Low-frequency stability determination method for grid-connected photovoltaic power generation system under high-speed phase locking

Abstract

The present invention relates to the technical field of stability determination of grid-connected photovoltaic power generation systems, and discloses a method for determining the low-frequency stability of a grid-connected photovoltaic power generation system under high-speed phase locking, comprising: S1, establishing a linearized state space model for a grid-connected photovoltaic power generation system with reactive power support capability; S2, based on the linearized state space model, establishing a block diagram model of the grid-connected photovoltaic power generation system according to the connection relationship between each input parameter and output parameter in the linearized state space model; S3, performing time scale decomposition on the block diagram model of the grid-connected photovoltaic power generation system to obtain a stage simplified model for low-frequency stability analysis of the grid-connected photovoltaic power generation system; S4, based on the stage simplified model for low-frequency stability analysis of the grid-connected photovoltaic power generation system, obtaining a method for determining the low-frequency stability of the grid-connected photovoltaic power generation system through a mathematical stability criterion. The present invention effectively reduces the complexity and calculation amount of stability analysis.

Description

Technical Field

[0001] The present invention relates to the technical field of stability determination of a grid-connected photovoltaic power generation system, and in particular to a method for determining low-frequency stability of a grid-connected photovoltaic power generation system under high-speed phase locking. Background Art

[0002] In recent years, my country's photovoltaic power generation has developed rapidly, and the installed capacity of photovoltaic power generation grid-connected has ranked first in the world. With the expansion of the scale of photovoltaic power generation grid-connected, the changes in the dynamic characteristics of its VSC control system may cause system oscillation problems.

[0003] At present, in the study of the stability of grid-connected photovoltaic power generation systems, the open-loop mode resonance principle and analysis method reveal the general mechanism of subsynchronous and supersynchronous oscillations caused by photovoltaic grid connection from the perspective of dynamic interaction between photovoltaic power generation subsystems and the rest of the power system. Existing research conclusions show that the stability problem of grid-connected photovoltaic power generation systems under weak connection conditions is mainly related to their converter control systems, and most of the current research focuses on the stability problems caused by the phase-locked loop. Although a large number of studies have shown that the stability of grid-connected photovoltaic systems is closely related to the dynamics of the phase-locked loop, it is impossible to determine whether the stability problems of such systems are only caused by the dynamics of the phase-locked loop. The influence of the parameter setting of the power outer loop control link in the converter control system of the photovoltaic power generation system on the system stability still needs to be further explored. Summary of the invention

[0004] The present invention provides a low-frequency stability judgment method for a grid-connected photovoltaic power generation system under high-speed phase locking, which is used to explore the specific influence of parameters in a power outer loop control link in a converter control system of the photovoltaic power generation system on the stability of the photovoltaic power generation system.

[0005] The present invention is achieved through the following technical solutions:

[0006] A method for judging low-frequency stability of a grid-connected photovoltaic power generation system under high-speed phase locking comprises:

[0007] S1. Establish a linearized state space model for a grid-connected photovoltaic power generation system with reactive power support capability;

[0008] S2. Based on the linearized state space model, a block diagram model of the grid-connected photovoltaic power generation system is established according to the connection relationship between each input parameter and output parameter in the linearized state space model;

[0009] S3, performing time scale decomposition on the block diagram model of the grid-connected photovoltaic power generation system, ignoring the influence of the high-frequency band dynamic characteristics of the grid-connected photovoltaic power generation system, and obtaining a stage simplified model for low-frequency stability analysis of the grid-connected photovoltaic power generation system;

[0010] S4. Based on the stage simplified model of the low-frequency stability analysis of the grid-connected photovoltaic power generation system, through mathematical stability criteria, the parameters for judging the low-frequency stability of the grid-connected photovoltaic power generation system with reactive power support capability under high-speed phase locking and the method for judging the low-frequency stability of the grid-connected photovoltaic power generation system according to the parameters are obtained.

[0011] As an optimization, the mathematical stability criterion is the Routh-Hurwitz criterion.

