With a global compound annual growth rate
of 32 percent over the past five years, wind power is the
world’s fastest growing energy source, and wind turbines are
currently installed or being installed in more than thirty
states nationwide.
The American Wind Energy Association
(AWEA) projects that more than 6,000 MW of wind energy
generating capacity will have been installed in the
United States by the end of 2003, which can produce
enough electricity for 1.5 million U.S. households.1 Applications include utility-scale central generation,
grid-interconnected, and separately derived systems
through distributed generation, battery charging, and
direct-drive water pumping for irrigation systems. With
this increased use of wind energy products comes the
increased probability that sooner, rather than later,
authorities having jurisdiction will be faced with the
responsibility of inspecting a wind turbine
installation.
The following article will provide
authorities with information regarding the construction,
ratings, markings and safety certification of wind
turbines, along with the knowledge of how UL
certification services for wind turbines can facilitate
the inspection of these machines.
The
Basics
At
a fundamental electrical level, a wind turbine is no
different than a reciprocating engine generator set. A
fuel source (wind) is converted into mechanical energy
(turning the rotor, low-speed shaft, gear box and
high-speed shaft), which in turn is converted into
electrical energy (via the generator). What sets a wind
turbine apart from a diesel generator set is the fact
that all of the machinery and most of the electrical
equipment are located on top of the tower, which in the
case of the largest available turbines today, can be as
high as 110 meters (approximately 360 feet).
Photo
1
However, there are a number of
distinct characteristics of wind turbines that bear
mentioning here, as they relate to the unique
terminology used in the wind industry and the
determination of a suitable installation. The following
definitions are taken from IEC 61400-1, the Standard
Wind Turbine Generator Systems, Part 1: Safety
Requirements, 2nd edition, dated 1999-02.
Photo
2
Nacelle is the housing that
contains the drive-train and other elements on top of a
horizontal axis wind turbine tower. Refer to figure 1
for a simplified wind turbine nacelle configuration.
Swept area is the projected area
perpendicular to the wind direction that a rotor will
describe during one complete rotation.
Reference wind speed (Vref)
is the basic parameter for wind speed used for defining
IEC wind turbine classes. A turbine designed for a wind
turbine class with a reference wind speed Vref is designed to withstand climates for which the extreme
10 min. average wind speed with a recurrence period of
fifty years at turbine hub-height is lower than or equal
to Vref.
Extreme wind speed is the highest
average wind speed, averaged over t seconds, that
is likely to be experienced within a specified time
period of N years ("recurrence period": N
years). The IEC standards use recurrence periods of N =
50 years and N = 1 year and averaging time intervals of t = 3 s and t = 10 min. The turbine is designed
using extreme wind speeds for design mechanical load
cases. The actual measured average wind speed at the
specific installation site must be less than the design
extreme wind speed to ensure a suitable turbine
installation.
IEC wind turbine classes are
standardized wind regime parameters segmented by wind
speed and turbulence values. Refer to table 1 for
details.
Wind turbine designs that do not
conform to a standardized class are considered special
designs and are marked as Class S. The wind regime
parameters for which a Class S turbine is designed must
be specified by the turbine designer and marked on the
product by the manufacturer.
Figure
1
UL classified wind turbines are
required to be marked with the following information:
• Reference wind speed, Vref
• Hub-height operating wind speed
range, Vin – Vout (these are the
lowest mean wind speeds at hub-height at which the wind
turbine starts to produce power and highest mean wind
speed at hub-height at which the wind turbine is
designed to produce power respectively)
• Operating ambient temperature
range
• IEC wind turbine class
• Rated voltage at the wind turbine
terminals
• Frequency at the wind turbine
terminals or frequency range in the case that the
nominal variation is greater than 2 percent.
Table
1
This information, coupled with the
complete product certification information, should
enable the authority having jurisdiction to verify a
suitable installation of the wind turbine with respect
to mechanical, electrical and personnel safety hazards.
While, a number of country specific
technical product standards for wind turbines exist,
there is currently no US National Standard for these
products. UL is currently working with AWEA to publish
US National Standards for wind turbines. Underwriters
Laboratories Inc. (UL) has developed a certification
program for wind turbines that is rooted in the
international IEC 61400 Series of Standards for Wind
Turbines, but incorporates site-specific technical
requirements that are dictated by the model U.S.
installation codes, such as NFPA 70, the National
Electrical Code (NEC). The IEC 61400 series
of standards contain detailed mechanical and electrical
safety construction requirements and safety and
performance testing for wind turbines.
UL currently classifies these wind
turbines for compliance with the IEC 61400 series of
standards. In addition to meeting the product
requirements set forth in the IEC standards, UL
evaluates the influence of the model electrical and
building codes, as well as the relevant OSHA personnel
safety codes, in the product evaluation. Detailed review
of the manufacturer’s installation, operation and
maintenance instructions is also included in the UL
classification evaluation. The end result is a certified
product that can be installed in accordance with
applicable model codes, i.e., no different than any
other type of permanently installed equipment certified
by UL.
