CN113468718A - Method for extracting key influence factors of high-frequency negative damping of modular multilevel converter - Google Patents
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Abstract
The invention discloses a method for extracting key influence factors of high-frequency negative damping of a modular multilevel converter, which comprises the following steps: establishing a detailed impedance model of an MMC alternating current side under constant power control, wherein the impedance model comprises an MMC main circuit, a controller, a positive and negative sequence separation algorithm, voltage feedforward, control delay and other links; analyzing the impedance characteristics of the high-frequency band based on the impedance model to obtain a high-frequency negative damping frequency range, namely a potential high-frequency resonance risk frequency band; and (3) providing a concept of parameter damping sensitivity, analyzing the damping sensitivity in the negative damping frequency band, and extracting key factors influencing the high-frequency negative damping characteristic. The extraction method provided by the invention can be used for quantitatively analyzing the influence of each parameter on the high-frequency negative damping characteristic, extracting the key factors with larger influence degree in a specific frequency band, and has a larger application value for the high-frequency resonance inhibition analysis of the MMC.
Description
Technical Field
The invention relates to the technical field of power electronics, in particular to a method for extracting key influence factors of high-frequency negative damping of a modular multilevel converter.
Background
In recent years, the modularized multi-level converter has the advantages of low switching frequency, high waveform quality, good controllability, easiness in expansion and the like, is widely applied to occasions of offshore wind power direct current output, alternating current power grid interconnection and the like, and the flexible direct current transmission technology based on the modularized multi-level converter is rapidly developed due to the advantages of convenience in pressurization and capacity expansion, strong system fault ride-through capability, weak power grid networking, island power supply and the like. However, in practical engineering, the high frequency oscillation phenomenon is frequent, and therefore, it is necessary to analyze the mechanism of the generation of the high frequency oscillation in order to propose an effective oscillation suppression strategy.
The existing research has shown that the main reason for generating the high-frequency oscillation phenomenon after the modular multilevel converter is connected to the grid is that the grid impedance presents a partial capacitive area in a high-frequency band when considering a long transmission line, while the ac-side impedance of the modular multilevel converter presents a periodic negative resistance inductance in the high-frequency band, and the grid impedance and the ac-side impedance may generate the high-frequency oscillation phenomenon after being connected to each other. In order to analyze the mechanism of the high-frequency oscillation phenomenon in detail, most of the existing researches analyze the intersection point of the amplitude-frequency characteristic of the impedance and the phase margin of the corresponding phase-frequency characteristic from the perspective of the amplitude and the phase of the impedance to judge the stability of the system, and qualitatively analyze the influence of different control links and parameters based on the amplitude-frequency and phase-frequency characteristics of the impedance. At present, most researches show that links of controlling link delay and voltage feedforward have large influence on high-frequency impedance characteristics, and links of a phase-locked loop, a control outer loop, a current inner loop, a circulating current suppressor and the like have small influence on the high-frequency impedance characteristics.
However, the existing research judges the influence degree of each link parameter from a qualitative angle, can only subjectively judge the size of the curve change degree according to the impedance characteristic, cannot quantitatively analyze the influence degree between different links, and cannot accurately extract the key influence factors in a specific frequency band. In addition, there is little study to analyze the influence of negative-sequence current control on the high-frequency impedance characteristics.
Disclosure of Invention
The invention provides a method for extracting key influence factors of high-frequency negative damping of a modular multilevel converter aiming at the problems in the prior art, which can quantitatively analyze the influence of different link parameters on damping characteristics and accurately extract the key influence factors in a specific frequency band.
In order to solve the technical problems, the invention is realized by the following technical scheme:
the invention provides a method for extracting key influence factors of high-frequency negative damping of a modular multilevel converter, which comprises the following steps:
s11: establishing an impedance model of an MMC alternating current side under constant power control;
s12: analyzing the impedance characteristic of the high-frequency band based on the impedance model established in S11 to obtain a high-frequency negative damping frequency range, namely a potential high-frequency resonance risk frequency band;
s13: and (5) analyzing the damping sensitivity in the negative damping frequency band obtained in the step S12, and extracting key factors influencing damping by comparing the damping sensitivity of different parameters in the high-frequency negative damping frequency band.
