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Scientifically Recognize the Zero Earth Voltage Problem of UPS Power Supply in Data Machine Room
- Apr 26, 2018 -

I. Introduction


For a long time, in the process of designing, building, and applying power in the data center of the domestic computer room, "zero voltage" has been overwhelmed and even became the primary indicator of the quality of the power supply of the computer room. This trend has intensified in recent years. It is incredible that this anti-science "zero voltage" has actually been written into certain national standards. For example, a GB-class room design specification requires "zero for UPS system". The effective value of the ground voltage is controlled within the range of less than 2V, etc., etc. Many manufacturers and users are accustomed to attribute various problems occurring in the data system to zero ground voltage. At present, the influence of the domestic industry on the excessively high "zero data voltage" and "statistical data" on IT equipment such as mainframes, minicomputers, servers, disk storage devices, network routers, and communication equipment can be summarized as follows:


1. It may lead to fatal damage to the microprocessor chip of the microprocessor in IT equipment.


2. Probability of crashes in IT equipment may increase;


3, may increase the bit error rate of network transmission and slow down the network speed;


4, may cause damage to the storage device, data errors and so on.


5, some well-known IT vendors stipulate zero voltage is greater than 1V not to boot and so on.


However, when we look at the international IEC and UL power supply standards, there is no such term as zero-ground voltage, and no article on the impact of zero voltage on IT load has been retrieved from the IEEE article. It is interesting that the author once accompanied European and American power experts to visit some data room users in China. Some users raised the issue of zero voltage. Poor experts who engaged in decades of power supply and participated in the drafting of UL power standards in the United States simply did not understand it. After repeated explanations, he basically understood the meaning of the so-called "zero voltage", but he was surprised to ask him: "In China, is there any conclusive evidence of the impact of this voltage on IT load?"


Although there is no solid scientific basis for the influence of zero-ground voltage on IT loads (mostly confusing ground potential with zero-earth voltage), many domestic users have to solve this terrible and mysterious “zero-ground voltage” problem. But he did not hesitate to invest a lot of money. For example, a telecommunications data room purchases dozens of transformer cabinets at the input of each floor to reduce ground voltage. This not only results in a lot of waste of resources, but also significantly increases the operating costs of the equipment room, making it less profitable. The IDC business is even worse, but also reduces the reliability of the power supply system room.


For this reason, the author thinks that systematically discusses the “zero voltage” generation mechanism of the power supply system of the equipment room, especially the impact of IT load, so that the design, construction, and users of the data center power supply in the computer room have a scientific “zero voltage” problem. The recognition is very necessary.


Second, zero ground voltage generation mechanism


In the 380V AC power supply system, due to the need for line protection, the center point of the three-phase four-wire system is usually directly grounded through the grounding device. Figure 1 shows a typical architecture diagram of the current data room power distribution system. The system is usually equipped with one or more 10KV/380V △/Yo transformers. The center point on the side of Yo is directly grounded through the grounding network, as shown in Figure 1. point.


From the transformer to the IT load, for the sake of safe operation and maintenance management, the line in this distance is usually divided into three levels of distribution bus, that is, the UPS input distribution bus or the mains input bus L1 (including diesel generator Input after switching), UPS output distribution bus L2, floor distribution bus L3, floor distribution and then split to the column head cabinet (also has floor power distribution and column head cabinet into one), and then single-phase access Rack PDUs power IT loads.


In this way, there is a long transmission distance from the secondary grounding point G of the transformer to the neutral input point N of the IT load. After the load is put into operation, due to the asymmetry of the three-phase voltage and phase of the power grid, all levels The asymmetry of the load of each phase of the distribution bus and the non-linear characteristics of each single-phase load, there will be a large number of three-phase unbalanced current and 3N subharmonic current flowing back to the transformer through the neutral line. At point G, due to the existence of the line impedance, the current flowing through the neutral line generates a voltage difference with respect to the reference point G at each point of the zero line, which is the so-called “zero earth voltage”. The zero-ground voltage is essentially nothing else with other voltages, just the voltage drop on the zero line.


Since the line impedances of the power distribution busbars to the transformer grounding point G are different at each level, the zero-line current flowing in each stage of the zero line is also different, which forms different zero-ground voltage points, as shown in FIG. 1 . However, data room users usually care about the following zero voltage points:


1, UPS input zero voltage-U N1-G


2, UPS output zero ground voltage -U N2-G


3, floor distribution cabinet output zero ground voltage-U N3-G


However, the zero-earth voltage-U N-G at the cabinet end of an IT load that is the most "lethal" to IT load is often overlooked.



