WO2011023238A1 - A method and a device for overvoltage protection, and an electric system with such a device - Google Patents

A method and a device for overvoltage protection, and an electric system with such a device Download PDF

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Publication number
WO2011023238A1
WO2011023238A1 PCT/EP2009/061214 EP2009061214W WO2011023238A1 WO 2011023238 A1 WO2011023238 A1 WO 2011023238A1 EP 2009061214 W EP2009061214 W EP 2009061214W WO 2011023238 A1 WO2011023238 A1 WO 2011023238A1
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WO
WIPO (PCT)
Prior art keywords
electric system
surge
switching means
overvoltage
surge arrester
Prior art date
Application number
PCT/EP2009/061214
Other languages
French (fr)
Inventor
Jose Nunes
Jonas Karlsson
Original Assignee
Abb Technology Ag
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Abb Technology Ag filed Critical Abb Technology Ag
Priority to EP09782404.9A priority Critical patent/EP2474086B1/en
Priority to US13/392,913 priority patent/US8643995B2/en
Priority to PCT/EP2009/061214 priority patent/WO2011023238A1/en
Priority to CN200980161130.0A priority patent/CN102484419B/en
Priority to KR1020127005030A priority patent/KR101374968B1/en
Publication of WO2011023238A1 publication Critical patent/WO2011023238A1/en

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Classifications

    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
    • H02M1/00—Details of apparatus for conversion
    • H02M1/32—Means for protecting converters other than automatic disconnection

