Transformers are
backbone of electrical systems. Transformers are used for both stepping up and
stepping down the voltage. While procurement of Distribution Transformers
there are following technical aspects which should be considered :-
For Tan-delta, Loss angle test visit link:-
Below are the technical
specifications of Oil type 400 KVA transformer :-
1. Transformer voltage ratio should be 11 KV/ 433 V
2. Vector group should be Dyn11
3. Transformer Impedance at 75 degree Celsius
should be 5%
4. Transformer-400 KVA maximum no load losses should 0.7 KW
5. Transformer-400 KVA Maximum full load losses should be
5.1 KW
6. Transformer Oil Temperature rise without
enclosure should be 35 Deg C max over ambient 40 Deg C
7. Transformer Winding Temperature rise without
enclosure should be 40 Deg C max over ambient 40 Deg C
8. Current density of LT and HT winding
should be 3 A/mmsq for 400 KVA Transformer.
9. Tapping on HT winding should be off load with +/-
5% in 2.5% step
10. Design clearances for Phase to phase for 11 KV system should
be 180 mm and Phase to earth for 11KV system should be 120 mm for 400 KVA Transformer.
11. Design clearances for Phase to phase for 433 KV system should be
25 mm and Phase to earth for 433KV system should be 25 mm for 400 KVA Transformer.
12. Transformer Type should be Double Copper wound, three
phase, oil immersed, with ONAN cooling
13. Voltage variation on Supply side should be +/- 10%
14. Frequency variation on Supply Side should be +/- 5%
15. Insulation level for one minute power frequency withstand voltage
should be 3KV for 433V system & 28KV for 11KV system for 400 KVA transformer.
16. Insulation level for Lightning impulse withstand voltage
should be 75KV peak for 11KV system for 400 KVA transformer.
17. Short circuit withstand level should be 3 Sec with 5% impedance
18. Noise level for Transformer should not exceed limits as per NEMA
TR-1 with all accessories running measured as per IEC 551 / NEMA standard
running measured as per IEC 551 / NEMA standard
For Transformer Oil Filteration process visit link:-
19. Transformer tank design should be as below:-
Tank
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Type
Tested Design
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Design
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a)
Completely sealed type with corrugated fins and with/without conservator tank
as per site requirements
b)
Completely oil filled or N2 cushion at top filled with positive pressure. N2
shall be technical grade in accordance with IS:1747
c)
With bolted / welded cover
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Plate /
Corrugated fin / tank features
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a)
Adequate for meeting mechanical & electrical withstand requirements, as
per applicable standard.
b) The
tank and its sealing (gaskets, o-rings, etc.) shall be of adequate strength
to withstand positive and negative pressures built-up inside the tank while
the transformer is in operation. The maximum pressure generated inside the
tank should not exceed 40kPa, positive or negative.
c)
Corrugated fins shall be built up of CRCA sheets of minimum 1.2mm thick.
d) The
corrugated tank wall shall ensure sufficient cooling of the transformer and
compensate for the changes in the oil volume during operation.
e)
The transformer shall be capable of giving continuous rated output, without
exceeding the specified temperature rise.
f)
Internal clearance of tank shall be such that, it shall facilitate easy
lifting of core with coils from the tank and HV & LV bushings mounted on
Top cover.
g)
All joints of tank and fittings shall be oil tight. The tank design shall be
such that the core and windings can be lifted freely with cover. The tank
plate shall be of such strength that the complete transformers when filled
with oil may be lifted bodily by means of lifting lugs.
h) Tanks
with corrugations & without conservator shall be tested for leakage at a
pressure as per the applicable standard.
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Material
of Construction
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Mild steel
plate with low carbon
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Plate
Thickness
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To meet
the requirements of pressure and vacuum type tests as per CBIP manual
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Welding
features
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a) All
seams and joints shall be double welded
b) All
welding shall be stress relieved for sheet thickness greater than 35 mm
c) All
pipes, stiffeners, welded to the tank shall be welded externally
d)
All corrugated fins or expansion bellows provided shall be double welded.
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Tank
features
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a) Bottom
with stiffeners & adequate space for collection of sediments
b) No
external pocket in which water can lodge
c) Tank
bottom with welded skid base
d)
Strength to prevent permanent deformation during lifting, jacking,
transportation with oil filled.
e) Minimum
disconnection of pipe work and accessories for cover lifting
f)
Tank to be designed for oil filling under vacuum g) Tank cover fitted with
lifting lug
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20. Transformer oil should be Class 1 new mineral insulating oil,
shall be certified not to contain PCBs. Naphthalene base with antioxidant
inhibitor.
21. Transformer winding should have following:-
Winding
|
||
Material
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Electrolytic
Copper
|
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Maximum
Current Density allowed
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Maximum 3
amp / sqmm
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Winding
Insulating material
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Class A,
non catalytic, inert to transformer oil, free from compounds liable to ooze
out, shrink or collapse.
