A method for suppressing subsynchronous oscillations in a flexible HVDC system

Abstract

The present invention discloses a method for suppressing subsynchronous oscillations in a flexible direct current (HVDC) transmission system. The method comprises obtaining operating parameters of the HVDC transmission system; obtaining d-axis current and q-axis current injected into a HVDC converter station; isolating the DC components of the obtained d-axis current and q-axis current through a high-pass filter; filtering the d-axis current and q-axis current after HVDC isolation through the high-pass filter to remove high-frequency components; multiplying the d-axis current and q-axis current after HVDC isolation through the high-pass filter by a gain resistor and limiting their amplitudes; superimposing a compensation voltage after the amplitude limit with the actual d-axis and q-axis voltages and transmitting the result to an outer loop voltage controller; and calculating a reference output voltage value of a three-phase converter and inputting the result into an existing three-phase converter control system to suppress subsynchronous oscillations in the HVDC transmission system. The algorithm of the present invention is simple, reliable, easy to implement, does not require additional peripheral hardware circuits, is low-cost, highly reliable, and has good stability. It also has certain versatility.

Description

Technical Field

[0001] The present invention relates to a high voltage direct current (HVDC) flexible power transmission system based on a voltage source converter (VSC), including technologies such as the flexible HVDC system accommodating wind farms and flexible DC grids, and more specifically to a method for suppressing subsynchronous oscillations in the flexible HVDC system. Background Art

[0002] Energy and environmental pollution issues have captured the attention of countries around the world. Reducing fossil energy consumption and improving the efficiency of renewable energy utilization are key areas of active effort. With the rapid development of renewable energy sources such as hydropower, wind power, and photovoltaics, traditional AC and UHVDC transmission technologies have shown limitations when delivering renewable energy to remote or offshore areas, making them incapable of meeting the requirements for efficient and reliable power transmission. However, the flexible and controllable VSC-HVDC technology can efficiently deliver these renewable energy sources to load centers, improving their utilization efficiency. Figure 1 Shown is the main circuit structure of the renewable energy base transmitted through the flexible DC transmission system.

[0003] When VSC-HVDC systems incorporate renewable energy or provide voltage support to weak AC grids, the outer loop typically employs a constant AC voltage control method. When providing AC voltage support to renewable energy bases in remote areas or offshore platforms, the converter station itself typically generates a rated 50Hz AC voltage to supply the renewable energy base. On the other hand, when providing voltage support to weak AC grids, the converter station control system typically utilizes a phase-locked loop (PLL) to synchronize the AC grid and thereby couple with the weak AC grid. Regardless of the power supply method, flexible DC converter stations are subject to the risk of oscillation instability within the subsynchronous frequency range. Therefore, improving the damping performance of flexible DC converter stations within the subsynchronous frequency range to mitigate the risk of subsynchronous oscillations is crucial for improving system reliability.

[0004] Therefore, the present invention proposes a method for suppressing subsynchronous oscillations in a flexible DC transmission system. The implementation of this method is as follows: Figure 2 As shown, there are two parts, the d-axis and the q-axis, and the relevant parameters of the two parts can be the same or different. Summary of the Invention

[0005] The object of the present invention is to provide a method for suppressing subsynchronous oscillations in a flexible direct current transmission system with low cost, high reliability and good stability.

[0006] The method for suppressing subsynchronous oscillations in a flexible direct current transmission system provided by the present invention comprises the following steps:

[0007] S1. Obtaining operating parameters of the flexible HVDC system;

[0008] S2. Obtaining the d-axis current and q-axis current injected into the flexible DC converter station;

[0009] S3. The d-axis current and q-axis current obtained in step S2 are isolated from the DC component by a high-pass filter, thereby obtaining a d-axis current without a DC component and a q-axis current without a DC component;

[0010] S4. The d-axis current without a DC component and the q-axis current without a DC component obtained in step S3 are filtered out by a low-pass filter to remove the high-frequency component, thereby obtaining a d-axis current containing only an oscillating component and a q-axis current containing only an oscillating component;

