GENERATOR & TRANSFORMER CONNECTIONS

By: Er. RAVINDER SHARMA

Three phase connections: Three phases are interconnected in two

fashions: A. Star or Y connection B. Delta or D connection

In Y connection similar ends are connected whereas in D dissimilar ends of coil are connected.

In Y I line=I phase & Vline=Root 3XVphase. In D I line=root 3XI phase & Vline=Vphase. Y gives 3ph 4W system. N is grounded which is used for E/F protection. D is used in transformers for small low operating voltages

& induction motors.

Conversion of Balanced load system from Y to D:Any balanced Y-connected system can

be replaced with eq. D & vice-versa the relation is:

Zd = 3Zs or Zs = Zd/3

If one of the branch of balanced 3ph Y-connected system get removed or dis connected, reduction of load shall be 50% whereas in D it will be 33.33%.

Three phase Transformer Connections:

There are 4 ways in which windings of transformers can be connected:

1. Y-Y 2. D-D 3. Y-D 4. D-Y

1. Star-Star (Y-Y) connection: Most economical for small I rating, HV

transformers. N/phase & insulation required is less as

Vp=Vline/root3. There is no ph dis between pry & sec windings.

Three phase Transformer Connections (Contd.) :

2. Delta-Delta (D-D) Connection: Generally used in systems carrying large

currents on low voltage. When one of the ph gets removed/open in

D-D operation continuous on what is called open delta.

X-sec. Of cond. Is reduced as Ip=Iline/root3.

Three phase Transformer Connections (Contd.) :

3. Star-Delta (Y-D) connection: Principally used for voltage step down. Neu of pry is earthed. Line voltage ratio is 1/rt3 times of turn

ratio of the transformer.4. Delta-Star (D-Y) Connection:

Principally used to step up the voltage. Line voltage ratio is rt3 times of

transformer turn ratio & sec voltage leads pry by 30 deg.

Sec. Neutral is earthed to provide 3-ph 4w system.

TERTIARY WINDING:

In D-D,D-Y,Y-D systems all Voltages are balanced & there is not floating of neutral which only persists in Y-Y connection.

Sometimes 3 winding Trf are mfg which have Y-Y main windings & 3rd winding is used to make a closed D. It stabilize the neutral of both pry & sec wdgs. This wdg also carries 3rd harmonic currents.

Generator Connections:Principally generators are star

connected.Neutral point of generator is

earthed through NGT or NGRNeutral grounding provides ease

for E/F protection.