Protection and control equipment for multi-terminal DC power transmission systems

Abstract

Provided is a protection control device for a multi-terminal DC power transmission system that makes it possible to minimize chances of shutdown of a renewable energy power generation plant, adequately separate the section of a DC line of the multi-terminal DC power transmission system where a DC accident has occurred, and maintain the operation of the entire system. A protection control device for a multi-terminal DC transmission system in which a wind farm converter station equipped with an AC / DC converter that performs AC / DC conversion of wind farm power is connected to other AC / DC converter stations via a plurality of DC lines including DC breakers, the protection control device being characterized in having a function of suppressing power flowing into the DC line from the AC / DC converter station to which a wind farm is connected in the event of an accident on the DC line.

Description

[Technical Field]

[0001] The present invention relates to a protection and control device for a multi-terminal DC power transmission system that connects a power generation system such as a renewable energy power source via a power converter and a DC transmission line. [Background technology]

[0002] High-Voltage Direct Current (HVDC) transmission systems are used for long-distance power transmission and to improve the efficiency of undersea power transmission. Since general power systems are AC systems, in DC transmission systems, AC power is converted to DC using an AC-DC converter before being transmitted.

[0003] In the past, AC / DC converters used in DC transmission systems were mainly externally commutated converters using thyristors, but in recent years, the introduction of self-commutated AC / DC converters using insulated gate bipolar transistors (IGBTs) has been increasing.

[0004] Modular multilevel converters (MMCs) are increasingly being used as AC / DC converters for self-commutated DC power transmission systems, as they have fewer harmonics and are suitable for larger capacity and higher voltages.

[0005] In addition, while point-to-point transmission, which connects two points one-to-one, has traditionally been the mainstream form of transmission for DC power transmission systems, in recent years DC power transmission systems have become larger in scale and capacity, and so-called multi-terminal DC power transmission systems, which connect three or more points, have been attracting attention. One operational challenge of such multi-terminal DC power transmission systems is the continued operation of the entire system in the event of a DC line fault.

[0006] The basic concept of continuing operation of the entire system is the same as that of conventional AC systems, which is to isolate the faulty section and continue operation using the remaining healthy sections. In this respect, unlike AC systems, multi-terminal DC transmission systems are interconnected via AC-DC converters, accident After the fault is removed, it is required that control be restored by the AC / DC converter as quickly as possible. In addition, to protect the AC / DC converter from overcurrent and overvoltage and to prevent the accident from spreading, it is generally desirable to be able to isolate the faulted section within a few milliseconds after the accident occurs and continue operation with the remaining system.

[0007] A known method for removing DC faults and maintaining system operation in a multi-terminal DC power transmission system is, for example, that described in Patent Document 1. Patent Document 1 describes a system including: switches installed to separate a power transmission path into sections; a detection unit that detects the presence of a fault; and a control unit that controls the opening and closing of the switches based on the detection results of the detection unit; wherein, when the detection unit detects a fault, the control unit performs an interruption operation on the switches to separate the faulted section from the DC power transmission path; after the interruption operation, the control unit performs a closing operation on one of the switches that performed an interruption operation due to the fault; when the detection unit detects a continuation of the fault after the closing operation of the one switch, the control unit performs a re-interruption operation on the one switch that performed the closing operation; and, if the detection unit does not detect a continuation of the fault after the closing operation of the one switch, the control unit restores an operation to restore the faulted section by returning all of the switches that performed an interruption operation to separate the faulted section from the DC power transmission path to a closed state. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] Patent No. 6517589 Summary of the Invention [Problem to be solved by the invention]

[0009] On the other hand, when protecting a multi-terminal DC power transmission system, it is possible that a switch that does not actually need to be opened to clear a fault may be opened or closed by mistake due to a setting error in the fault detection unit or a communication error between the fault detection unit and the switching unit. Hereinafter, in this specification, "the erroneous opening or closing of a switch or circuit breaker that does not need to be opened to clear a fault" will be referred to as a "malfunction."

