Electrical surge, as its name implies, is the instantaneous peak value exceeding the stable value, which includesSurge voltageandSurge current。
Surge is also calledBurst, as the name implies, it is the instantaneous overvoltage that exceeds the normal working voltage.In essence, surge is a kind of intense pulse occurring in only a few millionths of a second.Possible causes of surge include heavy equipment, short circuit, power switching or large engine.Products with surge arresting devices can effectively absorb sudden huge energy to protect connected equipment from damage.
Surge current refers toPower SupplyAt the moment of connection orCircuitThe current generated under abnormal conditions is far greater than the steady state currentpeak current Or overload current.
In electronic design, surge mainly refers to the power generated at the moment when the power supply (only the power supply) is just turned onpulse, because the linearity of the circuit itself may be higher than the pulse of the power supply itself;Or because the power supply or other parts of the circuit are subject to their own or external sharp pulse interference, it is called surge. It is likely to burn out the circuit at the moment of surge, such as PN junction capacitance breakdown, resistance burn out, etcSurge protection is a protection circuit designed by using the sensitivity of linear components to high frequency (surge). The simple and commonly used ones are parallel capacitors and series inductors.
Cause
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The source of power supply system surge is divided into external (caused by lightning) and internal (start stop and fault of electrical equipment, etc.).
External causes
Lightning surge overvoltage
The surge caused by lightning is the most harmful. During lightning discharge, dangerous over-voltage may be generated within 1.5-2KM around the lightning strike.The (external) surge caused by lightning stroke is characterized by single-phase pulse type and huge energy.The voltage of external surge can rapidly rise from hundreds of volts to 20000 volts in a few microseconds, which can transmit a considerable distance.According to ANSI/IEEE C62.41-1991, the instantaneous surge can be up to 20000 V, and the instantaneous current can be up to 10000 A.According to statistics, the surge outside the system mainly comes from lightning and other system shocks, accounting for about 20%.
(1) Induced lightning surge overvoltage: the high-speed changing electromagnetic field generated by lightning strikes. The electric field radiated by lightning acts on the conductor and induces very high overvoltage. This kind of overvoltage has a very steep front and decays rapidly.
(2) Direct lightning surge overvoltage: direct lightning strikes on the power grid. Due to the huge instantaneous energy and strong destructive power, no equipment can protect direct lightning strikes.
(3) Lightning conduction surge overvoltage: it is conducted from a distant overhead line. Since the equipment connected to the power network has different suppression capabilities for overvoltage, the energy of conducted overvoltage will decrease with the extension of the line.
(4) Oscillating surge overvoltage: the power line is equivalent to an inductance, and there is a distributed capacitance between the earth and adjacent metal objects, forming a parallel resonant circuit. In TT and TN power supply systems, at the moment of single-phase ground fault, due to the resonance of high-frequency components, a very high overvoltage occurs on the line, mainly damaging the secondary instrument.
Direct lightning strike is the most serious event, especially if the lightning strike strikes the overhead transmission line near the user's inlet.When these events occur, the voltage of overhead transmission lines will rise to hundreds of thousands of volts, usually causing insulation flashover.The transmission distance of lightning current on the power line is one kilometer or more, and the peak current near the lightning strike point can reach 100kA or more.The current of low-voltage line at the user inlet can reach 5kA to 10kA per phase.In areas with frequent lightning activities, power facilities may suffer several direct lightning strikes every year, causing serious lightning currents.However, the above events rarely occur in areas where underground power cables are used for power supply or where lightning activities are not frequent.
The probability of indirect lightning stroke and internal surge is high, and most of the damage of electric equipment is related to them.Therefore, the focus of power supply surge prevention is to absorb and suppress the surge energy.
