Innovations - UHV DC
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Innovations - UHV DC
Throughout the world, the demand for power continues to grow at a scale and speed never imagined in the past. For various reasons we are also witnessing a strong push for renewable energy sources (RES) with power generation becoming increasingly distributed and a growing number of generation facilities located far away from load centers. At the same time, demanding economic objectives as well as obligations to reduce greenhouse gases have to be met.
To meet all these demands one-to-one, Siemens has taken great efforts to overcome the limitations presented up to now in the technology available for high voltage direct current (HVDC) power transmission.
Thanks to R&D efforts, Siemens is able to produce the entire range of components required for 800-kV DC power transmission and supply complete Ultra High Voltage Direct Current (UHV DC) systems from a single source.
The first 800 kV UHV DC system, ordered by the China Southern Power Grid Co. in Guangzhou, took up commercial operation of the first pole in December 2009 and commenced with commercial service by mid-2010. It allows the country to tap more hydropower instead of adding new coal plants. The CO2 emissions offset by this amount to an impressive 33 million tons at the Yunnan-Guangdong project alone.
The Ultra High Voltage Direct Current (UHV DC) converter station is designed based on the proven technology and the project experience in the ±500 kV, 3000 MW HVDC systems.
However, if the classical configuration of one 12-pulse bridge per pole in common bipolar HVDC systems would be employed in UHV DC systems with very high ratings and transportation limitations, the increased voltage and power stress would bring unconquerable challenges to design of key equipment, such as, converter transformer.
The solution for such a UHV DC station design is a series connection of two 12-pulse bridges with 400 kV rated voltage each to reach the rated dc voltage of 800 kV per pole.
An example of the station layout and the converter arrangement with two 400 kV systems in series in each pole for n-1 redundancy is given in the following figures.
A converter station links the DC transmission line at each end to the AC grids. It consists of a number of components which have reached a high degree of maturity. However, for UHV DC applications, innovative solutions have been implemented to fully meet the extended requirements for ultra high voltage bulk power transmission.
An UHV DC converter performs the AC to DC conversion and vice versa. It consists of a number of thyristor modules which are connected in twelve-pulse groups. In the figures, the arrangement of thyristor valve towers and a view of the new 6-inch thyristor (8 kV, 4.5 kA) is shown.
Excerpts of the impressive tests of the DC valve support structure are also highlighted in the figures.
Valve Hall Configuration – for 800 kV HVDC
A major benefit of this solution will be a smaller size of the converter transformers, when transportation restrictions exist. Furthermore, it increases the redundancy of the transmission: each of the four converters of plus and minus pole can be bypassed and the assigned DC line will be operated at a 400 kV reduced voltage level.
Even with two thyristor valve groups per pole the overall arrangement and single-line diagram (SLD) will not be significantly different from other bipolar schemes with only one valve group per pole. Furthermore also UHV DC schemes with ±800 kV DC can be arranged with only one valve group per pole.
Dielectric Testing of the Valve-Support Structure and the new 6’’ Thyristor
Assembly of the DC Valve Tower
As far as DC converter valves and associated equipment are concerned, the design of creepage distance is not a problem as such equipment will be installed indoors in the converter valve hall such as in the existing projects. The valve hall provides a controlled environment. For UHV DC equipment inside the valve hall the same specific creepage distance can be selected as for the existing 500 kV DC equipment.
Converter transformers are one of the very important components for UHV DC applications. It is well understood that the existing technology and know-how used in converter transformers can manage higher DC voltages, yet there are critical areas which needed careful consideration and intensive testing in order to keep electrical stress at a safe level. Above all, the windings and the internal transformer part of bushings on the valve side of the converter transformers with the barrier systems and cleats and leads required very careful attention.
“Snapshots” from DC Transformer Testing
The converter transformers connect each pole of the UHV DC converter to the AC grid in an economical way. With UHV DC, transformers exhibit very impressive dimensions, especially their bushings. Local transportation restrictions and converter configuration determine their type and size; e.g. for the world’s first UHV DC project Yunnan-Guangdong in China, a total of 48 transformers (plus 8 spare units) has been provided.
The UHV DC voltage divider from Siemens provides the DC voltage measuring signal to the UHV DC control system. It is based on technology used in high voltage test fields all over the world. The high accuracy of the divider is linear for almost the entire operating voltage and large ambient temperature range. The challenge of developing an UHV DC voltage divider was coping with the high internal and external dielectric stress which is reflected in the dimension of the end-to-end composite insulator and the huge corona shield.
The UHV DC bypass switch provides an option for more flexibility of the whole transmission scheme. For example, for bipolar 800 kV DC transmission, two 400 kV converters at each pole can be operated in series, and each one can be bypassed without interruption of power whenever required.
HVDC is operated as a current-sourced converter. A current-sourced converter requires a reactor as a smoothing element for the UHV DC current. It also helps reduce harmonics, together with the UHV DC filters. For UHV DC, an air-core reactor design is applied which is insulated to the ground by large composite post insulators.
The UHV DC disconnectors must provide safe isolation of all the equipment in case of system shut-down, including maintenance. The mechanical layout shows these requirements in an impressive way.
Insulation level coordination of the protection levels of the UHV DC system determines the reliability of the whole transmission scheme. Surge arresters are the key elements for system security, e.g. in case of line faults caused by lightning strokes, which is a typical and unavoidable natural phenomenon.
The converter station includes additional UHV DC equipment as follows:
UHV DC wall bushing:
similarly impressive as the transformer bushings on account of its extended dimensions for the required insulation levels
Hybrid optical UHV DC measuring system:
ohmic shunt for measuring UHV DC current on high-voltage potential transmitted to ground via fiber optics
UHV DC PLC capacitor:
is used to prevent high frequency noise from entering the DC overhead line and provides a connection path for the DC line fault locator signal
UHV DC post insulators:
different technologies on the market all with silicone housings providing hydrophobic behavior on the surface of the insulators which greatly reduces the risk of flashovers due to pollution.