Power TransformerINTRODUCTION Transformer is a vital link in a
power system which has made possible the power generated at low
voltages (6600 to 22000 volts) to be stepped up to extra high
voltages for transmission over long distances and then transformed
to low voltages for utilization at proper load centers. With this
tool in hands it has become possible to harness the energy
resources at far off places from the load centers and connect the
same through long extra high voltage transmission lines working on
high efficiencies. At that, it may be said to be the simplest
equipment with no motive parts. Nevertheless it has its own
problems associated with insulation, dimensions and weights because
of demands for ever rising voltages and capacities. In its simplest
form a Transformer consists of a laminated iron core about which
are wound two or more sets of windings. Voltage is applied to one
set of windings, called the primary, which builds up a magnetic
flux through the iron. This flux induces a counter electromotive
force in the primary winding thereby limiting the current drawn
from the supply. This is called the no load current and consists of
two componentsone in phase with the voltage which accounts for the
iron losses due to eddy currents and hysteresis, and the other 90
behind the voltage which magnetizes the core. This flux induces an
electro-motive force in the secondary winding too. When load is
connected across this winding, current flows in the secondary
circuit. This produces a demagnetising effect, to counter balance
this the primary winding draws more current from the supply so that
IP.NP = IS.NS Where Ip and Np are the current and number of turns
in the primary while IS and NS are the current and number of turns
in the secondary respectively. The ratio of turns in the primary
and secondary windings depends on the ratio of voltages on the
Primary and secondary sides. The magnetic core is built up of
laminations of high grade silicon or other sheet steel which are
insulated from each other by varnish or through a coating of iron
oxide. The core can be constructed in different ways relative to
the windings.
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CONSTRUCTION 1- Transformer Core Construction in which the iron
circuit is surrounded by windings and forms a low reluctance path
for the magnetic flux set up by the voltage impressed on the
primary. Fig (1), Fig. (6) and Fig. (7) Shows the core type
Fig (1) core type The core of shell type is sh own Fig.(2),
Fig.(3), Fig.(4), and Fig.(5), in which The winding is surrounded
by the iron Circuit Consisting of two or more paths through which
the flux divides. This arrangement affords somewhat Better
protection to coils under short circuit conditions.
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In actual construction there are Variations from This simple
construction but these can be designed With such proportions as to
give similar electrical characteristics.
Fig (2) shell type
Fig.(3) Single phase Transformer Fig. (4) Single phase
Transformer .
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Fig. (5) 3- phase Transformer Shell type
Fig. (6) 3- phase Transformer core type
Fig. (7) Cross section of a three-phase Distribution Transformer
(Core Type) Three-phase Transformers usually employ three-leg core.
Where Transformers to be transported by rail are large capacity,
five-leg core is used to curtail them to within the height
limitation for transport. Even among thermal/nuclear power station
Transformers, which are usually transported by ship and freed from
restrictions on in-land transport, gigantic
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Transformers of the 1000 MVA class employ five-leg core to
prevent leakage flux, minimize vibration, increase tank strength,
and effectively use space inside the tank. Regarding single-phase
Transformers, two-leg core is well known. Practically, however,
three leg cores is used, four-leg core and five-leg core are used
in large capacity Transformers. The sectional areas of the yoke and
side leg are 50 % of that of the main leg; thus, the core height
can be reduced to a large extent compared with the two leg core.
For core material, high-grade, grain oriented silicon steel strip
is used. Connected by a core leg tie plate fore and hind clamps by
connecting bars. As a result, the core is so constructed that the
actual silicon strip is held in a sturdy frame consisting of clamps
and tie plates, which resists both mechanical force during hoisting
the core-and-coil assembly and short circuits, keeping the silicon
steel strip protected from such force. In large-capacity
Transformers, which are likely to invite increased leakage flux,
nonmagnetic steel is used or slits are provided in steel members to
reduce the width for preventing stray loss from increasing on metal
parts used to clamp the core and for preventing local overheat. The
core interior is provided with many cooling oil ducts parallel to
the lamination to which a part of the oil flow forced by an oil
pump is introduced to achieve forced cooling. When erecting a core
after assembling, a special device shown in Fig. (8) Is used so
that no strain due to bending or slip is produced on the silicon
steel plate.
Fig (8)
Fig (9) The steel strip surface is subjected to inorganic
insulation treatment. All cores employ miter-joint core
construction. Yokes are jointed at an angle of 45 to utilize the
magnetic flux directional characteristic of steel strip. A
computer-controlled automatic machine cuts grain-oriented silicon
steel strip with high accuracy and free of burrs, so that magnetic
characteristics of the grain-oriented silicon steel remains
unimpaired. Silicon steel strips are stacked in a circle-section.
Each core leg is fitted with tie plates
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on its front and rear side, with resin-impregnated glass tape
wound around the outer circumference. Sturdy clamps applied to
front and rear side of the upper and lower yokes are bound together
with glass tape. And then, the resin undergoes heating for
hardening to tighten the band so that the core is evenly clamped
Fig. (9). Also, upper and lower clamps are connected by a core leg
tie plate; fore and hind clamps by connecting bars. As a result,
the core is so constructed that the actual silicon strip is held in
a sturdy frame consisting of clamps and tie plates, which resists
both mechanical force during hoisting the core-and-coil assembly
and short circuits, keeping the silicon steel strip protected from
such force. In large-capacity Transformers, which are likely to
invite increased leakage flux, nonmagnetic steel is used or slits
are provided in steel members to reduce the width for preventing
stray loss from increasing on metal parts used to clamp the core
and for preventing local overheat. The core interior is provided
with many cooling oil ducts parallel to the lamination to which a
part of the oil flow forced by an oil pump is introduced to achieve
forced cooling. When erecting a core after assembling, a special
device shown in Fig. (8) Is used so that no strain due to bending
or slip is produced on the silicon steel plate. 2 - Winding Various
windings are used as shown below. According to the purpose of use,
the optimum winding is selected so as to utilize their individual
features. 1 - Helical Disk Winding (Interleaved disk winding) In
Helical disk winding, electrically isolated turns are brought in
contact with each other as shown in Fig. (10) Thus, this type of
winding is also termed "interleaved disk winding." Since conductors
1 - 4 and conductors 9 - 12 assume a shape similar to a wound
capacitor, it is known that these conductors have very large
capacitance. This capacitance acts as series capacitance of the
winding to highly improve the voltage distribution for surge.
Unlike cylindrical windings, Helical disk winding requires no
shield on the winding outermost side, resulting in smaller coil
outside diameter and thus reducing Transformer dimension.
Comparatively small in winding width and large in space between
windings, the construction of this type of winding is appropriate
for the winding, which faces to an inner winding of relatively high
voltage. Thus, general EHV or UHV substation Transformers employ
Helical disk winding to utilize its features mentioned above. 2 -
Continuous Disk Winding This is the most general type applicable to
windings of a wide range of voltage and current Fig. (11). this
type is applied to windings ranging from BI L of 350kV to BI L of
1550kV. Rectangular wire is used where current is relatively small,
while transposed cable Fig. (12) is applied to large current. When
voltage is relatively low, a Transformer of 100MVA
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or more capacity handles a large current exceeding 1000A. In
this case, the advantage of transposed cable may be fully
utilized.
Fig. (10)
Fig. (11). Continuous Disk Winding
Fig. (12) Transposed conductor construction Diagram Further,
since the number of turns is reduced, even conventional continuous
disk construction is satisfactory in voltage distribution, thereby
ensuring adequate dielectric characteristics. Also, whenever
necessary, potential distribution is improved by inserting a shield
between turns. 3 - Helical windings For windings of low voltage
(20kV or below) and large current, a helical coil is used which
consists of a large number of parallel conductors piled in the
radial Direction and wound. Adequate transposition is necessary to
equalize the share of current among these parallel conductors. Fig
(12) illustrates the transposing procedure for double helical coil.
Each conductor is transposed at intervals of a fixed number of
turns in the order shown in the figure, and as a result the
location of each conductor opposed to the high
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voltage winding is equalized from the view point of magnetic
field between the start and the end of winding turn.
Fig. (13) double helical coil 3 - Tank. The tank has two main
parts: a The tank is manufactured by forming and welding steel
plate to be used as a container for holding the core and coil
assembly together with insulating oil. The base and the shroud,
over which a cover is sometimes bolted. These parts are
manufactured in steel plates assembled together via weld beads. The
tank is provided internally with devices usually made of wood for
fixing the magnetic circuit and the windings. In addition, the tank
is designed to withstand a total vacuum during the treatment
process. Sealing between the base and shroud is provided by weld
beads. The other openings are sealed with oil-resistant synthetic
rubber joints, whose compression is limited by steel stops. Finally
the tank is designed to withstand the application of the internal
overpressure specified, without permanent deformation.
Fig (14) Power Transformer 30 MVA 132 / 11 KV
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b - Conservator The tank is equipped with an expansion reservoir
(conservator) which allows for the expansion of the oil during
operation. The conservator is designed to hold a total vacuum and
may be equipped with a rubber membrane preventing direct contact
between the oil and the air.
