JP2024007268A - Charging current compensation method for power cable and power generation system using power cable - Google Patents

Abstract

PROBLEM TO BE SOLVED: 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

COPYRIGHT: (C)2024,JPO&INPIT

Description

Embodiments of the present invention relate to a charging current compensation method for a power cable and a power generation system using the power cable.

In order to utilize renewable energy, large-capacity offshore wind power generation systems are planned to transmit power using long-distance power cables ranging from several tens of kilometers to more than 100 kilometers, for example. As the power transmission voltage, for example, an ultra-high voltage of 187 to 275 kV or an extra high voltage of 66 to 154 kV is used. In such high-voltage, long-distance cable power transmission, there are several technical issues related to charging current. This technical problem includes, for example, the generation of large amounts of reactive power that adversely affects the power system, and the generation of voltage increases due to the Ferranti effect.

Therefore, as a technique for solving the above-mentioned technical problems, a technique is known in which a shunt reactor for compensating for reactive power is connected to the starting end, the terminal end, or both ends of a power cable. Since the charging current flowing through the power cable has a 90° leading phase, the shunt reactor provides a 90° lagging phase current. Thereby, reactive power can be reduced and voltage rise due to the Ferranti effect can be suppressed.

Unexamined Japanese Patent Publication No. 2015-80404

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 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

However, if the power cable is long distance, it is highly likely that simply connecting a shunt reactor to the power cable will not be able to sufficiently reduce the charging current. Therefore, an oversized power cable is required. In this case, it can be assumed that a power cable cannot actually be manufactured, making it difficult to realize power transmission. Furthermore, even if this were possible, the cost of the power cable would be extremely high.

Therefore, an object of the present invention is to provide a charging current compensation method for a power cable that can reduce the cable size, and a power generation system using the power cable.

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 grid. This charging current compensation method uses a reactive power adjustment device installed at the starting end of the first power cable that is electrically connected to the power grid to adjust the reactive power generated in the first power cable, and The charging current is compensated using a first shunt reactor installed at the terminal end of the first power cable electrically connected to the charging current and having a compensation rate for the charging current set to 50%.

According to this embodiment, it is possible to reduce the cable size.

FIG. 1 is a wiring diagram showing a schematic configuration of a power generation system according to a first embodiment. It is a figure which shows an example of the relationship between the electric current which flows into a cable, and a required cable size. 7 is a diagram showing the distribution of charging current in the first power cable in Comparative Example 1. FIG. 7 is a diagram showing the distribution of charging current in the first power cable in Comparative Example 2. FIG. 7 is a diagram showing the distribution of charging current in the first power cable in Comparative Example 3. FIG. 7 is a diagram showing the distribution of charging current in the first power cable in Comparative Example 4. FIG. FIG. 6 is a diagram showing the distribution of charging current in the first power cable when the sum of the compensation factors of the first shunt reactor and the second shunt reactor is 60%. FIG. 6 is a diagram showing the distribution of charging current in the first power cable when the sum of the compensation factors of the first shunt reactor and the second shunt reactor is 75%. FIG. 6 is a diagram showing the distribution of 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%. FIG. 3 is a diagram showing the relationship between transmitted power and required cable size for the first power cable. 3 is a wiring diagram showing a schematic configuration of a power generation system according to Comparative Example 5. FIG. FIG. 3 is a wiring diagram showing a schematic configuration of a power generation system according to a third embodiment. FIG. 3 is a three-line wiring diagram inside a tapped first shunt reactor. 7 is a wiring diagram showing a schematic configuration of a power generation system according to modification 2. FIG. 7 is a diagram showing the distribution of charging current in the first power cable in Modification 2. FIG.

Embodiments of the present invention will be described below with reference to the drawings. The embodiments described below are not intended to limit the invention.

(First embodiment)
FIG. 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 the present embodiment is an example of an offshore wind long distance cable power generation system using the first power cable 10 and the second power cable 20. This power generation system 1 includes a first power cable 10, a generator 11, a step-up substation 12, an interconnection substation 13, a second power cable 20, and a bus bar 30.

As the generator 11, for example, a wind power generator that generates electricity by rotating a windmill installed on the ocean can be applied. Note that the generator 11 is not limited to a wind power generator, and may be, for example, a solar power generator that generates power using a solar panel installed on the ocean.

Further, although 20 generators 11 are shown in FIG. 1, 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 connected generators 11 to one second power cable 20 and the number of second power cables 20 is not particularly limited.

