CN112653133A - Power grid simulator based on nine-switch converter and application method thereof - Google Patents
Power grid simulator based on nine-switch converter and application method thereof 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
- H02J3/001—Methods to deal with contingencies, e.g. abnormalities, faults or failures
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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
- H02J3/38—Arrangements for parallely feeding a single network by two or more generators, converters or transformers
- H02J3/381—Dispersed generators
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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
- H02M5/00—Conversion of ac power input into ac power output, e.g. for change of voltage, for change of frequency, for change of number of phases
- H02M5/40—Conversion of ac power input into ac power output, e.g. for change of voltage, for change of frequency, for change of number of phases with intermediate conversion into dc
- H02M5/42—Conversion of ac power input into ac power output, e.g. for change of voltage, for change of frequency, for change of number of phases with intermediate conversion into dc by static converters
- H02M5/44—Conversion of ac power input into ac power output, e.g. for change of voltage, for change of frequency, for change of number of phases with intermediate conversion into dc by static converters using discharge tubes or semiconductor devices to convert the intermediate dc into ac
- H02M5/453—Conversion of ac power input into ac power output, e.g. for change of voltage, for change of frequency, for change of number of phases with intermediate conversion into dc by static converters using discharge tubes or semiconductor devices to convert the intermediate dc into ac using devices of a triode or transistor type requiring continuous application of a control signal
- H02M5/458—Conversion of ac power input into ac power output, e.g. for change of voltage, for change of frequency, for change of number of phases with intermediate conversion into dc by static converters using discharge tubes or semiconductor devices to convert the intermediate dc into ac using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only
- H02M5/4585—Conversion of ac power input into ac power output, e.g. for change of voltage, for change of frequency, for change of number of phases with intermediate conversion into dc by static converters using discharge tubes or semiconductor devices to convert the intermediate dc into ac using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only having a rectifier with controlled elements
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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
- H02J2203/00—Indexing scheme relating to details of circuit arrangements for AC mains or AC distribution networks
- H02J2203/20—Simulating, e g planning, reliability check, modelling or computer assisted design [CAD]
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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
- H02J2300/00—Systems for supplying or distributing electric power characterised by decentralized, dispersed, or local generation
- H02J2300/20—The dispersed energy generation being of renewable origin
- H02J2300/22—The renewable source being solar energy
- H02J2300/24—The renewable source being solar energy of photovoltaic origin
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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
- H02J2300/00—Systems for supplying or distributing electric power characterised by decentralized, dispersed, or local generation
- H02J2300/20—The dispersed energy generation being of renewable origin
- H02J2300/28—The renewable source being wind energy
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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
- Y02E10/00—Energy generation through renewable energy sources
- Y02E10/50—Photovoltaic [PV] energy
- Y02E10/56—Power conversion systems, e.g. maximum power point trackers
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Abstract
The invention provides a power grid simulator based on a nine-switch converter and an application method thereof. In the current transformation unit, a nine-switch converter topology is adopted to replace a traditional back-to-back twelve-switch converter topology, and an upper bridge arm of the nine-switch converter is connected with three-phase alternating current through a three-phase transformer; the lower bridge arm is connected with a simulator load through an LC filter. The control unit adopts a same-frequency modulation mode to realize the output amplitude and the phase-adjustable voltage of the nine-switch converter according to the topological structure and the mathematical model of the nine-switch converter. The power grid simulator can simulate and output standard power grid voltage, and can simulate common faults such as power grid voltage amplitude drop, phase offset and the like. Compared with the traditional power grid simulator based on the back-to-back converter, the power grid simulator based on the back-to-back converter can save 3 switching tubes, reduce the size of the converter, reduce the system cost, and is simple in structure and easy to realize and control.
Description
Technical Field
The invention relates to a power grid voltage generating device, in particular to a power grid simulator based on a nine-switch converter and an application method thereof.
Background
The grid-connected operation of distributed power generation systems such as wind power generation and solar photovoltaic power generation is acknowledged by experts of scholars to save cost, reduce energy consumption and improve the economy, stability and flexibility of a power system. However, as a large-scale distributed system is connected to a power grid, negative influences of the distributed power generation system on the power grid are gradually highlighted, for example, faults of voltage amplitude drop, sudden rise, voltage phase shift and the like occur in the power grid. In the research of the integration of the distributed system into the power grid, it is very important to know the operation characteristics of the power system under the faults. However, since the goal of the grid is to provide a standard three-phase sinusoidal voltage, various forms of grid faults are rare and, within the scope of laboratory conditions, these faults are difficult to reproduce by the grid alone. Therefore, there is a need for a device, i.e. a grid simulator, that is capable of reproducing grid faults in accordance with test requirements.
