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As the main power supply equipment of modern communication system and computer network, there are more than ten output electrical indicators of UPS. This paper discusses the output power factor (PF) in more detail and introduces the test method of this indicator.

The output power factor of the UPS is one of the technical indicators that most users are concerned about, because the high and low power factor of the UPS will directly affect the driving capability of various loads (such as inductive, capacitive and rectifying nonlinear loads). The output capacity of the AC power supply device is expressed in volt-amperes (VA), that is, the product of the effective value of the output AC voltage of the power supply device and the current effective value, which is what we call the apparent power PS.

The output capacity of the UPS is expressed in terms of apparent power VA. All UPSs indicate the output power factor while indicating the output capacity. At present, the output power factor of imported or domestic UPS sold in the domestic market is generally between 0.6 and 0.8. For UPS output power factor, there are some incomplete understandings or inappropriate evaluations among some users and UPS vendors. Some UPS users or sales personnel believe that the product of the output capacity PS and the power factor PF is the actual output power of the UPS or the output active power P, that is, P = PS × PF. This understanding and interpretation of the output power factor is not wrong, but it is still not comprehensive, ignoring the other side of the UPS output capability, that is, the output power of the reactive power PQ. Most of the AC power loads in modern computer network systems and automatic control systems are non-linear loads, of which the rectifying nonlinear load is the first. In the automatic control system, there are often inductive nonlinear loads with iron cores, such as transformers. , AC motor, etc. When these power loads are working normally, not only the active power P is required, but also the UPS must provide the reactive power PQ required for the load in the state where the output voltage waveform is not significantly distorted to ensure the normal operation of the power load. The reactive power PQ provided by the UPS to the load is provided by harmonic currents other than the fundamental current.

Each AC power load has different power factor expressions depending on its impedance characteristics. There are two expressions of power factor: phase shift power factor cosφ and distortion power factor PFD.

The phase shift power factor is typically generated on a linear load, such as a capacitive or coreless inductive load. Since the phase of the sinusoidal voltage and the sinusoidal current are different, a phase shift power factor is generated. The cosine of the phase angle φ is the phase shift power factor, as shown in Fig. 1. It can be seen from the figure that although the voltage u and the current I have a phase difference, both are sinusoidal waves, and there is no additional harmonic current in the current waveform due to the load.

Figure 1 Schematic diagram of linear load phase shift power factor

The distortion power factor is mainly generated by diode rectification, thyristor rectification, and inductive nonlinear loads with iron cores. The phase shift power factor of diode rectification and core inductive nonlinear load is generally high. For example, the phase shift power factor of AC asynchronous motor is generally around 0.9, and the phase shift power factor of diode rectified nonlinear load is generally up to 0. 98~0.99. However, due to the operation of these two loads, a large harmonic current will be generated, as shown in Figure 2. Since there is harmonic current in the load and there is no harmonic voltage corresponding to it, the harmonic current has zero average power in one cycle of the input voltage, and the harmonic current is only reactive power exchange between the UPS output and the load. . In particular, the harmonic current generated by the diode rectified nonlinear load is almost equal to the fundamental current.

The distortion power factor is defined as:

PFD=VI1/VIT=I1/IT=I1/√I12+I22+I32+I42+...... (1)

Where I1 - the effective value of the fundamental current;

IT—the total harmonic current rms value including the fundamental current;

It can be seen from equation (1) that the distortion power factor PFD=1 when the effective value of each fundamental current without the fundamental current I1 is zero. The distortion power factor PFD of the diode rectified nonlinear load is =0. 6 to 0.7. From equation (1), it can be inferred that the sum of the rms currents of the harmonic currents other than the fundamental wave is 1 of the rms value of the fundamental current. 02 to 1. 33 times.

