Two‐Level Voltage Source Inverter

This chapter focuses on pulse width modulation (PWM) schemes for the highpower two‐level inverter, where the device switching frequency is normally below 1 kHz. A carrier based sinusoidal PWM (SPWM) scheme is reviewed, followed by a detailed analysis of space vector modulation (SVM) algorithms. The conventional SVM scheme usually generates both even‐ and odd‐order harmonic voltages. The chapter analyses mechanism of even‐order harmonic generation, and presents a modified SVM scheme for even‐order harmonic elimination. The primary function of a voltage source inverter (VSI) is to convert a fixed DC voltage to a three‐phase AC voltage with variable magnitude and frequency. The dwell time for the stationary vectors essentially represents the duty‐cycle time of the chosen switches during a sampling period. The dwell time calculation is based on 'volt‐second balancing' principle, the product of the reference voltage and sampling period equals the sum of the voltage multiplied by the time interval of chosen space vectors.

For most medium voltage high power applications, Neutral Point Clamped (NPC) three-level inverter is a preferred choice due to its advantages such as low cost, light weight and compact size. Space vector modulation is widely used in real-time digital control for power converters. It is especially suitable for use in NPC multilevel inverters due to its good harmonic profile, flexibility and easy digital implementation. This thesis focuses on the space vector modulation for high power three-level NPC inverter, where the switching frequency of the semiconductor devices is nominally below 1 OOOHz to reduce the power loss of the switching devices. The conventional space vector modulation (SVM) scheme for the three-level NPC inverter produces even order harmonics in the output voltages, which are not desirable for most industrial applications. In this thesis, the mechanism of even order harmonic generation is analysed. A new space vector modulation scheme, which can eliminate all the even order harmonics, is proposed. The performance of the new design is investigated and simulation results are provided for the verification purpose. The harmonic and THD profiles are compared with those of the conventional SVM scheme. The elimination of even order harmonics is achieved at the expense of a slight increase in the switching frequency. The proposed space vector modulation scheme can be applied to other types of converters for the even order harmonic elimination. An algorithm is developed to mitigate the neutral point potential deviation, which is a common problem in the NPC inverters. The simulation results show that this algorithm is suitable for both conventional and the proposed space vector modulation schemes.

Current source inverter (CSI) offers key merits of direct output current control, voltage gain operation, current limiting ability, and inherent short-circuit protection. The modulation and control for medium voltage high power application are implemented at low device switching frequency in order to attain high efficiency and to reduce cooling requirements. At low switching frequency operation, the conventional modulation techniques such as space vector modulation (SVM) and sinusoidal pulse width modulation (SPWM) cannot maintain low total harmonic distortion (THD). Synchronous optimal pulse width modulation (SOPWM) is a low frequency modulation technique that offers lower THD without compromising on device switching frequency. However, this technique requires a separate conversion technique for current source converters to incorporate additional switching constraints of CSI. Whereas, the SVM technique directly implements the switching constraints of CSI without any conversion technique and in addition SVM technique provides better dynamic performance compared to low frequency modulation techniques. Hence, the goal of this paper is to propose a hybrid modulation technique that combines the SVM technique and SOPWM technique. Three methods have been proposed for developing hybrid modulation scheme based on SVM and SOPWM. Performance of the proposed hybrid modulation is validated using experimental results.

A voltage source inverter with variable frequency and variable voltage is applied for three-phase interior permanent magnet synchronous motor (IPMSM) drive for variable speed application. To obtain the required output voltage in the line side of the inverter a suitable pulse width modulation technique is used. The most prominent PWM techniques are Sinusoidal pulse width modulation (SPWM) and Space vector pulse width modulation (SVPWM). SPWM is simple and easy method to generate pulses for inverter, whereas SVPWM is an accurate method and generate less harmonic distortion compared to SPWM. However there is a lot of complexity involved in calculation of angle and sector as the level increases with SVPWM. To overcome this a similar and easy method known as unified pulse with modulation also know as carrier based space vector pulse width modulation technique(CBSVPWM is described using the concept of effective time. By using this method the inverter output voltage is directly synthesized by the effective times and the voltage modulation task can be greatly simplified. In this paper an IPMSM drive using CBSVPWM is simulated using MATLAB/SIMULINK software and its performance is compared with SPWM by varying modulation index (MI). The simulation study reveals that CBSVPWM simple and easy method which utilizes dc bus voltage more effectively and generates less THD compared to SPWM.

