Power oscillation suppression method and system for multi-network-construction-type converter parallel system

Abstract

The invention discloses a power oscillation suppression method and a power oscillation suppression system for a multi-construction-network type converter parallel system, which relate to the technical field of converter control and comprise the following steps of: acquiring voltage, current and voltage frequency output by a three-phase converter at a grid-connected point in the multi-construction-network type converter parallel system; calculating active power and reactive power and inputting the active power and the reactive power into the virtual oscillator; according to the voltage frequency and the rated frequency, system power reference compensation is obtained through an introduced proportional differential controller, then the active power and an active power reference value are combined, frequency deviation is obtained through an active droop link of a virtual oscillator, and then a voltage phase angle is obtained through calculation; meanwhile, the reactive power is subjected to a reactive power control link of the virtual oscillator to obtain a voltage amplitude; and calculating a system reference voltage according to the voltage amplitude and the voltage phase angle, and performing real-time adjustment on the output of the three-phase converter through voltage and current double-closed-loop adjustment and PWM (Pulse Width Modulation) control so as to suppress the frequency oscillation of the system.

Description

Technical Field

[0001] The present invention relates to the technical field of converter control, and in particular to a method and system for suppressing power oscillation for a multi-network-forming converter parallel system. Background Technique

[0002] The statements in this part merely provide background technical information related to the present invention and do not necessarily constitute prior art.

[0003] With the continuous increase in the installed capacity ratio of new energy dominated by wind and light, the power system shows characteristics such as a significant reduction in inertia and frequent weak grid conditions, and the system stability is severely threatened. Therefore, new network-forming control technologies represented by droop control, virtual synchronous machine control, and virtual oscillator control (VOC) have become research hotspots. Among them, as a time-domain control strategy, VOC adjusts the frequency and voltage by simulating the dynamic characteristics of a weak nonlinear oscillator, improves the dynamic performance of the system, enables the synchronization between converters not to rely on inertia, and can achieve synchronization at any power angle. Therefore, VOC has obvious advantages in self-synchronization, power sharing, etc. compared with droop control and virtual synchronous machine control, and this control strategy has attracted much attention.

[0004] However, after new energy equipment based on VOC control is connected to the grid, due to the lack of mechanical inertia of a synchronous motor itself, when encountering load fluctuations or inconsistent line impedances, the frequency and output active power of the parallel system are prone to transient power oscillations, which is not conducive to the stable operation of the system. Currently, the power oscillation suppression strategy based on VOC is the key to the multi-machine parallel grid connection of network-forming converters, which mainly focuses on dynamic parameter optimization and increasing transient damping. Among them, the dynamic parameter optimization scheme adjusts parameters such as the oscillation amplitude and frequency response coefficient of VOC in real time to match the dynamic requirements of the system. This method makes full use of the advantage of the reconfigurable parameters of VOC, but its control law often depends on non-linear functions designed empirically and lacks strict support from Lyapunov stability theory, making it difficult to achieve precise quantitative control of the power dynamic process; transient damping suppresses power oscillations by introducing a virtual damping term, usually constructing a virtual damping using a state observer. This method avoids direct differential operations, but increases the order of the VOC model, affects the dynamic characteristics of the system, and needs to balance between steady-state accuracy and transient robustness. Summary of the Invention

[0005] To solve the deficiencies of the above-mentioned existing technologies, the present invention provides a method and system for suppressing power oscillation in a multi-configuration network converter parallel system. By frequency feedback and introducing a proportional derivative controller and dynamically adjusting system parameters, stable operation of multiple parallel new energy grid-connected converters is achieved. This method is simple, practical, and has strong stability, and can effectively solve the problem of power oscillation in the parallel connection of configuration network converters.

[0006] In the first aspect, the present invention provides a method for suppressing power oscillation in a multi-configuration network converter parallel system.

[0007] A method for suppressing power oscillation in a multi-configuration network converter parallel system includes: Collect the voltage, current, and voltage frequency output by the three-phase converter at the grid connection point in the multi-configuration network converter parallel system; Calculate the active power and reactive power of the three-phase converter based on the voltage and current and input them into the virtual oscillator; Based on the voltage frequency and the rated frequency, through the introduced proportional derivative controller, obtain the system power reference compensation. Then, combine the active power and the active power reference value, and through the active droop link of the virtual oscillator, obtain the frequency deviation, and further calculate the voltage phase angle; at the same time, the reactive power passes through the reactive power control link of the virtual oscillator to obtain the voltage amplitude; Calculate the system reference voltage based on the voltage amplitude and the voltage phase angle. Based on the system reference voltage, through voltage-current double closed-loop regulation and PWM control, the output of the three-phase converter is adjusted in real time to suppress the system frequency oscillation.

