Tibet is often referred to as the "Roof of the World" and not without reason. Standing more than three miles above sea level, it is surrounded by imposing mountain ranges that harbor some of the world's highest summits, with several of them making the top ten list. Mount Everest, located on the border with Nepal, is at 8,848 meters the highest mountain on earth. Extreme cold, permafrost and high ultraviolet levels only add to the challenges of the location.
The Qinghai-Tibet HVDC (high-voltage direct current) project is built by the State Grid Corporation of China. The ±400 kV (kilovolt) portion of the line starts from Geermu, Qinghai Province, and ends in Lhasa, Tibet, with a total length of 1038 kilometers. What makes this portion of the project especially noteworthy is that it is among the world's highest overhead transmission lines and also the longest HVDC line ever built at such altitudes. For example, it traverses terrain that averages 4,500 meters (nearly 15,000 ft) with the highest point reached at the dramatic 5300-meter mountain pass at Tanggula. It is therefore critical that the high-voltage equipment, in particular its external insulation, meets the demands imposed by such altitudes and climatic conditions.
ABB has supplied more than 900 high-voltage capacitor units for the converter station in Lhasa, Tibet at an altitude of above 4,000 meters. These units were delivered from the company's manufacturing facility in Xi'an, central China, in a record delivery time to ensure installation before the cold winter season set in. Moreover, the modular design of the units helped reduce installation time, and cuts maintenance costs.
Enhancing power quality
AC (alternating current) power typically has two components - active power that contributes to useful work at the end user load and reactive power which occurs when voltage and current are not in phase. While reactive power is required by electrical machines to create a magnetic field it does not contribute to any useful work. In fact excessive reactive power in a network reduces the quality of power. The effects include taking away capacity to channel active power, voltage drop across the line which can lead to equipment failure and increased transmission and distribution losses.
In a HVDC transmission link a number of components such as the HVDC converters and transformers draw reactive power from the network leading to poor power quality. ABB supplied filter capacitor banks and shunt capacitor banks at a voltage a level of 220 kV for the Lhasa converter station. The capacitors installed close to the converters and transformers generate reactive power locally and as less reactive power is drawn from the grid power quality improves. This leads to higher supply network reliability and availability as well as energy efficiency.
Robust and tough equipment for extreme conditions
To ensure optimal performance in the extreme Tibetan conditions, all capacitors units have integrated bushings. The vacuum impregnation and painting processes were also specially developed for this project. All units went through the standard ABB quality tests, and this product has already demonstrated high quality and reliability in all climates from the Arctic cold to the tropical heat.