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15. The Insulation of High-voltage

Transformers: Some Aspects of Current

Research

15. The Insulation of High-voltage

Transformers: Some Aspects of Current

Research전기공학과 이 승 수

15.1 Introduction

15.1.1 Transformer insulation : basic requirement

Transformer

Specified maximum voltage

Over-voltage of indeterminate magnitudes

Lightning and switching operation

Insulating materials

Thermal, mechanical and other environmental conditions

- 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.

- 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.

Insulation of transformer

15.1 Introduction

15.1.2 Dielectric strength

- usually in transformers, two or three dielectric materials are used in combination

- dielectric failure, if it occurs, results from the highest local dielectric stress, or voltage gradient, in an insulating medium

Insulation assembly

Maximum stress

Damage breakdown High-voltage test

Transformer oil

Normal stress

Residual air pocket Regional high stress

If discharges do occur in oil regions, theirconsequent high energy may cause deterioration of the surrounding dielectric

15.1 Introduction

15.1.3 voltage conditions

Service voltage Over-voltage

1/50 us ‘full’ impulse ‘chopped’ impulse

Lightning surge Switching surge

Transformer insulation

Power-frequencytest

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 Cg 와 연속적인 coil 사이의

캐패시터 Cs 의 비율로 서지 전압의 초기 전압 분배

-두 캐패시터가 일정하고 권선 길이 l 에 연관되고

‘space constant’ 라고하면 초기 전압은 다음과 같이

나타낼 수 있다 .

s

g

C

C

/l

15.3 Transient overvoltages

15.3.1 surge analysis

- 4-uS 간격으로 전압을 인가

- 20 uS 에서 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 impedance 의 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

- 인가된 파형의 모습 유지로 절연 파괴 유무 판단

-A, B, C 세 지점에서 인가된 파형과 다름

- low-voltage recurrent surge technique 에 의해

절연파괴 의심 지역을 발견

-A 지점에서는 발견 실패

- B, C 지점에서 피크치 발견 , 절연파괴 지점과 일치

-이론적인 분석과 계산

- 실제 모델에 low-voltage impulse 인가 , 측정

- high-voltage production impulse test

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 uS 에서 다소 과도한 전압 측정

- 부분방전은 impulse test 동안 발견할 수 있고

실제 절연파괴 전의 high stress 의 증거로 명확함