CN111244980A - Power electronic transformer nonlinear control method based on MMC structure - Google Patents
Power electronic transformer nonlinear control method based on MMC structure Download PDFInfo
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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
- H02J3/26—Arrangements for eliminating or reducing asymmetry in polyphase networks
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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
- H02J3/12—Circuit arrangements for ac mains or ac distribution networks for adjusting voltage in ac networks by changing a characteristic of the network load
- H02J3/16—Circuit arrangements for ac mains or ac distribution networks for adjusting voltage in ac networks by changing a characteristic of the network load by adjustment of reactive power
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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
- H02M3/00—Conversion of dc power input into dc power output
- H02M3/22—Conversion of dc power input into dc power output with intermediate conversion into ac
- H02M3/24—Conversion of dc power input into dc power output with intermediate conversion into ac by static converters
- H02M3/28—Conversion of dc power input into dc power output with intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode to produce the intermediate ac
- H02M3/325—Conversion of dc power input into dc power output with intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode to produce the intermediate ac using devices of a triode or a transistor type requiring continuous application of a control signal
- H02M3/335—Conversion of dc power input into dc power output with intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode to produce the intermediate ac using devices of a triode or a transistor type requiring continuous application of a control signal using semiconductor devices only
- H02M3/33569—Conversion of dc power input into dc power output with intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode to produce the intermediate ac using devices of a triode or a transistor type requiring continuous application of a control signal using semiconductor devices only having several active switching elements
- H02M3/33576—Conversion of dc power input into dc power output with intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode to produce the intermediate ac using devices of a triode or a transistor type requiring continuous application of a control signal using semiconductor devices only having several active switching elements having at least one active switching element at the secondary side of an isolation transformer
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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
- H02M7/00—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output
- H02M7/02—Conversion of ac power input into dc power output without possibility of reversal
- H02M7/04—Conversion of ac power input into dc power output without possibility of reversal by static converters
- H02M7/12—Conversion of ac power input into dc power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode
- H02M7/21—Conversion of ac power input into dc power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal
- H02M7/217—Conversion of ac power input into dc power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only
- H02M7/219—Conversion of ac power input into dc power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only in a bridge configuration
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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
- 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
- H02M7/53—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 using devices of a triode or transistor type requiring continuous application of a control signal
- H02M7/537—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 using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only, e.g. single switched pulse inverters
- H02M7/5387—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 using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only, e.g. single switched pulse inverters in a bridge configuration
- H02M7/53871—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 using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only, e.g. single switched pulse inverters in a bridge configuration with automatic control of output voltage or current
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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
- H02M1/0067—Converter structures employing plural converter units, other than for parallel operation of the units on a single load
- H02M1/007—Plural converter units in cascade
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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
- Y02E40/30—Reactive power compensation
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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
- Y02E40/50—Arrangements for eliminating or reducing asymmetry in polyphase networks
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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
- 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 relates to a power electronic transformer nonlinear control method based on an MMC structure, which comprises the following steps: s1, establishing input stage AC side and DC side mathematical models; s2, obtaining an input stage alternating current side mathematical model under a dq two-phase coordinate system through coordinate conversion; s3, respectively constructing an input stage inner ring control strategy, an outer ring control strategy and a circulating current suppression strategy of the MMC power electronic transformer based on a Lyapunov function and by combining an input stage direct current side mathematical model and an input stage alternating current side mathematical model under a dq two-phase coordinate system; s4, constructing a middle isolation stage phase-shifting voltage-regulating control strategy; and S5, constructing an output stage inner ring control strategy and an output stage outer ring control strategy. Compared with the prior art, the method has the advantages that the input stage, the intermediate isolation stage and the output stage of the MMC power electronic transformer are respectively provided with the corresponding control strategies, so that the comprehensive and effective control can be performed when the power grid fails, the voltage stability can be improved, and the power supply and electric energy quality can be ensured.
Description
Technical Field
The invention relates to the technical field of MMC power electronic transformer control, in particular to a power electronic transformer nonlinear control method based on an MMC structure when the voltage of a power grid is unbalanced.
Background
In the electric power system, the transformer is the most reliable and widely used electric equipment, and in recent years, with the rapid development of distributed energy, the nonlinear load used by these electric power consumers in industry, business and residence is increasing, and the traditional distribution transformer lacks flexibility and bidirectional energy control capability, so that the requirement of the current electric power system cannot be met.
Compared with the traditional Transformer, a Power Electronic Transformer (PET) adopting a high-frequency Transformer has incomparable advantages in the aspects of voltage sag compensation, instantaneous voltage regulation, Power factor correction, harmonic suppression and the like, and meanwhile, the PET has the advantages of small size and light weight; the Modular Multilevel Converter (MMC) technology has the advantages of a large number of output levels, good electromagnetic compatibility, low harmonic content, low requirement on voltage resistance of a switching device, small switching loss and the like. In medium and high voltage power grids, MMC-PET obtained by combining MMC technology and Power Electronic Transformers (PET) has become a trend for development at home and abroad. However, when the voltage of the power grid fails, for the MMC-PET, the current and power of the external ac side will fluctuate, and the voltage of the dc side will also fluctuate, which will finally seriously affect the voltage stability of the power system and the quality of the power supplied to the low-voltage side power grid.