According to the research results of the power grid simulator for years at home and abroad, the power grid simulator devices can be summarized into three types: impedance-based versions, transformer-based versions, and power electronics conversion-based versions. The power grid simulator based on the power electronic conversion form is flexible in operation and powerful in function, and can meet testing requirements under various fault conditions.
At present, the power grid simulator topological structure based on the power electronic conversion form is used more as follows:
1. the back-to-back type twelve-switch converter topology is characterized in that a three-phase PWM rectifier is adopted in a rectification link, and a three-phase PWM inverter is adopted in an inversion link;
2. the three-phase separation structure topology, the three-phase PWM rectifier is adopted in the rectification link, and 3 single-phase inverters are adopted in the inversion link. Meanwhile, in order to avoid the occurrence of short circuit at the direct current side, a transformer is required to be used for isolation;
3. the fundamental and harmonic decomposition type structure topology is characterized in that the whole system is divided into a fundamental wave part and a harmonic part, the fundamental wave rectification module and the harmonic rectification module adopt three-phase PWM rectifiers, and the fundamental wave inversion module and the harmonic inversion module respectively adopt 3 single-phase inverters, which is equivalent to two three-phase separation structure topologies;
4. the multi-level fundamental-harmonic decomposition type structure is characterized in that the whole system is also divided into a fundamental wave part and a harmonic wave part, a fundamental wave generation module adopts a high-power low-switching-frequency three-level back-to-back system, and a harmonic wave generation module adopts a low-power high-bandwidth three-level back-to-back system;
5. the multilevel DC-DC structure is mainly characterized in that a plurality of power modules form a power converter, and the power converter has three phases in total. A power module comprises a power frequency diode, a DC-DC converter and a single-phase H-bridge inverter.
The topological structure and the control are complex, the number of switching devices is large, the size is large, and the manufacturing cost is high.
Disclosure of Invention
The invention aims to provide a power grid simulator based on a nine-switch converter, which can not only operate in four quadrants to realize the feedback of electric energy to a power grid, but also provide three-phase power grid voltage output and simulate various power grid fault conditions such as voltage drop, phase offset and the like; compared with the topological structure, the power grid simulator based on the nine-switch converter is lower in cost, smaller in size and less in loss.
The invention adopts the following technical scheme:
a power grid simulator based on a nine-switch converter comprises an input unit, an output unit, a current transformation unit and a control unit, wherein the output end of the input unit is connected with the output unit through the current transformation unit; the current transformation unit adopts a nine-switch current transformer; the control unit obtains an inverter side PWM control signal by detecting the inverter side three-phase voltage, performing coordinate transformation and PI control regulation, thereby controlling the on and off of a nine-switch converter power device in the converter unit and generating the required simulated power grid voltage.
Further, the input unit comprises a three-phase transformer and an inductor L1; the output unit comprises an LC filter and an output port.
Furthermore, the nine-switch converter is provided with three parallel-connected bridge arms, each bridge arm comprises three switch tubes connected in series, and the three switch tubes of the upper bridge arm are S respectivelyA、SB、SC(ii) a Three switching tubes of the lower bridge arm are respectively SU、SV、SW(ii) a The remaining switch tubes of the three switch tubes of each bridge arm except the two switch tubes are respectively SAU、SBV、SCW(ii) a The above 9 switch tubes constitute the main circuit of the converter, the upper output ports of the nine-switch converter constitute the rectification side, the three-phase transformer T of the input unit is connected with the power grid, the other lower output port constitutes the inversion side, and the analog signals are output through the LC filter of the output unit.
An application method of the power grid simulator based on the nine-switch converter comprises the following steps:
firstly, establishing mathematical models of a rectification side and an inversion side according to the topology of a nine-switch converter by using a switch function method;
step two, obtaining the specific working state of the whole nine-switch converter according to the mathematical model obtained in the step one, and selecting a same-frequency mode as a modulation mode of the nine-switch converter;
and step three, according to the same-frequency modulation mode selected in the step two and by combining with a corresponding control mode, obtaining output voltage with high response speed and high quality, thereby improving the efficiency and reliability of the power grid simulator.
