An analysis method of wind turbine frequency modulation control on power system power angle stability

Abstract

The present invention discloses a method for analyzing the power angle stability of a power system by frequency modulation control of a wind turbine, which relates to the field of power grid control. The key points of the technical solution are: constructing a first mathematical model for analyzing the power angle stability of a two-machine system of a power system; connecting the wind turbine under the MPPT control strategy to the sending end area of ​​the power system to construct an extended two-machine system of the power system; presetting the output reduction coefficient of the synchronous generator at the sending end, and determining the second mathematical model of the extended two-machine system according to the output reduction coefficient and the first mathematical model; comparing the first mathematical model and the second mathematical model, analyzing the first analysis result of the power angle stability of the power system during a fault period and after the fault is cleared when the wind turbine changes the structural parameters of the power system and adjusts the output. The present invention realizes the analysis of the power angle stability of the power system during the fault period and the early stage after the fault is cleared when the wind turbine is additionally controlled by frequency modulation.

Description

Technical Field

[0001] The present invention relates to the field of power grid control, and more specifically, to a method for analyzing the effect of wind turbine frequency modulation control on power system power angle stability. Background Art

[0002] The share of new energy units represented by wind power in the system is gradually increasing. The "low inertia, weak damping" characteristics of the new power system accompanying this process are highlighted, and power fluctuations caused by factors such as load switching and system failures are becoming more frequent and severe. When a low proportion of new energy units are connected to the system, all the system frequency regulation tasks are undertaken by traditional synchronous generators, and power electronic equipment such as wind turbines are in maximum power point tracking (MPPT) control, making full use of natural resources for power generation. With the continuous increase in the penetration rate of new energy units, the use of traditional synchronous generators alone to adjust power output can no longer meet the system frequency stability requirements, and power electronic equipment such as wind turbines are required to participate in grid frequency regulation. The essence of wind turbines participating in frequency regulation is to reduce the unbalanced energy generated in the system when frequency fluctuations occur. While providing frequency support, it also affects the rotor movement of synchronous generators, making the system power angle stability problem during faults more complicated, which brings great challenges to the stable operation of the power grid.

[0003] Wind turbines (i.e. wind turbines) are different from traditional synchronous generators in millisecond-level control dynamic characteristics, which makes wind turbine frequency control exist in multiple time scales. Therefore, after a large disturbance occurs, the frequency regulation strategy of the wind turbine will affect the stability of the transient power angle of the system. As the proportion of wind power increases, the impact of the frequency regulation control of the wind turbine will become more obvious. In the prior art, the equal area rule is extended to analyze the impact of additional frequency regulation control of wind turbines on the stability of the transient power angle of the system through the fault limit removal time and limit removal angle. When the wind turbine is connected to the sending end area of ​​the system, its frequency regulation control will improve the stability of the system power angle, while when it is connected to the receiving end area of ​​the system, it is not conducive to the stability of the system power angle. However, the current analysis of the system power angle stability in the prior art ignores the study of the stability of the system power angle during the fault period and the early stage after the fault is cleared when the wind turbine is additionally controlled.

[0004] Therefore, an analysis method for the effect of wind turbine frequency modulation control on the power system power angle stability is needed to analyze the impact mechanism of wind turbine frequency modulation control on the system power angle at various stages of system failure, and provide a theoretical reference for subsequent power system protection setting work. Summary of the invention

[0005] The purpose of the present invention is to provide a method for analyzing the power angle stability of an electric power system affected by frequency modulation control of a wind turbine set. The present invention introduces the output reduction coefficient of a synchronous generator into a first mathematical model to achieve power angle stability analysis modeling of a system with a high proportion of new energy access, i.e., a second mathematical model. By comparing the first mathematical model and the second mathematical model, the analysis results of the power angle stability of the electric power system during a fault and after the fault is cleared are analyzed when the wind turbine set changes the structural parameters of the electric power system and / or adjusts the output.

[0006] The above technical objectives of the present invention are achieved through the following technical solutions:

[0007] The present application provides a method for analyzing the effect of wind turbine frequency modulation control on power system power angle stability, the method comprising:

[0008] Constructing a first mathematical model for analyzing the stability of the power angle of the two-machine system of the power system, wherein the first mathematical model represents the correlation between the power angle of the two-machine system, the unbalanced power of the synchronous generator and the speed difference of the synchronous generator;

[0009] Connect wind turbines under the MPPT control strategy to the sending end area of ​​the power system to build an extended two-machine system of the power system;

[0010] Preset a reduction coefficient of the sending-end synchronous generator, and determine a second mathematical model of the extended two-machine system based on the reduction coefficient and the first mathematical model, wherein the reduction coefficient refers to the ratio of the change in the output of the synchronous generator to the output of the wind turbine after the wind turbine is connected to the power system, and the second mathematical model represents the correlation between the power angle of the extended two-machine system, the output of the wind turbine, the unbalanced power of the synchronous generator and the speed difference of the synchronous generator;

[0011] By comparing the first mathematical model with the second mathematical model, a first analysis result of the power angle stability of the power system during a fault and after the fault is cleared is analyzed when the wind turbine generator system changes the structural parameters of the power system and adjusts the output.

[0012] In one implementation, a first mathematical model for analyzing the power angle stability of a two-machine system of a power system is constructed, specifically:

[0013] According to the time constant of the moment of inertia, power angle, mechanical power and electromagnetic power of the synchronous generator at the sending end and the synchronous generator at the receiving end, the rotor motion equation of the two-machine system model is established;

[0014] According to the power angle difference, speed difference, angular velocity rating, mechanical power and electromagnetic power of the two-machine system, the two-machine system is transformed into a single-machine infinite system;

[0015] Based on the power angle characteristic curve of the single-machine infinite system, the power angle at each moment when a fault occurs, the fault is cleared, and the fault is cleared is characterized; the power angle at each moment when a fault occurs, the fault is cleared, and the fault is cleared is characterized;

[0016] According to the mechanical power, electromagnetic power and power angle of the single-machine infinite system at each moment when the fault occurs, the fault is cleared, and the fault is cleared, the residual acceleration area of ​​the synchronous generator of the two-machine system model is determined, that is, the first mathematical model, where the expression of the first mathematical model is: P m Represents the mechanical power of a single infinite system, P e represents the electromagnetic power of a single-machine infinite system, δ0 represents the power angle of the synchronous generator that fails at time t0, δ d Indicates that after the fault is cleared, t d The power angle of the synchronous generator at time , H represents the time constant of the moment of inertia, ω0 is the rated value of the angular velocity, and Δω represents the speed difference of the synchronous generator.

[0017] In one implementation, a reduction coefficient of the sending-end synchronous generator is preset, and a second mathematical model of the extended two-machine system is determined based on the reduction coefficient and the first mathematical model, specifically:

[0018] When the wind turbine is connected to the sending end area of ​​the power system, the load of the power system will not be changed, but the output of the synchronous generator will be reduced. The time constant of the moment of inertia of the power system will remain unchanged before and after the wind turbine is connected. After the wind turbine is connected, the mechanical power and electromagnetic power of the synchronous generator of the expanded two-machine system will be calculated based on the output reduction coefficient.

