General Interview Questions and Answers on Electrical Power System:

Question Bank :

1. Electrical power system consist of 4 major categories:
   a. Generation System
   b. Transmission System
   c. Distribution System
   d. Utility System
2. Majority of electricity (65-70%) is produced by steam turbine plants and fuel is Coal or Nuclear.
3. Hydro electric generation (25-30%) forms the 2nd largest means of generating electricity.
4. Renewable sources of energy fall in a very small range (2-5%).
5. Renewable sources of energy – solar, wind, ocean, bio gas, geothermal, etc.
6. Gas Turbines are used during short periods of high demand for Peaking.
7. Large generators voltage rating or generation is @ 13.8kV to 24kV voltage levels.
8. Generator voltage is stepped up to transmission voltage level using transformers.
   (Because …
   a. Generation and Distribution stations are far away.
   b. There will be huge I2R losses (transmission losses or copper losses) if the voltage level is low.
   c. Transformers can transform this energy to higher voltage levels, without much loss.)
9. Transmission voltage levels in the range from 115kV to 765kV.
10. Standard transmission voltages are 115kV, 138kV, 230kV, 345kV, 500kV and 765kV.
11. At distribution station, the transformer steps down the voltages.
12. Low voltage ranges from 34.5kV to 138kV at distribution station.
13. Distribution standard voltages are 4.16kV, 12.47kV, 13.2kV, 13.8kV and 34.5kV i.e. range is from 5kV to 34.5kV.
    Why we cannot use higher voltages directly?
    a. Difficult to have equipments with such a high insulation rating.
    b. Not economical.
14. Distribution transformers are used to further step down to utilization voltage levels, usually at 600V.
15. Standard utilization voltages are 480Y/277V, 460V, 208Y/120V, 240V, and 120V.
16. Higher utilization voltages – 6.9kV and 4.16kV are standard voltages for supplying large industrial motor loads.