1. What is a surge?
A surge, also known as a surge, is an instantaneous overvoltage that exceeds the normal operating voltage. Essentially, a surge is a violent pulse that occurs in just a few millionths of a second. Surges can be caused by heavy equipment, short circuits, power switching, or large engines. Products containing surge arrest devices can effectively absorb bursts of tremendous energy to protect connected devices from damage.
2. Characteristics of surges
Surges occur for a very short period of time, in the order of a few picoseconds. When a surge occurs, the amplitude of the voltage and current is more than twice the normal value. Because the input filter capacitor charges quickly, this peak current is much greater than the steady-state input current. The power supply should limit the level of surge that AC switches, rectifier bridges, fuses, and EMI filters can withstand. Repeatedly switching the loop on and off, the AC input voltage should not damage the power supply or cause the fuse to blow.
This phenomenon usually lasts only a few nanoseconds to a few milliseconds
The voltage and current values at the time of the surge are more than twice the normal value.
3. Manifestation of surges
Surges are common in power distribution systems, which means they are everywhere. The main manifestations of surge in the power distribution system are:
Voltage fluctuations
Under normal working conditions, the machine will automatically stop or start
Among the electrical equipment are air conditioners, compressors, elevators, pumps, or motors
Computer control systems often reset for no reason
Motors often have to be replaced or rewound
Electrical equipment has a shortened lifespan due to faults, resets, or voltage issues
The effects of surges on sensitive electrical and electronic equipment are of the following types:
destruction
Voltage breakdown semiconductor devices
Destroy the metallized surface layer of the component
Destruction of printed circuit board printed circuits or contact points
Destruction of triac/thyristors...
interference
Lock-up, thyristor or triacs are out of control
Partial destruction of data files
There was an error in the data processing program
Errors and failures in receiving and transmitting data
Unexplained failures...
Premature aging
Premature aging of parts and components and greatly shortening the life of electrical appliances
The output sound quality and picture quality are reduced
4. The source of the surge
Taking the power distribution system as a reference, surges can be divided into two types: out-of-system and in-system. According to statistics, surges outside the system mainly come from lightning and other system impacts, accounting for about 20%; Surges in the system are mainly due to the impact of the electrical load inside the system, which accounts for about 80%.
• External — mainly lightning strikes
• Interior – Switches on and off electrical equipment, etc
Thunder:
1. Direct lightning strike, lightning strikes on lightning rods, lightning belts and a certain part of buildings or oil refining towers.
2. Lightning electromagnetic radiation; The powerful magnetic field at the point of the lightning strike radiates in all directions.
Even if the lightning strike does not directly hit the building, it will cause damage to the microelectronic equipment in the building, because as long as the center point of the lightning strike occurs within a radius of 2Km from the building, an extremely strong electromagnetic field will be generated in the space within this range, and all the power supply lines, networks and signal lines that pass through this electromagnetic field will generate a surge voltage on the line due to electromagnetic induction, and enter the equipment input port in the building along the line, thereby destroying the electronic equipment.
3. The shunt of lightning current on the power supply and signal line;
4. Lightning induction: The lightning current forms a strong alternating magnetic field around the lightning current during the discharge process from the down conductor, and the induced voltage is generated on the metal conductor within the magnetic field.
5. Local high potential formed at the lightning strike site.
6. Lightning intrusion.
When direct lightning strikes the power line or down the lead wire to channel the lightning current, the lightning overvoltage will be generated on the power line and a strong electromagnetic pulse will be generated around the power cable, and all kinds of power, signal and control lines within the range of this electromagnetic pulse will induce overvoltage, and this part of the overvoltage will be transmitted to the back-end equipment along various lines, thereby causing malfunction or damage to the equipment.
Intra-Grid Surge:
(1) Input and removal of large power loads;
Air conditioners, compressors, pumps, or motors
(2) input and removal of perceptual load;
(3) The input and removal of power factor compensation capacitors
(4) Short-circuit fault
(5) Mechanical contacts
Mechanical switches include switching contacts, button switches, buttons, and potentiometers with switching relays
5. Classification of surges
Protection against lightning surges
1. Lightning surge immunity test standard for electronic equipment
The national standard for lightning surge immunity test of electronic equipment is GB/T17626.5 (equivalent to the international standard IEC61000-4-5).
