4373

IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 9, No. 2, JUNE 1999

Diagnosis of electric cable insulation by High Tc SQUID
S. Tanaka and K. Inadomi*
Ecological Engineering, Toyohashi University of Technology, Toyohashi &chi 441-8580, Japan--Abstract
We have developed the wide band
electronics for SQUID and measured short pulse
currents simulating a signal i n a degraded
power electric cable. The SQUID driving
electronics had a bandwidth as wide as 1.4 MHz
and i t could successfully detect currents of 3pA with the width of 120 nsec. These results
suggested that SQUIDS can be applied to thepartial discharge measurement of power cables.

I. INTRODUCTION
An installed electric power cable such as a CV
Grosslinked polyethylene insulated poly~inylchloridesheathed) cable becomes degraded by continuous use formany years. The degradation occurs at a part of the cableinsulation. This phenomena is initiated by a small defectsuch as a void [l]. Then the small defect extends andcreates a current pass called "tree". Partial discharges inthe tree expand the tree itself. Finally, it triggers a fatal
breakdown. Therefore it is important to detect the defects
at the initial stage. This kind of electric insulation
diagnosis of power cable is one of non destructive
evaluation. Generally, the partial discharge measurement
by a radio receiver, a ultrasonic microphone and animpedance meter are commonly used for the diagnosis of
the cable [2]. However, it is quite difficult to define thelocation where the small discharge OCCUIS, because thedischarge current is as small as several nano amperes andits pulse width is several 10 nano seconds.
High Tc SQUID magnetometer has a high sensitivity
and a high space resolution. It is possible to apply theSQUID to the cable diagnosis. Unfortunately, you cannot obtain a wide band SQUID driving electronics to detectsuch a small current with small pulse width on the market.
Thus we have developed the wide band electronics forSQUID and measured simulated short pulse currents byusing the system.

11. EXPERIMENTAL

A . Estimation of magnetic field associated with adischarge current
We estimated the magnetic field intensity as a functionof spacing from a wire associated with a current to a sensor.
The configuration of the SQUID sensor and a current isshown in Fig. 1. The intensity of the field generated by aManuscript received September 15, 1998
*present address Canon Corp.

current is calculated by a Biot-Savart Law,

BZ =P&
2 4 x 2 +2 )where po is permeability of vacuum and I is current
associated with a discharge. Bz vs. spacing X between
current and a sensor for several values of the currents isshown in Fig. 2. It shows that the magnetic field
associated with the current of 1OnA is about 0. lpTesla at adistance of 20mm from the current. It can be detected bya SQUID sensor.

Fig. 1 Configuration of Current direction and SQUID sensor.
Direction of magnetic field which can be detected by theSQUID is B,.

020406080100Spacing X [ mm ]120140Fig. 2 Magnetic field vs. spacing for several currents.
(ZOmm, X: 1 to 130mm) Magnetic field associated with
the Current of lOnA is about 0. lpTesla at a distance of 20mmfrom the current.

B. SQUID Driving Electronics Box
Pulse width of a discharge is about several lOnanoseconds. The band width of the measurement systemshould be wide enough to detect the pulse. Modulation
frequency of conventionally obtainable FLL (Flux LockedLoop) system on the market is about 1MHz. So we havemade non-modulated FLL with APF (Additional Positive1051-8223/99$10.00 0 1999 IEEE

4374Feedback) technology [3],[4].
APF improves theeffective flux-voltage transfer function and the noiseproperties. In an APF scheme, SQUID is connected to a
resistance Ra and a inductance La as shown in Fig. 3.
Effective flux-voltage transfer function dV/4, is given

bydvdv-=-4d L

I-B,

where dV/@ is transfer function of the SQUID without
APF andintegrator was set at 0.9psec. Feedback resistor was
5.1kOhms. The feedback range was -4040 to 404
9. A
signal input coil (1 turn Cu wire) was magnetidly
coupled to the SQUID. The coil was connected to adigital pulse generator (BNC Model 7010) via a resistor fora signal current measurement. This system was poweredby an acid battery. Noise properties of the flux locked
SQUID was measured by HP35670A dynamic signal
analyzer. To avoid ambient magnetic field the dewar issurrounded b three mu-metal shielck. The flux noise was
5Op40/Hz1 / l and almost flat in range from l O O H z to100kHz The field resolution was 1.0 PT/HZ’’~at whitelevel. Sinusoidal input currents were pushed into the
input coil to measure frequency response. Output voltage
signal of buffer amplifier was measured.

