WO2011003435A1

WO2011003435A1 - An arrangement and a method for monitoring the status of dc-side reactors in an hvdc transmission

Info

Publication number: WO2011003435A1
Application number: PCT/EP2009/058474
Authority: WO
Inventors: Fredrik Jansson; Krister Nyberg; Lars-Erik Juhlin
Original assignee: Abb Technology Ag
Priority date: 2009-07-06
Filing date: 2009-07-06
Publication date: 2011-01-13

Abstract

An arrangement for monitoring the status of two smoothing reactors (7, 8) in a back-to-back-station having line-commutated thyristor converters (4, 5) and a DC-link (6) interconnecting the converters and provided with smoothing reactors (7, 8) and a grounded mid point (9), comprises means (10, 11) configured to measure the voltages across the two reactors and means (12-15) configured to filter out at least one of the dominating harmonic voltage components resulting from the commutations of said converters on the DC-side thereof from voltage values delivered by said first means for use thereof for determining the status of the smoothing reactors.

Description

AN ARRANGEMENT AND A METHOD FOR MONITORING THE STATUS OF DC-SIDE REACTORS IN AN HVDC TRANSMISSION

TECHNICAL FIELD OF THE INVENTION AND BACKGROUND ART 0 The present invention relates to an arrangement for monitoring the status of two smoothing reactors in a back-to-back-station interconnecting two alternating voltage power networks and having a line-commutated thyristor converter connected to each said network and a DC-link interconnecting said converters and 5 provided with two said smoothing reactors, one for each said converter, and a grounded mid point between said reactors, as well as a method for monitoring the status of such smoothing reactors. 0 Back-to-back-stations of this type are used for interconnecting two alternating voltage power networks, which may carry voltages of different levels and then being connected to said network through transformers. In operation one of the converters will function as rectifier and the other as inverter, and this will 5 change depending upon the direction of the power flow between the two alternating voltage networks.

It is pointed out that the invention is not restricted to any levels of the voltages or currents of such power networks intercon- 0 nected by said station.

The proper function of said smoothing reactors is of importance for avoiding disturbances on said network. However, an even bigger reason to monitor the status of these smoothing reactors 5 is that a degraded such reactor may cause a fire totally de- stroying the reactor and possibly also associated equipment if not appropriate actions are taken in due time.

Such a reactor degradation will cause a reduction of the imped- ance of the reactor, i.e. a reduction of the inductance of the reactor. However, such an inductance reduction may not be detected by measuring the DC-current flowing through the reactors. SUMMARY OF THE INVENTION

The object of the present invention is to provide an arrangement of the type defined in the introduction making it possible to determine occurrence of a degradation of a said smoothing reac- tor.

This object is according to the invention obtained by providing such an arrangement with:

• first means configured to measure the voltages across said two reactors,

• second means config ured to filter out at least one of the dominating harmonic voltage components resulting from the commutations of said converters on the DC-side thereof from voltage values delivered by said first means,

• third means configured to carry out a comparison the voltage level of said at least one dominating harmonic voltage component belonging to the voltage across one of said reactors with the voltage level of the corresponding at least one dominating harmonic voltage component belonging to the voltage across the other of said reactors, and

• a device configured to determine the status of said smoothing reactors based on the result of said comparison.

Thus, the present inventors have realized that the dominating harmonic voltage components resulting from the commutations of the converters on the DC-side thereof may be used to indi- rectly establish a value of the inductance of one reactor with respect to the inductance of the other reactor and by that determine the status of said smoothing reactors. This means that it will be possible to determine if one of the smoothing reactors is degraded in spite of the impossibility to use the main electric component, the direct current, on the DC-side of the converters, where the smoothing reactors are located.

