New energy station multi-band oscillation risk online assessment and early warning method and storage medium

Abstract

The invention discloses a new energy station multi-band oscillation risk online evaluation and early warning method and a storage medium, and the method comprises the steps: carrying out the frequency sweeping to obtain the frequency domain impedance characteristics of fans and photovoltaic equipment of various models in a new energy station under different typical working conditions, and carrying out the frequency sweeping to obtain the frequency domain impedance characteristics of a power grid under different typical working conditions; the power flow level of the new energy station and the power grid side is monitored online, the working condition types of a new energy unit and the power grid are judged according to online monitoring data, and a new energy station equivalent impedance model is established according to the impedance characteristics of fans and photovoltaic equipment of various models of the new energy station and the station topology. And on-line evaluation of oscillation risks is carried out based on the Nyquist criterion. And selecting whether to send an oscillation alarm signal based on the evaluation result. According to the method, the multi-band oscillation risk of the new energy station can be effectively evaluated and early warned, and safe and stable operation of the new energy station is ensured.

Description

Technical Field

[0001] The present application relates to the field of power systems, and specifically to an online assessment and early warning method for multi-band oscillation risks of new energy stations and a storage medium. Background Art

[0002] The large-scale access of new energy equipment to the power grid has made the multi-modal weak damping characteristics of the new power system significant. The modal diversity characteristics have led to an increasingly serious risk of multi-band oscillations in the new power system. Since the geographical locations of my country's new energy bases and load centers are inversely distributed, large-scale new energy transmission via high-voltage AC and DC has become the norm, which has aggravated the complexity of the power system grid. The short-circuit ratio of the power grid varies greatly under complex working conditions. In addition, the external characteristics change under the random fluctuations of new energy output. The stability of the new energy grid-connected system has become increasingly critical. Assessing the potential multi-band oscillation risks of the power system under different operating conditions is crucial to ensuring the safe and stable operation of the new power system.

[0003] At present, the assessment and early warning strategies for the oscillation risks of new energy sites are often targeted at a single frequency band, ignoring the differences in online assessments and response measures under multiple frequency bands, and lack a complete assessment process. It is impossible to achieve oscillation risk assessment and early warning at different levels before and after the event. For scenarios where large-scale new energy sites are integrated into complex AC and DC power grids, the existing technology is not sufficient to assess the multi-band oscillation risks well, and cannot provide accurate early warning for multi-band oscillations that may occur at the system level.

[0004] Therefore, it is necessary to study an online assessment and early warning method and storage medium for multi-band oscillation risks of new energy stations, define risk assessment indicators using the frequency domain impedance model of new energy stations, and provide a feasible solution for online assessment and early warning of oscillation risks under multiple operating conditions of new energy stations. Summary of the invention

[0005] In view of the shortcomings of the existing technology, this application provides an online assessment and early warning method for multi-band oscillation risk of new energy stations and a storage medium, the purpose of which is to warn of potential multi-band oscillation risks under different working conditions and ensure the safe consumption of electric energy in new energy stations. The technical solution is as follows:

[0006] A method for evaluating oscillation risk after considering the power loss of the collector network, etc., characterized by: frequency sweeping to obtain the frequency domain impedance characteristics of various types of wind turbines and photovoltaic equipment in new energy stations under different typical working conditions, frequency sweeping to obtain the frequency domain impedance characteristics of the power grid under different typical working conditions, online monitoring of the current level of new energy stations and the power grid side, judging the type of working condition of the new energy units and the power grid side according to the online measurement data, establishing an equivalent impedance model of the new energy station according to the impedance characteristics of various types of wind turbines and photovoltaic equipment in the new energy station and the station topology, and online evaluation of the oscillation risk based on the Nyquist criterion. Choose whether to send an oscillation alarm signal based on the evaluation results. This application can effectively evaluate and warn of the multi-band oscillation risks of new energy stations and ensure the safe and stable operation of new energy stations.

