A method for constructing a safe working domain of a grid-connected converter system

Abstract

A method for constructing a safe working domain of a grid-following converter system comprises the following steps: firstly, small signal modeling is performed on the power loop, current loop and power grid circuit of the grid-following converter, and working point variables are introduced by using the dynamic characteristics of a phase-locked loop and a power loop, so as to establish a multi-input and multi-output small signal model of the grid-following converter system containing working point variables; then, the small signal model is used to derive the power loop control analytical expression of the power reference disturbance value and the output current disturbance value considering the influence of the phase-locked loop and the power grid strength, so as to obtain a single-input single-output closed-loop transfer function that can characterize the stability of the grid-following converter system; then, the initial value, analysis range and analysis step of the working point variable are determined, and the given value of the working point variable is substituted into the common denominator polynomial of the single-input single-output closed-loop transfer function to calculate the closed-loop poles, and the stability of the grid-following converter system under different working points is determined by judging the position of the poles, and thus a safe working domain diagram of the grid-following converter system in the full working range is drawn.

Description

Technical Field

[0001] The present invention relates to the technical field of power system converter control, and in particular to a method for constructing a safe working domain of a grid-connected converter system. Background Art

[0002] The stability of the grid-connected converter system is affected by the control loop. Based on the state space method and the impedance method, a large number of studies have been conducted to analyze the stability of the current loop. When the state space method in the time domain is used, the state space small signal model must be established first, and then the eigenvalue analysis method is used to study the stability of the current loop. When the system involves too many variables, the state matrix order is too high and the solution is very complicated. Compared with the state space method, the impedance method is simpler to model and has higher computational efficiency. Commonly used impedance models include ordered impedance models, and dq The impedance model in the coordinate system. The impedance method establishes the impedance model of the converter, compares it with the grid impedance, and uses the Nyquist criterion to perform stability analysis based on the obtained impedance. However, the current loop usually needs to be coordinated with the power loop. Ignoring the outer power loop and only considering the stability analysis of the inner current loop may be inaccurate. Therefore, it is necessary to comprehensively consider the power loop and the current loop for a more comprehensive analysis.

[0003] Typical power loops include grid-building type and grid-following type. Grid-building type control includes droop control, virtual synchronous machine control, etc., and grid-following type control includes power control, etc. On the one hand, the grid-building type has attracted widespread attention due to its excellent characteristics of supporting system voltage and frequency. Comparing the impedance characteristics of the grid-building type and the grid-following type, the grid-building type has a smaller impedance amplitude and a perceptual phase. Its stability under weak grid conditions is better than that of the grid-following type, while the inverter controlled by the grid-following type may become unstable when connected to a strong grid. On the other hand, the grid-following type technology is mature and widely used in power grids. It will be dominant for some time to come. It is of great significance to study the stability of the grid-following type converter.

[0004] Many studies only consider a single operating point in the stability analysis of the power conversion system. However, in actual operation, the system operating point has a strong time-varying characteristic. The stability under a single operating point cannot reflect the working characteristics of the entire operation cycle. In order to analyze the impact of the operating point on stability, there are two technical routes in existing research. One is to select multiple discrete operating points for analysis separately to expand the analysis range of stability, such as analyzing the change law of the positive or negative impedance resistance caused by wind speed changes, and then analyzing the impact of wind speed variables on system stability; the second is to introduce the operating point information into the modeling and use the model containing the operating point information to analyze the system stability. For example, wind speed, light intensity, etc. are introduced as operating point variables into the impedance modeling process, a binary model containing operating point variables is established, and then the stability of the system in the full wind speed and light intensity interval is analyzed using a three-dimensional Bode diagram and an amplitude phase contour diagram. The existing analysis method is only applicable to the analysis of a single operating point variable. When there are multiple operating point variables, it is difficult to simultaneously analyze the impact of multiple operating point variables on system stability due to the limitation of dimensionality. Summary of the invention

[0005] The technical problem to be solved by the present invention is to provide a method for constructing a safe operating domain of a grid-following converter system. The method utilizes the dynamic characteristics of a phase-locked loop and a power loop to introduce operating point variables into the modeling process, obtains a variable coefficient transfer function that can characterize the stability of the system, and depicts the safe operating domain of the system, so as to simply and effectively obtain the stability of the grid-following converter system under time-varying working conditions.

