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

Patent metadata

Field	Value
Publication number	US-9310399-B2
Application number	US-201213492364-A
Country	US
Kind code	B2
Filing date	Jun 8, 2012
Priority date	Dec 11, 2009
Publication date	Apr 12, 2016
Grant date	Apr 12, 2016

Abstract

A fiber-optic current sensor includes a measuring unit having a light source and a light detector, and a sensing head having a sensing fiber wound around a conductor and a retarder connected to the sensing fiber. The scale factor as a function of current of the fiber-optic current sensor is described by the product of two scaling functions fe′ and fs′ for the measuring unit and the sensing head, respectively. The data describing the scaling functions fe′, fs′ is stored in a memory of the measuring unit, while the data describing the scaling function fs′ is also stored in a memory of the sensing head. Providing two such memory devices allows to store the scaling functions fe′ and fs′ separately, thereby turning the control unit as well as the sensor head into easily replaceable modules.

First claim

What is claimed is:

1. 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 )· f e ′( S )· f s ′( S )

wherein:
f e ′(S) is a scaling function of the measuring unit depending on the signal S;
f s ′(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 f s ′ is defined by Δφ/(4φ F ), where 4φ F is a defined phase shift.

2. The method of claim 1 , comprising:
determining the measuring unit scaling function f e ′ independently from the sensor head scaling function f s ′.

3. 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 )· f e ′( S )· f s ′( S )

wherein:
f e ′(S) is a scaling function of the measuring unit depending on the signal S;
f s ′(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 f e ′ is a function of the signal S as well as of temperature T e , wherein the measuring unit scaling function f e ′ is determined by:
measuring, for at least one current value, the signal S as a function of current at a given temperature T o , to obtain the measuring unit scaling function f e ′(S, T o );
measuring, for at least one temperature other than the given temperature T o , the signal S at a given current I o , to obtain the function f e ′ (S o , T e ); and
approximating the measuring unit scaling function f e ′ by f e ′(S, T e )=f e ′(S, T o )·f e ′(S o , T e )/f e ′(S o , T o ).

4. The method of claim 1 , wherein the measuring unit scaling function f e ′ is a function of the signal S as well as of temperature T e , and
wherein the measuring unit scaling function f e ′ is determined by:
measuring, for a plurality of current and temperature values, values of the signal S; and
storing the values of the measuring unit scaling function f e ′ in a look-up table.

5. The method of claim 1 , comprising: the step of determining the measuring unit scaling function f e ′ by:
connecting the measuring unit to a reference sensor head having a known sensor head scaling function f s ′, and
measuring the signal S for at least one current sensed by the reference sensor head.

6. The method of claim 5 , wherein the sensor head scaling function f s ′ of the reference sensor head is equal to 1, independent of the current I.

7. The method of claim 5 , wherein the measuring unit comprises a phase modulator, and wherein the measuring unit scaling function f e ′ is determined by:
connecting the measuring unit to 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.

8. The method of claim 7 , wherein the measuring unit scaling function f e ′ is approximated by f e ′(S)=1/[f e ′(S/k)], with f e =Δφ(4φ F ),

Δϕ
=

arctan
⁢

2
⁢
U

1
-

U
2

,

⁢

U
=

tan
⁢

⁢

(

2
⁢

φ
F

)

cos
⁢

⁢
γ

,

⁢

cos
⁢

⁢
γ

=

(

1
⁢

/

⁢
h

)

,

and
4φ F =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 the measured interference fringe visibility, and
wherein h is an offset from unit of function f e at temperature Te.

9. The method of claim 8 ,
wherein the measured parameter h is measured from the interference fringe visibility.

10. The method of claim 8 , wherein the measured parameter h is determined by:
connecting the measuring unit to a sensor head having a sensing head scaling function f s ′=1 and having a coil wound around a conductor;
sending the current I through the conductor for measuring the sensor signal S and determining the parameter h from the signal S.

11. The method of claim 8 , wherein the measured parameter h is determined by determining a polarization cross-talk in the measuring unit.

12. The method of claim 1 , comprising:
determining the sensor head scaling function f s ′ by connecting the sensing head to a measuring unit having a known measuring unit scaling function f e ′ and measuring the signal S by applying a range of currents and/or temperatures T s to the sensor head.

13. A method for operating a fiber-optic current sensor meas

Assignees

• Bohnert Klaus
• Frank Andreas
• Wüest Robert
• Abb Research Ltd

Inventors

• Bohnert Klaus
• Frank Andreas
• Wüest Robert

Classifications

• G01R35/00

  Testing or calibrating of apparatus covered by the other groups of this subclass · CPC title

• G01R15/246 Primary

  based on the Faraday, i.e. linear magneto-optic, effect · CPC title

• G01R15/247

  Details of the circuitry or construction of devices covered by G01R15/241 - G01R15/246 · CPC title