Solving EMC problems in Field BUS installations

• With commissioning of a small combined heat and power plant in the customers installation, the whole control system of the building failed with field bus errors. It was an installation with M-Bus, but also some Profibus and Modbus controllers.
• The field bus errors caused some wrong measurements and control signals and parts of the plant were shut down for safety reasons.
• Analysis of the system integrators and the supplier of the power plant led to no result.
• Finally, PQP has performed an EMC fault analysis and uncovered the causes of the problems up to the root. Subsequently, the system fault has been eliminated by improving the installation and a small passive filter.

In the first step, the low-voltage network and the affected equipment has been checked for visible errors in the installation. The first mistakes regarding a correct EMC installation came to light (see example images).

Examples:

In addition we performed short measurements with a special power quality analyzer to get an overview of the overall situation of the power supply.

It was good to see, that the CHP was causing voltage distortion (see picture with voltage and current waveform).

The limits of EN 61000-2-4 class 2 were violated for high order harmonics in the voltage.

In a secondary analysis with the power quality analyzer and additional instruments, the causative components could be identified quickly on the combined heat and power plant (CHP).

For the complete clarification of the cause-effect relationships, the interference effects has been traced to the affected bus components and documented all mistakes in the installation.

On the basis of our detailed report and its proposed solutions, the suppliers were able to repair the most important defects in the installation of the M-Bus, Profibus and Modbus-components by themselves.

With regard to the damage caused by perturbations of the CHP system and its faulty installation, our customer decided for a reconstruction of the wiring and the installation of a special passive filter. In this project step, we recommend our customer potential suppliers, prepared the inquiry documents and advise in the selection of the technically and economically best solution. The results speak for themselves:

Voltage without filter (left) and with filter (right):

Example of further measures:

Based on our report further improvements were made.

Afterwards, we assessed the improvements made in the installation and carried out a verification measurement on the voltage supply of the building. The filter function was verified too.

The field bus faults are completely eliminated now.