DK2885801T3 - Separation device for a High Voltage Direct Current - Google Patents

Description

The present invention relates to a disconnecting assembly for a high-voltage direct-current network. Moreover, the present invention relates to a method for operating a disconnecting assembly for a high-voltage direct-current network.

High-voltage networks usually make use of disconnecting assemblies with which the current in the fault situation (and also operating currents) can be switched off. Particularly for switching off direct currents in high-voltage networks, mechanical switches cannot form the switching process by themselves. The reason for this is that the arc voltage that usually arises, at the contacts of the mechanical switch, is too low relative to the driving network voltage. Therefore, so-called hybrid switches or hybrid DC circuit-breakers, consisting of a combination of power electronic and mechanical switches, are proposed for interrupting direct currents. In this case, the electric current is passed by the mechanical switch in the normal state. During a switch-off process, the current firstly commutates from the mechanical switch to the semiconductor branch. In order to ensure the quenching of the arc in the mechanical switch, the semiconductor switch must firstly be switched to the on state. After the quenching of the arc and a time for recovery of the isolation path, the semiconductor switches are correspondingly driven such that a voltage builds up which is used for switching off the current in the DC network. However, for this sequence it is necessary for the arc voltage generated in the mechanical switch to be high enough in order that the commutation process can actually take place. In particular, the threshold voltage of massively series-connected semiconductor switches used nowadays has to be overcome statically. In addition, the voltage drops occurring across the leakage inductances in the commutation circuit also have to be taken into account for the dynamic process.

In the case of disconnecting assemblies used nowadays, by way of example, 10 mechanical switches are connected in series for switching off direct currents in a high-voltage network having an electrical voltage of 300 kV. Said mechanical switches are usually embodied as vacuum interrupters. Each of the vacuum interrupters generates an arc voltage of approximately 30 volts after the opening of the contacts, which corresponds to a total commutation voltage of 300 volts. Given a network voltage of 300 kV, by way of example, 140 semiconductor switches in the form of IGBTs having a respective reverse voltage of 3.3 kV have to be connected in series in order to be able to build up an adequate reverse voltage. Each of the semiconductor switches has a forward voltage of approximately 3 volts, which corresponds to a minimum commutation voltage of 420 volts. In this exemplary arrangement, therefore, the current would not commutate from the mechanical switch to the semiconductor branch and it would therefore not be possible for the current to be switched off.

In this context, WO 2011 057 675 A1 discloses an apparatus for switching off a direct current in a high-voltage network comprising a first disconnecting device, which comprises a plurality of series-connected semiconductor switches. A second disconnecting device, which comprises a series circuit formed by a mechanical switch and a semiconductor switch, is connected in parallel with the first disconnecting device. Furthermore, the publication by J. Håffner and B. Jacobson "Proactive Hybrid HVDC Breakers - A key innovation for reliable HVDC grids" presented at "The electric power system of the future Integration supergrids and microgrids", International Symposium in Bologna, Italy, 2011, describes a disconnecting assembly in which a first disconnecting device comprises a plurality of series-connected semiconductor switches. The second disconnecting device connected in parallel with the first disconnecting device comprises, as mechanical switch, a so-called fast disconnector, which is connected in series with an auxiliary switching device comprising a semiconductor switch. Corresponding driving of the semiconductor switch in the auxiliary switching device makes it possible to build up an electrical voltage which considerably exceeds the voltage drop in the first disconnecting device and thus allows the process of commutation to the first disconnecting device. What is disadvantageous about such an assembly is that the current in normal operation always flows through the semiconductor switch in the auxiliary switching device and losses are permanently generated, therefore, which require corresponding permanent cooling outlay. Furthermore, in the case of failure of the entire disconnecting assembly and an unaffected short-circuit current associated therewith, the auxiliary switching device can be destroyed because the short-circuit current then flowing exceeds the saturation current of the semiconductor switch in the auxiliary switching device. A further possibility is to build up a negative voltage in the semiconductor branch of the first disconnecting device for the purpose of commutation. This is possible, for example, if so-called full-bridge circuits comprising energy stores, for example capacitors, are used in the semiconductor branch. However, this requires the use of full-bridge modules each having four IGBT branches, for example.