Preferably, the S11 specifically includes:
based on a harmonic state space method, considering the internal harmonic dynamics of the MMC, writing a small-signal harmonic state space equation of the MMC main circuit in a row mode, wherein the small-signal harmonic state space equation comprises 4 state variables of bridge arm circulation current, upper/lower bridge arm equivalent capacitance voltage and alternating current side current;
establishing a relation between a main circuit and a controller according to a modulation function and a control structure;
disturbance voltage is injected into the alternating current side, the small-signal harmonic state space equation is solved, alternating current side disturbance current is obtained, alternating current side impedance is solved, and therefore the MMC alternating current side detailed impedance model under constant power control is obtained.
Preferably, the control part of the impedance model considers links of a power outer loop, a positive/negative sequence current inner loop, a phase-locked loop, a loop current suppressor, a positive and negative sequence separation algorithm, and voltage feedforward and control delay; the positive and negative sequence separation algorithm can extract positive/negative sequence current for the inner loop control of the positive/negative sequence current.
Preferably, the S12 specifically includes: drawing a real part and an imaginary part of the MMC impedance characteristic based on the MMC alternating current side detailed impedance model; the real part numerical value represents the damping magnitude, the positive and negative of the damping are determined through the positive and negative of the real part, and a negative damping frequency band in a high frequency band is obtained and is a potential high-frequency oscillation risk frequency band.
Preferably, the S13 specifically includes: the parameter damping sensitivity is defined as:
the damping sensitivity characterizes a specific frequency point fkProcess parameter thetaiThe magnitude of the degree of influence of the change in (b) on the damping value, wherein ZrealFor the real part of the MMC impedance, i.e. damping in S12, Delta thetaiTaken as θi1% of the total amount of the particles is a slight variation of the parameter; the damping sensitivity is in the dimension of omega/theta1％Wherein theta1％The damping variable quantity is a constant dimension and is the damping variable quantity when the parameter value changes by 1%; by comparing the damping sensitivity of different parameters in the high-frequency negative damping frequency band, key factors influencing damping are extracted.
The second objective of the invention is to provide a system for extracting key influence factors of high-frequency negative damping of a modular multilevel converter, which comprises:
the model establishing module is used for establishing an impedance model of the MMC alternating current side under the control of fixed power;
a risk frequency band analysis module for analyzing the impedance characteristic of the high frequency band based on the impedance model established by the model establishing module to obtain a high frequency negative damping frequency range, namely a potential high frequency resonance risk frequency band;
and the damping sensitivity analysis module is used for carrying out damping sensitivity analysis in the negative damping frequency band obtained by the risk frequency band analysis module, and extracting key factors influencing damping by comparing the damping sensitivity of different parameters in the high-frequency negative damping frequency band.
The third objective of the present invention is to provide a terminal for extracting key influencing factors of high-frequency negative damping of a modular multilevel converter, which includes a memory, a processor, and a computer program stored in the memory and capable of running on the processor, wherein the processor is configured to execute any one of the methods for extracting key influencing factors of high-frequency negative damping of a modular multilevel converter when executing the program.
A fourth object of the present invention is to provide a computer-readable storage medium, on which a computer program is stored, which when executed by a processor is configured to execute any of the above described modular multilevel converter high frequency negative damping key influencing factor extracting methods.
Compared with the prior art, the embodiment of the invention has at least one of the following advantages:
(1) according to the method and the system for extracting the key influence factors of the high-frequency negative damping of the modular multilevel converter, the high-frequency negative damping frequency band can be accurately given through the analysis of the real part and the imaginary part of the detailed impedance model, and the potential high-frequency resonance risk frequency band is determined;
(2) according to the method and the system for extracting the key influence factors of the high-frequency negative damping of the modular multilevel converter, the positive-negative sequence separation algorithm and the negative sequence current control are considered, and the influence of the negative sequence current control on the high-frequency damping characteristic can be analyzed;
(3) according to the method and the system for extracting the key influence factors of the high-frequency negative damping of the modular multilevel converter, the influence of different control parameters on the high-frequency negative damping characteristic can be quantitatively analyzed by defining the concept of parameter damping sensitivity, and the key influence factors of a high-frequency negative damping frequency band can be extracted by accurately reflecting the influence degree of the parameters according to the sensitivity value.
