CN113098244B - Bridge arm reactance unit of MMC (modular multilevel converter) - Google Patents
Bridge arm reactance unit of MMC (modular multilevel converter) Download PDFInfo
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
- H02M1/00—Details of apparatus for conversion
- H02M1/12—Arrangements for reducing harmonics from ac input or output
- H02M1/126—Arrangements for reducing harmonics from ac input or output using passive filters
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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
- H02J3/002—Flicker reduction, e.g. compensation of flicker introduced by non-linear load
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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
- H02J3/01—Arrangements for reducing harmonics or ripples
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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
- H02J3/36—Arrangements for transfer of electric power between ac networks via a high-tension dc link
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- 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
- Y02E40/00—Technologies for an efficient electrical power generation, transmission or distribution
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- 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]
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Abstract
The invention discloses a bridge arm reactance unit of an MMC transverter, which comprises: the bridge type multi-phase bridge-arm converter comprises a reactance circuit and an RC damping network circuit, wherein the first end of the reactance circuit is connected with one-phase upper bridge arm unit or one-phase lower bridge arm unit of the MMC converter, and the second end of the reactance circuit is connected with a valve-side bus of the converter transformer; the first end and the second end of the RC damping network circuit are respectively connected with the third end of the reactance circuit and correspondingly connected with the second end; the bridge arm reactance unit is used for determining component parameters of the bridge arm reactance unit based on a preset converter control method, an inductance distribution coefficient constraint condition, a harmonic impedance constraint condition and a fundamental wave impedance ratio constraint condition, so that the real part of equivalent harmonic impedance of the alternating current side of the MMC converter under a target frequency band is a positive value, the harmonic impedance characteristic of the converter is improved, the alternating current harmonic impedance of the MMC converter is effectively corrected, negative damping is eliminated, potential high-frequency harmonic oscillation is inhibited, and the dynamic control performance of the bridge arm reactance unit is kept to the maximum extent.
Description
Technical Field
The invention relates to the technical field of high-frequency harmonic oscillation suppression of a flexible direct current converter, in particular to a bridge arm reactance unit of an MMC converter.
Background
The flexible direct-current power transmission technology based on the Modular Multilevel Converter (MMC) is widely applied to the fields of new energy access, weak grid asynchronous interconnection, urban direct-current power distribution and the like, and has wide application prospects. The high-frequency harmonic oscillation problem is one of core challenges in flexible direct current transmission engineering application, and a generation mechanism of the high-frequency harmonic oscillation problem is that nyquist stability conditions are not met between alternating current network impedance and MMC alternating current side equivalent impedance under partial frequency bands, so that a small signal instability phenomenon is generated under corresponding frequency harmonic excitation, namely high-frequency harmonic oscillation occurs. Further theoretical analysis shows that the characteristic of local frequency band negative damping (the real part of impedance is less than zero) caused by the time delay of the MMC control system is a core inducement for forming a resonance risk frequency band. Aiming at the high-frequency harmonic oscillation phenomenon of a flexible-straight system, the existing solution ideas include the following three categories: the method comprises the following steps: the filter (including a linear low-pass filter and a specially designed nonlinear filter) is added in the voltage feedforward loop, the interaction effect of a high-frequency disturbing signal and a control loop is eliminated, the equivalent impedance characteristic of the MMC on the alternating current side presented in the high-frequency band is improved, but the application effect of the MMC is limited by the filter characteristic, if the traditional linear low-pass filter is adopted, the MMC is influenced by the attenuation characteristic, the impedance characteristic of the middle-frequency band (particularly the frequency band near the cut-off frequency of the filter) is possibly deteriorated while the impedance characteristic of the high-frequency band is improved, and a new oscillation frequency point is caused; if a specially designed nonlinear filter is adopted, the dynamic characteristic of the system is sacrificed to a certain extent; the second method comprises the following steps: when the resonance is detected, an active damping control strategy aiming at a resonance frequency band is adopted, but the effect is limited by the performance of a high-pass/band-pass filter selected by an active damping control loop and the self control time delay of the loop, so that the impedance characteristic of a specific frequency band is improved, the impedance characteristics of other frequency bands are possibly deteriorated, and secondary resonance is generated, thereby causing chain response; the third method comprises the following steps: the PI parameter design of the optimization control loop has the limitations that the small signal characteristics of a power grid and a current converter in actual engineering are changed along with the change of working conditions, the harmonic oscillation risk is difficult to avoid completely through theoretical analysis in a control parameter design stage, and the control parameter design is highly conservative and the dynamic characteristic of a system is sacrificed due to the consideration of excessive stability constraint.
