HVDC Transmission Systems: An Overview

What is HVDC? HVDC stands for High Voltage Direct Current and is today a well-proven technology employed for power transmission all over the world. In total about 70,000 MW HVDC transmission capacity is installed in more than 90 projects. The development of the HVDC technology started in the late 1920s, and only after some 25 years of extensive development and pioneering work the first commercially operating scheme 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. At that time mercury arc valves were used for the conversion between AC and DC, and the control equipment was using vacuum tubes. A significant improvement of the HVDC Technology came around 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 Classic HVDC Transmission Using HVDC to interconnect two points in a power grid, in many cases is the best economic alternative, and furthermore it has excellent environmental benefits. The HVDC technology (High Voltage Direct Current) is 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 can not be used. In a high voltage direct current (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.

Why HVDC?: It’s Advantages The vast majority of electric power transmissions use three-phase alternating current. The reasons behind a choice of HVDC instead of AC to transmit power in a specific case are often numerous and complex. Each individual transmission project will display its own set of reasons justifying the choice of HVDC, but the most common arguments favoring HVDC are: 1. Lower investment cost 2. Lower losses 3. Asynchronous interconnections 4. Controllability 5. Limit short circuit currents 6. Environment

In general terms the different reasons/ADVANTAGES for using HVDC can be divided in two main groups, namely: HVDC is necessary or desirable from the technical point of view (i.e. controllability). HVDC results in a lower total investment (including lower losses) and/or is environmentally superior. HVDC transmission for lower investment cost

Relative Cost of AC versus DC For equivalent transmission capacity, a DC line has lower construction costs than an AC line: A double HVAC three-phase circuit with 6 conductors is needed to get the reliability of a two-pole DC link. DC requires less insulation ceteris paribus. For the same conductor, DC losses are less, so other costs, and generally final losses too, can be reduced. An optimized DC link has smaller towers than an optimized AC link of equal capacity.

Typical tower structures and rights-of-way for alternative transmission systems of 2,000 MW capacity.

AC versus DC (continued) The cost advantage of HVDC increases with the length, but decreases with the capacity, of a link. For both AC and DC, design characteristics trade-off fixed and variable costs, but losses are lower on the optimized DC link. The time profile of use of the link affects the cost of losses, since the MC of electricity fluctuates. Interest rates also affect the trade-off between capital and operating costs.

HVDC technology. The conceptual design of the classic HVDC converter stations of today dates back from the mid 1970's, when thyristor valves were taking over in place of the mercury arc valves. But there has been a dramatic development in the performance of HVDC equipment and systems.

TYPES OF HVDC SYSTEMS Three types of dc links are considered in HVDC applications. Homopolar Link

Applications of HVDC HVDC is particularly suited to undersea transmission, where the losses from AC are large. First commercial HVDC link (Gotland 1 Sweden, in 1954) was an undersea one. Back-to-back converters are used to connect two AC systems with different frequencies – as in Japan – or two regions where AC is not synchronized – as in the US. HVDC links can stabilize AC system frequencies and voltages, and help with unplanned outages. A DC link is asynchronous, and the conversion stations include frequency control functions. Changing DC power flow rapidly and independently of AC flows can help control reactive power. HVDC links designed to carry a maximum load cannot be overloaded by outage of parallel AC lines.

Renewable Energy & HVDC HVDC seems particularly suited to many renewable energy sources: Sources of supply (hydro, geothermal, wind, tidal) are often distant from demand centers. Wind turbines operating at variable speed generate power at different frequencies, requiring conversions to and from DC. Large hydro projects, for example, also often supply multiple transmission systems.

New Technologies Needed? For transfers of 5,000 MW over 4,000 km, the optimum voltage rises to 1,000–1,100 kV. Technological developments in converter stations would be required to handle these voltages. Lower line losses would reduce the optimum voltage. However, environmentalist opposition and unstable international relations may be the biggest obstacle to such grandiose schemes.

HVDC links in India

CONCLUSION: HVDC transmission system is suitable for transmission of power over long range by overhead line and submarine cable with minimum cost ,good performance and reliability.

REFERENCES 1..HVDC Power Transmission Systems - Technology and System Interactions, K.R. Padiyar, John Wiley & Sons, 1990, ISBN 0-470-21706-5. 2..HVDC and FACTS Controllers Applications of Static Converters in Power Systems Vijay K. Sood 3…High Voltage Direct Current Transmission, 2nd Edition, The Institution of Electrical Engineers, J. Arrillaga, 1998, ISBN 0-85296-941-4.