Systems and methods for detecting insulation defects in an underground power cable
A system for detecting insulation defects in an underground power cable comprising one or more single conductors surrounded by an electrically conducting shield. The system comprises one or more external sensors clamped onto the outside of or arranged in the proximity of the power cable. The sensors are configured to provide two or more current measurements from the outside of the power cable without being electrically connected to any of the one or more conductors of the power cable. A signal processing unit of the system is adapted to use a mathematical statistical model that processes measurements made by the sensors to identify if the current measurements are caused by a partial discharge event from a leakage structure in the power cable.
1 . A system for detecting insulation defects in an underground power cable, having at least one conductor and being surrounded by an electrically conducting shield, the system comprising:
two or more sensors clamped onto an outside of or arranged in a proximity of the underground power cable, wherein the two or more sensors are configured to provide one or more current measurements from the outside of the underground power cable without being electrically connected to the at least one conductor of the underground power cable; and
a signal processing unit adapted to use a mathematical statistical model that processes measurements made by the two or more sensors to identify if the one or more current measurements are caused by a partial discharge event from a leakage structure in the underground power cable, the mathematical statistical model configured to make a linear projection of a current in the at least one conductor and the electrically conducting shield, wherein the mathematical statistical model is defined as:
Y t =F t (θ t )+ε t ε t ~δ 1 ( V t )
θ t =g t (θ t-1 )+ϑ t ϑ t ~δ 2 ( W t )
where Y t is a vector determining an observed process at time t comprising observed data from the two or more sensors;
θ t is a vector determining a latent stochastic process at time t comprising latent process data;
F t is a regression matrix which determines a linear relation between the latent stochastic process and the observed process at time t;
g t is an evolve matrix which determines a transition from time t−1 to time t in the latent stochastic process;
δ 1 and δ 2 are stochastic noise vectors of the observed process and the latent stochastic process respectively;
ε t and ϑ t are zero mean multivariate Gaussian distributed noise vectors of the observed process and the latent stochastic process respectively;
V t is an observation variance-covariance matrix; and
W t is an evolution variance-covariance matrix.
2 . The system according to claim 1 , wherein the underground power cable comprises at least two conductors.
3 . The system according to claim 1 , wherein the two or more sensors are spaced apart along a length of the electrically conducting shield of the underground power cable.
4 . The system according to claim 1 , wherein the two or more sensors comprise a main sensor member and one or more additional sensor members arranged around a circumference of the electrically conducting shield of the underground power cable, wherein the main sensor member and the one or more additional sensor members are tangentially spaced apart.
5 . The system according to claim 1 , further comprising a calibration unit configured to carry out a calibration of the two or more sensors based on physical placement on the underground power cable and environment.
6 . The system according to claim 5 , wherein the calibration unit is physically separated from the two or more sensors.
7 . The system according to claim 5 , wherein the calibration unit is integrated in each of the two or more sensors.
8 . The system according to claim 5 , wherein the calibration unit is configured to calibrate the two or more sensors upon movement of a main sensor member and several additional sensor members of the two or more sensors along a periphery of the underground power cable.
9 . The system according to claim 1 , wherein the two or more sensors comprise an energy harvester.
10 . The system according to claim 9 , wherein the energy harvester comprises a thermoelectric generator or an electric-field energy harvesting device.
11 . The system according to claim 1 , further comprising:
a communication unit extending from at least one of the two or more sensors towards ground level; and
an antenna configured to transmit wireless signals;
wherein the communication unit and the antenna cooperate to wirelessly transmit measurements made by the two or more sensors.
12 . The system according to claim 1 , further comprising a shield structure surrounding the two or more sensors and an entire circumference of the underground power cable where the two or more sensors are positioned, wherein the shield structure is an electromagnetic field shield.
13 . The system according to claim 1 , wherein the processing unit comprises a peak detector configured to analyze the one or more current measurements and detect any current peaks.
14 . The system according to claim 1 , wherein the processing unit comprises a high pass filter configured to high pass filter the one or more current measurements.
15 . The system according to claim 1 , wherein the processing unit comprises an algorithm configured to automatically identify if the one or more current measurements are caused by a partial discharge event from a leakage structure in the underground power cable.
16 . A method for detecting insulation defects in an underground power cable, having at least one conductor and being surrounded by an electrically conducting shield, the method comprising:
clamping two or more sensors onto an outside of the underground power cable or arranging the two or more sensors in a proximity of the underground power cable, wherein the two or more sensors provide one or more current measurements from the outside of the underground power cable without being electrically connected to the at least one conductor of the underground power cable;
applying a signal processing unit to process data using a mathematical statistical model which processes measurements made by the two or more sensors to identify if the one or more current measurements are caused by a partial discharge event from a leakage structure in the underground power cable, wherein the mathematical statistical model is used to make a linear projection of current in the at least one conductor and the electrically conducting shield, wherein the mathematical statistical model is defined as:
Y t =F t (θ t )+ε t ε t ~δ 1 ( V t )
θ t =g t (θ t-1 )+ϑ t ϑ t ~δ 2 ( W t )
where Y t is a vector determining an observed process at time t comprising observed data from the two or more sensors;
θ t is a vector determining a latent stochastic process at time t comprising latent process data;
F t is a regression matrix which determines a linear relation between the latent stochastic process and the observed process at time t;
g t is an evolve matrix which determines a transition from time t−1 to time t in the latent stochastic process;
δ 1 and δ 2 are stochastic noise vectors of the observed process and the latent stochastic process respectively;
ε t and ϑ t are zero mean multivariate Gaussian distributed noise vectors of the observed process and the latent stochastic process respectively;
V t is an observation variance-covariance matrix; and
W t is an evolution variance-covariance matrix.
17 . The method according to claim 16 , wherein the underground power cable comprises two or more conductors.
18 . The method according to claim 16 , further comprising establishing a connection between each of the two or more sensors and the signal processing unit.
19 . The method according to claim 16 , further comprising a step of calibrating the two or more sensors.
20 . The method according to claim 16 , further comprising arranging a main sensor member and one or more additional sensor members of the two or more sensors around a circumference of the electrically conducting shield of the underground power cable such that the main sensor member and the one or more additional sensor members are tangentially spaced apart.