HK1179769B - Switching module for use in a device to limit and/or break the current of a power transmission or distribution line - Google Patents

Description

Switching module in a device for limiting and/or interrupting the current of a power transmission or distribution line

The invention relates to a switching module arranged in a device for limiting and/or breaking a current flowing through a power transmission or distribution line, wherein the switching module comprises: at least one power semiconductor switching element, a gate (gate) unit arranged to switch the at least one power semiconductor switch on and off, respectively, in dependence on a switch control signal, and a storage capacitor arranged to provide power to a power supply input of the gate unit.

The invention originally originates from the field of High Voltage Direct Current (HVDC) breakers and current limiters, i.e. switching devices, which are capable of limiting and/or breaking a direct current flowing through an electric power transmission line, wherein the line is at a voltage level above 50 kV. However, the invention is also applicable to circuit breakers for medium voltage direct current power distribution, i.e. for a DC voltage range between about 1kV and 50kV, and the bidirectional embodiments of the invention are even applicable to circuit breakers for AC power transmission and distribution at any voltage level.

In EP0867998B1, a solid state DC circuit breaker is described comprising a parallel connection of at least one main power semiconductor switch and a non-linear resistor operating as a surge arrester. When operating the dc breaker to interrupt the dc current in the dc transmission or distribution line, the at least one main power semiconductor switch commutates (communtates) the dc current into the non-linear resistor, which then reduces the dc current by dissipating the energy stored in the dc line. In PCT/EP2009/065233 a further solid state dc breaker is proposed, which in parallel with the parallel connection of the main power semiconductor switch and the surge arrester comprises a series connection of a mechanical high speed switch and at least one auxiliary power semiconductor switch.

In practice, such solid-state dc circuit breakers need to comprise a large number of main power semiconductor switches connected in series, in order to be applicable to the voltage levels of a dc power transmission or distribution system, since the individual power semiconductor switches have a rather low voltage rating. With HVDC voltage levels of hundreds of thousands of volts, the number of main power semiconductor switches connected in series can easily reach several hundred.

In connection with the invention, the main power semiconductor switches of the dc circuit breaker or dc current limiter and the auxiliary power semiconductor switches which may be present each represent a switching module, i.e. they comprise, in addition to one or more power semiconductor switching elements, a gate unit and an energy storage capacitor. Such a switching module is described in detail in EP0868014B1, for example, in which a storage capacitor is connected to the power supply input of a gate unit via a dc/dc converter. The energy stored in the capacitor is converted via the dc/dc converter into the dc supply voltage required by the gate unit for its usual operation of switching on and off the at least one power semiconductor switching element. The storage capacitor is itself connected to a so-called high voltage primary circuit, i.e. it is connected to the same circuit, and thus to the same high voltage level, as the at least one power semiconductor switching element of that particular switching module. The energy storage capacitor is charged whenever at least one power semiconductor switching element is in a blocking state, i.e. a non-conducting-switching state.

With regard to the dc breaker and the dc current limiter comprising main and/or auxiliary power semiconductor switches, this known way of supplying power to the gate unit of the switching element appears to be problematic, since under normal operating conditions the dc breaker or the dc current limiter should be conducting for a long time, preferably a year or more, without any need for switching operation. Thus, at least a portion of their power semiconductor switching elements are permanently conducting and therefore do not provide a blocking state that would allow the required charging or recharging of the corresponding storage capacitor. This makes it difficult to ensure that sufficient power can be supplied to the gate unit with the power semiconductor switching element operated. In addition, bringing the dc circuit breaker into operation generally means that the corresponding power transmission or distribution line is then disconnected from the dc grid, leaving the primary circuit at zero voltage. Thus, charging or recharging of the energy storage capacitor of the switching module of the direct current circuit breaker is possible only during the rare and short periods when the circuit breaker is open. If repeated and periodic charging of the energy storage capacitor cannot be guaranteed, the reliability of the corresponding dc circuit breaker or dc current limiter is significantly reduced.

From medium voltage converter applications, different solutions are known for supplying the gate unit of a power semiconductor switch with electrical power, wherein a remote supply is used via a pulse transformer, i.e. the supply operates independently of the primary circuit. However, for design and cost reasons, this solution cannot be applied to high voltage levels, since the insulation of each pulse transformer needs to withstand at least the nominal dc voltage, which for high voltage applications means hundreds of thousands of volts. In the case of high-voltage direct-current circuit breakers, overvoltage stress during the opening action requires an insulation level of even almost twice the direct-current voltage.

The object of the present invention is to introduce a solution for a switch module in a dc breaker or a dc current limiter, in particular for HVDC, by means of which the reliability of the switch module, and thus also the reliability of the dc breaker or the dc current limiter, is improved.

This object is achieved by a switching module further comprising a power converting component arranged to receive power via an optical power signal, to convert the optical power signal into an electrical power signal and to provide the electrical power signal to an energy storage capacitor.

According to the invention, the supply of the gate unit is made independent of the voltage conditions in the primary circuit by providing an optical power supply to the storage capacitor. Thus, the charging and recharging of the storage capacitor can take place periodically at predetermined time intervals, so that it can always be ensured that sufficient power is available to the gate unit to operate the corresponding power semiconductor switching element or elements whenever required. Thus, the operability and reliability of a dc circuit breaker or a dc current limiter comprising such a switch module is significantly improved. The insulation problem of the pulse transformer solution described above is overcome since the electrical power signal is replaced by an optical power signal (i.e. light, preferably laser light) transmitted via a fiber optic cable.

