Substation system

Abstract

In a substation system having a Power Voltage Transformer (PVT) connected to a three-phase AC power grid, the overcurrent on the secondary side of the PVT is reduced. [Solution] In a substation system (1) connected to a three-phase AC power system (SYS), each phase winding in the Y-connected secondary winding (112) of a three-phase transformer PVT unit (11), which is composed of PVTs, converts the primary-side phase voltage supplied from the power system (SYS) to each phase winding in the primary winding (111) of the three-phase transformer PVT unit (11) to a secondary-side phase voltage lower than the primary-side phase voltage. The three-phase transformer PVT unit (11) outputs the line-to-line voltage between one phase and another on the secondary side of the substation system (1) as the secondary-side line-to-line voltage. In the substation system (1), an auxiliary transformer (13) located downstream of the three-phase transformer PVT unit (11) converts a target voltage corresponding to the secondary-side line-to-line voltage to a voltage lower than the target voltage.

Description

[[Technical Field]]

[0001] The following disclosure relates to a substation system. [[Background Art]]

[0002] Various technical studies have been conducted on each device belonging to the power reception and transformation field. For example, the following Non-Patent Document 1 studies gas VT, which is a voltage transformer (VT) for instrumentation mounted on a Gas Insulated Switchgear (GIS).

[0003] It is known that the ferroresonance phenomenon in a device having an iron core (e.g., a transformer) may cause malfunctions in the device or surrounding devices. Therefore, Non-Patent Document 1 discusses the generation mechanism of the ferroresonance phenomenon in gas VT and countermeasures against the ferroresonance phenomenon. [[Prior Art Literature]] [[Non-Patent Literature]]

[0004] [[Non-Patent Document 1]] Hirotoshi Kojima, Toshihiro Kubo, Soichi Nakajima, Kazuhiro Kuroda, "Analysis and Countermeasures for Ferroresonance of Gas VT", Nissin Electric Technical Report Vol.56, No.2 (November 2011) [[Summary of the Invention]] [[Problem to be Solved by the Invention]]

[0005] In recent years, in a substation system that converts voltage supplied from a power grid by a power transformer, it has been proposed to use PVT (Power Voltage Transformer) as the power transformer. PVT is a special type of instrument voltage transformer that has a larger capacity than a conventional instrument voltage transformer.

[0006] As described below, in a substation system having a PVT connected to a three-phase AC power grid, there is a risk of large overcurrents occurring on the secondary side of the substation system. Therefore, one aspect of this disclosure aims to reduce such overcurrents. [Means for solving the problem]

[0007] A substation system according to one aspect of this disclosure is a substation system connected to a three-phase AC power grid, wherein the PVT (Power Voltage) The substation system comprises a three-phase transformer PVT unit, which is composed of a transformer, and the three-phase transformer PVT unit comprises a primary winding located on the primary side of the substation system and a secondary winding located on the secondary side of the substation system, wherein the primary winding is connected to the power grid, and the secondary winding is Y-connected, and each phase winding in the secondary winding converts the primary side phase voltage supplied from the power grid to each phase winding in the primary winding to a secondary side phase voltage lower than the primary side phase voltage, and the three-phase transformer PVT unit outputs the line voltage between one phase and another on the secondary side of the substation system as the secondary side line voltage, and the substation system further comprises an auxiliary transformer located downstream of the three-phase transformer PVT unit, which converts a target voltage corresponding to the secondary side line voltage output from the three-phase transformer PVT unit to a voltage lower than the target voltage. [Effects of the Invention]

[0008] According to one aspect of this disclosure, in a substation system having a PVT connected to a three-phase AC power grid, it is possible to reduce overcurrent on the secondary side of the substation system. [Brief explanation of the drawing]

[0009] [Figure 1] This shows an example configuration of a substation system as a reference example. [Figure 2]Examples of voltage vectors in a reference configuration, assuming a typical ungrounded three-phase AC power system, are shown. [Figure 3] This shows examples of voltage vectors in a reference configuration when a single-line-to-ground fault occurs in an ungrounded three-phase AC power system. [Figure 4] This shows an example configuration of a substation system in Embodiment 1. [Figure 5] Examples of voltage vectors in Embodiment 1 are shown, assuming a typical ungrounded three-phase AC power system. [Figure 6] Examples of voltage vectors in Embodiment 1 when a single-line-to-ground fault occurs in an ungrounded three-phase AC power system are shown. [Figure 7] An example configuration of the substation system in Embodiment 2 is shown. [Figure 8] An example configuration of a substation system in Embodiment 3 is shown. [Modes for carrying out the invention]

[0010] [Reference form] Prior to describing the substation system in Embodiment 1, a reference embodiment will be described. For the sake of clarity, components having the same function as those described in the reference embodiment will be denoted by the same reference numerals in subsequent embodiments, and their descriptions will not be repeated. Furthermore, for the sake of simplification, explanations of matters similar to those in the prior art will be omitted as appropriate.

[0011] In this specification, each component and each numerical value is merely illustrative unless otherwise specified. Therefore, for example, unless otherwise specified, the positional and connection relationships of each component are not limited to the examples in each figure. In this specification, the term "connected" means "electrically connected" unless otherwise specified.

[0012] (Example configuration of substation system 1X as a reference form) FIG. 1 shows an example configuration of a substation system 1X as a reference embodiment. FIG. 1 schematically shows a single-line diagram of the substation system 1X. The substation system 1X is an example of a conventional substation system. Specifically, the substation system 1X is an example of a substation system having a PVT connected to a three-phase AC power system.

[0013] The power system SYS in FIG. 1 is an example of a three-phase AC power system. In the example of FIG. 1, the power system SYS is equivalently represented as an AC power source. In the reference embodiment, a case where the power system SYS is an ungrounded three-phase AC power system will be described. As used herein, the term "ungrounded three-phase AC power system" means a power system that includes three-phase AC transmission lines and has an ungrounded neutral point.

[0014] For example, depending on the design specifications of a hydroelectric power plant operated by an electric power company, it may be required to provide an ungrounded power system. Accordingly, an example of the substation system 1X is a system that receives electric power generated by a hydroelectric power plant.

[0015] However, as described later, the power system SYS only needs to be a three-phase AC power system, and does not necessarily need to be an ungrounded power system. Therefore, it should be noted that the substation system according to one aspect of the present disclosure is not necessarily limited to a system that receives electric power generated by a hydroelectric power plant.

[0016] The substation system 1X is a single-phase AC system. The 1X includes a PVT 11X, a protection device 12X, and an auxiliary transformer 13X. The PVT 11X, the protection device 12X, and the auxiliary transformer 13X are all single-phase devices. Accordingly, for distinction from three-phase devices, the PVT 11X may be referred to as a single-phase PVT, the protection device 12X may be referred to as a single-phase protection device, and the auxiliary transformer 13X may be referred to as a single-phase auxiliary transformer.

[0017] PVT11X includes a primary winding 111X and a secondary winding 112X. The primary winding 111X is located on the primary side of the power transformation system 1X. The primary side of the power transformation system 1X can be rephrased as the high-voltage side of the power transformation system 1X. The primary winding 111X is connected to one of the three-phase alternating current transmission lines of the power system SYS. Therefore, the primary winding 111X acquires the alternating current voltage of a predetermined one phase among the three phases of the power system SYS as the primary-side phase voltage of PVT11X. In the reference embodiment, the primary-side phase voltage of PVT11X is denoted as V1.

[0018] In the reference embodiment, a case where the line-to-line voltage in the three-phase alternating current transmission line of the power system SYS is 66 kV is exemplified. Therefore, in the reference embodiment, the voltage to ground of each phase transmission line of the three-phase alternating current of the power system SYS is 66 / √3 kV (≒38.1 kV). Accordingly, the rated value of V1 in the example of the reference embodiment is 66 / √3 kV.

