DEVICE FOR TEMPORARILY TAKING OVER ELECTRICAL CURRENT FROM AN ENERGY TRANSFER OR DISTRIBUTION DEVICE, WHEN NEEDED

Abstract

A device (2) for the on-demand commutation of an electrical current from a first line branch (14, 3; 36) to another, second line branch (4; 41; 71) is created, which has a number of power semiconductor switching elements (7; 47; 53), which are arranged in series and/or parallel to one another in the second line branch (4; 41; 71), and a control unit (18; 51) for controlling the number of power semiconductor switching elements (7; 47; 53). The control unit (18; 51) is adapted to apply to each of the number of power semiconductor switching elements (7; 47; 53) an increased control voltage (VGE) whose level is above the maximum permissible control voltage specified for continuous operation, in order to switch on or maintain the conduction of the number of power semiconductor switching elements and to cause an increased current flow through it, whose current rating is at least double the nominal operating current. The control unit (18; 51) is further adapted to switch off the number of power semiconductor switching elements after a respectively provided short switch-on duration by switching off the control voltage (VGE) again while they conduct an increased current flow. The device (2) can thus be designed for a higher power in operation, or, at a given operating power, the semiconductor area and size of the device (2) can be reduced.

Claims

1. Device for the on-demand short-term transfer of an electrical current from an energy transmission or distribution device (14; 36) with at least one line branch (3; 41; 71) that is connected to the energy transmission or distribution device (36; 14), with a number of power semiconductor switching elements (7; 47; 53) arranged in series and/or parallel to one another in the at least one branch (3; 71; 41), and with a control unit (18, 51) for controlling the number of power semiconductor switching elements (7; 47; 53), whereby the control unit (18, 51) is arranged to apply to the number of power semiconductor switching elements (7; 47; 53) an increased control voltage (VGE), the size of which is above the specified maximum control voltage for continuous operation, to switch on the number of power semiconductor switching elements (7; 47; 53) and cause an increased current flow through the latter, the current strength of which is at least double the nominal operating current, and whereby the control unit (18, 51) is further adapted to control the number of power semiconductor switching elements (7; 47; 53) by switching off the control voltage (VGE) again while they conduct an increased current flow.

2. A device according to claim 1, whereby the positive control voltage is selected such that it is briefly more than 1.5 times, up to 4 times or even higher than thew maximum permissible control voltage specified by the manufacturer.

3. A device according to claim 1 or 2, whereby the current strength to be switched off is above the short-circuit current (ISC) at the recommended positive control voltage (VGE), and is up to 27 or more times the specified nominal current (IC nom).

4. A device according to any one of the preceding claims, characterized in that the energy transmission or distribution device (14, 36) is a high-voltage direct current (HVDC) transmission line and/or each of the number of power semiconductor switching elements (7; 47; 53) is formed from an IGBT.

5. A device according to any one of the preceding claims, characterized in that it is part of a hybrid direct current (DC) power switch (1), which has a main current branch (3) series-connected to the energy transmission or distribution device (14), which has a series circuit comprising an electronic auxiliary switch (11) and a fast mechanical disconnecting switch (12), and a bypass branch (4), which is connected in parallel to the main current branch (3), and an electronic main switch (6), which has the number of power semiconductor switching elements (7).

6. A device according to claim 5, characterized in that the electronic auxiliary switch (11) has one or more series-connected power semiconductor switching elements (13) arranged overall for a lower nominal power and for lower conduction losses than the series and/or parallel circuit of the number of power semiconductor switching elements (7) of the electronic main switch (6). A device according to claim 5 or 6, characterized in that the electronic main switch (6) has a series circuit with at least 2 or more, and for high-voltage applications at least 10 or dozens of power semiconductor switching elements (7).

7. A device according to any one of claims 5-7, characterized in that, in normal operation, the electronic auxiliary switch (11) is switched on conductively, the fast mechanical disconnecting switch (12) is closed and the current flows through the main current branch (3), and, in the case of detecting a fault in the energy transmission or distribution device (14), the electronic auxiliary switch (11) is first opened and the number of power semiconductor switching elements (7) of the electronic main switch (6) are energized with the increased control voltage (VGE) to commutate the current to the bypass branch (4), then the fast mechanical switch (12) is opened and then the electronic main switch (6) is opened to interrupt the increased current flow in the bypass branch (4).

8. A device according to any one of claims 1-4, characterized in that it is part of a power converter (33, 37; 39) with a number of parallel-connected branches (41a-c), whereby each branch (41a-c) has at least two series-connected electronic switches (47; 52; 59), which are controlled by the control unit (51) according to a determined pulse pattern to convert a first power with first current and voltage characteristics into a second power with second current and voltage characteristics.

9. A device according to claim 9, characterized in that the at least two series-connected electronic switches (47; 52; 59) in each case has a number of series- and/or parallel-connected power semiconductor switching elements (47; 53) that are controlled in the normal operation of the power converter with the increased control voltage (VGE) according to the determined pulse pattern.

