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Converter Transformers for HVDC Transmission
HVDC Transformers are key components of HVDC-stations. HVDC-converter and -inverter-stations terminate long-distance DC transmission lines or DC sea cables.
HVDC Transformers react as coupling elements between the connected AC grids and high power rectifiers, and are necessary for adapting the voltage. They insulate the rectifier itself from the AC grid and generate a phase shift.
New HVDC applications are seen between existing, well-established networks. A back-to-back link can easily combine two different AC networks together, even with different frequencies. Both network users are able to take advantage of balanced load / output due to different time zones or different living be hav iors that are causing delays in peak load.
Such solutions can also help customers to strengthen and optimize their networks. The changing environment provides more and more opportunities to HVDC solutions, and Siemens is prepared to provide its customers with either our solution or with converter transformers as a loose component.
Depending on the transferred power, HVDC Transformers can be 3-phase or 1-phase and can have one or two valve windings per phase. The valve windings are exposed to AC and DC stress and therefore a special insulation design is necessary. Aditionally, the load current contains a considerable amount of harmonics that leads to higher losses and higher noise. Special DC and polarity reversal tests have to be performed to check these windings.
Besides the standard parameters of each transformer, special parameters must be known for the design of an HVDC Transformer. These parameters are normally determined by the designer of the HVDC-station in consultation with the transformer designer in order to reach a cost-effective design for the HVDC Transformer and the whole HVDC-station.
Siemens Transformer plant Nuremberg built an HVDC-transformer for the offshore platform HelWin1 that links the two offshore wind farms Nordsee Ost and Meerwind to the mainland. It converts up to 576 MW to direct-current that is then transmitted to the mainland 85 km away. The low-loss direct- current is then converted on shore and fed into the German power grid.
Find out more at the reference page for the HelWin1-project here.
In 2009 Siemens built the world‘s largest HVDC Transformer with a capacity of 800kV for converter stations in China. Compared to the previously used 500 kV level, 800 kV represents a 60% increase in rated voltage – an unequaled rise in voltage level. Power losses in the new 800 kV systems are also extremely low, which means power can be increased to 7000 MW and above, per system. Taking the reduction in losses into account, UHVDC in this application can save in the region of 33 million tonnes of CO2 emissions each year. With a length of more than 2000 kilometers and a transmission level of 6400 MW this line is at the moment the longest and powerful HVDC-transmission-line in the world.
Transmitting energy over a long distance through nearly unaccessible terrain is a classic challenge in power transmission, and it is the field in which HVDC proves its high efficiency best. A characteristic example is the Three Gorges Project. The landline HVDC system covers a distance of more than 1,000 kilometers. Siemens contributed 14 HVDC transformers, 283.7 MVA, as well as three smoothing reactors, 0.27 H, 500 kV, for the Zhengping converter station, which is situated between Shanghai and Nanjing.
In 2005 the longest submarine HVDC link worldwide started its commercial operation. The 295 km long Basslink crosses the Bass strait, delivers hydro - electric power from Tasmania to the Australian State of Victoria, and also allows the import of base load from Victoria to Tasmania. The project was approved after extensive environmental impact studies in 2002, and consortium leader Siemens consigned the 500 MW HVDC system in 2005. Siemens provided eight HVDC Transformers rated at 196 MVA each.