Power capacitors for HVDC
Minimizing energy losses
The future belongs to low-loss high-voltage DC transmission (HVDC), and EPCOS power capacitors are indispensable for stabilizing efficient HVDC systems.
An efficient alternative to the widespread older three-phase technology has been available for the transmission and distribution of electrical power since the mid-1970s, namely high-voltage DC transmission (HVDC). Essentially two groups can be distinguished within this technology:
- Conventional HVDC transmission
- Multi-level VSC (voltage-sourced converter) HVDC transmission
In the meantime, HVDC technologies are being implemented in more and more power transmission projects. Compared with three-phase technology, HVDC not only reduces transmission losses – especially over long distances – but it also helps to save costs elsewhere, such as in the installation of the transmission lines, as HVDC transmission also requires much lower material outlays.
Three-phase technology approaches its limits
Three-phase technology with frequencies of 50 or 60 Hz has already been used for energy transmission over several decades. Its main strength is the simple transformation of the voltage to the most diverse voltage levels in order to bridge even longhaul routes. In addition, even very high AC voltages can be simply switched without generating electric arcs, and rugged asynchronous motors can be supplied directly with DC current.
However, three-phase networks also have a critical weakness: their relatively high losses. The finite conductivity of the cables causes resistive losses that are additionally increased by the skin effect that occurs in AC transmission: the current flows only through the outer part of the conductor so that the whole cross-section of the line is not used to transport the power. This is a great disadvantage particularly on long link sections. The same applies when more extensive interconnected networks are set up.
But PFC losses also occur in AC systems due to the inductive and capacitive parasitic effects, and these are increased even more by the transformer inductances. Depending on the line lengths and the effect of the parasitic effects, up to ten percent of the electrical energy can be converted to power losses in this way. In addition, AC networks operating at different frequencies (e.g. 50 and 60 Hz) cannot be directly coupled.
HVDC halves losses and needs only two lines
In order to avoid the many drawbacks of three-phase technology, power utilities worldwide are increasingly opting for HVDC transmission. Its great advantage is its significantly lower power losses, which are only three percent for a line length of 1000 km. In comparison, AC systems lose at least six percent over the same route. So HVDC can avoid losses of about 120 MW when 4000 MW is to be transported.
Another advantage of HVDC technologies is that they need fewer lines. Three-phase systems need at least three lines. For reliability reasons, two redundant systems, each comprising three lines in parallel, are often run in practice. In contrast, HVDC needs only two lines. Above all, in overhead power lines with masts and cantilevers, HVDC power lines need less land, which means that the costs can be greatly reduced. But for underground and marine cables too, which take up a relatively small amount of area, the fewer HVDC lines needed represent a cost factor. In particular, underground and marine cables can be used in three-phase technology only for routes up to 50 km due to their relatively high parasitic capacitance.
EPCOS power capacitors are key components
HVDC technologies only became feasible thanks to the development of power semiconductors with correspondingly high dielectric strengths of several hundred kV for rectification at the beginning of the transmission route and inversion at its end – the feed point into the supply network. Thyristors were initially used, but were increasingly replaced by series-connected IGBTs offering better controllability. In addition to these semiconductor components, EPCOS power capacitors play a decisive role in HVDC systems.
MKV and MKK capacitors for conventional HVDC technologies
Conventional HVDC technologies use six‑pole rectification at the start of the transmission route. Power capacitors are needed here to protect the thyristors from overvoltages: they operate in this case as snubber capacitors.
The B25990T5165A000 type from the EPCOS power capacitor portfolio is very well suited for this purpose. It has a capacitance of 1.6 μF and a dielectric strength of 5100 V AC. Manufactured in conventional MKV technology, the capacitor winding consists of bilaterally metalized paper. A polypropylene film between the paper layers acts as the dielectric.
For new projects with comparable electrical parameters, MKK technology rated at 1.4, 1.6, 2.0, 2.4 and 4 μF is used. In this case, the metalization is applied directly onto the propylene dielectric. These capacitors are oil-free. These EPCOS MKK capacitors are particularly suited for 2-level VSC HVDC systems. Figure 1 shows their block diagram.
|Figure 1: Block diagram of a 2-level VSC HVDC converter|
Up to the present, some thirty projects worldwide employ conventional HVDC technologies with a total transmission volume of over 100 GW (Figure 2). On average, several hundred capacitors are used in such systems.
