5.1. Prototype

4. Control strategy

140 Solar Panels and Photovoltaic Materials

There is only one single switch in the proposed DPP converter, whereas it has three outputs for PV1–PV3. Hence, the DPP converter needs to be properly controlled so that shaded substrings only receive a current from the DPP converter while no currents flow toward unshaded ones. To this end, the current sensorless ΔV-controlled equalization strategy [11] is employed.

The mechanism of the ΔV-controlled equalization is illustrated in Figure 9(a). The proposed single-switch DPP converter can be equivalently depicted as a single-input multi-output converter with Ve and blocking diodes. Based on the ΔV-controlled equalization, the DPP converter is operated so that the voltage difference among substrings ΔV = VH – VL (where VH and VL are the highest and lowest substring voltages, respectively) is controlled to be a non-zero positive value. In this scenario, the DPP converter supplies the equalization current only for the shaded substring PV1 whose voltage is equal to the output voltage of the DPP converter Ve. Meanwhile, unshaded

substrings' voltages are higher than Ve, and therefore, currents do not flow toward them.

set to be slightly greater than zero to be unaffected by noise.

Figure 9. (a) Mechanism and (b) block diagram of ΔV-controlled equalization.

The control block diagram of the ΔV-controlled equalization is illustrated in Figure 9(b). All the substring voltages are individually measured to calculate ΔV. The reference of ΔV, ΔVref, is A 30-W prototype of the proposed single-switch DPP converter for standard 72-cell PV panels comprising three substrings was built, as shown in Figure 10. Table 1 enlists the circuit elements used for the prototype. The prototype was operated at a switching frequency of 100 kHz, and a TMS320F28335 control card (Texas Instruments) was used to implement the ΔV-controlled equalization.
