With the advent of summer, thunderstorms are frequent in Guangdong Province. A group of young people set foot in Conghua City, Guangdong Province, ready to catch thunder and lightning with human power!What kind of means will they take to fight against Thor? Can mysterious weapons successfully ignite thunder?What gave them such great courage, and what dangers and challenges would they face?The wind and clouds are unpredictable. Can the most powerful current on the earth be captured by human beings in time?
Although this iron house looks simple, its all metal shell and good underground grounding grid form a simpleFaraday cage。In the Faraday cage, the current will choose a conductor better than the person - metal to flow directly into the ground. Because there is no high and low potential difference in the Faraday cage, there will be no current passing through the body, and there will be no electric shock.The young people in the iron house use this principle to ensure that they will not be in danger when lightning strikes.
Thunder and lightning is one of the most serious meteorological disasters.Due to the lack of preparedness, thousands of people are killed by lightning every year in the world. In China alone, the number of people killed by lightning every year exceeds 1000, and the economic loss reaches billions of yuan.The lightning generation speed is extremely fast, and it will randomly search for targets in a moment to carry out destructive strikes.This powerful force is fleeting from occurrence to end, making it more mysterious.The study of lightning has a long history, but there are still many unsolved mysteries.Just waiting and observing natural lightning can no longer satisfy people's urgent desire to explore lightning.The scientific community has come up with the idea of artificial lightning triggering. Through such field experiments, it can simulate the real lightning environment, explore deeper physical performance of lightning, improve the lightning positioning system, and test the lightning protection performance of various electronic equipment in a short distance.
introduce
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In order to improve system operation safety, lightning protection must be very reliable, so lightning current must be tracked and measured online.Use 89C51singlechipRealize online lightning current monitoring and measurementLightning arresterLightning current at iron tower, etc.When lightning occursLightning arresterWhen the lightning current at the iron tower reaches a certain amplitude, the lightning current tracking measurement system will record the time, polarity and amplitude of lightning occurrence.This measurement method is determined bysinglechipAutomatic control, users can easily view the recorded data by pressing the key.The data measured by the lightning current tracking and measurement system can provide scientific basis for the operation of equipment in the system, and can also provide direct data for the national lightning current research, so as to facilitate more in-depth research on lightning current.
Hardware composition
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The hardware of the lightning current tracking and measurement system is composed ofCurrent transformer, analog signal processing circuitMCU controller, A/D conversion chip, independentClock circuitI2C bus datastorageSee Figure 1 for the composition of watchdog circuit, LCD circuit and other peripheral circuits.Please log in to: Power Transmission and Distribution Equipment Network for more information
1.1 Current transformer
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Generally, the "hollow" transformer without iron core is used to measure the impulse current, and the secondary coil is surrounded by the conductor carrying the measured current, so as to avoid the impact of defects caused by the high-frequency component of the impulse currentMeasurement accuracyAnd high di/dt can also induce enough measurement signals [1].The secondary coil for this measurement is“Rogowski coil”The appearance of the shielding box is shown in Figure 2, where 1 is Rogowski coil, 2 is iron shielding box, 3 is measured object, 4 is slotted to cut off the circulating current, and 5 is slotted to cut off the magnetic bypass.?
Rogowski coil mainly considers the following issues:?
① The frequency band of lightning current is very wide (0~several hundred kHz), so the equivalent inductance must be increased and the equivalent resistance must be reduced.Make the wire diameter of the wound coil as large as possible, and find the appropriate number of turns of the coil to increase the equivalent inductance.?
② In order to make the subsequent circuit linear, the linearity of the current output from Rogowski coil must be good.?
③ High voltage transmission can reduce interference. To avoid the interference of lead bifurcation at the coil outlet and external magnetic field, turn the lead at one end of the coil back into the coil, and then come out from the other end, so that the leads at both ends can be combined as shown in Figure 3;At the same time, the whole coil shall be shielded to prevent unnecessary capacitive coupling between the coil and the measuring circuit.To avoid the formation of short circuit turns at the coil of the shield, a small gap should be made on the shield as shown in Figure 3, so that the main magnetic flux enters the coil.?Please log in:Transmission and distribution equipment networkBrowse for more information
1.2 Analog signal processing circuit
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① Polarity transformation of current.In order to adapt the measurement to the input voltage of different polarity, a polarity conversion circuit must be added so that the subsequent circuit can be measured at the same pole.?
