AU2011214562A1

AU2011214562A1 - Surge-limiting device for direct current networks

Info

Publication number: AU2011214562A1
Application number: AU2011214562A
Authority: AU
Inventors: Michael Morsch; Reinhard Vogel
Original assignee: Siemens AG
Current assignee: Siemens AG
Priority date: 2010-02-15
Filing date: 2011-01-27
Publication date: 2012-08-02
Anticipated expiration: 2031-01-27
Also published as: KR101864184B1; DE102010001924B4; AU2011214562B2; EP2537219A1; BR112012020480A2; KR20130009969A; ES2595382T3; DE102010001924A1; WO2011098359A1; EP2537219B1

Abstract

A surge-limiting device (10) according to the invention comprises a varistor (11) that is connected in series to a parallel circuit (12) consisting of a preferably electronic switch (13) and an electronic component (14) with a surge-limiting characteristic, preferably a Zener diode characteristic, or multiple electronic components (14) which are connected in series and each of which has a surge-limiting characteristic, preferably a Zener diode characteristic. It is thus possible to reduce comparatively high amounts of energy in high driving currents and comparatively small differences between the permissible maximum voltage and the normal operational voltage of the network (1) in order to thus ensure the protection of consumers, in particular voltage-sensitive consumers, such as power electronic converter circuits for example.

Description

PCT/EP2011/051128 / 2009P20242WO Description Surge-limiting device for direct current networks The invention relates to a surge-limiting device for direct current networks as claimed in the preamble of claim 1. Energy storage in direct current (DC) voltage networks is preferably based on the use of batteries or accumulators (referred to in the following collectively as "batteries" for simplicity). In order to protect batteries of said type as well as to protect individual loads, use is made of protective devices which enable selective clearing of a short circuit. In order to achieve this, the battery itself is on the one hand assigned protection elements which trip with a delay in the event of a short circuit, such as e.g. delayed-action electromechanical circuit breakers with overcurrent detection and a delay time of e.g. > 100 ms. The individual loads, on the other hand, are assigned protection elements which operate without a delay or at least have a significantly smaller delay, e.g. instantaneously-acting electromechanical circuit breakers having a total tripping time of approx. 30 ms or fusible cutouts having a tripping time of < 1... 40 ms that is dependent on the short-circuit current, such that the selectivity of the short-circuit clearing can be achieved. However, in high-power direct current networks, such as are employed in submarines for example, in which the energy is stored in high-performance batteries, the effective time constant at which the current increases in the event of a short circuit is so small, e.g. < 1.. .3 ms, that even when instantaneously-acting protection elements are used the short circuit current approximately or fully attains the value of PCT/EP2011/051128 / 2009P20242WO 2 the stationary short-circuit current. Furthermore, due to the very low internal ohmic resistance of the batteries, the power cables, distributing busbars, etc., said battery-driven short circuit current is very large and can amount to several 10 kA. In the event of a short circuit in an outgoing load circuit, the instantaneously-acting load protection element then disconnects upon detecting said large current and prevents further damage due to overcurrents in the affected outgoing load circuit. However, due to the energy stored in the parasitic inductances of the battery, the cable runs, etc., the short-circuit current fed by the energy store or voltage source continues to flow until said energy has been dissipated or e.g. transferred into capacitive energy stores. In this case only a small part of the energy stored in the parasitic inductances is dissipated in the electric arc of the opening contacts or fusible conductors of the load protection elements. The energy from the short-circuit current that has not been dissipated in the arc now flows onward via the network to the other loads connected there and can lead there to significant overvoltages, with the potential to cause damage in particular to voltage-sensitive loads. Such voltage-sensitive loads are in particular power electronic converters and actuators having electrolytic capacitors on the DC side which usually, for cost, space and availability reasons, cannot be dimensioned for withstanding significant overvoltages. With such power converters, the maximum permissible voltage value with overvoltage often lies only approx. 33% above that in the fault-free state of the onboard electrical network. High requirements therefore exist in respect of an effective, rapid PCT/EP2011/051128 / 2009P20242WO 3 and narrowly toleranced limiting of the occurring overvoltages. It is already known from WO 2006/003191 Al to limit overvoltages caused in onboard DC electrical networks by energy feedbacks from loads, e.g. by energy feedbacks from DC motors or generators, at exceptionally low load levels of the onboard electrical network in such a way that if an excessively high supply voltage occurs a voltage detection circuit will drive an IGBT which in turn withdraws energy from the network by way of an ohmic resistor. The devices known as "brake choppers", as are typically used today in practically all pulse converters having network-side diode injection, also operate according to the same functional principle, because the braking energy from the electrical machine cannot be fed back into the network, but is converted into heat in the DC intermediate circuit in the ohmic resistor which is arranged therein and which is connected into the circuit as necessary. Such an arrangement is suitable particularly for such overvoltage events which result from energy feedbacks that are comparatively long-lasting, but in terms of current magnitude are limited to a few hundred amperes, as occur in the energy feedback from electrical machines. However, in order to dissipate energy from short-lived overvoltage pulses in the millisecond range, which result from very high driving currents in the range of several 10 kA, many such circuits would have to be connected in parallel on account of the limited current-carrying capacity of deactivatable power semiconductor switches IGBTs or even IGCTs, which necessitates a high investment of technical resources.