[0019] According to the mechanical power and electromagnetic power of the synchronous generator of the extended two-machine system, the time constant of the moment of inertia of the synchronous generator, the speed difference and power angle difference of the extended two-machine system, a single-machine infinite system of the extended two-machine system is constructed;

[0020] Construct high-proportion wind power access. When the power system has not yet recovered stability, the output of the synchronous generator is affected by the output of the wind turbine and the changes in the structural parameters of the power system. When it is only affected by the changes in the structural parameters, that is, when the wind turbine does not participate in the system frequency regulation, the two-machine system is extended to be equivalent to the electromagnetic power of the synchronous generator during the single-machine infinite system fault, and the electromagnetic power of the synchronous generator after the fault is cleared;

[0021] According to the single-machine infinite system mechanical power, the electromagnetic power of the synchronous generator during the fault period, and the electromagnetic power of the synchronous generator after the fault is cleared, the residual acceleration area of ​​the synchronous generator of the extended two-machine system, that is, the second mathematical model, is determined, where the expression of the second mathematical model is:

[0022] β represents the output reduction coefficient of the preset synchronous generator at the sending end, δ w represents the power angle difference, H1 represents the time constant of the moment of inertia of the synchronous generator at the sending end, H2 represents the time constant of the moment of inertia of the synchronous generator at the receiving end, δ d,w Indicates that after the wind turbine is connected, d System power angle at the moment, Indicates that after the wind turbine is connected, d The speed difference of the synchronous generator is kept constant.

[0023] In one implementation, the first analysis result is specifically:

[0024] According to the expressions of the first mathematical model and the second mathematical model, it is concluded that the wind turbine changes the structural parameters of the power system or adjusts the output, which affects the electromagnetic power output of the synchronous generator during the fault and after the fault is cleared, thereby affecting the power angle stability of the power system. Under different wind turbine control strategies, when a fault occurs, the different outputs of wind turbines have different effects on the stability of the first swing power angle of the power system.

[0025] In one implementation, the method further includes:

[0026] A distribution coefficient of unbalanced frequency regulation power of the wind turbine set is preset, and when constructing the additional frequency regulation control of the wind turbine set based on the second mathematical model and the distribution coefficient, a third mathematical model of the two-machine system is expanded, wherein the distribution coefficient represents the proportion of the change in the output of the synchronous generator at the sending end caused by the frequency regulation output of the wind turbine set after the fault to the frequency regulation output;

[0027] The second analysis result of the power angle stability of the power system during the fault period and after the fault is cleared when the wind turbine generator set is additionally frequency-controlled is analyzed according to the third mathematical model.

[0028] In one implementation, during a power system fault, based on the equivalent external characteristics of the wind turbine, a negative impedance model is connected to the two-machine system model during the fault period to determine the equivalent resistance and equivalent reactance, wherein the equivalent resistance is related to the bus voltage at the wind turbine export and the active power output of the wind turbine, and the equivalent reactance is related to the bus voltage at the wind turbine export and the reactive power output of the wind turbine;

[0029] During the power system fault, when the wind turbine adopts droop control, the remaining acceleration area of ​​the power system is determined according to the distribution coefficient of the frequency modulation power of the unbalanced wind turbine and the second mathematical model, that is, the third mathematical model. The expression of the third mathematical model is:

[0030] Among them, β′ represents the distribution coefficient, δ c ,w,f represents the wind turbine after adopting droop control c System power angle at the moment, It means that the wind turbine adopts droop control after t c The speed difference of the synchronous generator at each moment, H1 represents the time constant of the moment of inertia of the synchronous generator at the sending end, and H2 represents the time constant of the moment of inertia of the synchronous generator at the receiving end.

[0031] In one implementation, the second analysis result is specifically: when the wind turbine is connected to the sending end area, the allocation coefficient is 1, and the second mathematical model and the third mathematical model are combined to obtain that when the wind turbine is connected to the sending end area, the droop control of the wind turbine will reduce the speed difference of the synchronous generator of the power system at the time of fault removal;

[0032] When the wind turbine is connected to the receiving end area, the allocation coefficient takes the value of 1. Combining the second mathematical model and the third mathematical model, it is concluded that when the wind turbine is connected to the receiving end, the droop control of the wind turbine will increase the speed difference of the synchronous generator of the power system when the fault is cleared.

[0033] In one implementation, the method further includes: modeling based on the equivalent external characteristics of the wind turbine generator set in the early stage after the fault is cleared, analyzing the impact of the additional frequency regulation control of the wind turbine generator set on the speed difference between the sending and receiving ends of the extended two-machine system during the dynamic process of power system fault recovery, and obtaining a third analysis result of the power angle stability of the power system in the early stage after the fault is cleared by the wind turbine generator set frequency regulation control.

[0034] In one implementation, based on the equivalent external characteristics of the wind turbine generator set in the early stage after the fault is cleared, the negative impedance model is connected to the two-machine system during the power system fault recovery period to calculate the mutual impedance between the sending-end synchronous generator and the receiving-end synchronous generator;

[0035] The power angles of the two synchronous generators of the wind turbine under the MPPT control strategy are calculated according to the DC power flow method, and the power angles of the two synchronous generators under droop control are determined. Based on the power angle difference of the two synchronous generators under the MPPT control strategy and droop control, the power angle difference of the two-machine system during the fault recovery period of the negative impedance model connected to the power system is determined;

[0036] According to the power angle difference of the two-machine system during the recovery period of the power system fault connected to the negative impedance model, the distribution coefficient of the frequency regulation power imbalance of the wind turbine group is calculated;

[0037] According to the distribution coefficient, mechanical power and electromagnetic power of the wind turbine frequency modulation power imbalance, the residual acceleration area of ​​the power system is calculated when the wind turbine additional droop control passes through the same power angle difference after the power system fault is cleared. The calculation formula of the residual acceleration area is: in, Indicates that the motor group adopts droop control after t d The speed difference of the synchronous generator is kept constant.

[0038] In one implementation, the third analysis result includes:

[0039] In the early stage after the power system fault is cleared, when the distribution coefficient of wind turbine frequency regulation power imbalance is β'

[0040] In the early stage after the power system fault is cleared, when the distribution coefficient of the wind turbine frequency regulation power imbalance β'>H1 / (H1+H2), the additional droop control will increase the rate at which the speed difference of the power system drops to 0, that is, when the power angle first swing is positive, the degree of speed suppression on the sending-end synchronous generator is greater than that on the receiving-end synchronous generator. The power system passes through the same acceleration area. In the early stage after the fault is cleared, the power angle first swing amplitude of the power system with additional droop control of the wind turbine is smaller than the power angle first swing amplitude of the power system under the MPPT control strategy.

[0041] Compared with the prior art, the present invention has the following beneficial effects:

[0042] 1. The present invention introduces the output reduction coefficient of the synchronous generator into the first mathematical model, thereby realizing the system power angle stability analysis modeling based on relative kinetic energy for high-proportion new energy access, that is, the second mathematical model. By comparing the first mathematical model and the second mathematical model, the analysis results of the power angle stability of the power system during the fault period and after the fault is cleared are analyzed when the wind turbine group changes the structural parameters of the power system and / or adjusts the output.

