Introduction:
A manufacturing facility faced an extended power outage due to severe weather conditions. The facility relied on a backup power system consisting of a diesel generator and an Uninterruptible Power Supply (UPS) to ensure uninterrupted power supply during such events. However, during the prolonged power outage, the UPS system failed to accept power from the generator and continued to run on batteries, resulting in the discharge of all batteries and posing potential risks to critical equipment.
Timeline of Events:
1- Power Outage Begins:
A severe storm caused widespread damage to the power grid, resulting in a complete power outage in the area. The facility's main power was disrupted, triggering the automatic startup of the backup diesel generator.
2- Generator Activation:
The diesel generator, equipped with an automatic transfer switch (ATS), promptly responded to the power loss by supplying electricity to the facility. The generator was well-maintained and regularly serviced.
3- UPS System Behavior:
As per its configuration, the UPS system should have detected the generator's power and seamlessly switched from the utility power input to the generator input to protect the critical equipment. However, during this power outage, the UPS system failed to recognize the generator's frequency and did not switch over to generator power due to the fluctuation in generator frequency.
4- Battery Power Running:
Since the UPS system was unable to accept generator power, it continued to operate on battery power. Initially, the battery runtime was sufficient to sustain the facility's critical loads for a few minutes.
5- Prolonged Outage and Frequency Fluctuations:
The power outage lasted longer than anticipated, extending into several hours. Throughout this extended period, the frequency output of the generator exhibited fluctuations due to load harmonics and potential issues in the generator's voltage regulation.
6- UPS Rejection of Generator Power:
The UPS system's sensitivity settings were not adequately adjusted to tolerate the frequency fluctuations at that rate, leading it to interpret the generator's frequency variations as indicative of an unstable power source. Consequently, the UPS system refused to switch to generator power and remained in battery mode.
7- Reduced Backup Runtime:
As the power outage persisted and the UPS continued to rely on battery power, the backup runtime began to deplete rapidly. The UPS system, which was not designed for prolonged operation on battery power alone, started alarming for a few minutes before shutting down, resulting in the critical load being dropped.
Resolution:
Upon realizing the UPS system's inability to accept generator power, the facility's maintenance team and UPS vendor were notified. They promptly investigated the issue and identified that the UPS system's sensitivity settings were set outside the acceptable range, causing it to become overly responsive to frequency fluctuations. The team adjusted the sensitivity settings to allow for minor fluctuations within acceptable limits. Additionally, they established communication between the generator interface relay board and the UPS interface relay board, enabling the UPS to recognize when it was running on the generator and permitting it to tolerate the frequency deviation range while powered by the generator.
Post-Resolution:
After implementing these adjustments, the backup power system underwent another round of testing. The UPS successfully accepted power from the generator and operated as intended. Furthermore, the facility chose to install a harmonic filter and recalibrate the generator regulator. It's worth noting that certain UPS systems can introduce input harmonics that result in generator frequency fluctuations. To mitigate this issue, steps were taken to minimize these induced harmonics by either installing a harmonic filter or adjusting the UPS's internal harmonic filter to engage at specific load percentages.
Conclusion:
The UPS system's failure to accept generator power during an extended power outage due to frequency fluctuations underscored the significance of accurate sensitivity settings. Regular maintenance, load testing, and compatibility checks between the generator and UPS system are vital to guarantee reliable backup power during prolonged outages. Through swift troubleshooting and adjustments, the facility effectively resolved the issue and bolstered the overall reliability of its backup power infrastructure.
