Power conversion device

Abstract

The present invention provides a power conversion device capable of continuing operation even if instantaneous voltage drop due to a system accident occurs during interconnection operation. To do so, a first voltage command generation unit (307) generates a first voltage command V1 from a reactive power command Qo and reactive power Qout obtained by measurement and calculation. A current command generation unit (308) generates a current command I* from a measurement voltage Vout and the first voltage command V1. A current command restriction unit (309) applies a constraint to and outputs the current command I*. A second voltage command generation unit (310) generates a second voltage command V* from the constrained current command and a measurement current Iout. A control circuit (3) controls a power conversion circuit (2) on the basis of the second voltage command V*.

Description

Power Conversion Device

[0001] The present invention relates to a power conversion device.

[0002] Patent Literature 1 discloses a power conversion device that can eliminate the need to change the control method, such as current control or voltage control, when switching from independent operation to grid-connected operation in a combined power generation system such as a microgrid. The power conversion device calculates a command value for the output current of a power converter from an internal phase difference angle calculated from an active power control loop and an internal electromotive voltage calculated from a reactive power control loop, and controls the power converter using the command value.

[0003] International Publication No. 2013 / 008413

[0004] "Grid Interconnection Regulations (JEAC 9701-2019)" Grid Interconnection Special Committee of the Japan Electric Association

[0005] For example, the power conversion device for a combined power generation system disclosed in Patent Document 1 uses a power converter that does not require a change in control method when interconnected to a grid, thereby enabling the construction of an easy-to-use independent power supply system. Meanwhile, Non-Patent Document 1 specifies "Requirements for the continued operation performance of distributed power sources during grid disturbances, which are necessary to ensure power quality" (Fault Ride Through: FRT) for grid-connected power sources. That is, continued operation performance during a fault is required to prevent simultaneous parallel-off of power generation equipment or continued output reduction due to widespread instantaneous voltage drops and frequency fluctuations caused by a grid fault. Therefore, in order for a power conversion device for a combined power generation system to operate while connected to a commercial grid, it is necessary to satisfy the continued operation performance during a fault.

[0006] However, when a power conversion device for a combined power generation system experiences a sudden change in grid voltage due to a momentary fault or other reason during grid-connected operation, its output current and power may be disrupted. A sudden change in grid voltage also causes a sudden change in the active power output of the power conversion device for a combined power generation system, which may result in a change in the frequency command value. This may result in changes in the frequency difference and phase difference with the grid, which may cause output disruption. Furthermore, if the internal electromotive force command value cannot be changed quickly enough in response to a sudden change in grid voltage, an overcurrent may occur due to the potential difference between the internal electromotive force command value and the grid voltage, which may result in equipment shutdown to protect the equipment during grid-connected operation. In other words, there is a problem in that it is difficult to continue operation during a fault.

[0007] Therefore, one object of the present invention is to provide a power conversion device that can continue to operate even if a momentary voltage drop occurs due to a grid fault during grid-connected operation.

[0008] The above and other objects and novel features of the present invention will become apparent from the description of this specification and the accompanying drawings.

[0009] A power conversion device according to one embodiment includes a power conversion circuit that converts DC power to AC power, and a control circuit that controls the power conversion circuit. The control circuit has an active power command unit, an angular frequency command generation unit, a phase command generation unit, a reactive power command unit, a first voltage command generation unit, a current command generation unit, a current command constraint unit, and a second voltage command generation unit. The active power command unit determines an active power command. The angular frequency command generation unit generates an angular frequency command from the active power command and an output active power obtained by measurement and calculation. The phase command generation unit generates a phase command from the angular frequency command. The reactive power command unit determines a reactive power command. The first voltage command generation unit generates a first voltage command from the output reactive power obtained by measurement and calculation and the reactive power command. The current command generation unit generates a current command from the measured voltage and the first voltage command. The current command constraint unit constrains and outputs the current command. The second voltage command generation unit generates a second voltage command from the constrained current command and the measured current. The control circuit then controls the power conversion circuit based on the second voltage command.

[0010] According to the embodiment, even if a momentary voltage drop occurs due to a grid fault during grid-connected operation, operation can be continued.

[0011] FIG. 1 is a schematic diagram showing a configuration example of a power conversion device according to a first embodiment. FIG. 2 is a block diagram showing a detailed configuration example of each unit in FIG. 1. FIG. 3 is a schematic diagram showing a configuration example of a power conversion device according to a second embodiment. FIG. 4 is a timing chart showing an example of an active power command and an angular frequency command generated in FIG. 3. FIG. 5 is a schematic diagram showing an example of a current command constraining method in a current command constraining unit in a third embodiment. FIG. 6 is a block diagram showing a detailed configuration example of each unit in FIG. 1 or FIG. 3 in a power conversion device according to a fourth embodiment. FIG. 7 is a block diagram showing a detailed configuration example of each unit in FIG. 1 or FIG. 3 in a power conversion device according to a fifth embodiment. FIG. 8 is a schematic diagram showing a partial configuration example in a power conversion device according to a sixth embodiment.

[0012] Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. In all drawings for explaining the embodiments, the same components are generally designated by the same reference numerals, and repeated description thereof will be omitted.

[0013] (First embodiment) <Outline of power conversion device> Fig. 1 is a schematic diagram showing an example of the configuration of a power conversion device according to a first embodiment. The power conversion device 1 shown in Fig. 1 is configured, for example, by a housing in which various internal components are mounted. As part of the internal components, the power conversion device 1 includes a power conversion circuit 2, a control circuit 3, and an AC filter 4. However, the AC filter 4 may be an external component. The power conversion circuit 2 is, for example, a three-phase full-bridge inverter that converts DC power into AC power. The AC terminal of the power conversion circuit 2 is connected to an AC system 5 and an AC load 6 via the AC filter 4.

