A method and device for passive damping of low voltage side of transformer to suppress subsynchronous resonance

Abstract

The application provides a transformer low-voltage side passive damping method and device for inhibiting subsynchronous resonance, comprising: collecting electrical parameters related to a power supply side, a transformer and a load side respectively; determining a corresponding damping resistance value from the collected electrical parameters according to a preset constraint condition; detecting whether subsynchronous resonance occurs at a preset detection point, and when detecting that subsynchronous resonance occurs, opening a preset switching element and putting in a damping resistance with the same damping resistance value as the damping resistance value; wherein the damping resistance is arranged at a low-voltage side outlet end of the partial transformer and is connected in series with the transformer, the damping resistance is connected in parallel with the switching element at both ends, and the switching element is initially set to a closed state. The application can effectively inhibit subsynchronous resonance caused by super / subsynchronous interharmonics generated by a power electronic device cluster accessing a power grid, and realizes reasonable selection of a damping resistance value, which can improve the subsynchronous resonance inhibition effect and will not affect the normal operation of a system and a load.

Description

Technical Field

[0001] This invention relates to the field of passive damping technology for suppressing subsynchronous resonance, and in particular to a passive damping method and apparatus for suppressing subsynchronous resonance on the low-voltage side of a transformer. Background Technology

[0002] With the continuous development of power system electrification, the large-scale grid connection of various power electronic loads has profoundly changed the dynamic characteristics of power systems. Due to factors such as the nonlinear characteristics, differences in output impedance, and control strategies of various power electronic devices, deeper levels of dynamic behavior and interactive coupling occur between multiple power electronic devices and between power electronic devices and the system. This easily leads to subsynchronous resonance problems, resulting in overvoltage or overcurrent phenomena, and in severe cases, damage to power equipment, threatening the safe and stable operation of the system. Researching how to effectively suppress subsynchronous resonance is of great significance for ensuring the healthy development of power systems.

[0003] Implementing grid-side mitigation is a crucial method for suppressing harmonic resonances, primarily encompassing two categories: active damping and passive damping. Active damping methods achieve this by optimizing controller parameters or constructing new control loops; however, these control loops are typically complex, increasing system design difficulty and cost. In contrast, passive damping methods do not suffer from these issues, are easier to implement, and are less expensive. However, commonly used LCL passive filters inherently possess resonance risks and cannot effectively suppress subsynchronous resonances caused by supersynchronous / subsynchronous interharmonics generated by power electronic device clusters connected to the grid. Therefore, designing passive damping methods and supporting devices to suppress subsynchronous resonances is an urgent problem to be solved. Summary of the Invention

[0004] The purpose of this invention is to propose a passive damping method and device for suppressing subsynchronous resonance on the low-voltage side of a transformer, thereby solving the technical problem of how to effectively suppress subsynchronous resonance in the power system caused by super / subsynchronous interharmonics generated by the grid connection of power electronic device clusters.

[0005] On the one hand, a passive damping method for suppressing subsynchronous resonance on the low-voltage side of a transformer is provided, including:

[0006] Collect relevant electrical parameters from the power supply side, transformer side, and load side respectively;

[0007] The corresponding damping resistance value is determined from the collected electrical parameters according to the preset constraints.

[0008] The subsynchronous resonance is detected at a preset detection point. When the subsynchronous resonance is detected, the preset switching element is turned on and a damping resistor with the same value as the damping resistor is applied.

[0009] The damping resistor is located at the low-voltage side output terminal of the partial transformer and connected in series with the transformer. The switching element is connected in parallel across the two ends of the damping resistor, and the switching element is initially set to a closed state.

[0010] Preferably, the electrical parameters include at least the short-circuit capacity of the power supply system, the rated capacity of the transformer, the short-circuit loss and the percentage of short-circuit voltage, the cross-sectional area of ​​the conductor on the load side, and the length of the line.

[0011] Preferably, determining the corresponding damping resistance value from the collected electrical parameters according to preset constraints includes:

[0012] Using the electrical parameters, determine whether the input damping resistor value meets the preset interharmonic current peak suppression rate and preset voltage loss.

