Abstract:
A disconnecting device for interrupting a direct current between a direct current source and an electric apparatus, having a current-conducting mechanical switch, a power electronics unit connected thereto, and an energy store which is charged by an arcing voltage generated on the switch by an arc as the switch is being disconnected. A pulse generator that is connected to the energy store triggers at least one semiconductor switch of the power electronics unit in such a way that the power electronics unit short-circuits the switch and the arc is extinguished.

Description:
[0001]    This nonprovisional application is a continuation of International Application No. PCT/EP2015/068590, which was filed on Aug. 12, 2015, and which claims priority to German Patent Application No. 10 2014 015 643.5, which was filed in Germany on Oct. 24, 2014, and which are both herein incorporated by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    Field of the Invention 
         [0003]    The invention relates to a disconnecting device for direct current interruption between a direct current source and an electrical apparatus, comprising a current-conducting mechanical switch and a power electronics unit connected thereto, and an energy store, which is charged by an arcing voltage generated on the switch by an arc as the switch is being disconnected. In this case, a direct current source is understood to be a photovoltaic generator (PV generator, solar system), and an electrical apparatus is, in particular, an inverter. 
         [0004]    Description of the Background Art 
         [0005]    From DE 20 2008 010 312 U1, a photovoltaic system (PV system) or a solar system with a so-called photovoltaic generator is known, which in turn has solar modules which are combined into groups of partial generators, which in turn are connected in series or are present in parallel strings, wherein the direct current power of the photovoltaic generator is fed into an alternating voltage network via an inverter. Since as a result of the system, a PV system continuously provides, on the one hand, an operating current and an operating voltage in the range between 180V (DC) and 1500V (DC), and since on the other hand, for example for installation, assembly or service purposes or in particular for general personal protection, reliable separation of the electrical components or devices from the photovoltaic system acting as a direct current source is desired, a corresponding disconnecting device must be capable of performing an interruption under load, i.e. without previously switching off the direct current source. 
         [0006]    For load separation, a mechanical switch (switching contact) can be used with the advantage that a galvanic separation of the electrical apparatus (inverter) from the direct current source (photovoltaic system) is produced when the contact has been interrupted. If, on the other hand, power semiconductor switches are used for load separation, unavoidable power losses occur in the semiconductors even during normal operation. In addition, no galvanic separation and therefore no reliable personal protection is ensured with such power semiconductors. 
         [0007]    DE 102 25 259 B3 discloses a plug connector designed as a load breaker which, in the manner of a hybrid switch, comprises a semiconductor switching element in the form of, for example, a thyristor in the housing of the inverter, and main and auxiliary contacts which are connected to photovoltaic modules. The main contact leading in an unplugging operation is connected in parallel with the trailing auxiliary contact, which is connected in series with the semiconductor switching element. In this case, the semiconductor switching element is controlled for purposes of arc prevention or arc suppression by periodically switching it on and off. 
         [0008]    A hybrid electromagnetic direct current switch with an electromagnetically actuated main contact and an IGBT (insulated gate bipolar transistor) can also be provided as a semiconductor switch for direct current interruption (DE 103 15 982 A2). However, such a hybrid switch has an external energy source for operating a power electronics unit with a semiconductor switch. 
         [0009]    WO 2010/108565 A1, which corresponds to U.S. Pat. No. 8,742,828, discloses a hybrid disconnecting switch with a mechanical switch or disconnecting element, and a semiconductor electronics which is connected in parallel therewith and which essentially comprises at least one semiconductor switch, preferably an IGBT. The semiconductor electronics does not have an additional energy source and, with the mechanical switch closed, is current-blocking, that is to say virtually current-free and voltage-free. The semiconductor electronics gains the energy required for its operation from the disconnecting device, that is, from the breaker system itself, for which purpose the energy of the arc arising when the mechanical switch is opened is used. In this case, the semiconductor electronics is connected to the mechanical switch on the trigger side in such a way that the arcing voltage switches the semiconducting electronics in a conducting manner via its switching contacts as a result of the arc. 
         [0010]    As soon as the semiconductor electronics is switched to conduct current, the arc current starts to commutate from the mechanical switch to the semiconductor electronics. The corresponding arcing voltage or the arc current in this case charges an energy store in the form of a capacitor, which discharges specifically to produce arc-free switching-off of the semiconductor electronics, whilst generating a control voltage. The predetermined time duration or time constant and thus the charging duration of the energy store or capacitor determines the duration of the arc. Following the charging process, a timing element starts, during which the semiconductor electronics is controlled arc-free and current-blocking. The time duration of the timer is set to a secure deletion. 
