System interconnection inverter and system frequency variation suppression method

Abstract

Provided is a system interconnection inverter and a system frequency variation suppression method that can suppress variation in system frequency accompanying rapid changes in load and output variation in a renewable energy power source in a short time. The system interconnection inverter is interconnected with a power system connected with a synchronous generator and operates by following an output active power command generated by a VSG control function, thereby being able to suppress variation in system frequency, in which the control circuit of the inverter drives an inverter main circuit in accordance with an output active power command represented by the sum of an output active power set value of the inverter, a value obtained by multiplying a deviation between system frequency and a reference frequency by a simulated damping coefficient, and a value obtained by multiplying a differential value of system frequency by a simulated inertia coefficient, and the control circuit of the inverter includes an adjustment unit that is adjusted in such a manner that the simulated inertia coefficient after the system frequency reaches a maximum point of frequency variation is smaller than the simulated inertia coefficient before the system frequency reaches the maximum point of frequency variation.

Description

TECHNICAL FIELD

[0001] The present application relates to a system interconnection inverter and a system frequency fluctuation suppression method for suppressing fluctuation of system frequency by a so-called virtual synchronous generator control (VSG) function. BACKGROUND

[0002] An inverter for converting direct current output from a renewable energy power source such as a solar power generation device into alternating current and supplying it to a power system is generally operated in synchronization with the system frequency by PLL (Phase Locked Loop) control. It is well known that, unlike a synchronous generator having a rotating body, an inverter is a stationary type electrical device composed of power semiconductor switching elements, and therefore does not have a function of suppressing fluctuation of system frequency caused by inertial force of the rotating body.

[0003] In particular, if the amount of introduction of renewable energy power sources increases and the number of synchronous generators decreases in proportion, there is a concern that the system frequency will greatly fluctuate due to rapid changes in load and output fluctuation of renewable energy power sources.

[0004] Therefore, as a virtual synchronous generator control (VSG), an inverter is known to have an analog inertial force to realize a frequency fluctuation suppression function of a synchronous generator, thereby seeking to stabilize the system.

[0005] Figure 6 is a schematic configuration diagram of a system interconnection system including an inverter having the above-described VSG function.

[0006] In Figure 6 , a synchronous generator 30 is connected to a power system 10, thereby supplying alternating current to a load 60. In addition, a PLL circuit 20 is connected to the power system 10, and a system frequency f g and its differential value (df g / dt) detected by the PLL circuit 20 are input to an inverter 50 via a signal line 21. Here, the inverter 50 includes a main circuit composed of power semiconductor switching elements for performing direct current / alternating current conversion and a control circuit thereof.

[0007] A renewable energy power source such as a solar power generation device 40 is connected to the direct current input side of the inverter 50, and the power system 10 is connected to the alternating current output side.

[0008] Note that Figure 6 the synchronous generator 30, the inverter 50 (and the solar power generation device 40) in are not limited to a single case, and in a case where a plurality of synchronous generators, a plurality of inverters, and the power system 10 are interconnected, they are illustrated as each simplified device.

[0009] In the above configuration, the inverter 50 suppresses variation of the system frequency f g accompanying a sharp change in the load or the like by the VSG function shown below.

[0010] That is, for example, as shown in mathematical expression 18 of Non-Patent Literature 1, the effective power (command) output by the inverter 50 is calculated by the following mathematical expression 1.

[0011] (Mathematical Expression 1)

[0012]

[0013] In mathematical expression 1, each symbol represents the following content:

[0014] P inv : Output effective power (command) of the inverter 50;

[0015] P0: Set value of the output effective power of the inverter 50;

[0016] k vd : Analog damping coefficient;

[0017] k vi : Analog inertia coefficient;

[0018] f g : Actual system frequency;

[0019] f0: Reference frequency of the system (for example, 50 (Hz) or 60 (Hz)).