[0019] The secondary winding 112X is located on the secondary side of the power transformation system 1. The secondary side of the power transformation system 1 can be rephrased as the low-voltage side of the power transformation system 1X. The secondary winding 112X converts the primary-side phase voltage of PVT11X into the secondary-side phase voltage of PVT11X. In the reference embodiment, the secondary-side phase voltage of PVT11X is denoted as V2. In the reference embodiment, the turns ratio between the primary winding 111X and the secondary winding 112X is set such that V1>V2 is satisfied.

[0020] As described above, PVT11X is designed to step down V1 to V2. In the example of the reference embodiment, a case where the rated value of V2 is 210 V is exemplified. As described above, PVT11X in the reference embodiment steps down V1, which is a high voltage, to V2, which is a low voltage. V2 only needs to be set as a voltage suitable for a second load LD2 described later.

[0021] The protective device 12X is located downstream of the PVT 11X. The protective device 12X is connected to the secondary winding 112. If an abnormality occurs in either the power system SYS or the substation system 1X, an overcurrent may flow on the secondary side of the PVT 11X as a fault current. Therefore, the protective device 12X is provided to interrupt this overcurrent.

[0022] The type of protective device 12X is not particularly limited, as long as it can interrupt overcurrents according to predetermined conditions. An example of protective device 12X is an MCCB (Molded Case Circuit Breaker). Another example of protective device 12X is a fuse. In Figure 1, an MCCB is shown as an example of protective device 12X.

[0023] Node Nd1 in Figure 1 is the secondary interconnection point of the substation system 1X. The protective device 12X can interrupt overcurrent at node Nd1. Therefore, the protective device 12X can protect the first load LD1 and the second load LD described below from overcurrent.

[0024] In the example shown in Figure 1, the substation system 1X includes a first load LD1 and a second load LD. In this reference configuration, the rated voltage of the second load LD2 is higher than the rated voltage of the first load LD1. For example, the rated voltage of the second load LD2 is 200V. Therefore, the second load LD2 is directly connected to node Nd1. Consequently, the second load LD2 is supplied with V2 from the secondary winding 112X via node Nd1.

[0025] On the other hand, as mentioned above, the rated voltage of the first load LD1 is lower than the rated voltage of the second load LD2. For example, the rated voltage of the first load LD1 is 100V. Therefore, an auxiliary transformer 13X is provided to step down the aforementioned V2 to a 100V class voltage suitable for the first load LD1. The first load LD1 is connected to node Nd1 via the auxiliary transformer 13X.

[0026] The auxiliary transformer 13X comprises a primary winding 131X and a secondary winding 132X. The rated capacity of the auxiliary transformer 13 is less than the rated capacity of the PVT 11X. The primary winding 131X is connected to node Nd1. Therefore, V2 is supplied from the secondary winding 112X of the PVT 11X to the primary winding 131X of the auxiliary transformer 13X via node Nd1. The secondary winding 132X of the auxiliary transformer 13X is connected to the first load LD1.

[0027] The secondary winding 132X converts the primary phase voltage V2 of the auxiliary transformer 13X to a secondary phase voltage of the auxiliary transformer 13X that is lower than V2. In the reference configuration, the secondary phase voltage of the auxiliary transformer 13X is denoted as Vaux. In the reference configuration, the turns ratio of the primary winding 131X and the secondary winding 132X is set such that V2 > Vaux. For example, Vaux = 105V. Thus, in this example of the reference configuration, the auxiliary transformer 13X steps down 210V to 105V.

[0028] (An example of the behavior of substation system 1) Figure 2 illustrates the voltage vectors in a reference configuration when the power system SYS is normal. In this specification, the three phases on the primary side of a substation system (e.g., substation system 1X) are denoted as phase A, phase B, and phase C, respectively. Phase A may be referred to as the first phase on the primary side of the substation system. Phase B may be referred to as the second phase on the primary side of the substation system. Phase C may be referred to as the third phase on the primary side of the substation system.

[0029] In this specification, the three phases on the secondary side of the substation system are denoted as phase a, phase b, and phase c, respectively. Phase a corresponds to phase A. For this reason, phase a may also be referred to as the first phase on the secondary side of the substation system. Phase b corresponds to phase B. For this reason, phase b may also be referred to as the second phase on the secondary side of the substation system. Phase c corresponds to phase C. For this reason, phase c may also be referred to as the third phase on the secondary side of the substation system.

[0030] In Figure 2, the symbol 201 indicates the voltage vectors of each phase of the power system SYS when the power system SYS is functioning normally. As mentioned above, the primary winding 111X of PVT11X is connected to the power system SYS. Therefore, the voltage of the power system SYS corresponds to the primary voltage of PVT11X. In the example in Figure 2, the primary winding 111X is connected to the B-phase transmission line of the power system SYS. Therefore, in the example in Figure 2, V1 is equal to the B-phase voltage (phase voltage of the B-phase) of the power system SYS. Thus, as an example, the magnitude of V1 in the example in Figure 2 is 66 / √3kV.

[0031] In Figure 2, the symbol 202 indicates the voltage vector of the secondary voltage of PVT11X when the power system SYS is normal. In the example in Figure 2, the neutral point on the secondary side of PVT11X is assumed to be grounded. As shown in Figure 2, the only V2 mentioned above is the b-phase voltage (the phase voltage of the b-phase). As mentioned above, in the reference configuration, V2 is proportional to V1. In one example in the reference configuration, V2 is {210 / (66000 / √3)} times V1. Therefore, in this example, when the power system SYS is normal, V2 is 210V.

[0032] Figure 3 illustrates the voltage vectors in a reference configuration when a single-line-to-ground fault occurs in the SYS power system. Figure 3 is a counterpart to Figure 2. The example in Figure 3 illustrates a case where a ground fault occurs in the A-phase transmission line of the SYS power system.

[0033] In Figure 3, reference numeral 301 indicates the voltage vectors of each phase of power system SYS when a ground fault occurs in the A-phase transmission line of power system SYS. When a ground fault occurs in the A-phase transmission line of power system SYS, the B-phase voltage of power system SYS becomes equal to the line-to-line voltage between the A-phase and B-phase of power system SYS.

[0034] Therefore, if a ground fault occurs in the A-phase transmission line of the SYS power system, V1 will rise compared to when the SYS power system is operating normally. Specifically, the magnitude of V1 in the example of reference numeral 301 is √3 times (≒1.73 times) the magnitude of V1 in the example of reference numeral 201. For example, if the magnitude of V1 in the example of Figure 2 is 66 / √3kV, then the magnitude of V1 in the example of Figure 3 will be 66kV.

[0035] In Figure 3, the reference numeral 302 indicates the voltage vector of the secondary voltage of PVT11X when a ground fault occurs in the A-phase transmission line of power system SYS. In the example of reference numeral 302, as V1 rises, V2 also rises compared to when power system SYS is operating normally. Specifically, the magnitude of V2 in the example of reference numeral 302 is equal to √3 times the magnitude of V2 in the example of reference numeral 202.

[0036] As an example, the size of V2 in the example in Figure 3 is: 66kV × {210 / (66000 / √3)} = 210 × √3V ≈ 363V Thus, if the magnitude of V2 in the example in Figure 2 is 210V, then the magnitude of V2 in the example in Figure 3 will be 210 × √3V.

[0037] As described above, when a single-line-to-ground fault occurs in the power system SYS, V2 rises along with V1. Consequently, as V2 rises, the current flowing from the secondary winding 112X to the protective device 12X also increases. Specifically, when a single-line-to-ground fault occurs in the power system SYS, this current increases by √3 times compared to when the power system SYS is operating normally.

