Autonomous Inverter Research Articles

MATHEMATICAL MODEL OF THE FREQUENCY CONVERTER SYSTEM OPERATION IN A SHIP'S AUTONOMOUS POWER SYSTEM

The article shows the process of building a model of the frequency converter control system for a ship's autonomous power system. This task was solved to simplify the mathematical description of the frequency converter for the ship's autonomous power system. As a result, a mathematical set of probable combinations of operating states of power switches and the corresponding values of generalised voltage vectors of the autonomous inverter using pulse width modulation were obtained. The modelling carried out on the basis of this data confirmed the correspondence of the obtained values of the phase voltage of the autonomous inverter to real energy processes. It is shown that it is important to further study the technical and methodological factors of increasing energy efficiency and the accuracy of the mathematical description of real links of power systems, which will ensure the process of a more qualitative study of energy processes in them.

Selection of output inverter for microgrid

The paper considers the problem of a microgrid output inverter selecting, when information about operating conditions is uncertain. The following factors are considered as sources of uncertainty in information about operating conditions: the possibility of the load increasing in the area, for which the microgrid is being designed, the prospects for its integration into the local power supply system, based on the microgrid, operating in the given region, as well as possible changes in the element base due to import substitution.
 A system of criteria and a methodology for optimal selection have been developed, based on a combination of multi-vector optimization approaches and the hierarchy analysis method. The quality criterion of generated energy, the criterion of reliable operation in conditions of information uncertainty and the criterion of economic indicators were cho- sen as vector criteria. The total number of scalar criteria that are components of the specified vector criteria is seven. At the first stage of selection, an algorithm is used for expert assessment of the importance of vector criteria and their scalar components, as well as values of these criteria for compared alternatives, using of the hierarchy analysis method. At the second stage of selection, considered inverters are ranked according to a global criterion, calculated based on results of the first stage. The technique is demon- strated by choosing between three well-known schemes of autonomous inverters: a voltage inverter with PWM-shaping of the output curve, a current inverter with a variable struc- ture and a group multilevel inverter based on a universal source of levels, when they are operating in autonomous or integrated into the system of other microgrid modes. The de- scribed selection technique can be used, when other microgrid conversion equipment de- vices are selecting.

RESEARCH OF VALVE MOTOR CONVERTER CONTROL METHODS

A technique for selecting the electrical equipment of a valve motor with machine commutation has been developed, which takes into account the ways of controlling the inverter. The installed capacity of the electrical equipment of the valve motor (VD) is determined by technological requirements and depends on the structure of the converter, the method of commutation of the converter keys and the methods of controlling the inverter. The use of autonomous inverters with artificial switching, the keys of which are switched due to the reactive energy accumulated in additional switching devices, increases the utilization factor of the synchronous machine. However, in this case, the circuit becomes more complicated and the installed power of the converter increases. The use of IGBT transistors and IGCT thyristors leads to a significant increase in the cost of the equipment.
 To reduce the influence of the rectifier VD on the network and match the drive voltage with the supply voltage, a matching transformer and reactive power compensation device may be needed. The amount of power of this equipment is affected by the method of controlling the rectifier, the nature of the engine load and the range of speed changes.
 Voltage regulation of the rectifier can be symmetrical or asymmetrical. Several methods of asymmetric regulation are known, among which one of the most expedient is alternating. In the case of alternating control in the three-phase bridge circuit, an artificial zero circuit is created, as a result of which the duration of the current flowing through the secondary windings of the transformer becomes less than 2/3 of the half-cycle of the supply voltage. In addition, the energy indicators also increase, in particular, the reactive power consumption decreases and the shift and power coefficients increase.

Autonomous Solar Inverters

New structural and schematic solutions for autonomous inverters of solar power plants, made on the basis of single-three-phase transformers with a rotating magnetic field, are proposed to improve the operational and technical characteristics of solar photovoltaic installations generating three-phase voltage. The features of their work on voltage conversion and stabilization are disclosed.