Disclosure of Invention
The invention aims to overcome the defects in the prior art and provide a nonlinear control method for a power electronic transformer based on an MMC structure, which improves the voltage stability of an alternating current side and a direct current side when the voltage of a power grid is unbalanced and ensures the quality of power supply electric energy by respectively constructing corresponding control strategies for an input stage, an intermediate isolation stage and an output stage.
The purpose of the invention can be realized by the following technical scheme: a power electronic transformer nonlinear control method based on an MMC structure comprises the following steps:
s1, establishing an input-stage AC side and DC side mathematical model according to a topological structure of the MMC power electronic transformer, wherein three phases on the input-stage AC side are composed of an upper bridge arm and a lower bridge arm, each bridge arm is composed of a plurality of sub-modules, bridge arm inductors and bridge arm equivalent resistors which are sequentially connected in series, and each sub-module is composed of two half bridges composed of two IGBTs with anti-parallel diodes and a capacitor which are connected in parallel;
s2, performing coordinate conversion on the input stage alternating current side mathematical model to obtain the input stage alternating current side mathematical model under a dq two-phase coordinate system;
s3, respectively constructing an input stage inner ring control strategy, an outer ring control strategy and a circulating current suppression strategy of the MMC power electronic transformer based on a Lyapunov function and by combining an input stage direct current side mathematical model and an input stage alternating current side mathematical model under a dq two-phase coordinate system;
s4, constructing a middle isolation stage phase-shifting voltage-regulating control strategy according to the topological structure of the MMC power electronic transformer, wherein the middle isolation stage is specifically an active double-bridge DC/DC converter;
s5, respectively constructing an inner ring control strategy and an outer ring control strategy of an output stage according to the topological structure of the MMC power electronic transformer, wherein the output stage is specifically a three-phase full-bridge voltage type converter.
Further, step S1 is specifically based on Kirchhoff' S law to establish an input stage ac side and dc side mathematical model, where the input stage ac side mathematical model specifically is:
Ls＝LT+L/2
Rs＝RT+R/2
wherein ,uva、uvb、uvcThree-phase alternating voltage, u, for an MMC input stagea、ub、ucIs a three-phase voltage, i, on the mains sidea、ib、icFor three-phase currents on the mains side, Rs、LsRespectively an equivalent resistance, an equivalent inductance, R of the transmission lineT、LTRespectively is equivalent resistance and equivalent inductance of a side line of the power grid, and R, L is bridge arm resistance and bridge arm inductance of the MMC respectively;
the input stage direct current side mathematical model specifically comprises:
wherein ,udcIs the DC voltage of the MMC input stage, ujp、ujnVoltage of upper and lower arms of j phase, icirjIs a circulation of j phases, ijp、ijnThe current of the upper bridge arm and the current of the lower bridge arm of the j phase are respectively.
Further, the input stage alternating current side mathematical model in the dq two-phase coordinate system in step S2 specifically is:
where ω is the AC system angular frequency at the grid side, ud、uqD-axis component and q-axis component i of three-phase alternating voltage at power grid side under two-phase rotating coordinate systemd、iqRespectively d-axis component and q-axis component u of three-phase current at power grid side under two-phase rotating coordinate systemsd、usqThe three-phase alternating voltage input side of the MMC is respectively the d-axis component and the q-axis component under a two-phase rotating coordinate system.
Further, the step S3 specifically includes the following steps:
s31, determining electromagnetic transient equations of the input stage of the MMC power electronic transformer at the alternating current side and the direct current side in a positive sequence system according to the input stage direct current side mathematical model and the input stage alternating current side mathematical model in the dq two-phase coordinate system;
s32, obtaining a switching function of the input stage of the MMC power electronic transformer under the positive sequence system based on a Lyapunov function and electromagnetic transient equations of the input stage of the MMC power electronic transformer at the alternating current side and the direct current side in the positive sequence system, wherein the switching function is an inner ring control strategy and a voltage outer ring control strategy of the input stage positive and negative sequence current of the MMC power electronic transformer, and the voltage outer ring control strategy adopts constant direct current voltage and constant reactive power control;
s33, aiming at circulating current components on an input stage bridge arm of the MMC power electronic transformer, a double-frequency circulating current mathematical model is constructed, namely the input stage circulating current suppression strategy of the MMC power electronic transformer is obtained.
