POWER QUALITY IMPROVEMENT USING MULTILEVEL INVERTER BASED HYBRID POWER FILTER

Abstract

Modern civilization has become reliant on the unremitting supply of clean electrical power. But the power supplied by the grid and transmission system may not be always clean and continuous. Therefore, it necessitates the elimination or minimization of disturbances causing impurity. The first step of providing the power quality solution is to understand the types of power quality problems in the received supply and the nature of the loads to be coupled. Recently, the sophisticated and advanced technology is being implemented more due to its immense benefits in terms of lifestyle, business, infrastructure, and health. Accepting these aids, however, intensifies reliance on electrical power and often that power has to be completely free from interruption and disruption of the system to function efficiently. The ac distribution systems have experienced high harmonic pollution due to the wide use of power electronic loads that are nonlinear. These nonlinear loads generate harmonics which degrade the distribution systems and may affect the communication and control systems. The nonlinear load causes distortion and nonlinearity in current and/or voltage waveform for system. This deviation of current from the ideal sinusoidal waveform is described as harmonic and interharmonic current. Many of these new devices are more sensitive to the voltage quality than conventional linear loads. Nonlinear equipment such as induction machines, transformers, electric arc furnaces, welding machines, fluorescent lamps (with magnetic ballasts), ac/dc drives, and battery chargers are also accountable for the generation of harmonics in electric power systems. Besides, there are different types of power disturbances ranging from few micro seconds to few seconds duration problems which lead to waveform distortion. The harmonic distortion of voltage and current waveform are controlled by the harmonic regulations and guidelines such as IEEE-519-1992 and IEC 61000. These guidelines promote best practice in the design of both power systems and nonlinear equipment. A harmonic survey is being conducted at different commercial and high power industrial loads to know the existing level of harmonics in the system and a brief report is presented. The harmonic distortion instigates various undesirable effects in power system especially reduces system efficiency, causes low power factor, deteriorates the performance of induction motors, increase losses in transformer and line, forces resonance phenomenon, neutral burning, mal-operation of relays, blowing of fuses, and interference with nearby communication networks. Harmonic filters, in general, are designed to reduce the effects of harmonic penetration in power systems and they should be installed when it has been determined that the recommended harmonic content has been exceeded. There are two approaches to reduce the effect of the harmonic distortion, namely active filtering approach and passive filtering approach. The passive filter method is conventional, which have the drawbacks of large size, resonance, and fixed compensation. Moreover, to filter out manifold harmonics, ii multiple filters are required in parallel combination. The performance of passive filter is not much satisfactory due to the variable operating conditions. Active Power Filters (APFs) are feasible alternatives to passive filters and emerged as more efficient solution to the harmonics. The contemporary active filters have better performance, fast response, more adaptability, and flexibility as compared to passive elements. In the active filtering approach, the harmonic currents/voltages produced by the nonlinear loads are extracted, and their opposites are generated and injected into the power line using a power converter. Several active filtering approaches based on different circuit topologies and control theories have been suggested by researchers. The active filter mainly consists of a single high rating PWM power converter which takes care of all the harmonic components in the distorted signal. The main advantage of active filters over passive ones is their fine response to changing loads and harmonic variations. In addition, a single active filter can compensate more than one harmonic, and improve or mitigate other power quality problems such as flicker. For applications where the system configuration and/or the harmonic spectra of nonlinear loads change, active elements may be used instead of the passive components to provide dynamic compensation. An APF is implemented when the order numbers of harmonic currents are varying. This may be due to the nature of nonlinear loads injecting time-dependent harmonic spectra (e.g. variable speed drives) or may be caused by a change in the system configuration. The structure of an active filter may be that of series or parallel architectures. The proper structure for implementation depends on the types of harmonic sources in the power system and the effects that different filter solutions would cause to the overall system performance. The series topology is mainly applied for voltage harmonic elimination, whereas for current harmonic minimization the shunt topology is used. The shunt topology is more popular as compared to others due to its performance and ease of implementation for current harmonic elimination. In this thesis the current harmonics are required to be minimized, therefore, the shunt topology is identified for implementation and modifications. APFs rely on active power conditioning to compensate undesirable harmonic currents replacing a portion of the distorted current wave stemming from the nonlinear load. This is achieved by producing harmonic components of equal amplitude but opposite phase angles, which cancel the injected harmonic components of the nonlinear loads. APFs are expensive compared with their passive counterparts and are not feasible for small facilities. The main drawback of active filters is that