Third, the zero-ground voltage at the input point of the IT load cabinet is the "worst" zero-ground voltage


Data room users are usually very concerned about the zero-ground voltage level at the UPS output terminal, and are also very concerned about the zero-ground voltage level of the floor-level power distribution cabinets, but they never care about the zero-ground voltage level at the input of the IT load equipment inside the cabinet. If zero-ground voltage really affects the IT load, no matter what measures you take to reduce the ground voltage at the output of the UPS or the floor output distribution cabinet, as long as the zero-ground voltage UN-G2 at the input of the IT load device is not less than At 1V, its "serious harm" still exists. The zero-ground voltage at the input end of the IT load cabinet is the superposition of all the UPS input zero line voltage drop, the UPS output zero line voltage drop, and the floor voltage drop of the floor voltage distribution. It can be said that the zero-earth voltage is the most outpost “hardest hit area”.


1, UPS output zero ground voltage -U N2-G


The UPS output zero ground voltage equals the UPS input zero ground voltage plus the zero line voltage gain generated by the UPS, ie U N2-G=UNI-G+UN-UPS


For different UPSs, whether it is a modern high-frequency machine or an old-fashioned power-frequency machine UPS to be eliminated, the internal zero line and the ground line are straight through; as long as the output filter is properly designed, the UPS generates its own output. The UVS N of the neutral voltage gain can be well suppressed. On the other hand, if the design is not good, both UPSs will have a higher zero-ground voltage gain. If Eaton IGBT rectified 9395 UPS, its zero-ground voltage gain is even better than the same frequency power frequency machine.


2, UPS floor output zero voltage on the distribution cabinet - U N3-G


The zero-ground voltage of the floor distribution output is equal to the zero voltage of the UPS output plus the zero line voltage gain between the UPS output and the floor distribution cabinet, ie, U N3-G=UN2-G+UN3-N2=UNI-G+UN-UPS+UN3-N2


The level of zero-ground voltage output from the floor distribution cabinet is often the zero voltage of the ground that the user of the data engine room cares about. When the transmission distance between the UPS and the floor distribution cabinet is long, the zero-ground voltage at the output of the UPS has been smaller than 1V, but the zero-ground voltage of floor power distribution output is still as high as 3~5V. In order to eliminate this problem, many superstitious zero-earth voltage users take a △/Yo isolation transformer in the floor distribution cabinet and re-ground the center point of the transformer output, ie, form a new ground point G2 and a new one close to 0V. Ground voltage.


3, IT load input zero ground voltage


As far as the current data center equipment room is concerned, single-phase power distribution is usually used between the floor output power distribution cabinets and the load cabinets, so that the neutral current in this distribution section is equal to the rack load current I4. The neutral voltage gain generated between the electricity and the IT load is UN-N3=I4*ZN-N3. Since I4 is large, and the power distribution circuit is fine, this voltage may still be greater than 1V. For example, for a cabinet with a load of 3500 W, if the power distribution to the cabinet of the floor distribution cabinet is 2.5 mm2 and the cable length is 20 m (assumed to be the farther cabinet), the zero line resistance is 0.15Ω, full-load neutral current of 16A, the resulting neutral pressure drop of 2.4V.


For the system in which the isolation transformer is installed 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 of the input terminal of the IT equipment and the new ground point G2, which is also equal to the zero line. The neutral pressure drop is 2.4V.


It can be seen that even if the transformer is configured on the floor and the zero-ground voltage at the floor power distribution output is equal to 0V, the zero-ground voltage of the actual IT load input still reaches 2.4V, which is much greater than 1V.



For a system with no isolating transformer installed in the floor distribution cabinet, the zero-ground voltage at the IT load input is equal to the potential difference between the N point of the IT equipment input and the original grounding 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+2.4V


The zero-ground voltage at the actual IT load input at this time is obviously much higher than 2.4V.


Fourth, zero ground voltage impact on IT load


From the previous analysis, it can be seen that for the actual input of the IT load in the data room, the zero-ground voltage is very difficult to eliminate as zero as the “ghost”, unless an isolation transformer is added to each IT cabinet, which is obviously very absurd Measures. So is zero ground voltage really affecting the IT load?


