Control device, power conversion device

Abstract

To provide a control device capable of satisfying both a request for early detection of isolated operation and a request for ensuring follow-up to system changes.SOLUTION: In a control device (20) for controlling an inverter unit 10, a first reactive power calculation unit (22) calculates a first reactive power that is injected to detect isolated operations. A second reactive power calculation unit (23) calculates a second reactive power that is injected to stabilize the power system (3). A reactive power injection volume calculation unit (24) calculates the amount of reactive power injected into AC power output from the inverter unit (10) based on the first reactive power calculated by the first reactive power calculation unit (22), a change in the first reactive power, the second reactive power calculated by the second reactive power calculation unit (23), and a change in the second reactive power.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present disclosure relates to a control device and a power conversion device that control output power of a distributed power source. [Background technology]

[0002] In recent years, distributed power systems such as photovoltaic power generation systems and power storage systems have become widespread. A distributed power system requires a power conditioner to convert DC power output from a distributed power source into AC power and output the converted AC power to a commercial power system (hereinafter simply referred to as the system). A power conditioner is required to be equipped with an islanding detection function (see, for example, Patent Document 1). The islanding detection function is a function that disconnects a distributed power source from a distribution line to which the distributed power source is connected to the system when power transmission to the distribution line stops. The islanding detection function prevents charging from the distributed power source in a place that should be voltage-free, and prevents electric shock to workers and equipment failure.

[0003] The JEM1498 standard specifies an active islanding detection method to prevent mutual interference of active signals for islanding detection when multiple power conditioners are connected to the grid. This active islanding detection method employs a feedback method with step injection, which encourages frequency shift during islanding by injecting reactive power using the frequency feedback function and reactive power step injection function, thereby detecting islanding at an early stage.

[0004] In recent years, power conditioners (also called smart inverters) have been developed that are equipped with a function to control reactive power output based on reactive power and power factor command values ​​from an upper server and a function to autonomously control reactive power output based on the state of the system voltage and system frequency in order to stabilize the system. For example, IEC61850-90-7 standardizes a method for remotely controlling the active and reactive power output of a power conditioner for distributed power sources according to the system frequency and system voltage. Volt-Var control and dynamic reactive current control are known as control methods for autonomously controlling reactive power output according to the system state. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] JP 2020-78134 A Summary of the Invention [Problem to be solved by the invention]

[0006] In power conditioners equipped with a reactive power injection function for grid stabilization, the reactive power injection for grid stabilization affects the reactive power injection for islanding detection, and islanding may not be detected within the specified time limit.When in islanding mode, the frequency deviates from the reference frequency (50Hz / 60Hz) due to the injection of reactive power, but when reactive power for grid stabilization is injected in the opposite direction, the frequency does not change as intended, and the detection of islanding may be delayed.

[0007] In response to this, it is possible to set the ramp rate of reactive power injection for grid stabilization to a slope that does not affect islanding detection, but in that case, the amount of change in reactive power injection for grid stabilization becomes small, and the ability to follow grid changes decreases.

[0008] The present disclosure has been made in consideration of these circumstances, and has an objective to provide a control device and a power conversion device that meet both the demands for early detection of islanding operation and the demands for ensuring the ability to follow changes in the system. [Means for solving the problem]

[0009] In order to solve the above problems, a control device according to an embodiment of the present disclosure is a control device that controls an inverter unit that converts DC power to AC power and outputs the converted AC power to a power grid, and includes a first reactive power calculation unit that calculates a first reactive power to be injected to detect isolated operation, a second reactive power calculation unit that calculates a second reactive power to be injected to stabilize the power grid, and a reactive power injection amount calculation unit that calculates an injection amount of reactive power to be injected into the AC power output from the inverter unit based on the first reactive power calculated by the first reactive power calculation unit, an amount of change in the first reactive power, the second reactive power calculated by the second reactive power calculation unit, and the amount of change in the second reactive power. Effect of the Invention

[0010] According to the present disclosure, it is possible to satisfy both the requirement for early detection of islanding and the requirement for ensuring the ability to follow changes in the system. [Brief description of the drawings]

[0011] [Figure 1] 1 is a diagram for explaining a power conversion device according to an embodiment; [Diagram 2] FIG. 4 is a diagram illustrating an example of the configuration of a first reactive power calculation unit according to the embodiment. [Diagram 3] FIG. 13 is a diagram illustrating an example of frequency deviation-reactive power characteristics. [Figure 4] FIG. 4 is a diagram illustrating an example of the configuration of a second reactive power calculation unit according to the embodiment. [Diagram 5] 5(a)-(c) are diagrams for explaining a specific example of a reactive power injection function for stabilizing a power grid. [Figure 6] FIG. 13 is a diagram for explaining yet another specific example of the reactive power injection function for stabilizing the power system. [Figure 7] 7(a)-(b) are diagrams for explaining the problems that arise when the reactive power injection function for detecting an isolated operation and the reactive power injection function for stabilizing the grid are used together. [Figure 8] 10 is a flowchart for explaining a method for calculating an injection amount of a target reactive power according to a comparative example. [Figure 9] 5 is a flowchart for explaining a method for calculating an injection amount of a target reactive power according to an embodiment. [Figure 10] FIG. 13 is a graph illustrating a method for determining a target reactive power according to a fourth specific example. [Figure 11] 11(a)-(b) are diagrams showing specific examples of a case in which a first reactive power injected in a stepwise manner by the reactive power injection function for detecting isolated operation interferes with a second reactive power injected in accordance with an externally set reactive power command value. [Figure 12] 12(a)-(d) are diagrams showing specific examples of reactive power determination methods in the states shown in FIGS. 11(a)-(b). [Figure 13] 10 is a diagram for explaining the significance of setting a second observation condition that is satisfied earlier in time than a first observation condition of a system state used for calculating a first reactive power. FIG. [Figure 14] 14(a)-(c) are diagrams showing specific examples of change conditions when the second reactive power is corrected based on the second measurement value of the frequency deviation. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0012] 1 is a diagram for explaining a power conversion device 2 according to an embodiment. The power conversion device 2 converts DC power supplied from a distributed power source 1 into AC power, and outputs the converted AC power to a grid 3 or a load 4 via a distribution board (not shown). The distributed power source 1 corresponds to a solar cell, a storage battery, a fuel cell, or the like. When a solar cell is used, a DC / DC converter (boost chopper) is provided between the solar cell and an inverter unit 10 of the power conversion device 2, and the DC / DC converter executes MPPT (Maximum Power Point Tracking) control.

[0013] A plurality of distributed power sources 1 may be connected in parallel to the power conversion device 2. For example, a solar cell and a storage battery may be connected in parallel. In this case, a DC / DC converter is provided between the solar cell and the inverter unit 10, and between the storage battery and the inverter unit 10.

[0014] The power conversion device 2 includes an inverter unit 10, a control unit 20, a system frequency detection unit 31, a system voltage detection unit 32, and a communication unit 40. The control unit 20 includes a reactive power injection unit 21, a current control unit 25, and an isolated operation detection unit 26. The reactive power injection unit 21 includes a first reactive power calculation unit 22, a second reactive power calculation unit 23, and a reactive power injection amount calculation unit 24.

[0015] The configuration of the control unit 20 can be realized by a combination of hardware and software resources, or by hardware resources alone. Analog elements, microcomputers, DSPs, ROMs, RAMs, ASICs, FPGAs, and other LSIs can be used as hardware resources. Programs such as firmware can be used as software resources.