Further, in step S31, the electromagnetic transient equations of the input stage of the MMC power electronic transformer at the ac side and the dc side in the positive sequence system are specifically:
wherein ,
the d-axis component and the q-axis component of the alternating-current side voltage under the positive sequence system,
is the reference value of the d and q axis components of the three-phase current at the power grid side under the positive sequence system, Sdref and SqrefAre respectively a switching function Sd、SqIs determined by the reference value of (a),
is a reference value for the voltage on the dc side,
the reference value of the direct current side current is obtained, N is the number of sub-modules in the input-stage bridge arm, and C is the capacitance value in the sub-modules;
the switching function of the input stage of the MMC power electronic transformer in the positive sequence system in step S32 is specifically:
wherein ,
d-axis switching function and q-axis switching function of the input stage of the MMC power electronic transformer in the positive sequence system respectively, α and β are outer loop control coefficients udcactIs the actual value of the DC side voltage, gamma0Is an intermediate variable, x1、x2 and x3Are all the variables of the state, and are,
respectively representing d-axis components and q-axis components of three-phase current on the power grid side under a positive sequence system in a two-phase rotating coordinate system;
the second harmonic circulation mathematical model in the step S33 specifically includes:
wherein ,
is the voltage of the zero-sequence component of the double frequency circulation in the j phase,
is a zero-sequence circular current component,
is a zero sequence circulating component command value, kpFor the circulating current suppression proportionality coefficient, kiThe integral coefficient is suppressed for the circulating current.
Further, the step S4 specifically includes the following steps:
s41, obtaining an output active power model of the active double-bridge DC/DC converter according to the structure of the active double-bridge DC/DC converter:
wherein ,LlIs leakage inductance of the high frequency transformer, fsIs the switching frequency of the switching tube, udcLD is the duty ratio of the high-voltage side single-phase bridge type full-control converter;
s42, converting the low-voltage side direct current voltage udcLAnd an output voltage reference value
After difference is made, the phase shift reference value theta is obtained through the first static-error-free PI controller*The phase-shifting voltage-regulating control strategy is used as a switching tube trigger signal of the DC/DC converter, namely the phase-shifting voltage-regulating control strategy of the intermediate isolation stage.
Go toStep by step, the phase shift reference value theta in the step S42*The method specifically comprises the following steps:
wherein ,kp1Is the first non-static PI controller proportionality coefficient, ki1Is the first quiet error-free PI controller integral coefficient;
further, the step S5 specifically includes the following steps:
s51, establishing a stable mathematical model of the three-phase full-bridge voltage type current converter under a dq two-phase coordinate system;
s52, constructing an MMC-PET output stage positive and negative sequence current inner loop control strategy and an outer loop control strategy adopting constant alternating voltage and constant reactive power control aiming at the positive and negative sequence current of an output stage based on a Lyapunov function and a steady state mathematical model of the three-phase full-bridge voltage type current converter.
Further, the steady-state mathematical model of the three-phase full-bridge voltage type converter in the step S51 is specifically:
wherein ,ucd、ucqBridge arm midpoint voltage u of three-phase full-bridge voltage type current converterA、uB、uCD, q axis components, i, in a two phase rotating coordinate systemsd、isqD-axis component and q-axis component u of output current of the three-phase full-bridge voltage type current converter under a two-phase rotating coordinate systemsd、usqThree-phase voltage u respectively output by three-phase full-bridge voltage type current converterA0、uB0、uC0D, q axis components, R, in a two phase rotating coordinate systemdIs the equivalent resistance, L, of a three-phase full-bridge voltage type current converterdIs the equivalent inductance of the three-phase full-bridge voltage type current converter.
Compared with the prior art, the input stage, the intermediate isolation stage and the output stage of the MMC power electronic transformer respectively construct corresponding control strategies to carry out comprehensive and effective control when the power grid fails, wherein for the input stage, the voltage and the current input by the power grid are subjected to positive-negative zero sequence separation to respectively control the positive and negative zero sequences of the current, an inner ring control strategy of the input stage positive-negative sequence current based on a Lyapunov function is constructed, a zero sequence current PI controller is designed, an outer ring control strategy of fixed direct current voltage and reactive power control is adopted to reduce the influence of the power grid voltage fault on the direct current voltage, and an input stage circulation suppression strategy is constructed to improve the integral dynamic performance of the MMC power electronic transformer, namely the stability of direct current voltage output;
for the intermediate isolation level, the invention adopts a control strategy of phase-shifting voltage regulation, and further reduces the influence of voltage fluctuation at the direct current side of the input level of the MMC power electronic transformer on the low-voltage side power grid when the power grid voltage fails;
for the output stage, the invention designs an output stage positive and negative sequence current inner ring control strategy based on a Lyapunov function aiming at the positive and negative currents generated when the power grid at the output side is in fault, and adopts constant alternating voltage control and constant reactive power control to carry out outer ring control, so as to further reduce the influence of the fluctuation of the reactive power on the power grid at the low voltage side when the power grid is in fault, improve the stability of alternating voltage output and effectively ensure the quality of electric energy.
