Galvanically Isolated Modular Converter

The ever increasing penetration of renewable energy systems in the distribution grids will inevitably require profound changes in the grid infrastructure. One emerging direction, the medium voltage dc (MVdc) grids, is the cornerstone of this work. They are foreseen for offshore wind parks and onshore large scale renewables collection grids, future on-board ship power systems, repurposed ac lines with increased transport capacity, etc. The shift to dc offers energy savings and reduced impact on the landscape. This thesis provides insights on the presumed most suitable conversion topology between a MVdc and a LVac grid, which is not expected to disappear in the near future. The conversion is characterized by a large voltage ratio between the two terminals. A modular multilevel converter (MMC) with an integrated magnetic component, the galvanically isolated modular converter (GIMC), is proposed and preferred for efficiency and cost reasons over a solid-state transformer, whose efficiency is heavily penalized by the inverter stage on the low voltage side. The thesis opens on a detailed benchmark of the performances of various control, modulation and branch balancing methods, with a focus on medium voltage applications. Extensive simulations are carried out to support the discussion. In case of an application with fast dynamics, the closed-loop control method, comprising energy balancing controllers, offers by far the best performances. For the modulation and branch balancing methods, it was concluded that, as long as both the number of cells per branch and average cell switching frequency are not very low, PWM methods based on the reference branch voltage, rather than the number of inserted cells per branch, feature lower voltage errors. From there, the two GIMC variants sharing the same three-windings transformer, the interleaved GIMC and stacked GIMC, are analyzed. This solution does not suffer from dc bias in the magnetic device, unlike the open-end windings MMC. Since the obtained model is identical to the one for the conventional dc/3-ac MMC, the same control algorithms can be applied without restriction. The volume and efficiency comparison against the conventional case with discrete air-core inductors, supported by FEM simulations, quantifies the benefits of the proposal. It is concluded that the gains are marginal for the considered modest power ratings (0.5 MVA). However, the magnetic design is considerably simplified. Larger gains are expected for designs with higher branch inductance values, since the stacking of the primary windings gives easy access to high leakage inductances. A generic and versatile losses estimation method for modular converters is introduced. Compared to detailed switched simulations, the simulation times are improved by two orders of magnitude, if the assumptions to neglect the branch current ripple and branch capacitor voltage spread hold. The estimation error is below 2 % in the considered comparison. At last, the design of a 0.5 MVA converter prototype connected to 10 kVdc with 96 cells is presented. The cell, with a dedicated Flyback-based auxiliary cell power supply from its dc-link and protection circuits for a cell bypass in case of over-current or -voltage, along with the electric design of the cabinet hosting one converter phase-leg, are verified experimentally. The cell and phase-leg layout provide a platform for further research activities

Galvanically Isolated Modular Converter

Direct current (DC) electrical grids are already a reality in low voltage (LV) telecom distribution systems and point-to-point high voltage DC transmission. Medium voltage (MV) domain, despite its big potential, still suffers from a lack of suitable conversion and protection technologies. This study presents a bidirectional, galvanically isolated, high power converter for interface of emerging MVDC grids with readily available LVAC grids. To achieve high conversion efficiency, the integration of a line frequency transformer into the structure of the modular multilevel converter (MMC) is analysed and described in a systematic manner. Two configurations of the galvanically isolated modular converter: (i) interleaved and (ii) stacked, are derived and presented. Differences and similarities, compared to the classical MMC, are presented on the system design level, while control performances are evaluated by means of simulations

State-space modeling of modular multilevel converters including line frequency transformer

This paper presents a detailed state-space modeling of a Modular Multilevel Converter (MMC) in combination with line frequency transformer (LFT). The classical MMC topology is compared with the recently proposed Open-End Winding MMC (OEWMMC) topology, which integrates the LFT in the converter arm. The line frequency transformer parameters are added in the model, some of which are often neglected in the available literature. Both models are verified and compared by means of numerical simulations in open-loop. Despite integration attractiveness offered by the OEWMMC topology, there are also some inherent drawbacks affecting the overall system sizing that are discussed in the paper