[0014] The AC system 5 is, for example, a 50 Hz, three-phase, 200 V commercial system. The AC load 6 is, for example, a consumer-owned device such as an air conditioner. Here, multiple AC loads 6 are collectively described as one load, but in detail, there may be multiple loads. The AC filter 4 is composed of an inductor and an X capacitor. Although not shown, a relay may be installed at the interconnection point with the AC system 5 so that the power conversion device 1 and the AC load 6 can be disconnected from the AC system 5.

[0015] The DC end of the power conversion circuit 2 is connected to a DC power supply 7. The DC power supply 7 is, for example, a secondary battery such as a lithium-ion battery, a nickel-metal hydride battery, or a lead-acid battery, a capacitor, or a renewable energy power supply such as solar power generation. Note that instead of the DC power supply 7, a power supply in which a device capable of storing energy in a form other than electrical energy, such as a compressed air energy storage (CAES) or a flywheel, is connected to a device that converts that energy into electrical energy may be used.

[0016] A voltage meter 10 is installed on the AC system 5 and AC load 6 side of the AC filter 4. A current meter 11 is installed on the power conversion circuit 2 side of the AC filter 4. The control circuit 3 receives the measured AC voltage value from the voltage meter 10 and the measured AC current value from the current meter 11. The control circuit 3 includes a measurement unit 301, a power calculation unit 302, an active power command unit 303, an angular frequency command generation unit 304, a phase command generation unit 305, a reactive power command unit 306, a first voltage command generation unit 307, a current command generation unit 308, a current command constraint unit 309, a second voltage command generation unit 310, and a main circuit control unit 311.

[0017] Specifically, the control circuit 3 may be realized by, for example, a microcontroller including a processor and a memory. In this case, the measurement unit 301 may be realized by, for example, an analog-to-digital converter built into the microcontroller. The other units may be realized by the processor executing a program stored in the memory. Alternatively, the control circuit 3 may be realized by a programmable logic device such as an FPGA (Field Programmable Gate Array). In this case, the measurement unit 301 may be realized by, for example, an analog-to-digital converter built into the FPGA. The other units may be realized by logic circuits programmed within the FPGA.

[0018] The measurement unit 301 calculates the AC voltage measurement value and the AC current measurement value as V ｏｕｔ and I ｏｕｔ The measurement unit 301 outputs V ｏｕｔ , I ｏｕｔ When outputting, measurement noise may be removed using a low-pass filter or the like before outputting. The time constants of the low-pass filter may be set separately for the voltage and the current.

[0019] The power calculation unit 302 calculates the V output from the measurement unit 301. ｏｕｔ , I ｏｕｔ From this, the output effective power P ｏｕｔ and reactive power Q ｏｕｔ In the embodiment, the active power P ｏｕｔ Regarding the reactive power Q, the power flow discharging the DC power supply 7 is positive. ｏｕｔ For example, the three-phase voltage and three-phase current V ｏｕｔ , I ｏｕｔ can be converted into two-phase voltages and two-phase currents in the α-β coordinate system by a three-phase to two-phase transformation.

[0020] The active power command unit 303 receives an active power command value P ｏ Based on the active power command P ｏ In the embodiment, P ｏ =P ｏ The active power command value Pｏ The active power command value P' can be received via wired or wireless communication from a higher-level command device, such as an energy management system, that is provided outside the power conversion device 1. The reception frequency can be set arbitrarily, for example, every minute. ｏ ' may be received aperiodically or may be arbitrarily input by the user of the power conversion device 1.

[0021] The angular frequency command generator 304 generates an active power command P ｏ and the effective power P ｏｕｔ The angular frequency command ω is calculated by droop control based on the deviation from ＊ Generates the angular frequency command ω ＊ is expressed by the following formula (1): ｐ is the droop coefficient of the active power-angular frequency change command. ｏ is a reference angular frequency, for example, 2π×50.

[0022] Here, the angular frequency command ω ＊ In order to give a pseudo inertia to the droop control part, a first-order lag filter or a first-order lag-lead filter may be set. For example, when a first-order lag-lead filter is set, the angular frequency command ω ＊ is expressed by the following formula (2): １ , T ２ is the time constant.

[0023] For example, in formula (2), P ｏｕｔ <P ｏ When ω ＊ >ω ｏ Therefore, by increasing the angular frequency, the effective power P ｏｕｔ On the other hand, P ｏｕｔ >P ｏ When ω ＊ <ω ｏ Therefore, by lowering the angular frequency, the effective power P ｏｕｔ This droop control setting and filter result in a behavior that reduces ω ＊ The gradual change in the voltage allows the behavior of a virtual synchronous generator to be simulated.

[0024] The phase command generation unit 305 generates an angular frequency command ω ＊ is integrated to obtain the phase command θ ＊ The reactive power command unit 306 generates a reactive power command Q ｏ The reactive power command Q is generated and output. ｏ is, for example, the rated voltage V ｏ (for example, 200 V) and the output of the measuring unit 301, V ｏｕｔ Deviation V ｏ -V ｏｕｔ , proportional gain K ｑ (K ｑ The reactive power command unit 306 generates the reactive power command value Q ｏ ' is used as the reactive power command Q ｏ It may be output as

[0025] In addition, the reactive power command unit 306 outputs a reactive power command Q ｏ is input from the outside of the power conversion device 1, ｏ ', the rated power of the power conversion device 1, and a desired power factor. In this case, behavior equivalent to power factor control is possible. Furthermore, the reactive power command unit 306 determines the active power command P ｏ The reactive power command Q is calculated from the rated power of the power conversion device 1 and the desired power factor. ｏ may be determined.

