Tong W. Wind Power Generation and Wind Turbine Design

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Design and Development of Megawatt Wind Turbines 235

greater than the upper sections, but the average value yields an alternative view for

the lowest possible tower mass design goal; i.e. target the average tower mass per

meter for the solid line or lower.

The turbine designer must keep a large number of additional tower design goals

in perspective when searching for the overall cost-effective solution. Some of these

considerations include:

Dynamics, structural, machinery vibration damping and seismic 1.

Internals, electrical cables, climb systems, platforms, packaged power modules 2.

(PPMs) or down-tower assemblies (DTAs)

Installation, erection methods, joints, fasteners and crane requirements 3.

Future large turbine tower technology will likely incorporate lightweight space-

frame construction incorporating multiple support legs that are spread apart. These

conﬁ gurations must remain simple; yet meet all of the design goals while comply-

ing with personnel health and safety requirements. Additionally, one should not

underestimate the internals as these can signiﬁ cantly affect overall tower design;

e.g. potential impact of a welded studs, Kt (stress concentration factor) on the

tower plate thickness speciﬁ cation.

The use of hybrid materials and structural design conﬁ gurations should become

more prevalent for larger turbine sizes. Better tower and foundation integration

should increase overall functional capabilities for lower total cost.

4.4.16 Structural bolted connections

Construction of today’s WTs uses a large number of structural bolted assemblies

including:

Blade attachment 1.

Hub attachment 2.

Drivetrain components 3.

Bedplates 4.

MH and yaw bearing attachment 5.

Tower sections 6.

Tower to foundation attachment 7.

Flanged joints and machine elements are bolted together with pre-loads such

that the ﬂ ange does not separate. The ﬂ ange friction takes all shear force and

the bolts are only in tension. In practice, small amounts of bending moment

due to ﬂ ange machining, separation and misalignment can exist which needs

to be accounted.

To ﬁ nish a bolted connection, the torque process is the most commonly applied

process. However, the torque process works against the frictional resistance on the

bolt or nut face and in the thread resulting in inaccuracies of up to 100%. Another

popular method is called “turn of the nut” or turn-angle. Bolts that are tensioned

using the turn-angle method are one step better than the torque process. When the

turn-angle process is used, the initial tension is provided by torque. The torque part

of the process involves friction and provides about 20 − 30% of the maximum value.

236 Wind Power Generation and Wind Turbine Design

The angular turn speciﬁ ed by the designer provides the ﬁ nishing tension and

results in ﬁ nished inaccuracies of about 25 − 35%.

There has been huge progress in the past few years for pre-loading, locking and

inspecting fasteners used in large WT design. Companies such as ITH-GmbH [ 30 ]

offer a complete suite of bolt tensioning solutions using more accurate stretch

type-tensioning tools. Bolt stretching processes are used for clamp-length ratio of

1:3 or more and for pre-tensioning large size bolts (e.g. M24 and greater). The

clamp-length ratio is the ratio of the bolt diameter to the clamp length of the joint.

Stretching procedures are best when (1) a high degree of accuracy (5 − 10%) is

required or (2) when several bolts have to be pre-tensioned simultaneously.

Future large turbine structural bolted connection technology will probably not

be too much different from today.

4.4.17 Fire detection system

Future large turbine ﬁ re detection technology will probably not change much

from today, with the exception of better integration into the turbine control sys-

tem such that predictive capability may be able to prevent a ﬁ re from happening

to begin with.

4.5 Electrical

The electric power system can be classiﬁ ed into two main categories; (1) the tur-

bine including the MV transformer and (2) the collection system and substation

to the point of delivery to the grid. Future large turbine electrical system technol-

ogy may incorporate elements of high voltage DC (HVDC) collection, centralized

power conditioning and superconducting or quantum wire electricity transmission

(for the right cost).

4.5.1 Turbine – generator and converter

The generator is where electricity production and the customer’s revenue stream

begin. As such, doing this efﬁ ciently for a justiﬁ able cost is of prime importance

and must be reliable and sustainable over the lifetime of the turbine.

Figure 33 presents the mass for the three main elements of today’s typical WT

power system that operates at relatively LV level (i.e. 575 − 690 VAC) between the

up-tower generator and the down-tower power conditioning and MV step-up trans-

former. The mass for these components are in the same order of magnitude, with

the LV cable mass roughly 2/3 of the mass of the generator and converter. The

converter mass can vary widely depending on the particular converter topology,

which can have a big impact on the amount of reactor mass required (reactor mass

being copper and steel, and often dominating total mass).

