CN107887902B - Inverter stability analysis method and system during remote severe voltage drop in weak network - Google Patents

Inverter stability analysis method and system during remote severe voltage drop in weak network Download PDF

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CN107887902B
CN107887902B CN201711001812.8A CN201711001812A CN107887902B CN 107887902 B CN107887902 B CN 107887902B CN 201711001812 A CN201711001812 A CN 201711001812A CN 107887902 B CN107887902 B CN 107887902B
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inverter
inverter system
quasi
stability
state model
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CN107887902A (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

Abstract

The invention discloses a method and a system for analyzing the stability of an inverter when a far-end severe voltage drops in a weak network. According to the method, an alternating current power grid Thevenin is equivalent, and an inverter system quasi-steady-state model considering a typical low voltage ride through control strategy during a fault period is established; analyzing whether a system has a balance point by using a phase plane method, and establishing a stability criterion of the inverter system under the quasi-steady state model; under the condition that a balance point exists, a system small signal model is established to analyze the small interference stability of the system; and finally, judging whether the inverter system is stable or not when the far-end severe voltage drops by combining the stability criterion and the small-interference stability analysis. The method analyzes the instability mechanism of the inverter connected into the high-inductance power grid when the far-end severe voltage drops from the large interference and small interference angles, has clear theory, and can be applied to the stability analysis of the inverter system when the far-end severe voltage fault occurs in the new energy power station grid-connected system with the inverter as an interface.

Description

Inverter stability analysis method and system during remote severe voltage drop in weak network
Technical Field
The invention relates to an inverter of power equipment, in particular to an inverter stability analysis method and system when a far-end severe voltage in a weak grid drops.
Background
With the access of a large amount of power generated by renewable energy sources such as photovoltaic energy, wind energy and the like to a power system, low-voltage ride-through capability is provided for the power generation grid connection of the renewable energy sources at home and abroad. In addition, during the voltage drop, the grid-connected guide rules require that a wind power plant and a photovoltaic power station inject reactive current in a certain proportion to the voltage drop to support the voltage of a power grid, and the residual capacity is sent out in an active mode to prevent the power grid from generating large active shortage to influence the frequency stability of the system.
Grid-connected guidance generally assumes that an alternating current power grid is a strong grid (i.e., a line inductance value is small), and requires that an inverter has a certain low-voltage ride-through capability. However, renewable energy grid connection in China has the characteristics of long distance, large scale and high concentration, and a weak power grid (namely a large line inductance value) is usually arranged on the power grid side. Different from a voltage drop fault at a near-end public connection point in a strong network, when the voltage drop fault occurs at a far-end power grid side in a weak power grid, the voltage drop at the end of an inverter is small under the influence of a high inductive reactance value of a line, and a typical low-voltage ride-through control strategy possibly has an inapplicable risk, so that the inverter has a new instability problem. The problems of small interference and large interference stability during low-voltage ride through of the inverter in the high-inductance power grid need to be continuously and deeply researched.
At present, researches on the inverter low-voltage ride-through process mainly focus on small interference stability analysis, and in the aspect of small interference stability analysis, an article about considering the influence of the introduction of inverter terminal voltage dynamics in an inverter low-voltage control strategy on system small interference stability is not reported yet; the research on the large interference stability is relatively few, and the research ignores the problem of the large interference stability of the inverter possibly caused by the typical low-voltage ride-through control strategy when the voltage drop at the far end of the high-inductance power grid is serious (the voltage drop is considered to be below 0.2pu and is serious).
Disclosure of Invention
In order to solve the problems in the prior art, the invention provides a method for analyzing the stability of an inverter when the far-end severe voltage drops in a weak network, which can clearly explain the instability mechanism of the inverter when the far-end severe voltage drops, namely, the stability of the inverter system during the fault period is analyzed from the two aspects of large interference stability and small interference stability.
