Method for compensating charging current of power cable and power generation system using power cable

Abstract

To provide a charging current compensation method for a power cable capable of reducing a cable size.SOLUTION: A charging current compensation method according to an embodiment compensates for a charging current of a first power cable that transmits power generated by a generator to a power system. With this charging current compensation method, by using a reactive power adjustment apparatus installed on a starting end side of the first power cable electrically connected to the power system, reactive power generated in the first power cable is adjusted, and the charging current is compensated by using a first shunt reactor that is installed on a terminal end side of the first power cable electrically connected to the generator and in which a compensation rate for the charging current is set to 50%.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] An embodiment of the present invention relates to a method for compensating for a charging current of a power cable and a power generation system using a power cable. [Background technology]

[0002] To utilize renewable energy, large-capacity offshore wind power generation systems are planned to transmit electricity over long distances, for example, several tens of kilometers to over 100 kilometers, using power cables. The transmission voltages used are, for example, ultra-high voltages of 187 to 275 kV or extra-high voltages of 66 to 154 kV. Such high-voltage, long-distance cable transmission poses several technical challenges related to charging current. These challenges include, for example, the generation of large reactive power that adversely affects the power system and the occurrence of voltage increases due to the Ferranti effect.

[0003] One known technique to solve the above technical problems is to connect a shunt reactor to the start, end, or both ends of a power cable to compensate for reactive power. Since the charging current flowing through the power cable has a 90° leading phase, the shunt reactor supplies a 90° lagging phase current. This reduces reactive power and suppresses voltage increases due to the Ferranti effect. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Unexamined Japanese Patent Publication No. 2015-80404 [Non-patent literature]

[0005] [Non-Patent Document 1] Wojciech Wiechowski et al, Power System Technical Performance Issues Related to the Application of Long HVAC Cables, CIGRE Technical Brochure 556, 2013,pp.13-23 [Non-patent document 2] Maria Zouraraki et al, Hornsea projects 1 and 2 - Design and Optimization of the Cables for the World Largest Offshore Wind Farms, 10th International Conference on insulated power Cables,2019 Summary of the Invention [Problem to be solved by the invention]

[0006] However, when the power cable is long, simply connecting a shunt reactor to the power cable is unlikely to be enough to reduce the charging current. Therefore, an excessively large power cable would be required. In this case, it is conceivable that the power cable cannot be manufactured in reality, making it difficult to realize power transmission. Moreover, even if it were possible to realize this, the cost of the power cable would be extremely high.

[0007] Therefore, an object of the present invention is to provide a method for compensating for the charging current of a power cable, which makes it possible to reduce the cable size, and a power generation system using the power cable. [Means for solving the problem]

[0008] A charging current compensation method according to one embodiment compensates for the charging current of a first power cable that transmits power generated by a generator to a power grid. This charging current compensation method uses a reactive power regulating device installed at the starting end of the first power cable electrically connected to the power grid to regulate the reactive power generated in the first power cable, and compensates for the charging current using a first shunt reactor installed at the ending end of the first power cable electrically connected to the generator, with a compensation rate for the charging current set to 50%. [Effects of the Invention]

[0009] According to this embodiment, it is possible to reduce the cable size. [Brief explanation of the drawings]

[0010] [Figure 1] 1 is a wiring diagram showing a schematic configuration of a power generation system according to a first embodiment. [Figure 2] FIG. 10 is a diagram showing an example of the relationship between the current flowing through a cable and the required cable size. [Figure 3] FIG. 10 is a diagram showing the distribution of charging current in a first power cable in Comparative Example 1. [Figure 4] FIG. 10 is a diagram showing the distribution of charging current in a first power cable in Comparative Example 2. [Figure 5] FIG. 10 is a diagram showing the distribution of charging current in a first power cable in Comparative Example 3. [Figure 6] FIG. 10 is a diagram showing the distribution of charging current in a first power cable in Comparative Example 4. [Figure 7] FIG. 10 is a diagram showing the distribution of the charging current in the first power cable when the sum of the compensation rates of the first shunt reactor and the second shunt reactor is 60%. [Figure 8] FIG. 10 is a diagram showing the distribution of the charging current in the first power cable when the sum of the compensation rates of the first shunt reactor and the second shunt reactor is 75%. [Figure 9]FIG. 10 is a diagram showing the distribution of the charging current in the first power cable when the sum of the compensation rates of the first shunt reactor and the second shunt reactor is 90%. [Figure 10] FIG. 10 is a diagram showing the relationship between transmission power and required cable size for the first power cable. [Figure 11] FIG. 10 is a wiring diagram showing a schematic configuration of a power generation system according to a fifth comparative example. [Figure 12] FIG. 10 is a wiring diagram showing a schematic configuration of a power generation system according to a third embodiment. [Figure 13] This is a three-wire wiring diagram inside the first shunt reactor with tap. [Figure 14] FIG. 10 is a wiring diagram showing a schematic configuration of a power generation system according to a second modification. [Figure 15] FIG. 10 is a diagram showing the distribution of charging current in a first power cable in Modification 2. DETAILED DESCRIPTION OF THE INVENTION

[0011] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings. The present invention is not limited to the following embodiments.

[0012] (First embodiment) 1 is a wiring diagram showing a schematic configuration of a power generation system according to a first embodiment. The power generation system 1 according to this embodiment is an example of an offshore wind power long-distance cable power generation system using a first power cable 10 and a second power cable 20. The power generation system 1 includes the first power cable 10, a generator 11, a step-up substation 12, an interconnection substation 13, the second power cable 20, and a bus bar 30.

