The delta-delta, the delta-Y, and the Y-Y connections are the most generally used; they are illustrated in figure below. The Y-delta and delta-delta connections may be used as step-up transformers for moderate voltages.

                      Standard 3-phase/3-phase transformer systems.
The Y-delta has the advantage of providing a good grounding point on the Y-connected side which does not shift with unbalanced load and has the further advantage of being free from third-harmonic voltages and currents; the delta-delta has the advantage of permitting operation in V in case of damage to one of the units.

Delta connections are not the best for transmission at very high voltage; they may, however, be associated at some point with other connections that provide  means for properly grounding the high voltage system; but it is better, on the whole, to avoid mixed systems of connections. The delta-Y step-up and Y-delta step-down connections are without question the best for high voltage transmission systems.

They are economical in cost, and provide a stable neutral whereby the high-voltage system may be directly grounded or grounded through resistance of such value as to damp the system critically and prevent the possibility of oscillation.

The Y-Y connection (or Y-connected autotransformer) may be used to interconnect two delta systems and provide suitable neutrals for grounding both of them. A Y-connected autotransformer may be used to interconnect two Y systems which already have neutral grounds, for reasons of economy.

In either case, a delta-connected tertiary winding is frequently provided for one or more of the following purposes. In stabilization of the neutral, if a Y-connected transformer (or autotransformer) with a delta connected tertiary is connected to an ungrounded delta system (or poorly grounded Y system), stability of the system neutral is increased.

That is, a single-phase short-circuit to ground on the transmission line will cause less drop in voltage on the short-circuited phase and less rise in voltage on the other two phases. A 3-phase three-leg Y-connected transformer without delta tertiary furnishes very little stabilization of the neutral, and the delta tertiary is generally needed.

Other Y connections offer no stabilization of the neutral without a delta tertiary. With increased neutral stabilization, the fault current in the neutral on single-phase short circuit is increased, and this may be needed for improved relay protection of the system.

Third-harmonic components of exciting current find a relatively low impedance path in a delta tertiary on a Y-connected transformer, and less of the third-harmonic exciting current appears in the connected transmission lines, where it might cause interference with communication circuits. Failure to provide a path for third-harmonic current in Y-connected 3-phase shell-type transformers or banks of single-phase transformers will result in excessive third-harmonic voltage from line to neutral.

The bank of a 3-phase, three-legged core-type Y-connected transformer acts as a delta winding with high impedance to the other windings. As a consequence, there is very little third-harmonic line-to-neutral voltage and a separate delta tertiary is not needed to reduce it. An external load can be supplied from a delta tertiary. This may include synchronous or static capacitors to improve system operating conditions.

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