WO2012003857A1

WO2012003857A1 - Interface arrangement between ac and dc systems

Info

Publication number: WO2012003857A1
Application number: PCT/EP2010/059565
Authority: WO
Inventors: Staffan Norrga; Anshuman Shukla
Original assignee: Abb Research Ltd
Priority date: 2010-07-05
Filing date: 2010-07-05
Publication date: 2012-01-12

Abstract

The invention concerns an interface arrangement (20) between an AC system (S1) and a DC system (S2). The arrangement comprises a pulse width modulation controlled voltage source converter (28) having a DC side galvanically coupled to a set of poles in the DC system and an AC side galvanically coupled to the AC system, a set of smoothing reactors, each coupled to a corresponding DC pole, at least one filter (32, 34) with filter elements set for removing a frequency component at three times the fundamental frequency of an AC voltage appearing at the AC side of the converter and a control unit (30) controlling the converter using PWM. In the conversion, the control unit injects the frequency component at three times the fundamental frequency and each filter is provided on the DC side of the converter and includes one smoothing reactor in parallel with a capacitor.

Description

INTERFACE ARRANGEMENT BETWEEN AC AND DC SYSTEMS

FIELD OF THE INVENTION The present invention generally relates to power transmission systems. More particularly the present invention relates to an interface arrangement for connection between an AC system and a DC system. BACKGROUND

Interface arrangements are known to be connected between an Alternating Current (AC) system, often denoted AC grid and a Direct Current (DC) system, like a High Voltage Direct Current (HVDC) system. Such an arrangement typically includes a pulse width modulation (PWM) controlled voltage source converter for

conversion between AC and DC and having a DC side coupled to the DC system and an AC side coupled to the AC system. The arrangement also often includes a transformer having a primary side connected to the AC system and a secondary side coupled to the converter.

In order to improve the efficiency of the converter, it is also known to add a zero sequence third harmonic component to the AC voltage using pulse-width

modulation and the converter will also generate the 3^rd harmonic on the AC side. If the amplitude of the harmonic component is 1/6 of the amplitude of the fundamental component, the output voltage range of the converter can be extended to its maximum limit. This increases the efficiency of the converter and is therefore important in power transmission situations. However, this harmonic component has to be separated from the AC system. In a grid-connected converter, where the grid connection is made via a transformer, this third harmonic will cancel if the neutral point on at least one side of the transformer is ungrounded.

However, in order to reduce costs and reduce losses it is sometimes of interest to remove the transformer, i.e. to let the converter be galvanically coupled to the AC system.

If transformers are not used other ways of removal of the third harmonic components are required. Some such other types of removal are for instance described in WO 2004/017505, where a zero-sequence reactor is used between an AC system and a converter, and in WO

2009/149755, where a filter in the form of a parallel LC circuit between the AC system and the converter is used .

Filters in relation to removal of third harmonics without transformers is also described by Vithayathil, Mittlestadt and Bjorklund in "DC Systems with

Transformerless Converters", IEEE Transactions on Power Delivery, Volume 10, No. 3, 1994. This document

discusses the removal of 3^rd harmonic currents in relation to a converter that is a six point bridge. In order to suppress the 3rd harmonic currents the

document discusses blocking these with a filter on the AC side but also on the DC side.

Finally GB 2459764 also discusses placing a harmonics removal filter on the DC side of a converter, where a part of a smoothing reactor is a part of this filter. The only type of converter described is a six-pulse bridge. A transformer is furthermore employed between converter and AC system. The filter is provided for stopping harmonic currents from reaching a DC system.

However, there is still room for improvement in

relation to removal of such added third order harmonics when connecting a voltage source converter to an AC network without a transformer.

SUMMARY OF THE INVENTION

The present invention addresses this situation. The invention is thus directed towards providing an

alternative zero sequence third harmonic removal in the absence of a transformer.

This object is according to the invention achieved through an interface arrangement for connection between an AC system and a DC system and comprising

a pulse width modulation controlled voltage source converter for conversion between AC and DC, the

converter having a DC side galvanically coupled to a set of poles in the DC system and an AC side

galvanically coupled to the AC system,

a set of smoothing reactors, each coupled to a

corresponding DC pole,

at least one filter with filter elements being set for removing a frequency component at three times the fundamental frequency of an AC voltage appearing at the AC side of the converter, and a control unit configured to control the converter using pulse width modulation,

wherein the control unit in controlling the pulse width modulation is configured to inject said frequency component at three times the fundamental frequency in the conversion and each filter that is set for removing frequency components at three times the fundamental frequency is provided on the DC side of the converter and includes one smoothing reactor in parallel with a capacitor.

