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SLD of interconnected system vs isolated system

Short answer

In an interconnected system, several generating stations and load centers are linked by high-voltage tie-lines forming a grid that enables power exchange. An isolated system consists of one or a few plants supplying their own local loads without grid connection, offering lower reliability and requiring larger reserve.

Draw a typical single line diagram of interconnected power system. How is isolated power system different from interconnected power system?

Single line diagram of an interconnected power system

Several generating stations and load centres are tied together through a high-voltage transmission grid so that any plant can supply any load.

 [G1]11kV-[11/132]--+===132 kV===+--[11/132]-[G2]
                    |            |
               Grid S/S A   Grid S/S B
                    |            |
                 [132/33]     [132/33]---->Tie to
                    |            |       neighbour grid
                 33 kV        33 kV
                    |            |
                 [33/11]      [33/11]
                    |            |
                 11 kV feeders  11 kV feeders
                    |            |
                 [11/0.4]     [11/0.4]
                    |            |
                 400/230 V    400/230 V loads

Typical levels in Nepal: generation 11 kV; transmission 400/220/132 kV; sub-transmission 66/33 kV; distribution 11 kV and 400/230 V.

Isolated vs interconnected

An isolated system is a single plant (or small group) supplying only its own local load with no link to other systems, e.g. a micro-hydro feeding a village. An interconnected system shares generation and load across a grid.

BasisIsolated systemInterconnected system
StructureOne or few plants feed their own loadMany plants and areas linked by tie-lines (grid)
ReliabilityPlant outage interrupts supplyOther plants supply the load
Reserve neededLarge reserve in each plantShared reserve, less total
Frequency/voltageVaries widely with loadStable, grid is stiff
EconomyCannot use cheapest plant alwaysEconomic dispatch of plants
Fault levelLowHigh (needs larger breakers)
ExampleMicro-hydro village gridNepal INPS linked with India

Thus an interconnected system gives higher reliability, lower reserve and cheaper operation, while an isolated system is simpler but less reliable and less economical.

✓ Answer checked by Claude Opus 5.5 (high effort)

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