HomeMy WebLinkAboutThayer Lake Hydropower Project 12kV Transmission Line Design Basis - Dec 2020 - REF Grant 7050825electric Power Systems
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Consulting Engineers
Inside Passage Electric Cooperative
Thayer Creek Hydro 12kV Line Project
Electrical Design Basis
EPS Project #: 20-0092
Dec 30, 2020
Submitted by: Electric Power Systems, Inc.
Prepared by: Matthew S. Williams, PE
Summary of Changes
Revision Number
Revision Date
Revision Description
1
2020-12-30
Initial Submission with 95% design
2
2020-12-30
Revised per client comments
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Electrical Design Basis
1 Summary...........................................................................................................................4
2 Design Standards and Codes.......................................................................................... 4
3 Project Scope Description............................................................................................... 4
4 Overhead vs. Underground Construction Considerations ............................................ 4
5 Design Basis..................................................................................................................... 6
5.1 Loading Criteria......................................................................................................... 6
5.2 Clearances..................................................................................................................6
5.3 Structures and Foundations..................................................................................... 7
5.4 Line Conductor.......................................................................................................... 8
5.5 Insulators and Hardware........................................................................................... 8
5.6 Guys and Anchors..................................................................................................... 8
5.7 Grounding.................................................................................................................. 8
5.8 Communications........................................................................................................9
6 Right -of -Way and Easements.......................................................................................... 9
7 Other Considerations....................................................................................................... 9
8 Cost Estimate................................................................................................................... 9
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Summa
This report presents the design standards, design criteria and planned equipment and materials
for the construction of the 12-kV tie line to connect the Thayer Creek Hydro project into Angoon,
Alaska on Admiralty Island. It also addresses considerations of overhead vs underground
construction for the project. Estimated electrical construction costs will be provided as an
addendum to this report.
2 Design Standards and Codes
The reference design standards, codes, and guidelines utilized in the development of the
electrical design of the project include, but are not limited to the following:
C2-2017 National Electrical Safety Code (NESC) — 2017 edition
RUS 1782F-804 Specifications and Drawings for 12.5/7.2 kV Line Construction
ANSI American National Standards Institute
NEMA National Electrical Manufacturer's Association
3 Project Scope Description
This project consists of a 12kV line extension to tie the Thayer Creek Hydro project into the
Angoon electrical distribution system. It consists of a proposed underground bore beneath
Kootznahoo Inlet to Turn Point, and then a 12 kV line extension over six miles in length from the
Inlet to the proposed Thayer Creek hydro site. An access road is planned to be constructed
from the Inlet to the site; the line is intended to follow the access road as close as possible to
allow for easier access for both initial construction and line maintenance.
4 Overhead vs. Underground Construction Considerations
As part of our basis of design, EPS has performed a basic analysis of the relative benefits of
constructing the line from Kootznahoo Inlet to Thayer Creek with overhead construction vs
underground construction. Specifically, we wanted to look at efficacy, reliability, maintenance,
and costs.
For efficacy, or in other words, effectiveness of the construction type, we usually consider things
like energy losses during operation, installation cost, and maintenance costs. Overhead lines
generally have higher impedances than equivalent underground lines and therefore higher
operational losses; however, the costs of these difference in line losses is usually negligible
compared to the costs associated with overall installation and maintenance.
When considering reliability, there are two factors to consider: quantity of outages, and the cost
and time of outages. Typically, the overhead lines end up with more outages from external
events such as wind or trees. Overhead lines also lend themselves to easier troubleshooting
and shorter outages; time to find and repair overhead line outages is typically a matter of hours.
When faults occur on underground lines it can be days to find and repair an outage.
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Maintenance on costs on overhead lines tend to skew heavily towards the cost of clearing;
these lines require clearing to be performed on a regularly scheduled interval. In addition, visual
inspections of the line are used to identify any areas where poles, crossarms or insulators might
need replacement. As visual inspection of underground lines is limited to the above ground
equipment, maintenance for underground lines tends to take place in response to outages,
unless visible damage or disrepair is noted on the above ground equipment and pads.
When comparing the initial construction cost of underground construction vs. overhead
construction, underground construction is more expensive than its equivalent overhead.
Typically, we expect underground construction to be on the order of one -and -a -half to three
times the cost of overhead construction. Specifically, for this project, one item that we anticipate
factoring into the underground construction cost is rock excavation. We anticipate the
construction to encounter considerable rock; this will drive up the cost of trenching which is one
of the highest cost line items in underground construction. For this project, we estimate that the
underground construction would be approximately 2.5 times the estimated overhead
construction cost.