It may be mentioned that DE 198 29 966 A1 describes an ar- rangement of a similar but other type being arranged to measure voltages across components connected in series and compare these voltages for determining the status of these components. However, it is in that case possible to measure the main current/voltage for making said determination. Accordingly, this document represents the state of the art of unbalanced protection, but the principal thereof is not applicable to monitoring the status of the smoothing reactors in a back-to-back-station to which this disclosure relates. According to an embodiment of the invention said device is configured to determine that the status of one of said smoothing reactors is degraded if said comparison results in a difference of said dominating harmonic voltage component levels exceeding a threshold value . A rel ia ble determination that one of said smoothing reactors is degraded may by this be made early enough for protecting equipment of the station and enabling repair/replacement of the degraded reactor at a minimum of disturbances of the operation of the station. According to another embodiment of the invention said device is configured to use a said threshold value which is 5-25% more preferred 8-18% and most preferred 1 0-15%, especially approximately 10%, of the value of the dominating harmonic component voltage compared having the highest value. It has been found that a threshold value of said difference within these ranges is suitable to use for obtaining a notice of a degraded smoothing reactor early enough for avoiding severe inconveniences.

According to another embodiment of the invention the arrange- ment comprises alarm means configured to be triggered to deliver an alarm signal upon determination by said device that the status of one of said smoothing reactors is degraded. Thus, appropriate measures to addressing the problem with the degraded reactor may then be taken before the degradation will have seri- ous consequences.

According to another embodiment of the invention said third means is configured to carry out a comparison by calculating the ratio U-|/U₂, in which Ui and U₂ are the voltages of said at least one dominating harmonic component of the voltage across a first and a second of said smoothing reactors, respectively, and deliver this ratio value to said device configured to determine the status of the smoothing reactors based on this ratio value. It has been found that basing said determination on a said voltage ratio will be a reliable method of discovering a degradation of one of the smoothing reactors in due time, so that for instance an alarm signal may be delivered as soon as the degradation is disturbing the proper function of the station. According to another embodiment of the invention said third means is configured to form both the ratio U-|/U₂ and U₂/Ui and deliver the values thereof to said device configured to determine the status of the smoothing reactors based on these ratio values. This way of proceeding makes it possible to unambiguously determine which one of the smoothing reactors is the one degraded, since one of these two ratios would under certain circumstances be difficult to use if one of the voltages Ui and U₂ is very low. According to another embodiment of the invention said third means is configured to carry out said comparison by forming the difference Ui - U₂, in which Ui and U₂ are the voltages of said at least one dominating harmonic voltage component of the voltage across a fi rst a n d a secon d of sa i d smooth i n g reactors , respectively, and to deliver the difference value thus formed to said device configured to determine the status of said smoothing reactors based on the value of the voltage difference Ui - U₂. Th is constitutes another possible option to carry out said comparison for determining the status of the smoothing reactors. According to another embodiment of the invention the arrangement comprises fourth means configured to measure the overlap angle of current and voltage of said converters, and said third mea n s i s co nfi g u red to se l ect th e d o m i n ati n g h a rmon i c component of said voltages across the smoothing reactors to be su bjected to said comparison based upon the value of said overlap angle. This makes it possible to utilize the dominating harmonic voltage component being the strongest one for the overlap angle presently used increasing the reliability of the status determination.

According to another embodiment of the invention the arrangement is configured to monitor the status of said two smoothing reactors in a back-to-back-station having 12-pulse converters, and said second means is configured to filter out the 12^th and/or 24^th harmonic voltage component for said comparison . It has been found that these two harmonic voltage components are dominating and most suited to be used for obtaining proper monitoring of the status of said smoothing reactors in the case of 12-pulse converters in said station.

According to another embodiment of the invention the arrangement is configured to monitor the status of said two smoothing reactors in a back-to-back-station having 6-pulse converters, and said second means is configured to filter out the 6^th and/or 12^th harmonic voltage component for said comparison . It has been found that these two harmonic voltage components are dominating and most suited to be used for obtaining proper monitoring of the status of said smoothing reactors in the case of 6-pulse converters in said station. The invention also relates to a method for monitoring the status of two smoothing reactors in a back-to-back-station according to the appended independent method claim. The features and advantages of this method and the methods according to embodiments of the invention defined in dependent claims appear clearly from the above discussion of the arrangement according to the invention.

Furthermore, the invention relates to a back-to-back-station provided with an arrangement for monitoring the status of said two smoothing reactors thereof according to the present invention, in which said station according to another embodiment of the invention is configured to interconnect alternating voltage power networks carrying a voltage exceeding 50 kV, preferably exceeding 100 kV. Accordingly, the present invention relates to a use of an arrangement for monitoring the status of two smoothing reactors in a back-to-back-station of the type defined above.

