CN113054640B - Direct current converter parallel system stability criterion method based on impedance decomposition - Google Patents
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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
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- H02J1/10—Parallel operation of dc sources
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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
- H02J1/00—Circuit arrangements for dc mains or dc distribution networks
Abstract
The invention discloses a stability criterion method for a parallel system of direct current converters based on impedance decomposition, and belongs to the technical field of power systems. The stability criterion method specifically comprises the following steps: firstly, determining a direct current bus voltage mathematical model at a common analysis node in a direct current converter parallel system; then, based on the Nyquist stability theorem, a method for judging the system stability according to the source load cascade system impedance is provided; and finally, determining a dominant subsystem causing instability at the instability frequency of the source load cascade system, and correcting the stability of the source load cascade system. The method can realize the judgment of the impedance stability of the direct current power distribution system in a wide frequency domain range, analyze the system stability from two angles of positive and negative damping and a resonance source, explain the instability from the aspect of physical essence and provide guidance for impedance remodeling.
Description
Technical Field
The invention belongs to the technical field of power systems, and relates to a stability criterion method of a parallel system of direct current converters based on impedance decomposition.
Background
In a dc microgrid, there are a large number of power electronic components, motors, and loads that are connected to a dc bus via a power electronic converter. Due to the influence of the switching characteristic and closed-loop control in the operation process of a switching power supply device consisting of switching power electronic components, the input and output impedance characteristics of a power electronic power supply or a load device present nonlinear characteristics. In particular, to ensure a fast response characteristic, the power electronic load presents its own input impedance characteristic with a negative impedance characteristic due to the introduction of negative feedback. With the increase of power electronic devices in a direct current distribution system, the interaction between subsystems with different input and output impedance characteristics aggravates the impedance stability analysis complexity of the direct current microgrid, so that the stability of the whole system is difficult to analyze and ensure. When disturbance occurs in a complex direct current distribution system, the voltage oscillation of the direct current microgrid may be unstable or even breakdown, so that the stability analysis of the direct current microgrid is very important. An important feature of the oscillation or instability due to impedance interaction is that even if a single converter in the system is stable, the oscillation or even instability of the entire dc bus voltage occurs spontaneously. In the research of the phenomenon of oscillation or instability caused by impedance mismatching, the impedance stability analysis method is a common analysis means, and compared with a state space analysis method, the impedance stability analysis method has the advantages of clear physical concept, flexible modeling method and capability of directly measuring the impedance characteristic of a port.
In the conventional stability determination of a direct current power distribution system, the impedance characteristic of a source converter is generally represented by a bode graph, and the direct current power distribution system can determine the stability of the direct current power distribution system by determining whether a power output impedance and a load input impedance amplitude-frequency characteristic curve are connected or not and determining a phase difference at a connection position. The stability criterion requires that the source converter impedance amplitude is greater than the load converter impedance amplitude, or the source converter impedance amplitude and the load converter impedance amplitude are out of phase by no more than 180 degrees within the frequency range that the source converter impedance amplitude is less than the load converter impedance amplitude; the stability criterion is a non-sufficient stability criterion, the stability is judged by combining the amplitude and the phase difference, the intuitiveness of the stability criterion needs to be enhanced, the essence of impedance mismatching and instability cannot be accurately reflected, and especially when a plurality of converters are connected in a direct-current power distribution system, the stability analysis and judgment are more difficult to guarantee.
Disclosure of Invention
The invention provides a stability criterion method of a direct current converter parallel system based on impedance decomposition, which solves the problems that in the prior art, impedance analysis is low in accuracy in a direct current power distribution system, a physical mechanism is unclear, a system weak link is difficult to find, and positive and negative damping in instability oscillation is difficult to quantify due to impedance mismatching.
In order to solve the technical problems, the technical scheme adopted by the invention is a method for judging the stability of a parallel system of direct current converters based on impedance decomposition, which mainly comprises the following steps:
step S1: determining a direct-current bus voltage mathematical model at a common analysis node in a direct-current converter parallel system;
step S2: based on the Nyquist stability theorem, a method for judging the stability of the system according to the impedance of the source-load cascade system is provided, and the instability frequency of the cascade system is determined;
step S3: and determining a dominant subsystem causing instability at the instability frequency of the source load cascade system, and correcting the stability of the source load cascade system.
