Voltage drop musings [RE-wrenches]

Tue May 1 17:57:12 PDT 2001

"How often will a given PV system produce
rated power?"
--Depends on what one means by "rated power."  In my real world, the module
manufacturers' STC (standard test conditions) of 1000 watts insolation per
square meter with the cells at 25C does not exist 'cept maybe at higher
elevations in the winter time when the ambient temp is brrrrr cold.  Using
the P(VUSA)TC rating for PV output is more realistic in terms of actual
power produced from an on-site PV array.  For those of you Wrenches out
there interested, I have an e-copy of a October 2000 report on 16 70kW and
above grid-connected PV systems from around the country.  There is a lot of
very interesting info on the costs and performance of such systems.  Most
pertinent to this discussion, they provide the STC rating of the system (DC)
and then the actual (PTC) AC output.  The averaged efficiency (PTC/STC) of
these 16 large systems was 77.6 percent.  If anyone out there is interested
in receiving a copy of the report, send me an email.

"If the system is a non-battery line tie, the inverters I am familiar with
are MPPT based.  This means the system voltage will be at about 17 volts
per panel, or 68 volts total.  Can I expect the PV system to put out rated
power only if the sun is perpendicular to the panels?  I should calculate
wire sizing based on 68 volts, not 48, right?"
--If you're using a wonderful Trace ST series inverter, the DC voltage is
going to be much more than 68, right?  So, you must be talking about a
SW4048 or 5548 with battery back-up since Trace no longer makes the
battery-less UPV SW inverters.  Unless you go for a 54 volt battery bank
you're not going to see an array voltage anywhere close to 68 if the system
is operating normally.  Besides, the SW shuts down on over-voltage if the DC
voltage goes above 68.  (I did this on several occasions based on the
suggestions from one of the Trace techies who told me that with a battery
voltage of 54 as opposed to 48, the array would spend more time at the MPPT
voltage and thereby deliver more kWhs to the customer.) So, no, I don't
think that you should use  68 volts in your calculations for a 48 volt
nominal system.  While using 48 volts in your calculations is on the
conservative side, I think that it's better to oversize the wire rather than
undersize it.  And it sounds like anyone who would shoot for a 1 percent
voltage drop would likely agree, right William?

Anyway, those are some of my thoughts on the matter.

marco
proVision technologies
Hilo, Hawai'i

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