[0012] As an optimization, in S1, the grid-connected photovoltaic power generation system includes: a photovoltaic array, a VSC control system, a first filter inductor X, and a plurality of photovoltaic arrays connected in series in the direction of current flow. F , the second filter inductor X L and an external power system, a DC capacitor C is provided between the high voltage output terminal and the low voltage output terminal of the photovoltaic array dc .

[0013] As an optimization, the VSC control system consists of a current inner loop control module, a power outer loop control module and a phase-locked loop.

[0014] As an optimization, the linearized state space model is specifically:

[0015]

[0016] ΔP dc =V bd0 ΔI d +V bq0 ΔI q +I d0 ΔV bd +I q0 ΔV bq ;

[0017] ΔQ d =V bq0 ΔI d -V bd0 ΔI q +I d0 ΔV bq -I q0 ΔV bd ;

[0018]

[0019]

[0020]

[0021]

[0022] Among them, C dc and Vdc Respectively represent the DC capacitance and its terminal voltage; V bd , V bq , I d , I q Respectively represent the PCC point voltage (V bd is the real voltage, V bq is the imaginary voltage) and the dq axis component of the output current; P dc Indicates the active power input to the VSC, Q d Represents the reactive power output by VSC; V d , V q Represents the dq axis component of the VSC AC side terminal voltage; K p1 (K i1 ), K p2 (K i2 ), K p3 (K i3 ) and K p4 (K i4 ) represent the proportional (integral) parameters of the DC voltage control outer loop, reactive power control outer loop, d-axis current control inner loop, and q-axis current control inner loop, respectively, while x1, x2, x3, and x4 represent the output of the integral link in the above control links; X F is the filter reactance. The subscripts “r” and “0” represent the reference value and steady-state value of the corresponding variable, “Δ” represents the change, ω0 represents the steady-state value of the synchronous speed of the grid-connected photovoltaic power generation system, V dr , V qr , I dr , I qr They are the reference values ​​of the VSC AC side terminal voltage and the output current dq axis components respectively.

[0023] As an optimization, the simplified model of the stage is:

[0024] in,

[0025]

[0026] Among them, X L Represents the reactance of the transmission line between the photovoltaic power generation system and the external AC power system, P0 = V bd0 I d0 +V bq0 I q0 , is the steady-state value of the active power output by the grid-connected photovoltaic power generation system.

[0027] As an optimization, a 11 、a 12 、a 13 The specific formula is:

[0028]

[0029]

[0030]

[0031] As an optimization, in S4, the parameters for determining the low-frequency stability of the grid-connected photovoltaic power generation system with reactive power support capability under high-speed phase locking include: the steady-state transmission power P0 of the grid-connected photovoltaic power generation system, the steady-state voltage amplitude V0 at the grid connection point (PCC point), the second filter inductance X L , DC capacitor C dc And the steady-state voltage value of the DC capacitor V dc0 .

[0032] As an optimization, in S4, the method for determining the low-frequency stability of the grid-connected photovoltaic power generation system according to the parameters is specifically:

[0033] When the parameters meet and When the grid-connected photovoltaic power generation system has high low-frequency stability.

[0034] As an optimization, Z js Specifically:

[0035]

[0036] z a =K p1 K p2 (K p1 K i2 +K i1 K p2 );

[0037]

[0038]

[0039] Wherein, V0 represents the steady-state value of the voltage amplitude at the PCC point.