When the voltage and current of the electrical load have both a phase difference φ and a harmonic current, the power factor is called the total power factor PFT. The relationship between the total power factor PFT and the phase shift power factor cosφ and the distortion power factor PFD is:

PFT=cosφ×PFD (2)

Equation (2) applies to all types of loads. The output power factor indicated by the UPS is the total power factor PFT, and cosφ is also used to represent the total power factor of the UPS. This can only be said to be a general understanding of the cosφ power factor. UPS is now an important AC power supply device that should meet different impedance characteristics or requirements for a certain impedance characteristic load. That is, while providing active power, it is also necessary to provide the reactive power required by the load. Therefore, the output power factor of the UPS is not only an indicator for indicating the output active power, but also an indicator indicating that the UPS outputs reactive power. After a large number of tests, it is found that some small-capacity (3kVA ~ 5kVA) UPSs meet the standard requirements for output active power and output voltage waveform distortion when using resistive load test. However, when the diode rectification non-linear load test conforming to its output power factor is used, the UPS not only displays the overload alarm but also significantly increases the distortion of the output voltage waveform, and the UPS generates electromagnetic oscillation and squeak. This phenomenon indicates that the UPS is not enough to provide the harmonic current required by the load, resulting in the UPS and the load not working properly.

It can be seen that when evaluating the UPS output capability, it is not only possible to test the output active power of the UPS with a resistive load, but also to use a diode-rectified non-linear load compatible with the UPS output power factor, an inductive load with a core, and a capacitive The load is tested separately for the output power factor. Only in this way can the UPS drive the load of various impedance characteristics fully.

The evaluation of how to evaluate the output power factor of the UPS is mainly based on the impedance characteristics of the load carried by the UPS. It cannot be generalized. Generally, the load of UPS of medium and small capacity (about 20kVA or less) is mostly PC, small LAN and server or small computer. The input circuit of these loads is generally diode rectified nonlinear load, and the phase shift power factor cosφ can be as high as 0.98~ 0.99, but the distortion power factor PFD is low, generally only about 0.65, so the total power factor of this type of load is 0.6 to 0.7. When the UPS is selected, the output power factor is 0. 6 to 0.7, which is suitable for ensuring that the output capacity meets the load requirements. For large UPS, the load situation is more complicated, and the distribution of load impedance characteristics of the three-phase output is not the same, so the output power factor of the UPS should be selected according to the specific conditions of the load. Now there are also UPSs with strong output adaptability, and the output power factor range can be 0~1. That is to say, the output of such a UPS can be from 100% reactive power to 100% active power. But the cost and price of this UPS are more expensive.

The test of UPS output power factor is more complicated, so only some large professional manufacturers are likely to conduct comprehensive testing of this indicator. If the UPS's output power factor specification does not indicate "leading" or "lag" afterwards, it means that the UPS is suitable for inductive or capacitive loads. The output power factor is followed by the “advanced” for capacitive loads and vice versa for inductive loads. Most of the load on the UPS is an inductive or diode-rectified non-linear load. The output power factor test method for these two load conditions is briefly described below.

The inductive load test circuit with iron core is shown in Figure 3. In the figure, L is the inductor with iron core, and R is the sum of the inductor resistance and the series load resistance. The phase angle φ of the current and voltage in the load circuit is determined by the resistance R and the inductance L, that is, φ=arctgωL/R. Adjusting the resistance R or the inductance L value changes the phase angle φ such that cosφ is equal to the output power factor of the UPS. The load impedance Z = √R2+(ωL)2, the current in the load I=U/Z, and the apparent power on the load PS=U×I=U2/Z. When the load capacity and power factor meet the test conditions, use the power harmonic analyzer to observe whether the output voltage, frequency and voltage waveform distortion of the UPS meet the standard requirements. The allowable dissipated power WR of the resistor R in the test circuit should satisfy WR>U/Z×R, and the cross-sectional area of ​​the inductor coil conductor can be calculated by 5A/mm2 reference.

The test circuit for the diode rectified nonlinear load is shown in Figure 4. The resistor RS in the figure is the voltage drop of the analog power line. At the same time, the power factor of the nonlinear load can be changed within a small range by adjusting the values ​​of RS, capacitor C and load resistor RL. When the time constant of RL and C is 0. 15s, the power on the RS is 4% of the apparent power, the power factor of this non-linear load is 0.7. When the UPS output capacity and the power factor of the non-linear load meet the test conditions, the power harmonic analyzer is used to observe whether the output voltage, frequency and voltage waveform distortion of the UPS meet the standard requirements.

The main capacity load of the UPS used in the communication industry is the tube rectification non-linear load, so the diode rectification non-linear load used in the UPS output capability test is closer to the actual use state of the UPS.