[0043] 2. The present invention introduces the distribution coefficient of the unbalanced wind turbine frequency modulation power, combines the fault attributes and the equivalent external characteristics of the wind turbine set, improves the residual acceleration area, and realizes the analysis of the impact of the wind turbine set frequency modulation control during the fault on the unbalanced power of the synchronous generators at the sending and receiving ends of the extended two-machine system, and the analysis of the speed difference between the sending and receiving ends of the extended two-machine system, and obtains the impact mechanism of the wind turbine frequency modulation during the fault on the transient power angle stability of the system. BRIEF DESCRIPTION OF THE DRAWINGS

[0044] The drawings described herein are used to provide a further understanding of the embodiments of the present invention, constitute a part of this application, and do not constitute a limitation of the embodiments of the present invention. In the drawings:

[0045] Figure 1 A schematic flow chart of a method for analyzing the power angle stability of a power system by frequency modulation control of a wind turbine set provided by an embodiment of the present invention is shown; ​

[0046] Figure 2 A schematic diagram of the structure of a two-machine system model provided by an embodiment of the present invention is shown;

[0047] Figure 3 A schematic diagram of the structure of a single-machine infinite system provided by an embodiment of the present invention is shown;

[0048] Figure 4 An equivalent circuit diagram of an extended two-machine system during a fault period provided by an embodiment of the present invention is shown;

[0049] Figure 5 An equivalent circuit diagram of an extended two-machine system in the initial stage of fault clearing provided by an embodiment of the present invention is shown;

[0050] Figure 6 A diagram of an extended two-machine system provided by an embodiment of the present invention is shown;

[0051] Figure 7 A simulation diagram of the frequency, power output and export bus voltage of a wind turbine generator set access point provided by an embodiment of the present invention is shown;

[0052] Figure 8 A comparison diagram of early parameters after a fault is cleared is shown in an embodiment of the present invention;

[0053] Fig. 9 A rotation speed comparison simulation diagram of a wind turbine generator set before and after frequency modulation provided by an embodiment of the present invention is shown;

[0054] Fig.10 A simulation diagram comparing the power angles of the system before and after frequency modulation of a wind turbine set provided by an embodiment of the present invention is shown. DETAILED DESCRIPTION

[0055] 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.

[0056] In various embodiments of the present application, the expression "or" or "at least one of B or / and C" includes any combination or all combinations of the words listed at the same time. For example, the expression "B or C" or "at least one of B or / and C" may include B, may include C, or may include both B and C.

[0057] It should be understood that terms such as "first" and "second" are used for descriptive purposes only and should not be understood to indicate or imply relative importance or implicitly indicate the number of technical features indicated. Therefore, features defined as "first" and "second" may explicitly or implicitly include one or more of the features.

[0058] The technical terms appearing in the present invention are explained. In wind power generation, DFIG is the abbreviation of the first letters of the "double-fed induction generator". Uses of double-fed induction motors: Mainly used in wind power generation. In a wind power generation system, the wind drives the blades to rotate, and the blades drive the motor to rotate. However, since the wind speed is not constant, it is sometimes large and sometimes small, and the rotation speed of the blades is also sometimes fast and sometimes slow. In this case, the AC power generated by the general generator is an electric energy with an unstable frequency and amplitude. However, with a double-fed induction motor, it can draw power from the power grid when the wind speed is low to ensure a constant speed. When the wind speed is high, the excess electric energy can be stored in a large capacitor to ensure that the generated power has a balanced frequency and amplitude.

[0059] The Maximum Power Point Tracking (MPPT) system is an electrical system that adjusts the working state of the electrical module to enable the photovoltaic panel to output more electrical energy. It can effectively store the direct current generated by the solar panel in the battery, and can effectively solve the living and industrial electricity consumption in remote areas and tourist areas that are not covered by conventional power grids without causing environmental pollution.

[0060] Please refer to Figure 1 , provides a method for analyzing the effect of wind turbine frequency modulation control on power system power angle stability, the method comprising:

[0061] S101, constructing a first mathematical model for analyzing the stability of the power angle of a two-machine system of the power system, wherein the first mathematical model represents the correlation between the power angle of the two-machine system, the unbalanced power of the synchronous generator and the speed difference of the synchronous generator.

[0062] It is understandable that the first mathematical model includes multiple mathematical models for solving the residual acceleration area of ​​the traditional power system. The traditional two-machine system of the power system, such as Figure 2 As shown, the active power flow flows from the sending-end synchronous generator G1 to the receiving-end synchronous generator G2.

[0063] S102, connecting the wind turbines under the MPPT control strategy to the sending end area of ​​the power system to construct an extended two-machine system of the power system.

[0064] In this embodiment, the MPPT control strategy is a well-known technology for those skilled in the art, and this application does not provide any unnecessary explanation. Since the wind turbine is connected to the sending end area of ​​the power system, the two-machine system has changed as a whole, so an extended two-machine system is constructed. The wind turbine refers to a doubly fed induction generator (DFIG), which is the same as below.

[0065] S103, preset a reduction coefficient of the sending-end synchronous generator, and determine a second mathematical model of the extended two-machine system based on the reduction coefficient and the first mathematical model, wherein the reduction coefficient refers to the ratio of the change in the output of the synchronous generator to the output of the wind turbine after the wind turbine is connected to the power system, and the second mathematical model represents the correlation between the power angle of the extended two-machine system, the output of the wind turbine, the unbalanced power of the synchronous generator and the speed difference of the synchronous generator.

[0066] In this embodiment, the synchronous generator output reduction coefficient β is defined, which represents the ratio of the output change of the synchronous generator G1 to the output of the wind turbine after the wind turbine is connected to the system. The wind turbine is directly connected to the system without changing the load in the system, but the output of the synchronous generator is reduced. The inertia time constant of the system remains unchanged before and after the wind turbine is connected. The second mathematical model includes multiple mathematical models for solving the residual acceleration area of ​​the power system connected to the wind turbine.

[0067] S104, comparing the first mathematical model with the second mathematical model, and analyzing a first analysis result of the power angle stability of the power system during a fault and after the fault is cleared when the wind turbine generator system changes the structural parameters of the power system and adjusts the output.

[0068] In this embodiment, the wind turbine changes the structural parameters of the power system or adjusts the output, which will affect the electromagnetic power output of the synchronous generator during the fault and after the fault is cleared, thereby affecting the stability of the system power angle. Among them, different wind turbine control strategies will have different DFIG output conditions after the fault occurs, and the impact on the stability of the system's first swing power angle will also be different.

[0069] To summarize, the analysis method provided in this embodiment, by introducing the reduced output coefficient of the synchronous generator into the first mathematical model, realizes the system power angle stability analysis modeling for a high proportion of new energy access based on relative kinetic energy, that is, the second mathematical model. By comparing the first mathematical model and the second mathematical model, the analysis results of the power angle stability of the power system during the fault and after the fault is cleared are analyzed when the wind turbine changes the structural parameters of the power system and / or adjusts the output.

