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Insulating Materials for Power Apparatus
Prof. Kee-Joe Lim
[email protected], 261-2424
School of Electrical Engineering
Chungbuk National University
http://www.cbucc.com
2016/6/14
contents
1. 진공 및 기체 절연재료
2. 액체 절연재료
3. 고체 절연재료
4. 기타
2
3
• Less use of insulating materials-compact design
& cost reduction
• Lower loss insulation systems-high efficiency
• More environmentally friendly use of insulating
materials including recycle systems
Insulation tech. trend
4
Condition in service V(AC+LI/SI, DC+AC/LI/SI), T(Tm, heat
cycle), M(stress, vib.), etc
Field distribution
analysis; Emax,
-mathematical method
-indirect method
-conformal mapping
-analog method(electrolytic tank)
-capacitance network
-numerical methods(FDM, FEM, CSM, BEM.)
Field grading/stress
control; Emax,
electrode grading, permittivity grading,
Condenser grading, resistivity grading
Criteria of insulation
design of materials
BDS=f(AC/DC/IMP, T, t, M..)
V/TET
Prototyping
Test BD, V/TET, pd…
Insulation Design Process
5
Vacuum as a insulator
p
T
N
1
• =0.4mm@1mBar, 400m@10-6mBar
• Advantage; self-repairing, low permittivity, no dielectric
loss, non-inflammable, rather high BDS
• Disadvantage; relatively high cost(hermetic tightness),
support electrode and solid insulator
•Ex. ; x-ray tube, VI
6
Breakdown mechanism of Vacuum
• breakdown caused by field emission
Field enhancement factor
• breakdown caused by particles
• combination of the two mechanisms
EE 0
w
h2
7
Vacuum insulation in practice
• Electrodes - SUS, Titanium (hard, insulating oxide film)
- Submicron mirror finish, ultrasonic wash, assembled in clean room,
avoid contamination.
• Conditioning - to further remove irregularities at electrode surface
- conditioned by application of low-energy breakdown
- HV, increasing in discrete step,
- BD current limited less than 1A
- microprotrusions and microparticles are evaporated
8
Vacuum insulation in practice
• gap distance - for gaps in order of 1 - 2mm; BDS ~ 50 – 100 kV/mm, depending
on metals ( SUS-59, Al – 30..)
- gaps larger than 2 mm; BDS ~ 20 – 30 kV/mm
• vacuum pressure - for small gap(<2mm) and for low pressure(10-4 – 10-7 Torr)
breakdown level is independent of pressure
- for larger gap and above 10-4 Torr Breakdown level fall off
- operating pressure range of VI 10-3 – 10-8 Torr
• electrode geometry and area • geometry is not important
• area influence is a significant influence on BD at shorter
gap(<1mm)
9
Vacuum insulation in practice
• AC BD -Short gap(under 2mm); electron emission BD, 20 ~ 30 kV/mm
-Larger gap; microparticle BD, voltage is dominant factor
• Impulse BD -Short gap(under 2mm); electron emission BD, 30 ~ 40 kV/mm
-Larger gap;
•Interface with solid insulators(glass, ceramics)
- BDV will be substantially lower than in a plain vacuum
- BD is caused by complicated emission process
dcVb mmkVc /30
mmkVc /60
10
• process and avalanches
• feedback by the - process
•feedback by several mechanism
)exp(E
BpAp
deqNJ 0
d
d
d
d
e
eqN
e
eqNJ
1)1(100
)1
1ln(
d
)1
1ln(
d
BD of Gas at low pressure -Townsend Theory
11
BD of Gas at low pressure -Townsend Theory
1. Creation of starting electrons by light or by cosmic irradiation
2. Multiplication of electrons leading to avalanche
3. Feedback by a process that new electrons from the cathode
)
11ln(
d
Cathode Argon Hydrogen Air Oxygen
Al 0.12 0.1 0.035 0.1
Cu 0.06 0.05 0.025 0.065
Fe 0.06 0.06 0.02 0.06
(non-uniform field, low pressure)
12
Paschen’s law
)1
1ln(
d
)( pdfVd
)1
1ln(0
dx
d
In case of non-homogeneous field
mmatmpd 5
13
Gas at high pressure –streamer mechanism
BD characteristics over 5 atmmm • time to breakdown
• dependency of BDV on cathode materials
• shape of BD channels
Streamer mechanism • -process, avalanche + space charge
14
Raether E
Exx r
cc lnln7.17
p
d
P
E
pd lnln5.14ln
Meek
(Non-uniform field)
15
SF6, electron detachment
(at higher (pd))
16
BDS of SF6 , Air Simplified design approach(SF6)
dp
VE
nc
Ec ; breakdown voltage grad.