Definitions

  • the present invention relates to a method for protecting an electric system against overvoltage occurrences, the electric system being adapted to be subjected to voltages. Further, the present invention relates to a device for protecting an electric system against overvoltage occurrences, the electric system being adapted to be subjected to voltages, which device comprises a plurality of surge arresters and comprises detection means for detecting overvoltage occurrences in the electric system. The present invention also relates to an electric system comprising a device as mentioned above.
  • VSC Voltage Source Converter
  • HVDC High Voltage Direct Current
  • An electric system including a wind power park or wind farm is an electric system where overvoltages can be especially adverse.
  • the wind turbines of an offshore wind farm are connected to a local AC bus which is connected to a first VSC-converter, a second VSC-converter is connected to an onshore AC grid, and a DC link for transmitting the power generated by the offshore wind farm to the onshore AC grid is connected between the first and the second VSC-converter.
  • a 3-phase fault in the AC-system on the inverter side which in- eludes said second VSC-converter and the AC grid, will cause high overvoltages on the rectifier side, which includes said first VSC-converter and the DC link, i.e.
  • WO 2008/131799 A1 discloses a number of solutions to protect an electric system against overvoltages, the electric system including a wind farm and a DC link for the transmission of power from the wind farm to an AC grid, where the DC link at each end is connected to a VSC-converter.
  • a DC chopper which is composed of a switchable resistor, is connected to the DC link.
  • the AC voltage in the local AC bus, to which the wind farm is connected is modified by controlling the VSC-converter on the rectifier side.
  • WO 02/50972 A1 discloses a VSC-converter for converting direct voltage into alternating voltage and vice versa, in which at least one of the current valves of the VSC-converter is provided with a circuit for overvoltage protection connected in parallel with the current valve, which circuit comprises a series connection of a surge arrester.
  • the object of the present invention is thus to provide an improved overvoltage protection for an electric system.
  • the above-mentioned object of the present invention is attained by providing a device for protecting an electric system against overvoltage occurrences, the electric system being adapted to be subjected to voltages, which device comprises a plurality of surge arresters and comprises detection means for detecting overvoltage occurrences in the electric system, wherein the surge arresters of said plu- rality of surge arresters are connected in series, said plurality of surge arresters comprises a first surge arrester which is connectable to ground and a second surge arrester which is connectable to the electric system which is to be protected, the device comprises a switching means connected in parallel with at least one surge arrester of said plurality of surge arresters, and the switching means is adapted to be open when no overvoltage occurrence is detected and adapted to close upon overvoltage occurrence detection and short-circuit the surge arrester with which it is connected in parallel.
  • the switching means upon detection of overvoltages in the electric system, the switching means will short circuit the surge arrester with which it is connected in parallel, whereby energy in the electric system is "absorbed” and the overvoltages are further reduced to lower levels.
  • the overvoltage protection of the electric system is efficiently improved.
  • the device according to the present invention is applied to the electric system in the above-mentioned example involving the wind farm, the wind turbines do not have to be brought to a standstill if a corresponding fault occurs. Instead, energy is absorbed by the innovative device and the overvoltages are reduced to lower levels.
  • the innovative device can be connected to the system at several positions, and in the detailed de- scription of embodiments, several examples are disclosed.
  • the protective level When having a surge arrester which provides a fixed protective level as disclosed in the prior art, the protective level is limited by the maximum continuous operation voltage and the protective level is relatively high. This increases the wear on the surge arrester.
  • the protective level of the device is "adjusted" to a lower level upon overvoltage detection, and the electric system can continue to operate when overvoltages are detected, and the wear on the surge arrester is also reduced.
  • the other components of the electric system can also be dimensioned to a lower protective level, which provides cost savings.
  • the switching means By the term “open” with regard to the switching means is meant that the switching means is in the open or non-conductive state, in which state the switching means is substantially non-conductive and does not conduct any current, and the switching means can be said to be turned off.
  • the switching means By the term “closed” with regard to the switching means is meant that the switching means is in the closed or con- ductive state, in which state the switching means is conductive and conducts current, and the switching means can be said to be turned on.
  • the switching means is in the form of a fast switch, advantageously with an operate time, or closing time, of less than 2 ms, advantageously approximately 1 ms.
  • suitable switching means known to the per- son skilled in the art can be used for the device according to the present invention, e.g. a semiconductor element of turn-off type, such as an IGBT (Insulated Gate Bipolar Transistor) or a GTO (Gate Turn-Off Thyristor), or a switching device of the type CapThorTM.
  • the detection means can be in the form of a conventional detec- tion unit or equipment known to the skilled person.
  • Each surge arrester, or lightning arrester, per se can be structured in various suitable ways known to the skilled person.
  • the plurality of surge arrester includes two or more surge arresters.
  • the device comprises two surge arrester units.
  • the detection means comprises measuring means for measuring the voltages applied to the electric system.
  • the measuring means can be in the form of a conventional measuring unit or equipment known to the skilled per- son.
  • the overvoltage protection for the electric system is further improved.
  • the device comprises control means for controlling the switching means, the control means being connected to the detection means, and the control means are adapted to control the switching means to close upon detection of overvoltage occurrence.
  • the control means can be in the form of a control unit or control device including suitable processing means, e.g. a CPU.
  • the surge arrester with which the switching means is connected in parallel, has a lower knee-point voltage in relation to the other surge arresters of said plurality of surge arresters.
  • the knee-point voltage of the surge arrester with which the switching means is connected could be of a normalized voltage value of 0.8 p.u. (per unit), and the other surge arrester could have a knee-point voltage of 1.0 p.u.
  • the first surge arrester has a lower knee-point voltage in relation to the second surge arrester. Tests performed by the inventors have shown that this embodiment provides an advantageous overvoltage protection. According to yet another advantageous embodiment of the device according to the present invention, where the first surge arrester has a lower knee-point voltage in relation to the second surge arrester, the first surge arrester has a lower knee-point voltage in relation to the other surge arresters of said plurality of surge arresters. Tests performed by the inventors have shown that this embodiment provides an advantageous overvoltage protection.
  • the switching means is connected in parallel with the first surge arrester.
  • the switching means By connecting the switching means in parallel with the first surge arrester, which is connectable to ground, instead of connecting the switching means in parallel with a surge arrester situated at a higher potential, i.e. closer to the electric system, the switching means requires less insulation.
  • the switching means can be less complicated in its structure and consequently less expensive, whereby the overvoltage protection of the electric system is further improved.
  • the above-mentioned object of the present invention is also attained by providing a method for protecting an electric system against overvoltage occurrences, the electric system being adapted to be subjected to voltages, the method comprising the following steps: connecting a plurality of surge arresters in series; connecting a first surge arrester of said plurality of surge arresters to ground; con- necting a second surge arrester of said plurality of surge arresters to the electric system; detecting overvoltage occurrences in the electric system; and when overvoltage occurrence is detected, short-circuiting at least one surge arrester of said plurality of surge arresters by means of a switching means which is connected in parallel therewith.
  • the overvoltage protection for the electric system is efficiently improved for reasons mentioned above.
  • the method is characterized by keeping the switching means open when no overvoltage occurrence is detected, and by closing the switching means upon overvoltage occurrence detection to effect said short-circuiting of the surge arrester.
  • the method is characterized by short-circuiting the first surge arrester, upon overvoltage occurrence detection, by means of the switching means which is connected in parallel therewith.
  • the step of detecting overvoltage occurrences comprises measuring the voltages applied to the electric system.
  • the present invention provides an electric system adapted to be subjected to voltages, comprising at least one device for protecting the electric system against overvoltage occurrences, wherein said at least one device comprises the features mentioned in any of the claims 1 to 7, or the features mentioned in connection with any of the above-mentioned embodiments of the device according to the present invention.
  • the electric system comprises a line-commutated converter to which the device is connected, and the device is adapted to protect the line-commutated converter against overvoltage occurrences.
  • the device can be directly connected to the line-commutated converter, or indirectly connected to the line-commutated converter, for example via a system component or unit, for example a transformer.
  • the electric system comprises a VSC-converter to which the device is connected, and the device is adapted to protect the VSC- converter against overvoltage occurrences.
  • VSC stands for Voltage Source Converter.
  • the device can be directly connected to the VSC-converter, or indirectly connected to the VSC-converter, for example via a system component or unit, for example a transformer and/or an inductor/reactor.
  • the electric system comprises a power transmission line to which the device is connected, and the device is adapted to protect the power transmission line against overvoltage occurrences.
  • the DC link connected between a rectifier side converter and an inverter side converter is an example of a power transmission line to which the device according to present invention may be connected.
  • the device can be directly connected to the power transmission line, or indirectly connected to the power transmission line, for example via a system component or unit.
  • the device according to the present invention can also be applied to other electric systems, which advantageously are used for power transmission.
  • the electric system is advantageously adapted to be subjected to medium and/or high voltage, e.g. 10-1000 kV.
  • Fig. 1 is a schematic circuit diagram illustrating an embodiment of the device according to the present invention
  • Figs. 2-4 are schematic circuit diagrams illustrating embodiments of the electric system according to the present invention, where the electric system includes two VSC-converters;
  • Figs. 5-6 are schematic circuit diagrams illustrating embodiments of the electric system according to the present invention, where the electric system includes two line-commutated converters; and Fig. 7 is a schematic flow chart illustrating an embodiment of the
  • Fig. 1 illustrates an embodiment of the device 102 for protecting an electric system 104 against overvoltages according to the present invention, the electric system 104 being adapted to be subjected to voltages, for example voltages of 10- 800 kV.
  • the device 102 comprises two surge arresters 106, 108 which are connected in series.
  • a first surge arrester 106 is connectable to ground 110 and a second surge arrester 108 is connectable to the electric system 104 which is to be protected.
  • the first surge arrester 106 has a knee-point volt- age which is lower than that of the second surge arrester 108, for example 0.8 p.u. compared to 1.0 p.u.
  • the device 102 may be provided with conventional connection means for connecting the device 102 to ground 110 and to the electric system 104, respectively.
  • Each surge arrester 106, 108 of the device 102 may be of a conventional type, such as a zinc oxide surge arrester, which is also denominated MOV (Metal Oxide Varistor), and normally conducts a very low current, but when the voltage across the surge arrester 106, 108 exceeds a certain level it will conduct a substantially increased current.
  • MOV Metal Oxide Varistor
  • the device 102 includes detection means 1 12 for detecting overvoltages in the electric system, and the detection means 1 12 may be in the form of a conventional detection unit or equipment known to the skilled person.
  • the detection means 112 comprises measuring means 1 14 connectable to the electric system 104 for measuring the voltages applied to the electric system 104.
  • the measuring means 114 may be in the form of a conventional measuring unit or equipment known to the skilled person.
  • the device 102 comprises a switching means 1 16, in the form of a fast switch, connected in parallel with the first surge arrester 106.
  • the switching means 1 16 is adapted to be open when no overvoltage is detected and adapted to close upon overvoltage detection and short-circuit the first surge arrester 106 with which it is connected in parallel.
  • the switching means 1 16 has an operate time of approximately 1 ms, and the switching means 1 16 can be in the form of a semiconductor element of turn-off type, such as an IGBT (Insulated Gate Bipolar Transistor) or a GTO (Gate Turn-Off Thyristor), or a switching device of the type CapThorTM.
  • IGBT Insulated Gate Bipolar Transistor
  • GTO Gate Turn-Off Thyristor
  • CapThorTM CapThor
  • the device 102 comprises control means 1 18 for controlling the switching means 116, and the control means 1 18 are connected to both the detection means 1 12 and the switching means 1 16.
  • the control means 1 18 are adapted to control the switching means 1 16 to close upon detection of overvoltages, and adapted to control the switching means 1 16 to open and be open when no overvoltages are detected.
  • the control means 1 18 can be in the form of a control unit or control device including suitable processing means, e.g. a CPU.
  • the device 102 may also be provided with more than two surge arresters.
  • Fig. 2 illustrates a first embodiment of the electric system according to the present invention which is adapted to be subjected to voltages, which electric system comprises a wind farm 202 connected to an AC grid 204, as it is known in prior art.
  • the wind farm 202 which can be located offshore, includes a plurality of wind turbines 206 and is coupled to a local AC bus 208, the local AC bus 208 being connected via a first transformer 210 and a first inductor/reactor 212 to a first VSC-converter 214.
  • the first VSC-converter 214 is connected via a DC link 216 to a second VSC-converter 218 and the VSC-converter 218 is connected via a second inductor/reactor 220 and a second transformer 222 to the AC grid 204, which can be located onshore.
  • the first VSC-converter 214 converts AC to DC
  • the second VSC-converter 218 converts DC to AC.
  • the VSC-converters 214, 218 may be of conventional type known to the skilled person, and the function of the VSC- converters 214, 218 is well known to the person skilled in the art and is not more closely described here.
  • the electric system also includes a device 102 as disclosed in Fig. 1.
  • the device 102 is adapted to protect the first VSC- converter 214 against overvoltages.
  • the device 102 according to the present invention is connected to the electric system on the rectifier side at the position A between the local AC bus 208 and the first transformer 210. If an AC fault occurs in the AC grid 204, the rectifier side will be subjected to overvoltages which are detected by the device 102, and in the device 102, the switching means 1 16 will short circuit the surge arrester 106 with which it is connected in parallel, whereby energy in the electric system is "absorbed" and the overvoltages are reduced to lower levels. Hereby, the wind turbines 206 do not have to be brought to a standstill.
  • Fig. 3 illustrates a second embodiment of the electric system according to the present invention which corresponds to the electric system as illustrated in Fig. 2, but where the device 102 as disclosed in Fig. 1 is connected differently.
  • the device 102 according to the present invention is instead connected to the electric system on the rectifier side at the position B between the first transformer 210 and the first inductor/reactor 212.
  • Fig. 4 illustrates a third embodiment of the electric system according to the present invention which corresponds to the electric system as illustrated in Fig. 2, but where the electric system is provided with two devices 102 as disclosed in Fig. 1 which are connected differently.
  • the two devices 102 according to the present invention are connected to the DC link 216 at the position C.
  • the device according to the present invention it also possible to connect the device according to the present invention to the electric system at the position D between the second inductor/reactor 220 and the second transformer 222, or at the position E between the second transformer 222 and the AC grid 204.
  • the connection of the device 102 to the electric system may also be possible.
  • the first, second and third embodiments of the electric system as illustrated in Figs. 2-4 can also be combined in various ways.
  • the first embodiment of Fig. 2 can for example be supplemented with a second device 102 connected as illustrated in Fig. 3, or be supplemented with two devices 102 connected as illus- trated in Fig. 4, or be supplemented with devices 102, one of which being connected as illustrated in Fig. 3, and the others being connected as illustrated in Fig. 4.
  • the second embodiment of Fig. 3 can also be supplemented with devices 102 connected as illustrated in Fig. 4.
  • Fig. 5 illustrates a fourth embodiment of the electric system according to the present invention which is adapted to be subjected to voltages.
  • the electric system of Fig. 5 corresponds to the electric system as illustrated in Fig. 2, but includes two line-commutated converters (LCC) 502, 504 instead of the two VSC- converters 214, 218, and the two inductors/reactors 212, 220 are excluded.
  • the device 102 as disclosed in Fig. 1 is connected to the electric system on the rectifier side at the position A between the local AC bus 208 and the first transformer 210.
  • the LCC-converters 502, 204 may be of conventional type known to the skilled person, and the function of the LCC-converters 502, 204 is well known to the person skilled in the art and is not more closely described here.
  • Fig. 6 illustrates a fifth embodiment of the electric system according to the present invention which corresponds to the electric system as illustrated in Fig. 5, but where the electric system is provided with two devices 102 as disclosed in Fig 1 which are differently connected.
  • the two devices 102 according to the present invention are con- nected to the DC link 206 at the position C.
  • the device according to the present invention it also possible to connect the device according to the present invention to the electric system at the position E between the second transformer 222 and the AC grid 204.
  • other positions for the connection of the device 102 may also be possible.
  • the fourth and fifth embodiments of the electric system as illustrated in Figs. 5-6 can also be combined, i.e. the fourth embodiment of Fig. 5 can be supplemented with devices 102 connected as illustrated in Fig. 6.
  • the device 102 can be connected to electric systems including fewer, or more, VSC-converters or LCC-converters, or to other systems without any VSC-converters or LCC-converters.
  • Fig. 7 shows a schematic flow chart illustrating an embodiment of the method according to the present invention, for protecting an electric system against overvoltages.
  • the method comprising the following steps: two surge arresters are connected in series, wherein a first surge arrester of said two surge arresters is connected to ground, and a second surge arrester of said two surge arresters is connected to the electric system which is to be protected, at step 701.
  • the voltages applied to the electric system are measured, at step 702. Overvolt- age occurrences are monitored and detected, at step 703. If overvoltages are detected, i.e.
  • the first surge arrester is short-circuited by means of switching means which is connected in parallel therewith, by closing the switching means if it is open, at step 704. If no overvoltages are detected, the switching means is opened if closed, at step 705.

