PV Powered inverters-II [RE-wrenches]

[Michael Welch]

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Hi gang. Brian Dupin of PV Powered asked me to post this on his behalf. I am working with them to get a manufacturing rep on this list so they won't need a go-between.

Bill Brooks wrote at 05:44 PM 07/12/2004:
  
>Ryan,
>
>I think we need a clarification on the message from PV Powered. A single circuit breaker cannot provide backfeed protection to two strings coming into a PV array. The only reason you can put to strings on an SB2500U is because it has been listed for NO backfeed. 15A of backfeed doesn't obsolve you of overcurrent protection on the strings unless you only have one string. Not many 2.8 kW systems only have one string.
>
>Bill.

Wrenches,

I apologize for the delayed reply from PV Powered. The following statement addresses the backfeed/OCP issues as discussed in the series of emails attached below:  

The PVPowered family of inverters is a non-backfeed inverter consistent with letter and intent of NEC Article 690-9, Exception b.  This exception states that an overcurrent device shall not be required for circuit conductors sized in accordance with Section 690-8 (b) and located where there is no external sources such as parallel connected source circuits, batteries, or backfeed from inverters... 

The PVPowered family of inverters includes many levels of protection which prevent the backfeeding of power from the AC utility side of the inverter to the DC (faulted side of the inverter.  These levels of protection include direct measurement of reverse power as well as sympathetic faults monitoring. Mechanisms are described below.

1)  Direct measurement and disconnect in the event of reverse power flow from AC to DC side.
2)  Redundant AC line side fuses which disconnect the inverter from the utility in the event that reverse power is not picked up.  This protection is available even in the event of a two point failure (DC fault and failure of reverse power detection).
3)  Faults which occur on the DC side are likely to bring the DC bus voltage to a level below the normal operating range of the PVPowered inverter.  This will cause the inverter to disconnect immediately from the utility on an undervoltage fault. Backfeed is not possible beyond this low voltage condition.
4)  In the event the fault occurs when the inverter is not operational, i.e. at night, the reduced voltage level described in 3) will prevent the inverter from connecting to the utility during morning startup sequence. In this case again no backfeed is possible.

PVPowered uses a redundant disconnect method on the AC side to connect/disconnect the inverter to the utility.  This provides an extra measure of disconnect assurance.  The features described herein have been tested in the field and as part of the PVPowered’s UL 1741 certification testing.

Thanks for all your work and enthusiasm Wrenches! 
This is a great forum and we (PV Powered) are honored to be included.

Best Regards,
Brian Dupin

-----------------------

Ryan,

I think we need a clarification on the message from PV Powered. A single circuit breaker cannot provide backfeed protection to two strings coming into a PV array. The only reason you can put to strings on an SB2500U is because it has been listed for NO backfeed. 15A of backfeed doesn't obsolve you of overcurrent protection on the strings unless you only have one string. Not many 2.8 kW systems only have one string.

Bill Brooks

-----------------------

Marco and Wrenches,

I apologize for not posting this earlier, PV Powered responded to the question and I failed to post it for them. The following is their response to the issue of back feeding and the over current protection for multiple strings. Feel free to contact me directly with any questions. 

Ryan Mayfield
Energy Outfitters

-----------------------

Ryan,

Our inverters do not back feed. The inverter will only operate if a number of safety conditions on the PV system and electrical grid are met, if not, the inverter will never turn on. Also, the inverters come with a DC circuit breaker mounted inside them; thus, there is no need for external PV fusing.  The DC circuit breakers handle PV over current protection.

Best regards,
Brian Hoffman
PV Powered 

-----------------------

Wrenches:

I am not sure the no fuses argument applies to the 2800.  We had to run special tests to show this was not a problem.  We have redundant reverse current protection and this was a key point.  If current can flow from the inverter to the array then OC protection is required on every string.  The calculations for this are in a recent post of mine and on our web site in a tech note.

Best Regards,

John Berdner

-----------------------

We've installed about a half dozen of the PVP 2.8 kW inverters so far.

Nice unit.

There should be no back feed current from the inverter to the DC side, but like the SMA inverters, if you have to combine more than 2 strings, I think separate OCP for the input strings would usually be required.

/wk

-----------------------

Has no one out there installed a 2.8 kW PV Powered inverter and doesn't anyone know if a individual fuses are needed for each series string connected to the inverter? Bob M....can you find out from the manufacturer if this inverter can back feed from the AC side to the DC side?

thanks,
marco

Marco Mangelsdorf
President
ProVision Technologies, Inc.
69 Railroad Avenue, A-7
Hilo, Hawai'i 96720
(808) 969-3281, 934-7462 facsimile
www.provisiontechnologies.com 

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