Advancements & Future Trends in Protection of Wind Energy

Authors: Mital Kanabar, Jorge Cardenas and David Menéndez, GE Digital Energy, USA

Worldwide wind energy capacity has reached 336,327 MW by the end of 2014, out of which 17,613 MW were added in the first six months of 2014. Total wind energy generated of approximately 640 TWh, which represents 4% share of the global electricity demand. With high penetration of wind energy in different types/configurations of power electronic converters, protection engineers have been facing challenges due to fault current contribution by the wind generators. Various international technical committees/working groups are focusing on detailed modeling of wind turbine generator systems of different types for transients and dynamic stability studies; short-circuit current contributions; advanced protection functions; robust & easy to deploy communication systems; and ancillary services from wind farms.

Wind Turbine (Drive Train) Mechanical Protections & Condition Monitoring
Figure 2 shows a typical wind turbine drive train inside the nacelle. Drive train consists of: Hub, main bearing, main shaft, gearbox, brake, high-speed shaft and the generator. A wind turbine gearbox should be robust enough to handle the frequent changes in torque caused by changes in the wind speed. In direct drive wind turbines the gearbox does not exists as direct drive system connects the rotor directly to a permanent-magnet generator. Turbine side - advanced protection & condition monitoring:

  • Torque-limiting device is a mechanical slip device to provide a maximum limit for the forward & reverse torques, which is mechanical side protection against generator short circuits, emergency stops and other transient load events
  • Accelerometers are mounted on the gearbox and generators casings to continuously monitor vibration signature at each location. Nacelle accelerators mounted in the transverse and fore-aft directions can detect rotor mass imbalance and blade pitch errors
  • Measurements of lubricant oil pressure and debris concentrations to ensure proper lubrication and wear failure
  • Bearing resistance monitored by measuring the motor current in the case of electrical pitch drives
  • An encoder or inductive proximity sensors to measure the rotor speed in order to facilitate over-speed alarm or trip
  • Aerodynamic parking brake system is used as a backup system once the turbine is stall/stopped

Other mechanical and electrical protection systems (i.e. corrosion, lightning protection, etc.) are also part of wind turbines.

Advancements in Wind Generator Protection
A wind turbine generator is normally an induction generator with a power electronic converter and associated control system. Some protection functions, which may be supported by converter control system, are:

  • Over/Under voltage (27/59)
  • Over/Under frequency (81O/U)
  • Current limiting function, while controlling active and reactive power

In addition, a dedicated digital multi-functional wind generator protection relay is provided for advanced protections & enhanced co-ordination. Asynchronous wind generators operate over the wider frequency range (typical slip 1% to 10%), and hence digital wind generator protection relay should allow wide range of frequency measurement 2Hz - 90 Hz, with a frequency tracking up to 70 Hz.

Advanced time over current elements can be applied for enhanced co-ordination with a transformer fuse and short length cable capability curve (e.g. phase, neutral and negative sequence elements 51P/ N/_2 and 50P/N/_2). Customized/editable inverse curves are available in modern relays with 80 operating points (between 1.03-20 pu) and 40 reset points (0.1-0.98 pu) as shown in Figure 4. Time curve can be combination of very inverse to extremely inverse and then to definite-time for effective coordination with fuse and cable capability curve. In addition to fundamental phasor, RMS based Time Over Current (TOC) may be preferred in case presence of significant harmonics in the wind farm.

Asynchronous wind turbine generator capability curves are different from the conventional synchronous generator and should be obtained from the wind turbine-generator manufacturer. On the other hand, some grid codes specify the requirements for operating ranges. Wind generator relay may provide flexibility of multiple-stages of pick-ups and time delays for various over/under frequency and voltage; as well as sensitive (active and reactive) power directional.  
Other advanced wind generator protections may include: phase reversal, low power factor, loss of reactive power support, breaker failure, VT Fuse Fail, etc.

Non-electric protection functions can also be achieved within the same wind generator protection relay with analog (dcmA) measured quantity from specific application based transducers (vibration or hall-effect sensors) in addition to RTDs for thermal protection. User defined thresholds can be applied to the comparator (over/under) for achieving unconventional protection functions, e.g. bearing over temperature, bearing vibration, Tachometer over-speed, etc. Different setting groups in a digital protection relay can be used to change multiple protection elements based on remote signal or user-customized logic (e.g. circuit reconfiguration).

Usually, the unit step-up transformer has a fuse at the MV side. However, for larger transformers (>1MVA) or when transformer is mounted in the nacelle with an integrated circuit breaker, dedicated transformer protection functions, such as differential and phase/ground time-overcurrent are also applied.

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