② Current peak holding.fromRogowski coilThe current taken out of is changing very quickly (μ s). In order to measure the current value, a peak holding circuit is used to maintain the current peak.?
③ Amplitude transformation of current.Some amplitude conversion circuits are adopted to reduce the too large current, so as to meet the sampling requirements of ADC and facilitate the processing of subsequent circuits.?
④ Polarity signal acquisition.The polarity acquisition circuit is used to record the polarity of the current and send it to the subsequent circuit for processing.?
⑤ Give the next level of digital circuitcontrol signal。After the current amplitude is sampled and processedcontrol signalThat is, when the current amplitude is sampled and processedsinglechipSend a control signal. After receiving this control signal, the SCM sends a sampling signal to the ADC to make it work.?
1.3 Single chip controller
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89C51singlechipIt is an 8-bit MCU.useCMOS process, and Intel's 8051singlechipFully compatible with [2] in pin function, with 256 bytes of RAM, which can be used to store original data.The 4KB EPROM can be used to store the original data.It has 8-bit parallelI/O interfaceP0～P3，Each port can be used as input or output.Two timers can control the computer.It also has 5Interrupt sourceInterrupt control system, on-chip oscillator and clock generation circuit.?
1.4 Reset and watchdog circuit
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When the power supply is powered on or offMAX813LAs a power supply monitoring and watchdog circuit, it can provide a reset level, and when its WDI pin cannot collect the flip signal, it will send a 200ms reset pulse.Use P1.2 pin timing (≤ 1.5s) to send voltage overturn signal to the watchdog circuit. When external interference or other reasons causecrashOr when the system is not working properly, the watchdog will automatically sendReset signal。?
1.512 bit successive approximation fast AD converter
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ThisconverterThe maximum conversion speed is 35 μ s, and the conversion accuracy is ≤ 0.05%,AD574The chip is equipped with a three state output buffer circuit, which can directly connect with various typical 8-bit or 16 bitmicroprocessorConnected and used with CMOSTTL levelCompatible.When an external interrupt occurs to 89C51, 89C51 sendsAD574One signal, AD574analog signalIt is converted into digital signal for 89C51 processing.?
1.6 Independent clock circuit
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The clock/calendar circuit adopts DS1302 high performance, low power consumption with RAMReal time clockThe chip provides time and other information, records the time of lightning occurrence, and can automatically adjust the problem in 2000 and the month with<31 days.The DS1302 has a slow charging function and can operate independently. In this way, even if the main power supply fails, it can still be powered by the standby power supply, ensuring the normal operation of the DS1302.?
1.7I2C data memory
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The 24LC65 used is 8K × 8 (64K bit) serial EEPROM (electrically erasable PROM) with a wide operating voltage range (2.5V to 6.0V).24LC65 adopts a 2-wire interface that can be interoperated with I2C, greatly reducing the system interface.?Source:Transmission and distribution equipment network
1.8 Data display
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16 × 2（characterThe character type LCD module (× lines) has low power, long life and high reliability, which can enable users to observe the recorded lightning current data intuitively.?
Programming
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The program design is mainly composed of the main programInterrupt service program, numerical processing program.When programming, we focus onsinglechipCorrect initialization, data acquisition and processing, data storage and display, etc.?
main program
The main program mainly completes the initialization of the system: sends the voltage flip signal to the watchdog circuit regularly;LCD display data;Query the status of the reset key and the LCD page turning key. See Figure 4 for the flow chart.?
First initialize each register andstackAddress, and then initialize 8155storageRead the information in, and query the status of the reset button and the LCD page turning button after displaying them respectively.?
