Multilevel Inverter Topologies for MVDC

Concern over climate change is driving a growth in renewable generators, such as solar and wind farms, and this is creating a problem for electricity distribution network operators. Electricity networks are traditionally organised with centrally located generators, such as coal, oil, gas and nuclear power plants, with the power transported via a national transmission system to substations which interface with the local distribution network. That distribution network has feeders which transport the power radially outwards to homes and businesses for consumption. However, much of the renewable generation is not centrally located, such as solar panels, but is instead connected to the distribution network and are known as distributed generators (DGs). This shift away from the traditional system combined with the increasing demand for electricity because of emerging technologies, such as electric vehicles, is resulting in a need for network reinforcement. Traditional network reinforcement is expensive and uncertainty surrounding future DG projects creates difficulties when planning such reinforcement projects. Smart grid technologies, such as the soft-open point (SOP), have been proposed as a method of accommodating more DGs without requiring expensive reinforcement projects. A SOP is a way of connecting two normally unconnected feeders which allows for the control of the power flow between the feeders. This means that on a particularly windy day the power from a wind farm can be shared among different feeders, thus reducing the strain on the network. A SOP consists of two AC/DC converters, one at either end, separated by a medium voltage (tens of kV) DC-link. This concept can be extended to a multi-terminal network, a medium voltage DC (MVDC) network with multiple connections with the AC distribution network to provide further flexibility and reinforcement.
In order to successfully employ MVDC networks, it is important to understand the various DC/AC converter designs and assess their relative benefits. There is currently no consensus regarding the optimum converter designs for use in MVDC systems; therefore, further research is needed to identify which converter designs provide the best performance in terms of cost and efficiency. Improving efficiency has obvious benefits in terms of life-time cost but also for the cooling requirements of the substation. Limited space at substations requires very compact designs so minimisation of the required energy storage components, such as capacitors, and device count is also important. Recent research in this field has identified some new converter designs with DC fault ride-through capability, the ability of a converter remain operation and support the grid even during a DC fault. DC fault ride-through capability has been shown to reduce the DC circuit breaker requirement in DC systems and thus reduce cost.
This project will involve the investigation of new converter designs for MVDC systems and the comparison of these with conventional designs, with the aim of identifying the most promising designs for use in MVDC systems. This project will also consider the utilisation of converters in MVDC systems including features such as DC fault ride-through capability and how this will impact the wider network. A mixture of analytical, simulation, and experimental research techniques will be employed in order to achieve these aims. Analysis of converter designs will be used to assess the capacitor sizing requirement and device count of converters, while modelling and simulation will be used to show the operation of the converter and can be used to estimate the efficiency. Experimental realisation and prototyping of converters will provide validation of the research.