CN112865505B - Double-frequency circulating injection method for inhibiting MMC bridge arm power fluctuation under fault - Google Patents

Double-frequency circulating injection method for inhibiting MMC bridge arm power fluctuation under fault Download PDF

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CN112865505B
CN112865505B CN202110151941.5A CN202110151941A CN112865505B CN 112865505 B CN112865505 B CN 112865505B CN 202110151941 A CN202110151941 A CN 202110151941A CN 112865505 B CN112865505 B CN 112865505B
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CN112865505A (en
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许建中
邓伟成
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North China Electric Power University
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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
    • H02M1/32—Means for protecting converters other than automatic disconnection
    • 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
    • H02J3/001—Methods to deal with contingencies, e.g. abnormalities, faults or failures
    • 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
    • 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
    • H02M7/00—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output
    • H02M7/42—Conversion of dc power input into ac power output without possibility of reversal
    • H02M7/44—Conversion of dc power input into ac power output without possibility of reversal by static converters
    • H02M7/48—Conversion of dc power input into ac power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode
    • H02M7/483—Converters with outputs that each can have more than two voltages levels
    • 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 double frequency circulation injection method for inhibiting MMC bridge arm power fluctuation under a fault, which comprises the following steps: establishing an optimized objective function by taking the fundamental frequency and the double-frequency fluctuation component of the instantaneous power of the bridge arm when the alternating-current single-phase fault occurs as an inhibition target; carrying out global optimization by taking the objective function minimization as an objective to obtain the optimal amplitude and phase of double frequency circulation injection; and generating a reference signal according to the amplitude and the phase of the optimal double frequency circulation injection, and combining a modulation algorithm to realize the injection of the double frequency circulation. The method can effectively reduce the fluctuation of the capacitor voltage of the sub-module during the fault period and reduce the maximum voltage born by the MMC sub-module during the fault period.

Description

Double-frequency circulating injection method for inhibiting MMC bridge arm power fluctuation under fault
Technical Field
The invention relates to the technical field of flexible direct current power transmission, in particular to a double-frequency circulating current injection method for inhibiting the power fluctuation of an MMC bridge arm under a fault.
Background
Flexible direct current transmission (VSC-HVDC) is a new generation of direct current transmission technology following alternating current transmission, conventional direct current transmission. The flexible direct current transmission technology has the characteristics of independent active and reactive power adjustment, strong weak power grid access and low voltage ride through capability, low alternating current filtering, reactive power compensation requirements and the like. The Modular Multilevel Converter (MMC) has the characteristics of flexible control, module expansibility, low switching frequency, low harmonic content and the like. At present, MMC structures are adopted in Shanghai south-Virginia flexible direct current engineering, nanao three-terminal flexible direct current engineering, zhoushan five-terminal flexible direct current engineering, xiamen +/-320 kV flexible direct current demonstration engineering and Zhang Bei direct current power grid engineering which are built in China.
Ac single-phase earth faults are the most common and frequent type of fault occurring in electrical power systems. When an alternating-current single-phase earth fault occurs, the dynamic characteristic of the MMC can be greatly changed, the voltage fluctuation of the sub-module capacitor can be inevitably increased, and the overvoltage of a device can be generated, so that the damage of a power electronic device is caused. And the voltage fluctuation of the capacitor of the submodule is positively correlated with the power fluctuation of the bridge arm. Therefore, a method for restraining the power fluctuation of the bridge arm of the MMC under the alternating-current single-phase earth fault is explored, the reliability of the MMC is improved, and the method has important engineering significance.
Disclosure of Invention
The invention aims to provide a double-frequency circulating current injection method for inhibiting the power fluctuation of an MMC bridge arm under a fault, which can effectively inhibit the power fluctuation of the bridge arm of a converter valve, further reduce the capacitor voltage fluctuation of a submodule during the fault and reduce the maximum voltage borne by the MMC submodule during the fault.
