Charging and discharging system, and method for controlling the charging and discharging system

Abstract

This invention provides a charging and discharging system and a control method thereof that can reliably satisfy the FRT function even when the AC / DC converter and DC / DC converter are spatially separated. [Solution] The charging and discharging system has a DC bus 9 that converts DC from an electric vehicle 6, which acts as an energy storage device, into DC of different voltages using a DC / DC converter 4, and supplies the DC from the DC / DC converter to an AC / DC converter 2. The AC / DC converter has the function of converting the DC supplied from the DC bus into AC and supplying power to the power system 1. When supplying power, the DC / DC converter stops operation or limits output based on the voltage rise of the DC bus.

Description

[Technical Field]

[0001] The present invention relates to a charge-discharge system and a control method for a charge-discharge system. [Background Art]

[0002] As renewable energy, power supply to grid power from power generation means such as photovoltaic power generation and wind power generation is widely performed. In addition, studies on power supply from storage batteries of electric vehicles to power grids for the future are also progressing.

[0003] However, when the grid voltage instantaneously drops due to a lightning strike or the like, it is feared that a dropout phenomenon may occur in power converters used for power supply to grid power. Therefore, facilities that supply power to the power grid via power converters are required to have an FRT (Fault Ride Through) function that continues operation even during an instantaneous voltage drop when a grid abnormality occurs.

[0004] One example of a specific regulation is the Grid Interconnection Code (JEAC9701-2024) by the Japan Electrical Association, a general incorporated foundation. According to this regulation, when the voltage of the power grid instantaneously drops, the power converter side is also required to continue operation. This is because if a large number of power converters stop operating, there is a risk that the power balance will be disrupted.

[0005] As an example of compliance with the FRT function, there is Patent Document 1. [Prior Art Documents] [Patent Documents]

[0006] [Patent Document 1] Japanese Unexamined Patent Publication No. 2018-7458 [Summary of the Invention] [Problem to be Solved by the Invention]

[0007] Implementing FRT support requires a short response time. Therefore, it is necessary to address control delays caused by communication between devices.

[0008] Patent Document 1 discloses a power converter 5, as shown in Figure 1, etc. In this power converter 5, the generator-side power converter 3 and the grid-side power converter 4 are controlled by a single power converter control unit 6.

[0009] Therefore, since the FRT compatibility of the generator-side power converter 3 and the grid-side power converter 4 is controlled by instructions from the same power converter control unit 6, there is no need to consider communication delays between the generator-side power converter 3 and the grid-side power converter 4.

[0010] On the other hand, in recent years, there have been cases where equipment corresponding to the grid-side power converter 4, such as an AC / DC converter, and equipment corresponding to the generator-side power converter 3, such as a DC / DC converter, are installed spatially separated and controlled by separate control devices. A specific example is a charging and discharging system for electric vehicles. For example, in a parking lot, the DC / DC converter is placed near the parking space where the electric vehicle to be charged is parked, while the AC / DC converter is placed at a distance so as not to encroach on the parking space.

[0011] In such cases, increased communication delay is unavoidable between the AC / DC converter and the DC / DC converter due to the physical distance between them. As a result, depending on the installation location and grounding distance, there is a risk that the FRT function may not be fully realized.

[0012] Such challenges and solutions are neither disclosed nor suggested in Patent Document 1.

[0013] Therefore, the objective of this invention is to provide a charging and discharging system that can reliably satisfy the FRT function even when the AC / DC converter and DC / DC converter are spatially separated.

[0014] Further objectives of this application will become clear throughout the entire specification below. [Means for Solving the Problem]

[0015] An example of means for solving the above problem is as follows.

[0016] A charge-discharge system comprising: a DC bus that converts direct current from a power storage means into direct current of a different voltage by a DC / DC converter, and supplies the direct current from the DC / DC converter to an AC / DC converter, wherein the AC / DC converter has a function of converting the direct current supplied from the DC bus into alternating current and feeding the alternating current to a power system, and during the power feeding, the DC / DC converter stops operation or limits output based on a rise in voltage of the DC bus. [Effects of the Invention]

[0017] According to the present invention, even when an AC / DC converter and a DC / DC converter are spatially spaced apart, a charge-discharge system that can reliably satisfy the FRT function can be provided.

