HVDC transmission schemes

Abstract

The present invention provides a high voltage direct current (HVDC) transmission system (300, 600) comprising: a first station (102) comprising series-connected first and second HVDC converters (110, 130); a second station (104) comprising series-connected third and fourth HVDC converters (150, 170), wherein a neutral node (164) coupling the third HVDC converter (150) to the fourth HVDC converter (170) is coupled to earth; a first transmission line (200) connecting a positive node (114) of the first HVDC converter (110) to a corresponding positive node (154) of the third HVDC converter (150), wherein a first pole (240) of the system (300, 600) comprises the first HVDC converter (110), the third HVDC converter (150) and the first transmission line (200); a second transmission line (210) connecting a negative node (138) of the second HVDC converter (130) to a corresponding negative node (178) of the fourth HVDC converter (170), wherein a second pole (250) of the system (300, 600) comprises the second HVDC converter (130), the fourth HVDC converter (170) and the second transmission line (210); characterised in that: a neutral node (124) connecting the first HVDC converter (110) to the second HVDC converter (130) is coupled to a parallel combination of a resistance (310) and a neutral bus ground switch (312) for coupling the neutral node (124) to earth.

Claims

1. A high voltage direct current transmission system comprising: a first station comprising first and second HVDC converters coupled in series via a first disconnector and a second disconnector; a second station comprising series-connected third and fourth HVDC converters, wherein a neutral node coupling the third HVDC converter to the fourth HVDC converter is coupled to earth; a first transmission line connecting a positive node of the first HVDC converter to a corresponding positive node of the third HVDC converter, wherein a first pole of the system comprises the first HVDC converter, the third HVDC converter and the first transmission line; a second transmission line connecting a negative node of the second HVDC converter to a corresponding negative node of the fourth HVDC converter, wherein a second pole of the system comprises the second HVDC converter, the fourth HVDC converter and the second transmission line; wherein a neutral node between the first disconnector and the second disconnector is coupled to a parallel combination of a resistance and a neutral bus ground switch for coupling the neutral node to earth.

2. The HVDC transmission system according to claim 1, wherein: the first station comprises a first link coupled at a first end to the first transmission line and coupled at a second end to the second transmission line, the first link comprising series-connected first and second high-speed switches, wherein a third node connecting the first and second high-speed switches is coupled to the neutral node of the first station, and wherein the first and second high-speed switches are selectively operable to couple either the first transmission line or the second transmission line to the neutral node of the first station; the second station comprises a second link coupled at a first end to the first transmission line and coupled at a second end to the second transmission line, the second link comprising third and fourth high-speed switches, wherein a fourth node connecting the third and fourth high-speed switches is coupled to the neutral node of the second station, and wherein the second and third high-speed switches are selectively operable to couple either the first transmission line or the second transmission line to the neutral node of the second station.

3. The HVDC transmission system according to claim 1 wherein the neutral node of the second station is coupled to a surge arrestor for coupling the neutral node to earth.

4. The HVDC transmission system according to claim 1 wherein the first, second, third and fourth HVDC converters comprise line commutated converters.

5. The HVDC transmission system according to claim 1 wherein the first, second, third and fourth HVDC converters comprise voltage source converters.

6. The HVDC transmission system according to claim 1 further comprising a dedicated metallic return coupling the neutral node of the first station to the neutral node of the second station.

7. A method for reconfiguring the system of claim 6 on detection of a fault to ground in one of the first, second, third or fourth HVDC converters, the method comprising: detecting the fault; isolating the HVDC converters of the faulted pole containing the HVDC converter in which the fault occurred from respective AC transmission terminals; and opening the neutral bus ground switch so as to cause leakage current in the system to flow through the resistance.

8. The method according to claim 7, further comprising: coupling the transmission line of the faulted pole to the neutral nodes of the first and second stations, such that the coupled transmission line provides a return path for current in the healthy pole; and isolating the HVDC converter in which the fault occurred from the HVDC converters of the healthy pole and from the coupled transmission line.

