HVDC > HVDC Light > Differences from HVDC Classic
HVDC Light main circuit | 
Classical HVDC main circuit |
Power range
Classical
HVDC is most cost effective in the high power range, above approximately some 250 MW. HVDC Light, on the other hand, comes in unit sizes ranging from a few tens of MW. In the upper range, the technology now reaches 1,200 MW and ±320 kV.
DC transmission circuit
The DC transmission circuit for classical HVDC can be an overhead line or a DC cable. The cables are normally mass-impregnated (MIND) cables suitable for HVDC with a copper conductor and insulation made of oil impregnated paper. The vast majority of classical DC cables are submarine cables.
The DC transmission circuit for HVDC Light is made by extruded polymer cables both for land transmissions (underground) and across water (submarine). HVDC Light is by nature bipolar. The DC circuit is not connected to ground. Therefore two cables are needed. HVDC Light can also be build with over head lines - exemple
Caprivi Link Interconnector
Modular
HVDC Light is based on a modular concept with a number of standardized sizes for the converter stations. Most of the equipment is installed in enclosures at the factory. Conventional HVDC is always tailor made to suit a specific application.
Converter station circuits (see image)
HVDC Light converter stations are Voltage Source Converters (VSCs) employing state of the art turn on/turn off IGBT power semiconductors. (IGBT = Integrated Gate Bipolar Transistors.)
Therefore the circuit is quite different from conventional HVDC:
| Function | Classical HVDC | HVDC Light |
| Converter valves | Thyristor | IGBT |
 | | |
Connection
valve - AC grid | Converter transformer | Series reactor
(+ transformer) |
| | |
| Filtering and reactive compensation | 50% in filters and shunt capacitors | Only small filter |
| | |
| DC current smoothing | Smoothing reactor + DC filter | DC capacitor |
| | |
| Telecom between converter station controls | Needed | Not needed |
| | |
Independence of AC network
HVDC Light does not rely on the AC network’s ability to keep the voltage and frequency stable. Unlike conventional HVDC, the short circuit capacity is not important. HVDC Light can feed load into a passive network (i.e. lacking synchronous machines)!
Active and reactive power control
Classic HVDC terminals can provide limited control of reactive power by means of switching of filters and shunt banks and to some level by firing angle control. But this control requires additional equipment and therefore extra cost.
The HVDC Light control makes it possible to create any phase angle or amplitude, which can be done almost instantly. This offers the possibility to control both active and reactive power independently. In fact the same converter can be used as a SVC and it is then called
SVC Light. It is extremely effective to eliminate flicker e.g. from arc furnaces.
Low power operation and power reversal
Unlike conventional HVDC converters, that normally has a 5% minimum current, the HVDC Light converter can operate at very low power, and even at zero power. The active and reactive power are controlled independently, and at zero active power the full range of reactive power can be utilized.
Active power transfer can be quickly reversed by HVDC Light without any change of control mode, and without any filter switching or converter blocking. The power reversal is obtained by changing the direction of the DC current and not by changing the DC voltage as for conventional HVDC.