3 phase

The "Y" configuration: each leg of the "Y" is one phase. The phases of the voltages are, nominally, 120 degrees in relation to each other. They are connected together at the common point which is the neutral. A proper ground conductor should always be included whenever power is conveyed over wires.

...unless it is high voltage transmission, in which case it is more economical to transmit the power with 3 wires instead of 4 .

On Mon, 30 Apr 2007 20:11:30 GMT Beachcomber wrote: | On Mon, 30 Apr 2007 09:25:29 -0700, SparkyGuy | wrote: | |>> I was told that a three phase Y connection has three hot legs, one |>> ground and a neutral. Is this true? |>> |>> thanks for your help, |>> Ryan |>

|>The "Y" configuration: each leg of the "Y" is one phase. The phases of the |>voltages are, nominally, 120 degrees in relation to each other. They are |>connected together at the common point which is the neutral. A proper ground |>conductor should always be included whenever power is conveyed over wires. |>

| | ...unless it is high voltage transmission, in which case it is more | economical to transmit the power with 3 wires instead of 4 .

Given that with the high diversity of the very large number of loads a transmission line would serve, the power would almost entirely be transmitted on 3 wires even if a 4th was present (e.g. if Y instead of D). Another question is whether the source transformer for such a transmission line is wired Y or wired D. At this scale, they sure would be expected to make this choice correctly.

High voltage lines are supplied from a Y source (eg transformer LV most often delta, HV Y). There may or may not be a neutral carried along but usually a multigrounded neutral is carried overhead, serving for lightning shielding. Delta supplied transmission systems pretty well disappeared about 75 years ago. Part of the problem is that with an ungrounded system, atmospheric conditions mayuse the line to ground voltage to float up to a point where a flashover to ground occurs. There can be enough capacitive follow through current to cause severe arcing and damage but such currents are hard to detect. Rather than use some grounding technique such as zig-zag transformers, it is simpler to go with a Y system. Ground fault detection is simpler. There are also economic advantages-particularly in HV transformers.

| High voltage lines are supplied from a Y source (eg transformer LV most | often delta, HV Y). | There may or may not be a neutral carried along but usually a multigrounded | neutral is carried overhead, serving for lightning shielding. | Delta supplied transmission systems pretty well disappeared about 75 years | ago. Part of the problem is that with an ungrounded system, atmospheric | conditions mayuse the line to ground voltage to float up to a point where a | flashover to ground occurs. There can be enough capacitive follow through | current to cause severe arcing and damage but such currents are hard to | detect. Rather than use some grounding technique such as zig-zag | transformers, it is simpler to go with a Y system. Ground fault detection is | simpler. There are also economic advantages-particularly in HV transformers.

So why not corner grounded?

Higher insulation to ground for the ungrounded legs and more expensive transformers. (going LV delta to HV Y has $ advantages). Higher ground currents in normal operation and the most common fault becomes line to line rather than line to ground.

Corner grounded secondaries are very popular where you have motor only loads like a sewer pump station. It allows the installer to use 2 pole equipment for 3 phase loads. The problem is, you don't get L/N voltage for 120v loads. As long as the station is designed with this in mind there is really no problem there. You just use all 240v equipment. There is also a confusion factor for the unqualified but unqualified people should not be there anyway.

| Higher insulation to ground for the ungrounded legs and more expensive | transformers. (going LV delta to HV Y has $ advantages). Higher ground | currents in normal operation and the most common fault becomes line to line | rather than line to ground.

OK, so what does the other end of the transmission line have? Also Y on the HV side and D on the LV side? Or is it delta loaded? I would think at least some transmission lines have to be designed for possible reverse usage, and I would think switching transformers to switch direction would be a pain and a lot of $$$.

On Tue, 08 May 2007 11:35:54 -0400 snipped-for-privacy@aol.com wrote: | On Tue, 08 May 2007 05:27:36 GMT, "Don Kelly" wrote: | |>Higher insulation to ground for the ungrounded legs and more expensive |>transformers. (going LV delta to HV Y has $ advantages). Higher ground |>currents in normal operation and the most common fault becomes line to line |>rather than line to ground. |>

|>-- |>

There are corner grounded 120V systems. They just happen to typically be (or once were) on railroad cars. But if you needed 120V and needed to do three phase in 2-pole circuits, and didn't mind to heavy up the neutral for the single phase loads, why not do a CG120 system?