[0020] According to mathematical expression 1, the output effective power P inv of the inverter 50 is calculated by adjusting the amount from the set value P0of the output effective power by the sum of the analog damping component (second term on the right) according to the frequency deviation (f g - f0) of the variation of the system frequency and the analog damping component (third term on the right) according to the differential value (df g / dt) of the system frequency f g . By operating the inverter 50 with this value P inv as the output effective power command, it is possible to suppress the variation of the system frequency f g .

[0021] Note that, as the above analog damping coefficient k vd and the analog inertia coefficient k vi , a fixed value that is appropriately selected is used in the past.

[0022] In addition, in Patent Literature 1, as the same VSG function as Non-Patent Literature 1, the following is described: by a generator inertia force generating portion provided to a control circuit of an inverter, a frequency variation suppression amount equivalent to an inertia force of a synchronous generator is calculated based on a phase difference with a system voltage obtained by delaying a response of a PLL circuit at the time of a sharp change in a load, and this is added to an effective power target value of the inverter, thereby improving a decrease in a system frequency.

[0023] (Prior Art Documents)

[0024] (Patent Literature)

[0025] Patent Literature 1: Japanese Patent Application Publication No. 2019-3454 (

[0026] to

[0033] , Figure 1 , Figure 2 and the like)

[0026] (Non-Patent Literature)

[0027] Non-Patent Literature 1: "Grid Tied Converter with Virtual Kinetic Strage", IEEE, 2009 SUMMARY

[0028] (Problems to be Solved by the Invention)

[0029] According to the technology disclosed in Non-Patent Literature 1, the system frequency f g is controlled in such a manner that the frequency deviation (f g converges to the reference frequency f0after the frequency deviation (f vi is set to a fixed value, and thus there is a problem that the convergence speed of the system frequency f g is slow.

[0030] Note that in Patent Literature 1, a specific method for recovering the varied system frequency f g in a short time at the time of a sharp change in a load is not disclosed.

[0031] Therefore, the present application has an object to provide a system interconnection inverter and a system frequency variation suppression method that can suppress a variation in a system frequency accompanying a variation in an output of a renewable energy power source due to a sharp change in a load in a short time.

[0032] (Method for Solving the Technical Problem)

[0033] To solve the above problems, the system interconnection inverter of claim 1 is an inverter that is interconnected with a power system to which a synchronous generator is connected and that is capable of suppressing variation in system frequency by operating in accordance with an output active power command generated by a virtual synchronous generator control function, and in a system interconnection inverter in which a control circuit for controlling a main circuit of the inverter turns on and off a semiconductor switching element of the main circuit in accordance with the output active power command represented by the sum of an output active power set value of the inverter, a value obtained by multiplying a deviation of a system frequency from a reference frequency by a simulated damping coefficient, and a value obtained by multiplying a differential value of the system frequency by a simulated inertia coefficient,

[0034] The control circuit includes an adjustment unit that is adjusted so that the simulated inertia coefficient after the system frequency reaches a point of maximum variation in frequency is smaller than the simulated inertia coefficient before the system frequency reaches the point of maximum variation in frequency.

[0035] The system interconnection inverter of claim 2 is the system interconnection inverter described in claim 1, in which a renewable energy power source is connected as a direct current power source of the inverter.

[0036] The system interconnection inverter of claim 3 is the system interconnection inverter described in claim 1 or 2, in which variation in system frequency that accompanies variation in a load connected to the power system or variation in output of a renewable energy power source connected as a direct current power source of the inverter is suppressed.

[0037] The system interconnection inverter of claim 4 is the system interconnection inverter described in any one of claims 1 to 3, in which the control circuit sets the simulated inertia coefficient after the system frequency reaches the point of maximum variation in frequency based on inertia of the synchronous generator before the system frequency reaches the point of maximum variation in frequency and inertia of the entire power system after the system frequency reaches the point of maximum variation in frequency.

