Asymmetric fault transient stability analysis method and system based on network-forming inverter

Abstract

The invention discloses an asymmetric fault transient stability analysis method and system based on a network-forming inverter. Relates to the technical field of power system transient stability analysis. According to the scheme, a mathematical model of hybrid synchronous control before and after a fault is derived according to a power grid system model of a hybrid synchronous control inverter connected to a power grid, and a transient instability mechanism based on the hybrid synchronous control inverter under an asymmetric fault is analyzed from the aspect of legal quantity by means of a power angle curve and a phase plane. The influence rule of the positive and negative sequence current on the stability is disclosed; based on mathematical modeling, a power angle curve and a phase diagram method, the influence of positive and negative sequence component coupling under an asymmetric fault is considered, and transient instability mechanism analysis of hybrid synchronous control is realized; the boundary of instability caused by positive and negative sequence current can be determined, a converter transient stability analysis method considering positive and negative sequence coupling caused by asymmetric faults under network construction type control is perfected, and a reference is provided for improving the transient synchronization stability of the converter under large disturbance; meanwhile, the researched current loss stability boundary can provide reference for a current limiting reference value.

Description

Technical Field

[0001] The present invention relates to the technical field of transient stability analysis of power systems, and in particular to a method and system for transient stability analysis of asymmetric faults based on a grid-connected inverter. Background Art

[0002] With the increasing penetration of new energy in modern power grids, the power electronics characteristics of power systems are more prominent, and the grid structure and operation mode of power systems are undergoing profound changes. Voltage source converters (VSC) can realize bidirectional power flow and four-quadrant operation. They are often used as the interface between asynchronous machine power sources and large power grids and are widely used in power grids. Compared with the low inertia of grid following control (GFLC), grid forming control (GFMC) technology can give full play to the advantages of VSC's flexibility and controllability, and show good adaptability and stability to weak and even extremely weak power grids. It can independently and quickly build system voltage in weak power grids, compensate for the inherent inertial damping characteristics of the system, and provide reliable voltage and frequency support for the system. However, the characteristic of grid-type control is to actively establish the frequency by simulating the rotor motion equation of the traditional synchronous machine. Therefore, it will encounter various stability problems existing in the synchronous machine. Under large interference, grid-type control may also have synchronous instability (i.e. transient instability) similar to that of the synchronous generator. Moreover, due to the limitations of power electronic equipment, it is impossible to provide short-circuit current like a synchronous generator. Therefore, the controller must set a current limiting link, which will result in a low transient stability margin of the grid-type voltage source grid-connected converter and a high risk of system instability.

[0003] In view of the transient instability phenomenon under symmetrical faults, a large number of literatures have analyzed and proposed some transient stability enhancement control strategies through equal area criterion, Laplace transform, transfer function, phase plane method, etc., which greatly improved the transient stability of the inverter under large disturbances. However, compared with symmetrical faults, asymmetrical faults are more common. When an asymmetrical fault occurs, there is positive and negative sequence coupling in the grid voltage and current, and due to the effect of current limiting, the voltage source characteristics of the converter are switched to current source characteristics, which makes the synchronous stability of the converter and the transient instability process more complicated. Some studies have derived the dynamic equations of the grid-type control synchronization unit based on the sequence network model, and then used the phase diagram method to perform transient stability analysis, but the influence of positive and negative sequence coupling has not yet been considered. Summary of the invention

[0004] The technical problem to be solved by the present invention is that the transient instability mechanism of a grid-type hybrid synchronous control inverter under an asymmetric fault is unclear. The purpose of the present invention is to provide an asymmetric fault transient stability analysis method and system based on a grid-type inverter, and to realize the transient instability mechanism analysis of the hybrid synchronous control by considering the influence of the coupling of positive and negative sequence components under an asymmetric fault.

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

[0006] This solution provides an asymmetric fault transient stability control method based on a grid-connected inverter, including:

[0007] Collect the power data of the hybrid synchronous control inverter export under normal operation and fault operation of the power grid;

[0008] Based on the power data, a grid system model of a hybrid synchronous control inverter connected to the grid is constructed, and the power angle δ, angular velocity and voltage amplitude U as state variables to construct the state space equation of the power grid system before the fault occurs;

[0009] Based on the power grid system model and the state space equation before the fault, the hybrid synchronous control mathematical model of the power grid system with asymmetric faults is derived;

[0010] Based on the power angle curve and phase plane method, the hybrid synchronous control mathematical model is qualitatively analyzed from both qualitative and quantitative aspects, and the conclusion of the influence relationship of system transient stability is obtained;

[0011] Based on the conclusion of the transient stability impact relationship of the system, the power grid is regulated to maintain the transient stability of asymmetric faults.

[0012] A further optimization scheme is that the power grid system model of the hybrid synchronous control inverter connected to the infinite power grid is constructed based on the power data, including the method:

[0013] A grid system model in which a grid-building inverter is connected to an infinite grid is constructed, and the grid system model is controlled by a hybrid synchronous control strategy with positive and negative sequence decoupling; the control strategy includes: the positive sequence loop of the grid system model is controlled by hybrid synchronous control, the reactive loop is controlled by reactive power-voltage droop, the inner loop of the grid system model is controlled by a voltage and current dual loop, and the negative sequence loop of the grid system model is phase-locked by a phase-locked loop and direct current control;

[0014] The power data is input into a power grid system model to obtain a positive sequence voltage phase, a negative sequence voltage phase, an expected positive sequence voltage, and an expected negative sequence voltage.

[0015] A further optimization scheme is that the power grid system model includes a main circuit part and a control circuit part;

[0016] The main circuit part is used to output the grid voltage, the voltage and current of the common coupling point;

[0017] The control circuit part includes: a power calculation link, a positive sequence control loop and a negative sequence control loop;

[0018] The power calculation link calculates the active power P output by the hybrid synchronous control inverter at the common coupling point based on the voltage and current at the common coupling point. e and reactive power Q;

[0019] The positive sequence control loop and the negative sequence control loop are based on the active power P e The phase and voltage information is obtained by using the reactive power Q, and then the expected positive sequence voltage and the expected negative sequence voltage are calculated. The expected positive sequence voltage and the expected negative sequence voltage are demodulated to obtain the control signal of the main circuit part;

[0020] The positive sequence control loop comprises: a hybrid synchronous control loop, a reactive power loop, a voltage inner loop and a current inner loop.