[0016] The inverter unit 10 converts the DC power supplied from the distributed power source 1 into AC power, and outputs the converted AC power to the load 4 or the grid 3. If the distributed power source 1 includes a storage battery, the inverter unit 10 can also convert the AC power supplied from the grid 3 into DC power, and output the converted DC power to the storage battery.

[0017] The system frequency detection unit 31 detects the frequency of the system 3 by hardware and outputs it to the control unit 20. Specifically, the system frequency detection unit 31 converts a sine wave system voltage into a square wave system voltage. The system frequency detection unit 31 determines the period as the time difference between the intermediate value between the falling edge and rising edge of the square wave and the intermediate value between the next falling edge and rising edge. The system voltage detection unit 32 detects the voltage of the system 3 by hardware and outputs it to the control unit 20.

[0018] The communication unit 40 receives a command value of reactive power or power factor for system stabilization from an external server 5 via a network. The external server 5 may be, for example, a server managed and operated by an aggregator that provides a demand response service, a VPP (Virtual Power Plant) service, etc. The communication unit 40 outputs the command value received from the server 5 to the control unit 20. The communication unit 40 may be provided inside or outside the power conversion device 2. Note that when the power conversion device 2 and the external server 5 are not connected, the communication unit 40 can be omitted.

[0019] 2 is a diagram showing an example of the configuration of the first reactive power calculation unit 22 according to the embodiment. The first reactive power calculation unit 22 includes a first frequency deviation calculation unit 221, a harmonic voltage calculation unit 222, a reactive power oscillation suppression control unit 223, a first active standby release condition determination unit 224, a first step injection condition determination unit 225, a frequency feedback injection reactive power calculation unit 226, a step injection reactive power calculation unit 227, and a reactive power determination unit 228.

[0020] The first frequency deviation calculation unit 221 calculates the frequency deviation of the system voltage. In this embodiment, the frequency deviation is calculated in accordance with the JEM1498 standard. The first frequency deviation calculation unit 221 calculates the frequency deviation based on the difference between the past and most recent cycles of the system voltage detected by the system frequency detection unit 31. The first frequency deviation calculation unit 221 updates the cycle data for each cycle. The current cycle is called the most recent cycle. As the past cycle, a moving average value for 320 ms at a point in time 200 ms before the most recent cycle is used. As the most recent cycle, a moving average value for 40 ms after the most recent cycle is used.

[0021] The first frequency deviation calculation unit 221 calculates the moving average value and the frequency deviation every 5 ms. The first frequency deviation calculation unit 221 outputs the calculated frequency deviation to the frequency feedback injection reactive power calculation unit 226, the reactive power oscillation suppression control unit 223, and the first step injection condition determination unit 225.

[0022] The frequency feedback injection reactive power calculation unit 226 calculates reactive power for frequency feedback injection based on the frequency deviation calculated by the first frequency deviation calculation unit 221, with reference to the frequency deviation-reactive power characteristics.

[0023] 3 is a diagram showing an example of frequency deviation-reactive power characteristics. The frequency feedback injection reactive power calculation unit 226 changes the gain used when calculating reactive power when the frequency deviation reaches the boundary of ±0.01 Hz. When the system frequency is 50 Hz, a frequency deviation of ±0.01 Hz corresponds to a periodic deviation of ±4.0 μs. When the system frequency is 60 Hz, a frequency deviation of ±0.01 Hz corresponds to a periodic deviation of ±2.8 μs.

[0024] The example shown in FIG. 3 shows a case where the rated capacity of the power conversion device 2 is 4 kVA and the upper and lower limits of the reactive power injection amount are set to ±0.25 pu, and the upper and lower limits of the reactive power injection amount are ±1 kvar. pu (per unit) indicates a ratio to the rated capacity. The upper and lower limits of the reactive power injection amount by frequency feedback may be set within the range of ±0.25 pu. Reactive power injection by frequency feedback is performed within a half cycle of the system frequency after the detection of the frequency deviation. Also, if the reactive power oscillation suppression control unit 223 has determined that the active function is in standby state (details will be described later), the reactive power injection amount by frequency feedback is set to 0 p.u. regardless of the frequency deviation.

[0025] The harmonic voltage calculation unit 222 calculates the harmonic voltage of the system voltage. In this embodiment, the harmonic voltage is calculated in accordance with the JEM1498 standard. The second to seventh harmonics are used to calculate the harmonic voltage. The following discrete Fourier analysis may be used to calculate the harmonic voltage. A[h]=Σ(x[nT]cos[2πhf1(nT)]) / (M / 2) (Equation 1) B[h]=Σ(x[nT]sin[2πhf1(nT)]) / (M / 2) (Equation 2) H arm [h]=√(A 2 [h]+B 2 [h])...(Formula 3) n: Sampling points (M) n=0,1,2,...,M-1 f1: Fundamental frequency f1=1 / (MT)=f s / M f s :Sampling frequency T: Sampling interval T=1 / f s h: Harmonic order h=0,1,2,...,M / 2 x[nT]: System voltage A[h]: Amplitude of the h-th cosine B[h]: Amplitude of h-th sine H arm [h]: h-th harmonic voltage (peak voltage)

[0026] The harmonic voltage calculation unit 222 outputs the calculated harmonic voltage to the first step injection condition determination unit 225, the first active standby release condition determination unit 224, and the second reactive power calculation unit .

[0027] The step injection function is a function that changes the frequency by injecting step-like reactive power in order to enable detection of islanding even when the output power of the power conversion device 2 and the power consumption of the load 4 are in balance. If islanding occurs when the output power of the power conversion device 2 and the power consumption of the load 4 are in balance, the frequency feedback function alone cannot change the frequency, and the step injection function is required.

[0028] If the frequency deviation is within 0.01 Hz and all of the following conditions (Equation 4) to (Equation 9) are satisfied, the first step injection condition determination unit 225 instructs the reactive power calculation unit for step injection 227 to start step injection. If the frequency deviation is not within 0.01 Hz or if any one of the following conditions (Equation 4) to (Equation 9) is not satisfied, the first step injection condition determination unit 225 does not instruct the reactive power calculation unit for step injection 227 to start step injection. (Nh0-Nh_ave)>2V (Formula 4) (Nh1-Nh_ave)>2V (Formula 5) (Nh2-Nh_ave)>-0.5V (Formula 6) -0.5V<(Nh3-Nh_ave)<0.5V (Formula 7) -0.5V<(Nh4-Nh_ave)<0.5V (Formula 8) -0.5V<(Nh5-Nh_ave)<0.5V (Formula 9) Nhi: Second to seventh harmonic voltage effective values ​​before i cycles Nh_ave: Average value of three harmonic voltages from three cycles ago to five cycles ago

[0029] Furthermore, the first step injection condition determination unit 225 may instruct the reactive power calculation unit for step injection 227 to start step injection when the frequency deviation is within 0.01 Hz and all of the conditions in the following (Equation 10) to (Equation 15) are satisfied. When the frequency deviation is not within 0.01 Hz or when any one of the conditions in the following (Equation 10) to (Equation 15) is not satisfied, the first step injection condition determination unit 225 does not instruct the reactive power calculation unit for step injection 227 to start step injection. (Nb0-Nb_ave)>2.5V (Formula 10) (Nb1-Nb_ave)>2.5V (Formula 11) (Nb2-Nb_ave)>-0.5V (Formula 12) -0.5V<(Nb3-Nb_ave)<0.5V (Equation 13) -0.5V<(Nb4-Nb_ave)<0.5V (Equation 14) -0.5V<(Nb5-Nb_ave)<0.5V (Equation 15) Nbi: Fundamental voltage before i cycles Nb_ave: Average value of three fundamental voltages from 3 to 5 cycles ago

[0030] In addition, when the reactive power oscillation suppression control unit 223 judges that the active function is in standby state (details will be described later), no step injection is performed even if the fundamental wave voltage conditions of (Equation 10) to (Equation 15) above are satisfied.