If the status of the LCD page turning key is "1", call the LCD page turning display subroutine.First read the group value, then read the group number, polarity, current amplitude, time and other data, and call the LCD display subroutine. At the same time, pay attention to adding a delay and give the watchdog a command CPLP1.2 every 1.5s to ensure that the watchdog can work normally.
If foundReset keyIt has been pressed and needs to be queried repeatedly for several times to ensure that it is not affected by misoperation. At this time, the data can be clearedstorageThe number of times the data has been recorded and a series of flags in.
Service interruption
The current level analog processing circuit will send an interrupt request signal when data arrives. The interrupt flow chart is shown in Figure 5.WhensinglechipWhen an external interrupt signal is detected, it will turn to servicesubroutine。Interrupting servicesubroutineTo complete A/D conversion, data conversion, storage and other operations.Service interruptionsubroutineIt is also the core of software design. After an external interrupt is detected, the following transactions should be handled:?
① A/D conversion.The signal of the front analog processing circuit is an analog quantity, which must be processed by A/D before sending the signal to the microcontroller.?
② Data transformation.Because the former analog processing circuit has processed the amplitude, it needs to transform the data.
③ Data storage.In order to prevent accidental power failure and facilitate query, the number, current polarity, amplitude, occurrence time and other values must be stored.
Numerical processing
In order to ensure the accuracy of data, numerical processing must be carried out.When the interrupt trigger circuit sends an interrupt signal, the interrupt trigger circuitAD574Collect data once and store it.According to the previous acquisition andstorageDataLagrange interpolation polynomialOperation, i.e. pn (x)?
Because the amplitude conversion circuit is used, the correction coefficient should be proposed according to the measured results and multiplied in the program.
3. Reliability measures taken and measured data
The following measures have been taken in the design: ① The watchdog circuit has the function of automatic reset. ②During software design, a series of judgments and checks are carried out on the status of the reset button to ensure correct inquiry and prevent misoperation. ③E2PROM connected by I2C bus can save field information under abnormal conditions, so the reliability of the whole system can be greatly improved.
Indoor magnetic field influence
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introduction
The conductive frame of a building is often used as its lightning protection systemDownlead。When the building is struck by lightning, the lightning current flowing through the conductive frame may generate transient electromagnetic field indoors, which may interfere with the normal operation of sensitive electronic equipment.Therefore, the analysis of indoor electromagnetic environment during lightning stroke is of great significance to the accurate assessment of lightning hazard.There have been many studies on numerical simulation and model test of indoor electromagnetic field distribution during lightning strike [1,4], in which lightning current is usually simulated as a double exponential wave with wave head time>2 μ s.However, some observation results show that the measured lightning current waveform may be much steeper, and it is not uncommon for the wave head time<1 μ s [2-3].The existing literature on the impact of lightning current waveform on indoor magnetic field distribution is still lack of research.This paper uses this numerical simulation method to analyze it.?
1 Numerical simulation method and its experimental verification
The conductive framework of the building can be seen as a framework composed of interconnected branch conductors, as shown in Figure 1.When the size of the building is much smaller than the equivalent wavelength of lightning current wave, the indoor electromagnetic field can be regarded as a quasi-static field.Each branch conductor is segmented in the conductive frame model, and each segment is represented by a lumped parameter π - shaped equivalent circuit.After establishing the equivalent circuit model of the conductive frame, the current distribution in the frame can be calculated by the circuit method, and thenMaxwell equationAnd Biot Safat law to solve the magnetic field distribution in the building [4].On this basis, the numerical simulation software for calculating the indoor magnetic field distribution is compiled.?
The conductive frame model shown in Figure 1 uses round steel with a diameter of 1cm to form a 3 × 4 × 3 conductor grid, and each section is 0.6m long.Apply a test current of 7.8/16 μ s with a peak value of 1.7kA.The current of each branch is measured with Rogowski coil, and the space magnetic field is measured with self-made magnetic field measurement coil. After verification, the linear error of the measurement system is less than 5%.The corresponding double exponential pulse current waveform parameters are substituted into the simulation program to obtain the calculated values.Figure 2 compares the measured and calculated values of the vertical branch shunt coefficient (the ratio of the branch current to the peak value of the total injection current) in the second layer.The difference between measured and calculated values is less than 10%.To reduce the influence of lead current (introduced from the Y-axis direction), measure the magnetic field in the Y-direction.Figure 3 is on the points where Z=90cm (the second layer) and X=3cmMagnetic inductionThe measured value is consistent with the calculated value.?
