WO2020232824A1

WO2020232824A1 - Sag domain recognition method considering uncertainty of load sensitivity

Info

Publication number: WO2020232824A1
Application number: PCT/CN2019/097841
Authority: WO
Inventors: 李新; 武利会; 罗容波; 邱太洪; 范心明; 董镝; 宋安琪; 陈邦发; 王俊波; 李慧
Original assignee: 广东电网有限责任公司; 广东电网有限责任公司佛山供电局
Priority date: 2019-05-22
Filing date: 2019-07-26
Publication date: 2020-11-26
Also published as: CN110082643B; CN110082643A

Abstract

A sag domain recognition method considering the uncertainty of load sensitivity, comprising the following steps: performing power flow calculation to acquire the voltage amplitude of a system node before a fault; calculating sequence impedances; calculating a residual voltage of each bus fault; obtaining upper and lower limits of an uncertainty interval of a sensitive load according to an existing voltage tolerance curve to form a discriminant matrix; calculating a line critical point according to the discriminant matrix and the lower and upper limits of the uncertainty interval of the sensitive load; and traversing all lines and all uncertain spaces, and outputting two sag domain recognition results corresponding to the upper and lower limits of the uncertainty interval of the sensitive load. Considering the uncertainty interval of the sensitive load, a sag domain that is closer to an actual project is described, overcoming the defect that the existing sag domain recognition method fails to consider that sensitive loads have different tolerance to voltage sags, and fixing the sag domain by means of a definite boundary fails to take all sag domain critical points into account.

Description

A method of sag zone identification considering the uncertainty of load sensitivity

Technical field

The present invention relates to the technical field of identifying the voltage sag region of a power system, and more specifically, to a method for identifying the sag region considering the uncertainty of load sensitivity.

Background technique

Load sensitivity refers to the sensitivity of the load to power quality problems such as voltage quality. When the power quality provided to the load is poor, the load’s ability to withstand interference and still work normally. The lower the capacity, the higher the sensitivity. In the actual power system During operation, the load sensitivity is uncertain; the sag domain refers to when a short-circuit fault occurs in the power system, the fault causes the voltage amplitude of the sensitive point to drop below a certain voltage tolerance threshold, making the sensitive load unable to work normally In order to reduce the adverse effects of voltage sags on sensitive loads and realize the reliable and high-quality operation of the power grid, the collection of fault points has important guiding significance.

Strictly speaking, the sag domain is not an area in a strict sense, but a collection of points (substations) and lines (lines and cables), but the area enclosed by the envelope is more conducive to the visualization of the concept, which can be clear Whether the electrical components belong to the sag domain.

At present, a variety of sag zone identification methods have been proposed at home and abroad, mainly including fault point method, critical distance method, analytical method and other methods. The above methods have more or less defects in the sag zone identification calculation, such as Chen Tiemin , Yang Honggeng mentioned in the "Voltage Sag Evaluation Based on Improved Fault Point Method" published by "Power Automation Equipment" (2008, 28(6)) that the accuracy of the fault point method in the sag domain identification calculation is difficult to guarantee and requires a lot of The information of the fault point can improve the calculation accuracy; and the current sag domain calculation methods are all based on traditional short-circuit calculations, and the algorithm considers few influencing factors, resulting in a large difference between the pre-evaluation results of the sag domain and the actual results. There is a method to calculate an exact sag region boundary. The size of the sag region is mainly determined by the voltage sag tolerance of the equipment. Generally, although the voltage sag tolerance of sensitive loads is considered to be a specific However, due to the different operating modes and load conditions of the equipment, the withstand capability of sensitive equipment may be different. In this case, the voltage sag withstand characteristics of the sensitive load cannot be described by a fixed voltage sag withstand curve. The location of the voltage sag tolerance curve is uncertain, and the corresponding voltage critical point cannot be fully considered. Therefore, the fixed sag domain boundary will have certain limitations at this time.

Summary of the invention

The present invention overcomes the disadvantages that the existing sag region identification method does not take into account the different tolerance of sensitive loads to voltage sags, fixes the sag region with a clear boundary and cannot take all voltage critical points into consideration. The present invention is based on Based on the upper and lower bounds of the variation range of the sensitive load voltage tolerance curve interval, a sag region identification method considering the uncertainty of load sensitivity is proposed.

