Virtual impedance control method for low-frequency oscillation suppression of multi-virtual synchronous machine interaction system

Abstract

The invention discloses a virtual impedance control method for low-frequency oscillation suppression of a multi-virtual synchronous machine interaction system, and belongs to the technical field of power control. The method comprises the following steps: constructing a plurality of groups of virtual synchronous machines, and obtaining characteristic parameters including virtual impedance of each virtual synchronous machine; based on local active power feedback, constructing a local flushing filter through a Laplacian operator; and constructing a self-adaptive virtual impedance item according to the local flushing filter and the local active power, and embedding the self-adaptive virtual impedance item into each virtual synchronous machine. According to the method, virtual impedance is constructed based on a local active power item filtered by a flushing filter, and an inverter is controlled, so that the output equivalent inductive reactance of a local virtual synchronous motor is adaptively changed; according to the method, the output active power can be calculated only through local voltage and current, and low-frequency oscillation caused by interaction of multiple virtual synchronous machines can be effectively suppressed on the premise that neighbor frequency information does not need to be obtained based on communication, so that the reliability of the system is greatly improved.

Description

Technical Field

[0001] This invention relates to a virtual impedance control method, specifically a virtual impedance control method for suppressing low-frequency oscillations in a multi-virtual synchronous machine interactive system, belonging to the field of power control technology. Background Technology

[0002] Microgrids are widely used to facilitate the large-scale integration of renewable energy sources such as solar power, wind power, fuel cells, and storage units. Microgrid operation can be divided into grid-connected mode and islanded mode. In grid-connected mode, with the support of the grid, frequency / voltage stability and power balance can be guaranteed. However, in islanded mode, due to the widespread use of power electronic converters and the reduction in the rotational inertia of synchronous generators, microgrids lack sufficient inertia to support frequency stability. Insufficient inertia leads to excessively high frequency minimums and excessively high frequency change rates, making the stability of microgrids more vulnerable.

[0003] In microgrids, VSG control can improve the inertia of converter systems, mitigate the adverse effects of high-proportion power electronic converters on system frequency stability, and effectively avoid unnecessary load shedding or large-scale power outages, thus gaining widespread application. However, VSG control also introduces the oscillation characteristics of synchronous motors into the system. The inertia coefficient and damping coefficient have a significant impact on system oscillations. Therefore, power and frequency oscillations during transient processes can be suppressed by adjusting the inertia and damping coefficients. However, the internal coupling of multiple VSG systems makes power oscillations more severe. Existing research proposes centralized and distributed control schemes to suppress low-frequency oscillations, but communication increases costs, and communication data packet loss and single-point failures also reduce the oscillation suppression effect. Summary of the Invention

[0004] To address the problems existing in the prior art, the present invention aims to provide a virtual impedance control method for suppressing low-frequency oscillations in multi-virtual synchronous machine interactive systems. This method constructs a virtual impedance based on a flushing filter derived from local active power, and controls the inverter to adaptively change the output equivalent inductive reactance of the local virtual synchronous machine. The present invention only requires obtaining the local voltage and current to further calculate the output active power. Furthermore, without needing to obtain neighbor frequency information through communication, it can effectively suppress low-frequency oscillations caused by the interaction of multiple virtual synchronous machines, thereby significantly improving the reliability of the system.

[0005] To achieve the above technical objectives, the present invention provides a virtual impedance control method for suppressing low-frequency oscillations in a multi-virtual synchronous machine interactive system, comprising:

[0006] Step S1: Construct multiple virtual synchronizers and obtain the characteristic parameters of each virtual synchronizer, including virtual impedance;

[0007] Step S2: Based on local active power feedback, construct a local flushing filter using the Laplace operator;

[0008] Step S3: Couple the local flushing filter obtained in step S2 with each virtual synchronizer in step S1 to obtain the final filter.

[0009] The key technical feature of the technical solution provided by this invention is that the local flushing filter constructed based on the local active power feedback can effectively suppress low-frequency oscillations in a multi-virtual synchronous machine interactive system without relying on the communication process during the control process. Especially during the system islanding operation, it greatly improves the robustness and reliability of the system.

[0010] As a preferred embodiment, the multiple virtual synchronizers include two or more virtual synchronizers arranged in parallel in a star topology.

[0011] As a preferred embodiment, the characteristic parameters include: angular frequency, phase angle, active reference power, active output power, reactive output power, virtual impedance, total virtual reactance, initial virtual inductance, virtual inductance, adaptive virtual reactance coefficient, and flushing filter cutoff frequency.