Drawings
FIG. 1 is a schematic flow diagram of the process of the present invention;
FIG. 2 is a topological diagram of an MMC power electronic transformer;
FIG. 3(a) is a general control block diagram of an input stage of the MMC power electronic transformer of the present invention;
FIG. 3(b) is a block diagram of the circulating current suppression control of the input stage of the MMC power electronic transformer of the present invention;
FIG. 3(c) is a block diagram of the intermediate isolation stage control of the MMC power electronic transformer of the present invention;
FIG. 3(d) is a control block diagram of the output stage of the MMC power electronic transformer of the present invention;
FIG. 4 is a waveform of an input stage MMC sub-module capacitance voltage during normal operation of the MMC power electronic transformer of the present invention;
FIG. 5 is an input stage MMC circulating current waveform during normal operation of the MMC power electronic transformer of the present invention;
FIG. 6 is an MMC-PET input side grid voltage waveform of target 1 of an embodiment of the present invention;
FIG. 7 is an MMC-PET input side grid current waveform of target 1 of the present invention;
FIG. 8 is an MMC-PET input stage power waveform of target 1 of an embodiment of the present invention;
FIG. 9 is a MMC-PET input stage DC-side voltage waveform of target 1 of an embodiment of the present invention;
FIG. 10 is a DC voltage waveform of the MMC-PET isolation stage output of target 1 of the present invention;
FIG. 11 is an MMC-PET output stage output power waveform of target 1 of an embodiment of the present invention;
FIG. 12 is a MMC-PET output stage output voltage waveform of target 1 of an embodiment of the present invention;
FIG. 13 is a MMC-PET output stage output current waveform of target 1 of an embodiment of the present invention;
FIG. 14 is an MMC-PET input side grid voltage waveform of target 2 of an embodiment of the present invention;
FIG. 15 is an MMC-PET input side grid current waveform of target 2 of an embodiment of the present invention;
FIG. 16 is an MMC-PET input stage power waveform of target 2 of an embodiment of the present invention;
FIG. 17 is a MMC-PET input stage DC-side voltage waveform of target 2 of an embodiment of the present invention;
FIG. 18 is a DC voltage waveform of the MMC-PET isolation stage output of target 2 of an embodiment of the present invention;
FIG. 19 is an MMC-PET output stage output power waveform of target 2 of an embodiment of the present invention;
FIG. 20 is a MMC-PET output stage output voltage waveform of target 2 of an embodiment of the present invention;
FIG. 21 is a graph of the MMC-PET output stage output current waveform of target 2 of the present invention.
Detailed Description
The invention is described in detail below with reference to the figures and specific embodiments.
Examples
As shown in fig. 1, a power transformer nonlinear control method based on an MMC structure includes the following steps:
s1, establishing an input stage AC side and DC side mathematical model according to the topological structure of the MMC power electronic transformer;
s2, performing coordinate conversion on the input stage alternating current side mathematical model to obtain the input stage alternating current side mathematical model under a dq two-phase coordinate system;
s3, respectively constructing an input stage inner ring control strategy, an outer ring control strategy and a circulating current suppression strategy of the MMC power electronic transformer based on a Lyapunov function and by combining an input stage direct current side mathematical model and an input stage alternating current side mathematical model under a dq two-phase coordinate system;
s4, constructing a middle isolation stage phase-shifting voltage-regulating control strategy according to the topological structure of the MMC power electronic transformer;
and S5, respectively constructing an inner ring control strategy and an outer ring control strategy of an output stage according to the topological structure of the MMC power electronic transformer.
The topological structure of the MMC power electronic transformer is shown in figure 2, three phases on the AC side of an input stage are composed of an upper bridge arm and a lower bridge arm, each bridge arm is composed of a plurality of sub-modules, bridge arm inductors and bridge arm equivalent resistors which are sequentially connected in series, and each sub-module is composed of two half bridges composed of IGBT with anti-parallel diodes and a capacitor which are connected in parallel;
the intermediate isolation stage is specifically an active double-bridge DC/DC converter;
the output stage is a three-phase full-bridge voltage type converter, namely a three-phase inverter.
The method is applied to the embodiment, and the specific process comprises the following steps:
step one, establishing an input stage mathematical model of the MMC power electronic transformer: respectively constructing mathematical models of an AC side and a DC side of the MMC according to the topological structure of the MMC based on a Kirchoff law;
step two, two-phase rotation coordinate conversion: according to a coordinate transformation theory, transforming a mathematical model on the AC side of the MMC into an AC side mathematical model under a dq two-phase rotating coordinate system;
designing an inner ring control strategy of the input stage of the MMC power electronic transformer based on the positive and negative sequence currents of the input stage of the MMC-PET based on a Lyapunov function under the voltage fault of a power grid;
analyzing the value range of the outer ring control coefficient to cope with the influence of the discrepancy between the reference value given by the voltage outer ring and the actual value;
designing a positive, negative and zero sequence 2-frequency doubling circulation controller for inhibiting positive and negative zero sequence double frequency circulation in an MMC-PET input stage bridge arm;
step six, a control strategy of the intermediate stage of the MMC power electronic transformer under the power grid voltage fault is as follows: the control strategy of phase-shifting voltage regulation is adopted, so that the influence of voltage fluctuation on the direct current side of the input stage of the MMC-PET on the system is further reduced when the voltage of a power grid fails;
and seventhly, designing an MMC-PET output stage positive and negative sequence current inner ring control strategy based on a Lyapunov function for positive and negative currents generated when a power grid at an output side of the MMC-PET output side of the three-phase voltage type full-bridge inverter fails, wherein an outer ring is controlled by constant alternating voltage and constant reactive power.
