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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
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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
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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)
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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
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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)
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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
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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
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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 …
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Distribution lines
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three-phase four-wire multi-grounded Y
three-phase Delta (under replacement)
[Blume]
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240/120 V single-phase distribution transformers
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[Blume]
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208/120 V three-phase distribution transformers
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[Blume]
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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
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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)
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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)
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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…
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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
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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
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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
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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
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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
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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