Grid stability with renewables? Yes, Wind can!

Windflow’s power-train and its synchronous generator provide physical inertia for grid stability.

As renewable energy is added to modern grid systems, which have traditionally relied on large spinning turbines with directly grid-connected synchronous generators1) from hydro, coal and thermal power stations contributing physical inertia2) to the grid, the requirement for an inertial response from renewables has been brought into sharp focus.  This has been highlighted by the experience in the South Australian blackout during 2016, and is an issue under discussion by grid operators and regulators internationally.  Most countries view their fastest and easiest renewable addition to be solar, however over-reliance will reduce grid stability in case of any shocks to the grid.

Windflow Technology Ltd (WTL), New Zealand's only designer & manufacturer of wind turbines, announces the licensing opportunity for their synchronous generating power-train technology.

A primary advantage that synchronous generators deliver is physical inertia which instantly resists speed changes in a grid shock, such as the failure of a large power plant or a transmission line.  Generator speed determines grid frequency, and vice versa for all the generators on a synchronous grid.  The larger the shock or the lower the inertia, the faster the rate of change of frequency.  Therefore grids need inertia for stability.

What does physical inertia2) do?  It enables grid frequency to be stable.  Any device or system with physical inertia has the momentum to “ride through the bumps”, without slackening speed.  By contrast, controlling a system with little inertia involves working very fast with the “gas pedal” to alternatively squirt in more power, or reduce the amount squirted in, depending on whether the system has slowed down or sped up.  In an electricity system, inertia normally comes from synchronous generators, which inherently (through electro-magnetic reaction) combine their physical inertia in an instantaneously co-ordinated way, even when hundreds of kilometres apart.  But most wind turbines’ generators are not synchronous like this, so the increase of renewable energy is causing system inertia to reduce.  With a low inertia electricity system, multiple generators would have to “work the gas pedal” in an instantaneously co-ordinated way, to maintain a steady frequency.  This is unproven with current power electronic converter (PEC) wind turbine technology, which has zero inertia connected to the grid and has a good chance of undershooting, overshooting, or getting out of rhythm and losing control, causing blackouts.  By contrast, Windflow’s system enables wind turbines to drive synchronous generators.

Most wind turbine systems provide asynchronously generated energy which contribute no inertia because their spinning masses are decoupled electronically from the grid.  While asynchronous wind turbines can in principle provide Fast Frequency Response “FFR” (sometimes referred to as "synthetic inertia", see inertia2) below) to respond to grid shocks via their power electronics, this relies on a mixture of power stored in the flywheel energy of the wind turbine and any ability to increase output power (for example if the turbine is running significantly derated).  Solar panels are less able to provide FFR due to the lack of a spinning mass and can only provide it if they are running significantly derated.

Windflow’s power-train enables its synchronous generator to be directly connected to the grid, providing inertia for grid stability the same as traditional power generators, without power electronics.  This delivers direct resistance to frequency change as opposed to only providing the proposed response of FFR that most existing wind turbine systems expect to offer.

Windflow’s system has been IEC certified by Lloyds Register and has a track-record of more than 600 turbine-years of synchronous wind power operation.  The cost-effectiveness and track record of Windflow’s system differentiate it from a small number of other synchronous wind turbine systems that have been tried.  It enables FFR using the wind turbine rotor’s flywheel energy (and any reserve power if the turbine is running significantly derated) as well as the direct generator inertia.  It also provides the full range of attributes that synchronous generators provide to the grid:

  • Voltage support through a full range of reactive power capabilities.
  • System strength by providing huge “short-circuit currents” (5-10 times rated) to rectify voltage faults.3)
  • Synchronous condenser mode even with no wind.4)
  • Controllable power level and ramp rates.

Windflow welcomes the Media Release: Blueprint for a world-class electricity system, 09 June 2017 by the Australian Government’s Chief Scientist, Dr. Alan Finkel, of his final report Independent Review into the Future Security of the National Electricity Market - Blueprint for the Future (PDF, 2.96 MB) recommending a range of changes to the Australian electricity market to improve system security.  These Recommendations (PDF, 51 KB) include requirements for minimum amounts of inertia and all of the above attributes that synchronous generators have traditionally provided, and Windflow’s system provides within the wind industry.