[0021] A further optimization scheme is that the power angle δ and angular velocity The state space equation of the power grid system before the fault occurs is constructed by taking the voltage amplitude U as the state variable, including the method:

[0022] The active component of the voltage at the common coupling point U q Added as feedback to the active loop of the hybrid synchronous control loop, and resisted the frequency deviation based on the phase angle difference, to obtain the control law of the hybrid synchronous control loop;

[0023] According to the reactive power droop control structure, the control law of the reactive power loop is obtained;

[0024] Based on the control law of the hybrid synchronous control loop and the control law of the reactive loop, the active power P output by the hybrid synchronous control inverter at the grid connection point is e And reactive power Q are used to obtain the state space equation of the power grid system before a fault occurs.

[0025] A further optimization scheme is that the state space equation of the power grid system before a fault occurs includes:

[0026]

[0027] Among them, the three-dimensional state variable x1 = δ, the three-dimensional state variable Three-dimensional state variable x3 = U; represents the first-order derivative of the three-dimensional state variable x1; represents the first-order derivative of the three-dimensional state variable x2; represents the first-order derivative of the three-dimensional state variable x3; J is the virtual inertia; D is the damping coefficient; K Qis the reactive integral coefficient; Q ref is the given value of reactive power; U g is the amplitude of the grid voltage; X g1 is the reactance between the common coupling point and the fault point; X g2 is the reactance between the fault point and the large power grid; R g is the resistance on the grid side; t represents time; P ref is the active power reference value.

[0028] The further optimization scheme is to derive the hybrid synchronous control mathematical model of the asymmetric fault of the power grid system according to the power grid system model and the state space equation before the fault, including the following methods:

[0029] Construct positive sequence outer loop motion equation and negative sequence outer loop motion equation based on power grid system model;

[0030] According to the sequence domain equivalent circuit of single-phase grounding fault, the positive sequence voltage and negative sequence voltage of the common coupling point are derived and calculated.

[0031] The positive-sequence voltage and negative-sequence voltage are respectively substituted into the positive-sequence outer-loop motion equation and negative-sequence outer-loop motion equation of the power grid system model to obtain the state space equation after the fault occurs.

[0032] The further optimization scheme is that the state space equation after the fault occurs is:

[0033]

[0034] in, represents the first-order derivative of y1; represents the first derivative of y2; represents the first derivative of y3; y1 = δ p , where δ p It is the output power angle of the positive sequence active loop; where ω p is the output angular velocity of the positive sequence active loop; y3=U p , expressed as the positive sequence voltage amplitude at the common coupling point; is the output positive sequence active power; U dp and U qp are the d-axis and q-axis components of the positive sequence voltage at the common coupling point, I dp and I qp are the d-axis and q-axis components of the positive sequence current at the common coupling point respectively; reactance X1 = Z1sinΦ1, resistance R1 = Z1cosΦ1, reactance X2 = Z2sinΦ2, resistance R2 = Z2cosΦ2, reactance X4 = Z4sinΦ4, resistance R4 = Z4cosΦ4, reactance X5 = Z5sinΦ5; resistance R5 = Z5cosΦ5; Z i =Zi ∠Φ i , i=1,2 represents the equivalent impedance when the positive sequence injection current and the negative sequence injection current act alone in the expression of the positive sequence voltage at the common coupling point under fault, Z3=Z3∠Φ3 represents the coefficient before the grid voltage act alone in the expression of the positive sequence voltage at the common coupling point;; Z i =Z i ∠Φ i , i=4,5 represents the equivalent impedance when the positive sequence injection current and the negative sequence injection current act alone in the negative sequence voltage expression of the common coupling point under fault, Z6=Z6∠Φ6 represents the coefficient before the grid voltage act alone in the negative sequence voltage expression of the common coupling point; U gabc Indicates the amplitude of the grid voltage.

[0035] A further optimization scheme is to qualitatively analyze the hybrid synchronous control mathematical model from both qualitative and quantitative aspects based on the power angle curve and phase plane method to obtain the conclusion of the transient stability influence relationship of the system, including the following methods:

[0036] Obtain the equivalent active power given value P' of the hybrid synchronous control mathematical model based on the state space equation after the fault occurs ref , output positive sequence active power P + ' e , and the equivalent damping coefficient D peq ;

[0037] Based on the equivalent active power given value P' ref , output positive sequence active power P + ' e and the equivalent damping coefficient D peq Draw the power angle curve after a single-phase grounding fault;

[0038] According to the power angle curve after a single-phase grounding fault, the correlation between the positive sequence current and the given value of equivalent active power, and the correlation between the negative sequence current and the given value of equivalent active power are analyzed.

[0039] The further optimization scheme is that the equivalent active power given value P' ref And output positive sequence active power P + ' e , equivalent damping coefficient D peq Obtained according to the following formula:

[0040]

[0041] Where η = arctan((D p K p -I qp ) / I dp ), Φ0=Φ3+η; Φ0 represents The angle corresponding to the maximum.

[0042] As a preferred method, the phase plane method is used to obtain the quantitative conclusion of the transient stability influence relationship of hybrid synchronous control under asymmetric faults, and the nonlinear state space equations are solved using the MATLAB "ode45" toolkit;

[0043] Based on the state space equation before the fault occurs, the initial value x1=δ=0 is given, x3=U=311V, solve the state space equation before the fault in 0~0.5s;

[0044] When a single-phase grounding fault occurs at t=0.5s, the state space equation is executed, and the solution result based on the above 0.5s is used as the initial state at the time of the fault.

[0045] According to the phase diagram after a single-phase grounding fault, the quantitative relationship between the positive sequence d-axis current, the q-axis current, the negative sequence d-axis current and the transient stability is analyzed.

