Tech Tidbits, vol. 2 - Potential Electrical Protective

Relay Problems

Every piece of electrical equipment on every rig is installed with a device or devices of some type to

prevent damage, ranging from the simplest, fuses, to complex protective relays. These devices are

used to isolate equipment from the problem when conditions exist that can cause damage, including

under and over voltage, current, frequency, load imbalance and some combinations of the above.

While conducting generator testing on one rig recently, a generator tripped off-line on two occasions

due to differential current. The load imbalance in this case was a difference of 1.65 MW between

transformer phases, which would cause a fire. However, both times, no problem was identified. The

drilling contractor contacted the manufacturer, GE, and learned of potentially defective protective

equipment.

WEST confirmed that differential current tripping problems

have been documented from Multilin SR 489 relays that were loaded with a particular version of

firmware. Only relays with certain serial numbers are affected. Accordingly, those rigs that utilize

these Multilin SR 489 protective relays should review the relay serial numbers to understand

susceptibility to false trips from their installed firmware. Two of the six installed on the referenced rig,

including the one responsible for the false trips, fit the criteria requiring the upgraded firmware. Please

see the GE Multilin Service Bulletin for more information.