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Diesel Generator + UPS Combined Power Supply for Data Centers: Watch Out for These Common High-Frequency Faults

Time : 2026-09-30

Watch Out for These Common High-Frequency Faults.jpg

Introduction

Using diesel generator sets paired with UPS for backup power supply is a mature power solution for data centers. However, mismatched load characteristics between the two equipment easily cause various tricky on-site faults. In most cases, neither the generator nor the UPS itself is broken; the problem lies in compatibility issues. This article sorts out the most frequent fault symptoms and simple troubleshooting ideas encountered on site.

Fault 1: Voltage fluctuation, overvoltage and burnt AVR when generator supplies UPS

Symptoms: When the generator is no-load or lightly loaded with UPS, the output voltage fluctuates up and down with swinging voltmeter readings. In severe cases, the automatic voltage regulator (AVR) burns out directly. Causes: Lightly loaded UPS generates leading capacitive reactive power. Ordinary AVR is designed for inductive loads. Faced with leading capacitive reactive power, the excitation regulation will lose control. Solutions: Avoid connecting UPS to the generator under no-load condition; connect inductive loads such as air conditioners first; adopt AVR with anti-capacitive load capability; enlarge the generator capacity to 1.5–2 times the load.

Fault 2: UPS frequently switches to battery mode while the generator is running

Symptoms: The diesel generator starts and outputs power normally, but the UPS repeatedly reports input power failure and switches to battery mode continuously, resulting in heavy battery discharge. Causes: The UPS detects distorted waveform or slight frequency fluctuation from the generator and judges the input power as unqualified. Harmonic interference will also trigger protection. Solutions: Increase generator capacity; install input reactor; inspect the governor board to stabilize generator frequency.

Fault 3: Generator overheating without overload alarm

Symptoms: The actual active power load is low, but the generator temperature rises sharply and the winding overheats. Causes: UPS rectifier generates massive harmonic current. Harmonics do not produce active power yet greatly increase the stator copper loss and cause severe heating of the generator. Solutions: Do not select generator merely based on active power; install reactors or filters; prevent UPS from running under extremely low load for a long time.

Fault 4: Generator trips immediately at closing when connected to UPS

Symptoms: The generator works normally alone. Once closing the UPS input switch, the unit trips instantly due to overcurrent protection. Causes: At UPS power-on moment, the input filter capacitor produces huge inrush current and leads to instantaneous overcurrent. Solutions: Switch UPS to bypass mode or adopt staged load switching; avoid switching on multiple UPS units simultaneously; increase generator capacity.

Fault 5: Severe frequency fluctuation, UPS reports frequency out-of-limit alarm

Symptoms: Voltage seems normal, but frequency drifts continuously and triggers UPS alarm. Causes: The governor board of diesel engine cannot respond fast enough to sudden changes of non-linear loads. Fluctuating UPS loads lead to delayed speed adjustment of diesel engine. Solutions: Maintain the governor board and actuator; avoid sharp sudden load changes; set staged power-on for UPS units.

Fault 6: Abnormal oscillation after mains power restoration and switching

Symptoms: After mains power recovers and the system switches back to mains, short-time voltage disturbance occurs and partial equipment restarts unexpectedly. Causes: Improper ATS switching time sequence causes logic conflict among generator, UPS and mains power and leads to temporary circulating current. Solutions: Adjust ATS switching delay to match UPS operating mode and prevent temporary circulating current.

Practical Summary

Most faults of generator + UPS system in data centers are not hardware damage but compatibility problems. Remember these 3 practical principles:

  • Do not save on capacity. The generator capacity is recommended to be 1.5–2 times the total UPS capacity.
  • Avoid long-term UPS operation under load rate lower than 40%.
  • Power-on sequence: connect inductive loads to generator first, then switch on UPS. Never connect UPS directly to no-load generator.

Media Contact

Shanghai Outevo Machinery Co.,Ltd.

Web: www.outevo.com

Tel: +8613280091753

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