The standard is mainly to simulate the various situations generated by indirect lightning strikes:
(1) Lightning strikes the external line, and a large amount of current flows into the external line or ground resistance, resulting in the interference voltage.
(2) Indirect lightning strikes (such as lightning strikes between or within clouds) induce voltage and current on external lines.
(3) Lightning strikes objects adjacent to the line, and a strong electromagnetic field builds up around it, inducing a voltage on the external line.
(4) Interference introduced when lightning strikes the adjacent ground and the ground current passes through the common grounding system.
In addition to simulating lightning strikes, the standard also simulates interference (voltage transients caused by switching during switching) introduced by switching operations in substations and other situations, such as:
(1) Interference generated during the switching of the main power supply system (such as the switching of capacitor banks).
(2) Interference in the same power grid when some smaller switches near the equipment are jumping.
(3) Switch thyristor equipment with resonant circuits.
(4) Various systemic faults, such as short circuit and arcing faults between equipment grounding networks or grounding systems.
The standard describes two different waveform generators:
One is the waveform produced by lightning strike induced on the power line;
The other is the waveform generated by induction on the communication line.
Both types of lines are empty lines, but the impedance of the lines is different: the induced surge waveform is narrower (50uS) and steeper (1.2uS) on the power line, while the induction of the induction is wider on the communication line, but the leading edge is slower. Later, we mainly analyze the circuit by using the waveform induced by lightning strikes on the power line, and also briefly introduce the lightning protection technology of the communication line.
2. Simulate the working principle of lightning surge pulse generation circuit
The diagram above simulates the surge voltage generated in the transmission line when lightning strikes the power distribution equipment, or the counterattack high voltage generated by the lightning current through the common ground resistance after the lightning lands. The energy of a single pulse at 4kV is 100 joules.
Cs in the figure is the energy storage capacitor (about 10uF, equivalent to the thundercloud capacitor);
US is a high-voltage power supply;
Rc is the charging resistor;
Rs is the pulse duration formation resistance (discharge curve formation resistance);
Rm is the impedance matching resistance and Ls is the current rise to form the inductance.
The lightning surge immunity test has different parameter requirements for different products, and the parameters in the above figure can be slightly changed according to the requirements of product standards.
Basic parameter requirements:
(1) Open circuit output voltage: 0.5~6kV, divided into 5 levels of output, the last stage is determined by the user and the manufacturer through negotiation;
(2) Short-circuit output current: 0.25~2kA, for different levels of testing;
(3) Internal resistance: 2 ohms, additional resistance 10, 12, 40, 42 ohms, for other different levels of testing;
(4) Surge output polarity: positive/negative; When the surge output is synchronized with the power supply, the phase shift is 0~360 degrees;
(5) Repetition rate: at least once per minute.
The severity rating of the lightning surge immunity test is divided into 5 levels:
Level 1: A better protected environment;
Level 2: Somewhat protected environment;
Level 3: Ordinary electromagnetic harassment environment, no special installation requirements for equipment, such as industrial workplaces;
Level 4: Severely harassed environments, such as civil empty overhead lines, unprotected high-voltage substations.
Class X: Determined by the user in consultation with the manufacturer.
The 18uF capacitor in the figure can be selected differently depending on the severity level, but when it reaches a certain value, it basically does not have much significance.
10 ohm resistance and 9uF capacitor, according to different severity levels, the selection value is also different, the minimum resistance can be selected as 0 ohms (the American standard is like this), 9uF capacitor can also be selected very large, but after a certain value, basically there is not much meaning.
3. Common mode surge suppression circuit
Surge protection is designed to assume that the common mode and differential mode are independent of each other. However, these two parts are not really independent, as common-mode chokes can provide considerable differential-mode inductance. This part of the differential mode inductance can be simulated by a discrete differential mode inductance.
