CN114123356B - Power curve optimization method and system for improving frequency modulation capability of doubly-fed wind turbine generator - Google Patents

Power curve optimization method and system for improving frequency modulation capability of doubly-fed wind turbine generator Download PDF

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CN114123356B
CN114123356B CN202111361593.0A CN202111361593A CN114123356B CN 114123356 B CN114123356 B CN 114123356B CN 202111361593 A CN202111361593 A CN 202111361593A CN 114123356 B CN114123356 B CN 114123356B
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power
curve
reference value
doubly
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CN114123356A (en
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周仪旎
朱东海
邹旭东
胡家兵
康勇
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Huazhong University of Science and Technology
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    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
    • H02J3/00—Circuit arrangements for ac mains or ac distribution networks
    • H02J3/38—Arrangements for parallely feeding a single network by two or more generators, converters or transformers
    • H02J3/46—Controlling of the sharing of output between the generators, converters, or transformers
    • H02J3/48—Controlling the sharing of the in-phase component
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
    • H02J3/00—Circuit arrangements for ac mains or ac distribution networks
    • H02J3/24—Arrangements for preventing or reducing oscillations of power in networks
    • H02J3/241—The oscillation concerning frequency
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
    • H02J3/00—Circuit arrangements for ac mains or ac distribution networks
    • H02J3/38—Arrangements for parallely feeding a single network by two or more generators, converters or transformers
    • H02J3/381—Dispersed generators
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
    • H02J2300/00—Systems for supplying or distributing electric power characterised by decentralized, dispersed, or local generation
    • H02J2300/20—The dispersed energy generation being of renewable origin
    • H02J2300/28—The renewable source being wind energy
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E10/00—Energy generation through renewable energy sources
    • Y02E10/70—Wind energy
    • Y02E10/76—Power conversion electric or electronic aspects

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  • Engineering & Computer Science (AREA)
  • Power Engineering (AREA)
  • Control Of Eletrric Generators (AREA)
  • Wind Motors (AREA)

Abstract

The invention discloses a power curve optimization method and a system for improving frequency modulation capacity of a doubly-fed wind turbine, wherein the power curve optimization method comprises the following steps: when a frequency accident occurs in the system, switching the doubly-fed wind turbine from an MPPT control mode to a short-time overload control mode to realize frequency modulation; in a short-time overload control mode, controlling an output active power reference value to be increased to a power limit corresponding to an initial rotating speed under a torque limit condition, then reducing a mechanical power value captured by a fan corresponding to a point C along a parabolic line BM, and finally reducing a maximum wind energy tracking power corresponding to a point D along an arc line CA; the parabolic line BM takes the point M as the vertex, passes through the point B, the opening is upward, the arc line CA passes through the point C and the point A, and an intersection point except the end point exists between the parabolic line BM and the MPPT curve; point C is the intersection of parabolic BM and the mechanical power curve; the point D is the intersection point of the arc CA and the MPPT curve except the end points; the invention improves the lowest point of frequency drop, has stronger frequency supporting capability, and also avoids the problem of secondary drop of the power grid frequency.

Description

Power curve optimization method and system for improving frequency modulation capability of doubly-fed wind turbine generator
Technical Field
The invention belongs to the technical field of wind power generation, and particularly relates to a power curve optimization method and system for improving frequency modulation capacity of a doubly-fed wind turbine generator.
Background
The doubly-fed wind turbine generator set has the advantages of bidirectional energy flow, wide speed regulation range, active and reactive decoupling control, rotor excitation and the like, and becomes a widely used fan type in the current market. With the improvement of the permeability of wind power in a power grid, strict grid-connected specifications of wind power units are provided for maintaining safe and stable operation of a power system in various countries, wherein the inertia corresponding capacity of the wind power units is of great concern. However, since doubly-fed wind turbines typically operate on a maximum wind energy tracking (Maximun Power Point Tracking, MPPT) curve, direct coupling between doubly-fed wind turbine rotational speed and grid frequency is no longer possible. Therefore, when the frequency of the power grid falls, the wind turbine generator hardly provides inertia for the power grid. Currently, in order to suppress the frequency drop during frequency accidents, the prior research generally adopts a virtual inertia control method to optimize a power curve, namely, the active power output is regulated to release the kinetic energy of a rotor, so that the rotor has the similar capacity of suppressing the frequency drop as a synchronous machine.