[0046] The present solution also provides an asymmetric fault transient stability analysis system based on a grid-type inverter, which is used to implement the asymmetric fault transient stability analysis method based on a grid-type inverter as described above. The system includes:

[0047] A collection module, used to collect power data of the hybrid synchronous control inverter outlet under normal operation and fault operation of the power grid;

[0048] The model building module is used to build a grid system model of a hybrid synchronous control inverter connected to the grid based on the power data, and to use the power angle δ, angular velocity and voltage amplitude U as state variables to construct the state space equation of the power grid system before the fault occurs;

[0049] A derivation module, used for deriving a hybrid synchronous control mathematical model when the power grid system has an asymmetric fault according to the power grid system model and the state space equation before the fault;

[0050] An analysis module, used to qualitatively analyze the hybrid synchronous control mathematical model from both qualitative and quantitative aspects based on a power angle curve and a phase plane method, and obtain a conclusion on the influence relationship of system transient stability;

[0051] A control module is used to control the power grid based on the transient stability influence relationship conclusion of the system to maintain the transient stability of asymmetric faults. Compared with the prior art, the present invention has the following advantages and beneficial effects:

[0052] The present invention provides a method and system for transient stability analysis of asymmetric faults based on a grid-connected inverter; based on a grid system model of a hybrid synchronous control inverter connected to a grid, a mathematical model of hybrid synchronous control before and after a fault is derived, and the transient instability mechanism of the hybrid synchronous control inverter under an asymmetric fault is analyzed from the legal qualitative aspect with the help of a power angle curve and a phase plane, and the influence of positive and negative sequence currents on stability is revealed; based on mathematical modeling, power angle curves and phase diagrams, the transient instability mechanism analysis of hybrid synchronous control is realized by considering the influence of the coupling of positive and negative sequence components under an asymmetric fault; the boundary of instability caused by positive and negative sequence currents can be determined, and the transient stability analysis method of the converter considering the positive and negative sequence coupling caused by asymmetric faults under grid-connected control is improved, providing a reference for improving the transient synchronous stability of the converter under large disturbances; at the same time, the studied current instability boundary can provide a reference for the current limiting reference value. BRIEF DESCRIPTION OF THE DRAWINGS

[0053] 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:

[0054] Figure 1 It is a flow chart of a method for transient stability control of asymmetric faults based on a grid-connected inverter;

[0055] Figure 2 It is a schematic diagram of the power grid system model;

[0056] Figure 3 It is a schematic diagram of the equivalent circuit of the hybrid synchronous control inverter of the power grid system under asymmetric AC fault;

[0057] Figure 4 It is a schematic diagram of the hybrid synchronous control structure;

[0058] Figure 5 It is a schematic diagram of the equivalent circuit of a hybrid synchronous control inverter of a power grid system under a single-phase grounding fault;

[0059] Figure 6 It is a schematic diagram of the equivalent circuit of a hybrid synchronous control inverter of a power grid system under a two-phase short-circuit grounding fault;

[0060] Figure 7 It is a schematic diagram of the equivalent circuit of the hybrid synchronous control inverter of the power grid system under a two-phase short circuit fault;

[0061] Figure 8 It is a schematic diagram of the power angle curve after a single-phase grounding fault;

[0062] Fig. 9 HSC phase diagram for different positive sequence d-axis current magnitudes injected into the system after a fault occurs;

[0063] Fig.10 To inject different I into the system after a single-phase grounding fault occurs qp Schematic diagram of the phase diagram;

[0064] Fig.11 To inject different I into the system after a single-phase grounding fault occurs dn Phase diagram of

[0065] Fig.12 To inject different I into the system after a single-phase grounding fault occurs qn Phase diagram of

[0066] Fig.13 For the first case, different I qp Schematic diagram of the phase diagram experimental results;

[0067] Fig.14 For the second case, different I qp Schematic diagram of the phase diagram experimental results;

[0068] Fig.15 For the third case, different I qp Schematic diagram of the phase diagram experimental results;

[0069] Fig.16 For the fourth case, different I qp Schematic diagram of the phase diagram experimental results. DETAILED DESCRIPTION

[0070] 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.

[0071] Example 1

[0072] This embodiment provides an asymmetric fault transient stability control method based on a grid-connected inverter, such as Figure 1 As shown, including:

[0073] Step 1: Collecting power data of the hybrid synchronous control inverter outlet under normal operation and fault operation of the power grid; the power data includes three-phase voltage and three-phase current;

[0074] Step 2: Based on the power data, a grid system model of a hybrid synchronous control inverter connected to the grid is constructed, and the power angle δ, angular velocity and voltage amplitude U as state variables to construct the state space equation of the power grid system before the fault occurs;

[0075] The construction method of the power grid system model specifically includes:

[0076] A grid system model in which a grid-building inverter is connected to an infinite grid is constructed, and the grid system model is controlled by a hybrid synchronous control strategy with positive and negative sequence decoupling; the control strategy includes: the positive sequence loop of the grid system model is controlled by hybrid synchronous control, the reactive loop is controlled by reactive power-voltage droop, the inner loop of the grid system model is controlled by a voltage and current dual loop, and the negative sequence loop of the grid system model is phase-locked by a phase-locked loop and direct current control;

[0077] The three-phase voltage and three-phase current data are input through a hybrid synchronous control with positive and negative sequence decoupling and a negative sequence DSOGI-PLL to obtain the positive sequence voltage phase and negative sequence phase, and the desired positive sequence voltage m is obtained through inner loop control. dp 、m qp and the expected negative sequence voltage m dn 、m qn .