Another under consideration is the overall life cycle of the installation. For underground
construction, we typically assume an overall project life of 30 years before the system will
require replacement. For overhead line construction, a project life cycle of 50 years is typically
assumed.
Finally, we considered a line design where the line was installed partially underground in
segments where it might be feasible to do so. However, each of these transitions from
overhead to underground and back up again will add two additional riser structures, with their
additional cost. The risers also include associated terminations of the underground cable for
transition to and from overhead construction, which are additional points of failure on the
system, as underground cable failure most frequently occurs at terminations and splices. Each
termination point is also a potential point of additional line impedance. Also, and perhaps as
critically, underground cable adds a capacitive effect to the line. Due to the smaller nature of this
hydro generation, adding this additional capacitance may require installation of line reactance to
offset this capacitance, which also adds line losses, additional cost, and maintenance.
Therefore, primarily because of installation cost, but also for ease of troubleshooting and
maintenance, we have selected overhead construction as the basis of design for the entirety of
the section of line between Turn Point and Thayer Creek on this project.
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5 Design Basis
The following are the design criteria that the overhead line design is based upon.
5.1 Loading Criteria
The overhead line will be designed for the following loading criteria using NESC Grade B
construction:
1) NESC Heavy Loading (NESC Rule 250B)
A 40 mph wind (4 psf), with'/ -inch of radial ice, at 0°F, with NESC Grade B load and
strength factors as summarized in the table below:
Load Factors/Strength Factors
Item
Wind Loads
Wire Tension
Loads
Vertical Loads
Wood Structures
2.5/0.65
1.65/0.65
1.5/0.65
Guy Wire
2.5/0.9
1.65/0.9
1.5/0.9
Anchors and Support
Hardware
2.5/1.0
1.65/1.0
1.5/1.0
2) Extreme Wind Loading (NESC Rule 250C)
A 110 mph (36.9 psf) with no ice, at 40°F. This will be applied with a 1.0 load factor
and strength factors of 0.75 for wood poles, 0.9 for guys, and 1.0 for anchors and
hardware.
5.2 Clearances
Ground Clearances for this project will be based on NESC minimum clearance
requirements with supplemental good practice adders included. The clearance condition
will be the cable or conductor sag at either 32 degrees F with 1/2" radial ice (57pcf) or
sag at the maximum operating temperature; whichever results in the greater sag.
The maximum operating temperature for the communications cables and 12.5 kV
conductor will be 120 degrees F (minimum allowed per NESC requirements).
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The following table summarizes the NESC minimum clearances for the conductors of
this project, as well as our recommended supplemental adders:
Comm over
Comm over
12.5kV over
12.5kV over
Areas
Roads and
Areas
Roads and
Accessible to
Land
Accessible to
Land
Pedestrians
Traversed by
Pedestrians
Traversed by
Only
Vehicles
Only
Vehicles
NESC minimum
clearance (from
15.5 ft
9.5 ft
14.5 ft
18.5 ft
Table 232-1
Extra Clearance
for construction
variations, snow,
2.5 ft
2.5 ft
2.5 ft
2.5 ft
sag increase, etc
TOTAL DESIGN
18 ft
12 ft
17 ft
21 ft
CLEARANCE
Line design will mostly assume that vehicle traffic is possible (with the exception of steep
slopes), so ground clearance will be based upon those columns above. In addition,
wherever the line crosses the roadway, a minimum of 20 feet of road clearance to all
conductors and comm cables will be designed for.
5.3 Structures and Foundations
Basic structures for this line will consist of single wood pole structures using crossarm
construction. Pole top assemblies will consist of RUS based configurations. Raptor
protection configurations will be used based upon the RUS "P" units, including using 10
foot arms with a pole top center phase to provide additional spacing between
conductors. Wood crossarms will be used for tangent and small angle applications;
fiberglass deadend arm assemblies will be used for deadends and larger angles.
Wood Poles will be Douglas Fir, pressure treated with pentachlorophenal. Anticipated
minimum pole class will be class 3; pole lengths are expected to be 45 feet minimum, up
to 60 feet if the terrain of the routing dictates.
Foundations will be direct embedded and backfilled with native or imported granular
material. Rock is expected to be encountered along the route; rock drilling with lower
embedment depths will be used in areas with subsurface solid rock conditions.
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5.4 Line Conductor
The overhead line conductor selected for this project will be #1/0 ACSR bare conductor.
