I am trying to study an ever-on IGBT. in other words, IGBT is tried to be on in a long time. To this purpose, a +15V constant dc voltage is applied to the gate-emitter. But, when connecting a +2V voltage to the collector-emitter in this condition, voltmeter show Vce equal to 0.05V! It should be equal to 2V, shouldn't it? What is the problem?
You didn't actually connect 2V to the collector-emitter, obviously. You connected a power supply that had 2V out until it was connected, then it dropped.
So the IGBT is pulling down whatever power supply you have. Which is to be expected, you're shorting it out with the IGBT. Either that or the IGBT might burn out, 2V is a fairly high voltage for an IGBT Vce(on).