JP6472723B2 - Power transmission system and power transmission method - Google Patents

Description

  Embodiments described herein relate generally to a power transmission system including a power transmission cable group and a power transmission method using the power transmission system.

  The electric power generated at the power plant is transmitted to the demand area. Especially, when the demand area is a remote island or the like, the overhead transmission line cannot be used unlike the power transmission to the demand area existing on land. Instead of overhead transmission lines, transmission cables laid on the sea floor are used. In recent years, such a submarine power transmission cable is required to have a maximum length of several tens of kilometers.

  On the side where the power plant exists (the mainland side) as seen from the power transmission cable, this power is usually sent from the power system consisting of the power plant and the transmission line through a step-down at the substation (hereinafter referred to as the first substation). Connection is made to the transmission cable. On the other hand, on the side where the demand area exists from the power transmission cable (the remote island side), the power transmission cable is connected to another substation (hereinafter referred to as the second substation) and further stepped down before being sent to the demand area.

  In general, a plurality of transformers that step down at the first substation are provided, and a necessary number of transformers are operated in parallel connection according to a necessary power transmission situation (the number of transformers = the number of banks). In general, a plurality of power transmission cables are also provided in parallel, and a necessary number of power transmission cables are operated according to a necessary power transmission situation (the number of power transmission cables = the number of lines). The transformer that steps down at the second substation is also connected and operated in the same way as the transformer at the first substation (number of transformers = number of banks).

  In the second substation, when any one of the transformers shifts to the operating state, the corresponding circuit breaker is closed and the transformer is charged, but the excitation current flows into the charged transformer. . In general, the excitation current immediately after the voltage application is distorted including a direct current component and can be generated at a much larger value than the normal state (excitation inrush current). This is because the voltage is applied so that the moment when the transformer core is saturated and the moment when the transformer core is not saturated are alternately repeated depending on the phase of charging. It is. This is a normal phenomenon as a transformer alone.

  Next, when the transformer of the second substation is shifted to the operation state, there is a unique phenomenon that occurs when there is a transformer that is already in operation in parallel with this transformer. That is, along with the generation of the magnetizing inrush current in the transformer that has been newly put into operation, the transformer that is already in operation is also attracted with a large magnetizing inrush current that has the polarity reversed. This phenomenon is caused by a resistance component existing in the power supply system, but is a phenomenon peculiar when a plurality of transformers are connected in parallel at the second substation.

  When a magnetizing inrush current is attracted to the transformers connected in parallel, the primary side of the second substation has a distorted overvoltage including low-order harmonics (hereinafter simply referred to as “distorted overvoltage”). It should be noted that there is a phenomenon that may occur). If this distorted overvoltage causes problems such as damage to equipment for the power transmission system, overheating of the transmission cable, harmonic disturbance of these equipment / equipment, destabilization of the supply voltage to the demand area, etc. Cost.

  The generated overvoltage resonates and increases when the resonance frequency of the power transmission system matches the harmonic frequency of the power frequency. Therefore, if the resonance frequency can be designed so as to be excluded from the harmonic frequency of the power frequency, the resonance frequency can be reduced to an extent that there is no problem. Further, if the resonance frequency is a relatively high frequency, the occurrence of overvoltage can be accommodated in a very short time without any problem. However, when the power transmission cable is a long distance such as several tens of km, the feasibility of such a response becomes very poor.

  That is, when the power transmission cable is a long distance such as several tens of km, the ground capacitance is large. And since the number of operation banks of the transformer used in the power transmission system and the number of operation lines of the power transmission cable are not constant, the effective ground capacitance varies. As a result, a large number of resonance frequencies are generated according to the difference in operation state, and even a resonance frequency having a frequency that matches the lower harmonic of the power frequency may be generated. In other words, the longer the transmission cable is, the lower the resonance frequency and the greater the number, so it is not possible to prevent the occurrence of overvoltage that is a problem by removing the resonance frequency of the power transmission system from the harmonic frequency of the power frequency. It becomes unrealistic.

Yodogawa et al., Power Equipment Course 5 "Transformers", Nikkan Kogyo Shimbun, 1966 Nishiwaki et al., Resonant overvoltage due to induced inrush current of parallel-connected transformer, 2009 IEEJ National Convention, 7-125

  A problem to be solved by the present invention is to provide a power transmission system including a group of power transmission cables and a power transmission method using the same, which can prevent the occurrence of overvoltage even when the power transmission cable is a long distance. It is.

  The power transmission system of the embodiment includes a power transmission cable group, a transformer group, and a circuit breaker group. One end of the transmission cable group can be connected to the substation voltage output from the first substation that is the substation on the side where the power plant exists, and the second substation that is the substation on the side where the demand area exists In order to supply power from the first substation, a plurality of power transmission cables having a length of 10 km or more, the other end of which is connected to the second substation, are electrically connected in parallel to each other.

  In addition, the transformer group is formed by electrically connecting a plurality of transformers for transforming the power supplied from the power transmission cable group provided in the second substation. The circuit breaker group is provided in the second substation so as to be electrically located between the power transmission cable group and each transformer of the transformer group.

