Power conditioner and frequency control method

Abstract

A power conditioner according to the present invention comprises: an inverter that converts direct-current power outputted from a battery to alternating-current power and supplies the alternating-current power to a power system; and a control device that controls the inverter. The control device calculates a frequency variation amount for the alternating-current power on the basis of a differential power that is the difference between the alternating-current power and a target power. The control device also calculates a correction amount for correcting the frequency variation amount such that the frequency variation amount is below a threshold value. The control device also sets a target frequency at the frequency obtained by adding the rated frequency of the inverter to the difference between the frequency variation amount and the correction amount. The control device also outputs instructions to the inverter such that the frequency of the alternating-current power approaches the target frequency.

Description

Power conditioner and frequency control method

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

[0002] Patent Literature 1 discloses a power system stabilization system for stabilizing a power system. Specifically, the power system stabilization system calculates the frequency change rate of each generator in the event of a hypothetical fault. Then, the power system stabilization system calculates the range of the frequency change rate up to its stability limit based on the generator with the largest frequency change rate. Furthermore, the power system stabilization system calculates the amount of change in active power of a power converter based on the range of the frequency change rate up to its stability limit and registers the calculated amount of change in active power in a database. As a result, if a hypothetical fault actually occurs, stabilization can be achieved in the event of an actual fault by implementing a predetermined stabilization measure.

[0003] Japanese Patent Publication No. 2023-060571

[0004] Consider a case where the frequency of the AC power output from a power conditioner (specifically, an inverter) drops due to a power grid fault. In this case, the AC power increases as the frequency of the AC power decreases, and it is assumed that the AC power exceeds the rated power of the inverter. Furthermore, due to the characteristics of the inverter, the ratio of the inverter's output voltage to its frequency is controlled to be constant. For example, if the frequency of the AC power decreases, the inverter's output voltage also decreases due to the inverter's characteristics. On the other hand, since the AC power increases, the inverter's output current becomes an overcurrent state.

[0005] According to Patent Document 1, when the frequency of AC power output from an inverter fluctuates, control is performed to stabilize the AC power. However, since the frequency remains low, the output voltage of the inverter also remains low, and there is a risk that the output current of the inverter will become an overcurrent state.

[0006] One object of the present disclosure is to provide a technology that can prevent the output current of an inverter from becoming an overcurrent state when the frequency of AC power output from the inverter fluctuates.

[0007] A first aspect of the present disclosure relates to a power conditioner. The power conditioner includes an inverter that converts DC power output from a battery into AC power and supplies the AC power to a power grid, and a control device that controls the inverter. The control device calculates a frequency change amount of the AC power based on a differential power that is the difference between the AC power and a target power. The control device also calculates a correction amount for correcting the frequency change amount so that the frequency change amount is less than a threshold. The control device then sets a frequency obtained by adding a rated frequency of the inverter to the difference between the frequency change amount and the correction amount as a target frequency. The control device also issues an instruction to the inverter to bring the frequency of the AC power closer to the target frequency.

[0008] A second aspect of the present disclosure relates to a frequency control method, which includes: converting DC power output from a battery into AC power using an inverter, and supplying the AC power to a power grid; calculating a frequency change amount of the AC power based on a differential power that is a difference between the AC power and a target power; calculating a correction amount for correcting the frequency change amount so that the frequency change amount is less than a threshold value; setting a frequency obtained by adding a rated frequency of the inverter to the difference between the frequency change amount and the correction amount as a target frequency; and issuing an instruction to the inverter to bring the frequency of the AC power closer to the target frequency.

[0009] According to the present disclosure, a frequency change amount of AC power is calculated based on a differential power, which is the difference between AC power and a target power. Then, a correction amount is calculated to correct the frequency change amount so that the frequency change amount is less than a threshold value. Furthermore, a frequency obtained by adding the rated frequency of the inverter to the difference between the frequency change amount and the correction amount is set as a target frequency, and an instruction is issued to the inverter to bring the frequency of the AC power closer to the target frequency. As a result, when the frequency of the AC power output from the inverter fluctuates, the frequency of the AC power is corrected, and the AC power is also corrected so that the differential power becomes smaller. Therefore, it is possible to prevent the output current output from the inverter from becoming an overcurrent state, and it is possible to stabilize the power system.

