The Concept Of Power Control

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02 Nov 2017

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CHAPTER 2

Power Electronics has modernized the concept of power control for power conversion and for control of electrical motor drives. It upholds an intelligent and corporeal solution to achieve the objectives of flexible and efficient control of various electrical machinery and systems. With the development of power semiconductor technology, the power handling capabilities and the switching speed of the power devices have improved extremely well. The development of microprocessors and microcontrollers technology has a great impact on the power control and also synthesizing the control strategy for the power semiconductors that can be regarded as the muscle and the microelectronics that has the power and intelligence of a human brain. Power electronic technology strives to replace the conventional electromagnetic power conversion systems with their semiconductor counterparts not only to enhance performance but also to devise its miniature version.

Inverters form an important class of power electronic circuits, which converts DC power to AC power. Vintage inverters, realized with silicon controlled rectifiers (SCR), needed bulky compounds for the communication of the SCRs. With the advent of power semiconductor technology, modern power devices such as BJTs, MOSFETs and IGBTs replaced the SCRs at low and medium power level, as these devices do not require the complex communication circuitry to turn them off.

Latest enhancements in power electronics have made the multilevel concept viable. The multilevel inverters have strained remarkable interest in the power industry. They present a new set of features that are well suited for utility and drive applications.

As a major source supplying power for industries, the motors effectively transferring electrical energy to mechanical energy without noise or exhaust gas from engine power systems are divided into two categories, the DC motors and the AC motors, according to the power supplied. Due to its inherent advantages such as simple mathematic modeling and easy control, the DC motor universally employed, needs carbon brushes and a commutator for commutation by virtue of which smooth torque is generated. During the commutation, brushes easily get worn out due to the vastly generated sparks and even the same may cause difficulties in its maintenance. On the other hand, with advantages in high starting torque, low cost, and easy maintenance, the AC motor, without problems of carbon brushes and commutator, has become the main tool in current applications of industries on the heels of fast development or progress in semiconductor technologies. The main reason is due to the high computation capability of the development of micro-processors such as Digital Signal Processing (DSP), Field Programmable Gate Array (FPGA), etc.

It realizes exact control of a three phase AC motor`s dynamic effects without restriction in the conventional V/F control. Moreover, the vector control based AC motor behaves like a DC shunt motor. However, despite its advantages such as simple structure and low price, an induction motor used in vector control has complexity as to estimate the rotor magnetic axis due to uncertainty in rotor resistance and the direct torque control (DTC) the other hand is different from the FOC. DTC does not aim at replacing the dynamics of a DC motor. Hence, it does not need complicated algorithms to decouple the torque and flux components as in the DC motor. The DTC only aims to produce a fast torque response. In addition, DTC requires only one parameter, stator resistance, among a motor`s all parameters during a control operation. However, this is not the case in the FOC, which depends on not only the stator resistance but also the rotor resistance.

In this thesis, a new switching table for Direct Torque Control (DTC) of a 3-level, 5-level, 7-level and 9-level Neutral Point Clamped (NPC) Voltage Source Inverter (VSI) fed induction motor drive is proposed. The proposed DTC IM drive is capable to control both electromagnetic torque and stator flux with very low ripple with high degrees of freedom. As the number of levels increasing, the %THD in the motor line voltage decreased. Besides, as the number of levels increased the torque ripple is reduced to minimum and the stator flux ripple is also minimized. This thesis, proposes high dynamic performance, good stability and precision in a multi-level inverter fed DTC IM drive. Control analysis and simulation studies using MATLAB/Simulink software, Fuzzy Logic Control (FLC) and space vector modulation for an induction motor with direct torque control are presented to show the effectiveness of the multilevel inverters.

2.2 Review of previous Work

Generally, the AC motor driver system is categorized into the scalar control and the field oriented control in general.[1]-[2]. Referred to as voltage/frequency (V/F control), Scalar control uses the pulse width modulation (PWM)directly to control a frequency converter`s output voltages for indirect control of a motor`s rotational speeds even though controlled rotational speeds scalar control method has drawbacks such as small starting torques, slow response rates, poor accuracy, and influences from the load.

ABB Ltd., In1983 designed AC drivers based on direct torque control [8]–[33], but not on field oriented control [5]-[7]. The ABB drivers fulfill industrial control requirements without decoupling flux and torque components via complicated mathematical operations as needed in FOC [41]-[43] . In contrast to the hysteresis, comparator employed in the early direct torque control for the determination of voltage vectors [15][18][24], fuzzy control or the neural network are used in direct torque control [22]-[26].But they had worse efficiency in control in comparison to the method of applications of a hysteresis comparator significantly [24].According to different conditions, a different voltages vector selection table is designed to satisfy the requirements [44]-[46]. However, it complicates the hardware circuit and loses the original easy implementation of direct torque control, because of using a sophisticated switching table [10]. Moreover, to simply the implementation of DTC, the stator resistance is low enough when calculating the stator flux and is generally ignored [47]-[49].

However, the stator resistance deviates from its normal value because it is subjected to the temperature rise, which results in error in the calculated stator flux. As a result, the issue with respect to changes in stator resistance caused by the temperature rising effect needs to be further investigated.

As for the issue of an induction motor`s parameters changed by the temperature rising effect, some strategies were recommended to solve the problems in other papers, i.e., the new dynamic parameter estimator based on a motor`s mathematic model in general [53]-[55]. But, they made overall control system complicated. Other strategies, such as the fuzzy control or the neural- fuzzy control in estimations are simpler than the new – type estimator based on a motor`s mathematical model [57]-[61]. However, it led to much computation load in the system based on a single chip digital signal processor (DSP) for fuzzy control or neural-fuzzy control.

Through the selection of a voltage vector table [27], the conventional direct torque control method having only six voltage vectors for the switching makes power transistors fail to generate sinusoidal voltage and causes the motor to create ripples during vector switching. Referring to space vector modulation (SVM) [32]-[37] for synthesis of more vectors than the conventional six, Habetler [29] provides SVMPWM for the synthesis of more voltage vectors and used the deadbeat current and flux control [29]-[31] to calculate properly switching vectors. However, this method is complicated and hardly realized. To expand sectors from original 6 to 12 or more vectors for effective reduction of torque and flux ripples, the discrete space vector modulation (DSVM) [38]-[43] with high-level hysteresis and multiple switching status were proposed . But the reduction of torque ripples limited. [62]-[65] is to adjust slip velocities by controlling flux frequency with added forecasted flux errors for determining a proper command vector. Due to failure of effectively solving the torque ripples, cirrincione presented a new strategy [66] for reducing currents and torque`s harmonic waves. Lots of literatures solving the same issue [67]-[76] forecasted torque change in accordance with the estimated torque and rotational speeds. Then, they used voltage vector synthesized from SVPWM to control torque ripples.

Several modulation schemes have been suggested in the literature to control the three-level inverters [77]-[95]. These PWM schemes are either carrier based or space vector based. Apart from the control of the output voltage, the PWM schemes attempt to restrain the neutral point fluctuations. In space vector based modulation schemes, the property of redundancy of inverter states for a given space vector location is exploited to achieve this objective.

A control strategy based on Direct Self Control, taking a special coupling condition to replace a three-level inverter with two two-level inverters into account, is reported in [77].

A simple on-line calculation scheme, which achieves the highest possible sampling frequency without overloading the inverter (Switching Frequency Optimal PWM control), has been suggested in [78]. Independent hysterisis comparator based controllers have also been suggested to regulate the direct and the quadrature axis components of the three-phase output voltages [79] [80].

A space vector based PWM method, which achieves the objectives of avoiding the neutral point fluctuations along with the narrow pulse, which is shorter than the minimum on/off time of the GTO thyristors has been proposed in [82]. An optimal PWM strategy, in which the harmonic loss is minimized, has been investigated in [83].

Common-mode voltage produced by conventional PWM inverters is a major cause of premature motor bearing failures. A modulation scheme to eliminate the common-mode voltages in three-level inverter has been proposed in [84]. This PWM scheme stems from the fact that there are certain switching states, which do not generate common-mode voltage. Only these switching states are used in the proposed PWM method to eliminate the common-mode voltage.

Different types of two-phase bipolar and unipolar PWMs in application to the control of three-level inverters have been described in [87]. A comprehensive investigation on the neutral point fluctuations has been presented in [88]. A space vector based PWM method is presented in [89], in which the pulse widths of the gate signals are directly derived from the phase reference voltages by simple algebraic equations. A method to compensate the DC-link voltage unbalance is proposed in [91] to ensure that the inverter output voltage exactly follows the reference voltage.

The inbuilt relationship between the SPWM and the space vector modulation for three-level inverters has been investigated in [93]. It has been shown that these two schemes function homogeneously through proper choice of common-mode injections in the case of SPWM or dwell times in the corresponding superfluous switching states in the case of Space Vector Modulation scheme.

A few interesting variants of the three-level inverters are proposed in [96]-[99].

A technique to parallel two NPC three-level inverters using a current sharing reactor has been proposed in [97] and [98] to achieve a larger capacity inverter. Various modulation strategies are devised to improve the spectral performance of such an inverter.

A new multilevel PWM inverter topology is proposed in [99]. An inner clamping capacitor is added in this circuit configuration to balance the voltage across the DC-link capacitors dynamically. The inner clamping capacitor also serves as the blocking voltage of the inner devices at turnoff. Also, bi-directional current is achieved through it. A new low-loss snubber circuit, which includes an over-voltage clamping circuit for a three-level GTO inverter is presented in [100].

The NPC configuration can be extended to realize inverter with higher number of levels. An -level diode clamp inverter typically consists of capacitors on the DC bus and produces levels of the phase voltage [101]-[103].

A variety of PWM strategies for multilevel inverter configurations have been proposed [104-[114]. A comprehensive spectral analysis for multilevel inverters is provided and the analytical expressions of the spectral components of the output waveforms in all the operating conditions are presented in [104]. In [105], multilevel inverters are studied and modeled from a control point of view.

A balancing control strategy to minimize the voltage differences among the DC-link capacitors of the generalized - level inverter is proposed in [106]. In the carrier based modulation scheme described in [107], it has been shown that rotation of the modulating waveform through different bands of carrier waveforms using line-line redundant voltage states make use of all the levels in the inverter, even during low modulation index operation. A modified carrier-based space vector modulated PWM method to suppress the voltage fluctuations of DC-link capacitors in the diode clamped four level inverter with passive rectifier is proposed in [108]. In [110], carrier-based multilevel PWM schemes are presented to minimize switch utilization.

An equivalence between the carrier-based PWM and space vector modulated PWM strategies for multilevel inverters is established in [114]. A new diode clamping multilevel inverter configuration has been proposed in [115], which works without the series association of the clamping diodes. With this structure, not only the main switches are clamped by the clamping diodes, but also the clamping diodes mutually clamp each other. Hence the large RC networks to ensure proper voltage sharing among series diodes are eliminated.

An interesting variant of the series connected H-bridge configuration is reported in [112]. It has been shown in [112] that the performance attributes of output waveform in terms of number of levels can be enhanced by using unequal, binary weighted DC power supplies in each phase. For example, it is possible to obtain a seven-level waveform with only two DC power supplies per phase, which are in the ratio 2:1. Another approach to realize a space vector with a high-resolution using series connected H-bridge inverters with asymmetric DC voltage sources has been described in [123].

A carrier wave based space vector PWM method for cascaded multilevel H-bridge inverter system is proposed in [124]. This method exploits the phase voltage redundancies to ensure an identical utilization of the devices. A multilevel space vector PWM technique based on phase- shift harmonic suppression has been presented in [125]. It has been shown that a suitable phase- shift between reference vectors of the two cells enhances the amplitude of the fundamental nearly twice and reduces main harmonics through vector summation of voltages of the cells.

V.T.Somasekhar, K.Gopakumar proposed a multilevel inverter topology cascading 3-phase two 2-level inverters in the year 2003 [137].

Statement of the problem

A classical DTC drive system, which is based on fixed hysteresis bands for both torque and flux controllers, suffers from a varying switching frequency, which is a function of the motor speed, stator/rotor fluxes, and stator voltage; it is also not constant in a steady state. Variable switching frequency is undesirable as controlling of the same becomes difficult. At minor speeds, a substantial level of acoustic noise is observed largely due to the stumpy inverter switching frequency. However, the high frequency is limited by the switching characteristics of the power devices. Hence, there would be huge torque ripples and hazy waveforms in currents and fluxes.

For these reasons, in this thesis, the space vector pulse width modulation based algorithms are projected and executed for multilevel inverter fed DTC IM drive. These space vector based algorithms not only produce the preferred fundamental frequency voltages, but also get rid of the harmonics up to the utmost extent and as a result trim down the total harmonic distortion (THD). Thus the space vector generation with a higher resolution is obtained for an induction motor with direct torque control generating the motor phase voltage waveform very smooth and close to sinusoidal. This eventually improves dynamic performance of a multilevel inverter DTC IM drive with reduced steady state ripples and low harmonic distortion accomplished.

Main contribution of the thesis

In this thesis, the various space vector pulse width modulation based algorithms for multilevel inverter fed DTC induction motor drive are proposed and implemented. The performance of these algorithms are evaluated in terms of inverter output voltage, current waveforms, total harmonic distortion, speed of DTC induction motor and torque ripples.

The following algorithms have been proposed in this thesis:

1. The space vector pulse width modulation (SVPWM) algorithm for a three-level inverter fed DTC induction motor is proposed and is used for analyzing two-level and three-level inverters. The voltage vector assortment procedure, switching time calculation and switching pattern invention for a three level inverter is detailed. The same algorithm is applied to DTC IM drive with new switching table in order to contribute to the reduction of switching power losses and to prove the advantages of a three level inverter carrying voltage with contents of less harmonic injection as against a two-level inverter which showed comparatively higher harmonic profile. The above method has been discussed in detail and results are presented and investigated.

2. The space vector pulse width modulation for multilevel using fractal approach has been proposed and implemented also for a five-level inverter fed DTC IM drive. As the levels of inverter increased, the sector identification and switching vector determination and dwelling time calculation became more complex with the computational complexity and the execution time increased. This method is mainly based on the information that the switching vector depiction of any multilevel inverter has an innate fractal structure which is triangular and also the basic unit of the hexagonal space vector switching states of a five-level inverter.

The above modulation scheme is applied to a five level inverter fed DTC IM drive to generate the optimal twelve sector switching table. This switching table ensures the dynamic performance of the proposed drive. The results are presented and investigated. The complexity and feasibility of this algorithm have been discussed.

3. A qualitative space vector pulse width modulation algorithm is proposed and implemented for Multi point clamped multilevel inverter fed DTC IM drive. In this technique, the duty cycles of reference voltage vectors are exacted hence for identifying the location of the reference voltage vector in each region. The apt switching sequence of the region and calculation of the turning ON times for each state is predicted. This scheme can be expanded to high-level inverters too. This algorithm is applied to a multilevel inverter fed DTC IM drive has been discussed in detail and results have been presented and analyzed for seven-level inverter. The total harmonic distortion have been calculated and compared with lower levels also.

4. The space vector pulse width modulation for multilevel using fractal approach has been proposed and implemented for nine-level inverter fed DTC IM drive and fuzzy logic control of nine level inverter fed DTC IM drive. As the level of inverter enhances, the sector recognition and switching vector evaluation and dwelling time calculation becomes more composite. The computational difficulty and the implementation time increase. This method is chiefly based on the truth that the switching vector representation of any multilevel inverter has an inbuilt fractal structure, which is the basic unit of this structure being the triangle shaped by the vertices of three adjacent inverter voltage space vectors. The proposed method uses plain arithmetic for evaluating the sector and look up tables, hence fractal approach is applied for multilevel inverters by means of SVPWM. The results are presented and investigated. The complexity and feasibility of this algorithm have been discussed.

Organization of thesis report

The whole thesis is organized in the following manner.

Chapter-1 proposes a brief introduction about variable speed drive and also explains what DTC is; why and how it has been evolved; the basic theory behind its success; the features and benefits of this technology and also literature review on pulse width modulation, sinusoidal pulse width modulation, space vector pulse width modulation for DTC IM drive and various topologies of multi-level inverters. Besides, the literature review of existing modulation scheme is also discussed in this chapter.

Chapter-2 proposes the declaration of the problem, the main role of the thesis and the organization of thesis.

Chapter-3 proposes the principle of direct torque controlled induction motor drive, supplied by space vector pulse width modulation (SVPWM) algorithm for a two-level and three-level NPC inverter. The method directly controls the stator flux linkage and electromagnetic torque by the selection of the optimum inverter switching modes and the selection is made to limit the flux and torque errors inside the equivalent flux and torque hysteresis bands to obtain fast torque reply. Also the simulated results that are the waveforms of the current, motor speed, torque, stator flux linkage etc.

Chapter-4 proposes the power circuit configuration of a five-level MPC voltage source inverter, SVM algorithm for implementation of five-level inverter and DTC using five-level MPC inverter fed VSI. This method directly controls the stator flux linkage and electromagnetic torque by the selection of the optimum inverter switching modes and the selection is made to restrict the flux and torque errors within the corresponding flux and torque hysteresis bands to obtain a fast torque response. Also the simulated results that are the waveforms of the current, motor speed, torque, stator flux linkage etc. are shown and compared with the lower levels.

Chapter-5 proposes the power circuit configuration of seven-level MPC inverter, qualitative space vector pulse width modulation algorithm for neutral point clamped seven-level inverter. In this method, the duty cycles of reference voltage vectors are accurated accordingly for recognizing the location of the reference voltage vector in each region. The suitable switching sequence of the region and calculation of the turning ON times for each state is predicted. Based on the above modulation, a seven level inverter topology is applied to DTC IM drive. The method directly controls stator flux linkage and the electromagnetic torque by the opting the optimum inverter switching modes and the selection is made to limit the flux and torque errors within the corresponding flux and torque hysteresis bands to get fast torque response. Also the simulated results that are the waveforms of the current, motor speed, torque, stator flux linkage etc. are shown and compared with 2-level, 3-level, 5-level and 7-levels.

Chapter-6 proposes the power circuit configuration of a nine-level MPC inverter, inherent fractal structure space vector pulse width modulation algorithm for multi point clamped nine-level inverter. This method is primarily based on the detail that the switching vector representation of any multilevel inverter has a natural fractal structure, which is the essential unit with whose structure being triangular, formed by the vertices of three adjacent inverter voltage space vectors. Based on the above modulation, power circuit topology is applied to DTC IM drive. This method straight away controls the stator flux linkage and electromagnetic torque by the option of the optimum inverter switching modes and however, the selection is made to check the flux and torque errors within the corresponding flux and torque hysteresis bands to attain fast torque response. Also the simulated results that are the waveforms of the current, motor speed, torque, stator flux linkage etc. are shown and compared with the lower levels.

The above method automation is done by using the Fuzzy Logic Controller (FLC) of nine-level inverter fed DTC IM drive which is proposed in this chapter, and compared.

Chapter-7 gives conclusions of the thesis work followed by suggestions for future work.



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