Further, the step S1 is specifically:
determining a common analysis node for a plurality of direct current converters accessed to a direct current distribution network; dividing the direct current converter into a source converter and a load converter at a common analysis node according to the power flow direction, and further simplifying the direct current distribution network into a source-load cascade system of a source subsystem and a load subsystem; the dc bus voltage V at the common analysis nodebus(s) is a mathematical model of
In the formula, Vo1(s) is the open-circuit output voltage of the source converter, Zin(s) is the load subsystem input impedance, Zos(s) is the equivalent impedance of the source subsystem, and the mathematical model is
Zos(s)＝Zo1(s)+Zline1(s)
In the formula, Zo1(s) is the output impedance of the source converter, Zline1(s) is the line impedance of the source transformer connected to the common analysis node.
Further, the step S2 is specifically:
step S21: the method comprises the steps that a direct-current bus voltage stability criterion at a common analysis node of a source-load cascade system is equivalent to a cascade system impedance stability criterion;
step S22: and carrying out stability criterion on the impedance of the cascade system by using the Nyquist stability judgment characteristic to determine the instability frequency of the cascade system.
Further, the step S21 is specifically: the stability judgment of the Nyquist curve applied to the source load cascade system needs to ensure the stability of the converter, namely the equivalent impedance Z of the source subsystemos(s) and load subsystem input impedance Zin(s) without the right half-plane pole, the stability of the source-to-charge cascade system depends on
The dc bus voltage V at the common analysis nodebusThe mathematical model of(s) can be further simplified to
Wherein Tm(s) ═ Zos(s)/Zin(s) is the open loop transfer function, is the DC bus voltage V of the entire cascade systembus(s) minimum loop gain; then the stability of the source-load cascade system is takenDependent on (1+ T)m(s)), for (1+ T)m(s)) applying the Cauchy argument theorem, wherein the amplitude-phase characteristic curve does not comprise an origin, which indicates that the source-load cascade system is stable; and (1+ T)m(s))＝(Zin(s)+Zos(s))/Zin(s) simultaneous load converter input impedance Zin(s) self-stabilizing, so that the DC bus voltage V at the common analysis nodebus(s) stability is dependent on Zin(s)+ZosAnd(s) the source load cascade system impedance application stability criterion can be equivalently established by connecting the source subsystem port equivalent impedance and the load subsystem input impedance in series.
Further, the step S22 is specifically: the nyquist stability judging characteristic is applied to the impedance of the cascade system which is equivalent to the series connection of the equivalent impedance of the port of the source subsystem and the input impedance of the load subsystem, and the essential condition for stabilizing the cascade system is that no reactance part crosses zero in the negative damping frequency range.
Further, the step S3 is specifically: the method comprises the steps of carrying out impedance frequency domain decomposition on source subsystem equivalent impedance and load subsystem input impedance at the instability frequency to obtain the specific proportion of positive and negative damping and the proportion of imaginary part capacitive inductive reactance of the source subsystem impedance and the load subsystem impedance at the instability frequency of the source-load cascade system impedance, further determining a leading subsystem causing instability at the instability frequency of the cascade system, and realizing stability correction of the cascade system by adopting at least one of positive damping of the source subsystem and the load subsystem.
Furthermore, the positive damping of the added source subsystem and the load subsystem is a passive damper formed by connecting a capacitor, an inductor and a resistor in series at the port of the source converter.
According to the technical scheme of the embodiment of the invention, the impedance stability judgment of the direct current power distribution system can be realized in a wide frequency domain range, and the system stability is analyzed from two angles of positive and negative damping and a resonant source, wherein the negative damping corresponds to an impedance characteristic real part, the resonant source corresponds to an impedance characteristic imaginary part zero crossing point, and the instability is explained from the aspect of physical essence. Through the quantification of positive and negative damping of the source-load cascade subsystem, the contribution degree of each converter to the negative damping and the analysis of the influence factors of the resonant source can be determined, so that accurate resonant frequency is obtained, and guidance is provided for impedance remodeling.
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In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the embodiments or the prior art will be briefly described below, it is obvious that the drawings in the following description are only some embodiments of the present invention, and for those skilled in the art, other drawings can be obtained according to the drawings without creative efforts.
