CN112701711A

CN112701711A - Method for detecting and evaluating operation and maintenance safety of important line of AC-DC combined construction converter station

Info

Publication number: CN112701711A
Application number: CN202011374395.3A
Authority: CN
Inventors: 李靖翔; 赖皓; 尹海涛; 邓健俊; 刘羽超; 左干清; 赵明; 熊双成; 雷庆山; 雍育烨; 李双杰; 颜志敏; 李宏昌; 肖志超; 潘立邦; 肖一鹏; 吕圣琪
Original assignee: Guangzhou Bureau of Extra High Voltage Power Transmission Co
Current assignee: Guangzhou Bureau of Extra High Voltage Power Transmission Co
Priority date: 2020-11-30
Filing date: 2020-11-30
Publication date: 2021-04-23
Anticipated expiration: 2040-11-30
Also published as: CN112701711B

Abstract

The invention discloses an operation and maintenance safety detection and evaluation method for an important line of an AC-DC combined construction converter station, which comprises the steps of establishing a local system equivalent model for a target line to obtain a line impedance index and a through current safety margin index; performing commutation failure sensitivity and line power transfer detection analysis on a target line to obtain a commutation failure sensitivity index and a line power transfer index; respectively detecting and analyzing the overload rate and overload margin, the line power deviation, the branch sensitivity and the line temperature rise of a target line to obtain an overload rate and overload margin index, a line power deviation index, a branch sensitivity index and a line temperature rise index; and comprehensively considering all the indexes by adopting a fuzzy multi-target decision method to select the optimal evaluation indexes at different moments to evaluate the importance of the line. The method selects and evaluates the importance index with the highest satisfaction degree, and is used for guiding operation and maintenance personnel and a monitoring center to make the most reasonable selection.

Description

Method for detecting and evaluating operation and maintenance safety of important line of AC-DC combined construction converter station

Technical Field

The invention relates to the technical field of electric power, in particular to a method for detecting and evaluating operation and maintenance safety of an important line of an AC/DC combined construction converter station.

Background

With the rapid development of national economy and the rapid improvement of the working degree, the power utilization requirements of users are rapidly developed, and the scale of a power system is increased day by day. Meanwhile, the objective condition of asymmetric and uneven resource distribution inevitably causes the development of the power grid structure to be different from place to place, and the condition of 'backbone' lines exists in the district power transmission. The whole power grid structure, even the power grid of a small area, has more complex structure, and from the system concept, the more complex the system, the higher the probability of element failure, and the wider the chain reaction influence range caused by the failure of a single element, which can be verified from some cases of serious social influence caused by large-area power failure in recent years abroad.

At present, super-huge converter stations are all alternating current-direct current combined building stations, and are mostly local power supply supporting points, and the line fault leads to the influence that the parcel and even the whole power grid are involved to be serious, but the fault of not all lines can lead to serious consequence, therefore should have the differentiation to the operation and maintenance degree of different lines. Meanwhile, because the power grid system mode is influenced by load and has certain time characteristics, if the power consumption in spring festival is low along with the increase of the power consumption in summer, even the power consumption per day has peak-valley difference, and the individual transformer substations and line maintenance work are carried out, the safety detection and importance evaluation of the line should not be unchanged, but should embody the difference of adaptability.

Disclosure of Invention

The invention aims to overcome the defects of the prior art and provides a method for detecting and evaluating the operation and maintenance safety of an important line of an AC/DC joint construction converter station so as to accurately and effectively detect and analyze the safety and the importance of the line.

In order to achieve the purpose, the technical scheme of the invention is as follows:

a method for detecting and evaluating operation and maintenance safety of an important line of an AC/DC combined construction converter station comprises the following steps:

establishing a local system equivalent model for a target line to detect and analyze the line blocking and the passing current safety margin of the target line so as to obtain a line impedance index and a passing current safety margin index;

performing commutation failure sensitivity and line power transfer detection analysis on a target line to obtain a commutation failure sensitivity index and a line power transfer index;

respectively detecting and analyzing the overload rate and overload margin, the line power deviation, the branch sensitivity and the line temperature rise of a target line to obtain an overload rate and overload margin index, a line power deviation index, a branch sensitivity index and a line temperature rise index;

and comprehensively considering all the indexes by adopting a fuzzy multi-target decision method to select the optimal evaluation indexes at different moments to evaluate the importance of the line.

Compared with the prior art, the invention has the beneficial effects that:

the method detects and analyzes the safety of the line from the time and space dimensions, thereby carrying out evaluation and analysis on the safety and the importance of the line relatively comprehensively, simultaneously applying the fuzzy theory to multi-index evaluation of the importance of the line, selecting and evaluating the importance index with the highest satisfaction degree through a multi-objective decision method, and guiding operation and maintenance personnel and a monitoring center to make the most reasonable selection.