[0006] In order to solve the above technical problems, the present invention adopts the following technical methods: a method for constructing a safe working domain of a grid-connected converter system, comprising:

[0007] Step S1, small signal modeling is performed on the power loop, current loop and grid circuit of the grid-following converter, and the working point variable is introduced by using the dynamic characteristics of the phase-locked loop and the power loop, so as to establish a multi-input multi-output small signal model of the grid-following converter system containing the working point variable, and the intermediate variables involved in the multi-input multi-output small signal model are all represented by functions containing the working point variable;

[0008] Step S2, using the obtained multi-input multi-output small signal model, deriving the power loop control analytical expression of the power reference disturbance value and the output current disturbance value considering the influence of the phase-locked loop and the grid strength, and obtaining the single-input single-output closed-loop transfer function that can characterize the stability of the grid-following converter system;

[0009] Step S3, determine the initial value, analysis range and analysis step of the working point variable, substitute the given value of the working point variable into the common denominator polynomial of the single-input single-output closed-loop transfer function to calculate the closed-loop poles, and determine the stability of the grid-following converter system under different working points by judging the position of the poles. When the grid-following converter system remains stable, its corresponding working point is within the safe working domain, thereby drawing a safe working domain diagram of the grid-following converter system in the entire working range.

[0010] Furthermore, in step S1, when establishing a multi-input multi-output small signal model of the grid-following converter system, it includes:

[0011] S101, using a phase-locked loop structure based on a synchronous reference system; defining the actual grid-connected point voltage phase angle as , the phase-locked loop output voltage phase angle is , and The difference is , The corresponding coordinate system is the actual Coordinate system, The corresponding coordinate system is controlled Coordinate system, take Representing reality Physical quantities in the coordinate system, Delegate Control Physical quantity in coordinates, superscript " " represents the conjugate of a physical quantity, Representing reality Small perturbation in the coordinate system, Delegate Control Small disturbance in the coordinate system; determine the output angle of the phase-locked loop The expression is:

[0012] (1)

[0013] In the formula, represents a phase-locked loop proportional-integral controller, represents the phase-locked loop ratio parameter, represents the phase-locked loop integral parameter, represents the variable parameter in the Laplace transform, Represents the imaginary part of the grid connection point voltage; Indicates the rated angular frequency;

[0014] S102, determine the dynamic characteristics of the converter side in actual The coordinate system is:

[0015] (2)

[0016] (3)

[0017] In the formula, is the converter output voltage vector, , for of Axis component, for of Axis component, is an imaginary unit; is the current vector of the converter grid-connected point; is the voltage vector of the converter grid connection point; is the grid voltage vector; is the impedance corresponding to the filter inductor, , They are the filter inductor and the parasitic resistance of the filter inductor respectively; is the grid impedance, , are grid inductance and grid resistance respectively;

[0018] S103, determining whether the converter current loop controller and the power loop controller are in control In the coordinate system it is expressed as:

[0019] (4)

[0020] (5)

[0021] In the formula, represents the modulation voltage of the converter; Indicates the reference value of the converter current loop; , are the active power and reactive power output values ​​of the converter respectively; , are the reference values ​​of active power and reactive power of the converter respectively; represents the PI regulator of the current loop, Represents the current loop proportional parameter, Represents the current loop integral parameter; represents the PI regulator of the power loop, , Indicates the power loop ratio parameter, Represents the power loop integral parameter; is the conjugate value of the power reference value, Output power conjugate value.