[0038] The system interconnection inverter of claim 5 is the system interconnection inverter described in claim 4, in which

[0039] The inertia of the synchronous generator before the system frequency reaches the point of maximum variation in frequency is calculated using the mechanical input variation of the synchronous generator at the point of time before the system frequency reaches the point of maximum variation in frequency, the load variation that is independent of the system frequency, the damping coefficient of the synchronous generator, the simulated damping coefficient, the deviation of the system frequency from the reference frequency, the differential value of the system frequency, and the simulated inertia coefficient.

[0040] The system frequency variation suppression method of claim 6 is a method of suppressing variation in system frequency by operating an inverter connected to a power system interconnection with a synchronous generator and including a virtual synchronous generator control function,

[0041] In the system frequency variation method of operating the inverter in accordance with an output active power command represented by the sum of an output active power set value of the inverter, a value obtained by multiplying a deviation of system frequency from a reference frequency by a simulated damping coefficient, and a value obtained by multiplying a differential value of system frequency by a simulated inertia coefficient,

[0042] The simulated inertia coefficient after the system frequency reaches the point of maximum frequency variation is adjusted to be smaller than the simulated inertia coefficient before the system frequency reaches the point of maximum frequency variation, thereby generating the output active power command after the point of maximum frequency variation.

[0043] In the system frequency variation suppression method of claim 7, in a case where the deviation of system frequency from the reference frequency is smaller than a first threshold value, the inverter is operated in accordance with an output active power command represented by the sum of an output active power set value of the inverter and a value obtained by multiplying the deviation of system frequency from the reference frequency by a simulated damping coefficient,

[0044] The output active power command after the point of maximum frequency variation is generated in accordance with the processing described in claim 6 after the time when the deviation of system frequency from the reference frequency exceeds the first threshold value.

[0045] The system frequency variation suppression method of claim 8 is the system frequency variation suppression method described in claim 6 or 7, which suppresses variation in system frequency accompanying variation in a load connected to the power system or variation in output of a renewable energy power source connected as a direct current power source of the inverter.

[0046] (EFFECTS OF THE INVENTION)

[0047] According to the present application, the output active power command of the inverter is generated by changing the simulated inertia coefficient after the time when the system frequency changes to reach the point of maximum frequency variation to a prescribed value, thereby enabling the varying system frequency to be restored in a short time, which contributes to early stabilization of the system. BRIEF DESCRIPTION OF DRAWINGS

[0048] Figure 1 is a configuration diagram of a system interconnection inverter of an embodiment of the present application.

[0049] Figure 2 is an explanatory diagram of a point of maximum frequency variation of an embodiment of the present application.

[0050] Figure 3is a diagram showing the state of recovery of the system frequency in the case where the analog inertia coefficient is adjusted in the embodiment of the present application and in the case where the analog inertia coefficient is fixed.

[0051] Figure 4 is a flowchart showing the method of calculating the output effective power command in the embodiment of the present application according to the change in the system frequency.

[0052] Figure 5 is a waveform chart showing one example of the case where the system frequency varies in the embodiment of the present application.

[0053] Figure 6 is a schematic configuration diagram of a system interconnection system including an inverter having a VSG function.

[0054] BRIEF DESCRIPTION OF REFERENCE NUMERALS

[0055] 10: power system

[0056] 20: PLL circuit

[0057] 21: signal line

[0058] 30: synchronous generator

[0059] 40: solar power generation device

[0060] 50: inverter

[0061] 60: load

[0062] 71, 74: subtraction operation section

[0063] 72: differentiation unit

[0064] 73: addition operation section

[0065] 80: current reference value generation section

[0066] 90: current control section

[0067] 91, 92, 93: coordinate conversion section

[0068] 94: modulation signal generation section

[0069] 100: inverter main circuit

[0070] 101: direct current power supply

[0071] 102: direct current intermediate capacitor

[0072] 103: current detector DETAILED DESCRIPTION

[0073] Hereinafter, the embodiment of the present application will be described with reference to the drawings.

