Collective protection system

Abstract

To provide a collective protection system with a reduced cost of long distance power transmission using an AC cable.SOLUTION: According to one embodiment, a collective protection system includes a second collective protection device including a second acquisition unit that acquires current values on a secondary side and an in-station load facility side in a first boosting transformer and current values on a primary side, a secondary side, and an in-station load facility side in a second boosting transformer, a second detection unit that detects occurrence of an electrical accident on the basis of the current values acquired by the second acquisition unit, and a second control unit that outputs a shutdown command when the second detection unit detects an electrical accident. Further, the second detection unit detects an electrical accident in a power transmission path between the first boosting transformer and the second boosting transformer on the basis of the difference between current flowing into the power transmission path and current flowing out from the power transmission path. Further, the second detection unit detects an electrical accident in the second boosting transformer on the basis of the ratio between current flowing through a primary winding and current flowing through a secondary winding in the second boosting transformer.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] SUMMARY OF THE DISCLOSURE The present invention relates to a global protection system. [Background technology]

[0002] A characteristic of offshore wind power generation transmission and transformation facilities is the long transmission distance from the offshore wind turbines to the grid connection point. In particular, when the capacity of the wind turbines becomes large, they are often connected to the grid at ultra-high voltage (275kV, etc.), and the transmission distance on land is often also long.

[0003] In recent years, from the standpoint of maintaining the landscape, etc., underground power transmission is sometimes used instead of overhead transmission. In this case, a power transmission cable insulated by a coating is used for power transmission instead of a bare wire.

[0004] When the transmission distance is long in an ultra-high voltage grid connection, a 154kV cable may be selected as the transmission cable. Since the charging current of the transmission cable increases in proportion to the distance and voltage, the resulting voltage fluctuation and the effect of reactive power must be taken into consideration. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] JP 2019-154321 A Summary of the Invention [Problem to be solved by the invention]

[0006] The longer the transmission cable, the greater the impact of the charging current, and it is expected that costs will increase to take measures such as installing protective devices. On the other hand, in order to popularize renewable energy power generation, it is desirable to reduce equipment costs, and each operator is also working to reduce equipment costs for offshore wind power generation.

[0007] Therefore, an embodiment of the present invention provides a collective protection system that reduces costs in long-distance power transmission using AC cables. [Means for solving the problem]

[0008] According to one embodiment, the general protection system includes a second general protection device including a second acquisition unit that acquires current values ​​on the secondary side and the station load equipment side of the first step-up transformer and current values ​​on the primary side, the secondary side, and the station load equipment side of the second step-up transformer, a second detection unit that detects the occurrence of an electrical accident based on the current values ​​acquired by the second acquisition unit, and a second control unit that outputs a cutoff command when the second detection unit detects an electrical accident. Furthermore, the second detection unit detects an electrical accident in the transmission line based on a difference between a current flowing into the transmission line between the first step-up transformer and the second step-up transformer and a current flowing out of the transmission line. Furthermore, the second detection unit detects an electrical accident in the second step-up transformer based on a ratio between a current passing through a primary winding and a current passing through a secondary winding in the second step-up transformer. [Brief description of the drawings]

[0009] [Figure 1] 1 is a schematic configuration diagram of a power generation and transformation system in a first embodiment. [Diagram 2] FIG. 2 is a schematic configuration diagram of a power generation and transformation system in a comparative example. [Diagram 3] This is an example of where a circuit breaker opens when an electrical accident occurs. [Figure 4] FIG. 2 is an example of a block diagram of a first collective protection device in the first embodiment. [Diagram 5] FIG. 4 is an example of a block diagram of a second collective protection device in the first embodiment. [Figure 6] 13 is an example of a flowchart when an electrical accident is detected by the second collective protection device in the first embodiment. [Figure 7] FIG. 11 is a schematic configuration diagram of a power generation and transformation system according to a second embodiment. [Figure 8] FIG. 13 is an example of a block diagram of a third collective protection device in the second embodiment. [Figure 9] FIG. 11 is a schematic configuration diagram of a power generation and transformation system according to a third embodiment. [Figure 10] FIG. 13 is a hardware configuration diagram of a first collective protection device in a fourth embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0010] Hereinafter, an embodiment of the present disclosure will be described with reference to the drawings. The present embodiment does not limit the present invention. The drawings are schematic or conceptual, and the ratio of each part is not necessarily the same as the actual one. In the specification and drawings, the same elements as those described above with respect to the previously mentioned drawings are given the same reference numerals, and detailed description is omitted as appropriate. In the following description, in a three-winding transformer, the primary winding side which is the low voltage side of the transformer is called the primary side, and the secondary winding side which is the high voltage side is called the secondary side. In addition, the tertiary winding side which supplies power to the station load equipment side is called the station load equipment side. In addition, in the following, the transformation ratio of the step-up transformer is expressed as 66 kV / 154 kV. In this case, the voltage of the primary winding of the step-up transformer is expressed as 66 kV, and the voltage of the secondary winding is expressed as 154 kV.

[0011] (First embodiment) FIG. 1 is a schematic configuration diagram of a power generation and transformation system in the first embodiment.

[0012] In the power generation and transformation system 100 of Fig. 1, the offshore wind turbine 50 is connected to a primary substation 70 and a secondary substation 80, and the generated power is boosted (two-stage boost) at each substation before being connected to the power system. The primary substation 70 and the secondary substation 80 are connected by a 154 kV power cable 12 buried underground. In the case of long-distance cable transmission, the capacitance of the 154 kV power cable 12 becomes very large, and shunt reactors 13 and 14 are provided in the primary substation 70 and the secondary substation 80 to reduce the leading charging current. The primary substation 70 is also called a landing substation, and the secondary substation 80 is also called an interconnection substation. The 154 kV power cable 12 is an example of a power transmission line.

[0013] For the sake of explanation, in this example, the power generation and transformation system 100 refers to the offshore wind turbine 50, the primary substation 70, the 154 kV power cable 12, the secondary substation 80, and the cables and power generation and transformation equipment included in these sections.

