Line commutated converters

Abstract

In the field of line commutated converters, for use in high voltage direct current (HVDC) power transmission, a line commutated converter comprises a plurality of converter limbs that extend between first and second DC terminals. Each converter limb includes first and second limb portions which are separated by an AC terminal. The first limb portions together define a first limb portion group and the second limb portions together define a second limb portion group. Each limb portion includes at least one switching element that is configured to turn on and conduct current when it is forward biased and it receives a turn on signal and to naturally turn off and no longer conduct current when it is reverse biased and the current flowing through it falls to zero. The converter also includes a control unit.

Claims

1. A line commutated converter, for use in high voltage DC power transmission, comprising: a plurality of converter limbs extending between first and second DC terminals, each converter limb including first and second limb portions separated by an AC terminal, the first limb portions together defining a first limb portion group and the second limb portions together defining a second limb portion group, each first and second limb portions including at least one switching element configured to turn on and conduct current when it is forward biased and it receives a turn on signal and to naturally turn off and no longer conduct current when it is reverse biased and the current flowing through it falls to zero; and a control unit programmed to, in use, control successive switching of switching elements whereby a first switching element in the first limb portion group and a second switching element in the second limb portion group and a different converter limb to the first switching element connect two corresponding AC terminals in series between the first and second DC terminals, the control unit being further programmed to send a third switching element in the first limb portion group a turn on signal whereby the third switching element turns on and begins to conduct current while the current flowing through the first switching element begins to fall to zero and the first switching element prepares to naturally turn off, and to subsequently send a fourth switching element in the second limb portion group and a different converter limb to the third switching element a turn on signal whereby the fourth switching element turns on and begins to conduct current while the current flowing through the second switching element begins to fall to zero and the second switching element prepares to naturally turn off, and the control unit checking for an abnormal current flow associated with the first switching element during a finite monitoring period to establish whether correct natural turn off of the first switching element occurs, wherein the finite monitoring period commences when the first switching element becomes forward biased, and the control unit checking for the abnormal current flow further comprises: receiving a signal indicating that the first switching element becomes forward biased; and sending a current detector a turn on signal based on the signal, wherein the current detector is electrically interconnected with the first switching element via the AC terminal.

2. The line commutated converter according to claim 1, wherein the control unit checks for abnormal current flow prior to the second switching element becoming forward biased.

3. The line commutated converter according to claim 2, wherein the control unit checks for abnormal current flow in a form of current of the same polarity as that previously flowing through the first switching element, and the control unit establishes that correct natural turn off of the first switching element has not occurred if the abnormal current flow of such same polarity current is observed.

4. The line commutated converter according to claim 1, wherein the finite monitoring period ends when the control unit sends the turn on signal to the fourth switching element.

5. The line commutated converter according to claim 4, wherein the control unit checks for abnormal current flow in a form of any current flow, and the control unit establishes that correct natural turn off of the first switching element has not occurred if the abnormal current flow of any current is observed.

6. The line commutated converter according to claim 1, wherein a respective limb connector extends from each AC terminal to, in use, connect the AC terminal with a corresponding phase of a multi-phase AC network, and the control unit checks for abnormal current flow in a limb connector electrically interconnected with the first switching element via a corresponding AC terminal.

7. The line commutated converter according to claim 1, wherein in an event that correct natural turn off of the first switching element does not occur the control unit alters timing of the next turn on signal it sends to the switching element.

8. The line commutated converter according to claim 7, wherein the control unit brings forward the timing of the next turn on signal it sends to the switching element.

9. The line commutated converter according to claim 8, wherein the control unit brings forward the timing of the next turn on signal it sends to the switching element by up to 6 electrical degrees.

10. The line commutated converter according to claim 8, wherein the control unit brings forward the timing of the next three turn on signals it sends to respective switching elements.

