Complete method for judging whether power frequency on-line UPS needs derating operation.
Definition of derating: under non-standard working conditions, the long-term allowable output power of UPS is less than the rated kVA/kW of nameplate, and the maximum load must be limited; If any judgment condition is met, it needs to be derated, and the most severe derating coefficient is taken by combining multiple factors. The power frequency generator is equipped with a power frequency isolation transformer, which is more sensitive to temperature rise, insulation and harmonics. The derating requirement is stricter than that of high frequency UPS, and it is implemented according to GB/T 7260.3 and IEC62040-3.
I. Judgment Dimension 1: Ambient temperature (the most common reason for derating)
Standard reference condition
The rated capacity of most power frequency UPS is applicable: the ambient temperature is 0 ~ 40℃ without derating (coefficient = 1.0).
decision rule
Environmental temperature > 40℃ must be derated.
40 ~ 45℃: for every 1℃ increase, the capacity decreases by 2% ~ 3%.
> 50℃: Long-term full load is easy to protect transformer and power tube from overheating, so it must be greatly derated.
The equivalent high temperature of cabinet with poor heat dissipation should also be derated.
Closed machine room, blocked air inlet and outlet, multiple UPS attached side by side, aging fans, actual internal temperature rise far exceeding ambient temperature measurement, measured radiator temperature is high, even if the room temperature is ≤40℃, derating operation is recommended.
Low temperature (< 0℃) is generally not derated, but the battery is limited, which does not affect the power derating of UPS host.
Second, judgment dimension 2: installation altitude (insulation+heat dissipation double derating)
Benchmark condition
Altitude ≤1000m: no derating.
Decision rule (power frequency transformer is more sensitive to air gap insulation)
1000 ~ 2000 m: every 100m increase will be reduced by 0.5% ~ 1%.
> 2000 m general experience: 1% reduction for every 100m increase.
Example: at an altitude of 3000m, the derating coefficient is ≈0.9, and the maximum allowable load of 100kVA UPS is 90kVA.
Principle: The air at high altitude is thin → the heat dissipation becomes worse+the external insulation breakdown margin decreases, and the winding gap margin of power frequency transformer is smaller, and the derating range is usually larger than that of high frequency machine.
Third, judgment dimension 3: load nature (power factor+nonlinear harmonic)
1. The load power factor does not match (active power/apparent power must be calculated)
Rating of power frequency UPS: kVA (apparent) and rated power factor cos φ (common 0.8 and 0.9).
Formula: rated active power PN=SN×cosφ0
Judge:
The load cos φ is less than the rated power factor of UPS, and the apparent power exceeds the standard, so it must be derated.
Example: 100kVA UPS cosφ=0.8 (rated active power 80kW); The load cosφ=0.7, and the apparent ≈ 114kVA > 100kVA with 80kW active power, so it is necessary to reduce the rating and limit the current.
2. Nonlinear load (forced derating due to excessive harmonic current THDi)
Nonlinear loads such as rectifiers, frequency converters and switching power supplies generate a large number of 3rd, 5th and 7th harmonics:
The manufacturer's nominal 100% non-linear load can be fully loaded without derating;
If not marked, when the total harmonic current THDI is greater than 30%, it usually needs to be derated by 10% ~ 25%;
The three-phase load of three-phase power frequency UPS is seriously unbalanced (unbalance degree > 15%), and the zero-sequence current heats up in the power frequency transformer, so it must be derated.
4. Judgment Dimension 4: Abnormal input mains voltage
Long-term deviation from the rated input range of UPS (380V three-phase conventional 15%)
Continuous low voltage: the rectifier side current increases and the heat rises, and the input current is too large under the same output power, so it needs to be derated;
Continuous ultra-high voltage: the voltage tolerance of rectifier devices is insufficient, and the manufacturer's manual specifies the derating curve corresponding to the overpressure interval.
The input voltage fluctuates frequently, the harmonic distortion THDu is too high, and the rectifier bridge heats up, so it is suggested to reduce the rating appropriately.
V. Judgment Dimension 5: Measured temperature rise of the equipment itself (actual judgment is the most intuitive)
Determination of infrared temperature measurement in on-site operation with load;
Temperature of winding and core of power frequency transformer > 110℃ (upper limit of insulation class B);
The shell temperature of inverter IGBT and rectifier module continues to be > 75℃;
The temperature rise slope under full load operation is > 5℃/10 min, and the temperature continues to climb;
Meet any one, regardless of whether the environmental parameters are up to standard, it must be run at reduced load to prevent insulation aging from burning the transformer.
VI. Judgment Dimension 6: Special derating of parallel/redundant systems
Multiple power frequency UPS are connected in parallel with uneven current, and the output current deviation of a single machine is more than 10%, so the unit with heavy load needs to be derated;
N+1 redundant long-term single machine with full load and long-term overload operation must be derated to match the capacity of single machine.
VII. Comprehensive Judgment Steps (Field Operation Process)
Check UPS nameplate: list of rated capacity SN, rated power factor and manufacturer's derating curve;
Actual measurement: ambient temperature, installation altitude, total apparent/active power of load, load power factor, current harmonic THDi and three-phase unbalance;
Calculate the temperature derating coefficient Kt, altitude derating coefficient Kh and load derating coefficient KL respectively;
Comprehensive allowable capacity: allowed;
If the actual load apparent power > allowed → must be derated; On the contrary, it can run at full load.
