PQ4Wind: Power Quality Testing and Impedance Model Validation for Main Converters of Wind Turbines

At a glance

• An energy system fundamentally based on renewable energies poses new challenges for the stability of the power supply grid.
• Grid instability is a product of the harmonics caused by wind turbines and other renewable energy generation systems among other things.
• At present, there are no suitable measuring procedures available for investigating and validating the effects of wind turbines on the grid stability. As such, optimizing the impact said systems have on grid stability is no mean feat.
• The focus of the PQ4Wind project coordinated by the Fraunhofer IWES is on setting up a test facility for the evaluation of the stability-impacting characteristics of the frequency converter as the central, electrotechnical component of wind turbines.

The challenge

The transition of the energy system comprising a few central power plants to a decentralized system with a great number of distributed power generating systems presents grid stability among other things with as yet unresolved challenges. Wind turbines generate so-called harmonics – waves with a frequency a number of times that of the fundamental frequency – in the grid, which can put the stability of the grid at risk. The frequency-dependent, non-linear impedance characteristics of wind turbines are one reason for the formation of stability-endangering harmonics. Until now, the available measuring procedures have not permitted generally applicable evaluation of the impedance and harmonics behavior, with the result that optimization of the turbines in terms of their impact on grid stability is often not possible.

The solution

The PQ4Wind research project is dedicated to the development of a test facility for wind turbine frequency converters in the multi-megawatt range with the aim of being able to precisely determine harmonics, validate the impact of the turbines on the grid stability and prevent the formation of harmonics. The frequency converter converts the current generated by the generator with variable voltage and frequency into grid-compatible 50 Hz. As a central link between the generator and the grid, it plays a significant role in determining the turbine’s impedance and harmonic behavior.

The added value

In addition to setting up the test facility, the PQ4Wind project is pursuing other goals. For example, the test procedures required for determining the harmonic behavior and measuring the impedance of the wind turbine are also being developed. The measurement results will be used to determine the validity of simulation models of the frequency converters. In addition, test procedures for the validation and optimization of future grid-forming control procedures for wind turbines are being developed and the basis for a component-based certification of the electrical characteristics of power generation systems is established.