Description

(1) There now follows a brief description of preferred embodiments of the invention, by way of non-limiting example, with reference being made to the following figures in which:

(2) FIG. 1 shows a schematic view of a line commutated converter according to an embodiment of the invention;

(3) FIGS. 2(a) to 2(c) illustrate normal operation of the line commutated converter shown in FIG. 1;

(4) FIG. 3 illustrates operation of a control unit within the line commutated converter shown in FIG. 1 to establish whether correct natural turn off of a first switching element within the converter has occurred; and

(5) FIG. 4 illustrates operation of an alternative control unit within the line commutated converter shown in FIG. 1 to establish whether correct natural turn off of a first switching element within the converter has occurred.

(6) A line commutated converter according to a first embodiment of the invention is designated generally by reference numeral 10, as shown in FIG. 1.

(7) The line commutated converter 10 includes three converter limbs 12A, 12B, 12C, each of which corresponds to a respective phase A, B, C of an AC network 14 with which the converter 10 is, in use, operatively connected. Other embodiments of the invention may include fewer than or more than three converter limbs according to the commensurate number of phases included in an associated AC network.

(8) Each converter limb 12A, 12B, 12C extends between first and second DC terminals 16, 18 which, in use, are operatively connected with a DC network 20. Each converter limb 12A, 12B, 12C also includes first and second limb portions 22, 24 which are separated by a corresponding AC terminal 26A, 26B, 26C. The first limb portions 22 together define a first limb portion group 28 and the second limb portions 24 together define a second limb portion group 30.

(9) A respective limb connector 32A, 32B, 32C extends from each AC terminal 26A, 26B, 26C to, in use, connect the AC terminal 26A, 26B, 26C with a corresponding phase A, B, C of the multi-phase, i.e. three-phase, AC network 14. In the embodiment shown each limb connector 32A, 32B, 32C incorporates the winding of a transformer (not shown) and so takes the form of a respective limb winding 34A, 34B, 34C.

(10) In the embodiment shown each limb portion 22, 24 includes a single switching element 36.sup.1, 36.sup.2, 36.sup.3, 36.sup.4, 36.sup.5, 36.sup.6, although more switching elements may optionally be included in each limb portion 22, 24. Each switching element 36.sup.1, 36.sup.2, 36.sup.3, 36.sup.4, 36.sup.5, 36.sup.6 is a thyristor which is configured to turn on and conduct current when it is forward biased and it receives a turn on signal, and to naturally turn off and no longer conduct current when it is reverse biased and the current flowing through it falls to zero. Other semiconductor switching elements may, however, be used, as may other switching elements such a gas tube switching devices.

(11) In addition the converter 10 includes a control unit 38 that is programmed to control successive switching of the switching elements 36.sup.1, 36.sup.2, 36.sup.3, 36.sup.4, 36.sup.5, 36.sup.6.

(12) Also, each limb connector 32A, 32B, 32C, i.e. each limb winding 34A, 34B, 34C, includes a corresponding current detector 40A, 40B, 40C which is able to both identify when current is flowing through the corresponding limb connector 32A, 32B, 32C as well as establish the polarity, i.e. direction, of current flowing therethrough.

(13) In use, the control of successive switching of the switching elements 36.sup.1, 36.sup.2, 36.sup.3, 36.sup.4, 36.sup.5, 36.sup.6 by the control unit 38 leads to a first switching element 36.sup.1, 36.sup.2, 36.sup.3, 36.sup.4, 36.sup.5, 36.sup.6 in the first limb portion group 28, and a second switching element 36.sup.1, 36.sup.2, 36.sup.3, 36.sup.4, 36.sup.5, 36.sup.6 in the second limb portion group 30 and a different converter limb 12A, 12B, 12C to the first switching element, connecting two corresponding AC terminals 26A, 26B, 26C, i.e. the AC terminals 26A, 26B, 26C with which the aforementioned first and second switching elements 36.sup.1, 36.sup.2, 36.sup.3, 36.sup.4, 36.sup.5, 36.sup.6 are associated, in series between the first and second DC terminals 16, 18.

