CN107887902A - Inverter stability analysis method and system when distal end fatal voltage falls in weak net - Google Patents

Inverter stability analysis method and system when distal end fatal voltage falls in weak net Download PDF

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CN107887902A
CN107887902A CN201711001812.8A CN201711001812A CN107887902A CN 107887902 A CN107887902 A CN 107887902A CN 201711001812 A CN201711001812 A CN 201711001812A CN 107887902 A CN107887902 A CN 107887902A
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mrow
inverter
msub
inverter system
stability
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CN107887902B (en
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徐谦
辛焕海
戴攀
章枫
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Zhejiang University ZJU
State Grid Corp of China SGCC
Economic and Technological Research Institute of State Grid Zhejiang Electric Power Co Ltd
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Zhejiang University ZJU
State Grid Corp of China SGCC
Economic and Technological Research Institute of State Grid Zhejiang Electric Power Co Ltd
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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
    • 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
    • H02J3/12—Circuit arrangements for ac mains or ac distribution networks for adjusting voltage in ac networks by changing a characteristic of the network load
    • H02J3/16—Circuit arrangements for ac mains or ac distribution networks for adjusting voltage in ac networks by changing a characteristic of the network load by adjustment of reactive power
    • 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
    • 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]
    • 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
    • Y02E40/00—Technologies for an efficient electrical power generation, transmission or distribution
    • Y02E40/30—Reactive power compensation

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  • Engineering & Computer Science (AREA)
  • Power Engineering (AREA)
  • Inverter Devices (AREA)

Abstract

Inverter stability analysis method and system when falling the invention discloses distal end fatal voltage in a kind of weak net.The present invention is equivalent by AC network Dai Weinan, and the inverter system quasi steady state model of typical low voltage crossing control strategy is considered during establishing failure；It whether there is equalization point using phase plane method analysis system, establish inverter system stability criterion under quasi steady state model；In the case of it equalization point be present, system small-signal model analysis system small signal stability is established；Stability criterion and analysis on Small Disturbance Stability are finally combined, judges whether inverter system is stablized when distal end fatal voltage falls.The inventive method accesses the inverter instability Mechanism of high induction reactance power network when falling respectively from big interference and small interference angle parsing distal end fatal voltage, it is theoretical clear, can be applied to using inverter as interface new energy power station grid-connected system distal end occur fatal voltage failure when, inverter system stability analysis.

Description

Inverter stability analysis method and system when distal end fatal voltage falls in weak net
Technical field
The present invention relates to power equipment inverter, inverter when distal end fatal voltage falls in specifically a kind of weak net Method for analyzing stability and system.
Background technology
As the renewable energy power generations such as photovoltaic, wind energy largely access power system, both at home and abroad to renewable energy power generation Grid-connected proposition should possess low voltage ride-through capability.In addition, during Voltage Drop, grid-connected directive/guide requires wind power plant, photovoltaic plant With the proportional reactive current of Voltage Drop to support line voltage, residual capacity is then sent in the form of active for injection, It is stable that system frequency is influenceed to prevent power network from big active vacancy occur.
Grid-connected directive/guide is generally used as premise by strong net (i.e. circuit induction reactance value is smaller) of AC network, it is desirable to which inverter possesses Certain low voltage ride-through capability.However, China's regenerative resource grid-connected the characteristics of remote, extensive, high concentration is presented, electricity Net side is usually light current net (i.e. circuit induction reactance value is larger).Different from Voltage Drop failure at near-end points of common connection in strong net, In light current net when Voltage Drop failure occurs in distal end grid side, influenceed by the high induction reactance value of circuit, inverter end voltage falls Fall smaller, typical low voltage crossing control strategy there may be inapplicable risk so that inverter has unstability new problem.Close Small interference and Large disturbance stability problem are still needed and continue deeper into research during inverter low-voltage crossing in high induction reactance power network.
