EECE 887 Distribution EECE 887 Distribution System EngineeringSystem Engineering

CHAPTER 1 CHAPTER 1

Power Delivery SystemsPower Delivery Systems

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SUBSYSTEMS

GenerationTransmissionSubtransmissionDistribution

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Billions of Dollars

Generation -- 40%Transmission -- 20%Distribution -- 20%

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MISSION

Reach every customerMeet demands of customersSupply reliable powerProvide fluctuation free voltage Provide power with low harmonicsKeep cost low

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Hierarchical Structure

Source: Willis

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VOLTAGE LEVELS

Generation: 4 kV to 34.5 kV Transmission: 138 kV to 1100 kV Subtransmission: 34.5 kV to 161 kV Distribution (Primary): 4 kV to 34.5

kV (Most common is 12.47 kV) Distribution (Secondary): 120/240 V

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OBSERVATIONS

Transformers separate the levels Equipment size and capacity

decrease as we move down into the system. However, number of equipment increases.

Net capacity increases Reliability drops as we move closer

to customers

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Distribution Service Transformer

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Service Transformer Schematic

Source: Gonen

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Reliability Statistics

Source: Willis

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Further Observations

Transmission Systems:– Very high capacity– Networked for high reliability– Designed to provide system stability

Subtransmission Systems:– Medium capacity– Networked for high reliability– Designed for stability as well as power delivery

needs

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Further Observations

Distribution Substations:– Link between transmission and

distribution– 1 to 6 transformers of 5 MVA to 150 MVA

(generally more than 1 for contingency)– The substation also has high and low

voltage buses, circuit breakers, metering and protective equipment, and a control house.

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Further Observations

Feeders:– Overhead lines mounted on wooden poles

or underground cables (3 to 10 times more expensive than overhead)

– Underground cables used for aesthetics and in dense urban areas.

– 2 to 12 feeders per substation– 2 MVA to 30 MVA per feeder– Radial arrangement

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Dendrillic Configuration

Source: Willis

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Components of Distribution System (Primary side)

Primary trunkMain branches

– Trunk and main branches are usually three-phase

Switches– Normally Closed (NC)– Normally Open (NO)– NO switches are closed during emergencies

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Underground feeder get-away to prevent congestion of wires

Laterals– Branches off of main feeders– one or two phase– Different phases are tapped alternately

to maintain balance– 10 kVA to 2 MVA– Overhead or underground (buried

directly)

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Secondary System

Close to customers at utilization voltage

RadialFeeds 1 to 12 customers (4 to 6

is more common)

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Distribution Secondary

Source: Willis

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What is Distribution System?

– It consists of feeders, service transformers, and secondary system.

– In some cases distribution substation is also included.

– Other equipment: Voltage regulators and tap changers to

maintain voltage levels Capacitors to compensate inductive loads