Power system and control method

Abstract

To effectively suppress harmonics resonance in a power conversion device of a power system.SOLUTION: In a power system, a power conversion device includes: a switch for switching electrical connection with an AC power supply on a power system side; a power converter including a switching element for switching connection with a DC device that is a device operated by DC power; and a converter control unit for giving an operation command to the switching element. The power system includes a harmonics detection unit for detecting harmonics in the power system and a command function unit. The command function unit is connected to a plurality of power conversion devices to output an operation state command to the power conversion device. When detecting that the harmonics exceed a predetermined criterion, the command function unit outputs an operation state change command for changing the operation state to a predetermined operation state for harmonics suppression to the plurality of power conversion devices and gradually reduces the number of power conversion devices to which the operation state change command is output after detecting that the harmonics becomes equal to or lower than the predetermined criterion.SELECTED DRAWING: Figure 4

Description

[Technical Field]

[0001] FIELD OF THE INVENTION Embodiments of the present invention relate to a power system and a control method. [Background technology]

[0002] 2. Description of the Related Art Conventionally, a power system includes a plurality of power conversion devices that convert AC power and DC power, for example. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Patent No. 6921631 Summary of the Invention [Problem to be solved by the invention]

[0004] In power conversion devices that are connected to a grid (electric power system), harmonic resonance occurs due to the interaction between the system impedance caused by the wiring and power load on the grid side and the control of the power conversion device, causing operation instability and, in some cases, leading to a protective shutdown. For example, in systems such as microgrids and offshore wind power generation systems in which many power conversion devices are interconnected, there is a high risk of serious resonance problems occurring due to their interaction.

[0005] Furthermore, although the power conversion device is equipped with a harmonic filter, when the operating state changes, the system function of the harmonic filter changes, which may cause harmonic resonance.

[0006] Therefore, an object of this embodiment is to provide a power system that can effectively suppress harmonic resonance in a power conversion device. [Means for solving the problem]

[0007] According to an embodiment, the power system includes a plurality of power conversion devices that convert AC power to DC power and vice versa. Each power conversion device includes a switch that switches an electrical connection with an AC power source on a power grid, a power converter including a switching element that switches a connection with a DC device that operates on DC power, and a converter control unit that issues an operation command to the switching element. The power system also includes a harmonic detection unit that detects harmonics in the power system, and a command function unit that is connected to the plurality of power conversion devices and outputs an operating state command to the power conversion devices and, when the harmonic detection unit detects that the harmonics exceed a predetermined standard, outputs an operating state change command to the plurality of power conversion devices to change their operating states to a predetermined operating state for harmonic suppression, and, after the harmonic detection unit detects that the harmonics have subsequently fallen below the predetermined standard, gradually reduces the number of power conversion devices to which the operating state change command is output. [Brief explanation of the drawings]

[0008] [Figure 1] FIG. 1 is a diagram showing the overall configuration of a power system according to an embodiment. [Figure 2] FIG. 2 is a diagram showing combination patterns of the placement locations of each component in the embodiment. [Figure 3] FIG. 3 is a graph showing the relationship between an increase in the number of switching-operated units and suppression of harmonics in the embodiment. [Figure 4] FIG. 4 is a flowchart showing the operation of the power system according to the embodiment. [Figure 5] FIG. 5 is a diagram showing the overall configuration of the power system of the first modification. [Figure 6] FIG. 6 is a diagram showing the overall configuration of a power system according to the second modification. [Figure 7] FIG. 7 is a flowchart showing the operation of the power system of the third modification. [Figure 8] FIG. 8 is a flowchart showing the operation of the power conversion device of the fourth modification. [Figure 9]FIG. 9 is a flowchart showing the operation of the power system of the sixth modification. [Figure 10] FIG. 10 is a flowchart showing the operation of the power system of the tenth modification. DETAILED DESCRIPTION OF THE INVENTION

[0009] Hereinafter, embodiments and modifications of the power system and control method of the present invention will be described with reference to the accompanying drawings. Note that, hereinafter, a power load will be simply referred to as a load. Also, a power system may be simply referred to as a system. Also, in the description of Modification 1 and subsequent modifications, duplicated descriptions of matters similar to those previously described will be omitted as appropriate.

[0010] (Embodiment) Fig. 1 is an overall configuration diagram of a power system S according to an embodiment. The power system S may be, for example, a large-scale battery system with input and output on the order of several tens of megawatts (MW), a solar power generation system, a wind power generation system, or a load system. The power system S includes an AC power source 1, a system configuration 2, a plurality of power conversion devices 3 (PCS: Power Conditioning System), a DC device 4 provided for each power conversion device 3, and a control system 5. Note that information communication between the components is performed, for example, via a wire.

[0011] The AC power supply 1 generates AC power.

[0012] The system configuration 2 is wiring, loads, etc. on the system side, and generates a system impedance.

[0013] The power conversion device 3 is provided at the interconnection point between the AC system and the DC device 4 (such as a storage battery, a generator, or a load), and converts AC power supplied by the AC system into DC power supplied by the DC device 4, and vice versa. The power conversion device 3 includes a voltage transformer VT, a switch CB, an interconnection inductor I, a harmonic filter F, a current transformer CT, a converter control unit 31, and a power converter 32.

