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

Technical Field

[0006] , , , ,

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

Background Art

[0002] Patent Document 1 discloses a power conversion device (power conditioner). Specifically, when the reverse voltage on the AC side of the inverter is equal to or greater than a predetermined value, the power conditioner determines that the inverter is operating alone and controls the inverter to stop.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] Consider a case where the impedance of the three-phase line in the power system becomes unbalanced. In this case, in Patent Document 1, there is a possibility that the reverse voltage on the AC side of the inverter becomes equal to or greater than a predetermined value, and it is erroneously determined that the inverter is operating alone.

[0005] One object of the present disclosure is to provide a technique capable of appropriately determining whether an inverter is operating alone.

Means for Solving the Problems

[0006] The first aspect of this 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 system, and a control device that controls the inverter. The control device generates a reverse-sequence current command to cause a reverse-sequence AC current to flow from the inverter based on a first difference voltage, which is the difference between the reverse-sequence voltage on the AC side of the inverter and the voltage obtained by removing the high-frequency components of the reverse-sequence voltage. The control device then controls the inverter to stop if a second difference voltage, which is the difference between the reverse-sequence voltage command obtained by multiplying the reverse-sequence current command by the impedance characteristics between the inverter and the power system and the reverse-sequence voltage, satisfies a threshold condition.

[0007] A second aspect of this 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 the positive-sequence voltage on the AC side of the inverter is within a predetermined range if the first differential voltage, which is the difference between the negative-sequence voltage on the AC side of the inverter and the voltage obtained by removing the high-frequency components of the negative-sequence voltage, is greater than zero. If the positive-sequence voltage is determined to be within the predetermined range, the control device controls the inverter to stop.

[0008] A third aspect of this 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 AC power to a power grid, and a control device that controls the inverter. The control device calculates a first differential voltage, which is the difference between the reverse-sequence voltage on the AC side of the inverter and the voltage obtained by removing the high-frequency components of the reverse-sequence voltage, and controls the inverter to stop if the positive-sequence voltage and reverse-sequence voltage on the AC side of the inverter satisfy predetermined conditions.

[0009] A fourth aspect of this disclosure relates to a power control method. The power control method includes: using an inverter to convert DC power to AC power and supply AC power to a power system; generating a reverse-sequence current command to cause a reverse-sequence AC current to flow from the inverter based on a first difference voltage, which is the difference between the reverse-sequence voltage on the AC side of the inverter and the voltage obtained by removing the high-frequency component of the reverse-sequence voltage; and controlling the inverter to stop if a second difference voltage, which is the difference between the reverse-sequence voltage command obtained by multiplying the impedance characteristics between the inverter and the power system and the reverse-sequence voltage, satisfies a threshold condition.

[0010] A fifth aspect of this disclosure relates to a power control method. The power control method includes: using an inverter to convert DC power to AC power and supply AC power to a power system; determining whether a first differential voltage, which is the difference between the reverse-sequence voltage on the AC side of the inverter and the voltage obtained by removing the high-frequency component of the reverse-sequence voltage, is greater than zero; if it is determined that the first differential voltage is greater than zero, determining whether the positive-sequence voltage on the AC side of the inverter is within a predetermined range; and if it is determined that the positive-sequence voltage is within a predetermined range, controlling the inverter to stop.

[0011] The sixth aspect of this disclosure relates to a power control method. The power control method includes: using an inverter to convert DC power to AC power and supply AC power to a power system; calculating a first differential voltage, which is the difference between the reverse-sequence voltage on the AC side of the inverter and the voltage obtained by removing the high-frequency component of the reverse-sequence voltage; and controlling the inverter to stop if the positive-sequence voltage and reverse-sequence voltage on the AC side of the inverter satisfy predetermined conditions. [Effects of the Invention]

[0012] From the first perspective, a reverse-sequence current command is generated from the inverter to supply a reverse-sequence AC current based on a first differential voltage, which is the difference between the reverse-sequence voltage on the AC side of the inverter and the voltage obtained by removing the high-frequency components of the reverse-sequence voltage. Then, if the second differential voltage, which is the difference between the reverse-sequence voltage command obtained by multiplying the impedance characteristics between the inverter and the power system by the reverse-sequence current command, satisfies a threshold condition, control is performed to shut down the inverter. This allows for proper determination of whether the inverter is operating in an isolated state. Furthermore, if the inverter is operating in an isolated state, it becomes possible to shut down the inverter early. Thus, safety is ensured.

