Power exchange system

Abstract

To provide a power transfer system which can stop operation of slave units when a master unit is down while power transfer is balanced between a plurality of slave units.SOLUTION: A power transfer system S includes: a master unit 1 which is connected to a DC bus B and controls bus voltage of the DC bus B; a slave unit 2A which is connected to the DC bus B and supplies power to the DC bus B; and a slave unit 2B which is connected to the DC bus B and receives power from the DC bus B. The master unit 1 measures a transfer amount A1 of current or power between itself and the DC bus B, and when a measurement result of the transfer amount A1 is less than a threshold which is previously set, controls the bus voltage so as to be prescribed first set voltage, and when the measurement result of the transfer amount A1 is equal to or more than the threshold, controls the bus voltage to be prescribed second set voltage. The slave unit 2A measures the bus voltage, and when the measurement result of bus voltage is the first set voltage, performs operation to temporarily increase / decrease a transfer amount A2 between the slave unit 2A and the DC bus B.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a power sharing system having a DC bus.

Background Art

[0002] A power sharing system including a master unit connected to a DC bus to control the voltage of the DC bus (hereinafter referred to as "bus voltage") and a plurality of slave units connected to the DC bus is known (see, for example, Patent Document 1). The master unit is constituted by, for example, an AC-DC conversion device connected to a commercial AC power supply, and the slave unit is constituted by, for example, a DC-DC conversion device connected to a distributed power source such as a solar power generation device, an electric vehicle, or a stationary energy storage device. Generally, when the master unit fails (i.e., becomes inoperable), the slave unit is configured to automatically stop when the bus voltage becomes a voltage outside the rated voltage range in order not to continue the operation of the slave unit.

[0003] Referring to FIG. 6, a conventional power sharing system will be described. The power sharing system shown in FIG. 6(A) includes a master unit 101 and two slave units 102. Of the two slave units 102, one operates as a first slave unit (hereinafter referred to as "slave unit 102A") that supplies power to the DC bus B, and the other operates as a second slave unit (hereinafter referred to as "slave unit 102B") that receives power supply from the DC bus B. When the power demand of the slave unit 102B is larger than the power supply amount of the slave unit 102A, the master unit 101 maintains the bus voltage by supplying power to the DC bus B.

[0004] FIG. 6(B) shows changes in the amount of current transfer A1 between the master unit 101 and the DC bus B, the bus voltage, and the amount of power transfer A2 between the slave units 102A and 102B and the DC bus B. The transfer amount A1 is positive for the amount of current flowing from the master unit 101 to the DC bus B, and negative for the amount of current flowing from the DC bus B to the master unit 101. Also, the transfer amount A2 is positive for the amount of power supplied by the slave unit 102 to the DC bus B, and negative for the amount of power received by the slave unit 102 from the DC bus B.

[0005] As shown in Figure 6(B), at time Ta1, when the master unit 101 is supplying power to the DC bus B, the amount of current exchanged between the master unit 101 and the DC bus B, A1, is positive, and the bus voltage is configured to be maintained within the rated voltage range (i.e., above a predetermined lower limit voltage Vmin and below a predetermined upper limit voltage Vmax). When the master unit 101 goes down at time Ta2, power is no longer supplied from the master unit 101 to the DC bus B, causing the bus voltage to drop. The slave units 102A and 102B are configured to stop operating at time Ta3, when the bus voltage falls below a predetermined lower limit voltage Vmin. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Patent No. 6481938 [Overview of the project] [Problems that the invention aims to solve]

[0007] However, when the power exchange between multiple slave units 102A and 102B is balanced, the amount of current A1 exchanged between the master unit 101 and DC bus B is 0, and as shown in Figure 6(C), the bus voltage does not fluctuate even if the master unit 101 goes down at time Ta4. Therefore, there was a problem in that the slave units 102A and 102B continued to operate without stopping. This continued operation of the slave units 102A and 102B caused inconveniences such as the inability of users of the slave units 102A and 102B to detect the abnormality (downtime) of the master unit 101.