Drawings

Fig. 1 is a flowchart of an operation and maintenance safety detection and evaluation method for an important line of an ac-dc combined-construction converter station according to an embodiment of the present invention;

FIG. 2 is a diagram of a system equivalent model;

FIG. 3 is a schematic diagram of single phase ground fault boundary conditions;

FIG. 4 is a flow chart of a hazardous line identification algorithm;

FIG. 5 is a diagram of an equivalent circuit model of branch i-j.

Detailed Description

Example (b):

the technical solution of the present invention is further described below with reference to the accompanying drawings and examples.

Referring to fig. 1, the method for detecting and evaluating the operation and maintenance safety of the important line of the combined ac and dc converter station mainly includes the following steps:

101. establishing a local system equivalent model for a target line to detect and analyze the line blocking and the passing current safety margin of the target line so as to obtain a line impedance index and a passing current safety margin index;

102. performing commutation failure sensitivity and line power transfer detection analysis on a target line to obtain a commutation failure sensitivity index and a line power transfer index;

spatial position distribution characteristics are associated with specific geographic positions, environmental conditions (earthquake areas, ice disaster areas, mountain fire areas and the like) and network topological structures, and can be attributed to spatial dimension indexes. Thus, the important finger scale of the space dimension can be obtained through the above two steps.

103. Respectively detecting and analyzing the overload rate and overload margin, the line power deviation, the branch sensitivity and the line temperature rise of a target line to obtain an overload rate and overload margin index, a line power deviation index, a branch sensitivity index and a line temperature rise index;

104. the power transmission of the power grid line changes along with the load of a user, and is an index related to the real-time running state, and the load change has time characteristics, so that the importance index of the time dimension of the line can be obtained through the detection and analysis of the steps.

Comprehensively considering all the indexes by adopting a fuzzy multi-target decision method to select optimal evaluation indexes at different moments to evaluate the importance of the line

Therefore, the method detects and analyzes the safety of the line from the time and space dimensions, so that the safety and the importance of the line can be evaluated and analyzed comprehensively, meanwhile, a fuzzy theory is applied to multi-index evaluation of the importance of the line, and the importance index with the highest satisfaction is selected and evaluated through a multi-objective decision method and used for guiding operation and maintenance personnel and a monitoring center to make the most reasonable selection.

The above-described respective indices are further explained below:

1) the equivalent model analysis indexes comprise a line impedance index and a through current safety margin index

A local system equivalent model can be established, as shown in fig. 1, the equivalent of a 500kV main frame of a super-huge east converter station is taken as a system power supply, the length difference of 220kV outgoing lines represents the magnitude of line impedance, the reaction is the electrical tightness degree associated with the east station, the impact of the tripping on the east converter station on the station is different, including voltage drop, the disturbance degree on the power supply in the station, the influence on the direct current commutation failure, etc., in fig. 1, Xs is the system equivalent impedance. Based on this, the following indicators can be proposed:

I. line impedance index

The lengths of the lines are different, that is, the line impedance values are different, the losses of the lines are different, the voltage drop of the lines are different, and the degree of closeness of the connection with the east converter station system is different, so that a function reflecting the magnitude of the line impedance can be researched, for example, if the line impedance is an X line i, a line impedance function K impedance (f) (xi) can be used for reflecting the different degrees of the influence of the line impedance on the east converter station, the smaller the line impedance K impedance is, the closer the connection of the point on the line to the station is, the higher the importance of the line is, the greater the influence on the station is, and the smaller the opposite is.

II. Safety margin index of current passing through

If near-region fault tripping occurs in 220kV, for example, a near-region single-phase earth fault, fault through current generated by the fault through current passes through a main transformer, irreversible deformation or even fault can be caused to a main transformer winding when the fault through current is serious, in addition, due to the fact that a transformer is impacted by external short-circuit current, a transformer coil is contracted and expanded to generate oil flow surge, gas protection action can be caused under the double action of vibration of a transformer body, the power failure range is expanded, and the stability and safety of a system are threatened.

Different outgoing lines and different fault conditions lead to different generated through currents, and different through currents have different influence degrees on the main transformer. Through the allowable magnitude of the through current of the main transformer, the line importance of different outgoing lines under the power failure condition under different 220kV wiring modes and different load rates can be evaluated. If the function of the transmission power P and the current I of the line is I-through ═ f (P) in the known connection mode, the importance of the line at the current transmission power level can be evaluated by using the current safety margin K-through.

In the formula I_{Allow for}Is a through current allowed by the main transformer, I_{At present}Is the current actual operating current, and the function I ═ F (P) represents the expression function of the passing current I which changes correspondingly with the power P;

obviously, the smaller the line importance. The influence degree of the current power level of the line on the main transformer can be visually quantized by providing the K-crossing, and further, the maximum transmission power of the line in operation can be arranged in a suggested line scheduling mode under different wiring modes.

2) Sensitivity index of commutation failure

The alternating current system fault often causes the phase change voltage drop of the networked inverter stations, and further causes the phase change failure, and the phase change failure becomes the most common fault of the receiving end of the high-voltage direct current system. In recent years, with the rapid development of alternating current and direct current grid connection, the failure of phase commutation of a direct current system caused by the failure of the alternating current system gradually increases, and is more frequently caused by thunderstorm weather. The single commutation failure does not cause direct current locking, but the commutation failure moment can cause adverse effects of reduction of direct current power and direct current voltage, increase of direct current, direct current magnetic biasing of a converter transformer and the like, and impact is generated on an alternating current system. More seriously, the continuous commutation failure will cause the direct current to stop running, and the simultaneous commutation failure of multiple loops of direct current may cause huge impact on the power grid.

There are many reasons for phase commutation failure, but ac voltage sag is the most common cause. The relation between the alternating current system fault and the commutation failure is quantitatively evaluated by researching the relation between the alternating current voltage drop amplitude, the drop duration, the fault phase number and the like and the commutation failure, and a fault sensitivity index K of the commutation failure to the line is provided based on the relation for evaluating the importance of the line.

In a traditional voltage sag evaluation algorithm, only voltage sag amplitude and frequency of voltage sag occurrence are generally concerned, and the problem of voltage sag duration is ignored, so that a power grid node voltage sag evaluation algorithm considering line protection and reclosing action characteristics and considering the duration is provided. The most common single-phase line short-circuit fault is taken as an example for description.

When a single-phase grounding (phase a is taken as an example) fault occurs in the system as shown in fig. 3, the ABC three-phase voltage current phasor at the fault point m satisfies the fault boundary condition shown in the following formula.

The single-phase earth fault belongs to an asymmetric fault type, and an earth point at a fault position similar to a two-phase short circuit earth fault provides a path for a zero-sequence current component, so that a positive-sequence component, a negative-sequence component and a zero-sequence fault current component are contained in the system. After the above three-phase boundary condition is decomposed by applying a symmetric component method, the fault boundary condition expressed by the sequence component of the fault point m is as follows:

three-phase rotation factor is introduced by using sequence impedance Zmm and ohm's law

The three-phase voltage at any point g of the line can be obtained. The following formula:

wherein

The three-phase voltage phasor at the fault point before the system is in fault and when the system is in normal operation can be calculated according to the distance parameters at two ends of the line at the fault point. It can be seen that the only variables in the fault voltage expression at the time of system network configuration and operating parameter determination are the voltage parameter representing the location of the fault point on the line and the fault transition impedance Zmm. If a metallic fault occurs, i.e., if the impedance Zmm is 0, the change in fault voltage is only related to the position parameter of the fault point on the line.

Therefore, according to the voltage expression, the corresponding region where the transmission line is located can be reversely solved according to different voltage drop degrees, if the voltage drop amplitude allowed by operation in the converter station is given, which section of range on the line can be determined, even which section of line roadbed tower should be in key operation and maintenance, the operation and maintenance frequency and the operation standard of the key line region can be properly increased, and the operation and maintenance of the transmission line can be differentiated.

The concept of the DC voltage drop factor is combined with voltage type and arc-quenching angle type commutation failure judgment standards to quickly judge whether the commutation failure of the DC occurs after the AC failure. For any contact point (which may be an alternating current bus of a converter station) j in the system, when the node m fails, the direct current voltage sag factor DCVDF based on the extended impedance matrix is as follows:

wherein the content of the first and second substances,

is a transimpedance matrix between nodes j, m,

transpose the self-impedance matrix between nodes m. If the current before and after the fault at the fault point m has the variable quantity of

The voltage variation at the fault point is:

by combining the two formulas, the voltage variation of the inversion side alternating current bus node of the converter station after the fault is obtained as follows:

it is clear that,

and

the two different voltage drop conditions are described, wherein the former is a voltage drop condition of a certain fault point on a line, and the latter is a special case for describing a node voltage drop condition in a system.

According to the actual operation analysis of engineering, the critical voltage drop and the critical extinction angle are widely used as the rapid judgment standard of the commutation failure caused by the fault of the alternating current side in the current research.

The voltage type rapid judging method comprises the following steps: the critical voltage drop criterion is related to the rated voltage and the transmission capacity of direct current, and the criterion that the voltage drop of a converter bus is more than 30 percent can be generally taken as the criterion. If it is

For the post-fault voltage of the bus-bar of the converter station,

is a pre-fault voltage, then:

thus, it is possible to provide

Is the criterion of commutation failure.

