CN118040706A - Power system transient overvoltage analysis method and system based on short circuit ratio - Google Patents
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Abstract
The invention provides a method and a system for analyzing transient overvoltage of a power system based on a short circuit ratio, which belong to the technical field of transient overvoltage calculation and comprise the following steps: expressing transient overvoltage at the converter bus according to a tide calculation equation; using a power factor to represent a new energy single-station short-circuit ratio calculation formula; substituting a new energy single-station short-circuit ratio as a constraint condition into a tide calculation equation to calculate a quantitative value of the transient overvoltage; when the line resistance is not considered and the line resistance is considered, respectively analyzing the quantitative relation between the transient overvoltage at the converter bus and the short-circuit ratio; and when the reactive compensation quantity in the converter station is a set multiple of the active power transmitted by the direct current line during normal operation, determining the relation between the short circuit ratio and the transient overvoltage.
Description
Technical Field
The invention belongs to the technical field of transient overvoltage calculation, and particularly relates to a new energy grid-connected system transient overvoltage analysis method based on a short circuit ratio.
Background
The statements in this section merely provide background information related to the present disclosure and may not necessarily constitute prior art.
With the increase of the grid-connected access ratio of new energy sources such as photovoltaic power generation, wind power and the like, the stability problem caused by large-scale new energy source development is outstanding, wherein the transient overvoltage problem is a main constraint for restricting the large-scale development of the new energy source.
The fault with the highest occurrence frequency in the direct current transmission system is commutation failure. At the early stage of commutation failure fault occurrence, the AC power grid of the transmitting end converter station can absorb a large amount of reactive power, the voltage of the transmitting end power grid can be greatly reduced, after a period of time, the converter station cuts off the reactive power compensation device and has time delay, a large amount of reactive power is injected into the transmitting end power grid, the voltage of the transmitting end power grid is increased, and therefore the transient voltage of the transmitting end power grid can be characterized by being reduced firstly and then increased.
When the short-circuit fault occurs in the transmitting-end alternating-current system, the voltage change condition of the transmitting-end power grid is similar to that of the commutation failure fault.
Unlike the above two faults, when a dc blocking fault occurs, a large amount of reactive surplus in the converter station is instantaneously injected into the feed-end system, the voltage of the feed-end is rapidly increased, and the 'first-decrease-then-increase' change process does not occur.
For the direct current power transmission end network which is connected in a high proportion in a concentrated way, as the new energy source has the function of high/low voltage crossing, in the fault process, when the voltage of the machine end of the new energy source reaches the threshold value of the high/low voltage crossing, the active and reactive power output of the new energy source can change, even the network is disconnected due to the failure of the voltage crossing, and the transient overvoltage can be aggravated.
Research shows that the relation between the generation of transient overvoltage and the power grid strength of new energy grid connection points is extremely large, and the weaker the power grid strength is, the greater the risk of the occurrence of transient overvoltage and other stability problems is.
The inventor finds that, in terms of research results on the aspect of the generation mechanism of the transient overvoltage of the transmitting end at present, when a plurality of researches are carried out to analyze the transient overvoltage problem, the transient overvoltage problem is analyzed according to different faults of the system, the relation between the transient overvoltage and the fault factors cannot be obtained directly, and the transient overvoltage research under the complex faults and the transient overvoltage comparison analysis under the different fault forms are less. Secondly, the quantitative analysis of the transient overvoltage of the transmitting end is mainly focused on the aspect of deriving the transient overvoltage and influence factors, when the short-circuit ratio constraint is adopted, the calculation formula is complex, the calculation complexity is extremely high when the multi-feed system is considered, and when the transient overvoltage of the multi-feed system is analyzed, the characteristic of the transient overvoltage of the transmitting end is mainly qualitatively researched through a simulation experiment.
Disclosure of Invention
In order to overcome the defects of the prior art, the invention provides a power system transient overvoltage analysis method based on a short circuit ratio, which has important significance for planning and running of a new energy grid-connected system.