[0014] The switch CB switches the electrical connection (continuity) with the AC power source 1 on the power grid side. The switch CB is a semiconductor or mechanical type, and switches the electrical connection between the power conversion device 3 and the power grid by opening and closing based on an operating state command from the control system 5.

[0015] The converter control unit 31 executes various controls such as giving operation commands to the switching elements of the power converter 32. The converter control unit 31 generates gate commands and the like for controlling the switching operation of the power converter 32 using an AC voltage obtained by a voltage transformer VT connected to a system interconnection point P between the system configuration 2 that generates a system impedance and the interconnection inductor I, an AC current obtained by a current transformer CT provided between the interconnection inductor I and the power converter 32, and a command signal from a command function unit 52 of the control system 5.

[0016] Specifically, the converter control unit 31 includes a current control unit, a converter control signal generation unit, a harmonic detector, etc. The converter control unit 31 executes a program stored in a storage unit of a hardware processor such as a CPU, for example, to perform general proportional integrals on the grid-tie point voltage and AC current in the current control unit based on the power command value and switching command from the control system 5, thereby generating a switching signal.

[0017] The harmonic detector extracts the maximum value of harmonic amplitude within a specified frequency range by, for example, performing a Fourier transform on the detected value of the grid-tie point voltage or by using a counter to observe the oscillation period. The extracted harmonic detection value is then transmitted to the control system 5. While wired communication provides stable communication, wireless communication eliminates the need for cable maintenance. The communication function may be built into the control device of the power conversion device 3, or a separate board may be installed. The frequency range is set to the range subject to harmonic regulations stipulated in standards or a range where a high risk of harmonic generation has been identified through prior analysis. Alternatively, the harmonic detector may be retrofitted to the outside of the power conversion device 3.

[0018] The converter control unit 31 realizes each function by, for example, a hardware processor such as a CPU (Central Processing Unit) executing a program (software) stored in a storage unit (not shown). Furthermore, some or all of the functions may be realized by hardware such as an LSI (Large Scale Integration), an ASIC (Application Specific Integrated Circuit), or an FPGA (Field Programmable Gate Array), or may be realized by a combination of software and hardware.

[0019] The power converter 32 includes a switching element that switches the connection with the DC device 4. The power converter 32 converts AC power and DC power mutually based on the control of the converter control unit 31. The power converter 32 is a circuit configured using self-extinguishing switching elements such as IGBTs (Insulated Gate Bipolar Transistors) and MOSFETs (Metal-Oxide-Semiconductor Field Effect Transistors).

[0020] The DC device 4 is a device that operates on DC power, such as a storage battery, a generator, or a load.

[0021] The control system 5 transmits an opening / closing signal for the switch CB, a power command value for the power conversion device 3, and an operating state command, which is a switching command, calculated from the input / output power of the entire power system S and the needs of the DC device 4 connected to the DC side of the power conversion device 3. The control system 5 includes a harmonic detection unit 51 and a command function unit 52.

[0022] The harmonic detection unit 51 receives information such as voltage, current, and values ​​based on the voltage and current as the system state. This information may be obtained through communication from each power conversion device 3, or may be obtained from a separate detector.

[0023] The harmonic detector 51 detects harmonics in the power system S. For example, based on input information (such as harmonic detection values) from the converter controller 31 or the like, the harmonic detector 51 detects whether the multiple power conversion devices 3 are in a harmonic exceeding standard state, in which harmonic components in a predetermined frequency range of the AC voltage and / or AC current exceed a predetermined standard, or in a harmonic normal state, in which the harmonic components are equal to or less than the predetermined standard. Note that the threshold value for determining whether the harmonic exceeding standard state has occurred and the threshold value for determining whether the harmonic normal state has occurred may be the same or different (details will be described later).

[0024] The command function unit 52 is connected to the plurality of power electronics devices 3 and outputs operating state commands (original commands unrelated to harmonic suppression) to the power electronics devices 3. When the harmonic detection unit 51 detects a harmonic excess state, the command function unit 52 outputs operating state change commands to the plurality of power electronics devices 3 to change their operating states to predetermined operating states for harmonic suppression. After the harmonic detection unit 51 subsequently detects a harmonic normal state, the command function unit 52 gradually reduces the number of power electronics devices 3 to which operating state change commands are output. In other words, the number of power electronics devices 3 to which operating state change commands are output is reduced stepwise, rather than all at once. This makes it possible to prevent harmonics from increasing again.

[0025] Furthermore, for example, when the harmonic standard is exceeded, the command function unit 52 outputs a switching operation command to the power conversion device 3 in a standby state where the switch CB is in a conducting state and the switching element is in a switching stopped state, as an operation state change command, to switch the switching element. The power conversion device 3 that has received the switching operation command is operated, for example, with substantially zero power. These will be described in detail below.

[0026] During operation, each power conversion device 3 is roughly in either a standby state (where the switch CB is in a conducting state and the switching elements are in a non-switching state) or a switching operation state (where switching operation is being performed for power conversion). When the harmonic standard is exceeded, a switching operation command is output as an operation state change command to the standby power conversion device 3, causing it to perform switching operation. This makes it possible to suppress harmonics. However, since using a current of the normal magnitude during operation, for example, during this switching operation would be disadvantageous in terms of power loss, it is preferable to operate the power conversion device 3 in a substantially zero-power operation, such as zero current or minute power.