[0010] It is a diagram for explaining an overview of a power conversion system. It is an explanatory diagram showing a specific example of a control device according to an embodiment. It is an explanatory diagram showing a specific example of a control device according to an embodiment. It is a flowchart showing an example of processing by a control device according to an embodiment. It is an explanatory diagram showing an example of an output result of an inverter according to an embodiment.

[0011] A power conditioner and a frequency control method according to an embodiment of the present disclosure will be described with reference to the accompanying drawings. Elements common to the drawings will be designated by the same reference numerals, and duplicated descriptions will be omitted.

[0012] 1. Overview of the Power Conversion System Fig. 1 is a diagram for explaining an overview of a power conversion system 1. The power conversion system 1 includes a battery 11, a power conditioner 10, a transformer 20, and a power grid 30. The power conditioner 10 includes an inverter 12 and a control device 100.

[0013] The battery 11 is an electricity storage device that stores electricity generated by renewable energy, such as solar power, wind power, and hydropower.

[0014] The inverter 12 is a device that converts DC power output from the battery 11 into AC power and supplies the AC power to the power grid 30 via a transformer 20. An example of the inverter 12 is a voltage-controlled GFM inverter.

[0015] The control device 100 is connected to the inverter 12 and controls the inverter 12. The output voltage Vs and output current Io output from the inverter 12 are input to the control device 100. The control device 100 calculates the AC power output from the inverter 12 based on the output voltage Vs and the output current Io.

[0016] The output voltage Vs and output current Io input to the control device 100 are detected values ​​of the output voltage Vs (hereinafter referred to as the detected Vs value) and the output current Io (hereinafter referred to as the detected Io value). The detected Vs value and the detected Io value are detected by a detector (not shown) provided between the power conditioner 10 and the power grid 30. In the example shown in FIG. 1 , the detected Vs value and the detected Io value are detected between the power conditioner 10 and the transformer 20. However, this is not limiting. The detected Vs value and the detected Io value may be detected, for example, between the transformer 20 and the power grid 30.

[0017] Furthermore, the output voltage Vs output from the inverter 12 is made up of three-phase voltages (Vsu, Vsv, Vsw), and the output current Io output from the inverter 12 is made up of three-phase currents (Iou, Iov, Iow). That is, the above-mentioned Vs detection value includes the Vsu detection value, the Vsv detection value, and the Vsw detection value, and the above-mentioned Io detection value includes the Iou detection value, the Iov detection value, and the Iow detection value.

[0018] The control device 100 executes VSG (Virtual Synchronous Generator) control 101 based on the input Vs detected value and Io detected value. A VSG is a virtual synchronous generator that simulates the dynamic characteristics of a synchronous generator in the inverter 12. In other words, the VSG control 101 refers to controlling a virtual synchronous generator. Furthermore, parameters that indicate the dynamic characteristics of a synchronous generator are used in the VSG control 101. Examples of such parameters include an inertia constant M and a damping constant D.

[0019] The control device 100 performs frequency control 102, which controls the frequency of AC power (composed of output voltage Vs and output current Io) output from the inverter 12, based on the target frequency Fref obtained by the VSG control 101. Specifically, in the frequency control 102, the control device 100 issues an instruction to the inverter 12 to bring the frequency of the AC power closer to the target frequency Fref. An example of calculating the target frequency Fref will be described in detail later.

[0020] As a result, even if the frequency fluctuates due to a grid fault, the inverter 12 can output AC power with a stable frequency. Therefore, the inverter 12 can continue to operate, preventing the power grid 30 from becoming unstable. Note that functional requirements for continuing inverter operation to prevent the power grid 30 from being simultaneously disconnected in the event of a grid fault are defined. These functional requirements are called Fault Ride Through (FRT) requirements. Examples of FRT requirements include that the frequency of the AC power is within a specified range and that the rate of change of the frequency of the AC power is within a reference value. According to this embodiment, the frequency of the AC power is controlled based on the FRT requirements. For example, when the frequency of the AC power is decreasing and the power value of the AC power is increasing, the frequency of the AC power is controlled.

