The installation of a large number of HVDC systems is expected to assist the transition to a low carbon energy supply by integrating RES. Furthermore, the high level of interconnection reduces intermittency issues and enhances the security and the long term sustainability of electricity supply [4].
The number of HVDC projects has increased significantly in recent years. Approximately 100 HVDC installations are in operation or planned for the future transmitting more than 80 GW of power [30]. A summary of the most important HVDC projects in the UK, in Europe and in the world is presented here.
1.2.2.1 HVDC Projects in the UK
Considering the geographical position of the UK as well as the predicted large penetration of offshore wind, several HVDC links have been built for connecting the island to continental Europe and to Ireland as well as to reinforce the existing AC grid. The HVDC systems linking the UK to different countries are IFA, BritNed and East-West interconnectors, as shown in Figure 1.5. The HVDC systems connecting different parts of the UK electrical network are the
Western Link and the Eastern Link (planned).
IFA (Interconnector France Anglaterre) is one of the oldest HVDC project in the UK. It is a joint agreement between National Grid and RTE (the French TSO) and it was commissioned in 1986. The interconnector is rated at 2000 MW and it is 70 km long [4]. BritNed is a joint project between National Grid and TenneT (the Dutch TSO). It has been operational since 2011 and connects the Isle of Grain in the UK to Maasvlakte in the Netherlands. It is a 260 km long interconnector rated at 1000 MW [4]. The East-West interconnector has been in operation since 2012 and it is owned by EirGrid (the Irish TSO). It connects Ireland with the UK (Deeside) via a 130 km long link rated at 500 MW [4].
Netherlands Denmark Belgium Iceland N Ireland Norway France Ireland Existing In development Future potential
Figure 1.5: HVDC schemes in the UK [4].
The Western HVDC Link connects the renewable generation from Scotland with the large demand of England transferring bulk power across the country. It is rated at ±300 kV, 2200 MW and it is 420 km long. The project, currently under construction, will be completed by 2016 [31]. The Eastern Link is a proposal for a 2000 MW HVDC link that would upgrade the existing infrastructure in Scotland’s North East coast and deliver low carbon renewable power to households and businesses across Britain. Its completion date is due to be 2018 [31].
Apart from the existing HVDC schemes, a large number of potential future interconnector opportunities are being investigated by National Grid, as it is illustrated in Figure 1.5. These include a link between the UK and Belgium (1000 MW, 150 km long, completed by 2018), a link between the UK and Norway (1400 MW, 750 km long, completed by 2020), a link between the UK and Denmark (1000 MW, 600 km long, completed by 2020) and another HVDC link between the UK and France (1000 MW, 200 km long, completed by 2020) [4].
1.2.2.2 HVDC Projects in Europe
In Europe the number of HVDC projects has increased significantly in the last decade. A summary of the operational and planned HVDC schemes is detailed in [32]. Among the more notable HVDC projects in Europe are INELFE and NordBalt.
INELFE (France-Spain ELectrical INterconnection) is a mixed-capital corporation between the Spanish (Red Eléctrica de España REE) and the French (Reseau Transport d’Electricite RTE) TSOs and it aims to increase the electrical security along with the quality of supply. The link, rated at ±320 kV, 2000 MW, connects Spain to France through the Pyrenees and it has been in operation since 2014 [32].
NordBalt, also known as SwedLit, is an HVDC link (submarine cable) connecting Lithuania to Sweden. It aims to promote trading between Baltic and Nordic electricity markets as well as to increase power system reliability in both countries. It is rated at ±150 kV, 700 MW and it is expected to be operational in 2015 [32].
1.2.2.3 HVDC Projects Worldwide
Figure 1.6 indicates the number of HVDC projects per region. The U.S. and China, as Europe, are the countries with the largest presence of HVDC schemes in the world. Unlike in the U.S. and in Europe, where HVDC links usually connect different AC grids, in China they are included within the massive national power systems.
In the U.S., one of the first HVDC projects was the Pacific DC inter-tie (1970), which links Oregon to California. It is a 1362 km long corridor rated at ±500 kV, 3100 MW [33]. One of the most recent HVDC projects is the Transbay Cable (2010) which connects Pittsburgh to San Francisco. It is rated at ±200 kV and 400 MW and it is the first link to use IGBT Modular Multi-level Converter technology developed by Siemens [34].
In China, the installation of HVDC links is experiencing a significant boost. Since the power plants (hydro and wind farms) are usually located close to the generation sources and far away from the urban areas, the power needs to be transmitted over very long distances, leading to a growing demand for HVDC infrastructures. Among the Chinese HVDC projects, the most notable are: the Three Gorges-Guangdong (2004), which transfers bulk power from the Three Gorges hydro power plant to the Guandong area for 940 km and it is rated at ±500 kV, 3000 MW; the Three Gorges-Shangai (2006), which connects the same hydro power plant to Shangai via a 1060 km long HVDC link rated at ±500 kV, 3000 MW; and the Jinping-Sunan (2013), which is one of the biggest HVDC schemes in the world and it is rated at ±800 kV, 7200 MW and is 2090 km long [35]. The Zhoushan (2013) and the Nan’ao (2014) schemes are among the largest multi-terminal DC grids worldwide. The Zhoushan project connects the islands located nearby the Chinese South-East coast with the onshore grid via a five terminal scheme. The power rating ranges from 100 MW to 400 MW and the common DC voltage is ±200 kV. The Nan’ao project consists of four terminals (three onshore stations and one offshore wind farm) characterized by a DC voltage of ±160 kV. The terminals power ratings vary among 60 MW and 240 MW [35].