2 Power Supplies - Relay momentarily drops out . . . 1

[pmover]

Once again, bear with me as I'm a gearhead, not a sparky . . .

There are 2, 24-vdc power supplies providing electrical power to each respective turbine/compressor unit and to other critical electrical systems.

The power supply to the other critical electrical systems is via a relay. The relay transfers electrical power from one power supply (i.e. primary supply) to other power supply (i.e. secondary) in the event of a low voltage output of primary power supply.

The problem is that when the relay transfers electrical power from primary to secondary, the other critical electrical systems will momentarily loose incoming electrical power. This momentary loss of electrical power will allow a relay within the critical electrical system to drop (senses loss of incoming electrical power), thus shutting down all operations.

What modifications can be made to avoid this momentary power loss and avoid the relay in the critical electrical system from dropping out?

Eventually, a dedicated 24-vdc power supply will be installed to provide power to the critical electrical systems (via UPS), but in the meantime (i.e. short-term operational solution) what modifications can be accomplished to sustain operations?

Other options and suggestions are acceptable.

Thanks!
-pmover

 

[Comcokid]

How much is the maximum current used by the 24 volt control/electrical systems? Since that may a little difficult for a mechanical to measure or determine, then what is the current rating of the power supplies?

What I'm getting at here is that if the control systems just use a few amps at 24 VDC, then a large capacitor may hold up the 24V supply long enough for the switch-over to take place (50 to 100 msec). Otherwise, if the control system takes a few tens of amps, a hold-up capacitor may become far too large to be a solution.

 

[pmover]

Comcokid,

ok, thanks, ya made me chuckle a little . . .

yes, we measured the amp draw at start-up and i recall the amp draw for the 24-vdc critical system to be about 3-amps. i do not recall the normal load amp draw (too many glasses of wine . . .). i think the fuse size is 2 or 3-amps.

a local, smart electrician mention the capacitor option, but not too sure about it as he expressed some uncertainity (low degree of confidence - a gearhead factor too!). how is the capacitor size determined? is there a method/technique to test the capacitor option to ensure a high reliability (short-term basis)? Lastly, how would the capacitor be connected to the electrical system?

Thanks!! Hope this helps!
-pmover

 

[ScottyUK]

Decouple the power supplies from each other using blocking diodes. It's a standard technique for building a no-break redundant power scheme on DC systems.

The first page of this article shows the principle - it was harder than I expected to find an image, or I'm just using the wrong search phrase.


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If we learn from our mistakes I'm getting a great education!

 

[pmover]

ScottyUK,

i somewhat did not want to discuss or previously mention blocking diodes we believe they are troublesome in this installation. for 25+ years, blocking diodes were in service.

This particular location has a history of electrical problems with the 24-vdc power system, whether or not associated with blocking diodes, I do not know.

This past May-June, we encountered problems & measured 40+ volts dc output at the turbines and back at power supply connections. somehow, someway, both 24-vdc power systems supplemented one another and sent 40+ volts to the turbine units. The 40+ volts dc can result in electrical component failure on these units; a rather $ repair. The same 40+ volts dc was measured at the critical electrical system, which resulted in relay and light failures.

by design, both turbine units have isolated grounding systems (floating grounds). so powering both units from one power supply can be advantageous until VRLA battery replacement is completed (if needed).

i decided to completely isolate the 24-vdc power supply to each respective unit (no longer load sharing capabilities), keep the floating ground, and then install a relay to power the 3rd load (critical electrical system), which draws electrical power from either 24-vdc power supply.

Oh heck, you mean to use the blocking diodes on the critical electrical system only! i will look into this option later this week. Weidmuller has blocking diodes that may work, but we cannot afford to have the high voltages.

yes, blocking diodes are likely the best option, but these devices (?) MAY have caused problems that were very troublesome. yes, too many headaches for this gearhead.

ok, thanks ScottyUK!
-pmover

 

[ScottyUK]

Hi pmover,

If you need a true galvanic separation then ComcoKid has likely offered the alternative: a big reservoir cap sized to bridge the 30ms (?) or so to get a confirmed open from one relay or contactor and close the other. Time will depend on the relay or contactor: a 10A PCB-mounted relay will move a lot quicker than a 1200A bar & shaft contactor.

FWIW, I have seen some 220V appear in some unexpected places when one 110V battery had a positive pole earth fault and the other had a negative pole earth fault. Bad things started happening - the equipment should have been ok in this state, but a few initially inexplicable generator trips occured until we realised what was happening and implemented some circuit mods. I can relate to the once-bitten, twice-shy sentiment.


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If we learn from our mistakes I'm getting a great education!

 

[Comcokid]

How big a capacitor gets into questions about what is the drop-out voltage of your relays, or the most sensitive item in your controls.

With a 3-amp load at 24 volts (equivalent to 8 Ohms), and assuming the 24Volt components operate properly down to 16 volts, then 10,000uF will hold for 30 msec.

2-amp load, 10,000uf will hold up for about 50msec

1-amp load, 10,000uF will hold up for about 100msec

Given the other issues of your post, I would suggest a 50 volt minimun rating on the capacitor. This capacitor will be about the size of a coffee cup. When you turn-on the system, the power supply will pull more current for a half-second as the cap charges, but any transients on your 24 volts will certaintly be filtered away.

Rather than solve it with the standard natural log time constant formula, I just sketched it in a circuit simulator (spice program) to get approximates.

 

[pmover]

Thanks Comcokid!

ok, now I've a better understanding of the capacitor size and capacity. Yes, the load and relay data (transition time) will be a factor in determining the capacitor size. i'll investigate further into the math behind the capacitor sizing - sort of scrape the rust of thy brain as mid-80s electrical classes were a long time ago . . . an RC circuit analysis i believe.

thanks and i agree on the minimum voltage (50-vdc). fyi, we just replaced a 4200uF capacitor at 50-volt, so i have an idea on the size (about 1.25-inch diameter and 6-inches long).

i'm not too concerned about the additional load imposed at start-up as the current fuse is a slow-burning fuse, but a matter that does need to be verified.

Thanks fellas! Ya done good!!!
-pmover

 

[Skogsgurra]

You may run into a problem with welded relay contacts if you add a large capacitor. It will usually not be a problem if your sources have small capacitors since the total "impedance" will be low.
But, if your DC sources are accumulators or if they have large capacitors (they probably have), you will have to watch out for that problem. It will probably be worse at first switch-on than during transfer caused by low voltage.
I would prefer to use diodes. Best solution.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[ScottyUK]

I've previously solved the problem Gunnar identifies using a resistor and a diode in parallel: the resistor limits the charging current for the cap, but the diode allows the cap to discharge at whatever rate the load demands. It's certainly not the solution for every situtation but it could be appropriate here.


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If we learn from our mistakes I'm getting a great education!