Overall conclusion of derated operation on the life of power frequency online UPS
Reasonable derating (load rate 40% ~ 70%) will significantly prolong the service life of UPS. Excessive derating (load < 20%) will bring negative problems such as increased wear and tear of some components and decreased reliability. The power frequency machine relies on core devices such as power frequency transformer, rectifier bridge, IGBT, electrolytic capacitor, battery, fan, power device, etc., and different components are obviously differentiated by derating.
1. Moderate derating (optimal load range: 40% ~ 70% of rated capacity)-positive gain, the core reason for prolonging life.
1. Power semiconductor (rectifier, inverter IGBT/SCR, thyristor)
When the device works at full load, the conduction loss and switching loss of the device are large, and the junction temperature increases; After derating, the working current is greatly reduced, and the junction temperature of the chip is obviously reduced. The lifetime of semiconductor devices follows Arrhenius' law: the aging rate of devices doubles every 10℃ increase in temperature. Lower steady-state temperature rise will greatly slow down the fatigue of semiconductor package solder layer and chip aging, and greatly reduce the failure rate of power tube.
Power frequency UPS is mostly thyristor rectifier structure, and the thyristor generates serious heat under high current, especially the derating effect on prolonging the life of thyristor.
2. Power frequency isolation transformer (symbol part of power frequency machine)
Transformer loss includes copper loss (heating of winding resistance)+iron loss. The load current decreases → copper loss decreases significantly, the temperature rise of winding decreases, and the aging speed of insulating paint and interlayer insulation slows down; At the same time, the winding will not be in the cycle of high temperature thermal expansion and cold contraction for a long time, which can eliminate the hidden dangers of winding insulation cracking and turn-to-turn short circuit, and greatly improve the service life of the transformer in dozens of grades.
3. DC bus, filter electrolytic capacitor (short life wearing parts of the whole machine)
The life of electrolytic capacitor is determined by working temperature+ripple current. At full load, the ripple current of inverter and rectifier side is the largest, and the electrolyte inside the capacitor volatilizes fastest when heated. After derating, the ripple current decreases, the temperature of capacitor shell decreases, and the drying rate of electrolyte slows down, which is one of the most effective means to extend the life of capacitor. The DC capacitor of power frequency machine has high cost, difficult replacement and high derating benefit.
4. Cooling fan and air duct system
The load generates less heat, so the fan does not need to run at full speed for a long time, the bearing wears and the fan blades age slowly; Some temperature-controlled fans even run intermittently at low speed, so the service life of fans is doubled, and the risk of overheating shutdown caused by fan failure is reduced.
5. Battery pack (UPS battery)
UPS inverter has low on-load power, smaller battery discharge current and shallower discharge depth. High current discharge will lead to vulcanization, heating and active material shedding of battery plates; Under derating condition, the discharge rate of the battery is low, the charging recovery is more sufficient, and the cycle life of the battery can be improved by more than 30%. At the same time, the load of UPS charging module is light, the charging voltage and current are more stable, and the battery damage caused by overcharge is reduced.
6. Internal bus, copper bar and terminal
When the working current decreases, the heat generated by the contact resistance decreases, and the terminal is not easy to be oxidized, heated and ablated, which reduces the hidden dangers of virtual connection and ignition and improves the long-term operation stability of the whole machine.
2. Excessive derating (load rate < 20% of rated capacity)-negative losses are generated and the life of local devices is shortened.
The operating point of inverter becomes worse, and the proportion of no-load loss increases.
No matter how much power frequency online UPS is loaded, the rectifier and inverter circuits always work online, and their no-load power consumption is fixed. When the load is extremely low, the efficiency of the whole machine drops, and useless heat is concentrated in UPS's own circuit. When the inverter works in the nonlinear range of light load, the output waveform distortion increases slightly, but the harmonic loss increases.
The heat dissipation system is insufficient, and the internal heat accumulation is silted.
The cooling duct and radiator of high-power industrial frequency UPS are designed according to full load heating. When the load is very light, the heat of the whole machine is small, the fan runs at a low speed/stops, the heat in the enclosed space inside the chassis cannot be discharged in time, and the heat is accumulated around the transformer and capacitor, and the ambient temperature remains high, which accelerates the evaporation of electrolyte and insulation aging ("canned effect").
System circulation and no-load harmonic loss increase.
The working point of switching devices in power frequency inverter is unstable under extremely light load, and the proportion of switching loss increases; In the scene of multi-machine parallel operation, light load is easy to produce circulation between units, which will continue to cause useless power consumption and heat generation.
The risk of long-term floating and overvoltage of battery increases.
When the UPS rectifier charger is under light load, the DC side demand current is extremely low, and the charging voltage stabilizing circuit is prone to high voltage. The battery is at a high floating charge voltage for a long time, which will accelerate the grid corrosion and water loss and shorten the battery life.
III. Long-term Full Load/Overload Operation (Comparative Reference)
Long-term operation with load > 80%: the temperature rise of the whole machine is high for a long time, and all power devices, capacitors, transformers and batteries are accelerated to age; Short-term overload will bring thermal shock, easily trigger overheating protection and permanent damage of devices, which is the main cause of premature failure of UPS.
Fourth, the engineering practical recommended interval
Priority of life extension: UPS maintains 50% ~ 60% rated capacity for a long time, with the longest comprehensive life, giving consideration to device loss and heat dissipation balance;
Minimum safety load: it should not be less than 25% for a long time, and the machine room must be equipped with small load permanent load to avoid no-load/extremely light load operation;
Long-term operation with a load rate of > 80% is strictly prohibited, and allowance for capacity expansion and impact load is reserved.
V. Supplement: Difference in derating sensitivity between power frequency machines and high frequency machines.
The power frequency machine comes with a heavy power frequency transformer, which has large heat capacity and strong short-term full load tolerance, but the insulation aging is irreversible at long-term high temperature; Therefore, the power frequency machine is more suitable for moderate derating operation, and the service life obtained from derating is more obvious than that of high frequency UPS.
