5 renewable plant design

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

© 2016 General Electric International, Inc. All rights reserved. Not for distribution without permission.

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
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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

© 2016 General Electric International, Inc. All rights reserved. Not for distribution without permission.

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

© 2016 General Electric International, Inc. All rights reserved. Not for distribution without permission.

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
© 2016 General Electric International, Inc. All rights reserved. Not for distribution without permission.

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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