Why should we use a star-star connection?

 OQ: 

For the most part you usually shouldn’t, except under special circumstances.

A star-star connection is a topology used to build a three phase transformer. Both the primary and secondary windings in the transformer are connected to their source and load lines, respectively, in a star configuration, also known as a Y, configuration. The other possibility is a the Delta connection. Here’s a diagram to make the difference clear:

The star connection gives you a true neutral (N) and always produces a four wire circuit. The delta connection is what Navy ships use. The delta topology does not give you a true ground, so one of the phases can short to ground (e.g. battle damage), which comes with a lot of fireworks and transient currents that pop breakers, but when things settle out again, you can reset the blown breakers, replace the bad fuses, and keep operating.

Stepping voltages up and down and moving electrical power around is always done with transformers that use one of these topologies: (star-star or delta-delta) or else mixes them in one of two ways (delta-star or star-delta) Each of these four transformer designs has advantages and disadvantages. But for the answer the question, we will discuss only the star-star.

Here is how a star-star transformer is wired:

|———————————— Source ———————————-|

| ———————————— Load ————————————-|

The star-star transformer’s advantages:

1. The primary and secondary voltages are always in phase.
2. The currents flowing through the primary windings is always the same as the currents in the source.
3. The currents flowing though the secondary windings is always the same as the currents n the load.
4. The voltage between the phases is always SQRT(3) * winding voltage
5. The voltage of the Neutral wire is always zero volts with respect to ground, so long as the loads on the three phases is balanced.

The star-star disadvantages all rear their ugly heads when the load is not balanced. They are:

1. If the load is unbalanced, N is never zero volts with respect to ground. In fact, N’s phase and voltage with respect to ground move around as a function of the unbalanced loads of each of the phases.
2. If the load is unbalanced, the voltages of the three phases are unequal.
3. If the load is unbalanced, this configuration generates a lot of radio frequency noise that interferes with nearby communication lines. For example, telephone lines can’t be run in parallel with lest inductive coupling ruin their signals.

In practice, loads are never very balanced. Because of that, the star-star configuration is just not used in common practice (but I don’t know anything about high tension long distance power transmission — never got involved in it).

EDIT: What are the special conditions where a star-star transformer might be preferable? When the load is always balanced! One example is stepping down high voltage to drive a 3-phase motor. The motor’s current will be proportional to load, and the stead-state current in the three phases is always equal due to the nature of 3-phase electrical motors.