Magnetic Circuit:
The law for magnetic circuits is
MMF = Flux x Reluctance.
Total mmf for the complete magnetic circuit is given by
F= ( B1/ μ1) l1 + ( B1/ μ1) l1 + ........
where l1 , l2 etc. are magnetic flux path lengths in the various parts of the magnetic circuit.
Transformers:
The transformer consists of the following elements:
(i) Magnetic circuit consisting of limbs (core), yokes and clamping structures,
(ii) Electric circuit consisting of low voltage winding, high voltage winding and tertiary winding,
(iii) Dielectric circuit consisting of insulation in different forms,
(iv) Tank and accessories such as oil, cooling devices, conservators, breathers, terminal insulators and lead emf equation is given by
E1 = 4.44 f T1 B Ai volts,
where Ai is the cross-sectional area of the core in sq-m.
The output equation is given as
Q = 2.22 f Bm Kw δ A i Awvolt amperes
where Kw is the space factor of the window, Aw is the net window area and δ is the current density.
All the conducting parts and the magnetic parts of the transformer must be well insulated from each other. The insulation between core and LV winding is provided by an insulating cylinder of paper bakelite or press board. Similar insulation is used between LV and HV winding. The end insulation is provided to insulate the windings from the yokes. Partitions of press board are provided between HV windings of different legs, proper insulation is provided between the coils, turns and layers of the winding.
Design and Constructional Features of a Transformer
(A) Design of transformer:
1. Specification of transformer:
Rating according to standard loading type, frame size, temperature rise, efficiency.
2. Performance of Transformer:
Transient behavior, system considerations, no load characteristics, short circuit characteristics, mechanical forces, optimization.
3. Manufacture of transformer:
Material procurement, handling and processes, skill and talent, transportation to site.
(B) Constructional features of transformer:
1. Transformer Cores:
Material, core and yoke, laminations, insulation, clamping, flux density, window, space optimization, core loss, weight.
2. Transformer Windings:
Type, current density, conductors, turns, layers, strands, tappings, insulation, transposition, resistance, l2R loss, leakage reactance, mechanical forces, thrust block, shielding, insulation gradient, weight.
3.Transformer Frame:
Size, stiffness, fixing, tank, heat exchanger, conservator, bushings, relays, weight.
Insulation Thickness :
|
Rated
KVA
|
Highest
voltage of the transformer
|
Thickness
of insulation cylinder in mm
|
Thickness
of oil duct in mm
|
Insulation
from yoke to the winding in mm
|
||
|
|
LV
and HV
|
Between
phases
|
LV
and HV
|
Between
phases
|
|
|
|
Up to 100 kVA
100 to 500 kVA
500 to 1000 kVA
|
3.3 and 6.6 KV
|
3
3
3
|
2
2
2
|
6
9
11
|
8
8
8
|
22
22 to 31
31 to 52
|
|
Up to 6500 kVA
500 to 1000 kVA
|
11 kV
|
3.5
5.0
|
2.5
2.5
|
10
12
|
13
13
|
35
35 to 50
|
|
Up to 500 kVA
500 to 1000 kVA
|
|
5.0
6.0
|
3
3
|
20
20
|
25
25
|
55
70
|
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