PCT/EP2011/051128 / 2009P20242WO 4 Another generally conventional solution possibility for limiting overvoltages consists in using voltage-dependent resistors, called "varistors", for protection against overvoltages. In the context of power converter circuitry they are preferably used for limiting lightning and switching voltages. However, limiting voltage surges to a value approx. 1.33 times that of the rated steady-state operating voltage is not achievable in this way, because the limiting characteristic curve of such components is not steep enough. The use of Zener diodes ("Z diodes"), which have a very steep limiting characteristic curve, would suggest itself as suitable for voltage-limiting applications in which the maximum permissible voltage lies only slightly above the value of the rated steady-state operating voltage. However, such components are only available in the low power range (up to several watts) and consequently cannot be used in the field of high-power DC networks for limiting high-energy overvoltage pulses. To sum up, it can be stated that no satisfactory and technically simple solution is known in the prior art for limiting overvoltages that result in high-power DC networks from the deactivation of short-circuit currents in the kA range to a voltage level lying only marginally above the rated operating voltage. It is therefore the object of the present invention to disclose a surge-limiting device for direct current networks which enables comparatively high amounts of energy in the case of high driving currents and comparatively minor differences between permissible maximum voltage and normal operating PCT/EP2011/051128 / 2009P20242WO 5 voltage of the network to be dissipated in order thereby to ensure the protection of loads, in particular voltage sensitive loads such as e.g. power electronic converter circuits. This object is successfully achieved by means of a surge limiting device as claimed in claim 1. Advantageous embodiments are the subject matter of the respective dependent claims. A surge-limiting device according to the invention comprises a varistor which is connected in series with a parallel circuit consisting of a preferably electronic switch and an electronic component having a voltage-limiting characteristic, preferably a Z diode characteristic, or a plurality of series-connected electronic components, each having a voltage-limiting characteristic, preferably a Z diode characteristic. An electronic component having a Z diode characteristic is understood in this context to mean a component which in relation to its current-voltage characteristic curve has an identical characteristic to a Z diode. By connecting the varistor in series with the electronic component(s) having the voltage-limiting characteristic it is possible to steepen the voltage-limiting characteristic curve of the solution according to the invention compared with the use of the varistor alone in the low leakage current range. With the aid of the switch, which in the closed state, i.e. when allowing current through, bridges the electronic component(s) having the voltage-limiting characteristic, it is possible to activate or deactivate the steepening of the voltage-limiting characteristic curve of the solution PCT/EP2011/051128 / 2009P20242WO 6 according to the invention compared with the use of the varistor alone. The closing of the switch can be effected for example by means of a control device which closes the switch, i.e. switches to allowing current through, if the voltage to be limited and/or the current through the series circuit consisting of varistor and the electronic component(s) having the voltage-limiting characteristic exceeds a predetermined value, since the current through the two series-connected components is determined by the sum of the voltage-dependent resistances of said two components and by the voltage applied to said series circuit. The opening of the switch can be effected for example by the switch being embodied in such a way that it opens automatically, or by the control device or - in the case of a thyristor switch for example - a quenching device for the switch current being configured in such a way that it opens the switch if the voltage that is to be limited drops below a predetermined value. The voltage and/or current value at which the switch opens are/is preferably chosen such that a repeat tripping of the surge-limiting device after switchover to the then once again present series circuit consisting of varistor and the electronic component(s) having the voltage-limiting characteristic can be ruled out. Closing the switch causes a change in the voltage-limiting properties of the circuit in such a way that a resulting tripping overvoltage of the surge-limiting device is reduced PCT/EP20ll/051128 / 2009P20242WO 7 by the Z diode voltage and consequently the voltage-limiting value of the varistor or varistors can be reached. It is therefore possible to steepen the voltage-limiting characteristic curve compared with use of a varistor alone in the low leakage current range, with the result that an overvoltage can be limited by means of the circuit according to the invention to a value which lies only marginally above the rated steady-state operating voltage, e.g. to approx. 