Common mode - Nieuwe ontwikkelingen en normontwikkeling - FHI, federatie van technologiebranches

Figuur 1: Verschil tussen een differential mode en common mode circuit.

Figure 1: Difference between a differential mode and common mode circuit.

Introduction

Common mode is a major source of interference, and the challenges associated with it are only increasing with the advent of DC grids and the proliferation of switched-mode power supplies. But first, what is common mode?

Difference between differential mode and common mode

To go into this, it is easier to start with differential mode. Differential mode is about the voltage difference between two or more conductors, with the exception of the ground conductor. This voltage difference can be intended to transfer energy or a signal. In addition, there can also be unwanted differential mode signals (interference signals). Common mode looks at the difference between the average voltage on the conductors of a certain circuit and ground. In most cases, this is an interference signal.

Figure 1: Difference between a differential mode and common mode circuit.

Source of common mode

A common mode voltage occurs when the average output voltage of a device is unequal to the reference potential (usually PE). For example, the average of the three output voltages of a generator (L1, L2 and L3) is normally 0 V. However, this does not apply to a frequency converter in which a sine wave is created from a DC voltage using 6 semiconductors. Here, an output can be switched to the plus or minus of the intermediate circuit. The average for a three-phase output is therefore never equal to 0 V, as can be seen in Figure 2.

Figure 2: Origin of common mode voltages.

L1	L2	L3	Ucm
+½ Udc	+½ Udc	+½ Udc	+½ Udc
+½ Udc	+½ Udc	-½ Udc	+⅙ Udc
+½ Udc	-½ Udc	-½ Udc	-⅙ Udc
-½ Udc	-½ Udc	-½ Udc	-½ Udc

Besides the fact that common mode is generated by an average voltage that is not equal to 0 V, common mode is also created by the high dv/dt that occurs during switching of the semiconductors.

Risks of common mode

In networks where the neutral point is connected to earth, so-called TN networks, this need not be a problem, as long as the common mode currents are kept out of sensitive circuits. This can normally be done by filters at the input of a common mode source and by providing defined common mode paths, for example by cable screens that are earthed on both sides.

Figure 3: Defined common mode path in a network with a grounded star point.

In networks where the star point is not connected to earth, so-called IT networks, two problems arise.

While in grids with an earthed star point the common mode voltage is mainly visible on the secondary side of the grid, where for example a motor is connected that can be designed to tolerate high common mode voltages, this is not the case in grids with a floating star point. In these grids the common mode voltage is determined by the ratio between the total capacitance to earth on the grid side and on the secondary side. These capacitances consist of filter capacitors to earth and parasitic capacitances between for example cables and their environment/screen, and the capacitance in motors between their windings and the stator. If this capacitance is relatively large, the impedance, and therefore the common mode voltage, is low. On the other side of the common mode source the capacitance must then be relatively small, the impedance high, and as a result the common mode voltage also high. In this case the common mode voltage can also be largely present on the grid side of the common mode source, whereby other devices connected to the grid are loaded with this common mode voltage. This again introduces common mode currents that can cause damage.

Figure 4: Distribution of common mode voltage between primary and secondary side of common mode source.

2. In addition, in grids with a floating star point, the capacitance to earth is often limited due to earth fault detection systems and fire risks. This means that it is much more difficult to lead common mode currents back to their source via a desired path. The common mode current will seek out a path itself, and this path may coincide to a greater or lesser extent with signal circuits, which can cause interference or damage small filter capacitors in power supply circuits.

Figure 5: Common mode current paths in a network with a floating neutral point.

New technical developments

With the introduction of new energy sources, such as fuel cells, solar panels and generators that run at variable speed, energy storage in batteries and flywheels, and the focus on minimizing energy loss, it is becoming increasingly interesting to work with DC grids. However, in order to convert this DC voltage into other DC voltages or AC voltages, switching converters are needed, which can therefore generate common mode. In addition, many (smaller) transformers are being replaced by switching power supplies, which can also cause common mode.

This has two major consequences. Firstly, the common mode disturbance increases and secondly, it is more common to use a frequency converter as a microgrid converter, which does not feed a motor but a variety of small AC loads. Motors can still be designed to handle common mode voltage with better insulation, but for this quantity of small AC loads this is much more difficult.

Challenges in standards development

This has received particular attention from two technical committees within the IEC, because they also work extensively with networks with a floating neutral point. These are:

TC18: Electrical installations on ships and on mobile and fixed units on water
TC82: Solar energy systems

Together with TC77 (Electromagnetic Compatibility) and CISPR (International Special Committee on Radio Interference) these committees are looking for ways to define common mode in standards. The problem with the current CISPR standards is that they are not really suitable to say anything about common mode levels in floating networks or for equipment intended for floating networks. CISPR uses LISNs (Line Impedance Stabilisation Networks) which have large capacitances to earth. By connecting these devices the ratio of capacitance to earth between the input and output of the common mode source is changed and therefore also the measured voltage. In addition, experience has shown that connecting LISNs and CISPR probes regularly leads to damage when they are connected to floating networks. The common mode currents are so large that LISNs burn out, or the common mode voltage, especially between 2 and 150 kHz, is so high that CISPR 16 probes become defective. In addition, the LISNs can also influence integrated ground fault monitoring in inverters in such a way that the inverters switch off for safety reasons.

New developments

In order to find a way to say something about the height of common mode signals, new measurement methods are being looked at. For example, TC18 is working on measurement networks that have a high impedance to earth in order to minimize the influence of the common mode voltage. TC82 looks at measurements with low-frequency magnetic antennas in which the impedance to earth is not also influenced.

Conclusion

With the continuous development in electrical engineering, new challenges continue to arise in the field of EMC. Within the IEC, a team of stakeholders is working on new methods and limits to deal with these new electrical engineering developments. It is important that a balance remains between reliable equipment and additional costs for additional measures in the equipment and measurements.

Standardization work

If you are interested in standardization work, you can find more information here: https://www.nen.nl/normontwikkeling