[0031] When receiving an instruction to start step injection from the first step injection condition determination unit 225, the reactive power calculation unit 227 for step injection calculates reactive power for step injection. Reactive power for step injection is injected within half a cycle of the system frequency after the step injection condition is satisfied. The injection time of reactive power for step injection is set to 3 cycles or less. The upper limit of the amount of reactive power injected for step injection is set to 0.1 pu. Reactive power for step injection is injected in a direction that delays the current phase as viewed from the power conversion device 2 (in the direction of decreasing frequency).

[0032] The reactive power determination unit 228 determines a first reactive power from the reactive power for frequency feedback injection and the reactive power for step injection, and outputs it to the reactive power injection amount calculation unit 24 and the second reactive power calculation unit 23.

[0033] The reactive power oscillation suppression control unit 223 performs control to suppress oscillation of reactive power injected by frequency feedback. Specifically, the reactive power oscillation suppression control unit 223 controls state transition between an active function normal state and an active function standby state. In the active function standby state, the amount of reactive power injected by frequency feedback is set to zero.

[0034] When a large number of power conversion devices 2 are connected to the system 3, voltage flicker (fluctuations in system voltage of about 6 to 7 Hz) may occur due to injection of reactive power by frequency feedback, depending on the line impedance of the distribution line and the consumer load. When a sign of flicker occurrence due to oscillation of reactive power is detected, the reactive power oscillation suppression control unit 223 sets the amount of reactive power injection by frequency feedback to 0.

[0035] When the reactive power oscillation suppression control unit 223 detects a sudden frequency change or continued low amplitude frequency fluctuation as disturbance detection, it transitions from the active function normal state to the active function standby state. The reactive power oscillation suppression control unit 223 continuously monitors the sudden frequency change and the low amplitude frequency fluctuation while frequency feedback control is being executed, regardless of whether the active function is in the normal state or the active function standby state. The reactive power oscillation suppression control unit 223 judges the sudden frequency change and the continuation of the low amplitude frequency fluctuation every 5 ms in synchronization with the calculation of the frequency deviation.

[0036] The reactive power oscillation suppression control unit 223 detects a sudden frequency change when the polarity of the frequency deviation is reversed or becomes 0 after the frequency deviation exceeds ±0.16 Hz. When the reactive power oscillation suppression control unit 223 detects a sudden frequency change, it transitions from the active function normal state to the active function standby state, and increments the value of a standby transition counter (upper limit value=100, lower limit value=0) that manages the number of standby transitions by one. If the value of the standby transition counter is 2 or less, the reactive power oscillation suppression control unit 223 sets the value of the standby transition counter to 3. Note that when a sudden frequency change is detected in the active function standby state, the reactive power oscillation suppression control unit 223 does not increment the value of the standby transition counter.

[0037] When the reactive power oscillation suppression control unit 223 detects a frequency deviation exceeding 0.01 Hz and having the opposite polarity to the detected frequency deviation within 165 ms after detecting a frequency deviation exceeding ±0.01 Hz, the reactive power oscillation suppression control unit 223 increments the value of the low amplitude frequency deviation counter by 1. When the value of the low amplitude frequency deviation counter reaches 12, the reactive power oscillation suppression control unit 223 detects the continuation of low amplitude frequency fluctuation. Upon detecting the continuation of low amplitude frequency fluctuation, the reactive power oscillation suppression control unit 223 returns the value of the low amplitude frequency deviation counter to 0 and increments the value of the standby transition counter by 1.

[0038] When the reactive power oscillation suppression control unit 223 does not detect a frequency deviation exceeding 0.01 Hz and having the opposite polarity to the detected frequency deviation within 165 ms after detecting a frequency deviation exceeding ±0.01 Hz, it resets the value of the low amplitude frequency deviation counter to 0.

[0039] After detecting a frequency deviation exceeding ±0.01 Hz, the reactive power oscillation suppression control unit 223 resets the value of the low amplitude frequency deviation counter to 1 when the frequency deviation becomes ±0.01 Hz and exceeds 0.01 Hz again with the same polarity as when it exceeded ±0.01 Hz.

[0040] The first active standby release condition determination unit 224 determines whether or not a condition for returning to the active function normal state (a release condition for the active function standby state) is satisfied in the active function standby state.

[0041] The first active standby release condition determination unit 224 transitions from the active function standby state to the active function normal state when the harmonic voltage calculated by the harmonic voltage calculation unit 222 satisfies all of the conditions of the following (Equation 16)-(Equation 17). (Nh0-Nh_ave0)>α...(Equation 16) (Nh1-Nh_ave0)>α...(Equation 17) Nhi: Second to seventh harmonic voltage effective values ​​before i cycles Nh_avei: The average value of three harmonic voltages from three cycles before to five cycles before i cycles α is a threshold value of the latest cycle calculated by auto-tuning, and is defined by the following (Equation 18). α=max[(Nhi-Nh_avei)]+1 (Equation 18) α is the voltage value obtained by adding 1 to the maximum value of (Nhi-Nh_avei) (if the grid frequency is 50 Hz, i = 5 to 50; if the grid frequency is 60 Hz, i = 6 to 60) in the period from 1 second before the latest cycle to 0.1 second before (900 ms). The minimum value of α calculated by auto-tuning is 2 [V].

[0042] The first active standby release condition determination unit 224 calculates the harmonic voltage effective value deviation (Nhi-Nh_avei) for each predetermined period (for example, 100 ms). When calculating the threshold value α at 100 ms intervals, the first active standby release condition determination unit 224 holds the maximum value for the 100 ms period as X0. The first active standby release condition determination unit 224 holds the maximum value for the 100 ms period for the past 900 ms (X0 to X -8 ) and calculates the largest 100 ms period maximum value. The first active standby release condition determination unit 224 uses the calculated 100 ms period maximum value as the 100 ms period maximum value for the next 100 ms period.

[0043] The maximum value of the 100 ms period (X -1 ~X -9 ) is the maximum value (X0 to X) of the past 900 ms (including the maximum value of the current 100 ms) -8 ) is the same interval. Therefore, the maximum value of the 100 ms period in the past 900 ms (X0 to X -8 ) is calculated and taken as the maximum value after the next 100 ms period.

[0044] The first active standby release condition determination unit 224 sets the threshold value α to (the maximum value of the 100 ms period for the past 900 ms)+1 [V], excluding the maximum value of the 100 ms period of the most recent 100 ms period.

[0045] In addition, the first active standby release condition determination unit 224 also transitions from the active function standby state to the active function normal state if the value of the standby transition counter is 2 or less and both the sudden frequency change and the continued low-amplitude frequency fluctuation are not detected for 5 seconds.

[0046] In addition, the first active standby release condition determination unit 224 also transitions from the active function standby state to the active function normal state if the value of the standby transition counter is 3 and both the sudden frequency change and the continued low-amplitude frequency fluctuation have not been detected for 5 minutes.

[0047] When both the sudden frequency change and the continuation of the low amplitude frequency fluctuation are not detected for 5 minutes, the first active standby release condition determination unit 224 decrements the value of the standby transition counter by 1. The value of the standby transition counter may be cleared to 0 after the power conversion device 2 is stopped or when the power conversion device 2 is restarted.