Further, the first step specifically includes: defining the switch state of the switch tube as follows:
wherein x, a, b and c respectively correspond to A, B, C three arms of the equivalent rectifying side part of the corresponding nine-switch converter, and the A, B, C three-phase output voltage is Uau、Ubu、UcuAnd the inductances at the output terminals are respectively Lau、Lbu、LcuThe rated voltage of each phase of analog output is Ula、Ulb、UlcThe current flowing through each phase is iau、ibu、icuAccording to the switching mode of the switching device and an equivalent rectification side loop equation, an equivalent rectification side mathematical model expression of the nine-switch converter is obtained as follows:
the equivalent inverting side switching function of the nine-switch converter is as follows:
wherein x, a, b and c respectively correspond to A, B, C three bridge arms of the corresponding nine-switch inversion side part, and A, B, C three-phase output voltages are respectively Ual、Ubl、UclTo transportThe output end inductances are respectively Lal、Lbl、LclThe rated voltage of each phase of analog output is Ula、Ulb、UlcThe current flowing through each phase is ial、ibl、iclAccording to the switching mode of the switching device and the equivalent inversion side loop equation, the equivalent inversion side mathematical model expression of the nine-switch converter is obtained as follows:
according to the kirchhoff current law, obtaining the direct current side stroke of the nine-switch converter:
wherein idcu、idclThe direct current input current i of the equivalent rectifying side and the inversion side of the nine-switch converter respectivelydcThe total current flows through the direct-current side capacitor.
Furthermore, in the second step, the modulation modes of the nine-switch converter are divided into a CF mode and a DF mode according to whether the voltage frequencies of the two output ports are the same.
Further, in the CF mode, the frequencies of the output voltages of the two ports are the same, and only the amplitude and the phase are different; modulating wave signals U of upper and lower output ports of the nine-switch converterrefH、UrefLCommon triangular wave modulation; if U isrefHIf the carrier wave is larger than the carrier wave, the corresponding switch tube SA、SB、SCConducting, otherwise, turning off; if U isrefLIf the carrier wave is larger than the carrier wave, the corresponding switch tube SU、SV、SWTurning off, otherwise, turning on; and the driving signal of the middle tube of each bridge arm is determined by the XOR of the upper and lower switching tubes of the corresponding phase.
Further, the third step is specifically:
the inverter side of the nine-switch converter is controlled by a double closed loop, wherein the voltage of a direct current bus controls an outer loop, and after a reference value of the voltage of the direct current bus is compared with an actually measured value, d-axis and q-axis current instruction values are respectively generated by a PI controller and given power factors;
and inputting a standard power grid current instruction value or a power grid fault working condition current instruction value to be simulated into a current inner loop formed by a decoupling PI controller to generate corresponding d-axis and q-axis modulation signals, finally obtaining a three-phase modulation signal through Park inverse transformation, and finishing output control on the inversion side of the nine-switch converter according to a PWM (pulse-width modulation) method.
Compared with a back-to-back twelve-switch converter topology adopted by a traditional power grid simulator, the power grid simulator has the beneficial effects that:
according to the invention, firstly, mathematical models of a rectification side and an inversion side are given according to a topological structure of the nine-switch converter, then a same-frequency modulation mode and a corresponding control strategy are adopted, and finally, amplitude and phase-adjustable voltage can be output at the nine-switch inversion side, so that common faults such as power grid voltage drop, phase offset and the like are simulated, and the function of a power grid simulator is integrally realized; from their respective circuit topologies, mathematical models and control strategies, it can be verified that the back-to-back twelve-switch converter and the nine-switch converter: the power grid simulator based on the nine-switch converter can save 3 switch tubes, reduce the size of the converter, reduce the system cost, has a simple structure, is easy to realize and control, and can simulate common faults such as power grid voltage drop and phase deviation.
Drawings
FIG. 1 is a block diagram of one embodiment of a nine-switch converter based grid simulator according to the present invention;
FIG. 2 is a circuit topology diagram of one embodiment of a grid simulator based on a nine-switch converter according to the present invention;
FIG. 3 is an equivalent rectification side topology diagram of a nine-switch converter of the present invention;
FIG. 4 is an equivalent inverter side topology diagram of a nine-switch converter of the present invention;
fig. 5 is a diagram illustrating a process of generating switching states of a nine-switch converter according to the present invention.
Detailed Description
In order to make the objects, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be 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 some, but not all, embodiments of the present invention. All other embodiments, which can be obtained by a person skilled in the art without any inventive step based on the embodiments of the present invention, are within the scope of the present invention.
