With the rapid development and wide application of computer industry, communication industry, various office equipment and precision electronic instruments, the demand for UPS is increasing, and the requirements are getting higher and higher. With the continuous improvement of manufacturing technology, the improvement of control technology and the continuous development of power devices, UPS has developed from an initial backup power supply to a kind of stable voltage, frequency stabilization, anti-electromagnetic and radio frequency interference, and anti-voltage surge. Power protection system with other functions. Technical indicators such as UPS efficiency, noise, volume, dynamic response speed, and voltage stabilization accuracy have been greatly improved. The overall efficiency of large and medium-sized UPS has reached more than 93%. The mean time between failures MTBF has reached more than 100,000 hours, and the overload capacity and non-linear load capacity have also been greatly improved.
UPS plays a key role in the power supply of computers and their networks, communications, finance, medical care, electric power, transportation, national defense, universities, research institutes and other departments. The loads of UPS are mainly small computers, servers and their peripheral equipment, as well as precision electronic equipment such as CT, etc. The input power of these electrical equipment is mostly single-phase high-frequency switching power supply, and the input end of the power supply generally does not have power factor correction. The single-phase capacitor filter rectifiers (as shown in Figure 1), and these rectifiers are a kind of inductive non-linear load for UPS.
For the main circuit of the single-phase capacitive filter rectifier shown in Figure 1 (a), because the filter capacitor with larger capacity is behind, it is easy to cause a misunderstanding that the nature of this load is capacitive. The current leads the voltage. It’s not exactly the case. Because in the case of rectification, only when the instantaneous value of the sine wave voltage is higher than the voltage on the capacitor, the filter capacitor can begin to charge, forming a charging current ic.
The expression of charging voltage Uc is
In the formula, Uc is the charging voltage on the filter capacitor; Um is the peak value of the input sine wave voltage; θ is the phase angle at which the rectifier diode starts to conduct. Depending on the situation, the value of θ is also different; τ is the capacitor charging time constant, τ =CR, where C is the capacitance value of the filter capacitor Cd, R is the resistance value of the equivalent resistance of the charging circuit; t is the required charging time.
Figure 1 (b) shows the working waveform of the filter capacitor charging in a steady state. Charging starts at point a, and the phase angle at this time is θ.
It can be seen from the working waveform shown in Figure 1 (b) that the magnitude of the rise of the charging current ic depends on the voltage difference △U=U-Uc and the equivalent resistance value R of the charging circuit. R is a constant value, so charging The maximum value ICm of the current ic depends entirely on △U.
From equation (2), it can be seen that the time when the maximum value of charging current ic ICm appears is a function of t (at this time △U = △Um, when the impedance of the capacitor Cd is smaller than the value of the load RL, ic rises faster).
Equation (2) shows that the time when the current peak appears is also a function of the phase angle θ at which the diode starts to conduct. The relationship between the time t and θ when the current peak appears is shown in Table 1, and the functional relationship is
|Initial phase angle θ (°)||30°||45°||50°||60°||70°||80°||85°||88°|
|Initial phase angle θ (rad)||π/6||π/4||5π/18||π/3||7π/18||4π/9||17π/36||22π/45|
|Time t (ms) when the peak charging current appears||4.17||5.0||5.28||5.83||6.38||6.94||7.22||7.39|
|Current pulse widthτ (ms)||3~4||3~4||3~4||3~4||3~4||3~4||3~4||3~4|
It can be seen from Table 1-1 that before θ=π/4, the peak value of charging current leads the peak value of voltage u, which is capacitive; when θ=π/4, it is resistive; when θ=π/4 ears Then it becomes lagging. And the phase angle of the current peak of almost all capacitor filter rectifier circuits satisfies θ>π/4, even after π/3. Therefore, generally speaking, the rectifier filter load is lagging.
It can also be seen from Table 1-1 that the width of the charging current pulse is between 3 ~ 4ms, and the calculation formula is
In the formula, τ is the approximate width of the charging current pulse; t is the moment when the peak of the current pulse appears; tθ is the starting time of the pulse; tD is the width of the trailing edge of the current pulse.
From the above description, it is proved that the load of UPS is a lagging nonlinear load, and the smaller the width of the current pulse, the smaller the power factor; the larger the displacement angle of the current pulse, the smaller the power factor.
The following introduces an important performance parameter of UPS, namely the current crest factor. As mentioned above, the main loads of UPS power supply are small computers, servers and their peripheral equipment. The input ends of these electrical equipment are generally single-phase capacitive filter full-wave rectifiers without power factor correction function (as shown in Figure 1 ), this kind of rectifier (proved above) is a non-linear load with hysteresis (inductive) of UPS.
In order to measure the ability of UPS to drive nonlinear loads, the concept of current crest factor is proposed. The so-called current crest factor is a parameter used to measure the drive capability of the UPS to nonlinear loads. The nonlinear load circuit of a typical UPS is shown in Figure 2. In the figure, Cd is a DC filter capacitor with a capacitance of C, and RL is the equivalent DC input resistance of the electrical equipment. When the input voltage is a sine wave, the input current waveform is shown in Figure 3. Suppose Ip is the peak value of the periodic non-sine wave current, and Ia is the effective value of the non-sine wave current, then the current crest factor is
The experiment shows that when the distortion power factor PFD=0.7, K=3~3.5. If the UPS peak current output capability is strong, as the load distortion power factor decreases, the K value can reach about 5, but at this time the load distortion power factor PFD≈0.5. When the capacitance value of the filter capacitor Cd is multiplied by the resistance value of the inverter equivalent input resistance RL, when RLC=0.15s, the input distortion power factor of a single-phase input UPS without power factor correction is about 0.7. However, when the load current changes, the input distortion power factor will also change. Therefore, the UPS input current crest factor K≥3 is required.
The output current crest factor of the UPS is determined by the design of the inverter power circuit and output circuit, but the designer and product inspection agency must have corresponding test methods to verify whether the design meets the relevant standards.