To achieve the above object, one or more embodiments of the present invention provide the following technical solutions:
In a first aspect, a method for analyzing transient overvoltage of a power system based on a short circuit ratio is disclosed, comprising:
Expressing transient overvoltage at the converter bus according to a tide calculation equation;
using a power factor to represent a new energy single-station short-circuit ratio calculation formula;
substituting a new energy single-station short-circuit ratio as a constraint condition into a tide calculation equation to calculate a quantitative value of the transient overvoltage;
when the line resistance is not considered and the line resistance is considered, respectively analyzing the quantitative relation between the transient overvoltage at the converter bus and the short-circuit ratio;
And when the reactive compensation quantity in the normal working converter station is a set multiple of the active power transmitted by the direct current line, determining the relation between the short circuit ratio and the transient overvoltage.
As a further technical scheme, the transient overvoltage expression formula at the converter bus contains transient voltage rise per unit value at the fault converter bus;
And the short-circuit capacity and reactive variable quantity during faults are used for representing the transient voltage rise per unit value at the fault current conversion bus.
As a further technical solution, the transient overvoltage at the converter bus is expressed as:
。
as a further technical scheme, the transient voltage rise per unit value at the fault current conversion bus is
In the formula, delta Qd is reactive variable quantity of a converter bus in fault, and Sac is short-circuit capacity of the system.
As a further technical solution, by a short-circuit ratio calculation formula, the single-station short-circuit ratio can be expressed as:
In the formula, uac is equivalent power supply phase voltage generated by new energy of a direct current transmission end, equivalent line impedance between an equivalent power supply of an alternating current power grid of the transmission end and a converter busbar is Z=R+jX, P d is active power sent out by a direct current line, and phi is a power factor angle of a converter station.
As a further technical solution, the transient overvoltage at the converter bus can be expressed as:
。
in a second aspect, a new energy grid-connected system transient overvoltage analysis system based on short circuit ratio constraint is disclosed, comprising:
A transient overvoltage representation module configured to: expressing transient overvoltage at the converter bus according to a tide calculation equation;
A short ratio calculation module configured to: using a power factor to represent a new energy single-station short-circuit ratio calculation formula;
A quantitative value calculation module of transient overvoltage configured to: substituting a new energy single-station short-circuit ratio as a constraint condition into a tide calculation equation to calculate a quantitative value of the transient overvoltage;
A quantitative relationship analysis module configured to: when the line resistance is not considered and the line resistance is considered, respectively analyzing the quantitative relation between the transient overvoltage at the converter bus and the short-circuit ratio;
A relationship determination module of short ratio and transient overvoltage configured to: and when the reactive compensation quantity in the converter station is a set multiple of the active power transmitted by the direct current line during normal operation, determining the relation between the short circuit ratio and the transient overvoltage.
The one or more of the above technical solutions have the following beneficial effects:
According to the technical scheme of the embodiment, the method for analyzing the transient overvoltage of the power system based on the short circuit ratio is independent of a model and line parameters of an actual interconnection system, only the proportion of reactive compensation quantity and active output of new energy in each converter station and the power factor of the system after reactive compensation are required to be set, wherein when the active output of the new energy is selected as a reference value, the analysis of the transient overvoltage of the system can be realized only by knowing the reactive compensation quantity, and therefore, the method can be applied to the online judgment of the transient voltage stability of the new energy grid-connected system.
According to the technical scheme of the embodiment, reactive compensation quantity in each converter station is needed during quantitative analysis, so that the method can be applied to determining reactive compensation strategies based on transient overvoltage stabilization of a new energy grid-connected system.
According to the technical scheme of the embodiment, the calculated short-circuit ratio index can reflect the stability degree of the new energy grid-connected system and can judge the strength degree of the system under the constraint condition of the short-circuit ratio besides the calculation of the short-circuit ratio of the new energy grid-connected system.
According to the technical scheme of the embodiment, quantitative analysis of the transient overvoltage of the system under the constraint of the short circuit ratio is realized, the severity of different faults can be ordered according to the magnitude of the short circuit ratio after the faults through analysis of the quantitative relation between the short circuit ratio and the transient overvoltage and calculation of the short circuit ratio, and therefore the method can also be applied to fault screening of the power system.
The technical scheme of the embodiment can realize the early discrimination of the transient overvoltage of the system, so that the method can also be applied to the safety early warning of the interconnection system.
