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CONSIDERATION OF ON-LINE PROCESSING OF HIGH VOLTAGE POWER TRANSFORMERS

by Victor Sokolov, ZTZ Service – Ukraine Ben Taylor, Velcon Filters Inc.-USA

INTRODUCTION The global task of the electric power industry in the near term outlook will be to manage the serviceability of a huge transformer population that has already been in service for 25-40 years. The basic problem is to ensure that appropriate actions are taken to promote the longest possible service life under any operating conditions. Apparently, after long-term operation the condition of transformer insulation should be substantially changed. Experience with assessment of the condition of aged 110-500 kV Power transformers has shown [1] that after 25-30 years in average each aged transformer may have about 3 latent defects in the main tank and 47% defects have been attributed to impairment of the conditions of insulation due to contamination with water, particles and oil aging products. On the other hand, failure statistic has shown [2] that a stable high rate (15-20%) of failures is attributed to the impairment of the conditions of major and minor insulation especially due to reducing the impulse withstands strength. Hence, reconditioning of “dielectric life” of a transformer might be anticipated as an efficient means to prevent unexpected failure and to extend transformer life. In recent years there has been considerable interest in the subject of on-line processing of power transformers, particularly in reclamation of oil, drying out and regeneration of insulation. Plain economic benefit encourages fast developing processing techniques [3,4,6]. CIGRE WG 12.18 “Transformer Life Management” has studied possible condition of aged transformers, dangerous effect of insulation degradation factors as well as different techniques of insulation rehabilitation and reconditioning including On-line Processing This paper presents some theoretical and practical aspects and basic motivation of On-Line processing based on the Cigre analysis and studies and practical experience of ZTZ-Service and Velcon Filters, Inc.

INSULATION RECONDITIONING AS A PRIMARY MEAN TO EXTEND LIFE The basic philosophy of the loading guides considers that ” the life of the transformer is the life of paper”. However, there is little information available about transformers that have failed, primarily due to thermal degradation of insulation material. Only 3-5 % of total failures of aged transformers are associated with overheating or wear out of winding conductor’s insulation [2 ]. Experience has shown that “dielectric life” of HV transformer could approach its end faster than “thermal life”. Even local overheating of windings coils results often in dielectric mode failure, namely, in carbonization of oil and a disk-to-disk flashover.

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Failure statistic [1] has shown that about 15-20 % of failures are attributed to the impairment of the conditions of major and minor insulation especially due to reducing the impulse withstands strength. The conditions of 106 aged (23-39 years in service), 110-500 kV, power transformers assessed by ZTZ-Service utilizing the functional-based methodology [2] have shown the following trends: • In average each aged transformer may have about 3 latent defects in the main tank • 47% of these defects have been attributed to impairment of the conditions of

insulation due to contamination with water, particles and oil aging products. • About 27% of defects have been attributed to localized oil overheating that

accordingly may cause abnormal contamination of insulation. • About 28% of defects have been attributed to mechanical weakness of winding and

core clamping. Therefore removing water, particles, and aging by-product might be an effective mean to avoid equipment failures and extent the life. A transformer insulation rehabilitation program aims to restore or rectify the dielectric safety margin and slow down the rate of further insulation deterioration. The following objectives of dielectric system processing should be distinguished: 1. Reconditioning and reclaiming the naturally deteriorated transformers, namely: Aged oil; contamination of cellulose insulation with oil aging products, saturation of cellulose with air, moisture, or particle contamination. 2. Reconditioning or rectifying the transformer being in a defective condition, namely:

• Having a source of gas generation (e.g., localized overheating) • Having source of particle generation, e.g. carbon, metal or fibers • Having severe moisture contamination of solid insulation • Having severe insulation contamination with sludge or other aggressive oil aging

products

It is always important to distinguish between natural deterioration (under impact of temperature, oxygen, mechanical friction, ingress of air and moisture through the breathing system provided by design) and abnormal deterioration when a defect is involved. In the latter case identification of the defect and its correction is important. DEGRADATION FACTORS CONSIDERATION Water, particles of different origin, and oil aging products are agents of degradation, which can shorten transformer life significantly under impact of thermal, electric, electromagnetic and electrodynamic stresses. One should consider that practically all impurities are distributed in certain proportions between oil and solid insulation. Solid insulation is not only a reservoir of absorbed moisture but also contains a significant amount of gases and oil aging products. Water The main source of water contamination is atmospheric moisture. The main mechanism of water penetration in transformers is through poor seals by the viscous flow of wet air created by a total pressure gradient. Typical leaks are the top seal of draw-lead