(14) For example, as shown in FIG. 2(a), initially a first switching element 36.sup.1 in the first limb portion group 28 and a second switching element 36.sup.2 in the second limb portion group 30 connect two of the AC terminals 26A, 26C, and thereby two of the phases A, C of the AC network 14, in series between the first and second DC terminals 16, 18.

(15) The control unit 38 is further programmed, in use, to send a third switching element 36.sup.3 in the first limb portion group 28 a turn on signal whereby the third switching element 36.sup.3 turns on and begins to conduct current while the current flowing through the first switching element 36.sup.1 begins to fall to zero and the first switching element 36.sup.1 prepares to naturally turn off, as shown schematically in FIG. 2(b).

(16) The control unit 38 is also programmed, in use, to subsequently send a fourth switching element 36.sup.4 in the second limb portion group 30 and a different converter limb 12A to the third switching element 36.sup.3 a turn on signal whereby the fourth switching element 36.sup.4 turns on and begins to conduct current while the current flowing through the second switching element 36.sup.2 begins to fall to zero and the second switching element 36.sup.2 prepares to naturally turn off, as shown schematically in FIG. 2(c).

(17) Accordingly, during further operation of the converter 10 other first and second switching elements 36.sup.3, 36.sup.4, 36.sup.5, 36.sup.6 successively connect two corresponding AC terminals 26A, 26B, 26C, and thereby two corresponding phases A, B, C of the AC network 14, between the DC terminals 16, 18, and thereafter prepare to naturally turn off.

(18) It follows that the checking described below in relation to an example first switching element 36.sup.1, is carried out by the control unit 38 in relation to each switching element 36.sup.1, 36.sup.2, 36.sup.3, 36.sup.4, 36.sup.5, 36.sup.6 as and when they are successively expected to naturally turn off during the continued and ongoing operation of the converter 10.

(19) By way of example, the control unit 38 checks for an abnormal current flow associated with the first switching element 36.sup.1 during a finite monitoring period 42, 42 as illustrated in FIG. 3, to establish whether correct natural turn off of the first switching element 36.sup.1 occurs.

(20) The finite monitoring period 42, 42 commences 44 when the first switching element 36.sup.1 becomes forward biased 46, i.e. when the voltage V.sub.1 across the first switching element 36.sup.1 (as defined at that particular time by the phase to phase voltage V.sub.AB between phases A and B of the AC network 14) reverses and becomes positive with respect to the first switching element 36.sup.1.

(21) In the embodiment shown the monitoring period 42, 42 ends 48 when the second switching element 36.sup.2 becomes forward biased 50, i.e. when the voltage across the second switching element 36.sup.2 (as defined at that time by the phase to phase voltage V.sub.CA between phases C and A of the AC network 14) reverses and becomes positive with respect to the second switching element 36.sup.2. In this manner the control unit 38 checks for abnormal current flow prior to the said second switching element 36.sup.2 becoming forward biased 50.

(22) More particularly, the control unit 38 checks for abnormal current flow in the form of current of the same polarity as that previously flowing through the first switching element 36.sup.1. More particularly still, the control unit 38 uses the current detector 40A in the phase A limb connector 32A, i.e. the current detector 40A electrically interconnected with the first switching element 36.sup.1 via the corresponding phase A AC terminal 26A, to check for such abnormal current flow.

(23) In this regard the current I previously flowing through the first switching element 36.sup.1 (as shown in FIG. 2(b)), i.e. as the first switching element 36.sup.1 prepares to naturally turn off, has a positive polarity 52.

(24) Following correct natural turn off of the first switching element 36.sup.1, e.g. during a first finite monitoring period 42, the current I flowing through the phase A current detector 40A (which arises because of the fourth switching element 36.sup.4 being turned on and starting to conduct, i.e. as shown in FIG. 2(c)) has a negative polarity 54.

(25) Such current flow is therefore not considered an abnormal current flow by the control unit 38 and so the control unit 38 does not indicate that a commutation failure has occurred with respect to the first switching element 36.sup.1, i.e. does not indicate that correct natural turn off of the first switching element 36.sup.1 has failed to occur.