Internal causes
Operating surge overvoltage
The cause of internal surge is related to the start and stop of equipment inside the power supply system and the failure of power supply network operation:
In the power system, due to the operation of the circuit breaker, load input and removal, or system fault and other internal state changes of the system, the system parameters change, resulting in the internal electromagnetic energy conversion or transmission transition process of the power system, there will be over-voltage in the system.The surge in the system mainly comes from the impact of power load inside the system, accounting for about 80%.The causes of internal overvoltage in the power system can be roughly divided into:
(1) Input and removal of large power load;
(2) Input and removal of inductive load;
(3) Switching on and switching off of power factor compensation capacitor
(4) Short circuit fault
The power supply system will bring internal surge due to the start and stop of high-power equipment, line fault, switching action, operation of frequency conversion equipment and other reasons, which will bring adverse effects to electrical equipment.Especially the computer, communication and other microelectronic equipment bring fatal impact.Even if there is no permanent damage to the equipment, the abnormality and pause of the system operation will bring serious consequences.For example, nuclear power plants, medical systems, large-scale factory automation systems, securities trading systems, telecommunications exchanges, network hubs, etc.
Surge performance
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Surge generally exists in the distribution system, that is to say, surge is everywhere.Surge in power distribution system mainly includes:
- Voltage fluctuation
- Under normal working conditions, the machine and equipment will automatically stop or start
- There are air conditioners, compressors, elevators, pumps or motors in the electrical equipment
- Computer control system is often reset without reason
- The motor often needs to be replaced or rewound
- Shorten service life of electrical equipment due to fault, reset or voltage problems
Characteristics of surge
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The time of surge generation is very short, about picosecond.When the surge occurs, the amplitude of voltage and current exceeds twice of the normal value.Because the input filter capacitor charges rapidly, the peak current is far greater than the steady input current.The power supply shall limit the surge level that AC switch, rectifier bridge, fuse and EMI filter can withstand.Repeatedly switch the loop and AC input voltage shall not damage the power supply or cause the fuse to burn out.
Hazards of surge
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Failure principle and phenomenon
The causes of surge (transient pulse) include lightning, poor grounding, inductive load switching, mains fault removal andstatic electricityDischarge (ESD) may result in data loss (or damage) or even equipment damage.Among them, lightning is the most destructive.The lightning strike and the surge caused by the instantaneous discharge or arc discharge generated by the contact switch are:
Flashover: leave obvious arc marks on the damaged parts;
Corona: on the surface of the insulator, there are obvious signs of electric erosion, and the insulation of the corroded part drops;
IC and other components of control circuit are damaged;
Rectifier elements and voltage stabilizing elements of general electronic equipment and household appliances are damaged;
Grounding fault causes equipment electrification (single-phase grounding): causes equipment phase to phase short circuit (motor phase to phase short circuit).
Hazard classification
The hazards of surge are mainly divided into two types: catastrophic hazards and cumulative hazards.
Catastrophic hazard:If a surge voltage exceeds the bearing capacity of the equipment, the equipment will be completely damaged or its life will be greatly reduced.
Generally, the insulation voltage of the motor is twice the normal working voltage plus about 1000V, so the insulation voltage of the 220V motor is generally 1500V.The surge constantly impacts the insulation layer of the motor, causing the insulation layer to be broken down.
Cumulative hazard:Multiple small surge accumulation effectssemiconductorThe decline of device performance, equipment failure and shortening of service life will eventually lead to shutdown or productivity decline.
