CN113193598B - New energy grid-connected inverter transient stability margin evaluation method - Google Patents
New energy grid-connected inverter transient stability margin evaluation method 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
- H02J3/38—Arrangements for parallely feeding a single network by two or more generators, converters or transformers
- H02J3/381—Dispersed generators
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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
- H02J3/001—Methods to deal with contingencies, e.g. abnormalities, faults or failures
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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
- H02J2300/22—The renewable source being solar energy
- H02J2300/24—The renewable source being solar energy of photovoltaic origin
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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
Abstract
The invention discloses a new energy grid-connected inverter transient stability margin evaluation method, which is used for converting a three-phase voltage signal of a grid-connected point to obtain two-phase direct current quantityU td 、U tq And will beU tq As an input signal for a phase locked loop; sequentially calculating the output angular frequency of the phase-locked loop and the transient stability margin of the inverter switched from the normal operation stage to the fault continuation stage、Equivalent limit cut angle、The equivalent limit removal time is compared with the actual fault detection delay, and whether the fault is unstable or not can be judged by switching from the normal operation stage to the fault continuous stage; and if the transient state is not unstable, calculating the transient state stability margin in the fault continuous stage and judging whether the transient state is unstable or not. And after the fault is cleared, calculating the transient stability margin switched from the fault continuous stage to the fault recovery stage and judging whether the transient stability margin is unstable or not. The method can judge whether the new energy grid-connected inverter has transient instability in different fault stages and quantitatively calculate the transient stability margin value.
Description
Technical Field
The invention relates to a transient stability margin evaluation method for a new energy grid-connected inverter, which is suitable for the new energy grid-connected inverter under the symmetrical short-circuit fault of an alternating current power grid.
Background
With the rapid increase of installed capacity of power generation of new energy sources such as wind energy, photovoltaic and the like, the permeability level of the new energy sources is continuously improved, but the new energy sources and loads are often in a serious reverse distribution characteristic, and a large-scale new energy power generation base is usually located in a remote area and has small short-circuit capacity. When an alternating current power grid has a serious short-circuit fault, the new energy grid-connected inverter is very likely to have a transient instability phenomenon, even a large-scale new energy base is disconnected, and the safe and stable operation of the power grid is seriously influenced. Therefore, the problem that the transient stability margin of the new energy grid-connected inverter during the short-circuit fault of the power grid can be quantitatively evaluated is a key problem in the development of the new energy power generation technology at present. Currently, scholars at home and abroad have carried out a series of related researches, such as the following published documents:
[1] the transient stability analysis of the whole power conversion wind power generation set is [ J ] in China Motor engineering, 2017,37(14): 4018-.
[2]Jinxin Pei,JunYao,Ruikuo Liu,et al.Characteristic analysis andrisk assessment for Voltage-frequency coupled transient instability oflarge-scale grid-connected renewable energy plants during LVRT[J].IEEE Transactions on Industrial Electronics,2020,67(07):5515-5530。
The literature [1] analyzes the influence of factors such as power grid strength, fault point port characteristics and phase-locked loop bandwidth on the transient stability margin of the new energy grid-connected inverter by using an equal area criterion, and provides a method for improving the transient stability margin of the new energy grid-connected inverter by reducing the phase-locked loop bandwidth, but the literature does not quantize the transient stability margin. Document [2] utilizes a vector diagram analysis method to analyze the power grid voltage drop degree, an active reactive current instruction of a wind power grid-connected system and transmission line impedance from the perspective of voltage vector amplitude-frequency coupling, which are main reasons influencing transient synchronization stability of the wind power grid-connected system, and provides a method for evaluating a line resistance pressure drop angle margin, which quantitatively evaluates the transient stability margin of the system, but the method only aims at a fault duration stage and is difficult to evaluate the transient synchronization stability of the fault full stage. Actually, the transient response of the new energy grid-connected inverter in the whole fault process can be divided into different stages, and the transient stability margin of each stage needs to be quantitatively evaluated.
Disclosure of Invention
In view of the above disadvantages in the prior art, an object of the present invention is to provide a transient stability margin evaluation method for a new energy grid-connected inverter at different fault stages, where the method can determine whether a transient instability of the new energy grid-connected inverter occurs at different fault stages, and quantitatively calculate a transient stability margin value of the new energy grid-connected inverter.
