ieee-deis (jun2006) electrical breakdown in hv winding insulations

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„Electrical Breakdown in High-Voltage Winding Insulations of DifferentManufacturing Qualities “

R. Vogelsang,andT. Weiers,andK. Fröhlich,

andR. Brütsch.

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May/June 2006 — Vol. 22, No. 3

F E A T U R E A R T I C L E F E A T U R E A R T I C L E F E A T U R E A R T I C L E F E A T U R E A R T I C L E F E A T U R E A R T I C L E

D

The results show that electrical tree

propagation is the main electrical

degradation mechanism that leads to

breakdown of the main wallinsulation, and that poor

impregnation of the insulation and

type of taping can reduce time to

breakdown significantly.

Introductionue to an increasing demand for new, or the refurbish-

ment of, power stations, high-voltage rotating machines

continue to play a significant role in generating electrical en-

ergy. An important component of these machines is their electri-

cal insulation. Although much research has been done to im-

prove the material, about a quarter of all failures still are related

to insulation problems [1] – [3]. The state of the art of winding

insulation is a composite of mica, binder resin, and a support

material, the so called mica tapes. A peculiarity of the insulation

is that the tapes are intermediate products, which usually are

produced by a different company than the manufacturer of the

final insulation or the entire machine. Such a discontinuity in

the production process results in many different types of insula-

tion systems and a variation in manufacturing quality [4] – [6].

Research to improve winding insulation is very complex be-

cause a large number of factors influence insulation life, among

them the raw materials, size of the mica tapes, geometrical char-

acteristics of the bar and taping- or manufacturing techniques

[6]. Additionally, the different service stress factors—electrical,

thermal, mechanical, and ambient load—also influence the life-

time of the insulations [6] – [10]. Although electrical degrada-

tion and breakdown in polymers have been studied extensively,the process that leads to failure in the composite structure of

industrially made winding insulations has not yet been conclu-

sively described. Especially, the influence of manufacturing qual-

ity is not fully understood. This is mainly because reliable re-

search on industrially made winding insulation is complex due

to the already-mentioned large number of influencing factors.

Additionally, test samples are expensive and their number often

is very limited, particularly when they come directly from the

production line of different companies.

Electrical Breakdown in High-Voltage

Winding Insulations of Different

Manufacturing Qualities

Key Words: high-voltage rotating machine insulation, ground (main) wall insulation, VPI,resin-rich, machine-taped insulation, hand-taped insulation, electrical treeing, time to

break down

R. Vogelsang, T. Weiers, K. Fröhlich,

and R. Brütsch

In order to provide a basis for material improvements, the first

aim of this work is to describe the influence of manufacturingquality on time to breakdown of high-voltage winding insula-

tion. Because tests accredited under IEEE Standard 1043 re-

quire much effort and are very expensive, a second aim of the

work is to use the knowledge about the main electrical degrada-

tion mechanism to develop a method that allows testing of such

insulation systems with less effort. The new test method should

be developed to allow investigation of industrially manufac-

tured insulation systems from the production line and should be

easily usable by industrial companies.



6 IEEE Electrical Insulation Magazine

Structure and Manufacture of High-Voltage

Winding InsulationsThe main wall insulation for high-voltage (hv) rotating ma-

chines consists of three basic components—a barrier that is resis-

tant to partial discharges and tree growth, a support material that

gives mechanical strength, and a binder resin that fills any voids

between the mica and the support material. These materials are

produced as tapes and are wrapped around the surface of the

conductor to form the insulation [4], [5]. The wrapping process is

made either by hand or by machine as shown in Figures 1 and 2,respectively.

When the bar is fully covered by the tape, any spaces between

the mica tapes are filled with binder resin in order to reduce air

inclusions. For this purpose, two main technologies, resin-rich

and vacuum-pressure impregnation (VPI) are used [4], [5]. When

the insulation has been formed, the final bar is finished with a

corona protection for the slot- and end-winding coatings [4], [7].

Resin-rich bars are pressed under high temperatures and high

pressure. Therefore, the bars have an almost rectangular shape, a

tight arrangement of the mica tapes, and the whole cross section

of the insulation is impregnated with similar quality [4], [5]. A

disadvantage of the resin-rich process is the machine effort, which

is high especially for bent end-winding sections [4].