The technical scheme adopted by the invention is as follows: the method for analyzing the stability of the inverter when the far-end severe voltage drops in the weak grid comprises the following steps:
carrying out Thevenin equivalent processing on an alternating current power grid, and establishing an inverter system quasi-steady-state model considering a typical low voltage ride through control strategy during a fault period;
analyzing whether the inverter system has a balance point by using a phase plane method, and establishing an inverter system stability criterion under a quasi-steady-state model of the inverter system;
aiming at the condition that the inverter system has a balance point during the fault period, a system small signal model is established to analyze the small interference stability of the inverter system;
and judging whether the inverter system is stable when the far-end severe voltage drops by combining whether a balance point exists or not, a stability criterion and small-interference stability analysis.
In addition to the above technical solution, the quasi-steady-state model of the inverter system includes a network-side quasi-steady-state model f (I)d,Vt) Control quasi-steady-state model g (I) for 0 and low voltage ride throughd,Vt)＝0，IdRepresenting the d-axis current, V, of the filter inductortRepresenting the inverter terminal voltage.
As a further supplement to the above technical solution, the network side quasi-steady state model f (I)d,Vt) 0 is expressed as:
Figure BDA0001443547410000021
wherein E represents the equivalent voltage of the network side during the fault, X∑Indicating the total inductive reactance of the network side line, IdRepresenting the d-axis current, I, of the filter inductormaxRepresenting the maximum delivered current capacity of the inverter.
As a further supplement to the above technical solution, the low voltage ride through control quasi-steady state model g (I)d,Vt) 0 is expressed as:
Figure BDA0001443547410000031
wherein k represents the gain coefficient of the low voltage ride through control strategy, IdRepresenting the d-axis current, I, of the filter inductormaxRepresenting the maximum delivered current capacity of the inverter.
As a supplement to the above technical solution, the method for analyzing whether the inverter system has a balance point by using the phase plane method and establishing the inverter stability criterion under the quasi-steady state model includes:
1) drawing a quasi-steady state model I of the inverter system during the fault periodd-VtPhase plane characteristic curve, including network side quasi-steady state model Id-VtPhase plane characteristic curve and low voltage ride through control quasi-steady state model Id-VtA phase plane characteristic curve, and a network side quasi-steady state model I is drawn according to the established model formulas of the network side quasi-steady state model and the low voltage ride through control quasi-steady state modeld-VtPhase plane characteristic curve and low voltage ride through control quasi-steady state model Id-VtA phase plane characteristic curve;
2) judging network side quasi-steady state model Id-VtPhase plane characteristic curve and low voltage ride through control quasi-steady state model Id-VtWhether two curves of the phase plane characteristic curve have an intersection point: if the intersection point exists, the inverter system has a balance point, the intersection point is used as the balance point, and if the intersection point does not exist, the inverter system does not have the balance point;
3) then, a stability criterion is established as represented by the following formula:
Figure BDA0001443547410000032
in the formula, I is more than or equal to 0d≤E/X∑，IdRepresenting the d-axis current, I, of the filter inductormaxRepresenting the maximum transmission current capacity of the inverter, k representing the gain coefficient of a low-voltage ride-through control strategy, E representing the equivalent voltage of the network side during the fault period, and X∑And represents the total inductive reactance of the network side line.
The left side of the inequality is a network side Id-VtInverter terminal voltage V in phase plane characteristicstExpression, inverter low voltage ride through control I on the rightd-VtInverter terminal voltage V in phase plane characteristicstAnd (5) expressing.
In the range of a network side operation area, filtering inductance d-axis current I under any inverter dq coordinate systemdAnd when the inequality of the stability criterion is met, the inverter system is unstable.