FIG. 1 is a flow chart of a method for determining stability of a parallel system of DC converters based on impedance decomposition;
FIG. 2 is a diagram of a simple DC power distribution system topology;
FIG. 3 is a diagram of the topology partitioning source-load subsystem cascade shown in FIG. 2;
FIG. 4 is a simplified topological diagram of the source-to-load subsystem cascade system of FIG. 3;
FIG. 5 is a diagram of the Nyquist curve of the source-load cascade system impedance versus the real and imaginary parts of the impedance;
FIG. 6 is a schematic diagram of a destabilizing frequency range using a conventional stability criterion based on the topology shown in FIG. 3;
FIG. 7 is a graph of real and imaginary values of the source-charge system after a real and imaginary part solution;
FIG. 8 is a graph of real and imaginary values after decomposition of the cascaded system impedance and real and imaginary components;
fig. 9 is a graph of real and imaginary values of the impedance of the cascaded system after impedance reshaping and after real and imaginary part decomposition.
Detailed Description
The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the drawings in the embodiments of the present invention, and it is obvious that the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.
As shown in fig. 1, a flowchart of a method for determining stability of a parallel dc converter system based on impedance decomposition according to this embodiment mainly includes the following steps:
step S1: mathematical model for determining direct-current bus voltage at common analysis node in direct-current converter parallel system
For stability judgment of a plurality of converters connected to a direct-current power distribution network, a direct-current power distribution system is generally divided into a source subsystem and a load subsystem in a cascade mode, and points of the direct-current power distribution system divided into the source subsystem and the load subsystem are defined as common analysis nodes. In this embodiment, the stability analysis node of the dc power distribution system is set as the common analysis node due to the line impedance. The common analysis node is not selected uniquely, and is characterized in that port characteristics of each converter and line impedance are conveniently established. But the mathematical model of the impedance established via the common analysis node is relevant only for the selected analysis node. As shown in fig. 2, which is a topology structure diagram of a simple cascade system of a dc converter, converter # 1 is a voltage source converter, and is used for maintaining the bus voltage of a dc distribution network stable and providing energy for a dc load; vin1Is the input voltage of converter 1#, r1Is the energy storage inductor series resistance, L, of transformer 1#1Is the energy storage inductance, R, of transformer 1#cf1Is the resistance, C, of the filter capacitor of converter 1#f1Is the filter capacitance of converter 1#, rLine1Is the line impedance resistance, L, of converter 1#Line1Is the line inductance of converter # 1. The converter 2# is a constant power load based on a Buck circuit, and the constant power load of the direct current distribution system is simulated through the output voltage of the constant Buck circuit. Wherein R iscf2Is the filter capacitor series resistance, C of converter No. 2f2Is the filter capacitance of converter 2#, r2Is the energy storage inductor resistance, L, of converter # 22Is the energy storage inductor, C, of converter 2#2Is an energy storage inductor output filter capacitor, R2Is a load resistor combined with a control loop constant load resistor R2The voltage value of the Buck converter enables the Buck converter to embody constant power characteristics. In the dc converter simple cascade system topology shown in figure 2 with the load converter access points as common analysis nodes,converter # 1 is the source converter and converter # 2 is the load converter, depending on the common analysis node power flow direction. The output impedance of the source converter is Zo1(s) the line impedance of the source converter connected to the common analysis node is Zline1(s) input impedance of the load converter is Zin(s) since the source converter is connected directly to the common analysis node via the line impedance, i.e. the energy of converter 1# is passed through the line impedance Zline1(s) sending out, so that the converter 1# and the line impedance form a source subsystem of the cascade system, the topology structure of the simple cascade system of the direct current converter shown in fig. 2 can be simplified into the source load cascade subsystem shown in fig. 3, and then the mathematical model of the equivalent impedance of the source subsystem is as follows:
Zos(s)＝Zo1(s)+Zline1(s) (1)