In a preferred embodiment of the invention, the optical power signal is a low power signal of less than 1 watt. Since low power devices typically feature greater reliability than devices for higher power levels, the use of a low power optical power supply helps to further improve the reliability of the switch module.

If a low power optical power supply is used, some measures need to be taken to keep the internal power requirements of the gate cells at a low level. A preferred way to achieve this is to reduce the number of functions performed by the gate unit to a minimum level.

As mentioned in the introduction, today's dc breakers or dc current limiters applicable to medium and high voltage levels of dc power distribution and transmission systems need to contain a rather large number of switch modules connected in series. In a series connection, the problem of equal voltage distribution during dynamic and transient processes is important in order to avoid undesired voltage stress to some of the switching modules (due to different switching characteristics of the power semiconductor switching elements of different switching modules).

One of the functions that a known gate unit performs in high voltage converter valves, where a plurality of power semiconductor switches are connected in series and each power semiconductor switch is equipped with its own gate unit, is to ensure equal voltage distribution between the switches connected in series during dynamic and transient processes.

According to a preferred embodiment of the invention, this function is performed not by the gate unit but by an RCD snubber circuit (snubber circuit) which is contained in the switching module and connected in parallel with the at least one power semiconductor switching element. The RCD snubber circuit includes at least one resistor, at least one capacitor, and at least one diode. RCD buffers are known in the art and are disclosed, for example, in WO 96/27230. An RCD snubber can be provided for unidirectional and bidirectional switching modules (see below), wherein a unidirectional RCD snubber comprises a series connection of a capacitor and a diode for one current direction, the diode being connected in parallel with a resistor, and a bidirectional RCD snubber additionally comprises a series connection of a capacitor and a diode for the other current direction, the diode being connected in parallel with a resistor again.

During the switching-off of the power semiconductor switching element, the current flowing through the switching element is commutated into the at least one snubber capacitor via one of the snubber diodes (which corresponds to the current direction). As described in the introduction above, a dc circuit breaker typically comprises a plurality of groups of series-connected switching modules which together are connected in parallel with a non-linear resistor operating as a surge arrester. A dc current limiter comprises a plurality of such sets. When operating a dc breaker or a dc current limiter, the switching modules of the groups are switched off simultaneously. Thus, the same commutation of the current into the snubber circuit occurs for all the series-connected switching modules of each group. As a result, the buffer capacitors of each group are charged until the sum of the buffer capacitor voltages of each group is high enough for the group's discharger to receive (take over) the current. When these groups of switch modules are turned on again, the snubber capacitors are discharged via the corresponding snubber resistors. This results in some losses, which are, however, of no importance in the application of dc breakers and dc current limiters, due to the infrequent occurrence of operating actions.

In addition to equal dynamic voltage distribution, the RCD buffer has some additional advantages. Due to the presence of the at least one capacitor in the RCD buffer, the rate of rise of the voltage across the corresponding at least one power semiconductor switching element is limited. Thus, individual switching characteristics (like, for example, individual turn-off delays of the power semiconductor switching elements, which may differ between switching modules connected in series) no longer have significance.

In addition, the limited voltage rise rate in combination with the parallel connection of the IGBT or BIGT modules described below shows its advantages, and furthermore, because the different switching delays no longer have a greater significance, the risk of damaging high-frequency oscillations between the modules is eliminated. In general, it can be stated that due to the RCD buffer it is possible to connect IGBT or BIGT modules in series and/or in parallel with each other, while at the same time no complex and power demanding gate units need to be provided to handle (tap care of) uniform voltage distribution and possible high frequency oscillations.

Another advantage of the RCD snubber is that when the power semiconductor element is turned off, the snubber capacitor causes the voltage to start at zero, i.e. the switching is performed at zero voltage. As a result, during switching off and thus during operation of the dc breaker or the dc current limiter, less transient losses are generated. The reduced losses allow for higher off-currents and/or a larger number of repeated switching events before the thermal limit of the power semiconductor switching element is reached.

In a further embodiment of the invention, the gate unit is connected to the gates of the power semiconductor switching elements via an H-bridge, which generates and outputs the bipolar dc voltage required to drive the gate of at least one of the power semiconductor switching elements, wherein the H-bridge is supplied with a unipolar dc voltage and outputs a symmetrical bipolar dc voltage, for example plus or minus 15V. According to this embodiment, the gate unit is capable of operating internally with a unipolar direct voltage, contrary to the gate unit known from EP0868014B1, which operates internally with a bipolar direct voltage and thus with two internal power supplies. Using a single polarity operating dc voltage even further reduces the internal power requirements of the gate unit and makes it even more suitable for use at low supply levels. When two internal power supplies are used instead of an H-bridge, the gate unit can generate an asymmetric dc voltage, e.g., +18 volts and-5 volts.

In another embodiment of the invention, the switch module further comprises a control signal detector arranged to separate an electrical control signal from the received electrical power signal and to provide the electrical control signal to the gate unit. In other words, the control signal is incorporated into the same optical signal, which control signal comprises, inter alia, a switching control signal to initiate the gate unit to switch on or off the at least one power semiconductor switching element. The optical signal also contains a power signal, and the control signal is still contained in the electrical power signal after signal conversion by the power conversion means. In this way, the need for additional fiber optic cables is eliminated.

According to another embodiment of the invention, the gate unit of the switch module is arranged to generate status information on the functionality of at least one of the elements of the switch module, and the switch module further comprises a signal conversion component arranged to convert the status information into an optical information signal and to send the optical information signal to the central control unit. The status information is optically transmitted due to the fact that: the switch module is set at high voltage levels up to several hundred thousand volts in dc circuit breaker or dc current limiter applications. The use of optical communication simplifies the design and improves the reliability of the communication system.

