Transformer rating and cooling methods

like all other electrical machines, transformers rating is limited by temperature rise, which depends on losses. in transformer there are mainly two type of losses

1. copper losses - depends on current through windings and resistance of winding (I²R). keeping resistance constant this part is restricted by current.

2. iron losses - depends on core (size, material, lamination thickness), flux density and power frequency. here physical properties of core and power frequency is constant, so iron losses will depend on flux density. which will depend on voltage applied.

in this way transformer heat generated will depend on voltage applied on transformer and current flowing through windings, it has nothing to do with power factor. so we may find transformer ratings such as 300VA, 315KVA, 100 MVA ....

Further transformer rating can be improved if we maintain the temperature in limits. For cooling of transformers various methods are used:

• Oil Natural Air Natural (ONAN): Oil circulation and air circulation is natural. Oil circulates inside transformer by covection. Air circulates outside transformer by convection.
• Oil Natural Air Forced (ONAF): Oil circulation is natural but  air is forced with the help of fans installed near radiators.
• Oil Forced Air Forced (OFAF): Oil is circulated with the help of cooling pumps installed between transformer main tank and radiators. Air is forced with the help of fans installed near radiators.
• Oil Directed Air Forced (ODAF): Oil is circulated with the help of cooling pumps installed between transformer main tank and radiators, and direction of oil is directed to flow inside winding. Air is forced with the help of fans installed near radiators.

For example one transformer may have rating like 100MVA OFAF / 70MVA ONAF / 50MVA ONAN. This means this transformer can deliver 100MVA if both pumps and fans are in service, 70MVA if only fans are in service and 50MVA if no pumps or fans are in service. Generally, starting of fans and pumps is automated with temperature operated switches. These switches are installed for measurement of transformer oil temperature. 

For example, for a typical transformer, if temperature rises above 70⁰C Fans will starts. If temperature rises further above 75⁰C pumps also starts. If temperature further rises above 100⁰C transformer trips. if temperature falls below 70⁰C pumps stops. If temperature falls below 60⁰C fans also stops.  

Transformer vector group

Understanding vector groups:

The vector groups are used for three phase transformers for representing their winding connections. For understanding vector groups, let us assume the following winding connections:

Now we have to find vector group of the above transformer. The currents on primary side are IA, IB, IC and currents on secondary side (after delta formation) are Ia, Ib, Ic. Now let us assume currents inside secondary windings are Ia', Ib', Ic'. Being wound on same core limb and subjected to same magnetic flux:

Ia' will be in phase to IA
Ib' will be in phase to IB
Ic' will be in phase to IC

Let us derive secondary current for phase-A for this case: 

Ia= Ia'- Ib'

Now representing this in vector form (vector displacement of Ib' is 120 deg from Ia') :

So, Ia is derived in above figure. As Ia is leading IA by 30 deg. the vector group is d11. Phase rotation is always anti-clockwise. (international adopted convention)

See the vector groups below:

In the vector group:

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.

Winding connection designations:
High Voltage Always capital letters
Delta - D
Star - Y
Interconnected star - Z
Neutral brought out - N

Low voltage Always small letters
Delta - d
Star - y
Interconnected star - z
Neutral brought out - n

For auto transformer - a

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°)

Examples:
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°
YNa0d11 : Star connected HV winding with neutral brought out, auto transformer with 0° displacement. LV winding delta conected leading HV by 30°

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.