Page 102 - Offshore Electrical Engineering Manual

CHAPTER
7
Direct Current Switchgear

INTRODUCTION
New variable drive equipment is likely to be via variable-frequency inverters, and
therefore this chapter applies mainly to existing equipment found on offshore
installations.
The direct current (DC) switchgear used offshore is unlikely to be of very high
rating, but there are a number of areas where it is likely to be used as follows:

Drilling equipment 1
• Rotary table motor drives
• Drawworks drives
• Top drill drives
• Mud pump drives
• Cement pump drives
DC-secure power supplies
• Level zero isolation for batteries
Floating installations
• Thruster drives
• Anchor winches

This list is typical, not comprehensive, as every installation will be different.

SWITCHING DIRECT CURRENTS

Switching in DC circuits is more onerous because unlike alternating currents, there
is no natural current zero at any time, and it is invariably necessary to break the
full voltage by increasing the arc resistance using suitably rated arc chutes to cool
and quench the arc. In other words, the DC must be forced to zero by the circuit
breaker.
In most designs the fault current is used in blowout coils to magnetically pull the
arc into the arc chutes, so that the higher the fault current the stronger the magnetic
field produced. In early designs, there tended to be a minimum ‘critical current’
required to pull the arc into the arc chute, and so ‘low’ fault currents could leave the

1 As mentioned earlier, in more modern rigs, inverter-fed variable-frequency induction motor drives
will be used.

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