The arc quenching angle type rapid judgment method comprises the following steps:

wherein kj is the converter transformation ratio of the inverter station; idj is DC; XLj is commutation reactance; ULj is an effective value of the voltage of the inversion station conversion bus; β j is the trigger advance angle. Considering that the inverter arc-quenching angle cannot be smaller than 7 °, there are:

if the voltage value after the fault is obtained by the two criterions

The corresponding sensitivity index may be defined as:

the smaller the real-time K conversion of the line is, the smaller the safety margin is, the higher the probability of phase change failure possibly occurring after the fault is, and the importance of the corresponding line is to be improved. The index of the sensitivity system K is provided, the safety margin that the distance may cause the commutation failure under the current voltage level and if the system trip or the line trip occurs is quantitatively and intuitively reflected, and the method has important guiding significance for the real-time monitoring of the operation and maintenance of the converter station. After line generation mode adjustment, transmission load and station voltage level (actually, power grid voltage level) are changed, the safety margin conditions of corresponding lines can be quickly reflected, and the importance of the lines can be divided according to different margins, namely, key operation and maintenance can be carried out differently.

3) Line power transfer indicator

The power system has a large power failure accident, and huge loss is brought to social economy. These accidents are usually caused by the failure of one or several elements in the grid, which are at the limit of operation, causing a wide diversion of the power flow, thus stressing part of the lines, causing the protection to act in succession, eventually forming a cascading failure. Research shows that in the initial stage of cascading failure, the active power flow of only part of lines changes rapidly, and the active power flow of most lines changes little or even basically unchanged. The lines with the rapid change of the active power flow are often closely connected with bus nodes at two ends of a cut-off line, and are the dangerous lines which are most prone to have the interlocking overload tripping. And (3) a research method comprehensively considering network topological relation, power flow distribution and size.

The complex network of the power grid can be described by a complex authorized network with n contacts and E edges, and is described by a graph G and an edge-authorized adjacency matrix W.

G＝[V，E] (1-12)

When wij are all 1, the equations (1-13) (1-14) represent the model of the unlicensed network.

And the weighting network model takes the line reactance as the side weight, and reflects the topological structure characteristic of the power grid. In the weighted network model of the power grid, the side weights are:

where Xij is the line Lij reactance.

And the weighted power grid model with the inverse line load rate as the side weight reflects the difference of the power flow distribution of the line characteristics and the limit of the transmission power limit. In the weighted grid model, the side weights are:

after a certain line in the power grid is disconnected, the line which is directly or indirectly connected with two ends of the disconnected line and is greatly influenced by active power flow is defined as a dangerous power transmission line.

It is assumed that the line Li is overloaded,

the active power flow of Lj for the line before the dc Li disconnection,

for the active power flow, P, of line Lj after the disconnection of branch Li_i ⁰Is the active power flow before the disconnection of the direct current Li. Active power flow distribution coefficient of Li of cut-off line

Can be expressed as:

active power flow distribution coefficient K_{Is divided into}The influence severity of the normal line Lj after the line Li breaking accident is represented. Obviously, K_{Is divided into}The larger the active power flow transferred to the line Lj, the more serious the influence, and therefore the higher the importance of the line, whereas the smaller the influence, the lower the importance of the line.

By using the Floyd algorithm, dangerous lines are taken as search targets in different weighted network models, and dangerous lines considering network topology characteristics and power flow characteristics are identified, as shown in fig. 4, so that the importance of the lines can be evaluated.

4) Overload rate and overload margin index

In the actual power grid operation, more attention is paid to whether the line of the section flow is in line overload or not, including the line power condition under the normal operation condition and the N-1 condition. For example, in a certain power flow section, an overload fixed value of a certain line is defined as pass, and when the actual running power is greater than the overload fixed value, the line is judged to be overloaded. The identification method is simple and visual, is favorable for quick judgment of a dispatcher, and has certain experience and overabundance in section selection. Calculating the power actually transmitted between each generator and each load on each line by adopting a rapid analysis method of power composition of the power transmission line based on graph theory according to the current power flow; then, the lines are classified by adopting clustering analysis based on the sum of squared deviations method, and the ranking of the lines according to the degree of similarity of the power composition of the overloaded lines is obtained. And after the circuit is overloaded, sequentially comparing the current directions of the circuit and the overloaded circuit in the loop to judge whether the circuit belongs to the parallel power transmission section of the overloaded circuit. And finally, judging the range of the parallel power transmission section of the overload circuit by calculating the active power flow distribution coefficient of the overload circuit which causes the power flow increase after the overload circuit is cut off. The power flow section selected by the method can reflect the complex internal influence of the line on the system.

The overload setting value is generally determined by considering the thermal stability limit of the line or the condition that the overload does not occur on another line after the section N-1. Therefore, the determination of the transmission section and the overload value thereof are a key index for measuring the importance of the line. In order to quantify and provide better visual judgment for operation and maintenance personnel, an overload margin index K is provided, which is used for measuring the degree of the current line power P and the overload power value P, and can be expressed by an absolute value or a percentage:

the greater the K is, the weaker the importance of the line is, the longer the operation and maintenance period of the line can be adjusted, and the smaller the K is, the stronger the importance of the line is, and the operation and maintenance interval of the line should be shortened.