[0074] Figure 1 is a configuration diagram showing a system-interconnected inverter of the present embodiment including an inverter main circuit and a control circuit thereof, and Figure 6 The same parts are given the same numbers.

[0075] Figure 1 In the system frequency f g The input subtraction operation section 71 calculates the deviation (f g -f0) from the reference frequency f0. Then, the multiplication operation result of this deviation (f g -f0) and the above-mentioned analog damping coefficient k vd is input to the addition operation section 73.

[0076] In addition, the system frequency f g is multiplied by the analog inertia coefficient k vi which is a variable value, and the multiplication operation result is differentiated by the differentiation unit 72 and input to the addition operation section 73.

[0077] The addition operation section 73 adds the above-mentioned two inputs and inputs to the subtraction operation section 74, and the subtraction operation section 74 subtracts the output of the addition operation section 73 from the output active power set value P0 of the inverter 50 to obtain the output active power command P inv . This output active power command P inv is input to the current reference value generation section 80.

[0078] The inactive power command Q inv is input to the current reference value generation section 80 together with the output active power command P inv , and the current reference value generation section 80 generates the d-axis and q-axis current command values i dref , i qref based on the input and outputs to the current control section 90.

[0079] On the other hand, on the direct current input side of the three-phase inverter main circuit 100 composed of power semiconductor switching elements, a direct current power source 101 corresponding to a renewable energy power source is connected in parallel to a direct current intermediate capacitor 102, and the alternating current output side of the inverter main circuit 100 is connected to the power system 10.

[0080] The respective phase output currents i a , i b , i c of the inverter main circuit 100 are detected by the current detectors 103, and then converted to the d-axis and q-axis currents i d , i qand input to the current control section 90, and each phase output voltage v a , b , c converted to d-axis, q-axis voltages v d , q by the coordinate conversion section 92 and input to the current control section 90.

[0081] Note that p is a phase angle used for coordinate conversion.

[0082] The current control section 90 generates d-axis, q-axis modulation commands m d , q based on each of the input signals of the current and the voltage. d , q The modulation commands m a , b , c are input to the inverter main circuit 100. In the inverter main circuit 100, the semiconductor switching elements are turned on and off by PWM control using the modulation signals m a , b , c , and the like, whereby direct current of the direct current power supply 101 is converted to alternating current and power is supplied to the power system 10.

[0083] Here, a motion equation (swing equation) in the case where the synchronous generator 30 is modeled is shown as mathematical expression 2.

[0084] (Mathematical expression 2)

[0085]

[0086] In mathematical expression 2, each symbol represents the following:

[0087] M: inertia of the synchronous generator (simplified model in which synchronous generators connected to the power system are collectively regarded as one inertia);

[0088] f m : rotational frequency of the synchronous generator (rotor);

[0089] ΔP m : mechanical input variation of the synchronous generator;

[0090] ΔP e : electrical output variation of the synchronous generator.

[0091] From the above mathematical expression 1, the variation ΔP inv of the effective power output by the inverter can be expressed by mathematical expression 3.

[0092] (Mathematical Formula 3)

[0093]

[0094] In addition, as shown in mathematical formula 4, the effective power variation ΔP inv The electrical output variation ΔP of synchronous generator 30 e The sum of the effective power variation ΔP of the load 60 L Equal to each other, the effective change in electricity ΔP L To be independent of system frequency f g Load variation ΔP L0 The attenuation coefficient D and frequency deviation (f) of synchronous generator 30 g The sum of the products of -f0).

[0095] (Mathematical Expression 4)

[0096] ΔP inv +ΔP e =ΔP L =ΔP LO +D(f g -f0)

[0097] Here, if f in mathematical expression 2 m ≒f g Based on mathematical equations 2-4, the amplified motion equation of the synchronous generator 30 is mathematical equation 5.

[0098] (Mathematical Formula 5)

[0099]

[0100] In mathematical formula 5, (M+k vi ) is the overall inertia coefficient of the system (including the inertia of the inverter output), (D+k) vd ) represents the overall attenuation coefficient of the system.

