CN117117795A

CN117117795A - Fault protection method and device for HVDC converter station

Info

Publication number: CN117117795A
Application number: CN202210529966.9A
Authority: CN
Inventors: 马茨·安德森
Original assignee: Hitachi Energy Switzerland AG
Current assignee: Hitachi Energy Co ltd
Priority date: 2022-05-16
Filing date: 2022-05-16
Publication date: 2023-11-24

Abstract

A fault protection method, system and apparatus for a high voltage direct current, HVDC, converter station are provided. The fault protection method for the high-voltage direct current (HVDC) converter station comprises the following steps: acquiring the current sum of each arm of the STATCOM; determining a fault from the sum of currents; and causing a protection action to be performed in the HVDC converter station based at least in part on the determined fault.

Description

Fault protection method and device for HVDC converter station

Technical Field

The present disclosure relates to a method and apparatus for fault protection of a high voltage direct current, HVDC, converter station.

Background

For HVDC converter stations, there may be significant transient cell voltages upon failure. Transient overvoltages may require expensive alternative methods and complex converter station configurations if not relieved in any other way. Thus, there is a need for a method, system and apparatus for fault protection of HVDC converter stations.

Disclosure of Invention

It is therefore an object of the present disclosure to provide a method, system and apparatus for fault protection of a high voltage direct current HVDC converter station.

According to one aspect of the present disclosure there is provided a fault protection method for a high voltage direct current, HVDC, converter station comprising a three-phase connected static synchronous compensator, STATCOM, and a three-phase connected converter, LCC, each limb of the three-phase connected STATCOM being connected in parallel with a respective limb of the three-phase connected LCC, the method comprising: acquiring the current sum of each arm of the STATCOM; determining a fault from the sum of currents; and causing a protection action to be performed in the HVDC converter station based at least in part on the determined fault.

According to another aspect of the present disclosure there is provided a fault protection device for a high voltage direct current, HVDC, converter station comprising a three-phase connected static synchronous compensator, STATCOM, and a three-phase connected commutation converter, LCC, each limb of the three-phase connected STATCOM being connected in parallel with a respective limb of the three-phase connected LCC, the device comprising: acquisition means for acquiring a sum of currents of the respective arms of the STATCOM; determining means for determining a fault from the sum of currents; and protection means for causing a protection action to be performed in the HVDC converter station based at least in part on the determined fault.

According to another aspect of the present disclosure, a fault protection device for a high voltage direct current, HVDC, converter station is provided comprising one or more processors configured to perform the steps of a fault protection method for a high voltage direct current, HVDC, converter station according to embodiments of the present disclosure.

According to another aspect of the present disclosure, there is provided a high voltage direct current, HVDC, converter station comprising: a static synchronous compensator STATCOM with three phases connected; a three-phase connected inverter LCC, each arm of the three-phase connected STATCOM being connected in parallel with a respective arm of the three-phase connected LCC; and one or more processors configured to perform the steps of the fault protection method for a high voltage direct current, HVDC, converter station according to embodiments of the present disclosure.

According to another aspect of the present disclosure, there is provided a computer readable medium having instructions stored thereon, which when executed by one or more processors, cause the one or more processors to perform the steps of a fault protection method for a high voltage direct current, HVDC, converter station according to embodiments of the present disclosure.

According to a further aspect of the present disclosure, there is provided a computer program product comprising a computer program which, when executed by one or more processors, causes the one or more processors to perform the steps of a fault protection method for a high voltage direct current, HVDC, converter station in accordance with embodiments of the present disclosure.

Drawings

Embodiments of the present disclosure are illustrated by way of example, and not by way of limitation, in the figures of the accompanying drawings in which like reference numerals refer to similar elements.

Fig. 1 exemplarily shows a topology of an HVDC converter station according to an embodiment of the present disclosure.