Disclosure of Invention
Therefore, the technical problem to be solved by the invention is to overcome the defects that the dynamic characteristic of a system needs to be sacrificed and the harmonic oscillation risk cannot be avoided under the full target frequency band when a high-frequency resonance suppression circuit of a flexible direct current converter in the prior art suppresses resonance, thereby providing a bridge arm reactance unit of an MMC converter.
In order to achieve the purpose, the invention provides the following technical scheme:
the embodiment of the invention provides a bridge arm reactance unit of an MMC (modular multilevel converter), which comprises: the bridge type multi-phase bridge-arm converter comprises a reactance circuit and an RC damping network circuit, wherein the first end of the reactance circuit is connected with one-phase upper bridge arm unit or one-phase lower bridge arm unit of the MMC converter, and the second end of the reactance circuit is connected with a valve-side bus of the converter transformer; the first end and the second end of the RC damping network circuit are respectively connected with the third end of the reactance circuit and correspondingly connected with the second end; the bridge arm reactance unit is used for determining component parameters of the bridge arm reactance unit based on a preset converter control method, an inductance distribution coefficient constraint condition, a harmonic impedance constraint condition and a fundamental wave impedance ratio constraint condition, so that the real part of the equivalent harmonic impedance of the alternating current side of the MMC converter under a target frequency band is a positive value and has a positive damping characteristic.
In one embodiment, the reactive circuit comprises: the harmonic inductor and the fundamental inductor are connected in series, and then two ends of the harmonic inductor and the fundamental inductor are respectively and correspondingly connected with the upper bridge arm unit or the lower bridge arm unit of one phase and a valve side bus of the converter transformer.
In one embodiment, the harmonic inductance parameter and the fundamental inductance parameter are determined based on the inductance distribution coefficient and the total inductance of the bridge arm.
In one embodiment, the RC damping network circuit is formed by at least one RC damping network element, wherein each RC damping network element comprises: when the RC damping network circuit is composed of an RC damping network unit, two ends of the damping capacitor and the damping resistor which are connected in series are respectively connected with a connection point of a harmonic inductor and a fundamental inductor which are connected in series, and an upper bridge arm unit or a lower bridge arm unit of one phase; when the RC damping network circuit is composed of at least two RC damping network units, a first end of a plurality of damping capacitors connected in series is connected with a connection point of a harmonic inductor and a fundamental inductor connected in series, a second end of the plurality of damping capacitors connected in series is connected with an upper bridge arm unit or a lower bridge arm unit of one phase through a damping resistor, and the connection point of the plurality of damping capacitors connected in series is connected with the upper bridge arm unit or the lower bridge arm unit of one phase through a damping resistor.
In one embodiment, the inductance distribution coefficient constraint condition is:
where α is the inductance distribution coefficient, kvhMaximum harmonic content, f, of the output voltage of the MMC converterhFor the considered harmonic frequency lower limit, I, of the output voltage of the MMC converterh-limThe harmonic current effective value upper limit of the bridge arm allowed by the MMC converter.
In one embodiment, the harmonic impedance constraints are:
Re[ZMMC(s)|s＝j2πf]>0
wherein Z isMMCThe equivalent harmonic impedance of the AC side of the MMC converter is shown, and f is the target harmonic frequency.
In one embodiment, the harmonic impedance is determined by a plurality of transfer functions, harmonic inductance, fundamental inductance, damping resistance, damping capacitance, and converter transformer equivalent leakage inductance of a preset converter control method.
In one embodiment, the predetermined converter control method is a control method based on a voltage outer loop and a current inner loop, and the harmonic impedance is calculated by the following formula:
wherein G isPIIs the transfer function of the current inner loop PI regulator; gd1And Gd2Respectively reflecting the transfer functions of equivalent time delays of the current inner loop and the voltage feedforward link; gfilterA transfer function of a second-order linear low-pass filter added in the voltage feedforward loop and used for inhibiting harmonic oscillation; z is a linear or branched memberlink-eqFor equivalent connection impedance, Z, between the MMC converter and the point of common connectionLIs the complex impedance of the reactance units of the bridge arm, LTThe equivalent leakage inductance of the converter transformer is obtained.
