1 October 2005
Lightning and surge protection in emissions monitoring
At any given moment, there are over 1,800 thunderstorms occurring.
By Donald Long
The 1,000-ft stacks at electrical power generating plants are huge lightning rods reaching up into the sky.
The very same stacks, selective catalytic reaction (SCR) beds, ammonia injectors, and precipitators, only now carrying expensive nitrogen oxide/sulfur dioxide (NOx/SO2) emissions monitoring equipment and associated power and instrumentation cabling, take on nature's fury in the form of potentially 200,000-ampere bolts of lightning.
How can one effectively deal with this phenomenon? How does lightning get into the continuous emissions monitoring system (CEMS)? Is this the only form of surge? Is proper grounding the answer?
Everywhere one looks today, HDTVs in homes, ignition control in automobiles, and laptops and PCs at the office, there are microprocessors and ICs (integrated circuits) hard at work digitizing data, controlling devices, and communicating information.
For the most part, this is good. Except this low voltage, high intelligence equipment—more specifically, expensive control instrumentation and NOx and SO2 monitoring equipment now common to the power generation industry—is quite susceptible to both externally and internally generated voltage transients and surges.
This begs the question: Why would knowledgeable managers/operators of power plants install NOx/SO2 monitoring equipment that can readily cost up to $100,000 or more without providing the same or better protection that is now commonly afforded home computers or entertainment systems?
Is lightning a real concern?
In a word, YES! The National Oceanic and Atmospheric Administration (NOAA) reports at any single second there are over 1,800 thunderstorms occurring around the globe, and in that same instant, on average, lightning has struck somewhere in the U.S. six times. Further, each specific lightning strike may consist of up to 16 individual side surges.
The vast majority of distributed instruments in power plants such as analyzers, temperature, and pressure transmitters, current to pneumatic (I/P) valves, recorders, and others are all connected to the input/output (I/O) control elements in the form of distributed control systems (DCS), programmable logic controllers (PLC), and the like via copper conductors.
These copper conductors form the metallic highways upon which surges may travel throughout the entire control network. Lightning may enter this highway system several ways:
Direct attachment: This is the most obvious method and often the most damaging. A strike on the electrical cabling on the stack can destroy CEMS instruments at both ends of the cable run. Where there is significant risk of a direct strike, external structural protection based on lightning rods and grounding conductors can be of assistance.
Capacitive/inductive coupling: Because the underside of a highly charged thundercloud carries a tremendous negative potential, an enormous electric field couples into the instrument/power cables at the moment of discharge. Likewise the high current (in tens to hundreds of kilo amperes) flowing down the ground paths may easily be coupled inductively into the instrument cables routed alongside.
Resistive coupling: This is easily the most common form of insertion of damaging energy into the control and monitoring circuitry. If an ideal low resistance, single point, grounding system existed in the plant, then resistive coupling would be a non-issue. Since this is rarely the case in practice because of the relative size, complexity, and disparity of ages of different parts of the plant, instrument, and grounding systems tend to have multipoint grounding systems.
"Realistic" grounding system
Slight differences in resistances to true power ground between different points within the grounding network result in voltage potentials ranging from several hundred volts to tens of thousands of volts depending on the amount of current induced by the surge.
These potentials readily appear across the I/O and communications lines interconnecting the control network. Since instrumentation, I/O modules, and the like rarely have in excess of 1,500 volts isolation, they are susceptible to damage even by modestly low-level transient surges.
Catastrophic or clear blue sky
Less than 5% of continuous emissions monitoring systems failures are due to catastrophic causes such as lightning strikes, transmission faults, or brownouts.
The remaining 95% of failures are due to repeated degradation of the equipment from transients that fall into a category between safe or normal and catastrophic operation of a microprocessor or an IC.
Visualize the relative effects of destructive surges riding on sample waveforms on the pin of an IC or microprocessor of the typical NOx/SO2 CEMS.
The result of repeated attack on solid state devices by transients shown at the degradation level are unexplained "clear blue day" failures of the monitor and significantly reduce mean-time-between-failures (MBTF) of the system.
Microprocessors are subject to weakening damage from reverse voltage/currents as little as one micro joule. Integrated circuits are subject to damage from as little reverse voltages as 10 micro joules. To conceptualize a micro joule, imagine standing a quarter on edge and permitting it to fall onto its side. This relatively tiny expenditure of mechanical energy is equivalent to approximately 20 micro joules of electrical energy, a factor of two and 10 times more energy respectively than is required to damage the IC and the microprocessor.
There are a number of devices available to protect against voltage transients and surges. Some have high-energy capabilities but poor response times while others have a much quicker speed of response but low energy capabilities. What this means is no single device achieves an ideal state of protection for the exposed NOx/SO2 monitor and associated CEMS instrumentation.
As a result, the only practical solution is to use a combination of devices, drawing on the strengths of each, in a circuit known as a "reset table" hybrid surge protector. An example of this would be to employ both a zener diode with extremely fast response time to the leading edge of the typical "eight microsecond rise-20 microsecond fall" surge pulse in combination with a gas discharge tube having very high-energy capability to carry the relative bulk of the balance of the current impulse. The critical and harmful "let-through voltage" of this type of device remains in a safe zone in comparison to single component protectors that only offer a modicum of protection.
AC power protection concept
Up to this point, the discussion has been primarily addressing DC, signal, and communications types of circuits between the NOx/SO2 monitors and the control room. What about AC power circuits?
First, what equipment uses AC power in plants? Certainly, DC power supplies that provide the 24 VDC for most of the control and instrument circuitry use AC power. Controllers such as PLC's, industrial computers, etc., all use AC power.
One must treat AC power with a zoned approach beginning at the entry, the main power-distribution panel level, where massive surges can appear. A properly sized primary power protector at this point limits the externally generated let-through to a manageable level. It also protects the UPS system's relatively vulnerable solid-state switching network.
