CN113419588A - Two-stage cascade converter system stability analysis method based on impedance characteristics - Google Patents
Two-stage cascade converter system stability analysis method based on impedance characteristics Download PDFInfo
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- CN113419588A CN113419588A CN202110746828.1A CN202110746828A CN113419588A CN 113419588 A CN113419588 A CN 113419588A CN 202110746828 A CN202110746828 A CN 202110746828A CN 113419588 A CN113419588 A CN 113419588A
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- G05F—SYSTEMS FOR REGULATING ELECTRIC OR MAGNETIC VARIABLES
- G05F1/00—Automatic systems in which deviations of an electric quantity from one or more predetermined values are detected at the output of the system and fed back to a device within the system to restore the detected quantity to its predetermined value or values, i.e. retroactive systems
- G05F1/10—Regulating voltage or current
- G05F1/46—Regulating voltage or current wherein the variable actually regulated by the final control device is dc
- G05F1/56—Regulating voltage or current wherein the variable actually regulated by the final control device is dc using semiconductor devices in series with the load as final control devices
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Abstract
The invention discloses a method for analyzing the system stability of a two-stage cascade converter based on impedance characteristics, which is also suitable for the research of the overall stability of three-stage PET grid connection. It comprises the following steps: establishing a state space small signal unified mathematical model of a cascade H-bridge PET and an inverter by taking a three-level cascade PET as a research object, and evaluating the stability difference of the system according to an impedance stability criterion; the influence of the controller parameters on the stability of the PET equivalent circuit model is analyzed and researched, and the parameter range and the stability boundary are further determined; and a simulation model of the cascade H-bridge type PET-inverter is established, and the fact that the change of the proportional coefficient of the inverter can influence the stability of a two-stage converter system by influencing an input impedance frequency characteristic curve is pointed out, so that a theoretical basis is provided for setting a reasonable stability margin. The method can accurately judge the near steady state of the system, improves the stability and robustness of the system, and has wide application range.
Description
Technical Field
The invention relates to a method for analyzing the system stability of a two-stage cascade converter based on impedance characteristics, which is also suitable for analyzing the integral grid-connected stability of a three-stage power electronic transformer.
Background
With the introduction of energy source internet and the continuous development of power electronic technology, the limitations of the traditional power converter in the aspects of energy configuration range, comprehensive control capability, electric energy quality and the like gradually appear. As a novel power conversion device, the power electronic transformer not only can realize voltage grade conversion and electrical isolation of the traditional transformer, but also has the advantages of electric energy quality isolation, active alternating current and direct current energy management, reactive compensation, higher controllability and the like. A three-stage module cascade Power Electronic Transformer (PET) is one of the most widely studied PET topologies, and has a good application background in distributed power generation systems and distributed energy storage systems due to its good control characteristics. However, such PET has more power modules and a complex structure, and the interaction between subsystems easily causes system instability and power quality problems.
At present, most of researches on the stability problem of power electronic transformers adopt an impedance modeling analysis method. Unlike inverters and DC/DC converters, PET is a typical multiple-input multiple-output system, and this type of circuit topology includes two DC buses on the high and low voltage sides, and the control is more complicated, so it is extremely complicated to analyze the impedance characteristics. In the prior art, a CHB and DAB two-stage system of PET can be used as a whole for stability analysis. However, as the number of submodules increases, the stability problem of PET has to be further investigated. Therefore, a unified mathematical model of the PET equivalent circuit small signal needs to be established, the modeling difficulty is reduced, the parameter range and the stable boundary are determined, and the system stability is improved.
Disclosure of Invention
In view of the above technical problems, the present invention provides a method for analyzing system stability of a two-stage cascaded converter based on impedance characteristics, which takes a three-stage cascaded PET as an example.
The technical scheme for solving the technical problems is as follows:
establishing a modeling general formula of a PET equivalent circuit full-system linearization state space model, wherein the modeling general formula comprises two modules of a cascade H-bridge type PET and an inverter;
analyzing and researching the influence of the controller parameters on the stability of the PET equivalent circuit model, and further determining the parameter range and the stability boundary;
and evaluating the stability difference of the system by adopting an impedance stability criterion, thereby providing a theoretical basis for setting a reasonable stability margin.
