Answer Posted / dhiren
What do you mean by vector group of a transformer?
Ans:-
The vector group denotes the way in which the windings of a
transformer are connected and the phase position of their
respective voltage vectors.
The following examples are for two winding three phase
transformers:
So, what does Dd0, Dyn11, YNd5 etc. mean?
First symbol/symbols, capital letters: HV winding
connection.
Second symbol/symbols, small letters: LV winding
connection.
Third symbol, number: Phase displacement expressed as the
clock hour number.
The ANSI notations are not identical to the IEC marketplace
(See ANSI/IEEE Std C57.70). For the IEC marketplace the
winding connections are categorized as follows:
a) Delta (D,d)
b) Start (Y,y)
c) Interconnected Star (Z,z)
d) Open (III, iii)
Winding connection designations
High Voltage Always capital letters
Delta - D
Star - S
Interconnected star - Z
Neutral brought out - N
Low voltage Always small letters
Delta - d
Star - s
Interconnected star - z
Neutral brought out - n
Phase displacement
Phase rotation is always anti-clockwise. (international
adopted convention)
Use the hour indicator as the indicating phase displacement
angle. Because there are 12 hours on a clock, and a circle
consists out of 360°, each hour represents 30°.
Thus 1 = 30°, 2 = 60°, 3 = 90°, 6 = 180° and 12 = 0° or
360°.
The minute hand is set on 12 o'clock and replaces the line
to neutral voltage (sometimes imaginary) of the HV winding.
This position is always the reference point.
Because rotation is anti-clockwise, 1 = 30° lagging (LV
lags HV with 30°)and 11 = 330° lagging or 30° leading (LV
leads HV with 30°)
Dd0
Delta connected HV winding, delta connected LV winding, no
phase shift between HV and LV.
Dyn11
Delta connected HV winding, star connected LV winding with
neutral brought out, LV is leading HV with 30°
YNd5
Star connected HV winding with neutral brought out, delta
connected LV winding, LV lags HV with 150°
Some extra comments:
The phase-bushings on a three phase transformer are marked
either ABC, UVW or 123 (HV-side capital, LV-side small
letters)
Two winding, three phase transformers can be devided into
four main categories (Clock hour number and phase
displacement of those most frequently encountered in
practice in brackets)
Group I - (0 o'clock, 0°) - delta/delta, star/star
Group II - (6 o'clock, 180°) - delta/delta, star/star
Group III - (1 o'clock, -30°) - star/delta, delta/star
Group IV - (11 o'clock, +30°) - star/delta, delta/star
(Minus indicates LV lagging HV, plus indicates LV leading
HV)
Group I
Example: Dd0 (no phase displacement between HV and LV)
The conventional method is to connect the red phase on A/a,
Yellow phase on B/b, and the Blue phase on C/c. Other phase
displacements are possible with unconventional connections
(for instance red on b, yellow on c and blue on a) By doing
some unconventional connections externally on one side of
the trsf, an internal connected Dd0 transformer can be
changed either to a Dd4(-120°) or Dd8(+120°) connection.
The same is true for internal connected Dd4 or Dd8
transformers.
Group II
Example: Dd6 (180° displacement between HV and LV)
By doing some unconventional connections externally on one
side of the trsf, an internal connected Dd6 transformer can
be changed either to a Dd2(-60°) or Dd10(+60°) connection.
Group III
Example: Dyn1 (-30° displacement between HV and LV)
By doing some unconventional connections externally on one
side of the trsf, an internal connected Dyn1 transformer
can be changed either to a Dyn5(-150°) or Dyn9(+90°)
connection.
Group IV
Example: Dyn11 (+30° displacement between HV and LV)
By doing some unconventional connections externally on one
side of the trsf, an internal connected Dyn11 transformer
can be changed either to a Dyn7(+150°) or Dyn3(-90°)
connection.
Additional Note
By doing some unconventional connections externally on both
sides of the trsf, an internal connected groupIII or
groupIV transformer can be changed to any of these two
groups. Thus, an internal connected Dyn1 transformer can be
changed to either a: Dyn3, Dyn5, Dyn7, Dyn9 or Dyn11
transformer, by doing external changes on both sides of the
trsf. This is just true for star/delta or delta/star
connections.
Changes for delta/delta or star/star transformers between
groupI and groupII can just be done internally.
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