Fig. 2 schematically illustrates a fault protection method for a high voltage direct current HVDC converter station according to an embodiment of the present disclosure.

Fig. 3 illustrates an exemplary three-dimensional overview of a converter according to an embodiment of the disclosure.

Fig. 4 illustrates an exemplary ground fault scenario in accordance with an embodiment of the present disclosure.

Fig. 5A-5D illustrate exemplary simulation results for an inverter according to an embodiment of the present disclosure.

Fig. 6 illustrates an exemplary two-dimensional overview of a converter according to some embodiments of the present disclosure.

7A-7C illustrate exemplary simulation results for comparison of different protection actions according to embodiments of the present disclosure.

Fig. 8 schematically illustrates a fault protection device for a high voltage direct current HVDC converter station according to an embodiment of the present disclosure.

Detailed Description

In HVDC converter stations, a line-commutated converter (LCC) converter is often used. LCCs are used to convert electrical energy from alternating current to direct current or vice versa. The term commutation means that the conversion process depends on a stable line voltage, the ac system to which the converter is connected having a distinct zero crossing point for flow commutation from one switching element to the other.

Fig. 1 shows a high-level overview of a topology 100 for an HVDC converter station comprising a STATCOM and an LCC, herein abbreviated as "lcc+statcom topology". A static synchronous compensator (static synchronous compensator, STATCOM) is a fast acting device capable of providing or absorbing reactive current and thereby regulating the voltage at the grid connection point. In the provided lcc+statcom topology, for each LCC converter a corresponding STATCOM converter is connected in parallel.

Such a topology may be for HVDC or UHVDC converter stations.

Referring to FIG. 1, it is to be understood that the transformer connection of the LCC valve may be any of a variety of ways, including but not limited to the standard Y/Y and Y/D connections illustrated.

It will also be appreciated that the converter may be constituted by a common 12-pulse group, but is not limited thereto, and may be any kind of converter, including any type of rectifier and inverter known to those skilled in the art to be useful in HVDC converter stations.

In this connection, a method 200 for fault protection of a high voltage direct current, HVDC, converter station is presented. The HVDC converter station comprises a three-phase connected static synchronous compensator, STATCOM, and a three-phase connected commutation converter, LCC, each limb of the three-phase connected STATCOM being connected in parallel with a respective limb of the three-phase connected LCC.

Referring to fig. 2, at step 210, the method includes obtaining a sum of currents of respective arms of the STATCOM.

At step 220, the method includes determining a fault from the sum of the currents.

At step 230, the method includes causing a protection action to be performed in the HVDC converter station based at least in part on the determined fault.

According to the method 200, fast fault detection and effective protection may be achieved for the lcc+statcom topology disclosed herein.

Determining faults based on the sum of currents of each arm of the STATCOM may take into account the sum of three-phase currents into and out of the STATCOM. It is to be appreciated that the current may be monitored or determined by any means known to those skilled in the art, and the present disclosure is not limited thereto.

It is also understood that the protection action may be any protection action known to those skilled in the art, including, but not limited to, performing a delay, tripping the AC circuit breaker, bypassing the fault branch (e.g., by closing the corresponding bypass switch), latching (e.g., latching after a time delay) the valve, and the like.

According to an embodiment of the present disclosure, determining a fault from the sum of currents may include: in response to determining that the sum of currents is above a first threshold, the fault is determined to be a ground fault.

When a ground fault occurs, there will be a significant current imbalance at the time of the ground fault due to the additional current in one of the three phases. By comparing the sum of the currents with a threshold value, i.e. a first threshold value, it is easy to detect when the threshold value is exceeded and thus a ground fault occurs.

In some embodiments of the present disclosure, the first threshold may be substantially zero. This is because in steady state the sum of the three arms of the STATCOM current is almost zero. In other embodiments, the first threshold may be a value slightly greater than zero. Therefore, the ground fault can be recognized efficiently and quickly.

