An oscillation suppression method based on control bandwidth switching

Abstract

The application discloses an oscillation suppression method based on control bandwidth switching, which is applied to one or more voltage source converters of a flexible direct current power transmission system, frequency switching set value Fset is determined according to converter impedance characteristics of a first set of low control bandwidth parameters and a second set of high control bandwidth parameters, it is judged whether the oscillation frequency F of the current operation mode is greater than the frequency switching set value Fset, if yes, the controller of the converter is set to the low control bandwidth parameters, otherwise, the controller of the converter is set to the high control bandwidth parameters. By using the oscillation suppression method, the converter presents positive resistance near the oscillation frequency, additional equipment is not needed, and tripping caused by oscillation can be avoided.

Description

Technical Field

[0001] This invention belongs to the field of flexible DC transmission technology in power systems, and specifically relates to an oscillation suppression method. Background Technology

[0002] Flexible direct current (DC) transmission technology has been widely applied both domestically and internationally. However, with the increasing number of DC projects in operation, stability issues have become increasingly prominent. Oscillations caused by flexible DC systems are one of the most pressing problems to be solved. High-frequency oscillations occurred during the trial operation of the Zhoushan Wutong DC project and the Luxi DC project, as well as during the commissioning of the Chongqing-Hubei DC project. Once oscillations occur, they can range from affecting the power quality of the system to causing control and protection system failures, system shutdowns, or even equipment damage.

[0003] From the perspective of impedance characteristics, oscillations are caused by insufficient damping near the system's resonant point. Since harmonic sources are widespread in power systems, it is difficult to completely eliminate harmonics. Under weak or negative damping conditions, harmonics are amplified, leading to oscillations. Related research indicates that control link delay is a significant cause of high-frequency oscillations after flexible DC systems are connected to the AC grid. Reducing control link delay should be a crucial means of suppressing high-frequency oscillations. However, control delay is affected by hardware and architecture, making it difficult to reduce indefinitely. Another effective method for suppressing oscillations is to improve system damping. Currently, most scholars utilize additional damping control strategies to suppress various oscillation problems, but the operating modes in actual systems are complex and varied, making it difficult for additional damping control strategies to adapt to all operating modes. The literature "Research on High-Frequency Oscillation Suppression Strategies for Flexible DC Transmission Systems" (Proceedings of the CSEE, Vol. 41, No. 11, 2021) also proposes using a second-order RLC high-pass filter to suppress high-frequency oscillations, but this increases equipment costs. Currently, there are no reports of methods that actively suppress oscillations by utilizing the difference in damping characteristics of the converter itself at medium and high frequencies. Summary of the Invention

[0004] The purpose of this invention is to provide an oscillation suppression method based on control bandwidth switching. This method utilizes the difference in damping characteristics between the intermediate frequency and high frequency of the converter under high and low control bandwidths. The control bandwidth is switched according to the oscillation frequency of the operating mode, so that the converter exhibits positive resistance near the oscillation frequency, thereby suppressing oscillation and avoiding tripping caused by oscillation.

[0005] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0006] An oscillation suppression method based on control bandwidth switching, applied to one or more voltage source converters in a flexible DC transmission system, includes the following steps:

[0007] 1) Select two sets of converter control parameters, one set of which is the low control bandwidth parameter and the other set of which is the high control bandwidth parameter, and proceed to step 2).

[0008] 2) Based on the converter impedance characteristics of the two sets of control parameters, determine the frequency switching setpoint Fset, and proceed to step 3).

[0009] 3) Determine the oscillation frequency F of the current operating mode, and proceed to step 4).

[0010] 4) When the oscillation frequency F is greater than the frequency switching setpoint Fset and the duration exceeds the delay setpoint td, proceed to step 5); otherwise, proceed to step 6.

[0011] 5) Set the converter controller to low control bandwidth parameters and proceed to step 3).

[0012] 6) Set the converter controller to high control bandwidth parameters and proceed to step 3).

[0013] Furthermore, in step 1), the low control bandwidth parameter and the high control bandwidth parameter are determined based on system research.

[0014] Further, in step 2), the frequency switching setpoint Fset is determined by selecting a frequency switching setpoint Fset that is less than or equal to the frequency in the high control bandwidth where the impedance angle crosses 90 degrees, based on the converter impedance characteristics under the low and high control bandwidth of the controller.