[0101] In this embodiment, for example, the system frequency f changes with load variations. g As the reference frequency f0 decreases, by reducing the system frequency f g The convergence value f is greater than the predicted value. ∞ Self-converging value f in a small region ∞ During the period of departure, the simulated inertia coefficient k is made vi The larger value results in a higher overall inertia coefficient (M+k) of the system as shown in mathematical formula 5. vi ) is a relatively large value; in addition, it is close to the convergence value f. ∞ During the period, make the simulated inertia coefficient k vi The smaller the value, the smaller the overall inertia coefficient (M+k) of the system.vi is smaller, thereby making the system frequency f g recover faster. Note that the inertia coefficient (M + k vi ) of the entire system and the inertia M of the synchronous generator are positive values.

[0102] In other words, the system frequency f g is changed in a region smaller than the convergence value f ∞ , the simulated inertia coefficient k g is made larger until the system frequency f vi reaches the maximum point of the frequency variation amount, and the simulated inertia coefficient k ∞ is made smaller during the period in which the system frequency f vi approaches the convergence value f vi .

[0103] Thus, by adjusting the simulated inertia coefficient k vi of the inverter, the system frequency f g can be recovered in a short time compared to the case in which the simulated inertia coefficient k g is made a fixed value as in the past.

[0104] At the maximum point of the frequency variation amount, the differential value (df g / d t ) of the system frequency f g is zero, and thus the mathematical expression 5 becomes a mathematical expression 6. In the mathematical expression 6, f min is the minimum value of the frequency at the maximum point of the frequency variation amount, and t2 is the time thereof.

[0105] (Mathematical expression 6)

[0106] ΔP LO = - (D + k vd ) (f min - f O ) + ΔP m (t2)

[0107] Figure 2 is the diagram of the above f min , t2, and the like at the maximum point of the frequency variation amount, and also shows the time tl before t2 and the frequency fl and the frequency change rate (df1 / dt) as the differential value thereof.

[0108] Next, the convergence value f vi of the system frequency necessary when the simulated inertia coefficient k ∞ is adjusted as described above is estimated as follows.

[0109] First, the variation amount ΔF of the rotational frequency of the synchronous generator 30 and the above load variation amount ΔP L0There is a relationship of mathematical expression 7 between them. In this mathematical expression 7, s is a Laplacian operator, and G(s) is a transfer function.

[0110] (Mathematical expression 7)

[0111] ΔF(s) = G(s) ΔP L0 (s)

[0112] From this mathematical expression 7 and the final value theorem, a converged value f ∞ of the system frequency can be estimated by mathematical expression 8.

[0113] (Mathematical expression 8)

[0114] Steady state value

[0115] Here, Figure 3 is a graph showing the recovery state of the system frequency f g in the case where the analog inertia coefficient k vi is adjusted as described above (this embodiment) and in the case where it is fixed (prior art). As is apparent from Figure 3 , in this embodiment, the time until the system frequency f g is recovered to within the prescribed range with the converged value f ∞ as the center is shorter than in the prior art, and thus, it is possible to contribute to early stabilization of the system frequency f g .

[0116] Next, an arithmetic method of the output effective power of the inverter corresponding to the variation state of the system frequency f g will be described based on Figure 4 , Figure 5 .

[0117] Figure 4 The flowchart of the control circuit of the inverter shown in Figure 1 is a flow executed by the control circuit of the inverter in a prescribed control period, and the present time is set to k, and the time one control period before is set to (k-1).

[0118] In addition, Figure 5 is a waveform chart showing one example of the variation of the system frequency f g .

[0119] Hereinafter, a case where the system frequency f g decreases will be described, but the present application can also be applied to a case where the system frequency f g increases.