[0048] The reactive power oscillation suppression control unit 223 and the first active standby release condition determination unit 224 output the occurrence of a state transition to the frequency feedback injection reactive power calculation unit 226, the first step injection condition determination unit 225 and the second reactive power calculation unit .

[0049] 4 is a diagram showing a configuration example of the second reactive power calculation unit 23 according to the embodiment. The second reactive power calculation unit 23 includes a grid stabilization reactive power calculation unit 231, a second frequency deviation calculation unit 232, a condition determination unit 233, and a grid stabilization reactive power correction unit 234. The condition determination unit 233 includes a change direction determination unit 233a, a frequency deviation condition determination unit 233b, a second step injection condition determination unit 233c, and a second active standby release condition determination unit 233d. The grid stabilization reactive power calculation unit 231 calculates reactive power for preventing an increase and decrease in the grid voltage and stabilizing the grid voltage.

[0050] 5(a)-(c) are diagrams for explaining a specific example of a reactive power injection function for system stabilization. FIG. 5(a) shows an example in which a command value for reactive power is received from the server 5 to control the output of reactive power. Note that a user may set the command value for reactive power from a remote control setting device. In the example shown in FIG. 5(a), when the command value for reactive power is changed to a lower value, the system stabilization reactive power calculation unit 231 gradually reduces the output of reactive power to the changed command value according to the set ramp rate [var / s].

[0051] Fig. 5(b) shows an example of Volt-Var control that changes the output of reactive power according to the system voltage detected by the system voltage detection unit 32. In Volt-Var control, capacitive (leading) reactive power is injected within a certain range where the system voltage is lower than the reference voltage (200V in Fig. 5(b)). Conversely, inductive (lagging) reactive power is injected within a certain range where the system voltage is higher than the reference voltage.

[0052] 5(c) shows an example of dynamic reactive current control that changes the output of reactive current in response to a change in the system voltage detected by the system voltage detection unit 32. In the dynamic reactive current control, a leading reactive current is caused to flow to the system 3 in response to a sudden drop in the system voltage. Conversely, a lagging reactive current is caused to flow to the system 3 in response to a sudden rise in the system voltage.

[0053] Fig. 6 is a diagram for explaining yet another specific example of the reactive power injection function for grid stabilization. Fig. 6 shows an example in which a command value for the power factor is received from the server 5 to control the output of reactive power. Note that the user may set the command value for the power factor from a remote control setting device. The grid stabilization reactive power calculation unit 231 fixes the power factor to the set value and controls the output of reactive power.

[0054] 6, when the command value of the power factor is changed to a lower value, the system stabilization reactive power calculation unit 231 gradually increases the reactive power output to a value corresponding to the changed power factor according to the set ramp rate [var / s]. Since the power factor is fixed, when the active power decreases due to a load fluctuation or the like, the system stabilization reactive power calculation unit 231 decreases the reactive power output according to the power factor, and when the active power increases, the system stabilization reactive power calculation unit 231 increases the reactive power output according to the power factor.

[0055] Other components of the second reactive power calculation unit 23 will be described later, returning to Fig. 1. The description will be continued on the assumption that the reactive power calculated by the system stabilization reactive power calculation unit 231 is output as it is to the reactive power injection amount calculation unit 24 in Fig. 1 as the second reactive power.

[0056] The reactive power injection amount calculation unit 24 calculates the injection amount of reactive power (hereinafter referred to as target reactive power as appropriate) to be injected into the AC power output from the inverter unit 10, based on the first reactive power calculated by the first reactive power calculation unit 22 and the second reactive power calculated by the second reactive power calculation unit 23. In the simplest example, the reactive power injection amount calculation unit 24 adds the first reactive power calculated by the first reactive power calculation unit 22 and the second reactive power calculated by the second reactive power calculation unit 23 to calculate the injection amount of the target reactive power.

[0057] The current control unit 25 generates a current command value based on the final reactive power injection amount obtained by adding the target reactive power injection amount input from the reactive power injection amount calculation unit 24 to the reactive power injection amount according to the set operating power factor.

[0058] As basic control, the current control unit 25 generates a current command value so that the input voltage (voltage of the DC bus) of the inverter unit 10 maintains the target voltage. In other words, the current command value is generated so that the amount of power input from the distributed power source 1 to the power conversion device 2 and the amount of power output from the power conversion device 2 to the load 4 or the grid 3 are balanced. The current control unit 25 adjusts the current command value generated by basic control in accordance with the final injection amount of reactive power. The current control unit 25 outputs the adjusted current command value to the drive unit of the inverter unit 10 to control the duty ratio of multiple switching elements included in the inverter unit 10.

[0059] The islanding operation detection unit 26 monitors the frequency of the system 3, and when a change in the system frequency exceeds a threshold value for detecting an islanding operation, it determines that an islanding operation state has occurred and stops the inverter unit 10. The islanding operation detection unit 26 may detect an islanding operation using a change in frequency detected by the system frequency detection unit 31, or may detect an islanding operation using a frequency deviation calculated by the first frequency deviation calculation unit 221.

[0060] Figures 7(a)-(b) are diagrams for explaining the issues that arise when the reactive power injection function for islanding detection and the reactive power injection function for system stabilization are used together. Figure 7(a) is a diagram showing an example of the transition of frequency and reactive power injection amount when only the reactive power injection function for islanding detection is operating. When the injection of leading reactive power by frequency feedback starts upon detection of frequency deviation, the increase of the system frequency is promoted. When the frequency change exceeds a threshold value, islanding is detected.

[0061] FIG. 7(b) is a diagram showing an example of the transition of the frequency and the reactive power injection amount when both the reactive power injection function for islanding detection and the reactive power injection function for system stabilization are operating. As in FIG. 7(a), the injection of leading reactive power by frequency feedback control is started upon detection of the frequency deviation. In the example shown in FIG. 7(b), the injection of lagging reactive power by Volt-Var control is started in response to the increase in the system frequency and system voltage due to the injection of leading reactive power by frequency feedback control. This injection of lagging reactive power by Volt-Var control offsets a part of the injection amount of leading reactive power by frequency feedback control. This slows down the change in the system frequency, and the detection of islanding is delayed. In other words, the injection of reactive power for system stabilization affects the injection of reactive power for islanding detection, and there are cases where islanding cannot be detected within the specified time limit.

[0062] In response to this, it is conceivable to set the ramp rate of reactive power injection for system stabilization to a gradient that does not affect islanding detection. The ramp rate is a parameter that specifies the rate of change [var / s] for suppressing abrupt changes in reactive power. However, if the rate of change of reactive power is suppressed by the ramp rate, the amount of change in reactive power injection for system stabilization becomes smaller, and the ability to follow system changes decreases. This embodiment provides a power conversion device 2 in which measures have been taken against the decrease in the ability to follow system changes in the reactive power injection function for system stabilization.

[0063] First, an example will be described in which a measure against a decrease in the ability to follow a system change in a reactive power injection function for system stabilization is taken by the reactive power injection amount calculation unit 24. As described above, the reactive power injection amount calculation unit 24 calculates a target reactive power based on the first reactive power calculated by the first reactive power calculation unit 22 and the second reactive power calculated by the second reactive power calculation unit 23.

[0064] In the first calculation method of the target reactive power, the reactive power injection amount calculation unit 24 calculates the target reactive power by adding the first reactive power calculated by the first reactive power calculation unit 22 and the second reactive power calculated by the second reactive power calculation unit 23.

