WO2010097122A1

WO2010097122A1 - A modular voltage source converter

Info

Publication number: WO2010097122A1
Application number: PCT/EP2009/052416
Authority: WO
Inventors: Jean-Philippe Hasler; Falah Hosini; Jan R. Svensson
Original assignee: Abb Technology Ltd
Priority date: 2009-02-27
Filing date: 2009-02-27
Publication date: 2010-09-02

Abstract

The present invention relates to a modular voltage source converter (VSC) comprising one or more phases (L1, L2, L3). Each of the phases comprises two or more converter cell modules 16 connected to each other, wherein at least two of the converter cell modules 16 in a phase comprises one inductor 17 each to be used for connection to another converter cell module 16 of that phase. The invention further relates to a converter cell module 16 for a voltage source converter.

Description

Title

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 modular voltage source converter and to a converter cell module for a voltage source converter according to the preambles of the independent claims.

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. 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 feasible. A simple, straightforward and cost-effective way of power quality improvement in such cases is to install equipment especially developed of the purpose in the 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.

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 offer 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 behavior. One example where the compact STATCOM has proven to be very useful is in the steel making industry. An electric arc furnace (EAF) is a piece of equipment needed to make steel products. For the grid owner and for the supplier of electricity, the EAF user is a subscriber to power, i.e. a customer, but in the worst case also a polluter of the grid. Out of the EAF may well come an abundance of distortion such as voltage fluctuations, harmonics and phase asymmetry. Also, the grid may be subject to carrying large amounts of reactive power, which is unintended and gives rise to transmission and distribution losses as well as impedes the flow of useful, active power in the grid. An electric arc furnace is a heavy consumer not only of active power, but also of reactive power. Also, the physical process inside the furnace (electric melting) is erratic in its nature, with one or several electrodes striking electric arcs between furnace and scrap. As a consequence, the consumption especially of reactive power becomes strongly fluctuating in a stochastic manner. The voltage drop caused by reactive power flowing though circuit reactances in the electrodes, electrode arms and furnace transformer therefore becomes fluctuating in an erratic way, as well. This is called voltage flicker and is visualized most clearly in the flickering light of incandescent lamps fed from the polluted grid. The problem with voltage flicker is attacked by making the erratic flow of reactive power through the supply grid down into the furnaces decrease. This is done by measuring the reactive power consumption and generating corresponding amounts in the compact STATCOM and injecting it into the system, thereby decreasing the net reactive power flow to an absolute minimum. As an immediate consequence, voltage flicker is decreased to a minimum, as well.

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 the furnace refractory.

To parry the 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 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. In the compact STATCOM, the VSC uses a switching frequency greater than 1 kHz. The AC voltage across the reactor at full reactive power is only a small fraction of the AC voltage, typically 15%. This makes the compact STATCOM close to an ideal tool for fast reactive power compensation.

For the compact STATCOM, the IGBT has been chosen as the most appropriate power device. IGBT allows connecting in series, thanks to low delay times for turn-on and turn- off. It has low switching losses and can thus be used at high switching frequencies. 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 (GTOs), Integrated Gate Commutated Thyristors (IGCTs), MOSFETs or any self commutated device.

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.

The converter topology for a compact STATCOM may be a two level configuration. 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. One such converter topology is shown in fig. 1.

An alternative to series connection of valve positions to achieve the necessary voltage rating is to connect converter cells in series. In this way smoother AC current and AC voltage waveforms are possible to obtain with lower switching frequency and minimal filtering. One such arrangement is series connection of single phase full-bridge converters, which sometimes are referred to as chain-link cells. A chain-link based converter comprises a number of series-connected cell modules, each cell comprising a capacitor, besides the valves. The DC-capacitor of each such cell module is rather big compared to the above described two-level static compensator, when seen in relation to the total effect of the system.

A chain-link cell module may consist of four IGBT positions and a DC link Capacitor bank as shown schematically in figure 2. Each of the three VSC phases consists of a number of chain-link cells, here shown in series in the general diagram of figure 3 for a delta connected arrangement. The phases can also be connected in an Y-arrangement.

