BRIEFS
Date
FROM THE McGRAW-EDISON POWER CAPACITOR PLANT
GREENWOOD, SOUTH CAROLINA
August 1989
Issue
16
Evaluation Of Changes In Dielectric Stress Levels
Introduction
Changes i n c a p a c i t o r design s t r e s s l e v e l s have t r a d i t i o n a l l y been a cause f o r
concern among e v a l u a t i n g and s t a n d a r d s e n g i n e e r s .
The p e r c e p t i o n t h a t higher
stress l e v e l s t r a n s l a t e i n t o lower o v e r v o l t a g e p e r f o r m a n c e c a p a b i l i t i e s i s o n l y
t r u e i f a manufacturer i n c r e a s e s d e s i g n stress w i t h o u t a matching i n c r e a s e i n
product s t r e n g t h . This i s s u e of t h e Kilovar B r i e f s d i s c u s s e s what stress is, and
how t o e v a l u a t e design changes i n stress l e v e l s . Maintenance o f a manufacturer's
h i s t o r i c a l s a f e t y margins
t h e d i f f e r e n c e between s t r e s s and s t r e n g t h
is t h e
key i s s u e .
-
Stress
-
-
A Definition
A c a p a c i t o r , reduced t o i t s b a s i c form, c o n s i s t s of p a r a l l e l aluminum f o i l p l a t e s
s e p a r a t e d by a d i e l e c t r i c material. I n modern power c a p a c i t o r s , t h i s d i e l e c t r i c
If
c o n s i s t s t y p i c a l l y of two s h e e t s o f polypropylene f i l m .
See Figure 1A.
manufacturing p r o c e s s e s and raw m a t e r i a l s were p e r f e c t i n n a t u r e , t h e s e would be
t h e sole components o f t h e d i e l e c t r i c system. However, t h e n a t u r e of real world
p r o c e s s e s and m a t e r i a l s r e s u l t i n a d i e l e c t r i c system t h a t appears more l i k e t h a t
shown i n F i g u r e 1B.
Figure 1A
I d e a l D i e l e c t r i c System
Cross-Secti on
Figure 1B
Real i s t i c D i e l e c t r i c System
Cross-Secti on
The r e s u l t i n g s p a c e s , o r voids, which e x i s t between t h e l a y e r s o f f i l m and f o i l
have a much lower e l e c t r i c a l s t r e n g t h t h a n t h a t o f t h e f i l m , and w i l l l i k e l y
breakdown e l e c t r i c a l l y upon e n e r g i z a t i o n of t h e c a p a c i t o r . This w i l l c a u s e t h e
c a p a c i t o r t o f a i l r a p i d l y . To a c h i e v e a s u c c e s s f u l c a p a c i t o r design, a d i e l e c t r i c
f l u i d i s introduced t o t h e c a p a c i t o r . F l u i d has a h i g h e r d i e l e c t r i c s t r e n g t h t h a n
a i r , b u t s u b s t a n t i a l l y lower t h a n t h a t of t h e f i l m . The purpose of t h i s f l u i d is
t o occupy t h e s e v o i d s , preventing an e l e c t r i c a l f a i l u r e of t h e u n i t .
COOPER POWER SYSTEMS
Post Office Box 1224. Greenwood. SC 29648
Dielectric S t r e s s is defined as t h e v o l t a g e a c r o s s t h e two f o i l - p l a t e s divided by
t h e d i e l e c t r i c ' s t h i c k n e s s . At this p o i n t , two d i f f e r e n t methods o f c a l c u l a t i n g
stress must be evaluated. The f i r s t , r e p r e s e n t e d by Equation 1, includes o n l y t h e
f i l m t h i c k n e s s . For t h e p a s t 25 years, t h i s h a s been t h e o n l y way f i l m stress h a s
been computed. More r e c e n t l y a second method, r e p r e s e n t e d by Equation 2, which
i n c l u d e s b o t h t h e f i l m and t h e f l u i d i n t h e c a l c u l a t i o n has become popular.
Voltage Between F o i l s
Thickness of Film S h e e t s
E2=
Voltage Between F o i l s
Thickness of Film S h e e t s + Uniform F l u i d Layer Thickness
Equation 1
Equation 2
For b o t h Equations 1 and 2, t h e v o l t a g e i s commonly measured i n v o l t s , and t h e
m a t e r i a l t h i c k n e s s i n m i l s (thousands of an i n c h ) , g i v i n g t h e f a m i l i a r v o l t s / m i l
u n i t s of stress.
The Confusion of Including F l u i d Layers
A s can b e seen i n Figure lB, t h e s i z e of t h e spaces t o be occupied by t h e
d i e l e c t r i c f l u i d are not c o n s t a n t . A t some p o i n t s , t h e r e is a r e l a t i v e l y t h i c k
f l u i d l a y e r and a t o t h e r l o c a t i o n s , t h e f i l m and f o i l s a l l come i n d i r e c t c o n t a c t
with each o t h e r leaving a n e g l i g i b l e f l u i d l a y e r .
From t h i s , one can make two
o b s e r v a t i o n s:
1.
The v o l t a g e s t r e s s c a l c u l a t e d by E is dependent e n t i r e l y upon what
2
v a l u e i s assumed f o r t h e f l u i d - l a y e r .
2.
Where t h e f i l m and f o i l do come i n c o n t a c t with each o t h e r , t h e
true f i l m stress i s r e p r e s e n t e d by E (which i s a c t u a l l y a s p e c i a l
1
case of E ).