their rating is sometimes very close to the load (up to 80% in some typical applications), and thus it becomes a costly option for power quality improvement. Moreover, a single APF might not provide a complete solution in many practical applications due to the presence of both voltage and current quality problems. The combination of high power and high switching frequency results in excessive amounts of power losses. Furthermore, the reliability of the existing active filters is a major concern, as iii the failure of converter results in no compensation at all. In such cases, a more complicated filter design consisting of two or three passive and/or active filters (called a Hybrid Power Filter (HPF)) is recommended. APF or HPF topology selection depends upon several factors such as harmonic distortion, kVA rating and cost of passive filter components, displacement factor, and losses in APF including switching ripple filters, ability to provide harmonic isolation between supply and load, and control complexity. In addition to the advancement in power electronics and technology, the power and voltage requirement of the industrial applications also reached a higher level and better power quality in terms of minimum harmonic distortion with unity power factor. The conventional two-level inverters are found to be incapable of satisfying this demand; therefore multilevel inverters are being used. The Multilevel Inverter (MLI) allows the expansion of system rating with increased efficiency and better performance. The MLI can generate approximately sinusoidal voltages at low switching frequency with reduced switching stress and negligible electromagnetic interference and common-mode voltage. The rating of MLI is increased by adding more voltage levels without increasing individual device rating with reduced harmonics at the output voltage. Mostly, the active harmonic filter uses a standard two-level Voltage Source Inverter (VSI) for low power, low voltage applications. However, for medium/high voltage applications, MLI VSIs provide more advantages. The two-level inverter has many disadvantages such as it requires coupling transformer, large smoothing reactors, snubber components, complicated control and higher cost when applied at a medium or high voltage system. As compared to the two-level inverter, MLI provides many advantages such as no coupling transformer, small smoothing reactor, less switching stress, modular structure for medium or high voltage application. Furthermore, MLI has lesser harmonic distortion for more number of voltage levels to the reasonable increase in cost. The MLIs are mainly classified in three topologies: a) Diode Clamped Multilevel Inverter (DCMLI) b) Flying Capacitor Multilevel Inverter (FCMLI) c) Cascade H-bridge Multilevel Inverter (CHBMLI). The selection of MLI topology and number of levels depends upon the system parameters, such as voltage/power rating, and cost. Among the available MLI topologies, the CHBMLI topology has comparatively simple control and modular layout for medium/high power applications with the least number of components for a given level. The researchers have invariably utilized, modified, tested, and implemented the various MLI configurations for a large number of applications for medium/high power and medium/high voltage systems for power quality enhancement. The harmonic filtering and minimization, power factor improvement, reactive power compensation, static var compensation, and drives are among the main applications. The MLI mainly as the harmonic filter is focused and analysed for the thesis. iv The literature study indicates that the CHBMLI is the candidate topology for harmonic filtering and reactive power compensation at high-power, medium-voltage systems. As, the CHBMLI has modular units for each level, that allows the flexibility and simple extension of levels at the output. The CHBMLI requires the minimum number of components by eliminating the clamping components such as diodes and capacitors, which are necessary for DCMLI and FCMLI configurations respectively. These advantages have motivated the author of the thesis to exploit the CHBMLI scheme. However, the requirement of separate dc source for each unit of each phase is the main drawback of CHBMLI, as it complicates the voltage balancing with the increase in output levels. Recently, Artificial Intelligence (AI) based controllers are used in harmonic filtering using MLI with higher efficiency and more dynamics; the researchers have reported various methods in the literature for various applications. The single-phase as well as three-phase MLIs have been implemented using a Fuzzy Logic Controller (FLC), Neural Network (NN), and/or Genetic Algorithm (GA) based controllers. These AI based controllers are employed to control the voltage, frequency, and/or current for power quality improvement by reducing harmonic distortions. In most of the reported methods, the AI technique is used for dc voltage control for three or five-level MLI and uses traditional compensating techniques such as Instantaneous Power Theory (IPT) or Synchronous Reference Frame (SRF) and PI controllers with its inherent limitation of fine tuning. These drawbacks are subdued by using the AI for the controllers of the multilevel harmonic filter as described in this work. In which an AI controller based three-phase, five-level, multilevel Shunt Active Power Filter (SAPF) is designed developed to improve percentage Total Harmonic Distortions in the source current (%THDi The described harmonic filter has an eleven-level, CHB as a filter and AI based control scheme. As compared with the two-level filter, the method achieves better compensation performance by using smaller filter inductors, with reduced switching stress at higher switching frequency and higher voltage. However, filters based on MLIs are generally more expensive and more complicated to control as it has more number of inverter switching states. Moreover, the application of these filters in medium and high voltage with