Electric power generated by the plurality of generators 11 is first collected into the plurality of second power cables 20, and then collected from each second power cable 20 to the bus bar 30. The power collected on the busbar 30 is supplied to the step-up substation 12.

Step-up substation 12 includes a first shunt reactor 121 and a step-up transformer 122 . One end of the first shunt reactor 121 is connected to the terminal end of the first power cable 10. On the other hand, the other end of the first shunt reactor 121 is grounded. In the first shunt reactor 121, the compensation rate for the reactive current flowing through the first power cable 10 is set to 50%.

One end of the step-up transformer 122 is connected to the bus bar 30. On the other hand, the other end of the step-up transformer 122 is connected to the terminal end of the first power cable 10. The step-up transformer 122 steps up the voltage value of the power supplied from the bus bar 30 to a voltage value suitable for power transmission through the first power cable 10 . The boosted power is transmitted by the first power cable 10 and supplied to the interconnection substation 13.

The interconnection substation 13 includes 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. On the other hand, the other end of the second shunt reactor 131 is grounded. In the second shunt reactor 131, the compensation rate for reactive current is set to a lower value than the compensation rate of the first shunt reactor 121.

One end of the interconnection transformer 132 is connected to the starting end of the first power cable 10. On the other hand, the other end of the interconnection transformer 132 is connected to the power system 40 of the power company. The interconnection transformer 132 transforms the power transmitted by the first power cable 10. The transformed power is supplied to the power system 40.

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

A feature of a large-capacity offshore wind power generation system is that a large number of generators 11 are installed, and the total power transmission capacity is large, ranging from 100 MW to over 1000 MW, which is equivalent to a nuclear power plant.Therefore, the cross-sectional area of the first power cable 10 used is For example, the distance may become large, from several hundred ^mm2 to several thousand ^mm2 , and the length may become long, from several tens of kilometers to about 100 km.

The first power cable 10 may be a submarine cable or a land cable depending on the location. In either case, since it is a long-distance cable, the charging current becomes large, and when the distance is several tens of kilometers, it exceeds the current corresponding to the generated power. For example, in the case of system condition 1, the effective current is 546A, whereas the charging current, which is the reactive current, is 948A. In this case, the charging current is 1.74 times the effective current. Further, the combined current obtained by adding the effective current and the cable charging current is 1094 A, which is about twice the effective current.

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 must be larger than the square of the current, so it must be at least as large as the square of the current. A cross-sectional area of 2 squared, that is, four times or more, is required. Additionally, the first power cable 10 may actually require a cross-sectional area significantly greater than four times due to skin and proximity effects.

FIG. 2 is a diagram showing an example of the relationship between the current flowing through the cable and the required cable size. In FIG. 2, the current on the horizontal axis indicates the current flowing through the first power cable 10, and the cable size on the vertical axis indicates the cross-sectional area of the first power cable 10 required to flow the current.

As shown in FIG. 2, for example, if the current increases from 900A to approximately 1000A to 1000A, the cable size must increase by 43%. In other words, if the current can be reduced by 10% from 1000A to 900A, the cable size can be reduced by 43%.

Additionally, as the current increases from 800A to approximately 20% to 1000A, the cable size must increase 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%. Although the current reduction is 20%, the cable size reduction is 67%, more than three times as effective. This is because cables have a characteristic that their size increases rapidly in a large current region.

In the case of the above-mentioned system condition 1, the combined current, that is, the current flowing through the first power cable 10, is about 1000A. In this case, as shown in FIG. 2, the required cable size is approximately 5000 ^mm2 . In reality, cables with such a cross-sectional area are not manufactured, so it is difficult to transmit the power from the generator 11 to the power grid 40. Even if it could be realized, the cost of the first power cable 10 would be extremely high. Therefore, for example, instead of using one first power cable 10, it is necessary to connect several first power cables 10 in parallel. In this case as well, the cost will increase.

Therefore, reducing the charging current of the first power cable 10 greatly contributes to reducing the cost of the first power cable 10. Further, the size of the first power cable 10 can be set within a manufacturable 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 the rapid increase in cable size in the high current range.

