Mitigating the effect of low inertia and low short-circuit level in HVDC-rich AC grids

可再生能源，特别是海上风力发电，将成为英国能源转型的重要组成部分，以满足其能源需求，同时减少碳排放。高压直流输电将是将海上风力发电整合到英国交流电网以及连接欧洲其他交流电网的关键技术。线路整流变换器（LCC） HVDC特别适用于大容量电力传输，电压源变换器（VSC） HVDC特别适用于将海上电力接入低惯性、低短路水平的交流电网。基于VSC技术的多终端高压直流输电网络和直流电网将在整个北海开发，形成未来的超级电网，以增加海上风电传输的灵活性、冗余性和经济可行性。传统的同步发电机将越来越多地被采用电力电子变流器和高压直流输电的可再生能源发电所取代。这使得系统惯性和短路水平显著降低。特别是在英国，大规模的海上风力发电和与其他欧洲国家电网的互联将形成一个富含hvdc的交流电网。这将导致交流电网具有低故障电路和低惯性，这将给交流系统的运行带来一系列挑战，如对现有继电保护的潜在影响；LCC高压直流频率失稳与换相失效。这个拟议的项目将研究通过电力电子转换器供电的高压直流交流电网的低惯性和低短路水平的行为。挑战将是HVDC链路提供系统支持的能力可能（同时）受到这些对电网的影响的不利影响。LCC高压直流可以提供人工惯性，但对电网的短路率要求较高，才能正常工作。在交流故障和故障后恢复过程中，LCC的惯性支持能力往往在最需要的时候受到限制。VSC高压直流控制对交流电网的依赖性较小。然而，由于直流线路的电阻非常低，直流故障很容易在整个高压直流电网中传播，从而影响到所有端子的交流侧。在直流故障发生时，注入交流电网的实际功率以及VSC高压直流电网的惯性支持将在急需维持系统频率的时刻失去。与此同时，从VSC高压直流电网对交流电网的无功支持也将完全或部分失去，这取决于转换器的拓扑结构。将调查高压直流变流器的惯性支持，不同类型的变流器减轻低惯性效应的能力，以及它们通过交流侧（用于点对点高压直流链路）或通过直流侧（用于同一直流电网内的变流器）的协调。卡迪夫大学的硬件在环（HIL）平台由高压直流电网试验台、实时数字模拟器（RTDS）和功率放大器组成，将在高压直流电网和高压直流转换器控制的建模和测试中发挥关键作用，以减轻低惯性和低短路水平的影响。通过本项目，深入了解高压直流线路丰富的交流电网运行特点。将建立解决方案，以提高系统惯性和短路水平。通过高压直流可以将更多的可再生能源整合到交流电网中，而不会降低系统性能。该项目的研究成果将通过工业伙伴、国际学术协会、会议和期刊出版物传播。

Renewable power, particularly offshore wind power, will be a major element of the UK's transition to meet its energy demands while reducing carbon emissions. HVDC will be the key technology for integrating offshore wind power into the UK AC grid and for interconnecting other AC grids in Europe. Line commutated converter (LCC) HVDC is particularly suitable for bulk power transfer while voltage source converter (VSC) HVDC is particularly suitable for connecting offshore power into AC grids with low inertia and low short-circuit level. Multi-terminal HVDC networks and DC grids based on VSC technology will be developed across the North Sea to form a future SuperGrid to increase the flexibility, redundancy and economic viability of offshore wind power transmission.Conventional synchronous generators will be replaced increasingly by renewable generation with power electronic converters and HVDC transmission. This causes significant reduction of system inertia and short-circuit level. Particularly in the UK, large scale offshore wind power and interconnection with grids in other European countries will lead to an HVDC-rich AC grid. This will result in AC grids with low fault-circuit and low inertia which will present a series of challenges for AC system operation such as the potential impact on existing relaying protection; frequency instability and commutation failure of LCC HVDC.This proposed project will look at the behaviour of low-inertia and low short-circuit level in HVDC-rich AC grids supplied through power electronic converters. The challenges will be that the capability of HVDC links to provide the system support could be (at the same time) adversely affected by these effects on the grids. LCC HVDC can provide artificial inertia but requires high short-circuit ratio of the grid to work properly. During AC fault and post-fault restoration, the inertia support capability of the LCC would be limited at the very time it is most needed. VSC HVDC control is less dependent to AC grids. However a DC fault can be easily propagated across whole HVDC grid due to the very low resistance of DC lines, which in turn affects the AC sides of all terminals. During the DC fault, the real power injected into AC grids as well as the inertia support from VSC HVDC grids would be lost at the very moment it is much needed to maintain the system frequency. At the same time, reactive power support to AC grids from VSC HVDC grids would also be lost completely or partially depending on converter topologies. Investigations will be undertaken of the inertia support from HVDC converters, on the capabilities of the different types of converters to mitigate low-inertia effects and on their coordination through the AC side (for point-to-point HVDC links) or through the DC side (for converters within the same DC grid).The hardware-in-loop (HIL) platform at Cardiff University, which consists of a HVDC grid test rig, a real time digital simulator (RTDS) and a power amplifier, will play a key role in the modelling and testing of HVDC-rich grids and HVDC converter control for mitigating low-inertia and low short-circuit level effects.Through this project, in-depth understanding of operation characteristics of AC grids which are rich in HVDC links will be achieved. Solutions will be founded to enhance the system inertia and short-circuit level. More renewable power through HVDC can be integrated into AC grids without deteriorate the system performance. The research outputs of this project will be disseminated through industrial partners, international academic associations, conferences and journal publications.