[0012] As a preferred embodiment, the construction process of the local flushing filter is as follows:

[0013] Step S2-1: Under the premise that the line impedance is highly inductive, the output power of the i-th virtual synchronous machine in the multiple virtual synchronous machines can be expressed as the node injection power, which is expressed as:

[0014] Formula 1: ;

[0015] Step S2-2: Obtain the first-order derivative of the node injection power with respect to time, which is expressed as:

[0016] Formula 2: ;

[0017] Step S2-3: Based on the phase difference between nodes and the negligible change in the output voltage amplitude of the virtual synchronizer, Equation 2 is simplified, and the result is:

[0018] Formula 3: ;

[0019] Formula 4: ;

[0020] Formula 5: ;

[0021] Step S2-4: Based on the principle of approximate replacement between the differentiating element and the flushing filter, and according to the results of step S2-3, obtain the flushing filter. The process is as follows:

[0022] Formula 6: ;

[0023] In equations 1-6, b ij =V i V j B ij B ij It is the mutual admittance between node i and node j. V j Let δ be the output voltage amplitude of the j-th inverter. j The output phase angle of the j-th inverter; For flushing the filter, For the Laplace operator, ω c This indicates the cutoff frequency of the flushing filter.

[0024] When indirectly obtaining the difference between its own frequency and the external frequency by using the derivative of the local active signal, directly using a pure differentiator can easily introduce high-frequency noise. Therefore, in designing the adaptive virtual impedance stage, this invention uses a flushing filter to approximate the differentiating stage, thereby effectively suppressing the influence of high-frequency measurement noise.

[0025] As a preferred embodiment, the process of embedding the adaptive virtual impedance term into each virtual synchronizer is as follows: after obtaining the virtual impedance of each virtual synchronizer, a local flushing filter is introduced and coupled into an adaptive virtual impedance, which is then substituted into the impedance term of each virtual synchronizer to obtain the desired result.

[0026] As a preferred embodiment, the expression for the virtual impedance of each virtual synchronizer is:

[0027] Formula 7: ;

[0028] The construction process of the adaptive virtual impedance is as follows:

[0029] Formula 8: ;

[0030] Formula 9: ;

[0031] In equations 7-9, For the first Virtual impedance of a virtual synchronous motor For the first The total virtual reactance of a virtual synchronous motor. For the first The initial virtual inductance value of a virtual synchronous motor. For flushing filters, c i ω is the adaptive virtual reactance coefficient of the virtual synchronous motor. c This indicates the cutoff frequency of the flushing filter.

[0032] To facilitate a further understanding of the technical effects of the solution provided by this invention, taking the simplest dual virtual machine synchronous system as an example, after load fluctuations, when the angular frequency of virtual synchronous machine 1 is greater than that of virtual synchronous machine 2, the active power after high-pass filtering is greater than 0, the total virtual inductance decreases, and according to the power transfer equation, the output power of virtual synchronous machine 1 will increase, and the output angular frequency of virtual synchronous machine 1 will decrease; when the angular frequency of virtual synchronous machine 1 is less than that of virtual synchronous machine 2, the active power after high-pass filtering is less than 0, the total virtual inductance increases, and according to the power transfer equation, the output power of virtual synchronous machine 1 will decrease, and the output angular frequency of virtual synchronous machine 1 will increase; that is to say, the introduced adaptive virtual inductance term will promote the consistency of the output angular frequencies of the two virtual synchronous machines, thereby suppressing the low-frequency oscillations caused by the interaction of the two virtual synchronous machines.

[0033] Compared with the prior art, the beneficial technical effects of the technical solution provided by the present invention are as follows:

[0034] 1) The method provided by this invention constructs a virtual impedance based on the local active power obtained after the flushing filter, and controls the inverter to adaptively change the output equivalent inductive reactance of the local virtual synchronous motor. This invention only needs to obtain the local voltage and current to further calculate the output active power. Without the need to obtain neighbor frequency information based on communication, it can also effectively suppress low-frequency oscillations caused by the interaction of multiple virtual synchronous motors, thereby greatly improving the reliability of the system.

[0035] 2) In the technical solution provided by the present invention, the local flushing filter constructed based on the local active power feedback can effectively suppress low-frequency oscillations in a multi-virtual synchronous machine interactive system without relying on the communication process during the control process. Especially during the system islanding operation, it greatly improves the robustness and reliability of the system. Attached Figure Description

[0036] Figure 1 This is a star topology diagram of the VSG parallel system used in Embodiment 1 and Comparative Example 1 of the present invention;

[0037] Figure 2 This is a schematic diagram of the virtual impedance control method for suppressing low-frequency oscillations in a multi-virtual synchronous machine interactive system proposed in Embodiment 1 of the present invention;

[0038] Figure 3 This is a waveform diagram of the multiple virtual synchronous machines after a sudden load disturbance in Embodiment 1 of the present invention;

