Investigations on Applications of Current Error Space Phasor Based Hysteresis Controllers in Converters of Active Power Filters

Abstract

In last few decades, automation at various levels has been one of the prime factors for industrial growth throughout the world. Advancement in power semiconductor device technology, availability of fast & e cient power electronic devices (static switches) at higher power rating, and advent of fast digital signal processors have made it pos- sible to implement control and automation for sustained industrial growth. Power electronic devices based converters have found numerous applications like, induction furnace, arc furnace, variable frequency drives for motor control, welding, and many more. Most of these power electronic converters are considered as non-linear loads that inject current harmonics in the power system, and hence deteriorate quality of power. At the same time, on the other hand, usage of power quality sensitive loads like computers, process controls, medical equipment, communication equipment, etc., has increased. Power quality is a set of parameters related to properties of power supply. In a broader perspective, power quality includes considerations regarding di erent aspects of relia- bility of electrical power supply such as distortion, phase unbalance, line interruptions, amplitude variations, frequency changes, icker, and transients, etc. But while nar- rowing down, the focus of power quality revolves around distortion in waveforms of voltage (at system level) and current (at equipment level). Due to the intensive use of power converters and other non-linear loads in industry and by consumers in general, an increasing deterioration of the power systems voltage and current waveforms is observed. As the operation of customer equipment (e.g., power electronic converters) can cause power disturbances, the consumers of utility power should also play an active role in the mitigation of power quality problems, especially at equipment level. Conventionally, passive lters have been used to eliminate harmonics but due to their inherent drawbacks, they have been replaced by active power lters, as active lters have superior ltering characteristics and dynamic response compared to passive l- ters. Active power lters are basically of three types - Shunt, Series and Hybrid active power lter. Shunt active power lter is used to mitigate current harmonics, while series active power lter is used to eliminate voltage harmonics and hybrid is either combination of both shunt and series or active power lter and passive lter. The proposed research work is focussed on shunt active power lters. Till now, in the eld of active power lters main focus of research has been on de- velopment of novel techniques for reference compensating current generation in order to improve the compensation characteristic and to extend the application of active power lter from harmonic compensation to harmonic damping, harmonic isolation, voltage regulation, etc. Conventionally for the control of active power lters, amongst di erent types of current controllers, hysteresis current controllers are widely used, due to their inherent implementation simplicity and fast dynamic response. But they su er from drawbacks like, higher number of switching and selection of non adjacent (random) voltage vectors, limit cycle oscillations, overshoot in current error and gen- eration of sub-harmonic components in current. This is mainly because of lack of coordination between individual hysteresis controller placed for individual phase of the lter. Current error space phasor based hysteresis controller is more popularly used in vari-able frequency drive application of induction motors. This is basically to control the voltage source inverter with current controlled pulse width modulation for high performance drives employing induction motors. This controller allows the current error space phasor to move within a speci ed xed boundary. The proposed research work is focused on development of self-adaptive current controller which overcomes the drawbacks of random voltage vector selection, and limit cycle oscillations encoun- tered in conventional hysteresis controllers used for shunt active power lter. The proposed research work presents a current error space phasor based hysteresis controller for two-level shunt active power lter, which allows precise compensation of harmonic currents produced by non-linear loads. The proposed controller is self- adaptive in nature and does not require any particular calculation of point of com- mon coupling voltage vector because of proper sector change detection logics used. Detailed modelling and performance analysis of the proposed controller for SAPF with di erent compensating current generation methods (e.g., instantaneous reactive power theory, synchronous reference frame theory, dc-link voltage regulation, Fryze current computation), along with simulation results, is presented in the thesis. The proposed controller uses two hysteresis bands. Inner hysteresis band is used for re- gion detection. Region detection logic enables switching of unique voltage vector of shunt active power lter which keeps the current error space phasor well within the prescribed hexagonal boundary. Outer hysteresis band is used for appropriate sec- tor change detection. The performance of controller for shunt active power lter is tested for balanced as well as unbalanced mains voltage and is found to be satisfac- tory. E ect of dc-link voltage variations, rapid load changes on the performance of the controller is also analyzed. Under steady-state and transient conditions controller performance is found fast and precise. Hence the proposed current controller gives generalized solution for any method of reference compensating current generation. The shunt active power lter based on the proposed controller provides adequate compensation for mitigation of harmonics. For shunt active power lter developed with proposed controller using outer hysteresis band for detecting necessary sector changes (keeping track of movement of reference voltage vector of shunt active power lter), during each sector change, the current error space phasor moves out of the hexagonal boundary to hit the outer hysteresis band. Because of a total of six sectors in voltage space phasor structure, it happens six times in one fundamental cycle of supply for two-level converter based shunt active power lter. This puts slight limitation on the harmonic elimination in the supply current. Here in order to further improve the distortion in supply current, current error is restricted within hexagonal boundary even during appropriate sector changes by avoiding outer hysteresis band. The outer hysteresis band is eliminated and shunt active power lter compensating currents are generated with the help of two di erent logics- Sector change detection by zero crossing detection of voltage at the point of common coupling and Sector change detection by instantaneous values of voltages at the point of common coupling. Compensation provided by shunt active power lter with both schemes of sector change detection is satisfactory. The same current controller with outer hysteresis band for sector change detection is extended for high power applications of shunt active power lter by using multi-level converter topologies for shunt active power lter. This performance of current er- ror space phasor based hysteresis controller is studied for Three-Level Neutral Point Clamped and Flying Capacitor converters used in shunt active power lter, which al- lows precise compensation of harmonic currents produced by nonlinear loads. Issues of dc-link voltage imbalance for neutral point clamped converter based shunt active power lter and clamping capacitor voltage imbalance are addressed and solutions are provided. Proposed ying capacitor shunt active power lter operates e ectively keeping the current error within the desired hexagonal boundary. A laboratory scale prototype model of the proposed controller based two-level shunt active power lter is developed using DSP TMS320LF2407A. IGBT based two-level converter is fabricated and used as SAPF. Three-phase diode bridge recti er with capacitor lter and resistive load is used as non-linear load. Reference compensating currents are generated by Fryze current computation technique using DSP. Experi- mental results for the proposed controller based shunt active power lter, both using outer hysteresis band as well as without using outer hysteresis band for sector change detection is presented and the performance is found satisfactory. The compensation capability of the proposed controller is demonstrated for steady state as well as tran- sient conditions of operation. This research has further opened-up the scope for developing better current con- trollers for multi-level converter based shunt active power lters for more precise compensation of harmonics (i.e., for more smoother source currents)