[0038] Thus, when a single-line-to-ground fault occurs in the power system SYS, an overcurrent is generated on the secondary side of the substation system 1X due to the rise in V1. For example, an overcurrent with a magnitude √3 times the rated current value flows through the protective device 12X. Therefore, the protective device 12X operates to interrupt the overcurrent. As a result, the power supply from PVT 11X to the first load LD1 and the second load LD2 is stopped.

[0039] (Another example of the behavior of substation system 1X) Overcurrents may also occur on the secondary side of the substation system 1X due to factors other than single-line-to-ground faults in the power system SYS. For example, such overcurrents may be caused by residual charge on the busbar of the power system SYS.

[0040] As an example, consider a case where the power supply from power system SYS to substation system 1X is stopped due to some abnormality in power system SYS. In this case, a circuit breaker (not shown in Figure 1) located upstream of substation system 1X is opened. However, since the busbar of power system SYS has capacitance, charge remains on the busbar even after the circuit breaker has opened.

[0041] Therefore, a DC voltage originating from the residual charge on the busbar of power system SYS is applied to the substation system 1X through the transmission lines of power system SYS. Then, a current originating from this DC voltage flows into the substation system 1X.

[0042] Specifically, a DC voltage originating from the residual charge on the busbar of power system SYS is applied to the primary winding 111 of PVT11X as the primary voltage of substation system 1X. As a result, a secondary voltage of substation system 1X corresponding to this primary voltage is generated on the secondary winding 112X of PVT11X. Then, the voltage generated on the secondary side of substation system 1X is applied to the auxiliary transformer 13X. Consequently, a current originating from the DC voltage originating from the residual charge on the busbar of power system SYS flows through both PVT11X and the auxiliary transformer 13X.

[0043] Thus, in the substation system 1X, a secondary current corresponding to the primary current of the substation system 1X is generated in the secondary winding 112X of the PVT 11X. A portion of this secondary current then flows into the auxiliary transformer 13X. As described above, in the substation system 1X, a portion of the residual charge on the busbar of the power system SYS is discharged via the auxiliary transformer 13X.

[0044] The voltage applied to the auxiliary transformer 13X causes an increase in the magnetic flux density in the auxiliary transformer 13X. When the auxiliary transformer 13X reaches a magnetic saturation state, a large current may flow through it. Thus, a large current may be generated in the auxiliary transformer 13X due to magnetic saturation. Therefore, when the auxiliary transformer 13X reaches a magnetic saturation state, the current flowing through the protective device 12X may be larger than before the auxiliary transformer 13X reached a magnetic saturation state.

[0045] Based on the above, it is conceivable that residual charge on the busbar of the power system SYS may cause the auxiliary transformer 13X to reach a magnetic saturation state, resulting in an overcurrent on the secondary side of the substation system 1X. If the overcurrent is large, the protective device 12X will operate to interrupt the overcurrent. As a result, the power supply from PVT 11X to the first load LD1 and the second load LD2 will be stopped.

[0046] [Embodiment 1] As described above, in conventional substation systems (e.g., substation system 1X in the reference configuration), there is a risk that protective devices located on the secondary side of the substation may activate as a result of an overcurrent occurring on the secondary side of the substation. If the magnitude of the overcurrent can be reduced, it will not necessarily be required to activate the protective devices even if such an overcurrent occurs.

[0047] Therefore, for example, even if a single-line-to-ground fault occurs in a power system SYS, power can continue to be supplied to loads located on the secondary side of the substation system. Thus, the convenience of users utilizing those loads can be improved.

[0048] (Overview of Substation System 1) Figure 4 shows an example configuration of the substation system 2 in Embodiment 1. Substation system 1 is an example of a substation system newly created by the inventors of the present application, taking into account the aforementioned problems in conventional substation systems. Therefore, Figure 4 is a counterpart to Figure 1. In the following description, the term "overcurrent" refers to the overcurrent on the secondary side of the substation system, unless otherwise inconsistent with the context.

[0049] Embodiment 1 also illustrates the case where the power system SYS is an ungrounded three-phase AC power system. Substation system 1 is similar to substation system 1X in that it is connected to the three-phase AC power system SYS. However, unlike substation system 1X, substation system 1 receives three phases of AC power from the power system SYS.

[0050] Therefore, the substation system 1 is equipped with a three-phase transformer PVT unit 11 instead of a PVT 11X (single-phase PVT). In the example in Figure 4, the three-phase transformer PVT unit 11 converts the voltage of each phase of the power system SYS to a lower voltage. The three-phase transformer PVT unit 11 is composed of PVTs. Thus, the substation system 1 is also an example of a substation system having PVTs connected to a three-phase AC power system SYS.

[0051] Embodiment 1 illustrates a case where a three-phase transformer PVT unit 11 is composed of three single-phase PVTs. The three-phase transformer PVT unit 11 in the example in Figure 4 has a primary winding 111, a secondary winding 112, and a tertiary winding 113. Therefore, for example, by preparing three single-phase PVTs each having a primary winding, a secondary winding, and a tertiary winding, and combining the three single-phase PVTs, the three-phase transformer PVT unit 11 in the example in Figure 4 can be realized.

[0052] The substation system 1 in the example shown in Figure 4 includes a resistor 14. The resistor 14 is connected to the tertiary winding 113. A detailed explanation of the tertiary winding 113 and the resistor 14 will be provided later.

[0053] An example of a single-phase PVT constituting the three-phase transformer PVT unit 11 in Embodiment 1 is a single-phase PVT having (i) a primary winding with a rated phase voltage of 66 / √3kV, (ii) a secondary winding with a rated phase voltage of 210 / √3V (≒121.2V), and (iii) a tertiary winding with a rated phase voltage of 400 / 3V (≒133.3V). Embodiment 1 mainly illustrates the case where the three-phase transformer PVT unit 11 is configured by combining three such single-phase PVTs.

[0054] The primary winding 111 is connected to the power system SYS. The primary winding 111 is located on the primary side of the substation system 1. The primary side of the substation system 1 can be rephrased as the high-voltage side of the substation system 1. Each phase winding in the primary winding 111 is supplied with the primary side phase voltage from the power system SYS.

[0055] As shown in Figure 4, Embodiment 1 illustrates a case where the primary winding 111 is Y-connected. However, it should be noted that the connection method of the primary winding 111 is not necessarily limited to Y-connection. For example, the primary winding 111 may be Δ-connected. As another example, the primary winding 111 may be V-connected. The primary winding 111 only needs to be connected to the transmission lines of each phase of the power system SYS.

[0056] The secondary winding 112 is located on the secondary side of the substation system 1. The secondary side of the substation system 1 can be rephrased as the low-voltage side of the substation system 1. As shown in Figure 4, the secondary winding 112 is Y-connected. By using the Y-connected secondary winding 112, it becomes possible to output the secondary side line voltage, which will be described later, from the three-phase transformer PVT unit 11.

[0057] Each phase winding in the secondary winding 112 converts the primary phase voltage supplied from the power system SYS to each phase winding in the primary winding 111 to a secondary phase voltage that is lower than the primary phase voltage.

[0058] In Embodiment 1, the A-phase winding of the primary winding 111 is connected to the A-phase transmission line of the power system SYS, the B-phase winding of the primary winding 111 is connected to the B-phase transmission line of the power system SYS, and the C-phase winding of the primary winding 111 is connected to the C-phase transmission line of the power system SYS.

[0059] Therefore, in Embodiment 1, the A-phase voltage supplied from the A-phase transmission line of the power system SYS to the A-phase winding in the primary winding 111 is denoted as V1A. The B-phase voltage supplied from the B-phase transmission line of the power system SYS to the B-phase winding in the primary winding 111 is denoted as V1B. The C-phase voltage supplied from the C-phase transmission line of the power system SYS to the C-phase winding in the primary winding 111 is denoted as V1C.