[0070] In one embodiment, a first mathematical model for analyzing the power angle stability of a two-machine system of a power system is constructed, specifically:

[0071] According to the time constant of the moment of inertia, power angle, mechanical power and electromagnetic power of the synchronous generator at the sending end and the synchronous generator at the receiving end, the rotor motion equation of the two-machine system model is established;

[0072] According to the power angle difference, speed difference, angular velocity rating, mechanical power and electromagnetic power of the two-machine system, the two-machine system is transformed into a single-machine infinite system;

[0073] Based on the power angle characteristic curve of the single-machine infinite system, the power angle at each moment when a fault occurs, the fault is cleared, and the fault is cleared is characterized; the power angle at each moment when a fault occurs, the fault is cleared, and the fault is cleared is characterized;

[0074] According to the mechanical power, electromagnetic power and power angle of the single-machine infinite system at each moment of fault occurrence, fault clearance and after fault clearance, the residual acceleration area of ​​the synchronous generator of the two-machine system model is determined, that is, the first mathematical model, where the expression of the first mathematical model is: P m Represents the mechanical power of a single infinite system, P e represents the electromagnetic power of a single-machine infinite system, δ0 represents the power angle of the synchronous generator that fails at time t0, δ d Indicates that after the fault is cleared, t d The power angle of the synchronous generator at time , H represents the time constant of the moment of inertia, ω0 is the rated value of the angular velocity, and Δω represents the speed difference of the synchronous generator.

[0075] Specifically, Figure 2 As shown in Figure 1, the active power flow flows from the sending-end synchronous generator G1 to the receiving-end synchronous generator G2. The rotor motion equations of the sending-end and receiving-end synchronous generators in the two-machine system are

[0076] Where: H1, H2 are the time constants of the moment of inertia of the synchronous generators G1, G2; δ1, δ2 are the power angles of the synchronous generators G1, G2; P m1 , P m2 is the mechanical power of synchronous generators G1 and G2; P e1 , P e2 is the electromagnetic power of synchronous generators G1 and G2.

[0077] The power angle difference between the two systems is δ = δ1-δ2, and the speed difference is Δω=ω1-ω2(3), ω0 is the rated value of angular velocity.

[0078] Transformation of two-machine system into single-machine infinite system

[0079]

[0080] in,

[0081]

[0082]

[0083]

[0084] The power angle characteristic curve of the traditional two-machine system is as follows: Figure 3 As shown, PI , P II , P III They correspond to the generator power characteristic curves before, during and after the fault of the equivalent single-machine system. c The fault is cleared at the moment, and the generator power angle reaches δ c At t d At the moment, the generator power angle reaches δ d .

[0085] The remaining acceleration area of ​​the generator (the total acceleration area minus the deceleration area that has been passed), that is, the first mathematical model can be expressed as:

[0086] Fault Clear c At the moment, the speed difference between the two systems reaches the maximum value, S remain The maximum acceleration area corresponding to the first swing angle of the system, t c The lower the speed difference of the synchronous generators at the moment, the smaller the acceleration area during the fault. After the fault is cleared, the speed difference between the two systems decreases. When the speed difference is reduced to zero, the system first swing power angle reaches the maximum value. The shorter the time for the synchronous generator speed difference to drop to 0, the greater the absolute value of the speed difference change rate. It can be seen from the rotor motion equation of formula (1) that, under the same power angle difference, the larger the system deceleration area, the stronger the system power angle first swing stability.

[0087] In one embodiment, a reduction coefficient of the sending-end synchronous generator is preset, and a second mathematical model of the extended two-machine system is determined based on the reduction coefficient and the first mathematical model, specifically:

[0088] When the wind turbine is connected to the sending end area of ​​the power system, the load of the power system will not be changed, but the output of the synchronous generator will be reduced. The time constant of the moment of inertia of the power system will remain unchanged before and after the wind turbine is connected. After the wind turbine is connected, the mechanical power and electromagnetic power of the synchronous generator of the expanded two-machine system will be calculated based on the output reduction coefficient.

[0089] According to the mechanical power and electromagnetic power of the synchronous generator of the extended two-machine system, the time constant of the moment of inertia of the synchronous generator, the speed difference and power angle difference of the extended two-machine system, a single-machine infinite system of the extended two-machine system is constructed;

[0090] Construct high-proportion wind power access. When the power system has not yet recovered stability, the output of the synchronous generator is affected by the output of the wind turbine and the changes in the structural parameters of the power system. When it is only affected by the changes in the structural parameters, that is, when the wind turbine does not participate in the system frequency regulation, the two-machine system is extended to be equivalent to the electromagnetic power of the synchronous generator during the single-machine infinite system fault, and the electromagnetic power of the synchronous generator after the fault is cleared;

[0091] According to the single-machine infinite system mechanical power, the electromagnetic power of the synchronous generator during the fault period, and the electromagnetic power of the synchronous generator after the fault is cleared, the residual acceleration area of ​​the synchronous generator of the extended two-machine system, that is, the second mathematical model, is determined, where the expression of the second mathematical model is:

[0092] β represents the output reduction coefficient of the preset synchronous generator at the sending end, δ w represents the power angle difference, H1 represents the time constant of the moment of inertia of the synchronous generator at the sending end, H2 represents the time constant of the moment of inertia of the synchronous generator at the receiving end, δ d,w express, express.

[0093] In this embodiment, a DFIG under MPPT control is introduced to construct an extended two-machine system, and mathematical modeling and theoretical research are performed on the transient power angle stability of the extended two-machine system based on relative kinetic energy. The steps are as follows:

[0094] Construct a mathematical model of DFIG connected to a two-machine system. Define the synchronous generator output reduction coefficient β, which represents the ratio of the output change of the synchronous generator G1 to the DFIG output after DFIG is connected to the system. The wind turbine is directly connected to the system, which does not change the load in the system, but reduces the output of the synchronous generator. The inertia time constant of the system remains unchanged before and after DFIG is connected. 1,w =H1,H 2,w =H2. After wind power is connected, the mechanical power of the synchronous generator is P m,w and electromagnetic power P e,w for,

[0095]

[0096] Expand the speed difference Δω between the two systems w =ω 1w -ω 2w , power angle difference δ w =δ 1w -δ 2w ,at this time,

[0097]

[0098] The expanded two-machine system is equivalent to a single-machine infinite system, as shown in the following formula:

[0099]

[0100] in,

[0101]

[0102]

[0103] Constructing a high proportion of wind power centralized access, when the system has not yet recovered stability, the output of the synchronous generator is affected by the DFIG output and the changes in system structure parameters.

[0104]

[0105] When only affected by structural changes, the two-machine system is equivalent to a single-machine infinite system.

[0106]

[0107] Electromagnetic power of synchronous generator after fault clearing

[0108]

[0109] Then t d The remaining acceleration area of ​​the generator at the moment, that is, the second mathematical model is:

[0110]

[0111] In one embodiment, the first analysis result is specifically:

[0112] According to the expressions of the first mathematical model and the second mathematical model, it is concluded that the wind turbine changes the structural parameters of the power system or adjusts the output, which affects the electromagnetic power output of the synchronous generator during the fault and after the fault is cleared, thereby affecting the power angle stability of the power system. Under different wind turbine control strategies, when a fault occurs, the different outputs of wind turbines have different effects on the stability of the first swing power angle of the power system.

