Fault handling in a terminal of a multi-terminal high-voltage direct current transmission system

Abstract

The invention relates to a method for handling faults in a terminal (11) of a multi-terminal high-voltage direct current transmission system (1). In said method, the DC voltage arising at a DC voltage connection (25) of the terminal (11) is continuously measured so as to form a DC voltage measurement value (Vdc) and information about said DC voltage measurement value (Vdc) is temporarily stored for a predetermined period of time. If a fault arises in an AC grid (21), a DC voltage measurement value which was measured before the fault arose is used as a setpoint value for a DC voltage controller (429) and the DC voltage controller (429) is used to regulate the DC voltage arising at the DC voltage connection (25) to said setpoint value.

Description

[0001] Description

[0002] Fault handling at a terminal of a multi-terminal high-voltage direct current transmission system

[0003] The invention relates to a method for error handling in a terminal of a multi-terminal high-voltage direct current transmission system and to such a terminal.

[0004] In the future, multi-terminal high-voltage direct current transmission systems will be used for power transmission. Two or more alternating current networks, for example, can be connected to such a multi-terminal high-voltage direct current transmission system. In particular, the alternating current networks each represent a power transmission network. The two or more alternating current networks are therefore coupled to one another via the multi-terminal high-voltage direct current transmission system. If a fault occurs in one of the alternating current networks, this can lead to the effects of the fault also occurring in another of the alternating current networks, for example in the form of voltage fluctuations, current fluctuations and / or power fluctuations. These effects on the other alternating current network, in which no fault has occurred, are undesirable.

[0005] The invention is based on the object of specifying a method and a terminal with which, when a fault occurs in an alternating current network that is connected to a multi-terminal high-voltage direct current transmission system, the effects of the fault on another alternating current network can be minimized.

[0006] This object is achieved according to the invention by a method and by a terminal according to the independent patent claims. Advantageous embodiments of the method and the terminal are specified in the dependent patent claims. A method for error handling in a terminal of a multi-terminal high-voltage direct current transmission system is disclosed, wherein

[0007] - the terminal has an AC voltage connection and a DC voltage connection,

[0008] - the AC terminal is connected to an AC network and the DC terminal is connected to a DC network of the multi-terminal high-voltage direct current transmission system,

[0009] - the terminal has a power converter designed to convert direct current from the direct current network into alternating current from the alternating current network and / or vice versa, and

[0010] - the terminal has an energy converter for converting electrical energy into thermal energy (by means of at least one electrical resistance element), wherein in the method

[0011] - the DC voltage occurring at the DC voltage connection of the terminal is continuously measured to form a DC voltage measurement value and information about this DC voltage measurement value is temporarily stored for a predetermined period of time, and

[0012] - when a fault occurs in the AC network, a DC voltage measured before the fault occurs is used as a setpoint for a DC voltage regulator and the DC voltage occurring at the DC voltage connection is regulated to this setpoint by means of the DC voltage regulator.

[0013] In other words, after the occurrence of the fault, the DC voltage occurring at the DC voltage connection is regulated to the DC voltage that existed before the occurrence of the fault. This reduces the effects of the fault on the DC voltage network of the multi-terminal high-voltage DC transmission system. This also reduces the effects of the fault on another AC network connected to the multi-terminal high-voltage DC transmission system, such as an AC network connected to a different terminal. To achieve this, information on continuously measured DC voltage values ​​is buffered for a predetermined period of time. In particular, the DC voltage value measured at a time can also be buffered for a predetermined period of time.This means that even after the error has occurred, an error-free DC voltage value or DC voltage measurement is available in order to regulate the DC voltage to this value.

[0014] In the event of a fault, the DC voltage is regulated to the value that existed before the fault occurred using the temporarily stored information. In particular, the DC voltage measured before the fault occurred can be reconstructed from the temporarily stored information and used as the setpoint. However, the DC voltage measured value can also be temporarily stored so that it is immediately available and can be used as the setpoint.

[0015] The procedure can be carried out in such a way that

[0016] - the DC voltage occurring at the DC voltage connection is regulated to the setpoint by controlling the energy converter in such a way that the energy converter converts electrical energy transmitted from the DC network to the terminal (which cannot be absorbed by the AC network due to the fault) into heat in a controlled manner, if such electrical energy is transmitted to the terminal.

