How to handle high in-rush current / startup current

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  #1  
Old 04-20-19, 03:53 AM
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How to handle high in-rush current / startup current

Hi All!

This is my first post here, and apologies in advance if I miss out on certain information / rules.

We have installed an outdoor LED screen, which uses 124 numbers of AC-DC transformers (Output - 5 V, 60 A, 300 W).

The main power supply to this screen is 3-phase, and it has a circuit breaker of 63 A. The problem we are facing is that on some days, when the power is switched on, the circuit breaker trips. This happens only on startup. Once the screen starts, there is no problem.

I have attached a PDF file with further details such as the line diagram and the transformer rating. Is the circuit breaker tripping certainly because of a high in-rush current? If yes, is there a way of controlling this or reducing the in-rush current?

Any leads will be appreciated.

Thanks!
 
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  #2  
Old 04-20-19, 04:02 AM
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Hi, have you contacted the mfg. about the problem? Seems like an install that size there should be tech support for it.
Is this a new installation?
Geo
 
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Old 04-20-19, 05:27 AM
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There are work around solutions but there is a design or installation problem. Contact manufacturer first.
 
  #4  
Old 04-20-19, 10:03 AM
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It's not clear from the diagram whether the switching supplies are evenly distributed between the three phases. While it may not be the only factor causing the the breaker to trip, unbalanced loading could aggravate the situation.
 
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Old 04-20-19, 11:35 AM
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Supplies appear to be single phase input. That would mean that your load is not balanced evenly. You need to try dividing the fourth line across the other three so that one line is on each phase instead of one phase having two of the lines.
 
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Old 04-20-19, 12:37 PM
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Typically it is better to let the screen stay on but just make the screen go black. That way the fans continue to run and keep and prevent any condensation from forming on the internal parts of the sign.

This is mostly a US site but I would think similar rules may apply. You want to size the circuit conductors and circuit breaker to 125% of the full load. Your load is 60 amps so you need to size the circuit at 75 amps. I don't know what size breakers you have available as we do not have 63 amp breakers, but a 63 amp breaker is too small for your load.
 
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Old 04-20-19, 03:10 PM
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Even sub 100W switching supplies can be rated at 10A surge at power on. You have many of these turning on at once. If you catch the switch on time at the high side of the AC cycle, you will sustain a very large initial surge current.

As noted, a number of ways to fix this. Sequenced turn on, Use a series R that is auto switched out in a couple of power line cycles, PTC's at each SMPS; and I'm sure other ways.
Is there a power contactor used? You might consider a scheme where you have multiple sets of SMPS connected to the previous set by a contactor. So, the main contactor only turns on a few, then the output of these turns on another contactor for the second set, and so on. It will be more of a wave of power, rather than a spike of surge.
 
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Old 04-20-19, 11:01 PM
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Hi! We have assembled the screen from parts sourced from different suppliers. We are in touch with the transformer manufacturer, but they have not been able to help so far.
 
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Old 04-20-19, 11:04 PM
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@engr3000 and joed: This is a good point, the load may indeed be unbalanced. Out of the 4 branches we have, 2 branches are connected on a single phase. Maybe we should split the 4th branch across all 3 phases as joed mentioned.
 
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Old 04-20-19, 11:07 PM
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@Tolyn Ironhand: Thanks! I was wondering if keeping the power supplies and fans switched on 24/7 would create problems in terms of life of the parts? This is again a good suggestion as we can easily schedule the screen to be black, but just worried about the life of the power supplies / fans.

Also, I am curious as to how you were able to calculate the load to be 60 A? If thats the case, we could have a breaker of 100 A with a 25 sq.mm. wire.
 
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Old 04-20-19, 11:14 PM
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@telecom guy: Surge current really seems to be the issue here! I am going to look into the ideas you have mentioned here.
To add to your point, do you think it would be viable to sequentially activate the 3 AC phases using a timer? For example, phase 1 is activated first, followed by phase 2 then 3.
 

Last edited by screen23; 04-21-19 at 01:52 AM.
  #12  
Old 04-21-19, 04:02 AM
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Sequencing each phase separately as a unit will probably not work. When the phase is finally switched on, all of the loads (transformers, etc.) on that phase are still going to draw their respective startup (inrush) currents at the same time.

Could you use a breaker designed to take a larger and brief startup load without tripping?
 
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Old 04-21-19, 05:04 AM
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Just as a test you could disconnect the fourth branch to see if the system reliably turns on without tripping the breaker. If the breaker doesn't trip then a suggestion would be to delay the connection of the fourth branch. With this approach the maximum startup surge current on any phase would be 25 percent less than if you evenly distributed the switching power supplies on all 3 phases. This of course assumes that it's just a surge problem and the extra load from the fourth branch on the one phase is OK once everything is turned on.
The delayed connection of the fourth branch could be automatic using a timer or with the manual activation of a switch depending on your preferences.
 

Last edited by engr3000; 04-21-19 at 05:28 AM.
  #14  
Old 04-21-19, 05:12 AM
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I calculated the max current to be 23 per branch using the max output of 300 per supply.
31 x 300 = 9300 watts

watts out = watts in so current = 9300/400 volts = 23.25 amps

That is the max amps if the power packs were running at full 100 watts.
 
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Old 04-21-19, 08:39 AM
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Yeah, balance the phases, even during any type of sequenced start up. And, a HACR rated breaker would be a good idea, to increase trip delay.
I'm not really enamored with only a single breaker at the front end of this power supply string. I would encourage perhaps 6 banks of PS, each breakered.
 
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Old 04-21-19, 11:07 AM
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"We have assembled the screen from parts sourced from different suppliers." This is your problem. Engineering takes the specifications for each part and creates a circuit connecting the parts such that the specifications of each part are met. This obviously hasn't been done. You are getting suggestions and SWAGs that may/ may not get the devices orchestrating. I suggest you hire an electrical engineer to design the circuits that connects the parts.
 
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Old 04-21-19, 11:35 AM
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There is only one way in handling the startup of 124 switching power supplies and that is delayed startup. An electrical engineer will point you in that direction or specify different power supplies.

I use delayed startup on many of my commercial sound systems.
 
  #18  
Old 04-22-19, 01:10 AM
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Hi Pete!

We did make a mistake here by not consulting an electrical engineer beforehand, and I think our best course of action at this stage would be to delay the startup as you suggested.

We have another screen that has 50 switching power supplies and does not trip, so I'm thinking to split up the 124 power supplies into two startups - 62 and 62 using an automatic timer. That said, would you be able to recommend the time duration between the two startups?

Thanks!
 
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Old 04-22-19, 05:00 AM
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As I previously suggested, check the specifications for the LED array. It they don't mention sequencing on the power supplies, contact the LED array manufacturer or proceed at your own risk.
 
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