[0011] S5. The current component value obtained in step S4 is multiplied by the gain resistor and its amplitude is limited to obtain a compensation voltage for suppressing subsynchronous oscillations;

[0012] S6. The compensation voltage after the limit value obtained in step S5 is superimposed with the actual d-axis and q-axis voltages and sent to the outer loop voltage controller;

[0013] S7. Calculate the reference output voltage value of the three-phase converter according to the superimposed voltage value obtained in step S6;

[0014] S8. Input the reference output voltage value obtained in step S7 into the control system of the existing three-phase converter, thereby suppressing subsynchronous oscillation of the flexible HVDC system.

[0015] The step S2 of obtaining the d-axis current and q-axis current injected into the flexible DC converter station is specifically to obtain the instantaneous current i on the AC side of the three-phase converter. sa (t), i sb (t) and i sc (t), and the instantaneous voltage u on the AC side of the three-phase converter sa (t),u sb (t) and u sc (t), and adopt Park coordinate transformation to obtain the d-axis current i sd (t), d-axis voltage u sd (t), q-axis current i sq (t) and q-axis voltage u sq (t);i sa (t) is the instantaneous current value of phase A on the AC side of the three-phase converter, i sb (t) is the instantaneous current value of phase B on the AC side of the three-phase converter, i sc(t) is the instantaneous current value of phase C on the AC side of the three-phase converter, u sa (t) is the instantaneous voltage value of phase A on the AC side of the three-phase converter, u sb (t) is the instantaneous voltage value of phase B on the AC side of the three-phase converter, u sc (t) is the instantaneous voltage value of phase C on the AC side of the three-phase converter.

[0016] The instantaneous current and instantaneous voltage on the AC side of the three-phase converter are obtained, and the rotating coordinate transformation is performed to obtain the d-axis current, d-axis voltage, q-axis current and q-axis voltage. Specifically, the d-axis current i is calculated using the following formula: sd (t), q-axis current i sq (t), d-axis voltage u sd (t), q-axis voltage u sq (t):

[0017]

[0018]

[0019] Where i sa (t) is the instantaneous current value of phase A on the AC side of the three-phase converter; i sb (t) is the instantaneous current value of phase B on the AC side of the three-phase converter; i sc (t) is the instantaneous current value of phase C on the AC side of the three-phase converter; u sa (t) is the instantaneous voltage value of phase A on the AC side of the three-phase converter; u sb (t) is the instantaneous voltage value of phase B on the AC side of the three-phase converter; u sc (t) is the instantaneous voltage value of phase C on the AC side of the three-phase converter; θ PLL Output phase value of the phase-locked loop of the three-phase inverter.

[0020] In step S3, the d-axis current and the q-axis current obtained in step S2 are isolated from the DC component by a high-pass filter, thereby obtaining a d-axis current without a DC component and a q-axis current without a DC component. Specifically, the d-axis current i obtained in step S2 is sd (t) and q-axis current i sq (t), converted to s-domain form, i sd (s) and i sq (s) After passing through a first-order high-pass filter, the d-axis current without a DC component and the q-axis current without a DC component are obtained, and the output value y hpf_d (s) and y hpf_q (s) are:

[0021]

[0022] Where isd (s) and i sq (s) are the d-axis current i sd (t) and q-axis current i sq (t) is a physical quantity in the s domain, s is the Laplace operator, ω hpf is the bandwidth of the first-order high-pass filter.

[0023] In step S4, the d-axis current without a DC component and the q-axis current without a DC component obtained in step S3 are filtered out with a low-pass filter to remove the high-frequency component, thereby obtaining a d-axis current containing only an oscillating component and a q-axis current containing only an oscillating component. Specifically, the d-axis current without a DC component and the q-axis current without a DC component obtained in step S3 are passed through a first-order low-pass filter to obtain a d-axis current containing only an oscillating component and a q-axis current containing only an oscillating component, and the output value y 1pf_d (s) and y 1pf_q (s) are:

[0024]

[0025] Where ω lpf is the bandwidth of a first-order low-pass filter.

