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Typical rapid transient electrical applications are often characterized as "mission critical." In many cases, an engineer or test technician may have just one viable opportunity to capture invaluable diagnostic data for the prevention of damage to machinery, systems and equipment. In many cases, improper data collection could add millions of dollars to project costs and also cause significant personal injury. As such, this very short-duration event requires a high-speed, high-accuracy, isolated data acquisition system to ensure measurement safety, accuracy and reliability.
Whether the measurement system is used to help protect spacecraft from pre-launch lightning strike damage or to ensure the continuous operation of a major power grid, it is essential for an engineer to fully understand the demands and risks of the measurement; ultimate testing goals; and the true compatibility of specified hardware and software for meeting these objectives. The team at HBM, Inc. (www.hbm.com) has nearly 40 years of field-proven experience in the successful design, development and manufacture of ultra-high speed data acquisition devices, known as the Genesis HighSpeed product family, for the reliable measurement of such extreme rapid single-shot electrical events.
At the time of measurement, rapid transient electrical applications commonly include the presence of high-voltage conditions, such as those created by a cloud-to-ground lightning strike. Other environmental considerations are both low- and high-temperature extremes; the presence of humidity and moisture, as well as dirt, dust and other contaminants; and the risk of equipment exposure to inclement weather conditions. These conditions create a definitive need for a system that is electrically isolated to be safe for the user and from equipment damage, rugged enough to withstand potentially aggressive outdoor conditions, and yet with the necessary high-accuracy at extreme high speeds to meet customer application challenges. This article shall present a series of considerations and application examples for the successful implementation of such ultra high-speed data acquisition devices.
Lightning Strike Protection
As a naturally occurring electrical discharge, lightning travels at different rates and voltage levels depending upon the conductivity of the medium through which it is traveling. The U.S. Department of Energy has reported the speed of lightning as 93,000 miles per second, with other estimates as high as nine million miles per hour. The speed of lightning at the time of detection may be also affected by its overall stage of detection. For example, a downward strike event tends to travel much slower than its returning upstroke. Potential damage caused by lightning strikes can take less than one second to occur, with resultant damages to critical systems and equipment taking months or years to repair.
With its proximity to the equator, NASA Kennedy Space Center in Cape Canaveral, Florida, USA (28° 36′ 30.23″ N; 80° 36′ 15.64″W) is an ideal locale for the launches of both manned and unmanned spacecraft, as well as future-generation rockets. The earth's natural rotation at that point provides spacecraft with an extra natural upward push, which ultimately reduces fuel requirements to space. At the same time, Kennedy Space Center is plagued by one of the highest rates of lightning strikes to ground per square kilometer in the United States. In 2009, it was estimated that the NASA Space Shuttle Endeavour launch pad area was struck a minimum of 11 times on the lightning mast and water tower, leading to costly launch delays. Thus, lightning is considered a formidable risk to launch operations, as spacecraft are highly vulnerable to damage caused by high-induced strike currents and voltages.
Spacecraft are initially assembled inside the large Vehicle Assembly Building and transported to the launch pad on special heavy-duty transporters, or Mobile Launcher Platforms, for final prelaunch preparations and mission checks. A spacecraft is vulnerable to lightning strike damage from the moment it emerges from the Vehicle Assembly Building until final launch. During this time, it is important to continuously monitor numerous points to identify any potential negative induced area lightning effects.
To ensure the safety and effectiveness of planned spacecraft and future-generation rocket launches, NASA designed its own proprietary lightning monitoring system. Using a series of unique high-precision transient recorders and digitizer transmitters, the system could work alongside a secondary lightning protection system, with both components remaining effective at each critical spacecraft launch point. Design of the NASA Kennedy Space Center lightning protection system incorporated the use of tall towers, supporting metal cables that could intercept lightning strikes and divert the current away from the spacecraft launch vehicle. Two launch pads were protected in the testing area. Launch Pad 39A, used during active manned Shuttle launches, incorporated one lightning protective device on top of the pad, while Launch Pad 39B, designed for next-generation launches, features three 180-meter high lightning protection towers.
Special environmental considerations at Kennedy Space Center included the humid surrounding climate of the state of Florida, leading to a system requirement for high corrosion and moisture resistance, as well as suitable protection from other environmental contaminants. In addition, the risk of damages caused by the high shock and vibration levels and ambient temperatures typically present during launch required a system that met specific MIL-SPEC standards. Transmitter input had to be solely DC-powered with an effective switch to battery operation and complete system isolation while lightning was in the area. Equally important was the ability to switch to a DC charging circuit via remote control after a thunderstorm for continuous system monitoring.