[0008] In the second aspect, the present invention provides a system for suppressing power oscillation in a multi-configuration network converter parallel system.

[0009] A system for suppressing power oscillation in a multi-configuration network converter parallel system includes: A signal acquisition module for collecting the voltage, current, and voltage frequency output by the three-phase converter at the grid connection point in the multi-configuration network converter parallel system; A calculation module for calculating the active power and reactive power of the three-phase converter based on the voltage and current and inputting them into the virtual oscillator; A compensation module for obtaining the system power reference compensation based on the voltage frequency and the rated frequency through the introduced proportional derivative controller. Then, combine the active power and the active power reference value, and through the active droop link of the virtual oscillator, obtain the frequency deviation, and further calculate the voltage phase angle; at the same time, the reactive power passes through the reactive power control link of the virtual oscillator to obtain the voltage amplitude; An adjustment module is used to calculate the system reference voltage based on the voltage amplitude and voltage phase angle. Based on the system reference voltage, through double closed-loop regulation of voltage and current and PWM control, the output of the three-phase converter is adjusted in real time to suppress the system frequency oscillation.

[0010] In a third aspect, the present invention also provides an electronic device, including: a memory for storing executable instructions; a processor for implementing the above-mentioned power oscillation suppression method for a multi-networked converter parallel system when executing the executable instructions stored in the memory.

[0011] In a fourth aspect, the present invention also provides a computer-readable storage medium storing executable instructions for causing a processor to implement the above-mentioned power oscillation suppression method for a multi-networked converter parallel system when executing the executable instructions.

[0012] In a fifth aspect, the present invention also provides a computer program product, which includes executable instructions stored in a computer-readable storage medium; wherein, when a processor of an electronic device reads and executes the executable instructions from the computer-readable storage medium, the above-mentioned power oscillation suppression method for a multi-networked converter parallel system is implemented.

[0013] The above one or more technical solutions have the following beneficial effects: 1. The present invention provides a power oscillation suppression method and system for a multi-networked converter parallel system. In a converter parallel system with different line impedances, by frequency feedback and introducing a proportional-derivative controller and dynamically adjusting system parameters, stable operation of multiple parallel new energy grid-connected converters is achieved. Among them, a frequency feedback strategy is introduced, and a proportional-derivative (PD) controller dynamically corrects the power reference value according to the feedback frequency, so as to introduce a leading damping effect in the initial stage of power fluctuation, which can significantly suppress the transient fluctuation degree of active power and improve the dynamic stability of the system; this derivative control only acts on the transient process and automatically exits at steady state, which not only suppresses the oscillation but also avoids introducing steady-state error, ensuring that the system quickly returns to steady-state operation after dynamic disturbance.

[0014] 2. The present invention can directly construct a control compensation amount through the frequency feedback strategy, without relying on a non-linear function designed based on experience or a complex state observer, which can effectively simplify the controller parameter tuning process and improve engineering applicability.

[0015] Advantages of additional aspects of the present invention will be partially given in the following description, partially become obvious from the following description, or be understood through the practice of the present invention. Description of the Drawings

[0016] The accompanying drawings of the specification, which form a part of the present invention, are used to provide a further understanding of the present invention. The schematic embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute an improper limitation of the present invention.

[0017] Figure 1 It is a topology diagram of a multi-converter parallel system with inconsistent line impedances in an embodiment of the present invention; Figure 2 It is a power oscillation suppression control structure diagram of a grid-forming three-phase converter parallel system based on a virtual oscillator in an embodiment of the present invention; Figure 3 It is a comparison diagram of the active power waveforms of the improved strategy and the original strategy proposed in an embodiment of the present invention. Detailed implementation manners

[0018] It should be noted that the following detailed description is exemplary only and is for the purpose of describing the specific implementation manners, aiming to provide a further explanation of the present invention and not intending to limit the exemplary embodiments according to the present invention. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by those of ordinary skill in the technical field to which the present invention belongs. In addition, it should also be understood that when the terms "comprising" and / or "including" are used in this specification, they indicate the presence of features, steps, operations, devices, components, and / or combinations thereof.

