A Study on Reliability Assessment for Offshore Wind Farm

Configurations

Je-Seok Shin

, Wook-Won Kim

, In-Su Bae

and Jin-O Kim

Department of Electrical Engineering, Hanyang University, Seoul, Korea

Department of Electrical Engineering, Kangwon University, Samcheok, Korea

Keywords: Reliability Assessment, Offshore Wind Farm, Cost/Reliability Analysis, PNDR (Power Not Delivered

Ratio), EEND (Expected Energy Not Delivered).

Abstract: Due to environment concern, fossil resource exhaustion issue and so forth, an attention on the use of

renewable energy is being increased sustainably and various types of renewable energy are being developed.

In particular, wind power plant is one of the most used resources among them. The recent trend in

development of wind power is the large-scale offshore wind farm. However, the burden of investment for

offshore wind power is still considerable so that comprehensive evaluation must be performed in the

planning stage. For the evaluation, this paper introduces the concept and method to assess offshore wind

farm according to their configurations, in the reliability aspect.

1 INTRODUCTION

Wind energy is one of the most used renewable

energy resources. The related technology has

advanced and a penetration of wind power has being

increased sustainably. Furthermore, an installation

capacity of a wind power plant has being increased.

There are two types of wind power developments

according to the size. One is used as a small scale

distributed generator, and the other is a large-scale

wind farm. In addition, wind power plant can be

developed at onshore or offshore. The recent trend is

the offshore wind farm (OWF) which can gather

more wind energy and avoid several problems that

occur at onshore, such as noise pollution, destruction

of the environment and concerns related with

construction. However, compared with onshore,

OWF has crucial disadvantages which are the more

expensive investment and maintenance costs, and

the fault effects lasted for a longer time due to a

difficulty of geographical access. Therefore, OWF

operator has a considerable burden on investment for

OWF, so OWF operator has to determine a

configuration and design for OWF comprehensively,

in order to achieve the economic feasibility. The

second one of disadvantages is evaluated by a cost

through a reliability assessment, and the reliability

and economic analysis can be performed using the

result of the reliability assessment. For this, the

following indices are redefined in order to represent

the results of reliability assessment. PNDR is the

power not delivered ratio, EEND is the expected

energy not delivered. EENDC is the cost on EEND.

The rest of the paper is organized as follows; In

section 2, basic compositions of OWF are described.

In section 3, a method to evaluate a reliability of

OWF according to wind power configurations is

introduced. In section 4, brief case studies are

performed in order to demonstrate the introduced

method. Finally, section 5 contains a conclusion of

this paper.

2 OFFSHORE WIND FARM

Basic composition of large scale offshore wind farm

is represented in Fig.1, which consists of wind

turbines, inner grid, offshore substations and

external grid.

Figure 1: Basic Composition of OWF.

198

Shin J., Kim W., Bae I. and Kim J..

A Study on Reliability Assessment for Offshore Wind Farm Conﬁgurations.

DOI: 10.5220/0004960801980202

In Proceedings of the 3rd International Conference on Smart Grids and Green IT Systems (SMARTGREENS-2014), pages 198-202

ISBN: 978-989-758-025-3

 2014 SCITEPRESS (Science and Technology Publications, Lda.)

An electrical energy produced at wind turbines is

aggregated thorough inner grid. And then, a voltage

of the energy is made higher in order to transfer

efficiently toward onshore at transformers in

offshore substation. Finally, the energy is delivered

toward onshore through external grid.

At the planning stage for OWF, the locations of

wind turbines and offshore substations are

preferentially decided considering a geographical

condition, in order to gather more wind energy. And

then, the design and configuration for inner grid,

offshore substation and external grid are decided.

There are diverse alternatives based on how to

connect wind turbines and how to configure

substation and external grid. Each alternative of

OWF has had different performance on any failure

in the OWF and it is indicated as the difference in

results of reliability evaluation. When the planning

for OWF is performed, considerations for each

component are as follows.

In case of designing inner grid, there are the

following considerations;

- the voltage of inner grid

- transfer capacity of cables

- the number of feeders(wind turbines per feeder)

- the layout for inner grid: the radial, star and ring

types;

Figure 2: Layout of radial structure.

Figure 3: Layout of star structure.

