CN111313438B - High-frequency oscillation suppression method and system for flexible direct current transmission system - Google Patents

Abstract

The invention relates to a method and system for suppressing high-frequency oscillation of a flexible DC power transmission system. The present invention suppresses high-frequency oscillation by adding a first-order low-pass filter link to the terminal voltage feedforward link of the fundamental frequency current loop. The suppression scheme of the present invention adds a first-order high-pass filter link and a virtual resistance adjustment link modulation on the basis of it. channel, separating the high-frequency component in the current forward channel of the fundamental frequency current controller and performing amplitude modulation in combination with a virtual resistor, which is used to offset the high-frequency component of the PI channel current signal. Compared with the original suppression measures, the present invention can greatly reduce the risk of high-frequency oscillation in the system, and solve the problem that the original suppression scheme cannot complete the suppression effect under certain working conditions.

Description

A method and system for suppressing high-frequency oscillation of a flexible direct current transmission system

technical field

The invention belongs to the flexible direct current transmission system high-frequency oscillation suppression method and system thereof in the field of flexible direct current transmission, and more specifically relates to a flexible direct current transmission system high-frequency oscillation suppression method and system based on an additional current control branch.

Background technique

As a power transmission technology based on voltage source converters and controllable shutdown devices, flexible direct current transmission has been rapidly developed and applied by virtue of its many advantages in power transmission. Modular Multilevel Converter (MMC), as an important component of large-capacity flexible DC transmission system, has many advantages such as flexible structure, strong controllability, and good output waveform quality. It is widely used in asynchronous grid transmission projects

During the operation of flexible DC converters, due to the fast control characteristics of power electronic equipment, the connected AC system is prone to broadband unstable oscillations, including low-frequency oscillations, sub-supersynchronous oscillations, and high-frequency oscillations. Reports on low-frequency and sub-supersynchronous oscillations have become more common, and high-frequency oscillations have also received more attention in recent years. High-frequency oscillation will not only reduce system stability and cause large-scale accidents, but at the same time, large voltage and current stress will bring greater challenges to the safety of equipment in the power grid.

At present, there are two main methods for suppressing high-frequency oscillations of flexible DC transmission systems. One is to suppress high-frequency oscillations by adding a low-pass filter to the terminal voltage feedforward link of the fundamental frequency current loop. The lower suppression effect is not obvious; another way is to suppress high-frequency oscillation by adding a filter device on the AC side, which will increase the construction cost of the MMC.

To sum up, it is an urgent problem to propose an optimal suppression measure for the high-frequency oscillation of the flexible direct transmission system.

Contents of the invention

Aiming at the deficiencies of the prior art, the present invention aims to propose a method for suppressing high-frequency oscillation of a flexible direct current transmission system. The invention solves the optimal restraint measures for high-frequency oscillation of the flexible direct current transmission system, thereby making up for the deficiency of the existing restraint measures.

Another object of the present invention is to provide a system used in the high-frequency oscillation suppression method of the flexible DC transmission system. The system used in the present invention is simple and convenient without increasing the construction cost of the MMC.

In order to achieve the above object, the present invention proposes a method for suppressing high-frequency oscillation of a flexible direct current transmission system, comprising the following steps:

S1) Determine the current reference signal under the rotating dq coordinates of the MMC equipment fundamental frequency current controller in the flexible direct current transmission system;

S2) Sampling and obtaining the AC current and voltage of the MMC equipment, and performing park transformation to obtain the voltage and current signals under the rotating dq coordinates;

S3) Take the current reference signal and the current signal difference under the rotating dq coordinates, and through the PI adjustment link, obtain the first internal potential modulation signal;

S4) Take the current signal under the rotating dq coordinates, and pass through the first-order high-pass filter link and the virtual resistance adjustment link to obtain the potential modulation signal in the second circuit;

S5) Take the voltage signal under the rotating dq coordinates, and obtain the potential modulation signal in the third circuit through a first-order low-pass filtering link;

S6) Obtain a comprehensive internal potential modulation signal from the three internal potential modulation signals in S3 to S5, and perform inverse park coordinate transformation on the comprehensive internal potential modulation signal to obtain the internal potential modulation signal under the static coordinates;

S7) Combining the DC voltage signal and the internal potential modulation signal under the static coordinates to obtain the MMC equipment modulation signal, which acts on the MMC equipment. Compared with the original control system output modulation signal, the effect of adding the second and third internal potential modulation signals will be Effectively suppress high-frequency oscillation of the system.

