Transient stability improving method of SVG and VSG series-parallel system and computer medium

Abstract

The invention discloses a transient stability improving method for an SVG and VSG series-parallel system. The method comprises the following steps: S1, calculating a first curve; the first curve comprises a system power angle characteristic curve considering the VSG reactive ring influence; s2, obtaining a second curve according to the first curve; the second curve comprises a system power angle characteristic curve considering a VSG reactive ring under the condition of voltage drop; s3, obtaining a limit cutting angle set under the first condition according to the first curve and the second curve in combination with an equal-area method; the first condition comprises selecting an SVG output current value and a VSG damping coefficient value from a preset parameter set as system parameters; s4, obtaining a limit removal time set according to the limit removal angle set in combination with the fitting curve of the VSG power angle and time of the system; and S5, obtaining the lowest required cut-off time under the actual voltage drop condition of the system, and obtaining the selectable ranges of the SVG output current value and the VSG damping coefficient value according to the limit cut-off time set and the lowest required cut-off time.

Description

Technical Field

[0001] The present invention relates to the technical field of power systems, and particularly to a method for improving the transient stability of an SVG and VSG hybrid system and a computer medium. Background Art

[0002] In recent years, large-scale new energy power stations have begun to be connected to the power grid. In order to improve the damping and inertial support of new energy power stations for the power system, VSG (Virtual Synchronous Generator) technology has been applied. When large disturbances occur in the power system, there is a transient instability phenomenon in VSG, which severely restricts the support ability of new energy power stations for the power system under faults. If only considering the parameter design optimization and control strategy improvement of VSG itself, it is often limited by the capacity of VSG, and the control is complex, making it difficult to improve the transient stability of VSG. Therefore, existing new energy power stations are often equipped with a hybrid system composed of SVG (Static Var Generator) and VSG. SVG has a fast response speed and can provide reactive power support to stabilize the grid connection point voltage, thereby improving the transient stability of VSG.

[0003] However, in the prior art, there is a lack of research on characterizing the boundary of the transient stability region of the SVG and VSG hybrid system. Most scholars use the Lyapunov method or the phase plane analysis method to analyze the stability of the system under large disturbances. However, for the study of the transient stability of the hybrid system, the existing methods tend to be qualitative analysis, with difficulties in quantification, making it difficult to determine the coordinated control quantity and being unfavorable for practical operation in engineering. In order to better quantify the transient stability of the system, some scholars use the equal area method to calculate the boundary of the VSG transient stability region. However, in order to simplify the analysis process, the existing methods usually do not consider the influence of the VSG damping coefficient and the voltage dynamic change problem of the VSG output voltage during the transient process, resulting in inaccurate region characterization and making it difficult to carry out coordinated regulation of the SVG and VSG hybrid system.

[0004] Therefore, there is an urgent need for a new technical solution to solve the technical problem of how to improve the transient stability of the SVG and VSG hybrid system. Summary of the Invention

[0005] The present invention provides a method for improving the transient stability of an SVG and VSG hybrid system and a computer medium, so as to solve the technical problem of how to improve the transient stability of the SVG and VSG hybrid system.

[0006] To achieve the above object, the present invention provides a method for improving the transient stability of an SVG and VSG hybrid system, including the following steps:

[0007] S1. Calculate the first curve; the first curve includes the power angle characteristic curve of the hybrid system considering the influence of the VSG reactive power loop.

[0008] S2. Obtain the second curve based on the first curve; the second curve includes the power angle characteristic curve of the hybrid system considering the influence of the VSG reactive power loop under the voltage dip with a preset percentage of dip.

[0009] S3. Obtain the set of critical clearing angles under the first condition by combining the first curve and the second curve using the equal - area method; the first condition includes selecting the SVG output current value and the VSG damping coefficient value from the preset parameter set as the parameters of the hybrid system.

[0010] S4. Obtain the set of critical clearing times by combining the set of critical clearing angles with the third curve; the third curve includes the fitting curve of the VSG power angle and time of the hybrid system.

[0011] S5. Obtain the minimum required clearing time under the actual voltage dip condition of the hybrid system, obtain the optional range of the SVG output current value and the VSG damping coefficient value based on the set of critical clearing times and the minimum required clearing time, and select the SVG output current value and the VSG damping coefficient value as the parameters of the hybrid system according to the optional range.

[0012] Preferably, S1 includes:

[0013] S11. Calculate the power angle characteristic curve of the hybrid system under normal conditions.