[0007] The present invention provides a method for scanning frequency to obtain impedance characteristics of new energy equipment, comprising the following steps:

[0008] Step 1: Establish steady-state operating points for new energy equipment such as wind turbines, photovoltaics and SVG;

[0009] Step 2: Inject harmonic voltage signals into the grid connection points of wind turbines, photovoltaics, SVG and other new energy equipment in steady-state operation. The frequency of the harmonic voltage signal is 2.5-1000Hz, and the amplitude of the harmonic voltage signal is 1%-5% of the fundamental frequency voltage amplitude in steady state. Among them, the harmonic signal is injected once every 0.1Hz from 2.5 to 10Hz, the duration of each injection of the harmonic signal is 10s, and the signal amplitude is 1% of the fundamental frequency voltage amplitude; the harmonic signal is injected once every 1Hz from 10 to 100Hz, the duration of each injection of the harmonic signal is 1s, and the signal amplitude is 3% of the fundamental frequency voltage amplitude; the harmonic signal is injected once every 10Hz from 100 to 1000Hz, the duration of each injection of the harmonic signal is 0.1s, and the signal amplitude is 5% of the fundamental frequency voltage amplitude. At the same time, sample the three-phase voltage and current of the grid connection point. The minimum sampling rate must ensure the integrity of the currently injected harmonic signal, that is, it must meet the Shannon sampling law under the extraction of the currently injected harmonic signal;

[0010] A simplified statement of Shannon's sampling theorem is: to extract a component with a frequency of x, the number of sampling points per second must be more than 2x.

[0011] Step 3: Perform Fourier analysis on the voltage and current sampling results in step 2, extract the voltage signal and current signal in each injected harmonic frequency domain after Fourier transform, calculate the impedance of the new energy equipment at each injected harmonic frequency according to the voltage signal and current signal in each injected harmonic frequency domain, and thus obtain the impedance characteristics of the new energy equipment.

[0012] The present invention provides a method for dividing typical working conditions of new energy equipment and power grid in an electric power system, comprising the following steps:

[0013] Step 1: For photovoltaic and wind turbines in new energy equipment, different active output and reactive output are used as the basis for typical working conditions;

[0014] Step 2: For SVG in new energy equipment, different reactive outputs are used as the basis for typical working conditions.

[0015] Step 3: For the power grid, the typical operating conditions are divided based on normal operation, peak load, valley load, different N-1 faults in the vicinity, and possible AC / DC operation mode switching of the power grid.

[0016] Furthermore, the present invention provides a new energy station impedance equivalent method, comprising the following steps:

[0017] Step 1: Select the low-voltage bus connected to the step-up transformer of the new energy station as the reference bus, and select the number of collector lines connected to the reference bus as the number of links;

[0018] Step 2: Under each link, the impedance of the collector line of each link is equalized by the principle of equal power loss, and the impedance of the new energy equipment under each link is equalized by capacity weighting;

[0019] The calculation method of the equal power loss method is:

[0020] The equal power loss method is used for the collector line. For each selected link, the power loss calculation formula is as shown in formula (1) and formula (2).

[0021]

[0022] Where n is the number of new energy devices on the link, the subscript i represents the i-th new energy device, Z is the impedance of the new energy device, I represents the output current of the new energy device, ΔS detail is the total active power loss of the new energy station before the equivalent value, ΔS eq is the total active power loss of the new energy station after equalization. The equalization target is that the active power loss before and after equalization is the same, that is, ΔS detail =ΔS eq .

[0023] The calculation method of the capacity weighted method is:

[0024] The capacity-weighted equivalent is adopted for new energy equipment. For new energy equipment of the same model, the equivalent impedance calculation formula of the converter is shown in formula (3); the box transformer only considers the leakage magnetic branch and ignores the excitation branch. The equivalent inductance calculation formula of the box transformer is shown in formula (4); the equivalent post-filter capacitance is the sum of the filter capacitances of all new energy equipment on the link, and the equivalent filter capacitance calculation formula is shown in formula (5).

