ABB has successfully type-tested the world's most powerful direct current (DC) transformer bushing developed for the ultra-high voltage direct current (UHVDC) transmission link project. This is an important milestone in the development of the longest, most powerful and highest capacity power transmission links in the world, being driven by China.
The dynamic short circuit test is the most difficult test a transformer can be put through, where it is subject to higher fault levels than it would face in the power network. Passing this test proves the transformer's robustness to withstand rigours in the field, validating design excellence and flawless production.
Bushings are a key component of the UHVDC converter transformers that will play an important role in enabling unprecedented amounts of power to be transmitted over ever-longer distances with minimal electrical losses.
The first application for the 1,100 kv DC bushing will be for the Changji-Guquan UHVDC transmission link in China, which will transmit 12,000 megawatts of hydro-generated electricity at 1.1 million volts (1,100 kV DC) over a distance of 3,000 kilometers. This is a 50 percent increase on the previous record - also set by ABB - of 8,000 MW at 800 kV DC over 2,000 km for the Xiangjiaba-Shanghai UHVDC link, also in China, which has been in operation since 2010.
Overcoming the challenges
Rising far above the ground, and probably the most noticeable feature of the transformer, the bushing is an insulated device that allows an electrical conductor to pass safely through a grounded conducting barrier such as the case of the transformer. It connects the HVDC power line to the transformer, managing the electricity when it is converted from direct current to alternating current and lowering its voltage for use in the local transmission network.
There were many technical challenges to overcome when designing and manufacturing a bushing to handle 50 percent more electrical energy at a higher voltage than was previously possible. New concepts in insulation, thermal and mechanical design had to be developed to withstand the extreme performance requirements of operating safely and reliably at 1,100 kV.
The bushing is 40 percent longer, three times heavier and has a 30 percent larger diameter than the 800 kV DC bushing - dimensions which pushed even the world's most advanced production and testing facilities to the limit. During type-testing, the bushings had to withstand the potentially disabling effects of extreme operating conditions - air temperatures as low as -42°C, huge surges of power from lightning strikes, sudden rises in temperature from current, and earthquakes among others.
Speed was critical. This new transmission voltage for bushings and converter transformers had to be designed, manufactured and tested within only 18 months.
Continuous innovation
Over the past decade, ABB has continuously extended the limits of bushing and converter transformer capacity, from 6,400 MW to 10,000 MW* at 800 kV DC and now to 12,000 MW at 1,100 kV DC - all world-firsts. In addition, ABB has developed the world's first ultrahigh-voltage bushings and transformers for 1,200 kV AC and the first transformer and bushings to connect UHVDC to UHVAC. These innovations enable huge amounts of energy to be transported over immense distances in China, India, Africa and the Americas.
* In 2016, ABB won orders to supply UHVDC converter transformers and bushings for two UHVDC transmission links in China that will deliver 10,000 MW of power at 800 kV DC. Both links are expected to start operating later this year.
