MLI BASED ACTIVE POWER FILTER FOR POWER QUALITY IMPROVEMENT

Abstract

Electrical power system is designed to operate at 50 or 60Hz. However, the nonlinear loads connected to the power system generate harmonic currents. The power system harmonics are not new phenomenon. Concern over harmonic distortions has flowed during the history of electrical power systems. Conventionally, the saturated iron in transformers, induction machines, electric arc furnaces, welding equipment and fluorescent lamps, 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 converter utilizing switching devices are being widely used in domestic, commercial and industrial applications, ranging from few watts to megawatts. However, these converters suffer from the drawbacks of harmonic generation and reactive power flow from the source and offer highly nonlinear characteristics. The generation of harmonics and reactive power flow in the power systems has given rise to ‗Electric Power Quality problems‘. These power quality problems are reflected in the system in the form of reduced efficiency of equipment, deteriorated performance of electrical machines, interference with nearby communications networks, neutral burning, mal-operation of relays, blowing of fuses and so on. Thus there is a growing concerned of power quality with proliferation of ac/dc converters in adjustable-speed drives, power supplies, SMPS, DC motor drives, and so on. Harmonic regulation or standard guide lines such as IEEE-519-1992, IEC 1000-4-7, AS 2279 and IEC 6100, etc. are currently used to keep current and voltage harmonics level within the standard limits. In addition, the equipment designed based on these standards can improve the performance of power system. To meet these requirements, passive filters, active filters and their combination have been used with two level inverters in order to improve power quality. Conventionally, passive filters have been used for the power quality improvement. Though passive compensations is a simple approach, but they have several drawbacks such as inability to provide dynamic compensation, bulky size, cost, resonance problem, separate filters for each harmonics, etc. and this fact has attracted the attention of power engineers to develop dynamic and adjustable solutions to power quality problems. One modern and very promising group of solutions that deals with load current and/or supply voltage imperfections is active power filter (APF). It is well known that high performance and cost-effective inverter is a prerequisite for the realization of an APF. These inverters can be broadly categorised into two classes, namely, voltage source inverter (VSI) and current source inverter (CSI). A critical comparison of the performance of VSI and CSI when used as a power circuit of APF is beyond the scope of this report. However, in the present work, VSI has been considered as a power circuit for APF as it has higher market penetration and a more noticeable development on VSI has ii taken place over the last decade, in comparison to CSI topologies. The high harmonic content of the output voltage makes basic six-pulse (two-level) VSI impractical for direct use in high-power, medium-voltage applications. Instead of using filters and connecting several switching devices in series to achieve the required voltage level, several alternative possible solutions are reported in the literature and can be broadly categorized into two groups: multipulse and multilevel inverters. The first one requires bulky phase-shifting transformers and therefore, its application is limited to high-power, high-voltage systems. The second approach, multilevel inverters, uses the concept of addition of multiple small voltage levels for achieving the required voltage level with the help of additional switching devices and few components like diodes or capacitors. This approach does not require bulky phase shifting transformers and hence these topologies are best suited for medium-power applications. The common multilevel inverters (MLI) topologies are the diode-clamped (DCMLI), flying capacitor (FCMLI), and cascaded H-bridge multilevel inverters (CHBMLI). The performance of APF depends on the control strategy used for its implementation. The control algorithm proposed in this work aims to eliminate harmonics, compensate reactive power as well as control and balance the dc capacitor voltages of the APFs in steady-state and in transient conditions. The load harmonic currents have been derived by using the measured voltages at point of common coupling (PCC), load currents, and the dc bus voltages of the capacitors of the APF using proposed control strategies based on adaptive neural network such as least mean square (LMS), anti-Hebbian and anti-Hebbian based on TLS (Total least square). The controlling performance of the APF depends on the controller design. However it is difficult to develop the mathematical model of APF with multilevel inverters under parametric varying conditions. Therefore, there is great tendency to use unconventional controllers (intelligent controller) namely type-2 fuzzy logic controller. The type-2 fuzzy logic controller (T2FLC) is one of the intelligent controllers which handles the uncertainty in a better way and has been used in many of the application. The T2FLC, which is a highly nonlinear dynamic controller and does not require the mathematical model of system, not only has a strong adaptive and learning ability but also has a good processing and nonlinear mapping capabilities with large time delay and uncertain conditions. These characteristics result in meeting the control requirement of three level active power filter for three phase three wire system. A computer simulation study under different load condition has been carried out to verify the performance of the three level DCMLI APF for harmonic elimination and reactive power compensation. The simulation study of the entire system has been carried out in MATLAB/Simulink environment. Various