Of all the 10-turbine analysis group component mass characteristics plotted as

a function of net rated power, the generator is the only one to exhibit a negative

squared term for the curve ﬁ t (i.e. diminishing mass increase for larger sizes). The

10-turbine analysis group varies gearbox ratio in order to hold the generator shaft

speed constant. This is consistent with increasing generator torque rating and the

corresponding electrical loading of the machine. This loading (electric current per

Design and Development of Megawatt Wind Turbines 237

unit surface area of the rotor − stator air gap) also tends to increase resulting in

increased torque density (Nm/kg) and hence the convex curve shape.

An alternative to this trend would be if the gear ratio were to be ﬁ xed instead of

varied. Generator rotational speeds would then decrease with increasing power

rating, which would increase the generator rated torque faster than the power rating.

For this scenario, the curve shape would tend away from the convex shape [ 31 ].

Figure 33 : Generator, converter and LV cable – 10-turbine analysis results.

Figure 34 : Generator mass – 10-turbine analysis compared to industry study set.

238 Wind Power Generation and Wind Turbine Design

The two shaded regions shown in Fig. 34 are for DD and geared (i.e. high speed)

type generators. The lower bound represents the expected mass assuming historical

technology progress with increasing MW rating, and the upper bound is for straight

scaling of today’s technologies. The shaded symbols at the 10 MW rating reﬂ ect

recent public announcements and are unproven at this time. With respect to Fig. 34 :

DD generators using today’s conventional wire-wound construction are more •

than ﬁ ve times heavier than the geared generator alone.

The linear ﬁ ts projected from industry mass data form the bottom edge of the •

shaded ranges. These mass trends should be achievable for continued technol-

ogy advancements with increased MW rating (i.e. new technology is needed to

avoid mass increase for straight scaling of today’s technology originally

developed at lower MW ratings).

When the mass for the gearbox is included with the geared generator (a •

fairer comparison), the DD and geared conﬁ guration have the same order of

magnitude mass.

The geared conﬁ guration (i.e. GB + Gen) is lighter than DD below 5 MW and •

heavier above 5 MW.

For a 10-MW size WT, an advanced DD using a projection of technology •

improvement from today’s wire wound know-how, is at ﬁ rst a reasonable choice

of about 150 tonnes. High temperature superconducting (HTS) DD generators

(shaded circular symbols) are reported to be on the order of 20% lighter and

advanced superconducting (shaded triangular symbols) are reported to be 50%

lighter still, bringing them to almost the same mass as the best high-speed

generator alone (i.e. without the GB accounted).

4.5.2 Turbine – electrical transformer

Should the electrical transformer be located up-tower or down-tower? The

up-tower advantage is higher voltage and lower current for a given power level

which translates into lower cost (i.e. smaller diameter) cable in the tower. The

downside is that a higher voltage level results in more stringent design and envi-

ronmental health and safety (EHS) requirements. Many turbines today employ

relatively LV (e.g. 575 − 690 V) between the generator and a MV transformer.

A down-tower transformer can be located inside the tower or installed on a con-

crete pad just outside the base of the tower. The vast majority of MW WTs installed

in the U.S. utilize the later design.

MV/LV transformers are a key component of wind plant. Traditionally WT and

MV/LV transformers have been protected using fused switches on and MV side and

circuit breakers on the LV side. This solution has worked well with rated

powers up to around 1.5 MW. WT ratings have grown well above 2 MW and WT

distribution voltages up to 36 kV, normal for offshore, are now gaining interest for

onshore application. Switch solutions have reached their limit for these higher volt-

age levels due to the nominal current ratings of the available fuse elements. There

are advantages for using circuit breakers for the protection of MV/LV transformers.

The use of modern digital protection relays associated with circuit breakers have

Design and Development of Megawatt Wind Turbines 239

advantages in terms of increased availability, more efﬁ cient maintenance and

reduced downtime.

