Hi Bill,
There are many places to take this measurement and we pick the terminal lugs
on the solar breaker for convenience. Another obvious point (as I mentioned)
is the L1-L2 terminals on the inverter output. A third would be the load
jaws of the meter.
>From my way of thinking,
(1) the service transformer, service wiring, meter, service disconnect
constitute one (active) impedance
(2) the distribution buss and its house loads constitute a second (passive)
impedance
(3) the inverter output circuit including AC wiring, AC disco and back-fed
breaker constitute the third impedance.
It is a combination (series-parallel) of all three that determine the
impedance seen from the inverter side.
Another reason for making the measurement at the back-fed breaker terminal
lugs, is that the voltage drop if the inverter output circuitry is not
included. If one is interested in the condition of the distribution panel
buss, and utility side of the system, this is where one wants to make the
measurement.
If you want to include the AC output wiring, then the L1-L2 terminals on the
inverter output are the best place to make the measurement.
I am glad to hear that dV~3V-ac is nominal for 4000 kW-ac and relatively new
200A service.
- Peter
Peter T. Parrish, Ph.D., President
California Solar Engineering, Inc.
820 Cynthia Ave., Los Angeles, CA 90065
CA Lic. 854779, NABCEP Cert. 031806-26
peter.parr...@calsolareng.com
Ph 323-258-8883, Mobile 323-839-6108, Fax 323-258-8885
-----Original Message-----
From: re-wrenches-boun...@lists.re-wrenches.org
[mailto:re-wrenches-boun...@lists.re-wrenches.org] On Behalf Of Bill Brooks
Sent: Friday, December 10, 2010 2:53 PM
To: 'RE-wrenches'
Subject: Re: [RE-wrenches] Driving Point Impedance of the Point ofConnection
of a Grid-tied PV System
Peter,
Are you making your measurement before and after the inverter breaker? I
believe what you are attributing to the breaker is actually the voltage rise
from the service transformer, through the service conductors, through the
service panel. A 3-Volt drop is common when adding a 4kW load to a
distribution transformer.
Actually, without knowing the load on the transformer to start with, you
have several variables. What is likely happening is that the transformer is
providing power to the grid on a sunny day before turning off the inverter
at say, 2kW. Turn off the inverter, now you instantly turn the inverter from
2kW sourcing to the grid to a 2kW load. The voltage drop, just on the
transformer can be a volt or two in the direction of power flow (transformer
losses).
Let me know if you took the service transformer out of the equation.
Bill.
-----Original Message-----
From: re-wrenches-boun...@lists.re-wrenches.org
[mailto:re-wrenches-boun...@lists.re-wrenches.org] On Behalf Of Peter
Parrish
Sent: Friday, December 10, 2010 9:07 AM
To: 'RE-wrenches'
Subject: [RE-wrenches] Driving Point Impedance of the Point of Connection of
a Grid-tied PV System
I wanted to share with the Wrenches a post-install measurement that we make
and file along with the other routine measurements (e.g. V(+) to V(-), V(+)
to Gnd, V(-) to Gnd, Pac, time or date, date, ambient temp, etc).
This is the AC voltage at the distribution panel with and without the
inverter operating. For example, let's say we have a 5.5 kW PV system. A
clear day in late Fall around Noon might result in 4 kW-ac. We measure the
voltage at the distribution panel with both system(s) operating and then
throw the AC disco and re-measure. We typically see something around 3 Vac
difference.
Among other things this measurement provides evidence that a grid tied
inverter raises its output voltage/current to the point needed to inject its
AC power through the back-fed circuit breaker onto the panel (and from there
into the house loads and possibly the grid).
One can calculate the "effective resistance" at the backfed breaker, using
the formula:
R=(2*Vac*dV/dP), where Vac~240V, dV~3V and dP~4000W. In this example the
result is 0.36 ohms. And this is close to what we see for relatively new
200A 120/250 split phase panel.
>From our way of thinking, this number represents the impedance seen at the
back-fed solar breaker which is in turn a function of the resistance of the
breaker, the contact resistance of the breaker-buss connection, the
resistance of the panel distribution buss, the total resistance of the house
loads and the utility feed. The lower this number is the better. We have
seen as much as 1.2 ohms in an old 100A Zinsco panel, and this causes us
concern.
Is anyone aware of an article which discusses the effective circuit model of
a back-fed, grid-tied PV system? Or the inferences that one can make from
this measurement? What is the normal range of this impedance; or any other
similar measurements (such as taking these measurements at the L1-L2
terminal bock of the inverter proper which would include the voltage drop
for the inverter-panel circuit)?
- Peter
Peter T. Parrish, Ph.D., President
California Solar Engineering, Inc.
820 Cynthia Ave., Los Angeles, CA 90065
CA Lic. 854779, NABCEP Cert. 031806-26
peter.parr...@calsolareng.com
Ph 323-258-8883, Mobile 323-839-6108, Fax 323-258-8885
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