[0027] Furthermore, in order to avoid deteriorating the original performance of the power system S, priority is given to the original operating state commands (commands related to the switches CB, GB (Gate Block: non-switching state), power command values, power supply degradation status, etc.), and when the original operating state command of the power conversion device 3 during switching operation for harmonic suppression changes, the power conversion device 3 is immediately controlled in accordance with the changed operating state command.

[0028] Furthermore, when the harmonic standard exceeding state is reached, the command function unit 52 simultaneously switches all (or one or more) of the power conversion devices 3 in standby mode to switching operation. After that, when the harmonic normal state is reached, the command function unit 52 gradually reduces the number of power conversion devices 3 switched to switching operation (for example, by one or more at a fixed interval).

[0029] In addition, when the harmonic standard is exceeded, the command function unit 52 may be configured to output a switching operation command to a power conversion device 3 selected from the power conversion devices 3 in standby state and with the switch CB in a conducting state based on a priority corresponding to the state of the connected DC device 4.

[0030] For example, the DC device 4 is a storage battery. In this case, the command function unit 52 calculates charging / discharging needs based on the current state of charge of the connected storage battery and the deterioration characteristics related to the state of charge, selects a power conversion device 3 with a higher priority as the charging / discharging needs increase, and outputs a switching operation command to the selected power conversion device 3. For example, if a storage battery is less deteriorated when its charging rate is 80%, its charging needs (priority) increase as its charging rate decreases below 80%, and its discharging needs (priority) increase as its charging rate increases above 80%.

[0031] Furthermore, for example, the DC device 4 is a generator. In this case, the command function unit 52 calculates the power generation efficiency from the current operating state of the generator, selects the power conversion device 3 with a higher power generation efficiency as having a higher priority, and outputs a switching operation command to the selected power conversion device 3. For example, in the case of a PV (Photovoltaic: solar power generation) or a wind power generator, the closer to the maximum efficiency operating point, the higher the priority.

[0032] Also, for example, the DC device 4 is a power load. In this case, the command function unit 52 calculates the power supply needs from the current operating state of the load, selects the power conversion device 3 as having a higher priority as the power supply needs increase, and outputs a switching operation command to the selected power conversion device 3. For example, if the load is a temperature adjustment device, the power supply needs (priority) increase as the difference between the current temperature and the set temperature increases.

[0033] Furthermore, for example, the command function unit 52 selects a power conversion device 3 on the basis that the smaller the wiring impedance from the location where the harmonic detector that detected the harmonics is connected to the power conversion device 3, the higher the priority, and outputs a switching operation command to the selected power conversion device 3.

[0034] The above describes the case where the operating state change command is a switching operation command. However, the operating state change command is not limited to this and may also be, for example, a command to change from current control mode to voltage control mode. In the current control mode, the current of the AC power supply 1 is feedback controlled. In the voltage control mode, the current of the AC power supply 1 is not directly feedback controlled, but the phase of the voltage output to the AC power supply side is adjusted in synchronization with the voltage of the AC power supply 1.

[0035] In this case, when the harmonic standard is exceeded, the command function unit 52 outputs an operating state change command to the power conversion device 3 operating in the current control mode to change from the current control mode to the voltage control mode, which also makes it possible to suppress harmonics.

[0036] Furthermore, as an emergency measure, the command function unit 52 may open the switches CB of the power conversion devices 3 to which the operating state change command is output due to the harmonic standard exceeding state when the number of power conversion devices 3 to which the operating state change command is output due to the harmonic standard exceeding state is equal to or greater than a predetermined threshold and the harmonic standard exceeding state does not transition to a harmonic normal state within a predetermined period. In other words, if harmonic resonance continues even after changing the operating states of a certain number of power conversion devices 3, some or all of the multiple switches CB are opened to prevent the unstable state from continuing. This, for example, prevents the harmonic filter F from acting on the grid side, thereby restoring the grid's harmonic stability.

[0037] The combination of the locations of the harmonic detector, harmonic detection unit 51, and command function unit 52 is not limited to that shown in Fig. 1 or the above description. For example, the combination shown in Fig. 2 is possible.

[0038] 2 is a diagram showing combination patterns of the locations of the components in the embodiment. For example, pattern 1 is a case where all components are provided in the power conversion device 3. Furthermore, pattern 2 is a case where the command function unit 52 is provided in the intermediate control system.

[0039] The intermediate control system may be a separate system that also serves as a relay between the power conversion device 3 and the control system 5. If the intermediate control system is installed near the power conversion device 3, local communication between them becomes possible, and advantages such as high communication speed, short communication cycle, high stability, and low power consumption can be expected.

[0040] Furthermore, when wireless communication is used between the control system 5 and the intermediate control system or between the power conversion devices 3 because the distance is long, low-speed communication (mobile radio waves) is used, for example.

