Cooling,Maintenance,D iagnostics & Testing of transformers
S S CHOPADE
Why Transformer Cooling? Heat is one of the most common destroyers of Transformers Only 100C above the transformer rating will cut transformer life by 50% What are the basic types of transformer coolings?
“A” indicates air, “FA” indicates Forced air, “O” indicates oil, “FO” indicates Forced oil.
Liquid-Immersed Transformer Cooling System Class OA: Oil-Immersed, self-cooled. ◦ Transformer windings and core are immersed in some type of oil and are self-cooled by natural circulation of air around the outside enclosure. ◦ Fins or radiators may attached to the enclosure to aid in cooling
Dry Type Transformer Cooling System Class AA are ventilated self cooled transformers Class AFA transformers are self cooled (A) & additionally cooled by forced circulation of air (FA) Class AA/FA are ventilated, self cooled. In addition, they have fans providing additional forced-air cooling. Fan may be wired to start auto when temp. reaches a pre-set value. These transformer generally have a dual load rating, one for AA and larger load rating for FA.
Class OA/FA: Liquid-immersed, self cooled/forced air cooled. Same
as OA above, with the addition of fans. Fans are usually mounted on radiators. The transformer typically has two load ratings, one with the fans off (OA) and a larger rating with fans operating (FA). Fans may be wired to start automatically at a pre-set temperature.
Class
OA/FA/FA: Liquid-immersed, selfcooled/forced air -cooled. Same as OA/FA above with an additional set of fans. There typically will be three load ratings corresponding to each increment of cooling. Increased ratings are obtained by increasing cooling air over portions of the cooling surfaces. Typically there are radiators attached to the tank to aid in cooling. The two group of fans may be wired to start automatically at pre-set level temperature increases. There are no oil pumps. Oil flow through the transformer
Liquid-immersed, Air cooled/Forced Liquid-cooled.
There
are two classes in this section 1. Class OA/FA/FOA: Liquid-immersed, selfcooled/forced air-cooled/forced liquid, and forced air-cooled. Windings and core are immersed in some type of oil. The transformer has selfcooling (OA) natural ventilation, forced aircooling FA (fans), and forced oil-cooling (pumps) with additional forced air-cooling (FOA) (more fans). The transformer has three load ratings corresponding to each cooling step.
Class OA/FOA/FAO: liquid-immersed, selfcooled/forced oil, and forced air-cooled/forced oil, and forced air-cooled. Cooling controls are arranged to start only part of the oil pumps and part of the fans for the first load rating/temperature increase, and the remaining pumps and fans for the second load rating increase.
INSPECTIONS OF OIL-FILLED TRANSFORMER A transformer maintenance program must be based on thorough routine inspections. These inspections must be in addition to normal daily/weekly data gathering trips to check oil levels and temperatures. Some monitoring may be done remotely using SCADA systems, but this can never substitute for thorough inspections by competent maintenance or operations people.
Testing and Maintenance of High-Voltage Bushings. Caution: Do not test a bushing while it's in its wood shipping crate, or while it is lying on wood. Wood is not as good an insulator as porcelain and will cause the readings to be inaccurate. Keep the test results as a baseline record to compare with future tests.
If
the oil level is low and there is no visible external leak, there may be an internal leak around the lower seal into the transformer tank. If possible, re-fill the bushing with the same oil and carefully monitor the level and the volume it takes to fill the bushing to the proper level. If it takes more than one quarter, make plans to replace the bushing. The bushing must be sent to the factory for repair or it may be junked; it cannot be repaired in the field.
Corona
(air ionization) may be visible at tops of bushings at night, especially during periods of rain, mist, fog, or high humidity. At the top, corona is considered normal; however, as a bushing becomes more and more contaminated, corona will creep lower and lower. If the bushing is not cleaned, flashover will occur when corona near the grounded transformer top. If corona seems to be lower than the top of the bushing, clean the bushing as quickly as possible.
Oil Preservation Sealing Systems The
purpose of sealing systems is to prevent air and moisture from contaminating oil and cellulose insulation. Sealing systems are designed to prevent oil inside the transformer from coming into contact with air. Air contains moisture, which causes sludging and an abundant supply of oxygen. Oxygen in combination with moisture causes greatly accelerated deterioration of the cellulose. This oxygen-moisture combination will greatly reduce service life of the transformer.
