Protection for an HVDC network

A method of protecting a high-voltage network comprising the steps for maintaining first controlled switches closed and second controlled switches open; measuring voltage and current on high-voltage interfaces; communicating the direction of the current to the other end of a high-voltage line; for each node: identifying a fault; verifying that the current is lower than the current interruption capability of the high-voltage interface switch and opening this switch.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is the United States National Phase of PCT Patent Application No. PCT/FR2017/052407 filed on 11 Sep. 2017, which claims priority to French Patent Application No. 1658595 filed 14 Sep. 2016, both of which are incorporated herein by reference.

The invention relates to high-voltage direct current, generally denoted by the acronym HVDC, transmission and/or distribution networks. The invention relates, in particular, to the selectivity and the continuity of service for an HVDC network when a fault occurs.

HVDC networks are notably envisaged as a solution to the interconnection of disparate or non-synchronous electricity production sites which are springing up with the development of renewable energies. HVDC networks, rather than AC technologies, are notably envisaged for the transmission and the distribution of energy produced by offshore wind farms, owing to the lower line losses and the absence of incidence of stray capacitances of the network over long distances. Such networks typically have voltage levels of the order of 50 kV or more.

For the point-to-point transmission of electricity, a sectioning may be carried out by means of an end-of-line converter, equipped with a circuit breaker on the AC side. On the other hand, the sectioning can no longer be implemented by such a converter in multipoint or multi-node transmission. The interruption of the DC current in such networks is a crucial issue directly conditioning the feasibility and the development of such networks. Indeed, the occurrence of a short-circuit at a node propagates very rapidly throughout the whole network. In the absence of a fast enough current interruption within the node, the short-circuit current continues to increase and may reach several tens of kA in a few ms. The short-circuit current may then exceed the current interruption capability of the DC circuit breakers of the various nodes. The short-circuit current could also damage the power electronics used in the AC/DC converters within the nodes of the network.

The known strategies for protection of such networks are based on the use of ultra-fast DC circuit breakers. Such circuit breakers prove to be both extremely costly and technologically complicated. Furthermore, such strategies are based on ultra-fast identification algorithms and relays not yet available on the market.

The invention aims to overcome one or more of these drawbacks. The invention is notably aimed at optimizing the selectivity and the continuity of service of the high-voltage network in the case of a fault, using electrical equipment at a reasonable cost. The invention thus relates to a method of protecting a high-voltage DC electrical network, such as defined in the appended claim1.

The following various features may also be combined with the features of the dependent claims, where each of these features may be combined with the features of claim1without establishing an intermediate generalization.

The invention also relates to a high-voltage DC electrical network, such as defined in the appended claims.

FIG. 1is a simplified schematic representation of one example of a high-voltage DC network1comprising interconnection nodes10,20and30. The simplified network1illustrated here comprises high-voltage lines120,130and230. The network1here is illustrated in a simplified manner in a unipolar configuration. The line120is for connecting the interconnection nodes10and20, the line130is for connecting the interconnection nodes10and30, and the line230is for connecting the interconnection nodes20and30. Each interconnection node comprises an interface for connection to high-voltage lines, and an interface for connection to a local network. Converters16,26and36of the multi-level modular type or MMC (for Modular Multi-Level Converter) are connected to the respective local network connection interface of the interconnection nodes10,20and30. The converters16,26and36are of the half-bridge type. The converters16,26and36are associated with local AC networks or equipment (for example electrical generators such as wind farms, tidal power stations, nuclear power stations, thermal power stations or else photovoltaic generators, or local transport or consumer networks). The converters16,26and36control, in a manner known per se, the flow of power between their AC interface and their DC interface.

The MMC converter16is connected to the local network interface of the interconnection node10via a protection circuit. This protection circuit comprises a switch11connected to the local network interface of the interconnection node10. The protection circuit furthermore comprises a split circuit connected in series with the switch11, between a DC input of the converter16and the local network interface of the interconnection node10. The split circuit comprises first and second branches connected in parallel. The first branch comprises a switch12connected in series with a current limiter13. The second branch comprises a switch14connected in series with a current limiter15.