At present, the research on inverter low voltage crossing process is concentrated mainly on Small signal stability analysis, in small interference In terms of stability analysis, on considering that inverter end voltage is dynamically introduced to the small interference of system in inverter low-voltage control strategy There is not been reported for the article of stability influence；Research in terms of Large disturbance stability is relatively fewer, and these researchs all have ignored height When induction reactance power network distal end fatal voltage falls (here it is considered that Voltage Drop falls to below 0.2pu for fatal voltage), typical case is low Inverter Large disturbance stability problem caused by voltage ride-through control strategy is possible.
The content of the invention
To solve the above-mentioned problems of the prior art, the present invention provides inverse when distal end fatal voltage falls in a kind of weak net Become device method for analyzing stability, it can clearly explain inverter instability Mechanism when distal end fatal voltage falls, i.e., respectively from big dry Disturb the stable and aspect of small interference stability two and analyze stability during fault of dc-to-ac converter system.
The technical solution adopted by the present invention is as follows：Inverter stability analysis side when distal end fatal voltage falls in weak net Method, including：
AC network is subjected to Dai Weinan equivalent process, typical low voltage crossing control strategy is considered during establishing failure Inverter system quasi steady state model；
It whether there is equalization point using phase plane method analysis inverter system, establish inverter system quasi steady state model subinverse Become device system stability criterion；
In the case that inverter system has equalization point during failure, system small-signal model analysis inverter is established System small signal stability；
With reference to the presence or absence of equalization point, stability criterion and analysis on Small Disturbance Stability, judge that inverter system is tight in distal end Whether stablize during weight Voltage Drop.
As the supplement of above-mentioned technical proposal, described inverter system quasi steady state model includes network side quasi steady state model f(Id,Vt)=0 and low voltage crossing control quasi steady state model g (Id,Vt)=0, IdRepresent filter inductance d shaft currents, VtRepresent inverse Become device terminal voltage.
As the further supplement of above-mentioned technical proposal, described network side quasi steady state model f (Id,Vt)=0 is expressed as：
Wherein, E represents network side equivalent voltage during failure, X∑Represent the total induction reactance of network side circuit, IdRepresent filtered electrical Feel d shaft currents, ImaxRepresent inverter maximum delivery current capacity.
As the further supplement of above-mentioned technical proposal, described low voltage crossing control quasi steady state model g (Id,Vt)=0 It is expressed as：
Wherein, k represents low voltage crossing control strategy gain coefficient, IdRepresent filter inductance d shaft currents, ImaxRepresent inverse Become device maximum delivery current capacity.
It is described to whether there is equalization point using phase plane method analysis inverter system as the supplement of above-mentioned technical proposal, And inverter stability criterion under quasi steady state model is established, particular content is：
1) inverter system quasi steady state model I during failure is drawnd-VtPhase plane characteristic curve, including network side quasi-steady state Model Id-VtPhase plane characteristic curve and low voltage crossing control quasi steady state model Id-VtPhase plane characteristic curve, respectively basis The model formation for network side quasi steady state model and low voltage crossing the control quasi steady state model established draws network side quasi-steady state Model Id-VtPhase plane characteristic curve and low voltage crossing control quasi steady state model Id-VtPhase plane characteristic curve；
2) network side quasi steady state model I is judgedd-VtPhase plane characteristic curve and low voltage crossing control quasi steady state model Id- VtWhether characteristic two curves of phase plane have intersection point：With the presence of intersection point then inverter system equalization point, using intersection point as Equalization point, then equalization point is not present in inverter system to no intersection point；
3) then establish below equation expression stability criterion：
In formula, 0≤Id≤E/X∑, IdRepresent filter inductance d shaft currents, ImaxRepresent inverter maximum delivery current capacity, k Low voltage crossing control strategy gain coefficient is represented, E represents network side equivalent voltage during failure, X∑Represent that network side circuit is total Induction reactance.