In the transformer group, there are a normal transformer that does not have a priority relationship for operation and one priority operation transformer that is operated in preference to the normal transformer. Equal normal transformer and the respective number of turns N and the number of turns N ₁ of the priority operational transformer, a flux peak value [Phi _P at the rated operation in the normal transformers each core, at rated operation in the iron core of the priority operating transformer It is equal to the magnetic flux peak value Φ _P1 of. The priority operation transformer has a relationship of 0.80Φ _S1 ≦ Φ as the relationship between the saturation magnetic flux value Φ _S1 in the iron core of the priority operation transformer and the magnetic flux peak value Φ _P1 at the rated operation in the iron core of the priority operation transformer. _P1 ≦ 0.90Φ _S1 is satisfied. Usually transformer, respectively, as the relationship between the magnetic flux peak value [Phi _P at the rated operation in the normal transformers each core saturation flux value [Phi _S and iron center in meets Φ _S ≧ 2.55Φ _P .

1 is a system diagram (corresponding to a single phase) illustrating a configuration of a power transmission system according to a first embodiment. The relationship between “excitation current” and “magnetic flux” in the normal transformers 22, 23, and 24 shown in FIG. 1 and the relationship between “excitation current” and “magnetic flux” in the priority operation transformer 21A shown in FIG. FIG. The characteristic view which shows the relationship of "excitation current" vs. "magnetic flux" in the priority operation transformer 21A shown in FIG. FIG. 2 is a waveform diagram showing the relationship among voltage, magnetic flux, and excitation current when a voltage phase of 0 degrees is applied without residual magnetic flux in the priority operation transformer 21A shown in FIG. The characteristic view which shows the relationship of "excitation current" versus "magnetic flux" in the normal transformers 22, 23 and 24 shown in FIG. FIG. 2 is a waveform diagram showing the relationship among voltage, magnetic flux, and excitation current when the normal transformers 22, 23, and 24 shown in FIG. The display figure which shows each case of the operation | movement assumed in the power transmission system shown in FIG. The graph which shows the relationship between the transmission cable length and the resonance frequency calculated by each case shown to FIG. 5A. The characteristic view explaining the influence of the residual magnetic flux in the normal transformers 22, 23, and 24 shown in FIG. The connection diagram explaining the mitigation element of the residual magnetic flux in the normal transformers 22, 23, and 24 shown in FIG. FIG. 5 is a system diagram (corresponding to a single phase) illustrating a configuration of a power transmission system according to a second embodiment. FIG. 5 is a system diagram (corresponding to a single phase) illustrating a configuration of a power transmission system according to a third embodiment. FIG. 9 is a system diagram (corresponding to a single phase) illustrating a configuration of a power transmission system according to a fourth embodiment. FIG. 9 is a system diagram (corresponding to a single phase) illustrating a configuration of a power transmission system according to a fifth embodiment. FIG. 7 is a system diagram (corresponding to a single phase) illustrating a configuration of a power transmission system according to a sixth embodiment.

(First embodiment)
Based on the above, the power transmission system of the embodiment will be described below with reference to the drawings. FIG. 1 shows the configuration of the power transmission system of the first embodiment, and only a portion corresponding to a single phase among the three phases is shown for the sake of simplicity of explanation. The power plant 101, the transmission line 102, and the substation 10 shown in the figure are system facilities that exist as preconditions, and the remaining power transmission cables 11, 12 and the substation 20 correspond to this power transmission system. Here, the substation 20 includes transformers 21 </ b> A, 22, 23, 24, circuit breakers 31, 32, 33, 34, circuit breaker switching controller 41, high-voltage side bus 107, and low-voltage side bus 108.

  The electric power generated at the power plant 101 is transmitted to the demand area. Since an overhead power transmission line cannot be used for power transmission to a remote island assumed as a demand area, a power transmission cable group (power transmission cables 11 and 12) laid on the seabed is used instead. Here, as the power transmission cables 11 and 12, cables having a long distance of 10 km or more are considered.

  On the side where the power plant 101 exists as viewed from the power transmission cables 11 and 12 (the mainland side), a substation 10 (hereinafter also referred to as a first substation) is connected from a power supply system composed of the power plant 101 and the power transmission line 102. ), The voltage is stepped down by the transformers 104 and 105 via the bus 103 and connected to the power transmission cables 11 and 12 via the bus 106. That is, the substation voltage output from the substation 10 is connected to the power transmission kales 11 and 12.

  On the other hand, on the side where the demand area exists (the remote island side) when viewed from the power transmission cables 11 and 12, the power transmission cables 11 and 12 are connected to the bus 107 of another substation 20 (hereinafter also referred to as a second substation). After being further stepped down, it is sent to a demand place (not shown) via the bus 108. That is, the other ends of the transmission cables 11 and 12 are connected to the substation 20 in order to supply power from the first substation 10 to the second substation 20.

  In general, a plurality of transformers 104 and 105 for stepping down in the first substation 10 are provided, and a necessary number of transformers are operated in parallel connection according to a necessary power transmission situation (the number of transformers = banks). Number; here 2 banks). A plurality of power transmission cables 11 and 12 are also generally provided in parallel, and a necessary number of power transmission cables are operated from among them according to a necessary power transmission situation (number of power transmission cables = number of lines; here, two lines). In order to set the number of banks and the number of lines to be operated, a circuit breaker is individually provided, although not shown.

  The transformers 21A, 22, 23, and 24 (= transformer group) included in the second substation 20 are electrically connected in parallel to each other in order to transform the power supplied from the power transmission cables 11 and 12, respectively. (The number of transformers = the number of banks; here, four banks), and the same number as the first substation 10 is used in that a necessary number of transformers are operated in parallel according to a necessary power transmission situation. Therefore, the circuit breakers 31 to 34 (breaker group) of the substation 20 are electrically located between the power transmission cables 11 and 12 and the transformers of the transformer groups 21A, 22, 23, and 24, respectively. Is provided.

