ANALYSIS AND CONTROL OF MULTILEVEL INVERTER FOR POWER QUALITY IMPROVEMENT

Abstract

Modern power distribution systems face two challenges from increasing electronic-based loads and integrating renewable energy sources. Distribution networks experience problems with current-based power quality as a result of these power-related electronic devices. Power quality problems include issues like poor power factor, load imbalance, harmonics in the supply current, voltage regulation, and others. To mitigate these issues, a shunt active power filter (SAPF) is used. This can be done by a number of controllers. In the case of a normal grid condition, the unit-in-template approach can be utilized to extract the synchronization signals. When the grid deteriorates due to voltage sag, swell, distortion, and DC offset problems present in the grid. Therefore, it is also essential to employ effective PLL algorithms to estimate synchronization template signals.. The grid-connected system is connected to the point of common coupling (PCC) via a voltage source converter, and the non-linear loads are also connected. Due to their efficiency and compactness, the power electronic-based loads are growing every day. Reactive power is drawn by the non-linear loads, which lowers the quality of the power and introduces harmonics into the system, which increases losses. To address the aforementioned problems, SAPF is built and configured to perform power factor correction, reactive power compensation, and harmonic mitigation. In order to enhance the power quality in distribution systems, it is advised to comply to a range of international standards based on IEEE and IEC. IEEE-519 standards for grid current harmonic mitigation should be followed. According to the IEEE-519 standard, the Total Harmonic Distortion Factor in grid current should be less than 5%. In addition, the IEEE-1547 standard mentions the allowable harmonic content that can be injected by PV systems when connected to the grid. In this thesis work, the SAPF is created for single-phase distribution systems utilizing Matlab Simulink software. Dynamic load conditions evaluate the effectiveness. In a single phase system, a 5-level cascaded H-Bridge Multilevel Inverter replaces the standard 2-level inverter. MLI has a number of benefits over a normal 2-level inverter since it switches operated at a lower frequency and experiences fewer switching losses. The harmonic content decreases as the number of levels rises. The CHB-MLI system has the necessary flexibility vi thanks to its modular design. A phase-shifted pulse width modulation technique produces the gate pulses by comparing the generated reference currents with the actual supply current signals. The performance of the SAPF is dependent on the different control algorithms used to extract the harmonics from the non-linear load current. In the literature, various conventional and adaptive control techniques are addressed. The performance of the control algorithm is assessed using a number of important considerations, such as the degree of computational complexity, the degree of filtration, oscillations, overshoot, settling time, mean square error, phase-locked loop (PLL) requirement, memory requirement, and the THD obtained in grid current and convergence behavior of fundamental load current under dynamic conditions. To enhance the power quality of distribution networks, SAPF needs to be appropriately controlled for further processing and control. Its controller is based on extracting the fundamental component of load current for the generation of reference current. This thesis involves designing new control algorithms to retrieve the fundamental load current component. This study proposes, develops, and tests a variety of control algorithms. The SAPF efficiency is also impacted by how precisely the control algorithm extracts the reference current and compensates for the harmonics produced by the load. The areas of active noise cancellation, signal enhancement, noise filtering, echo cancellation, etc., have all been covered by adaptive signal processing techniques over the past few decades. Least Mean Square (LMS), Synchronous Reference Frame Theory (SRFT), and Notch Filter are common algorithms used for the estimation of fundamental component of load current components. A method of parallel tangent (PARTAN-LMS) algorithm has been developed to operate under distorted grid conditions, and some advanced adaptive control algorithms, such as Normalized Least Mean Absolute Third (NLMAT), Normalized Huber (NHuber), and Robust Shrinkage Affine Projection Sign Algorithm (RSAPS), have also been implemented. The grid must be synchronized with power electronic converters for effective control. The voltage-based electrical quality of Indian systems can occasionally be problematic. Harmonics, phase angle jumps, frequency shifts, voltage sag, swell, and DC offset are vii common issues with voltage signals used for synchronization. To resolve these voltage based power quality problems, open-loop and closed-loop synchronization approaches can be applied. Phase-locked loops are a frequently employed part of synchronization techniques. The phase-locked loop technique is required for quick and accurate detection of the grid phase angle and frequency for integration. The chosen method of synchronization has an impact on the stability and accuracy of the control system. Grid collapse could result from inaccurate PLL information. It is necessary to look for an appropriate PLL that can be applied in real-time, has a simpler structure, and can address the weak grid problems. For the 5-Level CHB-MLI controller to operate, the synchronization controller also keeps track of grid voltage, phase, and frequency information. This thesis presents three grid-synchronization techniques for single-phase systems: Synchronous Reference Frame Theory (SRF-PLL), Second Order Generalized Integrator (SOGI-PLL), and Third Order Sinusoidal Integrator (TOSSI-PLL). SRF-PLL is the traditional and frequently employed PLL under typical circumstances. However, this PLL does not accurately track frequency and phase angle when the grid is distorted. Therefore, finding alternatives to SRF-based PLLs is essential. For handling various grid anomalies, orthogonal signal generator (OSG) based PLL has been widely used and implemented in recent years. Additionally, compared to SRF-PLL, its tracking capability is quite good and accurate. The most commonly used OSG- PLL is SOGI-PLL, but its performance deteriorates especially in DC-offset conditions; therefore, one more PLL, i.e., TOSSI-PLL, has been implemented to tackle grid abnormalities which exhibits good performance as compared to SRF-PLL and SOGI-PLL. The findings of extensive mathematical modeling, simulations and experiments have been presented in this thesis. The effectiveness of the synchronization algorithms has been demonstrated through simulation and experimental testing under various grid voltage conditions such as voltage sag and swells, phase shift, and DC offset and also tested under sudden load variations In grid-connected systems, photovoltaic (PV) based voltage source converters (VSC) serve the dual purposes of bidirectional active power transfer to the grid and load. They can be controlled to achieve grid current balancing, harmonic reduction, reactive power balance, viii and improvement of the supply side power factor to unity. The PV source can be integrated through a DC-DC boost converter using the double-stage topology, or it can be linked directly to the VSC at its DC link using the single-stage topology. Fewer devices are required in a single stage, and it offers improved control. Additionally, because of independent Maximum Power Point Tracking, it has an increased operation region and more flexibility. In this thesis, a single-stage, single-phase grid-connected system has been presented. The PV arrays are connected to the DC link side of the 5-Level CHB-MLI. The proposed system can operate in two modes, day and night. During the day, the active power is injected into the grid, and during the night, the solar arrays are disconnected; the SAPF will do the harmonics compensation and make the system power factor unity. The system is tested under distorted grid conditions. Two PLLs viz. SRF-PLL and Modified Notch Filter SOGIPLL (MNFSOGI-PLL) is implemented to generate the unit template and reference current. The SRF-PLL fails to work in distorted grid conditions, but MNFSOGI-PLL exhibits good performance under the distorted grid, especially to handle DC offset. To enhance the quality of the power, offer reactive power compensation, and inject active power into the grid, these system configurations will be implemented and analyzed using simulation models and experimental prototypes.