Alternative configurations for induction-generator based geared wind turbine systems for reliability and availability improvement

8/14/2019 Alternative configurations for induction-generator based geared wind turbine systems for reliability and availability

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Alternative configurations for induction-generator based geared wind turbine

systems for reliability and availability improvement

M. EL-Shimy

Electric Power and Machines Department, Faculty of Engineering, Ain Shams University, 11517, Cairo, Egypt

[email protected];[email protected];Mobile phone: 002 0105639589

Abstract- The main objectives of this paper are to study and

improve the reliability and the structural availability of WTG

systems. Due to limitations on the availability and accuracy of

failure and repair data, the scope of the study is limited to the

main items comprising the electrical subsystems of the induction

generator based WTG systems. However, induction generator

based WTG systems are the most widely used systems in wind

power generation and the mechanical subsystems of such

generating systems are almost identical. Previous studies show

that the electronic subassemblies in WTG systems are among the

main causes of the reduction of the overall system availability.

Hence, the proposed alternative configurations are based on

either redundancy in the converter subassemblies (active and

standby redundancy) or converter bypass during converter

failure and repair times. Operational limitations of the proposedconfigurations as well as some previously proposed

configurations are discussed. Suitability of the proposed

configurations for offshore applications is considered. It is found

that the squirrel-cage induction generator (SCIG) with full-scale

converter (FSC) and active-redundant converter configuration is

the optimal WTG system for offshore applications. However,

attempts should be made to improve the maintainability of such a

configuration.

Index Terms -Wind power; Reliability; DFIG; SCIG; BDFIG

I. INTRODUCTION

Referring to the rotational speed, wind turbine (WT)

concepts can be classified into fixed speed, limited variablespeed and variable speed. For variable speed wind turbines,

based on the rating of the power converter related to the

generator capacity, they can be further classified into wind

generator systems with a partial-scale and a full-scale power

electronic converter. In addition, considering the drive train

components, the wind turbine concepts can be classified into

geared-drive and direct-drive wind turbines. In geared-drive

wind turbines, one conventional configuration is a multiple-

stage gear with a high-speed generator; the other one is the

multibrid concept that has a single-stage gear and a low-speed

generator. Extended details about wind turbine concepts and

their comparison can be found in [1-5].

The multiple-stage geared drive DFIG concept is stilldominant in the current market. Additionally, the market

shows interest in the direct-drive or geared-drive concepts

with a full-scale power electronic converter. Current

developments of wind turbine concepts are mostly related to

offshore wind energy; variable speed concepts with power

electronics will continue to dominate and be very promising

technologies for large wind farms [1]. Geared wind turbine

systems with induction generators have been shown to be the

most common configurations (more than 55%) used for large

wind turbines [2] where DFIG based system is the more

common configuration among them [3].

Compared with the DFIG system, the Brush-less Doubly-

Fed Induction Generator (BDFIG) does not require slip rings;

however, it requires double stator windings, with a different

number of poles in both stator layers. The second stator layer

generally has lower copper mass, because only a part of the

generator nominal current flows in the second winding. This

second stator winding is connected through a power electronic

converter, which is rated at only a fraction of the wind turbine

rating [1]. One of the main reasons for lower reliability of the

system with the DFIG, in comparison to SCIG based fixed

speed systems, is the presence of brushes in the configuration.

With the advent of BDFIG technology, this drawback could beovercome in future years [2]. Test results from prototype of

BDFIG indicate that it is a valid alternative to the DFIG for

future wind turbines; however, the machine operation

principle and its assembly are relatively complex [1, 2].

To understand WT reliability, we need to break down the

WT system into subsystems and in turn, subsystems are

divided into subassemblies [2-6]. A subsystem of WT system

could for example be the drive train, consisting of rotor hub,

shaft, bearing, gearbox, couplings, and generator. Components

that constitute a subsystem are subassemblies such as the

gearbox. Fig. 1 illustrates a typical configuration and main

components of horizontal axis geared wind turbine system. It

is depicted from Fig. 1 that a wind turbine system consists ofseveral components. A component can be considered as a

subsystem if it is divided into its constituting items. For

example, the converter of the DFIG system can be considered

as a subsystem consisting of four subassemblies namely, the

rotor side converter (RSC), the grid side converter (GSC), the

DC link, and the control unit (CU) [2].

