CN107706909B - Oscillation center identification system and method based on frequency characteristics under multi-frequency oscillation - Google Patents

Oscillation center identification system and method based on frequency characteristics under multi-frequency oscillation Download PDF

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CN107706909B
CN107706909B CN201710840778.7A CN201710840778A CN107706909B CN 107706909 B CN107706909 B CN 107706909B CN 201710840778 A CN201710840778 A CN 201710840778A CN 107706909 B CN107706909 B CN 107706909B
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CN107706909A (en
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马静
李沛
康文博
刘静
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North China Electric Power University
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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
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Abstract

The invention belongs to the technical field of stability analysis of power systems, and particularly relates to an oscillation center identification system and method based on frequency characteristics under multi-frequency oscillation, wherein the system comprises a data acquisition module, a step-out/oscillation center crossing judgment module, a step-out/oscillation center position identification module and a result output module; the data acquisition module is used for acquiring network structure parameters and bus frequency on each side in the system; the step-out/oscillation center crossing judging module judges whether the step-out/oscillation center crosses or not so as to judge whether the step-out/oscillation center is in the protection range of the T-connection line or not; and the step-out/oscillation center position identification module identifies the step-out/oscillation center position according to the characteristic that the frequency change condition of the side bus is opposite to the frequency change condition of the rest two sides bus when the step-out/oscillation center crossing judgment module judges that no crossing phenomenon exists, namely the step-out/oscillation center is positioned at one side of the three-end system, and sends the analysis result to the result output module.

Description

Oscillation center identification system and method based on frequency characteristics under multi-frequency oscillation
Technical Field
The invention belongs to the technical field of stability analysis of power systems, and particularly relates to an oscillation center identification system and method based on frequency characteristics under multi-frequency oscillation.
Background
When the operation of the power system is severely disturbed, the generator in the system cannot keep relatively stable, out-of-step oscillation occurs, and in order to prevent the breakdown of the power system, on one hand, the system needs to be split into a plurality of subsystems on an out-of-step section to keep stable operation; on the other hand, it is necessary to prevent a relay protection device in the grid from malfunctioning due to oscillation, thereby expanding the disturbance influence range. The accurate positioning and tracking of the out-of-step/oscillation center is an important aspect of electric power system oscillation research, can provide an important reference basis for an electric power system out-of-step separation device, and is also one of important judgment bases for protecting oscillation locking. With the increasing expansion of the operation scale of the power grid, the connection between the regional power grids is tighter, and when a fault occurs in the power system and further oscillation is caused, the step-out/oscillation center is not fixed at a certain point on the line any more and can move among the regional power grids. In a multi-frequency oscillation scene, the change rule of the electrical quantity and the like are greatly changed, and the traditional positioning method based on the two-machine system model is not applicable to the situation of step loss/oscillation center movement in the multi-frequency oscillation scene.
At present, most of research on the method for identifying and tracking the oscillation center is based on two-machine system models, and the two types are mainly divided into two types, one is to realize step-out/oscillation center positioning by using local information, such as based on an apparent impedance track, an apparent impedance angle or
Figure BDA0001409473810000011
The method and the like can accurately position the oscillation center. The other type is to realize positioning by utilizing wide area information, at present, on the basis of an identification method based on the electric quantity frequency characteristics of the oscillation center of a two-machine system model, partial scholars have developed the research of a step-out/oscillation center positioning method under a multi-frequency oscillation scene, analyze the frequency distribution characteristics under the multi-frequency oscillation scene and provide a positioning method of the step-out center, however, the assumed condition is that all system impedances and line impedances are equal and have larger difference with the actual situation.
Disclosure of Invention
In view of this, the present invention provides an oscillation center identification system based on frequency characteristics under multi-frequency oscillation and a method thereof.
