Limitations in Applying the Existing LISN Topologies for Low Frequency Conducted Emission Measurements and Possible Solution

Abstract

The use of power electronic converters to interface renewable sources and intelligent loads to electricity distribution systems is increasing at a rapid rate as they bring flexibility and control to the system. However, they also bring an increased level of conducted emissions (CE) to the system due to their switching behaviour -usually at a few tens of kilohertz. The increased emissions are seen particularly in the low-frequency range (2-150 kHz) and it may possibly impair the operation of information and communication technology (ICT) equipment connected to the same system. It is therefore essential to assure accurate measurement of low frequency emissions from a particular piece of equipment, to ensure it meets standards for electromagnetic compatibility (EMC). According to EMC standards, CE are usually measured by using a Line Impedance Stabilization Network (LISN). However, the standard LISN bandwidth does not fully cover this low-frequency range (2-150 kHz), resulting in inaccurate measurement and poor repeatability. This paper examines this issue, by investigating the limitations in using the CISPR 16-1-2 LISN topology for CE measurement at low frequency in a grid-tied inverter system, and by proposing a possible solution, which requires adding extra components to the LISN circuit.

Waveform and Spectral Characteristics of Supraharmonic Unsymmetrical Conducted EMI of Switched-Mode Power Supplies

“…An anti-aliasing low-pass filter R 1 C 1 , which was designed starting from a maximum frequency of 1 MHz (positioning its cutoff frequency one octave above at 2 MHz, which was very compatible with the said sampling frequency of 10 MHz) and exploiting the input capacitance of the DSO channel (found to be quite accurate and stable at 15 pF), thus resulting in a resistive value R 1 = 1.5 kΩ (negligible in terms of the voltage drop in series to the 1 MΩ input resistance of the DSO); • A high-pass filter C 2 R 2 that further attenuated the 50 Hz mains fundamental and its harmonics and designed around a resistive value large enough not to disturb the impedance matching and to remarkably influence the low-pass filter (set to 5 kΩ) and a resulting series capacitance C 2 with a negligible reactance over the desired frequency interval. The use of a high-pass filter is supported by the suggested increase of the decoupling factor in [26] and confirmed by [27] for similar measurements.…”

Black-box Modeling of Converters in Renewable Energy Systems for EMC Assessment: Overview and Discussion of Available Models

“…[48] used an LISN to provide a stable external impedance that was not affected by possible variations in the grid impedance. The analysis showed that stable impedance conditions could be achieved in the frequency bandwidth of the emissions [58] . However, for frequencies below the minimum frequency for CE measurements (i.e., 9 kHz or 150 kHz, depending on the standard), significant variations were observed.…”

“…Owing to the fluctuations in the grid impedance [59] , the unstable impedance provided by the (50 μH+5 Ω) || 50 Ω LISN in the CE measurement of a grid-tied inverter system was presented in Ref. [58]. To investigate how this effect may degrade the effectiveness of the black-box model, the same LISN was used here to measure the CE from the boost converter, as shown in Fig.…”

“…The impact on the model accuracy also depends on the relative values between the converter and LISN impedances. This issue was severe if the emissions were measured on the AC side of inverters equipped with LCL filters, with an impedance comparable to the LISN impedance [58] . The manuscript presents a review of available modeling techniques for power converters targeted at EMI prediction, with a particular focus on behavioral modeling strategies, which are more suitable for EMC analyses of complex systems involving power converters, such as in renewable energy systems.…”

Enhanced Impedance Measurement to Predict Electromagnetic Interference Attenuation Provided by EMI Filters in Systems with AC/DC Converters

“…Figure 6a shows the diagram of a typical setup of a motor drive system, which includes a variable-frequency drive, a three-phase induction motor, and cables. The AC power from the grid is transferred to the motor drive system through a line impedance stabilization network (LISN), which provides a stable impedance regardless of possible variations of the power grid impedance in the specified frequency range [29]. Figure 6b shows a picture of the motor drive system along with the proposed single-probe setup.…”