       SUMMARY OF THE INVENTION 
       [0011]    It is therefore an object of the invention to provide a particularly suitable disconnecting device (hybrid switch or electronics) for direct current interruption between a direct current source, for example, a photovoltaic generator, and an electrical apparatus, for example, an inverter, with the highest possible switching capacity and in particular the highest possible drive speed, i.e. very fast control of the power electronics unit of the disconnecting device. 
         [0012]    In an exemplary embodiment, a disconnecting device, which can also be referred to below as a hybrid switch, has a current-conducting mechanical switch and a power electronics unit connected thereto, as well as an energy store, which is charged by an arcing voltage generated on the switch by an arc as the switch is being disconnected. The hybrid switch further comprises a pulse generator, which is also referred to in the following as a pulse generator circuit, which is connected to the energy store, which can be designed as a capacitor. The pulse generator controls the at least one semiconductor switch of the power electronics unit in such a way that it short-circuits the switch, which leads to the extinction of the arc. 
         [0013]    In an embodiment, the pulse generator (the pulse generator circuit) comprises a semiconductor switch which is connected to the energy store and is switched to conducting when the charging voltage of the energy store reaches a set or adjustable voltage value, which is also referred to hereinafter as operating voltage. This semiconductor switch of the pulse generator is suitably a thyristor. On a voltage tap connected downstream of this semiconductor switch of the pulse generator, on the control side, the power electronics unit taps a control pulse, preferably generated by the operating voltage. In other words, the pulse generator is connected on the control side via this voltage tap to the control side of the power electronics unit, i.e. to the at least one semiconductor switch, so that the latter is activated when the control pulse or control signal of the pulse generator is switched, i.e. is switched to conducting, and then short-circuits the mechanical switch, in particular its switching contacts or its corresponding contact connections. The pulse generator generates only a single control pulse per switching operation, that is, a single pulse. 
         [0014]    In an embodiment, the pulse generator has a capacitor connected to the energy store. This capacitor, which is connected, for example, to a discharge resistor, is preferably used as a timer, or triggers a timer, after which lapse of time a shutdown, that is to say, a switching off of the control pulse and, consequently, the power electronics unit, takes place. 
         [0015]    In addition, the pulse generator comprises a semiconductor circuit with, for example, a plurality of semiconductor switches which are connected to further components, for example, resistors, and to at least one Zener diode. Suitably, the pulse generator comprises at least one thyristor and at least one transistor (bipolar transistor, MOS-transistor, PMOS-transistor). In conjunction with the capacitor of the pulse generator, the semiconductor circuit causes the control pulse, which is present at the voltage tap, to be switched off when, as a result of charging the capacitor of the pulse generator, the switching voltage of a semiconductor switch, which can be in the form of a MOS-transistor or PMOS-transistor and is connected to the voltage tap, is achieved. 
         [0016]    In an embodiment of the power electronics unit, this has a first and a second semiconductor switch in each case in the form of an IGBT with a free-wheeling diode. These semiconductor switches are switched into a first or a second current path, to which a first or second switching connection of the mechanical switch is connected. Diodes are suitably connected to these current paths, which together with the free-wheeling diodes serve to rectify the arcing voltage. By means of the two semiconductor switches, the power electronics unit can be used bi-directionally, and, independently of the potential (positive or negative) at the respective switching connection of the mechanical switch, both semiconductor switches are always controlled. 
         [0017]    The power electronics unit are suitably assigned a driver stage with transistors connected to a complementary output stage, which are connected on the collector-emitter side to the energy store and are connected on the base side to the voltage tap of the pulse generator. This output stage acts as a current amplifier when the pulse generator delivers the control pulse to the control inputs (bases) of the driver transistors. This results in a quick recharging of the IGBTs of the power electronics unit, which in turn allows for a particularly fast switching process. Suitably, the capacitor of the driver circuit provides the recharge current. 