The system used in the high-frequency oscillation suppression method of the flexible direct current transmission system of the present invention includes:

a. The actual MMC equipment equivalent module 1/(R+Ls), Z(s) represents the result of the actual equipment mathematization;

b. The reference signal acquisition module is used to acquire the base frequency current controller d and q axis current reference signals _idref and _iqref of the MMC equipment in the flexible direct current transmission system;

c. Park coordinate transformation and reverse park coordinate transformation module, used to convert a mathematical transformation of the coordinate system where the variable is located;

d. The actual signal acquisition module is used to obtain the three-phase AC voltage and current on the MMC valve side through sampling, and perform park coordinate transformation to obtain d and _{q axis current and voltage signals id, i q ,} u _d , u _q ;

e. The first internal potential modulation signal generation module takes the difference between the d and q axis current reference signals i _dref and i _qref of the fundamental frequency current controller and the d and q axis current signals i _d and i _q obtained by sampling, and passes through In the PI link, the output is used as the potential modulation signal in the first circuit;

f. The first-order high-pass filter link G _hpf is used to filter out signal components below the set cut-off frequency;

g. The virtual modulation resistor r _vir is used to modulate the gain of the channel signal;

h. Delay centralized equivalent link e ^-sTd is used for equivalent delay of all parts of the system;

i. The second internal potential modulation signal generation module takes the d and q axis current signals i _d and i _q , and obtains the second internal potential modulation signal through the first-order high-pass filter G _hpf and the virtual modulation resistor r _vir ;

j. The first-order low-pass filter link G _lpf is used to filter out signal components above the set cut-off frequency;

k. The third-channel internal potential modulation signal generating module takes the d and q-axis voltage signals u _d and u _q , and obtains the third internal potential modulation signal through the first-order low-pass filter link G _lpf ;

l. The signal synthesis calculation module synthesizes the obtained three-way internal potential modulation signals, wherein the symbols of the first and third internal potential modulation signals are determined in conjunction with the control system itself, and the symbols of the second internal potential modulation signal are the same as the first The sign of the modulated signal is the same as that of the channel modulation signal, which is used to cancel the high frequency component of the current signal;

m. Modulation signal acquisition module, the valve modulation signal acting on the MMC equipment is acquired from the internal potential modulation signal and DC voltage signal obtained through reverse park transformation, and the addition of the second and third internal potential modulation signals will effectively suppress the system High frequency oscillation.

Through the above technical solutions of the present invention, compared with the prior art, the present invention has the following beneficial technical effects:

1) Compared with the first existing suppression measure, the present invention suppresses high-frequency oscillation by adding a first-order low-pass filter to the terminal voltage feedforward link of the fundamental frequency current loop, and the suppression scheme of the present invention adds an additional The first-order high-pass filter link and the virtual resistance adjustment link modulate the channel, separate the high-frequency component in the current forward channel of the fundamental frequency current controller and perform amplitude modulation in combination with the virtual resistance, which is used to offset the high-frequency component of the PI channel current signal. Compared with the original suppression measures, the present invention can greatly reduce the risk of high-frequency oscillation in the system, and solve the problem that the original suppression scheme cannot complete the suppression effect under certain working conditions;

2) Compared with the second existing suppression measure, the present invention suppresses high-frequency oscillation by adding a filter device on the AC side, and the suppression scheme of the present invention does not need to increase the construction cost of the MMC.

