# Problem caused by thyrestor firing



## a.atalla (Apr 22, 2010)

hello every body 
this is my first topic in your forums 
i faced a strange problem I never had before , I work in a drilling rig we have DC motors driven by SCR system (AC to DC using thyristor bridge)

the rooms and offices takek power from the main busbar (600 volts) through a transformer (600:380 volts)

when those motors starts we here noise in the florscent lamps at the bed rooms and we had alot of computer power supply damages 

the wave forms of the voltage at the office before and after starting the motors as follow 






so does the ripples in the second picture cause to burn all the power supplies or it is normal???

NOtice :: the element that damages in all the computer power supplies is the smoothing capacitor after the diode bridge


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## ElectricArcher (Feb 25, 2010)

Where are you taking the reading from on your oscope?


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## a.atalla (Apr 22, 2010)

i took it from a socket in my office


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## wildleg (Apr 12, 2009)

what is the input voltage rating for the power supplies that are burning up.


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## MDShunk (Jan 7, 2007)

The peaks of the waveforms still look about the same. I'm a bit puzzled about your power supply issue, myself.


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## micromind (Aug 11, 2007)

A line reactor installed ahead of the DC drive would smooth a lot of that out.

Switching power supplies don't handle the spikes on that waveform very well. 

Rob


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## a.atalla (Apr 22, 2010)

wildleg said:


> what is the input voltage rating for the power supplies that are burning up.


100-240 volts 50-60 HZ most of them 



MDShunk said:


> The peaks of the waveforms still look about the same. I'm a bit puzzled about your power supply issue, myself.


the peaks is fine and that leads an acceptable RMS value for the volts 




micromind said:


> A line reactor installed ahead of the DC drive would smooth a lot of that out.
> 
> Switching power supplies don't handle the spikes on that waveform very well.
> 
> Rob


so u think those spikes may damage the capacitor if they work on them along time 
cause i notice that the power supplies that not damaged yet make noise so i expect that they will follow the others very soon 

now i load all the offices and rooms on a separate 380 volt emergency generator till we find what is going on


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## Electric_Light (Apr 6, 2010)

That's only a snap shot over a short period of time and standby UPS output is dirtier than that, so I'm suspecting you're experiencing intermittent voltage spikes not shown in capture. On a 230v line, the peak voltage is 325v. With your motor drive running, you're running slightly over 400v peak, which will cause the capacitors to charge up past 400v. Computer PSUs are usually designed for up to 264v rms or 370v peak. It's possible that you're going even higher than that as the motor control cycles through its process. 

The noise you hear from power supplies and ballasts is from harmonics. You'll hear a similar sound from the PC power supply when its running on a normal UPS (not true sinewave) too. 

As someone mentioned, placing a line reactor right before the drive should reduce spikes considerably and a reactor before your IT equipment should reduce surge from getting to them. 

How many kVA is your computer load? If it's not huge, a mechanical MG-set is something worth looking at as it will completely isolate the load from power quality issues and the flywheel will also serve as a UPS during split-second power interruptions.


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## varmit (Apr 19, 2009)

A line reactor or isolation transformer on the DC drive would help eliminate the " noise" that is appearing on the sine waves. Is this a new problem or has it always done this? How large of a drive/motor is this? If the motor is large, there could be enough voltage drop to create problems. If this system has ran for a while and this problem only began recently, the DC drive could be on the verge of failure. As the drive components begin to near failure, USUALLY the line disturbances will worsen.


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## John Valdes (May 17, 2007)

Why don't you put the office on a separate isolation transformer? I do not think line reactors will help as they only condition the power going to the drive.

I worked in a plant that had 18 DC motors with 200 HP being the smallest. Each one had an isolation XFMR right in front of the drive. We never used line reactors on DC loads only VFD's. There was never had any issues up front in the office. The office was fed from a separate XFMR.


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## Electric_Light (Apr 6, 2010)

John Valdes said:


> Why don't you put the office on a separate isolation transformer?


From the way OP written, It already is. if the drive is on 600v AC bus, and as the OP described, the office is on the same bus, but stepped down at the 600delta-380Y230v transformer, the transformer is already isolating it. 

If the machine is in 380v and office runs 230v, then a separate transformer would help.