[0040] Compared with the prior art, the present invention has the following advantages and beneficial effects:

[0041] The present invention establishes a transfer function for a grid-connected photovoltaic power generation system with reactive power support capability and establishes its block diagram model on this basis, intuitively revealing the connection relationship between the various components of the grid-connected photovoltaic power generation system and the conversion relationship between the various input and output quantities;

[0042] For grid-connected photovoltaic power generation systems with reactive power support capabilities, the derived reduced-order simplified model for low-frequency stability analysis ignores the impact of the dynamic characteristics of the high-speed phase-locked loop and the VSC current control inner loop on the dynamic characteristics of the low-frequency band, effectively reducing the complexity of the block diagram model and the amount of calculation for simulation analysis;

[0043] The obtained low-frequency stability judgment method does not need to establish a linear model of the grid-connected photovoltaic power generation system. It only needs to substitute the steady-state quantities such as the steady-state voltage and active output of the corresponding grid-connected photovoltaic power generation system and the PI parameters in the VSC control system into the calculation. The low-frequency stability of the corresponding grid-connected photovoltaic power generation system is judged by whether the steady-state criterion is met, which effectively reduces the complexity and calculation amount of the stability analysis. BRIEF DESCRIPTION OF THE DRAWINGS

[0044] In order to more clearly illustrate the technical solutions of the exemplary embodiments of the present invention, the following briefly introduces the drawings required for use in the embodiments. It should be understood that the following drawings only illustrate certain embodiments of the present invention and should not be regarded as limiting the scope. For those of ordinary skill in the art, other relevant drawings can be obtained based on these drawings without creative work. In the drawings:

[0045] Figure 1 It is a structural diagram of the grid-connected photovoltaic power generation system in the present invention;

[0046] Figure 2 It is the control strategy diagram of VSC power outer loop and current inner loop;

[0047] Figure 3 It is the relative position diagram between the public xy coordinate system and the dq coordinate system of the power grid;

[0048] Figure 4 This is the working principle diagram of the phase-locked loop;

[0049] Figure 5 It is a block diagram model of the grid-connected photovoltaic power generation system in the present invention;

[0050] Figure 6 This is a nonlinear simulation result diagram of the grid-connected photovoltaic power generation system in the present invention.

[0051] Marks and corresponding parts names in the attached drawings:

[0052] 1- PV array, 2- VSC control system, 2.1- current inner loop control module, 2.2- power outer loop control module, 3- external power system. DETAILED DESCRIPTION

[0053] In order to make the objectives, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with embodiments and drawings. The exemplary embodiments of the present invention and their description are only used to explain the present invention and are not intended to limit the present invention.

[0054] Example

[0055] A method for judging low-frequency stability of a grid-connected photovoltaic power generation system under high-speed phase locking comprises:

[0056] S1. Establish a linearized state space model for the grid-connected photovoltaic power generation system with reactive power support capability.

[0057] like Figure 1 As shown, in this embodiment, in S1, the grid-connected photovoltaic power generation system includes: a photovoltaic array 1, a VSC control system 2, a first filter inductor X, and a plurality of photovoltaic arrays connected in series in the direction of current flow. F , the second filter inductor X L and an external power system 3, a DC capacitor C is provided between the high voltage output terminal and the low voltage output terminal of the photovoltaic array 1 dc .

[0058] The stability problem of the grid-connected photovoltaic power generation system is mainly related to the VSC control system. For the grid-connected photovoltaic power generation system with reactive power support capability, its VSC vector control system (i.e., VSC control system 2) is composed of three parts: the current inner loop control module 2.1, the power outer loop control module 2.2, and the phase-locked loop.

[0059] For the DC capacitor C dc ,have:

[0060]

[0061] Among them, C dc is the DC capacitance C dc The capacitance value, V dc is the DC capacitance C dc The voltage value, P pv is the active power output by the photovoltaic array, P dc The active power input to the VSC control system is equal to the active power output from the PCC point in terms of value, so:

[0062] P dc =V d I d +V q I q =V bd I d +V bq I q (2)

[0063] Among them, V d is the real part of the output voltage of the VSC control system, V q is the imaginary part of the output voltage of the VSC control system, I d is the imaginary part of the output current of the VSC control system, I q is the imaginary part of the output current of the VSC control system, V bd is the real part of the output voltage at the PCC point, V bq is the imaginary part of the output voltage at the PCC point, and the VSC control system outputs reactive power Q d It can be expressed as:

[0064] Q d =V bq I d -V bd I q (3)

[0065] For the first filter inductor, we have:

[0066]

[0067] Among them, ω0 represents the steady-state value of the synchronous speed of the power system, X F is the inductance value of the first filter inductor.

