The zone concept : design of low-voltage installations considering the spread of high frequency harmonics

Sammanfattning: The use of electric power has become a backbone of the modern society. Together with the growth in possible applications of electric power, we also see a growth in the number of devices and the interest of using electrical energy as an environmentally-friendly alternative to other sources of energy. Electrical equipment and the use of electricity develops towards more energy-efficient and high-technology products, This implies at the same time that the power quality is impacted as well. The accelerated use of power electronics, harmonics (LF harmonics, up to about 2 kHz) became an important subject for both engineers and researchers. This included origin, impact and propagation of harmonics. The existing situation is that HF harmonics (above 2 kHz) are taking over the role of LF harmonics as an area of concern in the power system as well as for end-user equipment. The driving forces behind the increasing levels of HF harmonics are the increased introduction of energy-efficient and high-technology products and, ironically enough, the limitation of LF harmonics among others using active converters. The research group at Luleå University of Technology, to which the author of this thesis belongs, is one of the few groups in the world that has studied different aspects of HF harmonics. A number of reasons have been identified for the lack of research on this subject: the fast development in technology; the lack of suitable measurement equipment; but most importantly the fact that this subject requires input from different engineering disciplines: electronics; electromagnetic compatibility (EMC); and power engineering. The last few years have however seen a fast growth in interest in the area, with several companies and universities contacting our group.The resonance phenomena, one of the main areas of concern for LF harmonics, are also a concern of HF harmonics but obviously at higher frequencies where experience and suitable measurement equipment was missing. Resonance phenomena in this frequency range have been identified by other researchers, but in relation to dc distribution or in relation to the testing of equipment. Common-mode resonance phenomena in the (ac) low-voltage distribution are treated in this thesis, whereas differential-mode resonances are studied in a parallel project.The aim of this thesis is to show the origin, propagation, impact and treatment of HF harmonics when modern equipment is connected to the low-voltage power distribution. The description is based on the existing knowledge on LF harmonics and the ongoing work, globally, on EMC, end-user equipment and power distribution.The title of the thesis "The zone concept - Design of low-voltage installations considering the spread of High- frequency harmonics" has been chosen to emphasize the need for electromagnetic compatibility and high power quality. This can only be achieved through interplay between end-user equipment and the electrical installation.An essential part in the zone concept is the limitation of the spread of HF harmonics through series filters, which may be power transformers, and through shunt elements to prevent high levels of disturbance especially due to resonances. Such shunt elements close the current path by lowering the impedance in the HF harmonic frequency range and can help in preventing resonance phenomena by introducing active losses. It is further shown in this thesis that many of the means to limit HF harmonics, e.g. transformers, can also be used to limit LF harmonics.