Determination method of optimal load interval of power frequency on-line UPS
Core conclusion: the optimal load of power frequency on-line UPS is generally 40% ~ 70% of rated capacity, and it is preferably controlled at about 50% in engineering; Less than 30% belongs to light load, and more than 80% belongs to heavy load, so try to avoid it. Let's talk about how to determine quantitatively, instead of just remembering empirical values.
First, distinguish two key parameters.
Rated capacity of UPS (kVA): apparent power, nominal nameplate.
Active power kW = kVA × power factor The common output power factor of power frequency machines is 0.8 (old models), and some new models are 0.9 and 1.0; Must be based on the nameplate of this machine.
Load calculation should use active power matching, not just kVA.
Second, the four core bases for determining the optimal load interval
1. UPS overall efficiency curve (the core basis)
The power frequency machine includes: rectifier, inverter, power frequency isolation transformer and charger.
Load < 30%: the efficiency of the whole machine is obviously reduced, the iron loss of the transformer is high, the heat is high and the energy consumption is high; Harmonic and DC ripple will become worse under light load of some models.
Load 40% ~ 70%: the efficiency of the whole machine reaches the peak range, and the working points of inverter and transformer are the best, and the heating is the least.
Load > 80%: the efficiency begins to slowly decrease, the device temperature rises rapidly, and the overload margin decreases; Once the load suddenly increases, it is extremely easy to protect.
The most rigorous way: check the efficiency-load ratio curve of the manufacturer's sample, and the load range corresponding to the highest point of the curve is the exclusive optimal range of this model (different brands of power frequency machines are slightly different).
2. Battery discharge capacity and backup time constraints
Too heavy load (> 80%): the battery discharge current is large, the voltage drops quickly, the actual backup time is greatly shortened, the battery heating is intensified, and the service life is shortened.
Too light load (< 30%): although the backup time becomes longer, the battery is easy to be sulfated with long-term small current floating charge and light load discharge, especially for lead-acid batteries; At the same time, UPS itself has a high proportion of no-load loss, which is uneconomical.
If the backup time is very long, the load rate should not be too high; If the backup time is short, it can be biased towards the upper limit of the interval, but long-term > 75% is not recommended.
3. Overload margin and reliability (redundancy)
Power frequency online UPS has short-term overload capability (for example, 125% load for 10min and 150% load for 30s, depending on the model).
Long-term operation load rate shall be reserved to prevent load impact and later expansion.
Single UPS: the recommended upper limit should not exceed 70%;
Parallel system (N+1): it can be improved appropriately, but it is still recommended to be ≤70% for a long time, and it cannot be loaded by overload capacity for a long time.
4. Harmonic and output voltage stability index
Power frequency machine with nonlinear load (server, switching power supply):
When the load is too low, the nonlinear load harmonics are easier to amplify, and the output waveform distortion rate THDv increases;
When the load is 40%~70%, the inverter has the best voltage stabilization and harmonic suppression ability.
Third, the calculation steps (practical operation)
Count the total active power P_load(kW) of all loads, distinguish linear/nonlinear loads, and count them into the maximum simultaneous workload (not simply adding the equipment nameplates, taking the simultaneous coefficient).
Obtain UPS nameplate: rated kVA, output PF, and calculate rated active power of UPS:
\ (p _ {rated} = s _ {rated} \ timespf _ {out} \) 3. Load rate
\ (loadrate = \ frac {p _ {load}} {p _ {rated}} \ Times 100 \% \) 4. Compare the efficiency curve to determine whether it falls in the optimal range; At the same time, check the battery backup time, temperature rise and overload margin.
Example: power frequency UPS 100kVA, PF=0.8, rated active power 80kW, load 40kW → load rate 50%, which falls in the optimal range.
Fourth, boundary judgment and engineering trade-offs
Load rate < 30%: long-term operation is not recommended.
Problems: low efficiency, battery aging risk, and poor harmonic characteristics.
Countermeasures: Replace UPS； with smaller capacity; Multiple parallel machines quit one.
30% ~ 40%: operational, but not optimal.
40% ~ 70% is the best
70% ~ 80%: short-term operation is allowed, long-term operation is not recommended, and only when there is no load expansion and no impact load can it be considered.
> 80% long-term operation is not recommended: high temperature, insufficient backup time and high failure risk.
V. Correction of Special Working Conditions
Low temperature environment: the battery capacity decreases, the optimal load range is lowered, and the upper limit is reduced to about 60%;
There is a big impact on the load (motor, contactor): the upper limit of load rate is reduced, and the impact margin is reserved;
Charge and discharge of high frequency secondary battery: the load should not be too low to avoid sulfide of lead-acid battery;
Built-in isolation transformer of power frequency machine: Compared with high frequency machine, the iron loss under light load is greater, and it is more taboo to keep the load below 30% for a long time.
Six, a sentence summary to determine the logic.
Read the manufacturer's efficiency curve first and take the load range with the highest efficiency; Combined with battery backup time, load impact and capacity expansion requirements, the general optimal range of the project is 40% ~ 70%, and the recommended design point load rate is 50%.