In order to take advantage of differential mode inductance, common mode and differential mode should not be done at the same time during the design process, but should be done in a certain order. First, common-mode noise should be measured and filtered out. By using a Differential Mode Rejection Network, the differential mode component can be eliminated, so that common-mode noise can be measured directly. If a common-mode filter is designed so that the differential-mode noise does not exceed the allowable range at the same time, then the mixed noise of the common-mode and differential-mode should be measured. Because the common-mode components are known to be below the noise tolerance, only the differential-mode components are exceeded, which can be attenuated by the differential-mode leakage inductance of the common-mode filter. For low-power power systems, the differential-mode inductance of the common-mode choke is sufficient to solve the differential-mode radiation problem because the source impedance of the differential-mode radiation is small, so only a very small amount of inductance is effective.
To suppress the surge voltage below 4000Vp, generally only need to use LC circuit for current limiting and smoothing filtering, and reduce the pulse signal to the level of 2~3 times the average value of the pulse signal as much as possible. Since L1 and L2 have 50 cycles of grid current flowing, the inductor is easily saturated, so L1 and L2 generally use a common mode inductor with a large leakage inductance.
It is used in AC and DC, usually we are common in power supply EMI filters and switching power supplies, while the DC side is rare, and it can be seen in automotive electronics that it is used on the DC side.
Common mode inductors are added to eliminate common mode interference on parallel lines (two-wire and multi-wire). Due to the imbalance of the impedance of the two wires on the circuit, the common-mode interference is finally reflected in the differential mode. It is difficult to filter with differential mode filtering.
Where do common mode electrometry devices need to be used? Common-mode interference is usually electromagnetic radiation, space coupling, so whether it is AC or DC, you have long-term transmission, it involves common-mode filtering, you have to add common-mode inductance. For example, there are many USB cables, so add a magnetic ring on the line. Switching power inlet, AC power is transmitted over a long distance, so it needs to be added. Usually, the DC side does not need to be teletransmitted, so it does not need to be added. There is no common-mode interference, adding it is a waste, and there is no gain to the circuit.
The design of power filters can often be considered in terms of both common mode and differential mode. The most important part of the common mode filter is the common mode choke, compared with the differential mode choke, a significant advantage of the common mode choke is that its inductance value is extremely high, and the size is small, an important problem to consider when designing the common mode choke is its leakage inductance, that is, differential mode inductance. In general, the method of calculating the leakage inductance is to assume that it is 1% of the common mode inductance, but in fact, the leakage inductance is between 0.5% ~ 4% of the common mode inductance. When designing chokes for optimal performance, the impact of this error may not be negligible.
The importance of leakage sensing
How is leakage inductance formed? A tightly wound toroid, wound around a full circle, even without a core, has all the magnetic flux concentrated in the coil "core". However, if the toroidal coil is not wound for a full circle, or if it is not wound tightly, then the magnetic flux will leak out of the core. This effect is proportional to the relative distance between the turns and the permeability of the spiral core. Common mode chokes have two windings that are designed so that the current they flow through is conducted in the opposite direction along the coil core, resulting in a magnetic field of 0. If, for safety reasons, the coils on the core are not double-wired, there is a considerable gap between the two windings, this will naturally cause a magnetic flux "leak", which means that the magnetic field is not really zero at the various points of interest. The leakage inductance of a common-mode choke is a differential-mode inductance. In fact, the magnetic flux associated with differential mode must leave the core at some point, in other words, the magnetic flux forms a closed loop outside the core, not just inside the toroidal core.
In general, CX capacitors can withstand 4000Vp differential-mode surge voltage impulses, and CY capacitors can withstand 5000Vp common-mode voltage impulses. By correctly selecting the size of the L1, L2, CX2, and CY parameters, it is possible to suppress common-mode and differential-mode surge voltages of 4000Vp or less. However, if the two CY capacitors are installed in the whole line, the total capacity cannot exceed 5000P, and if the surge voltage is to be suppressed more than 4000Vp, a capacitor with higher withstand voltage and a surge suppression circuit with limiting function should be selected.
