CN114123244B - Power grid frequency characteristic calculation method considering wind-storage-direct combined frequency modulation - Google Patents
Power grid frequency characteristic calculation method considering wind-storage-direct combined frequency modulation Download PDFInfo
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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
- H02J3/24—Arrangements for preventing or reducing oscillations of power in networks
- H02J3/241—The oscillation concerning frequency
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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
- H02J3/04—Circuit arrangements for ac mains or ac distribution networks for connecting networks of the same frequency but supplied from different sources
- H02J3/06—Controlling transfer of power between connected networks; Controlling sharing of load between connected networks
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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
- H02J3/28—Arrangements for balancing of the load in a network by storage of energy
- H02J3/32—Arrangements for balancing of the load in a network by storage of energy using batteries with converting means
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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
- H02J3/36—Arrangements for transfer of electric power between ac networks via a high-tension dc link
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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
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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
- H02J2203/00—Indexing scheme relating to details of circuit arrangements for AC mains or AC distribution networks
- H02J2203/20—Simulating, e g planning, reliability check, modelling or computer assisted design [CAD]
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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
- 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
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- 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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- 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
- Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
- Y02E60/60—Arrangements for transfer of electric power between AC networks or generators via a high voltage DC link [HVCD]
-
- 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
- Y02E70/00—Other energy conversion or management systems reducing GHG emissions
- Y02E70/30—Systems combining energy storage with energy generation of non-fossil origin
Abstract
The invention discloses a power grid frequency characteristic calculation method considering wind-storage-direct combined frequency modulation, which is based on a traditional thermal power, wind power, energy storage and flexible direct current transmission combined frequency modulation strategy, and comprises the steps of firstly establishing a wind-storage-direct combined frequency modulation frequency response model and analyzing and deducing frequency characteristics; secondly, calculating a frequency change time domain model, and quantitatively calculating a frequency characteristic set A of the power system under wind-storage-direct combined frequency modulation according to the frequency change time domain model, wherein the frequency characteristic set A comprises a frequency change rate, a maximum frequency deviation and a frequency peak time; and finally, calculating a frequency characteristic set B comprising a steady-state frequency error and an initial frequency change rate according to the frequency change frequency domain model. The frequency characteristic calculation method can be provided under the background that the 'double-high' characteristic of the novel power system is increasingly remarkable, and has a promotion effect on the frequency stability analysis of the power system.
Description
Technical Field
The invention belongs to the field of power system automation, and relates to a wind-storage-direct combined frequency modulation-based power grid frequency characteristic quantitative calculation method.
Background
In recent years, with the low carbonization transformation of energy structures, the access of high-proportion new energy and high-proportion power electronic equipment becomes a remarkable feature of a modern power system. On the power supply side, most of renewable energy sources represented by wind power are connected with the power electronic interface, so that the power electronic equipment is widely applied; on the power transmission side, the power transmission technologies such as flexible direct current power transmission and the like which are raised at the end of the 20 th century promote the continuous improvement of the duty ratio of power electronic equipment in a power transmission network; on the load side, a power electronic interface is often used for a new load represented by a new energy automobile. The power electronization in many aspects of source network load becomes the development trend of a modern power system, and is a great change of the power system on equipment.
However, the differentiation of the frequency modulation resource structures of the power grids in different areas makes the heterogeneity of the frequency modulation characteristics of each node more obvious in the frequency dynamic process. In a dual-high power system, the characteristics of a transmitting end and a receiving end are presented at two sides of a cross-region tie line, and the frequency dynamic process of the regional power grid is obviously different due to the difference of equivalent inertia and frequency modulation resources of the power grids at the two sides. On the other hand, the equipment adopting the power electronic converter interface lacks of the inertia of the traditional generator although the equipment can provide faster primary frequency support, and after the equipment fails, the frequency fluctuation amplitude and the frequency change rate are large, so that a series of frequency stability problems are easily caused.
At present, a frequency modulation strategy controlled by power electronic equipment provides a flexible control means for novel power system frequency modulation, but the current power system frequency situation research does not stand on quantitative calculation and joint simulation verification of frequency characteristics, so how to quantitatively analyze the frequency characteristics of the power system becomes a research hotspot. Along with the gradual perfection of a dual-high power system, the frequency characteristic calculation flow method of the system has promotion significance for the frequency stability analysis, frequency modulation unit control, resource allocation optimization and the like of the novel power system.
