Starting electro-motor loads in battery-less off-grid photovoltaics using a super-capacitor based charge-pump

Abstract

Starting Electromotor (EM) loads with off-grid photovoltaics (PV) is always challenging. Because their starting current makes the PV voltage fall, leading to converter instability. A practical solution is using batteries as a backup, but the repeating inrush current lowers the battery lifespan. This paper proposes an electrical charge pump based on a super-capacitor-bank (SCB) to fix the stability issue in off-grid battery-less photovoltaics. It contains an SCB connected to the DC-link of the inverter via a bi-directional DC/DC converter. The SCB is first pre-charged from the PV. Then at the moment of starting an electromotor load (EM), it supplies the DC-link capacitor with a pulse current. The main challenge here is to control the charging/discharging current of the SCB. In the proposed method, the system model is first developed by circuit analysis. Then a pre-defined charging pattern is obtained to charge the SCB from the zero to nominal voltage with a controlled current with no transient inrush. When injecting the pulse current as a charge pump (SCB discharging mode), constant duty-cycle switching for a pre-defined period is applied to ensure the converter stability in boost mode. To get the best results, a time shift is implemented between the pulse current injection and the load start-up, as a pre-charging phase. Experimental results from a 100 w system prototype running a 70 w universal motor show that a 2 Amp/200 ms pulse current, starting 50 ms before the load start-up, keeps the DC-link voltage deviation under 10 %, and no current transient is observed.