--- title: "Basic structure of power system" subject: "Power System Analysis I" chapter: 1 key: "semester_4/power_system_analysis_i:basic-structure-power-system" source: https://www.bctengineer.com/q/semester_4/power_system_analysis_i/basic-structure-power-system site: BCT Engineering --- # Basic structure of power system Power System Analysis I · Chapter 1: General Background - Asked in: 2072 Magh, 5 marks - Video: [Basic Structure of Power System - Power System Engineering - Power System Engineering 1](https://www.youtube.com/watch?v=Tkl6Va7JHj0) — Ekeeda - 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/basic-structure-power-system - Whole chapter: https://www.bctengineer.com/md/semester_4/power_system_analysis_i/chapter-1 ## Question Describe the basic structure of a power system with a clear diagram. ## Short answer A power system has generating stations, transmission, sub-transmission, primary and secondary distribution, plus protection and control. Generation is at 11 kV, stepped up to 132/220/400 kV, reduced to 66/33 kV, then 11 kV, then 400/230 V for consumers. ## Answer A power system is the network that **generates** electrical energy, **transmits** it over long distances and **distributes** it to consumers, with protection and control at every stage. ```text [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 ``` #### Parts of the structure **1. Generating stations** - Hydro, thermal, nuclear, solar or wind plants generate 3-phase AC at 6.6–25 kV (Nepal: mostly hydro at 11 kV). - A step-up transformer raises the voltage for transmission. **2. Transmission system** - Extra-high voltage lines (132, 220, 400 kV in Nepal; up to 765 kV and above elsewhere) carry bulk power to load centres. - Lines are interconnected to form a **grid**, allowing power exchange and shared reserve. **3. Sub-transmission system** - At grid substations the voltage is reduced to 66 or 33 kV and carried to distribution substations in towns and districts. - Large industries may be supplied directly at this level. **4. Primary distribution** - Distribution substations (33/11 kV) feed 11 kV feeders through the load area. - Medium industries take supply at 11 kV. **5. Secondary distribution** - Distribution transformers (11/0.4 kV) supply LT distributors at 400 V 3-phase and 230 V single-phase for homes, shops and small industries. **6. Protection and control** - Circuit breakers, relays, lightning arresters at all substations; load dispatch centre (SCADA) monitors and controls the whole system, keeping frequency and voltage within limits. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. A power system consists of which three main functions? A. Generation, rectification and storage B. Transmission, metering and billing C. Generation, transmission and distribution D. Distribution, storage and inversion Answer: C. It generates, transmits over long distances and distributes to consumers. (Key point: Three main functions) 2. What happens at grid substations in the structure? A. Voltage is reduced to 66 or 33 kV for sub-transmission to distribution substations B. Voltage is raised to 765 kV C. Power is converted to DC D. Generation takes place Answer: A. Grid substations step down to 66/33 kV which feed distribution substations. (Key point: Sub-transmission) 3. Which supplies homes in Nepal's secondary distribution? A. 33/11 kV transformers giving 11 kV B. 220/132 kV transformers C. Generator terminals directly D. 11/0.4 kV transformers giving 400 V 3-phase and 230 V single-phase Answer: D. Distribution transformers 11/0.4 kV feed LT distributors. (Key point: Secondary distribution) 4. Why are transmission lines interconnected into a grid? A. To increase line loss B. To remove the need for protection C. Power exchange and shared reserve D. To lower frequency Answer: C. Interconnection allows power exchange between areas and sharing of reserve. 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