As a supplement to the above technical solution, when there is a balance point in the inverter system during the fault, the method for establishing the system small signal model to analyze the stability of the small interference of the inverter system includes:
at a balance point, carrying out linearization on a quasi-steady-state model of the inverter system to construct a small signal model
Figure BDA0001443547410000041
The delta x represents the state variable of the state,
Figure BDA0001443547410000042
represents the differential of the state variable; a represents a coefficient matrix of a system small signal model, the characteristic root of the coefficient matrix A is analyzed, and whether the inverter system is small in interference and stable is judged;
when the real part of the maximum characteristic root is larger than zero, the inverter system is considered to be unstable, and an unstable pole exists; when the real part of the maximum characteristic root is equal to zero, the inverter system is considered to be critically stable; and when the real part of the maximum characteristic root is less than zero, the inverter system is considered to be stable, and an unstable pole does not exist.
As a supplement to the above technical solution, the combination of the balance point, the stability criterion, and the small disturbance stability analysis determines whether the inverter system is stable when the far-end severe voltage drops, and the specific contents are as follows:
when the inverter system has no balance point or the stability criterion is constantly met in the range of the network side operation area during the fault, the inverter system is unstable;
when a balance point exists in the inverter system during the fault or a stability criterion is not constantly met in the range of a network side operation area, and an unstable pole exists in a small signal model of the inverter system, the inverter system is unstable;
when a balance point exists in the inverter system during the fault or the stability criterion is not constantly met in the range of the network side operation area, and an unstable pole does not exist in the small signal model of the inverter system, the inverter system is considered to be stable.
Another object of the present invention is to provide a system for analyzing inverter stability in case of a remote severe voltage drop in a weak grid, comprising:
an inverter system quasi-steady state model building module: carrying out Thevenin equivalent processing on an alternating current power grid, and establishing an inverter system quasi-steady-state model considering a typical low voltage ride through control strategy during a fault period;
a stability criterion establishing module: analyzing whether the inverter system has a balance point by using a phase plane method, and establishing an inverter system stability criterion under a quasi-steady-state model of the inverter system;
a small interference stability analysis module: aiming at the condition that the inverter system has a balance point during the fault period, a system small signal model is established to analyze the small interference stability of the inverter system;
inverter system stabilizes the judgement module: and judging whether the inverter system is stable when the far-end severe voltage drops by combining whether a balance point exists or not, a stability criterion and small-interference stability analysis.
The invention has the beneficial effects that: the method analyzes the instability mechanism of the inverter connected into the high-inductance power grid when the far-end severe voltage drops from the large interference and small interference angles, has clear theory, and can be applied to the stability analysis of the inverter system when the far-end severe voltage fault occurs in the new energy power station grid-connected system with the inverter as an interface.
Drawings
FIG. 1 is a block diagram of an exemplary low voltage ride through control and equivalent circuit for an inverter of the present invention;
FIG. 2 shows an inverter system I according to the inventiond-VtA phase plane diagram;
FIG. 3 shows an inverter system I for different voltage dips according to the present inventiond-VtA phase plane diagram;
FIG. 4 shows an example of X in the simulation verification of the present invention∑And when k is 4 and E is 0.1pu, the voltage response curve of the inverter terminal is shown.
FIG. 5 shows an example of X in the simulation verification of the present invention∑And when k is 8 and E is 0.1pu, the voltage response curve of the inverter terminal is shown.
Detailed Description
The invention is described in further detail below with reference to the drawings and the detailed description.
Example 1
The embodiment provides an inverter stability analysis method in the case of a remote severe voltage drop in a weak grid.
The low voltage ride through control mode and the structure of the inverter are shown in fig. 1, and the definitions and physical meanings of some variables are shown in table 1 below. The Low Voltage Ride-Through (LVRT) control strategy comprises the following steps: the current control link and the current dq axis control instruction.
TABLE 1 symbolic definition and description of partial system variables in the drawings of the present invention
Figure BDA0001443547410000061
A current control link dynamic mathematical model:
Figure BDA0001443547410000062
in the formula: the first term on the right of the equation is a conventional current inner loop PI control link, the second term on the right of the equation is a current feedforward compensation term, and the third term on the right of the equation is voltage feedforward.
