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POWER FREQUENCY TESTS POWER FACTOR–VOLTAGE TEST: In this test, bushing is set up as in service or immersed in oil. It is connected such that the line conductor goes to the high-voltage side and the tank or earth portion goes to the detector side of the high-voltage Schering-bridge. Voltage is applied up to the line value varied in steps and then reduced. The capacitance and power factor (or tan δ) are recorded at each step and their characteristics are drawn. This is a normal routine test but sometimes may be conducted on percentage basis. INTERNAL OR PARTIAL DISCHARGE TEST This test is used to find the deterioration or failure due to internal discharges caused in the composite insulation of the bushing. The voltage versus discharge magnitude as well as the quadratic rate, gives an excellent record of the performance of the bushing in service. This is a routine test for high-voltage bushings. MOMENTARY WITHSTAND TEST AT POWER FREQUENCY: This is done as per the Indian Standard Specifica...

  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...

                          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 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 ...

 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 ...

 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...

 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...

 A simple measuring circuit for the measurement of resistance is shown in Fig 1. The galvanometer is first calibrated using a standard resistance of 1 to 10 MΩ (±0.5% or ±1%) and a standard universal shunt is used with the galvanometer. The deflection in cm per microampere of current is noted. The specimen (Rp) is inserted in the circuit and maintaining the same supply voltage, the galvanometer current is observed by adjusting the universal shunt. E - dc stabilized power supply 500 to2000v  V- voltmeter  G- galvanometer 10^-8 to 10^-9A/cm deflection Rsh- Universal shunt Rp-specimen   The resistance of the specimen is given by  𝑅𝑃=𝑉(𝐷×𝐺) Where  D – deflection in cm (with specimen) G – galvanometer sensitivity. 𝐺=(𝑉𝑅𝑠)×(1𝑛)×(1𝐷𝑠) n = the universal shunt ratio, Ds = deflection in cm with the standard resistance in position, Rs = standard resistance used for calibration, and V = the supply voltage Deflection with the specimen in the circuit, will ch...

The shape of specimen and the arrangement of electrode  should be such  that the resistivity can be easily calculated .For a solid specimen, a flat plate with plane and parallel surfaces usually circular are preferred. The specimens are normally in the form of disc of 5 to 10 cm diameter and 3 to 12 mm thickness . A three -electrode arrangement as shown is used for measurement of resistance The electrode which covers the surface of the specimen is called the ‘unguarded’ electrode and is connected to the high-voltage terminal. ➢ The third electrode surrounding other measuring electrode is connected to a suitable terminal of the measuring circuit. ➢ The width of this guard electrode must be at least twice the thickness of the specimen, and the unguarded electrode must extend to the outer edge of the guard electrode. The gap between the guarded and guard electrodes should be as small as possible. The effective diameter of the guarded electrode is greater than the actual diameter...