Question

Write a literature review on the different topologies of power converters and why matrix converter is the best out of all power converters. List down references as well. Also, explain the advantages of applying a matrix converter to a wind turnine system. And show connection of matrix converter to a wind turnine system on Simulink.

Answer 1

The converter acts as the link or the transforming stage between the power source and the power supply output. There are several kings of converters based on the source input voltage and the output voltage and these falls into four categories namely the AC to DC converter known as the rectifier, the AC to AC clycloconverter or frequency changer, the DC to DC voltage or current converter, and the DC to AC inverter.

AC to DC Converters:

During the first half of this century, the ignitron, which is a voltage controlled gas tube, was widely used for controlled rectification. With the advent of the thyristor, the ignitron has been replaced not only in rectification circuits but also in various other power-control equipment. In all subsequent treatment, the words ‘converter’ and ‘rectifier’ will be treated as synonymous terms. The advantages of using the thyristor as an element for controlled rectification are as follows. (a) A firing circuit (or a gate control circuit) can be constituted with the help of digital electronic devices, a recent trend being the use of microprocessors for this purpose. These IC chips are mounted on printed circuit cards which can be conveniently housed in card cages. (b) Thyristorized circuits are compact and need very little maintenance. (c) By properly changing the firing instant, thyristorized rectifier circuits can also be used for inversions. This mode of operation is adopted in applications, such as hoist control, that involve regenerative braking. (d) A controlled rectifier is an important component of a dc drive, which consists of a dc motor, a rectifier, and thyristorized power-control circuits together with relevant firing circuitry. Dc drives are driven by rectifiers whereas ac drives are controlled by inverters, ac controllers, or cycloconverters. In dc drives the speed of the motor is controlled by varying the armature voltage. On the other hand, speed control of ac drives involves the variation of both the magnitude and frequency of the applied voltage. Though the commutator of a dc motor makes it a costlier proposition, the precision obtained with it for speed control is higher than that obtained with an ac drive. When used for rectification purposes, a thyristor does not need a separate circuit for turning off. This is because, in the negative half-cycle, the polarity of the anode-to-cathode voltage applied to the device is reversed and this turns the thyristor off in a natural manner. (e) Thyristorized drives may be conveniently converted into closed-loop speed control systems, thus deriving all the advantages of such control. In these systems, a signal proportional to the actual shaft speed of the drive is compared with a signal that represents the desired speed. The error so obtained adjusts the firing angle to restore the speed to the nominal value. The following are some of the drawbacks associated with thyristorized rectifiers. (a) The waveform of the voltage obtained after rectification is of a pulsating nature and therefore contains harmonics. The harmonic content increases with the firing angle. Such a rectified voltage is not suitable for certain load circuits. To obtain a fairly constant voltage at the load, smoothing (or filter) circuits have to be employed, but these add to the cost of the rectifier. (b) The harmonics cause excessive heating due to increased iron losses in the armature of dc motors. Therefore, special precautions like providing highquality insulation and extra cooling arrangements become necessary. (c) When the load current of a rectifier is made constant by means of smoothing inductors, the thyristors in each phase carry rectangular pulses of current. Consequently, the current in the primary of a rectifying transformer is usually not sinusoidal. This causes some derating of the rectifier transformer. It thus becomes imperative that a transformer of appropriate capacity, over and above the load power that is supplied, be installed after an assessment of the extent of derating. (d) Thyristorized circuits are subject to overvoltages and overcurrents. This may be, to a large extent, due to switching transients caused by the presence of smoothing inductors, capacitors, and transformers in the circuit. For safeguarding the circuits against such transients, protective features and also some redundancy of devices are essential. However, these add to the cost of the circuitry. (e) The presence of harmonics causes an increase in the RMS value of the input current and this causes additional heat dissipation in the conductors. The effective resistance also increases due to the skin effect caused by the harmonic frequencies. Thus there is an increase in the overall ohmic loss, resulting in a decrease of the power factor presented to the input supply system. In spite of the above-mentioned drawbacks, thyristorized converters have become popular because of their compactness as well as other merits enumerated earlier. They have found extensive use in all types of industry involving speed and power control. After going through this chapter the reader should • know the different configurations of single- and three-phase, half-wave controlled rectifiers as well as the topologies of single- and three-phase, full-wave controlled rectifiers, • understand how the concept of single- and three-phase rectifiers can be generalized to n-phase rectifiers and how analyses are made with different types of loads, become familiar with the inverting mode of a converter, • know how a quantitative expression for the ripple factor can be obtained, • realize that overlapping of conduction occurs for thyristors in an n-phase rectifier when the source inductance has a significant magnitude, and know how the average voltage output of the rectifier gets reduced because of this, • know how to evaluate the rectifier efficiency and derating factorfor different rectifier configurations, • get acquainted with dual converters and their operation with simultaneous and non-simultaneous control, • become familiar with different methods for braking of motors, and • understand how the power factor of a rectifier can be improved by using common forced commutation methods.