[0065] 8 is a flowchart for explaining a method for calculating the injection amount of target reactive power according to the comparative example. The first reactive power calculation unit 22 calculates the first reactive power based on the reactive power for frequency feedback injection, or based on the reactive power for frequency feedback injection and the reactive power for step injection (S10). The second reactive power calculation unit 23 calculates the second reactive power for stabilizing the system voltage (S11). The reactive power injection amount calculation unit 24 calculates the target reactive power based on the first reactive power calculated by the first reactive power calculation unit 22 and the second reactive power calculated by the second reactive power calculation unit 23 (for example, by adding them together) (S12).

[0066] 9 is a flowchart for explaining a method for calculating the injection amount of target reactive power according to the embodiment. The first reactive power calculation unit 22 calculates the first reactive power based on the reactive power for frequency feedback injection, or based on the reactive power for frequency feedback injection and the reactive power for step injection (S20). The second reactive power calculation unit 23 calculates the second reactive power for stabilizing the system voltage (S21).

[0067] The reactive power injection amount calculation unit 24 calculates the amount of change in the first reactive power calculated by the first reactive power calculation unit 22 (S22). The reactive power injection amount calculation unit 24 calculates the amount of change in the second reactive power calculated by the second reactive power calculation unit 23 (S23). The reactive power injection amount calculation unit 24 identifies the relationship between the change direction of the first reactive power and the change direction of the second reactive power (S24). The reactive power injection amount calculation unit 24 calculates a target reactive power based on the first reactive power, the second reactive power, and the relationship between the change directions of both (S25). Below, a specific example of a method for determining the target reactive power taking into account the relationship in the change directions will be described.

[0068] In a first specific example, when the first reactive power and the second reactive power are changing in the same direction, the reactive power injection amount calculation unit 24 selects the reactive power with the larger change between the first reactive power and the second reactive power as the target reactive power.

[0069] In the second specific example, when the first reactive power and the second reactive power change in the same direction, the reactive power injection amount calculation unit 24 selects the first reactive power as the target reactive power from among the first reactive power and the second reactive power.

[0070] In the third specific example, when the first reactive power and the second reactive power change in the same direction and the reactive power with the larger change amount of the first reactive power and the second reactive power is within the range of the upper limit and lower limit of the first reactive power, the reactive power injection amount calculation unit 24 selects the reactive power with the larger change amount. When the first reactive power and the second reactive power change in the same direction and the reactive power with the larger change amount of the first reactive power and the second reactive power exceeds the range of the upper limit and lower limit of the first reactive power, if it exceeds in the leading direction, the reactive power injection amount calculation unit 24 determines the upper limit value of the first reactive power as the target reactive power, and if it exceeds in the lagging direction, the lower limit value of the first reactive power as the target reactive power.

[0071] In this embodiment, the upper and lower limits of the first reactive power are set to ±2.5 pu (±1 kvar in the example shown in Fig. 3 above). That is, when the reactive power having the larger change amount between the first reactive power and the second reactive power exceeds the upper or lower limit of the first reactive power, the target reactive power is clipped to +1 kvar or -1 kvar.

[0072] In the fourth specific example, the reactive power injection amount calculation unit 24 selects the first reactive power when the absolute value of the first reactive power is changing in an increasing direction (an increasing direction in a leading phase or a decreasing direction in a lagging phase), and selects the second reactive power when the absolute value of the first reactive power is changing in a decreasing direction (a decreasing direction in a leading phase or an increasing direction in a lagging phase).

[0073] 10 is a graph showing a method for determining a target reactive power according to a fourth specific example. When the first reactive power is in an increasing direction with a leading phase (1) or in a decreasing direction with a lagging phase (2), the first reactive power is selected. When the first reactive power is in a decreasing direction with a leading phase (3) or in an increasing direction with a lagging phase (4), the second reactive power is selected.

[0074] In the fifth specific example, the reactive power injection amount calculation unit 24 selects the first reactive power when the first reactive power and the second reactive power are changing in the opposite directions and the absolute value of the first reactive power is changing in the increasing direction (the increasing direction in the leading phase or the decreasing direction in the lagging phase), and selects the second reactive power when the absolute value of the first reactive power is changing in the decreasing direction (the decreasing direction in the leading phase or the increasing direction in the lagging phase). When the first reactive power and the second reactive power are changing in the opposite directions, the target reactive power is determined by any of the first to third specific examples.

[0075] Next, an example will be described in which a measure against a decrease in the ability to follow a system change in a reactive power injection function for system stabilization is taken by the second reactive power calculation unit 23. In the example shown below, the reactive power injection amount calculation unit 24 includes a calculation method of a target reactive power according to a comparative example shown in Fig. 8, and may adopt any of the calculation methods of the target reactive power described above.

[0076] The second reactive power calculation unit 23 calculates the second reactive power so that the reactive power injection function for detecting isolated operation is affected by interference from the reactive power injection function for stabilizing the grid. The second reactive power calculation unit 23 imposes a limit on the calculated value of the second reactive power according to the observation condition of the grid state. When the first reactive power and the second reactive power are changing in opposite directions, the second reactive power calculation unit 23 imposes a limit on the calculated value of the second reactive power according to the observation condition of the grid state, and when the first reactive power and the second reactive power are changing in the same direction, the second reactive power calculation unit 23 does not impose a limit on the calculated value of the second reactive power.

[0077] When the second reactive power calculation unit 23 imposes a limit on the calculated value of the second reactive power, the second reactive power calculation unit 23 may set the calculated value of the second reactive power to 0var. When the second reactive power calculation unit 23 imposes a limit on the calculated value of the second reactive power, the second reactive power calculation unit 23 may impose a limit on the calculated value of the second reactive power so as not to exceed an amount of change that can ignore the effect on the islanding operation detection function. When the second reactive power calculation unit 23 imposes a limit on the calculated value of the second reactive power, the second reactive power calculation unit 23 may impose a limit on the calculated value of the second reactive power in a stepwise manner within a range from an unlimited amount of change to an amount of change that can ignore the effect on the islanding operation detection function.

[0078] As an amount of change that can be used to ignore the effect on the islanding detection function, for example, a value approved by the General Rules for Grid Interconnection Test Methods can be used. The General Rules for Grid Interconnection Test Methods stipulate that the power factor switching test by power flow should be performed in 80 to 100 seconds to switch between a power factor of 1 and a power factor of 0.8. In this case, the amount of reactive power change between a power factor of 1.0 during forward flow and a power factor of 0.8 during reverse flow is the maximum amount of change.

[0079] When the power factor is 1.0 (maximum power factor) during forward flow, the reactive power is 0 p.u. = 0 var, and when the power factor is 0.8 (minimum power factor) during reverse flow, the reactive power is 0.6 pu. From the above, the amount of change in reactive power per system cycle is expressed by the following (Equation 19). 0.6pu÷50Hz÷80s=0.00015pu / s (Equation 19) 0.6 (=0.6-0.0) pu is the maximum reactive power change 50 Hz has fewer cycles than 60 Hz, and 80 s is the shortest change time, so the maximum change amount is 0.00015 pu / s.

[0080] 0.00015 pu is significantly smaller than the ±0.25 pu upper and lower limits of the amount of reactive power injection by frequency feedback and the 0.1 pu upper limit of the amount of reactive power injection by step injection, and is a change amount whose impact on the islanding detection function can be ignored.

[0081] A specific description will be given below with reference back to Fig. 4. The system stabilization reactive power calculation unit 231 calculates the second reactive power based on an externally set command value of reactive power, an externally set command value of power factor, Volt-Var control, or dynamic reactive current control.

