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In the case of high-frequency UPS with medium and high power, the system efficiency when the battery is discharged is not a matter of reducing 2 by the problem proposer. Generally, in the high-frequency UPS models with medium and high power, the virtual power supply can no longer meet the requirements of high-current output. At this time, the capacitor can only be used as a supplement to the excessive internal resistance of the battery when the load changes suddenly, which can not give the frontier current problem. The following large current still depends on the large-capacity battery pack, as shown in Figure 12. Whether it is the high-frequency model UPS with two DC power supplies shown in FIG. 12a) or the high-frequency model UPS with only one DC power supply shown in FIG. 12 (b), almost at least 32 12V batteries are used in series Or a battery series solution with similar voltage. The rated voltage of these battery packs is much higher than the peak voltage of AC 220V 310V. Therefore, after the mains power is cut off, the charging link has also stopped working, and the set backup time is maintained by the energy of the battery itself until the battery voltage drops to the inverter shutdown voltage level. At this time, the shutdown voltage level is generally 320 to 332V, which is the same as the work of the industrial frequency model UPS inverter, so these 2 ties do not exist. What really exists is the output transformer of the power frequency model UPS, see Figure 12 (c). This transformer occupies nearly two-thirds of the power frequency machine UPS space and more than 2 power consumption. If you have to say "fatal", you should go to the power frequency model UPS. In fact, some people just make a fuss. Although the power frequency UPS consumes a lot of power, it has been working very well after so many years of use, and no one said it was a fatal problem.
The external transformer of high frequency model ups will damage the load
Why you need to connect an isolation transformer
The elimination of the output isolation transformer is a major feature of high-frequency UPSs, and it is also a major advantage because it reduces the initial price of system power consumption, volume, and weight. Some people have to add the removed transformer. Of course, some users here also have such requirements, but most of the users' requests are caused by misleading by some manufacturers. It is said that the purpose is to reduce zero ground. Voltage. Nonetheless, some of the problem proposers are not at ease, saying that "zero ground voltage is still high, and it continues to jeopardize the safe operation of electrical equipment." Then for the time being, add an isolation transformer to the high-frequency model UPS, as shown in Figure 13 (a), take a look at this conclusion. You can compare the two circuits in Figure 13 (a) and (b). Now that the output of the two inverters is connected to the transformer, it can be seen that the working mode of the two inverters is the pulse width modulation type, and the modulation frequency is also similar, which can be said to be the same. Therefore, there is no difference in the work of the inverter power tube; in order to send the sine wave voltage to the load, a low-pass filter must be installed to filter out the high-frequency components during modulation, only A 50Hz sine wave can pass, and it can be seen from the figure that both of them have this filtering link, but the harmonic filter of the high frequency model UPS is before the transformer, and the harmonic filter of the power frequency model UPS one After the transformer, it means that the two links are not only the same, but also the same. The difference is the position of the filter link and the transformer. In this way, it can be seen that in the high-frequency model UPS, high-order harmonics are filtered out before the transformer, and return to the negative end of the DC BUS through the zero line, that is, the high-order model UPS high-order harmonics The wave didn't touch the primary winding of the transformer at all. The high-frequency harmonics of the power frequency model UPS are filtered out behind the transformer, in other words, they are filtered near the load end. This presents a problem. According to the author of the question, the zero ground voltage formed by the higher harmonics near the load cannot be added to the load, nor does it affect the T operation of the load; instead, it is far away from the load. The zero ground voltage formed by the subharmonics will definitely be added to the load and continue to jeopardize the safe operation of the load. The same circuit principle will instead produce two different results, I do n’t know whether it is analyzed or measured. Good Elephants do not make sense from graph theory.
Some places say that the high-frequency model UPS will bring hidden dangers to the equipment after the transformer is added. It is also said that when the high-frequency UPS "output power failure or flash failure occurs for some reason," the external isolation transformer will appear "flyback transient spike voltage", which is enough to burn IT equipment. When the input suddenly restores power, it will cause a "severe overload" of the parallel system, and so on. What is puzzling is that the same power supply link and the same function is that the power frequency model is replaced by the high frequency model. The difference is only one word, and the results of the two are different. Does it mean that the power frequency model UPS will not have output power failure or flash failure? Even if there is a failure, will the transformer produce a "flyback type transient spike voltage"? When the input suddenly restores power, the power frequency machine Type UPS will not cause serious overload of parallel system. Could it be said that the influence of the external isolation transformer is inherent in high-frequency UPS? For high-frequency UPS, there is no need for an additional transformer. First of all, as mentioned before, the zero-earth voltage is not a source of thousands of disturbances. Furthermore, there is no channel to pass the zero-earth voltage. It is the constant touch interference that affects the electrical equipment. How does the common mode interference break the electrical equipment? Figure 14 shows the schematic diagram of the normal mode interference and the common mode interference. If the interference voltage is effective, there must be energy. Here The energy is the power multiplied by the current and the voltage, that is, the interference source and the interfered object (electric equipment) must form a current loop. It can be seen from the figure that the normal mode interference current is formed by the voltage between the live and neutral lines, and can form a current loop with the power supply and the load. The common-mode voltage (here, zero-ground voltage) is the voltage between the zero line and the ground line. It does not form a closed-loop circuit of current with the electrical equipment. Neither the voltage nor the current reaches the channel of the electrical equipment. What about interference? What about "harming the safe operation of these electrical equipment".
