CN117313433A - Method for calculating transient area coefficient of current transformer for protection - Google Patents
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- G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
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Abstract
The invention belongs to the technical field of power engineering, and discloses a method for calculating the transient area coefficient of a current transformer for protection, which is used for calculating the instantaneous value of primary short-circuit current when a power grid is short-circuited, determining the accurate limit value of a protection device according to the action requirement of the protection device, calculating the magnetic flux peak value of the current transformer within the accurate limit value of the protection device, and determining the initial angle of the least favorable fault; and calculating to obtain a transient coefficient according to the appointed primary time constant, the appointed secondary time constant and the least favorable fault initial angle, then adjusting the transient coefficient by a simulation test, and finally obtaining the transient area coefficient of the current transformer by considering the safety margin. According to the invention, the transient area coefficient of the current transformer for protection is determined according to the actual requirement of the protection device, which is beneficial to reducing the size of the iron core of the current transformer for protection, improving the field intensity distribution of the current transformer and improving the operation reliability of the current transformer.
Description
Technical Field
The invention belongs to the technical field of power engineering, and particularly relates to a method for calculating transient area coefficients of a current transformer for protection.
Background
Along with the continuous increase of the capacity of the power system, the short-circuit current of the system is synchronously increased, and higher requirements are put forward on the transient protection of the system and the protection performance of the current transformer. The technical requirements for the current transformer and the relay protection are all proposed by users, and basically work cycle and load are given, so that the proposed partial parameters are too simple or conservative, and especially the parameters proposed for the current transformer for transient protection are too high, so that the product size of the current transformer is increased, the cost is increased unnecessarily, the difficulty of the manufacturing process is increased, and the insulation reliability of the transformer is affected. The failure cause of the inverted current transformer is mainly that the secondary parameter selection is not in accordance with the reality, so that the product design is too conservative, the iron core size is bigger, the main insulation processing difficulty is increased, and the hidden defect is caused.
Along with the development of new relay protection technologies such as smart power grids, computers and the like, the load of a current transformer is continuously reduced, and along with the different protection modes and protection algorithms, the time for requiring the current transformer to guarantee errors is also different, and the parameters directly influence the size of the iron core of the current transformer. It is not reasonable to continue to design the current transformer with a practice that requires only the same duty cycle.
Disclosure of Invention
The invention aims to provide a method for calculating the transient area coefficient of a current transformer for protection, which is used for determining the accurate limit value set time of a protection device according to the action requirement of the protection device, calculating the magnetic flux peak value of the current transformer within the accurate limit value set time and determining the least favorable fault initial angle; and calculating to obtain a transient coefficient according to the appointed primary time constant, the appointed secondary time constant and the least favorable fault initial angle, then adjusting the transient coefficient by a simulation test, and finally obtaining the transient area coefficient of the current transformer by considering the safety margin.
The technical scheme of the invention is as follows: the method for calculating the transient area coefficient of the current transformer for protection comprises the following steps:
step S1: calculating a primary short-circuit current instantaneous value when the power grid is short-circuited;
step S2: determining the accurate limit value of the protection device for a specified time according to the action requirement of the protection deviceSpecifying time according to the exact limit value>Determining the most unfavorable fault initial angle in the size range, wherein under the condition of the most unfavorable fault initial angle, the magnetic flux peak value of the current transformer reaches the maximum value;
step S3: transient coefficient K tf Is calculated by the numerical value of: according to a specified primary time constant T p Time constant T of the second order s And the most unfavorable fault initiation angleDetermining a time-dependent curve of the magnetic flux of the current transformer by using a numerical calculation method, and calculating a transient coefficient K tf I.e. the ratio of the maximum magnetic flux to the peak value of its alternating current component;
step S4: based on temporaryCoefficient of state K tf Performing simulation test on the current transformer according to the numerical calculation result, simulating current transformation of the current transformer and action conditions of the protection device under various working conditions, and verifying whether the performance parameters of the current transformer meet the functional requirements of the relay protection device;
step S5: according to the simulation test result, the transient coefficient K is adjusted tf The current transformer meets the functional requirements of the relay protection device under various adverse working conditions, and the transient coefficient K is determined tf Then, considering the safety margin M to obtain the transient area coefficient K of the current transformer td =K tf ×M。
Further preferably, in the step S2, the accurate limit value is set for a predetermined timeThe three ranges are divided according to size:
when (when)The most unfavorable failure initiation angle +.>The method comprises the following steps:
；
wherein e is the bottom of natural logarithm, θ is the initial angle of failure, t is time, ω is angular frequency;
when (when)The most unfavorable failure initiation angle +.>Calculated from the following formula:
；
when (when)Transient coefficient at this timeK tf The maximum value is not changed along with the initial fault angle theta, and the initial fault angle theta takes any value.
Further preferably, the magnetic flux in the core may be calculated according to a flux difference equation:
；
wherein,is the magnetic flux of the secondary turn chain at the moment t +.>Is the secondary turn chain magnetic flux at the time t-1, < >>Is the secondary current at t-1 time, +.>For time interval +.>Is the transformation ratio of the current transformer, < >>Is the sum of the secondary winding resistance and the rated resistive load.
