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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Abstract
本发明公开了一种基于阻抗特性的两级级联变换器系统稳定性分析方法，该方法同样适用于三级式PET并网整体稳定性研究。它包括以下步骤：以三级式级联型PET为研究对象，建立了级联H桥型PET和逆变器的状态空间小信号统一数学模型，根据阻抗稳定性判据评估系统稳定性差异；分析研究了控制器参数对PET等效电路模型稳定性的影响，进一步确定参数范围以及稳定边界；并通过建立了级联H桥型PET‑逆变器的仿真模型，指出逆变器比例系数的变化会通过影响输入阻抗频率特性曲线来影响两级变换器系统稳定性，进而为设置合理的稳定裕度提供理论基础。本发明可准确判断系统的临近稳定状态，提高了系统的稳定性与鲁棒性，且适用范围广。The invention discloses a stability analysis method of a two-stage cascade converter system based on impedance characteristics, and the method is also applicable to the research on the overall stability of a three-stage PET grid connection. It includes the following steps: taking the three-stage cascaded PET as the research object, establishing the state-space small-signal unified mathematical model of the cascaded H-bridge PET and the inverter, and evaluating the system stability difference according to the impedance stability criterion; The influence of the controller parameters on the stability of the PET equivalent circuit model was analyzed and studied, and the parameter range and stability boundary were further determined; and the simulation model of the cascaded H-bridge PET-inverter was established, and the proportional coefficient of the inverter was pointed out. The change will affect the stability of the two-stage converter system by affecting the input impedance frequency characteristic curve, thus providing a theoretical basis for setting a reasonable stability margin. The invention can accurately judge the near stable state of the system, improve the stability and robustness of the system, and has a 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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Cited By (4)
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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 |
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| 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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Application publication date: 20210921 |