The reason why the maximum supply power is different between summer and winter is that a periodic test for the power stations is carried out in winter with less power demand in {HEAD ~ preparation} for summer with {TAIL ~ more power demand} 
with
The reason why the maximum supply power is different between summer and winter is that a periodic test for the power stations is carried out in winter with less power demand in { HEAD ~ preparation } for summer with { TAIL ~ more power demand }
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD OTH OTH REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL