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FEATURE SURGE & CIRCUIT PROTECTION


What can be done about electromagnetic interference?


grounded, and the cables themselves must be shielded with a conductive braid or covered with a conductive foil. Interference is thus absorbed by the cable shield and the feedthrough, and discharged through the grounded frame. Systems such as those from Roxtec, which bundle several cables into a single frame and whose modules are divided, save space and are easy to install. To determine the optimal EMC cable entry seal, the manufacturer or operator has to decide which frequency range requires attenuation and what level of attenuation must be achieved. Attenuation is measured in decibel units, whereby the decibel’s curve is exponential rather than linear. A three- decibel increase in attenuation doubles the attenuation of interfering frequencies, while an increase of 30 decibels increases attenuation by one thousand times. This means it takes relatively little effort to significantly improve attenuation. It is also important to gain an overview of


Electromagnetic compatibility, or EMC, is still a complete mystery to many in the wind turbine and power converters industry. Matthias Brück, Roxtec, explains how EMC is a highly important aspect of electromagnetic interference, as the larger the converter, the more electromagnetic signals are released


P


owerful transmitters, lightning strikes and circuit switching are typical causes


of electromagnetic interference. The signals emitted by the power converters of a wind turbine, for example, can interfere with flight and marine radars, as well as wireless communication systems. Under certain conditions they can even cause interference within the wind turbine itself. The amount of power being converted dictates the amount of electromagnetic radiation, so interference from large wind turbines is, therefore, particularly high. There are currently no laws that regulate standards for electromagnetic interference emissions from wind turbines. However, Technical Guideline 9 (TR9), issued by the Fördergesellschaft Windenergie (FGW), the German association for the development of wind energy, provides the industry with an important preventive aid. This association outlines a measurement procedure for wind turbines and solar energy facilities that operators can use to confirm that their plants are not sources of electrical noise. Manufacturers can reduce electromagnetic interference by designing EMC versions of


14 NOVEMBER 2016 | ELECTRICAL ENGINEERING


converter cabinets, and by using cable transits equipped with electromagnetic shielding. It is best to incorporate these modifications at the beginning, but converters can be retro-fitted, if needed. Cabinets must be built like Faraday cages so that interference is not released to the outside environment. The cabinet doors should be fitted with EMC conductive gasket and ventilation openings should be covered with honeycomb vents. The fine metal mesh weakens outgoing electromagnetic waves, which causes them to scatter. This scattering process, referred to as attenuation, causes frequencies to be dispersed. Cable entry seals with special EMC shielding, such as those manufactured by Roxtec, can be used to combine efficient EMC shielding with, for example, an IP standard that prescribes water and gas tightness. The task of the cable feedthrough is to absorb frequencies in order to keep them out of the facility and to protect the environment from interference. For this to happen, the frame of the transit must be made of a conductive material and be


diverse EMC functions and products. For example, a Roxtec cable entry seal labelled PE (potential equalisation) provides an important basic EMC function, namely potential equalisation. It protects susceptible electronic components against overvoltage, providing ‘inside’ protection. At the same time, its foil also grounds any low frequency interference on the cable’s shielding. A Roxtec cable transit labelled BG (bonding and grounding) provides, in addition to potential equalisation and protection against conducted interference, a high current-carrying capacity and can divert short circuits or lightning strikes directly. This system is equipped with copper braid instead of foil and is very easy to install. Due to design both PE and BG products from Roxtec have a much better high frequency performance than traditional grounding by wire, so called pigtals. This is critical in EMC applications. A Roxtec seal labelled ES (electromagnetic


shielding) provides protection against both incoming and outgoing interference. Its conductive foil enables potential equalisation and offers protection against conducted interference, while a conductive barrier attenuates higher frequency interference in the atmosphere. This is how cell towers and radar units are protected against interference from signals emitted by converters.


Roxtec www.roxtec.com


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