Typical rapid transient electrical applications are often characterised 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 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 has nearly 40 years of experience in the design, development and manufacture of ultrahigh-speed data acquisition devices, known as the Genesis HighSpeed product family, to reliably measure 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 cloud-to-ground lightning. Other environmental considerations are both low- and high-temperature extremes, humidity and moisture, as well as dirt, dust and other contaminants, and the risk of equipment exposure to inclement weather.
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.
As a naturally occurring electrical discharge, lightning travels at different rates and voltages depending on the conductivity of the medium through which it is travelling. The US Department of Energy has reported the speed of lightning as 150,000 k/s, with other estimates as high as 145 million km/h.
The speed of lightning at the time of detection may be 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 a second to occur, with resultant damage to critical systems and equipment taking months or years to repair.
With its proximity to the equator, NASA Kennedy Space Center in Cape Canaveral is a good locale for launching 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. At the same time, the centre is plagued by one of the highest rates of lightning strikes to ground per square kilometre in the US.
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 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 lightning monitoring system. Using a series of high-precision transient recorders and digitiser transmitters, the system could work alongside a secondary lightning protection system, with both components remaining effective at each spacecraft launch point.
Design of the 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 m high lightning protection towers.
Special environmental considerations at the centre 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 fibre-optic cable effectively supported a distance of up to 12 km between numerous measuring points.
IRIG time codes were used to achieve synchronisation between multiple mainframes. Fibre-optic transmitters were linked to a receiver which accepted up to four units for singlemode fibre-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 analyse and generate automated reports for each lightning event. Effective multipoint monitoring allowed the identification of locations where high-induced currents may have occurred due to lightning induced rapid transients.
As a result of technology integration, NASA was able to outfit the centre with an effective lightning strike monitoring and protection system that 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.
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 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.
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 organised into levels by their required amount of power transport capacity: low voltages to several 10 kV; medium voltages to several 100 kV; and high voltages of over 100 kV.
The levels are interconnected by a series of substations which rely on transformers, circuit breakers, surge arrestors, isolators, switchgear and other equipment to ensure safe and reliable electricity transport.
The nature of the power grid set-up 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 favourable cost of ownership for installed product throughout its useful service life.
Globally recognised testing standards describe the proper steps for high-voltage test set-ups 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 specialised 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 under-performance, 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 analysed 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 reduced and results in more cost-effective measurements, including type testing and final testing, with significantly minimised risk of operator error.
For automated high-voltage component test set-ups, the company offers the ISOBE5600t/m, a high-voltage fibre-optic isolated data acquisition system with lightning impulse analysis software. The system is designed to meet the highest possible grade reference digitiser standards.
Impulse attenuators interface between the user’s voltage dividers and the ISOBE5600t transmitter input. The 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 storing test waveforms and results for further analysis and automated reporting. A user-selectable limit checking feature also increases testing efficiencies. The ISOBE5600t/m allows testing 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 individualised per test object measurements and brings them into the same report for process optimisations. 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.
The incorporation of these and other data acquisition systems allows for the integration of new high-voltage grid systems and components, as well as the upgrading of legacy systems, with added safeguards for continuous, uninterrupted power service.
Data acquisition systems are also used to support next-generation ‘smart grid’ development. As system concepts are developed, a growing need exists for testing and supporting high-voltage component parts, such as switchgear devices, transformers, surge arrestors, cables, isolators and other products, to support newly developed infrastructure.
R&D test laboratory requirements for smart grid technology are also increasing. In addition, with the advent of a new and more efficient infrastructure for energy delivery, the formal establishment of new testing standards and procedures is also likely, bringing with it new applications for the company’s high-voltage data acquisition technologies.
Traditional circuit-breaker systems operate on the premise of electrical contacts moving away from one another, thereby creating an electrical arc. Another high-voltage testing area for the company is the measurement of interruption phenomena affecting breaker performance and operation, a term known as circuit zero (CZ).
These measurements are commonly used as a research tool to help better understand and improve the descriptive mathematical model of the electric arc itself, while identifying dominant parameters for successful current interruption, such as pressure, temperature, ion density, plasma flow and other parameters, leading to circuit-breaker design improvements and the achievement of greater interruption capability.