The expression "coupled" used is intended to cover the possibility of an indirect connection between two elements. There may thus be one or more elements placed between two elements defined as being coupled to each other. The expression "connected" is on the other hand intended to mean a direct connection of two entities to each other without any entity between them. The invention has a number of advantages. The invention also enables removal of zero sequence third harmonics without the use of a transformer and without the need for 3^rd harmonic filters in phase connections between the AC system and the converter. Through providing at least one filter on the DC side of the converter the number of filter elements needed are reduced. As a part of such a filter is made up of a smoothing reactor, which is normally provided anyway, the number of additional elements provided for the filtering are limited even further. The invention therefore also provides a substantial cost saving. Compared to the ac- side blocking filters where at least three capacitors and three inductors are required, only two capacitors and maximum up to two inductors are required in the arrangement of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will in the following be

described with reference being made to the accompanying drawings, where fig. 1 schematically shows a DC system being coupled to an AC system via an interface arrangement according to the invention,

fig. 2 schematically shows the structure of a filter provided in the interface arrangement,

fig. 3 schematically shows a first type of converter that can be used in the interface arrangement,

fig. 4 schematically shows a second type of converter that can be used in the interface arrangement,

fig. 5 schematically shows a third type of converter that can be used in the interface arrangement, and fig. 6 shows an inductor in series with the filter that is used in one variation of the invention.

DETAILED DESCRIPTION OF THE INVENTION

In the following, embodiments of the invention will be described .

The present invention is directed towards providing an arrangement for interfacing a Direct Current (DC) system with an Alternating Current (AC) system, which systems may both be power transmission systems. The DC system can for instance be a High Voltage Direct Current (HVDC) power transmission system and the AC system may be a Flexible Alternating Current

Transmission System (FACTS) . However these types of systems are mere examples of such systems and should not be considered as a requirement. The invention can also be applied in for instance power distribution systems .

Fig. 1 schematically shows the interface arrangement 20 according to a first embodiment of the invention for connection between an AC system SI and a DC system S2. The AC system SI is in the example given in fig. 1 a three-phase AC system and includes three conductors 10, 12 and 14 to which the DC system S2 is coupled. It should be realized that the AC system is not limited to three-phase AC systems. The DC system S2 in turn includes a set of poles, which set here includes two poles 16 and 18. These two poles are coupled to the AC system conductors 10, 12 and 14 via the arrangement 20. In the example in fig. 1 there is a first and a second pole 16 and 18 and therefore the DC system is a bipole system. It should however be realized that the

invention can also be used with a monopole system. It should here furthermore be realized that both the DC and AC system could include a lot more elements than the poles and conductors shown. However, these are not central for the understanding of the present invention and have therefore been omitted. In order to enable the DC system S2 to be coupled to the AC system SI, the arrangement 20 includes a

converter 28 for conversion between AC and DC. The converter 28 may function as a rectifier and/or inverter. The converter 20 is typically a voltage source converter and can be of a number of various types, of which some will be described later on. The converter 28 is furthermore a pulse width modulation (PWM) controlled converter.

The converter 28 therefore has a DC side which is coupled to the set of poles and in this first

embodiment to the first pole 16 via a first filter 32 and to the second pole 18 via a second filter 34. These filters are series connected between the converter and the poles. In this first embodiment of the invention there is one filter per pole. This means that each pole is coupled to the converter via a corresponding filter. As an alternative it is possible that there is only one filter coupled between the converter and the DC system and therefore one filter between one pole and the converter and no filter between the other pole and the converter. The converter also has an AC side for being coupled to the AC system.

The interface arrangement 20 furthermore comprises a set of smoothing reactors, each connected to a

corresponding DC pole. However, in this first

embodiment of the invention they are all part of the filters on the DC side of the converter.

In fig. 1 there are a number of parallel intermediate electrical connections interconnecting the AC side of the converter 28 with the AC system SI. These

intermediate connections are sometimes denoted an AC filter busbar. The intermediate connections are

provided as parallel conductors that are at least two in number and in this case three. The intermediate connections are thus connected to the converter 28 and lead from this converter towards the AC system. In fig.l there is one reactor 22, 24 and 26 in each such intermediate connection. It may here also be mentioned that these reactors may be omitted. These reactors serve as a voltage harmonic attenuator to attenuate the high frequency voltage harmonics that the converter may generate. These reactors may in some instances be mandatory for supporting a difference in instantaneous voltage between the AC system SI and the AC side of the converter to allow a finite power exchange between the converter and this system SI. Since the converter 28 is coupled to the AC system via the intermediate

connections that only include the reactors 22, 24 and

26, it is clear that the AC side of the converter 28 is galvanically coupled to the AC system SI. There is thus no transformer provided between the converter 28 and AC system SI. The interface device 20 is thus transformerless. The DC side is also galvanically coupled to the DC system.