[0041] Furthermore, when a power unit is provided as in Modification 1 described below, for example, an intermediate control system and a harmonic detector may be provided for each power unit, and the determination and execution of operation switching may be completed within the power unit.

[0042] Next, the relationship between an increase in the number of switching operation units and the suppression of harmonics will be described with reference to Fig. 3. Fig. 3 is a graph showing the relationship between an increase in the number of switching operation units and the suppression of harmonics in an embodiment. In the graph of Fig. 3, the vertical axis represents the detected value of harmonics (hereinafter simply referred to as "detected value"). For example, the detected current value or voltage value is analyzed using a Fourier transform or the like, and data indicating the level of harmonics is obtained as the detected value. The horizontal axis represents the passage of time, and the numerical value at the top represents the rate of switching from standby state to switching operation.

[0043] The detected value at the 0.0% switching rate indicates the maximum value of the harmonic component of the voltage at the grid-tie point, analyzed under harsh conditions where harmonic resonance is likely to occur. It can be seen that the occurrence (degree) of harmonic resonance is suppressed as the operation switching rate increases.

[0044] Next, the operation of the power system S will be described with reference to Fig. 4. Fig. 4 is a flowchart showing the operation of the power system S of the embodiment. In steps S1 to S4, the control system 5 executes the processes of steps S2 and S3 for each power electronics device 3.

[0045] In step S2, the command function unit 52 determines whether or not the power conversion device 3 is in a standby state, and if Yes, skips step S3, and if No, proceeds to step S3.

[0046] In step S3, if the target power conversion device 3 has already output an operating state change command (switching operation command) for harmonic suppression, the command function unit 52 stops outputting the operating state change command. In other words, the original operating state command is given priority. For example, if the operating state command that was originally in a standby state changes to a state other than the standby state, such as opening the circuit breaker CB, while the operating state change command is being output, the command function unit 52 stops outputting the corresponding operating state change command.

[0047] Next, in step S5, the harmonic detection unit 51 aggregates the harmonic detection values ​​from the harmonic detectors and compares them with a predetermined reference value in step S6. The predetermined reference value is set to a value that satisfies regulations, such as those stipulated in standards. If the harmonic detection value exceeds the reference value, it is determined that the harmonic standard is exceeded, and if it is equal to or less than the reference value, it is determined that the harmonic standard is normal. If the harmonic standard is exceeded, the process proceeds to step S7, and if the harmonic standard is normal, the process proceeds to step S8.

[0048] Next, the command function unit 52 sets an operating state change command to switch the switching element to a switching operating state based on the operating state command, the harmonic state determined by the harmonic detection unit 51, and priorities according to the power supply and load states, etc.

[0049] Then, in step S7, the command function unit 52 outputs an operation state change command to all the power conversion devices 3 whose switches CB are in the conductive state and are in the standby state.

[0050] In step S8, the command function unit 52 gradually reduces the number of power conversion devices 3 that are in switching operation due to the output of an operation state change command. At this time, the selection of the power conversion device 3 to stop switching operation is determined based on priorities according to the states of the power supplies and loads connected to the DC side.

[0051] In this way, according to the power system S of this embodiment, when a harmonic excess state is detected, the power electronics devices 3 in standby mode are put into switching operation, and then when a harmonic normal state is detected, the number of power electronics devices 3 that are returned to standby mode is gradually increased, thereby making it possible to effectively suppress harmonic resonance. Specifically, this is as follows.

[0052] By switching the power conversion device 3 from a standby state to a switching operation, a resistance component is equivalently added to the characteristics of the harmonic filter F, thereby achieving the effect of suppressing harmonic resonance.

[0053] Furthermore, if the standby power conversion devices 3 are all switched to switching operation at the same time, the effect of suppressing harmonic resonance is maximized, and harmonic resonance can be suppressed quickly.

[0054] In addition, it is possible to suppress harmonics to below a specified value by switching some of the power conversion devices 3 in a standby state to switching operation. Then, in a normal harmonic state, by gradually reducing the number of power conversion devices 3 operating in switching operation by an operation state change command, it is possible to further reduce unnecessary switching operation, and it is possible to suppress an increase in the power consumption and failure rate of the power conversion devices 3.

[0055] Furthermore, since harmonic resonance is suppressed by switching the operation of the power converter 32, faster switching operation is possible compared to mechanical disconnection using the switch CB, and harmonics are suppressed more quickly.

[0056] Furthermore, the harmonic detector can be installed externally, making it easy to retrofit to existing systems.

[0057] Furthermore, since harmonic resonance is suppressed by utilizing surplus power conversion devices 3, there is no need to impose restrictions on the number of operating units, etc., and therefore there is no degradation in the performance of the power system S. For example, there is no need for operation restrictions to avoid a specific operating state as in Patent Document 1. Details are as follows.

[0058] In the technology of Patent Document 1, in order to avoid specific operating conditions that increase the possibility of harmonic resonance, operational constraints are imposed on the system, such as opening a switch and isolating the power conversion device from the grid under the corresponding operating conditions. Such operational constraints prevent the system from achieving its full potential. Furthermore, with the ever-changing state of the grid, there is a problem in that it is not possible to respond to the occurrence of harmonics under operating conditions other than those avoided by the constraints.

