Measurement of the frequency-dependent grid impedance

Due to a steady increase in DC loads with power supply units, frequency converters for power and speed control or other non-linear loads, the harmonic levels in a network can increase to such an extent that it causes a disturbance or an intolerable load on other components.

Therefore, it makes sense to be able to evaluate the extent to which a new installation will have an impact on the grid already in the planning phase. Although the current spectrum of the components is usually known by the manufacturer, in practice this information is often not sufficient to ensure a reliable assessment of the grid perturbations.

The reason why in most cases a reliable assessment of grid perturbations is not possible in the planning phase of a system is that the frequency-dependent grid impedance is not known. Lastly, the voltage at a node is determined by the grid voltage of the upstream grid and the voltage change (voltage increase or decrease) caused by the plant to be connected. This voltage change results from the product of the system current and the system impedance at the connection point of the system.

In standards, technical connection guidelines and grid codes circumvent this problem by specifying emission limit values depending on the system current (low voltage) or on the short-circuit power of the grid (high and very high voltage). The short-circuit power (50 Hz magnitude) depends on the grid impedance at 50 Hz and describes the extent to which a drawn current leads to a voltage change. For this reason, this is a sensible and, above all, pragmatic approach, but the system current, like the short-circuit power, only refers to the 50 Hz mains frequency, so that the voltage change at higher frequencies is not taken into account. Therefore, a measurement of the actual grid impedance is of utmost interest and is recommended by us to enable a detailed assessment.

Our services:

As a result of research work carried out by Hubert Göbel GmbH, a measuring system was developed with which an on-site measurement of the network impedance in the low voltage can be carried out.

The measuring system consists of an excitation unit, a recording measuring device and an evaluation software. To determine the frequency-dependent grid impedance, the excitation unit is connected to the customer's low-voltage grid.

With the help of loads that can be switched on and off, which are located in the excitation unit, the customer's grid is excited. The grid impedance is determined by the evaluation software from the recorded time curves of voltage and current at the input of the excitation unit.

Your advantages:

  • Early and reliable assessment of grid perturbations already in the planning stage: operational safety and legal security.
  • Early identification of the need for remedial measures (e.g. filters)


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