In order to achieve the above technical effects, the technical solution of the present invention is as follows:

The method includes the following steps:

S1: Calculate the power flow of the system to obtain the voltage amplitude U _pref during normal operation of each node of the system before the short-circuit fault;

S2: Set the short-circuit fault node K and the sensitive load access node m, calculate the zero sequence, positive sequence, and negative sequence transfer impedance between point K and point m, and the input impedance of point K and the voltage amplitude before the fault;

S3: Calculate the residual voltage amplitude U _{mag of} each bus according to the voltage amplitude of each node of the system before the fault and the input impedance of the fault point K;

S4: Obtain the upper limit U _max and the lower limit U _{min of the} uncertainty interval of the voltage withstand capability curve by knowing the variation range of the uncertainty interval of the voltage withstand capability curve, combined with the residual voltage amplitude U _{mag of} each busbar fault, Form the discriminant matrix BUS and LINE;

S5: Let the voltage tolerance value U _{th of the} load carried by node m take the lower limit value U _{min of the} uncertainty interval, calculate the critical point of the line according to the value of the discriminant matrix LINE, and determine the inclusion of the line in the sag domain ；

S6: Judge whether all lines of the system have been traversed and all the uncertain spaces have been traversed, if yes, output the sag domain recognition result; if not all lines have been traversed, return to step S4 for the next line recognition calculation; if all lines have been traversed Line, but has not traversed all the uncertain spaces, let U _th take the upper limit U _{max of the} uncertain interval, and return to step S4 to continue the calculation;

S7: Count two calculations, obtain sag domain 1 corresponding to U _min and sag domain 2 corresponding to U _max , and obtain the final sag domain identification result.

Preferably, the sequence transfer impedance between the faulty node K and the sensitive load access node m in step S2 is:

Z _mK,0 =Z _mF,0 +p(Z _mT,0 -Z _mF,0 )

Z _mK,1 =Z _mF,1 +p(Z _mT,1 -Z _mF,1 )

Z _mK,2 =Z _mF,2 +p(Z _mT,2 -Z _mF,2 )

Among them, Z _mK,0 , Z _mK,1 , Z _mK,2 are the sequence transfer impedances between the faulty node K and the sensitive load access node m, where Z _mK,0 is the zero sequence transfer impedance, Z _{mK, 1} is the positive sequence transfer impedance, Z _{mK, 2} is the negative sequence transfer impedance; Z _mF,0 , Z _mF,1 , Z _{mF, 2} are the sequences between the first-end bus node F of the sensitive load node m and the load point m Transfer impedance, where Z _mF,0 is zero sequence transfer impedance, Z _mF,1 is positive sequence transfer impedance, Z _mF,2 is negative sequence transfer impedance; Z _mT,0 , Z _mT,1 , Z _mT,2 are sensitive Each sequence transfer impedance between load node m tail bus node T and load point m, where Z _mT,0 is zero sequence transfer impedance, Z _mT,1 is positive sequence transfer impedance, Z _mT,2 is negative sequence transfer Impedance; p is the unit value of the distance from the fault point K to the bus head node F;

The input impedance of the fault node K is:

Z _KK,0 =p ² (Z _FF,0 +Z _TT,0 -2Z _FT,0 -Z _C,0 )+p[Z _C,0 -2(Z _FF,0 -Z _TT,0 )]+ Z _{FF, 0}

Z _KK,1 =p ² (Z _FF,1 +Z _TT,1 -2Z _FT,1 -Z _C,1 )+p[Z _C,1 -2(Z _FF,1 -Z _TT,1 )]+ Z _{FF, 1}

Z _KK,2 =p ² (Z _FF,2 +Z _TT,2 -2Z _FT,2 -Z _C,2 )+p[Z _C,2 -2(Z _FF,2 -Z _TT,2 )]+ Z _FF,2

In the formula, Z _KK,0 , Z _KK,1 , Z _KK,2 are the sequence input impedances of the faulty node K, where Z _KK,0 is the zero sequence impedance, Z _KK,1 is the positive sequence input impedance, and Z _{KK ,2} is the negative sequence input impedance; Z _FF,1 , Z _FF,2 Z _FF,0 , Z _FF,1 , Z _FF,2 is the input impedance of the first bus node F at the fault point k, where Z _{FF, 0} is the zero sequence input impedance, Z _FF,1 is the positive sequence input impedance, Z _FF,2 is the negative sequence input impedance; Z _TT,0 , Z _TT,1 , Z _TT,2 are the sequences of the tail bus node T Input impedance, where Z _TT,0 is zero sequence input impedance, Z _TT,1 is positive sequence input impedance, Z _TT,2 is negative sequence input impedance; Z _C,0 , Z _C,1 , Z _C,2 are The sequence impedances on the line FT, where Z _C,0 is the zero sequence impedance, Z _C,1 is the positive sequence impedance, Z _C,2 is the negative sequence impedance, and p has the same meaning as described above;

The voltage amplitude before short-circuit fault occurs at node K is expressed as:

U _K,pref =U _F,pref +p(U _T,pref -U _F,pref )

Among them, U _K,pref represents the voltage before the short-circuit fault occurs at node K; U _F,pref represents the node voltage amplitude of the bus node F before the fault; U _T,pref represents the node voltage amplitude of the bus node T before the fault; p and The meaning mentioned above is the same.

