An Interleaved Soft Switched High Step-Up Boost Converter With High Power Density for Renewable Energy Applications

In this article, a novel soft switched interleaved boost structure with a simple auxiliary circuit is proposed which is suitable for stand-alone loads or ac grid applications. In this topology, coupled inductors and switched capacitor cells of parallel modules are merged to obtain high voltage conversion ratio. The converter also has the capability of adding extra switched capacitor cells to attain very high voltage gain. To provide soft-switching condition in the wide range of output power, a new zero-voltage transition auxiliary circuit is employed which is responsible for soft switching of both phases and benefits from low conduction losses, the minimum number of semiconductor elements, and only one auxiliary gate-driver. These merits provide very high efficiency at both full-load and light loads. More importantly, no auxiliary magnetic components are utilized by taking advantage of the leakage inductance of coupled inductors for the resonant network. All semiconductor components operate under soft switching alleviating the reverse recovery problem and switching losses. Besides, the converter benefits from common ground between input and output which simplify voltage feedback. The experimental results of the interleaved converter prototype with 400-V output voltage at 400 W and 100 kHz switching frequency are provided. The full load efficiency of 98% was achieved and the power density was observed 1.9 W/Cm3

Preparation of Power Distribution System for High Penetration of Renewable Energy

Part I: Dynamic Voltage RestorerIn the present power grids, voltage sags are recognized as a serious threat and a frequently occurring power-quality problem and have costly consequence such as sensitive loads tripping and production loss. Consequently, the demand for high power quality and voltage stability becomes a pressing issue. Dynamic voltage restorer (DVR), as a custom power device, is more effective and direct solutions for "restoring" the quality of voltage at its load-side terminals when the quality of voltage at its source-side terminals is disturbed. In the first part of this thesis, a DVR configuration with no need of bulky dc link capacitor or energy storage is proposed. This fact causes to reduce the size of the DVR and increase the reliability of the circuit. In addition, the proposed DVR topology is based on high-frequency isolation transformer resulting in the size reduction of transformer. The proposed DVR circuit, which is suitable for both low- and medium-voltage applications, is based on dc-ac converters connected in series to split the main dc link between the inputs of dc-ac converters. This feature makes it possible to use modular dc-ac converters and utilize low-voltage components in these converters whenever it is required to use DVR in medium-voltage application. The proposed configuration is tested under different conditions of load power factor and grid voltage harmonic. It has been shown that proposed DVR can compensate the voltage sag effectively and protect the sensitive loads.Following the proposition of the DVR topology, a fundamental voltage amplitude detection method which is applicable in both single/three-phase systems for DVR applications is proposed. The advantages of proposed method include application in distorted power grid with no need of any low-pass filter, precise and reliable detection, simple computation and implementation without using a phased locked loop and lookup table. The proposed method has been verified by simulation and experimental tests under various conditions considering all possible cases such as different amounts of voltage sag depth (VSD), different amounts of point-on-wave (POW) at which voltage sag occurs, harmonic distortion, line frequency variation, and phase jump (PJ). Furthermore, the ripple amount of fundamental voltage amplitude calculated by the proposed method and its error is analyzed considering the line frequency variation together with harmonic distortion. The best and worst detection time of proposed method were measured 1ms and 8.8ms, respectively. Finally, the proposed method has been compared with other voltage sag detection methods available in literature.Part 2: Power System Modeling for Renewable Energy IntegrationAs power distribution systems are evolving into more complex networks, electrical engineers have to rely on software tools to perform circuit analysis. There are dozens of powerful software tools available in the market to perform the power system studies. Although their main functions are similar, there are differences in features and formatting structures to suit specific applications. This creates challenges for transferring power system circuit models data (PSCMD) between different software and rebuilding the same circuit in the second software environment. The objective of this part of thesis is to develop a Unified Platform (UP) to facilitate transferring PSCMD among different software packages and relieve the challenges of the circuit model conversion process. UP uses a commonly available spreadsheet file with a defined format, for any home software to write data to and for any destination software to read data from, via a script-based application called PSCMD transfer application. The main considerations in developing the UP are to minimize manual intervention and import a one-line diagram into the destination software or export it from the source software, with all details to allow load flow, short circuit and other analyses. In this study, ETAP, OpenDSS, and GridLab-D are considered, and PSCMD transfer applications written in MATLAB have been developed for each of these to read the circuit model data provided in the UP spreadsheet. In order to test the developed PSCMD transfer applications, circuit model data of a test circuit and a power distribution circuit from Southern California Edison (SCE) - a utility company - both built in CYME, were exported into the spreadsheet file according to the UP format. Thereafter, circuit model data were imported successfully from the spreadsheet files into above mentioned software using the PSCMD transfer applications developed for each software. After the SCE studied circuit is transferred into OpenDSS software using the proposed UP scheme and developed application, it has been studied to investigate the impacts of large-scale solar energy penetration. The main challenge of solar energy integration into power grid is its intermittency (i.e., discontinuity of output power) nature due to cloud shading of photovoltaic panels which depends on weather conditions. In order to conduct this study, OpenDSS time-series simulation feature, which is required due to intermittency of solar energy, is utilized. In this study, the impacts of intermittency of solar energy penetration, especially high-variability points, on voltage fluctuation and operation of capacitor bank and voltage regulator is provided. In addition, the necessity to interpolate and resample unequally spaced time-series measurement data and convert them to equally spaced time-series data as well as the effect of resampling time-interval on the amount of error is discussed. Two applications are developed in Matlab to do interpolation and resampling as well as to calculate the amount of error for different resampling time-intervals to figure out the suitable resampling time-interval. Furthermore, an approach based on cumulative distribution, regarding the length for lines/cables types and the power rating for loads, is presented to prioritize which loads, lines and cables the meters should be installed at to have the most effect on model validation

