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Tdworld 13113 Hgue Titelbild Xl

On-Load Tap Changers Ensure Constant Voltage in China's UHVDC Network

April 26, 2018
Changji to Guquan 1100 kV UHVDC project is largest UHVDC project in world.

The 1100 kV UHVDC project from Changji to Guquan is the largest UHVDC project in the world. As the backbone of the Chinese electricity supply system, the ultra-high-voltage network will feature a transmission capacity of 12 GW and will transport electricity over a record-breaking distance of 3,284 kilometers from western and northern China to the urban areas in the east and south.

In this network, 56 VACUTAP VR on-load tap-changers are used to ensure voltage conditions stay constant. The transformer for the UHVDC comes from Siemens. It is the most powerful HVDC transformer in the world and was recently tested successfully in Nuremberg, Germany. This super transformer has enabled Siemens to reach an entirely new dimension of high-voltage direct-current transmission. For the first time ever, direct current can be transmitted at an immense voltage of 1,100 kilovolts.

The ultra-high-voltage technology can reduce the power lost during long-distance transmission. At the start and end of each high-voltage line, direct and alternating current can be converted in current-transformer stations. The transformer supplies energy with high current and high voltage to the transformer station. To minimize power loss over the enormous distance of more than 3000 km and to maximize transmission efficiency, the electricity must be transmitted with a minimal current strength. This is only possible with a high transmission voltage. Therefore, the transformer is to realize a transmission voltage of ± 1100 kV. The higher the transmission voltage, the lower the transmission losses. The transformer, which was developed specifically for this project, is the largest single-phase transformer ever built. The transformer's on-load tap-changer ensures voltage regulation in the process.

In view of these significant technical challenges, the Chinese network operator State Grid Corporation (SGCC) put its trust in the vacuum technology of Maschinenfabrik Reinhausen (MR) for the on-load tap-changers. The VACUTAP VR, the most powerful vacuum tap-changer from MR, is used here. A total of 56 VACUTAP VR units will be installed in the transformers of the power converter stations on the UHVDC line.

The UHVDC project is a real challenge. With the development of its ultra-high voltage network, China is leading the world. Nowadays, UHVDC transmission is not only playing a significant role in alleviating the shortage of electricity and reducing environmental impacts in the central and eastern regions of China, it is also playing an increasingly important role in promoting reconfiguration of the energy infrastructure and alleviation of smog.

The Changji – Guquan transmission line boasts the world's highest transmission voltage, the largest transmission power and the longest transmission distance, and is setting new technological standards. The project is an important milestone for the continuous improvement of the state electricity network and has set the bar in electricity transmission technology higher. This heralds a new era in ultra-high-voltage transmission. The UHVDC grid is an important example for the development of the Global Energy Interconnection, a global energy grid proposed by the State Grid Corporation of China.

The Chinese power supply industry wants to develop the UHVDC power grid further, promote the more intensive use of renewable energies and, ultimately, ensure that the whole of China benefits from a safe, stable and environmentally sound power supply. With its reliable products, MR will continue to make a contribution to the UHVDC projects in China, thus enabling the country to create a cleaner and better future.

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