Method and system for suppressing low-frequency oscillation of a parallel structure of multiple virtual synchronous generators

Abstract

The present invention discloses a method and system for suppressing low-frequency oscillation of a parallel structure of multiple virtual synchronous generators. By feeding forward the high-frequency component of the differential of the electromagnetic power output by the VSG and multiplying it with an additional virtual damping proportional to inertia to form a virtual damping torque, the damping of the system during transient state can be effectively improved without affecting the steady state, so as to suppress the low-frequency power oscillation when the load of the VSG in parallel changes. The method of the present invention does not require communication and does not need to monitor the operating state of the VSG in real time to change parameters, which is convenient and feasible and the system has high reliability.

Description

Technical Field

[0001] The present invention relates to the field of multi-inverter parallel control, and particularly to a method and system for suppressing low-frequency oscillation of a parallel structure of multiple virtual synchronous generators. Background Art

[0002] Adopting distributed power generation has many advantages, which can make full use of rich clean and renewable resources in various places and enhance power supply reliability. However, because new energy in various forms such as wind energy and photovoltaic energy is easily affected by the external environment and has problems such as uncontrollable output, if these distributed energy sources are directly connected to the large power grid, it will bring great challenges to the safe and stable operation of the power grid, and the concept of microgrid emerges as the times require.

[0003] Different from a large-capacity power system with a large number of synchronous generators with relatively large inertia, a microgrid may exhibit low-inertia characteristics. Especially when distributed power sources with power electronic interfaces account for a large share, the low inertia of the system will pose a threat to the independent operation stability of the microgrid. In order to enable new energy microgrids to have the frequency modulation and voltage regulation capabilities of traditional generators, domestic and foreign scholars have proposed the virtual synchronous generator (VSG) control technology, that is, making traditional inverters have the inertia and damping characteristics of synchronous generators, providing inertia and damping support for the power system, so that under the condition of load disturbance, voltages, frequencies, etc. in the system are maintained in a relatively stable state. However, when multiple virtual synchronous generators are connected in parallel, if the load power changes suddenly, due to the different inertias of multiple virtual synchronous generators, power seizure will occur, which will trigger low-frequency oscillation of the system and pose a serious threat to the stability of the microgrid. Therefore, suppressing low-frequency oscillation of multiple parallel virtual synchronous generators has become a research hotspot.

[0004] CN113659618A provides a control method for a virtual synchronous generator. This control method adjusts the moment of inertia by real-time monitoring the output power and angular frequency state of the system. This method has complex requirements for program writing, may have delays, and the selection of the preset threshold directly determines the effectiveness of this method, resulting in poor suppression effect. Summary of the Invention

[0005] The technical problem to be solved by the present invention is to provide a method and system for suppressing low-frequency oscillation of a parallel structure of multiple virtual synchronous generators in view of the deficiencies of the prior art, suppress the frequency-power oscillation generated when the system load changes, improve the damping of the parallel system, and enhance the stability of the system.

[0006] To solve the above technical problems, the technical solution adopted by the present invention is: a method for suppressing low-frequency oscillation of a parallel structure of multiple virtual synchronous generators. The parallel structure of multiple virtual synchronous generators includes n paralleled virtual synchronous generators. The virtual synchronous generator includes a DC output module, an inverter, and a filter circuit connected in sequence. The filter circuits of all virtual synchronous generators are connected to a three-phase load. The method includes the following steps:

[0007] 1) At the starting point of each sampling period, sample the three-phase filter capacitor voltages u a 、u b 、u c and the three-phase output currents i a 、i b 、i c on the AC output side of the inverter respectively;

[0008] 2) Use the three-phase filter capacitor voltages u a 、u b 、u c and the three-phase output currents i a 、i b 、i c to calculate the active power P e and reactive power Q e output by the inverter; Obtain the actual output angular frequency ω through the phase-locked loop for the three-phase filter capacitor voltages u a 、u b 、u c ;