The technical scheme of the invention is realized as follows:
a new energy grid-connected inverter transient stability margin evaluation method is used for judging the transient stability of the new energy grid-connected inverter in different fault stages, and is characterized in that: the method comprises the following specific steps:
A1) the new energy grid-connected inverter adopts a generator convention, and utilizes a grid voltage d-axis orientation mode to collect a grid-connected point three-phase voltage signal U t Converting the three-phase static coordinate system into a two-phase rotating coordinate system to obtain two-phase direct current quantity U td 、U tq ；
A2) Will U tq As an input signal of the phase-locked loop, the phase-locked loop output angular frequency ω is calculated according to the following formula PLL ：
ω PLL ＝k pp U tq +k ip ∫U tq dt+ω g
Wherein k is pp And k ip Proportional and integral coefficients, omega, of the phase-locked loop, respectively g Is the grid angular frequency rating;
A3) on the basis of the step A2), quantifying the transient stability margin of the new energy grid-connected inverter when the new energy grid-connected inverter is switched from the normal operation stage to the fault continuation stage according to the following formula:
wherein S is acc1 、S dec1 And
respectively representing the equivalent acceleration area, the equivalent deceleration area and the equivalent energy consumption of the new energy grid-connected inverter switched from the normal operation stage to the fault continuation stage; delta d axis of phase-locked loop coordinate system and power grid voltage U g The phase angle difference of (1), namely the equivalent power angle; delta. for the preparation of a coating 0 Is the equivalent power angle, delta, of the new energy grid-connected inverter at the initial moment of the fault detection stage 1 Is the equivalent power angle, delta, of the new energy grid-connected inverter at the initial moment of the fault duration stage max1 The upper limit value omega of the equivalent power angle of the new energy grid-connected inverter in the fault continuation stage b Is the angular frequency base value, U gf Is the voltage amplitude of the grid fault point, L g And R g Respectively the equivalent inductance and resistance of the point-to-point transmission line,
and
respectively an active current instruction and a reactive current instruction during low voltage ride through of the new energy grid-connected inverter, I dm For the current limiting value, S, of the grid-side converter (GSC) of the new energy grid-connected inverter nm The value is a transient stability margin evaluation value of switching the new energy grid-connected inverter from a normal operation stage to a fault continuation stage;
A4) according to the step A3), calculating the equivalent limit cut-off angle of the new energy grid-connected inverter according to the following formula
A5) Obtained according to step A4)
Calculating new energy grid-connected inverse according to the following formulaFault detection limit times of converters, i.e. equivalent limit cut-off times
A6) The transient stability criterion for switching the new energy grid-connected inverter from the normal operation stage to the fault continuation stage is as follows:
wherein, t delay Delay for actual fault detection, if t delay When the stability criterion is not met, the new energy grid-connected inverter generates a transient instability phenomenon;
A7) on the basis that the new energy grid-connected inverter meets the stability criterion obtained in the step A6), calculating the transient stability margin of the new energy grid-connected inverter in the fault continuation stage according to the following formula:
wherein S is acc2 、S dec2 And
respectively representing the equivalent acceleration area, the equivalent deceleration area and the equivalent energy consumption of the new energy grid-connected inverter in the fault continuation stage; delta 2 Is a fault continuation phase omega PLL And omega g When the equivalent power angle is equal, the equivalent power angle of the new energy grid-connected inverter is obtained; delta min1 The lower limit value delta of the equivalent power angle of the new energy grid-connected inverter in the fault continuation stage k An equivalent power angle S corresponding to a balance working point of the new energy grid-connected inverter in the fault continuation stage dm Transient stability of new energy grid-connected inverter in fault continuation stageMargin evaluation value, S dm The larger the system stability margin is, the higher the transient stability is;
A8) the transient stability criterion of the new energy grid-connected inverter in the fault continuation stage is as follows:
when the new energy grid-connected inverter meets the stability criterion, the new energy grid-connected inverter can smoothly reach and stably operate in a low-voltage ride-through control mode in a fault continuous stage;
A9) after the fault is cleared, the transient stability margin of the new energy grid-connected inverter switched from the fault continuous stage to the fault recovery stage can be quantified according to the following formula:
wherein S is acc3_4 、S dec4 And
respectively representing the equivalent acceleration area, the equivalent deceleration area and the equivalent energy consumption of the new energy grid-connected inverter switched from the fault continuous stage to the fault recovery stage; delta 4 Is the equivalent initial power angle, delta, of the new energy grid-connected inverter in the fault recovery stage e Is the equivalent power angle delta corresponding to the balance working point of the new energy grid-connected inverter in the fault recovery stage max2 An upper limit value U of an equivalent power angle of the new energy grid-connected inverter at a fault recovery stage gn For mains voltage rating, U tdn To a grid point voltage U t The d-axis component of the nominal value,
a power command value for a fault recovery phase; s. the rm The value is a transient stability margin evaluation value S of switching the new energy grid-connected inverter from a fault continuous stage to a fault recovery stage rm Larger, new energy grid-connected inverterThe larger the transient stability margin is, the smaller the instability risk is;
A10) the transient stability criterion for switching the new energy grid-connected inverter from the fault continuous stage to the fault recovery stage is as follows:
when the new energy grid-connected inverter meets the stability criterion, the new energy grid-connected inverter can be successfully recovered to a stable operation state after the fault is cleared.