Further preferably, the transient coefficient K is based on the magnetic flux difference equation tf The numerical calculation steps of (a) are as follows:
step S31: assigning a value to the variable;
step S32: preparing an initial state;
step S33: for a given primary time constant T p And calculating a primary current at a specified time point by the fault initial angle theta;
step S34: calculating primary short-circuit current instantaneous value curve curves of different fault initial angles;
step S35: calculating a magnetic flux curve of each fault initial angle theta;
step S36: selecting a maximum magnetic flux for each time point from all the calculated magnetic flux curves;
step S37: in the time interval from t=0 to the actual point in time, the maximum magnetic flux at each point in time is found;
step S38: calculating transient coefficient K tf Is the ratio of the maximum magnetic flux to the peak value of its alternating current component.
Further preferably, the angle step of the different fault initiation angles is 10 °.
Further preferably, the electric network is short-circuited by a primary short-circuit current instantaneous valueWherein I psc The method is characterized in that the method is a primary symmetrical short-circuit current effective value, e is the bottom of natural logarithm, θ is the initial angle of fault, t is time, and ω is angular frequency.
According to the invention, the transient area coefficient of the current transformer for protection is determined according to the actual requirement of the protection device, which is beneficial to reducing the size of the iron core of the current transformer for protection, improving the field intensity distribution of the current transformer and improving the operation reliability of the current transformer.
Drawings
FIG. 1 shows a transient area coefficient K of a protective current transformer based on an accurate limit value for a specified time td The accurate calculation method implements a flow chart;
fig. 2 is a circuit diagram for numerically calculating a transient coefficient.
Detailed Description
The invention is further elucidated in detail below in connection with the accompanying drawings.
Referring to fig. 1, a method for calculating transient area coefficient of a current transformer for protection comprises the following steps:
step S1: and calculating the instantaneous value of the short-circuit current when the power grid is short-circuited. Collecting primary symmetrical short-circuit current effective value I in power grid psc Time constant of once T p Time constant T of the second order s Calculating the instantaneous value of primary short-circuit current when the power grid is short-circuited according to the parameters such as the initial fault angle theta and the likeWhere t is time and ω is angular frequency.
Step S2: determining the accurate limit value of the protection device for a specified time according to the action requirement of the protection deviceSpecifying time according to the exact limit value>And determining the most unfavorable fault initial angle in the range of the magnitude, wherein the magnetic flux peak value of the current transformer reaches the maximum value under the condition of the most unfavorable fault initial angle.
Specifying an accurate limit value for a timeThe three ranges are divided according to size:
(1) When (when)The most unfavorable failure initiation angle +.>The method comprises the following steps:
；
wherein e is the base of natural logarithm;
(2) When (when)The most unfavorable failure initiation angle +.>Calculated from the following formula:
；
(3) When (when)Transient coefficient K at this time tf The maximum value is not changed along with the fault initial angle theta, and the fault initial angle theta can take any value.
Step S3:transient coefficient K tf Is a numerical calculation of (2). According to a specified primary time constant T p Time constant T of the second order s And the most unfavorable fault initiation angleThe method comprises the steps of determining a time-dependent magnetic flux change curve of a current transformer by using a numerical calculation method in consideration of special working conditions such as iron core saturation, fault initial angle reduction range, reclosing and the like, and calculating a transient coefficient K tf I.e. the ratio of the maximum magnetic flux to the peak value of its alternating current component.
According to the circuit diagram of figure 2 for calculating the transient coefficients by the numerical method,exciting inductance for t moment, < >>For secondary winding resistance>For a rated resistive load, the following system of equations may be established:
；
；
；
wherein,for the primary current at time t, < >>For t time instant secondary current, +.>For the exciting current at time t>Is the transformation ratio of the current transformer, < >>Is the secondary electromotive force at the moment t +.>The magnetic flux is the secondary turn chain magnetic flux at the moment t;
for the linear part, by L m The magnetization curve represented, then:
；
when the number of the pins is small,r is the sum of the secondary winding resistance and the rated resistive load s =R ct +R b, T s =L m /R s The method can obtain:
；
the above equation can be converted into a magnetic flux difference equation, and the magnetic flux in the core can be calculated according to the magnetic flux difference equation:
；
wherein,is the secondary turn chain magnetic flux at the time t-1, < >>Is the secondary current at t-1 time, +.>Is a time interval.
According to the magnetic flux difference equation, transient coefficient K tf The numerical calculation steps of (a) are as follows:
step S31: assigning a value to the variable;
step S32: preparing an initial state;
step S33: for a given primary time constant T p And calculating a primary current at a specified time point by the fault initial angle theta;
step S34: calculating primary short-circuit current instantaneous value curves of different fault initial angles, wherein the angle step length of the different fault initial angles is 10 degrees;
step S35: calculating a magnetic flux curve of each fault initial angle theta;
step S36: selecting a maximum magnetic flux for each time point from all the calculated magnetic flux curves;
step S37: in the time interval from t=0 to the actual point in time, the maximum magnetic flux at each point in time is found;
step S38: calculating transient coefficient K tf Is the ratio of the maximum magnetic flux to the peak value of its alternating current component.
