Power conditioner

Abstract

This power conditioner is provided with 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 controller that controls the inverter. The controller calculates a differential impedance being a difference between a positive phase impedance obtained by dividing a d-axis positive phase voltage included in a positive phase voltage on the AC side of the inverter by a d-axis positive phase current included in a positive phase current, and a reverse phase impedance obtained by dividing a d-axis reverse phase voltage included in a reverse phase voltage on the AC side of the inverter by a d-axis reverse phase current included in a reverse phase current. Further, the controller stops the inverter if a solo operation condition is met where the absolute value of the differential impedance falls below a threshold value.

Description

Power conditioner

[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] When the impedance of a three-phase line in a power system becomes unbalanced, the negative-phase voltage on the AC side of the inverter increases. On the other hand, when the inverter is operating in islanded mode, the negative-phase voltage on the AC side of the inverter increases. That is, the negative-phase voltage increases in both cases. In this case, Patent Document 1 may erroneously determine that the inverter is operating in islanded mode even if the impedance of the three-phase line is unbalanced.

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

[0006] One 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 controller that controls the inverter. The controller calculates a differential impedance, which is the difference between a positive-sequence impedance obtained by dividing a d-axis positive-sequence voltage included in a positive-sequence voltage on the AC side of the inverter by a d-axis positive-sequence current included in a positive-sequence current, and a negative-sequence impedance obtained by dividing a d-axis negative-sequence voltage included in a negative-sequence voltage on the AC side of the inverter by a d-axis negative-sequence current included in the negative-sequence current. Furthermore, the controller stops the inverter when an islanding operation condition is met, in which the absolute value of the differential impedance is less than a threshold value.

[0007] According to the present disclosure, a differential impedance is calculated as the difference between a positive-sequence impedance obtained by dividing a d-axis positive-sequence voltage included in the positive-sequence voltage on the AC side of the inverter by a d-axis positive-sequence current included in the positive-sequence current, and a negative-sequence impedance obtained by dividing a d-axis negative-sequence voltage included in the negative-sequence voltage on the AC side of the inverter by a d-axis negative-sequence current included in the negative-sequence current. The inverter is then shut down when an islanding condition is met, in which the absolute value of the differential impedance is less than a threshold value. This allows appropriate determination of whether the inverter is in islanding operation. Furthermore, if the inverter is in islanding operation, the inverter can be shut down early. Therefore, safety is ensured.

[0008] 1 is a diagram for explaining an overview of a power conversion system according to an embodiment. FIG. 2 is a block diagram showing a first functional example of a controller according to an embodiment. FIG. 3 is a block diagram showing a second functional example of a controller according to an embodiment. FIG. 4 is a block diagram showing a first specific example of an islanding operation suppression function unit according to an embodiment. FIG. 5 is a block diagram showing a second specific example of an islanding operation suppression function unit according to an embodiment. FIG. 6 is a block diagram showing a specific example of an islanding operation detection unit according to an embodiment. FIG. 7 is a diagram for explaining an example of a detection result of islanding operation according to an embodiment. FIG. 8 is a diagram for explaining an example of a detection result of islanding operation according to an embodiment. FIG. 9 is a diagram for explaining an example of a detection result at the time of unbalance according to an embodiment.

[0009] A power conditioner 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.

[0010] 1. Overview of the 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 controller 100.

[0011] 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.

[0012] 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 the transformer 20. The inverter 12 is, for example, a voltage-controlled grid forming (GFM) inverter.

[0013] The controller 100 is connected to the inverter 12 and controls the inverter 12. The controller 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 controller 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 controller 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.

[0014] The controller 100 generates a pulse width modulation signal (PWM signal) for controlling the output voltage Vs of the inverter 12 based on the Vs detection value and the Io detection value, and outputs the PWM signal to the inverter 12 so that the inverter 12 operates in accordance with the PWM signal.

[0015] Consider the islanding operation of the inverter 12. For example, if a circuit breaker 40 installed in a substation 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 the human body, mechanical failure, or overcurrent during reclosing. Therefore, when the inverter 12 is operating in an islanded state, 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.

[0016] 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, which further increases the negative-phase-sequence voltage Vns. In addition, the increase in the negative-phase-sequence voltage Vns also increases the negative-phase-sequence current Ino.

[0017] Consider a case where the impedance of the three-phase line on the system 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 operating in an isolated mode and when the impedance of the three-phase line is unbalanced.