As shown in fig. 1, one embodiment of a grid simulator based on a nine-switch converter includes four units: the power grid comprises an input unit 10, an output unit 20, a variable flow unit 30 and a control unit 40, wherein the input end of the input unit 10 is connected with a power grid, the output end of the input unit 10 is connected with the output unit 20 through the variable flow unit 30, and the control unit 40 is connected with the variable flow unit 30 and the output unit 20.
Referring further to fig. 2, the input unit 10 includes a three-phase transformer T and an inductor L1; the output unit 20 includes an LC filter and an output port; an upper bridge arm of the nine-switch converter is connected with three-phase alternating current through a three-phase transformer T, and a lower bridge arm of the nine-switch converter is connected with a simulator load through an LC filter.
The converter unit 30 adopts a nine-switch converter to replace the existing back-to-back twelve-switch converter; the control unit 40 detects the three-phase voltage at the inversion side of the nine-switch converter in the converter unit 30 through a mutual inductor, and obtains an inversion side PWM control signal through coordinate transformation and PI control regulation, so that the on and off of the power devices of the nine-switch converter in the converter unit 30 are controlled, and the required simulated power grid voltage is generated; meanwhile, in order to control the direct-current side voltage of the nine-switch converter to be a constant value, the direct-current side voltage of the nine-switch converter needs to be introduced into a control loop; to ensure power balance, current control is introduced. On the whole, the control unit 40 realizes a function of controlling the on and off of the nine-switch converter power device in the converter unit 30 with the required grid voltage as a target, so that the converter unit 30 can output the required grid voltage on one hand, and the dynamic response speed and the output voltage quality of the whole converter unit 30 can be ensured on the other hand.
As shown in fig. 2, the power grid simulator topology based on the nine-switch converter of the present invention is shown, the converter unit 30 is a nine-switch converter, the nine-switch converter has three parallel-connected bridge arms, each bridge arm includes three switch tubes connected in series, and the three switch tubes of the upper bridge arm are respectively SA、SB、SC(ii) a Three switching tubes of the lower bridge arm are respectively SU、SV、SW(ii) a The remaining switch tubes of the three switch tubes of each bridge arm except the two switch tubes are respectively SAU、SBV、SCW. The above 9 switching tubes constitute the main circuit of the converter, the upper output ports of the nine-switch converter constitute the rectification side, which is connected to the grid through the three-phase transformer T of the input unit 10, and the other lower output port constitutes the inversion side, which outputs the analog signal through the LC filter of the output unit 20.
The embodiment of the invention also provides an application method of the power grid simulator based on the nine-switch converter, the power grid simulator based on the nine-switch converter is used as a power grid voltage simulator in the research of distributed power generation, can be used for simulating and outputting standard power grid voltage, and can also simulate common faults such as power grid voltage drop and phase offset, and the method comprises the following specific steps:
firstly, a switching function method is utilized to establish mathematical models of a rectifying side and an inverting side according to the topology of a nine-switch converter.
An equivalent rectification side model of the nine-switch converter is shown in fig. 3, and a lower bridge arm comprises three switching tubes SU、SV、SWAnd (3) fully conducting, and defining the switching state of the switching tube as follows:
where, x ═ a, b, and c correspond to three arms A, B, C of the equivalent rectification side portion of the corresponding nine-switch converter, respectively. A. B, C three-phase output voltages are respectivelyUau、Ubu、UcuAnd the inductances at the output terminals are respectively Lau、Lbu、LcuThe rated voltage of each phase of analog output is Ula、Ulb、UlcThe current flowing through each phase is iau、ibu、icu. According to the switching mode of the switching device and the equivalent rectification side loop equation, the equivalent rectification side mathematical model expression of the nine-switch converter can be obtained as follows:
an equivalent inversion side model of the nine-switch converter is shown in fig. 4, and an upper bridge arm comprises three switching tubes SA、SB、SCAnd (4) fully conducting. The switching function of the equivalent inverting side of the nine-switch converter is defined as:
wherein x, a, b and c respectively correspond to A, B, C three bridge arms of the corresponding nine-switch inversion side part, and A, B, C three-phase output voltages are respectively Ual、Ubl、UclAnd the inductances at the output terminals are respectively Lal、Lbl、LclThe rated voltage of each phase of analog output is Ula、Ulb、UlcThe current flowing through each phase is ial、ibl、icl. According to the switching mode of the switching device and the equivalent inversion side loop equation, the equivalent inversion side mathematical model expression of the nine-switch converter can be obtained as follows:
according to kirchhoff current law, the direct current side stroke of the nine-switch converter can be obtained:
wherein idcu、idclThe direct current input current i of the equivalent rectifying side and the inversion side of the nine-switch converter respectivelydcThe total capacitance flows for the dc side capacitance.
And step two, obtaining the specific working state of the whole nine-switch converter according to the mathematical model obtained in the step one, and selecting a same-frequency mode as a modulation mode of the nine-switch converter.
According to whether the voltage frequencies of the two output ports are the same or not, the modulation modes of the nine-switch converter can be divided into a common Frequency mode (CF) and a pilot Frequency mode (DF).
The embodiment of the invention selects the CF mode, the frequencies of the output voltages of the two ports are the same, and only the amplitude and the phase are different. As shown in FIG. 5, wherein SXRepresenting the drive signal of the upper switch, SYIndicating the down switch drive signal, SXYRepresenting the mid-way switch drive signal. Modulating wave signals U of upper and lower output ports of the nine-switch converterrefH、UrefLCommon triangular wave modulation; if U isrefHIf the carrier wave is larger than the carrier wave, the corresponding switch tube SA、SB、SCConducting, otherwise, turning off; if U isrefLIf the carrier wave is larger than the carrier wave, the corresponding switch tube SU、SV、SWTurning off, otherwise, turning on; and the driving signal of the middle tube of each bridge arm is determined by the XOR of the upper and lower switching tubes of the corresponding phase. To avoid two modulated waves UrefH、UrefLThe phenomenon that three switching tubes of the same bridge arm are simultaneously turned off caused by crossing needs to introduce proper direct current offset into a modulation signal, so that the voltage of an upper bridge arm is always higher than that of a lower bridge arm, and the voltage drop fault of a power grid can be simulated.
And step three, according to the same-frequency modulation mode selected in the step two and by combining with a corresponding control mode, obtaining output voltage with high response speed and high quality, thereby improving the efficiency and reliability of the power grid simulator.
Specifically, the inverter side of the nine-switch converter adopts double closed-loop control, as shown in the control unit in fig. 2, wherein the dc bus voltage controls the outer loop by referring to the dc bus voltage reference value
And the measured value VdcAfter comparison, d-axis current command values I are respectively generated through a PI controller and given power factorsrdAnd q-axis current command value Irq；
The standard power grid current instruction value or the power grid fault working condition current instruction value to be simulated is used
And
and inputting a current inner ring formed by a decoupling PI controller to generate corresponding d-axis and q-axis modulation signals, finally obtaining three-phase modulation signals through Park inverse transformation, and finishing output control on the inversion side of the nine-switch converter according to a PWM (pulse-width modulation) method.
By combining a same-frequency modulation mode and a corresponding control mode, the power grid simulator based on the nine-switch converter can realize the functions of outputting standard power grid voltage and simulating voltage drop and phase offset faults.
According to the invention, mathematical models of a rectification side and an inversion side are given according to a topological structure of the nine-switch converter, then a same-frequency modulation mode and a corresponding control strategy are adopted, and finally, amplitude and phase-adjustable voltage can be output at the nine-switch inversion side, so that common faults such as grid voltage drop, phase offset and the like are simulated, and the function of a grid simulator is integrally realized; from their respective circuit topologies, mathematical models and control strategies, it can be verified that the back-to-back twelve-switch converter and the nine-switch converter: the power grid simulator based on the nine-switch converter can save 3 switch tubes, reduce the size of the converter, reduce the system cost, has a simple structure, is easy to realize and control, and can simulate common faults such as power grid voltage drop and phase deviation.
The above description is only an 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 (8)
1. The utility model provides a power grid simulator based on nine switching converters which characterized in that: the converter comprises an input unit, an output unit, a converter unit and a control unit, wherein the output end of the input unit is connected with the output unit through the converter unit, and the control unit is connected with the converter unit and the output unit;
the current transformation unit adopts a nine-switch current transformer;
the control unit obtains an inverter side PWM control signal by detecting the inverter side three-phase voltage, performing coordinate transformation and PI control regulation, thereby controlling the on and off of a nine-switch converter power device in the converter unit and generating the required simulated power grid voltage.
2. The grid simulator based on the nine-switch converter as claimed in claim 1, wherein: the input unit comprises a three-phase transformer and an inductor L1; the output unit comprises an LC filter and an output port.
