Transformerless grid-connected inverters for photovoltaic systems

The increasing demand for electrical power, along with the decreasing stock of traditional energy sources, has caused a growing interest towards microgeneration from renewable power sources. In particular, photovoltaic energy (PV) has witnessed an increasing attention and the scientific community has concentrated its efforts in order to develop innovative solutions for the integration of PV systems into the existing distribution grid. In this thesis PV converters for grid-connected systems without galvanic insulation were studied. A particular solution of a multilevel-based converter was employed in a PV application and its behavior was thoroughly analyzed. Extensive simulations and experimental results confirmed the feasibility and the good performance of the proposed solution

Current-type Power Hardware in the Loop (PHIL) evaluation for smart transformer application

The development of power electronics and power systems due to the massive integration of renewable energy sources is challenging the distribution grids. Among several concepts, the Smart Transformer (ST), a solid-state transformer with advanced control and communication capabilities, has been investigated by several researchers. A great challenge of this kind of system is the possibility to test the effectiveness of the physical system under a broad spectrum of operating conditions. For this reason, the Power Hardware in the Loop (PHIL) concept can be adopted to emulate the behavior of a distribution grid connected to the ST. In this case, because the low-voltage stage of the ST is voltage controlled, the test setup must be current-controlled. In this paper, the current-controlled PHIL setup is analyzed. The theorethical analysis is carried out and preliminary results obtained with the PHIL facilities are presented, highlighting how the current-controlled PHIL can be an effective means to study the ST

Development of a high force density, actuation drive for an aerospace application

This paper deals with the design, implementation and experimental validation of a complete, electrical drive system, for linear actuation in an aerospace environment, with the main requirements being high force/power density, reliability, availability and fault tolerance. A directly-driven, linear, high force density, tubular motor supplied from a matrix converter for a rotorcraft application is specifically considered. Design aspects of both motor and converter are presented, while an overview of the implementation aspects for the complete drive system is also given. Precise control structures are proposed and tested. Experimental results are presented and compared to the calculated design values. This work shows how the combination of high performance components can achieve a high performance drive in terms of power density and force density

A smart transformer-rectifier unit for the more electric aircraft

In the framework of the More Electric Aircraft (MEA), an efficient and flexible power distribution system is of paramount importance. Considering the presence of both AC and DC loads at multiple voltage levels, the distribution system of the most modern aircrafts is intrinsically hybrid. In this scenario, the different buses are connected by AC/DC converters. The simplest approach is to use a Transformer-Rectifier Unit (TRU) based on a low-frequency transformer followed by passive rectifiers to perform the AC/DC conversion. This solution, however, is intrinsically uni-directional, introduces current harmonics in the AC side and can have a considerable size. This paper proposes the use of a Smart-TRU, based on a Cascaded H-Bridge topology and a multi-port DC/DC converter, to solve the issues of the traditional TRU, increasing the controllability of the system. Experiments show how the proposed STRU is resilient to faults in the AC side

Impact of Modulation Methods on the Trade-Off between Investment and Operation Costs of a Medium-Voltage MMC-based STATCOM

The Modular Multilevel Converter (MMC) has become a preferred topology in HVDC applications due to its full controllability and the huge number of voltage steps. The excellent waveform generation, even at low switching frequencies, makes the MMC also very attractive for medium-voltage applications. In this context, both the converter design and the modulation methods need to be properly studied. Minimum switching frequencies are achieved by appropriate modulation, however, sufficient energy needs to be stored in the capacitors. This is particularly a challenge for STATCOM applications because the stored energy in the system needs to be controlled from the ac grid. In this paper different modulation methods with various capacitor designs are investigated for this application to find an optimum trade-off between capacitor design (investment costs) and switching frequency (operation costs). The appropriate MMC design and operation have been proven by simulation and experimental results showing excellent waveforms without additional filtering

Robust stability analysis of LCL filter based synchronverter under different grid conditions

Synchronverters have gained interest due to their capability of emulating synchronous machines (SMs), offering self-synchronization to the grid. Despite the simplicity of the control structure, the adoption of an LCL-filter makes the overall model complex again, posing questions regarding the tuning of the synchronverter and its robustness. The multi-inputs multi-outputs (MIMO) formulation of the problem requires multivariable analysis. In this paper, the effects of control parameter and grid conditions on the stability of the system are investigated by means of structured singular values (SSV or μ-analysis). A step-by-step design procedure for the control is introduced based on a linearized smallsignal model of the system. Then the design repercussions on the stability performance are evaluated through the performed robustness analysis. The developed linearized model is validated against timedomain simulations and laboratory experiments. These have been carried out using a power hardwarein- the-loop (PHIL) test bench, which allows to test the synchronverter under different grid conditions. As a conclusion the paper offers a simple guide to tune synchronverters but also a theoretical solid framework to assess the robustness of the adopted design

A modular speed-drooped system for high reliability integrated modular motor drives

Future transportation challenges include a considerable reduction in pollutant emissions at a time when significant increase in demand is predicted. One of the enabling solutions is the electrification of transport systems as this should lead to improved operability, fuel savings, emission reduction, and maintenance. While state-of-the-art technology has demonstrable benefits there needs to be considerable advancement to meet future transportation affordability and emission targets. Primarily, electrical drives need an improved power density, an increased reliability, and a reduced specific cost. For this reason, integrated modular motor drives (IMMDs) present an attractive solution. Modularity leads to redundancy and easier integration. This paper presents a novel speed-drooped control system applied to motors fed by modular paralleled converters. This control technique allows precise speed regulation and power sharing among different segments showing improved fault tolerance and reliability. The design procedure and the power sharing dynamic have been presented and analyzed by means of MATLAB/Simulink and validated in a 3-kW experimental rig, showing good agreement with the expected performance