The distinction between inter-area and local modes of oscillation in power systems is based on frequency, but the exact frequency range can vary slightly depending on the specific power system. Here's a breakdown:

Inter-area modes (0.1 - 0.8 Hz typical): These oscillations involve large groups of generators in different areas of the power system swinging against each other.

Imagine two regions connected by a weak transmission line. A disturbance in one region can cause generators there to slow down, while generators in the other region speed up, creating an oscillating exchange of power.

The lower frequency reflects the inertia of the large machines involved and the weaker coupling between the areas.

Local modes (0.7 - 2 Hz typical): These oscillations involve the interaction between machines within a power plant or a small group of plants.

They might be caused by sudden changes in load within a plant or issues with control systems.

The higher frequency is due to the smaller masses of the machines involved and the stronger electrical connections within a plant.

So, while the 1 Hz boundary isn't a strict cutoff, it provides a general guideline for understanding the nature of the oscillations. Inter-area modes are slower due to the larger areas and weaker connections involved, while local modes are faster due to the smaller, more tightly coupled components.

Reasons for Low Frequency Inter-area modes (0.1 - 0.8 Hz typical):

• So, Inter-Area Modes Involve interactions over wide geographical regions with large inertia, leading to slower oscillations. while Local Modes Involve interactions within smaller, localized areas with less inertia, resulting in faster oscillations.