The switch11here is a circuit breaker of the mechanical type. The switch11is notably selected so as to provide a current interruption capability between the interconnection node10and the converter16. The current limiter13and the current limiter15here are of the superconducting short-circuit current limiter or SCFCL type. The switches12and14here are fast-switching controlled isolators. The high-voltage line120is connected to the interconnection node10via a switch112. The switch112here is a circuit breaker of the mechanical type. Although not shown, a fast-switching controlled isolator may be connected in series with the switch112between the high-voltage line120and the interconnection node10. The high-voltage line130is connected to the interconnection node10via a switch113. The switch113here is a circuit breaker of the mechanical type. Although not shown, a fast-switching controlled isolator may be connected in series with the switch113between the high-voltage line130and the interconnection node10.

The MMC converter26is connected to the local network interface of the interconnection node20via a protection circuit. This protection circuit comprises a switch21connected to the local network interface of the interconnection node20. The protection circuit furthermore comprises a split circuit connected in series with the switch21, between a DC input of the converter26and the local network interface of the interconnection node20. The split circuit comprises first and second branches connected in parallel. The first branch comprises a switch22connected in series with a current limiter23. The second branch comprises a switch24connected in series with a current limiter25.

The switch21here is a circuit breaker of the mechanical type. The switch21is notably selected so as to provide a current interruption capability between the interconnection node20and the converter26. The current limiter23and the current limiter25here are of the superconducting short-circuit current limiter or SCFCL type. The switches22and24here are fast-switching controlled isolators. The high-voltage line120is connected to the interconnection node20via a switch212. The switch212here is a circuit breaker of the mechanical type. Although not shown, a fast-switching controlled isolator may be connected in series with the switch212between the high-voltage line120and the interconnection node20. The high-voltage line230is connected to the interconnection node20via a switch223. The switch223here is a circuit breaker of the mechanical type. Although not shown, a fast-switching controlled isolator may be connected in series with the switch223between the high-voltage line230and the interconnection node20.

The MMC converter36is connected to the local network interface of the interconnection node30via a protection circuit. This protection circuit comprises a switch31, connected to the local network interface of the interconnection node30. The protection circuit furthermore comprises a split circuit connected in series with the switch31, between a DC input of the converter36and the local network interface of the interconnection node30. The split circuit comprises first and second branches connected in parallel. The first branch comprises a switch32connected in series with a current limiter33. The second branch comprises a switch34connected in series with a current limiter35.

The switch31here is a circuit breaker of the mechanical type. The switch31is notably selected so as to provide a current interruption capability between the interconnection node30and the converter36. The current limiter33and the current limiter35here are of the superconducting short-circuit current limiter or SCFCL type. The switches32and34here are fast-switching controlled isolators. The high-voltage line130is connected to the interconnection node30via a switch323. The switch323here is a circuit breaker of the mechanical type. Although not shown, a fast-switching controlled isolator may be connected in series with the switch323between the high-voltage line230and the interconnection node30. The high-voltage line130is connected to the interconnection node30via a switch313. The switch313here is a circuit breaker of the mechanical type. Although not shown, a fast-switching controlled isolator may be connected in series with the switch313between the high-voltage line130and the interconnection node30.

With current limiters13,15,23,25,33and35of the superconducting type, the latter have a potential difference of zero between their terminals when they are in the superconducting state, which therefore allows the losses induced in each branch, under normal operation of the network1, to be limited.

The controlled switches11,112,113,21,212,223,31,313and323are advantageously mechanical circuit breakers, notably owing to the low on-line losses that they are capable of generating.

The current limiters15,25and35are dimensioned so as to maintain the short-circuit current flowing through them at a level lower than the current interruption capability of the switches112and113,212and223,313and323, respectively. The current limiters15,25and35thus guarantee the effective opening of the switches112and113,212and223,313and323, respectively, in the case of occurrence of a short-circuit.

Similarly, the current limiters13,23and33are dimensioned so as to maintain the short-circuit current flowing through them at a level lower than the current interruption capability of the switches112and113,212and223,313and323, respectively. The current limiters13,23and33thus guarantee the effective opening of the switches112and113,212and223,313and323, respectively, in the case of occurrence of a short-circuit.