The number of cells in series in each phase is proportional to the AC voltage rating of the system and can, for high AC voltage systems, consequently include a large number of cells.

A line inductor is needed in each phase in series with the converter as shown in figure 3. The size of these high voltage inductors is considerable and there are only few manufactures available on the market which renders the inductors expensive. The features of these inductors are additionally dependent upon the different system AC voltage of the installation.

The single line inductor solution has thus inter alia the following drawbacks:

• High cost

• Large footprint with need for foundations etc. • A concentrated noise generating source

• Needs to be put on electrical insulators

• In some applications the inductor needs forced cooling

• The inductor is constructed with air core which in turn can create interference with other equipment and therefore needs a certain free distance to other equipment Summary of the invention

The object of the present invention is to remove the above stated drawbacks, and to present a new structure for a voltage source converter (VSC) based upon a modular cell topology that is considered for the next generation of compact STATCOMs.

The present invention thus relates to a modular converter cell with distributed line inductor and is based upon the modular nature of the chain-link topology.

The object is achieved by a modular voltage source converter according to the present invention, wherein the converter comprises one or more phases (Ll, L2, L3). Each of the phases comprises two or more converter cell modules connected to each other, wherein at least two of the converter cell modules in a phase comprise one inductor each to be used for connection to another converter cell module of that phase.

The invention further comprises a converter cell module for a voltage source converter comprising one or more phases (Ll, L2, L3). Each of the phases comprises two or more converter cell modules connected to each other, wherein the modules further comprise one inductor each to be used for connection to another converter cell module.

According to the present invention, the modular voltage source converter is provided with a distributed line inductor such that the converter cell modules are connected to each other via an inductor as shown in figure 4.

Thus, the present invention overcomes the above stated problems associated with single line inductors and provides inter alia the following advantages:

• The inductor construction is standardized for the cell module voltage and current.

• The inductor can be manufactured by any typical inductor and transformer supplier. • The noise source is distributed into small units that easily can be controlled and shielded if needed. • The construction allows the use of magnetic cores such as iron powder with distributed air gap leading to lower interference and reducing size.

• The cost of the inductor will be low due to the larger number and standard manufacturing techniques. • No additional insulation is needed for the inductors as they are on the same potential as the converter cell modules.

• To reduce costs further, the inductors may be cooled using the same cooling system as the converter cell modules.

• The inductor is mounted directly in the valve hall as part of the stack of converter cell modules.

• Since standard sizes of inductors are used, a smaller number of such inductors can be stored as spare parts without major cost penalty.

• The present invention additionally simplifies the main component construction for the converter.

Preferred embodiments are set forth in the dependent claims.

Short description of the appended drawings

Figure 1 illustrates a prior art two-level static compensator. Figure 2 illustrates a cell module of a chain-link voltage source converter.

Figure 3 shows a general single line diagram for a delta connected arrangement comprising one reactor and a number of chain-link cell modules in series for each phase.

Figure 4 schematically illustrates an embodiment of the VSC according to the present invention provided with distributed line inductors. Figure 5 schematically illustrates a further embodiment of the VSC according to the present invention provided with distributed line inductors and parallel converter cell modules.

Figure 6 schematically illustrates an embodiment of the VSC according to the present invention with converter cell modules arranged in half-bridge connection. Figure 7 schematically illustrates a further embodiment of the VSC according to the present invention with converter cell modules arranged in half-bridge connection. The present invention will now be described in detail by references to the appended drawings.

Detailed description of preferred embodiments of the invention

Figure 1 illustrates a prior art two-level static compensator 1 without any transformers to step down the power network voltage. The static compensator 1 comprises a VSC 2 connected at its DC side to a capacitor 3 and at its AC-side to a power network 8, also denoted grid.