[0025]

[0026] C eq =nC eq (5)

[0027] Where n is the number of new energy devices in the same link, Z eq is the impedance of all new energy converters on the same link after equalization, L eq is the inductance of all new energy boxes on the same link after they become equal. C eq It is the capacitance of all new energy filter capacitors on the same link after they have equal values. wt is the impedance of a single new energy converter, L wt is the inductance of a single new energy box, C eq It is a filter capacitor for a single new energy device.

[0028] Step 3: Generate an equivalent impedance model of the new energy station according to the equivalent impedance model of each link of the new energy station.

[0029] Furthermore, the present invention provides an oscillation risk assessment index and method based on the Nyquist criterion, comprising the following steps:

[0030] Step 1: According to the Nyquist criterion, calculate the ratio of the grid side impedance to the new energy station impedance. The calculation formula is:

[0031]

[0032] Where Z grid (s) is the grid side impedance, Z station (s) is the impedance of the renewable energy station, G(s) is the calculated Nyquist transfer function, and s is the Laplace operator.

[0033] Step 2: Plot G(s) on the complex plane, and construct the oscillation risk assessment index MD based on the distance between each point of the G(s) curve and the point (-1, j0). The calculation method of the oscillation risk assessment index MD is:

[0034] G(s)=G x (s)+jG y (s) (7)

[0035]

[0036] Among them G x (s) is the x-axis coordinate of a point on the Nyquist curve, G y (s) is the y-axis coordinate of a point on the Nyquist curve, j is the imaginary unit, and min() means taking the minimum value.

[0037] Definition D setTo meet the minimum distance between G(s) and (-1, j0) to ensure stability, when the oscillation risk evaluation index D is less than D set If the value is higher than 0, it is judged that there is an oscillation risk; otherwise, there is no oscillation risk.

[0038] The present invention designs an online assessment and early warning method for multi-band oscillation risks of new energy stations and a storage medium. The method takes into account the detailed topology of the new energy station collection network for equivalence, reduces the calculation amount of assessment and time domain simulation, and has high impedance equivalence accuracy. It performs online oscillation risk assessment based on the impedance characteristics obtained based on offline frequency scanning, and has good feasibility. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] In order to more clearly illustrate the embodiments of the present application or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art are briefly introduced below.

[0040] Figure 1 This is an implementation flow chart in the embodiment of the present application;

[0041] Figure 2 It is a schematic diagram of the before and after of the topology of the power collection network of the new energy station in the example of this application;

[0042] Figure 3 The impedance characteristic result diagram of the new energy device obtained by frequency sweeping in the embodiment of the present application;

[0043] Figure 4 The impedance characteristic calculation result of the new energy station after equalization in the embodiment of the present application, wherein the solid line is the impedance of the detailed model, the dashed line is the impedance of the new energy station after equalization by the method of this paper, and the dot-dash line is the equivalent impedance without considering the collector network topology;

[0044] Figure 5 A schematic diagram of the Nyquist criterion in an embodiment of the present application;

[0045] Figure 6 It is a Nyquist curve diagram in the embodiment of the present application. DETAILED DESCRIPTION

[0046] Example 1

[0047] Reference Figure 1 , Figure 2 As shown in the figure, an 11-node new energy station is simulated, including 10 wind turbines, which are connected to the grid-connected point bus through two links.

[0048] The present invention provides an online assessment and early warning method for multi-band oscillation risks of new energy stations, comprising three main links: frequency scanning, typical working condition division and monitoring, and online assessment and early warning of oscillation risks. The method is characterized in that the method comprises the following steps, and the following steps are performed sequentially.

[0049] Step 1: Sweep the frequency to obtain the frequency domain impedance characteristics of various types of wind turbines and photovoltaic equipment in new energy stations under different typical working conditions, and sweep the frequency to obtain the frequency domain impedance characteristics of the power grid under different typical working conditions.

