HVDC - What and Why?

HVDC stands for high voltage direct current, a well-proven technology used to transmit electricity over long distances by overhead transmission lines or submarine cables

HVDC stands for high voltage direct current, a well-proven technology used to transmit electricity over long distances by overhead transmission lines or submarine cables. It is also used to interconnect separate power systems, where traditional alternating current (AC) connections cannot be used.

In an HVDC system, electric power is taken from one point in a three-phase AC network, converted to DC in a converter station, transmitted to the receiving point by an overhead line or cable and then converted back to AC in another converter station and injected into the receiving AC network. Typically, an HVDC transmission has a rated power of more than 100 MW and many are in the 1,000 – 3,000 MW range.

With an HVDC system, the power flow can be controlled rapidly and accurately in terms of both power level and direction. This possibility is often used to improve the performance and efficiency of the connected AC networks. ABB pioneered HVDC technology and is the world leader in the HVDC field. In total, about 130,000 MW HVDC transmission capacity is installed in more than 140 projects worldwide. ABB has delivered more than 75 of these projects with a total transmission capacity of almost 60,000 MW. The largest bipole delivered by ABB to date is 6,400 MW. There are three different categories of HVDC transmission projects:

- Point-to-point transmission

- Back-to-back stations

- Multi-terminal systems.

The first commercial HVDC scheme, based on mercury arc valves was commissioned in 1954. This was a link between the Swedish mainland and the island of Gotland in the Baltic sea. The power rating was 20 MW and the transmission voltage 100 kV There was a significant improvement in HVDC technology in 1970 when thyristor valves were introduced in place of the mercury arc valves. This reduced the size and complexity of HVDC converter stations substantially. The use of microcomputer control equipment in today’s projects has also contributed to HVDC’s current success as a powerful alternative to AC power transmission.

In 1995 ABB announced a new concept for HVDC converter stations, HVDC with Capacitor Commutated Converters (CCC), which further improves the performance of HVDC transmissions. And in 1997 a completely new converter and DC cable technology called HVDC Light was introduced.

WHY HVDC?

The reasons for selecting HVDC instead of AC for a specific project are often numerous and complex. The most common arguments in its favour are:

1. Lower investment cost

2. Long distance water crossing

3. Lower losses

4. Asynchronous interconnections

5. Controllability

6. Limited short-circuit currents

7. Environment.

In general, the different reasons for using HVDC fall into two main groups, namely: - HVDC is necessary or desirable from the technical point of view (that is controllability). - HVDC results in a lower total investment (including lower losses) and/or is environmentally superior.

In many cases, projects are justified by a combination of benefits from the two groups. Environmental aspects are also increasingly important and HVDC has the advantage of a lower environmental impact than AC since the transmission lines are much smaller and need less space for the same power capacity. One of the most important differences between HVDC and AC is the possibility to accurately control the active power transmitted on a HVDC line. This is in contrast to AC lines, where the power flow cannot be controlled in the same direct way. The controllability of the HVDC power is often used to improve the operating conditions of the AC networks where the converter stations are located.

Another important property of an HVDC transmission is that it allows the interconnection of asynchronous networks.

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