the source-charge cascade subsystem shown in fig. 3 can be further simplified to the source-charge subsystem impedance cascade topology shown in fig. 4, where Vo1(s) is the open-circuit output voltage of the source converter, Ibus(s) is the DC bus current, Zin(s) is the equivalent input impedance of the load subsystem, then the DC bus voltage V at the load connection, i.e. the common analysis nodebus(s) is a mathematical model of
Step S2: based on Nyquist stability theorem, method for judging system stability according to source load cascade system impedance is provided
DC bus voltage V at common analysis node of source-load cascade systembus(s) is a unique feature that characterizes DC power distribution systems, thus VbusAnd(s) stable, the whole source load cascade system is stable. The stability judgment of the Nyquist curve applied to the source load cascade system needs to ensure the stability of the converter, namely the output impedance Z of the source subsystemos(s) and load subsystem input impedance Zin(s) without the right half-plane pole, the stability of the source-to-charge cascade system is dependent on
The dc bus voltage V at the common analysis nodebusThe mathematical model of(s) can be further simplified to
Wherein Tm(s) ═ Zos(s)/Zin(s), which is the open loop transfer function, is the minimum loop gain of the overall cascade system dc bus voltage vbus(s). The stability of the source-load cascade system depends on (1+ T)m(s)), for (1+ T)m(s)) applying the Cauchy amplitude-angle theorem, wherein the amplitude-phase characteristic curve does not comprise an origin, and the source-load cascade system is stable. And (1+ T)m(s))＝(Zin(s)+Zos(s))/Zin(s) simultaneous load converter input impedance Zin(s) self-stabilizing, so that the DC bus voltage V at the common analysis nodebusThe stability(s) can be equivalent to applying a stability criterion to the impedance of the source-load cascade system without influencing the number of molecular zeros of the source-load cascade system.
The source load cascade system impedance can be equivalent to the series connection of the output impedance of a source subsystem port and the input impedance of a load subsystem, and the mathematical model is as follows:
Zsum(s)＝Zos(s)+Zin(s) (4)
converting the system stability from complex frequency domain to frequency domain, as shown in FIG. 5, a point at an arbitrary frequency ω on the Nyquist curve of the impedance of the source-charge cascade system can be decomposed into the real part and imaginary part of the impedance
R＝|Zsum(jω)|cos(∠Zsum(jω)) (5)
X＝|Zsum(jω)|sin(∠Zsum(jω)) (6)
The real part R is an impedance resistance component of the source load cascade system, positive damping is positive damping, and negative damping is negative damping; and the imaginary part X is a reactive component of the source charge cascade system, positive values are inductive reactance, and negative values are capacitive reactance.
Based on the nyquist stability judging characteristic, the sufficient criterion for obtaining the stability of the source-load cascade system is as follows: at impedanceCharacteristic ZsumAnd zero-crossing points of imaginary parts are not arranged in the range of the negative real parts of (j omega). If, at R<When X is 0 and X is 0, the source load cascade system is in an unstable state, and the unstable frequency is the corresponding frequency ω at X is 01. I.e. the essential condition for the stability of the source-to-load cascade system is the absence of zero crossings of the reactive part in the negatively damped frequency range.
In other words, an essential condition for the stability of the source-load cascade system is that there are no zero crossings of the reactive part in the negatively damped frequency range.
Step S3: determining the dominant subsystem causing instability at the instability frequency of the source load cascade system, and correcting the stability of the cascade system
For instability frequency omega1Source subsystem output impedance Zos(jω1) Load subsystem input impedance Zin(jω1) And carrying out impedance frequency domain decomposition.
Then the real part Ro and the imaginary part Xo of the source subsystem are:
Ro＝|Zos(jω1)|cos(∠Zos(jω1)) (7)
Xo＝|Zos(jω1)|sin(∠Zos(jω1)) (8)
the real part Ro is a source subsystem impedance resistance component, a positive value is positive damping, and a negative value is negative damping; the imaginary part Xo is the reactive component of the source subsystem, positive values are inductive reactance, and negative values are capacitive reactance.
Real part R of load subsysteminWith the imaginary part XinComprises the following steps:
Xin＝|Zin(jω1)|sin(∠Zin(jω1)) (9)
Rin＝|Zin(jω1)|cos(∠Zin(jω1)) (10)
wherein the real part RinIs a load subsystem impedance resistance component, positive values are positive damping, and negative values are negative damping; imaginary part XinFor the load subsystem reactive components, positive values are inductive reactance and negative values are capacitive reactance.
