1. Interactive Inverters
Maree

2. Inverters - a rehash
part of a modern remote area power system
DC (ELV) to AC(240 V)
allows use of normal 240 V equipment
two types:
rotary inverters (mechanical)
DC motor directly coupled to an AC alternator
pure sine wave
very good quality power
high mechanical loses -> inefficient
not common today
electronic inverters (solid state)
240 V 50Hz AC output
rapidly switch DC polarity to produce AC -> fed to a transformer to
provide required output voltage
transformer output is square wave - used directly or fed through a
filter circuit to produce wave form more like sine wave -> filter
process causes loses (inefficiencies)
advances in inverter design allow for the production of true sine
wave inverters which produce sine waves directly rather than
filtering a square wave -> greater efficiencies.

3. Features of modern inverters
Automatic stand-by mode
when no load is connected
saves on power consumption
senses load usually between 5 - 20 watts
Thermal and short circuit shut down
protection against overload faults
sometimes solid state protection, sometimes circuit breakers
Reverse polarity protection
protects unit against inadvertent reversal of input connections
Over and under voltage shutdown
protect from damage from supply voltage outside normal operating
range
protect battery against excessive discharge
Quartz controlled output frequency
accurate frequency control for devices that are frequency sensitive
Regulated output voltage
protects against variation in output voltage which may damage some
appliances

4. Factors when selecting an inverter
type of wave form required by load
motors designed for sine wave -> heat, inefficiencies
computers and some switching speed controllers need sine wave
motors generally ok with square wave
microwave ovens need sine wave
output rating and efficiency
-> size and cost
one inverter for lot or square wave for most + smaller sine wave for
PC, microwave.
(sine wave inverters more expensive and have lower surge rating)

5. Inverter sizing
generally specified in terms of continuous output in W or VA
efficiency graph over a range of loads
overload capacity also quoted as % or figures given for 5 minute or 1/2
hour overload capacities (generally 200-400% of continuous rating)
look at continuous rating in relation to average load sizer and take into
account peak load for surge capacity. Peak load generally far more than
average load (genset may be more economic)
daily load figure will need to be broken up into a profile in order to obtain a
picture of power usage across the day to obtain a figure on which to base
the continuous rating of inverter. Can be done by breaking up power
usage into one hour blocks -> inverter continuous rating based on largest
one hour block. Overload figures are for periods up to 1/2 hour, loads longer
than 1/2 hour must be treated as continuous.
inverter efficiency must be taken into account when sizing system.
Modern inverters 80-90% from ~30-100% of rated output. Efficiencies fall off
as load approaches zero or rises above continuous rating.

6. Factors when choosing an inverter
Wattage - continuous, intermittent (1/2 hr) and surge rating
Output wave shape - as mentioned previously
Frequency and distortion - within .01% for frequency and < 4% harmonic
distortion
Autostart - standby mode when no load
Minimum input current - 0.6 A
Input voltage - 10-16 V
Output voltage - maintain AC voltage 3-5%
Efficiency - preferably have access to the full graph, not just "peak
efficiency" figure
Safety - AS 3100, AS 3108, C Tick, 'MEN' safety switch compatible
Does it have all the displays you want; data download facility
Warranty min 5 year and how hard is it to act on warranty?

7. Terminology
It can be a little confusing...
Grid-interactive inverters
Interactive inverter / inverter charger
Grid-feed inverters (one-way)
Grid-tie (US)

8. Grid-interactive inverters
Don't generate own frequency - synchronized to grid
simpler than RAPS (no batteries)
very strict standards re: voltage, frequency and wave form (harmonic
content and radio interference)
must be disconnected from grid if repairs are being carried out to grid
safeguards built in if grid goes down or other faults occur (including anti-
islanding protection - injecting small pulses that are slightly out of phase
with the AC electrical system in order to cancel any stray resonances that
may be present when the grid shuts down)
usually specified by max output and PV array connected must have output
V and current within limits of inverter input
DC input about 150-400V / 134-286V
max input current rating (-> size of modules or number of strings that can
be connected)

9. Interactive inverter chargers
Selectronic describes its off-grid/leasure inverters as either interactive inverter
chargers or stand-alone inverters .
"Selectronic Interactive Inverter Chargers use a battery to store energy for
periods when power demand is low. When the demand is high, or when the
battery requires recharging, the inverter automatically starts a generator."

10. Grid-interactive inverters vs grid-feed
inverters
Grid feed inverters send the power
from solar PV panels direct to the
grid. They do not use a battery bank
and therefore they do not give you
any power back up in the event of a
grid power failure.
Grid interactive inverters perform the
same function as grid feed inverters,
however they allow power to flow
'both ways'. They also incorporate a
battery bank and have an automatic
built in charger. This type of system
gives you back up power in the event
that the grid fails or goes out of
tolerance in terms of its voltage and
frequency.

11. Some manufacturers...
Selectronic
SMA (Sunnyboy)
Fronius
Xantrex
Latronics
Outback Power Systems
Kaco