CN111313438A - High-frequency oscillation suppression method and system for flexible direct current transmission system - Google Patents

High-frequency oscillation suppression method and system for flexible direct current transmission system Download PDF

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CN111313438A
CN111313438A CN202010246922.6A CN202010246922A CN111313438A CN 111313438 A CN111313438 A CN 111313438A CN 202010246922 A CN202010246922 A CN 202010246922A CN 111313438 A CN111313438 A CN 111313438A
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internal potential
path
modulation signal
signal
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CN111313438B (en
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周剑
林磊
刘洪涛
韦超
朱建行
谢惠藩
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Huazhong University of Science and Technology
China Southern Power Grid Co Ltd
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Huazhong University of Science and Technology
China Southern Power Grid Co Ltd
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    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
    • H02J3/00—Circuit arrangements for ac mains or ac distribution networks
    • H02J3/24—Arrangements for preventing or reducing oscillations of power in networks
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
    • H02J3/00—Circuit arrangements for ac mains or ac distribution networks
    • H02J3/36—Arrangements for transfer of electric power between ac networks via a high-tension dc link
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
    • H02J3/00—Circuit arrangements for ac mains or ac distribution networks
    • H02J3/36—Arrangements for transfer of electric power between ac networks via a high-tension dc link
    • H02J2003/365—Reducing harmonics or oscillations in HVDC
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
    • Y02E60/60—Arrangements for transfer of electric power between AC networks or generators via a high voltage DC link [HVCD]

Abstract

The invention discloses a high-frequency oscillation suppression method and a high-frequency oscillation suppression system for a flexible direct current transmission system. The invention adds a first-order low-pass filter link to the end voltage feedforward link of the fundamental frequency current loop to suppress the high-frequency oscillation, and adds a first-order high-pass filtering link and a virtual resistance adjusting link modulation channel on the basis of the suppression scheme, separates the high-frequency component in the current forward channel of the fundamental frequency current controller and combines the virtual resistance to perform amplitude modulation, so as to counteract the high-frequency component of the current signal of the PI channel. Compared with the original suppression measures, the method can greatly reduce the risk of high-frequency oscillation of the system and solve the problem that the original suppression scheme cannot achieve the suppression effect under certain working conditions.

Description

High-frequency oscillation suppression method and system for flexible direct current transmission system
Technical Field
The invention belongs to a high-frequency oscillation suppression method and a high-frequency oscillation suppression system of a flexible direct current transmission system in the field of flexible direct current transmission, and particularly relates to a high-frequency oscillation suppression method and a high-frequency oscillation suppression system of the flexible direct current transmission system based on an additional current control branch.
Background
As a power transmission technology based on a voltage source type converter and a controllable turn-off device, flexible direct current power transmission is rapidly developed and applied by virtue of various power transmission advantages. The Modular Multilevel Converter (MMC) is used as important component equipment of a high-capacity flexible direct-current power transmission system, has the advantages of flexible structure, strong controllability, good output waveform quality and the like, and is widely applied to large-scale, long-distance and asynchronous networking power transmission projects
In the operation process of the flexible direct current converter, due to the rapid control characteristic of power electronic equipment, the connected alternating current system is easy to generate broadband unstable oscillation phenomena including low-frequency oscillation, sub-super-synchronous oscillation and high-frequency oscillation. Reports on low-frequency and sub-supersynchronous oscillations have become common, while the problem of high-frequency oscillations has gained more attention in recent years. High-frequency oscillation can not only reduce system stability and cause a large range of accidents, but also bring great challenges to equipment safety in a power grid due to large voltage and current stress.
The method mainly comprises two methods for inhibiting the high-frequency oscillation of the flexible direct current transmission system at present, wherein one method is to add a low-pass filter to a terminal voltage feedforward link of a fundamental frequency current loop to inhibit the high-frequency oscillation, but the inhibition effect of the method is not obvious under partial working conditions; another method is to add a filter device on the ac side to suppress high frequency oscillation, which will increase the construction cost of the MMC.