[0026] The first voltage command generator 307 generates a reactive power command Q ｏ and reactive power Q ｏｕｔ The first voltage command V １ The current command generator 308 calculates the first voltage command V １ and AC voltage measurement value V ｏｕｔ and impedance Z, the AC terminal voltage of the power conversion circuit 2 is the first voltage command V １ The AC current flowing when ＊ Generate.

[0027] Equation (3) is a phasor representation, and V １ = (V １ , 0). In practice, the current command generator 308 calculates the d-axis component and the q-axis component in the dq transformation. That is, I ＊ = (I ｄ ＊ , I ｑ ＊ ) is calculated using the following equations (4) and (5). Here, the impedance Z is expressed by the resistance r and the reactance x. Also, V ｏｕｔ = (V ｏｕｔ＿ｄ , V ｏｕｔ＿ｑ ) dq transformation, the phase command θ ＊ is used.

[0028]

[0029] The impedance Z, in other words, the virtual impedance value, is determined based on the impedance Z of the AC filter 4, which is known in advance. ｆｉｌｔｅｒ =r ｆｉｌｔｅｒ +jx ｆｉｌｔｅｒ The impedance Z of the AC filter 4 may be used. ｆｉｌｔｅｒ In addition, the impedance Z inside the synchronous generator is ｖｉｒｔｕａｌ In this case, Z=Z ｆｉｌｔｅｒ +Z ｖｉｒｔｕａｌ = (r ｆｉｌｔｅｒ +r ｖｉｒｔｕａｌ ) + j(x ｆｉｌｔｅｒ +x ｖｉｒｔｕａｌ )

[0030] The current command constraint unit 309 constrains the current command I generated by the current command generation unit 308. ＊ = (I ｄ ＊ , I ｑ ＊ ) is constrained. For example, the current command constraint unit 309 sets limiters for the d-axis component and the q-axis component so that the upper and lower threshold currents are the upper and lower limits. In this case, the upper threshold current is, for example, the rated current of the power conversion circuit 2. The lower threshold current is, for example, the value obtained by multiplying the rated current of the power conversion circuit 2 by −1.

[0031] The second voltage command generator 310 generates the current command (I ｄ ＊ , I ｑ ＊ ) and the measured AC current I ｏｕｔ Phase command θ ＊ The d-axis component and q-axis component (I ｏｕｔ＿ｄ , I ｏｕｔ＿ｑ ) based on the second voltage command V ＊ = (V ｄ ＊ , V ｑ ＊ Specifically, the second voltage command generator 310 calculates the constrained current command I for each of the d-axis component and the q-axis component. ＊ = (I ｄ ＊ , I ｑ ＊ ) and the measured AC current I ｏｕｔ = (I ｏｕｔ＿ｄ , I ｏｕｔ＿ｑ ) and perform PI control.

[0032] Here, the gain of the PI control is the proportional gain K ｖｐ , integral gain K ｖｉ Then, the second voltage command V ＊ = (V ｄ ＊ , V ｑ ＊ ) are expressed by the following equations (6) and (7).

[0033]

[0034] Furthermore, the second voltage command generating unit 310 performs non-interference control to generate the second voltage command V ＊ = (V ｄ ＊ , V ｑ ＊ ) may be generated. When the decoupling control is added, the second voltage command V ＊ = (V ｄ ＊ , V ｑ ＊ ) are expressed by the following equations (8) and (9).

[0035]

[0036] The main circuit control unit 311 outputs a second voltage command V ＊ = (V ｄ ＊ , V ｑ ＊ ) as the phase command θ ＊ Then, an inverse dq transform is performed using the voltage command generator 311, and an inverse three-phase to two-phase transform is performed to generate a voltage command for each of the three phases (R, S, T). The main circuit control unit 311 generates a gate signal G for the switching element of the power conversion circuit 2 based on the generated voltage command. ｐｗｍ (R, S, T) is generated and drives the power conversion circuit 2.

[0037] <Details of the Power Conversion Device> Fig. 2 is a block diagram showing a detailed configuration example of each part in Fig. 1. In Fig. 2, a current command generation unit 308 generates a measured AC voltage V in an α-β coordinate system obtained by three-phase to two-phase conversion. ｏｕｔ＿α , V ｏｕｔ＿β , the phase command θ ＊ Using the AC voltage measurement value V in the dq coordinate system ｏｕｔ＿ｄ , V ｏｕｔ＿ｄ The current command generator 308 converts the converted AC voltage measurement value V ｏｕｔ＿ｄ , V ｏｕｔ＿ｄ Then, the current command generating unit 308 calculates the current command I based on the equations (4) and (5). ＊ d-axis component I ｄ ＊ and the q-axis component I ｑ ＊ Calculate the following.

[0038] The second voltage command generator 310 calculates the AC current measurement value I in the α-β coordinate system obtained by three-phase to two-phase transformation. ｏｕｔ＿α , I ｏｕｔ＿β , the phase command θ ＊ Using the AC current measurement value I in the dq coordinate system ｏｕｔ＿ｄ , I ｏｕｔ＿ｄ The second voltage command generator 310 converts the converted AC current measurement value I ｏｕｔ＿ｄ , I ｏｕｔ＿ｄThen, in this example, the second voltage command generating unit 310 generates the second voltage command V based on the above-described equations (8) and (9). ＊ The d-axis component of V ｄ ＊ and the q-axis component V ｑ ＊ Calculate the following.