Disclosure of Invention
In order to solve the technical problems, the invention provides a power grid frequency characteristic calculation method considering wind-storage-direct combined frequency modulation, and solves the technical problems of lack of a power system frequency characteristic calculation method under traditional thermal power, wind power, energy storage and flexible direct current transmission combined frequency modulation.
The invention relates to a power grid frequency characteristic calculation method considering wind-storage-direct combined frequency modulation, which comprises the following steps:
step 1, establishing a wind-storage-direct combined frequency modulation power system frequency response model and calculating frequency characteristics according to a traditional thermal power, wind power, energy storage and flexible direct current transmission frequency response model;
step 2, calculating to obtain a power system frequency change time domain expression, and calculating a power system frequency characteristic set A under wind-storage-direct combined frequency modulation based on a power system frequency change time domain model, wherein the power system frequency characteristic set A comprises a frequency change rate, a maximum frequency deviation and a frequency peak time;
and step 3, obtaining a frequency variation frequency domain expression according to a frequency characteristic function of the power system, and calculating a power system frequency characteristic set B under wind-storage-direct combined frequency modulation based on a power system frequency variation S domain model, wherein the power system frequency characteristic set B comprises a steady-state frequency error and an initial frequency variation rate.
Further, in the step 1, based on the frequency response transfer function of each frequency modulation unit, providing a frequency modulation service according to each generated power, and obtaining a frequency response model of the electric power system;
the frequency response transfer function of the wind-storage-direct combined frequency modulation power system is as follows:
G system (s)＝G R (s)+G WEDC (s)
＝G R (s)+G W (s)+G E (s)+G DC (s)
wherein G is system (s) is a frequency response transfer function of the wind-storage-DC combined frequency modulation power system under the condition of no disturbance, G R (s) is a conventional thermal power frequency response transfer function, G W (s) is a wind power frequency response transfer function, G DC (s) is a flexible DC power transmission frequency response transfer function, G WEDC (s) is a frequency response transfer function under the combined frequency modulation of wind power, energy storage and flexible direct current transmission;
wherein G is WEDC (s) satisfies:
G WEDC (s)＝G W (s)+G E (s)+G DC (s)
the frequency response model of each frequency modulation unit is as follows:
a. the traditional thermal power frequency response model adopts a low-order linear transfer function:
r is primary frequency modulation sagging coefficient, delta P R For generating power increment of thermal power generating unit, F H Is the work coefficient of a high-pressure cylinder of a prime motor, T R Is the reheat time constant, k m Is a mechanical power factor; Δf is the power system frequency variation;
b. the wind power frequency additional control adopts inertia control and pitch angle control in a typical frequency control method to establish a wind power frequency response model;
the wind power inertial response transfer function is:
wherein T is ω Is the rotor inertia response time constant; k (k) df Is the inertial response coefficient; ΔP ω The amount of power variation provided for rotor inertia control;
the pitch angle control frequency response transfer function is:
wherein T is β Is a pitch response time constant; k (k) pf Is a primary frequency modulation coefficient; ΔP β Providing a power variation for pitch control;
the wind power frequency response model combines inertial response control and pitch angle control, and the frequency response transfer function of the wind turbine generator set is as follows:
ΔP W the power output variable quantity of the wind power frequency modulation unit;
c. the energy storage unit is added with a frequency additional controller at the outer ring control side of the DC/DC converter, and the energy storage frequency response transfer function is as follows:
k in E Adding control proportionality coefficient for energy storage frequency, T E For controlling the response time constant for the inner loop, ΔP E Transmitting a power variation for the energy storage system;
d. the flexible direct current transmission provides frequency support through the VSC converter station, and the flexible direct current transmission frequency response model is as follows:
wherein alpha is the droop coefficient of flexible direct current transmission frequency control, T DC Is the inertia link time constant, deltaP DC Is a flexible direct current transmission power increment.
