Alternating current and direct current disc protection method for preventing fire disaster of traction substation

Abstract

The invention discloses an alternating-current and direct-current disc protection method for preventing a fire disaster of a traction substation, which comprises the following steps: parameter configuration and equipment connection: determining a protection constant value of a newly added alternating-current and direct-current disc protection device, connecting a sensor to a corresponding position of a monitored alternating-current and direct-current disc, and providing a current signal to be detected for the alternating-current and direct-current disc protection device; aC / DC disc input / output branch monitoring: using an AC / DC disc protection device to comprehensively monitor the AC / DC disc input / output branch; the protection device acts: according to the detected current waveform data, the alternating current and direct current disc protection device uses a built-in protection strategy to control protection switches in the alternating current and direct current discs to act, the corresponding switches are tripped off, and a power supply for supplying power to the alternating current and direct current discs is cut off; and fault information recording: if the alternating current and direct current disc protection devices act, recording related information of the fault, and uploading the related information to an integrated automation system. The method can greatly improve the safety and reliability of the traction substation.

Description

Technical Field

[0001] The invention relates to the technical field of railway power supply system safety, and in particular to an AC and DC disk protection method for preventing fires in traction substations along railways. Background Art

[0002] With the continuous expansion of the railway network and the popularization of high-speed trains, higher requirements are placed on the quality and stability of power supply. As an important hub for power transmission, the safety and reliability of traction substations directly affect the normal operation of the entire railway system. Traditional traction substation protection measures are mainly focused on transformers and feeders, while the safety of low-voltage AC and DC panels is often ignored. This is mainly because low-voltage equipment is usually considered to be a relatively safe area and rarely encounters serious failures like high-voltage equipment. But in fact, in recent years, many traction substations have successively experienced secondary cable and low-voltage circuit fires caused by strong current intrusion. Investigations have found that most of these problems are caused by the lack of effective protection measures for AC and DC panels. Specifically, when high voltage is connected to low voltage due to external strong current intrusion or lightning strikes, the AC and DC panel air switches may all trip, causing the entire AC and DC panel to lose power, resulting in unprotected operation of high-voltage equipment. In addition, since the transformer used is not disconnected, it will continue to provide high current to the fire point until the equipment is completely burned out. Such a chain reaction not only destroys the normal working order of the traction substation, but also increases the possibility of fire spread, threatening the safety of surrounding areas. Summary of the invention

[0003] The technical problem to be solved by the present invention is how to provide an AC and DC disk protection method for preventing traction substation fires by performing refined management and intelligent protection of the AC and DC disks, which can greatly improve the safety and reliability of the traction substation.

[0004] In order to solve the above technical problems, the technical solution adopted by the present invention is: a method for protecting AC and DC disks for preventing fire in traction substations, comprising the following steps: S1, parameter configuration and equipment connection: determine the protection setting of the AC and DC disk protection devices, connect the sensors to the corresponding positions of the monitored AC and DC disks, and provide the current signals to be measured for the AC and DC disk protection devices; The S1 specifically includes the following steps: S1-1, configure protection parameters: according to the load of secondary equipment in traction substation, calculate the current increment and current waveform peak of each output branch of AC panel at maximum load, the current increment and current change rate of each output branch of DC panel at maximum load, and determine the protection setting of AC and DC panel protection devices in combination with field test data and reliability coefficient; S1-2, connect the sensor: install the high-precision current transformer and Hall current sensor to the corresponding positions of the AC and DC disks respectively, and ensure that the communication between them and the AC and DC disk protection devices is smooth; S1-3, power connection: connect the battery in the DC disk to the power supply end of the AC and DC disk protection devices, and use the battery to power the AC and DC disk protection devices; S2, AC and DC input and output branch monitoring: Use AC and DC protection devices to fully monitor the AC and DC input and output branches; The S2 specifically includes the following steps: The AC and DC disk protection device obtains real-time current waveform data from high-precision current transformers and Hall current sensors in real time, and calculates the current waveform peak K of each AC branch. u With current increment And the current change rate of each DC branch With current increment ; S3, protection device action: The AC and DC disk protection devices use the built-in protection strategy to control the action of the protection switches in the AC and DC disks according to the detected current waveform data, trip the corresponding switches, and cut off the power supply to the AC and DC disks; S4: Fault information recording: If the AC and DC disk protection devices are activated, relevant information of the fault is recorded and uploaded to the integrated automation system.