The system comprises:
the system comprises a data acquisition module, a step-out/oscillation center crossing judgment module, a step-out/oscillation center position identification module and a result output module; the data acquisition module is used for acquiring network structure parameters and bus frequency on each side in the system; the step-out/oscillation center crossing judging module judges whether the step-out/oscillation center crosses or not so as to judge whether the step-out/oscillation center is in the protection range of the T-connection line or not; and the step-out/oscillation center position identification module identifies the step-out/oscillation center position according to the characteristic that the frequency change condition of the side bus is opposite to the frequency change condition of the rest two sides bus when the step-out/oscillation center crossing judgment module judges that no crossing phenomenon exists, namely the step-out/oscillation center is positioned at one side of the three-end system, and sends the analysis result to the result output module.
The method comprises the following steps:
step 1: three side power supplies are connected to one point O through three lines to construct a three-machine equivalent system model;
step 2: obtaining the phase angle difference of equivalent potentials at any two sides according to the phase relation of the potential oscillation initial time of the equivalent power supply at each side;
and step 3: according to the kirchhoff current law, listing an O-point current equation, deducing to obtain an instantaneous value expression of each side current, and combining a trigonometric function relationship to obtain current frequency characteristics under a multi-machine oscillation scene;
and 4, step 4: analyzing the current frequency characteristics of any point p to obtain the voltage frequency characteristics under the multi-machine oscillation scene, and analyzing to obtain the relationship between the voltage frequency distribution and the potential phase angle, the relative angular velocity and the specific position;
and 5: obtaining a deviation boundary of an oscillation center according to the continuous change process of the voltage frequency, wherein a deviation farthest point is a step-out center;
step 6: if the frequency change of a certain bus is severe compared with that of other buses, or the waveforms of two adjacent frequency change sections of the bus are reverse, the out-of-step center is crossed, the oscillation center identification is not needed, otherwise, the identification is carried out according to the step 7;
and 7: and detecting the frequency of the bus on each side, if the frequency change of one side is consistent and opposite to that of the other side, the oscillation center appears on the other side, and determining the position of the oscillation center point of the system under the multi-machine oscillation scene according to the intensity of the frequency change.
The invention has the beneficial effects that: the invention provides a voltage and current expression based on any point in a system, deduces a function expression of voltage and current frequency of the voltage and current expression, analyzes the change situation and the distribution situation of the voltage and current frequency in the system under a multi-machine oscillation scene, researches the relation between the frequency distribution of the system and a step-out/oscillation center, and provides an oscillation center identification tracking scheme based on the change situation of the bus frequency of the system. Simulation results based on DIgSILENT/PowerFactory show that the scheme provided by the invention can effectively track the position of the oscillation center of the multi-frequency oscillation system and can lay a foundation for out-of-step disconnection and protection of oscillation locking.
Drawings
Fig. 1 is a structural diagram of an oscillation center identification system based on frequency characteristics in a multi-frequency oscillation scene provided by the invention.
Fig. 2 is a configuration diagram of a T-connection system protection device in the embodiment of the present invention.
Fig. 3 is a phase relationship of equivalent power supply potentials on respective sides at the initial timing of oscillation in the embodiment of the invention.
FIG. 4 is a plot of delta during oscillation in an embodiment of the present invention12The phase relation of the equivalent power supply potential on each side when approaching 0.
FIG. 5 is a delta in oscillation in an embodiment of the invention13The phase relation of the equivalent power supply potential on each side when approaching 0.
Fig. 6 is a graph showing frequency changes at points a in the embodiment of the present invention.
FIG. 7 is a graph showing p during oscillation in an embodiment of the present inventionADot voltage waveform diagrams.
Fig. 8 is a graph showing frequency changes at points B in the embodiment of the present invention.
Fig. 9 is a graph showing frequency changes at points C in the embodiment of the present invention.
Fig. 10 is a frequency versus power angle distribution plot at a bus in an embodiment of the invention.
Fig. 11 is a system relative power angle simulation curve in an embodiment of the invention.
Fig. 12 shows the subsystem 1 side current in the embodiment of the present invention.
Fig. 13 is a subsystem 2 side current in an embodiment of the invention.
Fig. 14 shows subsystem 3 side currents in an embodiment of the invention.
Fig. 15 is a graph showing frequency fluctuations at each side system bus in the embodiment of the present invention.
Fig. 16 is a voltage change curve of the monitoring point in the embodiment of the present invention.
Detailed Description
The embodiments are described in detail below with reference to the accompanying drawings.
