THREE-PHASE HIGH POWER FACTOR CONVERTER

Abstract

Power systems are designed to operate at frequencies of 50 or 60 Hz. However, certain types of loads produce harmonic currents in the power system. The power system harmonics are not a new phenomenon. Concern over harmonic distortions has ebbed and flowed during the history of electrical power systems. Traditionally the saturated iron in transformers and induction machines, electric arc furnaces, welding equipment, fluorescent lamps (with magnetic ballasts), etc. have been responsible for the generation of harmonics in electric power systems. Most of these equipments also cause the flow of reactive component of current in the system. In recent years, many power electronic converters utilizing switching devices are being widely used in domestic, commercial and industrial applications, ranging from few watts to MWs. However these converters suffer from the drawbacks of harmonic generation and reactive power flow from the source and offer highly non-linear characteristics. The generation of harmonics and reactive power flow in the power systems has given rise to the 'Electric Power Quality' problems. These power quality problems are reflected in the system in the form of reduced efficiency of rectifiers, deteriorated performance of induction motors, interference with nearby communications networks, neutral burning, mal-operation of relays, blowing of fuses and so on. Thus there is a growing concern of power quality with the proliferation of AC/DC converters in adjustable-speed drives, power supplies, SMPS, DC motor drives, and so on. The standards like IEEE-519-1992, IEC 1000-4-7, AS 2279 and IEC 61000 series have been introduced to help combat the power quality problems and are being enforced on the consumers. IEEE 519-1992 Recommended Practices and Requirements for harmonic control in electric power systems provide guidelines for determining what are acceptable limits. To meet these requirements, passive and active filters have been used in combination with the conventional AC/DC converters. Classically, shunt passive filters consisting of tuned LC filters and/or high pass filters are used to suppress the harmonics. However, these filters suffer from the problem of bulkiness, high cost, fixed compensation, and they can also excite resonance phenomena with the system parameters. Moreover, to filter out all harmonics, one needs as many tuned filters as the number of harmonics to be eliminated. On the other hand, Active Power Filters (APFs) have been researched to compensate harmonics as well as reactive power requirements of the non-linear loads. But the rating, cost and complexity are the main drawbacks of APFs. In some cases, the rating of an APF is as high as that of the load. A new breed of power converters has been made possible mainly because of the use of modern high-speed, solid-state, self-commutating power semiconducting devices such as power MOSFETs, GTOs, IGBTs, IGCTs, etc. and are specifically known as Switched-Mode Rectifiers (SMRs), Power Factor Correction Converters (PFCs), High Power Factor Converters (HPFCs) and PWM Rectifiers. These improved power quality AC/DC converters are included as an inherent part of the AC-DC conversion system which produces better power quality on the line-side and load-side of converters with higher efficiency and reduced size. With remarkable development and advances in switching speed and power handling capacity of power semiconducting devices, various topologies of HPFCs have been researched and developed. Various linear and non-linear controllers and modulation techniques have been implemented for the control and improved performance of these converters. Research has also been done in the area of voltage and current sensorless techniques for these converters. The advent of high-speed processors, DSPs, and recently FPGAs has made possible the realization of complex and computation-intensive algorithms in real time. Although three-phase two-level high power factor converters have been extensively researched, for medium and high voltage and high power applications multi-level converters have been found to offer a viable solution. This has prompted the researchers to investigate multi-level converters and these converters have been a subject of intensive research in recent past. However the application of multilevel converters for rectification purposes needs to be thoroughly investigated. A grave problem with multilevel converters has been the DCbus capacitor voltage unbalancing. Efforts have been made to address this issue. Some techniques have been suggested wherein the use of additional hardware results in the balancing of capacitor voltages, but this adds to the bulkiness, complexity and cost. It has also been suggested to devise a suitable switching sequence for capacitor voltage balancing, but this does not result in perfect balancing of capacitor voltages under all conditions. The phenomenon of capacitor voltage unbalancing in these converters needs to be thoroughly studied and understood in order to devise a technique for addressing this burning issue with multi-level converters. The complexity of space vector pulse width modulation (SVPWM) algorithm with the intensive computations needs a high-speed DSP for its implementation. The conventional computation-intensive SVM modulator needs to be replaced by ANN-based modulator where no intensive on-line computations are required and the SVM can be implemented easily on a low-cost, smaller size Application-specific IC chip (ASIC). Moreover, some effort is needed in the area of converter modeling and control design, particularly for operation under distorted input voltage conditions. In order to improve the performance of multilevel rectifiers, the present work is carried out. In this dissertation, a three-phase, high power factor multilevel AC/DC Converter is designed, developed and investigated for the improved power quality. The power quality problems introduced by conventional AC/DC converters in a power system are discussed first. Classical solutions and the compensation of harmonics and reactive power by passive filters and active power filters are briefly discussed. Acomprehensive literature survey on the three-phase, high power factor converters for power quality improvement is given. Design and development of basic three-phase, neutral-point clamped high power factor