10. A device according to claim 9 or 10, characterized in that the power converter (33, 37; 39) is a chopper resistor device (67) for protection against surge voltage in a DC intermediate circuit (48) of the power converter (33, 37; 39), whereby the chopper resistor device (67) is connected in parallel to the one or the number of parallel branches (41a-c) of the power converter, and has a series circuit of at least one resistor (68) and one electronic switch (69), whereby the electronic switch (69) is formed from the number of series- or parallel-connected power semiconductor switching elements (7; 47) which, when the voltage in the DC voltage intermediate circuit (48) exceeds a predetermined threshold, are energized for a short time, in a pulsed manner, and if necessary repeatedly with the increased control voltage (VGE).

11. A device according to any one of the preceding claims, characterized in that the control unit (18; 51) is adapted to energize the number of power semiconductor switching elements (7; 47; 53) between the switch-on and final switch-off with a control voltage (VGE) on at least one intermediate level, which is below that of the increased control voltage during switch-on, but above the maximum permissible control voltage specified for continuous operation.

12. Use of a series and/or parallel circuit of a number of power semiconductor switching elements (7; 47; 53), in particular IGBTs, in a device (1; 39; 67) for the on-demand short-term transfer of a current from an energy transmission or distribution device (14; 36) with at least one line branch (3; 41; 71) that is connected to the energy transmission or distribution device (14; 36) and in which the series and/or parallel circuit of the number of power semiconductor switching elements (7; 47; 53) is arranged, whereby the number of power semiconductor switching elements (7; 47; 53) are energized in operation with an increased control voltage (VGE), the size of which is above the maximum permissible control voltage specified for continuous operation, in order to switch on the number of power semiconductor switching elements (7; 47; 53) and cause an increased current flow through them, the current strength of which corresponds to at least double the nominal operating current, and whereby the number of power semiconductor switching elements (7; 47; 53) are switched off again after a switch-on time by switching off the control voltage (VGE) while they conduct an increased current flow.

13. Use according to claim 13, characterized in that the power semiconductor switching elements (7; 47; 53) are IGBTs, and the size of the increased control voltage (VGE) depends on a predetermined total lifetime until a breakthrough of a gate oxide layer between a gate and an emitter of the power semiconductor switching elements (7; 47; 53), an expected total switch-on time of the power semiconductor switching elements as the sum of the estimated switch-on times of all expected switch-on processes and an estimated reduction of the lifetime based on the expected switch-on processes and briefly increased working temperatures is selected to achieve the predetermined lifetime without causing damage to the power semiconductor switching elements (7; 47; 53).

14. Use according to claim 13 or 14, characterized in that the series and/or parallel circuit in a hybrid direct current (DC) power converter (1), preferably for high-voltage direct current (HVDC) transmission applications, is used, whereby the hybrid DC main switch (1) has a main current branch (3) series-connected to the energy transmission or distribution device (14, 36), having a series circuit made from an electronic auxiliary switch (11) and a fast mechanical disconnecting switch (12), and a bypass branch (4) connected in parallel to the main current branch (3) and containing an electronic main switch (6), comprising the series and/or parallel circuit of the number of power semiconductor switching elements (7), whereby the series and/or parallel circuit has at least 2 or more, and for high-voltage applications at least 10 or dozens of power semiconductor switching elements (7).

15. Use according to claim 13 or 14, characterized in that it is used in a power converter (33, 37; 39), preferably for high-voltage DC applications, whereby the power converter (33, 37; 39) has one or more parallel-connected branches (41a-c), whereby each branch (41a-c) has at least two series-connected electronic switches (47; 52; 59), which are energized according to a predetermined pulse pattern to convert a first power with first current and voltage characteristics into a second power with second current and voltage characteristics, and whereby the at least two series-connected electronic switches (47; 52; 59) each have a number of series- and/or parallel-connected power semiconductor switching elements (47; 53), which, in the normal operation of the power converter, are energized with the increased control voltage (VGE) according to the determined pulse pattern.

16. Use according to claim 13 or 14, characterized in that it is used in a two-stage or multistage power converter (33, 37; 39), whereby the power converter (33, 37; 39) has at least one DC voltage intermediate circuit (48), one or more branches (41a-c) connected in parallel to the at least one DC voltage intermediate circuit (48) and to each other, with at least two series-connected electronic switches (47; 52; 59), which are energized according to a predetermined pulse pattern in order to convert a first power with first power and voltage characteristics into a second power with second current and voltage characteristics, and a chopper resistor device (67) for protection against surge voltage in the at least one DC voltage intermediate circuit (48) which is connected in parallel to the at least one DC voltage intermediate circuit (48) and has a series circuit with at least one resistor (68) and one electronic switch (69) which is formed from the series and/or parallel connection of a number of power semiconductor switching elements (47; 53) which, when the voltage in the DC voltage intermediate circuit (48) exceeds a predetermined threshold voltage, are energized for a short time, and in a pulsed manner, and repeatedly if necessary, with the increased control voltage (VGE).