It will be appreciated that the first threshold value may be set to other values depending on the conditions or parameters of the converter to be protected, on experience, simulation or experimentation, etc. of a person skilled in the art. For example, the first threshold value may be set to a value significantly larger than zero to avoid false tripping or the like caused by, for example, an inrush current, and the present disclosure is not limited thereto.

It should also be appreciated that monitoring of the current may be performed by any means known to those skilled in the art, and the present disclosure is not limited thereto.

Fig. 3 further illustrates an exemplary three-dimensional overview of a combined LCC and STATCOM converter in accordance with an embodiment of the present disclosure.

In an exemplary embodiment, the STATCOM shown may be a Y-connected STATCOM, with the neutral point not grounded. Each LCC valve may be connected in parallel with a corresponding STATCOM. This arrangement of STATCOM can reduce cost, loss and simplify the overall valve hall layout.

However, it has been found that a ground fault at the midpoint of the Y-connection of the STATCOM may result in a very high cell voltage. If not relieved in any other way, costly solutions may need to be employed in STATCOM, such as higher cell capacitance, more series cells and high energy rated arrester stacks, etc. Thus, according to one aspect of the present disclosure, it is an object to quickly detect faults at a neutral bus.

In some embodiments where the STATCOM is a wye-connected STATCOM, the sum of currents may be obtained by monitoring the current of each phase of the wye-connected STATCOM. In some cases this may be done by monitoring the current on a common neutral bus, where all three phases of STATCOM are combined on the bus.

In the case of Y-connected STATCOM, the neutral bus may be a very critical point of failure. For a normal Y-connected STATCOM, the voltage potential to ground at neutral bus 310 will be relatively small. However, for the topologies referred to herein, the dc voltage offset to ground will become progressively higher due to the parallel connection with the LCC converter. This also means that if a ground fault occurs at this time, a high dc voltage will drive a large fault current. This is illustrated in fig. 4, where the 6 pulse train operates as an inverter.

Referring to fig. 4, an exemplary ground fault scenario is illustrated. As shown in fig. 4, the neutral bus failed 410,6 pulse train operates as an inverter and valve V2 is conducting current. It should be understood that this is merely exemplary and that aspects of the present disclosure are not limited thereto.

During operation, one of the three upper valves V2, V6 or V4 will remain locked in the on state, because the voltage at its anode is higher than the voltage at its cathode, and thus current will be injected into the ground fault. In this example, valve 2 is conducting current before the ground fault occurs. The fault current may be large.

Simulation results from lcc+statcom converters operating as inverters are shown in fig. 5A-5D. Specifically, fig. 5A shows the maximum cell voltage, fig. 5B shows the STATCOM arm sum current, fig. 5C shows the DC (direct current) line voltage, and fig. 5D shows the DC current into and out of the 12 pulse group.

As can be seen from fig. 5A-D, in steady state the sum of STATCOM arm currents IA, IB and IC is almost zero. However, when a ground fault occurs, a significant current imbalance occurs at the time of the ground fault due to the additional current in one of the three phases (in this case, the IC). The cell voltage will increase rapidly due to the additional current of the conducting phase.

In some cases, a compromise must be made between fast protection and stable system operation without undesirable nuisance tripping. In some cases, one phase current measurement may be lost in the active control and protection system, while the same measurement is still working properly in the standby control and protection system. Some further considerations may be taken into account in this regard.

As shown in fig. 5C, there is a significant DC undervoltage when a fault occurs. In some optional embodiments of the present disclosure, the method 200 may further include determining that the DC under-voltage is below a second threshold. In this case, causing the protection action to be performed may include causing the protection action to be performed in response to the determined fault and in response to determining that the DC under voltage is below a second threshold.

This may be done by monitoring the line voltage on the DC side of the HVDC converter and comparing the monitored line voltage with a nominal value. It should be appreciated that the monitoring, comparison, or other determination of the DC undervoltage may be performed by any means known to those skilled in the art, and the present disclosure is not limited thereto.

