redundant supply vs UPS

  • Thread starter A. Edmundo Vega de la Garza
  • Start date
A

Thread Starter

A. Edmundo Vega de la Garza

Hi, partners

I am seeking advantages and disadvantages of using 24 VCD redundant supply instead 120vca
UPS to DCS, ESD, F&G, etc.

thanks in advance
 
V

Vitor Finkel

IMHO, that's an easy one.

If you try to design an AC supply that has no common failure mode, you can't.
All AC suplies systems I've seen so far, carry at least one "part",
component or module, where a single failure may compromise the entire
system supply.

Using DC allow you to do least expensive systems, that are quite fool proof
in the sense that any single ( or even multiple ) component, module, part,
etc. may fail without shutting down or endangering your installation. I've
never seen an AC scheme capable of doing that.

The better way to look into this matter is to try to design some elementary
schematics using AC and DC and comparing their "visible" reliability
failure points.
If you need some basic schematics to play around, let me know.

Vitor Finkel
Rio de Janeiro Brazil
 
J
You don't mention what your specific application is.

Typical refinery/chemical applications that I have seen use both.

The 110 (or 240) VAC is used to keep HMI-type equipment up as well as
possibly power the main control system, ESD system, etc.

The 24 VDC systems are typically much smaller and limited to providing power
for the 24 VDC field loops.

There is not really a correct answer. Take a look at how well rationalized
various voltages are in the plant, what the emergency shutdown philosophy
for the plant is, what the maintenance philosophy for the plant is, etc.
 
We use both. Each serves a purpose. If your 24VDC power supply fails a UPS
won't help but a redundant PS will. Likewise, if your power goes down
redundant, a 24VDC power supply does nothing, but a UPS gives you time to
take action.

Mike Ryan
Aerojet Fine Chemicals
 
J

Johan Bengtsson

A set of redundant power supplies (connected together with ordinary
diodes) does not basically solve the same problem as an UPS.
I would say that the best thing to have if you absoultely MUST have
power is (as suggested before by someone) something like this:

1. Have two (or more) power supplies, connect with diodes.
Preferably power supplies built in different ways.
2. Feed one (up to all BUT one) thru an UPS.
3. As an option you can add a set of batteries connected tru a diode
too. The voltage have to be less than the ordinary power supplies but
high enough for the load.


In this way you will have:
1. Protection of power fail by using the UPS
2. Protection if any power supply fail (at least one other way of supply)
3. Some common mode failure protection. If the types of power supply
differ it should be less probable that both fail at the same moment if
something happens. If the UPS fail thare is a way to pass it.

You will NOT get:
1. Load balancing, the diodes will make the power source with the higest
voltage the one used.
2. One common mode failure exists, the diodes. If a diode break you may
either not notice until it is too late or have the power drop because of
it. Properly dimensioned diodes should be long lived though.
3. Overload protection. A short circuit may be able to sink more current
than you want it to be able to. Especially if you use batteries too


A note: you must continously monitor all the power sources before the
diodes and action if any one fail, otherways you loose your redundancy.

Johan Bengtsson
P&L, the Academy of Automation
Internet: http://www.pol.se/
 
V

Vitor Finkel

At 07:39 17/11/99 -0500, Paul Gruhn wrote:
>DC supplies can be dioded together without any kind of switching mechanism
>to introduce potential failures.

You've said it Paul,

That's the most important feature, that no AC supply can compete with.
Besides that, different power Supplies, from different AC sources, and
2 battery sets complete a "minimum guaranteed" non interruptible supply,
almost without any possible common failure mode.

Vitor Finkel
P.O. Box 16061
22222.970 Rio de Janeiro Brazil
 
V

Vitor Finkel

At 07:49 18/11/99 +0300, Patrick McGinnis wrote:
>"A. Edmundo Vega de la Garza" <[email protected]> [wrote]
>>I am seeking advantages and disadvantages of using 24 VCD redundant supply
instead 120vca
UPS to DCS, ESD, F&G, etc.<<
>>
>Vitor Finkel [email protected]
> [responded]
>
>IMHO, that's an easy one.
>
>If you try to design an AC supply that has no common failure mode, you
>can't.
>All AC suplies systems I've seen so far, carry at least one "part",
>component or module, where a single failure may compromise the entire
>system supply.
>
>Using DC allow you to do least expensive systems, that are quite fool proof
>in the sense that any single ( or even multiple ) component, module, part,
>etc. may fail without shutting down or endangering your installation. I've
>never seen an AC scheme capable of doing that.
>
>The better way to look into this matter is to try to design some elementary
>schematics using AC and DC and comparing their "visible" reliability
>failure points.
>If you need some basic schematics to play around, let me know.
>
> And [I comment]
>
> Edmundo and Vitor,
>
> I believe Edmundo is seeking a secure power source for his DCS/ESD,
>etc.

Aren't we all ?

>and that must deal with his ability to control the security of the
>primary energy source.

Absolutelly !

>Component level concern is not involved as long as
>quality equipment with certified mtbf is used.

