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CHAPTER 4: LIFE EXTENSION
4.1 Power Transformers
4.1.1 Internal Insulating System and Components
4.1.3.2 Instruments and Gauges
4.1.3.3 Oil Preservation Equipment
It may be important that oil preservation equipment function properly so that oxygen and moisture are prevented (to an extent) from entering the paper and oil insulating system.
4.1.3.3.1 Expansion Tanks with Rubber Bags
Sargent and Lundy [84] reported that such equipment requires little maintenance. The oxygen and moisture contents from the periodic oil tests may be checked periodically for trends to determine whether the levels are increasing. A possible cause may be gasket leaks,but such trends may indicate that the bag has a leak or that air may be slowly permeating the rubber.
The system may not be operating properly,if the oil level gauge does not indicate the correct level and/or there may be a rupture in the bag, which may require replacement.
4.1.3.3.2/nsu/ationDry Out Systems
Van Wyk [85] reported that there are a number of effective transformer insulation off- line dry-out techniques may be used. These techniques are briefly discussed below.
The vapour phase dry-out system requires that the transformer be de-tanked in a workshop environment and inserted into a vapour phase autoclave. The tank is then washed with kerosene vapour heated to 180 "C, before drawing a vacuum.At this temperature the impregnated oil is moves out of the solid insulation, thus enabling moisture to be extracted from the paper in a period of approximately three days.
The conventional site dry-out system is a mobile plant that circulates, treats and heats the oil (and therefore the transformer insulation) to a temperature of 80 "C.
Thereafter, the oil is drained. A vacuum is then drawn on the drained transformer tank at approximately 4000 m3 per hour at a pressure of 10-3 kilopascals. The elevated temperature and reduced pressure on the paper insulation causes the moisture in the insulation to boil off and is removed by the vacuum plant. The complete process may take up to six weeks.
The low frequency heating (LFH) system for site or workshop dry-out of power transformers uses a solid-state low frequency converter, a controlled (pulsed) three phase current that is injected into the transformer primary, with the secondary short circuited.At the same time a conventional dry-out plant described above is used to circulate, treat and heat the oil. By heating the transformer insulation from within by the LFH plant and clrculatinq oil,the heating time is significantly reduced. Thereafter, the transformer oil is drained and a vacuum is drawn on the tank, exactly as previously described, to remove moisture from the paper insulation. This process may take 4 weeks to complete.
These off-line dry-out systems require that the transformer be taken out-of-service. It may be for this reason that an on-line insulation dry-out system may be favoured. The developer [86] reported the following with regard to an on-line dry-out system i.e.
The Dry Keep.The Dry Keep is a device that is fitted in the transformer oil-circulating path. It comprises of two passive cartridges containing a high technology sponge with the ability of removing dissolved moisture continuously from the oil. The device is capable of reducing moisture levels from 50 ppm to 10 ppm in transformer oils. The continuous extraction of moisture from the oil ensures the removal of the moisture in
the paper as well. This process may occur as a result of the diffusion of moisture from the paper insulation to the oil to maintain the associated equilibrium as moisture may be removed from the oil.
4.1.3.3.3 Gas Cushion Oil Preservation
These systems may result in supersaturating of the oil with nitrogen when the temperature of loaded transformers may be decreased rapidly by dropping the load in cold weather and/or during rain.Sargent and Lundy [84] reported that most utilities have replaced the pressure controls that allowed the pressure to increase up to 6 psig before the system started to release the nitrogen into the atmosphere. The pressure controls that were changed released nitrogen at around 3 to 3.5 psig.
Sargent and Lundy also reported that for some critical power transformers, the nitrogen system has been replaced with expansion tanks having rubber bags. The reason for this change was due to the following concerns, briefly discussed below.
If the nitrogen cushion designs are not properly maintained, this could result in the failure of the transformer. If the nitrogen bottle becomes empty and not replaced, the pressure in the gas space may become negative causing gas bubbles to evolve from the saturated oil.
4.2 Instrument Transformers
This section focuses on certain maintenance and life extension practices for instrument transformers.There may be very little that could be done to extend the life of the internal components of instrument transformers. The required maintenance and life extension practices differ for different brands of instrument transformers.The following is presented for guidance on such transformers.
4.2.1 Current Transformers
Sargent and Lundy [84] reported that manufacturers of current transformers state in their instruction manuals that oil samples are not to be taken. However, EPRI [20]
concluded that the history of current transformer operation indicated that the oil deteriorated in some current transformers,which would mean that the oil should be tested at predetermined intervals. EPRI also concluded that the deterioration would cause an increase in the power factor of the oil particularly at elevated temperatures after being in service for several years.This deterioration is believed to be related to the use of oils that were not suitable for HV application.
EPRI [17] reported that oil power factor'sas high as 15% at 100 DC were measured in some experimental high voltage current transformers. The same researcher concluded that if the power factor is as high as this value, it would be difficult to remove all of the contaminated oil from the insulation. Based on the review presented in chapter 2, x wax is known to be a polymer formed by partial discharges in oil-paper systems. EPRI [17] reported that if such polymers formed, it may be impossible to remove the wax from the insulation and that the presence of the wax may contribute to higher partial discharge levels.
EPRI [17] concluded that there is a high probability that the x wax exists in the insulation, if the following conditions exist:
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• Dissolved hydrogen of 1000 ppm or higher and
• Oil power factor at 100 °C greater than 5%.