[0120] In Figure 4 , first, it is judged whether the system frequency f gwhether the absolute value of the deviation of the system frequency f from the reference frequency f0 (for example, 50 (Hz)) is smaller than a preset first threshold value cl (step Sl). This step Sl is to determine whether the system frequency f is largely reduced (or increased). g

[0121] the system frequency f g is not largely reduced, the effective power command of the normal mode is output to the inverter (Sl YES, S8), and in the case where the system frequency f is largely reduced (Sl NO), the process proceeds to step S2.

[0122] In step S2, the analog inertia coefficient of the inverter is set to a first analog inertia coefficient k vi1 The effective power command P inv (k) at the present time k is calculated by mathematical expression 1 and input to the inverter.

[0123] Next, it is determined whether the absolute value of the difference between the system frequency f k at the present time k and the system frequency f k－1 at the time (k - 1) is smaller than a second threshold value c2 (S3). This step S3 is to determine whether the system frequency f g has reached the point of maximum frequency variation Figure 5 .

[0124] Note that the size relationship between the first threshold value cl and the second threshold value c2 is such that cl » c2 (for example, cl = 0.3, c2 = 0.001) as exemplified in Figure 5

[0125] the system frequency f g has not reached the point of maximum frequency variation, the process returns to step S2 (S3 NO), and if the system frequency f has reached the point of maximum frequency variation, the converged value f ∞ is estimated by the above-described method (S3 YES, S4).

[0126] Next, it is determined whether the sign of f k - f k－1 is changed (S5). This step S5 is to determine the point at which the system frequency f g begins to return.

[0127] In the case where the sign of f k - f k－1 is not changed (S5 NO), the effective power command P inv (k) at the present time k is calculated by mathematical expression 1 and input to the inverter (S9) as in step S2.

[0128] In the case where the sign of f k - f k－1 is changed (S5 YES), the process proceeds to step S6.​When the sign changes (S5YES), determine the system frequency f. g Is it in comparison to the convergence value f? ∞ Within a small range, the convergence value f approaches the convergence value. ∞ The direction changes (S6).

[0129] Then, the system frequency f g Towards the convergence value f ∞ When the direction changes (S6YES), the inverter's analog inertia coefficient is set to a value higher than the first analog inertia coefficient k. vi1 The small second simulated inertia coefficient k vi2 (k vi2 <k vi1 And 0 < k vi2 And calculate the effective power command P at the current time k using mathematical formula 1. inv (k) and input to the inverter (S10).

[0130] Furthermore, it does not converge to a value f. ∞ In the case of a change in direction (S6NO), similar to steps S2 and S9, the effective power command P at the current time k is calculated using mathematical formula 1. inv (k) and input to the inverter (S7).

[0131] It should be noted that as long as the system frequency f g If the frequency is not restored to the reference frequency f0, then execute. Figure 4 Either step S7 or S10 in the above steps. Additionally, if the system frequency f... g Once the reference frequency f0 is restored, the process following step S1 is repeated sequentially.

[0132] As described above, by performing operations related to the system frequency f g The above-described processing, corresponding to the changing state, enables the system frequency f to be adjusted. g After reaching the point of maximum frequency variation, it approaches the convergence value f. ∞ The recovery speed is relatively fast when the direction is changed.

[0133] Make the system frequency f g The action of restoring the reference frequency f0 is carried out by the inertial force of the synchronous generator 30. In many cases, even if the system frequency f0 is restored... g Even if changes occur, the adjustment function will not take effect immediately. Therefore, in order to suppress the system frequency f, the VSG function of this embodiment will remain in effect until the aforementioned adjustment function of the synchronous generator 30 is effective. g The changes are effective.

[0134] Next, regarding the setting in Figure 4 In step S10, the effective power command P is calculated.inv (k) the second simulated inertia coefficient k at time t vi2 (k) the second simulated inertia coefficient k at time t vi = k vi2 ) the method of finding the numerical range of the simulated inertia coefficient k vi will be described.