[0005] The beneficial effect of adopting the above technical solution is that the method introduces a new AC and DC disk protection device, combines a high-precision current transformer and a Hall current sensor for current sampling, and realizes real-time monitoring of the large current and its sudden change in the input and output branches of the AC and DC disks. When an abnormal condition is detected, the protection device can control the circuit breaker on the DC disk to close again, giving other protection devices a chance to be powered again, so that they can work again and respond to the large current and its sudden change at the fault point in time, and then cut off the fault circuit to prevent fire. In addition, for different situations, the present invention also provides corresponding protection strategies to ensure that the development of fire can be effectively prevented under any circumstances. This method greatly enhances the safety performance of the traction substation and reduces the risk of fire caused by electrical faults. BRIEF DESCRIPTION OF THE DRAWINGS

[0006] The present invention will be further described in detail below in conjunction with the accompanying drawings and specific embodiments.

[0007] Figure 1 is a main flow chart of the method described in the embodiment of the present invention; Figure 2 It is a wiring principle block diagram of the AC and DC disk protection devices in the method described in the embodiment of the present invention; Figure 3 It is an action logic diagram of the DC protection link of the AC and DC disk protection device in the method described in the embodiment of the present invention; Figure 4 It is an action logic diagram of the AC protection link of the AC and DC disk protection device in the method described in the embodiment of the present invention; Figure 5 It is a current waveform and its increment and change rate waveform diagram within 100m before and after the secondary DC circuit is short-circuited in an embodiment of the present invention; Figure 6 It is a waveform diagram of the current waveform and its increment and peak degree change within 100m before and after the secondary AC circuit is short-circuited in the embodiment of the present invention. DETAILED DESCRIPTION

[0008] The following is a clear and complete description of the technical solutions in the embodiments of the present invention in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0009] In the following description, many specific details are set forth to facilitate a full understanding of the present invention, but the present invention may also be implemented in other ways different from those described herein, and those skilled in the art may make similar generalizations without violating the connotation of the present invention. Therefore, the present invention is not limited to the specific embodiments disclosed below.

[0010] like Figure 1 As shown, the embodiment of the present invention discloses a method for protecting AC and DC disks for preventing fire in traction substations, comprising the following steps: S1, parameter configuration and equipment connection: determine the protection setting value of the AC and DC disk protection device, connect the sensor to the corresponding position of the monitored AC and DC disk, and provide the AC and DC disk protection device with a test signal; S2, AC and DC disk input and output branch monitoring: Use AC and DC disk protection devices to fully monitor the AC and DC disk input and output branches; S3, protection device action: The AC and DC disk protection devices use the built-in protection strategy to control the action of the protection switches in the AC and DC disks according to the detected current waveform data, trip the corresponding switches, and cut off the power supply to the AC and DC disks, that is, shut down the fire source; S4: Fault information recording: If the AC and DC disk protection devices are activated, relevant information of the fault is recorded and uploaded to the integrated automation system.

[0011] Furthermore, the S1 parameter configuration and device connection specifically include the following steps: S1-1, configure protection parameters: according to the load of secondary equipment such as main transformer protection, feeder protection and dynamic lighting system of traction substation, calculate the current increment and current waveform peak of each output branch of AC disk at maximum load, the current increment and current change rate of each output branch of DC disk at maximum load, and determine the protection setting of AC and DC disk protection devices in combination with field test data and reliability coefficient. The wiring diagram of AC and DC disk protection devices is as follows: Figure 2 As shown, power can be taken from the battery of the DC disk to ensure continuous and stable power supply even under extreme conditions. This can prevent the protection device from losing power due to the tripping of the DC circuit breaker, so that the AC and DC disk protection devices can always maintain effective monitoring of the AC and DC output branches to avoid greater losses.

