I. What is UPS derated operation?
Derated operation = the actual on-load power is lower than the rated output power of UPS for a long time, which also includes two situations: the manufacturer actively sets the upper power limit and the high temperature/aging/fault protection actively limits the output power.
Example: 10kVA UPS, which only carries 3~5kW load for a long time, or the maximum output of 7kVA is limited by internal protection, all belong to derating.
Second, the positive beneficial effects (main benefits)
The output load margin is sufficient and the output is more stable.
Load shock and starting current will not trigger UPS overload and current limiting protection;
The output voltage and frequency ripple of inverter are smaller, the waveform distortion is lower and the power supply quality is better.
UPS can actually output peak power.
Rated power is a long-term continuous output capacity, and the short-term overload margin is larger after derating.
For example, 10kVA UPS allows 125% overload for 10 seconds, and when it is used with light load derating, it is more difficult to skip load shutdown when the load is suddenly added.
The heating of the device decreases, and the attenuation of long-term available output capacity slows down.
The heating value of IGBT, transformer and bus capacitor of inverter is greatly reduced, and the temperature rise is low;
The aging speed of components slows down, and the long-term rated output capacity declines more slowly;
Power derating protection will not be triggered due to overheating, and UPS will be prevented from actively lowering the maximum output power.
Sufficient redundancy, good capacity expansion and fault tolerance
When the load is added later, the UPS does not need to be replaced immediately, and there is still surplus output power to undertake the new equipment.
Third, negative adverse effects (disadvantages of light load and excessive derating)
1. The power factor and efficiency become worse, and the invalid loss increases.
UPS has the best efficiency range (usually 50%~80% rated load)
Deep derating of load < 30%: the conversion efficiency of the whole machine is obviously reduced, and the proportion of its own power consumption is increased;
Some power frequency UPS and models with power factor correction, the input power factor becomes worse under extremely light load, and the reactive power loss of the front-end distribution cabinet increases.
2. The inverter operating point shifts, and the output harmonic and DC components slightly rise.
When switching power supply UPS is derated under extremely light load, the switching tube works intermittently, and the output waveform THD (total harmonic distortion rate) will rise slightly, which has the risk of weak interference to precision analog equipment.
3. The battery discharge utilization rate becomes worse (output is limited in backup mode)
When the mains power is cut off and the battery is powered by inverter:
Deep derating and light load, small battery discharge current and low discharge efficiency;
Some UPS are designed to shift the boost regulation strategy under extremely light load, and the actual backup time is less than the theoretical calculation value under the same battery capacity, which seems to have surplus power and the effective battery life is reduced.
4. Some models have light load protection and circulation problems (especially parallel UPS).
If multiple UPS parallel systems have excessive derating and uneven load distribution:
With the increase of parallel circulation, individual machines passively bear additional power and locally overload in phase change;
Very few UPS have light load sleep logic. When the load is too low, an inverter stops, and instantaneous switching will cause short-term disturbance of output.
5. Occupancy cost and waste of power resources
Long-term low-load derating operation of high-power UPS leads to high investment in equipment procurement, power distribution in computer room and air conditioning cooling, and the power supply cost per load rises.
Iv. active protection derating (high temperature/fault causes UPS to limit its own power)
This kind of abnormal derating directly limits the maximum available output power:
The ambient temperature is too high, and the internal dust accumulation leads to poor heat dissipation → the UPS internal control panel triggers overheating protection, forcing down the maximum output power;
Capacitor aging, abnormal module → system derating and current limiting to prevent device burning;
Consequence: The load with the original rated energy band can't move now. If the load is slightly larger, it will give an overload alarm or even bypass the jump.
V. Summary of recommendations
Reasonable derating (50%~80% rated power of load)
The output stability, overload capacity and equipment life are fully optimized, which is a conventional and reasonable configuration in the industry;
Excessive deep derating (load < 30%)
Lower output efficiency, worse harmonics and discounted battery life do more harm than good;
Fail-safe automatic derating
It belongs to the fault state, and the maximum output power is forced to be compressed, so there is a risk of insufficient load and power failure, so it is necessary to investigate the heat dissipation and hardware faults.