ここで、
Ｃ_１：第１電力ケーブルの対地静電容量（F／ｍ）
ｆ：周波数（Ｈｚ）
Ｈ_１：第１電力ケーブルの補償率（％）
Ｉ_Ｃ１：第１電力ケーブル全長に対する充電電流（Ａ）
ｌ_１：第１電力ケーブルの長さ（ｍ）
Ｑ_Ｃ１：第１電力ケーブルの充電容量（ｖａｒ）
Ｑ_ＳＲ１：第１分路リアクトルの容量（ｖａｒ）
Ｑ_ＳＲ２：第２分路リアクトルの容量（ｖａｒ）
Ｖ：線間電圧（Ｖ） Here, the compensation rate of the charging current of the first power cable 10 will be explained. The charging capacity Q _C1 , the charging current I _C1 , and the charging current compensation rate H ₁ of the first power cable 10 are expressed by the following equations (1) to (3), respectively.

here,
C ₁ : Ground capacitance of the first power cable (F/m)
f: Frequency (Hz)
_H1 : Compensation rate (%) of the first power cable
I _C1 : Charging current (A) for the entire length of the first power cable
l ₁ : Length of the first power cable (m)
Q _C1 : Charging capacity of the first power cable (var)
Q _SR1 : Capacity of the first shunt reactor (var)
Q _SR2 : Capacity of second shunt reactor (var)
V: line voltage (V)

FIG. 3 is a diagram showing the distribution of charging current within 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. The current distribution in 10 shows only the reactive current.

In FIG. 3, the charging current, that is, the leading current whose phase is 90 degrees ahead of the phase of the active current, is 100% at the interconnection transformer 132 side of the first power cable 10, that is, at the starting end of the first power cable 10. Flow into. On the other hand, on the step-up transformer 122 side, that is, at the end of the first power cable 10, the lead current becomes zero. Since 100% of the charging current flows through the starting end of the first power cable 10, the reactive current flowing through the first power cable 10, that is, the charging current is not reduced at all.

FIG. 4 is a diagram showing the distribution of charging current within the first power cable 10 in Comparative Example 2. In Comparative Example 2, while the second shunt reactor 131 is connected to the starting end of the first power cable 10, the first shunt reactor 121 is not connected to the terminal end of the first power cable 10.

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 power system 40 and interconnection transformer 132 becomes zero. However, 100% of the charging current flows at the interconnection transformer 132 side of the first power cable 10, that is, at the starting end. On the other hand, on the step-up transformer 122 side, that is, at the end of the first power cable 10, the charging current becomes zero.

In Comparative Example 2, the lead current flowing through the power system 40 and the interconnection transformer 132 is compensated. That is, in this compensation method, the leading reactive power flowing to the power system 40 and the interconnection transformer 132 is eliminated. Therefore, it is possible to eliminate reactive power that adversely affects the power system 40, and to suppress voltage increases due to the Ferranti effect. However, since the reactive current flowing through the first power cable 10, that is, the charging current, is not reduced, the cable size cannot be reduced.

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

In Comparative Example 3, the first shunt reactor 121 compensates for 100% of the charging current to the first power cable 10. Therefore, similarly to Comparative Example 2, the reactive current flowing through the power system 40 and the interconnection transformer 132 becomes zero. However, a 100% delayed current flows at the step-up transformer 122 side of the first power cable 10, that is, at the terminal end. Therefore, even with this compensation method, although the lead current flowing through the power system 40 and the interconnection transformer 132 is compensated for, the reactive current flowing through the first power cable 10 is not reduced. That is, although it is possible to eliminate reactive power that adversely affects the power system 40 and suppress the voltage rise due to the Ferranti effect, it is not possible to reduce the cable size.

FIG. 6 is a diagram showing the distribution of charging current within the first power cable 10 in Comparative Example 4. In Comparative Example 4, the first shunt reactor 121 is connected to the terminal end of the first power cable 10, and the second shunt reactor 131 is connected to the starting end of the first power cable 10. Further, 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.

In Comparative Example 4, as shown in FIG. 6, the distribution of leading current and lagging current, that is, reactive current, leads by 70% at the starting end of the first power cable 10 and lags by 30% at the terminal end of the first power cable 10. %become. 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 allows 70% of the charging current to flow therethrough. According to this compensation method, the reactive charge capacity can be reduced compared to Comparative Example 1, Comparative Example 2, and Comparative Example 3 described above. However, the effect of reducing the magnitude of the charging current flowing through the first power cable 10 is limited.

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%, the compensation rate of the first shunt reactor 121 is set to 50%. By setting this, the cable size of the first power cable 10 can be further reduced than in Comparative Example 4. The reason for this will be explained below.