[0012] S1-2, connect the sensor: install the high-precision current transformer and Hall current sensor to the corresponding positions of the AC and DC disks respectively for measuring AC current and DC current, and ensure smooth communication between them and the AC and DC disk protection devices.

[0013] High-precision current transformer: The theoretical basis of this transformer is the principle of electromagnetic induction. Its characteristic is that it has high linearity when converting large current on the primary side into small current on the secondary side. Where i1 is the instantaneous current on the secondary side (A); i2 is the instantaneous current on the primary side (A); n T is the current transformer ratio.

[0014] The output value of the high-precision current sensor is in ampere level, so its output end can be directly connected to AC and DC disk protection devices.

[0015] Hall current sensor: The measurement theory of this sensor is the Hall effect principle, that is, the current flowing through the current end of the Hall element will be converted into a Hall voltage through the magnetic field effect in the normal direction of the element plane. Since the voltage is proportional to the input current, the input current can be measured by measuring the electromotive force. The specific formula is: In the formula, u H is the Hall voltage; R H is the Hall coefficient; i con is the current passing through the Hall element; B is the magnetic induction intensity; d is the thickness of the Hall element.

[0016] The Hall current sensor is an integration of the Hall element and its signal linear amplification circuit. If its magnetic core ring is directly placed on the wire to be measured, the measured large current i is proportional to the voltage output value u of the Hall current sensor, that is, In the formula, the conversion factor k tran It is a physical quantity related to vacuum magnetic permeability μ0, amplifier circuit gain A, Hall element sensitivity K, and air gap length L.

[0017] S1-3, check the power connection: confirm that the connection between the battery and the AC and DC disk protection devices is firm and reliable to ensure a stable voltage supply under any circumstances.

[0018] Furthermore, the monitoring of the input and output branches of the S2 AC and DC disks includes: After completing the setting of the fixed value, the AC and DC disk protection devices enter the normal working state and begin to fully monitor the input and output branches of the AC and DC disks. At this stage, the AC and DC protection devices continuously obtain real-time current data from high-precision current transformers and Hall current sensors, and calculate the current waveform peak K of the AC disk output branch. u With current increment , the current change rate of the DC disk output branch With current increment The calculation result is compared with the preset current protection setting. If the current is within the normal range, the AC and DC protection devices will continue to monitor; if the real-time calculation result is detected to exceed the preset protection setting, the AC and DC panel protection devices will immediately send a trip command to the corresponding switch to cut off the power supply of the AC and DC panels to prevent fire.

[0019] Furthermore, the specific protection strategy in S3 is as follows: When strong current is connected to AC and DC boards, there are two cases to deal with: Case 1: Current protection of the output branch of the AC and DC panels. If a strong current intrusion occurs and a fault occurs, the line overvoltage will cause all circuit breakers to trip. At this time, the AC and DC panel protection devices automatically identify this situation and control the corresponding circuit breakers on the DC panel to close again. The key to this step is to give the feeder or the transformer used and other protection devices a chance to be re-energized so that they can work again, monitor the large current and its sudden change at the fault point, and then the AC and DC panel protection devices will act to cut off the fault circuit and prevent fire.

[0020] Taking into account the different output current standards of DC and AC disks, the protection algorithms of AC and DC disk protection devices are designed separately.