[0017] As a result, the DC voltage in the DC network drops. The energy converter is therefore advantageously used to influence the DC voltage, in particular to regulate the DC voltage. In particular, electrical energy that cannot be absorbed by the AC network due to the fault is converted into heat. Such electrical energy is also referred to below as "excess electrical energy". Such excess electrical energy would lead to an (undesirable) increase in the DC voltage if it is not converted into heat. The method can be carried out in such a way that

[0018] - the DC voltage occurring at the DC voltage connection is only regulated to the setpoint by means of the DC voltage regulator if the AC network in which the fault occurs is an AC network into which electrical energy is fed via the AC voltage connection or from which electrical energy is taken via the AC voltage connection. In other words, the DC voltage occurring at the DC voltage connection is only regulated to the setpoint by means of the DC voltage regulator if the AC network in which the fault occurs is electrically connected to the terminal for the purpose of feeding energy into the AC network or for the purpose of taking energy from the AC network.

[0019] The procedure can also be carried out in such a way that

[0020] - the level of the alternating voltage at the AC voltage terminal is monitored, and the occurrence of a fault in the AC network is detected when the level of the alternating voltage falls below a predetermined value. Advantageously, this allows the occurrence of a fault in the AC network to be detected in a simple manner.

[0021] The procedure can be carried out in such a way that

[0022] - the predetermined time period is between 0.5 s and 10 s, in particular between 1 s and 5 s. This ensures that, when the error occurs, information is available about at least one DC voltage measurement value that existed before the error occurred. The predetermined time period can be, for example, 0.5 s, 1 s, 2 s, 5 s or 10 s.

[0023] The procedure can be carried out in such a way that

[0024] - the information about the DC voltage measurement value (or about several DC voltage measurements) is buffered for the predetermined period of time by means of a delay element, in particular a first-order delay element. Such a first-order delay element can also be referred to as a PTI element. The method can also be carried out in such a way that

[0025] - the energy converter comprises several energy converter modules, each of which comprises an electronic switch and an electrical resistance element. This allows the amount of electrical energy converted into heat to be easily adjusted (scalable); the conversion of electrical energy into heat energy is therefore adjustable / scalable.

[0026] The procedure can be carried out in such a way that

[0027] - the alternating current network is an onshore alternating current network. The method is particularly advantageous for transmitting electrical energy from, for example, a wind farm located at sea to an onshore alternating current network (onshore alternating current network).

[0028] In this method, a unit for feeding in renewable energy can advantageously be connected to the multi-terminal high-voltage direct current transmission system. Such a unit could be, for example, a wind farm or a solar farm. The amount of renewable energy generated depends on factors that are difficult or impossible to influence, such as wind strength or solar radiation. Therefore, the amount of renewable energy generated cannot simply be reduced in the short term. This makes the method particularly advantageous for transmitting renewable energy.

[0029] Furthermore, a terminal of a multi-terminal high-voltage direct current transmission system is disclosed, wherein

[0030] - the terminal has an AC voltage connection and a DC voltage connection,

[0031] - the AC voltage connection is connected to an AC network and the DC voltage connection is connected to a DC network of the multi-terminal high-voltage direct current transmission system, - the terminal has a power converter which is designed to convert direct current of the DC network into alternating current of the AC network and / or vice versa,

[0032] - the terminal has an energy converter for converting electrical energy into thermal energy (by means of at least one electrical resistance element),

[0033] - the terminal has a measuring device for continuously measuring a DC voltage occurring at the DC voltage connection of the terminal to form a DC voltage measurement value and a storage device for temporarily storing information about this DC voltage measurement value for a predetermined period of time, and

[0034] - the terminal has a DC voltage regulator which is designed to use a DC voltage measured before the occurrence of the fault as a setpoint when a fault occurs in the AC network and to regulate the DC voltage occurring at the DC voltage connection to this setpoint.

[0035] This can

[0036] - the DC voltage regulator is designed to regulate the DC voltage occurring at the DC voltage connection to the setpoint value by the DC voltage regulator controlling the energy converter in such a way that the energy converter converts electrical energy transmitted from the DC network to the terminal (which cannot be absorbed by the AC network due to the fault) into heat if such electrical energy is transmitted to the terminal.

