Investigation on Influences of Electricmagnetic Fields and Causes of Breakdown in MVDC System

Concerns in electromagnetic fields induced by commonly used electric devices have been increasing at a fast rate recently. Electromagnetic waves with low frequency (50∼60Hz) are widely known to affect human body negatively. In Republic of Korea, restriction on magnetic field is stated to be below
$83.3\mu\mathrm{T}$
, followed by the standard set by International Commission On Non-Ionizing Radiation Protection (ICNIRP), which is very lenient compared to many other countries resulting in frequent civil complaints [5]. The standards therefore need modifications, but they should be re-established based on circumstances and purposes including methods of measuring electromagnetic fields, biological impacts on human beings, and degree of exposure. Instead of imposing stricter regulations, converting AC based system to DC system can minimize the detrimental effects, as AC cable system induce alternating magnetic field which is a main cause of the so-called ‘harmful’ electromagnetic field. Standards of electrical installations in South Korea state overhead lines should be installed 10 meters above the ground for safety issues. However, when DC voltage is applied, it is far safer than the AC circuit which could allow lower installation height to be accepted. Hence, in this document, electromagnetic fields generated under DC medium voltage are measured to suggest lower standards for the installation height. In addition, characteristics of partial discharge and wave propagation in the MVDC circuits are investigated using an artificial discharge model in order to examine mechanism of system breakdown. The model analyses deviations in distortion of partial discharge waves, which are measured by an oscilloscope, arising from impedance differences between underground cables and overhead lines, as well as joints and terminations.