[0013] If the condition is met, the damping resistance value at this time will be output; otherwise, the damping resistance value at this time will be ignored.

[0014] Preferably, the preset interharmonic current peak suppression rate should be as large as possible, including:

[0015]

[0016] Where α is the peak suppression rate of harmonic phase current between power supply side buses, I smax I is the peak value of the harmonic phase current between the power supply side buses before the series damping resistor. s ′ max This represents the peak value of the harmonic phase current between the power supply side buses after the series damping resistor is connected.

[0017] Preferably, the preset voltage loss cannot exceed a certain threshold, including:

[0018]

[0019] Where ΔU% is the voltage drop rate of the damping resistor, I lmax U is the maximum effective value of the phase current on the load side. lN Z is the rated voltage on the load side, β is the threshold voltage drop rate of the damping resistor, and Z is the voltage drop rate of the damping resistor. T Z is the equivalent impedance of the transformer referred to the low-voltage side. l R is the equivalent impedance of the line on the load side. d It is a damping resistor.

[0020] Preferably, detecting whether subsynchronous resonance occurs includes:

[0021] Collect three-phase current waveform data of the power supply side bus;

[0022] Perform a fast Fourier transform on multiple cycles in the three-phase current recording data to obtain the peak values ​​of each interharmonic current within a preset frequency band.

[0023] Based on the preset response time, the time window is slid backward by several cycles, and then a fast Fourier transform is performed on the subsequent multiple identical cycles to obtain the peak values ​​of each interharmonic current within the preset frequency band, and the difference is calculated to determine the change in the peak values ​​of each interharmonic current.

[0024] The change in the peak value of each interharmonic current is repeatedly calculated. When the change in the peak value of the interharmonic current obtained from three consecutive calculations exceeds the preset threshold, it is considered that a subsynchronous resonance has been detected.

[0025] On the other hand, a passive damping device for suppressing subsynchronous resonance on the low-voltage side of a transformer is also provided, for implementing the passive damping method for suppressing subsynchronous resonance on the low-voltage side of the transformer, comprising:

[0026] The system comprises a detection unit, a transmission unit, a control unit, a switching element, and a damping resistor, wherein the detection unit, the transmission unit, the control unit, and the switching element are connected in sequence, and the switching element is connected in parallel across the damping resistor.

[0027] The detection unit detects whether subsynchronous resonance occurs. When subsynchronous resonance is detected, the control unit opens a preset switching element and puts in a damping resistor with the same value as the determined damping resistor.

[0028] The damping resistor value is determined from the collected electrical parameters according to preset constraints. The electrical parameters are the electrical parameters related to the power supply side, transformer and load side respectively. The damping resistor is set at the low-voltage side output terminal of the transformer and connected in series with the transformer. The switching element is connected in parallel across the two ends of the damping resistor and is set to be in the closed state initially.

[0029] Preferably, it further includes:

[0030] When the detection unit detects the occurrence of subsynchronous resonance, it generates a subsynchronous resonance signal and transmits it to the transmission unit.

[0031] When the transmission unit receives the subsynchronous resonance signal from the detection unit, it transmits the signal to the control unit.

[0032] When the control unit receives the subsynchronous resonance signal transmitted from the transmission unit, it sends a command to the switching element to control the switching element to open.

[0033] Preferably, the preset constraints include:

[0034] The preset interharmonic current peak suppression rate should be as high as possible, including:

[0035]

[0036] Where α is the peak suppression rate of harmonic phase current between power supply side buses, I smax I is the peak value of the harmonic phase current between the power supply side buses before the series damping resistor. s ′ max The peak value of the harmonic phase current between the power supply side buses after the series damping resistor is connected;

[0037] In addition, the preset voltage loss must not exceed a certain threshold, including:

[0038]

[0039] Where ΔU% is the voltage drop rate of the damping resistor, I lmax U is the maximum effective value of the phase current on the load side. lN Z is the rated voltage on the load side, β is the threshold voltage drop rate of the damping resistor, and Z is the voltage drop rate of the damping resistor. T Z is the equivalent impedance of the transformer referred to the low-voltage side. l R is the equivalent impedance of the line on the load side. d It is a damping resistor.

