water treatment plant protection

TRANSIENT LIGHTNING PROTECTION FOR WATER/WASTEWATER TREATMENT PLANTS

Michael Nager

Phoenix Contact Inc. PO Box 4100

Harrisburg, PA 17111 Abstract This paper will discuss methods used for successfully protecting power distribution systems, field mounted instrumentation and control systems from the damaging effects of lightning strikes.

Wastewater plants have several physical locations that are at high risk for lightning strikes. Electric and electronic equipment in these areas is prone to malfunction because of electrical surges present on the distribution system. These areas include pump lift stations, RTUs (remote terminal units), elevated tanks, radio antennas. The equipment most at risk includes variable speed motor drives, process transmitters, communication equipment, and DCS or PLC controllers.

Using a case history example, we will explore the need to develop a systematic approach to lightning protection as suggested by the IEEE (Institute of Electrical and Electronic Engineers). Special consideration will be placed on IEEE Category C locations that have the potential for direct lightning strikes and which represent the biggest concern for wastewater facilities. Category C locations are outdoors or have electrical wires entering from the outside. The intent of this paper is to introduce the topic of surge suppression at water/wastewater plants and to inform the user of new technologies available to combat surges. Keywords Lightning, transient, over-voltage, surge suppression Introduction Increasing demand for reliable and cost effective control of water/wastewater systems is driving the industry towards microprocessor based instrumentation. The environmental conditions are considered extremely harsh and equipment used must survive for years without failure. A large portion of electronic equipment failure is attributed to transient over-voltages that can damage electrical and electronic systems. Preventing damage from over-voltages, and especially lightning, can be a daunting task. Equipment is often mounted outside and is at the highest risk of transient damage. Direct lightning strikes are a possibility but more damage occurs annually because of indirect hits. Every copper wire and metal surface is capable of transmitting transients into

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Copyright (c) 2000 Water Environment Federation. All Rights Reserved.

electronic equipment. Nearby lightning strikes can also dramatically increase the ground voltage potential causing a "reverse" hit as equalizing current flows between the different grounds. Evaluating Risk Both the geographic location of the plant and the location of the equipment at the plant must be considered to understand the level of risk due to lightning. Risk from Plant Location The geographic location of a water/wastewater plant indicates the relative risk the plant faces against lightning transients. The National Weather Service tracks thunderstorm activity across the country and produces what is called an isokeraunic chart (See Figure 1 "Mean Number of Thunderstorm Days Per Year"). The chart shows how many thunderstorm days per year occur in different regions of the country and with this knowledge one can implement an appropriate protection plan.

Figure 1 "Mean Number of Thunderstorm Days Per Year"

As indicated by the chart, the southwest and southeast portions of the country are literal hotbeds of lighting activity with 90, 100, or even 130 days per year with lightning activity. Water/wastewater plants in these regions generally are well aware of the lightning risk and have already taken measures to limit the damage that may occur. In other regions of the country, like the Mid-Atlantic States, there is less lightning. However, it should be noted that when compared to the whole world, this region is considered a "moderate severe" lightning area. Portions of Pennsylvania for example will experience between 40 and 50 lightning days per year. This roughly translates to 1

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lightning strike per year hitting every 20 to 25 acres of ground (See Table 1 "Relationship Between Thunderstorm Days and Flash Densities"). Since most treatment plants cover significant area, it is more than likely that at least one hit will occur.

Table 1 “Relationship Between Thunderstorm Days and Flash Densities” Risk from Equipment Location The power distribution system provides a path for direct and indirect lightning energy to damage electrical equipment. Damage to transformers, motors and wiring is common with high-energy surges and is responsible for causing downtime. The ANSI/IEEE C62.41-191 standard classifies equipment into three broad locations. Category C locations are those that are either outside or with a direct connection to the outside. Category C locations therefore are those most at risk of experiencing high energy transients. In fact, in such a location, one must expect a minimum of 10,000 amps of surge current resulting from lightning. Categories B and A are inside buildings to varying degrees and therefore are shielded from the highest energy levels. (See Figure 2 "IEEE Lightning Threat Categories")

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Figure 2 "IEEE Lightning Threat Categories"

Protecting equipment located in Category C locations is necessary. Energy that manages to get into a facility or equipment can not only cause damage directly but can link onto nearby conductors causing secondary damage to computers, data and phone lines, and remote monitoring equipment. Traditional Technology -- MOVs and SADs In the United States, the usual components used for protecting service entrances or the AC power to outdoor equipment are metal oxide varistors (MOVs) and silicon avalanche diodes (SADs). Evaluating surge suppression for AC power applications can be difficult. Manufacturers of surge suppression devices write their specifications differently so that direct comparisons are hard to make. Assumptions and omissions in the written data lead many to conclude that surge suppression manufacturers simply play "specman-ship" with little regard to real life experience. This is particularly true when comparing devices of that use different technologies.

