Remarks: New AC Pressure Test, IEC 60298: For switchgear, which are 80% of values detailed of IEC60694. (TABLE 3.7). 2) Reduced Value AC Pressure Test, for Previously In Service for IEC60298 -Switchgear. System voltage * 1.1. (TABLE3.7). 3) DC Pressure Test, Phase – Earth. System voltage * sqrt(2) * 1.1) / sqrt (3). IEC60298 (Table 3.3) DC Pressure Test, Phase – Phase. System voltage * sqrt(2) * 1.1 IEC60298. (Table 3.3) AC Pressure Test, BS 159. For Equipment at site. (2Vn +2kV). For test exceeded 1 minute, use Table 3.6.

Very Low Frequency Test (VLF) VLF stands for Very Low Frequency. VLF is generally considered to be 0.1 Hz and lower. It’s just an AC hi-pot but with a lower frequency output. The only way to field test high capacitance loads with AC voltage, like cables and motors/generators, is to use a VLF AC hi-pot. The lower the frequency, the less current and power needed to test high capacitance loads.

Very Low Frequency Test (VLF) cont- Example: Xc (capacitive reactance) = 1/2πfC. If Capacitance for 11kV cables = 0.0002 µF/m. Cable length 10000 meters, total Capacitance = 2µF The capacitive reactance at 50 Hz is 1592 ohms. To apply the IEC recommended of 22kV test voltage, it would require a power supply rated for 13.82 amps, or kVA. Obviously not practical for field use.

Very Low Frequency Test (VLF) cont- By using VLF testing set, at 0.1Hz Capacitive Reactance Xc = 759,774 ohm 22kV test voltage would draw only 0.028 Amps or only 0.616kVA

INSUALTION TEST OF ELECTRICAL EQUIPMENT Power factor or Dissipation Factor test The objectives of these tests are to determine the following characteristic of the insulation such as the AC dielectric loss, dielectric power factor and dissipation power factor. Dissipation factor is also known as tan delta. The test also reveals the moisture and void in the insulation. INSUALTION TEST OF ELECTRICAL EQUIPMENT

INSUALTION TEST OF ELECTRICAL EQUIPMENT Theory PF and DF test In a perfect capacitor, the voltage and current are phase shifted 90 degrees and the current through the insulation is capacitive. If there are impurities in the insulation, like those mentioned above, the resistance of the insulation decreases, resulting in an increase in resistive current through the insulation. It is no longer a perfect capacitor. The current and voltage will no longer be shifted 90 degrees. It will be something less than 90 degrees. INSUALTION TEST OF ELECTRICAL EQUIPMENT

INSUALTION TEST OF ELECTRICAL EQUIPMENT Power Factor Test The Power Factor in a circuit is defined as the ratio of power to volt-amperes (W/VA). Power Factor is also defined as the cosine of the angle between the voltage and current in a circuit, with the angle typically referred to as ‘theta’. This angle is typically very close to 90 degrees for capacitive (insulation) circuits. Tan Delta Test INSUALTION TEST OF ELECTRICAL EQUIPMENT Dissipation Factor is defined as the ratio of power to reactive volt-amperes (W/VAr) in a circuit. The Dissipation Factor is equal to the tangent of ‘Delta’, where ‘Delta’ is the angle of 90 deg minus ‘theta’

Basic trigonometry yields the following relationships: and the dissipation factor is defined as:

Equipment for PF and DF testing

Application Examples of PF and DF tests for two Winding Transformer

Schering’s Bride for Dissipation Factor Test Schering’s Bridge technique are often used in DF and PF Testing set. Rx = (R2 x C1)/C3 Cx = (R1/R2) x C3 Dissipation Factor= ωCxRx

AC vs DC Hi-Pot Test AC testing has some advantages over DC testing as it stresses the insulation equally in both polarities, whereas DC testing stresses the insulation in single polarity only. However, AC can have a downfall if the device under test is capacitive. The reactive current (caused by the capacitance) is typically much higher than the real current, resulting in the total leakage current to be comprised mostly of reactive current.

INSUALTION TEST OF ELECTRICAL EQUIPMENT AC vs DC Hi-Pot Test (continue)- Performing a DC test may be a better option than running the device at high unsafe currents. Performing a DC test may take longer because the device needs to charge, but once fully charged, the leakage is true leakage. DC is also used in non-destructive testing of devices for predicting when a breakdown occurs. This is done by raising the test voltage in small increments and waiting for the charge current to reduce after each step. INSUALTION TEST OF ELECTRICAL EQUIPMENT

Summary of Insulation Solid Insulation Testing The insulation resistance (IR) and polarization index (PI) tests universally are primarily intended to reveal moisture within the insulation, or surface contamination that could lead to destructive tracking Internal voids or insufficient resin penetration are not readily discoverable through IR testing. Such a test goes by various names: power factor, dissipation factor, "tan delta," loss tangent, dielectric loss factor, or "Doble Test."

Insulating Oils, Fluids Insulating oils, fluids, and gases are used as dielectrics in the electrical equipment and apparatus. The liquids used in the transformers are mineral oil and synthetic fluids, such as askarel, silicone and hydrocarbon. Importance characteristics of these liquids are: Low viscosity- an ability for the oils to move freely. High Flash Point- min temperature of the oils where vapour consisting of flammable is produce.

Oil Deterioration The deterioration of insulating oil, fluids, and gases is due to contamination, overheating, electrical stress, and oxidation. Moisture is the most common contaminant which adversely affects the insulating properties of these liquids. Moisture contamination is the most common cause of deterioration in the insulating quality of oil. This contamination can be readily corrected by purification. A slow but more serious deterioration, the formation of acids and sludge, is caused by oxidation. Thus, the exclusion of oxygen is of prime importance.

Oil Deterioration Water can be present in oil in a dissolved form, as tiny droplets mixed with the oil (emulsion), or in a free state at the bottom of the container holding the oil. Demulsification occurs when the tiny droplets unite to form larger drops, which sink to the bottom and form a pool of free water. The effect of moisture on the insulating properties of oil depends upon the form in which the moisture exists. A very small amount of emulsified water has a marked influence in reducing dielectric strength of oil.

Testing of Insulation Fluids Colour/visual Dielectric Strength Acidity Dielectic Dissipation Test Moisture Content

COLOR TEST This test consists of transmitting light through oil samples and comparing the colour observed with a standard colour chart. Colour Remarks 1 Cloudy Sludge 2 Dark Yellow Heat 3 Black Arc and Carbonised 4 Green Cooper Salts dissolve into oil

DIELECTRIC TEST The dielectric test simply consists of placing a liquid sample from the transformer or (circuit breaker) in a cup containing two electrodes of specified gap (12.5 – 13.00 mm) High voltage is then applied to the sample. The test is repeated for a least five different samples to determine the average dielectric strength. Average breakdown Voltage > 30 kV - PASS

ACIDITY TEST The acidity test measures the content of acids formed by oxidation. The acids are directly responsible for sludge formation. These acids precipitate out, as their concentration increases, and become sludge. They also react with metals to form another form of sludge in the transformer. The acid number of the neutralization number is the milligrams (mg) of potassium hydroxide (KOH) required to neutralize the acid contained in 1 g of transformer liquid. KOH level < 0.5mg KOH/g oil - PASS

WATER CONTENT TEST Three methods are used to conduct this test. Methods A and C utilize iodine present in a titration solution while Method B electrically generates the iodine in the equipment Moisture content(ppm) < 15 (72.5 -170 kV) –PASS < 10 (170kV) -PASS