9. The method according to claim 8, further comprising: closing the neutral bus ground switch.

10. A method for reconfiguring the system of claim 1 on detection of a fault to ground in one of the first, second, third or fourth HVDC converters, the method comprising: detecting the fault; isolating at least the HVDC converters of the faulted pole containing the HVDC converter in which the fault occurred from respective AC transmission terminals; opening the neutral bus ground switch so as to cause leakage current in the system to flow through the resistance; coupling the transmission line of the faulted pole to the neutral nodes of the first and second stations, such that the coupled transmission line provides a return path for current in the healthy pole in which the fault did not occur; and isolating the HVDC converters of the faulted pole from the HVDC converters of the healthy pole and from the coupled transmission line.

11. The method according to claim 10 further comprising: after isolating the HVDC converters of the faulted pole from the HVDC converters of the healthy pole and from the coupled transmission line, closing the neutral bus ground switch.

12. A HVDC converter station comprising: first and second HVDC converters coupled in series via a first disconnector and a second disconnector, the first HVDC converter comprising a positive node configured to be coupled to a first transmission line, the second HVDC converter comprising a negative node configured to be coupled to a second transmission line, wherein: a neutral node between the first disconnector and the second disconnector is coupled to a parallel combination of a resistance and a neutral bus ground switch for coupling the neutral node to earth.

13. The HVDC converter station according to claim 12, further comprising: a link configured to be coupled at a first end to the first transmission line and configured to be coupled at a second end to the second transmission line, the first link comprising series-connected first and second high-speed switches, wherein a node connecting the first and second high-speed switches is coupled to the neutral node, and wherein the first and second high-speed switches are selectively operable to couple either the first transmission line or the second transmission line to the neutral node of the first station.

14. The HVDC converter station according to claim 12, wherein the first and second HVDC converters comprise line commutated converters.

15. The HVDC converter station according to claim 12, wherein the first and second HVDC converters comprise voltage source converters.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Embodiments of the invention will now be described, strictly by way of example only, with reference to the accompanying drawings, of which:

(2) FIG. 1 is a schematic representation of a bipolar link HVDC transmission system;

(3) FIG. 2 is a schematic representation of the HVDC transmission system of FIG. 1 after reconfiguration to act as a monopolar link HVDC transmission system;

(4) FIG. 3 is a schematic representation of an alternative bipolar link HVDC transmission system;

(5) FIGS. 4a-4d are schematic representations of the HVDC transmission system of FIG. 3 following detection of a fault;

(6) FIG. 5 is a flow diagram illustrating steps in a method for reconfiguring the HVDC transmission system of FIG. 3;

(7) FIG. 6 is a schematic representation of a bipolar link HVDC transmission system with a dedicated metallic return (DMR); and

(8) FIGS. 7a-7d are schematic representations of the HVDC transmission system of FIG. 5 following detection of a fault; and

(9) FIG. 8 is a flow diagram illustrating steps in a method for reconfiguring the HVDC transmission system of FIG. 6.

DETAILED DESCRIPTION

(10) Referring now to FIG. 3, a HVDC transmission system is shown generally at 300. The system 300 shares many common elements with the system 100 described above and illustrated in FIG. 1, so common reference numerals have been used to denote common elements of the systems 100 and 300. For the sake of clarity and brevity only those features of the system 300 that differ from the system 100 will be described here.

(11) The system 300 is similar to the system 100, with the exception that the link 126 of the first station 102 includes first and second high-speed switches 302, 304 in place of the seventh and eighth disconnectors 128, 144 of the first station 102 of the system of FIG. 1. Similarly, the link 166 of the second station 104 includes third and fourth high-speed switches 306, 308 in place of the seventh and eighth disconnectors 168, 184 of the second station 104 of the system of FIG. 1. The high-speed switches 302, 304 can be selective opened and closed to selectively couple the first or second transmission line 200, 210 to the neutral node 124 of the first station 102 in order to reconfigure the system 300, whilst the high-speed switches 306, 308 can be selective opened and closed to selectively couple the first or second transmission line 200, 210 to the neutral node 164 of the second station 104 in order to reconfigure the system 300.

(12) Additionally, the first station 102 includes a resistor 310 of high resistance (of the order of 50-1000Ω, for example) and a neutral bus ground switch (NBGS) 312 connected in parallel between its neutral node 124 and earth. The resistor 310 provides a first, high resistance, path to earth, whilst the NBGS 312, when closed, provides a second, much lower resistance, path to earth than the resistor 310, such that when the NBGS 312 is closed the first path is ineffective. In normal operation of the system 300 the NBGS 312 is closed.

