The communication industry is a major application of UPS. As the power supply core of the entire equipment room, the primary problem of UPS is reliability, then safety, green, and energy saving. It is meaningless to talk about green and energy saving.
In a standard computer room power supply system, including transformers, diesel generators, ATS, TVSS, low-voltage power distribution cabinets, DC switching power supplies, UPS, batteries and other components, the most important power supply unit is UPS. The computer is the load of the UPS, and the UPS is also a load for the utility grid. The ideal UPS should be purely resistive to the grid, that is, its input power factor is preferably 1, so that it has no pollution to the grid. However, the reality is that the traditional UPS generally uses a 50Hz low-frequency thyristor rectifier, which generates a large amount of harmonic pollution to the mains. The problem facing all users is to control harmonic pollution, just like treating chemical plants to discharge sewage.
Harmonic is very harmful. It does not work, but it occupies a lot of power grid resources; harmonics cause the voltage distortion of the grid to increase, and the harmonics increase, which causes problems between the UPS and the generator. It is usually required that the power of the generator is 2 to 3 times that of the UPS, otherwise it will not operate. The author has also encountered actual cases in some communication departments, because some brands of UPS current harmonic feedback pollution is too large, causing the user's switch often trips inexplicably.
The composition of harmonics is very complex, and the control of harmonics is far from being as simple as increasing the power factor. Many people think that it is ok to add a few capacitors, but that is only for loads like motors. For non-linear loads such as UPS rectifiers, pure capacitance compensation results can be counterproductive, which may lead to further increases in distortion. Ordinary LC filters can only filter harmonics of a specific frequency. For example, a 5th filter can only be most effective for 5th harmonics, but has little effect on other odd harmonics. The principle of harmonic control is "who pollutes, Who governs."
Many UPSs on the market today use a 6-pulse rectifier, which requires the addition of an LC filter. With the addition of the filter, the input current harmonic distortion can be reduced to 7%, but this is only when the UPS is fully loaded. When the load is halved, the UPS input harmonic distortion will rise to more than 15%. Other UPS manufacturers offer a 12-pulse rectifier, which is a more conventional rectifier structure that adds a 30 compared to a 6-pulse rectifier. Phase shifting transformer and a set of 6 pulse rectifiers. Twelve thyristors are sequentially turned on in one cycle, so they are generally referred to as 12 pulses. After the 12-pulse rectifier is installed with the input filter, the UPS input current harmonic distortion can be reduced to 5% at full load, but this is only at full load. When the load is halved, the harmonic distortion of the UPS will rise to 15%. the above.
Therefore, there are several ways to control UPS harmonic pollution:
(1) 6-pulse rectifier + input filter;
(2) 12-pulse rectifier;
(3) 12-pulse rectifier + input filter;
(4) Active filter.
All of these methods have a common shortcoming, that is, pollution after treatment. Since the UPS uses the structure of the thyristor rectifier, it inevitably causes harmonic pollution, and then the user is forced to spend a lot of money to control the harmonic pollution, which is really a very uneconomical vicious circle.
If the UPS does not produce or rarely produces harmonic pollution, then the user does not need to spend money to manage it, then is there such a UPS? Yes, Eaton Axel's online UPS all use TGRT rectification, It fundamentally suppresses current harmonic pollution and reduces it to less than 3%. For the UPS field, this structure is an effective solution for harmonics. Now many users are still accustomed to using the number of pulses of the rectifier to evaluate the quality of the UPS. When choosing a UPS device, they will care about how many pulses the IGBT rectifier is. Simply put, the IGBT rectifier uses a pulse width modulation method. The IGBT chopping frequency reaches 8300 Hz, so it can be called a rectifier of 8300 pulses. Of course, this is much better than the so-called 12-pulse rectifier. Using an oscilloscope to observe the UPS input voltage and current waveforms, you will see that these waveforms are very pure sine waves. There is no feedback pollution to the grid. It is an active harmonic control method.
Today's computers and servers are very different from before. Blade servers with extremely high power density have become very popular. Servers generally use dual power supplies, and UPS power systems generally use "1+1" redundant parallels. Therefore, the load rate of the UPS single unit is relatively low, and the load of the 80% UPS is below 50%, or even less than 25%, and it is in the state of a large horse-drawn car. In this way, the UPS full load efficiency in the UPS indicator that everyone sees is of no practical significance to the user, because in reality the UPS does not reach full load, and it is good to use 50%. Therefore, the overall efficiency of the UPS at light load is more meaningful to everyone. At present, UPS manufacturers have claimed that their products can achieve 95% overall efficiency at full load, and can still reach 95% at half load, and even at 25% load, the overall efficiency can still reach 94%.
95% of the overall efficiency, provided that the UPS input power factor is greater than 0.99, the input harmonic distortion is less than 3%, and 95% can be achieved when the load is only 50%. Other UPS manufacturers' products, if they want to achieve the same specifications, generally need to add a large number of options, such as input filters, 12-pulse rectifiers, etc., but each additional option will further reduce the overall efficiency of the UPS: 12-pulse rectifier Will reduce the system efficiency by 2%, the active filter will reduce the system efficiency by 4%, so in the actual measurement, the overall efficiency of many 12-pulse UPS is less than 88%. Many people only pay attention to the UPS price itself when purchasing UPS. I don't know if the cost of using UPS is high.
Some UPS manufacturers now adopt a bypass energy-saving mode. The principle is that if the bypass voltage is within a certain range, the UPS is directly powered by the bypass. As a result, the efficiency of the UPS is indeed very high, but this efficiency is at the expense of load safety. Because the UPS rectifier inverter at this time does not have power supply, various transient disturbances in the power grid directly reach the load, the UPS does not play an isolation role, and when the utility power is cut off, the UPS switching has a time interruption. The power supply can be fully protected against the load only when the UPS is in online double conversion mode.
At present, UPS itself can provide analog dummy load test. The principle is that a full load test can be generated inside the UPS. The user can perform a full-load battery discharge test. In this way, all the links in the system can be tested, and the potential hidden dangers caused by the weak and loose wiring inside the UPS can be eliminated. While discovering hidden dangers for users, it can also save various expenses such as dummy load lease, electricity cost and cable connection cost.
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