[0014] In the power generation and transformation system 100 of FIG. 1, 66 kV of power generated by the offshore wind turbine 50 is boosted to 154 kV at the primary substation 70. The boosted power is then transmitted to the secondary substation 80 via the 154 kV power cable 12, and is boosted to 275 kV at the secondary substation 80. The power boosted to 275 kV is then connected to a power system owned by a power company or the like. A centralized protection system 1 is constructed between the primary substation 70 and the secondary substation 80, and when the centralized protection system 1 detects an occurrence of an equipment abnormality, such as an electrical accident in a transformer or power cable in the protected section, it opens a circuit breaker (CB) and separates the offshore wind turbine 50 and the like from the power generation and transformation system 100. This separates the power generation and transformation system 100 from the power system, making it possible to prevent the spread of an electrical accident. The 66 kV / 154 kV step-up transformer 10 is an example of a first step-up transformer, and the 154 kV / 275 kV step-up transformer 11 is an example of a second step-up transformer.

[0015] In this configuration, the central protection system 1 releases the leading charging current in the energized 154 kV power cable 12 by opening the circuit breaker 25 on the secondary side of the 154 kV / 275 kV step-up transformer 11 in the secondary substation 80 or the circuit breaker 24 on the secondary side of the 66 kV / 154 kV step-up transformer 10 in the primary substation 70.

[0016] In this embodiment, the section to be protected by the integrated protection system 1 will be described as extending from the secondary side of the 66 kV / 154 kV step-up transformer 10 installed in the primary substation 70 to the circuit breaker 25 on the secondary side of the 154 kV / 275 kV step-up transformer 11 installed in the secondary substation 80. This protected section includes various facilities such as the 154 kV power cable 12 connecting the two substations, the above-mentioned 154 kV / 275 kV step-up transformer 11, and the on-site load facilities in the secondary substation 80.

[0017] In this example, a plurality of offshore wind turbines 50 connected in series are connected in parallel to each other to form a group of offshore wind turbines 50. Electricity generated by the offshore wind turbines 50 is transmitted to a primary substation 70 via an undersea cable.

[0018] The primary substation 70 receives the power generated by the offshore wind turbine 50. The received power is stepped up from a voltage of 66 kV to 154 kV, and transmitted to the secondary substation 80. The main equipment installed in the primary substation 70 is a 66 kV / 154 kV step-up transformer 10, a shunt reactor 13, a first central protection device 2, a circuit breaker 22, a circuit breaker 23, a circuit breaker 24, and a disconnecting switch 26. The circuit breaker 24 is installed on the secondary side of the 66 kV / 154 kV step-up transformer 10. The primary substation 70 is configured such that no circuit breaker for the shunt reactor 13 is provided between the shunt reactor 13 and the electric circuit, but only the disconnecting switch 26 is provided.

[0019] In this example, the 66kV / 154kV step-up transformer 10 is a three-winding transformer, and has a secondary winding for stepping up the voltage from 66kV to 154kV, and a tertiary winding for transforming the voltage to be supplied to the load equipment in the primary substation 70. The 66kV / 154kV step-up transformer 10 adopts a resistance grounding method among neutral grounding methods, and is connected to a neutral grounding resistor (NGR). The primary substation 70 is also provided with a 66kV / 154kV transformer protection device (not shown) for protecting the section from the circuit breaker 23 on the primary side of the 66kV / 154kV step-up transformer 10 to the circuit breaker 24 on the secondary side.

[0020] Furthermore, the circuit breakers 22 and 23 separate the group of offshore wind turbines 50 from the power generation and transformation system 100. Furthermore, the circuit breaker 24 separates the offshore wind turbines 50 and the like from the 154 kV power cable 12.

[0021] The first collective protection device 2 detects the occurrence of an electrical accident from the current and voltage values ​​at each measurement point and opens the circuit breaker 24. To obtain the current value at each measurement point, a value converted from a large current to a small current by a current transformer (CT) is used, and to obtain the voltage value at each measurement point, a value converted from a high voltage to a low voltage by a voltage transformer (VT). These values ​​may also be obtained using, for example, a voltage and current transformer (VCT) or the like.

[0022] In this embodiment, the first general protection device 2 obtains current and voltage values ​​from a current transformer 31 and a voltage transformer 41 provided on the secondary side of the 66 kV / 154 kV step-up transformer 10, and obtains current and voltage values ​​from a current transformer 30 and a voltage transformer 40 provided on the in-station load equipment side of the transformer. The first general protection device 2 also obtains a current value from a current transformer 32 provided between the shunt reactor 13 and the electric circuit.

[0023] The first general protection device 2 also obtains the current value and the like on the primary side of the 154kV / 275kV step-up transformer 11 from the second general protection device 3 described later. The first general protection device 2 compares the current value on the secondary side of the 66kV / 154kV step-up transformer 10, the current value on the in-house load equipment side, and the current value and the like on the primary side of the 154kV / 275kV step-up transformer 11 to detect whether there is any current leakage. In this example, the first general protection device 2 uses the values ​​of the current transformers 31 and 32 as the current value on the secondary side of the 66kV / 154kV step-up transformer 10. The first general protection device 2 also uses the value of the current transformer 30 as the current value flowing on the in-house load equipment side. Moreover, the first general protection device 2 uses the values ​​of the current transformers 33 , 34 , etc. acquired by the second general protection device 3 as the value of the current flowing through the primary side of the 154 kV / 275 kV step-up transformer 11 .

[0024] The first collective protection device 2 can detect a current leakage from the difference between the current value flowing into the 154kV power cable 12 and the current value flowing out of the 154kV power cable. For example, if the sum of the current flowing into and out of the 154kV power cable 12 does not become 0, it is found that a current leakage has occurred. For example, this current leakage can be detected by the current transformer 34 on the primary side of the 154kV / 275kV step-up transformer 11. In this way, the first collective protection device 2 has a function of a current differential relay and detects an electrical accident in the 154kV power cable 12.

[0025] The first and second general protection devices 2 and 3 are connected to a network 6 via communication devices 4 and 5, respectively, and constantly transmit and receive measurement information such as current and voltage values ​​acquired by each device via the network 6. This network 6 can be, for example, a power safety communication network such as a wired line or a wireless line, or a general public line. The first and second general protection devices 2 and 3 can use, for example, a PCM (Pulse Code Modulation) method to realize the function of a current differential relay.

[0026] When there is a current leak, the first collective protection device 2 outputs a breaker command to disconnect the circuit breaker 24. In this way, when an electrical accident occurs, the offshore wind turbine 50 and the like are disconnected from the power grid.