(13) In normal operation the system 300 operates in the same manner as the system 100 described above, with the exception that the first and second high speed switches 302, 304 of the link 126 of the first station 102 (which replace the seventh and eighth disconnectors 128, 144 of the link 126 of the first station 102 of the system 100 of FIG. 1) are open, and the third and fourth high speed switches 306, 308 of the link 166 of the second station 104 (which replace the seventh and eighth disconnectors 168, 184 of the link 166 of the second station 104 of the system 100 of FIG. 1) are also open.

(14) As indicated above, the NBGS 312 is closed in normal operation of the system.

(15) If a single phase fault to ground develops in one of the HVDC converters 110, 130, 150, 170 during operation of the system 300, a number of actions are taken, as will be described below with reference to FIGS. 4a-4d, which are schematic representations of the system 300 of FIG. 3 following detection of a fault to ground in the third HVDC converter 150. It will be appreciated of course that the system 300 operates in the same way if a fault is detected in any of the other HVDC converters 110, 130, 170.

(16) As in the system 100 of FIG. 1, following detection of the fault in the third HVDC converter 150, the first, second, third and fourth AC breakers 112, 132, 152, 172 are opened, in response to commands issued by the first and second controllers 106, 108, to isolate the first, second, third and fourth HVDC converters 110, 130, 150, 170 from their respective AC transmission terminals.

(17) Alternatively, instead of opening all of the AC breakers 112, 132, 152, 172, only the AC breakers related to the faulted pole (in this case the first and third AC breakers 112, 152 of the first pole 240) are opened. The AC breakers related to the healthy pole (in this case the second and fourth AC breakers 132, 172 of the second pole 250) remain closed but the DC power in the healthy pole is ramped down to minimise the DC current flowing in the healthy pole.

(18) With the relevant ones of the first, second, third and fourth HVDC converters 110, 130, 150, 170 isolated in this way stored energy circulates in the system 300. Because of the fault to ground in the third HVDC converter 150, leakage current flows through the earth, second low resistance path provided by the closed NBGS 312, fourth disconnector 122, first HVDC converter 110, first and second disconnectors 116, 118 of first station 102, first transmission line 200, and first and second disconnectors 156, 158 of second station 104, as shown by the dashed arrow in FIG. 4a.

(19) After the relevant ones of the first, second, third and fourth AC breakers 112, 132, 152, 172 have been opened to isolate the relevant ones of the first, second, third and fourth HVDC converters 110, 130, 150, 170 from their respective AC transmission terminals, the NBGS 312 is opened in response to a command issued by the first controller 106, effectively activating the first, high resistance, path to earth containing the resistor 310, as shown in FIG. 4b.

(20) The system 300 is then reconfigured to operate in a monopolar link configuration, by closing the first high-speed switch 302 of the link 126 of the first station 102 and closing the third high-speed switch 306 of the link 166 of the second station 104 in response to commands issued by the first and second controllers 106, 108, so as to couple the first transmission line 200 to the neutral nodes 124, 164 of the first and second stations 102, 104.

(21) The second, fourth, fifth and sixth disconnectors 118, 136, 140, 142 of the first station 102 and the first, fourth, fifth and sixth disconnectors 156, 176, 180, 182 of the second station 104 remain closed, such that the first transmission line 200 can be used as a current return path for the second pole 250, as shown in FIG. 4c.

(22) Once the system 100 has been reconfigured in this way, the second and fourth AC breakers 132, 172 are closed (if they had previously been opened), or the DC power in the healthy pole is ramped up again (if it had previously been ramped down), in response to commands issued by the first and second controllers 106, 108 to re-couple the second and fourth HVDC converters 130, 170 to their respective AC transmission terminals if necessary, and allow current to flow in the system 100 in the direction indicated by the solid arrows in FIG. 4c, with the first transmission line 200 being used as a current return path.

(23) Leakage current arising due to the fault is now able to flow through the earth, first high resistance path provided by resistor 310, fourth disconnector 122, first HVDC converter 110, first and second disconnectors 116, 118 of first station 102, first transmission line 200, and first and second disconnectors 156, 158 of second station 104, as shown by the dashed arrow in FIG. 4c. However, due to the high resistance of the resistor 310 the leakage current that flows is very small.

