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Riding out the gridlock

The rapid expansion of wind energy is putting a severe strain on the electricity distribution networks. Wind turbines have to stay grid-compatible

There are dates you never forget – like 14 August 2003. Shortly after 4 p.m., the electricity grid in the US north- east and the adjoining Canadian province of Ontario collapsed entirely. The blackout, which went down in history as one of the most extensive ever to occur in the United States, cut off the power to 50 million people.

 

As is true for many countries around the world, the energy supply in the United States is faced with a major transformation. Wind energy is increasingly beginning to replace the conventional power generation technologies, with the installed wind power capacity already exceeding 40 gigawatts. The utilisation of wind energy requires that wind turbines operate in a gridcompatible manner and that they are able to deal with grid faults.

 

GL Renewables Certification has now revised its “GL Wind Technical Note 065” on the grid compatibility of electrical power generation systems – which includes wind turbines – and adapted its technical requirements to reflect the needs of the North American market. The design criteria, tests and certifications can therefore be applied to 60-hertz technology.

 

Balancing the Grid

“Even if widespread grid failures such as the 2003 blackout are absolute rarities, the grid operator must be able to rely on the wind turbines to get the grid up and humming again in the event of a problem,” says grid expert Tobias Gehlhaar of GL Renewables Certification. “The grid operators keep generation capacity in reserve to prevent a blackout. And if a grid fault does arise, the wind turbines must carry on running – called riding through the lowvoltage fault – and must supply power to stabilise the grid during the fault and again after the fault”.

 

To keep a grid operating robustly, the dispatchers in the grid control centres have to keep the system frequency within a narrow band. In normal cases, it is sufficient to connect or disconnect electrical consumers, hence balancing out the operational parameters. It must be possible to control wind farms to meet the grid requirements – i.e. reduce their output when the frequency is too high.

 

Both external and internal factors can upset the balance. If the grid should happen to collapse, modern wind turbines must remain online and able to deliver active current immediately, which was not a requirement in previous years. “Some grid operators also require that the turbines be able to deliver reactive power,” says Gehlhaar.

 

The requirements set by the various grid operators with respect to the grid compatibility of wind turbines tend to differ in terms of the details. This turns the certification of wind turbines in the United States into an extremely demanding task. “The situation in the United States is certainly complicated,” agrees Charlie Smith, CEO of the Utility Wind Integration Group (UWIG).

 

This interest group says that there are as many as 178 grid operators in the U.S.A. Requirements for the grid technology have thus far only been formulated in FERC (Federal Energy Regulatory Commission) Order 661-A for the low-voltage grid. At the National Environment Research Council (NERC), a task force is working on a grid code for the transmission lines. In the estimation of the UWIG, it could take years before this project is completed.

Calculated Short-Circuit Nevertheless, it is possible to proceed without a uniform national grid code. GL Renewables Certification offers type certification for wind turbines as well as project certification for wind farms worldwide as a system service. “For this, we need the precise grid code for that special region,” GL expert Gehlhaar points out. The service offering of GL also includes the validation of software models.

 

In the American wind energy industry, this usually involves the simulation of a grid failure, with a subsequent test of the turbine’s grid compatibility. With a mobile “fault ridethrough” test container of GL Garrad Hassan America, these simulations can be checked in the field with a “real emergency”. A short-circuit is produced on purpose, which in reality would be equivalent to a grid fault. The turbine then runs under no-load conditions and must withstand this situation without difficulty. According to Gehlhaar, clear-cut criteria have been set. “We look at the final result. The plant must not go into overspeed and fall down”.