energies Article
On the Feasibility of Gap Detection of Power Transformer Partial Discharge UHF Signals: Gap Propagation Characteristics of Electromagnetic Waves Xiaoxing Zhang 1, * 1 2
*
ID
, Guozhi Zhang 1 , Yalong Li 1 , Jian Zhang 1 and Rui Huang 2
School of Electrical Engineering, Wuhan University, Wuhan 430072, China;
[email protected] (G.Z.);
[email protected] (Y.L.);
[email protected] (J.Z.) Shandong Electric Power Research Institute, State Grid Shandong Electric Power Company, Jinan 250002, China;
[email protected] Correspondence:
[email protected]; Tel.: +86-027-6877-3771
Received: 24 August 2017; Accepted: 29 September 2017; Published: 2 October 2017
Abstract: This study analyzed the transformer electromagnetic gap propagation characteristics. The influence of gap size is also analyzed, and the results experimentally verified. The obtained results indicated that the gap propagation characteristics of electromagnetic wave signals radiated by the partial discharge (PD) source in different directions are substantially different. The intensity of the electromagnetic wave in the gap reaches a maximum at a gap height of 1 cm; and inside the gap, the intensity of the electromagnetic wave depicted an increasing trend at the tail area of the gap. Finally, from the obtained results, some suggestions on where to install sensors in practical systems for ultra high frequency (UHF) PD signal detection in the transformer gap are provided. The obtained results confirmed the feasibility of using this approach. These results can be seen as a benchmark and a challenge for further research in this field. Keywords: power transformer; partial discharge (PD); UHF detection; gap; propagation characteristics
1. Introduction Power transformers are expensive devices that play an important role in power systems. As an effective means of insulation degradation detection, the ultra high frequency (UHF) method has been extensively used in the detection of partial discharge (PD) in transformers [1,2]. Currently, UHF antenna sensors are primarily inserted into transformers via handholes [2] and oil drain valves [3] or placed outside by the addition of a dielectric window [4–6]. Built-in antenna sensors can effectively shield the sensor from the electromagnetic interference in the surrounding environment, but two problems exist: (1) the built-in antenna sensors will change the internal structure of the transformer, which may damage the uniform distribution of the internal electromagnetic field; (2) for transformers in operation, electric power companies are always unwilling to allow retrofitting. The method of adding a dielectric window can only occur when the transformer is being manufactured or during an overhaul. Accordingly, this dielectric window method cannot be used for the PD detection of transformers in operation. In China, power transformers (except for barrel type transformers produced by companies such as ABB and Siemens [7]) commonly comprise a top lid and a tank or a tank and a pedestal. The top lid and tank or the tank and pedestal are sealed by an insulating gasket through bolts, forming a non-ferromagnetic material connection gap that surrounds the transformer (Figure 1). Therefore, when PD defects exist in transformer, the electromagnetic wave radiated by the PD source will leak out through the joint gap, which provides the feasibility for the PD detection of transformer by detecting the leakage of electromagnetic signals through the gap. Energies 2017, 10, 1531; doi:10.3390/en10101531
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(a) (a)
(b) (b)
Figure 1. Schematic of transformer gap: (a) Gap of top lid and tank and (b) Gap of tank and Figure 1. Schematic of transformer gap: (a) Gap of top lid and tank and (b) Gap of tank and pedestal.
pedestal. Figure 1. Schematic of transformer gap: (a) Gap of top lid and tank and (b) Gap of tank and pedestal.
Scholars both in China and abroad have studied the gap detection of transformer PD
Amplitude/(V) Amplitude/(V)
Scholars both have studied studiedthethe detection of transformer Scholars bothininChina China and and abroad abroad have gapgap detection of transformer PD PD electromagnetic waves. Reference [8] proposed the PD detection method by detecting electromagnetic waves. Reference [8] proposed the PD detection method by electromagnetic detecting electromagnetic waves. Reference [8] proposed the PD detection method by detecting electromagnetic wave leakage, and the propagation characteristics of the electromagnetic wave electromagnetic wave leakage, and the propagation characteristics of the electromagnetic wave wave leakage,aand propagation electromagnetic through gap of 46 cm through gapthe of 46 cm × 2 cmcharacteristics × 1 cm (widthof×the height × depth) are wave simulated and aanalyzed. through a gap of 46 cm × 2 cm × 1 cm (width × height × depth) are simulated and analyzed. × 2 cm × 1 cm[9,10] (width × height depth) are simulatedthe andPD analyzed. Reference [9,10] proved Reference proved the × feasibility of detecting electromagnetic wave signals of the Reference [9,10] proved the feasibility of detecting the PD electromagnetic wave signals of feasibility of detecting theaPD electromagnetic wave signals of transformers a gap the with an transformers through gap with an equiangular spiral antenna. Referencethrough [11] showed transformers through a gap with an equiangular spiral antenna. Reference [11] showed the successful use antenna. of gap detection in the and the reference [12] showed the detection use in laboratory, as and equiangular spiral Reference [11]field showed successful use of gap in the field successful use of gap detection in the field and reference [12] showed the use in laboratory, as shown in Figure 2. reference showed shown[12] in Figure 2. the use in laboratory, as shown in Figure 2.
UHF sensor UHF sensor
Time/(s) Time/(s)
(b) (b)
Amplitude Amplitude
（30mV/div） （30mV/div）
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Vivaldi antenna Vivaldi antenna
（300ns/div） （300ns/div）
t t
(c) (c)
(d) (d)
Figure 2. Gap detection of PD UHF signal in the field and in the laboratory: (a) Sensors installed in
Figure 2. Gap detection inthe thefield fieldand and laboratory: (a) Sensors installed in Figure 2. Gap detectionofofPD PDUHF UHF signal signal in in in thethe laboratory: (a) Sensors installed in the gap of the high voltage reactor in the field; (b) Signals by UHF sensor installed in the gap of the gap of the high voltagereactor reactor in in the field; (b) Signals byby UHF sensor installed in theingap ofgap the of the the the gap of the high voltage the field; (b) Signals UHF sensor installed the high voltage reactor in the field; (c) Vivaldi antenna placed in the gap of transformer in the voltage reactor in field; the field; (c) Vivaldi antenna placedthe in the of gap of transformer the highhigh voltage reactor in the (c) Vivaldi antenna placed transformer in theinlaboratory; laboratory; (d) Signals by the Vivaldi antenna placed in the in gap of gap transformer in the laboratory. laboratory; the Vivaldi antenna placed gap of transformer in the laboratory. (d) Signals by (d) theSignals Vivaldibyantenna placed in the gap in ofthe transformer in the laboratory.