[0037] This can

[0038] - the DC voltage regulator must also be designed in such a way that it only regulates the DC voltage occurring at the DC voltage terminal to the setpoint if the AC network in which the fault occurs is an AC network into which electrical energy is fed via the AC voltage terminal or from which electrical energy is taken via the AC voltage terminal. The terminal can

[0039] - have a monitoring device designed to monitor the level of the alternating voltage at the alternating voltage terminal and to detect when the level of the alternating voltage falls below a predetermined value. It is then recognized that the fault is in the alternating current network.

[0040] The terminal can be designed so that

[0041] - the memory device comprises a delay element, in particular a first-order delay element.

[0042] The terminal can also be designed so that

[0043] - the energy converter comprises a plurality of energy converter modules, each of the energy converter modules comprising an electronic switch and an electrical resistance element.

[0044] Such a terminal can also be called a high-voltage direct current transmission station.

[0045] An example application is the transmission of energy generated offshore (for example by means of a wind farm) (especially renewable) to at least two onshore alternating current networks by means of a multi-terminal high-voltage direct current transmission system.

[0046] The DC voltage regulator can be implemented in particular in a control of the energy converter.

[0047] The method and the terminal have the same or similar advantages .

[0048] The invention is explained in more detail below using exemplary embodiments. The same reference numerals refer to the same or similarly acting elements. Figure 1 shows an exemplary embodiment of a multi-terminal high-voltage direct current transmission system with four terminals, in

[0049] Figure 2 shows the multi-terminal high-voltage direct current transmission system with exemplary energy flows entered, in

[0050] Figure 3 shows an embodiment of an energy converter with several energy converter modules, in

[0051] Figure 4 shows an exemplary control circuit for regulating the DC voltage occurring at the first terminal, and in

[0052] Figure 5 shows an exemplary process sequence using a flow chart.

[0053] Figure 1 shows an exemplary embodiment of a multi-terminal high-voltage direct current transmission system 1 with four terminals. This multi-terminal high-voltage direct current transmission system 1 has a first terminal 11, a second terminal 12, a third terminal 13 and a fourth terminal 14. A first alternating current network 21 is connected to an alternating voltage connection 17 of the first terminal 11. A first alternating current source AC1 and a first network impedance Zgridl of the first alternating current network 21 are shown as a model. A direct current network 28 is connected to a direct voltage connection 25 of the first terminal 11. The direct current network 28 connects the direct voltage connections of all four terminals to one another. The direct current network 28 has a first direct current conductor 29 and a second direct current conductor 30.In the exemplary embodiment, the first DC conductor 29 is a positive DC conductor, and the second DC conductor 30 is a negative DC conductor. Furthermore, the first terminal 11 has an energy converter 31. The energy converter 31 has a resistance element 34. The energy converter 31 is connected in parallel to the DC voltage terminal 25. The energy converter 31 is thus connected between the first DC conductor 29 and the second DC conductor 30.

[0054] The first terminal 11 has a power converter 39. The power converter 39 connects the AC voltage connection 17 to the DC voltage connection 25. The power converter 39 is designed to convert the alternating current present at the AC voltage connection 17 into the direct current present at the DC voltage connection 25 and / or vice versa. The power converter 39 can, in particular, be a modular multilevel power converter (MMC).

[0055] The fourth terminal 14 is similar to the first terminal

[0056] 11. An AC voltage connection of the fourth terminal 14 is electrically connected to a second AC network 44. A second AC source AC2 and a second grid impedance Zgrid2 of the second AC network 44 are modeled. The DC network 28 is connected to a DC voltage connection of the second terminal 14. The fourth terminal 14 has a further energy converter 48 and a further power converter 52.

[0057] The second terminal

[0058] 12 and the third terminal 13. The second terminal 12 has a power converter, but no energy converter. The third terminal 13 also has a power converter, but no energy converter. A first wind farm 58 is connected to an AC voltage connection of the second terminal 12; a second wind farm 60 is connected to an AC voltage connection of the third terminal 13. The second terminal 12 is therefore electrically connected to the first (offshore) wind farm 58; the third terminal 13 is electrically connected to the second (offshore) wind farm 60.