Then source-load cascade systemZsum(jω1) At the destabilizing frequency omega1Can also be controlled by the source subsystem and the load subsystem at frequency ω1Real and imaginary components, and
R＝Ro+Rin (11)
X＝Xo+Xin (12)
the specific proportion of positive and negative damping and the proportion of imaginary part capacitive inductive reactance of the source subsystem impedance and the load subsystem impedance at the source-load cascade system impedance instability frequency can be obtained.
The stability of the system can be realized by changing the negative damping degree of the cascade system into positive damping at the instability frequency, and the stability correction of the cascade system can be realized by increasing at least one positive damping of the source subsystem and the load subsystem according to the actual damping condition of the source-load subsystem.
In the embodiment, negative damping at the instability frequency is adopted, a passive damper is connected in series at the port of the source converter, and the passive damper can form appropriate passive damping by using the combination of a capacitor, an inductor and a resistor according to the requirement of impedance remodeling. The requirements for the passive damper are that the passive damper must exhibit a positive damping characteristic at the destabilizing frequency, and that the positive damping value must be greater than the negative damping value at the destabilizing frequency.
The topology of the simple cascade system of the dc converter shown in fig. 2 is shown in fig. 6 by using the conventional impedance criterion, wherein (a) is the instability range using the conventional stability criterion, and (b) is the phase frequency band diagram around the instability frequency, so that the source load impedance characteristic curve in (b) has three intersections, and in combination with the phase curve, it can be known that the phase difference of the cascade system crosses 180 ° between the intersections 1 and 2, and then a instability point exists in the frequency range (670Hz-710Hz) between the intersections 1 and 2. Over a frequency range greater than intersection point 3, the phase does not cross 180 ° and no instability is present. There is therefore a point of instability in the entire cascade system, which is located between the points of intersection 1 and 2.
The impedance characteristic curve of the source-to-charge cascade system is decomposed in the frequency domain, so that the resistance component and the reactance component of the source-to-charge converter impedance in the frequency domain can be obtained, and the result in the frequency domain is shown in fig. 7, wherein (a) is the real part characteristic in the full frequency band, and (b) is the impedance characteristic in the frequency band of 600Hz to 850 Hz.
Impedance Z of the cascade systemsumThe result of(s) decomposition in the frequency domain is shown in fig. 8, where (a) is a full-band impedance characteristic curve, and (b) is an amplified impedance characteristic curve obtained by amplifying the 600-850Hz band in (a), the resistance component and the reactance component are known from (b), the imaginary zero-crossing points are three, which are respectively the intersection points 1, 2, 3, the real zero-crossing point is the intersection point 4, and the whole system exhibits negative damping in the frequency band from 600Hz to 690 Hz. In this frequency band, only the imaginary part of the intersection point 1 crosses zero. Therefore, the intersection 1 is a point satisfying the system instability, and is a system instability point.
It can be known from fig. 7 and 8 that the entire cascade system exhibits negative damping near the instability frequency band, in this frequency band (600Hz-690Hz), the real part in (b) of the corresponding source converter in fig. 6 is positive, i.e. positive damping, while the real part of the corresponding load converter is negative, i.e. negative damping, but the absolute value of the negative damping is greater than that of the positive damping of the source converter, so that the cascade system exhibits negative damping in this frequency band as a whole. It can be further stated that the instability in this frequency band is mainly caused by the negative damping characteristic of the load converter in this frequency band.
By performing positive damping remodeling on the source converter, the positive damping of the source converter at the instability frequency is improved, and further the positive damping of the whole cascade system is improved. After reshaping, the real and imaginary parts of the cascade system are shown in fig. 9; by positive damping reshaping the real part is made positive at zero crossings of the imaginary part, i.e. near the frequency points where the imaginary part is zero, the real part is larger than zero. The nyquist curve of the impedance of the cascade system crosses the positive real axis from the right side of the origin and thus does not surround the origin, so that the stability of the cascade system can be ensured.
All the embodiments in the present specification are described in a related manner, and the same and similar parts among the embodiments may be referred to each other, and each embodiment focuses on the differences from the other embodiments.
The above description is only for the preferred embodiment of the present invention, and is not intended to limit the scope of the present invention. Any modification, equivalent replacement, or improvement made within the spirit and principle of the present invention shall fall within the protection scope of the present invention.