Preferably, the busbar fault residual voltage amplitude U _mag described in step S3:

Among them, U _mag represents the residual voltage amplitude of the bus fault;

Is the voltage amplitude of each phase of bus m when a fault occurs at point K,

Indicates the voltage amplitude of phase A,

Indicates the amplitude of phase B voltage,

It represents the voltage amplitude of phase C; U _A,m,pref is the voltage amplitude of phase A before the fault at point m; a represents the rotation factor e ^j120° ;

Represents the voltage amplitude at point m when the bus node K fails.

Preferably, the upper and lower limits of the uncertainty interval corresponding to the uncertainty curve of the voltage sag tolerance of the sensitive load described in step S4 are U _max and U _{min respectively} , and the discriminant matrices BUS and LINE are formed as follows:

Among them, U _th represents the voltage withstand capability of the load carried by node m; ΔU _mag,n represents the difference between the bus residual voltage amplitude U _mag and the voltage withstand capability U _th ; for i∈(1,...,n), When the value of ΔU _mag,i is less than or equal to zero, BUS _i takes 1, which means that the bus i is included in the sag domain; when the value of ΔU _mag,i is greater than zero, BUS _i takes 0, which means that the bus i is not included in the sag domain. The LINE value is the sag domain discrimination parameter of the line. The value of LINE _i is 0, which means that bus i is not in the sag domain; the value of LINE _i is 1, which means that the bus i contains a critical point, and the value of LINE _i is 2. Indicates that there are two critical points on the bus i;

Is the sag zone discrimination parameter of bus F at both ends of line i;

Is the sag zone discrimination parameter of bus T. When one of them is set to 1, LINE _{i is set to} 1, which means that the line has a critical point; when both are set to 0, LINE _i is set to be 0, which means the line The line has no critical points; if both are 1, then LINE _i takes the value 2, which means the line has two critical points.

Preferably, the upper limit U _{max of the} uncertainty curve of the voltage sag tolerance of the sensitive load represents the boundary between the immune zone and the uncertainty zone; the lower boundary U _min represents the boundary between the uncertainty zone and the failure zone.

Preferably, the process of step S5 is:

S501: determining whether the value of the LINE _i 0, if the value of the LINE _i 0, i indicates the bus dips located outside, the next line is performed is determined; the value of the LINE if _i is not 0, obtained by cubic spline interpolation Voltage sag amplitude curve, and execute step S502;

S502: Determine whether the value of LINE _i is 1, if the value of LINE _i is 1, find the critical point; if the value of LINE _i is not 1, go to step S503;

S503: Calculate the maximum point p _max of the voltage sag amplitude on the line and the residual voltage equation function f |f(pmax)| by using the golden section search method;

S504: After obtaining p _max , judge the size of |f(p _max )| and U _th . If |f(p _max )| is less than U _th , then the entire line is included in the sag domain, and return to step S501 to continue execution; If |f(p _max )| is greater than U _th , the critical point is determined according to the critical point determination method where the LINE _i value is 1.

Preferably, in step S502, if the value of LINE _i is 1, the method for obtaining the critical point is:

S5021: Use the three points p=0, p=0.5, and p=1 to perform spline interpolation, and obtain the residual voltage equation function as

Define U _th =f(p _i ), where x ₁ , x ₂ , and x ₃ are the coefficients of the function, and the independent variable corresponding to the p _i function. At this time, p _{i is} taken as p _i =0, p _i =0.5 and p _i =1;

S5022: Calculate |f(0.5)|. The three points described in step S5021 correspond to the short-circuit voltage amplitude of the first node of the line, the short-circuit voltage amplitude of the tail node of the line and |f(0.5)|, and the residual voltage equation function is calculated as 0 The root p _ic on ≤p _ic ≤1;

S5023: Use the root of the residual voltage equation to define the initial iteration value, determine the two initial iteration values near the critical point, take [p _ic ,p _ic +Δp] or [p _ic -Δp,p _ic ], △p= 0.01 is the initial value points p _from and p _end of the secant iteration to find the critical point; the iteration process is as follows:

p _new = p _end -[f(p _end )-U _th ](p _end -p _from )/[|f(p _end )|-|f(p _from )|]

p _from = p _end

p _end = p _new

Among them, p _from and p _end are the two initial value points of the iteration; p _new is the new iteration value of the critical point; f(p _end ) represents the residual pressure amplitude of the point p _end ; f(p _from ) represents the point p _from The residual voltage amplitude; U _th represents the voltage withstand capability of the load carried by node m;

The convergence conditions are:

||f(p _new )|-U _th |＜tol

Among them, tol is the accuracy of convergence; both p _new and U _th have the same meaning as described above.

S5024: Use Newton's method to obtain accurate critical point values.