The so-called suppression is simply to reduce the amplitude of the spike pulse a little, and then convert it into another waveform output with a relatively wide pulse width and flatter amplitude, but its energy is basically unchanged.
The capacity of the two CY capacitors is generally very small, the stored energy is limited, and its effect on the common mode suppression is not very large, therefore, the common mode surge suppression mainly depends on the inductors L1 and L2, but because the inductance of L1 and L2 is also limited by the volume and cost, it is generally difficult to do a large amount, so the above circuit has a very limited effect on the lightning common mode surge voltage suppression.
In Figure (a), L1 and CY1, L2 and CY2 suppress the two common-mode surge voltages respectively, and only one of them needs to be calculated. Ø To calculate L1 accurately, it is necessary to solve a set of second-order differential equations, and the results show that the capacitor charge is carried out according to the sinusoidal curve, and the discharge is carried out according to the cosine curve. However, this calculation method is more complicated, and a simpler method is used here.
Hypothetically, the common-mode signal is a square wave with an amplitude of Up and a width of τ, and the voltage at both ends of the CY capacitor is Uc, and the measured current through the inductor is a sawtooth wave with a width equal to 2τ:
The current flowing through the inductor is:
The maximum current flowing through the inductor is:
The average current flowing through the inductor during 2τ is:
From this, it can be obtained that the voltage change of CY capacitance during 2τ is:
The above formula is the calculation formula for calculating the parameters of inductance L and capacitance CY in the common-mode surge suppression circuit, where Uc is the voltage at both ends of the CY capacitor and the output voltage of the surge suppression circuit, ∆Uc is the voltage change at both ends of the CY capacitor, but because the period of the lightning pulse is very long and the duty cycle is very small, it can be considered that Uc = ∆Uc, Up is the peak value of the common-mode surge pulse, q is the charge stored in the CY capacitor, τ is the width of the common-mode surge pulse, L is the inductance, and C is the capacitance.
According to the above formula, assuming that the peak voltage of the surge is up = 4000 Vp, the capacitor C = 2500 p, and the output voltage of the surge suppression circuit Uc = 2000 Vp, the value of the inductor L is 1H. Obviously, this value is very large and difficult to achieve in practice, so the ability of the above circuit to suppress the common mode of lightning is very limited, and this circuit needs to be further improved.
The differential mode surge voltage suppression mainly depends on the selection of parameters such as filter inductance L1 and L2, and filter capacitor CX, L1, L2 filter inductance and CX filter capacitor in the figure, which can also be calculated by the following formula.
However, L in the above equation should be equal to the sum of the two filter inductors L1 and L2, C=CX, and Uc should be equal to the output voltage of differential mode suppression. Generally, the output voltage of differential mode suppression should not be greater than 600Vp, because the maximum withstand voltage of many semiconductor devices and capacitors is around this voltage, and after the two filter inductors of L1 and L2 and the filtering of CX capacitors, the amplitude of the lightning differential mode surge voltage is reduced, but the energy is basically not reduced, because after the filtered wave, the pulse width will increase, and once the device is broken down, most of them cannot be restored to the original state.
According to the above formula, assuming that the peak surge voltage is Up=4000Vp, the pulse width is 50uS, and the output voltage of the differential mode surge suppression circuit is Uc=600Vp, the LC value is 14mH×uF. Obviously, this value is still relatively large for the surge suppression circuit of general electronic products, in contrast, it is more advantageous to increase the inductance than to increase the capacitance, so it is best to choose an inductor with 3 windows, using silicon steel sheet as the core, and the inductance is relatively large (greater than 20mH) as the surge inductance, the common mode and differential mode inductance of this inductance are very large, and it is not easy to be saturated. By the way, the electrolytic filter capacitor behind the rectifier circuit also has the function of suppressing surge pulses, and if this function is also included, the output voltage Uc cannot be selected as 600 Vp, but only the maximum withstand voltage of the capacitor Ur (400 Vp).