Compared with the prior art, the invention has the following beneficial effects:
the transient stability margin value of the new energy grid-connected inverter can be quantitatively calculated according to the power grid fault degree, the control characteristics and the operation condition of the new energy grid-connected inverter and the impedance of a power transmission line, can be used for transient instability criterion and transient stability margin evaluation of the new energy grid-connected inverter in different fault stages, and provides guiding suggestions for large-scale development of the new energy grid-connected inverter.
Drawings
Fig. 1 is a schematic diagram of fault stage division of a new energy grid-connected inverter.
Fig. 2 is a diagram illustrating a control structure of a typical pll.
The time domain simulation results of the new energy grid-connected inverter when the fault detection delay is 5ms and 10ms are respectively given by the working condition 1 and the working condition 2 in fig. 3.
The current commands are given in condition 3 and condition 4 of fig. 4, respectively
And
and time domain simulation results of the new energy grid-connected inverter.
The power is given in the case of operating conditions 5 and 6 of fig. 5, respectivelyInstructions
And
and time domain simulation results of the new energy grid-connected inverter.
Detailed Description
The following detailed description of specific embodiments of the invention refers to the accompanying drawings.
Fig. 1 is a schematic diagram of fault stage division of a new energy grid-connected inverter. Fig. 2 is a diagram illustrating a control structure of a typical pll.
The method comprises the following specific implementation steps:
A1) the new energy grid-connected inverter adopts a generator routine and utilizes a grid voltage d-axis orientation mode to acquire a grid-connected point three-phase voltage signal U t Converting the three-phase static coordinate system into a two-phase rotating coordinate system to obtain two-phase direct current quantity U td 、U tq ；
A2) Will U tq As an input signal of the phase-locked loop, the phase-locked loop output angular frequency ω is calculated according to the following formula PLL ：
ω PLL ＝k pp U tq +k ip ∫U tq dt+ω g
Wherein k is pp And k ip Proportional and integral coefficients, omega, respectively, of the phase-locked loop g Is the grid angular frequency rating;
A3) on the basis of the step A2), quantifying the transient stability margin of the new energy grid-connected inverter switched from the normal operation stage to the fault continuation stage according to the following formula:
wherein S is acc1 、S dec1 And
respectively characterize the new energyThe source grid-connected inverter is switched to the equivalent acceleration area, the equivalent deceleration area and the equivalent energy consumption of the fault continuation stage from the normal operation stage; delta d axis of phase-locked loop coordinate system and power grid voltage U g The phase angle difference of (2), namely the equivalent power angle; delta 0 Is the equivalent power angle, delta, of the new energy grid-connected inverter at the initial moment of the fault detection stage 1 Is the equivalent power angle, delta, of the new energy grid-connected inverter at the initial moment of the fault continuation stage max1 The upper limit value omega of the equivalent power angle of the new energy grid-connected inverter in the fault continuation stage b Is the angular frequency base value, U gf Is the voltage amplitude of the grid fault point, L g And R g Respectively the equivalent inductance and resistance of the point-to-point transmission line,
and
respectively an active current instruction and a reactive current instruction during low voltage ride through of the new energy grid-connected inverter, I dm For the current limiting value, S, of the grid-side converter (GSC) of the new energy grid-connected inverter nm The transient stability margin evaluation value is used for switching the new energy grid-connected inverter from a normal operation stage to a fault continuation stage;
A4) according to the step A3), calculating the equivalent limit cut-off angle of the new energy grid-connected inverter according to the following formula
A5) Obtained according to step A4)
Calculating the fault detection limit time of the new energy grid-connected inverter according to the formula, namely equivalent limit cutting time
