high frequency voltage stress - ieeeewh.ieee.org/r10/taiwan/pses/archive/2012_04_27/ieee...
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High Frequency Voltage StressPresented by: Flore ChiangDate: March 30, 2012
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Now the additional data is available!
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ground rules:
1. intro to PD.
2. experimental results.
3. comparison with current practice.
4. why different?
5. how air and solid dielectrics behave under high frequency stress?
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IEC 60664‐4:2005 (2nd Ed.)
Any type of periodic voltages with a fundamental frequency
(f0) above 30 kHz and up to 10 MHz
Key phenomenon = partial discharge (PD)
Its deteriorating effect is aggravated roughly in proportionally
to the frequency
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partial discharge (PD)is a localized discharge within a solid or fluid dielectric system, restricted to only a part of dielectric material thus only partially bridging the electrodes, and is typically observed: in cavities, voids (bubbles) or gaps;
between interfaces of different dielectric properties;
at sharp electrode edges or protrusions.
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Electric arc video
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glow dischargedark discharge
Townsend regions normal glow abnormal glow
arc discharge
V
I
non‐thermal thermalbreakdown
voltage
glow‐to‐arctransition
background
saturation
nA µA mA A kA
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Slow motion video of lightning
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field homogeneity
inhomogeneous field (point‐to‐plane)……………….Case Ahomogeneous field (plane‐to‐plane)…………….. ….Case Bapproximately homogeneous field when the radius of curvature of the conductive parts is equal
or greater than 20 % of the associated clearance.
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figure A.1 — breakdown at high frequency in air at atmospheric pressure,
homogeneous field, 50 Hz – 25 MHz
breakdown voltage (Ub) is frequency‐dependent.
Ub degrades at 900 kHz, i.e., critical frequency.
worst‐case scenario at 2.5 MHz, i.e., f (min)
~80% retention rate
little effect on small clearances.
f (kHz)
Ub (kV peak)
Larger Clearances (d)
d = 4.0 mm
d = 3.5 mm
d = 3.0 mm
d = 2.5 mm
d = 2.0 mm
d = 1.5 mm
d = 1.0 mm
d = 0.5 mm
f (mains) 50 Hz
f (critical) 900 kHz
f (minimum) 2.5 MHz
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inhomogeneous field distribution• the corona discharge phenomenon in
inhomogeneous fields (point‐to‐plane) is much more intense than homogeneous (plane‐to‐plane) that can be observed by naked eye.
• the worst‐case Ub is about 50% of that at power frequency.
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0
10
20
30
40
50
60
330 400 500 600 800 1000 1200 1500 1600 1800 2000 2500 3000 4000 5000 6000 8000 10000120001500020000
Clearances (m
m)
Required Withstand Voltage or PEAK working voltage (V peak)
Method 1 (B/S)
Method 2 (B/S)
Method 3 (B/S)
Method 4 (B/S)
Method 1 (R)
Method 2 (R)
Method 3 (R)
Method 4 (R)
Legacy B/S
Legacy R
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electrical breakdown in gases, i.e., clearancestypically takes more than 100 μs to develop. power frequency (10 ms):
• a.c. (peak) and d.c. are virtually identical (98.77%).
high frequencies (16.7 μs):
• insufficient time to constitute complete breakdown.
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–
–
–
– +
+
+ionized air particle
–
ionization region
EC
+–
+–+–+–
excited air molecule
–
–
–
––
–
+
+
+ ++
electron
neutral air particles
if the clearances are large or at high frequencies, the ions might get trapped, resulting in the gradient distortion and consequent field strength weakening.
pseudo or phantom electrode
+
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figure C.3 — breakdown at high frequency, solid insulation; d = 0.75 mm; comparison on short‐time breakdown field strength Eb:
Eb (kV peak per mm)
60 Hz1 MHz100 MHz
f (kHz)
Higher Frequencies
50/60 Hz = 1 e
1 MHz = 0.66
100 MHz = 0.013
the bottom has not yet been reached!?
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solid insulation• compared to air insulation, solid insulation provides
at least a ten‐fold increase in electric strength.• however, in practice, a PD can occur in embedded
voids or air gaps in solid dielectrics at a PD‐inception voltage far below its breakdown voltage, reducing its voltage withstand ability, and likely resulting in the complete destruction of most solid dielectrics.
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modeling a gas‐filled void
• εr (air) = 1.0006;
• εr (PC, polycarbonate) = 2.3;
• εr (FR‐4) = 4.4;
C (remainder) >> C (void)
C (void) >> C (series)
for any dielectric of the same size:
V (void) = εr * V (dielectric)
111
1
r
)()(
)()(
td
VCC
CVVvoidseries
seriesvoid
dAC 0r
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1
6
7
2
5
8
3
4
void‐to‐tree transition
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the experiment
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Figure B.3 — Breakdown Voltage (Ub)Ub is less relevant to frequency factor
larger creepage distances do not add to the breakdown voltage, Ub
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0
5
10
15
20
25
30
100 125 160 200 300 400 500 600 700 800 900 1000 1100 1200 1300 1400 1500 1600 1700 1800
Creepage Distances (mm)
RMS Working Voltage (V r.m.s.) or PEAK Working Voltage (V peak)
Legacy (B/S)
30 kHz < f ≤ 100 kHz
100 kHz < f ≤ 200 kHz
200 kHz < f ≤ 400 kHz
400 kHz < f ≤ 700 kHz
700 kHz < f ≤ 1 000 kHz
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surface breakdown, i.e., creepage distancesunlike clearances, creepage distances and solid insulation are NOT replenishable — permanent damage such as puncture or carbonized tracking, is likely. in this sense, clearances are less vulnerable to PD.
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please humor me while I speculate…what does it mean by saying “creepage distances must be equal to or greater than clearances?”
geometry?geometry?
however, from this aspect it is difficult to understand what is the relationship between CL and CR.
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I would say when we’re talking about the creepage distances we have to keep in mind that we re talking about clearances as well.
air
solid insulation
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how creepage distances and clearancesinteract with each other?
air
solid insulation
–– –
–
–
15.5 eV(N2)
8.5 eV(PE)
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謝謝大家的時間!