CONVERTER ARRANGEMENT AND METHOD OF OPERATION FOR SAID CONVERTER ARRANGEMENT

Abstract

A device controls a power flow in an AC network and has a series converter with a DC side to connect to a DC link and an AC side to connect to the AC network via a series transformer. The device further has a bridging arrangement between the series transformer and the series converter configured to bridge the series converter. The bridging arrangement contains at least one bridging branch having a switching unit with antiparallel thyristors and a resistance in series with the switching unit. Furthermore, a method of operation operates the device.

Claims

1-12. (canceled)

13. A device for controlling a power flow in an AC network, the device comprising: a series transformer; a series converter with an AC side to connect to the AC network via said series transformer; and a bridging configuration connected between said series transformer and said series converter and configured to bridge said series converter, wherein said bridging configuration contains at least one bridging branch having a switching unit with antiparallel thyristors and a resistance connected in series with said switching unit.

14. The device according to claim 13, wherein: said series transformer has primary windings configured to be connected in series with corresponding phase lines of the AC network; said series transformer has secondary windings connected to each other in a delta connection and thus forming respective delta branches; and said at least one bridging branch is one of three bridging branches, each of said bridging branches is disposed in parallel to a respective delta branch of said respective delta branches.

15. The device according to claim 13, wherein said at least one bridging branch is one of a plurality of bridging branches, each of said bridging branches contains an arrester in parallel to said switching unit and said resistance.

16. The device according to claim 14, further comprising at least one bypass switch for bypassing at least one of said primary windings of said series transformer.

17. The device according to claim 13, further comprising: a shunt transformer; a DC link; and a shunt converter with an AC side to connect, via said shunt transformer, to the AC network, and a DC side to connect, via said DC link, to a DC side of said series converter.

18. The device according to claim 13, wherein said resistance (29) has a resistor element.

19. The device according to claim 18, wherein said resistor element is configured so that said at least one bridging branch has a residual voltage of at least 1 kV.

20. The device according to claim 13, wherein said at least one bridging branch is one of a plurality of bridging branches, each of said bridging branches further contains an inductance disposed in series with said switching unit.

21. The device according to claim 13, wherein said series converter is a modular multilevel converter.

22. The device according to claim 18, wherein said resistor element is a dry type resistor element.

23. A method of operation for a device for controlling a power flow in an AC network, which comprises the steps of: providing a series converter with an AC side to connect to the AC network via a series transformer; providing a bridging configuration between the series transformer and the series converter, the bridging configuration being configured to bridge the series converter, wherein the bridging configuration contains at least one bridging branch having a switching unit with antiparallel thyristors and a resistor element connected in series with the switching unit; and detecting a fault status and in a case of an internal or an external fault: blocking of the series converter; bridging the series converter by means of the bridging configuration; and deblocking of the series converter.

24. . The method according to claim 23, wherein the series converter is a modular multilevel converter and the method further comprises balancing the series converter after the deblocking.

25. . The method according to claim 23, which further comprises detecting if a fault is present and deciding, based on a measurement of an AC line current, whether the internal fault or the external fault is present.

Description

[0031] In the following the invention is explained in more detail in accordance with embodiments shown in the FIGS. 1 to 3.

[0032] FIG. 1 schematically shows an embodiment of a device according to the invention;

[0033] FIG. 2 shows a flow diagram of a first embodiment of a method according to the invention;

[0034] FIG. 3 shows a flow diagram of a second embodiment of a method according to the invention.

[0035] FIG. 1 shows a device for controlling a power flow in an AC network 2 (e.g. a power supply network), wherein the AC network comprises an AC link with three phase lines 2a, 2b and 2c. The device 1 is a universal power flow controller (UPFC). The device 1 comprises a series converter 3 with an AC side 4 and a DC side 5. On its AC side the series converter 3 has three AC terminals 4a, 4b, 4c. The series converter 3 is configured to be (and under operation condition is) connected to the AC network 2 via a series transformer 6. The DC side 5 of the series converter 3 has two DC terminals 5a, 5b (e.g. positive and negative terminals) configure do be connected to a DC link 7. The series converter 3 is a modular multilevel converter (MMC). It comprises three phase modules 8a-c and six converter arms (valves) 9a-f. Every converter arm 9a-f extends between one of the DC poles or terminals 5a,b and one of the AC terminals 4a-c. Each converter arm 9a-f comprises an arm inductance L and a number of switching modules 10 connected in series. In accordance with the embodiment shown in FIG. 1 all switching modules are configured alike, which however is not necessary in general. The number of switching modules 10 in every converter arm 9a-f is in general arbitrary (not restricted to three as shown in the figure) and can be adapted to the given application. According to the example of FIG. 1 the switching modules 10 are so-called half-bridge switching modules. The switching module 10 comprises two terminals X1, X2 to connect e.g. to further, neighboring switching modules. The switching module 10 further comprises two semiconductor switches 11, 12 of the turn-off type with a freewheeling diode D in antiparallel. An energy storage element (capacitor) 13 is connected in parallel to the series connection of the semiconductor switches 11,12. By a proper control of the switches 11, 12 a voltage across the terminals X1, X2 can be achieved which equals to the voltage of the capacitor 13 or a voltage (substantially) equal to zero. Instead of a half-bridge circuit, any (or even all) of the switching modules can comprise any other suitable circuit. An example is the full-bridge configuration known from the prior art.

[0036] The device 1 further comprises a shunt converter 14 which is a voltage source converter (e.g. an MMC). The shunt converter 14 is on its AC side 16 connectable (connected in normal operation) to an AC network (e.g. the AC network 2) via a shunt transformer 15. The shunt converter 14 is furthermore connected, on its DC side 17, to the series converter 3 via the DC link 7. A startup circuit 18 is provided between the shunt transformer 15 and the shunt converter 14.