Drawings
Embodiments of the invention are further described below with reference to the accompanying drawings:
fig. 1 is a block diagram of a constant power control structure of a modular multilevel converter according to an embodiment of the present invention;
fig. 2 is a flowchart of a method for extracting key influence factors of high-frequency negative damping of a modular multilevel converter according to an embodiment of the present invention;
fig. 3 is a graph of ac-side high frequency damping characteristics of a modular multilevel converter according to an embodiment of the present invention;
fig. 4 is a graph of a sensitivity analysis of a high-frequency negative damping parameter of a modular multilevel converter according to an embodiment of the invention.
Detailed Description
The present invention will be described in detail with reference to specific examples. The following examples will assist those skilled in the art in further understanding the invention, but are not intended to limit the invention in any way. It should be noted that variations and modifications can be made by persons skilled in the art without departing from the spirit of the invention. All falling within the scope of the present invention. The parts not described in detail in the following embodiments can be implemented by using the prior art.
The embodiment of the invention provides a method for extracting key influence factors on high-frequency negative damping of a modular multilevel converter, which comprises the steps of establishing an impedance model on an alternating current side of an MMC under constant power control, and analyzing high-frequency band impedance characteristics based on the established impedance model to obtain a high-frequency negative damping frequency range, namely a potential high-frequency resonance risk frequency band; and then, analyzing the damping sensitivity in the obtained negative damping frequency band, and extracting key factors influencing damping by comparing the damping sensitivity of different parameters in the high-frequency negative damping frequency band. The method can quantitatively analyze the influence of each parameter on the high-frequency negative damping characteristic and extract the key factors with larger influence degree in a specific frequency band.
Referring to fig. 1 to fig. 4, the method for extracting the key influence factor of the high-frequency negative damping of the modular multilevel converter is described in detail in the present embodiment, which takes the modular multilevel converter under constant power control as an example for description.
Referring to fig. 1, the modular multilevel converter under constant power control of the present embodiment includes: the control system comprises an active/reactive outer ring, a positive sequence current ring, a negative sequence current ring, a double-frequency circulating current suppressor, a phase-locked ring, a decoupling item of current ring and circulating current suppression, a control delay and a voltage feedforward item, and PI controllers in various control links. After the signal passes through each control link, a modulation reference voltage v under dq coordinate axes is generatedsref_p、vsref_n、vcrefFinally, obtaining the modulation reference voltage v under the total three-phase coordinate through superposition of park transformation phasessabc. The embodiment is based on the control block diagram and is modularized in analysisA method for extracting key influence factors of high-frequency negative damping is provided according to the damping characteristic of an alternating current side of a multilevel converter.
Referring to fig. 2, the method for extracting key influence factors of high-frequency negative damping of the present embodiment includes the following steps:
s11: establishing a detailed impedance model of an MMC alternating current side under constant power control, wherein the impedance model comprises an MMC main circuit, a controller, a positive and negative sequence separation algorithm and a voltage feedforward and control delay link;
s12: analyzing the impedance characteristic of the high-frequency band based on the impedance model established in S11 to obtain a high-frequency negative damping frequency range, namely a potential high-frequency resonance risk frequency band;
s13: and (4) a concept of parameter damping sensitivity is provided, damping sensitivity analysis is carried out in the negative damping frequency band obtained in S12, and key factors influencing the high-frequency negative damping characteristic are extracted.