[0059] In the exemplary embodiment, the first AC network 21 and the second AC network 44 are each onshore AC networks, i.e., the first AC network 21 and the second AC network 44 are located on land (and not at sea). In the exemplary embodiment, the first AC network 21 and the second AC network 44 are operated by different network operators. If a fault occurs in the first AC network 21 (for example, a short circuit between two AC voltage lines), significant transient events can also occur in the second AC network 44, for example, fluctuations in the active power and reactive power during and shortly after the occurrence of the fault. These fluctuations in the "healthy" (i.e., fault-free) second AC network 44 are not welcome by the network operator of this second AC network 44 and should be kept as low as possible.

[0060] The energy converter 31 is arranged on the DC side of the power converter 39. It can be connected between the two poles 29, 30, as in the exemplary embodiment; however, it can also be connected between one of the poles and a neutral conductor or between one of the poles and earth potential. The energy converter 31 serves to absorb any surplus of generated electrical energy ("excess electrical energy") and convert it into thermal energy. Such excess electrical energy is, in particular, electrical energy that cannot be transmitted to the AC grid for a short time due to the fault. For example, the energy converter 31 can be designed such that it can absorb and convert the excess electrical energy for a few seconds.

[0061] In particular, the first terminal 11 and the fourth terminal 14 can each be operated using two different control methods. However, both terminals must not use the same control method at the same time. The first control method controls the DC voltage Vd of the DC network. This controls the converter energy. A terminal operated using the first control method supplies the electrical energy required by the rest of the multi-terminal high-voltage DC transmission system or receives the electrical energy supplied by the rest of the multi-terminal high-voltage DC transmission system.

[0062] The second control method regulates the active power transmitted to the connected AC grid. If the converter energy leaves the permissible range, an energy control is activated, which adjusts the reference value for the active current and thus brings the converter energy back within the permissible range.

[0063] Figure 2 shows an exemplary operating state of the multi-terminal high-voltage direct current transmission system 1. The first wind farm 58 generates an electrical output of 900 MW; the second wind farm 60 generates no electrical output (0 MW). The electrical output generated by the first wind farm 58 is divided into a first portion with a size of 400 MW and a second portion with a size of 500 MW. The first portion (400 MW) is transmitted from the second terminal 12 to the first terminal 11 and from the first terminal 11 to the first AC grid 21. The first AC grid 21 transmits this first portion on to consumers (not shown).

[0064] The second portion (500 MW) is transmitted from the second terminal 12 to the fourth terminal 14 and from the fourth terminal 14 to the second AC network 44. The second AC network 44 transmits this second portion on to consumers not shown. Figure 3 shows a detailed exemplary embodiment of the energy converter 31. The energy converter 31 is connected between the first positive DC conductor 29 and the second negative DC conductor 30. The energy converter 31 has two inductors in the form of choke coils 303, a first energy converter module 306, a second energy converter module 309, a third energy converter module 312 and a fourth energy converter module 316. Each of the energy converter modules 306, 309, 312 and 316 includes an electronic switch 322 and the resistance element 34.For each of the energy converter modules, the electronic switch can be switched independently so that the electrical current flows through the respective resistance element and the electrical energy in this resistance element is converted into thermal energy. The energy converter module is then switched on or active. Depending on the number of switched-on / active energy converter modules, a different amount of energy is converted into thermal energy. This allows the level of energy conversion to be adjusted / scalable. The second energy converter 48 has a similar structure.

[0065] Figure 4 shows an exemplary embodiment of a control loop 401 for regulating the DC voltage Vd. The DC voltage Vd is the DC voltage of the DC network 28 of the multi-terminal high-voltage DC transmission system 1. The upper part of Figure 4 shows the first terminal 11 with the power converter 39 and the energy converter 31. The energy converter 31 is intended to be activated when an undesirably high DC voltage Vd occurs in the DC network.