4. Commonly used devices for lightning surge impulse voltage suppression
Lightning protection devices mainly include ceramic gas discharge tubes, zinc oxide varistors, semiconductor thyristors (TVS), surge suppression inductance coils, Class X surge suppression capacitors, etc., and various devices should be used in combination.
There are many types of gas discharge tubes, the discharge current is generally very large, up to tens of kA, the discharge voltage is relatively high, the discharge tube takes a certain amount of time from ignition to discharge, and there is residual voltage, and the performance is not very stable; The volt-ampere characteristics of the zinc oxide varistor are better, but due to the limitation of power, the current is relatively smaller than that of the discharge tube, and the breakdown voltage value will drop or even fail after being broken down by lightning overcurrent for many times; The volt-ampere characteristics of semiconductor TVS tubes are the best, but the power is generally very small and the cost is relatively high; The surge suppression coil is the most basic lightning protection device, and in order to prevent the flow of AC saturation through the power grid, a three-window iron core must be selected; X capacitors are also necessary, and capacitors with large allowable ripple currents should be selected.
Gas discharge tubes
Gas discharge tube refers to the lightning arrester or antenna switch tube used for overvoltage protection, with two or more electrodes in the tube, filled with a certain amount of inert gas. Gas discharge tube is a kind of intermittent lightning protection element, which is used in the lightning protection of communication systems.
The working principle of the discharge tube is that when a certain voltage is applied between the two poles of the discharge tube, an uneven electric field is generated between the poles: under the action of this electric field, the gas in the tube begins to be free, when the applied voltage increases to make the field strength between the poles exceed the dielectric strength of the gas, the gap between the two poles will break down the discharge, and the original insulation state will be transformed into a conductive state, and the voltage between the two poles of the discharge tube will be maintained at the residual voltage level determined by the discharge arc after conduction, and this residual voltage is generally very low, As a result, the electronic devices connected in parallel to the discharge tube are protected from overvoltage.
Some gas discharge tubes are encapsulated shells with glass as the tube. Some use ceramics as the encapsulated shell, and the discharge tube is filled with inert gas with stable electrical properties (such as argon and neon, etc.), and the discharge electrodes of the commonly used discharge tube are generally two or three, and the electrodes are separated by inert gas. According to the setting of the number of electrodes, the discharge tube can be divided into two poles and three pole discharge tubes.
The ceramic diode discharge tube is composed of pure iron electrodes, nickel-chromium-cobalt alloy caps, silver-copper welding caps and ceramic tube bodies. The discharge electrode in the tube is coated with radioactive oxide, and the inner wall of the tube is also coated with radioactive elements to improve the discharge characteristics. The discharge electrode mainly has two kinds of structures: rod-shaped and cup-shaped, in the discharge tube of the rod-shaped electrode, a cylindrical hot screen is installed between the electrode and the wall of the tube body, and the hot screen can make the ceramic tube body heat tend to be uniform, so as not to cause local overheating and cause tube fracture. The heat screen is also coated with radioactive oxide to further reduce discharge dispersion. In the discharge tube of the cup-shaped electrode, a molybdenum mesh is installed at the mouth of the cup, and cesium is installed in the cup, which also reduces the discharge dispersion.
The three-pole discharge tube is also composed of pure iron electrodes, nickel-chromium-cobalt alloy caps, silver-copper welding caps and ceramic tube bodies. Unlike the two-pole discharge tube, a nickel-chromium-cobalt alloy cylinder is added to the three-pole discharge tube as the third pole, that is, the grounding electrode.
Main parameters:
(1) DC breakdown voltage. This value is determined by the application of a voltage value with a low rise rate (dv/dt = 100 V/s).
(2) Impulse (or surge) breakdown voltage. It represents the dynamic characteristics of the discharge tube and is usually determined by a voltage value with a rise rate of dv/dt = 1kV/us.
(3) Nominal impulse discharge current. 8/20us waveform (8us leading-edge, 20us half-peak duration) rated discharge current, typically 10 discharges.
(4) Standard discharge current. Through the rated effective value of 50Hz AC current, the time of each discharge is specified to be 1s, and the discharge is 10 times.