For VPI insulations, the whole bar is first set under vacuum,

followed by impregnation with resin under high pressure. An

advantage of VPI insulations is that the mica tapes are not sharply

bent during the pressure process. In addition, cavities at the bars

also are filled, especially when applying global VPI, in which

the whole machine is impregnated and cavities between the bars

and the slots of the machine also are filled with resin. Therefore,

the VPI-process is very suitable to impregnate bars with a com-

plex form. However, the resin may not impregnate all voids in

the insulation, and resin bubbles or voids are likely to form cracks

as the resin shrinks during cure. A disadvantage of the global VPI

process is its requirement for large and expensive equipment [4],[5]. After forming the insulation, the final bar is finished with a

corona protection for the slot- and end-winding coatings [4], [7].

The internal structure of the winding insulation with half-lap

tape wrapping and conductive coating is illustrated in Figure 3.

The combination of mica tapes and impregnation determine

the properties of the insulation [4] – [7], [11]. As a result of the

many combinations of mica, support material, resin and impreg-

nation, there is a great variety of mica tapes available on the

market [4] – [7].

Determining Tree Propagation by a TestArrangement with an Embedded Electrode

A. BasicsWhen voltage is applied, the breakdown process can be di-

vided into “tree inception” and “tree propagation” [6], [8], [12] –

[16]. As shown in Eqn. (1), the time to breakdown, Tbd

, depends

on the time interval for tree inception, Tti, and tree propagation,

Ttp:

T bd

= T ti + T

tp(1)

Electrical treeing is a process by which the insulation mate-

rial is eroded by discharges taking place over a long period [13].

However, the start of the erosion begins with inception of dis-

charges in the bulk, at the inner conductor or at the surface of theinsulation [7] – [10],[17]. Generally, it can be said that the result-

ing time to breakdown of the insulation is long when tree incep-

tion and tree propagation time intervals are long, and it is short

when both intervals are short. In a hv winding insulation, tree

inception is mainly influenced by the coating or the conductor,

and tree propagation is mainly dependant on the insulation ma-

terial.

B. Measurement of Tree Propagation by a Test

Arrangement with an Embedded ElectrodeAs outlined theoretically, the tree propagation time repre-

sents a measure for the material’s insulation resistance againsttreeing. Therefore, the insulation properties can be determined

by merely measuring the time interval a tree needs to propagate

through the material. In order to determine that value, an ar-

rangement with an embedded electrode was developed, which

causes tree inception immediately after voltage application. The

electrode is a copper sheet of 20 x 20 mm2, with a thickness of 0.2

mm, which was embedded in the insulation by the insulation

manufacturer. In contrast to former arrangements [18], the em-

bedded eledctrode was set directly in the insulation in order to

Figure 3. Structure of a winding insulation.

Figure 1. Mica tape

wrapping by hand [5].

Figure 2. Mica tape wrapping

by machine [5].




avoid surface discharges. The principle design of the arrange-

ment is illustrated in Figure 4, and an actual cross-section is

shown Figure 5. The advantages of the arrangement with an em-

bedded electrode are that no surface discharges occur and that

the insulation properties are tested directly [6].

The edges of the embedded copper sheet have a radius of <10

µm, which ensures that the tree incepts immediately (Tti < 30 s)

after voltage application. Because the time to tree inception is

practically zero, the total measured time to breakdown is the

time for the tree to propagate through the material. Therefore,

such tests are referred to as “Electrical treeing tests” [6].

For all tests, insulation bars with a length of 1 m and a con-

ductor cross section of 8 x 39 mm2 were used. In each bar, four

electrodes were uniformly embedded into the insulation. In or-

der not to propagate in neighboring tree channels, the electrodes

were placed >100 mm from the end of the bar and > 200 mm

separation from each other [6].

C. Reference to Conventional Tests and Degradation Mechanism in the Insulation

To give a reference to current test standards, the results of the

electrical treeing tests were compared to results with the same

type of bars prepared according to IEEE Standard 1043 [19],

[20]. Tests according to the standards usually are made with three

times the rated voltage, and they are referred to as “3Un-tests”.

Samples for the 3Un-tests were prepared with outer conductive-

and field-grading coating [6], [11]. In the tests, three different

insulation systems were chosen. For better comparability, the

size of the insulation bar, insulation thickness of 2 mm, tempera-

ture of 20 ± 5°C, and voltage of 32 kV rms were kept the same for

each insulation system. There were 30 test specimens for the

3Un-tests, and 12 for the electrical treeing tests. Detailed infor-

mation about the test setup is given in [6], [8].