Impact on equipment
Presence of surge:Surge is ubiquitous in distribution system, that is to say, surge is everywhere.Surge in distribution system mainly includes:
voltage fluctuation
Under normal working conditions, the machine and equipment will automatically stop or start
Electric equipment includes air conditioner, compressor, elevatorpumpOr motor and computer control system often reset without reason
The motor needs to be replaced or rewound frequently
electricalShorten service life of equipment due to fault, reset or voltage problems
Impact of surge on equipment:The impact of surge on sensitive electronic and electrical equipment has the following types:
destruction
Voltage breakdown semiconductor device
Damaging the metallized surface of components
Destroy the printed circuit or contact point of the printed circuit board
Destroy the three terminal double thyristor element/thyristor
interfere
Locked, thyristor or three terminal bidirectional silicon controlled element out of control
The data file is partially damaged
Data handler error
Errors and failures in receiving and transmitting data
Faults with unknown causes
Premature aging
Parts are aging ahead of time, and the service life of electrical appliances is greatly shortened
The output sound quality and picture quality decrease[1]
Surge protection
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Surge protective device, also calledLightning arresterIt is an electronic device that provides security protection for various electronic equipment, instruments and communication lines.When the electrical circuit or communication line suddenly generates peak current or voltage due to external interference, the surge protector can conduct shunt in a very short time, so as to avoid the damage of surge to other equipment in the circuit.
Basic elements
Discharge gap
(also called protection clearance):
It is generally composed of two metal bars exposed in the air with a certain gap between them. One metal bar is connected to the power phase line L1 or zero line (N) of the required protection equipment, and the other metal bar is connected to the ground wire (PE). When the instantaneous overvoltage strikes, the gap is broken down, leading part of the overvoltage charge to the ground,The voltage rise on the protected equipment is avoided.The distance between the two metal bars of the discharge gap can be adjusted as required, and the structure is simple. Its disadvantage is that the arc extinguishing performance is poor.The improved discharge gap is an angle gap, and its arc extinguishing function is better than the former. The arc is extinguished by the electrodynamic force F of the circuit and the rise of the hot gas flow.
Gas discharge tube
It is composed of a pair of mutually separated cold negative plates sealed in glass tubes or ceramic tubes filled with a certain amount of inert gas (Ar).In order to improve the triggering probability of the discharge tube, there is also a trigger aid in the discharge tube.This kind of gas discharge tube has two pole type and three pole type,
The technical parameters of gas discharge tube mainly include: DC discharge voltage Udc;Impulse discharge voltage Up (generally Up ≈ (2-3) Udc;Power frequency current In;Impulse current Ip;Insulation resistance R (>109 Ω);Inter electrode capacitance (1-5PF)
The gas discharge tube can be used under DC and AC conditions, and the DC discharge voltage Udc selected is as follows: used under DC conditions: Udc ≥ 1.8U0 (U0 is the DC voltage under normal operation of the line)
Use under AC conditions: U dc ≥ 1.44Un (Un is the effective value of AC voltage under normal operation of the line)
Varistor
It is a metal oxide semiconductor nonlinear resistance with ZnO as the main component. When the voltage acting on its two ends reaches a certain value, the resistance is very sensitive to the voltage.Its working principle is equivalent to the series parallel connection of multiple semiconductor P-N.Varistors are characterized by good nonlinear characteristics (I=nonlinear coefficient α in CU α) and large current capacity (~2KA/cmtwo）Normal leakage current is small (10-7～10-6A），Low residual voltage (depending on the working voltage and current capacity of the varistor), fast response time to transient overvoltage (~10-8s），No freewheeling.
The technical parameters of varistor mainly include: voltage sensitive voltage (i.e. switching voltage) UN, reference voltage Ulma;Residual voltage Ures;Residual pressure ratio K (K=Ures/UN);Maximum flow capacity Imax;Leakage current;Response time.
The use conditions of the varistor are: voltage: UN ≥ [(√ 2 × 1.2)/0.7] Uo (Uo is the rated voltage of the power frequency power supply)
Minimum reference voltage: Ulma ≥ (1.8~2) Uac (used under DC conditions)
Ulma ≥ (2.2 ～ 2.5) Uac (used under AC conditions, Uac is AC working voltage)
The maximum reference voltage of the varistor shall be determined by the withstand voltage of the protected electronic equipment. The residual voltage of the varistor shall be lower than the loss voltage level of the protected electronic equipment, that is, (Ulma) max ≤ Ub/K, where K is the residual voltage ratio and Ub is the loss voltage of the protected equipment.