By further taking into account the DC undervoltage, the risk of false tripping can be reduced. It is understood that the DC undervoltage may be monitored by any means known to those skilled in the art, and the present disclosure is not limited thereto. In some cases, the second threshold may be set to a value slightly below 100% of the rated voltage. The second threshold value may also be set to other values according to the conditions or parameters of the current transformer to be protected, according to experience, simulation or experiment, etc. of those skilled in the art. As further non-limiting examples, the second threshold may be set to 95%, 90%, 80% of the rated voltage, or the like. It is readily understood that the second threshold may be set to a value lower than the rated voltage to ensure reliability of fault detection and reduce undesired false protection, and that the threshold may also be adjusted according to actual needs, and that the present disclosure is not limited thereto.

As shown in fig. 5D, there is a significant difference in the dc current flowing into and out of the 12 pulse group when a fault occurs. In some other additional or alternative embodiments of the present disclosure, method 200 may further include determining that a difference in current into and out of the LCC is above a third threshold, wherein causing the protection action to be performed may include causing the protection action to be performed in response to the determined fault and in response to the difference in current into and out of the LCC being above the third threshold.

By taking into account the current difference into and out of the pulse train, the risk of false tripping can also be reduced. It should be appreciated that this may be accomplished by monitoring the current into and out of the pulse set (e.g., 12 pulse set), and the current difference may be monitored and calculated by any means known to those skilled in the art, and the disclosure is not so limited.

As shown in fig. 5D, the current difference between the incoming and outgoing current is almost zero under normal conditions. However, when a ground fault occurs, there may be a significant difference. In some cases, the third threshold may be set to zero or a value slightly greater than zero. In other cases, the third threshold value may be set to other values according to experience, simulation or experimentation by a person skilled in the art or according to conditions or parameters of the current transformer to be protected, etc. It is readily understood that in order to ensure reliability of fault detection and reduce unwanted false protection, the third threshold may be set to a value greater than zero, and that this threshold may also be adjusted as needed in practice, and the disclosure is not limited thereto.

As described above, or as can be seen in fig. 5A, the maximum cell voltage will increase substantially during a fault. In some additional or alternative embodiments of the present disclosure, the HVDC converter station may further comprise a respective lightning arrester placed on each phase of the STATCOM. The presence of the lightning arrester may protect the unit from overvoltage during a fault.

Detailed information referring to fig. 6, a two-dimensional overview of the lcc+statcom topology is shown. In fig. 6, a lightning arrester 610 is shown. In such an embodiment, transient overvoltage may be reduced by placing a lightning arrester on the STATCOM valve.

Referring back to fig. 2, at step 230, a protection action is performed based on the determined fault. As described above, the protection action may be any protection action known to those skilled in the art, and the present disclosure is not limited thereto.

Some exemplary protection actions or sequences thereof will now be described in particular.

For some HVDC and UHVDC applications, series converter protection and action may be used. The main difference compared to a single 12-pulse group may be that the same pole has another 12-pulse group available and thus there is a corresponding bypass switch (BPS) that can be closed.

For example, for ground faults in converters, the following protection may be employed: valve short circuit protection (Valve Short Circuit Protection, VSCP) or inverter differential protection (Converter Differential Protection, CDP). For example, in accordance with some embodiments of the present disclosure, the CDP may act for certain specific fault conditions based on the difference in DC current flowing into and out of the 12 pulse groups. The protection action of the CDP on the UHVDC may be referred to as S-block. The action of this type of latching sequence may be as follows.

As a rectifier, the lockout sequence may include:

-immediate deferral;

-tripping an ac circuit breaker;

-closing a bypass switch (BPS) if the pole is still running with another 12 pulse group; and

locking after 30 milliseconds.