[0018] Further means and effects of the present invention will become apparent throughout the following entire specification. [Brief Description of the Drawings]

[0019] [Figure 1] FIG. 1 is a schematic configuration diagram of a charging system for an electric vehicle. [Figure 2] FIG. 2 is an operational conceptual diagram of an example of the charge-discharge system. [Figure 3] FIG. 3 is a sequence diagram of an example of the charge-discharge system. [Figure 4A] FIG. 4 is an excerpted view of the DC bus voltage 20 in FIG. 2. [Figure 4B] FIG. 5 is an explanatory diagram of an assumed event. [Figure 4C] FIG. 6 is an explanatory diagram of control of the DC bus voltage 20 in Embodiment 2. [Figure 5] FIG. 7 is a sequence diagram of an example of the charge-discharge system. [Figure 6] FIG. 8 is a comparative example corresponding to a part of FIG. 2. [Figure 7] FIG. 9 is an operational conceptual diagram of an example of the charge-discharge system. [Figure 8] It is a sequence diagram of an example charge-discharge system. [Figure 9] It is an operation conceptual diagram of an example charge-discharge system. [Figure 10] It is a sequence diagram of an example charge-discharge system. MODE FOR CARRYING OUT THE INVENTION

[0020] Hereinafter, embodiments according to the present invention will be described with reference to the drawings. Examples

[0021] FIG. 1 is a schematic configuration diagram of the charge-discharge system.

[0022] First, description will be given from the perspective of when charging an electric vehicle. When charging the electric vehicle, alternating current from a power grid 1 is input to an AC / DC converter 2 via a power line 8. In the AC / DC converter 2, the alternating current is converted into direct current. The converted direct current is supplied to a DC / DC converter 4 via a DC bus 9.

[0023] The AC / DC converter 2 is controlled by an AC / DC converter control device 3.

[0024] The DC / DC converter 4 converts direct current into another direct current by means of a converter or the like. The DC / DC converter 4 is controlled by a DC / DC converter control device 7. DC / DC conversion usually converts the direct current into direct current of a different voltage.

[0025] The direct current output from the DC / DC converter 4 is supplied to a charging stand 5. Charging power is supplied from the charging stand 5 to the electric vehicle 6 via a charging cable 91. The AC / DC converter control device 3 and the DC / DC converter control device 7 are connected by a signal transmission means 90.

[0026] In Figure 1, the DC / DC converter 4 consists of four units, 4A to 4D, each connected to a corresponding charging station 5A to 5D. Similarly, the corresponding electric vehicles 6A to 6D are charged via corresponding charging cables 91A to 91D.

[0027] However, there is no particular limit to the number of DC / DC converters 4, as long as there are multiple units.

[0028] Next, we will explain from the perspective of discharge from or power supply from an electric vehicle. When an electric vehicle discharges or power is supplied, power is supplied from the electric vehicle 6 to the charging station 5 via the charging cable 91. Normally, this is done by supplying a DC voltage from the battery built into the electric vehicle.

[0029] DC power is supplied from the charging station 5 to the DC / DC converter 4. The DC / DC converter 4 performs DC / DC conversion in response to a request from the connected AC / DC converter 2 and supplies DC power to the AC / DC converter 2 via the DC bus 9.

[0030] The AC / DC converter 2 converts the AC / DC to alternating current with the frequency and voltage specified by the power system 1 and supplies power to the power system 1.

[0031] This invention aims to address, in particular, the challenges associated with the discharge of electricity from electric vehicles or the supply of electricity from electric vehicles.

[0032] For the purpose of simplifying explanation, electric vehicles are used throughout this specification. However, this application also applies to various types of energy storage and power generation equipment other than electric vehicles, as long as they are power sources that supply electricity to the power grid. In that case, the charging station 5 can be any means of connection.

[0033] For example, this includes cases where solar power generation or wind power generation are applied in a manner corresponding to the electric vehicle in Figure 1. It also includes cases where a large-scale energy storage facility is used instead of the electric vehicle in Figure 1. Furthermore, it includes cases where the large-scale energy storage facility is installed between the power grid and the solar or wind power generation facilities.

[0034] Figure 2 is a conceptual diagram of an example of a charging and discharging system. The following section explains how to respond when an abnormality occurs in the power grid voltage when power is supplied to the power grid from a power source, such as an electric vehicle.

[0035] The AC / DC converter control 100 is an operation related to the control of the AC / DC converter 2 by the AC / DC converter control device 3.

[0036] The DC / DC converter control 200 is an operation related to the control of the DC / DC converter 4 by the DC / DC converter control device 7.

[0037] Initially, the system voltage 10 is at a normal value under the AC / DC converter control 100. This section explains what happens when the voltage drops significantly at time T1, for example, due to a momentary power outage.

[0038] The DC bus voltage 20 is the voltage of DC bus 9. At T1, a sharp drop in the power system voltage makes it difficult to supply power to the system voltage side. This is because supplying power from the AC / DC converter 2 to power system 1 requires the voltage, frequency, and phase to be within specified ranges, and disturbances in the power system make it difficult to comply.