Fig. 1 is a structural diagram of an oscillation center identification system based on frequency characteristics in a multi-frequency oscillation scene, which includes a data acquisition module, an out-of-step/oscillation center crossing discrimination module, an out-of-step/oscillation center position identification module and a result output module that are connected in sequence.
Figure 2 is a model of a multi-frequency oscillation equivalent three-machine system,
Figure BDA0001409473810000041
back side equivalent electromotive forces, Z, of the bus1, the bus2 and the bus3 respectively1、Z2、Z3Respectively, equivalent system impedance, Z, corresponding to the backside1O、Z2OAnd Z3OThe equivalent impedances of bus1, bus2 and bus3 to point O, respectively. R1, R2, R3 respectively represent protection devices each side of which is mounted on the bus bar side or the outlet line. In the analysis process, the specified current flows from the bus to the line to be positive, and impedance angles of upper and lower-stage elements of the system are equal.
FIG. 3 is the phase relationship of the initial oscillation time of the equivalent power supply potential on each side, where ω is the angular frequency of the A-side power supply and Δ ω is12Is the equivalent potential of the B side
Figure BDA00014094738100000410
Relative to the angular frequency of the A-side supply potential, Δ ω13Is C-side equivalent potential
Figure BDA0001409473810000042
Angular frequency relative to the a-side supply potential. In the oscillation process, if the A-side power supply potential is taken as a reference, the B-side equivalent potential is at any moment
Figure BDA0001409473810000043
Equivalent potential of C side
Figure BDA00014094738100000411
Relative to the equivalent potential of the A side
Figure BDA0001409473810000044
The phase angle difference is respectively as follows:
Figure BDA0001409473810000045
in the formula, delta02And delta03Are respectively vibrationOscillation initial time
Figure BDA0001409473810000046
Relative to
Figure BDA0001409473810000047
The phase angle of (c). The system on the A side represents a system formed by a circuit of the generator G1 and G1, the system on the B side is a system formed by a circuit of the generator G2 and G2, and the system on the C side is a system formed by a circuit of the generator G3 and G3.
Current frequency characteristics in a multi-frequency oscillation scenario:
the O point current equation is listed according to kirchhoff's current law as follows:
Figure BDA0001409473810000048
in the formula:
Figure BDA0001409473810000049
is a voltage of point O, ZAΣ＝Z1+Z1OFor the combined impedance of the A-side system, ZBΣ＝Z2+Z2OFor the combined impedance of the B-side system, ZCΣ＝Z3+Z3OThe impedance is synthesized for the C-side system.
The voltage phasor expression at O point according to equation (2) is:
Figure BDA0001409473810000051
and deducing the current phasor of each side system according to the voltage at the point O and the equivalent potential expression of each side system. Taking side a as an example, the current expression of side a is:
Figure BDA0001409473810000052
in the analysis process, the power supply potential on each side is set as E, and the instantaneous value expression of the current on the A side can be written according to the formulas (1) and (4) as follows:
Figure BDA0001409473810000053
from the trigonometric function relationship, equation (5) can be rewritten as:
Figure BDA0001409473810000054
as can be seen from the equation (6), the frequency characteristic of the current under the multi-machine oscillation scene is different from the frequency characteristic of the current under the two-machine oscillation scene, and the current frequency is not the average value of the power supply frequencies at the two sides, but is an amplitude value
Figure BDA0001409473810000055
At a frequency of
Figure BDA0001409473810000056
And an amplitude of
Figure BDA0001409473810000057
At a frequency of
Figure BDA0001409473810000058
Are superimposed.
In connection with equations (5) and (6), the current frequency is analyzed and discussed as follows:
1) when delta12The first partial amplitude of equation (6) approaches 0
Figure BDA0001409473810000059
Also approaches zero, the dominant frequency of the current is
Figure BDA00014094738100000510
At this time, the phase relationship of the potentials on the respective sides of the system can be regarded as the oscillation of the a-side and B-side power supplies with respect to the C-side power supply, as shown in fig. 4.