Description of drawings

Fig. 1 is a flow chart of a method for suppressing high-frequency oscillation of a flexible direct current transmission system provided by the present invention;

Figure 2 is a structural diagram of three-phase MMC equipment and its equivalent AC network;

Fig. 3 is a structural block diagram of a system used in a method for suppressing high-frequency oscillation of a flexible direct current transmission system provided by the present invention;

Fig. 4 is the MMC valve side three-phase alternating current waveform and its Fourier decomposition result figure when high-frequency oscillation occurs in the system of the research object of the present invention under a certain working condition;

Fig. 5 is the MMC valve side three-phase AC voltage waveform and its Fourier decomposition result when high-frequency oscillation occurs in the system under a certain working condition of the research object of the present invention;

Figure 6 shows the steady-state AC current waveforms of the system with different filter parameters under the first existing high-frequency oscillation suppression measure of the flexible DC transmission system;

Fig. 7 is the steady-state AC current waveform of the system with different filter parameters under the high-frequency oscillation suppression measures provided by the present invention.

Detailed ways

In order to make the object, technical solution and advantages of the present invention more clear, the present invention will be further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described here are only used to explain the present invention, not to limit the present invention.

In order to achieve the above purpose, the present invention provides a method for suppressing high-frequency oscillation of a flexible direct current transmission system, as shown in Figure 1, comprising the following steps:

S1) Determine the current reference signal under the rotating dq coordinates of the MMC equipment fundamental frequency current controller in the flexible direct current transmission system;

Specifically, the control method adopted is direct current control, using double closed-loop control or fundamental frequency current single-loop control. Under double closed-loop control, the power outer loop output can be used as the current reference value of the current inner loop. Loop control, the current loop reference value is directly calculated from the required transmission power.

S2) Sampling and obtaining the AC current voltage of the MMC equipment and performing park transformation to obtain the voltage and current signals under the rotating dq coordinates;

Specifically, the AC current here specifically refers to the AC current on the valve side of the MMC equipment, and the AC voltage sampling point is set on the valve side or grid side of the three-phase transformer.

As shown in Figure 2, it is a three-phase MMC physical equipment and its equivalent AC network structure diagram, including:

a. The ideal DC voltage source U _dc provides DC voltage support for MMC equipment;

b. The physical structure of the system MMC equipment, including bridge arm reactance L and equivalent resistance R;

c. Transformer equivalent impedance Z and network equivalent impedance Z _net ;

d. Ideal three-phase AC power supply;

e. Equip valve side AC current and voltage sampling points i _abc and u _abc .

S3) Take the current reference signal and the current signal difference under the rotating dq coordinates, and obtain the first internal potential modulation signal through the PI adjustment link, where the current signal difference is the difference between the current reference signal and the current signal;

Specifically, the following calculation formula is satisfied:

Among them, i _dref and i _qref respectively represent the d and q axis current reference values of the base frequency current controller, and i _d and i _q represent the d and q axis current sampling values of the base frequency current controller respectively. It should be noted that here d , The sampled value of the q-axis current is the value after the park transformation and then passed through the equivalent link e ^-sTd of the system delay concentration, the same is true for the sampled value of the voltage in the following steps, e _dref1 and e _qref1 represent the internal potential modulation signals d, q respectively The reference value of the first channel signal of the axis is obtained from the difference between the current reference signal of the d and q axes and the current signals of the d and q axes obtained by sampling, and is obtained through the output of the PI link, and _ki and k _p respectively represent the integral and proportional adjustment coefficients of the PI link , the adjustment coefficients of the d and q axis controllers can take the same value here.

S4) Take the current signal under the rotating dq coordinates, and obtain the potential modulation signal in the second circuit through the first-order high-pass filter and the virtual resistance adjustment link;

Specifically, the generation of the signal includes the following two steps:

S41) Take the d-axis and q-axis current signals and pass through a first-order high-pass filter. This step separates the high-frequency components above the cut-off frequency of the d-axis and q-axis currents through high-pass filtering, and satisfies the following calculation formula:

S42) Gain modulation is performed on the internal potential modulation signal d obtained in S41, and the second signal reference value intermediate signal of the q axis, satisfying the following calculation formula:

In the above calculation formula, e _dref2 and e _qref2 respectively represent the reference value of the internal potential modulation signal d and the second signal of the q axis, ω _c is the set first-order high-pass filter link G _hpf cut-off frequency, r _vir is the virtual modulation resistor, for The channel high-frequency signal amplitude is modulated to generate the reference value of the second channel signal of the internal potential modulation signal d and q axis. The main function of this channel is to offset the high-frequency components of the first signal of the internal potential modulation signal d and q-axis. The value of the virtual modulation resistor r _vir mainly refers to the gain of the high-frequency component of the current signal by the PI channel.

