If the frequency of Generator 1 were to be maintained constant regardless of its load, if Generator 1 were made infinitely stiff, its frequency droop under load is eliminated, and the bus frequency remains constant
(flat) regardless of the total load.
Ship constant frequency generator
The constant frequency generator is known as the isosynchronous generator. Most governors have a set point adjustment that allows for varying the no-load speed of the prime mover. By adjusting this set point, one can adjust the frequency or the load shared by one generator with the other operating in parallel.
In the isosynchronous generator, this adjustment is done by an automatic
control system with precise compensation feedback. We note two points here when operating two generators in parallel:
1. Only one generator can be isosynchronous. Two isosynchronous generators cannot work in parallel since they would have no intersection that makes the common point of stable operation. In parallel, they would conflict with each other, could overload, and self-destroy in a continuous search of an intersection.
2. Beyond the initial load sharing, all additional load is taken by the isosynchronous generator, whereas the drooping generator load remains constant. This is analogous to two mechanical springs sharing a load, with one soft spring drooping with load and the other spring infinitely stiff like a solid metal block (we may call it isoshape spring). Regardless of the initial load sharing, any additional load will be taken by the solid metal block (isoshape spring) without additional drooping of either spring.
Isosynchronous (constant frequency) generator on ship
For this reason, the load shared by the isosynchronous generator is much greater than that by the drooping generator. Therefore, the prime mover governor for an isosynchronous generator is specifically designed for isochronous operation at any load from zero to 100% load.
Such governor assures that the prime mover shares the load proportional to the generator kW rating by using direct measurement of kW.
It provides rapid response to load changes, stable system operation, ability for paralleling dissimilar-sized engine-generator sets, and fine speed regulation under 0.25%.
Droop is inherent in all prime mover speed controls, but in an isochronous generator, it is recovered in a short time.
It is a temporary droop of transient nature more commonly called the compensation.
This gives bus frequency 60 Hz from no load to full load.