performance indices such as THD, power factor, active and reactive powers have been investigated. From these studies it has been observed that the adaptive control schemes with proposed controllers can compensates the reactive iii power of the load and makes the harmonic currents to be less and within the limits imposed by IEEE–519–1992. The selection of individual inverter topologies for APF applications depends on their performance, cost, size, and implementation issues. DCMLI topology appears to be the most suited for APF applications. But, the large number of power components and voltage unbalance problem at higher levels limits the DCMLI for low power rating applications. On the other hand, Cascaded H-bridge multilevel inverter (CHBMLI) is one of the next generation multilevel inverters intended for high or medium-voltage power conversion without the requirement of line-frequency transformers. The CHBMLI is based on cascade connection of multiple single-phase H-bridge converter cells or chopper cells per leg. The least, low cost, modular structure, easy expansion to any number of levels, high fault tolerance component requirement and absence of bulky complex input transformer and without requirement of pre-charging circuit for the capacitor voltages make CHBMLI best suited for APF applications. The fuzzy logic controller is promising solution for five levels CHBMLI based APF. Because, they provide a high degree of robustness and immunity to external disturbances. Furthermore, they can be configured to be self-learning and adaptive. However, they require substantial computational power, due to the complex decision-making processes. For example, conventional FLC involves fuzzification, rule-base storage, inference mechanism, and defuzzification operations. For better accuracy in control, a larger set of rules is required, which results in longer computational time. However, this may not be practical because there are many implementation aspects that must be addressed, namely, sampling time and dc voltage excursion. Apart from these constraints, it is known fact that FLC requires simpler mathematics and offers a higher degree of freedom in tuning its control parameters compared to other nonlinear controllers. Most conventional FLCs use the error and the change of error as fuzzy input variables regardless of the complexity of controlled plants. These conventional FLCs came from the concepts of linear PD and PI control schemes. Such FLCs are suitable for simple lower order plants. However, in case of complex large order plants, such as multilevel inverter, all of the states are required to implement state feedback-based FLCs. Then, the design of an FLC is very difficult due to an increased number of fuzzy control rules as well as tuning parameters. Therefore, it is necessary to design an FLC that has a simple control structure and is computationally efficient compared to the conventional FLC. In the presents work, the simplified approach to design a fuzzy logic controller, known as a single-input fuzzy logic controller (SIFLC). The simplification converts two input fuzzy logic controllers to a single input known as signed distance. In this method, the SIFLC control surface can be approximated by a simple piecewise linear (PWL) function, which results in a significant simplification of the design and parameter tuning. The individual iv dc voltage regulation of CHBMLI based APF with anti-Hebbian based on total least square using SIFLC and PI controller has been carried out for balancing the voltages of the floating dc capacitors to their corresponding reference values. Computer simulation studies under different load conditions have been carried out to verify the performance of an APF for harmonic elimination and reactive power compensation. The simulation study of the entire system has been carried out in MATLAB/Simulink environment. The PI and SIFLC have been used to regulate the dc voltage of APF. Extensive simulation studies have been carried out to investigate the performance of the three phase three wire APF in normal voltage condition. The simulation studies have been performed for both steady-state and transient conditions with different non-linear loads. The capacitor voltage balance behaviour has been observed among the individual dc capacitors of the APF. Various performance indices such as THD, power factor, active and reactive power have been investigated. From these studies it has been observed that the PI and SIFLC with the anti-Hebbian based on total least square algorithm completely compensates the reactive power of the load and makes the harmonic currents to be less and within the limits imposed by IEEE–519–1992. The three-phase, four-wire distribution systems have been widely applied to deliver electric power to three-phase four wire loads. The asymmetrical distribution of larger number of single-phase loads, ranging from few watts to MWs results in voltage and current unbalance in the three phase four wire (3P4W) electrical distribution system. They are responsible for excessive neutral current. The problems associated with the excessive neutral current are: overloading of distribution feeders and transformers, ground voltage fluctuation, flat-topping of voltage waveform and wiring failure etc. Therefore, these excess neutral currents must be compensated for the reliable operation of the 3P4W electrical distribution system. In recent years, a number of APF schemes have been reported for simultaneous compensation of reactive power, source current harmonics and neutral current in 3P4W electrical distribution systems. They are: (1) split capacitor (2) three H-bridge topology and (3) four-leg topology. However, these schemes were complicated in control and require large volt-ampere rating inverter. Different transformer topologies such as zigzag, star-delta, Scott-connected and star/hexagon-connected have also been used in recent years to attenuate the neutral current on the utility sides due to