Global average temperatures are setting new records compared to the previous decade, primarily due to the accumulation of greenhouse gases such as CO₂. The growth potential in emerging economies worsens the current global situation. These CO₂ emissions are needed to be curtained and efforts are underway. The emissions target set by the International Maritime Organization seems far-fetched given the forecast for the growth of the shipping sector. Although electrification has proven to reduce emissions, the energy efficiency metrics (EEDI, EEXI) are incapable of providing the quantitative benefits that can be realized by switching to a bipolar DC (BiDC) grid when compared with the AC grid.
This thesis aims to evaluate the feasibility of BiDC grids over the complete operating profile of the ship by comparing ten arrangements, five for AC and five for DC, with the Key Performance Indicators (KPIs). The arrangements are two diesel generators (AC1 & DC1), two diesel generators and battery (AC2 & DC2), two diesel generators, battery, and shore power (AC3 & DC3), one diesel generator and battery (AC4 & DC4), one diesel generator, battery, and shore power (AC5 & DC5). The comparison is made for key performance indicators like CO₂ emissions, fuel consumption, electrical power requirement, propulsion power cable losses, capital costs, operating costs, propulsion system weight, and load carrying capacity of the ship. In this study, a ferry is chosen to compare the two grid architectures.
To establish the required background knowledge for the reader of this thesis, the literature on the current emission regulations, the different propulsion systems, and the differences in AC and DC grids are presented. The ship’s propulsion and auxiliary power demand are not freely available and are in the possession of the ship operator. Therefore, first, a preliminary methodology was developed for estimating the ship’s mechanical power requirements. Second, the ship’s electrical power requirements are estimated from the understanding of the literature. The cable power losses are also calculated for the AC and BiDC grid topology. Third, the diesel generator model was developed from the data points of a marine diesel generator for the AC and BiDC grid operation, i.e., the fixed speed and variable speed operation. Finally, an optimization problem was formulated for all the ten arrangements for minimising the CO₂ emissions over the complete operational profile of the ship.
Overall, it was observed that the minimum emissions achieved by BiDC grids, when compared with AC grids for all the arrangements, the emissions from the former were lower. Furthermore, these results depend on the operating profile of the ship, the shore infrastructure, the diesel generator and battery configuration and their technical parameters. However, these findings demonstrate the possible potential of BiDC grids in emission reduction from the shipping sector.

The increasing penetration of the distributed energy sources and the increasing load on the AC grid demands for integration of DC grids in AC distribution network. This has resulted in the requirement of power electronic converters such as DC/AC converters. For different operating conditions of the DC/AC converter it is not recommended for the converter to operate in open loop. Hence, while designing the control for the converter there are challenges such as the presence of unbalance on the grid leading to higher losses, offsetting of AC voltages at the output and grounding of the converter leading to higher common mode voltage and currents. This thesis explores the control strategy that can be implemented on the converter to mitigate the unbalance in different operating conditions and compares different modulation techniques to reduce the common mode voltage and currents.
To begin with the islanded operation is considered for the converter and the voltage control is implemented on this type of converter. Further a modification on this type of control is proposed for the operation of converter under overcurrent scenario. Subsequently the control strategy for grid connected operation is analyzed such that the issue of unbalanced currents on the grid is mitigated. Three different operating modes are considered under grid connected operation for validating the control strategy proposed. Following the controlled operation in islanded and grid connected scenario, the active methods for reducing the common mode voltage are reviewed by comparing different modulation techniques.
On the discussed control strategies for different operating conditions of the DC/AC converters, an optimized implementation of the control on the microcontroller is presented. Other functionalities such as protection of the converter against unwanted actions is investigated through implementation of state machine. The results of the controlled DC/AC converter operating in islanded operation is presented and modifications in the existing PCB are discussed to make it compatible to operate under grid connected condition. The operation of control strategies implemented is validated for the real time application of the converter with unbalanced load conditions.

Global electricity usage has been increasing exponentially over the last 40 years and with the current pace of population and economic growth, the same trend will continue in the coming years. However, the current electricity generation is predominantly fossil fuels based, which makes the overall process unsustainable and polluting, and is contributing directly to the menace of global warming. The need of the hour is to transition to renewable and sustainable sources of electricity generation such as solar, wind etc.

However, the major bottleneck in the large-scale implementation of RESs is the intermittent availability and their integration into existing AC grids as the RESs are predominantly DC sources. In order to tackle the issue of intermittency various energy storage solutions are being developed. Further, as a solution to the issue of integrating RESs into the current electricity distribution system, DC distribution grids are being developed.

The DC distribution architecture is of two types, Unipolar and Bipolar distribution systems characterized by the number of wires used for power transmission; 2 wires for unipolar and 3 wires for Bipolar. Out of the two, Bipolar systems have inherent advantages of flexibility, stability and efficiency over unipolar systems.