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bushings, the seals in explosion vents, and leaks through poor sealing of nitrogen blanketed transformers. Large amounts of rainwater can be sucked into a transformer in a very short time (several hours), when there is a rapid drop of pressure (after a rapid drop of temperature that can be induced by rain) combined with insufficient sealing. Aging can produce a substantial amount of water only if insulation is subjected to elevated temperature and destructed significantly. In this case water is removed basically from the vicinity of the hot spots in the winding. Excessive moisture is inherent to transformers with open-breathing preservation system or to those that have insufficient sealing. Distribution of the moisture in the course of the transformer life is kept quite non-uniform. Most of the water is stored in so-called “cold thin structures”, namely in the thin pressboard barriers that operate at bulk oil temperature. Water content in turn-insulation is substantially lower than in pressboard barriers due to higher temperature. Solid insulation is a water accumulator and the main source of oil contamination in an operating transformer. Oil is a water-transferring medium. Water is usually present in the oil in a soluble or dissolved form but may also be present as a form adsorbed by “polar” aging products and called “bound water”. It has been found that as temperature increases, some bound water can be converted into soluble water. Test results of the water content of aged oil sampled from three power transformers are shown in Table 1. After heating the oil at 100°C for 4-6 hours the water content in oil increased significantly. A similar phenomenon has been observed in bushing and current transformer oils. Most likely, the dissolved polar compounds in the oil are the source of this additional water. TABLE 1 Transformation of Bound Water to Soluble Water from Aged Oil (blanked samples)

Water content ppm Type of oil

Properties Before

heating After heating at 100°C for 4-6 hours

25MVA, 110 kV 11 years

Acidity=0.038mg KOH/g IFT=32.0dynes/cm Saponification number= 0.097 mg KOH/g

29 40

40.5 MVA 110 kV 18 years

Acidity=0.133 mg KOH/g IFT=23.18dynes/cm Saponification number= 0.44 g KOH/g PF=10.8 % at 90 C

25.8 50

150 MVA 220 kV 25 years

Acidity=0.055 mg KOH/g IFT=28.8 dynes/cm Saponification number= 0.138 g KOH/g

17.7 32

† Tests performed in ZTZ – Service Material Lab

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Particle contamination The particles in oil range from microscopic to visible range. Large particles usually settle down. Time constant of particle sedimentation depend on oil viscosity. Hence an oil sample taken from a transformer at high temperature may contain only small suspended particles. Suspended particles are usually those above 0.45 µm. The visible range starts at about 50 µm [10]. Classification of typical particles in transformer oil is suggested in Table 2 Manufacturing contaminants: Cellulose fibers, iron, aluminum, copper and other particles resulting from manufacturing processes are naturally present in the transformer’s oil. Non-conductive mode particles presumable would be present in a 5 to 50 micron range – easily removable with 0.5 micron filters Dress and test dirt : This type of contaminant gets in the transformer tank during bushing installation, oil filling, from cooling system, etc. Size range probably from 5 to 100 microns. Sometimes, the filter itself can supply particles, especially if the paper and the oil are somewhat wet. Aged oil: During utilization at normal and overload temperatures oil slowly forms sludge particles, "polymeric" in nature. Based on Velcon Filters research these could be one to five microns in size and this contamination is difficult to remove by common filtration medias. Aging destruction of cellulose insulation would result in fibers partition. Localized oil overheating: Over 500°C would be a symptom of forming carbon. Any transformer (shunt reactor) that has a source of localized oil heating may be at a time a source of carbon generation. Clay particles as well as carbon are difficult to remove using conventional filter medias.

TABLE 2 Particle nature and mode classification Particle origin

Mode of contaminant

Contaminants resulting from manufacturing processes

Cellulose fibers, sand iron, aluminum, copper.