Phase-Disposition PWM Based Active Voltage Control of Seven-Level Nested Neutral-Point-Piloted (NNPP) Inverters

This paper introduces logic-equations for active voltage balancing of seven-level nested neutral-point-piloted (NNPP) inverters. It measures the load current and voltage of floating-capacitors (FCs). These instantaneous measurements are then translated into logic-variables on basis of their reference values. Next, the interpreted logic-variables are employed in a set of logic-equations that are specifically derived for the 7-level NNPP inverters. These equations aim to actively balance the FC voltages at their target operating levels and to generate the reference voltage-level according to the adopted PWM strategy. As an outstanding advantage, the proposed logic-equations-based active-voltage balancing method eliminates all the complicated and time-consuming computations that are necessary in the conventional active balancing approaches since it is not utilizing or optimizing any cost/energy functions. The experimental results obtained from a 7-level prototype of the NNPP converter are provided to verify the logic-equations and active control method

Flying-Capacitor Voltage-Balancing Control in Five-Level Active Neutral-Point-Clamped (A-NPC) Converters Using Phase-Disposition PWM

A new control technique for five-level active neutral-point-clamped (A-NPC) inverters is proposed in this paper. The introduced method is based on a set of simple and effortless logic-variables and logic-equations. It actively balances the voltage of the flying-capacitor (FC) at its requisite value, and controls the 5-level A-NPC converter\u27s synthesized voltage according to phase-disposition pulse-width-modulation (PD-PWM) strategy. These logic-variables and equations are derived using the instantaneous measurements of output current and FC\u27s voltage. The regulation of the FC\u27s voltage at its reference operating level and generating the commanded voltage-levels of PD-PWM modulator at output with the less computational requirements are the control targets. The logic-equations and the proposed active balancing technique are validated using the experimental results which are obtained from the prototype of a five-level A-NPC inverter

Medium-voltage DC grid connection using modular multilevel converter

In recent years medium voltage direct-current (MVDC) grids have gained significant attention. This paper presents a new isolated modular multilevel converter (MMC)-based dc/dc converter for connecting such networks. The proposed converter can connect DC grids with different voltage levels and control power flow among them. The proposed isolated dc/dc converter has fewer components compared to conventional isolated MMC-based dc/dc converter. A major challenge in MMC-based dc/dc converters is balancing capacitor voltages in each arm and phase. To this end, a closed loop control is proposed. Simulation results using Matlab/SIMULINK is provided to demonstrate the dynamic response under step changes of reference powers

A new 2-cell shunt active power filter with compensation principle based on synchronous reference frame

In this paper a new 2-cell shunt active power filter (APF) is proposed. The proposed APF is capable of eliminating current harmonics and compensating reactive current, drawn by both nonlinear and linear loads. An online control method based on the discrete nonlinear model of multicell inverters is applied to APF. The main idea of the controller is the tracking problem of the reference current and the reference capacitor voltage by state variables. The state variables are controlled using an optimal feedback in a very fast and efficient manner. Moreover, a novel extraction system based on the synchronous reference frame is adopted as a part of the control system, and provides the required signals for harmonic filtering and reactive power compensation. The compensation principle is presented in details. The circuit is simulated using PSCAD/EMTDC software and simulation results are presented to validate the effectiveness of the novel control scheme and proposed APF

Novel multi-terminal MMC-based dc/dc converter for MVDC grid interconnection

Multi-terminal modular multilevel converter (MMC)-based dc/dc converter with the medium-frequency ac-link transformer is an attractive and feasible solution for interconnecting medium-voltage direct-current (MVDC) grids. This study proposes a novel multi-terminal dc/dc converter which features significantly reduced number of components compared with the conventional converter topology. The proposed dc/dc converter is capable of interconnecting multiple MVDC grids with different voltage levels as well as controlling the power flow between them. A combination of sine wave modulation scheme and phase disposition level-shifted pulse-width modulation is developed and employed for voltage balancing and controlling the power flow in the proposed converter. Several simulations, experimental and real-time hardware-in-the-loop case studies are carried out to demonstrate the dynamic response and capacitor voltage balancing capability of the proposed MMC and dc/dc converter under the developed control strategy