Page 120 - Offshore Electrical Engineering Manual

CHAPTER
10
Motor Control Gear

INTRODUCTION
This chapter deals with the design and operation of motor control gear but assumes
some familiarity with low-voltage switchgear and motor control centres (MCCs).
Motor protection is covered in PART 4 Chapter 3 and requirements for large pro-
cess drives are covered in PART 2 Chapter 12.

LOW-VOLTAGE SWITCHGEAR AND MOTOR CONTROL CENTRES
FOR OFFSHORE USE

It is recommended that low-voltage switchgear and control gear for use offshore
should be continuously rated for operation at the maximum ambience with module
ventilation failed. This may require allowing a little more space in the switchroom
but will provide operators with more flexibility during upset conditions. In highly
ambient climates, this may not be possible, in which case some form of backup ven-
tilation for critical switchrooms is recommended.
Other design issues which will help operators are as follows:

• Contactor thermal ratings should not include any ‘heat sink’ effects obtained by
using oversized cables.
• Ensure (by testing if necessary) that contactor coils will maintain closure of
contactors down to at least 80% of nominal voltage.
• Provide reacceleration timers with adjustment facilities so that the timers may
be set based on the results of fault clearance simulation studies.
• In low-voltage switchgear, arc faults may be prolonged and damaging.
Therefore arc fault detection is recommended, such as ABB’s REA or Siemens’
Siprotec systems.

Star delta starters are frequently used offshore to reduce transient currents, in the
same way as they are used onshore. However, because of the additional switching
involved, they are less reliable and may require more maintenance.
Auto transformer starting are used offshore but should be used sparingly because
of the additional weight and space they take up. Using an electronic soft starter may
be a better option.

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