[0009] 3) Subtract ω from ω n , subtract Q e from Q ref , and obtain the output mechanical power P m and the reference voltage E through droop control; ω n is the power frequency, and Q ref is the reference reactive power;

[0010] 4) Calculate the phase angle θ using the output mechanical power P m and the output active power P e ;

[0011] 5) Pass the reference voltage E and the phase angle θ through a three-phase waveform generator and park transformation to obtain the three-phase voltage reference value u dq_ref , and perform park transformation on the three-phase filter capacitor voltages u a 、u b 、u c , the three-phase output currents i a 、i b 、i c respectively to obtain u dq 、idq , and then u dq , i dq After double closed-loop control of voltage and current and through inverse Park transformation, the PWM modulation wave signal u r_abc is obtained;

[0012] 6) Perform carrier stacking PWM modulation on u r_abc and the triangular carrier wave to obtain the duty cycle signal of the inverter switching tube,

[0013] Control the turn-on and turn-off of the inverter switching tube through the drive protection circuit of the inverter.

[0014] By performing high-pass filtering on the derivative of the output active power of the main body inverter to extract the high-frequency component and adding an additional virtual damping term, the present invention not only does not need to rely on communication stability to meet the communication requirements between parallel virtual synchronous generators, nor does it require a cumbersome calculation process, and can effectively increase the damping of the oscillation component in the transient stage without affecting the steady state. The method of the present invention effectively suppresses the oscillation phenomenon caused by different inertias during load mutation and enhances the stability of the microgrid system.

[0015] In step 2), the calculation formulas for the output active power P e , reactive power Q e are as follows:

[0016] P e = u a i a + u b i b + u c i c ;

[0017] Q e = u a i c + u b i a + u c i b .

[0018] In step 3), the calculation formulas for the output mechanical power P m and the reference voltage E are as follows:

[0019]

[0020] where k d and k q are the droop coefficients of active power - frequency and reactive power - voltage respectively, ω n is the power frequency, Q ref is the reference reactive power, P ref is the reference active power, and E0 is the reference voltage.

[0021] In step 4), the calculation formula of the phase angle θ is: θ = ∫ωdt; where the output active power P e has the following relationship with the angular frequency ω: J is the moment of inertia of the inverter, D is the damping coefficient of the inverter, a is the coefficient of the high-pass filter, b is the reciprocal of the time constant, and s is the complex frequency.

[0022] In step 5), the expression of the voltage-current double closed-loop control is as follows:

[0023]

[0024] where L is the sum of the equivalent inductor of the inverter and the filter inductor in the filter circuit, and C is the filter capacitor. U dref , U qref respectively represent the d-axis and q-axis reference voltages; U d , U q respectively represent the actual d-axis and q-axis output voltages; I d , I q respectively represent the actual d-axis and q-axis output currents; k p1 , k i1 , k p2 , k i2 respectively represent the parameters of the PI controllers of the voltage outer loop and the current inner loop; U d0 , U q0 are the outputs of the voltage-current double closed-loop control, and after inverse Park transformation, they enter the PWM generator as modulation wave signals.

[0025] In the present invention, the DC output module is one of a photovoltaic module, an energy storage module, and a DC power supply.

[0026] As an inventive concept, the present invention also provides a low-frequency oscillation suppression system for a parallel structure of multiple virtual synchronous generators, including a memory, a processor, and a computer program stored on the memory; the processor executes the computer program to implement the steps of the above method of the present invention.