In summary, it is an urgent need to provide an optimal suppression measure for high-frequency oscillation of a flexible-direct power transmission system.
Disclosure of Invention
Aiming at the defects of the prior art, the invention aims to provide a high-frequency oscillation suppression method for a flexible direct current transmission system. The invention solves the optimization suppression measure of the high-frequency oscillation of the flexible direct current transmission system, thereby making up the defects of the existing suppression measure.
The invention also aims to provide a system for the high-frequency oscillation suppression method of the flexible direct current transmission system, which is simple and convenient and does not need to increase the MMC construction cost.
In order to achieve the purpose, the invention provides a high-frequency oscillation suppression method for a flexible direct current transmission system, which comprises the following steps:
s1) determining a current reference signal under rotation dq coordinates of an MMC equipment base frequency current controller in the 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) current signals under the rotating dq coordinate are taken, and a second path of internal potential modulation signals are obtained through a first-order high-pass filtering link and a virtual resistance adjusting link;
s5) voltage signals under the rotating dq coordinate are taken, and a third path of internal potential modulation signals are obtained through a first-order low-pass filtering link;
s6) obtaining a comprehensive internal potential modulation signal from the three paths of internal potential modulation signals in S3-S5, and performing reverse park coordinate transformation on the comprehensive internal potential modulation signal to obtain an internal potential modulation signal under a static coordinate;
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 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 compared with the action of the original control system output modulation signal.
The system used by the high-frequency oscillation suppression method of the flexible direct current transmission system comprises the following steps:
a. the actual MMC equipment equivalent module 1/(R + Ls) and Z(s) represent the result of actual equipment after mathematic;
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 systemdref、iqref；
c. The park coordinate transformation and reverse park coordinate transformation module is used for converting 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-voltage signals id、iq、ud、uq；
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 controllerdref、iqrefAnd d and q axis current signals i obtained by samplingd、iqThe output is used as a first path of internal potential modulation signal through a PI link;
f. first-order high-pass filtering section GhpfFor filtering out signal components below a set cut-off frequency;
g. virtual modulation resistor rvirFor modulating the gain of the channel signal;
h. delayed central equivalent link e-sTdThe 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 id、iqIs processed by a first-order high-pass filtering step GhpfAnd a virtual modulation resistance rvirObtaining a second path of internal potential modulation signal;
j. first-order low-pass filtering element GlpfFor 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 voltage signals ud、uqThrough a first-order low-pass filtering step GlpfObtaining 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.
Through the technical scheme of the invention, compared with the prior art, the invention has the following beneficial technical effects:
1) compared with the first existing suppression measure, the method adds a first-order low-pass filter link to the terminal voltage feedforward link of the fundamental frequency current loop to suppress the high-frequency oscillation, adds a first-order high-pass filtering link and a virtual resistance adjusting link modulation channel on the basis of the suppression scheme, separates the high-frequency component in the current forward channel of the fundamental frequency current controller, and combines a virtual resistance to perform amplitude modulation to counteract the high-frequency component of the PI channel current signal. Compared with the original suppression measures, the method can greatly reduce the risk of high-frequency oscillation of the system and solve the problem that the original suppression scheme cannot achieve the suppression effect under certain working conditions;
2) compared with the second existing suppression measure, the high-frequency oscillation suppression method has the advantages that the filtering device is additionally arranged on the alternating current side to suppress the high-frequency oscillation, and the MMC construction cost is not required to be increased according to the suppression scheme.
