Industrial UPS & High Frequency UPS & Modular UPS , Who is the best ?

At present, UPS products have been applied in the industry for more than 50 years, and they have made outstanding contributions to ensuring the uninterrupted operation of key equipment and businesses. With the continuous advancement of information technology construction, more and more scenarios need UPS protection, and its role becomes more and more important. There are three types of products in the current market: industrial frequency machine, high frequency tower machine, and high frequency modular UPS. This article aims to help users identify the development trend of UPS products by explaining the development history of UPS and comparing the advantages and disadvantages of various types of UPS.

The development from Industrial Power Frequency UPS to high frequency tower UPS

Industrial Power frequency UPS structure UPS technology appeared in the 1970s, and was named because its rectifier operating frequency was consistent with the grid frequency. Subject to the development of semiconductor technology at the time, the withstand voltage of the IGBT device in the inverter could only be 600V, so the bus voltage was limited, and the inverter output voltage could not be 380V; and the inverter of the commercial frequency machine was a full-bridge circuit. Because it is a three-phase hot wire, it cannot meet the requirements of single-phase IT load and three-phase four-wire load, and Δ-Y conversion must be performed. In order to solve these problems, the manufacturer added a transformer to the inverter output of the commercial frequency machine to boost and generate the neutral to make the output voltage meet the requirements of the load. This is the real purpose of the built-in transformer of the commercial frequency machine. Figure-1 shows a typical topology of a power frequency machine.

By the 1990s, the third-generation trench IGBT was launched, and its withstand voltage was increased to 1200V, which promoted the innovation of UPS technology. The high-frequency step-up circuit on the rectifier side can increase the bus voltage to about 700V, the inverter output voltage can reach 380V, and the output transformer can be eliminated. And this kind of rectifier inverter circuit works at high frequency (more than a few kHz) and UPS without output transformer is called high frequency UPS. Figure-2 shows a typical high-frequency machine topology.

Comparison between high frequency UPS and industrial frequency UPS

1. Industrial Power frequency UPS has low input power factor and high harmonics

Industrial Power frequency UPS adopts thyristor semi-controlled rectification. The input power factor of 6-pulse rectifier UPS is less than 0.7 and the harmonic is as high as 30%. The input power factor of 12-pulse rectifier UPS is only 0.8 and the harmonic is as high as 15%. Harmonic treatment measures, the highest power factor can only be improved to 0.95. In contrast, the high-frequency machine uses IGBT-PFC full-control rectification, the input power factor industry can achieve 0.99, and the harmonic current is less than 3%. Severe harmonic pollution may not only interfere with the inability of other equipment to malfunction and cause control and protection devices to malfunction, but also directly lead to a significant increase in investment: customers need to purchase additional harmonic processing equipment to reduce harmonics; if the front-end is connected to a diesel generator for backup The generator capacity must be configured to be 2-3 times the UPS capacity. At the same time, the pre-stage power distribution devices, cables, etc. need to be increased by about 20%, while the high-frequency machine only needs to configure the front-end generator capacity to 1.2- 1.5 times is enough, the power distribution capacity and UPS capacity remain the same or slightly higher.

2.Industrial Power frequency UPS has large power consumption

There are three factors that cause Industrial Power frequency UPSs to be less efficient than high frequency UPSs. First, the Industrial Power frequency UPS rectification is a step-down topology, and the device has a large working current. Both the linear loss and the square loss of the internal circuit are higher than the high frequency machine. Second, the Industrial Power frequency UPS is higher than the high frequency machine due to the need for boosting the output. One built-in output transformer reduces the efficiency of theIndustrial Power frequency UPS by about 2% -3%. Third, in practical applications, in order to increase the input power factor to above 0.95 and reduce the harmonic pollution injected into the power grid, the Industrial Power frequency UPS must also An external 5th or 11th harmonic filter will reduce the efficiency by 2% -3% again. According to the statistics of the current network operation of a British operator and a Spanish operator, the efficiency of the Industrial Power frequency UPS is generally about 85%, compared with the high frequency operation efficiency of about 92% and the modular operation efficiency of about 96%, resulting in a large number of Energy loss. Taking a 400kW load as an example, the Industrial Power frequency UPS will consume 410,000 kWh more power per year than the high frequency UPS and nearly 580,000 kWh more power than the modular UPS per year. In addition, the Industrial Power frequency UPS has problems such as high harmonics, low power factor, etc. leading to increased losses in distribution cables.

