ENTSO-EVoltage Source Converters (VSC)
Overview
Voltage Source Converters (VSCs) are self-commutated converters based on semiconductors, as insulated gate bipolar transistors (IGBTs), that can conduct or isolate independently on the current and voltage cycle. VSCs can generate Alternating Current (AC) voltages without the need to rely on an AC system. This allows for independent rapid control of both active and reactive power and black start capability. VSCs maintain a constant polarity of the Direct Current (DC) voltage and control the DC current flow. This ability enables the application of cross-linked polyethylene (XLPE) cables.
Technology Types
VSCs can be classified with respect to the converter technology types, which have evolved over time:
- Two-Level VSC, which was the earliest technology;
- Three Level Diode Neutral Point Clamped (NPC) or Three Level Active NPC;
- Two Level with Optimum Pulse-Width Modulation (OPWM);
- Cascaded-two Level Converter (CTL); and
- Modular Multi-Level Converter (MMC), which is the latest and most advanced technology used for HVDC transmission. MMC differentiates further into the so-called Half Bridge type and Full Bridge type MMC.
Benefits
Compared to the thyristor-based current source converters provide, the VSC technology has many advantages, as follows:
- Reactive voltage support;
- Grid forming capability (GFM);
- Operation in short circuit weak networks;
- Lower harmonics;
- Lower footprint; and
- No commutation failures.
In offshore projects, MMC is becoming the preferred power electronic converter for converting between AC and DC as it has several benefits:
- the ability to reverse the power flow without reversing the polarity of the DC voltages by DC current reversal, which makes the application of high voltage direct current (HVDC) XLPE cable possible;
- modularity and scalability, which makes it easy to realise other voltages; and
- the inherent capability of storing energy internally in the converter. This can benefit the system in which it is connected and enables the drastic reduction of operating losses of the converter stations by avoiding the need for the high frequency switching of the semi-conductor devices.
More complex HVDC grids are possible only using VSC MMC technology.
Current Enablers
Horizon Europe Project HVDC WISE “Reliable and resilient AC & DC grid design to accelerate the integration of renewables across Europe”. The project will propose a number of reliable and resilient HVDC grid configurations that will lay the groundwork for a future integrated European transmission system.
Horizon Europe Project InterOPERA “Enabling interoperability of multi-vendor HVDC grids” should deliver an alignment between manufacturers and users on the interoperability of multi-terminal and multi-vendor HVDC converters, including HVDC circuit breakers. The project is a key milestone for building more complex interconnected HVDC-Grids.
R&D Needs
Several R&D activities, listed below, can contribute to further improving the technology:
- the development of VSC for multi-terminal, multi-vendor, with fault separation devices (see also Factsheet on the HVDC circuit breakers);
- the integration of the many HVDC links into a dispatching centre, including dynamic security assessment and quasi-online stability assessment;
- studies on harmonic interactions between HVDCs and other active components such as neighbouring STATCOMs or conventional components such as capacitor banks or synchronous condensers to enhance the coordinated use of multiple devices and area voltage regulation, including instability detection technologies;
- basic research on failure propagation in extended HVDC networks;
- the combination of VSC converters with storage for inertia and GFM support;
- the further development and standardisation of the control and protection, including neutral interoperability tests;
- the further development and standardisation of the software interfaces, including neutral interoperability tests; and
- demonstrators for the above-mentioned abilities.
The technology is in line with milestone “Demonstration of interoperability of HVDC converters” under Mission 2 of the ENTSO-E RDI Roadmap 2024-2034.