The current dq axis control command is:
Figure BDA0001443547410000071
Figure BDA0001443547410000072
in the formula: i isqref0For q-axis current control commands in pre-fault control mode, VtIs the inverter terminal voltage amplitude, k is the gain factor,
Figure BDA0001443547410000073
is d-axis current control command upper limit, ImaxThe inverter current capacity (taken as 1.22pu here without loss of generality).
When constructing a quasi-steady-state model of the inverter system, the following assumptions are made:
assume that 1: dynamic processes of filter inductance, capacitance and network line are ignored, and dynamic processes of inverter inner loop control (i.e. I)d＝IdrefAnd Iq＝IqrefTrue).
Assume 2: under the condition of a remote grid voltage severe drop fault, the inverter assumes that the output of the inverter reaches the maximum current amplitude I for supporting the gridmax。
Under the above assumed conditions, when a serious voltage drop fault occurs in the far-end power grid, the quasi-steady state equation at the network side is as follows:
Figure BDA0001443547410000074
considering that the filter capacitor is relatively small, in quasi-steady state, the current flowing through the capacitor is relatively small and is approximately ignored, i.e. it is assumed that
Figure BDA0001443547410000075
Converting equation (4) to the phase-locked loop PLL coordinate system:
Figure BDA0001443547410000076
in the formula:
Figure BDA0001443547410000077
the d-axis current I of the filter inductor can be obtained by simplifying equation (5)dAnd the current I of the q-axis of the filter inductorqExpression:
Figure BDA0001443547410000081
when the network side operates in a quasi-steady state, the voltage amplitude | E of the q axis of the network sideqE | ≦ E, as derived from equation (6), the maximum allowable current for the d-axis cannot exceed E/X∑。
Since the inverter is considered to have a full current-generating capacity when a severe voltage drop occurs on the far-end grid side, the filter inductance d-axis current I is formulated by equations (3) and (6)dAnd inverter terminal voltage VtThe relation of (1):
Figure BDA0001443547410000082
from equations (2) and (3), the filter inductor d-axis current IdVoltage V at inverter terminaltControl, i.e. VtIs an independent variable, IdFor the dependent variable and for the convenience of description, the relationship described by the equation (7) will be referred to as network side Id-VtPhase plane characteristics.
Similarly, the filter inductor d-axis current I is obtained by equations (2), (3) and (4)dAnd inverter terminal voltage VtThe relation of (1):
Figure BDA0001443547410000083
here, formula (8) is inverter LVRT control Id-VtPhase plane characteristics.
The intersection of equations (7) and (8) is the system balance point during a fault, as shown in FIG. 2. As can be seen from fig. 2, when the network side operates in the section a-B, the inverter outputs a d-axis current command in the section a-c, and the d-axis current on the filter inductor increases; when the network side operates in the A-C section, the inverter outputs a d-axis current instruction in the A-b section, and the d-axis current on the filter inductor is reduced. This shows that the inverter LVRT control strategy is negative feedback, which is beneficial to the system to stably operate at point a.
From equations (7) and (8), the system balance point and the far-end grid-side voltage drop degree E and the fault position X during the fault period∑And is related to the gain factor k. The change in the voltage drop level E will be specifically described here.
FIG. 3 shows the line impedance X∑Inverter system I with different voltage drops, with gain factor k of 4, 0.5pud-VtAnd (4) a phase plane diagram, wherein a dashed curve represents a network side operation boundary.
As can be seen from fig. 3, as the voltage drop degree is increased, the quasi-steady-state operation area on the network side is reduced, the maximum allowable current of the d-axis of the filter inductor is reduced, the system tends to have no balance point, and the stability margin of the inverter is reduced. For example, when the network-side voltage drops to E ═ 0.05pu, the network-side quasi-steady-state operation region is a-b segment, and the inverter LVRT control mode corresponds to the d-axis current control command being d-E segment, so that the filter inductor d-axis current increases and exceeds the maximum allowable current Idmax0.10pu, so the system has no balance point and the system is unstable.