Waveform simulation
In order to analyze the influence of lightning current waveform on indoor magnetic field during lightning stroke, the six lightning current models in Table 1 are used for simulation calculation.Models A and B are based on the lightning current model proposed in reference [2], which ignores the impact of building height.A. The wave heads of B and C are similar in time and different in shape, as shown in Figure 4.The current of C decays slowly after the wave peak, and B has a large DC component. C, D, E, and F are double exponential waves, the main difference is that the wave head time is different.?
3 Influence of lightning current waveform on indoor magnetic field distribution
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The conductive framework is the building shown in Figure 1. Assume that each branch conductor is 10m long, 2cm in diameter, and the resistivity ρ=9.8 × 10-8 Ω· m,Grounding resistance0.25 Ω, the lightning current is introduced from a corner of the roof.The six lightning current models in Table 1 are used to calculate the current distribution on each branch conductor and the magnetic field distribution on the planes with heights of 5, 15 and 25m.The current waveform on the branch conductor under model A, B and C is roughly the same, with strong high-frequency oscillation.Figure 5 compares the current waveform on the 10th branch conductor (dotted line position in Figure 1) calculated with model B and D.The high-frequency oscillation on the current waveform with model D is much smaller.
The magnetic field distribution calculated with different lightning current waveforms is shown in Table 2.The magnetic field distribution on the three floors calculated with models A, B and C is roughly the same.Figure 6 shows the magnetic field distribution of the second layer calculated with B.Compare the magnetic field distribution calculated with C, D, E and F in Table 2. Under the same lightning current peak value, the shorter the wave head time, the greater the indoor magnetic field intensity.The influence of lightning current waveform on the magnetic field distribution is caused by the catadioptric reflection of current wave in the conductive frame.Therefore, the head time of lightning current has a greater impact on the magnetic field distribution, while the shape of lightning current waveform has a small impact.In addition, when the wave head time is large, the magnetic field decreases significantly with the decrease of the floor.When the wave head time is less than 1 μ s, the magnetic field distribution between layers is close.At this time, although the current distribution tends to balance with the decrease of the floor, the high-frequency oscillation on the lower layer branch conductor intensifies, leading to the increase of the magnetic field intensity in the lower layer space, making the magnetic field distribution between the layers close.?
In order to further understand the impact of lightning current wave head time on the magnetic field distribution, C, D, E, F are used for calculation on the 1 × 2 × 2 building conductive frame model (the length of branch conductor is 10m).Figure 7 shows the distribution of the magnetic field at each point along X=0.5m at the height of 15m (the second layer) and 5m (the first layer) (taking the ground directly below the lightning strike point as the coordinate origin). The four curves in the figure correspond to models C, D, E and F from top to bottom. The indoor magnetic field intensity increases obviously with the decrease of the wave head time.
conclusion
The simulation results of indoor magnetic field distribution under different lightning current waveforms show that when the lightning current wave head time is similar, the shape of the lightning current waveform has little influence on the current distribution and spatial magnetic field distribution on the conductive framework of the building, but the impact of the lightning current wave head time is greater, and the shorter the wave head time, the greater the indoor magnetic field.The double exponential lightning current waveform can be used in the estimation of indoor magnetic field distribution during lightning stroke, but when the wave head time>2 μ s is used, the possible indoor magnetic field level will be underestimated due to insufficient wave head steepness.?
assignment problem
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In order to carefully analyze lightning strikes on buildingsLightning protection deviceThe distribution of lightning overvoltage and lightning current in low-voltage power supply system has been discussed warmly in domestic lightning protection academic circles.This kind of discussion is very important for us to study IEC lightning protection code and formulate our building lightning protection code.It is quite natural that everyone is engaged in different jobs and professions, as well as the resulting different views.We should discuss these issues constructively based on the principle of seeking truth from facts.