[0013] From a second perspective, if the first differential voltage, which is the difference between the reverse-sequence voltage on the AC side of the inverter and the voltage obtained by removing the high-frequency components of the reverse-sequence voltage, is greater than zero, it is determined whether the positive-sequence voltage on the AC side of the inverter is within a predetermined range. If it is determined that the positive-sequence voltage is within the predetermined range, control is implemented to stop the inverter. This allows for an appropriate determination of whether the inverter is operating in isolation. Furthermore, if the inverter is operating in isolation, it becomes possible to stop the inverter early. Thus, safety is ensured.

[0014] From a third perspective, the reverse-phase voltage and high-frequency components of the reverse-phase voltage on the AC side of the inverter are removed. taden The first differential voltage, which is the difference from the voltage, is calculated. Furthermore, if the positive-sequence voltage and negative-sequence voltage meet predetermined conditions, control is implemented to shut down the inverter. This allows for proper determination of whether the inverter is operating independently. Moreover, if the inverter is operating independently, it can be shut down early. Therefore, safety is ensured.

[0015] From the fourth perspective, the same effect can be obtained as from the first perspective.

[0016] From the fifth perspective, the same effect can be obtained as from the second perspective.

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

Brief Description of Drawings

[0018] [Figure 1] It is a diagram for explaining the outline of the power conversion system according to Embodiment 1. [Figure 2] It is a block diagram showing a functional example of the control device according to Embodiment 1. [Figure 3] It is a block diagram showing a detection example of single operation according to Embodiment 1. [Figure 4] It is a diagram for explaining a specific example of the characteristics of single operation according to Embodiment 1. [Figure 5] It is a flowchart showing a processing example of the control device according to Embodiment 1. [Figure 6] It is a block diagram showing a detection example of single operation according to Embodiment 2. [Figure 7] It is a flowchart showing a processing example of the control device according to Embodiment 2. [Figure 8] It is a block diagram showing a detection example of single operation according to Embodiment 3. [Figure 9] It is a flowchart showing a processing example of the control device according to Embodiment 3.

Modes for Carrying Out the Invention

[0019] Referring to the accompanying drawings, a power conditioner and a power control method according to an embodiment of the present disclosure will be described. In addition, the same reference numerals are given to common elements in each figure, and redundant explanations are omitted.

[0020] 1. Embodiment 1 1-1. Outline of the Power Conversion System Figure 1 is a diagram illustrating the overview of power conversion system 1. 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 is composed of an inverter 12 and a control device 100.

[0021] The DC power supply 11 is an energy storage device (e.g., a solar cell module) that stores electricity generated from renewable energy sources. Examples of renewable energy sources include solar power, wind power, hydropower, etc.

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

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

[0024] The control device 100 generates a current command to control the output current Io of the inverter 12 based on the detected Vs value and the detected Io value. 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 according to the PWM signal.

[0025] Let's consider the islanding operation of inverter 12. For example, if a circuit breaker 40 installed in a substation on the grid side opens due to a grid fault, the AC power output from inverter 12 and the power of load 50 will match. This state is called "islanding operation". If inverter 12 continues to operate islanding, electric shock to people, machine failure, or overcurrent may occur when reclosing the circuit may occur. Therefore, if inverter 12 is operating islanding, it is necessary to stop inverter 12. Load 50 is, for example, a device connected to power conditioner 10 and consumed in a factory or other place where power conditioner 10 is installed. Load 50 is typically composed of a resistive load R, an inductive load L, and a capacitive load C.

