Grid-tie Inverters and Generators

February 7, 2012

Grid-Tied PV Systems and Generators

The question keeps coming up; “Will a generator keep the grid interactive, non-battery based PV array operating during utility power outages?” In theory, it seems like a logical idea since the generator operates at 240 volt 60 Hz and this is what the inverter is looking for in order to continue operating.

The problem is, the generator cannot absorb the excess energy from the PV inverter output, nor can the generator react quickly enough
to the fluctuating output of the PV inverter. With a grid connection there is a certain amount of buoyancy or capacitance in the grid to allow for varying electrical energy.

A second problem is how the generator will react to another AC source; it may shut down or it may be damaged by the other AC source.

The solution is the same as it has been since the start of PV energy systems, battery storage. There are many new product advancements in
battery storage or energy storage and the future looks bright. For now, the best solution for backup power during a utility outage is one of two options:

  1. An AC generator can be set up as backup power with a transfer switch that senses the loss of utility  power. If a grid interactive, non-battery based PV array is operating when utility power is down, it will go into a standby status until the utility power returns to normal.
  2. Another option is to use a battery based, grid interactive PV system that can provide backup power during utility outages; a generator can be tied in with the battery-based inverter to help charge the batteries during extended cloud cover and rainy conditions.

If option #2 is your preference, you will find that battery based inverters are quite different from utility interactive inverters. If you presently have an SMA utility interactive inverter, you can add the SMA Island battery based inverter to the system and have the best of both worlds.

For other utility, non-battery based interactive inverters, you either need to change out the inverter to one that is battery based with the utility interactive feature or just use the generator as the backup.

Kelly Provence
IREC/ISPQ Certified Master PV Trainer

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