Further advantages as well as advantageous features of the invention will appear from the following description.

BRIEF DESCRIPTION OF THE DRAWINGS

With reference to the appended drawings, below follows a specific description of embodiments of the invention cited as exam- pies.

In the drawings:

Fig 1 is a very schematic view illustrating a combination of arrangements according to different embodiments of the invention, Fig 2 illustrates a 12-pulse thyristor converter typically used in back-to-back-stations to which the present invention applies,

Fig 3 is a graph of the two dominating harmonic voltage components resulting on the DC-side of a converter shown in Fig 2 versus the overlap angle of current and voltage,

Figs 4

and 5 are graphs of alarm versus signal level for an additive noise and a subtractive noise, respectively, for the case of using a voltage ratio for determining degradation of a smoothing reactor, and

Figs 6

and 7 are graphs corresponding to Figs 4 and 5, respectively, for the case of using a voltage difference for determin- ing degradation of a smoothing reactor.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION Fig 1 illustrates very schematically a back-to-back-station 1 interconnecting two alternati ng voltage power networks 2 , 3 , which may carry voltages of different levels, such as for instance 240 kV on the network 2 and 315 kV on the network 3, which means that transformers not shown are arranged at the interconnections of these networks to the station . The station h a s two l i n e-commutated thyristor converters 4, 5, which typically may have the appearance of a 12-pulse thyristor converter as shown in Fig 2. A DC-link 6 interconnects the two converters and is provided with two smoothing reactors 7, 8, one for each said converter, and a grounded mid point 9 between these reactors. It is of importance to monitor the status of the two smoothing reactors 7, 8 for the reasons thoroughly explained further above. An arrangement for taking care of this and in accordance with the present invention will now be described.

This arrangement comprises first means 10, 1 1 configured to measure the voltages across the two reactors 7, 8. These voltage levels are delivered to second means 12-15, in which the means 12 and 13 are configured to filter out the 12^th harmonic voltage component from the respective voltage measured and 14 and 15 to filter out the 24^th harmonic voltage component from said voltage.

Third means are configured to carry out a comparison of the voltage level of said harmonic voltage components belonging to voltages across one of the reactors with the voltage level of the corresponding harmonic voltage component belonging to the voltage across the other of the reactors, and it is here shown how for the 12^th harmonic voltage component these third means may be either a means 16 configured to carry out a comparison by calculating the ratio U-|/U₂, in which Ui and U₂ are the voltages of said harmon ic voltage components of the voltages across the smoothing reactor 7 and 8, respectively. The ratio value thus obtained is then delivered to a device 17 configured to determine the status of the smoothing reactors based on this ratio value. Said third means 16 may then be configured to form both the ratio U-|/U₂ and U₂/Ui , which enables the device 1 7 to determine which one of the smoothing reactors is degraded irrespectively of a very low voltage signal coming from that reactor.

A corresponding third means 16' and device 17' are illustrated for evaluating the 24^th harmonic voltage components, but it is emphasized that the function of the means 16 and 16' as well as the function of the devices 1 7 and 17' may be taken care of by one single said means and device. Furthermore, it is for the 12^th harmonic voltage component illustrated an alternative to said third means at 16" configured to carry out said comparison by forming the difference Ui - U₂ and deliver this difference value to said device 17 configured to de- termine the status of the smoothing reactors based upon this value.

Said device 17, 17' is configured to determine that the status of one of said smoothing reactors is degraded if said comparison results in a difference of the harmonic voltage component levels exceeding a threshold value, in which this threshold value may be set to be 5-25%, more preferred 8-18% and most preferred 10-15%, especially approximately 10% of the value of the dominating harmonic voltage component compared to the highest value of the two components.

The arrangement also comprises alarm means 18 configured to be triggered to deliver an alarm signal upon determination by said device 17, 17' that the status of one of said smoothing re- actors is degraded.

