Impact of Grid Voltage and Grid-Supporting Functions on Efficiency of Single-Phase Photovoltaic Inverters

Abstract

High penetrations of rooftop photovoltaic (PV) systems are creating several challenges for distribution network operators, most critically overvoltage during periods of peak solar generation. To assist with voltage regulation across distribution networks, grid-supporting functions, such as Volt–Watt ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$P(V)$</tex-math></inline-formula> ) and Volt–VAr ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$Q(V)$</tex-math></inline-formula> ) response modes, are now part of many grid connection standards for PV inverters. The constantly varying voltage profile during daylight hours, particularly in networks with high penetrations of PV systems, requires an inverter operation to vary from nonunity power factor with connection voltages that depart from their nominal values. However, current discussions on PV inverter efficiency focus on calculations and measurements under a unity-power-factor operation while assuming nominal voltage conditions. This article measures and analyzes the efficiency of commercial PV inverters across a more comprehensive and realistic range of voltage and power factors. The impact of grid-supporting modes on PV inverter efficiency is also evaluated experimentally. The definitions of the European and California Energy Commission efficiencies are expanded from a single value to an efficiency curve accounting for the impact of the grid voltage and the power factor. A <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">voltage-weighted PV inverter efficiency</i> metric is proposed that collectively considers the combined impact of solar irradiance, grid-supporting functions, and grid voltages. Experimental measurements from eight commercial PV inverters demonstrate that PV inverters under abnormal grid voltage conditions and with grid-supporting functionalities show lower efficiency values.