Fiber-optic current sensing using a sensor with exchangeable sub-modules

A method for operating a fiber-optic current sensor measuring a current I in a conductor, wherein the current sensor includes:

• a sensor head having a sensing fiber wound around the conductor and a retarder connected to the sensing fiber;
• a measuring unit having a light source and a light detector;
• and
• a polarization maintaining fiber connecting the sensor head to the measuring unit,
• wherein the method comprises:
• sending light from the light source through the sensor head;
• measuring, by means of the light detector, a signal S derived from light returning from the sensor head;
• and
• calculating the current I from
• I=(S/g)·fe′(S)·fs′(S)
• wherein:
• fe′(S) is a scaling function of the measuring unit depending on the signal S;
• fs′(S) is a scaling function of the sensor head depending on the signal S;
• and
• g is a constant independent of the signal S,
• wherein the measuring unit scaling function fe′ is a function of the signal S as well as of temperature Te, wherein the measuring unit scaling function fe′ is determined by:
• measuring, for at least one current value, the signal S as a function of current at a given temperature To, to obtain the measuring unit scaling function fe′(S, To);
• measuring, for at least one temperature other than the given temperature To, the signal S at a given current Io, to obtain the function fe′ (So, Te);
• and
• approximating the measuring unit scaling function fe′ by fe′(S, Te)=fe′(S, To)·fe′(So, Te)/fe′(So, To).

A method for operating a fiber-optic current sensor measuring a current I in a conductor, wherein the current sensor includes:

• a sensor head having a sensor fiber wound around the conductor and a retarder connected to the sensing fiber;
• a measuring unit having a light source and a light detector;
• and
• a polarization maintaining fiber connecting the sensor head to the measuring unit,
• wherein the method comprises:
• sending light from the light source through the sensor head;
• measuring, by means of the light detector, a signal S derived from light returning from the sensor head;
• and calculating the current I from I=(S/g)·fe′(S)·f′(S) wherein:
• fe′(S) is a scaling function of the measuring unit depending on the signal S;
• fs′(S) is a scaling function of the sensor head depending on the signal S;
• and
• g is a constant depends on the signal S,
• wherein the sensor head scaling function fs' is approximated by fs′(S)=1/[fe(S/k)], with fe=Δφ(4φF), k is a constant, and
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• unit, and wherein 4φF denotes a magneto optic phase shift in the sensing fiber, (2φF)2 denotes a Faraday phase shift, δ and δi denote the birefringent phase retardation in the sensing fiber due to bend-induced and intrinsic birefringence, respectively, β and βi are the angles between a principal axis of the polarization-maintaining fiber and slow axes of a bend-induced and an intrinsic birefringence, respectively, of the sensing fiber, “ε” is an offset of a retarder from 90° retardation.

A method for operating a fiber-optic current sensor measuring a current I in a conductor, wherein the current sensor includes:

• a sensor head having a sensing fiber wound around the conductor and a retarder connected to the sensing fiber;
• a measuring unit having a light source and a light detector;
• and
• a polarization maintaining fiber connecting the sensor head to the measuring unit,
• wherein the method comprises:
• sending light from the light source through the sensor head;
• measuring, by means of the light detector, a signal S derived from light returning from the sensor head;
• and
• calculating the current I from
• I=(S/g)·fe′(S)·fs′(S)
• wherein:
• fe′(S) is a scaling function of the measuring unit depending on the signal S;
• fs′(S) is a scaling function of the sensor head depending on the signal S;
• and
• g is a constant independent of the signal S,
• wherein the measuring unit comprises a phase modulator, and
• wherein the sensing head scaling function fs′ is determined by:
• connecting the sensing head to a measuring unit having a known measuring unit scaling function fe′;
• operating the modulator of the measuring unit for introducing a phase shift between light polarizations generated by the measuring unit, thereby generating interference fringes at the light detector;
• and
• measuring an interference fringe visibility of the interference fringes.

A fiber-optic current sensor for measuring a current I in a conductor, the sensor comprising:

• a sensor head having a sensing fiber wound around the conductor and a retarder connected to the sensing fiber;
• a measuring unit including a light source and a light detector;
• and
• a polarization maintaining fiber connecting the sensor head to the measuring unit;
• a first memory attributed to the measuring unit for storing the measuring unit scaling function fe′;
• and
• a second memory attributed to the sensor head for storing the sensor head scaling function fs′, wherein:
• the light source is configured to send light through the sensor head;
• the light detector is configured to measure a signal S derived from light returning from the sensor head, and
• the current I is calculated from
• I=(S/g)·fe′(S)·fs′(S)
• wherein:
• fe′(S) is a scaling function of the measuring unit depending on the signal S;
• fs′(S) is a scaling function of the sensor head depending on the signal S;
• and
• g is a constant independent of the signal S,
• wherein the measuring unit is configured to measure a phase shift Δφ in the sensing fiber of the sensor head, and the sensor head scaling function fs′ is defined by Δφ/(4φF), where 4φF is a defined phase shift.

