Power conditioner and frequency control method

Abstract

A power conditioner is provided with: an inverter that converts DC power output from a battery into AC power and supplies the AC power to a power system; and a control device that controls the inverter. The control device calculates the frequency change amount of the AC power on the basis of differential power, which is the difference between the AC power and the target power. In addition, the control device calculates a correction amount for correcting the frequency change amount so that the frequency change amount is less than a threshold value. Further, the control device sets, as a target frequency, a frequency obtained by adding a rated frequency of the inverter to a difference between the frequency change amount and the correction amount. Further, the control device instructs the inverter such that the frequency of the AC power approaches the target frequency.

Description

Technical Field

[0001] The present disclosure relates to techniques for controlling the frequency of a power regulator. Background Art

[0002] Patent document 1 discloses an electric power system stabilization system for stabilizing an electric power system. Specifically, the electric power system stabilization system calculates the frequency change rate of each generator when an accident is assumed. Then, the electric power system stabilization system calculates the range up to the stability limit of the frequency change rate based on the generator with the largest frequency change rate. Furthermore, the electric power system stabilization system calculates the change in the effective power of the power converter based on the range up to the stability limit of the frequency change rate and registers it in a database. Thus, in the event that the assumed accident actually occurs, stabilization can be achieved during the actual accident by executing a predetermined stabilization measure.

[0003] Prior art literature

[0004] Patent Literature

[0005] Patent Document 1: Japanese Patent Application Publication No. 2023-060571 Summary of the invention

[0006] Problems to be solved by the invention

[0007] Consider a case where the frequency of the AC power output from the power conditioner (specifically, the inverter) decreases due to a system accident in the power system. In this case, it is assumed that the AC power increases as the frequency of the AC power decreases, and the AC power exceeds the rated power of the inverter. In addition, in the characteristics of the inverter, the ratio of the output voltage and frequency of the inverter is controlled to be constant. For example, when the frequency of the AC power decreases, the output voltage of the inverter also decreases according to the characteristics of the inverter. On the other hand, since the AC power increases, the output current of the inverter becomes an overcurrent state.

[0008] According to Patent Document 1, when the frequency of the AC power output from the inverter changes, control is performed to stabilize the AC power. However, since the frequency remains reduced, the output voltage of the inverter also becomes low, and the output current of the inverter may become an overcurrent state.

[0009] One object of the present disclosure is to provide a technology that can suppress an output current of an inverter from becoming an overcurrent state when the frequency of AC power output from an inverter varies.

[0010] Means for solving problems

[0011] 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 system; and a control device that controls the inverter. The control device calculates a frequency change amount of the AC power based on a difference between the AC power and a target power, that is, a differential power. The control device calculates a correction amount for correcting the frequency change amount such that the frequency change amount is less than a threshold value. Further, the control device sets, as a target frequency, a frequency obtained by adding a difference between the frequency change amount and the correction amount to a rated frequency of the inverter. And the control device gives an instruction to the inverter so that the frequency of the AC power approaches the target frequency.

[0012] A second aspect of the present invention relates to a frequency control method. The frequency control method includes: using an inverter to convert DC power output from a battery into AC power and supply the AC power to a power system; calculating a frequency change amount of the AC power based on a difference between the AC power and a target power, that is, a differential power; calculating a correction amount for correcting the frequency change amount such that the frequency change amount is less than a threshold value; setting, as a target frequency, a frequency obtained by adding a difference between the frequency change amount and the correction amount to a rated frequency of the inverter; and giving an instruction to the inverter so that the frequency of the AC power approaches the target frequency.

[0013] Advantages of the Invention

[0014] According to the present disclosure, a frequency change amount of the AC power is calculated based on a difference between the AC power and a target power, that is, a differential power. Then, a correction amount for correcting the frequency change amount such that the frequency change amount is less than a threshold value is calculated. Further, a frequency obtained by adding a difference between the frequency change amount and the correction amount to a rated frequency of the inverter is set as a target frequency, and an instruction is given to the inverter so that the frequency of the AC power approaches the target frequency. Thereby, when the frequency of the AC power output from the inverter changes, the frequency of the AC power is corrected, and the AC power is also corrected so as to reduce the differential power. Therefore, it is possible to suppress the output current output from the inverter from becoming an overcurrent state and stabilize the power system. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Figure 1 is a diagram for explaining the outline of a power conversion system.

[0016] Figure 2 is an explanatory diagram showing a specific example of a control device according to an embodiment.

[0017] Figure 3 is an explanatory diagram showing a specific example of a control device according to an embodiment.

[0018] Figure 4 is a flowchart showing a processing example of a control device according to an embodiment.

