[RE-wrenches] Driving Point Impedance of the Point of Connection of a Grid-tied PV System

[Jeff Irish]

We usually measure this input resistance into the grid (it's not a full impedance measurement) at the MSP lugs.  Experience has shown that less than 150 mohm is usually OK, above 250 mohm starts to get troublesome.  The cause for a high resistance is most often undersized conductors from the utility transformer to the meter and MSP.  Depending on your utility and area, these conductors may be the responsibility of the homeowner or the utility.  If the resistance is much above 250 mohm you'll find the voltage seen at the MSP will vary widely with load, whether or not a PV system is present.

Jeff Irish

-----Original Message-----
From: re-wrenches-bounces at lists.re-wrenches.org [mailto:re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of Peter Parrish
Sent: Friday, December 10, 2010 12:07 PM
To: 'RE-wrenches'
Subject: [RE-wrenches] Driving Point Impedance of the Point of Connection ofa 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.parrish at calsolareng.com  
Ph 323-258-8883, Mobile 323-839-6108, Fax 323-258-8885

 

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