An electronic trip unit and method for a three-phase ac circuit breaker

Abstract

The application discloses an electronic tripping device and method suitable for a three-phase alternating current circuit breaker, which comprises a power taking transformer, a sampling sensor, a power processing circuit, a sampling processing circuit, a control circuit and a tripping circuit, and the method comprises setting a rated current level I r , a long time delay protection level Class , a short time delay tripping current I sd , collecting data and transmitting the data to the control circuit to calculate the effective value of the three-phase current; determining whether a phase current is greater than a limited current value and whether the three-phase is balanced; if yes, tripping; determining whether the current is greater than I sd , starting a timer, and determining whether the current is greater than I sd ‑I r、 total heat resistance Q 0 , and controlling the tripping of the circuit breaker. The application realizes the isolation of a main circuit and a tripping controller; the three sampling modules of the application have an "or" logical relationship, and the reliability of the tripping mechanism is high; the application adopts a critical heat resistance principle to realize a inverse time limit protection algorithm, and realizes the real-time protection and high reliability requirements.

Description

Technical Field

[0001] This invention relates to the field of circuit breaker technology, and in particular to an electronic trip device and method suitable for three-phase AC circuit breakers. Background Technology

[0002] As a motor protection device, AC circuit breakers are used in ship power distribution systems and carry out motor protection functions. When an overload fault occurs in the power distribution system, the tripping mechanism inside the circuit breaker will identify the fault and control the action mechanism to perform tripping protection. Traditional tripping mechanisms identify fault modes by the performance of materials under different overload currents. (1) The principle of the thermal tripping device is that when the current passes through the tripping device, the thermal element heats up, the bimetallic strip deforms due to heat, and when the deformation reaches a certain degree, it strikes the traction rod, thereby driving the mechanism to act. (2) The thermal-magnetic tripping device adds an electromagnetic coil to the thermal tripping device, and the short-circuit current instantaneously drives the armature to trip. (3) The hydraulic electromagnetic tripping device is a tripping device that uses damping fluid and a movable iron core to obtain inverse time-delay action characteristics. Among them, method (1) is mainly to prevent the motor from overheating. After the trip, the motor must go through a heat dissipation process to reduce the heat accumulation value to below the set value before it can be allowed to start again; method (2) has a relatively large action value error, low sensitivity, and is easily affected by the environment; method (3) is greatly affected by the installation method.

[0003] Chinese patent CN101615778A discloses an intelligent trip unit, which uses a PIC16F977A microcontroller as the main control unit. The overload long delay protection is determined by the energy formula Q=I2TL with an error of ±10%. It has a rated current level and a short delay protection level, but its functionality is poor and its reliability is insufficient. Summary of the Invention

[0004] The purpose of this invention is to provide an electronic trip unit suitable for three-phase AC circuit breakers. It employs a critical heat resistance algorithm to achieve three-stage protection and three-phase unbalanced protection, with nine adjustable long-delay and short-delay protection current levels.

[0005] The technical solution of this invention: An electronic trip unit suitable for three-phase AC circuit breakers, comprising a current transformer, a sampling sensor, a power processing circuit, a sampling processing circuit, a control circuit, and a tripping circuit, wherein:

[0006] The current transformer is used to provide electrical energy to the circuit;

[0007] The sampling sensor is used to collect current data from the main circuit;

[0008] The power processing circuit is used to provide a stable control voltage for the circuit;

[0009] The sampling and processing circuit is used to process the acquired signals and transmit them to the control chip;

[0010] The control circuit is used to implement the logic functions of the electronic trip unit;

[0011] The tripping circuit is used to realize the tripping action of the trip unit.

[0012] The aforementioned electronic trip unit for three-phase AC circuit breakers includes a heat resistance judgment module in its control circuit, which is used to determine whether to perform a tripping action based on the heat resistance.

[0013] The aforementioned electronic trip unit for three-phase AC circuit breakers, specifically the heat resistance judgment module, is based on the total heat resistance Q0, the heat generation q in one cycle, and the heat dissipation q in one cycle. d Calculate the current remaining heat resistance of each phase; determine whether the current critical heat resistance of each phase is less than or equal to zero. If so, control the circuit breaker to trip; otherwise, proceed to the next cycle calculation.

[0014] The electronic trip unit applicable to three-phase AC circuit breakers described above also includes a current judgment module in its control circuit, which is used to determine whether to perform a tripping action based on the current.

