Military Embedded Systems

Unique COTS application of the double-conversion on-line UPS


May 16, 2009

Michael A. Stout

Falcon Electric, Inc.

Unique COTS application of the double-conversion on-line UPS

Engineers often only implement the use of double-conversion, on-line UPS technology when they have a battery backup requirement. The technology frequently offers many more capabilities that are not often realized or understood. The capabilities may vary from UPS manufacturer to manufacturer and may not be readily clear in the manufacturer's datasheets or sales materials. When understood, the true capabilities of the on-line UPS may be a real problem-solver for the systems design engineer.

As our military and government continue facing the difficulties of protecting citizens and soldiers, an emphasis is placed on rapid implementation of newly developed technology. One example is the military’s push for immediate technology-based systems for the Improvised Explosive Device (IED) problem in Iraq and Afghanistan. More recently, Homeland Security has contracted with companies to develop advanced equipment to protect U.S. borders. These systems, along with a multitude of others, must be deployed domestically and throughout the world. To speed their development and deployment, engineers have incorporated into these systems COTS computers, power supplies, and other electronics that are designed to operate from 120 VAC domestic power. At first, their choices of power-compatible equipment may seem limited as the domestically available COTS equipment may have to operate from international voltages and frequencies. Due to the sophisticated design of this equipment, ultra-clean, reliable power is demanded.

Embedding existing COTS double-conversion on-line Uninterruptible Power Supply (UPS) technologies can assist equipment manufacturers and engineers with a low-cost solution to these power problems. This type of UPS technology is very flexible as it can provide no-break power backup, in addition to offering superior AC output voltage regulation; surge, transient, and overvoltage protection; frequency regulation; frequency conversion; input power factor correction; and with some models, phase conversion.

Overview of double-conversion on-line UPS design

The double-conversion on-line UPS is designed with a continuous-duty inverter circuit. The inverter produces new sinewave output power while operating from the incoming utility source, or when not available, from its internal or external battery sources. The incoming Alternating Current (AC) utility or generator power is rectified to an unregulated Direct Current (DC) and filtered. The batteries are also connected at this point. The unregulated DC from the utility or the DC from the batteries is fed to a regulating boost chopper or DC-DC converter circuit. This provides regulated power at a suitable voltage to the input of the DC-AC inverter circuit. The DC-AC inverter circuit then regenerates clean, tightly regulated, true sinewave AC power for use by the connected equipment. An additional bypass circuit is provided, allowing incoming utility power to be “bypassed” to the UPS output during overload conditions or in the event of a UPS failure.

Convert polluted power to clean power

The double-conversion on-line UPS is ideal for solving power problems emanating from utility or generator power: Its design allows for the conversion of “polluted” power to clean power.

Figure 1: A typical "polluted" AC power waveform from a low-cost generator.

Figure 2: The clean output waveform of an on-line UPS that was powered by the polluted generator power in Figure 1.

Figure 1 shows a typical “polluted” AC power waveform from a low-cost generator. Figure 2 shows the actual clean output waveform of an on-line UPS that was powered by the polluted generator power in Figure 1. Notice the:

1. Voltage dropouts

2. Noise and harmonics

3. Waveform distortion

4. Frequency instability [A and B durations between AC sinewaves (frequency) vary.]

No-break power capabilities

When the utility power is lost, the connected battery supply simply takes over as the energy source without the switch-over voltage dropouts associated with other low-cost off-line or line-interactive UPS designs.

The UPS output shown in Figure 1, 4A and 4B, defines a time duration of 1 Hz of AC power. For 60 Hz domestic power, the duration of 1 Hz is 16.66 milliseconds. A true double-conversion, on-line UPS has no break in output power should a loss of utility power occur as the battery simply takes over as the power source. Additionally, as the on-line UPS has a continuous-duty inverter circuit, extended or long-term battery mode operation is not a problem.

AC output voltage regulation

Because the on-line UPS is regenerating new AC power while operating from both utility and battery sources, a continuous ±2 percent or better output voltage regulation is typically maintained. When the UPS is operating from a utility or generator source, the ±2 percent output voltage regulation is maintained over the entire input voltage range of the UPS. This assures protection against sustained utility brownouts and overvoltage conditions while maintaining the optimum voltage and power levels to the connected critical equipment.

