Four ways to defeat the three zombies from UPS

If "high energy consumption", "high cost" and "carbon emissions" are compared to the three "zombies" of green data centers, then what are the "plants" that can defeat these terrifying "zombies"? The answer is obvious, of course, it is the green energy-saving means of the data center. The connotation of green data center is very rich. It refers to the IT system, machinery, lighting and electrical equipment in the data room that can achieve maximum energy efficiency and minimize environmental impact.

So for users, what technologies and products can realize the construction of green data centers? What are the main obstacles in building green data centers? Which green energy-saving technologies and products will users pay for? We will sort out the development status of green energy-saving technologies in various fields for you, and recommend products and technologies that can achieve green energy saving.

This article introduces green energy-saving technologies and methods related to UPS (Uninterrupted Power Supply). The purpose is to help enterprise users achieve maximum energy efficiency and minimize environmental impact, defeat the three major "zombies" in data centers, and build a green and energy-saving data center.

Optimize UPS load efficiency curve to reduce energy loss

Generally, the electricity we use is AC, which needs to be converted into DC to supply the internal components of the server. At present, UPSs are all online double-conversion structures, and the rectifier and inverter have power losses when they are working. Taking a 400KVA UPS as an example, each kilowatt-hour is calculated at 0.95 yuan. For every 1% increase in UPS efficiency, the annual electricity bill saved is (400KVA×0.8)×0.01×24×365×0.95=26630.4 yuan. Therefore, how to improve the working energy efficiency of UPS can save a lot of electricity bills for a data center. At the same time, improving UPS efficiency is the most direct way to reduce the energy consumption of the entire computer room.

Therefore, when purchasing UPS, you should try to purchase a UPS with higher efficiency. Of course, the high efficiency of UPS is not just high full-load efficiency, but requires a higher efficiency curve, especially in the 1+1 parallel system. According to the system planning, the capacity of each UPS shall not be greater than 50%. If the efficiency is only below 90%, even if the full-load efficiency reaches more than 95%, it is meaningless. Therefore, the UPS must adopt specific measures to optimize the efficiency curve so that the UPS efficiency can reach a higher efficiency at a lower load.

The so-called harmonics are caused by the fact that the power line has a certain impedance, which is equivalent to a passive network composed of resistance, inductance and capacitance. Due to the non-sinusoidal current generated by the non-linear load, the current and voltage in the circuit are distorted. In the UPS industry, we call the models with high input power factor and low input current harmonics as green power supplies.

The hazards of harmonics include: causing additional losses and heating of electrical components (such as capacitors, transformers, motors, etc.); increasing the temperature of electrical components, reducing efficiency, accelerating insulation aging, and reducing service life; interfering with the normal operation of equipment; increasing reactive power factors, and reducing the active capacity of power equipment (such as transformers, cables, and distribution equipment); low power supply efficiency; resonance, especially when the oil generator is generating electricity; circuit breakers tripping, fuses blowing, and equipment damage for no reason. UPS is a nonlinear load for the power grid, and it will generate a large amount of harmonics when it is working. Taking a UPS equipped with a 6-pulse rectifier as an example, its input power factor is generally around 0.75, and the harmonics are greater than 30%.

At present, there are four solutions for large UPS input harmonic current suppression:

Solution 1: Use 6-pulse UPS + active harmonic filter, input current harmonics <5% (rated load), input power factor 0.95. Although this configuration has very good input indicators, the technology is still immature, and there are problems such as miscompensation and overcompensation, which may cause the main input switch to trip or be damaged.

Solution 2: Use 6-pulse UPS + 5th harmonic filter. If the UPS rectifier is a three-phase full-controlled bridge 6-pulse rectifier, the harmonics generated by the rectifier account for nearly 25-33% of all harmonics. After adding the 5th harmonic filter, it is reduced to less than 10%, and the input power factor is 0.9, which can partially reduce the harm of harmonic current to the power grid. In this configuration, the input current harmonics are still too large, and the generator capacity ratio is required to be more than 1:2, and there is a hidden danger of abnormal increase in generator output.

Solution 3: A pseudo 12-pulse solution using a phase-shifting transformer + 6-pulse rectifier is composed of two 6-pulse rectifier UPSs: a standard 6-pulse rectifier and a 30-degree phase-shifting transformer + 6-pulse rectifier. The pseudo 12-pulse rectifier UPS appears to have a full-load input current harmonic of 10%. This configuration has a serious single-point failure. When a UPS fails, the system input harmonic current increases sharply, seriously endangering the safety of the power supply system.

Solution 4: Use 12-pulse UPS + 11th harmonic filter. If the UPS rectifier is a three-phase full-controlled bridge 12-pulse rectifier, the harm to the power grid caused by harmonic current content can be basically eliminated after adding the 11th harmonic filter. The price is much cheaper than that of active filter. Use 12-pulse UPS + 11th harmonic filter, the input current harmonic is 4.5% (rated load), and the input power factor is 0.95. This configuration is the most mature and reliable solution in the UPS industry, and the generator capacity requirement is 1:1.4.

The above technologies, performance and investment comparison are shown in Table 1. You can choose the appropriate solution according to your actual needs:

When purchasing a UPS, we also need to pay attention to the working mode of the UPS. Clever use of some working modes can help us easily achieve the goal of energy saving and consumption reduction.

1. Three-phase high-power UPS energy-saving technology with self-aging mode

Traditional UPS aging process: The input of UPS is connected to the mains, and the output is connected to a resistive load, so that the load rate of UPS is between 95% and 100%, and the mains is disconnected after continuous power supply for 24 hours. The result of this aging process is that a large amount of electrical energy is converted into heat energy that cannot be recovered.