[0037] A central control unit CU is provided and configured to control the series converter 3 and the shunt converter 14 by means of controlling the respective semiconductor switches.

[0038] The series transformer 6 comprises primary windings 19a-c which are connected in series with the respective phase lines 2a-c of the AC link 2. Each of the primary windings 19a-c can be bypassed by a respective bypass switch (a circuit breaker) 20-22. The series transformer 6 further comprises three secondary windings 23a-c connected to each other in a delta connection forming three delta branches 24a-c.

[0039] In addition, the device 1 comprises a bridging arrangement 25 to bridge the series converter 3 in case of a fault. The bridging arrangement 25 comprises three bridging branches 26a-c each connected in parallel to one of the delta branches 24a-c. A first bridging branch 26a comprises a switching unit 27 with two antiparallel thyristors 28a, b. The first bridging branch 26a comprises further a resistance 29 (a resistor element) and an inductance 30 arranged in series with the switching unit 27. A MOV-arrester 31 is provided in parallel with the series circuit of said resistance 29, inductance 30 and switching unit 27. The second bridging branch 26b and the third bridging branch 26c are arranged in a similar manner.

[0040] In FIG. 2 a flow diagram shows a method of operating a device for controlling power flow in an AC network, for example the device of FIG. 1. Said method of operation comprises in particular a recovery sequence of the series converter in case of a fault. The embodiment of FIG. 2 shows the method steps performed in case of an external fault. In the following all numerals referring to aspects of the device correspond to those used in FIG. 1.

[0041] At a time t0 a fault occurs and is detected by a suitable detection device. The series transformer 6 is protected against high voltage by arresters 31.

[0042] At a time t1, ca. 1-50 microseconds after t0, the converter is actively blocked, e.g. via a converter current protection.

[0043] At a time t2, ca. 1.5-2 ms after t0, the thyristors 28a,b of the bridging arrangement 25 are actively switched on (‘fired’). Accordingly, the series converter is protected via the bridging arrangement 25. The fault current is commutated from the converter to the bridging branches 26a-c (in particular through the thyristors) and flow continually through said bridging branches until the fault is cleared. Transients occurring at t2 are largely suppressed due to the presence of the resistor elements 29 in the bridging branches.

[0044] At a time t3, ca. 20-30 ms after t0, the series converter is actively deblocked. Before deblocking of the series converter a valve current through the thyristors and a converter current through the series converter are measured and compared with a respective threshold in order to decide whether the deblocking can be initiated.

[0045] At a time t4, approx. 50-150 ms after t0, the fault is cleared via a line circuit breaker. From t4 on a normal line current will flow through the bridging branches. The balancing of the series converter 3 is actively started.

[0046] At a time t5 the thyristors are blocked (achieved at a current zero crossing by not actively providing a firing pulse). The line current is commutated from the bridging branches to the series converter 3. Transients occurring at t5 are largely suppressed due to the presence of the resistor elements 29 in the bridging branches.

[0047] At a time t6, approximately 100-200 ms after t0, the series converter returns to normal operation.

[0048] In FIG. 3 a flow diagram shows a method of operating a device for controlling power flow in an AC network, for example the device of FIG. 1. Said method of operation comprises in particular a recovery sequence of the series converter in case of a fault. The embodiment of FIG. 3 shows the method steps performed in case of an internal fault. In the following all numerals referring to aspects of the device correspond to those used in FIG. 1.

[0049] At a time s0 a fault occurs and is detected by a suitable detection device. The series transformer 6 is protected against high voltage by arresters 31.

[0050] At a time s1, ca. 1-50 microseconds after s0, the converter is actively blocked, e.g. via a converter current protection.

[0051] At a time s2, ca. 1.5-2 ms after s0, the thyristors 28a,b of the bridging arrangement 25 are actively switched on (‘fired’). Accordingly, the series converter is protected via the bridging arrangement 25. The fault current is commutated from the converter to the bridging branches 26a-c (in particular through the thyristors) and flow continually through said bridging branches until the fault is cleared. Transients occurring at s2 are largely suppressed due to the presence of the resistor elements 29 in the bridging branches.

[0052] At a time s3, ca. 35-50 ms after s0, the bypass switches 20-22 is switched on, so that the series converter 3 and the series transformer 6 are both protected via the bypass switches. The fault current is commutated from the bridging branches to the bypass switches and flows continually through the bypass switches until the fault is cleared.

[0053] At a time s4 the thyristors 28a,b in the bridging branches are blocked (this can be achieved at a current zero crossing by not actively providing a firing pulse). The line current is commutated from the bridging branches to the series converter 3.

[0054] At a time s5, approx. 50-150 ms after s0, the fault is cleared via a line circuit breaker.

[0055] At a time s6, ca. 300-1000 ms after s0, the AC line is energized, and a nominal power is transmitted.

[0056] At a time s7, the series converter 3 is actively deblocked, the thyristors in the bridging branches are actively fired and opening of the bypass switches 20-22 is initiated (in the indicated sequence). Before deblocking of the series converter a valve current through the thyristors and a converter current through the series converter are measured and compared with a respective threshold in order to decide whether the deblocking can be initiated.

[0057] At a time s8 the line current flows through the bridging branches 26a-c and no current flows through the bypass switches 20-22. This is confirmed to the central control unit CU by a suitable measurement set up. The balancing of the series converter 3 is actively started.

[0058] At a time s9 the thyristors of the bridging arrangement are blocked. Transients occurring at s9 are largely suppressed due to the presence of the resistor elements 29 in the bridging branches.

[0059] At a time s10, approximately 100-200 ms after s0, the series converter returns to normal operation.