Frequency and Voltage are two important parameters which decide the healthiness of the electrical grid at a particular instant of time.
Frequency fluctuations occur in the electrical grid when there is a mismatch between the amount of power generation and amount of power demand. Both under frequency and over frequency operation is harmful to the operation of power system.
In an electrical grid, over frequency operation is a rare phenomenon. Most of the cases we observe frequency drop phenomenon due to lack of power generation supporting the demand. Hence frequency in the power system drops below the rated.
When the frequency tends to drop governor systems connected to the Turbo-Generators in the electrical grid will act and allow more steam to flow to the turbine and increase the speed. However, there is a limitation for this band. Beyond the certain limit of operation if grid frequency is drop Islanding operation is employed in the grid.
When the frequency of the overall electrical grid is declining below the rated at a predetermined frequency, the Electrical grid is divided into a number of small islands which contains some generation units and load centers come out of the grid and operate together until the normal frequency is restored. This method of splitting the electrical grid into small Islands is to protect the healthy part of the grid.