Drawings
Fig. 1 is a flow chart of a high-frequency oscillation suppression method for a flexible direct current transmission system provided by the invention;
FIG. 2 is a three-phase MMC rig and its equivalent AC network structure diagram;
fig. 3 is a block diagram of a system for suppressing high-frequency oscillation of a flexible direct current transmission system according to the present invention;
FIG. 4 is a graph of the waveform of three-phase AC current at the valve side of an MMC valve and its Fourier decomposition result when a system of an object under study generates high-frequency oscillation under a certain working condition;
FIG. 5 shows a waveform of three-phase AC voltage at the valve side of an MMC valve when a system of an object under study generates high-frequency oscillation under a certain working condition and a Fourier decomposition result thereof;
FIG. 6 is a steady AC current waveform of the system with different filter parameters under the high-frequency oscillation suppression measure of the first flexible DC power transmission system;
FIG. 7 shows the steady-state AC current waveform of the system with different filter parameters under the high frequency oscillation suppression provided by the present invention.
Detailed Description
In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention is described in further detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention.
In order to achieve the above object, the present invention provides a method for suppressing high-frequency oscillation of a flexible dc power transmission system, as shown in fig. 1, including the following steps:
s1) determining a current reference signal under rotation dq coordinates of an MMC equipment base frequency current controller in the flexible direct current power transmission system;
specifically, the adopted control mode is direct current control, 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 and obtained by the required transmission power.
S2) sampling to obtain alternating current voltage of the MMC equipment and performing park conversion to obtain voltage and current signals under a rotating dq coordinate;
specifically, the alternating current refers to the alternating current on the valve side of the MMC equipment, and the alternating voltage sampling point is arranged on the valve side or the network side of the three-phase transformer.
As shown in fig. 2, the structure diagram of a three-phase MMC physical equipment and its equivalent ac network includes:
a. ideal dc voltage source UdcProviding direct-current voltage support for MMC equipment;
b. the system MMC is provided with a physical structure which comprises a bridge arm reactance L and an equivalent resistance R;
c. transformer equivalent impedance Z and network equivalent impedance Znet；
d. An ideal three-phase AC power supply;
e. device valve side alternating current voltage sampling point iabc、uabc。
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, wherein the current signal difference value is the difference between the current reference signal and a current signal;
specifically, the following calculation formula is satisfied:
Figure BDA0002434206730000051
wherein idrefAnd iqrefRespectively representing d-axis and q-axis current reference values, i, of the fundamental frequency current controllerdAnd iqRespectively representing d-axis and q-axis current sampling values of a base frequency current controller, and it should be noted that the d-axis and q-axis current sampling values are values after park conversion and are concentrated in an equivalent link e after system delay-sTdThe voltage sample values obtained in the following step are also the same, edref1And eqref1Respectively 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 value 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 a PI link, and outputting kiAnd kpRespectively represent PI link integral and proportional adjustment coefficients, and the adjustment coefficients of the d-axis controller and the q-axis controller can take the same values.
S4) current signals under the rotating dq coordinate are taken, and a second path of internal potential modulation signals are obtained through first-order high-pass filtering and virtual resistance adjustment links;
specifically, the generation of the path of signal includes the following two steps:
s41) taking d-axis and q-axis current signals, and performing first-order high-pass filtering, wherein the step separates high-frequency components above the set cut-off frequency of the d-axis and q-axis currents through the high-pass filtering, and the following calculation formula is satisfied:
Figure BDA0002434206730000061
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:
Figure BDA0002434206730000062
in the above calculation formula edref2And eqref2Respectively representing the reference values, omega, of the second path signals of the d and q axes of the internal potential modulation signalcFor a set first-order high-pass filtering element GhpfCut-off frequency, rvirAnd modulating the amplitude of the channel high-frequency signal to generate a second path of signal reference value of an internal potential modulation signal d and a q axis for a virtual modulation resistor. 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 signalvirThe value is mainly referred to the gain of the PI channel to the high-frequency component of the current signal.
S5) voltage signals under the rotating dq coordinate are taken, and a third path of internal potential modulation signals are obtained through a first-order low-pass filtering link;
specifically, the cut-off frequency of the first-order low-pass filter element may be the same as the cut-off frequency of the first-order high-pass filter element in S41.
Specifically, the steps S3 to S5 may not be in the order.
