Page 124 - Offshore Electrical Engineering Manual

CHAPTER

Power Electronics 11

(Semiconductor

Equipment)

INTRODUCTION

Since the first edition of this book, solid-state devices have improved significantly in
their capability to handle higher powers and voltages.
Although thyristors are still in use for such applications as integral cycle fired
heater controllers, new variable-frequency converters operating alternating current
(AC) induction motors are now more likely to be utilised for drilling motor drives.
The heart of these drives is usually made up of a set of high-voltage insulated gate
bipolar transistors, using deionised water cooling.
Power MOSFETs (metal oxide semiconductor field-effect transistors) are now
commonly used in switched-mode power supplies for computers and small control
panels. Secure supply systems now have more intelligent monitoring, with better
diagnostics, so that, for example, the need for replacement of electrolytic capacitors
can be spotted earlier.

ENVIRONMENTAL CONDITIONS
These devices are able to operate satisfactorily in air-conditioned pressurised mod-
ules, using the same conditions as would be provided for controlgear and switchgear.
Where full convertor systems are used, harmonics can be regulated and kept
within industry-accepted limits.

UNINTERRUPTIBLE AND SECURE POWER SUPPLIES
Because of the need to maintain control of the process, and to provide electrical sup-
plies for statutory safety equipment such as navaids and marine and helicopter radios,
whether generation is available or not, a number of battery-fed secure supply systems
are required on every offshore installation.
The grouping of these supplies so that they are fed from a common battery/
charger system should be considered carefully to avoid common mode failures, i.e.,
situations where several vital supplies are lost owing to the failure of a common
component. Emergency lighting fed from a central battery is particularly prone to

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