[0027] Compared with the prior art, the beneficial effects of the present invention are: by extracting the high-frequency components of the derivative of the output active power P e and adding virtual damping for feedforward, the communication requirements between the parallel virtual synchronous generators are reduced, thereby reducing the dependence on communication stability; effectively increasing the damping of the oscillation components in the transient stage without affecting the steady state; effectively suppressing the oscillation phenomenon caused by different inertias during load mutation, and enhancing the stability of the microgrid system. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1Structural diagram of the virtual synchronous generator parallel low-frequency oscillation suppression system according to an embodiment of the present invention;

[0029] Figure 2 Control block diagram of the virtual synchronous generator parallel low-frequency oscillation suppression method according to an embodiment of the present invention;

[0030] Figure 3 Simulation case without adding the suppression method of this embodiment;

[0031] Figure 4 Simulation case with the suppression method of this embodiment of the present invention added. Specific implementation manner

[0032] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are some, but not all, of the embodiments of the present invention. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0033] In this article, the terms "comprising", "including", and other similar words are intended to represent a logical relationship and should not be regarded as representing a spatial structure relationship. For example, "A includes B" is intended to mean that logically B belongs to A, rather than indicating that B is located inside A in terms of space. Additionally, the meanings of the terms "comprising", "including", and other similar words should be regarded as open-ended rather than closed. For example, "A includes B" is intended to mean that B belongs to A, but B does not necessarily constitute all of A, and A may also include other elements such as C, D, and E.

[0034] Embodiment 1

[0035] As Figure 1 shown, the structure of the virtual synchronous generator parallel low-frequency oscillation suppression system according to Embodiment 1 of the present invention includes two parts: a module structure and a control system. The module structure is n (n = 1, 2, 3...), and the virtual synchronous generator structure modules are connected in parallel to the PCC terminal. The virtual synchronous generator structure module includes a stable DC output module, an inverter circuit, and a filter circuit. The virtual synchronous generator structure module is connected to a three-phase load through a transmission line via the PCC terminal. The DC output module U dc can be a photovoltaic module, an energy storage module, or a DC power source in the microgrid system. R is the equivalent resistance of the inverter, L is the sum of the equivalent inductance of the inverter and the filter inductance, C is the filter capacitor, and R g and L g are the line resistance and inductance respectively. u abc , i abc are the three-phase output voltage and three-phase output current of the inverter; Pe , Q e is the active power output and the reactive power output, P ref , Q ref are the active power reference value and the reactive power reference value; U 0_abc is the reference three-phase voltage, which becomes u dq_ref after the park transformation and serves as the reference voltage for the voltage loop control. The control system includes an outer loop of VSG, a three-phase sine wave generator, a double closed-loop control of voltage and current, and PWM control to obtain the trigger pulses of each switching tube of the inverter circuit, controlling the turn-on and turn-off of the switching tubes of the inverter circuit.

[0036] Figure 2 This is the control block diagram of Embodiment 1 of the present invention. At the start of each sampling period, the three-phase output voltage u abc and the three-phase output current i abc are sampled respectively. The data converted by the AD converter is sent to the DSP controller for processing, and the output active power P e and the reactive power Q e are obtained through calculation; among them, the calculation formulas for the output active power P e and the reactive power Q e are:

[0037] P e = u a i a + u b i b + u c i c ;

[0038] Q e = u a i c + u b i a + u c i b ;

[0039] The three-phase output voltage u abc sampled is sent through a phase-locked loop to obtain the actual angular frequency ω; the active power P e , the reactive power Q e and the angular frequency ω are sent into the improved VSG outer loop controller to obtain the output reference voltage E and the phase angle θ; the expression of the improved VSG outer loop controller is as follows:

[0040]

[0041]

[0042] where k d and k qare the droop coefficients of active power - frequency and reactive power - voltage respectively; ω n is the power frequency, with a value of 50 Hz; Q ref is the reference reactive power, P ref is the reference active power, and E0 is the reference voltage.

[0043] In this embodiment, the calculation formula for the phase angle θ is: θ = ∫ωdt.