In the first step, mathematical model expressions of an alternating current side and a direct current side of an input stage of the MMC power electronic transformer are as follows:
wherein ,Ls＝LT+L/2，Rs＝RT+R/2，
in the formula ：uva、uvb、uvcThree-phase alternating current voltage at the input side of the MMC; ua, ub and uc are three-phase voltages on the side of the power grid; i.e. ia、ib、icThree-phase current at the side of the power grid; rs、LsRespectively representing equivalent electricity of transmission lineResistance and equivalent inductance; rT、LTRespectively equal resistance and equal inductance of a side line of the power grid; r, L are bridge arm resistance and bridge arm inductance of MMC respectively.
wherein ,
in the formula ：icirjA circulating current of j (j ═ a, b, c) phase; u. ofjp、ujnThe voltages of the upper bridge arm and the lower bridge arm of the j phase are respectively; r, L are bridge arm resistance and bridge arm inductance of MMC respectively.
In the second step, two-phase rotational coordinate transformation is performed. The AC side mathematical model of the input stage of the MMC power electronic transformer under the dq two-phase rotating coordinate system is as follows:
in the formula: omega is the angular frequency of the alternating current system at the side of the power grid; u. ofd、uqThe components of d and q axes of three-phase alternating voltage on the power grid side under a two-phase rotating coordinate system are respectively; i.e. id、iqRespectively representing d-axis components and q-axis components of three-phase current on the power grid side under a two-phase rotating coordinate system; u. ofsd、usqThe three-phase alternating voltage input side of the MMC is respectively the d-axis component and the q-axis component under a two-phase rotating coordinate system.
In the third step, an inner ring control strategy of the MMC-PET input stage positive and negative sequence current of the input stage of the MMC power electronic transformer based on the Lyapunov function under the grid voltage fault is designed:
the electromagnetic transient equations of the AC side and the DC side of the input stage of MMC-PET are as follows:
wherein ,
in the formula ,
is the steady state value of the switching function; Δ d, Δ q are the ripple components of the switching function.
When a power grid fails, the MMC-PET input stage aims to stabilize the output direct-current voltage, restrain the output direct-current voltage from generating large fluctuation and maintain the output reactive power close to the target value. For a positive-negative sequence system, the control target of the controller is to make the positive sequence currents of the d and q axes track the given current reference values, when the controller is stabilized at the d and q axes positive sequence current stabilization reference values, the electromagnetic transient equations of the input stage of the MMC-PET on the alternating current side and the direct current side in the positive sequence system are respectively as follows:
in the formula ,
reference values of d and q axis components of three-phase current at the power grid side are obtained; sdref、SqrefReference values for d and q axis components of the switching function;
is a reference value of the DC side voltage;
is a direct current reference value.
Can obtain the product
Let the state variables of the system be:
let the impedance on the DC side be XdcObtainable idc＝udc/XdcThe MMC-PET input stage model based on the Lyapunov function under the positive sequence is as follows:
according to the Lyapunov stabilization theory, the system, whether linear or nonlinear, is globally and gradually stabilized when the following conditions are met: 1. v (0) ═ 0;
2. for any x ≠ 0, v (x) > 0;
3. for any x ≠ 0,
4. when | | x | | tends to infinity, v (x) tends to infinity.
Let the Lyapunov function of the MMC-PET input stage in positive sequence be taken as:
the derivation is performed on the above formula to obtain:
namely, the method comprises the following steps:
selecting:
wherein α and β are outer loop control coefficients.
The switching function of the MMC-PET input stage based on the Lyapunov function under the positive sequence system can be obtained as follows:
a control block diagram corresponding to this switching function is shown in fig. 3 (a).
In step four, the value ranges of the two coefficients α and β are analyzed to cope with the influence of the discrepancy between the reference value given by the voltage outer loop and the actual value.
Suppose the expected value of the system at time t is
Actual value of
The derivative of the chosen Lyapunov function is:
according to the above formula can be converted into:
wherein α < 0 and β < 0, if both:
then
Negative determination, hypothesis
Under imprecise control, this can be done
The transformation is:
respectively ordering:
the following can be obtained:
in the case of the above formula, the compound is,f1(m1,m3) And f2(m2,m3) While greater than 0, then
And (4) negative determination.
Let m3＝h1m1、m3＝h2m2Then, then
Order to
In the formula, function λ1(r1,γ1,h1) To relate to h1A quadratic function of (d) when h1＝(1+γ1)/(2γ1) The quadratic function takes the minimum value, which is:
λ1min＝Rs+r1[1-(1+γ1)2/(4γ1)]
to make f1(m1,m3) Meets the positive definite condition, and the system can be gradually stabilized to obtain gamma1The value range is as follows:
in the formula ,γ0＝1+2Rs/r1。
When gamma is1The smaller the interval of parameter changes that can stabilize the system, the larger the interval is.
If the uncertain interval of the parameters is 1-epsilon < gamma1< 1+ ε, a maximum of α can be obtained:
similarly, the maximum value of β that can be obtained is:
according to the above, the value ranges of α and β under inaccurate control can be determined to ensure the stability of the system.