Reference [13] investigated the propagation characteristics of electromagnetic waves through a gap length of 1.5 cm and a gap height of 0.1 cm (0.3, 0.5, 0.8, and 1 cm) by simulation and verified that
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3 of 16 investigated the propagation characteristics of electromagnetic waves through a gap length of 1.5 cm and a gap height of 0.1 cm (0.3, 0.5, 0.8, and 1 cm) by simulation and verified that a high gap leads to a strong electromagnetic wave passing through. Moreover, Reference [13] a high gap leads to a strong electromagnetic wave passing through. Moreover, Reference [13] affirmed affirmed that when the height is greater than a certain value, the intensity of the electromagnetic that when the height is greater than a certain value, the intensity of the electromagnetic radiation by radiation by gap tends to be stable. However, Reference [13] does not take the gap depth into gap tends to be stable. However, Reference [13] does not take the gap depth into consideration. In the consideration. In the real transformer gap, along with reflection loss and transmission loss, both real transformer gap, along with reflection loss and transmission loss, both diffraction and reflection diffraction and reflection occur, leading to unstable change of energy distribution. Reference [14] occur, leading to unstable change of energy distribution. Reference [14] validated that the transformer validated that the transformer tank has a serious attenuation effect on the electromagnetic wave tank has a serious attenuation effect on the electromagnetic wave propagating through the gap and propagating through the gap and other parts. Reference [7] analyzed the propagation other parts. Reference [7] analyzed the propagation characteristics of 0- to 2-GHz electromagnetic characteristics of 0- to 2-GHz electromagnetic waves in a flange gap by using plate waveguide waves in a flange gap by using plate waveguide theory and magnetic field attenuation theory of theory and magnetic field attenuation theory of electromagnetic wave gap propagation. electromagnetic wave gap propagation. Although the method of detecting PD electromagnetic signals through the transformer gap has Although the method of detecting PD electromagnetic signals through the transformer gap has been successfully used in China, the electromagnetic gap propagation characteristics and the been successfully used in China, the electromagnetic gap propagation characteristics and the influence influence of gap size have not been studied. This research aims to further improve the efficiency of of gap size have not been studied. This research aims to further improve the efficiency of gap detection gap detection of PD UHF signals. The propagation characteristics of the electromagnetic wave of PD UHF signals. The propagation characteristics of the electromagnetic wave radiated by the radiated by the PD source in different directions and the influence of the gap height and depth on PD source in different directions and the influence of the gap height and depth on the propagation the propagation characteristics are simulated and analyzed by the finite-difference time-domain characteristics are simulated and analyzed by the finite-difference time-domain technique and then technique and then experimentally verified. Finally, suggestions on where to install sensors for experimentally verified. Finally, suggestions on where to install sensors for UHF signal detection UHF signal detection in the transformer gaps in practical systems are provided, which generates a in the transformer gaps in practical systems are provided, which generates a solid foundation for solid foundation for future research. future research.
2. in Gaps Gaps 2. Propagation Propagation Characteristics Characteristics of of Electromagnetic Electromagnetic Waves Waves in The propagationprocess processofofPD PDelectromagnetic electromagnetic wave through transformer gap results in The propagation wave through the the transformer gap results in edge edge diffraction the electromagnetic at the gap entrance and the In gap exit. In given addition, diffraction of the of electromagnetic wave atwave the gap entrance and at the gapatexit. addition, that given that the size of the transformer gap is similar to the wavelength of electromagnetic wave the size of the transformer gap is similar to the wavelength of electromagnetic wave radiated by the radiated bythe the PD source, theelectromagnetic diffraction of wave the electromagnetic will occur through during the PD source, diffraction of the will occur duringwave the propagation the propagation through the transformer gap. transformer gap. 2.1. 2.1. Edge Edge Diffraction Diffraction In waves, edge diffraction willwill occur because of the In the the propagation propagationprocess processofofelectromagnetic electromagnetic waves, edge diffraction occur because of presence of the gap edge and as a result, the electromagnetic wave signals will no longer follow the presence of the gap edge and as a result, the electromagnetic wave signals will no longer geometric optics optics theorytheory whichwhich is only usedused to study incidence, follow geometric is only to study incidence,reflection reflectionand anddiffraction diffraction of of electromagnetic waves[15]. [15]. electromagnetic will away deviate away from the original electromagnetic waves TheThe electromagnetic wave wave will deviate from the original propagation propagation pathto and to the geometric Figure 3 showsdiagram a schematic diagram of path and diffract thediffract geometric shadow area.shadow Figure 3area. shows a schematic of straight edge straight edge diffraction geometry. diffraction geometry. F
θ S2
Q S1
S
φ incident plane
diffraction plane
Figure Figure 3. 3. Schematic Schematic diagram diagram of of straight straight edge edge diffraction diffraction geometry. geometry.
Here Here SS is is the the source source point, point, Q Q is is the the edge edge diffraction diffraction point, point, F F is is the the field field point, point, S1 S1 is is the the distance distance between the source sourcepoint pointand andthe the diffraction point, S2the is distance the distance between the diffraction between the diffraction point, andand S2 is between the diffraction point point and the field point, φ is incidence angle, θ is wedge angle. At the diffraction point Q, the initial value of the diffraction field Ed(Q) can be expressed as follows:
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and the field point, φ is incidence angle, θ is wedge angle. At the diffraction point Q, the initial value of the diffraction field Ed (Q) can be expressed as follows: Ed (Q) = D · Ei (Q),
(1)
where D is the dyadic diffraction coefficient matrix and the order of matrix D is different in different coordinate systems. The field value of field point F can be obtained from Equation (1) [16]: d
Ed (F) = Ei (Q) · Ad (S2) · e−jks ,
(2)
where Ad (S2) is the diffusion factor, which indicates the amplitude change of the field along diffraction ray. When the incident wave is a plane wave which can be used to roughly analyze spherical wave in the far field zone: 1 Ad (S2) = √ , (3) S2 where S2 is the distance between the diffraction point and the field point. 2.2. Kirchhoff Diffraction Theory When the electromagnetic wave passes through a gap or aperture, diffraction will occur, thereby changing the direction of the electromagnetic wave propagation. Kirchhoff’s theory is the basis of studying electromagnetic diffraction. The Kirchhoff equation is employed to represent the scalar field in the enclosed region with its edge value. On the assumption of passive closed region V, and the scalar field generated by the current and the magnetic flux source at observation point P(r) is ψ, and then the scalar field ψ satisfies the Helmholtz equation: ∇2 Ψ(r) + k2 Ψ(r) = 0 (4) Through Green’s second theorem: ψ(r) = −
I
ejkR 1 R [∇0 Ψ(r0 ) + jk(1 + j ) Ψ(r0 )]gen0 dS0 4ΠR kR R
(5)
If the center of the infinite large screen has a small hole, then volume V0 is the right side of the screen space, and the boundary conditions are as follows: small hole surface—S3, the right side of the screen surrounded by the infinite space hemisphere—S4. Point r0 is a point on S4: 0
ejkr ψ(r ) = f(θ , ϕ ) 0 , r 1 ∂ψ(r0 ) 0 0 0 = − e g ∇ ψ ( r ) = jk − ψ (r0 ), n ∂r0 r0 0
0
0
(6) (7)
Equations (6) and (7) are integrated into Equation (5), and the calculation is follows:
1 ∇ ψ(r ) + jk 1 − j 0 kr 0
0
R 0 ψ(r ) · e0 ≈ 0, R
(8)
Therefore, the field at any point r in region V0 is generated only by the sub-wave source on S3. The integral in Equation (5) only need to be performed on S3: ψ(r) = −
I S0
ejkR 1 R ∇0 ψ(r0 ) + jk 1 + j ψ(r0 ) · e0n dS0 , 4πR kR R
(9)
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Considering the far-field diffraction (Fraunhofer diffraction), if the observation point on the right side of the screen is far away, then Equation (9) can be simplified to the following Considering the far-field diffraction (Fraunhofer diffraction), if the observation point on the right form [17]: side of the screen is far away, then Equation (9) can be simplified to the following form [17]:
e-jkr ψ ( r ) = − e−jkr∫I e − jkgr′ 0e′n ∇ ' ψ ( r′ ) + jke′n ψ ( r′ ) dS′ , ψ(r) = − 4πr S0 e−jkgr en0 ∇0 ψ(r0 )+jken0 ψ(r0 ) dS0 , 4πr
S0
(10) (10)
2.3. 2.3. Propagation Propagationof of Electromagnetic Electromagnetic Waves Waves in in Transformer Transformer When When electromagnetic electromagneticwaves wavesradiated radiatedby byaa PD PD source source propagate propagate from from the the transformer transformer internal internal area to the transformer gap, edge diffraction occurs at the edge of gap. The edge area to the transformer gap, edge diffraction occurs at the edge of gap. The edge diffraction diffraction will will cause cause electromagnetic electromagnetic wave wave to to diverge diverge from from the the original original propagation propagation path path and and converge, converge, and and the the electric field strength of the convergence point will be strengthened. Figure 4 is a schematic electric field strength of the convergence point will be strengthened. Figure 4 is a schematic diagram diagram of the electromagnetic wave incidence in the transformer gap,the where the line dashed line is the of the electromagnetic wave incidence in the transformer gap, where dashed is the original original propagation path of the electromagnetic wave, andis the line is one of the actual propagation path of the electromagnetic wave, and the red line one red of the actual propagation paths propagation paths after diffraction, and point 2 is the convergence point. after diffraction, and point 2 is the convergence point. transformer gap
2 gap edge transformer internal
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transformer exterior
Figure Figure4.4.Schematic Schematicdiagram diagramof ofincidence incidenceof ofelectromagnetic electromagneticwave wave in in transformer transformer gap gap entrance. entrance.