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More electrically oriented readers are encouraged to contact the author and request a copy of an article entitled "Evaluating Surge Protection Devices For Lightning Protection" by Patrick McCurdy published in the April 2000 issue of Power Quality Magazine. It discusses this topic in detail and provides a clear explanation of surge suppression specifications and examines the common misconceptions when comparing surge suppressors. New Technology -- Triggered ARC GAPS Although the phrase "lightning fast" is used to denote something that happens exceedingly rapidly, lightning is actually considered a slow moving transient. Comparing the duration of lighting to others like the switching transients from the electric utility or from an inductive load suddenly being switched off, lightning lasts for a long time. The long duration of lightning provides more time for the energy to do its damage. In fact, the IEC has adopted a special waveform to more accurately depict lightning transients. The duration of the waveform is 250 microseconds compared with 20 microseconds commonly referenced by domestic surge suppression manufacturers. The long duration of a lightning transient literally cook the traditional components because they convert a portion of the energy to heat. The European market has developed and used a different technology that provides superior lightning protection. The device is wired in parallel from each line to ground. Inside, a small distance separates two pieces of metal (See Figure 3 "Triggered Arc Gap"). During normal operation, the circuit is open and there is no current flow. During a transient, the voltage will increase rapidly and the device will fire at 900 volts, diverting energy away from the protected equipment and into the ground.

Figure 3 "Triggered Arc Gap"

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The triggered arc gap provides protection on AC power lines and can be used in both single phase and three phase applications. The advantage of the arc gap is that it can handle very high energy levels. The most common reference waveform used in the United States is called the (8/20)us waveform (pronounced the eight by twenty microsecond waveform). It allows MOV or SAD based devices to specify very high surge current handling capability. In Europe, the (10x350)us waveform is used to compare surge suppression components that are to be considered lightning arresters. The long duration of this test waveform more closely resembles the long duration surge that is caused by lightning. The length of the surge produces great hit in MOV based protection modules and reduces both the maximum current that can be passed through the device as well as its usable lifespan. If one compares area under the curve as a measure of energy, compare the difference between a 40mm MOV-based device and one that uses an arc gap. Figure 4 8/20 µµµµs waveform vs. 10/350 µµµµs waveform.

Figure 4 8/20 µµµµs waveform vs. 10/350 µµµµs waveform

Case History A water treatment plant in the Florida panhandle has 250 lift stations serving the residents with potable water. Looking at the isokeraunic chart, we can see that this region of the country experiences 90 to 100 thunderstorm days per year. This translates into about 50 lightning strikes per year hitting every square kilometer or 140 strikes per square mile. This places the plant in an extremely hazardous location for lightning activity. Experience has confirmed the threat and it is not uncommon for damage to occur at different locations, especially in the summer months. In fact, the maintenance personnel know when they will be called upon for odd hour work simply by listening to the weather forecast!

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The plant was located in a high-risk geographic location--how about the equipment at the plant. A typical lift-station consists of a pump used to move water throughout the system. The pump is connected to a motor that uses both a soft starter and a variable speed drive. A programmable logic controller (PLC) is used to control the starting and stopping of the pump. Various analog and digital signals representing level, flow and pressure are inputs to the PLC, while an analog output determines pump speed. In the spring of 1998, one of the lift stations was hit by lightning. It contained no surge suppression. The following equipment was destroyed: Soft Motor Starter $1,966 All PLC I/O Boards $1,141 150 Hp Drive $7,000 VFD $6,850 --------------------------- --------- Total $16,957 While the exact labor costs and time required replacing the above equipment is not known, the utility generally values its maintenance labor at $30.00 per hour. It is not hard to see that just material costs associated with a lightning strike are prohibitive and should be avoided whenever possible. If the lift station was wastewater the negative effects from the community could also be a concern. Solution To protect any piece of equipment or facility, establish a zone of protection around it and protect everywhere copper conductors enter or exit the zone. This lift station was protected both with AC power protection (Triggered Arc Gaps) and I/O protection modules. For the 12 months since it was installed there has been no additional damage caused by over-voltages. The particular triggered arc-gap is used is mounted in a control panel and provides three phase protection. The arc gap devices are located inside an electrical enclosure and are mounted on the load side of the service entrance. The system consists of a series connection of the arc gap device and fuse. The circuit is then wired in parallel to the electrical service. This means that during normal operation, no load current passes through the surge suppression components. Only during a surge, does the arc-gap conduct the transient current to ground. (See Figure 5 "Mounted Triggered Arc Gap Devices").

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Figure 5 "Mounted Triggered Arc Gap Devices" In addition, it was realized that the PLC was still at risk from surges. While the power line was protected, a transient path still exists through the I/O wiring that connects the I/O cards to the process transmitters. Surge suppressors were used to stop transients from entering into the control system. Field mounted surge suppressors were also installed to protect the transmitters from the effects of over-voltages. Conclusions The geographic location of the plant and the location of the equipment can roughly determine risk to lightning and other over-voltage transients. AC power lines are likely to bring high energy, long duration lightning transients into the facility. Triggered arc gaps have been designed to handle this energy better than traditional MOV or SAD technology. By establishing a zone of protection around the lift station, the users of the plant ensure that the equipment inside is safe. In addition to protecting the AC power lines, every other electrical conductor entering the lift station should be treated as a possible path for lightning and other over-voltages. A zone of protection that prevents transients from entering on data lines, antennas and I/O lines should be considered mandatory. References McCurdy, Patrick. (2000) "Evaluating Surge Protection Devices for Lightning Protection," Power Quality Assurance, Vol. 11, No. 4, pp. 28 -33. IEEE Standards Collection (1992) "Surge Suppression," C62.

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water treatment plant protection

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