A Non-Isolated DC-DC Converter With An Ultra-High Voltage Ratio for Offshore Power Applications

Abstract

To maximise renewable energy generation, different forms of renewable power systems will need to be utilised. With this comes the opportunity to minimise capital expenditure
and spatial usage via co-location. A promising co-location opportunity comes from the integration of the offshore wind farms with wave energy converters. Where floating or submerged wave energy converters are placed within the space between offshore wind turbines, to optimise marine spatial usage and utilise the same electrical infrastructure. To achieve this integration, the low output voltage of the wave energy converters would need to be stepped up to a medium voltage compatible with the inter-array voltage of an offshore wind farm. Conventionally, power converters which utilise step-up transformers have been adopted to produce similar forms of voltage conversion. While effective for offshore wind applications, complications arise from the mechanical constraints and environmental hazards associated with the use of a transformer on a floating or submerged platform. To study this issue, the work presented explored a strategy to achieve low to medium voltage conversion without the need for a step-up transformer. This strategy involved the exploration and development of transformerless DC-DC converters capable of high and ultra-high voltage conversion ratios, which produced an output voltage multiple times
greater than the given input voltage.

Conventionally, to achieve a high voltage conversion ratio (> 10) in generic DC-DC converters, designs either require the use of excessive duty ratios (> 0.75) or the integration of voltage multiplying sub-circuits. Excessive duty ratios are often associated with reduced operational power efficiency, which limits the maximum amount of energy that
may be extracted from a power system. Conversely, voltage multiplying sub-circuits increase circuit complexity while maintaining high operational efficiency. Therefore, voltage
multiplication sub-circuits are often the chosen strategy to achieve high voltage conversion ratios when the maximisation of energy extraction is of high priority. To electrically integrate low-voltage wave generation technologies into medium-voltage offshore wind farm infrastructure, ultra-high voltage conversions (> 40) would be required. To achieve this, combinations of voltage multiplication techniques demonstrated within the literature were explored and evaluated for suitability.

Three transformerless DC-DC converters were sequentially conceptualised and developed. The operational analysis of all designs was presented and then subsequently validated via simulated and experimental demonstration. The novel continuous operation of a 1 kW scalable bipolar switched capacitor-based boost converter capable of a high voltage
conversion ratio at a 98.2 % power efficiency was initially proposed. The experimental converter demonstrated a gain of ±10 via the step up of a 100 V input into a ±1 kV output. Following this, a scalable unipolar switch capacitor boost submodule was combined with a new method of scaling the voltage lift switched inductor topology to achieve
ultra-high voltage conversion. The device demonstrated a voltage conversion ratio of 41 with a 100 V input and 4.1 kV output, at 97.5 % efficiency when operating at 1 kW. Finally, the bipolarity was reintegrated into the design to demonstrate an innovative DC-DC converter capable of a greater voltage conversion ratio than the previously demonstrated converters and those reported in the literature for continuous, not isolated, operation. The peak operating efficiency was recorded during a 2 kW test with an input of 100 V and output of ±4 kV where the converter achieved an efficiency of 95.7 %. To demonstrate the ability of the converter to achieve a continuous medium voltage output from a range of low voltage inputs, two DC sweeps were conducted at 1 kW. During the unipolar DC sweep, the positive output of the converter was referenced to the negative output. From
this operation, the converter produced a single 4.7 kV output for an input voltage range of 40 - 125 V. During the bipolar DC sweep, the two outputs of the converter were referenced to a common ground. From this operation, the converter produced a positive and negative output of ±4.7 kV output for an input voltage range of 70 - 200 V. When
considering both unipolar and bipolar operation, the converter demonstrated a voltage conversion ratio range of 24 - 118.

Opportunities for power efficiency optimisation were identified based on the simulated
and experimental findings. A particular focus was on the reduction of power and voltage losses in switched capacitor voltage multipliers. From this, key trade-offs were identified between both types of losses and capacitor sizes. The greatest unrealised pathway for power loss reduction originated from the active switching devices. A focus on the reduction of conduction-based losses led to the selection of devices with low on-state resistances. Due to this, the greatest proportion of power dissipation originated from switching-based losses. From this, strategies for the reduction of switching losses were identified as a key area
for future research. Further opportunities for development were identified based on the experimental findings, strategies for integration into a solid-state transformer, allowing for bidirectional power flow, upscaling operation power and potential alternative applications were outlined and discussed.