1.33 times the value of the rated steady-state operating voltage. Because of the strongly nonlinear voltage-dependent resistance profile of the varistor, the leakage current through the varistor and the electronic switch can very quickly rise to a large current value and consequently a high-energy overvoltage pulse can be dissipated. Following the dissipation of the overvoltage pulse and decaying of the leakage current through the varistor, the current can once more be conducted by the switch to the electronic component(s) having the voltage limiting characteristic, as a result of which the tripping overvoltage defined by means of the series connection of the varistor with the electronic component(s) having the voltage limiting characteristic, and consequently the initial state of the surge-limiting device, is reset automatically. The invention accordingly combines the properties of varistors for limiting voltage pulses having high energy content in the kJ range, the properties of electronic components having a voltage-limiting characteristic, such as e.g. Z diodes for steep voltage limiting in the low current range and power ratings in the range of several watts, and the switching and current-conducting capability of switches, in particular PCT/EP2011/051128 / 2009P20242WO 8 electronic switches, such as e.g. thyristors, in the range from several 10 kA to 100 kA. A tripping overvoltage for surge limiting which is less than or equal to a maximum permitted overvoltage of the direct current network is preferably defined by means of the series connection of the varistor with the electronic component(s) having the voltage-limiting characteristic. In this case the varistor itself can have a rated steady-state operating voltage which is less than the rated voltage of the direct current network. Preferably the varistor has a limiting voltage which, in the case of the leakage current that is to be expected, is less than or equal to a maximum permitted overvoltage of the direct current network. According to a particularly advantageous embodiment of the invention, the number of series-connected electronic components having the voltage-limiting characteristic is chosen such that the sum of the rated steady-state operating voltages of the electronic components having the voltage limiting characteristic and the rated steady-state operating voltage of the varistor is equal in size to the rated voltage of the direct current network. In this case the switch can be actuated particularly easily and quickly by means of an electrical resistor which is connected in series with the electronic component(s) having the voltage-limiting characteristic and which is dimensioned such that, upon a tripping overvoltage of the surge-limiting device being reached, a current flowing through the resistor PCT/EP2011/051128 / 2009P20242WO 9 is so large that there occurs at the resistor a voltage drop which is sufficient to switch the switch into the through connected state, in particular to fire a thyristor. It is particularly advantageous if the switch is embodied as a thyristor. The current flowing through the electronic component(s) having the voltage-limiting characteristic in the event of an overvoltage can then also be used at the same time for firing the thyristor. This can be accomplished for example - as described hereintofore - with the aid of the voltage drop at an electrical resistor connected in series with the electronic component(s) having the voltage-limiting characteristic. After being fired and consequently switched to a state allowing current to flow through, the thyristor bridges the electronic component(s) having the voltage-limiting characteristic, so no more firing current can flow once the thyristor has been fired, with the result that following dissipation of an overvoltage pulse and decaying of the leakage current through the varistor under the holding current of the thyristor the latter can automatically return to the blocking state once more, in which case the component(s) having the voltage-limiting characteristic which is (are) connected in parallel with the thyristor limits (limit) the returning blocking voltage to the value of the limiting voltage. In the case of a thyristor switch, a suitable quenching device can also be present for quenching or assisting in the quenching of the thyristor switch.