[0026] When inverter 12 is operating independently, the reverse-phase voltage Vns rises slightly, causing inverter 12 to... Inverse-sequence current Ino When the solution is injected, the reverse-phase voltage Vns rises further.

[0027] On the other hand, consider the case where the impedance of the three-phase transmission line on the grid side is unbalanced. In this case, since the three-phase transmission line is not perfectly balanced, the reverse-phase voltage Vns will rise. In other words, the reverse-phase voltage Vns will rise both when inverter 12 is operating independently and when the impedance of the three-phase transmission line is unbalanced. Therefore, a mechanism is needed to distinguish between these situations.

[0028] According to the control device 100, a reverse-sequence current command is generated from the inverter 12 to supply a reverse-sequence AC current based on a first differential voltage, which is the difference between the reverse-sequence voltage Vns on the AC side of the inverter 12 and the voltage obtained by removing the high-frequency component of the reverse-sequence voltage Vns. Then, the control device 100 controls the inverter 12 to stop if the second differential voltage, which is the difference between the reverse-sequence voltage command obtained by applying the reverse-sequence current command to the impedance characteristics between the inverter 12 and the power system 30 and the reverse-sequence voltage Vns, satisfies a threshold condition. This allows for appropriate determination of whether the inverter 12 is operating in isolation. Furthermore, if the inverter 12 is operating in isolation, it is possible to stop the inverter 12 early. Thus, safety is ensured. Details of the control device 100's processing will be described later.

[0029] 1-2. Examples of control devices 1-2-1. Example Configuration The control device 100 has hardware that implements various functions. The hardware includes processing circuits capable of high-speed calculations. Examples of processing circuits include FPGAs (Field Programmable Gate Arrays) and ASICs (Application Specific Integrated Circuits). In addition to processing circuits, the hardware may also include a memory device and an arithmetic unit (e.g., CPU, GPU) that executes programs stored in the memory device.

[0030] 1-2-2. Functional Examples Figure 2 is a block diagram showing an example of the 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] The inverter operation control unit 110 generates a reverse-phase current command to supply reverse-phase AC current from the inverter 12 based on the Vns detection value. The reverse-phase current command is a reverse-phase current command for the d-axis component. value Indref (hereinafter, d-axis negative sequence current command value (referred to as Indref) and the q-axis component of the reverse-sequence current command value Inqref (hereinafter, q-axis negative sequence current command value It includes at least one of the following: (referred to as Inqref) and

[0032] Furthermore, the inverter operation control unit 110 performs islanding detection processing. In islanding detection processing, the inverter operation control unit 110 determines whether the inverter 12 is operating in an island state based on the reverse-phase current command, the Vs detection value, and the Io detection value. If it is determined that the inverter 12 is operating in an island state, the inverter operation control unit 110 outputs a gate block signal gtb to the output control unit 140 to stop the inverter 12. Details of the islanding detection processing will be described later.

[0033] Furthermore, the Vs detection value input to the inverter operation control unit 110 may be obtained, for example, via a PLL (Phase Locked Loop) provided in the control device 100. A PLL is a circuit that synchronizes the phase of the input voltage signal and the output voltage signal. This allows the phase of the Vs detection value and the output voltage Vs of the inverter 12 to be synchronized, and the inverter 12 and the power system 30 to be properly connected.

[0034] The power control unit 120 calculates the command value Pref for active power (hereinafter referred to as the active power command value Pref) and the command value Qref for reactive power (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 reverse-phase current command (at least one of the d-axis reverse-phase current command value Indref and the q-axis reverse-phase 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 according to the current command Iref. When the gate block signal gtb is active, the output control unit 140 issues an instruction ins to the inverter 12 to stop it. On the other hand, when the gate block signal gtb is inactive, the output control unit 140 issues an instruction ins to the inverter 12 to operate according to the PWM signal.

