METHODS OF CONTROLLING AN ELECTRICAL SYSTEM TO EXTINGUISH AN ELECTRIC ARC, AND ELECTRICAL SYSTEMS

Abstract

An electrical system forming part of a solar power plant is described. The electrical system includes a plurality of photovoltaic (PV) panels, a power converter, and a controller. In response to a detected electric arc on the DC side of the power converter, the controller is configured to enable a short circuit state of the power converter by controlling semiconductor switches of the power converter (e.g., turning on some or all of the semiconductor switches) to create a short circuit between DC input terminals of the power converter. The short circuit path though the power converter will extinguish the detected electric arc in the connected DC circuit.

Claims

1. A method of controlling an electrical system, the electrical system comprising: a direct current DC power source; and a power converter including at least one controllable semiconductor device, each semiconductor device including at least a controllable semiconductor switch, the power converter having DC input terminals connected to the DC power source by means of a DC circuit; wherein the method comprises: in response to a detected electric arc on the DC side of the power converter, enabling a short circuit state of the power converter by controlling the at least one semiconductor switch of the power converter to create a short circuit between the DC input terminals.

2. A method according to claim 1, further comprising the step of maintaining the short circuit state of the power converter for a period of time.

3. A method according to claim 1, wherein the power converter is initially in a converter on state and in response to the detected electric arc is (i) transitioned directly from the converter on state to the short circuit state, or (ii) transitioned from the converter on state to a converter off state and then from the converter off state to the short circuit state.

4. A method according to claim 1, wherein the power converter comprises an DC/AC converter having at least one AC terminal and a plurality of phase legs, and wherein during the short circuit state, semiconductor switches of at least one phase leg are controlled to turn on so that a short circuit current flows between the DC input terminals of the power converter.

5. A method according to claim 4, wherein the electrical system includes an AC circuit connected to the AC terminal(s) of the DC/AC converter and connectable to an AC power network or utility grid, wherein the AC circuit includes an AC switch, and wherein the method further comprises opening the AC switch before the power converter is transitioned to the short circuit state.

6. A method according to claim 5, wherein the power converter further comprises a DC/DC converter, and wherein during the short circuit state, at least one semiconductor switch of the DC/DC converter is controlled to turn on so that a short circuit current flows between the DC input terminals of the power converter.

7. A method according to claim 6, wherein the short circuit state of the DC/AC converter is enabled after the short circuit state of the DC/DC converter is enabled.

8. A method according to claim 1, wherein the DC circuit includes a DC link with a DC switch, and wherein the method further comprises opening the DC switch before the power converter is transitioned to the short circuit state.

9. A method according to claim 8, wherein, with the DC switch open, the method further comprises the: if the DC link voltage exceeds a voltage threshold: discharging the DC link until the DC link voltage does not exceed the voltage threshold, and transitioning the power converter to the short circuit state.

10. A method according to claim 8, further comprising closing the DC switch after the power converter has been transitioned to the short circuit state.

11. An electrical system comprising: a DC power source; a power converter including at least one controllable semiconductor device, each semiconductor device including at least a controllable semiconductor switch, the power converter having DC input terminals connected to the DC power source by means of a DC circuit; and a controller; wherein the controller is configured to: in response to a detected electric arc on the DC side of the power converter, enable a short circuit state of the power converter by controlling the at least one semiconductor switch of the power converter to create a short circuit between the DC input terminals.

12. An electrical system according to claim 11, wherein the controller is further configured to maintain the short circuit state of the power converter for a period of time.

13. An electrical system according to claim 11, wherein the power converter comprises a DC/AC converter having at least one AC terminal.

14. An electrical system according to claim 13, wherein the DC circuit includes a DC link with a DC switch, and wherein the electrical system further comprises an AC circuit connected to the AC output terminal(s) of the DC/AC converter and connectable to an AC power network or utility grid.

