Efficient Control of Power Conversion Interfaces for Solar Grid-Tie Inverter

Abstract

Amongst the available renewable sources, solar photovoltaic (PV) energy sources is becoming dominant due to its long operational life, lesser emission and decreasing installation and maintenance costs. The grid integration of PV sources eliminates the requirement of additional storage and provides support for the peak loads. With the increasing number of PV sources integrated to the grid, the standards for grid integration are continuously revised in order to ensure that only stable, safe, e cient and reliable systems are integrated with the grid. The present standards recommend the sources to stay connected to provide support to the grid especially during voltage sags. Due to this reason, developing improved topologies and control strategies for power conversion interfaces (PCI) that can perform satisfactorily under varying grid conditions. Several control techniques for PCIs exist in literature based on the pulse width modulation (PWM). Amongst these, the hysteresis control (HC) exhibits superior performance when compared to other conventional PWM techniques. The HC is one of the most simple modulation techniques especially for grid tie inverters (GTIs). It regulates the ripple current output of GTI within xed hysteresis limits. This results in a varying switching frequency which is not implicitly known prior to control implementation. Hence, HC of conventional PCI for GTI is hindered by its varying switching frequency, requirement of high precision AC current sensor and undecided switching intervals. Due to these reasons, HC is also not often used in novel PCI topologies, despite all its advantages. In this thesis, the above mentioned shortcomings are discussed along with their solutions so as to improve the HC. These contributions can provide assistance to researchers and engineers in design and implementation of applications such as GTIs using HC. The output lter design, switching device selection, switching and conduction loss calculations of GTIs are predominantly dependent on the switching frequency. Hence, estimating the minimum and maximum switching frequencies is essential for a reliable system design. The existing estimations for HC assume linear ripple current. Since the frequency variation is large in HC, this assumption is invalid for the range of low frequencies. Inaccurate estimation of switching frequency can have considerable e ect on system design. In this thesis, a more precise and generalized expression to estimate the switching frequency of multilevel GTI is obtained by time-domain analysis. The accurate estimation results in the improvement of system design which is demonstrated with an example of a second order lter. iii The e ect of changes in system parameters on the switching frequency is also analysed to determine the operating point for an accurate system design. One other limitation of HC is the requirement of high precision AC current sensors. Though no current sensorless HC can be found in the literature, the current emulation technique of eliminating current sensor may be modi ed and implemented for HC. However, the computational requirement for such a control is high. To this end, an AC current sensoreless HCC for two-level GTI is developed by formulating the switching intervals as a function of known system variables. All real time conditions such as non-linearity of ripple current, dynamic changes in operating conditions and e ects of digital sampling are considered while developing the algorithm of the proposed AC current sensorless HC. Due to the uncertain switching intervals of HC, its use in the relatively novel PCIs are not vastly explored. Hence in this work, the closed loop controls for Z-source inverter (ZSI) and microinverter ( I) using HC are presented. A single stage PCI with independent input and output side controls can be achieved using ZSI. The shoot through interval is an important parameter which decides the boost ratio of ZSI. For ZSI with HC, determining the shoot through interval is di cult due to the undecided switching intervals. Hence, in this thesis, a detailed analysis of ZSI as a single-stage PCI is discussed. A closed loop control using HC with shoot through error estimation is developed based on this analysis. AC modules and the associated PIC, commonly referred to as Is have been gaining attention due to the advantage of eliminating partial shading in series connected PV modules. The prime features of a I are high e ciency, high gain ratio with less number of switching components and preferably without using coupling inductors or transformers, simple control and reduced THD. Keeping these in mind, an e cient I with pseudo DC-link (pDC-l) for grid integration of AC module is proposed as a part of this work. A high gain buck-boost Z-source converter (ZSC) is used to obtain a pDC-l from a single PV module. An unfolding circuit follows the ZSC. It operates at fundamental frequency which reduces the switching losses without a ecting the THD. A simple closed loop control using voltage HC is developed to obtain a pseudo DC-link voltage. A current HC ensures power balance by controlling the current to the inverter bridge. Simulations and experimental validations are carried out to verify the theoretical analysis and ensure the proposed controls achieve the standards of grid integration of PV systems. The simulation studies are carried out in MATLAB while ensuring the real time operating conditions.