Fig. (15)
Fig. (16) 4 - Handling devices: Various parts of the tank are
provided with the following arrangements for handling the
Transformer. - Four locations (under the base) intended to
accommodate bidirectional roller boxes for displacement on rails. -
Four pull rings (on two sides of the base) - Four jacking pads
(under the base) - Tank Earthing terminals: The tank is provided
with Earthing terminals for Earthing the various metal parts of the
Transformer at one point. The magnetic circuit is earthed via a
special external terminal. 5 - Valves: The Transformers are
provided with sealed valves, sealing joints, locking devices and
position indicators. The Transformers usually include: 163
- Two isolating valves for the "Buchholz" relay. - One drainage
and filtering valve located below the tank. - One isolating valve
per radiator or per cooler. - One conservator drainage and
filtering valve. And when there is an on-load adjuster: - Two
isolating valves for the protection relay. - One refilling valve
for the on-load tap-changer. - One drain plug for the tap-changer
compartment. 6 - Connection Systems Mostly Transformers have
top-mounted HV and LV bushings according to DIN or IEC in their
standard version. Besides the open bushing arrangement for direct
Connection of bare or insulated wires, three basic insulated
termination systems is available. Fully enclosed terminal box for
cables Fig. (17&18) Available for either HV or LV side, or for
both. Horizontally split design in degree of protection IP 44 or IP
54. (Totally enclosed and fully protected against contact's With
live parts, plus protection against drip, splash, or spray water.)
Cable installation through split cable glands and removable plates
facing diagonally downwards. Optional conduit hubs suitable for
single-core or three-phase cables with solid dielectric insulation,
with or without stress cones. Multiple cables per phase are
terminated on auxiliary bus structures attached to the bushings
removal of Transformer by simply bending back the cables.
Fig. (17)
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Fig. (18) HV Side 300 KV
Fig. (19) LV Side (11KV) connection terminal 3-cable for each
phase 7 - The dehydrating breather The dehydrating breather is
provided at the entrance of the conservator of oil immersed
equipment such as Transformers and reactors. The conservator
governs the breathing action of the oil system on forming to the
temperature change of the equipment, and the dehydrating breather
removes the moisture and dust in the air inhaled and prevents the
deterioration of the Transformer oil due to moisture absorption.
Construction and Operation See Fig. (20) The dehydrating breather
uses silica - gel as the desiccating Agent and is provided with an
oil pot at the bottom to filtrate the inhaled air. The
specifications of the dehydrating breather are shown in Table (1)
and the operation of the component parts in Table (2).
165
Fig. (20) Dehydrating breather 166
1. Case 2. Peep window 3. Flange 4. Oil pot 5. Oil pot holder 6.
Breathing pipe 7.Filter 8. silica-gel 9.Absorbent 10. Oil
(Transformer oil) 11. Wing nut 12.Cover 13. Suppression screw 14.
Set screw 15. Oil level line (Red
Table - 1Type Weight of desiccating agent 4.5 kg Desiccating
agent
FP4.5A
Material --- Silica-gel (Main component SiO2) Shape, Size ---
spherical, approx. 4 5 Mixed ratio --- white silica-gel 75% blue
silica-gel 25%
Table - 2Item Silicagel Blue silica -gel Action Removes moisture
in the air inhaled by the Transformer Or reactor. In addition to
the removal of moisture, indicates the Extent of moisture
absorption by discoloration. (Dry condition) (Wet condition ) Blue
------ Light purple ----- Light pink Removes moisture and dust in
the air inhaled by: the Transformer or reactor. In addition, while
it is not performing breathing action, it seals the desiccating
agent from the outer air to prevent unnecessary moisture Absorption
of the desiccating agent. Absorbs dust and deteriorated matter in
the oil pot, to Maintain the oil pot in a good operating
condition.
Oil pot
Oil and filter
absorbent
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Bushing Having manufactured various types of bushings ranging
from 6kV-class to 800kVclass, Toshiba has accumulated many years of
splendid actual results in their operation. Plain-type Bushing
Applicable to 24 kV-classes or below, this type of bushing is
available in a standard series up to 25,000A rated current.
Consisting of a single porcelain tube through which passes a
central conductor, this bushing is of simplified construction and
small mounting dimensions; especially, this type proves to be
advantageous when used as an opening of equipment to be placed in a
bus duct Fig. (21).
Fig. (21) 24 KV Bushing Oil-impregnated, Paper-insulated
Condenser Bushing
Fig. (22) 800 KV bushing The oil-impregnated, paper insulated
condenser bushing, mainly consisting of a condenser cone of
oil-impregnated insulating paper, is used
For high-voltage application (Fig. 22&23). This bushing, of
enclosed construction, offers the Following features: High
reliability and easy maintenance.
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Partial discharge free at test voltage. Provided with test
tapping for measuring electrostatic capacity and tan . Provided
with voltage tapping for connecting an instrument Transformer if
required.
Fig. (23) Bushing type GOEK 1425 for direct connection of 420 KV
Power Transformer to gas insulated Switchgear or high voltage
cable
Fig. (24) Cut away view of Transformer bushing type GOE
Construction of Cable Connection and GIS Connection Cable
Connection In urban-district substations connected with power
cables and thermal power stations suffered from salt-pollution,
cable direct-coupled construction is used in which a Transformer is
direct-coupled with the power cable in an oil chamber. Indirect
connection system in which, with a cable connecting chamber
attached to the Transformer tank, a coil terminal is connected to
the cable head through an oil-oil bushing in the cable connection
chamber. Construction of the connection chamber can be divided into
sections. Cable connections and oil filling can be separately
performed upon completion of the tank assembling.
169
Fig. (26) Indirect Cable Connection GIS (Gas Insulated
Switchgear) Connection There is an increasing demand for GIS in
substations from the standpoint of site-acquisition difficulties
and environmental harmony. In keeping with this tendency, GIS
connection-type Transformers are ever-increasing in their
applications. The SF6 gas bus is connected directly with the
Transformer coil terminal through an oil-gas bushing. Oil-gas
bushing support is composed of a Transformer-side flange and an SF6
gas bus-side flange, permitting the oil side and the gas side to be
completely separated from each other.
Fig. (27) Direct GIS Connection
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Buchholz Relays The following protective devices are used so
that, upon a fault development inside a Transformer, an alarm is
set off or the Transformer is disconnected from the circuit. In the
event of a fault, oil or insulations decomposes by heat, producing
gas or developing an impulse oil flow. To detect these phenomena, a
Buchholz relay is installed. Buchholz Relay The Buchholz relay is
installed at the middle of the connection pipe between the
Transformer tank and the conservator. There are a 1st stage contact
and a 2nd stage contact as shown in Fig. (28). the 1st stage
contact is used to detect minor faults. When gas produced in the
tank due to a minor fault surfaces to accumulate in the relay
chamber within a certain amount (0.3Q-0.35Q) or above, the float
lowers and closes the contact, thereby actuating the alarm
device.
Fig. (28). Buchholz Relay
The 2nd stage contact is used to detect major faults. In the
event of a major fault, abrupt gas production causes pressure in
the tank to flow oil into the conservator. In 171
this case, the float is lowered to close the contact, thereby
causing the Circuit Breaker to trip or actuating the alarm device.
Temperature Measuring Device Liquid Temperature Indicator (like BM
SERIES Type) is used to measure oil temperature as a standard
practice. With its temperature detector installed on the tank cover
and with its indicating part installed at any position easy to
observe on the front of the Transformer, the dial temperature
detector is used to measure maximum oil temperature. The indicating
part, provided with an alarm contact and a maximum temperature
pointer, is of airtight construction with moisture absorbent
contained therein; thus, there is no possibility of the glass
interior collecting moisture whereby it would be difficult to
observe the indicator Fig. (30&31). Further, during remote
measurement and recording of the oil temperatures, on request a
search coil can be installed which is fine copper wire wound on a
bobbin used to measure temperature through changes in its
resistance. Winding Temperature Indicator Relay (BM SERIES) The
winding temperature indicator relay is a conventional oil
temperature indicator supplemented with an electrical heating
element. The relay measures the temperature of the hottest part of
the Transformer winding. If specified, the relay can be fitted with
a precision potentiometer with the same characteristics as the
search coil for remote indication.
Fig. (29) Construction of Winding Temperature Indicator
Relay
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Fig (30) Oil Temperature Indicator
Fig. (31) Winding Temperature Indicator The temperature sensing
system is filled with a liquid, which changes in volume with
varying temperature. The sensing bulb placed in a thermometer well
in the Transformer tank cover senses the maximum oil temperature.
The heating elements with a matching resistance is fed with current
from the Transformer associated with the loaded winding of the
Transformer and compensate the indicator so that a temperature
increase of the heating element is thereby proportional to a
temperature increase of the winding-over-the maximum- oil
temperature. Therefore, the measuring bellows react to both the
temperature increase of the winding-over-the-maximum-oil
temperature and maximum oil temperature. In this way the instrument
indicates the temperature in the hottest part of the Transformer
winding. The matching resistance of the heating element is preset
at the factory.
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Pressure Relief Device When the gauge pressure in the tank
reaches abnormally To 0.35-0.7 kg/cm.sq. The pressure relief device
starts automatically to discharge the oil. When the pressure in the
tank has dropped beyond the limit through discharging, the device
is automatically reset to prevent more oil than required from being
discharged.
Fig. (32) Pressure Relief Device Cooling System METHODS OF
COOLING The kinds of cooling medium and their symbols adopted by
I.S. 2026 (Part 11)-1977 are: (a) Mineral oil or equivalent
flammable insulating liquid O (b) Non flammable synthetic
insulating liquid L (c) Gas G (d) Water W
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(e) Air A The kids of circulation for the cooling medium and
their symbols are: (a) Natural N (b) Forced (Oil not directed) F
(c) Forced (Oil directed) D Each cooling method of Transformer is
identified by four symbols. The first letter represents the kind of
cooling medium in contact with winding, the second letter
represents the kind of circulation for the cooling medium, the
third letter represents the cooling medium that is in contact with
the external cooling system and fourth symbol represents the kind
of circulation for the external medium. Thus oil immersed
Transformer with natural oil circulation and forced air external
cooling is designated ONAF. For oil immersed Transformers the
cooling systems normally adopted are: 1- Oil Immersed Natural
cooled Type ONAN. Fig. (33 & 34) In this case the core and
winding assembly is immersed in oil. Cooling is obtained by the
circulation of oil under natural thermal head only. In large
Transformers the surface area of the tank alone is not adequate for
dissipation of the heat produced by the losses. Additional surface
is obtained with the provision of radiators. 2. Oil Immersed Air
Blast - Type ONAF Fig. (35 & 36) In this case circulation of
air is obtained by fans. It becomes possible to reduce the size of
the Transformer for the same rating and consequently save in
cost.