15. A solar power plant comprising the electrical system according to claim 11, wherein the DC power source comprises one or more photovoltaic PV panels.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0069] FIG. 1 is a schematic diagram showing a solar power plant according to the present disclosure;

[0070] FIG. 2 is a schematic diagram showing part of an alternative DC circuit for the solar power plant of FIG. 1;

[0071] FIG. 3 shows a first example of a DC/AC converter with a two-level VSC topology with three phase legs that may be implemented as a single-stage power converter in the solar power plant of FIG. 1;

[0072] FIG. 4 shows a second example of a DC/AC converter with a three-level NPP VSC topology with three phase legs that may be implemented as a single-stage power converter in the solar power plant of FIG. 1;

[0073] FIG. 5 shows a first example of a two-stage power converter including a DC/DC converter and a DC/AC converter with a two-level VSC topology with three phase legs that may be implemented in the solar power plant of FIG. 1;

[0074] FIG. 6 shows a second example of a two-stage power converter including an alternative DC/DC converter and a DC/AC converter with a two-level VSC topology with three phase legs that may be implemented in the solar power plant of FIG. 1;

[0075] FIG. 7 is a flowchart of a method of extinguishing an electric arc caused by a short circuit fault in the solar power plant of FIG. 1;

[0076] FIG. 8 shows an arrangement for the first example of the DC/AC converter of FIG. 3 where three parallel short circuit current paths are provided simultaneously through all three phase legs during the short circuit state;

[0077] FIG. 9 shows an arrangement for the second example of the DC/AC converter of FIG. 4 where two parallel short circuit paths are provided simultaneously through all three phase legs during the short circuit state;

[0078] FIG. 10 shows an arrangement for the first example of the two-stage power converter of FIG. 5 where a short circuit current path is provided through the DC/DC converter during the short circuit state;

[0079] FIG. 11 shows an arrangement for the first example of the two-stage power converter of FIG. 5 where a short circuit current path is provided through the DC/DC converter and a parallel short circuit path is provided through one phase leg of the DC/AC converter during the short circuit state;

[0080] FIG. 12 shows an arrangement for the first example of the two-stage power converter of FIG. 5 where three parallel short circuit current paths are provided simultaneously through all three phase legs of the DC/AC converter during the short circuit state;

[0081] FIG. 13 shows an arrangement for the second example of the two-stage power converter of FIG. 6 where a short circuit path is provided through the DC/DC converter during the short circuit state; and

[0082] FIG. 14 shows an arrangement for the second example of the two-stage power converter of FIG. 6 where a short circuit current path is provided through the DC/DC converter and a parallel short circuit path is provided through one phase leg of the DC/AC converter during the short circuit state.

DETAILED DESCRIPTION

[0083] FIG. 1 shows a solar power plant 1 according to the present disclosure.

[0084] The solar power plant 1 includes a power converter 2 with two DC input terminals 4 and three AC output terminals 6.

[0085] The DC input terminals 4 are connected to a plurality of photovoltaic (PV) panels 8 by means of a DC circuit 10. The DC circuit 10 includes a DC link 12 having positive and negative DC conductors that are connected to the DC input terminals 4, and one or more capacitors 14 connected between the positive and negative DC conductors. The DC link 10 also includes a DC switch 16.

[0086] The DC circuit 10 includes a DC bus 18 with positive and negative DC conductors connected to the DC link 12. A plurality of DC input strings 20a, 20b, . . . , 20n are connected in parallel to the DC bus 18. Each DC input string 20a, 20b, . . . , 20n includes positive and negative DC conductors and is connected to one or more PV panels 8. In particular, any suitable number of PV panels 8 may be connected together in series with the first PV panel being connected to the positive DC conductor of the DC input string 20a, 20b, . . . , 20n and the last PV panel being connected to the negative DC conductor or vice versa. The number of PV panels 8 connected to each DC input string 20a, 20b, . . . , 20n will depend on the design of the solar power plant.