[0113] Specifically, by comparing the second mathematical model of the extended two-machine system with the first mathematical model of the traditional two-machine system, it can be seen from the above equations (8) and (17) that when the DFIG changes the system structural parameters or adjusts the output, it will affect the electromagnetic power output of the synchronous generator during and after the fault is cleared, thereby affecting the stability of the system power angle. Among them, different wind turbine control strategies will result in different DFIG output conditions after the fault occurs, and the impact on the stability of the system first swing power angle is also different.

[0114] In this embodiment, in the above Figure 1 Based on the method shown, the method also includes:

[0115] A distribution coefficient of unbalanced frequency regulation power of the wind turbine set is preset, and when constructing the additional frequency regulation control of the wind turbine set based on the second mathematical model and the distribution coefficient, a third mathematical model of the two-machine system is expanded, wherein the distribution coefficient represents the proportion of the change in the output of the synchronous generator at the sending end caused by the frequency regulation output of the wind turbine set after the fault to the frequency regulation output;

[0116] The second analysis result of the power angle stability of the power system during the fault period and after the fault is cleared when the wind turbine generator set is additionally frequency-controlled is analyzed according to the third mathematical model.

[0117] Specifically, the method provided in this embodiment is based on the modeling of the equivalent external characteristics of DFIG during the fault period, and by analyzing the influence of wind turbine frequency modulation on the system reactance parameters and DFIG output, the influence of wind turbine frequency modulation control on the system power angle during the fault period is obtained. By introducing the unbalanced wind turbine frequency modulation power distribution coefficient, combining the fault attributes and the equivalent external characteristics of DFIG, the residual acceleration area theory is improved, and the analysis of the influence of wind turbine frequency modulation control on the unbalanced power of the synchronous generators at the sending and receiving ends of the extended two-machine system during the fault period and the analysis of the speed difference between the sending and receiving ends of the extended two-machine system are realized, and the influence mechanism of wind turbine frequency modulation on the stability of the transient power angle of the system during the fault period is obtained.

[0118] In one embodiment, during a power system fault, based on the equivalent external characteristics of the wind turbine generator set, a negative impedance model is connected to a two-machine system model during the fault period to determine an equivalent resistance and an equivalent reactance, wherein the equivalent resistance is related to the bus voltage at the wind turbine generator set outlet and the active power output of the wind turbine generator set, and the equivalent reactance is related to the bus voltage at the wind turbine generator set outlet and the reactive power output of the wind turbine generator set;

[0119] During the power system fault, when the wind turbine adopts droop control, the remaining acceleration area of ​​the power system is determined according to the distribution coefficient of the frequency modulation power of the unbalanced wind turbine and the second mathematical model, that is, the third mathematical model. The expression of the third mathematical model is:

[0120] Among them, β′ represents the distribution coefficient, δ c ,w,f represents the wind turbine after adopting droop control c System power angle at the moment, It means that the wind turbine adopts droop control after t c The speed difference of the synchronous generator at each moment, H1 represents the time constant of the moment of inertia of the synchronous generator at the sending end, and H2 represents the time constant of the moment of inertia of the synchronous generator at the receiving end.

[0121] Specifically, based on the DFIG equivalent external characteristics during the fault period, the negative impedance model is connected to the two-machine system during the fault period, such as Figure 4 As shown. Equivalent resistance r and wind turbine output bus voltage u s And wind turbine output active power P g The equivalent reactance x is related to the bus voltage u at the wind turbine generator output. s And wind turbine output reactive power q g Related.

[0122]

[0123]

[0124] When DFIG adopts MPPT control, the remaining acceleration area of ​​the two-machine system is expanded. Since the fan output is decoupled from the system frequency and is not affected by faults, the fan mechanical power and output electromagnetic power are equal. mw -P ew = 0, from equation (17) we can get equation (20), that is

[0125] The residual acceleration area of ​​the two-machine system is expanded when the fan adopts droop control. The unbalanced fan frequency modulation power distribution coefficient β' is defined, which represents the proportion of the change in the output of the synchronous machine at the sending end caused by the fan frequency modulation output after the fault to the fan frequency modulation output. The electromagnetic power output of the synchronous machine during the fault is

[0126] The maximum remaining acceleration area of ​​the system is

[0127]

[0128] On the basis of the above method, the second analysis result is specifically as follows: when the wind turbine is connected to the sending end area, the value of the allocation coefficient is 1. Combining the second mathematical model and the third mathematical model, it is concluded that when the wind turbine is connected to the sending end area, the droop control of the wind turbine will reduce the speed difference of the synchronous generator of the power system at the time of fault removal; when the wind turbine is connected to the receiving end area, the value of the allocation coefficient is 1. Combining the second mathematical model and the third mathematical model, it is concluded that when the wind turbine is connected to the receiving end, the droop control of the wind turbine will increase the speed difference of the synchronous generator of the power system at the time of fault clearing.

[0129] In this embodiment, during a power system fault, a three-phase short circuit divides the power system into two independent circuits. When a wind turbine is connected to the sending end area, the frequency modulation output only affects the sending end area, and has no effect on the synchronous generator at the receiving end, that is, the distribution coefficient is 1. Combining the second mathematical model and the third mathematical model, it is concluded that when the wind turbine is connected to the sending end area, the droop control of the wind turbine will reduce the speed difference of the synchronous generator of the power system at the time of fault removal, thereby improving the power angle stability of the power system.

[0130] When the wind turbine is connected to the receiving end area, during the fault period, the frequency modulation output of the wind turbine is affected by the synchronous generator in the receiving end area, but has no effect on the synchronous generator in the sending end area, that is, the distribution coefficient is 0. Combined with the second mathematical model and the third mathematical model, it is concluded that when the wind turbine is connected to the receiving end, the droop control of the wind turbine will increase the speed difference of the synchronous generator of the power system at the moment of fault clearing, and reduce the power angle stability of the power system.

[0131] Furthermore, by comparing the fan output after additional frequency modulation control, the influence on the speed difference between the sending and receiving ends of the extended two-machine system when the fault is removed is analyzed, and the influence on the first swing amplitude of the transient power angle of the system is obtained. The specific influences are as follows:

[0132] During the fault period, the three-phase short circuit divides the system into two independent circuits. When connected to the sending end area, the DFIG frequency modulation output only affects the sending end area, but has no effect on the synchronous generator at the receiving end, that is, β'=1, that is

[0133]

[0134] From formulas (20), (22) and (23), it can be seen that when the fan is connected to the sending end, during the fault period, the droop control of the fan will reduce the speed difference of the system synchronous machine at the moment of fault removal and improve the system power angle stability.

[0135] When the wind turbine is connected to the receiving end area, during the short circuit fault, the wind turbine frequency modulation output affects the synchronous machine in the receiving end area, but has no effect on the synchronous machine in the sending end area, that is, β'=0, that is

[0136]

[0137] It can be seen from formulas (20), (22) and (24) that when DFIG is connected to the receiving end, during the fault period, the droop control of the wind turbine will increase the speed difference of the synchronous generator of the system at the moment of fault clearing, and deteriorate the power angle stability of the system.

