Mart Coenen, EMCMCC
Nearly all electronic mains connected hardware (PV, UPS, SMPS, PWM motor drives, LED drivers, AC/DC chargers, etc.) creates electromagnetic interference (EMI), which affect the performance of other devices in the surrounding area (even while sharing the same wall outlet) and can cause malfunctions. When designing equipment, electronics engineers also need to guarantee their designs do not create a high noise level that can affect other devices, starting at the mains frequency harmonics till 2 – 2.4 kHz (differential) and then from 9 (150) kHz onwards to 30 MHz (asymmetric).
If not done correctly, the energy optimization i.e., the utilization of the mains distribution network at 50/60 Hz is at stake. This optimization is often done by adding large capacitor banks at the feeding point to create a power (quality) factor near to one. These energy optimization developments on the mains distribution network side; and the use of mains entry filters in nearly any electric and electronic system causes heavy resonances on that mains distribution network too.
At the low frequencies, 50/60 Hz, the wall outlet mains impedance is expected to follow the average impedance according to IEC 61000-4-7, being (0.1 + j∙0.15) Ohm for which the reactive part is equivalent to (j∙w) ∙ 0.5 µH. The 0.5 µH inductance primarily stems from the stray inductance of the mid-to-low voltage power transformer and the wiring inductance towards the point of connection i.e., AC mains wall outlet. This also implies that the mains distribution network impedance increases with frequency, even at the lower mains harmonics.
Real absolute impedance values of the mains distribution network vary between less than 1 Ohm and more than 1 kOhm, in the frequency band 9 kHz to 30 MHz, though the products (as designed for) are ‘just’ evaluated against the standardized 50 Ohm, in parallel with (50 µH + 5 Ohm), according the definition of the Artificial Mains Network (AMN) or Line Impedance Stabilizing Network (LISN): IEC 55016-1-2, as being the statistical average of all mains impedances occurring at wall outlets (based on measurements taken in the 80-ties).
On the other hand, the property also called electromagnetic compatibility (EMC) is at stake too. Failure to address electronic interference in the later stages of a device's development; even after proto release, production, sales and installation can lead to prolonged delays down the road that cost both time and money or create safety hazards. Most electronic devices need to withstand a degree of EMI, whether or not to meet the public or industry standards but when these are evaluated under ‘false’ conditions, formal product releases don’t guarantee unhampered performances while these products being used in ‘any’ application environment.
To reduce EMI in a design, electronics engineers first need to understand where it comes from and how it may affect the device. But even after assessing the situation, engineers are usually left with more questions than answers when it comes to mitigating EMI and noise. What should I do to improve my design? What types of filters should, or could I use? What components do I want to evaluate and against which conditions?
On the other hand, those power engineers responsible for power quality: EN 50160, IEC 61000-3-x and IEEE 1159 of the mains distribution networks should keep these ‘self-infected’ causes in mind too. The mains distribution network is primarily intended for 50/60 Hz energy transportation, but all types of power line communication (PLC) devices as well as internet over powerlines at Gb/s are becoming state-of the art too, all in parallel with the interferences as described above. Till now, the conducted emission requirements in the frequency band 2 kHz till 150 kHz are an ‘Eldorado’ for power conversion systems as no requirements apply, except for lighting devices above 25 Watt, which have their requirements from 9 kHz onwards.
In Europe, an inventory has been made about the ‘Eldorado’ band and levels up to 30 volts, superimposed on the nominal 230 volts AC mains, were measured. These levels are beyond the mains voltage tolerance level of +/- 5 or +/- 10 % and may be present for a long period of time, for example when stemming from a PV converter during a sunny day. The resulting problem is fivefold:
1. There are no formal conducted RF emission requirements in this band, as these start at 150 kHz. High-power devices are exempted anyhow e.g. IEC 61800-2 (2021) for AC supplied PWM drives. Requirements for contactless charging (WPT) have recently been released: IEC 61980-1, edition 1.0 2015-07, International Standard, Electric vehicle wireless power transfer (WPT) systems – Part 1. General requirements also defined in IEC/ EN 55011. The running in parallel contactless power transfer data exchange is regulated by ETSI under the RED. Even power line communications (PLC) are operated in this band with allowed levels up to 10-volt w.r.t. conducted emissions. Also, inductive cooking operates between 25 -50 kHz.
2. There are limited conducted RF immunity requirements: IEC 61000-4-16:2015 Electromagnetic compatibility (EMC) - Part 4-16: Testing and measurement techniques - Test for immunity to conducted, common mode disturbances in the frequency range 0 Hz to 150 kHz and Electromagnetic compatibility (EMC) - Part 4-19: Testing and measurement techniques - Test for immunity to conducted, differential mode disturbances and signalling in the frequency range 2 kHz to 150 kHz at AC power ports. Complementary immunity is also required for power line communication (PLC) devices.
3. While using power factor compensating capacitor banks to achieve power efficiency, the resonances caused in the mains distribution network start at several kHz onwards. The many mains filters used with all electrical and electronic equipment together with some inter-wiring inductance cause additional resonances all over the frequency band with high quality.
4. Most inductive and capacitive sensor systems operate in the frequency band 9 – 150 kHz.
5. With most mains filters, X-capacitors are used of which their losses are low at 50/60 Hz, but these dissipative losses increase significantly at higher frequencies. These extra losses mean extra heat, means less lifetime. A blown-up X-cap typically means that a mains fuse is blown out due to a short-circuit and will also result in a transient or sag of the mains voltage.
Click image to enlarge
Figure 1. Resonances may appear as an increased voltage or as an increased current to the associated circuit.
With the above, the resulting damage will be different due to overvoltage (dielectric breakdown) or due to overcurrent (thermal overload). An additional effect may be audible noise due to the fact that the resonant frequency mixes together with the operating frequency of the non-linear power converter down into the audible frequency band.
A general solution is to avoid the high(er) frequency short-circuiting of transmission lines by using capacitor banks for power factor control with additional (lossy) inductances. This also applies to the selection of mains filters which, in 99,9% of the COTS cases, show the X-cap directly across the AC-mains input terminals, either with or without a common-mode choke. The common-mode choke is transparent for the differential mode impedances anyhow aside some stray inductance (typically less than 1 % of the common-mode inductance value: k > 0,99) from it. The dielectric losses from the cable insulation; rubber, PVC, as well as the skin-effect losses of the copper wiring used can be ignored while determining the resonance frequencies
Simple SPICE calculations e.g. LT-Spice (Freeware), will show these resonances at instant taking into account the cable length: 1 µH/m as rule of thumb for the inductance of a single wire. With mains installations, the mutual coupling between the phase and neutral wire can be set at 0,5. Even so for the coupling towards the protective earth (PE) wiring (when present). X-capacitor values can be measured (when disconnected from the power mains) with a simple multimeter having this option or an LCR-meter.