[0087] Each DC input string 20a, 20b, . . . , 20n is provided with at least one fuse 22a, 22b, . . . , 22n for over-current protection. The fuses 22a, 22b, . . . , 22n are shown for completeness and it will be readily appreciated that they may be omitted completely in those arrangements where the power converter 2 is used as the only way of extinguishing an electric arc caused by a short circuit fault in the DC circuit 10. In other arrangements, the fuses 22a, 22b, 22n may be retained and the operation of the power converter 2 may be coordinated with conventional protective measures.

[0088] The AC terminals 6 of the power converter 2 are connected to an AC circuit 24. The AC circuit 24 is a three-phase AC circuit and includes AC filters 26, an AC switch 28 and a transformer 30. The transformer 30 includes a primary winding that is connected to the AC terminals 6 of the solar inverter 2 and a secondary winding that is connected to an AC power network or utility grid 32.

[0089] FIG. 2 shows part of an alternative DC circuit which is similar to the DC circuit shown in FIG. 1 and like components have been given the same reference numbers. The alternative DC circuit uses combiner boxes. A plurality of primary DC input strings 20′a, 20′b, . . . , 20′n are connected in parallel to a combiner box 34a. Each primary DC input string 20′a, 20′b, . . . , 20′n includes positive and negative DC conductors and is connected to a plurality of series-connected PV panels 8a, 8b, . . . , 8n. The number of PV panels 8a, 8b, . . . , 8n connected to each DC input string 20′a, 20′b, . . . , 20′n will depend on the design of the solar power plant.

[0090] The primary DC input strings 20′a, 20′b, . . . , 20′n are connected in parallel to a DC bus 36 within the combiner box 34a. Additional combiner boxes 34b, . . . , 34n are also shown schematically and are connected to series-connected PV panels in the same way.

[0091] A secondary DC input string 20a, 20b, . . . , 20n is connected to each combiner box 34a, 34b, . . . , 34n. In particular, each secondary DC input string 20a, 20b, . . . , 20n is connected in parallel between the respective DC bus 36 of the combiner box 34a, 34b, . . . , 34n and the DC bus 8 of the DC circuit. Each secondary DC input string 20a, 22b, 22n includes a fuse (only the fuse 22a in secondary DC input string 20a is shown for clarity) which can be omitted completely in some arrangements. In FIG. 2 the fuses are shown to be located inside the combiner boxes 34a, 34b, . . . , 34n because this is typical, but the fuses may also be located outside the combiner boxes in some arrangements.

[0092] The power converter 2 may be a DC/AC converter (e.g., a solar inverter) that includes a plurality of controllable semiconductor switches, e.g., IGBTs, and anti-parallel connected diodes, which have suitable voltage and current ratings and which are arranged in a suitable topology. FIG. 3 shows a two-level VSC topology with three phase legs 38a, 38b and 38c. FIG. 4 shows a three-level NPP VSC topology with three phase legs 38a, 38b and 38c. It will be readily understood that other topologies may be used in a practical implementation of the power converter 2. Each phase leg 38a, 38b and 38c includes a pair of semiconductor switches connected in series between a positive DC rail 40 and a negative DC rail 42 of the power converter 2. The positive and negative DC rails 40, 42 are connected to or define the DC input terminals 4 of the power converter 2 and are connected to the DC link 12. Each phase leg 38a, 38b, 38c also defines a respective AC output terminal 6 of the power converter 2 and is connected to a corresponding phase (i.e., U, V and W) of the three-phase AC circuit 24. In the case of the three-level NPP VSC topology shown in FIG. 4, the pair of semiconductor switches that are connected in series between the positive and negative DC rails 40, 42 define a first (or “vertical”) branch. Each phase leg 38a, 38b and 38c also includes semiconductor switches in a second (or “horizontal”) branch that is connected between the point of connection of the semiconductor switches in the first branch and an intermediate DC point (labelled NP) of the DC link 12. A first capacitor 14a is connected between the intermediate DC point and the positive DC rail 40 and a second capacitor 14b is connected between the intermediate point and the negative DC rail 42.

