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Abstract

Content

Introduction

On the modern world economy, energy is a system-forming industry, which has been developing at a faster pace in the last 15 years. The growth of the population, as well as the development of scientific and technological progress, leads to an exponential growth in the demand for energy. At present, the main raw material used to produce energy in the world is the extracted from the depths of the earth – hydrocarbon fuel (oil, gas and coal). Increasing the need for energy necessitates an increase in hydrocarbon production, which results in the depletion of their reserves. The widespread use of hydrocarbon fuels in all sectors of the economy and the lack of a real alternative to their replacement, make the country's economy vulnerable to risks associated with natural, climatic and economic problems. In addition, the use of this type of fuel causes a significant number of environmental problems, the negative effect of which is already evident on a global scale, as a result of which the development of alternative energy sources is one of the current directions in building the energy future. [1]

1.Theme urgency

The urgency of work is determined by the transition of modern industrial production to the use of energy-saving technologies. The use of a doubly fed induction generator (DFIG) in modern automated systems allows to significantly improve the technical and economic performance of various process units. For example, one of the most promising areas is the use of wind energy in an asynchronous machine with direct power from the stator side and power supply through a reversible converter on the rotor side [2]:

One of the main directions of the transition is the replacement of existing adjustable electric drives of direct current to AC drives that have the best mass, energy and dynamic characteristics. The most widespread among AC drives was an electric drive based on an induction motor with a squirrel-cage rotor with a frequency converter in the stator circuit. The use of the vector control mode in this electric drive makes it possible to divide the flow control channels and the electromagnetic moment, which makes it possible to build subordinate control systems similarly to a DC electric drive. However, an electric drive based on an induction motor with a squirrel cage rotor has a number of disadvantages, such as: low overload capacity at rotor speeds above the nominal, low speed regulation range down from the nominal speed sensor on the motor shaft and the inability to control the gliding energy. The range of speed regulation down from nominal without a sensor on the motor shaft usually does not exceed 1:50, which is connected with the complexity of ensuring the operation of the vector control system at low stator current frequencies. To ensure deep regulation of the speed down from the rated speed, it is necessary to use a sensor on the shaft, which reduces the reliability of the electric drive. Elimination of the above deficiencies is possible in doubly fed induction generator.

2. Goal and tasks of the research, the planned results

The master's work is devoted to the study of a double-feeding machine with direct power supply from the stator side and power supply through a reversible converter from the rotor side.

Research objective:  description of the operation principles of a TIR-based power plant for the use of wind energy.
To achieve this goal, it is necessary to solve the following tasks:

3. Theoretical part

3.1 General information about the doubly fed induction generator (DFIG)

Before proceeding to a specific description of the mathematical model of a DFIG, let us consider what a machine is. DFIG — structurally it is an asynchronous machine with a phase rotor having a separate power supply of the stator and rotor windings, the sum (difference) of the frequencies of the power current being a multiple of the shaft rotation frequency. For example, if the stator winding of the motor is energized at 50 Hz, and the winding of the phase rotor is & mdash; frequency of 10 Hz, then the rotational speed (with bipolar windings) of the rotor can be, depending on the order of rotation of the rotor phases, 40 or 60 revolutions per second. But the principle of the DFIG action corresponds to a synchronous machine, since the currents in the rotor are obtained not by sliding the latter relative to the stator field, but by supplying a current from an external source. DFIG can operate both in the motor and in the generator mode [3].

The disadvantages of dual-power machines are:

Benefits of dual power machines:

3.2 The choice of engines and their characteristics

When carrying out the investigation of a dual-power machine, in order to simplify the calculations and analyze the results, we take any asynchronous machine with a phase rotor as the subject of the study. For example, let's take AD of type AOK2-51-4T2. Its technical data is given in the table below 1.1.