[0060] In Embodiment 1, the a-phase winding in the secondary winding 112 corresponds to the A-phase winding in the primary winding 111. Therefore, in Embodiment 1, the a-phase voltage generated in the a-phase winding of the secondary winding 112 is denoted as V2a. As can be understood from the above explanation, V1A is converted (stepped down) to V2a by the a-phase winding in the secondary winding 112. For example, if V1A = 66 / √3kV, then V2a = 210 / √3V. This correspondence between the values ​​of V1A and V2a also applies to the relationship between V1B and V2b, and the relationship between V1C and V2c, as described below, as long as there is no contradiction in content.

[0061] In Embodiment 1, the b-phase winding in the secondary winding 112 corresponds to the B-phase winding in the primary winding 111, and the c-phase winding in the secondary winding 112 corresponds to the C-phase winding in the primary winding 111. Therefore, in Embodiment 1, the b-phase voltage generated in the b-phase winding of the secondary winding 112 is denoted as V2b, and the c-phase voltage generated in the c-phase winding of the secondary winding 112 is denoted as V2c. Similar to the above example for the a-phase winding, V1B is converted to V2b by the b-phase winding in the secondary winding 112, and V1C is converted to V2c by the c-phase winding in the secondary winding 112.

[0062] As described above, the secondary winding 112 generates the secondary phase voltage of the three-phase AC. However, as mentioned in the reference configuration, if a single-line-to-ground fault occurs in the power system SYS, the rise in the primary phase voltage in the substation system 1 will cause a rise in the secondary phase voltage in the substation system 1.

[0063] Therefore, the three-phase transformer PVT unit 11 is configured to output the line voltage between one phase and another phase on the secondary side of the substation system 1 as the secondary line voltage. For example, the three-phase transformer PVT unit 11 outputs the line voltage between phase a and phase b on the secondary side of the substation system 1 as the phase a-phase b line voltage. The phase a-phase b line voltage is an example of a secondary line voltage.

[0064] In Embodiment 1, the line voltage between phase a and phase b is denoted as V2ab. As an example, V2ab is: V2ab = V2a - V2b This can be expressed as follows. As shown in Figure 5 below, when the power system SYS is functioning normally, there is a phase difference of 120° between V2a and V2b. Therefore, the magnitude of V2ab is equal to √3 times the magnitude of V2a. Thus, as an example, V2ab = 210V.

[0065] As an example, the three-phase transformer PVT unit 11 may be equipped with a terminal box having output terminals for each phase winding of the secondary winding 112. In this case, for example, the above-mentioned V2ab can be obtained by using the terminal box to draw V2a from the a-phase output terminal of the secondary winding 112 and also by drawing V2a from the b-phase output terminal of the secondary winding 112.

[0066] Refer to Figure 4 again. Substation system 1 is equipped with protective device 12 in place of protective device 12X. Protective device 12 is located downstream of the three-phase transformer PVT unit 11. In the example in Figure 4, protective device 12 is connected to the secondary winding 112.

[0067] Furthermore, the substation system 1 is equipped with an auxiliary transformer 13 in place of the auxiliary transformer 13X. The auxiliary transformer 13 is located downstream of the protective device 12. Therefore, the protective device 12 is located downstream of the three-phase transformer PVT unit 11 and upstream of the protective device 12. The auxiliary transformer 13 is equipped with a primary winding 131 and a secondary winding 132.

[0068] The auxiliary transformer 13 may be a single-phase auxiliary transformer or a three-phase auxiliary transformer. Therefore, if the auxiliary transformer 13 is a single-phase auxiliary transformer, the primary winding 131 is a single-phase primary winding and the secondary winding 132 is a single-phase secondary winding. On the other hand, if the auxiliary transformer 13 is a three-phase auxiliary transformer, the primary winding 131 is a three-phase primary winding and the secondary winding 132 is a three-phase secondary winding.

[0069] In addition, whether the auxiliary transformer 13 is a single-phase auxiliary transformer or a three-phase auxiliary transformer, the protective device 12 is a three-phase protective device (three-phase MCCB).

[0070] Node Nd2 in Figure 4 is the secondary interconnection point of the substation system 1. The protective device 12 can interrupt overcurrent at node Nd2. In the example in Figure 4, the second load LD2 is directly connected to node Nd2. Therefore, the secondary line voltage described above is supplied to the second load LD2 via node Nd2. This secondary line voltage is an example of a target voltage, which will be described later. Therefore, the second load LD2 in the example in Figure 4 may be a load with a rated voltage of, for example, 200V, just like in the example in Figure 1. However, the second load LD2 in the example in Figure 4 differs from the second load LD2 in the example in Figure 1 in that it may be a three-phase load.

[0071] The rated capacity of the auxiliary transformer 13 is smaller than the rated capacity of the three-phase transformer PVT unit 11. The primary winding 131 is connected to node Nd2. Therefore, the target voltage described above is supplied to the primary winding 131 via node Nd2. The secondary winding 132 of the auxiliary transformer 13 is connected to the first load LD1. Thus, in the example in Figure 4, the first load LD1 is connected to node Nd2 via the auxiliary transformer 13.

[0072] The auxiliary transformer 13 is configured to convert the target voltage (in the example of Embodiment 1, the voltage between the secondary windings) to a voltage lower than the target voltage. Therefore, the turns ratio of the primary winding 131 and the secondary winding 132 is set to step down the target voltage. In one configuration example in Figure 4, the rated voltage of the primary winding 131 is 210V, and the rated voltage of the secondary winding 132 is 105V. Therefore, as an example, the auxiliary transformer 13 steps down the target voltage of 210V to 105V. Thus, the first load LD1 in the example of Figure 4 may also be a load with a rated voltage of, for example, 100V, just like in the example of Figure 1. However, the first load LD1 in the example of Figure 4 differs from the first load LD1 in the example of Figure 1 in that it may be a three-phase load.

[0073] As described above, each component located downstream of the secondary winding 112 in the three-phase transformer PVT unit 11 may be a single-phase device or a three-phase device. If each component located downstream of the secondary winding 112 is a single-phase device, the three-phase transformer PVT unit 11 only needs to be configured to output one secondary side-to-line voltage. For example, the three-phase transformer PVT unit 11 only needs to be configured to output the above-mentioned V2ab as the secondary side-to-line voltage.

[0074] On the other hand, if each component located downstream of the secondary winding 112 is a three-phase device, the three-phase transformer PVT unit 11 is configured to output three secondary line voltages. In this case, for example, the three-phase transformer PVT unit 11 outputs the a-phase to b-phase line voltage, as well as the b-phase to c-phase line voltage and the c-phase to a-phase line voltage. The b-phase to c-phase line voltage is the line voltage between the b-phase and c-phase on the secondary side of the substation system 1. The c-phase to a-phase line voltage is the line voltage between the c-phase and a-phase on the secondary side of the substation system 1.

[0075] In Embodiment 1, the line voltage between phase b and phase c is denoted as V2bc, and the line voltage between phase c and phase a is denoted as V2ca. As an example, V2bc and V2ca are, V2bc = V2b - V2c V2ca = V2c - V2b It can be expressed as follows. As can be seen from Figure 5 below, the magnitudes of V2bc and V2ca are equal to the magnitude of V2ab mentioned above. Therefore, as an example, V2ab = V2bc = V2ca = 210V. Also, as can be seen from Figure 5, there is a phase difference of 120° between V2ab, V2bc, and V2ca.

[0076] (Each voltage vector in Embodiment 1) Figure 5 illustrates the voltage vectors in Embodiment 1 when the power system SYS is functioning normally. Figure 5 is a counterpart to Figure 2 in the reference embodiment. In Figure 5, reference numeral 501 indicates the voltage vector of each phase of the power system SYS when the power system SYS is functioning normally. As described above, the primary winding 111 of the three-phase transformer PVT unit 11 is connected to each phase transmission line of the power system SYS. Therefore, the voltage of each phase of the power system SYS corresponds to the primary side voltage of each phase of the three-phase transformer PVT unit 11. In other words, the phase voltages shown in the example of reference numeral 501 indicate the phase voltages (V1A to V1C) on the primary side of the substation system 1.

