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Hello! Welcome to visitUnderstand the problem of "zero ground voltage" of UPS power supply in data room scientifically-INDUSTRY-News-Anhui Accord Science And Technolgy Co.,LtdThe official website!
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Update time:2019-12-04   Click on the quantity:

  One, the introduction

  For a long time, in the design, construction and application process of data center power supply in domestic computer room, "zero-ground voltage" has been fabulously touted, and even become the primary indicator of power supply quality in computer room. This trend intensified in recent years, incredibly the anti-science "zero voltage" should be written into the some national standards, such as a gigabyte computer room design specification requirements of "UPS power supply system of zero voltage RMS control in less than 2 v range", etc., many manufacturers and users are used to all kinds of problems in the data system caused by the voltage to zero. At present, the domestic industry according to the "statistical data" "zero ground voltage" too high on IT equipment, such as mainframe, minicomputer, server, disk storage equipment, network routers, communications equipment can be summarized as the following:

  1. May cause "inexplicable" fatal damage to microprocessor CPU chips in IT equipment;

  2. Increase the probability of IT equipment crash accidents;

  3. It may cause the increase of the bit error rate of network transmission and slow down the network speed;

  4. It may cause damage to storage equipment, data error, etc.

  5. Some well-known IT manufacturers stipulate that the zero-to-ground voltage greater than 1V will not be turned on.

  However, when looking at the international IEC and UL power standards, the term "zero-ground voltage" is not found at all, and a search of the IEEE papers has not found any "relevant literature on the impact of zero-ground voltage on IT load". IT is interesting to note the author had accompanied the power of the European and American experts visit some Chinese data room users, some users put forward the problem of zero voltage, the poor have decades of power and participate in the United States UL standard drafting experts simply can't understand, after repeated explanation to basic understand the meaning of the so-called "zero voltage", but he was surprised to ask: "in China, there are a voltage conclusive evidence of the impact of the IT load?" .

  Although there is no solid scientific evidence on the impact of zero-ground voltage on IT load (most of which confuse ground potential with zero-ground voltage), many users in China are willing to invest a lot of money to solve this terrible and mysterious "zero-ground voltage" problem. As a communication data rooms bought dozens of input transformer tank placed in each floor room to reduce the zero voltage, this not only lead to a lot of waste of resources, greatly increased the operation cost of the room, make originally is not IDC business profit added to the gloom, but also reduce the reliability of the equipment room power supply system.

  Therefore, the author believes that IT is necessary to systematically discuss the generation mechanism of "zero-ground voltage" of power supply system in computer room, especially the influence on IT load, so that the design and construction of power supply in computer room data center and users have a scientific understanding of "zero-ground voltage".

  The generation mechanism of zero ground voltage

  In a 380V ac power supply system, the center point of the three-phase four-wire system is usually directly grounded through a grounding device for line protection. FIG. 1 shows the typical architecture diagram of the distribution system in the current data room. One or more 10KV/380V △/Yo transformers are usually configured in the system.

  From transformer to the load between IT, for the sake of safety, operation and maintenance management is usually the distance of the circuit is divided into three distribution bus, namely the UPS input distribution bus or mains input bus L1 (including diesel generator after switch input), UPS output distribution bus L2, floor distribution bus L3, distribution and shunt into the column head tank floor (or the distribution into the column header tank floor), then the IT load single-phase access rack PDU power supply.

  So, from the secondary side of transformer ground G to the IT load the zero line of the input point between N, there is a long transmission distance, when the load of operation due to the power grid of three-phase voltage, phase asymmetry, asymmetric distribution of each phase load bus at all levels and the existence of the nonlinear characteristics of the single phase load factor, there will be a large number of three-phase unbalance current and 3 N time harmonic current through the zero line flows back to the transformer ground point G, due to the existence of the line impedance, the zero line current through the zero line of each point produced relative to the reference point G voltage difference, this is the so-called "zero voltage". Zero ground voltage is essentially nothing special to any other voltage, just a voltage drop on the zero line.

  Due to the different line impedance between the distribution bus at all levels and the point G at the connection point of the transformer, the current flowing through the zero line at each level is also different, thus forming different zero ground voltage points, as shown in figure 1. However, data room users are usually concerned about the following zero ground voltage points:

  1. UPS input zero ground voltage -u n1-g

  2. UPS output zero ground voltage -u n2-g

  3. Floor distribution cabinet outputs zero-ground voltage -u n3-g

  However, the zero-ground voltage-u n-g on the IT load cabinet, which is the most lethal for IT load, is often overlooked.

