A Method to Enhance the Global Efficiency of High-Power Photovoltaic Inverters Connected in Parallel

Abstract

Central inverters are usually employed in large photovoltaic farms because they offer a good compromise between costs and efficiency. However, inverters based on a single power stage have poor efficiency in the low power range, when the irradiation conditions are low. For that reason, an extended solution has been the parallel connection of several inverter modules that manage a fraction of the full power. Besides other benefits, this power architecture can improve the efficiency of the whole system by connecting or disconnecting the modules depending on the amount of managed power. In this work, a control technique is proposed that maximizes the global efficiency of this kind of systems. The developed algorithm uses a functional model of the inverters’ efficiency to decide the number of modules on stream. This model takes into account both the power that is instantaneously processed and the maximum power point tracking (MPPT) voltage that is applied to the photovoltaic field. A comparative study of several models of efficiency for photovoltaic inverters is carried out, showing that bidimensional models are the best choice for this kind of systems. The proposed algorithm has been evaluated by considering the real characteristics of commercial inverters, showing that a significant improvement of the global efficiency is obtained at the low power range in the case of sunny days. Moreover, the proposed technique dramatically improves the global efficiency in cloudy days.

Experimental Study of the Parameter Mismatch Effects on the Low Frequency Circulating Currents of Parallel Three Phase Inverters

“…Then, if there is a mismatch between the current or the inductance of the grid filters, there will be an imbalance between the voltage inverters → V A1 and → V A2 , and circulating currents will appear, as (1) expresses. Usually, the sum of the phase currents in a three-phase inverter is z several inverters are connected in parallel, the sum of the phase current ( 5) and (6), where 𝐼 𝑂1 and 𝐼 𝑂2 define the zero-sequence or homopolar c currents, respectively.…”

“…In a three-phase system such as that represented in Figure 2, the cir follow (9), where k represents the harmonic order; ω1 represents the grid harmonic); and φAk, φBk, and φCk are the phase angle of each one of the k [29]. Usually, the sum of the phase currents in a three-phase inverter is zero (4), but when several inverters are connected in parallel, the sum of the phase currents is expressed by ( 5) and (6), where → I O1 and → I O2 define the zero-sequence or homopolar component of the currents, respectively.…”

“…Another benefit of paralleling converters is that they allow for sharing of the current among the modules, so low-power devices that have a higher switching frequency and reduced inductor sizes can be used [4]. Finally, in some applications, such as high-power central inverters for photovoltaic farms, the connection in parallel of inverters that manage a fraction of the full power allows for the connection/disconnection of parallel modules, increasing the efficiency in low power conditions [5,6].…”