It seems that there is a lack of profound knowledge in electronics.
I still believe that there is a bug in the (ideal) transformer.
At the end of this message I cited the last statement about the ideal transformer.
This statement is wrong!
The ideal transformer is defined by the governing equations
turn ratio N:=N2/N1
u2 = N * u1
i1 = -N * i2
These simple equations can be looked up in
1) Leon O. Chua: Linear and Nonlinear Circuits. McGraw Hill, 1987.
2) Jiri Vlach: Computer Methods for Circuit Analysis and Design. VNR, 1994.
and in many other books.
Due to the definition above, the ideal transformer does NOT
act as a short circuit, not even for dc!
A REAL transformer acts as a short circuit for dc only!
For ac signals it acts not as a short circuit, as practice shows us.
Even on the cited webpage (http://qucs.sourceforge.net/tech/node48.html)
it is stated "And in matrix representation this is for DC and for AC simulation".
The current which flows into, e.g., port 1 is only defined by the resistance
connected to port 2 (and, for the real transformer, some current due to the
stray field of the transformer).
The ideal transformer is not only a perfect current transformer,
but ALSO a perfect voltage transformer!
The equations which can be found on the cited webpage
can mislead the reader. They try to make a link to the
scattering parameters. In my humble opinion, this was didactically
As a result, there is still a bug in the ideal transformer!!!
Having investigated this, I think there is not a bug after all, and instead there is a misunderstanding of what the ideal transformer component actually does.
The ideal transformer acts as a perfect current transformer, the current in the primary is mutiplied on the secondary. If you just put a voltage source across the terminals, it acts like a short circuit, so there is infinite current. At the same time in this case, there is no resistance, and therefore no voltage drop across the tranformer inputs, hence the output voltage is also zero.
See here for more information:
The ideal transformer component does not reflect a real physical transformer device, a real device must have winding resistance and inductance, and only works with AC voltages. An ideal transformer can transform DC current, which is physically impossible in the real world (maybe some exotic superconducting device I'm not aware of).
What changed in Qucs 0.0.15 -> 0.0.16 as far as I can tell was that the behaviour was actually corrected for DC simulation.