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IMPULSE TESTS:

1. IMPULSE WITHSTAND VOLTAGE TEST : This test is done by applying standard impulse voltage of specified value under dry conditions with both positive and negative polarities of the wave. If five consecutive waves do not cause a flashover or puncture, the insulator is deemed to have passed the test. If two applications cause flashover, the object is deemed to have failed. If there is only one failure, additional ten applications of the voltage wave are made. If the test object has withstood the subsequent applications, it is said to have passed the test. 2. IMPULSE FLASHOVER TEST: The test is done as above with the specified voltage. Probability of failure is determined for 40% and 60% failure values or 20% and 80% failure values. The average value of the upper and the lower limits is taken. The insulator surface should not be damaged by these tests, but slight marking on its surface or chipping off of the cement is allowed. 3. POLLUTION TESTING: The normal type...

PARTIAL DISCHARGE REPRESENTATION:

The weak points in an insulation like voids, cracks, and other imperfections lead to internal or discharges in the insulation and as they are small, were not revealed in capacitance measurements but revealed as power loss components, contributing for an increase in the dissipation factor ‘PARTIAL DISCHARGES’ in a course of time reduces the strength of insulation leading to a total or partial failure or breakdown of the insulation Consider Ca as capacitor with a void inside the insulation. Capacitance of the void is represented by a capacitor in series with the rest of the insulation capacitance (Cb). The remaining void-free material is represented by the capacitance Cc When the voltage across the capacitor is raised, a critical value is reached acr...

PARTIAL DISCHARGE MEASUREMENTS

PARTIAL DISCHARGE PHENOMENON: TERMINOLOGY USED 1. Electrical Discharge : The movement of electrical charges through an insulating (dielectric) medium, initiated by electron avalanches. 2. Partial Discharge: An electrical discharge that partially bridges the dielectric or insulating medium between two conductors. Examples are: internal discharges, surface discharges and corona discharges. ➢ Internal discharges are discharges in cavities or voids which lie inside the volume of the dielectric or at the edges of conducting inclusions in a solid or liquid insulating media. ➢ Surface discharges are discharges from the conductor into a gas or a liquid medium and form on the surface of the solid insulation not covered by the conductor. ➢ Corona is a discharge in a gas or a liquid insulation around the conductors that are away or remote from the solid insulation. 3. Discharge Inception (Applied) Voltage : It is the lowest voltage at which discharges of specified magnitude will recur when ...

DIELECTRIC RESPONSE IN TIME VARYING (AC) FIELDS

In dielectric materials, the polarization P, the electric field E and the flux density D are related by the equation 𝐷=𝜀𝑜𝐸+𝑃=𝜀𝑜[1+𝜒]𝐸⁄ where, χ – dielectric susceptibility of the material with a varying electric fields E(t), The polarization P induces current in a dielectric due to charge migration whenever an electric field is suddenly applied. With dc, if the material has a conductivity σ, then the current density obtained is σE(t) and the polarization displacement current will be δD(t)/δt. Hence, the total current density produced is 𝑗(𝑡)=𝜎𝐸(𝑡)+𝛿𝐷(𝑡)𝛿𝑡=𝜎𝐸(𝑡)+𝜀𝑜𝛿𝐸(𝑡)𝛿𝑡+𝛿𝑃(𝑡)𝛿𝑡 = {𝜎+𝜀𝑜(1+𝜒)𝛿(𝑡)+𝑓(𝑡)}𝐸(𝑡) Where , δ(t) – instantaneous impulse response f(t) – the further response obtained. Hence, the polarization current obtained in terms of the geometrical capacitance C0 (without material) is, 𝑖(𝑡)=𝐶𝑜𝑉[𝜎𝑜𝜀𝑜+(1+𝜒)𝛿(𝑡)+𝑓(𝑡)] where, V – applied voltage that produces the electric field E(t) If the equations are transformed into ...

MEASUREMENT OF DIELECTRIC CONSTANT AND LOSS FACTOR

Insulating substances will have a dielectric constant greater than unity and dielectric loss when subjected to ac voltages. These two quantities, depend on the magnitude of the voltage stress and on the frequency of the applied voltage. The microscopic properties of the dielectric are described by combining the variation of the two quantities into one ‘complex quantity’ known as ‘complex permittivity’ and can be determined at various frequencies. A capacitor connected to a sinusoidal voltage source v = v0 exp (jωt) with an angular frequency ω= 2πf stores a charge Q = C0v and draws a charging current 𝐼𝑐 = 𝑑𝑄 𝑑𝑡 = 𝑗𝜔𝐶𝑜𝑣. When the dielectric is vacuum ➢ C0 is the vacuum capacitance or geometric capacitance of the capacitor, and the current leads the voltage vc by 90°. If the capacitor is filled with a dielectric of permittivity ε', ➢ Capacitance is increased to 𝐶 = 𝐶0𝜀/ 𝜀0 = 𝐶0𝐾/ where K’ is the relative dielectric constant of the material with respect to vacuum.Unde...

LOSS OF CHARGE METHOD

Current-time characteristic of the discharge current is obtained by placing the switch B in position 2. The slope of these current time characteristic gives the time constant τ = CR, where C is the capacitance of the specimen. The volume resistance of the specimen R can thus be obtained from the relationship by measuring τ and C. Alternatively, a capacitance previously charged (C1) may be discharged through the specimen by inserting it in place of the battery E. By knowing the value of the capacitance C1 which is very large compared to the specimen capacitance (≈10 μF) and knowing the current-time characteristic of the discharge current, its time constant can be calculated. Hence, the resistance of the specimen is obtained from the relationship 𝑅=𝜏/𝐶1 where C1 – capacitance used τ – time constant. If initial voltage V0 on the capacitor is measured and the voltage V across it at any time τ Then the unknown resistance R can be calcul...