Fig 2 illustrates schematically the structure of a so-called 12- pulse thyristor converter 4 typically used in a back-to-back-station of this type and the commutations of which results in domi- nating harmonic voltage components on the DC-side 6' thereof in the form of 12^th and 24^th harmonic voltage components. The voltage of these components versus said overlap angle OA is shown in Fig 3 for a firing angle of said thyristors of 17.5°. The solid lin e i s th e 1 2^th harmonic voltage component and the dashed line the 24^th harmonic voltage component. It is shown that the signal from the 12^th harmonic voltage component is for most overlap angles the strongest one, but for an overlap angle in the region of 15° it may be more suited to use the 24^th harmon i c voltag e com pon ent for ch ecki n g th e statu s of the smoothing reactors. The arrangement according to the invention may for this sake include fourth means 19 very schematically illustrated in Fig 1 configured to measure the overlap angle of current and voltage of the converters and deliver information thereabout to said third means configured to select the harmonic voltage component of the voltages across the smoothing reactor to be subjected to said comparison based upon the value of said overlap angle.

Fig 4 illustrates alarm A versus signal level S for an alarm level B set to correspond to an inductance reduction of 10% of one of the smoothing reactors compared to the inductance of the other smooth i n g reactor for th e case of u si n g the voltag e rati o approach for said comparison. The dashed line N illustrates an additive noise. The line 21 corresponds to the case that the two smoothing reactors have the same inductance, and the line 22 corresponds to the case of a difference of said inductances below said threshold value, but the line 23 corresponds to said difference being above 10%. Thus, it is important to require a minimum signal level for triggering the alarm, since this would otherwise be triggered for conditions corresponding to the lines 21 and 22, in which the status of the smoothing reactors would be acceptable.

Fig 5 illustrates the appearance of the same lines as shown in Fig 4 when the assumed noise N is instead subtractive. It ap- pears that also in this case it is necessary to require a minimum signal level for triggering the alarm.

Figs 6 and 7 are graphs corresponding to Figs 4 and 5, respectively, for the case of using the method of forming a voltage dif- ference instead. The lines 21 and 22 as well as 22' correspond to differences in inductance level of the two smoothing reactors being below 10% and thus acceptable, whereas the lines 23-25 correspond to unacceptable such levels. It is shown that it is necessary to require a certain signal level for triggering the alarm for avoiding false alarm. The invention is of course not in any way restricted to the embodiments described above, but many possibilities to modifications thereof will be apparent to a person skilled in the art without departing from the scope of the invention as defined in the appended claims.

It is pointed out that the arrangement may be configured to monitor the status of smoothing reactors in a back-to-back-station having other types of line-commutated thyristor converters, such as 6-pulse converters.

Claims

1 . An arrangement for monitoring the status of two smoothing reactors (7, 8) in a back-to-back-station (1 ) interconnecting two alternating voltage power networks (2, 3) and having a line- commutated thyristor converter (4 , 5) connected to each said network and a DC-link (6) interconnecting said converters and provided with two said smoothing reactors, one for each said converter, and a grounded mid point (9) between said reactors, characterized in that said arrangement comprises:

• first means ( 1 0, 1 1 ) configured to measure the voltages across said two reactors,

• second means (12-15) configured to filter out at least one of the dominating harmonic voltage components resulting from the commutations of said converters on the DC-side thereof from voltage values delivered by said first means,

• third means (16, 16', 16") configured to carry out a comparison the voltage level of said at least one dominating harmonic voltage component belonging to the voltage across one of said reactors with the voltage level of the corresponding at least one dominating harmonic voltage component belonging to the voltage across the other of said reactors, and

• a device (17, 17') configured to determine the status of said smoothing reactors based on the result of said comparison.

2. An arrangement according to claim 1 , characterized in that said device (17, 17') is configured to determine that the status of one of said smoothing reactors (7, 8) is degraded if said comparison results in a difference of said dominating harmonic volt- age component levels exceeding a threshold value.

3. An arrangement according to claim 2, characterized in that said device (17, 17') is configured to use a said threshold value which is 5-25%, more preferred 8-18% and most preferred 10- 15%, especially approximately 10% , of the value of the domi- nating harmonic component voltage compared having the highest value.

4. An arrangement according to claim 2 or 3, characterized in that it comprises alarm means (18) configured to be triggered to deliver an alarm signal upon determination by said device (17, 17') that the status of one of said smoothing reactors is degraded.

