I just tried to Transwarp from the Sol system to Eta Eridani using my Advanced Heavy Cruiser Retrofit and once the countdown hit 1 the nacelles started sparking with energy discharges and the ship made a sound similar to the original Excelsior in Star Trek 3 and my transwarp failed. I still got a cooldown but no transwarp jump.
I believe it's a 1% chance of failure. I've been flying the Excelsior for months, and I've had one failed transwarp jump the entire time. It's funny when it happens though, apart from having to now fly all the way across the galaxy. XD
This character is why I don't play my Romulan any more. Tovan Khev is NOT my BFF! Get him off my bridge!