Both theory and simulation experiments can prove the stability mechanism analysis, but the simulation still finds that even if the inverter system has a balance point under a quasi-steady-state model, the system also has the risk of instability. The reason for this is that even if there is a balance point, the small disturbance stability margin of the balance point is low, so that the system still has a stability problem. The method mainly aims at the situation that the current of a filter inductor d axis is not 0 corresponding to a balance point in a fault period, and the analysis system is small in interference and stable.
Considering that no literature exists in small disturbance stability analysis, the influence of inverter dq axis control command change in a low-voltage control strategy on system small disturbance stability is considered, the invention provides a linearization expression of the inverter dq axis control command:
Figure BDA0001443547410000091
in the formula: the upper right label "0" indicates the value of the variable at the equilibrium point. Obviously at steady state, there is an equality relationship
Figure BDA0001443547410000092
Substituting equation (12) yields:
Figure BDA0001443547410000093
combining the inverter linearization model and the control command dynamics equation (13), the small signal model of the inverter system can be obtained:
Figure BDA0001443547410000101
TABLE 2 dominant unstable poles and corresponding equilibrium points
Figure BDA0001443547410000102
By calculating the characteristic value of the A matrix under the condition that the d-axis current component of various equilibrium points is greater than 0, at least one real unstable pole far away from the origin usually exists. The partial balance point d-axis current component greater than 0 is listed in table 2, wherein the values of the main parameters of the system are shown in table 3. As can be seen from table 2, when the far-end voltage drops severely, the d-axis component at the equilibrium point is usually much smaller than the q-axis component. It is found from equation (13) that since the d-axis component is much smaller than the q-axis component, the coefficient
Figure BDA0001443547410000103
Will be large, which results in a relevant Δ I in the A matrixdrefIs much larger than the remaining elements, resulting in a real unstable pole of the a matrix far from the origin.
The specific embodiment of the invention is as follows:
to verify the validity of the theoretical analysis described above, the system shown in FIG. 1 was simulated in a MATLAB/Simulink environment. The normal control mode of the inverter is an active/alternating voltage control mode, and the system stably runs at P before a faulte0.6pu and VtWhen t is 0.3s, a far-end voltage drop fault occurs. The main parameters are detailed in table 3.
Table 3 Simulink simulation parameters
Rated voltage/V of system 380
Rated frequency/Hz of system 50
System rated capacity/kVA 10
Filter inductance Lf/pu 0.15
Filter capacitor Cf/pu 0.05
Damping resistor Rd 0.1
Current inner loop parameter kp_acc、k i_acc 1、15
Phase-locked loop parameter kp_pll、ki_pll 80、3500
Feed forward gain coefficient af 40
FIG. 4 shows the line impedance X∑And when the gain coefficient is equal to 4 and the voltage drop degree is equal to 0.1pu, the voltage waveform response curve of the inverter terminal is obtained. Quasi-steady state model I of inverter system in FIG. 3d-VtNo intersection point exists in the inverter system quasi-steady state model in the phase plane characteristic curve, and the system is unstable. It can be seen from fig. 4 that the inverter terminal voltage oscillations are unstable, which is consistent with the theoretical analysis.
FIG. 5 shows the line impedance X∑When the gain coefficient is k equal to 8 and the voltage drop degree is E equal to 0.1pu, the voltage waveform response curve of the inverter terminal is obtained. From table 2, it can be known that there is a balance point in the quasi-steady-state model of the inverter system, but the small disturbance is unstable. Fig. 5 can see that the inverter terminal voltage oscillations are unstable, which is consistent with the theoretical analysis.
In addition, as can be seen from fig. 4 and 5, the inverter terminal voltage dynamics during the destabilization is at VtThe oscillation up and down of 0.9pu indicates that the instability mode appears as frequent switching of the inverter control mode between the normal control mode and the low voltage ride through control mode.