[0021] (1) DC disk output branch current protection algorithm This current protection adopts a combined current change rate With current increment The transient protection can reflect both the speed and range of current change. When applied, the two must cooperate with each other and the starting conditions must be the same. Start value E. When the combined protection is started, the two enter their own delay phase (T set1 、T set2 ), that is, whichever protection reaches the predetermined action condition first will be used to control the action of the protection switch. In each DC power branch of the DC disk, the combined protection consists of two units: instantaneous tripping and delayed tripping. The action equations and protection setting calculation methods of the two are as follows: ① Action equation of instantaneous tripping: In the formula, is the initial current increment; is the starting value of the current increment; is the delay time of current change rate protection; T set1 It is the delay setting value of the current change rate protection.

[0022] Calculation of setting value of instantaneous tripping (the AC and DC protection devices used are regarded as a load of DC operating power supply): In the formula, is the maximum rate of change of load current; is the maximum increment of load current; k 1.rel , k 2.rel is the reliability coefficient, generally k 1.rel =k 2.rel =1.2~1.4.

[0023] ② Action equation of delayed tripping: In the formula, F is Protected return value; is the delay time of current change rate protection; T set2 It is the delay setting value of the current increment protection; The delayed arrival time T set2 The current increment; for Protected return value.

[0024] Calculation of setting value of time-delay tripping (the AC and DC protection devices used are regarded as a load of DC operating power supply): In the formula, k 3.rel , k 4.rel is the reliability coefficient, generally k 3.rel = k4.rel =0.8.

[0025] Figure 3 Provide the action logic of the DC protection link of AC and DC disk protection devices.

[0026] (2) AC disk output branch current protection algorithm The current protection adopts the combined current waveform peak K u With current increment The power frequency cycle quantity protection algorithm is used to accurately capture the fault large current and its mutation characteristics. It refers to the difference between the effective value of the current in this power frequency cycle and the effective value of the current in the previous power frequency cycle. The power frequency cycle is 20ms.

[0027] The action equation of the AC disk output branch is: Where M is the setting value of the current effective value increment within the power frequency cycle; N is the setting value of the current waveform peak.

[0028] Calculation of the setting value of the output branch of the AC disk (the DC disk and its DC power supply link are regarded as a load of the AC disk): In the formula, is the maximum increment of load current; k1 is the reliability coefficient, generally k1 = 1.2.

[0029] The following is a brief summary of the current waveform peak K u The calculation method of .

[0030] In any power frequency cycle, the current of each output branch of the AC disk can be regarded as a random event. If the probability density function of the random variable x is , then the k-th central moment of the random variable x can be written as In the formula, is the mean of the random variable x.

[0031] The fourth-order central moment is a numerical feature that measures the steepness of a random variable. If the frequency distribution of the sample points follows the normal distribution law, then its fourth-order central moment is equal to 0. The steeper the distribution of the variable, the greater its fourth-order central moment is greater than 0; the flatter the distribution of the variable, the less its fourth-order central moment is less than zero. Dividing the fourth-order central moment by the fourth power of the mean square error gives a dimensionless quantity and is defined as the kurtosis parameter, that is: For the sample data of the AC disk output current, the discrete model of the kurtosis parameter can be written as = n ( n + 1 ) ∑ i = 1 n ( x i − μ ) 4 − 3 [ ∑ i = 1 n ( x i − μ ) 2 ] 2 ( n − 1 ) ( n − 1 ) ( n − 2 ) ( n − 3 ) s 4 In the formula, s 4 is the fourth power of the sample standard deviation; n is the number of sample points.

[0032] Although the output current signal of the AC disk does not follow the standard normal distribution law, the present invention draws on its idea of ​​measuring the steepness of the waveform. According to the discrete model of the kurtosis parameter of the normal distribution function, it is known that the kurtosis parameter of a standard sine wave with a power frequency period of 20ms is a certain value of -1.5076 regardless of its amplitude. For the convenience of calculation, it is defined as: Thus, for any current waveform with a power frequency period of 20ms, the peak degree K of the standard sine wave is u is 0, the kurtosis K of other quasi-sine waves u will deviate from 0, and the more obvious the deviation from the sine wave is, the higher the kurtosis K u The further away from 0.

