Resonance suppression device, power transmission system, and resonance suppression method

Abstract

To provide a resonance suppression device, a power transmission system, and a resonance suppression method capable of suppressing an increase in harmonic voltage expansion multiple and suppressing harmonic interference when AC cable power transmission is used.SOLUTION: An AC power transmission system according to an embodiment includes a first transformer connected to a power supply of a power system and transforming a voltage to supply power to the power supply, and a first AC cable connected to the first transformer. In the AC power transmission system, a harmonic resonance suppression device includes a first capacitor and a setting unit. The variable capacity capacitor is connected to the AC power transmission system. The setting unit sets the capacitance of the first capacitor to a value such that the resonant frequency in the AC power transmission system does not resonate with any of the third, fifth, and seventh odd harmonics.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] FIELD Embodiments of the present invention relate to a resonance suppression device, a power transmission system, and a resonance suppression method. [Background technology]

[0002] It is known that long-distance AC cable transmission is often used for offshore wind power generation. In such transmission systems using long-distance AC cables, the large capacitance of the cables is known to cause harmonic resonance and harmonic overvoltages.

[0003] Generally, the harmonics that occur in AC transmission systems and cause harmonic overvoltages are mainly odd-order harmonics such as the third, fifth, and seventh harmonics. When such harmonic overvoltages occur, there is a risk of overheating of cables, increased noise in transformers, overheating damage to iron cores, and harmonic interference in the loads within the power plant. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Patent No. 5814716 [Patent Document 2] Patent No. 6786423 Summary of the Invention [Problem to be solved by the invention]

[0005] However, if the inductance L is increased to avoid resonance with odd harmonics, there is a risk that the voltage magnification factor will increase.

[0006] The present embodiment has been made in consideration of such problems, and provides a resonance suppression device, a power transmission system, and a resonance suppression method that can suppress an increase in the voltage amplification factor and suppress harmonic interference when AC cable transmission is used. [Means for solving the problem]

[0007] According to this embodiment, in an AC power transmission system including a first transformer connected to a power source of a power system and transforming a voltage to supply power to the power source, and a first AC cable connected to the first transformer, a harmonic resonance suppression device includes a first capacitor and a setting unit. The first capacitor is connected to the AC power transmission system. The setting unit sets the capacitance of the first capacitor to a value that causes the resonant frequency in the AC power transmission system to not resonate with any of the third, fifth, and seventh odd-order harmonics. [Effects of the Invention]

[0008] According to this embodiment, it is possible to suppress an increase in the voltage amplification factor and also to suppress harmonic interference when AC cable power transmission is used. [Brief explanation of the drawings]

[0009] [Figure 1] FIG. 1 is a single-line diagram of an example of a power transmission system having a resonance avoidance unit. [Figure 2] FIG. 2 is a diagram showing a configuration example of a resonance avoidance unit. [Figure 3] FIG. 2 is a block diagram showing an example of the configuration of a calculation device. [Figure 4] FIG. 10 is a diagram showing an example in which the resonance avoidance unit is disposed in a second substation 4. [Figure 5] FIG. 10 is a diagram showing an example of a frequency and a voltage amplification factor when a resonance avoidance unit is not provided. [Figure 6] FIG. 10 is a diagram showing an example in which the resonance frequency is shifted by the resonance avoidance unit. [Figure 7] A diagram showing the voltage magnification factor when a capacitor or series reactor is installed. [Figure 8] FIG. 10 is a diagram showing an example of the configuration of a resonance avoidance unit according to a second embodiment. [Figure 9] FIG. 10 is a diagram showing an example of the configuration of a resonance avoidance unit according to a third embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0010] Hereinafter, a resonance suppression device, a power transmission system, and a resonance suppression method according to embodiments of the present invention will be described in detail with reference to the drawings. Note that the embodiments described below are examples of embodiments of the present invention, and the present invention should not be interpreted as being limited to these embodiments. Furthermore, in the drawings referred to in this embodiment, identical parts or parts having similar functions are given the same or similar reference numerals, and repeated explanations thereof may be omitted. Furthermore, for convenience of explanation, the dimensional ratios of the drawings may differ from the actual ratios, and some components may be omitted from the drawings.