[0008] The present invention has been made in view of the above circumstances, and aims to provide a power sharing system that can stop the operation of multiple slave units when the master unit goes down while the power exchange between multiple slave units is balanced. [Means for solving the problem]

[0009] To solve the above problems, the power exchange system according to the present invention comprises a master unit connected to a DC bus and controlling the bus voltage of the DC bus, a first slave unit connected to the DC bus and supplying power to the DC bus, and a second slave unit connected to the DC bus and receiving power from the DC bus, wherein the master unit measures the amount of current or power exchanged with the DC bus, controls the bus voltage to a predetermined first set voltage if the measurement result of the amount of current or power exchanged is less than a preset threshold, controls the bus voltage to a predetermined second set voltage if the measurement result of the amount of current or power exchanged is greater than or equal to the threshold, and at least one of the first slave unit and the second slave unit measures the bus voltage, and if the measurement result of the bus voltage is the first set voltage, performs an operation to temporarily increase or decrease the amount of power exchanged with the DC bus.

[0010] According to the above configuration, if the master unit is down, the master unit cannot control the bus voltage. Therefore, at least one of the first and second slave units can temporarily increase or decrease the amount of power exchanged with the DC bus, thereby fluctuating the bus voltage to stop the slave unit from operating. Thus, even if the master unit goes down while the power exchange between multiple slave units is balanced, the slave units can still be stopped from operating.

[0011] In the above power exchange system, it is preferable that the first slave unit temporarily increases or decreases the amount of power exchanged with the DC bus by temporarily reducing the amount of power supplied to the DC bus when the measurement result of the bus voltage is the first set voltage.

[0012] In the above power exchange system, it is preferable that the second slave unit temporarily increases the amount of power it receives from the DC bus, thereby temporarily increasing or decreasing the amount of power exchanged with the DC bus, when the measurement result of the bus voltage is the first set voltage.

[0013] In the above power exchange system, it is preferable that at least one of the first slave unit and the second slave unit repeatedly performs an operation to temporarily increase or decrease the amount of power exchanged with the DC bus at predetermined time intervals.

[0014] In the above power exchange system, it is preferable that the master unit controls the bus voltage to the second set voltage only when the measurement result of the amount of current or power exchanged with the DC bus is above the threshold for a longer period of time than the time during which the amount of power exchanged between at least one of the first slave unit and the second slave unit and the DC bus temporarily increases or decreases. [Effects of the Invention]

[0015] According to the present invention, a power sharing system can be provided that can stop the operation of multiple slave units when the master unit goes down while the power exchange between the slave units is balanced. [Brief explanation of the drawing]

[0016] [Figure 1] This is a schematic diagram of a power exchange system according to one embodiment of the present invention. [Figure 2] This is a flowchart illustrating the operation of the master unit according to the same embodiment. [Figure 3] (A) and (B) are flowcharts illustrating the operation of the slave unit according to the same embodiment. [Figure 4] (A) and (B) are timing charts showing the changes in the amount of current exchanged between the master unit and the DC bus, the bus voltage, and the amount of power exchanged between the slave unit and the DC bus in the power exchange system according to the same embodiment. [Figure 5] (A) and (B) are timing charts showing the changes in the amount of current exchanged between the master unit and the DC bus, the bus voltage, and the amount of power exchanged between the slave unit and the DC bus in the modified power exchange system. [Figure 6](A) is a schematic configuration diagram of a conventional power sharing system, and (B) and (C) are timing charts showing changes in the amount of current transferred between the master unit and the DC bus, the bus voltage, and the amount of power transferred between the slave unit and the DC bus in the conventional power sharing system.

Embodiments for Carrying Out the Invention

[0017] Referring to the drawings, a power sharing system S according to an embodiment of the present invention will be described. As shown in FIG. 1, the power sharing system S includes an AC line L, a DC bus B, a master unit 1, and a plurality of slave units 2. The power sharing system S shares power between the master unit 1 and the slave units 2 and among the plurality of slave units 2 via the DC bus B.

[0018] The AC line L is a wiring for distributing AC power supplied from a commercial AC power source P. The AC line L is connected to the master unit 1.