  Of the transformers belonging to the transformer group, the transformer 21A is a priority operation transformer, and the remaining transformers 22, 23, and 24 are normal transformers. The priority operation transformer is a transformer in which the operation order is given priority over the normal transformer. That is, when starting operation with only one transformer in the second substation 20, the transformer 21A is used, and in such a case, the other transformers 22 to 24 are not used.

  For this reason, the circuit breaker switching control unit 41 of the substation 20 switches from the open state to the closed state in the circuit breaker 31 connected to the priority operation transformer 21A among the circuit breakers 31 to 34. It has at least a control function to be performed only when all of the circuit breakers 32 to 34 connected to 24 are in an open state. Conversely, switching from the closed state to the open state in the circuit breaker 31 may be determined independently of the open / closed state of the circuit breakers 32-34, but when the circuit breaker 31 is closed again, the above condition is satisfied. Do so to meet. In addition, it is not necessary to provide an operational priority relationship between the normal transformers, and the control function of the circuit breaker switching control unit 41 is adapted to this.

  2 shows the relationship between “excitation current” versus “magnetic flux” in the normal transformers 22, 23, and 24 shown in FIG. 1, and “excitation current” vs. “magnetic flux” in the priority operation transformer 21A shown in FIG. "Is shown in comparison with the relationship. In FIG. 2, those relationships in the normal transformers 22, 23, 24 are drawn by solid lines, and the same relationship in the priority operation transformer 21 </ b> A is drawn by broken lines.

For convenience of explanation, the relationship in the priority operation transformer 21A will be described first. In the priority operation transformer 21A, 0.80Φ _S1 ≦ Φ _P1 ≦ 0.90Φ _S1 is satisfied as a relationship between the saturation magnetic flux value Φ _S1 in the iron core and the magnetic flux peak value Φ _P1 in the rated operation in the iron core. . In general, in a transformer, such a relationship between the saturation magnetic flux value Φ _S1 and the flux peak value Φ _P1 during rated operation is an ordinary relationship. By setting in this way, it is possible to avoid an increase in size of the transformer by reducing the core cross-sectional area while preventing saturation of the core during rated operation (steady state).

On the other hand, in the normal transformers 22 to 24, as shown in the figure, the relationship between the saturation magnetic flux value Φ _S in each iron core and the magnetic flux peak value Φ _P at the rated operation in the iron core is Φ _S ≧ 2.55Φ. _P is satisfied. Therefore, in the normal transformers 22 to 24, the relationship between the “excitation current” and the “magnetic flux” at the rated operation (steady state) is as shown by the thick line in the figure, and there is a large margin until the iron core is saturated. There is.

  3A shows the relationship between “excitation current” and “magnetic flux” in the priority operation transformer 21A shown in FIG. 1, and FIG. 3B shows the residual magnetic flux in the priority operation transformer 21A shown in FIG. The waveform shows the relationship among voltage, magnetic flux, and excitation current when the voltage phase is 0 °. T0 to t4 in FIG. 3A correspond to times t0 to t4 in FIG. 3B, respectively.

In the priority operation transformer 21A, when a voltage phase is applied (voltage applied) at 0 degrees without residual magnetic flux, the magnetic flux increases from time t0 to time t1, but unlike the voltage phase 90 degrees, Somewhere in between, the iron core reaches saturation (magnetic flux at this time = Φ _S1 ) (see t0 to t1 in FIG. 3A, “voltage” and “magnetic flux” in FIG. 3B). After this saturation (magnetic flux = Φ _S1 ), the relationship between the “excitation current” and the “magnetic flux” becomes a very small slope as the saturation region, and as shown in FIG. 3A, a very large current at time t1. (= See “Excitation Current” in FIG. 3B).

  Next, from time t1 to time t2, the value of the magnetic flux returns from the saturated state to zero. Thereafter, the operation is repeated in substantially the same manner, and the states change from t2 to t3 and from t3 to t4 in FIGS. 3A and 3B. In any case, it will be in a steady state, and the saturation will be eliminated by the hysteresis characteristic with the origin as the symmetry point in FIG. 3A.

  As shown in “Excitation Current” in FIG. 3B, the excitation current immediately after the voltage application in the priority operation transformer 21A is distorted including the direct current component and can be generated at a much larger value than the normal state (excitation inrush current: For example, the excitation inrush current is about 760 A, which is three digits larger than the normal excitation current 0.8 A). This is because, as described above, the charging occurs so that the moment when the transformer core is saturated and the moment when the transformer core is not saturated are alternately repeated according to the charging phase, and at the moment when the transformer is saturated, the transformer is equivalently This is to act as an air-core reactor. This is a normal phenomenon as a transformer alone.

Note that the worst condition for increasing the magnetizing inrush current is when a residual magnetic flux exists in the iron core and is turned on at a voltage phase of 0 degrees. In this case, at time t0 corresponds to the point of the residual magnetic flux [Phi _R1 shown in Figure 3A, that the excitation from the point indicated by t1, t3 in Fig. 3A by being initiated is further moved to the right Because.

In the priority operation transformer 21A, as described above, a large magnetizing inrush current may occur at the time of power application. However, since the priority operation transformer 21A is operated with priority over other transformers (normal transformers 22 to 23), when power is applied to the priority operation transformer 21A, it is already in an operation state. There is no parallel transformer. That is, even when the saturation magnetic flux value Φ _S1 of the priority operation transformer 21A is set to a value according to the normal design, it is possible to avoid a situation in which a large magnetizing inrush current is induced in the other transformers 22 to 24.