[0021] Furthermore, the control device 100 may perform voltage control 103, which controls the output voltage Vs from the inverter 12 based on the target voltage Vref obtained by the VSG control. For example, when a voltage drop in the AC voltage of the power grid 30 is equal to or greater than a predetermined value due to fluctuations in the AC voltage, the control device 100 instructs the inverter 12 to adjust the output voltage Vs to approach the target voltage Vref. This allows the inverter 12 to output a stable voltage even if a low voltage ride-through (LVRT) occurs along with frequency fluctuations. This prevents the power grid 30 from becoming unstable.

[0022] 2. Examples of Control Devices 2-1. Configuration Examples The control device 100 has hardware that realizes various functions. The hardware may be a processing circuit, or a computer that executes programs stored in a storage device using a CPU. Examples of processing circuits include FPGAs (Field Programmable Gate Arrays) and ASICs (Application Specific Integrated Circuits).

[0023] 2-2. Example of Calculation of Target Frequency FIG. 2 is an explanatory diagram showing a specific example of the function of the control device 100 according to the embodiment. FIG. 2 shows an example of calculation of the target frequency Fref in the VSG control 101. Specifically, in the VSG control 101, the control device 100 calculates the difference between the AC power P output from the inverter 12 and the target power Pref as the differential power ΔP. However, the target frequency Fref calculated in the example shown in FIG. 2 is a dimensionless value based on the rated frequency, and dimensionless values ​​are also used for the target power Pref and the AC power P. To make these powers dimensionless, for example, the rated power is used. In other words, values ​​when the rated power is set to 1 are used as the values ​​of the target power Pref and the AC power P in the calculation shown in FIG. 2.

[0024] Then, the control device 100 calculates the frequency change ΔFc of the AC power P based on the differential power ΔP. Specifically, the control device 100 sets the frequency obtained by multiplying the differential power ΔP by a parameter indicating the dynamic characteristics of the synchronous generator as the frequency change ΔFc. For example, when the inertia constant is H, the damping constant is D, and the unit time is s, the frequency change ΔFc of the AC power P is expressed by the following equation (1):

[0025]

[0026] Furthermore, the control device 100 calculates a correction amount ΔFfrt for correcting the frequency change amount ΔFc so that the frequency change amount ΔFc is less than a threshold value. An example of calculating the correction amount ΔFfrt will be described in detail later.

[0027] Furthermore, the control device 100 determines whether correction of the frequency change amount ΔFc is necessary based on a predetermined condition. The predetermined condition is when the frequency change amount ΔFc is greater than a predetermined value and the AC power P is greater than the inverter 12's allowable overpower, i.e., the maximum power that is allowable to be output in excess of the rated power for a short period of time. The allowable overpower is set to, for example, 130% of the rated power. If the predetermined condition is met, the control device 100 determines that correction of the frequency change amount ΔFc is necessary. In this case, the control device 100 subtracts the correction amount ΔFfrt from the frequency change amount ΔFc. On the other hand, if the predetermined condition is not met, the control device 100 determines that correction of the frequency change amount ΔFc is not necessary. In this case, the control device 100 does not subtract the correction amount ΔFfrt from the frequency change amount ΔFc. In the example shown in FIG. 2 , if it is determined that correction of the frequency change amount ΔFc is not necessary, the value of the correction amount to be subtracted from the frequency change amount ΔFc is set to zero.

[0028] The control device 100 then sets the second frequency F2 obtained by adding the rated frequency Fn of the inverter 12 to the first frequency F1, which is obtained by subtracting the correction amount ΔFfrt from the frequency change amount ΔFc, as the target frequency Fref. The rated frequency Fn is a reference value for calculating the dimensionless target frequency Fref as described above, and is set to 1. The rated frequency before dimensionlessization is, for example, 50 Hz or 60 Hz.