(5) The maximum single impulse discharge current. The single maximum discharge current for an 8/20us current wave.
(6) Power frequency current value. The single maximum discharge current for an 8/20us current wave. For 50 Hz alternating current, the maximum current that can withstand 9 consecutive cycles is the effective value.
(7) Insulation resistance. The single maximum discharge current for an 8/20us current wave. For 50 Hz alternating current, the maximum current that can withstand 9 consecutive cycles is the effective value.
(8) Capacitance. The capacitance between the electrodes of the discharge tube is generally between 2~10pF, which is the smallest among all transient interference absorption devices.
Metal oxide varistors
Varistors are generally pressed with zinc oxide as the main component, plus a small amount of other metal oxides (particles), such as co., tung, fierce, bismuth, etc. Since two objects with different properties are combined together to form a PN junction (diode), a varistor is equivalent to a large number of PN junctions connected in series.
5. Ultra-high surge voltage suppression circuit
Example 1
The above figure is an electrical schematic diagram that can resist strong lightning surge pulse voltage, in the figure: G1 and G2 are gas discharge tubes, which are mainly used to suppress high-voltage common-mode surge pulses, and also have the ability to suppress high-voltage differential mode surge pulses; VR is a varistor, which is mainly used to suppress high-voltage differential mode surge pulses. After G1, G2 and VR suppression, the amplitude and energy of the common-mode and differential-mode surge pulses are greatly reduced.
The breakdown voltage of G1 and G2 can be 1000Vp~3000Vp, and the varistor voltage of VR is generally 1.7 times of the maximum power frequency voltage.
G1 and G2 will produce follow-up current after breakdown, and a fuse must be added to prevent the follow-up current from being short-circuited over the ambassador line.
Example 2
Two varistors VR1, VR2 and a discharge tube G3 are added, the main purpose is to strengthen the suppression of common mode surge voltage, because the varistor has leakage current, and the general electronic products have strict requirements for leakage current (less than 0.7mAp), so a discharge tube G3 is added in the figure, so that the leakage current of the circuit to the ground is equal to 0. The breakdown voltage of G3 is much smaller than the breakdown voltage of G1 and G2, and the breakdown voltage of varistor VR1 or VR2 can be selected correspondingly low after the leakage isolation of G3, and VR1 and VR2 also have a strong suppression effect on the differential mode surge voltage.
Example 3
G1 is a three-terminal arrester, which is equivalent to installing two two-terminal arresters in one housing, and can be used to replace the G1 and G2 arresters in the above two examples. In addition to the two-terminal and three-terminal discharge tubes, there are also four-terminal and five-terminal discharge tubes, and the purpose of each discharge tube is not exactly the same.
Example 4
Two varistors (VR1 and VR2) are added, the main purpose of which is to isolate the follow-up current generated after the breakdown of G1 to prevent the follow-up current from being over-exposed to the input circuit from short-circuiting, but because the maximum peak current of VR1 and VR2 is generally only a few tenths of G1, the suppression ability of this example to ultra-high surge voltage is much worse than that of example 3.
Example 5 Lightning protection device is made directly on the PCB board
Discharge lightning protection device is directly made on the PCB board, can replace the lightning protection discharge tube, can suppress tens of thousands of volt common mode or differential mode surge voltage impact, the distance between the electrodes of the lightning protection device is generally strict, when the input voltage is AC110V, the distance between the electrodes can be selected 4.5mm, when the input voltage is AC220V, 6mm can be selected;
Example 6: The PCB board air gap discharge device replaces the discharge tube
The air gap discharge device is directly made on the PCB board, the normal discharge voltage is 1000~1500V per millimeter, the discharge voltage of 4.5mm creepage distance is about 4500~6800Vp, and the discharge voltage of 6mm creepage distance is about 6000~9000Vp.
6. Connection of various lightning protection devices
The installation sequence of lightning protection devices should not be mistaken, the discharge tube must be at the front, followed by the surge suppression inductor and varistor (or discharge tube), and then the semiconductor TVS thyristor or Class X capacitor and Class Y capacitor.
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