S5) Take the voltage signal under the rotating dq coordinates, and obtain the potential modulation signal in the third circuit through a first-order low-pass filtering link;

Specifically, the cutoff frequency of the first-order low-pass filter may be the same as the cutoff frequency of the first-order high-pass filter in S41.

Specifically, the above steps S3 to S5 may be in no particular order.

S6) Obtain a comprehensive internal potential modulation signal from the three internal potential modulation signals in S3 to S5, and perform inverse park coordinate transformation on the comprehensive internal potential modulation signal to obtain the internal potential modulation signal under the static coordinates;

Specifically, the integrated internal potential modulation signal is obtained from the three internal potential modulation signals, wherein the signs of the first internal potential modulation signal and the third internal potential modulation signal are determined in combination with the control system itself, and the second internal potential modulation signal symbol is the same as the first internal potential modulation signal The same sign, used to cancel the high frequency components in the current signal.

The park coordinate transformation and inverse park coordinate transformation mentioned in the above steps are currently the most commonly used coordinate transformation for analyzing the operation of synchronous motors, which was proposed by American engineer R.H.Park in 1929. The park transformation projects the a, b, and c three-phase variables onto the rotating direct axis (d axis), the quadrature axis (q axis) and the zero axis (0 axis) perpendicular to the dq plane, that is, the abc coordinate system is transformed to the dq coordinates system, the inverse park transformation is the opposite.

In the application of high-voltage and large-capacity MMC flexible DC transmission system, the number of modules in each bridge arm reaches hundreds, and multiple links of control cannot be completed by a single controller. Considering the reliability of power system control and protection devices and the requirements of device redundancy, high-voltage DC power sampling, pole control, valve control and other links are all separately grouped into screens, with many control and protection devices and communication data between different control and protection devices. These reasons make it difficult for the control link delay to be large. Generally, the measured delay can reach hundreds of microseconds. Here, the centralized module e ^-sTd is used to represent the system long link delay, and T _d is the delay.

S7) Combine the DC voltage signal and the internal potential modulation signal under the static coordinates to obtain the MMC equipment modulation signal, which acts on the MMC equipment. Compared with the original control system output modulation signal, the present invention adds the second and third internal potential modulation signals. The function will effectively suppress the high-frequency oscillation of the system.

The original control system has no second modulation signal, and there is no low-pass filter link on the third modulation signal channel. The above-mentioned MMC equipment modulation signal includes the modulation signal of the upper bridge arm of each phase and the modulation signal of the lower bridge arm. For the modulation signal of the upper bridge arm of each phase, it can be obtained by subtracting the internal potential modulation signal of the corresponding phase from the 0.5 times the DC voltage signal. For each phase The modulation signal of the upper bridge arm can be obtained by adding 0.5 times the DC voltage signal to the internal potential modulation signal of the corresponding phase.

The structural block diagram of the system used in the high-frequency oscillation suppression method of a flexible DC transmission system provided by the present invention is shown in Figure 3, wherein the equipment part corresponds to the MMC physical equipment shown in Figure 2, which is the result of its mathematicization, specifically include:

a. The actual MMC equipment equivalent module 1/(R+Ls), Z(s) represents the result of the actual equipment mathematization;

b. The reference signal acquisition module is used to acquire the base frequency current controller d and q axis current reference signals _idref and _iqref of the MMC equipment in the flexible direct current transmission system;

c. Park coordinate transformation and reverse park coordinate transformation module, used to convert a mathematical transformation of the coordinate system where the variable is located;

