Multilevel Inverter Topologies for MVDC

对气候变化的担忧正在推动太阳能和风电场等可再生发电机的增长，这给配电网络运营商带来了一个问题。传统上，电力网络由位于中央的发电机组成，如煤炭、石油、天然气和核电站，电力通过国家输电系统输送到与当地配电网络连接的变电站。该配电网络有馈线，将电力径向向外输送到家庭和企业消费。然而，许多可再生能源发电并不位于中心位置，例如太阳能电池板，而是连接到配电网络，被称为分布式发电机(DG)。这种对传统系统的转变，加上电动汽车等新兴技术对电力的需求不断增加，导致需要加强网络。传统的网络加固成本很高，而且围绕未来DG项目的不确定性给规划此类加固项目带来了困难。智能电网技术，如软开关点(SOP)，已被提出作为一种无需昂贵的加固工程即可容纳更多分布式电源的方法。SOP是一种连接两个通常不连接的馈线的方式，它允许控制馈线之间的功率流。这意味着在特别多风的日子里，风电场的电力可以在不同的馈线之间共享，从而减轻网络的压力。SOP由两个AC/DC转换器组成，两端各一个，由一条中压(几十千伏)的直流线路隔开。这一概念可以扩展到多端网络，即具有与交流配电网络的多个连接的中压直流(MVDC)网络，以提供进一步的灵活性和加固。为了成功应用MVDC网络，了解各种DC/AC转换器设计并评估其相对优势非常重要。目前还没有关于用于MVDC系统的最佳转换器设计的共识；因此，需要进一步研究以确定哪种转换器设计在成本和效率方面提供最佳性能。提高效率不仅在寿命成本方面有明显的好处，而且对变电站的冷却要求也有好处。变电站有限的空间要求非常紧凑的设计，因此最大限度地减少所需的储能元件(如电容器)和设备数量也很重要。最近这一领域的研究发现了一些新的换流器设计，具有直流故障穿越能力，即使在直流故障时换流器仍能保持运行并支持电网的能力。直流故障穿越能力已被证明可以减少直流系统对直流断路器的要求，从而降低成本。该项目将研究用于MVDC系统的新的变流器设计，并将这些设计与传统设计进行比较，目的是确定最有希望用于MVDC系统的设计。该项目还将考虑在MVDC系统中使用变流器，包括直流故障穿越能力等功能，以及这将如何影响更广泛的网络。为了实现这些目标，将使用分析、模拟和实验研究技术的混合。对转换器设计的分析将用于评估电容器大小要求和转换器的设备数量，而建模和模拟将用于显示转换器的运行情况，并可用于估计效率。转换器的实验实现和原型将为研究提供验证。

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.