The existing inertial control is mainly divided into the following two types: one type is Frequency response based inertial control (Frequency-Based Inertial Control, FBIC); the other is a step-wise inertial control (Stepwise Inertial Control, SIC). Although FBIC can raise the frequency nadir, it relies on frequency measurement to result in a slower response of the system because it requires an additional frequency loop. And the SIC response speed is high, more rotor kinetic energy can be released, and the lowest frequency point is effectively improved. However, the deceleration curve adopted by the conventional SIC in the rotor deceleration process does not pass through the deceleration lowest point of the rotating speed, and there is a problem that excessive release of the kinetic energy of the rotor may be caused. Meanwhile, the power is reduced as much as possible in the rotating speed recovery process, but the problem of secondary frequency drop is caused by overlarge instantaneous power reduction.
In order to solve the problem of cutting machine and secondary frequency drop caused by excessive kinetic energy release of the rotor in the conventional SIC, a part of scholars have studied improved SIC control strategies such as short-time power support (TFS). The method redesigns a deceleration curve in a deceleration stage, so that the deceleration curve passes through a deceleration lowest point to avoid excessive release of the kinetic energy of the rotor; in order to meet the requirement that the release amount of the kinetic energy of the rotor reaches a certain standard and the lowest point of frequency drop is improved, the deceleration curve adopted by the method is added with delta P (shown in figure 7) and is maintained for a period of time; in order to avoid serious secondary frequency drop caused by the reduction of output power when the traditional SIC exits frequency modulation, TFS adopts linear drop, but the time of the deceleration stage is longer, and the risk of secondary frequency drop still exists. The longer time of the frequency support in the deceleration stage also can lead to longer time of the recovery of the rotating speed, the rotating speed can not be recovered quickly, the maximum wind energy can not be tracked quickly, and the wind energy utilization rate is lower.
Disclosure of Invention
Aiming at the defects or improvement demands of the prior art, the invention provides a power curve optimization method and a system for improving the frequency modulation capacity of a doubly-fed wind turbine, which are used for solving the technical problem of secondary frequency drop in the prior art when inertia control is performed.
To solve the above-mentioned problems, in a first aspect, the present invention provides a power curve optimization method for improving the frequency modulation capability of a doubly-fed wind turbine, in which the doubly-fed wind turbine operates in an MPPT control mode and operates at a point a (ω) in a steady state before a frequency accident occurs in a power system 0 ，P A )；ω 0 At an initial rotational speed, P A Tracking power for maximum wind energy corresponding to the initial rotating speed;
when the frequency accident occurs in the power system, judging whether the frequency deviation of the power system is larger than a preset deviation, if so, executing the following steps:
s1, controlling an output active power reference value of a doubly-fed wind turbine generator to increase to an initial rotating speed omega under a torque limit condition 0 Corresponding power limit P Tlim0 The rotor of the doubly-fed wind turbine enters a deceleration stage so as to release kinetic energy stored by the rotor; point B is (omega) 0 ，P Tlim0 )；
S2, a deceleration stage: continuously controlling the output active power reference value to reduce the output active power reference value to a mechanical power value P captured by the fan corresponding to a point C along a parabola BM C The method comprises the steps of carrying out a first treatment on the surface of the At this time, the deceleration phase is ended, and the acceleration phase is entered; wherein, during the deceleration stage, the output active power reference value is always greater than the mechanical power; the parabolic line BM is a parabolic line which takes the point M as the vertex, passes through the point B and has an upward opening, and is used for representing the change relation of the output active power reference value in the deceleration stage along with the real-time rotating speed; point M is (omega) min ，P MPPTmin )，ω min At the lowest rotation speed, P MPPTmin Omega on MPPT curve min The corresponding minimum output power; point C is the intersection of parabolic BM and the mechanical power curve;
s3, acceleration stage: continuously controlling the output active power reference value to reduce the active power reference value to the maximum wind energy tracking power P corresponding to the point D along the arc CA D Switching to an MPPT control mode to realize rotation speed recovery; wherein, in the acceleration stageIn the process, the active power reference value is always smaller than the mechanical power; arc CA is an arc having an intersection point with the MPPT curve except for the end point, and passing through point C and point A, for representing the point of the secondary P C Reduced to P D Outputting the change relation of the active power reference value along with the real-time rotating speed in the process; point D is the intersection of arc CA and MPPT curve except for the end points.
Further preferably, on the parabolic BM, the active power reference value P is output ref The relation with the real-time rotation speed omega is as follows:
wherein omega C The rotational speed corresponding to point C.
Further preferably, the arc line CA is a lower semicircle with a line segment CA as a diameter, and the active power reference value P is output on the arc line CA ref The relation with the real-time rotation speed omega is as follows:
wherein P is 0 For the initial rotational speed omega on the MPPT curve 0 The corresponding output power; omega C The rotation speed corresponding to the point C; omega D The rotational speed corresponding to point D.