Description

[0030] Further advantageous details of embodiments of the invention result from the dependent claims, the drawings and the associated description. The invention is described in more detail below with reference to a drawing, which shows exemplary, non-limitative embodiments of the invention, whereby identical reference numerals are used in all figures to designate identical elements. Description:

[0031] FIG. 1 shows a hybrid direct current (DC) power converter, comprising a number of series-connected power semiconductor switching elements, according to a first embodiment of the invention, in a greatly simplified representation;

[0032] FIG. 2 shows measured voltage and current processes in the switching of a power semiconductor switching element, which is usable e.g. in the hybrid DC power switch according to FIG. 1 to illustrate the principle of embodiments of the invention;

[0033] FIG. 3 shows a schematic representation of an output characteristic curve of an IGBT illustrating a so-called “SURGE” operation (overload operation) in a greatly simplified schematic representation;

[0034] FIGS. 4a and 4b show voltage and current processes similar to FIG. 2 to illustrate further developments of embodiments of the invention, in a greatly simplified representation;

[0035] FIG. 5 shows a wind farm with a high-voltage direct current transmission (HVDC) system with devices for on-demand short-term transfer of an electrical current from an energy transmission or distribution device according to further embodiments of the invention.

[0036] FIG. 6 is a schematic block diagram of an exemplary power converter according to an embodiment of the invention, in a greatly simplified schematic representation;

[0037] FIG. 7 shows a parallel circuit of a number of power semiconductor components which can be used according to the invention in one of the devices or one of the systems according to FIGS. 1 to 6; and

[0038] FIG. 8 shows a circuit configuration of a submodule with power semiconductor components which can be used according to the invention in one of the devices or one of the systems according to FIGS. 1 to 6.

[0039] FIG. 1 shows a hybrid direct current (DC) power converter 1, which forms a first preferred embodiment of a device 2 for on-demand short-term transfer or commutation of an electrical current according to the invention. The hybrid DC power switch 1 can be used as a so-called “DC breaker”, e.g. in high-voltage direct current transmission (HVDC) systems, such that, in the case of a short-circuit in a downstream part of the DC network, the defective part can be isolated quickly and safely to continue to maintain the function of other parts of the system. Existing mechanical DC disconnecting switches are not able to completely interrupt the DC within a few milliseconds, as is required in HVDC applications. They also require complex steps to prevent or extinguish arcing when opening the disconnecting switch. Semiconductor-based DC breakers can be switched very quickly and without arcing. The hybrid DC power switch 1 overcomes these shortcomings and is able, in high-voltage applications of 50 kV or more, e.g. in an HVDC system with a network voltage of 300-400 kV, to reliably interrupt currents in the kA range, e.g. up to 10 kA or more.

[0040] The hybrid DC power switch 1 has a main current path 3 and a bypass branch 4 parallel to this. The bypass branch 4 has a main electronic switch 6, which here comprises a number of mutually series-connected power semiconductor switching elements 7. According to the DC voltage of the application and the performance data of the power semiconductor switching elements 7 used, the series circuit can comprise several tens or even several hundred power semiconductor switching elements 7. As illustrated, power semiconductor switching elements 7 are preferably provided for both current flow directions, whereby bidirectional power semiconductor switches can be used here which are commonly available in different configurations. If the polarity of the current is determined, then conventional unidirectional switches can also be used.

[0041] The semiconductor-based main switch 6 is divided into number of sections with 8 individual surge voltage or surge current arresters 9, which are arranged parallel to the number of series-connected power semiconductor switching elements 7 and formed from nonlinear resistors.

[0042] The main current branch 3 has a series circuit of an electronic auxiliary switch 11 and a fast mechanical disconnecting switch 12. The electronic auxiliary switch 11 is also semiconductor-based and has a number of power semiconductor switching elements 13 which are preferably also equipped for bidirectional current transmission. The auxiliary switch 11 is adapted for significantly lower blocking voltage than the main switch 6 and, for this purpose, preferably has a series circuit of power semiconductor switching elements 13, the number of which is significantly lower than in the main switch 6. Optimally, only a single switch is used in series. It can be designed for lower voltage than the switch 7. If the polarity of the current and thus the direction of energy flow is determined, then a conventional unidirectional switch can also be used. The switch 11 may also consist of elements other than the elements 7, e.g. GCTs.

[0043] The fast mechanical disconnecting switch 12 is capable of fast switching in the millisecond range and can be based e.g. on a known gas-insulated switching technology, etc. As FIG. 1 further shows, in an energy transmission or distribution device, represented here by a transmission line 14, in which the hybrid DC power switch 1 is inserted, a residual current protection switch 16 can further be provided, which serves to trigger the hybrid DC power switch 1 to interrupt a residual current flowing in the line 14. Further, a current-limiting choke 17 can be provided to limit the rise in current in the case of a short-circuit, e.g. in a DC power network or a busbar, to which the hybrid DC power switch 1 is connected.

[0044] As can be further seen in FIG. 1, a control unit 18 is provided which serves to control the components of the hybrid DC power switch 1 in operation. The control unit 18 can monitor and control the overall operation of the hybrid DC power switch 1 and may for this purpose e.g. monitor the current flow through the line 14 with a current sensor 19. The control unit 18 could also monitor currents in the main current branch 3, and the bypass branch 4, as well as the voltage potentials at these on demand (not illustrated). The control unit could itself be controlled by a superordinate control device (not illustrated in detail here), which can take over the task of monitoring and controlling the hybrid DC power switch 1, while the control unit 18 can only generate and create the appropriate control signals for the power semiconductor switching elements 7, 13. The control unit 18 could also be subdivided into several control subunits assigned to the individual power semiconductor switching elements 7 or to a group of them (not illustrated).

