Performance Analysis of Neutral Point Clamped and Cascade H-Bridge Multilevel Converter Interfaced With Solar Photovoltaic System to Reduce Total Harmonic Distortion

Present day industrial applications require higher power apparatus for power conversion. At medium voltage grid, to connect only one power semiconductor switch directly is a not practically successful concept to overcome this multilevel power converter has been studied as an alternative in high power and medium voltage applications. Renewable energy sources like photovoltaic, wind, fuel cells can be conveniently interfaced to a multilevel converter system. Solar photovoltaic system becomes more popular among other renewable energy system, therefore Power electronics devices have been become an indispensable part for renewable energy system. This paper demonstrates comprehensive analysis and comparison of the Total harmonics distortion (THD) in detail of multilevel converter for different levels by modelling and simulation when interfaced with Solar Photovoltaic (PV) system. The results presented give an understanding for the implementation of the multilevel converter in the solar PV system. The modular structure for the multi level inverters allows extending higher number of levels easily for future studies with less complexity comparably. The simulation results are carried out using two different topologies for multilevel converter, neutral point clamped (NPC) and cascade H-bridge (CHB). The main focus of this paper is to present physical modelling and simulation of solar PV system and multilevel converter for different levels. The results displayed are such that the output gets smoother as the levels increases and the total harmonics distortion decreases. The solar PV system used is modelled in SIMCAPE library. All the simulation studies are carried out under MATLAB/Simulation environment

Variable-Angle Phase-Shifted PWM for Multilevel Three-Cell Cascaded H-bridge Converters

Multilevel cascaded H-bridge converters have become a mature technology for applications where high-power medium ac voltages are required. Normal operation of multilevel cascaded H-bridge converters assumes that all power cells have the same dc voltage, and each power cell generates the same voltage averaged over a sampling period using a conventional phase-shifted pulse width modulation (PWM) technique. However, this modulation method does not achieve good results under unbalanced operation per H-bridge in the power converter, which may happen in grid-connected applications such as photovoltaic or battery energy storage systems. In the paper, a simplified mathematical analysis of the phase-shifted PWM technique is presented. In addition, a modification of this conventional modulation method using variable shift angles between the power cells is introduced. This modification leads to the elimination of harmonic distortion of low-order harmonics due to the switching (triangular carrier frequency and its multiples) even under unbalanced operational conditions. The analysis is particularized for a three-cell cascaded H-bridge converter, and experimental results are presented to demonstrate the good performance of the proposed modulation method

Performance Analysis of H-Bridge Inverter Integrated For Renewable Energy Sources

An optimized third harmonic compensation strategy is proposed to improve the linear modulation range of single-phase inverter. The method injects the minimum amount of positive third harmonic into inverter by keeping modulation waveform amplitudes of over-modulation cells just being unity, then compensates same amount of negative third harmonic and properly distributes it to the normal cell. It has been observed and the simulation results are discussed. The two-level inverter has the lowest cost and weight in comparison with the other topologies. Hence it has very high THD and it is not practical to have an output voltage with high such THD. The design of the 5-level multilevel inverters seems to be better than the inverters. The proposed method is verified by the combination of battery storage power, capacitor bank and the solar PV Cell

Multilevel Inverter by using Switching Capacitor in PV Grid

Abstract: In this paper, we are using multilevel inverter by using switching capacitor for PV grid. Now a day's multilevel inverters have become more popular over the years in electric high power application with the promise of less disturbances and the possibility to function at lower switching frequencies than ordinary two-level inverters. So by cascading multilevel inverter the output voltage will be increases. By using multilevel inverter the cost will be less. It will be required less space. Modulation strategies, component comparison and solutions to the multilevel voltage source balancing problem will also be presented in this work. By using the multilevel inverter we can reduce the total harmonic distraction compare to the other sources in PV grid for DC to AC converter. The proposed method is simulated in Matlab Simulink

Multilevel Converter Topologies for Utility Scale Solar Photovoltaic Power Systems

Renewable energy technologies have been growing in their installed capacity rapidly over the past few years. This growth in solar, wind and other technologies is fueled by state incentives, renewable energy mandates, increased fossil fuel prices and environmental consciousness. Utility scale systems form a substantial portion of electricity capacity addition in modern times. This sets the stage for research activity to explore new efficient, compact and alternative power electronic topologies to integrate sources like photovoltaics (PV) to the utility grid, some of which are multilevel topologies. Multilevel topologies allow for use of lower voltage semiconductor devices than two-level converters. They also produce lower distortion output voltage waveforms. This dissertation proposes a cascaded multilevel converter with medium frequency AC link which reduces the size of DC bus capacitor and also eliminates power imbalance between the three phases. A control strategy which modulates the output voltage magnitude and phase angle of the inverter cells is proposed. This improves differential power processing amongst cells while keeping the voltage and current ratings of the devices low. A battery energy storage system for the multilevel PV converter has also been proposed. Renewable technologies such as PV and wind suffer from varying degrees of intermittency, depending on the geographical location. With increased installation of these sources, management of intermittency is critical to the stability of the grid. The proposed battery system is rated at 10% of the plant it is designed to support. Energy is stored and extracted by means of a bidirectional DC-DC converter connected to the PV DC bus. Different battery chemistries available for this application are also discussed. In this dissertation, the analyses of common mode voltages and currents in various PV topologies are detailed. The grid integration of PV power employs a combination of pulse width modulation (PWM) DC-DC converters and inverters. Due to their fast switching nature a common mode voltage is generated with respect to the ground, inducing a circulating current through the ground capacitance. Common mode voltages lead to increased voltage stress, electromagnetic interference and malfunctioning of ground fault protection systems. Common mode voltages and currents present in high and low power PV systems are analyzed and mitigation strategies such as common mode filter and transformer shielding are proposed to minimize them