#1/0 AWG, 6/1 stranding ACSR
Code Name "Raven"
Weight: 0.145 Ibs/ft.
Diameter: 0.398 inches
Rated Tensile Strength: 4,380 Ibs
Tensions will be designed in accordance with the tension limitations defined in the
NESC. In addition, a low temperature vibration control limitation of 18% rated strength
at 23 IF will be used to eliminate the requirement for vibration dampeners.
5.5 Insulators and Hardware
Polymer pin -type 15kV insulators will be used for tangent and small angle applications.
Pin insulators will not be loaded to more than their maximum reference cantilever load
(approximately 50% of their Specified Cantilever Load) under NESC Heavy and extreme
loading.
Polymer suspension -type insulators will be used for dead-end pole top assemblies.
Suspension insulators will be loaded to not more than their Routine Test Load
(approximately 50% of their Specified Mechanical Load) under NESC Heavy and
extreme loading.
5.6 Guys and Anchors
Guys and anchors on wood pole structures will be designed to take the entire load in the
direction in which they act; the pole will act as a strut only. Anchors and guy hardware
will be sized to meet or exceed the rated breaking strength of the guy strand. Guys will
be bonded to the pole grounds.
3/8" EHS guy strand will be used. Anchors will be plate anchors, or grouted rock anchors
in locations where sub -surface rock is encountered.
5.7 Grounding
Pole grounding will be designed for all pole locations with downguys, transformers,
risers, reclosers, and other similar equipment installed. The grounding will consist of a
5/8" diameter ground rod driven at the pole, attached to a ground wire which will run the
majority of the length of the pole up to the neutral conductor. Pole hardware, including
communications hardware and messengers, will be bonded to the pole ground. In
addition to the specific locations noted above, pole grounds will be located such that
there are always four grounds in each mile of line to meet the requirements of an
effectively grounded neutral.
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5.8 Communications
In addition to the 12kV line conductors, this project will be designed with an underbuild
fiber optic cable for system communications. The fiber optic cable will consist of a 24
count cable, which should be more than adequate for the requirements of this project.
The fiber will be single -mode, gel -free, all -dielectric outdoor aerial cable lashed to a W
EHS messenger wire. The communications cable and messenger will be located below
the neutral conductor with enough separation to maintain it as a communications space
on the pole.
In addition to the general line design criteria noted above, the fiber optic cable design will
allow for slack cable storage periodically along the route. The design will be based upon
providing 150 feet of slack cable approximately every 1500 feet along the route, in
accordance with industry standards.
6 Right -of -Way and Easements
All poles, conductors, equipment, downguys and anchors for this project will fall within the
proposed 200' right-of-way for the roadway and power line; no additional easement is expected
to be required.
7 Other Considerations
In addition to the materials noted above in the design criteria, we have planned for installation of
two reclosers, one near the power house, and one near the Inlet crossing. Overhead solid
dielectric pole mounted reclosers with associated SEL controllers and power supply
transformers are planned. These reclosers will provide operational switching capabilities as well
as line protection for the project.
Finally, the design is based upon close proximity to the access road as noted above; the nature
of the access road project is such that final alignment of the access road can be modified all the
way through the construction process. Therefore, although our design is based upon the
current access road design, it is anticipated that design will need to be reviewed and modified
based upon the final constructed alignment and grading of the roadway.
8 Cost Estimate
An engineer's estimate for the electrical construction cost of the line has been included as an
addendum to this report. Note that a separate cost for the Kootznahoo Inlet crossing was prepared
earlier in April of 2020, and is not included again in this estimate for the line construction.
End of Electrical Design Basis.
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Thayer Creek Hydro Project
12kV Tie Line - Turn Pt to Thayer Creek Powerhouse
Estimate Summary Sheet
Item Description Item Subtotal Cost
1) 12.5 kV Overhead Line Costs
$ 2,650,822
Poles and Foundations
$
531,621
Conductor
$
703,937
Pole Top Assemblies
$
184,276
Guys, Anchors, Grounding
$
231,612
Fiber Optic Underbuild
$
659,624
Reclosers, Risers and Miscellaneous
$
339,753
2) Construction Management, Engineering, and Staking
$ 345,000
Construction Management Costs
$
250,000
Final Design Engineering
$
50,000
Field Staking for Construction
$
45,000
Subtotal Cost Before Contingency $ 2,995,822
Contingency Factor (10%)
$ 299,582
Total Estimated Cost for Overhead 12 kV Line $ 3,295,405