Preferably, the steps of calculating the maximum value point p _max of the voltage sag amplitude value on the line and the residual voltage equation function f described in step S503 are as follows:

S5031: According to the golden ratio point

Define two initial value points p ₁ , p ₂ , the expression of the two initial value points is:

S5032: Calculate the residual pressure equation function values corresponding to p ₁ and p ₂ |f(p ₁ )| and |f(p ₂ )|;

S5033: Determine whether |f(p ₁ )| is greater than or equal to |f(p ₂ )|, if |f(p ₁ )|≥|f(p ₂ )|, perform the following assignment iteration:

p _b = p ₂

p ₂ =p ₁ ;

If |f(p ₁ )|<|f(p ₂ )|, then perform the following assignment iteration:

p _a = p ₁

p ₁ =p ₂ ;

Preferably, the method for obtaining the critical point according to the value of LINE _{i in} step S504 is:

S5041: Use the three points p=0, p=p _max and p=1 to perform spline interpolation, and obtain the residual voltage equation function as

Define U _th =f(p _i ), where x ₁ , x ₂ , and x ₃ are the coefficients of the function, and the independent variable corresponding to the p _i function. At this time, p _{i is} taken as p _i = 0 and p _i = p _max And p _i = 1;

S5042: Calculate |f(p _max )|, the three points described in step S5042 correspond to the short-circuit voltage amplitude of the first node of the line, the short-circuit voltage amplitude of the tail node of the line, and |f(p _max )| to obtain the residual voltage equation function The root p _ic on 0≤p _ic ≤1;

S5043: Use the root of the residual voltage equation to define the initial iteration value, determine the two initial iteration values near the critical point, take [p _ic ,p _ic +Δp] or [p _ic -Δp,p _ic ], △p= 0.01 is the initial value points p _from and p _end of the secant iteration to find the critical point; the iteration process is as follows:

p _from = p _end

p _end = p _new

The convergence conditions are:

||f(p _new )|-U _th |＜tol

Among them, tol is the convergence accuracy, and both p _new and U _th have the same meaning as described above.

S5043: Use Newton's method to obtain the exact value of the critical point when |f(p _max )| is greater than U _th .

Compared with the prior art, the technical solution of the present invention has the beneficial effect that: the method of the present invention is based on the uncertainty interval of the existing voltage withstand curve, selecting the upper limit and the lower limit of the uncertainty interval to obtain critical points, respectively, Describe the sag region that is closer to the actual project, overcome the existing sag region identification method that does not take into account the different tolerance of sensitive loads to voltage sags, fix the sag region with a clear boundary, and cannot make all sag regions critical Points to consider the drawbacks.

Description of the drawings

Fig. 1 shows a flowchart of a method for identifying a sag domain according to an embodiment of the present invention.

Figure 2 shows the IEEE30 node system diagram.

Figure 3 shows a schematic diagram of a failure of the line FT.

Figure 4 shows the voltage sag tolerance curve of sensitive loads.

Figure 5 shows a schematic diagram of the golden section search method.

Figure 6 shows a schematic diagram of reading the critical point sag domain results.

Figure 7 shows a schematic diagram of reading the results of two critical point sag regions.

Figure 8 shows a schematic diagram of the sag domain recognition result of the IEEE30 node system.

Detailed ways

The attached drawings are only for illustrative purposes, and cannot be understood as a limitation of this patent;

In order to better illustrate this embodiment, some parts of the drawings may be omitted, enlarged or reduced, and do not represent the size of the actual product;

For those skilled in the art, it is understandable that some well-known structures in the drawings and their descriptions may be omitted.

The technical solution of the present invention will be further described below in conjunction with the drawings and embodiments.

The flow diagram of the implementation method of this embodiment is shown in Fig. 1. Taking the IEEE30 node system shown in Fig. 2 as an example, the fault type of the system is a single-phase grounding fault. As shown in Fig. 1, the sag domain calculation process as follows:

S1: Perform power flow calculation according to the existing network parameters and operating data of the system shown in Figure 2 to obtain the voltage amplitude U _{pref of} each node of the system before the short-circuit fault in normal operation;

S2: As shown in Figure 3, on the line FT, set the short-circuit fault node K and the sensitive load access node m, calculate the zero sequence, positive sequence, negative sequence transfer impedance between K and m, and the input impedance of K , Voltage amplitude before failure:

The sequence transfer impedance between the fault node K and the sensitive load node m is:

Z _mK,0 =Z _mF,0 +p(Z _mT,0 -Z _mF,0 )

Z _mK,1 =Z _mF,1 +p(Z _mT,1 -Z _mF,1 )

Z _mK,2 =Z _mF,2 +p(Z _mT,2 -Z _mF,2 )

The input impedance of the fault node K is:

U _K,pref =U _F,pref +p(U _T,pref -U _F,pref )

S3: Calculate the residual voltage amplitude U _{mag of} each bus according to the voltage amplitude of each node of the system before the fault and the input impedance of the fault point K.