[0033] Figure 4 Provide the action logic of the AC protection link of the AC and DC disk protection devices.

[0034] Case 2: Current protection of AC and DC disk bodies.

[0035] When strong electric intrusion occurs and the circuit breaker does not operate, if the power frequency periodic current increment and peak degree of the DC disk input branch (AC and DC disk connecting branch) suddenly increase while the DC output current does not change significantly, the AC and DC disk protection device determines that the DC disk body has a fault and disconnects the DC disk input branch switch and the battery switch.

[0036] When a strong current intrusion occurs and the circuit breaker does not operate, if the effective value of the AC input current I ac.in The situation where the current is much larger than the sum of the effective values ​​of each AC output current, that is, The AC and DC disk protection device determines that there is a fault in the AC disk body and disconnects the AC disk incoming line switch QF1 and the switch of the DC disk input branch.

[0037] (1) Test results of DC disk output branch 1) Typical Cases Transition resistance is the main factor affecting the output branch current value and its waveform characteristics of the DC disk. Figure 5 The current waveform within 100ms before and after the DC output branch is short-circuited and its 4ms current increment and change rate waveform are given (the transition resistance is 2Ω).

[0038] Conclusion: When the DC output branch is short-circuited, the current rises rapidly from 2.0A to 51.6A, the 4ms current increment of 0A increases rapidly to 27.3A, and the current change rate rises rapidly from -1.1 A / ms to 10.0 A / ms. The changes in these three indicators are very obvious.

[0039] 2) Multiple case analysis Table 1 gives the calculation results of the current increment and current change rate within 100ms before and after the DC disk output branch short-circuit fault under different transition resistances.

[0040] Table 1 Calculation results of current increment and change rate within 100ms before and after the DC output branch short circuit fault Transition resistance (Ω) 4ms current increment (A) Current change rate (A / ms) Transition resistance (Ω) 4ms current increment (A) Current change rate (A / ms) 2 [0.00，27.30] [-1.08，10.04] 18 [0.00，5.58] [-0.90，7.47] 4 [0.00，19.83] [0.00，9.16] 20 [0.00，5.03] [-0.90，7.28] 6 [0.00，15.08] [-1.09，8.59] 22 [0.00，4.58] [-0.90，7.10] 8 [0.00，11.95] [-1.09，8.36] 24 [0.00，4.20] [-0.91，6.92] 10 [0.00，9.79] [-0.88，8.28] 26 [0.00，3.88] [-0.91，6.75] 12 [0.00，8.25] [-0.88，8.07] 28 [0.00，3.60] [-0.91，6.58] 14 [0.00，7.11] [-0.89，7.86] 30 [0.00，3.37] [-0.91，6.46] 16 [0.00，6.25] [-0.89，7.66] 32 [0.00，3.16] [-0.91，6.40] Analysis results: Although the transition resistance has a great influence on the short-circuit current, the 4ms increment and change rate of the fault current are significantly higher than those of the load current. Therefore, the current protection algorithm of the AC and DC protection devices can quickly and accurately identify the fault behavior of the DC disk output branch.

[0041] (2) Test results of AC output branch 1) Typical Cases Similarly, transition resistance is the main factor affecting the output branch current value and its waveform characteristics of the AC disk. Figure 6 The current waveform and its increment and peak value waveform within 100ms before and after the AC output branch is short-circuited are given (the transition resistance is 2Ω).

[0042] Conclusion: At the moment when the output branch of the AC disk is short-circuited, the power frequency periodic current increment of the branch current increases from 0A to 34.26A, and the kurtosis parameter fluctuates rapidly from 0 to 28.79. The changes in these two indicators are very significant.

[0043] 2) Multiple case analysis Table 2 gives the calculation results of the current increment and waveform peak within 100ms before and after the AC output branch short-circuit fault under different transition resistances.

