Power overvoltage protection scheme for small capacity UPS

In the fields of construction, communication, electricity, etc., overvoltage protection has become an indispensable part, but the overvoltage protection technology and application of UPS (uninterruptible power supply) as a power supply system are still not properly understood, and are even ignored. This article combines the actual situation and proposes appropriate solutions for the overvoltage protection needs in UPS applications.

1. Changes in the concept of overvoltage protection

When lightning strikes in the distance, the lightning surge is transmitted to the equipment through the power grid or communication lines. Although it may not immediately damage the equipment, it will cause cumulative damage to the equipment. In addition, with the rapid development of the economy, the possibility of equipment being disturbed by other surges on the line (such as the operational overvoltage phenomenon caused by the start-up of various power equipment to the power grid) is also very high, which may have a greater impact on the equipment.

Therefore, it is obviously incorrect to simply and intuitively conclude that "there is no need for overvoltage protection if there is no lightning". It can be said that the current overvoltage protection work has shifted from traditional lightning protection to comprehensive protection against direct lightning, lightning electromagnetic pulses, ground potential counterattacks and operational overvoltage.

2. Misunderstandings about Lightning Protection in UPS Applications

2.1 Myth 1: “Lightning arrester” is only for lightning protection

In the actual application of UPS, this situation often occurs: the sky is clear and no lightning can be felt, but the built-in "lightning arrester" of UPS is damaged. The user says that there is a problem with the quality of the UPS machine, but the UPS itself can still continue to work normally.

If there is no heavy power equipment nearby, it may not be easy to convince users with "operation overvoltage". In fact, there are many statistics and reports on various voltage surges on such ordinary low-voltage distribution lines abroad. For example, a statistic from the United States shows that within 10,000 hours, the number of surges of various voltage values ​​between lines exceeded the original working voltage by more than 800 times, of which more than 300 times exceeded 1000V.

It is conceivable that it is entirely possible for a "lightning arrester" to operate or be damaged without the action of lightning.

2.1 Misconception 2: Cheap “Lightning Arrester” also protects against lightning

Many users, considering relevant regulations, require UPS to be equipped with "lightning arresters" at a lower price. Some manufacturers, in order to "meet" user requirements, simply install a small varistor and call it "lightning protection". In fact, a varistor with a small current capacity can only have a certain overvoltage protection effect. If lightning protection is really needed, sufficient current capacity devices and related costs must be considered.

3. UPS overvoltage protection requirements

As a power supply system, UPS must have line connections from multiple aspects, including AC input of mains power, AC output of UPS, communication interface, etc. Strictly speaking, overvoltage protection should be set for these three ports. This article mainly discusses the operation overvoltage protection of AC ports. UPS overvoltage protection has two meanings: on the one hand, various surges or voltage spikes from the outside have a certain impact on UPS, and protection is required; on the other hand, these surges or voltage spikes may affect the load through UPS, and protection is also required when necessary.

4. Power overvoltage protection features of small capacity UPS

Data centers or control centers equipped with large UPSs usually have buildings or computer rooms that are equipped with relatively complete lightning protection systems, so the residual value of overvoltage reaching the UPS end is not high. However, the operating environment of small UPSs is relatively poor. In addition to lightning protection, surge protection against operating overvoltages on the surrounding power grid must also be considered.

On the other hand, large UPS has more cost margin and protection schemes are easy to implement; while small UPS is cost-constrained and the protection measures and devices that can be used are limited.

5. Power overvoltage protection scheme for small capacity UPS

The effectiveness and cost of overvoltage protection measures are closely related to the selection of devices and solutions. Selecting SPD devices with lower operating voltage and larger current capacity can reduce the residual voltage, but if the operating voltage is too low, the SPD device will frequently operate and fail prematurely due to unstable power supply, and if the current capacity is too large, the protection cost will be too high. Usually, small-capacity UPS is not mainly concerned with lightning protection, but protection against power supply operation overvoltage.

5.1 Early solutions

In early designs, due to cost considerations, small UPSs were similar to other common power products, and generally used 14D471 zinc oxide varistor (MOV) for overvoltage protection on the 220Vac input EMI.

The general 14D471 varistor product has a current capacity of approximately 6kA (8/20μs, once), which is not a problem in areas with stable power grids. However, in areas with unstable power grids, 14D471 varistors are more easily damaged. This is because compared with lightning surges, the operating overvoltage surge has a lower amplitude but a longer duration and is periodic. For varistors with smaller current capacity, the heat absorbed by the surge accumulates continuously and cannot be dissipated in time, so they are very easy to be damaged.

5.2 Solution Improvement

One solution is to increase the current capacity of the MOV, for example, choose 20D471, 25D471 or even 32D471 MOV devices to increase the current capacity to about 10kA to 25KA (8/20μs, once). In this way, it can withstand long-term or periodic overvoltage energy discharge and keep the residual voltage on the line at a low level. However, this will greatly increase the protection cost (tens of times increase).

Another solution is to increase the action voltage of MOV, for example, select MOV devices such as 14D561 or 14D621 to increase the action voltage from 470V to 560V or 620V. In this way, without changing the current capacity, the action probability and energy discharge time of MOV are greatly reduced without increasing the cost. However, this will increase the residual voltage on the line.

Gas discharge tube (GDT) is a new type of SPD device suitable for use because of its relatively low price. Compared with MOV, GDT has the following important features:

A). GDT has better repetitive discharge characteristics than MOV and is less prone to damage.

B). MOV is a clamping type component, while GDT is a short-circuit type component. Once the GDT is activated, it will be in a low-resistance state close to a short circuit. Its short-circuit action may last for about half a cycle (10ms) until it passes through the zero point. Therefore, gas discharge tubes generally need to be used in conjunction with short-circuit protection devices (such as fuses or circuit breakers, etc.).

C). The operating voltage accuracy of GDT is lower than that of MOV. Usually, the operating voltage accuracy of MOV is ±10%, while the operating voltage accuracy of GDT is ±20%.

For outdoor UPS, due to frequent lightning surges and operating overvoltages, and considering that the recovery of short-circuit protection devices is not convenient, it is generally not appropriate to directly use gas discharge tubes as overvoltage protection devices.

5.3 Combination Solution

Since MOV and GDT have different performance characteristics, their applications are also quite different. An ideal overvoltage protection device requires low leakage current, fast action response, low residual voltage, and not easy to age, but the existing single device cannot fully meet the requirements.

In order to combine the characteristics of the two devices, the two devices can be used in combination to give full play to the strengths of each device.

When the two devices are used in series, the leakage current of MOV is larger than that of GDT, while GDT does not have this problem; however, GDT has the problem of following current. When used in series with MOV, MOV has a certain current limiting effect on it and can interrupt the following current in time.

In practical applications, it can be improved by connecting capacitors at both ends of the discharge tube. When a surge occurs, the initial charging state of the capacitor is equivalent to a short circuit, causing the MOV to conduct first, and the capacitor also serves as the energy storage element of the GDT; after the capacitor is charged, the GDT conducts and forms a discharge circuit for the capacitor.

In order to reduce the residual voltage amplitude at the load end, it is also necessary to add a level of SPD at the output end of the UPS, thus forming a two-level SPD protection network. SPD1 is the first-level overvoltage protection device, and has a higher residual voltage when a surge invades, while SPD2 is the second-level overvoltage protection device, and has a lower residual voltage.

6. Conclusion

The failure of overvoltage protection devices is also a UPS failure, which will also bring great inconvenience to the use and maintenance of the UPS. Under low-cost conditions, choosing and designing appropriate overvoltage protection measures has become an important part of modern UPS applications.