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TRANSFORMERS
by
Er. Y.S. JADAUN
1.0
PRINCIPAL
 Laminated core
 Primary winding
 Secondary winding
dφ
where;
N1 & N2 = No. of turns in Prim. Sec.
dt
m = Max. flux in core, weber
If V1 = Vimsin2ft
then  = msin2ft
Bm = Flux density in web/sqm
Hence e1 = N1 m 2f cos2ft
A = net cross section area of core (Sq. Tesla)
Maximum value (r.m.s)
F = Frequency
2π
E1 
φ m fN 1  4.44φ m fN 1  4.44 B m AfN 1
2
Similarly, E2 = 4.44m fN2 = 4.44 Bm AfN2
e1  N 1
1.1
IDEAL TRANSFORMER
E2 N2

 k , E 1 I1  E 2 I 2
E1 N1
I1
I2
Actual Transformer: (i) iron loss (hysteresis loss) and eddy current losses.
(ii) copper loss.
(i)
Iw = Iron loss and small copper loss
(ii)
I = wattles component
(iii) I = I0 sin 0
I0 small
Hence no load input
 Iron loss

I0  I2μ  I2w
I2’ = I2
, 2 = 2,
but in opposite direction.
I2 = load component of primary current.
Hence, magnetic losses are same.
N1I2 = N1I1 = N2I2
I
N
Or, 1  2  K
I 2 N1
(a) At No Load
1.2
Transformer Having Winding Resistance But No Magnetic Leakage
V2 = E2 – I2R2
1.3
(b) On Load
,
E1 = V1 – I1R1
Magnetic Linkage
 Ideal case: all the flux linked with primary also linked with secondary winding.
 Practically: not possible, hence self excitation.
 Leakage could be avoided if primary and secondary occupy the some space.
Physically not possible.
 Minimization of leakage may be done by placing windings concentrically.
1.3.1 Regulation
 If primary voltage V1 constant when transformer loaded, V2 drops (log PF)
V  V2
% Regulation  2
X100
V2
where: V2, = Secondary voltage at no load
V2 = secondary voltage at full load
% Regulation at any P.F. = (R Cos + X Sin) 
where, R = percentage resistive drop
X Cosθ  R Sinθ 2
200
X = percentage reactive drop
Cos  = P.F. log (Note: for leading P.F.,  will change to -)
1.4
Transformer Losses
1.4.1 No Load Losses:
 Hysteresis loss, Wh = Kh f Bm2 watts.
 Eddy current losses, We = Ke Kf Bm watts
Where: Kh = Hysteresis constant
Ke = Eddy current constant
Kf = Form factor
 To reduce losses: Use
 Core steel with high silicon constant.
 Thin lamination.
 Core losses = No load input power of a transformer
1.4.2 Load Losses
 Mainly due to ohmic resistance of transformer and include stray losses (due to
stray flux in mechanical structure and winding conductor).
 Measured by short circuit test.
1.5
EFFICIENCY
% Efficiency 
Output
Input  Losses
X100 
X100
Input
Input
Losses
X100
Input
 Condition of maximum efficiency: Occurs when Iron loss = copper loss
Iron loss
 Load corresponding of maximum efficiency 
X Full load
Full load copper loss
1
2.0
TWO WINDING TRANSFORMER
 Total weight is proportional to N1I1 + N2I2
 Working: as above.
3.0
AUTO TRANSFORMER
 Single continuous winding.
 Secondary side voltage is obtained by tapping the winding.
 Used where transformer ratio differs slightly from unity.
 Part winding carries current I1 and remaining portion I2 – I1 where I2 > I1.
 Total weight of copper is proportional to {(N1 – N2) I1 + N2 (I2 – I1)}.
 Weight of copper compared to that in two winding Transformer.