Const.(kV/inch/ psia or kV/mm/bar)
negative polarity impulse - 7.45 kV/mm/bar
positive polarity impulse – 8.25 kV/mm/bar
V ; impulse breakdown voltage(kV)
p ; absolute gas pressure(psia)
d ; gap length(inches)
; utilization factor
n ; pressure exponent=0.86
17
• The electrodes are polished , and cleaned with solvents
• The surface of the solid insulators are cleaned with
solvents
• The construction is dried by evacuating, filling with dry gas,
evacuating, etc
• The electrode are conditioned by repeated breakdown
Compressed gas insulation
18
Liquid as a dielectric
• Advantage; - Recovery after BD
- Low permittivity(2~2.5), low loss(tanδ~10-4)
- BDV better than gas
- Cooling better than gas
• Disadvantage
- container, solid insulators is required
- Combustible
• Used in combination with other materials; board+oil(TR),
paper+oil(HV cable, bushing)
19
Breakdown mechanism of liquids
• Electronic breakdown
• Suspended solid particle mechanism
• Cavity breakdown
• Electroconvection and electrohydrodynamic model of
dielectric breakdown
* Static electrification in power TR
강성화, 임기조; 변압기유의 유동대전현상에
관한 연구, 전기학회논문지 39권6호(1990)
BTA
20
Breakdown in practice; particles
The insulating oil used in practice are never free of impurities.
Dust, particles, fibers, metal slivers, and tiny air bubbles
are always present.
21
Liquid insulation of power TR
1. Barrier(board)-9 layers
2. Oil channel, 6~8mm, cooling winding
3. Stress cone at the edge of HV winding, 4 oil layer
4. Equipotential line at board, tangential fields along the board, bushing
22
Insulation within winding in power TR
• Capacitance network under impulse voltage
• Cs; inter-layer capacitance, Cg; cap. Layers/ earth
• With impulse voltage, non-linear potential distribution
• Linear distribution approaches with increasing Cs HV
Cs
Cg
l
23
Solids as an insulator
• Advantage; self-supporting, no tank, high BDS
• Disadvantage; not self-healing, higher permittivity and
losses, often combustible, less effective cooling
• Major insulation materials; epoxy resin(polymer concrete),
porcelain, glass, mica(with resin), synthetic rubber(EPR,
SIR), polyester molding compound(BMC, SMC), PC,
fluoroplastics(PTFE), phenolics, polyethylene(XLPE),
paper(OI,RI)
• Layered insulation; composite or laminated dielectric
structure, capacitor network; For improving strength proper
selection and placement of insulation are important
24
BD mechanism in Solid insulator
• Short –time mechanism
- Electronic breakdown
• Intrinsic breakdown( )
• Electron avalanche breakdown ( )
• Zener breakdown( )
-Thermal breakdown ( )
- Electromechanical breakdown ( )
0
d
Eb
0
d
Eb
0
T
Eb
0
T
Eb
0
T
Eb
2)( EgradTkdivdt
dTCv 0Ediv
d
dY
d
V 02 ln)(2
1 )
2
1exp()( 2/1
0
Y
d
VE m
m
0
d
Eb
0
T
Eb
tan
42 cT
TV
25
BD mechanism in Solid insulator
• Long –time mechanism
- BD caused by PD in cavities
- BD caused by inclusions of foreign particles
Process; Field concentration, impact of electrons and
ions, erosion of cavity wall(bombardment, local heating,
chemical reaction, and etc), pit formation, narrow
channel formed(pit growing, field strength at tip of pit
approaches intrinsic BDS), channel growing and
widening, treeing, breakdown
26
BD mechanism in Solid insulator
• Voltage life, BD caused by PD
in cavities
ncEL EnkL lnln