[0018] Consider the positive-sequence voltage Vps. When the inverter 12 is operating in an isolated state, the positive-sequence voltage Vps fluctuates within the same voltage range as in normal operation. On the other hand, when the impedance of the three-phase line on the system side is unbalanced, the positive-sequence voltage Vps fluctuates and does not fluctuate within the same voltage range as in normal operation.

[0019] As described above, the electrical characteristics differ when the inverter 12 is in islanded operation from when the impedance of the three-phase line on the grid side is unbalanced. Furthermore, when the inverter 12 is in islanded operation, the electrical characteristics are such that the negative-phase voltage Vns and the negative-phase current Ino increase. It is preferable to utilize these characteristics to appropriately determine whether the inverter 12 is in islanded operation, and to stop the inverter 12 early if it is determined that the inverter 12 is in islanded operation.

[0020] The power conditioner 10 (controller 100) calculates a differential impedance, which is the difference between a positive-sequence impedance obtained by dividing a d-axis positive-sequence voltage included in the positive-sequence voltage Vps on the AC side of the inverter 12 by a d-axis positive-sequence current included in the positive-sequence current Ipo, and a negative-sequence impedance obtained by dividing a d-axis negative-sequence voltage included in the negative-sequence voltage Vns on the AC side of the inverter 12 by a d-axis negative-sequence current included in the negative-sequence current Ino. Furthermore, when an islanding condition is met in which the absolute value of the differential impedance is less than a threshold, the inverter 12 is stopped. 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. Therefore, safety is ensured. Details of the processing by the controller 100 will be described later.

[0021] 2. Controller 2-1. Configuration Example The controller 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) that executes a program stored in the storage device.

[0022] 2-2. Functional Examples There are two functional examples (first functional example and second functional example) for realizing the processing of the controller 100. Below, the first functional example and the second functional example will be described.

[0023] 2 is a block diagram showing a first functional example of the controller 100 according to the embodiment. The controller 100 includes an abc / dq axis calculation unit 110, an islanding operation suppression function unit 120, an islanding operation detection unit 130, a VSG control unit 140, a voltage adjustment unit 150, a voltage control unit 160, a dq / abc axis conversion unit 170, a current control unit 180, and a PWM control unit 190.

[0024] The abc / dq-axis calculation unit 110 converts each of the positive-sequence voltage Vps (detected Vps value), negative-sequence voltage Vns (detected Vns value), positive-sequence current Ipo (detected Ipo value), and negative-sequence current Ino (detected Ino value) from the abc-axis to the dq-axis. The detected Vps value after conversion to the dq-axis is referred to as a d-axis positive-sequence voltage Vpd and a q-axis positive-sequence voltage Vpq. The detected Vns value after conversion to the dq-axis is referred to as a d-axis negative-sequence voltage Vnd and a q-axis negative-sequence voltage Vnq. The detected Ipo value after conversion to the dq-axis is referred to as a d-axis positive-sequence current Ipd and a q-axis positive-sequence current Ipq. The detected Ino value after conversion to the dq-axis is referred to as a d-axis negative-sequence current Ind and a q-axis negative-sequence current Inq.

[0025] The Vs detection value input to the abc / dq axis calculation unit 110 may be acquired, for example, via a PLL (Phase Locked Loop) provided in the controller 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.

[0026] When the inverter 12 is in islanded operation, the islanding suppression function unit 120 suppresses the negative-sequence current Ino on the AC side of the inverter 12. Specifically, the islanding suppression function unit 120 generates a negative-sequence current command Ins_ref for controlling the negative-sequence current Ino based on the Vns detection value and the q-axis negative-sequence current Inq. The negative-sequence current command Ins_ref includes a d-axis negative-sequence current command Indref and a q-axis negative-sequence current command Inqref. The negative-sequence current command Ins_ref is a three-phase component and is also referred to as a three-phase negative-sequence current command Ins_ref. Details of the islanding suppression function unit 120 will be described later.

[0027] The islanding operation detection unit 130 detects the islanding operation of the inverter 12. Specifically, the islanding operation detection unit 130 determines whether the inverter 12 is in islanding operation based on the d-axis positive-sequence voltage Vpd, the d-axis negative-sequence voltage Vnd, the d-axis positive-sequence current Ipd, and the d-axis negative-sequence current Ind. If it is determined that the inverter 12 is in islanding operation, the islanding operation detection unit 130 enables a gate block signal gtb for stopping the inverter 12. If it is determined that the inverter 12 is not in islanding operation, the islanding operation detection unit 130 disables the gate block signal gtb for stopping the inverter 12. Details of the islanding operation detection unit 130 will be described later.