[0013] For example, a wind power generator that generates electricity by rotating a wind turbine installed on the ocean can be applied as the generator 11. Note that the generator 11 is not limited to a wind power generator, and may be, for example, a solar power generator that generates electricity using solar panels installed on the ocean.

[0014] 1 illustrates 20 generators 11, but the number of generators 11 is not particularly limited. Furthermore, although four generators 11 are connected to one second power cable 20, the number of generators 11 connected to one second power cable 20 and the number of second power cables 20 are not particularly limited.

[0015] The electric power generated by the multiple generators 11 is first collected in the multiple second power cables 20, and then collected from each second power cable 20 to a bus bar 30. The electric power collected in the bus bar 30 is supplied to a step-up substation 12.

[0016] The step-up substation 12 has a first shunt reactor 121 and a step-up transformer 122. One end of the first shunt reactor 121 is connected to the end of the first power cable 10. The other end of the first shunt reactor 121 is grounded. In the first shunt reactor 121, the compensation rate of the reactive current flowing through the first power cable 10 is set to 50%.

[0017] One end of the step-up transformer 122 is connected to the bus 30. Meanwhile, the other end of the step-up transformer 122 is connected to the end of the first power cable 10. The step-up transformer 122 steps up the voltage value of the power supplied from the bus 30 to a voltage value suitable for transmission through the first power cable 10. The stepped-up power is transmitted through the first power cable 10 and supplied to the interconnection substation 13.

[0018] The interconnection substation 13 has a second shunt reactor 131 and an interconnection transformer 132. The second shunt reactor 131 is an example of a reactive power adjustment device that adjusts the reactive power generated in the first power cable 10. One end of the second shunt reactor 131 is connected to the starting end of the first power cable 10. Meanwhile, the other end of the second shunt reactor 131 is grounded. The compensation rate of the reactive current of the second shunt reactor 131 is set to a value lower than the compensation rate of the first shunt reactor 121.

[0019] One end of the interconnection transformer 132 is connected to the starting end of the first power cable 10. Meanwhile, the other end of the interconnection transformer 132 is connected to the power grid 40 of the power company. The interconnection transformer 132 transforms the power transmitted through the first power cable 10. The transformed power is supplied to the power grid 40.

[0020] In the power generation system 1 configured as above, for example, the following system condition 1 is assumed. (System condition 1) (1) Transmission power: The total power generation capacity of Generator 11 is 260MW. (2) Transmission voltage of first power cable 10: 275 kV (3) Length of the first power cable 10: 100 km (4) Type of first power cable 10: cross-linked polyethylene cable, copper conductor, 1200 mm 2 , 3 hearts, (5) Capacitance of the first power cable 10: 0.149 μF / km (6) Frequency: 50Hz

[0021] A large-capacity offshore wind power generation system is characterized by the installation of a large number of generators 11, with a total transmission capacity ranging from 100 MW to over 1000 MW, equivalent to that of a nuclear power plant. Therefore, the cross-sectional area of ​​the first power cable 10 used is several hundred mm. 2 to several thousand mm 2 The main features of these waves are that they become larger and longer, with lengths ranging from several tens of kilometers to 100 kilometers.

[0022] Depending on the location, the first power cable 10 may be a submarine cable or a land cable. In either case, the charging current becomes large as the cable becomes long distance, and at distances of several tens of kilometers, it exceeds the current corresponding to the generated power. For example, in the case of system condition 1, the active current is 546 A, while the charging current, which is the reactive current, is 948 A. In this case, the charging current is 1.74 times the active current. Furthermore, the combined current of the active current and the cable charging current is 1094 A, which is approximately twice the active current.

[0023] The cross-sectional area of ​​the first power cable 10 used to transmit the power generated by the generator 11 to the power grid 40 needs to be at least the square of the current, so the cross-sectional area needs to be at least 2.5 times the size capable of carrying the active current, i.e., at least four times the size. Furthermore, due to the skin effect and proximity effect of the first power cable 10, the cross-sectional area that is actually required may be significantly more than four times the size.

[0024] Fig. 2 is a diagram showing an example of the relationship between the current flowing in a cable and the required cable size. In Fig. 2, the current on the horizontal axis represents the current flowing in the first power cable 10, and the cable size on the vertical axis represents the cross-sectional area of ​​the first power cable 10 required to carry that current.

[0025] As shown in Figure 2, if the current increases by approximately 10% from 900 A to 1000 A, the cable size must be increased by 43%. In other words, if the current can be reduced by 10% from 1000 A to 900 A, the cable size can be reduced by 43%.

[0026] Furthermore, if the current increases by approximately 20% from 800A to 1000A, the cable size must be increased by 67%. In other words, if the current can be reduced by 20% from 1000A to 800A, the cable size can be reduced by 67%. Despite the current reduction being only 20%, the cable size reduction is 67%, which is more than three times the effect. This is because a cable has a characteristic that its size increases rapidly in the high current range.

[0027] In the case of the above system condition 1, the combined current, i.e., the current flowing through the first power cable 10, is about 1000 A. In this case, as shown in FIG. 2, the required cable size is 5000 mm. 2In reality, cables with such a cross-sectional area are not manufactured, so it is difficult to transmit the power of the generator 11 to the power grid 40. Even if it were possible to achieve this, the cost of the first power cable 10 would be extremely high. Therefore, for example, it would be necessary to connect several first power cables 10 in parallel instead of one first power cable 10. This would also increase the cost.