  In general, attraction of the magnetizing inrush current occurs as follows. When a voltage is newly applied to another transformer in a state where there is an operating transformer connected in parallel, a magnetizing inrush current starts to flow through the other transformer. There is no immediate effect on the parallel transformer. The excitation inrush current is maximized when the voltage is applied at a phase of 0 °, and the current at that time has an alternating current component and a direct current component, both of which have large values that reach several times the rated current. Here, since there is an electrical resistance on the side of the power supply system as seen from the transformer group, a voltage drop occurs for a direct current. Therefore, a voltage including this DC divided voltage is applied to the parallel transformer, and the parallel transformer is gradually demagnetized by this DC divided voltage. Excitation current is induced. Since the electrical resistance that causes a DC voltage drop increases as the transmission line becomes longer, such induction of the exciting DC current is likely to occur in a system having a long-distance transmission line.

  In this embodiment, when power is applied to the priority operation transformer 21A, there is no transformer that is already in operation and is connected in parallel. Therefore, there is no phenomenon of induction of magnetizing inrush current with respect to the other transformers 22 to 24 due to the priority operation transformer 21A. Therefore, a distorted overvoltage may be generated on the primary side of the second substation 20. Absent. Therefore, due to the priority operation transformer 21A, damage to power transmission system equipment (phase adjusting capacitors, etc.), overheating of power transmission cables 11 and 12, harmonic disturbance of these equipment and facilities, supply to demand areas Does not cause problems such as instability of voltage.

  The above is an explanation that there is no phenomenon of induction of magnetizing inrush current with respect to the other transformers 22 to 24 due to the priority operation transformer 21A, but another phenomenon caused by the other normal transformers 22 to 24. The presence or absence of the phenomenon of attraction of magnetizing inrush current to the transformer is described below.

  4A shows the relationship between “excitation current” and “magnetic flux” in the normal transformers 22, 23, 24 shown in FIG. 1, and FIG. 4B shows the normal transformers 22, 23 shown in FIG. 24, the relationship between the voltage, magnetic flux, and excitation current when the voltage phase is 0 degrees without residual magnetic flux is shown as a waveform. T0 to t4 in FIG. 4A correspond to times t0 to t4 in FIG. 4B, respectively.

  Normally, in the transformers 22, 23, and 24, when the voltage phase is zero (voltage applied) with no residual magnetic flux, the magnetic flux increases from time t0 to time t1, but even in this case, the iron core Saturation is not reached (see t0 to t1 in FIG. 4A, “voltage” and “magnetic flux” in FIG. 4B). Next, from time t1 to time t2, the value of the magnetic flux returns to zero. Thereafter, the operation is repeated in substantially the same manner, and the states change from t2 to t3 and from t3 to t4 in FIGS. 4A and 4B. In addition, in any case, it falls within a steady state, and in FIG. 4A, a hysteresis characteristic with the origin as a symmetric point is drawn (indicated by a thick line).

As shown in “Excitation Current” in FIG. 4B, the excitation current at the start of voltage application in the normal transformers 22, 23, and 24 remains small. This is a direct effect of setting the saturation magnetic flux value Φ _S larger in the normal transformers 22, 23, and 24 than in the priority operation transformer 21A. More specifically, the saturation magnetic flux value Φ _S in each iron core of the normal transformer in FIG. 4A is related to the magnetic flux peak value Φ _P during rated operation in the iron core, as already described, Φ _S ≧ 2 .55Φ _P is satisfied.

The basis for calculating Φ _S ≧ 2.55Φ _P is as follows. The value Φ _S1 (see FIG. 2) of the saturation magnetic flux according to a normal design with respect to the magnetic flux peak value Φ _P at the rated operation is 1 / 0.90 to 1 / 0.80 times, that is, Φ _S1 = 1.11Φ. _{P 1 to} 1.25Φ _P. For values [Phi _S1 of saturation magnetic flux in the normal design, the value [Phi _S of saturation magnetic flux in the above normal transformer 22-24, also prevent saturation of the iron core in the worst condition is set to more than double (2 + k) ing.

Here, “2” of (2 + k) times is assumed to be 0 degree as the worst case of the applied potential phase. Even if the alternating amplitude of the magnetic flux is completely shifted to one polarity, it is somewhat until saturation. This is a value corresponding to a margin (see FIG. 3B). “K” is an additional value assuming that magnetic flux remains in the iron core and excitation starts from that state as a further adverse condition. k is the ratio of the values of the residual magnetic flux for the magnetic flux peak value [Phi _P at the rated operation, is estimated to be 0.95 at the maximum. If there is some degaussing factor in the power transmission system, it is possible that k has decreased to about 0.30.

Based on the above, from Φ _S ≧ (2 + k) Φ _S1 , it is possible to calculate that the condition that Φ _S ≧ 2.30Φ _S1 is necessary by using a loose value 0.30 as k (which is not loose as k) A value of 0.95 is sufficient). And since Φ _S1 = 1.11Φ _{P to} 1.25Φ _P , the condition of Φ _S ≧ 2.55Φ _P can be finally derived as necessary using the looser Φ _S1 = 1.11Φ _P ( It is sufficient if it is derived using Φ _S1 = 1.25Φ _P which is not loose).

  In this configuration, when power is normally applied to the transformers 22 to 24, even if there are transformers that are already in operation and are connected in parallel with the transformers 22-24, there is no adverse effect on those transformers. Absent. This is because no excitation inrush current is generated even when power is normally applied to the transformers 22 to 24, and naturally no large excitation inrush current is induced in other transformers.