Windflow’s system has recently been extended with a new patented system to enable broad-band variable-speed operation which wind turbines require in lower wind climates.  Windflow CEO Geoff Henderson stated “Our patent protected technology, which is scalable to both the mid-size and multi-MW turbine market, eliminates use of power electronics and results in significant noise, weight and cost reductions. Further, and as evidenced by demand for renewable energy to assist grid stability, Windflow’s proven designs and technology are an idea whose time has come”.  A cursory examination on grid stability makes it abundantly clear that there is market confusion on these issues.  Difficulties are now beginning to be apparent in China, EU, Australia and the Pacific Islands which are investing heavily in renewables including solar.

As the world moves to renewable energy, it will become an issue for all nations as the 21st century progresses.

Windflow is actively seeking partnerships with wind turbine manufacturers who are at the forefront of wind energy and interested to license Windflow’s proven power-train technology.  Please see our Windflow Synchronous Power-Train Licensing Brochure.

In addition, please see the paper Field Experience with Synchronous Wind Turbines in New Zealand and Scotland by Geoff Henderson, presented at the “16th International Workshop on Large-Scale Integration of Wind Power” 25 - 27 October 2017 in Berlin (http://windintegrationworkshop.org).

Windflow is also pleased to communicate directly with grid operators, consultants, academics, journalists and other parties interested in issues in grid stability.  For further information please contact info@windflow.co.nz.

Author: Geoff Henderson - Date: 4.8.2017 - Updated: 14.12.2017

1)   Synchronous generator:   The term “synchronous generator” in this article refers to synchronous generators which are directly connected to the grid (i.e. hydro and thermal plants as well as Windflow’s turbines).  Please note, that a few competitors’ wind turbines use synchronous generators, which are connected to the grid via power electronic converters/inverters (PEC), therefore can not provide physical inertia to stabilize the grid.
      In other publications, the terms “synchronous machines” or “synchronous turbines” might be used instead of the term “synchronous generators”. 
2)   Inertia:   The term “inertia” in this article refers to “physical inertia”, which is the inertia provided by the spinning mass of generators, which are directly connected to the grid, therefore inherently help to stabilize the grid (i.e. hydro and thermal plants as well as Windflow’s turbines).  Most competitor’s wind turbines use generators, which are connected to the grid via power electronic converters/inverters (PEC) and do not provide physical inertia.
      Inertia should not be confused with “synthetic inertia” which is fundamentally different (and also relatively untested), being a form of fast frequency response (FFR), which is essentially a governor response (involving measure and control steps which take time) rather than an inertial reaction that slows the rate of change of frequency, providing time for a governor response to stabilize the grid.
      “Physical inertia from synchronous machines plays an important role in slowing the rate of change of frequency when there is a mismatch between supply and demand, allowing time for frequency control mechanisms to respond.”  (Australian Chief Scientist, Dr. Alan Finkel, “Independent Review into the Future Security of the National Electricity Market - Blueprint for the Future”, June 2017)
      In other publications, the terms “mechanical inertia”, “rotational inertia” or “synchronous inertia” might be used instead of the term “physical inertia”.
3)   Windflow’s system strength:   In comparison to Windflow’s system strength by providing huge “short-circuit currents” (5-10 times rated) to rectify voltage faults, other wind turbines with power electronic converters/inverters (PEC) may provide “short-circuit currents” of only about up to 1.5 times rated, due to current limitations of transistors.
      In other publications, the terms “fault current” might be used instead of the term “short-circuit current”.
4)   Windflow’s synchronous condenser mode:   For example, Windflow’s synchronous condenser mode provided reactive power which generated up to 1/3 of the revenue generated by the Gebbies Pass turbine (due to incentives offered in 2003 for reactive power by the network operator) in New Zealand.  Windflow’s synchronous wind turbine can also import reactive power to reduce line voltage, as in in 2006/2007, when 5 turbines imported kVArs up to 130% of the turbine rating at the Te Rere Hau wind farm to control voltage on an 11 kV line.  (“Field Experience with Synchronous Wind Turbines in New Zealand and Scotland”, Windflow Technology Limited, Geoff Henderson, 27.10.2017, Berlin).