Wednesday, October 14, 2015

Transformer technical Specifications; 990 KVA Transformer Technical Specifications

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 :-

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Below are the technical specifications of Oil type 990 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 maximum no load losses should 1.4 KW
5.       Transformer Maximum full load losses should be 10.8 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
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
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
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
16.   Insulation level for Lightning impulse withstand voltage should be 75KV peak for 11KV system
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

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19.   Transformer tank design should be as below:-

Type Tested Design

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

Plate / Corrugated fin / tank features
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.

Material of Construction
Mild steel plate with low carbon

Plate Thickness
To meet the requirements of pressure and vacuum type tests as per CBIP manual

Welding features
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.

Tank features
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
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:-


Electrolytic Copper

Maximum Current Density allowed
Maximum 3 amp / sqmm

Winding Insulating material
Class A, non catalytic, inert to transformer oil, free from compounds liable to ooze out, shrink or collapse.

Winding Insulation

Design features
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.
22.   Transformer Bushings and Terminations should have following technical specifications:-

Bushings and Terminations

Type of HV side bushing
Epoxy cast bushing, 630 Amp, interface type ‘C’ as per EN50180 and EN50181.

Type of LV side bushing
Indoor, Epoxy resin cast, 1kv voltage class and creepage 31mm/KV

Essential provision for LV side line bushing
It shall be complete with copper palm suitable for tinned copper busbar of size 100x12 mm

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

Arcing Horn
As per site requirements

Termination on HV side bushing
Cable connection by screened separable connector kit.

Termination of LV side bushing
Bus bar connections

Minimum creepage distance of all bushings and support insulators.

Protected creepage distance
At least 50 % of total creepage distance

Continuous Current rating
Minimum 20 % higher than the current corresponding to the minimum tap of the transformer

Rated thermal short time current
26.3kA for 3 sec

Bushing terminal lugs in oil and air
Tinned copper
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
Resin Cast Ring type suitable for outdoor use
CT ratio
 990KVA -1500/5 Amp

24.    Transformer Name plate should consists of following Parameters:-

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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)    1.2/50┬Ás 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:-


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

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