[0050] The present invention provides a method for scanning frequency to obtain impedance characteristics of new energy equipment, comprising the following steps:

[0051] (1) Establish steady-state operating points for new energy equipment such as wind turbines, photovoltaics, and SVG;

[0052] (2) Inject harmonic voltage signals into the grid connection points of new energy equipment such as wind turbines, photovoltaics and SVG in steady-state operation. The frequency of the harmonic voltage signal is 2.5 to 1000 Hz, and the amplitude of the harmonic voltage signal is 1% to 5% of the fundamental frequency voltage amplitude in steady state. Among them, the harmonic signal is injected every 0.1 Hz from 2.5 to 10 Hz, the duration of each injection of the harmonic signal is 10 s, and the signal amplitude is 1% of the fundamental frequency voltage amplitude; the harmonic signal is injected every 1 Hz from 10 to 100 Hz, the duration of each injection of the harmonic signal is 1 s, and the signal amplitude is 3% of the fundamental frequency voltage amplitude; the harmonic signal is injected every 10 Hz from 100 to 1000 Hz, the duration of each injection of the harmonic signal is 0.1 s, and the signal amplitude is 5% of the fundamental frequency voltage amplitude. At the same time, sample the three-phase voltage and current of the grid connection point. The minimum sampling rate needs to ensure the integrity of the currently injected harmonic signal, that is, it needs to meet the Shannon sampling law under the currently injected harmonic signal;

[0053] (3) Performing Fourier analysis on the voltage and current sampling results in (2) to extract the voltage signal and current signal in each injected harmonic frequency domain after Fourier transform, and calculating the impedance of the new energy device at each injected harmonic frequency based on the voltage signal and current signal in each injected harmonic frequency domain, thereby obtaining the impedance characteristics of the new energy device.

[0054] For this embodiment, the wind turbine impedance characteristic obtained by frequency sweep is as follows: Figure 3 shown.

[0055] Step 2: Online monitoring of the power flow levels at the renewable energy stations and the grid side, and judging the operating conditions of the renewable energy units and the grid side based on the online measurement data.

[0056] A method for dividing typical working conditions of new energy equipment and power grid in a power system includes the following steps:

[0057] (1) For photovoltaic and wind turbines in new energy equipment, different active and reactive outputs are used as the basis for typical working conditions;

[0058] (2) For SVG in new energy equipment, different reactive outputs are used as the basis for typical working conditions.

[0059] (3) For the power grid, the typical operating conditions are divided based on normal operation, peak load, valley load, different N-1 faults in the vicinity, and possible AC / DC switching of the power grid.

[0060] For this embodiment, 8 typical operating conditions of the wind turbine, 6 typical operating conditions of the SVG, and 20 grid-side operating conditions are selected. The frequency is swept for each operating condition of the wind turbine, SVG, and grid, and the frequency domain impedance data is recorded.

[0061] Then, using the monitoring device installed in the power system, on the new energy station side, the output active power and reactive power of the photovoltaic and wind turbines of the new energy station are monitored in real time, and the output reactive power of the SVG of the new energy station is monitored in real time to determine the working condition of the new energy equipment and record it; on the power grid side, the flow level of the key nodes and transmission lines of the power grid is monitored in real time, and the power grid fault events are monitored in real time to determine the working condition of the power grid and record it.

[0062] Step 3: Establish an equivalent impedance model of a new energy station based on the impedance characteristics of various types of wind turbines and photovoltaic equipment in the new energy station and the station topology;

[0063] For new energy equipment, the capacity-weighted equivalent is adopted. For the same type of wind turbine, its equivalent impedance is shown in formula (1). The box transformer only considers the leakage magnetic branch. The box transformer equivalent inductance is shown in formula (2). The filter capacitor equivalent capacitance is shown in formula (3).

[0064]

[0065]

[0066] C eq =nC eq (3)

[0067] Where n is the number of new energy devices in the same link, Z eq is the impedance of all new energy converters on the same link after equalization, L eq is the inductance of all new energy boxes on the same link after they become equal. C eq It is the capacitance of all new energy filter capacitors on the same link after they have equal values. wt is the impedance of a single new energy converter, L wt is the inductance of a single new energy box, C eq It is a filter capacitor for a single new energy device.