[0019] Embodiment 1 For a multi-converter parallel system with inconsistent line impedances as shown in Figure 1 this embodiment provides a grid-forming converter parallel power oscillation suppression method based on frequency feedback, including the following steps: Step S1, collect the voltage, current, and voltage frequency output by the three-phase converter at the grid connection point in the multi-grid-forming converter parallel system; Step S2, calculate the active power and reactive power of the three-phase converter according to the voltage and current and input them into the virtual oscillator.

[0020] Specifically, as shown in Figure 1 the three-phase grid-connected converter uses a voltage-source three-phase bridge inverter circuit as the main circuit, uses a DC voltage source DC as the inverter input signal, converts the DC voltage source into a three-phase AC voltage source through a three-phase six-arm circuit, and accesses the power grid through a three-phase filter circuit. On this basis, as shown in Figure 2 for each three-phase grid-connected converter in the distributed generation (DG) branch, collect the voltage and current output by the three-phase converter (i.e., the voltage and current output at the point of common coupling PCC), and at the same time collect the output voltage frequency of this point of common coupling as feedback.

[0021] Further, according to the collected voltage and current, calculate the active power and reactive power of the three-phase converter. Among them, first perform Clark transformation and Park transformation on the collected voltage and current output by the three-phase converter. After the transformation, the three-phase voltage and current signals are converted into a two-phase rotating coordinate system (i.e., abc / dq conversion) to obtain the two-phase voltage and current, which is convenient for the signal to perform PI control. The conversion model is as follows: , , (1) In the above formula, and are the voltage and current output by the three-phase converter respectively, is the phase angle of the converter output voltage.

[0022] After calculation, the voltage and current in the two-phase rotating coordinate system are obtained. According to the two-phase voltage and current, calculate the active power and reactive power of the three-phase converter respectively. The calculation formulas are: (2) After that, introduce the active power and reactive power into the virtual oscillator, and perform real-time adjustment on the converter output through voltage feedback and current feedback to achieve the stable operation of the system. Among them, the virtual oscillator, as a non-linear controller, its dynamic mathematical model is expressed in polar coordinates as: (3) (4) In the above formula, , is the given voltage reference value; is the natural resonance frequency, that is, the rated frequency; is the output voltage of the virtual oscillator; is the resonance capacitor; is the convergence coefficient; is the voltage and current proportionality coefficient; and are the active power reference and reactive power reference of the converter respectively; is the line impedance angle; , represent the derivative values of the two-phase voltages. The superscript represents the derivative value of the corresponding parameter; represents the voltage difference between the two-phase voltages.

[0023] Usually, the line impedance is equivalent to a pure inductor, that is , according to the voltage amplitude and phase angle expression (4), its dynamic mathematical model can be simplified as: (5) In the above formula, and represent the derivative values of voltage amplitude and phase angle; and are the active power and reactive power output by the converter respectively, and are calculated from the three-phase converter voltage and current.

[0024] Therefore, after substituting and into the VOC control model (5) and through the double closed-loop regulation of voltage and current, the grid-forming control of the converter based on the virtual oscillator can be realized. Preferably, considering that in an inductive network, there is coupling between the d axis and the q axis signals in the rotating coordinate system and precise control cannot be achieved, decoupling of the signals is also performed when introducing the double closed-loop of voltage and current.

[0025] However, considering that during the grid connection process of new energy equipment based only on VOC control above, the active power output of the parallel system is prone to transient power oscillation. Therefore, in this embodiment, the above VOC control method is improved. By applying this control method to a multi-converter parallel system, the output frequency of the converter is obtained in real time as feedback, and a proportional-derivative controller is introduced to dynamically adjust the system power reference and increase the system damping. Since the derivative link only acts on the dynamic process of the system, power oscillation can be suppressed without affecting the system steady state, and stable operation of multiple parallel grid-connected converters for new energy is achieved.

[0026] That is, under weak grid conditions, the control objectives of the three-phase grid-forming converter parallel system are: obtaining stable output voltage and output current, suppressing the transient fluctuations of active power and frequency, and achieving stable operation of the multi-converter parallel system. The above control objectives can be transformed into: (6) wherein, " " represents "achieve tracking".

