Design and development of megawatt wind turbines - Wind Power Generation and Wind Turbine Design - page 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