[0039] Power continues to be supplied from the DC / DC converter 4 to the AC / DC converter 2 via the DC bus 9. As a result, the DC bus voltage 20 begins to rise.

[0040] Figure 6 is a comparative example. It is an explanatory diagram of what happens when no appropriate action is taken after the DC bus voltage 20 begins to rise in Figure 2.

[0041] The DC bus voltage 20 begins to rise sharply from the rated voltage at T1 and exceeds the overvoltage level 21 in a short time. As a result, there is a risk of damage to equipment such as the AC / DC converter 2 and the DC / DC converter 4. In the case of Figure 6, the DC bus voltage 20, which has exceeded the overvoltage level 21, remains at a high voltage.

[0042] Subsequently, at T2, once the system voltage 10 returns to normal, the DC bus voltage 20 will gradually decrease over time.

[0043] In any case, if appropriate measures are not taken, the DC bus voltage will exceed the overvoltage level, potentially causing equipment damage.

[0044] This problem is particularly pronounced in configurations where multiple DC / DC converters 4 are connected to a single AC / DC converter via a shared DC bus 9, as shown in Figure 1. Since the DC / DC converter control devices 7A to 7D operate and are controlled independently for each DC / DC converter 4A to 4D, this can lead to unmanageable situations. Furthermore, even if one of the control devices is designated as the master unit, communication between the control devices is essential, making it unavoidable that communication delays will cause delays in resolving the issue.

[0045] One of the objectives of this application is to avoid the incident shown in Figure 6, which is a comparative example, and to prevent equipment damage.

[0046] Figure 2 illustrates an example of a solution.

[0047] The DC / DC converter control unit 200 detects the start of the rise in the DC bus voltage 20, and the DC / DC converter control unit 7 does this independently. Because it does not involve communication, the rise in the DC bus voltage 20 can be detected shortly after the start of the rise, for example, as shown by X1 in the figure.

[0048] As soon as the DC / DC converter control device 7 detects an increase in the DC bus voltage 20 at X1, it generates an output limiting signal 80 or instructs the DC / DC converter 4. At this point, it is T1'', which includes a very small time delay after the detection at X1. Upon receiving the output limiting signal 80, the DC / DC converter 4 cuts the DC / DC converter discharge current 70. At T1'''', the current cut is implemented or output limiting begins.

[0049] As a result, the current supplied from the DC / DC converter 4 to the DC bus 9 is also cut or limited, so the voltage rise of the DC bus 9 stops and remains constant. This corresponds to the flat portion between R1 and T2 in the DC bus voltage 20 in Figure 2.

[0050] This prevents the DC bus voltage 20 from rising above the overvoltage level 21, as shown in the comparative example in Figure 7, and thus prevents equipment damage.

[0051] Next, we will explain the case where the system voltage 10 returns to normal at time T2.

[0052] To satisfy the FRT condition, it is necessary to restore at least 80% of the output power before the momentary power outage within 0.1 seconds from this T2 point.

[0053] As soon as the AC / DC converter 2 detects the restoration of the grid voltage 10, it immediately resumes supplying power to the power system 1. Therefore, the DC bus voltage 20, which is the power source for the AC / DC converter 2, begins to decrease from time T2. If the DC bus voltage 20 drops too low, it will become impossible to supply power to the power system 1.

[0054] In response, the DC / DC converter control device 7, upon detecting a drop in the DC bus voltage 20 at X2, releases the output limit signal 80 or instructs the DC / DC converter 4 to terminate the output limit. At this point, T2'' occurs, with a very small time delay after detection at X2. Upon receiving the release of the output limit signal 80, the DC / DC converter 4 resumes normal operation. As a result, the DC / DC converter discharge current 70 begins to rise towards its rated value from T2'''', reaching the rated or commanded value at T4.

[0055] The decrease in the DC bus voltage 20 due to its use by the AC / DC converter 2 and the increase in the DC bus voltage 20 due to the restarted DC / DC converter discharge current 70 will eventually balance out, and the DC bus voltage 20 will return to its rated voltage.

[0056] To avoid an excessive drop in the DC bus voltage 20 and to satisfy the FRT condition, the DC / DC converter discharge current 70 must be within time TX from time T to time T3, when it reaches 80% of its rating. For the FRT condition to be satisfied, this time TX must be within 0.1 seconds.

[0057] This condition of being within 0.1 seconds is so short that it is highly susceptible to the effects of communication delays.

[0058] In particular, as explained in the problem the invention aims to solve, when the DC / DC converter and AC / DC converter are placed far apart, communication delay due to the physical distance becomes a major problem.