[0082] The change direction determination unit 233a compares the change direction of the first reactive power calculated by the first reactive power calculation unit 22 with the change direction of the second reactive power calculated by the grid stabilization reactive power calculation unit 231. When the change directions of the two are opposite, the grid stabilization reactive power correction unit 234 corrects the second reactive power calculated by the grid stabilization reactive power calculation unit 231 to a value with a lower degree of influence on the isolated operation detection function based on a second observation condition that is more relaxed than the first observation condition of the grid state used to calculate the first reactive power. The second observation condition is set to a condition that is activated earlier in time than the first observation condition of the grid state used to calculate the first reactive power. Specific examples of the first observation condition and the second observation condition will be described later.

[0083] When the first reactive power and the second reactive power change in opposite directions and the system state satisfies the second observation condition, the system stabilization reactive power correction unit 234 limits the amount of change in the second reactive power so as not to exceed an amount of change that can ignore the effect on the islanding detection function. The system stabilization reactive power correction unit 234 outputs the corrected second reactive power to the reactive power injection amount calculation unit 24.

[0084] When the change direction of the first reactive power and the change direction of the second reactive power are the same, the system stabilization reactive power correction unit 234 does not correct the second reactive power calculated by the system stabilization reactive power calculation unit 231 regardless of the second observation condition of the system state. The system stabilization reactive power correction unit 234 outputs the second reactive power calculated by the system stabilization reactive power calculation unit 231 to the reactive power injection amount calculation unit 24 as it is.

[0085] 11(a)-(b) are diagrams showing specific examples of a case in which a first reactive power injected in a stepwise manner by the reactive power injection function for detecting isolated operation interferes with a second reactive power injected in accordance with an externally set reactive power command value.

[0086] Figures 12(a)-(d) are diagrams showing specific examples of reactive power determination methods in the states shown in Figures 11(a)-(b). Figure 12(a) shows an example of calculating target reactive power by adding the second reactive power calculated by the grid stabilization reactive power calculation unit 231 directly to the first reactive power calculated by the first reactive power calculation unit 22. In this example, the maximum amount of change in the second reactive power calculated by the grid stabilization reactive power calculation unit 231 is reflected directly in the target reactive power.

[0087] 12(b) shows an example in which the amount of change in the second reactive power calculated by the system stabilization reactive power calculation unit 231 is limited and added to the first reactive power calculated by the first reactive power calculation unit 22. In this example, when the second observation condition for the system state is satisfied, the amount of change in the second reactive power is limited to an amount of change that allows negligible effect on the islanding detection function (0.00015 [pu / s] in the above example).

[0088] 12(c) shows another example in which the amount of change in the second reactive power calculated by the system stabilization reactive power calculation unit 231 is limited and added to the first reactive power calculated by the first reactive power calculation unit 22. In this example, a plurality of stepwise second observation conditions are set. When the second observation condition of each step is satisfied, the amount of change in the second reactive power is stepwise limited within a range from unlimited to a change amount where the effect on the islanding detection function can be ignored.

[0089] 12(d) shows yet another example in which a change amount of the second reactive power calculated by the system stabilization reactive power calculation unit 231 is limited and added to the first reactive power calculated by the first reactive power calculation unit 22. In this example, the second reactive power is limited to 0 var when a second observation condition of the system state is satisfied.

[0090] FIG. 13 is a diagram for explaining the significance of setting a second observation condition that is satisfied earlier in time than the first observation condition of the system state used for calculating the first reactive power. There is a time lag between when a change actually occurs in the system state and when the injection amount of the first reactive power for detecting islanding changes in response to the change. The greater the time lag, the more the effect of injecting the first reactive power for detecting islanding is reduced when the second reactive power in the reverse direction is injected for system stabilization. In that case, the risk of not being able to detect islanding early and correctly increases.

[0091] Returning to FIG. 4, in a scene where the first reactive power is calculated by the step injection function, the first observation condition indicates the start condition of the step injection. The second observation condition may be set to the same condition as the first observation condition, or may be set to a condition that is activated before the first observation condition. A plurality of second observation conditions may be set in stages. In this case, the maximum value of the amount of change in the second reactive power is reduced in stages as the system state approaches the start condition of the step injection (first observation condition). This makes the amount of change in the second reactive power more gradual as the system state approaches the first observation condition from the second observation condition. A specific example will be described below.

[0092] The second step injection condition determination unit 233c determines that the second observation condition is satisfied when the frequency deviation is within 0.01 Hz and all of the conditions in the following (Equation 20)-(Equation 21) are satisfied. The second step injection condition determination unit 233c may also determine that the second observation condition is satisfied when the frequency deviation is within 0.01 Hz and the condition in the following (Equation 20) is satisfied. Note that the following (Equation 20) may be replaced with the following (Equation 22). (Nh0-Nh_ave)>2V (Equation 20) (Nh1-Nh_ave)>-0.5V (Formula 21) (Nh0-Nh_ave)>1V (Formula 22)

[0093] In either case, compared to the first observation condition, which requires that all of the above-mentioned conditions (Equation 4) to (Equation 9) be satisfied, the conditions regarding time and voltage are relaxed, and the second observation condition is activated before the first observation condition.

[0094] In a situation where the first reactive power is calculated by the frequency feedback function, a measurement value having a higher sensitivity to frequency changes is used for the second measurement value of the frequency deviation used under the second observation condition than the first measurement value of the frequency deviation used under the first observation condition. In other words, the second measurement value can detect frequency fluctuations in system 3 earlier than the first measurement value.

[0095] A plurality of stepped values ​​having different sensitivities to frequency changes may be calculated as the second measurement value of the frequency deviation used under the second observation condition. The closer the calculated second measurement value is to the first measurement value, the smaller the maximum change amount of the second reactive power is set. As a result, the closer the second measurement value is to the first measurement value, the more gradual the change amount of the second reactive power becomes. Hereinafter, a specific example will be described.

[0096] The above-described first frequency deviation calculation section 221 calculates a first measurement value of the frequency deviation based on the difference between a moving average value for 320 ms at a point in time 200 ms before the latest period and a moving average value for 40 ms before the latest period.

[0097] In response to this, the second frequency deviation calculation unit 232 calculates the second measurement value of the frequency deviation based on the difference between the moving average value for 320 ms at a time point 100 ms past the latest cycle and the moving average value for 40 ms from the latest cycle. Alternatively, the second frequency deviation calculation unit 232 may calculate the second measurement value of the frequency deviation based on the difference between the moving average value for 100 ms at a time point 200 ms past the latest cycle and the moving average value for 40 ms from the latest cycle. Alternatively, the second frequency deviation calculation unit 232 may calculate the second measurement value of the frequency deviation based on the difference between the moving average value for 100 ms at a time point 100 ms past the latest cycle and the moving average value for 40 ms from the latest cycle. In either case, the second measurement value has a higher sensitivity to frequency changes in system 3 than the first measurement value.

[0098] The frequency deviation condition determination unit 233b refers to the frequency deviation-reactive power characteristics and calculates reactive power for frequency feedback injection based on the frequency deviation calculated by the second frequency deviation calculation unit 232. The frequency deviation condition determination unit 233b compares the current calculation value of the first reactive power with the calculation value of the reactive power calculated based on the second measurement value of the frequency deviation calculation. For the calculation value of the current first reactive power, the output value of the reactive power calculation unit 226 for frequency feedback injection or the output value of the reactive power determination unit 228 may be used.