[0015] The aforementioned electronic trip unit for three-phase AC circuit breakers, specifically the current judgment module, calculates the effective values ​​of the three-phase currents based on data collected by the sampling and processing circuit; determines whether any phase current exceeds a predetermined current value; if so, trips; otherwise, proceeds to the next step; determines whether the three-phase currents are balanced; if so, trips; otherwise, proceeds to the next step; and determines whether the current exceeds I... sd If the conditions are met, the timer is started. When the timer value exceeds the specified time, the tripping mechanism is activated; otherwise, proceed to the next step. Check if the current is less than I. sd -I r If the condition is met, the timer will be turned off and cleared; otherwise, the next cycle calculation will begin.

[0016] A tripping method for an electronic trip unit applicable to a three-phase AC circuit breaker: Step 1: Initialize the peripherals by reading the inputs of the three encoders and setting the corresponding rated current level I. r Long-delay protection class (Class), short-delay trip current (I) sd The program enters a waiting state;

[0017] The second step is that after the A / D converter acquires data for one cycle, the flag is set to 1, the acquired data is transmitted to the control circuit, and the ADC continues to acquire data for the next cycle.

[0018] The third step is for the control circuit to perform FFT calculations on the data of phases A, B, and C to calculate the effective values ​​of the current in the three phases.

[0019] Step 4: Based on the effective value of the current, the control circuit determines whether the current of any phase exceeds the limit current value. If so, the circuit trips; otherwise, it continues to the next step.

[0020] Step 5: Determine if the three phases of the current are balanced. If so, the trip unit will trip; otherwise, proceed to the next step.

[0021] Step 6: Determine if the current is greater than I. sd If the conditions are met, the timer is started. When the timer value is greater than the limit value, the trip is controlled; otherwise, proceed to the next step.

[0022] Step 7: Determine if the current is less than I. sd -I r If the condition is met, the timer is turned off and cleared; otherwise, proceed to the next step.

[0023] Step 8: The control circuit calculates the total heat resistance Q0, the heat generated in one cycle q, and the heat dissipation in one cycle q. d Calculate the current remaining heat capacity of each phase separately:

[0024] Q i+1 =Q i -q i+1

[0025] In the formula: Q i+1 For the current remaining heat resistance, Q i q represents the remaining heat tolerance from the previous cycle. i+1 The heat generated during this cycle;

[0026] Step 9: Determine if any of the three current critical heat resistance values ​​are less than or equal to zero. If so, control the circuit breaker to trip; otherwise, proceed to step 2.

[0027] The specific steps of step eight in the above-described electronic tripping method applicable to three-phase AC circuit breakers are as follows:

[0028] Step 1: Calculate the critical heat resistance Q0 of the circuit breaker:

[0029]

[0030] Parameters k and n are determined based on the inverse time-delay curve, Class is determined by the long-delay protection level, and Δt is the sampling time of one cycle, which is 20ms in this design.

[0031] Step 2, Judgment: If the current current value I f >1.2I r Then calculate the heat generation q over one cycle, if the previous current value I...f <I r Then calculate the heat dissipation q over one cycle. d :

[0032]

[0033]

[0034] K d It is the scattering heat number, which can be determined according to requirements;

[0035] Step 3: Calculate the current heat tolerance Q i+1 :

[0036] Q i+1 =Q i -q i+1 Or Q i+1 =Q i +q di+1 .

[0037] Beneficial effects: Compared with the prior art, the present invention has the following advantages:

[0038] 1. This invention uses a current transformer to draw power. The output of the transformer is rectified to charge the energy storage capacitor. After being conditioned by the power supply circuit, it forms the power supply required by each module of the trip unit. No auxiliary power interface is required, thus achieving isolation between the main circuit and the trip controller.

[0039] 2. This invention uses a Rogowski coil as a sampling sensor. The Rogowski coil has no hysteresis effect, no magnetic saturation effect, small size, simple structure, easy processing, and good linearity. Using the Rogowski coil as a sampling transformer for detecting the main circuit current eliminates the problems of magnetic core saturation and magnetic core nonlinearity. At the same time, it has a large current measurement range, good linearity, and light weight.

[0040] 3. The present invention adopts a separate design for the power transformer and the sampling sensor, which not only meets the power supply requirements for the normal operation of the tripping mechanism, but also ensures that within the protection range, the power transformer and the sampling sensor are respectively responsible for providing power and collecting the main circuit current signal.