3. Industrial Power frequency UPS is large and heavy

Because the power frequency machine uses low-frequency components and is equipped with an output transformer, the volume and weight of the UPS has greatly increased. Compared with a brand 400kVA industrial frequency machine and a high frequency machine, the weight of the industrial frequency machine is 2.2 times that of the high frequency machine, and the volume is 1.5 times that of the high frequency machine. In actual transportation, there may be a small room door or aisle, and the elevator load. Insufficient, floor load-bearing and other problems, in some cases even need to use cranes to load and unload, and then break through the wall to install power frequency UPS, greatly increasing the transportation time and cost.

4. Industrial Power frequency UPS has no advantage in reliability compared to high frequency machine

The main difference between the Industrial Power frequency UPS and the high frequency machine is reflected in the rectifier and the transformer. Industrial Power frequency UPS rectifiers use SCR devices, with low voltage stress and high current stress. High frequency UPS mainly use IGBT devices, with low current stress and high voltage stress. SCR and IGBT are currently mature devices. As long as they are properly applied, there will be no difference in reliability. In fact, the inverter part of the Industrial Power frequency UPS also uses IGBT, which does not reduce the reliability of the Industrial Power frequency UPS, and there is no evidence that the inverter is a weak link of the Industrial Power frequency UPS. From the perspective of topology, industrial frequency machines use phase-controlled rectification + full-bridge inverter, and high-frequency machines generally use high-frequency rectification + half-bridge inverter. These topologies are all very common topologies in power electronics technology, there is no question of who is more reliable in principle, and its reliability depends on the level of design.

For transformers, the industry often hears many of its so-called advantages, such as strong anti-shock capability and reduced zero-ground voltage, but is this really the case?

First, strong overload capacity and strong load shock resistance. Overload capacity is one of the key indicators required in IEC62040-3. Its strength can be measured by actual data. Table-1 shows the overload capacity of industrial frequency machines and high frequency machines of the same manufacturer. From Table-1, it can be seen that there is no difference in overload capacity between the two types of models.

The output transformer does not enhance the anti-shock capability of the industrial frequency machine. The imagination that the transformer can enhance the anti-shock capability comes from the inductance characteristic of the transformer. The ability of the inductor to smooth the current can smooth the current waveform and delay the current shock when the load current surges. But in fact the ability of the inductor to smooth the current is directly proportional to its own inductance. The power transformer output transformer has a small transformation ratio, and the excitation inductance of the transformer output winding will not be too large. It is very easy to saturate under the impact of large currents, and it is difficult to have a significant buffer effect on the impact of the inverter. According to the characteristics of traditional transformers transmitting energy and the principle analysis of magnetic devices, when energy shock occurs at the back-end load, that is, the output side of the transformer, the peak excitation current at the back end will be directly reflected to the front end to the UPS before the energy transmission capacity of the transformer reaches the upper limit of saturation. The IGBT produces a shock, and the front end will receive a larger inrush current than the output due to the transformer's ratio problem, and the damage will be more serious. Moreover, due to the hysteresis characteristics of the transformer in the power frequency system, it is difficult to monitor the dynamic response of the subsequent stage in real time. When a sudden change occurs in the back end of the transformer and it is fed back to the previous stage, the system will take tens or even hundreds of milliseconds to delay the relevant actions compared to the high-frequency machine without the transformer. At this time, the inrush current flowing through the IGBT is enough to damage the UPS and even cause it. Fire.

Secondly, it cuts off the dangerous DC voltage during the shoot-through fault of the IGBT tube of the inverter. PIndustrial Power frequency UPS'transformers can indeed prevent DC from passing to the secondary side, but high frequency UPS can avoid the danger of DC dangerous voltage to the load through fast detection and protection measures. When a high-frequency inverter inverts a certain IGBT and has a shoot-through fault, the UPS controller can immediately detect the abnormal output current, and quickly cut off the path from the dangerous DC voltage to the output port through measures such as rectifier unit shutdown and output port fuse protection. During the protection process, the voltage output to the load port is approximately 400V DC for several ms. For IT loads powered by switching power supplies, the input allowable voltage can reach 276Vac, and the voltage after rectification is also about 400Vdc. The device selection is based on the bus voltage selection. At this time, the 400Vdc of the input port will not exceed the device's tolerance, and it is impossible to cause damage to the device. For a commercial frequency machine, loading a DC voltage on its primary side will cause a sharp increase in current and a rapid rise in temperature, which may cause more serious failures such as fire.

