1、 AbstractDifferent methods have different applicability and a single test is not a full,objective,realistic way to reflect the operating state of the switchgear.Transient Earth Voltage detection has been widely promoted in the regular testing of the switchgear by virtue of the advantages of non-powe
2、r failure detection,high sensitivity,strong anti-interference ability and non-invasion,etc.However,the research on the detection mechanism,influencing factors and propagation characteristics of TEV method is still in its initial stage.On the basis of theoretical analysis of partial discharge transmi
3、ssion characteristics and influencing factors,established the finite element model of the switch cabinet and deduced the TEV transmission characteristics and relationship between partial discharge source and its influenced factors.The conclusion was confirmed by using the online measured TEV data.In
4、dex TermsSwitchgear;insulation deterioration;Transient earth voltage;partial discharge I.INTRODUCTION The operation of power equipment is a dynamic process.Due to various factors,such as electricity,heat,mechanical force,environment,etc.,insulation materials gradually deteriorate during operation.On
5、ce the fault occurs,it will inevitably lead to power failure in some or all areas,causing huge direct and indirect economic loss and social impact.Equipment insulation deterioration is the main cause of equipment failure in substation.A lot of data show that about 70%of power equipment failure is ca
6、used by the deterioration of insulation performance.The results show that partial discharge is an important symptom and main form of insulation failure.With the development of insulation deterioration,the level of partial discharge will be gradually strengthened.Therefore,if the local discharge situ
7、ation of the equipment can be grasped as soon as possible,it can ensure sufficient reaction time,and make effective prediction and maintenance for the power equipment.It can reduce the economic loss and improve the reliability of the system by withdrawing from operation before serious failure occurs
8、 in the equipment.As the current latest switchgear detection technology,transient earth voltage(TEV)method with high sensitivity,strong anti-interference ability,has become main detection method for metal enclosed switchgear on substation After years of development,TEVs test technology has been matu
9、re in the field of charge detection in the switchboard.However,the research on relationship between the characterization,influencing factors and different detection amounts is still not deep enough.II.PD CHARACTERIZATION OF TEV As switchgear with partial discharge occurs,can produce high-frequency e
10、lectromagnetic wave,on the basis of electromagnetic theory,high-frequency electromagnetic waves exist when it enters the conductor skin effect and attenuation in the conductor soon,only exists in the conductor surface.Electromagnetic waves cannot penetrate the closed metal cabinet,forming a pulse cu
11、rrent to spread on the inner surface of the metal shell of the switch cabinet.When the metal cabinet body is completely closed,electromagnetic waves are shielded by the grounding metal shell of the equipment,and the discharge signal cannot be detected outside the switch cabinet.The actual metal clos
12、ed switchgear always has joints and insulation gaskets,the high frequency electromagnetic signal can be transmitted to outside of the switchgear from the shielding that discontinuous.Surface current is formed on the outer surface of the metal cabinet and propagates along the outer surface while a ce
13、rtain transient voltage pulse signal is formed on the outer surface of the metal cabinet.In practice,high-frequency electromagnetic waves have skin-collecting effect when they enter the conductor.It decays quickly in a conductor,only on the surface of the conductor.The distance that electromagnetic
14、waves can penetrate into the conductor is called penetration depth,and the calculation formula of penetration depth of high-frequency signals is:f/1=(1)In equation(1),represents the penetration depth and its unit is m.f is the frequency of electromagnetic wave,its unit is Hz;is metal permeability,it
15、s unit is H/m;is metal conductivity,its unit is S/m.For metal closed switchgear,the higher the electromagnetic wave frequency is,the smaller the Transmission Characteristics and Influencing Factors of TEV in High V oltage Switchgear Jing XIE,Hongchun SHU,Qi SONG,Linglin LUO 2018 China International
16、Conference on Electricity Distribution Tianjin,17-19 Sep.2018CICED2018 Paper No.201804190000003 Page1/6 207penetration depth is.For an ideal conductor,the conductivity is approximately infinite,and the penetration depth is zero.It means that the high frequency electromagnetic wave cannot enter the i
17、deal conductor;only can propagate in the surface of the conductor and forms the surface current.In order to better study the relationship between partial discharge intensity and transient earth voltage,the electromagnetic waves excited by partial discharge can be regarded as the generation of dipole
