Common Mode Problematic of Solar Inverter Systems

In the medium and high power range (starting from about 5kW) mostly three-phase Solar inverter systems are used. The PWM-systems reduce the harmonics of the mains current but insert a high frequency voltage between the DC-link and the neutral point of the AC-side, which is called common mode disturbance voltage here. This voltage is shown to be a four-level signal whose amplitude is in the range of the DC-link voltage. Its frequency is at least switching frequency and its rate of rise depends on the switching speed of the power semiconductors used. The required switching frequency for low distortion input currents and the resulting switching speed of modern semiconductors lead to excessive earth currents. Especially big solar fields show a considerable ground (earth) capacitance leading to problematic leakage current in DC-slink side. Both countermeasures, the reduction and the short connection of the parasitic earth capacitances (as well as the insertion of a differential inductor in both cases) lead to non trivial resonance problems. The problems are described in detail and a solution is proposed. Key-Words: Common Mode Disturbances , Solar Inverter, PWM Inverter