Method for judging grid oscillation when multiple grid-connected inverters are running in parallel

Abstract

The invention discloses a method for judging whether an electric grid oscillates when multiple machines of a grid-connected inverter are in parallel operation. The method is applicable to a system in which n LCL-type grid-connected inverters are in parallel operation. The method comprises: establishing a mathematical model of an LCL-type grid-connected inverter, deriving a resonant frequency of the system when multiple machines are in parallel, wherein the resonant frequency comprises a fixed resonant angular frequency and a variable resonant angular frequency; collecting a three-phase voltage at a common coupling point, performing positive and negative sequence separation, phase locking, and Park transformation to obtain d-axis and q-axis voltage components; filtering out a DC component and other frequency fluctuation components in the d-axis and q-axis voltage components to obtain d-axis and q-axis voltage amplitudes at the resonant frequency; obtaining a voltage amplitude at the resonant frequency according to the d-axis and q-axis voltage amplitudes, performing low-pass filtering and normalization processing, and comparing the voltage amplitude with a specific threshold value to judge whether resonance occurs at the resonant frequency, and then suppressing the resonance in a targeted manner to ensure stable operation of the system.

Description

Technical Field

[0001] The present invention relates to the field of new energy technology, and in particular to a method for judging whether an electric grid oscillates when multiple grid-connected inverters are running in parallel. Background Art

[0002] At present, LCL filters are widely used in grid-connected (grid) inverters. Since it is a third-order low-damping system, resonance problems are difficult to avoid. When multiple inverters are connected to the same common coupling point (Pcc), the grid-connected inverter and the grid will interact, resulting in more serious and complex resonance phenomena, which will lead to system instability and power quality pollution, which is extremely harmful to the system. In addition, the equivalent grid impedance at the access point of the grid-connected inverter often changes with changes in factors such as line impedance, the number of grid-connected units, and the system operation mode, resulting in fluctuations in grid strength and a weak grid situation. At this time, it is more difficult to suppress the resonance of multiple grid-connected inverters in parallel, so it is particularly important to suppress the resonance peak of multiple parallel LCL grid-connected inverters under weak grids.

[0003] The existing harmonic extraction methods (methods for determining whether the power grid is oscillating) mainly include the indirect resonant extraction method and the direct resonant extraction method. Although the direct resonant extraction method can detect the resonant frequency in the grid-connected inverter system, it has great limitations in its application: the traditional Fourier analysis method has the characteristics of large amount of calculation and relatively complex, so it is generally only used to detect the resonant frequency of a single grid-connected inverter system. In the indirect resonant extraction method, the phase-locked loop is very sensitive to the dynamic response of the system and has poor anti-interference ability. If the number of grid-connected inverters in the system changes or the grid impedance value changes, the dynamic performance of the PLL will also deteriorate, so its stability is not good.

[0004] Therefore, it is necessary to design a method for determining whether a power grid oscillates when multiple grid-connected inverters are running in parallel, so as to solve the problems existing in the above-mentioned prior art. Summary of the invention

[0005] The purpose of the present invention is to solve the shortcomings of the prior art and to propose a method for determining whether an oscillation occurs in a power grid when multiple grid-connected inverters are running in parallel.

[0006] In order to achieve the above object, the present invention adopts the following technical solutions:

[0007] A method for judging whether a power grid oscillates when multiple grid-connected inverters are running in parallel is applicable to a system in which n LCL-type grid-connected inverters are running in parallel, and comprises the following steps:

[0008] Step 1: Establish a mathematical model of the LCL type grid-connected inverter and derive the system resonant frequency when multiple machines are connected in parallel, wherein the resonant frequency includes a fixed resonant angular frequency and a variable resonant angular frequency;

[0009] Step 2: Collect the three-phase voltage at the common coupling point, separate the positive and negative sequences, lock the phases, and perform Park transformation to obtain the d and q axis voltage components;

[0010] Step 3: Filter out the DC component and other frequency fluctuation components in the d-axis and q-axis voltage components to obtain the d-axis and q-axis voltage amplitudes at the resonant frequency;

[0011] Step 4: Obtain the voltage amplitude at the resonant frequency according to the d-axis and q-axis voltage amplitudes, perform low-pass filtering and normalization processing, and compare it with a specific threshold to determine whether resonance occurs at the resonant frequency;

[0012] Here, n is an integer greater than or equal to 2.

