Extreme Design: When radar failure at 12,000 feet is not an option

(Editor's note: this is the latest in our on-going Extreme Design series, where a single design or test objective—not low cost—dominates the engineering challenge. To see all the previous entries in the Extreme Design series, click here.)

High above the Fryingpan Valley in Colorado looms Red Table Mountain, a massive 18-mile long sandstone ridge protruding sharply from the valley below. The mountain top is accessible by a 4-wheel drive vehicle just two months of the year, a bumpy ride up a single lane road so rough it can wear out the wheels of an army jeep in a single trip. The rest of the year, the only way to the top is by helicopter. Here, at the clear altitude of 12,000 feet, FAA maintains a radar site for guiding aircraft over the Continental Divide. It is arguably one of the worst locations for a radar failure to happen, Figure 1.

Figure 1: FAA Radar Station at 12.000 feet,

Red Table Mountain, Colorado.

(To enlarge, click here.)

Most radar sites are operated on commercial power, and are backed up by a generator with a Power Conditioning System (PCS). The PCS supports the Critical Radar Loads during power bumps, power spikes, voltage sags and transitions from commercial to engine-generated power. The static switch and the PCS unit have to make the transition in less than 4 milliseconds to ensure uninterrupted function of the beacon, radio transmitters and other radar equipment.

The PCS at the Red Table Mountain site had a frustrating problem: PCS batteries were supposed to last up to 4 years, but they only lasted a year to year and a half. The frequent replacement of the batteries was costly and time-consuming. The site personnel suspected that the PCS unit was going to batteries frequently, without the engine sensing a power failure. It was not clear why this would be happening, so they requested assistance in identifying the problem.

The person sent to solve the puzzle was Jerry Taylor, an engineering technician with the FAA’s Western Service Area Technical Support Center in Denver, Figure 2. Taylor had worked as a resident engineer for a number of years, installing Power Systems all over the United States, including Alaska and Guam. He was often called to assist facilities in solving critical electrical issues.

Figure 2: Jerry Taylor, a support engineer for the FAA

Western Service Area, arrives on Red Table Mountain by helicopter

to help staff solve the frequent switching puzzle.

(To enlarge, click here.)

Before heading to Red Table, Taylor had asked for two PQubes to be delivered to the station. The PQube (www.pqube.com) is a tiny but highly accurate instrument for monitoring and recording power quality, Figure 3. Taylor knew it had the unique ability to automatically email all data, including waveform captures, to anyone anywhere. It could also email daily, weekly, and monthly reports about critical power parameters, including digital and analog data.

Figure 3: A typical PQube installation:

PQube unit (middle) with a power-supply module (right)

and a combined current/Ethernet module (left).

(To enlarge, click here.)

Taylor had not used the PQube before but a colleague at the FAA Training Academy in Oklahoma City had recommended the instrument. Engineers there used the PQube to verify 4 millisecond static switch transfers, which other instruments could not always capture. The colleague described the PQube as a plug-and-play device that requires minimal setup—just input power and a few CTs to begin monitoring and recording the system’s voltage and current.

Unlike the bulky and expensive portable testing equipment technicians typically brought with them to test for problems, the PQube seemed like the near-ideal instrument to send to Red Table. Technicians could have several for the price of their regular testing equipment, and could receive the data right away and anywhere.

Taylor’s intention was to leave the two PQubes at the site to collect data on the commercial incoming power, the transfer time to the engine generator during power failure, the time the generator was on, the quality of generator power, the transfer back to commercial power, the PCS input and output power, and the transfer times of the PCS. Two PQubes could do all that, and provide the digital, analog and waveform data in real time. By leaving them connected to the equipment over a period of time, Taylor hoped to gather enough information to see if any events or patterns appeared—something he couldn’t do with a portable analyzer in a day.

When he arrived, Taylor found that just one PQube had been sent, so he installed it on the PCS input. He also connected two temperature/humidity probes that had been purchased with the unit because he felt the room might be too hot for the PCS and batteries. He installed one probe in the PCS room and one in an adjacent room, using an extension cable. As soon as it was hooked up, the PQube began monitoring and recording power data. Taylor returned to his duty station.

Two weeks later, the site technicians forwarded to Taylor the data from the PQube’s SD card (.HTM summaries and .GIF and .CSV files he could open on any computer). The evidence was incontrovertible: In just two weeks, the PQube had detected  and recorded 30 power-quality events, noting the exact type, depth and duration of each event (Figure 4). The PCS interpreted all of these deviations from normal voltage as power outages. The system switched from commercial to battery power every single time, then switched back in a few seconds when it did not detect continuous outage.

Figure 4: HTM summary of data from the PQube

at Red Table Mountain.

(To enlarge, click here.)

The PQube data revealed a second problem. To prevent the battery from freezing, employees were careful to keep the door of the PCS room shut all the time—raising temperature in the room above 100° Fahrenheit (38° C)! Not only was the battery working overtime, it was working in an extremely overheated environment, well above its maximum rating.

Taylor kept digging into the data. The shape of the waveform, Figure 5, captured during the transfers alerted him to the possibility of bad capacitors in the PCS module—something the PCS manufacturer was quick to address. Graphs like these, generated by the PQube directly, can be emailed as attachments and can be viewed in any picture viewer program—no special software is required.

Figure 5a, 5b:  Waveform and RMS “snapshots” of a

waveshape change event like the one that

caused the PCS unit at Red Table Mountain to switch.

(To enlarge, click here and here.)

Armed with the information from the PQube, the engineers at the site began to implement small but important changes to reduce wear on the PCS unit. They installed a filtering system to improve ride-through capability, and they tightened the timing of the transfer based on the voltage sag information from the PQube. They also called the utility company and asked it to address the voltage sags on its end. Finally, they installed an exhaust fan and a thermostat in the PCS room, to maintain optimal temperature and humidity.

The Red Table technicians then followed up on Taylor’s original intention and installed a second PQube permanently on the output side of the PCS. This allows them to compare input and output data to make sure the PCS continues to function properly.

“The technicians do not want to give the PQubes back because they work so well and are so easy to use,” says Taylor about his experience with taking PQubes around different sites, Figure 6. “I tell them next time they talk about buying an expensive monitor to instead buy ten PQubes so all the technicians have them.”

Figure 6: A typical PCS system with PQube on DIN rail.

(To enlarge, click here.)

The cost of a PQube (right-side of) is a tiny fraction of the cost of a sophisticated backup system, and the knowledge and peace of mind it provides are absolutely priceless, says Taylor.

“What the PQube allows us to do is be proactive in how we deal with power problems,” he adds. “It’s like magic glasses: Suddenly you can see how much current you are drawing, what the voltage is doing, and see it as things are happening; the whole system becomes alive.”

About the author

Ani Dimusheva is a Marketing Communications Specialist at Power Standards Lab (PSL), Alameda, CA. (Jerry Taylor can be reached at jerry.taylor@faa.gov.)

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