Misunderstanding on "battery life shortening" theory of high frequency UPS

Time of issue:2022-04-19


1. Appearance and responsibility of UPS


UPS has a history of nearly 50 years, and high-frequency UPS has a history of more than 10 years. As the twin brother of computer, UPS has been responsible for protecting data since the day of birth. The early computer input system is not as convenient as it is now. The input data must first be converted into binary Boolean algebraic forms of 0 and 1. For example, 1 is represented by 01, 3 by 11, etc., and then these numbers are perforated on the paper tape, as shown in Figure 1. Generally, there are 8 perforation positions along the width direction on the paper tape. In this position, if there is a hole, it is 1, and if there is no hole, it is 0. After the paper tape is punched on the punch, it is sent to the photoelectric machine for input to the computer. The perforated paper tape moves under the irradiation of light. The place with holes is transparent and is received by the opposite photoelectric tube, which is 1. The place without holes is opaque, which is 0... So it is very complicated to input the problem into the computer. However, in the calculation process, once the mains power fails, the whole calculation would completely become blank. After the next mains power recovery, the whole photoelectric input process would have to be repeated, which is quite troublesome! Therefore, someone proposed that if a signal is given to the computer when the mains power is off, and then the power is supplied for 5S, let the computer save the on-site calculation results, and then calculate the last result after the next mains power is restored. So the first generation UPS came out with the mission of protecting data and uninterrupted power supply.


What was the first generation UPS like? For example, the 20kva UPS imported with the 1900 computer in 1967 installed a 5T heavy flywheel on the coaxial of an electric generator. When the mains power is cut off, the inertia of the flywheel drives the generator on the coaxial to maintain the power supply time of 5S. Later, it developed to charge the battery pack and drive the DC motor to rotate after rectifying the mains power, so as to drive the coaxial alternator. In this way, the time of uninterrupted power supply could be extended. This is also the first time that the battery pack has entered UPS as uninterrupted energy. By the 1970s, semiconductors had been developed, and the emergence of thyristor developed the original rotating generator UPS into static conversion circuit, but the battery pack was still retained as uninterrupted energy. So UPS began to develop in two directions: rotating generator UPS and static conversion UPS. At present, rotating generator UPS has 3000kva products, and static conversion high-frequency UPS also has 1200kva single machine products.


2. Current situation and development of UPS


At the beginning, the static conversion circuit is the power frequency type UPS, and the rectifier and inverter circuits are composed of thyristors, and the input and output have transformers. Because the UPS circuit technology at that time was still a full bridge circuit, and the voltage at the output end was AC, that is, the output lines were live lines, which could not meet the desire of UPS output zero line grounding required by the computer manufacturer, so the output isolation transformer had to be installed, which could not only meet the requirements of output grounding, but also bring convenience to the flexible configuration of the battery in front of the inverter. But this also increases the weight, volume and power consumption of UPS. With the development of electronic components and IT technology, data centers began to be established one after another, and the computer rooms of these centers began to become large users of electricity. Another weakness of power frequency UPS has been exposed, that is, the low input power factor. This is mainly due to the thyristor used in the input circuit. As the result of the destructive effect of the thyristor rectifier filter input circuit (commonly known as 6 pulses) on the grid voltage, its input power factor is only 0.8. In other words, if the UPS has a capacity of 300kVA, it can only effectively use the active power of 240kw of the municipal power supply, and the reactive power of 180kvar occupies the line of the power grid. In addition, it also forms external interference. If the generator is added in front, the capacity of the generator is at least three times that of UPS, that is, at least 900kVA, which brings a great burden to users. Therefore, it is necessary to improve the input power factor of UPS. The method is to configure active filter or upgrade 6-pulse rectifier to 12 pulse rectifier, and it is best to add 11 harmonic filter, which increases the burden of UPS. For example, after the 300kVA capacity of a 6-pulse rectifier UPS is increased to 12 pulse rectifier and 11 harmonic filter, it increases by 600kg, and the weight is increased from 1.6 tons to 2.2 tons, which is not a small cost.


The development of electronic components has opened a channel for the upgrading of UPS. At present, the weight of a 300kVA high-frequency model UPS with an input power factor of more than 0.99 is only about 0.8 tons, and the efficiency is nearly 5 percentage points higher than that of the former, which uccessfully solves the above difficulties and brings good news to the data center. This is the replacement product of power frequency UPS.


3. The debate between the two models


There is a proverb in China, which is called "the old does not go, the new does not come". The problem here is that the new has come and the old has not gone. The reason is that the old people are unwilling to go. Since you don't want to go, you have to give the reason why you don't want to go. The best way is to take the attack as the defense and accuse the deficiency of high-frequency UPS. Of course, the "arguments" on accusing the high-frequency UPS of no transformer, low reliability of IGBT and late appearance have been refuted by all parties, because the so-called arguments are basen on that they have a vague understanding of the basic functions of UPS, so they are untenable. In desperation, a new attack target was put forward: when the rectifier power of high-frequency UPS is insufficient, it needs to ask for energy from the battery to supplement, resulting in the shortening of battery life.


This argument is undoubtedly a heavy bomb in the eyes of laymen, but for people with a little knowledge of UPS, they have to shake their heads and sigh: because this is another basic conceptual problem. Why is UPS called uninterruptible power supply? Who is responsible for this uninterrupted function? It is clear from the previous description that the energy storage of the flywheel - kinetic energy is used at the beginning. Later, chemical energy was used, that is, the battery. A circuit without battery cannot be called UPS, and can only be regarded as AC voltage regulator or voltage stabilizing and frequency stabilizing source (CVCF). As shown in Figure 2, if the battery pack GB is removed, there will be no uninterrupted function. This battery pack is put here for uninterrupted function. No matter what the reason is: whether the input mains power is cut off or the rectifier energy is insufficient due to unstable input voltage, the battery pack should be managed as long as it affects the intermittent power supply. Someone said: when the rectifier power is insufficient, it is necessary to ask for energy from the battery to supplement, resulting in the shortening of battery life. This is the responsibility of the battery. You have to do it to shorten its service life. Otherwise, what do you want it to do! This is the first misunderstanding: do not understand the basic role of batteries.


The second misunderstanding is that when the rectifier power of power frequency UPS is insufficient, its battery pack will not supplement energy in order not to shorten its service life. Then what shall we do? Either shut down the UPS or send the insufficient energy directly to the load, regardless of whether the load works normally or not! Power frequency UPS will never work like this.


Therefore, when the rectifier power is insufficient, it is necessary to ask for energy from the battery to supplement, leading to the shortening of battery life. This will exist for both power frequency UPS and high-frequency UPS, and it must be done.