[0013] Furthermore, the derivation of the system resonant frequency when multiple machines are connected in parallel includes:

[0014] According to the control block diagram of a single LCL type grid-connected inverter under grid-connected current control, the closed-loop transfer function of a single LCL type grid-connected inverter is obtained;

[0015] Assuming that the circuit parameters of the LCL filters of n inverters are consistent, combined with the closed-loop transfer function of a single LCL grid-connected inverter, the closed-loop transfer function of the parallel operation system of n LCL grid-connected inverters is obtained, and the fixed resonant angular frequency ω is obtained according to the resonance condition. r1 , the variable resonant angular frequency ω r2 .

[0016] Furthermore, the closed-loop transfer function of the single LCL grid-connected inverter is (1):

[0017] (1);

[0018] Among them, i 2 is the grid-connected current, i 1 * For a given current, Z L1 , Z L2 , Z C are the component impedances of the LCL filter, Vpcc is the common coupling point voltage, G PI Represents the proportional and integral control expression, K pwm is the inverter equivalent gain, which takes a value of 1.

[0019] Furthermore, the closed-loop transfer function of the n LCL-type grid-connected inverter parallel operation system is as follows:

[0020] (2);

[0021] Where A=Z L1 +Z C +G PI , B=Z L2 (Z L1 +Z C +G PI )+Z C (Z L1 +G PI ), D = Z C ×G PI , Z g is the grid impedance.

[0022] Furthermore, the condition for system resonance to occur when multiple machines are connected in parallel is that the denominator of the equation (2) is 0.

[0023] Furthermore, the step 2 is specifically as follows:

[0024] The three-phase voltage at the common coupling point is collected to separate the positive and negative sequences. Then the positive sequence voltage component is phase-locked using a phase-locked loop based on the second-order generalized integral method to obtain the synchronous rotation angle α. After Park transformation, the d and q axis voltage components V dpcc 、V qpcc , as shown in formula (9), where V pccA 、V pccB 、V pccC is the three-phase voltage at the common coupling point;

[0025] (9).

[0026] Furthermore, the step 3 is specifically as follows:

[0027] Respectively, r1 , the said ω r2 Construct a bandwidth of ω for the center frequency c The unit gain bandpass filter is as shown in equation (10) and equation (11); the d-axis and q-axis voltage components are respectively passed through the bandpass filter BPF (ω) represented by equation (10) and equation (11) r1 )、BPF(ω r2 ) to filter out the DC component and other frequency fluctuation components, leaving V dpccwr1 and V qpccwr1 The voltage fluctuation, V dpccwr2 and V qpccwr2 The voltage fluctuation amount;

[0028] (10);

[0029] (11);

[0030] Among them, V dpccwr1、 V qpccwr1 After filtering is completed, ω r1 The voltage amplitude of d-axis and q-axis at dpccwr2、 V qpccwr2 After filtering is completed, ω r2 is the voltage amplitude of the d-axis and q-axis at , and s is a complex variable used to describe the response of the system in the complex domain.

[0031] Furthermore, the step 4 is specifically as follows:

[0032] The ω is calculated using the d and q axis voltage amplitudes. r1 and the ω r2 The voltage amplitude Vʹ at wr1 and Vʹ wr2 , respectively, are filtered by a low-pass filter, then normalized, and then compared with the specified thresholds, and finally determined in the ω r1 and the ω r2 Whether resonance occurs.

[0033] Further,

[0034] The Vʹ wr1 and Vʹ wr2 The low-pass filter is LPF (ω r1 )、LPF(ω r2 ), as shown in formula (14) and formula (15), respectively; the Vʹ wr1 and Vʹ wr2 The voltage amplitude after low-pass filter is V wr1 and V wr2 The standardization process refers to the process of normalizing the V according to formula (16) and formula (17). wr1 and the V wr2 Process to obtain k wr1 , k wr2 ;

[0035] (12);

[0036] (13);

[0037] (14);

[0038] (15);

[0039] (16);

[0040] (17);

[0041] Among them, V N is the grid phase voltage amplitude under normal conditions.