The inequality left side is network side Id-VtInverter end voltage V in phase plane characteristictExpression formula, the right are that inverter is low Voltage ride-through controls Id-VtInverter end voltage V in phase plane characteristictExpression formula.
In the range of network side operation area, as filter inductance d shaft currents I under any inverter dq coordinate systemsdMeet steady When determining the inequality of criterion, then inverter system unstability.
As the supplement of above-mentioned technical proposal, there is equalization point in described for inverter system during failure Under, system small-signal model analysis inverter system small signal stability is established, particular content is：
At equalization point, linearisation structure small-signal model is carried out to inverter system quasi steady state modelΔx Represent state variable,Represent the differential of state variable；A represents the coefficient matrix of system small-signal model, coefficient of analysis matrix A characteristic root, judge inverter system whether small interference stability；
When the real part of Maximum characteristic root is more than zero, then it is assumed that inverter system is unstable, unstable limit be present；When most When the real part of big characteristic root is equal to zero, then it is assumed that inverter system neutrality；When the real part of Maximum characteristic root is less than zero, then Think that inverter system is stable, in the absence of unstable limit.
As the supplement of above-mentioned technical proposal, described combination equalization point, stability criterion and analysis on Small Disturbance Stability, sentence Whether disconnected inverter system is stablized when distal end fatal voltage falls, and particular content is：
Equalization point is not present in inverter system during failure or stability criterion is permanent full in the range of network side operation area When sufficient, then inverter system unstability；
During failure there is equalization point in inverter system or stability criterion is impermanent full in the range of network side operation area When sufficient, and there is unstable limit in the small-signal model of inverter system, then inverter system unstability；
During failure there is equalization point in inverter system or stability criterion is impermanent full in the range of network side operation area When sufficient, and unstable limit is not present in inverter system small-signal model, then inverter system thinks stable.
Inverter stability analysis system when falling it is a further object of the present invention to provide distal end fatal voltage in a kind of weak net System, including：
Inverter system quasi steady state model establishes module：AC network is subjected to Dai Weinan equivalent process, establishes age at failure Between consider the inverter system quasi steady state model of typical low voltage crossing control strategy；
Stability criterion establishes module：It whether there is equalization point using phase plane method analysis inverter system, establish inverter Inverter system stability criterion under system quasi steady state model；
Analysis on Small Disturbance Stability module：In the case that inverter system has equalization point during failure, system is established Small-signal model of uniting analysis inverter system small signal stability；
The stable judge module of inverter system：With reference to the presence or absence of equalization point, stability criterion and analysis on Small Disturbance Stability, Judge whether inverter system is stablized when distal end fatal voltage falls.
The invention has the advantages that：The present invention is respectively from big interference and small interference angle parsing distal end fatal voltage The inverter instability Mechanism of high induction reactance power network is accessed when falling, it is theoretical clear, it can be applied to the new energy using inverter as interface When fatal voltage failure occurs for electric station grid connection system distal end, inverter system stability analysis.
Brief description of the drawings
Fig. 1 is the inverter typical case's low voltage crossing control block diagram and equivalent circuit of the present invention；
Fig. 2 is the inverter system I of the present inventiond-VtPhase-plane diagram；
Fig. 3 is the inverter system I being directed in the present invention under different degrees of Voltage Dropd-VtPhase-plane diagram；
Fig. 4 is X in simulating, verifying of the embodiment of the present invention∑=0.5pu, k=4, inverter end voltage responsive during E=0.1pu Curve map.
Fig. 5 is X in simulating, verifying of the embodiment of the present invention∑=0.55pu, k=8, inverter end voltage responsive during E=0.1pu Curve map.
Embodiment
The present invention is described in further detail with reference to Figure of description and embodiment.
Embodiment 1
The present embodiment provides inverter stability analysis method when distal end fatal voltage falls in a kind of weak net.