Finally, DE 694 08 811 T2 describes a direct-current circuit-breaker for high powers for use in a direct-current high-voltage line. In this case, a semiconductor element is connected in parallel with a mechanical switch. If the mechanical switch is opened, an arc voltage is present at the contacts of the mechanical switch. If said arc voltage exceeds a predefined limit value, a triggering signal for the semiconductor element is provided by means of a control pulse generator. As a result, the semiconductor element is closed and the current is thus passed through the semiconductor element.

It is an object of the present invention to provide a disconnecting assembly of the type mentioned in the introduction which can be operated more efficiently and more reliably.

This object is achieved by means of a disconnecting assembly comprising the features of Patent Claim 1 and by means of a method comprising the features of Patent Claim 10. Advantageous developments of the present invention are specified in the dependent claims.

The disconnecting assembly according to the invention for a high-voltage direct-current network comprises a first disconnecting device, which comprises a first semiconductor switching unit, and a second disconnecting device, which is connected in parallel with the first disconnecting device and which comprises a first mechanical switching unit and an auxiliary switching device connected in series with the first mechanical switching unit, wherein the auxiliary switching device comprises a second semiconductor switching unit, and wherein the auxiliary switching device comprises a second mechanical switching unit, which is connected in parallel with the second semiconductor switching unit.

The disconnecting assembly can be used in particular for switching off direct currents in high-voltage networks. The current in the fault situation and also operating currents can be switched off with the disconnecting assembly. Hereinafter, all instances of switching-off are designated as "fault situation" for simplification, although normal instances of switching off operating currents are also meant thereby. The first semiconductor switching unit of the first disconnecting device can comprise a plurality of semiconductor switches which are electrically connected in series. The respective semiconductor switches can be embodied as IGBT (Isolated Gate Bipolar Transistor), for example. The use of half-bridge modules is likewise possible here. The first mechanical switching unit in the second disconnecting device can be formed by a plurality of mechanical switches which are connected in series. Vacuum interrupters can preferably be used for the mechanical switches. In this regard, by way of example, ten vacuum interrupters can be connected in series, wherein each of the vacuum interrupters can isolate an electrical voltage of 30 kV. As a result, by way of example, a total reverse voltage of 300 kV can be provided by means of the first mechanical switching unit. The auxiliary switching device connected in series with the first mechanical switching unit comprises a parallel circuit formed by a second semiconductor switching unit and a second mechanical switching unit. The second mechanical switching unit can also comprise a plurality of mechanical switches which are connected in series. For the realization of the second semiconductor switching unit, it is possible to use one semiconductor switch, for example, which is embodied as an IGBT, for example. In practice, however, the series connection of a few semiconductor switches is expedient for reasons of redundancy. The use of two antiseries-connected IGBTs is also conceivable in the case of a bipolar active circuit. Moreover, a half-bridge or a full-bridge with connected capacitors can be used.

With the auxiliary switching device, when switching off the direct current, it is possible to generate the required electrical voltage in order to commutate the electric current from the second disconnecting device to the first disconnecting device. In normal operation, the electric current can flow through the first and second mechanical switching units. As a result, the losses can be significantly reduced and permanent cooling of the semiconductor switches can be completely dispensed with. Furthermore, the first and second mechanical switching units can be closed in the case of failure of the entire disconnecting assembly. Consequently, destruction of the semiconductor switching elements is ruled out in the case of a fault within the disconnecting assembly.

Preferably, the disconnecting assembly comprises a control device for driving the first mechanical switching unit, the second mechanical switching unit, the first semiconductor switching unit and the second semiconductor switching unit. The control device can comprise or be connected to a detection device with which a fault current in the high-voltage direct-current network can be detected. In the fault situation, the first and second mechanical switching units and also the first and second semiconductor switching units can be driven independently of one another. In the first semiconductor switch of the first disconnecting unit, the IGBTs can be driven individually, if appropriate. By means of corresponding arresters installed in parallel with said IGBTs, a switch-off back-EMF can thus be set in a variable fashion. The path can furthermore also be used for current limiting. Reliable operation of the disconnecting assembly can thus be ensured.