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Fig. (33) Oil Immersed Natural cooled ONAN
Fig. (34) Oil Immersed Natural cooled ONAN
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Fig. (35) Oil Immersed Air Blast - Type ONAF
Fig. (36) Oil Immersed Air Blast - Type ONAF 3. Oil Immersed
Water Cooled - Type ONWN In this case internal cooling coil is
employed through which the water is allowed to flow. Apparently
this system of cooling assumes free supply of water. Except at
hydropower stations this would off-set the saving in cost when
special means have to be provided for adequate supply of water. The
circulation of oil is only by convection currents. This type of
cooling was employed in older designs but has been almost abandoned
in favor of the Type OFWF discussed later. 4. Forced Oil Air Blast
Cooled - Type OFAF Fig. (37) In this system of cooling also
circulation of oil is forced by a pump. In addition fans are added
to radiators for forced blast of air. 5. Forced Oil Natural Air
Cooled - Type OFAN Fig. (38) In this method of cooling, pump is
employed in the oil circuit for better circulation of oil.
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Fig. (37) Forced-oil, Forced-air-cooled - Type OFAF
Fig. (38) Forced Oil Natural Air Cooled - Type OFAN 6. Forced
Oil Water Cooled - Type OFWF In this type of cooling a pump is
added in the oil circuit for forced circulation of oil, through a
separate heat exchanger in which water is allowed to flow. 7.
Forced Directed Oil and Forced Air Cooling -ODAF. 178
It should be remembered that Transformers cooling type OFAF and
OFWF will not carry any load if air and water supply respectively
is removed. It is quite common to select Transformers with two
systems of Cooling e.g., ONAN/ONAF or ONAN/OFAF or sometimes three
systems e.g., ONAN/ONAF/ OFAF. These determine the type of cooling
upto certain loading. As soon as the load exceeds a preset value,
the fans/pumps are Switched on. The rating of a Transformer with
ONAN/ONAF cooling may be written, say, as 45/60 MVA. This means
that so long as the load is below 45 MVA, the fans will not be
working. These are Switched on automatically when the load on the
Transformer exceeds 45 MVA. Type of cooling has a bearing on the
cost of the Transformer. It shall be appreciated that the ONAN
cooling has the advantage of being the simplest with no. fans or
pumps and hence no auxiliary motors. On smaller units say up to 10
MVA, saving in price in changing from ONAN cooling to other forms
of cooling is negligible. On bigger units not only there is a
saving in price but also the reduced weights and dimensions, with
other systems of cooling of Transformers, render the transport easy
and decrease the cost of Foundations etc. Site conditions sometimes
influence the preferred cooling arrangement. For example the
advantage of reduced price, dimensions and weight in case of type
OFWF can be fully realised only where water supply is readily
available. Where special arrangements have to be made for water
supply and disposal of the water, the installation costs for OFWF
Transformers may increase. INSULATING OIL (SPECIFICATIONS AND
DEHYDRATION AT SITE) In Transformers, the insulating oil provides
an insulation medium as well as a heat transferring medium that
carries away heat produced in the windings and iron core. Since the
electric strength and the life of a Transformer depend chiefly upon
the quality of the insulating oil, it is very important to use a
high quality insulating oil. The insulating oil used for
Transformers should generally meet the following requirements: (a)
Provide a high electric strength. (b) Permit good transfer of heat.
(c) Have low specific gravity-In oil of low specific gravity
particles which have become suspended in the oil will settle down
on the bottom of the tank more readily and at a faster rate, a
property aiding the oil in retaining its homogeneity. (d) Have a
low viscosity- Oil with low viscosity, i.e., having greater
fluidity, will cool Transformers at a much better rate. (e) Have
low pour point- Oil with low pour point will cease to flow only at
low temperatures. (f) Have a high flash point. The flash point
characterizes its tendency to evaporate. The lower the flash
point
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the greater the oil will tend to vaporize. When oil vaporizes,
it loses in volume, its viscosity rises, and an explosive mixture
may be formed with the air above the oil. (g) Not attack insulating
materials and structural materials. (h) Have chemical stability to
ensure life long service. Various national and international
specifications have been issued on insulating oils for Transformers
to meet the above requirements. The specifications for insulating
oil stipulated in Indian Standard 335: 1983 are given below.1 2 3 4
5 6 characteristic Appearance Density at 29.5C, Max Interfacial
tension at 270C, Min. Flash point Min. Pour Point Max. Corrosive
Sulphur (in terms of classification of copper strip). Electric
strength (breakdown voltage) Min. (a) New unfiltered oil (b) After
filtration Dielectric dissipation factor (tan ) at 90 C Max.
Specific resistance (resistivity): (a) At 9 0 C Min. (b) at 2 7 0 C
Min. Oxidation stability. (a) Neutralization value, after oxidation
Max. (b) Total sludge, after oxidation, Max. Presence of oxidation
inhibitor Water content, Max. Requirement The oil shall be clear
and transparent and free from suspended matter or sediments. 0.89
g/cm3 0.04 N/m. 104 C - 9 C Non-corrosive.
7
30 kV (rms) 60 kV (rms). 0.002 35 X1012
8 9
/ cm1012
1500 X
/ cm
10
0.4 mg KOH/g 0.10 percent by weight
11 12
The oil shall not contain antioxidant additives. 15 ppm
Gases analysis The analysis of gases dissolved in oil has proved
to be a highly practical method for the field monitoring of power
Transformers. This method is very sensitive and gives an early
warning of incipient faults. It is indeed possible to determine
from an oil sample of about one litre the presence of certain gases
down to a quantity of a few mm3 , i.e., a gas volume corresponding
to about 1 millionth of the volume of the liquid (ppm). The gases
(with the exception of N2 and O2) dissolved in the oil are derived
from the degradation of oil and cellulose molecules that takes
place under the influence of thermal and electrical stresses.
Different stress modes, e.g., normal operating
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temperatures, hot spots with different high temperatures,
partial discharges and flashovers, produce different compositions
of the gases dissolved in the oil. The relative distribution of the
gases is therefore used to evaluate the origin of the gas
production and the rate at which the gases are formed to assess the
intensity and propagation of the gassing. Both these kinds of
information together provide the necessary basis for the evaluation
of any fault and the necessary remedial action. This method of
monitoring power Transformers has been studied intensively and work
is going on in international and national organizations such as
CIGRE, IEC and IEEE. APPLICATION. The frequency with which oil
samples are taken depends primarily on the size of the Transformer
and the impact of any Transformer failure on the network. Some
typical cases where gas analysis is particularly desirable are
listed in the following: 1 - When a defect is suspected (e.g.,
abnormal noise). 2 - When a Buchholz (gas-collecting) relay or
pressure monitor gives a signal. 3 - Directly after and within a
few weeks after a heavy short circuit 4 - In connection with the
commissioning of Transformers that are of significant importance to
the network, followed by a further test some months later.
Different routines for sampling intervals have been developed by
different utilities and in different countries. One sampling per
year appears to be customary for large power Transformers (Rated
>= 300 MVA >= 220 kV). The routine that has been used over a
long period of time of checking the state of the oil every other
year by measuring the breakdown strength, the tan value, the
neutralization coefficient and other physical quantities is not
replaced by the gas analysis. Extraction and analysis To be able to
carry out a gas analysis, the gases dissolved in the oil must be
extracted and accumulated. The oil sample to be degassed is sucked
into a pre-evacuated degassing column. A low pressure is maintained
by a vacuum pump. To assure effective degassing (> 99 per cent),
the oil is allowed to run slowly over a series of rings which
enlarge its surfaces. An oil pump provides the necessary
circulation. The gas extracted by the vacuum pump is accumulated in
a vessel. Any water that may have been present in the oil is
removed by freezing in a cooling trap to ensure that the water will
not disturb the vacuum pumping. The volumes of the gas and the oil
sample are determined to permit calculation of the total gas
content in the oil. The accumulated gas is injected by means of a
syringe into the gas chromatograph, which analyses the gas sample.
The result is plotted on a recorder in the form of a chromatogram.
Using calibration gases it is possible to identify the different
peaks on a chromatogram. Recalculation of the height of a peak to
the content of this gas is done by comparison with chromatogram
deflections from calibration gases. With the composition of the gas
mixture and the total gas content in the oil sample known; the
content (in ppm) of the individual gases in the oil is obtained.
The following gases are analyzed: 1 - CARBON MONOXIDE CO
181
2 - CARBON DIOXIDE 3 - HYDROGEN 4 - ETHANE 5 - ETHENE 6 -
ACETYLENE 7 - METHANE 8 - PROPANE
CO2 H2 C2H6 C2H4 C2H2 CH4 C3H6
The detection limits depend partly on the total gas content; for
hydrocarbons (except methane) the limit lies below 0,5 ppm, for
hydrogen, methane and carbon monoxide about 5 ppm and for carbon
dioxide about 2 ppm. This high sensitivity is necessary in those
cases where it is desired to determine a trend in the gas evolution
at short sampling intervals, e.g., during a heat run test or when
oil samples are taken at intervals of only a few days.