[0028] Therefore, reducing the charging current of the first power cable 10 contributes greatly to reducing the cost of the first power cable 10. Furthermore, the size of the first power cable 10 can be made within a manufacturable cable size range. In this embodiment, the first shunt reactor 121 compensates for 50% of the charging current of the first power cable 10, and the combined current with the load current can be significantly reduced. This makes it possible to significantly reduce a sudden increase in cable size in the large current range.

[0029] Here, we will explain the compensation rate of the charging current of the first power cable 10. The charging capacity Q of the first power cable 10 C1 , charging current I C1 , and the compensation rate H1 of the charging current are expressed by the following formulas (1) to (3), respectively.

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[0030] Fig. 3 is a diagram showing the distribution of charging current in the first power cable 10 in Comparative Example 1. In Comparative Example 1, no shunt reactor is connected to either end of the first power cable 10. Note that the current flowing through the first power cable 10 includes an active current corresponding to the active power and a reactive current corresponding to the reactive power, but in Fig. 3 and subsequent figures, the current distribution in the first power cable 10 shows only the reactive current.

[0031] 3, 100% of the charging current, i.e., the leading current whose phase is 90° ahead of the active current, flows into the first power cable 10 on the interconnection transformer 132 side, i.e., the starting end of the first power cable 10. On the other hand, the leading current becomes zero on the step-up transformer 122 side, i.e., the ending end of the first power cable 10. Because 100% of the charging current flows into the starting end of the first power cable 10, the reactive current, i.e., the charging current, flowing through the first power cable 10 is not reduced at all.

[0032] 4 is a diagram showing the distribution of charging current in the first power cable 10 in Comparative Example 2. In Comparative Example 2, the second shunt reactor 131 is connected to the starting end of the first power cable 10, but the first shunt reactor 121 is not connected to the ending end of the first power cable 10.

[0033] In Comparative Example 2, the second shunt reactor 131 compensates for 100% of the charging current to the first power cable 10. Therefore, the reactive current flowing through the power grid 40 and the interconnection transformer 132 is zero. However, 100% of the charging current flows on the interconnection transformer 132 side of the first power cable 10, i.e., at the starting end. On the other hand, the charging current is zero on the step-up transformer 122 side, i.e., at the end of the first power cable 10.

[0034] In Comparative Example 2, the leading current flowing in the power grid 40 and the interconnection transformer 132 is compensated for. That is, with this compensation method, leading reactive power flowing in the power grid 40 and the interconnection transformer 132 is eliminated. This eliminates reactive power that adversely affects the power grid 40, and makes it possible to suppress voltage increases due to the Ferranti effect. However, the reactive current flowing in the first power cable 10, i.e., the charging current, is not reduced, and therefore the cable size cannot be reduced.

[0035] 5 is a diagram showing the distribution of charging current in the first power cable 10 in Comparative Example 3. In Comparative Example 3, a first shunt reactor 121 is connected to the terminal end of the first power cable 10, but a second shunt reactor 131 is not connected to the starting end of the first power cable 10.

[0036] In Comparative Example 3, the first shunt reactor 121 compensates for 100% of the charging current to the first power cable 10. Therefore, as in Comparative Example 2, the reactive current flowing through the power grid 40 and the interconnection transformer 132 is zero. However, 100% of the lagging current flows on the step-up transformer 122 side of the first power cable 10, i.e., at the terminal end. Therefore, even with this compensation method, although the leading current flowing through the power grid 40 and the interconnection transformer 132 is compensated for, the reactive current flowing through the first power cable 10 is not reduced. In other words, although the reactive power that adversely affects the power grid 40 can be eliminated and the voltage rise due to the Ferranti effect can be suppressed, the cable size cannot be reduced.

[0037] 6 is a diagram showing the distribution of charging current in the first power cable 10 in Comparative Example 4. In Comparative Example 4, a first shunt reactor 121 is connected to the terminal end of the first power cable 10, and a second shunt reactor 131 is connected to the starting end of the first power cable 10. The compensation rates of the first shunt reactor 121 and the second shunt reactor 131 are each set to 30% of the charging capacity of the first power cable 10. That is, in Comparative Example 4, the compensation rate is set to 60%, which is the sum of the compensation rate of the first shunt reactor 121 and the compensation rate of the second shunt reactor 131.

[0038] In Comparative Example 4, as shown in FIG. 6, the distribution of leading and lagging currents, i.e., reactive current, is 70% leading at the start of the first power cable 10 and 30% lagging at the end of the first power cable 10. The cable size of the first power cable 10 is determined by the maximum current. Therefore, in Comparative Example 4, the first power cable 10 needs to have a cable size that can carry 70% of the charging current. This compensation method can reduce the reactive charging capacity compared to Comparative Examples 1, 2, and 3 described above. However, the effect of reducing the magnitude of the charging current flowing through the first power cable 10 is limited.

[0039] In contrast, in this embodiment, even if the sum of the compensation rate of the first shunt reactor 121 and the compensation rate of the second shunt reactor 131 is 60%, by setting the compensation rate of the first shunt reactor 121 to 50%, it is possible to further reduce the cable size of the first power cable 10 compared to Comparative Example 4. The reason for this will be explained below.