[0045] The hybrid DC power switch 1 described so far functions as follows:

[0046] During normal operation, the fast mechanical disconnecting switch 12 is closed and the power semiconductor switching elements 13 of the electronic auxiliary switch 11 are also closed or switched on so as to be conductive. With the DC power switch 16 closed, the current then flows only through the main branch 4 and the switches 11 and 12 arranged therein. The electronic main switch 6 can be closed, i.e. switched on so as to be conductive, or opened, and thus rendered nonconductive. Since it has a much greater impedance than the auxiliary switch 11, then no current flows through the bypass branch 3.

[0047] When a direct current fault occurs, in particular a short-circuit in the downstream DC power network, the auxiliary switch 11 is energized by the control unit 18 or other control device to open or move into the nonconductive state. Thus, the current flowing through the main current branch 3 commutates to the bypass branch 4. Once the auxiliary switch 11 is not conductive within a few microseconds, and the entire current is commutated to the bypass current branch 4, the fast mechanical disconnecting switch 12 is opened. With the mechanical disconnecting switch 12 opened, the main switch 6 can then interrupt the fault current.

[0048] Once the main switch 6 is opened by the control unit 18, the current commutates to the parallel arresters 9 and is converted into heat. After the current has largely decayed, the DC power switch 16 is opened on demand to completely interrupt the current flow.

[0049] The mechanical disconnecting switch 12 isolates the auxiliary switch 11 with regard to the primary voltage across the main switch 6 while the current is interrupted. Thus, the required nominal or blocking voltage of the auxiliary switch 11 is reduced significantly. In the conductive state, the auxiliary switch 11 has forward voltages in the range of comparatively few volts, such that the transmission losses of the hybrid DC power switch 1 are greatly reduced compared to a purely semiconductor-based circuit breaker. The high power losses of the main switch 6 that apply only during the short duration of the commutation of the current in the main switch 6 and further on the arrester 9 in a triggering of the hybrid DC power switch 1, are insignificant.

[0050] To provide the required blocking voltage, the electronic power switch 6 requires a number of power semiconductor switching elements 7 that receive the current load in the case of short-circuit. In HVDC applications with a voltage of 300-400 kV and e.g. IGBT semiconductor switches as the power semiconductor switching elements 7 with performance data of e.g. 1.5 kA/3.3 kV, up to 200 IGBT switches or more are required for the implementation of the main switch 6. This represents an enormous semiconductor area, which increases the implementation costs enormously. Moreover, additional devices are required for cooling the hybrid power converter 1 and to monitor and control it. A hybrid DC power switch 1 for HVDC applications can proportionally achieve a high single-digit percentage easily on the overall power converter. The present invention makes it possible to significantly reduce this size and the effort and costs associated with the implementation and operation of the hybrid DC power switch 1.

[0051] Before discussing the invention in more detail, it should be noted that FIG. 1 illustrates IGBTs T as the power semiconductor switching elements 7. IGBTs are commonly known, are characterized by a particularly good switching behavior, good transmittance and a low control power and are used for different power ranges during operation. Each IGBT T has a control electrode, called gate G, and an emitter E and a collector C, as further electrodes. Advantageously, an internal free-wheeling diode is connected antiparallel to the IGBT T to which a current can be commutated if required. However, in principle, other power semiconductor components, e.g. BIGTs, MOSFETs, etc. are used according to the invention. The terms collector, emitter and gate used herein thus refer to the preferred use of IGBTs as the power semiconductor switching elements 7 (and 13), whereas for the person skilled in the art, the corresponding designations for terminals and electrodes of other similar semiconductor components are commonly used.

[0052] As mentioned above, the hybrid DC power switch 1, in particular its main switch 6, according to the invention must be optimized to the required semiconductor area, the space requirements and the associated implementation and operating costs. For this purpose, the control unit 18 is adapted to the power semiconductor switching elements 7 or IGBTs T of the main switch 6 on the triggering of the hybrid DC power switch 1 with an increased control voltage and gate-emitter voltage VGE, the height of which is above the maximum control voltage specified for continuous operation. In general, IGBTs are operated for continuous operation with a gate-emitter voltage of around 15V. According to the manufacturer's specifications, the gate-emitter voltage for continuous operation may typically not exceed 20V, as this could otherwise lead to a breakthrough of the thin insulating gate oxide layer under the gate G, causing destruction of the IGBT.

[0053] According to the invention, the IGBTs 7 of the main switch 6 for a commutation of the short-circuit current from the main current branch 3 to the bypass branch 4 can be operated by applying an increased gate-emitter voltage which is greater than the maximum permissible 20V and is e.g. 30V or 50V. If necessary, if statistically few short-circuits are reported in the DC power network, a gate-emitter voltage of 70V can also be selected for the operation of the main switch 6.

[0054] This increased control voltage has the result that each IGBT T of the main switch 6 can cause increased currents compared to the nominal operating current, which can be at least two times and even up to ten times or more the nominal operating current. After this increased current flows for only a short time after the commutation of the current from the main current branch 3 to the bypass branch 4 until the complete opening of the main switch 6, in general for a time shorter than a millisecond, or only a few milliseconds, the slight impairment of the insulating gate oxide layer in operation can be accepted, while still ensuring a sufficient lifetime of the hybrid DC power switch 1.

