Fig 1: Interconnection and load sharing
For alternators in parallel, change in ﬁeld excitation will mainly change the operating power factor of the generator and has primarily no eﬀect on the active power delivered by the generators (change in power factor will change the total current of an alternator thereby changing copper loss.
The output active power will alter through a very small amount). The control of active power shared between alternators is aﬀected by changing the input power to the prime mover. For example, in a thermal power station: having alternators driven by steam turbines, an increase of throttle opening and thus allowing more steam into the turbine will increase the power input; in a hydro station, the power input is controlled by water inlet into the turbine. The prime-mover, speed-load characteristics thus determine the load sharing between the alternators.
Consider for simplicity, a two machine case, consisting of two non-salient pole synchronous machines (generators) 1 and 2 respectively coupled to prime-movers 1 and 2 Fig. 1 shows the speed-load characteristics of the prime- movers. Assume that initially the two generators share equal active power and it is now required to transfer a certain amount of power from unit 1 to unit 2, the total power remaining constant.
The initial operating points are indicated on the characteristic by points b and c, the busbar speed (or frequency) being given by the point a. The load on each machine is PL. the total load being 2PL. To reduce the load on unit 1, its input is decreased (by reducing the throttle opening) so that the prime-mover characteristic is now given by 1′. The total load being constant, the loads shared by the machines are
the total load being PL1 PL2 = 2PL, and the bus frequency given by the point d is reduced. To maintain the bus frequency constant at its original value (given by point a) the input to unit 2 must be suitably increased so that its speed-load characteristic is given by 2′. The ﬁnal load sharing is thus given by