Suppression diode
The suppression diode has the function of clamping and voltage limiting. It works in the reverse breakdown area. Because of its advantages of low clamping voltage and fast action response, it is particularly suitable for being used as the last protection element in the multi-level protection circuit.The volt ampere characteristics of the suppression diode in the breakdown zone can be expressed by the following formula: I=CU α, where α is the nonlinear coefficient, for Zener diode α=7~9, and for avalanche diode α=5~7
Technical parameters of suppression diode mainly include:
(1) Rated breakdown voltage refers to the breakdown voltage under the specified reverse breakdown current (usually lma). The rated breakdown voltage of Zener diode is generally in the range of 2.9V~4.7V, while the rated breakdown voltage of avalanche diode is usually in the range of 5.6V~200V.
⑵ Maximum clamping voltage: it refers to the maximum voltage at both ends of the tube when it passes the large current of the specified waveform.
⑶ Pulse power: it refers to the product of the maximum clamping voltage at both ends of the tube and the equivalent current in the tube under the specified current waveform (such as 10/1000 μ s).
(4) Reverse displacement voltage: it refers to the maximum voltage that can be applied at both ends of the pipe in the reverse leakage area. Under this voltage, the pipe shall not be broken down.This reverse deflection voltage should be significantly higher than the peak value of the maximum operating voltage of the protected electronic system, that is, it should not be in a weak conduction state during the normal operation of the system.
⑸ Maximum leakage current: it refers to the maximum reverse current flowing through the pipe under the action of reverse displacement voltage.
(6) Response time: 10-11s
Choke coil
Choke coil is a common mode interference suppression device with ferrite core. It consists of two coils with the same size and number of turns symmetrically wound on the same ferrite ring core to form a four terminal device. It can suppress common mode signals by presenting a large inductance, but has little effect on differential mode signals by presenting a small leakage inductance.The choke coil used in the balanced line can effectively suppress the common mode interference signal (such as lightning interference), but has no impact on the differential mode signal transmitted normally by the line.
Choke coil shall meet the following requirements during fabrication:
1) The wires wound on the coil magnetic core shall be insulated from each other to ensure that there is no breakdown short circuit between coils under the action of instantaneous overvoltage.
2) When the coil flows through the instantaneous large current, the magnetic core shall not be saturated.
3) The magnetic core in the coil shall be insulated from the coil to prevent breakdown between them under the action of instantaneous overvoltage.
4) The coil shall be wound in a single layer as far as possible, which can reduce the parasitic capacitance of the coil and enhance the resistance of the coil to transient overvoltage.
1/4 wavelength circuit breaker
1/4 wavelength short circuiter is a microwave signal surge protector made according to the spectrum analysis of lightning wave and the standing wave theory of antenna feeder. The length of metal short circuiter in this protector is determined according to the size of 1/4 wavelength of working signal frequency (such as 900MHZ or 1800MHZ).For the working signal frequency, the impedance of the parallel short circuit rod is infinite, equivalent to an open circuit, which does not affect the transmission of the signal. But for lightning waves, because the lightning energy is mainly distributed below n+KHZ, the impedance of the short circuit rod for lightning waves is very small, equivalent to a short circuit, and the lightning energy level is discharged to the ground.