[0010] A malfunction of a switchgear can cause unnecessary disconnection of the DC grid in a multi-terminal DC transmission system, expanding the fault section and, in the worst case, causing the entire system to shut down. In particular, if a renewable energy power plant, such as a wind farm, is connected to at least one end of the multi-terminal DC transmission system, a malfunction of a switchgear can disconnect the renewable energy power plant from the multi-terminal DC transmission system, potentially leading to the detection of an islanding state at the renewable energy power plant, causing the power plant to disconnect and shut down.

[0011] In this case, it may take time to restore operation of a power plant that has been shut down, and even if the faulty line in the DC system of a multi-terminal DC transmission system is disconnected and the transmission system is restored, it will not be possible to quickly resume transmitting electricity from renewable energy power plants.

[0012] Therefore, in a DC transmission system in which a renewable energy power plant is connected to at least one end of the multi-terminal DC transmission system, it is desirable to avoid shutting down the renewable energy power plant as much as possible even if a switch malfunction occurs, and to properly isolate the section where a DC fault occurs on the DC line of the multi-terminal DC transmission system so that the system as a whole can continue to operate.Patent Document 1 does not mention the above-mentioned switch malfunctions or the issues and solutions that arise when a renewable energy power plant is connected.

[0013] In view of the above, the present invention aims to provide a protection and control device for a multi-terminal DC power transmission system that can avoid shutdown of a renewable energy power plant as much as possible, and that can properly isolate a section where a DC fault occurs on the DC line of the multi-terminal DC power transmission system, allowing the system as a whole to continue operating. [Means for solving the problem]

[0015] Book The invention describes a protection and control device for a multi-terminal DC power transmission system in which a wind farm converter station equipped with an AC / DC converter that converts wind farm power into AC / DC is connected to other AC / DC converter stations via multiple DC lines including DC circuit breakers, and detects that a DC fault has occurred in some of the multiple DC lines and that all of the multiple DC circuit breakers in the wind farm converter station are in an open state. When all of the DC circuit breakers are in an open state, A malfunctioning DC circuit breaker is identified based on the open / close status of the DC circuit breaker and the current or voltage information of the DC line, The protection and control device for a multi-terminal DC power transmission system is characterized in that it re-closes the DC circuit breaker of the DC line on the side where no DC fault has occurred, and when a DC fault occurs on part of the DC line, it suppresses and controls the power of the wind farm, and the wind farm converter station is connected to the ground via a braking chopper between the AC / DC converter and the DC line, and connects the braking chopper to the ground when it detects that all of the multiple DC circuit breakers are in the open state. [Effects of the Invention]

[0016] According to the present invention, when a DC fault occurs, the power transmitted from a renewable energy power generation device to a multi-terminal DC power transmission system is suppressed. Therefore, even when the faulted section is isolated to remove the DC fault, it is possible to avoid shutting down the renewable energy power generation device and the AC / DC converter station to which the renewable energy power generation device is connected, and to continue operation using as many healthy sections as possible. [Brief explanation of the drawings]

[0017] [Figure 1] 1 is a diagram showing an example of the configuration of a multi-terminal DC power transmission system and its control device according to an embodiment of the present invention; [Figure 2] 1 is a diagram showing a fault state of a multi-terminal DC power transmission system. [Figure 3] FIG. 2 is a diagram showing an operation flow of a protection control device for a multi-terminal DC power transmission system according to an embodiment of the present invention. [Figure 4] FIG. 10 is a diagram showing a simplified circuit configuration of a wind farm converter station and a wind farm in a situation where the wind farm converter station is disconnected. [Figure 5] FIG. 10 is a diagram showing the concept of change in DC voltage Vdc when active power Pac is changed when a braking chopper is not used. [Figure 6] FIG. 1 is a diagram showing the concept of changes in DC voltage Vdc and the temperature of a braking chopper when a braking chopper is used. [Figure 7] FIG. 10 is a diagram showing a simplified circuit of a wind farm converter station and a wind farm in a situation where the wind farm converter station is disconnected. [Figure 8] This figure shows the concept of the changes over time in DC voltage Vac, AC current Iac, and active power Pac when a method for lowering the output AC voltage of a wind farm converter station is implemented. DETAILED DESCRIPTION OF THE INVENTION

[0018] Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings. Note that the following embodiments show one embodiment of the present invention, and the present invention includes other embodiments as long as they do not deviate from the gist of the present invention.

