Alternative HVDC-systems

Asgeir E. Nilsen

My own background

I graduated from NTNU, Faculty of Electrical Engineering and Telecommunications, Electrical Engineering Section, in December 1996.

After serving in the military for one year, I started this study in January 1998. Thus, my work with the thesis is at a very early stadium.

Initiation and funding

The subject of research was suggested by associate professor Arne T. Holen and Dr.ing Thor Henriksen at Sintef Energy Research. The study is funded by a three year scholarship from the Research Council of Noway as a part of the program: "Basic Energy Research", and one year is founded by NTNU. The study is scheduled to be finished by December 2001.

Introduction

High Voltage Direct Current (HVDC) is today a well proven technology employed for power transmission all over the world. In total about 50.000 MW HVDC transmission capacity is installed in some 60 projects.

The HVDC technology is used to transmit electricity over long distances by overhead transmission lines or submarine cables. The reason why AC-transmission is not preferred, is of both economic and technical nature. For earth- and sea-cables HVDC-transmission is preferred when the length of the cables exceeds round 50 km, while for overhead lines the limit is 600 - 800 km.

HVDC is also used to interconnect separate power systems, where traditional alternating current (AC) connections can not be used.

Present HVDC-transmission technology was developed during a period from the end of the twenties and resulted in the first commercial transmission, Gotland in 1954. 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. There has been development and refinements of HVDC during the years such as lowering of losses, much more advanced control and protection, lower harmonics, lower audible sound etc., but basically it is still the same technology as in the first Gotland scheme. The most important achievements is the introduction of thyristor valves in the beginning of the 1970's. In the last decades we have seen an enormous development regarding semiconductor devices: Improvement in qualities like voltage and current capabilities, speed etc., and decreasing production costs. Thus the tendency is improvement compared to AC-solutions.

There are three different cathegories of HVDC transmissions:

1. Two terminal transmissions

2. Back-to-back stations

3. Multi-terminal systems

Most of the commissioned HVDC-projects are of the first category, but here are several back-to-back stations in operation in the world. In these installations both the rectifier and the inverter are located in the same station and are normally used in order to create an asynchronous interconnection between two AC networks, which could have the same or different frequencies.

A Multi-terminal HVDC transmission is an HVDC system with more than two converter stations. Such a system is more complex than an ordinary point-to-point transmission. In particular, the control system is more collaborate and the telecommunication requirements between the stations become larger. There are only two large scale multi-terminal HVDC system in operation in the world today. One of them is the 2000 MW Hydro Quebec - New England transmission built by ABB between 1987 and 1992. The operating experience of this transmission is very good and has proved that from a technical point of view there are no problems to connect several converter stations to the same HVDC transmission line.

The present technology has some inherent weaknesses, which to some extent limit the use of HVDC such as the need for rotating machines in the receiving network. It has not been feasible to use the present HVDC technology for small power transmission or distribution. As an answer to this problem, ABB has come up with the concept HVDC Light. The concept is an alternative to conventional a.c. transmission or local generation in many situation. The transmission technology is based on voltage source converters (VSC) with series connection of modern semiconductor device, the IGBT. Thus there is no need for rotating machines in the receiving network. This technology extends the economical power range for d.c. transmission downwards to just a few MW. Possible applications include the feeding of distant loads and the connection of distant generation plants.