[0066] The first alternating current network 21 is connected to the first terminal 11. The alternating voltage Va of the first alternating current network 21 is measured by means of a measuring transformer 404 to form alternating voltage measured values ​​Vac. The alternating voltage measured values ​​Vac are then transmitted to three different sections of the control loop 401. More precisely, the alternating voltage measured values ​​Vac are transmitted to a first monitoring device 411, a second monitoring device 412 and a third monitoring device 413. The first monitoring device 411, the second monitoring device 412 and the third monitoring device 413 are of similar construction and monitor the alternating voltage for the presence of an undervoltage. The monitoring devices therefore carry out undervoltage detection. The presence of an undervoltage is detected when the level of the alternating voltage falls below a predetermined value.Thus, if the AC voltage measurement values ​​Vac fall below the predetermined value, it is detected that a fault has occurred in the AC network 21. Such a fault (for example, a short circuit) generally results in a reduction in the AC voltage Va of the AC network 21. The first monitoring device 411 is arranged in a first section 421 of the control loop; the second monitoring device 412 is arranged in a second section 422 of the control loop, and the third monitoring device 413 is arranged in a third section 423 of the control loop.

[0067] If no fault occurs in the AC network 21, the control loop / regulation functions as follows: By means of a measuring device 407, the DC voltage Vd of the DC network 28 (which is present between the first DC conductor 29 and the second DC conductor 30) is measured to form a DC voltage measurement value Vdc. The DC voltage measurement value Vdc is compared with a DC voltage reference value Vdc*, and the deviation (difference) between the DC voltage reference value Vdc* and the DC voltage measurement value Vdc is formed. This deviation (Vdc* - Vdc) is fed to a DC voltage regulator 429. If the deviation ( Di f ference ) between the DC voltage reference value Vdc* and the DC voltage measurement value Vdc is greater than a permitted tolerance value ( " Margin " ), then the DC voltage regulator 429 outputs an energy converter reference current Ichop* which is fed to a modulator 431 .The tolerance value ("margin") can, for example, correspond to a single-digit percentage value of a nominal DC voltage Vdc_nom. The DC voltage reference value Vdc* represents a setpoint for the DC voltage Vd (DC voltage setpoint Vdc*).

[0068] The modulator 431 then controls the energy converter 31 such that a current corresponding to the energy converter reference current Ichop* flows through the energy converter 31 and a corresponding amount of electrical energy is converted into thermal energy. In the exemplary embodiment, the modulator 431 controls the energy converter 31 such that so many energy converter modules 306, 309, 312, 316 of the energy converter 31 are switched on by means of the electronic switch 322 that a current corresponding to the energy converter reference current Ichop* flows through the energy converter 31.

[0069] If a fault occurs in the AC network 21, the three monitoring devices 411, 412, and 413 detect this fault based on the voltage drop of the AC voltage of the first AC network and output a fault signal 435. This fault signal 435 is used in the first section 421, the second section 422, and the third section 423.

[0070] By means of the first section 421, it is achieved that the energy converter reference current Ichop* output by the DC voltage regulator 429 is only transmitted to the modulator 431 when the error signal 435 is present, i.e., when a fault has occurred in the AC network 21. This is achieved by means of a first signal selection device 434, which is controlled by the error signal 435. If the error signal 435 is active (error signal = 1), the signal selection device 434 forwards the energy converter reference current Ichop* output by the DC voltage regulator 429 to the modulator 431. If the error signal 435 is inactive (error signal=0), then the energy converter reference current Ichop* is set to zero (lchop*=0), whereupon the modulator controls the energy converter 31 in such a way that the energy converter does not perform any energy conversion (all energy converter modules are switched off by means of the electronic switch 322).This ensures that the energy converter of a terminal (here: the first terminal 11) is only activated and performs an energy conversion if a fault occurs in the AC network 21 connected to this terminal. In the exemplary embodiment, the energy converter 31 is therefore only activated and performs an energy conversion if a fault occurs in the first AC network 21 connected to the first terminal 11.

[0071] In the exemplary embodiment, the first portion of electrical energy (400 MW) is transmitted from the second terminal 12 to the first terminal 11. However, due to the fault in the first AC network 21, the first portion of electrical energy cannot be transmitted further to the first AC network 21. This first portion would lead to an increase in the DC voltage Vd in the DC network 28, which would also have undesirable effects on the second AC network 44. However, the first portion of electrical energy (400 MW) is converted into heat by means of the energy converter 31, so that the effects on the second AC network 44 are reduced.

