Harmonics affecting computers

[electrician1]

Working on offshore installation and we have our own power plant.
There is SCR power distribution for big dc motors and various equipment
During the last 3 months 6 computers have burned.They were all the same type and
were connected trough the same type of ups.diagnostic-mother board burned
I measured voltage supplied to computers
noticed that with SCR off (all DC motors stopped) voltage supplied to computers
looks ok and stable- THDr,THDf around 1.47% K factor=1.27 but during the operation, voltage is unstable with
increased 5th harmonic
THD factor increases to 11.90% , K factor=13.1 and computer power supply just follows voltage fluctuation on line and distribute unstable voltage to motherboard.
I read somewhere that THD factor should not be higher than 5% is that some rule or it depends about
power plant configuration and power distribution.
There is no any problem with other electronic equipment.May be will help if we just put power conditioners for all computers?
Thanks

 

[slavag]

Hi Electrician1
Only one recommendation.
Used UPS for computers and communication stuff.
Regards.
Slava

 

[slavag]

BTW. You can also use Active Harmonic Filters
Try google.
Regards.
Slava

 

[dpc]

The 5% THD comes from IEEE 519 and describes the acceptable limit for harmonics at the interface between a customer and a utility.

The 11.9% THD is somewhat high, but I'm a little skeptical that it would be causing motherboards to burn out. Anything power to the mobo of the computer is coming from the computer power supply. This should provide some buffering from the harmonics in the incoming power line. Any possibilities of transients on other incoming lines into the computer such as Ethernet, etc?

Putting the PCs on an active on-line UPS would probably be a good idea for a lot of reasons, as long as you have a contingency plan for dealing with failure of the UPS, such as a static bypass switch, and/or redundant UPS.

 

[jahrtrix]

Electrician1, DPC's skeptism of the harmonics burning
of the mother board seems right. I would question if
the DC supply is the old style 6 pulse or newer 12 pulse.
The 6 pulse was terrible at producing 5,7 harmonics and at
large magnitudes. The newer 12 pulse reduces these to a
more managable number.You mentioned you are at an offshore
installation. Are these not notorius for grounding issues?
If your ground grid is bad or your grounding system has
many additions you may have grounding issues. My suggestion
is look for the quick fixes first before looking at high
dollar installions. A UPS with all the goodies may fix it,
but a bad grounding system will still plague you.
Good luck.

 

[electrician1]

Thanks for quick reply’s
Computers are already connected to ups.Each of them has its own ups.
About DC power,yes it is old 6 pulse.I will have a look in all this

 

[pqguy]

Do you have communication lines between computers? If so, make sure you have common ground between the power & communication side. Single phase UPS typically have small MOV's on front end & they fail in a few months when subjected to industrial environment. If grounding is ok, try a TVSS on panel feeding computers.

 

[slavag]

Hi.
11.9% THD after UPS????
Regards.
Slava

 

[electrician1]

Yes all computers are connected to communication switch.communication switch has its own separate ups.Ground should be on the same potential as grounds for computers power!
Are you trying to say that some voltage spikes can be induced in net (CAT) cable and that can be the reason of burned motherboards.
Slavag, yes 11.9% THD is measured on ups output.Measurment is done with fluke scope meter 123.

 

[slavag]

Hi Electrician1.
I strongly recommend ask to PC and UPS mnf.
For me it's sounds as "serial/type problem" of this type PC's.(may be' PS' may be mother board.)
Regards.
Slava

 

[electrician1]

Hi Salvag
computers are dell GX 280 Optiplex
ups - Powerware3 105
That is what I was thinking.may be just serial/type problem.
But they have been working ok for almost 3 years and then suddenly they all burned in short period of 5-6 months!

 

[slavag]

Hi Electrician1.
I think it's serial/type problem.
Please see attached:
There was problem with blown capacitors in Dell GX270/GX280 computers (one of the reviews:

http://www.news.com/PCs-plagued-by-bad-capacitors/2100-1041-5942647.html)

You need call to DELL support.
Regards.
Slava

 

[LionelHutz]

The harmonics should not be affecting the power supply in the PC. The PC supply should convert the AC to raw DC and then have a switching supply that steps down the raw DC to the power supply voltages and this should easily filter out the harmonics.

One thing to note is that many of the cheap computer UPS's have a very distorted waveform when they are operating on battery. It's done this way because it is cheap to do and the manufacturer knows that the computer power supply usually does not care about the input voltage distortion.

It could be a matter of grounding as others have mentioned. I would look at installing a dedicated isolation transformer for the computer 120V and then installing isolated 120V circuits, including the ground by using isolated ground receptacles. In this way, you can run a dedicated insulated ground wire to the receptacle so all the computer grounds connect to the same common point. This ground point should be the point the neutral of the transformer is connected to.

The UPS's you are using sound like standby units that switch to battery when there is enough of a power problem. Change to online units and they will do an continuous AD-DC-AC conversion which will filter out the harmonics.

 

[jahrtrix]

Electrician1, Sola/Hevi-Duty has a product,a Constant
Voltage Power Conditioner. It is a do-all product that
may take care of your problem. The UPS only helps with
voltage sags, brown outs and power outages. CVPC's takes
care of everything else, but a faulty ground will still
bug you! Make sure the computer grounds are from the
computer and direct back to the generator frame. the
ground has to be isolated. Another issue may be a faulty
SCR not firing. A scope of the SCR power wave form may be neccessary.Scoping will let you see whats going on. As you said, It was working fine until a short while ago, thus my
statement of a grounding issue. Look at any new additions
to the electrical systems.

 

[pqguy]

Powerware 3105 is pure standby UPS with no power conditioning other than a few small MOV's. It produces square wave output when on battery, otherwise passes incoming power directly to load.

 

[electrician1]

Slavag as per your link from April.2003-March.2004 Dell manufactured motherboards for those computers with "faulty" components. We received our computers in November.2004 !
Any way different type of new computers is already installed.Lets see what will happen.
Also like LionelHutz mentioned my ups is stby unit.I will try to get a better type- line interactiv ups with integrated AVR.
Thanks to all for replys