Overvoltage refers to the long time voltage fluctuation phenomenon that the root mean square value of AC voltage rises at power frequency, exceeds 10% of the rated value, and lasts for more than 1 minute;The occurrence of overvoltage is usually the result of the instant of load switching.It occurs when inductive or capacitive load is connected or disconnected in normal use.
Overvoltage refers to the long time voltage fluctuation phenomenon that the root mean square value of AC voltage rises at power frequency, exceeds 10% of the rated value, and lasts for more than 1 minute;The occurrence of overvoltage is usually the result of load switching, such as cutting off a large capacity load or adding energy to the capacitor bank (overvoltage caused by excessive reactive power compensation).
Overvoltage type
The abnormal voltage rise that exceeds the working voltage under specific conditions in the power system belongs to an electromagnetic disturbance phenomenon in the power system.The insulation of electrical equipment must withstand the working voltage for a long time, and at the same time must be able to withstand a certain range of overvoltage, so as to ensure the safe and reliable operation of the power system.To determine the power system, it is necessary to study the causes of various overvoltages, predict their amplitudes, and take measures to limit themInsulation coordinationFor the manufacture of electrical equipment andpower system operation Are of great significance.
Main categories
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Overvoltage includes external overvoltage andInternal overvoltageTwo categories.
Secondary overvoltage protector
External overvoltageLightning overvoltage、Atmospheric overvoltage。Caused by the discharge of thunder clouds in the atmosphere to the ground.branchDirect lightning overvoltageandInduced lightning overvoltageTwo.The duration of lightning overvoltage is about tens of microseconds, with pulse characteristics, so it is often calledlightning impulse 。Direct lightning overvoltage is the overvoltage that occurs when lightning flashes directly hit the conductive part of electrical equipment.Electrified conductor in lightning flash, such asOverhead transmission lineConductor, called direct lightning stroke.Conductors that are normally grounded during lightning stroke, such asTransmission line towerIt is called counterattack to discharge the charged conductor after raising its potential.The amplitude of direct lightning overvoltage can reach millions of volts, which will damage the insulation of electrical facilities and cause short circuit grounding fault.Induced lightning overvoltage refers to the overvoltage induced on the ground near the electrical equipment during lightning flash, which is not directly struck by lightning due to the sharp change of space electromagnetic field during the discharge process (including secondary equipment and communication equipment).Therefore, overhead transmission lines need to be erectedLightning conductorandGrounding deviceAnd so on.Usually usedLine lightning withstand levelAnd lightning trip rateTransmission lineLightning protection capability.
Current transformer overvoltage protector
Overvoltage protector
Internal overvoltage Overvoltage caused by the change of internal operation mode of power system.yesTransient overvoltage、Switching overvoltageandResonance overvoltage。Transient overvoltage is caused byCircuit breakerOvervoltage occurs when the operation or short circuit fault makes the power system reach a certain temporary stability again after the transition process, which is also called power frequency voltage rise.Common ones are: ① no-load long line capacitance effect (Ferranti effect).Under the action of power frequency power supply, due to the accumulation of capacitance effect of long-distance no-load lines, the voltage distribution along the line is unequal, and the terminal voltage is the highest. ②Asymmetrical short circuit to ground.When phase a short circuit grounding fault occurs in three-phase transmission lines, the voltage on phase b and c will rise. ③Load shedding overvoltage, when the transmission line is forced to suddenly shed load due to failureemfOvervoltage caused by failure to adjust automatically in time.Switching overvoltage refers to the overvoltage with fast attenuation and short duration caused by circuit breaker operation or sudden short circuit. Common overvoltage includes: ① no-load line closing andReclosing overvoltage。②Cut off over-voltage of no-load line. ③Cut off over-voltage of no-load transformer. ④Arc grounding overvoltage.resonanceOvervoltage is the over-voltage caused by the resonance of energy storage components such as inductance and capacitor with the power frequency in some connection modes in the power system.Generally, it can be divided into: ① linear resonance overvoltage; ②Ferroresonance overvoltage。③Parametric resonance overvoltage.[1]
Overvoltage protector
Cause of overvoltage
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The sudden change of circuit state and electromagnetic state in power system is the fundamental cause of overvoltage.Overvoltage is divided into external overvoltage and internal overvoltage.Studying the cause of various overvoltage in power system, predicting its amplitude, and taking measures to limit it are the premise to determine the insulation coordination of power system, which is of great significance to the manufacturing of electrical equipment and the operation of power system.
