Transcript Switchgear03 Materials

Module 3
Basic materials & brief specification
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SHEET STEEL
:
The Sheet Steel used are of Cold Rolled base metal, commercial quality
type. 1.2mm, 1.5mm, 2.0mm and 3.0mm thick sheets are used in various
enclosures.
Two types of sheet steel are used viz., Hot dip galvanised and Electro
galvanised.
Hot dip galvanized sheets are produced on continuous galvanizing lines
which guarantee smooth and uniform quality, according to JIS G-3302 or
ASTM A-526.
The Electro galvanized sheets have coating thickness of 2.5 microns per
side, according to JIS G-3313 or ASTM A-591
Definition of steel
Steel is a material composed mainly of iron, with a carbon content lower than 2%
and which contains other elements.
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Classification of steel
Steels are classed according to their:
- chemical composition in:
- unalloyed steels,
- stainless steels,
- other alloyed steels.
- quality class (except for stainless steels) in:
- quality steels,
- special steels.
Stainless steels
Stainless steels are subdivided according to the following criteria:
- their nickel content:
- nickel < 2.5%,
- nickel > 2.5%,
- their principal feature:
- corrosion resistance,
- high temperature oxidation resistance,
- creep resistance.
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METAL TREATMENT :
Degreasing is carried-out on the sheet steel items by soaking in hot
chemical solution, at a temperature of 65 to 85 C, for a duration of 6 to 8
minutes. This removes linolin-based oils, grease etc. from the items.
Then the sheet steel is rinsed twice with flowing water at ambient
temperature, for about two minutes, to remove alkali solution deposits on the
surface.
After rinsing, Phosphate refiner is used , at ambient temperature for
about two minutes. This avoids formation of coarse phosphate coating and
ensures a fine-grained crystal coating with excellent adhesion characteristics.
Next, it undergoes zinc phosphating at a temperature of 54 C, for 5 – 10
minutes, to have a crystalline coating of Zinc phosphate.
Again, it is rinsed with water at ambient temperature and applied with
chromate solution for preserving the zinc phosphate on to the sheet,
thereby enhancing the anti-corrosion properties.
Finally, the sheet steel is applied with paint using Electro-static powder
coating. The powder is of polyester type, finishing to Light Grey Color RAL
7032, as a standard. The overall thickness of paint shall be maintained
at 70 to 80 microns.
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Reaction of Phosphating:
Fe + H3PO4 --- Fe(HPO4)2 + H2 >
Metal + Acid Phosphoric --- Primary metal Phosphate (O)
Iron Phosphate -- FePO4 (sludge) + H2O
3Zn(H2PO4)2 -- Zn3(PO4)2+4H2O
Zinc Primary Phosphate -- Hopite
2Zn(H2PO4)2+Fe – Zn2Fe(PO4)2+H2O
Phosphophyllite
After Phosphating -- Water rinsing is a must.
Cannot be allowed a gap of more than 2-3hrs. Before Painting;
Degree=60min.
Phosphating = 2.5 – 4gm/m2
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BUSBARS
Busbars are of High Conductivity
Hard Drawn Electrolytic Grade Copper .
The copper used for electrical purposes shall be of highest purity. In
general, its total content of impurities, including oxygen, shall be less than
0.1 per cent.
Copper of this type is known as “High Conductivity” Copper .
As per IEC, the following are the characteristics of Annealed copper:
At a temperature of 20 C, the resistance of a wire of uniform
section is 0.017241 Ohm/Mt./mm2.
At a temperature of 20 C, the density is 8.89 grammes per
cubic centimetre.
At a temperature of 20 C, the ‘constant mass’ temperature
coefficient of resistance is 0.00393 per C.
At a temperature of 20 C, the resistance of a wire of uniform
section one Metre in length and weighing one gramme is
0.15328 ohm.
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
For busbar purposes, hard drawn or medium hard drawn copper
conductors are preferable to annealed conductors on account of their
greater stiffness, strength, hardness and better surface finish.
The temperature at which copper starts to anneal is influenced by
the presence of impurities and the extent to which the metal has been
cold worked and usually a temperature of about 200 C is required.