d. The actual signal acquisition module is used to obtain the three-phase AC voltage and current on the MMC valve side through sampling, and perform park coordinate transformation to obtain d and _{q axis current and voltage signals id, i q ,} u _d , u _q ;

e. The first internal potential modulation signal generation module takes the difference between the d and q axis current reference signals i _dref and i _qref of the fundamental frequency current controller and the d and q axis current signals i _d and i _q obtained by sampling, and passes through In the PI link, the output is used as the potential modulation signal in the first circuit;

f. The first-order high-pass filter link G _hpf is used to filter out signal components below the set cut-off frequency;

g. The virtual modulation resistor r _vir is used to modulate the gain of the channel signal;

h. Delay centralized equivalent link e ^-sTd is used to equivalent delay of all parts of the system;

i. The second internal potential modulation signal generation module takes the d and q axis current signals i _d and i _q , and obtains the second internal potential modulation signal through the first-order high-pass filter G _hpf and the virtual modulation resistor r _vir ;

j. The first-order low-pass filter link G _lpf is used to filter out signal components above the set cut-off frequency;

k. The third-channel internal potential modulation signal generating module takes the d and q-axis voltage signals u _d and u _q , and obtains the third internal potential modulation signal through the first-order low-pass filter link G _lpf ;

l. The signal synthesis calculation module synthesizes the obtained three-way internal potential modulation signals, wherein the symbols of the first and third internal potential modulation signals are determined in conjunction with the control system itself, and the symbols of the second internal potential modulation signal are the same as the first The sign of the modulated signal is the same as that of the channel modulation signal, which is used to cancel the high frequency component of the current signal;

m. Modulation signal acquisition module, the valve modulation signal acting on the MMC equipment is acquired from the internal potential modulation signal and DC voltage signal obtained through reverse park transformation, and the addition of the second and third internal potential modulation signals will effectively suppress the system High frequency oscillation.

In the above steps, the signals and modules corresponding to the above systems are as follows:

S1) Determine the current reference signal under the rotating dq coordinates of the base frequency current controller equipped with the MMC in the flexible direct current transmission system; as shown in Fig. 3, the current reference signals _idref and i _qref of the base frequency current controller;

S2) Sampling and obtaining the AC current and voltage of MMC equipment and performing park transformation to obtain the voltage and current signals under the rotating dq coordinates; as shown in Figure 3, the d and q axis current and voltage signals obtained after the park transformation module and the delay concentration equivalent link e ^-sTd i _d , i _q , u _d , u _q ;

S3) Take the current reference signal and the current signal difference under the rotating dq coordinates, and through the PI adjustment link, obtain the potential modulation signal in the first road; as shown in Figure 3, the output channel labeled 1 after passing through the PI module;

S4) Take the current signal under the rotating dq coordinates, pass through the first-order high-pass filter link G _hpf and the virtual modulation resistor r _vir , and obtain the potential modulation signal in the second circuit; as shown in Figure 3, pass through the first-order high-pass filter link G _hpf and the virtual modulation resistor The channel labeled 2 after r _vir is output;

S5) Get the voltage signal under the rotating dq coordinates, and through the first-order low-pass filter link, obtain the potential modulation signal in the third road; as shown in Figure 3, after passing the first-order high-pass filter link G _hpf , the channel output with the label 3;

S6) The integrated internal potential modulation signal is obtained from the three internal potential modulation signals in S3 to S5, and the integrated internal potential modulation signal is transformed into inverse park coordinates to obtain the internal potential modulation signal under the static coordinates; , 3-channel output signal synthesis module and anti-park conversion module;

S7) Combining the DC voltage signal and the internal potential modulation signal under the static coordinates to obtain the sub-module valve modulation signal, which acts on the MMC equipment. Compared with the original control system output modulation signal, the second and third internal potential modulation signals are added. It will effectively suppress the high frequency oscillation of the system. As shown in Figure 3, the 1/(R+Ls) and Z(s) modules are equivalent models of MMC equipment.

