Is there any way to increase the exposure time longer than 30" in the Nikon D5100? I know there is
BULB, but is not working with self-timer.
You can use a wired remote release that has a built in intervalometer, such as this fairly inexpensive one or this one. Regardless of the brand name stamped on them, they all seem to be made identically.
You then set the camera to Bulb and let the timer in the remote open and close the shutter.