(26) In contrast, e.g. during a second finite monitoring period 42, if the current I flowing through the phase A current detector 40A has the same positive polarity 54 as the current I previously flowing through the first switching element 36.sup.1 it is considered by the control unit 38 to be an abnormal current flow 56. The control unit 38 therefore establishes 58 that correct natural turn off of the first switching element 36.sup.1 has not occurred, i.e. a commutation failure has occurred with respect to the first switching element 36.sup.1.

(27) Thereafter, i.e. in the event that correct natural turn off of the first switching element 36.sup.1 does not occur, the control unit 38 alters the timing of the next turn on signal it sends to a switching element 36.sup.1, 36.sup.2, 36.sup.3, 36.sup.4, 36.sup.5, 36.sup.6, and more particularly brings forward the timing of the turn on signal it sends to the fifth switching element 36.sup.5 by 6 electrical degrees. In other embodiments of the invention the control unit may bring forward the timing of the next turn on signal it sends by less than 6 electrical degrees.

(28) The control unit 38 may additionally bring forward the timing of two further turn on signals it sends to respective further switching elements 36.sup.1, 36.sup.2, 36.sup.3, 36.sup.4, 36.sup.5, 36.sup.6, e.g. a sixth switching element 36.sup.6.

(29) In an alternative embodiment of the invention, the control unit of the first line commutated converter 10 may instead check for abnormal current flow 42 in a different manner to that described herein above.

(30) More particularly, the alternative control unit may check for abnormal current flow during a different, further finite monitoring period 62 62, as illustrated in FIG. 4.

(31) The further finite monitoring period 62, 62 again commences 64 when the first switching element 36.sup.1 becomes forward biased 46, i.e. when the voltage V.sub.1 across the first switching element 36.sup.1 (as defined at that particular time by the phase to phase voltage V.sub.AB between phases A and B of the AC network 14) reverses and becomes positive with respect to the first switching element 36.sup.1. The further finite monitoring period 62, 62 differs, however, in that it ends 68 when the control unit 38 sends a turn on signal 70 to the fourth switching element 36.sup.4.

(32) In addition, the alternative control unit checks for abnormal current flow in the form of any current flow through the phase A current detector 40A in the phase A limb connector 32A, i.e. the limb connector 32A electrically interconnected with the first switching element 36.sup.1 via the corresponding phase A AC terminal 26A.

(33) In this regard, following correct natural turn off of the first switching element 36.sup.1, e.g. during a first further finite monitoring period 62, no current 72 flows through the phase A current detector 40A, not least because the fourth switching element 36.sup.4 has not yet been turned on.

(34) Since no current flow is observed by the alternative control unit, it does not indicate that a commutation failure has occurred with respect to the first switching element 36.sup.1, i.e. it does not indicate that correct natural turn off of the first switching element 36.sup.1 has failed to occur.

(35) In contrast, e.g. during a second further finite monitoring period 62, if any current 74 does flow through the phase A current detector 40A it is considered by the alternative control unit to be an abnormal current flow 56. The alternative control unit therefore establishes 58 that correct natural turn off of the first switching element 36.sup.1 has not occurred, i.e. a commutation failure has occurred with respect to the first switching element 36.sup.1.

(36) Thereafter, i.e. in the event that correct natural turn off of the first switching element 36.sup.1 does not occur, the alternative control unit again preferably alters the timing of the next turn on signal it sends to a switching element 36.sup.1, 36.sup.2, 36.sup.3, 36.sup.4, 36.sup.5, 36.sup.6, and more particularly preferably brings forward the timing of the turn on signal it sends to the fifth switching element 36.sup.5 by 6 electrical degrees.

(37) The alternative control unit may also additionally bring forward the timing of two further turn on signals it sends to respective further switching elements 36.sup.1, 36.sup.2, 36.sup.3, 36.sup.4, 36.sup.5, 36.sup.6, e.g. a sixth switching element 36.sup.6 and then to the first switching element.