Influence of derating operation on output power of power frequency online UPS
First make clear the concept: UPS derating operation = artificially controlling UPS loaded load < UPS rated capacity (for example, 100kVA UPS only carries 50 ~ 70kVA load for a long time), which is different from fault current limiting derating. The output power of power frequency on-line UPS (double conversion on-line, rectifier+inverter architecture) is divided into apparent power kVA and active power kW (limited by power factor).
First, the positive impact
The inverter output margin is sufficient, and the dynamic loading capacity is improved. The power frequency inverter, power tube and output transformer all have margins. After the downgrade:
It is not easy to trigger the inverter overload protection when the load is suddenly added and the impact load is started;
The transient drop of output voltage and frequency is smaller, the accuracy of voltage and frequency stabilization is better, and the waveform distortion rate is lower.
The loss of power frequency UPS mainly comes from inverter power devices, power frequency output transformers and reactors. Loss reduction and temperature rise reduction under low load;
The junction temperature of power devices is lower, and the continuous output ability of inverter is more stable;
Reduce the probability of high temperature triggering overheating protection and output power limit;
Prolong the service life of the whole machine and reduce the output parameter deviation caused by thermal drift.
The short-term overload capacity is more plentiful. The overload capacity of most power frequency UPS: 125% load for 10min, 150% load for 30s (different specifications). If it is close to full load, a slight impact load will easily trigger overload shutdown; During derating operation, the remaining capacity can absorb short-term impact and it is not easy to trigger the output limit.
The output of the battery is more stable in the discharge stage. When the commercial power is interrupted and the battery is powered by inverter, the battery voltage gradually drops, and the available output power of inverter will decrease with the battery voltage drop. Under the condition of full load, the battery is prone to power shortage in the later stage; Derated operation can delay the phenomenon of low-voltage power limit of battery, prolong backup time and avoid inverter shutdown due to insufficient power.
Second, the negative impact (easily overlooked)
The utilization rate of active power output by UPS decreases, and the system investment is wasted. The rated output of industrial frequency UPS has a fixed power factor (common 0.8)：100kVA UPS has a rated active power of 80kW. Long-term significant derating (for example, long-term load < 30% of rated capacity), equipment capacity is not utilized.
Light load brings problems related to output harmonics and power factor (the power frequency machine is slightly better than the high frequency machine, but it still exists). Under extremely light load condition, the inverter:
The working point of the switching device shifts, and the output voltage THD (harmonic distortion rate) of some models rises instead;
If the back-end load is nonlinear, the compensation effect of UPS internal filter loop becomes worse under light load.
General suggestions in the industry: 40% ~ 70% rated capacity of the best load range of power frequency UPS; Load < 20% is too light, so long-term operation is not recommended.
Under no-load/extremely light load, the voltage stabilization characteristics of some models become worse, and the excitation current of power frequency output transformer has obvious influence. Under extremely light load, the no-load voltage is higher, and the dynamic characteristics of voltage regulation are not as good as those in the middle load range.
Can't improve the rated maximum output capacity of UPS. Key points: derated operation can exceed the rated power. The derating is only "the working point moves down", and the hardware upper limit of UPS inverter and output transformer remains unchanged. Once the load exceeds the rated kVA/kW of the nameplate, it will still start overload current limiting, alarm and even shut down.
Third, special scenario: hardware derating caused by UPS's own fault (distinguished from artificial derating)
In case of automatic derating protection caused by equipment failure (module overheating, abnormal power device, damaged fan):
UPS actively limits the maximum output power;
At this time, the load capacity is permanently reduced, and once the load exceeds the upper limit after derating, it will be directly bypassed or shut down;
It belongs to abnormal working conditions, can't run for a long time, and needs maintenance.
Fourth, summarize and sort out
form
Influence of work conditions on output pow/performance
Reasonable derating (40% ~ 70% rated load) has good output voltage waveform quality, shock resistance, large overload margin, low temperature rise, and the highest reliability of output continuous operation.
Long-term close to full load (> 85%) leads to high heat, overload of impact load and insufficient power in the later period of battery discharge.
Excessive light load (< 20% rated) wastes capacity, and the harmonic index of some models becomes worse, so long-term operation is not recommended.
The maximum output power of automatic fault derating is forced to be limited, and there is a risk of downtime, which belongs to the fault state.
Supplementary engineering practical conclusion
Moderate derating design is recommended for power frequency on-line UPS (generally, the load of the whole machine is selected according to 60% ~ 70% rated capacity) in exchange for power supply reliability; However, long-term extreme light load is not recommended; At the same time, note: derating is only to optimize the operating conditions, and cannot change the upper limit of the rated output power of UPS hardware.