The low voltage crossing control mode and its structure of inverter are as shown in figure 1, definition and the physical significance of Partial Variable It is as shown in table 1 below.Wherein, low voltage crossing (Low Voltage Ride-Through, LVRT) control strategy includes：Electric current control Link processed and electric current dq axle control instruction two parts.
The symbol definition and explanation of part system variable in 1 accompanying drawing of the present invention of table
Current control link dynamic mathematical models：
In formula：Section 1 is conventional current inner ring PI controlling units on the right of equation, and equation the right Section 2 is current feed-forward Compensation term, equation the right Section 3 is electric voltage feed forward.
Electric current dq axle control instructions are：
In formula：Iqref0For q shaft current control instructions, V under control model before failuretFor inverter end voltage magnitude, k is increasing Beneficial coefficient,For the d shaft current control instruction upper limits, Imax(without loss of generality, it is taken as here for inverter current capacity 1.22pu)。
When building inverter system quasi steady state model, hypothesis below is done：
Assuming that 1：Ignore the dynamic process of filter inductance, electric capacity and network line, while ignore the control of inverter inner ring Dynamic process (i.e. Id=IdrefAnd Iq=IqrefSet up).
Assuming that 2：In the case where distal end line voltage seriously falls failure, inverter is in order to support power network, it is assumed that its output reaches Maximum current amplitude Imax。
Under the conditions of assumed above, when power network generation fatal voltage in distal end falls failure, the quasi-steady state equation of network side is：
Smaller in view of filter capacitor, during quasi-steady state, the electric current flowed through on electric capacity is smaller, and approximation is ignored, i.e., false If
Formula (4) is transformed into phase-locked loop pll coordinate system：
In formula：
Abbreviation equation (5) can obtain filter inductance d shaft currents IdWith filter inductance q shaft current electric currents IqExpression formula：
During network side quasi-steady running, due to network side q shaft voltage amplitudes | Eq|≤E, it can be obtained by equation (6), d axles are maximum Allow electric current no more than E/X∑。
Due to when distal end grid side occurs fatal voltage and fallen, it has been believed that inverter completely generates electricity current capacity, therefore by equation (3) and (6) arrange to obtain filter inductance d shaft currents IdWith inverter end voltage VtRelational expression：
From equation (2) and (3), filter inductance d shaft currents IdBy inverter end voltage VtControl, i.e. VtFor independent variable, IdFor dependent variable, for convenience of description, behind the relation of formula (7) description is referred to as network side Id-VtPhase plane characteristic.
Similarly, by equation (2), (3) and (4), filter inductance d shaft currents I is arranged to obtaindWith inverter end voltage VtPass It is formula：
It is that inverter LVRT controls I referred to here as formula (8)d-VtPhase plane characteristic.
System balancing point during equation (7) and the intersection point of (8) they are failure, as shown in Figure 2.As shown in Figure 2, network side is worked as When operating in section A-B, inverter output d shaft current instructions d shaft currents increase in A-c sections, filter inductance；When network side is run In A-C sections, inverter output d shaft current instructions d shaft currents in A-b sections, filter inductance reduce.This explanation inverter LVRT Control strategy is negative-feedback, is advantageous to system stable operation in A points.
From equation (7) and (8), system balancing point and distal end grid side Voltage Drop degree E, fault bit during failure Put X∑It is relevant with gain coefficient k.Here specifically illustrated with Voltage Drop degree E change.
Fig. 3 provides line impedance X∑Under=0.5pu, gain coefficient k=4, different degrees of Voltage Drop, inverter system Id-VtPhase-plane diagram, dashed curve represents network side running boundary in figure.
From the figure 3, it may be seen that as Voltage Drop degree is aggravated, network side quasi-steady running region reduces, and filter inductance d axles are most Big to allow electric current to reduce, system is intended to no equalization point, and inverter stability margin reduces.For example, work as network side Voltage Drop For E=0.05pu when, network side quasi-steady running region is a-b sections, and inverter LVRT control models correspond to the control of d shaft currents and referred to Make as d-e sections, cause filter inductance d shaft currents to increase over maximum allowed current Idmax=0.10pu, so as to which system does not have Equalization point, system unstability.
