Analysis and Experimental Verification of the EMI Signature of Three-Phase Three-Level TCM Soft-Switching Converter Systems

This paper investigates the Electromagnetic Interference (EMI) noise signature of three-phase three-level (3L) Triangular Current Mode (TCM)-modulated grid-tied Photovoltaic (PV) inverters that achieve full Zero Voltage Switching (ZVS) and thus minimal switching losses over the entire mains period and/or ensure > 99 % efficiency. Further required is a very high power density, which facilitates installation and is achieved with high switching frequencies > 100 kHz. The impact of the characteristic variation of f(sw) in all three phases and the therefore different instantaneous switching frequencies in each phase on the overall converter EMI noise signature is analyzed and it is found that the consideration of only one single phase is sufficient to characterize the noise emissions. Numeric approaches to estimate the detector output of EMI test receivers are compared and it turns out that the peak value of the noise voltage envelope is a useful measure to determine the required filter attenuation, provided the phase-shift of the harmonics is considered in the envelope detection. Finally, a hardware demonstrator of a 6.6 kW, > 99% efficiency three-phase 3L-TCM PV inverter with a power density of 6.2 kW/dm(3) (102 W/in(3)) is designed and the theoretical findings are verified. Moreover, the impact of parasitic capacitances from the switch-nodes and from the floating dc link to Protective Earth (PE) is thoroughly studied qualitatively and quantitatively with the result that these capacitances considerably reduce filter attenuation (35 dB at 150 kHz in the case at hand), requiring sufficient design margin.