Emerson Exchange 365

Noise between Delta V Control System Earth and Safety Earth - many VSDs

Hi Forum, in our new plant we are using hundreds of VSDs.  We will be bonding the main control system earths to the safety earth of the plant. We will be following correct bonding principles, but  I am concerned that noise generated from the many VSDs could interfere with the control system earth.  Many years ago in Thailand on a refinery project, we linked the safety to the control system earths via a choke.  I cannot remember the specification of the choke, or whether it is really necessary.  Any ideas?

David Tudor Evans

• 20 Jun 2019 1:41 AM
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10 Replies

• David Tudor Evans

  

  • 20 Jun 2019 3:29 AM

  Thanks Les. I am in Australia, can we maybe have an email discussion?

  Best regards

  Dave

  
  

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• Kit Chong Mak

  

  • 20 Jun 2019 4:36 AM

  In reply to David Tudor Evans:

  Hi guys, if you could include me in your discussion would be great as we are also planning to tie in multiple VSD systems to a new DCS system. I would like to learn from you guys any specific way we need to handle the grounding

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• David Tudor Evans

  

  • 20 Jun 2019 4:55 AM

  In reply to Kit Chong Mak:

  The main issue for us is to determine the frequencies of noise we are generating from our many ( varied) VSD's and then working out which inductance and or combination with a resistance / capacitance is going to provide the best filter.  This is a practical matter, and I am sure some vendor has already worked this out.

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• Kit Chong Mak

  

  • 20 Jun 2019 5:49 AM

  In reply to David Tudor Evans:

  Hi David,
  
  As i have little knowledge in this field, i would like to understand the issue more. The concern here is due to the shared ground between VFD and Control system which will potentially cause the EMI from the VFD to travel back to the Control system?

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• David Tudor Evans

  

  • 20 Jun 2019 5:55 AM

  In reply to Kit Chong Mak:

  Hi Kit,
  yes, that is the prime concern for us. We need to bond the earths together for safety, but the VSDs will generate a lot of noise. I am a process engineer so need some advice on the best electrical filtering techniques, given we will have various frequencies.

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• Sal Salamone

  

  • 20 Jun 2019 9:51 PM

  In reply to David Tudor Evans:

  Hello David and Kit,
  VFDs (or VSDs) make a lot of very high frequency noise, way above 50 or 60 Hz. High frequencies are blocked (filtered) by an inductor, but I do not think that's what you want to do on your ground line. Normally good VFDs have line reactors before the VFD and chokes on the motor leads as they leave the VFD. These protect the drive and motor but also reduce harmonics (high frequency) noise. Then you should use inverter duty cable between the VFD and the motor. Inverter duty cable has a ground cable next to or around each power cable (i.e. 3 of them) as it runs between the VFD and motor. All grounding goes back to the VFD and the VFD grounds to the Motor Control Center's ground. This MCC ground should not be the same as your DeltaV ground because the MCC ground ties to the Wye transformer's middle that is then grounded. That will end up being the building ground but hopefully somewhere other than where your DeltaV system gets grounded. The DeltaV Instrument and Safety ground should run separately until they tie together as the plant/earth ground point. As your wire gauge increases, keep it increasing (or remain the same) as you go toward the plant ground point; never decrease wire gauge or you'll develop reflective waves.
  Finally, run your low voltage perpendicular to your high voltage if they must cross. Try to minimize parallel runs since that's where the induced voltage will be generated. Having a "barrier" in a cable tray is not enough. Use separate trays and make your low voltage a covered tray (nearly totally enclosed) and made of galvanized steel not aluminum.
  I know it costs more up front but it will have huge payback a year later and on. If you can, have them drive a separate grounding rod for your control system ground.
  
  FYI, my background, I'm a Electrical Engineer who worked in paper mills for many years. The advice I have given is from that experience.
  Regards,
  Sal

  Sal Salamone

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• David Tudor Evans

  

  • 21 Jun 2019 1:24 AM

  In reply to Sal Salamone:

  Many thanks for your response Sal.
  
  So in essence, there is little point in putting a choke between the electrical and control systems earth. We have discussed internally and we will be installing a separate instrument earth pit near the switchrooms containing the VSDs. We will then be connecting the two together just before they enter the ground. I take your point about not reducing the earth wire gage. We are using high quality VSD, so expect noise on the motor leads to be adequately controlled.
  
  Best regards,
  Dave Evans

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• Les Ballard

  

  • 21 Jun 2019 1:40 AM

  Hi David,

   

  I would like to help with this concern. Please give me some time to figure out how to support Emerson’s engineers directly.

   

  I have been contacted by Emerson on how I need to communicate support. Hang tight my friend.

   

  Les Ballard

  Phone: 414-758-3870
  Email: les@basgap.com
  Web: basgap.com

   

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• Sal Salamone

  

  • 24 Jun 2019 1:05 PM

  In reply to David Tudor Evans:

  Hello David.
  Yes, in my experience a choke between the electrical and control system earth will do nothing. It is however VERY important to deal with the electrical noise at the source, and not just for the sake of the control system. The biggest violation I've seen is running VSD, or other 120VAC and above, in parallel (right next to) 24VDC and other buses, and then in second place, no ground at all (or two different grounds causing a ground loop).
  You're asking the important questions, good luck.

  Sal Salamone

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• David Tudor Evans

  

  • 25 Jun 2019 1:16 AM

  In reply to Sal Salamone:

  Hi Sal,
  thanks, yes discussing here with Electrical, we decided not to install the choke, but to connect the instrument and safety earth bars together in the switchrooms, just before the common earth pit. The VSD noise will be controlled by keeping motor leads (with individual screens) well away from any signal wiring. Thanks again for your advice.

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