ANALYSIS OF EMERGENCY MODES OF TRACTION INDUCTION ELECTRIC DRIVE

The operation process of the traction asynchronous electric drive of the rolling stock may be accompanied by the occurrence of emergency modes, which are characterized by current overloads and may lead to damaging power electric circuits or mechanical part of the electric drive. Therefore, studying electromagnetic processes that occur in emergency modes of operation is relevant, in particular, for developing systems aimed at diagnosing and protecting traction equipment. Ukrainian and foreign studies of emergency modes in an asynchronous electric drive were analyzed. The most frequent causes of emergency modes are failures in static converters, therefore it is advisable to conduct an analysis of electromagnetic processes specifically in autonomous inverters. Using Matlab/Simulink software environment a simulation model of the vector control system of the traction asynchronous electric drive with spatial vector pulse width modulation was developed, and the short-circuit modes of the power switch of the autonomous voltage inverter and the disappearance of the supply voltage were simulated. The oscillograms of electromagnetic processes in these modes were shown and compared with the nominal mode of operation. The simulation results show that the analyzed emergency modes, even with a short duration of less than 0.5 s, can lead to significant damage to the electric drive. Both cases lead to significant current overloads: in the case of a short circuit of the power switch, a three-fold excess of the phase current is observed, during recovery after a supply voltage failure, the phase currents increase to the starting level, as well as an almost instantaneous increase in the supply voltage on the input filter capacitor occurs. The Fourier analysis of the phase A current in the above emergency modes shows an increase in the coefficient of nonlinear distortions by 65 %, a decrease in the amplitude of the main harmonic to 60 %, and the appearance of a significant number of subharmonics. In the case of a short circuit, the amplitudes of the subharmonics even exceed the fundamental harmonic. In addition, the process of returning from the emergency mode to normal at the first moment of time was characterized by a reduced moment of resistance (20 % of the nominal), and then by an exit to the nominal parameters.

Computational methods for studying the thermal state of frequency-controlled asynchronous traction motors

The article discusses methods for calculating the thermal state of a frequency-controlled asynchronous traction motor, and provides calculations of the thermal state of an asynchronous motor powered by a semiconductor frequency converter with an autonomous inverter. Methods for assessing the thermal state of traction asynchronous electric motors of locomotives under various operating modes are considered. Computational methods for assessing the thermal state of asynchronous motor units allow you to continuously monitor the thermal state of an asynchronous traction motor and prevent possible emergencies in a timely manner.

CALCULATION OF SHORT-CIRCUIT CURRENTS IN MARINE ELECTRIC POWER SYSTEMS WITH DIRECT CURRENT POWER DISTRIBUTION

A method for calculating shock short-circuit currents in marine electric power systems with direct current distribution of electricity is presented in the paper. These systems are widely used in shipbuilding. The calculation of short-circuit currents in such systems has a number of features related to the operating modes of these networks. In the systems with direct current electricity distribution, the generation and consumption of electricity is carried out on alternating current, while the distribution is carried out on direct current. The current standards for calculating short-circuit currents in DC and AC systems in this case cannot be used for a number of reasons. These standards do not consider specific supply sources of short-circuit points (short circuit) and variable rotational speed of valve generators in the various operating modes. AC electric motors (ED), when they are mains powered with DC power distribution via autonomous inverters, supply the short-circuit point on the DC side. Capacitor banks and accumulator batteries connected to a system with direct current electricity distribution are additional sources for the short circuit site supply. The elements of a methodology for calculating short-circuit currents in marine power plants with direct current electricity distribution, taking into account all operating modes and additional sources of short-circuit location recharge, are proposed in the paper. A method for determining the maximum value of the shock current at the short circuit design point of the ship’s electric power system is proposed.

Стабилизация напряжения шины постоянного тока автономного инвертора в составе частотноуправляемого электропривода с накопителем энергии на суперконденсаторах

Рассмотрена проблема поддержания напряжения на шине постоянного тока преобразователя частоты с автономным инвертором, входящим в состав электропривода переменного тока с частотным способом регулирования скорости. Электропривод снабжен накопителем энергии на суперконденсаторах, сопряженных с шиной постоянного тока через реверсивный повышающе-понижающий преобразователь постоянного тока в постоянный ток. При решении проблемы использована концепция поддержания напряжения шины постоянного тока на заданном уровне путем компенсации тока нагрузки автономного инвертора внешним током накопителя энергии. Накопитель энергии снабжен одноконтурной следящей системой регулирования тока. Представлены несколько вариантов систем регулирования тока, построенных по принципу комбинированного управления. Цель работы - оценка точности стабилизации напряжения на шине постоянного тока автономного инвертора. Выполнен теоретический анализ коэффициентов ошибок систем регулирования тока. Проведено моделирование электропривода без учета и с учетом дискретности преобразователя накопителя энергии. Результаты исследования могут быть полезны при разработке систем регулирования накопителей энергии на суперконденсаторах для различных приложений.