DC-DC Converters:

A dc chopper is a power converter whose input is a fixed dc voltage and output a variable dc voltage. Dc choppers can be realized using thyristors, gate turn-off power transistors (GTOs), metal oxide semiconductor field effect transistors (MOSFETs), or integrated gate bipolar transistors. Dc choppers are used in variable speed dc drives because of high efficiency, flexibility, quick response, and capability of regeneration. Their applications include subway cars, machines hoists, forklift trucks, and mine haulers among others. Earlier, for traction purposes, series dc motors controlled by dc choppers were preferred because of the high starting torque provided by them. However, a series motor has the following limitations: (i) its field voltage cannot be easily controlled by power electronic converters, (ii) when field control is not employed the motor base speed has to be set equal to the highest desired speed of the drive; this implies using fewer field ampere turns and therefore lower torque per ampere at low speeds, and (iii) the implementation of regenerative braking is not easy. Besides being free from these drawbacks, a separately excited dc motor can also be operated to give the characteristics of a series motor. Hence the present trend is to use separately excited motors fed from dc choppers for traction as well as other applications. After going through this chapter the reader should • know the principle of a dc chopper, • understand the working of a step-down chopper and its application to a separately excited dc motor, • know the derivation of the expressions for its current and developed torque and also the determination of its speed–torque characteristics, get acquainted with the principle of a step-up chopper and its practical utility in the regenerative braking of dc motors, • become familiar with two kinds of two quadrant choppers, • get acquainted with the various methods of operation of a four-quadrant chopper, • understand how the speed–torque characteristic of a dc series motor can be drawn with the duty ratio as a parameter, and • become familiar with the various turn-off circuits employed in choppers.

DC-AC Converters:

Inverters are used for conversion of dc power into ac power of variable voltage and frequency. The phase-controlled rectifiers dealt with in Chapter 2 can be operated in the inverting mode with a firing angle greater than 90◦ so as to return power from the output (dc) side to the ac supply network; the type of commutation provided for the thyristors therein is ac line commutation. This operation, known as the inverting mode of a converter, is useful for limited applications such as cranes and hoists (which operate in all the four quadrants), as well as in load commutated inverters. It is evident that the above-mentioned mode is one of the modes of operation for a converter. As against this, the inverters discussed in this chapter are based on a different principle: they have a dc source and therefore need forced commutation circuits for the thyristors. The loads catered by them include, among others, various types of ac motors such as induction motors, synchronous motors, etc. In order to control the speed and also provide constant torque operation for these motors, it is necessary that the inverters incorporate voltage- as well as frequency control features. The output frequency of such static inverters is determined by the switching rate of the semiconductor devices, and hence they can be independently controlled. Constant torque operation can be implemented in an ac machine by maintaining a constant V /f ratio, and thus voltage control is an essential part of such a mode of operation. Voltage control also facilitates the regulation of ac machines operating under widely ranging load conditions. The ac output voltage of a power electronic inverter is usually non-sinusoidal and hence has a high harmonic content. These harmonics can be eliminated by means of appropriate filters, but the cost of the inverter increases with the sophistication demanded in the output. When the output frequency of the inverter varies over a wide range, the design of the filter becomes a formidable task. A decision regarding the incorporation of a filter should therefore be supported by economical justification. The technique of pulse width modulation (PWM) is beneficial for reducing harmonics and obtaining an output which is very nearly equal to the fundamental component of the desired output. Classification of Inverters 415 Dc choppers and inverters come under the broad title of dc line commutation because both have dc sources and need forced-commutation circuits. It is advantageous to use devices such as gate turn-off thyristors (GTOs), power transistors, and IGBTs for inversion purposes in view of the high switching speeds obtainable with them and also because no special commutation circuits are necessary as in the case of thyristors. For low-power applications that demand very high switching speeds, however, the metal oxide semiconductor field effect transistor (MOSFET) continues to be the preferred device. Apart from ac machines, other important applications of inverters are induction heating, uninterrupted power supplies (UPSs), and high-voltage dc transmission. Though dc drives are still being used in the industry, ac drives have an edge over them because of the absence of the costly and cumbersome commutator that is essential for the former. After going through this chapter the reader should • understand the principle of single-phase and three-phase inverters and gain acquaintance with their configurations, • know the operation as well as design of commutating elements for the simplest inverter, namely, the single-phase, parallel capacitor inverter, • gain acquaintance with the construction of voltage waveforms for a three-phase, six-step voltage source inverter (VSI) as well as the design of commutating elements for its McMurray and McMurray–Bedford versions. • become familiar with the principle and sequence of operation of the 120◦- mode VSI and the input circuit commutated VSI, • know the different methods for control of the VSI output, • become familiar with PWM inverters and their features, • understand the principles of operation of single-phase and three-phase current source inverters and their merits, and • gain acquaintance with the principle underlying the load commutated inverter and the control of its output quantities.

