MONITORING STATION INSTRUMENTATION AND MEASUREMENTS - Wind Resource Assessment: A Practical Guide to Developing a Wind Project - page 55

environments, resistance to saltwater exposure is important. Guy wires should be
secured with anchors that match the site's soil conditions (the vendor and installer
should advise on this), and all ground-level components should be clearly marked to
prevent accidents. Protection from lightning (with instruments including lightning rod,
cable, and grounding rod) is a must. In some locations, security measures to prevent
vandalism, theft, and unauthorized climbing may be required, as well as protection
against cattle or other large animals.
Further information about tower installation is provided in Chapter 5, and the
special requirements of offshore towers are discussed in Chapter 14.
4.8.2 Sensor Support Hardware
Anemometers and direction vanes are most often mounted on booms that extend
horizontally from the side of the tower. Near the end of the boom is attached a short
vertical mast, or pillar, to hold the sensor above the boom. The boom and pillar should
be long enough that the influence of both the tower and the boom on the speed and
direction readings is kept small, except when the sensors are directly downwind of
the tower. Chapter 5 provides guidelines for achieving this goal.
Anemometers can also be mounted vertically off the top of the tower. This has
the advantage of eliminating the problem of tower wake (or shadow), although care
must still be taken to use a long-enough vertical boom to minimize the influence of
the tower top on the observed wind speed. Since vertically mounted anemometers
experience different tower effects from horizontally mounted anemometers, the
vertically mounted configuration is not suitable for measuring wind shear. One
application of this configuration is turbine power performance testing, for which a
pair of hub-height vertically mounted anemometers is commonly deployed.
A further consideration is that the boom and vertical mounting pillar must be fixed
in such a way that the anemometer shaft is precisely vertical (for cup and sonic
anemometers and direction vanes) or precisely horizontal (for prop-vane anemome-
ters). A tilted or skewed boom or mast can result in an incorrect measurement of the
free-stream wind speed or direction.
It is advisable to use sensor support hardware that is able to withstand the same wind
and ice loading extremes as the tower and is not prone to wind-induced vibration. The
hardware should be protected against corrosion, especially in coastal environments.
Drainage holes in sensor housings need to be kept open to prevent water accumula-
tion and expansion in freezing conditions. Tubular (hollow) sensor booms and pillars
should be used instead of solid stock.
Guidance regarding instrument placement, including the heights of sensors and ori-
entation of booms with respect to prevailing wind directions, is provided in Chapter 5.
4.9 WIRING
The following are the guidelines for selecting the proper electrical wire or cable type:
• Use the proper class wire for the voltage and sensor type