[0011] (First embodiment) FIG. 1 is a single-line diagram of an example of a power transmission system 100 having a resonance avoidance unit 13 according to a first embodiment. The power transmission system 100 is an example of an offshore wind power generation facility and includes an AC power transmission system and multiple resonance avoidance units 13. Specifically, the AC power transmission system includes a first substation 2 that boosts an intermediate voltage to a system voltage to supply power to a power source 1 of the power system, multiple first transformers 3 provided in the first substation 2, a second substation 4, multiple second transformers 5 provided in the second substation 4, a wind power plant 6, multiple wind turbine transformers 7 and multiple wind turbine generators 8 provided in the wind power plant 6, multiple first cables 9, multiple second cables 10, a first voltage transformer 11, and a first harmonic voltage detector 12. The resonance avoidance unit 13 according to this embodiment corresponds to a resonance suppression device.

[0012] Furthermore, an inductance 14 corresponding to the short-circuit capacity of the power supply system 1 exists between the power supply 1 of the power system and the first substation 2. The resonance avoidance unit 13 will be described in detail later.

[0013] As shown in FIG. 1, in a first substation 2, power transmitted from a plurality of first cables 9 is boosted by a corresponding plurality of first transformers 3, and the power is transmitted to a power source 1 of the power system via a plurality of circuit breakers S10 provided on the high-voltage side circuits of the transformers 3 and a high-voltage side bus L10. Each of the plurality of circuit breakers S10 can be independently connected or disconnected. For this reason, it is possible to set the circuit breaker S10 corresponding to a first transformer 3 used for voltage transformation among the plurality of first transformers 3 to a connected state (on state), and to set the circuit breaker S10 corresponding to a first transformer 3 not in use to a disconnected state (off state). Information on these connection states is supplied to a resonance avoidance unit 13. Note that a load that is a source of harmonics is also connected to the power system.

[0014] The second substation 4 boosts the first wind turbine output voltage to an intermediate voltage using a plurality of second transformers 5. The intermediate voltage power boosted by the second substation 4 is transmitted to the first substation 2 via a plurality of first cables 9. The first cables 9 are, for example, underground cables, and are AC cables of 10 kilometers or more in length. These first cables 9 can also be installed as undersea cables. Note that in this embodiment, boosting or lowering the voltage by either the transformer 3 or the transformer 5 is referred to as "transformation." That is, in this embodiment, the case where the transformers 3 and the transformer 5 are used for boosting the voltage will be described, but this is not limited to this and the case where the transformers 3 and the transformer 5 are used for stepping down the voltage is also included.

[0015] The power of the first wind turbine output voltage is transmitted to a second transformer 5 via a low-voltage side bus L12 of a second substation 4.

[0016] The power of the first wind turbine output voltage is transmitted to the low-voltage side bus L12 via a plurality of second cables 10 and a plurality of circuit breakers S12 provided between the second cables and the low-voltage side bus L12. Each of the plurality of circuit breakers S12 is independently in a connected or disconnected state. For this reason, it is possible to set the circuit breaker S12 corresponding to the second cable 10 to be used for power transmission among the plurality of second cables 10 to a connected state (on state), and the circuit breaker S12 corresponding to the second cable 10 that is not used to a disconnected state (off state). Information on these connection states is supplied to the resonance avoidance unit 13. These second cables 10 can be arranged as, for example, submarine cables or underground cables.

[0017] In this way, in the AC power transmission system of this embodiment, the form of the connection system changes depending on the connection states of the circuit breakers S10 and S12. As described above, information on these changes in the form of the connection system is supplied to the resonance avoidance unit 13.

[0018] The wind power plant 6 uses multiple wind turbine transformers 7 to step up the second wind turbine output voltage to the first wind turbine output voltage and transmits the power to the second substation 4 via the second cable 10. Multiple wind turbine generators 8 transmit power to corresponding wind turbine transformers 7 among the multiple wind turbine transformers 7. Note that there are cases where multiple wind turbine transformers 7 and multiple wind turbine generators 8 are connected to one line of the second cable 10.

