Monday, 5 March 2012

 High technology combined with speed and cost of implementation is driving the deployment of equipment used by today’s military. After 9/11, the military has had to diversify and adjust to the realities of a global war on terror. Its power sensitive, high-tech equipment may be deployed throughout the world on land, sea and in the air at a moment’s notice to react to the challenges. In more and more cases, commercial off-the-shelf (COTS) hardware solutions are readily available for rapid deployment.
In the case of a computer system, it may only require installing the military’s proprietary software and is “good to go.” In other cases, the equipment may be of a sophisticated microprocessor-based design that demands a clean, reliable power source. Much of this equipment has been designed to operate from a very reliable, domestic 120-Vac, 60-Hz utility power.
The power generated in Afghanistan, Iraq, Europe, and many other countries is 220 to 240 Vac, 50 Hz. Aircraft and many military land-based mobile generator power systems supply 115-Vac, 400-Hz power. The situation gets even more complicated due to unreliable local generating sources inside the war zone or in poor third-world countries. Aircraft and land-based mobile generator systems are subject to disruptive power, frequency instabilities and harmonic distortion problems. Thankfully, today’s online uninterruptible power supplies (UPSs) and power distribution units (PDUs) can address unique power challenges in harsh power environments.
Frequency conversion
The solution to effectively and reliably power the equipment needs to be both flexible and multifunctional. In applications where the local utility source is reliable, the power solution has to convert the incoming voltage from 220 to 120 Vac, along with converting the ac-frequency from 50 to 60 Hz, if domestic equipment is to be connected. When NATO equipment is to be connected, 120-Vac power has to be converted to 220 Vac and the frequency to 50 Hz.
In locations where the utility or generator source suffers from poor frequency regulation, the solution has to provide a reasonable amount of immunity from frequency variations, harmonics, high-voltage transients and a wide input-voltage range, while maintaining a tight output-voltage regulation. Frequency conversion may also be required, such as with aircraft applications where an unstable 115-Vac, 400-Hz power is the only power source available.
In locations subject to complete power outages, the solution must include battery-backup capability to assure the equipment continues to operate reliably and prevent software damage. Also, extended battery banks may be needed to provide power for equipment used in remote locations where no other power is available.
To top off the list, the solution must be rugged and reliable enough to survive in a wide range of environments.
The solution chosen is usually is in the form of an off-the-shelf single-phase online UPS. From the power protection standpoint, the online UPS utilizes a suitable double-conversion design and is an effective solution that will provide clean, reliable power. However, there are several problems that may be encountered in this approach.
First, most off-the-shelf online UPSs available are designed to operate from a single-phase 50- or 60-Hz power source only and will not operate from a single or three-phase 400-Hz source. If the UPS operates on 400 Hz, it is typically a single-phase device and may create a phase imbalance, resulting in excessive harmonic distortion. Some manufacturers offer the option of three-phase to single-phase conversion, which eliminates these problems.
The next problem that may be encountered is with the electrical and mechanical construction of most of-the shelf UPS units. They typically have been designed for use in a computer room or office environment. These types of units cannot withstand the shock and vibration inherent in aircraft or mobile installations.
Additionally, internal components such as batteries may not meet the required altitude or temperature ratings. The internal UPS wiring and external line cord are constructed using poly vinyl chloride (PVC) insulation, which is banned from use on aircraft.
As the online UPS takes the incoming alternating current power, rectifies it to a steady regulated direct current, frequency variation, high-voltage transients, voltage sags and swells, and harmonic distortion, are removed. The clean regulated dc is then fed to a pulse-width-modulated inverter that recreates clean, tightly regulated ac-power at the desired frequency. If battery backup is desired, batteries are installed that provide the source of power when another ac-power source is disrupted or not available.
Some online UPS makers’ platforms are very versatile and can be configured in commercial desktop cabinets or rugged rackmount enclosures. They can be configured without batteries, with standard valve regulated sealed lead-acid batteries, or with specialized wide temperature range batteries.
Power distribution units
PDUs are equally important in the power scheme and complement UPSs. Usually considered for branching single power sources to downstream equipment, many installations of electronic equipment require more than one type of power source. In these cases, there can be advantages to integrating more than one power source, and even power conversion, into a single PDU. Starting with ac- and/or dc-power sources, there are four types of power conversion: ac/ac, ac/dc, dc/dc, and finally dc/ac conversion
Moreover, due to the large amount of COTS servers, networking and communications equipment that must be plugged in and powered at a given location, power distribution is a perpetual challenge. Most installations use one or more PDUs to feed power to each device in the system. The power connector styles of each piece of equipment in the system can vary with some using commercial styles, and some using MIL-STD styles.
Additionally, the required power configuration for each piece of equipment can vary from numerous ac configurations to various dc voltages. Since the PDU is a common point at which this equipment is tied together, it is often beneficial to resolve these challenges in the PDU itself. This is accomplished by using a modified COTS PDU to help integrate the various connector styles and power configurations required by the equipment. The PDU itself should be configured to provide the appropriate mating connectors for each piece of equipment, thus avoiding the need for adapter cables. If dc power is required for some equipment, the PDU can also be configured with an internal power supply, thus eliminating the need for external power supplies. Both of these approaches help to simplify the overall system.
Depending on the installation location and available power quality, extra power conditioning may be needed in the PDU. This includes transient-voltage suppression by installing a surge protection device. It may also include power line noise suppression by using an EMI filter. In multi-phase applications, the PDU should have some local monitoring capability to allow efficient installation without requiring user-provided power measurement tools. A local monitor provides input current values for each phase, offering an easy means to ensure the entire system is balanced during setup.
As mentioned before environmental issues are very important. In many cases, COTS components used within the PDU are analyzed and repackaged or replaced to add extended capability to meet adverse temperature, shock ,and vibration needs for the target installation. Other applications further complicate design considerations.
These harsh environments can be conducive to salt spray, EMI, humidity, sand blasts, icing, and freezing rain, which may further impact the enclosure design as well as internal component selection. In many cases, traditional electromechanical control devices used in COTS PDUs cannot meet the criteria, thereby requiring solid-state technology. However, these devices carry the challenge of managing additional thermal dissipation that can drive unique airflow requirements


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