Circuit-breaker manufacturers often rely on third-party test laboratory services for acceptance testing. The external test house acts as a credible, independent authority between buyer and seller, verifying product performance according to published specifications, with a buyer identifying individual specifications of interest for verification.
While international testing standards for circuit-breaker final products are well established, circuit zero testing standards can vary. As a result, the Arnhem, Netherlands-based KEMA High-Power Laboratory, one of the global experts in high-power acceptance testing, has established its own fully dedicated CZ test program.
To conduct this testing, use of a fibre-optic isolated digitiser with particular performance attributes, such as the HBM GEN 6600 HV, is recommended. The digitiser should ideally be placed as close as possible to the test article, to minimise required analog cable lengths and to ensure best results.
A suitable fibre-optic digitiser must also be able to deliver signal quality conditions for safe performance, as can be found within the existing KEMA system.
Among the necessary system performance attributes, high dynamic input range is important, as currents of 100 mA should be measured immediately following the interruption of many tens of kA of short-circuit current.
While the main frequency range of the application is typically only 50 or 60 Hz, other relevant processes occur on a submicrosecond scale, making required bandwidth another key specification, as well as vertical resolution.
As the system must also be able to reliably operate, undisturbed and uninterrupted, in the presence of naturally occurring transients, electromagnetic field immunity is also important. Further immunity against the fast or transient electrical events that can arise within both current and voltage during the switching progress is also critical.
High-voltage circuit-breaker test laboratory environments tend to see the most electromagnetic interference, as the highest voltages and currents occur simultaneously, also using the greatest amount of energy.
A typical acceptance test is designed to verify proper system operation under the most severe product operating conditions within a high-power test laboratory.
A system’s ability to pass under the worst possible conditions implies that its operation under less severe conditions, such as medium- and low-voltage tests, would not cause significant problems, as components are exposed to less stress and less energy is used to perform the testing itself.
The company’s GEN 6600 HV fibre-optic digitiser was fully tested by KEMA and passed at all levels of acceptance testing, including under extreme conditions, making it a viable option for high-voltage or CZ-related testing requirements.
Another rapid transient electrical application exists in the area of switchgear testing. This application calls for the use of equipment that can effectively measure low-, medium- and high-level energy values on an as-needed basis, with each requiring different isolation capabilities.
The system must also be able to offer accurate sequencing and timing control, so that in case of a failure within one segment, such as a short-circuit due to component breakdown, the rest of the grid may be protected by disconnecting the failed segment and interconnecting remaining, active segments.
Because of its complexity and wide energy measurement ranges, specific hardware and software must be used to produce accurate, safe and reliable test results.
Within this type of testing, high currents of up to hundreds of kA must be interrupted, while high voltages of up to several hundred kV are still present. Thus, hardware must have isolation, excellent electromagnetic field immunity and offer battery-powered operation, with supporting software that features strong data integrity, repeatability and optimal user test efficiencies.
A typical high-power switchgear test is ideally conducted using a battery-powered and fibre-optic isolated digitiser to obtain optimal signal quality, with appropriate voltage protection for personnel and equipment.
Successful switchgear testing requires the use of a data acquisition system, fibre-optic isolated digitiser, test sequencer and supporting analysis software. Due to the complexity and dangers associated with this type of testing, it is recommended for a user to source all these components from a single manufacturer, to ensure system compatibility, experienced technical support and necessary system calibrations.
The HBM Genesis HighSpeed data acquisition system, previously noted for use within one-shot lightning strike monitoring applications, has been used to support high-voltage switchgear testing.
In addition, the 6600 MV fibre-optic digitiser system offers an isolated power supply and small battery set-up for safe and effective medium-voltage testing requirements. For high-speed test sequencing, the fully fibre-optically isolated HBM BE3200 is suitable for switchgear applications, due to its fully user-synchronised timing pattern to the main generator, either via the external mains or derived from an internal timer, and application-specific extensions.
Kennedy Space Center
http://www.kennedyspacecenter.com/