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HVDC PLUS - The Siemens world class top innovation enables new applications of well proven HVDC transmission.
The innovative design of HVDC PLUS provides technical as well as economic benefits. Via implementation of a new concept of modular multilevel voltage-sourced converters, HVDC PLUS is the preferred solution when HVDC transmission needs to be combined with supporting functions such as AC voltage control or black-start capability or where space for converter stations is limited. It is ideal for connection of remote offshore platforms and wind farms to the main power grid as well as for power supply to megacities. HVDC PLUS improves the performance of the transmission grid with regard to system security. Dedicated functions make HVDC PLUS ready for future power system developments and HVDC grids.
Therefore HVDC PLUS offers a wide range of applications. Learn more about HVDC PLUS Applications from Siemens.
As a world leader in HVDC transmission technologies, Siemens introduced HVDC PLUS, an advanced and universally applicable solution for power transmission up into the gigawatt range.
HVDC PLUS offers significant technological benefits for transmission grids:
VSC Technology provides a straight forward AC side connection. The modular multilevel converter principle renders AC harmonic filters superfluous under typical network conditions. In symmetrical monopolar configuration a standard transformer design can be used without special requirements to withstand DC voltage or harmonic currents.
The modular rack-type converter arrangement provides flexibility with respect to building height versus length. For reducing its visual impact, it allows lower building heights as compared to conventional HVDC converters.
The converter modules are operated with a low switching frequency, resulting in low converter losses and lower operational costs accordingly.
The converter and the control equipment are designed with a high level of component redundancy for extremely high reliability.
The features of HVDC PLUS are:
As the power industry strives to reduce its environmental impacts by utilizing more renewable energy resources (e.g. large wind farms), transmission system owners face the difficulties of integrating these often remote and fluctuating energy sources into the grid.
HVDC PLUS is the key solution to these challenges which impede the take-up of renewable energies. In this regard, the possibilities of operating with low short-circuit levels, the small footprint, and the independent control of active and reactive power are particularly advantageous.
With HVDC PLUS, islanded networks, such as oil and gas platforms as well as mines, can be connected without the need for local generation, increasing the overall efficiency of the plant significantly. This contributes to reducing CO2 emissions.
Modular Multilevel Converters for HVDC PLUS have a typical 6-pulse bridge topology. One converter unit consists of three identical phase units, each comprising two valves – each valve contains a number of power modules supporting the full DC voltage, Ud.
Existing 2-level or 3-level converters using pulse width modulation (PWM) must switch the full DC voltage in large steps. This causes high harmonic distortion and high transient stresses, resulting in HF noise.
With the Siemens Modular Multilevel Converter approach each individual power module contributes only a small voltage step. The power modules are controlled individually to achieve the required sinusoidal AC and smooth DC side output voltage waveforms.
In this way, each module within the modular multilevel converter is a discrete voltage source in itself with a local capacitor to define its voltage step without creating ripple voltage distortion across the converter’s other phases.
Control of the power modules via PLUSCONTROL
The power modules are bi-directionally connected via optical fibres with a central control unit, the PLUSCONTROL. PLUSCONTROL was developed to provide the high performance requirements for HVDC Plus and has the following functions:
The current status of each power module as well as the measured capacitor voltage at this time is transferred by telegram via optical fibres to PLUSCONTROL. In the other direction, PLUSCONTROL sends control signals to the power modules, e. g. the switching of the IGBTs.
Main features of PLUSCONTROL include:
Four Quadrant Operation
Taking advantage of its fully controllable IGBT devices, the Modular Multilevel Converter is able to both absorb/generate active and reactive power independently (up to the converter rating), based on a typical characteristic, as shown in the figure. The output currents can be varied over the complete operating range in a smooth, linear fashion - operation down to zero DC current is also possible.
The HVDC PLUS half-bridge topology is preferably used for cable connections or in back-to-back configurations. The DC voltage of a half-bridge power module is always controlled in one polarity only. Its design and functionality is described in following figures:
Within the half-bridge type topology the power capacitor can be connected in one polarity to the terminals. Therefore the DC voltage is always higher than the AC voltage.
The converters of the VSC-based (voltage-sourced converter) cable connections like those in Trans Bay (USA), INELFE (France/Spain), and offshore projects in Germany are designed with half-bridge topology. This technology is highly efficient.
The HVDC PLUS modular multilevel converter (MMC) can be designed in full-bridge topology as shown in the figure:
In MMC full-bridge topology the power capacitors can be connected to the terminals in either polarity. Hence, the DC voltage is independent of the AC voltage and can be controlled to zero or even be entirely reversed maintaining current control on the AC and DC sides, also under short-circuit conditions.
The DC voltage can be controlled over a wide range, including both polarities. Such a topology is predestined for DC circuits with overhead lines and provides the following features:
The line protection system of the HVDC can differentiate between individual segments. The overhead line affected by a fault can be brought back into operation quickly while cable faults require some time to repair. The restart attempts in the case of overhead lines are made after fast short-time polarity reversal of the converter voltage. In doing so, current extinction and electric arc deionization are followed by a short break; the whole procedure can be repeated as often as required, typically up to three times. The break time and the rate-of-rise of the DC voltage during recovery can be variably adapted to the dielectric strength, e.g. the degree of pollution. Using this “adaptive” strategy, supply security can be significantly increased.
Siemens has been successfully using the full-bridge circuit for traction frequency converters Sitras® as well as for FACTS and industrial installations with the SVC PLUS® since 2009 in 85 reference converters with full bridge worldwide as of February, 2014.