[0027] The secondary substation 80 receives power transmitted from the primary substation 70 via the 154 kV power cable 12. The received power is stepped up from 154 kV to 275 kV and connected to the power grid. The main equipment installed in the secondary substation 80 is a 154 kV / 275 kV step-up transformer 11, a shunt reactor 14, a second central protection device 3, a circuit breaker 25, a disconnecting switch 27, and a disconnecting switch 28. The circuit breaker 25 is installed on the secondary side of the 154 kV / 275 kV step-up transformer 11. The secondary substation 80 is configured such that no circuit breaker is provided between the shunt reactor 14 and the electric circuit, but only the disconnecting switch 27 is provided. In addition, the secondary substation 80 is configured such that no circuit breaker is provided between the 154 kV / 275 kV step-up transformer 11 and the 154 kV power cable 12, but a disconnecting switch 27 is provided.

[0028] In this example, the 154kV / 275kV step-up transformer 11 is a three-winding transformer that has a secondary winding that steps up the voltage from 154kV to 275kV, and also has a tertiary winding that transforms the voltage to a voltage that supplies power to the load equipment in the secondary substation 80. The 154kV / 275kV step-up transformer 11 employs a neutral grounding system, and a direct grounding system rather than a resistance grounding system. This allows the second collective protection device 3 to more reliably detect a ground fault current when an electrical accident occurs, such as when a surge current flows in, compared to a resistance grounding system.

[0029] A circuit breaker 25 is provided on the secondary side of the 154 kV / 275 kV step-up transformer 11 to separate the power system from the equipment of the secondary substation 80.

[0030] The second collective protection device 3 detects the occurrence of an electrical accident from the current and voltage values ​​at each measurement point and opens the circuit breaker 25. The second collective protection device 3 obtains the current and voltage values ​​at each measurement point using a current transformer and a potential transformer. The second collective protection device 3 may also obtain these values ​​using, for example, a potential transformer.

[0031] In this embodiment, the second general protection device 3 obtains a current value from a current transformer 34 provided on the primary side of the 154 kV / 275 kV step-up transformer 11, and obtains a voltage value from a voltage transformer 42. The second general protection device 3 also obtains a current value from a current transformer 38 provided on the primary side of the transformer and a current transformer 37 provided on the in-house load equipment side, and obtains a voltage value from a voltage transformer 43. The second general protection device 3 also obtains a current value from current transformers 35 and 36 provided on the secondary side of the 154 kV / 275 kV step-up transformer 11, and obtains a voltage value from a voltage transformer 44. The second general protection device 3 also obtains a current value from a current transformer 33 provided between the shunt reactor 14 and the electric circuit.

[0032] Moreover, the second general protection device 3 receives measurement information such as current values ​​and voltage values ​​acquired by the first general protection device 2 via the network 6 .

[0033] Similarly to the first general protection device 2, the second general protection device 3 has a function of a digital current differential relay that detects whether there is a current leakage based on the difference between the current value flowing into the 154kV power cable 12 and the current value flowing out of the 154kV power cable 12. In this example, the current leakage is determined based on the values ​​of the current flowing through the primary side of the 154kV / 275kV step-up transformer 11, the current flowing through the in-station load equipment side, the current flowing through the secondary side of the 154kV / 275kV step-up transformer 11, the current flowing through the secondary side of the 66kV / 154kV step-up transformer 10, and the current flowing through the in-station load equipment side. For example, the second general protection device 3 detects a ground fault in the 154kV power cable 12 by the function of a digital current differential relay.

[0034] In this example, the second general protection device 3 uses the values ​​of current transformers 33, 34, and 38 as the current value on the primary side of the 154kV / 275kV step-up transformer 11. The second general protection device 3 uses the value of current transformer 37 as the current value flowing on the station equipment side. The second general protection device 3 uses the values ​​of current transformers 35 and 36 as the current value on the secondary side of the 154kV / 275kV step-up transformer 11. The first general protection device 2 uses the values ​​of current transformers 31 and 32 acquired by the first general protection device 2 as the current value on the secondary side of the 66kV / 154kV step-up transformer 10. The second general protection device 3 uses the value of current transformer 30 as the current value flowing on the station load equipment side of the 66kV / 154kV step-up transformer 10.

[0035] The current values ​​on the primary side and secondary side are different due to the transformation ratio of the 154kV / 275kV step-up transformer 11. Therefore, the second general protection device 3 converts, for example, the voltage values ​​acquired by the voltage transformers 42, 43, and 44 into current values ​​corresponding to those voltage values, and detects whether there is a current leak. Similarly, the second general protection device 3 converts the current value acquired from the first general protection device 2 into a current value corresponding to the voltage value detected by the voltage transformer 41, and detects whether there is a current leak.

[0036] The second collective protection device 3 also has a function of a ratio differential relay that compares the current value passing through the primary winding of the 154kV / 275kV step-up transformer 11 with the current value passing through the secondary winding, and detects the occurrence of an electrical accident when the ratio of these values ​​becomes equal to or exceeds a certain value. When the ratio of the current value passing through the primary winding of the 154kV / 275kV step-up transformer 11 with the current value passing through the secondary winding becomes equal to or exceeds a certain value, the second collective protection device 3 detects an electrical accident in the 154kV / 275kV step-up transformer 11 and outputs a breaker command to open the circuit breaker 25. For example, the second collective protection device 3 detects a short circuit accident in the 154kV / 275kV step-up transformer 11 by the function of a ratio differential relay.

[0037] FIG. 2 is a schematic diagram of a power generation and transformation system in a comparative example.

[0038] In the power generation and transformation system 100 in the comparative example, the first general protection device 2 and the second general protection device 3 are not provided. Instead, the 66kV / 154kV step-up transformer 10, the 154kV power cable 12, the 154kV / 275kV step-up transformer 11, the shunt reactor 13, and the shunt reactor 14 are each provided with a protection device. In the configuration of the comparative example, when an electrical accident occurs, the power generation and transformation system 100 can be cut off according to the section protected by each protection device, and the spread of the electrical accident can be suppressed, but the number of circuit breakers and protection devices increases, resulting in a significant increase in costs. In the comparative example, in order to separate the equipment of the secondary substation 80 from the 154kV power cable 12, the secondary substation 80 is provided with a circuit breaker 48 between the 154kV power cable 12 and the 154kV / 275kV step-up transformer 11. Moreover, the shunt reactor 13 and the shunt reactor 14 require a circuit breaker 46 and a circuit breaker 47 between the shunt reactor 13 and the electric circuit, respectively.

