Help needed - 2 input, 1 output relay following a specific logic

Thread Starter

gchoudhary

Joined Jul 30, 2022
1
Hi everyone,
I am using on-grid solar power plant to decrease my electricity consumption from the state electricity board.
The on-grid project works only when there is a constant supply from the electricity board.
When the supply is not available, the power plant goes into inoperative mode.
I want to use the supply from the power plant to charge my invertor when the state electricity board supply is unavailable.
I want to put a magnetic relay(automatic) which has two input.
Input 1 shall be State Electricity board.
Input 2 shall be the output of solar power plant.
The relay should work in such a way that when
Input 1 - ok, Input 2 - ok, Output = Input 1
Input 1 - NA, Input 2 - ok, Output = Input 2
Input 1 - ok, Input 2 - NA, Output = Input 1
Input 1 - NA, Input 2 -NA, Output = NA
Can you suggest me the type of relay to be used in this case?
and the exact product link, if this doesn't violate the forum guidelines.
 

sghioto

Joined Dec 31, 2017
3,359
Assuming the Inputs have a common connection a simple SPDT relay with a coil voltage equal to the Input 1 voltage should suffice.
Relay specs will depend on the load requirements on the Output.
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Last edited:

Irving

Joined Jan 30, 2016
3,004
I am using on-grid solar power plant to decrease my electricity consumption from the state electricity board.
The on-grid project works only when there is a constant supply from the electricity board.
When the supply is not available, the power plant goes into inoperative mode.
I want to use the supply from the power plant to charge my invertor when the state electricity board supply is unavailable.
If the solar power plant is inoperative when there is no grid supply how will it "charge my invertor" without a grid supply?
 

MisterBill2

Joined Jan 23, 2018
12,745
It seems that the package between the DC output of the solar cell array is set to only use the solar power as a backup.
Charging the battery pack that powers an inverter will need to access the DC output of the solar cell array, and convert that variable voltage (unknown) to the voltage needed to charge the battery pack. (also unknown). Thus the requirement is a bit more complex than a simple relay.
 

sghioto

Joined Dec 31, 2017
3,359
There won't be any output from the Solar inverter if there's no grid-supply to it. The inverter is sync'd to the grid AC.
That's the part that might be misunderstood. Why would the solar plant require grid supply?
This is one setup which would require a DPDT relay.
1659217035741.png
 

sagor

Joined Mar 10, 2019
719
TS states "on-grid" solar power plant. If that means grid-tied inverters (feeding the grid), there is no way they will run without a power line signal (voltage and frequency).
So, it is not really clear as to what the TS actually has...
 

MisterBill2

Joined Jan 23, 2018
12,745
I think that Sagor got it right. But of all the types of inverters that would be my last choice, because then the dumb converter is making the decisions, not me.
The TS mentioned a battery and an inverter, so the box with the solar array may not be needed.
My idea would be a switch-mode system to supply charging to the existing battery from the solar array when the external mains source is available and the battery is not fully charged, and then connect that "on line system when nothing else needs the solar array power. Then the TS has the option of using power from the inverter or the mains, or direct from the battery.
Certainly this will be a bit more complex than just a relay.
 

vu2nan

Joined Sep 11, 2014
264
The on-grid project works only when there is a constant supply from the electricity board. When the supply is not available, the power plant goes into inoperative mode.

I want to use the supply from the power plant to charge my invertor when the state electricity board supply is unavailable.
Hi Choudhary,

The above statements are contradictory.

When there is no grid power there's no solar power, either, to charge the back-up inverter battery.

The workable solution is to charge the back-up inverter battery when grid power is up, to back up essential loads when it is down.

Here's the block diagram.

1.png

And the back-up inverter wiring scheme.

2.png

Nandu.
 

Irving

Joined Jan 30, 2016
3,004
That will work. Also, if the output of the inverter is a pure sine wave then it could be connected to the solar inverter to jump-start that and generate power to charge the inverter batteries - no its not perpetual motion, they will run down eventually unless the solar array is much bigger than the load. My solar-inverter needs about 6W of 240vAC to sync to, but it must be a pure sine wave, and then will happily produce an output. Of course you MUST disconnect it from the grid as the power authorities don't like random power sources popping up and electrocuting their workforce after a fault!
 

eetech00

Joined Jun 8, 2013
3,330
Of course you MUST disconnect it from the grid as the power authorities don't like random power sources popping up and electrocuting their workforce after a fault!
In the US that is called a "Generator Interlock". The "Generator" part comes from its common use to switch between an AC generator power source and AC utility power source. The device is a mechanical interlock mounted in the breaker panel that prevents both sets of breakers from being activated at the same time. Of course, most standby backup systems come with their own "automatic" switch that does the same thing.
 

Ramussons

Joined May 3, 2013
1,305
Hi everyone,
I am using on-grid solar power plant to decrease my electricity consumption from the state electricity board.
The on-grid project works only when there is a constant supply from the electricity board.
When the supply is not available, the power plant goes into inoperative mode.
I want to use the supply from the power plant to charge my invertor when the state electricity board supply is unavailable.
I want to put a magnetic relay(automatic) which has two input.
Input 1 shall be State Electricity board.
Input 2 shall be the output of solar power plant.
The relay should work in such a way that when
Input 1 - ok, Input 2 - ok, Output = Input 1
Input 1 - NA, Input 2 - ok, Output = Input 2
Input 1 - ok, Input 2 - NA, Output = Input 1
Input 1 - NA, Input 2 -NA, Output = NA
Can you suggest me the type of relay to be used in this case?
and the exact product link, if this doesn't violate the forum guidelines.
You need to have 2 independent systems.
1) Solar Cells - Charge Controller - Battery - Std By Invertor. This is a typical setup - No Grid power -> Ups from Battery.
2) Solar Cells - Grid Invertor - Grid. Typical Grid Feed setup.
You need to have a Relay that disconnects the Charge Controller when Grid power is available.

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