[0022] Furthermore, in step S2, when deriving the power loop control analytical expression of the power reference disturbance value and the output current disturbance value considering the influence of the phase-locked loop and the grid strength, the following steps are included:

[0023] S201, determine the transformation relationship between the disturbance quantities of each voltage and current in the grid-connected converter system between different reference frames, which are:

[0024] (6)

[0025] In the formula, Represents the steady-state value vector of the grid-connected point current; is the steady-state value vector of the converter output voltage; is the grid-connected point voltage steady-state value vector;

[0026] S202, small disturbance analysis is performed on the dynamic characteristics of the converter side, the modulation voltage of the converter, and the reference value of the current loop, respectively, to obtain:

[0027] (7)

[0028] (8)

[0029] S203, determine that the converter output power is under control The perturbation relationship in the coordinate system is:

[0030] (9)

[0031] S204, using the conjugate relationship of complex numbers, determine the disturbance relationship of the converter phase-locked loop as:

[0032] (10)

[0033] (11)

[0034] In the formula, for of Axis component;

[0035] S205, the modulation voltage of the converter is changed from the control Convert the coordinate system to the actual In the coordinate system, the output voltage of the converter is connected with the modulation voltage through the delay link, and the following is obtained:

[0036] (12)

[0037] (13)

[0038] In the formula, is the PWM modulation gain, Represents the delay phase, The value is 1;

[0039] S206, combining the small disturbance relation of the converter side dynamic characteristics and the relation between the converter output voltage and the modulation voltage, obtain:

[0040] (14)

[0041] S207, combined equations (6)-(14), simplify the disturbance expression of the grid-connected point current to be:

[0042] (15)

[0043] (16)

[0044] S208, using the disturbance relation of the phase-locked loop, eliminate the Variables, reuse and The conjugate relationship of eliminates the ,get:

[0045] (17)

[0046] (18)

[0047] S209, according to equation (17) and equation (18), the power loop control analytical expression of the power reference disturbance value and the output current disturbance value is derived:

[0048] (19)

[0049] (20)

[0050] In the formula, , , , are four single-input single-output closed-loop transfer functions that characterize the stability of the grid-connected converter system; , Respectively represent the active and reactive power reference disturbance values ​​to the converter output current The influence of axis disturbance value; , Respectively represent the active and reactive power reference disturbance values ​​to the converter output current The influence of axis disturbance value; , , , The closed-loop poles of the common denominator polynomials are used to determine the stability of the grid-connected converter system.

[0051] Furthermore, step S3 includes the following steps:

[0052] S301, determining the operating point variables to be analyzed, and giving the initial value, analysis range, and analysis step length of each operating point variable;

[0053] S302, judging whether each working point is a meaningful point according to the circuit topology constraint relationship between the grid connection point voltage and the grid voltage; if the output power corresponding to the working point satisfies the circuit topology constraint, the working point is a meaningful point, and the next step of stability analysis is performed; if the output power corresponding to the working point does not satisfy the circuit topology constraint, the working point is a meaningless point, no stability analysis is required, and the stability analysis is performed. The operating point is marked as not satisfying the topological constraint; the expression of the circuit topological constraint relationship is as follows:

[0054] (twenty one)

[0055] (twenty two)

[0056] In the formula, , The grid voltage is axis, Axis component;

[0057] S303, substitute the given values ​​of the operating point variables that satisfy the circuit topology constraints and the initial parameters of the circuit into the common denominator polynomial of the four closed-loop transfer functions to solve the closed-loop poles. If all the poles are in the left half plane of the complex plane, it means that the grid-connected converter system is stable. Otherwise, it means the grid-connected converter system is unstable. Represents; stores the stability analysis results of the current working point;

[0058] S304, traversing all operating points that satisfy the circuit topology constraint, obtaining stability analysis results of all operating points, and storing them;

[0059] S305, according to the stability analysis results, the working point variables, and the circuit parameters, a safe working domain diagram of the system is drawn to obtain the safe working domain of the system in the entire working range; the entire analysis range can be divided into three domains, namely: safe working domain, corresponding to ; Instability domain, corresponding to ; meaningless domain, corresponding to .

[0060] Preferably, in step S303, formulas (2)-(16) and formulas (21)-(22) are combined to obtain the initial parameters of the circuit according to the given values ​​of the known operating point variables.

