WO2010118772A1

WO2010118772A1 - A converter structure and a modular voltage source converter

Info

Publication number: WO2010118772A1
Application number: PCT/EP2009/054465
Authority: WO
Inventors: Falah Hosini; Jan Svensson; Fredrik Tinglöw
Original assignee: Abb Technology Ag
Priority date: 2009-04-15
Filing date: 2009-04-15
Publication date: 2010-10-21

Abstract

The invention relates to a converter structure for a modular voltage source converter (VSC) comprising converter cell modules connected in series, wherein the converter structure comprises X converter cell modules accommodated in a housing A comprising a housing wall, wherein the housing wall is provided with at least one through-going contact means enabling series connection of converter cell modules of the converter structure inside the housing A to Y series connected converter cell modules accommodate in one or more housings B, C.... The present invention also relates to a modular VSC comprising one or more converter structures.

Description

Title

A converter structure and a modular voltage source converter

Field of the invention The invention generally relates to the field of power compensation in a high- voltage power network, and in particular to a converter structure according to the preamble of the independent claim.

Background of the invention Modern society relies heavily upon electricity. With deregulation and privatisation, electricity has become a commodity as well as a means for competition. Power quality, as a consequence, is coming into focus to an extent hitherto unseen. Disturbances emanating from any particular load will travel far, and, unless properly remedied, spread over the grid to neighbouring facilities. In some countries, the grid (network) becomes larger very quickly due to an increased industrialisation. Fast growing industries may also get a power demand for which the power equipment is not adapted for.

A traditional way to deal with the problem of poor or insufficient quality of power distribution is to reinforce the grid by building new lines, installing new and bigger transformers, or moving the point of common coupling to a higher voltage level.

Such measures, however, are expensive and time-consuming, if they are at all permitted. A simple, straightforward and cost-effective way of power quality improvement in such cases is to install equipment especially developed of the purpose in immediate vicinity of the source(s) of disturbance. As an additional, very useful benefit, improved process economy will often be attained enabling a profitable return on said investment.

A way to deal with the problem of a rapid increase of the power demand is to install oversized capacity from the very beginning. However, in many cases the grid owners do not want to make large investments directly to be able to install necessary equipment for future grid demand. Within flexible alternating current transmission systems (FACTS) a plurality of control apparatus are known. One such FACTS apparatus is the static compensator (STATCOM). A STATCOM comprises a voltage source converter (VSC) having an AC side connected to the AC network (transmission line) via an inductor in each phase. The DC side is connected to a temporary electric power storage means such as capacitors. In a

STATCOM the voltage magnitude output on the AC side is controlled thus resulting in the compensator supplying reactive power or absorbing reactive power from the transmission line. With zero active power transfer, the voltage over the DC capacitors is constant when assuming that the converter losses are negligible. The VSC comprises at least six self- commutated semiconductor switches, each of which is shunted by a reverse or anti- parallel connected diode. A STATCOM apparatus with no active power source can only compensate for reactive power, balancing load currents and remove current harmonics in point of common connection by injecting current harmonics with opposite phase.

By bringing together STATCOM and IGBT (Insulated Gate Bipolar Transistor) technologies, a compact STATCOM with reactive power compensation is obtained which offers possibilities for power quality improvement in industry and power distribution. This performance can be dedicated to active harmonic filtering and voltage flicker mitigation, but it also allows for the compact STATCOM to be comparatively downsized, its footprint can be extremely small. The grid voltage profile may be controlled according to a given optimal characteristic, and the result is an enhanced grid capacity with a more stable, strengthened and predictable behaviour. One example where the compact STATCOM has proven to be very useful is in the steel making industry, to reduce voltage flicker in an electric arc furnace (EAF). Important added benefits are a high and constant power factor, regardless of load fluctuations over furnace cycles, as well as a high and stable bus RMS voltage. These benefits can be capitalized as improved furnace productivity as well as decreased operational costs of the process in terms of lower specific electrode and energy consumption and reduced wear on furnace refractories.

To parry a rapidly fluctuating consumption of reactive power of the furnaces, an equally rapid compensating device is required. This is brought about with the state of the art power electronics based on IGBT technology. With the advent of such continuously controllable semiconductor devices capable of high power handling, VSCs with highly dynamic properties have become feasible far into the 100 MVA range.

The function of the VSC in this context is a fully controllable voltage source matching the bus voltage in phase and frequency, and with an amplitude which can be continuously and rapidly controlled, so as to be used as the tool for reactive power control.

The input of the VSC is connected to a capacitor, which is acting as a DC voltage source. At the outputs, the converter is creating a variable AC voltage. This is done by connecting the voltages of the capacitor or capacitors directly to any of the converter outputs using the valves in the VSC. In converters that utilise Pulse Width Modulation (PWM), the input DC voltage can be kept constant when creating output voltages that in average are sinusoidal. The amplitude, the frequency and the phase of the AC voltage can be controlled by changing the switching pattern.

