When two voltage source inverters are connected together on a common dc bus the inverter capacitors and the inductance of the dc bus bars form a resonant circuit. This paper explains how DC side harmonic currents are produced by PWM inverters and how they interact with the capacitors and cable inductances on the system. When multiple PWM drives are connected together there are multiple resonant frequencies and avoiding resonances may be impossible. A example design is done which shows that the inherent losses in Electrolytic capacitors can sometimes provide sufficient damping but that the optimum design of low loss Metalised film capacitors does not work with out additional damping resistors or eliminating the dc bus inductance. A full DC harmonic study is necessary combining all the harmonics sources and the multiple natural system resonances to have a better understanding of how much damping is required and where to add it.
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