Page 24 - Offshore Electrical Engineering Manual

Description of a Typical System 11

platform generation and distribution system via transformers. This main switchboard
then supplies a series of motor control centres, one or more of which contains a series
of silicon controlled rectifier (SCR) DC variable speed drive controllers. By using an
arrangement of DC contactors, these controllers may be assigned to various DC drive
motors from the driller’s console. As the SCR systems are phase angle controlled, a
certain variable amount of harmonics is generated, depending on the kind of drill-
ing operation being carried out. When the drilling rig is being fed from the platform
supply, the harmonics may affect certain sensitive equipment, such as secure supply
invertors. This subject is discussed in detail in PART 4 Chapter 9.