Table 1.1 – Engine specification №1 [4]

 

Фотография

To test the technique of implementing power regulators and speed, let's take another AD with a phase rotor of the type 4АНК280М8У3. Its characteristics are given in the table 1.2.

 

Table 1.2 – Engine specification №2 [5]

 

Фотография


3.3 Mathematical description of the induction motor

With vector control from the side of the rotor, one of the possible ones is the use of the control law ΨS=const.

We introduce generalized stress vectors US and UR, currents IS and IR, flux linking ΨS and ΨR stator and rotor with the help of a mathematical description, called the Park-Gorev equations.

Equations of the balance of stresses of the stator and rotor circuits have the form:

Фотография

 

The equations of the relationship between currents and flux linkages:

Фотография

 

Using the equations of stress balance of stator and rotor chains, as well as the equations of coupling between currents and flux linkages, we exclude vectors IS,ΨR using the equations:

Фотография

Фотография

 

As a result, after transformations, it is possible to obtain equations for the balance of stator and rotor voltages of an induction motor:

Фотография

Фотография

где – equivalent active resistance of an induction motor;
TS=LS/RS – rotary chain time constant of an induction motor.

We decompose the last equations into components taking into account condition Фотография. As a result, two systems of scalar equations can be obtained:

Фотография


Фотография


Фотография


The second equation is the condition for the exact orientation of the coordinate system d,q along the vector of the stator flux linkage.

Фотография

Фотография,
where ΨSq=0, kS=Lm/LS – coefficient of electromagnetic coupling of the stator.

The equation of the electromagnetic moment is obtained in the form:

Фотография


Using the equations obtained, as well as the equation of motion,

Фотография


It is possible to construct a structural scheme, where equivalent time constant of the electromagnetic circuits of the induction motor.


Фотография

Picture 1 – Block diagram of an asynchronous motor with control from the side of the rotor with regulation of the stator flux coupling



Фотография


Picture 2 – Components of the double-feed machine



Фотография


Picture 3 – The active power flux of the MIS during operation at the super-synchronous and subsynchronous speeds [6]

(animation: 4 frames, 7 repetition cycles, 27 kilobytes)

The maximum achievable dynamic characteristics of the drive, taking into account the given physical limitations of its energy channel, can be obtained only in systems with relay control principles, since they allow the maximum use of the available control resource in the system. Often this is done at the expense of the power indicators of the electric drive, for example, accompanied by an increase in the additional losses of the voltage inverter-motor system caused by high-frequency switching of the inverter's keys. The current relay circuit implements a sliding mode for monitoring instantaneous current error values, generates energy-efficient algorithms for switching the inverter's keys, is insensitive to engine parameters and to limited errors in measuring the vector of equivalent voltage, provides the maximum possible use of the control resource available in the drive in dynamic modes of large deviations current errors. The advantages of using relay controllers in the control system include:
• astaticism to parametric and coordinate perturbations;
• high accuracy and speed;
• The possibility of working in the energy saving mode due to the reduction of the number of commutations.

Conclusions

n accordance with the purpose and objectives of the master's work, theoretical studies of the dual-power machine were carried out. General information on the doubly fed induction generator (DFIG) and the coordinate systems used in modeling were described. The investigated engines, their characteristics and substitution circuits were also described, the mathematical description of the induction motor is given.

When developing a system for regulating reactive power and speed, a description of the functional diagrams of control systems and calculation of reactive power was given. Objects of regulation of reactive power and speed, as well as adjustment of regulators are described.

Based on the functional diagrams, the electric drive control system was developed and investigated. The study of the dynamic properties of the drive control system in typical operating modes was performed on a mathematical model using the MATLAB package and its SIMULINK application. The general description of the investigated model, as well as its individual elements, is given.

Also results of modeling in the certain mode of work are resulted.

At the time of writing this essay, the master's work is not yet complete. Approximate date of completion of the master's work: June 2017. The full text of the work and materials on the topic can be obtained from the author or his supervisor after the specified date.

References

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