Power conditioner and power control method

Abstract

This power conditioner comprises: an inverter that converts DC power supplied from a DC power source into AC power and supplies the AC power to a power grid; and a control device that controls the inverter. On the basis of a first differential voltage which is the difference between a reverse-phase voltage on the AC side of the inverter and a voltage obtained by removing the high-frequency component of the reverse-phase voltage, the control device generates a reverse-phase current command for causing reverse-phase AC current to flow from the inverter. The control device performs control so as to stop the inverter when a second differential voltage, which is the difference between the reverse-phase voltage and a reverse-phase voltage command obtained by multiplying characteristics of the impedance between the inverter and the power grid by the reverse-phase current command, satisfies a threshold value condition.

Description

Power conditioner and power control method

[0001] The present disclosure relates to a technique for controlling a power conditioner.

[0002] Patent Document 1 discloses a power conversion device (power conditioner). Specifically, when a negative-phase voltage on the AC side of an inverter is equal to or greater than a predetermined value, the power conditioner determines that the inverter is in islanding operation and controls the inverter to stop.

[0003] Japanese Patent No. 7151911

[0004] Consider a case where the impedance of a three-phase line in a power system is unbalanced. In this case, in Patent Document 1, the negative-phase voltage on the AC side of the inverter may exceed a predetermined value, and the inverter may be erroneously determined to be in islanding operation.

[0005] One object of the present disclosure is to provide a technique that can appropriately determine whether an inverter is in islanding operation.

[0006] A first aspect of the present disclosure relates to a power conditioner. The power conditioner includes an inverter that converts DC power supplied from a DC power source into AC power and supplies the AC power to a power grid, and a control device that controls the inverter. The control device generates a negative-phase-sequence current command for causing the inverter to supply a negative-phase AC current based on a first differential voltage that is a difference between a negative-phase-sequence voltage on the AC side of the inverter and a voltage from which high-frequency components of the negative-phase-sequence voltage have been removed. The control device then controls the inverter to stop when a second differential voltage that is a difference between the negative-phase-sequence voltage command obtained by multiplying the negative-phase-sequence current command by characteristics of impedance between the inverter and the power grid satisfies a threshold condition.

[0007] A second aspect of the present disclosure relates to a power conditioner. The power conditioner includes an inverter that converts DC power supplied from a DC power source into AC power and supplies the AC power to a power grid, and a control device that controls the inverter. The control device determines whether a positive-phase voltage on the AC side of the inverter is within a predetermined range when a first differential voltage, which is the difference between a negative-phase voltage on the AC side of the inverter and a voltage obtained by removing high-frequency components from the negative-phase voltage, is greater than zero. If the positive-phase voltage is determined to be within the predetermined range, the control device controls the inverter to stop.

[0008] A third aspect of the present disclosure relates to a power conditioner. The power conditioner includes an inverter that converts DC power supplied from a DC power source into AC power and supplies the AC power to a power grid, and a control device that controls the inverter. The control device calculates a first differential voltage that is a difference between a negative-phase voltage on the AC side of the inverter and a voltage from which high-frequency components have been removed from the negative-phase voltage, and controls the inverter to stop when the positive-phase voltage and negative-phase voltage on the AC side of the inverter satisfy a predetermined condition.

[0009] A fourth aspect of the present disclosure relates to a power control method. The power control method includes: using an inverter to convert DC power to AC power and supply the AC power to a power grid; generating a negative-phase-sequence current command for causing a negative-phase-sequence AC current to flow from the inverter based on a first differential voltage that is a difference between a negative-phase-sequence voltage on the AC side of the inverter and a voltage from which high-frequency components have been removed from the negative-phase-sequence voltage; and controlling the inverter to stop when a second differential voltage that is a difference between the negative-phase-sequence voltage command obtained by multiplying the negative-phase-sequence current command by characteristics of impedance between the inverter and the power grid satisfies a threshold condition.

[0010] A fifth aspect of the present disclosure relates to a power control method. The power control method includes: converting DC power to AC power using an inverter and supplying the AC power to a power grid; determining whether a first differential voltage, which is a difference between a negative-phase-sequence voltage on the AC side of the inverter and a voltage from which high-frequency components have been removed from the negative-phase-sequence voltage, is greater than zero; if it is determined that the first differential voltage is greater than zero, determining whether a positive-phase-sequence voltage on the AC side of the inverter is within a predetermined range; and if it is determined that the positive-phase-sequence voltage is within the predetermined range, controlling the inverter to stop.

