Grid impedance
The network impedance ( English grid impedance known) or even complex network resistance is the sum of all impedances in a public electric power grid or an electrical system. In addition to current and voltage, the network impedance depends largely on the frequency.
Determination of the exact network impedance
Measurements show that neither the location nor the type of network have a direct influence on the network impedance. The courses measured at network connection points differ greatly in terms of their location, but show very similar patterns in their course. The electrical properties of a network connection point (NVP) to the public electrical energy supply system can be described in a simplified manner according to Thévenin's theorem with a voltage source and a complex network impedance. Both parameters are frequency-dependent and vary with time within certain limits. The mains voltage is mostly known and can also be measured relatively easily. In some cases, the network impedance at an NVP can be analytically estimated at the nominal network frequency with the help of equipment characteristics. However, an exact determination of the network impedance can only be made through measurement-based identification. This applies in particular to frequencies above the nominal network frequency, since there analytical approaches are subject to large errors and do not correctly reflect the actual frequency curve of the network impedance. For this reason, there have been various research initiatives in recent years in order to be able to determine the network impedance precisely, since the knowledge of the network impedance is essential for a safe and reliable operation of a decentralized power supply.
The metrological determination of the frequency-dependent network impedance currently requires a spectral excitation of the network with current curves. These cause measurable voltage drops at the measuring point depending on the respective network impedance. The network impedance is determined by the corresponding current and voltage curves and the measured open circuit voltage. This current method of measuring or determining the network impedance is based on the principle of active excitation of the network to be examined.
Network impedance and effects on equipment
With the knowledge of the frequency-dependent network impedance, repercussions of emitted harmonic currents on harmonic network voltages can be explained. The network impedance is therefore an important basis for converting harmonic voltages into corresponding currents. This means that resonance points can be identified ex-ante and countermeasures can be initiated to protect equipment and the network. The latest research results have produced measuring devices which, in addition to power quality measurements ( voltage quality ), can also determine the frequency-dependent network impedance. Active methods, such as switching loads on and off, have proven their worth. Excitation signals are generated, the voltage response is measured and the frequency-dependent network impedance is calculated.
Outlook and increase in importance
Because many consumers and producers are connected to the public electrical power supply network via power electronic circuits, knowledge of the network impedance is becoming more and more important. In particular, regenerative generation units such as photovoltaics and wind turbines or electromobility contribute to this trend. The network integration of these decentralized units takes place predominantly in distribution network structures on the low and medium voltage level. When connecting systems with power electronic network coupling to the grid, the grid impedance at the respective connection point is of great importance. The network impedance not only determines the short-circuit power of the connection point, but is also an important parameter for the filter and controller design as well as for the evaluation of system perturbations in the form of flicker and harmonics. Up to now, the network impedance has been determined almost exclusively by means of analytical or numerical calculations with network simulation programs, which are based on equipment characteristics. If detailed network data are available, the network impedance at the nominal network frequency can be estimated with a good approximation. However, mains voltage and mains impedance at a connection point are time-dependent variables due to changes in the status of the network, which makes correct calculation more difficult. In particular, the frequency curve of the network impedance can only be modeled and calculated with insufficient accuracy in simulation programs. An exact determination of the time- and frequency-dependent network impedance can therefore only be done by measuring at the respective connection point.
Web links
- Expert network netzimpedanz.com
Individual evidence
- ↑ Measurement of network impedances.
- ↑ Determination of the network impedance in medium and high voltage networks with a high proportion of renewable energies , on hsu-hh.de
- ↑ Influence of network impedance and signal form , on vde-verlag.de
- ↑ morEnergy
- ↑ NetzHarmonie , on hsu-hh.de
- ↑ Power Quality , on tu-dresden.de