Designing HVDC grids for optimal reliability and availability performance

Summary

Since its modern re-incarnation, dating from the 1960’s onwards, High Voltage Direct Current (HVDC) schemes have principally been used as two-terminal schemes to connect remote energy sources to load centres or as interconnectors between asynchronous transmission networks. Although multi-terminal schemes, with three (or four) stations were built, the limitations of the prevailing Line Commutated Converter (LCC) technology meant that such schemes were the exception rather than the rule. With the advent of Voltage Source Converter (VSC) technology since the turn of the millennium, the application of multi-terminal schemes has become a more practical possibility and already there are schemes with up to five terminals in operation.

The functionality provided by VSC technology has opened up the possibility of building a HVDC grid, where multiple AC to DC converter stations are interconnected via a DC transmission network to provide controllable power flows over a large geographical region. There has been considerable interest in such a possibility, driven by the need to create wide area energy markets, to integrate remote renewable energy sources to the AC grids, and to provide power supplies to remote oil and gas installations.

This Technical Brochure addresses the question of how to design a DC grid to achieve optimal reliability and availability performance. The starting point was to consider a suitable metric which could be used to assess the reliability and availability performance of the grid. This is proposed to be the “energy not served by the grid”, i.e. a measure of the achieved energy capability of the grid under operational conditions, compared with its maximum designed capability.

Anticipating that a DC grid may not have an overall “architect”, the brochure considers the evolution of the grid from smaller radial and meshed multi-terminal systems and the factors which need to be considered when incorporating such building blocks to ensure that the grid can achieve high levels of reliability and availability. The technologies used at the AC to DC converter stations and at DC switching stations are discussed in the brochure in terms of their impact on the grid. The interconnecting medium between grid stations, whether overhead transmission lines, underground cables or submarine cables, will also have a major impact on the grid in terms of their susceptibility to internal or external faults.

The techniques use to protect the grid, to minimise potential wide area outages, will be critical to achieving high reliability and availability and an outline of the issues involved are presented in the brochure. The modes of operation of the individual stations on the DC grid will also have a considerable impact. Similarly the strategies adopted for maintenance of the equipment and the levels of spare parts holding need to be carefully considered for a DC grid.

The design of a DC grid will require analytical techniques to assess whether the evolving grid is able to achieve the desired levels of reliability and availability performance. The brochure includes an example of such a study, based on a deterministic evaluation of a DC grid test model, to illustrate the impacts of specific outage conditions on the energy not served by the grid. This study is presented as an illustrative example only and is not proposed as the methodology for future evaluations of DC grids, as other methods, including probabilistic techniques, may be adopted.

Additional informations

Keywords

hvdc, power electronics