[0061] The method for constructing a safe working domain of a grid-following converter system involved in the present invention uses the dynamic characteristics of a phase-locked loop and a power loop to introduce working point variables into the modeling process, obtains a variable coefficient transfer function that can characterize the stability of the system, and depicts the safe working domain of the system, thereby determining the stability of the grid-following converter system under time-varying working conditions. Compared with typical single-point or discrete-point modeling and analysis methods, the present invention takes into account the converter power outer loop control and introduces multiple working point variables, which can not only more comprehensively and accurately analyze the stability of the converter in the entire working range and obtain the safe working domain of the system, but also simultaneously analyze the influence of multiple variables such as grid impedance parameters, phase-locked loop parameters, grid voltage parameters, etc. on the stability of the system. BRIEF DESCRIPTION OF THE DRAWINGS

[0062] Figure 1 The structure and control circuit diagram of a typical grid-following current conversion system in the present invention;

[0063] Figure 2 It is a flow chart of the method for constructing a safe working domain of a grid-connected converter system involved in the present invention;

[0064] Figure 3 Schematic diagram of the coordinate system of the network variable system in the present invention;

[0065] Figure 4 Draw a diagram for a safe working area in an embodiment of the present invention;

[0066] Figure 5 Schematic diagram of closed-loop poles corresponding to working point A in an embodiment of the present invention;

[0067] Figure 6 Schematic diagram of closed-loop poles corresponding to working point B in an embodiment of the present invention;

[0068] Figure 7 Schematic diagram of closed-loop poles corresponding to working point C in an embodiment of the present invention;

[0069] Figure 8 Schematic diagram of closed-loop poles corresponding to working point D in an embodiment of the present invention;

[0070] Fig. 9 This is a test operation result diagram of the grid-following converter system at working point A in the implementation of the present invention;

[0071] Fig.10 This is a test operation result diagram of the grid-following converter system at working point B in the implementation of the present invention;

[0072] Fig.11 This is a test operation result diagram of the grid-following converter system at the working point C in the implementation of the present invention;

[0073] Fig.12This is a test operation result diagram of the grid-following converter system at the working point D in the implementation of the present invention. DETAILED DESCRIPTION

[0074] In order to facilitate the understanding of those skilled in the art, the present invention is further described below in conjunction with embodiments and drawings. The contents mentioned in the implementation modes are not intended to limit the present invention.

[0075] Figure 1 This is a typical grid-following converter system structure and control circuit diagram. The control circuit consists of a power loop and a current loop. Indicates that the converter output voltage is expressed as Indicates that the grid connection point voltage and grid connection point current are , , , They are the filter inductor and the parasitic resistance of the filter inductor respectively; , are grid inductance and grid resistance respectively; , are the active power and reactive power output values ​​of the converter respectively; , are the reference values ​​of active power and reactive power of the converter respectively; , Represent the converter output current axis, Shaft current reference value; , Represents the output voltage of the converter current loop controller axis, Axis reference value.

[0076] like Figure 2 As shown, the method for constructing a safe working domain of a grid-connected converter system provided by the present invention comprises the following steps:

[0077] Step S1, small signal modeling is performed on the control circuit (power loop, current loop) and the grid circuit of the grid-following converter, and the operating point variables are introduced by using the dynamic characteristics of the phase-locked loop and the power loop, so as to establish a multi-input multi-output small signal model of the grid-following converter system containing the operating point variables. The intermediate variables involved in the multi-input multi-output small signal model are all represented by functions containing the operating point variables.

[0078] In order to synchronize with the power grid, a phase-locked loop structure based on a synchronous reference frame (SRF-PLL) is used. Due to the existence of SRF-PLL, if there is a voltage disturbance at the grid connection point, it will cause the actual grid connection point voltage phase angle Phase angle with the phase-locked loop output voltage difference ,like Figure 3 As shown. The corresponding actual The coordinate system will be Defined Control The coordinate system is denoted as , The corresponding physical quantity in the coordinate system is superscript Label.

[0079] PLL output angle The expression is:

[0080] (1)

[0081] In the formula, represents a phase-locked loop proportional-integral controller, represents the phase-locked loop ratio parameter, represents the phase-locked loop integral parameter, represents the variable parameter in the Laplace transform, Represents the imaginary part of the grid connection point voltage; Indicates the rated angular frequency.