  Therefore, there is no phenomenon of generation of its own magnetizing inrush current due to the normal transformers 22 to 24 and induction of magnetizing inrush current with respect to other transformers. No overvoltage distorted on the side. Therefore, normally, due to the transformers 22 to 24, the power transmission system equipment (phase adjusting capacitors, etc.) is damaged, the power transmission cables 11 and 12 are overheated, harmonic disturbances of these equipment / equipment, and supply to demand areas. Does not cause problems such as instability of voltage.

Next, a more specific structural relationship between the priority operation transformer 21A and the normal transformers 22 to 24 will be supplementarily described. As on the relationship between structure, usually transformers 22-24 illustrating the relationship between the number of turns N ₁ of each of windings N and the priority management transformer 21A. Here, the “number of turns” is the number of turns on the same side (that is, the same one of the primary side and the secondary side) of each transformer. First, to make them equal:

In this case, assuming that the magnetic flux peak value Φ _P at the rated operation in the iron core of each of the normal transformers 22 to 24 is equal to the magnetic flux peak value Φ _P1 at the rated operation in the iron core of the priority operation transformer 21A, as follows. Can be configured. That is, the priority operation transformer 21A sets 0.80Φ _S1 ≦ Φ _P1 ≦ 0.90Φ _S1 as the relationship between the saturation magnetic flux value Φ _S1 in the iron core and the magnetic flux peak value Φ _P1 in the rated operation in the iron core. As a relationship between the value Φ _S of the saturation magnetic flux in the iron core of each of the normal transformers 22 to 24 and the value Φ _S1 in the priority operation transformer 21A, Φ _S ≧ 2.30Φ _S1 is satisfied. The latter relationship is based on the already described relationship of Φ _S ≧ (2 + k) Φ _S1 (using 0.30 as described above for k).

If normal transformers 22-24 is not set equal to the number of turns N ₁ of each of windings N and the priority management transformer 21A or less. In this case, the relationship between the saturation magnetic flux value Φ _S1 in the iron core of the priority operation transformer 21A and the magnetic flux peak value Φ _P1 in the rated operation in the iron core is 0.80Φ _S1 ≦ Φ _P1 ≦ 0.90Φ _S1 . Assuming this, it can be configured as follows. That is, SN ≧ 2.30S ₁ N as the relationship between the cross-sectional area S and the number of turns N of the core of each of the normal transformers 22 to 24 and the cross-sectional area S ₁ and the number of turns N ₁ of the core of the priority operation transformer 21A ₁ is satisfied.

In this case, since the normal transformer 22-24 winding number N ₁ of each of windings N and the priority management transformer 21A are different, when considering these different number of turns, wherein [Phi _S ≧ 2 already indicated .30Φ _S1 is changed to the formula Φ _S N ≧ 2.30Φ _S1 N ₁ for the same purpose. This is because, in general, a physical quantity obtained by multiplying the value of magnetic flux by the number of turns has a voltage dimension, and when transformers are connected in parallel, the voltage is common to each transformer.

In general, the value of the saturation magnetic flux is a product represented by saturation magnetic flux density × iron core cross-sectional area, and the saturation magnetic flux density is the same value if the core material is the same in the priority operation transformer and the normal transformer. Based on the above, SN ≧ 2.30S ₁ N ₁ is derived. In this case, the transformers 22 to 24 usually have a margin until the iron core is saturated. Therefore, an increase in the core cross-sectional area does not correspond to this, but the number of turns is also increased to increase the core cross-sectional area. It is shown that the increase in size of the transformer can be mitigated. If this increases the number of turns on either side, the transformer will correspondingly increase the number of turns on the other side.

  Next, when the power transmission cables 11 and 12 are long distances such as 10 km or more, the resonance frequency of the power transmission system is removed from the harmonic frequency of the power frequency, and the primary side of the second substation 20 is The difficulty of preventing the occurrence of overvoltage due to the magnetizing inrush current will be described supplementarily. In short:

  When the power transmission cables 11 and 12 are long distances such as several tens of km, the ground capacitance is large (for example, larger than about 2 μF). Since the number of operation banks of the transformers 104 and 105 used in the power transmission system and the number of operation lines of the power transmission cables 11 and 12 are not constant, the effective ground capacitance varies. As a result, a large number of resonance frequencies are generated according to the difference in operation state, and even a resonance frequency having a frequency that matches the lower harmonic of the power frequency may be generated. In other words, the longer the transmission cables 11 and 12 are, the lower the resonance frequency is, and the greater the number of the transmission cables 11, 12. It is not realistic to do.

The result of calculating this point will be described below. FIG. 5A shows each case of operation assumed in the power transmission system shown in FIG. 1. FIG. 5B shows the relationship between the transmission cable length and the resonance frequency calculated in each case shown in FIG. 5A. The resonance frequency f _R can be generally calculated by 1 / (2π√ (LC)). Here, as L, (1) the inductance L _{G of the} power supply system constituted by the power plant 101 and the transmission line 102, (2) the leakage inductance L _TR of the transformers 104 and 105, (3) the transmission cable 11, Twelve inductances L _C become elements, and the total value becomes larger than about 10 mH, for example. On the other hand, as C, the ground capacitance C _C of the power transmission cables 11 and 12 is an element, and the value thereof is larger than about 2 μF, for example, as described above.