[0019] The DC bus B is a wiring for sharing DC power and is a common line to which the master unit 1 and the plurality of slave units 2 are connected. A distributed power source D is connected to the DC bus B via the slave unit 2. The voltage of the DC bus B (hereinafter referred to as "bus voltage Vb") is preferably maintained within the rated voltage range during the operation of the master unit 1 and the slave units 2.

[0020] The master unit 1 is constituted by an AC-DC conversion device that mutually converts AC power and DC power. When the power demand of the second slave unit described later is larger than the power supply amount of the first slave unit described later, the master unit 1 converts the AC power supplied from the AC line L into DC power and supplies the power to the DC bus B. Also, when the power supply amount of the first slave unit described later is larger than the power demand of the second slave unit described later, the master unit 1 converts the DC power supplied from the DC bus B into AC power and supplies the power to the AC line L. The master unit 1 includes a current measurement unit 11 and a voltage control unit 12.

[0021] The amount of current exchanged between the master unit 1 and the DC bus B (hereinafter referred to as "exchange amount A1") changes according to the balance of power exchange between the slave units 2 (i.e., the balance between the power supply amount and power demand amount of the first and second slave units, as described later). When the master unit 1 is supplying power to the DC bus B, exchange amount A1 is the amount of current flowing from the master unit 1 to the DC bus B, and when the master unit 1 is receiving power from the DC bus B, it is the amount of current flowing from the DC bus B to the master unit 1. In this embodiment, the exchange amount A1 is defined as positive for the amount of current flowing from the master unit 1 to the DC bus B, and negative for the amount of current flowing from the DC bus B to the master unit 1.

[0022] The current measurement unit 11 periodically measures the amount of power being sent and received A1. When the balance between the power supply and power demand of the first and second slave units, as described later, is not maintained, the current measurement unit 11 measures an amount of power being sent and received A1 that is sufficiently greater than 0, and when the balance between the power supply and power demand of the first and second slave units, as described later, is maintained, it measures an amount of power being sent and received A1 that is close to 0.

[0023] The voltage control unit 12 controls the bus voltage Vb according to the measurement result of the transfer amount A1 by the current measurement unit 11. Specifically, if the measurement result of the transfer amount A1 is less than a preset threshold Th, the voltage control unit 12 controls the bus voltage Vb so that the bus voltage Vb becomes a predetermined first set voltage (hereinafter referred to as "set voltage V1"), and if the measurement result of the transfer amount A1 is greater than or equal to the threshold Th, the voltage control unit 12 controls the bus voltage Vb so that the bus voltage Vb becomes a predetermined second set voltage (hereinafter referred to as "set voltage V2") that is different from set voltage V1. Set voltages V1 and V2 are each within the rated voltage range (i.e., greater than or equal to a predetermined lower limit voltage Vmin and less than a predetermined upper limit voltage Vmax). In this embodiment, the voltage control unit 12 controls the bus voltage Vb so that the bus voltage Vb becomes set voltage V2 only if the measurement result of the transfer amount A1 is greater than or equal to the threshold Th for a longer period than the time during which the amount of power transferred between the slave unit 2 and the DC bus B temporarily increases or decreases (a predetermined time T described later). The threshold Th is a value close to 0.

[0024] Each of the slave units 2 is composed of a DC-DC converter that converts DC power to a desired DC power. Each of the slave units 2 is connected to a distributed power source D, such as a solar power generation system, an electric vehicle, or a stationary energy storage system. Each of the slave units 2 is equipped with a voltage measurement unit 21 and a power control unit 22.

[0025] Multiple slave units 2 can each perform charging and discharging operations independently. For example, at least one unit can operate as a first slave unit (hereinafter referred to as "slave unit 2A") that supplies power to the DC bus B, and at least one unit can operate as a second slave unit (hereinafter referred to as "slave unit 2B") that receives power from the DC bus B. In this embodiment, one of the two slave units 2 operates as a first slave unit that supplies power to the DC bus B, and the other operates as a second slave unit that receives power from the DC bus B. Slave unit 2 stops operating when the bus voltage Vb is no longer maintained within the rated voltage range, that is, when the bus voltage Vb falls below the lower limit voltage Vmin or becomes equal to or greater than the upper limit voltage Vmax.