Reliability is the probability of a subassembly to perform

its purpose adequately, under the operating conditions

encountered, for the intended period. Analytical methods are

available for evaluating reliability, depending on the data

available, the depth of study, and the expected accuracy of the

model [8, 9]. A reliability model can only provide correct

conclusions if accurate data are used [2]. Operational data willverify correctness of the predicted system lifetime. Statistical

data analysis may result in a component redesign or a changed

maintenance schedule [5].

The control unit inside the turbine regularly collects

operational statistics from wind power plants. Today, most

turbines are fitted with equipment that makes it possible to

collect the data remotely via modems or internet [5]. The basis

for developing and establishing a database for collecting

reliability and reliability-related data, for assessing the

EL-Shimy M. Alternative configurations for induction-generator based geared wind turbine systems for

reliability and availability improvement. MEPCON10 IEEE International Conference;

Dec. 19-21, 2010; Cairo, Egypt2010. p. 538 - 43.
mailto:[email protected]:[email protected]:[email protected]:[email protected]:[email protected]:[email protected]


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reliability of wind turbine components and subsystems and

wind turbines as a whole, as well as for assessing wind turbine

availability while ranking the contributions at both the

component and system levels is presented in [4, 9, 10].

Fig. 1:A typical configuration and main components of horizontal axis

geared wind turbine system [4]

Collecting accurate wind-turbine reliability data is

considered a challenging task [2, 5]. This was for several

reasons, e.g.,: no statistical data were collected, wind turbine

manufacturers refused to reveal data, data from different

designs could not be compared, or data retrieval was too

expensive to access [5]. Even if it is available, the field failure

data are usually tainted, incomplete, lack sufficient detail, or

do not satisfy the assumptions of a model selected for analysis[7]. In order to consider such an incompletion and obtain a

more accurate reliability growth of wind turbines, a general

three-parameter Weibull failure rate function is presented in

reference [7] to depict the reliability growth. The parameters

of this function are estimated by two techniques, maximum

likelihood and least squares. Similar results have been

achieved by the two techniques.

Despite the deficiencies of this data, reliability-growth

analysis methods allow the extraction of reliability trends over

an observed period [11]. The analysis can also differentiate

between subassemblies in a system subject to human-driven

reliability improvement and mature technology, and

subassemblies that are deteriorating, and characterized byincreasing failure intensity.

The main literature findings from the investigations of the

failure statistics of WT systems indicate the following [2-6]:

The gearbox is critical to the availability of the windturbine. Most of the gearbox failures are caused by

wear on the mechanical parts.

Direct drive WT systems are not necessarily morereliable than geared WT systems. Aggregate failure

intensities of generators and converters in direct drive

WT systems are greater than the aggregate failure

rate of gearboxes, generators, and converters in

geared WT systems.

The gears and the drive train are the components thatdemand the longest downtime per failure. Since drive

train and gearboxes seldom fail, one reason for the

long downtime could be that spare parts need to be

ordered, which could prolong the time for repair. Power electronic converters of direct and geared

drive WT system exhibit higher failure intensities

throughout their operation than converters in other

industries.

Although the fixed-speed wind turbine is lessaerodynamically efficient, its availability is higher,

when its reliability is taken into account, at least in its

electrical subassemblies.

If the wind power is to be competitive, the downtime

needs to be shortened and visits to the turbine should be kept

to a minimum [5]. This can be achieved through improvement

in WT system design, fault detection and monitoring, and

maintenance procedures. Better reliability of small windturbines could be achieved with grid-connected configurations

that require minimal power electronics [12-13].

The main objectives of this paper are to study and

improve the reliability and the structural availability of WTG

systems. Due to limitations on the availability and accuracy of

failure and repair data, the scope of the study is limited to the

main items comprising the electrical subsystems of the

induction generator based WTG systems. However, induction

generator based WTG systems are the most widely used

systems in wind power generation and the mechanical

subsystems of such generating systems are almost identical.

