In addition to the three main UPS technologies—Offline, Line-Interactive, and Online—there are several other UPS designs developed to meet specific power protection requirements. These designs are intended for specialized applications where higher efficiency, improved power quality, longer operational life, or greater reliability is required. Some of the commonly used alternative UPS designs include Hybrid UPS, Ferroresonant UPS, DC UPS, and Rotary UPS.
Hybrid Topology (Double Conversion on Demand)
A Hybrid UPS, also known as a Double Conversion on Demand UPS, is designed to combine the high efficiency of an Offline UPS with the advanced protection provided by an Online UPS. Although there is no official industry designation for this design, some manufacturers, such as UTL, refer to it as Double Conversion on Demand.
Under normal power conditions, a Hybrid UPS operates like an Offline (Standby) UPS. As long as the incoming power remains within a predefined voltage and frequency range, the UPS allows the connected equipment to receive power directly from the utility supply. Since the power does not continuously pass through the inverter, the UPS achieves very high operating efficiency and consumes less energy.
Whenever the incoming power moves outside the preset operating limits due to voltage fluctuations, frequency changes, or other electrical disturbances, the UPS automatically switches to Online (Double-Conversion) mode. In this mode, the UPS continuously converts AC power into DC and then back into AC, ensuring a stable and clean power supply.
During Double-Conversion operation, the UPS can correct voltage variations without immediately relying on battery power. It is also capable of filtering electrical noise from the power line and maintaining a stable output frequency, thereby providing the same high level of protection as a conventional Online UPS.
Features of a Hybrid UPS
| Feature | Details |
|---|---|
| Normal Operation | Functions like an Offline UPS for maximum efficiency. |
| Power Disturbance Mode | Automatically switches to Online Double-Conversion mode. |
| Efficiency | Very high due to direct utility operation under normal conditions. |
| Power Quality | Corrects voltage fluctuations, filters electrical noise, and stabilizes frequency. |
| Suitable Applications | Installations requiring both high efficiency and advanced power protection. |
Ferroresonant UPS
A Ferroresonant UPS operates similarly to a Standby UPS but uses a special ferroresonant transformer to provide continuous output filtering and eliminate transfer delays during power interruptions.
The ferroresonant transformer is specially designed to store electrical energy for a short period. When the utility power fails, the energy stored inside the transformer supplies the connected equipment while the UPS switches from mains power to battery operation. Because of this stored energy, the transfer time is effectively eliminated, allowing connected devices to continue operating without interruption.
Most Ferroresonant UPS systems achieve an efficiency of approximately 82% to 88% while providing excellent electrical isolation between the input and output power.
The transformer used in this UPS contains three separate windings. One winding receives the normal utility power, the second receives power from the rectified battery supply, and the third delivers AC output power to the connected load.
Earlier, the Ferroresonant UPS was one of the most widely used UPS technologies. Today, it is generally limited to systems with capacities of around 150 kVA and is mainly found in industrial environments such as oil and gas plants, petrochemical facilities, chemical industries, power utilities, and heavy manufacturing industries. These industries continue to use Ferroresonant UPS systems because of their rugged construction and reliable operation.
However, some Ferroresonant UPS systems that use controlled ferro technology may interact with power-factor correction equipment. This interaction can cause fluctuations in the UPS output voltage. Such problems can often be minimized by reducing the electrical load or by adding suitable linear loads to the system.
Features of a Ferroresonant UPS
| Feature | Details |
|---|---|
| Working Principle | Uses a ferroresonant transformer to store energy and filter output power. |
| Transfer Time | Effectively eliminated due to stored transformer energy. |
| Efficiency | Approximately 82%–88%. |
| Electrical Isolation | Excellent isolation between input and output. |
| Typical Capacity | Up to about 150 kVA. |
| Common Applications | Oil and gas, petrochemical, chemical, utility, and heavy industrial facilities. |
DC UPS
A DC UPS is designed specifically for equipment that operates directly on Direct Current (DC). Its working principle is very similar to that of an Online UPS, but it does not require an output inverter because the connected equipment already uses DC power.
If the battery voltage inside the UPS matches the voltage required by the connected device, the device’s own power supply may also become unnecessary. Since one or more stages of power conversion are eliminated, the overall efficiency of the system increases while the available backup time also becomes longer.
DC UPS systems are widely used in the telecommunications industry, where 48 V DC has become the standard power supply. This voltage is commonly referred to as a common battery system and is preferred because it has less restrictive safety requirements for installation in conduits and junction boxes.
