Dam Instrumentation - Transient Protection
Perth, Australia: New Victoria Dam, completed in 1991, is a roller-compacted concrete gravity dam built for the Perth metropolitan water supply system. The dam, which is 350 meters long and 52 meters high, retains a reservoir with a storage capacity of 9.2 GL.
Instrumentation at the dam included twenty-two VW piezometers, sixteen VW strain gauges, a hanging pendulum, 137 thermocouples, 23 RTDs, a water level recorder, a seepage weir float well, seven multi-channel data loggers, and a modem. Because installed sensors are generally inaccessible and non-replaceable, an extensive transient protection system was implemented.
Six Point Plan
The protection system followed the Australian Standard AS 1768-1991 and incorporated a six-point plan devised by ERICO International, a manufacturer of lighting protection products. Elements of the six point plan are:
- Capture the lightning strike on purpose-designed air terminals at preferred points.
- Conduct the lightning current safely to ground using purpose-designed ground conductors.
- Dissipate the lighting energy into the earth with minimal rise in ground potential.
- Create a low impedance, equipotential earth grounding system to eliminate earth loops and differentials.
- Protect equipment from surges and transients on incoming power lines.
- Protect equipment from surges and transients on incoming telecommunications lines
Installing the Instruments
A low impedance earth grounding system, which would serve as instrumentation earth, was established when construction started on the dam wall. As instruments were installed, they were connected to this grounding system for protection.
Later, when data loggers were installed in the main gallery, instruments were connected to the loggers, and the loggers were connected to instrumentation earth. Data loggers were powered by solar panels until mains power was brought in at the completion of the dam wall.
All electronic instruments except for thermocouples and RTDs were fitted with 3-state transient protection, with the ability to divert high levels of energy very quickly. Such devices, although more expensive than one or two-stage devices, would provide protection for more events.
Protection for Sensor Cables
The thermocouples and RTDs were left unprotected, partly because of expense and partly because their numbers provided enough redundancy so that some losses could be tolerated. This decision had unforeseen consequences. Cables from the thermocouples and RTDs carried transients from the first lighting strike directly to the data logging stations, blowing up the multiplexers to which they were connected. The sensors themselves survived. After this event, Shottky diodes were fitted to these sensors, and no further lighting damage has been reported.
Protection for Power Supplies
Once mains power became available, high performance power line filters were installed to protect electronic equipment from surges and transients on the 250 V mains power lines. However, the data loggers were not equipped with power line filters because they were run from a common DC supply via a transformer-rectifier unit, and it was thought that this unit would isolate any incoming voltage surges and protect the loggers. Unfortunately, the transformer-rectifier unit burned out, taking the power supplies for each logger with it. An investigation found that the mains supply, relayed to the dam via the pump station, was operating at 265 volts and was subject to surges as the pump switched on and off. The lesson learned was that surge protection had to be provided to anything that would be connected to mains power with surge protection. After surge protection was installed at each logger and the supply voltage for the pumps was restored to the proper level, no further problems occurred.
Protection for Telephone Lines
The loggers were eventually connected to a modem and a telephone line so that data could be retrieved by a remote PC. At the time the modem was installed, there were no Telstra (telephone company) approved devices for protecting a modem, so the telephone line leading to the modem was not protected. Within the next six months, the telephone line brought four transient events to the modem, destroying the modem and causing logger lockups. Shortly afterwards, modem protection devices came onto the market and when one was installed, problems with telephone telemetry were reduced.
The cost of lighting protection can be high, and there are no guarantees that lightning protection will work in every case. However, there is some empirical evidence that the system installed at New Victoria Dam works well. Since its implementation in 1991 and 1992, there have been many documented lightning strikes, but no sensors or equipment have been lost. Similar successes with the six-point system have been experienced at other dams as well.
Thanks to Tony Moulds of Geotechnical Instrumentation Services and Tony Watson of Water Corporation of Western Australia for contributing the information for this story. Photos are courtesy of Water Corporation and Tony Watson. The six point plan graphic is courtesy of ERICO Products Ltd.
Moulds, A and Watson, A: Lightning Protection for Dam Instrumentation, a Case Study of the New Victoria Dam, presented at the 1998 ANCOLD/NZCOLD Conference on Dams in Sydney, Australia and published in the conference papers. This is a PDF file.
Edwards, DW and Wherett, PM: A Six Point Protection Approach for Lighting Protection, Surge Protection, and Single Point Ground for Low Voltage Facilities. This is the source of the six-point plan mentioned in the paper above. This paper and others are available in PDF form from www.erico.com .
Shoup, Dale: Sensors in the Real World, Protecting Geotechnical Sensors and Cable from Lighting Damage. This paper concentrates on buried sensors and complements the two papers above.
Australian Standard NZS/AS 1768-1991: This document on lighting protection can be purchased at www.standards.com.au