PCT/EP2011/051128 / 2009P20242WO 10 Instead of a thyristor switch, the switch can alternatively also be embodied as a deactivatable power electronics component. According to a constructionally simple embodiment, the electronic component(s) having the voltage-limiting characteristic is (are) embodied in each case as a Z diode or a varistor. Instead of just the one varistor the surge-limiting device can also have a plurality of varistors connected in parallel with one another. Preferably the number of varistors is matched to a desired energy discharge capacity. The invention and further advantageous embodiments of the invention according to features of the dependent claims are explained in more detail below with reference to exemplary embodiments depicted in the figures, in which: FIG 1 shows a direct current network having a surge-limiting device according to the invention, FIG 2 shows a current-voltage characteristic curve of the surge-limiting device of FIG 1, and FIG 3 shows an exemplary extension of the surge-limiting device illustrated in FIG 1. FIG 1 shows a direct current network 1, for example a high energy direct current network of a submarine, comprising a battery 2 and two loads 3, 4 connected in parallel with the battery. The load 4 is a voltage-sensitive load such as e.g. a power electronics converter and actuator having electrolytic PCT/EP2011/051128 / 2009P20242WO 11 capacitors on the DC side. In order to protect the battery 2 and the individual loads 3, 4, the direct current network 1 has protective devices 5, 6 which allow selective clearing of a short circuit. Thus, for example, a protection element 5 which operates with a delay in the event of a short circuit is connected in series with the battery 2. The protection element 5 is, for example, a delayed-action electromechanical power switch having overcurrent detection and a delay time of e.g. > 100ms. The individual loads 3, 4, in contrast, are assigned protection elements 6 which operate instantaneously or at least have a significantly smaller delay than the protection elements 5. The protection elements 6 are, for example, instantaneously-acting power electromechanical power switches having a total tripping time of approx. 30ms (as shown) or fusible cutouts having a short-circuit-current-dependent tripping time of < 1 to 10 ms, such that a selectivity of the short-circuit clearing can be achieved. The effective time constant at which the current increases in the event of a short circuit is so small, e.g. < 1... 3 ms, such that even if instantaneously-acting protection elements are used the short-circuit current approximately or fully attains the value of the stationary short-circuit current. Furthermore, due to the very low internal ohmic resistance of the batteries, the power cables, distributing busbars, etc., said battery-driven short-circuit current is very large and can amount to several 10 kA. In the event of a short circuit in an outgoing load circuit, in this case a short circuit in the load 3, the associated instantaneously-acting load protection element 6 then disconnects upon detecting said large current and prevents further damage due to overcurrents in the affected outgoing load circuit.