[0019] The first potential transformer 11 steps down the voltage at the power receiving terminal (connection point) n10 of the first substation 2. The first harmonic voltage detector 12 detects the voltage of a predetermined harmonic of the stepped-down voltage.

[0020] Fig. 2 is a diagram showing an example of the configuration of the resonance avoidance unit 13. As shown in Fig. 2, the resonance avoidance unit 13 has a plurality of capacitors 15, a plurality of switches 16, a second potential transformer 17, a second harmonic voltage detector 18, and a calculation device 19. Note that the calculation device 19 according to this embodiment corresponds to the setting unit. Furthermore, the plurality of capacitors 15 and the plurality of switches 16 according to this embodiment correspond to first capacitors whose capacitances are variable. Furthermore, the capacitor 15 corresponds to the second capacitor.

[0021] The capacitor 15 may include a discharge resistor or a discharge coil for discharging the charge when the capacitor 15 is disconnected from the circuit. However, if a shunt reactor is connected in parallel to the capacitor 15, the discharge resistor or the discharge coil may be omitted.

[0022] One end of the switch 16 is connected to the low-voltage side output terminal n12 of the first transformer 3, and the other end is connected to one end of the capacitor 15. The switch 16 is placed in a connected state (on state) or a disconnected state (off state) under the control of the computing device 19. In this way, the resonance avoidance unit 13 can change the capacitance of the power receiving terminal n10 under the control of the computing device 19.

[0023] The second potential transformer 17 steps down the voltage at the low-voltage side output terminal n12 of the first transformer 3. The second harmonic voltage detector 18 detects the voltage of a predetermined harmonic of the stepped-down voltage.

[0024] 3 is a block diagram showing an example of the configuration of the arithmetic device 19. The arithmetic device 19 is, for example, a monitoring control panel capable of calculations, and is configured to include a CPU. The arithmetic device 19 has an acquisition unit 90, a first determination unit 101, a second determination unit 102, a circuit configuration determination unit 103, a capacitor connection number determination unit 104, and a capacitor connection command generation unit 105.

[0025] 3, the acquisition unit 90 is, for example, an input interface (I / F), and acquires operation signals from the first harmonic voltage detector 12 and the second harmonic voltage detector 18, and status signals from the plurality of circuit breakers S10 and the plurality of circuit breakers S12. Although multiple acquisition units 90 are shown in FIG. 3, the acquisition unit 90 may be configured with a single acquisition unit 90.

[0026] When the operation signal is input, the first determination unit 101 determines which first transformer 3 is in use. That is, the first determination unit 101 determines which first transformer 3 is in use according to the on / off information included in the status signals from the plurality of circuit breakers S10.

[0027] When the operation signal is input, the second determination unit 102 determines the second cable 10 that is being used. That is, the second determination unit 102 determines the second cable 10 that is being used according to the on / off information included in the status signals from the plurality of circuit breakers S12.

[0028] The circuit configuration determination unit 103 acquires information on the first transformer 3 being used from the first determination unit 101, and acquires information on the second cable 10 being used from the second determination unit 102. The circuit configuration determination unit 103 acquires information on the load and inductance 14 of the power system in advance.

[0029] This enables the circuit configuration determination unit 103 to generate a circuit configuration for the power transmission system 100. That is, the circuit configuration determination unit 103 can generate a circuit configuration corresponding to the connection topology of the AC power transmission system. More specifically, the circuit configuration determination unit 103 stores in advance the resistances Rn, capacitances Cn, and inductances Ln of the first transformer 3, the second transformer 5, the first cable 9, and the second cable 10, where n is a natural number corresponding to each component. This enables the circuit configuration determination unit 103 to calculate the resonant frequency of the circuit configuration in the power transmission system 100 and the capacitance of the capacitor 15 required to lower the resonant frequency.