[0093] With reference to FIGS. 5 and 6, the power converter 2 may be a two-stage power converter that includes a DC/DC converter 2a (e.g., a step-up or boost converter) and a DC/AC converter 2b (e.g., a solar inverter) connected together in series by a DC link. The DC/DC converter 2a has DC input terminals that define the DC input terminals 4 of the power converter 2 and are connected to the DC link 12. The DC/DC converter 2a steps-up (or “boosts”) the DC input voltage from the PV panels 8 and supplies the stepped-up DC voltage to the DC/AC converter 2b, which converts it into an AC voltage that may be exported to the AC power network or utility grid 32.

[0094] FIG. 5 shows a DC/DC converter 2a with a single controllable semiconductor switch 44, e.g., an IGBT or MOSFET, an inductance 46, a flyback diode 48. A capacitor 50 is part of the DC link that connects together the DC/DC and DC/AC converters 2a and 2b. The semiconductor switch 44 is connected between positive and negative rails 40, 42 that are shared by the DC/DC converter 2a and the DC/AC converter 2b. In this case, the shared positive rail 40 can be considered to include the inductance 46 and the flyback diode 48. FIG. 6 shows an alternative DC/DC converter 2a with three branches 52a, 52b and 52c. Each branch 52a, 52b and 52c includes a pair of semiconductor switches connected in series between the positive and negative DC rails 40, 42. The midpoint of each branch 52a, 52b and 52c is connected to the positive DC input terminal of the power converter 2 by means of an inductance 46. It can be seen that the DC/DC converter shown in FIG. 5 is a simplified version of the DC/DC converter shown in FIG. 6—it has a single branch where the upper semiconductor switch and its anti-parallel connected diode are replaced by the flyback diode 48.

[0095] The DC/AC converter 2b shown in FIGS. 5 and 6 is the same as the DC/AC converter shown in FIG. 3 and has a two-level VSC topology with three phase legs 38a, 38b and 38c.

[0096] It will be readily understood that other topologies may be used in a practical implementation of the power converter 2.

[0097] The semiconductor switches of the power converter 2 are controlled to switch on and off by a controller 54. In the case of the DC/AC converters shown in FIGS. 3 to 6, the controller 54 may control the semiconductor switches in the three phase legs 38a, 38b and 38c to turn on and off. In the case of the DC/DC converter 2a shown in FIG. 5, the controller 54 may control the semiconductor switch 44 to turn on and off. In the case of the DC/DC converter 2a shown in FIG. 6, the controller 54 may control the semiconductor switches in the three branches 52a, 52b and 52c to turn on and off. As described above, turning a semiconductor switch on means transitioning it to the conduction mode and turning a semiconductor switch off means transitioning it to the blocking mode. For a two-stage power converter, the controller 54 may be split into separate controllers for the DC/DC converter and the DC/AC converter, for example.

[0098] The controller 54 receives power from the DC link 12 through a power converter 56 or additionally or alternatively from an external power source 58.

[0099] The controller 54 receives current and voltage measurements from suitable sensors or transducers within the solar power plant 1. In particular, the controller 54 receives:

[0100] measurements of the DC link current and voltage from sensors 60, 62 shown in FIG. 1,

[0101] measurements of the current between the DC link and ground from sensor 64 shown in FIG. 1,

[0102] measurements of the current in the DC input strings from sensors 66 (note that in FIG. 1 only one sensor is shown for simplicity, but it will be understood that a corresponding sensor may be provided in each DC input string 20a, 20b, . . . , 20n), and

[0103] measurements of the current in each combiner box from sensors 68 (note that in FIG. 2 only one sensor is shown for simplicity, but it will be understood that a corresponding sensor may be provided in each combiner box 34a, 34b, . . . , 34n).