2) When delta13The second partial amplitude of equation (6) approaches 0
Figure BDA00014094738100000511
Approaches zero, thisThe dominant frequency of the time current is
Figure BDA00014094738100000512
At this time, the phase relationship of the potentials of the respective sides of the system can be regarded as the oscillation of the a-side and C-side power supplies with respect to the B-side power supply, as shown in fig. 5.
3) When delta2＝δ3When the system is in synchronization with the system on the B side and the system on the C side, the system is degraded into a two-machine system oscillation model, and the current frequency is
Figure BDA0001409473810000061
Voltage frequency characteristics in a multi-frequency oscillation scenario:
if the point p is any point in the side a system, the equivalent electrical distance from the point to the side a system is p × LAO, and p is any real number in the interval [0,1], the voltage expression of the point p is:
Figure BDA0001409473810000062
the A-side current expression (4) is substituted into an expression (7) and is transformed into the form of amplitude and phase angle:
Figure BDA0001409473810000063
in the formula: smAnd SnAre respectively as
Figure BDA0001409473810000064
The real part and the imaginary part of (c), the expression is:
Figure BDA0001409473810000065
Figure BDA0001409473810000066
is composed of
Figure BDA0001409473810000067
The equivalent phase angle of (c).
The angular frequency of the p-point voltage can be expressed as a derivative function of its phase angle, i.e.:
Figure BDA0001409473810000068
in the formula: Δ ω is the increment of the angular frequency.
From the above derivation, it can be seen that the voltage frequency distribution is not only related to the potential phase angle, relative angular velocity, but also to the location where the point is located in the system. Further observation has shown that the angular frequency increment Δ ω is inversely proportional to the square of the voltage amplitude at that point. Therefore, the frequency distribution is greatly affected by the system voltage distribution.
The frequency increment at a point in the system is inversely proportional to the square of the voltage amplitude at that point. At the step-out center, the voltage amplitude is zero, so that the frequency increment at the step-out center approaches infinity, and the frequency will be distorted at the step-out center. For other points in the system, the closer to the out-of-step center, the smaller the voltage amplitude of the point, and even close to zero, and thus the larger the frequency increment, the more drastic the frequency change.
The system power angle change is a continuous process, and therefore, the frequency change value calculated according to equation (9) is also continuous. The oscillation center moves continuously in the system, moves from the point O to a certain side to the farthest point, returns to the point O, and further moves to other system sides. The oscillation center has a definite offset boundary, and the boundary function value is only related to the structure parameter. During the oscillation, the a-side oscillation center moves to the farthest point ρAAnd then returning, the point is also the desynchronizing central point of the system in a certain oscillation mode.
Does not have Z in the formula (2)AΣ:ZBΣ:ZCΣThe system impedance angle is 86.5 ° at 10:5:4, and the farthest point ρ of the oscillation center on the a side can be calculated therefromAPosition p 0.6364. Let Δ ω12And Δ ω13When the voltage is larger than zero, the voltage frequency of each point in the A-side system can be obtained according to the formula (9), partial points of the A-side system are selected to be used as the frequency change chart as shown in figure 6, and the voltage waveforms of the points can be generated as shown in figure 7. It can be seen from fig. 7 that when t is 40s, ρAZero crossing of point voltage as loss of systemAnd (5) step center.
It can be seen from fig. 6 and 7 that the frequency changes of the system are opposite on both sides of the step-out center point. The frequency variation of each point in the system reaches the maximum value at the moment when the step-out center appears on the line. Meanwhile, the frequency variation ranges at p-0.8 and p-0.5 are significantly smaller than those at p-0.7 and p-0.6, and thus the frequency variation is more severe as the step-out center is closer.
Fig. 8 and 9 show the frequency variation of system partial points on the B side and C side, respectively. The farthest offset point rho of the oscillation center of the B side can be calculated by the same methodBPosition p is 0.7778, and the farthest point ρ is the center of oscillation on the C sideCPosition p-0.8750, p is pBAnd ρCBoth of which may be system out-of-sync centers. In order to observe the frequency change on both sides of the step loss/oscillation center, the frequencies of points where p is 0.7 to 1 on the B side and C side are made are shown in fig. 8 and 9. It can be seen that, during the period of time t being 0s to 22.8s, the frequency changes of each point in the B-side system are consistent, the desynchronizing center is not located on the side, but the frequency is obviously reversed between the position where p is 0.9 and the position where p is 1.0 in the C-side system, and the frequency fluctuates greatly, so that the desynchronizing center is located between the position where p is 0.9 and the position where p is 1.0 on the C-side.
