Method of over-voltage protection for small-capacity UPS uninterruptible power supply

by:Power Kingdom     2021-06-23
Tags: UPS uninterruptible power supply overvoltage protection has two meanings: on the one hand, various surges or voltage spikes from the outside have a certain impact on the UPS uninterruptible power supply and need to be protected; on the other hand, these surges or Voltage spikes may affect the load through the UPS uninterruptible power supply, and protection is also required if necessary. The data center or control center equipped with large UPS uninterruptible power supply generally has a relatively complete overall lightning protection system in its building or computer room, and the residual value of the overvoltage reaching the UPS uninterruptible power supply terminal is not high; while for the small UPS uninterruptible power supply The operating environment is relatively poor. In addition to lightning protection, it is also necessary to consider surge protection against operating overvoltages on the surrounding power grid.   The effect and cost of overvoltage protection measures have an important relationship with the choice of devices and solutions. Choosing an SPD device with a lower operating voltage and larger current capacity can reduce its residual voltage, but too low an operating voltage will cause SPD devices to fail prematurely due to the instability of the power supply, and a larger current capacity will cause excessive protection costs. .   Under normal circumstances, small-capacity UPS uninterruptible power supplies are mainly not considered for lightning protection, but for protection of power supply operating over-voltage.  In the early design, due to cost considerations, small UPS uninterruptible power supplies are similar to other ordinary power products. Generally, 14D471 zinc oxide varistors (MOV) are used for overvoltage protection on 200Vac input EMI. General 14D471 varistor products, its current capacity is about 6kA (8/20μs, once), which is no problem in areas where the power grid is stable, but in areas where the power grid is unstable, it is easier to use 14D471 varistors Damaged, this is due to the operating over-voltage surge compared with the lightning surge, although the amplitude is lower, but the duration is longer, and it is periodic. For the varistor with a small current capacity, it can absorb the wave. The surge of heat continuously accumulates and is too late to be dissipated, which is very easy to damage.   One solution is to increase the current capacity of the MOV. For example, select MOV devices of 20D471, 25D471 or even 32D471 to increase the current capacity to 10kA to 25kA (8/20μs, once). In this way, it can not only withstand a long time or periodic over-voltage energy discharge, but also keep the residual voltage on the line at a low level. However, this will greatly increase the cost of protection (a tens of times increase).   Another solution is to increase the operating voltage of the MOV. For example, use MOV devices such as 14D561 or 14D621 to increase the operating voltage from 470V to 560V or 620V. In this way, without changing the flow capacity, the MOV's action probability and energy-reduction time are greatly reduced without increasing the cost. However, this will increase the residual pressure on the line.   Gas Discharge Tube (GDT) is a new type of SPD device suitable for use, because its price is also relatively cheap. Compared with MOV, GDT has the following important characteristics:    (1) Compared with MOV, GDT has better repeated discharge characteristics and is not easily damaged.   (2) MOV is a clamp type component, while GDT is a short-circuit type component. Once the GDT is in a low-impedance state similar to a short-circuit, its short-circuit action may last about half a cycle (10ms), and it will not be interrupted until the zero-crossing point. Therefore, gas discharge tubes generally need to be used in conjunction with short-circuit protection devices (such as fuses or circuit breakers, etc.).   (3) The operating voltage accuracy of GDT is lower than MOV. Generally, the operating voltage accuracy of MOV is ±10%, while the operating voltage accuracy of GDT is ±20%.   For outdoor UPS uninterruptible power supplies, due to lightning surges and frequent operating overvoltages, considering the inconvenient recovery of short-circuit protection devices, it is generally not appropriate to directly use gas discharge tubes as overvoltage protection devices.  Because MOV and GDT have different performance characteristics, there is a big difference between what they should be. The ideal overvoltage protection device requires low leakage current, fast action response, low residual voltage, low aging, etc. However, the existing single device cannot fully meet the requirements.   Under the impact of a surge, the residual voltage of MOV and GDT devices is different.   In order to combine the characteristics of the two devices, the two devices can be used in combination to give full play to their respective strengths. If two devices are used in series, the leakage current of MOV is larger than that of GDT, but GDT does not have this problem; but GDT has the problem of following current. After being used in series with MOV, MOV has a certain current limit. Function, and can interrupt the following current in time.  In practical applications, the discharge tube is connected in parallel with a capacitor. When a surge occurs, the initial charging state of the capacitor is equivalent to a short circuit, so that the MOV is turned on first, and the capacitor is also used as the energy storage element of the GDT; when the capacitor is charged, the GDT is turned on and forms a discharge circuit for the capacitor.  In order to reduce the residual voltage amplitude on the load side, it is also necessary to add a level of SPD to the output side of the UPS uninterruptible power supply at the same time, thus forming a two-level SPD protection network. SPD1 is the first-level over-voltage protection device, which has a higher residual voltage when surge invades, while SPD2 is used as the second-level over-voltage protection, and its residual voltage is lower.
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