[0041] 4. This invention uses a sampling module to encapsulate the current-taking transformer and the sampling transformer, which reduces the size and complexity of the coil.

[0042] 5. The three sampling modules of this invention are connected by an "OR" logic relationship. As long as one of them is working normally, the power supply requirements of the entire tripping mechanism can be met, thereby improving the reliability of the tripping mechanism.

[0043] 6. The power processing circuit of the present invention uses capacitor energy storage and adopts capacitor voltage negative feedback control to obtain stable 5V and 3.3V voltages, and the output voltage is stable.

[0044] 7. This invention can achieve long-delay protection, short-delay protection, instantaneous protection, and three-phase imbalance protection for nine different current levels through an encoder, and has three motor tripping levels.

[0045] 8. This invention uses the critical heat resistance principle to implement the inverse time protection algorithm, which overcomes the shortcomings of traditional inverse time overcurrent protection that cannot accurately reflect the real fault state when the current changes rapidly. At the same time, the derivation process of this algorithm is simple, the calculation complexity is low, the operation time is short, and the protection accuracy is high, thus achieving the requirements of real-time protection and high reliability.

[0046] 9. This invention uses GD32F405RGT6 as the main control chip, which belongs to the Cortex-M4 architecture and contains a hardware floating-point unit (FPU). For single-precision floating-point multiplication operations, it is tens of times faster than M0 / M3.

[0047] The trip unit of this invention was compared with a product on the existing market, and the results are shown in the table below:

[0048]

[0049]

[0050] As shown in the table above, when the inverse time protection algorithm based on critical heat resistance is applied to long-delay protection, the relative error of the circuit breaker tripping time is as high as 9.1% and as low as 1.2%, with both errors being less than 10%. This demonstrates that the circuit breaker can accurately achieve long-delay tripping, with a small difference from the preset theoretical value. Attached Figure Description

[0051] Figure 1 This is the circuit schematic diagram of the present invention;

[0052] Figure 2 This is a schematic diagram of the power transformer and sampling sensor.

[0053] Figure 3 This is a schematic diagram of the power supply processing circuit;

[0054] Figure 4 This is a schematic diagram of the sampling and processing circuit;

[0055] Figure 5 This is a schematic diagram of the control circuit;

[0056] Figure 6 The flowchart of the inverse time-limit algorithm in Example 1 is shown below;

[0057] Figure 7 This is a flowchart of the tripping method in Example 1;

[0058] Figure 8 This is a schematic diagram of the tripping circuit.

[0059] In the diagram: 1-Power transformer, 2-Air gap, 3-Silicon steel core, 4-Sampling sensor, 5-Main circuit wire, 6-Sampling coil frame, 7-Power input interface, 8-Rectifier bridge, 9-MOSFET, 10-Energy storage capacitor, 11-Capacitor energy storage circuit, 12-5V power supply circuit, 13-3.3V power supply circuit, 14-Voltage feedback circuit, 15-Sampling interface, 16-Rotary encoder, 17-Microcontroller, 18-Transistor, 19-Trigger electromagnet coil. Detailed Implementation

[0060] The present invention will be further described below with reference to the accompanying drawings and embodiments, but this should not be construed as limiting the present invention.

[0061] Example 1. An electronic trip unit suitable for three-phase AC circuit breakers, comprising a current transformer, a sampling sensor, a power processing circuit, a sampling processing circuit, a control circuit, and a trip circuit, wherein:

[0062] The current transformer is used to provide electrical energy to the circuit;

[0063] The sampling sensor is used to collect current data from the main circuit;

[0064] The power processing circuit is used to provide a stable control voltage for the circuit;

[0065] The sampling and processing circuit is used to process the acquired signals and transmit them to the control chip;

[0066] The control circuit is used to implement the logic functions of the electronic trip unit;

[0067] The tripping circuit is used to realize the tripping action of the trip unit.

[0068] The specific circuit structure is shown in 2, 3, 4, 5 and 8. The current transformer 1 is wound on the silicon steel sheet core 3, and the middle of the current transformer 1 is an air gap 2.

[0069] The sampling sensor 4 is wound around the sampling coil frame 6 and mounted on the silicon steel sheet core 3;

[0070] The power processing circuit connects the current transformer 1 to the power input interface 7 and supplies power to the circuit through the rectifier bridge 8. It includes a capacitor energy storage circuit 11, a 5V power supply circuit 12, a 3.3V power supply circuit 13, and a voltage feedback circuit 14.