[0039] The main circuit control unit 311 outputs a second voltage command V ＊ = (V ｄ ＊ , V ｑ ＊ ), the phase command θ ＊ By performing an inverse dq transformation using α ＊ , V β ＊ Thereafter, although not shown in the figure, the main circuit control unit 311 calculates voltage commands for each of the three phases (R, S, T) by inverse three-phase to two-phase conversion, and generates gate signals Gpwm(R, S, T) which are PWM signals.

[0040] <Major Effects of the First Embodiment> As described above, the power conversion device 1 operates as a voltage source, but by providing the current command generation unit 308, the current command constraint unit 309, and the second voltage command generation unit 310, it becomes a voltage source incorporating current control. This makes it possible to continue operation even if an instantaneous voltage drop occurs due to a grid fault. In particular, by providing the current command constraint unit 309, it becomes possible to operate while suppressing overcurrent.

[0041] (Second embodiment) <Outline of power conversion device> Fig. 3 is a schematic diagram showing a configuration example of a power conversion device according to a second embodiment. In the second embodiment, descriptions of the same parts as in the first embodiment will be omitted as appropriate. The power conversion device 1 shown in Fig. 3 has a different configuration of an active power command unit 303 compared to the case of Fig. 1.

[0042] When the power conversion device 1 shown in FIG. 3 or FIG. 1 is operating in a grid-connected manner with respect to the AC grid 5, the power conversion device 1 ｏ As shown below, the effective power P ｏｕｔHowever, when a fault occurs in the AC system 5 and the voltage of the AC system 5 drops instantaneously, the effective power P ｏｕｔ That is, the power conversion device 1 reduces the active power command P ｏ As shown below, the effective power P ｏｕｔ In this case, in the formula (1) or the formula (2), P ｏｕｔ <P ｏ Therefore, ω ＊ >ω ｏ As a result, the power conversion device 1 increases the effective power P ｏｕｔ We try to increase the

[0043] Here, when the fault is removed from the AC system 5 and the voltage of the AC system 5 is restored, the power conversion device 1 generates an effective power P ｏｕｔ However, when the voltage is restored, the angular frequency command ω ＊ is the angular frequency ω of the AC system 5 ｏ For ω ＊ >ω ｏ As a result, the angular frequency difference and phase difference between the power conversion device 1 and the AC system 5 become large. ｏｕｔ becomes excessively large, and the active power command P ｏ Then, the effective power P ｏｕｔ If the rated power is exceeded, overcurrent and overload operation may occur.

[0044] Meanwhile, Non-Patent Document 1 defines "Requirements for the operation continuity performance of distributed power sources during grid disturbances, which are necessary to ensure power quality" (Fault Ride Through: FRT). According to this, grid-connected power sources are required to have the operation continuity performance during a fault to prevent disconnection or continuous output reduction due to widespread instantaneous voltage drops and frequency fluctuations caused by a grid fault. In order for the power conversion device 1 to operate in a grid-connected manner, it is necessary to satisfy this operation continuity performance during a fault. However, as described above, if an overcurrent or overload operation occurs after recovery from the instantaneous voltage drop, the power conversion device 1 may stop operating, and may not be able to satisfy the operation continuity performance during a fault.

[0045] Therefore, to prevent overcurrent and overload operation after voltage recovery, the angular frequency command ω ＊ As described above, when the voltage of the AC system 5 drops, the effective power P ｏｕｔ decreases, so the angular frequency command ω ＊ In order to reduce the fluctuation of the active power command P ｏ The value of can be reduced.

[0046] Generally, the line-to-line voltage V ｓ , receiving end line voltage V ｓ , reactance X between the sending end and the receiving end, and active power P in a three-phase AC with a phase difference δ is expressed as in equation (10).

[0047] Based on the equation (10), the active power command unit 303 shown in FIG. 3 calculates the active power command P ｏ That is, the active power command value P ｏ ' is the d-axis component V of the AC voltage measurement value ｏｕｔ＿ｄ However, V in equation (11) is corrected by the square of ｏ is the AC terminal rated voltage of the power conversion circuit 2.

[0048] <Details of the Active Power Command Unit> Fig. 4 is a timing chart showing an example of the active power command and the angular frequency command generated in Fig. 3. In Fig. 4, the horizontal axis represents time t, and from the top, the voltage (measured AC voltage) V of the AC system 5 is ｏｕｔ , active power P ｏｕｔ , active power command P ｏ , angular frequency command ω ＊ In addition, the active power command P ｏ and angular frequency command ω ＊ Regarding the graph, the solid line is an example of a waveform when the method of the second embodiment is applied, and the dashed line is an example of a waveform when the method of the second embodiment is not applied.

[0049] In FIG. 4, the AC voltage measurement value V ｏｕｔ When decreases, the active power P ｏｕｔWith the decrease of ｏ As a result, when the method of the second embodiment is applied, the value of the angular frequency command ω ＊ As a result, it is possible to suppress the increase in the angular frequency difference and phase difference between the power conversion device 1 and the AC system 5, and it is possible to suppress overcurrent and overload operation after voltage recovery.

