Subject: On-line Partial Discharge Testing of Medium voltage Cable
Performance Contract: Independent 3rd Part (NETA) Testing Agency
Purpose: to proactively Test and Measure the structural integrity of the MV cable’s insulation.
Case Study: Hotel Casino in Las Vegas loses entire facility power.
First, it is essential to draw attention to the fact that there are situations that occur where facilities maintenance managers have to arbitrate between various types of equipment maintenance services, and often, their choices are stunted due to a famished monetary budget. Equally constricting for a facilities maintenance manager is the challenge of persuading their Chief Financial Officer(s) for funding of the critical maintenance requirements of a facility, which are imperative for preemptively safeguarding against downtime.
Sometimes, a facility may seem functional to the less than keen eye; however, as the following story purports, in certain circumstances, it may be less appealing for the company to swallow the maintenance bills rather than risk downtime, but the potentially disastrous and often times potentially dangerous outcomes are the even bigger pill to swallow!
An example of such a failure in what may be considered uptime roulette occurred in a renowned hotel located on the Las Vegas strip. While leaving the hotel anonymous, the cautionary tale proceeds as follows:
Hotel Anonymous, a state-of-the-art facility, implemented a multitude of redundancy and contingency efforts to fortify the facility from calamitous events: Two separate vaults fed the building along with the installation of generators, UPS Systems, and Paralleling Switchgear to protect the facility from downtime. The only flaw with the hotel’s layout dealt with the side-by-side location of the vaults and the 15kV Medium Voltage Cable.
A new concept was introduced to Hotel Anonymous based on electrical theory. The new concept called for placing a monitoring device on the MV cable to ‘scope’ downstream (approximately 200 feet) of the cable to check its condition. Note that IEEE and NEC prescribe ‘Over-Potential’ (Hi-POT) testing and then incrementally for routine maintenance tests; however, there is debate about this type of testing causing damage to the insulation of the electrical cable causing what the industry nicknamed ‘electrical trees’ or ‘bow-ties’ where the patterns are perceived to be what they are named. The idea was to perform the ‘On-line Partial Discharge Testing’ as a noninvasive way to determine if the testing activity was causing additional structural damage to the cable.
It is speculated that during construction the cable may have been ‘nicked’ by a backhoe or a forklift during the installation phase of the building project. We may want to highlight, though, that this is an additional reason why the bend radius is critical for designing pathways through geometric changes throughout the facility.
The ‘Scoping’ conducted during the On-Line Partial Discharge Testing is designed to detect anomalies which can be recognized by some proprietary software that illustrates the sinusoidal waves much like an oscilloscope. This testing is relatively expensive and may be considered too large a pill to swallow for most people responsible for managing the CAP-EX and OP-EX budgets. In retrospect, it would have been far less costly for the hotel to pay for the testing than the eventual catastrophic event that took place a few years after the building was commissioned.
There was a photograph taken at night from a bird’s eye view after the Hotel was downed from the catastrophe showing the beautifully lit Las Vegas strip. Everything was illuminated except for a huge dark square which was where the hotel resided.
Water transitions to steam when it reaches its boiling point of approximately 212 degrees Fahrenheit while expanding at approximately 200% volume. Copper on the other hand, which is the typical metal used as for conductors, expands at a mind-blowing rate of 32,000%! Needless to say, when the heavy manhole covers sailed into the air (anywhere from 10 to 30 feet high), and the explosion of the MV Cable took out the second vault which was laying in close proximity to the defective cable, it did not matter there were Generators and UPS Systems for protection as the event took place upstream between the gear and the transformer.
If one were to calculate the number of people on the property who needed to have their rooms ‘comped’, and the number of dollars the Hotel lost in downtime, safety issues, repairs, and possible lawsuits, it makes the initial fee of the testing seem like a worthwhile investment as opposed to an investment to avoid.
I would recommend taking a longer look at the preventative maintenance options before being too quick to say “No” to the service provider!
Below is a PDF further explaining Non-invasive Electrical Testing: