Mike Dolt Enterprises
Mike Dolt Enterprises
User: dale87, Location: illinois
Asked on: Tuesday, December 17, 2024 at 7:45 pm EST
Answered on: Thursday, March 6, 2025 at 6:26 am EST
Question: Looking for some insight as to what happened at a local water plant. A scheduled outage from the utility was scheduled for maintenance. Plant operators stopped all their VFD's and across the line starters. They did not power down the drives, just stopped them before the outage, so they where setting there idle, powered up, with no run signal. After everything was stopped, maintenance gave the poco the okay to take the outage. Power company pulled the 4160V cut outs to their 300KVA Delta Wye 277/480 pad mount one at a time, did their work, and closed the cut outs 30 minutes later one at a time. No issues..... checked voltage at the transformer.... poco lets maintenance know they're done and can restart the plant. Maintenance walks into the plant to find a 75HP 480V ABB ACQ580 with 2 blown input fuses, and a 50HP 480V Toshiba P9 with 2 blown internal input fuses. Both drives where replaced with spares and the plant was up and running. Condition of the drives is unknown but assumed blown rectifiers. Other drives in the facility where not affected. Water plant blames poco...... Come to find out, a week prior to that, one of the bayonet fuses on the primary side of the pad mount was loose from the factory and had been arching to the point of failure, and one leg of the primary was lost resulting in the plant voltage going crazy and drives kicking out. Poco replaced the bayonet the same day, but didn't have the correct size, so had to take the above outage a week later to install the correct size fuse. Alot of finger pointing going around, plant maintenance says facility should have been completely disconnected from utility via main disconnect before power down, and that the fact that the 4160V cut outs where pulled and closed one at a time caused the issue. Poco explains to the facility that they could have a primary fuse blow or line snap during any storm that would, and probably has, recreate above scenario, and the drives should protect themselves and fault out. Open to any ideas as to what may have caused the issue. No line side reactors on either drive. No ferro resonance issue at the transformer. Drives are both about 2 years old.
Okay, this is a tricky situation with a lot of potential contributing factors. Here's a breakdown of possible causes and who might be at fault, based on the information provided
Possible Causes (and contributing factors):
1. Voltage Imbalance/Harmonics from Single-Phasing During Switching
This is the most likely culprit.
The Problem
When the power company opened and closed the 4160V cutouts one at a time, they created a brief period of single-phasing on the primary side of the transformer. This translates to a severe voltage imbalance and significant harmonic distortion on the secondary (480V) side feeding the plant. VFDs are highly susceptible to these types of power quality issues.
How it Damages VFDs
The rectifiers in VFDs convert AC to DC. Voltage imbalances can cause overcurrents in one or more rectifier diodes, leading to their failure. The blown input fuses are a telltale sign of this. Harmonics can also stress the rectifiers and capacitors.
Why Only Some VFDs Failed
Not all VFDs are created equal. Their sensitivity to voltage imbalance and harmonics can vary. Also, the location of the VFD within the electrical distribution system can affect the severity of the voltage imbalance it experiences. VFDs closer to the transformer (electrically speaking) may have experienced the worst of it.
Contributing Factor - Previous Fault
The arcing bayonet fuse a week prior likely stressed the VFDs and other electrical equipment in the plant. Even though the fuse was replaced, the prior stress could have weakened components, making them more vulnerable to the event during the outage.2.
Voltage Sag During Switching
Even without perfect single-phasing, the act of opening and closing the cutouts can cause a brief voltage sag (dip) on the 480V side. While VFDs should be able to ride through minor sags, a significant sag can cause them to trip or even damage components, especially if the VFD's DC bus capacitors aren't in perfect health.3.
Switching Transients/Surges
While less likely given the relatively slow switching of cutouts compared to breakers, there's still a possibility of switching transients (voltage spikes) being generated during the opening and closing of the cutouts. These transients, if large enough, could damage the VFDs, especially if they lack adequate surge protection.4.