Particularly in view of the rapid rate of power grid expansion in Asia, experts forecast an increase in installed power of around 350 GW by 2020. The principal application for conventional HVDC systems is low-loss power transmission over long distances, for instance to link metropolitan areas to remote hydroelectric plants. Another use for this technology is to link grids operating at different frequencies such as 50 and 60 Hz.
|Figure 2: Conventional HVDC transmission installations with EPCOS power capacitors|
Conventional HVDC technologies have a relatively simple design and two significant drawbacks. First, pulse-width modulation requires several filters on the output side. Any cost advantages resulting from the fewer required lines are consequently partially offset. Second, the older thyristor-based systems, which can only switch the power semiconductors on but not off, do not allow any active network control of the frequency and voltage.
New multi-level VSC HVDC transmission
The multi-level VSC HVDC technology was developed to enable switchable network links with HVDC technologies (Figure 3).
|Figure 3: Block diagram of a multi-level VSC HVDC system|
The decisive benefit of this IGBT module-based technology is that it enables the converters to be controlled actively. For this purpose, capacitive voltage dividers comprising EPCOS power capacitors are installed at the end of the HVDC line between the anode and the cathode and switched by the controlled IGBT modules. They are thus controlled to produce a step function of the output voltage that already closely approximates the desired sinusoidal curve. The number of filters required at the converter output is consequently reduced dramatically compared with solutions using conventional HVDC technology. Figure 4 shows six modules.
|Figure 4: Modular system for multi-level VSC HVDC transmission|
The use of capacitive voltage dividers leads to a need for more power capacitors; altogether up to 5000 capacitors per installation are required for new VSC systems. In return, the costs for filtering the converter outputs are greatly reduced, and even more important: the network link can be very well controlled and thus satisfies the key precondition for use in new energy transmission projects. In contrast to conventional HVDC technology, multi-level VSC HVDC systems are autonomous, i.e. they regulate and stabilize the network frequency and voltage.
MKK power capacitors for multi-level VSC HVDC
Dry EPCOS MKK power capacitors have been developed specially for multi-level VSC HVDC transmission systems. They offer capacitances of between 2000 and 10 000 μF and are designed for voltages of up to 3000 V DC. Current capabilities of over 700 A can be reached depending on the type. A low ESR is also critical in order to minimize losses. These EPCOS capacitors are rated at less than 0.2 mΩ. As they are connected in series in multi-level VSC installations, very narrow limits must be observed for their rated values in order to avoid voltage surges at individual modules. Over their entire operating life EPCOS capacitors have a tolerance of less than 3 percent from their initial capacitance.
Successful customer projects
EPCOS power capacitors for multi-level VSC HVDC systems have already proved themselves in several major Siemens projects. A start was made in 2010 with the Transbay project, a multi-level VSC HVDC transmission link spanning the length of the San Francisco Bay. The link to the wind farm projects BorWin2 and HelWin1 in the North Sea will follow in 2013. In the meantime, a further high-volume project has been completed: the link to the SylWin1 group of wind farms lying 70 kilometers west of the North Sea island of Sylt. Able to transmit some 860 MW, this is the highest capacity offshore link to date.
Multi-level VSC HVDC is finding increasing acceptance in linking offshore wind farms located 50 kilometers or further from the coast. DC transmission technology in combination with marine cables is the only meaningful option for such links for both technological and economical reasons. EPCOS power capacitors are also planned to be used in many forthcoming Siemens projects in the German Bight and off the east coast of England. Figure 5 shows the VSC HVDC projects established around the world using rugged EPCOS capacitors.
EPCOS power capacitors will also be used in the INELFE project (Interconnexion Electrique France Espagne) that Siemens is implementing in VSC HVDC technology. In this trans-European network link, up to 2000 MW will be transmitted at a voltage of ±320 kV. An underground cable will be used on this 65 km long route, part of which will run in a tunnel under the Pyrenees.
|Figure 5: Multi-level VSC HVDC projects using EPCOS power capacitors|
Worldwide presence for EPCOS power capacitors