[0015] Further, in step 3), the oscillation frequency F of the current operating mode is determined by calculating the oscillation frequency of the converter and the system under this operating mode offline.

[0016] Further, in step 3), the oscillation frequency F of the current operating mode is determined by real-time monitoring of the harmonic components in the AC voltage or current. When the maximum amplitude of the harmonic component is greater than the set value of the oscillation amplitude, the frequency corresponding to the harmonic component is the oscillation frequency F.

[0017] Furthermore, the oscillation amplitude setting value is determined based on system research.

[0018] Furthermore, in step 4), the delay setpoint td ranges from 1 microsecond to 1 second.

[0019] After adopting the above solution, the beneficial effects of the present invention are as follows:

[0020] 1. This invention utilizes the characteristic that the converter exhibits positive resistance at different frequency bands when the control bandwidth is high and the control bandwidth is low. The control bandwidth is switched according to the oscillation frequency of the operating mode, so that the converter exhibits positive resistance near the oscillation frequency, thereby suppressing oscillation and avoiding tripping caused by oscillation.

[0021] 2. This invention does not restrict the operating mode and has good adaptability;

[0022] 3. This invention does not require additional equipment and is highly economical. Attached Figure Description

[0023] Figure 1 In the diagram, (a) and (b) are a schematic diagram and an equivalent circuit diagram of the converter connected to the AC power grid, respectively.

[0024] Figure 2 Impedance curves of a converter with low control bandwidth and high control bandwidth are provided for an embodiment.

[0025] Figure 3 The flowchart illustrates an oscillation suppression method based on control bandwidth switching, as provided in this embodiment. Detailed Implementation

[0026] The technical solution of the present invention will be described in detail below with reference to the accompanying drawings and specific embodiments.

[0027] Figure 1 (a) is a schematic diagram of the converter connected to the AC power grid. The converter VSC is connected to the common coupling point PCC of the AC power grid and forms a series system with the AC power grid. Figure 1 In the diagram, (b) is the equivalent circuit diagram corresponding to (a). g Zg(s) is the harmonic voltage source of the AC system, Zg(s) is the impedance of the AC system, and V is the voltage source. vsc (s) is the harmonic voltage source of the converter VSC, Z vsc (s) represents the impedance of the converter VSC. According to the impedance principle, when Zg(s) + Z vsc (s) When the real part of a certain frequency is complex, there is negative resistance, which easily leads to oscillation. Therefore, to avoid oscillation, one method is to adjust the impedance characteristics of the converter at that frequency to reduce the negative resistance of the converter or even make it positive.

[0028] Figure 2The graphs show the impedance curves for a converter with low and high control bandwidths. The solid line represents the impedance characteristic curve for low control bandwidth, and the dashed line represents the impedance characteristic curve for high control bandwidth. The upper graph shows the impedance magnitude, and the lower graph shows the impedance angle. In the high control bandwidth section, the impedance angle crosses 90 degrees at 823 Hz, transitioning from positive to negative resistance with a gradually increasing negative resistance, decreasing after 1300 Hz, but still exhibiting increased negative resistance at 2000 Hz. In the low control bandwidth section, the impedance angle crosses 90 degrees at 220 Hz, transitioning from positive to negative resistance with a gradually increasing negative resistance, but exhibiting slight positive or negative resistance after 600 Hz. Since actual IGBTs, inductors, transformers, etc., all contain resistance, the converter will exhibit positive resistance. Therefore, when the oscillation frequency is below 823Hz, the converter generates a large positive damping when the controller uses a high control bandwidth, which can suppress the oscillation; when the oscillation frequency is above 600Hz, the converter has a small positive or negative resistance when the controller uses a low control bandwidth. Since the primary equipment in the AC system has appropriate resistance, oscillation is generally not easy to occur.

[0029] Figure 3 The flowchart illustrates an oscillation suppression method based on control bandwidth switching, applied to one or more voltage source converters in a flexible DC transmission system, including the following steps:

[0030] Step 201: Select two sets of converter control parameters, where the first set of control parameters is the low control bandwidth parameter and the second set is the high control bandwidth parameter, then proceed to step 202; the low control bandwidth parameter and the high control bandwidth frequency value are determined based on system research, and can be selected as needed. Figure 2 The two sets of converter control parameters are shown in the figure. The first set of controller parameters corresponds to the low control bandwidth controller parameters (solid line) and the second set of controller parameters corresponds to the high control bandwidth controller parameters (dashed line).