AC-AC Converters:

The main advantage of matrix converter is elimination of dc link filter. Zero switching loss devices can transfer input power to output power without any power loss. But practically it does not exist. The switching frequency of the device decides the THD of the converter. Maximum power transfer to the load is decided by nature of the control algorithm. Matrix converter has a maximum input output voltage transfer ratio limited to 87 % for sinusoidal input and output waveforms, which can be improved. Further, matrix converter requires more semiconductor devices than a conventional AC-AC indirect power frequency converter. Since monolithic bi-directional switches are available they are used for switching purpose.

Though ac voltage controllers, are useful as sources for ac drives, they suffer from the drawback that their output voltage waveforms have a high harmonic content. The voltage source and current source inverters dealt with in are free from this disadvantage, but they consist of two or more power conversion stages and each of these stages contributes to the overall conversion losses. This prompted the development of the cycloconverter, which is an ac to ac converter or a direct frequency changer. This chapter deals with the various constructional and operational aspects of line commutated cycloconverters. After going through this chapter the reader should • become familiar with the principle of the line commutated cycloconverter, • gain acquaintance with the simultaneous and non-simultaneous methods for the control of a cycloconverter and in the former case become familiar with the inverse cosine firing scheme, • know the effects of the circulating current that flows under the simultaneous control mode, • learn the derivation of the expressions for the load voltage, load current, and the input power factor and also for the different thyristor current ratings, • understand how the magnitude and frequency of the output voltage of a line commutated cycloconverter are controlled, • become familiar with two other configurations of direct frequency changers, namely, load commutated and forced-commutated cycloconverters, and • know the merits and drawbacks of line commutated cycloconverters as compared to voltage source inverters (VSIs).

Matrix Converters:

The matrix converter consists of 9 bi-directional switches that allow any output phase to be connected to any input phase. The input terminals of the converter are connected to a three phase voltage-fed system, usually the grid, while the output terminal are connected to a three phase current-fed system, like an induction motor might be. The capacitive filter on the voltage-fed side and the inductive filter on the current-fed side. With nine bi-directional switches the matrix converter can theoretically assume 512 (29 ) different switching states combinations. But not all of them can be usefully employed. Regardless to the control method used, the choice of the matrix converter switching states combinations (from now on simply matrix converter configurations) to be used must comply with two basic rules. Taking into account that the converter is supplied by a voltage source and usually feeds an inductive load, the input phases should never be short-circuited and the output currents should not be interrupted. From a practical point of view these rules imply that one and only one bi-directional switch per output phase must be switched on at any instant. By this constraint, in a three phase to three phase matrix converter 27 are the permitted switching combinations.

Advantages of Matrix converter:

The matrix converter has several advantages over traditional rectifier-inverter type power frequency converters. It provides sinusoidal input and output waveforms, with minimal higher order harmonics and no subharmonics; it has inherent bi-directional energy flow capability; the input power factor can be fully controlled. Last but not least, it has minimal energy storage requirements, which allows to get rid of bulky and lifetime-limited energy-storing capacitors. But the matrix converter has also some disadvantages. First of all it has a maximum inputoutput voltage transfer ratio limited to @ 87 % for sinusoidal input and output waveforms. It requires more semiconductor devices than a conventional AC-AC indirect power frequency converter, since no monolithic bi-directional switches exist and consequently discrete unidirectional devices, variously arranged, have to be used for each bi-directional switch. Finally, it is particularly sensitive to the disturbances of the input voltage system