[0071] The sampling processing circuit connects the sampling sensor 4 to the sampling interface 15, and the sampling signal is amplified and DC biased before being input to the ADC port of the microcontroller.

[0072] The control circuit initializes the parameters of the microcontroller 17 by adjusting the rotary encoder 16, thereby realizing the tripping logic function. The microcontroller 17 uses the GD32F405RGT6 chip.

[0073] The tripping circuit uses a transistor 18 to drive a tripping electromagnet coil 19 to achieve the tripping action.

[0074] The operating principle of the electronic trip unit is as follows:

[0075] 1. When the main circuit conductor 5 is energized, an induced magnetic field is generated in the silicon steel core 3, and an induced current is generated on the current transformer 1 and the sampling sensor 4.

[0076] 2. When the current transformer 1 is connected to the power input interface 7, the current in the main circuit conductor 5 increases. The current from the current transformer 1 is rectified by the rectifier bridge 8 and charges the energy storage capacitor 10. When the voltage of the energy storage capacitor increases to above 16.4V, the voltage feedback circuit 14 outputs a high level to drive the MOSFET 9 to conduct, short-circuiting the energy storage capacitor 11 and causing its voltage to drop. When the voltage of the energy storage capacitor is below 16V, the voltage feedback circuit outputs a low level to drive the MOSFET to turn off, and the energy storage capacitor continues to charge. Finally, the energy storage capacitor is maintained at around 16V, and stable 5V and 3.3V voltages are obtained through the 5V power supply circuit 12 and the 3.3V power supply circuit 13.

[0077] 3. The sampling sensor 4 is connected to the sampling interface 15. The 1.65V voltage obtained by the resistor voltage divider method is superimposed with the input signal of the sampling sensor 4 and output to the microcontroller 17 after passing through the power amplifier.

[0078] IV. Adjust the rotary encoder 16 to determine the long-delay protection level, motor tripping level, and short-delay tripping level. The microcontroller 17 reads the pin data of the rotary encoder 16, sets the initial parameters and initializes the function, and executes the program (e.g., ...). Figure 7 (As shown).

[0079] The aforementioned electronic trip unit for three-phase AC circuit breakers includes a heat resistance judgment module in its control circuit, which is used to determine whether to perform a tripping action based on the heat resistance.

[0080] The aforementioned electronic trip unit for three-phase AC circuit breakers, specifically the heat resistance judgment module, is based on the total heat resistance Q0, the heat generation q in one cycle, and the heat dissipation q in one cycle. d Calculate the current remaining heat resistance of each phase; determine whether the current critical heat resistance of each phase is less than or equal to zero. If so, control the circuit breaker to trip; otherwise, proceed to the next cycle calculation.

[0081] The electronic trip unit applicable to three-phase AC circuit breakers described above also includes a current judgment module in its control circuit, which is used to determine whether to perform a tripping action based on the current.

[0082] The aforementioned electronic trip unit for three-phase AC circuit breakers, specifically the current judgment module, calculates the effective values ​​of the three-phase currents based on data collected by the sampling and processing circuit; determines whether any phase current exceeds a predetermined current value; if so, trips; otherwise, proceeds to the next step; determines whether the three-phase currents are balanced; if so, trips; otherwise, proceeds to the next step; and determines whether the current exceeds I... sd If the conditions are met, the timer is started. When the timer value exceeds the specified time, the tripping mechanism is activated; otherwise, proceed to the next step. Check if the current is less than I. sd -I r If the condition is met, the timer will be turned off and cleared; otherwise, the next cycle calculation will begin.

[0083] The above-mentioned electronic trip unit for three-phase AC circuit breakers, in its first step, performs peripheral initialization by reading the inputs from the three encoders and setting the corresponding three parameters: rated current rating (I... r ), long-delay protection class (Class), short-delay trip current (I) sd The program enters a waiting state.

[0084] The second step is that after the A / D converter acquires data for one cycle, the flag is set to 1, the acquired data is transmitted to the control circuit, and the ADC continues to acquire data for the next cycle.

[0085] The third step is for the control circuit to perform FFT calculations on the data of phases A, B, and C to calculate the effective values ​​of the current in the three phases.