Further, calculating frequency characteristics, wherein the frequency characteristic transfer function of the wind-storage-direct combined frequency modulation power system is as follows:
wherein m=2h, h is the system inertia constant; ΔP L The disturbance of the system is large; K. η (eta) W 、η E 、η DC The non-negative frequency modulation coefficients of the conventional unit, wind power, energy storage and flexible direct current transmission respectively meet K+eta W +η E +η DC =1; g(s) is the power system frequency transfer function under disturbance.
Further, in the step 2, a frequency change rate, a maximum frequency deviation and a peak time thereof are calculated by using a frequency change time domain model of the power system under wind-storage-direct combined frequency modulation;
1) Power system frequency-varying time domain model:
Δf(t)＝L -1 [Δf(s)]＝L -1 [G s_WEDC (s)·ΔP L (s)]
2) Deriving the frequency change deltaf (t) to obtain a frequency change rate expression:
3) Let the frequency change rate expression be zero, i.e. μ (T) =0, to obtain the maximum frequency deviation T max Peak time t of the pulse p ：
Further, in the step 3, calculating a steady-state frequency error and an initial frequency change rate by using an S domain model of power system frequency change under wind-storage-direct combined frequency modulation;
1) Obtaining a frequency variation frequency domain expression according to the frequency characteristic transfer function of the power system:
2) Deducing a frequency domain model of the frequency error of the power system under wind-storage-direct combined frequency modulation by using a final value theorem, wherein a representation formula of the steady-state frequency error containing all frequency modulation coefficients is as follows:
the expression formula of the steady-state frequency error without the traditional thermal power generating unit frequency modulation coefficient is as follows:
3) Calculating disturbance delta P of power system under wind-storage-direct combined frequency modulation by using initial value theorem L The initial change rate per unit value of the time frequency is in the form of
Actual value form:
the beneficial effects of the invention are as follows: 1) The invention takes a double-high power system as a background, establishes a frequency response model and a characteristic model of the power system under combined frequency modulation, considers the traditional thermal power unit, considers new energy wind power, energy storage and flexible direct current transmission, and solves the problem of lack of a power system frequency characteristic calculation method when load disturbance occurs under the double-high background; 2) The invention discloses a calculation method of a power system based on a frequency change time domain model and a frequency change frequency domain model under wind-storage-direct combined frequency modulation, which provides a reference for calculation and analysis of frequency characteristics of a current novel power system.
Drawings
In order that the invention may be more readily understood, a more particular description of the invention will be rendered by reference to specific embodiments that are illustrated in the appended drawings.
FIG. 1 is a flow chart of the method of the present invention;
FIG. 2 is a flow chart of calculating the frequency characteristics of the power system under joint frequency modulation based on a power system frequency variation time domain model;
FIG. 3 is a flow chart of calculating the frequency characteristics of the power system under joint frequency modulation based on the power system frequency variation S domain model;
FIG. 4 is a graph of the frequency response model of the power system for wind-storage-DC combined frequency modulation.
FIG. 5 is a graph of power system frequency variation based on the frequency variation expression (10) with 25% frequency modulation factor for thermal power, wind power, energy storage, and flexible DC power transmission;
FIG. 6 is a graph of the frequency change of the power system based on the frequency change rate expression (12) with a thermal power, wind power, energy storage and flexible DC transmission frequency modulation factor of 25%;
fig. 7 is a three-dimensional graph of the relationship between the steady-state frequency error and the additional control scaling factor of the auxiliary frequency modulation unit when the steady-state frequency error is based on the steady-state frequency error formula (14) or (15) and the thermal power, wind power, energy storage and flexible direct current transmission frequency modulation factor is 25%.
Detailed Description
The invention will be further described with reference to the accompanying drawings and specific examples.
In the embodiment, a power system frequency control model comprising power, wind power, energy storage and flexible direct current transmission is established based on Matlab/Simulink; the energy storage is connected in parallel near the wind power outlet in a centralized way and is connected into the system through the power electronic converter; the energy storage type is selected from power type energy storage, such as a super capacitor, and is characterized by shorter response time and large power ratio, so that the requirement of wind power frequency modulation is met; the flexible direct current transmission model incorporates frequency additional control in view of sag characteristics. The frequency characteristic change of the power system under different working conditions is analyzed, the load is 1000MW, the rated power of wind power is 200MW, and the load disturbance is 60MW (0.06 p.u).
As shown in fig. 1-4, the method for calculating the frequency characteristics of the power grid by considering wind-storage-direct combined frequency modulation comprises the following steps:
and step 1, establishing a wind-storage-direct combined frequency modulation power system frequency response model and calculating frequency characteristics according to a traditional thermal power, wind power, energy storage and flexible direct current transmission frequency response model.
Step 2, calculating to obtain a power system frequency change time domain expression, and calculating a power system frequency characteristic set A under wind-storage-direct combined frequency modulation based on a power system frequency change time domain model, wherein the power system frequency characteristic set A comprises a frequency change rate, a maximum frequency deviation and a frequency peak time;
and step 3, obtaining a frequency variation frequency domain expression according to a frequency characteristic function of the power system, and calculating a power system frequency characteristic set B under wind-storage-direct combined frequency modulation based on a power system frequency variation S domain model, wherein the power system frequency characteristic set B comprises a steady-state frequency error and an initial frequency variation rate.
Further, in the step 1, based on the frequency response transfer function of each frequency modulation unit, providing a frequency modulation service according to each generated power, obtaining a frequency response model of the power system and calculating frequency characteristics;
1) Wind-storage-direct joint frequency modulation power system frequency response transfer function:
wherein G is system (s) is a frequency response transfer function of the wind-storage-DC combined frequency modulation power system under the condition of no disturbance, G R (s) is a conventional thermal power frequency response transfer function, G W (s) is a wind power frequency response transfer function, G DC (s) is a flexible DC power transmission frequency response transfer function, G WEDC And(s) is a frequency response transfer function under the combined frequency modulation of wind power, energy storage and flexible direct current transmission.
Wherein G is WEDC (s) satisfies:
G WEDC (s)＝G W (s)+G E (s)+G DC (s) (2)
the frequency response model of each frequency modulation unit is as follows:
a. the traditional thermal power frequency response model adopts a low-order linear transfer function:
r is primary frequency modulation sagging coefficient, delta P R For generating power increment of thermal power generating unit, F H Is the work coefficient of a high-pressure cylinder of a prime motor, T R Is the reheat time constant, k m Is a mechanical power factor; Δf is the power system frequency variation.
b. The wind power frequency additional control adopts inertia control and pitch angle control in a typical frequency control method to establish a wind power frequency response model;
the wind power inertial response transfer function is:
wherein T is ω Is the rotor inertia response time constant; k (k) df Is the inertial response coefficient; ΔP ω The amount of power variation provided for rotor inertia control;
the pitch angle control frequency response transfer function is:
wherein T is β Is a pitch response time constant; k (k) pf Is a primary frequency modulation coefficient; ΔP β Providing a power variation for pitch control;
the wind power frequency response model combines inertial response control and pitch angle control, and the frequency response transfer function of the wind turbine generator set is as follows:
ΔP W the power output variable quantity of the wind power frequency modulation unit is obtained.
c. The energy storage unit is added with a frequency additional controller at the outer ring control side of the DC/DC converter, and the energy storage frequency response transfer function is as follows:
k in E Adding control proportionality coefficient for energy storage frequency, T E For controlling the response time constant for the inner loop, ΔP E Transmitting a power variation for the energy storage system;
d. the flexible direct current transmission provides frequency support through the VSC converter station, and the flexible direct current transmission frequency response model is as follows:
wherein alpha is the droop coefficient of flexible direct current transmission frequency control, T DC Is the inertia link time constant, deltaP DC Is a flexible direct current transmission power increment;
2) Wind-storage-direct combined frequency modulation power system frequency characteristic transfer function:
wherein m=2h, h is the system inertia constant; ΔP L The disturbance of the system is large; K. η (eta) W 、η E 、η DC Frequency modulation coefficient (non-negative) of conventional unit, wind power, energy storage and flexible direct current transmission respectively and meets K+eta W +η E +η DC =1; g(s) is the power system frequency transfer function under disturbance.
