Measurement of converter impedances

In the lab a test set up is available to measure the input and output impedances of switching converters. One example is the input impedance of a grid-connected rectifier whose input impedance was measured in a range from 10 Hz up to 25 kHz. The result is shown in Fig. 3. It shows that the impedance of converters can change abruptly at high frequencies, which may cause oscillations between two or more converters connected to each other or to the same grid.

The test set up consists of the following components:

• A 15 kW three-phase mains simulation system (3x PAS5000 4-quadrant amplifier of Spitzenberger & Spies): this voltage amplifier can create voltages with adjustable amplitude, frequency and waveform. It can be used for the steady-state voltage to be applied on the converter terminals, as well as for small deviations, necessary to measure small-signal impedances. For single-phase applications, there is also a small version of this amplifier: PAS1000. 
• A network analyser HP-3562A, able to generate frequency sweeps, measure two channels, calculate impedances, amplitudes, phase angles…
• A power analyser Voltech PM3000A: to measure power, phase angles, total harmonic distortion, harmonics…

Fig. 4 shows the test set up with a full-bridge grid-connected converter as device under test.