Since the diameter of the 1/4 wavelength short circuit rod is usually several millimeters, it has good resistance to impact current, which can reach more than 30KA (8/20 μ s), and the residual voltage is very small. This residual voltage is mainly caused by the inductance of the short circuit rod. Its disadvantage is that the power frequency band is narrow, and the bandwidth is about 2%~20%. Another disadvantage is that it cannot add DC bias to the antenna feeder facilities,Some applications are restricted.[2]
Graded protection
Grading protection of surge protector (also called lightning arrester)
Since the energy of lightning stroke is very huge, it is necessary to release the energy of lightning stroke to the earth gradually through the method of grading release.The first level lightning arrester can discharge the direct lightning current, or discharge the huge energy conducted when the power transmission line is subjected to direct lightning strike. For places where direct lightning strike may occur, CLASS-I lightning protection must be carried out.The second level lightning arrester is the protection equipment for the residual voltage of the former level lightning arrester and the induced lightning strike in the area. When the former level has a large absorption of lightning strike energy, a part of the energy that is quite huge for the equipment or the third level lightning arrester will be transmitted, which needs further absorption by the second level lightning arrester.At the same time, the transmission line passing through the first level lightning arrester will also induce lightning electromagnetic pulse radiation LEMP. When the line is long enough, the energy of induced lightning becomes large enough, and the second level lightning arrester needs to further discharge the lightning energy.The third level lightning arrester protects LEMP and residual lightning strike energy passing through the second level lightning arrester.
First level protection
The purpose is to prevent the surge voltage from conducting directly from LPZ0 area into LPZ1 area, and limit the surge voltage of tens of thousands to hundreds of thousands of volts to 2500-3000V.
When the power lightning arrester installed at the low-voltage side of the incoming power transformer is used as the first level protection, it shall be a three-phase voltage switch type power lightning arrester, and its lightning current flow shall not be less than 60KA.This level of power supply lightning arrester shall be a large capacity power supply lightning arrester connected between each phase of the incoming line at the entrance of the user power supply system and the earth.Generally, this level of power supply lightning arrester is required to have a maximum impact capacity of more than 100KA per phase, and the required limiting voltage is less than 1500V, which is called Class I power supply lightning arrester.These electromagnetic lightning arresters are specially designed to withstand the large current of lightning and induced lightning and attract high-energy surges, which can shunt a large amount of surge current to the ground.They only provide limited voltage (when the impulse current flows through the power lightning arrester, the maximum voltage on the line is called the limited voltage), which is a medium level protection. Because Class I protectors mainly absorb large surge current, they cannot completely protect sensitive electrical equipment inside the power supply system.
The first level power supply lightning arrester can prevent 10/350 μ s and 100KA lightning waves, reaching the highest protection standard specified by IEC.Its technical reference is: lightning current is greater than or equal to 100KA (10/350 μ s);Residual voltage value is not more than 2.5KV;The response time is less than or equal to 100ns.
Second level protection
The purpose is to further limit the residual surge voltage through the first level lightning arrester to 1500-2000V, and implement equipotential connection for LPZ1-LPZ2.
When the power supply lightning arrester output from the distribution cabinet line is used as the second level protection, it should be a voltage limiting power supply lightning arrester, and its lightning current capacity should not be less than 20KA. It should be installed at the branch distribution point supplying power to important or sensitive electrical equipment.These power supply lightning arresters can absorb the residual surge energy of surge arresters passing through the power supply entrance of users more perfectly, and have a better suppression effect on transient overvoltage.The maximum impulse capacity of the power supply lightning arrester used here is required to be more than 45kA per phase, and the required limiting voltage should be less than 1200V, which is called Class II power supply lightning arrester.The power supply system of general users can meet the operation requirements of electric equipment if the second level protection is achieved
Level II power supply lightning arrester adopts Class C protector for full mode protection of phase medium, phase ground and medium ground. The main technical parameters are: lightning current capacity is greater than or equal to 40KA (8/20 μ s);Residual voltage peak value is not more than 1000V;The response time is not more than 25ns.
Third level protection
The purpose is to finally protect the equipment, reduce the residual surge voltage to within 1000V, so that the surge energy will not damage the equipment.
When the power lightning arrester installed at the AC power incoming line end of electronic information equipment is used as the third level protection, it shall be a series type voltage limiting power lightning arrester, and its lightning current capacity shall not be less than 10KA.