Specifically, S11 includes:
s111: based on the harmonic state space method, consider the inside harmonic developments of MMC, write the small-signal harmonic state space equation of MMC main circuit in a row, specifically do:
in the equation, a delta symbol represents a small signal component, a subscript x represents three phases of abc, a subscript s represents a numerical value of a variable in steady-state operation, and bold represents that the variable is represented in a harmonic matrix form; i.e. icx、
igxRespectively representing four state variables (bridge arm circulation, upper/lower bridge arm equivalent capacitance voltage and alternating current side current), vdcAnd vgxRespectively representing the voltage of a direct current side and the voltage of an alternating current side, R, L respectively representing the equivalent resistance and the inductance of a single bridge arm of the MMC, CarmRepresenting the equivalent capacitance of the sub-module, nux、nlxRespectively representing modulation functions (insertion indexes) of an upper bridge arm and a lower bridge arm of a single phase;
the modulation function can be expressed in terms of the respective modulation reference voltages as:
in the formula,. DELTA.vsref_px,Δvsref_nx,ΔvcrefxThe modulation reference voltages output by the positive sequence current loop, the negative sequence current loop and the double frequency circulating current suppressor under a three-phase coordinate system are respectively. Each modulation reference voltage can be represented by each state variable according to the control structure of fig. 1, so that the modulation function in the harmonic state space equation of the main circuit small signal can be replaced by the state variable;
according to the prior art, the harmonic state space equation of the system can be written as:
sX＝(A-Q)X+BU
wherein s represents the complex frequency in the frequency domain, X is the input variable (state variable) harmonic matrix, U is the output variable harmonic matrix, and A, Q, B is the coefficient harmonic matrix; the expression at steady-state after small signal linearization is:
Xp＝-(Ap-Qp)-1BpUp
in the formula Xp、UpHarmonic matrices representing small-signal input and output variables, respectively, Ap、Bp、QpAs harmonic matrices of small signal coefficients, packetsContains the linear harmonic components of small signal.
The final impedance solving method comprises the following steps: injecting small disturbance voltage V at AC sidep(embodied in U)pIn matrix), the current I on the alternating current side after disturbance voltage is injected can be obtained by solving the state space equation of the small signal harmonic wavepThen the MMC AC side impedance can be based on
The impedance is determined to vary with the frequency of the injection voltage.
S112: the impedance model control part considers links of a power outer loop, a positive/negative sequence current inner loop, a phase-locked loop, a loop current suppressor, a positive and negative sequence separation algorithm, and voltage feedforward and control delay; the positive and negative sequence separation algorithm can extract positive/negative sequence current for the inner loop control of the positive/negative sequence current. The positive and negative sequence separation algorithm specifically adopts a positive and negative sequence separation algorithm based on double synchronous coordinate system decoupling (DDSRF-PLL).
Specifically, S12 includes: analyzing the impedance characteristic of the high-frequency band based on the impedance model established in S11 to obtain a high-frequency negative damping frequency range, namely a potential high-frequency resonance risk frequency band; impedance Z in S11mmcCan be expressed in real/imaginary form: r + jX, wherein R is the real part of impedance, namely the damping of the modular multilevel converter, and jX is the imaginary part of impedance; by drawing a curve of the variation of the damping with the frequency, a high-frequency negative damping frequency range can be obtained.
Specifically, S13 includes: defining parameter damping sensitivity alpha based on high-frequency negative damping frequency band obtained by S12sensitivity(θi,fk)：
The damping sensitivity characterizes a specific frequency point fkProcess parameter thetaiThe degree of influence of the change of (c) on the damping value; wherein ZrealIs ZmmcThe real part of (i.e. damping R, Delta theta in S12)iTaken as θi1% of (A) is a parameterA minute amount of change of (a); the damping sensitivity is in the dimension of omega/theta1％Wherein theta1％The damping variable quantity is a constant dimension and is the damping variable quantity when the parameter value changes by 1%; frequency f to be analyzedkThe value range of (c) may be determined according to the negative damping frequency range in S12.
FIG. 3 shows a high-frequency damping characteristic curve of the present embodiment, and high-frequency negative damping regions are obtained according to the positive and negative of the damping, which are 892-1803 Hz and 2670-3000 Hz in the present embodiment. It should be noted that in practical engineering, there is a high-frequency resonance phenomenon of 3000Hz or more, so in this embodiment, only 3000Hz is analyzed, and a negative damping frequency band still exists above 3000 Hz.
FIG. 4 shows the sensitivity analysis of the high-frequency negative damping parameter of the present embodiment, and the negative damping frequency bands (892-1803 Hz and 2670-3000 Hz) obtained by combining the analysis of FIG. 3 can be seen that the key influence factors with large sensitivity (negative) values have control delay T within the 892-1803 Hz banddBridge arm inductance L and voltage feedforward coefficient Kf(ii) a In the 2670-3000 Hz range, the key influence factor is control delay TdThe bridge arm inductance L is consistent with the existing research and analysis. However, the analysis method can observe the key factors with larger influence degree more intuitively from the quantitative angle, and the influence of control delay on negative damping is the largest, the inductance of a bridge arm is the second order, the influence of feedforward is the smallest, and the frequency band width of the influence is gradually reduced.