[0068] The equal power loss method is used for the collector line, according to Figure 1 , we can write equation (4) and equation (5).

[0069]

[0070] Where n is the number of new energy devices on the link, the subscript i represents the i-th new energy device, Z is the impedance of the new energy device, I represents the output current of the new energy device, ΔS detail is the total active power loss of the new energy station before the equivalent value, ΔS eq is the total active power loss of the new energy station after equalization. The equalization target is that the active power loss before and after equalization is the same, that is, ΔS detail =ΔS eq .

[0071] In this embodiment, the equivalent front and back schematic diagram is as follows Figure 4 The solid line is the impedance of the detailed model, the dashed line is the impedance of the new energy station after the equivalent method in this paper, and the dotted line does not consider the collector network topology. It can be seen that this paper uses the equal power loss method for each link with good equivalent accuracy.

[0072] Step 4: Perform an online assessment of the oscillation risk based on the Nyquist criterion, and decide whether to send an oscillation alarm signal based on the assessment result.

[0073] According to the Nyquist criterion, the ratio of the grid-side impedance to the new energy station impedance is calculated as follows:

[0074]

[0075] Where Z grid (s) is the impedance characteristic of the grid side, Z station (s) is the impedance characteristic of the new energy station side, G(s) is the calculated target transfer function, G(s) is plotted on the complex plane, and the distance between each point of the G(s) curve and the point (-1, j0) is calculated. When the distance at a certain frequency is lower than the set value, it is determined that there is an oscillation risk at this frequency.

[0076] In this embodiment, the Nyquist criterion diagram is as follows: Figure 5 shown.

[0077] G(s) in formula (6) is expressed as formula (7), and the oscillation risk assessment index MD is defined as shown in formula (8).

[0078] G(s)=G x (s)+jG y (s) (7)

[0079]

[0080] Definition D set To meet the minimum distance between G(s) and (-1, j0) to ensure stability, when the oscillation risk evaluation index D is less than D setIf the value is higher than 0, it is judged that there is an oscillation risk; otherwise, there is no oscillation risk.

[0081] For this embodiment, D set If it is set to 0.2, an alarm signal will be triggered when MD is less than 0.2.

[0082] Draw the Nyquist curve as Figure 6 As shown by Figure 6 It can be seen that at this time, the distance between the Nyquist curve and the (-1.j0) point in the range of 99 to 105 Hz is less than 0.2, which does not meet the set safety margin, and at 102 Hz, the Nyquist curve intersects with the unit circle. According to the Nyquist criterion, oscillation at the corresponding frequency may occur at this time. Figure 6 In the medium frequency band of 99Hz to 105Hz, G(s) enters the safety margin circle. The system has an oscillation risk in this frequency band under this working condition. An alarm signal should be issued, and emergency control should be performed according to the size of the calculated oscillation risk index MD. For example, in this embodiment, when the frequency is 102Hz, G(s) has the shortest distance to (-1, j0), at which time MD=0.173<0.2, and the Nyquist curve surrounds (-1, j0) counterclockwise. When the power grid strength is large, there will also be a frequency point at which the MD corresponding to the frequency point is greater than 0.2. In the case of disturbance, the system will be accompanied by the harmonic component at this frequency, but the harmonic component is always attenuated and will not cause oscillation. Combining the above steps 1 to 4, the online evaluation and early warning of the multi-band oscillation risk of new energy stations can be realized.

[0083] The present invention designs an online assessment and early warning method for multi-band oscillation risks of new energy stations and a storage medium. The method can online assess the multi-band oscillation risks of new energy grid-connected systems, warn of possible oscillation accidents, improve the stability of new energy station operation, and ensure the safe consumption of new energy.