[0072] By means of the second section 422 it is achieved that the permitted tolerance value ("Margin") is only used when the error signal 435 is inactive (error signal = 0), i.e. when no error has occurred in the AC network 21. This is achieved by means of a second signal selection device 439 which is controlled by the error signal 435. If the error signal 435 is active (error signal = 1), then the tolerance value ("Margin") is set to zero (Margin = 0). If the error signal 435 is inactive (error signal = 0), then the predetermined tolerance value ("Margin") is used. In the third section 423, information about the respectively measured DC voltage value Vdc is continuously buffered in a memory device 410 for a predetermined period of time.If the error signal 435 is inactive (error signal = 0), the nominal DC voltage value Vdc_nom is forwarded to the DC voltage regulator 429 by means of a third signal selection device 445. However, if an error has occurred in the AC network 21, i.e. if the error signal 435 is active (error signal = 1), the third signal selection device 445 forwards the stored DC voltage measurement value Vdc_filt to the DC voltage regulator 429 instead of the nominal DC voltage value Vdc_nom. This has the consequence that, in the event of a fault in the AC network 21, the DC voltage value Vd that existed before the fault occurred is also used as the nominal DC voltage value for the DC voltage regulator 429 after the fault occurs. In the exemplary embodiment, the memory device 410 is designed as a delay element, in particular as a first-order delay element.This delay element represents a measuring filter.

[0073] If a fault has occurred in the AC network 21, the filtered DC voltage measurement value Vdc_filt (i.e., the buffered DC voltage measurement value Vdc_filt) is used as the nominal DC voltage value for the DC voltage regulator 429. The time constant of the delay element (which corresponds to the predetermined buffering time) can, for example, be between 0.5 s and 10 s, preferably between 1 s and 5 s. A possible value would be, for example, 2 s. In other embodiments, the memory device 410 can also be designed differently, for example as a memory cell or as a shift register.

[0074] In the method, the direct voltage Vd of the direct current network 28 (which is present between the first direct current conductor 29 and the second direct current conductor 30) is measured by means of a measuring device 407 to form direct voltage measured values ​​Vdc. The direct voltage measured values ​​Vdc are fed to the memory device 410. The memory device 410 stores information about the direct voltage measured values ​​Vdc for a predetermined period of time. The memory device 410 ensures that, when the error occurs, information is available about at least one direct voltage measured value Vdc that existed before the error occurred. In the event of a fault, i.e. after the error has occurred, a stored direct voltage measured value Vdc_filt is used as the nominal direct voltage value for the DC voltage regulator 429. The DC voltage regulator 429 therefore regulates the direct voltage Vd to the value that existed before the error occurred.The DC voltage Vd is an electrical quantity that couples the first AC network 21 to the second AC network 44 (see Figure 1). Because the DC voltage Vd is regulated back to the value it had before the fault occurred by means of the DC voltage regulator 429 after the fault occurs, the effects of the fault in the first AC network 21 on the second AC network 44 are comparatively small. The second AC network 44 is thus decoupled from the first AC network 21 with respect to the effects of the fault.

[0075] The control method described in connection with Figure 4 is, in particular, independent of the above-mentioned first control method and the second control method. The control method described in connection with Figure 4 can, in particular, be implemented as a control for the energy converter.

[0076] Figure 5 shows an exemplary process sequence in the form of a flow chart. The process steps 510 to 550 are executed.