[0086] Step 4: Based on the effective value of the current, the control circuit determines whether the current of any phase exceeds the limit current value. If so, the circuit trips; otherwise, it continues to the next step.

[0087] Step 5: Determine if the three phases of the current are balanced. If so, the trip unit will trip; otherwise, proceed to the next step.

[0088] Step 6: Determine if the current is greater than I. sd If the conditions are met, the timer is started. When the timer value is greater than the limit value, the trip is controlled; otherwise, proceed to the next step.

[0089] Step 7: Determine if the current is less than I. sd -I r If the condition is met, the timer is turned off and cleared; otherwise, proceed to the next step.

[0090] Step 8: The control circuit calculates the total heat resistance Q0, the heat generated in one cycle q, and the heat dissipation in one cycle q.d Calculate the current remaining heat capacity of each phase separately:

[0091] Q i+1 =Q i -q i+1

[0092] In the formula: Q i+1 For the current remaining heat resistance, Q i q represents the remaining heat tolerance from the previous cycle. i+1 The heat generated during this cycle;

[0093] Step 9: Determine if any of the three current critical heat resistance values ​​are less than or equal to zero. If so, control the circuit breaker to trip; otherwise, proceed to step 2.

[0094] The electronic trip unit applicable to three-phase AC circuit breakers described above, and the specific steps of step eight are as follows:

[0095] Step 1: Calculate the critical heat resistance Q0 of the circuit breaker:

[0096]

[0097] Parameters k and n are determined based on the inverse time-delay curve, Class is determined by the long-delay protection level, and Δt is the sampling time of one cycle, which is 20ms in this design.

[0098] Step 2, Judgment: If the current current value I f >1.2I r Then calculate the heat generation q over one cycle, if the previous current value I... f <I r Then calculate the heat dissipation q over one cycle. d :

[0099]

[0100]

[0101] K d It is the scattering heat number, which can be determined according to requirements;

[0102] Step 3: Calculate the current heat tolerance Q i+1 :

[0103] Q i+1 =Q i -q i+1 Or Q i+1 =Q i +q di+1 .

[0104] The specific derivation steps are as follows:

[0105] Step 1: Modify the inverse time-limited equation as follows:

[0106]

[0107] In the formula: t, I f I r These represent the protection device's operating time, fault current, and rated current, respectively.

[0108] Step 2: Determine parameters k and n based on the inverse time-limit curve;

[0109] Step 3: Determine the sampling time for each cycle, which is one power frequency cycle. Assume that the current in the main circuit remains constant during this time, and N cycles have elapsed before the circuit breaker trips. The heat generated in each cycle is q. Then:

[0110] Nq=Q0

[0111] The tripping time is:

[0112]

[0113] Further, the heat output q in each cycle is obtained:

[0114]

[0115] Step 4: Determine the total heat resistance Q0 and the heat generation q within the cycle based on the error requirements; when the loop current is greater than 1.2I... r The inverse-time protection is activated at a specific time, therefore the minimum tripping current is 1.2I. r At this time, the heat generated is at its minimum value q. min The relative error is at its maximum value. If we ensure this relative error is less than 1%, then the error will be even smaller than 1% when the current is higher. Because the microcontroller's ADC sampling value can only be an integer, while the actual value may be a decimal, the absolute error will not exceed 1. Therefore:

[0116]

[0117] Take q min =100, therefore the critical heat resistance Q0 of the circuit breaker is:

[0118]

[0119] The expression for calculating the heat q over time is:

[0120]

[0121] Step 5: Determine the circuit breaker protection return time characteristic. If a transient fault occurs and disappears before the protection trips, then when the current is less than the rated current setting, the heat dissipation is greater than the heat generation, and the heat resistance begins to increase. After each sampling cycle, the critical heat resistance of the circuit breaker increases gradually until it returns to Q0. The return time characteristic equation is:

[0122]

[0123] Step 6: Determine the heat dissipation q over one cycle. d Only when the loop current I f Less than I r Furthermore, the circuit breaker protection return function will only be executed when the current critical heat resistance of the circuit breaker is less than Q0, with the heat dissipation within the time period being q. d If the circuit breaker's critical heat resistance recovers from 0 to Q0 in a total of N cycles, then:

[0124] Nq d =Q0

[0125] The protection return time is:

[0126]

[0127] The heat dissipation is q d The expression is:

[0128]

[0129] Substituting the Q0 expression into the equation yields the heat dissipation over the time interval:

