Interactive InvertersMaree

Inverters - a rehashpart of a modern remote area power systemDC (ELV) to AC(240 V)allows use of normal 240 V equipmenttwo types:

rotary inverters (mechanical)DC motor directly coupled to an AC alternator

pure sine wavevery good quality powerhigh mechanical loses -> inefficientnot common today

electronic inverters (solid state)240 V 50Hz AC outputrapidly switch DC polarity to produce AC -> fed to a transformer to provide required output voltagetransformer 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.

Features of modern invertersAutomatic stand-by mode

when no load is connectedsaves on power consumptionsenses load usually between 5 - 20 watts

Thermal and short circuit shut downprotection against overload faultssometimes solid state protection, sometimes circuit breakers

Reverse polarity protectionprotects unit against inadvertent reversal of input connections

Over and under voltage shutdownprotect from damage from supply voltage outside normal operating rangeprotect battery against excessive discharge

Quartz controlled output frequencyaccurate frequency control for devices that are frequency sensitive

Regulated output voltageprotects against variation in output voltage which may damage some appliances

Factors when selecting an invertertype of wave form required by load

motors designed for sine wave -> heat, inefficienciescomputers and some switching speed controllers need sine wavemotors generally ok with square wavemicrowave ovens need sine wave

output rating and efficiency-> size and costone inverter for lot or square wave for most + smaller sine wave for PC, microwave.(sine wave inverters more expensive and have lower surge rating)

Inverter sizinggenerally specified in terms of continuous output in W or VAefficiency graph over a range of loadsoverload 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.

Factors when choosing an inverterWattage - continuous, intermittent (1/2 hr) and surge ratingOutput wave shape - as mentioned previouslyFrequency and distortion - within .01% for frequency and < 4% harmonic distortionAutostart - standby mode when no loadMinimum input current - 0.6 AInput voltage - 10-16 VOutput voltage - maintain AC voltage 3-5%Efficiency - preferably have access to the full graph, not just "peak efficiency" figureSafety - AS 3100, AS 3108, C Tick, 'MEN' safety switch compatibleDoes it have all the displays you want; data download facilityWarranty min 5 year and how hard is it to act on warranty?

Terminology

It can be a little confusing...

Grid-interactive invertersInteractive inverter / inverter chargerGrid-feed inverters (one-way) Grid-tie (US)

Grid-interactive invertersDon't generate own frequency - synchronized to gridsimpler 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 gridsafeguards 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 inputDC input about 150-400V / 134-286Vmax input current rating (-> size of modules or number of strings that can be connected)

Interactive inverter chargersSelectronic 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."

Grid-interactive inverters vs grid-feed invertersGrid 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.

Some manufacturers...SelectronicSMA (Sunnyboy)FroniusXantrexLatronicsOutback Power SystemsKaco