[0040] Preferably, the detection unit is specifically used to collect three-phase current recording data of the power supply side bus;

[0041] Perform a fast Fourier transform on multiple cycles in the three-phase current recording data to obtain the peak values ​​of each interharmonic current within a preset frequency band.

[0042] Based on the preset response time, the time window is slid backward by several cycles, and then a fast Fourier transform is performed on the subsequent multiple identical cycles to obtain the peak values ​​of each interharmonic current within the preset frequency band, and the difference is calculated to determine the change in the peak values ​​of each interharmonic current.

[0043] The change in the peak value of each interharmonic current is repeatedly calculated. When the change in the peak value of the interharmonic current obtained from three consecutive calculations exceeds the preset threshold, it is considered that a subsynchronous resonance has been detected.

[0044] In summary, implementing the embodiments of the present invention has the following beneficial effects:

[0045] The present invention provides a passive damping method and device for suppressing subsynchronous resonance on the low-voltage side of a transformer. Installing this device on the three-phase outgoing lines of the low-voltage side of some transformers increases system damping, effectively suppressing the emission of super- / subsynchronous interharmonics and the superposition effect of interharmonics from power electronic device clusters, thereby effectively suppressing the subsynchronous resonance peak value caused by super- / subsynchronous interharmonics. It allows for the rational selection of damping resistor values, improving the peak suppression rate of interharmonic currents, while controlling the voltage loss caused by the damping resistor within a reasonable range. This improves the subsynchronous resonance suppression effect without affecting the normal operation of the system and load. The invention has a simple design, is easy to implement, and saves costs, contributing to improved power system stability and power quality. Attached Figure Description

[0046] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, obtaining other drawings based on these drawings without creative effort still falls within the scope of the present invention.

[0047] Figure 1 This is a schematic diagram of the main flow of a passive damping method for suppressing subsynchronous resonance on the low-voltage side of a transformer, as described in an embodiment of the present invention.

[0048] Figure 2 This is a schematic diagram of a circuit topology at the interharmonic frequency after a passive damping resistor is connected in an embodiment of the present invention.

[0049] Figure 3 This is a schematic diagram of a passive damping device on the low-voltage side of a transformer for suppressing subsynchronous resonance, as described in an embodiment of the present invention.

[0050] Figure 4 This is a schematic diagram of the wiring of a 10kV substation system in an embodiment of the present invention.

[0051] Figure 5 This is a graph showing the variation of interharmonic phase current peak suppression rate and voltage loss rate under different damping resistors in an embodiment of the present invention.

[0052] Figure 6 The image shows a simulated waveform of the three-phase current of a 10kV busbar, illustrating the effect of suppressing subsynchronous resonance in an embodiment of the present invention. Detailed Implementation

[0053] To make the objectives, technical solutions, and advantages of the present invention clearer, the present invention will be further described in detail below with reference to the accompanying drawings.

[0054] like Figure 1The diagram shown is a schematic representation of an embodiment of a passive damping method for suppressing subsynchronous resonance on the low-voltage side of a transformer provided by the present invention. In this embodiment, the method includes the following steps:

[0055] Electrical parameters related to the power supply side, transformer, and load side are collected separately; understandably, relevant electrical parameters of the power supply side, transformer, and load side are surveyed and collected. In a specific embodiment, the electrical parameters include at least the short-circuit capacity of the power supply system, the rated capacity of the transformer, the short-circuit loss and the percentage of short-circuit voltage, and the cross-sectional area of ​​the conductor and the length of the line on the load side.

[0056] Furthermore, the corresponding damping resistance value is determined from the collected electrical parameters according to preset constraints; it is understood that the method for selecting the damping resistance value should satisfy the two constraints of interharmonic current peak suppression rate and voltage loss.

[0057] In specific embodiments, such as Figure 2 As shown, Z s Z represents the equivalent impedance of the power supply system, obtained from the rated voltage of the power supply and the short-circuit capacity of the system; T represents the transformer; Z represents the equivalent impedance of the power supply system. T The equivalent impedance of the transformer referred to the low-voltage side is obtained based on the transformer's rated capacity, short-circuit loss, and short-circuit voltage percentage; Z l The equivalent impedance of the load-side line is obtained based on the cross-sectional area of ​​the conductor and the length of the line on the load side. For power electronic device clusters, it serves as an equivalent interharmonic voltage source; R d is the damping resistor; S is the switching element.