[0011] A sixth aspect of the present disclosure relates to a power control method, the power control method including: converting DC power into AC power using an inverter and supplying the AC power to a power grid; calculating a first differential voltage that is a difference between a negative-phase-sequence voltage on the AC side of the inverter and a voltage from which high-frequency components have been removed from the negative-phase-sequence voltage; and controlling the inverter to stop when the positive-phase-sequence voltage and the negative-phase-sequence voltage on the AC side of the inverter satisfy a predetermined condition.

[0012] According to the first aspect, a negative-phase current command for causing a negative-phase AC current to flow from the inverter is generated based on a first differential voltage, which is the difference between a negative-phase voltage on the AC side of the inverter and a voltage from which high-frequency components have been removed from the negative-phase voltage. Then, when a second differential voltage, which is the difference between the negative-phase voltage command obtained by multiplying the characteristic of the impedance between the inverter and the power grid by the negative-phase current command, satisfies a threshold condition, control is performed to stop the inverter. This makes it possible to appropriately determine whether the inverter is in islanding operation. Furthermore, when the inverter is in islanding operation, it is possible to quickly stop the inverter. Therefore, safety is ensured.

[0013] According to the second aspect, when a first differential voltage, which is the difference between the negative-phase voltage on the AC side of the inverter and a voltage obtained by removing high-frequency components from the negative-phase voltage, is greater than zero, it is determined whether the positive-phase voltage on the AC side of the inverter is within a predetermined range. If it is determined that the positive-phase voltage is within the predetermined range, control is performed to stop the inverter. This makes it possible to appropriately determine whether the inverter is in islanding operation. Furthermore, if the inverter is in islanding operation, it is possible to stop the inverter early. Therefore, safety is ensured.

[0014] According to the third aspect, a first differential voltage is calculated, which is the difference between the negative-phase voltage on the AC side of the inverter and a first voltage obtained by removing high-frequency components from the negative-phase voltage. Furthermore, when the positive-phase voltage and the negative-phase voltage satisfy a predetermined condition, the inverter is controlled to stop. This makes it possible to appropriately determine whether the inverter is in islanding operation. Furthermore, when the inverter is in islanding operation, it becomes possible to stop the inverter early. Therefore, safety is ensured.

[0015] According to the fourth aspect, the same effect as that of the first aspect can be obtained.

[0016] According to the fifth aspect, the same effect as that of the second aspect can be obtained.

[0017] According to the sixth aspect, the same effect as that of the third aspect can be obtained.

[0018] FIG. 1 is a diagram for explaining an overview of a power conversion system according to a first embodiment. FIG. 2 is a block diagram showing an example of the functions of a control device according to the first embodiment. FIG. 3 is a block diagram showing an example of detection of an islanding operation according to the first embodiment. FIG. 4 is a diagram for explaining a specific example of the characteristics of an islanding operation according to the first embodiment. FIG. 5 is a flowchart showing an example of processing by a control device according to the first embodiment. FIG. 6 is a block diagram showing an example of detection of an islanding operation according to a second embodiment. FIG. 7 is a flowchart showing an example of processing by a control device according to the second embodiment. FIG. 8 is a block diagram showing an example of detection of an islanding operation according to a third embodiment. FIG. 9 is a flowchart showing an example of processing by a control device according to the third embodiment.

[0019] A power conditioner and a power control method according to an embodiment of the present disclosure will be described with reference to the accompanying drawings. Elements common to the drawings will be designated by the same reference numerals, and duplicated descriptions will be omitted.

[0020] 1. First Embodiment 1-1. Overview of Power Conversion System Fig. 1 is a diagram for explaining an overview of a power conversion system 1. The power conversion system 1 includes a DC power supply 11, a power conditioner 10, a transformer 20, a power grid 30, a circuit breaker 40, and a load 50. The power conditioner 10 includes an inverter 12 and a control device 100.

[0021] The DC power supply 11 is a power storage device (e.g., a solar cell module) that stores electricity generated by renewable energy, such as solar power, wind power, and hydropower.

[0022] The inverter 12 is a device that converts DC power output from the DC power supply 11 into AC power and supplies the AC power to the power grid 30 via a transformer 20. Examples of the inverter 12 include a current-controlled grid-following (GFL) inverter and a voltage-controlled grid-forming (GFM) inverter.

