IEEE Power & Energy Magazine - July/August 2019 - 21

nance costs grow exponentially once
the substations are transported offshore.
Onshore testing and verification prior to
transport is an important aspect. Much
of OSS maintenance resembles practices of the oil and gas industry, which
has extensive experience with offshore
structures and some electrical assets.
The challenge is determining typical
maintenance requirements for different

submerged substations. These technologies have the potential to reduce the cost
even further for offshore wind developments. New onshore substation designs
can utilize many of the OSS technological and economical improvements.
Engineers consider construction and
operational issues as part of the design
and installation of projects. This is particularly true for OSSs, where mainte-

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PANEL

PANEL

PANEL

PANEL

PANEL

PANEL

PANEL

PANEL

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Data

100

0.08

90

80

0.07

Run
Evaluation

70
0.06

PANEL

PANEL

0.09

60
0.05
50
0.04
40
0.03

Cumulative Percent

PANEL

Export and
View Results

Unavailability (hrs/year)

forced and maintenance outages. We see
the drive to reduce offshore installation
weights leading to tradeoffs between efficiency and amount of equipment. Cooling systems for HVdc converter stations
add weight and can lead to interesting
applications of dc GIS solutions for the
offshore substations (OSSs), which do not
require significant cooling systems.
The fifth article, by Vandad Hamadi,
Úna Brosnan, Ingar Loftus, and Gavin
Montgomery, who are all from the United
Kingdom, discusses the lifecycle optimized approach to OSS design in Europe.
OSSs have been built in Europe for a few
years and are now starting to be deployed
globally as offshore wind installations
continue to grow. They play an important
role as collector stations of offshore wind
resources by stepping up the voltage to onshore substation connections. The United
States expects offshore wind resources to
reach an installed capacity of 8.4 GW by
2030, with more than 30 GW in planning and development stages. Offshore
wind developers have experienced significant pressure to reduce overall installation
costs, including capital costs (CAPEX)
and operational costs. Lifecycle cost
management for OSSs affects revenue
models, which consider costs of transmission losses and reduced availability due to repairs. The overall CAPEX
costs of the HVdc installations become
more favorable than ac alternatives for
offshore projects that are far from land.
An innovative approach of the offshore
transmission module (OTM) strips down
the OSS equipment to a bare minimum.
The OTM weight reduction is achieved
by using automated controls. Integrated
offshore HVdc and HVac substations and
interlinked OSSs offer opportunities to
reduce overall costs, which could portend
offshore grids driven primarily by economic considerations. The technological
advances helping the industry to further
improve OSS design include increases in
array cable voltages, adopting midpoint
reactive compensation platforms, larger
wind turbines, structure standardization,
and the use of GIS equipment. Technologies currently under development include
low-frequency ac systems, floating substations, offshore development hubs, and

30

Component
Type

Name

Failure
Contirbution (%)

Failure Rate
(per year)

Avg Repair
Time (hrs)

Unavailability
(hrs/year)

Uptime (%)

0.02

Bus
Bus
Bus
Bus

MAIN BUS A
MAIN BUS B
MECH BUS A
MECH BUS B

12.4003
12.4003
12.4001
12.4001

0.00949048
0.00949048
0.009490247
0.009490247

7.289839
7.289839
7.2899071
7.2899071

0.06918407
0.06918407
0.069183016
0.069183016

0.9999921
0.9999921
0.9999921
0.9999921

0.01

Breaker
Branch
Branch
Breaker
Breaker

BL-7
C-1_A
C-1_B
BL-6
BL-6_A

5.76386
5.30718
5.30718
4.12983
4.12983

0.00085777
0.00282
0.00282
0.002400276
0.002400276

37.490168
10.5
10.5
9.599442
9.599442

0.032157953
0.02961
0.02961
0.023041312
0.023041312

0.99999633
0.99999662
0.99999662
0.99999737
0.99999737

Breaker
Breaker
Branch
Branch
Breaker
Breaker
Breaker
Breaker
Breaker

BL-17
BL-17_A
C-1_C
C-1
BL-10
BL-10_A
BL-8
BL-8_A
BL-9

2.70288
2.70288
2.65359
2.12287
1.35144
1.35144
1.35144
1.35144
1.35144

0.0026
0.0026

5.8
5.8
10.5
10.5
5.8
5.8
5.8
5.8
5.8

0.01508
0.01508

0.00141
0.001128
0.0013
0.0013
0.0013
0.0013
0.0013

0.014805
0.011844
0.00754
0.00754
0.00754
0.00754
0.00754

0.99999828
0.99999828
0.99999831
0.99999865
0.99999914
0.99999914
0.99999914
0.99999914
0.99999914

Breaker
Breaker
Breaker
Breaker

BL-9_A
BL-14
BL-14_A
BL-15

1.35144
0.991175
0.991175
0.991175

0.0013
0.002765
0.002765
0.002765

5.8
2
2
2

0.00754
0.00553
0.00553
0.00553

0.99999914
0.99999937
0.99999937
0.99999937

Breaker
Breaker
Breaker
Breaker
Breaker
Breaker
Breaker
Breaker

BL-15_A
BL-7_A
BL-16
BL-16_A
BL-16_B
BL-16_C
BL-2
BL-2_A

0.991175
0.991175
0.496484
0.496484
0.496484
0.496484
0.0833447
0.0833447

0.002765
0.002765
0.001385
0.001385
0.001385
0.001385
0.00093
0.00093

2
2
2
2
2
2
0.5
0.5

0.00553
0.00553
0.00277
0.00277
0.00277
0.00277
0.000465
0.000465

0.99999937
0.99999937
0.99999968
0.99999968
0.99999968
0.99999968
0.99999995
0.99999995

20

10

0.00

M

ain

Bu

s
B

2
us

B

3
us

Bu

W
sS

-1
BL

C

-A
_1

C

-B
_1

-6
BL

-4
BL

0

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IEEE Power & Energy Magazine - July/August 2019

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