Project study: Resonance suppression through a special active filter

Problem description

After installing a second photovoltaic system on a low-voltage grid with a high proportion of inverters, frequent faults were detected in the first and larger solar system. The inverters of this system, but also of the newer PV system, frequently switched off with the message “grid fault”.

During a grid quality measurement carried out by the local power supply company, high voltage distortions and exceeding of the limit values according to EN 50160 as well as EN 61000-2-2 could be determined at times. However, no cause was given or determined.

Since a CHP unit with a higher output is to be connected to this network and further disturbances were to be expected, we were commissioned with a network and harmonic analysis. The goal here is always the clear determination of the cause and the subsequent planning of suitable solutions.

Approach and analysis

First, we obtained an overview of the overall situation of the voltage supply by measuring with a special network analyzer. Already after connecting the first measurement to the transformer infeed, very high voltage distortions were detected (see the following picture of the voltage and current waveform).

Voltage distortions

Since this is a customer-owned grid, we applied the limit values of EN 61000-2-4 class 2 for the evaluation of the voltage quality. Of course, these limits were also exceeded, in some cases very significantly in the case of individual harmonics and interharmonics.

Two causes are already evident in the current and voltage waveforms. Firstly, the grid is almost exclusively loaded with 6-pulse converters (typical for frequency converters). And secondly, a clear resonance oscillation is recognisable. The latter can usually be seen as the main cause of fault messages from sensitive or particularly protected electrical equipment, such as solar inverters.

Parallel resonance arises as an interaction between transformer inductance and capacitive components in the low-voltage grid. In most cases, these are distributed over several systems and therefore cannot be specifically eliminated as the cause of the problem. During further measurements in this network, we were therefore able to identify several feeders that are involved in the resonance. On the one hand, these were mainly lighting systems and, on the other hand, the disturbed photovoltaic systems. In both cases, the use of unchoked or only slightly choked capacitors as main or filter components is typical. Thus, the disturbed system components are also involved in the disturbance itself. In addition, there was also a frequent shift of the resonance frequency, the cause of which was the switching on and off of the components involved.

The solution

Therefore, there were only two possible solutions. The first way would be a comprehensive restructuring of the network and therefore very time-consuming and expensive. The second way turned out to be feasible in the short term and was preferred by the customer.

As a short-term solution, a special active filter was installed and parameterized by us. Selection, installation, and commissioning were carried out in close cooperation with our customer and the supplier of the filter. The objective was to eliminate the resonance oscillation as the cause of the inverter’s fault messages and also the greatest risk of faults in the CHP to be installed later.

The results are clear:

At the time of commissioning, the resonance was not so pronounced. Therefore, the voltage in the image without the filter is also less distorted than in the first image. Nevertheless, the installed filter also caused a permanent and reliable suppression of the resonance oscillation in the course of the verification measurement over two weeks.

The remaining voltage distortions in the right picture are due to the frequency inverters and contain only slight exceedances of the limit values according to EN 61000-2-4. These do not lead to disturbances at the solar inverters but can be reduced to below the limit values by an additional filter if necessary.

All the systems, including the CHP unit that has since been put into operation, have been working trouble-free since the filter was put into operation.

Imprint:
PQ Professionals GmbH
Landsberger Street 4
04157 Leipzig
represented by the Managing Director Dipl.-Ing. (FH) Frank Strobel