[0068] For Figure 1 The grid-connected photovoltaic power generation system shown in FIG. 1 has a control strategy of the current inner loop control module 2.1 and the power outer loop control module 2.2 in the VSC control system 2 as shown in FIG. Figure 2 As shown. Figure 2 , we can get:

[0069]

[0070]

[0071] Where V dr , V qr , I dr , I qr are the reference values ​​of the VSC AC side terminal voltage and the output current dq axis components, V dcr and Q dr are the DC capacitor voltage reference value and the VSC output reactive power reference value, K p1-4 and K i1-4 They represent the proportional and integral parameters of the DC voltage control outer loop, the reactive power control outer loop, the d-axis current control inner loop, and the q-axis current control inner loop, respectively, and x 1-4 Then they are the outputs of the integral link in the above control link. Ignoring the dynamic characteristics of PWM, we have:

[0072]

[0073] The relative position between the power grid public xy coordinate system and the dq coordinate system is as follows Figure 3 shown. Figure 3 In, θ p represents the measured phase of the voltage at the photovoltaic power generation grid connection point in the dq coordinate system, and θ represents the actual phase of the voltage in the xy common coordinate system.

[0074] Depend on Figure 3 ,have:

[0075]

[0076] Similarly, there are:

[0077]

[0078] The voltage phase θ at the PCC point can be expressed as:

[0079]

[0080] The photovoltaic grid-connected power generation system 1 determines the relative position between the grid public xy coordinate system and the dq coordinate system through a phase-locked loop. The working principle of the phase-locked loop is as follows: Figure 4 As shown. Figure 4 ,have:

[0081]

[0082] Among them, K pp and K ip are the proportional and integral control parameters of the phase-locked loop control link, and x p It represents the output of the integral link of the phase-locked loop.

[0083] By linearizing equations (1) to (7), we can obtain:

[0084]

[0085] ΔP dc =V bd0 ΔI d +V bq0 ΔI q +I d0 ΔV bd +I q0 ΔV bq (13)

[0086] ΔQ d =V bq0 ΔI d -V bd0 ΔIq +I d0 ΔV bq -I q0 ΔV bd (14)

[0087]

[0088]

[0089]

[0090]

[0091] Among them, C dc and V dc Respectively represent the DC capacitance and its terminal voltage; V bd , V bq , I d , I q Respectively represent the terminal voltage at PCC point and the dq axis component of the output current; P dc Indicates the active power input to the VSC, Q d Represents the reactive power output by VSC; V d , V q Represents the dq axis component of the VSC AC side terminal voltage; K p1 , K p2 , K p3 and K p4 They represent the proportional parameters of the DC voltage control outer loop module, the reactive power control outer loop module, the d-axis current control inner loop module, and the q-axis current control inner loop module, respectively. i1 , K i2 , K i3 and K i4 They represent the integral parameters of the DC voltage control outer loop module, the reactive power control outer loop module, the d-axis current control inner loop module, and the q-axis current control inner loop module, respectively; x1, x2, x3, and x4 represent the outputs of the integral links in the DC voltage control outer loop module, the reactive power control outer loop module, the d-axis current control inner loop module, and the q-axis current control inner loop module, respectively; X F is the first filter reactance, subscript “r” and subscript “0” represent the reference value and steady-state value of the corresponding variable, “Δ” represents the change, ω0 represents the steady-state value of the synchronous speed of the grid-connected photovoltaic power generation system, V dr , V qr , I dr , I qr They are the reference values ​​of the VSC AC side terminal voltage and the output current dq axis components respectively.

[0092] S2. Based on the linearized state space model, a block diagram model of the grid-connected photovoltaic power generation system is established according to the connection relationship between each input parameter and output parameter in the linearized state space model.