Claims (4)
1. A stability criterion method for a parallel system of direct current converters based on impedance decomposition is characterized by mainly comprising the following steps of:
step S1: determining a direct-current bus voltage mathematical model at a common analysis node in a direct-current converter parallel system; defining points of a direct current power distribution system divided into a source subsystem and a load subsystem as common analysis nodes;
step S2: based on the Nyquist stability theorem, a method for judging the stability of the system according to the impedance of the source-load cascade system is provided, and the frequency causing the instability of the cascade system is determined;
step S3: determining a dominant subsystem causing instability at the instability frequency of the source load cascade system, and correcting the stability of the source load cascade system:
the step S2 specifically includes:
step S21: the method comprises the steps that a direct-current bus voltage stability criterion at a common analysis node of a source-load cascade system is equivalent to a cascade system impedance stability criterion;
step S22: carrying out stability criterion on the impedance of the cascade system by applying the Nyquist stability judging characteristic, and determining the frequency causing the instability of the cascade system;
the step S22 specifically includes: applying a nyquist stability judgment characteristic to a cascade system impedance which is equivalent to a series connection of a source subsystem port equivalent impedance and a load subsystem input impedance, wherein a sufficient condition for the stability of the cascade system is that no reactance part crosses zero in a negative damping frequency range;
the step S3 specifically includes: the method comprises the steps of carrying out impedance frequency domain decomposition on source subsystem equivalent impedance and load subsystem input impedance at the instability frequency to obtain the specific proportion of positive and negative damping and the proportion of imaginary part capacitive inductive reactance of the source subsystem impedance and the load subsystem impedance at the instability frequency of the source-load cascade system impedance, further determining a leading subsystem causing instability at the instability frequency of the cascade system, and realizing stability correction of the cascade system by adopting at least one of positive damping of the source subsystem and the load subsystem.
2. The method for judging the stability of the parallel system of the direct current converters based on the impedance decomposition as claimed in claim 1, wherein the step S1 is specifically as follows:
determining a common analysis node for a plurality of direct current converters accessed to a direct current distribution network; dividing the direct current converter into a source converter and a load converter at a common analysis node according to the power flow direction, and further simplifying the direct current distribution network into a source-load cascade system of a source subsystem and a load subsystem; the dc bus voltage V at the common analysis nodebus(s) is a mathematical model of
In the formula, Vo1(s) is the open-circuit output voltage of the source converter, Zin(s) is the load subsystem input impedance, Zos(s) is the equivalent impedance of the source subsystem, and the mathematical model is
Zos(s)＝Zo1(s)+Zline1(s)
In the formula, Zo1(s) is the output impedance of the source converter, Zline1(s) is the line impedance of the source transformer connected to the common analysis node.
3. The method for judging the stability of the parallel system of the direct current converters based on the impedance decomposition as claimed in claim 1, wherein the step S21 is specifically as follows: the stability judgment of the Nyquist curve applied to the source load cascade system needs to ensure the stability of the converter, namely the equivalent impedance Z of the source subsystemos(s) and load subsystem input impedance Zin(s) without the right half-plane pole, the stability of the source-to-charge cascade system depends on
The dc bus voltage V at the common analysis nodebus(s) is a mathematical model of
Wherein Tm(s) ═ Zos(s)/Zin(s) is the open loop transfer function, is the DC bus voltage V of the entire cascade systembus(s) minimum loop gain; the stability of the source-load cascade system depends on 1+ Tm(s), the Cauchy amplitude theorem is applied to the 1+ Tm(s), and the amplitude-phase characteristic curve does not include the origin, which indicates that the source-load cascade system is stable; and 1+ tm(s) ═ Zin(s)+Zos(s))/Zin(s) simultaneous load converter input impedance Zin(s) self-stabilizing, so that the DC bus voltage V at the common analysis nodebus(s) stability is dependent on Zin(s)+ZosAnd(s) the source load cascade system impedance application stability criterion can be equivalently established by connecting the source subsystem port equivalent impedance and the load subsystem input impedance in series.
4. The method as claimed in claim 1, wherein the step of increasing the positive damping of the source subsystem and the load subsystem is performed by connecting a passive damper formed by a combination of a capacitor, an inductor and a resistor in series at the port of the source converter.
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