The conventional two-level VSC 2 comprises three phase-legs Pl, P2, P3 (the phases are denoted Ll, L2, L3 when describing the present invention), each phase-leg consisting of two series-connected valves. The two valves of phase-leg Pl are indicated at reference numerals 9a, 9b. Each valve 9a, 9b in turn comprises a transistor with an anti-parallel diode, or rather, in order to manage high voltages, each valve comprises a number of series-connected transistors, for example IGBTs, each IGBT having an anti-parallel diode.

The VSC 2 is connected to the grid 8, in figure 1 comprising a three phase network, via a phase reactor 4, via an optional starting resistor 5 connected in parallel with a switch 6 and via an AC circuit breaker 7 in each phase. A starting resistor 5 is needed and may be used in series with each converter phase, if the current is too high for the converter. Each phase comprises such phase reactor, starting resistor (if needed), switch and circuit breaker. The respective phases are connected to the middle point of the respective phase-leg Pl, P2, P3, i.e. connected between the respective valves as illustrated in the figure. It is possible to reduce the number of components by equipping (if needed) only two of the phases with the starting resistor connected in parallel with the switch. Only one phase is described in the following in order to simplify the description, but it is understood that the phases are similar.

When the grid-connected VSC 2 is to be energized and started, the circuit breaker 7 is switched so as to provide a current path from the grid 8 through, if needed, the starting resistor 5, the phase reactor 4, and through the diodes of the VSC 2 so as to charge the capacitor 3. When the capacitor voltage has reached a predetermined level, the starting resistor 5 is short-circuited by closing the parallel-connected switch 6. As the starting resistor 5 is short-circuited, the capacitor voltage will increase a bit more and when it is high enough, the valves of the VSC 2 are deblocked and start to switch. The capacitor voltage is then controlled up to its reference value.

The starting resistor 5 is provided in order to protect the diodes of the VSC 2 from being damaged by a too high and/or too long-lasting current surge, which could occur upon closing the AC circuit breaker 7 without the use of the starting resistor 5.

The stress put on the valves, and in particular the diodes, of the VSC 2 depend on several factors, for example the size of the DC-side capacitor 3, the size of the phase reactors 4 and on the voltage levels of the power network 8.

Figure 2 illustrates one converter cell module, also denoted converter link or chain-link cell module, of a modular converter applicable in the present invention. The cell module 10 comprises four valves 11, 12, 13, 14, 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 11, 12, 13, 14 are connected in a full-bridge arrangement with a capacitor unit 15.

When constructing voltage source converters it is of major importance that the inductance is kept as low as possible on the DC-side to reduce voltage transients when switching and to keep down the switching losses. However, on the AC-side inductance is needed to limit the current and filter the current wave form.

The VSC according to the invention comprises one or more phases (Ll , L2, L3) wherein each of the phases comprises two or more converter cell modules 16 connected to each other. The inductance within each cell should be kept low. Advantageously, at least two of the converter cell modules 16 in a phase comprise one inductor 17 each to be used for connection to another converter cell module 16 of that phase. Examples of this embodiment can be seen from figures 4 to 7. Thus, if the inductance is distributed to several inductors 17 intended for lower voltage, it is possible to reduce costs as high voltage inductors are expensive, and also to reduce the total size of the VSC as high voltage inductors are quite large and bulky.

If only one inductor is used for each phase (as shown in figure 3), a large foundation is needed for the inductor as the footprint of the inductor is quite large. A large foundation is of course connected with questions such as where the foundation should be placed, and is not easily adjusted. By sub-dividing the inductance to several converter modules 16 in a phase, the problem of accommodating and locating the inductors is reduced, as the inductor 17 may be mounted directly in the valve hall as a part of the stack of the cell modules. It is also possible to place the modules 16 belonging to each phase distant from each other, in case it is not possible to accommodate the modules 16 close to each other. The modules 16 are interconnected by a bus-bar or by a cable. A bus-bar or a cable will introduce some inductance in between the modules 16. This is thus not a problem, as it is only desired to decrease the inductance inside the modules 16. The sub-dividing of the inductance also entails the positive effect that a latter placed module 16 obtains a protection from disturbances originating from former placed modules 16, if there is an inductor 17 placed in between.

