AC Coupling in Utility-Interactive and Stand-Alone Applications: Page 7 of 16
Inside this Article
NEED FOR STANDARDIZATION
A general lack of standardization is an issue throughout various segments of the PV industry, including ac-coupled applications. Successful ac coupling requires a delicate balance between energy production, battery charging and total load demand, and, ideally, communicating that balance across multiple brands of equipment. Ultimately, industry stakeholders need to develop a universal, interoperable method for ac coupling by adopting a standardized control structure to ramp up PV production as required.
One way to achieve this standardization is to use the structure codified in the newly adopted German standard VDE-AR-N 4105, which lays out a method of frequencydependent active power control. Designed for stabilizing the European utility grid during periods of excessive renewable generation, it outlines a structure whereby PV inverters decrease output at a defined rate as the grid’s frequency rises, ramp up production upon return to normal and soften the transition upon reconnect. Battery-based inverters could use this method to control the output of any number of VDE-AR-N 4105–compliant grid-direct inverters as required, providing designers with freedom of choice when selecting components for ac-coupled systems.
Schneider Electric Battery-Based and Grid-Direct Inverters
By James Goodnight, Schneider Electric
As interest in and deployment of ac-coupled PV systems has grown, Schneider Electric has directed product development and support resources toward these applications. Currently, many ac-coupled systems utilize equipment from multiple inverter manufacturers. This mix of products can create issues with system design, installation and operation and lead to warranty issues in some cases. Schneider Electric is one of two companies that designs and manufactures both utilityinteractive grid-direct inverters and battery-based inverter/ chargers for the North American solar market. Schneider Electric’s solar inverters now feature the Conext product range name, followed by letter designations for the various models. The Conext TX is our newest generation of residential griddirect inverters and replaces the GT inverter models. The Conext TX integrates with the Conext XW battery-based inverter/ charger to create an ac-coupled system.
CONEXT INVERTER OPERATION
In residential or light commercial utility-interactive ac-coupled PV systems, all energy sources and loads are connected directly to the ac bus. This system design topology has benefits over dc-coupled systems in some applications. In ac-coupled systems, the dc infrastructure is kept to a minimum by enabling higher voltage PV arrays (up to 600 Vdc). Higher voltage arrays minimize system material costs because smaller, lower-cost conductors, conduits and BOS components can be specified. These components are also more easily handled and installed in the field, which drives down installation labor cost.
Array-to-grid efficiency is improved in ac-coupled batterybased systems as well. In a utility-interactive ac-coupled system with battery backup, the array is connected directly to the utility grid through a grid-direct inverter. In a dc-coupled utility-interactive battery-based system, the array output is connected to the battery bank through a charge controller, which is then connected to the grid through an inverter/ charger. The ac-coupled design approach increases system efficiency by removing a conversion step when the utility grid is present.
Grid-present operation. On Schneider Electric’s Conext XW battery-based inverter/chargers, each of the two ac inputs is equipped with an input relay that closes only when the ac source is qualified and is within the parameters of the customer-adjustable voltage and frequency ranges. Closing the input relay connects the ac source directly to the inverter’s ac-output terminals. In this Pass-Through mode, the XW essentially behaves like any other load as it charges the battery bank using its multi-stage algorithm. This charging feature helps deliver good battery performance and operational life. In ac-coupled systems, if the grid ac voltage and frequency are within limits per UL 1741 and CSA C22.2 No. 107.1, Schneider Electric’s Conext TX grid-direct inverters synchronize with the utility-power reference and process power from the PV array. Local loads consume the energy from the PV array, including the XW charger as it charges the battery bank, and any excess is exported directly to the grid.