[0014] S12. Obtain the first curve by considering the influence of the VSG reactive power loop based on the power angle characteristic curve.

[0015] Preferably, S11 includes:

[0016] Obtain the current of the hybrid system according to the structure of the hybrid system, which can be expressed by the following relational formula:

[0017]

[0018] where, I vsg represents the VSG output current; E v represents the VSG output voltage; V pcc represents the grid - connected point voltage; I g represents the grid - side current; U g represents the grid voltage; I svg represents the SVG output current; Z f and Z g respectively represent the line impedances of the VSG and the grid - side.

[0019] Obtain the grid - connected point voltage V pcc according to the current of the hybrid system, which can be expressed by the following relational formula:

[0020]

[0021] Among them, δ represents the phase angle of the VSG output voltage; θ represents the phase angle of the SVG output current; j represents the imaginary unit in complex numbers.

[0022] For the port circuit, if the output voltage is U and the output current is I, the relationship between the output apparent power S, active power P, and reactive power Q can be expressed by the first relational expression, and the first relational expression includes:

[0023]

[0024] Among them, represents the conjugate complex number of the port circuit output current I.

[0025] According to the current of the series-parallel system, the grid connection point voltage V pcc and the first relational expression, it can be obtained that:

[0026] The active power output by the VSG can be expressed by the following relational expression:

[0027]

[0028] The active power transmitted by the VSG and SVG combined system to the power grid can be expressed by the following relational expression:

[0029]

[0030] Considering that the SVG is mainly used for outputting and absorbing reactive power, and the proportion of the output active power is extremely small, it is approximately considered that P e1 = P e2 ; According to P e1 and P e2 to obtain the phase angle θ of the SVG output current, it can be expressed by the following relational expression:

[0031]

[0032] Combining P e1 and θ, the power angle characteristic curve P e3 of the series-parallel system is obtained, and it can be expressed by the following relational expression:

[0033]

[0034] Preferably, S12 includes:

[0035] The VSG reactive-voltage loop control equation can be expressed by the following relational expression:

[0036] Q ref -Qe = -k q (U N - U * )

[0037] where k q represents the reactive power droop coefficient; Q ref represents the reactive power reference value; Q e represents the reactive power output by the VSG; U N represents the VSG voltage reference value; U * represents the VSG voltage command value.

[0038] Since the time scale of the VSG power loop is much larger than that of the voltage-current double closed loop, and the dynamic speed is more than ten times slower than that of the voltage-current inner loop, it is considered that the VSG output voltage E v can track the voltage command value U * without error, that is, E v = U * .

[0039] According to the VSG reactive-voltage loop control equation, the current of the hybrid system, the grid connection point voltage V pcc and the output power S of the VSG and SVG, the output voltage E v of the VSG in the hybrid system can be expressed by the following relational expression:

[0040]

[0041] Combining the output voltage E v of the VSG and the power angle characteristic curve P e3 of the hybrid system, the first curve P e is obtained and can be expressed by the following relational expression:

[0042]

[0043] Preferably, S3 includes:

[0044] It can be known from the equal area method that when the second condition is satisfied, the hybrid system can maintain transient stability. The second condition includes:

[0045]

[0046] where P ref represents the reference active power; D represents the VSG damping coefficient; P eN represents the VSG output power angle curve after the voltage dip, that is, the second curve; P e represents the first curve; δ 1 represents the VSG stable equilibrium power angle; δ 2 represents the VSG unstable equilibrium point power angle; δ CIt represents the fault clearing angle; t represents time, which is counted from the occurrence of the fault.

[0047] Transform dδ / dt into a function of δ:

[0048] The VSG active power - frequency control loop equation can be expressed by the following relational expression:

[0049]

[0050] Among them, J represents the VSG moment of inertia; dδ / dt represents the first - order derivative of the VSG power angle; d 2 δ / dt 2 represents the second - order derivative of the VSG power angle;

[0051] Solve the VSG active power - frequency loop control equation by the Runge - Kutta method to obtain the first result; the first result includes the numerical solutions of the output power, power angle, and first - order derivative of the power angle of the VSG with respect to time under different voltage sags.

