[en] This paper reports on the CERN SPS superconducting (s.c.) cavity (LEP s.c. cavity prototype), installed to investigate behavior in a real accelerator environment and to obtain some additional voltage for e⁺e^- acceleration when the SPS is working as LEP injector. In the SPS supercycle (14.4 s long) low intensity e⁺e^- beams (0.5 MA DC) and high intensity proton beams are accelerated alternately. During the lepton cycles, the s.c. cavity produces the highest possible voltage for e⁺e^- acceleration (8.5 MV total for the nominal field of 5 MV/m at 352 MHz). On the other hand, during the high intensity proton cycle the cavity impedance must be reduced drastically to avoid beam instabilities. This can be achieved by active beam compensation where the amplifier driven by a feedback system feeds a current into the cavity which just cancels the current of the beam. In this way the cavity voltage can be made negligibly small even in the presence of the strong proton beam current (0.2A DC)