Apparatus and method for quantizing remaining lifetime of transmission cable insulation

• US 5,241,184 A
• Filed: 09/26/1991
• Issued: 08/31/1993
• Est. Priority Date: 09/26/1991
• Status: Expired due to Fees

• Current Assignee: Electric Power Research Institute Incorporated
• Original Assignee: Electric Power Research Institute
• Inventors: Menzel, Erhard R.
• Primary Examiner(s): Fields, Carolyn E.
• Assistant Examiner(s): NOT, DEFINED
• Application Number:
  US07/766,653
• Time in Patent Office: 705 Days
• Field of Search: 250/458.1, 250/459.1, 250/461.1, 250/362, 250/302
• US Class Current: 250/458.1
• CPC Class Codes: G01N 2021/6417 Spectrofluorimetric devices
• CPC Class Codes: G01N 21/64 Fluorescence; Phosphorescence
• CPC Class Codes: G01N 21/952 Inspecting the exterior surface of cylindrical bodies or wires G01N21/956 takes precedence

Abstract

The remaining life of transmission cable insulation is quantized by irradiating the insulation with excitation energy causing it to emit a bulk fluorescence spectrum. The intensity of the bulk fluorescence spectrum is then measured at at least two wavelengths and an intensity ratio determined at these wavelengths. The magnitude of the resultant ratio correlates with the age and condition of the insulation under test, and allows estimation of the remaining transmission cable insulation lifetime. Such measurements may be made in situ if the transmission cable includes a projecting fiber optic cable having first and second ends. The first end of the fiber optic cable is embedded within the cable so as to touch the insulation, and the second end is adapted to receive energy causing the bulk fluorescence, and is further adapted to optically communicate the bulk fluorescence to an optical measurement system that determines intensities and ratios. In this fashion, the remaining lifetime of insulation within a transmission cable may be estimated, even though the transmission cable remains buried underground.

Claims

1. A method for quantizing the remaining life of insulation in a transmission cable, the method comprising the steps of:

irradiating the insulation with excitation energy at a first wavelength causing the insulation to emit a bulk fluorescence spectrum containing at least second and third wavelengths longer than said first wavelength;

measuring intensity of said bulk fluorescence spectrum at said second and third wavelengths;

determining a ratio of the measured intensities at said second and third wavelengths;

estimating from said ratio the remaining transmission cable insulation lifetime.

2. The method of claim 1, wherein said step of irradiating is performed by a laser.

3. The method of claim 1, wherein said first wavelength is about 514.5 nm.

4. The method of claim 1, wherein said ratio has a numerator representing bulk fluorescence intensity measured at a wavelength between about 550 nm to about 660 nm, and has a denominator representing bulk fluorescence intensity measured at a wavelength between about 600 nm and about 750 nm.

5. The method of claim 1, wherein for insulation including cross-linked polyethylene said ratio has a numerator representing bulk fluorescence intensity measured at a wavelength of about 580 nm, and has a denominator representing bulk fluorescence intensity measured at a wavelength of about 650 nm.

6. The method of claim 1, wherein for insulation including ethylene propylene rubber said ratio has a numerator representing bulk fluorescence intensity measured at a wavelength of about 540 nm, and has a denominator representing bulk fluorescence intensity measured at a wavelength of about 570 nm.

7. The method of claim 1, wherein said insulation includes a material selected from the group consisting of cross-linked polyethylene and ethylene propylene rubber.

8. The method of claim 1, wherein:

said step of irradiating includes transmitting said excitation energy to said insulation via an optical fiber cable, and said measuring step includes transmitting at least a portion of said bulk fluorescence to an optical measurement system via said optical fiber cable.

9. The method of claim 8, wherein said transmission cable is disposed underground, said method including:

said optical fiber cable communicating said excitation energy from a location above ground to said insulation, and

said optical fiber cable furthermore communicating said bulk fluorescence from said insulation to said location above ground.

10. The method of claim 8, wherein said transmission cable is disposed in a radiation environment, said method including:

said optical fiber cable communicating said excitation energy from a location external to said radiation environment to said insulation, and

said optical fiber cable furthermore communicating said bulk fluorescence from said insulation to said location external to said radiation environment.

11. The method of claim 8, wherein said estimating step includes comparing said ratio of measured intensities with a set of prerecorded intensity ratios and corresponding insulation lifetime values.

12. A system for testing insulation deterioration in a transmission cable, comprising:

a transmission cable including a length of electrically conductive line having a layer of insulation axially surrounding said line and an outer protective jacket axially surrounding said layer of insulation;

an optical source that produces an optical excitation signal;

a length of fiber optic cable having a first end embedded in said outer protective jacket so as to contact said layer of insulation and a second end coupled to said optical source, wherein said optical excitation signal is directed to said insulation, thereby producing bulk fluorescence in said layer of insulation; and

an optical measurement system, coupled to said second end of said fiber optic cable, that measures intensity of said bulk fluorescence at at least two wavelengths.

13. The system of claim 12, wherein said optical measurement system includes means for determining an intensity ratio of said at least two wavelengths;

wherein said intensity ratio is indicative of remaining transmission cable insulation lifetime.

14. The system of claim 12, wherein said electrically conductive line defines a longitudinal axis of the transmission cable, and said first end of said fiber optic cable is embedded within said outer protective jacket such that said length of fiber optic cable protrudes therefrom at an angle between about 10°

and about 90°

relative to said longitudinal axis.