Transmissão e distribuição de energia

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Transmission & Distribution
Kevin Davis
Ref: UNDERSTANDING ELECTRIC POWER SYSTEMS by Jack Casazza
Frank Delea (IEEE Press)
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HVAC Overhead Transmission Components
• Conductors; aluminium conductor steel reinforced (ACSR) type,
(stranded aluminium woven around a core of stranded steel) 
Bundled Conductor = two or more wires per phase
• Ground or shield wires; strung from the top of one transmission
tower to the next to shield the transmission line from lightning 
strokes.
• Insulators; made of polymer or porcelain and attached to the 
energized conductors from the supporting structures
• Support Structures; steel lattice tower (known as electricity 
pylons in Ireland). Purpose is to maintain a safe distance between 
conductors to ground and from each other. May need to carry two 
or more three phase circuit if land restrictions in place. Allowable 
sag in conductor determines spacing between towers.
• Land or right-of-way (R-O-W). Width of land increases with 
voltage level. Clearance determined by vegetation growth under 
lines
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220kV and 400kV Transmission Network Support Structures 
(Pylons)
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Insulators
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© KDavis CIT 2013 6Ref: www.fgg.uni-lj.si
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A one-line diagram (aka a single line diagram) is similar 
to a block diagram, except that the components are 
shown by their symbols rather than by rectangles. 
The symbols give us an idea of the nature of the 
components; consequently, one-line diagrams yield more 
information.
Multi-phase circuits may be presented as a:
• multi-line presentation wherein all phases are presented
• single-line presentation wherein all phases are represented by a 
single-phase circuit with indication of the number of phases
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Ref: Electric power systems: a conceptual 
introduction/by Alexandra von Meier
USA Electrical 
Power System 
Locations of 
Substations
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Transmission or Distribution SUBSTATIONS Functions
Functions depend on location of substation
• location where power from different generating 
stations are fed into the main transmission 
corridors.
• Tied lines together. 
• a terminus for interconnections with other 
systems.
• location of step down transformers from 
transmission to distribution systems
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Transmission or Distribution SUBSTATIONS Functions
• location of tie line change-over switchgear, to 
provide redundancy in the transmission paths.
• location for compensation devices (shunt or 
series reactors or capacitors )
• location where transmission lines can be 
isolated for maintenance or fault repair.
• location for protection, control, and metering
equipment.
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Distribution Substation
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Substation Equipment
• Bus : name given to the electrical structure to which all lines and 
transformers are connected. Open air Bus � lines separated by air 
(as in transmission lines). Enclosed buses have reduced spacing 
between lines and require an insulating gas such as sulphur 
hexafluoride (SF6). 
• Protective relays : monitor line voltages and currents and then 
these relays actuate circuit breakers.
• Circuit breakers ; open the circuit and extinguishing the arc using 
either oil, vacuum, air blast or sulphur hexafluoride (SF6). Rated by 
interrupting current ability. 
• Transformers : connect lines operating at two different voltage 
levels. Step –up transformer ; increases input voltage (Primary) to 
higher voltage at output (secondary). Step-down transformer 
provides the inverse voltage change. Need mechanism to remove 
heat such as external fins. Tap-changing and auto-transformers also 
used.
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Substation Equipment
• Disconnect switches : used to isolate a circuit. Not opened under 
load (no ability to break the arc). 
• Lightning arrestors: used to protect equipment from the effects 
of high voltage due to lightning stroke or a switching operation. 
Designed to flashover when the voltage exceeds
a pre-selected level (determined by the basic insulation level (BIL) 
of the transformer )
• Metering equipment : measure line and transformer loadings to 
ensure that these are within acceptable limits. 
• SCADA ; (System Control And Data Acquisition). Computer system 
to automation of substation operation and allow some remote 
monitoring \control.
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Substation Compensation Equipment
• Shunt reactors (reactors connected from the energized bus to 
ground): control high voltages that occur at night on lightly loaded 
transmission lines. 
• Series reactors : increase the impedance of the line, to decrease 
current levels in the event of short circuits
• Shunt capacitors ; provide VArs to the system to support voltage 
levels.
• Series capacitors : reduce the effective impedance of a very long 
transmission lines enabling more power to flow over the line.
In many systems these discrete components can be switched in and 
out using power semi-conductor devices such as thyristors or gate-
turn-off thyristors. Industry terms used
Static VAr Compensators (SVCs) or Static Compensators 
(STATCOMs). 
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Examples of Substation Breaker Arrangements
More examples on Page 81 of reference.
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Ref: Electric power systems: a 
conceptual introduction/by 
Alexandra von Meier
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Step-down
Transformer 
(20kV to 400V)
Mounted on 
a Pole
Distribution Network 
Equipment
At customer site
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Distribution Box
“Mini –Pillar”
Underground 
Cabling in a build up 
area
Allows the supply to 
a housing estate to 
be distributed to 
each individual 
house
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Final Grid Destination for Domestic Customers \consumers 
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Electricity Power lines of the Future?
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Underground Power Cables 
Weight of cables is not an issue but heat dissipation is a major issue 
due to insulation around the conductor