Nearly three years after it proved 1100 kV DC in principle, ABB has won orders worth over $300 million to supply its breakthrough technologies for the world’s first 1100 kV ultra-high-voltage direct current transmission link.

State Grid Corporation of China’s Changji- Guquan UHVDC link will transmit power from the Xinjiang region in the northwest, to Anhui province in eastern China and will set a new world record in terms of voltage level, transmission capacity and distance. It will be capable of transporting 12 000 MWe, which represents a 50 % increase in transmission capacity compared to the 800 kV DC links currently in operation.

This will also help extend the current notion of what represents long distance transmission from around 2000 km to over 3000 km. The link will play a key role in integrating remote renewables on a large scale, transmitting power over greater distances and facilitating a more interconnected national grid.

The order

ABB’s scope of supply include advanced converter transformers and associated components such as bushings and tap changers. ABB will also supply the HVDC converter valves, DC circuit breakers, wall bushings and capacitors as well as provide system design support. The transformers will be among the most powerful in the world, meeting, says ABB, the most stringent performance, reliability and safety standards currently in use.

Each transformer weighs 800 tons, and measures 32 metres in length. ABB’s transformer manufacturing and testing facility in Chongqing, as well as the local HVDC engineering and technology centre, will be actively involved in the delivery and execution of the project.

“China has major load centres in its eastern region, while a significant amount of its energy resources are in the west and northwest. The expansive geography and increased demand over the last decade have prompted the build-up of UHV capacity to transmit larger amounts of power over greater distances with minimum losses,” said Claudio Facchin, president of ABB’s Power Grids division. “Ultrahigh voltage technologies are a key focus area of our Next Level strategy, and our technology advancements in this area are making it possible to increase power transmission capacity and distance to an unprecedented level with minimal transmission losses.”

UHVDC transmission is the next step up from HVDC, a technology first introduced by ABB over 60 years ago. The world’s first 800 kV DC link to go into commercial operation was constructed in 2010, also by ABB and also in China for SGCC. This was the Xiangjiaba-Shanghai project. To date ABB has built or supplied over 70 such projects around the world with a combined transmission capacity of around 60 GW.

1100 kV component development

A few years later in July 2012 ABB announced that it had successfully tested the prototype of the world’s most powerful converter transformer, rated at a record 1100 kV. This was the starting point. By December 2012 ABB could announce that it was actively working on developing all the components required for a 1.1 million volt power transmission system, which represented ‘the biggest capacity and efficiency leap in over two decades’. Such a system could deliver in the order of 10 000 MW of power across several thousands of km with reduced, relatively low transmission losses.

Because of the extreme reliability demands placed on the equipment by these kinds of voltages,it is not advisable to introduce untested materials at this stage. More typically the industry introduces new materials at lower power ratings and later extends their use up the voltage scale. The R&D engineers have therefore been working with proven materials and methods, and focused on design of the transformer parts to increase performance.

ABB decided on an approach where as many of the subcomponents as possible were subject to high voltage testing early on in the R&D process. Since one of the longest lead items in the R&D process involves bushings, ABB chose to work with scale experiments conducted on 800 kV DC test voltage levels as a way of speeding up the R&D process. By applying this method, some of the subcomponents and their design principles had already been through the testing phase before entering final prototype testing.

Other components that became available during this process included ultrahigh- voltage by-pass switches, surge arresters, coupling capacitors and capacitor filters. All these components have been designed and tested for a switching impulse withstand level of >2.1 million volts, and for alightningimpulsewithstandlevelof>2.5 MV, the highest ever for products within this category. The protection levels of the arresters have been optimised to give a sufficient operational margin with respect to these levels. This has been made possible through the careful design of equipment to ensure its thermal performance, and the use of high-quality ZnO varistors.

Increasing the ground to earth clearance between live parts to take advantage of the insulating properties of air has meant that much of this equipment stands high above the ground, with capacitor filters at 26 metre, bypass switches at 14 to 16 m, surge arresters at 16 m and coupling capacitors at 18 m. These physical constraints have placed further requirements on the design of UHVDC products, which must be capable of withstanding seismic activities and associated mechanical load combinations.

Transformer design

Increasing the working voltage is to a large extent a matter of transformer dielectric design. It is not a matter of simply increasing clearances because the electric field stress governing insulation performance is non-linear in its behaviour. For this reason, all insulation design related to the increased valve side voltage of the transformers had to be reworked.

The overall structure of the transformer is similar to that used for previous voltage levels, as the application itself is similar, but the physical dimensions of the transformer and its parts grow substantially. Bushings becomes much larger as does the transformer tank , the core and windings.

The increase in physical size does itself pose a challenge since the mechanical structure holding the transformer together needs to upgraded as well. The overall effect is that the design of nearly all transformer was revisited either as a direct consequence of the increased voltage level or an indirect consequence of the increase in size. A transformer component which is especially critical is the bushing, which has increased significantly in size and the new bushings required what ABB is calling ‘a masterpiece in design’.

Kerr electro-optic method

One of the most important tools deployed in the R&D effort was the combination of Kerr electro-optic measurements with theoretical modelling and simulation. The behaviour of oil/cellulose insulation under DC-stress is very complex. The Kerr measurement allows the direct measurement of electric field stress in a dielectric liquid by measuring its optical properties using lasers. This has enabled ABB to measure the electric field distribution over time, and to create simulation models and engineering tools that then can be applied to the design of full- scale insulation systems.