Electric power is one of the basic raw materials for most industrial, commercial, financial and domestic processes. It is essential that the user highly trusts that the supplier company will supply the energy when it is needed and complying with the contracted specifications. Therefore, electric power, understood as an asset, should meet the appropriate quality requirements. This means that, for the correct operation of equipment and processes, the characteristics of the supplied voltage should lie within a certain range around their nominal values.

Electromagnetic disturbances compromise the quality of electricity supply. They can be classified according to their frequency content, as high or low frequency disturbances; according to their transmission medium, as radiated or conducted; or according to their behavior in time, as permanent or transitory. Among the low-frequency conducted disturbances, the main ones are the harmonics and interharmonics, voltage dips and short interruptions, voltage unbalance, variations of the fundamental frequency, and voltage fluctuations or flicker.

The proliferation of renewable energies due to the fostering of energy sources that favor environmentally sustainable development, has an adverse effect on the quality of the electricity supply. Specifically, the integration of wind power into the grid may cause disturbances such as voltage fluctuations, harmonic and interharmonic content and variations in the generated power..

The levels of quality in the electricity supply desired by the users do not have to coincide with those considered acceptable by the supplying company. The standardization of supply quality is therefore essential. The standards specify the design of procedures and systems suitable for the measurement of disturbances, and establish the acceptable emission, immunity and electromagnetic compatibility limits. There are two sets of fundamental standards for regulating the quality of electricity supply: the European standard EN50160 (Voltage Characteristics in Public Distribution Systems) and, in a more international context, the IEC 61000 (Electromagnetic Compatibility) series of standards. In the case of wind generation, the IEC 61400-21 standard (Measurement and assessment of power quality characteristics of grid connected wind turbines) establishes the requirements, in terms of quality of supply, that a wind turbine must fulfill in order to be connected to the grid.

This research topic started in the GSC group more than 30 years ago. During this time, work in this line has been oriented towards different aspects of the quality of electricity supply:

• Measurement and assessment of low frequency conducted disturbances in the power supply system, mainly harmonic content and flicker.
• Measurement and assessment of the quality of the electric power supply of wind turbines.
• Analysis of the sensitivity of new lighting technologies to voltage fluctuations.
• Automatic analysis of the waveform recorded by protection relays.
• Troubleshooting and fault characterization in medium voltage networks.

The extensive research work in this line has resulted in 6 doctoral thesis.

This research line is based on a solid relationship with companies in the electricity sector. Since the beginning, the research group has collaborated closely Iberdrola, one of the world leader distributor and supplier companies, with headquarters in Bilbao. Within this long-standing relationship of mutual trust, Iberdrola has been both a driving force of certain research activities of the GSC in this line and a beneficiary of some of its results. We also maintain a cooperative relationship with Ingeteam, a manufacturer of equipment and a provider of operation and maintenance services in the electrical engineering sector.

Research results in this line have been transferred to different international standardization forums. These contributions have been possible thanks to the active participation of different members of the GSC group in different international standardisation groups related to electricity supply quality (IEC TC 77 / SC 77A / WG 2, IEC TC 88 / WG 21, CIGRE WG C4.111 ), with a relevant influence on the latest versions of the standards IEC 61000-4-15, IEC 61400-21 and IEC 61457-1.