A6) The transient stability criterion for switching the new energy grid-connected inverter from the normal operation stage to the fault continuation stage is as follows:
wherein, t delay Delays for actual fault detection; if t delay The stability criterion is not met, and the new energy grid-connected inverter generates a transient instability phenomenon;
A7) on the basis that the new energy grid-connected inverter meets the stability criterion obtained in the step A6), calculating the transient stability margin of the new energy grid-connected inverter in the fault continuation stage according to the following formula:
wherein S is acc2 、S dec2 And
respectively representing the equivalent acceleration area, the equivalent deceleration area and the equivalent energy consumption of the new energy grid-connected inverter in the fault continuation stage; delta 2 Is a fault continuation phase omega PLL And omega g When the equivalent power angle is equal, the equivalent power angle of the new energy grid-connected inverter is obtained; delta min1 The lower limit value delta of the equivalent power angle of the new energy grid-connected inverter in the fault continuation stage k An equivalent power angle S corresponding to a balanced working point of the new energy grid-connected inverter in the fault continuation stage dm The value is the transient stability margin evaluation value S of the new energy grid-connected inverter in the fault continuation stage dm The larger the system stability margin is, the higher the transient stability is;
A8) the transient stability criterion of the new energy grid-connected inverter in the fault continuation stage is as follows:
when the new energy grid-connected inverter meets the stability criterion, the new energy grid-connected inverter can smoothly reach and stably operate in a low-voltage ride-through control mode in a fault continuous stage;
A9) after the fault is cleared, the transient stability margin of the new energy grid-connected inverter switched from the fault continuous stage to the fault recovery stage can be quantified according to the following formula:
wherein S is acc3_4 、S dec4 And
respectively representing the equivalent acceleration area, the equivalent deceleration area and the equivalent energy consumption of the new energy grid-connected inverter switched from the fault continuous stage to the fault recovery stage; delta 4 Is the equivalent initial power angle, delta, of the new energy grid-connected inverter in the fault recovery stage e Is the equivalent power angle, delta, corresponding to the balance working point of the new energy grid-connected inverter in the fault recovery stage max2 An upper limit value U of an equivalent power angle of the new energy grid-connected inverter at the fault recovery stage gn For mains voltage rating, U tdn To a grid point voltage U t The d-axis component of the nominal value,
is the power command value of the fault recovery stage; s. the rm The value is the transient stability margin evaluation value S of the new energy grid-connected inverter switched from the fault continuous stage to the fault recovery stage rm The larger the transient stability margin of the new energy grid-connected inverter is, the smaller the instability risk is;
A10) the transient stability criterion for switching the new energy grid-connected inverter from the fault continuous stage to the fault recovery stage is as follows:
when the new energy grid-connected inverter meets the stability criterion, the new energy grid-connected inverter can be successfully recovered to a stable operation state after the fault is cleared.
Description of the effects of the invention:
the time domain simulation results of the new energy grid-connected inverter when the fault detection delay is 5ms and 10ms are respectively given by the working condition 1 and the working condition 2 in fig. 3. When the voltage level of the power grid drops to 0.3p.u., the voltage level is calculated
As shown in condition 1 of FIG. 3, if the fault detection delay is 5ms, that is, the time delay is set to be less than the threshold
The transient stability criterion of switching the new energy grid-connected inverter from the normal operation stage to the fault continuation stage is met, the new energy grid-connected inverter can smoothly complete the stable transition of switching the new energy grid-connected inverter from the normal operation stage to the fault continuation stage, and the new energy grid-connected inverter enters a low-voltage ride-through mode; when the fault detection delay is increased to 10ms, i.e.
