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Designing for the real world
Designing the monitoring system of a small commercial PV installation
The owner of a solar PV system installed on the roof of a commercial building in the USA wanted a performance monitoring solution to keep tabs on his investment and after shopping around and looking at various suppliers, opted for the Green Energy Portal. This article describes the design of the monitoring solution for his PV plant and demonstrates how the Green Energy Portal was used to solve a problem with his inverter that was not reported by its built-in monitoring service. The system is rated at 100kWp, the PV panels are wired into five strings fed to a 5-string combiner box and uses a single inverter connected to the grid.
Outline of customer’s needs
For commercial systems the minimum monitoring solution is the Installers Kit that consists of a site unit (D-RTU) incorporating a high accuracy AC meter. The AC meter monitors the electrical network through current transformers behind the inverter (i.e. at the inverter output or at the point of grid connection).
Systems with an output of up to 700V can be connected to the meter without requiring an additional voltage transformer. This setup allows one to monitor the total system output on the AC side of the inverter or PV system. To ensure truly independent monitoring of the inverter efficiency one should, in addition to the AC meter, also install at least one current and voltage transformer on the DC side of the inverter. For information about the health and efficiency of the PV modules, additional DC monitoring devices or at least DC current transformers need to be installed at each string combiner box. In order to verify module efficiency and enable comparison of real and expected production, a reference panel (measuring irradiance and module temperature) should to be considered. All these field monitors are connected to the D-RTU which in turn is connected to the Internet via a standard LAN connection. >>
