Ronald Seeger, TDK EPCOS
The use of renewable energy is growing worldwide. Higher numbers of ever more complex systems and installations are leading to a growing need for EMC solutions – especially for new high-power converters. EPCOS components ensure that wind power plants do not introduce interference into the grid.
The rotors, generators, gearing and converters of wind turbines are undergoing a rapid rate of development to significantly boost the system performance. Thus, power outputs of 2.5 MW and more have already been attained. The use of frequency converters operating with pulse width modulation (PWM) for the transmission of the total output of the generator now enables a greater efficiency of the wind turbine. This is achieved by more flexible matching of its speed to the wind strength.
A further advantage is that frequency converters can also adjust the phase shift as needed. However, the switching operations of the converters produce interference in the range of the switching frequencies, which is between 1 kHz and 5 kHz, depending on the design. They will become even higher in future as a result of the further development of power semiconductors.
Every deviation from the ideal sinusoidal shape leads to losses in power grids, and ultimately also in the loads. The requirements on the power utilities in terms of preventing distortion and harmonics are correspondingly high.
The use of power chokes at the converter output is common practice and already allows a good approximation to the ideal sinusoidal shape. However, residual switching frequencies and their harmonics remain.
Chokes L1 to L3 and L1’ to L3’ (see Figure 1) carry the total load current of the converter. Their inductance and core material determine the filter effect of these chokes on the switching frequency. However, high inductance values necessitate a large choke volume, corresponding cooling measures and considerable costs. All these are limiting factors, especially for the design of converters of wind turbines.
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Figure 1: Output filtering of power converters
This situation can be remedied by a series resonant circuit with the combined EPCOS chokes L4 to L6 connected in series with EPCOS power capacitors of the B3236* or B2536* series (see Figure 1). This circuit ensures sufficient attenuation of the switching frequencies but is not exposed to the actual load current. To ensure that the circuit is not detuned and thus becomes ineffective, its chokes must not go to saturation up to the maximum load.
Benefits of ferrite cores
Ferrites are used as the core materials especially in transformers and chokes operating at frequencies of more than 50 kHz. This option is countered by efforts to replace ferrites by new materials with high saturation capacities. However, these improved materials require rare earth metals, which are expensive. This is especially disadvantageous when the materials are to be used in large cores of the type required for resonant circuit chokes for high powers.
Even in comparison with less expensive materials, ferrite cores have proved to be more cost-effective when used in resonant circuits at switching frequencies of between 2.5 kHz and 25 kHz. Grain-oriented silicon sheets or iron powder cause much higher losses due to the significant proportion of switching frequency current, resulting in overheating. Ferrite materials are also corrosion-resistant, as they are oxides.
By designing the chokes accordingly, the low saturation strength of the ferrites can be compensated so that it extends to double the rated current. Figure 2 shows the saturation curve of a new EPCOS resonant circuit choke for 200 A. Thanks to the EPCOS choke’s ferrite core, its inductance decreases by only about 2 µH, even at double the rated current.
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Figure 2: Saturation curve of a new EPCOS resonant circuit choke for 200 A
Lightweight aluminum windings save costs
The aluminum winding of these EPCOS chokes is not only more cost-effective than copper solutions but also saves weight. Thanks to the combination of an aluminum strip and copper connection angles, the critical contact between aluminum and copper is moved into the protected inner zone of the coil. Vacuum impregnation of the choke with high-grade epoxy resins ensures that the contact zone between copper and aluminum is completely protected from moisture and oxygen. Figure 3 shows an EPCOS three-phase ferrite choke.
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Figure 3: EPCOS three-phase ferrite choke for 130 A
A solvent-free impregnation resin prevents the occurrence of cracks during the entire operating life of the choke. Such cracks would allow moisture and oxygen to penetrate through to the contact point, leading to electrolytic corrosion of the terminals. Copper terminal rails prevent any electrolytic corrosion of the contacts even if the choke is exposed to salt water during operation. A frame made of a stainless steel sheet, which surrounds the entire core, secures the choke during transport and assembly. This minimizes any risk of damage before it is put into operation. As the frame is non-magnetic, no losses due to leakage fields occur.
Years of experience in the development and manufacture of EPCOS three-phase chokes with ferrite cores and aluminum strip windings have gone into the production of these new chokes. Compared with other solutions, they are able to withstand the extremely harsh operating conditions of wind power plants and also offer cost advantages.