Power System Fundamentals

1. Power System Fundamentals EE 317 Lecture 10 27 October 2010

2. Distribution System Overview • Role of the Substation • What is a Bus? • What is a Feeder? • What does Radial mean? • What are the methods used by engineers to increase reliability in the distribution system?

3. Bahria Town Grid Station

4. Roles of a Substation • What are they?

5. Voltage transformation • Why? • Typical levels 230,138,69, 34.5 or 23 kV in • And 12kV or 4 kV out

6. System Protection • Substation Breakers : what do they protect? • The Public • Substation Transformers • Switchgear

7. System Reliability • Isolate Faults: trip a feeder or a bus as opposed to an entire substation

8. TYPICAL Sub Station

9. Elements of sub station • Substations generally have switching, protection and control equipment and one or more transformers. • In a large substation,circuit breakers are used to interrupt any short-circuits or overload currents that may occur on the network. • Smaller distribution stations may use recloser circuit breakers or fuses for protection of distribution circuits. • Substations do not usually have generators, although a power plant may have a substation nearby. Other devices such as capacitors and voltage regulators may also be located at a substation.

10. Functional Types of sub stations • Transmission substation: • A transmission substation connects two or more transmission lines • Distribution substation: • A distribution substation transfers power from the transmission system to the distribution system of an area • Collector sub station: • In distributed generation projects such as a wind farm, a collector substation may be required • Switching sub station: • A switching substation is a substation which does not contain transformers and operates only at a single voltage level.

11. Layout of a Typical Substation • Hi-voltage Breaker • Switchgear • Station Transformer • Switchgear • Bus • Feeder Switchgear (and/or breakers) • Single primary feeders • Multiple primary feeders

12. High Voltage Breaker • A circuit breaker is an automatically-operated electricalswitch designed to protect an electrical circuit from damage caused by overload or short circuit. Its basic function is to detect a fault condition and, by interrupting continuity, to immediately discontinue electrical flow. Unlike a fuse, which operates once and then has to be replaced, a circuit breaker can be reset (either manually or automatically) to resume normal operation. Circuit breakers are made in varying sizes, from small devices that protect an individual household appliance up to large switchgear designed to protect high voltage circuits feeding an entire city.

13. Switch Gears • Switchgear for low voltages may be entirely enclosed within a building. For transmission levels of voltage (high voltages over 66 kV), often switchgear will be mounted outdoors and insulated by air, though this requires a large amount of space. Gas insulated switchgear used for transmission-level voltages saves space, although it has a higher equipment cost. Oil insulated switchgear generally takes up more space, and is being phased out to eliminate the risk of an oil spill.

14. Station Transformers • Station Transformers come in varying sizes, depending on site requirements, but are normally connected to the network through overhead connections which results in exposed HV or LV bushings that necessitate a fence or other protective enclosure for personnel safety and equipment security.

15. Primary Radial Feeders • Layout • 3  to any customer locations requiring it as well as to other key points to maximize reliability • Sectionalizer locations • Feeder Tie locations • Single phase to all other locations (balancing load connected to each phase)

16. Sectionalizing Switches • Switches WITHIN a feeder service area • Isolate Faults • Allow majority of feeder customers served to be put back into service during an extended repair

17. Feeder Tie Switches • Switches BETWEEN two different feeders to interconnect them • Allow an adjacent feeder region to be served by a feeder that is still in service when a substation problem or feeder fault which can be isolated from the substation side has disrupted service on an entire feeder

18. Distribution Service • Primary Feeder serves Distribution pots (or transformers): • Normally Fused • Switchable Transformer • Serves Secondary circuit • distribution voltage = 120/240 V • Houses/streetlights connected to secondary • Service Entrance Cable to Home/Meter

19. Distribution Transformers • Because of connection to overhead, open wire feeders these transformers are often exposed to lightning strikes • To minimize insulation breakdown and transformer failure lightning arrestors are added

20. Distribution Equipment • Lightning arrestor • Bushing Insert • 11-kV Insulator • Fused Cutout • Terminating Dead-End • 66- kV insulator • 3- 0.4 kV spacer • Secondary capacitor