N 1  N 2 I1  N 2 I 2  I1 
N 1 I1  N 2 I 2
2N 2
N1
2K
1
1
1 K
N2 I2
2
1
X
NI I1
where
I1 N 2

K
I 2 N1
i.e. Weight of copper in Auto-transformer = (1 – K) . W0
where W0 = weight of copper in two winding transformer.
Therefore, Saving
= W0 – Wauto
= W0 – W0 (1 – K) = KW0
Higher the value of K, higher is the saving.
4.0
THREE WINDING TRANSFORMER
 Two main windings and a tertiary winding.
 Used for:
 Supply of load which is not to be connected to secondary winding for
some reason.
 In star-star transformers, for allowing sufficient zero sequence current for
protection; to suppress harmonic voltages, to limit unbalanced current; to
limit voltage unbalance when main load asymmetrical winding is delta
connected.
 To interconnect 3 supply systems at different voltages (tertiary generally
delta winding).
 As voltage coil in testing transformer.
 Delta tertiary used in star-star transformer to limit disadvantages (when
loads unbalanced, third harmonic distortion) by circulating induced
currents in delta winding.
 Rating of tertiary:
 If for additional load, as per load and 3 phase dead short at terminals with
power from other 2 windings.
 If for stabilization as per thermal/mechanical stresses for short duration fault
currents. (1 Ph, line-ground fault most harmful).
5.0
PARALLEL OPERATION
 For Satisfactory Operation:
 Same phase sequence and zero relative displacement (essential)



Same polarity (essential)
Same voltage ratio (to close degree)
Same p.u. impedance (desired for better load division).
 Phase Sequence:
 Phase sequence decides the order in which phases reach their maximum
positive and negative voltages. If not identical, each pair of phases will be
short circuited in each cycle.
 Following transformers may be paralleled
Transformer 1: Yy
Yd
Yd
Transformer 2: Dd
Dy
Yz
 Polarity: wrong polarity will mean dead short circuit, n).
 Voltage Ratio:
 If not same, circulating currents in secondaries and therefore in primaries also.
 I2R losses, undesirable loading condition occur.
 Impedance
 Currents proportional to ratings, if impedance inversely proportional to ratings
and per unit impedance identical.
 If difference in quality factor (x/R ratio) of per unit impedance, divergence of
phase angle to two currents; hence power factor will be different.
6.0
TAP CHANGER
 It is essential to use tap changers so as to vary turns ratio to maintain the system
voltage within prescribed limits. Tap changers are of types, viz.


6.1
Off Load tap Changer
On Circuit Type
Off Load Tap Changer
 Comprises of three parts



Operating handle projecting outside the transformer.
Fixed contact with connecting terminal.
Moving contact system with insulating shaft.
 Winding Circuit Arrangements
 Double Compartment Type
 For large transformers.
 Separate tap selector and divertor switches.
a.
b.
c.
d.
e.
f.
M1 opens.
A1 opens: interruption of circulating current.
A2 closes: circulating current through R1 and R2.
M2 closes.
Complete operation in 40 to 80 ms.
For tap changing in opposite direction: sequence is reversed.
 Tapping on neutral end of High Voltage winding.
 Only on 3 pole tap changer required per transformer (3 ).
 Large regulating transformer
 3 single phase type required, coupled together, driven by 1 shaft.
OLTC with delta connected windings


6.2
A mechanical lock provided: prevent unauthorized operation.
Electromagnetic latching device/micro switch for interlocking C.B.
operation.
On Load Tap Changer



Changing turn ratio while load is being delivered.
Operating efficiency improved.
These possess an impedance: prevents short circuiting during changing operation.
 Classified as:
 Reactor Transition Type:
a. Center tapped reactor.
b. Large number of taps.
c. Shorter contact life due to long arching time.
d. Used in USA but not other countries in general.
 Resistor Transition Type:
a. Longer life of contact due to shorter arcing time (unity P.F.).
b. Resistance on transition tap.
 Single Compartment Type
6.3
Mannual and Electrical Operation
6.4
Automatic Control
6.5



With voltage relay.
Time delay to prevent hunting during transients.
Line drop compensation arrangements provided.