Identification of faults. The fault types that can and should be
identified are corona, electrical discharges, excessively hot metal
surfaces and fast degradation of cellulose. It is possible to
obtain an idea of the type of fault by using a diagnosis scheme. A
number of different schemes of this type have been prepared. To
avoid having to deal with the contents of the individual gases, one
frequently uses quotients between different gases. Some schemes
give an appearance of great precision, but certain care should be
observed when making assessments, until all factors influencing the
gassing rate are known. GAS ANALYSIS OF TRANSFORMER Type Of Gas
Caused By CARBON MONOXIDE, AGEING CO CARBON DIOXIDE, CO2 HYDROGEN,
ELECTRIC ARCS H2 ACETYLENE, C2H2 ETHANE, LOCAL C2H6 OVERHEATING
ETHENE, C2H4 PROPANE, C3H6 HYDROGEN, H2 CORONA METHANE, CH4 Gas
concentration limits used in the Interpretation of DGA data A
statistical survey concerning gas concentrations in Transformer Oil
using the results of that survey the following limits have been
set: 182
H2 CH4 C2H6 C2H4 C2H2 CO CO2
Threshold Limit 20 10 10 20 1 300 5000
Warning Limit 200 50 50 200 3 1000 20000
Fault Limit 400 100 100 400 10
Unit ppm ppm ppm ppm ppm ppm ppm
The limits above are for a Transformer which are open with a
breather and have no OLTC or has a separate conservator for the
OLTC. If the Transformer tank and the OLTC have a common
conservator the warning and fault limits are 30 ppm and 100 ppm
respectively for C2H2 Standard IEC 60475 Method of sampling liquid
dielectrics IEC 60422 Supervision and maintenance guide for mineral
Insulating oils in electrical equipment IEC 60567 Guide for the
sampling of gases and of oil from oil filled electrical equipment
and for the analysis of free and dissolved gases IEC 60599 Mineral
oil-impregnated electrical equipment in Service -Guide to the
interpretation of dissolved and Free gases analysis IEC 60296
Specification for unused mineral insulating oils for Transformers
and Switchgear ASTM Dl 17-96 Standard guide for sampling, test
methods, Specifications, and guide for electrical insulating oils
Of petroleum origin ASTM D923-97 Standard practices for sampling
electrical insulating liquids ASTM D3613-98 Standard test methods
of sampling electrical Insulting oils for gas analysis and
determination of Water content ASTM D36 12-98 Standard test method
for analysis of gases dissolved In electrical insulating oil by gas
chromatography ASTM D3487-88(1993) Standard specification for
mineral insulating oil Used in electrical apparatus
PARALLEL OPERATION OF THREE-PHASE TRANSFORMERS
183
Ideal parallel operation between Transformers occurs when (1)
there are no circulating currents on open circuit, and (2) the load
division between the Transformers is proportional to their kVA
ratings. These requirements necessitate that any - two or more
three phase Transformers, which are desired to be operated in
parallel, should possess: 1) The same no load ratio of
transformation; 2) The same percentage impedance; 3) The same
resistance to reactance ratio; 4) The same polarity; 5) The same
phase rotation; 6) The same inherent phase-angle displacement
between primary and secondary terminals. The above conditions are
characteristic of all three phase Transformers whether two winding
or three winding. With three winding Transformers, however, the
following additional requirement must also be satisfied before the
Transformers can be designed suitable for parallel operation. 7)
The same power ratio between the corresponding windings. The first
four conditions need no explanation being the same as in single
phase Transformers. The fifth condition of phase rotation is also a
simple requirement. It assumes that the standard direction of phase
rotation is anti-clockwise. In case of any difference in the phase
rotation it can be set right by simply interchanging two leads
either on primary or secondary. It is the intention here to discuss
the last two i.e., sixth and seventh conditions in detail.
Connections of Phase Windings The star, delta or zigzag connection
of a set of windings of a three phase Transformer or of windings of
the same voltage of single phase Transformers, forming a three
phase bank are indicated by letters Y, D or Z for the high voltage
winding and y, d or z for the intermediate and low voltage
windings. If the neutral point of a star or zigzag connected
winding is brought out, the indications are Y N or Z N and y n and
z n respectively. Phase Displacement between Windings The vector
for the high voltage winding is taken as the reference vector.
Displacement of the vectors of other windings from the reference
vector, with anticlockwise rotation, is represented by the use of
clock hour figure. IS: 2026 (Part 1V)-1977 gives 26 sets of
connections star-star, star-delta, and star zigzag, delta-delta,
delta star, delta-zigzag, zigzag star, zigzag-delta. Displacement
of the low voltage winding vector varies from zero to -330 in steps
of -30, depending on the method of connections. Hardly any power
system adopts such a large variants of connections. Some of the
commonly used connections with phase displacement of 0, -300, -180"
and -330 (clock-hour setting 0, 1, 6 and 11) are shown in Table (
below) Symbol for the high voltage winding comes first, followed by
the symbols of windings in diminishing sequence of voltage. For
example a 220/66/11 kV Transformer connected star, star and delta
and vectors of 66 and 11 kV windings having phase displacement of 0
and -330 with the reference (220 kV) vector will be represented As
Yy0 - Yd11.
184
If a pair of three phase Transformers have the same phase
displacement between high voltage and low voltage windings and
possess similar characteristics (Such as no load ratio of
transformation phase rotation, percentage impedance) these can be
paralleled with each other by connecting together terminals which
correspond physically and alphabetically. Thus taking the case of
two three phase Transformers having vector symbols Dd0 and Yy0,
these can be put into parallel operation by connecting H.V
terminals U1, V1 and W1 of one Transformer to HV terminals U1, V1
and W1 of the other Transformer. Similarly, low voltage terminals
U1V1 and of one Transformer should be connected to U1, V1 and W1
terminals of the second Transformer. Sometimes it may be required
to operate a three-phase Transformer belonging to one group with
another three-phase Transformer belonging to a different group.
This is possible with suitable changes in external connections. For
example, let us consider a three-phase Transformer with vector
symbol Dy1 and see how this can be operated in parallel with a
three-phase Transformer of similar characteristics but having
vector symbol Yd11. Referring to Table (below) the phasor diagrams
of the induced voltages in the h-v and l-v windings of the two
Transformers, with the phase sequence of the supply connected to
terminals U,V, W of the two being RYB in the anti-clockwise
direction are as shown in Figs. (39a) and (39b) respectively.
Fig. (39) Example of parallel operation of Transformers of
groups 3 and 4 (Transformers having symbols Dy 1 and Yd 11
operating in parallel
185
It may be seen from these diagrams that the phase displacement
between the induced voltages in the h-v and l-v windings is -30 in
the first Transformer and it is -330 in the second Transformer.
However, for the successful parallel operation of these
Transformers, the phase displacement must be the same in the two.
This can be achieved by interchanging externally two of the h-v
connections of the incoming Transformer to the supply, i.e., by
connecting 1V to bus B and 1W to bus Y as shown in Fig. (39c) by
full lines instead of Connecting 1V to bus Y and 1W to bus B as
shown in Fig (39b) by dotted lines.
Vector Group This results in the reversal from anticlockwise
direction to clockwise direction of the phase rotation of the
induced voltages as shown by arrows in Fig. (39c) and therefore
results in a phase displacement of -30 between the induced voltages
in the h-v and lv windings [see Fig. (39c)].
186
The change in two of external it-v connections of the second
Transformer thus brings it -30. The secondary voltages of this
Transformer, however, have a phase rotation reversed with respect
to that of the secondary voltages of the first Transformer. This
can be set right by changing again the two corresponding l-v
external connections, i.e., by connecting 2V to bus b and 2W to
busy as shown in Fig. (39c) instead of connecting 2V to busy and 2W
to bus b as shown in Fig. (39b). Thus Transformers connected in
accordance with clock hour No. 1 and 11 can be operated in parallel
with one another by interchanging two of the external h-v and also
the corresponding l-v connections of one Transformer. Transformers
connected in accordance with clock hour No. 0 and 6 however, cannot
be operated in parallel with one another without altering the
internal connections of one of them as change of external
connections only brings about change in phase rotation. The general
principle applying to the parallel operation of a three winding
Transformer with another three winding Transformer are the same as
those for the paralleling of two winding Transformers. However, to
obtain the same percentage impedance. Between the three pairs of
windings of the two (or more) Transformers (being paralleled) it is
imperative that the power ratio of the corresponding windings of
the Transformers should be the same, i.e.
( PH )1 ( PM )1 ( PL)1 = = ( PH ) 2 ( PM ) 2 ( PL) 2Where (PH)1
and (PH)2 represent the powers of the h-v windings (say primary),
(PM)1 and (PM)2 represent the powers of the medium voltage windings
(say secondary) and (PL)1 and (PL)2 represent the powers of the low
voltage windings (say tertiary) of the two Transformers labeled 1
and 2. This is proved below. Fig. (40) Shows two 3 winding
Transformers (represented by their equivalent circuits) connected
in parallel. The currents flowing in the various circuits and
windings are shown in the figure.
Fig (40) Shows two 3 winding Transformers (represented)
( ZH )1 ( ZM )1 ( ZL )1 = = ( ZH ) 2 ( ZM ) 2 ( ZL ) 2
187
Thus the power ratios of the corresponding windings are similar.
This as is evident also fulfils the second condition of same
percentage impedance. When Transformers which do not fulfilling
this condition are paralleled the operation may be satisfactory
without fulfilling the ideal conditions so long as the loads to be
carried do not overload either Transformer. Therefore, when new
three-phase 3 winding Transformers are to be purchased for parallel
operation with existing three-phase 3-winding Transformers the
purchase order must specify the power ratings of the various
windings of the existing Transformers along with other
specifications and indicate that the power ratios of the
corresponding windings of the various Transformers must be
identical failing which it will be impossible to design
Transformers with same percentage impedances for the corresponding
windings. Tap Changer The method to change the ratio of
Transformers by means of taps on the winding is as old as the
Transformer itself. From a very early stage, Transformers with a
turn ratio changeable within certain limits have been used for
electrical power transmission, since this is the simplest method to
control the voltage level as well as the reactive and active power
in electrical networks.
