It's not a matter of "if" but of “when” a wind turbine will see overvoltages and currents that must get bypassed to ground.
Carsten Wagener, Phoenix Contact GmbH & Co. KG
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