[0078] The power grid system model includes a main circuit part and a control circuit part; the main circuit part is used to output the grid voltage, the voltage and current of the common coupling point; the control circuit part includes: power calculation link, positive sequence control loop and negative sequence control loop; the power calculation link calculates the active power P output by the hybrid synchronous control inverter at the common coupling point according to the voltage and current of the common coupling point e and reactive power Q; the positive sequence control loop and the negative sequence control loop are based on the active power P e The phase and voltage information are obtained by using the reactive power Q, and then the expected positive sequence voltage and the expected negative sequence voltage are obtained. The expected positive sequence voltage and the expected negative sequence voltage are demodulated to obtain the control signal of the main circuit part;

[0079] The positive sequence control loop includes: a hybrid synchronous control loop, a reactive power loop, a voltage inner loop and a current inner loop.

[0080] Since asymmetric faults have negative sequence components, this scheme uses a hybrid synchronous control strategy with positive and negative sequence decoupling to control the power grid system model, that is, the positive sequence active outer loop of the power grid system model is composed of a positive and negative sequence decoupling module and an HSC control module, the reactive loop adopts reactive-voltage droop control, and the inner loop is controlled by a voltage and current dual loop; the negative sequence is controlled by a phase-locked loop and direct voltage control. Based on the three-phase voltage and three-phase current measured in step one, the positive sequence voltage phase and negative sequence phase are obtained through hybrid synchronous control with positive and negative sequence decoupling and DSOGI-PLL. The positive sequence current inner loop and the negative sequence current inner loop output the desired positive sequence voltage m respectively. dp 、m qp and the desired negative sequence voltage m dn 、m qn .

[0081] Depend on Figure 2-Figure 4 As shown in the figure, the control circuit mainly includes: power calculation link, positive sequence control loop and negative sequence control loop. The positive sequence control loop includes Figure 2 The hybrid synchronous control loop with positive and negative sequence decoupling, reactive loop, voltage and current inner loop shown; the negative sequence control loop includes Figure 4 The DSOGI-PLL phase-locked loop and current inner loop shown in the figure; d,q and I d,q They are the dq axis components of voltage and current in the dq coordinate system respectively. The power calculation link calculates the active power P output by the inverter at the grid connection point based on the voltage and current at the grid connection point. e and reactive power Q, and finally obtain the desired positive sequence voltage m through the positive and negative sequence loops abcp and negative sequence voltage m abcn , after PWM modulation, the main control switch tube S is obtained a1，2 , S b1，2 and S c1，2 control signal.)

[0082] With power angle δ, angular velocity The state space equation before the fault occurs is constructed with the voltage amplitude U as the state variable, where the initial quantity x1=δ=0, x3=U=311V.

[0083] The grid system model of the hybrid synchronous control inverter connected to the grid is constructed as follows Figure 2 As shown, it includes the main circuit part and the control circuit part. dc is the DC bus voltage; L f Composition filter inductor; X g1 , X g2 and R g is the reactance and resistance on the grid side; i 1a 、i 1b and i 1c are the A, B, and C three-phase currents at the output of the hybrid synchronous control inverter; u abc and i abc are the three-phase voltage and current at the common coupling point respectively; u gabc is the grid voltage; and They are the phasor representations of the voltage and current at the common coupling point respectively.

[0084] With power angle δ, angular velocity The state space equation of the power grid system before the fault occurs is constructed by taking the voltage amplitude U as the state variable, including the method:

[0085] Set the initial value x1 = δ = 0, x3=U=311V;

[0086] The active component of the voltage at the common coupling point U q Added as feedback to the active loop of the hybrid synchronous control loop, and resisted the frequency deviation based on the phase angle difference, to obtain the control law of the hybrid synchronous control loop;

[0087] Before the fault occurs, the inverter operates normally and there is no negative sequence component. The hybrid synchronous control structure is as follows Figure 3 As shown, the q-axis component U of the voltage at the common coupling point is q As a feedback quantity, it is added to the active loop of the hybrid synchronous control loop. The added voltage-based synchronous loop uses the phase angle difference to resist the frequency deviation. The control law of the hybrid synchronous control loop is expressed as:

[0088]

[0089] Among them, δ p =θ HSC -θ g , is the virtual power angle, θ HSC and θ g are the phase angle of the hybrid synchronous loop output and the phase angle of the power grid, ω HSC is the output angular velocity of the hybrid synchronous control inverter, ω0 is the angular velocity reference value, Δω VSG is the angular velocity deviation of the hybrid synchronous control inverter, Δω PLL is the angular velocity deviation of the PLL module. J is the virtual inertia, D is the damping coefficient, K p It represents the proportional coefficient of the PI controller in the PLL module, P ref is the active power reference value, U q is the q-axis component of the voltage at the common coupling point.

[0090] The active component U q Considered as 0, the control law of the reactive loop is obtained;

[0091] Due to the normal operation, it can be considered that U q =0, so in normal operation, the hybrid synchronous loop is approximately equivalent to the hybrid synchronous control inverter control mode, and the control law of the reactive loop is:

[0092]

[0093] Among them, K Q is the reactive integral coefficient; Q ref is the given value of reactive power; Q is the system output reactive power; U0 is the reference value of voltage amplitude at the common coupling point.

[0094] Based on the control law of the hybrid synchronous control loop and the control law of the reactive loop, the active power P output by the hybrid synchronous control inverter at the grid connection point is eAnd reactive power Q are used to obtain the state space equation of the power grid system before a fault occurs.

[0095] Depend on Figure 2 It can be calculated that the active power and reactive power output by the hybrid synchronous control inverter at the grid connection point are:

[0096]

[0097] Where U g For the amplitude of the grid voltage, define a three-dimensional state variable x1 = δ, x3=U, and substituting equation (3) into equation (1) and equation (2) can obtain the state space equation of the power grid system before the fault occurs:

[0098]

[0099] Among them, the three-dimensional state variable x1 = δ, the three-dimensional state variable Three-dimensional state variable x3 = U; represents the first-order derivative of the three-dimensional state variable x1; represents the first-order derivative of the three-dimensional state variable x2; represents the first-order derivative of the three-dimensional state variable x3; J is the virtual inertia; D is the damping coefficient; K Q is the reactive integral coefficient; Q ref is the given value of reactive power; U g is the amplitude of the grid voltage; X g1 is the reactance between the common coupling point and the fault point; X g2 is the reactance between the fault point and the large power grid; R g is the resistance on the grid side; t represents time; P ref is the active power reference value.