[0059] On the other hand, the method of this embodiment makes it possible to maximize the system's capabilities without the need for operational constraints as in the technology of Patent Document 1. Furthermore, even in the case of a constantly changing system state, it is possible to cope with the occurrence of harmonics under operating conditions other than those avoided by constraints.

[0060] (Variation 1) Next, Modification 1 will be described. FIG. 5 is an overall configuration diagram of a power system S of Modification 1. The power unit U includes a plurality of power conversion devices 3 in which the wiring impedance between the power conversion devices 3 is smaller than a predetermined reference value. The power unit U allows operation on a power unit basis by the control system 5, and operation switching on a unit basis by the command function unit. In other words, the command function unit outputs an operation state change command on a power unit basis. Specifically, it is as follows.

[0061] For example, an intermediate control system (not shown) equipped with a command function unit and a harmonic detector is provided in pair with a power unit U. The command function unit switches the output of an operating state change command corresponding to the power electronics devices 3 belonging to the same power unit U based on the harmonic state determined by the harmonic detection unit. For example, in the case of a harmonic standard exceedance state, the command function unit outputs an operating state change command to all operating state commands that are in a standby state, individually for each power unit. Furthermore, the number of power electronics devices 3 to which an operating state change command is output may be gradually increased. Furthermore, if the harmonic normal state is subsequently reached, the command function unit gradually reduces the number of units to which an operating state change command is output, individually for each power unit.

[0062] As described above, according to the first modification, operation switching for suppressing harmonic resonance is completed within the power unit U, which enables installation and maintenance on a power unit basis, thereby increasing versatility.

[0063] Furthermore, since the devices and apparatuses that make up one power unit U are all located within a relatively short distance, it becomes easier to introduce high-speed, highly reliable local wireless communication for information communication within the power unit U.

[0064] Furthermore, operation can be switched on an electric power unit basis, which increases the controllability and flexibility of operation switching as a system.

[0065] Furthermore, since the wiring impedance between the power conversion devices 3 within the power unit U is small and the interconnection point voltage of each power conversion device 3 can be treated as the same, it is possible to detect harmonics collectively for each power unit. In other words, there is no need to provide a harmonic detector for each power conversion device 3.

[0066] (Variation 2) Next, we will explain Modification 2. Fig. 6 is a diagram showing the overall configuration of a power system S of Modification 2. A harmonic detector 6 is paired with a control system 5 and connected to the AC side, and detects harmonics of the power conversion devices 3 belonging to the same power system S all at once.

[0067] As described above, according to the second modification, introduction costs can be reduced because only one harmonic detector 6 is required. Furthermore, by installing the harmonic detector near the control system 5, faster and more reliable information communication becomes possible.

[0068] (Variation 3) Next, we will explain Modification 3. Fig. 7 is a flowchart showing the operation of power system S (e.g., Fig. 1) of Modification 3. In step S11, harmonic detection unit 51 (Fig. 1) aggregates harmonic detection values ​​from the harmonic detectors and compares them with predetermined reference values ​​(upper limit reference value and lower limit reference value) with a hysteresis width.

[0069] The upper limit reference value is set to a value that satisfies the harmonic regulations stipulated in standards, etc. The lower limit reference value is set to a value higher than the harmonic detection value when all power conversion devices 3 in standby state are in switching operation as determined by preliminary analysis. If the harmonic detection value exceeds the upper limit reference value, it is determined to be in a harmonic standard exceeding state ("harmonic standard exceeding state" in step S11). If the harmonic detection value is equal to or less than the upper limit reference value and exceeds the lower limit reference value, it is determined to be in a harmonic maintenance state ("harmonic maintenance state" in step S11). If the harmonic detection value is equal to or less than the lower limit reference value, it is determined to be in a harmonic normal state ("harmonic normal state" in step S11).

[0070] In the case of a harmonic standard exceeding state, in step S13, the harmonic detection unit 51 outputs an operating state change command to all operating state commands of the power electronics devices 3 that are in a standby state. In the case of a harmonic maintenance state, if there is no change in the operating state command to the power electronics devices 3 (Yes in step S12), the harmonic detection unit 51 maintains (continues) the output of the current operating state change command in step S14. If there is a change in the operating state command to the power electronics devices 3 in the harmonic maintenance state (No in step S12) or in the case of a harmonic normal state ("harmonic normal state" in step S11), the harmonic detection unit 51 gradually reduces the number of power electronics devices 3 to which an operating state change command is to be output in step S15.

[0071] As described above, according to the third modification, by providing a hysteresis width to the reference value of harmonics, it is possible to prevent the operating state of the power conversion device 3 from being switched excessively in a short period of time.

[0072] Furthermore, adding a harmonic maintaining state can reduce the switching operations of the operating state of the power conversion device 3. Furthermore, since the power conversion device 3 in switching operation can be maintained in a reduced state, it is possible to reduce unnecessary switching operations and switching operations, thereby suppressing the failure rate of the power conversion device 3 and an increase in power consumption.

[0073] Furthermore, if the original operating state command of the power conversion device 3 changes, even if there are no switching operations for harmonic suppression, the harmonic standard exceeding state may not be reached, so the switching operations are reduced, thereby making it possible to avoid unnecessary switching operations.