In the fifth step, the process is carried out,
when the power grid fails, the circulating current components on the MMC-PET input stage MMC bridge arm are as follows:
in the formula ：ida、idb、idcIs a direct current component in the three-phase circulating current,
respectively the amplitudes of positive and negative zero sequence components in the double frequency circulation,
the initial phases of the positive and negative zero sequence components in the double frequency circulation are respectively.
After positive and negative sequence frequency doubling circulation flow control is put into use, the default is that the frequency doubling circulation flows of positive and negative sequences are both 0, so that the frequency doubling circulation flow only contains zero sequence components. The following mathematical model can be obtained:
in the formula ：ujp、ujnThe bridge arm voltages of j-phase upper and lower bridge arms respectively,
is the voltage of the zero-sequence component of the double frequency circulation in the j phase,
is a zero sequence circulating component.
Command value of zero sequence circulation component
The following controller for the zero sequence current component in the circulating current can be adopted:
a control block diagram of the above-described circulation suppression is shown in fig. 3 (b).
In step six, the active power transmitted by the DC/DC converter is:
in the formula ：udcThe direct current voltage is the direct current voltage of the MMC-PET high-voltage side; l islLeakage inductance of the high-frequency transformer; f. ofsIs the switching frequency of the switching tube; u. ofdcLDirect current voltage of low voltage side; d is the duty ratio of the high-voltage side single-phase bridge type full-control converter.
According to the formula, the duty ratio of the high-voltage side single-phase bridge type full-control converter can be changeddcLThe size of (2). By combining the output voltage with udcLAnd an output voltage reference value
After difference is made, the output result is a phase shift reference value theta through a PI controller without static difference*. The carrier phase shift PWM modulation is used for obtaining a switching tube trigger signal of the output DC/DC converter, which can be expressed as:
the control block diagram of the intermediate isolation stage is shown in fig. 3 (c).
In the seventh step, under the dq rotation coordinate system, the steady-state mathematical model of the three-phase full-bridge voltage type converter is as follows:
in the formula: angular frequency, u, of ac system on the side of the omega networkcd、ucqBridge arm midpoint voltage u of three-phase full-bridge voltage type current converterA、uB、uCD, q axis components, i, in a two phase rotating coordinate systemsd、isqD-axis component and q-axis component u of output current of the three-phase full-bridge voltage type current converter under a two-phase rotating coordinate systemsd、usqThree-phase voltage u respectively output by three-phase full-bridge voltage type current converterA0、uB0、uC0D, q axis components, R, in a two phase rotating coordinate systemdIs the equivalent resistance, L, of a three-phase full-bridge voltage type current converterdIs the equivalent inductance of the three-phase full-bridge voltage type current converter. Similarly, according to the third step and the fourth step, an inner ring control strategy based on the Lyapunov function of the MMC-PET output stage under the grid fault can be obtained, and a corresponding output stage control block diagram is shown in fig. 3 (d).
In order to verify the effectiveness of the method, according to the MMC-HVDC system, a simulation comparison experiment is carried out on the basis of a MATLAB/Simulink simulation model, and an experiment prototype is tested. The simulation main parameter settings are as in table 1:
TABLE 1
When the power grid normally operates, the voltage waveform of the capacitance of the input-stage MMC sub-module and the current waveform of the input-stage MMC ring are respectively shown in fig. 4 and fig. 5 when the MMC power electronic transformer normally operates. The target 1 set in the embodiment is that the grid voltage has a voltage three-phase voltage sag fault, the sag fault is 50% of the original grid voltage, and the target 2 is that the grid voltage has a voltage three-phase voltage sag fault, and the sag fault is 130% of the original grid voltage.
The specific simulation effect is as follows:
when the three-phase voltage of the power grid drops by 50%, fig. 6 shows the waveform of the power grid voltage at the input side of the MMC-PET when the target 1 fails, and the three-phase voltage sag fault occurs at 1.4s-1.5s, fig. 7 shows the waveform of the power grid current at the input side of the MMC-PET corresponding to the target 1 fault, fig. 8 shows the waveform of the power at the input stage of the MMC-PET corresponding to the target 1 fault, fig. 9 shows the waveform of the DC side of the input stage of the MMC-PET corresponding to the target 1 fault, fig. 10 shows the waveform of the DC voltage output by the isolation stage of the MMC-PET corresponding to the target 1 fault, fig. 11 shows the waveform of the output power by the output stage of the MMC-PET corresponding to the target 1 fault, fig. 12 shows the waveform of the output voltage by the MMC-PET corresponding to the target 1 fault, and fig.. As can be seen from the simulation fig. 6 to 13, when a voltage sag suddenly occurs, the output dc voltage of the MMC-PET input stage also decreases, and since the input stage of the present invention adopts constant dc voltage control and constant reactive power control, the output dc side voltage does not decrease to the original 50%, but only decreases by 7.4%, and reaches a stable state, and meanwhile, the reactive power of the input stage converter is stable and does not generate large fluctuation. During the voltage sag of the power grid, the rectifying stage of the MMC-PET can maintain certain active power transmission; because the constant direct current of the middle isolation stage is controlled, the target value of the control is half of the input value, and the control margin is certain, so that the control margin is not obviously reduced; in addition, because the output stage adopts constant alternating voltage and constant reactive power control and combines the energy storage function of the capacitor, under the condition of voltage sag, the active power and the reactive power output by the MMC-PET output stage are not reduced, and the output voltage and current are not greatly influenced, so that the fault low-voltage ride-through capability of a power grid is improved, and the power supply quality is ensured.