[0058] Specifically, using the electrical parameters, it is determined whether the input damping resistor value meets the preset interharmonic current peak suppression rate and preset voltage loss; if it meets the requirements, the damping resistor value at this time is output; if it does not meet the requirements, the damping resistor value at this time is ignored.

[0059] Among them, constraint one is to maximize the peak suppression rate of inter-harmonic phase currents between the power supply side buses after connecting the series damping resistor, that is, the preset peak suppression rate of inter-harmonic currents includes:

[0060]

[0061] Where α is the peak suppression rate of harmonic phase current between power supply side buses, I smax I is the peak value of the harmonic phase current between the power supply side buses before the series damping resistor. s ′ max This represents the peak value of the harmonic phase current between the power supply side buses after connecting the series damping resistor. smax and I s ′ max All in Figure 2 The reading was obtained at the test point shown.

[0062] Constraint condition two is that the voltage loss caused by the damping resistor cannot exceed a certain proportion of the rated voltage on the load side, that is, the preset voltage loss includes:

[0063]

[0064] Where ΔU% is the voltage drop rate of the damping resistor, I lmax U is the maximum effective value of the phase current on the load side. lN Z is the rated voltage on the load side, β is the threshold value for the voltage drop rate of the damping resistor, which is recommended to be 10% in practical applications. T Z is the equivalent impedance of the transformer referred to the low-voltage side. l R is the equivalent impedance of the line on the load side. d It is a damping resistor.

[0065] Furthermore, the occurrence of subsynchronous resonance is detected at a preset detection point. When subsynchronous resonance is detected, a preset switching element is opened, and a damping resistor with the same value as the damping resistor is engaged. The damping resistor is located at the low-voltage side output terminal of the partial transformer and connected in series with the transformer. The switching element is connected in parallel across the damping resistor, and the switching element is initially set to a closed state. It is understood that a selected damping resistor is connected in series with the three-phase output terminals on the low-voltage side of the partial transformer, and a switching element is connected in parallel across its terminals, initially set to a closed state. Specifically, as... Figure 2 As shown, both transformers are connected to interharmonic voltage sources on their low-voltage sides. A damping resistor R is selected and connected in series on the low-voltage side of one of the transformers. d and in the damping resistor R d A switching element S is connected in parallel across the two ends, and the switching element S is initially set to the closed state. When subsynchronous resonance is detected, the switching element S is opened, and the damping resistor R is engaged. d .

[0066] In a specific embodiment, detecting whether subsynchronous resonance occurs includes:

[0067] Collect three-phase current waveform data of the power supply side bus;

[0068] Perform a Fast Fourier Transform on multiple cycles in the three-phase current waveform data to obtain the peak values ​​of each interharmonic current within a preset frequency band; understandably, perform a Fast Fourier Transform on the first 10 cycles of the three-phase current waveform data to obtain the peak values ​​of each interharmonic current within the 0-100Hz frequency band.

[0069] Based on the preset response time, the time window is shifted backward by several cycles, and then a Fast Fourier Transform is performed on the subsequent identical cycles to obtain the peak values ​​of each interharmonic current within the preset frequency band. The difference is then calculated to determine the change in the peak values ​​of each interharmonic current. Understandably, based on the response time requirements in actual applications, the time window is shifted backward by several cycles, and then a Fast Fourier Transform is performed on the subsequent 10 cycles to obtain the peak values ​​of each interharmonic current within the 0-100Hz frequency band. The difference is then calculated to determine the change in the peak values ​​of each interharmonic current.

[0070] Repeatedly calculate the change in peak value of each interharmonic current. When the change in peak value of interharmonic current for three consecutive calculations exceeds the preset threshold, it is considered that subsynchronous resonance has been detected. Understandably, repeat the above steps. When the change in peak value of interharmonic current for three consecutive calculations exceeds 10%, it is considered that subsynchronous resonance has been detected.