UPS working mode with self-aging mode: no load is required during aging, the output of the UPS is directly connected to the grid, the inverter of the UPS is controlled to enter the current source mode, its output current tracks the grid voltage, the bypass is turned on, and the load rate of the inverter is controlled to be around 90%, thereby using about 10% of the energy consumption, directly saving about 90% of electricity, and achieving the purpose of energy saving and consumption reduction.

The general aging experiment is to connect the UPS directly to the resistive load for aging, which has a great energy consumption problem. Another method is to connect the UPS to an electronic load, which simulates the actual load to absorb electrical energy and then feeds this energy back to the grid. This method greatly reduces the loss of electrical energy, but there are still two loss points in essence, one is the loss of the UPS itself, and the other is the loss of the electronic load. Self-aging does not need to be connected to the electronic load, and there is no loss of the electronic load, which is more energy-saving.

The self-aging mode does not require the connection of resistive loads or electronic loads. After the UPS is powered on, it can be aged by itself through the monitoring settings. The principle of self-aging operation is: when the traditional UPS is running, the mains converts AC power into DC power through rectification PFC, and then converts DC power into AC power with stable voltage and frequency through the inverter voltage source for power supply. After the system is set to self-aging, the rectification PFC module part still performs AC power supply to the DC bus, while the inverter part is different. The inverter changes from the voltage source operation in the general mode to the current source operation, and is fed back to the power grid through the bypass according to a certain current size. The electric energy is rectified by the main line and then fed back to the power grid through the inverter current source to complete the aging of the rectification and inverter.

The new aging system sends 90% of the electricity back to the grid. Assuming that the power of UPS produced each year is 57600KW, the electricity saved is 57600*90%=51840Kw/year. According to the average UPS aging time of 8 hours, the electricity saved is: 51840*8=414720Kwh. The electricity saved is equivalent to about 414720Kwh/year*334g/Kwh=1242 tons/year of standard coal.

With the commissioning of UPS with self-aging mode, the traditional power aging method has been changed, and the aging process has entered the power production industry in an efficient and energy-saving way. The significant economic benefits have changed the concept that power aging is purely "burning electricity", making the power aging process a clean, environmentally friendly, and low-consumption new process. This project has a significant energy-saving effect. The biggest feature is that it saves a lot of electricity. Compared with the traditional power aging method, it can save 90% of electricity. It is in line with the basic national policy of energy conservation, and it also helps to alleviate the shortage of electricity in the region. It has a good application and promotion value.

2. Use ECO economic operation mode

The principle of ECO economic operation mode is to activate this function when the mains power environment is good, so that the UPS is directly powered by the static bypass. At this time, the inverter is in standby mode and works normally, but does not output energy. Once the mains power is abnormal, the UPS immediately switches to the inverter power supply state. The switching time is generally within 1ms. See Figure 3 for details. The blue is the input current waveform and the yellow is the output voltage waveform. Since the inverter is in standby mode at this time, its own loss is very small. At this time, the overall efficiency of the UPS can reach more than 97%, saving more than 3% power compared to the normal mode.

The following conditions must be met to use the ECO mode:

a) Static bypass must use two sets of high-reliability SCR transistors, and must not use a combination of contactor and SCR transistor, because when the contactor is closed, the contact point will spark, and generally it will not work normally after working hundreds of times. SCR transistors do not have this problem and can shorten the switching time.

b) It is recommended to use it in a better power environment, such as a first-level power supply unit.

Transformerless UPS equipment

The technological progress of power electronic equipment is closely related to the performance improvement of power devices and the continuous emergence of new devices. Over the past 50 years, with the advancement of power semiconductor devices, UPS equipment has evolved from multi-output power frequency transformers to single output power frequency transformers. The emergence of high-power IGBT devices with better performance and more advanced control technology has created material conditions for UPS equipment to fundamentally remove the output isolation transformer, making it have made great progress in high frequency, miniaturization, energy saving and green environmental protection. This is what people call "high-frequency machine". This model concentrates on the progress of UPS circuit technology and represents the development direction of UPS technology. Compared with traditional UPS with output transformer, it has made significant improvements and progress in further reducing size, reducing weight, improving performance, improving efficiency, and reducing costs.

It should be said that the use of output transformer is determined by the output circuit form of UPS inverter, but the existence of transformer is more disadvantageous than beneficial. Take a 400-watt power supply as an example. If the power supply claims that the conversion efficiency is 70%, that is to say, if 400 watts of AC power is input into the power supply, only 280 watts of power will be converted into DC power for server application, which is a waste of 120 watts of power.

The efficiency of traditional UPS is only 75-85%, but with transformerless models, it can be increased to over 88%. Therefore, choosing a transformerless UPS can make more efficient use of power, so that every kilowatt-hour of electricity is spent on system operation, thereby reducing electricity costs.

Currently, more and more manufacturers are launching transformer-free UPS devices, which can increase the efficiency of the whole machine to 90%.

Conclusion: To maximize UPS green energy saving and build a green energy-saving data center, in addition to paying attention to some of the UPS energy-saving technologies mentioned in this article, it is also necessary to strengthen the management of the data center life cycle, and reasonably plan and configure the entire data center life cycle, and effectively manage the power supply. Only in this way can we ultimately save direct and indirect battery investment and reduce the pollution of the entire computer room to the social environment.