Contaminants resulting from assembly in field- (Dress and test dirt)

sand, dust ,crumb of clay; calx, welding slag

Operating passive-mode contaminants: Oil aging Wear cellulose Peeling off of the paint

sludge particles ("polymeric" in nature) Cellulose fibers Polymer films, paint Crumb of sorbent

Operating active-mode contaminants Overheating of metals The carbon particles produced in the OLTC wear of bearings of the pumps peeling off metals (from coolers)

Soot, coke (films and three dimensional structures) Wear metals: copper or bronze, iron, aluminum Weld residue or arc debris (Organic and Metallic)

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DANGEROUS EFFECT OF DEGRADADION FACTORS The dielectric safety margin of both major and minor insulation of a transformer contaminated with water is still determined by the dielectric strength of the oil. Presence of bubbles in oil may cause occurrence of critical PD even at rated voltage. Sudden ingress of free water may cause failure of the transformer immediately. Presence of conductive particles including wet fibers of cellulose can reduce dielectric strength of oil and oil-barrier insulation noticeably (Fig1, see effect of aluminum). The dangerous effect of soluble water could be presented as a sharp reduction of dielectric strength of oil with increasing saturation percent due to the increasing conductivity of particles. Increase of the relative saturation e.g. above 50% results in increase of water content in precedently non-conductive fiber particles up to 6-7% causing a sharp reduction of dielectric strength of oil (Fig 1 see wet fibers). The fewer the particles, the weaker the effect of water on the dielectric strength of the oil. Hence removing particles could be a task of priority to maintain dielectric safety margin of insulation having an excessive level of moisture contamination. Efficient processing shall incorporate drying and filtering procedures simultaneously.

Effect of particles and oil relative saturation on dielectric strength of oil

FIGURE 1 Particle contamination is the main factor of degradation of dielectric strength of transformer insulation. The most dangerous particles are conductive mode particles (metals, carbon, wet fibers, etc.). Cigre WG 12.17 ”Particles in oil” collected approximately 50 major failures predominantly of transformers 400-800 kV that attributed to particles contamination [27 ]. TRAPPIING EFFECT OF TRANSFORMER COMPONENTS Particles existing in oil do not remain in oil due to the effect of gravity, oil flow, and particularly the effect of electrical and electromagnetic fields that attracts the conductive particles and simultaneously deposits them on the winding surfaces, pressboard barriers, and bushing porcelain. This phenomenon could be especially critical:

• for converter transformers where DC voltage reinforces particles attraction;

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6

8

10

12

50 100

Particle content Nu/ml

Wet fibres W≈6-7%

Aluminum

Oil Relative saturation, %

80

60

40

20

particles20 g/to

particles 50 g/to

Ebd %

Dry fibres W≤1%

20 40 60 80 100

Ebd KV/mm

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• for shunt reactors where high electromagnetic field strongly attracts particles depositing them on the barrier closest to the winding

• for EHV power transformers • for HV bushings that operate in contaminated oil

Experience has shown that dielectric withstand strength of the oil part of HV bushing could be very sensitive to contamination of transformer oil with conductive particles. There have been several documented cases associated with a deposit of carbon on the lower porcelain, which originated from the localized overheating of the core, and with deposits of iron particles on the porcelain surface, which originated from pump bearing wear. Some typical cases with sever contamination of insulation are shown in the Table 3. Electrical field around the bushing stimulates picking up the particles by the porcelain. Possible failure model of the stained 115 kV O plus CTM condenser bushings was discussed at the Doble Conference 2001 [24,25 ]. It was suggested, based on electrical field analysis, that the local increase of the electrical field intensity particularly in the “Bushing-Tank Wall” space could cause a concentration of conductive substances in this region. Accordingly, that could explain the formation of the semi-conductive streaks along the porcelain. Contamination of major insulation has been observed in the forms of the adsorption of oil aging products by cellulose, or deposits of conducting particles and insoluble aging products in areas of high electrical stresses. The surface contamination can cause a distortion of electrical field and a reduction in the electrical strength of the insulation system. TABLE 3 Cases with local contamination of transformer components

Case Origin of contamination Condition of insulation

1.Large UHV transformer [12 ]

Carbon from the LTC diverter switch escaping into the main tank

Trapping effect of electrical field Particle collected in area with high stress concentrating on the outer circumference of part duct barriers and inside the winding

2.Converter transformer 750 kV

Oil Tar infiltration Electrofilter effect of DC field Oily conductive mode residue on the barriers and valve winding

3.Shunt reactors 400 kV [26 ]

Aluminum particles due to mechanical attrition of aluminum shields

Trapping effect of electromagnetic field of winding Severe contamination of winding and pressboard sheets facing to winding Traces of PD

4.115 kV bushings [24]

Leaching substance of rubber gasket and high content of dissolved metals in transformer oil.

