The LIRA® technology


Introduction

The LIRA (Line Resonance Analysis) technology was developed by the Institute for energy Technology (IFE) in Halden, Norway in the early 2000’s. It was initiated by the need of non-destructive test and condition assessment methods for cables in nuclear power plants. Many cable condition assessment technologies in the market today are potential destructive, and cannot be regarded as an alternative test method for cables where cable destruction leads to time consuming and cost driving maintenance operations. Other technologies are non-destructive, but provide too little information and security related to the continued operation of the relevant cable.

The patented LIRA technology is a non-destructive test method, and will not damage any part of the cable or any equipment connected to the cable. The LIRA technology is based on the transmission line theory, through the estimation and analysis of the complex line impedance as a function of the applied signal frequency.

LIRA is a system for real-time diagnosis and condition based monitoring of installed electric cables. LIRA can measure cable lengths ranging from less than 50 meters to several hundred kilometres, depending by the cable design and signal attenuation.

LIRA can monitor the global, progressive degradation of the cable insulation due to harsh environment conditions (high temperature, humidity, radiation) and detect local degradation of the insulation material due to mechanical impacts or local abnormal environmental conditions. The LIRA technology is especially sensitive to small changes in the insulation material, which allows an early indication of challenged areas, detecting and localizing single and multiple faults and their severity.

LIRA measurements are recorded and stored and can later be played back and analysed off-site. The LIRA system consists of data acqusition hardware and proprietary software performing the failure analysis, degradation analysis and cable simulation. Extensive cable tests have shown that LIRA can detect the location of the challenged part with an estimation error better than 0.3% of the cable length.


Theoretical Background

The electrical transmission of signal and power is perhaps the most vital single contribution of engineering technology to modern civilization,
Robert A. Chipman, Transmission Lines, 1968 McGraw-Hill

 

Transmission_line_element.svg
Source: Wikipedia

The transmission line model represents a line segment where:

  • Rdx is the distributed line resistance (\Omega /m)
  • Ldx is the distributed line inductance (H/m)
  • Gdx is the distributed line conductance (S/m)
  • Cdx is the distributed line capacitance (F/m)

Transmission line theory describe the essential relationships in transmission lines where:

\frac{\delta V(x)}{\delta x}=-(R+j\omega L)I(x)
represents the line voltage, and

\frac{\delta I(x)}{\delta x}=-(G+j\omega C)V(x)
represents the line current.

Important line properties utilized by LIRA algorithms:

  • \gamma is the propagation constant (\gamma = \alpha + j\beta)
  • \alpha is the attenuation constant
  • \beta is the phase constant
  • Z_0 is the characteristic impedance

\gamma = \sqrt{(R+j\omega L)(G+j\omega C)} = \alpha + j\beta
 

Z_0 = \sqrt{\frac{(R+j\omega L)}{(G+j\omega C)}}

For further details and in depth descriptions of transmission line properties, please refer to the Wikipedia page for Transmission Lines.


Implementation

A transmission line is defined as a radio frequency conductor carrying current with frequency high enough that their wavelength must be taken into account.
Wikipedia

 

The LIRA hardware produces the complex impedance of the Device Under Test (DUT) over a wide frequency spectrum;

\vec{Z}(f)=R(f)+X(f)
The values make up a impedance vector consisting of:

  • f – Frequency
  • R – Impedance real part on the Re-axis
  • X – Impedance imaginary part on the Im-axis
Z-vector01
Vector representation of the complex impedance

This vector is input to the LIRA software which calculated the impedance amplitude;

|\vec{Z}| = \sqrt{R^2 + X^2}

and the impedance phase;

\theta=\arctan\left(\frac{X}{R}\right)
 

These results are plotted as a function of frequency, and they are the basis for any further calculation algorithms in the LIRA software;

Impedance amplitude spectrum
Impedance amplitude spectrum
Impedance phase spectrum
Impedance phase spectrum

 


Results and Applications

Spot signature

Representation of impedance variations as a function of cable length.

Spot signature of 29.7 km underground power cable. LIRA pinpoints 30 cable joints before the end termination.
Absolute spot signature of 30 km underground power cable. LIRA pinpoints 30 cable joints before the end termination.

In the above plot, we see the spot signature af a 29.7 km underground power cable. The peak at 29.7 km represents the end termination, while the 30 peaks along the length represents installation intersection joints. The two peaks at around 2.5 km represent intersection joints where the cable type between the joints is different from the other cable types. In this case the cable type between the joints are copper (Cu) core, while the rest of the sections are aluminum (Al) core.

Impedance variations can be plotted with a accuracy better than 0.3 % of the cable length. Impedance gain can be regarded as a severity indicator of the impedance change.

Delta-G

Global cable assessment and ageing indicator.

The LIRA software utilizes complex non-linear regression algorithms in order to estimate the cable attenuation over the measured frequency band. The frequency dependent attenuation is the basis for the Delta-G algorithm.
The dielectric loss increases with ageing of the dielectric material. Delta-G provides a numeric value for the cable condition similar to Tangens-\delta, but is 100 % non-destructive.

Balanced Termination Signature

Far end termination condition assessment.

The Balanced Termination Signature (BTS) function assesses the capacitance of the far end termination. E.g. water ingress into the termination will produce a different results compared with a dry termination.