By Jon Vaquerizo
Energy policies around the globe continue to focus on the increasing need for renewable energy, and the wind industry is at the lead of developing technologies. To remain competitive in the clean energy market, the Cost of Energy (CoE) must be considered, along with other main factors such as CAPEX, OPEX, reliability, and component lifetime.
Wind turbines include, in the nacelle, electric generators such as Doubly Fed Induction Generators [DFIG], Squirrel Cage Induction Generators [SCIG], and Permanent Magnet Synchronous Generators [PMSG]). One specific design has to perform under very different and demanding conditions in all kinds of environments: low/high temperature, standard/high altitudes, dust and desert conditions, low/high humidity levels, coastal areas with corrosion, and situations with chemical stressors. Even within the same wind farm, generators differ in performance depending on the power distribution layout. The life expectancy of components can be affected by external factors such as over-voltages. Additional operating inconsistencies in the field, like grid configuration, different frequency converter operation modes, and switching frequencies during synchronization/grid disconnection, are not always part of the specification requirements or insulation system evaluation standards.
The Electrical Insulation System (EIS) is critical in designing a generator, and a key factor in the life span of the component. Unfortunately, 95 percent of repairs cannot be addressed up-tower, so reliability is a must. It’s always a trade-off when weighing the cost of the EIS against the need for safety and reliability. In order to reduce the time-to-market, EIS standardization is based on minimum requirements based on common standards such as IEC, MEMA, IEEE, and UL/CSA. Again, many of these standards are developed under ideal conditions, and don’t account for all of the variables on site. This is where company experience comes into play, with the practice of total quality management (TQM), research and development (R&D) and continuous improvement (CI).
A standard strategy is to reduce the CoE for an existing wind turbine, by delivering more power to the grid in various conditions. Generator and turbine manufacturers understand this; both work towards optimizing the all electrical system Average Energy Production (AEP). The generator always works harder under more demanding conditions, so the electrical insulation system has a big impact on the generator’s lifetime, especially when considering today’s “new normal” expectation of a 25-year component lifespan.
Having an EIS certified by a 3rd party laboratory, such as UL, is normally required for a new listed generator – most customers also expect this. Remember that this evaluation is just a baseline for the correct behavior for the life of the equipment. Gathering data from the SCADA systems goes a long way in understanding the real operating condition of the generator onsite. R&D must regularly perform laboratory testing to obtain the different regression curves for the EIS under different conditions (Electrical Ageing [EA], Thermal Ageing [TA], Thermal & Electrical-Ageing [TEA] (image at left), Chemical Ageing [CA]. The Design Of Experiments (DoE) methodology also helps to identify the different effects of manufacturing parameters, design criteria, or new materials, all of which contribute to a reliable EIS.
In addition to the EIS electrical testing, complete diagnosis can be done using the following quantitative measurements: insulation resistance (IR), Surge Test, Dissipation Test, and Partial Discharges (PD). Individually, each test provides valuable information, but the total data offers a much better understanding of insulation behavior and evaluation during the previously mentioned tests. Macroscopic analysis (20x to 40x magnification) of coils tested – see image 2 – provides even more valuable information to help improve the manufacturing processes and materials.
With hundreds of generators reaching the 20-year point, it’s time for an end-of-life assessment. Decisions will be data driven, using all available monitoring equipment and techniques. Having a clear footprint of the generator status will improve the accuracy of generator “real life” consumption, and guide risk analysis for its life extension.
Is your generator EIS ready?
Jon Vaquerizo is Global Technical Manager for Indar Electric’s Wind Energy Business Unit.Indar Electric is a subsidiary of Ingeteam. With more than 75 years of industry experience (22 in the Wind Industry), the company has installed 29 GW in wind onshore to date. It maintains facilities in the USA (Milwaukee) and Spain (Beasain).
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