AC Coupling Guide

regular grid) and begins to inject power into the system. However, generators are not
designed to accept backfed power, so the concurrent operation of the AC source and PV
inverters must be disallowed. While the generator is running, the PV inverter must be
disconnected from the circuit and therefore PV harvest is only possible when the
generator is not running and XW Pro is grid-forming. This can be done with an interlocked
contactor whose coil is driven by the generator output. Normally-open contacts are
placed between the generator output and the AC2 (GEN) port of the XW Pro, and
normally-closed contacts placed between the PV inverters and the load panel.
Figure 5AC-coupled system with a generator
Recommended contactor part number is LC1D188G7.
Note: The power metering on the XW Pro may not work as expected when it is in grid
forming (invert mode) and power is flowing back into the batteries.
Operation with Legacy PV Inverters
For AC-coupled systems with legacy PV inverters without frequency-watt capability, the
battery inverter will have reduced ability to regulate the battery charge. When surplus
power from AC-coupled PV charges the batteries and the battery parameters (voltage,
current or SOC) reach limits, the battery inverter frequency is ramped up and the PV
inverter will cease producing power and disconnect. When the battery parameter which
caused frequency shift drops below the limit due to loss of PV charge power, its output
frequency returns to normal (50 or 60 Hz). This return to normal frequency causes the PV
inverter to initiate its reconnect delay timer (usually 5 minutes), and come online.
Depending on the available array power and loads, any surplus power will be used to
charge the batteries and the cycle will repeat. It is recommended to set the Absorbtion
Time limit to the minimum value to help reduce cycling. Where possible, it is generally
recommended to install smart PV inverters with frequency-watt capability for AC coupled
systems. However, operation with legacy PV inverters can be considered for grid-tied
applications with less frequent outages.
XW Pro AC Coupling Solutions Guide Planning and Installation
14 This manual is intended for use by qualified personnel 990-6421