[0093] Through the connection between the input and output of each part in the linearized state space model of the grid-connected photovoltaic power generation system, the transfer function of each part is derived and the connection relationship between the components is obtained.

[0094] Usually, the position of the photovoltaic grid-connected point voltage in the xy coordinate system under steady-state conditions is selected as the d-axis, so V bq0 =0, substituting into (13) and (14), we have:

[0095]

[0096]

[0097] From (12), (16) and (17), we can get:

[0098]

[0099] Among them, G1(s)~G4(s) respectively represent the transfer functions from the VSC input active power to the reference value of the d-axis component of the grid-connected point output current, from the reference value of the d-axis component of the grid-connected point output current to the d-axis component of the grid-connected point output current, from the VSC output reactive power to the reference value of the q-axis component of the grid-connected point output current, and from the reference value of the q-axis component of the grid-connected point output current to the q-axis component of the grid-connected point output current under linearization.

[0100] From equations (15) to (18), we can get:

[0101]

[0102] Depend on Figure 4 , we can get:

[0103]

[0104] Linearizing equations (8) to (10), we have

[0105]

[0106]

[0107]

[0108] in, is the voltage amplitude of the photovoltaic power generation grid-connected point under steady-state conditions.

[0109] From (24) and (25), we have:

[0110] ΔV bdq =f4Δθ p +F1ΔV xy (27)

[0111] ΔI xy =f5Δθ p +F2ΔI dq (28)

[0112] Where, ΔV bdq =[ΔV bd ΔV bq ] T is the column vector composed of the changes in the d-axis and q-axis components of the PCC point voltage, ΔI xy =[ΔI x ΔI y ] T It is a column vector composed of the changes in the x-axis and y-axis components of the output current at the PCC point, and f1~f5 are proportional coefficients.

[0113] From equations (19) to (23) and (26) to (28), the block diagram model of the photovoltaic power generation grid-connected system can be obtained as follows: Figure 5 shown.

[0114] After establishing the block diagram model of the grid-connected photovoltaic power generation system with reactive power support capability, Figure 5 The connection between the power control outer loop module, the current control inner loop module and the phase-locked loop in the VSC control system is intuitively revealed. In fact, the dynamic characteristics of the power outer loop control module and the current inner loop control module are at different time scales, and the dynamic response speed of the current control inner loop module is nearly ten times that of the power control outer loop module. Therefore, when conducting low-frequency stability analysis on a grid-connected photovoltaic power generation system with reactive power support capability, the influence of the current inner loop control module in the high-frequency band can be ignored.

[0115] S3, performing time scale decomposition on the block diagram model of the grid-connected photovoltaic power generation system, ignoring the influence of the high-frequency band dynamic characteristics of the grid-connected photovoltaic power generation system, and obtaining a stage simplified model for low-frequency stability analysis of the grid-connected photovoltaic power generation system;

[0116] Based on the different response speeds of the inner and outer loop control links in the VSC control system, the dynamic characteristics of the current control inner loop in the high-frequency band are ignored in the low-frequency stability analysis process; in addition, for the high-speed phase-locked photovoltaic power generation system, the influence of the high-frequency phase-locked loop dynamics is ignored, and a reduced-order simplified model dominated by the dynamic characteristics of the VSC power outer loop is obtained.

[0117] 1) Reduced-order simplified model of photovoltaic power generation system with reactive power support capability under high-speed phase locking

[0118] right Figure 5 When performing low-frequency stability analysis on the grid-connected photovoltaic system shown in the figure, the dynamic characteristics of the current inner loop control module can be ignored, so:

[0119]

[0120] Thus for Figure 5 From the block diagram model shown, we can obtain G3(s)=G4(s)=1.