A method for operating a fiber-optic current sensor measuring a current I in a conductor, wherein the current sensor includes:

• a sensor head having a sensing fiber wound around the conductor and a retarder connected to the sensing fiber;
• a measuring unit having a light source and a light detector;
• and
• a polarization maintaining fiber connecting the sensor head to the measuring unit
• wherein the method comprises:
• sending light from the light source through the sensor head;
• measuring, by means of the light detector, a signal S derived from light returning from the sensor head;
• and
• calculating the current I from
• I=(S/g)·fe′(S)·fs′(S)
• wherein:
• fe′(S) is a scaling function of the measuring unit depending on the signal S;
• fs′(S) is a scaling function of the sensor head depending on the signal S;
• and
• g is a constant independent of the signal S,
• wherein a phase shift Δφ is measured in the sensing fiber of the sensor head, and the sensor head scaling function fs′ is defined by Δφ/(4φF), where 4φF is a defined phase shift,
• wherein the method comprises determining the measuring unit scaling function fe′ independently from the sensor head scaling function fs′, and
• wherein the measuring unit scaling function fe′ is a function of the signal S as well as of temperature Te, wherein the measuring unit scaling function fe′ is determined by:
• measuring, for at least one current value, the signal S as a function of current at a given temperature To, to obtain the measuring unit scaling function fe′(S, To);
• measuring, for at least one temperature other than the given temperature To, the signal S at a given current Io, to obtain the function fe′ (So, Te);
• and
• approximating the measuring unit scaling function fe′ by fe′(S, Te)=fe′(S, To)·fe′(So, Te)/fe′(So, To).

A method for operating a fiber-optic current sensor measuring a current I in a conductor, wherein the current sensor includes:

• a sensor head having a sensing fiber wound around the conductor and a retarder connected to the sensing fiber;
• a measuring unit having a light source and a light detector;
• and
• a polarization maintaining fiber connecting the sensor head to the measuring unit,
• wherein the method comprises:
• sending light from the light source through the sensor head;
• measuring, by means of the light detector, a signal S derived from light returning from the sensor head;
• and
• calculating the current I from
• I=(S/g)·fe′(S)·fs′(S),
• wherein:
• fe′(S) is a scaling function of the measuring unit depending on the signal S;
• fs′(S) is a scaling function of the sensor head depending on the signal S;
• and
• g is a constant independent of the signal S,
• wherein the measuring unit scaling function fe′ is approximated by fe′(S)=1/[fe′(S/k)], with fe=Δφ/(4φF),
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• 4φF=4·V·N·I, where V is the Verdet constant of the sensing fiber, and N is the number of sensing fiber loops around the conductor,
• wherein k is a constant, and h is a measured parameter based on an interference fringe visibility of the light detector of the measuring unit.

A method for operating a fiber-optic current sensor measuring a current I in a conductor, wherein the current sensor includes:

• a sensor head having a sensing fiber wound around the conductor and a retarder connected to the sensing fiber;
• a measuring unit having a light source and a light detector;
• and
• a polarization maintaining fiber connecting the sensor head to the measuring unit,
• wherein the method comprises:
• sending light from the light source through the sensor head;
• measuring, by means of the light detector, a signal S derived from light returning from the sensor head;
• and
• calculating the current I from
• I=(S/g)·fe′(S)·fs′(S),
• wherein:
• fe′(S) is a scaling function of the measuring unit depending on the signal S;
• fs′(S) is a scaling function of the sensor head depending on the signal S;
• and
• g is a constant independent of the signal S,
• wherein the measuring unit comprises a phase modulator,
• wherein the measuring unit scaling function fe′ is determined by:
• connecting the measuring unit to a reference sensor head having a known sensor head scaling function fs′;
• measuring the signal S for at least one current sensed by the reference sensor head;
• operating the modulator for introducing a phase shift between light polarizations generated by the measuring unit, thereby generating interference fringes at the light detector;
• and
• measuring an interference fringe visibility of the interference fringes.

A fiber-optic current sensor for measuring a current I in a conductor, the sensor comprising:

• a sensor head having a sensing fiber wound around the conductor and a retarder connected to the sensing fiber;
• a measuring unit including a light source and a light detector;
• and
• a polarization maintaining fiber connecting the sensor head to the measuring unit;
• a first memory attributed to the measuring unit for storing the measuring unit scaling function fe′;
• and
• a second memory attributed to the sensor head for storing the sensor head scaling function fs′, wherein:
• the light source is configured to send light through the sensor head;
• the light detector is configured to measure a signal S derived from light returning from the sensor head, and
• the current I is calculated from
• I=(S/g)·fe′(S)·fs′(S)
• wherein:
• fe′(S) is a scaling function of the measuring unit depending on the signal S;
• fs′(S) is a scaling function of the sensor head depending on the signal S;
• and
• g is a constant independent of the signal S,
• wherein the measuring unit scaling function fe′ is a function of the signal S as well as of temperature Te, wherein the measuring unit scaling function fe′ is determined by:
• measuring, for at least one current value, the signal S as a function of current at a given temperature To, to obtain the measuring unit scaling function fe′(S, To);
• measuring, for at least one temperature other than the given temperature To, the signal S at a given current Io, to obtain the function fe′(So, Te);
• and
• approximating the measuring unit scaling function fe′ by fe′(S, Te)=fe(S, To)·fe(So, Te)/fe′(So, To).