[0027] To meet the above control objectives, considering that in the converter parallel system, due to reasons such as line impedance differences and load changes, it is difficult to balance between controllers, which causes power oscillation and further affects system stability and power quality. Therefore, the suppression strategy adopted in this embodiment is: Step S3: According to the voltage frequency and the rated frequency, through the introduced proportional-derivative controller, obtain the system power reference compensation, and then combine the active power and the active power reference value. Through the active droop link of the virtual oscillator, obtain the frequency deviation, and further calculate the voltage phase angle; at the same time, the reactive power passes through the reactive power control link of the virtual oscillator to obtain the voltage amplitude.

[0028] Specifically, for the distributed converters that generate active power oscillations, by collecting the output voltage as described above and current , the output active power is calculated. The power calculation method in the two-phase rotating coordinate system is given above, and the formula for calculating the active power in the three-phase coordinate system is: (7) The mathematical model of the introduced proportional-derivative controller is: (8) where is the input error signal, is the output signal, is the proportional coefficient, is the derivative coefficient.

[0029] Considering that in the case of a purely inductive line condition, the active power and frequency are approximately in a droop relationship. Therefore, as Figure 2 shown, after introducing the proportional-derivative controller, first, according to the collected voltage frequency and the rated frequency, through the introduced proportional-derivative controller, the system power reference compensation value is obtained, which is: (9) In the above formula, represents the derivative controller, which is equivalent to in formula (8), s represents the differential operation on the input deviation signal.

[0030] Since the derivative link of the above PD controller is proportional to the deviation change rate , the system damping can be increased by introducing the proportional-derivative controller, that is, the resistance in the system dynamic regulation process is increased. When the system frequency deviates from the reference value faster, the reverse regulation effect provided by the derivative link is greater, thus effectively suppressing oscillations.

[0031] After that, the active power and the system power reference compensation value are input into the active power droop link of the virtual oscillator model to compensate the system power reference compensation value to the active power reference value. In this way, the system power reference can be dynamically adjusted, and then the final angular frequency deviation of VOC is calculated, that is: (10) where is the proportional coefficient.

[0032] Next, subtract the angular frequency deviation from the rated angular frequency Obtain the system angular frequency , and obtain the output voltage phase angle through the integration link , that is: (11) Through the control method of the above formula (11), compared with the traditional control method of the voltage phase angle in formula (5), this embodiment can achieve effective system power suppression and realize fast and stable operation.

[0033] At the same time, the reactive power passes through the reactive power control link of the virtual oscillator, that is, the control of the voltage amplitude in the above formula (5), to obtain the corresponding voltage amplitude.

[0034] Step S4: Calculate the system reference voltage according to the voltage amplitude and voltage phase angle. Based on the system reference voltage, through voltage-current double closed-loop regulation and PWM control, the output of the three-phase converter is adjusted in real time to suppress the system frequency oscillation.

[0035] Finally, combine the system reactive power control loop to obtain the voltage amplitude , calculate the VOC system reference voltage , input it to the voltage-current double closed-loop, and through the PWM link, realize the system closed-loop control.

[0036] According to the above control strategy proposed in this embodiment, the system power suppression effect and dynamic performance can be improved by appropriately adjusting the controller coefficients. The effectiveness of the method proposed in this embodiment is further verified by the following examples.

[0037] Specifically, as Figure 1 shown in the three-phase converter parallel system, which is composed of three converters in parallel, and its common terminal is connected to the power grid. There are differences in line impedances, among which distributed power sources DG1 (5mH, 0.01Ω), DG2 (5mH, 0.05Ω), DG3(5mH, 0.1Ω). Figure 2 is the structure diagram of the oscillation suppression control of the grid-forming three-phase converter based on the virtual oscillator proposed in this embodiment. Among them, the virtual oscillator algorithm based on frequency feedback is as above, and the feedback frequency is compensated to the active power reference through the PD controller to achieve the purpose of suppressing the active power oscillation. The active power waveforms output by the improved strategy and the original strategy proposed in this embodiment are as Figure 3 shown. It can be seen that when the line impedances are inconsistent or the damping is weak, the active power fluctuates significantly, seriously affecting the stable operation of the system. After adding the power oscillation suppression method proposed in this embodiment, the amplitude of the power oscillation is significantly reduced during steady-state operation, and the system stability is enhanced. Through the comparison of the power waveforms, the effectiveness of this method is further verified.