[0031] Step 202: Based on the converter impedance characteristics of the two sets of control parameters, determine the frequency switching setpoint Fset, and proceed to step 203; the frequency switching setpoint Fset should be less than or equal to the frequency in the high control bandwidth where the impedance angle crosses 90 degrees. Figure 2 The high control bandwidth impedance angle shown by the dashed line crosses 90 degrees at 823Hz, and it is preferable to choose to switch the setting Fset to 800Hz.

[0032] Step 203: Determine the oscillation frequency F of the current operating mode, and proceed to step 204; The determination of the oscillation frequency F of the current operating mode can be achieved by calculating the oscillation frequency of the converter and the system under this operating mode offline; or by monitoring the harmonic components in the AC voltage or current in real time. When the maximum amplitude of the harmonic component is greater than the oscillation amplitude setting value, the frequency corresponding to the harmonic component is the oscillation frequency F. The oscillation amplitude setting value is determined based on system research.

[0033] Step 204: When the oscillation frequency F is greater than the frequency switching setpoint Fset and the duration exceeds the delay setpoint td, proceed to step 205; otherwise, proceed to step 206. The delay setpoint td ranges from 1 microsecond to 1 second, and is preferably selected as 20ms.

[0034] Step 205: Set the converter controller to low control bandwidth parameters, then proceed to step 203;

[0035] Step 206: Set the converter controller to high control bandwidth parameters, then proceed to step 203.

[0036] The above embodiments are merely illustrative of the technical concept of the present invention and should not be construed as limiting the scope of protection of the present invention. Any modifications made to the technical solutions based on the technical concept proposed in this invention shall fall within the scope of protection of this invention.

Claims

1. An oscillation suppression method based on control bandwidth switching, applied to one or more voltage source converters in a flexible DC transmission system, characterized in that, Includes the following steps: 1) Select two sets of converter control parameters, one set of which is the low control bandwidth parameter and the other set of which is the high control bandwidth parameter, and proceed to step 2). 2) Based on the converter impedance characteristics of the two sets of control parameters, determine the frequency switching setpoint Fset, and proceed to step 3). 3) Determine the oscillation frequency F of the current operating mode, and proceed to step 4). 4) When the oscillation frequency F is greater than the frequency switching setpoint Fset and the duration exceeds the delay setpoint td, proceed to step 5); otherwise, proceed to step 6. 5) Set the converter controller to low control bandwidth parameters and proceed to step 3). 6) Set the converter controller to high control bandwidth parameters and proceed to step 3).

2. The oscillation suppression method based on control bandwidth switching as described in claim 1, characterized in that, In step 1), the low control bandwidth parameter and the high control bandwidth parameter are determined based on system research.

3. The oscillation suppression method based on control bandwidth switching as described in claim 1, characterized in that, In step 2), the frequency switching setpoint Fset is determined by selecting a frequency switching setpoint Fset that is less than or equal to the frequency in the high control bandwidth parameter where the impedance angle crosses 90 degrees, based on the converter impedance characteristics under the low control bandwidth parameter and the high control bandwidth parameter of the controller.

4. The oscillation suppression method based on control bandwidth switching as described in claim 1, characterized in that, In step 3), the oscillation frequency F of the current operating mode is determined by calculating the oscillation frequency of the converter and the system under this operating mode offline.

5. The oscillation suppression method based on control bandwidth switching as described in claim 1, characterized in that, In step 3), the oscillation frequency F of the current operating mode is determined by real-time monitoring of the harmonic components in the AC voltage or current. When the maximum amplitude of the harmonic component is greater than the set value of the oscillation amplitude, the frequency corresponding to the harmonic component is the oscillation frequency F.

6. The oscillation suppression method based on control bandwidth switching as described in claim 5, characterized in that, The oscillation amplitude setting value is determined based on system research.

7. The oscillation suppression method based on control bandwidth switching as described in claim 1, characterized in that, In step 4), the delay setpoint td ranges from 1 microsecond to 1 second.

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