When the three-phase voltage of the power grid temporarily rises to 130% of the original voltage, fig. 14 shows a waveform of an input side power grid voltage of an MMC-PET corresponding to a target 2 fault, the temporary rising fault of the three-phase voltage occurs in 1.4s-1.5s, fig. 15 shows a waveform of an input side power grid current of the MMC-PET corresponding to the target 2 fault, fig. 16 shows a waveform of an input stage power of the MMC-PET corresponding to the target 2 fault, fig. 17 shows a waveform of an input stage direct current of the MMC-PET corresponding to the target 2 fault, fig. 18 shows a waveform of a direct current voltage output by an MMC-PET isolation stage corresponding to the target 2 fault, fig. 19 shows a waveform of an output stage power of the MMC-PET corresponding to the target 2 fault, fig. 20 shows a waveform of an output stage output voltage of the MMC-PET corresponding to the target 2 fault, and fig. 21 shows a. When the three-phase voltage of the power grid temporarily rises to 130% of the rated voltage value, as can be known from simulation graphs 14-21, when the voltage temporarily rises suddenly, the output direct-current voltage of the MMC-PET input stage also rises, and because the input stage adopts constant direct-current voltage control and constant reactive power control, the output direct-current side voltage does not rise to the original 150%, but only rises by 38.8%, and reaches a stable state, and meanwhile, the reactive power of the input stage converter is stable and does not generate large fluctuation; during the voltage sag of the power grid, the output voltage of the intermediate isolation stage is kept constant due to constant direct current control of the intermediate isolation stage; because the output stage controls the constant alternating voltage and the constant reactive power and combines the energy storage function of the capacitor, under the condition of voltage temporary rise, the active power and the reactive power output by the MMC-PET output stage do not greatly influence the output voltage and current in order to rise, the fault high-voltage ride through capability of a power grid is improved, and the power supply quality is effectively ensured.
Claims (9)
1. A power electronic transformer nonlinear control method based on an MMC structure is characterized by comprising the following steps:
s1, establishing an input-stage AC side and DC side mathematical model according to a topological structure of the MMC power electronic transformer, wherein three phases on the input-stage AC side are composed of an upper bridge arm and a lower bridge arm, each bridge arm is composed of a plurality of sub-modules, bridge arm inductors and bridge arm equivalent resistors which are sequentially connected in series, and each sub-module is composed of two half bridges composed of two IGBTs with anti-parallel diodes and a capacitor which are connected in parallel;
s2, performing coordinate conversion on the input stage alternating current side mathematical model to obtain the input stage alternating current side mathematical model under a dq two-phase coordinate system;
s3, respectively constructing an input stage inner ring control method, an outer ring control method and a circulating current suppression method of the MMC power electronic transformer based on a Lyapunov function and by combining an input stage direct current side mathematical model and an input stage alternating current side mathematical model under a dq two-phase coordinate system;
s4, constructing a middle isolation stage phase-shifting voltage-regulating control method according to the topological structure of the MMC power electronic transformer, wherein the middle isolation stage is specifically an active double-bridge DC/DC converter;
s5, respectively constructing an inner ring control method and an outer ring control method of an output stage according to the topological structure of the MMC power electronic transformer, wherein the output stage is specifically a three-phase full-bridge voltage type converter.
2. The method according to claim 1, wherein the step S1 is specifically based on Kirchhoff' S law to establish input stage ac side and dc side mathematical models, and the input stage ac side mathematical model is specifically:
Ls＝LT+L/2
Rs＝RT+R/2
wherein ,uva、uvb、uvcThree-phase alternating voltage, u, for an MMC input stagea、ub、ucIs a three-phase voltage, i, on the mains sidea、ib、icFor three-phase currents on the mains side, Rs、LsRespectively an equivalent resistance, an equivalent inductance, R of the transmission lineT、LTRespectively is equivalent resistance and equivalent inductance of a side line of the power grid, and R, L is bridge arm resistance and bridge arm inductance of the MMC respectively;
the input stage direct current side mathematical model specifically comprises:
wherein ,udcIs the DC voltage of the MMC input stage, ujp、ujnVoltage of upper and lower arms of j phase, icirjIs a circulation of j phases, ijp、ijnThe current of the upper bridge arm and the current of the lower bridge arm of the j phase are respectively.
3. The nonlinear control method for the power electronic transformer based on the MMC structure of claim 2, wherein in step S2, the mathematical model of the AC side of the input stage in dq two-phase coordinate system is specifically:
where ω is the AC system angular frequency at the grid side, ud、uqD-axis component and q-axis component i of three-phase alternating voltage at power grid side under two-phase rotating coordinate systemd、iqRespectively d-axis component and q-axis component u of three-phase current at power grid side under two-phase rotating coordinate systemsd、usqThe three-phase alternating voltage input side of the MMC is respectively the d-axis component and the q-axis component under a two-phase rotating coordinate system.
