Modular Multilevel Converter
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Modular Multilevel Converter
HVDC PLUS (VSC Technology)
HVDC PLUS is an Innovation in the field of HVDC transmission systems and opens up new fields of the proven HVDC technology.
Its innovative design allows technical as well as economical advantages. Via implementation of a new concept of modular multilevel voltage-sourced converters, HVDC PLUS is the preferred solution where shortage of space is a criterion. It is ideal for connection of remote offshore platforms and wind farms to the onshore grid as well as for power supply high-density areas such as mega cities. HVDC PLUS improves the performance of the transmission grid with regard to system security.
As an innovation leader in HVDC transmission, Siemens developed HVDC PLUS, an advanced and universally applicable solution for power transmission in the range up to thousand megawatts or higher.
HVDC PLUS offers significant technological benefits for transmission grids:
The VSC technology provides a straight forward AC side connection. The modular multilevel converter principle renders AC harmonic filters superfluous.
A standard transformer design can be used without special requirements to withstand DC voltage or harmonic currents.
The converter does not produce any significant high frequency noise, so outdoor installation of AC- and DC reactors (if necessary) and switchgear is feasible.
The modular rack-type converter arrangement provides flexibility with respect to building height versus -length. It allows to lower building height compared to conventional HVDC converters.
The converter modules are operated with a low switching frequency resulting in low converter losses.
The converter and the control equipment is designed with a high level of component redundancy.
As the power industry strives to reduce its environmental impacts by utilizing more renewable energy resources (e.g. large wind farms) the 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 possibility to operate with low short-circuit levels, the small footprint, and the independent control of MW and Mvar are of particularly advantages.
With HVDC PLUS, islanded networks, such as oil and gas platforms as well as mines, can be connected without the need for local generation which is less efficient. This helps reduce the CO2 emissions.
HVAC cable transmission systems have a limited transmission length, from both technical and economical points of view, whereas the HVDC PLUS has practically no restrictions. Therefore, it provides an optimal solution for longer transmission distances. The HVDC transmission is the solution for cable systems longer than 80 - 120 km.
HVDC PLUS, with its symmetrical monopole configuration, is a particularly attractive solution for cable applications. A wide range of economic cable designs is available in combination with HVDC PLUS. Typical applications include: energy platforms, offshore wind farms, island connections, urban in-feeds etc. which can be connected via submarine cables to the main grids.
DC Back-to-Back is frequently used to couple neighboring AC networks, e. g. two or more national grids, with different frequency or voltage control characteristics. In such cases a synchronous AC interconnection is not possible.
A Back-to-Back connection with HVDC PLUS will overcome the above mentioned difficulties: HVDC PLUS makes power flow. Furthermore, the interconnection with a Back-to-Back HVDC PLUS link will not increase the short-circuit power of the systems. In addition, it prevents the spread of cascading disturbances (firewall for Blackout prevention).
DC overhead lines use more narrow transmission corridors for the same power transfer when compared with their AC counterparts and this is often a crucial issue while opting for DC transmission. In the future DC may also be used to increase the power transfer through existing rights-of-way by converting AC lines to DC. HVDC PLUS with its flexible topology is an ideal converter to simplify this process.
All OHL sections are subjected to a higher risk of fault (from lightning strikes etc.) than the insulated cables and therefore HVDC PLUS has the capability to cope with faults on the DC line.
Offshore wind farms as well as oil and gas platforms are often located more than 150 km away from the coast line. They can be connected to the onshore grid via HVDC systems. From the electrical point of view, these offshore farms and platforms constitute weak isolated grids.
Connection of offshore wind farms and oil platforms via HVDC PLUS to the onshore grid helps overcome some challenges:
The HVDC PLUS represents the ideal power transmission system for such interconnections. It offers some outstanding advantages over the conventional HVDC: the design is very compact and synchronous condensers for voltage support are not necessary.
Moreover, Siemens has the experience and capability to carry out comprehensive system analysis as well as to design, deliver and maintain a complex transmission system with its typical sub-systems, offshore medium voltage distribution, HVDC or HVAC such as this system of transmission from offshore to onshore, FACTS devices and control as well as protection and communication systems.
With HVDC PLUS more than two AC systems can be interconnected via a DC transmission system, for the structure and controllability of the HVDC grid is much simpler with an HVDC PLUS than with the conventional HVDC. If the power flow direction has to be changed, a polarity reversal of the DC voltage is no longer necessary.
Multiterminal configurations can be implemented as a radial or a meshed system or in a combination of both. For megacities for example it offers an opportunity of an HVDC ring around an inner-city distribution network, using peripheral power plants for power supply.
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 tight spots like megacities. Trans Bay Cable LLC chose Siemens to meet that challenge in the area of San Francisco where the TransBay Cable project will form a milestone of HVDC PLUS technology. Read more about this first step into a smarter way of HVDC power transmission.
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In this standard 6-pulse bridge arrangement there are three identical phase units, each comprising two multi-valves – each multi-valve contains a sufficient number of modules to support the full DC voltage, Ud.
Existing 2-level or 3-level converters using Pulse Width Modulation (PWM) have to switch the full DC voltage in large steps. This causes high harmonic distortion and high transient stresses resulting in HF noise.
With the new multilevel approach individual module capacitors are uniformly distributed throughout the topology and each level is individually controlled to generate a small voltage step. In this way, each module within the 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. By incrementally controlling each step, an almost sinusoidal voltage is generated at the AC outputs of the “multi-valves”.
With the aim of control the submodules are bidirectionally connected via light conductors with a central control unit, the PLUSCONTROL. This PLUSCONTROL was developed especially for HVDC Plus and it has the following functions:
The current status of each submodule as well as the messured capacitor voltage at this time is transferred by telegram via light conductor to the PLUSCONTROL. Vice versa the PLUSCONTROL sends control signals to the submodules, e. g. the switching of the IGBTs.
Main features of the PLUSCONTROL are:
Taking advantage of its fully controllable IGBT devices, the VSC is able to both absorb/generate active and reactive power independently (up to the converter rating), refer to a typical characteristic, as shown in the figure. The VSC output currents can be varied over the complete operating range in a smooth, linear fashion - the operation down to zero DC current is also possible.
The IGBT technology based on Pulse Width Modulation (PWM) has higher losses in comparison with line-commutated thyristor technology. Typically the PWM IGBT switching frequency is 20 times higher which causes considerable power losses and this is an impediment for equipment aimed at the green, renewable sector. The multilevel approach provides lower switching frequencies (< 3 times of fundamental frequency), making the overall HVDC PLUS losses closer to the thyristor technology.