In the process of electromagnetic wave propagation inside the gap, given the blocking effect of In the process of electromagnetic wave propagation inside the gap, given the blocking effect of the gap wall, the electromagnetic wave will reflect on the inner surface of the gap. The reflection the gap wall, the electromagnetic wave will reflect on the inner surface of the gap. The reflection process increases the signal intensity in the gap, and the number of reflections increases with the process increases the signal intensity in the gap, and the number of reflections increases with the increase of the incident angle. In addition, given the existence of reflection loss and transmission increase of the incident angle. In addition, given the existence of reflection loss and transmission loss, the electromagnetic wave intensity will gradually decrease during the propagation process. loss, the electromagnetic wave intensity will gradually decrease during the propagation process. Without considering the leakage of electric field, the attenuation degree SE of the electromagnetic Without considering the leakage of electric field, the attenuation degree SE of the electromagnetic wave wave that propagates through gap is as follows [7]: that propagates through gap is as follows [7]:
c (1 + k) 2 SE = 27.3 c + 20lg (1 + k)2, dB , SE = 27.3b + 20lg 4k , dB,
(11) (11)
b 4k where b is the gap height, c is the gap depth, and k is the relative wave impedance of the gap. where b is theofgap c isofthe gap depth, andwhen k is the wave impedance of the gap.from a Because theheight, absence gap constraints, anrelative electromagnetic wave propagates Becausegap of to thethe absence of free gap space, constraints, when an electromagnetic wave propagates from transformer external the electromagnetic wave reflection process ends. Most a transformer gap to the external free space, the electromagnetic wave reflection process ends. Most of of the electromagnetic waves will continue to propagate in the free space along the original the electromagnetic waves will continue to propagate the free space along theDiffraction original propagation propagation path, thereby suddenly decreasing the in electric field intensity. and edge path, thereby suddenly decreasing the electric field intensity. Diffraction and edge diffraction at the diffraction at the edge of the gap exit, lead to electromagnetic wave divergence from the original edge of the gap exit, lead to electromagnetic wave divergence from the original propagation path to propagation path to the shadow area, thereby resulting in a further decrease in signal strength. the shadow thereby resulting in electromagnetic a further decrease in signal Figure is a where schematic Figure 5 is a area, schematic diagram of the wave in thestrength. transformer gap 5exit, the diagram of the electromagnetic wave in the transformer gap exit, where the dashed line is the dashed line is the original propagation path of electromagnetic wave, and the red line is oneoriginal of the propagation path ofpaths electromagnetic wave,and andedge the red line is one of the actual propagation paths after actual propagation after diffraction diffraction. diffraction and edge diffraction.
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shadow area area Figure 5. Schematic diagram of electromagnetic waveshadow in transformer gap exit. Figure 5. Schematic diagram of electromagnetic wave in transformer gap exit.
Figure 5. Schematic diagram of electromagnetic wave in transformer gap exit. 3. Simulation Experiment
3. Simulation Experiment
3. Simulation Experiment 3.1. Simulation Model
3.1. Simulation Model Depending the voltage level, product type, and manufacturer, the height and depth of gap 3.1. Simulationon Model Depending on the voltage level, product type, and manufacturer, and depth of gap between the top lid on and tank or between the tank and pedestal have athe fewheight differences. However, Depending the voltage level, product type, and manufacturer, the height and depth of gap an between the top lid and tank or between the tank and pedestal have a few differences. However, approximate size gap height approximately and the gap depth between the toprange lid andexists, tank orthe between the tankis and pedestal have a1–4 few cm, differences. However, an is an approximate size range exists, the gap height is approximately 1–4 cm, and the gap depth is approximately 2–18 cm. In the present study, a simulation model with a size of 1100 mm × 1100 approximate size range exists, the gap height is approximately 1–4 cm, and the gap depth is mm approximately 2–18 cm. In the present study, a simulation model with a size of 1100 mm × 1100 mm × 1080 mm is constructed on the actual of the size themm transformer approximately 2–18 cm.based In the present study, acharacteristics simulation model withgap a size of of 1100 × 1100 mm and × 1080 mm is constructed based on the actual characteristics of the gap size of the transformer and the × 1080 mm power is constructed based on the actual6). characteristics of the gap size of the transformer and the computing of the computer (Figure computing power of the computer (Figure 6). the computing power of the computer (Figure 6).
gap depth
top lid
gap depth D D
top lid
tank
air
air
PD source
PD source H H gap height gap height
tank
oil oil in in tank tank zz xx
yy Figure6.6. 6.Simulation Simulation model. Figure Simulation model. Figure model.
The values of H and D will vary with the difference of experimental items in the subsequent
The The values of H DD will experimental items in the subsequent values of and H and willvary varywith with the the difference difference ofofexperimental items in the subsequent sections of this paper. Accordingly, the specific values of the two are not directly provided in sections of this paper. Accordingly, Accordingly, the specific values of theof twothe aretwo not directly Figure 6. in sections of this paper. the specific values are notprovided directlyin provided Figure 6. Figure 7 shows the layout of the detection points, where detection point Nos. 1–4 are located in the Figure 6. Figure 7 shows the layout of the detection points, where detection point Nos. 1–4 are located in gap, and the distance between the adjacent detection points is ∆d which is different depending on the in Figure thedistance layout of the detection points,detection where detection 1–4isare located the gap,7 shows and the between the adjacent points ispoint which different ∆d Nos. research section. Detection point Nos. 5–7 are located inside the transformer for better understanding the gap, and on thethedistance the adjacent detection points inside is ∆dthewhich is different depending research between section. Detection point Nos. 5–7 are located transformer for the whole propagation process of electromagnetic waves from the transformer internal to transformer depending the researchthe section. pointprocess Nos. 5–7 located insidewaves the transformer better on understanding wholeDetection propagation of are electromagnetic from the for exterior, and the distance between the adjacent detection points is 70.7 mm, ∠α = 45◦ . Detection point transformer internal to transformer exterior, and the distance between the adjacent detection better understanding the whole propagation process of electromagnetic waves points from the Nos. 8–12 are located outside the transformer, and the distance between the adjacent detection points o is 70.7 mm, . Detection point Nos. 8–12 are located outside the transformer, and the ∠ α = 45 transformer internal to transformer exterior, and the distance between the adjacent detection points is 30 mm. distance the adjacent detection 30 mm. is 70.7 mm, between . Detection pointpoints Nos. is8–12 are located outside the transformer, and the ∠α = 45 distance between the adjacent detection points is 30 mm. o
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Figure 7. Layout of the detection point: (a) (b) X-axis positive view; (b) Z-axis positive view. Figure 7. Layout of the detection point: (a) X-axis positive view; (b) Z-axis positive view.
