Carsten Wagener
Phoenix Contact GmbH & Co. KG
Germany
Edward Doherty
Phoenix Contact USA
Harrisburg, Pa.
Wind energy has been growing at
a rate of 25 to 30% annually, with
installations in the U.S. now exceeding
10,000 MW in generation
capacity, according to the American
Wind Energy Association.
But a wind turbine typically measures
100-m high and sits out in
a field some distance away from
other tall objects. These conditions
make wind turbines tempting
targets for lightning strikes.
Operators of wind-turbine installations
(WTIs) of course want to
minimize downtime and repairs.
So well-designed surge protection
is a necessity for this equipment.
WTIs have a lot of technology
crammed into a small space. A
transformer station inside the
tower adapts the generator voltage
of 690 V to the grid operator’s
medium voltage, usually 3 to 6 kV.
The main switchgear elements
and a frequency converter that
synchronizes wind turbine output
to the 60-Hz system reside in the
low-voltage main distributor in
the base of the tower. This is also
where part of the control technology resides. The control technology
for sensors and actuators,
gear and generator monitoring,
and the motors for steering the
nacelle into the wind all reside in
the nacelle. In wind turbines that
have pitch systems, additional
control and motor technology
sits in the rotor hub for adjusting
the rotor blade angle.
Franklin-type lightning rods
protect WTIs against direct lightning
strikes. But complete lightning
protection in accordance
with IEC 61024-1 only comes
through the use of a lightning current
arrester. There are special
requirements for protecting the
power-supply system between
the generator and transformer
station at the 400/690-V level. It
consists of a Class One and Class
Two arrester wired in parallel.
A typical Class One arrester
using spark gap technology is
the Flashtrab+Ctrl. Its lightning
current carrying capacity (Iimp) is
50 kA (with a 10/350-μsec waveform)
per channel. These qualities
let the spark gap satisfy the
relevant requirements of the IEC
standard. It connects in parallel
with Class Two arresters based
on MOVs (metal-oxide varistors).
One such Class Two arrester is
called Valvetrab. Both the Class
One and Two components must
have an arrester-rated voltage (Uc) of 440 V.
These components function
on the principle of active energy
control. During a lightning strike
the Class Two arrester initially
handles all conduction because
of its rapid response time.
This device has a characteristic
curve that graphs conducted
current versus voltage. Designers
can use this information to determine
when the Class One arrester
should trigger so to keep from
exceeding the maximum permissible
energetic strain of the Class
Two device.
Thus once triggered, the Class
One arrester handles the massive
current from a lightning event,
preventing an overload of the
Class II arrester. This combination
and coordination of Class
One and Two arresters allows fast
response and high current handling
capability.
A Class II plug-in module can
be hot swapped when necessary.
A mobile testing device can be
used to log the status of the arrester
during maintenance work.
In some WTIs, surge voltage
arresters safeguard the I/O of
the measuring equipment leads
coming in from outside. These
carry signals that are important
for steering the blades into the
wind or for system start-up or
shutdown.
Typically the WTIs within a
wind park are networked to facilitate
the exchange of data. A master
system captures fault and status
reports and transmits these
to a main control center. The data
communications interfaces to the
telecommunications system are
equipped with pluggable surge
voltage protection devices.
For further reference:
IEC 61024-1-1:1993-09: Protection of
structures against lightning – Part 1:
General principles – Section 1: Guide
A: Selection of protection levels for
lightning-protection systems
IEC 61312-1:2002-06: Protection
against lightning electromagnetic
impulse – Part 1: General principles
IEC 61400-24: Wind-turbine generator
system – Part 24: Lightning protection
IEC 37A/139A/CDV IEC 61643-1/A2-
f2:2003-06: Surge-protective devices
connected to low-voltage power
distribution systems – Performance
requirements and testing methods