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Interconnected system SLD, isolated vs interconnected, advantages

Short answer

An interconnected system ties multiple plants and loads through a common transmission grid, whereas an isolated system has a single plant serving only its own local load without grid connection. Advantages include higher reliability, shared reserve, economic dispatch, better voltage and frequency stability, and use of diverse energy sources.

Draw a single line diagram of interconnected power system showing its essential parts and voltage levels. How is isolated power system different from interconnected power system? Write down advantages of interconnected power system.

Single line diagram with voltage levels

An interconnected power system links several generating stations and load centres through a common high-voltage transmission network (grid).

 [G] 11 kV       Step-up       Transmission
  |---->[11/220 kV]====== 220/400 kV ======+
                                            |
 [G] 11 kV                          [220/132 kV]
  |---->[11/132 kV]====== 132 kV ====== Grid S/S
                                            |
                              Sub-transmission 66/33 kV
                                            |
                                    [33/11 kV] S/S
                                            |
                              Primary distribution 11 kV
                                            |
                                    [11/0.4 kV] DT
                                            |
                         Secondary 400/230 V consumers

Essential parts: generators (11 kV), step-up transformers, transmission lines (400/220/132 kV), grid substations, sub-transmission (66/33 kV), distribution substations (33/11 kV), primary feeders (11 kV), distribution transformers (11/0.4 kV) and LT network (400/230 V), with breakers and protection at every stage.

Isolated vs interconnected

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

Advantages of interconnected system

  1. Higher reliability: If a plant fails, others supply the load.
  2. Less reserve capacity: Spinning reserve is shared, so installed capacity per area falls.
  3. Economic operation: Cheapest plants (e.g. run-of-river hydro) are loaded first; base and peak load plants are used optimally.
  4. Use of diverse sources: Wet-season hydro surplus can be exported and dry-season deficit imported (Nepal–India exchange).
  5. Load diversity: Peaks of different areas occur at different times, reducing total required capacity.
  6. Better voltage and frequency stability.
  7. Larger, more efficient units can be installed.
  8. Exchange of energy between utilities and countries.

Disadvantages to note: higher fault levels and the possibility of a disturbance spreading (cascade failure) unless protection and control are good.

✓ Answer checked by Claude Opus 5.5 (high effort)

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