[0026] In step S5, the current component value obtained in step S4 is multiplied by the gain resistor and its amplitude is limited to obtain a compensation voltage for suppressing subsynchronous oscillation; specifically, the y generated in step S4 is multiplied by the gain resistor. 1pf_d (s) and y 1pf_q (s) can be obtained after equivalent gain resistance compensation. com_d (s) and u com_q (s), whose expressions are:

[0027]

[0028] Where R com is the equivalent gain resistor, F com_d (s) and F com_q (s) The equivalent transfer functions of the subsynchronous oscillation suppression method on the d-axis and q-axis, respectively. However, in order to prevent the compensation voltage from being too large during dynamic and transient processes, it is necessary to limit it to obtain the compensation voltage, where the rules are as follows:

[0029]

[0030] In the formula The maximum value of voltage compensation is 5% of the AC rated voltage. The physical quantity containing (t) represents the instantaneous value of the corresponding physical quantity in the time domain.

[0031] The compensation voltage obtained in step S5 is specifically obtained by adding the compensation voltage value obtained in step S5 to the actual d-axis voltage u sd (s) and q-axis voltage u sq (s) are superimposed and sent to the AC voltage outer loop controller, where the superposition form is:

[0032]

[0033] Where u sd (s) is the actual voltage u on the d-axis sd (t) value in the s domain, u sq (s) is the actual voltage u on the q axis sq (t) value in the s domain; u com_d (s) and u com_q (s) is composed of y 1pf_d (s) and y 1pf_q (s) passes through the equivalent gain impedance R com The voltage value obtained after compensation.

[0034] The step S7 of calculating the reference output voltage value of the three-phase converter based on the superimposed voltage obtained in step S6 specifically includes the following steps:

[0035] A. Use the following formula as the frequency domain expression of the d-axis reference current and q-axis reference current output by the outer loop control part of the three-phase converter:

[0036]

[0037] Where i * sd (s) is the d-axis reference current output by the outer loop control part of the three-phase converter, i * sq (s) is the q-axis reference current output by the outer loop control part of the three-phase converter, G uac (s) is the AC voltage controller, U * s It is the AC voltage reference value, usually the rated value.

[0038] B. Use the following formula as the time domain expression of the d-axis reference voltage and q-axis reference voltage output by the inner loop control part of the three-phase converter:

[0039]

[0040] Where u * cd (t) is the d-axis reference voltage output by the inner loop control part of the three-phase converter; u * cq(t) is the q-axis reference voltage output by the inner loop control part of the three-phase converter; k pi k is the proportional coefficient of the AC current controller of the inner loop control part of the three-phase converter, ii is the integral coefficient of the AC current controller of the inner loop control part of the three-phase converter. The control function of the AC current controller of the inner loop control part of the three-phase converter is i sd (t) is the d-axis current obtained in step S2; i sq (t) is the q-axis current obtained in step S2; ω1 is the power frequency angular frequency; L1 is the inductance value of the VSC AC side inductor.

[0041] C. Perform an inverse coordinate transformation on the time domain expressions of the d-axis reference voltage and the q-axis reference voltage output by the inner loop control part of the three-phase converter obtained in step B, thereby obtaining the expression of the reference output voltage of the three-phase converter:

[0042]

[0043] Where u * ca (t) is the reference output voltage of phase A of the three-phase converter; u * cb (t) is the reference output voltage of phase B of the three-phase converter; u * cc (t) is the reference output voltage value of phase A of the three-phase converter; θ PLL is the phase-locked loop output phase of the three-phase inverter.

