Three-Phase Transformers Phasor Clock Diagrams

What in the world is a Three-phase transformer phasor clock diagram?"

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.

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

-------------- Sure, one can connect in such a way that the secondary a,b,c phase voltages lag or lead primary A,B,C by a number of different angles. However, life is a lot easier if a single standard is used, within a utility and between utilities and manufacturers. It can eliminate a lot of expensive mistakes. It is not a lack of knowledge of what is going on but rather a knowledge of what possible phasing problems (in differential protection for example or even fault analysis) can occur that leads to setting a standard. --

Don Kelly snipped-for-privacy@shawcross.ca remove the X to answer

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

Thanks for your clarifications. I think I figured out enough of what's going on for an exam.

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

I would rather do the same but effectively the manufacturers have come up with a standard so that you know what you are getting before you get it and can plan accordingly. In an academic situation- not of importance. I can calculate fault currents if I have information-. certainly knowing that the wiring is #12AWG Copper does allow use of standard information such as resistance per foot, which is needed for your household fault analysis- you don't go out and measure the resistance of the conductor but you do look up this information in a table. This transformer information is the same sort of thing-of importance to differential protection and in application of symmetrical components in fault analysis (unbalanced fault on the secondary of a 3 phase transformer- what is the primary current in each leg?).

Don Kelly snipped-for-privacy@shawcross.ca remove the X to answer

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

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

I would have thought that as well. But the jurisdiction's form indicates the fault current is to be at the "Load.Terminals".

Bill

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

------------------ 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.

----------------------- 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..

2/0 is probably in the main service feeder.

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

My guess is that this is for a multiunit apartment house.

All I plan on doing is to certify that the calculations going on the required form are correct.

--------------- 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. -

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