         [0018]    The, or each, semiconductor switch of the power electronics unit is connected in a suitable development (collector-base side) with a series circuit comprising a resistor and a diode. The potential between the diode and the resistor corresponds to the forward voltage of the semiconductor switch (IGBT), plus the saturation voltage of the diode. With this series circuit serving as a measuring circuit, knowledge of the semiconductor or IGBT characteristic can be used to make a statement about the current flow by means of the semiconductor switch, that is to say, by the corresponding power semiconductor. In this way, overcurrent detection is provided without an additional current measuring element in the form of, for example, a cost-intensive shunt. A protective circuit, which preferably comprises this series or measuring circuit, or also a separate protective circuit, switches off the control pulse of the pulse generator when the collector-emitter voltage of the semiconductor switch (IGBT) of the power electronics unit exceeds a threshold value. 
         [0019]    In the case of two semiconductor switches (IGBTs), two such protective circuits, preferably including a measuring circuit (series circuit having a diode and resistor), are provided for the respective IGBTs. The respective protective circuit causes a short-term, that is to say a sufficiently fast, switching-off of the power electronics unit in order to switch off the IGBTs for their protection in the event of a fault within preferably 10 μs. 
         [0020]    The separation device can comprises a power supply connected to the power electronics unit with at least one semiconductor switch in the form of an IGBT, which is connected to the energy store, said switch suitably being connected to a further semiconductor switch (NPN transistor) as well as to resistors and at least one Zener diode, and which for charging the energy store is controlled by means of the arcing voltage and—after reaching the operating voltage—by means of the subsequent charge interruption. 
         [0021]    By means of the pulse generator, which can produces only one single pulse per switching operation, a very fast control of the power electronics unit of a hybrid disconnecting device is achieved, and thus its switching capacity is particularly high, that is to say increased as compared to known disconnecting devices. In addition, by means of the protective circuit, a reliable overcurrent detection of the power electronics unit is made possible with a concurrent cost-saving measuring device in the form of the series connection of the diode and resistor. Furthermore, a particularly compact circuit configuration of the power electronics unit is made possible. 
         [0022]    The disconnecting device according to the invention can also be provided for direct current interruption in the DC voltage range up to 1500V (DC). In the preferred use of the additional mechanical disconnector, this autonomous, hybrid disconnecting device is therefore particularly suitable for reliable and non-contact galvanic direct current interruption between a photovoltaic system and an associated inverter as well as in connection with, for example, a fuel cell system or an accumulator (battery). 
         [0023]    Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes, combinations, and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0024]    The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus, are not limitive of the present invention, and wherein: 
           [0025]      FIG. 1  is a block circuit diagram of a hybrid disconnecting device arranged between a photovoltaic generator and an inverter, with a mechanical switch and a power electronics unit including a protective circuit, as well as a pulse generator and a power supply, 
           [0026]      FIG. 2  is a detailed circuit diagram of the disconnecting device with two semiconductor switches of the power electronics unit and its driver and protective circuits as well as the pulse generator and the power supply with a capacitor as an energy store, 
           [0027]      FIG. 3  illustrates the pulse generator as a partial circuit of the hybrid disconnecting device, 
           [0028]      FIG. 4  illustrates the power electronics unit with the drive output stage of one of the semiconductor switches as well as two contact connections of the mechanical switch as partial circuit of the hybrid disconnecting device, 
           [0029]      FIG. 5  illustrates the protective circuit with a measuring circuit for overcurrent detection as a partial circuit of the hybrid disconnecting device, 
           [0030]      FIG. 6  illustrates the power supply with a rectifier circuit as a partial circuit of the hybrid disconnecting device, and 
           [0031]      FIG. 7  is a circuit diagram according to  FIG. 2  of a hybrid disconnecting device with an alternative rectifier circuit. 
       
    
    
     DETAILED DESCRIPTION 
       [0032]      FIG. 1  schematically shows a disconnecting device  1 , which is connected in the exemplary embodiment between a photovoltaic generator  2  and an inverter  3 . The photovoltaic generator  2  can, in a manner not shown in detail, comprise a number of solar modules  4 , which are connected to one another in parallel to a common generator connection box, which basically serves as an energy collection point. 
         [0033]    In the main current path  4 , which represents the positive pole, the disconnecting device  1  comprises a switching contact  5 , which is referred to hereinafter as a mechanical switch, and a power electronics unit  6  connected in parallel therewith, and a pulse generator  7  which drives the latter. The disconnecting device  1  also comprises a protective circuit  8  and a power supply  9 . The mechanical switch  5  and the power electronics unit  6  as well as the pulse generator  7  driving them form an autonomous hybrid circuit breaker (hybrid switch). A further hybrid circuit breaker can be connected in a non-illustrated manner in the return line  10 , which represents the negative pole of the disconnecting device  1 —and thus of the overall system. Mechanically coupled switching contacts of a further mechanical disconnecting element can be arranged between the photovoltaic generator  2  and the inverter  3  in the lead line (main path)  4 , which represents the positive pole, and in the return line  10 , for a complete galvanic separation or direct current interruption. 