Theoretical and emulation experiment can prove aforementioned stable Analysis on Mechanism, but be also found that in emulating, even if inverter There is equalization point in system, system equally exists the danger of unstability under quasi steady state model.Even if its reason is equalization point be present, but The small interference stability nargin of equalization point is relatively low, therefore still such that stable problem occurs in system.Here mainly for consideration age at failure Between equalization point correspond to filter inductance d shaft currents be 0 situation, analysis system small interference stability.
In view of also considering that inverter dq axles control in low-voltage control strategy in Small signal stability analysis without document Influence of the instruction change to system small signal stability, the present invention provide inverter dq axle control instruction linearized expressions：
In formula：Upper right mark " 0 " represents the value of the variable at equalization point.During obvious stable state, equilibrium relationships be presentEquation (12) is substituted into obtain：
With reference to inverter inearized model and control instruction Dynamic Equation (13), inverter system small-signal mould can be obtained Type：
Table 2 dominates unstable limit and corresponding equalization point
Found by the A matrix exgenvalues in the case of calculating a variety of equalization point d shaft currents components more than 0, generally all can be extremely A unstable limit of real number away from origin less be present.Table 2 is set out partial equilibrium point d shaft current components and is more than as space is limited 0 situation, wherein system major parameter numerical value are as shown in table 3.As shown in Table 2, when distal end voltage seriously falls, equalization point d axles point Amount is generally much less than q axis components.Found by equation (13), because d axis components are much smaller than q axis components, coefficientCan very Greatly, this causes relevant Δ I in A matrixesdrefElement be much larger than remaining element, cause A matrixes occur away from origin real number not Stable polar point.
The specific embodiment of the present invention is as follows：
To verify the validity of theory analysis described above, system shown in Figure 1 is entered under MATLAB/Simulink environment Row emulation.Inverter ordinary control mode is " active/alternating voltage " control model, and system stable operation is in P before failuree= 0.6pu and VtDistal end Voltage Drop failure occurs when=1pu, t=0.3s.Major parameter refers to table 3.
The Simulink simulation parameters of table 3
System nominal voltage/V 380
System nominal frequency/Hz 50
System nominal capacity/kVA 10
Filter inductance Lf/pu 0.15
Filter capacitor Cf/pu 0.05
Damping resistance Rd 0.1
Current inner loop parameter kp_acc、ki_acc 1、15
Pll parameter kp_pll、ki_pll 80、3500
Feedforward gain coefficient af 40
Fig. 4 provides line impedance X∑=0.5pu, gain coefficient k=4, inversion when Voltage Drop degree is E=0.1pu Device terminal voltage waveform response curve.By inverter system quasi steady state model I in Fig. 3d-VtIt can be seen that in phase plane characteristic curve inverse Becoming under device system quasi steady state model does not have intersection point, system unstability.Inverter end voltage oscillation unstability as seen from Figure 4, this with Theory analysis is consistent.
Fig. 5 provides line impedance X∑=0.55pu, gain coefficient k=8, it is inverse when Voltage Drop degree is E=0.1pu Become device terminal voltage waveform response curve.Equalization point under inverter system quasi steady state model as shown in Table 2 be present, but small interference is unstable It is fixed.Fig. 5 can be seen that inverter end voltage oscillation unstability, and this is consistent with theory analysis.
In addition, inverter end voltage dynamic is in V during can be seen that unstability by Fig. 4 and Fig. 5t=0.9pu shakes up and down Swing, this explanation Failure Model shows as inverter control mode and frequently cut in ordinary control mode and low voltage crossing control model Change.