[0130]

Claims

1. An electronic trip unit suitable for three-phase AC circuit breakers, characterized in that: It includes a current transformer, a sampling sensor, a power processing circuit, a sampling processing circuit, a control circuit, and a tripping circuit, wherein: The current transformer is used to provide electrical energy to the circuit; The sampling sensor is used to collect current data from the main circuit; The power processing circuit is used to provide a stable control voltage for the circuit; The sampling and processing circuit is used to process the acquired signals and transmit them to the control chip; The control circuit is used to implement the logic functions of the electronic trip unit; The tripping circuit is used to realize the tripping action of the trip unit; The tripping method of the electronic trip unit includes the following steps: First, perform peripheral initialization, read the input of the three encoders, and set the corresponding rated current level I. r Long-delay protection class (Class), short-delay trip current (I) sd The program enters a waiting state; The second step is that after the A / D converter acquires data for one cycle, the flag is set to 1, the acquired data is transmitted to the control circuit, and the ADC continues to acquire data for the next cycle. The third step is for the control circuit to perform FFT calculations on the data of the three phases A, B, and C to calculate the effective value of the current in the three phases. Step 4: Based on the effective value of the current, the control circuit determines whether the current of any phase exceeds the limit current value. If so, the circuit trips; otherwise, proceed to the next step. Step 5: Determine if the three phases of the current are balanced. If so, the trip unit will trip; otherwise, proceed to the next step. Step 6: Determine if the current is greater than I. sd If the conditions are met, the timer is started. When the timer value is greater than the limit value, the trip is controlled; otherwise, proceed to the next step. Step 7: Determine if the current is less than I. sd -I r If the condition is met, the timer is turned off and cleared; otherwise, proceed to the next step. Step 8: The control circuit calculates the total heat resistance Q0, the heat generated in one cycle q, and the heat dissipation in one cycle q. d Calculate the current remaining heat capacity of each phase separately: ； In the formula: Q i+1 For the current remaining heat resistance, Q i q represents the remaining heat tolerance from the previous cycle. i+1 The heat generated during this cycle; Step 9: Determine if any of the three current total heat resistance values ​​are less than or equal to zero. If so, control the circuit breaker to trip; otherwise, proceed to step 2.

2. The electronic trip unit for three-phase AC circuit breakers according to claim 1, characterized in that: The control circuit includes a heat resistance judgment module, which is used to determine whether to perform a tripping action based on the heat resistance.

3. The electronic trip unit for three-phase AC circuit breakers according to claim 2, characterized in that: The heat resistance judgment module is based on the total heat resistance Q0, the heat generated q in one cycle, and the heat dissipation q in one cycle. d Calculate the current remaining heat resistance of each phase; determine whether the current critical heat resistance of each phase is less than or equal to zero. If so, control the circuit breaker to trip; otherwise, proceed to the next cycle calculation.

4. The electronic trip unit for three-phase AC circuit breakers according to claim 1, characterized in that: The control circuit also includes a current judgment module, which is used to determine whether to perform a tripping action based on the current.

5. The electronic trip unit for three-phase AC circuit breakers according to claim 4, characterized in that: The current judgment module specifically calculates the effective values ​​of the three-phase currents based on data collected by the sampling processing circuit; it determines whether any phase current exceeds a predetermined current value; if so, it trips; otherwise, it continues to the next step; it determines whether the three-phase currents are balanced; if so, it trips; otherwise, it continues to the next step; it determines whether the current is greater than I. sd If the conditions are met, the timer is started. When the timer value exceeds the specified time, the tripping mechanism is activated; otherwise, proceed to the next step. Check if the current is less than I. sd -I r If the condition is met, the timer will be turned off and cleared; otherwise, the next cycle calculation will begin.

6. The electronic trip unit for three-phase AC circuit breakers according to claim 1, characterized in that: The specific steps for step eight are as follows: Step 1: Calculate the total heat resistance Q0 of the circuit breaker: ； Parameters k and n are determined based on the inverse time-delay curve, Class is determined by the long-delay protection level, and Δt is the sampling time of one cycle; Step 2, Judgment: If the current current value I f >1.2I r Then calculate the heat generation q within one cycle, if the current value I f <I r Then calculate the heat dissipation q over one cycle. d : ； ； K d It is the scattering heat number; Step 3: Calculate the current heat tolerance Q i+1 : or .

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