The purpose of the invention is realized by the following technical scheme:
a double-frequency circulating current injection method for inhibiting MMC bridge arm power fluctuation under a fault is characterized by comprising the following steps:
establishing an optimized objective function by taking the fundamental frequency and the double frequency fluctuation component of the instantaneous power of the bridge arm when the alternating-current single-phase fault occurs as an inhibition target;
carrying out global optimization by taking the objective function minimization as an objective to obtain the optimal amplitude and phase of double frequency circulation injection;
and generating a reference signal according to the amplitude and the phase of the optimal double frequency circulation injection, and combining a modulation algorithm to realize the injection of the double frequency circulation.
It can be seen from the above technical solutions that the fundamental frequency and the double frequency fluctuation component of the instantaneous power are used as the suppression target, the amplitude and the phase of the injected double frequency circulating current are optimally designed, and finally the calculated double frequency circulating current is injected into the three phases by changing the modulation wave signal. The method can effectively inhibit the fluctuation of the bridge arm power of the converter valve, further reduce the fluctuation of the sub-module capacitance voltage during the fault period and reduce the maximum voltage born by the MMC sub-module during the fault period.
Drawings
In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings required to be used in the description of the embodiments are briefly introduced below, and it is obvious that the drawings in the description below are only some embodiments of the present invention, and it is obvious for those skilled in the art that other drawings can be obtained according to the drawings without creative efforts.
Fig. 1 is a flowchart of a double frequency circulating current injection method for suppressing a power fluctuation of an MMC bridge arm under a fault according to an embodiment of the present invention;
fig. 2 is a block diagram of the overall system control of the double frequency circulating injection method for suppressing the power fluctuation of the MMC bridge arm in case of a fault according to an embodiment of the present invention.
Detailed Description
The technical solutions in the embodiments of the present invention are clearly and completely described below with reference to the drawings in the embodiments of the present invention, and it is obvious that the described embodiments are only a part of the embodiments of the present invention, and not all embodiments. All other embodiments, which can be derived by a person skilled in the art from the embodiments of the present invention without making any creative effort, shall fall within the protection scope of the present invention.
The embodiment of the invention provides a double-frequency circulating current injection method for inhibiting MMC bridge arm power fluctuation under a fault, a flow chart and an overall system control block diagram are shown in figures 1-2, and the method mainly comprises the following steps:
1. and establishing an optimized objective function by taking the fundamental frequency and the double frequency fluctuation component of the instantaneous power of the bridge arm when the alternating-current single-phase fault occurs as an inhibition target.
In the embodiment of the invention, firstly, an expression of instantaneous power of a bridge arm considering double-frequency circulating injection under a single-phase earth fault is calculated, and then an optimized objective function is established.
1) And calculating an expression of the instantaneous power of the bridge arm considering double frequency circulating current injection under the condition of the single-phase earth fault.
When an alternating-current single-phase earth fault occurs (taking a-phase single-phase earth as an example), the three-phase upper bridge arm voltage is as follows:
Figure BDA0002932348830000031
wherein t is time, and omega is fundamental angular frequency; u shape dc Is a direct current voltage, U m Is the amplitude of the AC voltage u au_f 、u bu_f 、u cu_f Respectively representing the voltages of the upper bridge arms of a, b and c after the fault;
when an alternating current single-phase earth fault occurs, because a fault phase cannot transmit power, the transmission of 1/3 power is often reduced in consideration of reduced bridge arm current stress, and at the moment, the upper bridge arm current of three phases is as follows:
Figure BDA0002932348830000032
wherein, I m Is the amplitude of the alternating current,
Figure BDA0002932348830000035
is an initial phase angle, I 2 Showing the magnitude of the double frequency circulating injection,
Figure BDA0002932348830000033
represents the phase angle of the double frequency circulating current injection, m represents the modulation ratio, and the value is 2U m /U dc ，i au_f 、i bu_f 、i cu_f Respectively representing a, b and c three-phase upper bridge arm currents after the fault;
to simplify the representation, a frequency doubling circulating injection coefficient k is defined 2 Comprises the following steps:
Figure BDA0002932348830000034
when the combination of the voltage and the current of a three-phase upper bridge arm is combined with the occurrence of an alternating-current single-phase grounding fault, the expression of the instantaneous power of the three-phase upper bridge arm containing double frequency circulating current injection after the fault is obtained as follows: :
Figure BDA0002932348830000041
wherein, P au_f 、P bu_f 、P cu_f And respectively representing the instantaneous power of the upper bridge arm of the phases a, b and c after the fault.
