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 25-30years 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. 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.
1
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 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.
2
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 turninsulation 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
After heating at
heating
100°C for 4-6
hours
25MVA,
Acidity=0.038mg KOH/g
29
40
110 kV
IFT=32.0dynes/cm
11 years
Saponification number= 0.097 mg KOH/g
40.5 MVA
110 kV
18 years
150 MVA
220 kV
25 years
Acidity=0.133 mg KOH/g
IFT=23.18dynes/cm
Saponification number= 0.44 g KOH/g
25.8
50
Acidity=0.055 mg KOH/g
IFT=28.8 dynes/cm
Saponification number= 0.138 g KOH/g
17.7
32
PF=10.8 % at 90 C
† Tests performed in ZTZ – Service Material Lab
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].
3
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 is 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 micron 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
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)
sludge particles ("polymeric" in nature)
Cellulose fibers
Polymer films, paint
Crumb of sorbent
Soot, coke (films and three dimensional
structures)
Wear metals: copper or bronze, iron, aluminum
Weld residue or arc debris (Organic and Metallic)
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
4
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.
Ebd
Ebd
KV/mm
%
Fibras úmidas
W<ou=1%
6
4
20 g/to
60
10
8
partículas
80
Fibras Secas
40
W≈6-7%
Alumínio
50
100
Concentração Partículas Nu/ml
20
partículas
50 g/to
20
40
60
80
100
Saturação Relativa Óleo, %
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;
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
5
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
2.Converter
transformer 750 kV
Oil Tar infiltration
3.Shunt reactors
400 kV [26 ]
Aluminum particles due
to mechanical attrition of
aluminum shields
4.115 kV bushings
[24]
Leaching substance of
rubber gasket and high
content of dissolved
metals in transformer oil.
Transformer
200MVA
220 kV bushing
Transformer 1000
MVA,
500 kV bushing
Autotransformer
210 MVA,400KV
Carbon formation in the
place of localized core
heating
Carbon formation due in
the place of localized
core heating
Conductive oil byproduct from place of
Core lamination heating
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
Electrofilter effect of DC field
Oily conductive mode residue on the
barriers and valve winding
Trapping effect of electromagnetic field
of winding
Severe contamination of winding and
pressboard sheets facing to winding
Traces of PD
Trapping effect of bushing
Formation of conductive stain on the
bottom porcelain of the bushings
concentration of conductive substances
in the form of strips
Trapping effect of bushing
Carbon deposit of the bottom porcelain
surface
Trapping effect of bushing shield
Carbon deposit on the bottom shield
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
6
Insulation
condition
Stability to PD
actions
Time to flashover
PF,%
Resistivity
Ohm·cm
Surface
Resistivity
Ohm
Pressboard with
residue on the
surface
Without residue
flashover
immediately after
rise the voltage
12 min
1.21
4.3·1013
3·1013
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. (Victor, I agree but this statement needs further explanation)
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
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
Ubd, kV
Before WHRT (30C)
12
After heating( 65C)
40
Before WHRT (30C)
70
After heating( 65C)
24
Tests performed by the ZTZ-Service in-field Lab
14
30
73.6
36
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
7
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]
CHL − K oil ⋅ PFoil
,
Kp
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 =
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
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 :
8
15/12
16/13
Normal
High
Contamination level typical for transformer in service
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 inservice 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:
n(t )
t
= exp(−ξ ⋅ )
τ
n0
where
- initial concentration of contaminants (particles, water, gas, acids, etc)
- desirable final or current concentration
- Coefficient of purification effectiveness, 0 < ξ <1 - ratio between input and output
concentration or rate of removed contaminant per one pass
- time of processing
- time constant - with τ = V/Q
- oil volume in the transformer
- rate of flow
no
n(t)
ξ
t
τ
V
Q
Three parameters should be considered:
•
•
•
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;
9
•
•
•
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.
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
10
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.
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
11
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 )
Oils
TABLE 6 Water solubility of different oils
Content of
aromatic
Water saturation level (ppm)
Hydrocarbons
CA, %
20 °C
30°C
40 °C 50°C
60°C
1
2
3
4
5
8
16
21
42.8
46.8
56.2
75
65.47
72
86.1
111.7
97.5
108
128.3
162
141.6
158
186.5
230.2
201.2
225
265
320
70 °C
279
316
369.2
436
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.
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
12
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].
• 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 offline 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 byproducts
CONCLUSION
On-line processing of oil in load tap changer compartments has become a common
method for increasing reliabiliy 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
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processing be considered as an effective means of prolonging transformer life
and reliability."
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0D200332CF3/$File/1ZSC%20954003-006%20en%20Rev%203.pdf
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