What is the difference between wye and delta connections and why use both in a three phase electrical system?

Answer 1

WYE connections reference the line values of current and voltage to a "hard-wired" neutral. This means
(line voltage) = 1.732 * (phase voltage)
(line current) = (phase current)
This is a much easier system to use when manufacturing an electric machine ... there will be a "neutral" - complete with leads - that can be used when troubleshooting.

DELTA connections reference the line values of current and voltage to a "theoretical" neutral. This means
(phase voltage) = (line voltage)
(phase current) = (line current) / 1.732
Because of the "theoretical" nature of the neutral connection, the winding can experience far harsher effects from harmonics than the standard WYE version. Thus it isn't used as often in large machines ... although it's fairly common in smaller (<100 HP) designs.

Any 3-phase machine can be used in either format; in fact, most multi-speed windings employ one of each.

The simplest advantage may come when looking at transformers, though. With a WYE-DELTA transformer, there is no possibility of ground faults between the input and output sides (unless, of course, a major accident happens!). IT also has an immediate 1.732 ratio in terms of voltage/current ... even if the turn count is the same on both sides.

Answer 2

Wye Delta Connection

Answer 3

Check out this site

http://www.elec-toolbox.com/usefulinfo/xfmr-3ph.htm