[0044] The present invention provides a method for suppressing subsynchronous oscillations in a flexible direct current transmission system. The suppression method is simple and does not require additional peripheral hardware circuits. It only requires the d-axis and q-axis currents of the flexible direct current converter station to be processed by an algorithm to generate a compensation voltage. There are no excessive algorithm operation processes and no need to start a trigger signal. The method can be put into operation as the control system operates, and does not change the operating performance of the original control system under steady-state conditions. In addition, the method has certain versatility and is not only applicable to flexible direct current converter stations providing voltage support to renewable energy bases with passive properties, but also to flexible direct current converter stations providing support to weak alternating current power grids. Therefore, the present invention is low-cost, highly reliable, and has good stability. BRIEF DESCRIPTION OF THE DRAWINGS

[0045] Figure 1 This is a schematic diagram of the grid-connected transmission structure of an existing renewable energy base via a flexible DC converter station.

[0046] Figure 2 This is a schematic diagram of the control principle of the subsynchronous oscillation suppression strategy of the present invention, wherein the existing control method does not have steps ①, ②, ③, and ④.

[0047] Figure 3 Schematic diagram of the implementation steps of the subsynchronous oscillation suppression method of the present invention. DETAILED DESCRIPTION

[0048] like Figure 3 The method for suppressing subsynchronous oscillations in a flexible HVDC system provided by the present invention includes the following steps:

[0049] S1. Obtaining operating parameters of the flexible HVDC system;

[0050] S2. Obtain the d-axis current and q-axis current injected into the flexible DC converter station; specifically, obtain the instantaneous current i on the AC side of the three-phase converter sa (t), i sb (t) and i sc (t), and the instantaneous voltage u on the AC side of the three-phase converter sa (t),u sb (t) and u sc (t), and adopt Park coordinate transformation to obtain the d-axis current i sd (t), d-axis voltage u sd (t), q-axis current i sq (t) and q-axis voltage u sq (t);i sa (t) is the instantaneous current value of phase A on the AC side of the three-phase converter, i sb (t) is the instantaneous current value of phase B on the AC side of the three-phase converter, i sc (t) is the instantaneous current value of phase C on the AC side of the three-phase converter, u sa (t) is the instantaneous voltage value of phase A on the AC side of the three-phase converter, u sb (t) is the instantaneous voltage value of phase B on the AC side of the three-phase converter, u sc (t) is the instantaneous voltage value of phase C on the AC side of the three-phase converter.

[0051] Since proportional-integral control can only achieve zero steady-state error control of DC, three-phase VSCs generally control AC current in a synchronous rotating coordinate system. This requires the collected instantaneous three-phase current (i sa 、i sb 、i sc ) The AC quantity is converted into DC quantity through Park coordinate transformation; in specific implementation, the d-axis current i is calculated using the following formula sd (t), q-axis current i sq (t), d-axis voltage u sd (t) and q-axis voltage u sq (t).

[0052]

[0053]

[0054] Where i sa (t) is the instantaneous current value of phase A on the AC side of the three-phase converter; i sb (t) is the instantaneous current value of phase B on the AC side of the three-phase converter; i sc (t) is the instantaneous current value of phase C on the AC side of the three-phase converter; u sa (t) is the instantaneous voltage value of phase A on the AC side of the three-phase converter; u sb (t) is the instantaneous voltage value of phase B on the AC side of the three-phase converter; u sc (t) is the instantaneous voltage value of phase C on the AC side of the three-phase converter; θ PLL Output phase value of the phase-locked loop of the three-phase inverter.

[0055] S3. The d-axis current and the q-axis current obtained in step S2 are subjected to a high-pass filter to isolate the DC component, thereby obtaining a d-axis current without a DC component and a q-axis current without a DC component; specifically, the d-axis current and the q-axis current obtained in step S2 are subjected to a first-order high-pass filter to obtain a d-axis current without a DC component and a q-axis current without a DC component.