[0019] In the following explanation, unless otherwise specified, "AC" refers to three-phase AC. Furthermore, an AC / DC converter refers to a self-excited AC / DC converter that converts three-phase AC to DC. The part of the AC / DC converter that is connected to the three-phase AC is called the "AC side," and the part that is connected to the DC circuit is called the "DC side." Furthermore, dotted arrows in the diagrams indicate "communication of detection signals and command signals."

[0020] For ease of explanation, values ​​such as voltage and power will be described using numerical values. All voltage values ​​are expressed in arbitrary units, and the unit is [au]. Note that the numerical values ​​are used to illustrate an example of an embodiment of the present invention, and do not limit the embodiments of the present invention.

[0021] In addition, in the following description of the drawings, in order to indicate the closed and open states of a DC circuit breaker, a circuit breaker in the closed state is shown painted black, and a circuit breaker in the open state is shown painted white. [Example]

[0022] Hereinafter, an embodiment of the present invention will be described with reference to Figures 1 to 8. Figure 1 shows an example of the configuration of a multi-terminal DC power transmission system and its control device according to an embodiment of the present invention.

[0023] Figure 1 shows the configuration of a power system and its control device. The power system comprises a wind farm converter station 102W, an AC system-connected converter station 102A, and an AC system-connected converter station 102B, which are interconnected via DC transmission lines LD (LD1, LD2, LD3), forming a multi-terminal DC transmission system. The AC sides of each of the converter stations 102W, 102A, and 102B are electrically connected to a wind farm 103W, an AC system 104A, and an AC system 104B, respectively.

[0024] Each of the converter stations 102W, 102A, and 102B has almost the same configuration, except that a braking chopper 108W is additionally installed in the wind farm converter station 102W, and each of them includes an AC / DC converter 106 (106W, 106A, 106B) and a DC circuit breaker 107 (107WA, 107WB, 107AW, 107AB, 107BA, 107BW). The symbols W, A, and B attached to the AC / DC converter 106 and DC circuit breaker 107 distinguish the converter stations, and the two-digit symbols W, A, and B attached to the DC circuit breaker 107 indicate the connection relationship between the local end and the remote end of the DC transmission line by placing the local converter station first and the remote end converter station second.

[0025] The wind farm converter station 102W is further provided with a braking chopper 108W. Here, the braking chopper 108W is, for example, a series circuit of a switch and a resistor, which is connected to the ground on a DC line between the AC / DC converter 106W and the in-station bus, and has a function of switching the power consumed by the braking chopper 108W by switching the switch on and off and switching between a resistor-on state and a resistor-off state in response to commands from a control device 108W (described later).

[0026] The wind farm 103W is made up of a plurality of wind power generators 109, and FIG. 1 shows an example in which the wind farm 103W is made up of four wind power generators 109.

[0027] The above is the power system configuration of the multi-terminal DC power transmission system exemplified in Fig. 1, but the power system control device is configured to include an overall control device 100 that controls all converter stations, individual control devices 105 (105W, 105A, 105B) that control each converter station individually, and a wind farm control device 101 that controls wind farm 103W. In addition, various signals are sent and received via mutual communication between the overall control device 100 and the individual control devices 105 (105W, 105A, 105B), and mutual communication is also established between the individual control device 105W and the wind farm control device 101.

[0028] As a result, the status of each of the converter stations 102W, 102A, and 102B, including the status of the wind farm 103W, is grasped by the overall control device 100, and each of the converter stations 102W, 102A, and 102B, including the wind farm 103W, is controlled by control commands from the overall control device 100. The control objects of the individual control devices 105 (105W, 105A, 105B) are the AC / DC converters 106 and DC circuit breakers 107 within the respective stations, and the wind farm converter station 102W also performs switch opening and closing operations for the braking chopper 108W. The wind farm control device 101 controls multiple wind power generators 109 within the wind farm.

