[RE-wrenches] AC Coupled Re-visited

[Allan Sindelar]

Phil,

Good advice. Assuming 48V and a large system, what would you use as a
diversion load and controller in this application? As I understand it, this
would be a DC diversion load, such as a wind gennie air heater, that would
be gradual, resistive, and noncritical for occasional use in this rare
scenario.

 

Thank you.

 

Allan Sindelar

Allan at positiveenergysolar.com

NABCEP Certified Photovoltaic Installer

EE98J Journeyman Electrician

Positive Energy, Inc.

3201 Calle Marie

Santa Fe, New Mexico 87507

505 424-1112

www.PositiveEnergySolar.com <http://www.positiveenergysolar.com/> 

Kirpal, 

The biggest challenge when mixing brands is dealing with the 300 second
timeout after grid disturbance (what I call 5 minute PWM) make for less than
ideal system operation.  There needs to be a better way to regulate power
than kicking the string inverter offline.

Imagine, say, a 7kW array pumping energy into the home on a beautiful sunny
day.  The grid goes down, but the battery based inverter (BB) picks up the
slack and the grid-tied batteryless inverter (GT) stays online.  Home is
drawing less power than the array's output (~6kW worth), and the batteries
are full.  Battery voltage hits the regulation voltage, and the BB inverter
(or the voltage controlled switch) activates a relay to open the circuit to
the GT inverter, effecting a blackout on that circuit.  Suddenly the BB
inverter and the battery bank is hit with the full 6kW of load.  The
batteries are a little undersized because that was the easiest portion of
the system to cut costs on, and they're a little old and dried out from
years of sitting in an uncooled garage, so their voltage sags under the
load.  The voltage controlled switch senses the drop, and closes the relay
to the GT.  However, the GT has to stay offline for another 299 seconds and
the battery voltage continues to fall...

I've been worried about just this scenario for some time, especially as
systems age.  Therefore, I've been suggesting to anyone interested in using
mixed brands of inverters (or those who don't want to use RS485
communications with SB/SI combos) that they strongly consider installing a
diversion load and controller capable of absorbing at least the majority of
the expected surplus energy.  A blackout relay can be used as a secondary
control mechanism.  I think that this is going to provide the most reliable
operation, ultimately.

Phil Undercuffler

Director, Battery-based and Off-grid

Distribution Sales Group

Conergy 

 

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