[0077] In Figure 5, reference numeral 502 indicates the secondary voltage of the three-phase transformer PVT unit 11 when the power system SYS is operating normally. In the example of reference numeral 502, each phase voltage can be said to represent the phase voltages (V2a to V2c) on the secondary side of the substation system 1. In the example of Figure 5, it is assumed that the neutral point on the secondary side of the three-phase transformer PVT unit 11 is grounded.

[0078] As described above, the three-phase transformer PVT unit 11 differs from the PVT11X (single-phase PVT) in the reference configuration in that it is capable of outputting the secondary line voltages (e.g., at least one of V2ab to V2ca). For this reason, in the example of reference numeral 502, unlike the example of reference numeral 202 described above, the line voltages are further illustrated. In the example of reference numeral 502, the caption "phase voltage" indicates V2b, and the caption "line voltage" indicates V2ab.

[0079] Figure 3 illustrates the voltage vectors in Embodiment 1 when a single-line-to-ground fault occurs in the power system SYS. Figure 6 is a counterpart to Figure 5. Figure 6 is also a counterpart to Figure 3 in the reference embodiment. In the example of Figure 6, as in the example of Figure 3, a ground fault occurs in the A-phase transmission line of the power system SYS.

[0080] In Figure 6, reference numeral 601 indicates the voltage vectors of each phase of power system SYS when a ground fault occurs in the A-phase transmission line of power system SYS. As described in the reference configuration, when a ground fault occurs in the A-phase transmission line of power system SYS, the B-phase voltage of power system SYS becomes equal to the line voltage between the A-phase and B-phase of power system SYS.

[0081] Therefore, as can be understood from the explanation of Figures 2 and 3 above, when a ground fault occurs in the A-phase transmission line of the power system SYS, V1B increases compared to when the power system SYS is operating normally. Specifically, the magnitude of V1B in the example of reference numeral 601 is equal to √3 times the magnitude of V1B in the example of reference numeral 501.

[0082] In Figure 6, the reference numeral 602 indicates the voltage vector of the secondary voltage of the three-phase transformer PVT unit 11 when a ground fault occurs in the A-phase transmission line of the power system SYS. In the example of reference numeral 602, as V1B rises, V2b also rises compared to when the power system SYS is operating normally. Specifically, the magnitude of V2 in the example of reference numeral 602 is equal to √3 times the magnitude of V2b in the example of reference numeral 502.

[0083] However, as shown in Figures 5 and 6, V2ab remains unchanged before and after a ground fault occurs in the A-phase transmission line of the SYS power system. Similarly, V2bc and V2ca also remain unchanged before and after a ground fault occurs in the A-phase transmission line of the SYS power system. Thus, the magnitudes of V2ab to V2ca are all maintained at a constant value (e.g., 210V) before and after a ground fault occurs in the A-phase transmission line of the SYS power system.

[0084] Based on this, in Embodiment 1, the three-phase transformer PVT unit 11 is configured to output a secondary line-to-line voltage as described above. According to Embodiment 1, by supplying the secondary line-to-line voltage downstream of the secondary winding 112, it is possible to reduce the overcurrent caused by a single-line-to-ground fault in the power system SYS.

[0085] Therefore, in the substation system 1, the auxiliary transformer 13 is arranged such that a target voltage corresponding to the secondary line-to-line voltage output from the three-phase transformer PVT unit 11 is supplied to the auxiliary transformer 13. In this specification, the target voltage refers to the voltage on the secondary side of the substation system 1 that is subject to step-down by the auxiliary transformer 13.

[0086] In Embodiment 1, the simplest example is shown in which the target voltage is the secondary side line voltage itself. Thus, in the example in Figure 4, the primary winding 131 of the auxiliary transformer 13 is connected to the secondary winding 112 of the three-phase transformer PVT unit 11. Therefore, in the example in Figure 4, the auxiliary transformer 13 acquires the secondary side line voltage output from the three-phase transformer PVT unit 11 as the target voltage. Then, as described above, the auxiliary transformer 13 steps down the target voltage.

[0087] (effect) As described above, in the substation system 1 of Embodiment 1, a three-phase transformer PVT unit 11 is provided in place of the PVT 11X (single-phase PVT) in the substation system 1X of the reference embodiment. In the substation system 1 of Embodiment 1, the three-phase transformer PVT unit 11 outputs the secondary line-to-line voltage described above. Next, the auxiliary transformer 13 acquires a target voltage corresponding to the secondary line-to-line voltage and steps down the target voltage. The auxiliary transformer 13 can step down the target voltage to a voltage suitable for, for example, the first load LD1 described above.

[0088] Since the target voltage corresponds to the secondary line-to-line voltage, it can be said to be a voltage that is less susceptible to the effects of a single-line-to-ground fault in the three-phase AC power system SYS (ideally, a voltage that is unaffected by such a single-line-to-ground fault). By supplying the target voltage to the auxiliary transformer 13, it is expected that even if a single-line-to-ground fault occurs in the power system SYS, a relatively large overcurrent will not occur on the secondary side of the substation system 1.

[0089] Based on the above, Embodiment 1 makes it possible to reduce overcurrent on the secondary side of a substation system having a PVT connected to a three-phase AC power grid. Specifically, Embodiment 1 can reduce the overcurrent compared to a conventional substation system (e.g., reference example).

[0090] Therefore, according to Embodiment 1, the risk of unnecessary operation of the protective device 12 on the secondary side of the substation system 1 can be reduced. For this reason, even if a single-line-to-ground fault occurs in the power system SYS, for example, power can continue to be supplied to the load located on the secondary side of the substation system 1. Thus, the convenience of users using the load can also be improved.

[0091] (Supplement 1 concerning Embodiment 1) As described in the reference embodiment, magnetic saturation of an auxiliary transformer installed in a substation system can lead to the generation of overcurrent on the secondary side of the substation system. Considering this, in Embodiment 1, it is preferable that the substation system 1 is designed in such a way that magnetic saturation of the auxiliary transformer 13 does not occur as much as possible during operation of the substation system 1.

[0092] Therefore, as an example, in Embodiment 1, it is preferable that the three-phase transformer PVT unit 11 and the auxiliary transformer 13 are designed such that the magnetic flux density of the auxiliary transformer 13 during rated operation of the auxiliary transformer 13 is smaller than the magnetic flux density of the three-phase transformer PVT unit 11 during rated operation of the three-phase transformer PVT unit 11.

[0093] For this reason, it is preferable that the core material of each PVT constituting the three-phase transformer PVT unit 11 is the same as the core material of the auxiliary transformer 13. This is because the magnetic flux density at which magnetic saturation of the core occurs is mainly determined by the core material. By using cores of the same material for both the three-phase transformer PVT unit 11 and the auxiliary transformer 13, it becomes easier to design a three-phase transformer PVT unit 11 and an auxiliary transformer 13 that satisfy the above conditions regarding magnetic flux density.

[0094] If the magnetic flux density of the auxiliary transformer 13 during rated operation is less than the magnetic flux density of the three-phase transformer PVT unit 11 during rated operation, the risk of magnetic saturation of the auxiliary transformer 13 during operation of the substation system 2 is reduced. This is because, in this case, the three-phase transformer PVT unit 11 is more likely to reach a magnetic saturation state before the auxiliary transformer 13.

[0095] Furthermore, the method for determining the magnetic flux density of the three-phase transformer PVT unit 11 during rated operation is not particularly limited. Similarly, the method for determining the magnetic flux density of the auxiliary transformer 13 during rated operation is not particularly limited.