[0100] Step 3: Based on the power grid system model and the state space equation before the fault, derive the hybrid synchronous control mathematical model when the power grid system has an asymmetric fault; the specific method includes:

[0101] Construct positive sequence outer loop motion equation and negative sequence outer loop motion equation based on power grid system model;

[0102] According to the sequence domain equivalent circuit of single-phase grounding fault, the positive sequence voltage and negative sequence voltage of the common coupling point are derived and calculated.

[0103] The positive-sequence voltage and negative-sequence voltage are respectively substituted into the positive-sequence outer-loop motion equation and negative-sequence outer-loop motion equation of the power grid system model to obtain the state space equation after the fault occurs.

[0104] according to Figure 4 As shown, the control law of the hybrid synchronization loop under asymmetric fault can be obtained:

[0105]

[0106] in, is the output positive sequence active power. Since the output active power here uses the positive sequence active power, the positive sequence active power is U dp and U qp are the dq axis components of the positive sequence voltage at the common coupling point, I dp and I qp They are the dq axis components of the positive sequence current at the common coupling point respectively.

[0107] The negative sequence synchronization loop consists of three parts: DSOGI module, PNSC module and PLL module, and can be expressed by formula (6):

[0108]

[0109] Among them, θ n is the negative sequence voltage phase of the negative sequence phase locked loop output, ω n is the negative sequence angular velocity, k pn and k in They are the proportional coefficient and integral coefficient of the negative sequence phase-locked loop PI controller, U qn is the q-axis component of the negative sequence voltage at the common coupling point.

[0110] according to Figure 2 The equivalent circuit of the hybrid synchronous control inverter of the power grid system under the asymmetric AC fault is shown in the figure. Figure 5-Figure 7 The sequence domain equivalent circuits of the hybrid synchronous control inverter of the power grid system with different faults (single-phase grounding fault, two-phase short-circuit grounding fault and two-phase short-circuit fault) are given. Fp , U Fn , U F0 are the positive-sequence voltage phasor, negative-sequence voltage phasor and zero-sequence voltage phasor at the fault point respectively, I p and I n are the positive and negative sequence current phasors injected into the converter respectively. The current phase is determined by the synchronization loop and can be expressed as formula (7).

[0111]

[0112] Among them, θ Ip =arctan(I qp / I dp ), θ In =arctan(I qn / I dn), is the angle of the current in the dq coordinate system. From the equivalent circuit, it can be seen that the voltage at the common coupling point is affected by three sources, namely the injected positive sequence current source, the negative sequence current source and the grid side voltage source. The superposition principle can be used to calculate U p and U n The voltage is:

[0113]

[0114] Among them, Z i =|Z i |∠Φ i , i=1,2,3,4,5,6. The specific expressions are shown in the following table.

[0115] Table 1Z i Expressions under different asymmetric faults

[0116]

[0117] U g is the amplitude of the grid voltage. According to the principle of PARK transformation, equation (8) is transformed into the rotating coordinate system to obtain equation (9).

[0118]

[0119] In the formula, δ p =θ HSC -θ g ; δ n =θ n -θ g , we can get the q-axis component U of the positive and negative sequence voltage at the common coupling point qp , U qn They are:

[0120]

[0121] Substituting equation (10) into equation (5) and equation (6) respectively, we can obtain the dynamic equations of the positive sequence HSC and negative sequence phase locked loop:

[0122]

[0123] in, In order to calculate the output active power simply, the line resistance is not considered, and the positive and negative sequence coupling terms are ignored in equation (10) We can get:

[0124]

[0125] Combining equations (9), (11) and (12), we can obtain the state space equation after the fault occurs:

[0126]

[0127] in, represents the first-order derivative of y1; represents the first derivative of y2; represents the first derivative of y3; y1 = δ p , where δ p It is the output power angle of the positive sequence active loop; where ω p is the positive sequence active loop output angular frequency; y3=U p , expressed as the positive sequence voltage amplitude at the common coupling point; is the output positive sequence active power; U dp and U qp are the d and q axes of the positive sequence voltage at the common coupling point, I dp and I qp are the d-axis and q-axis components of the positive sequence current at the common coupling point respectively; reactance X1 = Z1sinΦ1, resistance R1 = Z1cosΦ1, reactance X2 = Z2sinΦ2, resistance R2 = Z2cosΦ2, reactance X4 = Z4sinΦ4, resistance R4 = Z4cosΦ4, reactance X5 = Z5sinΦ5; resistance R5 = Z5cosΦ5; Z i =Z i ∠Φ i , i=1,2 represents the equivalent impedance when the positive sequence injection current and the negative sequence injection current act alone in the expression of the positive sequence voltage at the common coupling point under fault, Z3=Z3∠Φ3 represents the coefficient before the grid voltage act alone in the expression of the positive sequence voltage at the common coupling point;; Z i =Z i ∠Φ i , i=4,5 represents the equivalent impedance when the positive sequence injection current and the negative sequence injection current act alone in the negative sequence voltage expression of the common coupling point under fault, and Z6=Z6∠Φ6 represents the coefficient before the grid voltage act alone in the negative sequence voltage expression of the common coupling point; as shown in Table 1. U gabc Indicates the amplitude of the grid voltage.

