Page 272 - Fluid Power Engineering

Deploying W i nd T urbines in Grid 239

lightning enters the main shaft, it finds a low impedance path. Most
modern turbines have carbon brushes to accomplish this. It avoids
passing lightning through the main bearings, which can cause severe
pitting in the rolling elements of the bearing. Since bearings are ex-
pensive to replace, alternate path are designed through the carbon
brushes. The height and shape of a turbine is such that not all light-
ning strikes start at the end of blades; side strikes to the blade and
strike to the tower are not uncommon. Although the blades are non-
conducting, water on the surface makes them conducting.
The second most common damage because of lightning is to the
control systems. This is primarily because the control systems are de-
signed for low voltages. High voltages and high currents enter the
control systems through the mechanisms described below; methods
to mitigate the damage are also described below: 16

Conduction primarily across improper insulation and small

air gaps. This can be minimized by designing a low-
impedance path for lightning current, proper insulation, and
good bonding methods. Good bonding ensures that all the
components and metal fixtures are at the same potential,
which eliminates jumping of lightning current.
Capacitive coupling occurs when lightning current carrying

conductors are in proximity to other conductors. This can be
minimized by separating the grounding cables from others
and shielding through metal enclosures.
Magnetic coupling is caused by high rate of change of current

flowing through conductors that cause strong and changing
magnetic field. This can induce damaging voltages into loops
of conductors. Metal enclosures, twisted cables, and avoiding
large loops can reduce magnetic coupling.

Transformers for Wind Applications
Step-up transformers in wind applications are different compared to
standard off-the-shelf transformers used for power applications. Sig-
nificant numbers of failures indicate that attention must be paid to the
uniqueness of the conditions in which wind application transformers
operate. The following are some of the differences: 17
Wind turbine step-up transformers (WTST) are subject to

loads that correspond to the average capacity factor of the
wind farm. That is, the transformer operates at 35 to 45% of
its rated capacity, on average. Traditional transformers are
subject to loads at the rated capacity or slightly higher. The
light loading leads to higher core losses and lower winding
losses.