I have my switchover set at 15 degrees but when it gets colder than 30 a lot of time my heat pump will run a few minutes. Then it will kick off and my auxiliary heat takes over. I would like my heat pump to run longer

Heat pumps are designed to run within there efficiency curve. Running a heat pump below that just wears out the compressor sooner since no heat benefit is derived.

Your adjustments should be within the efficiency curve for best and most economical performance.