The compensation rate H ₁ (%) of the first power cable 10 is determined by the capacity Q _SR1 of the first shunt reactor 121, the capacity Q _SR2 of the second shunt reactor 131, and the first It can be expressed using the charging capacity _QC1 of the power cable 10. Here, if the compensation rate of the first shunt reactor 121 is set to 50%, the capacity Q _SR1 of the first shunt reactor 121 is expressed by the following formula (4) by transforming the above formula (3). The capacity _QSR2 of the second shunt reactor 131 is expressed by the following equation (5).

In this embodiment, the compensation rates are unequal between the first shunt reactor 121 and the second shunt reactor 131. Furthermore, since the compensation rate of the first shunt reactor 121 is set to 50%, the compensation rate of the second shunt reactor 131 is H ₁ -50%.

FIG. 7 is a diagram showing the distribution of 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, since the compensation rate of the first shunt reactor 121 is set to 50%, the compensation rate of the second shunt reactor 131 is 10% (=60%-50%).

Further, the reactive current flowing through the power system 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%). On the other hand, the compensation rate at the terminal end of the first power cable 10 is also 50%. Accordingly, in this embodiment, the first power cable 10 only needs to have a cable size that allows 50% of the charging current to flow therethrough. 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.

FIG. 8 is a diagram showing the distribution of 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%.

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 25% (=75%-50%). Further, the reactive current flowing through the power system 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%). On the other hand, the compensation rate at the terminal 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 allows 50% of the charging current to flow.

In Comparative Example 4 described above, 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 ends up being 62.5% (25%+37.5%). Therefore, even if the sum of the compensation rates 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 compared to the fourth comparative example.

FIG. 9 is a diagram showing the distribution of 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%.

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 40% (=90%-50%). Further, the reactive current flowing through the power system 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%). On the other hand, the compensation rate at the terminal 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 allows 50% of the charging current to flow.

In Comparative Example 4 described above, 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 ends up being 55% (10%+45%). Therefore, even if the sum of the compensation rates 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 compared to the fourth comparative example.

ここで、
Ｉ_{Ｑ－ｍａｘ}： 第１電力ケーブル１０に流れる無効分電流の最大値（Ａ） According to the embodiment described above, 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 determined using the following equation (6).

here,
_IQ-max : Maximum value (A) of reactive current flowing through the first power cable 10

This current I _Q-max indicates the magnitude of the current flowing at the starting end and the terminal end of the first power cable 10, and the same magnitude of current flows at the starting end and the terminal end. As shown in Figures 7 to 9, the phase of the current flowing at the start end is opposite to the phase of the current flowing at the end, that is, the start end is a leading current of 90°, and the terminal end is a current of 90° delay. .

ここで、
Ｉ_Ｑ－ｘ： 第１電力ケーブルの始端から終端の途中の位置ｘにおける無効分電流（Ａ） The current _IQ-x at the midway position x of the first power cable 10 is expressed by the following equation (7).

here,
_IQ-x : Reactive current (A) at position x midway from the start end to the end of the first power cable

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 change depending on the position, as shown in FIGS. 7 to 9.

The effects of the first embodiment will be described below with reference to FIG. FIG. 10 is a diagram showing the relationship between transmitted power and required cable size for the first power cable 10. FIG. 10 shows the relationship based on the above-mentioned system condition 1. However, the power transmission capacity was varied from 180 to 300 MW instead of just 260 MW. Along with this, the required cable size changes and the capacitance also changes.

In FIG. 10, characteristic A represented by a dotted line represents 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-mentioned comparative example 4. The cable size characteristics are shown below. 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 Comparative Example 4 described above. Further, a characteristic C represented by a solid line indicates a cable size characteristic with respect to transmitted power in the first embodiment.

In FIG. 10, in characteristic B, when the transmitted power is 260 MW, a cable size of approximately 3000 mm ² is required. On the other hand, in this embodiment, as shown in characteristic C, the cable size required to transmit 260 MW is approximately 1000 mm ² . Therefore, the cable size is reduced to one-third of that of Comparative Example 4.

Further, in FIG. 10, in characteristic A, when the transmitted power is 200 MW, a cable size of approximately 1000 mm ² is required. On the other hand, in this embodiment, as shown in characteristic C, the cable size required to transmit 200 MW is approximately 400 mm ² . Therefore, the cable size can be reduced to 40% of Comparative Example 4.

