Everything said here is correct. The voltage source has precisely zero
output resistance, so it will sustain the driven voltage at whatever
current, no matter how hard the TVS tries to shunt it.
However, the other thing I find noteworthy is that your TVS is conducting
hundreds of amps even with the normal operating voltage (12.8V) when there
is no spike. I do not think you have set up its model parameters
correctly. Despite the fact that you have a TVS schematic symbol, the
parameters you gave it are for an ordinary silicon diode.
LTspice comes with two TVS diodes in its standard diode library. They use
different parameters than the ones you've specified.
Regards,
Andy
