Oscar E. Morel UtilX Corporation

Transcription

1 Oscar E. Morel UtilX Corporation

2 Time Domain Reflectometry (TDR) has been the preferred technique to assess: Cable length Splice number and spatial location, and Metallic neutral condition Tests for neutral corrosion are covered in the IEEE Std Guide for Detection, Mitigation and Control of Concentric Neutral Corrosion in MV Underground Cables.

3 Metallically shielded power cables are in essence uniform unbalanced transmission lines. Uniform cross section or controlled impedance, but dissimilar size conductors. The wavelength of propagating pulse is shorter than the length of the line but larger than the circumference of the insulation (TEM mode as opposed to TE or TM modes mode in waveguides).

4 VOP c 1 100% o r o r or VOP c 1 LC 100% ( o ) permittivity of free space = 8.9x10-12 F/m (ε r ) relative dielectric constant of material = 2.2 for XLPE (μ o ) permeability of free space = 4 x10-7 H/m (μ) relative permeability of material = 1 for XLPE (L) inductance (C) capacitance (c) speed of light VOP = 67% In reality the VOP is in the mid 50 s.

5 At a point of impedance change, the voltage and current propagating pulse must be same on both sides of the interface. Reflecting back a fraction of the incident pulse is the only way to maintain this balance: V inc V refl V trans and I inc I refl I trans

6 V V refl inc Z Z 2 2 Z Z 1 1 (ρ) reflection coefficient Three basic cases: 1. Cable termination is open (Z2 = and ρ = 1) 2. Cable termination is shorted (Z2 = 0 and ρ = -1) 3. Cable terminated with a matching impedance (Z1 = Z2 and ρ = 0)

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8 V 2 l ( l) Vo exp 2 2 ( ) attenuation factor ( ) standard deviation (l) Propagated length 2 erfc l 2

9 Bandwidth in Hz, given by: BW ( l) l 2 l ln(2) The effect of pulse dispersion was not included.

10 = 4x10-11 s/m Initial FMHW=2ns After 1,000ft FMHW=25ns High Resolution TDR 14-bit resolution [5V FS 0.3μV res.]

11 Coupling to companion transmission lines is a significant cause of pulse degradation, energy losses and ghost reflections. In tightly coupled transmission lines with open metallic shields (concentric neutrals, open copper tapes, corroded copper tapes) the signal in one trace will be affected by coupling to adjacent lines.

12 Resulting active line showing a series of ghost reflections produced by companion lines providing alternative propagation paths with different impedance or velocity of propagation (VOP).

13 TDR traces for a pulse injected on one of three lines (top), and trace of the pulse injected simultaneous on the three lines (bottom).

14 3-TDR in the presence of an added joint to one of the cable phases. Traditional single cable pulse injection (top). Note that the presence of the joint cannot be distinguished. 3-TDR trace more clearly reveals the presence of the joint (bottom).

15 As a cable ages, conduction between copper tape laps decreases and the shield current is forced to spiral down the cable. Helical shield current increases the cable inductance, increasing the cable impedance and reducing the VOP. High frequency attenuation also increases due to fluctuations in cable impedance.

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17 Applicable to energized cable systems fitted with elbows or T-bodies. Field tested for cables up to 450ft with and without splices. Clear detection of end of cable. Clear detection of traditional TDRdetectable splices.

18 206 2nsec pulse CapTDR trace of a cable section 206ft long terminated a one end with a 200A dead break elbow in a 4-point junction and at the opposite end with a live front termination at a cable riser pole. The CapTDR trace was made from the dead break elbow end.

19 303 25nsec pulse Traditional TDR trace showing splice 101feet into a 303ft long cable. This trace was taken was a 25nsec wide pulse.

20 nsec pulse 198 CapTDR trace of a cable section 303ft long terminated a both ends with load break elbow connectors. The traces were taken from both ends and the presence of a joint appears in both traces. From one end the joint shows at 99feet and from the opposite end it shows at 198feet; calculated total length 297ft compared to 302ft (2ns pulse).

21 1,000 25nsec pulse CapTDR trace for a 1,000ft cable. A pulse 25nsec wide was used to obtain the reflection. The reflections observed at shorter distance are believed to be effects of the cable being in a cable a reel.

22 The information presented is evidence of the continuous effort by our Engineering Department to improve the application of TDR to power cables: Impedance transition device Understanding pulse propagation in tape neutral cables. Using the 3-TDR concept to eliminate the effects of cross talk in close lines. Using capacitive coupled TDR to locate splices and cable section length in energized lines.