[0040] The compensation rate H1 (%) of the first power cable 10 is calculated by multiplying the capacitance Q of the first shunt reactor 121 by the equation (3) above. SR1 , the capacity Q of the second shunt reactor 131 SR2 , and the charging capacity Q of the first power cable 10 C1 Here, if the compensation rate of the first shunt reactor 121 is set to 50%, the capacitance Q of the first shunt reactor 121 can be expressed by modifying the above equation (3). SR1 , is expressed by the following equation (4), and the capacity Q of the second shunt reactor 131 is SR2 is expressed by the following equation (5).

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[0041] In this embodiment, the compensation rates are unequal between the first shunt reactor 121 and the second shunt reactor 131. In addition, since the compensation rate of the first shunt reactor 121 is set to 50%, the compensation rate of the second shunt reactor 131 is H1-50%.

[0042] 7 is a diagram showing the distribution of the charging current in the first power cable 10 when the sum of the compensation rates of the first shunt reactor 121 and the second shunt reactor 131 is 60%. In this embodiment, the compensation rate of the first shunt reactor 121 is set to 50%, so the compensation rate of the second shunt reactor 131 is 10% (=60%-50%).

[0043] Furthermore, the reactive current flowing through the power grid 40 and the interconnection transformer 132 is 40% (=100%-60%) of the charging current of the first power cable 10. Therefore, the compensation rate at the starting end of the first power cable 10 is 50% (=40%+10%). Meanwhile, the compensation rate at the ending end of the first power cable 10 is also 50%. As a result, in this embodiment, the first power cable 10 only needs to have a cable size that can carry 50% of the charging current. Therefore, in this embodiment, the cable size of the first power cable 10 can be significantly reduced compared to Comparative Example 4, which requires a cable size that is 70% of the charging current.

[0044] FIG. 8 is a diagram showing the distribution of the charging current in the first power cable 10 when the sum of the compensation rates of the first shunt reactor 121 and the second shunt reactor 131 is 75%.

[0045] In this embodiment, the compensation rate of the first shunt reactor 121 is set to 50%, and therefore the compensation rate of the second shunt reactor 131 is 25% (=75%-50%). Furthermore, the reactive current flowing through the power grid 40 and the interconnection transformer 132 is 25% (=100%-75%) of the charging current of the first power cable 10. Therefore, the compensation rate at the starting end of the first power cable 10 is 50% (=25%+25%). Meanwhile, the compensation rate at the ending end of the first power cable 10 is also 50%. Therefore, even if the total compensation rate is 75%, the first power cable 10 only needs to have a cable size that can pass 50% of the charging current.

[0046] In the above-described comparative example 4, the compensation rate of the first shunt reactor 121 and the compensation rate of the second shunt reactor 131 are equal to each other. Therefore, when the total compensation rate is 75%, the compensation rate of each shunt reactor is 37.5%. As a result, the compensation rate at the starting end of the first power cable 10 becomes 62.5% (25% + 37.5%). Therefore, even if the total compensation rate of the first shunt reactor 121 and the second shunt reactor 131 is 75%, the cable size of the first power cable 10 can be reduced more than in comparative example 4.

[0047] FIG. 9 is a diagram showing the distribution of the charging current in the first power cable 10 when the sum of the compensation rates of the first shunt reactor 121 and the second shunt reactor 131 is 90%.

[0048] In this embodiment, the compensation rate of the first shunt reactor 121 is set to 50%, and therefore the compensation rate of the second shunt reactor 131 is 40% (=90%-50%). Furthermore, the reactive current flowing through the power grid 40 and the interconnection transformer 132 is 10% (=100%-90%) of the charging current of the first power cable 10. Therefore, the compensation rate at the starting end of the first power cable 10 is 50% (=10%+40%). Meanwhile, the compensation rate at the ending end of the first power cable 10 is also 50%. Therefore, even if the total compensation rate is 90%, the first power cable 10 only needs to have a cable size that can pass 50% of the charging current.

[0049] In the above-described comparative example 4, the compensation rate of the first shunt reactor 121 and the compensation rate of the second shunt reactor 131 are each 45%. As a result, the compensation rate at the starting end of the first power cable 10 is 55% (10% + 45%). Therefore, even if the total compensation rate of the first shunt reactor 121 and the second shunt reactor 131 is 90%, the cable size of the first power cable 10 can be reduced more than in comparative example 4.

[0050] According to the above-described embodiment, the reactive current flowing through the first power cable 10 can be reduced to 50%, regardless of the compensation rate of the first power cable 10. That is, the reactive current flowing through the first power cable 10 can be calculated by the following equation (6).

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[0051] This current I Q-max indicates the magnitude of the current flowing at the start end and the end end of the first power cable 10, and the same magnitude of current flows at the start end and the end end. As shown in Figures 7 to 9, the phase of the current flowing at the start end is the opposite phase to the phase of the current flowing at the end end, that is, the current at the start end is leading by 90° and the current at the end end is lagging by 90°.

[0052] Current I at midpoint x in the first power cable 10 Q-x is expressed by the following equation (7).

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[0053] A negative sign indicates a 90° lagging current, and a positive sign indicates a 90° leading current. The magnitude and sign of the reactive current at any position vary depending on the position, as shown in FIGS.

[0054] The effects of the first embodiment will be described below with reference to Fig. 10. Fig. 10 is a diagram showing the relationship between the transmission power and the required cable size for the first power cable 10. Fig. 10 shows the relationship based on the above-mentioned system condition 1. However, the transmission capacity was changed not only to 260 MW but also to 180 to 300 MW. Accordingly, the required cable size and the capacitance also change.

