Line-commutated converters (LCCs) are the conventional, mature and well-established technology used to convert electric power from AC to DC or vice versa. The term line-commutated indicates that the conversion process relies on a stable line voltage, with clear zero-crossings of the AC system to which the converter is connected in order to have a flow commutation from one switching element to another.
In a current source converter (CSC), the DC current is kept constant with a small ripple using a large inductor. In practice, for most applications an LCC equals an CSC. The direction of power flow through a CSC is determined by the polarity of the DC voltage, whereas the direction of current flow remains the same. A LCC requires connection to a strong grid with sufficient short circuit power to avoid commutation faults and a synchronous voltage source in order to operate. In comparison to a VSC, it still allows for much higher power conversion capacities with lower losses, but would require converter stations with a larger ground footprint than the equivalent capacity VSC sites.
LCCs are differentiated in two technology types: Mercury-arc valves based LCC and Thyristor based LCC.
Mercury-arc valves based LCCs is a technology that was used until the 1970s. It is a type of cold cathode gas-filled tube made from a pool of liquid mercury. It was therefore self-restoring and long-lasting, capable of carrying high currents up to 1.8 kA at 450 kV but with high maintenance time and serious environ-mental hazard risks. The last operating mercury arc system was shut down in 2012.
Thyristors-based LCCs were first used in HVDC systems in the early 1970s while widely implemented since the 1960s, for both OHL and cable applications. A thyristor is a solid-state semiconductor device similar to the diode, but with an extra control terminal that is used to switch the device on at a defined instant. Additional passive components (e.g. grading capacitors and resistors), need to be connected in parallel with each thyristor to ensure a uniform share of the voltage between the thyristors across the valve. Initial designs used six pulse converters. Most modern designs apply twelve pulse converters.
Control and Telecommunications
Thyristors based LCCs are widely used for bulk power point-to-point (with overhead lines and or subsea cables), for back-to-back links between different synchronous networks and as an embedded link within a synchronous network.
Applications for CSC will most likely be outside Europe. In Europe, VSCs are expected to be the predominant technology for most applications.
This thyristor-based technology is characterised by its short time overload capability and its high efficiency, with a total operating power loss in a converter station of typically 0.7 – 0.8 % of the scheme rating.
TRL 9 – System ready for full scale deployment
Current fields of research: Virtual impedance control; Reduction of common-mode voltage, AC-side compensation; control algorithms in unbalanced grid voltages; natural sampling based space vector modulation; development of new types of semiconductors (SiC, diamond, etc.) which will allow the development of new thyristors; eco-conception of CSC (CO2 impact, noise, lifetime).
Variable for improvement: In the context of the Ultra-High HVDC (UHHVDC), development of air insulation of water cooled thyristor valves, transformer oil / paper insulation, development of modular design of UHHVDC converts (size and weight) to face transport constraints; shunt active power filter; operation in unbalanced grid conditions; synchronisation techniques; power density of converter; lower harmonics magnitudes.
Following many years of development, thyristor devices are able to operate at blocking voltages up to 8.5 kV and switch DC currents of up to 6.25 kA, which allowed LCC schemes to be installed up to 10 GW and ± 800 kV (7.2 GW already in operation in China), with a current rating amounting 5 kA.
In Europe, the largest LCC scheme is rated at 2,200 MW at ± 600 kV (Western Link in the UK, commissioned in 2018).
The maximum length for a line is 2,000 km, whereas for a cable this maximum length is 600 km.
Description
The HVDC submarine link came into operation in 2008 to connect the Norwegian and the Dutch grid.
Design
The HVDC system is bipolar with a total cable length of 580 km and is operated at ±450 kV.
Results
Transmission of 700 MW of active power across borders, facilitating cross border exchange.
Description
The Nemo Link consists of subsea and underground cables connected to a converter station and an electricity substation in each country, allowing electricity flows in either direction between the two countries for improved grid reliability, access to sustainable generation and electricity trading.
Design
The HVDC Nemo Link has a total cable length of 140 km (out of which 10 km underground and 130 km submarine) and is operated at ±400 kV.
Results
It started commercial operations on 31st January 2019 and is able to transmit 1000 MW of active power. ‘By enabling the market to react immediately to rapid changes in supply and demand, Nemo helps to better balance an energy system that is more reliant on intermittent wind and solar energy’, said Jon Butterworth, President of National Grid Ventures.
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