Y-Y three phase transformer
In three phase transformer, the most obvious way to transformer voltages and currents is to operate each phase as a single phase system. This requires three terminals for three phases and a common terminal for neutral of three phase systems. Each phase signal is transformed correspondingly from primary to secondary with a connection of phase to neutral. This connection is called as Y-Y or star-star connection.
Construction or connections of star to star(Y-Y) transformer
The star to star (Y-Y) transformer connection is shown in the figure. It consists of three primary windings and threes secondary windings with common neutral. It shows as if three two windings transformers connected each other. Both primary and secondary windings of transformers are connected between one phase X,Y,Z and a common neutral terminal N.
The winding connection vector diagram of Y-Y transformer is shown in figure. The direction of phase rotation is assumed to be counter clock wise i.e. that we can observe the connection that X phase is followed by Y phase and it is followed by Z phase. This vector is simple to remember as it is inverted Y which is specified in the Y-Y transformer. The same way for secondary winding of P,Q,R will be an inverted Y with a common terminal.
In this transformer connection each phase is 120 0 out of phase with the other two phases. This angle is represented as the angle between the legs of the inverted Y in the vector diagram of primary and secondary. Each phase is magnetically linked correspondingly with secondary phase as shown in the vector diagrams. Y-Y transformer connection is having a natural lead which may or may not be connected to ground.
Working Principle of start to star (Y-Y) Transformer
It works like three single phase transformers are connected in parallel with their neural is commonly connected. When the input is applied to one phase, it produces the magnetic flux. This magnetic flux induces a voltage in the corresponding secondary phase winding. Same way for all three phases and if any harmonic currents are presented in windings, those will get combined at the neutral and send it to the ground if it is connected to the ground.
Star-Star Transformer Harmonics
The input voltage or current is not purely sinusoidal; even if the exciting voltages are sinusoidal the transformer magnetizing currents are not sinusoidal. These magnetizing currents have significant number of odd harmonics. If three identical phases are excited by supply voltage equal magnitude with frequency 60 Hz, then the fundamental components are cancelled out each other at combined neutral point. Because those three phases are out off phase with 1200 and vector sum of these fundamental components will become zero. But the third, ninth and other zero sequence harmonic current components will be in phase at the neutral point. Therefore these components will combine at the neutral point and produces a zero sequence neutral current, provided if there is a path for the neutral current to flow. Otherwise these components introduce a distortion in induced voltage wave forms.
Zero Sequence Components in star to star transformer
The zero sequence component currents produces due to unbalanced odd harmonics presented in the star windings .If the both primary and secondary of star to star transformer connections not grounded then there will not be nay path for these zero sequence currents and the induced voltages will be distorted and they will not be sinusoidal.
Case 1: Star to Star (YY) Connection with Neutral of primary is brought out
The magnetizing currents are represented as ix, iy, iz with phase voltages are EX, EY, EZ in the circuit diagram. The secondary induced voltages are Ex, Ey, Ez. The zero sequence magnetizing currents of each phase are combined at the neutral and form a neutral current IN. This neutral current IN is brought back to source or it may rout to ground. If the input voltages are sinusoidal, then the induced voltages Ex, Ey, Ez are sinusoidal.
Case2: Star to star connection with primary and secondary neutral are open circuited
In this type of connections there will not be any path to the zero sequence component currents which may induce an unwanted voltage in the winding. Even if the input voltages are sinusoidal, the induced voltages will not be sinusoidal. he resulting voltage distortion is equivalent to a YY transformer with zero-sequence currents allowed to flow in the primary neutral with an imaginary superimposed primary winding carrying only the zero-sequence currents 180° out of phase with the normal zero-sequence currents.
The conclusion is that if a star to star (Y-Y) transformer does not have a neutral path for zero-sequence exciting currents, there will be harmonic voltages induced in the secondary even if the exciting voltage is purely sinusoidal.