[0121] When the phase-locked loop is in a high-speed phase-locked state, the dynamic characteristics of the phase-locked loop are in the high-frequency band. Therefore, the influence of the dynamic characteristics of the high-speed phase-locked loop can be ignored when analyzing the low-frequency stability of the grid-connected photovoltaic system, that is:

[0122] Δθ p =Δθ (30)

[0123] Substituting formula (29) into (17), we can obtain:

[0124]

[0125] Substituting formula (30) into (26), we can obtain:

[0126]

[0127] Substituting formula (31) into (19) and (20), we have:

[0128]

[0129]

[0130] Substituting the second equation in (25) into equations (33) and (34), we have:

[0131]

[0132]

[0133] Where, Q0 = I d0 V bq0 -I q0 V bd0 is the reactive power output of the photovoltaic power generation grid-connected point under steady-state conditions. Substituting equation (32) into equations (35) and (36), the obtained ΔP dc and ΔQ d Substitute the expression into the second equation in equation (12) and (16), and then combine it with the first equation in equation (16) to obtain:

[0134]

[0135] In the formula, ΔX lh3 =[ΔV dc Δx1 Δx2] T ,

[0136]

[0137] From equations (24), (31), (32) and (36), we can get:

[0138] ΔI xy =C lh3 ΔX lh3 +D lh3 ΔV xy (38)

[0139] Where:

[0140]

[0141]

[0142] Equations (37) and (38) constitute a reduced-order simplified model of a photovoltaic power generation system with reactive power support capability under high-speed phase locking on the DC voltage time scale.

[0143] The above steps establish a reduced-order simplified model of a photovoltaic power generation system with reactive power support capability under high-speed phase locking. L When connected to external power system 3 (such as Figure 1 As shown), the voltage equation of the transmission line can be obtained:

[0144]

[0145] Substituting formula (38) into formula (39), we can obtain:

[0146]

[0147] Substituting formula (40) into formula (37), we can obtain:

[0148]

[0149] In the formula, we have:

[0150]

[0151] in,

[0152]

[0153]

[0154]

[0155] Equation (41) is the reduced-order simplified model for low-frequency stability analysis of a photovoltaic grid-connected system with reactive power support capability under high-speed phase locking.

[0156] S4. Based on the stage simplified model of the low-frequency stability analysis of the grid-connected photovoltaic power generation system, through mathematical stability criteria, the parameters for judging the low-frequency stability of the grid-connected photovoltaic power generation system with reactive power support capability under high-speed phase locking and the method for judging the low-frequency stability of the grid-connected photovoltaic power generation system according to the parameters are obtained.

[0157] Usually, the position of the photovoltaic grid-connected PCC point voltage in the xy coordinate system under steady-state conditions is selected as the d-axis, then V bd0 =V0,V bq0 = 0. The characteristic equation corresponding to the reduced-order simplified model is |λI-A3|=0, which can be organized into a polynomial form:

[0158] a3λ 3 +a2λ 2 +a1λ+a0=0 (42)

[0159] In the formula, we have:

[0160] a3=1

[0161]

[0162]

[0163]

[0164] For the reduced-order system represented by equation (41), the necessary and sufficient condition for low-frequency stability is that all characteristic roots of the system are in the left half plane of the complex plane. Therefore, for the corresponding characteristic equation (42), the necessary and sufficient condition for the stability of the system can be obtained by the Routh-Hurwitz criterion:

[0165]

[0166] Substituting the expressions of the coefficients in the characteristic equation (42) into equation (43), the low-frequency stability criterion of the photovoltaic grid-connected system with reactive power support capability under high-speed phase locking can be obtained:

[0167] and

[0168] in:

[0169]

[0170] za =K p1 K p2 (K p1 K i2 +K i1 K p2 );

[0171]

[0172]

[0173] Formula (44) is a low-frequency stability judgment method derived from the Routh-Hurwitz criterion based on the reduced-order simplified model of the photovoltaic grid-connected system with reactive power support capability under high-speed phase locking on the DC voltage time scale. js The size of depends on the parameter setting of the photovoltaic grid-connected VSC power outer loop control system. It can be seen that for Figure 1 The low-frequency stability of the photovoltaic grid-connected system shown in FIG. 1 is closely related to the parameter setting of the VSC power outer loop control link when the influence of the high-frequency dynamic characteristics of the AC current time scale is ignored. In addition, it can be seen from equation (44) that the steady-state transmission power P0 of the photovoltaic grid-connected system, the steady-state voltage amplitude V0 of the grid-connected point, and the first filter inductance X L , DC capacitor C dc Its steady-state voltage value V dc0 These are important factors affecting the low-frequency stability of the system.