[0039] Furthermore, as described above, when the capacitance of the 154 kV power cable 12 becomes very large due to long-distance cable transmission, the shunt reactors 13 and 14 need to have a large capacity in order to ensure the breaker 48 and the like have the ability to open the leading charging current. On the other hand, a higher voltage type breaker may be adopted as the breaker 48 provided in the electric circuit in order to reduce the leading charging current. For example, a 300 kV gas circuit breaker (GCB) is adopted as the breaker 48 on the primary side of the 154 kV / 275 kV step-up transformer 11 provided in the secondary substation 80, which is expensive in terms of cost.

[0040] In the comparative example, the protected section of the protection device in the 66kV / 154kV step-up transformer 10 (also referred to as a 66kV / 154kV transformer protection device) is from the circuit breaker 23 on the primary side to the circuit breaker 24 on the secondary side of the 66kV / 154kV step-up transformer 10. Also, the protected section of the protection device in the 154kV power cable 12 (also referred to as a transmission line protection device) is from the circuit breaker 24 on the primary side to the circuit breaker 48 on the secondary side of the 154kV power cable 12. Also, the protected section of the protection device in the 154kV / 275kV step-up transformer 11 (also referred to as a 154kV / 275kV transformer protection device) is from the circuit breaker 48 on the primary side to the circuit breaker 25 on the secondary side of the 154kV / 275kV step-up transformer 11. The sections protected by the protective devices in the shunt reactors 13 and 14 are from the circuit breaker 46 to the ground point (not shown) and from the circuit breaker 47 to the ground point (not shown), respectively.

[0041] Figure 3 shows an example of where a circuit breaker opens when an electrical accident occurs.

[0042] FIG. 3A is an example diagram illustrating a location where the integrated protection system 1 in the first embodiment opens a circuit breaker when a ground fault occurs in the 154 kV power cable 12, and FIG. 3B is an example diagram illustrating a location where the power transmission line protection device in the comparative example opens a circuit breaker when an electrical fault occurs in the 154 kV power cable 12.

[0043] FIG. 3A illustrates an example in which the second protective device 3 detects a ground fault that occurs in the 154 kV power cable 12. In FIG. 3A, the 154 kV / 275 kV step-up transformer 11 employs a direct grounding method as its grounding method, so that the ground fault current flows from this grounding point to the earth. The second protective device 3 detects this current using a current transformer 34, and detects an electrical fault when the difference between the current flowing into and out of the 154 kV power cable 12 does not become zero. The second protective device 3 opens a circuit breaker 25 installed on the secondary side of the 154 kV / 275 kV step-up transformer 11.

[0044] 3B will explain an example in which a power transmission line protection device detects a ground fault in the 154 kV power cable 12. In Fig. 3B, the power transmission line protection device that protects the 154 kV power cable 12 detects a large current when a ground fault occurs, and detects an electrical fault when the difference between the current flowing into and out of the 154 kV power cable 12 does not become zero. The power transmission line protection device opens a circuit breaker 48 installed on the primary side of the 154 kV / 275 kV step-up transformer 11.

[0045] FIG. 4 is an example of a block diagram of a first collective protection device in the first embodiment.

[0046] In this embodiment, the first general protection device 2 includes a first acquisition unit 110, a first detection unit 120, a first control unit 130, a first storage unit 140, and a first communication unit 150.

[0047] The first comprehensive protection device 2 can be realized, for example, by installing a program for the first comprehensive protection device 2 in a PC (Personal Computer). A CPU (Central Processing Unit) in the first comprehensive protection device 2 executes the program for the first comprehensive protection device 2 to realize the functions of a first acquisition unit 110, a first detection unit 120, a first control unit 130, a first storage unit 140, and a first communication unit 150. The first storage unit 140 has an area for storing the acquired current value and voltage value, and is constructed, for example, on an auxiliary storage device on an HDD (Hard Disc Drive).

[0048] The first acquisition unit 110 acquires current values ​​and voltage values ​​from the current transformers 31, 32 and the voltage transformer 41 provided on the secondary side of the 66 kV / 154 kV step-up transformer 10, and acquires current values ​​and voltage values ​​from the current transformer 30 and the voltage transformer 40 provided on the in-station load equipment side of the transformer. The acquired values ​​are stored in the first storage unit 140. The current values ​​and voltage values ​​stored in the first storage unit 140 are used for, for example, log analysis and the like.

[0049] The first detection unit 120 detects the occurrence of an electrical accident based on the values ​​acquired by the first acquisition unit 110. In this example, a current leak is detected based on the current flowing through the secondary side and the on-site load equipment side of the 66 kV / 154 kV step-up transformer 10 and the current flowing through the primary side, the secondary side, and the on-site load equipment side of the 154 kV / 275 kV step-up transformer 11. For example, the first detection unit 120 calculates the difference between the current value flowing into and out of the 154 kV power cable 12 from these values, and detects the presence of a current leak if the difference is not zero.

[0050] When the first detection unit 120 detects the occurrence of an electrical accident, the first control unit 130 outputs a breaker command to open the circuit breaker 24. The circuit breaker 24 is opened by the breaker command.

[0051] The first communication unit 150 is connected to a second communication unit of the second general protection device 3 (described later) via the network 6. The first acquisition unit 110 acquires the current and voltage values ​​of the primary side and secondary side of the 154 kV / 275 kV step-up transformer 11, as well as the current and voltage values ​​of the in-station load equipment, from the second general protection device 3 via the first communication unit 150. The current and voltage values ​​related to the primary substation acquired by the first acquisition unit 110 are transmitted to the second communication unit of the second general protection device 3 via the first communication unit 150 and are used to detect the occurrence of an electrical accident in the second general protection device 3.

[0052] FIG. 5 is an example of a block diagram of the second collective protection device in the first embodiment.

[0053] In this embodiment, the second general protection device 3 includes a second acquisition unit 210, a second detection unit 220, a second control unit 230, a second storage unit 240, and a second communication unit 250.

[0054] The second comprehensive protection device 3 can be realized, for example, by installing a program for the second comprehensive protection device 3 on a PC, similar to the first comprehensive protection device 2. The CPU in the second comprehensive protection device 3 executes the program for the second comprehensive protection device 3 to realize the functions of the second acquisition unit 210, the second detection unit 220, the second control unit 230, the second storage unit 240, and the second communication unit 250. The second storage unit 240 has an area for storing the acquired current value and voltage value, and is constructed, for example, on an auxiliary storage device on a HDD.

[0055] The second acquisition unit 210 acquires current values ​​and voltage values ​​from the above-mentioned current transformers 33 to 37 and instrument transformers 42 to 44. The acquired values ​​are stored in the second storage unit 240. The current values ​​and voltage values ​​stored in the second storage unit 240 are used for, for example, log analysis.

