Question:When voltage increases then current also increases then what is the need of over voltage relay and over current relay? Can we measure over voltage and over current by measuring current only? 

Answer:No.We can't sense the over voltage by just measuring the current only because the current increases not only for over voltages but also for under voltage(As most of the loads are non-linear in nature). So,the over voltage protection & over current protection are completely different. Over voltage relay meant for sensing over voltages & protect the system from insulation break down and firing. Over current relay meant for sensing any internal short circuit, over load condition ,earth fault thereby reducing the system failure & risk of fire. So, for a better protection of the system.It should have both over voltage & over current relay.