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  • Emergency Protection Circuit Devices (AREA)
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Abstract

A method and a device (102) for protecting an electric system (104) against overvoltage occurrences, the electric system (104) being adapted to be subjected to voltages. The device (102) comprises a plurality of surge arresters (106, 108) and comprises detection means (112) for detecting overvoltage occurrences in the electric system (104). The surge arresters (106, 108) of said plurality of surge arresters are connected in series, said plurality of surge arresters comprising a first surge (106) arrester which is connectable to ground (110) and a second surge arrester (108) which is connectable to the electric system (104) which is to be protected. The device (102) comprises a switching means (116) connected in parallel with at least one surge arrester (106, 108) of said plurality of surge arresters, and the switching means (116) is adapted to be open when no overvoltage occurrence is detected and adapted to close upon overvoltage occurrence detection and short-circuit the surge arrester (106, 108) with which it is connected in parallel. An electric system comprising at least one such device (102).

Description

A METHOD AND A DEVICE FOR OVERVOLTAGE PROTECTION, AND AN ELECTRIC SYSTEM WITH SUCH A DEVICE
Technical Field
The present invention relates to a method for protecting an electric system against overvoltage occurrences, the electric system being adapted to be subjected to voltages. Further, the present invention relates to a device for protecting an electric system against overvoltage occurrences, the electric system being adapted to be subjected to voltages, which device comprises a plurality of surge arresters and comprises detection means for detecting overvoltage occurrences in the electric system. The present invention also relates to an electric system comprising a device as mentioned above.
Background of the Invention
There is a need for improved overvoltage protection for electric systems, for example electric systems including VSC-converters (VSC = Voltage Source Converter), electric systems including line-commutated converters, and/or electric systems including power transmission lines, for example, High Voltage Direct Current (HVDC) transmission systems.
An electric system including a wind power park or wind farm is an electric system where overvoltages can be especially adverse. Conventionally, the wind turbines of an offshore wind farm are connected to a local AC bus which is connected to a first VSC-converter, a second VSC-converter is connected to an onshore AC grid, and a DC link for transmitting the power generated by the offshore wind farm to the onshore AC grid is connected between the first and the second VSC-converter. A 3-phase fault in the AC-system on the inverter side, which in- eludes said second VSC-converter and the AC grid, will cause high overvoltages on the rectifier side, which includes said first VSC-converter and the DC link, i.e. said first VSC-converter and the DC link will be subjected to adverse overvoltages. As a consequence, the VSC-converter on the rectifier side may not be able to receive the power produced by the wind turbines, and the wind turbines have to be brought to a standstill, and when the fault is rectified, or repaired, the wind turbines have to be started up again, and this process results in substantial fall in power production. WO 2008/131799 A1 discloses a number of solutions to protect an electric system against overvoltages, the electric system including a wind farm and a DC link for the transmission of power from the wind farm to an AC grid, where the DC link at each end is connected to a VSC-converter. In one solution, a DC chopper, which is composed of a switchable resistor, is connected to the DC link. In another solution, the AC voltage in the local AC bus, to which the wind farm is connected, is modified by controlling the VSC-converter on the rectifier side.
It is known in prior art to use surge arresters to reduce overvoltages in an electric system in order to protect the electric system against overvoltage occur- rences. For example, WO 02/50972 A1 discloses a VSC-converter for converting direct voltage into alternating voltage and vice versa, in which at least one of the current valves of the VSC-converter is provided with a circuit for overvoltage protection connected in parallel with the current valve, which circuit comprises a series connection of a surge arrester.
However, when a surge arrester is used to protect an electric system according to prior art, for example the above-mentioned electric system including the wind farm, the protective level, or the knee-point voltage, of the connected surge arrester is adapted to the electric system and set to a fixed maximum operation voltage of the electric system, and this results in an inflexible overvoltage protec- tion.
The Object of the Invention
The object of the present invention is thus to provide an improved overvoltage protection for an electric system.
Summary of the Invention
The above-mentioned object of the present invention is attained by providing a device for protecting an electric system against overvoltage occurrences, the electric system being adapted to be subjected to voltages, which device comprises a plurality of surge arresters and comprises detection means for detecting overvoltage occurrences in the electric system, wherein the surge arresters of said plu- rality of surge arresters are connected in series, said plurality of surge arresters comprises a first surge arrester which is connectable to ground and a second surge arrester which is connectable to the electric system which is to be protected, the device comprises a switching means connected in parallel with at least one surge arrester of said plurality of surge arresters, and the switching means is adapted to be open when no overvoltage occurrence is detected and adapted to close upon overvoltage occurrence detection and short-circuit the surge arrester with which it is connected in parallel.
According to the present invention, upon detection of overvoltages in the electric system, the switching means will short circuit the surge arrester with which it is connected in parallel, whereby energy in the electric system is "absorbed" and the overvoltages are further reduced to lower levels. Hereby, the overvoltage protection of the electric system is efficiently improved. When the device according to the present invention is applied to the electric system in the above-mentioned example involving the wind farm, the wind turbines do not have to be brought to a standstill if a corresponding fault occurs. Instead, energy is absorbed by the innovative device and the overvoltages are reduced to lower levels. The innovative device can be connected to the system at several positions, and in the detailed de- scription of embodiments, several examples are disclosed.
When having a surge arrester which provides a fixed protective level as disclosed in the prior art, the protective level is limited by the maximum continuous operation voltage and the protective level is relatively high. This increases the wear on the surge arrester. In the solution according to the present invention, the protective level of the device is "adjusted" to a lower level upon overvoltage detection, and the electric system can continue to operate when overvoltages are detected, and the wear on the surge arrester is also reduced. The other components of the electric system can also be dimensioned to a lower protective level, which provides cost savings.
By the term "open" with regard to the switching means is meant that the switching means is in the open or non-conductive state, in which state the switching means is substantially non-conductive and does not conduct any current, and the switching means can be said to be turned off. By the term "closed" with regard to the switching means is meant that the switching means is in the closed or con- ductive state, in which state the switching means is conductive and conducts current, and the switching means can be said to be turned on.
Advantageously, the switching means is in the form of a fast switch, advantageously with an operate time, or closing time, of less than 2 ms, advantageously approximately 1 ms. Several suitable switching means known to the per- son skilled in the art can be used for the device according to the present invention, e.g. a semiconductor element of turn-off type, such as an IGBT (Insulated Gate Bipolar Transistor) or a GTO (Gate Turn-Off Thyristor), or a switching device of the type CapThor™. The detection means can be in the form of a conventional detec- tion unit or equipment known to the skilled person. Each surge arrester, or lightning arrester, per se can be structured in various suitable ways known to the skilled person.
The plurality of surge arrester includes two or more surge arresters.
According to an advantageous embodiment of the device according to the present invention, the device comprises two surge arrester units.
According to an advantageous embodiment of the device according to the present invention, the detection means comprises measuring means for measuring the voltages applied to the electric system. The measuring means can be in the form of a conventional measuring unit or equipment known to the skilled per- son. By this embodiment, the overvoltage protection for the electric system is further improved.