Tap-changer with single phase transformer
188
At the beginning of the development it was sufficient to have
tappings connected to bushings outside the Transformer tank, which
were connected according to the necessity of the network. A more
comfortable way was to connect the tappings to tap Switches today
called "off-circuit" or "no-load tap changers" - which could only
be actuated when the Transformer was de-energized. Obviously, this
simple device only permitted occasional corrections of the
Transformer ratio. It was not possible to control voltage drops
caused by load changes in the network. At that stage these
parameters could only be controlled at the generating plant. To
solve this problem, Switching devices were needed which permitted
the change of the turn ratio of Transformers under load condition,
i.e. Without interrupting the load current such Switching devices -
today called "on-load taps changers" (OLTC) were introduced to
Transformers more than 70 years ago. The demand for (OLTCs) came an
urgent necessity in the 1920ies, then power consumption took a
sharp upward trend, which required the interconnection and
expansion of the electrical networks. The very rapid development
brought, within a few years, solutions which were quite
satisfactory in regards to operating safety and efficiency. The
development of (OLTCs) was accelerated over the years due to the
steady increase of the transmission voltage and power. The
introduction of OLTCs improved the operating efficiency of
electrical systems considerably and this technique found acceptance
worldwide. In other industrialized countries the situation is
comparable. In general the percentage of Transformers equipped with
OLTCs is increasing with the increase of the load density and
interconnection of electrical networks. In addition. OLTCs applied
in industrial process Transformers as regulating units in the
chemical and metallurgical industry is another important field of
application. These range from some hundred to around 300,000
operations per year while the rated currents range from
approximately 50 to 3000 Amps. Today's state of the art OLTC has
reached such a high level of reliability that it is safe to state
that its mechanical life expectancy is equivalent to that of the
Transformer. Exceptions may be applications in industrial process
Transformers. However, even on such applications experience shows
that with proper maintenance several million operations can be
obtained. Table below shows a survey of the typical number of
operations for various applications. Transformer No of operation
data Power Power Voltage Current OLTC Per Transformer ring ring
ring Year MVA KV A Min Mean Max Generator 100 110 100 - 500 3000
10000 -1300 765 2000 Interconnection 200 110 300 - 300 5000 25000
-1500 765 3000 Distribution 15 - 400 60 - 525 50 - 1600 2000 7000
20000 189
Electrolysis Chemistry Arc furnace
10 - 300 20 - 110 50 - 3000 1000 30000 150000 0 1.5 - 80 20 -
110 50 - 1000 1000 20000 70000 2.5 - 20 - 230 50 - 1000 2000 50000
300000 150 0
The problem to be solved when changing taps under load is how to
connect the tappings of the Transformer winding successively to the
same output terminal without interrupting the load current. During
the load transfer operation between to adjacent taps, both taps
must be temporarily connected to the output terminal. To avoid a
short circuit of the winding transition impedances, which can be
reactors or resistors? Are inserted. Two basic principles have been
invented and are still used today - the slow motion reactor
Switching principle and the high speed resistor Switching
principle. Today both principles have been developed into reliable
OLTCs. The reactor type OLTC has its development origin in the USA,
hut also in Germany inventions were applied for a patent in 1905
and 1906. Because of the fact that the reactor Switching principle
causes a 90 degree phase shift between the Switched current and the
recovery voltage arising at the Switching distance, the reactor
type OLTC is less suitable for large step voltages. In addition to
this the costs of transition reactors increase considerably with
higher step voltages. Thus the reactor Switching principle over the
years has lost the remarkable importance it had in the beginning of
the OLTC development. In the late 1940 is many OLTC manufacturers
abandoned the production of OLTCs with this Switching principle.
However, in the USA the reactor principle is still used in a large
scale and reactor type OLTCs are still under production. The
high-speed resistor type OLTC has its origin in the invention of
Dr. Jansen of a diverter Switch and a tap selector. Which were
patented in 1926. The transition impedance is been carried out with
ohmic resistor with this principle the current Switched and the
recovery voltage are in phase. This lightens the quenching of the
arc in the current zero. The transition resistors hake to be
dimensioned only for a short-time loading which enables an economic
use of OLTCs in case of higher step voltages and power. Though the
reactor principle has also proven itself, its application is
limited to loner voltages, whereas the resistor principle dominates
in the high voltage field or in special applications like HVDC -
Transformers, Phase-Shifting Transformers or EHVTransformers. The
reactor principle OLTC in these fields can only be applied by mean
of booster Transformers. Which make its application more difficult
in regards to transport weight, transport size and profile and
overall economic considerations compared to the resistance
principle OLTC. DESIGN CONCEPTS OF ON-LOAD TAP-CHANGERS With an
on-load tap-changer the Transformer voltage ratio can be varied in
steps by adding or subtracting turns. For this purpose a
Transformer is furnished with a tapped winding and these taps are
connected to terminals on the tap-changer. The tap-changer provides
two basic functions.
190
Fig (2) Basic connection of a star-point linear regulation The
first is to select a Transformer tapping connection in an
open-circuit condition, the second is to divert or transfer power
to that selected tapping without interrupting the through-current.
The simplest type OLTC, the selector Switch, combines these two
functions into one device. Whereas separate selectors and diverter
or transfer Switches are used for higher power requirements.
Various tapping winding configurations are possible. The selection
function can be without change-over selector (linear). Or with
change-over selector (reversing or coarse / fine). A basic
connection of a star-point linear regulation is given in Fig (2).
The mechanical configuration of the tap selector can be designed as
a single or double multiway selector. The transfer of the load
current from the connected to the preselected trip is either
achieved by means of resistor transition or the alternative method.
Mainly used in the USA, reactor transition. In service, the
diverter or transfer Switch is required to make and break current
at a recovery voltage whose value is in the same order as the
voltage between two taps. The power transfer function can be
symmetrical or asymmetrical. The former providing similar Switching
conditions for advanced or retard power flow from the Transformer.
The action of the diverter or transfer Switch can be rotary or
oscillatory. All designs of tap-changers maintain direct mechanical
synchronism between the tap selector, change-over selector and the
diverter or transfer Switch. The transfer of electrical power
involves arcing in the oil and therefore contamination of the
insulating oil (the exception are OLTCs that use vacuum
interrupters as Switching devices). Therefore, the Switching
devices are located in their own Switching compartment to separate
the contaminated oil from the oil in the transformer main tank. To
fulfill this requirement several designs have been developed.
Selector Switches are designed for operation within an enclosure
inside the Transformer tank (in-tank type) or externally in a
separate oil-tilled housing bolted to the outside of the main
Transformer tank (compartment type). HIGH-SPEED RESISTOR TYPE
OLTC
191
The high-speed resistor type OLTC is designed either as a tap
selector and a diverter Switch, or as a selector Switch combining
the functions of the tap selector and diverter Switch into one
device.
Fig (3) Principle scheme of a selector Switch type OLTC The
latter is economical to manufacture, but certain inherent
limitations reduce the possible applications to small and medium
size Transformers with highest voltages of equipment of 132 kV and
rated-through currents in the range of 500 A to 600 A.
Fig (4) Principle scheme of a-tap selector and diverter Switch
type OLTC This type can only be built in one enclosure as mentioned
above and, therefore, the arc products are in contact not only with
wearing mechanical parts, but also with insulation subject to high
voltages. The selector Switch principle is represented in Fig. (3)
The OLTC comprising a tap selector and a diverter Switch lends
itself for any application up to the highest Transformer rating.
Line-end applications with highest voltages for equipment of 362 kV
and rated through-currents of 4500 A have been realized. Figure (4)
shows an OLTC comprising a tap selector and diverter Switch. With
the tap selector-diverter Switch concept the tap-change is affected
in two steps. The tap adjacent to the one in
192
service is pre-selected load free by the tap selector.
Thereafter the
Fig (4) Switching sequence for tap-changer on Switching from
position 6 to position 5. a) Position 6. Selector contact V lies on
tap 6 and selector contact H on tap 7. The main contact x carries
the load current. b) Selector contact H has moved in the no-current
state from tap 7 to tap 5. c) The main contact X has opened. The
load current passes through the resistor Ry and the resistor
contact y. d) The resistor contact u has closed. The load current
is shared between Ry and Ru The circulating current is limited by
the resistance of Ry + Ru. e) The resistor contact y has opened.
The load current passes through Ru and contact u f) The main
contact V has closed, resistor Ru. Has been short-circuited and the
load current passes through the main contact V. The tap-changer is
now in position 5.
193
Fig (45) Three-phase tap-changer type UCBRN 380/600, neutral
point design for 21 position with plus/minus Switching
194
Fig. (46) Motor-drive mechanism type BUE for UC tap-changer
Testing Tap-changers undergo type tests according to the
international standards for on-load tap-changers, IEC 214 the first
edition of which was published in 1966 and the most recent one in
1976. The tests on the tap-changer itself comprise: 1- Temperature
rise of contacts at 1.2 times the maximum rated through-current. 2-
Switching tests. 3- Short-circuit current tests. 4- Temperature
rise of transition resistors. 5- Mechanical tests. 6- Dielectric
tests. And for the motor-drive mechanism: 1- Mechanical load test.
2- Overrun test. 3- Degree of protection of motor-drive cubicle.
SF6 Transformer Introduction Demand for effective space utilization
is becoming increasingly stronger as a result of grade advancement
of commercial/industrial activities and urban life styles.