[0029] 2-3. Example of Calculation of Correction Amount of Frequency Change FIG. 3 is an explanatory diagram showing a specific example of the control device 100 according to the embodiment. FIG. 3 shows an example of calculation of the correction amount ΔFfrt for correcting the frequency change amount ΔFc. Specifically, in the VSG control 101, the control device 100 calculates the difference between the frequency F of the AC power P and the rated frequency Fn as the differential frequency ΔF. As the frequency F, a dimensionless value based on the actual measured value of the frequency of the AC power P is used. The actual measured value of the frequency of the AC power P is acquired, for example, via a PLL (Phase Locked Loop) provided in the control device 100. For example, when the PLL is locked, there is no difference between the actual measured value of the frequency of the AC power P and the reference frequency (e.g., 50 Hz or 60 Hz), and it can be confirmed that the frequency of the AC power P is not fluctuating. On the other hand, when the PLL is unlocked, a difference occurs between the measured frequency of the AC power P and the reference frequency, and it can be confirmed that the frequency of the AC power P is fluctuating.

[0030] Then, based on the differential frequency ΔF, the control device 100 calculates a power variation ΔPc of the AC power P. Specifically, the control device 100 sets the power obtained by multiplying the differential frequency ΔF by a parameter indicating the dynamic characteristics of the synchronous generator as the power variation ΔPc. For example, when the inertia constant is H, the damping constant is D, and the unit time is s, the power variation ΔPc of the AC power P is expressed by the following equation (2):

[0031]

[0032] The above-mentioned formula (2) can also be expressed as the following formula (3) or formula (4). Note that RoCoF (Rate of Change of Frequency) means the rate of change of frequency. The unit of RoCoF is Hz / s.

[0033]

[0034]

[0035] Furthermore, the control device 100 first subtracts the power change amount ΔPc from the target power Pref to obtain a first power P1, and then subtracts the allowable overpower Pmax of the inverter 12 from the first power P1 to obtain a second power P2. For example, if the allowable overpower Pmax is set to 130% of the rated power, the value of the allowable overpower Pmax is 1.3. The control device 100 sets the third power P3 obtained by multiplying the second power P2 by a predetermined coefficient as the correction amount ΔFfrt. The predetermined coefficient is expressed, for example, by the equation (1 / D) obtained by dividing 1 by a damping constant D.

[0036] 2-4. Processing Example Fig. 4 is a flowchart outlining a processing example of the control device 100 according to the embodiment.

[0037] In step S100, the control device 100 calculates the frequency change amount ΔFc of the AC power P based on the differential power ΔP, which is the difference between the AC power P and the target power Pref. Thereafter, the process proceeds to step S110.

[0038] In step S110, the control device 100 calculates a correction amount ΔFfrt for correcting the frequency change amount ΔFc so that the frequency change amount ΔFc is less than a threshold value. After that, the process proceeds to step S120.

[0039] In step S120, the control device 100 determines whether or not correction of the frequency change amount ΔFc is necessary based on predetermined conditions. If it is determined that correction of the frequency change amount ΔFc is necessary (step S120; Yes), the process proceeds to step S130. Otherwise (step S120; No), the process ends. The predetermined conditions are as described above.

[0040] In step S130, the control device 100 calculates the target frequency Fref. Specifically, the control device 100 sets the target frequency Fref to a frequency obtained by adding the rated frequency Fn of the inverter 12 to the difference between the frequency change amount ΔFc and the correction amount ΔFfrt. Thereafter, the process proceeds to step S140.

[0041] In step S140, the control device 100 issues an instruction to the inverter 12 to bring the frequency of the AC power P closer to the target frequency Fref.

[0042] 4. Example of Inverter Output Result FIG. 5 is an explanatory diagram showing an example of an output result of the inverter 12 according to the embodiment. FIG. 5(A) shows an example of a waveform of the frequency F when the frequency F of the AC power P fluctuates. FIG. 5(B) shows an example of a waveform of the AC power P before the frequency F of the AC power P is corrected. FIG. 5(C) shows an example of a waveform of the AC power P after the frequency F of the AC power P is corrected. Note that the vertical axes of the graphs shown in FIGS. 5(B) and 5(C) are expressed in PU (Per Unit), but may be expressed as a percentage. In the examples shown in FIGS. 5(B) and 5(C), the allowable overpower Pmax is expressed as "1.3 PU."