[0030] Note that, although the present embodiment illustrates a configuration example based on a single-line diagram, the present invention is not limited to this. For example, a configuration may be adopted in which the first cable 9 is connected to each of the multiple first transformers 3. In such a case, even if one of the multiple first transformers 3 is stopped for inspection, power transmission via the multiple first cables 9 is possible. In this case, the circuit configuration determination unit 103 acquires information on possible circuit configurations in advance, thereby being able to calculate the resonant frequency in the possible circuit configurations of the power transmission system 100 and the capacitance of the capacitor 15 required to lower the resonant frequency.

[0031] The capacitor connection command generating unit 105 generates operation signals for the plurality of switches 16 so that the capacitance matches the capacitor capacitance calculated by the circuit configuration determining unit 103. The operation signals control the connection state and non-connection state of each of the plurality of switches 16.

[0032] The resonance avoidance unit 13 can be placed at a position where it can contribute to changing the capacitance in the circuit configuration of the power transmission system 100. For this reason, for example, it may be placed in the second substation 4 as shown in Fig. 4. Fig. 4 is a diagram showing an example in which the resonance avoidance unit 13 is placed in the second substation 4.

[0033] 4, one end of the switch 16 is connected to the low-voltage side output terminal n14 of the second transformer 5, and the other end is connected to one end of the capacitor 15. Furthermore, the placement of the resonance avoidance unit 13 is not limited to one location, and it may be placed, for example, in both the first substation 2 (see FIG. 1) and the second substation 4 (see FIG. 4).

[0034] [Effect 1-1] When the capacitance of the capacitor calculated by the circuit configuration determining unit 103 is C and the inductance is L, the resonance frequency f is expressed by (Equation 1).

number

[0035] For example, assume that the first cable 9 is an extremely long cable of 40 km. In this case, the first cable 9 has the greatest effect on the resonant frequency and the voltage amplification factor.

[0036] For this reason, in the following explanation, the operation and effect of the present invention will be explained with a focus on the circuit between the power source 1 and the first cable 9 of the power system. Figure 5 is a diagram showing an example of the frequency and voltage magnification factor at the power receiving terminal (interconnection point) n10 (see Figure 1) when the resonance avoidance unit 13 is not provided. The horizontal axis represents the frequency, and the vertical axis represents the voltage magnification factor α. The voltage magnification factor α10 is when the short-circuit capacity is minimum, and the voltage magnification factor α12 is when the short-circuit capacity is maximum. Information regarding the minimum and maximum values ​​of the short-circuit capacity is generally notified in advance from the system side.

[0037] The system constants used to calculate the voltage magnification factor α10 and the voltage magnification factor α12 are as follows: (System Constants) (1) Power supply system: Voltage 275kV, maximum impedance 32mH, minimum impedance 16mH, rated frequency 50Hz (2) First transformer: High voltage side 275kV, low voltage side 154kV, short circuit impedance: 14% (3) Length of the first cable: 40 km Specifications of the first cable: 154kV, cross-linked polyethylene cable, 600mm2, 3-core, underground cable (4) Capacitance of the first 40km cable: 7.5μF

[0038] As shown in Fig. 5, when the capacity of the resonance avoidance unit 13 is set to 0, the power supply capacity of the system changes from maximum to minimum, and the resonant frequency of the system may become fifth order (250 Hz). When the resonant frequency of the system becomes fifth order (250 Hz), resonance occurs.

[0039] For this reason, the resonance avoidance unit 13 sets the capacitor capacitance C in accordance with the capacitor capacitance C and inductance L calculated by the circuit configuration determination unit 103. FIG. 6 is a diagram showing an example in which the resonance frequency is shifted by the resonance avoidance unit 13. The horizontal axis represents frequency, and the vertical axis represents voltage magnification factor α. Voltage magnification factor α14 is when the short-circuit capacitance is minimum, and voltage magnification factor α16 is when the short-circuit capacitance is maximum. Note that this is an example in which a total of 10 μF of capacitor capacitance (75 Mvar in total) is connected to the resonance avoidance unit 13 in a system configuration that results in the state of FIG. 5.