Example 2
The embodiment provides an inverter stability analysis system during a remote severe voltage drop in a weak grid, which includes:
an inverter system quasi-steady state model building module: carrying out Thevenin equivalent processing on an alternating current power grid, and establishing an inverter system quasi-steady-state model considering a typical low voltage ride through control strategy during a fault period;
a stability criterion establishing module: analyzing whether the inverter system has a balance point by using a phase plane method, and establishing an inverter system stability criterion under a quasi-steady-state model of the inverter system;
a small interference stability analysis module: aiming at the condition that the inverter system has a balance point during the fault period, a system small signal model is established to analyze the small interference stability of the inverter system;
inverter system stabilizes the judgement module: and judging whether the inverter system is stable when the far-end severe voltage drops by combining whether a balance point exists or not, a stability criterion and small-interference stability analysis.
The inverter stability analysis method of the present invention is described in detail above, and the principle and the embodiments of the present invention are illustrated herein by using specific examples, and the above description of the embodiments is only used to explain the method and the core idea of the present invention, but not to limit the present invention, and any modifications and changes made within the spirit of the present invention and the scope of the claims fall within the scope of the present invention.

Claims (7)

1. The method for analyzing the stability of the inverter when the far-end severe voltage drops in the weak grid comprises the following steps:
carrying out Thevenin equivalent processing on an alternating current power grid, and establishing an inverter system quasi-steady-state model considering a typical low voltage ride through control strategy during a fault period;
analyzing whether the inverter system has a balance point by using a phase plane method, and establishing an inverter system stability criterion under a quasi-steady-state model of the inverter system;
aiming at the condition that the inverter system has a balance point during the fault period, a system small signal model is established to analyze the small interference stability of the inverter system;
whether the inverter system is stable when the far-end severe voltage drops is judged by combining whether a balance point exists or not, a stability criterion and small-interference stability analysis;
the quasi-steady-state model of the inverter system comprises a network side quasi-steady-state model f (I)d,Vt) Control quasi-steady-state model g (I) for 0 and low voltage ride throughd,Vt)＝0，IdRepresenting the d-axis current, V, of the filter inductortRepresenting the inverter terminal voltage;
the network side quasi-steady state model f (I)d,Vt) 0 is expressed as:
Figure FDA0002474215630000011
wherein E represents the equivalent voltage of the network side during the fault, X∑Indicating the total inductive reactance of the network side line, IdRepresenting the d-axis current, I, of the filter inductormaxRepresenting the maximum delivered current capacity of the inverter.
2. The inverter stability analysis method of claim 1, wherein the low voltage ride through control quasi-steady state model g (I)d,Vt) 0 is expressed as:
Figure FDA0002474215630000012
wherein k represents the gain coefficient of the low voltage ride through control strategy, IdRepresenting the d-axis current, I, of the filter inductormaxRepresenting the maximum delivered current capacity of the inverter.
3. The inverter stability analysis method according to any one of claims 1-2, wherein the phase plane method is used for analyzing whether the inverter system has a balance point, and establishing an inverter stability criterion under a quasi-steady state model, specifically comprising:
1) drawing a quasi-steady state model I of the inverter system during the fault periodd-VtPhase plane characteristic curve, including network side quasi-steady state model Id-VtPhase plane characteristic curve and low voltage ride through control quasi-steady state model Id-VtA phase plane characteristic curve, and a network side quasi-steady state model I is drawn according to the established model formulas of the network side quasi-steady state model and the low voltage ride through control quasi-steady state modeld-VtPhase plane characteristic curve and low voltage ride through control quasi-steady state model Id-VtA phase plane characteristic curve;
2) judging network side quasi-steady state model Id-VtQuasi-stability of phase plane characteristic curve and low voltage ride through controlState model Id-VtWhether two curves of the phase plane characteristic curve have an intersection point: if the intersection point exists, the inverter system has a balance point, the intersection point is used as the balance point, and if the intersection point does not exist, the inverter system does not have the balance point;
3) then, a stability criterion is established as represented by the following formula:
Figure FDA0002474215630000021
in the formula, I is more than or equal to 0d≤EX∑，IdRepresenting the d-axis current, I, of the filter inductormaxRepresenting the maximum transmission current capacity of the inverter, k representing the gain coefficient of a low-voltage ride-through control strategy, E representing the equivalent voltage of the network side during the fault period, and X∑And represents the total inductive reactance of the network side line.