(24) Because the leakage current flowing in the first disconnector 116 and third disconnector 122 of first station 102 and in the second disconnector 158 and third disconnector 162 of the second station 104 is very small once the NBGS 312 has been opened, the second disconnector 158 of the second station 104 can now be opened, as shown in FIG. 4d, in response to a command issued by the second controller 108, to isolate the faulted third HVDC converter 150 from the first transmission line 200. The third disconnector 162 of the second station is also opened in response to a command issued by the second controller 108, to isolate the faulted third HVDC converter 150 from the fourth HVDC converter 170. The first and third disconnectors 116, 122 of the first station 102 are also opened, in response to a command issued by the first controller 106, to isolate the first HVDC converter 110 from the first transmission line 200 and the second HVDC converter 130. With the second and third disconnectors 158, 162 of the second station 104 and the first and third disconnectors 116, 122 open, no leakage current flows and the NBGS 312 can revert to its normal closed state, in response to a command issued by the first controller 106.

(25) In the system 300 the healthy pole (the second pole 250 in the example described above) is available for use in the monopole configuration of FIG. 4b very quickly after detection of a fault. This is because fewer disconnectors are required to be opened before the healthy pole 250 is available for use, since leakage current in the system 300 is greatly reduced by the resistor 310 and thus can safely be handled by the second disconnector 158 of the second station 104 after the system 300 has been reconfigured to operate in the monopole configuration, meaning that the first disconnector 116 and third disconnector 122 of first station 102 and the second disconnector 158 and third disconnector 162 of the second station 104 can be opened even while the healthy pole 250 is conducting. Thus the process of bringing the healthy pole back into service is not unduly limited by the time required to open the first disconnector 116 and third disconnector 122 of first station 102 and the second disconnector 158 and third disconnector 162 of the second station 104, since the healthy pole can be brought back into service while the first disconnector 116 and third disconnector 122 of first station 102 and the second disconnector 158 and third disconnector 162 of the second station 104 are closed, with the second disconnector 158 subsequently being opened. Further, the use of the high speed switches 302, 306 in the transfer buses 126, 166 facilitates the rapid reconfiguration of the system 300 into the monopole configuration.

(26) FIG. 5 is a flow diagram illustrating steps of a method 500 performed by the first and second controllers 106, 108 in order to reconfigure the system 300 to operate in a monopole configuration.

(27) In a first step 502, a fault to ground is detected in one of the HVDC converters 110, 130, 150, 170 of the system 300 by either the first controller 106 or the second controller 108. In the example discussed above the fault occurs in the third HVDC converter 150, which is part of the second station 104 and thus may be detected by the second controller 108. It will be appreciated that corresponding steps will be performed using the appropriate AC breakers, disconnectors and high-speed switches of the system 300 in the event that a fault is detected in one of the other HVDC converters 110, 130, 170.

(28) At step 504, the first and second controllers 106, 108 issue commands to the first, second, third and fourth AC breakers 112, 132, 152, 172 to cause them to open. Alternatively, the first and second controllers 106, 108 may issue commands to only the AC breakers related to the faulted pole (in this case the first and third AC breakers 112, 152 of the first pole 240) to cause them to open, whilst issuing commands to the HVDC converters 130, 170 of the healthy pole to ramp down their DC power to minimise the DC current flowing in the healthy pole.

(29) At step 506, the first controller 106 issues a command to the NBGS 312 to cause it to open.

(30) At step 508, the first and second controllers 106, 108 issue control signals to the first high-speed switch 302 of the link 126 of the first station 102 and the third high-speed switch 306 of the link 166 of the second station 104 to cause those switches to close, thus reconfiguring the system 300 to operate in a monopolar link configuration using the first transmission line 200 as a current return path as described above.

(31) At step 510 the first and second controllers 106, 108 issue commands to the second and fourth AC breakers 132, 172 to cause them to close, thereby re-coupling the second and fourth HVDC converters 130, 170 to their respective AC transmission terminals. Alternatively, if the second and fourth AC breakers were not opened, but instead the DC power in the healthy pole was ramped down following detection of the fault, the first and second controllers 106, 108 may issue commands to the HVDC converters 130, 170 of the healthy pole to ramp up their DC power, thereby increasing current flow in the healthy pole.