By providing the central control unit with status information, it becomes possible for the central control unit to handle each connected switch module individually, for example by sending back a control signal to start a specific test routine in case of reporting a suspicious status requiring further investigation. The central control unit may simultaneously generate the above mentioned control signals initiating the switching on and off of the at least one power semiconductor switching device. In dc breaker and dc current limiter applications, the switching of the switching modules can be delayed up to tens of microseconds until a sufficient number of switching modules are ready for switching, since the actual operation of the dc breaker or dc current limiter needs to occur less instantaneously than required in e.g. inverter applications. Thus, it can be ensured that: the switching modules are switched on or off as simultaneously as possible. In other words, by exchanging status information with the central control unit, it is possible to implement a "handshake" protocol between the central control unit and all the switching modules of the dc circuit breaker or of the dc current limiter, wherein all the gate units are equipped (arm) with the "handshake" protocol and the "handshake" protocol synchronizes all the gate units and issues the actual on or off control signals (only if all or (in the redundant case) enough switching modules are ready).

The at least one power semiconductor switching element of the switching module can be of different types and designs depending on the operational and cost requirements of the dc circuit breaker or dc current limiter in which the switching module is to be used. In the following some preferred types are briefly described, which are suitable for use in a unidirectional or bidirectional dc breaker or dc current limiter. In order to be applicable to a bidirectional dc circuit breaker or a dc current limiter, the unidirectional power semiconductor switching elements need to be doubled (duplicated), and the doubling needs to be set for the opposite current direction, i.e. in the direction anti-parallel or anti-series to the original power semiconductor switching elements.

In one unidirectional type of the at least one power semiconductor switching element, the switching element comprises a first module comprising a first parallel connection of a first IGBT or IGBTs and a first parallel connection of a first diode or diodes, wherein the diode or diodes are connected anti-parallel to the IGBTs or to the parallel connection of the IGBTs. Whether one or more parallel-connected IGBTs and diodes are used depends on the current level to be reached with the power semiconductor switching elements, i.e. the higher the number of parallel-connected IGBTs and diodes, which are controlled via the same gate unit, the higher the rated current.

The bidirectional power semiconductor switching element can be realized by connecting a suitable number of the above-mentioned modules in an anti-parallel or anti-series connection, wherein an anti-parallel connection is possible in case the IGBT has a reverse blocking capability. In other words, the switching module then also comprises at least a second module connected anti-parallel or anti-series to the first module, the second module comprising a second parallel connection of a second IGBT or IGBTs and a second parallel connection of a second diode or diodes, wherein the diode or diodes are again connected anti-parallel to the IGBTs or to the parallel connection of IGBTs.

In practice, the first and second modules may be based on different physical packaging concepts of the chips of IGBTs and diodes. Each module corresponds to one single package containing an integration of IGBTs and corresponding anti-parallel diodes, or all parallel-connected IGBTs of the same current direction are integrated in one package and all parallel-connected diodes of the same current direction are integrated in another package. The latter design will overcome the problems that may occur with the first design. In the first design, the differently packaged diodes may originate from different production cycles, and thus they may differ slightly in their characteristics, such as forward voltage drop. Since these diodes have a negative temperature coefficient, the different forward voltage drops may cause an undesired current flow between the diodes, which may lead to a so-called thermal runaway of the diode chip, i.e. an increase in temperature due to the current flow, which increases the current flow even further. When all parallel diodes of the power semiconductor switching elements of the same current direction are integrated in the same package (as proposed in the second design), it is ensured that their characteristics are matched as close to each other as possible, thereby minimizing the risk of thermal runaway.

In a particular embodiment of the above-described type of power semiconductor switching element, the diode is a line commutated diode. Usually, so-called fast recovery diodes are used as anti-parallel diodes of IGBTs, since they are particularly suitable for fast switching applications for which IGBTs are usually intended. However, in the case of dc breakers and dc current limiters, no fast switching action is required, so that line commutating diodes, such as are known from standard 50HZ rectifier applications, may be used instead. The losses of the first and second modules described above can be reduced due to the lower voltage drop of the line commutated diode compared to the fast recovery diode. In addition, the line-commutated diodes are less costly.

In an alternative unidirectional type of power semiconductor switching element, the switching element comprises a first module comprising a first reverse conducting IGBT or a first parallel connection of a plurality of reverse conducting IGBTs. In reverse conducting IGBTs, the IGBT and anti-parallel diode functions are directly integrated in one common chip. For example, reverse conducting IGBTs are described in european patent application 09159009.1 and are also referred to as dual mode insulated gate transistors (BIGTs). As mentioned above, the parallel connection of a plurality of such IGBTs provides a higher current rating of the power semiconductor switching elements.

The bidirectional power semiconductor switching element may be implemented by connecting two or more BIGT modules in an anti-series connection. It is therefore proposed that the above power semiconductor switching element further comprises a second module connected in anti-series connection with the first module, the second module comprising a second parallel connection of one second reverse-conducting IGBT or a plurality of reverse-conducting IGBTs.

The use of BIGT instead of separate IGBT and anti-parallel diodes includes several advantages.

One advantage is that the forward voltage drop of the integrated diode shows a positive temperature coefficient, so that possible thermal runaway problems are avoided.

In a particular embodiment of the BIGT based bidirectional dc breaker, the power semiconductor switching elements will each comprise an anti-series connection of two BIGTs, wherein the two BIGTs are vertically integrated in one and the same package. In a typical bidirectional dc breaker application, the current flows in the same direction for a considerable period of time, which in the case of a common IGBT with separate diodes means that the silicon area of the power semiconductor switching elements of the bidirectional dc breaker is only partially used. In contrast, due to vertical integration, the silicon area of a bidirectional BIGT package can be fully utilized, resulting in a smaller number of chips for the same current rating, or an increased current capacity for a given number of chips per package.

