A method for quickly identifying a high-frequency oscillation dominant link of a flexible direct current power transmission system

Abstract

The application discloses a flexible direct current power transmission system high-frequency oscillation leading link fast identification method, and the method comprises the following steps: step 1) obtaining AC system equivalent and flexible direct current operation control parameters; step 2) determining a flexible direct current operation and control mode strategy; step 3) constructing an MMC high-frequency impedance characteristic converter simplified model, and obtaining model impedance amplitude and phase characteristic curves; step 4) identifying high-frequency negative resistance inductance characteristics of a flexible direct current converter station according to the model impedance amplitude and phase characteristic curves; step 5) calculating MMC high-frequency impedance characteristic eigenvalues and sensitivity indexes of stable changes of the eigenvalues caused by parameter changes of each control link of the converter station and an AC system; and step 6) determining a leading link causing high-frequency oscillation according to the sensitivity indexes. The method considers that a simplified modular multilevel flexible direct current model is used to analyze high-frequency stability, and can flexibly adapt to complex and changeable operation modes of the flexible direct current.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of electrical engineering, and particularly relates to a flexible direct current power transmission system high-frequency oscillation dominant link fast identification method. BACKGROUND

[0002] In the past decade, high-voltage (MMC-HVDC) power transmission technology based on modular multilevel converter has been widely used in the grid connection of large-scale renewable power plants due to its advantages of weak grid interconnection and island power supply, and a large number of practical projects have been put into operation worldwide. However, with the increasing scale and complexity of power systems, high-frequency oscillation (HFR) phenomena occur frequently. For example, 1271Hz HFR and 700Hz and 1.8kHz HFR occurred in Luxi and Yuye projects, which poses a great challenge to the stability of power systems.

[0003] In order to analyze the stability of MMC-HVDC system, impedance-based stability criterion is often used, which requires the establishment of impedance models of MMC and interconnection system. Due to the complex internal structure and dynamic characteristics of MMC, scholars mainly use dynamic phasor, harmonic state space (HSS), harmonic linearization method for research, and establish detailed impedance models in a wide frequency range. However, when analyzing HFR problems, these detailed models are too complex, and simplified and accurate impedance models need to be selected to reduce the complexity of analysis.

[0004] Recent studies have shown that the external control loop, phase-locked loop (PLL), circulating current suppression control (CCSC), and the dynamics of MMC internal have little effect on the high-frequency impedance characteristics of MMC. Ignoring the control of current inner loop, control delay and feed-forward voltage, the simplified high-frequency model has been widely used in the analysis of HFR suppression methods. However, the impedance characteristics of two-level voltage source converter (VSC) under PQ control (grid-connected control) and AC voltage control (island control) are different, which is also meaningful for the analysis of MMC. Therefore, the simplified model should be distinguished according to the differences in high-frequency range under the two control strategies. SUMMARY

[0005] In order to solve the problems existing in the prior art, the present application provides a flexible direct current power transmission system high-frequency oscillation dominant link fast identification method to solve the above technical problems, which is realized by the following technical scheme:

[0006] The flexible direct current power transmission system high-frequency oscillation dominant link fast identification method, which comprises the following steps:

[0007] Step 1) Obtain the equivalent AC system and flexible direct current operation control parameters;

[0008] Step 2) Determine the flexible direct current operation and control mode strategy;

[0009] Step 3) constructing a simplified model of the MMC high-frequency impedance characteristics converter, obtaining the model impedance amplitude and phase characteristic curves;

[0010] Step 4) identifying the high-frequency negative resistance inductance characteristics of the HVDC converter station according to the model impedance amplitude and phase characteristic curves;

[0011] Step 5) calculating the MMC high-frequency impedance characteristic eigenvalues and the sensitivity index of the stable change of the eigenvalues caused by the parameter changes of each control link and the AC system;

[0012] Step 6) determining the dominant link causing high-frequency oscillation according to the sensitivity index.

[0013] The further design of the flexible DC power transmission system high-frequency oscillation dominant link fast identification method is that in step 1), the AC system equivalent and flexible DC operation control parameters include: converter station main circuit parameters, MMC controller parameters, and connected transformer and AC side impedance parameters.