[0052] Take the numerical solutions of the first - order derivative of the power angle and the power angle from the first result for 6 - order polynomial numerical fitting, and obtain the first function of the first - order derivative of the power angle with respect to the power angle under different voltage sags for different SVG injection currents according to the preset voltage sag conditions, which can be expressed by the following relational expression:

[0053]

[0054] Among them, P 0 、P 1 、P 2 、P 3 、P 4 、P 5 and P 6 are constants, obtained through function fitting;

[0055] For the set of limit clearing angles, substitute the first function into the second condition; when both sides of the relational expression of the second condition are equal, the fault clearing angle δ C is the limit clearing angle CCA, and the third condition is obtained:

[0056]

[0057] Calculate the set of limit clearing angles of the VSG under the first condition through the third condition and the first curve P e .

[0058] Preferably, S4 includes:

[0059] Take the numerical solution of the power angle with respect to time from the first result, fit the power angle and time of the VSG of the hybrid series - parallel system to obtain the third curve, which can be expressed by the following relational expression:

[0060] t = g(δ) = Q 0 +Q 1 *δ + Q 2 *δ 2 +Q 3 *δ 3 +Q 4 *δ 4 +Q 5 *δ 5 +Q 6 *δ 6

[0061] Wherein, Q 0 、Q 1 、Q 2 、Q 3 、Q 4 、Q 5 and Q 6 are constants, obtained by function fitting.

[0062] Substitute the set of critical clearing angles into the third curve to obtain the set of critical clearing times under the first condition, where the critical clearing time CCT = g(CCA).

[0063] When the fault clearing time is less than the critical clearing time CCT, the hybrid system remains transiently stable; when the fault clearing time is greater than the critical clearing time CCT, the hybrid system becomes transiently unstable; then the critical clearing time CCT is taken as the boundary of the transient stability interval of the hybrid system.

[0064] Preferably, S5 includes:

[0065] Obtain the low voltage ride-through time requirement of the hybrid system VSG according to the actual voltage dip situation, and get the minimum required clearing time T ref ; Take the minimum required clearing time T ref as the minimum clearing duration, and take the critical clearing time CCT as the maximum clearing duration; when CCT ≥ T ref , no control is required, and wait for the fault to disappear by itself.

[0066] When CCT < T ref , obtain the optional ranges of the SVG output current value and the VSG damping coefficient value according to the set of critical clearing times and the minimum required clearing time, and select the SVG output current value and the VSG damping coefficient value from the optional ranges as the parameters of the hybrid system for fault ride-through. After waiting for the fault to end, adjust the SVG output current value and the VSG damping coefficient value back to the original parameters.

[0067] The present invention also provides a computer medium, including a processor, a memory, and a computer program for implementing the method of the present invention.

[0068] The present invention has the following beneficial effects:

[0069] The method for improving the transient stability of the SVG and VSG hybrid system of the present invention takes into account the influence of the reactive power loop on the basis of the power angle characteristic curve of the hybrid system, so that this method can solve the problem that the traditional equal area method is difficult to handle the dynamic change of the VSG output voltage during the transient process. By calculating the set of critical clearing angles to obtain the set of critical clearing times, this method determines the boundary of the transient stability region of the hybrid system. By obtaining the minimum required clearing time under the actual voltage dip condition of the hybrid system and comparing it with the set of critical clearing times, this method obtains the optional ranges of the SVG output current value and the VSG damping coefficient value, and then can select parameters as the parameters of the hybrid system according to the optional ranges of the SVG output current value and the VSG damping coefficient value to improve the transient stability of the SVG and VSG hybrid system. The method of the present invention can break through the boundary of the transient stability region of a single VSG control, extend the VSG critical clearing time, and increase the VSG critical clearing angle, which is beneficial for new energy power stations to better cope with voltage dip faults.

[0070] In addition to the purposes, features and advantages described above, the present invention has other purposes, features and advantages. The following will refer to the accompanying drawings for a further detailed description of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0071] The drawings forming a part of this application are used to provide a further understanding of the present invention. The schematic embodiments and descriptions thereof of the present invention are used to explain the present invention and do not constitute an improper limitation of the present invention. In the drawings:

[0072] Figure 1 is a schematic flow chart of a preferred embodiment of the present invention.

[0073] Figure 2 is a schematic curve diagram before and after voltage dip of a preferred embodiment of the present invention.

[0074] Figure 3 is a schematic diagram of the set of critical clearing times of a preferred embodiment of the present invention.

[0075] Figure 4 is a schematic diagram of the optional ranges of the SVG output current value and the VSG damping coefficient value of a preferred embodiment of the present invention.

[0076] Figure 5 is a schematic diagram of the verification result of a single change in the VSG damping coefficient of a preferred embodiment of the present invention.