[0056] The second detection unit 220 detects the occurrence of an electrical accident based on the values ​​acquired by the second acquisition unit 210. In this example, the current leakage is detected based on the values ​​of the current flowing through the secondary side of the 66kV / 154kV step-up transformer 10 and the current flowing through the load equipment side of the station in addition to the current flowing through the primary side, the station equipment side, and the secondary side of the 154kV / 275kV step-up transformer 11. For example, the second detection unit 220 calculates the difference between the current value flowing into the 154kV power cable 12 and the current value flowing out from the 154kV power cable 12 from these values, and detects the occurrence of a current leakage if the difference is not zero. The second detection unit 220 also compares the current value passing through the primary winding of the 154kV / 275kV step-up transformer 11 with the current value passing through the secondary winding, and detects the occurrence of an electrical accident when this ratio becomes equal to or exceeds a certain value.

[0057] When the second detection unit 220 detects the occurrence of an electrical accident, the second control unit 230 outputs a breaker command to open the circuit breaker 25. The circuit breaker 25 is opened by the breaker command.

[0058] The second communication unit 250 is connected to the first communication unit 150 of the first general protection device 2 via the network 6. The second acquisition unit 210 acquires current values ​​and voltage values ​​on the secondary side of the 66 kV / 154 kV step-up transformer 10 as well as current values ​​and voltage values ​​on the in-station load equipment side from the first general protection device 2 via the first communication unit 150. The current values ​​and voltage values ​​related to the secondary substation 80 acquired by the second acquisition unit 210 are transmitted to the first communication unit 150 of the first general protection device 2 via the second communication unit 250 and are used to detect the occurrence of an electrical accident in the first general protection device 2.

[0059] FIG. 6 is an example of a flowchart when an electrical accident is detected by the second collective protection device in the first embodiment.

[0060] In this flowchart, the flow chart when the second general protection device 3 in the first embodiment detects an electrical accident will be described, but the first general protection device 2 performs current leakage detection according to a similar flow, so the description will be omitted. Also, this flowchart is periodically and repeatedly executed.

[0061] In step S1, the second acquisition unit 210 acquires current values ​​from the current transformers 33, 34, and 38 provided on the primary side of the 154 kV / 275 kV step-up transformer 11, and acquires a voltage value from the voltage transformer 42. The second acquisition unit 210 also acquires a current value from the current transformer 37 provided on the in-station load equipment side of the transformer, and acquires a voltage value from the voltage transformer 43. The second acquisition unit 210 also acquires current values ​​from the current transformers 35 and 36 provided on the secondary side of the 154 kV / 275 kV step-up transformer 11, and acquires a voltage value from the voltage transformer 44. The second acquisition unit 210 also acquires current values ​​and voltage values ​​on the secondary side of the 66 kV / 154 kV step-up transformer 10 from the first central protection device 2, and acquires current values ​​and voltage values ​​on the in-station load equipment side. In step S2, the second detection unit 220 calculates the difference between the current value flowing into and out of the 275 kV power cable 12 from the values ​​of the current flowing through the primary side, the current flowing through the station equipment side, and the current flowing through the secondary side of the 66 kV / 154 kV step-up transformer 10, in addition to the current flowing through the station load equipment side, and detects current leakage.

[0062] If the comparison of the current values ​​indicates that there is no current leakage (NO in step S2), in step S3, the second detection unit 220 compares the current value passing through the primary winding of the 154 kV / 275 kV step-up transformer 11 with the current value passing through the secondary winding. The second detection unit 220 may use a threshold value when calculating the ratio between the current value passing through the primary winding and the current value passing through the secondary winding. For example, this threshold value may be stored in advance in the second storage unit 240 and loaded into the memory when the program for the second collective protection device 3 is executed. If the comparison of the current values ​​indicates that the ratio is not equal to or greater than the threshold value (NO in step S3), the first collective protection device 2 ends the process. If the comparison of the current values ​​indicates that the ratio is equal to or greater than the threshold value (YES in step S3), in step S4, the second detection unit 220 detects that an electrical accident has occurred. For example, the second detection unit 220 detects that a short circuit has occurred in the 154 kV / 275 kV step-up transformer 11.

[0063] In step S5, the second control unit 230 outputs a shutoff command to open the circuit breaker 25, and ends the process. Also, in step S2, if the comparison of the current values ​​by the second detection unit 220 indicates that there is a current leak (YES in step S2), the process proceeds to step S4, and the second detection unit 220 detects that an electrical accident has occurred. For example, the second detection unit 220 detects that an electrical accident has occurred in the 154 kV power cable 12. Upon detection of the electrical accident, in step S5, similarly to the above, the second control unit 230 outputs a shutoff command to open the circuit breaker 25, and ends the process.

[0064] In this embodiment, an example of underground power transmission using a 154 kV power cable 12 has been taken up, but the collective protection system 1 can also be applied to overhead power transmission. In the case of overhead power transmission, bare wires are used, so the capacity of the shunt reactors 13 and 14 can be reduced or their installation can be omitted.

[0065] In addition, in this embodiment, only one power transmission system is taken up for the sake of explanation, but the integrated protection system 1 can also be applied to a case where two or three power transmission systems are multiplexed. For example, the integrated protection system 1 is adopted for each additional system.

[0066] In addition, in the present embodiment, the first collective protection device 2 opens the circuit breaker 24 as a tripping command, but the first collective protection device 2 may open the circuit breaker 25. In this case, the tripping command is transmitted via the network 6, for example.

[0067] Moreover, the secondary substation 80 may be configured without the current transformer 38. By not including the current transformer 38, the configuration of the secondary substation 80 can be further simplified. Since the second collective protection device 3 does not acquire the value of the current transformer 38, it does not distinguish between a short circuit fault occurring in the 154 kV / 275 kV step-up transformer 11 and a ground fault occurring in the 154 kV power cable 12. The second collective protection device 3 detects an electrical fault occurring in the power generation and transformation system 100 based on a comparison between the current value passing through the primary winding and the current value passing through the secondary winding of the 154 kV / 275 kV step-up transformer 11 in addition to the presence or absence of current inflow and outflow in the power generation and transformation system 100. When an electrical fault occurs, a maintenance worker investigates the fault and identifies the location of the fault.