[0099] When the absolute value of the reactive power calculated based on the second measurement value is greater than the absolute value of the current first reactive power, the system stabilization reactive power correction unit 234 corrects the second reactive power calculated by the system stabilization reactive power calculation unit 231 to a value with a lower degree of influence on the isolated operation detection function. That is, when the reactive power calculated based on the second measurement value is in a leading phase and increasing direction or in a lagging phase and decreasing direction with respect to the current first reactive power, the system stabilization reactive power correction unit 234 does not correct the second reactive power when the absolute value of the reactive power calculated based on the second measurement value is equal to or less than the absolute value of the current first reactive power.

[0100] Furthermore, when the absolute value of the reactive power calculated based on the second measurement value is greater than the absolute value of the current first reactive power by at least the first setting value, the system stabilization reactive power correction unit 234 may correct the second reactive power calculated by the system stabilization reactive power calculation unit 231 to a value with a reduced degree of influence on the isolated operation detection function. When the absolute value of the reactive power calculated based on the second measurement value is not greater than the absolute value of the current first reactive power by at least the first setting value, the system stabilization reactive power correction unit 234 does not correct the second reactive power.

[0101] Furthermore, when the absolute value of the second measurement value of the frequency deviation is greater than the absolute value of the first measurement value of the frequency deviation by at least a second set value, the system stabilization reactive power correcting unit 234 may correct the second reactive power calculated by the system stabilization reactive power calculating unit 231 to a value with a reduced degree of influence on the islanding operation detection function. When the absolute value of the second measurement value of the frequency deviation is not greater than the absolute value of the first measurement value of the frequency deviation by at least a second set value, the system stabilization reactive power correcting unit 234 does not correct the second reactive power.

[0102] 14(a)-(c) are diagrams showing specific examples of change conditions when the second reactive power is corrected based on the second measurement value of the frequency deviation. FIG. 14(a) shows an example of change conditions when the second reactive power is corrected when the absolute value of the reactive power calculated based on the second measurement value is even slightly larger than the absolute value of the current first reactive power. FIG. 14(b) shows an example of change conditions when the second reactive power is corrected when the absolute value of the reactive power calculated based on the second measurement value is larger than the absolute value of the current first reactive power by a first set value or more. FIG. 14(c) shows an example of change conditions when the second reactive power is corrected when the absolute value of the second measurement value of the frequency deviation is larger than the absolute value (current value) of the first measurement value of the frequency deviation by a first set value or more.

[0103] When the active function standby state is set by the reactive power oscillation suppression control described above, the second reactive power calculation unit 23 does not impose any restrictions on the calculated value of the second reactive power. When the active function normal state is set, the second reactive power calculation unit 23 corrects the calculated value of the second reactive power to a value with a reduced degree of influence on the isolated operation detection function as necessary.

[0104] When the switching condition for transitioning from the active function standby state to the active function normal state (the condition for releasing the active function standby state) is the first observation condition, the second observation condition may be set to the same condition as the first observation condition, or may be set to a condition that is activated before the first observation condition. A plurality of second observation conditions may be set in stages. In this case, the maximum amount of change in the second reactive power is reduced in stages as the system state approaches the first observation condition from the second observation condition. This makes the amount of change in the second reactive power more gradual as the system state approaches the first observation condition from the second observation condition. A specific example will be described below.

[0105] The second active standby release condition determination unit 233d releases the active function standby state when the harmonic voltage calculated by the harmonic voltage calculation unit 222 satisfies all of the above conditions (Equation 16)-(Equation 17). The first active standby release condition determination unit 224 uses three average harmonic voltage values ​​from 3 cycles ago to 5 cycles ago at the time point i cycles ago for Nh_avei, but the second active standby release condition determination unit 233d uses three average harmonic voltage values ​​from 2 cycles ago to 4 cycles ago at the time point i cycles ago for Nh_avei.

[0106] In the first active standby release condition determination unit 224, the threshold value α is set to (maximum value of 100 ms period for the past 900 ms) + 1 [V], excluding the maximum value of the 100 ms period of the most recent 100 ms, but in the second active standby release condition determination unit 233d, the threshold value α may be set to (maximum value of 100 ms period for the past 400 ms) + 1 [V], excluding the maximum value of the 100 ms period of the most recent 100 ms.

[0107] In addition, the second active standby release condition determination unit 233d may release the active function standby state when the harmonic voltage calculated by the harmonic voltage calculation unit 222 satisfies the condition of the above (Equation 16). It does not matter whether the condition of the above (Equation 17) is satisfied.

[0108] Furthermore, the second active standby release condition determination unit 233d may release the active function standby state when the harmonic voltage calculated by the harmonic voltage calculation unit 222 satisfies the condition of the following (Equation 23). (Nh0-Nh_ave0)>α-1 (Equation 23)

[0109] In any case, the second observation condition has a relaxed condition with respect to at least one of time, voltage, and autotuning, as compared with the first observation condition, and is activated before the first observation condition.

[0110] As described above, according to this embodiment, it is possible to prevent the reactive power injection for system stabilization from affecting the reactive power injection for islanding detection, making it impossible to detect islanding within a specified time limit. Furthermore, when the change direction of the first reactive power for islanding detection and the change direction of the second reactive power for system stabilization are the same, no limit is imposed on the injection amount of the second reactive power, and when the change directions of the first reactive power for islanding detection and the second reactive power for system stabilization are opposite, a limit is imposed on the injection amount of the second reactive power as necessary. This makes it possible to minimize the decrease in the tracking ability of the reactive power injection function for system stabilization to system changes. As described above, it is possible to simultaneously satisfy the demand for early detection of islanding operation and the demand for ensuring tracking ability to system changes.

[0111] The present disclosure has been described above based on the embodiments. The embodiments are merely examples, and it will be understood by those skilled in the art that various modifications are possible in the combination of each component and each processing step, and that such modifications are also within the scope of the present disclosure.

[0112] In the above-described embodiment, the calculation of the frequency deviation, the harmonic voltage, the calculation of the frequency feedback injection amount, the calculation of the step injection amount, the disturbance detection, and the release of the active function standby state are described as examples conforming to the JEM1498 standard. However, the present invention is not limited to the methods conforming to the JEM1498 standard, and a manufacturer's own method may be adopted.

[0113] The embodiment may be specified by the following items.