[0050] In FIG. 4, the AC voltage measurement value V ｏｕｔ Although the waveform shows an ideal one in which V quickly drops and recovers, in reality ｏｕｔ (V ｏｕｔ＿ｄ ) chattering may occur. ｏｕｔ This is directly used as the active power command P ｏ When used to correct the active power command P ｏ and angular frequency command ω ＊ Therefore, the active power command unit 303 determines the active power command P ｏ When correcting, the AC voltage measurement value V ｏｕｔ＿ｄ It is advisable to set a low-pass filter to remove vibrations caused by chattering before making corrections.

[0051] In addition, in equation (11), the d-axis component V ｏｕｔ＿ｄ was used, but the three-phase AC voltage measurement value V ｏｕｔ The effective value V ｏｕｔ＿ｒｍｓ and the d-axis component V ｏｕｔ＿ｄ and the q-axis component V ｏｕｔ＿ｑ In addition, in equation (10), the effective power P is calculated by multiplying the line-to-line voltage V ｓ , receiving end line voltage V ｓ Therefore, the active power command unit 303 outputs the second voltage command V ＊ The d-axis component of V ｄ ＊ Using the above, the active power command P is expressed by equation (12) instead of equation (11). ｏ may be calculated.

[0052] The active power command unit 303 also calculates the active power command P ｏAlternatively, the active power command unit 303 may constantly perform the calculation of the active power command P ｏ The calculation is performed using the AC voltage measurement value V ｏｕｔ＿ｄ or its effective value V ｏｕｔ＿ｒｍｓ , or the d-axis component V ｏｕｔ＿ｄ and the q-axis component V ｏｕｔ＿ｑ The detection may be performed only when the square root of the sum of the squares of the voltages is below a separately determined threshold voltage, that is, only when it is estimated that a fault has occurred in the AC system 5.

[0053] <Major Effects of the Second Embodiment> As described above, by using the power conversion device 1 according to the second embodiment, the same effects as those described in the first embodiment can be obtained. ＊ to suppress the change in the active power command P ｏ By correcting this, it is possible to suppress overcurrent and overload operation that may occur after recovery from a momentary voltage drop caused by a grid fault.

[0054] (Third embodiment) <Details of current command constraint unit> In the third embodiment, the description of the same parts as those in the above-mentioned embodiments will be omitted as appropriate. A schematic configuration example of the power conversion device 1 according to the third embodiment is the same as the configuration example shown in FIG. 1 or 3.

[0055] In FIG. 1 or 3, the current command constraint unit 309 determines the current command I generated by the current command generation unit 308. ＊ = (I ｄ ＊ , I ｑ ＊ 1, the current command constraint unit 309 sets limiters for each of the d-axis component and the q-axis component so that, for example, the rated current of the power conversion circuit 2 is the upper limit and the value obtained by multiplying the rated current by −1 is the lower limit, thereby constraining the d-axis component I ｄ ＊ and the q-axis component I ｑ ＊ Each of these can be limited to a rated current or less.

[0056] However, the d-axis component I ｄ ＊ and the q-axis component I ｑ ＊If we simply constrain the apparent current command √((I ｄ ＊ ) ２ + (I ｑ ＊ ) ２ ) may exceed the rated current. In this case, current control is performed according to the current command that exceeds the rated current, resulting in operation in an overcurrent state.

[0057] For example, when an accident occurs and the voltage of the AC system 5 drops instantaneously, the current command I ＊ d-axis component I ｄ ＊ and the q-axis component I ｑ ＊ In this case, the d-axis component I ｄ ＊ and the q-axis component I ｑ ＊ Even if each is individually constrained to be less than the rated current, the apparent current command √((I ｄ ＊ ) ２ + (I ｑ ＊ ) ２ ) becomes √2 times the rated current. Therefore, in the third embodiment, in order to suppress overcurrent, the current is restricted depending on whether the apparent current command exceeds the threshold current.

[0058] FIG. 5 is a schematic diagram showing an example of a current command constraint method in the current command constraint unit 309 in the third embodiment. The current command constraint unit 309 constrains the apparent current command √((I ｄ ＊ ) ２ + (I ｑ ＊ ) ２ ) is the threshold current I ｌｉｍ When the current exceeds the threshold, the current command I ＊ = (I ｄ ＊ , I ｑ ＊ ) into the current command I' ＊ = (I ｄ ' ＊ , I ｑ ' ＊) is restricted to

[0059]

[0060] The current command constraint unit 309 determines the current command I ＊ = (I ｄ ＊ , I ｑ ＊ ) is constrained to obtain the current command I ＊ = (I ｄ ＊ , I ｑ ＊ ) power factor is maintained, the apparent current command is ｌｉｍ This makes it possible to suppress overcurrent while maintaining the power factor.

[0061] Threshold current I ｌｉｍ is, for example, the rated current of the power conversion circuit 2. However, in order to allow temporary overload operation, a threshold current I ｌｉｍ may be set to, for example, 120% of the rated current of the power conversion circuit 2. ｌｉｍ may be changed as appropriate depending on the state of the AC grid 5 and the operating state of the power conversion device 1. As an example, the threshold current I ｌｉｍ is the measured AC voltage V ｏｕｔ When the threshold voltage is exceeded, the AC voltage measurement value V is set to 120% of the rated current of the power conversion circuit 2, etc. ｏｕｔ When the voltage Vcc is lower than the threshold voltage, the current Vcc may be set to the rated current of the power conversion circuit 2 .