[0014] The further design of the flexible DC power transmission system high-frequency oscillation dominant link fast identification method is that in step 2), the flexible DC operation and control mode strategy includes: island sending out constant AC voltage control mode and active and reactive power control mode connected with the AC power grid.

[0015] The further design of the flexible DC power transmission system high-frequency oscillation dominant link fast identification method is that step 3) is specifically:

[0016] For the island sending out constant AC voltage control mode, the MMC high-frequency impedance simplified model is set according to formula (1):

[0017]

[0018]

[0019]

[0020] In formula (1), Z dd is the ratio of the MMC high-frequency impedance frequency domain transfer function d-axis voltage to d-axis current, Z qq is the ratio of the MMC high-frequency impedance frequency domain transfer function q-axis voltage to q-axis current, Z dq is the ratio of the MMC high-frequency impedance frequency domain transfer function d-axis voltage to q-axis current, and Z qd is the ratio of the MMC high-frequency impedance frequency domain transfer function q-axis voltage to d-axis current; L eq is the equivalent reactance of the AC side system, R eq is the equivalent resistance of the AC side system; G dH(s) is the link delay transfer function, T d H(s) is the link delay time; H i k is the inner loop current PI control transfer function, k pi k is the inner loop current PI control proportional link, k ii k is the inner loop current PI control integral link, k f ω0 is the grid fundamental frequency, and s is a complex frequency.

[0021] According to formula (2), the MMC high-frequency impedance simplified model is set according to the active and reactive control mode of the flexible DC and AC power grid connection:

[0022]

[0023]

[0024]

[0025]

[0026] In the formula, H ctrl =H PQ H i G d C and D are both 2*2 order matrices, i gd , i gq are the dq axis currents of the AC system side, v gd , v gq are the dq axis voltages of the AC system side, H PQ k is the outer loop active and reactive control transfer function, k pi k is the outer loop active and reactive PI control proportional link, k iPQ k is the integral link of the outer loop controller k pPQ k is the proportional link of the outer loop controller.

[0027] The further design of the flexible DC power transmission system high-frequency oscillation dominant link fast identification method is that the model impedance amplitude and phase characteristic curve contains d-axis voltage and d-axis current impedance amplitude and phase, d-axis voltage and q-axis current impedance amplitude and phase, q-axis voltage and d-axis current impedance amplitude and phase, and q-axis voltage and q-axis current impedance amplitude and phase.

[0028] The further design of the flexible DC power transmission system high-frequency oscillation dominant link fast identification method is that the step 4) comprises: judging whether the impedance amplitude and phase characteristics curve of the MMC high-frequency impedance simplified model obtained in the above steps is less than 90° in terms of the d-axis voltage and the d-axis current impedance amplitude and phase, the d-axis voltage and the q-axis current impedance amplitude and phase, the q-axis voltage and the d-axis current impedance amplitude and phase, and the q-axis voltage and the q-axis current impedance amplitude and phase.

[0029] The further design of the flexible DC power transmission system high-frequency oscillation dominant link fast identification method is that the step 5) comprises: obtaining unstable characteristic roots by using an eigenvalue analysis method according to the MMC high-frequency impedance matrix; and researching the sensitivity of the characteristic roots caused by the parameter changes of each control link of the converter station and the AC system, that is, a sensitivity index:

[0030]

[0031] In the formula, p is the sensitivity index of each link of the flexible DC, λ i is the unstable characteristic root of the system, and x mmc is the control parameter of each link of the flexible DC.

[0032] The further design of the flexible DC power transmission system high-frequency oscillation dominant link fast identification method is that the step 6) of determining the dominant link causing the high-frequency oscillation specifically comprises: if the sensitivity index is greater than 0.5, the corresponding control link is the dominant link of the high-frequency oscillation.

[0033] The present application also provides an electronic device comprising a memory, a processor and a computer program, wherein the computer program is stored in the memory and is configured to be executed by the processor to realize the flexible DC power transmission system high-frequency oscillation dominant link fast identification method.

[0034] The present application also provides a computer readable storage medium, wherein the storage medium stores a computer program, and the computer program is executed by a processor to realize the flexible DC power transmission system high-frequency oscillation dominant link fast identification method.