It is puzzling that the same transformer connected to the output of the high-frequency model UPS inverter has so many "hidden dangers", while the output connected to the industrial frequency model UPS inverter has more excellent The ability to resist "impact" loads. In fact, this is the characteristic of the reactor or choke. Not to mention conceptual misunderstandings, let ’s consider this transformer to be inductive, and this inductance is used in a certain way: when used in the output of high-frequency UPS inverters, there will be damage to the electrical equipment. Excitation type transient spike voltage ", and the output of the UPS inverter used in power frequency models has more excellent resistance to" impact "load. Not only that, but also become a "50Hz filter connected between the UPS and the rectifier filter type nonlinear load, it will greatly improve the ability of the UPS to bear the inrush current with a peak ratio." It seems that this transformer has the function of "intelligence". However, the author encountered an example where the power supply system with the output connected to the transformer burned the UPS and the battery, and the industrial frequency machine was burned.
Example: A manufacturing plant in Beijing adopts a 60OkVAUPS power supply scheme, as shown in Figure 15. Five 15OkVA UPSs are used for "4 + 1" redundant parallel connection, and the output ends are five secondary windings of UPS output transformers connected in parallel. There is also a 30OkVA transformer in the load, which can be said to be layered fortification. However, due to the 30OkVA load transformer switch S closing during battery mode power supply, due to the instantaneous short circuit of the load transformer, the UPS was partially burned and the battery pack caught fire, and more than 70 100Ah batteries were burned in one fell swoop. The role of "buffer" and "filter".
It is worth mentioning that some people say that the transformer can resist interference, which is another basic conceptual problem. What devices can resist interference? Anyone with basic circuit knowledge knows that only non-linear devices or inertial devices can resist interference. The transformer is a non-linear core working in the linear region, which is why it does not distort the transmission waveform. The key to winding transformers is to make the leakage inductance as small as possible, and zero leakage inductance is the best. A good transformer is almost a fully linear device. The characteristic of a linear circuit is to transmit the waveform input without distortion. The waveform output is copied as it is. This can be detected with a dual trace oscilloscope. The transformer with large leakage inductance is a low-quality transformer because of its inductance, or even a substandard product, because it reduces the dynamic performance of the power supply output voltage.
Of course, in order to demodulate a sine wave from PWM, a special industrial frequency UPS output transformer intentionally leaves a little leakage inductance when winding the output transformer. The purpose is to use this leakage inductance and the capacitor behind the transformer to form an LC filter. But this leakage inductance is very small, to the extent that it does not affect the output dynamic performance of the UPS.
The transformer of the high-frequency model UPS is useless, its purpose is to introduce the so-called high performance of the output transformer of the industrial frequency model UPS. Some people say that the output transformer of this UPS is used to resist interference. What kind of interference is to be resisted? Is it interference from the front of the UPS output transformer or interference from the load? What is the purpose of anti-interference, is it to protect the load behind or to protect UPS inverter? To know that the output voltage of the UPS inverter is a good sine wave without interference. That there is "anti-" interference from the load. But the so-called interference from the load is caused by the normal work of the load. Because the load devices in the past were mostly rectified and filtered loads with low input power factor, which caused a certain degree of damage to the sine wave of the output voltage of the UPS, generally called "interference", and this so-called "interference" is after the load works Destroy the "result" of the voltage. The result of this damage voltage is the largest near the load. The farther the distance from the UPS output to the load, the thinner the wire, and the more contacts that pass, the greater the distortion; on the contrary, the distortion is the smallest at the UPS output, which does It is not the result of any transformer that can resist interference, but its original face. As shown in the two diagrams (a) and (b) in Figure 16, if two UPSs with the same power are carrying the same load, their UPS output ends are very good sine waves, and they become distorted waveforms at the load end. This is because the rectifier filter circuit of the load is not a sine wave current, but a pulse current that is an average or multiple of the effective value. This current must form a voltage drop on the transmission line and the distributed impedance of the transmission line. Because the pulse current is only on the sine voltage wave Is formed near the peak of, so this voltage drop is only formed near the peak. The peak value of the voltage wave reaching the load must be subtracted from the peak value of the output voltage, so that the distortion of truncation is formed. The waveform of the voltage at the output of the UPS cabinet depends on the size of the internal resistance of the UPS. Therefore, the large distortion at the load and the small distortion at the UPS have nothing to do with the transformer, and it is not a result of interference, nor is it the result of anti-interference of the transformer. And whether it is a power frequency type UPS or a high frequency type UPS, the results in this regard are the same. As for the "burr" on the cable between the UPS output and the load, it is also caused by the nonlinear waveform of the load and the transmission of the voltage, and it is not what is called interference.
At the output port of the UPS, the interference amplitude is very small, no need to resist. The purpose of anti-interference is nothing more than what to protect. There are only two goals to deal with this output transformer here: the front inverter and the back load equipment. It has been known in the past that this so-called interference is the result of the normal operation of the load, and it is within the normal working range, so there is no need to protect; the front inverter has capacitors in front of it, and the output voltage sine wave here is very good, there is no so-called "" "Interference" doesn't need to be targeted by the transformer. Therefore, the anti-interference of the transformer that is vigorously promoted here is "a false shot" and "no aim".
In short, in the market that devalues ​​high-frequency UPS models, some propagandists use the so-called "analysis" method or the performance of substandard products to create some so-called "potential" and "hidden danger" and other suspense to mislead users. These arguments are not lack of theoretical level and basic conceptual issues, but in any case, it should not mislead users, and it should not be contrary to the current national energy conservation and emission reduction policies.
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