[0077] Figure 6 is a schematic diagram of the verification result of a single change in the SVG output current of a preferred embodiment of the present invention.

[0078] Figure 7 Schematic diagram of the verification result of the method of the present invention in a preferred embodiment of the present invention. Detailed implementation manners

[0079] The embodiments of the present invention will be described in detail below with reference to the accompanying drawings. However, the present invention can be implemented in many different ways defined and covered by the claims.

[0080] In a preferred embodiment of the present invention, the transient stability of the hybrid system is judged by comparing the real-time monitored grid voltage of the hybrid system with the normal operating voltage. When the difference between the real-time monitored grid voltage and the normal operating voltage of the hybrid system is greater than a preset range, the transient stability of the hybrid system is improved according to the method of the present invention.

[0081] See Figure 1 , in a preferred embodiment of the present invention, a method for improving the transient stability of an SVG and VSG hybrid system is provided, including the following steps:

[0082] S1. Calculate the first curve; the first curve includes the power angle characteristic curve of the hybrid system considering the influence of the VSG reactive power loop.

[0083] S1 specifically includes:

[0084] S11. Calculate the power angle characteristic curve of the hybrid system under normal conditions. S11 specifically includes:

[0085] The current of the hybrid system is obtained according to the structure of the hybrid system, and can be expressed by the following relational expression:

[0086]

[0087] where, I vsg represents the VSG output current; E v represents the VSG output voltage; V pcc represents the grid connection point voltage; I g represents the grid side current; U g represents the grid voltage; I svg represents the SVG output current; Z f and Z g respectively represent the line impedances of the VSG and the grid side;

[0088] The grid connection point voltage V pcc is obtained according to the hybrid system current, and can be expressed by the following relational expression:

[0089]

[0090] where, δ represents the phase angle of the VSG output voltage; θ represents the phase angle of the SVG output current; j represents the imaginary unit in complex numbers;

[0091] For the port circuit, if the output voltage is U and the output current is I, the relationship among the output apparent power S, active power P, and reactive power Q can be expressed by the first relational expression, which includes:

[0092]

[0093] Among them, represents the conjugate complex number of the port circuit output current I;

[0094] According to the current of the series-parallel system and the grid connection point voltage V pcc and the first relational expression, it can be obtained that:

[0095] The active power output by the VSG can be expressed by the following relational expression:

[0096]

[0097] The active power transmitted by the VSG and SVG combined system to the power grid can be expressed by the following relational expression:

[0098]

[0099] Considering that SVG is mainly used for outputting and absorbing reactive power, and the proportion of the output active power is extremely small, it is approximately considered that P e1 = P e2 ; According to P e1 and P e2 The phase angle θ of the SVG output current is obtained and can be expressed by the following relational expression:

[0100]

[0101] Combined with P e1 and θ, the power angle characteristic curve P of the series-parallel system e3 is obtained and can be expressed by the following relational expression:

[0102]

[0103] S12. Considering the influence of the VSG reactive power loop, the first curve is obtained. S12 specifically includes:

[0104] The VSG reactive power-voltage loop control equation can be expressed by the following relational expression:

[0105] Q ref -Q e = -k q (U N -U * )

[0106] Among them, kq represents the reactive droop coefficient; Q ref represents the reactive power reference value; Q e represents the reactive power output of the VSG; U N represents the VSG voltage reference value; U * represents the VSG voltage command value;

[0107] Since the time scale of the VSG power loop is much larger than that of the voltage-current double closed loop, and the dynamic speed is more than ten times slower than that of the voltage-current inner loop, it is considered that the VSG output voltage E v can track the voltage command value U without error, that is, E * = U v ; * ;

[0108] When the line parameters, reactive droop coefficient, and SVG output current remain unchanged, the VSG output voltage is only related to the VSG phase angle. According to the VSG reactive-voltage loop control equation, the current of the hybrid system, and the grid connection point voltage V pcc and the output power S of the VSG and SVG, the output voltage E of the VSG in the hybrid system is obtained v , which can be expressed by the following relational formula:

[0109]

[0110] Combining the output voltage E of the VSG v and the power angle characteristic curve P of the hybrid system e3 , the first curve P e is obtained, which can be expressed by the following relational formula:

[0111]

[0112] S2. Obtain the second curve from the first curve; the second curve includes the power angle characteristic curve of the hybrid system considering the influence of the VSG reactive loop under the voltage drop with a preset drop percentage.