[0035] The present application has the following beneficial effects:

[0036] The flexible direct current power transmission system high-frequency oscillation dominant link fast identification method of the present application is based on the flexible direct current control mode to construct different modular multilevel (MMC) high-frequency impedance simplified models, obtain model impedance amplitude and phase characteristic curves, and identify the high-frequency negative resistance inductance characteristics of the flexible direct current converter station. The dominant link causing high-frequency oscillation is determined based on the sensitivity index under the root locus research method. This method considers the use of simplified modular multilevel flexible direct current models to analyze high-frequency stability, can flexibly adapt to the complex and variable operation mode of the flexible direct current, provides protection for fast and efficient suppression of high-frequency oscillation risk, and has important significance for the safe and stable operation of the power system. BRIEF DESCRIPTION OF DRAWINGS

[0037] Figure 1 The flowchart of the flexible direct current power transmission system high-frequency oscillation dominant link fast identification method of the present application is shown in the figure. DETAILED DESCRIPTION

[0038] The technical solutions in the embodiments of the present application are described clearly and completely below in combination with the drawings.

[0039] As Figure 1 The flexible direct current power transmission system high-frequency oscillation dominant link fast identification method of the present application includes the following steps:

[0040] Step 1) Obtain the equivalent of the alternating current system and the flexible direct current operation control parameters.

[0041] Step 2) Determine the flexible direct current operation and control mode strategy. In this embodiment, the flexible direct current operation and control mode strategy includes: island sending out fixed alternating current voltage control mode and active and reactive power control mode connected with the alternating current grid. The flexible direct current operation and control mode strategy is transmitted to the dispatching platform through the dispatching data network.

[0042] Step 3) Construct the MMC high-frequency impedance characteristic converter simplified model, and obtain the model impedance amplitude and phase characteristic curves.

[0043] Step 4) Identify the high-frequency negative resistance inductance characteristics of the flexible direct current converter station according to the model impedance amplitude and phase characteristic curves.

[0044] Step 5) Calculate the MMC high-frequency impedance characteristic eigenvalue and the sensitivity index of the stable change of the eigenvalue caused by the parameter change of each control link of the converter station and the alternating current system.

[0045] Step 6) Determine the dominant link causing high-frequency oscillation according to the sensitivity index.

[0046] In step 1) of the embodiment, the AC system equivalence and the flexible DC operation control parameters include: the main circuit parameters of the converter station (AC voltage, DC voltage, number of bridge arm sub-modules, converter transformer impedance, rated power, sub-module capacitance, bridge arm resistance and inductance, control link delay time T d ), the MMC controller parameters (PQ outer loop PI controller parameters, current inner loop PI controller parameters, circulating current suppression PI controller parameters and phase-locked loop PI controller parameters), the coupling transformer (ratio, leakage reactance) and the AC side impedance parameters (equivalent resistance R eq and reactance L eq ).

[0047] In step 3) of the embodiment, for the island sending-out fixed AC voltage control mode, the MMC high-frequency impedance simplified model is set according to formula (1):

[0048]

[0049]

[0050]

[0051] Z dd is the ratio of the d-axis voltage to the d-axis current of the MMC high-frequency impedance frequency domain transfer function, Z qq is the ratio of the q-axis voltage to the q-axis current of the MMC high-frequency impedance frequency domain transfer function, Z dq is the ratio of the d-axis voltage to the q-axis current of the MMC high-frequency impedance frequency domain transfer function, Z qd is the ratio of the q-axis voltage to the d-axis current of the MMC high-frequency impedance frequency domain transfer function; L eq is the equivalent reactance of the AC side system, R eq is the equivalent resistance of the AC side system; G d is the control link delay transfer function, T d is the control link delay time; H i is the inner loop current PI control transfer function, k pi is the inner loop current PI control proportional link, k ii is the inner loop current PI control integral link, K f is the voltage feedforward link coefficient, ω0 is the power grid fundamental frequency, and s is a complex frequency.

[0052] For the active and reactive power control mode of the flexible DC and AC power grid connection, the MMC high-frequency impedance simplified model is set according to formula (2):

[0053]

[0054]

[0055]

[0056]

[0057] In formula (2), H ctrl = H PQ H i G d is a control link transfer function, C and D are both 2*2 order matrices, i gd , i gq are dq-axis currents of an AC system side, v gd , v gq are dq-axis voltages of the AC system side, H PQ is an outer loop active and reactive power control transfer function, k pi is an outer loop active and reactive power PI control proportion link, k iPQ is an integral link of the outer loop controller k pPQ is a proportion link of the outer loop controller.

