That would be a Transzorb. Two types bi-directional and uni-directional... as per the 1N5555 its a 30Vdc working and 33Vdc minBkDn and 47.5Vdc max Vclamp device. So you will want most likely a uni-directional unit.... Diodes, Inc., General Instrument, and several others (like Panasonic) now make Transzorbs. The 15KP30A is rated for 15KW peak (10 x more power, is that enough???), 30Vdc working, 33.3V minBkDn and 50.7 max Vclamp.
IF you are working on Airborne equip specs then the 704-15K36 and the 704-15K36T are designed to protect the 28Vdc bus to mil spec's Mil-Std-704... use the non-T unit for short spikes and the T unit for "long surges or spikes" in excess of 1 milli-sec. Specs are 31.5Vdc working, 36V minBkDwn, and 51V max Vclamp...
Hope this helps... later