Step S4: based on transient coefficient K tf And (3) carrying out simulation test on the current transformer according to the numerical calculation result: according to the transient coefficient K tf And (3) carrying out parametric simulation test on transient coefficients of the current transformer by adopting digital simulation software and the relay protection device, simulating current conversion of the current transformer and action conditions of the protection device under various typical working conditions, and verifying whether performance parameters of the current transformer meet functional requirements of the relay protection device.
Step S5: according to the simulation test result, the transient coefficient K is adjusted tf The current transformer meets the functional requirements of the relay protection device under various adverse working conditions, and the transient coefficient K is determined tf Then, considering the safety margin M to obtain the transient area coefficient K of the current transformer td =K tf ×M。
Finally, it should be noted that: the above embodiments are only for illustrating the present invention and not for limiting the technical solution described in the present invention; thus, while the invention has been described in detail with reference to the various embodiments described above, it will be understood by those skilled in the art that the invention may be modified or equivalents; all technical solutions and modifications thereof that do not depart from the spirit and scope of the present invention are intended to be included in the scope of the appended claims.
Claims (6)
1. The method for calculating the transient area coefficient of the current transformer for protection is characterized by comprising the following steps of:
step S1: calculating a primary short-circuit current instantaneous value when the power grid is short-circuited;
step S2: determining the accurate limit value of the protection device for a specified time according to the action requirement of the protection deviceSpecifying time according to the exact limit value>Determining the most unfavorable fault initial angle in the size range, wherein under the condition of the most unfavorable fault initial angle, the magnetic flux peak value of the current transformer reaches the maximum value;
step S3: transient coefficient K tf Is calculated by the numerical value of: according to a specified primary time constant T p Time constant T of the second order s And the most unfavorable fault initiation angleDetermining a time-dependent curve of the magnetic flux of the current transformer by using a numerical calculation method, and calculating a transient coefficient K tf I.e. the ratio of the maximum magnetic flux to the peak value of its alternating current component;
step S4: based on transient coefficient K tf Performing simulation test on the current transformer according to the numerical calculation result, simulating current transformation of the current transformer and action conditions of the protection device under various working conditions, and verifying whether the performance parameters of the current transformer meet the functional requirements of the relay protection device;
step S5: according to the simulation test result, the transient coefficient K is adjusted tf The current transformer meets the functional requirements of the relay protection device under various adverse working conditions, and the transient coefficient K is determined tf Then, considering the safety margin M to obtain the transient area coefficient K of the current transformer td =K tf ×M。
2. The method for calculating transient area coefficient of current transformer for protection according to claim 1, wherein in said step S2, an accurate limit value is set for a predetermined timeThe three ranges are divided according to size:
when (when)The most unfavorable failure initiation angle +.>The method comprises the following steps:
；
wherein e is the bottom of natural logarithm, θ is the initial angle of failure, t is time, ω is angular frequency;
when (when)The most unfavorable failure initiation angle +.>Calculated from the following formula:
；
when (when)Transient coefficient K at this time tf The maximum value is not changed along with the initial fault angle theta, and the initial fault angle theta takes any value.
3. The method for calculating the transient area coefficient of the current transformer for protection according to claim 2, wherein the magnetic flux in the iron core is calculated according to a magnetic flux difference equation:
；
wherein,is the magnetic flux of the secondary turn chain at the moment t +.>Is the secondary turn chain magnetic flux at the time t-1, < >>Is the secondary current at t-1 time, +.>For time interval +.>Is the transformation ratio of the current transformer, < >>Is the sum of the secondary winding resistance and the rated resistive load.
4. A method for calculating a transient area coefficient of a current transformer for protection according to claim 3, wherein the transient coefficient K is based on a magnetic flux difference equation tf The numerical calculation steps of (a) are as follows:
step S31: assigning a value to the variable;
step S32: preparing an initial state;
step S33: for a given primary time constant T p And fault initiation angleCalculating a primary current at a specified time point;
step S34: calculating primary short-circuit current instantaneous value curves of different fault initial angles;
step S35: calculating each fault initial angleIs a magnetic flux curve of (2);
step S36: selecting a maximum magnetic flux for each time point from all the calculated magnetic flux curves;
step S37: in the time interval from t=0 to the actual point in time, the maximum magnetic flux at each point in time is found;
step S38: calculating transient coefficient K tf Is the ratio of the maximum magnetic flux to the peak value of its alternating current component.
5. The method for calculating the transient area coefficient of a current transformer for protection according to claim 4, wherein the angle step size of the different fault initiation angles is 10 °.
6. The method for calculating transient area coefficient of current transformer for protection according to claim 1, wherein said short-circuit current instantaneous value when said power grid is short-circuitedWherein I psc Is the effective value of the primary symmetrical short-circuit current, e is the bottom of natural logarithm, < >>For the initial angle of failure, t is time and ω is angular frequency.
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