corner grounded Delta system

[jcraft]

We have an old system in our plant that is the grounded B phase or the corner grounded delta. Whichever you want to call it. What we have is 4160V feeding 4 step down transformers that step it down to 230V Grounded B. When you open the secondary side of the 500kva transformers the center phase has a 500mcm bare copper jumper from it to the can of the transformer. My quesion is how does this phase just not short out as in any other system if you took it to ground. any thoughts, Thanks

 

[buzzp]

This is not the best answer I dont think. But three phase power does not return on a neutral leg. It returns on a hot leg. If you put a clamp on meter on the neutral on a balanced system, you should read zero amps. In a delta (corner grounded delta), the middle phase is grounded. Since no current flows to ground or the neutral, then attaching this wire has no affect other than still providing a return path to the transformer. It will also force a zero potential at this point which has no affect since a delta system has no ground reference (essentially floating at some arbitrary level but phase to phase voltages are known). If this were single phase then yes you would see some sparks.
I am sure someone will add to this or correct my thinking if I am wrong. And it is very possible that I am way off.

 

[ccjersey]

In the common single phase 120/240 centertapped house service transformer, the ends of two single phase 120V transformer secondary windings are grounded just like the "B" phase of the delta. The two 120's in series total 240 line to line. The center is grounded (with no sparks)so there is a ground reference and each line has 120V potential to that "neutral" point.

With some (mostly older)generators, the connections of the two single phase windings can be parallel 120V, series 240V or center tapped series (120/240). With the parallel 120V circuits, one end is selected and becomes the "neutral" and referenced to ground while the other is the "line" at 120V above neutral. The same could be done with the 240 series if desired to have 240 line-ground instead of the ususal 240 line-line, but if the center of the series windings is already grounded (120/240 connection), you cannot ground another point without sparks!

 

[cuky2000]

Q: How does this phase (corner grounded delta) just not short out …..?

Because there is no return path back to the source since the circuit is incomplete. Therefore, no short circuit current will flow.

Please notice that the corner grounded phase is a ground potential. Therefore, any other phase to ground fault will cause a phase-to-ground-to-phase fault.

 

[edison123]

cuky,

what is the advantage with corner grounded delta ?

* If you have never succeeded in anything in first attempt, skydiving isn't for you. *

 

[davidbeach]

what is the advantage with corner grounded delta ?

If you have a delta feeding a three wire system, and you want to ground it, a corner is the only place available.

 

[ccjersey]

grounding the delta limits overvoltage which can develop during arcing faults. ungrounded delta is used when continuous operation, even with ground faults on one phase, is important. The ground fault on the second damaged phase is the one which produces the sparks.

 

[HighPotter]

It is common for "station power" in power plants to be ungrounded. So if one phase becomes grounded, things keep cruising along.

If you use a grounded delta system, one phase to ground path and your sunk.

HP

 

[alehman]

The advantange that of a corner grounded system is cost. It provides the benefits of a grounded system without the need for a separate neutral conductor. Also, switches and overcurrent protection are needed only for two legs instead of three as in center grounded systems.

At the time these systems were used routinely, ungrounded delta systems were very common. Grounded systems have many advantages (discussed here previously). The cheapest way to obtain a grounded system for an ungrounded delta was to ground one phase.

 

[edison123]

Thx alehman.

* Homer Wisdom - "Operator, Give me the number for 911" *

 

[cuky2000]

What is the advantage with corner grounded delta?

a) From the economics point of view, the corner grounded delta save in wiring, hardware and installation cost comparing with respect 4 wires grounded systems.

b) During normal operation conditions, grounding a corner stabilize all the voltages to earth providing better voltage regulation to the system.

c) During transient conditions created by lightning, line surges, or unintentional contact with higher voltage lines, the overvoltage is limited to lower values. This translates in better chance for longer life of electrical equipment insulation.

Beware that better solutions are available for new installation