A third advantage is that the functionality of the diode can be monitored more easily in the case of a BIGT than in the case of a separate IGBT and diode.

In general, it is advantageous to provide the switching module with a further diode monitoring component which is arranged to perform a test of the blocking functionality of the anti-parallel diode or diodes and which is thereby capable of indicating whether the corresponding IGBT in the power semiconductor switching element is available for normal operation. This is recommended because it may happen in rare cases that one or more of the anti-parallel diodes break down when the corresponding IGBT is in the off or non-conducting state, which may have serious consequences. In fast switching applications it is possible to frequently test the blocking functionality of a diode when the corresponding IGBT is in a non-conducting state and no main current is flowing through the diode. However, in a dc breaker or a dc current limiter, where at least part of the IGBTs are continuously switched on, this is not equally possible for the corresponding diodes. Nevertheless, it is important to obtain information about defective diodes before the opening of the dc breaker or before the entering operation of the dc current limiter, since such defective diodes may cause fatal damage.

Therefore, for a switching module comprising separate IGBTs and diodes, it is suggested to provide a diode monitoring component adapted to monitor the blocking functionality of one or more diodes each time the corresponding IGBT is switched off and no main current flows through the diode or diodes to be monitored. In other words, the test is performed as often as possible, wherein for some dc breaker configurations this may mean that the test can only be performed during maintenance, while for other configurations, such as the dc breaker described in PCT/EP2009/065233, the test can be performed continuously for those switch modules that are not carrying (carry) primary current. The test consists of applying a small positive test voltage only in the forward direction of the IGBT that is turned off and checking if this voltage is maintained or if it is reduced, possibly even broken down (due to failure of the diode). If the latter occurs, for example by means of a gate unit, failure information can be generated and transmitted as an optical information signal to the central control unit. In connection with the above RCD buffer, another way of testing the functionality of the anti-parallel diode becomes possible: in a dc current limiter or dc breaker comprising a plurality of switch modules connected in series, this further test is performed when the dc breaker or dc current limiter is switched on and current flows in the forward direction through the series-connected IGBTs. To test the functionality of the diode, one or more of the series connected IGBTs can now be actively (activery) turned off for a very short time, preferably a few microseconds, until the current through the turned-off IGBT has started to divert to the corresponding RCD snubber circuit and until the voltage across the RCD snubber has started to rise slightly. Once a voltage rise is detected, the IGBT or IGBTs are switched on again, wherein the voltage rise can be detected in a simple manner by checking whether a predetermined voltage limit is exceeded, wherein the voltage limit is at a comparatively low voltage level, preferably of only a few hundred volts up to a few kilovolts. If a voltage rise cannot be detected, failure information is generated. In this way, testing of the diodes in the switch module is possible without hampering the operation of the dc breaker or the dc current limiter.

As becomes clear from the above, it is in general difficult for a switching module configuration with separate IGBTs and diodes to generate reliable information about the blocking capability of the diodes. In contrast, it is possible to detect a failure of the integrated diode function in the BIGT during the on and off states of the corresponding IGBT. During almost all operating states of the BIGT in dc breaker applications, a possible detection of a failing or failed diode in the BIGT is due to the fact that: a defective integrated diode function can be observed by a significant deterioration or even breakdown of the gate-emitter voltage of the corresponding IGBT. Thus, the increased gate emitter leakage current may be used as an indication or monitoring of irreversible impairment of the IGBT or diode function of the BIGT. Hence, the proposed diode monitoring means for a switching module comprising a BIGT is adapted to monitor the blocking functionality of one or more diode functions of a reverse conducting IGBT by generating failure information in case of a gate-emitter voltage breakdown across a reverse conducting IBGT that is switched on or off. Due to the possibility of performing the test in the on and off state of the BIGT, there are more opportunities to deduce information about the blocking capability of the diode function in the BIGT compared to solutions with separate IGBTs and diodes, thereby significantly improving the reliability of the dc breaker or dc current limiter.

In addition to the dynamic voltage distribution discussed above in connection with the RCD buffer, it is also advantageous if the steady-state voltage distribution of the series-connected switching modules is kept as equal as possible to avoid an increase in voltage stress on some modules. According to a further embodiment of the invention, it is therefore proposed that a non-linear voltage limiting resistor is connected in parallel with at least one power semiconductor switching element. Such a non-linear voltage limiting resistance not only ensures an equal steady-state voltage distribution, but also limits overvoltages (when the arresters of a group of series-connected switching modules receive current from the snubber circuit of the group). The arresters of a group of switching modules connected in series are also referred to as main arresters in the following. A further advantage of the non-linear voltage limiting resistance in the switching module is that it allows a reduction in the size of the snubber capacitor of the module, it enables a larger capacitor tolerance between different modules, and it simplifies the mechanical design of the current commutation path of the main arrester.

The invention and its embodiments will now be explained with reference to the accompanying drawings, in which:

figure 1 shows a first basic unit comprising a power semiconductor switching element arranged for unidirectional application,

figure 2 shows a second basic unit comprising a power semiconductor switching element arranged for bi-directional applications,

figure 3 shows a third basic unit comprising a power semiconductor switching element arranged for bi-directional applications,

figure 4 shows a fourth basic unit comprising a power semiconductor switching element arranged for bi-directional applications,

figure 5 shows a first example of a direct current breaker,

figure 6 shows a second example of a direct current breaker,

figure 7 shows an example of a dc current limiter,

figure 8 shows a first embodiment of a switch module,

figure 9 shows a second embodiment of a switch module,

figure 10 shows a third embodiment of a switch module,

figure 11 shows a fourth embodiment of a switch module,

figure 12 shows an arrangement of the switch modules and the central control unit of a direct current circuit breaker,

fig. 13 shows an arrangement of power semiconductor switching elements of a switching module.

