of the operating parameters in the frequency and temperature ranges should be no worse than 0.1%.
12. The functioning of the control system and the algorithms for regulating the modes of the keys should have universality, within the same class of induction MHD machines.
13. The operation of the control system should provide dual-frequency modes of linear induction machines, as well as combined switching on of two-phase and polyphase windings.
14. The control system must meet the special requirement for increased stability in highly unfavorable electromagnetic environments.
15. The interface of the control system and the protection and control subsystems must support standard data transfer protocols, including remote control.
Other requirements are formulated on the basis of a typical control strategy for a frequency electric drive, in the part that does not contradict the characteristic modes of asymmetric LIM. It is obligatory to take into account the specifics of each specific type of induction machine, the polarity of its inductor, the number of phases, characteristics of the windings and features of
the application technology [14]. The study of the traction characteristics of some types of LIM at different power frequencies is presented in the literature [15].
Conclusion. Metallurgical linear induction machines are a narrow group of induction devices with specific properties. The edge effects and the resulting electromagnetic asymmetry of the LIM inductors force us to turn to the development of power supplies built using power IGBT transistors. Traditional inverters of a frequency-controlled asynchronous electric drive are unsuitable for powering metallurgical linear MHD machines. But specialized sources, as a rule, have a control system that provides for stable operation in the low frequency range, with extreme performance characteristics, in extremely harsh operating conditions. The power switch control strategy, embedded in the program code in the controller, must provide for numerous critical modes: asymmetry, high reactivity, recuperation, no coasting and emergency power drop, which sharply distinguish a multiphase inverter from a typical frequency converter designed to control an electric drive.
References
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