Carrier-based sinusoidal pulse-width modulation techniques for flying capacitor modular multi-level cascaded converter

Carrier-based sinusoidal pulse-width modulation (PWM) techniques, such as phase disposoed PWM(PD-PWM) and phase shifted PWM (PS-PWM), are widely applied to control the modular multilevel cascaded converters (MMCC) having full H-bridge as sub-modules. This paper evaluates these PWM techniques when controlling a variant of the H-bridge MMCC, i.e. the MMCC five-level flying capacitor converter as sub-modules. This MMCC poses an extra challenge to PWM schemes; namely maintaining two inner floating capacitor voltage balancing. Two novel PWM techniques known as the swapped carrier PWM techniques are introduced for the control of this converter. The paper compares them with the two conventional ones using a performance metrics composed of voltage waveform performance, capability in natural flying capacitor voltage balancing, converter power loss, and switch utilisation. The results show that the proposed new PWM schemes outperform both conventional methods in both switching and conduction power losses and achieve similar performance like the PS-PWM under the three other metrics

Modulation for Cascaded Multilevel Converters in PV Applications with High Input Power Imbalance

Cascaded multilevel inverters, such as the cascaded H-Bridge (CHB) converter, are an attractive solution for multi-string photovoltaic (PV) systems, because they enable direct connection to the medium voltage grid and maximum power point tracking of multiple strings. As a challenge of the topology, the operation with high power imbalance in the strings is constrained by the over-modulation. This limitation is analyzed for sinusoidal modulation and the impact on the maximum power imbalance is demonstrated. For increasing the operating range with maximum power tracking in the strings, a discontinuous modulation with extended maximum power imbalance and reduced losses is proposed. The method is analyzed in terms of maximum power imbalance, efficiency and power quality. In addition, the method is validated on an experimental test bench

Beyond PWM: Active balancing, start-up and ZVS for multi-level converters with applications in renewable energy systems

This dissertation addresses many of the operational challenges of flying capacitor multi-level (FCML) converters and introduces new techniques to improve performance. First, start-up and a new method to extract power from within an FCML converter at a naturally occurring low-voltage node are investigated with a 5-level FCML converter. The introduction of the auxiliary power supplied induces an undesirable imbalance in the flying capacitors, which increases switch stress and induces additional harmonics in the output current. To reduce the induced imbalance, active balancing is implemented with a single voltage measurement of one specific flying capacitor. To correct for flying capacitor imbalance for more generic conditions, valley current detection has previously been proposed. However, prior work did not account for light load conditions, which we show to be challenging for traditional duty cycle compensation. Here we derive the light load conditions that lead to instability, and propose a new constant effective duty cycle compensation method for active balancing across the full load range. The effectiveness of the method is successfully demonstrated on a 4-level FCML prototype in light load and full load conditions. Quasi square wave zero voltage switching (ZVS) reduces switching losses and has been demonstrated at specific duty cycles for FCML converters in prior work. Here, we derive the fundamental limitations imposed by the FCML topology and show how the ZVS operating range can be extended through specific design choices. The results are demonstrated on an experimental 4-level hardware prototype, achieving step-down operation with a 1 kVdc input. The 4-level prototype also serves as an early stage proof of concept for high-voltage operation with 650 V GaN switches, as previous FCML designs utilized a higher number of FCML levels with lower voltage rated switches. As a final application of these techniques, a 1.5 kV, 15 kW, 3-phase solar photovoltaic (PV) inverter has been designed and implemented. The 5-level converter achieves a peak efficiency of 98.5% and demonstrates 3-phase operation with a Total Harmonic Distortion (THD) of less than 0.25%

Modulated model predictive control for a 7-level cascaded h-bridge back-to-back converter

Multilevel Converters are known to have many advantages for electricity network applications. In particular Cascaded H-Bridge Converters are attractive because of their inherent modularity and scalability. Predictive control for power converters is advantageous as a result of its applicability to discrete system and fast response. In this paper a novel control technique, named Modulated Model Predictive Control, is introduced with the aim to increase the performance of Model Predictive Control. The proposed controller address a modulation scheme as part of the minimization process. The proposed control technique is described in detail, validated through simulation and experimental testing and compared with Dead-Beat and traditional Model Predictive Control. The results show the increased performance of the Modulated Model Predictive Control with respect to the classic Finite Control Set Model Predictive Control, in terms ofcurrent waveform THD. Moreover the proposed controller allows a multi-objective control, with respect to Dead-Beat Control that does not present this capability