U _{mag is} calculated as follows:

Among them, U _mag represents the residual voltage amplitude of the bus fault;

Is the voltage amplitude of each phase of bus m when a fault occurs at point K,

Indicates the voltage amplitude of phase A,

Indicates the amplitude of phase B voltage,

Represents the voltage amplitude at point m when the bus node K fails.

S4: Obtain the upper limit U _max and the lower limit U _{min of the} uncertainty interval through the variation range of the uncertainty interval of the existing known voltage withstand capability curve, and combine the residual voltage amplitude U _{mag of} each busbar fault to form a discrimination matrix BUS and LINE;

As shown in Fig. 4, the voltage sag tolerance curve of sensitive load, the upper limit and lower limit of the uncertainty interval corresponding to the voltage sag tolerance curve of the sensitive load are U _max and U _{min respectively} , U _max represents The dividing line between the immune zone and the uncertain zone; U _min represents the dividing line of the failure zone in the uncertain zone. The discriminant matrices BUS and LINE are formed as follows:

Is the sag zone discrimination parameter of bus F at both ends of line i;

S5: Let the voltage tolerance value U _{th of the} load on the busbar m point take the lower limit value U _{min of the} uncertainty interval, and calculate the critical point of the line according to the value of the discriminant matrix LINE, and determine the inclusion of the line in the sag domain The situation, the process shown in Figure 1 is:

Referring to Fig. 1, in step S502, if the value of LINE _i is 1, the steps for obtaining the critical point are:

p _from = p _end

p _end = p _new

The convergence conditions are:

||f(p _new )|-U _th |＜tol

S5024: Use Newton's method to obtain accurate critical point values.

The steps of the |f(pmax)|golden section search method for calculating the maximum point p _max of the voltage sag amplitude on the line and the residual voltage equation function f described in step S503 are as follows, and the flowchart is shown in Figure 5:

S5031: According to the golden ratio point

p _b = p ₂

p ₂ =p ₁ ;

If |f(p ₁ )|<|f(p ₂ )|, then perform the following assignment iteration:

p _a = p ₁

p ₁ =p ₂ ;

S6: Determine whether all the lines have been traversed and whether all the uncertain spaces have been traversed, if yes, output the sag domain recognition result; if not all the lines have been traversed, return to step S4 to perform the next line calculation; if all the lines have been traversed, But without traversing all the uncertain spaces, let U _th take the upper limit U _{max of the} uncertain interval, and return to step S4 to continue the calculation;

According to the method proposed by the present invention, the voltage sag domain of the IEEE 30-node system as shown in Figure 2 is calculated, and the bus 20 is selected as the sensitive load connected to the bus node, and the fault type is a single-phase grounding fault. The voltage sag domain identification corresponding to other fault types will not be repeated in this specific embodiment. The upper limit U _max of the uncertainty space of the sensitive load voltage tolerance curve is 0.8, and the lower limit U _{min is} 0.7, where U The calculated value corresponding to _min is sag domain 1, and the calculated value corresponding to U _max is sag domain 2. The calculation results are shown in Table 1:

Table 1

As shown in Table 1, the content in the first column of Table 1 represents the line number at the beginning, the content in the second column represents the line end number; the content in the third column represents the number of calculated critical points corresponding to U _min , The content in the fourth column represents the number of calculated critical points corresponding to U _max , where 0 means that this section of line is not included in the sag domain; 1 means that this section of line has a critical point; 2 means that this section of line has Two critical points; 3 means that the line is completely contained in the sag zone; the contents of the fifth to eighth columns indicate the lines specified in the first and second columns of the table, in the sag zone 1 and sag The inclusion situation in domain 2. If the number of critical points is 0, that is, the start and end values are all 0, this section of the line is not included in the sag domain; if the number of critical points is 1, that is, the start and end values are [0, x] or [x,1], the reading result is shown in Figure 6, where x is the unit value of the distance from the beginning of the line to the critical point and the total length, the dotted line represents the sag zone, the start and end values When it is [0,x], the part of the line length from 0 to x belongs to the sag domain; when the start and end value is [x,1], the part of the line length from x to 1 belongs to the sag domain.

If the number of critical points is 2, and the starting point and ending point are [x1, x2], the reading result is shown in Figure 7, where x1 is the unit value of the distance from the beginning of the line to the first critical point and the total length Indicates that x2 is the unit value of the distance from the first end of the line to the second critical point and the total length. The dotted line represents the sag zone. The start and end values are [x1, x2], that is, when there are two critical points, the line length The part from 0 to x1 and from x2 to 1 belongs to the sag domain.

If the number of critical points is 3, then the entire line is in the sag domain, the starting point value is 0, and the end point value is 1, that is, the entire line belongs to the sag domain.