[0077] Process step 510 :

[0078] Continuous (repeated) measurement of the DC voltage Vd occurring at the DC voltage connection 25 of the terminal 11 to form a DC voltage measurement value Vdc. Process step 520:

[0079] Buffering information about the respective DC voltage measurement value Vdc for a predetermined period of time

[0080] Process step 530 :

[0081] Detecting the occurrence of a fault in the AC network 21

[0082] Process step 540 :

[0083] Using a DC voltage measurement value Vdc_filt measured and buffered before the fault occurred as a setpoint for a DC voltage regulator 429

[0084] Process step 550 :

[0085] Regulating the DC voltage Vd occurring at the DC voltage terminal 25 to the setpoint (by means of the DC voltage regulator 429)

[0086] The case was described in which a fault occurs in the first AC network 21 and in which the effects of this fault are reduced by means of the first terminal 11 (i.e. by means of the terminal to which the faulty first AC network 21 is connected). For this purpose, the energy converter 31 of this first terminal 11 and the control for this energy converter 31, which is also implemented in particular in the first terminal 11, are used. The further energy converter 48 of the fourth terminal 14 and the control for this further energy converter 48, which can also be arranged in particular in the fourth terminal 14, are not used for fault handling.

[0087] If, in another exemplary embodiment, the fault occurs in the second AC network 44, the described method runs in a similar manner with the additional energy converter 48 of the fourth terminal 14 and with the control system for this additional energy converter 48. With regard to the additional energy converter 48 and the associated control system, the fourth terminal 14 is thus designed in a similar manner to the first terminal 11.

[0088] A method and a terminal of a multi-terminal high-voltage direct current transmission system have been described which, in the event of a fault in an alternating current network connected to the multi-terminal high-voltage direct current transmission system, can be used to minimize the effects of the fault on another connected alternating current network. This is achieved by temporarily storing information on continuously measured direct voltage values ​​for the time after the potential occurrence of a fault. In the event of a fault, the temporarily stored information is used to regulate the direct voltage to the value that existed before the fault occurred.

[0089] Reference sign

[0090] I Multi-terminal high-voltage direct current transmission system

[0091] II first terminal

[0092] 12 second terminal

[0093] 13 third terminal

[0094] 14 fourth terminal

[0095] 17 AC voltage connection 21 first AC network

[0096] 25 DC voltage connection

[0097] 28 DC network

[0098] 29 first DC conductor

[0099] 30 second DC conductor

[0100] 31 energy converters

[0101] 34 resistance element 39 power converter 44 second AC network 48 further energy converter

[0102] 52 additional power converters

[0103] 58 first wind farm 60 second wind farm

[0104] 303 Choke coil 306 First energy converter module 309 Second energy converter module 312 Third energy converter module 316 Fourth energy converter module 322 Electronic switch

[0105] 401 Control loop 404 Transducer

[0106] 411 first monitoring device

[0107] 412 second monitoring device

[0108] 413 third monitoring device

[0109] 421 first section

[0110] 422 second section

[0111] 423 third section 429 DC voltage regulator

[0112] 431 Modulator

[0113] 435 Error signal

[0114] 434 first signal selection device 439 second signal selection device

[0115] 445 third signal selection device

[0116] AC1 first alternating current source

[0117] AG 2 second alternating current source Va alternating voltage

[0118] Vd DC voltage

[0119] Vac alternating voltage measurement value

[0120] Vdc DC voltage measurement

[0121] Zgridl first grid impedance Zgrid2 second grid impedance

Claims

Patent claims 1. A method for error handling in a terminal (11) of a multi-terminal high-voltage direct current transmission system (1), wherein - the terminal (11) has an AC voltage connection (17) and a DC voltage connection (25), - the AC voltage connection (17) is connected to an AC network (21) and the DC voltage connection (25) is connected to a DC network (28) of the multi-terminal high-voltage direct current transmission system (1), - the terminal (11) has a power converter (39) which is designed to convert direct current of the direct current network (28) into alternating current of the alternating current network (21) and / or vice versa, and - the terminal (11) has an energy converter (31) for converting electrical energy into thermal energy, wherein in the method - the DC voltage (Vd) occurring at the DC voltage connection (25) of the terminal (11) is continuously measured to form a DC voltage measured value (Vdc) and information about this DC voltage measured value (Vdc) is temporarily stored for a predetermined period of time, and - when a fault occurs in the AC network (21), a DC voltage measured value measured before the occurrence of the fault is used as a setpoint for a DC voltage regulator (429) and the DC voltage (Vd) occurring at the DC voltage terminal (25) is regulated to this setpoint by means of the DC voltage regulator (429).