[0023] The control device 100 is connected to the inverter 12 and controls the inverter 12. The control device 100 receives the output voltage Vs and output current Io output from the inverter 12. The output voltage Vs includes a positive-phase voltage Vps and a negative-phase voltage Vns. The output current Io includes a positive-phase current Ipo and a negative-phase current Ino. The output voltage Vs input to the control device 100 is, for example, a detected value of the output voltage Vs (hereinafter referred to as the Vs detected value). The Vs detected value includes a detected value of the positive-phase voltage Vps (also referred to as the Vps detected value) and a detected value of the negative-phase voltage Vns (also referred to as the Vns detected value). The output current Io input to the control device 100 is the detected value of the output current Io (also referred to as the Io detected value). The Io detection value includes a detection value of a positive-phase current Ipo (also referred to as an Ipo detection value) and a detection value of a negative-phase current Ino (also referred to as an Ino detection value). These detection values ​​are detected, for example, by a detector (not shown) provided between the inverter 12 and the transformer 20.

[0024] Based on the Vs detection value and the Io detection value, the control device 100 generates a current command for controlling the output current Io of the inverter 12. Then, the control device 100 generates a pulse width modulation signal (PWM signal) based on the current command and issues an instruction ins to the inverter 12 to operate in accordance with the PWM signal.

[0025] Consider the islanding operation of the inverter 12. For example, if a circuit breaker 40 installed in a substation or the like on the grid side is opened due to a grid fault or the like, the AC power output from the inverter 12 and the power of the load 50 will match. This state is called "islanding operation." If the inverter 12 continues to operate in an islanded state, it may cause electric shock to humans, mechanical failure, or overcurrent during reclosing. Therefore, when the inverter 12 is in islanding operation, it is necessary to stop the inverter 12. The load 50 is, for example, a device connected to the power conditioner 10 and consumed in a factory or the like where the power conditioner 10 is installed. The load 50 is typically composed of a resistive load R, an inductive load L, and a capacitive load C.

[0026] When the inverter 12 is in an isolated operation, a slight increase in the negative-phase-sequence voltage Vns causes the inverter 12 to inject the negative-phase-sequence voltage Vns, causing the negative-phase-sequence voltage Vns to further increase.

[0027] On the other hand, consider a case where the impedance of the three-phase line on the grid side is unbalanced. In this case, the three-phase line is not completely balanced, and the negative-phase voltage Vns rises. That is, the negative-phase voltage Vns rises both when the inverter 12 is in isolated operation and when the impedance of the three-phase line is unbalanced. Therefore, a mechanism that can distinguish between these situations is required.

[0028] The control device 100 generates a negative-phase current command for causing the inverter 12 to supply a negative-phase AC current based on a first differential voltage, which is the difference between the negative-phase voltage Vns on the AC side of the inverter 12 and a voltage obtained by removing high-frequency components from the negative-phase voltage Vns. The control device 100 then controls the inverter 12 to stop when a second differential voltage, which is the difference between the negative-phase voltage command obtained by multiplying the negative-phase current command by the impedance characteristics between the inverter 12 and the power grid 30, and the negative-phase voltage Vns, satisfies a threshold condition. This allows appropriate determination of whether the inverter 12 is in islanding operation. Furthermore, if the inverter 12 is in islanding operation, the inverter 12 can be stopped early. This ensures safety. Details of the processing performed by the control device 100 will be described later.

[0029] 1-2. Examples of Control Devices 1-2-1. Configuration Examples The control device 100 has hardware that realizes various functions. The hardware includes a processing circuit capable of high-speed calculations. Examples of the processing circuit include an FPGA (Field Programmable Gate Array) and an ASIC (Application Specific Integrated Circuit). In addition to the processing circuit, the hardware may also include a storage device and a computing unit (e.g., a CPU or GPU) that executes a program stored in the storage device.

[0030] 2 is a block diagram showing an example of functions of the control device 100 according to embodiment 1. The control device 100 includes an inverter operation control unit 110, a power control unit 120, a current control unit 130, and an output control unit 140.

[0031] Based on the Vns detection value, the inverter operation control unit 110 generates a negative-phase-sequence current command for causing a negative-phase AC current to flow from the inverter 12. The negative-phase-sequence current command includes at least one of a d-axis component negative-phase-sequence current command Indref (hereinafter referred to as the d-axis negative-phase-sequence current command Indref) and a q-axis component negative-phase-sequence current command Inqref (hereinafter referred to as the q-axis negative-phase-sequence current command Inqref).