The present invention suppresses high-frequency oscillation by adding a first-order low-pass filter to the terminal voltage feedforward link of the fundamental-frequency current loop. This suppression scheme adds a high-pass filter and a virtual resistance modulation channel on the basis of it, and separates the fundamental-frequency current control. The high-frequency component in the forward channel of the device current is combined with the virtual resistance for amplitude modulation, which is used to cancel the high-frequency component of the PI channel current signal. Compared with the original suppression measures, the present invention can reduce the risk of high-frequency oscillation in the system, and solve the problem that the original suppression scheme has no obvious effect under certain working conditions.

Finally, through MATLAB/SIMULINK simulation, the effect of a high-frequency oscillation suppression scheme for a flexible direct current transmission system provided by the present invention is verified.

Figure 4 shows the three-phase AC current waveform (top) and its Fourier decomposition results (bottom) on the side of the MMC valve when high-frequency oscillation occurs in the flexible DC transmission system under a certain working condition. The current waveform has obvious high-frequency components. The result of Lie analysis shows that the oscillation frequency of AC current is 1460Hz;

Fig. 5 is the MMC valve side three-phase AC voltage waveform (top) and its Fourier decomposition result figure (bottom) when high-frequency oscillation occurs in the system under a certain working condition of the research object of the present invention, and the voltage waveform also appears obvious high frequency Component, Fourier analysis results show that the voltage contains high-frequency components of the same frequency;

The system delay is set to 450μs, and the Fourier analysis of the waveform in the steady state shows that there are 1460Hz high-frequency harmonics in the current and voltage at the same time. Due to the limiting effect of the controller and the modulation part, the system does not diverge further and is finally stable. High frequency oscillation state.

Figure 6 is the AC current waveform in the steady state of the system after the voltage feedforward high-frequency filter is put into use, where Figure a) is the working condition without using the filter, and Figure b) is the cut-off frequency of the voltage feedforward first-order low-pass filter is 300Hz Working condition, c) is the working condition where the cut-off frequency of the voltage feedforward first-order low-pass filter is 200Hz, and d) is the working condition where the cutoff frequency of the voltage feedforward first-order low-pass filter is 100Hz. In this working condition, The high-frequency oscillation of the system can only be suppressed when the cut-off frequency of the voltage feedforward first-order low-pass filter is very low. Combined with the actual working conditions, on the one hand, the cut-off frequency of the filter should not be set too low; The filter cutoff frequency will cause the system dynamic characteristics to deteriorate and affect its normal operation. Fig. 7 is under the high-frequency oscillation suppression measure provided by the present invention, the system steady-state AC current waveform when different filter parameters: where Fig. a) is the working condition without putting in the filter, and Fig. b) is the voltage feed-forward first-order low-pass Filter and first-order high-pass filter cut-off frequency is 300Hz, c) is the working condition of voltage feed-forward first-order low-pass filter and first-order high-pass filter cut-off frequency is 200Hz, d) is voltage feed-forward first-order The cut-off frequency of the low-pass filter and the first-order high-pass filter is 100Hz, and the approximate value of the virtual resistance is (k _p +2). The stability of the system at the cutoff frequency of the filter is significantly improved, which reduces the risk of high-frequency oscillation in the system and reduces the difficulty of system parameter design. In summary, the present invention provides a high-frequency oscillation suppression scheme for a flexible direct current transmission system, and compared with existing suppression schemes, the system stability is further improved.

It is easy for those skilled in the art to understand that the above descriptions are only preferred embodiments of the present invention, and are not intended to limit the present invention. Any modifications, equivalent replacements and improvements made within the spirit and principles of the present invention, All should be included within the protection scope of the present invention.