Voltage at some distance point can be made constant irrespective of load.
Tap Changer Selection
 Voltage class of transformer winding and its rating.
 % voltage variation required.
 Maximum through current.
7.0
TYPES OF TRANSFORMERS
7.1
i.
ii.
iii.
iv.
v.
7.2
i.
ii.
8.0
8.1
Power Transformers
Two winding
Three winding
Auto
Step up
Step down
Instrument Transformers
Potential Transformer (P.T.) – also called voltage Transformer (V.T.).
Current Transformer (C.T.).
CONSTRUCTION
Core
Core diameter is adjusted to meet the guaranteed parameters and it depends on:
i.
Rating.
ii.
iii.
iv.
v.
vi.
8.2
i.
ii.
iii.
iv.
v.
vi.
8.3
% impedance between windings.
Basic insulation level (BIL).
Transport height.
Over-fluxing requirement.
Type of core and quality of steel.
Windings
Spiral: Medium current, low voltage (Tertiary winding of star/star/delta is
generally of this type).
Helical: High current, low voltage (generally for L.V. coils of large generator
transformer).
Reversed section (disk winding): low to medium current, high voltage (usually
up to 132 kV class windings and not for higher voltages due to impulse
distribution characteristic).
Parallel layer: for star connected transformers having graded insulation & for
voltages above 132 kV.
Tapered layer: use a number of concentric spiral coils arranged in layers.
Interleaved discs: for improved behaviour against impulse voltage.
Insulation
Minor: Generally paper insulation between different parts of one winding e.g.
between turns, layers etc.
ii.
Major: Generally press board cylinders separated by oil ducts. Insulation of
windings to earth & transformer to core, other windings of same phase (H.V.
to L.V.) and phase to phase.
i.
8.4
i.
ii.
8.5
Tank
8.6
i.
ii.
8.7
8.8
Tap Changer
Off – Circuit.
On – Load.
Bushings:
Porcelain (concentration of electric stress, up to 36 kV – other bulky).
Condenser (insulation wall thickness divided in to a number of capacitors by
concentric cylinders, outside porcelain).
Insulating oil
Cooling System and Arrangement
Cooling system
a. ONAN (Oil natural, Air natural).
b. ONAF (Oil natural, Air forced).
c. ONAF (OFWF (Oil forced, water forced).
d. ODAF (Oil directed, Air forced).
e. ODWF (Oil directed, Water forced).
ii.
Cooling Arrangement
a. With radiators.
b. Use of coolers.
i.
8.9
8.10
8.11
8.12
8.13
8.14
Conservator
Buchholz relay
Temperature indicator
Pressure relief valve
Oil level indicator
Cable sealing box
9.0
RATING
i.
ii.
iii.
iv.
Type of transformer.
Number of phases.
Frequency.
Rated power (kVA or VA & for taping ranges exceeding + 5%, required
power on extreme tapings).
v.
Rated voltage for each winding.
vi.
Connection symbol.
vii.
Requirement of on-load/off-load tap changers or links – number of tapings,
taping range, location of tapings, particular voltage required to be varied,
whether constant flux, variable flux/combined voltage variation.
viii. Impedance voltage at rated current and principal taping for different pairs of
windings and at least on extreme tapings in case of parallel operation, if
required.
ix.
In door or out door type.
x.
Type of cooling.
xi.
Temperature rises and ambient temperature conditions including altitude and
in case of water cooling, chemical analysis of water.
xii.
No. of cooling banks, spare capacity if any, with no. of stand by cooling
fans/pumps.
xiii. Highest system voltage for each winding.
xiv. Method of system earthing for each winding.
xv.
Whether windings shall have uniform or non-uniform insulation, if nonuniform – then power frequency with stand voltage of neutral and impulse
with stand level if an impulse test on neutral is required.
xvi. For windings having system highest voltage greater 300 kV, the method of
dielectric testing.
xvii. With stand voltage values constituting insulation of line terminals.
xviii. Limitation of transportation weight, moving dimensions and special
requirement, if any, of installation, assembly and handling.
xix. Whether stabilizing winding is required.
xx.
Over fluxing conditions/any other exceptional service conditions.
xxi. Loading combinations in case of multi-winding transformer and when
necessary, active and reactive outputs separately, especially in case of multiwinding autotransformer.