Transformers used in wind farm applications have been observed to fail at a

higher rate (independent of manufacturer) compared to their use in other forms of

power systems applications [ 32 ]. Some of the failure mechanisms include tran-

sient voltages on the LV side of the transformer causing overvoltages and abrupt

loss of voltage quite regularly. Transients generate voltage surges in the MV wind-

ing leading to dielectric failures and thermal stress. Transformers subjected to con-

tinuous high power levels are often subjected to periods of overload due to wind

gusts. This overloading can cause premature failure of the transformer. Some

transformers are installed in the nacelle and therefore are subject to vibrations

from the WT operation that may not be properly accounted in their design.

4.5.3 Turbine – grounding, overvoltage and lightning protection

Diverting lightning currents and conveying the energy safely to ground are accom-

plished by a lightning protection system [ 33 ]. The coupling effects of the high

and extremely broadband frequency current in lightning are neutralized by means

of screening. Surge voltages occurring on electric equipment are neutralized by

means of lightning arrestors or surge arrestors.

Lightning receptors on a WT blade are intended to act as Franklin rods, but

sometimes fail to intercept lightning strikes with subsequent damage and expen-

sive repairs. When blade lightning receptors works as intended, but main shaft

grounding brushes are inadequate, the current ﬂ ow through main shaft bearings

can cause signiﬁ cant damage [ 34 ].

4.5.4 Turbine – aviation/ship obstruction lights

It is necessary to account for these starting in the preliminary design to ensure

no issues later on. Integrating these into the design is not trivial – poor planning

can lead to costly nuisance issues in the ﬁ eld (e.g. cracking of light brackets or

power supply mounts). These lights are required by most permitting authorities.

However, in many instances, they are not required at every individual turbine for

a given project.

4.5.5 Collection and delivery – WPP electrical balance of plant

Balance of plant (BOP) is deﬁ ned to include the equipment and construction engi-

neering beyond the WT itself (i.e. everything else beyond the typical OEM “scope

of supply”). The wind park developer or the WT equipment customer normally

supplies the BOP.

Companies are beginning to offer turnkey solutions. An example for the electri-

cal portion is PACS Industries [ 35 ], who offer “Wind to Wire” electrical systems

that are fully integrated electrical gear and enclosures with features and services

such as:

Tower switchgear through 38 kV •

Collector switchgear through 38 kV •

Arc resistant switchgear through 38 kV •

240 Wind Power Generation and Wind Turbine Design

Outdoor electrical buildings •

Structural substations through 345 kV •

Installation engineering supervision •

Full equipment integration engineering services •

4.6 Controls

WT control systems enable and facilitate smooth operation of the WT and the

overall WPP. At the turbine level, the control system is responsible for reacting to

changes in local wind speeds to generate the optimum level of power output with

the minimum amount of loads. At the operational level, control systems collect

data for automating decisions and for a number of downstream analyses. They also

support communication and information ﬂ ow throughout the turbine and power

plant components. At the power plant level, control systems integrate all of the

above with grid conditions. Control systems continue to rapidly evolve, resulting

in steady performance enhancements from existing turbine hardware. These same

advancements are providing new opportunities for future large turbine designs.

4.6.1 Turbine control

A WT control system is used to control and monitor the turbine sub-systems to

ensure the life of components and parts, their reliability and meet the functional

performance requirements. The fundamental design goals are safety, reliability,

performance and cost. The turbine must produce the energy advertized for the

wind conditions actually experienced – and do so for the life of the machine at the

cost provisioned in the customer pro forma.

Variable-speed pitch controlled WTs are the most advanced control architecture

and state of the art for today’s MW WTs (see WT Type C, Fig. 9 ). To operate a

WPP under optimum conditions, the individual turbine rotor-generator speeds are

controlled in accordance with the local wind speeds using generator torque (elec-

tric current control) or blade pitch angle adjustment. Figure 35 illustrates the main

considerations for a typical control scheme used in today’s MW WTs.

A turbine is in the standstill state with the high-speed rotor brake applied when •

the turbine is down for maintenance.

The typical turbine condition with little or no wind is the idling mode with the •

blades feathered and the rotor near standstill or gently pin-wheeling.

The spinning state can best be characterized as increasing winds starting from •

dead calm and approaching cut-in wind speed, while the blades are pitched to

an intermediate blade angle (see sketch /B/ of Fig. 17 ) so that the rotor can

proceed to accelerate during the run-up condition.