Further, when the transmitted 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 scaled out from FIG. 10 and does not exist in the diagram. On the other hand, according to this embodiment, if the cable size is approximately 1200 mm ² , it is possible to transmit 270 MW.

Further, in FIG. 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 the present embodiment allows power transmission of 270 MW. However, power cannot be transmitted using the compensation method according to Comparative Example 4. Therefore, the compensation method according to this embodiment is superior to the compensation method according to Comparative Example 4.

Further, when the length of the first power cable 10 is 100 km and the transmitted power is 270 MW, the compensation method according to the present embodiment allows power to be transmitted through one line, that is, one first power cable 10. However, in the compensation method according to Comparative Example 4, even if the compensation rate is 90%, two lines, that is, two first power cables 10 are required. The cost of constructing a long-distance cable spanning several tens of kilometers is very large, so the difference in the number of cables becomes very large. In particular, in this embodiment, construction costs can be halved compared to Comparative Example 4.

Note that since the reactive power reduction amount Mvar value is proportional to the compensation rate, the reduction amount Mvar is the same in 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 _IQ-max of the reactive current flowing through the first power cable 10 is calculated by the above equation (6 ), a 50% reduction is possible. Therefore, according to this embodiment, it is possible to effectively reduce the cable size of the first power cable 10.

(Second embodiment)
First, with reference to FIG. 11, a compensation method of Comparative Example 5 to be compared with the second embodiment will be described. 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 of 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.

In the power generation system 200 shown in FIG. 11, the third shunt reactor 123 is connected to the bus bar 30. In the power generation system 200 , power generated by a large number of generators 11 is collected into a large number of second power cables 20 . Therefore, a large lead current flows due to the ground capacitance of the second power cable group, which generates lead reactive power _QC2 . Therefore, the capacity Q _SR3 of the third shunt reactor 123 is set to a size that compensates for the reactive power Q _C2 . The capacity Q _SR3 is expressed by the following equation (8) using the compensation rate H ₂ of the third shunt reactor 123.

ここで、
Ｈ_２：第２電力ケーブルの多回線の補償率（％）
Ｑ_Ｃ１：第１電力ケーブルの充電容量（ｖａｒ）
Ｑ_Ｃ２：第２電力ケーブルの多回線の充電容量（ｖａｒ）
Ｑ_ＳＲ３：第３分路リアクトルの容量（ｖａｒ）
Ｑ_ＳＲ１３：第２実施形態における第２分路リアクトルの容量（ｖａｒ） 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 removed from the power generation system 200 shown in FIG. 11. Further, in this embodiment, the compensation capacity Q _SR3 of the third shunt reactor 123 is added to the compensation capacity Q _SR1 of the first shunt reactor 121. That is, the capacity _QSR13 of the first shunt reactor 121 in this embodiment is expressed by the following equation (9).

here,
_H2 : Compensation rate (%) for multiple lines of the second power cable
Q _C1 : Charging capacity of the first power cable (var)
Q _C2 : Multi-line charging capacity of the second power cable (var)
Q _SR3 : Capacity of third shunt reactor (var)
Q _SR13 : Capacity (var) of the second shunt reactor in the second embodiment

According to this embodiment, the first shunt reactor 121 compensates for the charging current of the multiple lines of the second power cable 20, so that the charging current of the second power cable 20 flowing into the power system 40 can be sufficiently reduced. Thereby, reactive power that adversely affects the power system 40 can be reduced. Moreover, the Ferranti effect that 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 the extent that it does not affect the power system 40. Furthermore, not only the charging current of the first power cable 10 but also the charging current of the multi-line second power cable 20 can be sufficiently reduced.

In addition, according to the second embodiment, since the third shunt reactor 123 is eliminated, switching equipment for the third shunt reactor 123 is not required. Moreover, when 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 also becomes possible to reduce the installation space for the power generation system. Furthermore, it will also be possible to shorten the construction period for the power generation system.

(Third embodiment)
FIG. 12 is a wiring diagram showing a schematic configuration of a power generation system according to a third embodiment. In FIG. 12, the same components as those of 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.

The power generation system 3 according to this embodiment differs from the power generation system 1 according to the first embodiment in that it includes a tapped first shunt reactor 121A 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.

FIG. 13 is a three-line wiring diagram inside 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. Further, a line side terminal 121C connected to the first power cable 10 is provided at the other end of the winding. 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.

The method of designing the tap width will be explained below.