To understand whether the zero-ground voltage affects the IT load, the key question is whether the zero-ground voltage can be actually transmitted to the core components of the IT such as the CPU and the memory chip. In fact, by analyzing the internal structure of the IT load is not difficult to get, the voltage output by the UPS is only to supply power to the power module inside the IT load, and the output of this power module supplies power to the core components inside the IT. In this way, the influence of the zero-ground voltage on the IT load simplifies the problem that the voltage at zero ground affects the output of this power supply module.


The input power module inside the current IT load basically adopts two systems, namely ATX standard and SSI standard. The main circuit of these two kinds of power is shown as in Fig. 3.


Analysis of the working principle of this power supply can be seen, whether it is ATX or SSI power supply, UPS output 220V AC into the IT load inside, must be four levels of transformation, and finally converted into a stable 12V, 5V, 3.3V DC voltage , Provided to the IT load internal CPU, memory, storage devices, network communication chips and other "true load" use. This four-level transformation is shown in the figure below. They are:


The first level: bridge rectifier, the 220V AC into about 200 ~ 300V DC;


Second stage: high-frequency inverter, convert DC power into tens to hundreds of KHZ stabilized high-frequency AC power;


Third level: high-frequency isolation transformer, which steps down and isolates high-frequency alternating current;


Level 4: High-frequency rectifier, which converts stable high-frequency AC power into a stable DC 12V (or 5V, 3.3V) output.




1, zero ground voltage in the IT power transmission route


From the above figure, we can see that with 220V AC with a voltage of zero volts and ground, after entering the power source of the IT load, from the first to the second stage, maybe we can also "trace" the existence of this voltage trace, but after the third stage Later, due to the isolation of the transformer, this common-mode voltage is completely eliminated on the secondary side of the transformer. After the circuit has no zero line, only the positive and negative poles of the DC, so there is no longer a so-called zero ground voltage and Generated interference. In addition, both the ATX and SSI power supplies have conjugated reactors and Y capacitors at their input. This component can basically block common-mode zero-ground voltage beyond the first level of IT power.


It can be seen that after the zero-ground voltage enters the IT load, the communicator suppresses the commutation reactor and terminates at the front of the internal transformer. This does not reach the real IT internal CPU, RAM, EPROM, hard disk, etc. Therefore, no matter how high the zero-ground voltage is, it will have no effect on the data system at all.


It is necessary to point out that the 12V DC voltage output by the IT load power supply is obtained by the high-frequency transformation of the third-stage high-frequency inverter, and the conversion frequency is usually as high as 50KHZ~150KHZ, which is much higher than the frequency of the high-frequency UPS. , so high-frequency conversion is the fundamental IT power itself, IT load is not afraid of "high frequency."


2, "zero voltage" and "ground voltage"


"Zero ground voltage" has been widely known, and the concept of "ground voltage" seems a bit funny. However, if we can simply analyze the route of propagation of the phase and neutral lines within the IT load, we will come to a very surprising result. Since the transform structures of ATX and SSI are almost the same, we use the SSI standard power supply as an example.


The UPS with zero ground voltage outputs AC 220V voltage into the IT load power supply. After the positive half of the input power supply, after the second stage rectification, the phase line L is connected to the positive bus of the third stage high frequency inverter. The neutral line N is connected to the negative bus, as shown in Figure 4(a). In the negative half cycle of the input power supply, the opposite is true. The neutral line N is connected to the positive bus and the phase line L is connected to the negative bus. See Figure 4 ( b).


It can be seen that after the second stage of the IT load, the phase line and the zero line have exactly the same functions and flow lines. Thus, if the "zero voltage" is high, which will affect the normal operation of the IT load, it is undoubtedly high "ground voltage" will also have a fatal effect on the IT load. The zero-ground voltage can be made less than 1V or even equal to 0V by technical means. However, if we let the phase-to-ground voltage be controlled to be less than 1V, then the input of the IT load will be dead and the data room will be directly damaged. . Therefore, it can also be seen from this counterexample that it is a ridiculous concept to emphasize that the zero voltage is less than 1V!