Communications networks are furthermore created between various pieces of equipment.

A communications network (illustrated with a dashed-dotted line) is created at the interconnection node10between the switches11,112and113. A communications network (illustrated with a dashed-dotted line) is created at the interconnection node20between the switches21,212and223. A communications network (illustrated with a dashed-dotted line) is created at an interconnection30between the switches31,313and323.

A communications network (illustrated with a dashed line) is created between the interconnection node10, the switch11, the switch12, the switch14, the limiters13and15, and the converter16. A communications network (illustrated with a dashed line) is created between the interconnection node20, the switch21, the switch22, the switch24, the limiters23and25, and the converter26. A communications network (illustrated with a dashed line) is created between the interconnection node30, the switch31, the switch32, the switch34, the limiters33and35, and the converter36.

A communications network is created between the switches112and212. A communications network is created between the switches223and323. A communications network is created between the switches113and313.

In an initial configuration without any faults:a local control circuit19maintains the switches11,14,112and113closed, and maintains the switch12open;a local control circuit29maintains the switches21,24,212and223closed, and maintains the switch22open;a local control circuit39maintains the switches31,34,313and323closed, and maintains the switch32open.

The operation of the protection of the network1will now be detailed in a situation where a short-circuit to earth occurs on the line230(or a short-circuit between core and screen of a cable for example) dose to the switch323. The short-circuit current propagates throughout the network. The protection will aim to implement the following steps:identify the faulty high-voltage line;isolate the fault;re-establish the voltage level on the network;re-establish the flow of power.

The identification of the faulty line may be carried out as follows:a fault is detected in a non-synchronous manner within each interconnection node10,20and30. The detection of the fault is carried out in a manner known per se within each interconnection node by local voltage and current measurements;each converter16,26,36activates its internal protection. Since an MMC converter is not designed to withstand high short-circuit currents (an MMC converter is generally designed for a maximum current of 4 kA), the internal protection of each MMC converter16,26and36is activated as soon as the current passing through it exceeds a threshold. Each activated MMC converter16,26or36then no longer provides a voltage and power control;for each activated MMC converter16,26or36(with internal protection), the respective current limiter15,25or35then has a fault current flowing through it. This current limiter is then activated. The closer an MMC converter is to the location of the short-circuit, the faster the current flowing through it will increase. The respective current limiters15,25and35will thus be activated in a non-synchronous manner, as can be seen from the diagram inFIG. 2. The time t=0 corresponds to the occurrence of the fault short-circuit. The short-circuit current supplied by each MMC converter here falls below 2 kA once the corresponding current limiter has been activated. Owing to the current limitation, for each MMC converter16,26or36, a certain time is available for the identification of the faulty high-voltage line;measurements of voltage and current in these switches223and323and the use of the communications network between the switches223and323allows a short-circuit on the line230to be identified, and its proximity to the switch323to be identified. This identification may be carried out in a manner known per se from the document “Protection system for meshed HVDC network using superconducting fault current limiters” notably published by Justine DESCLOUX and Camille GANDIOLI, within a time of less than 10 ms.FIG. 5is a diagram of the currents through the switches223(dotted line) and323(continuous line) when the short-circuit occurs. It may be observed that these currents have relative amplitudes allowing both a short-circuit on the line230and the proximity of this short-circuit to the switch323to be identified for example, a control circuit of the node30receives the measurements coming from the switch223and from the switch323(at least the direction of the current flowing through these switches, advantageously the voltage and the current measured at these switches), so as to deduce from this that the line230is faulty and that this fault is close to the switch323. Alternatively, an algorithm for local detection of a fault in the high-voltage line may be used, in order for a node connected to this high-voltage line to supply a command to open the controlled switch of the other node connected to this high-voltage line. Independently, a control circuit of the interconnection node20receives the measurements coming from the switch223and from the switch323, or the command for the opening of the switch223, so as to deduce from this that the line230is faulty and that this fault is close to the switch323;

These steps for identification of the faulty line are carried out within the 10 ms following the occurrence of the short-circuit on the line230, owing to a simple communication chain and to an identification capacity based on a reduced number of measurements.

