High Definition – Modular Multilevel Converter

Published 29 April 2020

As offshore wind farms move further from the point of connection to our onshore grid, the industry is increasingly looking at alternative transmission techniques to address the inherent technical challenges that this added distance creates.

One very promising option is to use High Voltage Direct Current (HVDC) as this greatly reduces the losses in very long sub-sea cables. As wind turbines create Alternating Current (AC), a power electronic converter is required to convert between the AC power generated by the turbine and the DC power that will be transmitted to shore.

To date, the power converter topology of choice for offshore wind HVDC substations has been the Modular Multilevel Converter (MMC). At high voltages, this topology provides many advantages over other converters. However, at lower voltages, such as those found in wind turbines, a conventional MMC cannot create a smooth waveform due to the inherent limitation in the number of voltage levels that can be created. As a result, many advantages of the MMC are removed and it is rarely used at voltages under 100kV.

The Solution

To overcome this limitation and allow the many benefits of the MMC to be realised at lower voltages, ORE Catapult has developed a novel High Definition – Modular Multilevel Converter (HD-MMC) algorithm through computer simulations before demonstrating the HD-MMC and validating the models in joint projects with SINTEF, Tecnalia and IREC. This control algorithm can significantly improve the quality of the AC voltage waveform for a given converter and hence improve the converter’s efficiency and power density at lower voltages too.  Unlike other control algorithms that aim to achieve a similar effect, the HD-MMC developed by ORE Catapult does not increase converter losses and this opens it up to a wider range of applications

Through the IRPWind H2020-funded programme, the HD-MMC was proven to work within an actual MMC under common grid and generator operating conditions. When compared to the performance of a conventional MMC at the same voltage, the HD-MMC was found to reduce converter switching losses by up to 80%. As well as increasing efficiencies, this is a significant reduction, especially in applications where high operating frequencies are beneficial to reduce weight by minimising transformer sizes.  Given that a HVDC substation can cost around 15% of the overall wind farm, reducing the weight of components and the size (and therefore cost) of the substation will have a significant cost reduction impact.

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