[0026] The slave unit 2A is connected to a distributed power source D that can generate or discharge power, and converts the DC power generated or discharged by the distributed power source D into a desired DC power and supplies power to the DC bus B. A distributed power source D that can generate power can be, for example, a solar power generation device, and a distributed power source D that can discharge power can be, for example, an electric vehicle or a stationary energy storage device. Furthermore, if the distributed power source D connected to the slave unit 2A is rechargeable, the slave unit 2A can not only supply power to the DC bus B, but also receive power from the DC bus B, convert the DC power received from the DC bus B into a desired DC power, and charge the distributed power source D.

[0027] The slave unit 2B is connected to a rechargeable distributed power supply D, receives power from a DC bus B, and converts the DC power supplied from the DC bus B into a desired DC power to charge the distributed power supply D. The rechargeable distributed power supply D can be, for example, an electric vehicle or a stationary energy storage device. Furthermore, if the distributed power supply D connected to the slave unit 2B is capable of discharging, the slave unit 2B can not only charge the distributed power supply D, but also convert the DC power discharged by the distributed power supply D into a desired DC power to supply power to the DC bus B.

[0028] The amount of power exchanged between slave unit 2 and DC bus B (hereinafter referred to as "amount A2") changes depending on the operating mode of the distributed power supply D connected to slave unit 2. The amount A2 of power exchanged between slave unit 2A and DC bus B is the amount of power supplied by slave unit 2A, and changes depending on the amount of power that the distributed power supply D connected to slave unit 2A can generate or discharge. Similarly, the amount A2 of power exchanged between slave unit 2B and DC bus B is the amount of power demanded by slave unit 2B, and changes depending on the amount of power required to charge the distributed power supply D connected to slave unit 2B. In other words, if slave unit 2 is supplying power to DC bus B, the amount A2 of power exchanged is the amount of power that slave unit 2 supplies to DC bus B, and if slave unit 2 is receiving power from DC bus B, the amount of power that slave unit 2 receives from DC bus B. In this embodiment, the amount A2 transmitted and received is defined as the amount of power supplied by the slave unit 2 to the DC bus B as positive, and the amount of power supplied by the slave unit 2 to the DC bus B as negative.

[0029] The voltage measurement unit 21 periodically measures the bus voltage Vb. When the slave unit 2 is operating, the voltage measurement unit 21 measures the set voltages V1 and V2 as the bus voltage Vb. When the master unit 1 goes down and the bus voltage Vb can no longer be maintained, the voltage measurement unit 21 measures the bus voltage Vb when it is below the lower limit voltage Vmin or above the upper limit voltage Vmax.

[0030] The power control unit 22 controls the amount of power exchanged A2 according to the measurement result of the bus voltage Vb by the voltage measurement unit 21. Specifically, the power control unit 22 controls the amount of power exchanged A2 to temporarily increase or decrease it if the measurement result of the bus voltage Vb is the set voltage V1, that is, if the measurement result of the bus voltage Vb is not the set voltage V2. This control only needs to be performed by the power control unit 22 of at least one of the multiple slave units 2. In this embodiment, the power control unit 22 of the slave unit 2A that supplies power to the DC bus B controls the amount of power exchanged A2 to temporarily increase or decrease the amount of power exchanged between the slave unit 2A and the DC bus B by reducing the amount of power supplied to the DC bus B for a predetermined time T. Furthermore, the power control unit 22 repeatedly increases or decreases the amount of power exchanged A2 at predetermined time intervals while the measurement result of the bus voltage Vb is the set voltage V1.

[0031] Next, the operation of the master unit 1 will be explained with reference to Figure 2. As shown in Figure 2, the current measurement unit 11 of the master unit 1 measures the amount A1 being sent and received (step S1), and then the voltage control unit 12 of the master unit 1 determines whether the measurement result of the amount A1 being sent and received in step S1 is less than the threshold Th (step S2). Furthermore, if the amount A1 being sent and received is greater than or equal to the threshold Th (step S2: NO), the voltage control unit 12 of the master unit 1 determines whether a predetermined time T has elapsed since the amount A1 being sent and received became greater than or equal to the threshold Th (step S3).

