It is incredibly hard to plan for low-probability, highly catastrophic events. Let’s face it, competitive pressures tend to have us focus on the here and now, but electric utilities are adept at addressing potentially catastrophic events. Our response to Hurricanes Harvey and Irma is a good example. In preparing for catastrophic events, we must not only respond quickly, we also must design our systems to mitigate potential damage.
My former thesis advisor, Bill Black, is leading an IEEE Insulated Conductors Committee Working Group focusing on the mitigation of manhole explosions. This work started about eight years ago. At that time, the issue was so timely that he never had a problem getting experts to come in to discuss the problem of manhole events. Attendance ranged from about 40 to 80 participants.
Two years ago, the discussion group transitioned into a working group to write the IEEE "Guide for Smoke, Fire and Explosions in Underground Electrical Structures." Bill is seeing a lot of cooperation from utilities, and particularly from manufacturers, who are designing systems to mitigate the effect of manhole explosions and fires. And they are making progress. For example, when the group first started to meet, Bill tells me there were no mitigation devices on the market, and now there are four or five.
For most utilities, maybe only one in 1000 manholes has an event in a year. But with so many manholes in the U.S., this adds up to approximately 2000 manhole events per year, or 5.5 events per day. We won’t find a silver bullet to remediate the risk of manhole events because the causes are diverse and the number of manholes in the system is so high.
If there is an event, the severity of the outcome can vary widely. A typical manifestation of a minor event is smoke coming from the manhole; worse is a fire that can produce large quantities of smoke and flames extending from the manhole. The most severe manhole event is an explosion caused by combustible gases being ignited within the manhole, generating sufficient pressure to fling the manhole cover in the air. Maybe one-third of these events results in a fire or explosion.
Root causes of manhole events often can be a result of overheating or electrical failure of a splice or termination within the manhole. A high impedance cable fault within the duct work or a failure of switchgear within the structure can initiate a manhole event. Surveys show that 75% of the events are of this type. Because the issues are complex and the causes are many, addressing manhole events is not a topic any one utility should tackle on its own.
Utilities have performed simulated tests to determine internal pressures inside full-scale buried manholes. Intentional faults staged in high-voltage solid-dielectric cables resulted in measured pressure increases of 7 psi to 9 psi. Manhole explosions have launched manhole covers several stories high and are obviously quite a danger. Analytical results compare well with computer-predicted values when input data is selected to simulate experimental conditions.
Of course, the best outcome is to prevent an event from happening in the first place. This requires utilities to identify and eliminate cable designs that could lead to a manhole event; reduce the energy magnitude and duration generated by a fault with protective equipment; and ensure underground systems are operated within their design limits.
Field crews can also use thermal imaging to locate overheated cables and joints. Thermal imaging equipment can provide a means to reduce the probability of manhole events by instituting a replacement program for aging, overheated and degraded components. Even relatively new equipment that has been operated under harsh environmental conditions or heavily loaded conditions should be identified and considered for replacement. Test methods such as high-voltage AC partial-discharge tests, jacket integrity tests, megohmmeter tests and contact voltage tests may be considered to locate poor performing cables.
Today, OSHA-driven worker safety standards mandate the use of atmosphere analysis equipment prior to work crews entering a manhole or other enclosed space. If too much or too little oxygen is present, or if flammable gases or potential toxic air contaminants are detected, the source must be determined and eliminated before personnel are allowed to enter the manhole. Portable gas detectors suitable for the detection of combustible gases inside manholes are available from several manufacturers.
If arcing in a manhole can be detected before it can generate sufficient combustible gases, and before the gases reach a dangerous level, then an early warning of potential faults or gas explosions could reduce or eliminate some manhole events. A prototype piece of equipment has been developed and undergone initial testing. The device has the potential to recognize possible arcing from defective cables and accessories.
Vented duct systems are also now available that allow air to be collected from all levels of the manhole and expelled out of the manhole cover. Still some events could produce sufficient gases to move the cover. Specialty manhole covers are available to vent high-pressure gases that are generated inside the manhole while restraining the cover so that it does not become a projectile. ♦
Editor’s note: Bill Black is looking for more input for IEEE’s "Guide for Smoke, Fire and Explosions in Underground Electrical Structures," and the working group is encouraging increased participation. To get involved, please contact Bill at [email protected].