         [0034]    If the mechanical switch  5 , which then has current flowing through, is opened during operation, an arc LB forms between its switching contacts. By means of the arcing voltage caused thereby, a capacitor C 9  ( FIGS. 2 and 6 ) is charged as an energy store via the switching connections J 1  and J 2  shown in  FIG. 1 . As soon as its charging voltage reaches a certain voltage value, the pulse generator  7  activates the power electronics unit  6 , whereupon it short-circuits the switch  5  and the arc LB extinguishes. 
         [0035]    The power electronics unit  6  thereby suitably remains switched on for a certain time, i.e., for a set or adjustable time element, in order to enable a deionizing of the switching path. After the time period or the corresponding time element has elapsed, the pulse generator  7  switches off the power electronics unit  6 . An overvoltage occurring during the switching process is limited by a varistor R 5  ( FIGS. 2 and 4 ). During the switching process, the protective circuit  8  monitors the respective power semiconductor (IGBT) T 1 , T 2  of the power electronics unit  6  in order to avoid its destruction by an impermissibly high current. 
         [0036]      FIG. 2  shows the disconnecting device  1  in a detailed circuit diagram, wherein there, the different line types used in  FIG. 1  frame the components of the power electronics unit  6 , of the pulse generator, of the protective circuit  8  and of the power supply  9 . Since the power electronics unit  6  preferably has two semiconductor switches in the form of the shown IGBTs T 1  and T 2 , two protective circuits  8  and two driver circuits for the IGBTs T 1  and T 2  are also provided. For the sake of better clarity, only one of these circuits with its components is bordered by the corresponding line type. The individual sub circuits are shown separately in  FIGS. 3 to 6 . 
         [0037]    According to  FIGS. 2 and 3 , the pulse generator  7  comprises a semiconductor switch in the form of a thyristor T 4 , which is connected to the capacitor C 9  via the connection V 1 . The latter is connected on the anode side via a PMOS-transistor (P-channel metal-oxide-semiconductor transistor) Q 2 , i.e., via the collector-emitter path of the latter to the V 1  leading to the capacitor C 9 . The thyristor T 4  is connected on the control side via a PMOS-transistor Q 3  which is connected to resistors R 16  and R 17  as well as to a Zener diode D 11 . On the cathode side, the thyristor T 4  is connected via a resistor R 14  to a voltage tap S 1 , which is connected to ground via a resistor R 15 . Furthermore, the voltage tap S 1  is connected to ground via the drain-source path of a further transistor Q 4 , in the present case an NMOS or bipolar transistor. At the voltage tap S 1 , there is also the base or the gate of a further transistor (NMOS or bipolar transistor) Q 5 , which collector-emitter path is connected via resistors R 19 , R 20  as variable resistors and R 21 , as well as via a capacitor C 3 , which is connected in parallel with the resistor R 19  between the connection V 1  connected to the capacitor C 9  and ground. 
         [0038]    In parallel with the RC element R 19  and C 3 , a series circuit composed of a resistor R 32  and a Zener diode D 1  is arranged, to which the base of a PNP transistor Q 7  is connected on the cathode side. The control side of a further thyristor T 5  is connected via the transistor Q 7  and a resistor R 24  to the connection V 1  connected to the capacitor C 9 . The anode-cathode path of the thyristor T 5  is connected to ground between the connection V 1  connected to the capacitor C 9  and—via a resistor R 22 . A cathode-side tap of this thyristor T 5  is connected to the gate (base) of the transistor Q 4  via a resistor R 18  and to the gate (base) of the transistor Q 2  via a resistor R 13 . The circuit shown and described constitutes, in addition to the semiconductor switch T 4 , a correspondingly connected semiconductor circuit of the pulse generator  7 . The pulse generator  7  generates the or each control pulse P for the two IGBTs T 1 , T 2  of the power electronics unit  6 , as explained below. 
