Bipolar DC Grids on Ships: Challenges & Solutions for Voltage & Power Balance

Globally, the marine sector contributes to almost 3 % of greenhouse gas emissions. To reduce the impact of the shipping sector, the International Maritime Organisation (IMO) has developed several guidelines for ship manufacturers and operators. Hence, the industry needs an urgent push to improve operational efficiency on new and existing ships.

One of the ways to improve efficiency is by electrifying the ship's grids. Therefore, significant research is ongoing on DC grids on ships. When there is a + (P), neutral (Z), and - (N) voltage present in the grid, it is referred to as a Bipolar DC (BiDC) grid. Alternatively, the BiDC grids can only consist of two conductors - P and N. BiDC grids have several advantages compared to a unipolar DC (UDC) grid, such as higher power flow capability with the same or lower insulation costs and a higher number of voltage levels. A higher number of voltage levels is an interesting property for ships. In ships, there is a considerable disparity between the installed capacity of propulsion loads and other low-power hotel loads. In a BiDC grid, the propulsion loads can be connected between the P and N poles. However, as a Z conductor is available, half the pole-to-pole voltage is available for free. Hence, this lower voltage can power the lower power loads. Due to the lower voltage, the power converter design can be simplified or sometimes even eliminated. Furthermore, the grid can be more reliable because of two paths with lower voltages (PZ and ZN). In case of faults between a pole and a neutral, another pole can supply power to the critical loads.

All these benefits, however, come at a cost. When the loads connected between the pole and neutral draw different amounts of power from the grid, the voltage of the neutral conductor shifts. Hence, a balancing converter is required to equalize the voltage between the PZ and ZN poles. This thesis deals with designing and implementing balancing converters for the BiDC grid.

Design of balancing converters

Although several power electronic converter topologies exist, this thesis focuses on the series-resonant LC topology. Due to the resonant nature of the topology, it can have smaller sizes of passive components. Thus, the converter's weight is lower than other traditional topologies. Furthermore, several operating modes for this converter have been proposed in this thesis. Some operating modes and modulation schemes can guarantee zero voltage switching at the turn-on of all the switches. Hence, not only are the losses reduced, but there are lower electromagnetic emissions from the converters.

The thesis also proposes the design of buck-boost-based balancing converters for high-voltage applications. This is mainly done using series-connected MOSFETs.

Implementation of balancing converters

The power grids on ships come in two flavors: radial and zonal architecture. However, these two architectures are essentially the same in terms of applying balancing converters for the loads. Hence, any implementation works for the BiDC grids in both architectures. Furthermore, the thesis demonstrates that placing the converter near the loads improves system efficiency due to lower cable losses.

The thesis delves deeper into implementing multiple balancing converters in the grid. Depending upon the installed capacity and length of conductors, several balancing converters might be required. Subsequently, the balancing converters can be connected in parallel or series with the main bus. This designation is critical as the control of the converter differs depending on the connection type. Finally, to test the application of the balancing converter in the grid, several combinations of the balancing converter are simulated. These combinations are series-series, series-parallel, and parallel-parallel. Coordinated control for the different arrangements can be achieved without communication between the devices and multi-tier control schemes.