S6) obtaining a comprehensive internal potential modulation signal from the three paths of internal potential modulation signals in S3-S5, and performing reverse park coordinate transformation on the comprehensive internal potential modulation signal to obtain an internal potential modulation signal under a static coordinate;
specifically, the three paths of internal potential modulation signals obtain a comprehensive internal potential modulation signal, 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.
The park coordinate transformation and the inverse park coordinate transformation mentioned in the above steps are the most commonly used coordinate transformation for analyzing the operation of the synchronous motor at present, and are proposed by american engineers park (r.h. park) in 1929. The park transformation projects the three-phase variables a, b and c onto a rotating direct axis (d axis), a quadrature axis (q axis) and a zero axis (0 axis) perpendicular to the dq plane, namely, the abc coordinate system is transformed into the dq coordinate system, and the reverse park transformation is opposite to the direct axis.
In the application of a high-voltage large-capacity MMC flexible direct-current power transmission system, the number of modules of each bridge arm reaches hundreds, and a plurality of control links cannot be completed by a single controller. Considering the reliability of the control and protection device of the power system and the requirement of device redundancy, the links of high-voltage direct-current electric quantity sampling, pole control, valve control and the like are independently combined, the number of control and protection devices is large, and the number of communication data among different control and protection devices is large. Due to these reasons, the control link delay is difficult to be large, generally the actual measurement delay can reach hundreds of microseconds, and the centralized module e is used here-sTdRepresenting long-chain delay, T, of the systemdIs the magnitude of the delay.
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, the method provided by the invention has the advantages that the high-frequency oscillation of the system can be effectively inhibited by the action of adding the second path of internal potential modulation signal and the third path of internal potential modulation signal.
The original control system has no second path of modulation signals and no low-pass filtering link on the third path of modulation signals. The MMC equipment modulation signal comprises an upper bridge arm modulation signal and a lower bridge arm modulation signal of each phase, for each phase of the upper bridge arm modulation signal, an internal potential modulation signal of a corresponding phase is subtracted from a direct current voltage signal of 0.5 times, and for each phase of the upper bridge arm modulation signal, a direct current voltage signal of 0.5 times is added to an internal potential modulation signal of the corresponding phase.
A structural block diagram of a system used in the method for suppressing high-frequency oscillation of a flexible direct current transmission system provided by the present invention is shown in fig. 3, wherein an equipment portion corresponds to an MMC physical equipment shown in fig. 2, and is a result after mathematical transformation, specifically including:
a. the actual MMC equipment equivalent module 1/(R + Ls) and Z(s) represent the result of actual equipment after mathematic;
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 systemdref、iqref；
c. The park coordinate transformation and reverse park coordinate transformation module is used for converting 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-voltage signals id、iq、ud、uq；
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 controllerdref、iqrefAnd d and q axis current signals i obtained by samplingd、iqThe output is used as a first path of internal potential modulation signal through a PI link;
f. first-order high-pass filtering section GhpfFor filtering out signal components below a set cut-off frequency;
g. virtual modulation resistor rvirFor modulating the gain of the channel signal;
h. delayed central equivalent link e-sTdThe 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 id、iqIs processed by a first-order high-pass filtering step GhpfAnd a virtual modulation resistance rvirObtaining a second path of internal potential modulation signal;
j. first-order low-pass filtering element GlpfFor 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 voltage signals ud、uqThrough a first-order low-pass filtering step GlpfObtaining 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.
