CN113161983B - Power transmission line self-adaptive overload protection method considering dynamic thermal characteristics - Google Patents

Power transmission line self-adaptive overload protection method considering dynamic thermal characteristics Download PDF

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CN113161983B
CN113161983B CN202110336099.2A CN202110336099A CN113161983B CN 113161983 B CN113161983 B CN 113161983B CN 202110336099 A CN202110336099 A CN 202110336099A CN 113161983 B CN113161983 B CN 113161983B
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transmission line
power
power transmission
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overload
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CN113161983A (en
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欧阳金鑫
龙晓轩
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Chongqing University
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    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02H—EMERGENCY PROTECTIVE CIRCUIT ARRANGEMENTS
    • H02H3/00—Emergency protective circuit arrangements for automatic disconnection directly responsive to an undesired change from normal electric working condition with or without subsequent reconnection ; integrated protection
    • H02H3/08—Emergency protective circuit arrangements for automatic disconnection directly responsive to an undesired change from normal electric working condition with or without subsequent reconnection ; integrated protection responsive to excess current
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02H—EMERGENCY PROTECTIVE CIRCUIT ARRANGEMENTS
    • H02H3/00—Emergency protective circuit arrangements for automatic disconnection directly responsive to an undesired change from normal electric working condition with or without subsequent reconnection ; integrated protection
    • H02H3/006—Calibration or setting of parameters

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Abstract

The invention discloses a power transmission line self-adaptive overload protection method considering dynamic thermal characteristics, wherein when the current amplitude of a line is greater than the maximum allowable current of the line, protection is started, and the power of the line is measured once every delta t time; comparing the sampling power with the allowable long-term running power of the line, and if the sampling power is greater than the allowable long-term running power of the line, calculating the estimated allowable overload time of the current sampling; and then comparing the estimated allowable overload time of the sampling with the next sampling time, setting the protection action time if the estimated allowable overload time of the sampling is less than or equal to the next sampling time, and cutting off the overload circuit when the overload duration time meets the protection action time. The method can delay the protection measures to be taken on the overload line as far as possible under the condition of line safety, gives consideration to line safety and cascading failure risks, does not depend on communication, has a simple principle, is easy to realize, and has higher reliability and sensitivity.

Description

Power transmission line self-adaptive overload protection method considering dynamic thermal characteristics
Technical Field
The invention relates to the field of relay protection of power systems, in particular to a power transmission line self-adaptive overload protection method considering dynamic thermal characteristics.
Background
Exceeding the transmission line current to allow long term operation current will result in line overload. In a modern power system, due to the influences of factors such as equipment aging, severe weather, large-load mode operation and the like, a normally-operated line is easily converted into an overload state due to power flow transfer caused by the outage of a single fault element. At present, an electric transmission line is generally configured with overload protection as backup protection when the line fails. However, when the line is overloaded, the current is greater than the overload protection setting value, so that the protection action is caused to cut off the line, the trend is further transferred, and the cascading trip in a larger range is often caused, so that the cascading failure is caused, and huge social and economic losses are caused.
The act of improving overload protection to avoid protection during line overload is the most straightforward way to prevent line overload from causing nuisance trips. Some researchers have been working on preventing the malfunction of protection during overload by identifying the cause of the overload and locking the protection during overload of the line. However, the heating power during the line overload is higher than the heat dissipation power, and if the line overload still exists, the line temperature continuously rises, and the line temperature may exceed the maximum allowable temperature due to the locking of the over-protection, so that short-circuit fault is caused due to the increase of the sag, or the lead is damaged due to thermal oxidation, which causes a serious safety problem. In addition, if a fault occurs after protection locking, the fault can be difficult to reliably cut off, and a large safety risk is brought to the whole system.
Due to the thermal inertia of the transmission line, after the overload occurs, the temperature of the line generally changes continuously at a slow speed, and the temperature exceeds the maximum allowable temperature after the line is overloaded for a certain time. The time for the overload line temperature to reach the maximum allowable temperature is referred to as the allowable overload time. After the line is overloaded, the power flow distribution of the system is changed usually through power regulation and element switching, so that the overloaded line is recovered. If the power of the line can be recovered within the allowable overload time, the temperature can be prevented from exceeding the limit, and therefore cascading failure caused by line cutting is avoided. At present, the setting of the overload protection action time generally depends on operation experience, and the value is conservative, so that the line over-fast cutting caused by the conservative is a key factor for inducing the cascading failure. Therefore, the protection action is delayed within the overload allowable time of the line, sufficient conditions are provided for the control of the overload, and the method is the most effective mode for considering equipment safety and cascading failure defense.