PCT/EP2011/051128 / 2009P20242WO 12 However, due to the energy stored in the parasitic inductances of the battery, the cable runs, etc. (symbolized in this case for simplicity by an inductor L), the short-circuit current fed by the battery 2 continues to flow until said energy has been dissipated or e.g. transferred into capacitive energy stores. In this case only a small part of the energy stored in the parasitic inductances is dissipated in the electric arc of the opening contacts or fusible conductors of the load protection element 5. The energy from the short-circuit current that has not been dissipated in the arc now flows onward via the network 1 to the other loads connected there and can lead there to significant overvoltages, with the potential to cause damage in particular to the voltage sensitive load 4, which generally, for cost, space and availability reasons, is not dimensioned for withstanding significant overvoltages. A surge-limiting device 10 is therefore connected in parallel with the battery 2 and the loads 3, 4. The surge-limiting device 10 comprises a plurality of parallel-connected varistors 11 which are connected in series with a parallel circuit 12 consisting of an electronic switch 13 and one or more Z diodes 14 connected in series with a resistor 15. Varistors can also be used instead of Z diodes. The electronic switch 13 is embodied as a thyristor, the gate of which is connected via a line 16 to a line connection 18 between the series circuit of the Z diodes 14 and the resistor 15. A tripping overvoltage for surge limiting which is less than or equal to a maximum permitted overvoltage Umx of the direct current network 1 is defined by means of the series connection PCT/EP2011/051128 / 2009P20242WO 13 of the varistors 11 and the Z diodes 14. In this case the varistors 11 have a rated steady-state operating voltage UNV which is less than a rated steady-state operating voltage UN (e.g. UN = 750V) of the direct current network 1. The varistors 11 also have a limiting voltage UB which, in the case of an anticipated leakage current, is less than or equal to the maximum permitted overvoltage Umax of the direct current network 1. In this case the maximum permitted overvoltage Umax of the network 1 lies only marginally above the rated steady state operating voltage UN Of the network 1, e.g. only about 30% higher. The mode of operation of the surge-limiting device 10 is now explained with reference to FIG 2, which shows a current voltage characteristic curve K of the surge-limiting device 10. In this case the current I is represented logarithmically. The characteristic curve K consists of a first section Kl which is formed by the sum of the characteristic curves of the varistors 11 and the Z diodes 14. This is immediately followed by a second section K2 which is formed solely by the characteristic curve of the varistors 11. If the rated voltage UN is present at the surge-limiting device 10, an operating point A having a current IA of e.g. 1mA is established on the characteristic curve Kl. In this case the current IA flows through the varistors 11 and the Z diodes 14 as well as through the resistor 15. If there is an increase in the voltage U present at the surge limiting device 10, the operating point moves along the characteristic curve K1 in the direction of higher voltages and leakage currents until a tripping overvoltage defined by means of the series connection of the varistors 11 and the Z PCT/EP2011/051128 / 2009220242WO 14 diodes 14 is reached at a point B. A current IB (e.g. IB = 0.5 A) flowing through the resistor 15 is so large that the voltage present at the resistor 15 and consequently at the gate of the thyristor 13 fires the thyristor 13 and switches the latter to a state allowing current to flow through. The current flowing through the varistors 11 is then transferred to the thyristor 13. In this arrangement the resistor 15 and the line 16 serve as a control device 19 for switching the thyristor 13 to a state allowing current to flow through. Alternatively or in addition to the actuation of the electronic switch 13 by means of the current through the Z diodes 14 and the control device 19, the electronic switch 13 can also be actuated via a control device 20 by means of which a direct evaluation of the voltage U of the direct current network 1 that is present at the circuit arrangement 10 according to the invention and that is to be limited. At the moment when the current is transferred to the thyristor 13 and the latter bridges the Z diodes 14, the resulting tripping overvoltage of the surge-limiting device 10 is reduced by the voltage of the Z diodes 14 and a jump takes place from the operating point B of the characteristic curve K1 to an operating point C on the characteristic curve K2. The operating point C already lies in the voltage-limiting range of the varistors 11. As a result of the strongly nonlinear, voltage-dependent resistance profile of the varistors 11, the leakage current through the varistors 11 and the thyristor 13 can very rapidly rise to a large current value ID (e.g. ID = 1000 A) at the operating point D and the high-energy overvoltage pulse is dissipated. In this case the number of varistors 11 is matched to a desired energy discharge capacity.