5) Line power deviation index

Defining the line power deviation degree Kbias as:

where P is the line current power and P0 is the defined line initial power value.

The defined initial power is used as a reference, and the deviation and dispersion of the line power transmission influence the operation stability of a power grid of the system, so that the importance of the line can be reflected by researching the deviation index of the line transmission power.

6) Branch sensitivity index

When the transmission power of the line is different, the sensitivity degree of each branch in the net rack for the power loss change of different load nodes is different, which is caused by the attribute of the line parameter on one hand and the difference caused by the comprehensive multi-factor electrical quantity such as voltage caused by the power flow on the other hand, so that the relation between the line power value and the line sensitivity can be quantized by adopting a power transmission distribution function.

According to the classical power flow analysis theory, the active power and the reactive power of all branches included between a certain generator node and a load node pair in the network system are expressed as follows:

PGi and QGi in the formula (1) are active power and reactive power respectively, PLi and QLi are load node active power and reactive power, Ui and Uj are voltage per unit values of the nodes i and j, and theta ij represents a phase difference of the voltages Ui and Uj.

Wherein, the power in a certain branch i-j is:

in the formula (2), Pij and Qij are respectively the active power and the reactive power of the branch i-j; gij, bij, bi0 represent resistance, reactance, and admittance values, respectively. The branch equivalent mode represented by the equation is shown in fig. 5:

considering a high-voltage transmission line, adopting an ideal sensitivity assumed condition: the branch resistance is far smaller than the branch reactance; the phase angle difference of two nodes of the branch is very small; the per unit value of the node voltage is approximately 1. After simplification, the active power in the formula (2) can be expressed as:

in equation (3), xij is the reactance value of the branch i-j. It can be seen from formula (3) that the power of the branch in the high-voltage transmission line is determined only by the reactance of the branch and the voltage phase difference between the two nodes. In this case, a total of T load nodes are assumed, and one of the load nodes is represented by T. There are a total of L branches, one represented by L. Then the derivation equation is obtained:

wherein ρ l, t represents a power transmission distribution factor of the branch l, and the power transmission distribution factor reflects the power contribution degree of the branch l to the load node t to a certain extent; pl represents the real power exchanged by branch l and Pt represents the real power consumed by the load node. Substituting the formula (4) into the formula (3), and simplifying to obtain:

xit and Xjt represent the impedance values at corresponding locations in the grid impedance matrix, respectively. The sensitivity K of the branch i-j can thus be found to be (i, j):

wherein L is all branches; t is the set of load nodes; k (i, j) reflects the total efficiency of the branch to power generation load node to power exchange.

Sensitivity K_{Allergy (Miao)}The magnitude of (i, j) can reflect the change rate brought to the branch circuit by the change of the load loss, the sensitivity degree of each line in the power system is quantized, and the larger the numerical value is, the higher the line sensitivity is, and the greater the importance degree in the system is.

7) Line temperature rise indicator

The maximum allowable current-carrying capacity Imax is the current-carrying capacity when the temperature of the conductor reaches the maximum long-term allowable working temperature, and when the actual operation current-carrying capacity of the transmission conductor exceeds the maximum allowable current-carrying capacity Imax for a long time, the temperature of the conductor is too high, the insulation layer of the conductor is seriously damaged, and the potential safety hazard of a transmission line is brought. Therefore, the importance of the line under different operation loads along with the change of the temperature can be obtained by considering the real-time temperature rise or the temperature condition of the line, obviously, under the condition that the line is overloaded and the real-time temperature is higher, the safety margin of the maximum allowable current carrying of the line is considered to be smaller, the importance of the line is higher, the operation and the maintenance of the line are enhanced, and otherwise, the operation and the maintenance strength can be reduced. The importance index of the line temperature is considered, so that the method has important research significance, a foundation can be laid for the subsequent intelligent line temperature monitoring means application, and a quantitative rigorous judgment index is provided.

At present, a plurality of methods for calculating the maximum allowable current-carrying capacity of the power transmission line exist, IEEE 738-2006 overhead power transmission line current-carrying capacity calculation IEEE standard and a morgan calculation method are commonly used in China, but the methods are only suitable for the condition that the wind speed is not zero, when the wind speed and the wind direction change greatly, the IEEE standard and the morgan formula method are adopted to determine the conservative condition of the allowable current-carrying capacity and the actual operation scene are not consistent, so the required maximum allowable current-carrying capacity cannot represent the heat load capacity of the conductor under the actual operation condition, and the calculation method is complex. The line temperature calculation method considering the solar illumination change and convection conditions has the calculation result closer to the actual operation condition.

The heat transfer among strands of stranded wires and between the stranded wires and the air gaps in the power transmission conductor conforms to a three-dimensional steady-state heat conduction equation, and a heat conduction control equation in the conductor can be established as follows:

initial conditions: T0T 0.

In the formula: λ — the thermal conductivity of the material; t-temperature of the power transmission conductor; x, y, z space rectangular coordinates; eta 1 and eta 2 which are respectively the heating rate of the steel core and the aluminum wire in unit volume; c-specific heat capacity of the material; ρ is the density of the substance; τ -time; t0 — initial temperature of wire.

Due to the fact that

Q is the material calorific value and V is the material volume. For steel core material, because at the innermost part of the wire, the illumination and the convection can be ignoredThe influence of the current, the calorific value Q is only related to the current and the resistance of the steel core material. And the aluminum wire is arranged at the outer layer, so the heating value also considers the absorbed solar illumination heat Qs and the convection heat dissipation Qc, text [13 ]]The detailed calculation method is already described in the above, and is not described herein again. Therefore, it can be seen that the temperature of the wire is related to its current carrying capacity, and the above equation can be written as a function of the temperature and the current carrying capacity: t ═ f (i), and an allowable error δ is set_{Allow for}。

The maximum long-term allowable operating temperature Tmax of the wire is assumed to correspond to a current-carrying capacity Imax. Utilizing the dichotomy principle: (1) determining two load flow values I1, I2 as initial solution intervals so that the corresponding lead temperatures T1 and T2 meet the requirement of T1<Tmax<T2; (2) taking the intermediate value Im of the current-carrying capacity solution intervals I1 and I2 as (I1+ I2)/2, and solving the corresponding wire temperature Tm as f (Im) according to the formula (1-27); (3) if T1<Tmax<And Tm is used as a new solution interval, and I1 and Tm are used as a new solution interval, otherwise Tm and T1 are used as a new solution interval. (4) Determining | T-Tmax-<δ_{Allow for}And if so, outputting the current-carrying capacity, wherein the current-carrying capacity at the moment is the required maximum allowable current-carrying capacity. Otherwise, repeating the step 2 and the step 3 until the temperature value reaches the preset precision. By using the method, the maximum ampacity Imax under the condition of determining the maximum working temperature of the line can be calculated. In the case that the current operating current is I0, for the line operating temperature T0, the remaining safe operating margin KT index can be expressed as:

where, the function T ═ f (I) represents the function of the current I and the temperature T of the wire, and f (I)_max) Is the wire temperature at which the maximum current is allowed, f (I)₀) Is the wire temperature value at the present current.

Because the evaluation indexes have more dimensions, and the real-time state of the line changes, the evaluation of the line is not objectively and comprehensively described by adopting a single index along with the difference of the tidal current sections, but the best evaluation effect of a certain index on the current moment is difficult to determine when multiple indexes are adopted. Therefore, each index needs to be further weighted, that is, each index needs to be ranked in satisfaction (importance) in the user's mind. Because the weight of the index is influenced by the subjectivity of the actual using object, whether the index is important or not has the characteristic of being not quantifiable, and for the condition, a fuzzy theory can be adopted for analysis.

Fuzzy Theory (Fuzzy Theory) refers to the Theory that uses the basic concept of Fuzzy sets or continuous membership functions. In the fuzzy theory, the fuzzification of the target function refers to the construction of a membership function of the target function which can be used as a fuzzy evaluation index according to the satisfaction degree required by a decision maker, and the membership function can effectively compare and combine targets with different dimensions.

Membership degree model of each index

The fuzzification of the target function refers to the conversion of the target function in the form of a mathematical function according to the target type, the actual problem characteristics, the requirements and the like so as to unify the target function into the same dimension, thereby facilitating the further selection and decision. Generally, targets can be classified into fixed type and interval type, cost type, and benefit type.

Fixed object types, as their names imply, are better as the index is closer to a fixed value. The calculation formula of the relative dominance degree is as follows:

in the formula (f)_i ^*The optimal value of the ith target index fi is given in advance; wherein

In the above-mentioned indices, K_{Through the}、K_{Changeable pipe}、K_{For treating}、K_Deflection、K_TThe more closely these several indexes are from the allowable value or the set value, the higher the importance of the line, and the more the line reflected by the indexes is the object that the decision maker wishes to pay attention to, i.e. satisfiedSince the degree of blurring is higher, the blurring process may be performed by a fixed objective function.

For the cost-based target type, the smaller the index value of the target is required to be, the better. The calculation formula of the relative dominance degree is as follows:

μ_ij＝1-f_ij/(f_imax+f_imin) (1-30)

in the formula, fimax and fimin are the maximum and minimum values in the statistical data, respectively. Among the above-mentioned indexes, the three indexes of K impedance, K minute, and K sensitivity are considered to be that the higher the value thereof is (if the value is smaller, the function can be inverted), the higher the importance of the line is, and after inverting the objective function, the fuzzification processing can be performed with the cost-based relative dominance.