Some researchers have proposed to decide on the removal of the overload line by monitoring the temperature. However, the temperature measured by the line is mainly the surface temperature, the monitoring error is large, and in addition, the temperature rise of the line has obvious hysteresis, so that the protection cannot be timely performed, and the practicability of temperature monitoring is relatively limited. Since the ambient temperature changes during line overload are small, the allowable overload time of the line is mainly determined by the line parameters and power. Some researchers have proposed using different line power calculations to allow overload time to implement overload protection. However, the prior art uses constant overload power. During the overload of the line, the power distribution and the power size of the power grid are constantly changed by adjusting the topology of the power grid, the power output of the power supply and the like, the temperature rise speed of the overload line is changed, and the allowable overload time calculated by using the constant power has a large error, so that cascading failure caused by the excessively fast cutting-off of the line can be caused, or the line can be damaged by the delayed cutting-off.
In summary, how to determine the action time of protection by fully considering the dynamic change of the overload line power so as to ensure the safety of the equipment and the system becomes a problem which needs to be solved urgently by the technical personnel in the field.
Disclosure of Invention
Aiming at the defects of the prior art, the invention provides a power transmission line adaptive overload protection method considering dynamic thermal characteristics, which considers the dynamic property of a wire thermal balance process caused by power change during line overload, realizes accurate calculation of allowable overload time of a line, can adapt to the change of line power, delays taking protection measures for the overload line as far as possible under the condition of avoiding temperature rise exceeding the maximum allowable temperature, solves the problems of cascading failure caused by the over-load protection too fast action and line damage caused by overload protection locking in the prior art, and has the advantages of simple principle, easy realization and higher sensitivity and reliability.
In order to solve the technical problems, the invention adopts the following technical scheme:
a power transmission line self-adaptive overload protection method considering dynamic thermal characteristics comprises the following steps:
s101, sampling the current amplitude of the power transmission line in real time, judging that the power transmission line is overloaded when the current amplitude of the power transmission line is larger than the maximum allowable current amplitude of the power transmission line, and executing a step S102;
s102, overload protection is started, and power of the power transmission line is sampled once every delta t time;
s103, comparing the sampled power of the power transmission line with the allowable long-term operation power of the line, and if the power of the power transmission line is smaller than the allowable long-term operation power of the line, returning the protection and executing the step S101; if the power of the power transmission line is greater than or equal to the power of the line allowed to operate for a long time, executing step S104;
s104, calculating power change speed of the power transmission line based on the power of the power transmission line, and calculating and estimating the allowable overload time based on the power change speed of the power transmission line;
s105, comparing the estimated allowable overload time with the time of the power transmission line power sampled next time, if the estimated allowable overload time is larger than the time of the power transmission line power sampled next time, locking overload protection before the power transmission line power is sampled next time, and executing the step S102; if the predicted allowable overload time is less than or equal to the time of the power transmission line power sampled next time, executing the step S106;
and S106, calculating a protection action time setting value based on the estimated allowable overload time, and sending a tripping signal to cut off the overload line when the overload duration of the power transmission line is equal to the protection action time setting value.
Preferably, in step S104, the transmission line power variation speed v at the time of the kth sampling of the transmission line powerpkCalculated as follows:
Figure GDA0003616829060000031
in the formula, PL(k-1)And PLkThe k-1 th time and the k-th time are respectively the power of the power transmission line measured when the power of the power transmission line is sampled, and k is an integer greater than or equal to 2.
