Skip to Content
The security of power supply in terms of reliability and availability has the utmost priority when planning and extending power grids. The aspect of sustainability is gradually gaining in importance in view of such challenges as global climate protection and economical use of dwindling power resources.
HVDC Classic solutions from Siemens based on thyristor technology offer a power rating up to six gigawatts (GW) at a voltage level of ±600 kV and up to 10 GW at ±800 kV and provide:
Siemens as a pioneer in the history of DC technology has been one of the leading companies in the HVDC business for decades. Siemens is providing HVDC systems like HVDC Classic with the necessary features to overcome challenges in the power systems of the future.
The increasing demand for electric power in load centers, and a growing interest to use renewable energy resources usually generated far away from load centers calls for bulk power transmission. Siemens UHV DC technology takes power transmission to the next level and offers long-distance power transmission at a voltage level of ±800 kV for the first time ever (e.g. first UHV DC project Yunnan-Guangdong, China).
DC technology, especially HVDC Classic applications are available for:
HVDC Classic is a commonly used technology for bulk power transmission with a long history that provides the key to increased performance and robustness of the transmission grid, to keeping pace with the steadily growing energy demand, and to a highly economical method of CO2 emissions reduction.
HVDC Classic offers:
The most economical solution for long-distance bulk power transmission, due to lower losses, is transmission with High Voltage Direct Current (HVDC). A basic rule of thumb: for every 1000 kilometers the DC line losses are less than 3% (e.g. for 5000 MW at a AC voltage of 800 kV). Typically, DC line losses are 30–40% less than with AC lines, at the same voltage levels. HVDC systems from Siemens are the perfect solution for a low-loss power transmission.
The overload capabilities of thyristor-based HVDC systems typically depend on system and ambient temperature and on the availability of redundant cooling equipment.
Examples for typical short time overload and longtime overload capabilities are shown in the above figures
The resulting benefits are:
The next level of HVDC technology, Siemens ultra high voltage direct current (UHV DC) system, is characterized by:
The development of the 6-inch thyristor and the entire range of components required for UHV DC converter stations from Siemens offer the basis for shaping the future of bulk power transmission.
The UHV DC converter is an innovation within the HVDC systems form Siemens. It consists of a number of thyristor modules equipped with new Siemens 6-inch thyristors. For project conditions with transport limitations the valves are arranged in two series-connected 12-pulse groups. A major benefit of this layout is the relatively small size of the converter transformers. Furthermore, it increases the redundancy of the system, as each of the four converters of plus and minus poles can be bypassed and the assigned DC line can operate at a reduced voltage level of 400 kV.
Based on R&D efforts, Siemens is able to produce the entire range of components required for 800 kV DC power transmission itself and supply complete UHV DC systems from a single source.
The world's first UHV DC project Yunnan-Guangdong in China contracted to Siemens by China Southern Power Grid in 2007 started commercial operation of the first pole in December, 2009 and the complete bipole has been in operation since June 2010. In the same year the Xiangjiaba-Shanghai UHV DC, China with the first 6-inch thyristors from Siemens was also put into operation.
The rectifying process of the supplied AC voltage in the sending converter station, as well as the inverting process of the DC voltage in the receiving converter station, causes harmonic currents and reactive power unbalance within the AC grid. These effects can be reduced to acceptable limits by installing filters and capacitor banks. The picture shows a typical AC filter yard.
The design of AC filters and capacitor banks is suitable for the individual requirements of a converter station under the conditions of the connected AC grid.
Air insulated switchgear and outdoor installation of filter and compensation equipment is the common approach in the design of AC switchyards for converter stations. This is the most economical solution and is used in HVDC projects whenever possible.
Installations in densely populated areas or locations with other space restrictions may justify the use of the more expensive but highly compact gas insulated switchgear and, if necessary, indoor installation of shunt capacitors and AC filters can be arranged.
All in-house suppliers of switchgear systems and products, such as circuit breakers and surge arresters manufacture these systems and products at our large factory complex in Berlin.
Depending on the requirements for availability, the AC switchyard can be designed with a single or double busbar arrangement, and on request with transfer busbar or in a one-and-half circuit breaker scheme. The different degrees of redundancy allow for alternate use of equipment and maintenance or repair work during normal operation of the plant.
The WIN-TDC Control and Protection System plays an important role in the successful implementation of HVDC transmission systems. High reliability is guaranteed with a redundant and fault tolerant design. Flexibility (through a choice of optional control centers) and high dynamic performance were the prerequisites for the development of our control and protection system.
