Reverse Power Relay?

Juliet 15 · Jan 16, 2022

On my boat, I want to connect my DC-to-AC inverter to a 120vac bus normally powered by shore-side power (power grid).

I DON'T want that shore-side power to feed back into my inverter. When I connected the inverter output to the live, shore-powered bus the inverter alarms ("AC OVERLOAD") and tripped its breaker and the shore-side breaker.

I need something that will sense AC power at the bus and prevent the inverter from connecting unless the bus is cold.

Is what I need called a reverse power relay? If so, is it automatic? IOW, does the relay hold the inverter's connection at bay until the bus goes cold, then connect to it? Or does it connect manually, then trip when it senses current at the bus coming from elsewhere?

Thanks for your help.

pc1966 · Jan 16, 2022

Sounds like a similar requirement to an Automatic Transfer Switch for mains & backup generator.

Two inputs. Select mains if present, otherwise select generator and issue start command. In your case generator = inverter.

Juliet 15 · Jan 16, 2022

Yes, I agree. That would be great. Is that called a reverse power relay?

Juliet 15 · Jan 16, 2022

Or is the auto transfer switch something else?

Lucien Nunes · Jan 16, 2022

They are different things. If your inverter is a stand-alone unit that is not capable of grid-tied operation, you need an automatic transfer switch.

An automatic transfer switch chooses one of two sources according to priority. E.g. priority source: shoreline, backup source: inverter. If shoreline is live, it selects that, otherwise it selects inverter. It prevents the two sources ever being connected together.

Reverse power sensing is for sources that are allowed to be connected together, such as a grid-tied generator. When the generator is running it can feed into the grid, but if it malfunctions e.g. runs out of fuel, power flows 'in reverse' from the grid into the generator and the reverse power relay trips the connection. It does not prevent the two sources being connected together under normal conditions.

I hope your inverter is OK. Inverters that are not designed for grid-tied operation can be damaged or destroyed instantly if their output is connected to another source of energy.