[0056] Since the subsynchronous oscillation suppression method cannot introduce DC components in steady state, otherwise it will change the steady-state output value of the integrator in the AC voltage outer loop controller, which may affect the transient operation characteristics of the system. Therefore, it is necessary to perform DC isolation processing on the d-axis current and q-axis current obtained in step S2. Figure 2 Only a first-order high-pass filter DC isolation processing method is given. Other DC isolation processing methods include but are not limited to: second-order and above high-pass filters, band-pass filters, notch filters, sliding average methods, etc. A preferred method of the present invention is: in the s domain form, the d-axis current i obtained in step S2 is converted to sd (s) and q-axis current i sq (s) The output value after the first-order high-pass filter is y hpf_d (s) and y hpf_q (s).

[0057] The transfer function of the first-order high-pass filter in the s domain is ω hpf is the bandwidth of the first-order high-pass filter; the d-axis current y without DC component can be obtained hpf_d (s) and the q-axis current y without DC component hpf_q (s) is:

[0058]

[0059] Where isd (s) and i sq (s) are the d-axis current i sd (t) and q-axis current i sq (t) is a physical quantity in the s domain, s is the Laplace operator, ω hpf is the bandwidth of the first-order high-pass filter.

[0060] S4. The d-axis current without a DC component and the q-axis current without a DC component obtained in step S3 are filtered out by a low-pass filter to remove the high-frequency component, thereby obtaining a d-axis current containing only an oscillating component and a q-axis current containing only an oscillating component;

[0061] The d-axis current and q-axis current of the flexible DC converter station actually obtained often contain high-order harmonic components. In order to prevent them from entering the control system and to compensate for the phase characteristics of the output impedance of the flexible DC converter station, the current components after the DC isolation treatment in step S3 need to be sent to the low-pass filter. Figure 2 Only a first-order low-pass filter processing method is given. Other low-pass filter processing methods include but are not limited to: second-order and above low-pass filters, band-stop filters, notch filters, etc.

[0062] A preferred method of the present invention is to convert the d-axis current y without DC component in step S3 into hpf_d (s) and the q-axis current y without DC component hpf_q (s), and then sent to the first-order low-pass filter respectively, so that the output value y after passing through the first-order low-pass filter can be obtained 1pf_d (s) and y 1pf_q (s).

[0063] In specific implementation, the transfer function of the first-order low-pass filter in the s domain is ω lpf is the bandwidth of the first-order low-pass filter; the d-axis current y containing only the oscillation component can be obtained 1pf_d (s) and the q-axis current y containing only the oscillatory component 1pf_q (s) is:

[0064]

[0065] Where ω lpf is the bandwidth of a first-order low-pass filter.

[0066] S5. The current component value obtained in step S4 is multiplied by the gain resistor and its amplitude is limited to obtain a compensation voltage for suppressing subsynchronous oscillations; specifically, the d-axis current y generated in step S4 containing only the oscillation component 1pf_d (s) and the q-axis current y containing only the oscillatory component 1pf_q(s) can be obtained after equivalent gain resistance compensation. com_d (s) and u com_q (s), whose expressions are:

[0067]

[0068] Where R com is the equivalent gain resistor, F com_d (s) and F com_q (s) Equivalent transfer functions of the subsynchronous oscillation suppression method on the d-axis and q-axis respectively. In order to prevent the compensation voltage from being too large during dynamic and transient processes, it is necessary to com_d (s) and u com_q (s) is limited to obtain the compensation voltage, where the rules are as follows:

[0069]

[0070] Where, The maximum value of voltage compensation is 5% of the AC rated voltage. The physical quantity containing (t) represents the instantaneous value of the corresponding physical quantity in the time domain.

[0071] S6. The compensation voltage after the limit value obtained in step S5 is superimposed with the actual d-axis and q-axis voltages and sent to the outer loop voltage controller;

[0072] After the subsynchronous oscillation occurs, the power of the system will experience subsynchronous oscillation fluctuations. The essence of suppressing subsynchronous oscillations is to change the power absorbed or released by the flexible DC converter station. After the d-axis and q-axis compensation voltages are superimposed on the actual voltage, the essence is to change the output of the voltage controller, thereby changing the current inner loop reference value and current value, and finally changing the instantaneous power absorbed by the flexible DC converter station, thereby achieving the purpose of suppressing subsynchronous oscillations. Specifically, the compensation voltage value obtained in step S5 is superimposed on the actual d-axis voltage u sd and q-axis voltage u sq After superposition, the signals are sent to the AC voltage outer loop controller in the following superposition form:

