Measurement and evaluation of power quality
Nowadays, the electrical power supply in medium-high voltage is subject to a number of changes similar to those in low voltage.
One of these main changes is the shutdown of large conventional power plants. This results in a change not only in the active power load flow, but also in the reactive power load flow. As a consequence, the reactive power compensation demand in the grid changes accordingly. In order to be able to meet the reactive power demand, reactive power compensation systems (MSCDN systems, STATCOM systems, compensation reactors and rotating phase shifters) are being installed more and more frequently.
Another change is partial cabling in the grid. Even in extra-high voltage, where in the past almost only overhead lines were considered, more and more cable sections are currently being built. These cable sections have a significantly higher capacity coating compared to an overhead line with the same operating voltage and transmission power. One consequence of these new installations is that they change the network impedance both at 50Hz and at higher frequencies. As a result, there is a risk that a shift in the network resonances will result in a worsening of the harmonic situation in the network.
Another issue is the fact that the harmonic content tends to increase in the low voltage. Although the use of transformers provides galvanic isolation between the low voltage and the medium, high and extra-high voltages, there is some transfer of harmonics between the voltage levels.
In order to be able to ensure that the above-mentioned changes do not have a negative impact on the power quality, a measurement and assessment of the power quality situation should be carried out at relevant nodes in the network at regular intervals.
Furthermore, it is advisable to document concrete measurement results of the background harmonics in the technical specification for the invitation to tender for larger plants (e.g. HVDC plants and STATCOM plants), so that the manufacturer can take a realistic preload of the grid into account in the design of the plant.
During commissioning of power electronic systems, verification of compliance with emission limits (e.g. Upost-Upre measurement) takes place to ensure that the operation of the system does not cause any inadmissible network repercussions. For this purpose, too, a measurement and assessment of the power quality is a suitable instrument to demonstrate a manufacturer-neutral evaluation of compliance with the limit values.