high power level is justified in terms of cost and performance. The instantaneous power theory is modified and the reference/compensating component is extracted by using NN. Further, the scheme uses two separate FLCs, one is for regulating dc voltage (V) in high voltage system. The designed controller can be extended to any level of CHBMLI configuration and hence more versatile. The CHB topology using Level Shifted Pulse Width Modulation (LSPWM) is preferred over the other MLI topologies because of its comparatively simple control and modular layout. This scheme is advantageous as it uses AI, which is self-adaptive and self-adjusting and thereby less susceptible to change in system parameters. dc) and the other is for generating gate pulses for the IGBTs in the inverter circuit. In CHBMLI, the clamping diodes v are not required, which reduces the control complexity, therefore has a simple modular structure for expansion. The proposed active harmonic filter is a shunt/parallel connected, current harmonic compensating device, which injects the harmonic cancellation component in the power circuit at the Point of Common Coupling (PCC) and thereby reduces the harmonic distortion in current and also improves the power factor. The scheme has AI based controllers that are robust, do not necessitate a mathematical model, and accommodates unpredictability. In current controller scheme the reference signal is obtained by using the NN based Instantaneous Power Theory (IPT), which uses the basic IPT for its execution. The IPT uses Clarke/Inverse Clarke transformation and is based on time-domain. The IPT is more efficient and feasible as compared to other methods and can be applied to balanced/unbalanced systems under both steady state and/or transient condition. In the projected scheme, the NN is applied for harmonic component extraction. This controller requires only average/constant component and all others are undesirable and are to be eliminated. Traditionally a low pass filter is used for separating harmonic component, which requires tuning for changes in the system. The use of NN eliminates the repeated tuning and is self-adjusting according to the system parameter variation. The harmonic component with NN from active part is calculated. NN based controllers have self-adapting and high rated calculation characteristics that allow them to handle high nonlinearities and uncertainties to which systems are generally susceptible. A feed forward neural network is designed with three layers, the input layer, the hidden layer and the output layer respectively. The network is trained with large data of source current, reference dc voltage, power loss component and reference compensation current from a conventional PI method using tan sigmoidal and pure linear activation functions in the hidden and output layers respectively. The network is trained with Levenberg-Marquardt back-propagation (LMBP) algorithm. The NN based APF performance is evaluated through simulation results and it is found that the performance of APF improves with the use of NN as compared to that with conventional PI based controller. The fuzzy logic controlled dc voltage is calculated and active power loss is determined for reference estimation in coherence with harmonic components to be eliminated. This compensating component is then fed to the control circuit for harmonic filtering. The IPT gives satisfactory results, when supply voltages are sinusoidal, but in case of distorted mains voltage, prevalent in the most industrial power system, this generates errors in reference currents and limits the compensation of harmonics. In such cases the average power method is applied, which uses supply voltages through Phase Locked Loop (PLL) and load currents to calculate the instantaneous power and average power over one sixth of a cycle. This in turn is used to obtain the peak current component of fundamental load current. The loss component of APF is obtained by comparing the actual capacitor voltage and reference capacitor voltage. The peak component of reference source current is obtained by vi adding the peak value of the load current component and the peak value of current providing the APF losses which is obtained from PI controller. The MLIs are modulated by using multicarrier schemes, which are classified as Phase Shifted Pulse Width Modulation (PSPWM) multicarrier modulation or Level Shifted Pulse Width Modulation (LSPWM) multicarrier modulation. The PSPWM can be used for FCMLI and CHB, whereas LSPWM is topology independent. The technological growth in MLI configurations and increase in number of levels has forced the modification and expansion in modulation methods derived from conventional. These developments are aimed to utilize the additional switching state to compete with the added complexity. There has been a lot of progress in MLI modulation schemes for diverse applications that have correspondingly agitated the growth in modulation methods to achieve the specific objectives. The modifications are intended to generate the best stepped output voltage at all modulation indices with adaptable parameters such as magnitude, frequency, and phase. Further, the switching frequency has to be minimized for maximum efficiency and minimum switching stresses. Although, the trade-off between the appropriate modulation method with ease of control and less complexity is tricky, the researchers have tried to modify the conventional schemes for their best. The LSPWM is found to be a suitable modulation scheme for the presented work as it has the lowest harmonic distortion and lesser device switching frequency. To utilize these features it is required to overcome the basic drawback of uneven switching and unequal distribution of switching losses. The switching patterns are balanced by rotating the carriers. The basic LSWPM is described and modifications are proposed. There are several extended multicarrier LSPWM methods which have been discussed in literature by various researchers. In most of the