The converter topology for a compact STATCOM may be a two or three level configuration (NPC-converter). A two level configurated converter is shown in figure 1, and will now be explained. To be able to reach high power levels, each valve 1 has to handle a high voltage. In a compact STATCOM a number of IGBT components (or similar) are connected in series in each valve to obtain correct voltage durability. IGBT allows connecting in series, thanks to low delay times for turn-on and turn-off. Nowadays, devices are available with both high power handling capability and high reliability, making them suitable for high power converters. Instead of the IGBTs another possibility is to use Gate Turn-Off thyristors (GTO), Integrated Gate Commutated Thyristors (IGCT), MOSFET or any self commutated device. On the AC-side inductances 3 are used, before connection to e.g. a grid 2, and on the DC-side a capacitor 5. The compact STATCOM has to be constructed such that a minimal loop-inductance is achieved on the DC-side. This makes it very difficult to expand the converter, the compact STATCOM, to manage higher voltages and accordingly higher effects. Each schematic valve 1 (figure 2) comprises a number of series connected valves, called "positions l...n", shown in figure 3 as number 01... n. Each valve (if IGBTs) comprises a transistor 7 and a diod 6, and one phase of the converter is denoted 4 in figure 1. In a two-level converter the output of each phase can be connected to either the positive pole or the negative pole of the capacitor. The DC side of the converter is floating, or in other words, insulated relative to ground. The two-level topology makes two numbers of output voltage combinations possible for each phase on the AC-side.

As only a very small power is needed to control the IGBT, the power needed for gate control can be taken from the main circuit. This is highly advantageous in high voltage converters, where series connecting of many devices is used. At series connection of IGBTs, a proper voltage division is important. Simultaneous turn-on and turn-off of the series connected devices are essential.

Instead of expanding the compact STATCOM, which is difficult, one more converter could be used. Thus, the compact STATCOM will consist of several blocks. The drawback by using several blocks is that the footprint increases, and the compact STATCOM becomes large. Foundations have to be built for the new blocks, and as the blocks have to be insulated relative each other, areas around the blocks are dedicated for insulation which makes the blocks even more space demanding.

Another kind of VSC is a chain- link based converter comprising a number of series- connected cell modules, each cell comprising a capacitor, besides the valves. A chain-link cell module may consist of four IGBT positions and a DC link Capacitor bank as shown schematically in figure 4. Each of the three VSC phases consists of a number of chain- link cells, here shown in series in the general diagram of figure 5 for a Δ-connected arrangement. The phases can also be connected in Y-arrangement, as shown in figure 6. The number of cells in series in each phase is proportional to the AC voltage rating of the system and can consequently include a large number of cells.

The object of the present invention is to render it possible to easily adapt a voltage source converter to varied voltage levels and accordingly varying power demands. Summary of the invention

The above-mentioned object is achieved by a converter structure for a modular voltage source converter (VSC) comprising converter cell modules connected in series according to the independent claim. Thus, the converter structure comprises X converter cell modules accommodated in a housing A comprising a housing wall. The housing wall is provided with at least one through-going contact means enabling series connection of converter cell modules of the converter structure inside the housing A to Y series connected converter cell modules accommodated in one or more housings B, C...

Preferred embodiments are set forth in the dependent claims.

The modular VSC according to the invention may be used for example to control the voltage on the network (e.g. a transmission network, a sub transmission network or a distribution network), by consuming or injecting reactive power to the network.

Short description of the appended drawings

Figure 1 shows a circuit diagram for a two-level converter.

Figure 2 shows a schematic valve.

Figure 3 illustrates a number of positions l...n in a schematic valve. Figure 4 shows a converter cell module.

Figure 5 shows a three-phase modular converter connected in Y.

Figure 6 shows a three-phase modular converter connected in Δ.

Figure 7 shows a modular phase of a modular converter mounted in a housing.

Figure 8 shows a site layout of a modular STATCOM. Figure 9 shows a site layout of a modular STATCOM that manage twice the power compared with the modular STATCOM shown in figure 8.

Figure 10 shows a transformer suitable for a modular STATCOM, where the secondary windings are connected in parallel.

Figure 11 shows a transformer suitable for a modular STATCOM, where the secondary windings are connected in series.

The invention will now be described in detail in conjunction with the figures. Detailed description of preferred embodiments of the invention

The chain- link converter topology consists of converter cell modules 10 that are connected in series and builds up a phase (in figure 4 a converter cell module is shown). To obtain a three-phase converter, the phases can be connected in Y or Δ as shown in the figures 5 and 6. Hereinafter, this kind of converter is called a modular voltage source converter.