• Voltage life, BD caused by
inclusions of foreign particles
-Effect of field strength
-Effect of PD magnitude
10
100
10
Ebr
1 kV cm-1
1 ns 1 µs 1 ms 1 s 1 hr1 min 1 day 1 mo1 yr10 yrs
Water trees
Internal discharges
and electrical treesThermal
Electro-
mechanical
Intrinsic
Electronic
Time to breakdown
1 MV cm-1
Time to breakdown and the field at breakdown, Ebr, are interrelated and depend on the mechanism that causes
the insulation breakdown. External discharges have been excluded (based on L.A. Dissado and J.C. Fothergill,
Electrical Degradation and Breakdown in Polymers, Peter Peregrinus Ltd. for IEE, UK, © 1992, p. 63)
28
Voltage Endurance Test
• Voltage accelerated life test
• Constant stress accelerated life test
• Step stress accelerated life test
• Progressive stress accelerated life test
• Frequency accelerated life test
ncEL
k
ff ffttb )/(/ 00
29
Voltage Endurance Test
ncEL Life
Reaction rate
Accumulated damage
Constant
Step
progressive
0/ LEn
nELL 0
)1(0
1
ncL
E n
0/ LtE n
)1(
)1()(
0
n
knn
L
Et
1i
i
E
E
k
i
niL
Et
10
ctE
t
E
E
E
E
t
t
t
k
,
30
KVT m
FF
31
(120 V/h)
( 15 V/h )
( 3 V/h )
32
Solid insulation design guide
• Short- and long –time dielectric withstand capabilities
- Short-time capability; LI/SI BDS
- Long-time capability; 40 yr lifetime at rated max
operating voltage
• Volume and surface resistivity
• Dielectric loss (tan )
• Resistance to pd, treeing, tracking
• Thermal properties, thermal stability
• Mechanical properties
33
'
0
"
0tanr
r
]/[2
tan 32
0
'
0 mwE
W r
J
Dielectric loss(Absolute Tan value )
Dielectric loss; effect the power rating or harm the
economy of a power component
Tan
(x 10-4)
dielectrics Remarks(effect)
1 ~ 10 HV cable
insulation(low loss PE),
GIS(loss-free gas)
-no decrease of current rating
-no risk of thermal breakdown
-no cost of dielectric losses to be feared
20 ~ 50
OF insulation of
cable/TR , rubber
insulated cable, cable
accessories, bushing
-current rating effect, cost effect at
HV(400-700kV)
-ageing effect
>100 PVC insulation >6kV, current rating offset
-thermal breakdown
-losses are unacceptable
5/3
4
Combination of dielectrics ; interface perpendicular to field
• permittivity(ε) of paper insulation: 4.5 ( and Oil 2.2 )
• Registration
• Oil butt-gap
• Density and Porosity of the paper
- Density increase : permittivity increase and the breakdown strength
decreases
- Porosity increase : less paper fiber and breakdown strength decrease
• Viscosity of the oil
- the viscosity of the oil is decrease=breakdown strength is decrease
5/3
4
Combination of dielectrics ; interface parallel to field The actual breakdown voltage happens to be far lower than the ideal case
Methods for prevention
Recessed electrode Metalized surface of cavity
5/3
4
Combination of dielectrics ; interface parallel to field
(Joining dielectric surface)
weak spot occurs -> low dielectric strength
This problem has been solved by using elastic bodies of synthetic rubber
Stress cone, cable joint V-t of OIP
38
E-Field distribution
39 / 65
HVDC HVAC
Dominant E- field grading
Resistive (conductivity)
Capacitive (permittivity)
charges affecting field
Space charge Surface charge
-
Voltage experienced in service
DC + AC/LI/SI Polarity reversal
AC + LI/SI
Factors affecting field