Figure 4: Layout of ring structure.

In case of offshore substation, the following

factors are considered.

- the number of transformers

- a capacity of each transformer

In case of external grid, the factors to be considered

are same with case of offshore substation.

- the number of transmission lines

- the capacity of each transmission lines

3 RELIABILITY ASSESSMENT

ON OFFSHORE WIND FARM

In this paper, a quantitative reliability assessment is

performed in viewpoint of OWF operator. Therefore,

the goal of reliability assessment is to quantitate how

much an expected energy is delivered to onshore

from wind turbines considering interruptions in

OWF. For this, the following indices are redefined.

PNDR is defined as the ratio of a power not

delivered by any failure to a power in non-failure

state. EEND means the expected value of energy

which is not delivered, reflecting all failure states

defined at each component. At this time, the failures

that may occur at any component, affect to the next

component consecutively, so that failure states at

any component should contain the failure states

defined at the previous component. This fact is

represented in Fig. 5.

Figure 5: Failure States at each component.

3.1 PNDR at Wind Turbine

Although wind turbines are not related directly to

the ability to transfer energy toward onshore, faulted

wind turbines affect the transfer ability at next

component. Therefore, ratio of energy not produced

at wind turbine due to failure of wind turbines can

be evaluated like PNDR at wind turbine. PNDR at

wind turbine can be calculated as ratio of the number

of faulted wind turbines to the total wind turbines.

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199

3.2 PNDR at Inner Grid

PNDR at inner grid is influenced by sum of the

number of faulted wind turbines and disconnected

wind turbines according to locations where cable

faults occur. Because of cable failure with a very

low probability, two or more cable failures are

ignored in this paper.

There are two different approaches to calculate

PNDR at inner grid based on the layouts. In case of

a layout of radial structure, PNDR is calculated by

ratio of the net disconnected wind turbines to the

total wind turbines, the net disconnected wind

turbines mean the union set consisting of the faulted

wind turbines and the disconnected ones. However,

in case of a layout of ring structure which has the

redundant cables, a more complex approach is

applied. When a failure occurs at one among feeders

composing a ring structure, power disconnected by

the cable failure would detour through the other

feeders in the ring structure. All the remained wind

turbines in the ring structure, except for wind

turbines connected normally toward offshore

substation in the faulted feeder may be limited

according to wind speed. If power to be delivered

through non-faulted feeders is higher than their total

rated capacity, the rated power of the wind turbines

is restricted as shown in Fig. 6.

Figure 6: Normal/Restricted Output Characteristic of WT.

Therefore, in case of layout of ring structure,

relationship between power delivered through non-

faulted feeders and their total rated power is

considered as well as the net number of

disconnected wind turbines. Equation related to the

limited rated power of wind turbine is represented

by Eq (1).

()

if)

.()

()

() ()

if)

.() .()

IG IG

OS EG

WT WT

rNew OS EG

OS EG OS EG

WT WT WT WT

ACap

ACap ACap

nn f

nn f nn f





























(1)

where,

OS EG



means failure state at offshore

substation and external grid and contains failure

factors at formal components,

()

OS EG

ACapf



is total

capacity of non-faulted feeders in a ring structure,

is the rated power of wind turbine,

is the

number of wind turbines and

is the number of

the net disconnected wind turbines.

3.3 PNDR at Offshore Substation and

External Grid

Failure factors considered at two components affect

to PNDR of each component equally. Therefore,

Failure factors at two components are handled

together. Containing formal failure factors, failure

states can be defined by combination of faulted

transformers and external lines. If failures occur at

transformers and/or external lines, all wind turbines

may reduce their rated power only when power to be

delivered toward onshore from inner grid is higher

than the available capacity of offshore substation

and external grid. This process for determining the

limited rated power is same with case at inner grid

composed by ring structure.

After obtaining all PNDR under predefined

failure state at all components, an expected value of

PNDR is calculated by multiplying each PNDR and

the corresponding probability of failure state. And

then, an expected PNDR is used in order to evaluate

EEND. EEND is obtained by multiplying EPNDR

and an expected energy produced by the entire OWF

in non-failure state. This process is shown in Eq (2).