Fig. 1 shows a first base unit 6a comprising a power semiconductor switching element arranged for unidirectional application. The power semiconductor switching elements are an IGBT1 in a first current direction 4 and a freewheeling diode 2 connected in anti-parallel with the IGBT 1.

In fig. 2, a second basic cell 6b can be seen, which comprises a parallel connection of an IGBT1 of a first current direction 4 and an IGBT 3 of a second opposite current direction 5. The second base unit 6b is therefore suitable for bi-directional applications.

In fig. 3, a third basic cell 6c is shown, which comprises a series connection of an IGBT1 of a first current direction and an IGBT 3 of an opposite second current direction, in other words the third basic cell 6c is an anti-series connection of two IGBTs. Each IGBT has a freewheeling diode 2 and 7, respectively, connected in anti-parallel. The base unit 6c is suitable for bi-directional applications.

A fourth base unit 6d is shown in fig. 4. It comprises a reverse conducting IGBT, also called BIGT8 (dual mode insulated gate transistor), as a first current direction of the power semiconductor switching element and a reverse conducting IGBT, called BIGT9, of a second current direction in series with the BIGT 8. Therefore, BIGT8 and 9 are connected in anti-series, meaning that the fourth base unit 6d is also suitable for bidirectional applications.

According to an example described in fig. 5, the base units 6a-6d may be used in a dc breaker 14. The dc breaker 14 is suitable for medium or high voltage applications and it is connected in series with a dc power distribution or transmission line 13. The base unit 6a may be used in case the primary current in the line 13 only needs to be interrupted in one direction, whereas the base unit 6b or 6c or 6d may be used in case the primary current in the line 13 needs to be interrupted in both possible directions. The direct current breaker 14 comprises a main breaker 10, which main breaker 10 contains a series connection of several tens to several hundreds of base units 6-depending on the voltage level-and a non-linear resistor, which is also referred to as main arrester 11 and which is connected in parallel with the main breaker 10. In series with the dc breaker 14, a reactor 12 is provided for limiting the current rate in the line 13. Under normal operating conditions of the line 13, all IGBTs or BIGTs in the base unit 6 are switched on, i.e. the dc breaker 14 is conducting the primary current of the line 13. In case the primary current is to be interrupted, e.g. if a fault has occurred in the line 13, all IGBTs or BIGTs will be turned off simultaneously to commutate the primary current to the main discharger 11, which then reduces the current to zero.

Another example of a dc breaker 17 is shown in fig. 6, for which base units 6a-6d can be used. In addition to the main breaker 10 and the main arrester 11, a series connection of an auxiliary breaker 16 and a high-speed switch 15 connected in parallel with the main breaker 10 and the main arrester 11 is provided. The auxiliary breaker 16 comprises only one basic unit 6. The high speed switch 15 is a mechanical switch. In series with the dc breaker 17, the reactor 12 is again placed (place) for limiting the current rate.

It is interesting to note that in case the base unit 6 used in the dc breaker configuration of fig. 5 and 6 is a bidirectional base unit of type 6b, 6c or 6d, the same configuration is also suitable for use as an ac breaker for ac power distribution or transmission lines.

In fig. 7, an example for a dc current limiter 18 is shown, wherein the dc current limiter 18 comprises a series connection of a plurality of dc breakers 14. In other words, the dc current limiter 18 includes a plurality of groups of the base unit 6 connected in series, wherein each group includes the main discharger 11 connected in parallel with the base unit 6. A dc current limiter 18 is connected in series with the current rate limiting reactor 12 and with the dc power distribution or transmission line 13. In case the primary current in the line 13 is to be limited or reduced, a suitable number of dc breakers 14 are opened so that the corresponding non-linear resistance may consume some electrical energy which is not desired. In its simplest form, the direct current limiter should comprise two circuit breakers 14, hereinafter referred to as first and second circuit breakers. The protection level of the main discharger of the first circuit breaker corresponds to the nominal dc voltage level of the line 13. When the current through the line 13 is to be limited or reduced, the first circuit breaker will be opened. The protection level of the main discharger of the second circuit breaker can be set to a value lower than the nominal dc voltage level of the line 13, for example, 50% of the nominal dc voltage. After opening the first circuit breaker, the second circuit breaker can be used for interrupting the current in the line 13, also by opening the second circuit breaker.

The invention will now be further explained with reference to fig. 8 to 12. In order for the base unit 6 in the dc breaker or dc current limiter to be operated, a so-called gate unit is required which switches the corresponding IGBT or BIGT on or off in accordance with a control signal generated by the central control unit in dependence on the state of the line 13. The base unit 6 of the dc breaker 14 or 17 of the dc current limiter 18 therefore contains in practice not only power semiconductor switching elements. In fact, each base unit 6 may be replaced by a switch module 38, wherein the switch module 38 comprises, among others, a gate unit 31. Different embodiments of the switching module 38 will now be described, wherein for each embodiment the actually shown power semiconductor switching elements can be replaced by power semiconductor switching elements belonging to another suitable one of the base units 6a-6d, as well as by further combinations thereof, as explained below.