5. An arrangement according to any of the preceding claims, characterized in that said third means (16, 16') is configured to carry out a comparison by calculating the ratio Ui/U₂, in which Ui and U₂ are the voltages of said at least one dominating harmonic voltage component of the voltage across a first and a second of said smoothing reactors (7, 8), respectively, and deliver this ratio value to said device (17, 17') configured to determine the status of the smoothing reactors based on this ratio value.

6. An arrangement according to claim 5, characterized in that said third means (16, 16') is configured to form both the ratio U-|/U₂ and U₂/U₁ and deliver the values thereof to said device (17, 17') configured to determine the status of the smoothing reactors based on these ratio values.

7. An arrangement accordi ng to any of claims 1 -4, characterized in that said third means (16") is configured to carry out said comparison by forming the difference Ui - U₂, in which Ui and U₂ are the voltages of said at least one dominating har- monic voltage component of the voltage across a first and a second of said smoothing reactors, respectively, and to deliver the difference value thus formed to said device (17) configured to determine the status of said smoothing reactors based on the value of the voltage difference Ui - U₂.

8. An arrangement according to any of the preceding claims, characterized in that it comprises fourth means (19) configured to measure the overlap angle of current and voltage of said converters, and that said third means (16, 16') is configured to se- lect the dominating harmonic component of said voltages across the smooth i ng reactors to be su bjected to said comparison based upon the value of said overlap angle.

9. An arrangement according to any of the preceding claims, characterized in that it is configured to monitor the status of said two smoothing reactors (7, 8) in a back-to-back-station having 12-pulse converters, and that said second means (12-15) is configured to filter out the 12^th and/or 24^th harmonic voltage component for said comparison.

10. An arrangement according to any of claims 1 -8, characterized in that it is configured to monitor the status of said two smoothing reactors (7, 8) i n a back-to-back-station havi ng 6- pulse converters, and that said second means (12-15) is config- ured to filter out the 6^th and/or 12^th harmonic voltage components for said comparison.

1 1 . A method for monitoring the status of two smoothing reactors (7, 8) in a back-to-back-station (1 ) interconnecting two al- ternating voltage power networks (2, 3) and having a line-com- mutated thyristor converter (4, 5) connected to each said network and a DC-link (6) interconnecting said converters and provided with two said smoothing reactors, one for each said converter, and a grounded mid point (9) between said reactors,

characterized in that it comprises the steps:

a) measuring the voltages across said two reactors,

b) filtering out at least one of the dominating harmonic voltage components resulting from the commutations of said converters on the DC-side thereof from voltage values obtained by said measuring, c) comparing the voltage level of said at least one dominating harmonic voltage component belonging to the voltage across one of said reactors with the voltage level of the corresponding at least one dominating harmonic voltage compo- nent belonging to the voltage across the other of said reactors, and

d) determining the status of said smoothing reactors (7, 8) based on the result of the comparison in step c).

12. A method according to claim 1 1 , characterized in that it is in step d) determined that the status of one of said smoothing reactors is degraded if the comparison in step c) results in a difference of said dominating harmonic voltage component levels exceeding a threshold value.

13. A method according to claim 1 2, characterized in that it comprises a further step e) carried out after step d) of delivering an alarm signal upon determination in step d) that the status of one of said smoothing reactors is degraded.

14. A method according to any of claims 1 1 -13, characterized in that the comparison in step c) is carried out by calculating the ratio Ui - U₂, in which Ui and U₂ are the voltages of said at least one dominating harmonic voltage component of the voltage across a first and a second of said smoothing reactors, respectively, and that the determination of the status of the smoothing reactors in step d) is carried out based on this ratio value.

15. A back-to-back-station interconnecting two alternating volt- age power networks (2, 3) and having a line-commutated thy- ristor converter (4, 5) connected to each said network and a DC- link (6) interconnecting said converters and provided with two smoothing reactors (7, 8) , one for each said converter, and a grounded mid point (9) between said reactors, characterized in that said station is provided with an arrangement for monitoring the status of said two smoothing reactors thereof according to any of claims 1 -10.

16. A back-to-back-station according to claim 15, characterized in that it is configured to interconnect alternating voltage power networks (2, 3) carrying a voltage exceeding 50 kV, preferably exceeding 100 kV.