[0039] in, Figure 3 (a) is the waveform of the output active power. Figure 3 (b) is a frequency waveform diagram. Figure 3 (c) is the waveform diagram of the output reactive power;

[0040] Figure 4 This is a waveform diagram of the multiple virtual synchronous machines after a sudden load disturbance in Comparative Example 1 of the present invention;

[0041] in, Figure 4 (a) is the waveform of the output active power. Figure 4 (b) is a frequency waveform diagram. Figure 4 (c) is the waveform diagram of the output reactive power. Detailed Implementation

[0042] To make the objectives, technical solutions, and advantages of the present invention more apparent, exemplary embodiments according to the present invention will be described in detail below with reference to the accompanying drawings. Obviously, the described embodiments are merely a part of the embodiments of the present invention, and not all of the embodiments of the present invention. It should be understood that the present invention is not limited to the exemplary embodiments described herein. Based on the embodiments of the present invention described herein, all other embodiments obtained by those skilled in the art without inventive effort should fall within the protection scope of the present invention.

[0043] Example 1

[0044] This embodiment provides a virtual impedance control method for suppressing low-frequency oscillations in a multi-virtual synchronizer interactive system, specifically as follows:

[0045] Step S1: Construct multiple virtual synchronizers and obtain the characteristic parameters of each virtual synchronizer, including virtual impedance;

[0046] The multiple virtual synchronizers consist of 4 virtual synchronizers. Figure 1 The star topology shown is arranged in parallel;

[0047] The characteristic parameters include: angular frequency, phase angle, active reference power, active output power, reactive output power, virtual impedance, total virtual reactance, initial virtual inductance, virtual inductance, adaptive virtual reactance coefficient, and flushing filter cutoff frequency.

[0048] Step S2: Based on local active power feedback, construct a local flushing filter using the Laplace operator;

[0049] The construction process of the local flushing filter is as follows:

[0050] Step S2-1: Under the premise that the line impedance is highly inductive, the output power of the i-th virtual synchronous machine in the multiple virtual synchronous machines can be expressed as the node injection power, which is expressed as:

[0051] Formula 1: ;

[0052] Step S2-2: Obtain the first-order derivative of the node injection power with respect to time, which is expressed as:

[0053] Formula 2: ;

[0054] Step S2-3: Based on the phase difference between nodes and the negligible change in the output voltage amplitude of the virtual synchronizer, Equation 2 is simplified, and the result is:

[0055] Formula 3: ;

[0056] Formula 4: ;

[0057] Formula 5: ;

[0058] Step S2-4: Based on the principle of approximate replacement between the differentiating element and the flushing filter, and according to the results of step S2-3, obtain the flushing filter. The process is as follows:

[0059] Formula 6: ;

[0060] In equations 1-6, b ij =V i V j B ij B ij It is the mutual admittance between node i and node j. V j Let δ be the output voltage amplitude of the j-th inverter. j The output phase angle of the j-th inverter; For flushing the filter, For the Laplace operator, ω c Indicates the cutoff frequency of the flushing filter;

[0061] Step S3: Couple the local flushing filter obtained in step S2 with each virtual synchronizer in step S1 to obtain the final filter.

[0062] The process of coupling the flushing filter with each virtual synchronizer is as follows: after obtaining the active power of each virtual synchronizer, a local flushing filter is introduced and coupled into an adaptive virtual impedance.

[0063] Specifically, the expression for the virtual impedance of each virtual synchronizer is as follows:

[0064] Formula 7: ;

[0065] The coupling process of the adaptive virtual impedance is as follows:

[0066] Formula 9: ;

[0067] Formula 10: ;

[0068] For the first Virtual impedance of a virtual synchronous motor For the first The total virtual reactance of a virtual synchronous motor. For the first The initial virtual inductance value of a virtual synchronous motor. For flushing filters, c i ω is the adaptive virtual reactance coefficient of the virtual synchronous motor. c This indicates the cutoff frequency of the flushing filter.

[0069] Comparative Example 1

[0070] Comparative Example 1 employs the most common multi-group virtual impedance control method in the existing technology, and its expression is:

[0071] Formula 11: ;

[0072] In Equation 11, J i ω is the coefficient of inertia. i Let ω* represent the angular frequency of the i-th virtual synchronizer, and P represent the nominal angular frequency. * and P i These represent the active reference power and output power of the i-th inverter, respectively.

[0073] To verify the superior technical effect of the solution provided by the present invention, the control methods provided in Example 1 and Comparative Example 1 were used to perform virtual impedance control on a multi-virtual synchronous machine system composed of four virtual synchronous machines. The waveforms of the system after load sudden disturbance are shown below. Figure 3 and Figure 4 As shown. Figure 3 and Figure 4 Both load surge and load reduction disturbances were applied at t=15s and t=20s.

