INVESTIGATION ON CONVERTERS AND THEIR CONTROL TECHNIQUES FOR PV SYSTEM

Abstract

To meet high market expectation in terms of specification and performance, extensive research effort is being placed for superior PV converter design . This has played a significant role in pushing the manufacturers to invest in research & development to design a superior converter topology exclusively suited for PV application. Multilevel inverters are playing a vital role in recent development of PV converter design. Additionally, MLIs are being used in a variety of applications in recent years, including ac drives, microgrids, standalone solar PV systems, oil wells, static synchronous compensators, and others. Along with serving the core purpose of DC/AC conversion, these MLIs also provide improved power quality, which reduces further need for filtering. This guarantees optimal performance and lengthens the life span of sensitive apparatus connected through inverter. Few key advantages of MLIs are reduction of switching stress and uniform blocking voltage across its semiconductor switches. Conventionally three MLI topologies are well accepted through rigorous research and development. These MLI topologies are neutral point clamped (NPC) MLI, flying capacitor (FC) MLI, and cascaded H Bridge (CHB) MLI. For NPC MLI, stabilizing neutral voltage is challenging. Furthermore, for both NPC & FC MLI required number of components increases significantly with higher output resolution. For successful operation of CBH topology multiple isolated DC sources are needed. This makes it unattractive for industrial applications. Solution to this problem is to use floating capacitors but voltages of these capacitors need to be balanced properly. Thus, some major research objectives in the development of MLI topologies are realizing capability of producing high resolution output waveform with less components, simple circuit with preferably single DC source, simple control technique without any auxiliary sensing equipments e.t.c. Thus, various reduced switched inverter topologies are formulated with single or multiple DC sources. The DC sources of a MLI can operate in symmetrical or asymmetrical configuration. With asymmetrical configuration higher resolution of output waveform is possible without increasing the overall switching count. Therefore, different low and high frequency switching techniques are evaluated and studied for various reduced switched MLIs under this work. A novel artificial neural network (ANN) based selective harmonic elimination (SHE) technique is proposed to achieve significant power quality improvement. Another multii carrier pulse with modulation (MCPWM) strategy is formulated to achieve natural voltage balancing of floating capacitors for a reduced switched MLI. Furthermore, a grid connected modified 15-level MLI is proposed with active and reactive power control capability. A cross source based MLI is realized for 13-level of output feeding a single phase induction motor (IM) drive for agricultural solar pump application. A MCPWM based strategy is formulated for inverter control. The modulation technique takes care of frequency as well as fundamental voltage component control. This makes implementation of close loop V/f control of IM possible with variable environmental conditions. Few of the limitations associated with high frequency switching are high switching loss, common mode voltage, leakage current etc. Thus an ANN based low frequency SHE-PWM technique is proposed as an alternative. The proposed ANN like structure performs satisfactorily in solving higher number of transcendental SHE equations with good convergence. Thus, by achieving higher resolution (9 as well as 13 levels) at output waveform, better power quality is ensured. Practical feasibility of the system is also tested by incorporating a solar battery combination with it. This ensures all-day, all-weather operation of the system. A 7-level Packed U-cell type inverter is also investigated for IM based agricultural solar pump application. A MCPWM strategy is proposed that achieves natural voltage balancing of floating capacitors. It also incorporates close loop V/f control of IM. The system is further tested under practical application by incorporating battery-solar combination. A 9-level Packed U-cell topology with novel modulation strategy is proposed to achieve natural voltage balancing of three floating capacitors. This modulation strategy eliminates requirement of any auxiliary sensing equipment. In addition it also considers voltage ripple introduced by solar battery combination and performs optimally. Again, a grid connected 15-level modified MLI is proposed with active as well as reactive power control. This MLI topology is reduced switch type and quite compact in terms of its component count. The formulated MCPWM based modulation operates successfully under changing MI, frequency or loading condition. While connected with grid it effectively injects grid current with significantly low THD under variable active and reactive power requirements. Thus, the MLI is capable of maintaining IEEE 519 and IEEE 1547 grid current standards applicable for connecting distributed generating system with grid.