Previous studies show that the electronic subassemblies in

WTG systems are among the main causes of the reduction of

the overall system availability. Hence, the proposed alternative

configurations are based on either redundancy in the converter

subassemblies (active and standby redundancy) or converter

bypass during converter failure and repair times. Operational

limitations of the proposed configurations as well as some

previously proposed configurations are discussed. Suitability

of the proposed configurations for offshore applications is

considered.

II.WTGSUBASSEMBLIES AND RELIABILITY MODELLING

Three configurations are considered in this study, all of

them follow the variable speed WTG concept as shown in Fig.

2. The first configuration, shown in Fig. 2(a), is based on

DFIG with a partial-scale converter. The second configuration,shown in Fig. 2(b), is based on BDFIG with a partial-scale

converter. The third configuration, shown in Fig. 2(c), is based

on SCIG with a full-scale power converter. The considered

electrical subassemblies for each configuration, which are the

generators subassemblies,and the converters subassemblies

(the rotor or machineside converter (RSC or MSC), the

grid-side converter (GSC), the DC link, and the control unit

(CU)) are shown in Fig. 2. Failure and repair data for each of


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the considered subassemblies in each configuration are based

on a recent survey in the Manjil wind farm in Iran and are

obtained from [2].

Fig. 2:Configurations and subassemblies of the considered WTGs

For its simplicity and suitability to the considered

problem, reliability block diagram (RBD) modeling technique

is used to model the considered configurations. From

reliability point of view, the considered subassemblies of each

of the base configurations shown in Fig. 2 are connected in

series. Fig. 3 shows the RBD for the generator and converter

subsystems.

Fig. 3:RBD for various subsystems

Based on the reliability theory [8, 14] the following

formulae apply under steady state analysis regardless of the

distributions of failure and repair except for the case of

standby redundancy where each block must have

exponentially distributed active failure and repair times and

passive and switching failure rates assumed to be zero. Fig. 4

shows the basic RBD connections.

The failure () and repair () rates for basic RBD

configurations are calculated as follows. For series connected

items, shown in Fig. 4(a),

(1)

(2)

For active redundant items, shown in Fig. 4(b),

(3)

(4)

For standby redundant equal items system, shown in Fig. 4(c),

(5)

(6)

Fig. 4: Basic RBD connections. (a) Series, (b) Active redundancy, (c) Standby

redundancy

The failure and repair rates are the reciprocal of the mean-

time between failures (MTBF or m) and the mean-time to

repair (MTTR or r). The reliability and maintainability are

usually demonstrated by the values of failure and repair rates

respectively. The availability is calculated by

(7)

III.ANALYSIS OF WTGRELIABILITY DATA

Based on the failure and repair data [2], that are plotted in

Fig. 5, for configuration (a) of Fig. 2, it is depicted that the

subassemblies characterized by high failure rates (low

reliability) as in comparison to the rest of the considered

subassemblies, in descending order, are the RSC, the GSC,

and the brush gear. From availability point of view, it is

depicted from Fig. 5(c) that both the RSC and the GSC are

characterized by lower availability in comparison with the rest

of the subassemblies. The high maintainability characteristic

of the brush gear excluded it from being characterized by low

availability.

Configurations (b) and (c) of Fig. 2 do not include brush

gears and the characteristics of the subassemblies ofconfiguration (a) of Fig. 2 are applied to the subassemblies of

these configurations. Higher failure rate of the stator of the

BDFIG with respect to stators in other configurations is

assumed because of its double stator winding design. The

stator of the BDFIG is assumed, from reliability point of view,

to have failure and repair rates of two series connected stators

of the DFIG type.


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Fig. 5:Subassemblies characteristics of the DFIG with a partial-scale

converter configuration. (a) Failure rate (b) Repair rate (c) Availability

IV.ALTERNATIVE CONFIGURATION AND RESULTS

Configurations shown in Fig. 2 are referred to as the base

configurations. Alternative configurations are modified

versions of the base configurations. The considered

modifications are based on either redundancy in the converter

subassemblies (active and standby redundancy) or converter

bypass during converter failure and repair times.