Figures 35 and 36 can be cross-referenced to better understand the continued

sequence of control steps for the example case of increasing wind speeds (e.g. the

approach and development of a storm front):

Once the cut-in rotor speed has been achieved (condition (1) of Fig. 36 ), the •

blade pitch angle is advanced to the full operational position (see sketch /C/

of Fig. 17 ).

Design and Development of Megawatt Wind Turbines 241

As a result of this transition, and with increasing rotor torque, condition (2) is •

achieved and the breakers close – bringing the converter online and power

production begins.

Now in region 2 (RII) of the power curve, increasing wind speed will advance •

the turbine towards conditions (3) and (4). The blades remain in the full opera-

tional position (ﬁ xed) throughout this period, as the increasing wind speed and

increasing rotor speed (RPM) move together to achieve near optimal angle of

attack at each radial position along the blades.

Figure 36 : Power curve and operating characteristics.

Figure 35: Generalized WT control diagram.

242 Wind Power Generation and Wind Turbine Design

As the turbine approaches rated power or condition (4), the blades are com-•

manded to begin to pitch slightly back towards feather to lower the angle of

attack and reduce peak loads. This prescribed schedule is also known as a

“peak shaver.”

Implementation of a peak shaver comes with a reduction in generator output, so •

that the turbine designer must balance the beneﬁ ts for reducing peak loads,

material savings for the affected components, and the lower energy yield.

From conditions (2) • − (3), a torque command regulates the RPM for the optimal

rotor tip speed ratio (TSR). The torque command is equal to a prescribed

function of RPM.

From conditions (3) • − (4), a torque command regulates RPM based on the

converter setting and the generator current.

From condition (4) • − (4”), where (4”) is the turbine shutdown or cut-out wind

speed, a current control maintains rated power output and the blades are pitched

more and more towards feather to regulate the rotor RPM. This pitching is done

to unload the blades with higher and higher wind speed to reduce WT loads by

shedding the excess power that would have otherwise been captured (with

massive loads to the turbine) beyond the generator rating.

A typical cut-out wind speed for a modern MW WT is 25 m/s. When the •

machine reaches (4”) and shuts down, the blades are pitched to the full-feathered

position and the nacelle continues to be yawed into the wind. This keeps the

turbine in a low drag conﬁ guration to ride out the storm.

Referencing a WT designed to IEC TC1 criteria (as an example), wind speed •

increases beyond cut-out are provisioned throughout the turbine structure for

survival wind speeds of 50 m/s (10-min average) and 70 m/s (3-s average).

This is equivalent to hurricane intensities in the border region of category

II − III and category IV − V in accordance with the Safﬁ r-Simpson [ 36 ] scale,

respectively.

Boes and Helbig [ 37 ] give an alternative description of this process, and is pro-

vided here for additional context – “There are two modes of operation: speed

control at partial load operation (control of torque) and speed control at full load

operation (pitch control). Torque control: To achieve the optimum power yield, the

speed at partial load is adjusted to obtain an optimum ratio between the rotor speed

at the circumference and the wind speed. The blades are set to the maximum pitch.

The counter-torque at the generator controls the speed. Pitch control: After reach-

ing the maximum counter torque at the generator (nominal power) at nominal

wind speed, the speed cannot be maintained at the operating point by further

increasing the generator torque. Thus changing the pitch from the optimum value

reduces the aerodynamic efﬁ ciency of the blades. After reaching the maximum

generator torque the blade pitch thus controls the speed.”

Future control systems for large WT are likely to involve real-time measured

signals in combination with physics-based control environments. Turbines would

sense the actual imposed and reacted conditions and feed-forward these into

smart proactive control actions. This type of approach should permit lowering

Design and Development of Megawatt Wind Turbines 243

design margins and enable designs to use less material throughout the structure

and components.

4.6.2 SCADA hardware and software

The SCADA (Supervisory Control and Data Acquisition) system is the main sys-

tem to operate a wind plant. Wind SCADA systems need several hardware compo-

nents such as servers, modems, storage devices, etc. This rack-mounted hardware

is typically located in the wind plant control room; however smaller wind projects

may utilize standalone PCs. It provides the communication network and protocol

for information ﬂ ow between all components of the wind plant. At its simplest

level, the SCADA network connects and controls the WT generators and enables

collection of production and maintenance data.

