IGOR - Analysis of perturbations generated by inverters on LV distribution network

In case of distributed energy production and storage, the electrical energy is transferred from the source to the local distribution network through frequency converters, so called Injection Inverters.

The IGOR project investigates the Electromagnetic Compatibility between inverters, LV distribution network and consumer loads, in case of high density of distributed energy injection. Injection inverters generate harmonic waveforms in the range between 1 and 100 kHz. These harmonics result in power losses over the distribution lines, transformers and consumer loads. They can additionally interfere with network or load control signals. In a worst case the voltage waveform might become so distorted that injection inverters lose synchronicity with the network.

Our first objective was to define, understand and describe the potential problems resulting from parallel connection of several inverters in a close area. The resulting perturbations like resonance or beat frequencies were theoretically evaluated. A new method was developed to simulate the behavior of inverters together with the local electrical distribution network in frequency domain. The method is based on the interaction between spectral voltage or currents generated by inverters and the estimated complex harmonic impedance of the grid at the common point of connection.

The total complex grid impedance results from line, transformers, and load impedances. As a large part of the physical configuration of these elements is unknown, it is not possible to calculate the corresponding impedance. This impedance varies as well in time in function of the loads connected on the network. Therefore we concentrated our efforts on the “on-line” measurement of the grid impedance in amplitude and phase over the 1 to 150 kHz frequency range. Available equipment for the impedance measurement did not fulfill our requirements in terms of bandwidth, and portability.

The technique we developed makes it possible to realize a portable “on-line” impedance meter with medium to large bandwidth: a perturbation current is injected into the grid at the point of common connection (PCC) by a controlled current source. The perturbation current and the resulting voltage between phase and neutral are measured at the injection point. The grid impedance seen from the PCC at the perturbation frequency can be calculated as the complex ratio between voltage and current. A spectral response can be obtained by sweeping the perturbation frequency over a defined range.

Validation of our method was obtained by comparison between “on-line” measurement and calculation of complex harmonic impedance of our laboratory grid, of a lighting electronic ballasts or of the XTender battery inverter. Developed concepts for simulation or measurement offer state of the art tools for the analysis and prediction of EMC perturbations in case of energy injection into the local electrical distribution network. The gained expertise in a critical aspect of renewable energy deployment will be very valuable for consulting projects.