[0029] As mentioned above, in Figure 1 and other figures, the DC circuit breakers 107 (107WA, 107WB, 107AW, 107AB, 107BA, 107BW) are shown in white when open and in black ink when closed. Therefore, Figure 1 shows that the power system is healthy and all DC circuit breakers 107 (107WA, 107WB, 107AW, 107AB, 107BA, 107BW) are in the closed state.

[0030] In contrast, Figure 2 shows a state in which a DC fault 201 has occurred on the DC transmission line LD2. In this state, DC circuit breakers 107WB and 107BW at both ends of the DC transmission line LD2 have opened after detecting a fault on the DC line from a drop in DC voltage, for example. Furthermore, the other DC circuit breaker 107WA at the wind farm converter station 102W has also opened after detecting the effects of the DC voltage drop. The DC circuit breaker 107WA should not have opened, but rather opened due to a malfunction. As a result, the wind farm converter station 102W is unable to transmit power to the DC side that is equivalent to the power generated by the wind farm 103W.

[0031] Figure 3 is a diagram showing the operation flow of the protection and control device of the multi-terminal DC power transmission system according to the embodiment of the present invention. This diagram shows the process flow when a DC fault occurs in Figure 2, as a time-series process flow from top to bottom. From the left, the operation details of the overall control device 100, the operation details of the wind farm converter station 102W, and the operation details of the wind farm 103W are described in order. In Figure 3, the wind farm is abbreviated as WF.

[0032] In the following explanation, an embodiment of the present invention will be explained in chronological order, using as an example a case where a DC fault occurs at fault point 201 and DC circuit breaker 107WA malfunctions and opens, along with the operational flow of Figure 3.

[0033] First, we will explain the operation of the DC circuit breaker 107 immediately after the occurrence of a DC fault at the fault point 201. In step 301 of Fig. 3, a DC fault occurs at the fault point 201 of the DC transmission line LD2. At this time, a fault current flows from each of the converter stations 102W, 102A, and 102B toward the fault point.

[0034] In step 302, each of the AC / DC converter stations 102W, 102A, and 102B opens a DC circuit breaker to open the DC fault line LD2 based on information that can be detected at its own end, such as the current and voltage flowing through the LDs (LD1, LD2, and LD3). In this example, the converter stations 102W and 102B on both sides of the DC transmission line LD2 including the fault point 201 detect the fault, and the DC circuit breakers 107WB and 107BW on both ends of the DC transmission line LD2 are opened.

[0035] Up to this point, the protective action is correct when a DC fault occurs on the DC transmission line LD2. In the event of a DC fault occurring on the DC transmission line LD2, ideally, only the DC circuit breaker 107BW and the DC circuit breaker 107WB, which are the circuit breakers at both ends of the DC transmission line LD2 at the fault point 201, should be opened. However, here, it is assumed that in addition to the two DC circuit breakers 107BW and 107WB, the DC circuit breaker 107WA at the wind farm converter station 102W has also malfunctioned and opened.

[0036] In step 303, each of the converter stations 102W, 102A, and 102B transmits the open / closed state of the DC circuit breaker in its own station and information on the current and voltage of the DC transmission line to the overall control device 101. In this example, the converter stations 102W and 102B report the detection of a fault on the DC transmission line LD2 and the opened DC circuit breakers 107. At this time, the opened DC circuit breakers 107 reported by the wind farm converter station 102W include the DC circuit breaker 107WA that was opened due to a malfunction.

[0037] Hereinafter, the operations performed by the overall control device 100 will be described first. First, in step 304, the DC transmission line in which the fault has occurred is identified based on the open / closed state of the DC circuit breakers and the current and voltage information of the DC transmission lines LD (LD1, LD2, LD3) transmitted from each converter station 102W, 102A, 102B via information transmission to the overall control device 100. Methods for identifying the fault transmission line include, for example, a current differential method, which identifies the fault point using information from both ends of the transmission line. Here, it is assumed that the DC fault point identification process has identified that the fault has occurred at fault point 201.