[0071] like Figure 3 As shown, embodiments of the present invention also provide a passive damping device for suppressing subsynchronous resonance on the low-voltage side of a transformer, used to implement the passive damping method for suppressing subsynchronous resonance on the low-voltage side of a transformer, comprising:

[0072] The system comprises a detection unit, a transmission unit, a control unit, a switching element, and a damping resistor, wherein the detection unit, the transmission unit, the control unit, and the switching element are connected in sequence, and the switching element is connected in parallel across the damping resistor.

[0073] The detection unit detects whether subsynchronous resonance occurs. When subsynchronous resonance is detected, the control unit opens a preset switching element and puts in a damping resistor with the same value as the determined damping resistor.

[0074] The damping resistor value is determined from the collected electrical parameters according to preset constraints. The electrical parameters are the electrical parameters related to the power supply side, transformer and load side respectively. The damping resistor is set at the low-voltage side output terminal of the transformer and connected in series with the transformer. The switching element is connected in parallel across the two ends of the damping resistor and is set to be in the closed state initially.

[0075] In a specific embodiment, when the detection unit detects the occurrence of subsynchronous resonance, it generates a subsynchronous resonance signal and transmits it to the transmission unit; when the transmission unit receives the subsynchronous resonance signal transmitted by the detection unit, it transmits the signal to the control unit; when the control unit receives the subsynchronous resonance signal transmitted by the transmission unit, it sends a command to the switching element to control the switching element to open. It is understood that... Figure 3As shown, the output of the detection unit is connected to the input of the transmission unit, the output of the transmission unit is connected to the input of the control unit, and the output of the control unit is connected to the switching element. The switching element is connected in parallel across the damping resistor. The detection unit is used to detect whether subsynchronous resonance occurs on the power supply side. When subsynchronous resonance is detected, it transmits the subsynchronous resonance signal to the transmission unit. The transmission unit is used to transmit the subsynchronous resonance signal. When it receives the subsynchronous resonance signal from the detection unit, it transmits the signal to the control unit. The control unit is used to control the switching state of the switching element. When it receives the subsynchronous resonance signal from the transmission unit, it sends a command to the switching element to open it. The switching element is used to switch the damping resistor. Initially, the switching element is in a closed state, and the damping resistor is not connected. When it receives a command from the control unit, the switching element opens, and the damping resistor is connected. The damping resistor is connected in series to the three-phase output of the low-voltage side of part of the transformer to increase the system damping. When the switching element is closed, the damping resistor is not connected to the system; when the switching element is open, the damping resistor is connected to the system.

[0076] In this embodiment, the preset constraints include:

[0077] The preset interharmonic current peak suppression rate should be as high as possible, including:

[0078]

[0079] Where α is the peak suppression rate of harmonic phase current between power supply side buses, I smax I is the peak value of the harmonic phase current between the power supply side buses before the series damping resistor. s ′ max The peak value of the harmonic phase current between the power supply side buses after the series damping resistor is connected;

[0080] In addition, the preset voltage loss must not exceed a certain threshold, including:

[0081]

[0082] Where ΔU% is the voltage drop rate of the damping resistor, I lmax U is the maximum effective value of the phase current on the load side. lN Z is the rated voltage on the load side, β is the threshold voltage drop rate of the damping resistor, and Z is the voltage drop rate of the damping resistor. T Z is the equivalent impedance of the transformer referred to the low-voltage side. l R is the equivalent impedance of the line on the load side. d It is a damping resistor.

[0083] Specifically, the detection unit is used to collect three-phase current recording data of the power supply side bus;

[0084] Perform a fast Fourier transform on multiple cycles in the three-phase current recording data to obtain the peak values ​​of each interharmonic current within a preset frequency band.

[0085] Based on the preset response time, the time window is slid backward by several cycles, and then a fast Fourier transform is performed on the subsequent multiple identical cycles to obtain the peak values ​​of each interharmonic current within the preset frequency band, and the difference is calculated to determine the change in the peak values ​​of each interharmonic current.

[0086] The change in the peak value of each interharmonic current is repeatedly calculated. When the change in the peak value of the interharmonic current obtained from three consecutive calculations exceeds the preset threshold, it is considered that a subsynchronous resonance has been detected.