[0082] Take the superscript " " represents the conjugate of a physical quantity, represents the steady-state value of the variable, Representing reality Physical quantities in the coordinate system, Delegate Control Physical quantities under coordinates, control Small perturbation in the coordinate system and actual The corresponding small perturbation in the coordinate system The relationship between can be expressed as:

[0083] (twenty three)

[0084] The dynamic characteristics of the converter side are The coordinate system is:

[0085] (2)

[0086] In the formula, the vector (complex quantity) expression of electrical quantity is expressed as , is the converter output voltage vector, , for of Axis component, for of Axis component, is an imaginary unit; is the current vector of the grid-connected point of the converter; is the voltage vector of the converter grid connection point; is the grid voltage vector; the impedance corresponding to the filter inductor and the grid impedance are:

[0087] (3)

[0088] In the formula, is the impedance corresponding to the filter inductor, , They are the filter inductor and the parasitic resistance of the filter inductor respectively; is the grid impedance, , are the grid inductance and grid resistance respectively.

[0089] Modulation voltage of the converter (i.e. the reference input for generating the PWM signal) is determined by the current controller, and the current loop controller and the power loop controller control In the coordinate system it is expressed as:

[0090] (4)

[0091] Reference value of current loop Can be simplified to:

[0092] (5)

[0093] In the formula, represents the modulation voltage of the converter; Indicates the reference value of the converter current loop; , are the active power and reactive power output values ​​of the converter respectively; , are the reference values ​​of active power and reactive power of the converter respectively; represents the PI regulator of the current loop, , Represents the current loop proportional parameter, Represents the current loop integral parameter; represents the PI regulator of the power loop, , Indicates the power loop ratio parameter, Represents the power loop integral parameter; is the conjugate value of the power reference value, Output power conjugate value.

[0094] Grid-connected converter system The disturbance of the axis component is affected by its initial value and the PLL output disturbance Therefore, the operating point variable is introduced into the SRF-PLL and power loop to establish a variable coefficient small signal model and a small signal model of the grid-connected converter system containing the operating point variable.

[0095] Step S2, using the obtained multi-input multi-output small signal model, derive the power loop control analytical expression of the power reference disturbance value and the output current disturbance value considering the influence of the phase-locked loop and the grid strength, and obtain the single-input single-output closed-loop transfer function that can characterize the stability of the grid-following converter system.

[0096] The transformation relationships of the disturbance quantities of each voltage and current between different reference frames are:

[0097] (6)

[0098] In the formula, Represents the steady-state value vector of the grid-connected point current; is the steady-state value vector of the converter output voltage; is the grid-connected point voltage steady-state value vector.

[0099] By performing small disturbance analysis on the dynamic characteristics of the converter side, the modulation voltage of the converter, and the reference value of the current loop, we can obtain:

[0100] (7)

[0101] (8)

[0102] Output power is controlled The perturbation relationship in the coordinate system is:

[0103] (9)

[0104] Using the conjugate relationship of complex numbers, the perturbation relationship of the phase-locked loop is:

[0105] (10)

[0106] (11)

[0107] In the formula, for of Axis component.

[0108] The modulation voltage of the converter is changed from control Convert the coordinate system to the actual In the coordinate system, the output voltage of the converter is connected with the modulation voltage through the delay link, and the following is obtained:

[0109] (12)

[0110] (13)

[0111] In the formula, is the PWM modulation gain, represents the delay link, the present invention ignores the modulation and delay links, The value is 1.

[0112] Combining the small disturbance relation of the converter side dynamic characteristics and the relation between the converter output voltage and modulation voltage, we get:

[0113] (14)

[0114] S207, combined equations (6)-(14), simplify the disturbance expression of the grid-connected point current to be:

[0115] (15)

[0116] (16)

[0117] Using the perturbation relation of the phase-locked loop, we can eliminate Variables, reuse and The conjugate relationship of eliminates the ,get:

[0118] (17)

[0119] (18)

[0120] According to equations (17) and (18), the power loop control analytical expression of the power reference disturbance value and the output current disturbance value is derived:

[0121] (19)

[0122] (20)

[0123] In the formula, , , , are four single-input single-output closed-loop transfer functions that characterize the stability of the grid-connected converter system; , Respectively represent the active and reactive power reference disturbance values ​​to the converter output current The influence of axis disturbance value; , Respectively represent the active and reactive power reference disturbance values ​​to the converter output current The influence of axis disturbance value; , , , The closed-loop poles of the common denominator polynomials are used to determine the stability of the grid-connected converter system.

