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.

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.

The inverters and control methods are applied in more and more fields with power electronics development. The technology of current hysteretic band PWM is a kind of easy and reliable control method. But the harmonic distortion of load current is big amplitude around switching frequency side band. The paper introduce a Varied Hysteretic-band Current control technique(VHBCC), this method can spreads the spectrum contents of load current around switching frequency side band, reduces the harmonic distortion of load current . At last, The results of simulation prove the method feasible and effective.

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.

A new carrier-based discontinuous pulse-width modulation (PWM) scheme anchored on a novel space vector modulation methodology is proposed in this paper for four-leg converters. Using a space vector definition that includes the zero sequence voltage component and partitioning the feasible sixteen modes into three separate sets, the expressions for the modulation signals for the discontinuous carrier based PWM scheme are set forth. Significantly, the switching devices can be clamped either to the positive or negative rail for 120 degrees under all operating conditions of unbalanced three-phase voltages; ensuring the reduction of switching losses and the effective switching frequency of the inverters. The discontinuous PWM modulation methodology proposed which is shown by experimental results to synthesise desirable balanced or unbalanced three-phase voltage sets complements and further clarifies the results of the 3-dimensional space vector modulation scheme (3-D SVM) for four-leg converters reported in the literature.

A harmonic elimination pulse width modulation (PWM) strategy is proposed to minimize the switching losses by continuously tracking the load power angle and clamping the switches around the peak current regions for three-phase inverters. The proposed harmonic elimination PWM strategy is a two-phase modulation method, as any desired voltage space vector is contributed by two pairs of switches only with one pair of switches always inactivated. If the peak of the load current can be centered in the clamped area (or the clamped angle) where the switches are inactivated, then switching losses can be minimized. However, during operation, the power angle between the load voltage and current varies with loads. The peak of the load current may not always be centered in the clamped area. By measuring the load power angle on-line, the proposed PWM strategy adjusts the clamped angle so as to center the peak of the load current in the clamped area. To verify the proposed method, a separately excited synchronous motor served as the load, since its current can be arbitrarily controlled to be leading or lagging. Experiments have shown the effectiveness of the proposed approach in tracking the lower power angle variation and in minimizing switching losses.

Variable frequency and variable voltage are increasingly supplied by a three-level voltage source inverter for variable speed applications. The appropriate output voltage at the line side of the inverter is obtained using a suitable pulse width modulation technique. This chapter investigates the three-level Diode Clamped or Neutral Point Clamped topology, which is a popular multi-level topology. The modulation wave is generated using two Sine-triangle methods and two Space Vector Pulse Width Modulation methods. To produce the PWM signals for the three phases, these modulation waves are compared to a triangular carrier. Instead of the three phase modulating waves used in SPWM, a rotating reference voltage vector is used as a voltage reference in SVPWM methods. The magnitude and frequency of the line side's fundamental variable are controlled by the magnitude and frequency of the reference vector, respectively. Because of its higher dc bus utilization and lower harmonic distortion than SPWM, Space Vector Modulation has become a common PWM technique for three-phase Voltage Source Inverters. In MATLAB/SIMULINK software, the four PWM methods are simulated and compared for THD and Capacitor Balance. Embedded Editor and Matlab Editor Functions are used to build the programs. As compared to Sine PWM, simulation results show that Space vector PWM better utilizes dc bus voltage, produces less THD, and improves capacitor balance.

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.

Multilevel inverters are increasingly being used in high-power medium voltage applications due to their superior performance compared to two-level inverters. Among various modulation techniques for a multi-level inverter, the space vector pulse width modulation (SVPWM) is widely used. The complexity is due to the difficulty in determining the location of the reference vector, the calculation of on times, and the determination and selection of switching states.This paper proposes a general SVPWM algorithm for Multi-level inverters based on standard two-level SVPWM . To control multilevel converters, the pulse width modulation (PWM) strategies are the most effective, especially the space vector pulse width modulation (SVPWM) one, which has equally divided zero voltage vectors describing a lower total harmonic distortion (THD). In this work, a three and four-level inverter using space vector modulation strategy has been modeled and simulated with a passive R-L load. Simulation results are presented for various operation conditions to verify the system model.

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.

A propulsion system for hybrid electric vehicles and electric vehicles contains DC voltage supply, DC link capacitor, three phase voltage source inverter and a three phase AC machine. The key role of the inverter is to convert DC voltage to three AC voltages which commands the motor. The electrical machine is chosen to have the phase voltages in star connection and the common node of the phase voltages unconnected to ground. This paper contains a complete class of modulation techniques used to command the voltage source inverter. It consists in variation of potential of common node of electrical machine phases. This class of modulation techniques is simple to be implemented, offers a complete description of all possibilities to command the voltage source inverter and includes all other modulation techniques that could be found in literature. It is demonstrated analytically the inclusion in this class of some modulation techniques which could be found in literature such as sinusoidal pulse width modulation, third harmonic injection, and space vector pulse width modulation with sector selection.