[0058] Further, the model impedance amplitude and phase characteristic curves include d-axis voltage and d-axis current impedance amplitude and phase, d-axis voltage and q-axis current impedance amplitude and phase, q-axis voltage and d-axis current impedance amplitude and phase, and q-axis voltage and q-axis current impedance amplitude and phase.

[0059] Step 4) of the embodiment includes judging whether the d-axis voltage and d-axis current impedance amplitude and phase, the d-axis voltage and q-axis current impedance amplitude and phase, the q-axis voltage and d-axis current impedance amplitude and phase, and the q-axis voltage and q-axis current impedance amplitude and phase are less than 90° according to the MMC high-frequency impedance simplified model impedance amplitude and phase characteristic curves obtained in the above steps. If the phase judgment exceeds 90°, there is a negative resistance characteristic, that is, it is judged that the flexible direct current project has an oscillation risk; if the phase judgment is less than 90°, there is no negative resistance characteristic, that is, it is judged that the flexible direct current project has no oscillation risk.

[0060] Step 5) of the embodiment includes obtaining unstable eigenvalues by using an eigenvalue analysis method according to the MMC high-frequency impedance matrix; and researching the eigenvalue change sensitivity caused by the change of each control link of the converter station and the AC system parameters, that is, the sensitivity index:

[0061]

[0062] In the formula, p is the sensitivity index of each link of the flexible direct current, λ i is an unstable eigenvalue of the system, and x mmc is a control parameter of each link of the flexible direct current.

[0063] Step 6) of the embodiment is to determine the dominant link causing high-frequency oscillation, if the sensitivity index is greater than 0.5, the corresponding control link is the dominant link of high-frequency oscillation; the determined sensitivity index of the dominant link of high-frequency oscillation is sorted and collected, and the collected sensitivity index is used as the basis for subsequent research on high-frequency oscillation, so as to suppress the risk of high-frequency oscillation.

[0064] The embodiment also provides an electronic device, including a memory, a processor and a computer program, wherein the computer program is stored in the memory and is configured to be executed by the processor to realize the flexible HVDC system high-frequency oscillation dominant link fast identification method.

[0065] The embodiment also provides a computer readable storage medium, the storage medium stores a computer program, and the computer program is executed by a processor to realize the flexible HVDC system high-frequency oscillation dominant link fast identification method.

[0066] The flexible HVDC system high-frequency oscillation dominant link fast identification method of the embodiment constructs different modular multilevel (MMC) high-frequency impedance simplified models based on the flexible DC control mode, obtains model impedance amplitude and phase characteristic curves, and identifies the high-frequency negative resistance inductance characteristics of the HVDC converter station. The dominant link causing high-frequency oscillation is determined based on the sensitivity index under the root locus research method. The method considers the simplified MMC flexible DC model for analyzing high-frequency stability, can flexibly adapt to the complex and changeable operation mode of the flexible DC, provides guarantee for fast and efficient suppression of high-frequency oscillation risk, and has important significance for safe and stable operation of the power system.

[0067] The technical means disclosed in the present application scheme is not limited to the technical means disclosed in the above embodiments, but also includes the technical solutions composed of any combination of the above technical features. It should be noted that, for ordinary skilled persons in the art, without departing from the principles of the present application, a number of improvements and refinements can be made, which are also considered within the protection scope of the present application.