[0032] The voltage control unit 12 of the master unit 1 controls the bus voltage Vb to the set voltage V1 if the amount of power being transferred A1 is less than the threshold Th (step S2: YES), or if the amount of power being transferred A1 is greater than or equal to the threshold Th but a predetermined time T has not elapsed since it became greater than or equal to the threshold Th, i.e., if the amount of power being transferred A1 has not been greater than or equal to the threshold Th for a longer period than the predetermined time T (step S3: NO) (step S4). By controlling the bus voltage Vb in this way, when the amount of power being transferred A1 is close to 0, the bus voltage Vb becomes the set voltage V1. This state where the amount of power being transferred A1 is close to 0 indicates that the power transfer between the slave units 2A and 2B is balanced.

[0033] On the other hand, the voltage control unit 12 of the master unit 1 controls the bus voltage Vb to the set voltage V2 if a predetermined time T has elapsed since the amount of transmission / reception A1 became greater than or equal to the threshold Th, that is, if the amount of transmission / reception A1 has been greater than or equal to the threshold Th for a longer period than the predetermined time T (step S3: YES) (step S5). By controlling the bus voltage Vb in this way, if the amount of transmission / reception A1 is sufficiently greater than 0, the bus voltage Vb becomes the set voltage V2. This state in which the amount of transmission / reception A1 is sufficiently greater than 0 is a state in which the master unit 1 is supplying power to the DC bus B, or a state in which the master unit 1 is receiving power from the DC bus B.

[0034] Next, the operation of the slave unit 2A will be explained with reference to Figure 3(A). As shown in Figure 3(A), the voltage measurement unit 21 of the slave unit 2A measures the bus voltage Vb (step S11), and then the power control unit 22 of the slave unit 2A determines whether the measurement result of the bus voltage Vb in step S11 is the set voltage V1 or not (step S12).

[0035] If the bus voltage Vb of the slave unit 2A is not the set voltage V1 (step S12: NO), the power control unit 22 of the slave unit 2A determines that the bus voltage Vb is the set voltage V2, and the slave unit 2A continues to supply power to the DC bus B without performing the operations in steps S14 to S17 described later (step S13). In other words, the slave unit 2A continues to supply power to the DC bus B without temporarily increasing or decreasing the amount of power exchanged A2 between the slave unit 2A and the DC bus B.

[0036] Meanwhile, the power control unit 22 of the slave unit 2A temporarily increases or decreases the amount of power exchanged between the slave unit 2A and the DC bus B if the bus voltage Vb is set to the voltage V1 (step S12: YES) (step S14). Then, the voltage measurement unit 21 of the slave unit 2A measures the bus voltage Vb again (step S15), and the power control unit 22 of the slave unit 2A determines whether the measurement result of the bus voltage Vb in step S15 is within the rated voltage range (greater than or equal to the lower limit voltage Vmin and less than the predetermined upper limit voltage Vmax) (step S16).

[0037] If the bus voltage Vb is within the rated voltage range (step S16: YES), the slave unit 2A continues to supply power to the DC bus B (step S13), and if the bus voltage Vb is outside the rated voltage range (step S16: NO), it stops supplying power to the DC bus B (step S17). In other words, if the power control unit 22 of the slave unit 2A temporarily increases or decreases the amount A2 sent or received, and as a result the bus voltage Vb is not maintained within the rated voltage range, the slave unit 2A stops the power supply operation, and if the bus voltage Vb is maintained within the rated voltage range, the slave unit 2A continues to supply power to the DC bus B.

[0038] Next, the operation of the slave unit 2B will be explained with reference to Figure 3(B). As shown in Figure 3(B), the voltage measurement unit 21 of the slave unit 2B measures the bus voltage Vb (step S21), and then the power control unit 22 of the slave unit 2B determines whether the measurement result of the bus voltage Vb in step S21 is within the rated voltage range (greater than or equal to the lower limit voltage Vmin and less than the upper limit voltage Vmax) (step S22).

[0039] If the bus voltage Vb is within the rated voltage range (step S22: YES), the slave unit 2B continues charging the distributed power supply D (step S23), and if the bus voltage Vb is outside the rated voltage range (step S22: NO), it stops charging the distributed power supply D (step S24). In other words, similar to the slave unit 2A, if the bus voltage Vb is not maintained within the rated voltage range, the slave unit 2B stops charging, and if the bus voltage Vb is maintained within the rated voltage range, the slave unit 2B continues charging the distributed power supply D.