[0037] 1-2-3. Examples of detecting isolated operation Figure 3 is a block diagram showing an example of islanding detection processing. The islanding detection processing performed 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 reverse-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 the high-frequency components from the reverse-sequence voltage Vns on the AC side of the inverter 12. The voltage from which the high-frequency components of the reverse-sequence voltage Vns have been removed is also called the first voltage. A low-pass filter (LPF) can be used as a method to remove the high-frequency components of the reverse-sequence voltage Vns.

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

[0040] The impedance calculation unit 113 calculates the impedance between the inverter 12 and the power system 30 based on the positive-sequence voltage Vps and the positive-sequence current Ipo. The impedance is the value obtained by dividing the positive-sequence voltage Vps by the positive-sequence current Ipo. Alternatively, the impedance may be the 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 reverse-phase voltage command generation unit 114 calculates a reverse-phase voltage command by multiplying the reverse-phase current command obtained by the current command calculation unit 112 by the impedance characteristics obtained by the impedance calculation unit 113. The reverse-phase voltage command is a command value for supplying a reverse-phase AC voltage from the inverter 12.

[0042] The threshold determination unit 115 determines whether a threshold condition is met based on the second differential voltage, which is the difference between the reverse-phase voltage command obtained by the reverse-phase voltage command generation unit 114 and the reverse-phase voltage Vns. The threshold condition is a condition for determining whether the inverter 12 is operating in isolation. If it is determined that the threshold condition is met, i.e., the inverter 12 is operating in isolation, 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 met, i.e., the inverter 12 is not operating in isolation, the threshold determination unit 115 disables the gate block signal gtb. Disabling the gate block signal gtb is, for example, a value of zero. The threshold condition includes the absolute value of the second differential voltage being greater than or equal to a threshold, or the second differential voltage being outside a predetermined range. The threshold and predetermined range used in the threshold condition are values ​​that are designed and determined, for example, by the specifications of the power conversion system 1 (control device 100).

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

[0044] 1-2-4. Examples of characteristics of standalone operation Figure 4 is a diagram illustrating a specific example of the characteristics of islanding operation. Specifically, (A) in Figure 4 shows the observation point of the output voltage waveform of the high-frequency component removal unit 111. (B) in Figure 4 shows an example of the output voltage waveform result at the observation point when the impedance of the three-phase line on the grid side is unbalanced, and (C) in Figure 4 shows an example of the output voltage waveform result at the observation point during islanding operation. As shown in (A) in Figure 4, there are two observation points: voltage Va and voltage Vb. Voltage Va is the first voltage output from the high-frequency component removal unit 111. Voltage Vb is the first differential voltage obtained by differentiating the first voltage and the inverse-sequence voltage Vns.

[0045] As shown in (B) of Figure 4, if an imbalance occurs on the grid side, the reverse-sequence voltage Vns rises. Specifically, the reverse-sequence voltage Vns becomes greater than 0V. In this case, the voltage Vb (first differential voltage) rises to a value greater than 0V, and then decreases to 0V.

[0046] On the other hand, as shown in (C) in Figure 4, when islanding occurs, the reverse-phase voltage Vns rises. Specifically, the reverse-phase voltage Vns becomes greater than 0V. In this case, the voltage Vb (first differential voltage) rises to a value greater than 0V, then decreases, but does not return to 0V. In other words, the voltage value to 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 detecting islanding using this characteristic will be explained in Embodiment 2 of Section 2.

[0047] 1-3. Processing Example Figure 5 is a flowchart showing an example of processing by the control device 100 according to Embodiment 1. Specifically, Figure 5 provides a summary of an example of detecting isolated operation.

[0048] In step S100, the control device 100 removes the high-frequency component of the inverse-phase voltage Vns to generate a first voltage. The process then proceeds to step S110.