In the above steps, the signals and modules corresponding to the above system are as follows:
s1) determining a current reference signal under rotation dq coordinates of an MMC equipment base frequency current controller in the flexible direct current power transmission system; current reference signal i of a base frequency current controller as in fig. 3dref、iqref；
S2) sampling to obtain alternating current voltage of the MMC equipment and performing park conversion to obtain a voltage current signal under a rotating dq coordinate; for example, in FIG. 3, the equivalent element e is concentrated by the park transformation module and the delay-sTdD, q axis current and voltage signals i obtained laterd、iq、ud、uq；
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; as shown in fig. 3, the channel with the reference number 1 passes through the PI module and is output;
s4) taking current signals under the rotating dq coordinate, and processing the current signals through a first-order high-pass filtering link GhpfAnd a virtual modulation resistance rvirObtaining a second path of internal potential modulation signal; as shown in fig. 3 by the first-order high-pass filtering element GhpfAnd a virtual modulation resistor rvirThe channel with the rear label of 2 is output;
s5) voltage signals under the rotating dq coordinate are taken, and a third path of internal potential modulation signals are obtained through a first-order low-pass filtering link; as shown in fig. 3 by the first-order high-pass filtering element GhpfThe channel with the rear label of 3 is output;
s6) obtaining a comprehensive internal potential modulation signal from the three paths of internal potential modulation signals in S3-S5, and performing reverse park coordinate transformation on the comprehensive internal potential modulation signal to obtain an internal potential modulation signal under a static coordinate; a synthesis module and an inverse park transform module for outputting signals for channels 1, 2 and 3 as shown in FIG. 3;
s7) combining the direct-current voltage signal and the internal potential modulation signal under the static coordinate to obtain a sub-module valve modulation signal, acting on the MMC equipment, and compared with the modulation signal output by the original control system, adding the second path and the third path of internal potential modulation signal to effectively inhibit the high-frequency oscillation of the system. As shown in FIG. 3, the 1/(R + Ls) and Z(s) modules are equivalent models of MMC equipment.
According to the invention, a first-order low-pass filter is added in a terminal voltage feedforward link of a fundamental frequency current loop to suppress high-frequency oscillation, and a high-pass filtering and virtual resistance modulation channel is added on the basis of the suppression scheme, so that high-frequency components in a current forward channel of a fundamental frequency current controller are separated and amplitude modulation is carried out by combining with a virtual resistance, and the high-frequency components are used for offsetting current signals of a PI channel. Compared with the original suppression measures, the method can reduce the risk of high-frequency oscillation of the system and solve the problem that the effect of the original suppression scheme is not obvious under certain working conditions.
And finally, verifying the effect of the high-frequency oscillation suppression scheme of the flexible direct-current transmission system provided by the invention through MATLAB/SIMULINK simulation.
Fig. 4 is a graph (upper) of a three-phase alternating current waveform at the valve side of an MMC and a fourier decomposition result thereof (lower) when the flexible direct current transmission system generates high-frequency oscillation under a certain working condition, wherein the current waveform has obvious high-frequency components, and the fourier analysis result shows that the alternating current oscillation frequency is 1460 Hz;
FIG. 5 is a graph (upper) of a waveform of three-phase alternating-current voltage at the valve side of an MMC valve when a system of an object to be researched generates high-frequency oscillation under a certain working condition and a Fourier decomposition result (lower), wherein the voltage waveform also generates obvious high-frequency components, and the Fourier analysis result shows that the voltage contains the high-frequency components with the same frequency;
the system delay is set to be 450 mu s, the waveform under the steady state is subjected to Fourier analysis, so that 1460Hz high-frequency harmonic waves exist in the current and the voltage at the same time, and the system is not further dispersed due to the amplitude limiting effect of the controller and the modulation part and is finally in a stable high-frequency oscillation state.
Fig. 6 is an alternating current waveform in a steady state of the system after voltage feedforward high-frequency filtering, wherein a) is a working condition without a filter, b) is a working condition with a cut-off frequency of a voltage feedforward first-order low-pass filter of 300Hz, c) is a working condition with a cut-off frequency of a voltage feedforward first-order low-pass filter of 200Hz, and d) is a working condition with a cut-off frequency of a voltage feedforward first-order low-pass filter of 100 Hz. FIG. 7 shows the steady-state AC current waveform of the system with different filter parameters under the high-frequency oscillation suppression measure provided by the present invention: wherein, the graph a) is the working condition without putting into the filter, the graph b) is the working condition that the cut-off frequency of the voltage feedforward first-order low-pass filter and the first-order high-pass filter is 300Hz, c) is the working condition that the cut-off frequency of the voltage feedforward first-order low-pass filter and the first-order high-pass filter is 200Hz, d) is the working condition that the cut-off frequency of the voltage feedforward first-order low-pass filter and the first-order high-pass filter is 100Hz, and the approximatep+2), it can be seen that after the current high-pass filtering and virtual resistance modulation links are fed in on the basis of the original suppression measures, the system stability under the same filter cut-off frequency is obviously improved, the risk of high-frequency oscillation of the system is reduced, and the difficulty of system parameter design is also reduced. In summary, the invention provides a scheme for suppressing high-frequency oscillation of a flexible direct current transmission system, and compared with the existing suppression scheme, the stability of the system is further improved.