[0042] Further, the determination is made in the ω r1 and the ω r2 Whether resonance occurs includes:

[0043] If k wr1 ≥ɛ 1 , it means that in the ω r1 Oscillation occurs at

[0044] If k wr1 <ɛ 1 , it means that in the ω r1 No oscillation occurred;

[0045] If k wr2 ≥ɛ 2 , it means that in the ω r2 Oscillation occurs at

[0046] If k wr2 <ɛ 2 , it means that in the ω r2 No oscillation occurred;

[0047] Among them, ɛ 1 、ɛ 2 Represents the voltage oscillation coefficient, which shall not be less than 5%.

[0048] Compared with the prior art, the present invention provides a method for determining whether a power grid oscillates when multiple grid-connected inverters are running in parallel.

[0049] This method for judging the occurrence of grid oscillation when multiple machines of the grid-connected inverter are in parallel operation is used for the detection method for judging the occurrence of grid oscillation when multiple machines of the grid-connected inverter are in parallel operation. Taking the LCL type grid-connected inverter multi-machine parallel operation system as the research object, when the system is in closed-loop operation, the system will generate a fixed frequency resonance peak and a variable frequency resonance peak that changes with the number of parallel operation units and the impedance of the grid line. By further judging the voltage amplitude at the corresponding resonance frequency, it can be determined whether the resonance phenomenon occurs at this frequency, and then the resonance can be suppressed in a targeted manner to ensure the stable and safe operation of the system. BRIEF DESCRIPTION OF THE DRAWINGS

[0050] In order to more clearly illustrate the specific implementation methods of the present invention or the technical solutions in the prior art, the drawings required for use in the specific implementation methods or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are some implementation methods of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0051] Figure 1 It is a simplified circuit diagram of a system in which n LCL-type grid-connected inverters are connected in parallel;

[0052] Figure 2 It is the final simplified control block diagram corresponding to a single LCL type grid-connected inverter under grid-connected current control;

[0053] Figure 3 is to find the fixed resonant angular frequency ω r1 and variable resonant angular frequency ω r2 Block diagram of voltage amplitude at ;

[0054] Figure 4 It is a flow chart of the implementation steps of the present invention for determining whether an oscillation occurs in a power grid.

[0055] The symbols in the figure are defined as follows:

[0056] V dc1 、V dc2 ,......,V dcn are the DC voltages on the kth inverter side; L 11 , L 21 ,......,L n1 are the inverter side inductance of the LCL filter of the kth inverter; L 12 , L 22 ,......,L n2 are the grid-side inductance of the LCL filter of the kth inverter; C 1 , C 2 ,......,C n are the capacitances of the LCL filters of the kth inverter respectively; Vpcc is the voltage at the common coupling point; L g is the equivalent inductance of the power grid line; V g is the grid voltage; i 2 is the grid-connected current; G PI is the proportional integral control expression; K pwm is the inverter equivalent gain; Z L1 is the inductance L of the LCL filter 1 Impedance; Z C is the impedance of the capacitor C of the LCL filter; Z L2 is the inductance L of the LCL filter2 impedance. DETAILED DESCRIPTION

[0057] In order to make the purpose, technical solution and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with specific embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention. Example

[0058] The present invention provides a method for determining whether a power grid oscillates when multiple grid-connected inverters are running in parallel, which is suitable for Figure 1 The n LCL-type grid-connected inverters are shown in parallel operation system, and the specific connection relationship of the parallel operation system is as follows:

[0059] The DC source DCk is used as the input of the corresponding inverter k, and the output of the inverter k is connected to L k1 One end, L k1 The other end is connected to L k2 One end and C k One end, C k The other end of the ground, L k2 The other end is connected to the common coupling point Pcc, Pcc through L g Connect to the power grid;

[0060] Wherein, k is an integer from 1 to n, and n is an integer greater than or equal to 2; V dck is the DC voltage at the input side of the kth inverter (inverter k); L k1 are the inverter side inductance of the LCL filter of the kth inverter (inverter k); L k2 is the grid-side inductance of the LCL filter of the kth inverter (inverter k); C k are the capacitances of the LCL filter of the kth inverter (inverter k); Vpcc is the voltage at the common coupling point; L g is the equivalent inductance of the power grid line; V g is the grid voltage;

[0061] The method for determining whether a power grid oscillates comprises the following steps:

[0062] Step 1: Establish a mathematical model of the LCL grid-connected inverter and derive the system resonant frequency when multiple machines are connected in parallel. The resonant frequency includes a fixed resonant angular frequency and a variable resonant angular frequency:

[0063] Specifically, establish Figure 2 The mathematical model of the LCL grid-connected inverter is shown. The existing control method is used to derive the frequency at which the system may generate resonance when multiple LCL grid-connected inverters are running in parallel. The method is as follows:

[0064] according to Figure 2 The final simplified control block diagram of a single LCL type grid-connected inverter under grid-connected current control is shown, and the closed-loop transfer function of the single LCL type grid-connected inverter is obtained as formula (1).