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

[0050] In step S120, the control device 100 generates a reverse-phase current command to supply reverse-phase alternating current from the inverter 12. The process then proceeds to step S130.

[0051] In step S130, the control device 100 calculates the second differential voltage. The process then proceeds to step S140. The second differential voltage is the difference between the reverse-sequence voltage command, which is obtained by applying a reverse-sequence current command to the impedance characteristics between the inverter 12 and the power system 30, and the reverse-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 operating independently and executes control to stop the inverter 12.

[0054] 1-4. Effects According to the power conditioner 10 of Embodiment 1, the reverse-phase voltage on the AC side of the inverter 12 VnsBased on the first differential voltage, which is the difference between the first voltage obtained by removing the high-frequency components of the reverse-sequence voltage Vns, a reverse-sequence current command is generated to cause a reverse-sequence AC current to flow from the inverter 12. Then, the power conditioner 10 controls the inverter 12 to stop if the second differential voltage, which is the difference between the reverse-sequence voltage command obtained by applying the reverse-sequence current command to the impedance characteristics between the inverter 12 and the power system 30 and the reverse-sequence voltage Vns, satisfies the threshold condition. This makes it possible to appropriately determine whether the inverter 12 is operating in isolation. Furthermore, if the inverter 12 is operating in isolation, it becomes possible to stop the inverter 12 early. Thus, safety is ensured.

[0055] 2. Embodiment 2 2-1. Examples of detecting isolated operation Figure 6 is a block diagram illustrating an example of islanding detection according to Embodiment 2. Specifically, Figure 6 shows an example of islanding detection that utilizes the characteristic that the voltage Vb (first differential voltage) becomes greater than 0V when islanding is detected.

[0056] In Embodiment 2, the inverter operation control unit 110 includes a high-frequency component removal unit 111, a current command calculation unit 112, an island operation discrimination 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 Embodiment 1 described above, so their explanation is omitted. The island operation discrimination unit 117 and the condition determination unit 118 will be explained 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 operating in an islanding state. Alternatively, the islanding operation determination unit 117 may monitor the state of the voltage Vb (first differential voltage) for a certain period of time before determining 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 a certain period of time (see Figure 4(B)), the islanding operation determination unit 117 determines that the inverter 12 is not operating in an islanding state. On the other hand, if the voltage Vb (first differential voltage) has not transitioned to zero after a certain period of time (see Figure 4(C)), the islanding operation determination unit 117 determines that the inverter 12 is operating in an islanding state. Therefore, it is possible to appropriately determine whether the inverter 12 is operating in an islanding state or not.

[0058] 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 if the determination result of the islanding operation determination unit 117 is islanding. The predetermined range refers to, for example, the 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 by design, for example, the specifications of the power conversion system 1 (control device 100). For example, if it is determined that the positive-sequence voltage Vps is within the predetermined range, that is, if the positive-sequence voltage Vps is 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 islanding. On the other hand, if it is determined that the positive-sequence voltage Vps is not within the predetermined range, that is, if the determination result of the islanding operation determination unit 117 is "islanding," it is assumed that the impedance is unbalanced. In this case, the condition determination unit 118 determines that the inverter 12 is not islanding. This allows for a more accurate determination of whether the inverter 12 is operating independently or not.

[0059] Then, if the condition determination unit 118 determines that the inverter 12 is operating independently, it enables the gate block signal gtb to stop the inverter 12. If the condition determination unit 118 determines that the inverter 12 is not operating independently, it disables the gate block signal gtb.

[0060] 2-2. Processing Example Figure 7 is a flowchart showing an example of processing by the control device 100 according to Embodiment 2. Specifically, Figure 7 provides a summary of an example of detecting isolated operation.

[0061] In step S200, the control device 100 removes the high-frequency component of the inverse-phase voltage Vns to generate a first voltage. The process then proceeds to step S210.