[0019] Figure 5 It is an explanatory diagram showing an example of output results of the inverter according to the embodiment. DETAILED DESCRIPTION

[0020] The power conditioner and the frequency control method according to the embodiment of the present disclosure will be described with reference to the drawings. In addition, the same reference numerals are attached to the common elements in each figure, and the repeated description is omitted.

[0021] 1. Overview of power conversion system

[0022] Figure 1 1 is a diagram for explaining the outline of the power conversion system 1. The power conversion system 1 includes a battery 11, a power conditioner 10, a transformer 20, and a power system 30. The power conditioner 10 includes an inverter 12 and a control device 100.

[0023] The battery 11 is a power storage device that stores electricity generated by renewable energy. Examples of renewable energy include sunlight, wind power, and water power.

[0024] 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 system 30 via the transformer 20. As the inverter 12, a voltage-controlled GFM inverter is exemplified.

[0025] The control device 100 is connected to the inverter 12 and controls the inverter 12. The output voltage Vs and the output current Io outputted from the inverter 12 are inputted to the control device 100. The control device 100 calculates the AC power outputted from the inverter 12 based on the output voltage Vs and the output current Io.

[0026] In addition, the output voltage Vs and the output current Io input to the control device 100 are the detection value of the output voltage Vs (hereinafter referred to as the Vs detection value) and the detection value of the output current Io (hereinafter referred to as the Io detection value). The Vs detection value and the Io detection value are detected by a detector (not shown) provided between the power conditioner 10 and the power system 30. Figure 1 In the example shown, the Vs detection value and the Io detection value are detected between the power conditioner 10 and the transformer 20. However, the present invention is not limited thereto. The Vs detection value and the Io detection value may also be detected between the transformer 20 and the power system 30, for example.

[0027] In addition, the output voltage Vs output from the inverter 12 is composed of three-phase voltages (Vsu, Vsv, Vsw), and the output current Io output from the inverter 12 is composed 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.

[0028] The control device 100 performs VSG (Virtual Synchronous Generator) control 101 based on the input Vs detection value and Io detection value. VSG is a virtual synchronous generator that makes the inverter 12 simulate the dynamic characteristics of a synchronous generator. That is, VSG control 101 refers to controlling the virtual synchronous generator. In addition, parameters representing the dynamic characteristics of the synchronous generator are used in VSG control 101. As examples of such parameters, inertia constant M, brake constant D, etc. can be exemplified.

[0029] The control device 100 performs frequency control 102 for controlling the frequency of the AC power (composed of the output voltage Vs and the output current Io) output from the inverter 12 based on the target frequency Fref obtained by the VSG control 101. Specifically, the control device 100 instructs the inverter 12 in such a manner that the frequency of the AC power approaches the target frequency Fref in the frequency control 102. The details of the calculation example of the target frequency Fref will be described later.

[0030] Thus, even when the frequency fluctuates due to a system accident, the inverter 12 can output AC power with a stable frequency. Therefore, the operation of the inverter 12 can be continued, and the power system 30 can be prevented from becoming unstable. In addition, functional requirements for continuing the operation of the inverter are specified to avoid the entire power system 30 being decoupled when a system accident occurs. This functional requirement is called an FRT (Fault Ride Through) requirement. As FRT requirements, examples include the frequency of the AC power being within a specified range, the rate of change of the frequency of the AC power being within a reference value, and the like. According to this embodiment, the frequency of the AC power is controlled under the FRT requirement. For example, when the frequency of the AC power decreases and the power value of the AC power increases, the frequency of the AC power is controlled.

[0031] Moreover, the control device 100 can also perform voltage control 103 to control the output voltage Vs output from the inverter 12 based on the target voltage Vref obtained by VSG control. For example, when the voltage drop of the AC voltage of the power system 30 becomes equal to or greater than a specified value due to the fluctuation of the AC voltage, the control device 100 instructs the inverter 12 to make the output voltage Vs approach the target voltage Vref. Thus, even when LVRT (Low Voltage Ride Through) occurs together with the frequency fluctuation, the inverter 12 can output a stable voltage. Therefore, it is possible to prevent the power system 30 from becoming unstable.

[0032] 2. Examples of Control Devices

[0033] 2-1. Configuration Example

[0034] The control device 100 has hardware that implements various functions. The hardware can be either a processing circuit or a computer that executes a program stored in a storage device by a CPU. As the processing circuit, an FPGA (Field Programmable Gate Array), an ASIC (Application Specific Integrated Circuit), etc. can be exemplified.

[0035] 2-2. Calculation Example of Target Frequency

[0036] Figure 2 It is an explanatory diagram showing a specific example of the functions of the control device 100 of the embodiment. Figure 2 It shows a calculation example 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, in Figure 2 the example shown, the calculated target frequency Fref is a dimensionless value based on the rated frequency, and the target power Pref and the AC power P also use dimensionless values. The dimensionless values of these powers use the rated power, for example. That is, the values used for the target power Pref and the AC power P in the calculation shown in Figure 2 are the values when the rated power is set to 1.