[0055] 10, characteristic A represented by a dotted line indicates the cable size characteristic with respect to the transmitted power when the sum of the compensation rates of the first shunt reactor 121 and the second shunt reactor 131 is set to 75% in the above-described comparative example 4. Characteristic B represented by a dashed dotted line indicates the cable size characteristic with respect to the transmitted power when the sum of the compensation rates is set to 90% in the above-described comparative example 4. Characteristic C represented by a solid line indicates the cable size characteristic with respect to the transmitted power in the first embodiment.

[0056] In FIG. 10, in the characteristic B, when the transmission power is 260 MW, the 2 On the other hand, in this embodiment, as shown in characteristic C, the cable size required to transmit 260 MW is about 1000 mm. 2 Therefore, the cable size is one-third of that of Comparative Example 4.

[0057] In addition, in FIG. 10, in the characteristic A, when the transmission power is 200 MW, the 2 On the other hand, in this embodiment, as shown in characteristic C, the cable size required to transmit 200 MW is about 400 mm. 2 Therefore, the cable size can be reduced to 40% of that of Comparative Example 4.

[0058] Furthermore, when the transmission power is 270 MW, even if the compensation rate is increased from 75% to 90% in Comparative Example 4, the cable size that can transmit power is out of scale with respect to FIG. 10 and does not exist in the figure. On the other hand, according to this embodiment, the cable size is about 1200 mm. 2 If this is the case, 270MW of electricity can be transmitted.

[0059] 10, for power transmission in which the length of the first power cable 10 included in system condition 1 is 100 km, the compensation method according to this embodiment can transmit 270 MW. However, the compensation method according to comparative example 4 cannot transmit this power. Therefore, the compensation method according to this embodiment is superior to the compensation method according to comparative example 4.

[0060] Furthermore, if the length of the first power cable 10 is 100 km and the transmission power is 270 MW, the compensation method according to this embodiment allows power to be transmitted using one circuit, i.e., one first power cable 10. However, the compensation method according to Comparative Example 4 requires two circuits, i.e., two first power cables 10, even if the compensation rate is 90%. Since the construction costs of long-distance cables spanning several tens of kilometers are extremely high, the difference in the number of cables becomes very significant. In particular, this embodiment can halve the construction costs compared to Comparative Example 4.

[0061] Since the reactive power reduction amount Mvar is proportional to the compensation rate, the reduction amount Mvar is the same for both the compensation method according to the comparative example and the compensation method according to the present embodiment if the compensation rate is the same. In the compensation method according to the present embodiment, if the capacity of the first shunt reactor 121 is set to a compensation rate of 50%, the maximum value I of the reactive current flowing through the first power cable 10 is Q-max can be reduced by 50%, as shown in the above formula (6). Therefore, according to this embodiment, the cable size of the first power cable 10 can be effectively reduced.

[0062] (Second embodiment) First, a compensation method of Comparative Example 5 to be compared with the second embodiment will be described with reference to Fig. 11. Fig. 11 is a wiring diagram showing a schematic configuration of a power generation system according to Comparative Example 5. In Fig. 11, the same components as those in the power generation system 1 according to the first embodiment described above are denoted by the same reference numerals, and detailed description thereof will be omitted.

[0063] In the power generation system 200 shown in FIG. 11, a third shunt reactor 123 is connected to a bus 30. In the power generation system 200, the electric power generated by a large number of generators 11 is collected in a large number of second power cables 20. Therefore, a large leading current flows due to the capacitance to the ground of the second power cables, and this causes a leading reactive power Q C2 Therefore, the capacity Q of the third shunt reactor 123 SR3 is the reactive power Q C2 The capacitance Q is set to a value that compensates for SR3 is expressed by the following equation (8) using the compensation rate H2 of the third shunt reactor 123.

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[0064] Next, a second embodiment will be described. The power generation system according to this embodiment has a configuration in which the third shunt reactor 123 is eliminated from the power generation system 200 shown in FIG. 11. In this embodiment, the compensation capacity Q SR3 is the compensation capacitance Q of the first shunt reactor 121. SR1 That is, the capacitance Q of the first shunt reactor 121 in this embodiment is SR13 is expressed by the following equation (9).

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[0065] According to this embodiment, the first shunt reactor 121 compensates for the charging current of the multi-circuit second power cable 20, so that the charging current of the second power cable 20 flowing into the power grid 40 can be sufficiently reduced. This makes it possible to reduce reactive power that adversely affects the power grid 40. In addition, the Ferranti effect, which occurs when the charging current of the first power cable 10 passes through the interconnection transformer 132 and the step-up transformer 122, can be reduced to a level that does not affect the power grid 40. Furthermore, not only the charging current of the first power cable 10 but also the charging current of the multi-circuit second power cable 20 can be sufficiently reduced.

[0066] In addition, according to the second embodiment, since the third shunt reactor 123 is not required, switching equipment for the third shunt reactor 123 is not required. Furthermore, if the third shunt reactor 123 is oil-insulated, fire extinguishing equipment is also not required. Therefore, it is possible to reduce the cost required for installing the power generation system. It is also possible to reduce the installation space for the power generation system. Furthermore, it is also possible to shorten the construction period for the power generation system.