  From FIG. 5B, it can be seen that a large number of resonance frequencies are generated according to the difference in operation state, and that a plurality of resonance frequencies having frequencies close to the lower harmonics of the power frequency (60 Hz in this case) are generated as the cable length increases. . The overvoltage generated on the primary side of the substation 20 due to the magnetizing inrush current resonates and increases when the resonance frequency of the power transmission system matches the harmonic frequency of the power frequency. If the resonance frequency is a relatively high frequency, the occurrence of overvoltage can be accommodated in such a short time that there is no problem, but this indicates that this can hardly be realized. The resonance frequency changes to some extent depending on the state of the load connected to this system, but does not change greatly, and there is no difference that it exists in the vicinity of the low-order harmonic frequency of the power frequency.

Furthermore, the inductance L _G of the power supply system, the power supply system then updates to fluctuate if changed, the leakage inductance L _TR of the transformer 104, 105 also for example tap position changes if the change. Therefore, even if it is designed to remove the resonance frequency well so as to sew the constraint at the beginning, the state is not guaranteed.

Next, FIG. 6A explains the residual magnetic flux in the normal transformers 22, 23, and 24 shown in FIG. 1, and FIG. 6B shows the residual magnetic flux in the normal transformers 22, 23, and 24 shown in FIG. Describes magnetic flux mitigation factors. Residual magnetic flux [Phi _RR shown in FIG. 6A is estimated up to 0.95 relative to the magnetic flux peak value [Phi _P at rated operation. If there is some degaussing factor in the power transmission system, this ratio (= k) may be reduced to about 0.30.

  As a demagnetizing factor, as shown in FIG. 6B, a ground capacitance 51 that is normally present on the primary side of the transformer 24 (or 22, 23) can be cited. When the circuit breaker 41 is changed from a closed state to an open state due to the presence of the ground capacitance 51, a free vibration phenomenon occurs between the normal transformer 24 and the free vibration current is attenuated by a resistance component. Energy is lost. This becomes a demagnetizing factor.

Therefore, conversely, to measure the residual magnetic flux [Phi _RR in actual normal transformer can calculate the unknown k by using this. According to the calculated k, as shown in FIG. 6A, the saturation magnetic flux value Φ _S of the normal transformers 22 to 24 can be designed to satisfy Φ _S ≧ 1.11 (2 + k) Φ _P. . According to this, the value Φ _S of the saturation magnetic flux of the normal transformers 22 to 24 can be determined in accordance with the actual situation. When k = 0.30, the relationship of Φ _S ≧ 1.11 (2 + k) Φ _P is Φ _S ≧ 2.55Φ _P as already described.

(Second Embodiment)
Next, FIG. 7 shows the configuration of the power transmission system according to the second embodiment, and only a portion corresponding to a single phase of the three phases is shown for the sake of simplicity of explanation. In FIG. 7, the same components as those shown in FIG. In this embodiment, the priority operation transformer 21A is eliminated, and a normal transformer 21 is provided instead. The normal transformer 21 has the same configuration as the normal transformers 22 to 24. The circuit breaker switching control unit 41A has no restriction that the transformer 21 should be preferentially operated.

  This power transmission system does not generate a magnetizing inrush current when any normal transformer is used first. This point is clear from the points already explained. And when another normal transformer is charged after that, the magnetizing inrush current does not arise in the normal transformer. This is equally clear. Accordingly, regardless of the change in power and operation of the normal transformers 21 to 24, no magnetizing inrush current is generated, and no magnetizing inrush current is induced to another transformer.

  Therefore, there is no phenomenon of generation of its own magnetizing inrush current due to the normal transformers 21 to 24, and induction of magnetizing inrush current to other transformers, and thereby the primary side of the second substation 20A. No distorted overvoltage occurs. Therefore, the transformers 21 to 24 usually cause damage to the equipment for the power transmission system, overheating of the power transmission cables 11 and 12, harmonic disturbance of these equipment and facilities, and unstable supply voltage to the demand area. Do not cause problems such as.

  Further, since the priority operation transformer 21A is eliminated, the magnetizing inrush current itself does not occur regardless of the transformer. Generation of the magnetizing inrush current may cause a momentary voltage drop, but in this embodiment, such a voltage drop can also be prevented.

(Third embodiment)
Next, FIG. 8 shows a configuration of the power transmission system according to the third embodiment, and only a portion corresponding to a single phase among the three phases is shown for the sake of simplicity of explanation. In FIG. 8, the same components as those shown in the already described drawings are denoted by the same reference numerals, and the description thereof is omitted. In this form, there are solar power plants 66, 68 on the remote island side, and from these power plants via transmission lines 67, 69, substation 20A, transmission cables 11, 12, substation 10A, transmission line 65. It is considered that power can be supplied to the load 64 in the demand area on the mainland side. As the solar power plants 66, 68, there can be a wind power plant, an internal combustion power plant, and the like.

  In this configuration, the direction of power supply may be reversed from the configuration already described. For this reason, in the first substation 10A, the generation of the excitation inrush current or the excitation inrush current by switching the operation of the transformers 104A, 104B. Attraction may occur. Therefore, in this embodiment, as one configuration method, the saturation value of the iron core as already described in both the transformers 104A and 105A of the substation 10A is improved.

  Alternatively, if one of the transformers 104A and 105A is set to operate as a priority operation transformer, only the other transformer may be configured to improve the saturation value of the iron core. In this case, the switching of the voltage application of the transformers 104A and 105B is performed by controlling the circuit breakers 62 and 63 by the circuit breaker switching control unit 61. In that case, the circuit breaker switching control unit 61 switches from the open state to the closed state in the circuit breaker connected to the priority operation transformer, when the circuit breaker connected to the normal transformer is in the open state. Do as long as possible.