Preferably, in step S104, the predicted allowable overload time of the moment of sampling the power of the transmission line for the kth time
Figure GDA0003616829060000032
Calculated as follows:
Figure GDA0003616829060000033
wherein the coefficient xikCalculated by the following formula:
Figure GDA0003616829060000034
coefficient CkCalculated by the following formula:
Figure GDA0003616829060000035
coefficient eta1、η2、η3、η4Is calculated by the following formula:
Figure GDA0003616829060000036
Figure GDA0003616829060000037
Figure GDA0003616829060000038
Figure GDA0003616829060000039
in the formula, MLAnd NLThe convection heat dissipation coefficient and the radiation heat dissipation coefficient are respectively; rhoL、rL、cLAnd vLThe resistivity, the radius, the specific heat capacity and the density of the power transmission line are respectively; pL0、QL0And UL0Respectively the active power, the reactive power and the voltage of the power transmission line at the moment of overload; t isaIs the absolute temperature of the environment; t isc,maxThe maximum tolerance temperature of the power transmission line;
Figure GDA00036168290600000310
and
Figure GDA00036168290600000311
the k-1 th sampling transmission line and the k-th sampling transmission line are respectivelyAn estimated value of the transmission line temperature at power; v. ofpkThe power change speed of the power transmission line at the kth moment of sampling the power of the power transmission line; pL(k-1)And PLkThe k-1 th time and the k-th time are respectively the power of the power transmission line measured when the power of the power transmission line is sampled, and k is an integer greater than or equal to 2.
Preferably, the temperature of the transmission line at the kth time of sampling the power of the transmission line
Figure GDA0003616829060000041
The estimated value is calculated as follows:
Figure GDA0003616829060000042
in the formula, coefficient tkK Δ t, coefficient λ1、λ2、λ3Respectively as follows:
Figure GDA0003616829060000043
Figure GDA0003616829060000044
Figure GDA0003616829060000045
preferably, the temperature of the transmission line at the time of sampling the power of the transmission line for the 1 st time
Figure GDA0003616829060000046
Is determined in the following manner:
Figure GDA0003616829060000047
in the formula, PL1The power of the power transmission line is measured when the power of the power transmission line is sampled for the 1 st time; v. ofp1Is the 1 st samplingThe power transmission line power change speed at the moment of the power transmission line power is calculated by the following formula:
Figure GDA0003616829060000048
coefficient C1Calculated by the following equation:
Figure GDA0003616829060000049
in the formula (I), the compound is shown in the specification,
Figure GDA00036168290600000410
the estimated value of the temperature of the power transmission line at the moment of overload is calculated by the following formula:
Figure GDA00036168290600000411
preferably, in step S106, the protection operating time setting value is determined as follows:
protection action time setting value t during kth power sampling of power transmission lineop,kSetting by the following formula:
Figure GDA00036168290600000412
in the formula, KrelAs a reliability coefficient, the value thereof satisfies the following conditions:
Figure GDA00036168290600000413
compared with the prior art, the invention has the following beneficial effects:
(1) in the prior art, overload protection is realized by adopting a distance III section and a current III section, the action time of the overload protection is generally determined according to experience and is matched with the tail end of a line step by step according to fixed time delay, the action time lacks theoretical basis and has overlarge margin, so that the overload is cut off too fast, which is an important reason for triggering cascading failure of a power system. The overload protection action time is set by calculating the allowable overload time of the line, so that the removal of the overload line can be delayed as far as possible under the condition of ensuring the safety of the line, and sufficient time is provided for the regulation and control of the line overload, thereby reducing the risk of cascading failure of the power system.
(2) The prior art often uses empirical values for determining the allowable overload time for a line, or calculates the allowable overload time as the initial overload power, regardless of changes in line power. However, during the line overload period, the switching of the power supply of the power grid and the topology adjustment inevitably cause the line power to change significantly continuously, so that the temperature rise speed of the overload line changes, and further the dynamic change of the line overload time is caused. The invention realizes the accurate calculation of the overload time allowed in the dynamic change process of the line power, thereby implementing the accurate removal of the overload line.
(3) The invention realizes the evaluation of the safety of the line by allowing the accurate calculation of the overload time, thereby realizing the accurate protection of the overload line and considering the line safety and the cascading failure risk.
(4) The method can accurately and reliably identify the line overload and evaluate the line safety in real time to take targeted measures, improves the accuracy of protection actions and the safety of a power grid, does not depend on communication, has simple principle, is easy to realize, and has higher reliability and sensitivity.
