Hello Elis,
The ordering matters for the correct specification of the *tap ratio*.
>From the manual (p. 21--22):
"The transformer, whose tap ratio has magnitude $\tau$ and phase shift
angle $\theta_{shift}$, is *located at the from end of the branch*, as
shown in Figure 3-1".
If, for whatever reason, you wanted to swap FROM and TO labels, then the
values of the tap ratio would have to change accordingly (it's not hard to
figure out the formulas for this transformation).
Note also that the manual defines N as the *complex* value of the tap
ratio, so when you say N>1 or N<1, I assume you're thinking about its
modulus, tau. This value can be greater or less than 1, either way. And
MATPOWER doesn't care which side is the high-voltage or low-voltage one, as
long as your tap ratio is correctly modeled.
Hope it helps,
--
Jose L. Marin
Grupo AIA
2018-05-09 14:17 GMT+02:00 Elis Nycander <elisnycan...@gmail.com>:
> Hi,
>
> In the matpower branch model (Fig 3.1) the impedance is on the TO-bus
> side. However, since the impedance is in pu, for an ideal transformer we
> might as well move the pu impedance to the FROM-bus right? So in this case
> we can exchange the TO-bus and FROM-bus for any branch without effecting
> the results of a load flow.
>
> Q1. So when does the the ordering of the FROM and TO bus actually matter?
> Is there any case when we cannot swich the TO and FROM bus of a branch and
> still get the same result from a load flow or opf?
>
> Q2. From Figure 3.1 it looks as if the FROM bus is the high voltage bus
> (assuming N > 1). Is this always the case or can we have N < 1 just as well?
>
> Regards,
> Elis
>