[0087] In specific embodiments, such as Figure 4 As shown, a substation's 10kV busbar connects to two 10kV / 0.4kV transformers. Each transformer connects to two identical uninterruptible power supplies (UPS) to power the same load. The power electronic devices in the UPS generate 30Hz and 70Hz interharmonic currents, which are injected into the 0.38kV system and then flow into the 10kV system. Through interaction with the 50Hz power frequency, a subsynchronous resonance with a frequency of 20Hz is generated on the 10kV busbar. The system parameters are as follows: 10kV busbar short-circuit capacity: 200MVA; 10kV / 0.4kV transformer rated capacity: 2500kVA, short-circuit loss: 17.7kW, short-circuit voltage percentage: 7.5%; 0.38kV line conductor cross-sectional area: 50mm². 2 It is 100m long.

[0088] Based on the above system parameters, a simulation model of the system was built using the MATLAB / Simulink platform. According to the passive damping method and device for the low-voltage side of the transformer provided by this invention, a damping resistor is connected in series with the low-voltage side outgoing line of one of the transformers, and a switching element is connected in parallel across its two ends. This switching element is initially set to a closed state. Using the subsynchronous resonance detection method provided by this invention, the switching element is opened after subsynchronous resonance is detected, and the damping resistor is engaged. According to the damping resistor selection method provided by this invention, the damping resistor value is set to vary within the range of 0.01Ω to 0.02Ω. The peak suppression rate of the interharmonic phase current and the voltage drop rate corresponding to each resistance value are simulated and calculated. The results are as follows: Figure 5 As shown, the peak suppression rate of interharmonic phase current increases with the increase of the damping resistance. However, when the damping resistance value exceeds 0.015Ω, the voltage drop rate begins to exceed the recommended 10% threshold. Therefore, in this embodiment, a 0.015Ω damping resistance can achieve a good balance between increasing the peak suppression rate of interharmonic phase current and controlling the voltage drop rate, at which point the peak suppression rate of interharmonic phase current reaches 33%. Figure 6The image shows a comparison of the three-phase current waveforms of the 10kV bus before and after the 0.015Ω damping resistor is connected. It can be seen that the present invention has a good suppression effect on the subsynchronous resonance peak.

[0089] It should be noted that the apparatus described in the above embodiments corresponds to the method described in the above embodiments. Therefore, the parts of the apparatus described in the above embodiments that are not described in detail can be obtained by referring to the content of the method described in the above embodiments, and will not be repeated here.

[0090] In summary, implementing the embodiments of the present invention has the following beneficial effects:

[0091] The present invention provides a passive damping method and device for suppressing subsynchronous resonance on the low-voltage side of a transformer. Installing this device on the three-phase outgoing lines of the low-voltage side of some transformers increases system damping, effectively suppressing the emission of super- / subsynchronous interharmonics and the superposition effect of interharmonics from power electronic device clusters, thereby effectively suppressing the subsynchronous resonance peak value caused by super- / subsynchronous interharmonics. It allows for the rational selection of damping resistor values, improving the peak suppression rate of interharmonic currents, while controlling the voltage loss caused by the damping resistor within a reasonable range. This improves the subsynchronous resonance suppression effect without affecting the normal operation of the system and load. The invention has a simple design, is easy to implement, and saves costs, contributing to improved power system stability and power quality.

[0092] The above description discloses only preferred embodiments of the present invention and should not be construed as limiting the scope of the present invention. Therefore, equivalent variations made in accordance with the claims of the present invention are still within the scope of the present invention.