Further preferably, in step S1, the output active power reference value of the doubly-fed wind turbine generator is controlled to be suddenly increased to the initial rotational speed ω under the torque limit condition 0 Corresponding power limit P Tlim0 A place; wherein P is Tlim0 ＝T lim0 *ω 0 ；T lim0 Is the torque limit.
Further preferably, in step S3, after switching to the MPPT control mode, the output active power reference value is outputted from P along the MPPT curve D Rising to P A At this time, the doubly-fed wind turbine generator is operated again at the point A in a steady state.
Further preferably, before the frequency accident occurs in the power system, the doubly-fed wind turbine generator adopts a double closed-loop control mode of a power outer loop and a current inner loop; the double-fed wind turbine works in an MPPT control mode; the expression of the MPPT curve is as follows:
wherein P is MPPT The power of the wind energy tracking area is the maximum; ρ is the density of the fluid flowing through the fan rotor; a is that r Is the cross-sectional area through which wind energy passes; omega is the real-time rotation speed; r is the radius of the wind wheel; lambda (lambda) opt Is the tip speed ratio; c (C) pmax Is the maximum power coefficient.
In a second aspect, the present invention provides a power curve optimization system for improving frequency modulation capability of a doubly-fed wind turbine, including: the power curve optimization method for improving the frequency modulation capacity of the doubly-fed wind turbine generator comprises a memory and a processor, wherein the memory stores a computer program, and the processor executes the steps of the power curve optimization method for improving the frequency modulation capacity of the doubly-fed wind turbine generator provided by the first invention when executing the computer program.
In a third aspect, the present invention further provides a machine-readable storage medium storing machine-executable instructions that, when invoked and executed by a processor, cause the processor to implement the power curve optimization method for improving the frequency modulation capability of a doubly-fed wind turbine generator set provided by the first invention.
In general, through the above technical solutions conceived by the present invention, the following beneficial effects can be obtained:
1. the invention provides a power curve optimization method for improving the frequency modulation capability of a doubly-fed wind turbine, which is characterized in that when frequency drop is detected, an active power reference value is increased to a power limit corresponding to a torque limit, larger power is output in a short time, after the power grid frequency drop is effectively restrained, the output active power reference value is further controlled to be reduced to a mechanical power value P captured by a fan corresponding to a point C along a parabolic BM C Wherein the parabola BM is a parabola passing through the point B with the point M as the vertex and having an upward openingThe output active power reference value is reduced along the parabolic BM, excessive deceleration of the rotor can be avoided on the premise of ensuring that more kinetic energy of the rotor is released, compared with the existing TFS, the power reference value at the initial stage of the deceleration stage is increased to the power limit corresponding to the torque limit, the power reference value at the deceleration stage is higher than the power reference value at the deceleration stage of the TFS method, the lowest point of frequency drop is improved, frequency support is provided for a power grid, and meanwhile, frequency modulation can be withdrawn earlier on the premise of meeting the release amount of the kinetic energy of the rotor, the frequency change rate is positive, the time of the deceleration stage is shorter, and the drop of secondary frequency is greatly relieved.
2. According to the power curve optimization method for improving the frequency modulation capacity of the doubly-fed wind turbine, provided by the invention, the parabolic BM is adopted in the deceleration stage, and the output active power reference value is reduced to the mechanical power value captured by the fan corresponding to the point C, so that the other steady state is reached, and compared with the prior art, the method enters the acceleration stage earlier, so that the recovery process of the rotating speed of the rotor is faster, and the wind energy utilization rate is effectively improved.
3. The power curve optimization method for improving the frequency modulation capacity of the doubly-fed wind turbine generator set provided by the invention controls the output active power reference value in the acceleration stage so as to reduce the active power reference value to the maximum wind energy tracking power P corresponding to the point D along the arc CA, wherein the secondary frequency drop phenomenon is related to the time of exiting frequency modulation and the output power of the acceleration stage D Then, switching to an MPPT control mode, wherein an arc line CA is a lower semicircle taking a line segment CA as a diameter, and when electromagnetic power is decelerated by the arc line CA, the rotating speed of an intersection point C in an acceleration stage is higher, so that the rotating speed recovery time is reduced; meanwhile, the difference value between the mechanical power and the output power reference value at the initial stage after the frequency modulation is withdrawn is not large, and the occurrence of the phenomenon of secondary frequency drop is further relieved. Then the difference value between the mechanical power and the output power reference value is gradually increased, and the acceleration of the rotor is gradually increased, so that the recovery of the rotating speed is facilitated; based on the above, the invention can be more flexibly suitable for different permeability and wind speed conditions, and solves the technical problems that the prior art is difficult to simultaneously relieve the secondary frequency phenomenon and has shorter rotating speed recovery time.