According to the technical scheme of the embodiment, the relation between the generation of the transient overvoltage and the power grid strength of the new energy grid-connected point is fully considered, the transient overvoltage is quantitatively analyzed based on the short circuit ratio, a theoretical basis is provided for improving the transient stability of the new energy grid-connected system, meanwhile, the constraint condition can be used, the reactive compensation optimization strategy is further determined, and the method has important significance for planning and running of the new energy grid-connected system.
Additional aspects of the invention will be set forth in part in the description which follows and, in part, will be obvious from the description, or may be learned by practice of the invention.
Drawings
The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and together with the description serve to explain the invention.
FIG. 1 is a flow chart of a method of an embodiment of the present invention;
FIG. 2 is a schematic diagram of an equivalent circuit of a DC power transmission terminal according to an embodiment of the present invention;
FIG. 3 is a graph showing the relationship between transient overvoltage and short-circuit ratio when the line resistance is not considered in the embodiment of the invention;
FIG. 4 is a graph showing the relationship between transient overvoltage and short circuit ratio when the line resistance is considered and K 2 is more than 3 in the embodiment of the present invention;
FIG. 5 is a graph showing the relationship between transient overvoltage and short circuit ratio when the line resistance is considered in the embodiment of the invention.
Detailed Description
It should be noted that the following detailed description is exemplary and is intended to provide further explanation of the invention. Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs.
It is noted that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of exemplary embodiments according to the present invention.
Embodiments of the invention and features of the embodiments may be combined with each other without conflict.
Example 1
The embodiment discloses a power system transient overvoltage analysis method based on a short circuit ratio, which comprises the following steps:
s1: and according to a tide calculation formula, the transient overvoltage at the converter bus is represented.
S2: and the short-circuit capacity and reactive variable quantity during faults are used for representing the transient voltage rise per unit value at the fault current conversion bus.
S3: and (3) representing a new energy single-station short-circuit ratio calculation formula by using a power factor.
S4: substituting the short circuit ratio as constraint condition into a power flow calculation equation to calculate the quantitative value of the transient overvoltage.
S5: and analyzing the variation trend of the power factor and the short circuit ratio, and analyzing the variation of the transient overvoltage by taking the short circuit ratio as a core variable.
S6: and when the line resistance is not considered, analyzing the quantitative relation between the transient overvoltage and the short circuit ratio at the converter bus. Randomly selecting a power factor, and analyzing the relation between the transient overvoltage and the short circuit ratio by a mathematical method.
S7: and when the line resistance is considered, analyzing the quantitative relation between the transient overvoltage and the short circuit ratio at the converter bus. And assuming that the reactive compensation quantity in the converter station is K 1 times of the active power transmitted by the direct current line in normal operation, 0<K 1 is less than 1, simplifying the reactive compensation quantity by using a P d standard, and determining the relation between the short circuit ratio and the transient overvoltage through a mathematical relation.
So far, the transient overvoltage of the new energy grid-connected system based on the short circuit ratio constraint is successfully analyzed, and the quantitative relation between the short circuit ratio and the transient overvoltage of the new energy grid-connected system is obtained.
In one or more embodiments, in step S1, according to the load flow calculation formula, the transient overvoltage at the converter bus may be simply expressed as:
（1）
In the method, in the process of the invention, For transient overvoltage at the converter bus, U L is the transient voltage at the converter bus, U L is the change in the transient voltage after a fault, and represents per unit value.
In one or more embodiments, in step S2, the transient step-up per unit value at the faulty converter bus is:
（2）
in the formula, delta Qd is reactive variable quantity of a converter bus in fault, and Sac is short-circuit capacity of the system.
In one or more embodiments, in the step S3, the equivalent circuit is obtained from the dc power transmission end
（3）
In the formula, the power sent by the new energy power generation of the direct current end to the power transmission end converter station is P ac+jQac,Pd, which is the active power sent by the direct current line, Q d is the reactive power consumed in the converter station, Q c is the reactive compensation quantity in the direct current power transmission end converter station, and phi is the power factor angle of the converter station.