[0174] In the aforementioned parameters, 0 is added to the original parameter, representing the steady-state data of the original parameter.

[0175] Below Figure 1 Taking the grid-connected photovoltaic power generation system shown in the figure as an example, the correctness of the derived stability criterion formula (44) is verified. In this example, a single photovoltaic motor is used as the equivalent aggregation model of the grid-connected photovoltaic power station. The parameters of the photovoltaic motor are obtained by the capacity weighted average method. The external power system is an infinite power grid. All relevant parameters are in per-unit value, with the unit being pu. The PCC node is set as a balancing node, and its voltage is set to 1.0∠0°V.

[0176] In order to verify the correctness of formula (44) in judging the low-frequency stability of the photovoltaic grid-connected system with reactive power support capability under high-speed phase locking, the following four parameter settings are simulated:

[0177] Parameter setting case 1: Second filter inductor X L =0.4, the transmission power of the grid-connected photovoltaic power generation system P0 = 1.2, and the parameters of the photovoltaic power generation system VSC outer loop control module are set to K p1 =0.008, K i1 =15, Kp2 =0.02, K i2 =40, DC capacitance C dc =10, V dc0＝ 1;

[0178] Parameter setting case 2: K p1 =0.1, and the other parameters are the same as those in parameter setting case 1;

[0179] Parameter setting case 3: X L =0.3, and the other parameters are the same as those in parameter setting case 1;

[0180] Parameter setting case 4: P0=1.6, and the other parameters are the same as those of parameter setting case 3.

[0181] The calculation results of the stability criterion shown in formula (44) under the above parameter settings and the corresponding mode analysis results are shown in Table 1.

[0182] Table 1 Mode analysis and stability criterion calculation results

[0183]

[0184]

[0185] As shown in Table 1, under parameter setting conditions 1 and 4, there are X L P0 / V0>z js , that is, it does not satisfy the stability criterion (44). However, from the results of the mode analysis, it can be seen that the real part of the DC voltage oscillation mode of the system in these two cases is a positive number. From the mode analysis theory of low-frequency oscillation of the power system, it can be seen that the VSC outer loop control system is unstable at this time, which is consistent with the results of the stability criterion analysis. Similarly, under parameter setting conditions 2 and 3, there is X L P0 / V0<z js The real parts of the characteristic values ​​of the system's DC voltage oscillation mode and reactive power oscillation mode are both less than 0. The results of the mode analysis and stability criterion show that the VSC outer loop control system of the photovoltaic grid-connected system is stable under these two parameter settings.

[0186] The nonlinear simulation results under the above four parameter settings are as follows: Figure 6 As shown, the disturbance in the simulation is set to reduce the output power of the grid-connected photovoltaic power generation system by 5% at 0.2s and recover after 50ms. Figure 6 The nonlinear simulation results in are consistent with the stability judgment results of the stability criterion in Table 1. Figure 6From the nonlinear simulation results in , it can be seen that the stability criterion shown in formula (44) can effectively judge the low-frequency stability of the grid-connected photovoltaic system under high-speed phase-locking under different parameter settings.