As an inverter, the latch sequence may include:

-immediate deferral;

-tripping an ac circuit breaker;

-closing the BPS;

-issuing a Bypass Pair (BPP) command upon receipt of an early ac breaker on indication; and

-locking once the BPS is closed.

However, for the particular lcc+statcom topology disclosed herein, some undesirable and therefore non-optimal results may occur by using S-lock. For rectifiers, the inverter may latch up before the BPS is closed, while for inverters it may take a long time to command the BPP. For example, for an inverter, the indication of a BPP command typically requires about 14 milliseconds after tripping, while a latch-up may require about 65-70 milliseconds.

Accordingly, the following LCC protection operation for reducing the STATCOM cell voltage is further proposed.

According to an alternative embodiment of the present disclosure, causing the protection action to be performed may include: in response to determining that the LCC is operating as a rectifier, the valve is caused to latch after a bypass switch of the valve in a failed arm of the rectifier is closed.

In such a case, for the rectifier: when the BPS has been closed, the fault valve may be closed. As discussed above, such protection may be performed when it is determined that there is another pulse group running on the same pole for the failed arm.

The protection actions described above may be part of what is known as Y-block protection. In particular, for rectifiers, the Y-latch may include the following operations:

-immediate deferral;

-tripping an ac circuit breaker;

-closing a bypass switch (BPS); and

-blocking the converter after BPS closure.

According to other optional embodiments of the disclosure, causing the protection action to be performed may include: in response to determining that the LCC is operating as an inverter, a pair of valves in a fault arm of the inverter are immediately bypassed.

In such embodiments, bypassing two valves in the faulty arm of the inverter may also be referred to as issuing a bypass pair (BPP) command, commanding a BPP, or activating a bypass pair in the bridge (two valves in the same arm of the bridge). By commanding the BPP immediately, effective protection can be achieved. The BPP is active for the inverter at least because current can be diverted from the ground fault to the normal electrode.

The mentioned BPP protection action may be part of what is called Z-block protection. Specifically, Z-lock of the inverter may include the following operations:

-immediate deferral;

-tripping an ac circuit breaker;

-closing the BPS;

-immediately commanding BPP; and

-locking once the BPS is closed.

Fig. 7A-7C show simulated comparisons between S-latch initiated from CDP and Z-latch initiated from STATCOM arm current sum protection as described above for an inverter station. Specifically, fig. 7A shows a comparison of maximum cell voltages, fig. 7B shows a comparison of sum currents of STATCOM arms, and fig. 7C illustrates a comparison of DC line voltages. Curves 710, 720 and 730 are curves using CDP, and curves 715, 725 and 735 are curves using Z-lock protection as described above.

It should be understood that the arrangements shown in fig. 1, 3, 4 and 7 are illustrative. LCCs, STATCOMs, bus bars, and/or other devices can be constructed in any suitable manner as will be appreciated by those skilled in the art and the disclosure is not limited thereto.

According to one or more embodiments of the present disclosure, the reduction of the transient cell voltage may be accomplished by three-phase current sum protection.

According to one or more embodiments of the present disclosure, transient cell voltages may be reduced for lcc+statcom topologies and costs can be saved due to the relief of the need for, for example, higher cell capacitance, more cells in series, and/or high energy arrester stacks.

The present disclosure proposes an improved fault handling to reduce cell voltage in critical fault situations. According to one or more embodiments of the present disclosure, voltage reduction may be achieved by fast detection of faults and/or taking appropriate action in the HVDC converter.

According to one or more alternative embodiments of the present disclosure, the risk of false tripping according to the proposed primary protection may be minimized by taking into account additional criteria.

Table 1 shows an exemplary simulated peak cell voltage in STATCOM, where the communication delay between STATCOM and LCC is considered to be 1ms. It is to be understood that table 1 is merely exemplary and that the present disclosure is not limited thereto.

TABLE 1

With the proposed solution, one of the advantages is that the transient voltage can be reduced and that there is no need to increase the cell capacitance, which can be costly.

