Silicon Carbide-Based Uninterruptible Power Supplies For Data Centers
A technological leap in efficiency and reliability.
When it comes to energy requirements for today’s data centers, we find that there are ever increasing needs to be able to operate at higher efficiencies, maximize floor space, and reduce operating costs across the facility. These goals need to be achieved while still providing clean, reliable power to data center critical loads. It is our objective to show that through the use of emerging technologies in UPS double-conversion systems, these goals are becoming easier to achieve.
As you would expect, the data center challenges described above parallel the challenge in semiconductor manufacturing to maximize efficiency, reduce size, increase power quality, and reduce costs. Over the course of the last 25 years, researchers have learned that the use of wide bandgap (WBG) materials, such as silicon carbide (SiC), allows semiconductor components to be smaller, faster, more reliable, and more efficient than the existing silicon (Si) technology.1
WBG semiconductors are semiconductor materials, like SiC, that permit devices to operate at much higher voltages, frequencies, and temperature than conventional semiconductor materials. This allows more powerful electrical mechanisms to be built that are cheaper and more energy efficient. Bandgaps determine how a material’s electrons behave.
In solid-state-physics a bandgap, also called an energy gap, is an energy range in a solid where no electron states can exist. The bandgap generally refers to the energy difference (in electron volts) between the top of the valence band and the bottom of the conduction band in insulators and semiconductors (Table 1).
The term "wide bandgap" refers to higher-energy electronic band gaps, usually significantly larger than one electronvolt (eV). "Wide" bandgap typically refers to material with a bandgap of at least two or three eV, significantly greater than that of the commonly used semiconductor silicon (1.1 eV). The result is SiC, which has a specific forward conduction resistance 400 times lower than Si, thus reducing power losses.2 WBG semiconductors are considered by the U.S. Department of Energy to be a foundational technology that will transform multiple markets and industries, resulting in billions of dollars of savings for businesses and consumers when use becomes widespread.
WBG semiconductors permit devices to operate at much higher temperatures, voltages, and frequencies, making the power electronic modules using these materials significantly more powerful and energy efficient than those made from conventional semiconductor materials.3 In the 21st century, the integration of WBG technology will set a new course for all industries. The Institute of Electrical Engineers (IEEE) states silicon carbide may be to the 21st century what silicon was to the 20th century.4
The State Of The Union Today
With the use of conventional Si-based components, double-conversion uninterruptable power supplies (UPS) have characteristically been trapped in the range of 93% to 96% for operating efficiencies. In an attempt to increase efficiencies, UPS manufacturers have introduced what is called a “high efficiency” mode or ECO-mode. In order to achieve this high efficiency mode of operation, the use of the UPS internal bypass to supply power to the critical load under normal operating conditions is utilized. By running on bypass and eliminating power-conversion losses, you have raised the efficiency but subjected the critical operation to running on utility power. A study conducted by the Electric Power Research Institute (EPRI) concluded that a potential drawback existed. Enabling the high efficiency ECO-mode in double-conversion UPS leaves mission-critical loads exposed to power abnormalities like voltage sags and line noise.5
The EPRI study examined the reaction of a UPS operating in high efficiency mode to a 30% voltage sag for 1 cycle. The result was a 16.7ms event that created 20.9ms of unconditioned UPS output power where 15.6ms was a complete power outage.6 Analyzing the impact of this voltage sag using the Institute Of Electrical and Electronic Engineers (IEEE) orange book indicates that an outage longer than 8.35ms exceeds most computer equipment manufacturer’s stored energy capacity, causing the equipment to shut down.7 Moreover, the IEEE orange book indicates that a utility voltage swell of 115% for 16.7ms (1 cycle) will cause an equipment voltage breakdown, potentially leading to premature equipment failure.8
The EPRI findings demonstrate that while operating a UPS in high efficiency mode provides a 5% efficiency savings, it comes at the expense of reliability.9
Out With Silicon – In With Silicon Carbide
Let us introduce the game changer. The use of SiC components in double-conversion UPS technology allows the user to operate in the double-conversion mode and maintain higher efficiencies. A hybrid configuration of power-switching SiC-IGBTs produces a 45% reduction of power losses. A 100% SiC set of power-switching modules produces a 70% reduction of power losses (Figure 1).
The 70% reduction in power losses of SiC-based UPS directly leads to an increase in the double-conversion efficiency to 98.2%. This high efficiency not only occurs at high load situations, but also continues on into lower load situations (Figure 2).
It is a fact of life that most energized UPS modules are less than 50% loaded. With a UPS operating in ECO-mode, the EPRI reported efficiencies of up to 98%. Looking at the curves in Figure 2, we see that the efficiency benefits of SiC technology can be realized during lightly loaded situations. The SiC-based UPS is able to achieve the same high efficiencies while operating in double-conversion mode. The UPS can now provide the same energy savings while also providing the critical load with conditioned, reliable, high-quality power.
Another significant advantage of SiC-based UPS is the reduction in heat loss. At 100% load, a conventional Si-based UPS has a heat rejection of 69.2 kBtuhr, whereas a SiC-based UPS has a heat rejection of 42.0 kBtuhr. This results in a heat loss energy savings of 39.4% over the Si-based UPS.
The intrinsic characteristics of SiC-based semiconductors also allow them to operate at higher temperatures than conventional Si semiconductors. This reduction in UPS heat loss and the ability to operate at higher temperatures help customers to decrease their cooling costs, which accounts for approximately 35% of a data centers electrical energy consumption.
Size And Weight
When it comes to maximizing the available floor space in a data center, a SiC-based UPS offers a reduction in weight and size when compared to the conventional Si-based UPS. The weight of a 500kVA SiC-based UPS is 2,770 pounds, whereas the weight of a conventional UPS is 3,330 pounds, reducing the floor load by 17%. Moreover, a 500kVA SiC-based UPS occupies 14 ft2 compared to the conventional UPS that occupies 20 sq ft. This is a 30% reduction in the floor space required, increasing the available power capacity in a given area. The SiC-based 500kVA UPS has a power capacity of 36kVA/sq ft compared to 25kVA/sq ft for the conventional UPS. Comparing the power capacity on a 1MVA system, a SiC-based UPS system will require a floor space of 28 sq ft, and the conventional UPS will require a floor space of 40 sq ft. Consequently, the SiC-based UPS can deliver more power within a limited floor space.
In summary, SiC technology is an innovation that will establish a new trajectory for small devices and power electronics. Benefits include: increased system efficiency, lower cooling system requirements, operation at higher temperatures, and higher power density. With the integration of SiC-based technology into the UPS, the goals of data center operators to operate at higher efficiencies, maximize floor space, and reduce operating costs across the facility are being achieved. n