[0044] The three - phase reference voltage is obtained by passing E0 and the obtained phase angle θ through a three - phase waveform generator, and then the dq - axis reference voltages U dref and U qref are obtained through the park transformation. The three - phase output voltage u abc sampled is also transformed through the park transformation to obtain U d and U q ; the park transformation formula is:

[0045]

[0046] The differences are respectively taken between U d and U dref , and between U q and U qref , and after voltage - current double - closed - loop control and inverse park transformation, the PWM modulation wave signal u r_abc is obtained. The expression of voltage - current double - closed - loop control is:

[0047]

[0048] To prove the effectiveness of this implementation scheme, a two - VSG parallel system is built using simulink simulation software for simulation. Figure 3 is a simulation case without adding the suppression method of this embodiment. Figure 4 is a simulation case with the suppression method of this embodiment added. Among them, the initial active load is 20 kW. At 2.5 s, the load suddenly increases to 30 kW and decreases to 26 kW at 5 s. By comparing Figure 3 and Figure 4 it can be seen that after adding this suppression method, both the oscillation amplitude and time of the power are significantly reduced, proving the effectiveness of the suppression method in the embodiment.

[0049] Embodiment 2

[0050] Embodiment 2 of the present invention provides a low - frequency oscillation suppression system for a parallel structure of multiple virtual synchronous generators corresponding to Embodiment 1 above, including a memory, a processor, and a computer program stored on the memory; the processor executes the computer program on the memory to implement the steps of the method in Embodiment 1 above.

[0051] In some implementations, the memory may be a high-speed random access memory (RAM), and may also include non-volatile memory, such as at least one magnetic disk memory.

[0052] In other implementations, the processor may be various types of general-purpose processors such as a central processing unit (CPU) or a digital signal processor (DSP), which are not limited herein.

[0053] Those skilled in the art should understand that the embodiments of the present application can be provided as a method, a system, or a computer program product. Therefore, the present application can take the form of a complete hardware embodiment, a complete software embodiment, or an embodiment combining software and hardware aspects. Moreover, the present application can take the form of a computer program product implemented on one or more computer-usable storage media (including but not limited to magnetic disk memories, CD-ROMs, optical memories, etc.) containing computer-usable program code. The solutions in the embodiments of the present application can be implemented in various computer languages. For example, object-oriented programming languages such as Java and interpreted scripting languages such as JavaScript.

[0054] The present application is described with reference to the flowcharts and / or block diagrams of methods, devices (systems), and computer program products according to the embodiments of the present application. It should be understood that each flow and / or block in the flowchart and / or block diagram, and the combination of flows and / or blocks in the flowchart and / or block diagram, can be implemented by computer program instructions. These computer program instructions can be provided to the processor of a general-purpose computer, a special-purpose computer, an embedded processor, or other programmable data processing devices to generate a machine, such that the instructions executed by the processor of the computer or other programmable data processing devices generate means for implementing the functions specified in Figure 1 one flow or multiple flows and / or blocks Figure 1 one block or multiple blocks.

[0055] These computer program instructions can also be loaded onto a computer or other programmable data processing device, such that a series of operation steps are executed on the computer or other programmable device to generate a computer-implemented process, so that the instructions executed on the computer or other programmable device provide steps for implementing the functions specified in Figure 1 one flow or multiple flows and / or blocks Figure 1 one block or multiple blocks.

[0056] Although the preferred embodiments of the present application have been described, those skilled in the art can make additional changes and modifications once they learn the basic creative concept. Therefore, the appended claims are intended to be construed to include the preferred embodiments as well as all changes and modifications falling within the scope of the present application.

[0057] Obviously, those skilled in the art can make various changes and modifications to the present application without departing from the spirit and scope of the present application. Thus, if these modifications and variations of the present application fall within the scope of the claims of the present application and their equivalent technologies, the present application is also intended to include these modifications and variations.