Figure 6 compares the Weibull-plots for the times to break-

down of the electrical treeing tests and the 3Un-tests for the

various materials. The results show that, for all three materials,

the times to breakdown for electrical treeing and 3Un-tests are

not significantly different as the 95% confidence intervals of the

63% quantiles overlap. However, there is significant variationbetween the three materials. The longest times to breakdown

were measured with material 3, followed by material 2 and mate-

rial 1. The higher scatter in the treeing tests is explained by the

lower number of samples. The significant differences between

the times to breakdown of the three materials show that the tree

propagation is different for each insulation. Because they vary

in their material properties, it can be concluded that the treeing

test indicates significant differences in the insulation properties.

Because the tree inception time in the treeing tests is zero,

there is strong evidence that electrical tree propagation is a main

determining degradation process in the high voltage winding

insulation. Therefore, it must be considered that, from the begin-

ning of voltage application, tree propagation takes place in test

specimens, even in the bars that were prepared with an outer

coating. When treeing takes place, many channels propagate

through the material [6], [11]. Because the tree cannot easily

penetrate an intact mica tape, it mainly propagates along them

and to the next layer at regions of tape overlaps, which is shown

in Figure 7.

For application of the test, it must be mentioned that the elec-

trodes usually cannot be embedded during wrapping as described

previously. For already manufactured bars, the electrodes must

Figure 4. Principle of sample for tests with the embedded,

high-voltage electrode.

Figure 5. Photograph of specimen with embedded, high-

voltage electrode.

Figure 6. Weibull-plots for the times to breakdown for 3Un,

and treeing tests for three different winding insulations.




be embedded afterward. Therefore, a method was developed to

place the copper sheet in the insulation on a manufactured bar.

Because this is very relevant, tests with subsequently-introduced

electrodes at the High Voltage Laboratory of the ETH Zürich

were carried out. The times to breakdown show no significant

difference to those shown in Figure 6 [6]. Therefore, it is also

possible to implement the electrodes in already manufactured

bars.

Furthermore, treeing tests give a detailed answer for material

properties of the main wall insulation. They are not suited for

tests considering the entire bar with slot and endwinding relief

coatings as described in the IEEE Standard 1043 (such as cus-

tomer acceptance tests).

The advantages of treeing tests compared to conventional

tests are that they require much less material because fewer bars

are needed, they represent a material test because tree propaga-

tion through the insulation is measured, and that they allow

investigation of particular regions of the bar, such as end-wind-

ings, bends, or straight sections separately. Electrical treeing tests,

therefore, are recommended to complement conventional tests

in which only a low number of samples is accessible, such as for

random tests in the production line; determination of residual

dielectric strength of bars after service; investigating the ten-dency of some influencing factors in research laboratories; fin-

gerprint-tests for newly developed material compositions; and

to investigate the insulation properties at particular regions of

the bar in order to find weak spots in the insulation.

Influence of Manufacturing Quality on Time

to Breakdown of Winding Insulations

A. VPI InsulationThe VPI insulations tested were the same material with differ-

ent types of manufacturing, such as:

· reference manufacturing—taped by machine and supervisedby a skilled worker at the application laboratory of the mica

tape producer (12 specimens),

· industrially taped by machine in company no. 2 (4 speci-

mens),

· industrially taped by hand in company no. 1 (10 specimens).

Figure 8 shows the Weibull-plots for the times to breakdown

for the VPI insulations. The results in Figure 8 show that there is

no significant difference in the times to breakdown for the refer-

ence- and the machine-taped insulation. This proves that indus-

trial manufacturing with machine taping (company no. 2) is of ahigh quality. In contrast, significantly lower times to breakdown

were measured for the hand-taped material. The difference for

the 63% quantiles is more than two orders of magnitude. To find

the reason for such behavior, the microscopic structure of the

insulations was analyzed. Figure 9 shows a micrograph of the

reference insulation. Figure 10 is a micrograph of the machine-

taped insulation, and Figure 11 is a micrograph of the hand-

taped insulation.

The micrographs clearly show that the reference material is

without any voids or imperfections, similar to the industrially -

manufactured machine-taped material from company no. 2. In

contrast, the hand-taped material as manufactured by company

no. 1 shows delaminations in nearly every layer of the insula-

tion. The reason for such a difference is assumed to be the varia-

tion of the wrapping tension of the tapes. With hand taping, it is

not possible to have such a constant taping tension as with ma-

chine taping. This can lead to loose attachment of the tapes, and

the binder resin cannot fill the regions between the tapes suffi-

ciently. In that area, an electrical tree can propagate quickly, thus

causing very short times to breakdown. The break-down values

and the micrographs confirm a significant influence of the tap-

ing on the potential lifetime of winding insulations.