4. The power curve optimization method for improving the frequency modulation capability of the doubly-fed wind turbine generator is simple in design and has universality. And a large number of simulations are not needed, and then the optimal design of the power curve is carried out, so that the design is convenient.
Drawings
FIG. 1 is a block diagram of a double closed loop control strategy of a doubly-fed wind generator according to embodiment 1 of the present invention;
FIG. 2 is a graph showing the wind power generation system according to embodiment 1 of the present invention p -lambda graph;
FIG. 3 is a sectional view of an operating area of a variable speed wind power generation system according to embodiment 1 of the present invention;
FIG. 4 is a flowchart of a power curve optimization method for improving the frequency modulation capability of a doubly-fed wind turbine provided by embodiment 1 of the present invention;
FIG. 5 is a schematic diagram of a power curve optimized by the power curve optimization method according to embodiment 1 of the present invention;
fig. 6 is a specific control block diagram of a short-time overload control mode provided in embodiment 1 of the present invention;
fig. 7 is a schematic diagram of a power curve optimized by a TFS method according to embodiment 1 of the present invention;
FIG. 8 is an active power curve obtained by the power curve optimization method and the TFS method provided by the invention;
FIG. 9 is a graph of rotational speed obtained by the power curve optimization method and the TFS method provided by the invention;
fig. 10 is a frequency response curve obtained by the power curve optimization method and the TFS method provided by the present invention, respectively.
Detailed Description
The present invention will be described in further detail with reference to the drawings and examples, in order to make the objects, technical solutions and advantages of the present invention more apparent. It should be understood that the specific embodiments described herein are for purposes of illustration only and are not intended to limit the scope of the invention. In addition, the technical features of the embodiments of the present invention described below may be combined with each other as long as they do not collide with each other.
Example 1,
The doubly-fed wind power generation system adopts a double closed-loop control strategy of a power outer loop and a current inner loop as shown in figure 1. The output active power reference value is obtained by MPPT control when no frequency drop occurs, and when a frequency event occurs, the doubly-fed wind turbine generator is switched from an MPPT control mode to a short-time overload control mode to achieve frequency modulation. U in the figure sabc 、I sabc Respectively obtaining stator voltage and stator current values under a three-phase static coordinate system, and obtaining stator voltage U under a two-phase static coordinate system through coordinate transformation sα 、U sβ Stator current I sα 、I sβ The method comprises the steps of carrying out a first treatment on the surface of the The angular frequency of the grid-connected point voltage measured by the phase-locked loop is omega s Angular frequency ω of rotor r Slip angular frequency omega 2 ＝ω s -ω r Slip ratioSlip angular frequency omega 2 Integrating to obtain the slip angle theta 2 ，θ 2 For rotor current I rabc Transforming the coordinates to a dq coordinate system; p (P) sref 、Q sref Respectively an active power given value and a reactive power given value; stator active power P s And reactive power Q s From U sα 、U sβ I sα 、I sβ Obtaining the product.
Specifically, before the frequency accident occurs in the power system, the system operates normally, the doubly-fed wind turbine works in a maximum wind energy tracking (Maximun PowerPoint Tracking, MPPT) control mode, and the doubly-fed wind turbine operates at a point A (omega) 0 ，P A )；ω 0 At an initial rotational speed, P A Tracking power for maximum wind energy corresponding to the initial rotating speed; the output active power reference value changes along the MPPT curve; specifically, the expression of the MPPT curve (maximum wind energy tracking area power-rotation speed) is as follows:wherein P is MPPT The power of the wind energy tracking area is the maximum; ρ is the density of the fluid flowing through the fan rotor; a is that r Is the cross-sectional area through which wind energy passes; omega is the real-time rotation speed; r is the radius of the wind wheel; lambda (lambda) opt Is the tip speed ratio; c (C) pmax Is the maximum power coefficient.