Figure 7. Layout of the detection point: pulses (a) X-axis positive view; (b) Z-axis positive view. which is The PD source is simulated by Gaussian [18] loaded on the monopole antenna, 525, 500, andsource 500 mm the bottom, the left, and the inner of the oil antenna, tank, respectively, The PD is from simulated by Gaussian pulses [18]front loaded onwalls the monopole which is The PD source is simulated by Gaussian pulses [18] loaded on the monopole antenna, which is I τ where the pulse peak is 1 A, and the pulse width is 1 ns, shown in 1 ns (Figure 8). The 525, 500, and 500 mm from 0 the bottom, the left, and the front inner walls of the oil tank, respectively, 525, 500, and 500 mm from the bottom, the left, and the front inner walls of the oil tank, respectively, currentthe source ispeak in series 50 Ωpulse resistance which is 8). on the(Figure load is 8). 50 V in τ is voltage where the pulse peak is and 1 aA, and thewidth pulseand 1 in ns,1 ns shown in 1 ns The where pulse I0 isI 01with A, the τwidth is 1the ns,peak shown (Figure The current source fact. Although of Gaussian pulse bigger than real PD source, toisa50 bigger current source in value series with aand 50 Ωthe resistance and the peak voltage which the Vthe in is in series withisathe 50 Ω resistance peak is voltage which isthe on the load is 50isVon inleading fact.load Although value of detected signal by the point sensor, this can more clearly exhibit the gap distribution fact. Although thepulse valueisof Gaussian pulse bigger than the real source, leading to a bigger value of Gaussian bigger than the realisPD source, leading to aPD bigger value of detected signal characteristics of signal electromagnetic wave and the hasgap little effectclearly oncharacteristics the accuracy of propagation value detected the clearly point sensor, this can more exhibit theofgap distribution by the of point sensor, this canbymore exhibit distribution electromagnetic characteristics of electromagnetic wave. Figure 9 depicts the radiation pattern of the monopole characteristics of electromagnetic wave and has little effect on the accuracy of propagation wave and has little effect on the accuracy of propagation characteristics of electromagnetic wave. antenna. characteristics electromagnetic wave. Figure 9 depicts the radiation pattern of the monopole Figure 9 depictsofthe radiation pattern of the monopole antenna.
antenna.
i=I0 exp(-
4π(t-t 0 ) 2 ) τ2
1.0
i (A) i (A)
1.0
0.5 0.5
0.0 0.0
0.0 0.0
1.0x10-9
t (s)
1.0x10-9
2.0x10-9 2.0x10-9
t (s) Figure 8. Gaussian pulse.
Figure 8. Gaussian pulse.
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(a)
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Figure 9. Radiation pattern of monopole antenna: (a) Radiation pattern ( φ
(a)
(b)
= 0o ); (b) Radiation o ◦ ); = Figure 9. pattern of monopole antenna: (a) Radiation pattern ); (b) Radiation Figure 9. (Radiation of monopole antenna: (a) Radiation pattern (φ = (0φ (b)0Radiation pattern pattern θ Radiation = 0o ).pattern ◦ o (θ pattern = 0 ). ( θ = 0 ).
3.2. Analysis of Simulation Results 3.2. 3.2. Analysis Analysis of of Simulation Simulation Results Results 3.2.1. Propagation Characteristics in the Transformer Gap of the Electromagnetic Waves Radiated by 3.2.1. Characteristics in the PDPropagation Source in Different Directions 3.2.1. Propagation Characteristics in the the Transformer TransformerGap Gap of of the the Electromagnetic ElectromagneticWaves WavesRadiated Radiatedby by the PD Source in Different Directions the PD Source in Different Directions Figure 9 illustrates that the intensity of the electromagnetic wave radiated by the monopole Figure 9 illustrates that the intensity the in electromagnetic wave radiated by the monopole antenna in its vertical plane is greater thanof the parallel direction. Therefore, the PD Figure 9 illustrates that the intensity ofthat the electromagnetic wave radiated by when the monopole antenna in its vertical plane is greater than that in the parallel direction. Therefore, when the PD source source (antenna) is oriented in different directions, the PD source will produce low-intensity antenna in its vertical plane is greater than that in the parallel direction. Therefore, when the PD (antenna) is oriented insignals different the PD source will produce low-intensitycharacteristics electromagnetic electromagnetic wave ininadirections, specific Accordingly, the propagation in source (antenna) is oriented differentdirection. directions, the PD source will produce low-intensity wave signals in a specific direction. Accordingly, the propagation characteristics in transformer gaps transformer gaps of the electromagnetic signals radiated by PD source in different directions can be electromagnetic wave signals in a specific direction. Accordingly, the propagation characteristics in of the electromagnetic signalssource radiated by PD source in different directions can be studied by adjusting studied by adjusting direction. transformer gaps of the the PD electromagnetic signals radiated by PD source in different directions can be the PD source direction. From reasonable range of transformer studied bythe adjusting the PD source direction. gap height and depth, a 3 cm high and 5 cm deep the reasonable range of transformer gap height and depth, aThe 3 cm high and 5 cm deep gaptois gap isFrom selected the starting research object, where mm. is adjusted ∆d is 15 From the as reasonable range of transformer gap height and depth, asignal 3 cmsource high and 5 cm deep selected as the starting research object, where ∆d is 15 mm. The signal source is adjusted to the X-, Y-, the Y-, andasZ-axis positive directions separately, directions are the physical gapX-, is selected the starting research object, where ∆dofiswhich 15 mm.the The signal source is adjusted to and Z-axis positive directions separately, ofexperimental which the directions are the physical direction of antenna. direction of antenna. Figure 10 exhibits the results. the X-, Y-, and Z-axis positive directions separately, of which the directions are the physical Figure 10 exhibits the experimental results. direction of antenna. Figure 10 exhibits the experimental results.
56
gap
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Amplitude/V Amplitude/V
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X-axis positive direction Y-axis positive direction X-axispositive positivedirection direction Z-axis Y-axis positive direction Z-axis positive direction transformer exterior transformer exterior
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(a)of electromagnetic signals radiated by PD source in different (b) Figure directions: (a) Figure 10. 10.Distributions Distributions of electromagnetic signals radiated by PD source in different directions: Amplitude distribution; (b) Energy distribution. Figure 10. Distributions of electromagnetic signals radiated by PD source in different directions: (a) (a) Amplitude distribution; (b) Energy distribution. Amplitude distribution; (b) Energy distribution.