Claims (10)

1. A high-frequency oscillation suppression method for a flexible direct current transmission system is characterized by comprising the following steps:

s1) determining a current reference signal under rotation dq coordinates of an MMC equipment base frequency current controller in a flexible direct current power transmission system;

s2) sampling to obtain alternating current and voltage of the MMC equipment, and performing park conversion to obtain voltage and current signals under a rotating dq coordinate;

s3) obtaining a current reference signal and a current signal difference value under the rotating dq coordinate, and obtaining a first path of internal potential modulation signal through a PI (proportional integral) regulation link;

s4) taking a current signal under the rotating dq coordinate, and obtaining a second path of internal potential modulation signal through a first-order high-pass filtering link and a virtual resistance adjusting link;

s5) taking a voltage signal under the rotating dq coordinate, and obtaining a third path of internal potential modulation signal through a first-order low-pass filtering link;

s6) obtaining a comprehensive internal potential modulation signal by three paths of internal potential modulation signals in S3-S5, and performing inverse park coordinate transformation on the comprehensive internal potential modulation signal to obtain an internal potential modulation signal under a static coordinate;

and S7) combining the direct-current voltage signal and the internal potential modulation signal under the static coordinate to obtain an MMC equipment modulation signal, acting on the MMC equipment, and compared with the original control system for outputting the modulation signal, adding the second path and the third path of internal potential modulation signal to effectively inhibit the high-frequency oscillation of the system.

2. The method according to claim 1, wherein the control mode corresponding to the current reference signal in the dq coordinate of rotation of the fundamental frequency current controller is direct current control, and double closed-loop control or fundamental frequency current single-loop control is adopted, under the double closed-loop control, the power outer-loop output can be used as a current inner-loop current reference value, and if only the fundamental frequency current single-loop control is considered, the current loop reference value is directly calculated from the required transmission power.

3. The method according to claim 1, wherein the sampling is performed to obtain the MMC equipment ac current voltage and perform park transformation, wherein the ac current is specifically the MMC equipment valve side ac current, and the ac voltage sampling point is set on the three-phase transformer valve side or grid side.

4. The method for suppressing the high-frequency oscillation of the flexible direct current transmission system according to claim 1, wherein a first path internal potential modulation signal is obtained by taking a current reference signal and a current signal difference value under a rotating dq coordinate and passing through a PI (proportion integration) regulation link, and the following calculation formula is specifically satisfied:

wherein i _dref And i _qref Respectively representing d-axis and q-axis current reference values, i, of the fundamental frequency current controller _d And i _q Respectively representing fundamental frequency current controlD-axis and q-axis current sampling values, wherein the d-axis and q-axis current sampling values are values after park conversion and are subjected to a system delay concentration equivalent link e ^-sTd The voltage sample values obtained in the following step are also the same, e _dref1 And e _qref1 Respectively representing the reference values of the first paths of signals of the d and q axes of the internal potential modulation signal, obtaining the difference values of the current reference signals of the d and q axes and the current signals of the d and q axes obtained by sampling through the PI link output, and k _i And k _p The adjustment coefficients of the d-axis controller and the q-axis controller can take the same values.

5. The method for suppressing the high-frequency oscillation of the flexible direct current transmission system according to claim 1, wherein the current signal under the rotating dq coordinate is taken and subjected to first-order high-pass filtering and virtual resistance regulation links to obtain a second path of internal potential modulation signal, and the generation of the second path of internal potential modulation signal comprises the following two steps:

s41) taking d-axis and q-axis current signals, and performing first-order high-pass filtering, wherein high-frequency components above a cutoff frequency are set for the d-axis and q-axis currents in the step of separating the d-axis and q-axis currents through the high-pass filtering, and the following calculation formula is satisfied:

s42) carrying out gain modulation on the internal potential modulation signal d obtained in the S41 and the q-axis second-path signal reference value intermediate signal, and satisfying the following calculation formula:

e in the above calculation formula _dref2 And e _qref2 Respectively represent the reference values of the second path signals of the d and q axes of the internal potential modulation signal, omega _c For a set first-order high-pass filtering element G _hpf Cut-off frequency, r _vir Modulating the amplitude of the channel high-frequency signal to generate internal electricity for the virtual modulation resistorA potential modulation signal d and q axis second path signal reference value; the channel mainly acts as a virtual modulation resistor r for counteracting high-frequency components in the first path of signals of the d and q axes of the internal potential modulation signal _vir The value is mainly referred to the gain of the PI channel to the high-frequency component of the current signal.