[0096] For example, the magnetic flux density of the three-phase transformer PVT unit 11 during rated operation may be calculated using a predetermined formula or computer simulation based on the design specifications of the PVTs constituting the three-phase transformer PVT unit 11. Similarly, the magnetic flux density of the auxiliary transformer 13 during rated operation may be calculated using a predetermined formula or computer simulation based on the design specifications of the auxiliary transformer 13.

[0097] As another example, the magnetic flux density of the three-phase transformer PVT unit 11 during rated operation may be detected by a magnetic sensor provided in the substation system 1. The magnetic sensor for detecting the magnetic flux density of the three-phase transformer PVT unit 11 may be located outside the three-phase transformer PVT unit 11 or inside the three-phase transformer PVT unit 11.

[0098] Therefore, the magnetic flux density of the auxiliary transformer 13 during rated operation may be detected by another magnetic sensor provided in the substation system 1. The magnetic sensor for detecting the magnetic flux density of the auxiliary transformer 13 may be located outside the auxiliary transformer 13 or inside the auxiliary transformer 13.

[0099] (Supplement 2 concerning Embodiment 1) In the example shown in Figure 4 above, the three-phase transformer PVT unit 11 in Embodiment 1 has a tertiary winding 113. The side of the substation system 1 where the tertiary winding 113 is located may be referred to as the tertiary side of the substation system 1. The tertiary side of the substation system 1 may also be referred to as the zero-sequence side of the substation system 1.

[0100] In the example in Figure 4, the tertiary winding 113 is connected in an open delta configuration. The resistor 14 in the example in Figure 4 is connected to the tertiary winding 113. Specifically, the resistor 14 is connected to the open end of the open delta-connected tertiary winding 113.

[0101] In the tertiary winding 113 of the three-phase transformer PVT unit 11, a zero-sequence voltage is generated that corresponds to the primary-side phase voltage supplied from the power system SYS to each phase winding in the primary winding 111 of the three-phase transformer PVT unit 11. The zero-sequence voltage is also called the neutral point voltage.

[0102] As is known to those skilled in the art, zero-sequence voltage arises from an imbalance in the three-phase AC voltages on the primary side of the substation system 1. Therefore, when the power system SYS is functioning normally, the magnitude of the zero-sequence voltage in the tertiary winding 113 is 0. In other words, when the power system SYS is functioning normally, no zero-sequence voltage occurs in the tertiary winding 113.

[0103] On the other hand, if there is an imbalance in the three-phase AC voltage on the primary side of the substation system 1, a non-zero zero-sequence voltage will be generated in the tertiary winding 113. For example, if a single-line-to-ground fault occurs in the power system SYS, a zero-sequence voltage equal to three times the rated voltage of one phase of the tertiary winding 113 will be generated.

[0104] As an example, consider the case where a three-phase transformer PVT unit 11 is configured by combining three single-phase PVTs, each having a tertiary winding with a rated phase voltage of 400 / 3V, as described above. In this case, if a single-line-to-ground fault occurs in the power system SYS, a zero-sequence voltage of 400V (i.e., three times 400 / 3V) will be generated.

[0105] In the ungrounded substation system 1, there is a risk of neutral point instability occurring due to some kind of electrical shock. Neutral point instability is a phenomenon in which the zero-sequence voltage oscillates over a long period of time (see also Non-Patent Document 1 mentioned above). Therefore, in the example in Figure 4, a resistor 14 is provided to reduce the effect of neutral point instability.

[0106] When an imbalance occurs in the three-phase AC voltage on the primary side of the substation system 1, a zero-sequence current corresponding to the zero-sequence voltage flows through the tertiary winding 113. By connecting a resistor 14 to the tertiary winding 113, a zero-sequence current of a predetermined magnitude can be stably supplied through the resistor 14. As a result, unstable fluctuations in the zero-sequence voltage can be reduced.

[0107] In addition, setting the rated phase voltage of each phase winding in the tertiary winding 113 of the three-phase transformer PVT unit 11 to a reasonably high value is beneficial from the standpoint of reducing the magnitude of the zero-sequence current mentioned above. In conventional single-phase PVTs intended for use as instrument transformers, the rated phase voltage of the tertiary winding of the single-phase PVT is generally designed to be, for example, 110 / 3V (≒36.7V).

[0108] Therefore, from the standpoint of reducing the magnitude of the zero-sequence current, it is preferable that the rated phase voltage of each phase winding in the tertiary winding 113 of the three-phase transformer PVT unit 11 be set to a value that is somewhat higher than the rated phase voltage of the tertiary winding in a conventional single-phase PVT (e.g., 110 / 3V).

[0109] As an example, it is preferable that the rated phase voltage of each phase winding in the tertiary winding 113 of the three-phase transformer PVT unit 11 be set to a value of 200 / 3V or higher. Taking this into consideration, in one configuration example shown in Figure 4, a value of 400 / 3V is provided as the rated phase voltage of each phase winding in the tertiary winding 113 of the three-phase transformer PVT unit 11.

[0110] By setting the rated phase voltage of each phase winding in the tertiary winding 113 of the three-phase transformer PVT unit 11 to a value of 200 / 3V or higher, the magnitude of the zero-sequence current can be effectively reduced.

[0111] In addition, if the rated phase voltage of each phase winding in the tertiary winding 113 of the three-phase transformer PVT unit 11 is set to a value of 200 / 3V or higher, the magnitude of the zero-sequence current is reduced, making it possible to reduce the thickness of the wiring connecting the tertiary winding 113 and the resistor 14. As a result, the wiring work connecting the tertiary winding 113 and the resistor 14 is simplified, which is another advantage.

[0112] [Variation] (1) Note that the three-phase transformer PVT unit 11 only needs to be configured to output the secondary line voltage. Therefore, unlike the example in Figure 4, the three-phase transformer PVT unit 11 does not necessarily need to have a tertiary winding 113. From this, the substation system 1 does not necessarily need to have a resistor 14.

[0113] For example, a three-phase transformer PVT unit 11 without a tertiary winding 113 can be constructed by combining three single-phase PVTs (e.g., PVT11X in the reference configuration) that have primary and secondary windings but no tertiary winding. Therefore, for example, if the need to reduce the effects of neutral point instability in the substation system 1 is not considered to be very high, a three-phase transformer PVT unit 11 without a tertiary winding 113 may be adopted.

[0114] (2) The three-phase transformer PVT unit 11 does not necessarily have to be composed of three single-phase PVTs. For example, the three-phase transformer PVT unit 11 may be composed of one three-phase PVT.

[0115] However, it should be noted that PVTs available on the market as general-purpose products are generally single-phase PVTs. Therefore, the cost required to design and manufacture one three-phase PVT applicable to the substation system 1 is generally higher than the cost of combining three single-phase PVTs to form a three-phase transformer PVT unit 11. Thus, from a cost perspective, it is preferable to form a three-phase transformer PVT unit 11 using three single-phase PVTs.

[0116] [Embodiment 2] Figure 7 shows an example configuration of the substation system 2 in Embodiment 2. The substation system 2 is equipped with a voltage stabilizer 21. In this respect, the substation system 2 differs from the substation system 1.

[0117] The voltage stabilizer 21 is located on the secondary side of the substation system 2. In the example shown in Figure 7, the input of the voltage stabilizer 21 is connected to the protection device 12. The output of the voltage stabilizer 21 is connected to node Nd2. Thus, the voltage stabilizer 21 is located downstream of the three-phase transformer PVT unit 11 and upstream of the auxiliary transformer 13.

[0118] The voltage stabilizer 21 stabilizes the voltage input to its input section. The voltage stabilizer 21 then outputs the stabilized voltage from its output section. Therefore, the voltage output by the voltage stabilizer 21 may be called the stabilized voltage. In this way, the voltage stabilizer 21 converts the voltage input to it into a stabilized voltage.

