Lightning discharge and switching operation as a source of interference   The following describes how to use lightning discharge and switch as an interference source 1 Atmospheric overvoltage As a source of interference, lightning affects buildings and indoor electrical equipment and systems. Electrical surges originating in the atmosphere are almost always the result of direct/adjacent lightning strikes or distant lightning strikes. In the case of a direct lightning strike, the lightning strikes directly on a protected building; However, in the case of adjacent lightning strikes, lightning strikes on extended systems or pipelines (such as pipes, data transmission lines, or power lines) directly connected to the protected system. Lightning overhead lines are examples of distant lightning strikes. Lightning between clouds creates "reflected surges" (traveling waves) along the transmission line, while lightning in the surrounding area induces overvoltage.   1.1 Direct Lightning Strikes and Adjacent lightning Strikes The effect of the lightning current on the wires of the lightning channel and lightning protection system :(a) to produce a voltage drop on the impact grounding resistance of the grounding system; (b) The induction of surge voltage and current in the loop formed by the wires inside the building. Due to the voltage drop on the impact ground resistance, the lightning current is also discharged through the power line connected as a lightning protection equipotential connection measure. In particular, due to the magnetic interference radiation from lightning strikes, lightning strikes in the surrounding area induce surge voltages and currents in the device loop. If lightning strikes an overhead supply line, there is conducted surge voltage and current on the power supply inlet line. Intercloud lightning also produces conducting surge voltages and currents on power lines and other large wire systems due to radiation from electromagnetic interference. If accurate analysis is not possible or is too expensive, the partial lightning current on the power line from the struck building can be estimated in accordance with IEC 61312-1 and DIN VDE 0185 Part 103. Assume that 50% of the lightning current flows into the grounding system of the building and 50% is evenly distributed in the distant grounding supply system (such as pipes, power supplies, and communication lines). For simplicity, it is assumed that the lightning current is evenly distributed across the conductors (e.g. L1, L2, L3 and PEN of the power cable, or the four cores of the data cable) in each supply system. In Appendix C of DIN V ENV 61024-1(VDE V 0185 Part 100), there is a method for estimating the partial lightning current discharged through the inlet line (in the case of lightning protection systems). Accordingly, the lightning current will be distributed on the ground system, the external conductor and the incoming line (directly connected or connected through the arrester) as follows: The lightning current shared by each external conductor and conductor depends on the number of external conductors and conductors, their equivalent ground resistance, and the equivalent ground resistance of the grounding system. If the conductors used in an electrical or information system are not shielded or placed in metal tubes, the current shared by the conductors is It/n', where n' is the total number of conductors in the electrical or information system. 1.1.1. Voltage drop across the impact grounding resistance The maximum voltage drop uE on the impact ground resistance Rst of the struck building is calculated as the maximum i of the lightning current. This voltage drop uE is not dangerous to the protected system if an equipotential connection for lightning protection has been effectively established. At present, both domestic and international lightning protection standards require the implementation of integrated equipotential connection. In a synthetic equipotential connected system, all wires (incoming or outgoing) are connected to the ground system either directly or through spark gaps or surge protectors. During a lightning strike, the uE of the entire system will increase in potential, but within the system, there will be no dangerous potential difference. 1.1.2 Induced voltage in metal ring The maximum rising velocity of the lightning current (Δi/Δt, effective within Δt time) determines the peak value of the electromagnetic induced voltage in all open or closed device loops around the current-carrying conductor of the lightning current. In the design of the lightning protection system, the maximum value I/T 1 of the average rise rate of the given wave head current can be used (valid within the wave head time T1). When estimating the maximum induced square wave voltage U on a device loop (e.g., in a building), the loop is assumed to be near the lead down line of an infinitely long lightning current. The square wave voltage can be estimated for a square ring consisting of an infinite length of lightning current-carrying conductors and equipment wires (for example, the protective conductor of an electrical device connected to the lead line of a lightning protection system at the equipotential connection bar). For a square ring consisting of equipment wires insulated with an infinitely long lightning current carrying conductor, the square wave voltage can be found. In addition to the induction effect in the large metal ring caused by the equipment arrangement, the induction effect on the long and narrow ring of unshielded, layered stranded cable composed of parallel wires near the current-carrying wires of lightning current is also noteworthy. The induced voltage between the lines is called "transverse voltage". This voltage is particularly harmful to electronic devices. The square wave voltage can be found for a narrow loop of wires composed of the conductors of the equipment line parallel to the current carrying conductors of the infinite lightning current. The square wave voltage of a long wire frame composed of equipment wires perpendicular to the infinitely long lightning current carrying wire at a certain distance In contrast to the high voltage values in the large ring, there is only about 100V of induced voltage in the long narrow ring. But