Insulation coordination: used in power system to determineTransmission lineandElectrotechnical equipment Principles, methods and regulations of insulation level.The purpose of studying insulation coordination is to comprehensively consider the possible applied voltage that the electrical facilities may bear, the effectiveness of over-voltage protection devices, and the resistance characteristics of the insulation materials and insulation structures of equipment to various applied voltages, and to consider economic rationality to determine the insulation level of power transmission lines and electrical equipment.
The voltage acting on electrical equipment refers to the working voltage and various overvoltage under normal operating conditions, the latter includes temporary overvoltageSwitching overvoltage、Lightning overvoltageEtc.In order to economically and reasonably design the insulation of transmission lines and electrical equipment, special equipment and devices are generally used in power systems to limit overvoltage.
The insulation level is the test voltage that the electrical equipment can withstand:
① Short time power frequency withstand voltage value.
② Lightning impulse withstand voltage value.
③ Operating impulse withstand voltage value.
④ Power frequency test voltage value for a long time.
The waveform, value, application method, time, frequency, etc. of these test voltages are clearly specified in national standards.[1]
method
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Idiomatic usage
The method of insulation coordination is based on the concept of maximum overvoltage acting on equipment insulation and minimum insulation strength of equipment.The usage is simple and clear, but it is impossible to estimate the probability of insulation failure and the relationship between the probability and coordination coefficient, so this method has strict requirements on insulation.
The customary method of insulation coordination is based on the concept of "maximum overvoltage" and "minimum insulation strength" acting on equipment insulation.First, it is necessary to determine the most dangerous overvoltage that may occur on the equipment and the minimum withstand strength of the equipment insulation. Then, according to the operating experience, a coordination coefficient is selected as the ratio of these two voltages to compensate for the error in estimating the maximum overvoltage and the minimum withstand strength of the insulation and increase a certain safety margin. Finally, it is necessary to determine the voltage level that the equipment insulation should be able to withstand.The usage is simple and clear, but it is impossible to estimate the probability of insulation failure and the relationship between this probability and coordination coefficient, so this method has strict requirements on insulation.Due to the high cost of measuring the discharge probability of non self recovery insulation, the conventional method can only be used.For electrical equipment of 220 kV and below, the usual usage is still used.For example, power transformers are protected by lightning arresters.Lightning arrester limitLightning overvoltageThe capacity of lightning arrester is usually expressed by its protection level.The insulation level (BIL) of the transformer withstanding lightning impulse shall be higher than the protection level of the arrester, and the ratio of the two is called coordination coefficient.The value of coordination coefficient generally adopted in China is 1.4.For 500 kV transformers,International Electrotechnical Commission(IEC) stipulates that the coordination coefficient must be equal to or greater than 1.2.[1]
Statistical method
According to the statistical characteristics of overvoltage amplitude and insulation flashover voltage, the insulation failure rate is calculated.Change the sensitive influencing factors to make the failure rate acceptable, and reasonably determine the insulation level.The statistical method can not only quantitatively give the safety degree of insulation coordination, but also optimize the design according to the principle of minimizing the sum of equipment depreciation cost, operation cost and accident loss cost.The difficulty is that there are many random factors, and some statistical laws still need to be understood.
Based on the fact that overvoltage amplitude and insulation dielectric strength are random variables, the insulation failure rate is calculated according to the statistical characteristics of overvoltage amplitude and insulation flashover voltage.Change the sensitive influencing factors to make the failure rate acceptable, and reasonably determine the insulation level on the basis of technical and economic comparison.
This method can not only quantitatively give the safety degree of insulation coordination, but also optimize the design according to the principle of minimizing the sum of equipment depreciation cost, operation cost and accident loss cost.The statistical method based on the probability distribution of overvoltage amplitude has been studied much.
Reducing the insulation level in EHV power systems has significant economic benefits.The statistical characteristics of the insulation strength of self restoring insulation are relatively easy to obtain.Since the 1970s, statistical method has been recommended internationally for insulation coordination of self recovery insulation of EHV power systems.
The difficulty of statistical method is that there are many random factors, the statistical laws of some random factors still need to be accumulated and understood, and the data of low probability density is relatively difficult to obtain.Calculated failure rate,[2]It is usually much larger than the actual one, and needs to be constantly improved in application.
Simplified statistical method
For the convenience of calculation, it is assumed that the statistical distribution of overvoltage and insulation discharge probability obey normal distribution.International Electrotechnical Commission(IEC) and China's national insulation coordination standards, it is recommended to use the overvoltage with a probability of 2% as the statistical (maximum) overvoltage Uw, and then take the voltage with a flashover probability of 10% as the statistical withstand voltage Uw of the insulation. Under the condition of different statistical safety factors γ=Uw/Us, the insulation failure rate R can be calculated.According to the technical and economic comparison, the acceptable R value is determined by coordinating the cost and failure rate, and then the insulation level is determined according to the corresponding γ and Us.In order to facilitate calculation in practical application, it is assumed that the statistical distribution of overvoltage and insulation discharge probability obey normal distribution.
The simplified statistical method is as simple as the conventional method, and there are ready-made curves to check.Although the value of failure rate may not be very accurate, it is easy to compare schemes in engineering, so it is widely used.[2]
The purpose of studying insulation coordination is to comprehensively consider the possible applied voltage (working voltage and overvoltage) that the electrical facilities may bear.