[0038] Embodiment 2 This embodiment provides a power oscillation suppression system for a multi-configuration network converter parallel system, including: A signal acquisition module, configured to acquire the voltage, current, and voltage frequency output by the three-phase converter at the grid connection point in the multi-configuration network converter parallel system; A calculation module, configured to calculate the active power and reactive power of the three-phase converter according to the voltage and current and input them into the virtual oscillator; A compensation module, configured to obtain a system power reference compensation through the introduced proportional derivative controller according to the voltage frequency and the rated frequency, then combine the active power and the active power reference value, obtain a frequency deviation through the active droop link of the virtual oscillator, and further calculate the voltage phase angle; at the same time, the reactive power passes through the reactive power control link of the virtual oscillator to obtain the voltage amplitude; An adjustment module, configured to calculate the system reference voltage according to the voltage amplitude and the voltage phase angle, and based on the system reference voltage, perform real-time adjustment on the output of the three-phase converter through voltage-current double closed-loop regulation and PWM control to suppress the system frequency oscillation.

[0039] Embodiment III This embodiment provides an electronic device, including: a memory for storing executable instructions; a processor for implementing the above method provided in this embodiment when executing the executable instructions stored in the memory.

[0040] Embodiment IV This embodiment further provides a computer-readable storage medium storing executable instructions, which when executed by a processor, will cause the processor to execute the above method provided in this embodiment.

[0041] Embodiment V This embodiment provides a computer program product, which includes executable instructions, and the executable instructions are a kind of computer instructions; the executable instructions are stored in a computer-readable storage medium. When the processor of an electronic device reads the executable instructions from the computer-readable storage medium and the processor executes the executable instructions, the electronic device is caused to execute the above method provided in this embodiment.

[0042] The steps involved in Embodiments II to V above correspond to those in Method Embodiment I, and the specific implementation manners can refer to the relevant description part of Embodiment I. The term "computer-readable storage medium" should be understood to include a single medium or multiple media including one or more instruction sets; it should also be understood to include any medium that can store, encode, or carry an instruction set for execution by a processor and cause the processor to execute any method in the present invention.

[0043] Those skilled in the art should understand that the above-mentioned modules or steps of the present invention can be implemented by a general-purpose computer device. Optionally, they can be implemented by program codes executable by a computing device, so that they can be stored in a storage device and executed by the computing device, or they can be separately fabricated into individual integrated circuit modules, or multiple modules or steps among them can be fabricated into a single integrated circuit module for implementation. The present invention is not limited to any specific combination of hardware and software.

[0044] The above are only the preferred embodiments of the present invention. Although the specific implementation manners of the present invention have been described in conjunction with the accompanying drawings, it is not a limitation on the protection scope of the present invention. Those skilled in the art should understand that various modifications or deformations that can be made without creative efforts on the basis of the technical solutions of the present invention are still within the protection scope of the present invention.

Claims

1. A method for suppressing power oscillations in a multi-grid converter parallel system, characterized in that: include: Collect the voltage, current and voltage frequency output by the three-phase converter at the grid connection point in the multi-grid converter parallel system; According to the voltage and current, the active power and reactive power of the three-phase converter are calculated and input into the virtual oscillator; According to the voltage frequency and the rated frequency, the system power reference compensation is obtained through the introduced proportional differential controller, and then the active power and the active power reference value are combined, and the frequency deviation is obtained through the active droop link of the virtual oscillator, and then the voltage phase angle is calculated; at the same time, the reactive power is obtained through the reactive control link of the virtual oscillator to obtain the voltage amplitude; The system reference voltage is calculated according to the voltage amplitude and voltage phase angle. Based on the system reference voltage, the output of the three-phase converter is adjusted in real time through voltage and current double closed-loop regulation and PWM control to suppress system frequency oscillation.

2. The power oscillation suppression method for a multi-grid converter parallel system according to claim 1, characterized in that: Calculate the active power and reactive power of the three-phase converter based on the voltage and current, including: Perform Clark transformation and Park transformation on the voltage and current output by the three-phase converter, convert the three-phase voltage and current signals into a two-phase rotating coordinate system, and obtain the two-phase voltage and current; According to the two-phase voltage and current, the active power and reactive power of the three-phase converter are calculated respectively.