Figure 8 shows the schematic diagram of the sag region results of the IEEE30 node system considering the upper and lower bounds of the uncertainty interval of the sensitive load tolerance. See Figure 8. The resulting sag region is from the interval U _min to sag domain 1 and U _max to sag domain 2 The solid line corresponds to the boundary of sag zone 1 calculated from the lower limit of withstand capability U _min , and the dashed line corresponds to the boundary of sag zone 2 calculated from the upper limit of withstand capability U _max . Single-phase grounding faults outside the dashed line will not A voltage sag event occurs when the sensitive load is connected to the bus 20. A single-phase ground fault within the solid line will cause a voltage sag event on the bus 20, that is, the sag within the boundary of the sag domain 1 will definitely cause the sensitive load Affected, the sag that occurs outside the boundary of the sag zone 2 will definitely not affect the sensitive load, and the part in the middle of the two boundaries is an area of uncertainty, that is, it will not necessarily affect the sensitive load.

The same or similar reference numbers correspond to the same or similar parts;

The description of the positional relationship in the drawings is only for illustrative purposes, and cannot be understood as a limitation of the patent;

Obviously, the above-mentioned embodiments of the present invention are merely examples to clearly illustrate the present invention, and are not intended to limit the implementation of the present invention. For those of ordinary skill in the art, other changes or changes in different forms can be made on the basis of the above description. It is unnecessary and impossible to list all the implementation methods here. Any modification, equivalent replacement and improvement made within the spirit and principle of the present invention shall be included in the protection scope of the claims of the present invention.

Claims

A method for identifying the sag region considering the uncertainty of load sensitivity, characterized in that the method includes the following steps:

S1: Calculate the power flow of the system to obtain the voltage amplitude U pref during normal operation of each node of the system before the short-circuit fault;

S2: Set the short-circuit fault node K and the sensitive load access node m, calculate the zero sequence, positive sequence, and negative sequence transfer impedance between point K and point m, and the input impedance of point K and the voltage amplitude before the fault;

S3: Calculate the residual voltage amplitude U mag of each bus according to the voltage amplitude of each node of the system before the fault and the input impedance of the fault point K;

S4: Obtain the upper limit U max and the lower limit U min of the uncertainty interval of the voltage withstand capability curve by knowing the variation range of the uncertainty interval of the voltage withstand capability curve, combined with the residual voltage amplitude U mag of each busbar fault, Form the discriminant matrix BUS and LINE;

S5: Let the voltage tolerance value U th of the load carried by node m take the lower limit value U min of the uncertainty interval, calculate the critical point of the line according to the value of the discriminant matrix LINE, and determine the inclusion of the line in the sag domain ；

S6: Judge whether all lines of the system have been traversed and all the uncertain spaces have been traversed, if yes, output the sag domain recognition result; if not all lines have been traversed, return to step S4 for the next line recognition calculation; if all lines have been traversed Line, but has not traversed all the uncertain spaces, let U th take the upper limit U max of the uncertain interval, and return to step S4 to continue the calculation;

S7: Count two calculations, obtain sag domain 1 corresponding to U min and sag domain 2 corresponding to U max , and obtain the final sag domain identification result.
The method for identifying the sag domain considering the uncertainty of load sensitivity according to claim 1, wherein the sequence transfer impedance between the faulty node K and the sensitive load access node m in step S2 is:

Z mK,0 =Z mF,0 +p(Z mT,0 -Z mF,0 )

Z mK,1 =Z mF,1 +p(Z mT,1 -Z mF,1 )

Z mK,2 =Z mF,2 +p(Z mT,2 -Z mF,2 )

Among them, Z mK,0 , Z mK,1 , Z mK,2 are the sequence transfer impedances between the faulty node K and the sensitive load access node m, where Z mK,0 is the zero sequence transfer impedance, Z mK, 1 is the positive sequence transfer impedance, Z mK, 2 is the negative sequence transfer impedance; Z mF,0 , Z mF,1 , Z mF, 2 are the sequences between the first-end bus node F of the sensitive load node m and the load point m Transfer impedance, where Z mF,0 is zero sequence transfer impedance, Z mF,1 is positive sequence transfer impedance, Z mF,2 is negative sequence transfer impedance; Z mT,0 , Z mT,1 , Z mT,2 are sensitive Each sequence transfer impedance between load node m tail bus node T and load point m, where Z mT,0 is zero sequence transfer impedance, Z mT,1 is positive sequence transfer impedance, Z mT,2 is negative sequence transfer Impedance; p is the unit value of the distance from the fault point K to the bus head node F;

The input impedance of the fault node K is:

