Interconnected system and transmission planning

An interconnected system is one in which several generating stations and regional networks are tied together by transmission lines so that they work as one grid (for example the Integrated Nepal Power System, INPS, and its links with India).

Advantages

1. Higher reliability: if one plant trips, others supply the load through the ties.
2. Lower reserve capacity: spinning and standby reserve is shared, so total installed reserve is reduced.
3. Economic dispatch: cheapest plants (e.g. run-of-river hydro in the wet season) can be used fully; costly plants run less.
4. Use of load diversity: peaks of different regions occur at different times, so the combined peak is lower than the sum of peaks.
5. Larger, more efficient units can be installed because a single unit is a small part of the total.
6. Energy exchange/trade: surplus energy can be exported (e.g. Nepal to India in the monsoon) and deficits imported in the dry season.
7. Better frequency and voltage regulation because the grid is "stiffer".

Disadvantages

1. Higher fault level (SCMVA), so breakers of higher rating are needed.
2. Cascading failure: a disturbance in one area can spread and cause a wide-area blackout.
3. Complex operation: load-frequency control, tie-line scheduling and protection coordination are harder.
4. Stability problems: inter-area oscillations and transient stability limits on long ties.
5. High capital cost of tie lines and communication/SCADA systems.

Need of transmission system planning

Transmission planning decides what lines and substations to build, where, at what voltage and when, so that future generation can reach future load reliably and at least cost. It is needed because:

• Load growth: demand grows every year; the network must be expanded before it becomes overloaded.
• New generation: hydro sites in Nepal are far from load centres, so evacuation lines must be planned with the projects.
• Long lead time: lines take 5–10 years (survey, right-of-way, financing, construction).
• Economy: correct choice of voltage level, number of circuits and route minimises total cost (capital + losses).
• Reliability criteria: the grid should withstand single contingency (N-1) outages.
• Technical limits: thermal, voltage-drop and stability limits must be met for all future years.
• Coordination: with distribution planning, cross-border trade and environmental/right-of-way constraints.