Receive data from individual turbines •

Send control signals to individual turbines •

Provide real-time data monitoring •

Alarm checking and recording •

Provide capability for historical data analysis •

Ability to model system (region-plant-cluster-unit) in hierarchy •

It is used for conﬁ guration and commissioning of turbines, operation of the

turbines, troubleshooting, and reporting. Commissioning technicians, service

technicians, operators, owners, engineers and other experts use it [60].

SCADA knows everything that is going on in the wind farm: how much each

turbine is producing, the temperature inside and outside of each turbine, wind direc-

tion, and if a turbine needs service or repair. It even records if lightning has struck a

turbine. In the event the SCADA system detects a problem, it shuts down the machine

or machines automatically and notiﬁ es the plant operator. Controllers inside the tur-

bines also maintain the power quality of the electric current generated by the WT.

4.6.3 WPP control system

The WPP control system takes it to another level by integrating real-time grid

conditions together with electricity production to ensure stable operation. Power

quality is the stability of frequency and voltage and lack of electrical noise being

supplied to the grid. The WPP control system monitor power production, and

aggregates across the power plant and control regions ensuring power quality. It is

not uncommon today to have very large parks in excess of 200 MW or more, and

the WPP control system makes this possible. Examples of large projects include:

Horse Hollow Wind Energy Centre – The world's largest WPP with a capacity 1.

of 735.5 MW. It consists of 291 GE1.5 MW and 130 Siemens 2.3 MW WTs

spread over nearly 190 km

(47,000 acres) of land in Taylor and Nolan County,

Texas, owned and operated by NextEra Energy Resources (part of the Florida

Power & Light (FPL) group) [ 38 ].

Titan wind project, a 5050 MW project announced for South Dakota 2.

consisting of 2020 Clipper 2.5 MW WTs [ 39 ].

244 Wind Power Generation and Wind Turbine Design

4.6.4 Grid transient response

WT grid transient response relates to the operation control and protection of the

transmission and distribution networks, interconnects, generators and loads. This

is a very broad topic that has a number of speciﬁ c points to consider from the

WTG perspective. These include the fundamental frequency response and protec-

tion of the transmission and distribution system to faults that are also based on grid

transients. The WT and the WPP must be able to absorb voltage spikes caused by

lightning or switching events. This is addressed with proper grounding, overvoltage

and lightning protection of the equipment.

4.7 Siting

There are many considerations when deciding where to site a WPP. Installation for

ﬂ at terrain will often provide more favourable wind conditions and project eco-

nomics, while irregular or difﬁ cult terrain has more difﬁ cult WT placement and

increased installation cost. Turbine local geotechnical conditions are key when

designing WT foundations. The amount of energy produced for a given project

site is vital, but in many cases, more important are the regional electricity demand

and pricing structures that will determine the overall WPP proﬁ tability. Land use,

environmental regulations and permitting is the key when siting the individual

WTs, and accounting for these requirements early in the new product development

cycle will avoid costly miss-steps during production application.

4.7.1 Site-speciﬁ c loads analysis

Turbines are often designed to one of the IEC TCs [ 13 ]. Speciﬁ c sites may require

more detailed analyses to ensure design adequacy. Some permitting agencies may

require analyses certiﬁ ed by a licensed professional engineer for a speciﬁ c turbine

applied to the actual site conditions.

4.7.2 Foundations

Foundations are a crucial integral part of the overall MW WT design. They must

account for the highly variable geotechnical conditions encountered in normal

practice without adding unnecessary base cost. Foundations affect the natural fre-

quency of the overall WT design. Many OEMs chose not to include the foundation

in their scope of supply, but this does not mean that the foundation design can be

ignored from the overall turbine design ( Figs 37 and 38).

Figure 39 aggregates the major components of WTs into a single average tech-

nology trend derived from the 10-turbine analysis group. Against this backdrop the

nominal foundation mass and combined WT and foundation mass trends are plot-

ted with increasing WT size. The mass for some known foundations track reason-

ably well with the calculated “foundation only” trend. For a 10 MW machine these

results project nearly 6000 tonnes for a monolithic foundation. However, a design

goal of 4000 tonnes or below should be possible, especially considering numbers

of smaller individual foundations supporting a multi-leg tower structure instead of

a monolithic foundation. From Fig. 39 , a good ﬁ rst estimate for the monolithic