bidirectional rectifier using space vector pulse width modulation (SVPWM) technique is taken as the next investigative object. To analyze the performance of the NPC converter in steady-state and dynamic conditions, a complete mathematical model of the proposed converter using space vector PWM is developed and using this comprehensive mathematical model based on switching functions, the simulation model for the space-vector modulated converter is developed for evaluating the performance of converter under different operating conditions. A complete design of power circuit parameters such as the selection of boost inductors and DC bus capacitors is given. A comprehensive PI controller design for the outer-loop voltage controller and the inner-loop current controllers (using D-partition technique) for stable operation of converter under steady-state and dynamic conditions is given. The characteristic equations are derived in terms of controller parameters for synthesis. The probable stable region in the parametric plane is obtained with the help of D-partitioning technique. The stability of the region is confirmed by frequency scanning technique. To get the region of better stability, relative stability boundaries are drawn for different values of a. The final selection of controller parameters is carried out by the transient response of current-control loop and voltage-control loop, respectively. The designed power circuit parameters and PI controller parameters are validated through simulation results. The performance of the converter is evaluated using comprehensive simulation results for both rectification as well as inversion modes. The converter performance is also evaluated for sudden load changes and supply voltage fluctuations. The experimental validation of the simulation results is carried out through real-time simulation with the help of DSP of dSPACE. The real-time workshop (RTW) of MATLAB and real-time interface (RTI) feature of dSPACE results in the real-time simulation of the simulink model of converter (controller and modulator) which results in the generation of control pulses for all the devices of converter in real time. Waveforms such as source voltage, line-current, rectifier input voltage, and control pulses are recorded for performance investigation of the proposed high power factor converter. Moreover, in order to check the correct implementation of proposed control algorithm in real time, dSPACE ControlDesk animation feature is used for capturing the waveforms like sector identification, region selection, d- and q-components of source current, vector timings, voltage and current controller outputs, etc. The space vector PWM algorithm implemented in the three-phase NPC converter is considerably more complex owing to a large number of switching states compared to a twolevel converter. It requires complex and time-consuming online computation by a DSP. The artificial neural network (ANN)-based implementation of SVPWM for a three-phase, NPC bidirectional rectifier is performed as the next investigative step. A three-layer feedforward ANN is employed which receives the command voltage and angle information at the input and generates symmetrical PWM waves for three phases of the converter with the help of a single timer and some simple logic. The ANN-based converter performance is evaluated through detailed simulations and it is found that the ANN-based SVM modulator gives as good performance as the conventional SVM based modulator. The implementation ofANNbased modulator is advantageous as it does not require any on-line computation of vector timings, no look up table is required and the use of DSP can be avoided by using a dedicated Application-Specific IC chip (ASIC) for the given application. The excellent performance of the ANN-based converter is experimentally validated. Although multilevel converters produce excellent line-side and load-side performances such as nearly sinusoidal line-currents at unity power factor and regulated and reduced-rippled load voltage, the neutral point of the neutral-point clamped converters is prone to fluctuations due to the irregular charging and discharging of DC-bus capacitors. This causes the unbalanced DC-bus capacitor voltages which, if not balanced, cause large voltage stress on some devices which may exceed the manufacturers' specifications and may deteriorate the source current for large voltage unbalances, thus offsetting the advantages offered by these converters. To address this issue, a modified space vector modulation strategy (voltage balancing strategy) is designed and investigated in the next stage. The proposed algorithm identifies the switching states responsible for producing the capacitor voltage unbalance and then uses the redundant switching states for balancing the capacitor voltages. This algorithm perfectly balances the DC-bus capacitor voltages which IV further improves the performance of converter by reducing the line-current THD. The performance of converter using the proposed control algorithm is evaluated for fixed and variable supply voltage and load conditions, unbalanced load conditions and under the operation with distorted mains. The simulation results obtained are experimentally validated on the laboratory prototype. To summarize, a practical model (laboratory prototype) of a three-phase neutral-point clamped bidirectional rectifier is designed and developed. The performance of rectifier is thoroughly investigated in both rectification and inversion modes using the conventional SVPWM technique and modified SVPWM technique (using voltage balancing strategy for DC-bus capacitor voltage balancing). The designed power circuit and controller parameters are validated through the results obtained. ANN implementation of SVPWM technique is investigated and it makes possible the use of ASIC chips for practical implementation of ANN-based space vector modulator in three-level rectifiers. Extensive simulation and experimental results are discussed under different operating conditions to investigate the steady-state, transient state and dynamic performance of the converter. The performance of the proposed NPC converter is also investigated under distorted mains conditions. The simulation and experimental results show excellent performance of the proposed converter with proposed voltage balancing algorithm, in terms of line-side and load-side power quality