[0055] With the inventive step, the increase of the control voltage over the maximum allowable range specified for continuous operation, the hybrid DC power switch 1 can thus be used for higher transmitted DC voltages in the network. Vice versa, the number of similar power semiconductor switching elements 7 in the main switch 6 for the designed current load can be significantly reduced, and in particular can be reduced by half or even more.

[0056] This results in a corresponding reduction in the required semiconductor area and the associated costs of the main switch 6 as such and the hybrid DC power switch 1 in general. Also the effort and the costs associated with cooling, monitoring, control equipment, lines, etc., which are necessary for cooling, control and operation of the hybrid power converter 1 can be reduced significantly.

[0057] FIG. 2 shows measured processes of the voltage and current signals in switching an IGBT 7 on and off, as can be used in the main switch 6 of the hybrid DC power switch 1 of to FIG. 1, corresponding to the control method according to the invention. In particular, it shows a measurement of the switch-off of an IGBT specified on the nominal current IC nom=50 A at a collector current IC of 1,350 A, i.e. 27 times the nominal current. It shows the gate-emitter voltage VGE, the collector-emitter voltage VCE and the collector current IC over time t.

[0058] Before switching on, VGE=−5V, the DC link voltage is 2,000V. At t=12 μs, a positive gate voltage VGE=50V is applied. The current rises according to the inductance of the load. At t=56 μs, the switch-off signal is set. VGE drops to the Miller plateau, which now appears at around 30V with the increased current. The IGBT now starts to desaturate at around t=65 μs. According to the still rising current IC, VGE continues rising a little in the Miller plateau. The current IC likewise increases. Finally, a current IC of 1,350 A an is switched off successfully, which corresponds to 27 times the nominal current of 4.5 kV IGBT chips. At the end of the process, the gate voltage falls again to the applied −5V.

[0059] Similarly, the destruction limit of the IGBT is shifted upward during operation with increased gate voltage. FIG. 3 shows a schematic representation of an output characteristic curves of an IGBT, which shows that the so-called

[0060] SURGE operation (overload operation) of the IGBT at an increased gate voltage of e.g. 50V here falls within the saturation area. Accordingly, a safe distance must be maintained with regard to the respective application, which provides a sufficient distance from the destruction limit in the active area. Considering these criteria, a surge operation is made possible for transient overload cases with a “surge” current I Surge, which corresponds to 5 to 25 times the nominal current IN and a multiple of the short-circuit current ISC, at a gate-emitter voltage significantly higher than the gate-emitter nominal voltage.

[0061] FIGS. 4a and 4b show voltage and current processes similar to FIG. 2, but limited to the switch-off of an IGBT, in order to illustrate further developments of embodiments of the invention in a greatly simplified representation. FIG. 4a shows that a gate-emitter voltage VGE above the maximum permissible voltage for continuous operation of e.g. 35V is applied. Thus, e.g. five times the nominal operating current can be passed through the IGBTs of the main switch 6 of the hybrid DC power switch 1 according to FIG. 1. Nevertheless, this high current can be switched off safely.

[0062] As can be seen, the collector current IC falls relatively quickly, with a high slope, when the VGE is switched off. Further, it can be seen that the collector-emitter voltage VCE increases relatively quickly, with a high slope, up to the blocking voltage. Shortly before reaching the blocking voltage, VCE shows an interrupting voltage peak (voltage excess). The interrupting voltage is dependent on the leakage inductance and the thus induced voltage in this according to the switched di/dt and the switch-on voltage peak of the diode.

[0063] Such high collector current slopes di/dt, collector voltage slopes dv/dt and voltage excesses may damage the IGBT and cause increased switching losses. The control unit 18 can also arranged according to embodiments of the invention to optimize the di/dt and dv/dt processes, minimize surge voltages and reduce switching losses as well as to influence switch-on and switch-off times.

[0064] In a preferred embodiment, the control unit 18 for this purpose is arranged to control a power semiconductor switching element, e.g. the IGBT switch 7 of the main switch 6 of the hybrid DC power switch 1 according to FIG. 1, between the switch-on and the final switch-off with a control voltage on at least one intermediate level, which is below that of the increased control voltage during the switched-on state, but above the maximum permissible control voltage specified for continuous operation. For example, FIG. 4b illustrates an advantageous switch-off process for an IGBT, e.g. the main switch 6, in which firstly the increased gate-emitter voltage VGE of 50V is applied, and then an increased gate voltage above 20V is switched to, here e.g. 30V, before a voltage of −5V is finally switched to at the gate in order to switch off the switch permanently. The stepped switching of the gate voltage also allows a reduction in the collector current change speed di/dt and the voltage increase dvCE/dt. As can be seen from the slightly rising course of VCE shortly before its steeper rise, the intermediate stage at 30V further causes the IGBT to be desaturated more strongly, whereby both the IGBT collector current and the interrupting voltage peak of VCE are limited. The provision of one or more intermediate stages for the gate voltage VGE supports the safe switch-off of even very large currents, up to ten times the nominal operating current.