The converter bypass alternative configuration presented

in [2] allows the WTG system in the case of converter failure

to continue running and deliver power to the grid in a

temporarily fixed-speed operation mode instead of the default

variable-speed operation. However, the instantaneous bypass

of the converter may be practically impossible. From transient

response point of view, the converter bypass should not bedone simultaneously with the converter failure because of the

unpredictable response of the DFIG following blocking of the

RSC; this phenomena along with the factor affecting

successful restarting of the converter is fully covered in [15].

A bypass logic-control mechanism that considers the

operating point (the generator speed and powers) of the WTG

system at the instant of failure of the converter is required to

reduce the impact of the bypass on both the WTG and the

power system. Therefore, improved alternative configurations

that are not requiring transition to fixed-speed operation are

favorable. Despite these difficulties, the converter bypass

option is considered herein assuming that a negligible possible

trip and actuation-transition times, in comparison with the

converter downtime, are required to allow successful

transition to fixed-speed operation of the variable-speed WTG

system. Moreover, it is assumed that the bypass system is

100% reliable as in [2].

A. DFIG based WTG system

Five alternative configurations are considered for the

DFIG based WTG systems shown in Fig. 2(a), these

alternative configurations are listed in Table 1. It is depicted

from Table 1 that all alternatives exhibits higher availability

than the base-case configuration.

The alternative configuration (6-b), with a converter

bypass system and delta connected rotor winding, is

characterized by the lowest failure rate (highest reliability) and

the highest availability making it the optimal configuration for

offshore installations where minimum site visits are required.

However, such a configuration is not favorable because of theswitching and transition risks that are previously mentioned.

Therefore, attempts should be made to reduce or eliminate

such risks, for example, through an appropriate trip time to

allow successful transition from viable-speed to fixed speed

operation.

Among variable-speed alternative configurations, the

active-redundant configuration is characterized by the lowest

failure rate and highest availability. This may be the best

choice for offshore applications. However, either the active- or

standby- redundant RSC configurations may be economically

suitable for land-based installations. Although, the standby-

redundant RSC configuration exhibits higher repair rate

(maintainability), the reliability and availability of the active-

redundant RSC configuration are much better.

The highest repair rate (maintainability) is obtained with

the standby-redundant converter configuration. Both standby-

redundant converter and RSC configurations have the same

reliability. However, the standby-redundant converter

configuration exhibits higher availability and maintainability.

B. BDFIG based WTG system

Compared with the DFIG system, the BDFIG system does

not require slip rings; however, it requires double stator

windings, with a different number of poles in both stator

layers [1]. Therefore, it assumed from reliability point of view

that the stator of the BDFIG is consisted of two series

connected stators each having the same failure and repair ratesas the stator winding of the DFIG.

Five alternative configurations are considered for the

BDFIG based WTG systems shown in Fig. 2(b), these

alternative configurations are listed in Table 1. Apart from the

converter-bypass alternative which has lowest failure rate and

availability among all alternatives, the failure rates of all

configurations of the BDFIG-based WTG are lower (higher

reliability) than that for the DFIG-based WTG. This is because


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of the absence of the brush gear subassembly in the BDFIG

machine. However, inspecting Table 1 shows that the

availability of both systems is comparable. The comments

about the characteristics and the applications of the proposed

configurations of the BDFIG-based WTG system are similar

to those for the DFIG-based WTG configurations.

C. SCIG with a Full-Scale Converter (FSC) based WTG

systemUnlike the partial-scale converter based configurations,

where the RSC play the major role that facilitate variable

speed operation, the MSC and GSC are equally important in

configurations with full-scale converter. In addition, the

failure rate of the RSC in DFIG based configuration is twice

that for the GSC. However, both the MSC and the GSC in

SCIG with a full-scale converter configuration are of equal

failure rates [2]. Therefore, Configurations with redundant

MSC are not considered.