In the meantime, the transient stability criterion that the new energy grid-connected inverter is switched from the normal operation stage to the fault continuation stage is not met, and as shown in the working condition 2 of fig. 3, the new energy grid-connected inverter generates a frequency-raising step-out instability phenomenon in the process of entering the low voltage ride through mode.
FIG. 4 shows the current commands respectively
And
and time domain simulation results of the new energy grid-connected inverter. When electricity is generatedGrid voltage level drops to 0.3p.u., current command
Then, S is obtained by calculation dm ＝0.0312>0, meeting the transient stability criterion of the new energy grid-connected inverter in the fault continuation stage, as shown in the working condition 3 of fig. 4, the new energy grid-connected inverter can smoothly complete low voltage ride through in the fault continuation stage, and transient stable operation is realized; when the current is instructed
Then, calculating to obtain S dm ＝-0.8886<And 0, the transient stability criterion of the new energy grid-connected inverter in the fault continuation stage is not met, as shown in the working condition 4 of fig. 4, the new energy grid-connected inverter generates the phenomena of frequency reduction, step loss and instability in the fault continuation stage, and the low-voltage ride-through fails.
FIG. 5 shows that the power commands are respectively
And
and time domain simulation results of the new energy grid-connected inverter. When the voltage level of the power grid drops to 0.3p.u., a power instruction
Then, calculating to obtain S rm ＝0.1765>0, meeting a transient stability criterion that the new energy grid-connected inverter is switched from a fault continuation stage to a fault recovery stage, as shown in a working condition 5 of fig. 5, the new energy grid-connected inverter can be smoothly recovered to a stable operation state after the fault is cleared; when power command
Then, S is obtained by calculation rm ＝-0.3907<0, failure-free new energy grid-connected inverter slave fault supportAs shown in a working condition 6 of fig. 5, the transient stability criterion for switching from the continuous stage to the fault recovery stage is that the wind power grid-connected system has the frequency-increasing, step-losing and instability phenomenon in the power recovery process, and cannot be recovered to a rated operation state.
The method can judge whether the new energy grid-connected inverter has transient instability at different fault stages according to the grid voltage drop degree, controller delay, converter capacity limit of the new energy grid-connected inverter, active and reactive current instructions of the new energy grid-connected inverter at the fault continuation stage, power instructions of the new energy grid-connected inverter at the fault recovery stage, phase-locked loop parameters and transmission line impedance, quantitatively calculates the transient stability margin value of the new energy grid-connected inverter, and provides quantitative evaluation basis for the transient synchronization stability of the new energy grid-connected inverter.
Finally, it should be noted that the above-mentioned examples of the present invention are only examples for illustrating the present invention, and are not intended to limit the embodiments of the present invention. While the invention has been described in detail with reference to preferred embodiments, it will be apparent to those skilled in the art that various changes and modifications can be made therein without departing from the spirit and scope of the invention. It is not exhaustive here for all embodiments. All obvious changes and modifications of the present invention are within the scope of the present invention.