[0040] Next, with reference to Figures 4(A) and (B), an example of the operation of slave units 2A and 2B when master unit 1 goes down will be explained. Figures 4(A) and (B) show the change in transmission amount A1, bus voltage, and transmission amount A2 related to slave units 2A and 2B.

[0041] Figure 4(A) shows the change in the amount of current A1 exchanged between the master unit 1 and the DC bus B when the master unit 1 goes down, while the amount of current A1 exchanged between the master unit 1 and the DC bus B is 0 (i.e., the power exchange between the slave units 2A and 2B is balanced).

[0042] As shown in Figure 4(A), at time T1, the amount of current exchanged between the master unit 1 and the DC bus B, A1, is 0, and the bus voltage Vb is controlled to be the set voltage V1. Based on the fact that the set voltage V1 was measured as the bus voltage Vb at time T2, the slave unit 2A decreases the amount of current exchanged between the slave unit 2A and the DC bus B, A2, for a predetermined time T from time T2 to time T3, and increases or decreases the amount of current exchanged. At this time, the master unit 1, which is not down, supplies power to the DC bus B because the power demand of the slave unit 2B is greater than the power supply of the slave unit 2A, and maintains the bus voltage Vb as the set voltage V1 until the amount of current exchanged A1 exceeds the threshold Th, but the predetermined time T has elapsed.

[0043] When master unit 1 goes down at time T4, slave unit 2A reduces the amount of current A2 exchanged between slave unit 2A and DC bus B based on the set voltage V1 measured as the bus voltage Vb at time T5 after master unit 1 went down. At this time, since the downed master unit 1 cannot supply power to DC bus B and maintain the bus voltage Vb, the bus voltage Vb drops, and slave units 2A and 2B stop operating at time T6 when the bus voltage Vb falls below the lower limit voltage Vmin.

[0044] Figure 4(B) shows the change in the amount of current exchanged between the master unit 1 and the DC bus B, A1, when the master unit 1 goes down, and the master unit 1 is in a state where A1 is sufficiently greater than 0 (specifically, when the master unit 1 is supplying power to the DC bus B).

[0045] As shown in Figure 4(B), at time T7, the amount of current exchanged between master unit 1 and DC bus B, A1, is sufficiently greater than 0, and the bus voltage Vb is controlled to the set voltage V2. Even when slave unit 2A measures the set voltage V2 as the bus voltage Vb, it does not increase or decrease the amount of power exchanged between slave unit 2A and DC bus B, A2.

[0046] When master unit 1 goes down at time T8, power is no longer supplied from master unit 1 to DC bus B, causing the bus voltage Vb to drop. Slave units 2A and 2B stop operating at time T9 when the bus voltage Vb falls below the lower limit voltage Vmin.

[0047] In this embodiment, the following effects can be obtained. (1) Slave unit 2A measures the bus voltage Vb, and if the measured bus voltage Vb is the set voltage V1 (i.e., the measured amount of current exchanged between master unit 1 and DC bus B A1 is less than the threshold Th), it temporarily increases or decreases the amount of power exchanged between slave unit 2A and DC bus B A2. With this configuration, if master unit 1 is down, master unit 1 cannot control the bus voltage Vb, so slave unit 2A can change the bus voltage Vb by temporarily increasing or decreasing the amount of power exchanged between slave unit 2A and DC bus B A2, thereby stopping the operation of slave units 2A and 2B. Therefore, even if master unit 1 goes down while the power exchange between multiple slave units 2A and 2B is balanced, the operation of slave units 2A and 2B can be stopped.

[0048] Furthermore, if the master unit 1 is not down, even if the slave unit 2A temporarily increases or decreases the amount of power exchanged A2 between itself and the DC bus B, the master unit 1 maintains the bus voltage Vb, allowing slave units 2A and 2B to continue operating. Also, if the measured bus voltage Vb is the set voltage V2 (i.e., the measured amount of current exchanged A1 between the master unit 1 and the DC bus B is greater than or equal to the threshold Th), the bus voltage Vb can be stabilized by the slave unit 2A refraining from temporarily increasing or decreasing the amount of power exchanged A2 between itself and the DC bus B.