Embodiment 2
The present embodiment provides inverter stability analysis system when distal end fatal voltage falls in a kind of weak net, including：
Inverter system quasi steady state model establishes module：AC network is subjected to Dai Weinan equivalent process, establishes age at failure Between consider the inverter system quasi steady state model of typical low voltage crossing control strategy；
Stability criterion establishes module：It whether there is equalization point using phase plane method analysis inverter system, establish inverter Inverter system stability criterion under system quasi steady state model；
Analysis on Small Disturbance Stability module：In the case that inverter system has equalization point during failure, system is established Small-signal model of uniting analysis inverter system small signal stability；
The stable judge module of inverter system：With reference to the presence or absence of equalization point, stability criterion and analysis on Small Disturbance Stability, Judge whether inverter system is stablized when distal end fatal voltage falls.
The inverter stability analysis method of the present invention is described in detail above, specific case used herein The principle and embodiment of the present invention are set forth, the explanation of above example is used only to illustrate the side of the present invention Method and core concept, rather than limit the invention, in the protection domain of spirit and claims of the present invention, to this Any modifications and changes made are invented, both fall within protection scope of the present invention.

Claims (9)

  1. Inverter stability analysis method when 1. distal end fatal voltage falls in weak net, including：
    AC network is subjected to Dai Weinan equivalent process, the inversion of typical low voltage crossing control strategy is considered during establishing failure Device system quasi steady state model；
    It whether there is equalization point using phase plane method analysis inverter system, establish inverter under inverter system quasi steady state model System stability criterion；
    In the case that inverter system has equalization point during failure, system small-signal model analysis inverter system is established Small signal stability；
    With reference to the presence or absence of equalization point, stability criterion and analysis on Small Disturbance Stability, judge inverter system in the serious electricity in distal end Whether pressure is stablized when falling.
  2. 2. inverter stability analysis method according to claim 1, it is characterised in that described inverter system is accurate steady States model includes network side quasi steady state model f (Id,Vt)=0 and low voltage crossing control quasi steady state model g (Id,Vt)=0, IdTable Show filter inductance d shaft currents, VtRepresent inverter end voltage.
  3. 3. inverter stability analysis method according to claim 2, it is characterised in that described network side quasi-steady state mould Type f (Id,Vt)=0 is expressed as：
    <mrow> <mi>f</mi> <mrow> <mo>(</mo> <msub> <mi>I</mi> <mi>d</mi> </msub> <mo>,</mo> <msub> <mi>V</mi> <mi>t</mi> </msub> <mo>)</mo> </mrow> <mo>=</mo> <msqrt> <mrow> <msup> <mi>E</mi> <mn>2</mn> </msup> <mo>-</mo> <msubsup> <mi>X</mi> <mi>&amp;Sigma;</mi> <mn>2</mn> </msubsup> <msubsup> <mi>I</mi> <mi>d</mi> <mn>2</mn> </msubsup> </mrow> </msqrt> <mo>+</mo> <msub> <mi>X</mi> <mi>&amp;Sigma;</mi> </msub> <msqrt> <mrow> <msubsup> <mi>I</mi> <mrow> <mi>m</mi> <mi>a</mi> <mi>x</mi> </mrow> <mn>2</mn> </msubsup> <mo>-</mo> <msubsup> <mi>I</mi> <mi>d</mi> <mn>2</mn> </msubsup> </mrow> </msqrt> <mo>-</mo> <msub> <mi>V</mi> <mi>t</mi> </msub> <mo>=</mo> <mn>0</mn> <mo>,</mo> </mrow>
    Wherein, E represents network side equivalent voltage during failure, X∑Represent the total induction reactance of network side circuit, IdRepresent filter inductance d axles Electric current, ImaxRepresent inverter maximum delivery current capacity.