()Pr()

OS EG

OS EG OS EG

OWF

PNDR f f

EEND EE EPNDR

EENDC EEND asp

EPNDR













(2)

where,

Pr( )

OS EG



is probability of that the failure state,

OS EG



occurs.

OWF

means an expected annual

energy of OWF in non-failure state and

asp

is an

average settlement price of electrical energy

generated by wind turbine.

4 CASE STUDY

In the section, brief case studies are performed in

order to demonstrate the proposed method. OWF is

composed by 28 identical 5MW-wind turbines.

Wind speed model is applied identically by

historical data model obtained at the southwest coast

in Korea. The expected annual energy of the entire

OWF is 367,159.6MWh considering no failures.

Basic alternatives based on layouts of inner grid

are represented by Fig. 7-9.

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Figure 7: Basic Case 1.

Figure 8: Base Case2.

Figure 9: Base Case3.

Cable data in inner grid are represented in Table 1.

Table 1: Data for Cable in Inner Grid.

Cable type[mm2] 70 185 400 500

Available Capacity [MW] 13.0 21.5 31.5 35.5

Individual basic cases are also separated into the

four sub-cases according to the number and capacity

of transformers and external grid cables, as shown in

Table 2.

Table 2: Data for sub cases.

No. Sub Case -1 -2 -3 -4

Transformer

150MW

ｘ1

75MW

ｘ2

150MW

ｘ1

75MW

ｘ2

External

Grid Cable

150MW

ｘ1

150MW

ｘ1

75MW

ｘ2

75MW

ｘ2

For the simplicity, it is assumed that reliability

data for each component (Inner/External Cable,

Transformer) is identical. The reliability data for

individual components are shown in Table 3.

Table 3.

Components WT Inner Cable Trans.

External

Cable

Unavailability 0.016 0.00025 0.016 0.0025

The results for all cases are represented as the

following Table 4-6, where 0.2$/kWh is applied as

average settlement price of wind energy in order to

calculate EENDC.

Table 4: Result of Case 1.

Case 1

No.Sub- -1 -2 -3 -4

[%]

WT 1.60

Inner

Grid

1.70

OWF

3.51 2.17 3.30 1.95

EEND_OWF

[MWh]

12,887.3 7,967.4 12,116.3 7,159.6

EENDC [

2.58 1.59 2.42 1.43

Table 5: Result of Case 2.

Case 2

No.Sub- -1 -2 -3 -4

[%]

WT 1.60

Inner

Grid

1.68

OWF

3.50 2.15 3.29 1.94

EEND_OWF

[MWh]

12,850.6 7,893.9 12,079.6 7,122.9

EENDC [

2.57 1.58 2.41 1.42

Compared with Case 1, Case 2 has less value of

EPNDR and EENDC, respectively, due to a modified

radial structure.

Table 6: Result of Case 3.

Case 3

No.Sub- -1 -2 -3 -4

[%]

WT 1.60

Inner

Grid

1.60

OWF

3.41 2.06 3.20 1.85

EEND_OWF

[MWh]

12,520.1 7,563.5 11,749.1 6,792.5

EENDC [

2.50 1.51 2.35 1.36

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201

As shown in Table4-6, Case3-4 which has a

layout of ring structure for inner grid and two

transformers and external cables is the best

alternative in terms of reliability aspect. On the other

hand, Case 1-1 with a layout of typical radial

structure and one transformer and external cable is

the worst alternative. However, in particular, there is

more investment cost for constructing inner grid

which has ring structure. Therefore, the best

alternative for offshore wind farm has to be

determined considering not only reliability

assessment, but also economic assessment.

5 CONCLUSIONS

In this paper, a method to perform the reliability

analysis for offshore wind farm has been introduced.

Basic components of offshore wind farm are divided

into four components which are wind turbines, inner

grid, offshore substation and external grid.

According to a design or configuration for each

component, there are diverse alternatives for

offshore wind farm. The proposed method can

evaluate reliability at each component level step by

step, and then the results can be used for reliability

and economic assessment to determine the best

alternative for offshore wind farm.

As for future work, it is expected to study on

how to design optimally inner grid using the

suggested method in offshore wind farm.

ACKNOWLEDGEMENTS

This work was supported by the National Research

Foundation of Korea (NRF) grant funded by the

Korea government (MEST) (No. 2011-0017064).

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