Analysing the AC input part of this circuit also yields a more interesting result. Because of the complete symmetry of the input circuit, if we let the "zero voltage" be equal to AC 220V and the "phase voltage" equal to 0V, this The output of an IT power supply will operate normally without any influence. Therefore, theoretically, the safe zero voltage of the IT load should be AC 220V. The problem is that if the phase voltage is also equal to 220V, the phase zero voltage of the input IT load is equal to 0V or 440V, and the IT load appears. A power outage or high voltage accident! If we can design a "special UPS" with zero ground voltage, phase voltage and "phase zero voltage" equal to 220V to supply power to the IT load, the IT load will not be affected.



3, zero ground voltage impact on the server and other IT equipment and communications equipment test


The technical experts of China Telecom's electromagnetic protection support center and Huawei Technologies Co., Ltd. conducted zero-ground voltage scrambling tests on IT equipment such as servers and DTU data communications equipment, and conducted 121 on-grid equipment in more than 120 computer rooms of China Telecom. Sampling survey and research, the conclusions are as follows: (see References 1)


(1) From the scrambling test results for rack servers and blade servers, the zero voltage below 22V has no effect on these two servers.


(2) Zero voltage difference below 10V has no effect on DTU data communication equipment. However, in the case of a decentralized communication system, the zero-potential difference has an effect on data communication because the zero-potential difference causes a difference in ground potential between the device ports of the data communication line. (The author's note: According to the author's analysis of the circuit diagram given in the entire test report and report, to be precise, it should be when using RS232 and coaxial cable communication, due to the difference in ground potential leads to the impact of data communications The ground potential here is virtually independent of the zero-ground voltage of the input power supply, and they are two completely different concepts. In other words, if the ground potential of the two communication devices differs greatly, even if the input of the two communication devices is zero, A ground voltage equal to 0 will also have an effect on communication. In addition, if fiber optic communication is used, it will have no effect.)


(3) Through investigation of 122 on-net communication equipment rooms, under the condition that the normal operation of the equipment is ensured, the zero potential difference of the equipment is distributed below 10V. It is recommended that the zero-ground potential difference of the data communication equipment should be below 10V. ”


Sixth, the conclusion


From the type of UPS, no matter whether it is a modern high-frequency machine or a power-frequency machine UPS that will be completely eliminated, the zero line and the ground line are directly connected from the input end to the output end in the interior, and the mechanism of its generation and elimination is exactly the same. Can make it less than 1V, the key is whether the manufacturer is willing to invest in doing so.


If the user is concerned about the zero-ground voltage problem, then it should be concerned about the zero-ground voltage level of the IT load. That is the root cause of the "5 fatal problem" that is most likely to arise in the introduction. However, no matter what measures are taken to reduce the zero-ground voltage at the UPS output or at the floor distribution output, this voltage cannot be fundamentally reduced to less than 1V. Any practice that only guarantees the UPS output or the isolation transformer at the floor distribution end to achieve a zero-ground voltage of less than 1V is nothing more than self-deception.


Through the analysis of the IT load's four power converter stages, especially the high-frequency transformer converter stage, it can be seen that the zero-ground voltage does not have any effect on the output of the IT load power supply, and naturally it cannot be constructed on the data components of the IT load. The slightest impact. In addition, IT load power itself is an excellent "high frequency machine" power supply.


By comparing the technology of "zero voltage" and "ground voltage", it can be seen that the zero-ground voltage is the same as the ground voltage, and the voltage of 220V has no effect on the IT load. However, based on China Telecom's test data, it is very conservative to assume that the zero-earth voltage below 20V will not have any impact on modern IT loads (but it is necessary to pay attention to whether the phase-to-ground voltage at this time is normal).


Therefore, the last tip of the paper recommends that data room users should scientifically view the zero-ground voltage and its size, and go beyond the technical error of zero-ground voltage to avoid unnecessary waste and great damage to the reliability of the entire room power system.


references:


1. "The analysis of the influence of zero voltage on data communication equipment", Xie Qi Yu Pingfang Zheng Xiao


Scientifically understand the neutral-ground voltage of power system of data center


Eaton Power Quality (Shanghai) Co. Ltd. Wang Wei


Abstract : This paper analyzes the formation mechanism of the zero voltage in the power supply room of the data room, discusses the unavoidability of zero voltage generation and the possible impact on the IT load. It is recommended that the data room user should correctly view the zero voltage problem and step out of the zero voltage. The technical misunderstanding avoids unnecessary waste of resources.