[0113] S3. Obtain the set of critical clearing angles under the first condition by combining the first curve and the second curve using the equal area method; the first condition includes selecting the SVG output current value and the VSG damping coefficient value from the preset parameter set as the hybrid system parameters.

[0114] S3 specifically includes:

[0115] See the curves before and after the voltage drop in Figure 2 . In Figure 2 , the ordinate P e4 is the active power output of the VSG; the abscissa δ is the VSG power angle; P n = P ref-Ddδ / dt is the reference active power of the VSG equivalent; δ 1 represents the stable equilibrium power angle of the VSG; δ 2 represents the unstable equilibrium point power angle of the VSG; δ C represents the fault clearing angle; Curve Ⅰ is the output power angle curve of the VSG in the hybrid system under normal conditions; Curve Ⅱ is the output power angle curve of the VSG in the hybrid system under voltage dip.

[0116] Under normal conditions, the VSG operates at point A, and the power angle is δ 1 . When a voltage dip occurs, the output power angle curve of the VSG drops to Curve Ⅱ, and the operating point changes from A to B. According to the VSG active - frequency control loop equation, at this time P n -P e >0, the power angle accelerates and increases. When it reaches point C, the fault is cleared, and the power angle of the VSG is δ C . Between δ 1 and δ C , there is an accelerating area S 1 :

[0117]

[0118] where P eN represents the power angle curve of the VSG after the grid voltage dip.

[0119] After the fault is cleared, the power angle curve of the VSG in the hybrid system resumes to Curve Ⅰ. At this time, P n -P e <0, the power angle growth rate decreases, and the power angle δ of the VSG will continue to increase for some time. There are the following three cases:

[0120] (1) If when reaching δ 2 , the decelerating area is equal to the accelerating area S 1 , it indicates that when the power angle of the VSG reaches δ 2 , the power angle growth rate decreases to 0. At this time, δ C is the critical clearing angle CCA. After that, the power angle of the VSG decreases and finally returns to δ 1 .

[0121] (2) If when reaching δ 2 , the decelerating area S 2 > the accelerating area S 1 , it indicates that between δ C and δ 2 , the power angle growth rate of the VSG has decreased to 0, and the power angle returns to δ 1 .

[0122] (3) If when reaching δ 2 , the decelerating area S 2<Accelerating area S 1 , indicating that when the VSG power angle reaches δ 2 , the power angle acceleration is still greater than 0, the VSG power angle continues to increase, and transient instability occurs.

[0123] It can be seen from this that when the second condition is satisfied, the hybrid system can maintain transient stability. The second condition includes:

[0124]

[0125] Among them, P ref represents the reference active power; D represents the VSG damping coefficient; P eN represents the VSG output power angle curve after voltage dip, that is, the second curve; P e represents the first curve; δ 1 represents the VSG stable equilibrium power angle; δ 2 represents the VSG unstable equilibrium point power angle; δ C represents the fault clearing angle; t represents time, starting from the occurrence of the fault.

[0126] Convert dδ / dt into a function of δ:

[0127] The VSG active - frequency control loop equation can be expressed by the following relationship:

[0128]

[0129] Among them, J represents the VSG moment of inertia; dδ / dt represents the first derivative of the VSG power angle; d 2 δ / dt 2 represents the second derivative of the VSG power angle;

[0130] Solve the VSG active - frequency loop control equation by the Runge - Kutta method to obtain the first result; the first result includes the numerical solutions of the output power, power angle, and first derivative of the power angle of the VSG with respect to time under different voltage dips.

[0131] Take the numerical solutions of the first derivative of the power angle and the power angle from the first result for 6 - order polynomial numerical fitting, and obtain the first function of the first derivative of the power angle with respect to the power angle under different voltage dips for different SVG injection currents according to the preset voltage dip conditions, which can be expressed by the following relationship:

[0132]

[0133] Among them, P 0 、P 1 、P 2 、P 3 、P 4 、P 5 and P 6is a constant, obtained by function fitting;

[0134] For the set of critical clearing angles obtained by taking the limit, substitute the first function into the second condition; when the two sides of the relational expression of the second condition are equal, the critical clearing angle δ C is the critical clearing angle CCA, and the third condition is obtained:

[0135]

[0136] Through the third condition and the first curve P e Calculate the set of critical clearing angles of the VSG under the first condition.

[0137] S4. Obtain the set of critical clearing times according to the set of critical clearing angles in combination with the third curve; the third curve includes the fitting curve of the VSG power angle and time of the hybrid system.