[0067] (Third embodiment) Fig. 12 is a wiring diagram showing a schematic configuration of a power generation system according to the third embodiment. In Fig. 12, the same components as those in the power generation system 1 according to the first embodiment described above are denoted by the same reference numerals, and detailed description thereof will be omitted.

[0068] The power generation system 3 according to this embodiment differs from the power generation system 1 according to the first embodiment in that it has a first shunt reactor 121A with a tap instead of the first shunt reactor 121. Here, the internal configuration of the first shunt reactor 121A will be described with reference to Fig. 13 .

[0069] Fig. 13 is a three-wire diagram of the inside of the tapped first shunt reactor 121A. As shown in Fig. 130, a tap 121B is provided at one end of the winding of the first shunt reactor 121A. The other end of the winding is provided with a line-side terminal 121C that is connected to the first power cable 10. In the first shunt reactor 121A, the compensation capacity of the first shunt reactor 121A can be adjusted by adjusting the width of the tap 121B.

[0070] A method for designing the tap width will be described below.

[0071] The compensation rate H1 of the first power cable 10, the capacity Q of the first shunt reactor 121A SR1 , the capacity Q of the second shunt reactor 131 SR2 , and the reactive current I flowing through the first power cable 10 Q-max can be expressed by the formulas (3), (4), (5), and (6) described in the first embodiment, respectively.

[0072] However, strictly speaking, the charging capacity Q of the first power cable 10 C1 error ΔQ C1 (%) exists. Charging capacity Q C1 is expressed by the equations (1) and (2) described in the first embodiment, but the capacitance C1 to the ground of the first power cable 10 shown in the equations (1) and (2) has an error ΔC1 of about ±10% per unit length. Also, the length l1 of the first power cable 10 has an error Δl1 of about ±5%. Therefore, the charging capacity Q C1 Error ΔQ C1 is approximately ±15% at most.

[0073] In addition, the first shunt reactor 121A and the second shunt reactor 131 also have a manufacturing error ΔQ of about ±5% as specified in the JEC standard. SR1 , ΔQ SR2 There are each.

[0074] The true compensation rate of the first power cable 10 taking the above errors into consideration is H S1(%) is expressed by equation (10).

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[0075] The compensation rate H1 expressed by the formula (3) explained in the first embodiment is the compensation rate at the time of system design, that is, the design compensation rate, and is calculated by the earth capacitance C1, the length l1, and the capacitance Q SR1 , and capacitance Q SR2 Each value in the above is a design value. Therefore, the total error, which is the sum of the design values ​​with respect to the actual value, can be up to about ±20%. In this embodiment, the tap of the first shunt reactor 121A is set to ±25%, which is the total error of ±20% plus a margin based on a safety factor. For example, if the center value of the tap 12B shown in FIG. 13 is the capacity at the design compensation rate H1 and the width of one tap is 5%, five taps above and below it can adjust the capacity by ±25%.

[0076] The tap width Q indicates the range in which the capacity of the first shunt reactor 121A can be adjusted. tap satisfies the relationship expressed by the following formula (11).

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[0077] The error can be either positive or negative, so the respective errors, namely, error ΔC1, error Δl1, and error ΔQ SR2 and tap capacitance Q tap is marked with a ± sign.

[0078] The first shunt reactor 121A with tap has a tap capacity of ±Q tap For example, the center of the tap 121B is adjusted to the rated capacity Q SR1 In this case, the maximum capacity Q at the top of the tap SR1-max and the minimum capacitance Q at the bottom of the tap SR1-min are expressed by the following equations (12) and (13), respectively.

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[0079] The tap 121B may be provided to the first shunt reactor 121A, and does not need to be provided to the second shunt reactor 131.

[0080] The charging capacity Q of the first power cable 10 is calculated under system condition 2, which adds the following error conditions (7), (8), and (9) to system condition 1 described in the first embodiment. C1 , the capacity Q of the first shunt reactor 121A SR1 , the total capacitance of the error Q sum , and tap capacitance ±Q tap Adjustment capacity Q corresponding to tap1 When calculated, it is as follows: (System condition 2) (7)±Error ΔC1: ±10% (8)±ErrorΔl1: ±5% (9)±Error ΔQ SR1 : ±5% Q C1 :451Mvar Q SR1 :226Mvar Q sum :±90Mvar Q tap1 :±45 Mvar

[0081] According to this embodiment, as shown in equation (10), the regulating capacitance Q tap1 is the total capacitance Q sum That is, the adjustment capacitance Q tap1 , the total capacitance Q sum It is possible to reduce it to half of the original value.

[0082] In the power generation system 3 according to this embodiment, when designing the compensation rate H1 of the first power cable 10, the charging capacity Q C1 , capacity Q SR1 , and capacitance Q SR2is a design value that includes an error. However, in this embodiment, the first shunt reactor 121A has a tap 121B. Therefore, by measuring the error from the design value after the product is completed or after installation on site, it is possible to accurately achieve the target compensation rate H1 based on the actual measured value. In a long-distance cable system of several tens of kilometers or more, this error increases in proportion to the cable length. This can be achieved by adjusting the width of the tap 121B of the tapped first shunt reactor 121A.

[0083] In this embodiment, the tap width and number of taps of the tap 121B can be increased or decreased as needed. Also, if the system constants differ among the three phases, the taps can be positioned at different positions among the three phases.