Both external over-voltage and internal over-voltage are affected by many random factors. It is necessary to obtain data through calculation, simulation, on-site measurement and other ways in combination with the specific conditions of the power system, and use probability statistical methods to predict over-voltage.
In view of the cause of overvoltage, the power system must take protective measures to limit the amplitude of overvoltage.Such as installing lightning conductor, arresterreactorThe switch contact is added with a parallel resistance to reasonably implement insulation coordination and ensure the safe operation of the power system.
Overvoltage type
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Direct lightning overvoltageThe overvoltage that occurs when lightning flashes directly hit the conductive part of electrical equipment.Live conductors in lightning flash, such as overhead transmission line conductors, are called direct lightning strikes.Conductors that are normally grounded during lightning stroke, such as transmission line towerspotentialIt is called counterattack to discharge charged conductor after rising.The amplitude of direct lightning overvoltage can reach millions of volts, which will damage the insulation of electrical facilities and cause short circuit grounding fault.
Induced lightning overvoltageThe over-voltage induced on the ground near the electrical equipment during lightning flash due to the sharp change of space electromagnetic field during the discharge process, which is not directly affected by lightning (including secondary equipment, communication equipment).Induced lightning overvoltage mainly occurs on overhead transmission lines.
Lightning protection of transmission lineThe overhead transmission line stretches vertically and horizontally, and is most vulnerable to lightning strike, which is one of the main reasons for line failure. Lightning wires and grounding devices need to be erected for protection.Lightning protection capability of transmission lines is usually expressed by lightning withstand level and lightning trip rate of lines.Lightning withstand level refers to the maximum lightning current value (kA) that will not cause insulation flashover when the line is struck by direct lightning.
Once lightning overvoltage occurs on the transmission line, it will also spread along the line in the form of flowing waves, which may cause insulation damage accidents after invading the substation.The lightning overvoltage transmitted from the line is called lightning intrusion wave.Lightning arrester shall be used to weaken the lightning invasion wave to the extent that the insulation of electrical equipment can withstand.Magnetic steel bars, oscilloscopes and other observation and recording instruments are often installed in power systems to accumulate lightning overvoltage data.[1]
Overvoltage
Circuit diagram of voltage and surge overvoltage protection
Switching overvoltage
Overvoltage caused by circuit breaker operation or sudden short circuit in power system.Common switching overvoltages are as follows.
Figure 1 Closing and Reclosing Overvoltage of No load Line
① No load line closing and reclosing overvoltage: the transmission line has inductive and capacitive properties.The simplified equivalent circuit principle when the no-load line is closed is shown in Figure 1.In Figure 1, L is the equivalent inductance of the power supply and the line, C is the equivalent capacitance of the line, and e (t) is the AC power supply.When switch K is suddenly closed, a high-frequency oscillation transition process with angular frequency will occur in the circuit, and the voltage U on capacitor C (i.e. line)C(t) Possible maximum value, EmIs the amplitude of AC power supply voltage.If there is an initial voltage on capacitor C before closing, the overvoltage during oscillation after closing may also reach 3Em, lineAuto reclosingThis will happen when.
Cut off overvoltage of no-load line
② Cut off overvoltage of no-load line: no-load line belongs to capacitive load.The re ignition of AC arc during switching off causes more severe electromagnetic oscillation, resulting in overvoltage of the line.Time t1Power frequency currentExtinguish, and the line still maintains the residual voltage at this time;At t2-t3, the high-frequency arc is re ignited and extinguished for the first time, making the line voltage reach - 3Em after oscillation;At t4-t5, the arc is re ignited and extinguished for the second time to make the line voltage reach 5Em.This is done until the arc is no longer re ignited and the current is finally cut off.Switching off capacitors and other capacitive loads will cause overvoltage in the above process due to arc re ignition.
Overvoltage
③ Cut off the over-voltage of no-load transformer: the transformer is an inductive load and has an equivalent capacitance to the ground.When circuit breaker K suddenly cuts off the current, the current change rate is very large, resulting in very highInduced overvoltage。After the current is cut off, the residual electromagnetic energy in the transformer will charge the capacitance C to the ground again, forming an oscillation process, so there is an over-voltage, which is called cut-off over-voltage.Similar overvoltage will occur when the circuit breaker is operated to cut off other inductive loads.