[0104] The current and voltage measurements may be used by the controller 54 to detect a electric arc caused by a short circuit fault, to determine the location of the electric arc (e.g., within the DC circuit 10), and to determine when an electric arc caused by the short circuit fault has been extinguished by operating the power converter 2 according to the present disclosure. Other measurements or information 70 that may be received by the controller 54 may relate to electric power conditions or environmental conditions, e.g., temperature, light etc. that might indicate the existence of an electric arc. Such measurements or information might be provided by suitable sensors located, for example, within a housing or cubicle of the combiner boxes 34a, 34b, . . . , 34n shown in FIG. 2, or other parts of the DC circuit. The same measurements or information might also indicate the absence of an electric arc, and hence when an electric arc caused by the short circuit fault has been extinguished by the method of the present disclosure.

[0105] FIG. 7 is a flowchart of a method of extinguishing an electric arc according to the present disclosure.

[0106] The power converter 2 is initially in a converter on state (step 0), i.e., the power converter 2 is operating normally. The DC switch 16 and the AC switch 28 are closed.

[0107] An electric arc is detected in the solar power plant 1 (step 1). The electric arc may be detected, for example, using the current and voltage measurements provided to the controller 54 from the various sensors, from power imbalances within the electrical system, e.g., between the PV panels 8 and the power converter 2, or using measurements 70 of electric power conditions or environmental conditions, e.g., temperature, light etc., particularly within a housing or cubicle, that might indicate the existence of an electric arc.

[0108] At step 2, in response to the detection of the electric arc, the power converter 2 is transitioned to a converter off state (step 2) where the power converter remains enabled, but all of the semiconductor switches are turned off or are in a blocking mode. In the case of a two-stage power converter, this would include the semiconductor switch(es) of the DC/DC converter 2a and the semiconductor switches in the phase legs 38a, 38b and 38c of the DC/AC converter 2b. The DC switch 16 and the AC switch 28 are opened. Step 2 may represent a general protective measure that may be taken in response to any electric arc/short circuit fault in the solar power plant 1, for example.

[0109] The location of the electric arc within the solar power plant 1 may be known from the initial detection step (i.e., step 1). But if the location is not already known, the electric arc is discriminated in step 3 to determine its location within the solar power plant 1. In some cases, conventional protective measures may then be carried out to extinguish the electric arc and clear the short circuit fault. For the purposes of the following discussion, it will be assumed that the electric arc is located in the DC circuit 10, and in particular in the DC link 12 between the DC switch 16 and the DC bus 18 as indicated in FIG. 1. The electric arc caused by the short circuit fault is across the air gap between the positive and negative DC rails of the DC link 12. An electric arc caused by a short circuit fault at this particular location in the DC link 12 may be difficult to extinguish using conventional protective measures for the reasons described above.

[0110] The location of the electric arc may be determined by any suitable method, for example, using the current and voltage measurements provided to the controller 54 from the various sensors, power imbalances within the electrical system, e.g., between the PV panels 8 and the power converter 2, or using measurements of electric power conditions or environmental conditions, e.g., temperature, light etc., particularly within a housing or cubicle, that might indicate the existence of an electric arc and hence the location of the electric arc. For example, if currents are measured on both sides of the DC switch 16, i.e., by sensors 66 in the DC input strings 20a, 20b, . . . , 20n or the sensors 68 in the combiner boxes 34a, 34b, . . . , 34n on the PV panel-side of the DC switch and by sensor 60 in the DC link 12 (i.e., on the power converter-side of the DC switch) which provides a measurement of the DC link current, and there is a deviation in the current measurements, this might indicate that the short circuit fault causing the electric arc is on the PV panel-side of the DC switch. If there is no deviation in the current measurements on both sides of the DC switch 16, but spectral analysis of the DC link voltage (i.e., as measured by sensor 62) and/or the current measured by sensor 64 between the DC link 12 and ground shows high frequency components indicative of an electric arc, this might indicate that the short circuit fault causing the electric arc is on the power converter-side of the DC switch 6, for example.