Owing to the presence of the current limiters15,25and35interposed between the converters16,26and36and the high-voltage lines120,130and230, a current limitation is obtained for a time equal to at least 10 ms in order to allow the aforementioned steps for identification of the faulty line to be implemented.

Shortly after the appearance of the short-circuit, the respective voltages on the local network interfaces of the interconnection nodes10(continuous line),20(dotted line) and30(dashed line) fall abruptly to become close to zero after 10 ms, as illustrated inFIG. 4.

After the step for identification of the faulty high-voltage line, the step for isolation of the fault may be implemented as follows:a verification of the current interruption capabilities of the switches112,113,212,223,313and323. In the example illustrated, the fault may begin to be isolated starting from t=12 ms after the appearance of the fault, if the switches (or mechanical circuit breakers)112,113,212,223,313and323have a current interruption capability of 8 kA. Alternatively, the current flowing through each switch112,113,212,223,313and323may also be measured and it may also be determined from which point this current measured for a switch is lower than the current interruption capability. The current interruption capability of the switches of the high-voltage lines of the network1is dimensioned in a manner known per se as a function of the size of the network1and of the number of stations that are connected to it. Knowing the number of MMC converters connected to the network, the maximum possible fault current in the case of a fault may be determined, since it is at the most equal to the sum of the limitation currents of the current limiters connected in a branch in series with a closed switch. For example, if the current limiters13,23,33,15,25and35each have a limitation current equal to twice the nominal current ln of a current limiter, with a number N of MMC converters, the maximum fault current ldm is defined by ldm=N*2*ln. More generally speaking, each of the switches112,113,212,223,313and323will have a current interruption capability PdC equal to at least Σi=1NCli, with Clithe limitation current of a current limiter of the protection circuit of the MMC converter of index i.the fault current is then eliminated by controlling the opening of the switches223and323. Between the sending of the commands for opening the switches223and323and their effective opening (taking for example electromechanical delays associated with the interruption of the fault current into account), a delay of 17 ms is observed here. The opening of the switches223and323therefore takes place here at t=29 ms;the voltage on the network1is then restored. At this stage, the internal protection of the MMC converters16,26and36is activated. For MMC converters using three-phase rectifiers, these MMC converters can only recover their voltage and power control if the voltage on their DC input exceeds around 0.7 times the nominal voltage. The currents through the current limiters15,25and35fall progressively. At t=37 ms, the respective voltages on the local network interfaces of the interconnection nodes10,20and30reach 0.7 times the nominal voltage;the MMC converters16,26and36are respectively informed by the interconnection nodes10,20and30that the respective voltages on their local network interfaces are reaching 0.7 times the nominal voltage. The MMC converters16,26and36then recover their voltage control, so as to restore the voltage on the high-voltage lines120and130to the nominal value. The output voltage of the MMC converters16,26and36also progressively recovers to the nominal level.

The current limiters15,25and35are then again activated into the resistive state and traversed by nominal currents. These current limiters15,25and35cannot return to the superconducting state without interrupting their conduction. By measuring the current in each MMC converter16,26and36, and by measuring the voltage on the local network interface of the interconnection nodes10,20and30, it may be determined within each protection circuit that the short-circuit fault has been isolated. At this point, the switches12,22and32are closed. The currents from the MMC converters16,26and36then respectively flow through the current limiters13,23and33which are in the superconducting state. The currents from the MMC converters16,26and36then no longer flow through the current limiters15,25and35.

When the switches12,22and32are effectively closed, the output voltage of the MMC converters16,26and36is equal to the voltage on the respective local network interface of the interconnection nodes10,20and30. The power control of the MMC converters16,26and36may then be recovered. The flow of power through the high-voltage lines120and130may then also be restored.

After a safety time delay, the switches14,24and34may be opened, so that the current limiters15,25and35can progressively return to their superconducting state for a later use.

It is observed that the network1is now again functional with isolation of the fault after a period of 50 ms.

The use of fast switches in series in a branch with each of the current limiters notably allows a nominal current to be re-established for the MMC converters16,26and36in a reduced time.

Advantageously, the current limiters13and15,23and25, or33and35may use the same cooling tank, with a view to limiting their cost.