Zero ground voltage concept:
With the rapid development of computer technology, the working voltage of internal chips in computers is getting lower and the energy consumption is getting smaller and smaller. In order to ensure the normal operation of servers, new types of servers from various manufacturers have increasingly high requirements for zero ground voltage in the data center. What is the zero ground voltage promised by UPS manufacturers, which refers to the zero ground voltage without additional increase, that is, the zero ground voltage at the UPS output terminal when the zero ground voltage on the input distribution cabinet in the computer room is 0V. If the user’s own power distribution system does not meet the requirements, the zero line current or phase to phase high-order harmonic coupling will cause a certain voltage difference between the zero line and the ground line. In this way, at the input of the UPS, the zero ground voltage will rise and not meet the equipment requirements. This is not a problem that UPS itself can completely solve.
For example, when a certain computer room is working on the UPS bypass, the zero ground voltage of the distribution cabinet is 1.8V. When the UPS is inverted for power supply, the zero ground voltage of the computer room distribution is 2V, and the added zero ground voltage of the UPS is 0.2V. The UPS manufacturer’s claim that the zero ground voltage is lower than 1V refers to this added zero ground voltage. Because when UPS operates in dual conversion mode, the switching devices inside the UPS and some principles of EMC can cause zero ground induced voltage or high-frequency conduction voltage drop. The methods to reduce zero ground voltage include:
① Shortening the length of the neutral wire and increasing its cross-sectional area can reduce its reactance, thereby lowering the zero ground voltage. The advantage of this solution is its obvious effect. From the calculation formula of zero line reactance Zn=ρ L/S, when the line length L decreases and the cross-sectional area of the wire increases, Zn decreases accordingly, and the zero ground voltage also decreases. But due to the actual situation on site, it is not easy to achieve. It is necessary to fully consider in the initial design stage of the computer room, otherwise it is difficult to make changes. The cross-sectional area of the neutral and ground wires should be ensured to be at least 1.5 times that of the phase wire.
② For non isolated dual conversion pure online UPS, the zero line current at the input of the UPS during inverter operation should theoretically be 0V. However, the output neutral line current of the UPS is determined by whether the three phases are balanced, so it is advisable to evenly distribute the load among the three phases as much as possible to reduce the zero ground voltage.
③ Measure whether the grounding resistance meets national standards. For example, the grounding resistance of the grounding body should not exceed 4 Ω, and the wiring resistance of the joint grounding body should be less than 1 Ω. Check if the connection at the ground wire joint is loose or rusted, which increases the contact resistance.
④ Install an isolation transformer at the UPS load end and ground the isolated neutral wire, i.e. PPC or PDU precision distribution cabinet. The advantage of this solution is that it can effectively solve the problem of zero ground voltage at the load end. Because the isolated neutral wire is grounded, it can ensure that the zero ground voltage of the load approaches zero. Isolation transformer is a very mature product with a wide range of varieties that can meet the requirements of various power levels. It has a short supply cycle, low price, and is safe, reliable, and risk-free. At present, computer room users mostly use this power distribution method. However, it should be noted not to connect the load neutral line to the grid neutral line. At the same time, it is necessary to ensure that the grounding of the grounding bar meets the grounding specifications of the computer room and ensures reliability. The unreliability of the grounding bar may cause equipment damage or even personal injury.
⑤ It is a misconception that non completely isolated power frequency UPS can effectively solve zero ground voltage. Many users believe that there is an isolation transformer for the power frequency output, so reducing the zero ground voltage or directly reconnecting the output N line to the protective ground is actually a misunderstanding. Because the rectifier of the power frequency machine adopts SCR rectification, the DC bus voltage is mostly 400V, and the inverter AC voltage is around 160V, which needs to be stepped up to 220V by a transformer before use. Therefore, this transformer is essentially used for inverter voltage, and the output transformer of the power frequency machine is only installed in the inverter circuit, without an isolation transformer in the bypass. Therefore, the output N line of the power frequency machine is not isolated from the input N line, and the output N line cannot be directly short circuited to ground to reduce the zero ground voltage to zero. To completely solve the zero ground voltage problem, it is necessary to add an isolation transformer on the bypass, or follow method 4 to add an isolation transformer on the total output to achieve complete isolation of the primary and secondary N-wires.
The above methods 1-3 have a certain effect on reducing the zero ground voltage, but they cannot completely and effectively solve the problem of zero ground voltage.