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1 N u i2 Figure 10 exhibits the results of the simulation, where E nergy = throughout, K is ∑ K i=1 R N u2
1 i Figurenumber, 10 exhibits results of theofsimulation, where u Energy is sampling sampling N the is the number sampling point, i is the= sampling value, and RK is 50 Ω. The K ∑ R throughout, i=1 X-axis ofNFigure 10 is theofdetection shown in Figure 7. The intensity of 50 theΩ.electromagnetic number, is the number samplingpoint point, ui is the sampling value, and R is The X-axis of wave radiated by PD source in the different directions is significantly different. The intensity of the Figure 10 is the detection point shown in Figure 7. The intensity of the electromagnetic wave radiated electromagnetic wave radiated by the PD source, which is in theThe Y- and Z-axisofpositive directions, is by PD source in the different directions is significantly different. intensity the electromagnetic considerably larger than that of X-axis positive direction. Moreover, given the presence of reflection wave radiated by the PD source, which is in the Y- and Z-axis positive directions, is considerably larger and transmission losses, the intensity of the electromagnetic wave gradually decreases with the than that of X-axis positive direction. Moreover, given the presence of reflection and transmission increase propagation inside the gap. In the propagation process of the electromagnetic losses, theofintensity of the distance electromagnetic wave gradually decreases with the increase of propagation wave from the gap to the transformer exterior, the intensity of the electromagnetic wave distance inside the gap. In the propagation process of the electromagnetic wave from theradiated gap to by the PD source which is in the Yand Z-axis positive directions, suddenly decreased. In addition, the transformer exterior, the intensity of the electromagnetic wave radiated by the PD source which given the quantity of electromagnetic wave radiated by the PD source in X-axis positive direction is in the Y- and Z-axis positive directions, suddenly decreased. In addition, given the quantity of itself into the gap is radiated little, thebychange the electromagnetic wave intensity notthe evident the electromagnetic wave the PDofsource in X-axis positive direction itselfisinto gap is in little, process from the transformer gap to the transformer exterior. In the transformer exterior, the the change of the electromagnetic wave intensity is not evident in the process from the transformer intensity the electromagnetic emitted by the PD source in all directions decreases gradually gap to the of transformer exterior. Inwave the transformer exterior, the intensity of the electromagnetic wave with the increase of propagation distance. emitted by the PD source in all directions decreases gradually with the increase of propagation distance. From the the preceding preceding analysis, analysis, the the intensity intensity of of the the electromagnetic electromagnetic wave wave inside inside the the gap gap isis From significantly greater than the transformer exterior; therefore, inserting the antenna sensor into the significantly greater than the transformer exterior; therefore, inserting the antenna sensor into the transformer gap will considerably improve the detection sensitivity. In addition, given the very transformer gap will considerably improve the detection sensitivity. In addition, given the very largedifference differenceofofthe the electromagnetic wave propagation characteristics of different direction large electromagnetic wave gapgap propagation characteristics of different direction PD PD sources, each of the four gaps of transformer is required to set an antenna sensor to achieve full sources, each of the four gaps of transformer is required to set an antenna sensor to achieve full range range and the high-sensitivity detection of the electromagnetic wave. and the high-sensitivity detection of the electromagnetic wave. Forexample, example,as asshown shownin inFigure Figure11, 11,each eachgap gapof ofthe thetransformer transformerhas hasan anantenna antennasensor. sensor.For Forthe the For convenience of explanation, the sizes of four gaps are assumed to be the same. When the PD source convenience of explanation, the sizes of four gaps are assumed to be the same. When the PD source is is located in center the center oftransformer, the transformer, PD source in thedirection, X-axis direction, the sensors antenna located in the of the if the if PDthe source is in theisX-axis the antenna sensors in gaps 2 and 4 will have a high detection sensitivity due to the high intensity of the in gaps 2 and 4 will have a high detection sensitivity due to the high intensity of the electromagnetic electromagnetic waves; if the PD source is in the Y-axis, the antenna sensors in gaps 1 and 3 will waves; if the PD source is in the Y-axis, the antenna sensors in gaps 1 and 3 will have a high detection have a highand detection sensitivity; if the PD source is in the Z-axis,inthe antenna in gaps sensitivity; if the PD source isand in the Z-axis, the antenna sensors gaps 1, 2, 3,sensors and 4 will have1, 2, 3, and 4 will have the same detection sensitivity. Therefore, regardless of the PD source directions, the same detection sensitivity. Therefore, regardless of the PD source directions, when each gap of when each gaphas of an theantenna transformer has an antenna sensor at least twoa antenna sensors the transformer sensor invariably, at least twoinvariably, antenna sensors have high-sensitivity. have athe high-sensitivity. source is offone the of center of transformer, onewith of the two antenna When source is off theWhen centerthe of transformer, the two antenna sensors high detection sensors with high detection sensitivity will possess higher detection sensitivity because the distance sensitivity will possess higher detection sensitivity because the distance between the PD source and between thesensor PD source thethe antenna the antenna is no and longer same. sensor is no longer the same.
gap4 gap3
antenna sensor gap2
gap1 PD source
Z X
Y
Figure 11. Each gap of the transformer has an antenna sensor. Figure 11. Each gap of the transformer has an antenna sensor.
3.2.2. 3.2.2.Influence Influenceof ofGap GapHeight Heighton onGap GapPropagation PropagationCharacteristics Characteristicsofofthe thePD PDElectromagnetic ElectromagneticWaves Waves The layout scheme and and gap depth D remain unchanged. The PD The source orientedis Thedetection detectionpoint point layout scheme gap depth D remain unchanged. PDis source in Z-axis positive direction. The gap height H is adjusted to 4, 3, 2, and 1 cm separately. Out of oriented in Z-axis positive direction. The gap height H is adjusted to 4, 3, 2, and 1 cm separately. the range of range the transformer gap height, 0.5, and gap0.3heights areheights added are to Outreasonable of the reasonable of the transformer gap0.75, height, 0.75,0.3 0.5,cm and cm gap broaden the scope of research. The influence of gap height on the gap propagation characteristics of
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added to broaden the scope of research. The influence of gap height on the gap propagation characteristics of electromagnetic wave is studied by detecting the amplitude and cumulative electromagnetic is studied by detecting and cumulative energy of the electric field. energy of wave the electric field. Figure 12 depictsthe theamplitude experimental results.
Figure 12 depicts the experimental results.
(a)
(b) Figure 12. Distributions of PD electromagnetic signals in different height gaps: (a) Amplitude
Figure distribution; 12. Distributions of PD electromagnetic signals (b) Cumulative energy distribution. (a) Amplitude distribution; (b) Cumulative energy distribution.
in
different
height
gaps:
Figure 12 illustrates that, inside the gap, the intensity of electromagnetic wave shows an increasing trend firstly and then descending with the decrease of gap height. When the gap height Figure 12 illustrates that, inside the gap, the intensity of electromagnetic wave shows an is 1 cm, the intensity of the electromagnetic wave reaches the maximum. In the transformer exterior, increasing trend firstly and then with the decrease of gap height. When the gap the amplitude of electric field descending gradually diminishes with the decrease of gap height. Moreover, the height is 1 cm, variation the intensity of the electromagnetic wave reaches the maximum. In the transformer law of cumulative energy is affected by the accumulated energy inside the gap. Theexterior, tendencyof of signal intensity be inversely proportional to the gapdecrease height in the vicinity of the gap the amplitude electric field to gradually diminishes with the of gap height. Moreover, exit doeslaw not of exist; however, with the increase in the transmission distance, thisenergy trend is gradually the variation cumulative energy is affected by the accumulated inside the gap. reflected. The appearance of the maximum intensity of electromagnetic wave inside the gap with The tendency of signal intensity to be inversely proportional to the gap height in the vicinity of the decrease of gap height can be attributed to the following: diffraction at the gap entrance and the gap exit does not exist; however, with the increase in the transmission distance, this trend is reflection inside the gap will lead to an increase in the electromagnetic wave intensity. However, as gradually The appearance of thethe maximum of electromagnetic wave inside the the reflected. height of transformer gap decreases, number of intensity electromagnetic waves which enter the gap gap with the decrease of gapthereby heightweakening can be attributed to the of following: diffraction the gaptheentrance gradually decreases, the intensity electromagnetic wave.at When reinforcement of the diffraction reflection in is equivalent to the weakening effect caused by and reflection insideeffect the gap will lead toand an increase the electromagnetic wave intensity. However, the decrease of gap height, intensity the of the electromagnetic wave inside waves the gap which will reach the the gap as the height of transformer gapthe decreases, number of electromagnetic enter
gradually decreases, thereby weakening the intensity of electromagnetic wave. When the reinforcement effect of the diffraction and reflection is equivalent to the weakening effect caused by the decrease of gap height, the intensity of the electromagnetic wave inside the gap will reach the maximum. Moreover, with further reduction of the gap height, the weakening effect caused by the decrease of gap height is gradually larger than that of the diffraction at the gap entrance and reflection, the signal intensity inside the gap gradually decreased.