[0124] Step S3, determine the initial value, analysis range and analysis step of the working point variable, substitute the given value of the working point variable into the common denominator polynomial of the single-input single-output closed-loop transfer function to calculate the closed-loop poles, and determine the stability of the grid-following converter system under different working points by judging the position of the poles. When the grid-following converter system remains stable, its corresponding working point is within the safe working domain, thereby drawing a safe working domain diagram of the grid-following converter system in the entire working range.

[0125] Determine the operating point variables to be analyzed, and give the initial value, analysis range, and analysis step size for each operating point variable.

[0126] Since the values ​​of the operating points are assigned one by one according to the analysis range, there is a possibility that the given values ​​do not satisfy the circuit relationship, so it is necessary to determine whether the values ​​meet the circuit constraints.

[0127] For a given reference power (i.e., active power and reactive power reference values), when a steady state is reached, the converter output power can be considered to be approximately the reference power:

[0128] (twenty one)

[0129] Known grid voltage and the converter output power, the converter output terminal grid connection point voltage and the grid through impedance The topological constraint relationship between the connection point voltage and the grid voltage is:

[0130] (twenty two)

[0131] In the formula, , The grid voltage is axis, Axis component.

[0132] According to the circuit topology constraint relationship between the grid connection point voltage and the grid voltage, determine whether each working point is a meaningful point; if the output power (i.e., active power and reactive power output value) corresponding to the working point satisfies the circuit topology constraint, then the working point is a meaningful point, and the next step of stability analysis is performed; if the output power corresponding to the working point does not satisfy the circuit topology constraint, then the working point is a meaningless point, no stability analysis is required, and use The operating point is marked as not satisfying the topology constraints. and converter output power, the grid connection point voltage can also be solved through the circuit topology constraint relationship Axis steady state value .

[0133] Substitute the given values ​​of the operating point variables that satisfy the circuit topology constraints and the initial circuit parameters (when the given values ​​of the operating point variables are known, they can be combined into equations (2)-(16) and equations (21)-(22) to obtain the corresponding initial circuit parameters) into the common denominator polynomial of the four closed-loop transfer functions to solve the closed-loop poles. If all the poles are in the left half plane of the complex plane, it means that the grid-following converter system is stable. Otherwise, it means the grid-connected converter system is unstable. Indicates; stores the stability analysis results of the current working point.

[0134] All operating points that satisfy the circuit topology constraints are traversed, and stability analysis results of all operating points are obtained and stored.

[0135] According to the stability analysis results, working point variables and circuit parameters, the safe working domain diagram of the system is drawn to obtain the safe working domain of the system in the whole working range; the whole analysis range can be divided into three domains, namely: safe working domain, corresponding to ; Instability domain, corresponding to ; meaningless domain, corresponding to .

[0136] Compared with the prior art, the present invention improves the difficulties and shortcomings of typical stability analysis methods in modeling under variable working conditions: the state space method relies on the completeness and certainty of the converter system, and the modeling difficulty is high, while the impedance (admittance) method is usually based on a certain working point for modeling. The method proposed in the present invention uses the dynamic characteristics of the phase-locked loop and the power loop to introduce the working point variable into the modeling process, and obtains a variable coefficient transfer function that can characterize the stability of the system, and depicts the safe working domain of the system. In this way, the stability of the grid-connected converter system under time-varying working conditions can be followed, and the stability and stability margin information of the system can be obtained simply and intuitively. In addition to the working point parameters, the influence of changes in system circuit parameters, load switching (light load, overload), etc. on system stability can also be studied, which expands the application scope of the system safe working domain.

[0137] Select below Figure 1 Typical grid-connected converter system topology and control The method involved in the invention was simulated and tested. In the test, the system used a phase-locked loop for synchronization, power control, and selected the active power output value. and reactive power output is the working point variable. The active power output value analysis range is 0~50kW, with a change step of 1kW; the reactive power output value analysis range is 0~10kVar, with a change step of 1kVar.