[0059] Figure 3 is a sequence diagram of an example of the charge / discharge system in this embodiment. It is a sequence diagram corresponding to the conceptual operation diagram in Figure 2.

[0060] In Figure 3, the left side shows the sequence for AC / DC converter 2, and the right side shows the sequence for DC / DC converter 4.

[0061] As shown in Figure 2 at T1, a sharp drop in the grid voltage occurs (S11). The AC / DC converter 2 then immediately stops outputting to the power grid 1.

[0062] As shown in Figure 2, a sudden drop in the grid voltage causes the DC bus voltage to rise. The DC / DC converter 4 detects the rise in DC bus voltage (S12). As soon as the DC / DC converter 4 detects the rise in DC bus voltage, it starts output limiting (S13).

[0063] A distinctive feature of this sequence diagram is that, in the event of a sudden drop in grid voltage, AC / DC converter 2 and DC / DC converter 4 each perform their corresponding operations independently. In other words, in the event of a sudden drop in grid voltage, AC / DC converter 2 and DC / DC converter 4 can perform their corresponding operations without mutual communication.

[0064] In principle, by eliminating the need for communication altogether, it is possible to eliminate the effects of communication delays.

[0065] Therefore, the DC bus voltage 20 can be stabilized almost instantaneously, before it reaches the overvoltage level 21, thus reliably preventing equipment damage due to overvoltage.

[0066] Eventually, as shown in T2 of Figure 2, the system voltage returns to normal (S14). At this point, the AC / DC converter 2 immediately resumes outputting to the power system 1. Consequently, the DC bus voltage 20 also begins to decrease. However, the DC / DC converter 4 detects the voltage drop in the DC bus (S15). As soon as the DC / DC converter 4 detects the voltage drop in the DC bus, it releases the output limit (S16).

[0067] As a result, the discharge current from the DC / DC converter 4 is rapidly restored to its rated or commanded value, as shown in Figure 2.

[0068] As a result, the DC bus voltage 20 stabilizes at the rated voltage, and normal power supply operation resumes.

[0069] A distinctive feature of this sequence diagram is that, upon restoration of the grid voltage, AC / DC converter 2 and DC / DC converter 4 each perform corresponding operations independently. In other words, upon restoration of the grid voltage, AC / DC converter 2 and DC / DC converter 4 can perform corresponding operations without mutual communication.

[0070] In principle, by eliminating the need for communication altogether, it is possible to eliminate the effects of communication delays.

[0071] Therefore, it becomes possible to reliably meet the FRT requirements.

[0072] Unlike communication, the above method utilizes the physical characteristic that voltage and current do not have the concept of delay; in other words, they simultaneously possess the same value at each connected location. Therefore, by using the DC bus voltage as an operating trigger, AC / DC converter 2 and DC / DC converter 4 can achieve optimal response without communicating with each other.

[0073] According to this embodiment, a charging and discharging system that can reliably satisfy the FRT function can be provided even when the AC / DC converter and the DC / DC converter are spatially separated.

[0074] Furthermore, compared to methods using communication, it fundamentally eliminates the need for expensive communication devices and processing to reduce communication latency, thus enabling low-cost FRT support. [Examples]

[0075] This embodiment is an additional embodiment that can be performed in addition to Example 1.

[0076] Figure 2 illustrates an example where the DC bus voltage 20 remains nearly constant between T1 and T2.

[0077] Figure 4A is an excerpt of the DC bus voltage 20 from Figure 2. When the drop in the grid voltage is short, in other words, when the time between T1 and T2 is short, the situation is as explained in Figure 2. That is, the DC bus voltage 20 remains almost constant between T1 and T2.

[0078] On the other hand, if the grid voltage drops for a long period of time, in other words, if the time between T1 and T2 is long, the DC bus voltage 20 will gradually decrease due to natural discharge, etc. As a result, the system will detect the drop in the DC bus voltage 20, as shown in X2 in Figure 2, or will mistakenly determine that it has returned to a normal state. The DC / DC converter 4 will then restart operation, and the supply of the DC / DC converter discharge current 70 to the DC bus 9 will resume. However, at this time, the grid voltage is actually still decreasing. Therefore, the DC bus voltage 20 will rise sharply, and as shown in X1, the system will detect the voltage rise on the DC bus and restart the output limiting.

[0079] As a result, the DC bus voltage 20 will fluctuate in a zigzag pattern, for example, as shown as 20B in Figure 4B. This can, in some cases, lead to operational instability of the charging and discharging system.

[0080] Therefore, this embodiment is characterized by raising the DC bus voltage 20 to a predetermined value when a decrease in the power system voltage is detected.