[0111] At step 4, the method checks to see if the DC link voltage (UDC) exceeds a voltage threshold (UTH). The voltage threshold may be in the range between about 10 and about 100 VDC, for example. If the voltage threshold is not exceeded (i.e., UDC<UTH), the power converter 2 is transitioned from the converter off state to a short circuit state (step 5) as described in more detail below.

[0112] If the voltage threshold is exceeded (i.e., UDC>UTH) this indicates that the capacitors 14 in the DC link 12 are also charged up to the DC link voltage. This might typically be expected because the PV panels 8 were generating power and the DC switch 16 was closed prior to the electric arc being detected. If the power converter 2 was to be transitioned to the short circuit state, the capacitors 14 in the DC link 12 would also be short circuited and this would result in an unacceptably high short circuit current that could cause serious damage to the semiconductor switches. The capacitors 14 in the DC link 12 are therefore discharged (step 6), either passively through a discharge resistor (not shown) by waiting for a period of time, or actively by carrying out a suitable operation of the power converter, for example. Once the capacitors 14 are discharged and the voltage threshold is no longer exceeded (i.e., UDC<UTH), the power converter 2 may be safely transitioned to the short circuit state (step 5) and the DC switch 16 may be closed by the controller 40 (step 7) to place the short circuit current path(s) through the power converter—see below—in parallel with the electric arc.

[0113] In the case where the power converter 2 is a single-stage power converter, i.e., the DC/AC converter as shown in FIGS. 3 and 4, the power converter is transitioned to the short circuit state by turning on certain semiconductor switches in one or more of the phase legs 38a, 38b and 38c to provide one or more short circuit current paths between the positive and negative DC rails 40, 42, and consequently between the DC input terminals 4 of the power converter 2. FIG. 8 shows an arrangement for the two-level VSC topology shown in FIG. 4 where both of the semiconductor switches in all three of the phase legs 38a, 38b and 38c are turned on at the same time to provide three parallel short circuit paths P1, P2 and P3 through the phase legs. However, it is also possible for both of the semiconductor switches in just one of the phase legs 38a, 38b, 38c to be turned on to provide a short circuit path through that phase leg while the other semiconductor switches are turned off. A short circuit path may be provided on a phase leg-by-phase leg basis if this is advantageous, e.g., to prevent overheating of the semiconductor devices. It will be understood that there should be no intermission between the short circuit paths because this would lead to the capacitors in the DC link 12 being charged. So if the semiconductor switches in the phase legs 38a, 38b and 38c are turned on in a phase leg-by-phase leg basis, for example, it is important that there is some overlap between the short circuit paths. Other ways of sequentially creating short circuit paths may also be utilised, for example creating two parallel short circuit paths through two of the phase legs at a time.

[0114] FIG. 9 shows an arrangement for the three-level NPP VSC topology shown in FIG. 5 where certain semiconductor switches in the first and second branches of the first phase leg 38a and certain semiconductor switches in the first and second branches of the second phase leg 38b are turned on to provide a short circuit path P1 between the first and second DC rails 40, 42 that goes via the intermediate DC point. A corresponding short circuit path could then be provided through the second and third phase legs 38b and 38c, then through the first and third phase legs 38a and 38c, and so on. For completeness, a second short circuit path P2 is shown where both of the semiconductor switches in the first branch of the third phase leg 38c are turned on. Other ways of sequentially creating short circuit paths through one or more of the phase legs, without intermission, may also be utilised.

[0115] In the case where the power converter 2 is a two-stage power converter as shown in FIGS. 5 and 6, the power converter is transitioned to the short circuit state by:

[0116] turning on the semiconductor switch or certain semiconductor switches of the DC/DC converter 2a, and/or

[0117] turning on certain semiconductor switches of at least one of the phase legs 38a, 38b and 38c of the DC/AC converter 2b,

[0118] to provide one or more short circuit current paths between the positive and negative DC rails 40, 42, and consequently between the DC input terminals 4 of the power converter 2.