Traditionally, telecommunications equipment has primarily operated on DC power, whereas computers and servers have generally used AC power. However, considerable research has been conducted on using 48 V DC power for computer servers to reduce equipment costs and improve system reliability.
One limitation of lower-voltage DC systems is that they require higher electrical current to deliver the same amount of power compared to 115 V AC or 230 V AC systems. Higher current requires thicker conductors and may result in greater energy losses due to heat generation.
To overcome these limitations, some modern data centers are adopting 380 V High Voltage DC (HVDC) systems. Higher DC voltage reduces conductor size and improves efficiency, although it requires stricter electrical safety measures because of the increased voltage.
For low-power electronic devices operating on 5 V, certain portable battery banks can also function as a simple UPS by providing uninterrupted DC power during supply interruptions.
Features of a DC UPS
| Feature | Details |
|---|---|
| Output Power | Direct Current (DC). |
| Output Inverter | Not required. |
| Efficiency | Higher because fewer power conversions are involved. |
| Common Voltage | 48 V DC for telecommunications, 380 V DC for some data centers, and 5 V for portable devices. |
| Applications | Telecommunication equipment, computer servers, data centers, and portable electronics. |
Rotary UPS
A Rotary UPS is a mechanical type of UPS that stores energy in a large, continuously spinning flywheel. Instead of depending mainly on batteries, it uses the rotational energy of the flywheel to provide short-term backup power whenever the utility supply fails.
The heavy flywheel rotates continuously during normal operation, storing a large amount of kinetic energy. When a power outage occurs, the stored rotational energy continues to drive the system, supplying electricity to the connected load until the flywheel gradually slows down. In most cases, a flywheel-based UPS can provide backup power for approximately 10 to 20 seconds.
Although this backup duration is relatively short, it is generally sufficient to allow a standby diesel generator to start, stabilize, and begin supplying power to the load.
The flywheel also acts as an effective buffer against short-term power disturbances such as voltage spikes and voltage sags. Because the flywheel has a very high rotational inertia, sudden changes in electrical power have little effect on its speed, allowing it to maintain a stable output.
Rotary UPS technology is one of the oldest forms of uninterrupted power supply and was developed before the invention of vacuum tubes and integrated circuits. Since the flywheel rotates continuously during normal operation, Rotary UPS systems are considered to be continuously online.
These UPS systems are generally used only in installations requiring more than 10,000 watts of protection because of their higher installation cost. Larger flywheels or multiple flywheels connected together can increase both backup duration and system capacity.
Unlike battery-based UPS systems, Rotary UPS designs can mechanically connect the flywheel directly to a diesel engine through a transmission gearbox. During a power failure, the rotational energy stored in the flywheel can help start the diesel engine. Once the engine reaches normal operating speed, it can continue rotating the flywheel while simultaneously supplying electrical power.
Multiple flywheels can also operate together using mechanical countershafts without requiring separate motors and generators for each flywheel.
One of the major advantages of Rotary UPS systems is their ability to deliver extremely high current output. They can easily handle heavy inrush currents required for starting electric motors, compressors, MRI machines, cardiac catheterization laboratory equipment, and other inductive loads. They can also withstand short-circuit currents that are up to 17 times greater than those tolerated by many electronic UPS systems.
Rotary UPS systems are known for their exceptionally long service life, often lasting 30 years or more. However, because they contain mechanical components, they require periodic maintenance, including bearing replacement and other servicing. Large installations usually include redundant systems so that maintenance can be performed without interrupting operations.
Modern Rotary UPS systems incorporate technologies such as magnetic bearings and air-evacuated enclosures, which significantly reduce maintenance requirements while improving efficiency.
Configurations of a Rotary UPS
Rotary UPS systems can be designed in several different configurations depending on how the motor, generator, flywheel, and backup power source are arranged.
| Configuration | Description |
|---|---|
| Motor-Generator System | A motor mechanically drives a separate electrical generator. |
| Combined Synchronous Motor and Generator | A single machine performs both motor and generator functions using alternating rotor and stator windings. |
| Hybrid Rotary UPS | Similar to an Online UPS but replaces batteries with a flywheel. The rectifier drives a motor that spins the flywheel, while a generator uses the flywheel’s stored energy to supply the inverter. |
In Hybrid Rotary UPS systems, the motor-generator arrangement may use synchronous or induction machines. Depending on the design, the motor can be powered directly from the AC supply, through a double-conversion motor drive, or by a six-pulse inverter. Different configurations may also use batteries or independent electrically coupled flywheels as their short-term energy storage source, ensuring uninterrupted power until the primary backup system becomes fully operational.