The Role of Harmonics and Non-Sinusoidal Loads in a Wind Farm

Wind turbine transformers should be designed for the additional heating caused by harmonic loading and have an electrostatic shield between windings, neither of which are provided by conventional "off the shelf" transformer designs. Read on for more information on the role harmonics play in wind farm transformer design. Harmonics Basics Transformer design is based on the principal of creating a fluctuating magnetic field from a uniform sinusoidal input alternating voltage source to induce current flow in, and voltage potential across, an electrically separate conductor in that fluctuating field. A purely uniform sinusoidal wave form is possible only theoretically. In real world transmission and distribution power systems, voltage and current waves get distorted from the ideal. In fact, total harmonic distortion (THD) of 1 to 2 percent is common at the point of generation. Also, non-linear loads such as switching actions, rotating machinery, variable frequency drives, and electronic devices of all types add further distortion to the ideal wave shape. The cumulative distortions repeat every cycle, adding peaks that ride on the voltage and current waves and occur at other than the fundamental frequency of 60 Hertz. The harmonics creating these distortions are multiples of the fundamental frequency and are referred to by their multiple, for example, 3rd, 5th, and 7th. Dangers of Harmonics The danger from harmonics is that they increase the eddy and stray losses within the transformer. Eddy and circulating currents in a winding conductor causes additional heating which must be addressed with additional cooling. Otherwise, the additional heat can cause insulation degradation and lead to premature transformer failure. Harmonics in Wind Turbine Generators Wind turbine generators, like conventional generation sources, will produce generator-caused distortions, which result in harmonic wave forms. In addition to these harmonics caused by the generators, the turbine step-up transformers are managed with solid state controls that contribute their own form of damaging harmonics. Rectifier/Chopper Circuits Generator systems using rectifier/chopper circuits present particular problems for the transformer. Since harmonic losses for this configuration are multiples of the transformer's inherent winding eddy current losses, design steps are required to reduce the eddy losses to compensate for the harmonic currents. For rectifier/chopper turbine controllers, the transformers should be designed for harmonics similar to those for furnace or rectifier transformer applications. If not addressed by the transformer designer, the combined non-sinusoidal wave forms from the turbines and the switching induced harmonic wave forms will create excessive heating in the transformer. Shortened life spans and premature failures can result from using conventional distribution transformers which are not designed for this type of duty Harmonics contributed by rectifier/chopper type controllers contain high frequencies that can also affect other equipment on the grid if permitted to pass through the transformer. One example is that protective equipment may see this as a fault condition and attempt to disconnect the turbine. Though harmonic filtering is not specifically a function of the turbine step-up transformer, electrostatic shields located between the primary and secondary windings will act as a filter to prevent the transfer of dangerous harmonics onto the collector bus. Thus, an electrostatic shield should be considered mandatory for a turbine step-up transformer.