[0038] In step 305, the malfunctioning DC circuit breaker is identified from the fault point information identified in step 304 for identifying the DC fault point and the open / closed state of the DC circuit breaker transmitted from each converter station. Here, in step 304 for identifying the DC fault point, it has been identified that a fault has occurred at fault point 201, and it is known in advance that, ideally, only DC circuit breaker 107BW and DC circuit breaker 107WB, which are the circuit breakers at both ends of the DC transmission line at fault point 201, need to be opened in response to the DC fault that has occurred at fault point 201. Therefore, it is possible to identify the malfunctioning DC circuit breaker as DC circuit breaker 107WA.

[0039] In step 306, the overall control device 100 issues a command to each converter station to re-close the DC circuit breaker 107WB that has been found to have malfunctioned. In this example, the overall control device 100 issues a command to the wind farm converter station 102W to re-close the DC circuit breaker 107WA.

[0040] The above is a description of the operations carried out by the overall control device 100 in the embodiment of the present invention. Note that a time delay of several hundred milliseconds to several seconds occurs in the transmission and reception of operation information at each of the converter stations 102W, 102A, and 102B, and in the identification of a DC fault point or a malfunctioning DC breaker by the overall control device 100. However, the extent of the time delay can be determined by, for example, establishing in advance an operation rule for the overall control device 100 and each of the converter stations 102W, 102A, and 102B such that after the overall control device 100 detects a DC fault, a command is issued to each of the converter stations 102W, 102A, and 102B within a maximum time period. This makes it possible for the converter stations 102W, 102A, and 102B to know how long each of the converter stations 102W, 102A, and 102B needs to continue operating under autonomous control alone from the time the DC fault occurs until a command is received from the overall control device 100.

[0041] Next, an operation flow of the wind farm converter station and the wind farm in an embodiment of the present invention will be described. In step 308, the wind farm converter station determines whether it is disconnected from the multi-terminal DC power transmission system based on the open / closed state of the DC circuit breaker 107 of its own converter station. In this example, because both DC circuit breaker 107WA and DC circuit breaker 107WB are open, it can be determined that the wind farm converter station 102W is disconnected from the multi-terminal DC power transmission system.

[0042] In this state where the wind farm converter station 102W is disconnected from the multi-terminal DC transmission system, the power transmitted from the wind farm 103W to the wind farm converter station 102W accumulates in the wind farm converter station 102W, and eventually, due to electrical and thermal constraints of the converter station, the wind farm converter station 102W may stop operating.

[0043] For this reason, in an embodiment of the present invention, in order to extend the time until the wind farm converter station 102W stops operating as much as possible, processing of step 309 is performed to suppress the power transmitted from the wind farm 103W (output upper limit command to the wind farm converter station 102W), and processing of step 310 is performed to consume the power stored in the wind farm converter station 102W (braking chopper operation).

[0044] In the process of step 309 for suppressing the power transmitted from the wind farm 103W, an upper limit of output to the wind farm 103W is determined based on how long each converter station needs to continue operating under autonomous control alone from the time when the DC fault 201 occurs until a command is received from the overall control device 100. As a method for determining the upper limit of output to the wind farm 103W, for example, a method can be considered for determining the upper limit so as to avoid DC overvoltage at the wind farm converter station.

[0045] 4 and 5 are diagrams for explaining a conceptual diagram of a method for determining an output upper limit value. FIG. 4 is a simple circuit diagram of the wind farm converter station and the wind farm in a situation where the wind farm converter station is disconnected. In FIG. 4, the active power transmitted from the wind farm 103W to the wind farm converter station 102W is denoted as Pac, the active power transmitted from the wind farm converter station 102W to the DC transmission line LD2 is denoted as Pdc, the power consumed by the braking chopper 108W is denoted as Pbc, and the DC voltage on the DC side of the wind farm converter station 102W is denoted as Vdc. In a fault-continuing state, Pdc is 0.0 [au] because the wind farm converter station 102W is disconnected.

