by John Eder
A common frustration faced today by oil drilling professionals is the lost productivity and high maintenance costs attributed to electrical harmonics. A highly reliable and high quality power supply infrastructure is of vital importance to the success of oil and gas drilling operations. Proper grounding of electrical circuits, in conjunction with current harmonic minimizing techniques, can significantly reduce the costs associated with downtime and equipment failure caused by electrical harmonics.
Electrical harmonics are typically the result of non-linear loads on the power system. On oil drilling rigs, the bulk of the loads are non-linear in nature. These include top drives, mud pumps, centrifuges, etc. Additionally, the increase in variable frequency drive (VFD) use is becoming a major source for electrical harmonics.
High levels of electrical harmonic distortion can cause a multitude of problems, including transformer, motor or generator overheating, malfunction of electronic control equipment, interference with telephone circuits, etc. All of these compromise the efficiency of the operation.
Less intuitive is the fact that electrical disturbances on the power system can also result in catastrophic mechanical failure, specifically with regard to induced shaft voltages and bearing currents. Shaft voltage seeks a complete circuit through its bearings to ground, or through its outboard bearing and the connected machinery. Unless prevented from reaching high levels it can cause chemical changes in the insulating grease, breaking it down and thereby making the grease act like an electrolytic in a capacitor. Initially, this results in pitting within the raceways of the bearing. Over time, this leads to fluting and eventually complete bearing failure. Shaft voltage is a common phenomenon in all motors, but left unchecked and/or uncontrolled it can wreak havoc on operations.
Another phenomenon with VFDs is something called voltage reflection. Insulated gate bipolar transistors (IGBTs) let adjustable-speed drives turn voltage on and off 18,000 to 20,000 times a second. To do this means the voltage rise time is short, usually less than a microsecond. These short rise times combined with long power lines between the drive and controller can produce voltage reflections, also called reflected waves that have high peak voltages. If the voltages are large enough, they will generate potentially destructive stresses in the motor insulation. This phenomenon is not widespread, but users should be aware of it nonetheless.
While there are various methods to combat harmonic distortions within a system, the fundamental importance of proper grounding should not be overlooked. One of the largest potential causes of motor bearing damage due to bearing currents is inadequate grounding. The challenge is to provide a low impedance ground path for the voltage to flow to earth ground without passing through the motor bearings or other components. This can be accomplished by using insulated bearing and/or installing shaft grounding devices. Utilizing shielded cables between VFDs and motors is also beneficial.
Leviton is a leading provider of high-amperage electrical connectors used throughout the drilling rig site. Our Rhino-Hide® single pole connectors and panel receptacles are commonly used to connect top drives, mud pumps, etc. to the SCR house. In order to improve the integrity of the grounding system, we have also developed a patented ground pin that doubles as a mounting bolt for panel receptacles. This provides a convenient ground connection for the shielded cables used in this application. It also eliminates the need for separate ground lugs on the panel boards, where space is at a premium.
In addition to mitigating negative effects of electrical harmonics, let’s also remember that proper grounding of electrical circuits can provide invaluable protection against unwanted electrical noise such as spikes, surges, dips, and other known causes of circuit failure.