Third, the zero-to-ground voltage can be reduced. Many servers and other equipment have zero-ground voltage requirements, although the reason for this design has not been verified, in theory, the size of the zero-ground voltage will not affect the normal operation of IT equipment. In a data center, IT equipment is only allowed to use the TN-S or TN-CS power supply system, so the zero-ground voltage of the input port of the IT equipment is mainly from the neutral line ground point (TN-S system) or the neutral and ground separation point ( TN-CS system) The neutral impedance and neutral current of the IT input port are determined by the third harmonic current in the system. In the same system, both the power frequency machine and the high frequency machine will not affect the zero line impedance, and the zero line current and the third harmonic current are mainly related to the three-phase load configuration and load characteristics, that is, the type of UPS will not It has no obvious effect on the zero-ground voltage. What really determines the zero-ground voltage is the design of the power distribution system. If you need to improve the zero-ground voltage, it is best to start with the power distribution system and start to reduce the line impedance and neutral current. The most effective way to reduce the line impedance is to install an isolation transformer in the headboard of the load. It should be noted that in the application, there is a method of directly grounding the secondary side of the power frequency transformer, which is an unstandard method. The N line of the power frequency transformer is not isolated. For the TN-S system and the TN-C-S system where N and PE have been separated, re-grounding the N line will also cause current to flow through the PE line, which may interfere with the normal operation of the equipment. Neither the national standard nor the IEC standard allows such irregular practices.

And fourth, the transformer of the Industrial Power frequency UPS can play an isolation role, which can ensure personal safety. In order to ensure the smooth switching between the main and the side, the output N line of the Industrial Power frequency UPS is introduced by the bypass, that is, the transformer of the Industrial Power frequency UPS cannot play the role of electrical isolation and cannot be re-grounded. In scenarios where isolation is required, even if a power-frequency UPS is used, a bypass must be added to the transformer to isolate the N line to achieve true isolation.

In fact, the design of the transformer instead increases the risk of circulating current. Figure-3 shows the circulation path of the two types of models. The parallel connection of the Industrial Power frequency UPS is the direct parallel connection of the transformer. There are no device restrictions on the entire circuit, and the voltage deviation is easy to generate circulating current. The high-frequency machine has multiple diodes in the circulating current path, and the voltage difference of less than 2V does not form a circulating current at all.

5. Industrial Power frequency UPS increases user investment

Since the Industrial Power frequency UPS rectifier works at the mains frequency, a larger inductor energy storage is required. Its larger volume inductor and indispensable transformer are made of copper and magnetic materials, and the cost is difficult to drop. The price is generally more than 30% higher than that of high-frequency machines.

In summary, in terms of performance, reliability, and price, high frequency UPS have advantages over Industrial Power frequency UPS. Judging from the series of major manufacturers, the major manufacturers in the industry have not launched new Industrial Power frequency UPS models, and some manufacturers have fully turned to the development and sales of high-frequency UPS. The replacement of commercial Industrial Power frequency UPS with high frequency machines is a general trend.

Development from high-frequency tower UPS to modular UPS

Modular UPS has been around since the 1990s, but has been silent for a long time due to technical capabilities. Since 2000, due to the development of DSP, digital control and other technologies, the problem of parallel current sharing control of multi-power modules has been gradually solved, and modular UPS technology has begun to flourish. From 2009 to 2010, China Telecom carried out in-depth testing of modular UPS. According to feedback from actual users in various places, China Telecom believes that the mainstream modular UPS in the industry has met the requirements of the telecommunications industry, and began centralized procurement of modular UPS in late 2011 . China Mobile's modular UPS also uses a separate bidding section for centralized mining.

Comparison of modular UPS and high frequency tower UPS

1.High availability of modular UPS system

As a very important part of the current information system, a basic requirement of the power supply and distribution system is that the system must be able to work continuously. To achieve the goal of continuous work, first of all, the system should have high reliability, and secondly, the system must be able to be repaired quickly. If it cannot be repaired quickly, it may face the risk of paralysis of the entire system due to secondary failure, and the customer's load cannot guarantee continuous work.

