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Snubber for filter circuit Disclosure Number: IPCOM000243627D
Publication Date: 2015-Oct-07
Document File: 4 page(s) / 368K

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The Prior Art Database

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Sauro Macerini: AUTHOR

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Page 01 of 4


The output main filter of electronic converters, such as power inverters, may be affected by resonances in the filter current that sometimes are not easy to be damped with the PWM control of the power stage, because of limitations in the system bandwidth.

One typical case is shown on Fig. 1 where a three-phase inverter, such as a three-phase solar inverter, has the output filtering capacitors connected to the middle point of the DC link in front of the converter.

This kind of connection allows the suppression of the high-frequency common mode (CM) voltage between the DC input to ground; as a drawback, there's a current circulating on the electrical connection between the output capacitors and the DC link, which is flowing through the resonant LC circuit of the output main filter and is not damped.

Under distorted output voltage conditions, the undamped resonance may cause an oscillation in the current flowing to the middle point of the DC link with the amplitude reaching critical levels.

To avoid this inconvenient, a resistive impedance can be connected in series with the line where the oscillating current flows.

This line is carrying not only the resonance current, but also other components at different frequencies which are typical on this type of converters.

In particular there is a high-frequency component at the switching frequency of the converter which results from the filtering of the output, and a low-frequency component (usually f = 3xfgrid, so 150-180Hz).

© ABB Group October 2015

Snubber for filter circuit

Fig. 1

Page 02 of 4


These components are usually relatively high in amplitude and would cause an excessive power dissipation in the damping resistor introduced in series with the main filter circuit.

To mitigate this drawback, the damping resistor can be replaced by a complex impedance made with a RLC parallel circuit as shown in Fig. 2.

Fig. 2

The purpose of the complex impedance is offering a resistive behaviour in the frequency range around the main...