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Review of lecture 4

Review of lecture 4. Contents:. The Basic Parameters The Integrated Quantities The Inception and Distinction Voltages Waveform Characteristics of Discharges. Basic Parameters. Review of lecture 4. Integrated Quantities. u i and u e. For oil-impregnated paper, u i >u e.

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Review of lecture 4

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  1. Review of lecture 4 Contents: • The Basic Parameters • The Integrated Quantities • The Inception and Distinction Voltages • Waveform Characteristics of Discharges Basic Parameters

  2. Review of lecture 4 Integrated Quantities ui and ue For oil-impregnated paper, ui >ue

  3. Review of lecture 4 Estimation of ui for several typical internal discharges: • a dielectric with a flat cavity • a dielectric with a spherical cavity • the flat cavity in a cylindrical

  4. Review of lecture 4 Waveform Characteristics of Discharges Internal discharge Surface discharge Corona discharge

  5. Lecture 5 Dielectric Deterioration and PD Evaluation

  6. Contents 1. Mechanisms of DielectricAging by PD 局部放电老化机理 2. Process of DielectricAging by PD 局部放电老化过程 3. The Influence Factors on Aging Rate 影响老化的因素 4. PDEvaluation Approaches 局部放电评估方法

  7. 1. Mechanisms of Dielectric Aging by PD Electrical aging is a type of dielectric deterioration under the long-term electrical field and due to the fact of partial discharges. 电老化：在电场长期作用之下因局部放电而造成的绝缘性能劣化。 (1) Bombardment of charged particle 带电质点轰击 Broke the chemical bond and damage molecular structure of insulation material . (2) Thermal Effect 热效应 High temperature of discharge point accelerates thermal damage to materials (3) Resultant of Reaction 反应生成物 resultants, such as ozone, moisture, nitric acid, oxalic acid , erode materials. (4) Radiation Effect 辐射效应 visible light, ultraviolet radiation, X-radial,γ-radial embrittle materials. (5) Mechanical Effect 机械效应 Successive blasting discharges result in high gas pressure and crack the materials

  8. In nitrogen gas In air 2.Process of Dielectric Aging by PD 内部放电对聚合物的破坏 2.1 Damage of internal discharge to polymer 表面腐蚀 (1) Surface Erosion • A uniform surface erosion, which may be caused by bombardment of particle, thermal degradation, radiation and trapped electrons below surface, etc. 在放电的各种效应作用下，逐步腐蚀气隙表面 • Deep pits are formed and carbonized. Discharges become concentrated, some deep pits are formed, the discharges further concentrate and sometimes carbonization of the pits occurs. 放电集中的地方，形成一些深坑；放电进一步集中，并造成一些深坑碳化

  9. 2.Process of Dielectric Aging by PD 树枝放电的发展 (2) Propagation of electrical tree • The dielectric breaks down over a distance of some microns. The stress at the top of the pit approaches the breakdown strength over a distance of some microns owing to the field concentration around this sharp tip. 深坑端部一定距离内场强集中，达到了击穿场强，这些区域发生击穿。 • complete breakdown.The field concentration moves to the new tip and narrow carbonization channels propagate quickly through the dielectric, initiating complete breakdown. 击穿造成了新的深坑端部电场集中，碳化的狭窄通道很快在介质中发展，最后介质完全击穿。

  10. 2.Process of Dielectric Aging by PD Two configurations of electrical tree branched type brush type tree 刷状 丛林状 Simulation of electrical tree in XLPE cable

  11. 2.Process of Dielectric Aging by PD Development of treeing discharges （a） inception （b） branch growing （c） reaching to the opposite electrode （d） complete breakdown

  12. 2.Process of Dielectric Aging by PD 2.2 Damage of internal discharge to oil-paper (1)Discharges in void adjacent to the conductor attack the insulation. After some time, discharge penetrate the first layer. 在靠近导体的气隙处发生放电，轰击绝缘表面。一段时间之后，放电穿透第一层纸 (2) Surface discharges occur along the layers after penetrating the first few paper layers, and trees or carbonized tracks are formed. 沿着层间纸的表面发生沿面放电，形成树枝或碳化通道 (3)The tracks follow the weakest points in the insulation. 在绝缘纸存在薄弱点时，碳化轨迹容易穿透绝缘纸. (4)The tracks finally attain enormous lengths, ultimate breakdown occurs. 碳化通道达到一定的长度，击穿发生。