Claims

1. A method for suppressing low-frequency oscillations in a parallel structure of multiple virtual synchronous generators. The parallel structure of multiple virtual synchronous generators includes n parallel virtual synchronous generators. Each virtual synchronous generator includes a DC output module, an inverter, and a filter circuit connected in sequence. The filter circuits of all virtual synchronous generators are connected to a three-phase load. It is characterized in that, Including the following steps: 1) At the starting point of each sampling period, sample the three-phase filter capacitor voltages u a 、u b 、u c on the AC output side of the inverter, and the three-phase output currents i a 、i b 、i c respectively; 2) Using the three-phase filter capacitor voltage u a 、u b 、u c , three-phase output current i a 、i b 、i c Calculate the inverter output active power P e , reactive power Q e ; The three-phase filter capacitor voltage u a 、u b 、u c The actual output angular frequency ω is obtained through the phase-locked loop; 3) Subtract ω from ω n to obtain the difference, Q e and subtract Q from Q ref to obtain the output mechanical power P through droop control m and the reference voltage E; ω n is the power frequency, Q ref is the reference reactive power; 4) Use the output mechanical power P m and the output active power P e to calculate the phase angle θ; 5) The reference voltage E and phase angle θ are passed through a three-phase waveform generator and Park transformation to obtain the three-phase voltage reference value u dq_ref , and the three-phase filter capacitor voltages u a , u b , u c , the three-phase output currents i a , i b , i c are subjected to Park transformation to respectively obtain u dq , i dq . u dq , i dq are passed through a voltage-current double closed-loop control and through an inverse Park transformation to obtain the PWM modulation wave signal u r_abc ; 6) For u r_abc and the triangular carrier wave are subjected to carrier stacking PWM modulation to obtain the duty cycle signal of the inverter switching tube. Through the drive protection circuit of the inverter, the turning on and off of the inverter switching tube are controlled.

2. The method for suppressing low-frequency oscillation of the parallel structure of multiple virtual synchronous generators according to claim 1, characterized in that, In step 2), the calculation formulas for the output active power P e and the reactive power Q e are as follows: P e = u a i a + u b i b + u c i c ; Q e = u a i c + u b i a + u c i b .

3. The method for suppressing low-frequency oscillation of the parallel structure of multiple virtual synchronous generators according to claim 1, wherein, In step 3), the calculation formula for the output mechanical power P m and the reference voltage E is: where k d and k q are the droop coefficients of active power - frequency and reactive power - voltage respectively, ω n is the power frequency, Q ref is the reference reactive power, P ref is the reference active power, and E0 is the reference voltage.

4. The low-frequency oscillation suppression method for the parallel structure of multiple virtual synchronous generators according to claim 1, characterized in that, In step 4), the calculation formula for the phase angle θ is: θ = ∫ωdt; where the output active power P e has the following relationship with the angular frequency ω: J is the moment of inertia of the inverter, D is the damping coefficient of the inverter, a is the coefficient of the high-pass filter, b is the reciprocal of the time constant, and s is the complex frequency.

5. The method for suppressing low-frequency oscillation of the parallel structure of multiple virtual synchronous generators according to claim 1, characterized in that In step 5), the expression of the voltage and current double closed-loop control is as follows: Among them, L is the sum of the equivalent inductor of the inverter and the filter inductor in the filter circuit, C is the filter capacitor, U dref and U qref respectively represent the d-axis and q-axis reference voltages; U d and U q respectively represent the actual d-axis and q-axis output voltages; I d and I q respectively represent the actual d-axis and q-axis output currents; k p1 and k i1 and k p2 and k i2 respectively represent the parameters of the PI controllers of the outer voltage loop and the inner current loop; U d0 and U q0 are the outputs of the voltage-current double closed-loop control.

6. The low-frequency oscillation suppression method for the parallel structure of multiple virtual synchronous generators according to claim 1, characterized in that, The DC output module is one of a photovoltaic module, an energy storage module, and a DC power supply.

7. A low-frequency oscillation suppression system for a parallel structure of multiple virtual synchronous generators, comprising a memory, a processor, and a computer program stored on the memory; characterized in that, The processor executes the computer program to implement the steps of the method according to any one of claims 1 to 6.

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