In fig. 8a first embodiment 38a of a switching module is depicted, and in addition to IGBT1 and anti-parallel diode 2 it also comprises a first embodiment 30a of a gate drive module connected to the gate of IGBT 1. The gate driving module 30a includes a power conversion part in the form of a photodiode 20, a dc/dc converter 22, a storage capacitor 25, and a gate unit 31. The photodiode 20 is arranged to receive an optical power signal, to convert the optical power signal into an electrical power signal, and to provide the electrical power signal to the reservoir capacitor 25 via the dc/dc converter 22, thus charging or recharging the reservoir capacitor 25 from the power supply, independently of the state or switching conditions of the circuit, also referred to as the primary circuit, of which the IGBT1 and the diode 2 are part. The optical power signal is thus a low power signal below 1 watt.

The storage capacitor 25 is connected to the power input 29 of the gate unit 31 in order to provide the required energy to the gate driver and monitoring module 28 to drive the gate of the IGBT 1. In addition to the gate driver and monitoring module 28, the gate unit comprises a gate unit control module 27. The gate unit control module 27 receives electrical control signals from the control signal detector 23, wherein the control signal detector 23 is arranged to separate the electrical control signals from the electrical power signal output by the photodiode 20. The optical power signal received by the photodiode 20 therefore also contains an optical information signal, which is still present after conversion into an electrical signal. The photodiode 20 is connected to a central control unit 50 via a first fiber optic cable 51 (see fig. 12).

The gate unit control module 27 processes the electrical control signals and outputs the resulting turn-on command signal or turn-off command signal to the gate driver and monitoring module 28, the gate driver and monitoring module 28 thus causing the IGBT1 to turn on or off. The gate control module 27 also receives different information, such as information delivered by the gate driver and monitoring module 28 about the state of the IGBT1 and information delivered by the power monitoring unit 26 about the state of the elements contained in the power supply of the gate driver and monitoring module 28, i.e. about the state of the storage capacitor 25 and the dc/dc converter 22. These different information are processed by the gate unit control module 27 and then provided as status information via the signal transfer module 24 to the signal conversion means, which in this example is a light emitting diode 21. The light emitting diodes 21 are connected via a second fiber optic cable 52 to a central control unit 50 (see fig. 2) which adjusts the control signals sent to the photodiodes 20 via the optical power signals in response to the received status information.

A second embodiment of a switch module 38b is shown in fig. 9, where the second embodiment 38b comprises the same gate drive module 30a as the first embodiment 38 a. One detail of the gate driver and monitor module 28 is shown here, which is not shown in fig. 8. It can be seen from fig. 9 that the gate driver and monitoring module 28 and thus the gate unit 31 are connected via an H-bridge to the gate of at least one power semiconductor switching element, here an IGBT1 with an anti-parallel diode 2, wherein the H-bridge is supplied with a unipolar dc voltage of 15V and outputs a bipolar dc voltage of ± 15V. Thus, to some extent, the internal power requirements of the gate unit 31 are reduced.

In addition to the first embodiment 38a, the second embodiment 38b of the switching module also comprises a non-linear voltage limiting resistor 32 in parallel with the at least one power semiconductor switching element and an RCD snubber circuit comprising a series connection of a diode 33 and a capacitor 34 and a resistor 35 in parallel with the diode 33, wherein the RCD snubber circuit itself is also in parallel with the at least one power semiconductor switching element. The orientation of the diode 33 is the same as that of the IGBT 1. The RCD snubber circuit is primarily responsible for an equal dynamic voltage distribution in the series connection of a plurality of switch modules 38, as would be applicable for example to the dc breakers 14 and 17 or to the dc current limiter 18 when the base unit 6 is replaced by a switch module 38. The non-linear voltage limiting resistor 32 mainly ensures an equal steady-state voltage distribution in such a series connection of the switching modules 38.

In a third embodiment 38c of the switching module according to fig. 10, in addition to one IGBT1 and one diode 2 of the first and second embodiments (38 a and 38b, respectively, which together form a first module marked with the letter a, a second module marked with the letter b), a second IGBT1 and a second anti-parallel diode 2 are connected in anti-series connection with the first module. Therefore, the switch module 38c can be applied to a bidirectional dc breaker or a bidirectional dc current limiter.

As already described, additional and alternative combinations of IGBTs and diodes are possible. Fig. 13 shows an example in which the first and second modules each comprise not only one but two parallel-connected IGBTs 1a or 1b and respectively corresponding anti-parallel diodes 2a or 2 b. The physical encapsulation of the two modules can be in the form of one encapsulation for each pair of IGBT and corresponding diode, or in the form of a first encapsulation with all IGBTs 1a of a first current direction, a second encapsulation with all IGBTs 1b of a second, other current direction, and a third and fourth encapsulation with all diodes 2a and 2b, respectively, also according to their current direction. This latter type of packaging is depicted in fig. 13 by dashed lines, which provides a significant reduction in the risk of thermal runaway.

In addition to the first and second modules of IGBTs and diodes, a third embodiment 38c of a switching module includes a second embodiment 30b of a gate drive module, where this second embodiment 30b includes two additional cells not included in the first embodiment 30 a. One of the further units is a diode monitoring unit 37, the task of which is to monitor the blocking functionality of the diode 2 in the first module. This monitoring is done by applying a positive test voltage in the forward direction to the IGBT1a or 1b, respectively, whenever the IGBT1a or 1b is turned off and when no main current flows through the corresponding diode. By checking whether the test voltage is maintained, a failing (fail) or failed diode 2a or 2b, respectively, can be identified. Taking the example of a dc breaker 17, diode monitoring can be performed for the diodes in the main breaker 10 during normal operation, since the main or primary current flows through the auxiliary breaker 16 and the high speed switch 15 during this time.