  Three, IT load cabinet input point zero ground voltage is the "most terrible" zero ground voltage

  Data room users are usually very concerned about the zero-ground voltage level at the UPS output terminal and the zero-ground voltage level at the floor output distribution cabinet, but they never care about the zero-ground voltage level at the IT load equipment input terminal inside the cabinet. If the zero-ground voltage really has an impact on the IT load, no matter what measures you take to reduce the zero-ground voltage on the output end of the UPS and the floor output distribution cabinet, as long as the zero-ground voltage un-g2 at the input end of the IT load device is not less than 1V, its "serious harm" will still exist. And IT load cabinet input zero ground voltage is the superposition of all UPS input zero line voltage drop, UPS output zero line voltage drop and floor distribution zero line voltage drop, IT can be said to be the most outpost of zero ground voltage "disaster area".

  1. UPS output zero ground voltage -u n2-g

  The zero ground voltage output of UPS is equal to the zero ground voltage input of UPS plus the zero line voltage gain generated by UPS, that is, U n2-g = UNI -g + un-ups

  For different UPS, no matter it is the modern high-frequency machine or the old power frequency machine that will be eliminated, the zero line and the ground line are straight through in its internal. As long as the output filter is designed correctly, the zero-line voltage gain UUPS N generated by UPS can be well suppressed; otherwise, both kinds of UPS will produce higher zero-ground voltage gain. Such as Eaton IGBT rectifier 9395 UPS, its zero ground voltage gain is even better than the same capacity of the power frequency machine.

  2. Zero ground voltage -u n3-g on the output distribution cabinet of UPS floor

  The zero ground voltage of floor distribution output is equal to the zero ground voltage of UPS output plus the zero line voltage gain of UPS output to floor distribution cabinet, that is, U n3-g = un2-g + un3-n2 = uni-g + un-ups + un3-n2

  The zero-ground voltage output by the floor distribution cabinet is often the terminal zero-ground voltage concerned by users of the data room. When the transmission distance between the UPS and the floor distribution cabinet is very long, although the zero-ground voltage at the output of the UPS is less than 1V, the zero-ground voltage output by the floor distribution cabinet is still as high as 3~5V or more. In order to eliminate this problem, many users who believe in zero-ground voltage have to add a delta /Yo isolation transformer to the floor distribution cabinet and re-ground the center point of transformer output, that is, forming a new grounding point G2 and a new zero-ground voltage close to 0V.

  3. Zero-ground voltage at the input of IT load

  In terms of current data center machine room, the floor output power distribution cabinet to the load usually single-phase power distribution cabinet, so that when the distribution range of the zero line current is equal to cabinet load current i2, on the floor at this time IT occurs between the load distribution and zero voltage gain for the UN - N3 = I4 * zinc - N3, due to large I4, and distribution of circuit is fine, this still may be greater than 1 v voltage. For example, for a load of 3500 w cabinets, from if the floor distribution branch distribution to the cabinet of the cable is 2.5 mm squared, cable length of 20 m (assuming are relatively remote cabinet), as the zero line of resistance is 0.15 Ω, loaded with zero line current is 16 a, produces zero pressure drop of 2.4 V.

  For the system with isolating transformers in the floor distribution cabinet, see figure 2. At this time, the zero-ground voltage at the input of the IT load is equal to the voltage difference between the N point at the input of the IT equipment and the new contact point G2, and is also equal to the voltage drop of the zero line generated on the zero line of 2.4v.

  IT can be seen that even if the floor is equipped with transformers and the zero-ground voltage at the floor distribution output is equal to 0V, the zero-ground voltage at the actual IT load input is still up to 2.4v, much higher than 1V.

  For the system without isolating transformers in the floor distribution cabinet, then the zero-ground voltage at the input of IT load is equal to the potential difference between N points at the input of IT equipment and the original connection point G. According to figure 1, the corresponding zero-ground voltage is calculated as follows:

  UN - G = UNI - G + UN - UPS + UN3 - N2 + UN - N3 = UNI - G + UN - UPS + UN3 - N2 + 2.4 V

  The zero-ground voltage at the input of the actual IT load is obviously much higher than 2.4v.

  Iv. Impact of zero-ground voltage on IT load

  Previous analysis has shown that for the actual input side of the IT load in the data room, zero ground voltage is like a "ghost" that is very difficult to eliminate zero, unless you add an isolation transformer to each IT cabinet, which is obviously a very absurd measure. So does zero ground to ground really matter for IT load?

  To understand whether the zero-ground voltage has an impact on the IT load, the key question is whether the zero-ground voltage can really be transferred to the core components such as CPU and memory chip inside IT. In fact, by analyzing the internal structure of the IT load, IT is not difficult to obtain that the voltage output by UPS only supplies power to the power module inside the IT load, and the output of this power module supplies power to the core components inside IT. Thus, the effect of zero-ground voltage on the IT load is reduced to the effect of zero-ground voltage on the output of the power module.

