Well, there are already plenty of replies, but Internet bits are cheap. I'll try to make this sound educated.
If there is an inlet supplying pressure at 30psi, the pressure rise due to expansion of heated water will just push a tiny amount of water back up the pipe, and the pressure will stay at 30 psi. There seem to be just two things that might cause pressure to exceed 150psi (the setpoint of your relief valve) -- boiling and water hammer.
Water hammer in a home system happens when a valve or faucet is closed suddenly, causing a shock wave to travel backward through the pipe until it hits something, like an elbow. Just possibly, if your tank is actually filled solid with no trapped air, the pressure wave might exceed 150psi at the relief valve, causing it to open very briefly. The cure for this problem is an air chamber installed, usually, near the valve. The air will allow the shock wave to dissipate into the trapped air, eliminating it.
Far more likely is the possibility that the water is overheating, and occasionally exceeding the boiling temperature of water (at 30psi, that is about 250F). I don't know just how the temperature is sensed and controlled, but that is where I would look for the problem. That or just turn the thermostat down to 120F. That should be hot enough for normal purposes, dishwashing included. Oh by the way, boiling in the water heater could be very disastrous if the relief valve should stick shut, of if the boiling is rapid enough to overwhelm the relief. There is an impressive video somewhere on the Internet of a water heater launching itself a good distance into the air, powered by steam.
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