Specifically, as shown in FIG. 2, C of the wind power generation system is shown corresponding to different pitch angles beta p -lambda curve, wherein C p Is the capture coefficient of wind energy of the fan, lambda is the tip speed ratio, andwherein R is the radius of the wind wheel, and V is the wind speed. According to C p The lambda curve shows that when the pitch angle β=0° is adjusted and the tip speed ratio λ=λ is maintained opt When there is maximum power coefficient C pmax . The maximum mechanical power captured by the wind wheel is as follows:
and obtaining a P-omega curve according to the relation between the rotating speed and the power. At this time, the rotor will operate in the BC segment of the variable speed wind power system operating area as shown in fig. 3, i.e. within the maximum wind energy tracking area. When the wind driven generator stably operates at the point X, when the wind speed rises, the mechanical power changes along an aerodynamic curve X-Y-Z, the output power changes along an optimal power curve X-Z, and finally, the point Z reaches balance under the fresh air speed.
When a frequency accident occurs in the power system, judging whether the frequency deviation of the power system is larger than a preset deviation (in the embodiment, the preset deviation takes a value of 0.05 Hz), and if so, executing a power curve optimization method for improving the frequency modulation capability of the doubly-fed wind turbine generator so as to realize short-time overload control. Specifically, according to the related technical rule that the existing wind power plant is connected into a power system, inertia response is provided when the frequency change rate of the system is larger than the dead zone range (generally set to 0.05Hz/s, and the sampling period of the frequency change rate is not larger than 200 ms), so that the frequency deviation set value of the system is set to be 0.05Hz; and when detecting whether the frequency deviation is larger than the set value or not when the accident occurs, if so, switching from the MPPT control mode to the short-time overload control mode to realize frequency modulation.
Specifically, as shown in fig. 4, a flowchart of a power curve optimization method for improving the frequency modulation capability of a doubly-fed wind turbine is shown, and an obtained power curve schematic diagram is shown in fig. 5, where the method specifically includes the following steps:
s1, controlling an output active power reference value of a doubly-fed wind turbine generator to increase to an initial rotating speed (rotating speed at the moment of switching-in short-time overload control) omega under a torque limit condition 0 Corresponding power limit P Tlim0 The rotor of the doubly-fed wind turbine enters a deceleration stage so as to release kinetic energy stored by the rotor; point B is (omega) 0 ，P Tlim0 )；
Specifically, the output active power reference value of the doubly-fed wind turbine generator is controlled to be suddenly increased to the initial rotating speed omega under the torque limit condition 0 Corresponding power limit P Tlim0 At this time, an active power reference value P is output ref ＝P Tlim0 ＝T lim0 *ω 0 The method comprises the steps of carrying out a first treatment on the surface of the Wherein the torque limit T lim0 The value is 1.07p.u., the power limit P Tlim0 The limit value of (2) is 1.1p.u..
S2, a deceleration stage: continuously controlling the output active power reference value to reduce the output active power reference value to a mechanical power value P captured by the fan corresponding to a point C along a parabola BM C The method comprises the steps of carrying out a first treatment on the surface of the At this time, the deceleration phase is ended, and the acceleration phase is entered; wherein, during the deceleration stage, the output active power reference value is always greater than the mechanical power; the parabolic line BM is a parabolic line which takes the point M as the vertex, passes through the point B and has an upward opening, and is used for representing the change relation of the output active power reference value in the deceleration stage along with the real-time rotating speed; point M is (omega) min ，P MPPTmin )，ω min At the lowest rotation speed, P MPPTmin Omega on MPPT curve min The corresponding minimum output power; point C is the intersection of parabolic BM and the mechanical power curve;
specifically, in the deceleration stage, the output active power reference value-real-time rotation speed curve is the lowest through MPPT curve of the BC stage as shown in FIG. 5Point M (omega) min ,P MPPTmin ) And a power limit point B (ω) corresponding to the initial torque limit 0 ,P Tlim0 ) And a quadratic parabola with an upward opening. Excessive deceleration at this stage is avoided since the curve crosses the lowest point of the MPPT curve. Specifically, on parabolic BM, an active power reference value P is output ref The relation with the real-time rotation speed omega is as follows:
wherein omega C The rotational speed corresponding to point C.
It should be noted that the electromagnetic power of the BC-section rotor is greater than the mechanical power, which causes the rotor to slow down, releasing the kinetic energy stored by the rotation of the rotor, and the frequency drop of the system is supported for a short time. The reference value of the active power becomes smaller along with the decrease of the rotating speed, meanwhile, the mechanical power also decreases along an aerodynamic curve due to the decrease of the rotating speed until the reference value and the mechanical power are equal, and the frequency modulation process of the stage is ended. Since the curve of the BC stage passes the lowest point of the MPPT curve, it is ensured that the deceleration stage is not excessively decelerated.