[0032] The inverter operation control unit 110 also executes islanding operation detection processing. In the islanding operation detection processing, the inverter operation control unit 110 determines whether the inverter 12 is in islanding operation based on the negative-phase-sequence current command, the Vs detection value, and the Io detection value. If it is determined that the inverter 12 is in islanding operation, the inverter operation control unit 110 outputs a gate block signal gtb to the output control unit 140 to stop the inverter 12. The islanding operation detection processing will be described in detail later.

[0033] The Vs detection value input to the inverter operation control unit 110 may be acquired, for example, via a PLL (Phase Locked Loop) provided in the control device 100. The PLL is a circuit that synchronizes the phase of an input voltage signal with that of an output voltage signal. This allows the Vs detection value to be synchronized with the phase of the output voltage Vs of the inverter 12, thereby properly connecting the inverter 12 with the power grid 30.

[0034] The power control unit 120 calculates an active power command value Pref (hereinafter referred to as the active power command value Pref) and a reactive power command value Qref (hereinafter referred to as the reactive power command value Qref) based on a preset reference power.

[0035] The current control unit 130 generates a current command Iref for controlling the output current Io of the inverter 12 based on a negative-phase-sequence current command (at least one of a d-axis negative-phase-sequence current command value Indref and a q-axis negative-phase-sequence current command value Inqref), an active power command value Pref, and a reactive power command value Qref.

[0036] The output control unit 140 generates a PWM signal in accordance with the current command Iref. When the gate block signal gtb is valid, the output control unit 140 issues an instruction ins to the inverter 12 to stop the inverter 12. On the other hand, when the gate block signal gtb is invalid, the output control unit 140 issues an instruction ins to the inverter 12 to operate in accordance with the PWM signal.

[0037] 3 is a block diagram showing an example of the process for detecting islanding operation. The process for detecting islanding operation executed by the inverter operation control unit 110 includes a high-frequency component removal unit 111, a current command calculation unit 112, an impedance calculation unit 113, a negative-phase voltage command generation unit 114, a threshold determination unit 115, and a delay processing unit 116.

[0038] The high-frequency component removal unit 111 removes high-frequency components from the negative-phase-sequence voltage Vns on the AC side of the inverter 12. The voltage from which the high-frequency components have been removed from the negative-phase-sequence voltage Vns is also referred to as a first voltage. A low-pass filter (LPF), for example, is used to remove the high-frequency components from the negative-phase-sequence voltage Vns.

[0039] The current command calculation unit 112 generates a negative-phase current command (at least one of a d-axis negative-phase current command value Indref and a q-axis negative-phase current command value Inqref) for flowing a negative-phase AC current from the inverter 12 based on a first differential voltage, which is the difference between the negative-phase voltage Vns and the first voltage.

[0040] The impedance calculation unit 113 calculates the impedance between the inverter 12 and the power grid 30 based on the positive-sequence voltage Vps and the positive-sequence current Ipo. The impedance is a value obtained by dividing the positive-sequence voltage Vps by the positive-sequence current Ipo. Alternatively, the impedance may be a value obtained by dividing the negative-sequence voltage Vns by the negative-sequence current Ino. In this case, the negative-sequence voltage Vns and the negative-sequence current Ino are input to the impedance calculation unit 113.

[0041] The negative-phase-sequence voltage command generating unit 114 calculates a negative-phase-sequence voltage command by multiplying the negative-phase-sequence current command obtained by the current command calculating unit 112 by the impedance characteristics obtained by the impedance calculating unit 113. The negative-phase-sequence voltage command is a command value for causing the inverter 12 to supply a negative-phase AC voltage.

[0042] The threshold determination unit 115 determines whether a threshold condition is satisfied based on a second differential voltage, which is the difference between the negative-phase-sequence voltage command obtained by the negative-phase-sequence voltage command generation unit 114 and the negative-phase-sequence voltage Vns. The threshold condition is a condition for determining whether the inverter 12 is in islanding operation. If it is determined that the threshold condition is satisfied, i.e., if the inverter 12 is in islanding operation, the threshold determination unit 115 generates a gate block signal gtb to stop the inverter 12. On the other hand, if it is determined that the threshold condition is not satisfied, i.e., if the inverter 12 is not in islanding operation, the threshold determination unit 115 disables the gate block signal gtb. The gate block signal gtb is disabled by, for example, a value of zero. The threshold condition includes whether the absolute value of the second differential voltage is equal to or greater than a threshold or whether the second differential voltage is outside a predetermined range. The threshold and the predetermined range used in the threshold condition are, for example, values ​​determined by the design of the power conversion system 1 (control device 100).