In one embodiment, when the RC damping network unit is a first-order RC damping network circuit, the complex impedance of the reactance unit is:
wherein L isarm-aIs a harmonic inductance, Larm-bIs a fundamental wave inductance, CdTo damp the capacitance, RdThe damping resistor is n, and the order of the RC damping network circuit is n;
when the RC damping network unit is a second-order RC damping network circuit, the complex impedance of the reactance unit is:
wherein L isarm-aIs a harmonic inductance, Larm-bIs a fundamental wave inductance, CdTo damp the capacitance, RdAnd n is the order of the RC damping network circuit.
In one embodiment, the fundamental impedance ratio constraint is:
wherein beta is the fundamental impedance ratio, ZdampFor complex impedance of RC damping network，Larm-bThe inductance is a fundamental wave inductance, and A is a preset fixed numerical value;
the fundamental impedance ratio constraint is used to determine the parameters of the damping capacitance and damping resistance.
In one embodiment, when the RC damping network unit is a first-order RC damping network circuit, the complex impedance of the RC damping network is:
wherein, CdTo damp the capacitance, RdThe damping resistor is n, and the order of the RC damping network circuit is n;
when the RC damping network unit is a second-order RC damping network circuit, the complex impedance of the RC damping network is:
wherein, CdTo damp the capacitance, RdAnd n is the order of the RC damping network circuit.
The technical scheme of the invention has the following advantages:
according to the bridge arm reactance unit of the MMC converter, the first end of a reactance circuit is connected with one phase of an upper bridge arm unit or a lower bridge arm unit of the MMC converter, and the second end of the reactance circuit is connected with a valve side bus of a converter transformer; the first end and the second end of the RC damping network circuit are respectively connected with the third end of the reactance circuit and correspondingly connected with the second end; the bridge arm reactance unit is used for determining component parameters of the bridge arm reactance unit based on a preset converter control method, an inductance distribution coefficient constraint condition, a harmonic impedance constraint condition and a fundamental wave impedance ratio constraint condition, so that the real part of equivalent harmonic impedance at the alternating current side of the MMC converter under a target frequency band is a positive value and has a positive damping characteristic, the harmonic impedance characteristic of the converter is improved, the effective correction of the alternating current harmonic impedance of the MMC converter is realized, negative damping is eliminated, potential high-frequency harmonic oscillation is inhibited, and the dynamic control performance of the MMC converter is kept to the maximum extent.
Drawings
In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the embodiments or the prior art will be briefly described below, and it is obvious that the drawings in the following description are some embodiments of the present invention, and other drawings can be obtained by those skilled in the art without creative efforts.
Fig. 1 is a composition diagram of a specific example of a bridge arm reactance unit of an MMC converter according to an embodiment of the present invention;
fig. 2(a) is a specific circuit structure diagram of a specific example of a bridge arm reactance unit of an MMC converter provided in an embodiment of the present invention;
fig. 2(b) is a specific circuit structure diagram of another specific example of a bridge arm reactance unit of the MMC converter provided in the embodiment of the present invention;
fig. 3 is a block diagram of an equivalent effect of an inner loop of a current control of an MMC current converter according to an embodiment of the present invention;
fig. 4 is an equivalent circuit of the interaction of the power grid-MMC converter provided in the embodiment of the present invention;
fig. 5 is an equivalent ac harmonic impedance curve of an MMC converter (positive sequence) under different schemes provided by an embodiment of the present invention.
Detailed Description
The technical solutions of the present invention will be described clearly and completely with reference to the accompanying drawings, and it should be understood that the described embodiments are some, but not all embodiments of the present invention. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.
In the description of the present invention, it should be noted that, unless otherwise explicitly specified or limited, the terms "mounted," "connected," and "connected" are to be construed broadly, e.g., as meaning either a fixed connection, a removable connection, or an integral connection; can be mechanically or electrically connected; the two elements may be directly connected or indirectly connected through an intermediate medium, or may be connected through the inside of the two elements, or may be connected wirelessly or through a wire. The specific meanings of the above terms in the present invention can be understood in specific cases to those skilled in the art.
In addition, the technical features involved in the different embodiments of the present invention described below may be combined with each other as long as they do not conflict with each other.
Examples
The embodiment of the invention provides a bridge arm reactance unit of an MMC (modular multilevel converter), which is applied to the occasion of effectively inhibiting high-frequency harmonic oscillation on the AC side of the MMC, and as shown in figure 1, the bridge arm reactance unit 1 comprises: a reactance circuit 11, an RC damping network circuit 12.
