Three-Phase Transformers Phasor Clock Diagrams

I had apparently posted a question in the wrong group. My question is posted in http://groups.google.ca/group/sci.energy/browse_thread/thread/f9367327fcebdb57 #
Any help of commenting my confusion would be greatly appreciated. Thanks.
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What in the world is a Three-phase transformer phasor clock diagram?"
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wrote:

It refers to a "standard" phase shift for star delta transformers. As I recall, the standard is that primary A phase leads secondary A phase by 30 degrees. The way that the A phase as indicated on a drawing is not really that important- as long as it is marked -but there is likely a standard.
It is related to field wiring applications. If everyone does it this way, there is less likelihood of misconnection, particularly for some protective relaying.
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Don Kelly snipped-for-privacy@shawcross.ca
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wrote:

It sounds like one of those coo-book things that are needed only if you do not understand what is going on, I prefer working with fundamental principles rather than recipes.
Bill
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Quite right. Things like differential relay protection across transformer bank must have both sides connected for the same phase-shift. Gets really tricky when the power transformer is delta-wye and has a phase shift of its own :-). Mis-connections don't always show up until you start to significantly load the transformer, then you get protective trip and spend some downtime tracing it all out (btdt).
daestrom
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wrote:

Thanks for your clarifications. I think I figured out enough of what's going on for an exam.
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I am in a situation where (aside from being retired) I have little use for my PE license. Most of my need for a license has been to use the title of "Engineer." Rather than learning standards that I am never going to use, I would rather do the extra work required by starting from fundamentals.
I have a situation now, for example, where I am checking out someone's fault current calculations as a favor without charging. I do not have much experience with residential wiring, but I am willing to attest to accuracy of the calculations if I am provided with a schematic specifying all the conductors and their connections to equipment and each other.
Bill
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wrote:

I thought that residential fault analysis would assume zero ohms in the branch wiring. You can't really predict where the fault will occur so you assume it's immediately downstream of the circuit breaker. Only the supply transformer impedance and the service drop wiring go into the fault current. That's one reason why the utility doesn't put in really large transformers in developments. Too large a transformer and the available fault current is higher than 10kA and typical service panel equipment needs to be upgraded.
At least, that was my understanding....
daestrom
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I would have thought that as well. But the jurisdiction's form indicates the fault current is to be at the "Load.Terminals".
Bill
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wrote:

I suspect this would mean 'at the load terminals of the overcurrent protective device' (as in load side terminals and line side terminals), not 'at the terminals of the load'. I encourage you to use that interpretation if possible.
It would not make sense to include the branch wiring. This would quite simply be an incorrect way to carry out fault current calculations, in my opinion. It would be (theoretically) unsafe because it calculates fault currents lower than what the OCPD may be called on to interrupt. This is a life safety as well as a property damage issue. I suggest that it would in general be inappropriate for a PE to carry out a fault current calculation this way. One might get away with it because a fault that causes a catastrophic failure of an OCPD and thereby some collateral damage is probably not common in a typical residential application.
In your other post you mention watching out for the gotchas. Any running motors in a premises will contribute to fault current when a fault occurs. This contribution is often added to the utility contribution at the main service for simplicity.
I am a little surprised that a person would use the nameplate impedance of the utility transformer. What if the utility replaces it in the future with a lower impedance unit? Here we are required to assume certain lowish impedances for utility transformers. Plus infinite primary of course.
Ignoring phase angles of impedances is also not the greatest idea. It will tend to underestimate required interrupt ratings. Hopefully not by much in your application. It may end up being compensated for by the infinite primary assumption.
j
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Because of possible misinterpretation of mere words,I am asking for a schematic diagram of all the conductors and circuit breakers among other things. I would want to have branch breakers trip without tripping main breakers.
The jurisdiction wants to know what the transformer impedance is. Getting from nameplate information seems good enough for them. Who am I to argue. I expect that the conductor impedance will be high enough to limit current flow to a value breakers can handle. Transformers have leakage reactance!
Bill
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wrote:

--------------- Typically 5% (0.05 per unit)for distribution transformers under 150-200KVA -basically leakage reactance as you say. Max fault KVA for bolted fault will be its KVA rating divided by the per unit rating so that if the transformer is 100KVA the expected maximum bolted fault KVA will be something like 2000KVA for single phase. -
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That all seems reasonable enough, but you never know. I look upon this as n information gathering job to be performed by someone else. As a PE, all I have to do is take that information and certify that calculations are correct.
Bill
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wrote:

------------- Fair enough but if the information gatherer gives you something that is out of line for the size and type of transformer, it is nice to have an idea of what is reasonable. As a PE, I should think that questioning of out of normal data is a given.
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Everything seems to be reasonable. The size description of #2 instead of #2/0 is most likely a slip of the pen. The biggest unknown at this time is the short circuit properties of the supply transformer. I explained why that was important. So far, I have heard no more. That is OK with me.
Most of my electrical engineering has been in situations for which the Industrial Exemption is usually in force. I have nether pushed my PE except for some puffing when I did a bit of consulting.
Bill
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wrote:

------------------ Then it is safe to use to treat the "load terminals" as the house service panel, rather than at various points inside the house. With typical distribution transformers having about 5% impedance, the maximum fault current will be 20 times transformer KVA rating divided by rated voltage and this would ber good enough in practice. Since a fault on any circuit can occur just past the individual circuit breaker, this will be a good indication of worst case conditions.
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Don Kelly snipped-for-privacy@shawcross.ca
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By specification, I meant lengths and size of the conductors. The jurisdiction involved specifies the impedance of the wires (for a given size) including the effects of magnetic conduit. That makes my job one of checking multiplication and the like. In this case, they do not distinguish between resistive and inductive contribution to the impedance. The big problem is for the client to find out what transformer impedance is from the nameplate or the power company. The only real thing I have to do is watch out for the gotchas. For example, the client indicated #2 wire when he meant #2/0. Nevertheless, I would my comfort limit for anything much more complicated.
Bill
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wrote:

----------------------- Sorry- it looks like you are into something more than residential wiring when you meention 2/0. Just what are you looking at that requires this for internal wiring (and in raceways)? You may be getting into code /cookbook issues. I agree that the transformer impedance is the major factor and uncertainties in this may be greater than uncertainties in the conductor impedances..
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Don Kelly snipped-for-privacy@shawcross.ca
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