[0081] In Figure 4C, with respect to the DC bus voltage 20, 20A is the voltage corresponding to 20 in Figure 4A. In contrast, in this embodiment, as shown as 20C, the DC bus voltage 20 is made higher than the predetermined value ST1. At that time, the predetermined value ST1 is set to a value lower than the overvoltage level 21.

[0082] This reduces the number and frequency of fluctuations, as shown in Figure 4B, even when the grid voltage drops for extended periods, thereby improving the operational stability of the charge / discharge system.

[0083] It should be noted that this issue does not arise in the first place if the AC / DC converter 2 and the DC / DC converter 4 communicate to respond to a drop in the grid voltage. In Examples 1 and 2, this is a unique phenomenon that occurs because the DC / DC converter 4 determines its operation based on the DC bus voltage 20, without relying on communication.

[0084] Figure 5 is a sequence diagram of an example of the charge / discharge system in this embodiment. It corresponds to Figure 3. It is an example of the sequence for achieving the DC voltage of 20C shown in Figure 4B.

[0085] In Figure 5, the left side shows the sequence for AC / DC converter 2, and the right side shows the sequence for DC / DC converter 4.

[0086] As shown in Figure 2 at T1, a sharp drop in the grid voltage occurs (S21). In response, the AC / DC converter 2 limits its output to the power grid 1 to prevent it from becoming overcurrent (S22).

[0087] As shown in Figure 2, a sudden drop in the grid voltage causes the DC bus voltage to rise. The AC / DC converter 2 detects the sudden drop in grid voltage and sets the command value for constant DC bus voltage control to a state higher than the written fixed value ST1. (S22)

[0088] As soon as the DC / DC converter 4 detects that the DC bus voltage 20 has risen to a predetermined value ST1 or higher (S23), it starts limiting the output (S24).

[0089] Eventually, as shown in T2 of Figure 2, the system voltage returns to normal (S25). The AC / DC converter 2 then immediately resumes outputting to the power system 1. Consequently, the DC bus voltage 20 also begins to decrease. The control voltage value for the DC bus voltage 20 of the DC bus 9 is then returned to the rated value (S26).

[0090] The DC / DC converter 4 detects the voltage drop in the DC bus and detects that it has fallen below a predetermined value ST1 (S27). This also means that the DC bus voltage 20 has returned to normal. As soon as the DC bus voltage 20 returns to normal, the output limit is released (S28).

[0091] As a result, the discharge current from the DC / DC converter 4 is rapidly restored to its rated or commanded value, as shown in Figure 2.

[0092] In this embodiment as well, the effects of Example 1 can be achieved.

[0093] Furthermore, it ensures the operational stability of the charging and discharging system even when the grid voltage drops for an extended period.

[0094] In this embodiment, the absolute time of T2'''' is shifted slightly later compared to Embodiment 1. However, the DC bus voltage 20 is maintained at a higher level than in Embodiment 1 during the power grid voltage drop. As a result, the power supply from the AC / DC converter 2 to the power grid, which starts immediately after the power grid is restored, is powered by this higher DC bus voltage.

[0095] After the AC / DC converter 2 resumes supplying power to the power system, the resumption of supplying the DC / DC converter discharge current 70 to the DC bus 9 by the restart of the DC / DC converter 4 will result in a time delay of AD, for example, compared to the case of Embodiment 1. However, the power consumed by the AC / DC converter 2 during this AD period is higher than the predetermined value because the DC bus voltage 20 is set to 20C, so power supply is achieved without interruption by utilizing this. Therefore, in this embodiment as well, the FRT condition is substantially achieved. [Examples]

[0096] This embodiment can be applied alone or in combination with either Embodiment 1 or Embodiment 2.

[0097] In Examples 1 and 2, the charging and discharging system was capable of responding to a drop in the grid voltage 10 without communication between the AC / DC converter 2 and the DC / DC converter 4.

[0098] However, communication itself does not need to be absolutely eliminated as long as the communication delay is kept within a sufficiently small range and the FRT conditions are met.

[0099] Therefore, this embodiment describes an example of FRT support using communication, which is applicable when the AC / DC converter 2 and the DC / DC converter 4 are spatially separated, but the distance between them is not long.

[0100] Figure 7 is a conceptual diagram illustrating the operation of an example of a charge / discharge system. It corresponds to Figure 2.

[0101] A drop in the system voltage 10 is detected at T1, and the AC / DC converter control device 3 transmits an output limit signal to the DC / DC converter control device 7 at T1'. The output limit signal 30 transmitted from the AC / DC converter control device 3 is received by the DC / DC converter control device 7 at T1''. The time between T1' and T1'' is the communication delay time.

[0102] Upon receiving the output limiting signal 60, the DC / DC converter 4 stops supplying the DC / DC converter discharge current 70 to the DC bus 9.

[0103] Then, the system voltage 10 returns to normal at T2. The AC / DC converter control device 3 transmits an output limit release signal to the DC / DC converter control device 7 at T2'. The output limit release signal transmitted from the AC / DC converter control device 3, or the end of the output limit signal 30, is received by the DC / DC converter control device 7 at T2''. The time between T2' and T2'' is the communication delay time.

[0104] Upon receiving the output limit release signal, or when the received output limit signal 60 disappears, the DC / DC converter 4 resumes operation and resumes supplying the DC / DC converter discharge current 70 to the DC bus 9.

[0105] Figure 8 is a sequence diagram of an example of the charge / discharge system in this embodiment. It corresponds to Figure 3 or Figure 5.

[0106] In Figure 8, the left side shows the sequence for AC / DC converter 2, and the right side shows the sequence for DC / DC converter 4.

[0107] As shown in Figure 7, T1, a sudden drop in the grid voltage is detected (S31). The AC / DC converter 2 then immediately stops or suppresses its output to the power grid 1. The AC / DC converter control device 3 generates an output limit signal 30 and transmits this signal to the DC / DC converter control device 7 (S32).

[0108] The DC / DC converter control device 7, triggered by the received output limiting signal 60, stops supplying the DC / DC converter discharge current 70 from the DC / DC converter 4 to the DC bus 9. Alternatively, it starts output limiting (S33).

[0109] Eventually, as shown in T2 of Figure 7, the system voltage is detected to have returned to normal (S34). The AC / DC converter 2 then immediately resumes outputting to the power system 1. The AC / DC converter control device 3 transmits an output restriction release signal to the DC / DC converter control device 7 (S35).

[0110] The DC / DC converter control device 7 resumes supplying the DC / DC converter discharge current 70 from the DC / DC converter 4 to the DC bus 9, triggered by the disappearance of the output limiting signal or the reception of the output limit release signal. Alternatively, it releases the output limit (S36).

[0111] This embodiment can achieve the FRT condition when there is little delay due to communication.

[0112] Furthermore, the system voltage 10 monitoring function is provided only in the AC / DC converter control device 3 on the power system 1 side, and not in the DC / DC converter control device 7.

[0113] This simplifies the monitoring configuration for the grid voltage 10 and allows for the provision of a low-cost charge / discharge system that can fulfill the FRT function.

[0114] Furthermore, in Example 2, the hypothetical problems when communication is not used were explained using Figure 4B. In this example, by using communication, the hypothetical problems shown in Figure 4B can be eliminated, thereby improving the stability of the charging and discharging system. [Examples]

[0115] This embodiment is a modification of Embodiment 3.

[0116] Figure 9 is a conceptual diagram illustrating an example of a charge / discharge system. It corresponds to Figure 7. The differences between Figure 9 and Figure 7 will be explained below.

[0117] This embodiment is an example of a case where the decrease in the system voltage 10 is limited.

[0118] The AC / DC converter control device 3 determines the state of the grid voltage 10. If the grid voltage 10 exceeds the first determination value CV1, it is determined to be in a normal state. If the grid voltage 10 falls below the second determination value CV2, it is determined to be a momentary power outage, and the response described in Example 3 is taken.

[0119] On the other hand, when the system voltage 10 is between the first determination value CV1 and the second determination value CV2, that is, when the decrease in the system voltage 10 is limited, in this embodiment, power is supplied from the AC / DC converter 2 to the power system 1 at a voltage lower than that during normal power supply.

[0120] At T1, the AC / DC converter control device 3 determines a limited drop in grid power and reduces the power supply voltage to the power system 1. The degree of reduction may correspond to, for example, the value of the grid voltage 10, and may be set up to provide support to prevent further drops in the grid voltage.

[0121] Furthermore, the AC / DC converter control device 3 transmits an output value change signal to the DC / DC converter control device 7 at T1'. The output value change signal 31 transmitted from the AC / DC converter control device 3 is received by the DC / DC converter control device 7 at T1''. The time between T1' and T1'' is the communication delay time.

[0122] Upon receiving the output value change signal 61, the DC / DC converter 4 changes its operating state to low-level operation and reduces the DC / DC converter discharge current 70 supplied to the DC bus 9.

[0123] Eventually, the system voltage 10 returns to normal at T2.

[0124] At time T2', the AC / DC converter control device 3 stops transmitting the output value change signal to the DC / DC converter control device 7. The DC / DC converter control device 7 receives the termination of the output value change signal at T2''. The time between T2' and T2'' is the delay time due to communication.