[0068] According to this embodiment, the collective protection system 1 broadly defines the protected section in the power generation and transformation system 100 that performs two-stage boosting, and collectively monitors electrical accidents in that section, thereby enabling a significant reduction in equipment installation costs. For example, by using the collective protection system 1, the primary substation 70 and the secondary substation 80 can be configured to have disconnectors 26, 27, and 28 installed without the circuit breakers 46, 47, and 48, enabling cost reduction. Although there is a concern that the widespread spread of electrical accidents may occur when the protected section is broadened, the collective protection system 1 can limit electrical accidents that occur in the power generation and transformation system 100 to the range up to the secondary substation 80, thereby preventing the spread of the accidents to the power system.

[0069] In addition, according to this embodiment, the secondary substation 80 can improve the reliability of detection of earth fault current by the second central protection device 3 by using a direct grounding method for the grounding method of the 154 kV / 275 kV step-up transformer 11.

[0070] Second embodiment FIG. 7 is a schematic configuration diagram of a power generation and transformation system in the second embodiment.

[0071] In this embodiment, the electric power generated by the offshore wind turbine 50 is boosted in each of the primary substation 70, the secondary substation 80, and the tertiary substation 90, and then connected to the electric power grid.

[0072] The generated power is stepped up from 66 kV to 154 kV in primary substation 70, stepped up from 154 kV to 275 kV in secondary substation 80, and stepped up from 275 kV to 500 kV in tertiary substation 90. 275 kV / 500 kV step-up transformer 51 is an example of a third step-up transformer.

[0073] The tertiary substation 90 receives power transmitted from the secondary substation 80 via the 275 kV power cable 62, boosts the voltage from 275 kV to 500 kV, and connects it to the power grid. The main equipment installed in the tertiary substation 90 is a 275 kV / 500 kV step-up transformer 51, a shunt reactor 52, a third central protection device 7, a circuit breaker 59, a disconnecting switch 55, and a disconnecting switch 56. The circuit breaker 59 is installed on the secondary side of the 275 kV / 500 kV step-up transformer 51. The tertiary substation 90 is configured such that a circuit breaker for the shunt reactor 52 between the shunt reactor 52 and the electric circuit is not provided, but a disconnecting switch 55 is provided. In addition, the tertiary substation 90 is configured such that no circuit breaker is provided between the 275 kV / 500 kV step-up transformer 51 and the 275 kV power cable 62, but a disconnecting switch 56 is provided.

[0074] In addition, the 275kV / 500kV step-up transformer 51 employs a direct grounding method for the ground point on the primary winding side. When a ground fault occurs in the 275kV power cable 62, the ground fault current flows from this ground point to the earth. The third collective protection device 7 detects this current by the current transformer 63.

[0075] In this example, for the purpose of explanation, the power generation and transformation system 100 refers to the offshore wind turbine 50, the primary substation 70, the 154 kV power cable 12, the secondary substation 80, the 275 kV power cable 62, the tertiary substation 90, and the cables and power generation and transformation equipment included in these sections.

[0076] The primary substation 70 and the secondary substation 80 are mainly configured similarly to the first embodiment, and only the differences will be described. The secondary substation 80 is provided with a communication device 9, and the second centralized protection device 3 and the third centralized protection device 7 are connected to the network 6 via the communication devices 8 and 9, respectively, and constantly transmit and receive measurement information such as current values ​​and voltage values ​​acquired by each device via the network 6.

[0077] A shunt reactor 65 is provided on the secondary side of the secondary substation 80, and a disconnector 66 is provided between the shunt reactor 65 and the electric circuit. The second collective protection device 3 obtains the value of the current flowing through the shunt reactor 65 by a current transformer 67.

[0078] In this embodiment, an integrated protection system 1 is constructed between the primary substation 70, the secondary substation 80, and the tertiary substation 90, and when this integrated protection system 1 detects the occurrence of an abnormality, such as an electrical accident in a transformer or power cable in the protected section, it opens the circuit breaker 24, 25, or 59 and disconnects the offshore wind turbine 50, etc. from the power generation and transformation system 100. This disconnects the power generation and transformation system 100 from the power system, making it possible to prevent the spread of the electrical accident.

[0079] In this configuration, the third central protection device 7 opens the circuit breaker 59 on the secondary side of the 275 kV / 500 kV step-up transformer 51 in the tertiary substation 90, thereby releasing the leading charging current in the energized 275 kV power cable 62.

[0080] In the present embodiment, the first and second general protection devices 2 and 3 open the circuit breakers 24 and 25, respectively, as a tripping command. However, the first and second general protection devices 2 and 3 may open the circuit breaker 59. In this case, the tripping command is transmitted via the network 6, for example.

[0081] In this embodiment, the third general protection device 7 obtains a voltage value from a potential transformer 54 provided on the primary side of the 275 kV / 500 kV step-up transformer 51, and obtains a current value from current transformers 39, 53, and 63. The third general protection device 7 also obtains a current value from a current transformer 57 provided on the in-station load equipment side of the transformer, and obtains a voltage value from a potential transformer 58. The third general protection device 7 also obtains a current value from current transformers 60 and 64 provided on the secondary side of the 275 kV / 500 kV step-up transformer 51, and obtains a voltage value from a potential transformer 61.

[0082] Similarly to the second collective protection device 3, the third collective protection device 7 has a function of a digital current differential relay that detects whether there is any current leakage from the difference between the current value flowing into and out of the 275 kV power cable 62. In this example, the current leakage is found based on the values ​​of the current flowing through the primary side of the 275 kV / 500 kV step-up transformer 51, the current flowing through the station equipment side, the current flowing through the secondary side of the 275 kV / 500 kV step-up transformer 51, the current flowing through the secondary side of the 154 kV / 275 kV step-up transformer 11, and the current flowing through the station load equipment side.

[0083] In this embodiment, the section to be protected by the integrated protection system 1 is from the secondary side of the 66 kV / 154 kV step-up transformer 10 installed in the primary substation 70 to the circuit breaker 59 on the secondary side of the 275 kV / 500 kV step-up transformer 51 installed in the tertiary substation 90. This protected section includes the 154 kV power cable 12 connecting the primary substation 70 and the secondary substation 80, the 275 kV power cable 62 connecting the secondary substation 80 and the tertiary substation 90, the 154 kV / 275 kV step-up transformer 11, and the 275 kV / 500 kV step-up transformer 51, as well as various other facilities such as on-site load facilities in the secondary substation 80 and the tertiary substation 90.

