We well known that all the generating stations are interconnected by Transmission line and it is known as interconnected grid system.The grid system is made to share the load among all the power station generators.
In a typical grid system the frequency is maintained at 50HZ,it means the generating power and power requirement of the load is same.Hence in power stations the frequency is monitored continuously to find out the load requirement,if the frequency dips below 50HZ,means the generator shaft power increase and we have to give more input to generator to meetout the extra load put on the shaft.Hence the frequency is the measures of load,it is not constant,it is continuously changing depending on load, we are trying to bring it constant by maintaining generated power = Load requirement.
FOR THE GRID-: if the frequency of grid is increase by some another generator , at that time the power flows from that generator to all the other generators connected to the grid. means all the generators will act as a motor in their opposite direction which will be very dangerous. vice a versa for low frequency.
FOR THE SUPPLY-: we know that when we design any electrical equipment , we consider all the standard ratings on which we are use that equipment. so if we change the frequency , that affects losses(xmer ,power electronic ckt) , affects the speed of induction motor , also losses and torque of the induction motor.