[0125] Upon receiving the termination of the output value change signal, the DC / DC converter control device 7 restores the DC / DC converter 4 to normal operation. As a result, the supply of the DC / DC converter discharge current 70 to the DC bus 9 returns to its normal value.

[0126] Figure 10 is a sequence diagram of an example of the charge / discharge system in this embodiment. It corresponds to Figure 8.

[0127] In Figure 10, the left side shows the sequence for AC / DC converter 2, and the right side shows the sequence for DC / DC converter 4.

[0128] As shown in T1 of Figure 9, a limited drop in the grid voltage is detected (S41). The AC / DC converter 2 then suppresses its output to the power grid 1. The AC / DC converter control device 3 generates an output value change signal 31 and transmits this signal to the DC / DC converter control device 7 (S42).

[0129] The DC / DC converter control device 7, triggered by the received output value change signal 61, switches the operating state of the DC / DC converter 4 to low-level operation and reduces the supply of DC / DC converter discharge current 70 to the DC bus 9 (S43).

[0130] Eventually, as shown in T2 of Figure 9, the system voltage is detected to have returned to normal (S44). The AC / DC converter 2 then restores the output to the power system 1 to its normal value. The AC / DC converter control device 3 stops transmitting the output value change signal to the DC / DC converter control device 7 (S45).

[0131] The DC / DC converter control device 7, triggered by the disappearance of the output value change signal or the reception of a normal output instruction, restores the operating state of the DC / DC converter 4 to its normal state (S46). As a result, the supply of the DC / DC converter discharge current 70 to the DC bus 9 returns to its normal state. In this embodiment, when the decrease in the grid voltage 10 is limited, the AC / DC converter 2 and DC / DC converter 4 continue to operate at a lower-than-normal level even during the grid voltage drop. Therefore, when the grid voltage returns to its normal value, the charge / discharge system can be returned to normal operation in a shorter time than in Embodiment 3.

[0132] Each of the above embodiments has been described from the perspective of a charge / discharge system. Furthermore, insofar as the disclosed technical concept is applied, methods for controlling the charge / discharge system are also within the scope of disclosure of this specification.

[0133] Each of the above embodiments can be applied individually or in combination. In such cases, it is still within the scope of the disclosure of this application.

[0134] Furthermore, insofar as the technical concept disclosed in this specification is used, equivalent configurations obtained by modification, substitution, etc., are also within the scope of the disclosure in this application.

[0135] Furthermore, an example of the present invention described using the above embodiments can also be expressed as follows.

[0136] <Part 1> The system has a DC bus that converts DC from a power storage means into DC of different voltages using a DC / DC converter, and supplies DC from the DC / DC converter to an AC / DC converter, the AC / DC converter having the function of converting the DC supplied from the DC bus into AC and supplying power to the power grid. A charge / discharge system in which, during power supply, the DC / DC converter stops operation or limits output based on an increase in the voltage of the DC bus. <Part 2> The charging and discharging system according to <Part 1>, wherein when the voltage of the DC bus returns to a normal value, the operation of the DC / DC converter is restarted or the output limit is released. <Part 3> The charging and discharging system described in <Part 2>, wherein the voltage of the DC bus is increased to a higher value than normal when the voltage of the power system drops. <Part 4> The charge / discharge system described in <Part 3> is performed by changing the command value for constant voltage control of the AC / DC converter to raise the voltage value of the DC bus higher than normal. <Part 5> The charging and discharging system described in <Part 4>, which restores the voltage value of the DC bus to the rated value when the voltage of the power system returns to a normal value. <Part 6> A charge / discharge system according to any one of items <1> to <5>, wherein the AC / DC converter control device that controls the AC / DC converter transmits an output limit signal or an output value change signal to the DC / DC converter control device that controls the DC / DC converter when it detects a decrease in the system voltage of the power system. <Part 7> The AC / DC converter control device transmits an output limit release signal to the DC / DC converter control device, or stops transmitting an output limit signal, or stops transmitting an output value change signal, when the system voltage returns to a normal value, in the charge / discharge system described in <6>. <Part 8> The system has a DC bus that converts DC from a power storage means into DC of different voltages using a DC / DC converter, and supplies DC from the DC / DC converter to an AC / DC converter, the AC / DC converter having the function of converting the DC supplied from the DC bus into AC and supplying power to the power grid. A control method for a charge / discharge system in which, during the power supply, the DC / DC converter stops operation or limits output based on an increase in the voltage of the DC bus. <Part 9> A control method for a charge / discharge system according to <8>, wherein when the voltage of the DC bus returns to a normal value, the operation of the DC / DC converter is restarted or the output limit is released. <Part 10> A control method for a charge / discharge system according to <9>, wherein the voltage of the DC bus is increased to a higher value than normal when the voltage of the power system decreases. <Part 11> The control method for the charge / discharge system described in <10>, wherein the voltage value of the DC bus is increased to a higher level than usual by changing the command value of the constant voltage control of the AC / DC converter. <Part 12> A control method for a charge / discharge system described in <Part 11>, which restores the voltage value of the DC bus to the rated value when the voltage of the power system returns to a normal value. <Part 13> A method for controlling a charge / discharge system according to any one of items <8> to <12>, wherein the AC / DC converter control device that controls the AC / DC converter transmits an output limit signal or an output value change signal to the DC / DC converter control device that controls the DC / DC converter when it detects a decrease in the system voltage of the power system. <Part 14> The control method for a charge / discharge system according to claim 13, wherein the AC / DC converter control device transmits an output limit release signal to the DC / DC converter control device, or stops transmitting an output limit signal, or stops transmitting an output value change signal when the system voltage returns to a normal value. [Explanation of symbols]

