Methods and apparatus for limiting electrical current to a subsea petroleum installation

In a subsea oil production system including a plurality of oil well assemblies, a subsea electrical power distribution module for supplying alternating current to said oil well assemblies, and a power cable extending from said distribution module to one of said oil well assemblies, the improvement comprising:

• current sensing means for generating a first signal representative of the current amplitude in a current conductor of said power cable;
• logic means responsive to said first signal for generating a two-level control signal;
• protector means for limiting the current in said power cable, said protector means including
• (a) a first winding connected in series with said current conductor,
• (b) a second winding inductively coupled to said first winding, and
• (c) automatic variable impedance means responsive to said two-level control signal for substantially short circuiting said second winding when the current amplitude in said conductor becomes lower than a predetermined minimum, and thereafter substantially open circuiting said second winding when said current amplitude becomes substantially higher than a predetermined maximum;
• and with said logic means including
• means for providing a high reference signal related to said predetermined maximum and a low reference signal related to said predetermined minimum, and
• comparison means responsive to said first signal and said reference signals for producing a control signal of a first value when said first signal becomes larger than said high reference signal, maintaining said control signal at said first value irrespective of variations of said first signal until said first signal becomes smaller than said low reference signal, and producing a control signal of a second value from the time that said first signal becomes smaller than said low reference signal until said first signal again becomes larger than said high reference signal.

In a subsea oil production system which includes a plurality of distantly-spaced oil well assemblies each comprising equipment that requires electrical power, and a subsea electrical power distribution module comprising alternating current apparatus for supplying power to a plurality of said oil well assemblies, the combination of:

• a plurality of power cables extending respectively from said module to different ones of said oil well assemblies, with each of said power cables including at least one inductive coupling device which exhibits a relatively low terminating impedance when it is de-coupled and is vulnerable to being damaged if subjected to alternating currents exceeding a predetermined maximum;
• and
• current limiting means located adjacent said alternating current apparatus for sensing the current amplitude in a current conductor of one of said power cables and selectively introducing into and removing from said current conductor an auxiliary impedance of a magnitude sufficient to limit the conductor current to said predetermined maximum, said current limiting means comprising
• (a) sensing means for producing a line-current-representative signal representative of the current flowing in said current conductor,
• (b) means for providing a first reference signal and a second reference signal representative respectively, of a predetermined higher than normal power-cable current and a predetermined lower than normal power-cable current,
• (c) a transformer comprising a primary winding connected in series with said current conductor and a secondary winding inductively coupled to said primary winding,
• (d) comparison means for repeatedly comparing said line-current-representative signal with said first reference signal and with said second reference signal,
• (e) means for substantially open-circuiting said secondary winding automatically when said line-current-representative signal becomes larger than said first reference signal, and
• (f) means for thereafter substantially short-circuiting said secondary winding automatically when said line-current-representative signal becomes smaller than said second reference signal.

In a subsea oil production system including a plurality of oil well assemblies, a subsea electrical power distribution module for supplying polyphase alternating current to said oil well assemblies, at least first and second polyphase power cables extending respectively from said distribution module to first and second ones of said oil well assemblies, and at least one inductive coupling device connected in series with said first power cable, the improvement comprising:

• current sensing means for generating a first signal representative of the current amplitude in at least one of the phase conductors of said first power cable;
• logic means responsive to said first signal for generating a two-level control signal;
• and
• protector means responsive to said control signal for limiting the current in said first power cable, said protector means including
• (a) a plurality of first windings each connected in series with a respective one of the several phase conductors of said power cable,
• (b) a plurality of second windings each inductively coupled to a respective first winding, and
• (c) automatic variable impedance means responsive to said control signal for substantially short circuiting each of said second windings when the current amplitude in said at least one of said phase conductors becomes lower than a predetermined minimum, and thereafter substantially open circuiting each of said second windings when said current amplitude becomes higher than a predetermined maximum.

Apparatus for limiting current in an alternating current power system for supplying electrical power to a load, comprising:

• a primary winding connected in series with a current conductor of said system,
• a secondary winding inductively coupled to said primary winding;
• a current sensor for sensing the current amplitude in said power system and producing a first signal representative of said current amplitude;
• means for providing a first reference signal and a second reference signal representative, respectively, of a predetermined higher than normal power system current and a predetermined lower than normal power system current;
• comparison means for repeatedly comparing said first signal with said first reference signal and said second reference signal;
• means for substantially open-circuiting said secondary winding automatically in response to said comparison means when said first signal becomes larger than said first reference signal;
• and
• means for thereafter substantially short-circuiting said secondary winding automatically in response to said comparison means when said first signal becomes smaller than said second reference signal.