As shown in fig. 1, a first end of a reactance circuit 11 according to the embodiment of the present invention is connected to a midpoint of an upper arm unit or a lower arm of one phase of the MMC converter, and a second end thereof is connected to a valve-side bus of the converter transformer; the first end and the second end of the RC damping network circuit 12 are respectively connected to the third end and the second end of the reactance circuit 11.
According to the embodiment of the invention, according to different requirements of high-frequency resonance suppression, the bridge arm reactance unit 1 can be used independently as an independent oscillation suppression measure, and can also be used in combination with a linear low-pass filter in a voltage feedforward loop to realize a better oscillation suppression effect.
Each bridge arm reactance unit 1 in the embodiment of the invention is used for determining component parameters based on a preset converter control method, an inductance distribution coefficient constraint condition, a harmonic impedance constraint condition and a fundamental wave impedance ratio constraint condition, so that the real part of the equivalent harmonic impedance at the alternating current side of the MMC converter under a target frequency band is a positive value and has a positive damping characteristic.
Specifically, in the embodiment of the invention, under the condition that the total inductance value of a bridge arm is kept unchanged and the requirement of a system operation interval is met, a valve side alternating-current phase voltage rated value, the maximum harmonic content of the output voltage of an MMC converter, the lower limit of the harmonic frequency of the output voltage of the MMC converter and the effective value of the harmonic current of the bridge arm allowed by the MMC converter are used as input quantities of an inductance distribution coefficient constraint condition, a transfer function, a reactance circuit parameter and an RC damping network circuit parameter of a preset converter control method are used as input quantities of a harmonic impedance constraint condition, the RC damping network circuit parameter is used as an input quantity of a fundamental wave impedance ratio constraint condition, the three constraint conditions are combined, so that the real equivalent harmonic impedance part of the alternating-current side of the MMC converter under a target frequency band is a positive value and has a positive damping characteristic, the effective correction on the alternating-current harmonic impedance of the MMC converter is realized, negative damping is eliminated, suppressing potential high frequency harmonic oscillations.
In one embodiment, as shown in fig. 2(a) and 2(b), the reactance circuit 11 includes: harmonic inductor Larm-aFundamental wave inductor Larm-bWherein the harmonic inductance Larm-aAnd fundamental wave inductance Larm-bAnd two ends of the serial connection are respectively and correspondingly connected with the upper bridge arm unit or the lower bridge arm unit of one phase and the valve side bus of the converter transformer.
The embodiment of the invention is based on the inductance distribution coefficient alpha and the total inductance L of the bridge armarmDetermining harmonic wave inductance parameter and fundamental wave inductance parameter as shown in formula (1),
Larm-a＝(1-α)Larm,Larm-b＝αLarm α∈(0,1) (1)
the embodiment of the invention keeps the total inductance L of the bridge armarmThe numerical value is unchanged, and under the condition of meeting the requirement of a system operation interval, an inductance distribution coefficient constraint condition shown in a formula (2) is set:
where α is the inductance distribution coefficient, kvhMaximum harmonic content, f, of the output voltage of the MMC converterhFor the considered harmonic frequency lower limit, I, of the output voltage of the MMC converterh-limThe harmonic current effective value upper limit of the bridge arm allowed by the MMC converter.
In the embodiment of the invention, based on general working condition conditions, the inductance distribution coefficient alpha is about 0.9, which can meet the requirement.
In an embodiment, as shown in fig. 2(a) and 2(b), the RC damping network circuit 12 of the embodiment of the present invention is formed by at least one RC damping network unit, wherein each RC damping network unit includes: damping capacitor CdDamping resistor Rd。
Specifically, as shown in fig. 2(a), when the RC damping network circuit 12 is composed of one RC damping network element, the damping capacitance CdAnd a damping resistor RdThe two ends after series connection are respectively connected with the harmonic inductor Larm-aAnd fundamental wave inductance Larm-bAnd the connecting points after the series connection are connected with the upper bridge arm unit or the lower bridge arm unit of one phase.