It can here be mentioned that a number of units may be connected to these parallel intermediate connections. Surge arresters, and filters for filtering of high frequency components may for instance be connected between the intermediate connections and ground. There may furthermore be provided circuit breakers in the intermediate connections. However all these elements are not central for the present invention and are therefore omitted in fig. 1. Finally there is a control unit 30 configured to control the converter 28. The control, which is pulse width modulation (PWM) control, is indicated with a dashed arrow in fig. 1.

Fig. 2 shows one configuration of the first filter 32. The second filter may have the same configuration and the same values. Therefore this figure is applicable also for the second filter. The first filter 32 here includes a DC side smoothing reactor 38 with inductance L_d in parallel with a capacitor 36 with capacitance C_d- This parallel circuit is connected in series between the converter and the first pole. The values of the filter elements are selected for providing filtering at three times the fundamental frequency of an AC voltage at the AC side of the converter. This voltage appears on the intermediate connections i.e. at the fundamental frequency provided at the AC side of the converter. The frequency of this voltage is here also the frequency of the AC system.

Here the inductance of the reactor may first be

selected for providing a desired smoothing effect on the DC voltage and then the capacitor value selected to provide the desired filtering.

In order to obtain this filtering the values of the filter elements are chosen according to:

→c_d =

L_dC_d As mentioned earlier, the converter is a voltage source converter and may as such be of a number of different types. It may for instance be a two-level, a three- level or a multi-level converter, where a two-level converter 28A is schematically shown in fig. 3, a three-level converter 28B is schematically shown in fig. 4 and a multi-level converter 28C is schematically shown in fig. 5. Each such converter normally includes a number of phase legs, where there is one phase leg for each phase provided via the intermediate

connections. A converter thus includes at least two and in this case three phase legs. However, in fig. 3 - 5, only one such phase leg is shown. In fig. 3 the control unit 30 is also included.

As can be seen in fig. 3 depicting the two-level converter 28A, a phase leg PL of this converter

includes a number of series connected switching

elements, provided in the form of a transistor with anti-parallel diode. The switching elements are

connected in series between the two poles 16 and 18. In parallel with the phase leg PL there is a capacitor bank CB (here shown including two capacitors) . The midpoint of this capacitor bank CB is grounded while the mid point of the phase leg PL is connected to a first end of a phase reactor LCI having a phase

inductance. The second end of the phase reactor LCI is connected to a corresponding intermediate connection of the arrangement. The switching elements between the phase leg mid point and a pole here together make up a converter valve. There are thus two converter valves CV1 and CV2 in fig. 3. The phase reactor LCI here forms a pole to AC side inductance of the converter for both poles .

In operation the switching elements are controlled, typically by the control unit 30, using PWM for

obtaining an AC voltage at the second end of the phase reactor LCI having the frequency of the voltage at the intermediate connections. This frequency is the same frequency as the frequency of the AC system. The control will be described in more detail shortly.

The three-level converter 28B in fig. 4 resembles the two-level converter and in this example includes a phase leg with a first branch including four switching elements connected in series. The difference between the three- and the two-level converter is that there is a further branch of switching elements, here including two switching elements, connected in parallel with the two switching elements of the first branch provided adjacent and on opposite sides of the phase leg

midpoint. The midpoint of this further branch is furthermore grounded. The switching elements are here controlled by the control unit (not shown) , also using PWM.

The example of a multilevel converter 28C shown in fig. 5 does have a slightly different configuration. Each phase leg is made up of a series connection of cells, where each cell is made up of two series connected switching elements having a capacitor connected in parallel with both these elements. In this example the midpoint between two switching elements of a cell is connected to one end of the capacitor of a following cell. In this way the cells are connected in series between the two poles. In the phase leg, on opposite sides of the phase leg midpoint, there are furthermore provided reactors LCA and LCB . There is also in this example a capacitor bank with grounded midpoint

connected between the two poles. Also this converter is controlled using PWM.