[0043] In order to stabilize the operation of the inverter operation control unit 110, the delay processing unit 116 delays the timing of outputting the gate block signal gtb to the inverter 12. The delay processing unit 116 imparts a delay of, for example, several ms.

[0044] 1-2-4. Example of Islanding Operation Characteristics FIG. 4 is a diagram for explaining a specific example of the characteristics of islanding operation. Specifically, (A) in FIG. 4 shows the observation point of the output voltage waveform of the high-frequency component removal unit 111. (B) in FIG. 4 shows an example of the output voltage waveform at the observation point when the impedance of the three-phase line on the system side is unbalanced, and (C) in FIG. 4 shows an example of the output voltage waveform at the observation point during islanding operation. As shown in (A) in FIG. 4, there are two observation points: voltage Va and voltage Vb. Voltage Va is a first voltage output from the high-frequency component removal unit 111. Voltage Vb is a first differential voltage obtained by subtracting the first voltage from the negative-phase voltage Vns.

[0045] 4B, when an imbalance occurs on the grid side, the negative-phase-sequence voltage Vns increases. Specifically, the negative-phase-sequence voltage Vns becomes greater than 0 V. In this case, the voltage Vb (first differential voltage) increases to a value greater than 0 V, and then decreases to 0 V.

[0046] On the other hand, as shown in (C) of FIG. 4 , when islanding occurs, the negative-phase-sequence voltage Vns rises. Specifically, the negative-phase-sequence voltage Vns becomes greater than 0 V. In this case, the voltage Vb (first differential voltage) rises to a value greater than 0 V, then decreases, but does not return to 0 V. In other words, the voltage value at which the voltage Vb (first differential voltage) changes differs between when the impedance of the three-phase line on the grid side becomes unbalanced and when islanding is detected. An example of islanding detection utilizing this characteristic will be described in Section 2, Embodiment 2.

[0047] 5 is a flowchart showing an example of processing by the control device 100 according to embodiment 1. Specifically, Fig. 5 shows an outline of an example of detecting islanding.

[0048] In step S100, the control device 100 generates a first voltage by removing high-frequency components from the negative-phase voltage Vns. Then, the process proceeds to step S110.

[0049] In step S110, the control device 100 calculates a first differential voltage by subtracting the negative-phase-sequence voltage Vns from the first voltage, and then the process proceeds to step S120.

[0050] In step S120, the control device 100 generates a negative-phase current command for causing a negative-phase AC current to flow from the inverter 12. Thereafter, the process proceeds to step S130.

[0051] In step S130, the control device 100 calculates a second differential voltage. Then, the process proceeds to step S140. The second differential voltage is a voltage obtained by multiplying the characteristic of the impedance between the inverter 12 and the power grid 30 by the negative-phase-sequence current command and the negative-phase-sequence voltage Vns.

[0052] In step S140, the control device 100 determines whether the second differential voltage satisfies the threshold condition. If the second differential voltage satisfies the threshold condition (step S140; Yes), the process proceeds to step S150. Otherwise (step S140; No), the process ends.

[0053] In step S150, the control device 100 determines that the inverter 12 is in an isolated operation, and executes control to stop the inverter 12.

[0054] 1-4. Effects According to the power conditioner 10 of the first embodiment, a negative-phase current command for causing a negative-phase AC current to flow from the inverter 12 is generated based on a first differential voltage, which is the difference between the negative-phase voltage on the AC side of the inverter 12 and a first voltage obtained by removing high-frequency components from the negative-phase voltage Vns. The power conditioner 10 then controls the inverter 12 to stop when a second differential voltage, which is the difference between the negative-phase voltage command obtained by multiplying the impedance characteristics between the inverter 12 and the power grid 30 by the negative-phase current command, satisfies a threshold condition. This allows appropriate determination of whether the inverter 12 is in islanding operation. Furthermore, when the inverter 12 is in islanding operation, the inverter 12 can be stopped early. Therefore, safety is ensured.

[0055] 2. Embodiment 2 2-1. Example of Detection of Islanding Operation Fig. 6 is a block diagram showing an example of detection of islanding operation according to embodiment 2. Specifically, Fig. 6 shows an example of detection of islanding operation utilizing the characteristic that the voltage Vb (first differential voltage) becomes a value greater than 0 V when islanding operation is detected.