6. The method according to claim 1, wherein the voltage signal at the rotating dq coordinate is taken and is subjected to a first-order low-pass filtering to obtain a third internal potential modulation signal, wherein the cutoff frequency of the first-order low-pass filtering can be the same as the cutoff frequency of the first-order high-pass filter in S41.

7. The method according to any one of claims 1 to 6, wherein in steps S3 to S5, the three paths of internal potential modulation signals obtain a comprehensive internal potential modulation signal, and the comprehensive internal potential modulation signal is subjected to inverse park coordinate transformation to obtain an internal potential modulation signal under a static coordinate, wherein signs of the first path of internal potential and the third path of internal potential modulation signal are determined by the control system, and the sign of the second path of internal potential modulation signal is the same as that of the first path of internal potential modulation signal and is used for offsetting high-frequency components in the current signal.

8. The method according to claim 7, wherein the park coordinate transformation is a projection of the a, b, c three-phase variables onto a direct axis (d axis) of rotation, a quadrature axis (q axis) and a zero axis (0 axis) perpendicular to the dq plane, i.e. the abc coordinate system is transformed into the dq coordinate system, and the inverse park transformation is the inverse thereof.

9. The method according to claim 7, wherein the action of the additional second and third internal potential modulation signals is effective to suppress system high frequency oscillations compared to the modulation signal output by the original control system, where the original control system has no second modulation signal and the third modulation signal has no low pass filtering element in its path.

10. A system used in a high-frequency oscillation suppression method of a flexible direct current transmission system is characterized by comprising the following steps:

a. the actual MMC equipment equivalent modules 1/(R + Ls) and Z(s) represent the result of actual equipment after mathematization;

b. a reference signal obtaining module for obtaining the d and q axis current reference signals i of the MMC equipment base frequency current controller in the flexible DC power transmission system _dref 、i _qref ；

c. The park coordinate transformation and anti-park coordinate transformation module is used for transforming a mathematical transformation of a coordinate system where the variable is located;

d. the actual signal acquisition module is used for acquiring three-phase alternating current voltage current at the valve side of the MMC through sampling and performing park coordinate transformation to obtain d-axis and q-axis current and voltage signals i _d 、i _q 、u _d 、u _q ；

e. A first path internal potential modulation signal generation module for obtaining d and q axis current reference signals i of the base frequency current controller _dref 、i _qref And d and q axis current signals i obtained by sampling _d 、i _q The output is used as a first path of internal potential modulation signal through a PI link;

f. first-order high-pass filtering section G _hpf The filter is used for filtering signal components below a set cut-off frequency;

g. virtual modulation resistor r _vir For modulating the gain of the channel signal;

h. delayed central equivalent link e ^-sTd The method is used for delaying all parts of an equivalent system;

i. the second path of internal potential modulation signal generation module takes d and q axis current signals i _d 、i _q Is processed by a first-order high-pass filtering step G _hpf And a virtual modulation resistance r _vir Obtaining a second path of internal potential modulation signal;

j. first-order low-pass filtering element G _lpf For filtering out signal components above a set cut-off frequency;

k. a third path of internal potential modulation signal generation module for obtaining d and q axis voltagesSignal u _d 、u _q Through a first-order low-pass filtering step G _lpf Obtaining a third path of internal potential modulation signal;

the signal synthesis calculation module is used for synthesizing the obtained three paths of internal potential modulation signals, wherein the signs of the first path of internal potential and the third path of internal potential modulation signals are determined by combining the control system, and the sign of the second path of internal potential modulation signals is the same as that of the first path of internal potential modulation signals and is used for offsetting high-frequency components in the current signals;

and the modulation signal acquisition module is used for acquiring a valve modulation signal acting on the MMC equipment by using the internal potential modulation signal obtained through inverse park conversion and the direct-current voltage signal, and effectively inhibiting the high-frequency oscillation of the system by adding the action of the second path of internal potential modulation signal and the third path of internal potential modulation signal.

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