[0065] (1);

[0066] Among them, i 2 is the grid-connected current, i 1 * For a given current, Z L1 , Z L2 , Z C are the impedances of the components of the LCL filter (inductor L1, inductor L2, capacitor C), Vpcc is the voltage at the common coupling point, G PI Represents the proportional and integral control expression, K pwm K is the inverter equivalent gain, and its calculation formula is: pwm =Vdc / Vtri, Vdc is the DC voltage on the inverter input side, Vtri is the triangular carrier amplitude. The specific value is related to Vdc and Vtri, and is usually 1.

[0067] Assume that the circuit parameters of the LCL filter of n inverters are consistent, that is, L 11 =L 21 =……=L n1 =L 1 , L 21 =L 22 =……=L 2n =L 2 , C 1 =C 2 =……=C n =C. Combine Figure 1 By combining equation (1), the closed-loop transfer function of the parallel operation system of n LCL-type grid-connected inverters can be obtained as shown in equation (2).

[0068] (2);

[0069] Where A=Z L1 +Z C +G PI , B=Z L2 (Z L1 +Z C +G PI )+Z C (Z L1 +G PI ), D = Z C ×G PI , Z g is the grid impedance.

[0070] If resonance is to be generated, the denominator of equation (2) must be 0, that is, equation (3) or equation (4) must be satisfied.

[0071] (3);

[0072] (4);

[0073] The proportional and integral control expressions are:

[0074] (5);

[0075] Among them, K P is the proportionality coefficient, K i is the integration coefficient.

[0076] Solving equation (3) yields equation (6), ω r1 is a fixed resonant angular frequency.

[0077] (6);

[0078] Solving equation (4) yields equation (7), ω r2 It is a variable resonant angular frequency that changes with the number of units running in parallel and the impedance of the grid line.

[0079] (7);

[0080] In summary, when n LCL-type grid-connected inverters are connected in parallel, the resonant frequency calculation formula is as follows: (8).

[0081] (8);

[0082] When the number of parallel units n is known, and the grid line impedance parameter L g After confirmation, 1 is a fixed resonant frequency, fr 2 The variable resonant frequency is changed to a fixed resonant frequency. 1 With ω r1 The relationship is ω r1 =2πfr 1 ;fr 2 With ω r2 The relationship is ω r2 =2πfr 2 .

[0083] Step 2: Collect the three-phase voltage at the common coupling point, separate the positive and negative sequences, lock the phases, and perform Park transformation to obtain the d and q axis voltage components.

[0084] Specifically, the three-phase voltage at the common coupling point Pcc is collected, the positive and negative sequences of the three-phase voltage are separated, and then the positive sequence voltage component is phase-locked using a phase-locked loop based on the second-order generalized integration method to obtain the synchronous rotation angle α, and then the Park transformation is performed to obtain the voltage components of the d and q coordinate axes (d and q axis voltage components) V dpcc 、V qpcc , as shown in formula (9), where V pccA 、V pccB 、V pccC is the three-phase voltage at the common coupling point.

[0085] (9).

[0086] Step 3: Filter out the DC component and other frequency fluctuation components in the d-axis and q-axis voltage components to obtain the d-axis and q-axis voltage amplitudes at the resonant frequency.