[0056] In the second embodiment, the inverter operation control unit 110 includes a high-frequency component removal unit 111, a current command calculation unit 112, an islanding operation determination unit 117, a condition determination unit 118, and a delay processing unit 116. The high-frequency component removal unit 111, the current command calculation unit 112, and the delay processing unit 116 perform the same processing as in the first embodiment described above, and therefore their description will be omitted. The islanding operation determination unit 117 and the condition determination unit 118 will be described below.

[0057] The islanding operation determination unit 117 determines whether the voltage Vb (first differential voltage) is greater than zero. If it is determined that the voltage Vb (first differential voltage) is greater than zero, the islanding operation determination unit 117 determines that the inverter 12 is in islanding operation. The islanding operation determination unit 117 may monitor the state of the voltage Vb (first differential voltage) for a certain period of time and then determine whether the voltage Vb (first differential voltage) is greater than zero. For example, if the voltage Vb (first differential voltage) has transitioned to zero after the certain period of time has elapsed (see FIG. 4B), the islanding operation determination unit 117 determines that the inverter 12 is not in islanding operation. On the other hand, if the voltage Vb (first differential voltage) has not transitioned to zero after the certain period of time has elapsed (see FIG. 4C), the islanding operation determination unit 117 determines that the inverter 12 is in islanding operation. Therefore, it is possible to appropriately determine whether the inverter 12 is in islanding operation.

[0058] When the islanding operation determination unit 117 determines that the inverter 12 is in islanding operation, the condition determination unit 118 determines whether the positive-sequence voltage Vps on the AC side of the inverter 12 is within a predetermined range. The predetermined range refers to, for example, an estimated voltage range of the positive-sequence voltage Vps when the impedance of the three-phase line on the grid side is balanced (hereinafter referred to as the estimated positive-sequence voltage range). The predetermined range is determined, for example, by the design specifications of the power conversion system 1 (control device 100). For example, if the positive-sequence voltage Vps is determined to be within the predetermined range, i.e., the positive-sequence voltage Vps is determined to be within the estimated positive-sequence voltage range, it is assumed that the impedance is balanced. In this case, the condition determination unit 118 determines that the inverter 12 is in islanding operation. On the other hand, if the positive-sequence voltage Vps is determined to be within the predetermined range, i.e., the positive-sequence voltage Vps is determined to be outside the estimated positive-sequence voltage range, it is assumed that the impedance is unbalanced even if the islanding operation determination unit 117 determines that the inverter 12 is in islanding operation. In this case, the condition determination unit 118 determines that the inverter 12 is not in islanding operation, thereby more appropriately determining whether the inverter 12 is in islanding operation.

[0059] If the condition determination unit 118 determines that the inverter 12 is in islanding operation, it activates the gate block signal gtb for stopping the inverter 12. If the condition determination unit 118 determines that the inverter 12 is not in islanding operation, it deactivates the gate block signal gtb.

[0060] 2-2. Processing Example Fig. 7 is a flowchart showing a processing example of the control device 100 according to embodiment 2. Specifically, Fig. 7 shows an outline of an example of detecting islanding.

[0061] In step S200, the control device 100 generates a first voltage by removing high-frequency components from the negative-phase voltage Vns. Then, the process proceeds to step S210.

[0062] In step S210, the control device 100 calculates a first differential voltage, which is the difference between the negative-phase-sequence voltage Vns and the first voltage, and then the process proceeds to step S220.

[0063] In step S220, the control device 100 determines whether the first differential voltage is greater than zero. If it is determined that the first differential voltage is greater than zero (step S220; Yes), the process proceeds to step S230. Otherwise (step S220; No), the process ends.

[0064] In step S230, the control device 100 determines whether the positive-phase voltage Vps is within a predetermined range. If it is determined that the positive-phase voltage Vps is within the predetermined range (step S230; Yes), the process proceeds to step S240. Otherwise (step S230; No), the process ends.

[0065] In step S240, the control device 100 determines that the inverter 12 is in an isolated operation, and executes control to stop the inverter 12.

[0066] 2-3. Effects According to the power conditioner 10 of the second embodiment, when the first differential voltage, which is the difference between the negative-phase-sequence voltage Vns on the AC side of the inverter 12 and the first voltage obtained by removing high-frequency components from the negative-phase-sequence voltage Vns, is greater than zero, it is determined whether the positive-phase-sequence voltage Vps is within a predetermined range. When it is determined that the positive-phase-sequence voltage Vps is within the predetermined range, it is determined that the inverter 12 is in isolated operation, and control is performed to stop the inverter 12. This provides the same effects as those of the first embodiment described above.