[0138] Furthermore, based on the method provided in the above embodiment, the method also includes: modeling based on the equivalent external characteristics of the wind turbine set in the early stage after the fault is cleared, analyzing the influence of the additional frequency regulation control of the wind turbine set on the speed difference of the synchronous generators at the sending and receiving ends of the extended two-machine system during the dynamic process of power system fault recovery, and obtaining a third analysis result of the power angle stability of the power system in the early stage after the wind turbine set frequency regulation control is cleared.

[0139] This embodiment is based on the equivalent external characteristics of the DFIG in the early stage after the fault is cleared. The model is modeled to study the influence of the additional frequency regulation control of the wind turbine on the speed difference between the sending and receiving ends of the extended two-machine system during the dynamic process of system recovery, and the influence of the frequency regulation control of the wind turbine on the stability of the system power angle in the early stage after the fault is cleared is obtained.

[0140] In one embodiment based on the third analysis result, based on the equivalent external characteristics of the wind turbine generator set in the early stage after the fault is cleared, the negative impedance model is connected to the two-machine system during the power system fault recovery period to calculate the mutual impedance between the sending-end synchronous generator and the receiving-end synchronous generator;

[0141] The power angles of the two synchronous generators of the wind turbine under MPPT control are calculated according to the DC power flow method, and the power angles of the two synchronous generators under droop control are determined. Based on the difference in the power angles of the two synchronous generators, the power angle difference of the two-machine system during the fault recovery period of the negative impedance model access to the power system is determined;

[0142] According to the power angle difference of the two-machine system during the recovery period of the power system fault connected to the negative impedance model, the distribution coefficient of the frequency regulation power imbalance of the wind turbine group is calculated;

[0143] According to the distribution coefficient, mechanical power and electromagnetic power of the wind turbine frequency modulation power imbalance, the residual acceleration area of ​​the power system is calculated when the wind turbine additional droop control passes through the same power angle difference after the power system fault is cleared. The calculation formula of the residual acceleration area is: in, It means that the wind turbine adopts droop control after t d The speed difference of the synchronous generator is kept constant.

[0144] Specifically, based on the DFIG equivalent external characteristics in the early stage after the fault is cleared, the negative impedance model is connected to the two-machine system during the system recovery period. The structure of the system is as follows: Figure 5 shown.

[0145] The mutual impedance between synchronous machine G1 and synchronous machine G2 is:

[0146] It can be seen from formula (25) that when the equivalent reactance value of the wind turbine increases, the mutual reactance of the G1 to G2 synchronous machines will also increase.

[0147] Analyze the influence of wind turbine frequency modulation control on the unbalanced wind turbine frequency modulation power distribution coefficient β'. In the early stage after the fault is cleared, the power angle of the two synchronous machines of the wind turbine under MPPT control is: δ w =P e1w X+P ew X w -P L1 X L1 -P L2 X L2 (26)

[0148] Under droop control, the wind turbine has a power output increase of ΔP ew , the output power of the synchronous unit at the sending end increases by ΔPe1, and the output power of the synchronous unit at the receiving end increases by ΔP e2In the early stage after the fault is cleared, the system difference under different frequency regulation control of the wind turbine has a significant impact on the voltage at the access point of the wind turbine, affecting the reactive output of the wind turbine. Therefore, under different frequency regulation control strategies, the equivalent reactance of the wind turbine is different. Compared with MPPT control, the change in the equivalent reactance of the wind turbine increases the mutual reactance between the transmitting and receiving synchronous generators G1 and G2 by ΔX, and the power angle difference of the two-machine system is expanded to:

[0149] According to the difference between the left and right sides of equation (26) and equation (27), assuming that the power angle has not been opened, that is, δ w -δ′ w ≈0, so

[0150] 0=P e1w ΔX+ΔP e1 (X+ΔX)+ΔP ew X w (28)

[0151] From formula (28), we can know that the unbalanced distribution coefficient of wind turbine frequency modulation power is

[0152]

[0153] After the fault is cleared, compared with MPPT control, the system deceleration area of ​​the fan additional droop control when the same power angle difference is

[0154] In one embodiment, the third analysis result includes: in the early stage after the power system fault is cleared, when the distribution coefficient β' of the wind turbine frequency modulation power imbalance

[0155] In the early stage after the power system fault is cleared, when the distribution coefficient of the wind turbine frequency regulation power imbalance β'>H1 / (H1+H2), the additional droop control will increase the rate at which the speed difference of the power system drops to 0, that is, when the power angle first swing is positive, the degree of speed suppression on the sending-end synchronous generator is greater than that on the receiving-end synchronous generator. The power system passes through the same acceleration area. In the early stage after the fault is cleared, the power angle first swing amplitude of the power system with additional droop control of the wind turbine is smaller than the power angle first swing amplitude of the power system under the MPPT control strategy.

[0156] ​Specifically, the influence of frequency modulation control on the speed difference of the synchronous machines at the sending and receiving ends is analyzed, and the change of the first swing amplitude of the system transient power angle in the early stage after the fault is cleared is obtained.

[0157] After the fault is cleared, when β'

[0158] In the early stage after the fault is cleared, when β'>H1 / (H1+H2), it can be seen from formula (30) that the additional droop control will increase the rate at which the system speed difference drops to 0, that is, when the system power angle first swing is positive, the degree of speed suppression on the sending end synchronous machine is greater than that on the receiving end. The system passes through the same acceleration area. In the early stage after the fault is cleared, the power angle first swing amplitude of the DFIG additional droop control system is smaller than that of the wind turbine MPPT control system, which improves the power angle stability of the power system.

[0159] This embodiment further illustrates the technical solution of the present invention described in the above embodiment in combination with specific embodiments.

[0160] In DIgSILENT / PowerFactory, build Figure 6 In the extended two-machine system shown in the figure, the DFIG is connected to the sending end of the two-machine system, and the active outputs of the synchronous generators G1 and G2 are P G1 =720MW, P G2 = 600MW, consisting of 178 5WM double-fed wind turbines, the wind farm has an active output of P DFIG =217MW, the active loads of load nodes 7 and 9 are P L7 =500MW, P L9 =1000MW. A three-phase short circuit fault occurred at node 8 and lasted for 0.3s before the fault was cleared.

[0161] like Figure 7 As shown, Figure 7 In the comparison, the MPPT control of wind turbines without frequency regulation and the droop control with additional frequency regulation function are compared. Figure 7 As shown in (a), DFIG adopts droop control to effectively reduce the increase of system frequency. Figure 7 ​(b), 7(c), and 7(d) are the wind turbine output active power, reactive power, and bus voltage at the wind turbine grid connection point, respectively. During the fault period, the DFIG additional droop control reduces the active output and the grid connection point voltage drops severely. Compared with the MPPT control, the wind turbine reactive output and grid connection point voltage changes can be ignored at this time. In the early stage after the fault is cleared, the DFIG reactive output and grid connection point voltage under different wind turbine control strategies are different, and the wind turbine equivalent reactance value changes. At this time, the mutual reactance parameters of the synchronous generators at the sending and receiving ends of the extended two-machine system also change phase.