Trapping effect of bushing Formation of conductive stain on the bottom porcelain of the bushings concentration of conductive substances in the form of strips

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Transformer 200MVA 220 kV bushing

Carbon formation in the place of localized core heating

Trapping effect of bushing Carbon deposit of the bottom porcelain surface

Transformer 1000 MVA, 500 kV bushing

Carbon formation due in the place of localized core heating

Trapping effect of bushing shield Carbon deposit on the bottom shield

Autotransformer 210 MVA,400KV

Conductive oil by-product from place of Core lamination heating

Trapping effect of electrical field Deposit of contaminants of insulation surface. Traces of PD across the contaminated area at the top of winding

Deposits of oil sludge or conductive particles on the surface of barriers reduce breakdown voltage particularly under effect of switching surge impulse. Study of dielectric characteristics of the contaminated pressboard patterns taken from a converter transformer has shown (Table 4) that electrical field attracts predominantly conductive mode particles that reduce surface resistivity as many as ten times and cause a critical reduction of dielectric strength across the surface. TABLE 4 Effect of “polymeric” residue sediment on degradation of surface dielectric characteristics. Patterns of pressboard taken from converter transformer

Insulation condition

Stability to PD actions Time to flashover

PF,%

Resistivity Ohm·cm

Surface Resistivity Ohm

Pressboard with residue on the surface

flashover immediately after rise the voltage

1.21 4.3·1013 3·1013

Without residue 12 min 1.1 8.3·1013 2·1014

Tests performer by Transformer Research Institute (Zaporozhye)

For a transformer that has a source of particle generation, unless the source can be rectified or eliminated, only on-line filtration process should be considered as a solution to continued reliable operation. WHICH TRANSFORMER IS A CANDIDATE FOR PROCESSING The following conditions are ranked from lowest to highest from the point of view of improving the dielectric safety margin of the transformer: • Do not allow any bubbles in oil • Remove free water • Remove particles, particularly large and conductive ones • Dry wet insulation • Remove oil aging product Moisture identification

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Experience has shown the Water Heat Run Test [21] to be efficient at determining the presence of free water and dangerous moisture contamination of solid insulation. The method considers temperature migration of moisture and particles and utilizes the build up of water-in-oil with time, when a transformer is heated by load losses up to maximum operative top oil temperature (65-75C). Clear evidence of a defective condition of the transformer shows increasing moisture in oil content with temperature, and sharp reduction of the breakdown voltage due to increasing moisture content in particles. Table 5 presents the results of WHRT of two transformers. Both transformers had been considered normal based on conventional tests and found to be defective ones based on the WHRT results. Table 5. Change of moisture content and oil breakdown voltage while WHRT

Tested transformer Tests

Sampling 400MVA, 347kV, 180MVA, 220kV

Woil, ppm

Before WHRT (30C) After heating( 65C)

12 40

14 30

Ubd, kV Before WHRT (30C) After heating( 65C)

70 24

73.6 36

Tests performed by the ZTZ-Service in-field Lab

The temperature of the oil shall be high enough to “charge” moisture potential and to detect the level of questionable water contamination and to “discharge” insulation and allow extracting a sufficient amount of water from “wet zones”. The latest study of ZTZ-Service has shown that moisture status could be determined in terms of oil relative saturation, using on-line moisture sensor namely Domino [16 ]. To detect water contamination level over 1 % oil relative saturation shall be below 5%. Assuming initial water content in the oil 15 ppm we have that 5% of saturation could be if moisture saturation level is above 300ppm. Using oil saturation data suggested by Paul Griffin [14 ] ,we may determine that it corresponds to minimum oil temperature about 65C. Accordingly, in order to detect water contamination over 2%, oil relative saturation shall be below 8%, and corresponding minimum oil temperature about 55C. Increasing the oil moisture content with temperature in the range below 500C shows typically symptoms of free water. An average moisture content in the pressboard may be estimated using insulation Power Factor test value, especially CHL, considering relative portion of the oil in the space and the Power factor of the oil [ 15]

p

oiloilp K

PFKCHLPF ⋅−= ,

where PFp is power factor of pressboard; CHL-power factor of insulation space between winding at some elevated temperature PFoil-oil power factor at the same temperature; Koil and Kp- design parameters that determine the share of the oil and the pressboard in the space. If those parameters are unknown, we may assume that Koil=Kp=0.5 PFp≤ 0.5 % is characteristic of moisture content in the pressboard below 1% in the range of temperature 20-600C PFp > 0.7-1.0 % in the range of temperature 20-600C is characteristic of moisture content in the pressboard above 2 % Showings of free water or water content if paper over 2% might be a motivated basis for transformer processing