[0135] If the inertia coefficient of the entire system after the simulated inertia coefficient k vi is reset is set to the desired value M' (M' = M + k vi2 , and M' > 0), in order to improve the recovery speed of the system frequency f g , M' needs to be set smaller than the inertia coefficient of the entire system before the simulated inertia coefficient k vi is reset (M + k vi1 ), so that mathematical expression 9 holds.

[0136] (Mathematical expression 9)

[0137] k vi2 < k vi1 (O < k vi1 , O < k vi2 )

[0138] - M < k vi2 (k vi2 < O)

[0139] However, according to mathematical expression 9, if the inertia M of the synchronous generator 30 is not known, the simulated inertia coefficient k vi cannot be reset.

[0140] Therefore, by the following method, the numerical range for resetting the simulated inertia coefficient k vi can be found.

[0141] First, by substituting the above mathematical expression 6 into mathematical expression 5, mathematical expression 10 is obtained. In this mathematical expression 10, as shown in mathematical expression 10, t1, f1 are a certain time before the point of maximum frequency variation (time t2) and the system frequency at that time, and (df1 / dt) is the rate of change of frequency. Figure 2

[0142] (Mathematical expression 10)

[0143]

[0144] If mathematical expression 10 is transformed, mathematical expression 11 can be obtained. Note that k vi in the second term on the right side of mathematical expression 11 is the simulated inertia coefficient before the point of maximum frequency variation (corresponding to k Figure 4 ​k in steps S2, S7, S9 vi1 ).

[0145] M is found by this mathematical expression 11, and k vi2 is set again within the numerical range of M vi . Thus, even if k vi2 takes a negative value, k vi2 that satisfies k vi2 + M > 0 can be set again.

[0146] (Mathematical expression 11)

[0147]

[0148] In addition, as a method of setting again the simulation inertia coefficient k vi in the case where the inertia M of the synchronous generator 30 is not clear, for example, a power company that operates the power system calculates the inertia M of the synchronous generator, and transmits the value of the above mathematical expression 9 including the calculated value to the inverter side to set again.

Claims

1. A system interconnection inverter which is an inverter interconnecting with a power system to which a synchronous generator is connected and is capable of suppressing variation of system frequency by operating in accordance with an output effective power command generated by a virtual synchronous generator control function, a control circuit for controlling a main circuit of the system interconnection inverter turns on and off a semiconductor switching element of the main circuit in accordance with the output effective power command represented by a sum of an output effective power set value of the system interconnection inverter, a value obtained by multiplying a deviation of system frequency from a reference frequency by an analog damping coefficient, and a value obtained by multiplying a differential value of system frequency by an analog inertia coefficient, wherein the control circuit includes an adjustment unit which, in a case where the system frequency changes in a direction approaching a convergence value after the system frequency reaches a point of maximum variation amount, adjusts the analog inertia coefficient after the system frequency reaches the point of maximum variation amount to be smaller than the analog inertia coefficient before the system frequency reaches the point of maximum variation amount, on the basis of the following two mathematical expressions: k vi2 <k vi1 (0 <k vi1 ,0 <k vi2 In the case of) - M < k vi2 (k vi2 <0 cases), wherein k vi1 is an analog inertia coefficient of the system interconnection inverter before the system frequency reaches the maximum point of the frequency variation, k vi2 is an analog inertia coefficient of the system interconnection inverter after the system frequency reaches the maximum point of the frequency variation, M is the inertia of the synchronous generator, ΔP m is the mechanical input variation of the synchronous generator, ΔP L0 is the load variation independent of the system frequency, D is the damping coefficient of the synchronous generator, k vd is an analog damping coefficient of the system interconnection inverter, k vi is an analog inertia coefficient of the system interconnection inverter, t1 is the time before reaching the maximum point of the frequency variation, t2 is the time at which the maximum point of the frequency variation is reached, f0 is the reference frequency, f1 is the frequency corresponding to t1, the convergence value of the system frequency is estimated by the following mathematical expression: steady state value where s is the Laplace operator, G(s) is the transfer function, ΔP L0 is the load variation amount independent of the system frequency.

