My company does audio visual services. When we use a single 120/208 VAC Three-phase "Y" power source for lighting, audio and video we get noise caused by the lighting dimmers injected into the audio and video. We purchased a 30 KVA Isolation transformer. We tie-in power and run it first to the lighting dimmers then put the isolation transformer in line before powering up the audio and video equipment. It eliminates buzz in the audio and interference lines in the video. The transformer has three hot and one ground connections on the input side (no neutral on the input side) and on the output side a ground, neutral and three hot connections. Is it correct for me to wire the input ground to the transformer case and wire the output ground to the output neutral and not connect the output neutral and ground to the case or input ground?
Your situation is standard. Unless you have a specific permit allowing a floating Neutral on the secondary of the transformer, you MUST bond the Neutral connection on the transformer secondary to Earth ground. This is a simple bond to the case of the iso transformer and the incoming Earth conductor on the feeder provides the ground.
There are specific conditions where the Neutral is allowed to float. Primarily in recording studios and only with single-phase 120V secondary windings.
The reason that the isolation transformer is helping you with your audio and video power is that you are creating your own Neutral on the secondary of the iso transformer. The incoming Neutral is polluted with dimmer noise. Your isolation transformer allows you to create your own Neutral which doesn't have all that noise on it.