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1 . Occurrence of Discharge

1 . Occurrence of Discharge. 1.1 Partial discharge 1.2 Inception of internal discharge 1.3 Inception of surface discharge 1.4 Inception of corona discharge 1.5 Discharge by electrical treeing. 1.1 Partial discharge.

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1 . Occurrence of Discharge

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  1. 1 . Occurrence of Discharge 1.1 Partial discharge 1.2 Inception of internal discharge 1.3 Inception of surface discharge 1.4 Inception of corona discharge 1.5 Discharge by electrical treeing

  2. 1.1 Partial discharge • Electric discharge that do not completely bridge the electrodes are called partial discharges. • 방전크기는 작지만 점진적으로 발전하면 ultimate failure 로 유발가능 • 비파괴시험으로 방전의 진행을 detect 하는게 필수적이다. • 1.1.1 Classification • 1) Internal discharge (Figures 1.1 and 1.2) • 2) surface discharge (Figures 1.3) • 3) Corona discharge (Figures 1.4) • 4) Discharge in electrical trees (Figures 1.5)

  3. 1.1.2 Terminology • Partial discharge 대신에 미국, 영국(ionization) 프랑스(ionisation gazeuse) 독일(Ionisationsvorgange)라고함. • 파생어로는 ionization point , ionization livel • Corona 의 파생어 • 1.2 Inception of internal discharge • 1.2.1 Gas-filled cavities • Figure(1.2(a)-고체 절연체 전계 배 , (b) 배 ( c)유전체와 동일한 크기 (전계가 집중되는 위험이있음)

  4. Breakdown strength of the cavities (dimensions , type of gas , gas pressure) • Ex)a flat cavity of 0.1mm인 폴리에틸렌의 , =2.2 1atm 일때 fig 1.7에서 의 breakdown stress는 6.8kV/mm 이다. • inception stress 는 6.8/2.2 ~~3kV/mm • 부분방전 전계는 설계전계보다 낮아도 일어난다.(3.5 to 7kV/mm) • Breakdown voltage에 미치는 여러가지 요소중 예측치보다 낮은 BDS를 보이는데 그 예로 static charge in wall of cavity 에 의해 예측치는 20%만큼 감소한다.그의 방안으로 the wall을 도전성을 갖게한다.

  5. 1.2.2 Inclusions • Extruded plastics of cast resin 에는 various inclusions : dirt, paper of textile fibres and other foreign particles • inclusions에 의해 BDS는 감소하고 breakdown gas is formed and gas discharges occur. • Gooding and slade 의 metal splinters embedded in the dielectric  local field concentration • 1.2.3 oil-filled cavities • Oil-impregnated paper insulation 의 stress 는 gas-filled cavities와 유사하다.

  6. 1.3 Inception of surface discharge • Surface discharge may occur of there exists a stress component parallel to a dielectric surface.  bushings, ends of cables,the overhang of generator windings and where a discharge from outside touches the surface. • 1.3.1 Discharges in air • Figure 1.8 과 figure 1.9 에서의V와 에 의한 paschen curve 를 볼수있다.(table 1.1  dotted line)

  7. 1.3.2 Surface discharge under oil • Similar formula as for air • Contamination , moisture greatly affect the breakdown stress. • 공기중에서의 BDS 는 습도에 매우 민감하며, 절연유내의 실혐결과보다는 대개 낮은 특성을 보인다. • 1.3.3 Surface discharge under sulphur hexafluoride gas • 절연물에서의 particles 은 표면의 섬락과 부분방전의 원인이 된다. • Brokman 은 이러한 현상을 3가지 변수로 만들어보았다.

  8. (1) roughness of the surface • (2) semiconducting areas - contamination of the dielectric surface(Fig 1.11) • (3) Particle - shape and dielectric constant of particle(Fig 1.12) -particles with sharp edges are dangerous sources of discharge • The flashover voltage decreased with increasing

  9. 1.4 Inception of corona discharge • Corona discharge occur in gases at sharp points in the electrical field(Fig1.13) • Sharp edges , HV sides, laboratory floor… • 1.4.1 corona air • 1.4.2 corona in sulphur hexafluoride gas. • SF6하에서는 코로나 발생시 그의 부산물은 독성이 있다. • 1.5 discharge by electrical treeing • electrical trees start from defects in the insulation(Fig1.15)

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