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Chicago Secondary Network

An extensive complex electrical power network that serves the heart of the city ~ The Loop ~ providing the highest possible reliability. Distribution Engineering EE455 Topics covered so far? One topic

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Chicago Secondary Network

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    1. Chicago Secondary Network EE 455 Presentation Monday, October 14, 2002 G. Hillesland

    2. An extensive complex electrical power network that serves the heart of the city ~ The Loop ~ providing the highest possible reliability

    3. Distribution Engineering EE455 Topics covered so far? One topic “Transformer Connections” on three phase systems

    4. End of the line, before reaching customers equipment. Forerunners are Transmission and Generation.

    5. Function – to deliver a quality product in whatever amount customers require On 120 volt base - ±120 volts at the meter Frequency of 60 Hertz Very high reliability

    6. Involves equipment to take energy from the substation to the customer • Cable & Conductors; • Primaries & Secondaries, • Services & Meters; • Transformers & Regulators; • Relays & Fuses; • Capacitors & Insulators

    7. Involves a protection system, to minimize interruption of service to customers Fuses, Circuit Breakers, Lightening Arresters

    8. System requirements – Balanced 3 phase load

    9. Typical components in electrical service to a residence

    10. Distribution transformer, connected to one phase of a primary feeder 7.2/12.47 KV to 120/240 V Leg X1 120 V to midpoint Leg X2 120 V to midpoint Neutral – Midpoint connected to neutral and (earth) ground. Typical sizes 10 KVA, 15, 25, 37˝, 50, 75

    11. Secondary Conductors one or two spans either direction from transformer

    12. Service • Conductors from secondary • Customers service entrance • Meter • Customers service panel

    13. Protection components, to minimize impact of faults on customers Transformer Fuse Secondary Breaker Customers Fuses

    14. Susceptibility to interruptions Primary feeder – wind, ice, autos Transformer – lightening, squirrels Secondary Service

    15. Service to the Chicago Loop

    16. Physical Characteristics Large buildings No open spaces between Many customers in each block All facilities must be underground Streets and alleys all concrete

    17. Electrical Requirements • 3 phase, 208 V for air conditioning and ventilating, escalators and elevators • 120 volts for lights and outlets • Varying demands from 100 KW to 500 KW • Uninterruptible service

    18. Electrical System Chosen • 120/208 volt, grounded WYE system • Transformers in vaults under sidewalks • Primary – 12 KV dedicated feeders with paper insulated lead covered cable in concrete conduit and manholes • Secondary Mains – Large size insulated cable in duct runs between vaults • Services – Large size insulated cable in raceways or duct to customers switchgear

    19. Schematic Diagram of Secondary Network System

    20. Schematic Diagram of Secondary Network System

    21. Components of the network system Network transformers

    22. Transformer Primary Switch Secondary Protector

    23. Transformer • Basic transformation 12 KV – 120/208 compact, submersible • 3 phase units, 250, 500, 750, 1000, 1500, 2000, 2500 KVA • Liquid cooled – cooling tubes/fins • Askarel/silicone • Temperature indicator • Liquid level indicator • Oil sample valve

    24. High Voltage Switch • Compartment bolted to transformer • Contains a 3 position switch ? Closed – connects transformer leads to 12 KV cable ? Open – opens connection to 12 KV leads ? Ground – connects 12 KV phase leads to ground ? Switch is non-load break but can drop transformer charging current

    25. Secondary Protector • Bolted to transformer • Connects to transformer secondary leads and to secondary mains • Contains circuit breaker controlled by relays

    26. Reverse Power Relay (Master Relay) Prevents power from grid backfeeding into the transformer and primary cable Phasing Relay Prevents closing circuit breaker unless its unit has a leading phase angle and higher voltage than bus, so when closed, it will prick up load. Desensitizing Relay To prevent opening breaker under light load conditions and elevator regeneration activity circuit breaker interrupting rating

    27. Secondary Mains • 500 MCM cable in conduit under streets • Between network street vaults connections made in manholes in streets • Cable is insulated but no metallic sheath • Spiraled to reduce reactance (Quadraplex) • Ring bus in manholes • Limiters installed where fault current inadequate to burn off cable

    28. Services to Customers • Tapped off secondary mains • Sized for customers load and to minimize voltage drop • Installed in service conduit • Connected to main conduit • Limiters installed if service is too large to burn clear if faulted

    29. Meters Customers Entrance Equipment

    30. Extent of 120/208 Voltage Network System • Total of approximately 600,000 KVA separated into about 8 areas. Each area about 80,000 KVA • Average vault capacity about 4000 KVA • About 20 vaults in each area • Served with 4 to 8 12 KV dedicated lines

    31. Designed for Double Contingency Must carry load for loss of two most critical components during max load periods

    32. From 1936 to 1986 there had not been a network area outage

    33. Spot Networks • Advent of large, modern buildings • Much greater loads–10,000 to 60,000 per building • Too much for 120/208 network • Advent of 12 KV distribution within the building and 12 KV to 277/480 V vaults on site with max 4-2500 KVA, 12 KV – 277/480 volt • Networks confined to each vault

    34. Source for Low Voltage Networks • Original street network • 69 KV transmission system serving several 69/12 KV substations • Underground cable in conduit • Substations in large buildings

    35. 69 KV system fed from 138 KV system Jefferson TSS – 138 KV to 69 KV 800,000 KVA

    36. The electrical supply system to Downtown Chicago

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