On June 16, 2016, the Federal Energy Regulation Commission issued a final ruling “FERC 827” eliminating a previous exemption for wind generators from the requirement to provide reactive power as required by traditional power
generation assets.
You’ve probably all seen the Power Factor/Cappuccino Analogy. In simple terms you could say Wind Power until now has been allowed to operate on grids without the requirement to inject reactive power to reach appropriate PF, i.e. with more froth and less “active” cappuccino. Any good cappuccino, as the one illustrated, has froth. However, that is not what you’re buying, what you want is the active/functional part. When the grid’s reactive portion is getting too big, the consumer is not getting the full value. When there is too much froth, the full value is not provided. The solution is to compensate with Capacitive or Inductive reactive power to keep the Power Factor close to unity, typically between 0.95 leading and 0.95 lagging. For synchronous generators used in traditional power plants one adjust the generator’s excitation level to reach the required power factor. So far, wind generators have been exempt from this requirement.
As of last October, The FERC 827 ruling has been in effect which requires Wind Farms to provide dynamic reactive power within the range of 0.95 leading to 0.95 lagging at the high-side of the generator substation. The impact of this will depend on the type of Wind Turbine Generator used, e.g. Induction, Doubly Fed Induction Generator (DFIG), or Permanent Magnet with Full Power Converter, as well as always on site specific grid conditions.
Some of the key issues here are “dynamic” and “at the high-side of the generator substation”. As dynamic compensation is required, this cannot be completely addressed with Cap banks. In addition, Cap banks can only provide capacitive reactive power compensation. At the high end of the generator substation means that DFIGs with only a partial converter have a lesser capability to address this requirement.
As a consequence one of the three traditional methods of dynamic Power Factor compensations will be required in many wind farms.
Synchronous Condenser – A rotating synchronous machine providing spinning reserve and excellent voltage support in cases of 3 phase to ground faults in stiff grids
SVC – A Static VAr Compensation thyristor based device used for transmission lines and industrial applications such as electric arc furnaces creating extensive flicker. The SVC require additional filtering as the thyristor commutation causes harmonics.
Statcom – A Static Synchronous Compensator IGBT based Pulse Width Modulated device offering the fastest response/correction times, the smallest footprint, high efficiency and low harmonics. While there is no definite size limit a Statcom is typically the most cost effective choice up to about 50 MVAr.
In conclusion, Statcom is likely going to be the favored choice for most wind farms required to comply with FERC 827. For more details on the FERC 827 ruling and the actions wind developers and plant operators can take please see ABB’s latest white paper “New rules for interconnecting renewables: FERC 827 and the solutions to enable compliance”
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