[0044] Table 2 Calculation results of current increment and waveform peak within 100ms before and after the AC output branch short circuit fault Transition resistance (Ω) Current increment (A) Waveform kurtosis parameter Transition resistance(Ω) Current increment (A) Waveform kurtosis parameter 2 [0.00，34.26] [0.00，28.79] 18 [0.00，3.92] [0.00，2.67] 4 [0.00，17.44] [0.00，18.36] 20 [0.00，3.53] [0.00，2.31] 6 [0.00，11.68] [0.00，12.04] 22 [0.00，3.21] [0.00，2.02] 8 [0.00，8.78] [0.00，8.31] 24 [0.00，2.94] [0.00，1.79] 10 [0.00，7.03] [0.00，6.09] 26 [0.00，2.71] [0.00，1.60] 12 [0.00，5.87] [0.00，4.71] 28 [0.00，2.52] [0.00，1.44] 14 [0.00，5.03] [0.00，3.79] 30 [0.00，2.35] [0.00，1.31] 16 [0.00，4.40] [0.00，3.15] 32 [0.00，2.21] [0.00，1.19] Analysis results: Although the transition resistance has a great influence on the short-circuit current, the increment and waveform peak of the fault current are significantly higher than those of the load current. Therefore, the current protection algorithm of the AC and DC protection devices can quickly and accurately identify the fault behavior of the output branch of the AC disk.

[0045] S4: If the protection device is activated, the AC and DC disk protection devices will record the relevant information of the fault, such as the time, location, data, etc., and upload it to the integrated automation system for future query and analysis.

[0046] In summary, the method described in the present application can greatly improve the safety and reliability of traction substations by performing refined management and intelligent protection of AC and DC disks.

Claims

1. A method for protecting AC and DC disks for preventing fire in traction substations, characterized in that The steps include: S1, parameter configuration and equipment connection: determine the protection setting of the AC and DC disk protection devices, connect the sensors to the corresponding positions of the monitored AC and DC disks, and provide the current signals to be measured for the AC and DC disk protection devices; The S1 specifically includes the following steps: S1-1, configure protection parameters: according to the load of secondary equipment in traction substation, calculate the current increment and current waveform peak of each output branch of AC panel at maximum load, the current increment and current change rate of each output branch of DC panel at maximum load, and determine the protection setting of AC and DC panel protection devices in combination with field test data and reliability coefficient; S1-2, connect the sensor: install the high-precision current transformer and Hall current sensor to the corresponding positions of the AC and DC disks respectively, and ensure that the communication between them and the AC and DC disk protection devices is smooth; S1-3, power connection: connect the battery in the DC disk to the power supply end of the AC and DC disk protection devices, and use the battery to power the AC and DC disk protection devices; S2, AC and DC input and output branch monitoring: Use AC and DC protection devices to fully monitor the AC and DC input and output branches; The S2 specifically includes the following steps: The AC and DC disk protection device obtains real-time current waveform data from high-precision current transformers and Hall current sensors in real time, and calculates the current waveform peak K of each AC branch. u With current increment And the current change rate of each DC branch With current increment ; S3, protection device action: The AC and DC disk protection devices use the built-in protection strategy to control the action of the protection switches in the AC and DC disks according to the detected current waveform data, trip the corresponding switches, and cut off the power supply to the AC and DC disks; S4: Fault information recording: If the AC and DC disk protection devices are activated, relevant information of the fault is recorded and uploaded to the integrated automation system.

2. The AC and DC disk protection method for preventing traction substation fire according to claim 1, characterized in that: The AC / DC disk protection device in S3 compares the real-time calculation result with the preset protection setting value. If the real-time calculation result is within the normal range, the protection device will continue to maintain the monitoring state; if it is detected that the real-time calculation result exceeds the preset protection setting value, the AC / DC disk protection device will immediately send a tripping command to the corresponding switch to cut off the power supply of the AC / DC disk to prevent fire.