In one embodiment, a forward voltage of the second semiconductor switching unit is lower than an electrical voltage present at contacts of the second mechanical switching unit upon the opening of the second mechanical switching unit. An arc voltage can form upon the opening of the second mechanical switching unit in the auxiliary switching device. If the second mechanical switching unit is embodied as a vacuum interrupter, said arc voltage can be 30 volts, for example. This voltage suffices to overcome the forward voltage or threshold voltage of the second semiconductor switching unit switched in the forward direction. In this case, the second semiconductor switching unit can comprise only one or a few IGBTs electrically connected in series. Thus, in the ideal case, no arc arises at the second mechanical switching unit since the second semiconductor switching unit is switched in the forward direction faster than the mechanical switch is opened. On account of the low forward voltage of the second semiconductor switching unit, which can be just a few volts, for example, the minimum maintaining voltage for the arc cannot be attained. Furthermore, there is no need for an additional actuating device with which the electrical voltage at the second mechanical switching unit is monitored and with which the second semiconductor switching unit is opened by a corresponding control signal. Thus, a disconnecting assembly can be operated particularly efficiently.

In a further configuration, the control device is designed to close the first and second mechanical switching units for normal operation of the high-voltage direct-current network. Therefore, the electric current flows via the first and second mechanical switching units in normal operation. As a result, only low electrical losses arise and the cooling outlay can be reduced. Consequently, the disconnecting assembly can be operated particularly energy-efficiently.

In a further configuration, the control device is designed to open the second mechanical switching unit upon the presence of a fault situation in the high-voltage direct-current network. If the intention is to interrupt the direct current in the high-voltage network on account of a fault situation or fault current, the second mechanical switching unit can be opened as rapidly as possible. Consequently, the switching-off of the electric current can be reliably started.

Preferably, the control device is designed to switch the second semiconductor switching unit in the forward direction upon the presence of the fault situation. In this case, it can also be provided that the second semiconductor switching unit of the auxiliary switching device is switched on in normal operation. The second semiconductor switching unit is switched in the forward direction at the latest when the current in the high-voltage network is intended to be switched off. If the second mechanical switching unit is opened, an electrical voltage is present between its contacts. In this case it can also happen that an arc forms between the contacts of the second mechanical switching unit. The electrical voltage between the contacts suffices to overcome the threshold voltage of the second semiconductor switching unit. As soon as the electric current flows through the second semiconductor switching unit, a possible arc at the contacts of the second mechanical switching unit is quenched. The mechanical switching unit can then immediately take up electrical voltage.

In a further configuration, the control device is designed to open the first mechanical switching unit upon the presence of the fault situation simultaneously with the second mechanical switching unit or a predetermined time duration after the second mechanical switching unit. The first mechanical switching unit can be opened simultaneously with the second mechanical switching unit. In this case, it is also conceivable for the second mechanical switching unit to be opened a predetermined time duration, for example 0.1 to a few tens of milliseconds, after the second mechanical switching unit. This makes it possible to drive the first mechanical switching unit individually and independently of the second mechanical switching unit.

Preferably, the control device is designed to switch the second semiconductor switching unit to semiconductor switching unit-state operation after the opening of the first mechanical switching unit. In this regard, by way of example, with the second semiconductor switching unit formed by one or a plurality of IGBTs, a reverse voltage, for example of 2 kV, can be provided. If the first mechanical switching unit is opened, an arc can form between the contacts of the first mechanical switching unit and an arc voltage can thus be present between the contacts. The reverse voltage provided by the second semiconductor switching unit is in series with the arc voltage at the first mechanical switching unit. The reverse voltage generated at the second semiconductor switching unit is sufficient for commutating the electric current from the second disconnecting device to the first disconnecting device. As soon as the electric current flows through the first disconnecting device, the arc at the first mechanical switching unit can also be quenched and the actual switching-off of the electric current can begin.

As an alternative thereto, the control device can be designed to open the first mechanical switching unit only after an electric current has commutated from the second disconnecting device to the first disconnecting device. If the first mechanical switching unit is only opened if the electric current has completely commutated to the first disconnecting device or the first semiconductor switching unit, the first mechanical switching unit can be opened ideally in a manner free of current and no arc arises at the contacts of the first mechanical switching unit. Wear of the contacts of the first mechanical switching unit can be effectively prevented as a result.

The method according to the invention for operating a disconnecting assembly for a high-voltage direct-current network comprises providing a first disconnecting device, which comprises a first semiconductor switching unit, and providing a second disconnecting device, which is connected in parallel with the first disconnecting device and which comprises a first mechanical switching unit and an auxiliary switching device connected in series with the first mechanical switching unit, wherein the auxiliary switching device comprises at least one second semiconductor switching unit, and providing a second mechanical switching unit in the auxiliary switching device, which is connected in parallel with the second semiconductor switching unit.

The advantages and developments described above in association with the disconnecting assembly according to the invention can be applied in the same way to the method according to the invention.

The present invention will now be explained in greater detail with reference to the accompanying drawings, in which the single figure shows a schematic illustration of a disconnecting assembly for a high-voltage direct-current network.

The exemplary embodiment outlined in greater detail below constitutes one preferred embodiment of the present invention.

The figure shows a disconnecting assembly designated in its entirety by 10. The disconnecting assembly 10 can be used for a high-voltage network. The high-voltage network can have a rated voltage of 300 kV, for example. The disconnecting device 10 is connected to a line 26 of the high-voltage direct-current network. The disconnecting assembly 10 comprises a first disconnecting device 12 and a second disconnecting device 14, which are electrically interconnected in parallel. The first disconnecting device 12 comprises a first semiconductor switching unit 16. The first semiconductor switching unit 16 comprises a plurality of semiconductor switches or semiconductor components connected in series. The semiconductor switches can each be embodied as an IGBT. The second disconnecting device 14 comprises a first mechanical switching unit 18, which, in the present case, is represented by the series connection of two individual switches. The first mechanical switching unit usually comprises a plurality of vacuum interrupters which are connected in series. Furthermore, the second disconnecting device 14 comprises an auxiliary switching device 20, which is electrically connected in series with the first mechanical switching unit 18.

The auxiliary switching device 20 comprises a parallel circuit formed by a second mechanical switching unit 24 and a second semiconductor switching unit 22. The second mechanical switching unit 24 can be formed by one or a plurality of vacuum interrupters. The second semiconductor switching unit 22 can be formed by one or a plurality of IGBTs. The second semiconductor switching unit 22 can also be formed by two antiseries-connected IGBTs, by a half-bridge or a full-bridge with connected energy stores in the form of capacitors.

In normal operation of the high-voltage direct-current network, the first mechanical switching unit 18 and the second mechanical switching unit 24 are closed. The second semiconductor switching unit 22 as well can generally be switched on in normal operation of the high-voltage network. At the latest at the beginning of a required switching-off, the second semiconductor switching unit 22 is immediately switched in the forward direction. If a fault situation then occurs in the high-voltage direct-current network, the current flow in the electrical line 26 is intended to be switched off or interrupted. For this purpose, firstly the second mechanical switching unit 24 is opened. If the contacts of the second mechanical switching unit 24 are opened, an arc can form at the contacts. On account of the arc, an arc voltage of 30 volts, for example, is present between the contacts of the second mechanical switching unit 24. This electrical voltage suffices to overcome the threshold voltage of the second semiconductor switching unit 22. In the ideal case, no arc forms at the contacts of the second mechanical switching unit 24 since the second semiconductor switching unit 22 is switched in the forward direction faster than the second mechanical switching unit 24 is opened. Furthermore, owing to the low forward voltage of the second semiconductor switching unit 22, which can be just a few volts, for example, the required minimum maintaining voltage for an arc cannot be attained at the second mechanical switching unit 24.

At the same point in time or a predetermined time duration of, for example, 0.1 to a maximum of a few tens of milliseconds after the opening of the second mechanical switching unit 24, the first mechanical switching unit 18 is opened as well. As soon as the electric current flows through the second semiconductor switching unit 22, the arc at the contacts of the second mechanical switching unit 24 is quenched. The second mechanical switching unit 24 can then immediately take up electrical voltage.

Afterward, the second semiconductor switching unit 22 is driven in such a way that a high reverse voltage is generated at said unit. In this regard, by way of example, a reverse voltage of 2 kV can be provided with an IGBT. After the opening of the first mechanical switching unit 18, an arc usually forms at the contacts of the first mechanical switching unit 18 and an arc voltage is present between the contacts. The reverse voltage generated by the second semiconductor switching unit 22 is in series with said arc voltage. It suffices to commutate the electric current form the second disconnecting device 14 to the first disconnecting device 12. As soon as the electric current has commutated to the first disconnecting device 12 or the first semiconductor switching unit 16, the arc at the contacts of the first mechanical switching unit 18 is quenched and the actual switching-off of the electric current can begin.

In an alternative embodiment, the first mechanical switching unit 18 is opened only when the electric current has completely commutated from the second disconnecting device 14 to the first disconnecting device 12. In this regard, the first mechanical switching unit 18 is ideally opened in a manner free of current and no arc forms at the contacts of the first mechanical switching unit 18. Wear of the contacts of the first mechanical switching unit 18 on account of an arc can thus be prevented.