Additive, ageing, temperature, electrode materials, field etc
-
Time dependence dependent independent
2. HVDC 절연의 특징 / 전계해석
DC Insulation과 AC Insulation의 차이
40 / 65
• stressing at positive or at negative voltage
• polarity reversal
• stressing for a shorter or a longer period than the
saturation time of internal charge
• superposition with surges and polarity, AC
V Pure DC
t
q
2. HVDC 절연의 특징 / 전계해석
41 / 65
• distribution of the electric field • DC field-conductivity
• AC field-permittivity
• space charge, surface charge
• time-dependent field
• partial discharge behavior under DC
• dielectric loss and measurement • AC-tan d, DC-leakage current
• Breakdown mechanisms in DC • thermal breakdown is improbable
2. HVDC 절연의 특징 / 전계해석
DC Insulation과 AC Insulation의 차이
42 / 65
V
t
q
Electric field at different stages
I II III IV
II
IV I III Triple Maxwell
capacitor
2. HVDC 절연의 특징 / 전계해석
43 / 65
V
t
q
Electric field after polarity reversal
V VI VII
Stage V VI VII
Polarity reversal, Capacitive field, Surface charge
Old surface charges gradually disappear and new charges are formed
Resistive field(be similar to stage III)
Calculation capacitive field inc. effect of surface charge
V
2. HVDC 절연의 특징 / 전계해석
44 / 65
Electric field; effect of temp. (ex. OF cable)
Field distribution in paper insulated cable with a thermal gradient
A-concentrate the field at the higher resistive(cooler) outer part
B-actual field strength, relieve by the non-linearly of dielectric
C-capacitive distribution
D-difference between curve B and C is charge induced field D
E-field at polarity reversal is composed of the charge induced field and the
reserved capacitive field
2. HVDC 절연의 특징 / 전계해석
45 / 65
Example of a capacitive / resistive field in a
rectifier transformer
Capacitive field Resistive field
Equipotential lines are
compressed in the oil
(oil-low permittivity)
Equipotential lines are
compressed in the
barrier
(barrier-low conductivity)
Compression is mild
Compression is
excessive
Refraction of
equipotential lines at
the interface;the angle
is smallest in oil,
In the solid they bend
away from he interface
Refraction of
equipotential lines at the
interface;the angle is far
smaller in solid,
In the solid they bend
towards the interface
2
4
oil
barrier
verysmalloil
barrier
2. HVDC 절연의 특징 / 전계해석
46
Effect of dielectric coating
47
Effect of dielectric coating
48
Advanced insulation for GIS
49
50
51
친환경(eco-friendly) 전력기기 동향 1. 국제 환경규제
1972; UN환경선언(UN 환경회의)
1989;몬트리올 의정서(오존층파괴물질 금지)
1991;리우선언-Agenda21(지속가능개발)
1995;ISO14000(환경경영시스템)
1996;WTO(환경과 무역 다자간 회의)
1997;Kyoto protocol(지구온난화 물질감축)
2000;WEEE, RoHS, EUP directives
(전기전자제품에 대한 직접규제)
2. 대응
-정부; 기후변화협약대책위(국무총리), 에너지관리공단 기후대책총괄실, 기술개발지원, 법제화
-산업체;친환경제품 개발
유해물질free제품
Recycle 가능제품
환경부하 낮은 제품
-신 환경산업 대두
52
HV gas insulation technology, g3 ; Alstom
• At 2015 Hanover Fair, Alstom was presented two first HV
application
•
•1st application of pilot project- 420 kV g3-insulated busbar
-300 m, -25℃
• 2nd application – 245 kV CT type SKF using g3
- outdoor, -30℃
53
properties effect properties effect
유전특성
유전율 저하
절연파괴
단시간 불변 또는 약간상승
유전정접 저하 장시간 수배~백배향상,
상당향상
공간전하
전하축적 개시전계
저하 트리 길이
짧아짐
축적전하량 감소 내부분방전 상당향상
전도특성
도전율 저하/감소 경향(?)
내트랙킹성 상당향상
TSC 피크위치 고온으로
열전도도 증가
EL 발생전계 증가
유리전이온도 약간저하
Nanocomposite
54
Nanocomposite
55
경청해주셔서 감사합니다 !