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Winding
Insulation
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Uniform
|
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Design
features
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a) Stacks
of winding to receive adequate shrinkage treatment.
b)
Connections braced to withstand shock during transport, switching, short
circuit, or other transients.
c)
Minimum out of balance force in the winding at all voltage ratios.
d)
Conductor width on edge exceeding six times its thickness.
e)
Transposed at sufficient intervals.
f)
Coil assembly shall be suitably supported between adjacent sections by
insulating spacers + barriers.
g) Winding
leads rigidly supported, using guide tubes if practicable.
h)
Winding structure & insulation not to obstruct free flow of oil through
ducts.
i) Delta
connection shall be done using Flexible cable.
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22. Transformer Bushings and Terminations should have following
technical specifications:-
Bushings
and Terminations
|
||
Type of HV
side bushing
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Epoxy cast
bushing, 630 Amp, interface type ‘C’ as per EN50180 and EN50181.
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Type of LV
side bushing
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Indoor,
Epoxy resin cast, 1kv voltage class and creepage 31mm/KV
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|
Essential
provision for LV side line bushing
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It shall
be complete with copper palm suitable for tinned copper busbar of size 100x12
mm
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Essential
provision for LV side neutral bushing
|
In case of
neutral bushing the stem and bus bar palm shall be integral without bolted,
threaded, brazed joints. Bus bar size shall be 100x12 mm
|
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Arcing
Horn
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As per
site requirements
|
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Termination on HV side bushing
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Cable
connection by screened separable connector kit.
|
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Termination of LV side bushing
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Bus bar
connections
|
|
Minimum creepage distance of all bushings and
support insulators.
|
31mm/kv
|
|
Protected creepage distance
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At least
50 % of total creepage distance
|
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Continuous Current rating
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Minimum 20
% higher than the current corresponding to the minimum tap of the transformer
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Rated thermal short time current
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26.3kA for
3 sec
|
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Bushing terminal lugs in oil and air
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Tinned
copper
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23. Current Transformers used for Transformers should have following
Technical specifications:-
Current Transformers
|
All three
phases and neutral on LV side
|
Maintenance requirements
|
Replacement
should be possible by removing fixing nut of mounting plate without
disturbing LT bushing
|
Accuracy Class
|
5P10
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Burden
|
5VA
|
Type
|
Resin Cast
Ring type suitable for outdoor use
|
CT ratio
|
400KVA
-1000/5 Amp
|
24. Transformer Name plate should consists of following
Parameters:-
For Parallel Operation
of Transformers Visit link:-
Following details shall
be provided on rating and diagram plate as a minimum.
a) Type/kind of transformer with winding material.
b) IS/ IEC [R3] standard to which it is
manufactured.
c) Manufacturer's name.
d) Transformer serial number.
e) Month and year of manufacture.
f) Rated frequency in HZ.
g) Rated voltages in KV.
h) Number of phases.
i) Rated power in KVA.
j) Type of cooling (ONAN).
k) Rated currents in Amp.
l) Vector group connection symbol.
m) 1.2/50µs wave impulse voltage withstands level
in KV.
n) Power frequency withstands voltage in KV.
o) Impedance voltage at rated current and frequency
in percentage at principal, minimum and maximum tap
p) Load loss at rated current.
q) No-load loss at rated voltage and frequency
r) Continuous ambient temperature at which
ratings apply in deg c
s) Top oil and winding temperature rise at rated
load in deg c;
t) Winding connection diagram with taps and table
of tapping voltage, current and power
u) Transport weight of transformer
v) Weight of core and windings
w) Total weight
x) Volume of oil
y) Weight of oil
z) Name of the purchaser
25. Transformer to be designed for suppression of
3rd, 5th, 7th harmonic voltages and high frequency disturbances
26. Transformer core should have following technical
specifications:-
Core
|
|
Grade
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High grade
, non- ageing, low loss, high permeability, grain oriented, cold rolled
silicon steel lamination
|
Core
Design Features
|
a)
Magnetic circuit designed to avoid short circuit paths within core or to the
earthed clamping structures.
b)
Magnetic circuit shall not produce flux components at right angles to the
plane of lamination to avoid local heating.
c)
Least possible air gap and rigid clamping for minimum core loss and noise
generation.
d)
Adequately braced to withstand bolted faults on secondary terminals without
mechanical damage and damage/ displacement during transportation and
positioning.
e)
Percentage harmonic potential with the maximum flux density under any
condition limited to avoid capacitor overloading in the system.
f)
All steel sections used for supporting the core shall be thoroughly sand
blasted after cutting, drilling, welding.
g)
Provision of lifting lugs for core coil assembly.
h)
Supporting framework designed not to obstruct complete drainage of oil from
transformer
|
For Parallel operation
of Transformers visit link:-
For Tan-delta, Loss
angle test visit link:-
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