[0049] (2) When the measured bus voltage Vb of the slave unit 2A is set to the voltage V1, the slave unit 2A temporarily reduces the amount of power supplied to the DC bus B (i.e., for a predetermined time T), thereby temporarily increasing or decreasing the amount of power exchanged between the slave unit 2A and the DC bus B A2. With this configuration, it is possible to prevent the bus voltage Vb from becoming overvoltage compared to a configuration in which the slave unit 2A increases the amount of power supplied to the DC bus B.

[0050] (3) The slave unit 2A repeatedly increases or decreases the amount of power exchanged between the slave unit 2A and the DC bus B at predetermined time intervals. With this configuration, the repeated temporary increases and decreases in the amount of power exchanged A2 ensure that the operation of slave units 2A and 2B is reliably stopped when the master unit 1 goes down.

[0051] (4) If the measurement result of the amount of current exchanged between the master unit 1 and the DC bus B, A1, remains above a threshold Th for a longer period than the time during which the amount of power exchanged between the slave unit 2A and the DC bus B temporarily increases or decreases (i.e., a predetermined time T), the master unit 1 controls the bus voltage Vb to the set voltage V2. With this configuration, it is possible to prevent the bus voltage Vb from being controlled based on a temporary change in the amount of current exchanged A1 caused by the operation of the slave unit 2A.

[0052] The present invention is not limited to the embodiments described above, and the above configuration can be modified. For example, it can be implemented with the following modifications, or a combination of the following modifications can be used.

[0053] The master unit 1 may be configured to include a power measurement unit that measures the amount of power exchanged with the DC bus B. If the measurement result of the amount of power exchanged between the master unit 1 and the DC bus B is less than a preset threshold, the bus voltage Vb may be controlled to become the set voltage V1. If it is above the threshold, the bus voltage Vb may be controlled to become the set voltage V2. In other words, the configuration of the master unit 1 may be appropriately changed as long as the master unit 1 can measure the amount of current or power exchanged with the DC bus B and control the bus voltage Vb based on the measurement result of the amount of exchange.

[0054] If the measured bus voltage Vb of the slave unit 2A is equal to the set voltage V1, the slave unit 2A can also temporarily increase or decrease the amount of power exchanged between the slave unit 2A and the DC bus B by temporarily increasing the amount of power supplied to the DC bus B.

[0055] If the slave unit 2B measures the bus voltage Vb and the measured bus voltage Vb is equal to the set voltage V1, it may temporarily increase or decrease the amount of power exchanged with the DC bus B A2. In other words, it is sufficient for at least one of the slave units 2A or 2B to temporarily increase or decrease the amount of power exchanged A2.

[0056] Furthermore, it is preferable that the slave unit 2B temporarily increases the amount of power supplied from the DC bus B, thereby temporarily increasing or decreasing the amount of power exchanged with the DC bus B A2. With this configuration, it is possible to prevent the bus voltage Vb from becoming overvoltage compared to a configuration in which the slave unit 2B reduces the amount of power supplied from the DC bus B.

[0057] Furthermore, the slave unit 2B can temporarily increase or decrease the amount of power exchanged with the DC bus B by temporarily reducing the amount of power supplied from the DC bus B. An example of the operation of slave units 2A and 2B according to this modified example will be explained with reference to Figure 5.

[0058] Figure 5(A) shows the change in the amount of current A1 exchanged between the master unit 1 and the DC bus B when the master unit 1 goes down, while the amount of current A1 exchanged between the master unit 1 and the DC bus B is 0 (i.e., the power exchange between the slave units 2A and 2B is balanced).