As can be seen from fig. 6, 8 and 9, when the step-out/oscillation center passes through a certain point, the change of the frequency waveform before and after passing through the point is exactly opposite, for example, when p is 0.8 and p is 0.7 in the a-side system, when the step-out/oscillation center does not pass through the points, the change of the frequency is the same as that of the other points, when the step-out/oscillation center passes through the two points, the change of the frequency is opposite to that of the other two points, and when the step-out/oscillation center moves from the a-side system to the other sides of the system, the change of the frequency is the same as that of the other two points; before the step-out/oscillation center passes through a certain point, the point is always positioned on one side of the step-out/oscillation center, so that the change situations of the frequency waveforms are kept consistent, for example, the point p is 0.5 and the point p is 0.6 in an A-side system, the step-out/oscillation center does not pass through the points all the time, the frequency change situations of the two points are kept unchanged, the characteristic is called step-out/oscillation center crossing characteristic, and the crossing situation of the step-out/oscillation center can be judged by utilizing the characteristic.
Identifying the step-out/oscillation center position based on the bus frequency characteristics:
let Δ ω12And Δ ω13Are all larger than zero, can make delta12And delta13Under the condition of different power angle values, the voltage frequency change curves of different points in the system are shown in fig. 10, which sequentially represents the time delta12And delta13In the range of [ - π, π]When the frequency changes in the interval, the frequencies of the A side bus, the B side bus and the C side bus change. It can be found that when the step-out/oscillation center appears in the a-side system, the frequency change of the B-side system is consistent with that of the C-side system, but opposite to that of the a-side system; when the step-out/oscillation center appears in the B side system, the frequency change conditions of the A side system and the C side system are consistent, but are opposite to the frequency change condition of the B side system; when the step loss/oscillation center appears in the C-side system, the frequency change of the A-side system is consistent with that of the B-side system, but opposite to that of the C-side system.
As can be seen from equation (9), the frequency of the voltage at a certain point in a certain system is related to the frequency of the power supply and the frequency of the current at the certain point. Analysis of the current frequency characteristics in a multi-frequency oscillation scenario shows that the frequency of the system current is related to the frequency of the system power supply on each side and the relative positions of the three equivalent potentials (refer to fig. 4 and 5). Meanwhile, when the power angles of other two side power supplies are closer, the degree of deviation of the step-out/oscillation center to the side system is larger, and in the scene, the three-machine system can be similar to a two-machine system. For a T-connection multi-machine system, the relative power angle is changed constantly, so that the periodic short-time recombination of machine groups in an oscillation period can be regarded, and a two-machine oscillation mode with different machine group combinations is formed. If a certain side of the combination has a step-out center, the voltage frequency change conditions of the two systems in the combination are consistent and opposite to the voltage frequency change condition of the other side (single machine side) of the step-out center. Therefore, the frequency change condition of each side bus is monitored, and the section condition of the step-out center can be judged.
Aiming at a multi-frequency oscillation scene, the invention provides a system and a method for identifying a step-out/oscillation center based on system bus frequency change. The system and the method thereof have the following characteristics:
1) the conclusion that the system current frequency is constant in the oscillation of the two-machine system is not applicable under the T-connection multi-frequency oscillation scene. The T-connection system current is formed by overlapping two components with different frequencies, and different dominant frequencies can be presented in the power angle swinging process.
2) The frequency of each point in the multi-frequency oscillation system is determined by the power angle, the relative angular velocity and the position of the point, and the increment of the frequency is inversely proportional to the square of the voltage amplitude of the point.
3) When the step-out/oscillation center appears, the frequency variation of each point reaches an extreme value, and the frequency is distorted at the step-out/oscillation center. The closer the points are to the step-out/oscillation center, the more drastic the frequency change, and the frequency change of the points on both sides of the step-out/oscillation center is opposite.