There are two distinct types of wind
turbines: large, utility-scale turbines and small,
distributed generation turbines. All wind turbines
consist of a foundation, tower, and nacelle (containing
the generator, rotor and blades). Interconnected wind
turbines rely on an external inverter for safe
connection to the utility grid. For more information on
inverter safety and performance, refer to the article
titled "Utility Interactive Inverters—Key to
Unlocking Alternative Energy’s Future" in this
issue of IAEI News.
Small
Wind Turbines
Small
wind turbines are currently defined in IEC 61400-2 as
having swept areas less than 40m2, and are
typically installed alone or in a hybrid system with
solar panels or small diesel generators, and are owned
by private businesses or homeowners.
UL classifies these turbines under
the product category Wind Turbine Generating Systems,
Small (ZGYW). UL’s classification program for small
wind turbines uses a combination of the Standard for
Safety of Small Wind Turbines, IEC 61400-2, and the
relevant UL product standards for components of the
turbine. Consideration of the installation codes,
especially the NEC, is also included in the
certification process.
IEC 61400-2 contains requirements for
the electrical and mechanical safety of small wind
turbines. As with many products, the electrical
evaluation focuses on reducing electrical, fire and
shock hazards. The mechanical evaluation focuses on the
wind turbine’s ability to withstand the mechanical
stresses inherent in its operation. Turbines are rated
not only in power output (volts, amps or wattage) but
also with wind related ratings. Operational ratings
include cut-in and cut-out wind speeds, and extreme wind
speed ratings. This means that, in addition to proper
electrical installation (e.g., the output wiring is
sized correctly for the intended current), an AHJ will
need to confirm that the wind turbine is also installed
in a location with a wind resource that does not exceed
the design limits of the turbine. The wind resource is a
meteorological measurement of actual wind speeds
averaged over small time frames (typically 10 minutes)
and wind directions. Maximum wind speeds are also
captured for time periods as short as one second.
When an AHJ is asked to approve a
wind turbine installation they should consider (in
addition to the normal electrical and structural
installation requirements) both the average and 50-year
extreme wind speeds for the specific site. This
information should be readily available from the site
developer or turbine installer.
Large
Wind Turbines
Very
few permanently installed electrical products afford us
the chance to walk inside of the product enclosure, but
wind turbines not only afford us this opportunity, but
also require personnel to enter the product for
installation, operation, maintenance and servicing.
Large wind turbines are currently
defined in the IEC 61400 Series of Safety and
Performance Standards as having a swept area of greater
than 40m2, are typically installed in groups
called wind farms, and are owned and operated by
electrical utilities. Some of the larger wind turbines
have electrical lifts that can carry personnel to the
top of the tower, but most are still only provided with
a ladder for the service crew to ascend.
Since the physical size of these
products enables entry into the product, UL has
incorporated many of the NEC requirements for
electrical installations into the certification
evaluation. For example, although no working space
requirements are specified in IEC 61400-1, the
requirements specified in NEC 110.26 are verified
in the nacelle prior to extending UL certification for a
large wind turbine.
UL classifies large wind turbines
under the product category Wind Turbine Generating
Systems, Large (ZGYZ). UL also classifies sub assemblies
such as control panels under the product category Wind
Turbine Generating Systems Subassemblies (ZGZJ). The UL
guide information for these categories can be located in
the General Information for Electrical Construction
Equipment Directory (the White Book). The UL guide
information and classifications can also be located in
the UL Electrical Construction Materials Directory (the
Green Book) as well as online at www.ul.com/database and
entering the four-letter category code at the category
code search.
Summary
On
the surface, the inspection of a wind turbine can appear
to be a unique and vastly different task than that faced
by authorities having jurisdiction every day. But if you
delve deeper into the situation, the uniqueness of
inspecting a wind turbine diminishes quickly, with the
main difference between wind turbines and traditional
power generation products being the need to verify
suitability of the wind turbine’s design limits with
the installation site’s environmental characteristics.
UL certification services are
available for wind turbines, like most electrical
products. Certified small wind turbines are commercially
available and their installation is much the same as any
permanently installed electrical product. Large wind
turbines, due to their size and the ability for
personnel entry, may appear to be different, but they
too are subject to certification, rendering them no more
than a very large permanently installed product.
So the next, or first, time you are
faced with inspecting a wind turbine, look for the UL
mark signifying the product’s compliance with
international safety standards, and rest assured that
the partnership you have come to expect from UL also
applies to these new products.
To make electrical inspections on
wind turbines and all distributed generation equipment
easier for AHJs UL has included a list of all
distributed generation (DG) product categories in the
front of the General Information for Electrical
Equipment Directory (the White Book) on page xix. Also,
for more information on wind turbines and distributed
generation equipment in general, be sure to visit us on
the web at http://www.ul.com/dge/ .
Footnotes
1 American Wind Energy
Association, "Wind Energy Outlook 2003"
Bill Colavecchio has 16 years of
safety engineering experience as an employee of
Underwriters Laboratories Inc., working primarily in
power generation and power distribution equipment. In
2003, Colavecchio was appointed general manager of UL’s
Renewable Energy Strategic Business Unit and has been
instrumental in establishing safety certification and
performance verification programs for wind turbines, as
well as supporting photovoltaic and fuel cell technology
certification program development. Colavecchio has a
bachelor’s degree in electrical engineering from Duke
University
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