PCT/EP2011/051128 / 2009P20242WO 15 Because the thyristor 13 bridges the Z diodes 14, no more firing current can flow via the line connection 16 after the thyristor 13 has been fired. Following the dissipation of the overvoltage pulse and decaying of the leakage current through the varistors 11 under the holding current of the thyristor 13, the latter automatically returns to its blocking state. In this case the operating point moves on the characteristic curve K2 in the direction of decreasing voltages and leakage currents as far as the operating point E, at which the thyristor goes to the blocking state once more and the Z diodes 14 connected in parallel with the thyristor 13 take over the current from the thyristor 13. This leads to a jump from point E to point F on the characteristic curve Kl and the tripping overvoltage defined by means of the series connection of the varistors 11 and the Z diodes 14 is reset. In this case the Z diodes 14 limit the returning blocking voltage of the thyristor to the value of the Z diode voltage. When the voltage U present at the surge-limiting device 10 returns to the rated voltage UN Of the direct current network 1, the operating point then moves back once more from point F to point A. Thus, after the overvoltage pulse has dissipated, the initial state of the surge-limiting device 10 has been restored automatically. A prerequisite for the automatic quenching of the thyristor 13 is that the transition from the point E on the characteristic curve K2 to the point F on the characteristic curve K1 takes place at a current which is less than or equal to the holding current of the thyristor 13. In the event that this is not given or cannot be realized on account of the necessary choice of components and component data of the components used in circuit 10, in particular of the thyristor 13, quenching or PCT/EP2011/051128 / 2009P20242WO 16 quenching assistance is required for the thyristor 13, as indicated in an exemplary technical circuit solution shown in FIG 3. The circuit illustrated by way of example in FIG 3 represents an extension of the inventive circuit 10 by a quenching device 30. In this case the quenching device 30 is described only in principle and as an example of a multiplicity of possible technical solutions for quenching a thyristor current. A resistor 31 serves therein as a charging resistor for a quenching capacitor 32. In order to quench the thyristor 13, a switch 33 is closed, thus causing the energy stored in the quenching capacitor 32 to induce a current flow ILOE through the thyristor 13 in a direction opposite to that of the current IVAR which flows through the varistors 11 and the thyristor 13 at the intended quenching time. In this case the magnitude of the quenching current ILOE must preferably be greater than or approximately equal to the magnitude of the current IVAR at the time of quenching. The magnitude and duration of the requisite quenching current ILOE can in this case be achieved through suitable dimensioning of the quenching capacitor 32 and a current-limiting resistor 34. The switch 33 can preferably be implemented as an electronic semiconductor switch. The actuation of the switch 33 via a control line 35 can be effected by means of an evaluation of the current IVAR through the thyristor 13 in such a way that the switch 33 is closed and consequently the thyristor 13 quenched if the current IVAR through thyristor 13 falls below a defined minimum value. Alternatively or in addition thereto, the switch 33 can also be actuated by means of a control line PCT/EP2011/051128 / 2009P20242WO 17 37 by way of a direct evaluation, by means of the control device 20, of the voltage of the direct current network 1 which is present at the inventive circuit arrangement 10 and which is to be limited, or by way of an evaluation, by means of a control device 36, of the voltage drop across the thyristor 13 and appropriate actuation of the switch 33 via a control line 40, in that the thyristor 13 is quenched through closing of the switch 33 if a defined level of said voltage(s) is undershot. A different electronic switch, such as e.g. an IGBT, can also be used instead of a thyristor 13. In particular said component can also be an activatable and deactivatable switch whose switching state is controlled by means of a control device which senses the current flowing through the Z diodes 14 and through the electronic switch 13 and switches the switch 13 to a state allowing current to flow through if the current flowing through the Z diodes 14 exceeds a predetermined limit value or, alternatively, switches to a current blocking state if the switch current drops below a predetermined limit value. A desired summation voltage of the Z diodes 14 and consequently also the difference between the tripping voltage at low current (from the series connection of Z diodes 14 and varistors 11) and the limiting voltage at high current (through the varistors 11 alone) can be chosen based on the number of Z diodes 14 that are connected in series. By means of suitable configuration and dimensioning of the circuit elements it is possible to tailor the voltage-limiting function of the surge-limiting device 10 over a wide energy, voltage and current range to a desired tripping overvoltage, to a desired current to be leaked when a voltage-limiting PCT/EP2011/051128 / 2009P20242WO 18 event occurs, and to a desired limiting voltage at high leakage current, with voltage limiting also being possible when there are comparatively minor differences between permissible maximum voltage and normal operating voltage of the network.