18、.The electric dipole,that known as the high frequency carrier current element,is the basic unit of the antenna,also known as the electric basic oscillator.According to the principle of current continuity,at the same time,the electric dipoles accumulate time-varying charges of equal size and opposite
19、 direction.The electric dipole needs to meet the following conditions:(1).The length of current carrying line element is far less than the wavelength of high frequency electromagnetic wave,i.e:dl;(2).The cross section radius of current carrying line element is much smaller than that of high frequenc
20、y electromagnetic wave,i.e:a;(3).The current on the carrier line element is always has equal amplitude and same phase at any time.In the process of outward radiation,high frequency electromagnetic waves produced by electric dipoles can be classified into near radiation fields and far radiation field
21、s according to the distance from the field source.(1).Near radiation fields The field source is taken as the center,and within a range of wavelengths,it is called the near field,also known as the induction field.The field is greatly affected by the field source,and the field attenuates rapidly with
22、the increase of distance.(2).Far radiation fields With the field source as the center,a range beyond the wavelength is called the far field,also known as the radiation field.The field attenuation is slow and inversely proportional to the first power of the field source distance.The electric field an
23、d magnetic field are perpendicular to each other and both are perpendicular to the electromagnetic wave propagation direction,which belongs to TEM wave.The wave impedance is approximately equal to that in vacuum,i.e.:()00000377EZZH=(2)Electromagnetic waves generated by the deterioration of insulatio
24、n will be propagated in the inner space of the switch cabinet,and will be refracted and reflected in the switchgear.In order to consider the maximum field variables from partial discharge electromagnetic wave to the surface of the switchgear,we only need to consider the vertical component of electro
25、magnetic wave.The schematic diagram of electromagnetic wave refracting and reflecting on the surface of switchgear is shown in figure 1.xyzIkIHIETkTHTERkRHRE111222 Fig.1 Electromagnetic wave reflection In figure 1,medium is air,the characteristic constant of media is1,1 and 1 respectively;Medium is
26、metal shell of the switchgear,the characteristic constant of media is2,2 and 2 respectively.The propagation direction of the incident electromagnetic wave isIk,the corresponding electric field intensity and magnetic field intensity are respectively IEandIH;The direction of propagation of reflected e
27、lectromagnetic waves is Rk,the corresponding electric field intensity and magnetic field intensity are respectively REand RH;The direction of propagation of transmitted electromagnetic wave is Tk,the corresponding electric field intensity and magnetic field intensity are respectively TEand TH.From M
28、axwells equations,the wave impedance in medium and medium respectively is:010121IRIREEZHHZ=(3)The direction of electromagnetic wave propagation is perpendicular to the interface of media,and the direction of electric field and magnetic field is parallel to the interface of media.According to the con
29、nection condition of electromagnetic wave on the interface of different media:1212ttttEEHH K=(4)In formula(4),t represents the tangential component relative to the interface of the medium;n represents the normal component;K is the line density of conducting current on the interface,it indicates that
30、 the tangential component of the electric field intensity is always continuous,while the normal component of the magnetic field intensity is continuous in the region without conduction current.Because the electric field and magnetic field conductors of incoming,inverse and penetrating waves only hav
31、e tangential component,the electric field inside the conductor is zero,so there is:2018 China International Conference on Electricity Distribution Tianjin,17-19 Sep.2018CICED2018 Paper No.201804190000003 Page2/6 2080IRTIRTK+=+=EE EHHHHH(5)In formula(5),H is the magnetic field intensity that be super
32、imposed by incoming and reflected waves in medium I.Put formula(5)into formula(4),and you will get:=IRIR EEHH(6)According to the formula(6),the electromagnetic energy flow density of incident wave and reflected wave has the equal value in the opposite direction,so the total reflection happened on th
33、e surface of metal switchgear.The induction surface current flowing along the surface of the switch cabinet can be expressed as:nK=eH交 界 面(7)The induction surface current flowing along the surface of the metal switch cabinet will generate a transient earth voltage(TEV)on the metal shell of the cabin