A further additional unit of the second embodiment 30b of the gate drive unit is an auxiliary recharge circuit 36 which, in addition to the optical power supply, supplies energy to the storage capacitor 25 (whenever possible) and which takes its energy from the primary circuit to which the IGBT1 and the diode 2 are connected. However, as mentioned above, the opportunities for recharging from the primary circuit, i.e. when switching off the IGBT1 in a dc breaker or dc current limiter application, are typically very rare. Such monitoring and auxiliary recharging are both initiated by corresponding start signals sent from the gate driver and monitoring module 28 to the diode monitoring section 37 and the auxiliary recharging circuit 36, respectively. These starting signals can be generated internally in the switch module by the gate unit control module 27 or by the auxiliary recharging circuit 36 itself (in case it is intelligent enough to adapt to the conditions in the primary circuit), or they can be sent in the form of corresponding control signals from a central control unit 50 (see fig. 12) to the switch module via a first fiber optic cable 51 and then transmitted to the diode monitoring component 37 and the auxiliary recharging circuit 36 via the control signal detector 23, the gate unit control module 27 and the gate driver and monitoring module 28, respectively.

In fig. 11, a fourth embodiment 38d of a switch module is depicted. Here, the at least one power semiconductor switching element is an anti-series connection of two reverse conducting IGBTs according to the fourth base unit 6d, or in other words a series connection of BIGT8 of the first current direction and BIGT9 of the second current direction. Thus, the switch module is suitable for bi-directional applications. The non-linear voltage limiting resistor 32 is again placed in parallel with the anti-series connection of BIGT8 and 9 and the bi-directional RCD buffer circuit is connected in parallel with resistor 32. Bidirectional RCD buffer circuit: comprising a first parallel connection of a first diode 42 and a first resistor 40, wherein the first diode 42 is of a first current direction, a second parallel connection of a second diode 45 and a second resistor 41, wherein the second diode 45 is of a second current direction, a common capacitor 46 between and in series with the first and second parallel connections, a third diode 44 connected between the second diode 45 and the first diode 42 and having a direction from the cathode of the second diode 45 to the cathode of the first diode 42, and a fourth diode 43 connected between the second diode 45 and the first diode 42 and having a direction from the anode of the second diode 45 to the anode of the first diode 42. The gate drive module of the fourth embodiment 38d of the switch module is that of the third embodiment 30c and contains substantially the same elements as the second embodiment 30b except that the function of the diode monitoring means is different from the diode monitoring means 37 of fig. 10, since the blocking functionality of the integrated diode function of the BIGTs 8 and 9 is monitored together with the functionality of the IGBTs monitoring the BIGTs 8 and 9, respectively. The monitoring is performed during both the on and off states of the IGBTs of the respective BIGT, independently of the main or primary current. Failure information is generated if the gate-emitter voltage across the on or off reverse-conducting IGBT deteriorates or breaks down (which is detected by detecting increased gate-emitter leakage current).

The arrangement of a plurality of switching modules of the direct current circuit breaker, which switching modules consist of a gate drive unit 30, an IGBT1 and an anti-parallel diode 2, and a central control unit 50 has been mentioned earlier. The dc breaker further comprises a main arrester 11. The switch module of the dc circuit breaker can be of virtually any of the four types 38a-38d described above, or any other combination of possible embodiments of the main elements of the switch module, being at least one power semiconductor switching element, a gate drive unit, an optional RCD snubber circuit and an optional non-linear voltage limiting resistor. As can be seen in fig. 12, between the central control unit 50 and each gate drive unit 30, two fiber optic cables 51 and 52 are provided, wherein a first fiber optic cable 51 is used to transmit an optical power signal from the central control unit 50 to the respective gate drive unit 30, and wherein the optical power signal further comprises one or more control signals. A second fibre optic cable 52 is used for the transmission of status information in the form of optical information signals from the gate drive unit 30 to the central control unit 50.

Claims (19)

1. An arrangement (14, 17, 18) for limiting and/or breaking a current flowing through a power transmission or distribution line (13), the arrangement comprising at least one switch module, the at least one switch module comprising:

at least one power semiconductor switching element (1, 2; 8, 9),

-a gate unit (31) arranged to switch on and off the at least one power semiconductor switching element, respectively, in dependence on a switching control signal, and

an energy storage capacitor (25) arranged to provide power to a power supply input (29) of the gate unit,

a power conversion component (20) configured to: receiving power via an optical power signal, converting the optical power signal into an electrical power signal, and providing the electrical power signal to the energy storage capacitor and thereby charging or recharging the energy storage capacitor, wherein

-the switching module is arranged to separate an electrical control signal from the optical power signal and to provide the electrical control signal to the gate unit (31), wherein the electrical control signal comprises the switching control signal.

2. An arrangement (14, 17, 18) for limiting and/or breaking a current flowing through a power transmission or distribution line (13), the arrangement comprising at least one switch module, the at least one switch module comprising:

at least one power semiconductor switching element (1, 2; 8, 9) and at least one diode (2, 7) or diode function (8, 9), wherein the diode or diode function is connected in anti-parallel with the power semiconductor switching element;

-a gate unit (31) arranged to switch the at least one power semiconductor switching element on and off, respectively, in dependence on a switching control signal, and

an energy storage capacitor (25) arranged to provide power to a power supply input (29) of the gate unit,

a power conversion component (20) configured to: receiving an optical power signal, converting the optical power signal into an electrical power signal and providing the electrical power signal to the energy storage capacitor, an

-a diode monitoring component (37) adapted to: monitoring the blocking functionality of the one or more diodes or one or more diode functions to be monitored whenever the corresponding one or more power semiconductor switching elements are switched off and no main current flows through the one or more diodes or one or more diode functions to be monitored, wherein during the monitoring a positive test voltage is applied to the forward direction of the one or more switching elements and in case the test voltage is not maintained a failure information is generated.