  In this embodiment, overvoltage can be prevented from occurring in the first substation 10A. The first substation 10A in this description can be said to functionally correspond to the second substation 20 shown in FIG. 1 in view of the direction of power supply. In FIG. 8, the circuit breakers 31 to 34 and the circuit breaker switching control unit 41 </ b> A actually provided in the second substation 20 </ b> A are not shown for easy understanding in the explanation of the function.

(Fourth embodiment)
Next, FIG. 9 shows the configuration of the power transmission system according to the fourth embodiment, and only a portion corresponding to a single phase of the three phases is shown for the sake of simplicity of explanation. In FIG. 9, the same components as those shown in the already described drawings are denoted by the same reference numerals, and the description thereof is omitted. In this form, the configuration of the power system is different from that shown in FIG. 1 and the first substation 10 is not provided.

  That is, the power transmission cables 11 and 12 do not go through the substation, the power system by the power plant 71 and the power transmission line 72, the power system by the power plant 101 and the power transmission line 102, and the power by the power plant 73 and the power transmission line 74. It is directly connected to the three power systems of the system. In this case, when determining the resonant frequency of the system, the leakage inductance of the transformer in the first substation is removed as an element.

  That is, the resonance frequency is determined by the inductance of the power supply system and the inductance and capacitance of the power transmission cables 11 and 12, but there is still a possibility that the resonance frequency coincides with the lower harmonics of the power frequency. Will not change. In FIG. 9, there are three power supply systems, and there are seven types of combinations of inductances. When the power transmission cables 11 and 12 are combined with two lines, there are 14 types of resonance frequencies. Therefore, a design that removes the resonance frequency from the lower harmonic frequency of the power frequency is not realistic.

  Also in this embodiment, according to the configuration of the second substation 20A, the phenomenon of generation of its own magnetizing inrush current due to the normal transformers 21 to 24 and the induction of the magnetizing inrush current to other transformers are caused. Thus, a distorted overvoltage does not occur on the primary side of the second substation 20A. Therefore, the transformers 21 to 24 usually cause damage to the equipment for the power transmission system, overheating of the power transmission cables 11 and 12, harmonic disturbance of these equipment and facilities, and unstable supply voltage to the demand area. Do not cause problems such as.

(Fifth embodiment)
Next, FIG. 10 shows the configuration of the power transmission system according to the fifth embodiment, and only a portion corresponding to a single phase of the three phases is shown for the sake of simplicity. In FIG. 10, the same components as those shown in the already described drawings are denoted by the same reference numerals, and the description thereof is omitted. In this form, a plurality of substations on the remote island side are provided.

  That is, in FIG. 10, the bus 107 of the second substation 20A is connected to the third substation 81 and the fourth substation 82 via the power transmission lines 85 and 86, respectively. Here, the transformers in the substations 81 and 82 also adopt the same configuration as the transformers 21 to 24 in the substation 20A. According to this, in the third and fourth substations 81 and 82, the phenomenon of generation of its own inrush current due to the normal transformer and the induction of the inrush current of the other transformer are eliminated. Thus, it is possible to prevent the occurrence of overvoltage distorted on the primary side of those substations.

  Therefore, problems such as damage to the equipment for the power transmission system, overheating of the power transmission cables 11 and 12, harmonic disturbance of these equipment and facilities, and instability of the supply voltage to the demand area due to the normal transformer. Does not cause.

(Sixth embodiment)
Next, FIG. 11 shows a configuration of the power transmission system of the sixth embodiment, and only a portion corresponding to a single phase of the three phases is shown for the sake of simplicity of explanation. In FIG. 11, the same reference numerals are given to the same components as those shown in the already described drawings, and the description thereof is omitted. In this embodiment, a substation 91 is connected to the low-voltage bus 106 of the first transformer 10 via a power transmission line 96.

  Thus, even if the substation 91 is connected via the power transmission line 96 to the low voltage side bus 106 of the first transformer 10 to which one end of the power transmission cables 11 and 12 is connected, the second substation 20 There is no change in the effect of the configuration applied to the.

As described above, according to the power transmission system of each embodiment, at least one of the transformer groups is a priority operation transformer, and the remaining is a normal transformer. Here, Φ _S ≧ 2.55Φ _P is satisfied as the relationship between the saturation magnetic flux value Φ _S in each iron core of each normal transformer and the magnetic flux peak value Φ _P during rated operation in the iron core. That is, generally, the peak value of the magnetic flux at the rated operation as seen from the value of the saturation magnetic flux is set to a ratio of about 0.80 to 0.90, and this ratio is 0.392 (= 1 / 2.55) or less. This power transmission system set to 大 き な has a large margin between excitation in rated operation and saturation of magnetic flux.

  By setting in this way, the iron core will not be magnetically saturated at the start of voltage application even if the transformer normally has residual magnetic flux and the application phase at the start of voltage application is the worst condition. Therefore, it is possible to prevent the occurrence of a large magnetizing inrush current. As a result, excessive transformer inrush current is induced even when there are transformers that are already in operation (including priority operation transformers) in parallel with the transformers that have been charged. There is no. Therefore, the distorted overvoltage is prevented from occurring on the primary side of the second substation, and the occurrence of the overvoltage can be prevented even when the power transmission cable is a long distance.