2
Even i f more s o p h i s t i c a t e d c a l c u l a t i o n s a r e performed, which c a l c u l a t e s e p a r a t e
f i l m and f l u i d stresses based upon t h e r e l a t i v e d i e l e c t r i c c o n s t a n t s of t h e two
materials, t h e f a c t remains t h a t t h e r e s u l t i n g numbers are f u l l y dependant upon a n
assumed uniform f l u i d l a y e r t h i c k n e s s ; which b o t h common sense and r e a l i t y prove
does n o t e x i s t .
To a v o i d any confusion, stress must b e c a l c u l a t e d as p e r Equation 1. This is t h e
h i s t o r i c a l l y accepted method o f stress c a l c u l a t i o n used by both manufacturers and
u s e r s . The consequence of u t i l i z i n g a f l u i d l a y e r i n o n e ' s c a l c u l a t i o n s i s t o
lower t h e r e p o r t e d true f i l m stress which makes t h e a c c u r a t e comparison of d e s i g n s
impossible.
Design Evaluation T e s t s
A l l manufacturers conduct a b a t t e r y of tests, t h e purpose of which i s t o determine
whether o r n o t a p a r t i c u l a r d e s i g n w i l l perform p r o p e r l y under realistic o p e r a t i n g
c o n d i t i o n s . These tests i n c l u d e t h o s e c a l l e d f o r by i n d u s t r y s t a n d a r d s , as w e l l
as t h o s e s p e c i f i c tests performed by i n d i v i d u a l manufacturers t h a t t h e y f e e l are
necessary t o prove t h e i r design. The i n t e n t o f t h e s e t e s t s is t o e v a l u a t e whether
a c a p a c i t o r h a s s u f f i c i e n t s a f e t y margins i n i t s design t o allow r e l i a b l e long
term performance.
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Figure 2 i l l u s t r a t e s t h e relationship of DIV1 and d i e l e c t r i c system thickness.
The dashed l i n e indicates the nominal voltage rating of the capacitor (normalized
t o volts per mil).
Figure 2
DIV Design Curve With
Design S a f e t y M a r g i n I d e n t i f i e d
:
I
-Thin
Thick
Dielectric System Thickness
The safety factor i n a capacitor design i s denoted by the distance between the
rated s t r e s s dashed l i n e (calculated by E l ) and the location on the D I V curve
corresponding t o the design's d i e l e c t r i c thickness.
Increasing Stress
-
The Manufacturers Challenge
I f only the s t r e s s (by Equation 1) i s increased i n a capacitor design, the dashed
l i n e moves up. Unless a manufacturer changes the design i n such a way as t o
maintain the safety margin, a l l t h a t has been accomplished i s a reduction i n t h a t
margin. This type of approach i s a risky undertaking t h a t may lead t o poor
product performance.
The Cooper Power Systems Approach
I n 1985, McGraw-Edison (CPS) began the manufacture of the type EX capacitor. This
design, u t i l i z i n g an extended f o i l mechanical crimp construction, was a major
investment i n the future of capacitor technology. Prior t o t h i s , McGraw-Edison
capacitors u t i l i z e d tab s t y l e construction.
However, the mechanical and
e l e c t r i c a l discontinuities inherent i n a tab s t y l e design r e s t r i c t e d the a b i l i t y
t o move into advanced high performance d i e l e c t r i c systems.
The extended f o i l
mechanically crimped internal construction allowed the stacking factor t o be
increased with no corresponding s t r e s s increase.
A s a r e s u l t , there were
substantial improvements i n D I V (strength) along with reduction i n losses.
introduced i n 1987, represented a 7% increase i n
The EX-7 capacitor,
McGraw-Edison's design s t r e s s . To achieve t h i s s t r e s s increase, the following
changes were made i n our design.
1.
2.
3.
4.
5.
A new d i e l e c t r i c f l u i d , Edisol XT, was developed.
A new type of surface altered polypropylene film was developed
which permitted greater control of the stacking factor.
The stacking factor of the d i e l e c t r i c system was raised t o 89%.
Almost $1 million was spent on new impregnation equipment which
provides for improved process control.
2
The d i e l e c t r i c system thickness was reduced
.
1 D i s c h a r g e Inception Voltage - the voltage of which a capacitor goes i n t o corona
internally.
2 For
a more complete discussion of dielectric system thickness, see Kilovar Brief
Issue #7, dated June 1986.
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The effects these changes had on the DIV curve are shown in Figure 3.
Elevated D I V Curve
Due to Edisol XT
and Higher Stacking Factor
Figure 3
EX-7 Capacitor
and Increased Stress
Maintained
(EX)
I
I
I
E X - 7 EX
Thin
Thick
Dielectric System Thickness
To obtain outstanding field performance, it is essential to maintain historical
safety margins. Maintaining these safety margins while increasing dielectric
stress levels requires pushing the technology forward with design innovation.
Tighter QA procedures and improved housekeeping, though part of any efficient
manufacturing process, are not substitutes for the design innovations required
when an increase in a capacitor's stress level is being considered.
Conclusions
In evaluating the dielectric voltage stress levels in modern all film capacitors,
the following factors must be made part of the evaluation.
1.
Design stress should always be calculated as:
Stress =
Voltaqe Between Foils
Thickness of Dielectric Film Sheets
2.
Increases in a capacitor's stress level must be matched with
corresponding increases in the capacitor's DIV in order to maintain
historical safety margins. It is essential that the user becomes
familiar with the design innovation applied in the product.
3.
Design innovations to the dielectric system (film, foil and fluid)
are required to increase DIV and therefore to maintain the design's
historical safety margin. These innovations are only possible
through a long term, consistent R & D program.
4.
A manufacturer must test any new product to a set of design rules
which the manufacturer feels is required to prove the product's
performance under real life operation conditions. These tests are
what a manufacturer uses to evaluate designs and as such, must not
change. Doing so indicates a change in the manufacturer's safety
margin philosophy.
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