The converter cell module 10 illustrated in figure 4, also denoted converter link or chain- link cell module, may comprise four valves 15, 16, 17, 18, each valve including a transistor switch, such as an IGBT. In the following an IGBT is used as an example, but it is noted that other semiconductor devices could be used, for example gate turn-off thyristors (GTOs), Integrated Gate Commutated Thyristors (IGCTs), MOSFETs or any self commutated device. A free-wheeling diode, also denoted anti-parallel diode, is connected in parallel with each IGBT. The diode conducts in the opposite direction of the IGBT. The valves 15, 16, 17, 18 are connected in a full-bridge arrangement with a capacitor unit 19 in figure 4, but it is understood that they may as well be connected in a half-bridge arrangement.

The present invention relates to a converter structure 8, 12 for a modular voltage source converter (VSC) comprising converter cell modules 10 connected in series. The modular VSC comprises one or more phases Ll, L2, L3, wherein each of the phases comprises converter cell modules 10 connected in series. As explained before, it is difficult to expand an already installed prior art VSC, e.g. a compact STATCOM. The present invention provides a converter structure 8, 12 which makes it possible to expand an already installed modular VSC. The effect of the modular VSC is thus scalable by adding converter cell modules 10 to each phase to an already installed modular VSC.

In the modular VSC, a starting resistor together with a switch may be added in each phase Ll, L2, L3 of the Δ- or Y-connected converter cell module VSC to reduce stress of the diodes in the converter cell modules during energizing. The converter structure 8, 12 according to the invention, as shown in figures 7-9, comprises X converter cell modules 10 accommodated in a housing A comprising a housing wall 13, wherein the housing wall 13 is provided with at least one through-going contact means 9 enabling series connection of converter cell modules 10 of the converter structure inside the housing A to Y series connected converter cell modules 10 accommodated in one or more housings B, C... Thus, a VSC with the converter structure according to the invention may first be installed for a certain capacity, and when the demand for power increases, this demand may easily be met by connecting more housings B, C etc. with Y converter cell modules 10 to an already installed housing A with X converter cell modules 10. According to one embodiment, the converter structure comprises at least two series connected housings A, B.... with series connected converter cell modules 10. Each phase of the modular VSC, which consists of a number of converter cell modules connected in series, may thus be mounted in one or several housings as displayed in figure 8-9. The modular VSC may comprise one or several phases. The through-going contact means is e.g. cables for power transfer, and it is understood that the other housings B, C... also have this kind of through going contact means 9 through their housing wall 13, respectively. The housings A, B... may also have through-going means in the housing wall 13 for cooling facilities etc.

A STATCOM with a modular VSC which makes use of the converter structure according to the invention is hereinafter named "scalable STATCOM". A scaleable STATCOM which has expanded its installed voltage level is hereinafter named "expanded STATCOM".

The size of the housings A, B etc. may be standardized according to certain requirements, for example the maximum size of a container which is intended to be conveyed by shipping, truck transport or train. The invention makes it possible to make a prefabricated building component for a modular VSC, which makes installation procedures easier and minimizes costs.

It is thus possible to scale up the effect of an already installed modular VSC that comprise one or more phases Ll, L2, L3, where each of the phases comprising converter cell modules 10 connected in series to each other. The method of scaling up the modular VSC includes: adding Y converter cell modules 10 accommodated in at least one housing B, C... to a converter structure 8 of the modular VSC, and adapting other components of the modular VSC, such as transformer and insulation level, to the greater effect of the expanded modular VSC.

According to one embodiment, the housings A, B... are pileable on each other. Thus, the footprint of an expanded modular VSC may not be larger than the first installed modular VSC. In urban environments, lack of space may be a problem which may be relieved with the present invention.

The loop inductance of the prior art VSC shown in figures 1-3 should be small in order to limit over voltages and to be able to switch as fast as possible in order to decrease the losses. Therefore, the prior art VSC together with the DC capacitor 5 should be built in a compact way to reduce the commutation inductance, i.e., loop inductance. In a modular VSC according to the invention, it is important to keep loop inductance low inside the converter cell module 10. However, between the cell modules 10, the size of the inductance is not so critical. Therefore, the X converter cell modules 10 in one housing A may be connected to Y distant placed converter cell modules 10 in one or several housings B, C etc.

According to one embodiment, the modular VSC comprises three converter structures connected in a Y-configuration. Here, a converter structure comprises a phase. When a higher power is needed, the phases may according to one embodiment be connected in Δ and, therefore, more converter cell modules 10 must be connected in the phases since the voltage level is V3 times higher than for Y-connected phases. According to the invention, this may be accomplished by connecting Y series connected converter cell modules 10 accommodated in one or more housings B, C... Thus, with the same connection voltage level to either the secondary side of the grid connected transformers or when connecting directly to the grid, the power rating is increased V3 times. The connection AC voltage is the same for the original modular VSC and for the modular VSC connected in Δ. Thus, by only making minor changes to an already installed modular VSC, the power level may be increased.