[0028] The VSG control unit 140 executes VSG (Virtual Synchronous Generator) control. VSG is an example of a method for stabilizing the power grid 30, and refers to a virtual synchronous generator that simulates the dynamic characteristics of a synchronous generator in the inverter 12. In other words, the VSG control unit 140 controls a virtual synchronous generator. The VSG control unit 140 calculates a phase command value θref for the output voltage Vs of the inverter 12 using parameters that indicate the dynamic characteristics of the generator. Examples of the parameters include an inertia constant M and a damping constant D.

[0029] The voltage adjustment unit 150 has a function of stabilizing the output voltage Vs of the inverter 12. For example, when the load 50 or the power grid 30 fluctuates, the voltage adjustment unit 150 performs voltage adjustment to stabilize the output voltage Vs of the inverter 12. The voltage adjustment unit 150 is, for example, an automatic voltage regulator (AVR). In order to stabilize the output voltage Vs of the inverter 12, the voltage adjustment unit 150 calculates a d-axis voltage command value Vdref and a q-axis voltage command value Vqref. In the example shown in FIG. 2 , the q-axis voltage command value Vqref is set to zero.

[0030] The voltage control unit 160 generates a d-axis positive-sequence current command value Ipd_ref and a q-axis positive-sequence current command value Ipq_ref based on predetermined parameters. The predetermined parameters include a phase command value θref, a d-axis voltage command value Vdref, and a q-axis voltage command value Vqref. When the q-axis voltage command value Vqref is zero, the q-axis positive-sequence current command value Ipq_ref is zero.

[0031] The dq / abc-axis converter 170 generates a positive-sequence current command Ips_ref based on the d-axis positive-sequence current command value Ipd_ref and the q-axis positive-sequence current command value Ipq_ref. Specifically, the dq / abc-axis converter 170 converts the d-axis positive-sequence current command value Ipd_ref and the q-axis positive-sequence current command value Ipq_ref into the abc-axis. The positive-sequence current command Ips_ref is a three-phase component and is also referred to as a three-phase positive-sequence current command Ips_ref.

[0032] The current control unit 180 calculates a voltage command value Vref for controlling the output current Io of the inverter 12 based on a sum current command (also referred to as a current command value Iref) obtained by adding the three-phase negative-phase current commands Ins_ref and the three-phase positive-phase current commands Ips_ref. The current control unit 180 is also referred to as a current minor loop.

[0033] The PWM control unit 190 generates a PWM signal based on the voltage command value Vref and the gate block signal gtb. Specifically, when the gate block signal gtb is valid, i.e., when the inverter 12 is in an isolated operation state, the PWM control unit 190 generates a PWM signal for stopping the inverter 12. When the gate block signal gtb is invalid, i.e., when the inverter 12 is not in an isolated operation state, the PWM control unit 190 generates a PWM signal for controlling the inverter 12 based on the voltage command value Vref.

[0034] 2-2-2. Second Example FIG. 3 is a block diagram showing a second functional example of the controller 100 according to the embodiment. Here, only the configurations different from the first functional example described above will be described. Specifically, the difference between the first and second functional examples is the presence or absence of a current control unit 180 (current minor loop). In the first functional example, the current control unit 180 calculates a voltage command value Vref based on a current command value Iref obtained by injecting a negative-phase current command Ins_ref into a positive-phase current command Ips_ref. On the other hand, in the second functional example, the voltage command value Vref is obtained by injecting a negative-phase voltage command Vns_ref into a positive-phase voltage command Vps_ref. According to the second functional example, the current control unit 180 is not necessary. Below, the details of each configuration (islanding suppression function unit 120A, voltage control unit 160A, dq / abc axis conversion unit 170A, and PWM control unit 190A) that differs from the first functional example will be described.

[0035] When the inverter 12 is in islanded operation, the islanding operation suppression function unit 120A suppresses the negative-sequence voltage Vns on the AC side of the inverter 12. Specifically, the islanding operation suppression function unit 120A generates a negative-sequence voltage command Vns_ref for controlling the negative-sequence voltage Vns based on the Vns detection value and the q-axis negative-sequence current Inq. The negative-sequence voltage command Vns_ref includes a d-axis negative-sequence voltage command Vndref and a q-axis negative-sequence voltage command Vnqref. The negative-sequence voltage command Vns_ref is a three-phase component and is also referred to as a three-phase negative-sequence voltage command Vns_ref. Details of the islanding operation suppression function unit 120A will be described later.