The end of the deceleration stage is marked by determining whether the rotation speed has converged to the point C, in this embodiment, determining the deviation value |Δω| within a predetermined period (the value of 0.005s in this embodiment), if |Δω| is smaller than the predetermined deviation value (the value of 4.0x10 in this embodiment) -6 p.u.), it is determined that the system is able to reach another steady state value, and conversely, it is determined that the system is unable to reach another steady state value.
S3, acceleration stage: continuously controlling the output active power reference value to reduce the active power reference value to the maximum wind energy tracking power P corresponding to the point D along the arc CA D Switching to an MPPT control mode to realize rotation speed recovery; wherein, during the acceleration phase, the active power reference value is always smaller than the mechanical power; arc CA is an arc having an intersection point with the MPPT curve except for the end point, and passing through point C and point A, for representing the point of the secondary P C Reduced to P D Outputting the change relation of the active power reference value along with the real-time rotating speed in the process; point D is arc CIntersection points of a and MPPT curves except for the end points.
Preferably, in this embodiment, the arc line CA is a lower semicircle with a line segment CA as a diameter, and at this time, the active power reference value P is output on the arc line CA ref The relation with the real-time rotation speed omega is as follows:
wherein P is 0 For the initial rotational speed omega on the MPPT curve 0 The corresponding output power; omega C The rotation speed corresponding to the point C; omega D The rotational speed corresponding to point D.
It should be noted that, in order to avoid serious secondary frequency drop caused by excessive change of the active power reference value at the point C, the CD segment adopts a part of the arc CA to transition to the point D more gently.
At the point D, switching to MPPT control mode is started, and at this time, an active power reference value P is output ref The relation with the real-time rotation speed omega is as follows:
in summary, a specific control manner of the short-time overload control mode provided in this embodiment is shown in fig. 6.
Further, in order to illustrate the performance of the power curve optimization method for improving the frequency modulation capability of the doubly-fed wind turbine generator, the conventional short-time frequency support (Temporary Frequency Support, TFS) method with a better control effect is selected from a plurality of power curve optimization methods, and compared with the power curve optimization method provided by the invention.
Compared with the traditional SIC, the TFS method has the advantages that the lowest frequency point can be lifted, the secondary frequency drop amplitude is small and can be almost ignored, and the frequency recovery stability value is quick because the rotating speed is quick. The active power-rotation speed curve of the TFS method is shown in fig. 7, the system works at the point A in fig. 7 in the steady state, and after the frequency fluctuation is detected, an additional active power reference is set and maintainedFor a period of time t FN . Wherein t is FN Taking t according to the average value of the occurrence time of the lowest frequency point obtained by multiple simulation FN ＝2s。ω FN At t FN At the rotational speed. The active power reference value of BB' section is
Where n depends on the permeability, where n is 4 when the permeability is 10%; when the permeability is 20%, n is 3; when the permeability is 30%, n is 2; when the permeability is 20%, n is 1. Since the simulated condition permeability is about 10%, n is taken to be 4.
To converge the speed, the active power-speed curve is modified as shown in section B' C of fig. 7, and the active power reference value at this stage is:
in the acceleration stage, in order to avoid secondary frequency drop caused by excessive instantaneous change of active power, a TFS method adopts a CD section control mode, and the reference value of the active power in the stage is as follows:
if Δt is too large, the rotational speed recovery time may be too long, and if Δt is too small, the secondary frequency drop amplitude may be large. The TFS method literature indicates that by simulation analysis, two factors of the rotation speed recovery time and the secondary frequency drop amplitude are comprehensively considered, and the effect of taking 15s from the delta T is better.
Thereafter, the DA segment in fig. 7 is an MPPT curve.
From the implementation angle comparison TFS method and the provided power curve optimization method, the TFS needs to determine the duration t of the additional active power reference value of the BB' section by obtaining the average time of the occurrence of the frequency minimum point through a large number of simulations FN . and The delta T value is also required to be selected through a large number of simulation analysis, and the influence of two factors of the rotation speed recovery time and the secondary frequency drop is compromised to select the delta T, and the unsuitable control effect of the value selection can be influenced. The power curve optimization method does not need to carry out a large number of simulations before the active power-rotating speed curve is set, and the proposed strategy is more universal and easy to implement.
In addition to ease of implementation, the control effect of the present invention is more advantageous. In order to further explain the control effect of the power curve optimization method for improving the frequency modulation capability of the doubly-fed wind turbine set, the control effect is described below with reference to a specific embodiment:
in the embodiment, a simulation study is performed by taking a 1.5MW double-fed wind turbine generator set under 50 typical parameters as an example, and simulation parameters of the 1.5MW double-fed wind turbine generator set are shown in table 1. At t=45 s, a 50MW load is placed on the system, which causes a disturbance in frequency.
