Technical Cost of Operating a Photovoltaic Installation as a STATCOM at Nighttime

Reactive power support of photovoltaic farms has been under discussion in several countries. This kind of operation has been proposed because the photovoltaic farm converter is an expensive asset that is often used well below its power rating. This paper proposes a methodology for estimating the reactive power support capability and the associated technical cost of operating a photovoltaic installation as a STATCOM at nighttime. The technical cost is related to the need for buying active power from the grid to compensate for power losses. A precise estimation of this cost is of interest to both photovoltaic farm owners and grid operators to be able to evaluate the economic feasibility of this kind of operation. In order to illustrate the proposed methodology, a 1.5-MWp photovoltaic farm is considered. By evaluating the losses of each component (converter, tie reactor, filter, and transformer), a reactive power support capability map integrating the technical cost for each point of operation is obtained. The analysis outlines key points to operate a PV farm as a STATCOM at nighttime: the system must include a blocking diode, the capability map is asymmetric, an operation with variable dc-link voltage is desirable and can lead to savings of up to 8.9% in comparison with operation at fixed nominal dc-link voltage.