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maximum. Moreover, with further reduction of the gap height, the weakening effect caused by the Energies 2017, 10, 1531 decrease of gap height is gradually larger than that of the diffraction at the gap entrance and 11 of 16
reflection, the signal intensity inside the gap gradually decreased. 3.2.3. Influence of Gap Depth Gap Propagation Characteristics of the PD Electromagnetic Waves Waves 3.2.3. Influence of Gap Depth on on Gap Propagation Characteristics of the PD Electromagnetic The analysis the previous section corroborate that intensityof ofelectromagnetic electromagnetic waves The analysis resultsresults in theinprevious section corroborate that thetheintensity waves in the gap is maximized when the gap height H is 1 cm. Therefore, in this section, in the gap is maximized when the gap height H is 1 cm. Therefore, in this section, maintaining H to be maintaining H to be 1 cm and the PD source is in positive direction of Z-axis. Depth D of the 1 cm and the PD source is in positive direction of8,Z-axis. Depth D of the transformer adjusted transformer was adjusted to 18, 16, 14, 12, 10, 6, 4, and 2 cm separately, studying the was influence of to 18, 16, 14, 12, 8, 6, on 4, and 2 cmpropagation separately,characteristics studying theofinfluence of gap depth on the where gap propagation gap10, depth the gap the PD electromagnetic wave, the characteristics of the PD where the3.0, corresponding ∆dFigure is separately tothe be 5.4, 4.8, corresponding is separately to bewave, 5.4, 4.8, 4.2, 3.6, 2.4, 1.8, 1.2 cm. 13 exhibits ∆d electromagnetic experimental results. 4.2, 3.6, 3.0, 2.4, 1.8, 1.2 cm. Figure 13 exhibits the experimental results. 14
10 8 6 4
500
gap
2
6
5
4
3
2
400 300
gap
200 100
transformer exterior
transformer 0 internal 7
Deep 18cm Deep 16cm Deep 14cm Deep 12cm Deep 10cm Deep 8cm Deep 6cm Deep 4cm Deep 2cm
600
Energy/J
12
Amplitude/V
700
Deep 18cm Deep 16cm Deep 14cm Deep 12cm Deep 10cm Deep 8cm Deep 6cm Deep 4cm Deep 2cm
1
8
9
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10 11 12
transformer exterior
transformer internal 7
6
5
Detection point
4
3
2
1
8
9
10 11 12
Detection point
(a)
(b)
Figure 13. Distributions of PD electromagnetic signals in different depth gaps: (a) Amplitude
Figure 13. Distributions of PD electromagnetic signals in different depth gaps: (a) Amplitude distribution; distribution; (b) Cumulative energy distribution. (b) Cumulative energy distribution. Figure 13 illustrates that, inside the gap, the intensity of the electromagnetic waves shows a gradually increasing trend with the decrease of gap depth D. The intensity of electromagnetic wave Figure 13 illustrates that, inside the gap, the intensity of the electromagnetic waves shows has an increasing trend at the tail of gap, and the increasing trend is substantially evident with the a gradually increasing the decrease of gap depth D. be The intensity electromagnetic increase of the gaptrend depth.with Therefore, the antenna sensor should placed in the of entrance or the tail wave has an increasing trend at the tail gap, and andback the sections increasing trend is substantially evident area of transformer gap, and theof middle of the gap should be avoided as much aswith the improve PD detection In addition, the change of gap has a slight increasepossible of the to gap depth.theTherefore, thesensitivity. antenna sensor should be placed indepth the entrance or the tail on the intensity of the wave in transformer exterior. area of effect transformer gap, and theelectromagnetic middle and back sections of the gap should be avoided as much as
possible to improve the PD detection sensitivity. In addition, the change of gap depth has a slight effect 4. Comparison with Experiment on the intensity of the electromagnetic wave in transformer exterior. 4.1. Experimental Scheme
4. Comparison with Experiment
Based on the gap of a 10-kV oil-immersed transformer, the depth of gap is adjusted by different lengths of copper, and the height of the gap is adjusted by bolts. The antenna sensor for transmitting 4.1. Experimental Scheme and receiving electromagnetic wave signal is a monopole antenna (Figure 14a). And for the antenna, the radiation conductor line is oil-immersed 6 cm long; the square ground isthe 1 cmdepth wide. of gap is adjusted by different Based on the gap of a 10-kV transformer,
lengths of copper, and the height of the gap is adjusted by bolts. The antenna sensor for transmitting and receiving electromagnetic wave signal is a monopole antenna (Figure 14a). And for the antenna, the radiation conductor Energies 2017, 10, 1531 line is 6 cm long; the square ground is 1 cm wide. 12 of 16 30
U/V
20
10
0 0
20000
40000
Sampling point
(a)
(b)
Figure 14. Antenna sensor and pulse source: (a) Antenna sensor; (b) Pulse source.
Figure 14. Antenna sensor and pulse source: (a) Antenna sensor; (b) Pulse source.
From the radiation pattern of the monopole antenna, it’s easy to know that the radiation pattern of the PD source in the experiment is consistent with that of simulation experiment. That is, the capability to radiate electromagnetic wave in the vertical plane is robust. Therefore, the monopole antenna which radiates the electromagnetic wave, can be used to simulate the X-, Y-, Z-axis PD sources by changing its direction. Figure 15 shows the time-frequency domain results of the signal received by the antenna sensor in air environment without transformer tank.
0 0
20000
40000
Sampling point
(a)
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Figure 14. Antenna sensor and pulse source: (a) Antenna sensor; (b) Pulse source.
From the radiation pattern of the antenna, it’sit’s easy to know thatthat thethe radiation pattern of From the radiation pattern ofmonopole the monopole antenna, easy to know radiation the PDpattern sourceofinthe thePD experiment is consistent with that of simulation experiment. That is, the capability source in the experiment is consistent with that of simulation experiment. That is, the electromagnetic capability to radiate electromagnetic inisthe vertical plane is the robust. Therefore, the which to radiate wave in the verticalwave plane robust. Therefore, monopole antenna monopole antenna whichwave, radiates thebeelectromagnetic wave,the canX-, beY-, used to simulate the X-,by Y-,changing radiates the electromagnetic can used to simulate Z-axis PD sources Z-axis PD sources by changing its direction. Figure 15 shows the time-frequency domain results of its direction. Figure 15 shows the time-frequency domain results of the signal received by the antenna the signal received by the antenna sensor in air environment without transformer tank. sensor in air environment without transformer tank.
U/V
Amplitude/V
2
1
0
0
500
1000
1500
0.0
2.0x108
Sampling point
4.0x108
6.0x108
8.0x108
1.0x109
1.2x109
Frequency/Hz
(a)
(b)
Figure 15. Time-frequency signals received by Antenna sensor in air environment without
Figuretransformer 15. Time-frequency signals received by Antenna sensor in air environment without transformer tank: (a) Signal in time domain; (b) Signal in frequency domain. tank: (a) Signal in time domain; (b) Signal in frequency domain. With the comparison of the contents of simulation experiment and the combination of the operability of experiment, the following three parts are designed.
