Section 3TRANSMISSION & DISTRIBUTION

• US Bulk Power Transmission System

• Transmission & Distribution (T&D) Components

• New T& D Technologies

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T & D – Transmission

• Net Utility Investment (USA)Generation 55%Transmission 12%Distribution 29%Other 4%

• Bulk Power Transmission systems route power from Generating Plants to Distribution Systems

T & D – Transmission Grid • USA Bulk Transmission divided into three main power

gridsEastern InterconnectWestern InterconnectTexas Interconnect

• Connections between interconnectsEast/West-limited, Texas/others- DC onlyMexico linked to West & TexasCanada linked to East % West

T & D – Transmission Grid (cont) • Interconnects subdivided into ten NERC

(North American Electric Reliability Council) regions

• New England region served by NPCC(Northeast Power Coordinating Council)

• Interconnects also divided into 152 RTO’s (Regional Transmission Operators) andISO’s (Independent Switching Operators)

T & D – Transmission Systems

• The transmission system is the “trunk” of power delivery

• Power from many generators is pooled in the transmission system and each distribution system draws from this pool

• Transmission lines run not only from power plants to distribution systems but also they run between other transmission lines to provide redundancy of supplies and allow rerouting of power to avoid outages

T & D – Transmission System (cont)• Transmission System power transmitted usually

with “three phase” AC (69kv -765 kv)• DC transmission gaining greater use

(+/- 250kV - +/-500kV)• DC used between “asynchronous” grids• AC power needs power sources and lines to be

synchronized to facilitate safe and controlled switching to control flows

• Transmission lines also can carry data (PLC- Power Line Carrier and BPL- Broadband over Power Lines

T & D – Transmission System - Problem Issues

– Overloaded – increasing demand on grid– Power Quality issues

#1 –Surges caused by lightning strikes

Others-Voltage sags and swells, Impulse events, Decaying oscillating voltages, commutation notches, harmonic voltage

– System overloads results in brownouts, blackouts and rolling blackouts

T & D –Transmission Components• Transmission lines – OH (overhead)• Transmission lines- UG (underground)• Subtransmission lines• Substations– Change voltage (step-up or step-down)– Regulate voltage (keep within acceptable limits)– Switch T&D circuits into and out of grid system– Measure and meter– Connect generation plants to system– Make intersystem connections and switching– Connect to Distribution systems

T & D – Substations

• Four common types of SubstationStep-up transmissionStep-down transmissionDistributionUG Distribution

• Substations usually built with a “bus” , high voltage switches and ”circuit breakers”– usually multiples of both

T & D – Distribution Systems• Distribution System – originates at a distribution substation – includes the lines, poles, transformers, and all

other equipment needed to deliver electric service to consumers at the required voltages (voltages vary by customer types)

• Classification of customers

Industrial, Commercial, Residential, Transportation

T & D – Distribution Components– Substations– Distribution Feeder circuits– Switches– Protective equipment (circuit breakers, reclosers,

sectionalizers, fuses etc.)– Primary circuits (5kv -23kv AC)– Distribution transformers (OH &UG)– Secondary Lines ( 120-600v AC)– Services (120/240, 120/208, 277/480 volts AC)

T & D – New Technology

• Power Electronics (replace traditional devices such as switches, controllers, capacitors, and condensers – Often functions can be combined into one device

• DOE (Dept of Energy –federal) GridWorksProgram to support improving electric reliability by Modernization of key grid components –cables,

conductors, substations, protective systems and power electronics

T & D – New Technology(cont)• New Grid concepts– Minigrids– Power parks– DC microgrids– Flexible AC Transmission Systems– Electric load as a reliable resource• Price responsive load• Emergency responsive load

– EPRI (Electric Power Research Institute)says Greatest challenge facing T&D

“enhancing system reliability and performance while maximizing the utilization of power delivery assets”