[0084] FIG. 8 is an example of a block diagram of a third collective protection device in the second embodiment.

[0085] In this embodiment, the third general protection device 7 includes a third acquisition unit 310, a third detection unit 320, a third control unit 330, a third storage unit 340, and a third communication unit 350.

[0086] The third comprehensive protection device 7 can be realized, for example, by installing a program for the third comprehensive protection device 7 on a PC, similar to the first comprehensive protection device 2. The CPU in the third comprehensive protection device 7 executes the program for the third comprehensive protection device 7 to realize the functions of a third acquisition unit 310, a third detection unit 320, a third control unit 330, a third storage unit 340, and a third communication unit 350. The third storage unit 340 has an area for storing the acquired current value and voltage value, and is constructed, for example, on an auxiliary storage device on a HDD.

[0087] The third acquisition unit 310 acquires current values ​​and voltage values ​​from the above-mentioned current transformers 39, 53, 57, 60, 63, 64 and the instrument transformers 54, 58, 61. The acquired values ​​are stored in the third storage unit 340. The current values ​​and voltage values ​​stored in the third storage unit 340 are used for, for example, log analysis.

[0088] The third detection unit 320 detects the occurrence of an electrical accident based on the values ​​acquired by the third acquisition unit 310. In this example, the current leakage is obtained based on the values ​​of the current flowing through the primary side of the 275kV / 500kV step-up transformer 51, the current flowing through the secondary side, and the current flowing through the station equipment side, as well as the current flowing through the secondary side of the 154kV / 275kV step-up transformer 11 and the current flowing through the station load equipment side. For example, the third detection unit 320 calculates the difference between the current value flowing into and out of the 275kV power cable 62 from these values, and detects the occurrence of a current leakage if the difference is not zero. In addition, the third detection unit 320 compares the current value passing through the primary winding of the 275kV / 500kV step-up transformer 51 with the current value passing through the secondary winding, and detects the occurrence of an electrical accident when this ratio becomes equal to or exceeds a certain value.

[0089] When the third detection unit 320 detects the occurrence of an electrical accident, the third control unit 330 outputs a breaker command to open the circuit breaker 59. The circuit breaker 59 is opened by the breaker command.

[0090] The third communication unit 350 is connected to the second communication unit 250 of the second general protection device 3 via the network 6. The third acquisition unit 310 acquires current values ​​and voltage values ​​on the secondary side of the 154 kV / 275 kV step-up transformer 11 from the second general protection device 3 via the third communication unit 350, as well as current values ​​and voltage values ​​on the in-station load equipment side. The current values ​​and voltage values ​​related to the tertiary substation 90 acquired by the third acquisition unit 310 are transmitted to the second communication unit 250 of the second general protection device 3 via the third communication unit 350, and are used to detect the occurrence of an electrical accident in the second general protection device 3.

[0091] According to this embodiment, the collective protection system 1 broadly defines the protected section in the power generation and transformation system 100 that performs three-stage step-up, and collectively monitors electrical accidents in that section, thereby enabling a significant reduction in equipment installation costs. For example, by using the collective protection system 1, the tertiary substation 90 can be configured to install a disconnecting switch 56 without providing a circuit breaker between the 275 kV / 500 kV step-up transformer 51 and the 275 kV power cable 62. The tertiary substation 90 can also be configured to install a disconnecting switch 55 without providing a circuit breaker between the shunt reactor 52 and the electric circuit. Omission of the installation of these circuit breakers enables cost reduction. Although there is a concern that the widening of the protected section will expand the range of the electrical accident, the collective protection system 1 can limit the electrical accident that occurs in the power generation and transformation system 100 to the tertiary substation 90, thereby preventing the accident from spreading to the power system.

[0092] Third embodiment FIG. 9 is a schematic configuration diagram of a power generation and transformation system in the third embodiment.

[0093] In this embodiment, unlike the first embodiment, a secondary substation 80 is provided with a 154kV / 500kV step-up transformer 68. The configuration other than the 154kV / 500kV step-up transformer 68 is similar to that of the first embodiment, and therefore description thereof will be omitted. The 154kV / 500kV step-up transformer 68 is an example of a second step-up transformer.

[0094] The second collective protection device 3 detects the occurrence of an electrical accident from the current and voltage values ​​at each measurement point and opens the circuit breaker 25. In this way, even if the transformation ratios of the transformers are different, the collective protection system 1 can isolate the power generation and transformation system 100 from the power system by issuing a disconnection command, thereby preventing the spread of the electrical accident.

[0095] According to this embodiment, in a power generation and transformation system 100 that performs two-stage boosting, even if the boost pattern at the secondary substation 80 is different, the comprehensive protection system 1 broadly defines the section to be protected and performs comprehensive monitoring of electrical accidents in that section, thereby enabling a significant reduction in equipment installation costs.

[0096] (Fourth embodiment) FIG. 10 is a hardware configuration diagram of the first collective protection device in the fourth embodiment.

[0097] 10 includes a processor 452 such as a CPU, a main memory device 453 such as a RAM, an auxiliary memory device 454 such as a HDD, a network interface 455 such as a LAN (Local Area Network) board, a device interface 456 such as a memory slot or a memory port, and a bus 457 for connecting these devices to each other. The first overall protection device 2 is, for example, a computer such as a PC, and includes external input devices such as a keyboard and a mouse, and an output device such as an LCD (Liquid Crystal Display) monitor.

[0098] In this embodiment, a program for causing a computer to execute information processing of the first comprehensive protection device 2 is installed in the auxiliary storage device 454. The first comprehensive protection device 2 loads this program into the main storage device 453 and executes it with the processor 452. As a result, the functions of the first acquisition unit 110, the first detection unit 120, the first control unit 130, the first communication unit 150, and the first storage unit 140 shown in FIG. 4 are realized in the first comprehensive protection device 2, and comprehensive protection of the power equipment described in the first to third embodiments is possible. Note that data generated by this information processing is temporarily held in the main storage device 453 or stored and saved in the auxiliary storage device 454.

[0099] Moreover, the first storage unit 140 is constructed on the auxiliary storage device 454. The above-mentioned thresholds are stored in the auxiliary storage device 454. The thresholds are loaded into the main storage device 453 when this program is executed.

[0100] The first general protection device 2 and the second general protection device 3 are connected to the network 6 via a network interface 455. The first general protection device 2 controls the network interface 455 by the first communication unit 150 and receives the current value and voltage value acquired by the second general protection device 3.