[0073]

[0074] Where u sd (s) is the actual voltage u on the d-axis sd (t) value in the s domain, u sq (s) is the actual voltage u on the q axis sq (t) value in the s domain; u com_d (s) and u com_q (s) is composed of y 1pf_d (s) and y 1pf_q (s) through the equivalent gain resistor R comThe value that can be obtained after compensation.

[0075] S7. Calculate the reference output voltage value of the three-phase converter by adding the voltage values ​​obtained in step S6; specifically comprising the following steps:

[0076] A. Use the following formula as the frequency domain expression of the d-axis reference current and q-axis reference current output by the outer loop control part of the three-phase converter:

[0077]

[0078] Where i * sd (s) is the d-axis reference current output by the outer loop control part of the three-phase converter, i * sq (s) is the q-axis reference current output by the outer loop control part of the three-phase converter, G uac (s) is the AC voltage controller, U * s It is the AC voltage reference value, usually the rated value.

[0079] B. Use the following formula as the time domain expression of the d-axis reference voltage and q-axis reference voltage output by the inner loop control part of the three-phase converter:

[0080]

[0081] Where u * cd (t) is the d-axis reference voltage output by the inner loop control part of the three-phase converter; u * cq (t) is the q-axis reference voltage output by the inner loop control part of the three-phase converter; k pi k is the proportional coefficient of the AC current controller of the inner loop control part of the three-phase converter, ii is the integral coefficient of the AC current controller of the inner loop control part of the three-phase converter. The control function of the AC current controller of the inner loop control part of the three-phase converter is i sd (t) is the d-axis current obtained in step S2; i sq (t) is the q-axis current obtained in step S2; ω1 is the power frequency angular frequency; L1 is the inductance value of the VSC AC side inductor.

[0082] C. Perform an inverse coordinate transformation on the time domain expressions of the d-axis reference voltage and the q-axis reference voltage output by the inner loop control part of the three-phase converter obtained in step B, thereby obtaining the expression of the reference output voltage of the three-phase converter:

[0083]

[0084] Where u * ca (t) is the reference output voltage of phase A of the three-phase converter; u * cb (t) is the reference output voltage of phase B of the three-phase converter; u * cc (t) is the reference output voltage of phase A of the three-phase converter; θ PLL Output phase value of the phase-locked loop of the three-phase inverter.

[0085] The method of the present invention has the following advantages:

[0086] 1. The proposed method for suppressing subsynchronous oscillations in a flexible HVDC system can effectively suppress oscillations after subsynchronous oscillations occur in the system. If this strategy is continuously implemented, the risk of subsynchronous oscillations occurring in the system can be effectively reduced.

[0087] 2. The proposed subsynchronous oscillation suppression method is simple and does not require additional peripheral hardware circuits. It only requires the d-axis and q-axis currents of the flexible DC converter station to be processed by the algorithm to generate a compensation voltage. There are no excessive algorithm operation processes and it has the advantage of being easy to implement.

[0088] 3. The proposed subsynchronous oscillation suppression method does not require a trigger signal and can be put into operation as the control system operates. In the steady state, it does not change the operating performance of the original control system.

[0089] 4. The proposed subsynchronous oscillation suppression method has certain versatility. It is not only applicable to the flexible DC converter station providing voltage support to the passive renewable energy base, but also applicable to the flexible DC converter station providing support to the weak AC power grid.