Z KK,0 =p 2 (Z FF,0 +Z TT,0 -2Z FT,0 -Z C,0 )+p[Z C,0 -2(Z FF,0 -Z TT,0 )]+ Z FF, 0

Z KK,1 =p 2 (Z FF,1 +Z TT,1 -2Z FT,1 -Z C,1 )+p[Z C,1 -2(Z FF,1 -Z TT,1 )]+ Z FF, 1

Z KK,2 =p 2 (Z FF,2 +Z TT,2 -2Z FT,2 -Z C,2 )+p[Z C,2 -2(Z FF,2 -Z TT,2 )]+ Z FF,2

In the formula, Z KK,0 , Z KK,1 , Z KK,2 are the sequence input impedances of the faulty node K, where Z KK,0 is the zero sequence impedance, Z KK,1 is the positive sequence input impedance, and Z KK ,2 is the negative sequence input impedance; Z FF,1 , Z FF,2 Z FF,0 , Z FF,1 , Z FF,2 is the input impedance of the first bus node F at the fault point k, where Z FF, 0 is the zero sequence input impedance, Z FF,1 is the positive sequence input impedance, Z FF,2 is the negative sequence input impedance; Z TT,0 , Z TT,1 , Z TT,2 are the sequences of the tail bus node T Input impedance, where Z TT,0 is zero sequence input impedance, Z TT,1 is positive sequence input impedance, Z TT,2 is negative sequence input impedance; Z C,0 , Z C,1 , Z C,2 are The sequence impedances on the line FT, where Z C,0 is the zero sequence impedance, Z C,1 is the positive sequence impedance, Z C,2 is the negative sequence impedance, and p has the same meaning as described above;

The voltage amplitude before short-circuit fault occurs at node K is expressed as:

U K,pref =U F,pref +p(U T,pref -U F,pref )

Among them, U K,pref represents the voltage before the short-circuit fault occurs at node K; U F,pref represents the node voltage amplitude of the bus node F before the fault; U T,pref represents the node voltage amplitude of the bus node T before the fault; p and The meaning mentioned above is the same.
The method for identifying the sag region considering the uncertainty of load sensitivity according to claim 1, wherein the residual voltage amplitude U mag of the busbar fault in step S3:

Among them, U mag represents the residual voltage amplitude of the bus fault;
Is the voltage amplitude of each phase of bus m when a fault occurs at point K,
Indicates the voltage amplitude of phase A,
Indicates the amplitude of phase B voltage,
It represents the voltage amplitude of phase C; U A,m,pref is the voltage amplitude of phase A before the fault at point m; a represents the rotation factor e j120° ;
Represents the voltage amplitude at point m when the bus node K fails.
The method for identifying the sag region considering the uncertainty of load sensitivity according to claim 1, wherein the upper limit of the uncertainty interval corresponding to the uncertainty curve of the voltage sag tolerance of the sensitive load in step S4 The lower limits are U max and U min respectively , and the discriminant matrices BUS and LINE are formed as follows:

Among them, U th represents the voltage withstand capability of the load carried by node m; ΔU mag,n represents the difference between the bus residual voltage amplitude U mag and the voltage withstand capability U th ; for i∈(1,...,n), When the value of ΔU mag,i is less than or equal to zero, BUS i takes 1, which means that the bus i is included in the sag domain; when the value of ΔU mag,i is greater than zero, BUS i takes 0, which means that the bus i is not included in the sag domain. The LINE value is the sag domain discrimination parameter of the line. The value of LINE i is 0, which means that bus i is not in the sag domain; the value of LINE i is 1, which means that the bus i contains a critical point, and the value of LINE i is 2. Indicates that there are two critical points on the bus i;
Is the sag zone discrimination parameter of bus F at both ends of line i;
Is the sag zone discrimination parameter of bus T. When one of them is set to 1, LINE i is set to 1, which means that the line has a critical point; when both are set to 0, LINE i is set to be 0, which means the line The line has no critical points; if both are 1, then LINE i takes the value 2, which means the line has two critical points.
The method for identifying the sag region considering the uncertainty of load sensitivity according to claim 1, wherein the upper limit U max of the uncertainty curve of the sensitive load voltage sag tolerance in step S4 represents the immune region The boundary between the uncertainty zone and the uncertainty zone; the lower limit U min represents the boundary between the uncertainty zone and the failure zone.
The method for identifying the sag zone considering the uncertainty of load sensitivity according to claim 1, wherein the process of step S5 is:

S501: determining whether the value of the LINE i 0, if the value of the LINE i 0, i indicates the bus dips located outside, the next line is performed is determined; the value of the LINE if i is not 0, obtained by cubic spline interpolation Voltage sag amplitude curve, and execute step S502;

S502: Determine whether the value of LINE i is 1, if the value of LINE i is 1, find the critical point; if the value of LINE i is not 1, go to step S503;

S503: Calculate the maximum point p max of the voltage sag amplitude on the line and the residual voltage equation function f |f(pmax)| by using the golden section search method;