[0138] System control parameters and impedance parameters are shown in Table 1:

[0139] ;

[0140] Figure 4 This is a diagram of the system operating domain depicted by the method provided by the present invention in the experiment, wherein area (I) represents the safe operating domain, area (II) represents the unstable domain, and area (III) represents the meaningless domain.

[0141] In the stable domain, the grid voltage is different, and the maximum d-axis working current for stable operation of the system is also different. In order to verify the correctness of the stable domain division, different working points on the stable domain are selected and substituted into the physical model for verification. The selected working points are shown in Table 2:

[0142] ;

[0143] The closed-loop poles calculated using different operating points are as follows: Figure 5-8 The working point verification results are shown in Figure 9-12 As shown. Combined Figure 5-12 It can be seen that at the working point selected in the safe working region, the system can operate stably, while at the working point selected in the unstable region, the system three-phase current Distortion, active power output value Oscillation or even divergence, and stable operation cannot be achieved. Therefore, the accuracy of the safe working domain construction of the present invention can be proved.

[0144] The above embodiments are preferred implementation schemes of the present invention. In addition, the present invention may also be implemented in other ways. Any obvious replacement without departing from the concept of the present technical solution is within the protection scope of the present invention.

[0145] In order to make it easier for ordinary technicians in the field to understand the improvements of the present invention over the prior art, some drawings and descriptions of the present invention have been simplified, and for the sake of clarity, some other elements are omitted in this application document. Ordinary technicians in the field should realize that these omitted elements may also constitute the content of the present invention.

Claims

1. A method for constructing a safe working domain of a grid-connected power conversion system, characterized in that: include: Step S1, small signal modeling is performed on the power loop, current loop and grid circuit of the grid-following converter, and the working point variable is introduced by using the dynamic characteristics of the phase-locked loop and the power loop, so as to establish a multi-input multi-output small signal model of the grid-following converter system containing the working point variable, and the intermediate variables involved in the multi-input multi-output small signal model are all represented by functions containing the working point variable; S101, using a phase-locked loop structure based on a synchronous reference system; defining the actual grid-connected point voltage phase angle as , the phase-locked loop output voltage phase angle is , and The difference is , The corresponding coordinate system is the actual Coordinate system, The corresponding coordinate system is controlled Coordinate system, take Representing reality Physical quantities in the coordinate system, Delegate Control Physical quantity in coordinates, superscript " " represents the conjugate of a physical quantity, Representing reality Small perturbation in the coordinate system, Delegate Control Small perturbation in the coordinate system; Determine the phase-locked loop output angle The expression is: （1） In the formula, represents a phase-locked loop proportional-integral controller, represents the phase-locked loop ratio parameter, represents the phase-locked loop integral parameter, represents the variable parameter in the Laplace transform, Represents the imaginary part of the grid connection point voltage; Indicates the rated angular frequency; S102, determine the dynamic characteristics of the converter side in actual The coordinate system is: （2） （3） In the formula, is the converter output voltage vector, , for of Axis component, for of Axis component, is an imaginary unit; is the current vector of the grid-connected point of the converter; is the voltage vector of the converter grid connection point; is the grid voltage vector; is the impedance corresponding to the filter inductor, , They are the filter inductor and the parasitic resistance of the filter inductor respectively; is the grid impedance, , are grid inductance and grid resistance respectively; S103, determining whether the converter current loop controller and the power loop controller are in control In the coordinate system it is expressed as: （4） （5） In the formula, represents the modulation voltage of the converter; Indicates the reference value of the converter current loop; , are the active power and reactive power output values ​​of the converter respectively; , are the reference values ​​of active power and reactive power of the converter respectively; represents the PI regulator of the current loop, Represents the current loop proportional parameter, Represents the current loop integral parameter; represents the PI regulator of the power loop, , Indicates the power loop ratio parameter, Represents the power loop integral parameter; is the conjugate value of the power reference value, Output power conjugate value; Step S2, using the obtained multi-input multi-output small signal model, deriving the power loop control analytical expression of the power reference disturbance value and the output current disturbance value considering the influence of the phase-locked loop and the grid strength, and obtaining the single-input single-output closed-loop transfer function that can characterize the stability of the grid-following converter system; S201, determine the transformation relationship between the disturbance quantities of each voltage and current in the grid-connected converter system between different reference frames, which are: （6） In the formula, Represents the steady-state value vector of the grid-connected point current; is the steady-state value vector of the converter output voltage; is the grid-connected point voltage steady-state value vector; S202, small disturbance analysis is performed on the dynamic characteristics of the converter side, the modulation voltage of the converter, and the reference value of the current loop, respectively, to obtain: （7） （8） S203, determine that the converter output power is under control The perturbation relationship in the coordinate system is: （9） S204, using the conjugate relationship of complex numbers, determine the disturbance relationship of the converter phase-locked loop as: （10） （11） In the formula, for of Axis component; S205, the modulation voltage of the converter is changed from the control Convert the coordinate system to the actual In the coordinate system, the output voltage of the converter is connected with the modulation voltage through the delay link, and the following is obtained: （12） （13） In the formula, is the PWM modulation gain, Represents the delay phase, The value is 1; S206, combining the small disturbance relation of the converter side dynamic characteristics and the relation between the converter output voltage and the modulation voltage, obtain: （14） S207, combined equations (6)-(14), simplify the disturbance expression of the grid-connected point current to be: （15） （16） S208, using the disturbance relation of the phase-locked loop, eliminate the Variables, reuse and The conjugate relationship of eliminates the ,get: （17） （18） S209, according to equation (17) and equation (18), the power loop control analytical expression of the power reference disturbance value and the output current disturbance value is derived: （19） （20） In the formula, , , , are four single-input single-output closed-loop transfer functions that characterize the stability of the grid-connected converter system; , Respectively represent the active and reactive power reference disturbance values ​​to the converter output current The influence of axis disturbance value; , Respectively represent the active and reactive power reference disturbance values ​​to the converter output current The influence of axis disturbance value; , , , The closed-loop poles of the common denominator polynomials are used to determine the stability of the grid-connected converter system; Step S3, determine the initial value, analysis range and analysis step of the working point variable, substitute the given value of the working point variable into the common denominator polynomial of the single-input single-output closed-loop transfer function to calculate the closed-loop poles, and determine the stability of the grid-following converter system under different working points by judging the position of the poles. When the grid-following converter system remains stable, its corresponding working point is within the safe working domain, thereby drawing a safe working domain diagram of the grid-following converter system in the entire working range.