[0065] Yet other steps can be taken to support the safe switch-off of an IGBT, and thus e.g. the main switch 6 in FIG. 1, even at very high collector currents. For example, for the control of the gate and for the switch-on and switch-off with different gate resistances, various driver stages can be provided, which can be selected individually depending on the load current, voltage and temperature to prevent critical operating conditions, e.g. surge voltages, over-currents and oscillations, to optimize the switching operations dependent on time and to reduce switching losses. Further, a surge voltage limit (so-called active clamping) can be used to reduce the collector current change rate di/dt and also the maximum value of the collector-emitter voltage by a feedback of the collector-emitter voltage on the control input of the IGBT during the switch-off process. Further, the collector current change rate di/dt can be controlled or reduced actively to e.g. reduce the load on the free-wheeling diode. Likewise, the collector-emitter-voltage change rate can be controlled or reduced. A soft switch-off can also be accomplished by a continuous reduction of the gate-emitter voltage VGE.

[0066] Advantageously, the control voltage or gate-emitter voltage VGE to be selected can be selected appropriately according to the presettable lifetime and the estimated operating parameters. It is known that the lifetime until breakthrough of the gate oxide layer depends on the field strength at the oxide layer, the temperature of the oxide layer, the semiconductor area and the change in enthalpy from a stable state to an energized transition state for the breakthrough. The field strength at the oxide layer corresponds to the quotient of the applied gate voltage VGE and the oxide thickness of the gate oxide layer. Calculations and simulations have shown that doubling the gate voltage from usually 15V to 30V reduces the lifetime by a factor of around 300. In addition, the dependency of the lifetime of the gate oxide layer until the breakthrough of the gate-emitter voltage appears to be exponential. Thus, the amount of the increased control voltage and gate-emitter voltage VGE depending on a preset lifetime until the breakthrough of the gate oxide layer between the gate G and the emitter E of the power semiconductor switching elements 7 with an estimate of an expected total switch-on time of the power semiconductor switching elements 7 as the sum of the estimated switch-on times of all expected switch-on processes and considering an estimated reduction of the lifetime based on the expected switch-on processes can be selected such that the preset lifetime can be achieved with high probability, without leading to destruction of the power semiconductor switching elements 7. It can be estimated with consideration of the given or intended operating conditions whether, in the given application, a further increase in the gate-emitter voltage is possible, e.g. to +50V or even beyond.

[0067] FIG. 5 shows a further embodiment in which the invention can be used effectively. A wind farm 21 with a number of wind turbines 22 for power generation and a high-voltage direct current (HVDC) transmission system 23 is illustrated, which is used to transmit the power supplied from the wind farm to a downstream AC network 24, e.g. a power distribution network or a public supply network. As is generally known, each wind turbine 22 has rotor blades 26, which convert the kinetic energy of the wind into mechanical energy in a rotating shaft, which is connected to a generator 27. The generator 27 converts the mechanical energy of the shaft into electrical energy, which is then converted by means of power electronics, e.g. a full-scale converter or frequency inverter 28, and a downstream transformer 29, into a suitable voltage and a suitable current for feeding into a common busbar 31 of the wind turbines 22. The busbar voltage, which can be e.g. 33 kV, can then be transformed up by means of a power converter transformer, to e.g. ±150 kV for transmission over the HVDC system 23.

[0068] The HVDC system 23 basically comprises a rectifier 33, which converts the three-phase alternating voltage of the power converter transformer 32 into a DC voltage, which is transmitted through DC lines 36 of the HVDC system 23. For example, the lines 36 can transmit DC voltages of 300 kV or more over distances that can be hundreds of kilometers.

[0069] The transmitted power on the DC voltage side, which is a high voltage, is converted by an inverter 37 into an alternating voltage, which is a high voltage of e.g. 150 kV, which can be transformed via a power converter transformer 38 into an appropriate voltage of the downstream alternating-voltage network 24.

[0070] The rectifier 33 and the inverter 37 are power converters that must be designed for high performance. Various power converter topologies are known. An exemplary topology of a power converter 39 suitable for this purpose is shown in FIG. 6.

[0071] The power converter 39 here comprises three phase branches 41a, 41b, 41c, which extend between a positive power bus and a positive DC voltage terminal (“+”) 42 of the power converter 39 and a negative power bus and a negative DC voltage terminal (“−”) 43. Although here three phase branches 41a-c are illustrated, it is understood that, according to the number of phases in the respective application or circuit environment, only a single phase branch 41 or two or more than three phase branches can be present.

[0072] Each phase branch 41a-c has a first upper branch arm 44a in FIG. 6, and a second lower branch arm associated with it 44b, which are connected together at a connection point that defines the respective alternating voltage terminal 46a, 46b and 46c of the respective phase branch 41a, 41b and 41c. In each branch arm 44a, 44b of each phase branch 41a-c, a series circuit of a number of power semiconductor switching elements 47 (T1 . . . Tn) is arranged in each case, which are designed in common for the respective operating voltages and currents.

[0073] Parallel to the phase branches 41a-c, a DC voltage intermediate circuit 48 is provided, which is exemplified here by a DC capacitor C 49 connected between the DC voltage terminals 42, 43.

[0074] As illustrated, the power semiconductor switching elements 47 are formed advantageously from IGBTs, which are controlled by a control unit 51 in a specific pulse pattern according to a predetermined modulation method, e.g. by pulse width modulation (PWM), to convert the DC voltage Vdc in the DC voltage intermediate circuit 48 into a here three-phase alternating voltage vac on the alternating-voltage terminals 46a-c, or vice versa. With a variety of IGBTs per branch arm 44a,b, the power converter 39 can be designed with sufficient blocking capacity.

