ECE 5984: Power Distribution System Analysis

Lecture 1: Introduction to Power Distribution Systems Reference: Textbook, Chapter 1

Instructor: V. Kekatos

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The big picture

Distribution substations and feeders

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•  Switching

•  Voltage regulation

•  Voltage transformation

•  Protection

•  Metering §  analog current recordings §  meters on substation and/or feeder §  digital meters record min/max/avg of

current, voltage, power, power factor over 15 min, 30 min, or 1 hr

Switching and protective devices

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•  Switching devices §  manual switches

§  remote controlled switches

•  Protective devices §  circuit breakers

§  fuses §  overcurrent relays

§  reclosers §  sectionalizers

Photo courtesy of K. Schneider (PNNL)

Switches

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•  Simple devices; no control/communication

•  Cannot interrupt fault currents and lack synchronization equipment

Photo courtesy of K. Schneider (PNNL)

•  Isolate equipment for maintenance at substation, or reconfigure feeders

•  Located at substation or feeder

•  Manual control, or remote control via SCADA signals

•  Control equipment located at bottom of supporting structure or utility pole

Circuit breakers

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•  They can be operated remotely

•  Similar to switches, but can break fault currents

•  Located at substation due to size (rating)

•  Classified based on material used to quench arc (air, oil, vacuum, SF6)

•  Used to protect rather than disconnect

Photo courtesy of K. Schneider (PNNL)

Detailed structure

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138:69 kV

Distribution substation 15-200 MVA @ 2-46 kV •  Typical distribution feeders

4 MVA @ 4 kV 12 MVA @ 12.47 kV 20 MVA @ 23 kV 30 MVA @ 34 kV

[Glover, Sarma, Overbye]

69:14 kV

14:480/208 kV

•  sub-transmission is optional e.g., 115:4 kV [IEEE 123-bus]

•  Inline transformers 14:4 kV

Breaker-and-a-half scheme

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Voltage transformers

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On-Load Tap-Changing (OLTC) transformer §  a.k.a. Tap Changing Under-Load Transf. (TCUL)

§  located at the substation; can serve multiple feeders

§  maintains constant low-voltage side under varying distribution load or transmission-side conditions

§  can be substituted with transformer & regulator

In-line transformer and regulators

Distribution transformers

Primary and secondary distribution

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Distribution transformers (pole/pad-mounted) form the boundaries

Classes change: IEEE 1547 defines MV as 1-35 kV …

Distribution lines

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three-phase four-wire multi-grounded Y

three-phase Delta (under replacement)

[Blume]

240/120 V single-phase distribution transformers

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[Blume]

208/120 V three-phase distribution transformers

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[Blume]

480/277 V three-phase distribution transformers

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[Blume]

Small single-phase transformers provide 120 V from 480 V for lighting/office

Voltage regulation

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•  Capacitors: power factor correction (include switching and protective elements)

•  Voltage regulators: induction devices in shunt or series with regulated circuit for the control of its voltage

•  Voltage magnitude should lie within ±5% of nominal (114-126 for 120 V)

Radial feeders

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primary ‘main’ feeder: 2-30 MVA @ 2-46 kV

secondaries: 5-500 kVA @ 120-480 V

3, V, single-phase laterals

in-line transformers

protection devices

regulators and cap banks

distribution transformers 240/120 V 1-phase (split-phase) 208/120 V 3-phase 480/277/120 V 3-phase 400/230 V 3-phase (Europe)

Distribution feeder map

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•  Transformers (kVA rating, connection)

•  Shunt capacitors (kVAR rating, phase)

•  Voltage regulators (phase, ratios, compensator settings)

•  Lines (OH/UG, distance, conductor, phase)

•  Switches (NO/NC)

•  Geographical distances

•  Conductors (radius, diameter, resistance)

See e.g., the actual IEEE 123-bus benchmark…

Fuses

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•  Low-cost devices used to interrupt fault currents

•  Operate on an inverse time curve: the higher the fault current, the quicker the fuse will blow

Photo courtesy of K. Schneider

•  Once fuse interrupts overcurrent, it has to be manually replaced by a line crew

•  Fuse coordination: the practice of selecting fuse sizes so that fuse closest to the fault blows first

•  Fuse coordination requires knowledge of system load and gets complicated with distributed resources

Example: fuse protection

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•  Permanent line to ground fault on line 705-712

•  Fuse blows due to overcurrent, thus isolating single-phase lateral

•  Customers at node 712 and 742 call in to report power outage

•  Utility dispatches a line crew to investigate

•  Line crew locates fault and repairs condition

•  Line crew replaces blown fuse with a new one

•  Single-phase lateral us back to service Photo courtesy of K. Schneider

Protection relays

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•  Use local measurements to generate control signals

Photo courtesy of K. Schneider

•  Fuses measure only current; relays measure voltage and current so can also estimate - real and reactive power - sequence components - phasor measurements

•  They can be accessed remotely for maintenance and updates

Reclosers

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Photo courtesy of K. Schneider

•  Not needed in transmission or underground distribution systems

Operation

1.  Fault occurs 2.  Recloser interrupts fault current and remains

open for a time period (1-2 sec) to allow momentary faults to clear

3.  Recloser closes back into fault and sees if fault has cleared

4.  If fault has cleared, recloser stays closed; otherwise, recloser reopens

5.  Number of tries to reconnect is user-configurable (usually 3)

6.  After final ‘shot’, recloser locks open 7.  Utility crew must locally reset the unit

•  Designed to minimize number of customers affected by momentary fault

Sectionalizers

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Photo courtesy of K. Schneider

Operation

1.  Sectionalizer detects overcurrent but cannot interrupt fault

2.  It starts counting recloser shots 3.  During the second/third recloser shots, the

sectionalizer opens under no load

•  Operate on local measurements and with proper coordination of upstream reclosers

Recloser

Sectionalizer #1

Sectionalizer #2

Sectionalizer #3

•  Combination of reclosers and sectionalizers is ideal for system with permanent and temporary faults

Example: sectionalizer protection

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•  Permanent line to ground fault on line 710-735

•  Overcurrent causes recloser 730-709 to open

•  Sect. 708-733 detects overcurrent and prepares to open

•  Sect.738-711 does not detect overcurrent

•  Recloser waits and closes back in; repeats 2-3 times

•  Sect. 708-733 opens after 2nd or 3rd shot during no load

•  Recloser closes back in, sees no fault, and remains closed

•  Customers downstream of 708 report power outage and utility dispatches line crew

•  Line crew locates fault; repairs condition; and recloses sect.

•  Lateral back to service

Photo courtesy of K. Schneider