Support via remote access
All WIN-TDC components from the Human Machine Interface (HMI) workstations, the control and protection systems down to the state-of-the-art measuring equipment for DC current and voltage quantities are based on standard products with a product life cycle for the next 25 years.
Innovations have always been an important aspect in Siemens HVDC Projects. Developments such as water cooling for thyristor valves became state of the art in HVDC valve design, and are used today, as for example the Direct Light Triggered Thyristor with 8 kV blocking voltage and integrated forward protection. Valve towers including several valve modules are suspended from ceiling.
One valve module, including thyristors, RC snubber circuits and reactors is shown in the figure. Depending on the application, Siemens uses Direct Light Triggered Thyristors (LTT) or Electrical Triggered Thyristors (ETT).
Converter transformers are not only one of the major components in a converter station, but the most impressive in size. The key elements of winding type, electric losses and availability influence the dimensions, the weight and the costs of such a special transformer unit. A common solution is given by single-phase three-winding type converter transformers. The picture shows a 397 MVA / 1 phase / 3 winding transformer
The converter transformer's main task is to link the AC grid with the converter valves.
The choice of the suitable HVDC transformer design is largely dependent on the maximum permissible size (transportation restrictions) as well as spare part considerations.
HVDC converter transformers are often single phase three winding types. Large long distance HVDC systems sometimes have a one phase / two winding configuration.
The thyristors are stacked in the module with a heat sink on either side. The water connection to the heat sinks can be designed in parallel or series as shown in the figure. Siemens has used the parallel water cooling principle for more than 25 years. It provides all thyristors with the same cooling water temperature which allows a better utilization of the thyristor capability.
Siemens makes use of this principle, which offers the additional advantage that electrolytic currents through the heat sinks – the cause for electrolytic corrosion – can be avoided by placing grading electrodes at strategic locations in the water circuit. Nor does Siemens water cooling require any de-oxygenizing equipment.
Harmonic voltages which occur on the DC side of a converter station cause AC currents which are superimposed on the direct current in the transmission line. These alternating currents of higher frequencies can create interference in neighboring telephone systems despite limitation by smoothing reactors.
DC filter circuits, which are connected in parallel to the station poles, are an effective tool for combating these effects.
The configuration of the DC filters very strongly resembles the filters on the AC side of the HVDC station. There are several types of filter design. Single and multiple-tuned filters with or without the high-pass feature are common. One or several types of DC filter can be utilized in a converter station.
Smoothing reactor function:
Limits the rate of current rise
Limits DC current ripple
The smoothing reactor shown here is installed outdoors and operates under severe atmospheric conditions - its main data are:
|Total Height||13130 mm|
|Rated Voltage||500 kV DC|
|Rated Current||1800 A DC|
|Rated Voltage||500 kV DC|
|Rated Current||1800 A DC|
|Total Height||13130 mm|
The DC wall bushing is the connecting device between the DC yard (outdoor equipment) and the valve hall (indoor equipment). Due to the physical character of DC voltage, the DC bushing design is more sophisticated than the design of bushings for AC applications. Excellent experience is available even under highly polluted conditions using a composite wall bushing design with silicone rubber housings. The DC wall bushing for the Tian Guang project in the test lab is shown in the figure.
Depending on the arrangement and functional requirements of the HVDC link, different DC switches may be installed in the DC yard as:
Metallic Return Transfer Breaker (MRTB)
Earth Return Transfer Breaker (ERTB)
Neutral Bus Switch (NBS)
Neutral Bus Grounding Switch (NBGS)
DC Bypass Switch (BPS)
All these switches fulfill individual tasks allowing re-configuration of the dc circuit. Even though they do not provide DC fault current breaking capability, they typically require capabilities to commutate the DC currents from one path to another, e.g. from metallic return to earth return.
Some of the a.m. switches are equipment with auxiliary circuits assisting successful completion of the commutation process.
Insulation coordination means determination of adequate protection levels of the HVDC system and is essential for the reliability of the whole transmission scheme. Surge arresters are the key elements for system protection against overvoltages, for example, in case of lightning strikes, which are a typical and unavoidable natural phenomenon.
Since 1975, the proven Siemens HVDC technology has provided numerous comprehensive power solutions to customers all over the world. Take a look at our superior experience in the field of HVDC Classic and follow the steady improvements we developed over the years. Whether long-distance transmission schemes, sea cable interconnectors or back-to-back stations – we have the adequate solutions for all kind of challenges.