**1. Introduction**

Harmonics are caused by non-linear loads, which draw a non-sinusoidal current from a sinusoidal voltage source. Examples of such harmonic-producing loads, which are used extensively in the industry, include inverters, DC converters, electric arc furnaces, static VAR compensators, switch-mode power supplies (SMPS), and AC or DC motor drives. Other loads such as photocopiers, personal computers, laser printers, fax machines, battery chargers, fluorescent lamps, and UPSs are also a source of harmonics that can be found in the commercial sector [1, 2].

Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons

Large industrial loads are often connected to transmission networks due to large power requirements. Such loads are often non-linear and may include rolling mills driven by variable speed drives or could be an arc furnace. These non-linear loads are sources of harmonics. Harmonics that propagates from the industrial loads degrades the power quality at the electrical system. Harmonics can cause problems to different electrical equipment such as generators, motors, transformers, capacitors, and cables. In addition, it can lead to reduced capacity and efficiency of power systems.

The harmonic content in any network varies with time depending on the share of non-linear loads as well as system status. Examining temporal harmonic profile can help understanding system performance at different loading conditions. To examine the temporal profile, harmonics measurements were conducted at two industrial load grid stations of Oman Electricity Transmission Company (OETC) network in the Main Interconnected System (MIS) of Oman.