Overvoltage protection circuit
④ Arc grounding overvoltage: neutral point ungrounded system occursSingle phase ground faultOvervoltage caused by intermittent reignition of grounding arc.The grounding arc will go through the process of extinction and re ignition every time it passes zero point.Smaller arc current can extinguish itself without re ignition.A large arc current will re ignite stably and must be switched off.In the neutral point ungrounded system, the single-phase grounding current is capacitive, generally more than 10A, and the arc is not easy to extinguish itself, and is not enough to stably reignite, so intermittent reignition occurs.Each intermittent reignition of the arc will cause electromagnetic oscillation of the system, and the processes before and after the arc will affect each other, and the oscillation will strengthen gradually, causing overvoltage in the system.[2]
The insulation strength of electrical equipment must be able to withstand switching overvoltage of certain amplitude.The switching contact plus parallel resistance is mainly used to limit the amplitude of switching overvoltage, and lightning arrester can also be used for protection.Usually, a unipolar shock wave is used to equivalent the maximum peak value of switching overvoltage for withstand voltage test of electrical equipment.
Temporary overvoltage
Overvoltage occurs when the power system reaches a certain temporary stability again after the transition process due to circuit breaker operation or short circuit fault.The temporary overvoltage is mainly power frequency oscillation, which lasts for a long time and decays slowly, so it is also called power frequency voltage rise.Common temporary overvoltages are as follows.
① No load long line capacitance effect (Ferranti effect): transmission lines have distributed parameter characteristics such as inductance and capacitance.Under the action of power frequency power supply, the voltage distribution along the long distance no-load line is unequal and the terminal voltage is the highest due to the gradual accumulation of capacitance effect.When the length of EHV transmission line is more than 300km, the voltage rise at the end of no-load line caused by capacitance effect shall be considered.
② Asymmetrical short circuit grounding: when phase a short circuit grounding fault occurs in three-phase transmission lines, the voltage on phase b and c will rise, and its value can reach the phase voltage Uphα times of: Ub=Uc=αUph
α is called grounding coefficient, which is related to the ratio of zero sequence reactance X0 and positive sequence reactance X1 of the system at the fault point:
neutral pointGrounding system(X0/X1 ≤ 3), α is about 1.3;For neutral point ungrounded system, when │ X0/X1 │ tends to infinity, α tends to 1.
③ Load shedding overvoltage: a temporary overvoltage caused by the failure of the power supply line and the failure of the power supply electromotive force to automatically adjust the power supply electromotive force.In addition, power frequency or non power frequency resonance of power system, and nonlinearFerroresonanceIt also belongs to temporary overvoltage.
Overvoltage
The insulation strength of electrical equipment shall generally be able to withstand temporary overvoltage.EHV long-distance transmission line needs to be installedShunt reactorCompensate line capacitance effect to reduce temporary overvoltage.[3]
Resonance overvoltage
The over-voltage caused by the resonance of energy storage elements such as inductance and capacitance with the power frequency in some connection modes in the power system.Resonance overvoltage is generally divided into the following three types according to the causes.
①Linear resonance overvoltage: The inductance, capacitance and other parameters of the electrical equipment constituting the resonant circuit are constants and do not change with the voltage or current.For example, inductance and capacitance of transmission line, capacitor for line series compensation, arc suppression coil with linear excitation characteristics of iron core, etc.Resonant overvoltage is mainly caused by the circuit principle of series resonance.When the system forms a series circuit of inductance and capacitance under a certain wiring mode, and the natural vibration frequency of the circuit is exactly equal to or close to the power frequency, the series resonance phenomenon will occur, which will cause overvoltage on electrical equipment.
② Ferroresonance overvoltage:Inductive elementDue to the magnetic saturation phenomenon of the iron core, the inductance parameters change with the current (magnetic flux)Nonlinear inductance。For example,Electromagnetic voltage transformerThis is the element.A resonance phenomenon caused by the series connection of nonlinear inductance and capacitor is called ferromagnetic resonance, which will cause overvoltage in electrical equipment.Since the inductance in the ferromagnetic resonance circuit is not constant, the resonance frequency of the circuit is not a single value.The same circuit may produce power frequencyFundamental waveResonance, which may produceHigh order harmonics(such as 2nd, 3rd and 5th harmonics) or sub harmonics (such as 1/2, 1/3 and 1/5th harmonics).
③ Parametric resonance overvoltage:alternatorEquivalent inductance parameter occurs during rotationPeriodic changeIf capacitive load is connected, such as no-load transmission line, it will form resonance with the capacitor, and even without excitation, it can make the generator terminal voltage rise continuously, forming overvoltage.This phenomenon is also called generator self excitation overvoltage.The energy required for parametric resonance is provided by the mechanical work by periodically changing the inductance parameters.
Overvoltage
Increasing the damping of resonant circuit is the main measure to limit resonant overvoltage.We should also strive toSystem operation modeAvoid possible resonance overvoltage on the.[3]