Specifically, as shown in fig. 2(b), when the RC damping network circuit 12 is composed of at least two RC damping network elements, a plurality of damping capacitors CdThe first end and the harmonic inductor L after series connectionarm-aAnd fundamental wave inductance Larm-bA plurality of damping capacitors C connected in seriesdThe second end of the series connection passes through a damping resistor RdA plurality of damping capacitors C connected with the upper bridge arm unit or the lower bridge arm unitdThe connecting points after the series connection are all connected through a damping resistor RdAnd is connected with the upper bridge arm unit or the lower bridge arm unit of one phase. It should be noted that the reactance circuit and the RC damping network circuit 12 are connected to the same upper arm unit or the same lower arm unit of the same phase, and the number of the RC damping network units is selected as the damping order n, which can be 1-2 orders in consideration of the damping effect and the hardware cost, and a 2-order structure can be used to obtain a better resonance damping effect. In general, the initial design may consider the n-1 structure and independent use, and if the requirement cannot be met, the damping order is increased or the voltage feedforward filtering is added to the iterative design.
Damping capacitor CdThe method is a key parameter for determining the influence degree of the additional RC damping network circuit 12 on the bridge arm of the MMC converter under the fundamental wave frequency band, and specifically can be determined by the fundamental wave impedance ratio beta, and is defined as shown in a formula (3).
Wherein Z isdampIs the complex impedance of the RC damping network, and when the RC damping network unit is a first-order RC damping network unit, ZdampThe expression of (a) is:
when the RC damping network unit is a first-order RC damping network unit, ZdampThe expression of (a) is:
under general conditions, in order to reduce the influence of the damping circuit on the normal operation of the MMC converter and reduce the fundamental frequency loss, the fundamental wave impedance ratio constraint conditions are set as follows:
it should be noted that, although the fundamental wave impedance ratio β is required to be >100 in general, the actual value range of the fundamental wave impedance ratio β is determined in actual circumstances and is not limited herein.
In an embodiment of the present invention, the harmonic impedance is controlled by a plurality of transfer functions and harmonic inductances L of the predetermined inverter control methodarm-aFundamental wave inductor Larm-bDamping resistor RdDamping capacitor CdAnd determining equivalent leakage inductance of the converter transformer, wherein the preset converter control method can be a control method based on a voltage outer ring and a current inner ring, the current control inner ring is a leading factor influencing the high-frequency harmonic impedance characteristic of the MMC converter, influence of links such as a power/voltage outer ring and a phase-locked loop is ignored, an equivalent block diagram of the current inner ring is shown in fig. 3, and the MMC converter harmonic impedance Z based on the equivalent circuit shown in fig. 4 isMMCThe expression is shown in formula (7):
wherein G isPIIs the transfer function of the current inner loop PI regulator; gd1And Gd2Respectively reflecting the transfer functions of equivalent time delays of the current inner loop and the voltage feedforward link; gfilterA transfer function of a second-order linear low-pass filter which is added into the voltage feedforward loop and used for restraining harmonic oscillation; zlink-eqThe equivalent connection impedance between the MMC current converter and the grid-connected point.
The transfer function of the current inner loop PI regulator is expressed as
Wherein k isp、kiThe proportional coefficient and the integral coefficient of the current inner loop PI controller after per unit are respectively.
The transfer function of the delay element is expressed as:
Gd1(s)＝Gd2(s)＝e-sτ (9)
wherein τ is the control link average delay time.
If a second-order linear low-pass filter is introduced into the voltage feedforward loop, the following are provided:
wherein f iscIs the cut-off frequency; if voltage feedforward filtering is not used, then there is Gfilter＝1。
Neglecting winding resistance of converter transformer, equivalent connection impedance Zlink-eqCan be expressed as:
wherein Z isLIs the complex impedance of the reactance element, LTThe equivalent leakage inductance of the converter transformer is obtained. For first-order and second-order resonance suppression circuits, the complex impedance Z of the reactance units of the bridge armsLThe damping method is respectively shown in a formula (12) and a formula (13), wherein n is the order of the RC damping network circuit.
For a conventional bridge arm resonance suppression circuit, neglecting coil resistance, ZLIs shown as
ZL(s)＝sLarm (14)
According to the formula (14), by combining specific system parameters and performing per unit processing, equivalent harmonic impedance in an MMC frequency domain can be calculated by using MATLAB, and a delay link in calculation can be processed by pad approximation.