[0162] like Figure 8 As shown, Figure 8 In the early stage after the fault is cleared, the DFIG is additionally drooped, and the fan frequency modulation power unbalance distribution coefficient β' is related to the ratio of the inertia time constant of the sending-end synchronous generator set to the system inertia time constant, that is, H1 / (H1+H2). From the time when the fault is cleared to the time when the system power angle reaches the first swing, β'

[0163] like Fig. 9 As shown, Fig. 9 In the process, the frequency regulation of the wind turbine set reduces the unbalanced power of the sending-end synchronous generator of the extended two-machine system during the fault period, and suppresses the increase in the speed of the sending-end synchronous generator. However, it does not affect the electromagnetic power of the receiving-end synchronous generator. The speed of the receiving-end synchronous generator is the same as the speed of the wind turbine set during MPPT control. In the early period after the fault is cleared, during the first swing of the system power angle, the rate at which the speed difference between the sending and receiving-end synchronous machines drops to 0 is suppressed, that is, the speed reduction rate of the receiving-end synchronous machine is greater than the speed reduction rate of the sending-end synchronous machine. The speed reduction rate of the receiving-end synchronous generator is greater than the speed reduction rate of the sending-end synchronous generator, such as Fig. 9 Therefore, the additional frequency regulation of the wind turbine will reduce the speed difference between the synchronous generators at the sending and receiving ends of the extended two-machine system at the time of fault clearing, and reduce the amplitude of the first swing of the system power angle, as shown in Fig. 9 (b) The area is 1, but it also reduces the rate at which the speed difference between the synchronous generators at the sending and receiving ends of the extended two-machine system decreases to 0 after the fault is cleared, thereby increasing the amplitude of the system's first swing power angle, such as Fig. 9 (b) The area is 2.

[0164] like Fig.10 As shown, Fig.10 In, due to Fig. 9 Area 1 of (b) is larger than area 2, that is, the power angle reduced during the fault period is still larger than the power angle increased in the early stage after the fault is cleared. Therefore, the additional droop control reduces the amplitude of the system's first swing power angle.

[0165] ​In summary, combined with the simulation results, the correctness of the analysis method described in this embodiment is verified. Based on the analysis method, the following conclusions can be drawn about the transient power angle stability of the power system with additional frequency modulation of wind turbines:

[0166] (1) At different stages after the fault occurs, the wind turbine frequency control strategy affects the stability of the system power angle by changing the system reactance parameters and adjusting the wind turbine output to expand the electromagnetic power of the synchronous generators at the sending and receiving ends of the two-machine system.

[0167] (2) During the fault period, the wind turbine sets use additional frequency modulation control strategies. Compared with MPPT control, the system power angle difference is smaller, so the deviation of the power angle on the voltage at the wind turbine grid connection point can be ignored, and the output change of the wind turbine sets plays a leading role in the power change of the synchronous generator. When a three-phase short circuit occurs, the wind turbine sets adjust the frequency to reduce the output and increase the electromagnetic power of the synchronous generator in the access area. If the power angle swings positively and the wind turbine sets are connected to the sending end, the speed difference between the sending and receiving ends of the extended two-machine system will be reduced, and the acceleration area will be reduced. The opposite is true when connected to the receiving end.

[0168] (3) In the early period after the fault is cleared, when the wind turbine is additionally controlled with droop control relative to MPPT control, if the proportion of the output change of the sending-end synchronous generator to the wind turbine frequency modulation output reduction is greater than the proportion of the inertia time constant of the sending-end synchronous generator to the total inertia time constant of the system, it will be beneficial to reduce the amplitude of the system's first swing power angle. Otherwise, it will increase the amplitude of the system's first swing power angle.

[0169] The analysis method proposed in the present invention can obtain the influence mechanism of wind turbine frequency modulation control on system transient power angle stability at various stages of fault, and provide a theoretical reference for subsequent protection setting work.

[0170] 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. An analysis method of the effect of wind turbine frequency modulation control on the power system power angle stability. It is characterized in that Methods include: A first mathematical model for analyzing the power angle stability of a two-machine system of an electric power system is constructed, wherein the first mathematical model characterizes the correlation between the power angle of the two-machine system, the unbalanced power of the synchronous generator and the speed difference of the synchronous generator; wherein, the first mathematical model for analyzing the power angle stability of a two-machine system of an electric power system is constructed, specifically: based on the time constant of the moment of inertia, power angle, mechanical power and electromagnetic power of the synchronous generator at the sending end and the synchronous generator at the receiving end, the rotor motion equation of the two-machine system model is established; based on the power angle difference, speed difference, angular velocity rated value, mechanical power and electromagnetic power of the two-machine system, the two-machine system is converted into a single-machine infinite system; based on the power angle characteristic curve of the single-machine infinite system, the power angle at each moment when a fault occurs, the fault is cleared and the fault is cleared; based on the mechanical power, electromagnetic power and the power angle at each moment when a fault occurs, the fault is cleared and the fault is cleared, the residual acceleration area of ​​the synchronous generator of the two-machine system model is determined, that is, the first mathematical model, wherein the expression of the first mathematical model is P m Represents the mechanical power of a single infinite system, P e represents the electromagnetic power of a single infinite system, δ 0 Indicates that at t 0 The power angle of the synchronous generator at the moment of failure, δ d Indicates that after the fault is cleared, t d The power angle of the synchronous generator at the moment, H represents the time constant of the moment of inertia, ω 0 is the angular velocity rating, Δω represents the speed difference of the synchronous generator; Connect wind turbines under the MPPT control strategy to the sending end area of ​​the power system to build an extended two-machine system of the power system; The output reduction coefficient of the sending-end synchronous generator is preset, and the second mathematical model of the extended two-machine system model is determined according to the output reduction coefficient and the first mathematical model, wherein the output reduction coefficient refers to the proportion of the change in the output of the synchronous generator to the output of the wind turbine after the wind turbine is connected to the power system, and the second mathematical model represents the correlation between the power angle of the extended two-machine system, the output of the wind turbine, the unbalanced power of the synchronous generator and the speed difference of the synchronous generator; wherein the second mathematical model of the extended two-machine system model is determined as follows: When the wind turbine is connected to the sending end area of ​​the power system, the load of the power system will not be changed, but the output of the synchronous generator will be reduced. The time constant of the moment of inertia of the power system will remain unchanged before and after the wind turbine is connected. After the wind turbine is connected, the mechanical power and electromagnetic power of the synchronous generator of the expanded two-machine system will be calculated based on the output reduction coefficient. According to the mechanical power and electromagnetic power of the synchronous generator of the extended two-machine system, the time constant of the moment of inertia of the synchronous generator, the speed difference and power angle difference of the extended two-machine system, a single-machine infinite system of the extended two-machine system model is constructed; When constructing a high proportion of wind power access, the output of the synchronous generator is affected by the output of the wind turbine and the changes in the structural parameters of the power system before the power system is restored to stability. When it is only affected by the changes in the structural parameters, that is, when the wind turbine does not participate in the system frequency regulation, the extended two-machine system is equivalent to the electromagnetic power of the synchronous generator during the single-machine infinite system fault and the electromagnetic power of the synchronous generator after the fault is cleared. According to the mechanical power of the single-machine infinite system, the electromagnetic power of the synchronous generator during the fault and the electromagnetic power of the synchronous generator after the fault is cleared, the residual acceleration area of ​​the synchronous generator of the extended two-machine system is determined, that is, the second mathematical model, where the expression of the second mathematical model is: β represents the output reduction coefficient of the preset synchronous generator at the sending end, δ w Indicates the power angle difference, H 1 It represents the time constant of the moment of inertia of the synchronous generator at the sending end, H 2 represents the time constant of the moment of inertia of the synchronous generator at the receiving end, δ d,w Indicates that after the wind turbine is connected, d System power angle at the moment, Indicates that after the wind turbine is connected, d The speed difference of synchronous generator at all times; By comparing the first mathematical model and the second mathematical model, a first analysis result of the stability of the power angle of the power system during a fault and after the fault is cleared is analyzed when the wind turbine changes the structural parameters of the power system and adjusts the output; the first analysis result is specifically: according to the expressions of the first mathematical model and the second mathematical model, it is concluded that the wind turbine changes the structural parameters of the power system or adjusts the output, which affects the electromagnetic power output of the synchronous generator during and after the fault is cleared, thereby affecting the power angle stability of the power system, wherein under different wind turbine control strategies, when a fault occurs, the different outputs of the wind turbines have different effects on the stability of the first swing power angle of the power system.