[0059] As shown in Figure 5(A), at time T11, the amount of current exchanged between the master unit 1 and the DC bus B, A1, is 0, and the bus voltage Vb is controlled to be the set voltage V1. Based on the fact that the slave unit 2B measured the set voltage V1 as the bus voltage Vb at time T12, the slave unit 2B decreases the amount of current exchanged between the slave unit 2B and the DC bus B, and increases or decreases the amount of current exchanged A2, for a predetermined time T from time T12 to time T13. At this time, the master unit 1, which is not down, receives power from the DC bus B because the power supply amount to the slave unit 2A is greater than the power demand amount to the slave unit 2B, and maintains the bus voltage Vb as the set voltage V1 until the amount of current exchanged A1 exceeds the threshold Th, but the predetermined time T has elapsed.

[0060] When master unit 1 goes down at time T14, slave unit 2B reduces the amount of current A2 exchanged between slave unit 2B and DC bus B based on the set voltage V1 measured as the bus voltage Vb at time T15 after master unit 1 went down. At this time, since the downed master unit 1 cannot maintain the bus voltage Vb by receiving power from DC bus B, the bus voltage Vb rises, and slave units 2A and 2B stop operating at time T16 when the bus voltage Vb exceeds the upper limit voltage Vmax.

[0061] Figure 5(B) shows the change in the amount of current exchanged between the master unit 1 and the DC bus B, A1, when the master unit 1 goes down, and the master unit 1 is in a state where A1 is sufficiently greater than 0 (specifically, the master unit 1 is receiving power from the DC bus B).

[0062] As shown in Figure 5(B), at time T17, the amount of current exchanged between the master unit 1 and the DC bus B, A1, is sufficiently greater than 0, and the bus voltage Vb is controlled to the set voltage V2. Even when the slave unit 2B measures the set voltage V2 as the bus voltage Vb, it does not increase or decrease the amount of power exchanged between the slave unit 2B and the DC bus B, A2.

[0063] When master unit 1 goes down at time T18, power is no longer supplied to master unit 1 from DC bus B, causing the bus voltage Vb to rise. Slave units 2A and 2B stop operating at time T19 when the bus voltage Vb exceeds the upper limit voltage Vmax.

[0064] The power exchange system S may have three or more slave units 2, and two or more of the slave units 2 may operate as first slave units that supply power to the DC bus B, and two or more slave units 2 different from the first slave units may operate as second slave units that receive power from the DC bus B. In other words, slave unit 2A and slave unit 2B each only need to be composed of at least one slave unit 2. [Explanation of Symbols]

[0065] 1. Master unit 2 Handsets 2A First handset 2B Second slave unit B DC bus D Distributed power supply L AC line P Commercial AC power supply S Power exchange system

Claims

1. A master unit connected to a DC bus and controlling the bus voltage of the said DC bus, A first slave unit connected to the DC bus and supplying power to the DC bus, It comprises a second slave unit connected to the aforementioned DC bus and receiving power from the DC bus, The master unit measures the amount of current or power exchanged with the DC bus, If the measurement result of the amount of exchange is less than a preset threshold, the bus voltage is controlled to a predetermined first set voltage. If the measurement result of the amount of exchange is greater than or equal to the threshold, the bus voltage is controlled to a predetermined second set voltage. At least one of the first slave unit and the second slave unit measures the bus voltage, If the measured bus voltage is equal to the first set voltage, the amount of power exchanged with the DC bus is temporarily increased or decreased. A power exchange system characterized by the following.

2. If the measured bus voltage of the first slave unit is equal to the first set voltage, it temporarily reduces the amount of power supplied to the DC bus, thereby temporarily increasing or decreasing the amount of power exchanged with the DC bus. The power exchange system according to feature 1.

3. If the measured bus voltage of the second slave unit is equal to the first set voltage, it temporarily increases the amount of power supplied from the DC bus, thereby temporarily increasing or decreasing the amount of power exchanged with the DC bus. The power exchange system according to feature 1.

4. At least one of the first and second slave units repeatedly performs an operation at predetermined time intervals to temporarily increase or decrease the amount of power exchanged with the DC bus. The power exchange system according to feature 1.

5. The master unit controls the bus voltage to the second set voltage only if the measurement result of the amount of current or power exchanged with the DC bus is greater than or equal to the threshold for a longer period than the time during which the amount of power exchanged between at least one of the first slave unit and the second slave unit and the DC bus temporarily increases or decreases. A power exchange system according to any one of claims 1 to 4.

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