Is it possible to set exposure time greater than 30 seconds while using interval shooting? I know we could set the interval, number of intervals and number of shoots I each interval but i can't find anything to set the length of exposure. Please help!
No, the shutter will not go longer than 30 seconds unless you use Bulb shutter mode with an external timer that allows longer than 30 seconds. Then in Bulb mode, the timer can control it. There are several external such timers available for $20 to $25, search Amazon for Interval Timer D750.
If using the camera interval timer, you just set whatever shutter speed the camera can do, up to 30 seconds. Be aware that 30 seconds is actually 32 seconds, so the interval will need to be 33 seconds.
To avoid that causing a stir from readers, I should add that camera shutters MUST have speeds in the sequence of 1, 2, 4, 8, 16, 32 seconds, each exactly 2x longer, so that each stop is exactly 1 EV. Same for fractional speeds, the actual times must be 1, 1/2, 1/4, 1/8, 1/16, 1/32, 1/64, 1/128, 1/256, 1/512, 1/1024 seconds, etc. This is simply how digital works, each shutter speed is exactly 2x. Has to be that way. The marked numbers are simply nominal values, for the convenience of humans.