HVDC PLUS in full-bridge topology offers fast DC line fault clearing with a strategy similar to the proven HVDC “Classic” and is therefore the optimal solution for point-to-point radial DC transmissions. Read an Interview with Jörg Dorn, Head of the R&D department and Dietmar Retzmann, Director Technical Marketing and Innovations about HVDC solution strategies for fast and reliable clearance of DC line/cable faults.
The HVDC PLUS technology allows straightforward AC side connections to be made. Harmonic filtering is typically not needed.
For HVDC PLUS only one single circuit breaker is needed per converter. It is connected in series with the pre-charging unit between the power transformer and the converter.
In accordance with standard HV substation practice, the station can be safely isolated from both the AC network and DC circuit via circuit disconnectors. All insulated busbars can be automatically grounded with grounding switches for safety reasons. The external busbars are equipped with surge arresters to protect them against lightning strikes etc.
Voltage and current measuring devices, such as standard and special purpose VTs (Voltage Transformers) and CTs (Current Transformers) measure the primary AC voltage and currents plus secondary currents.
Control & Protection
Win-TDC is a recent innovation from Siemens in the field of HVDC control & protection and has already proven its benefits in existing HVDC projects and in a wide area of industrial applications. The system is based on the SIMATIC WinCC Human Machine Interface (HMI) and the SIMATIC TDC (Technology and Drive Control) control system which gives Win-TDC its name.
Compact design is provided by high integration. Fast communication links allow for an independent, central and redundant measuring system, resulting in a highly reliable design. The use of the systems, based on Microsoft Windows®, for all operator control, monitoring and engineering purposes enhances users’ acceptance and minimizes training time.
The C&P system for HVDC PLUS follows the well-proven Siemens three level hierarchical structure:
Operator Control and Monitoring Level
Control & Protection Level
The different levels are interconnected by powerful redundant serial communication links, such as Fast Ethernet (IEEE 802.3u) and PROFIBUS DP (EN 50170). The use of standardized communication protocols, e.g. TCP/IP and UDP, provides easy adaptation to specific requirements with the flexible implementation of additional features, such as remote access for maintenance purposes.
The heart of a converter station is the converter, in which the reliable and economical energy conversion of AC to DC or vice-versa takes place.
The self-commutated, voltage-sourced converters (VSCs) are provided in a modular multilevel-converter (MMC) configuration, developed by Siemens. The HVDC PLUS converter generates a nearly ideal sinusoidal waveform on the AC side and a smooth DC voltage without significant high frequency noise. Moreover, the MMC valves are operated at low switching frequencies contributing to low overall power losses.
Its modular converter design, incl. IGBTs, capacitors and diodes, in combination with a flexible control and protection system results in an excellent system scalability. The converter station layout can be perfectly adapted to the local requirements, e.g. the design can favor either a converter hall with a small footprint or a building with a low profile.
An important component of the HVDC converter station is the transformer. It is used as a coupling element between the connected AC grids and the converter and adapts the AC voltage. The encountered voltage and current loads in a symmetrical monopole configuration support the use of standard AC transformers. HVDC type transformers are installed for asymmetrical and bipolar configurations to handle the DC Voltage on the converter terminals of the transformer.
Asymmetrical Monopole / Bipole:
The cooling system is necessary to dissipate the thermal losses of the converter modules. A reliable converter cooling system is an important aspect to ensure the highest possible availability of the HVDC. Hence, consideration of an appropriate redundancy concept is essential.
The converter modules are cooled by a liquid closed loop. The water connection to the heat sinks can be designed in parallel or series. Siemens has successfully used the parallel water cooling principle for more than 25 years for thyristors. It has been adapted accordingly to the voltage-sourced converter technology. It provides all IGBT modules with the same cooling water temperature which allows a better utilization of their capability.
The disposal of the transported heat is typically carried out via the cooling system in the outdoor area as shown in the figure.
An alternative solution in case of space constraints offers the latest advanced technology from Siemens, the DC Compact Switchgear (DC CS). The DC CS at a voltage level of 320 kV DC is used for remote offshore applications, e. g. wind farms. Thanks to its compact design, the DC CS helps to reduce the HVDC system's space requirements.
Using the DC CS outdoors even in rough climates adds to this effect. In the near future the DC compact switchgear and transmission solutions will facilitate the realization of multi-terminal arrangements or DC grids, complementing the existing AC networks.
The DC Yard includes the following equipment:
DC Switchgear or DC Compact Switchgear
The nearly perfect shape of the DC voltage after conversion from AC typically eliminates the need for DC filters and smoothing reactors.
In accordance with standard HV substation practice, the station can be safely isolated from both the AC network and DC circuit via DC disconnectors as part of the DC Switchgear.
All insulated busbars can be automatically grounded with grounding switches for safety reasons.
The external busbar is equipped with surge arresters to protect them against lightning strikes etc.
Special purpose voltage transformers and current transformers are used for DC voltage and current measuring.
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.
Siemens´ HVDC PLUS with its multilevel converter provides the appropriate technology to connect remote energy sources to the grid or ensure power supply by means of HVDC technology, even in densely populated areas, such as megacities.
In the San Francisco area the TransBay Cable project forms a milestone in VSC technology with the world's first HVDC in MMC topology.
The world record in VSC transmission with two symmetrical monopoles and a capacity of 1000 MW each is held by the INELFE project connecting France and Spain. Siemens received the order for constructing the converter stations in 2011.
In 2014 Siemens received the fifth order in series from TenneT for offshore grid connections with HVDC PLUS. This makes Siemens the market leader in providing grid access for offshore wind farms using VSC transmission.
See the Siemens HVDC PLUS references and learn more about this first step into a smarter way of HVDC power transmission.