Photovoltaic (PV) inverters increase the line voltage in Distribution Grids (DG) by active power feed in. Today, modern PV inverters are also able to feed in reactive power to mitigate the above voltage rise. The favoured, cost effective implementation is the control of reactive power feed in according to the instantaneous measured line voltage. The stability of this decentralised Q(U) PV inverter closed-loop control is mandatory and analysed in this work. The DG operator must guarantee the voltage limits given in the regulatory framework. This is challenging due to fast changing solar irradiance, load flows and the interaction of an overlying automatic voltage control-loop of a connected substation. The performed tests in the AIT SmartEST laboratory resulted in very stable operation even at small Time Constants (TC) below 5 seconds of the Q(U) control parameter. As one test scenario, an abrupt rise of solar irradiance immediately followed by load drop is realized by use of the PV and load emulators. The PV inverter reduces the resulting voltage rise by increasing its reactive power, depending on the Q(U) control time constant. It was found that even at smaller Q(U) time constants than the typical applied values of 5 or 10 seconds no sign of instability arises. It is recommended to the DG operator to apply TC of 1 or 2 seconds of Q(U) control to minimise the duration of overvoltage condition during the transient voltage adjustment. Applying irradiance conditions of a typical cloudy day in the lab test yields 45% of the time the line voltage was above a given grid voltage limit, while applying Q(U) at TC of 1 second there was no occurrence of overvoltage. In detail at Q(U) TC setting of 20 seconds the overvoltage arises 3.4% of the total period and only 0.3% at TC 5 seconds. Only stable operation conditions were found including the automatic voltage control of the transformer sub-station at a typical setting of 10 seconds delay time of that sub-station control setting. Summarized, in combination with that delay time setting the smaller Q(U) time constant of the PV inverters below 5 seconds are beneficial due to the minimised overvoltage time.
In this paper we present our latest developments in bifacial 6” cells featuring polysilicon carrier-selective passivating contacts which can be seen as an upgrade to bifacial cell architectures such as PERC+ and PERT. In this study we present improvements of p+ polySi passivation on industrially relevant textured surfaces by means of advanced hydrogenation schemes, involving novel dielectric stacked layers, and modification of the B-profile leading to a record passivation level of ~8 fA/cm2 and 724 mV before firing and ~9 fA/cm2 and 722 mV after firing. The results promote industrial implementation of p+ polySi on 6” bifacial cells with excellent passivation properties and compatibility with screen-printed and fire-through metallization, especially as our latest results show Jo,contact being reduced down to 200 fA/cm2. Bifacial solar cells have been manufactured with industry compatible processing with front-and-rear passivating contacts with efficiencies above 20% in the first attempt and this development shows a potential of 23% with a bifaciality factor of 90%.
The simulation of the energy yield of Bifacial PV modules is not straightforward, since most of the light reaching the back side of the PV modules is scattered back from the ground. For tracking systems this is particularly challenging since the geometry is changing as the sun moves through the sky. In PVsyst, a simplified 2D model was introduced to describe bifacial horizontal single axis trackers with regular spacing. The approach uses view factors to model the fraction of light that is scattered back to the back side of the PV modules. The bifacial calculation includes direct and sky diffuse contributions on the back side, as well as ground scattering to the front and back side of the modules.