[0084] FIG. 2, which was explained in the first embodiment, shows the relationship between the current value and the cable size of the first power cable 10 under system condition 1. As shown in FIG. 2, the required cable size increases sharply as the current increases. This characteristic is due to the fact that the first power cable 10 generates heat in proportion to the square of the current, and the skin effect and proximity effect of the first power cable 10.

[0085] The proximity effect is particularly significant in three-core cables. Figure 2 shows that when the current increases by 10% around 1000A, the cable size must be increased by 43%. When the current increases by 20%, the cable size must be increased by 67%. Furthermore, Figure 2 shows that when the current approaches 1000A, no practical cable size exists.

[0086] As explained in the above equation (9), the compensation rate at the time of design includes an error of up to ±20%. Therefore, if a current larger than predicted flows through the first power cable 10, the first power cable 10 will overheat. As a result, the life of the first power cable 10 may be shortened or a ground fault may occur. Furthermore, selecting a cable size with a margin to cover this error would be disadvantageous in terms of cost. If the tap 121B is provided inside the first shunt reactor 121A as in this embodiment, the compensation rate of the first shunt reactor 121A can be set to exactly 50%. This allows the maximum value I of the reactive current flowing through the first power cable 10 to be reduced. Q-max can be reliably reduced to 50%.

[0087] According to the present embodiment described above, the tap 121B is provided in the first shunt reactor 121A, and an accurate compensation rate can be realized based on the actual measured values ​​of the cable charging capacity and the shunt reactor capacity. This allows for an economical cable size to be applied. Also, it is possible to prevent cable overheating accidents. Furthermore, it is possible to reduce the capacity of the first shunt reactor 121A with the tap.

[0088] (Variation 1) Modification 1 combines the second and third embodiments described above. That is, the power generation system according to this modification includes the first shunt reactor 121A with a tap described in the third embodiment. Furthermore, the capacity of this first shunt reactor 121A is set to a value obtained by adding 50% of the charging capacity of the first power cable 10 and the compensation capacity of the second power cable 20, as described in equation (8) of the second embodiment.

[0089] According to this modification, the third shunt reactor 123 (see FIG. 11) connected to the second power cable 20 is not required, which reduces the cost and installation space required for installing the power generation system and also shortens the construction period of the power generation system.

[0090] In addition, by using the first shunt reactor 121A with a tap, it is possible to realize an accurate compensation rate for the charging current according to the actual measured values ​​of the charging capacity of the first power cable 10 and the capacity of the first shunt reactor 121A.

[0091] (Variation 2) Fig. 14 is a wiring diagram showing a schematic configuration of a power generation system according to Modification 2. In Fig. 14, the same components as those in the power generation system 3 according to the third embodiment described above are denoted by the same reference numerals, and detailed description thereof will be omitted.

[0092] 14 differs from the power generation system 3 according to the third embodiment in that it has a reactive power compensator 14 instead of the second shunt reactor 131. The reactive power compensator 14 is connected to the interconnection point between the power grid 40 and an interconnection transformer 132.

[0093] When a large amount of active power is supplied from the power generation system 4 to the power grid 40 of the power utility company, it may be required to automatically supply required lagging and leading reactive power to the power grid 40. In this embodiment, a reactive power compensator 14 is installed to meet this requirement.

[0094] The reactive power compensator 14 is another example of a reactive power adjustment device that adjusts the reactive power generated in the first power cable 10. However, the reactive power compensator 14 can also output any reactive power Q in terms of phase lag or lead, or magnitude. The reactive power compensator 14 can also adjust the interconnection point voltage to any value, and can adjust these instantaneously (within a few cycles). The reactive power compensator 14 is configured, for example, with a separate-commutated SVC (Static Var Compensator) or a self-commutated STATCOM (Static Synchronous Compensator). A separate-commutated SVC uses semiconductor elements such as thyristors. On the other hand, a STATCOM uses semiconductor elements such as IGBTs (Insulated Gate Bipolar Transistors). The reactive power compensator 14 drives the semiconductor elements to supply leading or lagging reactive power required by the grid to the interconnection point. In addition, the reactive power compensator 14 can also have a voltage adjustment function of adjusting the voltage to an appropriate level by supplying leading reactive power whose phase is ahead of the active power when the interconnection point voltage is dropping, and by supplying lagging reactive power whose phase is behind the active power when the interconnection point voltage is rising.

[0095] This reactive power compensator 14 supplies the reactive power required by the power grid 40, thereby restricting the large reactive power generated in the first power cable 10, which is a long-distance cable, from flowing out to the power grid 40. As a result, from the viewpoint of reducing reactive power, the second shunt reactor 131 becomes unnecessary.

[0096] However, simply replacing the second shunt reactor 131 with the reactive power compensator 14 does not reduce the magnitude of the reactive current flowing through the first power cable 10.

[0097] Therefore, in this embodiment, a first shunt reactor 121A is connected to the terminal end of the first power cable 10, similarly to the third embodiment.

[0098] Fig. 15 is a diagram showing the distribution of the charging current in the first power cable 10 in Modification 2. In this modification as well, the compensation rate of the first shunt reactor 121A is 50% of the reactive current flowing through the first power cable 10. Therefore, as shown in Fig. 15, the reactive current can be reduced to 50%. Furthermore, since the first shunt reactor 121A has a tap 121B, the compensation rate can be adjusted to an accurate value.

[0099] According to the present modification described above, the reactive power compensator 14 takes measures to reduce reactive power, eliminating the need for the second shunt reactor 131. Furthermore, the first shunt reactor 121A takes measures to reduce the reactive current flowing through the first power cable 10, thereby reducing the reactive current by half. This allows the size of the first power cable 10 to be reduced.