B. Resin-Rich InsulationsTo consider the influence of manufacturing on the times to

breakdown of resin-rich insulations, two different materials were

chosen, each with a different type of tapings:

· reference manufacturing—taped by machine and supervised

by a skilled worker at the application laboratory of the mica

tape producer (8 specimens),

· same material as the first, but industrially taped by machine

in company no. 3 (8 specimens),

· material no. 2 of Figure 6 as reference manufacturing—taped

by machine and supervised by a skilled worker at the applica-

Figure 7. Tree propagation through winding insulation.

Figure 8. Weibull-plots for times to break down with a VPI.




tion laboratory of the producer of the mica tapes (12 speci-

mens),

· same material as the previous, but industrially taped by hand

in company no. 4 (10 specimens).

Figure 12 shows the Weibull-plots for the times to breakdown

of resin-rich insulations with the type of taping as a parameter.

The results show that there is no significant difference in the

times to breakdown for the machine-taped insulations. In con-

trast, the times to breakdown for the hand-taped material are

much lower than those of the reference specimens. The differ-

ence in the 63% quantiles is more than four orders of magnitude.

To identify reasons for this difference, the microscopic structure

of the insulations also was analysed. Figures 13–16 show micro-

graphs of the tested resin-rich insulations. The micrographs show

that the structure of both insulations with reference manufactur-

ing and the machine-taped material of company no. 3 are similar.

All materials have good adhesion of the tapes, and no voids or

delaminations are present within the insulation. In contrast, the

hand-taped insulation of material no. 2, company no. 4 shows

many voids in the insulation (Figure 16). Such a poor structure is

very likely responsible for the dramatic drop in the times to

breakdown shown in Figure 12. The reason is assumed to be the

poor pressing cycle. As shown for VPI insulations, hand-tapingcan cause poor quality with voids and/or delaminations. An elec-

trical tree propagates very quickly along such regions and leads

to short times to breakdown.

The results for resin-rich materials show that the type of manu-

facturing can have a significant influence on insulation life. It

can be seen that the sensitivity to poor preparation is much more

severe for resin-rich than for VPI insulations. For resin-rich mate-

rials, the binder resin must fill the voids that arise during wrap-

ping. If the volume of the voids is larger than the resin content in

the tapes, all the voids cannot be filled, thus leaving voids and

delaminations in the material. It is therefore concluded that tol-

Figure 9. Structure of VPI insulation: reference material.

Figure 10. Structure of VPI insulation: industrially

manufactured with machine taping (company no. 2).

Figure 11. Structure of VPI insulation: industrially

manufactured with hand taping (company no. 1).

Figure 12. Weibull-plots for the times to breakdown of resin-

rich materials: reference and industrially taped.




Figure 13. Structure of resin-rich insulation: reference

material.

Figure 14. Resin-rich insulation: industrially taped by

machine (company no. 3).

Figure 15. Structure of resin-rich insulation—material no. 2:

reference taping.

Figure 16. Resin-rich insulation of material no. 2: industrially

taped by hand (company no. 4) and poorly pressed.erances in taping are more critical for resin-rich than for VPI

insulations.

It also should be mentioned that 4 of the 10 samples in Figure

12 would have passed the 1-minute proof tests according to IEC

Standard 60243-1, [21], and IEC Standard 1212.2 [22]. Although

an estimation of the insulation life based on the power law may

not always lead to valid results [23], these insulations cannot be

expected to be reliable. Even if there would be no rapid electric

breakdown, additional thermal stresses would hasten the delami-

nation of loosely attached tape layers [6], and/or mechanical

vibrations contribute to crack formation near resin-filled cavi-

ties [6], [24], [25], thus causing a further reduction in the time to

breakdown.

ConclusionsFor the breakdown of high-voltage, winding insulation and

the influence of manufacturing quality on the time to break-

down, the following conclusions are made:

· Electrical tree propagation is the main electrical degradation

mechanism leading to breakdown of high-voltage, winding

insulation. To increase insulation life, tree inception should

be delayed, and tree propagation must be slowed down. A

delay of tree inception can be achieved by preventing voids

at the conductors. To slow down tree propagation, the mica

content of the insulation and the number of tape layers must

be as high as possible.