[0040] As shown in Figure 6, the peaks of the voltage magnification factor α10 and the voltage magnification factor α12 are both shifted to lower order than the fifth-order resonant frequency (250 Hz) of the system by the resonance avoidance unit 13. As such, it can be seen that the peaks of the voltage magnification factor α14 and the voltage magnification factor α16 fall within the range of third to fifth order at both the maximum and minimum short-circuit capacity, and do not resonate with either the third or fifth order. This avoids resonance.

[0041] In this embodiment, the resonant frequency is shifted between the third and fifth harmonics, but is not limited thereto. For example, the resonant frequency may be shifted between the second and third harmonics, or between the fifth and seventh harmonics. In this way, by designing the capacitance of the capacitor 15 and the number of resonance avoidance units 13 so that the resonant frequency of the system does not resonate with odd-order harmonics such as the third, fifth, and seventh harmonics in all possible circuit configurations of the power transmission system 100, it becomes possible to automatically adjust the resonant frequency.

[0042] [Effect 1-2] In the following explanation, the action and effect of the present invention will be explained by focusing on the circuit between the power source 1 of the power system and the first cable 9 under the same conditions as in [Action 1-1]. The voltage amplification factor α is expressed by (Equation 2), and it is possible to reduce the voltage amplification factor α by increasing the capacitance C.

number

[0043] Voltage magnification factor α18 is the case where the short-circuit capacity by the resonance avoidance unit 13 is minimum, and voltage magnification factor α20 is the case where the short-circuit capacity by the resonance avoidance unit 13 is maximum. On the other hand, voltage magnification factor α22 is the case where the short-circuit capacity when a series reactor with the same resonant frequency is installed is minimum, and voltage magnification factor α24 is the case where the short-circuit capacity when a series reactor is installed is maximum.

[0044] As shown in (Equation 2), when comparing the case where the resonance avoidance unit 13 is connected (when the capacitance C is increased) with the case where a series reactor is installed (when the inductance L is increased), it can be seen that the voltage amplification factor α is smaller when the resonance avoidance unit 13 is used than when a series reactor is used, both at the maximum and minimum short-circuit capacity. Figure 7 also shows similar results. Since it is generally difficult to reduce the series inductance of a circuit, resonance can be avoided by installing a series reactor and increasing its inductance. In this case, the voltage amplification factor α increases, as shown in (Equation 2). On the other hand, when the capacitance C is increased, it is possible to reduce the voltage amplification factor α, as shown in (Equation 2).

[0045] In this way, when suppressing (avoiding) resonance, adjusting with capacitance C reduces the voltage magnification factor α compared to adjusting with inductance. As can be seen from this, by using resonance avoidance unit 13 made up of a capacitor, it is possible to suppress the increase in voltage magnification factor α and also to suppress harmonic interference when power is transmitted via first AC cable 9. This makes it possible to prevent overheating damage to cables 9 and 10 and transformers 3, 5, and 7 in the offshore wind power generation system, and to prevent the expansion of harmonic voltages at interconnection point n10 where the offshore wind power plant is connected to the power system.

[0046] [Effect 1] As described above, according to this embodiment, the capacitance of the power transmission system 100 is adjusted by the resonance avoidance unit 13. This makes it possible to shift the resonant frequency to a frequency that does not resonate with odd-order harmonics such as the third, fifth, and seventh harmonics while suppressing an increase in the voltage magnification factor α.

[0047] Furthermore, the resonance avoidance unit 13 acquires in advance values ​​of capacitance, inductance, resistance, etc. for each combination of circuit configurations of the power transmission system 100. This makes it possible to automatically set the capacitance of the capacitor 15 of the resonance avoidance unit 13 according to the circuit configuration of the power transmission system 100.

[0048] (Second embodiment) The power transmission system 100 according to the second embodiment differs from the power transmission system 100 according to the first embodiment in that the capacitance of the capacitor of the resonance avoidance unit 13 is different. The following describes the differences from the power transmission system 100 according to the first embodiment.