Claims

1. A method for suppressing subsynchronous oscillations in a flexible HVDC system, comprising the following steps: S1. Obtaining operating parameters of the flexible HVDC system; S2. Obtaining the d-axis current and q-axis current injected into the flexible DC converter station; S3. The d-axis current and q-axis current obtained in step S2 are isolated from the DC component by a high-pass filter, thereby obtaining a d-axis current without a DC component and a q-axis current without a DC component; S4. The d-axis current without a DC component and the q-axis current without a DC component obtained in step S3 are filtered out by a low-pass filter to remove the high-frequency component, thereby obtaining a d-axis current containing only an oscillating component and a q-axis current containing only an oscillating component; S5. The d-axis current containing only the oscillating component and the q-axis current containing only the oscillating component obtained in step S4 are multiplied by the gain resistor and their amplitudes are limited to obtain a compensation voltage for suppressing subsynchronous oscillations; S6. The compensation voltage after the limit value obtained in step S5 is superimposed with the actual d-axis and q-axis voltages and sent to the outer loop voltage controller; S7. Calculate the reference output voltage value of the three-phase converter according to the superimposed voltage value obtained in step S6; S8. Input the reference output voltage value obtained in step S7 into the control system of the existing three-phase converter, thereby suppressing subsynchronous oscillation of the flexible HVDC system.

2. The method according to claim 1, characterized in that The step S2 of obtaining the d-axis current and q-axis current injected into the flexible DC converter station is specifically to obtain the instantaneous current i on the AC side of the three-phase converter. sa (t), i sb (t) and i sc (t), and the instantaneous voltage u on the AC side of the three-phase converter sa (t),u sb (t) and u sc (t), and adopt Park coordinate transformation to obtain the d-axis current i sd (t), d-axis voltage u sd (t), q-axis current i sq (t) and q-axis voltage u sq (t);i sa (t) is the instantaneous current value of phase A on the AC side of the three-phase converter, i sb (t) is the instantaneous current value of phase B on the AC side of the three-phase converter, i sc (t) is the instantaneous current value of phase C on the AC side of the three-phase converter, u sa (t) is the instantaneous voltage value of phase A on the AC side of the three-phase converter, u sb (t) is the instantaneous voltage value of phase B on the AC side of the three-phase converter, u sc (t) is the instantaneous voltage value of phase C on the AC side of the three-phase converter.

3. The method according to claim 2, characterized in that The step S2 of obtaining the d-axis current and q-axis current injected into the flexible DC converter station is specifically to calculate the d-axis current i using the following formula: sd (t), q-axis current i sq (t), d-axis voltage u sd (t), q-axis voltage u sq (t): Where i sa (t) is the instantaneous current value of phase A on the AC side of the three-phase converter; i sb (t) is the instantaneous current value of phase B on the AC side of the three-phase converter; i sc (t) is the instantaneous current value of phase C on the AC side of the three-phase converter; u sa (t) is the instantaneous voltage value of phase A on the AC side of the three-phase converter; u sb (t) is the instantaneous voltage value of phase B on the AC side of the three-phase converter; u sc (t) is the instantaneous voltage value of phase C on the AC side of the three-phase converter; θ PLL Output phase value of the phase-locked loop of the three-phase inverter.

4. The method according to claim 3, characterized in that In step S3, the d-axis current and the q-axis current obtained in step S2 are subjected to a high-pass filter to isolate the DC component, thereby obtaining a d-axis current without a DC component and a q-axis current without a DC component. Specifically, the d-axis current and the q-axis current obtained in step S2 are subjected to a first-order high-pass filter to isolate the DC component, thereby obtaining a d-axis current without a DC component and a q-axis current without a DC component.

5. The method according to claim 4, characterized in that The d-axis current and q-axis current obtained in step S2 are isolated from the DC component by a high-pass filter, thereby obtaining a d-axis current without a DC component and a q-axis current without a DC component. Specifically, the d-axis current i obtained in step S2 is sd (t) and q-axis current i sq (t), converted to s-domain form, i sd (s) and i sq (s) After passing through a first-order high-pass filter, the d-axis current without a DC component and the q-axis current without a DC component are obtained, and the output value y hpf_d (s) and y hpf_q (s) are: Where i sd (s) and i sq (s) are the d-axis current i sd (t) and q-axis current i sq (t) is a physical quantity in the s domain, s is the Laplace operator, ω hpf is the bandwidth of the first-order high-pass filter.