Each cell here provides a zero or a small voltage contribution. The switching elements of the cells are furthermore controlled by the control unit (not shown) so that the voltage at the phase leg midpoint resembles a reference AC voltage. This means that the cells are switched for providing a zero or the small voltage contribution, where the sum of the small voltage contributions of the cells together form an AC voltage resembling the reference AC voltage.

The PWM control of these types of converters is as such not new and known in the art.

However, in order to raise the efficiency of the converter it is also possible to add a zero sequence third harmonic to the AC voltage at converter AC side, i.e. to the AC voltage appearing on the intermediate connections. This third harmonic can be injected by the control unit using PWM, which is often called Third Harmonic Injection Pulse Width Modulation (3PWM) . This third harmonics component is then injected towards the AC system with an amplitude that as an example can be 1/6 of the amplitude of the fundamental component of the AC voltage. These injected harmonic sequences reduces the peak level of the voltage and therefore increases the modulation index with about 15%, which can be used for increasing the efficiency. However, this type of harmonic cannot be allowed to reach the AC system SI, where such harmonic is not used. This means that the zero sequence harmonic has to be removed.

In operation the converter converts the DC voltage to an AC voltage into which third harmonic zero-sequences have been added, which thus raises the efficiency of the converter. These injected third harmonic zero sequences are stopped from reaching the AC system by the first and second filters on the DC side of the converter. Even though these filters are provided on the DC side of the converter they still block the third harmonic zero sequences from reaching the AC system.

Furthermore as these filters employ smoothing reactors, which are inherently used anyway in almost every HVDC installation, these smoothing reactors are put to a dual use which leads to a saving of components.

In comparison, filters on the AC of the converter would require three filters, one for each phase. In relation to this there is thus a reduction in the number of filters required. Since the smoothing reactors are normally provided anyway, this means that the invention can be realized with one or a maximum of two additional capacitors .

If two filters are used, one in each pole, for blocking the third-harmonic, each filter will only receive half the current as compared to the filters required on an AC side, which also allows the capacitor rating to be lowered . It is possible to further vary the invention through placing a further inductor in series with a filter between the converter and a corresponding pole, . This situation is shown in fig. 6. This further inductor is used if it is desirable to prevent higher order

harmonics and also to limit the fault current. There may here be one further inductor provided for each filter .

The control unit may be provided as a computer or a processor with computer program memory including computer program code instructions causing the computer or processor to perform 3PWM control of the voltage source converter. As an alternative it may be realized through hardware .

From the foregoing description of different variations of the present invention, it should be realized that it is only to be limited by the following claims.

Claims

1. Interface arrangement (20) for connection between an AC system (SI) and a DC system (S2) and comprising:

a pulse width modulation controlled voltage source converter (28) for conversion between AC and DC, said converter having a DC side galvanically coupled to a set of poles in the DC system and an AC side galvanically coupled to said AC system, a set of smoothing reactors, each coupled to a corresponding DC pole,

at least one filter (32, 34) with filter elements (L_d, C_d) being set for removing a frequency

component at three times the fundamental frequency of an AC voltage appearing at the AC side of the converter, and

a control unit (30) configured to control the converter using pulse width modulation,

wherein the control unit in controlling the pulse width modulation is configured to inject said frequency component at three times the fundamental frequency in the conversion and each filter that is set for removing frequency components at three times the fundamental frequency is provided on the

DC side of the converter and includes one smoothing reactor (38) in parallel with a capacitor (36) .

2. Arrangement according to claim 1, wherein the capacitor of a filter has a value C_d set according to

C, =—— , where L_d is the inductance of the corresponding smoothing reactor.

3. Arrangement according to any previous claim, wherein each pole is coupled to the converter via a corresponding filter.

4. Arrangement according to any previous claim, wherein each filter that is set for removing frequency components at three times the fundamental frequency is coupled in series between the converter and a

corresponding pole.

5. Arrangement according to any previous claim, wherein the control unit is configured to inject the harmonics component with an amplitude that is 1/6 of the amplitude of the fundamental component of the AC voltage .

6. Arrangement according to any previous claim, further comprising a reactor (22, 24, 26) in each connection between the AC side of the converter and the AC system.

7. Arrangement according to any previous claim, wherein the voltage source converter is a two-level voltage source converter.

8. Arrangement according to any of claims 1 - 6, wherein the voltage source converter is a three-level voltage source converter.

9. Arrangement according to any of claims 1 - 6, wherein the converter is a multilevel voltage source converter .

10. Arrangement according to any previous claim, comprising at least one further inductor (40) in series with a corresponding filter set for removing a

frequency component at three times the fundamental frequency .