TRANSMISSION & DISTRIBUTION
HVDC Plus – a shore thing for Siemens?1 July 2007
Voltage source converter DC systems using IGBTs have long been employed in designs for industrial drives but not, until recently, in substantial power transmission systems. Now Siemens has introduced its take on this compact HVDC design capable of reactive and real power flow compensation, and it is aiming at an expanding market – carrying power to and from offshore installations such as wind farms and oil rigs.
Line commutated converter HVDC is generally considered to be suitable for power capacities of up to 1000 MW and interconnectors larger than 800 MW x 800 km are now run-of-the-mill, with ever increasing capacities being ordered for vast projects, particularly in China (see panel). But VSC based HVDC systems, which unlike line commutated systems can offer independent real and reactive power flow control, had until recently been limited to the lower end of the bulk power transfer range. Yet in the space of a few months that has all changed and VSC based HVDC?is now available for installations in the 1000 MW range.
Siemens PTD’s newly introduced HVDC concept is called HVDC?Plus and is suitable for 800 MW systems. This makes it a direct competitor to ABB’s HVDC Light of which the best known installation is probably the 350 MW Finland–Estonia subsea interconnector, Estlink. Less than a year ago ABB upgraded this HVDC?offering into the 1000 MW range.
Siemens’ unique selling point for HVDC?Plus is its innovative multilevel converter concept which offers, says the company, certain advantages compared to existing VSC solutions, namely lower losses owing to lower switching frequencies (Figures 2, 3 and 4), full modular design – hence straightforward scalability – and suitability for use with overhead lines.
The advantages already associated with VSC technology are well known – it operates with power semiconductors that have turn-on and turn-off capability, and so are independent of the AC system voltage, it allows better grid access to weak power networks and to passive networks, active and reactive power can be controlled independently of each other, it has excellent dynamic response owing to very fast control interactions with the converter, which is particularly important in the event of AC system faults and disturbances, it can help eliminate the commutation failures associated with line-commutated technology, it confers black start capability, ie the ability to start up a collapsed network, and it is compact compared to conventional line-commutated technology.
Thus far, HVDC Plus resembles existing VSC based solutions, but unlike two-level VSC, which has been the most widely used VSC arrangement, HVDC Plus is based on a modular multilevel converter (MMC) topology. Each converter leg in this arrangement can be used as a controllable voltage source (Figure 5). The total voltage of the two converter legs in a phase unit equals the DC voltage, and the voltage at the converter legs is further adjusted so as to provide the desired sine wave voltage at the AC terminal.
Siemens claims that MMC topology differs from already familiar VSC topologies in design, mode of operation, and protection capabilities:
•Its modular construction, in the power section and in control and protection, leads to excellent scalability and flexiblility in the overall design.
•In normal operation, no more than one level per converter leg switches at any given time. As a result, the AC voltages can be adjusted in very fine increments and a DC voltage with very little ripple can be achieved, which minimises the number of generated harmonics and in most cases eliminates the need for AC filters. Moreover, the small and relatively shallow voltage steps that do occur cause very little radiant or conducted high-frequency interference.
•The low switching frequency of the individual semiconductors results in very low switching losses. Total system losses are therefore relatively small for VSC technology, and efficiency is consequently higher (Figure 4).
•It utilises industrially proven standard components such as IGBT modules that are robust and highly reliable, and are widely used in other applications, such as traction drives. This results in a larger number of manufacturers as well as long-term availability and continuing development of these standard components.
•The resultant voltage and current loads support the use of standard AC transformers
•The achievable power range as well as the achievable DC voltage of the converter is determined essentially only by the performance of the control, ie by the number of submodules that can be operated. With the current design, transmission rates of 1000 MW or more could be achieved.
The basis of MMC
The most widely used current VSC technology is based on converters configured in what is known as two-level topology. This type of converter is usually connected to the three-phase network via a converter transformer. Connected to the DC terminals at the converter is a DC link capacitor that smoothes the DC voltage. The converter itself consists of six converter legs, which are connected to a six-pulse converter bridge.
Each converter leg consists of a switch and a free-wheeling diode. The switch handles current flow in one direction. In the other direction, the current flows through the diode. The type of switch most often used today is the insulated gate bipolar transistor, or IGBT. The converter configuration makes it possible to connect one of the two voltage poles of the DC link capacitor to each of the three AC terminals.
To render HVDC voltages controllable by semiconductors with a blocking ability of only a few kV, multiple IGBT diode pairs are connected in series – up to several hundred per converter leg, depending on the DC voltage. In this case, the individual IGBTs must switch simultaneously, with an accuracy in the microsecond range, to ensure uniform voltage distribution not only statically but also dynamically in all individual elements.