In the embodiment of the present invention, the fundamental inductance L is calculated by the following equations (1) to (14)arm-bHarmonic inductor Larm-aDamping capacitor CdThen, different damping resistances R can be repeatedly calculated in a proper range (10 omega-500 omega can be taken) by using MATLABdSelecting the optimal R according to the requirement of high-frequency resonance suppression by the equivalent impedance characteristic of the MMC converter under the valuedValue taking is carried out to ensure that the harmonic impedance constraint condition shown in the formula (15) is met in the target frequency band, namely the alternating current equivalent harmonic impedance Z of the MMC converter in the target frequency band is ensuredMMCIs positive and as far as possible by adjusting RdValue is taken so as to be Z in target frequency bandMMCThe phase angle of the harmonic wave is far away from a + 90-degree line and a-90-degree line, so that the harmonic wave stability of the system is improved to the maximum extent, the dynamic control performance of the system is kept to the maximum extent, and the high-frequency harmonic wave oscillation is effectively inhibited.
Re[ZMMC(s)|s＝j2πf]>0 (15)
Wherein f is the target harmonic frequency, and the target frequency range set here is 50 Hz-2500 Hz.
In order to further illustrate the application effect, an application case is given by combining a certain flexibility and straightness engineering. Designing a second-order RC damping network circuit according to the method and relevant engineering parameters, wherein alpha is 0.9, Cd＝500nF，RdThe improved arm reactance unit of 300 Ω is applied in conjunction with a 400Hz second order linear low pass filter in the voltage feed forward loop, and the relevant harmonic impedance correction effect is shown in fig. 5. Wherein, the curve # 1 and the curve #4 represent impedance characteristics under the condition of using a conventional bridge arm reactor and not adopting a voltage feedforward filtering (scheme one), the curve #2 and the curve #3 represent MMC alternating current equivalent impedance characteristics when the conventional bridge arm reactor is added into a voltage feedforward loop 400Hz low-pass filter (scheme two), and the curve #3 and the curve #6 represent alternating current equivalent impedance characteristics when an improved bridge arm reactance unit is adopted and matched with the voltage feedforward loop 400Hz low-pass filter (scheme three). Therefore, the negative damping phenomenon in the 50 Hz-2500 Hz full target frequency band can be completely eliminated only by the corresponding scheme III.
In the bridge arm reactance unit of the MMC converter provided by the embodiment of the invention, the first end of the reactance circuit is connected with one phase of upper bridge arm unit or lower bridge arm unit of the MMC converter, and the second end of the reactance circuit is connected with a bus on the valve side of the converter transformer; the first end and the second end of the RC damping network circuit are respectively connected with the third end of the reactance circuit and correspondingly connected with the second end; the bridge arm reactance unit is used for determining component parameters of the bridge arm reactance unit based on a preset converter control method, an inductance distribution coefficient constraint condition, a harmonic impedance constraint condition and a fundamental wave impedance ratio constraint condition, so that the real part of equivalent harmonic impedance at the alternating current side of the MMC converter under a target frequency band is a positive value and has a positive damping characteristic, the harmonic impedance characteristic of the converter is improved, the effective correction of the alternating current harmonic impedance of the MMC converter is realized, negative damping is eliminated, potential high-frequency harmonic oscillation is inhibited, and the dynamic control performance of the MMC converter is kept to the maximum extent.
It should be understood that the above examples are only for clarity of illustration and are not intended to limit the embodiments. Other variations and modifications will be apparent to persons skilled in the art in light of the above description. And are neither required nor exhaustive of all embodiments. And obvious variations or modifications of the invention may be made without departing from the spirit or scope of the invention.