The current carrying capacity of a busbar is usually determined by the
maximum temperature at which the bar is permitted to operate.
In British Standard 159, for Busbars and Busbar connections, it is
stated that the temperature rise of busbars and Busbar connections,
when carrying rated normal current at rated frequency shall not
exceed 50 C.
This temperature rise is based on an ambient temperature having a
peak value not exceeding 40 C and an average value not exceeding 35 C,
measured over a 24 hour period.
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In practice, these limitations on temperature rise may be relaxed for copper
busbars if suitable insulation materials are used.
A nominal rise of 60 C or more above an ambient of 40 C is allowed by
IEC 439 (BS 5486 Part 1) provided that suitable precautions are taken
viz., the rise is limited by the mechanical strength of the busbar material,
the effect on adjacent equipment, the permissible temperature rise of
insulating materials in contact with the bars, and the effect on apparatus
connected to the busbars.
The amount of heat generated in a copper conductor is proportional to its
resistance and to the square of the current flowing. Under short-circuit
conditions, the heat capacity of the bar itself play an important part in
regulating the temperature, but in normal circumstances, the temperature of
the conductor attains a steady value when the rate of heat generation is
equal to the rate of dissipation.
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The cooling effect of natural convection currents is greatest in the
case of a Rectangular bar when the bar is mounted on edge.
By increasing the thickness of a bar, for example from 4 in x ¼ in
to 4 in x ½ in, its resistance is reduced to half, but its current carrying
capacity is only increased by about 45 percent since the surface area is
increased marginally .
The dissipation of heat by radiation is increased in case of
busbars having dull black surface. Hence, a coat of paint , therefore ,
may be expected to increase the heat dissipation from a busbar and
thereby improve its current rating, by 20 to 25 percent.
When a number of conductors are used in parallel , the total current
capacity is less than the rating for a single bar times the number
of bars used. The following factors may be used to obtain the total
rating (DC) for conductors in parallel.
No. of laminations
Multiplying factor
2
3
4
1.7
2.2
2.5
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TINNING : The tinning of the surfaces is normally un-necessary ,
although advantages can be gained in certain circumstances.
For best results , the surfaces are to be tinned where the
busbars are subjected to corrosive atmospheres.
BOLT SIZES AND RECOMMENDED TIGHTENING TORQUE:
Used for
Bolt Size
Busbar Sizes, mm2
Torque, Nm
M6
15 x 5
M8
25 x 5 , 30 x 10,
40 x 5 , 40 x 10
40 x 5 , 40 x 10,
50 x 5 , 50 x 10,
60 x 10
80 x 10 , 100 x 10
120 x 10
M10
M12
,
15 x 10
8.47
20.41
40.22
68.85
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SLEEVES
The Busbars are sleeved using Heat shrinkable PVC sleeves.
The properties of sleeve is as follows:
Thickness
Di-electric withstand
Material
:
:
:
Shrinkage
:
Shrink temperature
:
0.2 + 0.03 mm
more than 7 kV
Poly Vinyl Chloride blended with suitable
additives and pigments.
25 to 30% along width
Max. 1% along length
140 – 175 C
Application
:
The busbars are inserted into selected
sleeve and Hot air is blown over the sleeve uniformly along the length of
the busbar using Air Gun
Usually Black colored sleeves are used, as a standard.
However ,
Red Yellow and Blue colored sleeves are also used on specific requirement.
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SELECTION
Busbar size , mm2
-----------------------------15 x 5
15 x 10
25 x 5
30 x 10
40 x 5
40 x 10
50 x 10
60 x 10
80 x 10
100 x 10
120 x 10
OF
SLEEVES :
Sleeve Flap size , mm
-------------------------------30
30
41
48
61
61
81
93
110
140
140
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METALLIC COATINGS – ELECTROPLATED ZINC COATINGS on IRON
or STEEL
Electroplating is achieved using three principal methods:
-Cyanide alkaline bath
-non-cyanide alkaline bath
-Acid bath
DESCRIPTION OF COATINGS:
Designation – Using four symbols : Zn X1 X2 / Fe ,
Where Zn – is the metal constituting the coating
X1 – minimal coating thickness
X2 – Finish symbol – A:Clear ; B-bleached ; N-Black;
C-indescent; D- Opaque
Fe – Support metal symbol
Thickness of Coating: The degree of protection afforded by Zinc coating is directly
proportional to its thickness. However, the maximum thickness is 25microns.