· The newly developed method with an embedded electrode

allows testing of insulation with far less effort than the stan-

dard test according to IEEE Standard 1043. It is recommended

that the new test be used for tests of insulation life in which

only small numbers of samples are available, which might bedue to time, financial, and/or material reasons.

· As impregnation of the insulation has a very significant effect

on the times to breakdown and hand taping can lead to weak

points inside the main-wall insulation, machine taping is rec-

ommended. If hand taping is used, VPI tapes should be ap-

plied as they are found to be less sensitive to lower taping

quality.

· Because manufacturing by different companies may reduce

the times to breakdown significantly, insulation quality



May/June 2006 — Vol. 22, No. 3 1

should be monitored by random tests in the production line.

As it requires much less material, tests with an embedded

electrode are recommended for this purpose, and the resulting

times to breakdown should be compared to reference values

given by the manufacturer of the mica tapes.

· At three times the rated voltage, breakdown in insulations

with poor quality occurred within a few minutes; but, in insu-

lations with good quality, it took up to 5000 hours. The dura-

tion of the standard 1-minute proof test therefore should be

extended to about 10 minutes. Insulations with poor quality,

thereby, could be identified, but high-quality insulations

would experience no significant degradation.

AcknowledgementsDr. Vogelsang gratefully acknowledges the support of the Swiss

Commission for Technology and Innovation (CTI), Von Roll Isola

AG and PD Tech Power Engineering AG. In particular, the inten-

sive cooperation and stimulating discussions with Dr. B. Fruth

(PD Tech Power Engineering AG, Stetten), Dr. T.H. Teich (High

Voltage Laboratory of the Swiss Federal Institute of Technology

Zurich – ETH) Prof. H. Sauvain (University of Applied Science

of Fribourg), and Dr. C.-E. Stephan (Alstom Power AG Birr, all

from Switzerland) are greatly appreciated.

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Ruben Vogelsang (M’01) was born in

1972 at Annaberg-Buchholz, Germany. He

studied electrical engineering at Dresden

University of Technology (TUD), Germany

and the University of Sheffield, England.

From 1999 to 2000, he worked at the TUD

on the development of polymeric cables for

voltage transmission. In 2000, Ruben

Vogelsang joined the Swiss Federal Institute

of Technology (ETH) Zurich, Switzerland,as a research and teaching assistant and completed his Ph.D. in

2004. In parallel to his work for the Ph.D. degree, he studied

business at the ETH Zurich and gained in 2004 the degree of

Master of Business Administration (MBA). Since 2005, Dr.

Vogelsang is working as R&D project manager in the generator

circuit breaker division at ABB Switzerland Ltd. in Zurich, Swit-

zerland. He is a member of IEEE and can be reached at

[email protected].




Tilman Weiers was born in 1978 in

Freiburg, Germany. From 1998, he studied

Physics at the University of Stuttgart and

graduated with a diploma (Dipl.-Phys.) in

2003. Hr then joined the Swiss Federal In-

stitute of Technology (ETH) as a research

and teaching assistant. He can be reached

at [email protected].

Prof. Dr. Klaus Fröhlich (F’02) was

born in 1945 in Salzburg, Austria. He re-

ceived the M.Eng. and Ph.D. degrees in tech-

nical science from the Vienna University of

Technology, Austria. After 11 years in

Switchgear and High Voltage Technology

with BBC (later ABB) in Switzerland, he

became a full professor at the Vienna Uni-

versity of Technology in 1990. Since 1997

he has been a full professor of High-Voltage Technology at the

Swiss Federal Institute of Technology Zurich, Switzerland. Klaus

Fröhlich is a Fellow of IEEE and chairman of CIGRE Study Com-

mittee A3 (High Voltage Equipment). He can be reached at

[email protected].

Rudolf Brütsch was born in 1943. He

studied Chemistry at the University of Bern

and received his Ph.D degree in 1972 from

the same university. Between 1972 and

1986, Dr. Brütsch worked in various R&D

positions on metal and ceramic materials.

In 1986 he joined Von Roll Isola AG in

Breitenbach, Switzerland and is today man-

ager for technical marketing of the busi-

ness unit—electrical of that company. Dr. Brütsch can be reached

at [email protected].

ieee-deis (jun2006) electrical breakdown in hv winding insulations

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