Multilevel Inverter Topologies for MVDC

对气候变化的担忧正在推动太阳能和风力发电场等可再生能源发电的增长，这给配电网运营商带来了一个问题。电力网络传统上是由集中分布的发电机（如燃煤、石油、天然气和核电站）组织起来的，电力通过国家输电系统输送到与当地配电网接口的变电站。该配电网有支线，将电力向外径向输送到家庭和企业供用户使用。然而，大部分可再生能源发电并不像太阳能电池板那样集中，而是连接到配电网络，被称为分布式发电机（dg）。这种从传统系统的转变，加上新兴技术（如电动汽车）对电力的需求不断增加，导致需要加强网络。传统的网络加固是昂贵的，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，如软开点（SOP），已经被提出作为一种不需要昂贵的加固工程就能容纳更多dg的方法。SOP是连接两个通常不连接的馈线的一种方式，它允许控制馈线之间的功率流。这意味着在一个特别有风的日子里，来自风力发电场的电力可以在不同的馈线之间共享，从而减少了网络的压力。SOP由两个AC/DC转换器组成，两端各一个，由中压（数十千伏）直流链路分开。这个概念可以扩展到一个多终端网络，一个与交流配电网有多个连接的中压直流（MVDC）网络，以提供进一步的灵活性和加强。为了成功地应用MVDC网络，了解各种DC/AC转换器设计并评估它们的相对优势是很重要的。目前，关于MVDC系统中使用的最佳转换器设计尚无共识；因此，需要进一步研究确定哪种转换器设计在成本和效率方面提供最佳性能。提高效率在全寿命成本和变电站的冷却要求方面都有明显的好处。变电站有限的空间需要非常紧凑的设计，因此最小化所需的能量存储组件，如电容器和设备数量也很重要。最近在该领域的研究已经确定了一些具有直流故障穿越能力的新型变流器设计，即使在直流故障期间变流器仍能保持运行并支持电网。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系统的新转换器设计，并将其与传统设计进行比较，目的是确定用于MVDC系统的最有前途的设计。该项目还将考虑MVDC系统中转换器的利用，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，将采用分析、模拟和实验研究技术的混合方法。转换器设计分析将用于评估转换器的电容器尺寸要求和器件数量，而建模和仿真将用于显示转换器的操作，并可用于估计效率。转换器的实验实现和原型设计将为研究提供验证。

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.