Claims

1. A surge-limiting device (10) for direct current networks (1), said device comprising a varistor (11), characterized in that the varistor (11) is connected in series with a parallel circuit (12) consisting of a preferably electronic switch (13) and an electronic component (14) having a voltage-limiting characteristic, preferably a Z diode characteristic, or a plurality of series-connected electronic components (14), each having a voltage-limiting characteristic, preferably a Z diode characteristic.

2. The surge-limiting device (10) as claimed in claim 1, characterized by a control device (19) which is configured in such a way that it closes the switch (13) if the current through the electronic component(s) (14) having the voltage limiting characteristic exceeds a predetermined value.

3. The surge-limiting device (10) as claimed in claim 1 or 2, characterized by a control device (20) which is configured in such a way that it closes the switch (13) if the voltage that is to be limited exceeds a predetermined value.

4. The surge-limiting device (10) as claimed in one of the preceding claims, characterized in that the switch (13) is embodied in such a way that it opens automatically, or in that the control device (19, 20) or a quenching device (30) for the switch current is configured in such a way that it opens the switch (13) if the voltage that is to be limited falls below a predetermined value.

5. The surge-limiting device (10) as claimed in one of the preceding claims, characterized in that the switch (13) is PCT/EP2011/051128 / 2009P20242WO 20 embodied in such a way that it opens automatically, or in that the control device (19, 20) or a quenching device (30) for the switch current is configured in such a way that it opens the switch (13) if the current through the switch (13) falls below a predetermined value.

6. The surge-limiting device (10) as claimed in one of the preceding claims, characterized in that a tripping overvoltage for surge limiting which is less than or equal to a maximum permitted overvoltage (Umax) of the direct current network (1) is defined by means of the series connection of the varistor (11) with the electronic component(s) (14) having the voltage limiting characteristic.

7. The surge-limiting device (10) as claimed in one of the preceding claims, characterized in that the varistor (11) has a rated steady-state operating voltage (UNV) which is less than the rated voltage (UN) of the direct current network (1).

8. The surge-limiting device (10) as claimed in one of the preceding claims, characterized in that the varistor (11) has a limiting voltage (UB) which is less than or equal to a maximum permitted overvoltage (Umax) of the direct current network (1) at an anticipated leakage current.

9. The surge-limiting device (10) as claimed in one of the preceding claims, characterized in that the number of series connected electronic components (14) having the voltage limiting characteristic is chosen such that the sum of the rated steady-state operating voltages of the electronic components (14) having the voltage-limiting characteristic and the rated steady-state operating voltage (UNV) of the varistor PCT/EP2011/051128 / 2009P20242WO 21 (11) is equal in size to the rated voltage (UN) of the direct current network (1).

10. The surge-limiting device (10) as claimed in one of the preceding claims, characterized by an electrical resistor (15) which is connected in series with the electronic component(s) (14) having the voltage-limiting characteristic and which is dimensioned such that, upon a tripping overvoltage of the surge-limiting device (10) being reached, a current flowing through the resistor (15) is so large that there occurs at the resistor (15) a voltage drop which is sufficient to switch the switch (13) into the through-connected state, in particular to fire a thyristor.

11. The surge-limiting device (10) as claimed in one of the preceding claims, characterized in that the switch (13) is embodied as a thyristor.

12. The surge-limiting device (10) as claimed in claim 11, characterized by a quenching device (30) for quenching a current through the thyristor.

13. The surge-limiting device (10) as claimed in one of the preceding claims, characterized in that the electronic component(s) (14) having the voltage-limiting characteristic is (are) in each case embodied as a Z diode or a varistor.

14. The surge-limiting device (10) as claimed in one of the preceding claims, characterized in that instead of just the one varistor (11) it has a plurality of varistors (11) connected in parallel with one another. PCT/EP2011/051128 / 2009P20242WO 22

15. The surge-limiting device (10) as claimed in claim 14, characterized in that the number of varistors (11) is matched to a desired energy discharge capacity.