[0062] <Major Effects of the Third Embodiment> As described above, by using the power conversion device 1 according to the third embodiment, it is possible to obtain the same effects as those described in the above-described embodiments. Furthermore, when a grid fault occurs, the apparent current is reduced to a value equal to or greater than the threshold current I ｌｉｍ When the current command I ＊ The value of the threshold current I ｌｉｍ By restricting the value of

[0063] (Fourth embodiment) <Details of the power conversion device> In the fourth embodiment, the description of the same parts as those in the above-mentioned embodiments will be omitted as appropriate. A schematic configuration example of the power conversion device 1 according to the fourth embodiment is the same as the configuration example shown in FIG. 1 or 3.

[0064] 6 is a block diagram showing a detailed configuration example of each unit in FIG. 1 or FIG. 3 in a power conversion device according to a fourth embodiment. In FIG. 6, the configuration example of the second voltage command generating unit 310 is different from that in FIG. 2, and further, a fault determination unit 315 is added. As described above, the second voltage command generating unit shown in FIG. 2 generates the second voltage command V in accordance with the equations (8) and (9). ＊ = (V ｄ ＊ , V ｑ ＊ ) is calculated. On the other hand, the second voltage command generating unit 310 shown in FIG. 6 calculates the AC voltage measurement value V ｏｕｔ By performing the feedforward of the second voltage command V ＊ = (V ｄ ＊ , V ｑ ＊ ) is calculated.

[0065]

[0066] In the equations (8) and (9) used by the second voltage command generating unit shown in FIG. 2, the voltage V ｏｕｔ = (V ｏｕｔ＿ｄ , V ｏｕｔ＿ｑ ) suddenly changes, the second voltage command V ＊ = (V ｄ ＊ , V ｑ ＊ ) does not change immediately. Therefore, immediately after the sudden change in the voltage of the AC system 5, the system side voltage (V ｏｕｔ＿ｄ , V ｏｕｔ＿ｑ ) and a second voltage command V corresponding to the voltage on the power conversion circuit 2 side. ＊ = (V ｄ ＊ , V ｑ ＊) occurs. This can be a cause of an overcurrent. Therefore, in order to suppress the potential difference immediately after the sudden change in voltage of the AC system 5, it is beneficial to perform feedforward of the measured voltage as shown in Equation (15) and Equation (16).

[0067] In FIG. 6, the AC voltage measurement value V ｏｕｔ = (V ｏｕｔ＿ｄ , V ｏｕｔ＿ｑ ) and a feedforward path to second voltage command generator 310, a low-pass filter 312 is applied to remove noise. A time constant Tv1 of, for example, 3 ms is set to low-pass filter 312. Furthermore, in FIG. 6, an accident determination unit 315 is provided. Accident determination unit 315 can also be applied to the configuration example shown in FIG. 1 or FIG. 3.

[0068] When it is determined that a grid fault has occurred, the fault determination unit 315 changes the time constant of the low-pass filter 312 from Tv1 to Tv2. The time constant Tv2 satisfies Tv2<Tv1 and is, for example, 1 ms. By reducing the time constant, voltage changes during a grid fault can be more quickly reflected in the generation of current commands and second voltage commands. Note that when Tv2=0 ms and it is determined that a grid fault has occurred, the measured voltage can be used directly for command generation and control without passing through the low-pass filter 312, i.e., by bypassing the low-pass filter 312.

[0069] The accident determination unit 315 determines, for example, the AC voltage measurement value V ｏｕｔ The d-axis component of V ｏｕｔ＿ｄ is the fault determination threshold voltage, for example, when it is less than 80% of the rated voltage, or when the apparent current command √((I ｄ ＊ ) ２ + (I ｑ ＊ ) ２ ) exceeds a fault determination threshold current, for example, 120% of the rated current, the fault determination unit 315 determines that a grid fault has occurred. ｏｕｔ The d-axis component of V ｏｕｔ＿ｄ is equal to or greater than the fault determination threshold voltage, and the apparent current command √((I ｄ＊ ) ２ + (I ｑ ＊ ) ２ ) is equal to or less than the fault determination threshold current, it is determined that no grid fault has occurred or that the grid fault has been removed.

[0070] In this way, the fault determination unit 315 determines whether a grid fault has occurred based on two conditions, voltage and current, on the fact that the voltage drops when a grid fault occurs and that the current command increases in response to the voltage drop. When the fault determination unit 315 determines that a grid fault has occurred, it reduces the time constant of the low-pass filter 312, thereby more quickly reflecting the voltage change during the grid fault in the generation of the current command and the second voltage command. This makes it possible to suppress overcurrent when a grid fault occurs and when the grid is restored from the fault.

[0071] <Major Effects of the Fourth Embodiment> As described above, the use of the power conversion device 1 according to the fourth embodiment also provides the same effects as those described in the previous embodiments. ＊ By generating the above, it is possible to suppress the potential difference immediately after a sudden voltage change in the AC system 5 due to a system fault, and to suppress overcurrent. Furthermore, by determining whether or not a system fault has occurred and changing the time constant of the low-pass filter 312 for the measured voltage in accordance with the determination result, it is possible to suppress overcurrent by prioritizing noise removal performance under normal circumstances and responsiveness when a system fault has occurred.

[0072] Fifth Embodiment <Details of the Power Conversion Device> In the fifth embodiment, the same parts as those in the above-described embodiments will not be described. A schematic configuration example of the power conversion device 1 according to the fifth embodiment is the same as the configuration example shown in FIG. 1 or 3.