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Particle Contamination Identification A number of methods could be suggested:

• Particles counting in the range of 2-150 µm; and that particles in the 2-10 µm range may detect and somewhat quantify carbon particles

• Microscopic examination to determine size, shape, metallic or nonmetallic nature • The difference between PF and Resistivity tests before and after oil filtration • Dissolve metal by means of atomic absorption spectroscopy • The difference in moisture content of oil before and after filtration

Possible presence of a source of particles contamination on the basis of DGA test and symptoms of wear of bearings of the pumps should be considered. Particle sedimentation and aforesaid trapping effect of electrical and electromagnetic field can cause some underestimation of contamination level. Repeated sampling after oil agitation may assist verification of the contamination level of a questionable transformer. ZTZ-Service uses some additional tests based on temperature response of insulation Power factor tests CH and CHL[15]. An unusual difference between two CH tests at different temperature could be a characteristic of excessive oil contamination, and reducing the CHL with temperature is typically symptom of insulation surface contamination. Denomination of typical contamination levels including possible dangerous levels has been advised by the WG 12.17 using ISO 4406 classification as the following : 15/12 Normal Contamination level typical for transformer in service 16/13 High Possible transformer malfunction High level means presents of 32000-64000 particles of 5 µm and above and over 8000 particles of 15 µm and above in 100 ml of oil. It’s apparent that improvement of transformer condition in-service is mandatory and on-line filtering process is particularly desirable. PARAMETERS OF PROCESSING The process of reconditioning a transformer by means of circulating the oil through processing equipment is of exponential mode and, irrespective of the type of purification, may be expressed by the equation:

)exp()(

0 τξ t

ntn

⋅−=

where no - initial concentration of contaminants (particles, water, gas, acids, etc) n(t) - desirable final or current concentration ξ - Coefficient of purification effectiveness, 0 <ξ <1 - ratio between input and

output concentration or rate of removed contaminant per one pass t - time of processing τ - time constant - with τ = V/Q V - oil volume in the transformer Q - rate of flow Three parameters should be considered:

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• Ratio of final and initial concentration of contaminants • Ratio of flow rate and total volume of oil in the transformer • Ratio of inlet and outlet concentration of contaminant per one pass of treatment

through processing machine

The most important parameter, which determines effectiveness of the process, is relative rate of contaminant removed per one pass, namely:

• Ratio of input and output water, • Ratio of particles, • Ratio of oil aging characteristics (neutralization number, interfacial tension, PF,

resistivity)

For example, if the system reduces the water content from the input 50 ppm to output 10 ppm per one pass with flow rate 2 m3 per hour, the time to reduce water to 10 ppm in the transformer of 20 m3 will take 20 hours. That is equal to processing two volumes of oil in the transformer. If processing equipment removes only 50 % of input contaminant per one pass, the time will be 32 hours. Another important parameter to be monitored is the ratio of flow rate and the volume of oil to be treated. Both of the above mentioned parameters are variable, which is why it is very important to properly arrange on-line monitoring of processing characteristics. The following approach might be suggested to optimize the process:

• Check the initial condition (concentration of contaminants to be removed) • Define the desirable final condition • Define the optimal parameters of processing: flow rate, temperature, that give the

maximal rate of removing contaminant; • Estimate the time of process • Evaluate the possible life of adsorbents and filter elements to be replaced during

the total time of processing; • Arrange monitoring of above mentioned basic parameters of processing and

auxiliary parameters (temperature, flow rate, vacuum) SAFETY ISSUES

The main disadvantage of on-line processing is a risk of failure due to unintentional impairment of the transformer condition. Recommendations for some safety measures: Minimize the risk of reducing the dielectric withstand strength due to possible introduction into the tank of foreign impurities The system shall not incorporate a vacuum process while the transformer is on-line. Do not allow air to permeate into the tank: Thoroughly remove air from lines Use a bypass system to allow for closed loop tests and adjustment of the machine before actual operation Do not allow oil to splash.

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Do not allow foam ingress into the tank (the oil degassifier) Reduce flow rate to let foam settle Do not process oil with excessive foaming tendency. Consider the presence of silicon.