On the integration of low frequency transformer into modular multilevel converter

Various high power applications require power converters with large voltage step ratios, easily achieved by inclusion of a single or multiple transformers, which provide galvanic isolation at the same time. This paper presents, in a systematic manner, the necessary steps for the integration of a Low Frequency Transformer (LFT) into the Modular Multilevel Converter (MMC). Unlike the classical MMC that requires an external transformer for galvanic isolation, this work considers a transformer integration at the arm level resulting in a complete replacement of the arm inductors. Such galvanically isolated modular converters can be realized either in interleaved or stacked arrangements. The properties of each variant are discussed and compared with the classical MMC with external LFT, on the system design level but also from the control point of view

Novel Insight into the Output Current Ripple for Multilevel and Multiphase Converter Topologies

When it comes to the computation / estimation of the switching losses in a multilevel converter, it is crucial to know the instantaneous value of the output current. If the average output current value is used instead, there is a non negligible error when the quality of the output current waveform is low. This paper addresses this issue with a generic and versatile fast numerical method for phase-disposition PWM that produces the exact switching pattern, hence the exact inductor flux, without having to run time-domain switched simulation(s), as the calculations are performed for the minimal set of points required to fully determine the inductor flux ripple at the switching instants

Virtual Submodule Concept Applied to the Modular Multilevel Converter

The design phase of a modular multilevel converter (MMC) needs to be supported by suitable tools for the submodule loss evaluation. This paper proposes a fast MMC cell loss calculation based on the virtual submodule (VSM) concept. Compared to other proposed tools, the impact of the circulating current control is directly taken into account and different modulation schemes can be easily compared. To verify the proposed concept, the results are compared with the losses obtained from a switched model with closed-loop control, where the analytical MMC key waveforms are approached in the steady state. The proposed method provides a great flexibility and a significant reduction of the simulation / computational time otherwise needed to evaluate SM losses under various operating conditions

Virtual Submodule Concept for Fast Semi-Numerical Modular Multilevel Converter Loss Estimation

During the design phase of a modular multilevel converter (MMC), an accurate loss evaluation of the submodule (SM) plays an important role. In this paper, a method based on the analytical description of the MMC key waveforms that allows to directly obtain the average semiconductor and capacitor losses that each SM will experience is introduced, under different operating conditions or control schemes. To verify the proposed concept, the results are compared with the losses obtained from a switched model with closed-loop control, where the analytical MMC key waveforms are approached in steady state. The proposed method provides a great flexibility and a significant reduction of the simulation / computational time otherwise needed to evaluate SM losses under various operating conditions

Oxygen consumption in offspring tawny owls Strix aluco is associated with colour morph of foster mother

In several colour polymorphic species, morphs differ in thermoregulation either because dark and pale surfaces absorb solar radiation to a different extent and/or because morphs differ in key metabolic processes. Morph-specific thermoregulation may potentially account for the observation that differently coloured individuals are frequently not randomly distributed among habitats, and differ in many respects, including behaviour, morphology, survival and reproductive success. In a wild population of the colour polymorphic tawny owl Strix aluco, a recent cross-fostering experiment showed that offspring raised and born from red mothers were heavier than those from grey mothers. In the present study, we tested in the same individuals whether these morph-specific offspring growth patterns were associated with a difference in metabolic rate between offspring of red and grey mothers. For this purpose, we measured nestling oxygen consumption under two different temperatures (laboratory measurements: 4 and 20°C), and examined the relationships between these data sets and the colour morph of foster and biological mothers. After controlling for nestling body mass, oxygen consumption at 20°C was greater in foster offspring raised by grey foster mothers. No relationship was found between nestling oxygen consumption and coloration of their biological mother. Therefore, our study indicates that in our experiment offspring raised by grey foster mothers showed not only a lower body mass than offspring raised by red foster mothers, but also consumed more oxygen under warm temperature. This further indicates that rearing conditions in nests of grey mothers were more stressful than in nests of red mother