[0075] According to the invention, the control unit 51 is here adapted to control the power semiconductor switching elements 47 of the respective series circuits in the respective branch arms 44a,b of the respective phase branches 41a-c respectively with a gate-emitter voltage, which is above the specified maximum control voltage for the power semiconductor switching elements 47 (gate-emitter voltage for IGBTs). At a maximum permissible gate-emitter voltage of 20V, the control unit 51 can here apply, in a similar manner to the embodiment according to FIG. 1, a gate-emitter voltage of e.g. 30V or, if the lifetime permits, 50V, in order to permit higher collector or branch arm currents that can be greater than twice, and even as much as five to ten times, the nominal operating current. In this way, the same power converter can be used for significantly higher powers, or vice versa, for a given nominal operating power, the number of power semiconductor switching elements 47 and the associated semiconductor area can be reduced. As a result, the implementation and operating costs of the power converter 39 and the system in which it is used, e.g. the HVDC system 23 according to FIG. 5, can be reduced.

[0076] Although FIG. 5 illustrates an HVDC system 23 for the coupling of the wind farm 21 to the network 24, an MV-DC system for the coupling of wind turbines or photovoltaic systems or drives can also be used according to the invention. In addition, the power converter used is not limited to the power converter type 6 specifically shown in FIG. 6. Rather, a variety of topologies of power converters with two or several stages can be used.

[0077] FIG. 7 shows a possible modification that can be applied to the power converter according to FIG. 6 that permits the advantageous use of the step according to the invention for the increase of the gate-emitter voltage beyond the permissible range. FIG. 7 illustrates a power semiconductor module 52 that can replace a series circuit made from power semiconductor switching elements 47 (T1 . . . Tn) in a respective branch arm 44a and 44b of each phase branch 41a-c. The power semiconductor module 52 has a parallel circuit made from semiconductor power switching elements 53 (T1 . . . Tn) connected in parallel to one another between a first terminal 54, with which all the collectors C of the power semiconductor switching elements 53 are connected, a second terminal 56, with which all the emitters E of the power semiconductor switching elements 53 are connected, and a common control or gate terminal 57. The power semiconductor module 52 may comprise any number of at least two mutually parallel IGBTs T1 . . . Tn.

[0078] As can further be seen in FIG. 7, a gate resistor can be arranged in each case between the gate electrode G of each IGBT T1 . . . Tn and the common control or gate terminal 57. These resistors 58 can be used for balancing of the circuit to cause all the IGBTs T1 . . . Tn to be able to be switched as simultaneously and uniformly as possible by the control unit 51. The gate series resistors can be arranged together with the IGBTs T1 . . . Tn 53 on the semiconductor chip or even outside of these chips.

[0079] As mentioned above, the power semiconductor module 52 can replace a respective series circuit of the number of power semiconductor switching elements 47 of the respective branch arms 44a and 44b. The series circuit of IGBTs is thus replaced by a parallel circuit, whereby each branch arm 44a,b of the power converter 39 is now designed for a larger current. Advantageously, by increasing the gate-emitter voltage VGE above the maximum permissible gate-emitter voltage according to the specification, each of the IGBTs T1 . . . Tn 53 in FIG. 7 are briefly loaded with an increased current that can be two or several, up to ten times the nominal current. Thus, the power converter can be designed for even better performance, or vice versa, for a given power, the number of parallel-connected IGBTs T1 . . . Tn 53 and the associated semiconductor chip area can be reduced, which lowers the implementation and operating costs.

[0080] In yet another optional modification, the parallel circuit of the IGBTs T1 . . . Tn according to FIG. 7 respectively can replace a single element of the power semiconductor switching elements 47 in FIG. 6. Thus, each branch arm 44a and 44b has a series circuit made from a number of parallel-connected IGBTs in each case. Such a power converter 39 is suitable for very high load currents and very high voltages, whereby the control according to the invention can be used particularly effectively here to reduce the space requirement, effort and costs.

[0081] FIG. 8 shows a bidirectional submodule 59 which can be used as another option in the power converter 39 according to FIG. 6 to create a modular multistage power converter. The submodule 59 has a full bridge configuration with a series circuit of IGBTs T1, T2 (53), each with antiparallel free-wheeling diodes D1, D2 and a second series circuit of IGBTs T3, T4 (53) with associated antiparallel free-wheeling diodes D3, D4, whereby both series circuits are connected to one another between a first DC voltage node 61 and a second DC voltage node 62. Further, a capacitor C serving as the energy storage 63 is connected in parallel to the two series circuits. The connection points between the IGBTs T1, T2 and T3, T4 of the respective series circuits form a first and second alternating voltage terminal 64 and 66.

[0082] The DC voltage vdc across the capacitor C is always positive due to the wiring of the free-wheeling diodes D1-D4 and may, according to the design and application, e.g. be between a few hundred volts and a few kV. The terminal voltage vac between the AC terminals 64, 66 of the submodule 59 can generally assume the values −vdc, +vdc or 0. The DC voltage vdc across the capacitor C can be larger or smaller as already described. A current can, in principle, flow through the submodule 59 in both directions.

