Merits and demerits of interconnected system

An interconnected power system is a system in which several generating stations and areas are linked together by transmission lines (tie-lines) and run in synchronism, so that all generators jointly supply the total load. An isolated system is a single station feeding its own load.

Merits (advantages)

1. Reliability and continuity: a failed unit's load is taken up by other stations.
2. Lower reserve requirement: reserve capacity is pooled for the whole system.
3. Load diversity: different areas peak at different times, so less total capacity is needed; better load factor.
4. Economy: cheapest generation is used first (economic dispatch); costly peaking plants run less.
5. Large efficient units can be installed.
6. Power exchange: surplus energy can be sold and deficit bought (e.g. Nepal–India cross-border trade).
7. Better frequency stability due to large total inertia.
8. Flexible maintenance scheduling.

Demerits (limitations)

1. Higher fault level: more sources feed a fault, so circuit breakers of higher rupturing capacity are needed.
2. Cascading failures: a disturbance in one area can spread and cause a wide-area blackout if not isolated quickly.
3. Stability problems: long tie-lines and many machines make transient and dynamic stability harder to maintain; inter-area oscillations can occur.
4. Complex control and protection: load-frequency control, voltage control, tie-line scheduling and protection coordination become complex.
5. Need for coordination among utilities or countries on frequency, scheduling and tariffs.
6. High capital cost of tie-lines, substations and communication (SCADA) systems.