Drawings
The accompanying drawings, which are incorporated in and constitute a part of this application, illustrate embodiments of the invention and, together with the description, serve to explain the invention and not to limit the invention. In the drawings:
FIG. 1 is a main circuit topology diagram of a power electronic transformer according to the present invention
FIG. 2 is a block diagram of DC voltage control proposed by the present invention
FIG. 3 is a block diagram of the inverter current single closed loop control proposed by the present invention
Detailed Description
The method comprises the following steps: establishing a cascade H-bridge type PET equivalent circuit as shown in figure 1; the equivalent circuit is characterized in that the isolation stage is equivalent to a first-order RL circuit by adopting a current average value equivalent model method, and the rectification stage and the isolation stage are integrated into a unified whole by eliminating intermediate variables, so that a cascade H-bridge type PET small signal mathematical model is established. The PET equivalent circuit small signal model is as follows:
in formulae (1) to (3), ed、eq、i1d、i1qD-axis and q-axis components of the ac voltage and the ac current, respectively; l issA network side inductor; u shapedc、CdcHigh-side voltage and high-side capacitance; u shape0、C0Respectively an isolation stage low-voltage side voltage and a low-voltage side capacitor,
and i0Respectively, a high-voltage side capacitor output current and a low-voltage side capacitor output current.
Step two: the control circuit adopts a simple open-loop control strategy as the isolation stage, so that the cascaded H-bridge type PET integral control strategy adopts a master-slave control strategy to maintain the voltage of a direct-current bus to be constant, namely, a direct-current voltage outer ring and power grid current inner ring control mode is adopted, as shown in figure 2; the output impedance under the working condition of the direct-current voltage closed-loop control mode can be obtained by combining the dynamic equation and the control circuit thereof
In the formula (4), Gu(s) is the voltage loop PI controller transfer function, Gi(s) is the current loop PI controller transfer function, A14To control the transfer function to the input current, A34To control the transfer function to the output voltage.
Step three: establishing a main circuit of a grid-connected inverter; wherein L is1Is an inverter side inductor, UinFor a DC bus input voltage, UgFor three-phase output voltage of the inverter ia、ib、icThree-phase currents of the inverter are respectively; and performing small disturbance and linearization processing near the static working point to obtain a small signal alternating current model under a dq coordinate system, namely the inverter state equation is as follows:
step four: in order to calculate the closed-loop input impedance of the inverter, a control block diagram of the inverter can be obtained on the basis of decoupling control and model simplification as shown in fig. 3, and the inverter control strategy in the invention has the advantages that: compared with the conventional double-loop control, the system stability can be realized by adopting a grid-connected inverter current single closed-loop control scheme; the input impedance of the inverter is derived further from the control circuit, i.e.
In the formula (6), Gci(s) is the transfer function of the inverter current loop PI controller, Dd＝Ugd/Uin，Kpwm＝Vin/VtriFor modulating wave to DC bus voltage U of inverterinThe transfer function of (2).
Step five: based on the state space method analysis, the cascade H-bridge PET output impedance and the inverter input impedance expression are deduced, and the influence of the dynamic response performance and the stability of the change system of the controller parameters is judged, so that the parameter range and the stability boundary can be further determined by analyzing and researching the difference of the controller parameters on the system stability.
Step six: the stability of the grid-connected system can be judged through the relation between Nyquist curves of two characteristic roots of the impedance ratio matrix and a (-1, j0) point, and control parameters can be selected in a proper range by utilizing an impedance stability criterion, so that the rapidity and the stability of the system are considered.
Step seven: in another embodiment of the present invention, system stability can be determined by determining whether the inverter output impedance is less than the load input impedance.
The following describes the optimized operation strategy of the present invention in further detail with reference to the accompanying drawings:
1) the method only needs to analyze and derive the small signal models of the cascaded H-bridge PET and the inverter by using a state space average value method, and does not need to analyze and derive each submodule, so that the complexity of the models is reduced, and the modeling difficulty is also reduced.