         [0039]    The two thyristors T 4  and T 5  of the pulse generator  7  are initially in the blocking state so that the gate of the transistor Q 2  is at ground potential. If the charging voltage of the capacitor C 5  and thus the operating voltage increases as a result of an arc LB arising during the disconnecting of the mechanical switch  5 , the negative gate-source voltage of the transistor Q 2  also rises, so that the latter is switched through and the anode of the thyristor T 4  has the potential of the operating voltage. If this voltage continues to rise, the Zener diode D 1  begins to go into the conducting state. The resulting current flow causes a voltage drop across resistor R 17 . If this voltage drop exceeds the threshold value of the base-emitter voltage of transistor Q 3 , the latter becomes conductive. In order to protect the transistor Q 3  from being destroyed, the current is limited by the resistor R 16 . This current leads to an ignition of the thyristor T 4 . The value of the resistor R 14  is substantially smaller than that of the resistor R 15  so that the potential between these two resistors R 14 , R 15  at the voltage tap S 1 , at which the control pulse P is tapped for the power electronics unit  6 , is only slightly below the operating voltage. 
         [0040]    As soon as the thyristor T 4  has fired, the transistor Q 5  turns on and the capacitor C 3  is charged via the resistors R 20  and R 21 . Since the capacitor C 3  is initially uncharged, the potential of the anode of the Zener diode D 12  is at operating voltage. By charging the capacitor C 3 , the potential shifts to ground. If this potential has decreased such that the Zener diode D 12  becomes conductive, a current will flow through the resistor R 23 . If the voltage drop across this resistor R 23  exceeds the threshold value of the base-emitter voltage of the PNP-transistor Q 7 , then the latter switches through. The resistor R 24  provides a current limitation and protects the transistor Q 7 . 
         [0041]    The current flowing through the transistor Q 7  leads to the ignition of the thyristor T 5  so that the potential at its cathode rises to the operating voltage minus the forward voltage. Thus, the transistor Q 4  also turns on and pulls the potential between the resistors R 14  and R 15  at the voltage tap S 1  to ground. In addition, transistor Q 2  now blocks and causes the thyristor T 4  to turn off. Thus, the transistor Q 5  also blocks and the capacitor C 3  is discharged via the resistor R 19 . The thyristor T 5  remains conductive until the capacitor C 9  is discharged. Since the capacitor is recharged during an arcing phase and also during the switching overvoltage, only a single control pulse is triggered. 
         [0042]    The power electronics unit  6  shown in  FIGS. 2 and 4  is assigned a driver stage  11 . The IGBTs T 1  and T 2  of the power electronics unit  6  form the lower part of a B 2  rectifier bridge. By using two power semiconductors with free-wheeling diode in the form of the IGBTs T 1  and T 2 , a bidirectionally usable circuit is achieved. If the illustrated switch or contact connection J 2  of the mechanical switch  5  has a positive potential and the other contact connection J 1  a negative potential, the current can flow through the IGBT T 2  and the free-wheeling diode of the IGBT T 1 . In the case of a reversed polarity, a current flow through the IGBT T 1  and the free-wheeling diode of the IGBT T 2  is possible. Since the control signal of an IGBT has no influence on its inverse operation, both IGBTs T 1  and T 2  of the power electronics unit  6  are always controlled. 
         [0043]    Since the driver circuits  11  of both IGBTs T 1  and T 2  are identically constructed, only one of the two driver circuits  11  is described below. The driver circuit  11  comprises an NPN-transistor Q 8  and a PNP-transistor Q 6 , which are connected to a complementary output stage. If the pulse generator  7  emits the control pulse P to the bases of the two transistors Q 6  and Q 8 , they act as a current amplifier and enable a quick recharge of the gate of the respective IGBT T 2 , T 1 . This results in a particularly rapid switching process. A capacitor T 5  of the driver circuit  11  provides the recharge current. The IGBT T 2  is attenuated by a resistor R 28  since, due to parasitic inductances and capacitances, oscillations can occur during the control of the respective IGBT T 2 . A Zener diode D 16  of the driver circuit  11  protects the gate of the IGBT T 2  from surges, if oscillations should still occur. Since due to the steep switching edge of the IGBT T 2  overvoltages can occur when switching inductive loads, the varistor R 5  limits the overvoltage in order to prevent destruction of the power semiconductors T 1 , T 2 . 
         [0044]      FIGS. 2 and 5  show the measuring and protective circuit  8  of the disconnecting device  1 . Although IGBTs as a semiconductor switch of the power electronics unit  6  are in principle short-circuit-proof, they must nevertheless be switched off in the event of a fault within 10 μs. The circuits  8  for monitoring or measuring the current of the two IGBTs T 1 , T 2  are of identical construction, so that  FIG. 5  again shows only one such circuit  8 . The measuring circuit essentially comprises a series circuit having a resistor R 27  and a diode D 3  connected between the gate and the collector of the IGBT T 2 . The control signal of the IGBT T 2  is applied to its collector-emitter path via the resistor R 27  and the diode D 3 . 