Those skilled in the art will appreciate that an MMC consists of six arms, phase a up, phase a down, phase b up, phase b down, phase c up, and phase c down. Each bridge arm comprises N sub-modules.
2) And establishing an optimized objective function by taking the fundamental frequency and the double-frequency fluctuation component of the instantaneous power of the bridge arm when the alternating-current single-phase fault occurs as an inhibition target. The main process is as follows:
A. and obtaining the fundamental frequency and double frequency fluctuation of the instantaneous power of the upper bridge arm of the fault phase when the alternating current single-phase fault occurs according to the instantaneous power of the upper bridge arm of the three phases containing double frequency circulation injection after the fault, thereby obtaining the fluctuation amplitude.
If the fault phase is a phase a, the fundamental frequency and the double frequency fluctuation of the instantaneous power of the bridge arm on the phase a are as follows:
Figure BDA0002932348830000042
wherein, P au_f1 And P au_f2 The fundamental frequency and the double frequency fluctuation of the instantaneous power of the upper bridge arm of the phase a are respectively;
to P au_f1 And P au_f2 Performing an identity transform to obtain:
Figure BDA0002932348830000051
Figure BDA0002932348830000052
the a and B parameters of the different subscripts referred to herein are intermediate parameters.
Obtaining P according to the identity transformation result au_f1 And P au_f2 The fluctuation amplitude is:
Figure BDA0002932348830000053
B. and obtaining fundamental frequency and double frequency fluctuation of instantaneous power of the non-fault two-phase upper bridge arm by adopting the same mode, and further obtaining the fluctuation amplitude.
The fundamental frequency and double frequency fluctuation of the instantaneous power of the bridge arm on the B phase are as follows:
Figure BDA0002932348830000054
wherein, P bu_f1 And P bu_f2 Fundamental frequency and double frequency fluctuation of instantaneous power of the bridge arm on the B phase are respectively;
to P bu_f1 And P bu_f2 Performing an identity transform to obtain:
Figure BDA0002932348830000055
Figure BDA0002932348830000056
obtaining P according to the identity transformation result bu_f1 And P bu_f2 The fluctuation amplitude is:
Figure BDA0002932348830000061
the fundamental frequency and double frequency fluctuation of the instantaneous power of the bridge arm on the C phase are as follows:
Figure BDA0002932348830000062
wherein, P cu_f1 And P cu_f2 Fundamental frequency and double frequency fluctuation of the instantaneous power of the C-phase upper bridge arm are respectively;
to P cu_f1 And P cu_f2 Performing an identity transform to obtain:
Figure BDA0002932348830000063
Figure BDA0002932348830000064
obtaining P according to the identity transformation result cu_f1 And P cu_f2 The fluctuation amplitude is:
Figure BDA0002932348830000065
C. establishing a target function according to the obtained fundamental frequency of the instantaneous power of the three-phase bridge arm and the fluctuation amplitude of the double frequency fluctuation
Figure BDA0002932348830000066
Figure BDA0002932348830000067
Wherein, the first and the second end of the pipe are connected with each other,
Figure BDA0002932348830000068
|P au_f1 i and I P au_f2 I is the fundamental frequency of the instantaneous power of the bridge arm on the phase a and the fluctuation amplitude of the double frequency fluctuation respectively; i P bu_f1 I and I P bu_f2 I is the fundamental frequency of the instantaneous power of the bridge arm on the b phase and the fluctuation amplitude of the double frequency fluctuation respectively; | P cu_f1 I and I P cu_f2 And l is the fundamental frequency of the instantaneous power of the bridge arm on the c phase and the fluctuation amplitude of the double frequency fluctuation respectively.