[0074] (Variation 4) Next, a description will be given of Modification 4. Fig. 8 is a flowchart showing the operation of the power conversion device 3 (for example, Fig. 1) of Modification 4. In step S21, the power conversion device 3 determines whether it is in a state exceeding the harmonic standard and in a standby state, and if Yes, proceeds to step S22, and if No, ends the process.

[0075] In step S22, the power conversion device 3 autonomously switches to the switching operation without waiting for a command for the switching operation determined by the command function unit 52.

[0076] In this way, according to variant example 4, when the power conversion device 3 enters a state in which the harmonic standard is exceeded, it autonomously switches from standby mode to switching operation, and since there is no influence from communication cycles or delays, the harmonic resonance suppression operation can be accelerated and the time in which the harmonic standard is exceeded can be shortened.

[0077] Furthermore, even if the power conversion device 3 cannot receive signals from the control system 5 or the intermediate control system due to some problem (such as a communication failure), harmonic resonance can be suppressed, thereby increasing the reliability of the system.

[0078] (Variation 5) Next, Modification 5 will be described. The power conversion device 3 that performs switching operation due to the harmonic standard exceeding state sets a corresponding power command so that the total power of the entire power system S becomes zero, for example. For example, one power conversion device 3 consumes power, and another power conversion device 3 outputs the same amount of power. Note that it is sufficient that the power and current of the power conversion device 3 alone are zero or so small that they are essentially zero, resulting in zero-power operation.

[0079] As described above, according to the fifth modification, by operating with zero power during switching operation, it is possible to suppress the occurrence of power loss and also to prevent deterioration of each element connected to the DC section of the power conversion device 3.

[0080] (Variation 6) Next, Modification 6 will be described. Fig. 9 is a flowchart showing the operation of the power system S of Modification 6. In the case of a harmonic standard exceeding state ("Harmonic standard exceeding state" in step S31), in step S32, the command function unit 52 gradually increases the operation state change command for the power conversion device 3 in the standby state by one or more units for the entire power system S or for each power unit U, thereby suppressing harmonic resonance. For example, for each power unit U, the operation state change command for the standby state is gradually increased by one or more units, thereby suppressing harmonics.

[0081] Furthermore, in the case of a normal harmonic state ("normal harmonic state" in step S31), in step S33, the command function unit 52 gradually decreases the operating state change command by one or more units at a time.

[0082] In this way, according to the sixth modification, for example, by switching the standby power conversion devices 3 one by one to switching operation, under the condition that the harmonic standard exceedance state can be avoided without switching all of them to switching operation, the number of devices to be switched to switching operation can be reduced. This makes it possible to avoid unnecessary switching operation. Also, it is possible to reduce the impact on the grid caused by sudden operation switching.

[0083] (Variation 7) Next, a description will be given of Modification 7. The command function unit 52 (for example, FIG. 1) selects a power conversion device 3 according to the priority given to a power conversion device 3 having a smaller wiring impedance from the connection point of the harmonic detector that detected the harmonics to the power conversion device 3, and outputs a switching operation command to the selected power conversion device 3.

[0084] In this way, the effect of suppressing harmonic resonance due to switching operation is enhanced according to Modification 7. This is because the smaller the impedance from the harmonic generation source of the power conversion device 3, the greater the effect of suppressing harmonic resonance due to switching operation.

[0085] (Variation 8) Next, an eighth modification will be described. Information communication between devices and apparatuses may be wireless. For example, all information communication between devices and apparatuses may be wireless. Furthermore, for example, information communication between devices and apparatuses installed in relatively short distances may be wired, and information communication between devices and apparatuses installed in long distances may be wireless. Furthermore, for example, information communication between devices and apparatuses requiring high-speed communication may be wired, and information communication between other devices and apparatuses may be wireless. Furthermore, the harmonic detection unit may transmit and receive information to and from the command function unit via wireless communication.

[0086] Wireless is inferior to wired in terms of communication speed and stability, but it is easy to introduce because it does not require communication cables. Wireless is also compatible with autonomous switching. In other words, with wireless, the possibility of communication failure due to surrounding conditions is higher than with wired, but even if communication is disabled, harmonics can be suppressed by autonomous switching if the autonomous switching method (variant 4) is adopted.

[0087] As described above, according to the eighth modification, by adopting wireless communication, it is possible to reduce the number of cables used for information communication, and the ease of installation and maintenance is improved.

[0088] (Variation 9) Next, a ninth modification will be described. In the ninth modification, a machine learning technique is used. An operating condition learning unit uses past operation data in advance to create a learning model that predicts the required number of power conversion devices 3 that output switching operation commands for harmonic suppression through machine learning. During learning, various data such as operating state commands, operating state change commands, detection values ​​from the harmonic detector, and determination results from the harmonic detection unit are used.

[0089] Then, during operation, when outputting a switching operation command to a power conversion device 3 in standby state, the command function unit 52 calculates the minimum number of power conversion devices 3 to which a switching operation command should be output in order to suppress harmonics based on the learning model, and outputs a switching operation command to that minimum number of power conversion devices 3.