[0046] Fig. 5 is a diagram showing the concept of change in the DC voltage Vdc on the DC side of the wind farm converter station 102W when the active power Pac transmitted from the wind farm 103W to the wind farm converter station 102W is changed without using the braking chopper 108W. The vertical axis of Fig. 5 represents the DC voltage Vdc [au], and the horizontal axis represents time.

[0047] In the explanation of Figure 5, the steady-state value of the DC voltage Vdc is 1.0 [au], the overvoltage upper limit value of the wind farm converter station 102W is 1.2 [au], and the time when the wind farm converter station 102W is disconnected from the multi-terminal DC transmission system is T0.

[0048] When the wind farm converter station 102W is disconnected from the multi-terminal DC power transmission system at time T0, the inflow of active power Pac causes energy to accumulate in the DC side capacitors and the stray capacitance of the DC transmission lines, causing the DC voltage Vdc to rise. The slope of the rise in the DC voltage Vdc at this time is determined by the magnitude of the active power Pac, and the greater the active power Pac, the greater the slope. In other words, the slope of the rise in the DC voltage Vdc can be slowed by lowering the active power Pac.

[0049] 5, an example of active power Pac=1.0 [au] is shown by a dotted line, and an example of active power Pac=0.5 [au] is shown by a solid line. The value of the DC voltage Vdc after the wind farm converter station 102W is disconnected is determined roughly by the product of the magnitude of the active power Pac and the time of disconnection, so if the active power Pac is halved, the time it takes for the wind farm converter station 102W to reach the overvoltage upper limit value roughly doubles.

[0050] Therefore, in this embodiment of the present invention, if the time it takes from the occurrence of a DC fault until a command arrives from the overall control device 100 is ΔTth, then the upper limit value of the active power Pac should be determined so that the time ΔT it takes for the DC voltage Vdc to rise from 1.0 [au] to 1.2 [au] is longer than ΔTth. This completes the description of the method for determining the output upper limit command value to the wind farm converter station 102W by the processing in step 309.

[0051] Furthermore, as in the embodiment of the present invention, it is preferable to use a braking chopper operation, which is the process of step 310 for consuming the power stored in the wind farm converter station 102W, in combination. If the wind farm converter station 102W is provided with a braking chopper 108W, the braking chopper 108W can be operated and turned on when the DC voltage Vdc rises, thereby consuming the energy stored on the DC side and reducing the slope of the DC voltage Vdc.

[0052] FIG. 6 shows the change in DC voltage Vdc when the braking chopper 108W is used in the upper part of FIG. 6, and the change in temperature of the braking chopper 108W at the same time in the lower part of FIG.

[0053] While the braking chopper 108W is turned on, power is consumed by the braking chopper 108W, suppressing an increase in the DC voltage Vdc, but the temperature rises due to heat generated by the braking chopper 108W. For example, if the upper limit value of the braking chopper DC voltage is 1.1 [au], the increase in the DC voltage Vdc can be suppressed by turning on the braking chopper DC voltage until the temperature of the braking chopper reaches the upper limit.

[0054] The above is an explanation of the principle of suppressing the rate of increase in the DC voltage Vdc due to the application of the braking chopper 108 W in the embodiment of the present invention. Because the power consumption of the braking chopper is determined by the resistance value and the DC voltage, by taking into account the operation of the braking chopper 108 W and calculating the time ΔT it takes for the DC voltage Vdc to increase from 1.0 [au] to 1.2 [au], it is possible to determine the active power Pac when the braking chopper 108 W is present using the same concept.

[0055] Note that the suppression of the rate of rise of the DC voltage Vdc by setting the output upper limit of the active power Pac and the suppression of the rate of rise of the DC voltage Vdc by the braking chopper can be used in combination. Also, in this explanation, a method for determining the upper limit of the active power Pac has been explained using DC overvoltage as an example, but the upper limit of the active power Pac can also be determined using a similar concept when considering constraints that depend on the time product of the active power Pac, such as thermal constraints at the converter station.