The compensation factor H ₁ of the first power cable 10, the capacity Q _SR1 of the first shunt reactor 121A, the capacity Q _SR2 of the second shunt reactor 131, and the reactive current I _Q-max flowing through the first power cable 10 are as follows: It can be expressed by Equation (3), Equation (4), Equation (5), and Equation (6) explained in the first embodiment, respectively.

However, strictly speaking, there is an error ΔQ _C1 (%) in the charging capacity Q _C1 of the first power cable 10. The charging capacity Q _C1 is expressed by the equations (1) and (2) described in the first embodiment, and the ground electrostatic charge of the first power cable 10 shown in the equations (1) and (2) The capacitance C ₁ has an error ΔC ₁ of about ±10% per unit length. Further, the length l ₁ of the first power cable 10 has an error Δl ₁ of about ±5%. Therefore, the error ΔQ _C1 in the charging capacity Q _C1 is approximately ±15% at maximum.

Further, the first shunt reactor 121A and the second shunt reactor 131 also have manufacturing errors ΔQ _SR1 and ΔQ _SR2 of about ±5%, respectively, as defined by the JEC standard.

The true compensation rate H _S1 (%) of the first power cable 10 in consideration of the above error is expressed by equation (10).

The compensation rate H ₁ expressed by the formula (3) described in the first embodiment is the compensation rate at the time of system design, that is, the design compensation rate, and includes ground capacitance C ₁ , length l ₁ , and capacitance Each value of Q _SR1 , capacity Q _SR2, etc. is a design value. Therefore, the total error, which is the sum of each design value with respect to the actual value, is about ±20% at maximum. In this embodiment, the tap of the first shunt reactor 121A is set to ±25%, which is the sum of ±20% of the total error and a margin based on the safety factor. For example, if the center value of tap 12B shown in FIG. 13 is the capacity at a design compensation rate of H ₁ and the width of one tap is 5%, if there are 5 taps above and below, the capacity can be adjusted by ±25%. I can do it.

The tap width Q _tap indicating the range in which the capacity of the first shunt reactor 121A can be adjusted satisfies the relationship expressed by the following equation (11).

Note that since the error can be on either the plus side or the minus side, each error, that is, the error ΔC ₁ , the error Δl ₁ , the error ΔQ _SR2 , and the tap capacitance Q _tap is given a ± sign.

The tapped first shunt reactor 121A is provided with a tap 121B whose tap capacity ±Q _tap can be adjusted. For example, if the center of the tap 121B is the rated capacity Q _SR1 , the maximum capacity Q _SR1-max at the top end of the tap and the minimum capacity Q _SR1-min at the bottom end of the tap are expressed by the following equations (12) and (13), respectively. be done.

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

Under system condition 2, which is obtained by adding the following error conditions (7), (8), and (9) to system condition 1 described in the first embodiment, the charging capacity Q _C1 of the first power cable 10, the first shunt The capacitance Q _SR1 of the reactor 121A, the total capacitance Q _sum of errors, and the adjustment capacitance Q _tap1 corresponding to the tap capacitance ±Q _tap are calculated as follows.
(System condition 2)
(7) ±Error ΔC ₁ : ±10%
(8) ±Error Δl ₁ : ±5%
(9) ±Error ΔQ _SR1 : ±5%
Q _C1 : 451Mvar
Q _SR1 : 226Mvar
_Qsum : ±90Mvar
Q _tap1 : ±45Mvar

According to the present embodiment, as shown in equation (10), the adjustment capacity Q _tap1 may be one half of the total capacity Q _sum . That is, it becomes possible to reduce the adjustment capacitance Q _tap1 to half of the total capacitance Q _sum .

In the power generation system 3 according to the present embodiment, when designing the compensation rate H ₁ of the first power cable 10, the charging capacity Q _C1 , the capacity Q _SR1 , and the capacity Q _SR2 are designed values that include errors. However, in this embodiment, the first shunt reactor 121A has a tap 121B. Therefore, by measuring the error value from the design value after the product is completed or installed on site, it is possible to accurately achieve the target compensation rate _H1 based on the actual measurement value. In long-distance cable systems 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.

In addition, in this embodiment, the tap width and the number of taps of the tap 121B can be increased or decreased as necessary. Furthermore, if the constants of the system are different between the three phases, the taps can be placed at different positions between the three phases.

FIG. 2 described in the first embodiment shows the relationship between the current value and cable size under system condition 1 with respect to the first power cable 10. According to FIG. 2, as the current increases, the required cable size increases rapidly. The reason for this characteristic is that the first power cable 10 generates heat according to the square of the current, and the skin effect and proximity effect of the first power cable 10.