k represents a weight coefficient of the secondary fluctuation of the capacitor voltage. Since the double frequency voltage fluctuation has less influence on the overall voltage fluctuation amplitude, the k value is set to 0.5 here. It should be noted that the value of k is not fixed, and can be adjusted appropriately according to actual conditions.
2. And carrying out global optimization by taking the objective function minimization as an objective to obtain the optimal amplitude and phase of the double frequency circulation injection.
In the embodiment of the invention, global optimization is carried out by taking objective function minimization as a target, and k is determined according to different modulation ratios m 2 、
Figure BDA0002932348830000071
Optimal values of the two variables. It should be noted that the present invention does not limit the optimization method, and the user can select the optimization method according to the actual situation.
It will be appreciated by those skilled in the art that since the objective function is based on the capacitor voltage fluctuation, its minimized solution can achieve a minimization of the capacitor voltage fluctuation, i.e., if the magnitude and phase of the optimal solution are injected, the objective can be achieved, in particular, by designing the parameter k 2 ，
Figure BDA0002932348830000072
Can realize suppression of fluctuation of the capacitor voltage.
3. And generating a reference signal according to the amplitude and the phase of the double frequency circulation injection, and combining a modulation algorithm to realize the injection of the double frequency circulation.
In the embodiment of the invention, the optimal amplitude and phase of double frequency circulating current injection are obtained, namely k is obtained 2 、
Figure BDA0002932348830000073
An optimal value, after which a reference signal i can be generated in conjunction with a reference signal generator 2fd_ref And i 2fq_ref (ii) a Wherein i 2fd_ref Reference signal, i, for d-axis double frequency circulating injection controller 2fq_ref Injecting a reference signal of the controller for q-axis double frequency circulation; the reference signal is injected into the controller through the double frequency circulation to obtain a double frequency circulation modulation voltage signal U cirj_ref Finally, the injection of the double frequency circulation is realized by changing the modulation wave of the modulation algorithm.
In the overall system control block diagram shown in FIG. 2, the positive and negative sequence dual-loop controllers in the upper dashed box are the targetA classic control scheme adopted during the front fault; wherein, T abc/dq The module is a dq transformation module; i all right angle d_refp And i q_refp Respectively as positive sequence dq axis current reference values, and generating a positive sequence differential mode voltage reference value e through dq conversion j_refp ；i d_refn And i q_refn Respectively negative sequence dq axis current reference values, and generating a negative sequence differential mode voltage reference value e through dq conversion j_refn ；I 2 And
Figure BDA0002932348830000074
showing the double frequency circulating current magnitude and phase. The lower dotted line frame shows the working process of the reference signal generator and the double frequency circulation injection controller, wherein the PI module is a proportional integral link; l is a radical of an alcohol arm Is the reactance value of the bridge arm; t is dq/abc The module is a dq inverse transformation module; i.e. i cira 、i cirb 、i circ Respectively measuring the a phase, the b phase and the c phase double frequency circulation; i all right angle 2fd And i 2fq D-axis double frequency circulation current and q-axis double frequency circulation current i 2fd_ref And i 2fq_ref Are the corresponding reference values; u. of cird_ref And u cirq_ref D-axis double-frequency circulating current modulation voltage and q-axis double-frequency circulating current modulation voltage are respectively generated into a double-frequency circulating current modulation voltage signal U through dq inverse transformation cirj_ref 。
Through the description of the above embodiments, it is clear to those skilled in the art that the above embodiments may be implemented by software, or by software plus a necessary general hardware platform. With this understanding, the technical solutions of the embodiments can be embodied in the form of a software product, which can be stored in a non-volatile storage medium (which can be a CD-ROM, a usb disk, a removable hard disk, etc.), and includes several instructions for enabling a computer device (which can be a personal computer, a server, or a network device, etc.) to execute the methods according to the embodiments of the present invention.