According to a further advantageous embodiment of the device according to the present invention, the device comprises control means for controlling the switching means, the control means being connected to the detection means, and the control means are adapted to control the switching means to close upon detection of overvoltage occurrence. The control means can be in the form of a control unit or control device including suitable processing means, e.g. a CPU. By this embodiment, the overvoltage protection for the electric system is further improved.
According to another advantageous embodiment of the device according to the present invention, the surge arrester, with which the switching means is connected in parallel, has a lower knee-point voltage in relation to the other surge arresters of said plurality of surge arresters. For example, the knee-point voltage of the surge arrester with which the switching means is connected could be of a normalized voltage value of 0.8 p.u. (per unit), and the other surge arrester could have a knee-point voltage of 1.0 p.u.
According to still another advantageous embodiment of the device according to the present invention, the first surge arrester has a lower knee-point voltage in relation to the second surge arrester. Tests performed by the inventors have shown that this embodiment provides an advantageous overvoltage protection. According to yet another advantageous embodiment of the device according to the present invention, where the first surge arrester has a lower knee-point voltage in relation to the second surge arrester, the first surge arrester has a lower knee-point voltage in relation to the other surge arresters of said plurality of surge arresters. Tests performed by the inventors have shown that this embodiment provides an advantageous overvoltage protection.
According to an advantageous embodiment of the device according to the present invention, the switching means is connected in parallel with the first surge arrester. By connecting the switching means in parallel with the first surge arrester, which is connectable to ground, instead of connecting the switching means in parallel with a surge arrester situated at a higher potential, i.e. closer to the electric system, the switching means requires less insulation. By this, the switching means can be less complicated in its structure and consequently less expensive, whereby the overvoltage protection of the electric system is further improved.
The above-mentioned object of the present invention is also attained by providing a method for protecting an electric system against overvoltage occurrences, the electric system being adapted to be subjected to voltages, the method comprising the following steps: connecting a plurality of surge arresters in series; connecting a first surge arrester of said plurality of surge arresters to ground; con- necting a second surge arrester of said plurality of surge arresters to the electric system; detecting overvoltage occurrences in the electric system; and when overvoltage occurrence is detected, short-circuiting at least one surge arrester of said plurality of surge arresters by means of a switching means which is connected in parallel therewith.
By the method according to the present invention, the overvoltage protection for the electric system is efficiently improved for reasons mentioned above.
According to an advantageous embodiment of the method according to the present invention, the method is characterized by keeping the switching means open when no overvoltage occurrence is detected, and by closing the switching means upon overvoltage occurrence detection to effect said short-circuiting of the surge arrester.
According to a further advantageous embodiment of the method according to the present invention, the method is characterized by short-circuiting the first surge arrester, upon overvoltage occurrence detection, by means of the switching means which is connected in parallel therewith.
According to another advantageous embodiment of the method according to the present invention, the step of detecting overvoltage occurrences comprises measuring the voltages applied to the electric system.
The positive technical effects of each of the above-mentioned embodiments of the method according to the present invention correspond to the effects mentioned above in connection with the corresponding embodiment of the device according to the present invention.
Further, the present invention provides an electric system adapted to be subjected to voltages, comprising at least one device for protecting the electric system against overvoltage occurrences, wherein said at least one device comprises the features mentioned in any of the claims 1 to 7, or the features mentioned in connection with any of the above-mentioned embodiments of the device according to the present invention.
According to an advantageous embodiment of the electric system according to the present invention, the electric system comprises a line-commutated converter to which the device is connected, and the device is adapted to protect the line-commutated converter against overvoltage occurrences. The device can be directly connected to the line-commutated converter, or indirectly connected to the line-commutated converter, for example via a system component or unit, for example a transformer.
According to a further advantageous embodiment of the electric system according to the present invention, the electric system comprises a VSC-converter to which the device is connected, and the device is adapted to protect the VSC- converter against overvoltage occurrences. VSC stands for Voltage Source Converter. The device can be directly connected to the VSC-converter, or indirectly connected to the VSC-converter, for example via a system component or unit, for example a transformer and/or an inductor/reactor.
According to another advantageous embodiment of the electric system according to the present invention, the electric system comprises a power transmission line to which the device is connected, and the device is adapted to protect the power transmission line against overvoltage occurrences. For example, the DC link connected between a rectifier side converter and an inverter side converter is an example of a power transmission line to which the device according to present invention may be connected. The device can be directly connected to the power transmission line, or indirectly connected to the power transmission line, for example via a system component or unit.
The device according to the present invention can also be applied to other electric systems, which advantageously are used for power transmission. The electric system is advantageously adapted to be subjected to medium and/or high voltage, e.g. 10-1000 kV.
Further advantageous embodiments of the device, the method and the electric system according to the present invention and further advantages with the present invention emerge from the detailed description of embodiments.
Brief Description of the Drawings
The present invention will now be described, for exemplary purposes, in more detail by way of embodiments and with reference to the enclosed drawings, in which:
Fig. 1 is a schematic circuit diagram illustrating an embodiment of the device according to the present invention;
Figs. 2-4 are schematic circuit diagrams illustrating embodiments of the electric system according to the present invention, where the electric system includes two VSC-converters;
Figs. 5-6 are schematic circuit diagrams illustrating embodiments of the electric system according to the present invention, where the electric system includes two line-commutated converters; and Fig. 7 is a schematic flow chart illustrating an embodiment of the
method according to the present invention.
Detailed Description of Preferred Embodiments
Fig. 1 illustrates an embodiment of the device 102 for protecting an electric system 104 against overvoltages according to the present invention, the electric system 104 being adapted to be subjected to voltages, for example voltages of 10- 800 kV. The device 102 comprises two surge arresters 106, 108 which are connected in series. A first surge arrester 106 is connectable to ground 110 and a second surge arrester 108 is connectable to the electric system 104 which is to be protected. In this embodiment, the first surge arrester 106 has a knee-point volt- age which is lower than that of the second surge arrester 108, for example 0.8 p.u. compared to 1.0 p.u. The device 102 may be provided with conventional connection means for connecting the device 102 to ground 110 and to the electric system 104, respectively. Each surge arrester 106, 108 of the device 102 may be of a conventional type, such as a zinc oxide surge arrester, which is also denominated MOV (Metal Oxide Varistor), and normally conducts a very low current, but when the voltage across the surge arrester 106, 108 exceeds a certain level it will conduct a substantially increased current.