[0138] S4 specifically includes:

[0139] Take the numerical solution of the power angle with respect to time from the first result, fit the VSG power angle and time of the hybrid system to obtain the third curve, which can be expressed by the following relational expression:

[0140] t = g(δ) = Q 0 +Q 1 *δ + Q 2 *δ 2 +Q 3 *δ 3 +Q 4 *δ 4 +Q 5 *δ 5 +Q 6 *δ 6

[0141] where Q 0 , Q 1 , Q 2 , Q 3 , Q 4 , Q 5 and Q 6 are constants, obtained by function fitting;

[0142] Substitute the set of critical clearing angles into the third curve to obtain the set of critical clearing times under the first condition, that is, the critical clearing time CCT = g(CCA) under different SVG injection currents and VSG damping coefficients. See the schematic diagram of the set of critical clearing times Figure 3 .

[0143] When the fault clearing time is less than the critical clearing time CCT, the hybrid system remains transiently stable; when the fault clearing time is greater than the critical clearing time CCT, the hybrid system is transiently unstable; then the critical clearing time CCT is taken as the boundary of the transient stability interval of the hybrid system.

[0144] S5. Obtain the minimum required clearing time under the actual voltage dip condition of the hybrid system, obtain the optional ranges of the SVG output current value and the VSG damping coefficient value according to the set of critical clearing times and the minimum required clearing time, and select the SVG output current value and the VSG damping coefficient value as the parameters of the hybrid system according to the optional ranges.

[0145] S5 specifically includes:

[0146] Obtain the VSG low voltage ride-through time requirement of the hybrid system according to the actual voltage dip condition, and obtain the minimum required clearing time T ref . In the preferred embodiment of the present invention, for type II faults, the VSG low voltage ride-through time requirements are shown in Table 1:

[0147] Table 1 VSG low voltage ride-through time requirement table

[0148]

[0149] Take the minimum required clearing time T ref as the minimum clearing duration, and take the critical clearing time CCT as the maximum clearing duration; when CCT ≥ T ref , no control is required and wait for the fault to disappear by itself;

[0150] When CCT < T ref , refer to Figure 4 , and obtain the optional ranges of the SVG output current value and the VSG damping coefficient value according to the set of critical clearing times and the minimum required clearing time. When CCT < T ref , it indicates that the hybrid system is transiently unstable, and it is necessary to restore CCT to be greater than or equal to T ref , so select the part of CCT ≥ T from Figure 3 , and the range of this part projected on the two-dimensional plane of the SVG output current value and the VSG damping coefficient value is the optional range of the SVG output current value and the VSG damping coefficient value. The schematic diagram of the optional range is shown in ref Figure 4 .

[0151] ​Select the SVG output current value and the VSG damping coefficient value from the optional range as the parameters of the hybrid system for fault ride-through. When selecting the SVG output current value and the VSG damping coefficient value from the optional range, flexible selection can be made according to the actual VSG energy storage capacity and the reactive power that the SVG can inject. After waiting for the fault to end, adjust the SVG output current value and the VSG damping coefficient value back to the original parameters.

[0152] In a preferred embodiment of the present invention, a computer medium is further provided, including a processor, a memory, and a computer program for implementing the method of the present invention.

[0153] Verification part:

[0154] Verify the influence of the method of the present invention on the transient stability of the VSG. See Table 2 for simulation data;

[0155] Table 2 System simulation parameters

[0156]

[0157]

[0158] Set the voltage dip to occur at 0.5 s with k = 0.3 and the fault time to 0.16 s. Take the VSG damping coefficient D = 3000 N·m·s / rad. See the simulation results in Figure 5 . After the fault at 0.16 s, the VSG and SVG hybrid system becomes unstable. Therefore, simply changing the VSG damping coefficient cannot keep the system stable.

[0159] Set the voltage dip to occur at 0.5 s with k = 0.3 and the fault time to 0.16 s. Take the SVG output current I svg = 15 A. See the simulation results in Figure 6 . After the fault at 0.16 s, the VSG and SVG hybrid system becomes unstable. Therefore, simply changing the SVG output current cannot keep the system stable either.

[0160] Set the voltage dip to occur at 0.5 s with k = 0.3 and the fault time to 0.16 s. Select Figure 3 the VSG damping coefficient D = 3000 N·m·s / rad and the SVG injection current value I svg = 15 A projected onto the two-dimensional plane of the optional range of the SVG output current value and the VSG damping coefficient value. See the simulation results in Figure 7 . After the fault at 0.16 s, the VSG and SVG hybrid system resumes stability, verifying the effectiveness of the coordinated control strategy.