The above description is only for the preferred embodiment of the present invention, but the scope of the present invention is not limited thereto, and any changes or substitutions that can be easily conceived by those skilled in the art within the technical scope of the present invention are included in the scope of the present invention. Therefore, the protection scope of the present invention shall be subject to the protection scope of the claims.

Claims (5)

1. A double-frequency circulating current injection method for inhibiting MMC bridge arm power fluctuation under a fault is characterized by comprising the following steps:
establishing an optimized objective function by taking the fundamental frequency and the double frequency fluctuation component of the instantaneous power of the bridge arm when the alternating-current single-phase fault occurs as an inhibition target;
carrying out global optimization by taking the objective function minimization as an objective to obtain the optimal amplitude and phase of double frequency circulation injection;
generating a reference signal according to the amplitude and the phase of the optimal double frequency circulation injection, and combining a modulation algorithm to realize the injection of the double frequency circulation;
before establishing an optimized objective function, an expression of bridge arm instantaneous power considering double frequency circulation injection under a single-phase earth fault is calculated, and the expression comprises the following steps:
when an alternating-current single-phase earth fault occurs, the three-phase upper bridge arm voltage is as follows:
Figure FDA0003953609630000011
wherein t is time, and omega is fundamental angular frequency; u shape dc Is a direct current voltage, U m Is the amplitude of the AC voltage, u au_f 、u bu_f 、u cu_f Respectively representing the voltages of the upper bridge arms of a, b and c after the fault;
when an alternating-current single-phase earth fault occurs, the current of an upper bridge arm of three phases is as follows:
Figure FDA0003953609630000012
wherein, I dc Is a direct current, I m Is the amplitude of the alternating current,
Figure FDA0003953609630000013
is an initial phase angle, I 2 Showing the magnitude of the double frequency circulating injection,
Figure FDA0003953609630000014
represents the phase angle of the double frequency circulating current injection, m represents the modulation ratio, and the value is 2U m /U dc ，i au_f 、i bu_f 、i cu_f Respectively representing a, b and c three-phase upper bridge arm currents after the fault;
defining a double frequency circulating injection coefficient k 2 Comprises the following steps:
Figure FDA0003953609630000015
when the combination of the voltage and the current of a three-phase upper bridge arm is combined with the occurrence of an alternating-current single-phase grounding fault, the expression of the instantaneous power of the three-phase upper bridge arm containing double frequency circulating current injection after the fault is obtained as follows:
Figure FDA0003953609630000021
wherein, P au_f 、P bu_f 、P cu_f Respectively representing the instantaneous power of the upper bridge arms of the phases a, b and c after the fault;
the establishing of the optimized objective function by taking the fundamental frequency and the double-frequency fluctuation component of the instantaneous power of the bridge arm when the alternating-current single-phase fault occurs as the suppression objective comprises the following steps:
obtaining fundamental frequency and double frequency fluctuation of instantaneous power of a fault phase upper bridge arm when an alternating current single-phase fault occurs according to the instantaneous power of the three phase upper bridge arm containing double frequency circulation injection after the fault, and further obtaining a fluctuation amplitude;
obtaining fundamental frequency and double frequency fluctuation of instantaneous power of a non-fault two-phase upper bridge arm by adopting the same mode, and further obtaining a fluctuation amplitude;
establishing a target function according to the obtained fluctuation amplitudes of the fundamental frequency and the double frequency fluctuation of the instantaneous power of the three-phase bridge arm
Figure FDA0003953609630000022
Figure FDA0003953609630000023
Wherein the content of the first and second substances,
Figure FDA0003953609630000024
|P au_f1 i and I P au_f2 I is the fluctuation amplitude of the fundamental frequency and the double frequency fluctuation of the instantaneous power of the bridge arm on the phase a respectively; i P bu_f1 I and I P bu_f2 I is the fluctuation amplitude of the fundamental frequency and the double frequency fluctuation of the instantaneous power of the bridge arm on the phase b respectively; i P cu_f1 I and I P cu_f2 And l is the fundamental frequency of the instantaneous power of the bridge arm on the c phase and the fluctuation amplitude of the double frequency fluctuation respectively, and k represents the weight coefficient of the secondary fluctuation of the capacitor voltage.