[0036] The voltage control unit 160A generates a d-axis positive-sequence voltage command value Vpd_ref and a q-axis positive-sequence voltage command value Vpq_ref based on the phase command value θref, the d-axis voltage command value Vdref, and the q-axis voltage command value Vqref. When the q-axis voltage command value Vqref is zero, the q-axis positive-sequence voltage command value Vpq_ref is also zero.

[0037] The dq / abc-axis converter 170A generates a positive-sequence voltage command Vps_ref based on the d-axis positive-sequence voltage command value Vpd_ref and the q-axis positive-sequence voltage command value Vpq_ref. Specifically, the dq / abc-axis converter 170A converts the d-axis positive-sequence voltage command value Vpd_ref and the q-axis positive-sequence voltage command value Vpq_ref into the abc-axis. The positive-sequence voltage command Vps_ref is a three-phase component and is also referred to as a three-phase positive-sequence voltage command Vps_ref.

[0038] The PWM control unit 190A generates a PWM signal based on a sum voltage command (also referred to as a voltage command value Vref) obtained by adding together the three-phase negative-phase voltage commands Vns_ref and the three-phase positive-phase voltage commands Vps_ref, and the gate block signal gtb. An example of the generation of the PWM signal is the same as the first functional example described above, and therefore will not be described here.

[0039] 4 is a block diagram showing a specific example of the islanding operation suppression function unit 120 according to the embodiment. The islanding operation suppression function unit 120 includes an LPF unit 121, a d-axis negative-phase-sequence current command calculation unit 122, and a negative-phase-sequence current control unit 123.

[0040] The LPF unit 121 generates a filtered voltage Vns_f by removing high-frequency components from the negative-phase-sequence voltage Vns (Vns detection value) on the AC side of the inverter 12. For example, a low-pass filter (LPF) is used to remove the high-frequency components from the Vns detection value.

[0041] The d-axis negative-sequence current command calculation unit 122 calculates the d-axis negative-sequence current command Ind_ref based on a differential voltage, which is the difference between the Vns detection value and the filter voltage Vns_f. Specifically, as shown in FIG. 4 , the d-axis negative-sequence current command calculation unit 122 calculates the d-axis negative-sequence current command Ind_ref corresponding to the differential voltage based on information such as a graph showing the relationship between the differential voltage and the d-axis negative-sequence current command Ind_ref. Note that when the differential voltage exceeds a reference voltage value, the d-axis negative-sequence current command Ind_ref may be set to output a maximum current Imax. The maximum current Imax is, for example, the rated value of the output current Io of the inverter 12.

[0042] The negative-sequence current control unit 123 calculates a negative-sequence current command Ins_ref for controlling the negative-sequence current Ino based on the d-axis negative-sequence current command Ind_ref and the q-axis negative-sequence current command Inq_ref. That is, the negative-sequence current control unit 123 performs negative-sequence current control to make the negative-sequence current Ino follow the d-axis negative-sequence current command Ind_ref and the q-axis negative-sequence current command Inq_ref. In the example shown in FIG. 4 , the q-axis negative-sequence current command Inq_ref is set to zero. In this case, the negative-sequence current control unit 123 performs negative-sequence current control to make the negative-sequence current Ino follow the d-axis negative-sequence current command Ind_ref.

[0043] 2-3-2. Second Functional Example FIG. 5 is a block diagram showing a specific example of the islanding suppression function unit 120A according to the embodiment. The only difference in configuration from the above-described islanding suppression function unit 120 is the negative-sequence current control unit 123A. Specifically, the negative-sequence current control unit 123A calculates a negative-sequence voltage command Vns_ref for controlling the negative-sequence voltage Vns based on the d-axis negative-sequence current command Ind_ref and the q-axis negative-sequence current command Inq_ref. That is, the negative-sequence current control unit 123A performs negative-sequence current control to make the negative-sequence current Ino follow the d-axis negative-sequence current command Ind_ref and the q-axis negative-sequence current command Inq_ref. In the example shown in FIG. 5, the q-axis negative-sequence current command Inq_ref is set to zero. In this case, the negative-sequence current control unit 123A performs negative-sequence current control to make the negative-sequence current Ino follow the d-axis negative-sequence current command Ind_ref.

[0044] 6 is a block diagram showing a specific example of the islanding operation detection unit 130 according to the embodiment. The islanding operation detection unit 130 includes a positive-sequence impedance calculation unit 131, a negative-sequence impedance calculation unit 132, and an islanding operation determination unit 133.