  The current IT load internal input power module basically adopts two standards, namely ATX standard and SSI standard. The main circuit of these two power sources is shown in figure 3.

  Analyzing the working principle of the power supply can be seen that both ATX and SSI power supply, UPS output 220 V alternating current into the IT load inside, must be the level 4 transform, finally converted to stable the 12 V, 5 V, 3.3 V, dc voltage, provided IT load within the CPU, memory, storage devices, network communication chips used "the real load". The four-step transformation is shown in the figure below, as follows:

  First stage: bridge rectifier, which converts 220V ac into about 200~300V dc;

  Second stage: high frequency inverter, which converts direct current into high frequency ac of tens to hundreds of KHZ stabilized voltage;

  Stage 3: high frequency isolating transformer, reducing voltage of high frequency ac and isolating;

  Level 4: high frequency rectifier, converting stable high frequency ac to stable dc 12V(or 5V, 3.3v) output.

  1. Transmission path of zero-ground voltage in IT power supply

  From above, have ShuFu zero voltage 220 v ac, into the IT load power, from the first to the second level, maybe we can "search" to the existence of this voltage traces, but after the third level, due to the effect of isolation of the transformer, the total mode voltage in transformer secondary side is completely eliminated, at the back of the circuit has no zero, only the positive and negative dc, there is no longer so called zero voltage and interference. In addition, both ATX and SSI power supplies are equipped with conjugated reactors and Y capacitors at their input terminals, which can basically block the zero-ground voltage of the common mode outside the first stage of the IT power supply.

  IT can be seen that after the zero-ground voltage enters the inside of the IT load, IT will end up in the front end of the internal transformer after being suppressed by the conjugated reactor, and cannot reach the power supply end of the CPU, RAM, EPROM, hard disk and so on inside IT. Therefore, no matter how high the zero-ground voltage is, IT is impossible to cause any impact on the data system.

  IT is necessary to point out that the 12V dc voltage output by the IT load power supply is obtained through the high-frequency transformation of the third-stage high-frequency inverter, whose transformation frequency is usually as high as 50KHZ~150KHZ, far higher than that of the UPS of the high-frequency machine. Therefore, high-frequency transformation is the root of the IT power supply itself, and IT load is not afraid of "high frequency".

  2. "zero-earth voltage" and "phase-earth voltage"

  While "zero-ground voltage" is widely known, the concept of "phase-ground voltage" seems a bit of a joke. However, if we could simply analyze how the phase and zero lines propagate within the IT load, we would get very surprising results. Since the transformation structure of ATX and SSI is almost the same, we take SSI standard power supply as an example to illustrate.

  After the UPS output AC 220V voltage with zero ground voltage enters the power supply of IT load, the phase line L is connected with the positive bus of the third-stage high-frequency inverter after the second stage of rectification in the positive half of the input power supply, while the zero line N is connected with the negative bus, as shown in figure 4(a). In the negative half cycle of the input power supply, it is just the opposite. The zero line N is connected with the positive bus, while the phase line L is connected with the negative bus, as shown in figure 4(b).

  Thus, after the second level of the IT load, the phase line and the zero line have exactly the same functions and flow lines. In this way, if a high "ground zero voltage" will affect the normal operation of the IT load, there is no doubt that a high "ground zero voltage" will also have a fatal impact on the IT load. We can make the zero-ground voltage less than 1V or even equal to 0V by technical means, but if we make the phase-ground voltage less than 1V, the input power of the IT load will run out and the data room will be directly paralyzed. Therefore, it can also be seen from this counterexample that it is an absurd concept to emphasize that zero ground voltage is less than 1V!

  An even more interesting result can be obtained by analyzing the AC input part of the circuit. Due to the complete symmetry of the input circuit, if we set the "zero-ground voltage" equal to AC 220V and the "phase-ground voltage" equal to 0V, the output of this IT power supply will work without any influence. Therefore, theoretically speaking, the safety zero-ground voltage of the IT load should be AC 220V. The problem is that if the phase-ground voltage is also equal to 220V, the phase-zero voltage of the input IT load will be equal to 0V or 440V, and the IT load will have a power failure or a high-voltage accident! If we can design a "special UPS" with zero ground voltage, phase earth voltage and "phase zero voltage" all equal to 220V to power the IT load, the IT load will not be affected.

  3. Test the impact of zero-ground voltage on servers and other IT equipment and communication equipment

  China telecom electromagnetic protection and support center and technical experts from huawei technologies co., ltd. communicate with servers and other IT equipment and DTU data

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