High Frequency Voltage Stress. Presented by: Flore Chiang Date: March 30, 2012

Transcription

1 High Frequency Voltage Stress Presented by: Flore Chiang Date: March 30, 2012

2 Now the additional data is available!

3 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?

4 IEC :2005 (2nd Ed.) Any type of periodic voltages with a fundamental frequency (f 0 ) above 30 khz and up to 10 MHz Key phenomenon = partial discharge (PD) Its deteriorating effect is aggravated roughly in proportionally to the frequency

5 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.

6 Electric arc video

7 V dark discharge glow discharge arc discharge breakdown voltage Townsend regions normal glow abnormal glow nonthermal thermal glow to arc transition saturation background na µa ma A ka I

8 Slow motion video of lightning

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10 field homogeneity inhomogeneous field (point to plane).case A homogeneous field (plane to plane)...case B approximately homogeneous field when the radius of curvature of the conductive parts is equal or greater than 20 % of the associated clearance.

11 figure A.1 breakdown at high frequency in air at atmospheric pressure, homogeneous field, 50 Hz 25 MHz Ub (kv peak) f (mains) f (critical) f (minimum) 50 Hz 900 khz 2.5 MHz d = 4.0 mm breakdown voltage (U b ) is frequency dependent. U b degrades at 900 khz, i.e., critical frequency. worst case scenario at 2.5 MHz, i.e., f (min) Larger Clearances (d) d = 3.5 mm d = 3.0 mm d = 2.5 mm d = 2.0 mm d = 1.5 mm ~80% retention rate d = 1.0 mm little effect on small clearances. d = 0.5 mm f (khz)

12 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 U b is about 50% of that at power frequency.

13 Clearances (mm) 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

14 electrical breakdown in gases, i.e., clearances typically takes more than 100 μsto 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.

15 + + + ionization pseudo or region phantom electrode + excited air molecule ionized air electron particle neutral air particles E C if the clearances are large or at high frequencies, the ions might get trapped, resulting in the gradient distortion and consequent field strength weakening.

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17 figure C.3 breakdown at high frequency, solid insulation; d = 0.75 mm; comparison on short time breakdown field strength E b : Eb (kv peak per mm) 50/60 Hz = 1 e 1 MHz = MHz = the bottom has not yet been reached!? Higher Frequencies 60 Hz 1 MHz 100 MHz f (khz)

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19 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.

20 modeling a gas filled void C r 0 A d ε r (air) = ; ε r (PC, polycarbonate) = 2.3; ε r (FR 4) = 4.4; C (remainder) >> C (void) C (void) >> C (series) for any dielectric of the same size: V (void) = ε r * V (dielectric) V ( void ) V C C ( series ) ( series ) C ( void ) V d r t 1

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22 1 void to tree transition

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25 the experiment

26 Figure B.3 Breakdown Voltage (U b ) U b is less relevant to frequency factor larger creepage distances do not add to the breakdown voltage, U b

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28 30 RMS Working Voltage (V r.m.s.) or PEAK Working Voltage (V peak) 25 Creepage Distances (mm) 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 khz

29 surface breakdown, i.e., creepage distances unlike 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.

30 please humor me while I speculate what does it mean by saying creepage distances must be equal to or greater than clearances? geometry? however, from this aspect it is difficult to understand what is the relationship between CL and CR.

31 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

32 how creepage distances and clearances air interact with each other? 15.5 ev (N 2 ) 8.5 ev (PE) solid insulation

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34 謝謝大家的時間!