[0045] The positive-sequence impedance calculation unit 131 calculates the positive-sequence impedance Rp based on the d-axis positive-sequence voltage Vpd and the d-axis positive-sequence current Ipd. Specifically, the positive-sequence impedance Rp is obtained by dividing the d-axis positive-sequence voltage Vpd by the d-axis positive-sequence current Ipd.

[0046] The negative-sequence impedance calculation unit 132 calculates the negative-sequence impedance Rn based on the d-axis negative-sequence voltage Vnd and the d-axis negative-sequence current Ind. Specifically, the negative-sequence impedance Rn is obtained by dividing the d-axis negative-sequence voltage Vnd by the d-axis negative-sequence current Ind.

[0047] The islanding operation determination unit 133 determines whether the inverter 12 satisfies the islanding operation conditions based on the positive-phase impedance Rp and the negative-phase impedance Rn. If the inverter 12 satisfies the islanding operation conditions, the islanding operation determination unit 133 activates a gate block signal gtb for stopping the inverter 12. If the inverter 12 does not satisfy the islanding operation conditions, the islanding operation determination unit 133 deactivates the gate block signal gtb. If the inverter 12 does not satisfy the islanding operation conditions, that is, if the gate block signal gtb is deactivated, the controller 100 controls the inverter 12 based on the voltage command value Vref.

[0048] The islanding operation condition includes the absolute value of the differential impedance, which is the difference between the positive-phase impedance Rp and the negative-phase impedance Rn, being less than a threshold value Rth. When the inverter 12 is in islanding operation, the absolute value of the differential impedance becomes less than the threshold value Rth, i.e., the absolute value of the differential impedance becomes small, because the negative-phase impedance Rn increases and becomes equal to the positive-phase impedance Rp.

[0049] Furthermore, the isolated-operation determination unit 133 includes a delay processing unit 134. The delay processing unit 134 delays the timing of outputting the gate block signal gtb to the inverter 12. The delay processing unit 134 is, for example, a delay element. The delay processing unit 134 imparts a delay of, for example, several ms.

[0050] 3. Example of Islanding Operation Detection Results FIGS. 7 and 8 are diagrams illustrating example islanding operation detection results according to the embodiment. As shown in FIG. 7 , when islanding of the inverter 12 occurs, the active power P and reactive power Q on the AC side of the inverter 12 change. In this case, as the negative-phase-sequence voltage Vns increases, the d-axis negative-phase-sequence current command Ind_ref calculated by the islanding operation suppression function unit 120 or the islanding operation suppression function unit 120A also increases. When the controller 100 detects islanding of the inverter 12, the gate block signal gtb becomes active, causing the inverter 12 to stop. As a result, the active power P and reactive power Q on the AC side of the inverter 12 transition to zero.

[0051] 8 shows an example of the result of the islanding operation detection unit 130. When islanding operation of the inverter 12 occurs, the positive-phase impedance Rp does not change, but the negative-phase impedance Rn increases. In this case, the absolute value of the differential impedance, which is the difference between the positive-phase impedance Rp and the negative-phase impedance Rn, becomes less than the threshold value Rth, and the gate block signal gtb is enabled (the value of the gate block signal gtb is 1). Therefore, the inverter 12 stops.

[0052] 4. Example of Detection Results During Unbalance FIG. 9 is a diagram illustrating an example of detection results during unbalance according to the embodiment. As shown in FIG. 9 , when an unbalance occurs on the grid side, the negative-phase-sequence voltage Vns increases. In this case, the negative-phase-sequence current I.sub.no increases to a value greater than 0 V and then decreases to near 0 V. The mechanism by which the negative-phase-sequence current I.sub.no increases and then decreases is achieved by removing high-frequency components from the negative-phase-sequence voltage Vns in the islanding suppression function unit 120 or the LPF unit 121 of the islanding suppression function unit 120A. In this way, even when an unbalance occurs on the grid side, the controller 100 can appropriately determine whether the grid side is unbalanced, i.e., whether the inverter 12 is operating in islanded mode. This prevents erroneous determination that the inverter 12 is operating in islanded mode.