6. The method according to claim 5, characterized in that In step S4, the d-axis current without a DC component and the q-axis current without a DC component obtained in step S3 are filtered out with a low-pass filter to remove the high-frequency component, thereby obtaining a d-axis current containing only an oscillating component and a q-axis current containing only an oscillating component. Specifically, the d-axis current without a DC component and the q-axis current without a DC component obtained in step S3 are passed through a first-order low-pass filter to obtain a d-axis current containing only an oscillating component and a q-axis current containing only an oscillating component, and the output value y 1pf_d (s) and y 1pf_q (s) are: Where ω lpf is the bandwidth of a first-order low-pass filter.

7. The method according to claim 6, characterized in that Step S5 is specifically: the y generated in step S4 is 1pf_d (s) and y 1pf_q (s) can be obtained after equivalent gain resistance compensation. com_d (s) and u com_q (s), whose expressions are: Where R com is the equivalent gain resistor, F com_d (s) and F com_q (s) are the equivalent transfer functions of the subsynchronous oscillation suppression method on the d-axis and q-axis, respectively. However, in order to prevent the compensation voltage from being too large during dynamic and transient processes, it is necessary to limit it to obtain the compensation voltage, where the rules are as follows: In the formula is the maximum value of voltage compensation; the physical quantity containing (t) represents the instantaneous value of the corresponding physical quantity in the time domain.

8. The method according to claim 7, characterized in that The compensation voltage obtained in step S5 is specifically obtained by adding the compensation voltage value obtained in step S5 to the actual d-axis voltage u sd (s) and q-axis voltage u sq (s) are superimposed and sent to the AC voltage outer loop controller, where the superposition form is: Where u sd (s) is the actual voltage u on the d-axis sd (t) value in the s domain, u sq (s) is the actual voltage u on the q axis sq (t) value in the s domain; u com_d (s) and u com_q (s) is composed of y 1pf_d (s) and y 1pf_q (s) passes through the equivalent gain impedance R com The voltage value obtained after compensation.

9. The method according to claim 8, characterized in that The step S7 of calculating the reference output voltage value of the three-phase converter based on the superimposed voltage obtained in step S6 specifically includes the following steps: A. Use the following formula as the frequency domain expression of the d-axis reference current and q-axis reference current output by the outer loop control part of the three-phase converter: Where i * sd (s) is the d-axis reference current output by the outer loop control part of the three-phase converter, i * sq (s) is the q-axis reference current output by the outer loop control part of the three-phase converter, G uac (s) is the AC voltage controller, U * s is the AC voltage reference value; B. Use the following formula as the time domain expression of the d-axis reference voltage and q-axis reference voltage output by the inner loop control part of the three-phase converter: Where u * cd (t) is the d-axis reference voltage output by the inner loop control part of the three-phase converter; u * cq (t) is the q-axis reference voltage output by the inner loop control part of the three-phase converter; k pi k is the proportional coefficient of the AC current controller of the inner loop control part of the three-phase converter, ii is the integral coefficient of the AC current controller of the inner loop control part of the three-phase converter. The control function of the AC current controller of the inner loop control part of the three-phase converter is G i (s)= i sd (t) is the d-axis current obtained in step S2; i sq (t) is the q-axis current obtained in step S2; ω1 is the power frequency angular frequency; L1 is the inductance of the VSC AC side inductor; C. Perform an inverse coordinate transformation on the time domain expressions of the d-axis reference voltage and the q-axis reference voltage output by the inner loop control part of the three-phase converter obtained in step B, thereby obtaining the expression of the reference output voltage of the three-phase converter: Where u * ca (t) is the reference output voltage of phase A of the three-phase converter; u * cb (t) is the reference output voltage of phase B of the three-phase converter; u * cc (t) is the reference output voltage of phase A of the three-phase converter; θ PLL Output phase value of the phase-locked loop of the three-phase inverter.

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