Historically, our national defense mine scholars have conducted a lot of lightning protection tests and studies, and put forward design ideas such as DBSGP (shunt, bonding, shielding, grounding, protection) system lightning protection theory, 6 design elements for building lightning protection, and 3 lines of defense (or 3 lightning protection subsystems) for overall and comprehensive lightning protection.Many of them have been written into GB50057-94《Code for design of lightning protection of buildings》For example, Item 1 of Paragraph 2 and Item 1 of Paragraph 3 of Article 3.3.9, that is, the incoming and outgoing lines shall be shielded by iron pipes.These lightning protection regulations are confirmed by sufficient test basis and long-term operation experience.IEC lightning protection specification is one of the advanced lightning protection specifications in the world. We should learn and digest it modestly.The principle of "bringing" should also take its advantages to make up for its shortcomings, and should not be "adopted equally".IEC61312-3; 2000, IDT Protection against Lightning Electromagnetic Pulses, Part III: Requirements for Surge Protectors. Figures B2, B7, B9, and B10 refer to the diffusion of lightning to other buildings and transformers outside.stayLightning counterattackUnder such circumstances, buildings and power supply lines connected by overhead lines are easy to expandLightning accidentThese analyses are all correct.These mine scholars in China have long analyzed and proposed a solution to the problem, that is, the provision that the incoming and outgoing lines use iron pipes as shielding sections.However, the above IEC lightning protection specifications do not give the analysis and calculation methods of overvoltage and overcurrent in the internal circuit when lightning strikes buildings.
Lightning protection deviceThe distribution of lightning current in is calculated according to the current source, and lightning current acts onGrounding deviceOvervoltage is generated, which becomes a new voltage source.Thereafter, the distribution and calculation of lightning strike back current should be calculated from the lightning strike back overvoltage, rather than from the estimation of 50% of grounding and 50% of line respectively.The assumption put forward in the IEC lightning protection specification is that the lightning current waveform will remain 10/350 μ s during the counterattack, "at the beginning of the impulse current, the current distribution is determined by the system inductance L (power supply)/L（Grounding device）Determination;At the tail of impulse current, the distribution of surge current is calculated as [I (grounding device)/I (power supply)] ∞ [R (power supply)/R (grounding device)].This assumption does not conform to the normal electrical principle, and it ignores the reflection process of lightning current in the grounding grid and the electromagnetic coupling process of electrical lines in the building.Corona phenomenon on the grounding body during lightning strikeGrounding resistanceThe value shows nonlinear characteristics, coupled with the inductance of the grounding body, and the electromagnetic oscillation process in the building structure, so the counterattack voltage of the grounding network cannot be maintained at 10/350 μ s.The shielding section of iron pipe with electromagnetic blocking effect is enough to make the coupling coefficient of electrical lines in the building reach 0.0001-0.01, and the calculation method of the overestimation is wrong.In the case of counterattack, SPDs at all levels are in equipotential and parallel state, and they are in high potential together.In this case, the sequence of their actions depends on which one starts fast and the starting voltage is low.The switch type SPDl can not start first because of its extended starting time and high starting voltage.
The decoupling inductance coil is connected behind the SPD1. Its voltage drop makes the voltage borne by the SPDl lower during the counterattack, and it cannot be started.The electronic triggered SPDl may solve the problem that it cannot be started in advance, but it may have the problem of chopping, that is, the sudden drop of voltage.Electronic instruments are particularly afraid of sudden voltage changes, and the winding of the transformer is also afraid of white intercepting voltage.The lightning protection designers should not be careless about these problems.The purpose of using zinc oxide MOV as SPD is to take advantage of its nonlinear resistance characteristics, which has the ability to prevent electromagnetic oscillation and limit voltage.From the comparison of various overvoltage protection schemes, it can be seen that it is better to properly increase the current capacity of voltage limiting SPD and expand its B value.The switching type SPDl will also encounter many troubles in the system circuit design, and it is not the optimal scheme in the measures to prevent lightning strike current.You are welcome to criticize and correct the above discussions.