Three alternative configurations are considered for the

SCIG with a full-scale converter based WTG systems shown

in Fig. 2(c), these alternative configurations are listed in Table1. The following are comments about the results shown in

Table 1.

Table 1:Alternative configurations for the DFIG and BDFIG based WTG systems

s.n Configuration

DFIG based system BDFIG based system SCIG and full-scale conv.

yr

yr A

yr

yr A

yr

yr A

Variable-speed alternative configurations

1 Base Case 0.870 74.938 0.989 0.790 67.254 0.988 0.970 69.027 0.986

2 Active-Redundant Converter 0.231 78.777 0.997 0.151 49.573 0.997 0.140 54.482 0.997

3 Active-Redundant RSC 0.474 77.007 0.994 0.394 62.629 0.994 -

4 Standby-Redundant Converter 0.870 120.416 0.993 0.790 107.400 0.993 0.970 118.527 0.992

5 Standby-Redundant RSC 0.870 98.084 0.991 0.790 87.711 0.991 -Fixed-speed alternative configurations

6 Conv. bypass6-a 0.220 77.458 0.997

0.120 49.399 0.998 0.120 49.398 0.9986-b 0.120 49.398 0.998

1. Y-connected rotor winding2. -connected rotor winding

Although the base-case configuration of the SCIG with

FSC system is characterized by highest failure rate and

lowest availability in comparison with the base case of all

other systems, it is characterized by lowest failure rate and

equal availability when there is an active redundancy in the

converter subsystem relative to similar converter

arrangement in the other configurations. This suggests thatthe SCIG with FSC and active-redundant converter

configuration is the optimal WTG system for offshore

applications. However, attempts should be made to improve

its maintainability.

V.CONCLUSIONS

This paper presents study, analysis, and improvement of

the reliability and availability of the most widely used

systems in wind power generation, which are the induction

generator based WTG systems. The availability and

accuracy of failure and repair data of the subassemblies of

WTG systems limit the study to the electrical subsystems.

However, the main outcomes are independent on thislimitation because the mechanical subsystems of the

considered WTG configurations are almost identical. Due to

their major effect on the availability of WTG systems,

alternative configurations for the converter subassemblies

are proposed in order to improve the systems reliability and

availability. The effect of the alteration on the systems'

configurations from the points of view of maintainability

and operational limitations are considered.

It is clarified that the converter-bypass technique results

on highest reliability and availability among the considered

alternatives; however, the probable instability of the WTG

system due to sudden blocking of the converter subsystem

hinder the practical implementation of such technique,

unless an appropriate trip time is considered to allow

successful transition from viable-speed to fixed speed

operation. By implementing proper switching logic, theconverter bypass alternative may be the optimal choice of

offshore applications. However, other configurations based

on redundancy of the converter subassemblies show

comparable reliability and availability levels without

hindering the variable-speed operation.

Several alternative configurations are demonstrated

along with numerical demonstration of their reliability,

availability, and maintainability. It is found that the SCIG

with FSC and active-redundant converter configuration is

the optimal WTG system for offshore applications.

However, attempts should be made to improve the

maintainability of such a configuration.

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M. EL-Shimy was born in Cairo in the Arab

Republic of Egypt. He completed his Electrical

Engineering B.Sc, M.Sc, and PhD degrees from

Faculty of Engineering Ain Shams University,Egypt, in 1997, 2001, and 2004 respectively. He

is now an associate professor in Department of

Electrical Power and Machines -Faculty of

Engineering Ain Shams University. He is a

consultant and trainer and a member of many

renewable energy associations. He teachesseveral undergraduates, graduate, and training

courses in Egypt Universities and outside. His

fields of interest include power system stability, power system equivalents,load aggregation, load signature, electric power distribution, optimal power

flow studies, flexible ac transmission systems (FACTS), power system

optimization, new energy resources, and power system reliability. For moredetails, please visit: http://shimymb.tripod.com
http://shimymb.tripod.com/http://shimymb.tripod.com/

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Alternative configurations for induction-generator based geared wind turbine systems for reliability and availability improvement