Concurrently, city construction facilities including buildings,
underground shopping areas, traffic systems, and public structures
are becoming larger in size and gaining in the degree of
complications. Since such facilities immensely contribute to
improving the efficiency of urban activities, the current trend
indicates the possibility of further expansion in the future. On
the other hand, accidents involving outbreaks of extensive fire and
other troubles are occasionally occurring in these large-sized
urban facilities, resulting in the creation of public voices
demanding improved fire or accident
195
preventive measures.
These construction facilities of cities represent high-valued
social assets. However, since a great number of citizens utilize
such .facilities day after day, it is quite essential to provide
effective means to eliminate outbreaks of fire. To achieve this
purpose, it is important to install modern fire-fighting systems
capable of coping with various causes of fire. At the same time,
Basically it is most important to eliminate the possible causes of
fire. The SF6 gas-insulated Transformers are designed to ideally
satisfy Non flammability-ensuring plans of power reception and
transformation systems installed in these urban facilities. Since
no oil for insulation is used, these Transformers can completely
free structures or adjacent rivers from oil contamination during
new installation work or system operation. In other words, the SF6
gas-insulated Transformers qualify themselves as truly "non
flammability-ensuring equipment" usable for power systems required
to prevent fires or accidents and eliminate pollution.
196
Features The SF6 gas-insulated Transformers offer excellent
insulation and cooling characteristics and thermal stability.
Additionally, these Transformers possess the following features
resulting from containing the active parts in a tank sealed with
nonflammable, harmless, and odorless SF6 gas. 1. High-level
stability Even should the actual Transformer develop an accident,
or should a fire break out on the installation environment,
combustion or an explosion will not occur. Since all live parts are
housed in grounded metal cases, maintenance and inspection can be
achieved easily and safely. 2. Outstanding accident preventive
characteristics Nonflammable structure employing no insulation oil
contributes to minimizing the scope of associated
accident-preventive facilities such as fireproof walls,
fire-fighting equipment, or oil tanks. 3. Compactness of substation
By directly coupling with gas-insulated Switchgear, substation
space can be minimized as the result of compact facilities. 4.
Simplified maintenance and long service life Because the
Transformers are completely sealed in housing cases, no contact
exists with exterior atmospheric air, thereby eliminating problems
of degradation or contamination triggered by moisture or dust
accumulation. Constant enveloping of components with inactive, dry
SF6 gas results in minimizing aging deterioration of insulating
materials and prolonging Transformer service life. 5. Easy, clean
installation SF6 gas can be quickly sealed into the Transformer
tank from a cylinder. Installation work never contaminates
surrounding areas, and ensures maintenance of a clean environment.
6. Ideal for high voltage systems By increasing the seal pressure,
SF6 gas Transformers offer insulation performance comparable to
that of oil-insulated types, being ideal for high voltages of 22 kV
to 154 kV. Applications The SF6 gas-insulated Transformers are
suitable for the following applications: 197
Locations where safety against fire is essential Buildings such
as hotels, department stores, schools, and hospitals Underground
shopping areas, underground substations Sites close to residential
areas, factories, chemical plants Locations where prevention of
environment pollution is specifically demanded Water supply source
zones, residential quarters, seaside areas Water treatment stations
Locations where exposure exists to high-level moisture or dust
accumulation Inside tunnels, industrial zones
Specifications and Ratings The SF6 gas-insulated Transformers
are manufactured under the following standard specifications. Table
1 Standard specifications
NOTES: 1. Mounting of on-load tap-changer is possible. The
voltage adjusting range in this case is 10 % of the rated voltage.
2. As for codes affixed to the primary tap voltage, F indicates
full-capacity taps and R indicates rated taps. 3. Consultation
regarding ratings other than the above is accepted. Quality
specifications The following specifications are provided to ensure
safe operation of gas-insulated Transformers. Withstand voltage
during zero gas gauge pressure No problem is caused by operation
under normal operating voltage. Permissible load under zero gas
gauge pressure No problem is caused by 50 % load continuous
operation. Permissible load under 1-series operation when 2-series
coolers are provided No problem is caused by 75 % load continuous
operation.
198
External Dimensions and Weight Figures below show external
dimensions and weight. Since external dimensions are subject to
change without notice, please obtain final confirmation from
approval drawings. Also,
199
Natural-cooled type
200
Natural-cooled type
NOTE: In case of 72.5 kV, GIS direct-coupling type, X size (up
to bushing Terminal end) becomes "the value in the above Table +
600 mm."
201
Forced-gas-circulated, natural-air-cooled type SF6 gas-insulated
Transformer
Forced-gas-circulated, forced-air-cooled type
202
Accessories SF6 gas temperature indicator (dial thermometer)
Measures temperature of SF6 gas sealed in Transformer tanks. Gas
temperature is measured by the heat sensing probe of a thermometer
inserted into the protective cylinder provided in the tank or on
the cover. Since this protective cylinder maintains air tightness
of the gas, the temperature indicator itself can be removed. The
temperature indicator is provided with alarm contacts and a pointer
for indicating maximum temperature.
Dial thermometer SF6 gas pressure gauge (compound gauge) This
gauge is used to measure the pressure of SF6 gas sealed in the
Transformer tank. The gauge is a compound type that measures both
positive and negative pressure, capable of measuring the positive
pressure up to 3.0 kg / and the negative pressure up to 760 mmHg.
Generally, only the positive pressure is indicated during
operation. Since vacuum suction is conducted when sealing SF6 into
the tank, the graduations for negative pressure are provided for
use during this gas sealing. The pressure gauge is provided with
alarm contacts that actuate at the upper limit of normal pressure
during operation.cm 2
203
Pressure gauge (compound gauge) Temperature compensating
pressure Switch Leakage is detected of SF6 gas sealed in the
Transformer tank. Pressure in the Transformer tank is compared with
pressure in the reference pressure chamber inserted into the
protective cylinder provided in the tank or on the cover.
Therefore, regardless of temperatures in the Transformer, SF6 gas
leakage is accurately detected and the alarm contacts are
actuated.
Temperature compensating pressure switch SF6 Gas Properties
Introduction SF6 is a combination of sulfur and fluorine its first
synthesis was realized in 1900 by French researchers of the
Pharmaceutical Faculty of Paris. It was used for the first time as
insulating material, In the United States about 1935. In 1953, the
Americans discovered its properties for extinguishing the electric
arc. This aptitude is quite remarkable. 204
Physical properties It is about five times heavier than air, and
has a density of 6.1 4kg / m3. It is colorless, odorless and
non-toxic. Tests have been carried out replacing the nitrogen
content of air by SF6 (the gaseous mixture consisted of 79 % SF6
and 24 % oxygen): five mice were then immersed in this atmosphere
for 24 hours, without feeling any ill effects. It is a gas which
the speed of sound propagation is about three times less than in
air, at atmospheric pressure. The interruption of the arc will
therefore be less loud in SF6 than in air. The dielectric strength
of SF6 in on average 2.5 times that of air, and, by increasing
pressure, it can be seen that the dielectric strength also
increases and than around 3.5 bar of relative pressure, SF6 has the
same strength as fresh oil. The principal characteristics of the
gas are as follows: Molar mass 146.078 Critical temperature 45.55C
Critical pressure 37.59 bars In short, SF6 at atmospheric pressure
is a heavier gas than air, it becomes liquid at 63.2C and in which
noise propagates badly. SF6 on the market SF6 which is delivered in
cylinders in liquid phase, contains impurities (within limits
imposed by IEC standards No. 376) Carbon tetra fluoride (CF4) 0.03
% Oxygen + nitrogen (air) 0.03 % Water 15 ppm C02 traces HF 0.3 ppm
SF6 is therefore 99.99 % pur. Chemical properties SF6 is a
synthetic gas which is obtained as we have just explained by
combination of six atoms of fluorine with one atom of sulfur:
S 2 + 6 F 2 2SF 6 + 524 KcalYou can see therefore that this
reaction is accompanied by an important release of heat. This
approximately similar to coal combustion. Given that the energy
released during synthesis is the same as is needed in order to
dissociate the final element, it can immediately be seen that: -
SF6 is a stable gas
205
- 524 k. calories are necessary for molecular breakdown, we can
there fore already expect that it will be a powerful cooling
agent:
6 F 2 S 2 + 2 SF 6 + 524 Kcal
The dissociation products before interruption of the arc At
normal temperature, the gas is stable, and does not react with its
environment. In contact with the parts where electric currents
circulate, the gas is heated to temperatures of around four hundred
degrees SF6 gives the following decomposition products: Thionyl
fluoride SOF2 Sulfur fluoride SO2F2 Sulfur tetra fluoride SF4
Sulfur deca fluoride S2F10 Thionyl tetra fluoride SOF4 SF6 also
reacts with the materials that are found in its environment: With
water (impurity in the gas), it gives hydrofluoric acid HF, With
air dioxide (impurity in the gas), it gives sulfur dioxide SO2,
With carbon dioxide (impurity in the gas), it gives carbon tetra
fluoride CF4, With the araldite casings which are high in silicon
dioxide, it gives silicon tetra fluoride SF4. The dissociation
products after interruption of an arc. An electric are develops
high temperatures which can reach 15000 C. At these temperatures,
many dissociation products that we have previously studied
disappear. It is thus that, besides the impurities of the gas
(water, air, carbon, and dioxide), there only remain: Sulfur
fluoride SO2F2 Carbon tetra fluoride CF4 Silicon tetra fluoride
SIF4 Sulfurous anhydride SO2. You can therefore see that a large
number of products have been dissociated by the electric arc. The
importance of the remaining products may be lessened by adding a
powder (alumina silicate). All these gases are heavier than air,
and May, under certain conditions is poisonous. SF6 Safety
precautions: Today there is no known dielectric and breaking agent
combined better than SF6 gas. Initial state In its initial state,
before it has undergone thermal stress (usually the electric arc);
SF6 is perfectly safe in normal conditions: - It is non-toxic, - It
is uninflammable, - It will not explode. This does not mean that no
precautions need to be taken: because of its lack of oxygen, this
gas will not support life. However, the concentration of SF6 would
have to be high, since the International electro technical
Commission (IEC) has shown that five mice left for 24 hours in an
atmosphere of 79 % SF6 and 21 % oxygen will not only remain alive
but will show no signs of abnormal behavior.