TSO Applications
Examples
| Location: Germany [2] | Year: expected in 2030 |
|---|---|
| Description: An innovative multi-terminal hub Heide is being built on the North Sea coast in Germany and is expected to be commissioned in 2030. This hub will be used to link two new 2 GW offshore grid connections and a planned underground cable with each other and with the AC grid. Until now, all direct current systems at sea and on land have been point-to-point connections. For the first time, an innovative 525 kV DC circuit breaker will be used, which paves the way towards a meshed direct current grid. | |
| Design: +/- 2000 MW, 525 kV, subsea cable and land cable with expected application of fault separation devices, bi-pole with dedicated metallic return. | |
| Results: In plan. | |
| Location: Germany [3] | Year: expected in 2027 |
|---|---|
| Description: Ultranet link is a roughly 340 km-long land connection using so-called hybrid overhead lines between Osterath and Philipsburg in western Germany. The benefit of the project is that it uses an existing AC circuit on the overhead line and converts it to a DC circuit. The 2 VSC per converter station are connected in parallel to provide the requested DC current per pole. Hence, per pole 4 converters are connected to create a multi-terminal network. The latter will be extended in future by adding a cable section between Osterath and Emden (project called A-Nord) of additional 2 converters, creating a multi-terminal network with 6 converters per pole. A novel full bridge VSC technology is applied to deal with atmospheric faults on overhead lines. | |
| Design: +/- 2200 MW, 380 kV, hybrid HVDC/HVAC overhead line, bi-pole with dedicated metallic return. | |
| Results: In plan. | |
| Location: UK, Denmark [4] | Year: 2023 |
|---|---|
| Description: The Viking Link will give the UK access to the west Denmark bidding area (DK1) of Nord Pool Spot and vice versa. The total length of the link is 765 km, which makes it the longest subsea HVDC link of the world. | |
| Design: +/- 1400 MW, 525 kV, subsea cable. | |
| Results: In operation with reduced capacity (800 MW) until the associated grid is completed in Denmark. | |
| Location: Germany, Belgium [5] | Year: 2020 |
|---|---|
| Description: The ALEGrO HVDC link is a 90 km-long land connection between Germany and Belgium. The HVDC Link supports energy trading between the countries. The project was supported by the European Commission from the Trans-European Energy Networks (TEN-E) programme. | |
| Design: +/- 1000 MW, 320 kV, land cable, symmetric monopole. | |
| Results: In operation | |
| Location: Denmark, Netherlands [6] | Year: 2019 |
|---|---|
| Description: The COBRA cable consists of two parallel, approx. 300 km long, DC submarine cables linking western Denmark and the Netherlands together. The project also comprises approx. 20 km of onshore cable. The cables are connected to converter stations built in Endrup, east of Esbjerg, Denmark and Eemshaven in the Netherlands, respectively. The VSC converter stations in Endrup and Eemshaven connect the 400 kV AC grid to the DC COBRA cable. | |
| Design: The system has a power transfer capacity of 700 MW and is operated at a voltage of +/- 320 kV. The total HVDC cable route is 329 km, of which 307 km are offshore and 22 km are onshore. Two cables are installed in parallel, making the total length of cables 658 km. | |
| Results: In operation. The COBRA connection contributes to the introduction of renewable power production in that both countries are able to manage much more sustainable production, i.e. buy and sell wind power and solar power across borders whenever there is a surplus situation in either one of the countries. At the same time, the connection ensures a high level of security of electricity supply, as increasing amounts of wind energy flows into the systems “as the wind blows”. The connection is designed to meet future requirements by presenting the opportunity for future offshore wind farms in the North Sea to be connected to the COBRAcable. Furthermore, the cable can be part of a future interconnected offshore electricity grid between the countries bordering the North Sea, capable of unpinning the expansion of wind power and strengthening the European electricity transmission grids. | |
| Location: France, Spain [7] | Year: 2015 |
|---|---|
| Description: The INELFE underground electrical interconnection is a joint project of RTE and REE that came into operation in 2015. It aims to increase the electricity capacity between France and Spain from 1,400 MW to 2,800 MW using VSC, thus enhancing commercial exchange. In addition, the power quality in the area was significantly improved. | |
| Design: The system has a total power transfer capacity of 2000 MW and is operated at a voltage of ±320kV. The entire 64.5 km link is undergrounded, which includes a 8.5 km tunnel through the Pyrenees. | |
| Results: Increased transmission capacity between Spain and France with flexible reactive power control, black-start capabilities and local power quality management. Prior to the construction of the new connection, surplus wind production generated in Spain could not be exported to the rest of the continent due to the limitations of cross-border capacity. The new infrastructure facilitates the incorporation of clean weather-dependent energy without putting the supply at risk. | |
| Location: Norway, Denmark | Year: 2014 |
|---|---|
| Description: The Skagerrak (SK) HVDC transmission system has been in operation since the 1970s and now comprises four HVDC links, which together provide a total of 1700 MW transmission capacity. | |
| Design: Of the 4 HVDC links, one is equipped with a VSC, the Skagerrak 4 link, for which the capacity of the VSC is 700 MW, with a rated voltage of 500 kV. | |
| Results: Successful demonstration of the black-start capabilities of the system as well as the energising of isolated AC grids. | |
Technology Readiness Level The TRL has been assigned to reflect the European state of the art for TSOs, following the guidelines available here.
- TRL 9 for VSC (symmetric Monopole no grid forming) at voltage rating 320 kV.
- TRL 6 for VSC (symmetric Monopole with grid forming) at voltage rating 320 kV.
- TRL 6-7 for VSC (Bipole with grid forming half and full bridge) at voltage rating 380 kV and 525 kV.
- TRL 5-6 for VSC MT single-vendor at voltage rating 380 kV and 525 kV.
- TRL 4-5 for VSC MT multi-vendor at voltage rating 380 kV and 525 kV.
- TRL 3-4 for VSC DC/DC converter at undefined voltage rating.
References and further reading
50Hertz and TenneT, press release. “50Hertz and TenneT to jointly bring wind power from the North Sea into the extra-high voltage grid for the first time.”
Amprion. “Project description: Ultranet.”
Energinet. “Cable to Great Britain.”
Elia Group, Press 11hermos. “Elia and Amprion launch ALEGrO, the first interconnector between Belgium and Germany.”
TenneT. “COBRAcable.”
Inelfe. “The Spain-France Underground Electrical Interconnection.”