[0037] Then, the control device 100 calculates the frequency change amount 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 representing the dynamic characteristics of the synchronous generator as the frequency change amount ΔFc. For example, when the inertia constant is H, the damping constant is D, and the unit time is s, the frequency change amount ΔFc of the AC power P is represented by the following formula (1).

[0038] [Number 1]

[0039]

[0040] Furthermore, the control device 100 calculates a correction amount ΔFfrt for correcting the frequency change amount ΔFc so that the frequency change amount ΔFc becomes smaller than a threshold value. Details of an example of calculating the correction amount ΔFfrt will be described later.

[0041] Furthermore, the control device 100 determines whether correction of the frequency change ΔFc is required based on prescribed conditions. The prescribed conditions refer to a situation where the frequency change ΔFc is larger than a prescribed value and the AC power P is larger than the allowable overpower of the inverter 12, that is, the maximum power allowed to be output in excess of the rated power if it is a short period of time. The allowable overpower is set to 130% of the rated power, for example. When the prescribed conditions are met, the control device 100 determines that correction of the frequency change ΔFc is required. In this case, the control device 100 subtracts the correction amount ΔFfrt from the frequency change ΔFc. On the other hand, when the prescribed conditions are not met, the control device 100 determines that correction of the frequency change ΔFc is not required. In this case, the control device 100 does not subtract the correction amount ΔFfrt from the frequency change ΔFc. Figure 2 In the example shown, when it is determined that the frequency change amount ΔFc does not need to be corrected, the value of the correction amount subtracted from the frequency change amount ΔFc is set to zero.

[0042] Then, the control device 100 sets the second frequency F2 obtained by adding the rated frequency Fn of the inverter 12 to the first frequency F1 obtained by subtracting the correction amount ΔFfrt from the frequency change ΔFc as the target frequency Fref. The rated frequency Fn is a reference value when calculating the dimensionless target frequency Fref as described above, and is set to 1. The rated frequency before dimensionless is, for example, 50 Hz or 60 Hz.

[0043] 2-3. Calculation example of frequency change correction amount

[0044] Figure 3 It is an explanatory diagram showing a specific example of the control device 100 according to the embodiment. Figure 3An example of calculating the correction amount ΔFfrt for correcting the frequency change ΔFc is shown. 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 is used with the actual measured value of the frequency of the AC power P as a reference. The actual measured value of the frequency of the AC power P is obtained, for example, via a PLL (Phase Locked Loop) provided in the control device 100. For example, when the PLL is locked, the difference between the actual measured value of the frequency of the AC power P and the reference frequency (for example, 50 Hz or 60 Hz) does not occur, and it can be confirmed that the frequency of the AC power P is unchanged. On the other hand, when the PLL is unlocked, the difference between the actual measured value of the frequency of the AC power P and the reference frequency occurs, and the frequency change of the AC power P can be confirmed.

[0045] Then, the control device 100 calculates the power change ΔPc of the AC power P based on the differential frequency ΔF. Specifically, the control device 100 sets the power obtained by multiplying the differential frequency ΔF by the parameter representing the dynamic characteristics of the synchronous generator as the power change ΔPc. For example, when the inertia constant H, the braking constant D, and the unit time s are set, the power change ΔPc of the AC power P is expressed by the following formula (2).

[0046] [Number 2]

[0047] ΔPc=(2Hs+D)×ΔF…(2)

[0048] The above equation (2) can also be expressed by the following equation (3) or equation (4). In addition, RoCoF (Rate of Change of Frequency) means the rate of change of frequency. The unit of RoCoF is Hz / s.

[0049] [Number 3]

[0050]

[0051] [Number 4]

[0052] ΔPc=2H×RoCoF+D×ΔF…(4)

[0053] Furthermore, the control device 100 first subtracts the power change amount ΔPc from the target power Pref to obtain the first power P1, and then subtracts the allowable overpower Pmax of the inverter 12 from the first power P1 to obtain the second power P2. For example, when 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 represented by the formula (1 / D) obtained by dividing 1 by the braking constant D, for example.

[0054] 2-4. Processing example

[0055] Figure 4 This is a flowchart schematically showing a processing example of the control device 100 according to the embodiment.

[0056] In step 100 , 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 .

[0057] In step S110 , control device 100 calculates correction amount ΔFfrt for correcting frequency change amount ΔFc so that frequency change amount ΔFc becomes smaller than a threshold value. Thereafter, the process proceeds to step S120 .