In terms of rapid repair, modular UPSs have inherent advantages. First of all, in terms of repair time, due to the fast plugging feature, the modular UPS can be replaced on site. The average repair time is within half an hour. Compared with the typical repair time of a traditional tower crane, which is 24 hours, the repair speed is obvious. Promotion. Secondly, in terms of repair quality, the repair form of the modular UPS is to replace the faulty module, while the traditional tower machine requires the original factory to send a professional engineer to the site to locate the fault, and then disassemble the machine to repair the faulty circuit and board. In addition, there is a communication and positioning process, which easily causes repeated work and affects the efficiency of troubleshooting.

Some users may question that the N + 1 architecture of the modular UPS is not as stable as the 1 + 1 parallel system. Indeed, in theory, the reliability of 1 + 1 in N + 1 parallel systems is definitely the highest. But it is often not so simple in actual scenarios:

First of all, this conclusion ignores the load rate situation. As a 1 + 1 parallel system, at most one UPS can be damaged. For the modular UPS system, taking 4 + 1 as an example, the reliability is 100% load. It is lower than 1 + 1, but at 75% load rate, the modular system actually becomes 3 + 2, and at 50% it becomes 2 + 3, and the reliability is much greater than 1 + 1 parallel. In common application scenarios, the UPS load rate is around 20-40%. In this case, the advantages of modularity have very obvious advantages.

Secondly, unlike the traditional single machine, the modular UPS can easily implement the redundancy mode of N + 2, N + 3, which can be achieved by adding only 1-2 modules, and the tower machine has to do more than this With the addition of a mainframe, machine transportation, site installation, wiring design, and corresponding power distribution and batteries need to be changed, resulting in a significant increase in investment.

In summary, the reliability of UPS modularity in practical scenarios is much higher than that of traditional tower-type parallel machines; coupled with the rapid maintenance and expansion of UPS, the availability of modular UPS is much higher than that of traditional tower-type machines.

2. Modular UPS has better scalability

Tower crane expansion requires the purchase of a whole new machine, installation of the machine in place, and bypassing other UPSs in the system to connect the new machine to the system. The entire step is not only high investment and long installation time, but also when the new machine is incorporated Because the entire system is in a bypass state, there is a risk that the load will be powered down if the utility power is interrupted.

However, as long as the power distribution system is planned in the early stage of modularization, it is possible to increase the load by adding modules, and to ensure uninterrupted power supply to the original load during the expansion process.

3. Modular UPS is easy to transport and install

The tower UPS needs to be installed and transported as a whole, and large single machines will be more difficult. For example, a UPS with a capacity of 400kVA generally weighs about 1500kg and has a volume of more than 3m3. Tower UPSs will be difficult to transport due to insufficient transportation channels and high weight. On the one hand, modular UPSs can separate modules and racks, and most of them Model racks can be transported separately, and the problems that tower UPSs may encounter will be solved.

4. Modular UPS has high practical operation efficiency

At present, both the high frequency tower UPS and the modular UPS can achieve a maximum efficiency value of 96%, but this can only be achieved when the load rate is above 50%. As mentioned earlier, due to system redundancy and advanced planning, the UPS load rate is about 20-40% under common operating conditions. The high-frequency tower machine can only achieve 94 ~ 95% efficiency under this working condition, and the mainstream modular UPS generally has the "module sleep" feature. On the basis of ensuring certain system redundancy, it can sleep a certain number of modules ( You can set it manually or automatically) to make the UPS system work in a more efficient area, that is, to maintain it near the highest efficiency point of 96%. Figure-4 shows the principle of hibernation to improve the load rate and operating efficiency.

In addition, some manufacturers consider that the module aging time may be different, and have further developed the "rotation sleep function": that is, the hibernation modules are rotated every set period to average the aging time of each module to improve the overall UPS system life. . Figure-5 shows the typical process of rotating hibernation.

V. concluding remarks

Since its birth, modular UPS has been designed to meet users' needs for the availability, reliability, maintainability, and energy saving of power supply systems. After long-term operation verification, the modular UPS does have a great advantage over traditional UPS systems in these aspects. With the continuous increase of energy costs and the further improvement of users' requirements for the flexibility and availability of power supply systems, modular UPS will be more widely used.