Bipolar dc grid is a promising solution for future power distribution grids for land, sea and space. These grids have higher power transfer capability when compared with 3 phase ac grids and unipolar dc grids. In case of future electric ships, the bipolar dc grids can help in improving the energy efficiency while augmenting the robustness of the grid. This is because, a bipolar dc grids can provide more voltage levels. However, when a load is connected between a pole and neutral, there can be a shift in the neutral voltage. Also, there will be a current flowing through the neutral conductor which reduces the overall distribution system efficiency. To mitigate these issues, balancing converters are necessary to be installed in a bipolar dc grid.
Series resonant dc-dc power converter is a promising topology for balancing a bipolar dc grid. This topology is favoured because of its quasi-resonant switching behaviour, smaller passive component size and lower voltage rating for the semiconductor switches (compared to buck-boost and other traditional topologies). If all the elements are considered lossless, the converter can easily balance the bipolar grid when the switching at the resonant frequency. In this case, whenever a voltage difference is present across the converter, a current would flow through the converter to remove this difference. However, in practical conditions, perfect balancing of the grid is not possible due to the voltage drop across the switches and passive elements of the converter itself. When there is a large difference in the voltage between the two poles and neutral, the converter can be used to balance the grid to a certain extent. However, depending upon the grid configuration and the amount of imbalance, the voltage difference between the two poles can be quite low. This would render the series resonant balancing converter ineffective. Hence, a different approach is required to extend the functionality of this converter.
Our invention proposes a novel control method of the series resonant power converter. Using this method, the converter can be utilized to balance the bipolar dc grid without any additional components. The principles of the control method are as follows. The converter consists of an inductor and a capacitor in its resonant tank. The capacitor is used to store the energy every half cycle from the high voltage pole and transfer it to the low voltage pole. This alternate connection of the resonant tank with the input and then to the output leads to the power transfer. The maximum voltage that the resonant capacitor can reach depends upon the pole to neutral voltage and hence, the maximum power transfer is limited by this voltage. To increase this voltage, the resonant tank is momentarily connected between the two poles of the grid. This leads to an increase in the voltage of the capacitor. The side effect of this strategy is that the current through the inductor also increases to very high levels. To mitigate this issue, the switching frequency of the converter is decreased to bring the converter in the capacitive region. This decreases the current through the inductor and also helps in balancing the grid. In conclusion, this invention aids in opening a paradigm for the implementation of resonant converter for balancing a bipolar grid.