(32) At step 512, the second controller 108 issues control signals to the second and third disconnectors 158, 162 of the second station 104 to cause the second and third disconnectors 158, 162 to open, thereby isolating the third HVDC converter 150 from the first transmission line 200 and from the fourth HVDC converter 170. The first controller 106 also issues control signals to the first and third disconnectors 116, 122 of the first station 102 to cause the first and third disconnectors 116, 122 to open, thereby isolating the first HVDC controller 110 from the first transmission line 200 and from the second HVDC converter 130.

(33) At step 510, the first controller 106 issues a command to the NBGS 312 to cause it to close.

(34) The principles discussed above are also applicable to bipolar configurations with a dedicated metallic return (DMR), as will now be explained with referent to FIGS. 6-8.

(35) Referring now to FIG. 6, a bipolar link HVDC transmission system with a dedicated metallic return is shown generally at 600. The system 600 shares many common elements with the system 300 described above and illustrated in FIG. 3, so common reference numerals have been used to denote common elements of the systems 300 and 600. For the sake of clarity and brevity only those features of the system 600 that differ from the system 600 will be described here.

(36) The system 600 is similar to the system 300, with the exception that the system 600 includes a dedicated metallic return (DMR) 610 which is operative to couple the neutral node 124 of the first station 102 to the corresponding neutral node 164 of the second station. The DMR 610 may be, for example, an overhead, underground or sub-sea transmission line of a conductive metallic material.

(37) A DMR transfer breaker (DMRTB) 612 is provided between the neutral node 124 and the link 126 of the first station 102, whilst a metallic return transfer breaker (MRTB) 614 is provided between a second end of the DMR 610 and the neutral node 124 of the first station 102. In normal operation of the system 600 (prior to any fault in the system 600) the DMRTB 612 is closed and the MRTB 614 is open.

(38) In normal operation the system 600 operates in the same manner as the system 300 described above.

(39) If a single phase fault to ground develops in one of the HVDC converters 110, 130, 150, 170 during operation of the system 600, a number of actions are taken, as will be described below with reference to FIGS. 7a-7d, which are schematic representations of the system 600 of FIG. 6 following detection of a fault to ground in the third HVDC converter 150. It will be appreciated of course that the system 600 operates in the same way if a fault is detected in any of the other HVDC converters 110, 130, 170.

(40) Following detection of the fault in the third HVDC converter 150, the first and third AC breakers 112, 152 are opened, in response to commands issued by the first and second controllers 106, 108, to isolate the first and third HVDC converters 110, 130 from their respective AC transmission terminals, thereby disabling the first pole 240, in which the fault occurred. Unlike in the system 300 of FIG. 3, the second and fourth AC breakers 132, 172 are not opened, but instead remain closed, permitting the healthy second pole 250 to continue to operate, with return current flowing in the DMR 510, as indicated by the solid arrow in FIG. 7a.

(41) With the first and third HVDC converters 110, 150 isolated in this way stored energy circulates in the system 600. Because of the fault to ground in the third HVDC converter 150, leakage current flows through the earth, second low resistance path provided by the closed NBGS 312, fourth disconnector 122, first HVDC converter 110, first and second disconnectors 116, 118 of first station 102, first transmission line 200, and first and second disconnectors 156, 158 of second station 104, as shown by the dashed arrow in FIG. 7a.

(42) The healthy pole 250 may now be reconfigured to operate as a monopole with metallic return, using the DMR 610 as a return path as shown in FIG. 7b. Thus the first controller 106 may issue control signals to the MRTB 614 and the DMRTB 612 to cause the MRTB 614 to close and subsequently to cause the DMRTB 612 to open, thus enabling the DMR 610 to provide a dedicated current return path between the first and second stations 102, 104.

(43) After the first and third AC breakers 112, 152, have been opened to isolate the first and third HVDC converters 110, 150 from their respective AC transmission terminals, the NBGS 312 is opened in response to a command issued by the first controller 106, effectively activating the first, high resistance, path to earth containing the resistor 310.

(44) The system 600 can now be reconfigured, if necessary, to use the first transmission line 200 as a current return path. In order to reconfigure the system 600 in this way, the first and second controllers 106, 108 issue commands to cause the first high-speed switch 302 of the link 126 of the first station 102 and the third high-speed switch 306 of the link 166 of the second station 104 to close. Additionally, the first controller 106 issues commands to the MRTB 614 to cause it to open, thereby isolating the DMR 610 from the first station 102 so that it can no longer act as a current return path. The first controller 106 then issue a command to the DMRTB 612 to cause the DMRTB 612 to close.