Quality of Coating: Coatings must be free of all defects such as spots, scoring,
marks,etc
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Hot Dip Aluminium Zinc (AZ) coated steel strips, sheets & blanks
Aluminium-zinc alloy coated products are mainly used in the building sector
(cladding and coverings). However this type of coating can be used to in
products: covering parts, structural parts (frames, supports, etc.) inside
devices.
The high aluminum content of this coating means that the sheet withstands
temperatures exceeding those of galvanised steel (> 300°C). This type of coating
yields a corrosion resistance 2 to 6 times greater than that of galvanised steel
for the same thickness.
It also has a better heat reflecting capacity than galvanised sheet (virtually
twice as high), thus enabling improved heat discharge. However the aluminium
content means that contact with certain products (copper, lead, unprotected
steel, etc.) must be avoided. The coating deposited on the surface of the
steel is an alloy containing 55% of aluminium, 1.6% of silicon and the top-up
in zinc.
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Just as for galvanised products, zinc provides steel with a sacrificial protection:
if the coating is damaged in a corrosive environment (e.g. during transport,
storage) zinc is attacked first (appearance of white rust), thus protecting the
iron. The aluminium increases corrosion resistance by the formation
on the surface of an insoluble alumina layer.
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"Belleville" conical spring washers
Description
These washers are described by their outer diameter and thickness,
completed by the surface treatment.
Materials
These spring washers are manufactured from steel strips in carbon steel
grades with a carbon content of > 0.60
Mechanical properties
These are obtained after quenching and tempering:
maximum hardness 48 HRC at all points of the washers.
The treatment can be followed by a surface hardening by shot blasting.
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ABS Acrylonitrile Butadiene Styrene
It is a amorphous Thermoplastic
Prorperties:
Electrical
Environment
Technology
Appearance
Comparative tracking index
600V
Moisture absorption
0.2%
Self thread
no unscrewing
Aesthetic appearance
very good
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What Is SF6?
Sulphur hexafluoride. SF6 is a gas that is used in electrical power equipment. It is
colourless, odourless, non-flammable and chemically stable. This means that at
room temperature it does not react with any other substance.
Pure SF6 is not poisonous. The gas is not dangerous to inhale, provided the
oxygen content is high enough. In principle you can inhale a mixture of 20%
oxygen and 80% SF6 without danger. SF6 is about 6 times heavier than air. That
means that it may collect in cable ducts or at the bottom of tanks.
Where and How Is SF6 Used?
SF6 is used as an insulating gas in substations, as an insulating and cooling
medium in transformers and as an insulating and arc quenching medium in
switchgear for high and medium voltage applications. These are all closed systems
which are extremely safe and unlikely to leak.
In electrical power systems, high and medium voltage switchgear is required to
cut off the power in case of a fault, in order to protect people and equipment.
When power is switched, an electric arc strikes between the circuit-breaker
contacts. Breakers filled with SF6 are electrically insulating and effectively control
arcing.
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What Is the Benefit of SF6 ?
There are two reasons for using SF6 in electrical equipment:
SF6 provides extremely good electrical insulation and very effectively quenches electric
arcs. These properties of SF6 make it possible to build electrical equipment and
apparatus that are compact, use a small amount of material, are safe and will last a long
time. At normal atmospheric pressure, SF6 has a dielectric withstand capability that is
2.5 times better than air. Usually the gas is used at 3-5 times atmospheric pressure and
then the dielectric properties are ten times better than for air.
SF6 insulates so well because it is strongly electronegative.
SF6 effectively controls circuit-breaker arcs because it has excellent cooling properties
at temperatures (1500-5000 K) at which the arcs extinguish (the gas uses energy when it
dissociates and therefore produces a cooling effect).
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