In another embodiment of the present invention, a system for extracting key influence factors of high-frequency negative damping of a modular multilevel converter is further provided, including: the model establishing module is used for establishing an impedance model of the MMC alternating current side under the control of fixed power; a risk frequency band analysis module for analyzing the impedance characteristic of the high frequency band based on the impedance model established by the model establishing module to obtain a high frequency negative damping frequency range, namely a potential high frequency resonance risk frequency band; and the damping sensitivity analysis module is used for carrying out damping sensitivity analysis in the negative damping frequency band obtained by the risk frequency band analysis module, and extracting key factors influencing damping by comparing the damping sensitivity of different parameters in the high-frequency negative damping frequency band.
The system for extracting the key influence factors of the high-frequency negative damping of the modular multilevel converter can be used for realizing the method for extracting the key influence factors of the high-frequency negative damping of the modular multilevel converter. The technology for realizing each module in this embodiment may adopt the technology of the corresponding step in any embodiment of the method for extracting the key influence factor of the high-frequency negative damping of the modular multilevel converter, and is not described herein again.
In another embodiment of the present invention, a terminal for extracting key influence factors of high-frequency negative damping of a modular multilevel converter is provided, which includes a memory, a processor, and a computer program stored in the memory and capable of running on the processor, and when the processor executes the program, the processor is configured to execute the method for extracting key influence factors of high-frequency negative damping of a modular multilevel converter according to any embodiment.
In another embodiment of the present invention, a computer readable storage medium is provided, on which a computer program is stored, which when executed by a processor is configured to perform the method for extracting the high frequency negative damping key influencing factor of the modular multilevel converter according to any of the embodiments.
It should be noted that, the steps in the method provided by the present invention may be implemented by using corresponding modules, devices, units, and the like in the system, and those skilled in the art may refer to the technical solution of the system to implement the step flow of the method, that is, the embodiment in the system may be understood as a preferred example for implementing the method, and details are not described herein.
Those skilled in the art will appreciate that, in addition to implementing the system and its various devices provided by the present invention in purely computer readable program code means, the method steps can be fully programmed to implement the same functions by implementing the system and its various devices in the form of logic gates, switches, application specific integrated circuits, programmable logic controllers, embedded microcontrollers and the like. Therefore, the system and various devices thereof provided by the present invention can be regarded as a hardware component, and the devices included in the system and various devices thereof for realizing various functions can also be regarded as structures in the hardware component; means for performing the functions may also be regarded as structures within both software modules and hardware components for performing the methods.
The embodiments were chosen and described in order to best explain the principles of the invention and the practical application, and not to limit the invention. Any modifications and variations within the scope of the description, which may occur to those skilled in the art, are intended to be within the scope of the invention.
Claims (10)
1. A method for extracting key influence factors of high-frequency negative damping of a modular multilevel converter is characterized by comprising the following steps:
s11: establishing an impedance model of an MMC alternating current side under constant power control;
s12: analyzing the impedance characteristic of the high-frequency band based on the impedance model established in S11 to obtain a high-frequency negative damping frequency range, namely a potential high-frequency resonance risk frequency band;
s13: and (5) analyzing the damping sensitivity in the negative damping frequency band obtained in the step S12, and extracting key factors influencing damping by comparing the damping sensitivity of different parameters in the high-frequency negative damping frequency band.
2. The extraction method for key influence factors of high-frequency negative damping of the modular multilevel converter according to claim 1, wherein the impedance model comprises an MMC main circuit, a controller, a positive-negative sequence separation algorithm, and a voltage feedforward and control delay link.
3. The method for extracting key influence factors of high-frequency negative damping of a modular multilevel converter according to claim 2, wherein the step S11 specifically comprises:
based on a harmonic state space method, considering the internal harmonic dynamics of the MMC, writing a small-signal harmonic state space equation of the MMC main circuit in a row mode, wherein the small-signal harmonic state space equation comprises 4 state variables of bridge arm circulation current, upper/lower bridge arm equivalent capacitance voltage and alternating current side current;
establishing a relation between a main circuit and a controller according to a modulation function and a control structure;
disturbance voltage is injected into the alternating current side, the small-signal harmonic state space equation is solved, alternating current side disturbance current is obtained, alternating current side impedance is solved, and therefore the MMC alternating current side detailed impedance model under constant power control is obtained.