With the comparison of the contents of simulation experiment and the combination of the (1) When the transformer is 3 cm high 5 cmare deep, the PD source is adjusted to X-, Y-, and operability of experiment, the gap following threeand parts designed. Z-axis positive directions separately (refer to the coordinate system in the simulation
(1)
(2)
(3)
to study gap 5propagation characteristics electromagnetic Whenexperiment) the transformer gaptheis 3transformer cm high and cm deep, the PD source of is adjusted to X-, Y-, and signals radiated by PD source in different directions. Z-axis positive directions separately (refer to the coordinate system in the simulation experiment) When the transformer gap is 5 cm deep and the PD source is oriented in Z-axis positive to(2)study the transformer gap propagation characteristics of electromagnetic signals radiated by direction (refer to the coordinate system in the simulation experiment), the gap height is set to PD source in different directions. be 4, 3, and 2 cm separately to study the influence of gap height on the gap propagation Whencharacteristics the transformer is 5 cm deep and the PD source is oriented in Z-axis positive direction of thegap electromagnetic wave. (3) When transformersystem gap is in 2 cm and the experiment), PD source is oriented Z-axis is positive (refer to thethe coordinate thehigh simulation the gapinheight set to be 4, 3, direction (refer to the coordinate system in the simulation experiment), the gap depth is set to and 2 cm separately to study the influence of gap height on the gap propagation characteristics of be 18, 14, 10, and 6 cm separately to study the influence of gap depth on the gap propagation the electromagnetic wave. characteristics of electromagnetic wave. When the transformer gap is 2 cm high and the PD source is oriented in Z-axis positive direction (refer to the coordinate system in the simulation experiment), the gap depth is set to be 18, 14, 10, and 6 cm separately to study the influence of gap depth on the gap propagation characteristics of electromagnetic wave. Energies 2017, 10, 1531
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4.2. Analysis of Experimental Results (1)
4.2. Analysis of Experimental Results
When the transformer gap is 3 cm high and 5 cm deep, Figure 16 shows the layout of detection
(1) When theexperiment, transformer gap is 3 detection cm high and 5 cmNos. deep,1–2 Figure 16 showsinthe of detection points in the where point are located thelayout gap and the distance points in the experiment, where detection point Nos. 1–2 are located in the gap and the between the adjacent detection points is 2 cm. Detection point No. 3 is located in the transformer distance between the adjacent detection points is 2 cm. Detection point No. 3 is located in the internal, and the distance from the tank wall is 2 cm. Detection point Nos. 4–6 are located in the transformer internal, and the distance from the tank wall is 2 cm. Detection point Nos. 4–6 are transformer and theexterior, distance between adjacent detection points is 2 points cm. is 2 cm. located inexterior, the transformer and the distance between adjacent detection transformer internal 2 3 2cm 1cm
gap 1 3cm 5cm
2cm
transformer exterior 4 5 2cm
6 2cm
Figure 16. Layout of detection the detection point of experiment (transformer gapis is high 3 cm and deep Figure 16. Layout of the point of experiment (transformer gap high 3 cm and deep 5 cm). 5 cm).
The signal source is adjusted to X-, Y-, and Z-axis positive directions separately. The electromagnetic wave signal is obtained according to the detection point layout in Figure 16. Figure 17 exhibits the experimental results.
transformer exterior 4 5 2cm
gap
transformer internal 2 3 2cm 1cm
1 2cm
3cm 5cm
6 2cm
Energies 2017, 10, 1531 Figure 16. Layout of the detection point of experiment (transformer gap is high 3 cm and deep 5 cm).
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The signal adjusted to X-, Y-, to andX-,Z-axis positive directions The electromagnetic Thesource signalissource is adjusted Y-, and Z-axis positive separately. directions separately. The electromagnetic wave according signal is obtained accordingpoint to thelayout detection point 16. layout in Figure 16. wave signal is obtained to the detection in Figure Figure 17 exhibits the Figure 17 exhibits the experimental results. experimental results. 14
X-axis positive direction Y-axis positive direction Z-axis positive direction
12
X axis positive direction Y axis positive direction Z axis positive direction
0.00015
Energy/J
Amplitude/mV
10 8
gap
6 transformer
transformer exterior
internal
0.00010
gap
transformer internal
transformer exterior
0.00005
4 2 3
2
1
4
5
6
3
2
Detection point
1
4
5
6
Dectection point
(a)
(b)
Figure 17. Distributions of electromagnetic signals radiated by PD source in different directions in
Figure 17. Distributions of electromagnetic signals radiated by PD source in different directions in experiment: (a) Amplitude distribution; (b) Cumulative energy distribution. experiment: (a) Amplitude distribution; (b) Cumulative energy distribution.
Figure 17 illustrates that, inside the gap, the intensity of the electromagnetic wave radiated by PD source in illustrates different directions is significantly The intensity the electromagnetic wave Figure 17 that, inside the gap, different. the intensity of the of electromagnetic wave radiated radiated by the PD source, which is in Yand Z-axis positive directions, is considerably larger than by PD source in different directions is significantly different. The intensity of the electromagnetic that of X-axis positive direction. In the propagation process of electromagnetic wave from gap to wave radiated by the PD source, which is in Y- and Z-axis positive directions, is considerably larger transformer exterior, the intensity of electromagnetic wave radiated by PD source in Y- and Z-axis than that of X-axis positive direction. In the propagation process of electromagnetic wave from positive directions, suddenly decreased. And the intensity change of the electromagnetic wave gapradiated to transformer exterior, the intensity of electromagnetic wave radiated by PD source in Y- and by the PD source in X-axis positive direction is not evident. Therefore, in the gap detection Z-axis positive directions, suddenly decreased. the intensity change of the of transformer PD electromagnetic wave signal,And inserting the antenna sensor intoelectromagnetic the transformer wave radiated by the PD sourceimprove in X-axis direction is not evident. in the gap detection of gap will considerably thepositive detection sensitivity. Each of the Therefore, four gaps of transformer is transformer PDbeelectromagnetic wave signal, inserting antenna sensor into the detection transformer gap will required to set with an antenna sensor to achieve fullthe range and a high-sensitivity of the electromagnetic wave. conclusion is consistent that of gaps the simulation resultsisshown in to be considerably improve theThis detection sensitivity. Each with of the four of transformer required Figure 10. set with an antenna sensor to achieve full range and a high-sensitivity detection of the electromagnetic
wave. conclusion is consistent of and the simulation results shown in (2) This When the transformer gap iswith 5 cmthat deep the PD source is oriented in Figure Z-axis 10. positive direction, the gap heights are 4, 3, and 2 cm separately. In addition, the electromagnetic wave
When the transformer gap isto 5the cmdetection deep and the PD source oriented in shows Z-axisthe positive signal is obtained according point layout in Figureis 16. Figure 18 direction, the gap heights are 4, 3, and 2 cm separately. In addition, the electromagnetic wave experimental results. signal is obtained according to the detection point layout in Figure 16. Figure 18 shows the Energies 2017, 10, 1531 14 of 16 experimental results. 0.00020
High 4cm High 3cm High 2cm
14 12
High 4cm High 3cm High 2cm
0.00015
10
gap
8 transformer
Energy/J
Amplitude/mV
(2)
transformer exterior
internal
0.00010
gap
transformer exterior
transformer internal
6 0.00005
4 3
2
1
4
Detection point
(a)
5
6
0
1
2
3
4
5
6
7
Detection point
(b)
Figure 18. Distributions of PD electromagnetic signals in different height gaps in experiment: (a)
Figure 18. Distributions of PD electromagnetic signals in different height gaps in experiment: Amplitude distribution; (b) Cumulative energy distribution. (a) Amplitude distribution; (b) Cumulative energy distribution.