Claims

1. A method for fast identification of high frequency oscillation dominant loop in a flexible HVDC power transmission system, characterized in that, The method comprises: Step 1) obtaining AC system equivalent and flexible DC operation control parameters; Step 2) determining flexible DC operation and control mode strategy; Step 3) constructing an MMC high-frequency impedance characteristic converter simplified model to obtain model impedance amplitude and phase characteristic curves; Step 4) identifying high-frequency negative resistance inductance characteristics of the HVDC converter station according to the model impedance amplitude and phase characteristic curves; Step 5) calculating MMC high-frequency impedance characteristic eigenvalues and sensitivity indexes of eigenvalue stable changes caused by parameter changes of each control link of the converter station and the AC system; Step 6) determining a dominant link causing high-frequency oscillation according to the sensitivity indexes. Step 3) specifically comprises: For island sending-out fixed AC voltage control mode, an MMC high-frequency impedance simplified model is set according to formula (1): In formula (1), Z dd is the ratio of d-axis voltage to d-axis current of the MMC high-frequency impedance frequency-domain transfer function, Z qq is the ratio of q-axis voltage to q-axis current of the MMC high-frequency impedance frequency-domain transfer function, Z dq is the ratio of d-axis voltage to q-axis current of the MMC high-frequency impedance frequency-domain transfer function, Z qd is the ratio of q-axis voltage to d-axis current of the MMC high-frequency impedance frequency-domain transfer function; L eq is the equivalent reactance of the alternating-current side system, R eq is the equivalent resistance of the alternating-current side system; G d is the control link delay transfer function, T d is the control link delay time; H i is the inner-loop current PI control transfer function, k pi is the inner-loop current PI control proportional link, k ii is the inner-loop current PI control integral link, K f is a voltage feedforward link coefficient, ω0 is a power grid fundamental frequency, and s is a complex frequency. For active and reactive power control mode of flexible DC and AC grid connection, an MMC high-frequency impedance simplified model is set according to formula (2): In equation (2), H ctrl =H PQ H i G d Let C and D be the transfer functions of the control loop, where C and D are both 2x2 matrices, and i gd i gq These are the dq-axis currents on the AC system side, v gd v gq These are the dq-axis voltages on the AC system side, H PQ Let k be the outer loop active and reactive power control transfer function. pi For the outer loop active and reactive power PI control proportional stage, k iPQ For the integral element of the outer loop controller, k pPQ This is the proportional element of the outer loop controller.

2. The method of claim 1, wherein the method is characterized by: In step 1), the AC system equivalent and flexible DC operation control parameters include: main circuit parameters of the converter station, MMC controller parameters, and connection transformer and AC side impedance parameters.

3. The method of claim 1, wherein the method is characterized by: In step 2), the flexible DC operation and control mode strategy includes: island sending-out fixed AC voltage control mode and active and reactive power control mode of flexible DC and AC grid connection.

4. The method of claim 1, wherein the method further comprises: The model impedance amplitude and phase characteristic curves include d-axis voltage and d-axis current impedance amplitude and phase, d-axis voltage and q-axis current impedance amplitude and phase, q-axis voltage and d-axis current impedance amplitude and phase, and q-axis voltage and q-axis current impedance amplitude and phase.

5. The method of claim 4, wherein the method further comprises: Step 4) includes: judging whether the d-axis voltage and d-axis current impedance amplitude and phase, the d-axis voltage and q-axis current impedance amplitude and phase, the q-axis voltage and d-axis current impedance amplitude and phase, and the q-axis voltage and q-axis current impedance amplitude and phase are less than 90° according to the MMC high-frequency impedance simplified model impedance amplitude and phase characteristic curves obtained in the above steps; if the phase judgment exceeds 90°, there is a negative resistance characteristic, that is, it is judged that the HVDC project has an oscillation risk; if the phase judgment is less than 90°, there is no negative resistance characteristic, that is, it is judged that the HVDC project has no oscillation risk.

6. The method of claim 1, wherein the method further comprises: Step 5) includes: obtaining unstable eigenvalues by eigenvalue analysis method according to the MMC high-frequency impedance matrix; and researching sensitivity indexes of eigenvalue changes caused by parameter changes of each control link of the converter station and the AC system, that is, sensitivity indexes: In the formula, p is the sensitivity index of each link of the flexible straight line, λ i is the unstable eigenvalue of the system, x mmc is the control parameter of each link of the flexible straight line.

7. The method of claim 1, wherein the method further comprises: In step 6), determining a dominant link causing high-frequency oscillation specifically comprises: if the sensitivity index is greater than 0.5, the corresponding control link is the dominant link of high-frequency oscillation.

8. An electronic device, comprising: The computer program is stored in the memory and is configured to be executed by the processor to realize the flexible DC power transmission system high-frequency oscillation dominant link fast identification method of any one of claims 1-7.

9. A computer-readable storage medium, characterized in that The storage medium stores a computer program, and the computer program is executed by the processor to realize the flexible DC power transmission system high-frequency oscillation dominant link fast identification method of any one of claims 1-7.

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