[0087] Specifically, with a fixed resonant angular frequency ω r1 (2πfr 1 ) and variable resonant angular frequency ω r2 (2πfr 2 ) is used as the center frequency to construct a unity gain bandpass filter with a bandwidth of ω c , as shown in equations (10) and (11). The obtained d and q axis voltage components V dpcc 、V qpcc The bandpass filter BPF (ω) represented by equations (10) and (11) is r1 )、BPF(ω r2 ) to filter out DC components and other frequencies (non-ω r1 , non-ω r2 frequency), only V dpccwr1 and V qpccwr1 Voltage fluctuation, V dpccwr2 and V qpccwr2 The voltage fluctuation amount; where V dpccwr1 After filtering is completed, ω r1 The d-axis voltage amplitude at qpccwr1 After filtering is completed, ω r1 The q-axis voltage amplitude at V dpccwr2 After filtering is completed, ω r2 The d-axis voltage amplitude at qpccwr2 After filtering is completed, ω r2 is the q-axis voltage amplitude at , and s is a complex variable used to describe the response of the system in the complex domain.

[0088] (10);

[0089] (11);

[0090] Step 4: Obtain the voltage amplitude at the resonant frequency according to the d-axis and q-axis voltage amplitudes, perform low-pass filtering and normalization processing, and compare the voltage amplitude with a specific threshold to determine whether resonance occurs at the resonant frequency.

[0091] Using the d and q axis voltage amplitudes V obtained above dpccwr1 and V qpccwr1 , V dpccwr2 and V qpccwr2 Calculate the fixed resonant angular frequency ω respectively r1 and variable resonant angular frequency ω r2 The voltage amplitude Vʹ at wr1 and Vʹ wr2 , filtered by a low-pass filter, then normalized, and then compared with the specified thresholds, and finally determined at a fixed resonant angular frequency ω r1 and variable resonant angular frequency ω r2 Whether resonance occurs at the location. Specifically:

[0092] By using equations (12) and (13), we can calculate the fixed resonant angular frequency ω. r1 and variable resonant angular frequency ω r2 The voltage amplitude Vʹ at wr1 and Vʹ wr2 , due to Vʹ wr1 and Vʹ wr2 They fluctuate according to their respective resonant cycles, so Vʹ needs to be wr1 and Vʹ wr2 Perform low-pass filtering, low-pass filter LPF (ω r1 )、LPF(ω r2 ) are shown in equations (14) and (15) respectively, and the filtered voltage amplitude V is obtained through a low-pass filter. wr1 and V wr2 Then, according to equations (16) and (17), V wr1 and V wr2 Standardize to get k wr1 , k wr2 .

[0093] (12);

[0094] (13);

[0095] (14);

[0096] (15);

[0097] (16);

[0098] (17);

[0099] Where V N is the grid phase voltage amplitude under normal conditions.

[0100] If k wr1 ≥ɛ 1 , it means that at a fixed resonant angular frequency ω r1 Oscillation occurs at

[0101] If k wr1 <ɛ 1 , it means that at a fixed resonant angular frequency ω r1 No oscillation occurred at

[0102] If k wr2 ≥ɛ 2 , it means that at the variable resonant angular frequency ω r2 Oscillation occurs at

[0103] If k wr2 <ɛ 2 , it means that at the variable resonant angular frequency ω r2 No oscillation occurred.

[0104] Among them, ɛ 1 、ɛ 2 represents the voltage oscillation coefficient, ɛ 1 The value is usually not less than 5%; 2 The value is usually not less than 5%.

[0105] This method for judging the occurrence of grid oscillation when multiple machines of the grid-connected inverter are in parallel operation is used for the detection method for judging the occurrence of grid oscillation when multiple machines of the grid-connected inverter are in parallel operation. Taking the LCL type grid-connected inverter multi-machine parallel operation system as the research object, when the system is in closed-loop operation, the system will generate a fixed frequency resonance peak and a variable frequency resonance peak that changes with the number of parallel operation units and the impedance of the grid line. By further judging the voltage amplitude at the corresponding resonance frequency, it can be determined whether the resonance phenomenon occurs at this frequency, and then the resonance can be suppressed in a targeted manner to ensure the stable and safe operation of the system.

[0106] The present invention is characterized in that it is only necessary to detect the voltage amplitude at the resonance frequency obtained by theoretical calculation to infer whether the power grid has resonance when multiple grid-connected inverters are operated in parallel.

[0107] Some key words involved in the present invention:

[0108] Grid-tied inverter: A grid-tied inverter is a special inverter that, in addition to converting DC into AC, can synchronize the frequency and phase of the AC output with the mains, so that the output AC can return to the mains. Grid-tied inverters are often used in applications where a DC voltage source (such as a solar panel or a small wind turbine) is connected to the grid.