[0119] As an example, any type of AVR (Automatic Voltage Regulator) may be used as the voltage stabilizer 21. With the voltage stabilizer 21, the voltage fluctuation rate of the stabilized voltage can be made smaller than the voltage fluctuation rate of the input voltage to the voltage stabilizer 21.

[0120] In the example shown in Figure 7, the voltage stabilizer 21 is connected to the secondary winding 112 of the three-phase transformer PVT unit 11 via the protection device 12. Therefore, the voltage stabilizer 21 can obtain the secondary side-to-line voltage output from the three-phase transformer PVT unit 11 as the input voltage to the voltage stabilizer 21.

[0121] Next, the voltage stabilizer 21 converts the secondary side line voltage, which is the input voltage, into a stabilized voltage. As described above, with the voltage stabilizer 21, the voltage fluctuation rate of the stabilized voltage is smaller than the voltage fluctuation rate of the secondary side line voltage.

[0122] In the example shown in Figure 7, the primary winding 131 of the auxiliary transformer 13 is connected to the output of the voltage stabilizer 21 via node Nd2. Therefore, the voltage stabilizer 21 can supply a stabilized voltage to the primary winding 131.

[0123] In the example of Figure 7, unlike the example of Figure 4, the primary winding 131 of the auxiliary transformer 13 is connected to the secondary winding 112 of the three-phase transformer PVT unit 11 via the voltage stabilizer 21. Thus, in Embodiment 2, unlike Embodiment 1, the auxiliary transformer 13 acquires the stabilized voltage as the target voltage. In this way, the target voltage in one aspect of the present disclosure may be the stabilized voltage.

[0124] As described above, in the substation system 2, the stabilized voltage is supplied to the auxiliary transformer 13 as a target voltage corresponding to the secondary line voltage. Therefore, the auxiliary transformer 13 steps down the stabilized voltage and supplies the stepped-down voltage to the first load LD1. Then, in the substation system 2, the stabilized voltage is supplied to the second load LD2 from the voltage stabilizer 21.

[0125] Therefore, according to the substation system 2, even if the voltage fluctuation rate of the secondary side line voltage increases due to, for example, a temporary malfunction of the three-phase transformer PVT unit 11, a stabilized voltage with a small voltage fluctuation rate can be supplied to each load on the secondary side of the substation system 2. Thus, according to embodiment 2, the convenience of the user using the load can be further improved.

[0126] [Embodiment 3] Figure 8 shows an example configuration of the substation system 3 in Embodiment 3. Substation system 3 is equipped with an auxiliary transformer 31 in place of the auxiliary transformer 13 of substation system 1. In this respect, substation system 3 differs from substation system 1.

[0127] Unlike auxiliary transformer 13, auxiliary transformer 31 is a primary-side tapped transformer. A primary-side tapped transformer is a transformer that has a voltage tap on its primary side. Auxiliary transformer 31 has a primary winding 311 and a secondary winding 312. In the example in Figure 8, the primary winding 311 has the aforementioned voltage tap.

[0128] With the auxiliary transformer 31, which is an auxiliary transformer with a primary tap, even if there is a fluctuation in the voltage input to the primary winding 311, a predetermined secondary voltage can be output to the secondary winding 312 by switching the voltage tap of the primary winding 311.

[0129] In Embodiment 3, as an example configuration, the auxiliary transformer 31 is shown to be a primary tapped transformer represented by the specification notation "F440-R420-F400V / 210V-105V". This specification notation means that the rated voltage value of the primary winding 311 of the auxiliary transformer 31 is 420V.

[0130] From this, an example of a single-phase PVT constituting the three-phase transformer PVT unit 11 in Embodiment 3 is a single-phase PVT having (i) a primary winding with a rated phase voltage of 66 / √3kV, (ii) a secondary winding with a rated phase voltage of 420 / √3V (≒242.5V), and (iii) a tertiary winding with a rated phase voltage of 400 / 3V. As an example, in Embodiment 3, three such single-phase PVTs are combined to constitute the three-phase transformer PVT unit 11. In this case, the three-phase transformer PVT unit 11 can supply a secondary side-to-line voltage of 420V (i.e., √3 times 420 / √3V) as the target voltage to the auxiliary transformer 31.

[0131] The above specification for the auxiliary transformer 31 indicates that when a voltage in the range of 400V to 440V is input to the primary winding 311 of the auxiliary transformer 31, it is possible to output 210V, which is the rated voltage of the secondary winding 312, to the secondary winding 312 by switching the voltage tap of the primary winding 311.

[0132] Furthermore, the specification notation for the auxiliary transformer 31 indicates that the secondary side of the auxiliary transformer 31 is provided with a secondary side reference voltage tap and a center tap (also called a midpoint tap). Therefore, in the example in Figure 8, the secondary winding 312 of the auxiliary transformer 31 has the aforementioned secondary side reference voltage tap and center tap. Thus, the secondary winding 312 in the example in Figure 8 is a center-tapped secondary winding.

[0133] According to the secondary side reference voltage tap, the voltage output to the secondary winding 312 (the secondary side voltage of the auxiliary transformer 31) can be drawn from the secondary winding 312. On the other hand, according to the center tap, half of the secondary side voltage of the auxiliary transformer 31 can be drawn individually from the secondary winding 312.

[0134] Therefore, in the example shown in Figure 8, both the first load LD1 and the second load LD2 are connected to the secondary side of the secondary winding 312, which is a center-tapped secondary winding. In the example shown in Figure 8, the secondary voltage of the auxiliary transformer 31 (e.g., 210V) is supplied from the secondary winding 312 to the second load LD2 via the secondary side reference voltage tap. On the other hand, half the value of the secondary voltage of the auxiliary transformer 31 (e.g., 105V) is supplied from the secondary winding 312 to the first load LD1 via the center tap.

[0135] In a substation system according to one aspect of this disclosure, for example, a temporary malfunction of the three-phase transformer PVT unit may cause the secondary line voltage of the three-phase transformer PVT unit to deviate from the rated value (420V in the example of Embodiment 3). However, in substation system 3, as long as the secondary line voltage is within the range of 400V to 440V, the auxiliary transformer 31 can supply a voltage of 210V to the second load LD2 and a voltage of 105V to the first load LD1.

[0136] Thus, in the substation system 3, fluctuations in the secondary line voltage in the three-phase transformer PVT unit 11 are offset by using an auxiliary transformer 31, which is an auxiliary transformer with a primary tap. Therefore, the third embodiment also further improves the convenience of users using each load in the substation system 3.

[0137] In addition, in substation system 3, both the first load LD1 and the second load LD2 are connected to the auxiliary transformer 31. As a result, in substation system 3, both (i) the first circuit connecting the first load LD1 and the auxiliary transformer 31, and (ii) the second circuit connecting the second load LD2 and the auxiliary transformer 31 are provided as ungrounded circuits. Consequently, in substation system 3, even if a ground fault occurs in at least one of the first and second circuits, the ground fault current generated in these circuits can be reduced.

[0138] [Embodiment 4] In the reference embodiment and embodiments 1 to 3, the case where the power system SYS is an ungrounded three-phase AC power system was described. As described above, according to one aspect of the present disclosure, it is possible to reduce the overcurrent on the secondary side of a substation system having a PVT connected to an ungrounded three-phase AC power system SYS.

[0139] However, the power system SYS may be any three-phase AC power system and does not necessarily have to be an ungrounded power system. For example, the power system SYS may be a resistance-grounded three-phase AC power system. In this specification, "resistance-grounded three-phase AC power system" means a power system that has three AC transmission lines and a neutral point that is grounded via resistors.

[0140] Therefore, a substation system according to one aspect of the present disclosure may be a substation system having a PVT connected to a resistance-grounded three-phase AC power system SYS. According to one aspect of the present disclosure, it is also possible to reduce overcurrent on the secondary side of a substation system having a PVT connected to a resistance-grounded three-phase AC power system SYS.