3. The grid simulator based on the nine-switch converter as claimed in claim 2, wherein: the nine-switch converter is provided with three parallel-connected bridge arms, each bridge arm comprises three switch tubes connected in series, and the three switch tubes of the upper bridge arm are S respectivelyA、SB、SC(ii) a Three switching tubes of the lower bridge arm are respectively SU、SV、SW(ii) a The remaining switch tubes of the three switch tubes of each bridge arm except the two switch tubes are respectively SAU、SBV、SCW(ii) a The above 9 switch tubes constitute the main circuit of the converterThe upper output port of the nine-switch converter forms a rectification side and is connected with a power grid through a three-phase transformer T of the input unit, the other lower output port of the nine-switch converter forms an inversion side, and analog signals are output through an LC filter of the output unit.
4. A method for applying a nine-switch converter based grid simulator according to any of claims 1-3, characterized in that: the method comprises the following steps:
firstly, establishing mathematical models of a rectification side and an inversion side according to the topology of a nine-switch converter by using a switch function method;
step two, obtaining the specific working state of the whole nine-switch converter according to the mathematical model obtained in the step one, and selecting a same-frequency mode as a modulation mode of the nine-switch converter;
and step three, according to the same-frequency modulation mode selected in the step two and by combining with a corresponding control mode, obtaining output voltage with high response speed and high quality, thereby improving the efficiency and reliability of the power grid simulator.
5. The method for applying the nine-switch converter based grid simulator according to claim 4, wherein: the first step specifically comprises: defining the switch state of the switch tube as follows:
wherein x, a, b and c respectively correspond to A, B, C three arms of the equivalent rectifying side part of the corresponding nine-switch converter, and the A, B, C three-phase output voltage is Uau、Ubu、UcuAnd the inductances at the output terminals are respectively Lau、Lbu、LcuThe rated voltage of each phase of analog output is Ula、Ulb、UlcThe current flowing through each phase is iau、ibu、icuObtaining the equivalent rectification side loop equation according to the switching mode of the switching deviceThe expression of an equivalent rectification side mathematical model of the nine-switch converter is as follows:
the equivalent inverting side switching function of the nine-switch converter is as follows:
wherein x, a, b and c respectively correspond to A, B, C three bridge arms of the corresponding nine-switch inversion side part, and A, B, C three-phase output voltages are respectively Ual、Ubl、UclAnd the inductances at the output terminals are respectively Lal、Lbl、LclThe rated voltage of each phase of analog output is Ula、Ulb、UlcThe current flowing through each phase is ial、ibl、iclAccording to the switching mode of the switching device and the equivalent inversion side loop equation, the equivalent inversion side mathematical model expression of the nine-switch converter is obtained as follows:
according to the kirchhoff current law, obtaining the direct current side stroke of the nine-switch converter:
wherein idcu、idclThe direct current input current i of the equivalent rectifying side and the inversion side of the nine-switch converter respectivelydcThe total current flows through the direct-current side capacitor.
6. The method for applying the nine-switch converter based grid simulator according to claim 4, wherein: in the second step, the modulation modes of the nine-switch converter are divided into a CF mode and a DF mode according to whether the voltage frequencies of the two output ports are the same or not.
7. The method for applying the nine-switch converter based grid simulator according to claim 6, wherein: in the CF mode, the frequencies of the output voltages of the two ports are the same, and only the amplitude and the phase are different; modulating wave signals U of upper and lower output ports of the nine-switch converterrefH、UrefLCommon triangular wave modulation; if U isrefHIf the carrier wave is larger than the carrier wave, the corresponding switch tube SA、SB、SCConducting, otherwise, turning off; if U isrefLIf the carrier wave is larger than the carrier wave, the corresponding switch tube SU、SV、SWTurning off, otherwise, turning on; and the driving signal of the middle tube of each bridge arm is determined by the XOR of the upper and lower switching tubes of the corresponding phase.
8. The method for applying the nine-switch converter based grid simulator according to claim 4, wherein: the third step is specifically as follows:
the inverter side of the nine-switch converter is controlled by a double closed loop, wherein the voltage of a direct current bus controls an outer loop, and after a reference value of the voltage of the direct current bus is compared with an actually measured value, d-axis and q-axis current instruction values are respectively generated by a PI controller and given power factors;
and inputting a standard power grid current instruction value or a power grid fault working condition current instruction value to be simulated into a current inner loop formed by a decoupling PI controller to generate corresponding d-axis and q-axis modulation signals, finally obtaining a three-phase modulation signal through Park inverse transformation, and finishing output control on the inversion side of the nine-switch converter according to a PWM (pulse-width modulation) method.
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