Do not allow particle ingress into the tank Consider reliable filtration

Do not allow moisture ingress into the tank

Consider static electrification. That is particularly important for transformers 160kV and above

Do not allow turbulence of oil Minimize the risk of losing oil during processing Consider minimal volume of oil in the transformer, taking into account possible loss of oil during reclamation (replacement of waste clay). Watch oil level; consider the oil level gauge. Consider arrangement of a metal standpipe to minimize the loss of oil Consider automatic shut down controls. Minimize the risk of failure during processing of a defective transformer In general, any defective transformer can be processed without de-energizing if adequate measures to prevent impairment of its condition are taken. However, lack of the necessary diagnostic characteristics often precludes the determination of the real technical condition of the unit. Consider possibility of overheating the transformer during the process Processes that need high temperature (drying out, insulation regeneration) may affect the thermal behavior of the transformer. Possible loss of paper life should be considered. TREATMENT METHODS ON ENERGISED TRANSFOMERS The following procedures have been experienced and may be performed on energized transformer • Drying of oil and insulation through drying of oil [3,4,5,17,] • Oil degassing [9,18] • Oil reclamation [6,7] • Oil filtering and purification of insulation through filtering of oil [28] • Regeneration (desludging) insulation using oil as a solvent [6,8] One can distinguish between passive and active methods of treatment: Active methods incorporate force moving the oil through filter, vacuum-degassing machine, fuller’s earth towers, etc. This approach gives the ability to monitor and accelerate the process.

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Passive methods incorporate typically a system of some cartridges filled with sorbent and connected to the tank or to the coolers . Efficiency of methods depend on physical effect chosen for processing Methods based on diffusion processes: reclamation, vacuum degassing-diffusion through oil film-, drying out of cellulose, etc. are more effective at high temperature; Methods based on adsorption processes: drying oil trough adsorption (e.g., paper) filter, restoration of color, etc. are more effective at low temperature Drying of oil and insulation through drying of oil On the basis of experience one can define two typical defective conditions of a transformer demanding drying out:

• Accumulation of free water on the bottom of the tank or coolers. Solid insulation is comparatively dry or wet locally. The quantity of free water amounts typically from 2-3 up to 10-13 gallons e.g.[23]

• Concentration of water in thin structure basically in the pressboard barriers

contacting the bulk of oil. This structure comprises typically 20-25%.[11] of the solid insulation mass. Assuming Water content in wet zones up to 3 - 4 % and total insulation mass of large power transformer in average 11,000 pounds we have that approximately 10-13 gallons of water should be removed to restore initial insulation condition.

ZTZ-Service experience with assessment of water content in power transformers [21,22] has shown that likely wet zones are pressboard barriers situated between outer winding and tank, particularly at the bottom. Quantity of excessive water is limited and typically only 6-9 gallons of water have to be extracted to restore residual water content on the level of 0.5%/

The process cellulose insulation drying needs in some elevated temperature (over 55-60C). In order to get moisture content below 1%, one must maintain of oil percent saturation below 5%. The water content in oil is directly proportional to the relative water concentration (relative saturation) up to the saturated level. That’s why the difference in solubility characteristics of the oils should be considered. Cigre WG 12.18 has suggested the solubility saturating level of oils depending on aromatic content (Table 6 )

TABLE 6 Water solubility of different oils Content of aromatic

Hydrocarbons

Water saturation level (ppm)

Oils CA, % 20 °C

30°C

40 °C

50°C

60°C 70 °C

1 5 42.8 65.47

97.5 141.6 201.2 279

2 8 46.8 72 108 158 225 316 3 16 56.2 86.1 128.3 186.5 265 369.2 4 21 75 111. 162 230.2 320 436

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7 There have been presented mainly three on-line dry-out techniques:

• Based on vacuum exposure • Utilizing the molecular sieves sorption capacity • Utilizing Superdri cartridges

The main advantage of vacuum technique is drying and degassing the oil simultaneously. In order to maintain percent saturation of effluent oil below 5% the residual pressure in vacuum system should be less than equilibrium level (300-400 Pa). The main advantage of adsorption technique is a high moisture capacity of molecular sieves even at high temperature. Table 7 shows that artificial adsorbent NaA can absorb at 50 C up to 16 g water per 100 gw of adsorbent maintaining oil percent saturation below 3%. TABLE 7 Sorption Capacity of the NaA Molecular Sieves , g/100g

Vapor Pressure, Temperature 0C

Pa psi 25 50 100

1.33 1.93·10-4 6.0 3.8 3.0

13.3 1.93·10-3 15 8.0 3.6

133 0.0193 18 16 3.6

The main advantage of Superdry technique is removing simultaneously dissolved moisture and particles. Typically one cartridge can remove up to 1.5 kg of water as well as a large quantity of particles. Oil Filtration

Experience has shown that filtration of the oil can effectively remove particles larger than a micrometer including coke, organic films, wear metals. However, effect of “agglomeration”, namely, “bunching” together of sub-micrometer substances like some carbon particles gives hope that most of dangerous contaminants can be removed in a course of on-line filtration.