the advantages of low cost, high reliability and simplified circuit connection. Among these, zigzag transformer based approach requires least kVA inverter rating. However, these configurations also have a low impedance path for zero-sequence voltage of the unbalanced utility, which will further cause a significant neutral current. Further, their attenuation characteristics are dependent on their locations, impedances of the transformers and utility voltage conditions. A reduced rating hybrid v topology comprising of a zigzag-delta transformer, a 3P3W active power filter (APF) and a single-phase APF had been demonstrated for the compensation of source neutral current and phase current harmonics, but, in this topology power factor and displacement factor are not improved. Also, the fundamental-frequency phase currents were also not made balanced. Single-phase powers APF can be combined with the zig-zag/ zero sequence transformers to advance the performance of the neutral current attenuation have been proposed in the literature. However, these schemes suffer from source phase current harmonics and unbalance. To address the above limitations, in the present work, two reduced rating hybrid 3P4W APF have been proposed. The proposed topologies of hybrid 3P4W active power filter comprises of three phase three wire APF, zero sequence transformer, ac power capacitor and single phase APF. 1. A 3P4W active power filter comprises of 3P3W APF, zero sequence transformer and single-phase APF. 2. A 3P4W hybrid active power filter comprises of ac power capacitors, 3P3W APF zero sequence transformer and single-phase APF. To show the effectiveness of the proposed topologies, extensive simulation studies have been carried out in MATLAB/Simulink® environment. The performances of the proposed schemes have been studied for reactive power compensation, harmonic elimination and neutral current attenuation under various loading and utility voltage conditions. From these studies it is observed that the proposed control schemes completely compensates the reactive power of the load and makes the harmonic currents to be reduced below the limits imposed by IEEE–519–1992. Further, it also attenuates the source neutral current to a very large extent. In order to further verify the simulation studies, a three-phase downscaled three level DCMLI and five level CHBMLI based APF has been designed, constructed, and tested to verify the viability and effectiveness of the control scheme with intelligent controller techniques. For this purpose, the following prototypes have been developed. 1. Three-level DCMLI based APF for three phase three wire system (LMS, anti-Hebbian and anti-Hebbian based on TLS with PI/type-2 fuzzy logic controller ) 2. Five level CHBMLI based APF for three phase three wire system(PI and SIFLC with anti-Hebbian based on TLS algorithm) 3. A 3P4W APF comprising of a zero sequence transformer, CHBMLI based 3P3W APF and series connected single-phase APF (anti-Hebbian based on TLS algorithm with SIFLC). 4. Another 3P4W hybrid APF comprising of a zero sequence transformer, 3P3W APF, AC capacitors and series connected single-phase APF (anti-Hebbian based on TLS algorithm with SIFLC). vi For hardware implementation, the IGBTs (IRG4PH40KD) have been used as the switching devices. Different hardware components as required for the operation of the experimental set-up such as pulse amplification, isolation circuit, dead-band circuit, voltage and current sensor circuits, and non-linear loads have been designed and developed. By using the Real-Time Workshop (RTW) of MATLAB and Real-Time Interface (RTI) feature of dSPACE-DS1104, the Simulink models of the various controllers of the prototypes have been implemented. The generated firing pulses have been given to the corresponding semiconductor devices of APF through isolation, delay, and pulse amplification circuits in real-time. An uncontrolled six pulse rectifiers with RL elements on their DC sides have been used as nonlinear loads. After compensation with APF, the source currents have been observed to be sinusoidal and their corresponding THDs have also been observed to be well within the limits of IEEE–519–1992 recommended value of 5%. The switching in response and the dynamic performance of APF for a step change in the load has been studied. A smooth control of dc voltages ensures the effectiveness of the DC voltage controllers. Initially, the simulation and experimentation of three level DCMLI based APF with PI and T2FLC with different adaptive neural network algorithm such as LMS, anti-Hebbian and Anti-Hebbian based on TLS has been carried out to study the performance of three level DCMLI APF for three phase three wire system. From the performance characteristics, it is found that the T2FLC with anti-Hebbian based on TLS adaptive algorithm is better for three level DCMLI APF. However, the T2FLC is very complex to implement with higher level inverter due to the association of length process such as fuzzification, rule base, inference and defuzzification. For this purpose, single input fuzzy logic controller is derived using Conventional FLC. The performance of five level CHBMLI based APF with SIFLC as well as PI controller is also studied with anti-Hebbian based on TLS algorithm. It found that the performance characteristics of CHBMLI based APF with the SIFLC is superior than PI controller. Therefore, for three phase three wire APF, the single input fuzzy logic controller is recommended if number of level in three phase APF is more than three. Finally, simulation of two 3P4W APF comprising three phase APF, zero sequence transformer, ac capacitors and single phase APF is carried out to study performance characteristics. The single input fuzzy logic controller integrated with anti-Hebbian based TLS algorithm is used for controlling three phase APF. The single phase APF control is implemented using PI controller. The simulated performance characteristics of 3P4W are also experimentally validated.