S504: After obtaining p max , judge the size of |f(p max )| and U th . If |f(p max )| is less than U th , then the entire line is included in the sag domain, and return to step S501 to continue execution; If |f(p max )| is greater than U th , the critical point is determined according to the critical point determination method where the LINE i value is 1.
The method for identifying the sag region considering the uncertainty of load sensitivity according to claim 6, wherein, in step S502, if the value of LINE i is 1, the method for obtaining the critical point is:

S5021: Use the three points p=0, p=0.5, and p=1 to perform spline interpolation, and obtain the residual voltage equation function as
Define U th =f(p i ), where x 1 , x 2 , and x 3 are the coefficients of the function, and the independent variable corresponding to the p i function. At this time, p i is taken as p i =0, p i =0.5 and p i =1;

S5022: Calculate |f(0.5)|. The three points described in step S5021 correspond to the short-circuit voltage amplitude of the first node of the line, the short-circuit voltage amplitude of the tail node of the line and |f(0.5)|, and the residual voltage equation function is calculated as 0 The root p ic on ≤p ic ≤1;

S5023: Use the root of the residual voltage equation to define the initial iteration value, determine the two initial iteration values near the critical point, take [p ic ,p ic +Δp] or [p ic -Δp,p ic ], △p= 0.01 is the initial value points p from and p end of the secant iteration to find the critical point; the iteration process is as follows:

p new = p end -[f(p end )-U th ](p end -p from )/[|f(p end )|-|f(p from )|]

p from = p end

p end = p new

Among them, p from and p end are the two initial points of the iteration; p new is the new iteration value of the critical point; f(p end ) represents the residual pressure amplitude of the point p end ; f(p from ) represents the point p from The residual voltage amplitude; U th represents the voltage tolerance of the load carried by node m;

The convergence conditions are:

||f(p new )|-U th |＜tol

Among them, tol is the accuracy of convergence; both p new and U th have the same meaning as described above.

S5024: Use Newton's method to obtain accurate critical point values.
The method for identifying the sag region considering the uncertainty of load sensitivity according to claim 6, characterized in that in step S503, the value of the maximum point p max of the voltage sag amplitude on the calculation line and the residual voltage equation function f f(pmax)|The steps of golden section search method are as follows:

S5031: According to the golden ratio point
Define two initial value points p 1 , p 2 , the expression of the two initial value points is:

S5032: Calculate the residual pressure equation function values corresponding to p 1 and p 2 |f(p 1 )| and |f(p 2 )|;

S5033: Determine whether |f(p 1 )| is greater than or equal to |f(p 2 )|, if |f(p 1 )|≥|f(p 2 )|, perform the following assignment iteration:

If |f(p 1 )|<|f(p 2 )|, then perform the following assignment iteration:

S5033: Determine whether |p b -p a |≤ε holds, ε represents the calculation setting accuracy; if |p b -p a |≤ε holds, use current p 1 and p 2 to calculate the maximum point P max : P max =(p b +p a )/2 and the corresponding residual pressure equation function value |f(P max )|.
The method for identifying the sag region considering the uncertainty of load sensitivity according to claim 6, wherein the method of obtaining the critical point according to the LINE i value of 1 in step S504 is:

S5041: Use the three points p=0, p=p max and p=1 to perform spline interpolation, and obtain the residual voltage equation function as
Define U th =f(p i ), where x 1 , x 2 , and x 3 are the coefficients of the function, and the independent variable corresponding to the p i function. At this time, p i is taken as p i = 0 and p i = p max And p i = 1;

S5042: Calculate |f(p max )|, the three points described in step S5042 correspond to the short-circuit voltage amplitude of the first node of the line, the short-circuit voltage amplitude of the tail node of the line, and |f(p max )| to obtain the residual voltage equation function The root p ic on 0≤p ic ≤1;

S5043: Use the root of the residual voltage equation to define the initial iteration value, determine the two initial iteration values near the critical point, take [p ic ,p ic +Δp] or [p ic -Δp,p ic ], △p= 0.01 is the initial value points p from and p end of the secant iteration to find the critical point; the iteration process is as follows:

p new = p end -[f(p end )-U th ](p end -p from )/[|f(p end )|-|f(p from )|]

p from = p end

p end = p new

Among them, p from and p end are the two initial value points of the iteration; p new is the new iteration value of the critical point; f(p end ) represents the residual pressure amplitude of the point p end ; f(p from ) represents the point p from The residual voltage amplitude; U th represents the voltage withstand capability of the load carried by node m;

The convergence conditions are:

||f(p new )|-U th |＜tol

Among them, tol is the convergence accuracy, and both p new and U th have the same meaning as described above.

S5043: Use Newton's method to obtain the exact value of the critical point when |f(p max )| is greater than U th .