[0137] 1: Power system 2: AC / DC converter 3: AC / DC converter control unit 4: DC / DC converter 5: Charging stand 6: Electric vehicles 7: DC / DC converter control unit 8: Power lines 9: DC bus 10: System voltage 20: DC bus voltage 21: Overvoltage level 30: Transmitted output limiting signal 31: Transmitted output value change signal 60: Received output limiting signal 61: Received output value change signal 70: DC / DC converter discharge current 80: Output limiting signal 90: Signal transmission means 91: Charging cable 100: AC / DC converter control 200: DC / DC converter control

Claims

1. The system has a DC bus that converts DC from a power storage means into DC of different voltages using a DC / DC converter, and supplies DC from the DC / DC converter to an AC / DC converter, the AC / DC converter having the function of converting the DC supplied from the DC bus into AC and supplying power to the power grid. A charge / discharge system in which, during power supply, the DC / DC converter stops operation or limits output based on an increase in the voltage of the DC bus.

2. The charge / discharge system according to claim 1, wherein when the voltage of the DC bus returns to a normal value, the operation of the DC / DC converter is restarted or the output limit is released.

3. The charge / discharge system according to claim 2, wherein the voltage of the DC bus is increased to a higher value than normal when the voltage of the power system decreases.

4. The charge / discharge system according to claim 3, wherein the voltage value of the DC bus is increased to a higher level than normal by changing the command value of the constant voltage control of the AC / DC converter.

5. The charge / discharge system according to claim 4, wherein the voltage of the DC bus is returned to the rated value when the voltage of the power system returns to a normal value.

6. The charging and discharging system according to any one of claims 1 to 5, wherein the AC / DC converter control device that controls the AC / DC converter transmits an output limit signal or an output value change signal to the DC / DC converter control device that controls the DC / DC converter when it detects a decrease in the system voltage of the power system.

7. The charging and discharging system according to claim 6, wherein the AC / DC converter control device transmits an output limit release signal to the DC / DC converter control device, or stops transmitting an output limit signal, or stops transmitting an output value change signal when the system voltage returns to a normal value.

8. The system has a DC bus that converts DC from a power storage means into DC of different voltages using a DC / DC converter, and supplies DC from the DC / DC converter to an AC / DC converter, the AC / DC converter having the function of converting the DC supplied from the DC bus into AC and supplying power to the power grid. A control method for a charge / discharge system in which, during the power supply, the DC / DC converter stops operation or limits output based on an increase in the voltage of the DC bus.

9. A control method for a charge / discharge system according to claim 8, wherein when the voltage of the DC bus returns to a normal value, the operation of the DC / DC converter is restarted or the output limit is released.

10. A control method for a charge / discharge system according to claim 9, wherein when the voltage of the power system drops, the voltage value of the DC bus is increased to a higher value than normal.

11. The control method for a charge / discharge system according to claim 10, wherein the voltage value of the DC bus is increased to a higher level than normal by changing the command value of the constant voltage control of the AC / DC converter.

12. The control method for a charge / discharge system according to claim 11, wherein the voltage of the power system returns to a normal value, and the voltage value of the DC bus is returned to the rated value.

13. A method for controlling a charge / discharge system according to any one of claims 8 to 12, wherein the AC / DC converter control device that controls the AC / DC converter transmits an output limit signal or an output value change signal to the DC / DC converter control device that controls the DC / DC converter when it detects a decrease in the system voltage of the power system.

14. The control method for a charge / discharge system according to claim 13, wherein the AC / DC converter control device transmits an output limit release signal to the DC / DC converter control device, or stops transmitting an output limit signal, or stops transmitting an output value change signal when the system voltage returns to a normal value.

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