Drawings
For purposes of promoting a better understanding of the objects, aspects and advantages of the invention, reference will now be made in detail to the present invention as illustrated in the accompanying drawings, in which:
fig. 1 is a schematic diagram of a power transmission line adaptive overload protection method taking dynamic thermal characteristics into consideration;
FIG. 2 is a schematic diagram of an exemplary power system;
FIG. 3 illustrates the system of FIG. 2 when the disclosed method is applied to adjust the power of each synchronous generator node under output according to a predetermined control strategy;
fig. 4 is a monitoring of line power overload when the method disclosed by the present invention is applied to the system of fig. 2.
Detailed Description
The present invention will be described in further detail with reference to the accompanying drawings.
As shown in fig. 1, the invention discloses a power transmission line adaptive overload protection method considering dynamic thermal characteristics, which comprises the following steps:
s101, sampling the current amplitude of the power transmission line in real time, judging that the power transmission line is overloaded when the current amplitude of the power transmission line is larger than the maximum allowable current amplitude of the power transmission line, and executing a step S102;
s102, starting overload protection, and sampling the power of the power transmission line once every delta t time;
s103, comparing the sampled power of the power transmission line with the allowable long-term operation power of the line, and if the power of the power transmission line is smaller than the allowable long-term operation power of the line, returning the protection and executing the step S101; if the power of the power transmission line is greater than or equal to the power of the line allowed to operate for a long time, executing step S104;
s104, calculating power change speed of the power transmission line based on the power of the power transmission line, and calculating and pre-estimating allowable overload time based on the power change speed of the power transmission line;
s105, comparing the estimated allowable overload time with the time of the power transmission line power sampled next time, if the estimated allowable overload time is larger than the time of the power transmission line power sampled next time, locking overload protection before the power transmission line power is sampled next time, and executing the step S102; if the predicted allowable overload time is less than or equal to the time of the power transmission line power sampled next time, executing the step S106;
and S106, calculating a protection action time setting value based on the estimated allowable overload time, and sending a trip signal to cut off the overload line when the overload duration of the power transmission line is equal to the protection action time setting value.
In specific implementation, in step S104, the power transmission line power change speed v at the kth moment of sampling the power transmission line powerpkCalculated as follows:
Figure GDA0003616829060000061
in the formula, PL(k-1)And PLkThe k-1 th time and the k-th time are respectively the power of the power transmission line measured when the power of the power transmission line is sampled, and k is an integer greater than or equal to 2.
In specific implementation, in step S104, the predicted allowable overload time of the moment of sampling the power of the power transmission line for the kth time
Figure GDA0003616829060000062
Calculated as follows:
Figure GDA0003616829060000063
wherein the coefficient xikCalculated by the following formula:
Figure GDA0003616829060000071
coefficient CkCalculated by the following formula:
Figure GDA0003616829060000072
coefficient eta1、η2、η3、η4Is calculated by the following formula:
Figure GDA0003616829060000073
Figure GDA0003616829060000074
Figure GDA0003616829060000075
Figure GDA0003616829060000076
in the formula, MLAnd NLThe convection heat dissipation coefficient and the radiation heat dissipation coefficient are respectively; rhoL、rL、cLAnd vLThe resistivity, the radius, the specific heat capacity and the density of the power transmission line are respectively; pL0、QL0And UL0Respectively the active power, the reactive power and the voltage of the power transmission line at the moment of overload; t isaIs the absolute temperature of the environment; t isc,maxThe maximum tolerance temperature of the power transmission line;
Figure GDA0003616829060000077
and
Figure GDA0003616829060000078
the estimated values of the temperature of the power transmission line are respectively the estimated values of the temperature of the power transmission line when the power of the power transmission line is sampled for the kth-1 time and the kth time; v. ofpkThe power change speed of the power transmission line at the kth moment of sampling the power of the power transmission line; pL(k-1)And PLkThe k-1 th time and the k-th time are respectively the power of the power transmission line measured when the power of the power transmission line is sampled, and k is an integer greater than or equal to 2.
In specific implementation, the temperature of the power transmission line during the kth sampling of the power transmission line
Figure GDA0003616829060000079
The estimated value is calculated as follows:
Figure GDA00036168290600000710
in the formula, coefficient tkK Δ t, coefficient λ1、λ2、λ3Respectively as follows:
Figure GDA00036168290600000711
Figure GDA00036168290600000712
Figure GDA00036168290600000713
in specific implementation, the temperature of the power transmission line during the 1 st sampling of the power transmission line
Figure GDA00036168290600000714
Determined in the following manner:
Figure GDA0003616829060000081
in the formula, PL1The power of the power transmission line is measured when the power of the power transmission line is sampled for the 1 st time; v. ofp1Calculating the power change speed of the power transmission line at the moment of the 1 st sampling of the power transmission line by the following formula:
Figure GDA0003616829060000082
coefficient C1Calculated by the following equation:
Figure GDA0003616829060000083
in the formula (I), the compound is shown in the specification,
Figure GDA0003616829060000084
the estimated value of the temperature of the power transmission line at the moment of overload is calculated by the following formula:
Figure GDA0003616829060000085
in specific implementation, in step S106, the protection operating time setting value is determined as follows:
protection action time setting value t during kth power sampling of power transmission lineop,kThe following formula is used for setting:
Figure GDA0003616829060000086
in the formula, KrelAs a reliability coefficient, the value thereof satisfies the following conditions:
Figure GDA0003616829060000087
taking the system shown in fig. 2 as an example, the nodes 1, 2, 13, 22, 23, and 27 are connected with synchronous generators (synchronous generators), and the capacities are 45MVA, 50MVA, 40MVA, 60MVA, 30MVA, and 40MVA, respectively. The long-term operation of the line allows the power to be 8.4MW, the maximum tolerant temperature is 70 ℃, and the line power sampling interval is 0.5 min. When the power system normally operates, the power of each node synchronous generator is 26.37MW, 9.68MW, 23MW, 11.59MW, 19.2MW and 26.91MW respectively, and a line L6-10The power of (2) is 7.03 MW.
As shown in fig. 3 and 4, according to the method disclosed in the present invention, at the time when t is 0, the line L is connected to the line L6-9And the operation is quitted due to the fault. The line L can be obtained by load flow calculation6-10The power rises to 12.38MW, exceeding the long-term operation allowed power, and overload occurs. And after the line is overloaded, each synchronous generator adjusts the output according to a preset control strategy. According toThe above steps, the line power sampled at each time and the estimated allowable overload time are shown in table 1. Line L before 6.5min6-10The power is greater than the long-term operation allowable power. The predicted dynamic allowable overload time before 6min is larger than the sampling interval, and the predicted dynamic allowable overload time at 6.5min is 6.85min and smaller than 7 min. Therefore, the set protection action time is infinite at each sampling before 6 min; and the set protection action time is 6.51min when sampling is carried out for 6.5min, and the circuit is removed when the protection is carried out for 6.51 min. If the protection action is not considered, the line L6-10The temperature reached 70 ℃ at 7.23min, and at this point the line power was 11.20MW, still under overload, and the line temperature continued to rise, resulting in line damage. The difference between the calculated value and the actual value of the estimated allowable overload time is only 0.38min, and the difference between the calculated value and the actual value of the estimated allowable overload time and the protection action time is 0.72 min. The self-adaptive overload protection considering the dynamic thermal characteristics of the power transmission line delays the removal of the line as much as possible under the condition of ensuring the safety of the line.
TABLE 1
Figure GDA0003616829060000091
Finally, it is noted that the above-mentioned embodiments illustrate rather than limit the invention, and that, while the invention has been described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.

Claims (6)

1. A power transmission line self-adaptive overload protection method considering dynamic thermal characteristics is characterized by comprising the following steps:
s101, sampling the current amplitude of the power transmission line in real time, judging that the power transmission line is overloaded when the current amplitude of the power transmission line is larger than the maximum allowable current amplitude of the power transmission line, and executing a step S102;
s102, overload protection is started, and power of the power transmission line is sampled once every delta t time;
s103, comparing the sampled power of the power transmission line with the allowable long-term operation power of the line, and if the power of the power transmission line is smaller than the allowable long-term operation power of the line, returning protection and executing the step S101; if the power of the power transmission line is greater than or equal to the power of the line allowed to operate for a long time, executing step S104;
s104, calculating power change speed of the power transmission line based on the power of the power transmission line, and calculating and pre-estimating allowable overload time based on the power change speed of the power transmission line;
s105, comparing the estimated allowable overload time with the time for sampling the power of the power transmission line next time, if the estimated allowable overload time is larger than the time for sampling the power of the power transmission line next time, locking overload protection before sampling the power of the power transmission line next time, and executing the step S102; if the predicted allowable overload time is less than or equal to the time for sampling the power of the power transmission line next time, executing the step S106;
and S106, calculating a protection action time setting value based on the estimated allowable overload time, and sending a trip signal to cut off the overload line when the overload duration of the power transmission line is equal to the protection action time setting value.