[0067] 3. Embodiment 3 3-1. Example of Detection of Islanding Operation Fig. 8 is a block diagram showing an example of detection of islanding operation according to embodiment 3. Specifically, Fig. 8 shows an example of detection of islanding operation using the characteristics of the positive-sequence voltage Vps or the negative-sequence voltage Vns.

[0068] In the third embodiment, the inverter operation control unit 110 includes a high-frequency component removal unit 111, a current command calculation unit 112, a condition determination unit 118, and a delay processing unit 116. The high-frequency component removal unit 111, the current command calculation unit 112, and the delay processing unit 116 perform the same processing as in the first embodiment, and therefore their description will be omitted. The condition determination unit 118 will be described below.

[0069] The condition determination unit 118 receives the positive-sequence voltage Vps and the negative-sequence voltage Vns. The condition determination unit 118 determines whether the positive-sequence voltage Vps and the negative-sequence voltage Vns on the AC side of the inverter 12 satisfy predetermined conditions. The predetermined conditions include the positive-sequence voltage Vps on the AC side of the inverter 12 being within a predetermined range and the negative-sequence voltage Vns on the AC side of the inverter 12 being greater than zero. The predetermined range refers to, for example, an estimated voltage range (estimated positive-sequence voltage range) of the positive-sequence voltage Vps when the impedance of the three-phase line on the grid side is balanced. The predetermined range is determined, for example, by the design specifications of the power conversion system 1 (control device 100).

[0070] For example, if the positive-sequence voltage Vps and the negative-sequence voltage Vns satisfy a predetermined condition, i.e., if the positive-sequence voltage Vps is within a predetermined range and the negative-sequence voltage Vns is greater than zero, it is assumed that the negative-sequence voltage Vns is rising and that the impedance of the three-phase line on the grid side is balanced. In this case, the condition determination unit 118 determines that the inverter 12 is in islanded operation. On the other hand, if the positive-sequence voltage Vps and the negative-sequence voltage Vns do not satisfy the predetermined condition, it is assumed that the impedance is unbalanced or that the positive-sequence voltage Vps and the negative-sequence voltage Vns are operating within a normal voltage range. In this case, the condition determination unit 118 determines that the inverter 12 is not in islanded operation. This allows an appropriate determination of whether the inverter 12 is in islanded operation.

[0071] If the condition determination unit 118 determines that the inverter 12 is in islanding operation, it activates the gate block signal gtb for stopping the inverter 12. If the condition determination unit 118 determines that the inverter 12 is not in islanding operation, it deactivates the gate block signal gtb.

[0072] 3-2. Processing Example Fig. 9 is a flowchart showing a processing example of the control device 100 according to embodiment 3. Specifically, Fig. 9 shows an outline of an example of detecting islanding.

[0073] In step S300, the control device 100 generates a first voltage by removing high-frequency components from the negative-phase voltage Vns. Then, the process proceeds to step S310.

[0074] In step S310, the control device 100 calculates a first differential voltage, which is the difference between the negative-phase-sequence voltage Vns and the first voltage, and then the process proceeds to step S320.

[0075] In step S320, the control device 100 determines whether the positive-sequence voltage Vps and the negative-sequence voltage Vns satisfy a predetermined condition. If it is determined that the positive-sequence voltage Vps and the negative-sequence voltage Vns satisfy the predetermined condition (step S320; Yes), the process proceeds to step S330. Otherwise (step S320; No), the process ends.

[0076] In step S330, the control device 100 determines that the inverter 12 is in an isolated operation, and executes control to stop the inverter 12.

[0077] Note that step S320 may be executed before step S300 or step S310. In other words, the control device 100 may be configured to execute steps S300 and S310, and steps S320 and S330 independently.

[0078] 3-3. Effects According to the power conditioner 10 of the third embodiment, a first differential voltage is calculated, which is the difference between the negative-phase-sequence voltage Vns on the AC side of the inverter 12 and a first voltage obtained by removing high-frequency components from the negative-phase-sequence voltage Vns. Furthermore, when the positive-phase-sequence voltage Vps and the negative-phase-sequence voltage Vns satisfy predetermined conditions, the power conditioner 10 determines that the inverter 12 is in isolated operation and controls the inverter 12 to stop. This provides the same effects as those of the first embodiment described above.