4. The inverter stability analysis method according to claim 3, wherein the d-axis current I is obtained in any inverter dq coordinate system within the network side operation regiondAnd when the inequality of the stability criterion is met, the inverter system is unstable.
5. The method for analyzing the stability of the inverter according to any one of claims 1 to 2, wherein when the inverter system has a balance point during a fault, a system small signal model is established to analyze the stability of the small disturbance of the inverter system, and the specific content is as follows:
at a balance point, carrying out linearization on a quasi-steady-state model of the inverter system to construct a small signal model
Figure FDA0002474215630000022
The delta x represents the state variable of the state,
Figure FDA0002474215630000023
represents the differential of the state variable; a represents a coefficient matrix of a system small signal model, the characteristic root of the coefficient matrix A is analyzed, and whether the inverter system is small in interference and stable is judged;
when the real part of the maximum characteristic root is larger than zero, the inverter system is considered to be unstable, and an unstable pole exists; when the real part of the maximum characteristic root is equal to zero, the inverter system is considered to be critically stable; and when the real part of the maximum characteristic root is less than zero, the inverter system is considered to be stable, and an unstable pole does not exist.
6. The inverter stability analysis method according to any one of claims 1-2, wherein the combination of whether there is a balance point, a stability criterion, and a small disturbance stability analysis determines whether the inverter system is stable when the far-end severe voltage drops, and the specific content is:
when the inverter system has no balance point or the stability criterion is constantly met in the range of the network side operation area during the fault, the inverter system is unstable;
when a balance point exists in the inverter system during the fault or a stability criterion is not constantly met in the range of a network side operation area, and an unstable pole exists in a small signal model of the inverter system, the inverter system is unstable;
when a balance point exists in the inverter system during the fault or the stability criterion is not constantly met in the range of the network side operation area, and an unstable pole does not exist in the small signal model of the inverter system, the inverter system is considered to be stable.
7. Inverter stability analysis system when serious voltage of distal end falls in weak net includes:
an inverter system quasi-steady state model building module: carrying out Thevenin equivalent processing on an alternating current power grid, and establishing an inverter system quasi-steady-state model considering a typical low voltage ride through control strategy during a fault period;
a stability criterion establishing module: analyzing whether the inverter system has a balance point by using a phase plane method, and establishing an inverter system stability criterion under a quasi-steady-state model of the inverter system;
a small interference stability analysis module: aiming at the condition that the inverter system has a balance point during the fault period, a system small signal model is established to analyze the small interference stability of the inverter system;
inverter system stabilizes the judgement module: whether the inverter system is stable when the far-end severe voltage drops is judged by combining whether a balance point exists or not, a stability criterion and small-interference stability analysis;
the quasi-steady-state model of the inverter system comprises a network side quasi-steady-state model f (I)d,Vt) Control quasi-steady-state model g (I) for 0 and low voltage ride throughd,Vt)＝0，IdRepresenting the d-axis current, V, of the filter inductortRepresenting the inverter terminal voltage;
the network side quasi-steady state model f (I)d,Vt) 0 is expressed as:
Figure FDA0002474215630000041
wherein E represents the equivalent voltage of the network side during the fault, X∑Indicating the total inductive reactance of the network side line, IdRepresenting the d-axis current, I, of the filter inductormaxRepresenting the maximum delivered current capacity of the inverter.
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