Xantrex Technology SW Series Using Multiple Inverters, “Series” Stacked Operation, Page 95

OPERATION

USING MULTIPLE INVERTERS

Multiple SW Series Inverter/Chargers can be used in the same system. There are some limitations to the design of the system for successful and reliable operation. For North American applications, the inverters can be used in a “series” configuration to operate 240 VAC loads and to connect to 120/240 VAC power systems. Series stacking of “E” versions (230 VAC / 50 Hz) would result in 230/460 VAC power.

Two inverters can also be connected together and operated in parallel to provide twice the power at the same output AC voltage. The inverters can operate in parallel as battery chargers from the same AC source connected to the same battery. The AC input terminals and DC terminals would then be all in parallel. The inverters will synchronize individually to the AC source and then connect.

“SERIES” STACKED OPERATION

As mentioned, in North America two inverters can be connected in a “series” stacked configuration to provide 120 and 240 VAC output.

When series stacked, you get twice the power of a single inverter available for operating 240 VAC loads, but only one inverter’s power is available for operating a single large 120 VAC load. Separate 120 VAC loads can be operated from either inverter. Do not connect the AC output terminals of the inverters in parallel - this will not work. A special stacking interface cable (SWI) is required to connect the series stacking ports of the inverter to ensure the output voltage waveforms of the two inverters are phase synchronized and locked 180 degrees from each other. This allows connection to 120/240 VAC generators and utility grids. The 120 VAC loads are split and connected to either inverter. Both of the inverters operate completely independently - except their frequency is locked. They do not operate as a master-slave device as previous inverters have. One inverter can be in battery charger mode while the other is inverting to power an AC load. This independent operation allows many new possible applications and abilities that were previously not possible.

When inverters are series stacked, they do coordinate all operations that affect 240 VAC loads. This requires that the inverters turn on together and switch to another AC source at the same time. If either inverter shuts down, both inverters are inoperable.

If no 240 VAC loads are required to be operated, then the need and benefits of stacking are reduced. Stacking adds complexity and will cause both of the inverters to shut down if either inverter goes into an error condition. With separate (un-stacked) inverters, only one will shut down if over-loaded, etc.

When two inverters are connected to a 120/240 VAC distribution system without using the stacking interface cable, the AC neutral wiring must be capable of handling twice the inverter output. This is due to the lack of synchronization between the inverters. If both inverters are operating in exact synchronization (as opposed to operating 180 degrees out of synch.), then the AC neutral will carry the current of both inverters, which was split among the two AC hot conductors. This often is not a problem since the AC distribution system may be oversized to handle the higher output level of a generator. When an AC source is connected to the inverters, the inverters will synchronize independently and transfer at different times. This is not a problem if no 240 VAC loads are connected to the output of the system.

WARNING: You must connect the two chassis of the inverters together and to the grounding system by their grounding lugs or a hazardous voltage may be present on the chassis of the inverters.

INPUT/OUTPUT BYPASS BREAKER SWITCH – 240 VAC LOADS

An inverter “bypass-breaker switch” diverts power around an inverter and is meant to be used only during times of maintenance or to remove the inverter out of the installation. There is a possibility with a multiple inverter installation stacked in “series” (120/240 VAC) with two bypass switches installed, that one inverter bypass breaker switch could be in the Bypass Operation position (power diverted around the inverter’s) and the other inverter bypass breaker switch could be in the Normal Operation position (power passing thru the inverter). The resulting output of the two bypass switches may be less than 240 VAC, which could cause problems with any 240 VAC loads that are not protected against an AC phase shift.

 2001 Xantrex Technology, Inc.	Telephone: 360/435-8826	SW Series Inverter/Charger
5916 - 195th Street N. E.	Fax: 360/435-2229	Part No. 2031-5
Arlington, WA 98223	www.traceengineering.com	Rev. C: February 2001

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