Merits and demerits of grid connected system

A grid-connected power system is one in which generating stations and loads of different regions are connected through a common transmission network (the grid) and operated together, e.g. the Integrated Nepal Power System (INPS) under NEA.

Merits

1. Reliability of supply: loss of one plant or line is covered by other sources through alternative paths.
2. Reduced reserve capacity: reserve is shared among all plants.
3. Economic operation: cheapest plants (run-of-river hydro) are loaded first; costly diesel/thermal used only at peak.
4. Diversity benefit: regional peaks occur at different times, so required installed capacity is lower.
5. Better frequency and voltage stability: large total inertia and high fault level make the system stiff.
6. Optimal use of hydro: surplus monsoon energy can be exported (e.g. to India) and dry-season deficits imported.
7. Allows larger, more efficient generating units and integration of distributed and renewable plants.

Demerits

1. Higher fault level, needing breakers of higher rupturing capacity.
2. Cascade tripping: a severe fault may spread and cause a nation-wide blackout.
3. Complex control and protection: load-frequency control, tie-line control, scheduling and SCADA are required.
4. Stability issues on long weak tie lines (inter-area oscillations).
5. High capital cost of transmission lines, substations and communication.
6. Small plants must follow strict grid code rules (frequency, voltage, protection settings).