Gaskets Gaskets
have several important jobs in sealing systems. A gasket must create a seal and hold it over a long period of time. It must be impervious and not contaminate the insulating fluid or gas above the fluid. It should be easily removed and replaced. It must be elastic enough to flow into imperfections on the sealing surfaces. It must withstand high and low temperatures and remain resilient enough to hold the seal even with joint movement from expansion, contraction, and vibration.
Transformer Oils Transformer
Oil Functions. Transformer oils perform at least four functions for the transformer. ◦ provides insulation ◦ provides cooling ◦ helps extinguish arcs. ◦ Oil also dissolves gases generated by oil degradation, moisture and gas from cellulose insulation, deterioration, gases and moisture from atmosphere to which oil is exposed to. • Close observation of dissolved gases in the oil, and other oil properties, provides the most valuable information about transformer health. Looking for trends by comparing information and understanding its meaning, is the most important transformer diagnostic tool.
Key Gas Method key
gas analysis method for interpreting dissolved gasses is set forth in IEEE key gases formed by degradation of oil and paper insulation are hydrogen (h2), methane (ch4), ethane (c2 h6), ethylene (c2 h4), acetylene (c2 h2), carbon monoxide (co), and oxygen (o2).
Except carbon monoxide and oxygen, all other gases are formed from the degradation of the oil itself. ◦ carbon monoxide, carbon dioxide (co2), and oxygen are formed from degradation of cellulose (paper) insulation. ◦ carbon dioxide, oxygen, nitrogen (n2 ), and moisture can also be absorbed from the air if there is a oil/air interface, or if there is a leak in the tank.
Additional transformer problems are listed below; there are many others. Gases
are generated by normal operation and aging, mostly H2 and CO with some CH4.
Operating
transformers at sustained overload will generate combustible gases. Problems with cooling systems, discussed in an earlier section, can cause overheating. A blocked oil duct inside the transformer can cause local overheating, generating gases.
Additional transformer problems are listed below; there are many others. An
oil directing baffle loose inside the transformer causes mis-direction of cooling oil. Oil circulating pump problems (bearing wear, impeller loose or worn) can cause transformer cooling problems. Oil level is too low this will not be visible if the level indicator is inoperative. Sludge in the transformer and cooling system. Circulating stray currents may occur in the core, structure, and/or tank. An unintentional core ground may cause heating by providing a path for stray currents. A hot-spot can be caused by a bad connection in the leads or by a poor contact in the tap changer. A hot-spot may also be caused by discharges of static electrical charges that build up on shields or core and structures which are not properly grounded.
Additional transformer problems are listed below; there are many others……………. Hot-spots may be caused by electrical arcing between windings and ground, between windings of different potential, or in areas of different potential on the same winding, due to deteriorated or damaged insulation. 14. Windings and insulation can be damaged by faults downstream (through faults), causing large current surges through the windings. Through faults cause extreme magnetic and physical forces that can distort and loosen windings and wedges. The result may be arcing in the transformer, beginning at the time of the fault, or the insulation may be weakened and arcing develop later.
Dissolved Gas Analysis Detection Limits. Hydrogen (H2) about 5 ppm Methane (CH4) about 1 ppm Acetylene (C2H2) about 1 to 2 ppm Ethylene (C2H4) about 1 ppm Ethane (C2H6) about 1 ppm Carbon monoxide (CO) and carbon dioxide (CO2) about 25 ppm Oxygen (O2 ) and nitrogen (N2) about 50 ppm These ratios and the resultant fault indications are based on large numbers of DGAs and transformer failures and
properties of Xmer Oil Viscosity
This determines the rate of cooling and varies with temp Insulating property This serves one of the basic purpose of using oil Flash point (Temp at which oil vapour at oil surface ignites) A flash point of not less than 160oC is usually demanded for safety reasons Fire point (temp at which oil ignites ) Again A fire point of not less than 200oC or above 25 % higher than flash point is usually demanded. Purity Must be free from sulfur to avoid corrosion of metal
3 Acid Number. Acid
number (acidity) is the amount of potassium hydroxide (KOH) in milligrams (mg) that it takes to neutralize the acid in 1 gram (gm) of transformer oil. The higher the acid number, the more acid is in the oil. It is recommended that the oil be reclaimed when it reaches 0.20 mg KOH/gm
Transformer testing Factory
tests Polarity test No load loss test Load loss test, short ckt test impedance calculation Dielectric test Separate source voltage withstand Induced overvoltage test Temp rise test Lightening impulse
Routine
tests CT testing (ratio,polarity & knee point if specified) Ratio test Winding resistance measurement Short ckt test Vector group test Magnetising current measurement Magnetic balance test
Oil deterioration
Major causes of oil deterioration are Oil in contact with air subjects to oxidation reactions ,Temp & presence of catalysts (Solid iron, copper etc) accelerates it , as a result oil darkens & its acidity increases Simultaneously its electrical properties such as dielectric dissipation factor(DDF) & resistivity falls considerably Ultimate effect can be formation of sludge too In some cases material used in the construction of oil may decompose to deteriorate the quality of oil.
