--- title: "SLD of interconnected system vs isolated system" subject: "Power System Analysis I" chapter: 1 key: "semester_4/power_system_analysis_i:interconnected-sld-vs-isolated" source: https://www.bctengineer.com/q/semester_4/power_system_analysis_i/interconnected-sld-vs-isolated site: BCT Engineering --- # SLD of interconnected system vs isolated system Power System Analysis I · Chapter 1: General Background - Asked in: 2077 Chaitra, 4+2 marks - Video: [Interconnected system [Hindi] | Power plant engineering interconnected system](https://www.youtube.com/watch?v=nTJT58kmKrM) — PK Classes - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_4/power_system_analysis_i/interconnected-sld-vs-isolated - Whole chapter: https://www.bctengineer.com/md/semester_4/power_system_analysis_i/chapter-1 ## Question Draw a typical single line diagram of interconnected power system. How is isolated power system different from interconnected power 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. ## Answer #### 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. ```text [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. | Basis | Isolated system | Interconnected system | |---|---|---| | Structure | One or few plants feed their own load | Many plants and areas linked by tie-lines (grid) | | Reliability | Plant outage interrupts supply | Other plants supply the load | | Reserve needed | Large reserve in each plant | Shared reserve, less total | | Frequency/voltage | Varies widely with load | Stable, grid is stiff | | Economy | Cannot use cheapest plant always | Economic dispatch of plants | | Fault level | Low | High (needs larger breakers) | | Example | Micro-hydro village grid | Nepal 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. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What is an isolated power system? A. One or few plants supplying only their own local load B. Many plants linked by a grid C. A system with infinite bus D. A system linked to India Answer: A. A micro-hydro village grid is an example. (Key point: Isolated system) 2. Which statement is correct about stability? A. Isolated systems are stiffer B. Interconnected grid is stiff; isolated system varies widely with load C. Both identical D. Interconnected varies widely Answer: B. A large grid absorbs load changes. (Key point: Frequency/voltage) 3. How do fault levels compare? A. High in isolated, low in interconnected B. Same C. Low in isolated, high in interconnected D. Zero in both Answer: C. Interconnection raises fault level, needing bigger breakers. (Key point: Fault level) 4. Why is interconnected operation cheaper? A. Plants never run B. Every plant runs at peak C. Fuel is free D. Economic dispatch loads the cheapest plants first Answer: D. Isolated systems cannot always use the cheapest plant. 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