[0073] 7 is a block diagram showing a detailed configuration example of each part in FIG. 1 or FIG. 3 in a power conversion device according to a fifth embodiment. In FIG. 1 or FIG. 3, when a fault occurs in the AC system 5 and a voltage imbalance occurs, the AC voltage measurement value V ｏｕｔ The d-axis component of V ｏｕｔ＿ｄ and the q-axis component Vｏｕｔ＿ｑ In this case, vibrations at a frequency twice the reference frequency, for example, 50 Hz, appear as negative-phase components. This causes the current command to oscillate, and a negative-phase current is output. As a result, the power conversion device 1 may have difficulty continuing to operate due to the occurrence of an overcurrent or the like. Therefore, it is beneficial to use the configuration shown in FIG. 7 to operate the power conversion device 1 so as not to output a negative-phase current.

[0074] 7, the second voltage command generating unit 310, like the case of FIG. 6, receives the AC current measurement value I in the α-β coordinate system from the current command restricting unit 309. ｏｕｔ = (I ｏｕｔ＿α , I ｏｕｔ＿β ), the phase command θ ＊ By performing a dq transformation using ｏｕｔ ＋ = (I ｏｕｔ＿ｄ ＋ , I ｏｕｔ＿ｑ ＋ Then, the second voltage command generating unit 310 calculates the deviation from the current command from the current command restricting unit 309 for each of the d-axis and q-axis components, and calculates the voltage command using the above-mentioned equations (15) and (16).

[0075] However, in the fifth embodiment, the calculation result is not the second voltage command but the positive-phase voltage command V ＋＊ = (V ｄ ＋＊ , V ｑ ＋＊ The main circuit control unit 311 determines the positive sequence voltage command V ＋＊ = (V ｄ ＋＊ , V ｑ ＋＊ ), the phase command θ ＊ By performing an inverse dq transformation using ＋＊ = (V α ＋＊ , V β ＋＊ ) to

[0076] The second voltage command generator 310 also calculates the AC current measurement value I in the α-β coordinate system. ｏｕｔ = (I ｏｕｔ＿α , I ｏｕｔ＿β ), the phase command θ＊ multiplied by -1 -θ ＊ By performing a dq transformation using ｏｕｔ － = (I ｏｕｔ＿ｄ － , I ｏｕｔ＿ｑ － ) is calculated. That is, the negative-phase-sequence current is calculated by such a calculation. In order to prevent the output of the negative-phase-sequence current, current control may be performed by setting the current command to 0.

[0077] Therefore, the second voltage command generator 310 calculates the negative-phase current command (0, 0) and the negative-phase measurement current I ｏｕｔ － = (I ｏｕｔ＿ｄ － , I ｏｕｔ＿ｑ － ) is taken for each d-axis / q-axis component, and the negative-phase voltage command V is calculated using the following equations (17) and (18). －＊ = (V ｄ －＊ , V ｑ －＊ Then, the main circuit control unit 311 calculates the negative-phase voltage command V －＊ = (V ｄ －＊ , V ｑ －＊ ), the phase command θ ＊ multiplied by -1 -θ ＊ By performing an inverse dq transformation using －＊ = (V α －＊ , V β －＊ ) to

[0078]

[0079] Second voltage command V after three-phase to two-phase conversion ＊ = (V α ＊ , V β ＊ ) is expressed as the positive sequence voltage command V ＋＊ = (V α ＋＊ , V β＋＊ ) and negative-phase voltage command V －＊ = (V α －＊ , V β －＊ ) and

[0080] Although not shown in the figure, the main circuit control unit 311 calculates the second voltage command V ＊ = (V α ＊ , V β ＊ ) is subjected to inverse three-phase to two-phase conversion to generate voltage commands for each of the three phases (R, S, T). Furthermore, the main circuit control unit 311 generates gate signals for the switching elements in the power conversion circuit 2 based on the voltage commands for each of the three phases, and drives the power conversion circuit 2.

[0081] <Major Effects of Fifth Embodiment> As described above, the use of the power conversion device 1 according to the fifth embodiment also provides the same effects as those described in the previous embodiments. ＋＊ and a negative-phase voltage command V that is controlled so that the negative-phase current becomes zero. －＊ The second voltage command V ＊ As a result, the power conversion device 1 can continue to operate even if an unbalanced fault occurs.

[0082] (Sixth embodiment) <Outline of power conversion device> Fig. 8 is a schematic diagram showing a partial configuration example of a power conversion device according to a sixth embodiment. The power conversion device 1 shown in Fig. 8 includes a user interface 316 in addition to the current command constraint unit 309 described in Fig. 5 and the fault determination unit 315 described in Fig. 6. The current command constraint unit 309, as described in Fig. 5, determines the threshold current I ｌｉｍ Based on this, the current command I ＊ The fault determination unit 315 determines whether or not a grid fault has occurred based on the fault determination threshold voltage or the fault determination threshold current, as described in FIG.

[0083] Here, the threshold current I ｌｉｍThe optimum values ​​of the fault determination threshold current and the fault determination threshold voltage may change depending on, for example, the state of the AC grid 5. The optimum value of the fault determination threshold voltage may also change depending on, for example, the impedance of the AC grid 5. Therefore, the user interface 316 allows the user 16 to change the threshold current I ｌｉｍ , the accident determination threshold voltage V ｔｈ or the fault determination threshold current I ｔｈ Accepts setting input.

[0084] In this example, the user interface 316 displays the setting screen 17 on the information terminal 15 of the user 16, and acquires the value input by the user 16 on the setting screen 17. Then, the user interface 316 calculates the acquired threshold current I ｌｉｍ is set in the current command constraint unit 309, and the acquired fault determination threshold voltage V ｔｈ or the fault determination threshold current I ｔｈ is set in the accident determination unit 315.