TABLE 1
Parameters (parameters) (symbol) Numerical value
Rated power P sN 1.5MW
Rated stator voltage U sN 690V
Rated grid frequency f sN 50Hz
Polar logarithm p sN 2 pairs of poles
Ratio of stator to rotor n sr 1:2.5
Stator resistor R s 0.023p.u.
Leakage inductance of stator L ls 0.18p.u.
Rotor resistor R r 0.016p.u.
Rotor side leakage inductance L lr 0.16p.u.
Mutual inductance L m 2.9
Rated DC bus voltage U dc 1200V
Wind speed V 8m/s
As shown in fig. 8, which is a waveform diagram of the change of the active power reference value with time, in combination with the rotation speed change curve shown in fig. 9, it can be seen that at an instant of the frequency drop, the power curve optimizing method provided by the present invention releases more energy in a short time compared with the TFS method, which can raise the frequency nadir in the frequency response curve shown in fig. 10.
Meanwhile, as can be seen from the rotation speed change curve shown in fig. 9, in the deceleration stage, the power curve optimization method provided by the invention has a larger rotation speed drop amplitude than that of the TFS method, but the recovery of the rotation speed in the acceleration stage is faster. On the one hand, the rotor releases more kinetic energy to support the frequency nadir. On the other hand, the frequency can restore the steady state value faster due to faster restoration of the rotation speed, and the utilization rate of wind energy is improved.
Further, as can be seen from fig. 10, the wind turbine generator adopts the TFS method, and the secondary frequency drop phenomenon lasts for a long period of time; however, when the power curve optimization method provided by the invention is adopted, almost no secondary frequency drop phenomenon occurs at the wind speed.
In summary, the power curve optimization method for improving the frequency modulation capability of the doubly-fed wind turbine generator provided by the invention can enhance the active supporting capability of the doubly-fed wind turbine generator under the disturbance of the power grid frequency, thereby improving the frequency stability of the system. The invention is suitable for providing short-time power support when the system frequency drops so as to improve the frequency transient stability of the system. When the system runs in a steady state, the doubly-fed wind turbine generator runs in a maximum wind energy tracking (Maximum PowerPoint Tracking, MPPT) mode; when the system frequency drops, judging whether the system frequency change rate is larger than the dead zone range, and when the system frequency change rate is larger than the dead zone range, switching the doubly-fed wind turbine generator from the MPPT control mode to the short-time overload control mode to realize frequency modulation, so that the output active power reference value is changed along the A-B-C-D-A in the graph 5; compared with the prior art, the power curve optimization method provided by the invention has four advantages: (1) The lowest point of frequency drop is improved, and stronger frequency supporting capability is achieved; (2) the problem of secondary dropping of the power grid frequency is avoided; (3) The rotating speed of the doubly-fed wind turbine generator is recovered faster, so that the wind energy utilization rate is improved; and (4) the design is simple, and the universality is realized.
EXAMPLE 2,
A power curve optimization system for improving frequency modulation capability of a doubly-fed wind turbine generator comprises: the power curve optimization method for improving the frequency modulation capacity of the doubly-fed wind turbine generator set provided by the embodiment 1 of the invention is implemented when the processor executes the computer program.
The related technical solution is the same as that of embodiment 1, and will not be described here in detail.
EXAMPLE 3,
A machine-readable storage medium storing machine-executable instructions that, when invoked and executed by a processor, cause the processor to implement a power curve optimization method for improving frequency modulation capability of a doubly-fed wind turbine provided by embodiment 1 of the present invention.
The related technical solution is the same as that of embodiment 1, and will not be described here in detail.
It will be readily appreciated by those skilled in the art that the foregoing description is merely a preferred embodiment of the invention and is not intended to limit the invention, but any modifications, equivalents, improvements or alternatives falling within the spirit and principles of the invention are intended to be included within the scope of the invention.