2. According to claim 1, a method for analyzing the effect of wind turbine frequency modulation control on power system power angle stability, It is characterized in that The method further comprises: A distribution coefficient of unbalanced frequency regulation power of the wind turbine set is preset, and when constructing the additional frequency regulation control of the wind turbine set based on the second mathematical model and the distribution coefficient, a third mathematical model of the two-machine system is expanded, wherein the distribution coefficient represents the proportion of the change in the output of the synchronous generator at the sending end caused by the frequency regulation output of the wind turbine set after the fault to the frequency regulation output; The second analysis result of the power angle stability of the power system during the fault period and after the fault is cleared when the wind turbine generator set is additionally frequency-controlled is analyzed according to the third mathematical model.

3. According to claim 2, a method for analyzing the effect of wind turbine frequency modulation control on power system power angle stability, It is characterized in that During the power system fault, based on the equivalent external characteristics of the wind turbine, the negative impedance model is connected to the two-machine system model during the fault period to determine the equivalent resistance and equivalent reactance, where the equivalent resistance is related to the wind turbine export bus voltage and the wind turbine output active power, and the equivalent reactance is related to the wind turbine export bus voltage and the wind turbine output reactive power. During the power system fault, when the wind turbine adopts droop control, the remaining acceleration area of ​​the power system is determined according to the distribution coefficient of the frequency modulation power of the unbalanced wind turbine and the second mathematical model, that is, the third mathematical model. The expression of the third mathematical model is: Among them, β′ represents the distribution coefficient, δ c ,w,f represents the wind turbine after adopting droop control c System power angle at the moment, It means that the wind turbine adopts droop control after t c The speed difference of synchronous generator at any moment, H 1 It represents the time constant of the moment of inertia of the synchronous generator at the sending end, H 2 Represents the time constant of the moment of inertia of the receiving-end synchronous generator.

4. According to claim 3, a method for analyzing the effect of wind turbine frequency modulation control on power system power angle stability, It is characterized in that The second analysis result is as follows: when the wind turbine is connected to the sending end area, the allocation coefficient is 1. Combining the second mathematical model and the third mathematical model, it is concluded that when the wind turbine is connected to the sending end area, the droop control of the wind turbine will reduce the speed difference of the synchronous generator of the power system at the time of fault removal; When the wind turbine is connected to the receiving end area, the allocation coefficient takes the value of 1. Combining the second mathematical model and the third mathematical model, it is concluded that when the wind turbine is connected to the receiving end, the droop control of the wind turbine will increase the speed difference of the synchronous generator of the power system when the fault is cleared.

5. According to claim 3, a method for analyzing the effect of wind turbine frequency modulation control on power system power angle stability, It is characterized in that The method also includes: modeling based on the equivalent external characteristics of the wind turbine generator set in the early stage after the fault is cleared, analyzing the impact of the additional frequency regulation control of the wind turbine generator set on the speed difference between the sending and receiving ends of the extended two-machine system during the dynamic process of power system fault recovery, and obtaining a third analysis result of the power angle stability of the power system in the early stage after the fault is cleared by the wind turbine generator set frequency regulation control.

6. The method for analyzing the effect of wind turbine frequency modulation control on power system power angle stability according to claim 5, It is characterized in that Based on the equivalent external characteristics of the wind turbines in the early stage after the fault is cleared, the negative impedance model is connected to the two-machine system during the power system fault recovery period to calculate the mutual impedance between the sending-end synchronous generator and the receiving-end synchronous generator. The power angles of the two synchronous generators of the wind turbine under the MPPT control strategy are calculated according to the DC power flow method, and the power angles of the two synchronous generators under droop control are determined. Based on the power angle difference of the two synchronous generators under the MPPT control strategy and droop control, the power angle difference of the two-machine system during the fault recovery period when the negative impedance model is connected to the power system is determined. According to the power angle difference of the two-machine system during the recovery period of the power system fault connected to the negative impedance model, the distribution coefficient of the frequency regulation power imbalance of the wind turbine group is calculated; According to the distribution coefficient, mechanical power and electromagnetic power of the wind turbine frequency modulation power imbalance, the residual acceleration area of ​​the power system is calculated when the wind turbine additional droop control passes through the same power angle difference after the power system fault is cleared. The calculation formula of the residual acceleration area is: in, Indicates that the motor group adopts droop control after t d The speed difference of the synchronous generator is kept constant.

7. The method for analyzing the effect of wind turbine frequency modulation control on power system power angle stability according to claim 6, It is characterized in that The third analysis results include: In the early stage after the power system fault is cleared, when the distribution coefficient of wind turbine frequency regulation power imbalance is β' <H 1 / (H 1 +H 2 ), the additional droop control reduces the rate at which the speed difference of the power system drops to 0, that is, when the power angle first swing of the power system is positive, the degree of speed suppression of the synchronous generator at the sending end is less than that of the synchronous generator at the receiving end. After the power system passes through the same acceleration area, the power angle first swing amplitude of the power system with additional droop control of the wind turbine is greater than the power angle first swing amplitude of the power system under the MPPT control strategy after the fault is cleared; In the early stage after the power system fault is cleared, when the distribution coefficient of wind turbine frequency regulation power imbalance β'>H 1 / (H 1 +H 2 ), the additional droop control will increase the rate at which the speed difference of the power system drops to 0, that is, when the power angle first swing is positive, the degree of speed suppression on the sending-end synchronous generator is greater than that on the receiving-end synchronous generator. The power system passes through the same acceleration area. In the early stage after the fault is cleared, the power angle first swing amplitude of the power system with additional droop control of the wind turbine is smaller than that of the power system under the MPPT control strategy.

Images