2. The power transmission line adaptive overload protection method considering dynamic thermal characteristics according to claim 1, wherein in step S104, a power transmission line power change speed v at a moment when the power transmission line power is sampled at the kth timepkCalculated as follows:
Figure FDA0003616829050000011
in the formula, PL(k-1)And PLkThe k-1 th time and the k-th time are respectively the power of the power transmission line measured when the power of the power transmission line is sampled, and k is an integer greater than or equal to 2.
3. The power transmission line adaptive overload protection method taking dynamic thermal characteristics into account of claim 1,characterized in that in step S104, the predicted allowable overload time of the moment of sampling the power of the power transmission line for the kth time
Figure FDA0003616829050000012
Calculated as follows:
Figure FDA0003616829050000013
wherein the coefficient xikCalculated by the following formula:
Figure FDA0003616829050000021
coefficient CkCalculated by the following formula:
Figure FDA0003616829050000022
coefficient eta1、η2、η3、η4Is calculated by the following formula:
Figure FDA0003616829050000023
Figure FDA0003616829050000024
Figure FDA0003616829050000025
Figure FDA0003616829050000026
in the formula, MLAnd NLThe convection heat dissipation coefficient and the radiation heat dissipation coefficient are respectively; rhoL、rL、cLAnd vLThe resistivity, the radius, the specific heat capacity and the density of the power transmission line are respectively; pL0、QL0And UL0Respectively the active power, the reactive power and the voltage of the power transmission line at the moment of overload; t isaIs the absolute temperature of the environment; t isc,maxThe maximum tolerance temperature of the power transmission line;
Figure FDA0003616829050000027
and
Figure FDA0003616829050000028
the estimated values of the temperature of the power transmission line are respectively the estimated values of the temperature of the power transmission line when the power of the power transmission line is sampled for the kth-1 time and the kth time; v. ofpkThe power change speed of the power transmission line at the kth moment of sampling the power of the power transmission line; pL(k-1)And PLkThe k-1 th time and the k-th time are respectively the power of the power transmission line measured when the power of the power transmission line is sampled, and k is an integer greater than or equal to 2.
4. The power transmission line adaptive overload protection method considering dynamic thermal characteristics of claim 3, wherein the power transmission line temperature at the kth time of sampling the power of the power transmission line
Figure FDA0003616829050000029
The estimated value is calculated as follows:
Figure FDA00036168290500000210
in the formula, coefficient tkK Δ t, coefficient λ1、λ2、λ3Respectively as follows:
Figure FDA00036168290500000211
Figure FDA00036168290500000212
Figure FDA00036168290500000213
5. the power transmission line adaptive overload protection method considering dynamic thermal characteristics of claim 3, wherein the power transmission line temperature at the time of sampling the power of the power transmission line for the 1 st time
Figure FDA0003616829050000031
Determined in the following manner:
Figure FDA0003616829050000032
in the formula, PL1The power of the power transmission line is measured when the power of the power transmission line is sampled for the 1 st time; v. ofp1Calculating the power change speed of the power transmission line at the moment of the 1 st sampling of the power transmission line by the following formula:
Figure FDA0003616829050000033
coefficient C1Calculated by the following equation:
Figure FDA0003616829050000034
in the formula (I), the compound is shown in the specification,
Figure FDA0003616829050000035
the estimated value of the temperature of the power transmission line at the moment of overload is calculated by the following formula:
Figure FDA0003616829050000036
6. the power transmission line adaptive overload protection method considering dynamic thermal characteristics according to claim 3, wherein in step S106, the protection action time setting value is determined in the following manner:
protection action time setting value t during kth power sampling of power transmission lineop,kThe following formula is used for setting:
Figure FDA0003616829050000037
in the formula, KrelAs a reliability coefficient, the value thereof satisfies the following conditions:
Figure FDA0003616829050000038
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