[0058] In step S120, the control device 100 determines whether correction of the frequency change amount ΔFc is required based on a predetermined condition. If it is determined that correction of the frequency change amount ΔFc is required (step S120; Yes), the process proceeds to step S130. Otherwise (step S120; No), the process ends. The predetermined condition is as described above.

[0059] In step S130, the control device 100 calculates the target frequency Fref. Specifically, the control device 100 sets the target frequency Fref as a frequency obtained by adding the difference between the frequency change amount ΔFc and the correction amount ΔFfrt to the rated frequency Fn of the inverter 12. The process then proceeds to step S140.

[0060] In step S140 , the control device 100 instructs the inverter 12 to bring the frequency of the AC power P close to the target frequency Fref.

[0061] 4. Example of inverter output results

[0062] Figure 5 It is an explanatory diagram showing an example of the output result of the inverter 12 according to the embodiment. Figure 5 (A) shows an example of a waveform of the frequency F when the frequency F of the AC power P varies. Figure 5 (B) shows an example of the waveform of the AC power P before the frequency F of the AC power P is corrected. Figure 5 (C) shows an example of the waveform of the AC power P after the frequency F of the AC power P is corrected. Figure 5 (B) and Figure 5 The vertical axis of the graph shown in (C) is expressed in PU (Per Unit), but it can also be expressed in percentage. Figure 5 (B) and Figure 5 In the example shown in (C), the permissible overpower Pmax is represented by "1.3PU".

[0063] Figure 5 (A) shows an example of a system failure occurring at time 2.0s. After that, the frequency F decreases from 50Hz to 47.5Hz. In this case, Figure 5 As shown in (B), the AC power P before the frequency F of the AC power P is corrected exceeds the allowable overpower Pmax. Figure 5 As shown in (C), the AC power P after the frequency F of the AC power P is corrected can be prevented from exceeding the allowable overpower Pmax.

[0064] In this way, when a system failure occurs and a change (e.g., a decrease in frequency) occurs in the frequency F of the AC power P, 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 can prevent the output current Io output from the inverter 12 from becoming an overcurrent.

[0065] 5. Effect

[0066] According to the power conditioner 10 of the present embodiment, the frequency change ΔFc of the AC power P is calculated based on the difference between the AC power P and the target power Pref, that is, the differential power ΔP. Then, the correction amount ΔFfrt for correcting the frequency change ΔFc in a manner that makes the frequency change ΔFc less than the threshold is calculated. Furthermore, the frequency obtained by adding the difference between the frequency change ΔFc and the correction amount ΔFfrt to the rated frequency Fn of the inverter 12 is set as the target frequency Fref, and the inverter 12 is instructed in a manner that the frequency F of the AC power P approaches the target frequency Fref. Thus, when the frequency F of the AC power P changes, the frequency F of the AC power P is corrected, and the AC power P is also corrected in a manner that makes the differential power ΔP smaller. Therefore, the output current Io output from the inverter 12 can be suppressed from becoming an overcurrent, and the power system 30 can be stabilized.

[0067] Description of Reference Numerals

[0068] 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 regulator, It is characterized in that have: an inverter that converts direct current power output from a battery into alternating current power and supplies the alternating current power to a power system; and a control device for controlling the inverter, The control device is configured as follows: The frequency change amount of the AC power is calculated based on the differential power which is the difference between the AC power and the target power. calculating a correction amount for correcting the frequency change amount so that the frequency change amount is smaller than a threshold value, setting a frequency obtained by adding a rated frequency of the inverter to a difference between the frequency change amount and the correction amount as a target frequency, An instruction is issued to the inverter so that the frequency of the AC power approaches the target frequency.

2. The power conditioner according to claim 1, It is characterized in that The control device is configured to correct the frequency change amount when the frequency change amount is larger than a predetermined value and the AC power is larger than an allowable overpower of the inverter.

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

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

5. The power conditioner according to claim 4, It is characterized in that The control device is configured to set, as the power variation, power obtained by multiplying the differential frequency by a parameter indicating a dynamic characteristic of the synchronous generator.

6. The power conditioner according to claim 1, It is characterized in that The control device is configured to perform voltage control so that the output voltage approaches a target voltage when a voltage drop of the output voltage becomes equal to or larger than a predetermined value due to a variation in the output voltage of the inverter.

7. A frequency control method, It is characterized in that The steps include: using an inverter to convert direct current power output from a battery into alternating current power and supply the alternating current power to a power system; calculating a frequency change amount of the AC power based on a difference between the AC power and the target power, that is, a differential power; calculating a correction amount for correcting the frequency change amount in such a manner that the frequency change amount is less than a threshold value; setting a frequency obtained by adding a rated frequency of the inverter to a difference between the frequency change amount and the correction amount as a target frequency; as well as An instruction is issued to the inverter so that the frequency of the AC power approaches the target frequency.

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