Influence of derating operation on service life of power frequency online UPS
Conclusion First, reasonable derating operation will significantly prolong the overall life of on-line UPS, while excessive full load/overload will greatly accelerate aging and shorten life.
First, the core principle (characteristics of power frequency models)
Power frequency transformer, power tube, rectifier bridge, capacitor and fan in power frequency UPS are the main wearing parts, and loss and heating are the core inducement of aging. Derating = actual load < rated capacity of ＜UPS, and the working load and temperature rise of the whole machine decrease.
Second, the influence of components
Power devices (IGBT, rectifier bridge, inverter module)
After derating, the conduction current decreases, the conduction loss and switching loss decrease greatly, and the junction temperature decreases obviously. The lifetime of semiconductor devices is negatively correlated with temperature. Every 10℃ decrease in temperature nearly doubles the lifetime, effectively reducing the risk of thermal fatigue and breakdown.
Power frequency main transformer/isolation transformer
Power frequency transformer is the core part of this model, which has large copper loss and iron loss and serious heating when it is fully loaded. After derating, the winding current is reduced, the temperature rise is lowered, the insulation layer is aging, the coil is loose, the probability of oil leakage is greatly reduced, and the life of transformer is most obviously improved.
Battery (the most easily worn part of UPS)
Discharge stage: the load current decreases, the battery discharge rate decreases, the high current polarization, plate vulcanization and heat generation decrease, and the cycle life of single battery is prolonged;
Charging stage: under light load, the rectifier charging load is low, the charging voltage/current is more stable, and the abnormal loss of overcharging and floating charging is reduced.
Note: Except for long-term no-load/extremely low load (< 10% rated), it will bring new problems.
Filter capacitance, bus capacitance
With the decrease of load, the current ripple decreases, the internal heat of the capacitor, the evaporation of electrolyte slows down, and the speed of bulging, leakage and capacity attenuation slows down.
Cooling system (fan, air duct)
The calorific value of the whole machine decreases, the fan speed decreases, the start-stop frequency decreases, and the bearing wear, noise and failure probability decrease.
Third, the actual effect of different load intervals
1. Recommended range: load rate 40% ~ 70% (optimal derating range)
Moderate load, lowest temperature rise and minimum device stress;
Considering both economy and service life, the overall service life of the whole machine is increased by 30%~60%, which is the standard working condition for long-term operation of power frequency UPS.
2. Light load derating: the load rate is 10% ~ 40%.
It is still better than full load, and the life of devices, transformers and batteries continues to extend;
Common faults of power frequency models: the inverter output waveform and reactive power compensation are slightly deviated under extremely light load, but it does not affect the service life.
3. Very low load: load rate < 10% (excessive derating)
Instead, it has a negative effect:
The working point of some old power frequency UPS inverter circuits deviates, harmonics and circulating current increase, and local hidden heating occurs;
Long-term shallow charge and shallow discharge+long-term low-current floating charge, plate passivation, active material attenuation, battery life is shortened;
The long-term low-speed/intermittent operation of the fan makes the dust accumulation in the air duct faster.
4. Full load/overload (load rate > 100%)
Bad working conditions: the whole machine is full of heat, the device is at the critical point of high temperature for a long time, the insulation is accelerated aging, the capacitor is rapidly attenuated, the battery is damaged by heavy current discharge, the service life is directly damaged by more than 50%, and it is easy to trigger failure and shutdown.
IV. Supplementary operation suggestions (practical operation)
Daily long-term operation, giving priority to controlling the load rate at 40%~70%, is the best scheme to extend the life of power frequency UPS;
Avoid extremely low load below 10% for a long time. If the field load is extremely small, the dummy load can be connected in parallel appropriately.
Long-term overload and instantaneous frequent impact load are strictly prohibited;
The derating operation is combined with regular dust removal, battery internal resistance inspection and temperature rise, and the life extension effect is better.
summary
Reasonable derating (40%~70% load): reduce the stress and temperature rise of devices in an all-round way, and greatly extend the life of UPS, batteries and transformers;
Slight derating (10%~40%): the advantages still outweigh the disadvantages, and the service life is better than full load;
Excessive derating (< 10%): The disadvantages outweigh the advantages, damaging the battery and some circuits;
Full load/overload: it seriously shortens the life of the whole machine and belongs to high-risk operating conditions.