2. Method according to claim 1, characterized in that - the DC voltage (Vd) occurring at the DC voltage connection (25) is regulated to the desired value by controlling the energy converter (31) in such a way that the energy converter (31) from the DC network (28) to the terminal (11) converts transmitted electrical energy into heat, if such electrical energy is transmitted to the terminal (11).

3. Method according to claim 1 or 2, characterized in that - the DC voltage occurring at the DC voltage connection (25) is only regulated to the desired value by means of the DC voltage regulator (429) if the AC network (21) in which the fault occurs is an AC network into which electrical energy is fed via the AC voltage connection (17) or from which electrical energy is withdrawn via the AC voltage connection (17).

4. Method according to one of the preceding claims, characterized in that - the level of the alternating voltage (Va) at the alternating voltage connection (17) is monitored and the occurrence of the fault in the alternating current network (21) is detected when the level of the alternating voltage falls below a predetermined value.

5. Method according to one of the preceding claims, characterized in that - the predetermined period of time is between 0.5 s and 10 s, in particular between 1 s and 5 s.

6. Method according to one of the preceding claims, characterized in that - the information about the DC voltage measurement value is buffered for the predetermined period of time by means of a delay element (410), in particular a first-order delay element.

7. Method according to one of the preceding claims, characterized in that - the energy converter (31) comprises a plurality of energy converter modules (306, 309, 312, 316), each of the energy converter modules (306, 309, 312, 316) comprising an electronic switch (322) and an electrical resistance element (34).

8. Method according to one of the preceding claims, characterized in that - the AC network (21) is a shore-side AC network.

9. Method according to one of the preceding claims, characterized in that - a unit (58, 60) for feeding in renewable energy is connected to the multi-terminal high-voltage direct current transmission system (1).

10. Terminal (11) of a multi-terminal high-voltage direct current transmission system (1), wherein - the terminal (11) has an AC voltage connection (17) and a DC voltage connection (25), - the AC voltage connection (17) is connected to an AC network (21) and the DC voltage connection (25) is connected to a DC network (28) of the multi-terminal high-voltage direct current transmission system (1), - the terminal (11) has a power converter (39) which is designed to convert direct current from the direct current network (28) into alternating current from the alternating current network (21) and / or vice versa, - the terminal (11) has an energy converter (31) for converting electrical energy into thermal energy, - the terminal (11) comprises a measuring device (407) for continuously measuring a DC voltage (Vd) occurring at the DC voltage connection (25) of the terminal (11) to form a DC voltage measurement value (Vdc) and a storage device (410) for temporarily storing information about this DC voltage measurement value (Vdc) for a predetermined period of time, and - the terminal (11) has a DC voltage regulator (429) which is designed to use a DC voltage measured before the occurrence of the fault as the setpoint value when a fault occurs in the AC network (21) and to regulate the DC voltage (Vd) occurring at the DC voltage terminal (25) to this setpoint.

11. Terminal according to claim 10, characterized in that - the DC voltage regulator (429) is designed to regulate the DC voltage (Vd) occurring at the DC voltage connection (25) to the desired value by the DC voltage regulator (429) controlling the energy converter (31) such that the energy converter (31) converts electrical energy transmitted from the DC network (28) to the terminal (11) into heat, if such electrical energy is transmitted to the terminal (11).

12. Terminal according to claim 10 or 11, characterized in that - the DC voltage regulator (429) is designed such that it only regulates the DC voltage occurring at the DC voltage connection (25) to the desired value if the AC network (21) in which the fault occurs is an AC network (21) into which electrical energy is fed via the AC voltage connection (17) or from which electrical energy is withdrawn via the AC voltage connection (17).

13. Terminal according to one of claims 10 to 12, characterized in that - the terminal (11) has a monitoring device (411, 412, 413) which is designed to monitor the level of the alternating voltage (Va) at the alternating voltage connection (17) and to detect when the level of the alternating voltage falls below a predetermined value.

14. Terminal according to one of claims 10 to 13, characterized in that - the memory device (410) has a delay element, in particular a first-order delay element.

15. Terminal according to one of claims 10 to 14, characterized in that - the energy converter (31) comprises a plurality of energy converter modules (306, 309, 312, 316), each of the energy converter modules (306, 309, 312, 316) comprising an electronic switch (322) and an electrical resistance element (34).

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