Alternative UPS Designs
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Hybrid topology / Double conversion on demand design
Recently there have been hybrid topology UPSs hitting the marketplace. These hybrid designs do not have an official designation, although one name used by HP and Eaton is Double Conversion on Demand. This style of UPS is targeted towards high efficiency applications while still maintaining the features and protection level offered by double conversion.
A hybrid (double conversion on demand) UPS operates as an off-line/standby UPS when power conditions are within a certain preset window. This allows the UPS to achieve very high efficiency ratings. When the power conditions fluctuate outside of the predefined windows, the UPS switches to online/double conversion operation.In double conversion mode the UPS can adjust for voltage variations without having to use battery power, can filter out line noise and control frequency. Examples of this hybrid/double conversion on demand UPS design are the HP R8000, HP R12000, HP RP12000/3 and the Eaton BladeUPS.
Ferro-resonant UPS design
Ferro-resonant UPS units operate in the same way as a standby UPS unit however they are online with the exception that a ferro-resonant transformer is used to filter the output. This transformer is designed to hold energy long enough to cover the time between switching from line power to battery power and effectively eliminates the transfer time. Many ferro-resonant UPSs are 82–88% efficient (AC-DC-AC) and offer excellent isolation.
The transformer has three windings, one for ordinary mains power, the second for rectified battery power, and the third for output AC power to the load.
This used to be the dominant type of UPS and is limited to around the 150 kVA range. These units are still mainly used in some industrial settings (Oil and Gas, Petrochemical, Chemical, Utility, and Heavy Industry markets) due to the robust nature of the UPS. Many ferro-resonant UPS’s utilizing controlled ferro technology may not interact with power-factor-correcting equipment.
DC Power design
A UPS designed for powering DC equipment is very similar to an online UPS, except that it does not need an output inverter, and often the powered device does not need a power supply. Rather than converting AC to DC to charge batteries, then DC to AC to power the external device, and then back to DC inside the powered device, some equipment accepts DC power directly and allows one or more conversion steps to be eliminated. This equipment is more commonly known as a rectifier.
Many systems used in telecommunications use 48 V DC power, because it is not considered a high-voltage by most electrical codes and is exempt from many safety regulations, such as being installed in conduit and junction boxes. DC has typically been the dominant power source for telecommunications, and AC has typically been the dominant source for computers and servers.
There has been much experimentation with 48 V DC power for computer servers, in the hope of reducing the likelihood of failure and the cost of equipment. However, to supply the same amount of power, the current must be greater than an equivalent 120 V or 240 V circuit, and greater current requires larger conductors and/or more energy to be lost as heat.
High voltage DC (380 V) is finding use in some data center applications, and allows for small power conductors, but is subject to the more complex electrical code rules for safe containment of high voltages.
Most switched-mode power supply (SMPS) power supplies for PCs can handle 325 V DC (230 V mains voltage × √2) directly, because the first thing they do to the AC input is rectify it. This does cause unbalanced heating in the input rectifier stage as the full load passes through only half of it, but that is not generally a significant problem. (Power supplies with a 115/230 V switch operate as a voltage doubler when in the 115 V position, which does require AC power, but the voltage doubler configuration also uses only half the rectifier, so it is certain to be able to handle the unbalance when operated from DC in
Rotary UPS design
A Rotary UPS uses the inertia of a high-mass spinning flywheel (Flywheel energy storage) to provide short-term ride-through in the event of power loss. The flywheel also acts as a buffer against power spikes and sags, since such short-term power events are not able to appreciably affect the rotational speed of the high-mass flywheel. It is also one of the oldest designs, predating vacuum tubes and integrated circuits.
It can be considered to be on line since it spins continuously under normal conditions. However, unlike a battery-based UPS, flywheel based UPS systems typically provide 10 to 20 seconds of protection before the flywheel has slowed and power output stops. It is traditionally used in conjunction with standby diesel generators, providing backup power only for the brief period of time the engine needs to start running and stabilize its output.
The Rotary UPS is generally reserved for applications needing more than 10,000 watts of protection, to justify the expense and benefit from the advantages rotary UPS systems bring. A larger flywheel or multiple flywheels operating in parallel will increase the reserve running time or capacity.
Because the flywheels are a mechanical power source, it is not necessary to use an electric motor or generator as an intermediary between it and a diesel engine designed to provide emergency power. By using a transmission gearbox, the rotational inertia of the flywheel can be used to directly start up a diesel engine, and once running, the diesel engine can be used to directly spin the flywheel. Multiple flywheels can likewise be connected in parallel through mechanical countershafts, without the need for separate motors and generators for each flywheel.
They are normally designed to provide very high current output compared to a purely electronic UPS, and are better able to provide inrush current for inductive loads such as motor startup or compressor loads, as well as medical MRI and cath lab equipment. It is also able to tolerate short-circuit conditions up to 17 times larger than an electronic UPS, permitting one device to blow a fuse and fail while other devices still continue to be powered from the Rotary UPS.
Its life cycle is usually far greater than a purely electronic UPS, up to 30 years or more. But they do require periodic downtime for mechanical maintenance, such as ball bearing replacement. In larger systems redundancy of the system ensures the availability of processes during this maintenance. Battery-based designs do not require downtime if the batteries can be hot-swapped, which is usually the case for larger units. Newer Rotary units use technologies such as Magnetic bearings and air-evacuated enclosures to increase standby efficiency and reduce maintenance to very low levels.
Typically, the high-mass flywheel is used in conjunction with a motor-generator system. These units can be configured as:
1. A motor driving a mechanically connected generator, 2. A combined synchronous motor and generator wound in alternating slots of a single rotor and stator, 3. A hybrid rotary UPS, designed similar to an online UPS except that it uses the flywheel in place of batteries. The rectifier drives a motor to spin the flywheel, while a generator uses the flywheel to power the inverter.
In case #3 the motor generator can be synchronous/synchronous or induction/synchronous. The motor side of the unit in case #2 and #3 can be driven directly by an AC power source (typically when in inverter bypass), a 6-step double-conversion motor drive, or a 6 pulse inverter. Case #1 uses an integrated flywheel as a short-term energy source instead of batteries to allow time for external, electrically coupled gensets to start and be brought online. Case #2 and #3 can use batteries or a free-standing electrically coupled flywheel as the short-term energy source.