Suitable methods for the generation of pulse patterns, such as pulse width modulation (PWM), can be used to approximate a desired voltage curve in the AC connection. The switching frequency of the IGBTs would be somewhere between 20 and 40 times the line frequency, ie in the range 1–2 kHz.
With this setup a relatively high proportion of harmonics is created in the generated AC voltage. At DC of several hundred kV, the voltage gradients resulting from switching are in the order of 100 kV/µs. The generation of harmonics and the emission of high-frequency radiation are undesirable consequences of this method, so there are opportunities for improvement, a point that Siemens is addressing with the installation of a 30 MW permanent test rig in Erlangen.
Both the voltage steps and the related voltage gradients can be reduced or minimised if the AC voltage generated by the converter can be selected in smaller increments than at just two levels. The finer this gradation, the smaller is the proportion of harmonics and the lower the emitted high-frequency radiation. Also, the switching frequency of individual semiconductor elements can be reduced, which, since switching events create losses in the semiconductors, can also effectively reduce converter losses.
Converters that can switch three different voltage levels at the AC terminal are referred to as three-level converters, those with more are called multilevel converters. Different multilevel topologies have been proposed but the one selected by Siemens’ engineers is to distribute the capacitors in the converter legs and to provide semiconductor elements at each capacitor so as to make it possible to switch the voltage of a given capacitor in a series connection on and off individually. This enables the converter to generate the AC voltages in small increments. This is the principle used as the basis for the HVDC Plus technology.
A converter in this context consists of three identical phase units that are connected in parallel on the DC side and to the three-phase system on the AC side.
The individual converter legs consist of submodules (Figure 6) connected in series. Each such submodule contains an IGBT half bridge as the switching element, a DC storage capacitor and electronics to control the semiconductors, for measuring the capacitor voltage, and for communicating with the higher-level control.
However meticulous the engineering, component faults occur, and when they do, operation of the system must not be impeded as a result. In the case of an HVDC transmission system this means that there must be no interruption of the energy transfer, and that the system will continue to operate until the next scheduled shut-down.
Redundant submodules are therefore integrated into the converter, and, in contrast to previous redundancy concepts, a unit can now be designed so that, upon failure of a submodule in a converter leg, the remaining submodules are not subjected to a higher voltage. The inclusion of the redundant submodules thus merely results in an increase of the number of submodules in a converter leg that deliver zero voltage at their output during normal operation.
In the event of a submodule failure during operation the fault is detected and the defective submodule shorted out by a reliable high-speed bypass switch. This provides fail-safe functionality, as the current of the failed module can continue to flow, and the converter continue to operate, without any interruption.
It is necessary to ensure, within certain limits, a uniform voltage distribution across the individual capacitors of the multilevel converter. With MMC topology this is achieved by periodic feedback of the current capacitor voltage to a central control unit. The time intervals between such feedback events are less than 100 µs.
HVDC Plus is considered suitable for several kinds of DC system:
• Cable transmission lines: here the use of XLPE cables, which are plastic-insulated, is advantageous, since the voltage polarity in the cable remains the same irrespective of the direction of current flow.
• Overhead transmission lines, because of the capability of managing DC side short circuits and prompt resumption of system operation.
• Back-to-back arrangements, that is, rectifiers and inverters in one station.
• The implementation of multi-terminal systems, which is relatively simple with HVDC Plus. In such systems, more than two converter stations are linked to a DC connection. It is possible to configure complete DC networks with branches and ring structures. Their possible future application was addressed in the development of HVDC Plus by pre-engineering the control strategies required for such systems.
Furthermore, the converters can also be used in MMC technology as STATCOMS, static synchronous compensators. They are also useful in markedly unbalanced networks, for instance in the presence of large single-phase loads.
Wind farm and oil rig applications
Wind farms offshore in the power range of 100 MW make particularly rigorous demands on power transmission. Some wind farms are located offshore over a hundred kilometres from the AC system on land, and could be grouped together off a single submarine cable system. Groups of offshore oil and gas rigs could likewise supplied with power from onshore networks. This generally exceeds the economic and technical limits of AC based cable transmission systems and calls for new DC transmission concepts.
Oil platforms, which have a high power demand, also require a high level of power quality of transmission if they are to be supplied from the mainland and not locally as in the past. Power delivery from the mainland not only increases the availability of the electric supply on the drilling rigs but also renders the maintenance and servicing work unnecessary for the small power plants currently used on the platforms and helps reduce emissions of CO2 and NOX.
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