3. An arrangement (14, 17, 18) for limiting and/or breaking a current flowing through a power transmission or distribution line (13), the arrangement comprising a series connection of a first switch module and at least one further switch module, wherein the first switch module comprises:

at least one power semiconductor switching element (1, 2; 8, 9) and at least one diode (2, 7) or diode function (8, 9), wherein the diode or diode function is connected in anti-parallel with the power semiconductor switching element;

-a gate unit (31) arranged to switch the at least one power semiconductor switching element on and off, respectively, in dependence on a switching control signal, and

an energy storage capacitor (25) arranged to provide power to a power supply input (29) of the gate unit,

a power conversion component (20) configured to: receiving an optical power signal, converting the optical power signal to an electrical power signal, and providing the electrical power signal to the energy storage capacitor,

an RCD snubber circuit connected in parallel with the at least one power semiconductor switching element, wherein the RCD snubber circuit comprises at least one resistor (35; 40, 41), at least one capacitor (34; 46) and at least one diode (33; 42, 45), the diode and the capacitor being connected in series with each other and the resistor being connected in parallel with the diode; and

-a diode monitoring component (37), the diode monitoring component (37) being adapted to: monitoring blocking functionality of the one or more diodes or the one or more diode functions in the switch module (38) by actively turning off the switch module (38) for a short period of time and then turning it on again when the apparatus is turned on and when current flows in a forward direction through the power switching elements of the first switch module and the at least one further switch module, and by generating a failure information if the voltage across the corresponding RCD buffer circuit does not exceed a predetermined voltage limit for the short period of time.

4. The apparatus of any of claims 1-3, wherein the optical power signal is a low power signal of less than 1 watt.

5. A device according to any of claims 1-3, wherein the gate unit (31) is arranged to generate status information on the functionality of at least one of the elements of the switch module, and wherein the switch module further comprises signal conversion means (21) arranged to convert the status information into an optical information signal and to send the optical information signal to a central control unit (50).

6. A device according to any of claims 1-3, comprising a first module as the at least one power semiconductor switching element, the first module comprising a first parallel connection of a first IGBT (1) or IGBTs (1a) and a first parallel connection of a first diode (2) or diodes (1b), wherein the diode or diodes are connected anti-parallel to the IGBT or IGBT parallel connection.

7. The device of claim 6, wherein the switching module further comprises a second module connected anti-parallel or anti-series to the first module, the second module comprising a second parallel connection (1b) of a second IGBT or IGBTs and a second parallel connection (2b) of a second diode or diodes, wherein the diode or diodes are connected anti-parallel to the IGBT or to the parallel connection (1b) of IGBTs.

8. The apparatus of claim 6, wherein the diode is a line commutated diode.

9. A device according to any of claims 1-3, comprising a first module as the at least one power semiconductor switching element, the first module comprising a first reverse conducting IGBT (8) or a first parallel connection of a plurality of reverse conducting IGBTs.

10. The arrangement of claim 9, further comprising a second module connected in anti-series with the first module, the second module comprising a second reverse conducting IGBT (9) or a second parallel connection of a plurality of reverse conducting IGBTs.

11. The arrangement of claim 10, wherein the first and second modules are integrated in one single semiconductor package, and wherein the package is provided with one common gate terminal and one common emitter terminal, which terminals are connected to the gate and emitter, respectively, of all reverse conducting IGBTs (8) in the package.

12. The arrangement according to any of claims 1-3, wherein the switching module further comprises a non-linear voltage limiting resistor (32) in parallel with the at least one power semiconductor switching element.

13. The device according to any of claims 1-3, wherein the switching module further comprises an auxiliary recharging circuit (36) adapted to receive electrical energy from a primary circuit to which the at least one power semiconductor switching element is connected and to provide the electrical energy to the energy storage capacitor (25).

14. The device according to any of claims 1-2, further comprising an RCD snubber circuit connected in parallel with the at least one power semiconductor switching element, wherein the RCD snubber circuit comprises at least one resistor (35; 40, 41), at least one capacitor (34; 46) and at least one diode (33; 42, 45), the diode and the capacitor being connected in series with each other and the resistor being connected in parallel with the diode.

15. The arrangement according to any of claims 1-3, wherein the gate unit (31) is connected to the gate of the at least one power semiconductor switching element (1, 2; 8, 9) via an H-bridge, wherein the H-bridge is powered by a unipolar direct voltage and outputs a bipolar direct voltage.

16. The apparatus of any of claims 2-3, wherein the switching module is further arranged to separate an electrical control signal from the optical power signal and to provide the electrical control signal to the gate unit (31).

17. The device according to claim 1, further comprising a diode monitoring component (37) adapted to monitor the blocking functionality of the diode or diode function in the first and/or second module, respectively.

18. The apparatus of claim 17, wherein the diode monitoring component (37) is adapted to: monitoring the blocking functionality of the diode function or diode functions of the reverse-conducting IGBT (8, 9) together with the monitoring of the functionality of the corresponding IGBT itself by generating failure information in case of gate-emitter voltage degradation or breakdown across the switched-on or switched-off reverse-conducting IGBT.

19. A device as claimed in claim 18, wherein the degradation or breakdown of the gate-emitter voltage is detected by detecting an increased gate-emitter leakage current of the reverse-conducting IGBT (8, 9).