Situation
An oil drilling company, based in Calgary, Alberta, builds and operates oil and natural gas drilling rigs that are used all over North America.
The Problem of Ground Faults
Many drill rigs are powered by up to four generators connected in parallel, and historically have used ungrounded electrical systems. An ungrounded system does not need to shut down when a ground fault occurs on one phase; the faulted phase goes to zero volts with respect to ground. The other two phases go to full phase-to-phase voltage above ground, but there are no large currents, nothing trips out, and the system continues to operate. Such a system has a number of serious disadvantages. Intermittent ground faults can create transient over-voltages throughout the system, which can endanger personnel and cause damage, including insulation failures on motors. And because there is no ground-fault current, when a ground fault occurs on an ungrounded system, it can be very difficult to locate — so difficult, in fact, that some users simply ignore a ground fault and keep operating. This works until a second ground fault occurs on another phase, at which point there is a phase-to-ground-to-phase fault with all the damage that can result.
Ground faults are common events on drill rigs. The rigs get very dirty, explains the company’s Electrical Manager, and operators wash them before moving them; there’s also rain and melting snow. “And then when they move,” he says, “that’s when you see ground faults.”
The primary method for locating a ground fault on an ungrounded system is a three-light indicator: three bulbs in a wye configuration, with one bulb connected to each phase and the center point connected to ground (on a 480 V system the bulbs are generally pairs of 240-volt bulbs wired in series). Normally the bulbs will glow dimly, but if one phase faults to ground, its bulb will go out and the other two will light up brightly. This indicates that a fault exists, but it gives no information about its location.
The Solution
The best solution to the problems of an ungrounded system is to switch to a high-resistance grounded (HRG) system in which the system neutral point is connected to ground through a neutral grounding resistor (NGR). (On a wye-connected system, it is the actual neutral; on a delta-connected system, it is a neutral created using a zig-zag transformer.) When a phase faults to ground, the neutral point rises to phase-to-neutral voltage above ground and a defined current flows through the resistor— usually about 5 amps. Because the system is no longer floating, the problem of transient overvoltages goes away. Also, the current through the NGR can be monitored to indicate that a ground fault is present, and to find the fault location.
Looking for alternatives, the company began considering switching to an HRG system for its drilling rigs in 2012 and they received a recommendation to contact Littelfuse. It started with one rig, putting in a zig-zag transformer and an NGR, and adding a Littelfuse SE-330 Neutral Grounding Resistor Monitor to detect ground faults and monitor the health of the resistor. They also installed a Littelfuse SE-701 Ground Fault Monitor on each of the main feeders to indicate which feeder has a ground fault. Both the SE-330 and the SE-701 connect to the rig’s control system to alert the operator and log the event.
With this combination of devices, any ground fault will be immediately indicated with an alarm on the operator’s display; the operator can tell not only the magnitude of the fault (anything from a current leak to a solid short) but also on which feeder it has occurred.
When a ground fault occurs and the affected feeder has been identified, the next step is to find which load has the actual fault. It would be possible to do this by checking each with a clamp-on ammeter, but because that would involve opening live electrical cabinets, the maintenance people generally prefer to simply turn off loads one at a time until the three lights on the local PGR-3100 all turn back on.