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Man dies when the oxygen level of the gas he is breathing falls
below 12 %. Precautions and hygiene The first recommendation is not
to smoke when SF6 gas is around. The heat given off by the
cigarette may decompose the gas. Your cigarette would then take on
a very strange taste also avoid operating combustion engines in
this gas. When the work positions are indoors, have ventilation and
/ or a system for detecting this halogen placed at the lowest
points of the installations. Remember that SF6 is a very heavy gas.
This device will warn you any gas leaks. Post-breaking state As we
seen at the beginning of this Chapter, the heat from the arc
modifies the SF6.This creates gaseous and solid decomposition
products. It is these products that need to be spoken about.
Certain of these gases are medically defined as being violent
irritants of the mucous membranes and of the lungs. In extreme
cases, they may cause pulmonary edema. The solid decomposition
products (whitish powder) an aggressive when the react with the
humidity of the mucous membranes and of the hands. Following this
rather unpleasant description of the SF6 after breaking we may
reassure ourselves on two counts: - For reasons of quantity - For
reasons of probability. Quantity. The volume of decomposed is
microscopic. This means that dangerous thresholds are rarely
reached, thanks in part to the molecular sieve which regenerates
the decomposition products to form pure SF6. This sieve is present
in all extinguishing chambers. Regeneration time is short, but
depends on the number of ampere being broken. The presence of
hydrogen sulphide, noticeable through its sickening smell, makes an
excellent alarm signal. The smell detection threshold is ten times
lower than the toxic threshold (1 ppm is detected by smell).
Probability. In normal operation, electric Switchgear using SF6 has
a leak rate guaranteed to be less than 1 % of the mass per year.
This makes any danger impossible in normal operation. The abnormal
situation is the risk of an appliance exploding. This is
fortunately extremely infrequent. And if by chance such an incident
accrued, the putrid smell would make us aware of it immediately.
Precaution and hygiene. If you were to find yourself in contact
with decomposed SF6 gas, you must leave your post and ensure that
the gas is eliminated by means of powerful ventilation. Once the
polluted gas has disappeared (when the smell becomes bearable) you
are still in contact with solid decomposition products. Operations
on the equipment must be carried out with a gas mask, gloves and
appropriate clothing. All this - together with the powders
themselves - shall be sent to a factory for dealing with dangerous
products.
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Any damage to the hands caused by these powders can be
neutralized by limewater. Conclusion It is important to point out
that sulfur hexafluoride does not bring about an increase in the
risks entailed in the work stations. This lack of specific danger
is furthermore confirmed by the fact that we have not had to record
any accident since 1960, the year in which SF6 was first used as a
breaking agent. As a matter of interest SF6 does not harm the ozone
layer. This is partly due to its weight. The electric arc The
creation of an arc Everyone has noticed that, when placing ones
hand near to a television screen, one feels a force which attracts.
There exists, in fact, in this apparatus, what one calls an
electric field. The latter is the source of an electric current,
for it is this that displaces the electrons in the conductors. An
electric field appears at the separation of the live contacts. Such
a field of a very great intensity will draw electrons at the hot
points of contacts. The electric arc has been born. If its own
energy is not sufficient, the arc will extinguish rapidly itself.
If, on the other hand, it is crossed by a strong current, it draws
throughout its own energy, which ensures the survival of the arc.
The electric arc: We have seen that the electric field was at the
origin of the displacement of electrons. When the contacts
separate, the electric field draws electrons to the hot points.
These electrons are going to circulate in surroundings which are
not conductive, which one calls dielectric, and will cause the
temperature of the surroundings to increase, if they are in
sufficient number. All bodies, under the influence of temperature,
end up by reaching their threshold of ionic dissociation. At this
moment, it parts with electrons, and becomes conductive. These
electrons themselves, and for the same reasons, will create others.
We have an avalanche, that is to say, creation of electrons, which
will accelerate. One can reach temperature of 15000 C. The value of
the thermal power can be 10MW. The electric arc is thus going to
follow the variations of alternating current, and thus, at regular
intervals, the arc will disappear and reappear immediately, if the
electrons have not been eliminated because in this case, the
surroundings remain conductive. In order to eliminate these
electrons, one could: - Rid oneself of them by some physical means,
like blow-out for example, - use dielectric with a very high speed
of recuperation (the case of SF6) - use a process to reduce the
temperature of the element (decompression, blowout, etc.) Out-off a
current If we perfect a system which allows cooling the arc
(turning arc, magnetic blow-out, mechanical or thermodynamic
blow-out, etc ...). One can well understand that the arc increasing
to temperatures of 1500C. Under the effect of current passing
through it, will see a temperature decrease as soon as the
alternating current starts its descent towards 0. The temperature
will decrease all the more rapidly as:
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- SF6 has two states of conduction, and appearance of the
resistive arc will bring about a fall in the intensity, and thus
its temperature, - SF6, as we have seen in its physical properties,
is a gas which Absorbs large quantities of energy when it
dissociates. The blow out of the arc will thus (mean) evacuate a
large quantity of energy. This lowering of temperature will make
the ionic recombination of the bodies and the dielectric will
recover its insulating properties which thus ensure interruption of
the current. Lastly the hydrofluoric acids attack all metals giving
metallic fluorides which are all very hydroscopic insulating
powders.
Fig (1) Disruptive voltage versus pressure
Fig (2) SF6 absolute pressure versus temperature with constant
volume mass (density)
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Electrical Substations Electrical Network comprises the
following regions: 1 - Generating Stations. 2 - Transmission
Systems. 3 - Distribution Systems. 4 - Load Points.
www.sayedsaad.com Functions of a Substation 1 - Supply of required
electrical power. 2 - Maximum possible coverage of the supply
network. 3 - Maximum security of supply. 4 - Shortest possible
fault-duration. 5 - Optimum efficiency of plants and the network. 6
- Supply of electrical power within targeted frequency limits,
(49.5 Hz and 50.5 Hz). 7 - Supply of electrical power within
specified voltage limits. 8 - Supply of electrical energy to the
consumers at the lowest cost. www.sayedsaad.com Substation Layouts
1. Switching requirements for normal operation. 2. Switching
requirements during abnormal operations, such as short circuits and
overloads. 3. Degree of flexibility in operations, simplicity. 4.
Freedom from total shutdown and permissible period of shutdown. 5.
Maintenance requirements, space for approaching various 6. Safety
of personnel. 7. Protective zones, main protection, back-up
protection 8. Bypass facilities. 9. Technical requirements such as
ratings, clearances, Earthing lightning protection, Noise, radio
interference, etc. 10. Provision for extensions, space requirement.
11. Economic considerations, availability, foreign exchange
involvement, cost of the equipment. 12. Requirements of network
monitoring, power line communication, data collection, Data
transmission etc. 13. Compatibility with ambient conditions. 14.
Environmental aspects, audible noise, RI, TI etc. 15. Long service
life, Quality, Reliability, and Aesthetics. www.sayedsaad.com
Essential Features for substation 1 - Outdoor Switchyard having any
one of the above. 2 - Bus-Bar schemes. 3 - High voltage Switchgear.
Medium voltage Switchgear, Low voltage Switchgear and control room.
4 - Office building.
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5 - Roads and rail track for transporting equipment. 6 -
Incoming line towers and outgoing line towers/cables. 7 - Store. 8
- Maintenance workshop (if required). 9 - Auxiliary power supply
Low voltage AC. 10 - Battery room and low voltage DC. Supply
system. 11 - Fire fighting system. 12 - Cooling water system;
drinking water system, etc. 13 - Station Earthing system. 14 -
Lighting protection system, overhead shielding. 15 - Drainage
system. 16 - Substation lighting system etc. 17 - Fence and gates,
Security system etc. www.sayedsaad.com SF6 Gas Insulated
Substations (GIS) 1. Introduction SF6 Gas Insulated Substations
(GIS) are preferred for voltage ratings of 72.5 kV, 145 kV, 300 kV
and 420 kV and above. In such a substation, the various equipments
like Circuit Breakers, Bus-Bars. Isolators, Load Break Switches,
Current Transformers, Voltage Transformers Earthing Switches, etc.
are housed in metal enclosed modules filled with SF6 gas. The SF6
gas provides the phase to ground insulation. As the dielectric
strength of SF6 gas provides the phase to ground insulation. As the
dielectric strength of SF6 gas is higher than air, the clearances
required are smaller. Hence, the overall size of each equipment and
the complete substation is reduced to about 10 % of conventional
Air-insulated substations. As a rule GIS are installed indoor.
However outdoor GIS have also been installed earlier.