[0049] Fig. 8 is a diagram showing an example of the configuration of the resonance avoidance unit 13 according to the second embodiment. As shown in Fig. 8, the capacitances of the capacitors 151 to 15n of the resonance avoidance unit 13 are made different from one another. In this way, the multiple capacitors 15 provided do not need to have the same capacitance, and they can also be configured with capacitors of different capacitances so as to achieve a design appropriate for the configuration of the target equipment.

[0050] [Effect 2] Using a plurality of capacitors 15 and switches 16, it is possible to adjust the resonant frequency of the system in the same manner as in the first embodiment. When the resonant frequency is to be shifted to a lower order, the number of parallel-connected capacitors is increased, and when the resonant frequency is to be shifted to a higher order, the number of parallel-connected capacitors is decreased.

[0051] [Effect 2] As described above, according to this embodiment, it is possible to change the number of capacitors 15 connected in parallel to an appropriate number so that the resonant frequency of the system does not resonate with odd-order harmonics such as the third, fifth, and seventh harmonics.

[0052] (Third embodiment) The power transmission system 100 according to the second embodiment differs from the power transmission system 100 according to the first embodiment in that the resonance avoidance unit 13 further includes a shunt reactor 20 connected in parallel with the capacitor. The differences from the power transmission system 100 according to the first embodiment will be described below.

[0053] Fig. 9 is a diagram showing an example of the configuration of a resonance avoidance unit 13 according to the third embodiment. As shown in Fig. 9, the resonance avoidance unit 13 further includes a shunt reactor 20 connected in parallel to each capacitor 15. The multiple capacitors 15 do not need to have the same capacity, and can be configured with capacitors of different capacities so as to achieve a design appropriate for the configuration of the target facility.

[0054] Capacitor 15 includes a discharge resistor or discharge coil for the purpose of discharging charge when it is opened from the circuit. However, if a shunt reactor is connected in parallel to capacitor 15, the discharge resistor or discharge coil can be omitted. The inductance value of shunt reactor 20 is set to a value that does not cause voltage fluctuations when the number of capacitors 15 connected is changed.

[0055] [Action 3] By simultaneously changing the number of parallel connections of multiple capacitors 15 and shunt reactors 20 using the switch 16, it is possible to adjust the resonant frequency of the system as in the first and second embodiments, and also to suppress the voltage rise caused by the capacitor 15.

[0056] [Effect 3] As described above, according to this embodiment, it is possible to change the number of parallel connections of capacitors 15 to an appropriate number so that the resonant frequency of the grid does not resonate with odd-order harmonics such as the third, fifth, and seventh harmonics, and it is also possible to suppress voltage increases due to changes in the number of connections of capacitors 15. Furthermore, the capacitance of capacitor 15 is automatically adjusted, and unlike the conventional method of increasing inductance L, the voltage amplification factor can be reduced by using a resonance avoidance unit formed by capacitor 15.

[0057] [Other embodiments]

[0058] Although several embodiments of the present invention have been described, these embodiments are presented as examples and are not intended to limit the scope of the invention. These novel embodiments can be embodied in various other forms, and various omissions, substitutions, and modifications can be made without departing from the spirit of the invention. These embodiments and their modifications are included within the scope and spirit of the invention, and are also included in the scope of the invention and its equivalents as defined in the claims.

[0059] There is no limit to the number of resonance avoidance units 13 that can be installed. In addition to the first substation 2 and the second substation 4, the resonance avoidance units 13 can also be installed in the wind power plant 6 or in the tertiary circuit of the first transformer 3 or the second transformer 5.

[0060] 1 and 4, the first substation 2 and the second substation 4 are substations equipped with transformers, but they may be configured as switchyards without transformers. Also, the number of substations is not limited to two, and three or more substations may be used.