[0114] [Item 1] A control device (20) for controlling an inverter unit (10) that converts DC power into AC power and outputs the converted AC power to a power grid (3), a first reactive power calculation unit (22) that calculates a first reactive power to be injected in order to detect an isolated operation; a second reactive power calculation unit (23) that calculates a second reactive power to be injected in order to stabilize the power system (3); a reactive power injection amount calculation unit (24) that calculates an injection amount of reactive power to be injected into AC power output from the inverter unit (10) based on the first reactive power calculated by the first reactive power calculation unit (22), an amount of change in the first reactive power, the second reactive power calculated by the second reactive power calculation unit (23), and the amount of change in the second reactive power; A control device (20) comprising: This makes it possible to prevent the islanding detection function from being impaired by the injection of the second reactive power for grid stabilization. [Item 2] When the first reactive power and the second reactive power are changing in the same direction, the reactive power injection amount calculation unit (24) selects the reactive power having a larger amount of change as the reactive power to be injected into the AC power. Item 1. The control device (20) according to item 1. This makes it possible to satisfy the requirements for both the islanding detection function and the grid stabilization function. Also, by injecting excess reactive power, which is the sum of the first reactive power and the second reactive power, the occurrence of flicker can be prevented. [Item 3] the reactive power injection amount calculation unit (24) limits the reactive power to be injected into the AC power to the upper limit value of the first reactive power when the reactive power selected as the reactive power to be injected into the AC power exceeds an upper limit value of the first reactive power, and limits the reactive power to be injected into the AC power to the lower limit value of the first reactive power when the reactive power selected as the reactive power to be injected into the AC power exceeds a lower limit value of the first reactive power. Item 3. The control device (20) according to item 2. This makes it possible to prevent excessive injection of reactive power. [Item 4] the reactive power injection amount calculation unit (24) selects the first reactive power as reactive power to be injected into the AC power when the first reactive power and the second reactive power are changing in the same direction. Item 1. The control device (20) according to item 1. This makes it possible to satisfy the requirements for both an islanding detection function and a system stabilization function. [Item 5] the reactive power injection amount calculation unit (24) selects the first reactive power as the reactive power to be injected into the AC power when the absolute value of the first reactive power is changing in an increasing direction, and selects the second reactive power as the reactive power to be injected into the AC power when the absolute value of the first reactive power is changing in a decreasing direction. Item 1. The control device (20) according to item 1. According to this, when the islanding operation detection function is operating in a direction that accelerates the frequency change, the islanding operation detection function is given priority, and when the islanding operation detection function is operating in a direction that converges the frequency change, the system stabilization function is given priority, thereby making it possible to optimally achieve both the islanding operation detection function and the system stabilization function. [Item 6] the reactive power injection amount calculation unit (24) selects the first reactive power as the reactive power to be injected into the AC power when the first reactive power and the second reactive power are changing in opposite directions and the absolute value of the first reactive power is changing in an increasing direction, and selects the second reactive power as the reactive power to be injected into the AC power when the absolute value of the first reactive power is changing in a decreasing direction. 5. The control device (20) according to any one of items 1 to 4. According to this, when the direction of change of the first reactive power and the direction of change of the second reactive power are opposite to each other, if the isolated operation detection function is operating in a direction that promotes the frequency change, the isolated operation detection function is given priority, and if the isolated operation detection function is operating in a direction that converges the frequency change, the system stabilization function is given priority, thereby making it possible to optimally achieve both the isolated operation detection function and the system stabilization function. [Item 7] an inverter unit (10) that converts DC power into AC power and outputs the converted AC power to a power system (3); A control device (20) according to any one of items 1 to 6 that controls the inverter unit (10); A power conversion device (2) comprising: This makes it possible to realize the power conversion device (2) in which the injection of the second reactive power for grid stabilization is prevented from impairing the islanding detection function. [Explanation of symbols]

[0115] 1 Distributed power source, 2 Power conversion device, 3 System, 4 Load, 5 Server, 10 Inverter unit, 20 Control unit, 21 Reactive power injection unit, 22 First reactive power calculation unit, 221 First frequency deviation calculation unit, 222 Harmonic voltage calculation unit, 223 Reactive power oscillation suppression control unit, 224 First active standby release condition determination unit, 225 First step injection condition determination unit, 226 Frequency feedback injection reactive power calculation unit, 227 Step injection reactive power calculation unit, 228 Reactive power determination unit, 23 Second reactive power calculation unit, 231 System stabilization reactive power calculation unit, 232 Second frequency deviation calculation unit, 233 Condition determination unit, 233a Change direction determination unit, 233b Frequency deviation condition determination unit, 233c Second step injection condition determination unit, 233d Second active standby release condition determination unit, 234 reactive power correction unit for system stabilization, 24 reactive power injection amount calculation unit, 25 current control unit, 26 isolated operation detection unit, 31 system frequency detection unit, 32 system voltage detection unit, 40 communication unit.

Claims

1. A control device that controls an inverter unit that converts DC power into AC power and outputs the converted AC power to a power grid, a first reactive power calculation unit that calculates a first reactive power to be injected in order to detect an isolated operation; a second reactive power calculation unit that calculates a second reactive power to be injected in order to stabilize the power system; a reactive power injection amount calculation unit that calculates an injection amount of reactive power to be injected into the AC power output from the inverter unit, based on the first reactive power calculated by the first reactive power calculation unit, an amount of change in the first reactive power, the second reactive power calculated by the second reactive power calculation unit, and the amount of change in the second reactive power; Equipped with the reactive power injection amount calculation unit, when the first reactive power and the second reactive power are changing in the same direction, selects the reactive power having a larger amount of change as the reactive power to be injected into the AC power. Control device.

2. the reactive power injection amount calculation unit, when the reactive power selected as the reactive power to be injected into the AC power exceeds an upper limit value of the first reactive power, limits the reactive power to be injected into the AC power to the upper limit value of the first reactive power, and, when the reactive power selected as the reactive power to be injected into the AC power exceeds a lower limit value of the first reactive power, limits the reactive power to be injected into the AC power to the lower limit value of the first reactive power. The control device according to claim 1 .

3. A control device that controls an inverter unit that converts DC power into AC power and outputs the converted AC power to a power grid, a first reactive power calculation unit that calculates a first reactive power to be injected in order to detect an isolated operation; a second reactive power calculation unit that calculates a second reactive power to be injected in order to stabilize the power system; a reactive power injection amount calculation unit that calculates an injection amount of reactive power to be injected into the AC power output from the inverter unit, based on the first reactive power calculated by the first reactive power calculation unit, an amount of change in the first reactive power, the second reactive power calculated by the second reactive power calculation unit, and the amount of change in the second reactive power; Equipped with the reactive power injection amount calculation unit, when the first reactive power and the second reactive power are changing in the same direction, selects the first reactive power as reactive power to be injected into the AC power; Control device.

4. A control device that controls an inverter unit that converts DC power into AC power and outputs the converted AC power to a power grid, a first reactive power calculation unit that calculates a first reactive power to be injected in order to detect an isolated operation; a second reactive power calculation unit that calculates a second reactive power to be injected in order to stabilize the power system; a reactive power injection amount calculation unit that calculates an injection amount of reactive power to be injected into the AC power output from the inverter unit, based on the first reactive power calculated by the first reactive power calculation unit, an amount of change in the first reactive power, the second reactive power calculated by the second reactive power calculation unit, and the amount of change in the second reactive power; Equipped with the reactive power injection amount calculation unit selects the first reactive power as the reactive power to be injected into the AC power when the absolute value of the first reactive power is changing in an increasing direction, and selects the second reactive power as the reactive power to be injected into the AC power when the absolute value of the first reactive power is changing in a decreasing direction. Control device.

5. A control device that controls an inverter unit that converts DC power into AC power and outputs the converted AC power to a power grid, a first reactive power calculation unit that calculates a first reactive power to be injected in order to detect an isolated operation; a second reactive power calculation unit that calculates a second reactive power to be injected in order to stabilize the power system; a reactive power injection amount calculation unit that calculates an injection amount of reactive power to be injected into the AC power output from the inverter unit, based on the first reactive power calculated by the first reactive power calculation unit, an amount of change in the first reactive power, the second reactive power calculated by the second reactive power calculation unit, and the amount of change in the second reactive power; Equipped with the reactive power injection amount calculation unit, when the first reactive power and the second reactive power are changing in opposite directions and the absolute value of the first reactive power is changing in an increasing direction, selects the first reactive power as the reactive power to be injected into the AC power, and when the absolute value of the first reactive power is changing in a decreasing direction, selects the second reactive power as the reactive power to be injected into the AC power. Control device.

6. an inverter unit that converts DC power into AC power and outputs the converted AC power to a power grid; The control device according to claim 1 , which controls the inverter unit; A power conversion device comprising:

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