[0090] In this way, according to the ninth modification, for example, operation switching can be performed without using detection of harmonics, and the decision on operation switching can be made quickly.

[0091] Furthermore, methods that gradually reduce the number of switching operation vehicles after simultaneous switching or gradually increase the number of switching operation vehicles are likely to result in excessive or insufficient operation switching, but operation switching based on learning results can avoid these issues.

[0092] In addition, by learning the conditions for generating harmonic resonance and the conditions for switching operation to suppress it, it is possible to avoid the generation of harmonics.

[0093] Therefore, for example, at the beginning of operation of the power system S, harmonics are suppressed using a method of gradually reducing the number of switching operation units after simultaneous switching, while accumulating operational data and carrying out the above-mentioned learning to create a learning model. Then, after the learning model is completed, harmonics can be efficiently suppressed by switching to a method that uses the learning model.

[0094] (Variation 10) Next, a description will be given of Modification 10. When the number of power electronics devices 3 to which an operating state change command is output due to a harmonic standard exceeding state is equal to or greater than a predetermined threshold value and the state does not transition from a harmonic standard exceeding state to a harmonic normal state within a predetermined period, the switches in the power electronics devices to which the operating state change command is output are opened.

[0095] 10 is a flowchart showing the operation of the power system S (for example, FIG. 1) of Modification 10. For example, if all the power electronics devices 3 in the standby state are put into switching operation in response to an operation state change command and are still in a state where the harmonic standard is exceeded (Yes in step S41), in step S42, the switch CB of the power electronics device 3 to which the operation state change command is output is opened, and the switching operation is stopped.

[0096] As described above, according to the modification 10, even if the occurrence of harmonic resonance caused by the circuit of the harmonic filter F included in the power conversion device 3 cannot be suppressed by switching to the switching operation, the harmonic resonance can be reliably suppressed. Therefore, it is possible to avoid the worst case scenario, such as operation being stopped due to a protection operation.

[0097] Although the embodiments and modifications of the present invention have been described, they are presented as examples and are not intended to limit the scope of the invention. These novel embodiments and modifications can be embodied in various other forms, and various omissions, substitutions, and modifications can be made without departing from the spirit of the invention. These embodiments and modifications are included within the scope and spirit of the invention, and are also included in the inventions and their equivalents as defined in the claims.

[0098] The program executed by the power system S of this embodiment can be provided by being recorded in an installable or executable file format on a computer-readable recording medium such as a CD (Compact Disc)-ROM (Read Only Memory), a flexible disk (FD), a CD-R (Recordable), or a DVD (Digital Versatile Disk).The program may also be provided or distributed via a network such as the Internet. [Explanation of symbols]

[0099] 1...AC power supply, 2...system configuration, 3...power conversion device, 4...DC device, 5...control system, 31...converter control unit, 32...power converter, 51...harmonic detection unit, 52...command function unit, S...power system

Claims

1. A power system including a plurality of power conversion devices that convert AC power and DC power mutually, The power conversion device is a switch that switches an electrical connection with an AC power source on the power grid side; a power converter including a switching element that switches a connection with a DC device that is a device that operates on DC power; a converter control unit that issues an operation command to the switching element, The power system includes: a harmonic detection unit that detects harmonics in the power system; a power converter connected to the plurality of power conversion devices and outputting an operating state command to the power conversion devices; When the harmonic detection unit detects that the harmonics exceed a predetermined standard, outputting an operating state change command to the plurality of power conversion devices to change the operating state to a predetermined operating state for harmonic suppression, and a command function unit that gradually reduces the number of power conversion devices to which the operating state change command is output after the harmonic detection unit detects that the harmonics have subsequently fallen below the predetermined standard.

2. The power system according to claim 1 , wherein the harmonic detection unit and the command function unit are provided in a control system that controls a plurality of the power conversion devices.

3. a plurality of the power conversion devices, each having a wiring impedance between the power conversion devices that is smaller than a predetermined reference value, are configured as one power unit; The power system according to claim 1 , wherein the command function unit outputs the operation state change command for each of the power units.

4. The power system according to claim 1 , wherein the harmonic detector is disposed at a position closer to the power grid than all of the power conversion devices.

5. The power system according to claim 1 , wherein the harmonic detector is provided for each of the power conversion devices.

6. The power system according to claim 3 , wherein the harmonic detector is disposed in an intermediate control system connected to each of the power units.

7. the harmonic detection unit is connected to the plurality of power conversion devices, and detects whether a harmonic standard exceeding state is occurring, in which harmonic components in a predetermined frequency range of an AC voltage and / or an AC current exceed the predetermined standard, or a harmonic normal state, in which the harmonic components are equal to or less than the predetermined standard; The command function unit When the harmonic standard is exceeded, outputting an operating state change command to the plurality of power conversion devices to change the operating state to the predetermined operating state; The power system according to claim 1 , wherein after the harmonic normal state is subsequently entered, the number of the power electronics devices to which the operation state change command is output is gradually reduced.