[0161] In summary, the transient stability improvement method for the SVG and VSG hybrid system of the present invention takes into account the influence of the reactive power loop based on the power angle characteristic curve of the hybrid system, enabling this method to solve the problem that the traditional equal area method is difficult to handle the dynamic change of the VSG output voltage during the transient process. By calculating the set of critical clearing angles to obtain the set of critical clearing times, this method determines the boundary of the transient stability region of the hybrid system. By obtaining the minimum required clearing time under the actual voltage dip condition of the hybrid system and comparing it with the set of critical clearing times, this method obtains the optional ranges of the SVG output current value and the VSG damping coefficient value, and then can select parameters as the parameters of the hybrid system according to the optional ranges of the SVG output current value and the VSG damping coefficient value to improve the transient stability of the SVG and VSG hybrid system. The method of the present invention can break through the boundary of the transient stability region of single VSG control, extend the VSG critical clearing time, and increase the VSG critical clearing angle, which is beneficial for new energy power stations to better cope with voltage dip faults.

[0162] The foregoing are only the preferred embodiments of the present invention and are not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A method for improving transient stability of a hybrid system of SVG and VSG, characterized in that: The following steps are involved: S1. Calculate a first curve; the first curve includes a power angle characteristic curve of a parallel-parallel system considering the influence of a VSG reactive ring; S2. Obtain a second curve according to the first curve; the second curve includes a power angle characteristic curve of a parallel-parallel system considering the influence of a VSG reactive ring under a voltage drop of a preset drop percentage; S3, according to the first curve and the second curve combined with the equal area method, obtain a set of limit resection angles under a first condition; the first condition includes selecting the SVG output current value and the VSG damping coefficient value from a preset parameter set as hybrid system parameters; S4, obtaining a limit removal time set according to the limit removal angle set and a third curve; the third curve includes a fitting curve of the VSG power angle and time of the hybrid system; S5. Obtain the minimum required cut-off time when the actual voltage of the hybrid system drops, obtain the optional range of the SVG output current value and the VSG damping coefficient value according to the limit cut-off time set and the minimum required cut-off time, and select the SVG output current value and the VSG damping coefficient value as parameters of the hybrid system according to the optional range.

2. The method for improving transient stability of a SVG and VSG hybrid system according to claim 1, characterized in that: The S1 includes: S11, calculating the power angle characteristic curve of the hybrid system in a normal state; S12. Obtain a first curve based on the power angle characteristic curve by taking into account the influence of the VSG reactive ring.

3. The method for improving transient stability of a hybrid system of SVG and VSG according to claim 2, characterized in that: The S11 includes: The current of the hybrid system is obtained according to the structure of the hybrid system, which can be expressed by the following relationship: Among them, I vsg Indicates VSG output current; E v Represents VSG output voltage; V pcc Indicates the grid connection point voltage; I g Indicates the grid side current; U g Indicates the grid voltage; I svg Indicates SVG output current; Z f and Z g Represent the line impedances on the VSG and grid sides respectively; The grid connection point voltage V is obtained according to the hybrid system current pcc , which can be expressed by the following relationship: Wherein, δ represents the VSG output voltage phase angle; θ represents the SVG output current phase angle; j represents the imaginary unit in the complex number; For the port circuit, if the output voltage is U and the output current is I, the relationship between the output apparent power S, the active power P and the reactive power Q can be expressed by a first relational expression, which includes: in, represents the conjugate complex number of the output current I of the port circuit; According to the hybrid system current and the grid connection point voltage V pcc And the first relationship can be obtained: The VSG output active power can be expressed by the following relationship: The VSG and SVG combined system transmits active power to the grid, which can be expressed by the following relationship: Considering that SVG is mainly used to output and absorb reactive power, and the output active power accounts for a very small proportion, it is roughly assumed that P e1 =P e2 According to P e1 and P e2 The SVG output current phase angle θ can be expressed by the following relationship: Joint P e1 and θ, the power angle characteristic curve P of the hybrid system is obtained e3 , which can be expressed by the following relationship:

