Wednesday, May 13, 2009

IEC Classification of Turbines: Selecting the right turbine for the site based on wind data

The International Electrotechnical Commission (IEC) creates and publishes standards for wind turbines among other electrical and electronics equipments. The IEC 61400 deals with wind turbine generators (WTG). This blog entry will explain turbine classes. Turbine classes are determined by three parameters the average wind speed, extreme 50-year gust, and turbulence. The following table explains the classifications.



WTG Class

I

II

III

IV

Vave average wind speed at hub-height
(m/s)

10.0

8.5

7.5

6.0

V50 extreme 50-year gust (m/s)

70

59.5

52.5

42.0

I15 characteristic turbulence Class A

18%

I15 characteristic turbulence Class B

16%

α wind shear exponent

0.20


For standards purposes, wind speeds are measured every 3 seconds, and every 10 minutes wind speed and standard deviation are recorded. For design load calculations purposes the wind speed over 10 minutes is assumed to be a Rayleigh distribution.

All wind speeds in the above table are at hub height. The extreme wind speed are based on the 3 second average wind speed. I15 Turbulence is the standard deviation of wind speed measured at 15 m/s wind speed.

As an illustration consider GE 1.5sle, a Class IIA WTG and GE 1.5xle a Class IIIB WTG. The Class IIA WTG has a rotor diameter of 77m and hub heights of 65m and 80m. It is designed for average wind speed at hub height of 8.5 m/s with turbulence of 18%.

The Class IIIB WTG has a rotor diameter of 82.5m and hub height of 80m. Because the Class IIIB WTG is designed for lower wind speed (7.5 m/s at hub height) and lower turbulence (16%), the design loads are going to be smaller, therefore its blades are larger and hub height is taller. Bigger rotors of Class IIIB WTGs therefore capture more wind energy and yield higher capacity factors compared to Class I or II WTG.

In conclusion, a wind resource assessment that is based on onsite wind measurements can provide not only the annual average wind speed, but also provide turbulence and extreme wind conditions. This data is necessary to select the class of a turbine. Wind data that is typically used for prospecting like reanalysis data and 10m airport wind data do not provide information about turbulence.

Article written by Dr. Pramod Jain

Email Pramod at pramod@frombeginningtowind.com

Visit Wind Energy Consulting and Contracting Inc.

26 comments:

  1. Hi Mr Pramod,

    I have collected info from EWEA web site some time back say 3 years according to which the class of as per IEC are as below.

    Wind Turbine Class I II III
    V ref in m/s 50 42.5 37.5
    A I ref(-) 0.16
    B I ref(-) 0.14
    C I ref(-) 0.12

    V ref – Reference Wind Speed Average over 10 min.
    A, B & C – Categories for high, medium and low turbulence characteristics
    I ref – Expected value of Turbulence intensity at 15 m/s.

    Hence, please confirm which are the correct values as per IEC.

    Ramesh Kumar.M

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