Improving the Quality of the Underwater Robot’s Contactless Battery Charging System

A special feature of the contactless battery charging system of an autonomous underwater robot is the use of a transformer with separating primary and secondary windings. As a result, a non-magnetic gap arises, which leads to the need to increase the primary current and the output current of the autonomous inverter. One of the ways to improve the quality of the system is the use of a resonant circuit at the inverter output in combination with the "soft switching" mode of its power switches. The use of resonance on the transformer secondary side also allows you to equalize the current loads of the primary and secondary windings. In this way, a minimum of losses in the inverter is achieved and the power transformer of the system is optimized. This allows you to reduce the size of the system while maintaining the transmitted power, or increase the transmitted power while maintaining the dimensions. The problem solved by using mathematical modelling with verification of the solution adequacy in a full-scale experiment.

Comparative Analysis of a Three-Level Autonomous Inverter and an Autonomous Inverter with a Transformer with a Rotating Magnetic Field in Emergency Modes of Functioning

Comparative Analysis of a Three-Level Autonomous Inverter and an Autonomous Inverter with a Transformer with a Rotating Magnetic Field in Emergency Modes of Functioning

SIMULATION OF COMMUTATION PROCESSES OF MULTI-LEVEL VOLTAGE INVERTER SWITCHES

An algorithm for controlling of commutator’s transistors of a single-phase multi-level voltage inverter is presented. The structure of the power circuit of the considered converter, in contrast to most existing circuits, does not depend on the levels number of the output curve due to the using of an universal source of levels, which are formed by output capacitors of two pulsed DC-converters. The commutator control algorithm ensures the formation of the required output curve of the inverter and excludes the occurrence of emergency situations during level commutating. Features of the converter and commutator operation in modes of active power transmission from pulse converters to the load and perception of the reactive power of the inverter load by them are considered.
 The commutator operation algorithm determines the sequence of control pulses, applied to the base of transistors, the sequence being synchronized with the process of the output voltage curve forming, as well as the direction of the inverter input current. At the same time, features of the level commutating are considered. They differ in the transition direction in value of level voltages: «up» from a lower value to a higher one and «down» from a higher value to a lower one. The sequence of supply and removal of pulses from control electrodes of switches and commutations caused by them, when the inverter input current is positive, are given. The similar commutating of levels is realized, when the inverter input current is negative. Wherein indexes of switches are rearranged in accordance with the direction replacement of their switching of the commutator circuit.
 A commutator model is realized using Micro-Cap 12 to demonstrate the operation of the algorithm. The transistor MJ15003 model is used as a commutator’s switch. In the model, output capacitors of pulse converters are represented by voltage sources, an autonomous inverter – by an active inductive load. The commutating from a higher voltage level to a lower one with a positive input current of the inverter is considered as an example. Simulation results confirm the performance of the algorithm.

Decentralized Optimal Control for Photovoltaic Systems Using Prediction in the Distribution Systems

The high penetration of photovoltaic (PV) systems and fast communications networks increase the potential for PV inverters to support the stability and performance of microgrids. PV inverters in the distribution network can work cooperatively and follow centralized and decentralized control commands to optimize energy production while meeting grid code requirements. However, there are older autonomous inverters that have already been installed and will operate in the same network as smart controllable ones. This paper proposes a decentralized optimal control (DOC) that performs multi-objective optimization for a group of PV inverters in a network of existing residential loads and autonomous inverters. The interaction of independent DOC groups in the same network is considered. The limit of PV inverter power factor is included in the control. The DOC is done by the power flow calculation and an autoregression prediction model for estimating maximum power point and loads. Overvoltage caused by prediction errors resulting in non-optimal commands from the DOC is avoided by switching to autonomous droop control (ADC). The DOC and ADC operate at different time scales to take account of communication delays between PV inverters and decentralized controller. The simulation of different scenarios of network control has proved the effectiveness of the control strategies.