remember this is the lateral voltage on the information system line, which is only 1-10V in normal operation and is connected to surge sensitive electronics. In a stranded line, especially a line with electromagnetic shielding, the induced square wave voltage is much smaller than the value calculated according to the above formula, and the transverse voltage of this amplitude is usually not dangerous. If the metal ring is short-circuited or its insulation is broken down by the induced square wave voltage U, there is an induced current i in the ring whose value can be calculated. Because the lightning current rises very fast, a rapidly changing magnetic field will be generated near the lightning channel or the current carrying conductor. The magnetic field in the building generates a surge voltage of up to 10, 00V in a wide "induction loop" formed by utilities such as power and information system wiring, water pipes and gas pipes. For example, a computer connected to a power and data system. After entering the building, the data cable is connected to the equipotential connection bar, and then the cable passes through the data cable socket into the computer. The power cable is also connected to the equipotential connection bar through the arrester, which supplies power to the computer through the electrical outlet. Since the power cord and data cable are installed independently, they can form an induction ring with an area of about 100m2. The open end of the ring is in the computer, and the surge voltage generated by magnetic induction in the ring is applied to the open end. Not only in the case of direct lightning strikes, but also in the case of adjacent lightning strikes, the ring can be induced by the surge voltage enough to cause equipment breakdown and sometimes even fire. The computer must be protected against these lightning surges "in place," meaning either on the device itself or directly at the power and data outlets (Section 5.8.2.3). 1.2 Remote Lightning strikes In the case of distant lightning strikes, the traveling wave surge travels along the road, or the lightning strikes near the protected system, thereby affecting the electromagnetic field of the protected system. Atmospheric overvoltage hazards in the 1890s demonstrated that electronic devices remain sensitive to induced or conducted surge voltages and currents up to 2km from the point of lightning strike (Section 2.1). This widespread hazard is due to the increasing sensitivity of high-tech equipment connected to cables that extend outside buildings and the growing use of sensitive networks. As technology has developed, the maximum allowable length of data lines connecting devices has increased rapidly. For example, the V2.4/V2.8 interface (used at the dawn of EDP) states that the electrical characteristics of line drivers allow for a direct cable connection up to about 15m in length. The available line drivers and interfaces allow direct connection of twin-core stranded cables up to about 1000m in length. When the lightning current flows in the cable, longitudinal and transverse voltages will be generated. The longitudinal voltage u1 generated between the core wire and the metal shield of the cable is applied to the insulation between the input end of the connected device and the grounded housing. The transverse voltage uq appears between the wires and puts pressure on the input circuit of the connected device. If the lightning current i2 is known, then the longitudinal voltage can be calculated from the coupling impedance R of the cable. 1.3 Coupling of surge current in signal line The following example shows how surge current is coupled to the signal lines of an extended system by resistive, inductive, or capacitive coupling. For example, consider the layout of device 1 in Building 1 and device 2 in building 2. The two devices are connected through signal cables. In addition, both devices are assumed to be connected to an equipotential bar (PAS) in their respective buildings by protective ground wires (PE). 1.3.1 Resistive coupling Lightning strikes building 1, creating a potential difference of about 100kV across the ground resistance RA1. The voltage of this amplitude is sufficient to flasmp the insulation distance between devices 1 and 2 so that the resistive cross-coupled surge current flows from PAS1 through Device 1 along the signal line to devices 2, PAS2, and RA2. The amplitude of the surge current, which peaks at several kA, depends on the relative values of the ohm resistance RA1 and RA2. 1.3.2 Perceptual coupling As mentioned before, the voltage in the metal ring is induced through the inductive field of the lightning channel or the lightning current carrying conductor. For example, the two core signal cables between devices 1 and 2 form an induction ring. If lightning strikes building 1, a transverse voltage of several thousand volts will be induced within the ring, producing a coupled current of up to several thousand amps. These induced voltages and currents are applied to the input or output of the device. Another example of emotional coupling that can occur. The signal line forms an induction ring with the ground. If lightning strikes building 1, a very high voltage (about 10kV) is induced on the ring, causing the insulation flashover of Equipment 1 and Equipment 2, generating coupling currents of thousands of amps. 1.3.3 Capacitive coupling If lightning strikes the ground or the lightning connector, the lightning channel or the lightning connector will rise to a very high voltage (about 100kv compared to its surroundings) due to the potential difference on the ground electrode resistance RA.   The signal line between devices 1 and 2 is capacitatively coupled to this lightning channel or receiver. The coupling capacitor is charged, causing an "injection" current (about 10A) to pass through the insulated flow of devices 1 and 2. 