2. The method for injecting the double-frequency circulating current for inhibiting the power fluctuation of the MMC bridge arm under the fault according to claim 1, wherein if the fault phase is a phase a, the fundamental frequency and the double-frequency fluctuation of the instantaneous power of the bridge arm on the phase a are as follows:
Figure FDA0003953609630000031
wherein, P au_f1 And P au_f2 The fundamental frequency and the double frequency fluctuation of the instantaneous power of the upper bridge arm of the phase a are respectively;
to P au_f1 And P au_f2 Performing an identity transform to obtain:
P au_f1 ＝A au_f1 sinωt+B au_f1 cosωt
Figure FDA0003953609630000032
P au_f2 ＝A au_f2 sin2ωt+B au_f2 cos2ωt
Figure FDA0003953609630000033
obtaining P according to the identity transformation result au_f1 And P au_f2 The fluctuation amplitude is:
Figure FDA0003953609630000034
3. the double-frequency circulating current injection method for inhibiting the MMC bridge arm power fluctuation under the fault according to claim 1 or 2, wherein if the fault phase is a phase, the fundamental frequency and double-frequency fluctuation of the instantaneous power of the bridge arm on the phase B are as follows:
Figure FDA0003953609630000035
wherein, P bu_f1 And P bu_f2 Fundamental frequency and double frequency fluctuation of B-phase upper bridge arm instantaneous power respectively
To P bu_f1 And P bu_f2 Performing an identity transform to obtain:
P bu_f1 ＝A bu_f1 sinωt+B bu_f1 cosωt
Figure FDA0003953609630000041
P bu_f2 ＝A bu_f2 sin2ωt+B bu_f2 cos2ωt
Figure FDA0003953609630000042
obtaining P according to the identity transformation result bu_f1 And P bu_f2 The fluctuation amplitude is:
Figure FDA0003953609630000043
4. the double-frequency circulating current injection method for inhibiting the MMC bridge arm power fluctuation under the fault according to claim 1 or 2, wherein if the fault phase is a phase, the fundamental frequency and double-frequency fluctuation of the C-phase upper bridge arm instantaneous power are as follows:
Figure FDA0003953609630000044
wherein, P cu_f1 And P cu_f2 The fundamental frequency and the double frequency fluctuation of the instantaneous power of the C-phase upper bridge arm are respectively;
to P cu_f1 And P cu_f2 Performing an identity transform to obtain:
Figure FDA0003953609630000045
Figure FDA0003953609630000046
Figure FDA0003953609630000047
Figure FDA0003953609630000048
obtaining P according to the identity transformation result cu_f1 And P cu_f2 The fluctuation amplitude is:
Figure FDA0003953609630000049
5. the double-frequency circulating current injection method for inhibiting the MMC bridge arm power fluctuation under the fault according to claim 1, wherein reference signals are generated according to the amplitude and the phase of the optimal double-frequency circulating current injection, and the injection of double-frequency circulating current is realized by combining a modulation algorithm, and comprises the following steps:
generating a reference signal according to the amplitude and the phase of the optimal double frequency circulating current injection and combining a reference signal generator;
the reference signal obtains a double frequency circulation modulation voltage signal through a double frequency circulation injection controller, and finally injection of the double frequency circulation is realized by changing a modulation wave of a modulation algorithm.
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