Further, in the step 2, a frequency change rate, a maximum frequency deviation and a peak time thereof are calculated by using a frequency change time domain model of the power system under wind-storage-direct combined frequency modulation;
1) Power system frequency-varying time domain model:
Δf(t)＝L -1 [Δf(s)]＝L -1 [G s_WEDC (s)·ΔP L (s)] (10)
in the example, the frequency modulation coefficients of traditional thermal power, wind power, energy storage and flexible direct current transmission are set to be 25%, the rest parameters are shown in an attached table A, and a frequency curve chart is obtained through simulation, as shown in fig. 5.
2) Deriving the frequency change deltaf (t) to obtain a frequency change rate expression:
3) Let the frequency change rate expression be zero, i.e. μ (T) =0, to obtain the maximum frequency deviation T max Peak time t of the pulse p ：
In the example, the frequency modulation coefficients of traditional thermal power, wind power, energy storage and flexible direct current transmission are set to be 25%, the rest parameters are shown in the attached table A, a frequency change rate simulation diagram is obtained through simulation, and the maximum frequency deviation and the peak time can be known from the diagram, as shown in fig. 6.
Table A system parameter set point
Further, in the step 3, calculating a steady-state frequency error and an initial frequency change rate by using an S domain model of power system frequency change under wind-storage-direct combined frequency modulation;
1) Obtaining a frequency variation frequency domain expression according to the frequency characteristic transfer function of the power system:
2) Deducing a frequency domain model of the power system frequency error under wind-storage-direct combined frequency modulation by using a final value theorem, wherein the representation modes are as follows:
wherein, the formula (14) is a steady-state frequency error containing all frequency modulation coefficients, and the formula (15) is a steady-state frequency error without the frequency modulation coefficients of the traditional thermal power generating unit; if only the relation between the frequency modulation coefficient of the irregular unit and the steady-state frequency error is examined, the formula (15) is selected, otherwise, the formula (14) is adopted.
In the example, the frequency modulation coefficients of traditional thermal power, wind power, energy storage and flexible direct current transmission are set to be 25%, the other parameters are shown in the attached table A, the quantitative relation between the steady-state frequency error and the control proportion coefficient of the auxiliary frequency modulation unit is simulated based on the formula (14), and the result is shown in figure 7.
3) Calculating disturbance delta P of power system under wind-storage-direct combined frequency modulation by using initial value theorem L The initial change rate per unit value of the time frequency is in the form of
Actual value form:
in the example, the frequency modulation coefficients of traditional thermal power, wind power, energy storage and flexible direct current transmission are set to be 25%, the rest parameters are shown in the attached table A, the power disturbance is-0.06 (p.u), and the initial change rate of the obtained frequency according to the formula (17) is-3 Hz s -1 The frequency change rate at time t=0 in fig. 6 is the same as the derived value.
The foregoing is merely a preferred embodiment of the present invention, and is not intended to limit the present invention, and all equivalent variations using the description and drawings of the present invention are within the scope of the present invention.
