Soft start

[Tired sparky]

Hi, I have a 1000w grid tie inverter that will turn on after my battery bank has been fully charged. This can happen at any time during the day so at times can be activated with up to 30 Amps when a relay clicks over.

So far this has been running ok for a year as the battery lasts a week or so between charges so the relay activates maybe 40 times a year? I don’t know if dumping higher current all at once will damage the inverter as they would seem designed to be on permanently and draw current slowly as the Sun rises, so I would like to add a soft start device to the mix?

It could do with being a cheap readily available addition and what I have seen so far are DC soft start motor controllers? Would any electronics buff know if this sort of thing will work before I order one?

Thanks.

[SunEagle]

So I guess my question is exact;y what does the circuit look like for your system and what brand and type of charge controller do you have. A quality CC will limit the amount of charging amps going to the battery and not over charge it. As well a quality inverter will limit the amount of current if the load is small.

I really don't think you have to ramp up the DC current using a soft start unless your load is big and you want to bring it on line slowly.

[bob-n]

Are you asking for a way to soft-start the transition to powering the grid or to powering the battery?

Is the inverter hard connected to the battery, or does the relay switch between battery and grid?

A sketch would really help.

Do you have a clamp ammeter? That might tell you how much current is being interrupted or activated when the relay switches.

[Mike90250]

A grid tie inverter, when connected to DC, will only power up it's internal electronics. Then it monitors the power line for 5 minutes before it begins inverting, so there should not be much of an initial surge..
If it's a legal GT inverter

[Tired sparky]

The grid tie inverter is by Eco-Worthy and is MPPT 1000 watts, probably a DIY Chinese made type? This puts the solar into the house as mains voltage. It comes on when a separate MPPT charge controller has charged up my batteries. A voltage sensor on the batteries controls a relay that flips between the two units as needed. Works great but I was worried if the relay flipped over from battery charging to grid tie when the solar was high it might suddenly pump a load of current into the inverter? I didn’t know if they can take that sort of inrush but thinking about it, these inverters have to deal with clouds passing and other things that cut the supply but not as fast as a relay does. As i said it’s worked for a year like this so maybe it won’t need any type of soft start?

[littleharbor]

My very first question would be, Is it UL listed? If not I would disconnect it.

[Tired sparky]

UL listed? Is that an American term? I’m in the UK but it’s CE certified and has anti island protection which I tested by cutting off my mains grid supply which instantly cut the solar to grid inverter.

[Tired sparky]

To be honest I would really appreciate a reply to JUST the question I asked and not be lectured to? Is that too hard? If you don’t know if a soft start motor controller would work then I would prefer anyone stay out the conversation please?
This happened almost a year ago when I posted questions on how to relay my system. I got lectures on safety, told it won’t work and if it does my house will burn down, I will be arrested for crimes against the national grid and hell will break open spewing forth demons!!
If this place is just for professionally installed super expensive solar systems regulated to every current law known and using top of the line expensive parts that I can’t get in my country then I will know to go elsewhere for information? Safety is safety I know, but why Is this forum even here if solar is so dangerous, shouldn’t we just buy and get installed commercial systems?

[Tired sparky]

I did appreciate the reply from Mike90250, and while I don’t know if my GT inverter can hold a candle to Midnight Solar units etc it gave some good info on the inner workings of ‘better’ units. Thank you.

[DanS26]

Try contacting this company....they have expertise in soft start engineering.......

http://www.hypereng.com/

[Tired sparky]

Thanks Dan, I’ll look into that but on first look at that link it seems like they might be for AC mains voltages?

[bob-n]

I'll try to help without lecturing.

I think that you are trying to switch high voltage, 30A DC safely. You have a relay to do that, but are concerned about inrush. I think that the current is bidirectional (sometimes charging, sometimes discharging the battery), which would make the problem tougher.

A soft start device would be a pulse-width modulation (PWM) regulator that ramps up current slowly. I know how to make one that can ramp up current in one direction, but it is more complex when you need it both ways. A soft start device like this will be complex and cost significant money. It has to process as much power as the inverter, so the size would be comparable to an inverter, but it isn't a high volume device, so cost will be higher. Ouch!

So far, nothing has failed, so probably inrush into the inverter isn't an issue. My concern would be relay arcing. In other words, long term life of the relay.

There is an inexpensive, efficient way to mitigate relay arcing problems. Put a series resistor and capacitor (RC snubber) across the relay contacts. In other words, buy two inexpensive components (resistor and capacitor) and wire them in series across the relay contacts. You need a relatively right sized resistor and capacitor for your situation but there is quite a bit of leeway.

If you can give me a bit more information, I can calculate the resistor and capacitor for you. We need to know the normal voltage coming into the relay. Is it 12V? 24V? Something else? When you are switching, what is the highest expected current flowing? Roughly how long is the wire from the source to the load? What is the voltage rating of the relay?

The wire length allows us to calculate the inductance. Together with the inductance, the current allows us to calculate the stored energy in the wire. Together with the voltage level, we can calculate the R and C value that will give a relatively small jump in voltage when the contacts open. Relative is in relation to the existing voltage. For example, if you are switching 48V DC and the relay is rated for 100V, you might allow the voltage to spike to 80V. If you are switching 12V and it is a 24V relay, you might allow the voltage to spike to 18V.

The relay won't switch often, so the resistor won't have to be a high power resistor. But it will have a lot of instantaneous power (a surge of energy), so can't be too small. Likewise, you need a capacitor that can handle inrush current, which suggests a metal foil capacitor rather than a metal film capacitor.

Is any of this helpful?

[Mike90250]

Here's a way to test it.
Charge up your batteries and clip a volt meter onto them - even better, clip a DC voltmeter to the DC terminals of the inverter.

disconnect the AC output of the inverter (island it)
Energize the DC, and measure the voltage at the inverter ( say it's 25.55V )
Energize the AC to the inverter, and see if the DC voltage there drops. A teeny drop would mean it's partly powered up and waiting. (25.54V) A larger drop (25.02V) would mean the inverter instantly started drawing power, and has short cut the safety standards.
A DC drop in 4 or 5 minutes (25.02V) would suggest it does behave properly and would not need a soft starter

(the voltages I list are wild guesses. a vary small volt drop is negligible. a larger drop indicates the inverter is consuming power
1,000w is about 42A at 24V )

[Tired sparky]

I thought I would just update this thread and also say thanks to those that helped.

The system I cobbled together has been running for the past 3 years and 2 since starting this thread. So far no problems have occurred.

So my question of do these Y&H 1000W 36-60 volt grid tie inverters mind an inrush current on startup seems to be no, as mine have now cycled over 150 times without issue!