8. the harmonic detection unit is connected to the plurality of power conversion devices, and detects whether the power conversion devices are in a harmonic exceeding reference state in which harmonic components in a predetermined frequency range of the AC voltage and / or AC current exceed an upper reference value, a harmonic normal state in which the harmonic components are equal to or lower than a lower reference value, or a harmonic maintaining state in which the harmonic components are equal to or lower than the upper reference value and exceed the lower reference value; The command function unit When the harmonic standard is exceeded, outputting an operating state change command to the plurality of power conversion devices to change the operating state to the predetermined operating state; The power system according to claim 1 , wherein the number of the power electronics devices to which the operation state change command is output is gradually reduced in the case of the harmonic normal state.

9. 9. The power system according to claim 7, wherein, when the harmonic standard exceeding state occurs, the command function unit outputs, to the power conversion device in a standby state in which the switch is in a conductive state and the switching element is in a switching stopped state, a switching operation command to switch the switching element as the operation state change command.

10. The command function unit, in the harmonic maintaining state, with respect to the power conversion device to which the operating state change command is output, If there is no change in the operating state command, the output of the current operating state change command is continued; When the operation state command is changed, the number of the power electronics devices to which the operation state change command is output is gradually reduced. The power system of claim 8 .

11. The power system according to claim 9 , wherein the power conversion device that receives the switching operation command operates at substantially zero power.

12. The command function unit 9. The power system according to claim 7, wherein, when the harmonic standard is exceeded, an operating state change command is output to all of the power conversion devices to change the operating state to the predetermined operating state.

13. The command function unit 9. The power system according to claim 7, wherein, in the case of the harmonic standard exceeding state, the operation state change command is output to a plurality of the power electronics devices, and at that time, the number of the power electronics devices to which the operation state change command is output is gradually increased.

14. 4. The power system according to claim 3, wherein the command function unit outputs the operation state change command to all the power units when the harmonic detection unit detects that harmonics exceed the predetermined standard.

15. 4. The power system according to claim 3, wherein when the harmonic detection unit detects that harmonics have exceeded the predetermined standard, the command function unit outputs the operating state change command to the plurality of power units, and at that time, gradually increases the number of power units to which the operating state change command is output.

16. 10. The power system according to claim 9, wherein, when the harmonic standard is exceeded, the command function unit outputs the switching operation command to a power conversion device selected from the power conversion devices in a standby state in which the switch is in a conductive state and the switching element is in a switching stopped state, based on a priority according to a state of the connected DC device.

17. the DC device is a storage battery; 17. The power system according to claim 16, wherein the command function unit calculates charging / discharging needs based on a current state of charge of the connected storage battery and a deterioration characteristic related to the state of charge, selects the power conversion device as having a higher priority as the charging / discharging needs are higher, and outputs the switching operation command to the selected power conversion device.

18. the DC device is a generator; 17. The power system according to claim 16, wherein the command function unit calculates a power generation efficiency from a current operating state of the generator, selects the power conversion device with a higher priority as the power generation efficiency is higher, and outputs the switching operation command to the selected power conversion device.

19. The DC device is a power load; 17. The power system according to claim 16, wherein the command function unit calculates power supply needs from a current operating state of the load, selects the power electronics device as having a higher priority as the power supply needs increase, and outputs the switching operation command to the selected power electronics device.

20. 17. The power system according to claim 16, wherein the command function unit selects the power conversion device by determining that the priority is higher when a wiring impedance from a detection location of harmonics to the power conversion device is smaller, and outputs the switching operation command to the selected power conversion device.

21. When the operating state command for the power conversion device to which the operating state change command is output changes from a standby state, The power system according to claim 1 , wherein the command function unit stops outputting the operation state change command to the power conversion device.

22. The power system according to claim 1 , wherein the harmonic detection unit transmits and receives information to and from the command function unit via wireless communication.

23. 10. The power system according to claim 9, wherein, when outputting the switching operation command to the power conversion device in a standby state in which the switch is in a conductive state and the switching element is in a switching stopped state in the harmonic standard exceeding state, the command function unit calculates a minimum value of the number of the power conversion devices to which the switching operation command is to be output in order to suppress harmonics based on a learning model created in advance by machine learning using past operation data, and outputs the switching operation command to the minimum number of the power conversion devices.

24. The command function unit 9. The power system according to claim 7 or 8, wherein, when the number of the power electronics devices to which the operating state change command is output due to the harmonic standard exceeding state is equal to or greater than a predetermined threshold value and the harmonic standard exceeding state does not transition to the harmonic normal state within a predetermined period, the switch in the power electronics device to which the operating state change command is output is opened.

25. A control method for a power system including a plurality of power conversion devices that convert AC power and DC power mutually, comprising: The power conversion device is a switch that switches an electrical connection with an AC power source on the power grid side; a power converter including a switching element that switches a connection with a DC device that is a device that operates on DC power; a converter control unit that issues an operation command to the switching element, The control method includes: an operation command step of outputting an operation state command to the plurality of power conversion devices; a harmonic detection step of detecting harmonics in the plurality of power conversion devices; When the harmonic detection step detects that the harmonics exceed a predetermined standard, outputting an operating state change command to the plurality of power conversion devices to change the operating state to a predetermined operating state for harmonic suppression, and a command function step of gradually reducing the number of power conversion devices to which the operating state change command is output after the harmonic detection step subsequently detects that the harmonics have fallen below the predetermined standard.

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