Evaluation matrix

The general comprehensive evaluation model can be described as:

in the formula, f (xj) ═ f1(xj)), f2(xj)), … fm (xj) — T denote target value vectors of the recipe xj. The ith target value of the recipe xj is denoted by fij (xj) (i1, 2 … m; j 1,2 … n). Although the power flow such as actual power, voltage and the like is continuously changed, for operation and maintenance personnel or a monitoring center, real-time data with high frequency does not need to be refreshed, and time axis data characteristics with an hour period can be adopted for decision making. For example, for a certain time in a day, the target value at the corresponding time can be obtained by using the historical data of the past tidal current in each hour through the index mentioned in the foregoing, that is, the objective function, and the target value is used as the data basis of the subsequent decision, so that the index data has historical contrast, the data dimension reduction can be effectively performed, the operation difficulty is reduced, and the conclusion obtained by the dimension reduction processing has no adverse effect on the actual operation and maintenance and monitoring. Therefore, the multi-target decision with low dimensionality and limited target value can be used as follows by adopting a decision matrix:

correspondingly, after the relative membership degree conversion is carried out on all the objective functions, a judgment matrix based on the membership degree can be obtained:

in the formula, mu_ijThe actual meaning of this is the satisfaction of the target value fi of the target function for the index Ki at the j-th moment. Therefore, by adopting the selected decision method, the index adopted by the line importance at a specific moment can be obtained through the evaluation matrix to evaluate the most effectively, and the most valuable evaluation index after n continuous moments can also be obtained.

Decision making method

The embodiment adopts a common fuzzy multi-target decision method: and (4) carrying out decision judgment by using a maximum method. If the scheme is

If so, then:

i. j is a node of the network system, and i-j is a branch connecting the nodes i and j of the network system;

when the scheme is

When the optimal time is reached, the corresponding index is the optimal index at the time, and the optimal index is obtained

Removing from the scheme set X, making a decision in the above formula, and so on, then all the scheme weights in the scheme set X can be ranked in high and low.

The above embodiments are only for illustrating the technical concept and features of the present invention, and the purpose thereof is to enable those skilled in the art to understand the contents of the present invention and implement the present invention accordingly, and not to limit the protection scope of the present invention accordingly. All equivalent changes or modifications made in accordance with the spirit of the present disclosure are intended to be covered by the scope of the present disclosure.

Claims

1. The method for detecting and evaluating the operation and maintenance safety of the important line of the AC/DC combined construction converter station is characterized by comprising the following steps of:

2. The method for detecting and evaluating the operation and maintenance safety of the important line of the AC-DC combined construction converter station according to claim 1, wherein the line impedance index is as follows:

if the line impedance is X-line i, the line impedance function K impedance ═ f (xi) can be used to reflect different degrees of influence of the line impedance on the converter station, and the smaller the line impedance K impedance is, the closer the point on the line is to the station, the higher the importance of the line is, the larger the influence on the station is, and vice versa.

3. The method for detecting and evaluating the operation and maintenance safety of the important line of the AC-DC combined construction converter station according to claim 1, wherein the indexes of the safety margin of the through current are as follows:

in a known connection mode, a function of transmission power P and a through current I of a line is I through ═ f (P), and importance of the line at the current transmission power level is evaluated by using a through current safety margin K through;

K_{through the}The smaller the value, the higher the line importance.

4. The method for detecting and evaluating the operation and maintenance safety of the important line of the AC-DC combined construction converter station according to claim 1, wherein the indexes of the overload rate and the overload margin are as follows:

P_{for treating}Is the overload power value, P is the current line power value

K_{For treating}The larger the line is, the weaker the importance of the line is, the operation and maintenance period of the line can be adjusted to be long, K_{For treating}The smaller the size, the more important the line is, and the shorter the operation and maintenance interval of the line should be.

5. The method for detecting and evaluating the operation and maintenance safety of the important line of the AC-DC combined construction converter station according to claim 1, wherein the line power deviation index is as follows:

defining the line power deviation degree Kbias as:

where P is the current power of the line, P₀Is a defined line initial power value;

with the defined initial power as a reference, the deviation and dispersion of the line power transmission affect the grid operation stability of the system.

6. The method for detecting and evaluating the operation and maintenance safety of the important line of the AC-DC combined construction converter station according to claim 1, wherein the line temperature rise index is as follows:

7. The method for detecting and evaluating the operation and maintenance safety of the important line of the AC-DC combined construction converter station according to claim 6, wherein the line temperature rise index is established in the following way:

the heat transfer among strands of stranded wires and between the stranded wires and the air gaps in the power transmission conductor conforms to a three-dimensional steady-state heat conduction equation, and the heat conduction control equation in the conductor is established as follows:

initial conditions: T0T 0;

in the formula: λ — the thermal conductivity of the material; t-temperature of the power transmission conductor; x, y, z space rectangular coordinates; eta₁、η₂The heating rates of the steel core and the aluminum wire in unit volume are respectively; c-specific heat capacity of the material; ρ is the density of the substance; τ -time; t0--the wire initial temperature;

due to the fact that

Q is the calorific value of the material and V is the volume of the material; the above equation is written as a function of temperature and current capacity: t ═ f (i), and an allowable error δ is set_{Allow for}；

