5. UPS testing
The purpose of testing UPS is to verify whether the actual technical specifications of UPS can meet the usage requirements. UPS testing generally includes two types: steady-state testing and dynamic testing. Steady state testing is the process of testing various phase voltages, line voltages, no-load losses, power factor, efficiency, output voltage waveform distortion, and output voltage frequency at the input and output terminals under no-load, 50% rated load, and 100% rated load conditions. Dynamic testing generally tests the changes in the output voltage waveform of the UPS during sudden load changes (usually selecting loads from 0 to 100% and from 100% to 0) to verify the dynamic characteristics and energy feedback path of the UPS.
(1) Steady state testing
The so-called steady-state testing refers to the testing of equipment when it enters the normal state of the system, which can generally measure waveform, frequency, voltage, and efficiency.
① Waveform testing. Usually, the waveform is observed to see if it is normal in both unloaded and fully loaded states. Measure the distortion of the output voltage waveform using a distortion meter. Under normal working conditions, connect a resistive load and measure the total harmonic content of the output voltage using a distortion meter, which should be less than 5%.
② Frequency testing. Generally, an oscilloscope can be used to observe the frequency of the output voltage and a power disturbance analyzer can be used for measurement. At present, the output voltage frequency of UPS generally meets the requirements. But when the frequency circuit and local oscillator of the UPS are not precise enough, it is also possible that the output voltage frequency of the UPS will change accordingly when the mains frequency is unstable. The accuracy of the output voltage frequency of UPS can generally reach ± 0.2% when synchronized with the mains power.
③ Voltage testing. The output voltage of UPS can be tested and judged through the following methods.
A. When the input voltage is 90% of the rated voltage and the output load is 100%, or when the input voltage is 110% of the rated voltage and the output load is 0, the output voltage should be maintained within the range of ± 3% of the rated value.
B. When the input voltage is 90% or 110% of the rated voltage and one phase of the output voltage is unloaded, and the other two phases are 100% loaded, the output voltage should be maintained within ± 3% of the rated value, and the phase difference should be maintained within 4 °.
C. When the input DC voltage of the UPS inverter changes by ± 15% and the output load changes from 0 to 100%, its output voltage should be maintained within the range of ± 3% of the rated value. This indicator appears to be repetitive with the previously mentioned indicators, but in reality it requires higher standards than the previous indicators. This is because when the input signal of the control system changes over a large range, it exhibits obvious nonlinear characteristics. To ensure that the output voltage does not exceed the allowable range, the circuit requirements are even higher.
④ Efficiency testing. The efficiency of UPS can be determined by measuring the output power and input power of the UPS. The efficiency of UPS mainly depends on the design of the inverter. Most UPS systems only have relatively high efficiency when operating at 50% to 100% load, and their efficiency drops sharply when operating below 50% load. The efficiency indicators provided by UPS manufacturers are mostly efficiency under rated DC voltage and rated load conditions. It is best for users to choose the relationship curve between efficiency and output power, as well as the efficiency when the DC voltage changes by ± 15%.
(2) Dynamic testing
① Sudden increase or decrease load testing. First, use a power disturbance analyzer to measure the phase voltage and frequency under no-load and steady-state conditions. Then, suddenly increase the load from 0 to 100% or suddenly decrease the load from 100% to 0. If the UPS output transient voltage is between -8% and 10% and returns to steady state within 20ms, then this UPS indicator is qualified; If the transient voltage output of the UPS exceeds this range, it will generate a large surge current, which is extremely detrimental to both the load and the UPS itself. Therefore, the UPS should not be selected.
② Conversion feature testing. This mainly tests the conversion characteristics when converting from inverter power supply to mains power supply or from mains power supply to inverter power supply. During testing, it is necessary to have a storage oscilloscope and a voltage regulator that can simulate changes in the mains power.
(3) Other routine tests
① Overload test. Overload testing is of great concern to users and an important indicator for measuring UPS. Overload testing is mainly used to test the overload capacity of the entire UPS system, ensuring that the UPS can maintain a certain period of time without damaging the equipment in case of overload during operation. Overload testing must be conducted according to equipment specifications and at room temperature below 25 ℃.
② Input voltage overvoltage and undervoltage protection test. This needs to be tested based on the allowable range of input voltage variation according to the device specifications. Generally, UPS allows input voltage to change by 10%. When the input voltage exceeds this range, an alarm should be triggered and the system should switch to battery power supply. The rectifier will automatically shut down; When the input voltage returns to the rated allowable range, the equipment should automatically resume operation, that is, the battery should be automatically released and switched to mains power operation. During the automatic activation and deactivation of the battery, the waveform of the UPS output power supply should remain unchanged.