4) When the step-out/oscillation center falls into one side of the three-terminal system, the frequency change condition of the bus at the side is opposite to the frequency change condition of the buses at the other two sides, and a system step-out/oscillation center identification scheme is formed based on the frequency change condition of the bus at the side.
Examples
A simulation system model built based on the DIgSILENT/PowerFactory platform is shown in FIG. 2, wherein the models of all lines of the system are the same, and the line positive sequence parameters are as follows: x1-0.4497 Ω/km R1-0.0529 Ω/km, c 1-0.0064 μ F/km, and the zero sequence parameters are: x0-0.6027 Ω/km R0-0.08145 Ω/km, c 0-0.004 μ F/km, line L1 length 80km, line L2 length 200km, line L3 length 120km, positive sequence equivalent impedance parameter 1.693+ j37.528 Ω of G1 system, and zero sequence impedance Z0-0.410 + j37.18.42 Ω; the positive sequence equivalent impedance parameters of the G2 system are as follows: the impedance Z1 is 1.21+ j57 Ω, the zero-sequence impedance Z0 is 0.6+ j9.091 Ω, and the positive-sequence equivalent impedance parameters of the G3 system are as follows: z1 is 0.77+ j43.235 Ω, and zero-sequence impedance Z0 is 0.6+ j29.097 Ω. For convenience of description, each side system is named by a generator number, namely: subsystem 1 represents a system formed by a side circuit of a generator G1 and a generator G1, and subsystem 2 and subsystem 3 represent a side circuit of a generator G2 and a generator G3 and a system formed by the side circuit of the generator G1. The simulation time is 6s in total, and the simulation system takes power frequency (50Hz) as the reference frequency.
In the simulation process, G3 is taken as a reference machine, the system stably operates in the initial state, the power angle of G1 relative to G3 is 20 degrees, and the power angle of G1 relative to G3 is 10 degrees. When t is 0.5s, the system starts to oscillate, and the relative power-angle variation curve of the system in the simulation is shown in fig. 11. As can be seen from the figure, after 0.5s, the power angle of G1 and the power angle of G2 gradually swing away from G3, and a three-machine oscillation state appears. The measured current waveform distribution in the subsystem 1, the subsystem 2 and the subsystem 3 during the oscillation process is shown in fig. 12, 13 and 14, respectively. Therefore, in the later period of system oscillation, the oscillation period is shortened, and the out-of-step speed is accelerated.
In the simulation process, frequency monitoring devices are respectively arranged on the Bus1, the Bus2 and the Bus 3. The frequency variation of each side bus is shown in fig. 15, and the frequency variation during the oscillation simulation process can be divided into 12 stages according to the results of the foregoing analysis, and each stage represents the process of step loss/oscillation center movement in different subsystems. Since the movement process is similar, only the first 10 stages are analyzed here, with a time period indicated between every two black dashed lines, which are numbered in fig. 15. In the stage 1, from t being 0.5s to 1.517s, the change of the measurement frequency at the Bus1 is consistent with the change of the measurement frequency at the Bus2, and the change of the measurement frequency at the Bus3 is opposite, and the step loss/oscillation center is positioned on the L3. The stage is divided into two parts, during the period that t is 0.5 s-1.036 s, the step-out/oscillation center shifts towards the inside of the subsystem 3, and the frequency shift of each side reaches the maximum value of the stage; thereafter, during the period t becomes 1.036s to 1.517s, the step-out/oscillation center starts to move from the inside of the subsystem 3 to O. In phase 2, the system out-of-sync/oscillation center is still shifted towards subsystem 3 and then back. In stage 3, the frequency change situation at the Bus of the subsystem 1 is changed to be consistent with the frequency change situation at Bus3, and in contrast to the frequency change situation at Bus2, the step-out/oscillation center is located on L2, the step-out/oscillation center shifts to the G2 side during the period of time t being 2.293 s-2.517 s, the frequency change reaches the maximum value of the period of time when t being 2.517s, and the step-out/oscillation center starts to shift from the L2 side to the O point during the period of time t being 2.517 s-2.817 s. Similarly, it can be analyzed that the step-out/oscillation center shifts to the L3 side in the 4 th stage and shifts to the L2 side in the 5 th stage, and thereafter, the step-out/oscillation center continuously moves back and forth on L2 and L3.
