The electrical properties of the dominating intrinsic-related defect centers in 6H-, 4H- and 3C-SiC (E-1/E-2, Z(1)/Z(2) and Z(1)/Z(2)(3C)) are investigated under illumination with photons of different energy by deep level transient spectroscopy. The E-1/E-2- and Z(1)/Z(2)-center are identified as negative-U-centers. The energy levels of the Z(1)/Z(2)-center in 4H-SiC are discussed in the framework of a configuration coordinate diagram. The optical ionization energies of the (Z(1)/Z(2))(-/0)-level in 4H-SiC and the (E-1/E-2)(-/0)-level in 6H-SiC are determined.