By a short-circuit ratio calculation formula, scalar quantities such as power factor, direct current output active power and the like are considered, the strength of the system is evaluated, and the connection between voltage and direct current output power and transient overvoltage is established, so that the single-station short-circuit ratio can be expressed as follows:
（4）
In the formula, uac is equivalent power supply phase voltage generated by new energy of a direct current transmission end, equivalent line impedance between an equivalent power supply of an alternating current power grid of the transmission end and a converter busbar is Z=R+jX, P d is active power sent out by a direct current line, and phi is a power factor angle of a converter station.
In one or more embodiments, in step S4, the formula is calculated according to the power flow
（5）
Is available in the form of
（6）
Will be
（7）
The quantitative relation between the transient overvoltage and the short-circuit ratio K can not be obtained by substituting the formula (6):
（8）
where φR is the power factor angle of the equivalent impedance on the line.
The relation between the short-circuit ratio K and the transient overvoltage U L is quantitatively expressed in the formula (8), and whether the transient overvoltage exceeds the safety requirement or not can be calculated only by the reactive compensation quantity Q c and the active P d sent out by direct current, and the system power factor at the moment, and the fault severity can be further judged according to the value of the transient overvoltage and the short-circuit ratio.
It can be seen from equation (4) that the change in power factor also affects the change in the short-circuit ratio at this time, and the effect of cos phi on the short-circuit ratio should be further analyzed.
In one or more embodiments, in step S5, when cos Φ increases, it means that reactive exchange of grid-connected points becomes smaller, so that system strength becomes larger, and the short-circuit ratio increases accordingly, so that cos Φ and the short-circuit ratio change trend coincide. Meanwhile, when cos phi changes, the change magnitude is 0.1 and 0.01, and when the short-circuit ratio changes, the change magnitude is 1 and 10, so that the influence of cos phi on the short-circuit ratio can be ignored when the influence of the short-circuit ratio on the transient overvoltage is studied, although the coupling relation exists between cos phi and the short-circuit ratio.
In one or more embodiments, in step S6, when the on-line resistance is ignored, equation (6) is further reduced to
（9）
Transient overvoltage at the converter bus can be expressed as
（10）
The formula (10) simplifies the calculation formula of the transient overvoltage, and the transient overvoltage can be calculated only by knowing the reactive compensation quantity Q c and the power factor.
When the line resistance is not considered, the active power flow on the line is small, namely the reactive power consumption is small, so that the reactive power exchange at the converter station is weak, the power factor is high, and the cos phi approaches to 1.
It can be seen from equation (10) that as the short-circuit ratio increases, the transient overvoltage decreases, and the transient overvoltage is inversely related to the short-circuit ratio and has a certain relationship with the power factor.
In step S7, a large amount of reactive compensation is required in the operation of the dc power transmission converter (rectification and inversion), assuming that the reactive compensation amount in the converter station is K 1 times of the active power transmitted by the dc line during normal operation, 0<K 1 <1, and the formula (8) is further simplified by using the reference P d
（11）
For the convenience of analysis, let
（12）
In the middle of
（13）
Deriving f (K) to obtain
（14）
The inflection point of f (K) is obtained as
（15）
Since a, b are functions of phi, the inflection point of f (K) and its monotonicity are analyzed, first the monotonicity of a (phi), b (phi) is analyzed.
Because 0.95< cos phi <1, cos phi monotonously increases, sin phi monotonously decreases, b (phi) monotonously decreases, and monotonicity of a (phi) is analyzed below.
Deriving a (phi) to obtain
（16）
Let the impedance ratio X/R=K 2, a (phi) <0, obtain
（17）
By limiting the line impedance ratio and the ratio of reactive compensation quantity in the converter station to the active power transmitted by the direct current line, the analysis of the relation between the transient overvoltage and the short circuit ratio is simplified, and a simplified thought is provided for transient overvoltage analysis.
As can be seen from querying the common line parameters, the line impedance ratio in the actual system is usually in the range 0<K 2 <2, so K 2 >3 is not a common case in the actual system, and therefore a detailed analysis is not performed.
When a (phi) monotonically increases, (1-a/K) 2 monotonically decreases with phi, f (phi) monotonically decreases with phi at this time by formula (12), and the relationship between transient overvoltage and phi exhibits a monotonically decreasing trend, so using the assignment method, two cases were concluded by analyzing cos phi=0.95 and cos phi=1, respectively: as the short-circuit ratio increases, the transient overvoltage decreases, and the transient overvoltage and the short-circuit ratio show a negative correlation.