[0061] In Figures 1 and 4, the first cable 9 has two lines and the second cable 10 has eight lines, but the number of lines in the power cable is not limited to this, and the length of the power cable is not limited either. [Explanation of symbols]

[0062] 1: power supply of power system, 2: first substation, 3: first transformer, 4: second substation, 5: second transformer, 6: wind power plant, 7: wind turbine transformer, 8: wind turbine generator, 9: first cable, 10: second cable, 11: first voltage transformer, 12: first harmonic voltage detector, 13: resonance avoidance unit, 14: inductance corresponding to short circuit capacity of power system, 15: capacitor, 16: switch, 17: second voltage transformer, 1 8: second harmonic voltage detector, 19: calculation device (setting unit), 20: shunt reactor, 100: power transmission system, 101: first judgment unit, 102: second judgment unit, 103: circuit configuration judgment unit, 104: capacitor connection number determination unit, 105: capacitor connection command generation unit, 151 to 15n: capacitors, α, α12 to α24: voltage magnification multipliers, C, Cn: capacitance, f: resonance frequency, L10: high voltage side bus, L12: low voltage side bus, L, Ln: inductance, n10: receiving terminal, n12: output terminal, R, Rn: resistor, S10, S12: circuit breaker.

Claims

1. An AC power transmission system including: a first transformer connected to a power source of a power system and transforming a voltage to supply power to the power source; and a first AC cable connected to the first transformer, a first capacitor having an adjustable capacitance connected to the AC power transmission system; a setting unit that sets a capacitance of the first capacitor to a value that causes a resonant frequency in the AC power transmission system not to resonate with any of the third, fifth, and seventh odd-order harmonics; Equipped with the setting unit increases the capacitance of the first capacitor so that a voltage amplification factor of the resonant frequency is reduced. Harmonic resonance suppressor.

2. 2. The harmonic resonance suppression device according to claim 1, wherein the setting unit increases the capacitance of the first capacitor so that the resonant frequency shifts to a lower frequency side that does not resonate with any of the third, fifth, and seventh odd-order harmonics.

3. 3. The harmonic resonance suppression device according to claim 2, wherein the setting unit increases the capacitance of the first capacitor so as to shift the resonance frequency between the second and third harmonic frequencies or between the fifth and seventh harmonic frequencies.

4. the first capacitor has a plurality of second capacitors connected in parallel via a plurality of switches that electrically connect or disconnect the first capacitor, The harmonic resonance suppression device according to claim 1 , wherein the setting unit sets the capacitance of the first capacitor by controlling connection states of the plurality of switches.

5. 5. The harmonic resonance suppression device according to claim 4, further comprising a plurality of shunt reactors connected in parallel to the plurality of second capacitors, respectively, via a plurality of switches.

6. The harmonic resonance suppression device according to claim 4 , wherein the plurality of second capacitors have different capacitances.

7. The harmonic resonance suppression device according to claim 1 , wherein the setting unit changes the capacitance of the first capacitor in accordance with a change in the form of a connection system of the AC power transmission system.

8. the AC transmission system further includes a second transformer connected to the first AC cable, the second transformer stepping up a wind turbine output voltage to an intermediate voltage; 2. The harmonic resonance suppression device according to claim 1, wherein the first capacitor is connected to at least one of the low-voltage side of the first transformer and the low-voltage side of the second transformer.

9. an AC power transmission system including a first transformer connected to a power source of the power system and transforming voltage to supply power to the power source, and a first AC cable of 10 kilometers or more connected to the first transformer; a capacitor having an adjustable capacitance that is connected to the AC power transmission system; a setting unit that sets the capacitance of the capacitor to a value that causes a resonant frequency in the AC power transmission system not to resonate with any of the third, fifth, and seventh odd-order harmonics; Equipped with the setting unit increases the capacitance of the capacitor so that a voltage amplification factor of the resonant frequency is reduced. Power transmission system.

10. A harmonic resonance suppression method in an AC power transmission system including a first transformer connected to a power source of the power system and transforming a voltage to supply power to the power source, and a first AC cable connected to the first transformer, the method comprising: A harmonic resonance suppression method, comprising: increasing the capacitance of a capacitor having an adjustable capacitance connected to the AC power transmission system so that a resonant frequency in the AC power transmission system does not resonate with any of the third, fifth, and seventh odd-order harmonics, and so that a voltage magnification factor of the resonant frequency is reduced.

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