Fig. 8 illustrates an apparatus 800 for fault protection of an HVDC converter station in accordance with some embodiments of the present disclosure. The HVDC converter station may comprise a three-phase connected static synchronous compensator, STATCOM, and a three-phase connected commutation converter, LCC, each arm of the three-phase connected STATCOM being connected in parallel with a respective arm of the three-phase connected LCC. The means comprise acquisition means 810, determination means 820 and protection means 830. The acquisition means 810 may be configured to acquire the sum of the currents of the individual arms of the STATCOM. The determining means 820 may be configured to determine the fault from the sum of currents. The protection device 830 may be configured to cause a protection action to be performed in the HVDC converter station based at least in part on the determined fault.

It should be appreciated that the various devices or modules of the device 800 shown in fig. 8 may correspond to the various steps in the method 200 described with reference to fig. 2. Accordingly, the operations, features and advantages described above with respect to method 200 apply to apparatus 800 and the apparatus or module therein. For brevity, certain operations, features and advantages are not described in detail herein.

According to another aspect of the present disclosure, there is provided a system for HVDC converter station fault protection comprising: a static synchronous compensator STATCOM with three phases connected; a three-phase connected inverter LCC, each arm of the three-phase connected STATCOM being connected in parallel with a respective arm of the three-phase connected LCC; and a controller comprising means for performing the steps of the fault protection method for a high voltage direct current, HVDC, converter station according to embodiments of the present disclosure.

According to another aspect of the present disclosure, a fault protection device for a high voltage direct current HVDC converter station is provided, the device comprising means for performing the steps of the fault protection method for a high voltage direct current HVDC converter station according to embodiments of the present disclosure.

In the foregoing description, various embodiments and aspects of the disclosure have been described with reference to the accompanying drawings. The foregoing description and drawings are illustrative of the present disclosure and are not to be construed as limiting the present disclosure. Numerous specific details are described to provide a thorough understanding of various embodiments of the disclosure. However, in some instances, well-known or conventional details are not described in order to facilitate a more concise discussion of embodiments of the present disclosure.

It will be understood that, although the terms first, second, third, etc. may be used herein to describe various elements, components, regions, layers and/or sections, these elements, components, regions, layers and/or sections should not be limited by these terms. These terms are only used to distinguish one element, component, region, layer or section from another. Thus, a first element, component, region, layer or section discussed above could be termed a second element, component, region, layer or section without departing from the teachings of the present disclosure.

Spatially relative terms, such as "below … …," "below … …," "below," "above … …," "above … …," and the like, may be used herein for ease of description to describe one element or feature's relationship to another element or feature as illustrated in the figures. It will be understood that spatially relative terms are intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. For example, if the device in the figures is turned over, elements described as "below" other elements or features would then be oriented "above" the other elements or features. Thus, the exemplary terms "below" and "beneath" can encompass both an orientation of above and below … …. For example, terms such as "before" or "before" and "after" or "after" may be similarly used to indicate the order in which light passes through the elements. The device may be otherwise oriented (rotated 90 degrees or in other directions) and the spatially relative descriptors used herein interpreted accordingly. Furthermore, it will be understood that when a layer is referred to as being "between" two layers, it can be the only layer between the two layers, or one or more intervening layers may also be present.

The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the disclosure. As used herein, the singular forms "a," "an," and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms "comprises" and/or "comprising," when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. As used herein, the term "and/or" includes any and all combinations of one or more of the associated listed items.

It will be understood that when an element or layer is referred to as being "on," "connected to," "coupled to," or "adjacent to" another element or layer, it can be directly on, connected to, coupled to, or adjacent to the other element or layer, or intervening elements or layers may also be present. In contrast, when an element is referred to as being "directly on," "directly connected to," "directly coupled to," or "directly adjacent to" another element or layer, there are no intervening elements or layers present. In no event, however, should be construed as requiring a layer to completely cover an underlying layer.