  4. 4. inverter stability analysis method according to claim 2, it is characterised in that described low voltage crossing control Quasi steady state model g (Id,Vt)=0 is expressed as：
    <mrow> <mi>g</mi> <mrow> <mo>(</mo> <msub> <mi>I</mi> <mi>d</mi> </msub> <mo>,</mo> <msub> <mi>V</mi> <mi>t</mi> </msub> <mo>)</mo> </mrow> <mo>=</mo> <mfenced open = "{" close = ""> <mtable> <mtr> <mtd> <mrow> <msub> <mi>I</mi> <mi>d</mi> </msub> <mo>-</mo> <msqrt> <mrow> <msubsup> <mi>I</mi> <mrow> <mi>m</mi> <mi>a</mi> <mi>x</mi> </mrow> <mn>2</mn> </msubsup> <mo>-</mo> <msup> <mi>k</mi> <mn>2</mn> </msup> <msup> <mrow> <mo>(</mo> <mn>0.9</mn> <mo>-</mo> <msub> <mi>V</mi> <mi>t</mi> </msub> <mo>)</mo> </mrow> <mn>2</mn> </msup> </mrow> </msqrt> <mo>=</mo> <mn>0</mn> <mo>,</mo> </mrow> </mtd> <mtd> <mrow> <mo>-</mo> <msub> <mi>I</mi> <mrow> <mi>m</mi> <mi>a</mi> <mi>x</mi> </mrow> </msub> <mo>&amp;le;</mo> <mo>-</mo> <mi>k</mi> <mrow> <mo>(</mo> <mn>0.9</mn> <mo>-</mo> <msub> <mi>V</mi> <mi>t</mi> </msub> <mo>)</mo> </mrow> <mo>&amp;le;</mo> <mn>0</mn> <mo>,</mo> </mrow> </mtd> </mtr> <mtr> <mtd> <mrow> <msub> <mi>I</mi> <mi>d</mi> </msub> <mo>=</mo> <mn>0</mn> <mo>,</mo> </mrow> </mtd> <mtd> <mrow> <mo>-</mo> <mi>k</mi> <mrow> <mo>(</mo> <mn>0.9</mn> <mo>-</mo> <msub> <mi>V</mi> <mi>t</mi> </msub> <mo>)</mo> </mrow> <mo>&lt;</mo> <mo>-</mo> <msub> <mi>I</mi> <mrow> <mi>m</mi> <mi>a</mi> <mi>x</mi> </mrow> </msub> </mrow> </mtd> </mtr> </mtable> </mfenced> <mo>,</mo> </mrow>
    Wherein, k represents low voltage crossing control strategy gain coefficient, IdRepresent filter inductance d shaft currents, ImaxRepresent inverter most Big conveying current capacity.
  5. 5. according to the inverter stability analysis method described in claim any one of 1-4, it is characterised in that described using equal Face method analysis inverter system whether there is equalization point, establish inverter stability criterion under quasi steady state model, particular content is：
    1) inverter system quasi steady state model I during failure is drawnd-VtPhase plane characteristic curve, including network side quasi steady state model Id-VtPhase plane characteristic curve and low voltage crossing control quasi steady state model Id-VtPhase plane characteristic curve, respectively according to built The model formation of vertical network side quasi steady state model and low voltage crossing control quasi steady state model draws network side quasi steady state model Id-VtPhase plane characteristic curve and low voltage crossing control quasi steady state model Id-VtPhase plane characteristic curve；
    2) network side quasi steady state model I is judgedd-VtPhase plane characteristic curve and low voltage crossing control quasi steady state model Id-VtPhase Whether two curves of flatness of the response curve have intersection point：With the presence of intersection point then inverter system equalization point, balance is used as using intersection point Point, then equalization point is not present in inverter system to no intersection point；
    3) then establish below equation expression stability criterion：
    <mrow> <msqrt> <mrow> <msup> <mi>E</mi> <mn>2</mn> </msup> <mo>-</mo> <msup> <msub> <mi>X</mi> <mi>&amp;Sigma;</mi> </msub> <mn>2</mn> </msup> <msubsup> <mi>I</mi> <mi>d</mi> <mn>2</mn> </msubsup> </mrow> </msqrt> <mo>+</mo> <msub> <mi>X</mi> <mi>&amp;Sigma;</mi> </msub> <msqrt> <mrow> <msubsup> <mi>I</mi> <mrow> <mi>m</mi> <mi>a</mi> <mi>x</mi> </mrow> <mn>2</mn> </msubsup> <mo>-</mo> <msubsup> <mi>I</mi> <mi>d</mi> <mn>2</mn> </msubsup> </mrow> </msqrt> <mo>&amp;le;</mo> <mo>-</mo> <mfrac> <mn>1</mn> <mi>k</mi> </mfrac> <msqrt> <mrow> <msubsup> <mi>I</mi> <mrow> <mi>m</mi> <mi>a</mi> <mi>x</mi> </mrow> <mn>2</mn> </msubsup> <mo>-</mo> <msubsup> <mi>I</mi> <mi>d</mi> <mn>2</mn> </msubsup> </mrow> </msqrt> <mo>+</mo> <mn>0.9</mn> <mo>,</mo> </mrow>