[0084] The above shows and describes the basic principles, main features and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The above embodiments and descriptions only describe the principles of the present invention. The present invention may be subject to various changes and improvements without departing from the spirit and scope of the present invention. These changes and improvements fall within the scope of the present invention. The scope of protection claimed by the present invention is defined by the attached claims and their equivalents.

Claims

1. A method for online assessment and early warning of multi-band oscillation risk in a new energy station, characterized in that: The steps include: Step 1: Sweep the frequency to obtain the frequency domain impedance characteristics of various types of wind turbines and photovoltaic equipment in new energy stations under different typical working conditions; Step 2: Sweep the frequency to obtain the frequency domain impedance characteristics of the power grid under different typical working conditions; Step 3: Online monitoring of the power flow level of new energy stations and grid side; Step 4: Determine the operating conditions of the new energy unit and the power grid based on the online measurement data; Step 5: Establish an equivalent impedance model of the new energy station based on the impedance characteristics of various types of wind turbines and photovoltaic equipment in the new energy station and the station topology; Step 6: Based on the Nyquist criterion, define the oscillation risk evaluation index and perform online evaluation of the oscillation risk; Step 7: Based on the evaluation results, determine whether there is a risk of multi-band oscillation. If there is an oscillation risk, send an alarm signal. Otherwise, repeat the above steps (3) to (7).

2. The online assessment and early warning method for multi-band oscillation risk of new energy stations according to claim 1 is characterized in that: The step one also includes the following steps: Step 1: Establish steady-state operating points for wind turbines, photovoltaics and SVG new energy equipment; Step 2: Inject harmonic voltage signals into the grid connection points of wind turbines, photovoltaics, SVG and other new energy equipment in steady-state operation. The frequency of the harmonic voltage signal is 2.5-1000Hz, and the amplitude of the harmonic voltage signal is 1%-5% of the fundamental frequency voltage amplitude in steady state. In particular, the harmonic signal is injected every 0.1Hz from 2.5 to 10Hz, and the duration of each harmonic signal injection is 10s, and the signal amplitude is 1% of the fundamental frequency voltage amplitude; the harmonic signal is injected every 1Hz from 10 to 100Hz, and the duration of each harmonic signal injection is 1s, and the signal amplitude is 3% of the fundamental frequency voltage amplitude; the harmonic signal is injected every 10Hz from 100 to 1000Hz, and the duration of each harmonic signal injection is 0.1s, and the signal amplitude is 5% of the fundamental frequency voltage amplitude. At the same time, the three-phase voltage and current of the grid connection point are sampled. The minimum sampling rate needs to ensure the integrity of the currently injected harmonic signal, that is, it needs to meet the Shannon sampling law under the extraction of the currently injected harmonic signal; Step 3: Perform Fourier analysis on the voltage and current sampling results in step 2, extract the voltage signal and current signal in each injected harmonic frequency domain after Fourier transform, calculate the impedance of the new energy equipment at each injected harmonic frequency according to the voltage signal and current signal in each injected harmonic frequency domain, and thus obtain the impedance characteristics of the new energy equipment.

3. According to the online assessment and early warning method for multi-band oscillation risk of new energy stations described in claim 1, it is characterized in that: The typical working conditions in step 1, step 2 and step 4 are formulated based on: For photovoltaic and wind turbines in new energy equipment, different active and reactive outputs are used as the basis for typical working conditions; For SVG in new energy equipment, different reactive outputs are used as the basis for typical working conditions; For the power grid, the typical operating conditions are divided based on normal operation, peak load, valley load, different N-1 faults in the vicinity, and possible switching of AC and DC operation modes of the power grid.

4. According to the online assessment and early warning method for multi-band oscillation risk of new energy stations described in claim 1, it is characterized in that: The step three also includes the following steps: For new energy sites, real-time monitoring of the output active power and reactive power of photovoltaic and wind turbines in new energy sites, real-time monitoring of the output reactive power of SVG in new energy sites, and the working conditions of new energy equipment are determined and recorded; for power grids, real-time monitoring of the flow levels of key nodes and transmission lines of the power grid, real-time monitoring of power grid fault events, real-time monitoring of the working conditions of the power grid and record them.

