The street scene shows damage caused by an explosion in a manhole as a result of gases generated by a low-voltage arcing fault.
North America, according to statistics published by the IEEE Insulated Conductors Committee, more than 32,000 km (19,884 miles) of low-voltage underground cables are installed annually. However, an Insulated Conductors Committee survey confirmed that some two-thirds of U.S. utilities are concerned about the reliability of their low-voltage underground cable networks.
The foremost cause of cable failures on the Hydro-Québec low-voltage underground cable system, where cables are installed in ducts, is attributable to mechanical damage, likely dating back to when the cable was installed. The damaged insulation in a conducting medium, such as contaminated water, may result in the development of an electrical arcing fault. This may then damage adjacent cables or equipment, jeopardize the security of the utility network and compromise public safety.
In certain conditions, the gases generated by the materials adjacent to the arcing fault can even result in violent explosions. A number of such major events occur in North America annually. Although these events are relatively infrequent on Hydro-Quebec’s low-voltage distribution system, their consequences could be serious for public safety and for the installations. The protection systems installed on the network, such as fuses and circuit breakers, are ineffective at eliminating this type of fault, which is often intermittent, develops randomly and may occur over a long period.
Several studies have concluded that there is no effective, economical, readily applicable method of verifying the condition of the insulation once the cables have been installed in ducts or directly buried. Dielectric tests are not effective for detecting broken insulation in a dry environment for example in cable ducts. Therefore to eliminate this problem a method is clearly needed to verify the mechanical integrity of the electrical insulation of cables particularly when installed in ducts before the circuit is commissioned. Hydro-Québec decided to develop a new test technique in collaboration with partners Dow Chemical and ndb Technologies.
The Pneumatic Test
The principle of the new insulation test technique for low-voltage cables consists of injecting compressed air into the cable core and then monitoring the pressure changes over a definite time period. A break in the insulation creates a leak, which is then detected by the drop in air pressure inside the cable. Based on this principle, an original test procedure was developed that optimizes the test duration and reliability of the diagnosis. The test is a two-step process: air injection and diagnosis. Compressed air p1is first injected at one end of the cable until the pressure p2at the other end starts rising. The time required for pressure p2starting to rise is specific to each cable. It increases with cable length and strand compaction, and decreases with conductor size. Then compressed air is injected simultaneously at both ends of the cable. The diagnosis “leak/no leak” is based on the pressure drop over time, which is monitored from the moment air injection ends. The test duration is a function of the specific time of each cable that is detected at the first step. This technique is applicable to bundled cables with several phase conductors that can be connected in a loop. However, a procedure for single-phase cables also is available.
The advantage of connecting several cables in a loop is that the ends of the loop are in the same location, making it possible to inject air and measure the pressure at both ends. In addition, all phases can be checked simultaneously, which considerably reduces the time required to perform the test.