Since last decades, the pulse width modulation (PWM) techniques have been an intensive research subject. Also, different kinds of methodologies have been presented on inverter switching losses, inverter output current/ voltage total harmonic distortion (THD), inverter maximum output of DC bus voltage. The Sinusoidal PWM is generally used to control the inverter output voltage and it helps to maintains drive performance. The recent years have seen digital modulation mechanisms based on theory of space vector i.e. Space vector PWM (SVPWM). The SVPWM mechanism offers the enhanced amplitude modulation indexes (MI) than sinusoidal PWM along with the reduction in the harmonics of inverter output voltage and reduced communication losses. Currently, the digital control mechanisms have got more attention than the analog counterparts, as the performance and reliability of microprocessors has increased. Most of the SVPWM mechanisms are performed by using the analog or digital circuits like microcontrollers and DSPs. From the recent study, analysis gives that use of Field Programmable Gate Arrays (FPGA) can offer more efficient and faster solutions. This paper discusses the numerous existing research aspects of FPGA realization for voltage source inverter (VSI) along with the future line of research.

Permanent magnet synchronous motor (PMSM) is the most widely used motor in electric vehicles (EV) due to its high power density and high efficiency. Conventional direct torque control (DTC) is developed to improve the performance of speed response of the PMSM drive but it suffers from higher torque ripples. Therefore a space vector modulation (SVM) based DTC has been developed to enhance the speed response. Due to the use of flux comparator and torque comparator conventional DTC has disadvantages such as higher torque and current ripples at steady state. To improve the performance of PMSM drive a space vector pulse width modulation is used with 3 level NPCI. To improve the performance of 2 level inverter in the PMSM drive is replaced with a neutral point clamped inverter (NPCI). Also, the switching selection strategy of SVM is modified using nearest three space vector pulse width modulation to reduce the total harmonic distortion (THD) in the inverter output. The simulation outcome proves that the SVM technique implementation in DTC for 3 level NPCI yields good performance for PMSM.KeywordsElectric vehicles (EV),voltage source inverter (VSI)Direct torque control (DTC)Neutral point clamped inverter (NPCI)Permanent magnet synchronous motor (PMSM)Space vector modulation (SVM)Total harmonic distortion (THD)

In this paper, Design and development of control circuit algorithm of 3-Φ Voltage Source Inverter (VSI) using Unipolar Sinusoidal Pulse Width Modulation (SPWM) techniques is proposed using STM32F407VGTx discovery board interfaced with MATLAB Simulink Environment. For implementing 3-Φ VSI, this method is most useful as it will eliminate complexity of control circuit algorithm occurred due to analog circuit, reduces cost, size of filter required, and improved overall inverter efficiency. Pulse width modulated voltage source inverter is preferred for high performance AC drives. The proposed control scheme and the result obtained by embedded simulations interfaced with STM32F4 microcontroller are presented and discussed. A simple hardware implementation is proposed and experimental results of SPWM based VSI are also shown. The Simulation results of the proposed scheme have excellent performance and compared with hardware results.

Modulation techniques for current source inverters (CSIs) have traditionally been derived from those used for voltage source inverters (VSIs), with space vector modulation (SVM) and pulse width modulation (PWM) being the most popular. Among these, PWM is the preferred choice due to its simplicity and ability to offer the same benefits as SVM, such as increased phase voltage. Ideally, PWM from VSIs could be directly applied to CSIs, but this is not feasible because CSIs require a holistic consideration of all switches on the upper (positive) and lower (negative) rails. In contrast, VSIs allow for per-phase modulation, which enables the straightforward use of a PWM modulator. In this paper, a PWM method tailored specifically for CSIs without any requirements typical for SVM, such as sector identification, is proposed. Using linear algebra, a technique for generating duty cycles averaged over a switching frequency period is derived. These duty cycles are then fed into a specially for CSIs proposed multi-threshold pulse width modulator, which finally produces the gate signals. To validate our method, both simulation and extensive experimental results are presented. The proposed approach is general and applicable to CSIs with any number of phases.

A method of deriving the switching instants of different space vectors in the direct torque control of induction motor drives using pulse width modulated voltage source inverters is described in this paper. The switching instants of different space vectors are determined to reduce torque ripple compared to the commonly used space vector concept based on the sinusoidal pulse width modulation technique. The time durations of two adjacent voltage space vectors and the corresponding zero voltage space vectors are determined in this method depending on maximum and average switching frequency, allowable for the switching devices in the three-phase inverter. The sampling rate of the impressed reference voltage vector (in other words the switching frequency of the carrier wave) is varied to keep the torque ripple within the specified limit, determined by maximum allowable switching frequency. This paper also deals with the optimum switching pattern to reduce the average switching frequency of the switching devices. A simulation program is developed in order to verify the validity of the theory.