  13. 3. The Influence Factors on Aging Rate 电场强度 3.1 Electrical field stress The principle of accelerating aging tests: (Lb, Lc, Ld) at higher stresses (Eb, Ec, Ed) to , extrapolate La at lower stress Ea 3.2 Voltage frequency 电压频率

  14. 3. The Influence Factors on Aging Rate 3.3 Situation of cavity and gas contents inside 气隙状态及成分 (a) Single flat cavity parallel with electric field. 单个扁平气隙，平行电场方向 单个扁平气隙，垂直于电场方向 (b) Single flat cavity , perpendicular with electric field. (c) Double cavities, in series in the direction of electric field. 两个串联气隙 (d) Double cavities, in parallel in the direction of electric field. 两个并联气隙 紧邻金属电极的气隙 (e) Cavities, adjacent to electrodes. （a）is more harmful than (b) （c）is more harmful than (d) （e）is more harmful than the cavities bounded by dielectric.

  15. 3. The Influence Factors on Aging Rate 3.4 Temperature and humidity 温度和湿度 k - aging rate T – temperature (K) Ea - activation energy (kJ/mol-1) Aa - pre-exponential factor • heat resistant grades of insulation 绝缘耐热等级 the upper limited operating temperature of insulation. 绝缘的最高允许工作温度。 机械应力 3.5 Mechanical force

  16. 4. PD Evaluation Approaches 4.1 Estimation by the basic parameters (1) Apparent charge, qa The maximum qa at 1~1.5 rated voltage of HV equipment should be smaller than the permissible discharge magnitude. 通常在1~1.5倍额定电压下测量设备视在放电量，需小于规定的允许放电水平 Disadvantages: (1) The qr is quite different with qr视在放电量和真实放电量有差别 (2) qa is often transient and fluctuant. qa是瞬变的和无规则波动的 (3) qa only indicates the maximum of discharge magnitude corresponding to the defects in the largest size. 仅代表最大缺陷中的最大放电量 (2) Discharge energy and power, W and P

  17. 4. PD Evaluation Approaches (3) The phase angle of pulses When partial discharges act more excitedly • the pulses occur at the wider range of phase angle. 放电相位变宽 • Inception pulses will shift to zero or a negative phase angle. 起始脉冲朝着过零点或者负半周移动 • Average phase angle of pulses in a power-frequency half cycle also becomes smaller. 平均放电相位也变小 (4) Ui and Ue Under the same test voltage, higher discharge magnitude or more pulses exist in the tested equipment with smaller inception and extinction voltages. 在同样的实验电压下，ui越低，放电量qa越大，放电次数n也就越多

  18. 4. PD Evaluation Approaches 4.2 Estimation by distribution (statistical) graphs 采用分布(统计)图谱评定 Every PD activity is a sequence of PD pulses qi(ti)

  19. q φ t 4. PD Evaluation Approaches 放电量-相位分布图 (1) q-φ graph， H(q, φ) The phase range is divided into 2n parts firstly, each part is a phase window. The average, (maximum, sum) pulse height distribution: the average (maximum, sum) amplitude in each phase window as a function of the phase angle, 统计每个相位窗中的放电量平均（最大，之和）值

  20. φ 4. PD Evaluation Approaches 放电次数-相位分布图 (2) n-φ graph， H(n, φ) The pulse count distribution: the number of observed discharges in each phase window as a function of the phase angle. 统计每个相位窗中的放电次数

  21. 4. PD Evaluation Approaches (3) n-q graph， H(n, q) The q range is divided into 2n parts firstly, each part is a q window. 将放电量等分为2n份，每一份为一个放电量窗 the number of observed discharges in each q window as a function of the discharge magnitude. 统计每个一个放电量窗中脉冲的个数 (4) w-n graph， H(w, n) the number of observed discharges in each w window as a function of the discharge energy.

  22. （a） sk>0 （b） sk=0 （c）sk<0 f(x) f(x) f(x) x x x 4. PD Evaluation Approaches 4.3 parameters extracting from graphs (statistic operators) 图谱的特征参数提取(统计算子) (1) Skewness, sk 偏斜度 (2) kurtosis, ku 陡峭度 (3)others 其他

  23. 4. PD Evaluation Approaches 4.3q-u hysteresis curve Q-u迟滞曲线 • The area surrounded by curve. Larger area means poorer performance to absorb gas 曲线包围面积，越大材料吸气性能越差 • Δu=ui-ue. Difference of ui and ue.

  24. The end

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