Need for interconnection into a grid

A power grid is formed by interconnecting generating stations and load centres by high-voltage transmission lines so that all plants operate in parallel and supply the combined load. This interconnection is needed for the following reasons.

1. Reliability and continuity: If one generator or plant trips, the others in the grid supply its load; consumers do not lose supply.
2. Reduced reserve capacity: Each isolated station needs its own standby units. In a grid, the spinning reserve is shared, so total installed capacity required is lower.
3. Economic operation: The most efficient and cheapest plants (e.g. run-of-river hydro) are run at full load and costly plants only at peak, i.e. economic load dispatch.
4. Load diversity: Peak demands of different regions occur at different times. Interconnection lowers the combined peak, so less capacity is needed (higher diversity factor).
5. Using remote resources: Hydro sites in mountains can supply cities far away; seasonal surplus in one area can supply another.
6. Seasonal and cross-border exchange: Nepal exports surplus hydro power in the wet season and imports in the dry season through 132/220/400 kV links with India.
7. Larger, efficient units: Grid can absorb large generating units, which have lower cost per kW.
8. Stable voltage and frequency: A large system is "stiff"; load changes cause negligible voltage and frequency variation.
9. Better use of transmission: Power can flow over parallel paths, reducing overload.

Why high voltage lines

Interconnecting lines carry large power over long distances. Since loss $\propto P^2R/V^2$ and capacity $\propto V^2$, only high voltages (132–400 kV) make such transfer efficient and stable.

 Plant A ====132 kV==== Plant B
    \                   /
     \== Load centre ==/
          ||
      Tie to neighbour grid

Drawbacks such as higher fault levels and spread of disturbances are handled by good protection and control, so interconnection is fully justified.