The device 102 includes detection means 1 12 for detecting overvoltages in the electric system, and the detection means 1 12 may be in the form of a conventional detection unit or equipment known to the skilled person. The detection means 112 comprises measuring means 1 14 connectable to the electric system 104 for measuring the voltages applied to the electric system 104. The measuring means 114 may be in the form of a conventional measuring unit or equipment known to the skilled person.
The device 102 comprises a switching means 1 16, in the form of a fast switch, connected in parallel with the first surge arrester 106. The switching means 1 16 is adapted to be open when no overvoltage is detected and adapted to close upon overvoltage detection and short-circuit the first surge arrester 106 with which it is connected in parallel. Advantageously, the switching means 1 16 has an operate time of approximately 1 ms, and the switching means 1 16 can be in the form of a semiconductor element of turn-off type, such as an IGBT (Insulated Gate Bipolar Transistor) or a GTO (Gate Turn-Off Thyristor), or a switching device of the type CapThor™. However, other switching means are possible.
The device 102 comprises control means 1 18 for controlling the switching means 116, and the control means 1 18 are connected to both the detection means 1 12 and the switching means 1 16. The control means 1 18 are adapted to control the switching means 1 16 to close upon detection of overvoltages, and adapted to control the switching means 1 16 to open and be open when no overvoltages are detected. The control means 1 18 can be in the form of a control unit or control device including suitable processing means, e.g. a CPU. The device 102 may also be provided with more than two surge arresters.
Fig. 2 illustrates a first embodiment of the electric system according to the present invention which is adapted to be subjected to voltages, which electric system comprises a wind farm 202 connected to an AC grid 204, as it is known in prior art. The wind farm 202, which can be located offshore, includes a plurality of wind turbines 206 and is coupled to a local AC bus 208, the local AC bus 208 being connected via a first transformer 210 and a first inductor/reactor 212 to a first VSC-converter 214. The first VSC-converter 214 is connected via a DC link 216 to a second VSC-converter 218 and the VSC-converter 218 is connected via a second inductor/reactor 220 and a second transformer 222 to the AC grid 204, which can be located onshore. The first VSC-converter 214 converts AC to DC, and the second VSC-converter 218 converts DC to AC. The VSC-converters 214, 218 may be of conventional type known to the skilled person, and the function of the VSC- converters 214, 218 is well known to the person skilled in the art and is not more closely described here.
The electric system also includes a device 102 as disclosed in Fig. 1. In the electric system of Fig. 2, the device 102 is adapted to protect the first VSC- converter 214 against overvoltages. In the first embodiment of the electric system according to the present invention shown in Fig. 2, the device 102 according to the present invention is connected to the electric system on the rectifier side at the position A between the local AC bus 208 and the first transformer 210. If an AC fault occurs in the AC grid 204, the rectifier side will be subjected to overvoltages which are detected by the device 102, and in the device 102, the switching means 1 16 will short circuit the surge arrester 106 with which it is connected in parallel, whereby energy in the electric system is "absorbed" and the overvoltages are reduced to lower levels. Hereby, the wind turbines 206 do not have to be brought to a standstill.
Fig. 3 illustrates a second embodiment of the electric system according to the present invention which corresponds to the electric system as illustrated in Fig. 2, but where the device 102 as disclosed in Fig. 1 is connected differently. In the second embodiment of the electric system shown in Fig. 3, the device 102 according to the present invention is instead connected to the electric system on the rectifier side at the position B between the first transformer 210 and the first inductor/reactor 212.
Fig. 4 illustrates a third embodiment of the electric system according to the present invention which corresponds to the electric system as illustrated in Fig. 2, but where the electric system is provided with two devices 102 as disclosed in Fig. 1 which are connected differently. In the third embodiment of the electric system shown in Fig. 4, the two devices 102 according to the present invention are connected to the DC link 216 at the position C. However, in some cases, it can be enough to connect only one device 102 to the electric system at the position C.
According to further aspects of the electric system according to the present invention, it also possible to connect the device according to the present invention to the electric system at the position D between the second inductor/reactor 220 and the second transformer 222, or at the position E between the second transformer 222 and the AC grid 204. However, other positions for the connection of the device 102 to the electric system may also be possible.
The first, second and third embodiments of the electric system as illustrated in Figs. 2-4 can also be combined in various ways. The first embodiment of Fig. 2 can for example be supplemented with a second device 102 connected as illustrated in Fig. 3, or be supplemented with two devices 102 connected as illus- trated in Fig. 4, or be supplemented with devices 102, one of which being connected as illustrated in Fig. 3, and the others being connected as illustrated in Fig. 4. The second embodiment of Fig. 3 can also be supplemented with devices 102 connected as illustrated in Fig. 4.
Fig. 5 illustrates a fourth embodiment of the electric system according to the present invention which is adapted to be subjected to voltages. The electric system of Fig. 5 corresponds to the electric system as illustrated in Fig. 2, but includes two line-commutated converters (LCC) 502, 504 instead of the two VSC- converters 214, 218, and the two inductors/reactors 212, 220 are excluded. In the electric system of Fig. 5, the device 102 as disclosed in Fig. 1 is connected to the electric system on the rectifier side at the position A between the local AC bus 208 and the first transformer 210. The LCC-converters 502, 204 may be of conventional type known to the skilled person, and the function of the LCC-converters 502, 204 is well known to the person skilled in the art and is not more closely described here.
Fig. 6 illustrates a fifth embodiment of the electric system according to the present invention which corresponds to the electric system as illustrated in Fig. 5, but where the electric system is provided with two devices 102 as disclosed in Fig 1 which are differently connected. In the fifth embodiment of the electric system shown in Fig. 5, the two devices 102 according to the present invention are con- nected to the DC link 206 at the position C. However, in some cases, it can be enough to connect one device 102 to the electric system at the position C.
According to further aspects of the electric system including two LCC-con- verters 502, 504, it also possible to connect the device according to the present invention to the electric system at the position E between the second transformer 222 and the AC grid 204. However, other positions for the connection of the device 102 may also be possible. The fourth and fifth embodiments of the electric system as illustrated in Figs. 5-6 can also be combined, i.e. the fourth embodiment of Fig. 5 can be supplemented with devices 102 connected as illustrated in Fig. 6.
It is to be understood that further alternative connections of the device 102 to an electric system are possible, and that the device 102 can be connected to electric systems including fewer, or more, VSC-converters or LCC-converters, or to other systems without any VSC-converters or LCC-converters.
Fig. 7 shows a schematic flow chart illustrating an embodiment of the method according to the present invention, for protecting an electric system against overvoltages. The method comprising the following steps: two surge arresters are connected in series, wherein a first surge arrester of said two surge arresters is connected to ground, and a second surge arrester of said two surge arresters is connected to the electric system which is to be protected, at step 701. The voltages applied to the electric system are measured, at step 702. Overvolt- age occurrences are monitored and detected, at step 703. If overvoltages are detected, i.e. if the voltages in the electric system exceed a certain level or threshold, the first surge arrester is short-circuited by means of switching means which is connected in parallel therewith, by closing the switching means if it is open, at step 704. If no overvoltages are detected, the switching means is opened if closed, at step 705.
As mentioned above, by the term "open" with regard to the switching means is meant that the switching means is in the open or non-conductive state, and by the term "closed" is meant that the switching means is in the closed or con- ductive state.
The invention shall not be considered limited to the embodiments illustrated, but can be modified and altered in many ways by one skilled in the art, without departing from the scope of the appended claims.

Claims

1. A device (102) for protecting an electric system (104) against overvoltage occurrences, the electric system (104) being adapted to be subjected to voltages, which device (102) comprises a plurality of surge arresters (106, 108) and comprises detection means (1 12) for detecting overvoltage occurrences in the electric system (104), characterized in that the surge arresters (106, 108) of said plurality of surge arresters are connected in series, in that said plurality of surge arresters comprises a first surge (106) arrester which is connectable to ground (1 10) and a second surge arrester (108) which is connectable to the electric system (104) which is to be protected, in that the device (102) comprises a switching means (1 16) connected in parallel with at least one surge arrester (106, 108) of said plurality of surge arresters, and in that the switching means (1 16) is adapted to be open when no overvoltage occurrence is detected and adapted to close upon overvoltage occurrence detection and short-circuit the surge arrester (106, 108) with which it is connected in parallel.
2. A device according to claim 1 , characterized in that the detection means (1 12) comprises measuring means (114) for measuring the voltages ap- plied to the electric system.
3. A device according to claim 1 or 2, characterized in that the device
(102) comprises control means (1 18) for controlling the switching means (1 16), the control means (1 18) being connected to the detection means (112), and in that the control means (1 18) are adapted to control the switching means (1 16) to close upon detection of overvoltage occurrence.
4. A device according to any of the claims 1 to 3, characterized in that the surge arrester (106, 108), with which the switching means (116) is connected in parallel, has a lower knee-point voltage in relation to the other surge arresters (106, 108) of said plurality of surge arresters.
5. A device according to any of the claims 1 to 4, characterized in that the first surge arrester (106) has a lower knee-point voltage in relation to the second surge arrester (108).
6. A device according to claim 5, characterized in that the first surge arrester (106) has a lower knee-point voltage in relation to the other surge arresters (108) of said plurality of surge arresters.
7. A device according to any of the claims 1 to 6, characterized in that the switching means (1 16) is connected in parallel with the first surge arrester (106).
8. An electric system (104) adapted to be subjected to voltages, comprising at least one device (102) for protecting the electric system (104) against overvolt- age occurrences, characterized in that said at least one device (102) com- prises the features mentioned in any of the claims 1 to 7.
9. An electric system according to claim 8, characterized in that the electric system (104) comprises a line-commutated converter (502, 504) to which the device (102) is connected, and in that the device (102) is adapted to protect the line-commutated converter (502, 504) against overvoltage occurrences.
10. An electric system according to claim 8 or 9, characterized in that the electric system (104) comprises a VSC-converter (214, 216) to which the device (102) is connected, and in that the device (102) is adapted to protect the VSC- converter (214, 216) against overvoltage occurrences.
1 1. An electric system according to any of the claims 8 to 10, characterized in that the electric system (104) comprises a power transmission line (216) to which the device (102) is connected, and in that the device (102) is adapted to protect the power transmission line (216) against overvoltage occurrences.
12. A method for protecting an electric system (104) against overvoltage occurrences, the electric system (104) being adapted to be subjected to voltages, the method comprising the following steps: ■ connecting (701 ) a plurality of surge arresters (106, 108) in series;
■ connecting (701 ) a first surge arrester (106) of said plurality of surge arresters to ground;
■ connecting (701 ) a second surge arrester (108) of said plurality of surge ar- resters to the electric system;
■ detecting (703) overvoltage occurrences in the electric system (104); and
■ when overvoltage occurrence is detected, short-circuiting (704) at least one surge arrester (106, 108) of said plurality of surge arresters by means of a switching means (1 16) which is connected in parallel therewith.
13. A method according to claim 12, characterized by keeping the switching means open (705) when no overvoltage occurrence is detected, and by closing (704) the switching means upon overvoltage occurrence detection to effect said short-circuiting of the surge arrester (106, 108).
14. A method according to claim 12 or 13, characterized by short-circuiting (704) the first surge arrester (106), upon overvoltage occurrence detection, by means of the switching means (116) which is connected in parallel therewith.
15. A method according to any of the claims 12 to 14, characterized in that the step of detecting overvoltage occurrences comprises measuring (702) the voltages applied to the electric system (104).
PCT/EP2009/061214 2009-08-31 2009-08-31 A method and a device for overvoltage protection, and an electric system with such a device WO2011023238A1 (en)

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EP09782404.9A EP2474086B1 (en) 2009-08-31 2009-08-31 A method and a device for overvoltage protection, and an electric system with such a device
US13/392,913 US8643995B2 (en) 2009-08-31 2009-08-31 Method and a device for overvoltage protection, and an electric system with such a device
PCT/EP2009/061214 WO2011023238A1 (en) 2009-08-31 2009-08-31 A method and a device for overvoltage protection, and an electric system with such a device
CN200980161130.0A CN102484419B (en) 2009-08-31 2009-08-31 Over-voltage protection method and equipment and there is the electric power system of equipment like this
KR1020127005030A KR101374968B1 (en) 2009-08-31 2009-08-31 A method and a device for overvoltage protection, and an electric system with such a device

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CN102484419B (en) 2015-12-02
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EP2474086A1 (en) 2012-07-11
KR101374968B1 (en) 2014-03-14
US20120218672A1 (en) 2012-08-30
CN102484419A (en) 2012-05-30
EP2474086B1 (en) 2016-04-20

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