Center-Tap DELTA Phase Shift 1

[tulum]

Can someone explain how to calculate the neutral angle in a center-tap grounded delta secondary?

I have been following an example in a book, and even though the a-b winding is center grounded (n'), they still use the phase shift from the original delta configuration (n - which is not center tapped)...

Would the addition of n' not change the phase shift??

Thanks in advance
Tulum

 

[DanDel]

I'm not sure what you mean by the 'neutral angle', since the neutral for this system is usually the grounded center tap.
The vectors for a center-grounded Delta system look like a 'T'. The zero point in the center of the circle represents the grounded center tap, two vectors at 180° to each other radiate from this point to represent the 'A' and 'B' phase voltages, and one vector from the center at a 90° angle to the other two vectors represent the 'C' phase voltage. If the system were a 240V Delta, for instance, the 'A' and 'B' phase vectors would be 120V, and the 'C' phase vector would be 208V.
Is this any help?

 

[tulum]

Dandel,

That is perfect.

I am using some notes that make no difference in reference to Van, Vbn, and Vcn ANGLES when calculating phase shift in an ordinary delta and a center-tap delta (They do change the Magnitude of the voltage however).

So what I gather from your comments is:

If my orignal secondary was a delta with positve sequence and Vab at 0 degrees, than:

Van = Vab<30,
Vbn = Vbc<30 +240,
Vcn = Vca<30 +120

If I now ground the center-tap on a-b (same xfmr):

Van = Vab/2<0,
Vbn = Vab/2<180,
Vcn = sqrt(Vac^2-Van^2)<90

This makes more sense...

Now, because the secondary has a different phase for the phase voltages...this does not get transformed to the primary right? I mean the line voltages are still the same (in magnitude and phase) and the the xfmr turns ratio is still a real number, so the line voltages on the primary will be the same, and the phase voltages will be offset (lead, I believe?) by 30 degrees.

Thanks for your help... Let me know if I am on the right track.

Also if you know where I can get some good info on tranformer connections and phase shift CALCULATIONS please let me know. My background is controls, so I am pretty "boggled" when trying to determine the effects of one connection to the next.



Regards,
Tulum

 

[busbar]

Be aware that some call out 240/120 4-wire-delta service with Bø [center bus] having the higher voltage to ground—consistent with recent editions of the National Electrical Code.

 

[DanDel]

tulum, I'm not sure where you get a neutral potential on your original connection without the grounded tap. Where is the neutral point?
The shift between the primary and secondary winding is soley based on the difference in primary and secondary connections. A delta-delta connected transformer will have no appreciable shift. What is the primary connection?

 

[tulum]

I am a little confused on this issue (as you can see).

This is a fictitious example. These types of question come up at work quite often and I don't like feeling like a tourist in a foreign country that can't speak the language!

The connection was delta-delta, and yes there is no real neutral point. However, in the manual it says that the line voltage equals the phase voltage. Furthermore, the symbol that is shown is a delta with a dotted line for the neutral (a-n) at 30 degrees between a-b and a-c to the center of the delta.

This makes me assume that Van may be calculated eventhough not really connected... to be used for phase reference?

Maybe you can explain the following:

1.

What happens if I bring a 3-wire service delta-delta from outside a building into a bulding to serve a Y UNGROUNDED load? What is the Phase to neutral voltage of the load... is it the line voltage of the delta secondary/sqrt(3)?
Poeple at work throw around the artificial neutral concept, which kinda makes sense.


2.

What happens if I bring a 3-wire service delta-delta from outside a building into a bulding to serve a Y GROUNDED load? What is the Phase to neutral voltage of the load... is it the line voltage of the delta secondary/sqrt(3)?

 

[rbulsara]

tulum:

Answer to your last post are yes. 1. yes and 2. yes.

 

[busbar]

One way of looking at it is, unlike a 4-wire wye system, in 4-wire delta service the neutral [at the electrical center of the delta] and the ‘center-tapped’ grounded-circuit conductor are not one in the same, but are at different points. A phase difference exists between Aø-N and Aø-G.

 

[stevenal]

Qualification to Rbulsara's post: If load is balanced. Unbalanced loading will cause the load neutral point to shift from the center of the delta.

Not much point in connection 2. Grounding the load nuetral does nothing to stabilize it, does nothing to assist overcurrent operation and probably violates most codes.

 

[DanDel]

tulum, if the load is Y, then that means you have four connections to make, otherwise it wouldn't matter if the load were Y or delta. If you connect it to a delta 3-wire source, you will leave the neutral point floating, and the phase-to-neutral voltages(measured at the load, since that is the only place where a neutral connection exists) will be based on the load imbalances, as already mentioned by stevenal. Grounding the neutral at the load does nothing but add some capacitive coupling into the mix, and that would only be involved at low current levels.
Yes, the line voltage equals the phase voltage in a delta system, because each line connection is directly across the (phase) winding. In a Wye system, the line voltage equals the phase voltage x 1.73, because the line is connected across two phases which are 120° apart. But these measurements are at the source; not at an artificially created neutral in the load.