[0109] Resonance: Resonance is also called "resonance". When an oscillating system is subjected to periodic external force, when the frequency of the external force is the same as or very close to the natural oscillation frequency of the system, the amplitude increases sharply. The frequency when resonance occurs is called "resonance frequency". In electrical engineering, the resonance phenomenon of an oscillating circuit. The resonance of an inductor and a capacitor in series is called "series resonance" or "voltage resonance"; the resonance of a parallel circuit is called "parallel resonance" or "current resonance".

[0110] Weak grid: Weak grid is a term in electrical engineering. Under the combined effects of nonlinear loads and line impedance, the grid in actual applications can no longer be ignored and becomes slightly inductive. When the location where the photovoltaic equipment is connected to the grid changes, the grid inductance relative to the common coupling point will also fluctuate. "Weak grid" is used to define this non-ideal grid.

[0111] Park transformation: Park transformation is a coordinate transformation method that can transform the three-phase current (i a ,i b ,i c ) is projected onto a two-phase rotating coordinate system (dq axis). This transformation has a wide range of applications in motor control, power electronics and other fields.

[0112] LCL filter: LCL filter is a filter used to reduce high-frequency interference in the circuit. It consists of an inductor (L), a capacitor (C) and an inductor (L) to form a series low-pass filter. The function of the LCL filter is to filter out high-frequency noise signals in the circuit, ensure the normal operation of the circuit and improve the stability and reliability of the system.

[0113] Common Coupling Point: The common coupling point is a common grounding point or connection point in the system, which is usually used to ensure the interconnection and communication between different circuits or devices. Specifically, the common coupling point is a ground lead or ground connection shared between different circuits. In circuit design, it is used to connect the common points of various circuits to provide a reference point to the ground, ensuring the normal signal reference and communication between various circuits.

[0114] Bandpass filter: A bandpass filter is a filter that selects signals within a specific frequency range and passes them through the filter while suppressing other frequencies. Specifically, a bandpass filter can transmit signals within the signal frequency range and reduce the amplitude of the signal at other frequencies to effectively filter out unwanted frequency components.

[0115] The above description is only a preferred specific implementation manner of the present invention, but the protection scope of the present invention is not limited thereto. Any technician familiar with the technical field can make equivalent replacements or changes according to the technical scheme and inventive concept of the present invention within the technical scope disclosed by the present invention, which should be covered by the protection scope of the present invention.

Claims

1. A method for judging whether a power grid oscillation occurs when multiple grid-connected inverters are operated in parallel, which is applicable to a system in which n LCL-type grid-connected inverters are operated in parallel, and is characterized in that: The method comprises the following steps: Step 1: Establish a mathematical model of the LCL grid-connected inverter and derive the system resonant frequency when multiple machines are connected in parallel. The resonant frequency includes a fixed resonant angular frequency ω r1 and variable resonant angular frequency ω r2 ; Step 2: Collect the three-phase voltage at the common coupling point Pcc, perform positive and negative sequence separation and phase locking, and perform Park transformation to obtain the d and q axis voltage components; Step 3: Filter out the DC component and other frequency fluctuation components in the d-axis and q-axis voltage components to obtain the d-axis and q-axis voltage amplitudes at the resonant frequency; Step 4: Obtain the voltage amplitude at the resonant frequency according to the d-axis and q-axis voltage amplitudes, perform low-pass filtering and normalization processing, and compare it with a specific threshold to determine whether resonance occurs at the resonant frequency; Wherein, n is an integer greater than or equal to 2; The mathematical model of the LCL type grid-connected inverter is established, and the system resonant frequency when multiple machines are connected in parallel is derived, including: According to the control block diagram of a single LCL type grid-connected inverter under grid-connected current control, the closed-loop transfer function of a single LCL type grid-connected inverter is obtained; Assuming that the circuit parameters of the LCL filters of n inverters are consistent, combined with the closed-loop transfer function of a single LCL grid-connected inverter, the closed-loop transfer function of the parallel operation system of n LCL grid-connected inverters is obtained, and the fixed resonant angular frequency ω is obtained according to the resonance condition. r1 , the variable resonant angular frequency ω r2 ; The closed-loop transfer function of the single LCL grid-connected inverter is (1): （1）； Among them, i2 is the grid-connected current, i1 * For a given current, Z L1 , Z L2 , Z C are the component impedances of the LCL filter, Vpcc is the common coupling point voltage, G PI Represents the proportional and integral control expression, K pwm is the inverter equivalent gain, which takes a value of 1.