The second zone for AC power protection is at the power distribution sub-panel. Here, the ideal AC power protector relies on high reliability, oversized metal oxide varistors that not only pick up the pass through surges from the C3 level protectors but also suppress the myriad of internally generated transients from such devices as arc welders, elevators, or motor switching. These are the types of surges that by themselves are not catastrophic but are insidiously degrading to computers, communications, and expensive NOx/SO2 monitoring types of equipment.
The last line of defense for AC protection is at the equipment level itself where the requirements are to suppress immediate area noise, UPS switching transients, arc welding surges, and equipment power switching. Ideally a surge device at this level will not only protect against surge transients but will also incorporate noise filters to minimize or eliminate RFI, hum, and ringing on the AC line that are often responsible for microprocessor malfunction.
To ground or not to ground
Grounding and bonding have been subjects of entire studies, books, workshops, seminars, and justifiably so. There exist more experts on this subject than any other. Rather than becoming immersed in such a controversial subject, focus on grounding specifically as it relates to CEMS surge devices.
There exist three primary requirements for the grounding of surge protection devices. They are in order:
Divert surge current as soon as possible. The longer an eight-to-20 microsecond, possibly 6,000V, surge transient exists in the monitoring system, the more the potential for damage exists. Hence, it is imperative to detect and divert that pulse to a safe ground plane just as quickly as possible.
Use dedicated low impedance connection. This bond to the ground plane cannot be overemphasized. Ideally, the resistance to the ground plane would be less than 0.1 ohms. The author recently visited a plant experiencing loss of instruments from severe lightning/surge problems in Cape Fear, N.C., where the measured resistance to the ground plane was 18 ohms.
A direct strike of 200,000 amperes to a lightning rod on their plant could easily produce a voltage across the entire building well over 3 million volts. By reducing their resistance to the ground plane to 0.1 ohms as recommended, the same lightning surge can now produce only a 20,000-volt pulse that is much easier to manage.
Ensure all systems connect to the same point—once. All the subsystems in the plant, instrumentation, communication, computers and control, and AC power, must connect to a single point ground system. This is "star point" grounding. Properly done, each subsystem ground is as short as possible and connects to the star point at only one point. Multiple paths to the ground plane from a subsystem inherently have different resistances. Different resistances to ground produce, again by Ohm's Law, different voltage potentials to the subsystem that result in transient surge damage to that system.
Star point grounding
Specific monitoring solutions
Focusing on the monitoring and control instrumentation required by NOx/SO2 analysis, we find, with the possible exception of the stack equipment, many similarities, problems, and solutions to transient surge elimination common to all process plants.
The elements primarily are:
Remote instruments and analyzers: The problem is fairly well defined. There are temperature and pressure transmitters, current to pneumatic valves (I/Ps), analyzers, and in-situ probes—to name a few—distributed in exposed locations throughout the typical generating plant. These expensive instruments, often with HART "smart" capabilities, require a minimum of two elements: power to operate and the means to communicate to a DCS or PLC I/O module located somewhere else in the plant. Manufacturers of these instruments rarely offer more than a minimum of surge protection as an option to the device.
DCS or PLC control system: This is where the control element, the brains of the system, resides. Here, the decisions to turn valves on or off, open or close ammonia injectors, and tune fuel/air ratios; based on primary sensor and analysis input information take place. Again, the DCS/PLC manufacturers offer only minimal surge protection devices.
The common element to both ends of the CEMS is the signal and power wiring. These are the metallic power and information highways just discussed. Once the surge, often up to 6,000V potential and often from several amps to tens of amperes, couples onto this highway, it travels, at microsecond speeds, in both directions on the conductor, seeking low resistance pathways to ground.
The electronics at both ends of the control loop provide these pathways. Having isolation resistances far less than the transient surge, they break down instantaneously, often permanently, under the high voltage/current and allow the current to dissipate through them to the ground plane. And by providing better grounding for the instruments at either end of the control/power loop, as many instrument manufacturers suggest or require, we have made the final destination of the surge pulse via the electronics that much more attractive!
What is the solution? Hybrid style surge protection devices installed at both ends of the loop and on every metallic conductor associated with those systems. A 1,300 megawatt coal-fired generating plant in the Ohio River Valley employed this concept and despite having previously installed improved grounding techniques, realized immediate relief to critical down times and loss of analyzers and DCS input modules.
Complete loop surge protection
Late on a Saturday night
If the NOx/SO2 instrument is expensive, microprocessor, or IC-based, critical to plant operations or reporting, exposed to lightning surges or other transients, has difficult or remote access, such as on a stack for repair or maintenance, then provide hybrid style transient surge protection.
Moreover, protect every metallic conductor—signal, power, and communications, no matter how large or small—to and from that device.
A good rule-of-thumb is to determine if a particular facility requires surge protection. If you have a structure subjected to lightning current surges and the CEMS instrument circuits includes a vertical displacement greater than 10 meters or a horizontal displacement of 100 meters, then you probably need surge protection.
How much does this type of effective, re-settable surge protection typically cost? It costs much less than the monitoring equipment on the stack or the I/O module at the controller end, or the resultant possible downtime. Moreover, it costs less than $200 for complete protection at both ends of the loop.
In the final analysis, Mother Nature inevitably will find a pathway to valued and critical CEMS instrumentation—most often late on a Saturday night.
Behind the byline
Donald Long (firstname.lastname@example.org) is an ISA member and has an electrical engineering degree. He worked as an instrumentation engineer on the Lunar Landing Team/Apollo Program. Now, he works for MTL in project management and applications engineering. This article is from his paper presented at the 46th Annual ISA/POWID/EPRI conference.