A beneficial experience of on-line LTC filtration system [28] suggest a future prospect fo implementation of similar system on Large Power Transformers There have been some technical problems with oil purification, which have to be considered: • Filter cartridge selection for oil processing is critical to achieving good results The

micron rating does not characterize a filter in a unique manner. Nominal filter ratings are based on gravimetric tests and applying efficiency, based on weight, which takes no regard of particle size. What one manufacturer calls a half-micron filter can be designated as a five-micron filter by another manufacturer. The Beta Ratio is a more precise definition of filter efficiency [28].

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• Filtering of small particles, especially carbon could be a subject of particular concern. Nominal 0.5µm or even 0.3µm cartridges should be used to remove carbon particles.

• Particle counting and microscopic analysis before and after filtration would support the selection of a proper cartridge.

• Removing small light particles (e.g. clay crumb) can also be a problem because they are floating in the oil following convective flow. This is a disadvantage in comparison with purification of the oil by draining all of it out of the transformer tank.

• Some filter (particularly paper) can be a source of particle generation itself. The useful life of the filter shall be considered, particularly for on-line applications.

• The possibility of cavitation and gas bubbles coming out of the oil at low pressure points in the system shall be particularly considered. Restrictions in the suction line, using a long length of hose or a small diameter of hose are common reasons for this.

• Filter systems should be checked for proper flow direction through the filter housing and cartridge. Proper matching of the filter with the pump flow rate is also critical to good filtration. Over-flowing a filter will reduce its efficiency and capacity.

Regeneration Similar to drying of oil, regeneration is a widespread process and can be performed for both off-line and on-line applications. On-line procedures are more efficient because of the possibility of using internal losses of a transformer to heat the oil. Passive mode permanent reclaiming systems filled with adsorbents (Silica-gel) have been specified in the former USSR for all Power Transformer above 4 MVA since early 60’s. This system was very beneficial: in many instances oil acidity in transformer population has been retained below 0.1 g/mg KOH Experience has shown a good efficiency of the so-called reclaiming without waste using Fuller’s earth reactivation technology [6]. There have been also some technical problems that have to be considered:

• A large amount of waste • Loss of oil during reclamation, which is more sensitive in the case of an

energized unit • Limited amount of oil processed with one charge • Risk of introducing clay crumbs into the tank (more critical for energized unit) • Risk of mechanical destruction of very aged paper layer being impregnated with

oil by-products

CONCLUSION On-line processing of oil in load tap changer compartments has become a common method for increasing reliability of the LTC. This paper presents compelling evidence for the application of on-line processing of transformer main tanks for not only the removal of moisture, but more importantly the removal of particles. Several case histories presented show that particles in the oil of the transformer were directly related to the early failure of the unit. Various methods of on-line processing have been discussed and as the population of transformers continues to age, it is suggested that the use of on-line processing be considered as an effective means of prolonging transformer life and reliability."

