By Vvon Engineering Team | Published | Updated
Why ordinary grid-following solar stops during an outage, how generator controls change the design, and what to specify for a separate backup system.
Solar EPC & street lighting RFQ checklist (TXT)
An ordinary grid-following rooftop PV system does not provide backup when the utility supply fails. Backup requires an appropriately designed system that isolates the utility and supplies selected loads; having panels or a net-metering connection alone is not enough.
This surprises people who have just spent money on a solar plant and then sat through an outage watching it do nothing. It is worth understanding why, because the reason also tells you what to do about it.
Unintentional islanding can endanger utility personnel and connected equipment. Grid protection and disconnection must follow the approved inverter configuration, applicable standards and utility requirements. Do not defeat or alter those protections to keep a plant running during an outage.
The same requirement explains a second thing owners notice. When the power comes back, the plant does not resume instantly. The inverter watches the returning supply for a set observation period, commonly a few minutes, and only reconnects if voltage and frequency have stayed inside limits throughout. On a day of repeated load-shedding those reconnection delays add up, and the plant's daily generation figure will look worse than the weather alone would explain.
Anti-islanding behaviour and the reconnection parameters are part of what the distribution utility approves when it grants the interconnection. They are set at commissioning, recorded, and they are not the contractor's to adjust afterwards.
Most Bangladeshi factories switch to a diesel or gas generator when the grid goes. This is where an ordinary grid-tied plant becomes an active problem rather than merely an idle one.
If a generator energises the same bus, a grid-following inverter may detect an acceptable voltage and frequency and reconnect. Without a coordinated control scheme, excess PV generation can produce reverse power or unstable operation. The effect depends on the generator, protection and controls; do not rely on an assumed trip or engine response.
There are three accepted ways to handle it, and every site with standby generation needs one of them shown on its single line diagram.
A backup-capable system uses an approved changeover or isolation arrangement and equipment designed to supply an islanded load. Not every product described as hybrid supports the same backup functions. Confirm the exact inverter, battery, neutral/earthing arrangement, transfer behaviour and supported generator interface.
Four things are worth knowing before assuming this solves everything.
Backing up an entire factory is rarely the right answer, and the cost of trying makes the whole project look bad. Decide instead what genuinely cannot stop: site and emergency lighting, security and access control, the server room and network, effluent treatment blowers where the biology depends on them, a cold room, the fire pump controller, and any process that spoils a batch if it halts mid-cycle.
Put that list on a dedicated backup board and size the hybrid inverter and battery around the board. Everything else stays on the grid-tied plant, which continues to do the job it is good at, which is producing energy cheaply when the grid is up.
One warning on cost. Separating those circuits onto a backup board means rewiring part of the distribution system, and on an existing factory that electrical work is frequently a larger line in the quotation than the battery itself. Get it surveyed and priced before deciding, because it changes which option is actually the cheapest.
Consult the relevant edition and the documents for the exact equipment and site: SREDA: Net Metering Guidelines and current application documents; NREL: photovoltaic and energy storage operation and maintenance.
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