Electrical equipment often operates under the working voltage of the power system, and will also be subject to various over-voltage effects.The insulation of electrical equipment has a certain tolerance to various applied voltages.When the insulation performance is damaged, it will cause equipment damage or even system power failure.In order to avoid the above losses, it is necessary to ensure that the electrical equipment has the specified insulation strength, which is the insulation level.To determine the insulation level, it is required to technically handle the mutual coordination among the action voltage, measures to limit overvoltage, and insulation withstand capacity, and also to economically coordinate the relationship between investment costs, maintenance costs, and accident loss costs to achieve better comprehensive economic benefits.[1]
Applied voltage
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The voltage acting on electrical equipment refers to the working voltage and various overvoltage under normal operating conditions.
When the power system is in normal operation, the power frequency voltage of each point is different.The voltage on the sending end equipment of the transmission line shall be higher than the voltage on the receiving end equipment.Therefore, for each rated voltage level of the power system, it is also necessary to specify a maximum operating voltage of the system.The maximum operating voltage shall be based on the energy distribution, transmission distance, power grid structure, power flow distribution, stability characteristicsReactive power compensation, economic operation, equipment insulation design and other comprehensive factors.For power systems of 220 kV and below in China, the maximum operating voltage is 15% higher than the rated voltage;For 330~500 kV EHV power systems, the maximum operating voltage is 10% higher than the rated voltage.
Overvoltage includes temporary overvoltageSwitching overvoltage、Lightning overvoltageEtc.Because of their different causes, they all have certain characteristics.From the perspective of insulation performance, in addition to the overvoltage amplitude, it is necessary to distinguish their waveform and the time process of voltage action.This is because the insulation strength has a volt second characteristic, and the ability to withstand voltage varies with the voltage waveform and action time.[2]
Classification of applied voltage
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The applied voltage can be divided into the following 5 types according to the waveform.
① Power frequency voltage under normal operating conditions.
② Temporary over-voltage: It is generally an undamped or weakly attenuated oscillation wave of several or hundreds of hertz, with a maximum duration of about 1 second.
③ Slow wave front overvoltage: double exponential or asymmetric attenuation oscillation wave with a wave head duration of tens to thousands of microseconds.The standard operating impulse test waveform of gap, insulator and external insulation of electrical equipment is generally 250 microseconds at the wave head/2500 microseconds at the wave tail.
④ Overvoltage before fast wave: unipolar double exponential wave with a wave head duration of a few tenths to tens of microseconds.The standard lightning impulse test waveform is 1.2 microseconds at the wave head/50 microseconds at the wave tail.
⑤ Steep wave front overvoltage: the duration of wave head is only tens of nanoseconds, followed by the superposition of several oscillation waves with frequencies ranging from several megahertz to 100 megahertz.The standard steep wave front impact test waveform is under preparation, and the wave front time is 15 nanoseconds.[2]
Voltage limit protection
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In order to economically and reasonably design the insulation of transmission lines and electrical equipment, special equipment and devices are generally used in power systems to limit overvoltage, referred to as overvoltage protection.
High pressure is usually usedShunt reactor、Static var compensatorLimit the value of power frequency voltage rise;Use fastrelay protectionReduce power frequency voltage rise and its duration;The overhead ground wire with good conductor can also reduce the power frequency voltage rise caused by ground fault in some cases.
In the EHV power system, the closing and reclosing of no-load lines can produce relatively high overvoltage, which occurs frequently and generally plays a decisive role in the insulation level of the power system.It is effective to improve the performance of circuit breaker, adopt closing parallel resistance, and reduce the different phases of three-phase closing;Large flow capacityMetal oxide arrester, can also be used as protectionSwitching overvoltageDevice.
Equipment in substationLightning overvoltageIt is protected by lightning arrester.The overvoltage on the equipment is related to the performance of the arrester, the incoming wave of the line, the wiring layout of the substation, etc.The insulation coordination of power system is formulated comprehensively, including the requirements for overvoltage protection measures.[2]
Insulation characteristics
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Insulation characteristics and insulation level
Insulation coordination
Electrical equipment insulation can be divided into two categories: self recovery insulation and non self recovery insulation.Self recovery insulation can recover itself after the insulation performance is damaged, generally referring to the air gap and external insulation in contact with air.The insulation performance of non self recovery insulation cannot be recovered by itself after discharge, which is usually equipment composed of solid medium and liquid mediumInternal insulation。[1]
The insulation strength of the equipment is related to the shape of electrodes, insulating materials, distance between electrodes, voltage waveform and other factors, and is dispersive.Therefore, in practical engineering, great importance is attached to the true insulation test of electrical equipment.
The insulation level of electrical equipment means that the equipment can withstand (withoutflashover, discharge or other damage).According to various applied voltages that electrical equipment may bear in the system, the characteristics of protective devices and the resistance characteristics of equipment insulation to various applied voltages, some test voltage values indicating the basic insulation level of equipment are formulated.They are: ① short-time power frequency withstand voltage value; ②Lightning impulse withstand voltage value; ③Switching impulse withstand voltage value; ④Power frequency test voltage value for a long time.
The waveform, value, application method, time and frequency of the above test voltage are clearly specified in national standards of various countries.[2]
