Power Converters Division
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Test Set-up for Measurement of Conducted EMI

The high-frequency switching power conversion systems are known to be more efficient, light-weight and compact than their line frequency and linear counterparts. However, the former generates electromagnetic interference (EMI). The EMI, transmitted in two forms – radiated and conducted, may disturb the normal operation of the other equipments in the vicinity. With increasing use of high-frequency switching power converters for accelerator and laser systems, it becomes important to limit the electromagnetic pollution to facilitate system integration and commissioning. There are various CISPR, IEC, VDE, FCC and Military standards that specify the maximum limit on conducted as well as radiated EMI. A set-up for the measurement of conducted EMI has been established in the Power Converters Division.

The equipment under test (EUT), (Indus-2 quadrupole magnet type – 3 power converter in the photograph) is placed on a 12 X 8 feet, 1.6 mm thick ground plane fabricated with aluminum sheets on wooden insulating supports of height 10 cm. For testing of table-top equipments, a wooden table is provided to keep the EUT 0.8 m above the ground plane as specified in the standard. The conducted noise is measured in terms of mains disturbance voltage across a 50 Ω termination impedance of an artificial mains network (AMN) or line impedance stabilization network (LISN) through which the AC mains power to EUT is provided. The LISN defines the line impedance to 50 Ω as seen by the EUT for measurement frequency range of 150 kHz to 30 MHz. The power converters are categorized as Group – 1, Class – A ISM equipments for which the limits for quasi-peak terminal disturbance voltage is 79 dbµV for frequency band of 150 kHz to 500 kHz and 73 dbµV for band of 500 kHz to 30 MHz. A 50 µH / 50 Ω, three-phase, four-wire LISN is installed at the setup for testing of EUTs powered from three-phase as well and single-phase AC mains. An attenuator inbuilt to LISN and additional transient pulse limiter are used for safe measurements on the spectrum analyzer. The interconnections of EUT to mains through LISN and to the load are made as recommended in the standards.

The test setup can also be used for initial functional tests of the power converters for which a three-phase 100 A auto-transformer, a 12 kVA isolation transformer is provided for reducing the mains voltage and electrical isolation. A low resistance (less than 1 Ω) high current (up to 200 A) load is also installed at the test site. Low conductivity water (LCW) is available for cooling of EUT and load.

The conducted EMI test set-up
The conducted EMI test set-up


In addition to the existing set up for measuring conducted EMI generated by power converters being fed by three phase input, a set-up has also been established for measuring and analysing the EMI generated by power converters which are fed by single phase. This setup consists of a Line Impedance Stabilizing Network (LISN) through which the AC mains power to equipment under test (EUT) is provided. The LISN defines the line impedance to 50Ω as seen by the EUT for measurement frequency range of 9 kHz to 30 MHz. It also consists of an EMI analyser having measurement frequency range of 9 kHz to 300 MHz. This analyser separates the conducted noise into two parts: common mode noise and differential noise which can be separately selected and displayed in a spectrum analyser, thereby facilitating respective filter design. Total EMI noise can also be seen directly by connecting spectrum analyser to LISN.

Photograph of LISN (bottom), EMI analyzer (middle) and spectrum analyzer (top).
Photograph of LISN (bottom), EMI analyzer (middle) and spectrum analyzer (top).


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