[0100] (Variation 3) In a power generation system that uses a long-distance cable such as the first power cable 10, resonance may occur. In this case, overvoltage and overcurrent may occur. Therefore, in this modification, resonance is avoided by setting the sum of the compensation rates of the first shunt reactor 121 and the second shunt reactor 131 to a value smaller than 100%.

[0101] The following explains that resonance may occur when the total compensation rate is 100%.

[0102] In the power generation system 1 shown in FIG. 1, the resonant frequency of the circuit formed by the first power cable 10, the first shunt reactor 121, and the second shunt reactor 131 is expressed by the following equation (14).

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[0103] If the compensation rate H1 of the above circuit is 100%, the following equation (15) holds.

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[0104] Furthermore, from equations (14) and (15), the following equation (16) is established.

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[0105] In other words, if the compensation rate is set to 100%, the resonant frequency of this circuit, f r In order to avoid this, even if the compensation rate is set to 95% to allow for some margin, various errors, namely ±ΔC1, ±Δl1, ±ΔQ SR1 , ±ΔQ SR2 There is an error of up to ±20%. As a result, even if you try to keep the actual compensation rate at 95% or less to avoid resonance, the error will cause it to fall into the resonance region. In other words, there is a possibility that the resonance frequency will match the commercial frequency. The resonance phenomenon occurs when the commercial frequency and the resonance frequency match, or almost match.

[0106] In contrast, as described in the third embodiment, if the first shunt reactor 121A with a tap is connected to the terminal end of the first power cable 10, the error can be corrected and the compensation rate can be accurately adjusted to, for example, 90%, thereby avoiding the resonance condition.

[0107] Furthermore, with the tapped first shunt reactor 121A, as shown in equation (10) described in the third embodiment, it is not necessary to correct the entire total capacity due to various errors; only half of that is required, so the shunt reactor capacity can be reduced. Under the above-described system conditions 1 and 2, the compensation rate of the first shunt reactor 121A is 50%, so the adjustment capacity of the tap 121B only needs to be ±45 Mvar, as described in the third embodiment.

[0108] According to the present modified example described above, the occurrence of resonance can be avoided by setting the sum of the compensation rates of the first shunt reactor 121A and the second shunt reactor 131 to 95% or less. Furthermore, since the first shunt reactor 121A is tapped, the capacity of the first shunt reactor 121A can be adjusted so that the sum of the compensation rates is exactly 95% or less.

[0109] Although several embodiments and modifications have been described above, these embodiments and modifications are presented only as examples and are not intended to limit the scope of the invention. The novel system described in this specification can be embodied in various other forms. Furthermore, various omissions, substitutions, and modifications can be made to the forms of the system described in this specification without departing from the spirit of the invention. The appended claims and their equivalents are intended to cover such forms and modifications that fall within the scope and spirit of the invention. [Explanation of symbols]

[0110] 1, 3, 4: Power generation system 10: First power cable 11: Generator 14: Var compensator 20: Second power cable 40: Power system 121, 121A: 1st shunt reactor 131: Second shunt reactor

Claims

1. A method for compensating a charging current of a first power cable that transmits electric power generated by a generator to an electric power grid, comprising: adjusting reactive power generated in the first power cable using a reactive power adjusting device installed on a starting end side of the first power cable electrically connected to the power grid; a first shunt reactor that is installed on a termination side of the first power cable electrically connected to the generator and has a compensation rate for the charging current set to 50%;

2. a second shunt reactor is installed as the reactive power regulating device, the compensation rate for the charging current being set to a value lower than the compensation rate of the first shunt reactor; 2. The method of claim 1, wherein the second shunt reactor is also used to compensate the charging current.

3. The electric power generated by the plurality of generators is collected in a second power cable installed on a terminal side of the first power cable, 2. The charging current compensation method according to claim 1, wherein a capacity of the first shunt reactor is set to a value obtained by adding 50% of a capacity of the first power cable and a capacity of the second power cable.

4. adjusting the capacity of the first shunt reactor with a tap; 2. The charging current compensation method according to claim 1, wherein a tap width Qtap indicating a range in which the capacitance can be adjusted satisfies a relationship expressed by the following formula: [Equation 1] Δl: error in the length of the first power cable ΔC １ : Error in capacitance of the first power cable per unit length ΔQ ＳＲ１ : Error in the capacity of the first shunt reactor

5. 2. The charging current compensation method according to claim 1, wherein a reactive power compensator that supplies reactive power required from the power system is installed as the reactive power regulating device.

6. 3. The charging current compensation method according to claim 2, wherein a sum of the compensation rate of the first shunt reactor and the compensation rate of the second shunt reactor is 95% or less.

7. A generator and a first power cable for transmitting the electric power generated by the generator to an electric power grid; a reactive power adjusting device that is installed at a starting end of the first power cable electrically connected to the power grid and adjusts reactive power generated in the first power cable; a one-shunt reactor installed at a terminal end of the first power cable electrically connected to the generator, the one-shunt reactor having a compensation rate for the charging current of the first power cable set to 50%; A power generation system comprising:

8. The power generation system according to claim 7 , wherein the power generator is a wind power generator that generates power by rotation of a wind turbine installed on the ocean, or a solar power generator that generates power using a solar panel installed on the ocean.

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