Claims (7)
1. The utility model provides a bridge arm reactance unit of MMC transverter which characterized in that includes: a reactive circuit, an RC damping network circuit, wherein,
the first end of the reactance circuit is connected with an upper bridge arm unit or a lower bridge arm unit of one phase of the MMC current converter, and the second end of the reactance circuit is connected with a bus on the valve side of the converter transformer; the first end and the second end of the RC damping network circuit are respectively connected with the third end of the reactance circuit and correspondingly connected with the second end;
the bridge arm reactance unit is used for determining component parameters of the bridge arm reactance unit based on a preset converter control method, an inductance distribution coefficient constraint condition, a harmonic impedance constraint condition and a fundamental wave impedance ratio constraint condition, so that the real part of the equivalent harmonic impedance of the alternating current side of the MMC converter under a target frequency band is a positive value and has a positive damping characteristic;
the constraint conditions of the inductance distribution coefficient are as follows:
where α is the inductance distribution coefficient, kvhMaximum harmonic content, f, of the output voltage of the MMC converterhFor the considered harmonic frequency lower limit, I, of the output voltage of the MMC converterh-limUpper limit of bridge arm harmonic current effective value, L, allowed by MMC converterarmThe total inductance of the bridge arm; vac-phNA rated value of the valve side alternating phase voltage;
the harmonic impedance constraints are as follows:
Re[ZMMC(s)|s＝j2πf]>0
wherein, ZMMCEquivalent harmonic impedance at the AC side of the MMC converter, and f is target harmonic frequency;
the preset converter control method is a control method based on a voltage outer ring and a current inner ring, and the harmonic impedance is calculated by the following formula:
wherein G isPIIs the transfer function of the current inner loop PI regulator; gd1And Gd2Respectively reflecting the transfer functions of equivalent time delays of the current inner loop and the voltage feedforward link; gfilterA transfer function of a second-order linear low-pass filter added in the voltage feedforward loop and used for inhibiting harmonic oscillation; z is a linear or branched memberlink-eqFor equivalent connection impedance, Z, between the MMC converter and the point of common connectionLIs the complex impedance of the reactance units of the bridge arm, LTEquivalent leakage inductance of the converter transformer;
the fundamental wave impedance ratio constraint conditions are as follows:
wherein beta is the fundamental impedance ratio, ZdampIs a complex impedance of RC damping network, Larm-bThe inductance is a fundamental wave inductance, and A is a preset fixed numerical value;
the fundamental impedance ratio constraints are used to determine parameters of the damping capacitance and damping resistance.
2. The MMC converter bridge arm reactance unit of claim 1, wherein said reactance circuit comprises: harmonic inductance, fundamental inductance, wherein,
and two ends of the harmonic inductor and the fundamental inductor which are connected in series are respectively and correspondingly connected with the upper bridge arm unit or the lower bridge arm unit of one phase and a valve side bus of the converter transformer.
3. The MMC converter bridge arm reactance unit of claim 2, wherein a harmonic inductance parameter, a fundamental inductance parameter are determined based on an inductance distribution coefficient, a bridge arm total inductance.
4. The MMC converter leg reactance unit of claim 2, wherein the RC damping network circuit is comprised of at least one RC damping network unit, wherein each RC damping network unit comprises: a damping capacitor, a damping resistor, wherein,
when the RC damping network circuit is composed of an RC damping network unit, two ends of the damping capacitor and the damping resistor which are connected in series are respectively connected with a connection point of a harmonic inductor and a fundamental inductor which are connected in series, and one phase upper bridge arm unit or lower bridge arm unit;
when the RC damping network circuit is composed of at least two RC damping network units, a first end of a plurality of damping capacitors connected in series is connected with a connection point of a harmonic inductor and a fundamental inductor connected in series, a second end of the plurality of damping capacitors connected in series is connected with an upper bridge arm unit or a lower bridge arm unit of one phase through a damping resistor, and the connection point of the plurality of damping capacitors connected in series is connected with the upper bridge arm unit or the lower bridge arm unit of one phase through a damping resistor.
5. The bridge arm reactance unit of an MMC converter according to claim 1, wherein said harmonic impedance is determined by a plurality of transfer functions, harmonic inductance, fundamental inductance, damping resistance, damping capacitance, converter transformer equivalent leakage inductance of said preset converter control method.
6. The MMC converter leg reactance unit of claim 1,
when the RC damping network circuit is a first-order RC damping network circuit, the complex impedance of the reactance unit is:
wherein L isarm-aIs a harmonic inductance, Larm-bIs a fundamental wave inductance, CdTo damp the capacitance, RdIs a damping resistor, n is the order of RC damping network circuit, LarmThe total inductance of the bridge arm;
when the RC damping network circuit is a second-order RC damping network circuit, the complex impedance of the reactance unit is as follows:
wherein L isarm-aIs a harmonic inductance, Larm-bIs a fundamental wave inductance, CdTo damp the capacitance, RdAnd n is the order of the RC damping network circuit.
7. The MMC converter leg reactance unit of claim 4,
when the RC damping network unit is a first-order RC damping network circuit, the complex impedance of the RC damping network is:
wherein, CdTo damp the capacitance, RdThe damping resistor is n, and the order of the RC damping network circuit is n;
when the RC damping network unit is a second-order RC damping network circuit, the complex impedance of the RC damping network is as follows:
wherein, CdTo damp the capacitance, RdAnd n is the order of the RC damping network circuit.
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