Pre-Existing VFD Issues
It's always possible that the failed VFDs were already weakened or had components nearing failure. The voltage imbalance event could have been the straw that broke the camel's back.5.
Grounding Issues
Poor or inadequate grounding can exacerbate the effects of voltage imbalances and transients. It can also contribute to harmonic problems.
Who's Potentially at Fault (and Why)
Power Company (Poco)
Primary Responsibility
They introduced the initial problem by opening and closing the cutouts one at a time. Standard utility practice, especially when dealing with sensitive equipment, should be to find a way to minimize voltage imbalances during switching. This might involve using different switching methods, providing temporary backup power, or requiring the customer to disconnect their equipment.
Knowledge of the Problem
The fact that the Poco knew about the previous arcing bayonet fuse failure is significant. They should have been more aware of the potential for problems when performing the scheduled outage.
Their Defense (and its Flaws)
While it's true that a primary fuse can blow or a line can snap during a storm, that's an unpredictable event. A scheduled outage should be handled with greater care to avoid unnecessary voltage disturbances. Also, while VFDs should have some level of protection, it's unreasonable to expect them to withstand severe and prolonged voltage imbalances.
Consider Mitigation
Power companies sometimes have procedures to mitigate these types of issues like: Install Temporary Load Bank to keep load balanced during switching Coordinate a complete facility shutdown so the line is de-energized
Water Plant
Potential for Mitigation
The plant maintenance team could have disconnected the main disconnect. However, this is not always practical, as many industrial processes take a long time to shut down and restart.
Responsibility for Protection
The plant has some responsibility to protect its own equipment. This could involve:
Line Reactors
Adding line reactors on the line side of VFDs can help to mitigate harmonics and reduce the impact of voltage transients.
Surge Protection
Installing surge protection devices (SPDs) at the main service entrance and at individual VFDs can help protect against voltage spikes.
Power Quality Monitoring
Implementing a power quality monitoring system can help identify and address power quality issues before they cause equipment damage.
Consulting with Power Quality Expert
Consider bringing in a power quality expert to assess the plant's electrical system and recommend solutions.*
Their Defense (and its Flaws)
While disconnecting the main disconnect is a good general practice, it's not always feasible. The plant is also relying on the power company to provide a reasonably stable power supply.
Recommendations
Documentation
Thoroughly document the entire event, including the timeline, the equipment that failed, and the power company's procedures. This documentation will be crucial for any claims or future discussions.2.
Root Cause Analysis
Conduct a thorough root cause analysis to determine the exact sequence of events that led to the VFD failures. This may involve analyzing power quality data, examining the failed VFD components, and reviewing the power company's switching procedures.3.
Negotiation with Power Company
Attempt to negotiate with the power company to cover the cost of the damaged VFDs. Emphasize the fact that the voltage imbalance was caused by their deliberate switching procedures.4.
Power Quality Assessment
Conduct a power quality assessment of the plant's electrical system. This assessment should identify any existing power quality issues and recommend solutions to mitigate them.5.
Protection Upgrades
Consider upgrading the plant's electrical protection systems, including adding line reactors, surge protection devices, and power quality monitoring equipment.6.
Communication
Improve communication between the plant and the power company. Ensure that both parties are aware of each other's needs and concerns.7.
Future Outage Planning
In the future, work with the power company to develop a detailed outage plan that minimizes the risk of voltage imbalances and other power quality issues. This plan may involve using different switching procedures, providing temporary backup power, or requiring the plant to disconnect its equipment.
In conclusion, the primary responsibility for the VFD failures likely lies with the power company due to the voltage imbalance caused by their switching procedures. However, the water plant also has some responsibility to protect its own equipment from power quality issues. A combination of negotiation, investigation, and protection upgrades will be necessary to resolve this issue and prevent similar problems in the future. Flag for review