When the condition of Ocpacl is satisfied, dV/4 isimproved. We used high Tc SQUIDS made by Sumitomo
Electric industries and set parameters, 8 Ohms for Ra and 5
C L A / ~forO 1/Mf to maximize dV/@,;
this condition
corresponds to paa.38. In our experiment, when pa wasincreased more than 0.4 by decreasing the value of Ra,
output voltage of the SQUID decreased Voltage-flux
properties of the SQUID with andwithout APF are shownin Fig. 4(a) and @) respectively. Flux to voltage transfer
function is improved from 1 5 ~ V / to@ ~1 9 ~ V / 4 , by using
the APF though this improvement is somewhat lower thanwhat we expected, To improve more, APF coil should becloser to the SQUID to increase mutual inductance. Thisproblem is discussed in elsewhere [4].

(a) with APF circuit. (vertical scale: 2pV/div),..APF.....Circuit...........‘V(b) without APF circuit. (vertical scale: 2pV/div)
Fig.3 Schematic drawing of AF’F(Additiona1 Positive feedback]circuit.

The SQUID driving circuit as shown in Fig.5 wasdesigned It is a non-modulation type [SI, [6]. Ultra low
noise operational amplifier AD797 (Analog Devices) wasused as a pre-amplifier, which measured input noisevoltage density was 1 . 3 n V / H ~ ~After/ ~ . the pre-amplifier
the signal was amplified by second and third amplifiers.
The band width of the amplifier including the pre-amplifier
was 1.7MHz at the gain of 52000. Time constant t ofFig.4 Voltage-flux properties of SQUID (a) with and (b)without APF. Flux to voltage transfer function is improved
from 15pV/1$~to 19pV/I$,by using an AF’F circuit.

The measured frequency response is shown in Fig. 6.
Bandwidth was about 1.4MHz and there was a peak at750kIIz Calculation by PSPICE (MicroSim) wasperformed . The calculated results are shown by dotted line
in Fig.6. There is no such a large peak on the calculation.
We think this peak is created by the buffer amplifier of thesystem because the buffer properties has not been reflected

4375

on the calculation.

The minimum detectable current for width of 120nsec was3pA.
Use of a digital oscilloscope is recommended
because this detectable currents are depend on the thresholdof the oscilloscope. To reduce the threshold level, thefield resolution of the SQUD should be improvedRecently high Tc SQUIDs with the resolution of 10-20
ff/H~’’~are available.
Using the high performance
SQUID, the threshold will be improved by about twoorden of magnitude. Another possibility to improve isapplying an averaging signal processing. Since practicalcurrents associated with partial discharge must be periodic
with power line frequency, the averaging technology may
be applicable. However, we have not done the feasibility
study of the averaging processing yet.

Bias Current

feedback

SQUID

coilPulse Generator(BNC-Model7010)

BufferFig.5 Schematic diagram of SQUID driving electronics. ’Iheband width of the amplifier including the pre-amplifier was1.7at a gain of 52000. Time constant t of integrator
was set at 0.9psec. A signal input coil was magneticallycoupled to the SQUID.

50ie3,.

\Calculation.

-50\‘i,\$;~.
scale: upper(a) input signal pulse width: 6 . 5 ~(vertical
SOWdiv, lower SOOmV/div, time scale: 20psec/div.)

.

, .

-1 00
10100

ik

10k

lOOk

Frequency

1M

10M

[Hz]

Fig. 6 Frequency response of flux locked SQUID. Filled
circles are measured, and dotted line is calculated Measured
bandwidth was 1.4MHz and a peak was found at 750kHz.

111. RE~ULTS
and DISCUSSION

A . Response for short pulse currents
Short pulse response of fhe system was tested.
Several different pulse currents were flown into the input
coil by a pulse generator. Output signal of the system
was measured by an analog oscilloscope. Typical input
and output signal waveforms are shown in Fig.8.
Although the output response for 6.5psec and 50pA is
square shape, the response for 900nsec and 20pA isdistorted and it looks like a kind of oscillation. This may
be because of restriction of bandwidth.
Minimum
detectable currents as a function of the pulse width, when
the threshold level was 5mV on the oscilloscope screen, isshown in Fig.& It shows that the detectable current
values are 2pA and the same in the range from 560nsec to6.5psec. This value correspond to 0.25nT for SQUID.