Claims (1)
1. A new energy grid-connected inverter transient stability margin evaluation method is used for judging the transient stability of the new energy grid-connected inverter in different fault stages, and is characterized in that: the method comprises the following specific steps:
A1) the new energy grid-connected inverter adopts a generator routine and utilizes a grid voltage d-axis orientation mode to acquire a grid-connected point three-phase voltage signal U t Converting the three-phase static coordinate system into a two-phase rotating coordinate system to obtain two-phase direct current quantity U td 、U tq ；
A2) Will U tq As an input signal of the phase-locked loop, the phase-locked loop output angular frequency ω is calculated according to the following formula PLL ：
ω PLL ＝k pp U tq +k ip ∫U tq dt+ω g
Wherein k is pp And k ip Proportional and integral coefficients, omega, respectively, of the phase-locked loop g Is the grid angular frequency rating;
A3) on the basis of the step A2), quantifying the transient stability margin of the new energy grid-connected inverter switched from the normal operation stage to the fault continuation stage according to the following formula:
wherein S is acc1 、S dec1 And Δ E DⅠ Respectively representing the equivalent acceleration area, the equivalent deceleration area and the equivalent energy consumption of the new energy grid-connected inverter switched from the normal operation stage to the fault continuation stage; delta d axis of phase-locked loop coordinate system and power grid voltage U g The phase angle difference of (2), namely the equivalent power angle; delta 0 Is the equivalent power angle, delta, of the new energy grid-connected inverter at the initial moment of the fault detection stage 1 Is the equivalent power angle, delta, of the new energy grid-connected inverter at the initial moment of the fault duration stage max1 The upper limit value omega of the equivalent power angle of the new energy grid-connected inverter in the fault continuation stage b Is the angular frequency base value, U gf Is the voltage amplitude of the grid fault point, L g And R g Respectively the equivalent inductance and resistance of the point-to-point transmission line,
and
respectively an active current instruction and a reactive current instruction during low voltage ride through of the new energy grid-connected inverter, I dm For the current amplitude limiting value S of the network side converter of the new energy grid-connected inverter nm Value as new energyThe transient stability margin evaluation value of the grid-connected inverter is switched from a normal operation stage to a fault continuation stage;
A4) according to the step A3), calculating the equivalent limit cut-off angle of the new energy grid-connected inverter according to the following formula
A5) Obtained according to step A4)
Calculating the fault detection limit time of the new energy grid-connected inverter according to the formula, namely equivalent limit cutting time
A6) The transient stability criterion for switching the new energy grid-connected inverter from the normal operation stage to the fault continuation stage is as follows:
wherein, t delay Delay for actual fault detection, if t delay The stability criterion is not met, and the new energy grid-connected inverter generates a transient instability phenomenon;
A7) on the basis that the new energy grid-connected inverter meets the stability criterion obtained in the step A6), calculating the transient stability margin of the new energy grid-connected inverter in the fault continuation stage according to the following formula:
wherein S is acc2 、S dec2 And Δ E DⅡ Respectively representing the equivalent acceleration area, the equivalent deceleration area and the equivalent energy consumption of the new energy grid-connected inverter in the fault continuation stage; delta. for the preparation of a coating 2 Is a fault continuation phase omega PLL And omega g When the equivalent power angle is equal, the equivalent power angle of the new energy grid-connected inverter is obtained; delta min1 The lower limit value delta of the equivalent power angle of the new energy grid-connected inverter in the fault continuation stage k An equivalent power angle S corresponding to a balance working point of the new energy grid-connected inverter in the fault continuation stage dm The value is the transient stability margin evaluation value S of the new energy grid-connected inverter in the fault continuation stage dm The larger the stability margin of the system is, the higher the transient stability is;
A8) the transient stability criterion of the new energy grid-connected inverter in the fault continuation stage is as follows:
when the new energy grid-connected inverter meets the stability criterion, the new energy grid-connected inverter can smoothly reach and stably operate in a low-voltage ride-through control mode in a fault continuous stage;
A9) after the fault is cleared, the transient stability margin of the new energy grid-connected inverter switched from the fault continuous stage to the fault recovery stage can be quantified according to the following formula:
wherein S is acc3_4 、S dec4 And Δ E DⅢ Respectively representing the equivalent acceleration area, the equivalent deceleration area and the equivalent energy consumption of the new energy grid-connected inverter switched from the fault continuous stage to the fault recovery stage; delta 4 Is equivalent to a new energy grid-connected inverter in the fault recovery stageInitial power angle, delta e Is the equivalent power angle, delta, corresponding to the balance working point of the new energy grid-connected inverter in the fault recovery stage max2 An upper limit value U of an equivalent power angle of the new energy grid-connected inverter at the fault recovery stage gn For mains voltage rating, U tdn To a grid point voltage U t The d-axis component of the nominal value,
is the power command value of the fault recovery stage; s rm The value is a transient stability margin evaluation value S of switching the new energy grid-connected inverter from a fault continuous stage to a fault recovery stage rm The larger the transient stability margin of the new energy grid-connected inverter is, the smaller the instability risk is;
A10) the transient stability criterion for switching the new energy grid-connected inverter from the fault continuous stage to the fault recovery stage is as follows:
when the new energy grid-connected inverter meets the stability criterion, the new energy grid-connected inverter can be successfully recovered to a stable operation state after the fault is cleared.
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