34、et.The approximate relationship between TEV value and field source intensity and source distance is given by theoretical derivation.A single pulse current of partial discharge can be simulated using the Gaussian pulse waveform given by equation(8),Where,0I represents the peak of pulse current,which
35、appears at the moment0tt=,is the constant that determines the pulse width.()2024()0ttI t Ie=(8)For the pulse current given in equation(8),according to the wave equation of vector magnetic potential,the relationship between vector magnetic potential and field source current is shown in equation(9).Wh
36、ere,R represents the distance to the source,l represents the length of the carrier line element,and satisfies that R l.2024()0 0044ttIIeRR=llA(9)The column coordinate system is established,and the high-frequency current-carrying line element is coincidence with the z-axis,so the vector magnetic pote
37、ntial direction is the same as that of the current-carrying wire,that is,only has z-direction component,0r=AA.Considering the hysteresis effect,the vector magnetic potential expression can be written as 2024()00(,)(,)4rttvzzIlrt rt er=AA e(10)According to=AB,in cylindrical coordinate system,you can
38、get:00 011 1rzrzrrr zr=e eeHB AA AA(11)Put formula(10)into formula(11),then you will get the magnetic field intensity of synthetic magnetic field in medium is：()()20204()0002 221(,)8=14zz zrrttvrt rrrrvt t rIlerv=AA AH ee ee(12)Substituting formula(7),it can be obtained that the surface current dens
39、ity of the metal switch cabinet is:()()40814202r(t-t)v-00nz 2 22r v t-t-rIl-erv=KeH e(13)If the contact area of capacitance sensor and metal cabinet in the detection device is s,the induced surface current is:()()2024()0002 22814rttvsrvt t rIli ds e srv=K(14)It is known that the detection impedance
40、of the detection device isKZ,by detecting the equipment,the current signal is converted to voltage signal,and the transient voltage is:()()()2024()0002 228,14rttvKKrvt t rI lsZu rt erv=(15)III.FACTORS AFFECTING TEV DETECTION VALUE In order to reflect the change of field quantity more directly,the si
41、mulation model of switch cabinet is built by using ANSOFT electromagnetic simulation software.3D modeling of the closed metal switchgear is carried out.The simulation model is shown in figure 2(a),and the segmented grid after meshing is shown in figure 2(b).In the simulation model,the material of th
42、e cabinet is steel,and there is a 2mm gap between the cabinet body and the cabinet door for electromagnetic wave to spread out.High frequency current-carrying coil is used to simulate the partial discharge generated after the failure in the switch cabinet.High frequency current-carrying line element
43、 is placed in the simulation model of the switch cabinet.The local magnification is shown in figure 3(a),and the segmented grid is shown in figure 3(b).2018 China International Conference on Electricity Distribution Tianjin,17-19 Sep.2018CICED2018 Paper No.201804190000003 Page3/6 209Voltage amplitud
44、e/mV(a)3d simulation model(b)model subdivision diagram Fig.2 High voltage switchgear 3d simulation model and subdivision diagram(a)3d simulation model(b)model subdivision diagram Fig.3 3d simulation model and subdivision diagram of High frequency carrier line element A.The influence of the distance
45、from PD equivalent power supply to the detector Detection impedance is50KZ=,Partial discharge pulse current amplitude0100 I mA=,when the pulse width is 10ns,it is approximately equivalent to the discharge power of 1000pc at the fault point.As put increasing distance between power supply and switch c
46、abinet metal shell,induction voltage pulse amplitude decreases,and put the distance between power supply and switch cabinet metal shell increased from 1 cm to 20 cm,the change of the voltage pulse as shown in figure 4,and voltage pulse amplitude change with the distance is shown in fig.5.Fig.4 Compa
47、rison of induced voltage pulse waves It can be seen from fig.4 that as the distance between the discharge source and the metal shell of the switch cabinet increase,the amplitude of the voltage pulse amplitude decreases rapidly.Near the discharge source,i.e.near radiation fields,the amplitude of indu
48、ction voltage pulse decreases significantly with the increase of distance.When the distance between the discharge power and the metal shell of switch cabinet increases from 20cm to 120cm,the voltage pulse waveform and voltage pulse amplitude change with the distance is shown in fig.6 and fig.7.Fig.5
49、 The voltage pulse amplitude varies with distance in near-zone field Fig.6 Induction voltage pulse waveform diagram in far zone field 20 40 60 80 100 12000.0050.010.015/m V Fig.7 The voltage pulse amplitude varies with distance in far zone field By contrasting the fig.7 with fig.5,it can be seen tha
50、t the amplitude of the induction voltage pulse changes as the distance between the discharge source and the metal shell of the switch cabinet increases relatively gently in far zone field.The induction surface current produced by electromagnetic wave on the metal shell of the switchgear can be captu