The last line of defenseA built-in power lightning arrester can be used in the internal power supply of the electrical equipment to completely eliminate the small transient overvoltage.The maximum impulse capacity of the power lightning arrester used here is 20KA or less per phase, and the required limiting voltage should be less than 1000V.It is necessary to have the third level protection for some particularly important or sensitive electronic equipment, and it can also protect the electrical equipment from the impact of transient overvoltage generated inside the system.
For the rectifier power supply used by microwave communication equipment, mobile station communication equipment and radar equipment, it is advisable to select DC power supply lightning arrester with suitable working voltage as the final protection according to the protection needs of its working voltage.
Level IV and above protection
According to the withstand voltage level of the protected equipment, if two levels of lightning protection can limit the voltage to be lower than the withstand voltage level of the equipment, only two levels of protection are required. If the withstand voltage level of the equipment is lower, four or more levels of protection may be required.The lightning current capacity of the fourth level protection shall not be less than 5KA.[3]
Classification of surge protective devices
Operating principle
1. Switch type: its working principle is that it presents high impedance when there is no instantaneous overvoltage, but once it responds to the lightning instantaneous overvoltage, its impedance will suddenly change to a low value, allowing lightning current to pass.The devices used for such devices include: discharge gap, gas discharge tube, thyristor, etc.
2. Voltage limiting type: its working principle is that when there is no instantaneous overvoltage, it is high impedance, but its impedance will continue to decrease with the increase of surge current and voltage, and its current and voltage characteristics are strongly nonlinear.Devices used for such devices include zinc oxide, varistor, suppression diode, avalanche diode, etc.
3. Shunt type or choke type
Shunt type: with protected equipmentparallel connectionIt presents low impedance for lightning impulse and high impedance for normal operating frequency.
Choke type: it is connected in series with the protected equipment, showing high impedance for lightning impulse and low impedance for normal operating frequency.
The devices used for such devices are: choke coil, high pass filter, low pass filter, 1/4 wavelength short circuiter, etc.
By use
⑴ Power protector: AC power protector, DCPower SupplyProtector, switching power protector, etc.
AC power lightning protection module is applicable to power protection of distribution room, distribution cabinet, switch cabinet, AC/DC distribution panel and other systems;There are outdoor input distribution boxes in the building, and distribution boxes on the building floor;Power type surge protector is used for low-voltage (220/380VAC) industrial power grid and civil power grid;In the power system, it is mainly used for the three-phase power input or output terminal in the power supply panel of the automation machine room and the main control room of the substation.Applicable to various DC power supply systems, such as DC distribution panel;DC power supply equipment;DC distribution box;Electronic information system cabinet;Output terminal of secondary power supply equipment.
(2) Signal protector: low-frequency signal protector, high-frequency signalProtector, antenna feeder protector, etc.
The scope of application of the network signal lightning arrester is used for lightning stroke and induced overvoltage protection caused by lightning electromagnetic pulse of 10/100Mbps SWITCH, HUB, ROUTER and other network equipment·Network switch protection of network computer room·Server protection of network computer room·Protection of other equipment with network interface in the network computer room·The 24 port integrated lightning protection box is mainly used for centralized protection of multiple signal channels in the integrated network cabinet and sub switch cabinet. The scope of application of the video signal lightning protection device is mainly used for point-to-point coordinated protection of video signal equipment, which can protect various video transmission equipment from inductive lightning strikes from signal transmission lines and hazards caused by surge voltage,It is also applicable to RF transmission under the same operating voltage.The integrated multi port video lightning protection box is mainly used for centralized protection of control equipment such as hard disk recorder and video cutter in the integrated control cabinet.[4]
Protector brand
Well known surge protector brand
Common lightning arresters on the market are:Bull ElectricAnti surge socket, LKX surge protector, ECS surge protector, Soule surge protector, ESP fuse surge protector, OBO surge protector, DEHN surge protector, PANAMAX surge protector, INNOVAT Ⅳ E surge protector, POLYPHASER surge protector.