4. The method according to claim 3, wherein the step S3 specifically comprises the following steps:
s31, determining electromagnetic transient equations of the input stage of the MMC power electronic transformer at the alternating current side and the direct current side in a positive sequence system according to the input stage direct current side mathematical model and the input stage alternating current side mathematical model in the dq two-phase coordinate system;
s32, obtaining a switching function of the input stage of the MMC power electronic transformer under the positive sequence system based on a Lyapunov function and electromagnetic transient equations of the input stage of the MMC power electronic transformer at the alternating current side and the direct current side in the positive sequence system, wherein the switching function is an inner ring control strategy and a voltage outer ring control strategy of the input stage positive and negative sequence current of the MMC power electronic transformer, and the voltage outer ring control strategy adopts constant direct current voltage and constant reactive power control;
s33, aiming at circulating current components on an input stage bridge arm of the MMC power electronic transformer, a double-frequency circulating current mathematical model is constructed, namely the input stage circulating current suppression strategy of the MMC power electronic transformer is obtained.
5. The method according to claim 4, wherein the electromagnetic transient equations of the input stage of the MMC power electronic transformer at the AC side and the DC side in the positive sequence system in the step S31 are specifically as follows:
wherein ,
the d-axis component and the q-axis component of the alternating-current side voltage under the positive sequence system,
is the reference value of the d and q axis components of the three-phase current at the power grid side under the positive sequence system, Sdref and SqrefAre respectively a switching function Sd、SqIs determined by the reference value of (a),
is a reference value for the voltage on the dc side,
the reference value of the direct current side current is obtained, N is the number of sub-modules in the input-stage bridge arm, and C is the capacitance value in the sub-modules;
the switching function of the input stage of the MMC power electronic transformer in the positive sequence system in step S32 is specifically:
wherein ,
d-axis switching function and q-axis switching function of the input stage of the MMC power electronic transformer in the positive sequence system respectively, α and β are outer loop control coefficients udcactIs the actual value of the DC side voltage, gamma0Is an intermediate variable, x1、x2 and x3Are all the variables of the state, and are,
respectively representing d-axis components and q-axis components of three-phase current on the power grid side under a positive sequence system in a two-phase rotating coordinate system;
the second harmonic circulation mathematical model in the step S33 specifically includes:
wherein ,
is the voltage of the zero-sequence component of the double frequency circulation in the j phase,
is a zero-sequence circular current component,
is a zero sequence circulating component command value, kpFor the circulating current suppression proportionality coefficient, kiThe integral coefficient is suppressed for the circulating current.
6. The method according to claim 1, wherein the step S4 specifically includes the following steps:
s41, obtaining an output active power model of the active double-bridge DC/DC converter according to the structure of the active double-bridge DC/DC converter:
wherein ,LlIs leakage inductance of the high frequency transformer, fsIs the switching frequency of the switching tube, udcLD is the duty ratio of the high-voltage side single-phase bridge type full-control converter;
s42, converting the low-voltage side direct current voltage udcLAnd an output voltage reference value
After difference is made, the phase shift reference value theta is obtained through the first static-error-free PI controller*The phase-shifting voltage-regulating control strategy is used as a switching tube trigger signal of the DC/DC converter, namely the phase-shifting voltage-regulating control strategy of the intermediate isolation stage.
7. The nonlinear control method for the power electronic transformer based on the MMC structure of claim 6, wherein in step S42, the phase-shifting reference value θ is*The method specifically comprises the following steps:
wherein ,kp1Is the first non-static PI controller proportionality coefficient, ki1Is the first quiet PI controller integral coefficient.
8. The method according to claim 1, wherein the step S5 specifically includes the following steps:
s51, establishing a stable mathematical model of the three-phase full-bridge voltage type current converter under a dq two-phase coordinate system;
s52, constructing an MMC-PET output stage positive and negative sequence current inner loop control strategy and an outer loop control strategy adopting constant alternating voltage and constant reactive power control aiming at the positive and negative sequence current of an output stage based on a Lyapunov function and a steady state mathematical model of the three-phase full-bridge voltage type current converter.
9. The method according to claim 8, wherein the steady-state mathematical model of the three-phase full-bridge voltage type converter in the step S51 is specifically:
wherein ,ucd、ucqBridge arm midpoint voltage u of three-phase full-bridge voltage type current converterA、uB、uCD, q axis components, i, in a two phase rotating coordinate systemsd、isqThe d-axis component and the q-axis component of the output current of the three-phase full-bridge voltage type converter under a two-phase rotating coordinate system are respectively,usd、usqthree-phase voltage u respectively output by three-phase full-bridge voltage type current converterA0、uB0、uC0D, q axis components, R, in a two phase rotating coordinate systemdIs the equivalent resistance, L, of a three-phase full-bridge voltage type current converterdIs the equivalent inductance of the three-phase full-bridge voltage type current converter.
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