Whereas, power plants using renewable energy sources were not too long ago considered exotic, today they are the new face of energy — the wind mill replacing the smokestack as the symbol of electric power generation. Spurred by governmental incentives, renewable energy sources are rapidly changing the nature and composition of power systems. They are still a fraction of the overall energy portfolio, but the renewables’ level of penetration of energy markets is growing. In most US RTOs and power pools, the queue for interconnection projects is dominated by renewables, primarily wind farms.

What is unique about wind farms?

The output from a wind farm depends primarily on the prevailing wind speed. The analogy to a coal power plant, for example, would be that the coal supply would vary on a minute-to-minute basis. This characteristic leads to certain features of wind farms that challenge the typical network integration methodologies.

• Control function needs to take into account the wind speed and direction and the dynamics of how wind is converted into electricity. For network analysis methods such as power flow and stability simulations, there are simplifying assumptions that are applied. But even then, specialized models are required. Existing simulation software may not have the structure and solution methodology to address wind farms adequately.
• Asynchronous operation. While most other supply sources in today’s power systems are synchronous machines, wind generators are typically induction machines which operate asynchronously. This difference has enormous implications on the nature of interconnected power systems and performance under disturbances and system events. Power systems depend on synchronism to operate multiple generators in parallel, providing capability to compensate for sudden changes in electrical demand, topography and supply. Asynchronous machine are more likely to disconnect when major disturbances occur in the neighboring facilities. This is somewhat mitigated by low voltage ride-through (LVRT) capability added to newer wind farms, but nonetheless, power system stability is generally decreased by the introduction of asynchronous machines.
• Variability: The dependable or available capacity of wind farms has a broader probability distribution than traditional types of power plants (for example, see The Coincidence of Wind, a Pterra technical article). Time-varying wind speeds result in minute-to-minute changes in output from the wind farm. Furthermore, each wind turbine in a wind farm sees a slightly different wind profile than the others. Power systems needs to be able to absorb these changes and still maintain frequency (or the balance of electric supply and demand).

System Impact

The first aspect of integrating wind farms into existing electric networks is to assess the system impact and to ensure that reliability criteria continue to be met. Among the technical aspects to consider are:

• In the steady-state, the wind farm is tested for any impacts on power flow and voltage during normal operations and under credible contingencies. This analysis may include any impacts on thermally or voltage limited interfaces. Mitigation of impacts may include transmission reinforcement, capacitor addition, remedial action schemes and other transmission planning solutions.
• Under short-circuit conditions, the wind farm contirbute to short circuit levels should not cause existing existing power system equipment, such as circuit breakers, to exceed their interrupting duties. Only certain types of wind turbines produce a fault contribution and the magnitude is small. But this test is still conducted as a pro forma for any new generation interconnecting to the grid.
• In stability analysis, the wind farm should not introduce instability to the existing grid. Further, the wind farm, should remain online for faults that do not directly isolate the farm. The typical wind generator is light enough not to impact most stability conditions. But the latter condition, requiring LVRT, is more challenging. Mitigation may take the form of additional equipment such as reactive power compensators.
• The operations of the wind turbines produce both voltage flicker and harmonic distortion. These can be measured and compared against accepted standards such as IEC-41000 or IEEE 519, and where the impacts are greater than what is allowed, mitigation in the form of harmonic filters may be applied.
• The wind turbines also introduce excitation currents that may excite resonant frequencies in steam generators. This may be accentuated if there is another source of non-fundamental frequency close by, such as a DC controller, SVC or series compensation. Torsional analysis may be needed to demonstrate potential impacts from torsional stresses.

Not a Synchronous Machine

As noted earlier, wind turbine generators (WTGs) are asynchronous machines. Conventional aspects of generator performance related to internal angle, excitation voltage, and synchronism are thus not applicable to WTGs. Hence, rotor swing curves and damping simulations, used to determine stability, cannot be monitored from the WTG. However, the WTGs’ impact on stability will show in the swing curves and damping of conventional synchronous machines – a form of stability by induction.

Typical dynamic simulation modeling for wind turbine generators (WTG) include:

• generator model – typically an induction machine, asynchronous
• excitation model – representing the method for field control, in those designs that have this feature
• pitch control model – for newer WTGs, this control provides for adjustment to high wind speeds
• wind turbine model – to capture the conversion of wind energy to electric energy
• generator protection – represents the relay package for under/over frequency and under/over voltage protection, among others
• low voltage ride through (LVRT) capability – represents the added capability to ride through low voltage conditions normally seen during fault conditions on the network
• wind gust model – representing response to sudden changes in wind

Not a regular induction machine.

Although WTGs are primarily induction generators, using a generic induction generator simulation model leads to pessimistic results. The field and turbine controls significantly alter the dynamic performance of the wind turbine, and have to be taken into account. At right is a comparison between a WTG model (red) and a conventional induction machine (green). The conventional machine has identical parameters to the WTG, but without the field and turbine controls. The figure shows the response to a fault at the machine terminals. The power swing is more severe and the voltage recovery is poorer in the plain induction generator model.

As a final note, we observe that wind farms are expected to meet the integration requirements that are currently applied to more traditional types of generators such as coal and other fossil-fueled steam plants. However, wind farms have very distinct characteristics that reliability criteria have not yet assimilated. This leads us to believe that the aspect of integrating wind farms is still evolving and that there is yet another chapter to blog about later. So please subscribe so that you don’t miss the next installment!

References

1. Sensitivity Analysis on Low-Voltage Ride-Through Requirements, ABB, 2004. A memo comparing simulation results using PSSE and PSLF.
2. Piwko, R.; Miller, N.; Sanchez-Gasca, J.; Xiaoming Yuan, Renchang Dai; Lyons, J., “Integrating Large Wind Farms into Weak Power Grids with Long Transmission Lines,” Transmission and Distribution Conference and Exhibition: Asia and Pacific, 2005 IEEE/PES, 15-18 Aug. 2005

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