Assuming that the maximum long-term allowable working temperature Tmax of the lead corresponds to the current-carrying capacity Imax, utilizing the dichotomy principle:

(1) determining two load flow values I1 and I2 as initial solution intervals, and enabling the corresponding lead temperatures T1 and T2 to meet the condition that T1< Tmax < T2;

(2) taking the intermediate value Im of the current-carrying capacity solution intervals I1 and I2 as (I1+ I2)/2, and solving the corresponding wire temperature Tm as f (Im) according to the formula (5);

(3) if T1< Tmax < Tm, taking I1 and Tm as a new solution interval, and otherwise, taking Tm and T1 as a new solution interval;

(4) determining | T-Tmax-<δ_{Allow for}If so, outputting the current-carrying capacity, wherein the current-carrying capacity at the moment is the required maximum allowable current-carrying capacity; otherwise, repeating the step 2 and the step 3 until the temperature value reaches the preset precision;

by using the method, the maximum ampacity Imax under the condition of determining the maximum working temperature of the line is calculated; while the current is I in the present operation₀For the line operating temperature T0, the remaining safe operating margin is K_TThe index can be expressed as:

8. the method for detecting and evaluating the operation and maintenance safety of the important line of the AC-DC combined construction converter station according to claim 1, wherein the line power transfer indexes are as follows:

it is assumed that the line Li is overloaded,

the active power flow of Lj for the line before the dc Li disconnection,

for the active power flow of line Lj after the disconnection of branch Li,

the distribution coefficient of the active power flow of the Li of the cut-off line is the active power flow before the direct current Li is cut off

Expressed as:

active power flow distribution coefficient K_{Is divided into}The influence severity of the normal line Lj after the Li break accident is represented, K_{Is divided into}The larger the active power flow transferred to the line Lj, the more serious the influence, and therefore the higher the importance of the line, whereas the smaller the influence, the lower the importance of the line.

9. The method for detecting and evaluating the operation and maintenance safety of the important line of the AC-DC combined construction converter station according to claim 1, wherein the branch sensitivity index is obtained by the following method:

the active power and reactive power of all branches included between a certain generator node and a load node pair in the network system are represented as follows:

p in formula (7)_GiAnd Q_GiRespectively active power, reactive power, P_LiAnd Q_LiIs the active power, reactive power, U, of the load node_iAnd U_jIs the voltage per unit value of nodes i and j, θ_ijRepresents a voltage U_iAnd U_jThe phase difference of (a);

wherein, the power in a certain branch i-j is:

in the formula (8), P_ijAnd Q_ijThe active power and the reactive power of the branches i-j are respectively; g_ij、b_ij、b_i0Respectively representing a resistance value, a reactance value and an admittance value;

adopting ideal sensitivity hypothesis conditions: the branch resistance is far smaller than the branch reactance; the phase angle difference of two nodes of the branch is very small; the per-unit value of the node voltage is approximately 1, and after simplification, the active power in the formula (8) is expressed as:

x in formula (9)_ijThat is the reactance value of branch i-j; from equation (9), the power of the branch in the high-voltage transmission line is determined only by the branch reactance and the voltage phase difference between the two nodes, and assuming that a total of T load nodes, T represents one of the load nodes, L branches and L represent one, a derivation equation is obtained:

where ρ is_l,tRepresenting a power transmission distribution factor of the branch I, wherein the power transmission distribution factor reflects the power contribution degree of the branch I to the load node t to a certain degree; p_lRepresenting the active power exchanged by branch l, P_tThe active power consumed by the load node is represented, and the formula (10) is substituted into the formula (9), and the reduction can be obtained:

X_itand X_jtThe impedance values respectively represent the corresponding positions in the impedance matrix of the power grid, and therefore the sensitivity Ksensitive (i, j) of the branch i-j can be obtained as follows:

wherein L is all branches; t is the set of load nodes; k (i, j) reflects the total efficiency of the branch to power generation load node to power exchange;

the sensitivity K of the sensitivity K sensitivity (i, j) can reflect the change rate of the change of the load loss to the branch circuit, the sensitivity degree of each line in the power system is quantized, and the larger the value of the sensitivity K sensitivity is, the higher the line sensitivity is, and the greater the importance degree in the system is.

10. The method for detecting and evaluating the operation and maintenance safety of the important line of the AC-DC combined construction converter station as recited in claim 1, wherein the step of comprehensively considering each index by adopting a fuzzy multi-objective decision method for all the obtained indexes to select the optimal evaluation indexes at different moments to evaluate the importance of the line comprises the following steps:

selecting a maximum method for decision judgment, and if the scheme is adopted

If so, then:

when the scheme is

Removing from the scheme set X, making a decision in the above formula, and so on, then deriving the high-low ordering of the weights of all schemes in the scheme set X.