2) The influence of the parameters of the controller on the stability of the system is analyzed, and the Nyquist curve analysis shows that the stability of the system is greatly influenced by the proportional coefficient of the inverter and is less influenced by the coefficient of the cascaded H-bridge type PET controller, so that the rapidity and the stability of the system can be considered only by selecting the control parameters in a proper range.
3) The patent stability criterion of the invention is simple, and the method is flexible, so the used mathematical modeling and control strategy can be widely applied to the overall stability analysis of the general two-stage converter, and the application range is wide.
Claims (2)
1. A two-stage cascade converter system stability analysis method based on impedance characteristics comprises the following steps:
establishing a modeling general formula of a PET equivalent circuit full-system linearization state space model, and simplifying the modeling general formula into two modules of a cascade H-bridge type PET and an inverter;
analyzing and researching the influence of the controller parameters on the stability of the PET equivalent circuit model, and further determining the parameter range and the stability boundary;
and evaluating the stability difference of the system by adopting an impedance stability criterion, thereby providing a theoretical basis for setting a reasonable stability margin.
2. The method for analyzing the stability of the two-stage cascaded transformer system based on the impedance characteristic as claimed in claim 1, wherein the step of taking the three-stage cascaded PET as a research object comprises:
the method comprises the following steps: a three-stage PET system is divided into two parts, wherein one part is a unified whole by eliminating intermediate variables to enable a rectification stage and an isolation stage to be unified, so that a cascade H-bridge type PET equivalent circuit model is established. And carrying out mathematical modeling on the equivalent cascade H-bridge PET according to a small signal linearization processing method.
Step two: the cascade H-bridge PET output impedance can be obtained by combining a small signal state equation and a control circuit thereof;
step three: carrying out small disturbance and linearization processing on the inverter, and establishing an inverter input impedance model;
step four: and according to the cascade H-bridge PET output impedance and the inverter input impedance expression, intuitively and clearly reflecting the trend of the system stability along with the change of control parameters by using a system zero pole diagram, and determining a parameter range and a stable boundary.
Step five: and determining the stability range of the control parameter according to the impedance stability criterion.
Step six: the stability of the system can be evaluated by judging whether the output impedance of the inverter is smaller than the input impedance of the load or not, and the quality of the optimization result is quantitatively judged according to the parameter range determined in the step five; according to the analysis, the parameter design is carried out on the input and output impedance model so as to ensure that the system maintains sufficient stability margin and improve the stability of the system.
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Cited By (2)
| Publication number | Priority date | Publication date | Assignee | Title |
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| CN116093917A (en) * | 2023-03-22 | 2023-05-09 | 广东工业大学 | Multi-control parameter optimization design method for energy storage power station |
| CN116644699A (en) * | 2023-05-29 | 2023-08-25 | 西南交通大学 | Cascade converter bus voltage stability analysis method based on voltage small signal disturbance |
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| CN110768299A (en) * | 2019-12-02 | 2020-02-07 | 湖南大学 | Sequence impedance modeling and stability analysis method of load virtual synchronous machine |
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| CN108767899A (en) * | 2018-06-19 | 2018-11-06 | 哈尔滨理工大学 | A kind of low pressure micro-capacitance sensor small disturbed stability analysis method |
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Cited By (4)
| Publication number | Priority date | Publication date | Assignee | Title |
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| CN116093917A (en) * | 2023-03-22 | 2023-05-09 | 广东工业大学 | Multi-control parameter optimization design method for energy storage power station |
| CN116093917B (en) * | 2023-03-22 | 2023-08-04 | 广东工业大学 | Multi-control parameter optimization design method for energy storage power station |
| CN116644699A (en) * | 2023-05-29 | 2023-08-25 | 西南交通大学 | Cascade converter bus voltage stability analysis method based on voltage small signal disturbance |
| CN116644699B (en) * | 2023-05-29 | 2023-12-29 | 西南交通大学 | Cascade converter bus voltage stability analysis method based on voltage small signal disturbance |
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