Elimination of EMC faults on bus systems

Incorrect wiring of an inverter causes problems

Problem description

• Immediately after commissioning an inverter-driven CHP unit, several bus systems failed in the customer’s system. M-Bus, Profibus and Modbus modules were affected.
• As a result of the bus malfunctions, there were constant error messages and false alarms in the process control and thus also failures of the entire plant.
• Analyses of the system suppliers of the control equipment as well as complaints to the supplier of the CHP with subsequent inspection by the manufacturer did not lead to any results.

Approach

In the first step, the low-voltage network and the affected installations were checked for visible faults in the installation. This revealed the first deficiencies with regard to an EMC-compliant installation.

Obvious wiring defect

Subsequently, an overview of the overall situation of the power supply was obtained by measuring with a special power analyser. This already revealed that clearly visible voltage distortions were caused by the CHP.

Current-voltage curve

The limit values of EN 61000-2-4 class 2 for harmonics are also exceeded.

During a further analysis with the grid analyser and additional measurement technology, the interfering components on the CHP were quickly identified.

In order to fully clarify the cause-effect relationships, the interference effect was traced back to the affected bus components and all defects in the installation were documented.

Problem solving

With the help of a detailed report and the solutions suggested in it, the suppliers were able to remedy individual deficiencies in the installation of the M-Bus, Profibus and Modbus components themselves.

In view of the network feedback caused by the CHP unit and its faulty installation, our customer decided to modify the cabling. In addition, an investment was made in a filter specially developed for the CHP unit. The results speak for themselves.

(1) Installation of a filter

Switching on a correctly designed filter (left image without filter, right image with filter).

(2) Correct wiring & cabling

The bus disruptions have since been completely eliminated.