Claims (3)
1. A power grid frequency characteristic calculation method considering wind-storage-direct combined frequency modulation is characterized by comprising the following steps:
step 1, establishing a wind-storage-direct combined frequency modulation power system frequency response model and calculating frequency characteristics according to a traditional thermal power, wind power, energy storage and flexible direct current transmission frequency response model;
step 2, calculating to obtain a power system frequency change time domain expression, and calculating a power system frequency characteristic set A under wind-storage-direct combined frequency modulation based on a power system frequency change time domain model, wherein the power system frequency characteristic set A comprises a frequency change rate, a maximum frequency deviation and a frequency peak time; the method comprises the following steps:
1) Power system frequency-varying time domain model:
Δf(t)＝L -1 [Δf(s)]＝L -1 [G s_WEDC (s)·ΔP L (s)]
2) Deriving the frequency change deltaf (t) to obtain a frequency change rate expression:
3) Let the frequency change rate expression be zero, i.e. μ (T) =0, to obtain the maximum frequency deviation T max Peak time t of the pulse p ：
Step 3, obtaining a frequency variation frequency domain expression according to a frequency characteristic function of the power system, and calculating a power system frequency characteristic set B under wind-storage-direct combined frequency modulation based on a power system frequency variation S domain model, wherein the power system frequency characteristic set B comprises a steady-state frequency error and an initial frequency variation rate and specifically comprises the following steps:
1) Obtaining a frequency variation frequency domain expression according to the frequency characteristic transfer function of the power system:
2) Deducing a frequency domain model of the frequency error of the power system under wind-storage-direct combined frequency modulation by using a final value theorem, wherein a representation formula of the steady-state frequency error containing all frequency modulation coefficients is as follows:
the expression formula of the steady-state frequency error without the traditional thermal power generating unit frequency modulation coefficient is as follows:
3) Calculating disturbance delta P of power system under wind-storage-direct combined frequency modulation by using initial value theorem L The initial change rate per unit value of the time frequency is in the form of
Actual value form:
2. the method for calculating the frequency characteristics of the power grid considering wind-storage-direct combined frequency modulation according to claim 1, wherein in the step 1, frequency modulation services are provided according to respective generated power based on frequency response transfer functions of all frequency modulation units to obtain a frequency response model of the power system;
the frequency response transfer function of the wind-storage-direct combined frequency modulation power system is as follows:
G system (s)＝G R (s)+G WEDC (s)
＝G R (s)+G W (s)+G E (s)+G DC (s)
wherein G is system (s) is a frequency response transfer function of the wind-storage-DC combined frequency modulation power system under the condition of no disturbance, G R (s) is a conventional thermal power frequency response transfer function, G W (s) is a wind power frequency response transfer function, G DC (s)Is a flexible DC power transmission frequency response transfer function, G WEDC (s) is a frequency response transfer function under the combined frequency modulation of wind power, energy storage and flexible direct current transmission;
wherein G is WEDC (s) satisfies:
G WEDC (s)＝G W (s)+G E (s)+G DC (s)
the frequency response model of each frequency modulation unit is as follows:
a. the traditional thermal power frequency response model adopts a low-order linear transfer function:
r is primary frequency modulation sagging coefficient, delta P R For generating power increment of thermal power generating unit, F H Is the work coefficient of a high-pressure cylinder of a prime motor, T R Is the reheat time constant, k m Is a mechanical power factor; Δf is the power system frequency variation;
b. the wind power frequency additional control adopts inertia control and pitch angle control in a typical frequency control method to establish a wind power frequency response model;
the wind power inertial response transfer function is:
wherein T is ω Is the rotor inertia response time constant; k (k) df Is the inertial response coefficient; ΔP ω The amount of power variation provided for rotor inertia control;
the pitch angle control frequency response transfer function is:
wherein T is β Is a pitch response time constant; k (k) pf Is a primary frequency modulation coefficient; ΔP β Providing a power variation for pitch control;
the wind power frequency response model combines inertial response control and pitch angle control, and the frequency response transfer function of the wind turbine generator set is as follows:
ΔP W the power output variable quantity of the wind power frequency modulation unit;
c. the energy storage unit is added with a frequency additional controller at the outer ring control side of the DC/DC converter, and the energy storage frequency response transfer function is as follows:
k in E Adding control proportionality coefficient for energy storage frequency, T E For controlling the response time constant for the inner loop, ΔP E Transmitting a power variation for the energy storage system;
d. the flexible direct current transmission provides frequency support through the VSC converter station, and the flexible direct current transmission frequency response model is as follows:
wherein alpha is the droop coefficient of flexible direct current transmission frequency control, T DC Is the inertia link time constant, deltaP DC Is a flexible direct current transmission power increment.
3. The method for calculating the frequency characteristics of the power grid considering wind-storage-direct combined frequency modulation according to claim 2, wherein the frequency characteristics are calculated, and the frequency characteristic transfer function of the wind-storage-direct combined frequency modulation power system is as follows:
wherein m=2h, h is the system inertia constant; ΔP L The disturbance of the system is large; K. η (eta) W 、η E 、η DC The non-negative frequency modulation coefficients of the conventional unit, wind power, energy storage and flexible direct current transmission respectively meet K+eta W +η E +η DC =1; g(s) is the power system frequency transfer function under disturbance.
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