[0119] In one arrangement, the power converter 2 may be transitioned to the short circuit state by initially turning on the semiconductor switch 44 of the DC/DC converter 2a shown in FIG. 5 or by initially turning on the lower semiconductor switch in one or more of the branches 52a, 52b and 52c of the DC/DC converter 2a shown in FIG. 6 such that one or more short circuit paths are provided initially through the DC/DC converter 2a only—see FIGS. 10 and 13 below. Normally, the power converter would only be transitioned to the short circuit state after the AC switch 28 has been opened. But in this case, the DC/DC converter 2a may be transitioned to the short circuit state before the AC switch 28 has been opened because the flyback diode 48 or the anti-parallel connected diode of the upper semiconductor device in the respective branch 52a, 52b and 52c of the DC/DC converter 2a provides a blocking or decoupling function. In particular, the DC/AC converter 2b is decoupled from the short circuit fault in the DC link 12 causing the electric arc. The short circuit path through the DC/DC converter 2a does not short the DC link between the DC/DC converter and DC/AC converter and inflowing fault currents from the AC power network or utility grid 32 are avoided. This is advantageous because the power converter 2 may start to divert short circuit current away from the electric arc before the AC switch 28 has been opened. The AC/DC converter 2b would remain in a converter off state (i.e., all of the semiconductor switches in the phase legs 38a, 38b and 38c would be turned off) and would continue to be connected to the AC power network or utility grid 32 until it is detected that the AC switch 28 has opened. After it has been detected that the AC switch 28 is open, the DC/AC converter 2b may be transitioned to the short circuit state by turning on certain semiconductor switches in one or more of the phase legs 38a, 38b and 38c to provide one or more short circuit current paths between the positive and negative DC rails 40, 42—see FIGS. 11 and 14 below.

[0120] FIG. 10 shows an arrangement where the semiconductor switch 44 is turned on to provide a short circuit path P1 through the DC/DC converter 2a. This could represent an arrangement where no short circuit path is provided through the DC/AC converter 2b, or the first step in the arrangement described above where a short circuit path is initially provided only through the DC/DC converter 2a while the DC/AC converter 2b remains in a converter off state until it is detected that the AC switch 28 is open.

[0121] FIG. 11 shows an arrangement where the semiconductor switch 44 is turned on to provide a short circuit path P1 through the DC/DC converter 2a and both of the semiconductor switches in the first phase leg 38a of the DC/AC converter 2b are turned on to provide a short circuit path P2 through the DC/AC converter 2b. This could represent an arrangement where the short circuit paths P1, P2 are provided simultaneously through the DC/DC converter 2a and the DC/AC converter 2b, i.e., where the semiconductor switch 44 and the semiconductor switches in the first phase leg 38a are turned on at substantially the same time, or the second step in the arrangement described above where the short circuit path P2 is subsequently provided through the DC/AC converter 2b after it has been detected that the AC switch 28 is open, i.e., where the semiconductor switch 44 is turned on first and then the semiconductor switches in the first phase leg 38a are turned on at a later time.

[0122] FIG. 12 shows an arrangement where the semiconductor switch 44 of the DC/DC converter 2a is turned off and where both of the semiconductor switches in all three of the phase legs 38a, 38b and 38c of the DC/AC converter 2b are turned on at the same time to provide three parallel short circuit paths P1, P2 and P3 through the phase legs. No short circuit path is provided through the DC/DC converter 2a.

[0123] FIG. 13 shows an arrangement where the lower semiconductor switch in the first branch 52a is turned on to provide a short circuit path P1 through the DC/DC converter 2a. This could represent an arrangement where no short circuit path is provided through the DC/AC converter 2b, or the first step in the arrangement described above where a short circuit path is initially provided only through the DC/DC converter 2a while the DC/AC converter 2b remains in a converter off state until it is detected that the AC switch 28 is open. It will be understood that the upper anti-parallel connected diode in the first branch 52a provides a blocking or decoupling function as described above. It will also be understood that the lower semiconductor switch in the second branch 52b and/or the lower semiconductor switch in the third branch 52c may also be turned on to provide additional parallel short circuit paths through the DC/DC converter 2a if appropriate. Other ways of sequentially creating short circuit paths through one or more of the branches 52a, 52b and 52c of the DC/DC converter 2a, without intermission, may also be utilised.