Claims (6)

1. A power curve optimization method for improving frequency modulation capability of a doubly-fed wind turbine generator is characterized in that before a frequency accident occurs in a power system, the doubly-fed wind turbine generator works in an MPPT control mode and runs at a point A (omega) in a steady state 0 ，P A )；ω 0 At an initial rotational speed, P A Tracking power for maximum wind energy corresponding to the initial rotating speed;
when the frequency accident occurs in the power system, judging whether the frequency deviation of the power system is larger than a preset deviation, if so, executing the following steps:
s1, controlling an output active power reference value of a doubly-fed wind turbine generator to increase to an initial rotating speed omega under a torque limit condition 0 Corresponding power limit P Tlim0 The rotor of the doubly-fed wind turbine enters a deceleration stage so as to release kinetic energy stored by the rotor; point B is (omega) 0 ，P Tlim0 )；
S2, a deceleration stage: continuously controlling the output active power reference value to reduce the output active power reference value to a mechanical power value P captured by the fan corresponding to a point C along a parabola BM C The method comprises the steps of carrying out a first treatment on the surface of the At this time, the deceleration phase is ended, and the acceleration phase is entered; wherein, during the deceleration stage, the output active power reference value is always greater than the mechanical power; the parabolic line BM is a parabolic line which takes the point M as the vertex, passes through the point B and has an upward opening, and is used for representing the change relation of the output active power reference value in the deceleration stage along with the real-time rotating speed; point M is (omega) min ，P MPPTmin )，ω min At the lowest rotation speed, P MPPTmin Omega on MPPT curve min The corresponding minimum output power; point C is the intersection of parabolic BM and the mechanical power curve;
s3, acceleration stage: continuously controlling the output active power reference value to reduce the active power reference value to the maximum wind energy tracking power P corresponding to the point D along the arc CA D Switching to an MPPT control mode to realize rotation speed recovery; wherein, during the acceleration phase, the active power reference value is always smaller than the mechanical power; arc CA is an arc having an intersection point with the MPPT curve except for the end point, and passing through point C and point A, for representing the point of the secondary P C Reduced to P D Outputting the change relation of the active power reference value along with the real-time rotating speed in the process; the point D is the intersection point of the arc CA and the MPPT curve except the end points;
on the parabolic BM, an active power reference value P is output ref The relation with the real-time rotation speed omega is as follows:
wherein omega C The rotation speed corresponding to the point C;
the arc line CA is a lower semicircle with the line CA as the diameter, and at the moment, an active power reference value P is output on the arc line CA ref The relation with the real-time rotation speed omega is as follows:
wherein P is 0 For the initial rotational speed omega on the MPPT curve 0 The corresponding output power; omega C The rotation speed corresponding to the point C; omega D The rotational speed corresponding to point D.
2. The power curve optimization method according to claim 1, wherein in step S1, the output active power reference value of the doubly-fed wind turbine generator is controlled to be suddenly increased to the initial rotational speed ω under the torque limit condition 0 Corresponding power limit P Tlim0 A place; wherein P is Tlim0 ＝T lim0 *ω 0 ；T lim0 Is the torque limit.
3. The power curve optimization method according to claim 1, wherein in step S3, after switching to the MPPT control mode, the output active power reference value is outputted from P along the MPPT curve D Rising to P A At this point, the doubly-fed wind turbine is again operating at point A in steady state.
4. The power curve optimization method according to claim 1, wherein before a frequency accident occurs in the power system, the doubly-fed wind turbine adopts a double closed-loop control mode of a power outer loop and a current inner loop; the double-fed wind turbine works in an MPPT control mode; the expression of the MPPT curve is as follows:
wherein P is MPPT The power of the wind energy tracking area is the maximum; ρ is the density of the fluid flowing through the fan rotor; a is that r Is the cross-sectional area through which wind energy passes; omega is the real-time rotation speed; r is the radius of the wind wheel; lambda (lambda) opt Is the tip speed ratio; c (C) pmax Is the maximum power coefficient.
5. The utility model provides a promote doubly-fed wind turbine generator system frequency modulation's power curve optimizing system which characterized in that includes: a memory storing a computer program, and a processor which when executing the computer program performs the steps of the power curve optimization method of any one of claims 1-4.
6. A machine-readable storage medium storing machine-executable instructions which, when invoked and executed by a processor, cause the processor to implement the power curve optimization method of any one of claims 1-4.
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CN110880795A (en) * 2019-11-19 2020-03-13 山东大学 Wind power frequency modulation control method and system based on overspeed fan release power boost
CN112332440A (en) * 2020-08-18 2021-02-05 华北电力大学（保定） Doubly-fed wind turbine generator inertia supporting frequency control method based on rotor kinetic energy
CN112821428A (en) * 2021-02-25 2021-05-18 南京理工大学 Wind turbine generator primary frequency modulation method based on short-time overload control

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CN110880795A (en) * 2019-11-19 2020-03-13 山东大学 Wind power frequency modulation control method and system based on overspeed fan release power boost
CN112332440A (en) * 2020-08-18 2021-02-05 华北电力大学（保定） Doubly-fed wind turbine generator inertia supporting frequency control method based on rotor kinetic energy
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