In particular, cables with a three-core structure are greatly affected by the proximity effect. In FIG. 2, near 1000A, if the current increases by 10%, the cable size needs to increase by 43%. Also, if the current increases by 20%, the cable size needs to increase by 67%. Furthermore, FIG. 2 shows that there is no practical cable size near 1000A.

As explained in equation (9) above, the compensation rate at the time of design includes an error of ±20% at maximum. Therefore, if a larger current than expected flows through the first power cable 10, the first power cable 10 will be overheated. 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 enough margin to cover this error will be disadvantageous in terms of cost. When the tap 121B is provided in the first shunt reactor 121A as in this embodiment, the compensation rate of the first shunt reactor 121A can be exactly 50%. Thereby, the maximum value _IQ-max of the reactive current flowing through the first power cable 10 can also be reliably reduced to 50%.

According to this 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 economical cable sizes to be applied. Moreover, cable overheating accidents can be prevented. Furthermore, the capacity of the tapped first shunt reactor 121A can be reduced.

(Modification 1)
Modification 1 combines the second and third embodiments described above. That is, the power generation system according to this modification includes the tapped first shunt reactor 121A described in the third embodiment. Further, as explained in equation (8) of the second embodiment, the capacity of the first shunt reactor 121A is equal to 50% of the charging capacity of the first power cable 10 and the compensation capacity of the second power cable 20. , is set to the value added.

According to this modification, the third shunt reactor 123 (see FIG. 11) connected to the second power cable 20 is not required, so the cost and installation space required for installing the power generation system are reduced, and further, It will also be possible to shorten the construction period for the power generation system.

In addition, by using the first shunt reactor 121A with a tap, an accurate charging current compensation rate can be realized according to the actual measured value of the charging capacity of the first power cable 10 and the capacity of the first shunt reactor 121A. be able to.

(Modification 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 of 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.

The power generation system 4 shown in FIG. 14 differs from the power generation system 3 according to the third embodiment in that it includes a reactive power compensator 14 instead of the second shunt reactor 131. The reactive power compensator 14 is connected to an interconnection point between the power system 40 and the interconnection transformer 132.

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

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 arbitrary reactive power Q in terms of phase delay and lead, and magnitude. Further, the reactive power compensator 14 can adjust the interconnection point voltage to any value, and can also adjust these instantaneously (within several cycles). The reactive power compensator 14 includes, for example, a separately excited SVC (Static Var Compensator) or a self-excited STATCOM (Static Synchronous Compensator). For example, a semiconductor element such as a thyristor is used in the separately excited type SVC. On the other hand, a semiconductor element such as an IGBT (Insulated Gate Bipolar Transistor) is used in the STATCOM. The reactive power compensator 14 drives the semiconductor element described above to supply leading reactive power or delayed reactive power requested from the grid to the interconnection point. Further, the reactive power compensator 14 supplies advanced reactive power whose phase is more advanced than the active power when the interconnection point voltage is decreasing, and supplies more advanced reactive power than the active power when the interconnection point voltage is increasing. By supplying delayed reactive power whose phase is delayed, it is also possible to provide a voltage adjustment function of adjusting the voltage to an appropriate level.

According to this reactive power compensator 14, by supplying the reactive power required by the power system 40, it is possible to prevent large reactive power generated in the first power cable 10, which is a long-distance cable, from flowing out to the power system 40. Can be restricted. As a result, the second shunt reactor 131 becomes unnecessary from the viewpoint of reducing reactive power.

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

Therefore, in this embodiment, the first shunt reactor 121A is connected to the terminal end of the first power cable 10, as in the third embodiment.

FIG. 15 is a diagram showing the distribution of charging current within the first power cable 10 in Modification 2. Also in this modification, 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%. Moreover, since the first shunt reactor 121A has the tap 121B, the compensation rate can be adjusted to an accurate value.

According to this modification described above, the reactive power compensator 14 takes charge of reducing reactive power, so the second shunt reactor 131 becomes unnecessary. Moreover, since the first shunt reactor 121A takes a measure to reduce the reactive current flowing through the first power cable 10, the reactive current can be halved. This makes it possible to reduce the size of the first power cable 10.

(Modification 3)
In a power generation system using a long distance cable like 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%.

Below, it will be explained that resonance may occur when the total compensation rate is 100%.