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Combined propulsion plants get wide spread on modern ships of foreign construction, while in the domestic shipbuilding industry there is practically no experience in the design and testing of these vessels. A distinctive feature of the combined propulsion plants is that the energy for the vessel movement is generated in them in two (or more) different types of ship engines - thermal and electric, working on a common propulsion unit - a propeller. Combined propulsion plants are complex electromechanical systems designed to ensure the movement of the vessel and the production of electricity in the various modes of operation. Combined propulsion plants conjoin the advantages of traditional propulsion systems with heat main engines and electric propulsion plants. A unified electric power system with a combined plant of a work boat is discussed in the paper. The work boat is the first vessel built in the Russian Federation with this type of propulsion system. The feature the combined propulsion plant installed on the work boat is the ability to operate the propeller electric motor in both propulsion and generator modes. The ship uses diesel generators, storage batteries and electric propulsion motor in the generator mode as the sources of electricity. The main modes of the vessel operation, the results of dock and sea trials of the unified electric power system and combined propulsion plant are briefly discussed in the paper. During the trials, the autonomous mode of operation of each electric power source on the auxiliary electric propulsion plant and ship consumers has been checked successfully. New method of synchronization of diesel-generator and autonomous inverter has been realized; it has high speed and accuracy that excludes appearance of current shot when power source connects to the main switchboard.

The Added Value of Coordinating Inverter Control

Coordinated photovoltaic inverter control with centralized coordination of curtailment can increase the amount of energy sent from low-voltage (LV) distribution networks to the grid while respecting voltage constraints. First, this paper quantifies the improvement of such an approach relative to autonomous droop control, in terms of PV curtailment and line losses in balanced networks. It then extends the coordinated inverter control to unbalanced distribution networks. Finally, it formulates a control algorithm for different objectives such as the fairer distribution of PV curtailment and rewarding PV customers for utilizing the excess power locally. The coordinated inverter control algorithm is tested on the 114-node and 906-bus LV European test feeders with cable sizes between 50mm^2 and 240mm^2 and validated with reference to OpenDSS. The results demonstrate that coordinated inverter control is superior when applied to high impedance LV networks and LV networks constrained by the distribution transformer capacity limits compared to autonomous inverters. On the 95mm^2 overhead line, it yields a 2% increase on average in the utilized PV output with up to 5% increase for some PV locations at higher penetration levels. Up to a 20% increase in PV hosting capacity was observed for location scenarios with PV system clustering.

Study of the operating modes of the high voltage power source for barrier type discharge excitation

The issues related to the features of operation and modes setting of a high-voltage switching power source based on a sequential autonomous resonant inverter with reverse diodes used to excite an atmospheric pressure barrier type discharge are discussed in the article. It is indicated that the characteristic features of the autonomous resonant inverters operation are the occurrence of damped voltage fluctuations in the LC circuit of the inverter, as well as the dependence of the output alternating voltage on the ratio of the inverter operating frequency (thyristor switching frequency) to the natural resonant frequency of the LC circuit. Depending on this ratio, the inverter can operate in discontinuous, boundary and continuous current mode. The amplitude and shape of the inverter output voltage were controlled using a 1:1000 voltage divider with a C1-65A oscilloscope. The shape of the gate trigger pulses was obtained using a C1-167 oscilloscope. It is established that when the gate trigger pulses are asymmetrical relative to each other due to the operation features of the step-up transformers, the value of the alternating high-voltage at the inverter output is insufficient to excite the atmospheric pressure barrier type discharge. In the case of the gate trigger pulses symmetry, the output voltage of the inverter stage reaches the values required for the breakdown of the dielectric medium. Oscillograms of the inverter output voltage while adjusting its operating frequency are obtained. It is shown that the amplitude value of the voltage at the gas-discharge load increases as the operating frequency of the inverter increases. For the gate trigger pulses frequency of 250 Hz the value of the inverter output voltage amplitude was 3.4 kV, for 460 Hz – 4.0 kV, and for 550 Hz – 4.2 kV.

Pulse-width modulation in the autonomous voltage inverter in identifying parameters of the three-phase asynchronous motor

This paper deals with control algorithm of a two-level autonomous voltage inverter with pulsewidth modulation in forming test phase voltages for identification of parameters of a three-phase asynchronous motor. Necessary conversion is carried out, the laws of change of phase voltages of an autonomous inverter are obtained. The feasibility of using third order active Butterworth filters for determining first harmonics of phase voltages is shown. By means of computer simulation and experimental research, it is revealed that the required shapes of phase voltages can be implemented in a twolevel autonomous voltage inverter with pulse-width modulation, and the presence of Butterworth filters does not introduce significant distortion and allows to obtain necessary phase voltage shapes in the generalized electric machine.