[0074] pass Figure 3 and Figure 4A comparison reveals that the control method provided in Comparative Example 1 exhibits severe low-frequency interactive oscillations, which are fully reflected in active power, frequency, and reactive power. This deficiency in frequency dynamic performance may further lead to system instability. However, the control method provided in Embodiment 1 of this invention significantly suppresses the low-frequency oscillations in power and frequency. This is mainly because the control method in Embodiment 1, after load disturbance, generates a high-pass filtered active signal with corresponding signs by comparing the local and system angular frequencies. This signal dynamically adjusts the virtual inductance value—a decrease in inductance increases output power and decreases the local angular frequency, while an increase in inductance decreases output power and increases the local angular frequency. This negative feedback mechanism promotes the convergence of the output angular frequencies of the parallel-operating virtual synchronous machines, thereby suppressing the low-frequency oscillations caused by the interaction of multiple virtual synchronous machines. Furthermore, this process further demonstrates that the control method provided by this invention can effectively suppress low-frequency oscillations in a multi-virtual synchronous machine interactive system without relying on communication to obtain neighbor frequency information. This indicates that even if the system is operating in an islanded manner, the control method provided by this invention can still provide excellent robustness and reliability.

Claims

1. A virtual impedance control method for suppressing low-frequency oscillations in a multi-virtual synchronous machine interactive system, characterized in that, include: Step S1: Construct multiple virtual synchronizers and obtain the characteristic parameters of each virtual synchronizer, including virtual impedance; Step S2: Based on local active power feedback, construct a local flushing filter using the Laplace operator; Step S3: Construct an adaptive virtual impedance term based on the local flushing filter and local active power, and embed it into each virtual synchronizer.

2. The virtual impedance control method for suppressing low-frequency oscillations in a multi-virtual synchronous machine interactive system according to claim 1, characterized in that: The multiple virtual synchronizers include two or more virtual synchronizers arranged in parallel in a star topology.

3. The virtual impedance control method for suppressing low-frequency oscillations in a multi-virtual synchronous machine interactive system according to claim 1, characterized in that: The characteristic parameters include: angular frequency, phase angle, active reference power, active output power, reactive output power, virtual impedance, total virtual reactance, initial virtual inductance, virtual inductance, adaptive virtual reactance coefficient, and flushing filter cutoff frequency.

4. The virtual impedance control method for suppressing low-frequency oscillations in a multi-virtual synchronous machine interactive system according to claim 1, characterized in that: The construction process of the local flushing filter is as follows: Step S2-1: Under the premise that the line impedance is highly inductive, the output power of the i-th virtual synchronous machine in the multiple virtual synchronous machines can be expressed as the node injection power, which is expressed as: Formula 1: ; Step S2-2: Obtain the first-order derivative of the node injection power with respect to time, which is expressed as: Formula 2: ; Step S2-3: Based on the phase difference between nodes and the negligible change in the output voltage amplitude of the virtual synchronizer, Equation 2 is simplified, and the result is: Formula 3: ; Formula 4: ; Formula 5: ; Step S2-4: Based on the principle of approximate replacement between the differentiating element and the flushing filter, and according to the results of step S2-3, obtain the flushing filter. The process is as follows: Formula 6: ; In equations 1-6, b ij =V i V j B ij B ij It is the mutual admittance between node i and node j. V j Let δ be the output voltage amplitude of the j-th inverter. j The output phase angle of the j-th inverter; For flushing the filter, For the Laplace operator, ω c This indicates the cutoff frequency of the flushing filter.

5. The virtual impedance control method for suppressing low-frequency oscillations in a multi-virtual synchronous machine interactive system according to claim 4, characterized in that: The process of embedding the adaptive virtual impedance term into each virtual synchronizer is as follows: after obtaining the virtual impedance of each virtual synchronizer, a local flushing filter is introduced based on the dynamic cooperative control of a single integrator, coupled into an adaptive virtual impedance, and substituted into the impedance term of each virtual synchronizer to obtain the result.

6. The virtual impedance control method for suppressing low-frequency oscillations in a multi-virtual synchronous machine interactive system according to claim 5, characterized in that: The expression for the virtual impedance of each virtual synchronizer is: Formula 7: ; The construction process of the adaptive virtual impedance is as follows: Formula 8: ; Formula 9: ; In equations 7-9, For the first Virtual impedance of a virtual synchronous motor For the first The total virtual reactance of a virtual synchronous motor. For the first The initial virtual inductance value of a virtual synchronous motor. For flushing filters, c i ω is the adaptive virtual reactance coefficient of the virtual synchronous motor. c This indicates the cutoff frequency of the flushing filter.

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