Subject: Electrical Machine-I

• This Lecture contains

​

• Three-phase Transformer

​

​

​

​

Syllabus.

• Three Phase Transformer:
• The three phase transformer are used throughout in industry to change values of three-phase voltage and current. Three phase power is the most common way in which power is produced.
• The basic working principle remain same as that of single phase transformer.

​

• Three Phase Transformer:
• As compared to a bank of three single phase transformer, the single unit of three-phase transformer occupies less floor space, weight reduction, cost reduction of about 15% and only one unit to be handled.
• The center lag formed, carries the flux produced by the three phase current 120^O apart, hence the sum of three fluxes is also zero. Hence It will make no difference if the common leg is removed.

​

• Three Phase Transformer connection:
• HV Side capital letter: D Y Z
• LV Side small letter: d y z
• Territory winding: y3 d3 z3

​

• Three Phase Transformer connection:
• Group 1
• Connection: Yy, Dd, Dz

​

• Three Phase Transformer connection:
• Group 2
• Connection: Yy, Dd, Dz

​

• Three Phase Transformer connection:
• Group 3
• Connection: Dy, Yd, Yz

​

• Three Phase Transformer connection:
• Group 4
• Connection: Dy, Yd, Yz

​

• Group 1 YY0 or YY12

​

​

• Group 1 YY6 or YY180

​

​

• Features in YY connection

​

• Reduction of phase voltage and hence Insulation requirement reduce.
• Phase current is higher and hence cross section area is more.
• Use of this connection in High voltage small current transformer

​

• Disadvantages
• As the transformer connection does not provide any closed path for 3^rd harmonic component of current emf is sinusoidal only when 3 limbed core type transformer is used.
• Problem of floating point neutral
• It cannot supply unbalanced loads when not used with 3 limbed core type transformer
• Source can supply only positive components

​

• Remedies

​

• Grounding primary and secondary neutrals along with source and load neutrals so that there is a closed path for 3^rd harmonic and zero sequence components.
• Using delta connection territory winding :
• Makes the emf more sinusoidal
• Stabilizes neutral point of YY transformer
• Helps YY transformer to supply unbalance load effectively

​

​

​

• Three Phase Transformer connection:
• Group 1
• Connection: Dd0, or Dd12

​

• Group 1 Dd0 or Dd12

​

​

• Group 1 Dd6

​

​

• DD connection

​

​

​

• More insulation required.

​

​

• Less cross section of conductor
• Less mech. Strength of winding

​

• a => C => Vr

​

• Can handle unbalance load

​

• YY connection

​

​

• Less Insulation required

​

​

​

• More cross section of conductor
• More mechanical strength of winding

​

• a => C => Vr
• Can’t handle unbalance load

​

​

​

• Three Phase Transformer connection:
• Group
• Connection: Dy11

​

• Group 2 Dy11

​

​

• Three Phase Transformer connection:
• Group
• Connection: Dy1

​

• Group 2 Dy1

​

​

• Three Phase Transformer connection:
• Group
• Connection: Yd11

​

• Group 2 Dy11

​

​

• Three Phase Transformer connection:
• Group
• Connection: Dy1

​

• Group 2 Dy11

​

​