4. The method for extracting key influence factors of high-frequency negative damping of the modular multilevel converter according to claim 3, wherein a control part of the impedance model considers a power outer loop, a positive/negative sequence current inner loop, a phase-locked loop, a circulating current suppressor, a positive/negative sequence separation algorithm, a voltage feedforward and control delay link; the positive and negative sequence separation algorithm can extract positive/negative sequence current for the inner loop control of the positive/negative sequence current.
5. The method for extracting key influence factors of high-frequency negative damping of a modular multilevel converter according to claim 3, wherein the step S12 specifically comprises:
drawing a real part and an imaginary part of the MMC impedance characteristic based on the MMC alternating current side detailed impedance model; the real part numerical value represents the damping magnitude, the positive and negative of the damping are determined through the positive and negative of the real part, and a negative damping frequency band in a high frequency band is obtained and is a potential high-frequency oscillation risk frequency band.
6. The method for extracting key influence factors of high-frequency negative damping of a modular multilevel converter according to claim 5, wherein the step S13 specifically comprises:
defining a parameter damping sensitivity alphasensitivity(θi,fk) Comprises the following steps:
the damping sensitivity characterizes a specific frequency point fkProcess parameter thetaiThe magnitude of the degree of influence of the change in damping value, wherein,
Zrealdamping in S12, the real part of the MMC impedance;
Δθitaken as θi1% of the total amount of the particles is a slight variation of the parameter;
the damping sensitivity is in the dimension of omega/theta1％Wherein theta1％The damping variable quantity is a constant dimension and is the damping variable quantity when the parameter value changes by 1%;
by comparing the damping sensitivity of different parameters in the high-frequency negative damping frequency band, key factors influencing damping are extracted.
7. The utility model provides a key influence factor extraction system of modularization multilevel converter high frequency negative damping which characterized in that includes:
the model establishing module is used for establishing an impedance model of the MMC alternating current side under the control of fixed power;
a risk frequency band analysis module for analyzing the impedance characteristic of the high frequency band based on the impedance model established by the model establishing module to obtain a high frequency negative damping frequency range, namely a potential high frequency resonance risk frequency band;
and the damping sensitivity analysis module is used for carrying out damping sensitivity analysis in the negative damping frequency band obtained by the risk frequency band analysis module, and extracting key factors influencing damping by comparing the damping sensitivity of different parameters in the high-frequency negative damping frequency band.
8. The method for extracting high-frequency negative damping key influencing factors of the modular multilevel converter according to claim 7, wherein the damping sensitivity analysis module comprises:
defining a parameter damping sensitivity alphasensitivity(θi,fk) Comprises the following steps:
the damping sensitivity characterizes a specific frequency point fkProcess parameter thetaiThe magnitude of the degree to which the change in damping value affects,wherein,
Zrealdamping in S12, the real part of the MMC impedance;
Δθitaken as θi1% of the total amount of the particles is a slight variation of the parameter;
the damping sensitivity is in the dimension of omega/theta1％Wherein theta1％The damping variable quantity is a constant dimension and is the damping variable quantity when the parameter value changes by 1%;
by comparing the damping sensitivity of different parameters in the high-frequency negative damping frequency band, key factors influencing damping are extracted.
9. A modular multilevel converter high frequency negative damping key contributor extraction terminal comprising a memory, a processor and a computer program stored on the memory and executable on the processor, wherein the processor when executing the program is configured to perform the method of any of claims 1 to 6.
10. A computer-readable storage medium, on which a computer program is stored which, when being executed by a processor, is adapted to carry out the method of any one of claims 1 to 6.
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| CN114421494A (en) * | 2022-01-11 | 2022-04-29 | 上海交通大学 | High-frequency oscillation suppression method and system for enhanced flexible direct current transmission system |
| CN116961031A (en) * | 2023-07-31 | 2023-10-27 | 长沙理工大学 | High-frequency oscillation frequency division suppression and parameter design method for flexible direct-current transmission system |
| CN116961031B (en) * | 2023-07-31 | 2024-05-10 | 长沙理工大学 | High-frequency oscillation frequency division suppression and parameter design method for flexible direct-current transmission system |
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