(45) The second, fourth, fifth and sixth disconnectors 118, 136, 140, 142 of the first station 102 and the first, fourth, fifth and sixth disconnectors 156, 176, 180, 182 of the second station 104 remain closed, such that the first transmission line 200 can be used as a current return path for the second pole 250, as indicated by the arrow in FIG. 7d. The system 600 thus operates in a monopolar link configuration.

(46) Leakage current is now able to flow through the earth, first high resistance path provided by resistor 310, fourth disconnector 122, first HVDC converter 110, first and second disconnectors 116, 118 of first station 102, first transmission line 200, and first and second disconnectors 156, 158 of second station 104, as shown by the dashed arrow in FIG. 7c. However, due to the high resistance of the resistor 310 the leakage current that flows is very small.

(47) Because the leakage current flowing in the first disconnector 116 and third disconnector 122 of the first station 102 and in the second disconnector 158 and third disconnector 162 of the second station 104 is very small once the NBGS 312 has been opened, the second disconnector 158 of the second station 104 can now be opened, in response to a command issued by the second controller 108, to isolate the faulted third HVDC converter 150 from the first transmission line 200. The third disconnector 162 of the second station is also opened in response to a command issued by the second controller 108 to isolate the third HVDC converter 150 from the fourth HVDC converter 130. The first and third disconnectors 116, 122 of the first station 102 may also be opened, in response to a command issued by the first controller 106, to isolate the first HVDC converter 110 from the first transmission line 200 and from the second HVDC converter 130. With the second and third disconnectors 158, 162 of the second station 104 and the first and third disconnectors 116, 122 of the first station 102 open, no leakage current flows, as shown in FIG. 7d, and the NBGS 312 can revert to its normal closed state, in response to a command issued by the first controller 106.

(48) FIG. 8 is a flow diagram illustrating steps of a method 800 performed by the first and second controllers 106, 108 in order to reconfigure the system 600 to operate in the monopole configuration of FIG. 7d in which the first transmission line 200 is used as a current return path.

(49) In a first step 802, a fault to ground is detected in one of the HVDC converters 110, 130, 150, 170 of the system 300 by either the first controller 106 or the second controller 108. In the example discussed above the fault occurs in the third HVDC converter 150, which is part of the second station 104 and thus may be detected by the second controller 108. It will be appreciated that corresponding steps will be performed using the appropriate AC breakers, disconnectors and high-speed switches of the system 600 in the event that a fault is detected in one of the other HVDC converters 110, 130, 170.

(50) At step 804, the first and second controllers 106, 108 issue commands to the first, and third AC breakers 112, 152 to cause them to open.

(51) At step 806, the first controller 106 issues a command to the NBGS 312 to cause it to open.

(52) In order to reconfigure the system 600 to use the first transmission line 200 as a current return path, at step 808 the first and second controllers 106, 108 issue commands to cause the first high-speed switch 302 of the link 126 of the first station 102 and the third high-speed switch 306 of the link 166 of the second station 104 to close. The first controller 106 also issues commands to the DMRTB 612, and the MRTB 614 to cause them to open.

(53) At step 810, the second controller 108 issues control signals to the second and third disconnectors 158, 162 of the second station 104 to cause the second and third disconnectors 158, 162 to open, thereby isolating the third HVDC converter 150. The first controller 106 may also issue control signals to the first and third disconnectors 116, 122 of the first station 102 to cause the first and third disconnectors 116, 122 to open, thereby isolating the first HVDC controller 110.

(54) At step 812, the first controller 106 issues a command to the NBGS 312 to cause it to close.

(55) The systems and methods described above are suitable for use with HVDC converters of different types. For example, the HVDC converters 110, 130, 150, 170 may be line commutated converters (LCCs) or voltage source converters (VSCs).

(56) As will be appreciated from the foregoing, the systems and methods described above provide a cost effective mechanism for rapidly reconfiguring bipolar HVDC transmission systems in the event that a fault is detected.

(57) It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims. The word “comprising” does not exclude the presence of elements or steps other than those listed in a claim, “a” or “an” does not exclude a plurality. Any reference signs in the claims shall not be construed so as to limit their scope.