[0124] FIG. 14 shows an arrangement where the lower semiconductor switch in the first branch 52a is turned on to provide a short circuit path P1 through the DC/DC converter 2a and where both of the semiconductor switches in the first phase leg 38a of the DC/AC converter 2b are turned on to provide a short circuit path P2 through the DC/AC converter 2b. This could represent an arrangement where the short circuit paths P1, P2 are provided simultaneously through the DC/DC converter 2a and the DC/AC converter 2b, i.e., where the lower semiconductor switch in the first branch 52a and the semiconductor switches in the first phase leg 38a are turned on at substantially the same time, or the second step in the arrangement described above where the short circuit path P2 is subsequently provided through the DC/AC converter 2b after it has been detected that the AC switch 28 is open, i.e., where the lower semiconductor switch in the first branch 52a is turned on first and then the semiconductor switches in the first phase leg 38a are turned on at a later time. Other ways of sequentially creating short circuit paths through one or more of the branches 52a, 52b and 52c of the DC/DC converter 2a and/or one or more of the phase legs 38a, 38b and 38c of the DC/AC converter 2b, without intermission, may also be utilised.

[0125] It will be readily understood that the arrangements shown in FIGS. 8 to 14 are only intended to show a range of different short circuit states for the power converter 2 and that other short circuit states are possible. For the purposes of the present disclosure, the critical feature is that at least one short circuit path is provided through the power converter when the short circuit state is enabled so that there is a short circuit between its DC input terminals.

[0126] The PV panels 8 are now short circuited at two parallel locations—namely by the electric arc caused by the short circuit fault in the DC link 12 and the deliberately-created short circuit current path(s) through the power converter 2. The short circuit current is therefore diverted away from the electric arc and through the power converter 2, which eventually results in the electric arc being extinguished.

[0127] At step 8, the method determines if the electric arc has been extinguished. For example, the methods used for electric arc detection may also be used to determine when the electric arc has been extinguished.

[0128] For example, if there is no longer a deviation in the currents measured on both sides of the DC switch 16, i.e., by sensors 66 in the DC input strings 20a, 20b, . . . , 20n or the sensors 68 in the combiner boxes 34a, 34b, . . . , 34n on the PV panel-side of the DC switch and by sensor 60 in the DC link 12 (i.e., on the power converter-side of the DC switch) which provides a measurement of the DC link current, this might indicate that the electric arc on the PV panel-side of the DC switch has been extinguished. If spectral analysis of the DC link voltage (i.e., as measured by sensor 62) and/or the current measured by sensor 64 between the DC link 12 and ground no longer shows high frequency components indicative of an electric arc, this might indicate that electric arc on the power converter-side of the DC switch 16 has been extinguished. Measurements of environmental conditions, e.g., temperature, light etc., particularly within a housing or cubicle, may indicate when the electric arc has been extinguished. For example, sensors might detect a sudden decrease in light intensity or temperature when the electric arc is extinguished.

[0129] If the electric arc has been extinguished, the short circuit state of the power converter 2 is disabled (step 9). For example, the power converter 2 is transitioned to the converter off state where it remains enabled, but all of the semiconductor switches are turned off or are in a blocking mode, or transitioned to another state.

[0130] At step 10, the method determines if the short circuit fault has been cleared. For example, the short circuit fault may have been cleared if the DC link voltage increases after the short circuit state of the power converter 2 is disabled. If the short circuit fault has cleared, the automatic re-start of the power converter 2 may be attempted, for example, and normal operation may eventually be resumed (step 11).

[0131] If the short circuit fault has not cleared, the DC switch 16 is opened (step 12) and the DC circuit 10 may be repaired (step 13).