[0085] This allows flexible adaptation to various system configurations to which the power conversion device 1 is applied. The setting screen 17 may be, for example, an operation panel or the like provided in the power conversion device 1. The setting items include a threshold current I ｌｉｍ , the accident determination threshold voltage V ｔｈ , fault determination threshold current I ｔｈ It may include, for example, the impedance Z as described in equations (4) and (5).

[0086] The invention made by the inventor has been specifically described above based on the embodiments, but the present invention is not limited to the above embodiments and can be modified in various ways without departing from the spirit of the invention. For example, the above-described embodiments have been described in detail to clearly explain the present invention, and the present invention is not necessarily limited to those including all of the described configurations. Furthermore, it is possible to replace part of the configuration of one embodiment with the configuration of another embodiment, and it is also possible to add the configuration of another embodiment to the configuration of one embodiment. Furthermore, it is possible to add, delete, or replace part of the configuration of each embodiment with other configurations.

[0087] 1...power conversion device, 2...power conversion circuit, 3...control circuit, 4...AC filter, 5...AC system, 6...AC load, 7...DC power supply, 301...measurement unit, 302...power calculation unit, 303...active power command unit, 304...angular frequency command generation unit, 305...phase command generation unit, 306...reactive power command unit, 307...first voltage command generation unit, 308...current command generation unit, 309...current command constraint unit, 310...second voltage command generation unit, 311...main circuit control unit, 315...fault determination unit

Claims

1. A power conversion device connected to a DC power source, comprising: a power conversion circuit having one end connected to the DC power source and the other end connected to an AC power source and an AC load; a measurement unit that outputs a measured voltage and a measured current on the AC side of the power conversion circuit; a power calculation unit that calculates an output active power and an output reactive power from the measured voltage and the measured current; an active power command unit that determines an active power command for the power conversion device; an angular frequency command generation unit that generates an angular frequency command from the output active power and the active power command; a phase command generation unit that generates a phase command from the angular frequency command; a reactive power command unit that determines a reactive power command from the measured voltage or an external input value; a first voltage command generation unit that generates a first voltage command from the output reactive power and the reactive power command; a current command generation unit that generates a current command from the measured voltage and the first voltage command; a current command constraint unit that constrains and outputs the current command; and a second voltage command generation unit that generates a second voltage command from the current command constrained by the current command constraint unit and the measured current, and controls the power conversion circuit based on the second voltage command. Power conversion device.

2. A power conversion device according to claim 1, wherein the current command generation unit uses a preset virtual impedance value to generate the current command by dividing the difference between the measured voltage and the first voltage command by the virtual impedance value.

3. A power conversion device according to claim 1, wherein the active power command unit receives an active power command value and outputs the received active power command value as the active power command.

4. A power conversion device according to claim 1, wherein the active power command unit receives an active power command value, corrects the received active power command value so as to suppress changes in the angular frequency command, and outputs the corrected value as the active power command.

5. A power conversion device according to claim 4, wherein the active power command unit corrects the input active power command value with the measured voltage and outputs the corrected value as the active power command.

6. A power conversion device according to claim 5, wherein the active power command unit calculates the square of the value obtained by dividing the measured voltage by the AC end rated voltage of the power conversion circuit, multiplies the calculated value by the input active power command value, and outputs the result as the active power command.

7. A power conversion device according to claim 1, wherein, when the square root of the sum of squares of the d-axis component and the q-axis component of the current command exceeds a preset threshold current, the current command constraint unit divides each of the d-axis component and the q-axis component of the current command by the square root of the sum of squares and then multiplies the result by the threshold current, and outputs the resultant value; and when the square root of the sum of squares is equal to or less than the threshold current, the current command generated by the current command generation unit is output as is.

8. The power conversion device according to claim 7, further comprising a user interface that accepts a user's input for setting the threshold current.

9. A power conversion device according to claim 1, wherein the second voltage command generation unit generates, as the second voltage command, a value obtained by adding the measured voltage to a voltage value calculated from the current command constrained by the current command constraint unit and the measured current.

10. A power conversion device according to claim 1, further comprising an accident determination unit that determines whether or not an accident has occurred in the AC power supply, wherein the current command generation unit, when the accident determination unit determines that no accident has occurred, applies a filter to the measured voltage to generate the current command, and when the accident determination unit determines that an accident has occurred, generates the current command without applying the filter to the measured voltage, or applies a filter having a time constant different from that of the filter to the measured voltage to generate the current command.

11. A power conversion device according to claim 10, wherein the fault determination unit determines that a fault has occurred when the measured voltage is equal to or lower than a preset threshold voltage, or when the root-sum-square of the d-axis component and q-axis component of the current command exceeds a preset threshold current.

12. The power conversion device according to claim 11, further comprising a user interface that accepts a user's input for setting the threshold voltage or the threshold current.

13. A power conversion device according to claim 1, wherein the second voltage command generation section: calculates a positive-sequence measured current consisting of a d-axis component and a q-axis component by performing a dq transform on the measured current using the phase command; generates a positive-sequence voltage command from the positive-sequence measured current and the current command constrained by the current command constraint section; calculates a negative-sequence measured current consisting of a d-axis component and a q-axis component by performing a dq transform on the measured current using a value obtained by multiplying the phase command by -1; generates a negative-sequence voltage command from the negative-sequence measured current and a preset negative-sequence current command value; and generates the second voltage command as the sum of the positive-sequence voltage command and the negative-sequence voltage command.

14. The power conversion device according to claim 13, wherein the second voltage command generation unit sets the negative-phase current command value to 0.

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