2. The method for determining whether a power grid oscillates when multiple grid-connected inverters are operating in parallel according to claim 1, characterized in that: The closed-loop transfer function of the n LCL-type grid-connected inverter parallel operation system is as follows: （2）； Where A=Z L1 +Z C +G PI , B=Z L2 (Z L1 +Z C +G PI )+Z C (Z L1 +G PI ), D = Z C ×G PI , Z g is the grid impedance.

3. The method for determining whether a power grid oscillates when multiple grid-connected inverters are operating in parallel according to claim 2, characterized in that: The condition for system resonance to occur when multiple machines are connected in parallel is that the denominator of equation (2) is 0.

4. The method for determining whether a power grid oscillates when multiple grid-connected inverters are operating in parallel according to any one of claims 1 to 3, characterized in that: The step 2 is specifically as follows: The three-phase voltage at the common coupling point is collected to separate the positive and negative sequences. Then the positive sequence voltage component is phase-locked using a phase-locked loop based on the second-order generalized integral method to obtain the synchronous rotation angle α. After Park transformation, the d and q axis voltage components V dpcc 、V qpcc , as shown in formula (9), where V pccA 、V pccB 、V pccC is the three-phase voltage at the common coupling point; （9）。 5. The method for determining whether a power grid oscillates when multiple grid-connected inverters are operating in parallel according to claim 4, characterized in that: The step 3 is specifically as follows: Respectively, r1 , the ω r2 Construct a bandwidth of ω for the center frequency c The unit gain bandpass filter is as shown in equation (10) and equation (11); the d-axis and q-axis voltage components are respectively passed through the bandpass filter BPF (ω) represented by equation (10) and equation (11) r1 )、BPF(ω r2 ) to filter out the DC component and other frequency fluctuation components, leaving V dpccwr1 and V qpccwr1 Voltage fluctuation, V dpccwr2 and V qpccwr2 The voltage fluctuation amount; （10）； （11）； Among them, V dpccwr1、 V qpccwr1 After filtering is completed, ω r1 The voltage amplitude of d-axis and q-axis at dpccwr2、 V qpccwr2 After filtering is completed, ω r2 is the voltage amplitude of the d-axis and q-axis at , and s is a complex variable used to describe the response of the system in the complex domain.

6. The method for determining whether a power grid oscillates when multiple grid-connected inverters are operating in parallel according to claim 5, characterized in that: The step 4 is specifically as follows: The ω is calculated using the d and q axis voltage amplitudes. r1 and the ω r2 The voltage amplitude Vʹ at wr1 and Vʹ wr2 , respectively, are filtered by a low-pass filter, then normalized, and then compared with the specified thresholds, and finally determined in the ω r1 and the ω r2 Whether resonance occurs.

7. The method for determining whether a power grid oscillates when multiple grid-connected inverters are operating in parallel according to claim 6, characterized in that: The Vʹ wr1 and Vʹ wr2 The low-pass filter is LPF (ω r1 )、LPF(ω r2 ), as shown in formula (14) and formula (15), respectively; the Vʹ wr1 and Vʹ wr2 The voltage amplitude after low-pass filter is V wr1 and V wr2 The standardization process refers to the process of normalizing the V according to formula (16) and formula (17). wr1 and the V wr2 Process to obtain k wr1 , k wr2 ; （12）； （13）； （14）； （15）； （16）； （17）； Among them, V N is the grid phase voltage amplitude under normal conditions.

8. The method for determining whether a power grid oscillates when multiple grid-connected inverters are operating in parallel according to claim 7, characterized in that: The determination in the ω r1 and the ω r2 Whether resonance occurs includes: If k wr1 ≥ɛ1, it means that in the ω r1 Oscillation occurs at If k wr1 <ɛ1, it means that in the ω r1 No oscillation occurred at If k wr2 ≥ɛ2, it means that in the ω r2 Oscillation occurs at If k wr2 <ɛ2, it means that in the ω r2 No oscillation occurred at Among them, ɛ1 and ɛ2 represent voltage oscillation coefficients, and their values ​​shall not be less than 5%.

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