15. The Insulation of Highvoltage Transformers: Some
Aspects of Current Research
전기공학과 이 승 수
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15.1 Introduction
15.1.1 Transformer insulation : basic requirement
Specified maximum voltage
Transformer
Over-voltage of
indeterminate magnitudes
Lightning and switching
operation
Insulation of transformer
- high-voltage windings increase the overall
dimensions
- Increased insulation space results in increased
size of the windings and core of the transformer
- since good electrical insulators are usually poor
heat-conductors, the conductors in windings
having a high insulation level must generally
be run at reduced current densities.
Insulating materials
Thermal, mechanical and
other environmental conditions
- the insulation itself, also, represents an
appreciable part of the cost of the materials
- there is thus a considerable incentive for using
the dielectric materials in a transformer with
maximum efficiency, thereby minimizing the
the quantities required.
15.1 Introduction
15.1.2 Dielectric strength
Insulation assembly
- usually in transformers, two or three dielectric
materials are used in combination
Transformer oil
Normal stress
- dielectric failure, if it occurs, results from the
highest local dielectric stress, or voltage gradient,
in an insulating medium
Residual air pocket
Regional high stress
Maximum stress
Damage
breakdown
High-voltage test
If discharges do occur in oil regions, their
consequent high energy may cause
deterioration of the surrounding dielectric
15.1 Introduction
15.1.3 voltage conditions
Transformer insulation
Service voltage
Over-voltage
Lightning surge
1/50 us ‘full’ impulse
‘chopped’ impulse
Switching surge
Power-frequency
test
15.2 Dielectric strength and dielectric field analysis
15.2.1 tests, test condition and measurements
- insulation tests may be in the nature of ‘withstand’ tests, to verify a specified minimum
dielectric strength as in commercial acceptance testing
Impulse test
- breakdown strength is conveniently determined by ‘front of wave’ impulse tests (in which
the test specimen break down on the rising front of the impulse)
-In impulse tests on dielectric samples with impulses of progressively increasing amplitude,
the increments of voltage, and the number of shot applied at each voltage level are significant
15.2 Dielectric strength and dielectric field analysis
15.2.1 tests, test condition and measurements
Switching surge
- generally considered to have durations of the order 50/1000 ~ 200/5000 us, it has been tacitly
assumed that the insulation strength lies between those for power-frequency and impulse
voltages
-To obtain consistent results in laboratory tests, solid insulation is dried and vacuum
impregnated, preferably under standardized conditions equal to the best commercial treatment
which can applied.
-Transformers in service may operate up to about 90℃ maximum oil temperature. Insulation
tests for design purposes may be made over a temperature range from ambient up to 90℃
or over
15.2 Dielectric strength and dielectric field analysis
15.2.2 Partial discharges
- at sufficiently high stress, discharges occur in oil or oil-immersed insulation even though no
gas cavities are initially present; they will, of course, do so much more readily if incomplete
impregnation has left left unfilled air pocket in the solid dielectric.
- fig 15.5 shows the effect on the surface of a
bakelized paper sheet after 5min gassing discharge
in an adjacent 3mm oil gap at the discharge
inception voltage
-It will be seen that a concentration of stress at the
edges of the recessed part of the latch resulted in
locally intensified discharge and marked increase in
damage
15.2 Dielectric strength and dielectric field analysis
15.2.2 Partial discharges
whilst the ‘radio influence voltage’(R.I.V) measurement does not give a correct quantitive
indication of the magnitude or energy of internal discharges in insulation
transformer (i) gave satisfactory performance ;
but in (ii) a flashover subsequently took place in oil,
at maximum over-voltage, due to inadequate
clearance from a temporary test bushing fitted to a
high-voltage cable terminating box
15.2 Dielectric strength and dielectric field analysis
15.2.3 development testing
- development testing usually involves carrying out breakdown tests on a number of more or
less simple insulation samples representing as closely as possible the insulation
-Measurements of breakdown strength on nominally identical insulation test pieces, again,
invariably show a considerable dispersion in the result
-On the basis of conventional ‘development testing’ (the combination of a large number of tests
on component parts of the insulation system, tests on more elaborate insulation assemblies,
and tests on complete full-scale prototype transformers) it is necessary to build up a
considerable ‘stock’ of design information; and such data can never really be complete so as
to cover all future designs.
15.2 Dielectric strength and dielectric field analysis
15.2.4 field analysis approach
- By ‘contouring’ of conductors to reduce stress concentrations
- by insertion of higher dielectric strength insulation at high stress points
- by selection of materials of appropriate permittivities to obtain more uniform voltage gradients
15.2 Dielectric strength and dielectric field analysis
15.2.4 field analysis approach
- Since the transformer winding are rotationally
symmetrical some small error is caused by ignoring
this factor
-By far the greatest error arises from the simulation
of the high-voltage lead
-The high-voltage lead itself on the other hand,
being in reality a comparatively small diameter
conductor which (away from the high-voltage winding)
would considerably disturb the natural field potential only
in its immediate vicinity, would be subjected to a high
local stress
15.3 Transient overvoltages
15.3.1 surge analysis
- Coil 과 earth 사이의 캐패시터 Cg와 연속적인 coil 사이의
캐패시터 Cs의 비율로 서지 전압의 초기 전압 분배
-두 캐패시터가 일정하고 권선 길이 l에 연관되고
‘space constant’
l /
라고하면 초기 전압은 다음과 같이
나타낼 수 있다.

Cg
Cs
15.3 Transient overvoltages
15.3.1 surge analysis
- 4-uS 간격으로 전압을 인가
- 20 uS에서 한계치
- 비슷한 속도로 되돌아옴.
15.3 Transient overvoltages
15.3.1 surge analysis
- ‘chopped wave’ impulse 인가
- 앞쪽 단은 캐패시터에 의해 감소
- 절연파괴를 방지하기 위해 coil edge 부분 강화
- 강화시킨 끝 부분에서 surge impedance의 증가로
인한 impulse 반사파 발생
15.3 Transient overvoltages
15.3.1 surge analysis
15.3 Transient overvoltages
15.3.2 methods of surge investigation
-이론적인 분석과 계산
- 실제 모델에 low-voltage impulse 인가, 측정
- high-voltage production impulse test
- 인가된 파형의 모습 유지로 절연 파괴 유무 판단
-A, B, C 세 지점에서 인가된 파형과 다름
- low-voltage recurrent surge technique에 의해
절연파괴 의심 지역을 발견
-A 지점에서는 발견 실패
- B, C 지점에서 피크치 발견, 절연파괴 지점과 일치
15.3 Transient overvoltages
15.3.2 methods of surge investigation
-Superimposed high-frequency disturbance or
‘smooth’ and identical current waveforms 에 의해
부분방전 발생
- 20 ~ 30 uS에서 다소 과도한 전압 측정
- 부분방전은 impulse test동안 발견할 수 있고
실제 절연파괴 전의 high stress의 증거로 명확함