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REFERENCES 1.V. Sokolov and Z. Berler Assessment of Power Transformer Insulation Condition Proceedings of the EIC/EMCWE’01 Conference, October 15-18,2001,Cincinnati, OH 2. V.V. Sokolov, Z. Berler, V. Rashkes ”Effective Methods of the Assessment of the Insulation System Conditions in Power Transformers: A View Based on Practical Experience”, Proceedings of the EIC/EMCWE’99 Conference, October 26-28,1999,Cincinnati, OH 3. Ben Taylor ,»Alternative Method to Heat and Vacuum for Drying Transformers, TechCon Conference ,1997 4. Sakkie van Wyk, Rotek Engineering(South Africa)“Dry out systems and techniques for Power Transformers” Proceedings of the EPRI 1998 Conference. 5. M.Girard (Electricity de France),”On-Line Processing of Transformer Oil, ”Minutes of the 1997 EuroDOBLE Colloquium” 6.B.Pahlavanpour,at al (National Grid ,UK)” Transformer life extension by in-situ oil reclamation”,Proceedings of the 4th International Conference on Properties and Application of Dielectric Materials,1994,Brisbane Australia 7. Ross Willoughby “Power Transformer Refurbishment Program”, Proceedings of 1997 Colloquium of Cigre SC12 Transformers, Sydney, October 1997 8. Alberto Herreсo Rocha, On-Line Reclamation of Power Transformer Insulating Oil, Proceedings of the Sixty-Sixth Annual International Conference of Doble Clients, 1999, Sec. 5-6 9. Larri Christodoulou, Bruce Baranowski, “On-Line Energized Oil Processing of Transformers a Perspective of System Manufacturer and Service Company”, Proceedings of the 1995 DOBLE Conference,Sec.5.7 10.T.V. Oommen, ”Particle Analysis on Transformer Oil for Diagnostic and Quality control Purposes”, Minutes of the 61st annual conference of Doble Clients, 1984,Sec 10-701 11. V.Sokolov, B Vanin, P. Griffin, Consideration on Moisture Distribution in Transformers, Proceedings of the Sixty-Sixth Annual International Conference of Doble Clients, 1999, Sec. 5-12. A. Carlson. Contamination within major insulation, Discussion Proceedings of 35th session of CIGRE, 1994 13. Abel Pereira,Bonnevile Power Administration. ”Safe handling Procedures for insulating oil with High Concentration of combustible gases”-Proceedings of the DOBLE 1996 Conference,Sec 5-7 14. P.Griffin, et al. Comparison of Water Equilibrium in Silicon and Mineral Oil Transformers. Minutes of the Fifty-Fifth Annual International Conference of Doble Clients, 1988, sec. 10-9.1. 15.V. Sokolov, B. Vanin. “Evaluation of Power Transformer Insulation through Measurement of Dielectric Characteristics”, Proceedings of the 63rd Annual International Conference of Doble Clients, 1996, sec. 8-7. 16. Lance R. Lewand and Paul J. Griffin, Development and Application of a continuous m\ Moisture-in Oil Sensor, Proceedings of the 2000 International Conference of Doble Clients - Sec 5-4

17. Jeffrey W. Ames, On-line removal of moisture from Power Transformers, Proceedings of the 2000 International Conference of Doble Clients, Sec 5-5 18. P.W.Brunson ,at.al.”On-Line degassung of EHV power transformers”,Minutes of the 1990 DOBLE Conference,Sec.6-11 19.J. Laakso, British Columbia Hydro and Power Authority,”Discussion of the P.Brunson paper”, 1990 Doble Conference, Sec.6.11.1A

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20.Minutes of the 1998 FALL Meeting of DOBLE Clients:Discussing experience with oil reclamation on energized transformers: 21. V.Sokolov, B.Vanin “Experience with in-field assessment of water contamination of large power transformers”, Proceedings of EPRI 1999 Conference 22. V.Sokolov, B.Vanin. In-Service Assessment of Water Content in Power Transformers. Proceedings of the Sixty-Second Annual International Conference of Doble Clients, 1995, sec. 8 23. P.Christensen and G.Ohlson. The Behavior of Moisture and Free water in Power Transformer. Proceedings of the Sixty-fifth Annual International Conference of Doble Clients, 1998, sec. 8-3. 24. Danny E. Bates Contamination of lower porcelain experienced on ABB type O PLUS CTM bushings, Proceedings of the Sixty-Eight Annual International Conference of Doble Clients, 2001, sec 8 25.Bent Hayman Discussion of the Danny Bates paper “Contamination of lower porcelain experienced on ABB type O PLUS CTM bushings”, Proceedings of the Sixty-Eight Annual International Conference of Doble Clients, 2001, sec 8 26.Victor V. Sokolov and Valery Shkrum, Experience with Assessment and Refurbishment of 400 kV Shunt Reactors, Proceedings of the Sixty-Fourth Annual International Conference of Doble Clients, 1997, Sec 8-7 27. “Effect of particles on transformer dielectric strength”, the Report of the Cigre WG12.17 Particles in oil 28.Ben Taylor, Mechanical Filtration of Insulating Oils, Proceedings of thee TechCon 2000, Feb. 2-3, Mesa, Arizona 29. Danny E. Bates On-Line LTC Oil Filtration and Temperature Monitoring. A Case study, Proceedings of the Sixty-Fifth Annual International Conference of Doble Clients, 1998, Sec. 8- 16