[0128] Step 4: Based on the power angle curve and phase plane method, the hybrid synchronous control mathematical model is qualitatively analyzed from both qualitative and quantitative aspects to obtain the conclusion of the transient stability influence relationship of the system; the specific methods include:

[0129] Obtain the equivalent active power given value P' of the hybrid synchronous control mathematical model based on the state space equation after the fault occurs ref , output positive sequence active power P + ' e , and the equivalent damping coefficient D peq ;

[0130] For the positive sequence hybrid synchronous control loop, it can be seen from formula (11) that the dynamic equation of the HSC control module is represented by a second-order nonlinear differential equation, which is similar to the dynamic equation of the hybrid synchronous control inverter of the power grid system. Here, formula (11) is rewritten in the form of the rotor motion equation to evaluate the transient stability of the HSC control module. From formula (11), we get:

[0131]

[0132] According to formula (14) and formula (12) and the synchronous machine rotor motion equation, the equivalent active power given value P' is obtained ref And output positive sequence active power P + ' e , equivalent damping coefficient D peq for:

[0133]

[0134] Where η = arctan((D p K p -I qp ) / I dp ), Φ0=Φ3+η, from the above formula we can see that P' ref It includes the dq axis components of positive and negative sequence currents, is δ HSC During the fault period, the converter should keep in sync with the grid, and having a balance point is a necessary condition for transient stability, that is, There is a solution, so the key to preventing transient instability is to ensure The maximum value is greater than P' ref )

[0135] Based on the equivalent active power given value P' ref , output positive sequence active power P + ' e and the equivalent damping coefficient D peq Draw the power angle curve after a single-phase grounding fault;

[0136] According to the power angle curve after a single-phase grounding fault, the correlation between the positive sequence current and the given value of equivalent active power, and the correlation between the negative sequence current and the given value of equivalent active power are analyzed.

[0137] According to formula (15), the power angle curve after a single-phase grounding fault can be drawn as follows: Figure 8 As shown, the red solid line represents P' ref , the red dashed line indicates P ref , the black dashed line indicates The blue solid line indicates The green shadow is the acceleration area S aca, the orange shadow is the deceleration area S dca , δ0 is the initial angle. After the fault occurs, the positive sequence current increases, from P ref Mutation to P' ref , the running point jumps from point a to point b, at point b P' ref Greater than Δω>0, so the continuous acceleration causes the HSC output frequency to be greater than the grid frequency. When δ increases to point c, At this time, Δω is still greater than 0, so δ continues to increase. Once δ exceeds point c, P' ref Less than Δω gradually decreases. When Δω=0, δ no longer increases. Since P' ref Less than Therefore, Δω continues to decrease, δ begins to decrease, moves to point c, and after several cycles of oscillation, finally stabilizes at point c, so point c is called the stable equilibrium point (SEP). If δ increases to point d and Δω is still greater than 0, δ will continue to increase through point d, resulting in transient instability, and point d is called the unstable equilibrium point (UEP).

[0138] From the above analysis, we can know that the positive sequence current is related to P' ref Positively correlated, negative sequence current and P' ref Therefore, increasing the negative sequence d-axis and q-axis currents or reducing the positive sequence d-axis or q-axis currents can reduce P' ref , only increasing the negative sequence current can ensure P m Reduce P' while keeping the same ref ,Depend on Figure 4 It can be seen that S aca Decrease, S dca However, it can be found from equation (15) that increasing the positive-sequence d-axis current and decreasing the positive-sequence q-axis current can increase The peak value (P m ), that is, by reducing the positive sequence q axis current and increasing the negative sequence dq axis current, it is beneficial to improve the transient stability of the system. In order to further analyze the influence of the positive sequence d axis current, let P m -P' ref to I dp Take the derivative.

[0139]

[0140] It can be seen from formula (13) that when the result of the above formula is greater than zero, it means that increasing I dpThe impact on the transient stability of the system is beneficial; otherwise, it is adverse. As can be seen from Table 1, |Z3| << X1. From the data in Table 2, it can be known that Equation (13) is less than 0. Therefore, increasing I dp will reduce the transient stability of the system.

[0141] For the negative-sequence synchronous loop, under steady-state conditions, there is U qn = 0. According to Equation (11), we get:

[0142] |Z4||I n |sin(Φ4 + θ In ) + |Z5||I p |sin(Φ5 + δ p + θ Ip - δ n ) + |Z6||U g |sin(Φ6 - δ n ) = 0 (15)

[0143] Since |Z4| >> |Z5|, ignoring the sequence network coupling in the above equation, the condition for the existence of the equilibrium point can be clearly expressed as:

[0144] |Z4|I n sin(Φ4 + θ In ) ≤ |Z6||U g | (16)

[0145] From the above equation, it can be obtained that it can be known that when the negative-sequence current injection exceeds I nmin , transient instability will occur. Substituting the parameters in Table 2, it can be known that the minimum value of I nmin is 20.93 A.

[0146] Similarly, taking the power angle δ, angular velocity and voltage amplitude U as state variables, the state-space equation after the fault occurs is constructed, where the initial quantity is the value at the moment before the fault in the state equation of Step 3. With the help of the phase diagram method, the influence law of positive and negative sequence currents on transient stability is quantitatively analyzed to obtain a quantitative conclusion.

[0147] Based on the state equations established in Step 3 and Step 4, the system parameters are shown in the following table:

[0148] Table 2 System Parameters

[0149]

[0150] The system operates normally within t = 0 - 0.5 s, and the state equation shown in Equation (4) is executed. The initial quantity x1 = δ = 0, x3=U=311V. At t=0.5s, a single-phase ground fault occurs, and the state equation shown in equation (13) is executed. The initial state of equation (13) is the state of equation (4) at t=0.5s. The result is Figure 9-12 As shown, Fig. 9 This is the HSC phase diagram when different positive sequence d-axis currents are injected into the system after a fault occurs. In the figure, when I dp =30A or 40A, the phase diagram is convergent, as shown by the blue and red lines, indicating that the system is stable. dp =54A, the phase diagram diverges, indicating that the VSC loses synchronization with the grid and the system is unstable. Fig. 9 It can be seen that increasing I dp It is detrimental to the stability of the system. dp When it increases to 54A, the system becomes unstable.

[0151] Fig.10 To inject different I into the system after a single-phase grounding fault occurs qp The phase diagram of Fig.10 It can be seen that when I dp =54A, the system becomes unstable. Under the condition of system instability, reduce I qp , as shown in the red and yellow lines in (b), when I qp = -5A or -10A, the phase diagram converges to 0 and the system returns to a stable state, which indicates that reducing I qp It is conducive to system stability. qp When it is reduced to -5A, the system is stable. Fig. 9 The analysis is consistent.

