Advantages of star to star (YY) Transformer
The star to star transformer has certain important advantages over other three phase transformer connections.
Primary and secondary circuits are in phase
The primary and secondary circuits are in phase that there is no phase angle displacement in star-star connections. This can be memorized easily by looking that primary and secondary have the same pattern of windings so there will not be any phase displacements. This property is mainly used for connecting two voltages together. For example let us suppose three systems are operating at 400 KV and 200 KV and 33KV. Then for connecting these systems together we can use Y-Y transformer using step up or step down configuration.
YY transformer needs lesser winding turns:
In star connection of windings as we know
That is line voltage or Phase to neutral voltage is 57% of the phase to phase voltage. So it requires lesser winding turns o produce the same amount of excitation as in the case of Delta winding connected across the phases.
Insulation levels can be reduced in YY connection:
If the neutral end of a Y-connected winding is grounded, then there is an opportunity to use reduced levels of insulation at the neutral end of the winding. A winding that is connected across the phases (Delta) requires full insulation throughout the winding.
It can also constructed as auto transformer by doing necessary modifications in connections with a feasibility of great saving coasts compared to two winding transformers.
Disadvantages of Star to star (Y-Y) transformer:
There are certain disadvantages of star to star transformer connections.
Harmonic effect in ungrounded star to star (Y-Y) connection:
The presence of third harmonic components in ungrounded star to star connection will result in over voltages in light load conditions. As we know in Y-Y connection the line voltage is 57% of phase to phase voltage. With sinusoidal voltage applied at the input then the third harmonic current components in ungrounded Y-Y circuit produces a 60% of effective total phase to phase voltage. So the total voltage is calculated as follows
E = (line voltage 2 + (0.6 * line voltage) 2) %
= (0.572 + (0.6 * 0.57)2) %
So due to the third harmonic components the resultant voltage is very high in ungrounded Y-Y connection.
Large Voltage drop for unbalanced loads:
There can be large voltage drop for unbalanced phase to neutral loads because voltage drop in phase to phase loads are caused due to leakage reactance of the transformer where as phase to neutral loads cause a voltage drop through the magnetizing reactance, which is 100 to 1000 times higher than the leakage reactance.
Thermal over heating:
Under certain conditions or faults, in Y-Y connection there will be a severe over heating of tank which will destroy the transformer quickly. This usually occurs with an open phase on the primary circuit and load on the secondary.
Ferro-resonance in Y-Y transformer connection:
Series resonance between the third harmonic magnetizing reactance of the transformer and line-to-ground capacitance can result in severe over voltages.
If the neutrals of the primary and secondary are both brought out, then a phase-to-ground fault on the secondary circuit causes neutral fault current to flow in the primary circuit. Ground protection laying in the neutral of the primary circuit may then operate for faults on the secondary circuit.
The obvious remedy for some of the disadvantages of the Y-Y transformer connection would be to simply solidly ground both the primary and secondary neutrals. In fact, this is standard practice for virtually all Y-Y transformers in systems designed by utility companies. Unfortunately, solidly grounding the neutrals alone does not solve the problem of tank overheating, Ferro-resonance, and operating primary ground protection during secondary faults.