4. The method for improving transient stability of a SVG and VSG hybrid system according to claim 3, characterized in that: The S12 includes: The VSG reactive-voltage loop control equation can be expressed by the following relationship: Q ref -Q e =-k q (U N -U * ) Among them, k q Represents reactive power droop coefficient; Q ref Indicates reactive power reference value; Q e Indicates the VSG output reactive power; U N Indicates VSG voltage reference value; U * Indicates the VSG voltage command value; Since the time scale of the VSG power loop is much larger than the voltage and current double closed loop, and the dynamic speed is more than ten times slower than the voltage and current inner loop, it is considered that the VSG output voltage E v The voltage command value U * Perform error-free tracking, that is, E v =U * ; According to the VSG reactive-voltage loop control equation, the hybrid system current, and the grid-connected point voltage V pcc The output voltage E of VSG in the hybrid system is obtained by summing the output power S of VSG and SVG. v , which can be expressed by the following relationship: The output voltage E of the VSG v And the power angle characteristic curve P of the hybrid system e3 , and obtain the first curve P e , which can be expressed by the following relationship:

5. The method for improving transient stability of a SVG and VSG hybrid system according to claim 4, characterized in that: The S3 includes: It can be seen from the equal area method that when the second condition is met, the hybrid system can maintain transient stability, and the second condition includes: Among them, P ref represents the reference active power; D represents the VSG damping coefficient; P eN represents the VSG output power angle curve after voltage drop, i.e. the second curve; P e represents the first curve; δ1 represents the VSG stable equilibrium power angle; δ2 represents the VSG unstable equilibrium point power angle; δ C represents the fault removal angle; t represents the time, which starts from the time when the fault occurs. Convert dδ / dt into a function of δ: The VSG active power-frequency control loop equation can be expressed by the following relationship: Where J represents the VSG moment of inertia; dδ / dt represents the first-order derivative of the VSG power angle; d 2 δ / dt 2 It represents the second derivative of VSG power angle; Solving the active power-frequency loop control equation of the VSG by the Runge-Kutta method to obtain a first result; the first result includes numerical solutions of the output power, power angle and first-order derivative of the power angle of the VSG with respect to time under different voltage drops; From the first result, the first derivative of the power angle and the numerical solution of the power angle are taken to perform a 6th-order polynomial numerical fitting, and the first function of the first derivative of the power angle with respect to the power angle under different voltage drops under different SVG injection currents is obtained according to the preset voltage drop conditions, which can be expressed by the following relationship: Among them, P0, P1, P2, P3, P4, P5 and P6 are constants obtained by function fitting; To find the limit removal angle set, substitute the first function into the second condition; when both sides of the relational equation of the second condition are equal, the fault removal angle δ C For the limiting resection angle CCA, the third condition is obtained: By the third condition and the first curve P e The set of limit resection angles of VSG under the first condition is calculated.

6. The method for improving transient stability of a SVG and VSG hybrid system according to claim 5, characterized in that: The S4 includes: The numerical solution of the power angle with respect to time is obtained from the first result, and the VSG power angle and time of the hybrid system are fitted to obtain the third curve, which can be expressed by the following relationship: t=g(δ)=Q0+Q1*δ+Q2*δ 2 +Q3*d 3 +Q4*d 4 +Q5*d 5 +Q6*d 6 Among them, Q0, Q1, Q2, Q3, Q4, Q5 and Q6 are constants obtained by function fitting; Substituting the limit resection angle set into the third curve, the limit resection time set under the first condition is obtained, wherein the limit resection time CCT = g(CCA); When the fault removal time is less than the limit removal time CCT, the hybrid system maintains transient stability; when the fault removal time is greater than the limit removal time CCT, the hybrid system is transiently unstable; the limit removal time CCT is used as the transient stability interval boundary of the hybrid system.

7. The method for improving transient stability of a SVG and VSG hybrid system according to claim 6, characterized in that: The S5 includes: According to the actual voltage drop situation, the low voltage ride-through time requirement of the hybrid system VSG is obtained, and the minimum required removal time T is obtained. ref ; The minimum required removal time T ref As the minimum resection time, the limit resection time CCT is taken as the maximum resection time; when CCT ≥ T ref When the fault occurs, no control is required and the fault can be eliminated automatically. When CCT<T ref When the fault is crossed, the optional range of the SVG output current value and the VSG damping coefficient value is obtained according to the limit cut-off time set and the minimum required cut-off time, and the SVG output current value and the VSG damping coefficient value are selected from the optional range as the parameters of the hybrid system for fault crossing. After waiting for the fault to end, the SVG output current value and the VSG damping coefficient value are adjusted back to the original parameters.

8. A computer medium, characterized in that The invention comprises a processor, a memory and a computer program for implementing the method according to any one of claims 1 to 7.

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