1.4 Amplitude of atmospheric overvoltage A distant lightning strike initially causes a surge of about 10kV, producing a relatively small numerical current. But a direct lightning strike has a much larger current with a much higher amplitude: 200kA of current (Class I protection) and peaks of hundreds of kilovolts. Low-voltage equipment usually can only withstand the impact breakdown voltage of thousands of volts, so it is vulnerable to tens of thousands of volts from distant lightning strikes or 100kV overvoltage from direct lightning strikes, and even be damaged. Some electronic devices may tolerate voltages as low as 10V. Therefore, the voltage value caused by atmospheric discharge is 100 to 10,000 times higher than the tolerable voltage of a low-voltage system containing electronic equipment. Therefore, these high-amplitude overvoltages must be reduced by protection measures or surge protectors to values significantly below the allowable impulse breakdown voltage/impulse flashover voltage. For reliable protection, even in the case of direct lightning strikes, the surge protector must be able to release a high level of lightning current without being damaged. 2 Operate the overvoltage Operating overvoltage can also affect low-voltage and secondary systems, especially when capacitive coupling is present. In some cases, the value of this operating overvoltage can exceed 15kV. The causes of these operating overvoltages are as follows: (a) Cut out no-load power lines (or capacitors). When the switch is on, the change in the instantaneous value of the supply voltage causes a high potential difference between the system and the cut line. This potential difference, built up in milliseconds, can cause reignition between the contacts of the switch, as if the contacts were closed again. The line voltage then equates to the instantaneous value of the supply voltage, and the arc between the switching contacts is extinguished. This process can be repeated many times. This process, in which the line voltage is equal to the instantaneous value of a certain supply voltage, produces an operating overvoltage characterized by attenuation oscillations in the order of several hundred kilohertz. The initial amplitude of this operating overvoltage is related to the potential difference between the switch contacts at the time of reignition and can be several times the rated supply voltage. (b) Cut out the no-load transformer. If a no-load transformer is removed from the grid, the energy of the magnetic field is loaded onto its own capacitance. The inductance - capacitance circuit then oscillates until all the energy is converted into heat through the resistance in the circuit, resulting in an operating overvoltage amplitude several times that of the rated supply voltage. (c) Ground fault in ungrounded network. If the grounding fault occurs in the external line of the ungrounded network, the ground potential of the whole system will change due to the voltage change of the grounding phase. If the grounding fault arc is extinguished, the effect is similar to that of cutting out a no-load line or capacitor: an operating overvoltage is generated with attenuated shocks. In addition to the above characteristics of grid operating overvoltages affecting low-voltage systems in a capacitive coupling manner, rapid changes in current can also generate surges in low-voltage systems through inductive coupling. This sudden change in current may be caused by heavy switching load, or may be caused by short circuit, ground fault or repeated ground fault. Operating overvoltage may also occur within the low voltage system itself for the following reasons: • Cut out inductors that are in parallel with the power supply, such as the coils or reactors of transformers, contactors and relays (in this case, the operating overvoltage is generated similar to the case above for cutting out no-load power transformers). • Remove inductors in the series arms of the current loop, such as the loop of wires, the series inductor, or the inductor itself (the current on the inductor cannot mutate when the circuit is disconnected, and the amplitude of the resulting operating overvoltage depends on the current value at the moment of disconnection). • Intentional cutting of a circuit by a switch, or unintentional tripping of a fuse or circuit breaker, or unintentional cutting of a wire prior to a natural zero crossing of the current (such disconnections result in a sharp change in current resulting in an operating overvoltage, usually attenuated and oscillating, with a amplitude several times the normal voltage of the system). • Phase control circuit, reversing effect of brush collector system, sudden unloading of motor and transformer, etc. Numerous measurements on various low-voltage networks have shown that the most notable surges are caused by the interfering radiation of the electric arc produced in the switch. Electromagnetic interference from power system operation is usually more frequent than lightning interference. For wideband conducted interference, high energy pulses and low energy pulses or different types of switching operation pulses are treated differently in EMC standards. Switching interference may be generated outside the building, through power lines, or inside the building. These two kinds of interference can be regarded as either a combination of surge voltage interference and surge current interference, as with lightning interference, or as applied surge voltage. Wide-band high energy conducted interference in the switching process can be treated equally with conducted lightning interference in the building (equipotential connections for appropriate lightning protection are arranged). Therefore, the VG standard specifies the corresponding peak interference by environment type. DIN VDE 0160 specifies the applied surge voltage due to the breaking process or overcurrent protection element. 0.1/1.3ms(0.1ms rising speed, 0.15ms wave head time), the surge voltage with a peak value of upeak is superimposed on the peak value of AC voltage uN/max. Wideband low energy operating voltage interference (i.e. pulse swarm) is specified in DIN VDE 0847 Part 4-4. The waveform is 5/50ns(5ns rise rate, wave head time about 7.4ns), the amplitude is related to the severity of the test, and is applied to power lines and communication lines in the form of pulse packets by coupling capacitors. In addition to conducted interference, the operation process itself generates considerable interference radiation (for example, arcing when a switch is disconnected), which induces more conducted interference.

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Post time: Feb-10-2023