Following contaminant may be found in oil in service • Water • Sediment (insoluble oxides,degradation product of solid or liquid insulating materials, Fibers) • precipitable sludge (Contaminants , Oil deterioration products) • Polar substances ( Oil soluble compound resulting from oxidation of oil) • Acids (Formed by CO2,CO & moisture in oil) • Dissolved gases • Light hydrocarbon (Formed by oil under influence of heat, Energy faults)
Tests on oil sample Visual
inspection (Colour & Odour) Cloudiness in oil may be due to suspended moisture or sediments such as iron-oxide or sludge Dark brown coloured oil may indicate the presence of dissolved bituminous compound Green coloured oil may indicate the presence of dissolved copper compounds & a rapid deterioration of oil may be expected Acrid acid smell indicates the presence of volatile acids which can cause corrosion
Electric
strength test A typical laboratory test set up is required for the test in which oil is subjected to high voltage with a fixed gap between electrode & voltage is increased till oil breaks down( a detail method is given in IS 6792-1972) permissible values for 2.5 mm gap upto 72.5 kV 30 kV min 72.5 to 145 kV 40 kV min Above 145 kV 50 kV min Water content test Presence of water in the oil may be detected by crackle test however exact PPM of water can be determined by titration method in laboratory. Permissible values are below 145 kV 35 ppm
Sludge test This is again a laboratory test which determines sludge & sediment in the oil by method of precipitation of oil sample with n-heptanesolution. The precipitated matter is then dissolved in the mixture of toluene, acetone and alcohol at 50oC till no more dissolution is there. The precipitated quantity is termed as sludge and the dissolved one represents sediments in the oil Permissible values are absolutely no sediments or precipitable sludge should be traced Dielectric dissipation factor test Permissible value for DDF( tan ) at 90oC is 0.005 (max) Specific resistance test Permissible value for resistivity is 0.1 X 1012 ohm-cm(min) at 90oC
Maintenance schedule - hourly Oil & Wdg temp check Check against reasonable rise Shutdown Xmer & investigate the matter Voltages & Load monitoring Check against the rated parameters Take the corrective action with the help of tap changer
Daily Maintenance schedule Oil
level in Xmer & OLTC Check against normal level Top up & investigate for the leakage Breather Check color of active agent if silicagel is pink , change by spare charge & old charge may be reactivated for further use
Quarterly Maintenance Schedule Bushing
Examine cracks & dirt deposited Clean or replace Oil in Xmer or OLTC Check for dielectric strength & water content Take suitable action to restore quality of oil Fan & Pumps Check bearing, examine contacts, check manual & auto P/U ckt. Replace faulty components
Half Yearly Maintenance Schedule Oil
cooler Test for pressure OLTC & driving gear Check for all moving contacts,brake mechanism
parts
Yearly Maintenance Schedule Insulation resistance Compare with the values at the time of commissioning Process if required Cable boxes Check for sealing arrangement for cable boxes,moisture condensate if any in air filled cable box Replace gasket if leaking
Drying out Drying
out is done when IR values or Moisture contents are not comparable with the standard value. It is done by circulation of hot oil through streamline filter - a machine incorporating oil heater & vacuum chamber It is preferable to lag or blanket Xmer tank to prevent loss of heat, radiation of from radiators may be prevented by lowering the oil level below the top inlet or by operating shut off valves if provided. Oil is drawn from bottom & let into the Xmer from top to remove settled impurities. At about 8 - 10 Hrs cycle is reversed Oil temperature should be of the order 60oC.In no case it is to be exceeded beyond 65oC & circulation
Oil Temp in oC IR value in M ohm
Drying out curve
Moisture coming out Time in Hrs