[0083] The submodule 59 can replace each of the power semiconductor switching elements 47 in the power converter 39 according to FIG. 6, whereby a modular multipoint power converter is created. Due to the modular design, the power converter is individually scalable for different powers and applications. The voltages and currents on the AC and DC side can be controlled and regulated in a highly dynamic way and largely decoupled from one another. Advantageously, the benefits are also increased even further here by brief energization with an increased gate-emitter voltage, which goes beyond the maximum permissible gate-emitter voltage of the IGBT. Vice versa, at the same power, the semiconductor area and size of the power converter 39 can be reduced, which allows the advantages of reducing the effort and the associated costs for the implementation and operation of the power converter 39. To prevent repetitions, reference is made to the above explanations in relation to the hybrid DC power switch according to FIG. 1, whereby the power converter 39 according to FIG. 6, which represents any rectifier (e.g. 33 in FIG. 5), inverter (e.g. 37 in FIG. 5) or frequency inverter (e.g. 28 in FIG. 5), here forms a device 2 for the on-demand short-term transfer of an electrical current according to the invention.

[0084] It must further be noted that the power converter according to FIG. 6 must be used with the inventive step of the increased gate-emitter voltage in principle only in applications in which it is only used relatively rarely or for a short time, such that the time to breakthrough of the gate oxide layer of the IGBTs does not exceed the desired lifetime. However, the power converter is suitable for numerous drive systems that are used only for a short time and/or rarely. The required semiconductor area, the size of the power converter 39 and its weight can be reduced significantly by the invention, which is extremely useful in many applications.

[0085] A further application of the invention to the power converter 39 can be seen in FIG. 5. FIG. 5 illustrates a so-called chopper resistor device 67, which is connected between the DC voltage lines 36. The chopper resistor device 67 is used to protect against surge voltage of the DC bus capacitance of the inverter 37. For this purpose, the chopper resistor device 67 has a series circuit of at least one resistor 68 and an electronic switch 69, which are arranged in a line branch 71 parallel to the inverter 37. In the present case, two such pairs of the resistor 68 and the electronic switch 69 are arranged symmetrically to a grounded midpoint between the DC lines 36 of the HVDC system 23. If a fault occurs in the AC power network 24, the switches 69 are energized in a pulse-like manner, possibly repeatedly, to dissipate the excess energy or consume it in the resistors 68, thus limiting a DC voltage rise in the intermediate circuit of the inverter 37 to safe levels.

[0086] Each electronic switch 69, represented only symbolically in FIG. 5, is preferably formed from a series circuit of power semiconductor switching elements, e.g. similar to the switching elements 7 and 47 in the electronic main switch 6 of the hybrid DC power switch 1 according to FIG. 1 or in the power converter 39 according to FIG. 6. In this way, the electronic switch 69 with the required blocking capacity, in particular for HVDC applications, can be created. The inventive step of increasing the energization or gate-emitter voltage VGE for the individual power semiconductor switching elements 7 and 47 of the switch 69 can here significantly reduce the required semiconductor area and the size of the chopper resistor device 67. Such a chopper resistor device 67 for HVDC applications, e.g. for the HVDC system 23 according to FIG. 5, can reach an enormous size. Advantageously, the invention can reduce this size to a fraction, e.g. one half or even less, of the normal size.

[0087] It should be noted that the chopper resistor device 67 is not limited to use in HVDC systems. It can be used e.g. as a brake chopper in drive systems to convert excess energy that flows in the form of a regenerative current from the motor back to the DC intermediate circuit into thermal energy via the braking resistor during braking. The electronic switch, e.g. 69, then switches on the connected resistor, e.g. 68, before the DC intermediate circuit voltage reaches an impermissible level for the components. Once the DC intermediate circuit voltage decreases again and becomes lower than the switch-on voltage, but higher than the network voltage, the switch 69 switches the resistor 68 off again. The process is repeated as soon as the voltage rises again. Even such brake choppers are used for a relatively short time and/or rarely, such that the inventive step of increasing the energization voltage can be used here advantageously over permissible limits.

[0088] The person skilled in the art will recognize that, in addition to the illustrated exemplary and particularly preferred applications, other applications will be apparent for the invention, in which a series and/or parallel circuit of power semiconductor switching elements can be energized during normal operation for a relatively short time and/or rarely and/or in a pulse-like manner, such that the use for voltage reduction according to the invention becomes feasible.

[0089] A device 2 for the on-demand commutation of an electrical current from a first line branch 14, 3; 36 to another, second line branch 4; 41; 71 is created, which has a number of power semiconductor switching elements 7; 47; 53, which are arranged in series and/or parallel to one another in the second line branch 4; 41; 71, and a control unit 18; 51 for controlling the number of power semiconductor switching elements 7; 47; 53. The control unit 18; 51 is adapted to apply to each of the number of power semiconductor switching elements 7; 47; 53 an increased control voltage VGE whose level is above the maximum permissible control voltage specified for continuous operation, in order to switch on or maintain the conduction of the number of power semiconductor switching elements and to cause an increased current flow through it, whose current rating is at least double the nominal operating current. The control unit 18; 51 is further adapted to control the number of power semiconductor switching elements according to a respectively provided short switch-on duration by switching off the control voltage

[0090] VGE again while they conduct an increased current flow. The device 2 can be designed for a higher power in operation or, at a given operating power, the semiconductor area and size of the device 2 can be reduced.