3. The AC and DC disk protection method for preventing traction substation fire according to claim 2, characterized in that: The protection strategy includes a current protection method for the DC disk output branch: This current protection adopts a combined current change rate With current increment The transient protection of Start value E; when the combined protection is started, the two protections enter their own delay phases respectively. The protection switch action is controlled by the condition that first reaches the predetermined action condition. For each branch, the combined protection includes two units: instantaneous tripping and delayed tripping. The action equations and protection setting values ​​are calculated as follows: ① Action equation of instantaneous tripping: ； In the formula, is the initial current increment; is the starting value of the current increment; It is the delay time of current change rate protection; T set1 It is the delay setting value of the current change rate protection; Calculation of setting value of instantaneous tripping: ； In the formula, is the maximum rate of change of load current; is the maximum increment of load current; k 1.rel , k 2.rel is the reliability coefficient; ② Action equation of delayed tripping: ； In the formula, F is Protected return value; It is the delay time of current change rate protection; T set2 It is the delay setting value of the current increment protection; The delayed arrival time T set2 The current increment; for Protected return value; Calculation of setting value of time-delay tripping: ； In the formula, k 3.rel , k 4.rel is the reliability coefficient.

4. The AC and DC disk protection method for preventing traction substation fires according to claim 2, characterized in that: The protection strategy includes the current protection method of the AC disk output branch: The current protection adopts the combined current waveform peak K u With current increment The power frequency periodic quantity protection method, in which the current increment It refers to the difference between the effective value of the current in this power frequency cycle and the effective value of the current in the previous power frequency cycle. The action equation of each output branch of the AC disk is: ； Where, M is the setting value of the current effective value increment within a single power frequency cycle; N is the setting value of the current waveform peak; Calculation of the setting value of each output branch of the AC panel: ； In the formula, is the maximum increment of load current; k1 is the reliability coefficient.

5. The AC and DC disk protection method for preventing traction substation fire according to claim 4, characterized in that: Current waveform peak K u The calculation method includes the following steps: In any power frequency cycle, the current of each output branch of the AC disk is regarded as a random event. If the probability density function of the random variable x is , then the k-order central moment of the random variable x is: ； In the formula, is the mean of the random variable x; The fourth-order central moment is a numerical feature that measures the steepness of a random variable. If the frequency distribution of the sample point obeys the normal distribution law, then its fourth-order central moment is equal to 0; if the variable distribution is steeper, its fourth-order central moment is greater than 0; if the variable distribution is flatter, its fourth-order central moment is less than zero; the fourth-order central moment is divided by the fourth power of the mean square error to obtain a dimensionless quantity and is defined as the kurtosis parameter, that is: ； For the current sample data of each output branch of the AC disk, the discrete model of the kurtosis parameter is: ； In the formula, s 4 is the fourth power of the sample standard deviation; n is the number of sample points; definition: ； For any current waveform with a power frequency period of 20ms, the peak value K of the standard sine wave is u is 0, the kurtosis K of other quasi-sine waves u will deviate from 0, and the more obvious the deviation from the sine wave is, the higher the kurtosis K u The further away from 0.

6. The AC and DC disk protection method for preventing traction substation fires according to claim 2, characterized in that: The protection strategy includes current protection methods for the AC and DC disk bodies: When strong current intrusion occurs and the circuit breaker does not operate, if the power frequency periodic current increment and peak degree of the DC disk input branch suddenly increase while the DC output current does not change significantly, the AC and DC disk protection devices determine that the DC disk body has a fault and disconnect the DC disk input branch switch and battery switch; When a strong current intrusion occurs and the circuit breaker does not operate, if the effective value of the AC input current I ac.in The situation where the current is much greater than the sum of the effective values ​​of each AC output current, that is, ； The AC and DC disk protection devices determine that there is a fault in the AC disk body and disconnect the AC disk incoming line switch QF1 and the switch of the DC disk input branch.

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