Since the exact position of the step-out/oscillation center cannot be accurately obtained according to the measurement, in order to verify the correctness of the above process, the offset boundary of the step-out/oscillation center on each side is selected as a monitoring point to monitor the change condition of the system voltage. By calculation, a point nod1 on the line L1 distant from the O point 36.32km, a point nod2 on the line L2 distant from the O point 118.88km, and a point nod3 on the line L3 distant from the O point 24.52km were respectively selected as monitoring points. Since the three voltage monitoring points are the deviation boundary points of the oscillation center, the voltage change condition of the point can effectively reflect the moving route of the oscillation center. During the simulation, the voltage change of the three voltage monitoring points is shown in FIG. 16.
According to the time division of fig. 15, the simulation process shown in fig. 16 is divided into 12 stages, and corresponding labels are made in the figure. It can be seen that each stage in fig. 15 can completely correspond to that in fig. 16, and the voltage variation of the monitoring point is completely consistent with the analysis conclusion of fig. 15. For example, in the stage 1, the step-out/oscillation center is located on the side of the subsystem 3 during the period t is 0.5s to 1.517s, the stage is divided into two parts, the voltage of each monitoring point is decreased during the period t is 0.5s to 1.036s, the voltage of the monitoring point nod3 on the side of the subsystem 3 is the lowest, thereafter, the voltage of each monitoring point is increased during the period t is 1.036s to 1.517s, the voltage of the node 3 is the lowest, and the step-out/oscillation center returns from the inside of the subsystem 3 to O. In the stage 2, the voltage change condition of each monitoring point is similar to that in the stage 1, and the system step-out/oscillation center still deviates to the subsystem 3 and then returns; in the 3 rd stage, the step-out/oscillation center is positioned at the side of the subsystem 2, the voltage at the monitoring point nod2 rapidly drops and falls to 0 in 2.517 s; other stages may perform similar analysis. The embodiment simulation result shows that the method for identifying the oscillation center based on the frequency characteristics in the multi-frequency oscillation scene can effectively identify the position of the step-out/oscillation center of the system by monitoring the change condition of the bus frequency of the system.
The above embodiments are only preferred embodiments of the present invention, but the scope of the present invention is not limited thereto, and any changes or substitutions that can be easily conceived by those skilled in the art within the technical scope of the present invention are also within the scope of the present invention. Therefore, the protection scope of the present invention shall be subject to the protection scope of the claims.

Claims (1)

1. An oscillation center identification method based on frequency characteristics under multi-frequency oscillation is characterized by comprising the following steps:
step 1: three side power supplies are connected to one point O through three lines to construct a three-machine equivalent system model;
step 2: obtaining the phase angle difference of equivalent potentials at any two sides according to the phase relation of the potential oscillation initial time of the equivalent power supply at each side;
and step 3: according to the kirchhoff current law, listing an O-point current equation, deducing to obtain an instantaneous value expression of each side current, and combining a trigonometric function relationship to obtain current frequency characteristics under a multi-machine oscillation scene;
and 4, step 4: analyzing the current frequency characteristics of any point p to obtain the voltage frequency characteristics under the multi-machine oscillation scene, and analyzing to obtain the relationship between the voltage frequency distribution and the potential phase angle, the relative angular velocity and the specific position;
and 5: obtaining a deviation boundary of an oscillation center according to the continuous change process of the voltage frequency, wherein a deviation farthest point is a step-out center;
step 6: if the frequency change of a certain bus is severe compared with that of other buses, or the waveforms of two adjacent frequency change sections of the bus are reverse, the out-of-step center is crossed, the oscillation center identification is not needed, otherwise, the identification is carried out according to the step 7;
and 7: and detecting the frequency of the bus on each side, if the frequency change of one side is consistent and opposite to that of the other side, the oscillation center appears on the other side, and determining the position of the oscillation center point of the system under the multi-machine oscillation scene according to the intensity of the frequency change.
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