The pulse width modulation (PWM) strategy employed in the voltage source inverter (VSI) not only control the magnitude of the output voltage but also the quality. Performance evaluations of such strategies are done in terms of fundamental voltage, total harmonic distortion (THD), switching losses etc. (primary indices) and also in terms of acoustic noise, electromagnetic interference (EMI), harmonic spread factor, distribution of harmonic power etc. (secondary indices). Multilevel inverter (MLI) has become unanimous choice in medium and high power applications due to their superior performance compared to three level inverters. The conventional Sub-Harmonic PWM (SHPWM) scheme and its variations offer the output voltage spectrum with high intensity harmonic components around the switching frequency; it will end with cluster harmonic with high acoustic noise. The first objective of this paper is to investigate harmonic spreading effects of existing multilevel inverter (MLI) strategies. Secondly the developing innovative PWM strategies for MLIs based on modified reference and carrier functions, which were proved for superior the primary indices at three-level VSI. Thorough simulation study of Pulse width modulation strategies such as SHPWM, inverted sine carrier PWM, MWM PWM, third harmonic injection PWM, triplen harmonic injection PWM, analog space vector PWM, trapezoidal PWM and discontinuous PWM for a cascaded multilevel inverter, are presented with results of primary and secondary indices. Hence, the PWM strategies of MLI are evaluated for harmonic spreading effect first time and a guide line for a beginner to select the PWM scheme for MLI fed drive systems is stenciled.

This paper presents a new, simple and generalized approach to calculate flying capacitor currents for an n-level flying capacitor converter without the use of look-up tables. The performance of the proposed method has been studied for a three-, four- and five-level flying capacitor converter in MATLAB/Simulink environment.

In this paper, a direct power control (DPC) is applied to a doubly fed induction generator (DFIG) based wind energy. The space vector modulation (SVM) algorithm has a constant switching frequency compared with the classical control; this advantage offers a good total harmonic distortion (THD) rate of the generated currents, reduces the oscillations of the calculated powers and makes the system more robust against significant disturbances. Combined with the direct power control, the system behavior will present a fast response time, which will guarantee a good tracking of the reference signal. The enhanced performances brought by this strategy are studied in the simulation results.

The Variable Frequency Drives (VFD) are a common type of controlling the speed of 3-phase induction motors which are based on two control modulation techniques of Voltage Source Inverter: Sinusoidal Pulse Width Modulation (SPWM) and Space Vector Pulse Width Modulation (SV-PWM). The SPWM control technique is known as an easier implementation, but the SV-PWM control technique can be distinguished by an easier digital realization and a better utilization of DC bus. All 3-phase PWM inverters produce a total harmonic distortion (THD) on its output, which causes electromechanical and thermal stresses. The aim of this paper is comparing the low order current harmonics while driving an unbalanced 3-phase induction motor (the case of deterioration of the insulation as a result of inter-turn short circuits between few turns of the same phase) using SPWM and SV-PWM two-level 3-phase Voltage Source Inverters (VSI). For this type of unbalanced 3-phase induction motor, the SVPWM and SV-PWM schemes are modeled and simulated using Matlab/Simulink software. A detailed comparative analysis of THD-phase currents is made for both modulation strategies at different switching frequencies while keeping the constant amplitude modulation index.

Conventional Sinusoidal Pulse Width Modulation (SPWM) topology is one of the essential Pulse Width Modulation (PWM) topologies used in Voltage Source Inverter (VSI) applications such as adjustable speed motor control, Uninterrupted Power Supplies (UPS) and Photovoltaic (PV) Systems. Although conventional SPWM is used in VSI applications, the output voltage waveform of VSI based on this topology contains a high content of harmonic distortion. Hence, this paper presents a modified SPWM topology with an Artificial Neural Network (ANN) technique. The ANN technique was applied to minimize the Total Harmonic Distortion (THD) present in the output voltage waveform of the VSI. The proposed system was designed and evaluated with MATLAB/Simulink software. Based on the comparative evaluation, the paper proves that the proposed system contributes to a 9.1 % reduction in the THD value of a three-phase two-level VSI's output voltage waveform.

In this thesis, a boost chopper using PV (Photovoltaics) system and PWM (Pulse Width Modulation) voltage type power converter were constructed to provide a pleasant environment to the patients in the hospital wards by controlling temperature, humidity and air-conditioning & heating. For the stable modulation of solar cell, synchronizing signal and control signal were processed using one chip microprocessor. Power converter system was constructed with booster chopper and voltage source inverter and test was carried out for both devices. Constant voltage control method was used to track a maximum power point at boost converter control. For the inverter control, synchronizing signal and control signal were processed by microprocessor for the stable modulation according to the switching theory of SPWM (Sinusoidal Pulse Width Modulation) and directions to each sector. Test was carried out for inverter control using SPWM control method. In addition, grid voltage was detected and this grid voltage and inverter output were operated at the same phase for the phase locking with PWM voltage source inverter so that surplus power could be linked to grid. This characteristic was applied on the temperature and humidity sensors in the general buildings and buildings having specific purposes such as hospitals. The good dynamic characteristic of inverter could be obtained by these applications. Also, PWM voltage inverter maintains a high power factor and low-frequency harmonic output so that power can be supplied in the load as well as system.