[0141] As another example, power system SYS may be a directly grounded three-phase AC power system. In this specification, "directly grounded three-phase AC power system" means a power system that has three AC transmission lines and a neutral point that is directly grounded. "Directly grounded neutral point" means a neutral point that is directly connected to the earth by a conductor with high electrical resistivity.

[0142] Therefore, a substation system according to one aspect of the present disclosure may be a substation system having a PVT connected to a directly grounded three-phase AC power system SYS. According to one aspect of the present disclosure, it is also possible to reduce overcurrent on the secondary side of a substation system having a PVT connected to a directly grounded three-phase AC power system SYS.

[0143] 〔summary〕 The substation system according to aspect 1 of this disclosure is a substation system connected to a three-phase AC power grid, wherein the PVT (Power Voltage) The substation system comprises a three-phase transformer PVT unit, which is composed of a transformer, and the three-phase transformer PVT unit comprises a primary winding located on the primary side of the substation system and a secondary winding located on the secondary side of the substation system, wherein the primary winding is connected to the power grid, and the secondary winding is Y-connected, and each phase winding in the secondary winding converts the primary side phase voltage supplied from the power grid to each phase winding in the primary winding to a secondary side phase voltage lower than the primary side phase voltage, and the three-phase transformer PVT unit outputs the line voltage between one phase and another on the secondary side of the substation system as the secondary side line voltage, and the substation system further comprises an auxiliary transformer located downstream of the three-phase transformer PVT unit, which converts a target voltage corresponding to the secondary side line voltage output from the three-phase transformer PVT unit to a voltage lower than the target voltage.

[0144] In the substation system according to Embodiment 2 of the present disclosure, in Embodiment 1, the magnetic flux density of the auxiliary transformer during rated operation of the auxiliary transformer may be smaller than the magnetic flux density of the three-phase transformer PVT unit during rated operation of the three-phase transformer PVT unit.

[0145] In the substation system according to Embodiment 3 of the present disclosure, in Embodiment 1 or 2, the three-phase transformer PVT unit may include a tertiary winding located on the tertiary side of the substation system, the tertiary winding may be in an open delta connection, and the substation system may include a resistor connected to the tertiary winding.

[0146] In the substation system according to aspect 4 of this disclosure, in any one of aspects 1 to 3, the rated phase voltage of each phase winding in the tertiary winding may be 200 / 3V or higher.

[0147] In the substation system according to Embodiment 5 of the present disclosure, in any one of Embodiments 1 to 4, the auxiliary transformer may acquire the secondary side line voltage output from the three-phase transformer PVT unit as the target voltage.

[0148] A substation system according to embodiment 6 of the present disclosure may include, in any one of embodiments 1 to 4, a voltage stabilizer located downstream of the three-phase transformer PVT unit and upstream of the auxiliary transformer, wherein the voltage stabilizer may convert the secondary line voltage output from the three-phase transformer PVT unit into a stabilized voltage, the voltage fluctuation rate of the stabilized voltage may be smaller than the voltage fluctuation rate of the secondary line voltage, and the auxiliary transformer may acquire the stabilized voltage as the target voltage.

[0149] The substation system according to embodiment 7 of the present disclosure may include a primary-side tapped transformer as the auxiliary transformer in any one of embodiments 1 to 6.

[0150] In the substation system according to aspect 8 of the present disclosure, in aspect 7, the primary tapped transformer may be equipped with a center tapped secondary winding.

[0151] In the substation system according to aspect 9 of the present disclosure, in any one of aspects 1 to 8, the three-phase transformer unit may be composed of three single-phase PVTs.

[0152] In the substation system according to aspect 10 of the present disclosure, in any one of aspects 1 to 8, the three-phase transformer unit may be composed of one three-phase PVT.

[0153] In the substation system according to aspect 11 of this disclosure, in any one of aspects 1 to 10, the power system may be an ungrounded power system.

[0154] In the substation system according to aspect 12 of this disclosure, in any one of aspects 1 to 10, the power system may be a resistance-grounded power system.

[0155] In the substation system according to aspect 13 of this disclosure, in any one of aspects 1 to 10, the power system may be a directly grounded power system.

[0156] [Additional Notes] One aspect of this disclosure is not limited to the embodiments described above, and various modifications are possible within the scope of the claims. Embodiments obtained by appropriately combining the technical means disclosed in different embodiments are also included within the technical scope of one aspect of this disclosure. [Explanation of Symbols]

[0157] 1,2,3 Substation System 11. Three-phase transformer PVT unit 111 Primary winding of a three-phase transformer PVT unit 112 Secondary winding of a three-phase transformer PVT unit 113 Tertiary winding of a three-phase transformer PVT unit 13. Auxiliary transformer 131 Primary winding of auxiliary transformer 132 Secondary winding of auxiliary transformer 14 resistor 21 Voltage Stabilizer 31. Auxiliary transformer (transformer with primary side tap) 311 Primary winding of auxiliary transformer (secondary winding of transformer with primary side tap) 312. Secondary winding of auxiliary transformer (center-tapped secondary winding of transformer with primary side tap) SYS power system

Claims

1. A substation system connected to a three-phase AC power grid, It is equipped with a three-phase transformer PVT unit, which is composed of PVTs (Power Voltage Transformers). The three-phase transformer PVT unit comprises a primary winding located on the primary side of the substation system and a secondary winding located on the secondary side of the substation system. The primary winding is connected to the power system, The aforementioned secondary winding is Y-connected, Each phase winding in the secondary winding converts the primary phase voltage supplied from the power system to each phase winding in the primary winding to a secondary phase voltage that is lower than the primary phase voltage. The three-phase transformer PVT unit outputs the line voltage between one phase and another phase on the secondary side of the substation system as the secondary line voltage. The aforementioned substation system further includes an auxiliary transformer located downstream of the three-phase transformer PVT unit, The auxiliary transformer is a substation system that converts a target voltage corresponding to the secondary line voltage output from the three-phase transformer PVT unit to a voltage lower than the target voltage.

2. The substation system according to claim 1, wherein the magnetic flux density of the auxiliary transformer during rated operation is smaller than the magnetic flux density of the three-phase transformer PVT unit during rated operation.

3. The three-phase transformer PVT unit is equipped with a tertiary winding located on the tertiary side of the substation system. The aforementioned tertiary winding is connected in an open delta configuration. The substation system according to claim 1, wherein the substation system comprises a resistor connected to the tertiary winding.

4. The substation system according to claim 3, wherein the rated phase voltage of each phase winding in the tertiary winding is 200 / 3V or higher.

5. The substation system according to claim 1, wherein the auxiliary transformer acquires the secondary side line voltage output from the three-phase transformer PVT unit as the target voltage.

6. The substation system includes a voltage stabilizer located downstream of the three-phase transformer PVT unit and upstream of the auxiliary transformer. The voltage stabilization device converts the secondary side line voltage output from the three-phase transformer PVT unit into a stabilized voltage. The voltage fluctuation rate of the stabilized voltage is smaller than the voltage fluctuation rate of the secondary side line voltage. The substation system according to claim 1, wherein the auxiliary transformer acquires the stabilized voltage as the target voltage.

7. The substation system according to claim 1, wherein the substation system includes a primary-side tapped transformer as the auxiliary transformer.

8. The substation system according to claim 7, wherein the primary tapped transformer is equipped with a center-tapped secondary winding.

9. The substation system according to claim 1, wherein the three-phase transformer PVT unit is composed of three single-phase PVTs.

10. The substation system according to claim 1, wherein the three-phase transformer PVT unit is composed of one three-phase PVT.

11. The substation system according to claim 1, wherein the power system is an ungrounded power system.

12. The substation system according to claim 1, wherein the power system is a resistance-grounded power system.

13. The substation system according to claim 1, wherein the power system is a directly grounded power system.

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