[0062] In step S210, the control device 100 calculates a first differential voltage by subtracting the inverse phase voltage Vns from the first voltage. The process then 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-sequence voltage Vps is within a predetermined range. If it is determined that the positive-sequence 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 operating independently and executes control to stop the inverter 12.

[0066] 2-3. Effects According to the power conditioner 10 of Embodiment 2, if the first differential voltage, which is the difference between the reverse-sequence voltage Vns on the AC side of the inverter 12 and the first voltage obtained by removing the high-frequency component of the reverse-sequence voltage Vns, is greater than zero, it is determined whether the positive-sequence voltage Vps is within a predetermined range. If it is determined that the positive-sequence voltage Vps is within a predetermined range, it is determined that the inverter 12 is operating independently, and control is performed to stop the inverter 12. This provides the same effect as in Embodiment 1 described above.

[0067] 3. Embodiment 3 3-1. Examples of detecting isolated operation Figure 8 is a block diagram showing an example of islanding detection according to Embodiment 3. Specifically, Figure 8 shows an example of islanding detection utilizing the characteristics of the positive-sequence voltage Vps or the negative-sequence voltage Vns.

[0068] In Embodiment 3, 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 Embodiment 1 described above, so their explanation is omitted. The condition determination unit 118 will be explained below.

[0069] The condition determination unit 118 receives the positive-sequence voltage Vps and the negative-sequence voltage Vns as input. The condition determination unit 118 determines whether the positive-sequence voltage Vps and negative-sequence voltage Vns on the AC side of the inverter 12 satisfy predetermined conditions. The predetermined conditions include that the positive-sequence voltage Vps on the AC side of the inverter 12 is within a predetermined range, and that the negative-sequence voltage Vns on the AC side of the inverter 12 is greater than zero. The predetermined range means, for example, the estimated voltage range of the positive-sequence voltage Vps when the impedance of the three-phase line on the grid side is balanced (estimated positive-sequence voltage range). The predetermined range is determined by design, for example, the 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 predetermined conditions, that is, 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 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 operating in isolation. On the other hand, if the positive-sequence voltage Vps and the negative-sequence voltage Vns do not satisfy predetermined conditions, it is assumed that the impedance is unbalanced, or that the positive-sequence voltage Vps and the negative-sequence voltage Vns are operating within the normal voltage range. In this case, the condition determination unit 118 determines that the inverter 12 is not operating in isolation. This allows for an appropriate determination of whether the inverter 12 is operating in isolation or not.

[0071] Then, if the condition determination unit 118 determines that the inverter 12 is operating independently, it enables the gate block signal gtb to stop the inverter 12. If the condition determination unit 118 determines that the inverter 12 is not operating independently, it disables the gate block signal gtb.

[0072] 3-2. Processing Example Figure 9 is a flowchart showing an example of processing by the control device 100 according to Embodiment 3. Specifically, Figure 9 provides a summary of an example of detecting isolated operation.

[0073] In step S300, the control device 100 removes the high-frequency component of the inverse-phase voltage Vns to generate a first voltage. The process then proceeds to step S310.

[0074] In step S310, the control device 100 calculates a first differential voltage by subtracting the inverse phase voltage Vns from the first voltage. The process then 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 predetermined conditions. If it is determined that the positive-sequence voltage Vps and the negative-sequence voltage Vns satisfy the predetermined conditions (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 operating independently and executes control to stop the inverter 12.

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

[0078] 3-3. Effects According to the power conditioner 10 of Embodiment 3, a first differential voltage is calculated, which is the difference between the reverse-sequence voltage Vns on the AC side of the inverter 12 and the first voltage obtained by removing the high-frequency component of the reverse-sequence voltage Vns. Furthermore, if the positive-sequence voltage Vps and the reverse-sequence voltage Vns satisfy predetermined conditions, the power conditioner 10 determines that the inverter 12 is operating independently and controls the inverter 12 to stop. This provides the same effects as in Embodiment 1 described above. [Explanation of symbols]