In order to protect the second semiconductor switching unit 22 against overvoltages and overcurrents, the second mechanical switching unit 24 of the auxiliary switching device 20 is closed again after the conclusion of the switching-off process. As a result, even in the event of failure to switch off the electric current in the first semiconductor switching unit 16 and/or the first mechanical switching unit 18, the second semiconductor switching unit 22 is protected against overvoltages and overcurrents. For protection against overvoltages or for protection of the semiconductors, arresters should be provided in parallel with each IGBT. In other words, the semiconductor switching units 16 and 22 can comprise an IGBT and an arrester connected in parallel with the IGBT. Voltage limiting methods that are customary in electrical engineering, such as e.g. the connection of voltage limiting components in parallel with the semiconductor switching unit 22, are also suitable for protecting the semiconductor switching unit 22 against overvoltages. Suitable voltage limiting components are, for example, avalanche diodes, zener diodes, or metal oxide varistors and so-called snubber circuits comprising capacitors, resistors and, if appropriate, diodes.

The disconnecting assembly 10 has the advantage that the electric current in the normal state flows exclusively via the first mechanical switching unit 18 and the second mechanical switching unit 24. Therefore, low losses occur and the cooling outlay can be kept low. Furthermore, in the case of failure of the disconnecting assembly 10, the first mechanical switching unit 18 and the second mechanical switching unit 24 can be closed again, as a result of which destruction of the second semiconductor switching unit 22 is ruled out in the case of a high short-circuit current. The fault rectification is then performed by an additional switch in the context of so-called backup protection.

Claims (10)

1. A high voltage DC network separation device (10) having - a first separation device (12) comprising a first semiconductor coupling unit (16), and - a second separation device (14) coupled in parallel with the first separation device (12), and comprising a first mechanical coupling unit (18) and an auxiliary coupling device (20) connected in series with the first mechanical coupling unit (18), wherein - the auxiliary device (20) comprises a second semiconductor coupling unit (22), characterized in that - the auxiliary coupling device (20 ) comprises another mechanical coupling unit (24) coupled in parallel with the second semiconductor coupling device (22). Separation device (10) according to claim 1, characterized in that the separating device (10) comprises a control device for controlling the first mechanical coupling unit (18), the second mechanical coupling unit (24), the first semiconductor coupling unit (16) and the second semiconductor coupling unit. (22). Separation device (10) according to claim 1 or 2, characterized in that a throughput voltage of the second semiconductor coupling unit (22) is less than an electrical voltage applied to the contacts of the second mechanical coupling unit (24) when the second mechanical coupling unit contacts (24) opens. Separation device (10) according to claim 2 or 3, characterized in that the control device is designed to close the first and second mechanical coupling unit (18, 24) for normal operation of the high voltage DC network. Separation device (10) according to one of claims 2 to 4, characterized in that the control device is designed to open the second mechanical coupling unit (24) in case of failure of the high voltage DC network. Separation device (10) according to one of claims 2 to 5, characterized in that the control device is designed to connect the second semiconductor unit (22) in the through direction in the event of failure. Separation device (10) according to one of claims 2 to 6, characterized in that the control device is designed to open the first mechanical coupling unit (18) in the event of failure simultaneously with the second mechanical coupling unit (24) or a predetermined period of time thereafter. second mechanical coupling unit (24). Separation device (10) according to one of claims 2 to 7, characterized in that the control device is designed to couple the second semiconductor coupling unit (22) to locking operation after opening the first mechanical coupling unit (18). Separation device (10) according to one of claims 2 to 8, characterized in that the control device is designed to first open the first mechanical coupling unit (18) after an electric current is commuted from the second separation device (14) to the first separation device (12). A method of operating a high voltage DC network separation device (10) by - providing a first separation device (12) comprising a first semiconductor coupling unit (16), and - providing a second separation device (14) coupled to it. first separation device (12), comprising a first mechanical coupling unit (18) and an auxiliary coupling device (20) connected in series with the first mechanical coupling unit (18), wherein the auxiliary device (20) comprises a second semiconductor coupling unit (22), characterized by - providing a second mechanical coupling unit (24) in the auxiliary coupling device (20) which is connected in parallel with the second semiconductor coupling device (22).