A large inductor is also a source for generation of disturbing noise, and often has to be shielded for reducing the noise. By having inductors 17 of smaller size, the noise source is distributed into small units that can easily be controlled and is more easily shielded if needed.

By having smaller inductors 17 instead of one large inductor for each phase also brings about a reduced need for forced cooling. The distributed inductors 17 may use the same cooling system as the converter cell modules 16, which may further decrease costs. According to one embodiment, all converter cell modules 16 of a phase comprise an inductor 17. Accordingly, by distributing the inductance between the cell modules 16, it is possible to mass-produce cell modules 16 with an inductance 17 standardized for the cell voltage and current and later assemble plurality of the cell modules 16 in a chain-link to obtain desired AC-voltage of the installation. It is accordingly not necessary to customize one large inductor for a specific installation; instead pre-fabricated cell-modules may be used.

The modular voltage source converter may thus be provided with a distributed line inductor so that the converter cell modules 16 are connected to each other via an inductor 17 as shown in the schematic figure 4. In the figure one inductor 17 is arranged between each converter cell module 16 , however, the inductor may be arranged between every second, every third, every fourth etc. converter cell module 16, as long as standard inductors may be used. Other combinations are of course possible, for example may a combination of the mentioned distributions of inductors be possible. Two inductors 17 may for example be distributed between five converter cell modules 16, e.g. one inductor 17 after three cell modules 16 and one inductor 17 after two cell modules 16 etc.

According to one embodiment, the converter cell modules 16 in one phase are made up of two or more parallel connected cells. All converter cell modules 16 in the phase then have to be either made of similar parallel connected cells or have similar current rating, as illustrated in figure 5, here illustrated with two parallel connected cell modules 16 and their adherent inductors 17 in series with two other parallel connected cell modules 16 and their adherent inductors 17. It is of course possible to increase the parallel connected cell modules 16 shown in figure 5 to several more parallel connected cell modules 16, e.g. there maybe three or four etc cell modules 16 with their adherent inductors 17 connected in parallel. This embodiment is advantageous if it is desired to have a high phase current, as it is possible to split the total current in one phase between the parallel connected cell modules 16. An alternative embodiment to connecting whole cell modules 16 in parallel is to connect valves in parallel in the cell modules 16. This will also increase the capability of the chain link configuration to handle high currents. All valves 11, 12, 13, 14 in one module then have to be connected in parallel with an equal number of valves, to get an even flow of the current. It is here possible to connect several valves, e.g. three or four, in parallel, to be able to handle higher currents. Thus, many alternative configurations are possible according to different design requirements. The modules and inductors in the figures are illustrating only some of a number of possible embodiments of the invention, and the cell modules 16 and sub-divided inductors 17 may be arranged in a plurality of different ways.

Preferably, the total inductance of the distributed inductors 17 arranged between converter cell modules 16 in one phase represents the total required inductance of that phase. By dividing the total inductance between several smaller units, the interference of the inductors with other components may be reduced by using magnetic cores such as iron powder with distributed air gap. The use of inductors with magnetic core instead of air core also entails the possibility of having inductors with a reduced size which of course is an advantage.

According to one embodiment, each of the converter cell modules 16 comprises four valves (11, 12, 13, 14) arranged in a full-bridge connection. Examples of such full-bridge connections can be seen from figures 2 to 5. It is thus possible to get plus-, zero- and minus-voltage levels from one module 16. According to another embodiment, each of the converter cell modules 16 comprises two valves 11, 14 arranged in a half-bridge connection. Examples of this embodiment can be seen from figures 6 and 7. Thus, it is possible to get zero- and plus-voltage levels or zero- and minus- voltage levels from the converter cell modules. The converter cell modules 16 comprising two valves 11, 14 arranged in half-bridge connection may also be arranged in parallel in principally the same way as described above with converter cell modules 16 comprising four valves 11, 12, 13, 14.