[0101] The program for the first collective protection device 2 can be installed, for example, by mounting an external device 458 on which the program is recorded to the device interface 456 and storing the program from the external device 458 in the auxiliary storage device 454. Examples of the external device 458 are a computer-readable recording medium and a recording device incorporating such a recording medium. Examples of the recording medium are a CD-ROM (Compact Disk Read Only Memory), a CD-R (Compact Disk Recordable), a flexible disk, a DVD-ROM (Digital Versatile Disk Read Only Memory), and a DVD-R (Digital Versatile Disk Recordable), and an example of the recording device is a HDD. Also, the program can be installed, for example, by downloading the program via the network interface 455.

[0102] According to this embodiment, the functions of the first general protection device 2 in the first to third embodiments can be realized by software. The second general protection device 3 or the third general protection device 7 can be realized in the same manner.

[0103] Although several embodiments have been described above, these embodiments are presented only as examples and are not intended to limit the scope of the invention. The novel first collective protection device 2 described in this specification can be embodied in various other forms. In addition, various omissions, substitutions, and modifications can be made to the form of the first collective protection device 2 described in this specification without departing from the gist of the invention. The appended claims and their equivalents are intended to include such forms and modifications that fall within the scope and gist of the invention. [Explanation of symbols]

[0104] 1: collective protection system, 2: first collective protection device, 3: second collective protection device, 4: communication device, 5: communication device, 6: network, 7: third centralized protection device, 8: Communication equipment, 9: Communication equipment, 10: 66kV / 154kV step-up transformer, 11: 154kV / 275kV step-up transformer, 12: 154kV power cable, 13: shunt reactor, 14: shunt reactor, 22: circuit breaker, 23: circuit breaker, 24: circuit breaker, 25: circuit breaker, 26: disconnecting switch, 27: Disconnector, 28: Disconnector, 30: Current transformer, 31: Current transformer, 32: Current transformer, 33: current transformer, 34: current transformer, 35: current transformer, 36: current transformer, 37: current transformer, 38: current transformer, 39: current transformer, 40: potential transformer, 41: potential transformer, 42: Potential transformer, 43: Potential transformer, 44: Potential transformer, 46: Circuit breaker, 47: Circuit breaker, 48: Circuit breaker, 50: Offshore wind turbine, 51: 275kV / 500kV step-up transformer, 52: Shunt reactor, 53: Current transformer, 54: Potential transformer, 55: Disconnector, 56: Disconnector, 57: current transformer, 58: voltage transformer, 59: circuit breaker, 60: current transformer, 61: Potential transformer, 62: 275kV power cable, 63: Current transformer, 64: Current transformer, 65: Shunt reactor, 66: Disconnector, 67: Current transformer, 68: 154kV / 500kV step-up transformer, 70: Primary substation, 80: Secondary substation, 90: Tertiary substation, 100: Power generation and transformation system, 110: first acquisition unit, 120: first detection unit, 130: first control unit, 140: first storage unit, 150: first communication unit, 210: second acquisition unit, 220: second detection unit, 230: second control unit, 240: second storage unit, 250: second communication unit, 310: third acquisition unit, 320: third detection unit, 330: third control unit, 340: third storage unit, 350: third communication unit 452: processor, 453: main memory device, 454: auxiliary memory device, 455: Network interface, 456: Device interface, 457: Bus, 458: External device

Claims

1. a second acquisition unit that acquires current values ​​on the secondary side and on the station load equipment side of the first step-up transformer and current values ​​on the primary side, secondary side, and on the station load equipment side of the second step-up transformer; a second detection unit that detects the occurrence of an electrical accident based on the current value acquired by the second acquisition unit; and a second control unit that outputs a shutoff command when the second detection unit detects an electrical accident. a second collective protection device; The second detection unit is detecting an electrical fault in the power transmission line between the first step-up transformer and the second step-up transformer based on a difference between a current value flowing into and a current value flowing out of the power transmission line between the first step-up transformer and the second step-up transformer; detecting an electrical fault in the second step-up transformer based on a ratio between a current value passing through a primary winding and a current value passing through a secondary winding of the second step-up transformer; Comprehensive protection system.

2. a first acquisition unit that acquires current values ​​on a secondary side and on a station load equipment side of the first step-up transformer and current values ​​on a primary side, a secondary side, and on a station load equipment side of the second step-up transformer; a first detection unit that detects the occurrence of an electrical accident based on the current value acquired by the first acquisition unit; and a first control unit that outputs a shutoff command when the first detection unit detects an electrical accident. Further comprising a first collective protection device; The first detection unit is detecting an electrical fault in the power transmission line between the first step-up transformer and the second step-up transformer based on a difference between a current value flowing into and a current value flowing out of the power transmission line between the first step-up transformer and the second step-up transformer; The collective protection system according to claim 1 .

3. The integrated protection system includes: a third acquisition unit that acquires current values ​​on the secondary side and on the station load equipment side of the second step-up transformer and current values ​​on the primary side, secondary side, and on the station load equipment side of a third step-up transformer; a third detection unit that detects the occurrence of an electrical accident based on the current value acquired by the third acquisition unit; and a third control unit that outputs a shutoff command when the third detection unit detects an electrical accident. a third collective protection device; The third detection unit is detecting an electrical fault in the power transmission line between the second step-up transformer and the third step-up transformer based on a difference between a current value flowing into and a current value flowing out of the power transmission line between the second step-up transformer and the third step-up transformer; detecting an electrical fault in the third step-up transformer based on a ratio between a current value passing through a primary winding and a current value passing through a secondary winding of the third step-up transformer; The collective protection system according to claim 2 .

4. The collective protection system according to claim 1 , wherein the second control unit outputs a breaker command to open a breaker on a secondary side of the second step-up transformer.

5. The collective protection system according to claim 2 , wherein the first control unit outputs a breaker command to open a breaker on a secondary side of the first step-up transformer.

6. The collective protection system according to claim 3 , wherein the third control unit outputs a breaker command to open a breaker on a secondary side of the third step-up transformer.

7. The central protection system according to claim 1 , wherein the primary side current value in the second step-up transformer includes a current value at a ground point of the second step-up transformer which is grounded by a direct grounding method.

8. 4. The central protection system according to claim 3, wherein the primary side current value in the third step-up transformer includes a current value at a ground point of the third step-up transformer which is grounded by a direct grounding method.

9. The collective protection system according to claim 1 , wherein no circuit breaker is provided between the second step-up transformer and the power transmission line.

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