Mike Dolt Enterprises
Mike Dolt Enterprises
User: jnh, Location: Northern illinois
Asked on: Tuesday, June 28, 2005 at 1:50 pm EDT
Answered on: Thursday, May 9, 2024 at 12:45 pm EDT
Question: I have an application where my 250 HP, 480V 18 pulse variable frequency drive keeps tripping on bus overvoltage. The electrical service feeding the mcc is from a 500 kVA transformer, 3 phase, 3 wire service comprised of 2 sets of (3-500kcmil, #2/0G) per phase. The VFD is fed from the MCC via a feeder breaker. The Utility put a meter on the secondary side of their 500 kVA transformer and noticed that for a split second the voltages in two phases become elevated and the third phase voltage drops, which is evident of an open neutral. What else could be causing this. The Utility did not seem to think it was their problem. What can be done to fix the problem?
Possible Causes
Loose or corroded neutral connection
A loose or corroded neutral connection can cause an open neutral, resulting in voltage imbalances.
Faulty neutral conductor
A damaged or undersized neutral conductor can also lead to an open neutral.
Unbalanced load
A heavily unbalanced load can cause voltage fluctuations, which can contribute to an open neutral condition.
Capacitor bank malfunction
A faulty capacitor bank can cause voltage distortions and imbalances, potentially causing an open neutral.
Harmonics
Excessive harmonics generated by the VFD or other equipment can interfere with neutral currents and cause voltage imbalances.
Steps to Resolve
1.
Verify the neutral connection
Inspect the neutral connection at the transformer, MCC, and VFD. Ensure it is tight and free of corrosion.2.
Check the neutral conductor
Inspect the neutral conductor for any damage or undersizing. Replace it if necessary.3.
Balance the load
Analyze the load distribution and make adjustments to balance the currents across all three phases.4.
Inspect the capacitor bank
Test the functionality of the capacitor bank and replace any failed capacitors.5.
Minimize harmonics
Implement harmonic filters or other mitigation strategies to reduce harmonic distortion.6.
Upgrade the VFD
Consider upgrading to a VFD with a lower harmonic profile or a built-in active front end (AFE) to minimize harmonics.7.
Contact the utility
If all other steps fail, contact the utility and provide them with detailed documentation of the issue. They may be able to diagnose and resolve the problem on their end.
Additional Considerations
Ground fault protection
Ensure that proper ground fault protection is in place to detect and isolate any potential ground faults.
Regular maintenance
Perform regular maintenance checks on all electrical components, including the VFD, MCC, and transformer, to prevent potential problems.
Load testing: Conduct load testing to verify that the system can handle the current loads without causing voltage imbalances or other issues. Flag for review