[0187] The specific implementation methods described above further illustrate the objectives, technical solutions and beneficial effects of the present invention in detail. It should be understood that the above description is only a specific implementation method of the present invention and is not intended to limit the scope of protection of the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A method for judging the low-frequency stability of a grid-connected photovoltaic power generation system under high-speed phase locking, characterized in that: include: S1. Establish a linearized state space model for a grid-connected photovoltaic power generation system with reactive power support capability; The grid-connected photovoltaic power generation system comprises: a photovoltaic array, a VSC control system, a first filter inductor X, and a second filter inductor X. F , the second filter inductor X L and an external power system, a DC capacitor C is provided between the high voltage output terminal and the low voltage output terminal of the photovoltaic array dc ; The VSC control system consists of a current inner loop control module, a power outer loop control module and a phase-locked loop; The linearized state space model is specifically: △P dc =V bd0 △I d +V bq0 △I q +I d0 △V bd +I q0 △V bq ; △Q d =V bq0 △I d -V bd0 △I q +I d0 △V bq -I q0 △V bd ; Among them, C dc and V dc Respectively represent the DC capacitance and its terminal voltage; V bd ,V bq ,I d ,I q Respectively represent the terminal voltage at PCC point and the dq axis component of the output current; P dc Indicates the active power input to the VSC, Q d Represents the reactive power output by VSC; V d ,V q Represents the dq axis component of the VSC AC side terminal voltage; K p1 ,K p2 ,K p3 and K p4 They represent the proportional parameters of the DC voltage control outer loop module, the reactive power control outer loop module, the d-axis current control inner loop module, and the q-axis current control inner loop module, respectively. i1 ,K i2 ,K i3 and K i4 They represent the integral parameters of the DC voltage control outer loop module, the reactive power control outer loop module, the d-axis current control inner loop module, and the q-axis current control inner loop module, respectively; x1, x2, x3, and x4 represent the outputs of the integral links in the DC voltage control outer loop module, the reactive power control outer loop module, the d-axis current control inner loop module, and the q-axis current control inner loop module, respectively; X F is the first filter reactance, subscript "r" and subscript "0" represent the reference value and steady-state value of the corresponding variable, "△" represents the change, ω0 represents the steady-state value of the synchronous speed of the grid-connected photovoltaic power generation system, V dr ,V qr ,I dr ,I qr are the reference values ​​of the VSC AC side terminal voltage and the output current dq axis components respectively; S2. Based on the linearized state space model, a block diagram model of the grid-connected photovoltaic power generation system is established according to the connection relationship between each input parameter and output parameter in the linearized state space model; S3, performing time scale decomposition on the block diagram model of the grid-connected photovoltaic power generation system, ignoring the influence of the high-frequency band dynamic characteristics of the grid-connected photovoltaic power generation system, and obtaining a stage simplified model for low-frequency stability analysis of the grid-connected photovoltaic power generation system; The simplified model of the stage is: in, Among them, X L Represents the reactance of the transmission line between the photovoltaic power generation system and the external AC power system, P0 = V bd0 I d0 +V bq0 I q0 , is the steady-state value of the active power output of the grid-connected photovoltaic power generation system; a 11 、a 12 、a 13 The specific formula is: S4. Based on the stage simplified model of low-frequency stability analysis of the grid-connected photovoltaic power generation system, through mathematical stability criterion, obtain parameters for determining the low-frequency stability of the grid-connected photovoltaic power generation system with reactive power support capability under high-speed phase locking and a method for determining the low-frequency stability of the grid-connected photovoltaic power generation system according to the parameters; The parameters for determining the low-frequency stability of the grid-connected photovoltaic power generation system with reactive power support capability under high-speed phase locking include: the steady-state transmission power P0 of the grid-connected photovoltaic power generation system, the steady-state voltage amplitude V0 of the PCC point, the second filter inductance X L , DC capacitor C dc And the steady-state voltage value of the DC capacitor V dc0 ; The method for determining the low-frequency stability of the grid-connected photovoltaic power generation system according to the parameters is specifically as follows: When the parameters meet and When , the grid-connected photovoltaic power generation system has high low-frequency stability; Z js Specifically: from a =K p1 K p2 (K p1 K i2 +K i1 K p2 ); Wherein, V0 represents the steady-state value of the voltage amplitude at the PCC point.

2. The method for judging the low-frequency stability of a grid-connected photovoltaic power generation system under high-speed phase locking according to claim 1, characterized in that: The mathematical stability criterion is the Routh-Hurwitz criterion.

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