    In formula, 0≤Id≤E/X∑, IdRepresent filter inductance d shaft currents, ImaxInverter maximum delivery current capacity is represented, k is represented Low voltage crossing control strategy gain coefficient, E represent network side equivalent voltage during failure, X∑Represent that network side circuit is always felt It is anti-.
  6. 6. inverter stability analysis method according to claim 5, it is characterised in that in network side operation area scope It is interior, as the electric current I of d axles under any inverter dq coordinate systemsdWhen meeting the inequality of stability criterion, then inverter system unstability.
  7. 7. according to the inverter stability analysis method described in claim any one of 1-4, it is characterised in that described is directed to event In the case that inverter system has equalization point during barrier, system small-signal model analysis inverter system small interference stability is established Property, particular content is：
    At equalization point, linearisation structure small-signal model is carried out to inverter system quasi steady state modelΔ x is represented State variable,Represent the differential of state variable；A represents the coefficient matrix of system small-signal model, coefficient of analysis matrix A Characteristic root, judge inverter system whether small interference stability；
    When the real part of Maximum characteristic root is more than zero, then it is assumed that inverter system is unstable, unstable limit be present；When maximum special When levying the real part of root equal to zero, then it is assumed that inverter system neutrality；When the real part of Maximum characteristic root is less than zero, then it is assumed that Inverter system is stable, in the absence of unstable limit.
  8. 8. according to the inverter stability analysis method described in claim any one of 1-4, it is characterised in that described combination is put down Weigh point, stability criterion and analysis on Small Disturbance Stability, judges whether inverter system is stablized when distal end fatal voltage falls, and has Hold in vivo and be：
    Inverter system is not present equalization point or when stability criterion perseverance meets in the range of network side operation area during failure, Then inverter system unstability；
    During failure there is equalization point in inverter system or stability criterion is not permanent in the range of network side operation area when meeting, And there is unstable limit in the small-signal model of inverter system, then inverter system unstability；
    During failure there is equalization point in inverter system or stability criterion is not permanent in the range of network side operation area when meeting, And unstable limit is not present in inverter system small-signal model, then inverter system thinks stable.
  9. Inverter stability analysis system when 9. distal end fatal voltage falls in weak net, including：
    Inverter system quasi steady state model establishes module：AC network is subjected to Dai Weinan equivalent process, examined during establishing failure Consider the inverter system quasi steady state model of typical low voltage crossing control strategy；
    Stability criterion establishes module：It whether there is equalization point using phase plane method analysis inverter system, establish inverter system Inverter system stability criterion under quasi steady state model；
    Analysis on Small Disturbance Stability module：In the case that inverter system has equalization point during failure, it is small to establish system Signal analysis based on model inverter system small signal stability；
    The stable judge module of inverter system：With reference to the presence or absence of equalization point, stability criterion and analysis on Small Disturbance Stability, judge Whether inverter system is stablized when distal end fatal voltage falls.
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