Advantageously, each valve (11, 12, 13, 14) comprises an insulated gate bipolar transistor (IGBT) with an anti-parallel diode. Consequently, an improved construction of a VSC including the advantages of using IGBT:s is achieved. By having distributed inductors 17 between the cell modules 16 connected in series to form a chain-link, no additional insulation is needed for the inductors 17 as they are on the same potential as the cells.

In one embodiment, the three phases (Ll , L2, L3) are connected in a delta configuration, as can be seen from figure 3. The three phases (Ll, L2, L3) may thus according to another embodiment of the invention be connected in a Y configuration.

The present invention further relates to a converter cell module 16 for a voltage source converter comprising one or more phases (Ll, L2, L3). Each of the phases comprises two or more converter cell modules 16 connected to each other. The module further comprises an inductor 17 to be used for connection to another converter cell module 16. Thus, an improved cell module is achieved that is a stand-alone unit capable of being directly coupled in series or parallel together with other improved cell modules to form a phase without the need of having an additional designed inductor for each phase. Reduced costs and place requirements follow, as the inductor may be manufactured by any conventional inductor and transformer supplier. The cost of the inductor may also be low, due to the larger number of orders of inductors of a specific size, and standard manufacturing techniques.

Preferably, the converter cell module 16 comprises four valves 11, 12, 13, 14 arranged in a full-bridge connection (see figures 2 to 5). According to another embodiment, the converter cell module comprises two valves 11, 14 arranged in a half-bridge connection (see figures 6 and 7). Each valve 11, 12, 13, 14 may comprise an insulated IGBT with an anti-parallel diode.

With the above described embodiments and examples it is beyond no doubt that the present invention may significantly simplify the main component construction of the modular VSC. The modular VSC may be used 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. 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 modular voltage source converter (VSC) comprising one or more phases

(Ll, L2, L3), each of said phases comprising two or more converter cell modules (16) connected in series to each other, c h a r a c t e r i z e d i n that at least two of said converter cell modules (16) in a phase comprises one inductor (17) each to be used for connection to another converter cell module (16) of that phase.

2. A modular voltage source converter according to claim 1 , wherein all converter cell modules (16) of a phase comprise an inductor (17).

3. A modular voltage source converter according to claim 1 , wherein every second converter cell module (16) of a phase comprises an inductor (17).

4. A modular voltage source converter according to claim 1, wherein every fourth converter cell module (16) of a phase comprises an inductor (17).

5. A modular voltage source converter according to any of the preceding claims, wherein the total inductance of the distributed inductors (17) arranged between converter cell modules (16) in one phase represents the total required inductance of that phase.

6. A modular voltage source converter according to any of the preceding claims, wherein each of said converter cell modules (16) comprises four valves (11, 12, 13, 14) arranged in a full-bridge connection.

7. A modular voltage source converter according to any of claims 1 to 5, wherein each of said converter cell modules (16) comprises two valves (11, 14) arranged in a half-bridge connection.

8. A modular voltage source converter according to any of claims 6 or 7, wherein each valve (11, 12, 13, 14) comprises an insulated gate bipolar transistor (IGBT) with an anti-parallel diode.

9. A modular voltage source converter according to any of the preceding claims, wherein said three phases (Ll, L2, L3) are connected in a delta configuration.

10. A modular voltage source converter according to any of claims 1 to 8, wherein said three phases (Ll, L2, L3) are connected in a Y configuration.

11. A converter cell module (16) for a voltage source converter comprising one or more phases (Ll, L2, L3), each of said phases comprising two or more converter cell modules (16) connected in series to each other, c h a r a c t e r i z e d i n that the module (16) further comprises an inductor (17) to be used for connection to another converter cell module (16).

12. A converter cell module (16) according to claim 11, wherein said converter cell module (16) comprises four valves (11, 12, 13, 14) arranged in a full-bridge connection.

13. A converter cell module (16) according to claim 11, wherein said converter cell module (16) comprises two valves (11, 14) arranged in a half-bridge connection.

14. A converter cell module (16) according to any of claims 12 or 13, wherein each valve (11, 12, 13, 14) comprises an insulated gate bipolar transistor (IGBT) with an anti-parallel diode.