[0152] Fig.11 To inject different I into the system after a single-phase grounding fault occurs dn Phase diagram, also in I dp =54A Under the condition of system instability, change I dn The value of I dn =11A or 20A, the system regains stability. Fig.11 As shown in the red and yellow lines, it indicates that the increase of I dn It is beneficial to improve the stability of the system. qp Increased to 11A, the system became stable.

[0153] Fig.12 To inject different I into the system after a single-phase grounding fault occurs qn Phase diagram, in I dp =54A Under the condition of system instability, increase I qn The value of I qn =13A or 20A, the phase diagram converges to zero and the system regains stability, as shown by the red and yellow lines, which indicates that increasing Iqn It is conducive to the stability of the system and the stability boundary is when I qn When increased to 13A.

[0154] Based on the above analysis method, experimental verification was carried out. Figure 13-Figure 16 For the experimental results, Fig.13 It shows that with I dp As the number of phases increases, the system is more likely to experience transient instability after a single-phase grounding fault occurs. dp When the current increases to 57A, the system becomes unstable, which is basically consistent with the theoretically calculated 54A. Fig.14 Indicates that the reduction of I qp It is beneficial to improve transient stability. qp When it is reduced to -3A, the system is stable after a single-phase grounding fault, which is basically consistent with the theoretically calculated -5A. Fig.15 It shows that with I dn The increase of I dn When it increases to 15A, the system changes from an unstable state to a stable state, which is basically consistent with the theoretically calculated 11A. Fig.16 Indicates that the increase in I qn It can also enhance the transient stability of the system. qn When it increases to 17A, the system becomes stable, which is basically consistent with the theoretically calculated 13A. Figure 13-Figure 16 It is further verified that reducing the dq-axis component of the positive-sequence current and increasing the dq-axis component of the negative-sequence current are beneficial to improving the transient stability after a single-phase grounding fault occurs.

[0155] Step 5: Based on the conclusion of the transient stability impact relationship of the system, the power grid is regulated to maintain the transient stability of asymmetric faults.

[0156] Example 2

[0157] This embodiment provides an asymmetric fault transient stability analysis system based on a grid-type inverter, which is used to implement the above-mentioned asymmetric fault transient stability analysis method based on a grid-type inverter. The system includes:

[0158] A collection module, used to collect power data of the hybrid synchronous control inverter outlet under normal operation and fault operation of the power grid;

[0159] The model building module is used to build a grid system model of a hybrid synchronous control inverter connected to the grid based on the power data, with power angle δ, angular velocity and voltage amplitude U as state variables to construct the state space equation of the power grid system before the fault occurs;

[0160] A derivation module, used for deriving a hybrid synchronous control mathematical model when the power grid system has an asymmetric fault according to the power grid system model and the state space equation before the fault;

[0161] An analysis module, used to analyze the hybrid synchronous control mathematical model from qualitative and quantitative aspects based on a power angle curve and a phase plane method, and obtain a conclusion on the influence relationship of system transient stability;

[0162] A control module is used to control the power grid based on the system transient stability impact relationship conclusion to maintain asymmetric fault transient stability.

[0163] 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 controlling transient stability of asymmetric faults based on a grid-connected inverter, characterized in that: include: Collect the power data of the hybrid synchronous control inverter export under normal operation and fault operation of the power grid; Based on the power data, a grid system model of a hybrid synchronous control inverter connected to the grid is constructed, and the power angle δ, angular velocity and voltage amplitude U as state variables to construct the state space equation of the power grid system before the fault occurs; Based on the power grid system model and the state space equation before the fault, the hybrid synchronous control mathematical model of the power grid system with asymmetric faults is derived; Based on the power angle curve and phase plane method, the hybrid synchronous control mathematical model is qualitatively analyzed from both qualitative and quantitative aspects, and the conclusion of the influence relationship of system transient stability is obtained; Based on the conclusion of the transient stability impact relationship of the system, the power grid is regulated to maintain the transient stability of asymmetric faults.

2. The asymmetric fault transient stability analysis method based on grid-connected inverter according to claim 1 is characterized in that: The power grid system model of the hybrid synchronous control inverter connected to the infinite power grid is constructed based on the power data, including a method: A grid system model in which a grid-building inverter is connected to an infinite grid is constructed, and the grid system model is controlled by a hybrid synchronous control strategy with positive and negative sequence decoupling; the control strategy includes: the positive sequence loop of the grid system model is controlled by hybrid synchronous control, the reactive loop is controlled by reactive power-voltage droop, the inner loop of the grid system model is controlled by a voltage and current dual loop, and the negative sequence loop of the grid system model is phase-locked by a phase-locked loop and direct current control; The power data is input into a power grid system model to obtain a positive sequence voltage phase, a negative sequence voltage phase, an expected positive sequence voltage, and an expected negative sequence voltage.

3. The asymmetric fault transient stability analysis method based on grid-connected inverter according to claim 2 is characterized in that: The power grid system model includes a main circuit part and a control circuit part; The main circuit part is used to output the grid voltage, the voltage and current of the common coupling point; The control circuit part includes: a power calculation link, a positive sequence control loop and a negative sequence control loop; The power calculation link calculates the active power P output by the hybrid synchronous control inverter at the common coupling point based on the voltage and current at the common coupling point. e and reactive power Q; The positive sequence control loop and the negative sequence control loop are based on the active power P e The phase and voltage information is obtained by using the reactive power Q, and then the expected positive sequence voltage and the expected negative sequence voltage are calculated. The expected positive sequence voltage and the expected negative sequence voltage are demodulated to obtain the control signal of the main circuit part; The positive sequence control loop comprises: a hybrid synchronous control loop, a reactive power loop, a voltage inner loop and a current inner loop.