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High voltage Gas Insulated Switch gear Type B95 Double Bus-Bar
(make Alostom) www.sayedsaad.com
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Single line diagram High voltage Gas Insulated Switch gear Type
B95 Double Bus-Bar (make Alostom) 1 Circuit Breaker . 2 Spring
Mechanism . 3 Disconnected . www.sayedsaad.com 4 Slow Earthing
Switch 5 Make Proof Earthing Switch. 6 Current Transformer. 7
Voltage Transformer. 8 HV cable connection. www.sayedsaad.com The
various modules of GIS are factory assembled and are filled with
SF6 gas at a pressure of about 3 kg/cm2. Thereafter, they a taken
to site for final assembly. Such substations are compact and can be
installed conveniently on any floor of a multistoried building or
in an underground substation. As the units are factory assembled,
the installation time is substantially reduced. Such installations
are preferred in cosmopolitan cities, industrial townships, etc.,
where cost of land is very high and higher cost of SF6 insulated
Switchgear (GIs) is justified by saving due to reduction in floor
area requirement. They are also preferred in heavily polluted areas
where dust, chemical fumes and salt layers can cause frequent
flashovers in conventional outdoor air-insulated substations
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GIS bay single Bus-Bar Make Mitsubishi 1- Circuit Breaker 2-
Disconnector Switch (GL-Type) 3- Disconnector Switch (GR-Type) 4-
Earthing Switch (GRE-Type) 5- 3-ph. Bus-Bar. 6- Current
Transformer. 7- Base. www.sayedsaad.com 8- Voltage Transformer. The
SF6 Gas Insulated Substations (GIs) contains the same Components as
in the conventional outdoor substations. All the live parts are
enclosed in metal housings filled with SF6 gas. The live parts and
supported on at resin insulators. Some of the insulators are
designed as barriers between neighboring modules such that the gas
does not pass through them. www.sayedsaad.com The entire
installation is sub-divided into compartments which are gas tight
with respect to each other. Thereby the gas monitoring system of
each compartment can be independent and simpler. The enclosures are
of non-magnetic material such as aluminum or stainless steel and
are earthed. Static O-seals placed between machined flanges provide
the gas tightness. The O-rings are placed in the grooves' such that
after assembly, the O-rings are squeezed by about 20 %. Quality of
material and dimension of grooves and O-seals are important to
ensure gas-tight performance. The GIs has gas-monitoring system.
The gas density in each compartment is monitored. If pressure drops
slightly, the gas is automatically tapped up with further gas
leakage, the low-pressure alarm is sounded or automatic tripping or
lock-out occurs www.sayedsaad.com Advantages of GIs and Application
Aspects: 1- Compactness. The space occupied by SF6 installation is
only about 8 to 10 % of that a conventional outdoor substation.
High cost is partly compensated by saving in cost of space. A
typical 214
420/525 kV SF6 GIs requires only 920 m2 site area against 30.000
m2 for a conventional air insulated substation. 2 - Choice of
Mounting Site. Modular SF6 GIS can be tailor made to Suit the
particular site requirements. This results is saving of otherwise
Expensive civil-foundation work. SF6 GIS can be suitably mounted
indoor on any floor or basement and SF6 Insulated Cables (GIC) can
be taken through walls and terminated through SF6 bushing or power
cables. 3 - Reduced Installation Time. The principle of building
block construction (modular construction) reduces the installation
time to a few weeks. Each conventional substation requires several
months for installation. In SF6 substations, the time-consuming
high cost galvanized steel structures are eliminated. Heavy
foundations for galvanized steel structures, www.sayedsaad.com
Equipment support structures etc are eliminated. This results in
economy and reduced project execution time. Modules are factory
assembled, tested and dispatched with nominal SF6 gas. Site
erection time is reduced to final assembly of modules.
www.sayedsaad.com 4 - Protection from pollution. The external
moisture. Atmospheric Pollution, snow dust etc. have little
influence on SF6 insulated substation. However, to facilitate
installation and maintenance, the substations are generally housed
inside a small building. 5- Increased Safety. www.sayedsaad.com As
the enclosures are at earth potential there is no possibility of
accidental contact by service personnel to live parts. 6 -
Explosion-proof and Fire-proof installation. Oil Circuit Breakers
and oil filled equipment are prone to explosion. SF6 breakers and
SF6 filled equipment are explosion proof and fire-proof..
www.sayedsaad.com Summary of Merits of SF6 GIS Safe Reliable Space
saving Economical Maintenance free Operating personnel are
protected by the earthed metal enclosures The complete enclosure of
all live parts guards against any Impairment of the insulation
system. SF6 Switchgear installations take up only 1/10 of the space
Required for conventional installations. High flexibility and
application versatility provide novel, and economic overall
concepts. An extremely careful selection of materials. an expedient
design and a high standard of manufacturing quality assure Long
service life with practically no maintenance requirement.
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Low weight
Low weight due to aluminum enclosure, correspondingly Low cost
foundations and buildings.
Quick site assembly ensured by extensive Shop assembled
preassembly and Testing of complete feeders or large units in the
factory. Disadvantages of GIS: www.sayedsaad.com 1- High cost
compared to conventional outdoor substation. 2 - Excessive damage
in case of internal fault. Long outage periods as Repair of damaged
part at site may be difficult. 3 - Requirement of cleanliness is
very stringent. Dust or moisture can cause internal flashovers.
www.sayedsaad.com 4 - Such substations are generally in door. They
need a separate building. This is generally not required for
conventional outdoor substations. 5 - Procurement of gas and supply
of gas to site is problematic. Adequate stock of gas must be
maintained. 6 - Project needs almost total imports including SF6
Gas. Spares conventional substation is totally indigenous up to 400
kV. Configuration of GIS: www.sayedsaad.com The GIS installations
are assembled from a variety of standard modules. Which are joined
together by flange connections and plug contacts on the Conductors.
So as to easily permit subsequent disassembly of individual
components. Gas-tight barrier insulators in the Switchgear sections
prevent neighboring Switchgear parts from being affected by
overhauls. Any maintenance and overhaul work on Switch contacts can
be done without removing the enclosure. With GIS installations, all
basic substation Bus-Bar schemes used, in conventional plant
constructions can be realized. Installations with single or
multiple Bus-Bar-also alternatively with a bypass bus-can be made
with the standard modules, including Bus-Bar sectionalizing with
disconnects and Breakers, and Bus-Bar coupling. The two-breaker.
One and-a-half circuit breaker and ring-bus systems can also be
realized economically. www.sayedsaad.com The essential parts of a
GIS are: 1 - Conductors which conduct the main circuit current and
transfer power these are of copper or aluminum tubes.
www.sayedsaad.com 2 - Conductors need insulation above grounded
enclosures. Conductors also need phase to phase insulation, In SF6
GIS these insulation requirements are met by cast resin insulators
and SF6 gas insulation. 3 - Gas filled modules have nonmagnetic
enclosures. Enclosures are of aluminum alloy or stainless steel.
Adjacent modules are joined by means of multi-bolts tightened on
flanges. Suitable neoprene rubber O ring gaskets are provided for
ensuring Gas-tight sealing joints. www.sayedsaad.com 4 - Various
circuit components in main circuit are: CB, Isolator, Earthing
Switches for conductors, CTs, VTs, cable-ends, Bushing-ends and
Bus-Bars. Each of these main components has its own gas -filled
metal enclosed module. 5 - Gas filling, monitoring system.
www.sayedsaad.com 6 - Auxiliary LV DC and LV AC supply system,
control, protection and Monitoring system. This is air-insulated
like in conventional sub-station. 216
The Bus-Bars are conducting bars to which various incoming and
outgoing bays are connected. In SF6 GIS the Bus-Bars are laid l
longitudinally in GIS hall. www.sayedsaad.com The bays are
connected to Bus-Bars cross-wise. Bus-Bars are either with a
three-phase enclosure or single phase enclosure. Alternatives of
Enclosures, Single three phase and three single enclosures
Three phase Single Enclosures Three phase and three single
enclosures The following alternatives are available to the
designers for configuration of GIs. 1. Separate enclosure for each
phase. This alternative was used for Components and Bus-Bars in
early GIs. Now it is used only for EHV and UHV, GIS. The GIS above
420 kV are generally with separate enclosure for each phase. 2.
Separate enclosure for components and a common single enclosure For
three phase enclosure for Bus-Bars. www.sayedsaad.com This
alternative is more widely used now for all GIS 3. Common single
enclosure for all three phases for components and For Bus-Bars. The
per cent trend is to use single three phase modules for components
and Bus-Bars for all GIS. The GIS developed during 1980s are with
this philosophy. www.sayedsaad.com Design Aspects The SF6 insulated
Switchgear contains the same components as a conventional outdoor
substation. Fig (1) illustrates the construction of typical
bay.
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Fig (1) Section of a 145 KV SF6 GIS with duplicate bus-bar 1 3-
phase Bus enclosure. 2 Isolator. 3 Earthing Switch. 4 C.B puffer
type. 5 CT's www.sayedsaad.com 6 Line Isolator. 7 VT.
www.sayedsaad.com 8 High Speed Earthing Switch. 9 Cable sealing
End. 10 Operating mechanism (cabinet). 11 Conductor tube. 12 Epoxy
partition fig. (2). All the live parts are enclosed in metal
housing filled with SF6 gas. Live parts are supported on cast resin
insulators. Some of the insulators are designed as barriers between
neighboring modules such that the gas does not pass through them.
The entire installation is sub-divided into compartments, which are
gas tight with respect to each other. Thereby the gas monitoring
system of each compartment can be independent and simple The
enclosures are of nonmagnetic material such as aluminum or
stainless steel and are earthed. The gas tightness is provided by
static O-seals placed between machined flanges. The O-rings are
placed in the grooves such that after assembly, the O-rings get
squeezed by about 20 %. Quality of material and dimension of groove
are important. Aluminum or stainless steel enclosures surround all
live parts. Enclosures are earthed. Pressurized SF6 gas provides
internal insulation between conductors and metallic enclosures. Fig
(2) below. High grade insulators of Epoxy partition resin give
support to active parts inside the enclosures and are also used as
barriers between adjacent gas filled compartments.
218
Fig(2) Epoxy partition resin Individual compartments (modules)
are connected by silver plated Plug contacts for current condu