We study the evolution of the ring current energy density during May 2-4, 1998 storm event as measured by Polar CAMMICE/MICS instrument and modelled by proton tracing in the guiding center approximation. Particle data from Polar shows that during the storm main phase protons with medium energies (20-80 keV) contribute more to the total ring current energy than the high energy protons (80-200 keV) whereas during the recovery phase high energies dominate. We trace protons with arbitrary pitch angles numerically in the guiding center approximation taking into account charge-exchange losses. Tracing is performed in the large-scale and smaller-scale time-dependent magnetic and electric field models. We model the substorm activity by several electric field pulses at times of the substorm onsets. It is shown that impulsive electric fields associated with substorms are effective in the proton transport and energization to higher energies more than 100 keV in the storm time ring current.