Commercial Wind Turbine Systems and Applications - Wind Turbine Technology: Fundamental Concepts of Wind Turbine Engineering - page 210

Medium-Scale Horizontal-Axis Wind Turbines

By the guidelines in Table 4-, medium-scale wind turbines are those with rotor di-
ameters between 2 and 45 m and/or power ratings between 40 and 999 kW. There are, of
course, many similarities between large-scale and medium-scale machines, since the dividing
line separating them is somewhat arbitrary. Also, they share a common technology base. In
general, more design variations in the overall system, as well as subsystems and components,
are found in medium-scale turbines. These include the number of blades, construction ma-
terials, types of power control, and drive train design. Table 4-4 contains a summary of the
characteristics of representative commercial medium-scale wind turbines.

Table 4-4. Configurations of Representative Medium-Scale HAWTs

Manufacturer EWT Goldwind Vergnet Windflow

Model Direct Wind 750 750 GEV MP 250 500

Rotor diam. 5.5 m 49 m 32 m 33.2 m

Rated power 750 kW 750 kW 275 kW 520 kW

Rotor location Upwind Upwind Downwind Upwind

No. of blades 3 3 2-teetered 2 - teetered

Blade material Fiberglass Fiberglass Fiberglass Wood/epoxy

Power control Full blade pitch Passive stall

(fixed pitch)

Full blade pitch Full blade pitch

Braking Mechanical 2 Mechanical

disks

Mechanical disk Mechanical disk

Overspeed Aerodynamic

pitching

Aerodynamic tip
pitching

Aerodynamic
pitching

Aerodynamic
pitching

Gearbox None Planetary, spur Planetary Planetary, parallel

(no. of stages) 0 2 2 4

Generator Direct drive

synchronous

Asynchronous 2 speed

asynchronous

Synchronous

Voltage, volts 690 690 690 45

Yawing system Active, electric Active, electric Passive Active, hydraulic

Tower type Tubular steel Tubular steel Guided/tilting

tubular steel

Tubular steel

The majority of medium-scale wind turbines presently feature a rotor located upwind
of the support tower. Two or, more often, three blades are used with full-span blade pitch
control, although stall-control, fixed-pitch systems also exist. Generator types include syn-
chronous, asynchronous, two-speed, and direct drive. Tubular steel towers dominate and
most have active yaw systems. The EWT Direct Wind 750 , shown in Figure 4-3 does not
have a gearbox but instead uses a direct-drive synchronous generator. The Vergnet 250
turbine shown in Figure 4-4 emphasizes reliability and survivability in hurricane zones. It
uses a guided tubular steel tower that can be lowered when the power station is threatened by
severe weather conditions.

Trends in Design of Medium-Scale HAWTs

Compared to multi-megawatt turbines, medium-scale machines offer potential econo-
mies associated with lower rotor costs per unit of swept area as well as easier transportation
and installation. The overriding advantage of medium-sized machines is lower development