[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. An inverter that converts DC power supplied from a DC power source into AC power and supplies the AC power to the power grid, The system comprises a control device for controlling the inverter, The control device is Based on a first differential voltage, which is the difference between the reverse-sequence voltage on the AC side of the inverter and the voltage obtained by removing the high-frequency component of the reverse-sequence voltage, a reverse-sequence current command is generated to cause a reverse-sequence AC current to flow from the inverter. The system is configured to control the inverter to stop if the second differential voltage, which is the difference between the reverse-sequence voltage command obtained by multiplying the impedance characteristics between the inverter and the power system by the reverse-sequence current command, satisfies a threshold condition. A power conditioner characterized by the following.

2. A power conditioner according to claim 1, The threshold condition includes the absolute value of the second differential voltage being greater than or equal to a threshold, or the second differential voltage being outside a predetermined range. A power conditioner characterized by the following.

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

4. A power conditioner according to any one of claims 1 to 3, The aforementioned reverse-sequence current command includes at least one of a reverse-sequence current command for the d-axis component and a reverse-sequence current command for the q-axis component. A power conditioner characterized by the following.

5. An inverter that converts DC power supplied from a DC power source into AC power and supplies the AC power to the power grid, The system comprises a control device for controlling the inverter, The control device is If the first differential voltage, which is the difference between the reverse-sequence voltage on the AC side of the inverter and the voltage obtained by removing the high-frequency component of the reverse-sequence voltage, is greater than zero, then it is determined whether the positive-sequence voltage on the AC side of the inverter is within a predetermined range. The system is configured to control the inverter to stop when the positive-sequence voltage is determined to be within the predetermined range. A power conditioner characterized by the following.

6. An inverter that converts DC power supplied from a DC power source into AC power and supplies the AC power to the power grid, The system comprises a control device for controlling the inverter, The control device is The first differential voltage is calculated, which is the difference between the reverse-sequence voltage on the AC side of the inverter and the voltage obtained by removing the high-frequency component of the reverse-sequence voltage. The inverter is controlled to stop if the positive-sequence voltage and negative-sequence voltage on the AC side of the inverter satisfy predetermined conditions. Configured to execute A power conditioner characterized by the following.

7. A power conditioner according to claim 6, The aforementioned predetermined conditions include the positive-sequence voltage being within a predetermined range and the negative-sequence voltage being greater than zero. A power conditioner characterized by the following.

8. Using an inverter, converts DC power to AC power and supplies the AC power to the power grid. Based on a first differential voltage, which is the difference between the reverse-sequence voltage on the AC side of the inverter and the voltage obtained by removing the high-frequency component of the reverse-sequence voltage, a reverse-sequence current command is generated to cause a reverse-sequence AC current to flow from the inverter. The control is to stop the inverter when the second differential voltage, which is the difference between the reverse-sequence voltage command obtained by applying the reverse-sequence current command to the impedance characteristics between the inverter and the power system, satisfies a threshold condition. including A power control method characterized by the following:

9. Using an inverter, converts DC power to AC power and supplies the AC power to the power grid. The first differential voltage, which is the difference between the reverse-sequence voltage on the AC side of the inverter and the voltage obtained by removing the high-frequency component of the reverse-sequence voltage, is determined to be greater than zero. If the first differential voltage is determined to be greater than zero, it is determined whether the positive-sequence voltage on the AC side of the inverter is within a predetermined range. If the positive-sequence voltage is determined to be within the predetermined range, the inverter is controlled to stop. including A power control method characterized by the following:

10. Using an inverter, converts DC power to AC power and supplies the AC power to the power grid. The first differential voltage is calculated, which is the difference between the reverse-sequence voltage on the AC side of the inverter and the voltage obtained by removing the high-frequency component of the reverse-sequence voltage. The inverter is controlled to stop if the positive-sequence voltage and negative-sequence voltage on the AC side of the inverter satisfy predetermined conditions. including A power control method characterized by the following:

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