Surge, transient, and overvoltage protection

The on-line UPS design places several layers of active electronic circuitry between the incoming utility power source and sensitive equipment. This results in a superior level of surge, transient, and overvoltage protection for equipment.

In contrast, off-line and line-interactive designs do not provide these active layers of electronics, depending instead on low-cost surge protection devices with limited capabilities. In overvoltage conditions, they switch to battery operation, limiting their operational time under this condition. In locations where the over- or under-voltage conditions repeatedly occur, the off-line and line-interactive UPS batteries can become totally discharged and render the UPS useless.

Frequency regulation

Most on-line UPS units ship configured with their output frequency set to be synchronized with the frequency of the incoming utility power. In most domestic applications this is acceptable. However, where the incoming power is provided by a generator, the UPS should be configured to have a fixed-frequency or nonsynchronized output. Generator sources are notorious for frequency drift and shifts, which, if passed through the UPS, can cause some equipment to operate unreliably. Configuring the on-line UPS for nonsynchronized operation provides protection against this problem. Meanwhile, since off-line and line-interactive UPSs pass incoming utility power directly through to the connected equipment, they are unable to provide any protection against incoming generator frequency problems.

Frequency conversion

Some on-line UPS models take the nonsynchronized mode of operation further and are designed to operate as a true frequency converter, providing 50, 60, and 400 Hz output frequency irrespective of the input frequency. On-line UPS technology has been used as 400 Hz to 60 Hz frequency converters in some KC-135 Stratotankers. They change the 400 Hz generated by the aircraft’s power to 60 Hz single-phase power for powering add-on computers and microwave-based electronic warfare subsystems. Off-line or line-interactive UPSs cannot be used for frequency conversion applications.

Input power factor correction

Most computer-based electronics today incorporate internal switch-mode power supplies. The input circuitry of these supplies typically consists of a basic rectifier and filter capacitor stage. One characteristic of these designs is a nonlinear current draw from the connected utility power. This is caused by the large input filter capacitors gulping charging current near the peaks of the AC waveform. In aircraft or buildings where multiple pieces of equipment are being powered from the same source or electrical panel, the current harmonics being generated by these nonlinear loads can rise to levels that cause excessive wiring heating or nuisance circuit breaker tripping. The harmonics can also be disruptive to networks, computers, and other sensitive electronics operating from the same power system.

A typical on-line UPS uses the same input rectifier and filter stage as a switch-mode power supply. However, many models are available with an additional input Power Factor Correction (PFC) circuit. The input PFC circuit senses the incoming voltage and current waveform and adjusts to maintain their phase relationships, eliminating most nonlinearities. Any equipment connected to the UPS output will also have its input power factor corrected.

Some military COTS requirements specify that equipment being supplied meets conducted emissions standard CE102. The standard states that the equipment will cause less than 3 percent current distortion (at any single harmonic) to be conducted back onto the incoming power source. Engineers should be aware that even though the COTS on-line UPS has input power factor correction, it probably will not meet the CE102 specifications due to the low-cost circuitry used.

Phase conversion

Some COTS on-line UPS models will accept a 400 Hz three-phase source while providing 60 Hz single-phase output. Small 400 Hz portable and aircraft-generation systems are sensitive to phase load imbalance. If an acceptable load balance is not maintained between the phases, power system harmonic levels can increase to a level that causes circuit breakers to “nuisance trip” and motors to overheat. Also, the harmonics can be disruptive to sensitive equipment operating from the same power bus. As more and more COTS single-phase equipment is installed as a low-cost solution, phase balancing is often not easily achievable. A COTS three-phase to single-phase UPS or phase converter is the ideal solution to these problems. These units allow the connection of one or more pieces of single-phase, 115-120 VAC, 50, 60, or 400 Hz equipment while drawing a balanced amount of power from a 400 Hz three-phase source.

Double-conversion: An effective remedy

Double-conversion on-line technology is a flexible COTS platform. Either used directly off-the-shelf or modified by the UPS manufacturer, it can provide a fast, cost-effective solution to difficult power problems that may not be easily solved by other methods.

Michael A. Stout is VP of Engineering at Falcon Electric, Inc. based in Irwindale, Calif. He is an authority on military and commercial computer power conversion and UPS systems, having more than two decades of experience. In his current position, Michael

specifies and designs new UPS and critical power system products and evaluates emerging technologies. He can be reached at [email protected].

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