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

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.

The proposed research programme will provide a significant technical advance in the areas of renewable energy sources and HVDC transmission. The provision of AC grid support (in terms of inertia and frequency response) will ensure safe development of an offshore DC grid without affecting the performance of the existing system. This, in turn, will reduce the need for further infrastructure reinforcements (due to low inertia and low short-circuit levels), allowing for a better grid integration of offshore renewable sources and, in the end, increasing the likelihood of meeting greenhouse gas emission policy directives by 2020 and 2050.

A significant economic investment has to be made in order to reinforce the existing transmission system to accommodate the power produced by renewables. However, the potential delays associated with planning and permission to build new AC lines would make it impossible to meet governmental targets on time. Therefore, investment in HVDC technologies is key and HVDC transmission is now a necessity in the UK. It is important to the government and UK citizens that public investment is kept to a reasonable level whilst significant accomplishments in increasing transmission capacity are achieved. Being able to achieve this without affecting system performance involves a deep understanding of the new technologies and their potential to provide system support. By providing ancillary services (control algorithms to provide inertia support), mathematical models and analysis tools, more effective system planning and operation may be achieved. This will lead ultimately to lower energy costs for the end user. It is clear that society can benefit directly from the improved performance of the transmission system operator.

The provision of ancillary services by HVDC converters, in terms of low inertia and low short-circuit level support, is not a standard industrial practice yet. National Grid, as transmission system operator in the UK, would benefit from this knowledge. A deep understanding of the operational issues associated with DC grids and with the future European SuperGrid is required. The research carried out in this project will allow National Grid to have a better understanding of the possibilities that HVDC technologies can provide for AC grid support and the operation of HVDC-rich AC grids. The simulation of case studies, analysis tools and the test platform provided by this proposed research project will complement National Grid's existing pool of resources so as to further its understanding of these possibilities. A strength of this proposal is its being supported by a manufacturer (Alstom Grid) and a consultancy company with an energy portfolio (PB Power). Cardiff University already works closely with both companies. Joint research projects have been carried out with Alstom Grid over a number of years; PB Power has provided Cardiff with a Visiting Professor, expert in HVDC technologies. Alstom can benefit directly from the novel ancillary services of HVDC converters and the understanding of potential control interactions within the converters, whilst PB Power may apply the knowledge generated by the proposed project and utilise the novel models when providing consultancy services to their clients. By making use of these unique and established relationships, outcomes of this project can be easily exploited.

The cooperation of the investigators of this project with other funded initiatives in this call, together with the organisation of workshops for students and researchers involved in the topic and other dissemination and engagement activities, will provide an excellent platform for national collaboration. This has the potential to influence industrial activities and international academia which will, in turn, maximise the potential and value of international activity in the field of HVDC as the need for improved transmission services becomes increasingly urgent.