Figure 18 depicts that, inside the gap, the intensity of electromagnetic wave shows an increasing trend with the decrease of gap height. In the transformer exterior, the amplitude of electric field gradually diminishes with the decrease of gap height. Moreover, the variation law of the cumulative energy is affected by the accumulated energy inside the gap. Owning to the limitation of the monopole antenna size, the propagation characteristics of the electromagnetic wave in the smaller height gaps cannot be experimentally verified. However, the conclusion drawn in the experiment is consistent with that of the simulation results shown in Figure 12.
Detection point
Detection point
(a)
(b)
Figure 18. Distributions of PD electromagnetic signals in different height gaps in experiment: (a) Amplitude distribution; (b) Cumulative energy distribution. Energies 2017, 10, 1531
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Figure 18 depicts that, inside the gap, the intensity of electromagnetic wave shows an Figure 18 depicts the gap, of thegap intensity of electromagnetic waveexterior, shows an increasing trend increasing trend that, with inside the decrease height. In the transformer the amplitude of with the decrease of gap height. In the transformer exterior, the amplitude of electric field gradually electric field gradually diminishes with the decrease of gap height. Moreover, the variation law of diminishes with energy the decrease of gap height. the energy variationinside law ofthe thegap. cumulative energy is the cumulative is affected by the Moreover, accumulated Owning to the affected by the accumulated energy inside the gap. Owning to the limitation of the monopole antenna limitation of the monopole antenna size, the propagation characteristics of the electromagnetic size, the propagation characteristics of the in the smaller the height gaps cannot be wave in the smaller height gaps cannot beelectromagnetic experimentally wave verified. However, conclusion drawn experimentally verified. However, drawn in the experiment is consistent with that of in the experiment is consistent withthe thatconclusion of the simulation results shown in Figure 12. the simulation results shown in Figure 12. (3) On the basis of ensuring the operability of the experiment, the gap height is set to be 2 cm, the (3) partial On thedischarge basis of ensuring operability of the experiment, thethe gapgap height is set 2 cm, source is the oriented in Z-axis positive direction, depths areto18,be14, 10, the partial discharge source is oriented in Z-axis positive direction, the gap depths are 18, 14, 10, and 6 cm separately, and the electromagnetic wave signals are obtained according to the and 6 cm point separately, and electromagnetic signals are obtained according detection layout in the Figure 19. Figure 20wave depicts the experimental results. to the detection point layout in Figure 19. Figure 20 depicts the experimental results. gap
transformer internal 4
2cm
2cm
5
1
2
3
transformer exterior
xcm
x/2cm
2cm
7
6
2cm
2cm
2cm
Figure 19. Layout of the detection point of experiment (transformer gap is (x + 4) cm high and 5 cm deep). deep).
Figure 19.1531 Layout of the detection point of experiment (transformer gap is (x + 4) cm high and 5 cm Energies 2017, 10, 15 of 16 16
Energy/J
Amplitude/mV
0.00025 Figure 20 illustrates that the propagation characteristics of electromagnetic wavesDeep in different Deep 18cm 18cm 14 Deep 14cm Deep depth gaps in the experiment are slightly different 0.00020 from the propagation characteristics14cm of the Deep 10cm Deep 10cm 12 electromagnetic wave in simulation experiment shown in Figure 13. In consideration of the existence Deep 6cm Deep 6cm 10 0.00015 of experimental errors, the following conclusions can still be drawn from the experimental results. 8 Inside the gap, the intensity of the electromagnetic wave shows a gradually increasing trend with 0.00010 6 gap the decrease of gap depth. In addition, the intensity of electromagnetic wave gap has an increasing trend transformer transformer transformer transformer 0.00005 at the tail gap. interior 4 of internal internal interior
2
0.00000
0 4
3
2
1
5
Detection point
(a)
6
7
4
3
2
1
5
6
7
Detection point
(b)
Figure 20. Distributions of PD electromagnetic signals in different depth gaps in experiment: (a) Figure 20. Distributions of PD electromagnetic signals in different depth gaps in experiment: Amplitude distribution; (b) Cumulative energy distribution. (a) Amplitude distribution; (b) Cumulative energy distribution.
5. Conclusions Figure 20 illustrates that the propagation characteristics of electromagnetic waves in different depth This study analyzed the gap propagation characteristics of electromagnetic wave radiated by gaps in the experiment are slightly different from the propagation characteristics of the electromagnetic the PD source in different directions and the influence of gap size, which were then experimentally wave in simulation experiment shown in Figure 13. In consideration of the existence of experimental verified. The following conclusions are obtained: errors, the following conclusions can still be drawn from the experimental results. Inside the gap, (1) The intensity the PD electromagnetic wave inside theincreasing gap is significantly than that in the intensity of theofelectromagnetic wave shows a gradually trend withgreater the decrease of gap the transformer exterior. Therefore, in the gap detection of transformer PD electromagnetic depth. In addition, the intensity of electromagnetic wave has an increasing trend at the tail of gap. wave signals, inserting the antenna sensor into transformer gap will considerably improve the 5. Conclusions detection sensitivity. (2) Inside the gap, the difference of electromagnetic wave intensity radiated by PDradiated source in This study analyzed the gap propagation characteristics of electromagnetic wave by different directions is significantly large. Therefore, each of the four gaps of transformer is the PD source in different directions and the influence of gap size, which were then experimentally required be set with an antenna to achieve full range and a high-sensitivity detection verified. The to following conclusions are sensor obtained: of the electromagnetic wave. (1) The intensity of the PD electromagnetic wave inside the gap is significantly greater than that (3) The gap height will affect the propagation of electromagnetic wave in the transformer gap. In in the transformer exterior. Therefore, in the gap detection of transformer PD electromagnetic the reasonable range of the gap height of the transformer, the intensity of the electromagnetic wave shows an increasing trend inside the gap with the decrease of gap height. (4) Inside the gap, the intensity of the electromagnetic wave shows a gradually increasing trend with the decrease of gap depth. Moreover, the intensity of electromagnetic wave has an increasing trend at the tail of gap. The antenna sensor should be placed in the entrance or the
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(2)
(3)
(4)
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wave signals, inserting the antenna sensor into transformer gap will considerably improve the detection sensitivity. Inside the gap, the difference of electromagnetic wave intensity radiated by PD source in different directions is significantly large. Therefore, each of the four gaps of transformer is required to be set with an antenna sensor to achieve full range and a high-sensitivity detection of the electromagnetic wave. The gap height will affect the propagation of electromagnetic wave in the transformer gap. In the reasonable range of the gap height of the transformer, the intensity of the electromagnetic wave shows an increasing trend inside the gap with the decrease of gap height. Inside the gap, the intensity of the electromagnetic wave shows a gradually increasing trend with the decrease of gap depth. Moreover, the intensity of electromagnetic wave has an increasing trend at the tail of gap. The antenna sensor should be placed in the entrance or the tail area of transformer gap and try to avoid the middle and back sections of transformer gap as much as possible.
Acknowledgments: The authors gratefully acknowledge the support of the National High-Tech Research and Development Plan of China (Grant No. 2015AA050204). Author Contributions: Xiaoxing Zhang conceived and designed the experiments; Guozhi Zhang, Yalong Li, Jian Zhang performed the experiments; Xiaoxing Zhang, Guozhi Zhang and Rui Huang analyzed the data; Xiaoxing Zhang and Guozhi Zhang wrote the paper. Conflicts of Interest: The authors declare no conflict of interest.
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