[0056] The above has described the output upper limit command to the wind farm 103W for suppressing the power transmitted from the wind farm 103W, and the operation of the braking chopper 108W for consuming the power stored in the wind farm converter station 102W.

[0057] Finally, in step 311, the output of the wind farm 103W is restricted based on the upper limit value of the active power Pac calculated in the output upper limit command to the wind farm 103W.

[0058] This concludes the explanation of the operational flow for reclosing the malfunctioning DC circuit breaker while avoiding shutdown of the wind farm converter station by suppressing the wind farm output even if a DC fault occurs and a DC circuit breaker malfunctions. Finally, the series of operational flows is completed in step 312.

[0059] The above has described how to respond when a DC fault occurs at fault point 201 and DC circuit breaker 107WA malfunctions and opens. Note that in the embodiment of the present invention, a method of using an output suppression command from wind farm 103W and a method of utilizing braking chopper 108W have been described in order to suppress power flowing from wind farm 103W into the DC transmission line, but the present invention can also be realized by a method of lowering the output AC voltage of wind farm converter station 102W.

[0060] 7 is a simplified circuit diagram of the wind farm converter station 102W and the wind farm 103W in a situation where the wind farm converter station 102W is disconnected. In Fig. 7, the voltage on the AC side of the wind farm converter station 102W is denoted as Vac, the AC current transmitted to the wind farm converter station 102W is denoted as Iac, the active power transmitted to the wind farm converter station 102W is denoted as Pac, and the active power transmitted from the wind farm converter station 102W to the DC transmission line is denoted as Pdc. When the power factor of the power transmitted to the wind farm converter station 102W is 1, the active power Pac is the product of the AC side voltage Vac and the AC current Iac.

[0061] 8 is a conceptual diagram of changes over time in AC voltage Vac, AC current Iac, and active power Pac when a method for lowering the output AC voltage Vac of the wind farm converter station 102W is performed at time T0. According to this, the wind farm 103W is operated to keep the AC current Iac constant, and control is performed at the wind farm converter station to lower the DC voltage Vac, thereby making it possible to lower the active power Pac.

[0062] Unlike the method using wind farm curtailment commands, no communication between the wind farm and the wind farm converter station is required while the wind farm converter station is isolated. [Explanation of symbols]

[0063] 10: Overall control device, 102A, 102B: AC system connection converter station, 102W: Wind farm converter station, 103W: Wind farm, 104A, 104B: AC system, 105A, 103B, 103W: Individual control device, 106A, 106B, 106W: AC / DC converter, 107AB, 107AW, 107BA, 107BW, 107WA, 107WB: DC circuit breaker, 108W: Braking chopper, 109: Wind power Power generation, 201: Fault point, 301: DC fault occurrence, 302: DC circuit breaker opening, 303: Information transmission to overall control device, 304: DC fault point identification, 305: Malfunctioning circuit breaker identification, 306: Circuit breaker closing command, 307: Circuit breaker closing, 308: Wind farm converter station disconnection detection, 309: Output upper limit command to wind farm, 310: Braking chopper operation, 311: Wind farm output suppression operation, 312: Completion

Claims

[Claim 1] A protection and control device for a multi-terminal DC power transmission system in which a wind farm converter station equipped with an AC / DC converter that converts power from a wind farm into AC / DC is connected to other AC / DC converter stations via a plurality of DC lines including DC circuit breakers, Detecting that a DC fault has occurred in some of the multiple DC lines and that all of the multiple DC circuit breakers at the wind farm converter station are in an open state, and when all of the DC circuit breakers are in an open state, identifying a malfunctioning DC circuit breaker from the open / close status of the DC circuit breaker and current or voltage information of the DC lines, and reopening the DC circuit breaker of the DC line on the side where the DC fault has not occurred; When a DC fault occurs in a part of the DC line, the power of the wind farm is suppressed and controlled, The wind farm converter station is connected to ground via a braking chopper between the AC / DC converter and the DC line, and the braking chopper is connected to ground when it detects that all of the multiple DC circuit breakers are in an open state.

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