The following is an example. By using MATLAB programming, irrespective of the line resistance, cos phi is randomly selected, and a curve of the relationship between the short-circuit ratio and the transient overvoltage is drawn as shown in fig. 3, so that as the short-circuit ratio increases, the transient overvoltage decreases, and the relationship between the transient overvoltage and the short-circuit ratio is negative.
When the line resistance is considered and K 2 >3, the transient overvoltage versus short circuit ratio is plotted at this time against the optional cos phi, K2 as shown in FIG. 4. As can be seen from querying the common line parameters, the line impedance ratio in the actual system is usually in the range 0<K 2 <2, so K 2 >3 is not a common case in the actual system, and therefore a detailed analysis is not performed.
Since the relationship between the transient overvoltage and the power factor angle phi shows a monotonically decreasing trend, the monotonicity of f (K) is analyzed by further analysis using an assignment method by analyzing two cases when cos phi=0.95 and cos phi=1, respectively. An image of transient overvoltage versus short circuit ratio for cos phi=0.95 and cos phi=1 is plotted as shown in fig. 5.
It can be seen from fig. 5 that as the short-circuit ratio increases, the transient overvoltage decreases, and the transient overvoltage exhibits a negative correlation with the short-circuit ratio.
In summary, in most cases, the transient overvoltage and the short-circuit ratio are inversely related, and if and only if K 2 >3 and the relationship shown in equation (17) is satisfied, further analysis is needed at this time, and it should be noted that this case is not in the usual range of the system.
Example two
It is an object of the present embodiment to provide a computer device comprising a memory, a processor and a computer program stored on the memory and executable on the processor, which processor implements the steps of the above method when executing the program.
Example III
An object of the present embodiment is to provide a computer-readable storage medium.
A computer readable storage medium having stored thereon a computer program which when executed by a processor performs the steps of the above method.
Example IV
The embodiment aims to disclose a new energy grid-connected system transient overvoltage analysis system based on short circuit ratio constraint, which comprises:
A transient overvoltage representation module configured to: expressing transient overvoltage at the converter bus according to a tide calculation equation;
A short ratio calculation module configured to: using a power factor to represent a new energy single-station short-circuit ratio calculation formula;
A quantitative value calculation module of transient overvoltage configured to: substituting a new energy single-station short-circuit ratio as a constraint condition into a tide calculation equation to calculate a quantitative value of the transient overvoltage;
A quantitative relationship analysis module configured to: when the line resistance is not considered and the line resistance is considered, respectively analyzing the quantitative relation between the transient overvoltage at the converter bus and the short-circuit ratio;
A relationship determination module of short ratio and transient overvoltage configured to: and when the reactive compensation quantity in the converter station is a set multiple of the active power transmitted by the direct current line during normal operation, determining the relation between the short circuit ratio and the transient overvoltage.
The steps involved in the devices of the second, third and fourth embodiments correspond to those of the first embodiment of the method, and the detailed description of the embodiments can be found in the related description section of the first embodiment. The term "computer-readable storage medium" should be taken to include a single medium or multiple media including one or more sets of instructions; it should also be understood to include any medium capable of storing, encoding or carrying a set of instructions for execution by a processor and that cause the processor to perform any one of the methods of the present invention.
It will be appreciated by those skilled in the art that the modules or steps of the invention described above may be implemented by general-purpose computer means, alternatively they may be implemented by program code executable by computing means, whereby they may be stored in storage means for execution by computing means, or they may be made into individual integrated circuit modules separately, or a plurality of modules or steps in them may be made into a single integrated circuit module. The present invention is not limited to any specific combination of hardware and software.
While the foregoing description of the embodiments of the present invention has been presented in conjunction with the drawings, it should be understood that it is not intended to limit the scope of the invention, but rather, it is intended to cover all modifications or variations within the scope of the invention as defined by the claims of the present invention.