Working with NASA, HBM incorporated the use of the Genesis HighSpeed, high-resolution data acquisition system with Perception software to facilitate review, control and analysis of captured induced current and voltage data at various points, with 0.1% full-scale accuracy and 25 MHz bandwidth. The system was housed in a corrosion-resistant 304 stainless steel package for high resistance to humidity, moisture and environmental contaminants. The use of fiber optic cable effectively supported a distance of up to 12 kilometers between numerous measuring points. IRIG time codes were used to achieve synchronization between multiple mainframes. Fiber optic transmitters were linked to a receiver which accepted up to four units for single mode fiber-optic transmission with 900 MS transient memory. Each measurement point included a remotely controlled test signal source for signal path verification, as well as the capability to analyze and generate automated reports for each lightning event. Effective multipoint monitoring allowed for the identification of locations where high-induced currents may have occurred due to lightning induced rapid transients.
As a result of successful technology integration, NASA was able to outfit Kennedy Space Center with a highly effective lightning strike monitoring and protection system that significantly reduced launch delays, by quickly identifying any potentially negative local effects from induced area lightning. The new system also helped to ensure the best possible pre-launch conditions for its spacecraft, ensuring their continued performance and integrity, while eliminating the possibility of further damages caused by rapid, single-shot lightning strikes.
High-Voltage Impulse Testing
The risk of lightning strike damage to machinery and equipment is not simply limited to the more extreme requirements of the space program. Such meteorological phenomena also pose significant risks for utilities and municipal power grids, for which damages to power masts, generators -or to the grid itself- can result in unforeseen power outages, costly and unpredictable downtime and grid blackout conditions. This ultimately increases overall power consumption requirements and decreases customer satisfaction.
In most cases, electricity is not produced at the same location where it is consumed. The power grid serves as the primary infrastructure by which power plants connect to end users. It is also the mechanism by which electrical energy is transported to utility consumers. Most power grids exist in the form of power lines installed onto towers, which are further organized into levels by their required amount of power transport capacity: low voltages (LV) to several 10 kV; medium voltages (MV) to several 100 kV; and high voltages (HV) of over 100 kV. Levels are interconnected by a series of substations which rely upon transformers, circuit breakers, surge arrestors, isolators, switchgear and other equipment to ensure safe and reliable electricity transport. The nature of the power grid setup itself leaves supporting substation components highly vulnerable to lightning strike damages. As hundreds of thousands of utility customers may all be linked within a single grid, the use of effective lightning testing is essential for sustained, continuous, efficient power grid operation.
With this requirement, an associated challenge for component manufacturers is to ensure development of a rigorously tested, highly rugged end product that can successfully withstand power grid conditions. Thus, the proper quality assurance testing and certification of transformers, surge arrestors, isolators and switchgear for their high-voltage survivability is vital. To stay globally competitive, each manufacturer must prove compliance with all relevant high-voltage testing standards, while adding minimal testing costs per component. In addition, the manufacturer must still be able to offer utility companies a favorable cost of ownership for installed product throughout its useful service life.
Globally recognized testing standards describe the proper steps for high-voltage test setups and procedures, as well as specific hardware and software requirements for accurate, repeatable data collection and results. Key criteria for such systems include high-resolution and accuracy, amplifier linearity, immunity against existing electromagnetic fields and grounding capabilities for safety.
High-voltage component testing requires specialized equipment, capable of producing lightning waveforms with known wave shapes and peak voltage levels of up to several MV. On the other side of the test object, equipment must be able to both measure wave shape and evaluate all relevant parameters according to appropriate standards. The more accurate an initial measurement, the greater likelihood exists that a manufacturer can avoid component under-testing or over-testing. Under-testing results in a greater risk of component underperformance, while over-testing results in a manufacturer having to offer the product to the marketplace with a non-competitive margin.
Another important aspect of optimal component testing is efficiency. For example, a three-phase transformer tested on all six phases/bushings (three inputs and three outputs) on a number of voltage levels and a number of different waveforms can easily result in 50 to 100 shots per test object. All waveforms are analyzed and documented within a report. By allowing for test sequences with automated analysis and limit testing, as well as automated report generation, overall test time is dramatically reduced and results in more cost-effective measurements, including type testing and final testing, with significantly minimized risk of operator error.
For challenging automated high-voltage component test setups, HBM offers the ISOBE5600t/m, a high-voltage fiber-optic isolated data acquisition system with HBM's own proprietary Lightning Impulse Analysis software. The system is designed to meet the highest possible grade reference digitizer standards. Impulse attenuators interface between the customer's voltage dividers and the ISOBE5600t transmitter input. The Lightning Impulse Analysis software evaluates captured data; tests for overshoot, oscillations and chopping; checks for limits; and allows for test sequences with automated storage. The software also allows for the storage of test waveforms and results for further analysis and automated reporting. A user-selectable limit checking feature also increases testing efficiencies. The ISOBE5600t/m allows for testing of several phases or bushings at positive and negative polarity, as well as at different voltage levels, for support of up to 100 measurements or more per test object. Test collections allow for individualized per test object measurements and brings them into the same report for process optimizations. A manual accept/reject verification is available after each measurement for real-time accuracy checks. Each test, whether a single measurement or a full data collection, is automatically available in the report generator. The user defines the report layout one time and gets test results, including Pass/Fail indication, at the click of a button. …../Continued on the next page.
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