2. The system interconnection inverter according to claim 1, wherein a renewable energy power source is connected as a direct current power source of the system interconnection inverter.

3. The system interconnection inverter according to claim 1 or 2, wherein variation of system frequency accompanying variation of a load connected to the power system or variation of an output of a renewable energy power source connected as a direct current power source of the system interconnection inverter is suppressed.

4. The system interconnection inverter according to claim 1 or 2, wherein the control circuit sets the analog inertia coefficient after the system frequency reaches the point of maximum variation amount on the basis of inertia of the synchronous generator before the system frequency reaches the point of maximum variation amount and inertia of the entire power system after the system frequency reaches the point of maximum variation amount.

5. The system interconnection inverter according to claim 4, wherein the inertia of the synchronous generator before the system frequency reaches the point of maximum variation amount is calculated using a mechanical input variation amount of the synchronous generator at a time before the system frequency reaches the point of maximum variation amount, a load variation amount independent of system frequency, a damping coefficient of the synchronous generator, the analog damping coefficient, a deviation of system frequency from a reference frequency, a differential value of system frequency, and the analog inertia coefficient.

6. A system frequency variation suppression method which suppresses variation of system frequency by operating a system interconnection inverter interconnecting with a power system to which a synchronous generator is connected and including a virtual synchronous generator control function, the method operates the system interconnection inverter in accordance with an output effective power command represented by a sum of an output effective power set value of the system interconnection inverter, a value obtained by multiplying a deviation of system frequency from a reference frequency by an analog damping coefficient, and a value obtained by multiplying a differential value of system frequency by an analog inertia coefficient, wherein In a case where the system frequency is changed in a direction approaching the convergence value after the system frequency reaches the point of maximum frequency variation, the output active power command after the point of maximum frequency variation is generated in such a manner that the simulated inertia coefficient after the system frequency reaches the point of maximum frequency variation is smaller than the simulated inertia coefficient before the system frequency reaches the point of maximum frequency variation, based on the following two mathematical expressions: k vi2 <k vi1 (0<k vi1 ,0<k vi2 ​ - M < k vi2 (k vi2 <0 cases), wherein k vi1 is an analog inertia coefficient of the system interconnection inverter before the system frequency reaches the point of maximum frequency variation, k vi2 is an analog inertia coefficient of the system interconnection inverter after the system frequency reaches the point of maximum frequency variation, M is the inertia of the synchronous generator, ΔP m is the mechanical input variation of the synchronous generator, ΔP L0 is the load variation independent of the system frequency, D is the damping coefficient of the synchronous generator, k vd is an analog damping coefficient of the system interconnection inverter, k vi is an analog inertia coefficient of the system interconnection inverter, t1 is the time before reaching the point of maximum frequency variation, t2 is the time at which the point of maximum frequency variation is reached, f0 is the reference frequency, f1 is the frequency corresponding to t1, The convergence value of the system frequency is estimated by the following mathematical expression: steady state value where s is the Laplace operator, G(s) is the transfer function, ΔP L0 is the load variation independent of the system frequency.

7. The system frequency variation suppression method according to claim 6, wherein, in a case where the deviation of the system frequency from the reference frequency is smaller than a first threshold value, the system interconnection inverter is operated in accordance with an output active power command represented by the sum of the output active power set value of the system interconnection inverter and a value obtained by multiplying the deviation of the system frequency from the reference frequency by a simulated damping coefficient, after the time when the deviation of the system frequency from the reference frequency exceeds the first threshold value, the system interconnection inverter is operated in accordance with the output active power command after the point of maximum frequency variation.

8. The system frequency variation suppression method according to claim 6 or 7, wherein variation of the system frequency accompanying variation of a load connected to the power system or variation of an output of a renewable energy power source connected as a direct current power source of the system interconnection inverter is suppressed.

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