Current thesis focuses on series connected switch configuration of voltage balancing converter for bipolar DC power distribution systems.

For this thesis, various topologies of voltage balancers for bipolar dc distribution systems were studied and a buck boost-based series switch-connected voltage balancer topology was chosen for the final design. For this topology, using MATLAB the parameterization and optimization of magnetic components of the converter were performed. The optimized configuration was then modeled and simulated using LTSpice.

Post this, a comparison between different methods adopted to account for the unbalanced voltage sharing across the series-connected switch configuration of the balancing converter owing to the non-linearities present in the circuit was done. Based on this study, the most prominent of the methods is then integrated to the LTSpice model.

The current energy systems with the mix of renewable and conventional sources are already working on their maximum capacity in order to meet the existing demand. In the near future to meet the burgeoning demand, the future electrical power system would be a mix of both AC and DC systems coexisting with each other. With the fast paced development of smart grids, decentralised system and micro grids, there is an increasing interest in the LVDC (Low Voltage Direct Current) distribution. Also there is increase in the loads in the residential and commercial sector side that require DC supply inherently. To tap in and realise the potential of the LVDC distribution systems, it requires the appropriate infrastructures to be developed that supports its assimilation faster. This thesis “The Design and development of LVDC cable” is a step forward in realising the potential that DC has in the LV distribution side. This research is done in collaboration between TU Delft and the Prysmian Group.

To begin with, two LVDC statespace based dynamic distribution system model namely: a neighbourhood distribution system and a ship distribution system are formed. The LVDC cables are incorporated as lines into these models. The sensitivity and stability analyses are performed by varying each of the active parameters individually, the influence that each of the active parameter has on node voltages, line currents and the stability of the system are studied and the trend is absorbed through this analysis. This analysis is set to be performed in both distribution systems under monopolar and bipolar configuration.

Subsequently, the short circuit analysis i.e the pole to pole and pole to neutral fault analysis is also performed on both the neighbourhood and ship distribution system. Here the cable’s active parameters that are expected to have an influence on the system performance and the line current during short circuit are taken into consideration. These active parameters are varied throughout this analysis and the resulting plots are further analysed to study their influence. These active parameters are cable resistance, cable inductance, cable capacitance, and cable length. The observations are tabulated for each of the active parameters individually. The generated results from this study would support in better assessment of cable’s active parameters influence on the short circuit current and designing the LVDC cables.

The accuracy of the dynamic model are validated by breaking the distribution system model into smaller sections and forthcoming results are compared to get the model accuracy. Finally based on the results from the sensitivity stability and short circuit analyses the optimal characteristics of the LVDC cables is formulated.

Roughly 840 million people, predominately from rural communities in sub-Sahara Africa, still lack access to electricity. The direct current (DC) microgrid is an emerging grid infrastructure that meshes efficiently with DC based technology such as photovoltaics, batteries, consumer electronics, and electric vehicles. This characteristic of DC microgrids designates them as a preferred solution for new grid infrastructures in rural electrification applications. In order to establish a DC microgrid, power electronic interfaces (PEI) are required for regulating power flow and interconnecting different grid components of the microgrid. In this thesis, the PEI (operating at 200-900 W) connecting a 350 V DC-microgrid to a solar home system (compatible with USB-C) is investigated. A unidirectional half-bridge LLC converter and a bidirectional dual active half-bridge (DAHB) converter (both utilizing gallium-nitride (GaN) transistors and planar transformers (PT)) are designed and tested. A working prototype of the half-bridge LLC converter with a center-tapped secondary is presented and the waveforms at maximum load and no-load conditions are analyzed. A complete design and efficiency approximation for a DAHB converter with a center-tapped secondary and active snubber circuits is included. In addition, simulations (in PLECS) of the designed DAHB converter provide waveform results for both the forward and reverse power flow direction. The results of the work discuss how high frequency operated, half-bridge isolated DC/DC topologies with GaN transistors and planar transformers are an excellent composition of technology for these rural electrification applications. The GaN transistor is most effective in a low voltage (up to 650 V), high performance scheme, and offers inherent benefits which allow for high frequency operation and thus, smaller passive components. Moreover, the effect of current collapse (an adverse effect in GaN transistors) is discussed and analyzed from a design standpoint. The benefits of planar transformers in low-medium power (up to 900 W) rural electrification applications and an in-depth PT design process are presented. Additionally, rural electrification safe extra-low voltage (SELV) standards require that PT designs must have reinforced (or double) isolation between primary and secondary windings. Taking this into consideration, two proposed PT configurations using a U-core and planar E-core, respectively, are compared. The main conclusions of this work aim to bridge the gap for the design and implementation of efficient DC microgrid variable power output converters for use in rural electrification applications.