ここで、
Ｃ_１： 第１電力ケーブルの静電容量（Ｆ）
Ｌ_１２： 第１分路リアクトルと第２分路リアクトルの並列のインダクタンス（Ｈ） In the power generation system 1 shown in FIG. 1, the resonant frequency of the circuit composed of the first power cable 10, the first shunt reactor 121, and the second shunt reactor 131 is expressed by the following equation (14). Ru.

here,
C ₁ : Capacitance (F) of the first power cable
_L12 : Parallel inductance (H) of the first shunt reactor and the second shunt reactor

ここで、
ｆ_０： 商用周波数（Ｈｚ） If the compensation rate _H1 of the above circuit is 100%, the following equation (15) holds true.

here,
_f0 : Commercial frequency (Hz)

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

That is, when the compensation rate is set to 100%, the resonant frequency f _r of this circuit becomes the commercial frequency f ₀ . In order to avoid this, for example, even if the design compensation rate is set to 95% with a margin in mind, various errors such as ±ΔC ₁ , ±Δl ₁ , ±ΔQ _SR1 will occur, as explained in the second embodiment. , ±ΔQ _SR2 exists, resulting in an error of about ±20% at maximum. As a result, even if an attempt is made to set the actual compensation rate to 95% or less in order to avoid resonance, the error will cause the compensation rate to fall into the resonance region. In other words, there is a possibility that the resonant frequency will match the commercial frequency. A resonance phenomenon occurs when the commercial frequency and the resonant frequency match or almost match.

On the other hand, as described in the third embodiment, if the tapped first shunt reactor 121A is connected to the terminal end of the first power cable 10, the error is corrected and the compensation rate is increased to, for example, 90%. Can be adjusted accurately. This allows resonance conditions to be avoided.

Further, according to 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, but only half of it. Therefore, the shunt reactor capacity can be reduced. Under system condition 1 and system condition 2 described above, the compensation rate of the first shunt reactor 121A is 50%, so the adjustment capacity of the tap 121B may be ±45 Mvar, as described in the third embodiment.

According to this modification 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. . Moreover, since the first shunt reactor 121A is equipped with a tap, the capacity of the first shunt reactor 121A can be adjusted so that the total compensation rate is exactly 95% or less.

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 herein can be implemented in a variety of other forms. Furthermore, various omissions, substitutions, and changes can be made to the form of the system described in this specification without departing from the gist of the invention. The appended claims and their equivalents are intended to cover such forms and modifications as fall within the scope and spirit of the invention.

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

Claims (8)

A method for compensating a charging current of a first power cable that transmits power generated by a generator to a power grid, the method comprising:
Adjusting the reactive power generated in the first power cable using a reactive power adjustment device installed on the starting end side of the first power cable electrically connected to the power system,
Compensating the charging current using a first shunt reactor installed at a terminal end of the first power cable electrically connected to the generator and having a compensation rate for the charging current set to 50%. , Charging current compensation method. A second shunt reactor is installed as the reactive power adjustment device, and the compensation rate for the charging current is set to a lower value than the compensation rate of the first shunt reactor;
The charging current compensation method according to claim 1, wherein the second shunt reactor is also used to compensate the charging current. Collecting the power generated by the plurality of generators to a second power cable installed at the terminal end of the first power cable,
The charging current compensation method according to claim 1, wherein the capacity of the first shunt reactor is set to a value that is the sum of 50% of the capacity of the first power cable and the capacity of the second power cable. Adjusting the capacity of the first shunt reactor with a tap,
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.

Δl: Error in the length of the first power cable ΔC ₁ : Error in the capacitance of the first power cable per unit length ΔQ _SR1 : Error in the capacitance of the first shunt reactor 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 adjusting device. The charging current compensation method according to claim 2, wherein the sum of the compensation rate of the first shunt reactor and the compensation rate of the second shunt reactor is 95% or less. generator and
a first power cable that transmits the power generated by the generator to the power grid;
a reactive power adjustment device installed on the starting end side of the first power cable electrically connected to the power system, and adjusting reactive power generated in the first power cable;
a 1-shunt reactor installed at a terminal end of the first power cable electrically connected to the generator, and having a compensation rate of 50% for the charging current of the first power cable;
A power generation system equipped with The power generation system according to claim 7, wherein the power generator is a wind power generator that generates power by rotating a windmill installed on the ocean, or a solar power generator that generates power using a solar panel installed on the ocean.