4. The asymmetric fault transient stability analysis method based on grid-connected inverter according to claim 3 is characterized in that: The power angle δ and angular velocity The state space equation of the power grid system before the fault occurs is constructed by taking the voltage amplitude U as the state variable, including the method: The active component of the voltage at the common coupling point U q Added as feedback to the active loop of the hybrid synchronous control loop, and resisted the frequency deviation based on the phase angle difference, to obtain the control law of the hybrid synchronous control loop; According to the reactive power droop control structure, the control law of the reactive power loop is obtained; Based on the control law of the hybrid synchronous control loop and the control law of the reactive loop, the active power P output by the hybrid synchronous control inverter at the grid connection point is e And reactive power Q are used to obtain the state space equation of the power grid system before a fault occurs.

5. The asymmetric fault transient stability analysis method based on grid-connected inverter according to claim 4 is characterized in that: The state space equations of the power grid system before a fault occurs include: Among them, the three-dimensional state variable x1 = δ, the three-dimensional state variable Three-dimensional state variable x3 = U; represents the first-order derivative of the three-dimensional state variable x1; represents the first-order derivative of the three-dimensional state variable x2; represents the first-order derivative of the three-dimensional state variable x3; J is the virtual inertia; D is the damping coefficient; K Q is the reactive integral coefficient; Q ref is the given value of reactive power; U g is the amplitude of the grid voltage; X g1 is the reactance between the common coupling point and the fault point; X g2 is the reactance between the fault point and the large power grid; R g is the resistance on the grid side; t represents time; P ref is the active power reference value.

6. The asymmetric fault transient stability analysis method based on grid-connected inverter according to claim 5 is characterized in that: According to the power grid system model and the state space equation before the fault, the hybrid synchronous control mathematical model of the power grid system asymmetric fault is derived, including the following methods: Construct positive sequence outer loop motion equation and negative sequence outer loop motion equation based on power grid system model; According to the sequence domain equivalent circuit of single-phase grounding fault, the positive sequence voltage and negative sequence voltage of the common coupling point are derived and calculated. The positive-sequence voltage and negative-sequence voltage are respectively substituted into the positive-sequence outer-loop motion equation and negative-sequence outer-loop motion equation of the power grid system model to obtain the state space equation after the fault occurs.

7. The asymmetric fault transient stability analysis method based on grid-connected inverter according to claim 6 is characterized in that: The state space equation after the fault occurs is: in, represents the first-order derivative of y1; represents the first derivative of y2; represents the first derivative of y3; y1 = δ p , where δ p It is the output power angle of the positive sequence active loop; where ω p is the output angular velocity of the positive sequence active loop; y3=U p , expressed as the positive sequence voltage amplitude at the common coupling point; P e + is the output positive sequence active power; U dp and U qp are the d-axis and q-axis components of the positive sequence voltage at the common coupling point, I dp and I qp are the d-axis and q-axis components of the positive sequence current at the common coupling point respectively; reactance X1 = Z1sinΦ1, resistance R1 = Z1cosΦ1, reactance X2 = Z2sinΦ2, resistance R2 = Z2cosΦ2, reactance X4 = Z4sinΦ4, resistance R4 = Z4cosΦ4, reactance X5 = Z5sinΦ5; resistance R5 = Z5cosΦ5; Z i =Z i ∠Φ i , i=1,2 represents the equivalent impedance when the positive sequence injection current and the negative sequence injection current act alone in the expression of the positive sequence voltage at the common coupling point under fault, Z3=Z3∠Φ3 represents the coefficient before the grid voltage act alone in the expression of the positive sequence voltage at the common coupling point;; Z i =Z i ∠Φ i , i=4,5 represents the equivalent impedance when the positive sequence injection current and the negative sequence injection current act alone in the negative sequence voltage expression of the common coupling point under fault, Z6=Z6∠Φ6 represents the coefficient before the grid voltage act alone in the negative sequence voltage expression of the common coupling point; U gabc Indicates the amplitude of the grid voltage.

8. The asymmetric fault transient stability analysis method based on grid-connected inverter according to claim 6 is characterized in that: The hybrid synchronous control mathematical model is qualitatively analyzed from both qualitative and quantitative aspects based on the power angle curve and phase plane method to obtain the conclusion of the transient stability influence relationship of the system, including the following methods: Obtain the equivalent active power given value P' of the hybrid synchronous control mathematical model based on the state space equation after the fault occurs ref , output positive sequence active power P + ' e , and the equivalent damping coefficient D peq ; Based on the equivalent active power given value P' ref , output positive sequence active power P + ' e and the equivalent damping coefficient D peq Draw the power angle curve after a single-phase grounding fault; According to the power angle curve after a single-phase grounding fault, the correlation between the positive sequence current and the given value of equivalent active power, and the correlation between the negative sequence current and the given value of equivalent active power are analyzed.

9. The method for transient stability analysis of asymmetric faults based on grid-connected inverters according to claim 8, characterized in that: Equivalent active power given value P' ref And output positive sequence active power P + ' e , equivalent damping coefficient D peq Obtained according to the following formula: Where η = arctan((D p K p -I qp ) / I dp ), Φ0=Φ3+η; Φ0 represents The angle corresponding to the maximum.

10. Asymmetric fault transient stability analysis system based on grid-connected inverter, characterized in that: The system is used to implement the asymmetric fault transient stability analysis method based on a grid-connected inverter according to any one of claims 1 to 9, and comprises: A collection module, used to collect power data of the hybrid synchronous control inverter outlet under normal operation and fault operation of the power grid; The model building module is used to build a grid system model of a hybrid synchronous control inverter connected to the grid based on the power data, and to use the power angle δ, angular velocity and voltage amplitude U as state variables to construct the state space equation of the power grid system before the fault occurs; A derivation module, used for deriving a hybrid synchronous control mathematical model when the power grid system has an asymmetric fault according to the power grid system model and the state space equation before the fault; An analysis module, used to qualitatively analyze the hybrid synchronous control mathematical model from both qualitative and quantitative aspects based on a power angle curve and a phase plane method, and obtain a conclusion on the influence relationship of system transient stability; A control module is used to control the power grid based on the system transient stability impact relationship conclusion to maintain asymmetric fault transient stability.

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