Voltage Source Converters (VSC) are a self-commutated converters to connect HVAC and HVDC systems using devices suitable for high power electronic applications, such as Insulated Gate Bipolar Transistors (IGBTs). VSCs are capable of self-commutation being able to generate 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 DC voltage for their building blocks, such as the 2-level or 3-level converter as well as the so-called “Modules” in a Modular Multi-Level Converter (MMC). The change of power flow direction is achieved by reversing the direction of the current. Therewith, VSCs are easier integrated in multi-terminal DC systems. VSC based HVDC systems offer a faster active power flow control with respect to the more mature CSC-HVDC, while also ensuring flexible and extended reactive power controllability at the two converter terminals.
VSC can be classified with respect to the converter technology types used, which have evolved over time:
- Two-Level VSC – earliest technology used
- 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)
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.
In offshore HVDC grids, MMC is becoming the preferred power electronic converter for converting between AC and DC since presenting several beneﬁts: (i) ability to reverse the power ﬂow without reversing the polarity of the DC voltages by DC current reversal; (ii) modularity and scalability features make it advantageous compared to other VSC topologies; (iii) its inherent capability of storing energy internally in the converter. This can beneﬁt the system in which it is connected and enables drastic reduction of operating losses of the converter stations, by avoiding the need for high frequency switching of the semi-conductor devices.
Components & enablers
- DC/DC converter
- Transformer (Optional Tapping in series/parallel)
- DC-link capacitors
- Passive high-pass filters
- Phase reactors
- DC cables
- DC breaker (Optional)
Advantages & field of application
The VSC technology provides several technical advantages, such as resilience to commutation failure, ancillary services and reactive power control (and consequently voltage control).
In comparison to the LCC technology, VSC has a shorter history , less operating experience, so far lower maximum voltages and power transfer capability. Although power and voltage ratings have risen dramatically in recent years, the overload capability of VSC technology remains low, limited by the capability of the IGBT devices.
VSC-HVDC is best suited to interconnect remote generation facilities (e.g. offshore wind far away from shore, typically >80 km) to the main power grid; perform a black start to start-up connected offshore wind farms or to re-energize network sections, to contribute to power system and voltages stability thanks to its fast reactive power flow and voltage control at its terminals.
It is expected to support Multi-Terminal HVDC (MTDC) applications forming the backbone of potential offshore grids (like the one in the North Sea) implementation.
To date most VSC applications use submarine or underground cables. The first multi-terminal systems are in operation (in China) and others are under construction (in Europe) and the number of commercial applications is rapidly growing.
The technology uses IGBT devices rated up to 6.5 kV and 2000 A. The largest scheme in service is the 2 x 1000 MW ±320 kV INELFE project (France – Spain) and the highest voltage is on the 700 MW Skagerrak Pole 4 project at 500 kV (Norway – Denmark).
The under construction North Sea Link (NSL) project between the UK and Norway will operate as a bi-pole at 1400 MW and ±525 kV. Further development involves a “full bridge” solution for more flexible operation of the VSC technology for overhead line links. The current “half bridge” solution cannot block DC fault currents and requires to be combined to a DC circuit breaker in series with the DC line.
Technology Readiness Level
Monopole and Bipole VSC:
- TRL 9-Ready for full scale deployment
- TRL 4-Technology validated in lab
VSC half bridge:
- TRL 9-Ready for full scale deployment (INELFE Project)
VSC full bridge:
- TRL 8-Incorporated in commercial design (ULTRANET project)
Research & Development
Current fields of research: Virtual impedance control, Grid-Current-Feedback Damping, advanced vector control, availability; improved efficiency of VSC converter based on new switching typologies; development of transformerless VSC converters; development of novel multilevel switching typologies (architecture and switching modes) to enhance transmission capacity; DC circuit breakers development for selective fault clearance; development of multiterminal VSC transmission.
Innovation priority: IGBTs, control algorithms, filters, overload capability.
Other: Future developments foresee an extended domain of use of HVDC technologies, such as far offshore and ultra-deep submarine connections (2000m+), ultra-high voltage and higher transmission distance, and combining HVDC and HVAC networks and related impact on reliability.
Best practice performance
Maximum capacity: 2000 MW per substation (bipole)
Voltage rating: 525 kV
Current rating: 2 kA
Longest distance: max. length of cable: 623 Km (NORD Link); expected to be more than 700 km in 2021 (North Sea Link)
Best practice application
Baixas (France) - Santa Llogaia (Spain)
The INELFE underground electrical interconnection is a joint project of RTE and REE came into operation in 2015. It aims to increase electricity capacity between France and Spain from 1,400MW to 2,800 MW using VSC  thus enhancing commercial exchange. Also the power quality in the area was significantly improved.
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.
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 dependant energy without putting the supply at risk.
Kristiansand (Norway) - Tjele (Denmark)
The Skagerrak (SK) HVDC transmission system is in operation since the 1970s and by now comprises four HVDC links which together provide a total of 1700 MW transmission capacity.
Out 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.
Successful demonstration of the black-start capabilities of the system as well as the energising of isolated AC grids.
Denmark (Endrup) – The Netherlands (Eemshaven)
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.
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.
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 more and more 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.
‘Aachen Liege Electricity Grid Overlay’ project should provide a transmission power capacity of 1000 MW. It will use an HVDC transmission technology as an underground cable: the AC Grid together with the DC transmission line will increase cross-border electricity flows.
The HVDC link requires a converter station positioned at each end of the line providing the switch from AC to DC, for which a VSC is implemented. The link will be positioned between the already existing 380 kV stations in Oberzier Germany and Lixhe Belgium and it will extend over a length of 90 km.
Increased security of supply and increased flexibility in cross-border load flow exchange.
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