5. The online assessment and early warning method for multi-band oscillation risk of new energy stations according to claim 1 is characterized in that: The step five also includes the following steps: Step 1: Select the low-voltage bus connected to the step-up transformer of the new energy station as the reference bus, and select the number of collector lines connected to the reference bus as the number of links; Step 2: Under each link, the impedance of the collector line of each link is equalized by the principle of equal power loss, and the impedance of the new energy equipment under each link is equalized by capacity weighting; Step 3: Generate an equivalent impedance model of the new energy station according to the equivalent impedance model of each link of the new energy station.

6. According to the online assessment and early warning method for multi-band oscillation risk of new energy stations described in claim 1, it is characterized in that: The calculation steps of the medium power loss method in step 2 of step 5 are as follows: The equal power loss method is used for the collector line. For each selected link, the power loss calculation formula is as shown in formula (1) and formula (2); Where n is the number of new energy devices on the link, the subscript i represents the i-th new energy device, Z is the impedance of the new energy device, I represents the output current of the new energy device, ΔS detail is the total active power loss of the new energy station before the equivalent value, ΔS eq is the total active power loss of the new energy station after equalization. The equalization target is that the active power loss before and after equalization is the same, that is, ΔS detail =ΔS eq .

7. According to the online assessment and early warning method for multi-band oscillation risk of new energy stations described in claim 1, it is characterized in that: The calculation steps of the capacity weighted method in the detailed step 2 of step 5 are as follows: The capacity-weighted equivalent is adopted for new energy equipment. For new energy equipment of the same model, the equivalent impedance calculation formula of the converter is shown in formula (3); the box transformer only considers the leakage magnetic branch and ignores the excitation branch. The equivalent inductance calculation formula of the box transformer is shown in formula (4); the equivalent post-filter capacitor is the sum of the filter capacitors of all new energy equipment on the link, and the equivalent filter capacitor calculation formula is shown in formula (5); C eq =nC eq (5) Where n is the number of new energy devices in the same link, Z eq is the impedance of all new energy converters on the same link after equalization, L eq is the inductance of all new energy boxes on the same link after they are equalized; C eq is the capacitance of all new energy filter capacitors on the same link after they have equal values; Z wt is the impedance of a single new energy converter, L wt is the inductance of a single new energy box, C eq It is a filter capacitor for a single new energy device.

8. According to the online assessment and early warning method for multi-band oscillation risk of new energy stations described in claim 1, it is characterized in that: The step six also includes the following steps: Step 1: According to the Nyquist criterion, calculate the ratio of the grid side impedance and the new energy station impedance. The calculation formula is: Where Z grid (s) is the grid side impedance, Z station (s) is the impedance of the new energy station side, G(s) is the calculated Nyquist transfer function, and s is the Laplace operator; Step 2: Plot G(s) on the complex plane, and use the distance D between each point on the G(s) curve and the point (-1, j0) as the oscillation risk assessment index. Define D set To meet the minimum distance between G(s) and (-1, j0) to ensure stability, when the oscillation risk evaluation index D is less than D set If the value is higher than 0, it is judged that there is an oscillation risk; otherwise, there is no oscillation risk.

9. According to the online assessment and early warning method for multi-band oscillation risk of new energy stations described in claim 1, it is characterized in that: The calculation method of the oscillation risk assessment index MD in the detailed step 2 of the step 6 is: G(s)=G x (s)+jG y (s) (7) Among them G x (s) is the x-axis coordinate of a point on the Nyquist curve, G y (s) is the y-axis coordinate of a point on the Nyquist curve, j is the imaginary unit, and min() means taking the minimum value.

10. A storage medium, characterized in that: It includes a stored program, which, when running, controls the device where the storage medium is located to execute the online assessment and early warning method for multi-band oscillation risks of new energy stations as described in any one of claims 1 to 9.

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