[0053] 5. Effects The power conditioner 10 (controller 100) calculates a differential impedance between a positive-sequence impedance Rp, which is obtained by dividing a d-axis positive-sequence voltage Vpd included in the positive-sequence voltage Vps on the AC side of the inverter 12 by a d-axis positive-sequence current Ipd included in the positive-sequence current Ipo, and a negative-sequence impedance Rn, which is obtained by dividing a d-axis negative-sequence voltage Vnd included in the negative-sequence voltage Vns on the AC side of the inverter 12 by a d-axis negative-sequence current Ind included in the negative-sequence current Ino. Furthermore, when an islanding condition is met in which the absolute value of the differential impedance is less than a threshold value Rth, the inverter 12 is stopped. 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.

[0054] Furthermore, the power conditioner 10 (controller 100) generates a voltage command value Vref for controlling the inverter 12 based on a three-phase negative-phase current command Ins_ref and a three-phase positive-phase current command Ips_ref generated based on predetermined parameters, or based on a three-phase negative-phase voltage command Vns_ref and a three-phase positive-phase voltage command Vps_ref generated based on predetermined parameters. Furthermore, when the inverter 12 satisfies the islanding operation condition, the inverter 12 stops. When the inverter 12 does not satisfy the islanding operation condition, the output voltage Vs of the inverter 12 is controlled based on the voltage command value Vref. This suppresses increases in the negative-phase voltage Vns and the negative-phase current Ino during islanding operation of the inverter 12 until the inverter 12 is determined to be in islanding operation and stops. Therefore, it is possible to suppress overcurrents associated with islanding operation of the inverter 12.

[0055] 6. Modification Example Consider a case where the absolute value of the differential impedance fluctuates and becomes less than the threshold value Rth. For example, if the fluctuation is due to an imbalance on the grid side, it is assumed that the absolute value of the differential impedance temporarily becomes less than the threshold value Rth, but then becomes equal to or greater than the threshold value Rth. Therefore, the islanding operation condition may include a state in which the absolute value of the differential impedance remains less than the threshold value Rth for a certain period of time. This allows the execution of the islanding operation determination unit 133 to be delayed until it is determined whether the fluctuation is due to an imbalance on the grid side when the absolute value of the differential impedance fluctuates. This makes it possible to more appropriately determine whether the inverter 12 is in islanding operation.

[0056] REFERENCE SIGNS LIST 1... power conversion system, 10... power conditioner, 11... DC power supply, 12... inverter, 20... transformer, 30... power system, 40... circuit breaker, 50... load, 100... controller

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 controller that controls the inverter, wherein the controller is configured to calculate a differential impedance that is the difference between a positive-sequence impedance obtained by dividing a d-axis positive-sequence voltage included in the positive-sequence voltage of the AC side of the inverter by a d-axis positive-sequence current included in the positive-sequence current, and a negative-sequence impedance obtained by dividing a d-axis negative-sequence voltage included in the negative-sequence voltage of the AC side of the inverter by a d-axis negative-sequence current included in the negative-sequence current, and to stop the inverter when an islanding operation condition is met in which the absolute value of the differential impedance is less than a threshold value.

2. A power conditioner according to claim 1, wherein the controller is configured to: calculate a d-axis negative-sequence current command based on the negative-sequence voltage; calculate three-phase negative-sequence current commands or three-phase negative-sequence voltage commands by performing negative-sequence current control that causes the negative-sequence current to follow the d-axis negative-sequence current command; generate voltage command values ​​for controlling the inverter based on the three-phase negative-sequence current commands and three-phase positive-sequence current commands generated based on predetermined parameters, or based on the three-phase negative-sequence voltage commands and three-phase positive-sequence voltage commands generated based on the predetermined parameters; and control the inverter based on the voltage command values ​​when the inverter does not satisfy the islanding operation condition.

3. A power conditioner according to claim 2, characterized in that the controller is configured to, in generating the voltage command value, generate the voltage command value based on an added current command obtained by adding the three-phase negative-sequence current commands and the three-phase positive-sequence current commands, or to generate as the voltage command value an added voltage command obtained by adding the three-phase negative-sequence voltage commands and the three-phase positive-sequence voltage commands.

4. A power conditioner according to claim 1, wherein the isolated operation condition includes a state in which the absolute value is less than the threshold value continuing for a certain period of time.

5. A power conditioner according to claim 2, characterized in that the d-axis negative-phase-sequence current command is calculated based on a differential voltage that is the difference between the negative-phase-sequence voltage and a voltage from which high-frequency components of the negative-phase-sequence voltage have been removed.

6. A power conditioner according to claim 2, wherein the predetermined parameters include a phase command value for the output voltage of the inverter, a d-axis voltage command value, and a q-axis voltage command value, which are calculated based on parameters indicating the dynamic characteristics of a synchronous generator.

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