methods as discussed in the literature for multicarrier modulation of MLI the switching sequence and repetition timings are variable and methods are applied and topology specific, which are difficult to extend to other types of MLI configuration. This thesis demonstrates the modified modulation scheme for a multilevel inverter (MLI) to equalize the switching pattern at all (high and low) modulation indices. The LSPWM performance is better than PSPWM. But LSPWM has an unequal switching frequency and unequal conduction period as its inherent disadvantages, which are undesirable for high power and/or high frequency applications. The demonstrated scheme is a modified LSPWM scheme for m-level MLI with (m-1) multicarrier. In this method the carriers are rotated/swapped cyclically for equalizing switching patterns and thereby balancing the conduction period for all the devices of the MLI at all modulation indices in (m-1)/2 cycle of modulating signal. The scheme has the advantage of both PSWPM and LSPWM as it has better harmonic performance and identical switching pattern for each device at high as well as low modulation indices. The scheme can be straightforwardly extended to any level of the inverter. The simulation study is done for three-phase, five-level and eleven-level CHBMLI in vii MATLAB/SIMULINK using simpower system tool box. The results are compared with traditional multicarrier PSPWM and LSPWM and with proposed rotated LSPWM for full, 1/2, 1/3, and 1/4 cycle rotation. The four types of rotation schemes are detailed and compared to show the viability and its effect on CHBMLI performance. The simulation results demonstrate the matching of switching pattern among the devices of each phase as compared to conventional LSPWM. Further, the CHBMLI is used as an active harmonic filter and requires dc voltage regulation. The five-level MLI has two series connected H-bridges per phase to be regulated. The batch control method is used for error adjustment, which reduces the complexity with less number of controllers. In batch control positive, negative, and zero errors are estimated for obtaining active power loss component corresponding to harmonics. The PI and fuzzy logic based controllers are used for comparison and simulation. Similarly, the eleven-level CHBMLI has five series connected H-bridges per phase. These voltages are controlled in a similar manner as that of five-level using batch control method. The designed controller adjusts the voltage magnitude and current error according to the existing magnitude of the corresponding harmonic component in current/voltage. This results in optimum dc side voltage and minimal converter losses. The information available on the magnitude of each harmonic component allows us to select the active filter parameter accordingly. This results in higher efficiency and superior performance. The performance of the method is investigated through simulation in MATLAB/Simulink, which is further verified by experimentation. The efficacy is demonstrated via exhaustive simulation and experimental results for different nonlinear loads and change in load. The simulation is carried out at a high voltage rating of the system; however the experimentation is carried out at low voltage due to practical limitations. The number of levels is also reduced to five for prototype preparation. The AI controller based three-phase five-level CHBMLI based harmonic filter is simulated in MATLAB/Simulink for performance description and implementation in the Institute laboratory. The simulation results show that the designed scheme is better in performance, economical as well as modular. The laboratory prototype at reduced rating of 100 V, 2 kVA is developed for the corroboration of simulation performance of the AI controller based rotated multicarrier LSPWM for CHBMLI as harmonic filter. The prototype arrangement consists of the following a) Three-phase, five-level CHBMLI with conventional and rotated multicarrier LSPWM. b) Three-phase, five-level CHBMLI with PI controller and conventional and rotated multicarrier LSPWM as shunt connected hybrid harmonic filter. c) AI controller based three-phase, five-level CHBMLI conventional and rotated multicarrier LSPWM as shunt connected hybrid harmonic filter. viii The prototype module has 24 IGBTs as switching devices that are used for 6 H-bridges of three-phase, five-level CHBMLI. Each switch is galvanically isolated, floating dc capacitor. The IGBT (IRG4PH40KD) is used as switching device for designing this prototype. Each phase has two series connected H-bridges, each H-bridge has two legs and each leg has two series connected IGBTs. The complete experimental scheme has dSPACE DS1103 interface, Host PC, current sensor, voltage sensor, isolation circuit, dead-band circuit, and driver circuit, which are designed and developed in the laboratory. The conventional three-phase nonlinear load and three-phase ac input are used for experimentation. Although the simulation is carried out for eleven-level CHBMLI, the laboratory prototype is developed for the five-level because of practical limitation of fabrication and design. The robust control algorithm is designed and developed through exhaustive simulation, which is independent of the number of levels of CHBMLI. The developed prototype performance is validated through comprehensive experimentation in the laboratory. The digital signal processor (DSP) DS1103 of dSPACE is used for the real-time simulation and implementation of control algorithm on hardware circuit. The control algorithm is designed in the MATLAB/Simulink. The Real-Time Workshop (RTW) of MATLAB is used to generate the optimized C-code for real-time implementation. The Real-time Interface (RTI) of dSPACE allows the interaction between virtual and real world for execution of the complete setup. The Multi-channel Analog to Digital Converter (ADC) is used to sense ac line currents, ac supply voltages, and dc capacitor voltages. An opto-isolated interface board is also used to isolate the entire DSP.