[0043] Fig. 5(A) shows an example in which a grid fault occurs at time 2.0 s. Thereafter, the frequency F drops from 50 Hz to 47.5 Hz. In this case, the AC power P before the frequency F of the AC power P is corrected exceeds the allowable overpower Pmax, as shown in Fig. 5(B). On the other hand, the AC power P after the frequency F of the AC power P is corrected can be prevented from exceeding the allowable overpower Pmax, as shown in Fig. 5(C).

[0044] In this way, when a grid fault occurs and the frequency F of the AC power P fluctuates (e.g., the frequency drops), the AC power P is controlled so as not to exceed the allowable overpower Pmax by correcting the frequency F of the AC power P. This makes it possible to prevent the output current Io output from the inverter 12 from becoming an overcurrent.

[0045] 5. Effects According to the power conditioner 10 of this embodiment, a frequency change ΔFc of the AC power P is calculated based on a differential power ΔP, which is the difference between the AC power P and the target power Pref. Then, a correction amount ΔFfrt is calculated to correct the frequency change ΔFc so that the frequency change ΔFc is less than a threshold value. Furthermore, a frequency obtained by adding the rated frequency Fn of the inverter 12 to the difference between the frequency change ΔFc and the correction amount ΔFfrt is set as the target frequency Fref, and an instruction is issued to the inverter 12 to bring the frequency F of the AC power P closer to the target frequency Fref. As a result, when the frequency F of the AC power P fluctuates, the frequency F of the AC power P is corrected, and the AC power P is also corrected so that the differential power ΔP is reduced. Therefore, it is possible to prevent the output current Io output from the inverter 12 from becoming an overcurrent, and it is possible to stabilize the power grid 30.

[0046] 1...power conversion system, 10...power conditioner, 11...battery, 12...inverter, 20...transformer, 30...power system, 100...control device, 101...VSG control, 102...frequency control, 103...voltage control

Claims

1. A power conditioner comprising: an inverter that converts DC power output from a battery into AC power and supplies the AC power to a power grid; and a control device that controls the inverter, wherein the control device is configured to: calculate a frequency change amount of the AC power based on a differential power that is the difference between the AC power and a target power; calculate a correction amount for correcting the frequency change amount so that the frequency change amount is less than a threshold value; set a frequency obtained by adding a rated frequency of the inverter to the difference between the frequency change amount and the correction amount as a target frequency; and issue an instruction to the inverter to bring the frequency of the AC power closer to the target frequency.

2. A power conditioner as claimed in claim 1, characterized in that the control device is configured to correct the amount of frequency change when the amount of frequency change is greater than a predetermined value and the AC power is greater than the allowable overpower of the inverter.

3. A power conditioner according to claim 1, characterized in that the control device is configured to set the frequency obtained by multiplying the differential power by a parameter indicating the dynamic characteristics of a synchronous generator as the frequency change amount.

4. A power conditioner as claimed in claim 1, characterized in that the control device is configured to: calculate a power change amount of the AC power based on a differential frequency which is the difference between an actual measured value of the frequency of the AC power and the rated frequency; and set as the correction amount a third power obtained by multiplying a second power obtained by subtracting an allowable overpower of the inverter from a first power obtained by subtracting the power change amount from the target power, by a predetermined coefficient.

5. A power conditioner according to claim 4, characterized in that the control device is configured to set the power obtained by multiplying the difference frequency by a parameter indicating the dynamic characteristics of a synchronous generator as the amount of power change.

6. A power conditioner as described in claim 1, characterized in that the control device is configured to perform voltage control so as to bring the output voltage closer to a target voltage when a voltage drop of the output voltage of the inverter due to fluctuations in the output voltage is equal to or greater than a predetermined value.

7. A frequency control method comprising: using an inverter to convert DC power output from a battery into AC power, and supplying the AC power to a power grid; calculating a frequency change amount of the AC power based on a differential power that is the difference between the AC power and a target power; calculating a correction amount for correcting the frequency change amount so that the frequency change amount is less than a threshold value; setting a frequency obtained by adding a rated frequency of the inverter to the difference between the frequency change amount and the correction amount as a target frequency; and issuing an instruction to the inverter to bring the frequency of the AC power closer to the target frequency.

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