Claims

1. A passive damping method for suppressing subsynchronous resonance on the low-voltage side of a transformer, characterized in that, include: Collect relevant electrical parameters from the power supply side, transformer side, and load side respectively; The corresponding damping resistance value is determined from the collected electrical parameters according to the preset constraints. Specifically, using the electrical parameters, it is determined whether the input damping resistor value meets the preset interharmonic current peak suppression rate and preset voltage loss; if it meets the requirements, the damping resistor value at this time is output; if it does not meet the requirements, the damping resistor value at this time is ignored. The subsynchronous resonance is detected at a preset detection point. When the subsynchronous resonance is detected, the preset switching element is turned on and a damping resistor with the same value as the damping resistor is applied. The damping resistor is located at the low-voltage side output terminal of part of the transformer and connected in series with the transformer. The switching element is connected in parallel across the two ends of the damping resistor, and the switching element is initially in a closed state. The preset interharmonic current peak suppression rate includes: in, The peak suppression rate of harmonic phase current between power supply side buses. The peak value of the harmonic phase current between the power supply side buses before the series damping resistor. The peak value of the harmonic phase current between the power supply side buses after the series damping resistor is connected; The preset voltage loss includes: in, The voltage drop rate of the damping resistor. I lmax This represents the maximum effective value of the phase current on the load side. U lN This is the rated voltage on the load side. This is the threshold value for the voltage drop rate of the damping resistor. Z T This is the equivalent impedance of the transformer referred to the low-voltage side. Z l The equivalent impedance of the line on the load side. R d It is a damping resistor.

2. The method as described in claim 1, characterized in that, The electrical parameters include at least the short-circuit capacity of the power supply system, the rated capacity of the transformer, the short-circuit loss and the percentage of short-circuit voltage, and the cross-sectional area of ​​the conductor and the length of the line on the load side.

3. The method as described in claim 1, characterized in that, The detection of whether subsynchronous resonance occurs includes: Collect three-phase current waveform data of the power supply side bus; Perform a fast Fourier transform on multiple cycles in the three-phase current recording data to obtain the peak values ​​of each interharmonic current within a preset frequency band. Based on the preset response time, the time window is slid backward by several cycles, and then a fast Fourier transform is performed on the subsequent multiple identical cycles to obtain the peak values ​​of each interharmonic current within the preset frequency band, and the difference is calculated to determine the change in the peak values ​​of each interharmonic current. The change in the peak value of each interharmonic current is repeatedly calculated. When the change in the peak value of the interharmonic current obtained from three consecutive calculations exceeds the preset threshold, it is considered that a subsynchronous resonance has been detected.

4. A passive damping device for suppressing subsynchronous resonance on the low-voltage side of a transformer, used to implement the method as described in any one of claims 1-3, characterized in that, include: The system comprises a detection unit, a transmission unit, a control unit, a switching element, and a damping resistor, wherein the detection unit, the transmission unit, the control unit, and the switching element are connected in sequence, and the switching element is connected in parallel across the damping resistor. The detection unit detects whether subsynchronous resonance occurs. When subsynchronous resonance is detected, the control unit opens a preset switching element and puts in a damping resistor with the same value as the determined damping resistor. The damping resistor value is determined from the collected electrical parameters according to preset constraints. The electrical parameters are the electrical parameters related to the power supply side, transformer and load side respectively. The damping resistor is set at the low-voltage side output terminal of part of the transformer and connected in series with the transformer. The switching element is connected in parallel across the two ends of the damping resistor. The switching element is set to be in the closed state initially. The preset constraints include: The preset interharmonic current peak suppression rate should be as high as possible, including: in, The peak suppression rate of harmonic phase current between power supply side buses. The peak value of the harmonic phase current between the power supply side buses before the series damping resistor. The peak value of the harmonic phase current between the power supply side buses after the series damping resistor is connected; In addition, the preset voltage loss must not exceed a certain threshold, including: in, The voltage drop rate of the damping resistor. I lmax This represents the maximum effective value of the phase current on the load side. U lN This is the rated voltage on the load side. This is the threshold value for the voltage drop rate of the damping resistor. Z T This is the equivalent impedance of the transformer referred to the low-voltage side. Z l The equivalent impedance of the line on the load side. R d It is a damping resistor.

5. The apparatus as described in claim 4, characterized in that, Also includes: When the detection unit detects the occurrence of subsynchronous resonance, it generates a subsynchronous resonance signal and transmits it to the transmission unit. When the transmission unit receives the subsynchronous resonance signal from the detection unit, it transmits the signal to the control unit. When the control unit receives the subsynchronous resonance signal transmitted from the transmission unit, it sends a command to the switching element to control the switching element to open.

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