2. The method for constructing a safe working domain of a grid-connected converter system according to claim 1, characterized in that: Step S3 includes the following steps: S301, determining the operating point variables to be analyzed, and giving the initial value, analysis range, and analysis step length of each operating point variable; S302, judging whether each working point is a meaningful point according to the circuit topology constraint relationship between the grid connection point voltage and the grid voltage; if the output power corresponding to the working point satisfies the circuit topology constraint, the working point is a meaningful point, and the next step of stability analysis is performed; if the output power corresponding to the working point does not satisfy the circuit topology constraint, the working point is a meaningless point, no stability analysis is required, and the stability analysis is performed. The operating point is marked as not satisfying the topological constraint; the expression of the circuit topological constraint relationship is as follows: （21） （22） In the formula, , The grid voltage is axis, Axis component; S303, substitute the given values ​​of the operating point variables that satisfy the circuit topology constraints and the initial parameters of the circuit into the common denominator polynomial of the four closed-loop transfer functions to solve the closed-loop poles. If all the poles are in the left half plane of the complex plane, it means that the grid-connected converter system is stable. express; Otherwise, it means that the grid-connected converter system is unstable. Represents; stores the stability analysis results of the current working point; S304, traversing all operating points that satisfy the circuit topology constraint, obtaining stability analysis results of all operating points, and storing them; S305, according to the stability analysis results, the working point variables, and the circuit parameters, a safe working domain diagram of the system is drawn to obtain the safe working domain of the system in the entire working range; the entire analysis range can be divided into three domains, namely: safe working domain, corresponding to ; Instability domain, corresponding to ; meaningless domain, corresponding to .

3. The method for constructing a safe working domain of a grid-connected converter system according to claim 2, characterized in that: In step S303, the initial parameters of the circuit are obtained by combining equations (2)-(16) and equations (21)-(22) according to the known given values ​​of the operating point variables.

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