Assesment of dynamic losses in power switches of autonomous inverters with different modulation algorithms

When designing an automated electric drive, it is required to provide the necessary quality indicators of speed control as much as possible with the lowest energy losses in it. Assessment of dynamic losses in power semiconductor switches of autonomous voltage inverters (AVI) is due to the need to select optimal control algorithms for them in order to increase energy efficiency in frequency-controlled asynchronous electric drive systems. To solve the set tasks, the method of mathematical modeling has been used. With the help of the developed program, an assessment of the dynamic losses in a high-speed electric drive during its operation in a steady state has been made. At a typical modulation frequency f = 6,000 Hz, which with scalar control algorithms provides a sufficient degree of approximation of the generated phase voltage envelopes to a sinusoidal form, and the use of pulse-width control (WIR) algorithms, significant dynamic losses are observed in the switches of an autonomous voltage inverter. In the course of using the WID algorithms, an accurate approximation of the sinusoidal shape is not required, which makes it possible to reduce the modulation frequency and, as a consequence, reduce the amount of dynamic losses in the AVI keys. Therefore, a comparative assessment of dynamic losses has been carried out for the scheme of a classic three-phased AVI using the algorithms of unipolar and bipolar pulse-width control. The obtained simulation results have shown that the unipolar WID algorithm is more energy-saving compared to the bipolar WID algorithm when used in high-speed electric drives.

Частотно-регульований електропривід для системи запуску газотурбінного двигуна військово-транспортного літака

З урахуванням специфіки режиму запуску військово-транспортного літака запропоновано його стартер-генератор виконувати на основі асинхронного двигуна змінного струму з модульним принципом регулювання частоти обертання. Система управління, що пропонується, має канали регулювання частоти та напруги живлення електродвигуна. Це дозволяє досягти оптимального співвідношення між динамічними та енергетичними характеристиками системи електроприводу. В результаті аналізу процесів в замкнених контурах напруги та частоти знайдені умови, які за допомогою функціонального перетворювача дозволяють підтримувати рівність між відносними значеннями частоти та напруги при постійному моменті навантаження, що забезпечує мінімальні втрати потужності в електродвигуні.

МОДЕЛИРОВАНИЕ ОДНОФАЗНОГО ДВУХУРОВНЕВОГО АВТОНОМНОГО ИНВЕРТОРА НАПРЯЖЕНИЯ С ШИРОТНО-ИМПУЛЬСНОЙ МОДУЛЯЦИЕЙ

The work is devoted to modeling the circuit of a single-phase two-level the autonomous inverter of the voltage with pulse-width modulation in the MATLAB environment. Pulse-width modulation generation unit developed. Static characteristics of the converter are obtained. With the help of a set of measuring instruments, the efficiency of the сonverter is measured under nominal operating conditions.

RESEARCH OF THE AUTOMATIC REGULATOR OF THE INPUT VOLTAGE STABILIZER OF AN AUTONOMOUS INVERTER


 
 
 This article presents the results of the study of the automatic regulator of the stabilizer. In the work, the study of the output voltage of an autonomous inverter was carried out by introducing a digital form of setting the insert of a stabilized voltage and a digital form of comparison with a reference voltage. The achieved high accuracy of voltage stabilization does not depend on the choice and careful adjustment of the elements. This made it possible to achieve stability of the voltage generator and the width of the sensitivity zone. The voltage regulation of the converter is carried out by changing the level setting associated with the shift of the insensitivity. The speed of development of a given control angle depends on the pulse repetition rate of the generator, which is limited only by the frequency properties of integrated circuits. Improvement of the control system of the autonomous inverter made it possible to increase the accuracy of stabilization of the output voltage of the inverter, which made it possible to simplify the adjustment of the control system for easy coordination with other software elements. Studies have shown that the voltage regulation of the converter is carried out by changing the level setting associated with a shift in the dead zone. The speed of development of a given control angle depends on the pulse repetition rate of the generator, which is limited only by the frequency properties of integrated circuits. And with the equality of the frequencies of the pulse generators, the minimum width of the dead zone and the change in the load from nominal to no-load, the stabilization accuracy of the output voltage of the inverter was 0.5%.