It will be understood by those skilled in the art that the foregoing is only a preferred embodiment of the present invention, and is not intended to limit the invention, and that any modification, equivalent replacement, or improvement made within the spirit and principle of the present invention should be included in the 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 the 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) current signals under the rotating dq coordinate are taken, and a second path of internal potential modulation signals are obtained through a first-order high-pass filtering link and a virtual resistance adjusting link;
s5) voltage signals under the rotating dq coordinate are taken, and a third path of internal potential modulation signals are obtained through a first-order low-pass filtering link;
s6) obtaining a comprehensive internal potential modulation signal from the three paths of internal potential modulation signals in S3-S5, and performing reverse park coordinate transformation on the comprehensive internal potential modulation signal to obtain an internal potential modulation signal under a static coordinate;
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 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 compared with the action of the original control system output modulation signal.
2. The method according to claim 1, wherein the control mode corresponding to the current reference signal in the dq coordinate of the 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 output of the power outer loop 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:
Figure FDA0002434206720000021
wherein idrefAnd iqrefRespectively representing d-axis and q-axis current reference values, i, of the fundamental frequency current controllerdAnd iqRespectively representing d-axis and q-axis current sampling values of a base frequency current controller, and it should be noted that the d-axis and q-axis current sampling values are values after park conversion and are concentrated in an equivalent link e after system delay-sTdThe voltage sample values obtained in the following step are also the same, edref1And eqref1Respectively 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 value 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 a PI link, and outputting kiAnd kpRespectively represent PI link integral and proportional adjustment coefficients, and 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 the step separates high-frequency components above the set cut-off frequency of the d-axis and q-axis currents through the high-pass filtering, and the following calculation formula is satisfied:
Figure FDA0002434206720000022
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:
Figure FDA0002434206720000031
in the above calculation formula edref2And eqref2Respectively representing the reference values, omega, of the second path signals of the d and q axes of the internal potential modulation signalcFor a set first-order high-pass filtering element GhpfCut-off frequency, rvirAnd modulating the amplitude of the channel high-frequency signal to generate a second path of signal reference value of an internal potential modulation signal d and a q axis for a virtual modulation resistor. 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 signalvirThe 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 in 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 the three paths of internal potential modulation signals in steps S3 to S5 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 in a stationary coordinate, wherein signs of the first path and the third path of internal potential modulation signals 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 canceling 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 module 1/(R + Ls) and Z(s) represent the result of actual equipment after mathematic;
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 systemdref、iqref；
c. The park coordinate transformation and reverse park coordinate transformation module is used for converting 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-voltage signals id、iq、ud、uq；
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 controllerdref、iqrefAnd d and q axis current signals i obtained by samplingd、iqThe output is used as a first path internal potential modulation signal through a PI link；
f. First-order high-pass filtering section GhpfFor filtering out signal components below a set cut-off frequency;
g. virtual modulation resistor rvirFor modulating the gain of the channel signal;
h. delayed central equivalent link e-sTdThe 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 id、iqIs processed by a first-order high-pass filtering step GhpfAnd a virtual modulation resistance rvirObtaining a second path of internal potential modulation signal;
j. first-order low-pass filtering element GlpfFor 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 voltage signals ud、uqThrough a first-order low-pass filtering step GlpfObtaining 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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