> I do not think line reactors will help as they only condition the power going to the drive.


It cushions the power line from sudden changes in SCR drive and reduces the amount of disturbance reflected back into the line.


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## chop347 (May 26, 2007)

*What Drive are you using?*

Atalla

I have been working on rig in the middle east for the last 10 years.
What you are seeing is very common.
As your drive is rated for 1600 amps and will be driving 2 500hp motors.

You have two options.

1). Run your office/camp off a different power supply.
2). Install a Isolation transformer. Usually at solace 1:1 ratio.

Rigs are always noisy but usually they just screw up UPS systems.
Did this problem just start as would indicate a problem with your drive.
Does it happen on all your MP drives or just one of them?
What type of drive are you using?


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## ko929 (Feb 6, 2009)

Im on a drillship and we use harmonic filters on the high voltage buses. 11kv to 600 v


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## wildleg (Apr 12, 2009)

ko929 said:


> Im on a drillship and we use harmonic filters on the high voltage buses. 11kv to 600 v


y'all have an automatic something like this, or something homemade and manual ?
http://www.nepsi.com/padmountedfilterbank.htm


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## a.atalla (Apr 22, 2010)

chop347 said:


> Atalla
> 
> I have been working on rig in the middle east for the last 10 years.
> What you are seeing is very common.
> ...


hi chop 
i make my report to the main office and i mentioned that we need a harmonic filter 
they send an engineer to mthe rig site ,he make some more grounding stuff and tighten all the connection

the problem sleep for the last few weeks but the monistor wakes up yesterday again 
nothing change ,my laptop charger and few mobile chargers damaged

the strange thing is that nothing in the operation change but suddenly things start burn 
that driving me crazy 

about ur questions 


> Does it happen on all your MP drives or just one of them?


it happens when both mud pumps work together 


> What type of drive are you using?


we use that Chinese copy of ROSS HILL SCR's its name is "BOOMAY"


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## chop347 (May 26, 2007)

*Change*

Atalla

Something always changes.
Did you move your rig?
Are you deeper in your hole?
Has your pump pressure come up?
Did you change assignments?
Did you move what bays your pumps are run on?
Can you figure out what pump causes it?
Is it a certain pump in a certain bay?


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## miller_elex (Jan 25, 2008)

That deviation almost hits 0 Volts when the SCR fires. All other rectifiers are firing at that time, trying to draw juice too.


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## Jlarson (Jun 28, 2009)

miller_elex said:


> That deviation almost hits 0 Volts when the SCR fires. All other rectifiers are firing at that time, trying to draw juice too.


Yep looks like it, I have seen the same thing before, lots of lighting and computer issues cause of it too.


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## Jlarson (Jun 28, 2009)

> ...other rectifiers can get no juice.


So you get power quality but spelling and grammar not so much huh? :laughing: :laughing:


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## Johnpaul (Oct 2, 2008)

Computers are as likely to be damaged by sags as surges and thrysistors provide zero protection. Best to use line conditioners that use an isolation transformer for supply feeds to electronic equipment. 

You mentioned the problems as being related to the motors kicking on and a motor will draw 3x as much current when starting up which is likely to cause a momentary voltage drop, and when they shut down you get the opposite with a voltage surge. 

Common situation with commercial refrigeration equipment with their large motor driven compressors. Computer gear needs to be on its own dedicated circuits with truly isolated grounds (which are often not done properly to provide true isolation). Current on the ground socket in an outlet is quite common and can easily damage computer equipment and can cause data com problems with an unstable reference point for the signal voltage.


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## got2climb (Aug 12, 2009)

Do not do isolated ground. That is nothing more than a dangerious myth, especaly where you are. A dedicated circuit not sharing the neutral can be beneficial. Follow what the others have said and put in either a isolated transformer or some type of line filter. miller_elex has nailed the problem and the ballasts on the florescent lights make it worse as they reach convergence due to the harmonics. Your computer hasn't a chance as now computers must accept power flaws. 

Extra grounding will never work either as the problem is also on the neutral. To save your computers a high end UPS with a line conditioner may be an answer. It needs to be listed for protection from brown outs. Tripp Lite probably has something in your voltage. :1eye:
*  *


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