  As mentioned above, although some embodiment of this invention was described, these embodiment is shown as an example and is not intending limiting the range of invention. These novel embodiments can be implemented in various other forms, and various omissions, replacements, and changes can be made without departing from the spirit of the invention. These embodiments and modifications thereof are included in the scope and gist of the invention, and are included in the invention described in the claims and the equivalents thereof.

  DESCRIPTION OF SYMBOLS 10 ... Substation (1st substation), 10A ... Substation, 11,12 ... Transmission cable, 20, 20A ... Substation (2nd substation), 21A ... Preferential operation transformer, 21,22,23 24, a normal transformer, 31, 32, 33, 34 ... circuit breaker, 41, 41A ... circuit breaker switching control unit, 51 ... ground capacitance, 61 ... circuit breaker switching control unit, 62, 63 ... circuit breaker, 64 ... Load, 65 ... Transmission line, 66, 68 ... Solar power plant, 67, 69 ... Transmission line, 71, 73 ... Power plant, 72, 74 ... Transmission line, 81, 82 ... Substation, 85, 86 ... Transmission line, 91 ... substation, 96 ... transmission line, 101 ... power plant, 102 ... transmission line, 103 ... high voltage side bus, 104, 105 ... transformer, 104A, 105A ... transformer, 106 ... low voltage side bus, 107 ... high-voltage bus, 108 ... low-voltage bus.

Claims (6)

One end can be connected to the substation voltage output from the first substation that is the substation on the side where the power plant exists, and the first substation to the second substation that is the substation on the side where the demand area exists A transmission cable group in which the other end is connected to the second substation for power supply from the substation, and a plurality of transmission cables having a length of 10 km or more are electrically connected in parallel to each other;
Wherein provided on the second converter station, anda transformer group connected in parallel to the plurality from one another electrically are transformer to transform the electric power supplied from the power transmission cable group,
In the transformer group, there is a normal transformer that does not have a priority relationship of operation with each other, and one priority operation transformer that is operated in preference to the normal transformer,
The normal transformer and each winding number N the priority management transformer windings N ₁ are equal,
The magnetic flux peak value Φ _P at the rated operation in the iron core of each normal transformer and the magnetic flux peak value Φ _P1 at the rated operation in the iron core of the priority operation transformer are equal,
The priority operation transformer has a relationship between the saturation magnetic flux value Φ _S1 in the iron core of the priority operation transformer and the magnetic flux peak value Φ _P1 at the rated operation in the iron core of the priority operation transformer , 0.80Φ _S1 ≦ Φ _P1 ≦ 0.90Φ _S1 is satisfied,
The through normal transformer, respectively, as the relationship between the magnetic flux peak value [Phi _P at the rated operation in the normal transformers each core saturation flux value [Phi _S and iron center in satisfies Φ _S ≧ 2.55Φ _P Power transmission system. Transmission system of the prior Symbol value [Phi as the relation _S and the value Φ _S1, Φ _S ≧ 2.30Φ _S1 is claim 1 are met. Before Symbol Normal sectional area S and the normal transformer each winding number N of the transformer each core, the priority management transformer core cross-sectional area S ₁ and the priority management transformer relationship between the number of turns N ₁ of The power transmission system according to claim ₁ , wherein SN ≧ 2.30S ₁ N ₁ is satisfied. A circuit breaker group provided in the second substation so as to be electrically located between each of the power transmission cable group and each transformer of the transformer group;
In the iron core of the normal transformer, and residual magnetic flux [Phi _RR remaining when breaker led to the normal transformer of the circuit breaker group is from a closed state to an open state, and the value [Phi _P The power transmission system according to claim 1, wherein when the ratio Φ _RR / Φ _P is represented by k, Φ _S ≧ 1.11 (2 + k) Φ _P is satisfied. A circuit breaker group provided in the second substation so as to be electrically located between each of the power transmission cable group and each transformer of the transformer group;
In the circuit breaker group, switching from the open state to the closed state in the circuit breaker connected to the priority operation transformer, all the circuit breakers connected to the normal transformer in the circuit breaker group are in the open state. The power transmission system according to claim 1, further comprising: a circuit breaker switching control unit that performs control so as to be performed only when it is configured. One end can be connected to the substation voltage output from the first substation that is the substation on the side where the power plant exists, and the first substation to the second substation that is the substation on the side where the demand area exists A transmission cable group in which the other end is connected to the second substation for power supply from the substation, and a plurality of transmission cables having a length of 10 km or more are electrically connected to each other in parallel; A transformer group provided in a substation, and a transformer group in which a plurality of transformers for transforming power supplied from the power transmission cable group are electrically connected to each other , and a power transmission method using a power transmission system,
In the transformer group, there is a normal transformer that does not have a priority relationship of operation with each other, and one priority operation transformer that is operated in preference to the normal transformer,
The normal transformer and each winding number N the priority management transformer windings N ₁ are equal,
The magnetic flux peak value Φ _P at the rated operation in the iron core of each normal transformer and the magnetic flux peak value Φ _P1 at the rated operation in the iron core of the priority operation transformer are equal,
The priority operation transformer has a relationship of 0.80Φ _S1 ≦ Saturation magnetic flux value Φ _S1 in the iron core of the priority operation transformer and magnetic flux peak value Φ _P1 in the rated operation in the iron core of the priority operation transformer. Operate to satisfy Φ _P1 ≦ 0.90Φ _S1 ,
The communication normal transformer, respectively, as the relationship between the magnetic flux peak value [Phi _P at the rated operation in the normal transformers each core saturation flux value [Phi _S and iron center in satisfies Φ _S ≧ 2.55Φ _P Power transmission method to be rated operation

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