Harmonics and Earthing System

Chandupp55,

Welcome to the forum with your first post. To get a good solution to your problem, as much information as you can give will help, including the type of interference, the type of panel which is having this interference problem, distance between them, power source for each, interconnections between them, if the VFD panel has more than one drive, when the interference occurs and its magnitude...

I will take a couple guesses--the interference is probably with an analog signal, and occurs when one (or more) drives are running; it disappears completely when all drives are off. Also, the panels are within a meter or less of each other, and some cables for signal and control run between them.

If this is so, then I would suggest five things to look into: 1) Ensure that the earthing (grounding or bonding) conductor that goes from the VFD to the motor is terminated inside the VFD panel ONLY on the earthing (grounding) terminal in the VFD and not on any other convenient earth/ground terminal in the VFD panel. 2) Make sure that all analog signal (and even discrete I/O signal cables) are run separately from the VFD line and load cables/conductors. This means that they are at right angles when they cross paths and otherwise at least 15cm (6") apart if running parallel. 3) Make sure that all analog signal cables are screened (shielded), with the screen (shield) properly terminated at one end only. 4) If noise persists, seriously consider using screened (shielded) cable for the load wiring from the VFD to the motor. 5) Consider a line reactor to minimize electronic noise being put back onto the incoming power line.

Further information will be helpful, particularly if any of my assumptions are wrong.

--JMM

Definitly agree with JMM with assumptions. If all of the VFD's are located in a single cabinet the grounding should all go to a single ground lug or busbar. This would be a point of common coupling (pcc) for the ground sytem just as the power delivery main breaker is the power pcc. If the grounding is left to panel ground screws or at the drive itself and assumption of grounding through the chassis. you will create ground loops, raising the potential through the porous material of the panel/cabinet.

If, the drives are in seperate enclosures with seperate power delivery disconnects (assumption) then insure the ground of the power delivery is run seperately along with the power from the source to the ground bus in the enclosure. If this is not done and grounds are linked between either drives or enclosures then run to either the source ground or ground-rod you will have created multiple groundloops, raising the potential of the overall circuit. This can be reduced by running seperate grounds to the enclosures from the source or driving seperate ground-rods. In either case check resistance to the source ground after connection.

Also, due to the newer drive IGBT switching, upsize the ground wire. If power delivery is #6, use #4 for system ground.

Again, agreeing with JMM on the signal runs to cross at 90-degrees OR at least 30-inches apart if runs are required in parallel. In any of the assumptions more information is required for final resolution.

What type of cables are you using between the VSD units and motors? If you look up the Olex cables site and check out the "Varolex" cables you will get a good insight into modes of interferance transfer and how to address it. You will note the cables have interstitial earths and that the total cross section is 3 times what would normally be used for motor cables and that the copper screen also has a large cross section. In bad cases it becomes necessary to isolate each drive via a specially constructed isolation transformer. Siemens are a good source of drive information to research.

Check with the VFD manufacturer as to the compliance to the latest local directives on harmonics and radio frequency interference. In some cases, replacement with a different make can be an acceptable solution.

Neutral-to-ground voltage is a function of the single phase loads connected elsewhere on the distribution system, and represents the neutral conductor resistance as presented to the single-phase loads. Be very surprised if this voltage is zero!

The increased Earth to Neutral voltage with VSDs is largely due to capacitive leakage of the fast rise time switching pulses through the dielectric to the motor frame and building structures generally. This has brought about a number of fixes including special cables, isolation transformers, VSD rated insulation and rotor shorting brushes to protect the bearings from spark erosion. Large complex installations such as Coal Wash Plants incorporate all the above. The capital cost is substantial but somewhere along the line there must be a ROI or no one would bother.