Embodiments of the present disclosure are described herein with reference to schematic illustrations of idealized embodiments (and intermediate structures) of the present disclosure. Thus, variations in the illustrated shapes, due to, for example, manufacturing techniques and/or tolerances, are to be expected. Thus, embodiments of the present disclosure should not be construed as limited to the particular shapes of regions illustrated herein but are to include deviations in shapes that result, for example, from manufacturing. Thus, the regions illustrated in the figures are schematic in nature and their shapes are not intended to illustrate the actual shape of a region of a device and are not intended to limit the scope of the present disclosure.

Unless defined otherwise, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this disclosure belongs. It will be further understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and/or the present specification and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein.

Claims

1. A fault protection method for a high voltage direct current, HVDC, converter station comprising a three-phase connected static synchronous compensator, STATCOM, and a three-phase connected commutation converter, LCC, each limb of the three-phase connected STATCOM being connected in parallel with a respective limb of the three-phase connected LCC, the method comprising:

acquiring the current sum of each arm of the STATCOM;

determining a fault from the sum of currents; and

based at least in part on the determined fault, causing a protection action to be performed in the HVDC converter station.

2. The method of claim 1, wherein determining a fault from the current sum comprises:

in response to determining that the sum of currents is above a first threshold, the fault is determined to be a ground fault.

3. The method of claim 2, wherein the first threshold is substantially zero.

4. A method according to any of claims 1-3, wherein the STATCOM is a Y-connected STATCOM, and wherein the current sum is obtained by monitoring the current of each phase of the Y-connected STATCOM.

5. A method according to any of claims 1-3, further comprising determining that a DC undervoltage is below a second threshold, wherein causing the protection action to be performed comprises: responsive to the determined fault and responsive to determining that the DC undervoltage is below the second threshold, causing the protective action to be performed.

6. The method of any of claims 1-3, further comprising determining that a difference in current into the LCC and out of the LCC is above a third threshold, wherein causing the protection action to be performed comprises: responsive to the determined fault and to a difference in current into and out of the LCC being above a third threshold, causing the protective action to be performed.

7. A method according to any of claims 1-3, wherein causing the protection action to be performed comprises:

in response to determining that the LCC is operating as a rectifier, after a bypass switch of a valve in a failed arm of the rectifier is closed, the valve is caused to latch.

8. A method according to any of claims 1-3, wherein causing the protection action to be performed comprises:

in response to determining that the LCC is operating as an inverter, a pair of valves in a fault arm of the inverter are immediately bypassed.

9. A fault protection device for a high voltage direct current, HVDC, converter station comprising a three-phase connected static synchronous compensator, STATCOM, and a three-phase connected commutation converter, LCC, each limb of the three-phase connected STATCOM being connected in parallel with a respective limb of the three-phase connected LCC, the device comprising:

acquisition means for acquiring a sum of currents of the respective arms of the STATCOM;

determining means for determining a fault from the sum of currents; and

protection means for causing a protection action to be performed in the HVDC converter station based at least in part on the determined fault.

10. A fault protection device for a high voltage direct current, HVDC, converter station comprising one or more processors configured to perform the steps of the method according to any of claims 1-8.

11. A high voltage direct current, HVDC, converter station comprising:

a static synchronous compensator STATCOM with three phases connected;

a three-phase connected inverter LCC, each arm of the three-phase connected STATCOM being connected in parallel with a respective arm of the three-phase connected LCC; and

one or more processors configured to perform the steps of the method according to any one of claims 1-8.

12. A computer-readable medium having instructions stored thereon, which when executed by one or more processors, cause the one or more processors to perform the steps of the method according to any of claims 1-8.

13. A computer program product comprising a computer program which, when executed by one or more processors, causes the one or more processors to perform the steps of the method according to any one of claims 1-8.