Claims (10)
1. A method for analyzing transient overvoltage of a power system based on a short circuit ratio is characterized by comprising the following steps:
Expressing transient overvoltage at the converter bus according to a tide calculation equation;
using a power factor to represent a new energy single-station short-circuit ratio calculation formula;
substituting a new energy single-station short-circuit ratio as a constraint condition into a tide calculation equation to calculate a quantitative value of the transient overvoltage;
when the line resistance is not considered and the line resistance is considered, respectively analyzing the quantitative relation between the transient overvoltage at the converter bus and the short-circuit ratio;
and when the reactive compensation quantity in the converter station is a set multiple of the active power transmitted by the direct current line during normal operation, determining the relation between the short circuit ratio and the transient overvoltage.
2. The method for analyzing the transient overvoltage of the power system based on the short circuit ratio according to claim 1, wherein the transient overvoltage expression formula at the converter bus contains a transient voltage rise per unit value at the fault converter bus;
And the short-circuit capacity and reactive variable quantity during faults are used for representing the transient voltage rise per unit value at the fault current conversion bus.
3. The method for analyzing transient overvoltage of a power system based on a short circuit ratio according to claim 1, wherein the transient overvoltage at the converter bus is expressed as:
；
In the method, in the process of the invention, For transient overvoltage at the converter bus, U L is the transient voltage at the converter bus, U L is the change in the transient voltage after a fault, and represents per unit value.
4. The method for analyzing transient overvoltage of power system based on short circuit ratio as claimed in claim 3, wherein the transient voltage rise per unit value at the fault current converting bus is
In the formula, delta Qd is reactive variable quantity of a converter bus in fault, and Sac is short-circuit capacity of the system.
5. The method for analyzing transient overvoltage of power system based on short-circuit ratio as claimed in claim 1, wherein the short-circuit ratio of single-station is expressed as:
In the formula, uac is equivalent power supply phase voltage generated by new energy of a direct current transmission end, equivalent line impedance between an equivalent power supply of an alternating current power grid of the transmission end and a converter busbar is Z=R+jX, P d is active power sent out by a direct current line, and phi is a power factor angle of a converter station.
6. A method of analyzing transient overvoltage in an electrical power system based on a short circuit ratio as claimed in claim 3, wherein the transient overvoltage at the converter bus is further expressed as:
；
K is the single-station short-circuit ratio, U L transient overvoltage, Q c is reactive compensation quantity, delta Qd is reactive change quantity of a converter bus in fault, P d is active power sent out by a direct-current line, phi is a power factor angle of the converter station, and Uac is equivalent power supply phase voltage of new energy power generation at a direct-current end.
7. The utility model provides a new forms of energy grid-connected system transient state overvoltage analysis system based on short circuit ratio constraint which characterized in that includes:
A transient overvoltage representation module configured to: expressing transient overvoltage at the converter bus according to a tide calculation equation;
A short ratio calculation module configured to: using a power factor to represent a new energy single-station short-circuit ratio calculation formula;
A quantitative value calculation module of transient overvoltage configured to: substituting a new energy single-station short-circuit ratio as a constraint condition into a tide calculation equation to calculate a quantitative value of the transient overvoltage;
A quantitative relationship analysis module configured to: when the line resistance is not considered and the line resistance is considered, respectively analyzing the quantitative relation between the transient overvoltage at the converter bus and the short-circuit ratio;
A relationship determination module of short ratio and transient overvoltage configured to: and when the reactive compensation quantity in the converter station is a set multiple of the active power transmitted by the direct current line during normal operation, determining the relation between the short circuit ratio and the transient overvoltage.
8. The new energy grid-connected system transient overvoltage analysis system based on the short-circuit ratio constraint according to claim 7, wherein the transient overvoltage expression formula at the converter bus contains a transient voltage rise per unit value at the fault converter bus;
And the short-circuit capacity and reactive variable quantity during faults are used for representing the transient voltage rise per unit value at the fault current conversion bus.
9. A computer device comprising a memory, a processor and a computer program stored on the memory and executable on the processor, characterized in that the processor implements the steps of the method of any of the preceding claims 1-6 when the program is executed.
10. A computer readable storage medium, on which a computer program is stored, characterized in that the program, when being executed by a processor, performs the steps of the method of any of the preceding claims 1-6.
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