[0079] 1... Power conversion system, 10... Power conditioner, 11... DC power supply, 12... Inverter, 20... Transformer, 30... Power system, 40... Circuit breaker, 50... Load, 100... Control device

Claims

1. A power conditioner comprising: an inverter that converts DC power supplied from a DC power source into AC power and supplies the AC power to a power grid; and a control device that controls the inverter, wherein the control device generates a negative-phase current command for causing a negative-phase AC current to flow from the inverter based on a first differential voltage that is the difference between a negative-phase voltage on the AC side of the inverter and a voltage from which high-frequency components have been removed of the negative-phase voltage; and is configured to control the inverter to stop when a second differential voltage that is the difference between the negative-phase voltage and a negative-phase voltage command obtained by multiplying the negative-phase current command by the characteristics of the impedance between the inverter and the power grid satisfies a threshold condition.

2. A power conditioner according to claim 1, wherein the threshold condition includes that the absolute value of the second differential voltage is equal to or greater than a threshold, or that the second differential voltage is outside a predetermined range.

3. A power conditioner according to claim 1, wherein the impedance is a value calculated based on the positive-sequence voltage and positive-sequence current on the AC side of the inverter and obtained by dividing the positive-sequence voltage by the positive-sequence current, or a value calculated based on the negative-sequence voltage and negative-sequence current on the AC side of the inverter and obtained by dividing the negative-sequence voltage by the negative-sequence current.

4. A power conditioner according to any one of claims 1 to 3, characterized in that the negative-phase-sequence current command includes at least one of a d-axis component negative-phase-sequence current command and a q-axis component negative-phase-sequence current command.

5. A power conditioner comprising: an inverter that converts DC power supplied from a DC power source into AC power and supplies the AC power to a power grid; and a control device that controls the inverter, wherein the control device is configured to determine whether or not a positive-phase voltage on the AC side of the inverter is within a predetermined range when a first differential voltage, which is the difference between a negative-phase voltage on the AC side of the inverter and a voltage from which high-frequency components have been removed of the negative-phase voltage, is greater than zero; and to control the inverter to stop when it is determined that the positive-phase voltage is within the predetermined range.

6. A power conditioner comprising: an inverter that converts DC power supplied from a DC power source into AC power and supplies the AC power to a power grid; and a control device that controls the inverter, wherein the control device is configured to: calculate a first differential voltage that is the difference between a negative-phase voltage on the AC side of the inverter and a voltage from which high-frequency components have been removed from the negative-phase voltage; and control the inverter to stop when the positive-phase voltage and negative-phase voltage on the AC side of the inverter satisfy predetermined conditions.

7. A power conditioner according to claim 6, wherein the predetermined conditions include that the positive-phase voltage is within a predetermined range and that the negative-phase voltage is greater than zero.

8. A power control method comprising: converting DC power into AC power using an inverter, and supplying the AC power to a power grid; generating a negative-phase current command for causing a negative-phase AC current to flow from the inverter based on a first differential voltage which is the difference between a negative-phase voltage on the AC side of the inverter and a voltage from which high-frequency components have been removed of the negative-phase voltage; and controlling the inverter to stop when a second differential voltage which is the difference between the negative-phase voltage and a negative-phase voltage command obtained by multiplying the negative-phase current command by characteristics of impedance between the inverter and the power grid satisfies a threshold condition.

9. A power control method comprising: converting DC power into AC power using an inverter, and supplying the AC power to a power grid; determining whether a first differential voltage, which is the difference between a negative-phase-sequence voltage on the AC side of the inverter and a voltage from which high-frequency components have been removed, is greater than zero; if it is determined that the first differential voltage is greater than zero, determining whether a positive-phase-sequence voltage on the AC side of the inverter is within a predetermined range; and if it is determined that the positive-phase-sequence voltage is within the predetermined range, controlling the inverter to stop.

10. A power control method comprising: converting DC power into AC power using an inverter and supplying the AC power to a power grid; calculating a first differential voltage that is the difference between a negative-phase-sequence voltage on the AC side of the inverter and a voltage from which high-frequency components have been removed from the negative-phase-sequence voltage; and controlling the inverter to stop when the positive-phase-sequence voltage and the negative-phase-sequence voltage on the AC side of the inverter satisfy a predetermined condition.

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