(b) input signal pulse width: 900ns. (vertical scale: upper
20pA/div, lower 5OOmV/div, time scale: 5pseddiv.)

Fig.7 Pulse response of the system. (a): for pulse width of
6.5psec and (b): for pulse width of 900nsec. Upper is input
signal and lower is output.

B. Cancellation of Magnetic Field from Power Line.
In an actual diagnosis, there are magnetic field noisefrom ac power line, which should be eliminated from thesignal due to the defects. A cancellation technique by
using two SQUIDS, one for detection of signals and noise,and the other for the noise, was tested. The experimental

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set up is shown in Fig.9. Power line currents were
simulated by a sinusoidal 70% current generated by an
oscillator. Two SQUIDS were used for this experiment
The output signal of each system was electricallysubtracted Frequency of 70 Hz was selected to isolate theresults clearly from 60 Hz noise associated with someother equipment in our lab.
Pulse current was not flown
in this experiment and only the 70Hz current was flown.
Output signals from two systems and the subtracted
signals from differential detector were measured by an
oscilloscope and a spectrum analyzer. The results fordifferent currents are shown in Fig.10. The 70Hz signalis reduced by a factor of 100. It is demonstrated that the70Hz noise can be electrically canceled by using twoSQUIDS.

-p“ 10I””“”’ “““‘I’ ””””’ “7System 2 Output

jDifferential Output

0200 400 600 800 1000
Simulated 70HZ Currents [pA]

Fig.10 Output signal of each system and the subtracted signal. 70Hz
signal is reduced by a factor of 100.

IV. SUMMARY

UJc

C

2

5085mc108c8n5.-E.r 0.1

E

10-7

10-6

10-5

10-4

10-3Input Pulse Width tw [sec]Fig.8 Minimum detectable currents as a function of the pulse

width when the threshold level was 5mV on the oscilloscope
screen.
Differential
ShuntSystem 2Generator

Oscilloscope

Spectrum
Analyzer

Oscillator

Current

Fig.9 experimental set up diagram for power line cable
cancellation test. Currents of power line were simulated bysinusoidal 70Hz current by an oscillator. Frequency of 70 Hz
was selected to isolate the results clearly from 60 Hz noise
associated with some other equipment.

A high Tc SQUID detection system for partial
discharge in a electric power cable was designed. TheSQUID driving electronics had a bandwidth as wide as 1.4MHz. Pulse currents with the width of 120 nsec weredetectable at the minimal FA. We think this value canlbe improved by using a higher performance SQUID.
Cancellation of magnetic field from power line was tested
by using two SQUIDS. The magnetic field was reduced byAlthough our experiments werea factor of 100.
preliminary, these results suggested that the SQUID can beapplied to the partial discharge measurement of power
cables.
ACKNOw LEDGMENTAuthors thank T.Mizoguchi for his supplemental
experiments.

REFERENCES[ 11 R.Bartnikas and E.J.McMahon, “Corona Measurement andInterpretation Engineering Dielectrics,” vol 1,ASTM, 1979.
[2] F.H.Kreuger, Partial Discharge Detection in High-Voltage
Equipment, Butterwoahs , 1989, pp. 36-128.
[3] H.Seppa, A.Ahonen, J.Knuutila, J.Simola, and V.Vilkman, “dcSQUID electronics based on adaptive positive feedback.:
experiments,” IEEE Trans. Magn., vol27, pp. 2488-2490,1991.
[4]AAdachi, K.Okajima, Y.Takada, S.Tanaka, H.Itozaki, H.Toyoda,H.Kado, “High Tc Superconducting Quantum Interference Device
with Additional Positive Feedback,” IEICE Trans. Electron., volE78-C, pp. 519-525,1995.
[5] H.C.Seton, J.M.S.Hutchison and D.M.Bussell, “ A Tuned SQUID
Amplifier for MRI Based on a DOIT Flux Locked Loop,” IEEETrans. on Appl. Supercond., vol7, pp. 3213-3216,1997.
[6] D.Drung, H.Mab, and H.Koch, “ A 5-MHz bandwidth SQUID
magnetometer with additional positive feedback,” Rev. Sci. Instrum.
V O 66,~ pp. 3008-3015,1995.