Excuse me ! I strongly disagree. I've never seen a single AC installation
that really can be considered "Uninterruptible". Try as want, and there
always be common failure points in AC supply. Weather you use different
sources, such as Mains Supply, Emergency On-Line Generators, battery
driven Inverters, etc. There always is at least a switching device,
that sooner or later will fail and you are going to loose power, usually
for quite some time, untill repairs ( or failed devices removal ) are made.

You can avoit that by criteriously using DC. Eliminating common failures
is usually very desirable, specially when disigning a SIS.

> Unless you are using a big battery bank to supply the 24 vdc and
>have absolutely no devices that rely on ac power, you must have reliable ac
>as your energy source for DCS/ESD.

Here you are absolutelly right. The size of battery banks ( 2 semi-banks,
so that you may remove one from service, while the other may feed you on
Emergencies for about 1/2 the time you figured both would feed you ). All
ESD/SIS components ( and why not BPCS, as well ) should be prefearably
DC feeded. "big battery bank" size depend upon how much time do you need
to run on them. On most cases your process equipment will be deenergized
anyway, so you don't need too much time just for SIS and control devices.

>Just feeding 24 vdc power supplies with
>generic ac line voltage does not cut-it. Once the ac fails, your system is
>dead. Various configurations of UPS (with battery backup) can be used. Your
>study must go back to the ac voltage source. Do you have any control over
>determining the reliability of the ac source?

No AC source is as reliable as desired for SIS purposes.
Frequently you may find more than one AC source available to feed your DC
ESD/control power. Besides that, the batteries should carry over between,
for instance "normal" AC supply and Emergency generators cut-in, or until
an orderly shut down and power down of ESD/SIS occurs.

>Whether it is from redundant
>switchgear in substation(s)? Or can you influence the arrangement of the
>incoming High Voltage utility feeds?. For example, if you are in a major
>hydrocarbon processing plant, you will want to determine if your utility
>service (15 - 115kV?) has the capability to bring redundant lines to your
>plant. Or, your substation may already have redundant 4kV switchgear feeding
>480 volt MCC's that can be used for separate feeders to a dual redundant
>UPS. If it is a truly secure installation, there will be a generator backed
>alternate source available to feed the UPS static switch.
>
> There are many variables, but again, unless you use batteries and
>have no need for ac to run your system when if the lights go out, your line
>ac fed 24 vdc power supplies are dead!

Right. You need the batteries, and design your control,shutdown systems to
run completelly on DC, just to assure that when lights go off, and all your
motors ( compressors, pumps, ventilators ) are off, the SIS and the operator
IHMs are still alive, at least for some time to guarantee safety and some
usefull housekeeping to make an easier, faster and safer start-up when AC
Power comes back.

> Good luck

You don't really need it, just do a propper design :)))

> Patrick McGinnis
> Saudi Arabia


Vitor Finkel [email protected]
P.O. Box 16061 tel (+55) 21 285-5641
22222.970 Rio de Janeiro Brazil fax (+55) 21 205-3339
 
A

Albion Zeglin

Except for lightning strike, or load failure. If you short out the system you
will still blow the breakers.
If it's this important make sure you have an offline backup.

Albion.
 
J

John G. Boland

Hello, the list!

<< I am seeking advantages and disadvantages of using 24 VCD redundant supply
instead 120vca UPS to DCS, ESD, F&G, etc. >>

There's been a lot of practical advice on this. Three observations, to avoid
trouble:

1) All of the suggestions are based on having at least one reliable, but
discontinuous, AC supply. Make sure that this is true - with an emergency
generator and transfer switch, for example, that FOR SURE will come on line
within a VERY conservative interval after mains failure. Test regularly
(weekly).

2) At some level, all of the suggestions transform AC to DC, support the DC
with interim (battery) energy storage, then transform the DC to AC to DC
(perhaps at board-level). WHERE the storage occurs is a logistical choice
that depends on your loads.

It is often most cost effective, lowest risk, and most maintainable to do
this at high voltage, as part of a multi-kW online UPS (i.e., continuously
supplying the load, except in case of failure, when it falls back to the
mains). In this case, the batteries and battery charging system are part of
the UPS reliability, and tend to be good.

On the other hand, if only a very small part of the control system is
involved and uninterruptible power does not need to be distributed, A local
battery bank might be better. MANY small banks are not a good idea, from a
maintenance and facility design (venting...) point of view. Battery and
battery charger choice is also a big issue, if you are going to "roll your
own".

Test regularly (quarterly) for battery draw-down (cell by cell) by failing
the charging - should be several times the generator startup time, typically
one hour.

3) In ANY case, minimize the number of power supplies, don't use connectors,
prevent unauthorized "taps", design your system to be tested while in
operation, and stick to the test schedule.

EVERY component in your system is going to fail. Someday. Probably when
stressed by failure of another component (shared loads) or transient (voltage
spike or dropout). Probably simultaneously with another component. Probably
at night, on a long weekend. Be sure you can live with what's left. Test.

Paranoia is pretty reasonable, once you think about it. 8^D

Test.

Good luck.

John G. Boland
 
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