Issues with the quality of electric supplies have been known since the beginning of electrification. The advent of power electronics, and more recently of inverters in connection with the multiplication of renewable energy sources, have led to an increase of non-linear loads and sources connected to the distribution grids. This produces harmonics of the 50 Hz sine waveform and, together with a wide range of other disturbances, can pose a risk to the safe operation of grids by stressing equipment connected to it. Hence, power quality (PQ) is sometimes referred to as the set of limits of electrical properties that aims at ensuring that electrical systems function in their intended manner without significant loss of performance or breakdown. Monitoring both the current and voltage contribution to power quality allows for the prevention of outages and damages, and for the analysis of issues in retrospect. The European standard EN 50160 specifies power quality in low- and medium-voltage grids, whereas the IEC standards IEC 61000-4-30, -4-7 and -4-15 define power quality quantities and methods.
Traditionally, power quality is analogue to electromagnetic compatibility but limited to 9 kHz. Disturbances of significant levels have been observed up to 500 kHz: supraharmonics up to 150 kHz and effects owing to narrow-band power line communication (PLC) up to 500 kHz. The low-voltage grid impedance is influenced by the impedance of devices or equipment connected to it, and as such is frequency dependent. This varying impedance has an impact on the grid stability, but also on the reliability of PLC signals, for instance, due to transmission losses. Even today, experimental instruments and methods that can precisely measure the line impedance are limited. Furthermore, various proposed methods tend to yield incompatible results. Consequently, there is a need to establish a metrological traceable grid impedance standard in order to compare different measurement techniques and open the road towards standardisation.
Some of the harmonic content of the power waveform is transferred to different voltage levels through power transformers. Instrument transformers that monitor current and voltage at power stations are designed for a nominal operation at 50 Hz and generally have an unknown frequency characteristic. It is assumed that some of the harmonics are transferred to the next voltage level, but this cannot be traced due to the inadequacy of instrument transformers. As these harmonics are not accounted for, they lead to uncontrolled parasitic power losses and a risk of premature aging of some sensitive components with limits designed for 50 Hz nominal operation. Typical components affected by harmonic energy content are transformers and shunt reactors, but this issue affects asset management more generally. Understanding propagation of harmonics is an important challenge to overcome when preventing detrimental effects. It requires the characterisation of the frequency transfer function of instrument transformers.
Grids which are weakly coupled to the main grid infrastructure, e.g., in railway infrastructure, large ships, or electric vehicle charging stations, sometimes operate at different frequencies (DC, 16.7 Hz) with compatibility requirements at interconnection or border nodes. In railway grids, transients and inrush currents need to be monitored closely in order to maintain voltage regulation, inverters produce high levels of harmonic distortion and meters need to be able to handle various frequency systems. So far, neither commercial power quality analysers nor standards comparable to IEC 61000-4-30 have been developed for these specific applications.