3. The power oscillation suppression method for a multi-grid converter parallel system according to claim 1, characterized in that: The mathematical model of the introduced proportional differential controller is: ； in, is the input error signal, is the output signal, is the proportionality coefficient, is the differential coefficient; According to the collected voltage frequency and rated frequency, the system power reference compensation is obtained through the introduction of the proportional differential controller, which is: ； in, Indicates the frequency of real-time acquisition; is the natural resonant frequency, i.e. the rated frequency; represents the derivative controller, s Indicates the differential operation of the input deviation signal.

4. The power oscillation suppression method for a multi-grid converter parallel system according to claim 1, characterized in that: The system power reference compensation combines the active power and the active power reference value, and obtains the frequency deviation through the active power droop link of the virtual oscillator, and then calculates the voltage phase angle, including: The system power reference compensation value and the active power are input into the active power droop link of the virtual oscillator. According to the active power reference value, the system power reference compensation value is compensated to the active power reference value. Combined with the proportional coefficient of the active power droop link, the frequency deviation is calculated and expressed as: ； in, is the proportionality coefficient, is the angular frequency deviation, is the system angular frequency; Indicates the active power output of the converter; Indicates the converter active power reference value, Indicates the system power reference compensation value.

5. The power oscillation suppression method for a multi-grid converter parallel system according to claim 4, characterized in that: Also includes: According to the rated frequency and frequency deviation, the system frequency is calculated, and the system frequency is then integrated to obtain the output voltage phase angle, which is expressed as: ； in, is the output phase angle, is the output voltage of the virtual oscillator, is the resonant capacitor, is the voltage and current proportionality coefficient, is the rated angular frequency, Indicates the frequency of real-time acquisition, Indicates the converter output active power, Indicates the converter active power reference value, is the proportionality coefficient, is the differential coefficient, s Indicates the differential operation of the input deviation signal.

6. The power oscillation suppression method for a multi-grid converter parallel system according to claim 1, characterized in that: The reactive power is passed through the reactive control link of the virtual oscillator to obtain the voltage amplitude, and the formula is: ； in, Indicates the reactive power output by the converter, For a given reference voltage value, is the output voltage of the virtual oscillator, is the resonant capacitor, is the convergence coefficient, is the voltage and current proportionality coefficient, Indicates the reference reactive power of the converter.

7. A power oscillation suppression system for a multi-grid type converter parallel system, characterized in that: include: A signal acquisition module is used to collect the voltage, current and voltage frequency output by the three-phase converter at the grid connection point in the multi-grid converter parallel system; A calculation module, used for calculating the active power and reactive power of the three-phase converter according to the voltage and the current, and inputting the calculated active power and reactive power into the virtual oscillator; The compensation module is used to obtain the system power reference compensation according to the voltage frequency and the rated frequency through the introduced proportional differential controller, and then combine the active power and the active power reference value to obtain the frequency deviation through the active power droop link of the virtual oscillator, and then calculate the voltage phase angle; at the same time, the reactive power is obtained through the reactive power control link of the virtual oscillator to obtain the voltage amplitude; The regulation module is used to calculate the system reference voltage according to the voltage amplitude and the voltage phase angle. Based on the system reference voltage, the output of the three-phase converter is adjusted in real time through voltage and current double closed-loop regulation and PWM control to suppress system frequency oscillation.

8. An electronic device, characterized in that: include: A memory for storing executable instructions; The processor is used to implement the power oscillation suppression method for a multi-grid-type converter parallel system as described in any one of claims 1 to 6 when executing the executable instructions stored in the memory.

9. A computer-readable storage medium, characterized in that: Executable instructions are stored, which are used to cause the processor to execute the executable instructions to implement the power oscillation suppression method for a multi-grid converter parallel system as described in any one of claims 1-6.

10. A computer program product, characterized in that The computer program product includes executable instructions stored in a computer-readable storage medium; When the processor of the electronic device reads the executable instructions from the computer-readable storage medium and executes the executable instructions, the power oscillation suppression method for a multi-grid-type converter parallel system described in any one of claims 1 to 6 is implemented.

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