Power Systems & Energy Course:
Renewable Plant Design
Jason MacDowell
Renewable Power Plant Elements
Collector Feeders
Substation
Wind Turbines
Zig-Zag
Transformer
Zig-Zag
Transformer
Zig-Zag
Transformer
Collector layout in large solar
plants are similar to wind plants
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5-2
Collector
Layout
Feeder Layout Constraints
• Number of WTGs/PV inverters on feeder or
feeder section
– Protection constraints
– Current limits
• Voltage regulation limits
• Loss optimization
• Minimize cable costs, product of
– Size
– Length
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5-4
Protection Constraints
• Protection can’t trip for
– Normal and abnormal operating currents
– Energization inrush currents
• More WTGs/Inverters = less sensitive protection
• May need to backup unit transformer fuses
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5-5
Feeder Current Limitations
• Circuit breakers
– Basic rating is 1200 A, higher ratings available
– Consider backup schemes, breaker may serve two feeders
• Other switchgear (switches, reclosers, interrupters, etc.)
– Typically 600 A rating
• Separable connectors
– 600 A for dead-break
– 200 A for loadbreak (simplifies O&M, more accessories available)
• Cables
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5-6
Cable Size Criteria
•
•
•
•
Current capacity (ampacity)
Economics, loss optimization
Voltage regulation
Short circuit withstand
– Phase conductor
– Neutral
– Sometimes additional separate neutral conductor
specified
• Practical considerations
– Large cables more awkward to handle, splice, etc.
– Large cables come in smaller lengths on reels
More splices = more cost + more opportunities for failure
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5-7
Cable Ampacity
• Cable ampacity does not increase proportional to
cable size
– Skin effect
– I2 relationship with loss; loss creates heat
– Larger cable does no proportionately increase heat
transfer
– Mutual heating constrains cable paralleling
• Ampacity of cables is highly dependent on thermal
conductivity of soil
– Many wind sites in desert and Great Plains areas have
terrible soil!
– Geophysical analysis is highly recommended
• Practical limit of 600 – 700 A in good soil
© 2016 General Electric International, Inc. All rights reserved. Not for distribution without permission.
5-8
Voltage Regulation
• Voltage drop/rise is the product of current and
impedance
– DV IpR + IqX
• Current is a function of units served from that point
• Impedance factors:
– Cable size (affects resistance most)
– Cable installation
Phase separation
Neutral bonding
– Overhead lines have substantially greater
impedance/1000’ than cables
© 2016 General Electric International, Inc. All rights reserved. Not for distribution without permission.
5-9
Cable Impedance – Affected by Installation
Less X, Less R
Simple Neutral Bonding
More X
Less R
Cross Bonding
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5-10
Loss Optimization
• Minimize route length
– Helps with cable cost and voltage regulation, too
• Optimal cable size
– Use economic loading tables (discussed later)
– These tables change by project
Energy prices
Financing structure
Long-term vs. short-term focused client
Expected return on investment
Wind curves
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5-11
Feeder Topologies - Simple Radial Strings
• The most common configuration
• Advantages
– No bifurcations
– Good for linear unit placements
• Disadvantages
– More current x distance product for 2-D unit layout
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5-12
Feeder Topologies - Bifurcated Radial Strings
• Advantages
– Allows one breaker to serve two “directions”
– Also good for linear unit placements
• Disadvantages
– Single bifurcation
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5-13
Feeder Topologies - Dendritic Radial Strings
• Advantages
– Less current x distance product for 2-D unit layout
• Disadvantages
– Many bifurcations
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5-14
Feeder Topologies - Looped Feeders
Advantages
• Allows continued production with cable or breaker outage
Disadvantage
• Cable length to close loop
• Cable and breaker needs to be overrated for 100% output
– Can operate with constrained output
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5-15
Ampacity Requirements for Looping
Normal
112 A
Outage of first
section
84 A
28 A
56 A
56 A
28 A
Normally
open
84 A
112 A
224 A
196 A
168 A
140 A
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5-16
Example – Long and Skinny
On the edge of plateau
Can be >10 mi (16 km) length
Mix of long overhead express feeders, shorter cable express
WTG in radial configuration on underground cable laterals
tapped from overhead or underground express feeders
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5-17
Example – Long and Skinny
On the edge of a plateau
16 mi (26 km) length
Long overhead express
feeders
WTG in radial
configuration on
underground cable
laterals tapped from
overhead express feeders
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5-18
Actual Layout in a Flat Area
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5-19
An Actual Bifurcated Design
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5-20
Insulation
Coordination
Wind Plant Insulation Coordination Process
Different than conventional; less flexibility in economic BIL
choices
• Select equipment insulation levels (BIL)
• Select and locate arresters to provide sufficient equipment
insulation protective margins, considering:
– Separation effects
– Lead length
• Determine arrester TOV and MCOV capabilities
• Evaluate TOV and MCOV of plant design
• Adjust design as necessary
– Control TOV
– Accept less insulation protective margin
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5-22
Surge Arresters – TOV Capability
Typical capability
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5-23
System
Grounding
What Is System Grounding?
Equipment Grounding
System Grounding
System grounding provides a reference point
for the three-phase set of voltages with
respect to ground potential
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