If the grid voltage for instance is 63 kV, the transformer has a secondary voltage equal to around 35 kV. When there is a need to increase the power, additional structures can be added to increase the modular VSC voltage, and thus the transformer could be removed and the scalable STATCOM connected directly to the grid.

If the grid voltage is very high, for instance 220 kV, the converter structure may according to one embodiment be connected to the transmission line via an adjustable transformer comprising a divided secondary winding. This adjustable transformer is illustrated in figures 10 and 11. There is preferably one adjustable transformer for each phase of the modular VSC. In one embodiment, the divided secondary windings in the adjustable transformer are connected in parallel. This embodiment is useful if it is desired to scale down the grid voltage from the transmission line. If the scaleable STATCOM is expanded, the divided secondary windings in the adjustable transformer may be connected in series. Note that the transformers must manage full power for the expanded STATCOM.

The site layout of the scaleable STATCOM may thus look as shown in figure 8. As shown in figure 8, three converter structures 8 accommodated in housings A are used for the three phases and a fourth housing 14 may according to one embodiment consists of additional equipment belonging to the modular VSC, such as valve cooling system, auxiliary system and control system. The additional equipment may thus be accommodated in more than one housing. To each phase a phase reactor is connected and moreover, a small filter is mounted that will reduce the amount of harmonics. These components of the scaleable STATCOM are schematically denoted 11 (also in figure 9). The passive filter may probably be removed since the output voltage of the modular VSC is almost sinusoidal. Furthermore, a transformer is connected between the grid and the scaleable STATCOM.

Assumed that the insulation level is high enough for the modular VSC shown in figure 8, i.e., for instance that the air distance in the housings is large enough to be able to handle a higher voltage, another housing B could be added and the converter cell modules 10 in each housing connected to create a phase that manage twice the voltage. The number Y of converter cell modules 10 in the housing B is in this case the same as the number X converter cell modules X in housing A. Thus, the following layout of the site is obtained, as shown in figure 9. The expanded STATCOM will have twice the power with the same footprint as the original layout. The expanded system has twice the number of converter cell modules as in the original case and therefore, the harmonics will be lowered compared with the original case and the filter requirements can probably be relaxed.

According to one embodiment, the insulation level of the structures in the housings A, B, C... is adapted to the final voltage level of the modular VSC in the expanded STATCOM. For example may the air distance from equipment inside the housings 8, 12 to the housing wall 13 be large enough to handle the final voltage level. The material the housings 8, 12 are made of, may also have insulating capabilities.

By using different power levels for the adjustable transformer connecting the grid and the Y- or Δ-connected phases, different power levels will be obtained for the first installation and for the expanded installations. Note that the insulation level for the expanded installations must be taken into consideration when designing the first installation.

The present invention is not limited to the above-described preferred embodiments.

Various alternatives, modifications and equivalents may be used. Therefore, the above embodiments should not be taken as limiting the scope of the invention, which is defined by the appending claims.

Claims

1. A converter structure (8, 12) for a modular voltage source converter (VSC) comprising converter cell modules (10) connected in series, c h a r a c t e r i z e d i n that said converter structure (8, 12) comprises X converter cell modules (10) accommodated in a housing A comprising a housing wall (13), wherein said housing wall (13) is provided with at least one through-going contact means (9) enabling series connection of converter cell modules (10) of said converter structure inside said housing A to Y series connected converter cell modules (10) accommodated in one or more housings B, C...

2. Converter structure according to claim 1, wherein said converter structure comprises at least two series connected housings (A, B....) with series connected converter cell modules 10.

3. Converter structure according to any of claims 1 or 2, wherein said housings

(A, B...) are pileable on each other.

4. A modular voltage source converter (VSC) comprising one or more converter structures according to any of the preceding claims.

5. Modular voltage source converter according to claim 4, comprising three converter structures connected in a Δ-confϊguration.

6. Modular voltage source converter according to claim 4, comprising three converter structures connected in a Y-configuration

7. Modular voltage source converter according to any of the claims 4 to 6, wherein the structures are connected to a transmission line via an adjustable transformer comprising a divided secondary winding.

8. Modular voltage source converter according to claim 7, wherein the divided secondary windings in the adjustable transformer are connected in parallel.

9. Modular voltage source converter according to claim 7, wherein the divided secondary windings in the adjustable transformer are connected in series.

10. Modular voltage source converter according to any of the claims 4 to 9, wherein additional equipment belonging to the modular VSC, such as valve cooling system, auxiliary system and control system, is accommodated in at least one separate housing.

11. Modular voltage source converter according to any of the claims 4 to 10, wherein an insulation level of the structures in the housings (A, B, C.) is adapted to a final total power level of the modular VSC.