         [0045]    The potential between the diode D 3  and the resistor R 27  corresponds to the forward voltage of the IGBT T 2 , plus the saturation voltage of the diode D 3 . Thus, knowing the IGBT characteristic, a statement can be made about the current flow through this power semiconductor T 2 . In order not to unnecessarily discharge the capacitor C 9  as energy store during the switching phase, the resistor R 27  is relatively high-resistance. In order to nevertheless enable rapid switching off in the event of a fault, a complementary output stage with correspondingly connected transistors Q 11  and Q 12  is connected downstream. A diode D 14  connected on the emitter side to the output stage allows the two measuring circuits D 3 , R 27  and D 4 , R 28  ( FIG. 2 ) to be connected in parallel. 
         [0046]    When the collector-emitter voltage of the IGBT T 2  exceeds a certain potential, a thyristor T 6  of the protective circuit  8  is triggered. The transistor Q 7  of the pulse generator  7  is thereby turned on, thus initiating the switching-off operation. A capacitor C 7  connected to ground on the control side of the thyristor T 6  and a resistor R 31  connected in parallel therewith form a filter in order, inter alia, to prevent the protective circuit  8  from tripping during the switch-on phase of the IGBT T 2 . The tripping voltage can be determined using the following formula: 
         [0000]        U   CE ( T 2)≧ U   BE ( Q 12)+ U   D ( D 14)+ U   Z ( D 13)+ U   zü ( T 6)− U   D ( D 3),
 
         [0047]    wherein U CE  is the collector-emitter voltage, U BE  is the base-emitter voltage, U D  is the forward voltage, U z  is the Zener voltage, and U zü  is the ignition voltage. 
         [0048]      FIGS. 2 and 6  show the circuit configuration of the power supply  9  of the disconnecting device  1 . The power supply  9  serves to charge the capacitor C 9  as an energy store and for protection against a switching overvoltage. The mechanical switch  5  ( FIG. 1 ) is located between the switch or contact connections J 1  and J 2 . As soon as the switch  5  opens the circuit, the arc LB is formed. The arcing voltage is rectified via diodes D 1 , D 2  connected in current paths  6   a  and  6   b  of the semiconductor switches (circuit breaker) T 1  and T 2  of the power electronics unit  6  and via the free-wheeling diodes of the IGBTs T 1  and T 2 , respectively. 
         [0049]    The power supply  9  comprises a semiconductor switch in the form of an IGBT T 7 , of which the gate is charged via resistors R 33  to R 37 . As soon as the gate-emitter potential of the thyristor T 7  is above the threshold voltage, IGBT T 7  turns on and the capacitor C 9  is charged. Connected to the IGBT T 7  is an NPN-transistor Q 15  in the manner shown in  FIG. 6 . On the emitter side, the transistor Q 15  is connected to ground via a Zener diode D 11 . When the potential of the capacitor C 9  reaches the value of the Zener diode D 19  plus the base-emitter threshold voltage of the transistor Q 15 , the latter becomes conductive and limits the gate-emitter voltage of the IGBT T 7 . The transistor then begins to block and the charging current of the capacitor C 9  is interrupted. The Zener diode D 19  also protects the gate of the IGBT T 7  and the transistor Q 15  from overvoltage. 
         [0050]    The disconnecting device  1  can also be operated with an upstream rectifier. A corresponding circuit is shown in  FIG. 7 . The individual sub circuits of the power supply  9 , the pulse generator  7  of the measuring and protective circuit  8 , and, in principle, the power electronics unit  6 , can be seen unchanged. In addition to the IGBT T 2  as semiconductor switch of the power electronics unit  6 , the diodes D 1 -D 4  inserted in the circuit shown in  FIG. 7  must be able to carry the entire current. In addition, the forward voltage in the switched-on state is comparatively high due to the series circuit of three semiconductors. 
         [0051]    The invention is not limited to the embodiments described above. Rather, other variants of the invention can also be derived from those skilled in the art without departing from the scope of the invention. In particular, all the individual features described in connection with the exemplary embodiments can also be combined with one another in a different manner without departing from the subject matter of the invention. 
         [0052]    The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are to be included within the scope of the following claims.