For a long time, in the process of design, construction and application of power supply in domestic computer room data center, "zero ground voltage" has been fooled, and even become the primary indicator of the quality of power supply in computer room. In recent years, this trend has become more and more intense. It is incredible that this anti-scientific "zero-ground voltage" has been written into some national standards, such as a GB-level computer room design specification requiring "the effective value of zero-ground voltage of UPS power supply system should be controlled within the range of less than 2V". Many manufacturers and users are accustomed to attributing all kinds of problems in data system to zero-ground. Voltage induced. At present, the influence of excessive zero-ground voltage on IT equipment, such as mainframe, minicomputer, server, disk storage device, network router and communication device, can be summarized as follows:

1. It may lead to the fatal damage of CPU chips of microprocessors in IT devices.

2. Increase the probability of IT equipment crash.

3. It may lead to the increase of BER and the slowdown of network speed.

4. It may cause damage to storage devices and data errors.

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

However, in view of the international IEC and UL power standards, there is no "zero ground voltage" at all, and no literature about "the impact of zero ground voltage on IT load" has been retrieved in IEEE articles. Interestingly, the author accompanied power experts from Europe and the United States to visit some users in China's data room. Some users raised the issue of zero-ground voltage. Unfortunately, these experts who worked on power supply for decades and participated in the drafting of UL power standards in the United States did not understand it at all. After repeated explanations, they basically understood the meaning of the so-called "zero-ground voltage", but he was surprised to ask, "In China." Is there any conclusive evidence of the effect of this voltage on IT load?

Although there is no definite scientific basis for the influence of zero-ground voltage on IT load (mostly confusing ground potential with zero-ground voltage), in order to solve this terrible and mysterious "zero-ground voltage" problem, many domestic users have spared no effort to invest a lot of money. For example, a communication data room purchased dozens of transformer cabinets and placed them at the input end of each floor to reduce the zero-ground voltage, which not only caused a lot of waste of resources, greatly increased the operating cost of the computer room, but also made the less profitable IDC business worse, and also reduced the reliability of the power supply system in the computer room.

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

2. Generation mechanism of zero-ground voltage

In 380V AC power supply system, due to the need of line protection, the central point of three-phase four-wire system is usually grounded directly through the grounding device. Figure 1 shows the typical structure of the distribution system in the current data room. One or more 10KV/380V /Yo transformers are usually installed in the system. The center point of Yo side is directly grounded through the grounding grid, and the G point in Figure 1.

From transformer to IT load, in order to ensure safe operation and maintenance and management, the lines in this distance are usually divided into three levels of distribution buses, i.e. UPS input distribution buses or municipal electrical input buses L1 (including diesel generator input after switching), UPS output distribution buses L2, floor distribution buses L3, floor distribution and then shunting to the header cabinet (also to distribute the floor distribution and header cabinet). Then the single-phase access rack PDU powers the IT load.

Thus, there is a long transmission distance between the secondary grounding point G of transformer and the zero line input point N of IT load. When the load is put into operation, there will be a large number of three-phase unbalanced currents and 3N harmonics due to the existence of factors such as three-phase voltage, phase asymmetry, phase load asymmetry of distribution buses at all levels and the non-linear characteristics of single-phase load. Current flows back to transformer's contact point G through zero-line current. Because of the existence of line impedance, the current flowing through zero-line generates voltage difference relative to reference point G at all points of zero-line, which is called "zero-ground voltage". Zero ground voltage is essentially nothing special with other voltages, just the voltage drop on the zero line.

Because the line impedance from all levels of distribution buses to transformer grounding point G is different, the zero line current flowing through each level of zero line is also different, which forms different zero-ground voltage points, as shown in Figure 1. However, data room users usually care 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. Output Zero Ground Voltage-U N3-G of Floor Distribution Cabinet

However, the zero-ground voltage-U-G at the cabinet end of IT load, which is the most "fatal" IT load, is often neglected.

3. The Zero-Ground Voltage at the Input Point of IT Load Cabinet is the "Most Terrible" Zero-Ground Voltage

Data room users are usually very concerned about the zero-ground voltage of UPS output terminal and the zero-ground voltage of floor output distribution cabinet, but they never care about the zero-ground voltage of IT load equipment input terminal inside the cabinet. If the zero-to-ground voltage really affects the IT load, no matter what measures you take to reduce the zero-to-ground voltage at the output end of UPS and the floor output distribution cabinet, as long as the zero-to-ground voltage at the input end of the IT load equipment is UN.-