ABSTRACT. A sensitive, directional coupling sensor attached to a high-voltage power transmission cable couples enough signal from a partial discharge to an ...
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A NOVEL CONCEPT FOR MONITORING PARTIAL DISCHARGE ON EHV-CABLE SYSTEM ACCESSORIES USING NO ACTIVE COMPONENTS AT THE ACCESSORIES * D. Pommerenke' and Keith Masterson'
'
Hewlettt Packard, USA 'National Institute for Standards and Technology, Boulder, CO, USA
ABSTRACT A sensitive, directional coupling sensor attached to a high-voltage power transmission cable couples enough signal from a partial discharge to an electrooptic modulator to measurably change the amplitude of an optical carrier. The demonstrated sensitivity and bandwidth lead to the design of a very robust system for measuring partial discharge that requires no power at the site of the sensor. System concept and experimental results are shown and future options discussed.
INTRODUCTION A cascade of breakdowns in a series of terminations on a 150 kV power transmission cable on one day in 1993 caused a major blackout in the Netherlands [ 11. Recently 240 kV cable joints failed in Singapore, probably due to thermal-mechanical stress which cannot be tested very well in accelerated tests due to long thermal time constants. In almost every case, partial discharge (PD) is a precursor to breakdown in polymer-insulated systems. Monitoring critical cable links will significantly reduce the risk of events such as those mentioned above. Several methods have been used to measure partial discharge. They include capacitive, inductive or directional coupling sensors at the accessories. Signals are amplified, pre-processed (e.g. with an oscilloscope) [2], and then transmitted either electrically or optically to a data analysis system. These techniques have been successfully applied during post-installation tests and in large-scale experiments for cable pre-qualifications [2]. Picocoulomb sensitivities were achieved, and it was possible to locate faulty cable joints before breakdown. But for continuous monitoring, an additional set of requirements need to be considered:
0
Reliability. High-voltage cable systems are, in general, very reliable. Thus, monitoring systems need to be even more reliable. The monitoring system must run on a minimal amount of service if it is intended to be used for many years. If the cable accessories such as joints and terminations that are being monitored are inaccessible (e.g., buried or under water), no components that are placed at the accessory should be allowed to fail. Sensitivity. The monitoring system must be sensitive enough to detect PD in a early stage. The
Dielectric Materials, Measurements and Applications Conference Publication No. 473,O IEE 2000
time from detection to actual breakdown must be maximised to allow further analysis and decision making. Accuracy. PD from sources like gas insulated switchgear systems or corona from terminations should not be confused with damaging internal PD, nor should damaging internal PD be erroneously regarded as harmless PD. 0 Power requirement. Often there is no low voltage electrical power at the accessories. Thus a monitoring system must either draw power from the electromagnetic field of the high-voltage current or be passive. While accuracy is achieved mainly by the sensor selection and post processing, reliability can only be achieved if active elements are avoided as much as possible at the monitored accessories. The technique we report uses an electrooptic modulator which requires no electrical power at the site of the accessory and meets these requirements. 0
PREVIOUS OPTICAL-FIBER MEASUREMENTS
BASED
It is desirable to use optical fiber in monitoring cable systems. Optical fibers allow data to be transmitted over long distances and avoid problems of electromagnetic interference. Operational parameters like current and voltage have been measured using direct optical modulation methods [3,4,5]. Reliability parameters can also be monitored via opticalfiber systems to some extent: Temperature, as an indicator for potential problems at conductor connections or as a help in maximising cable load, is measured using optical time-domain reflectometry (TDR). The backscattered signals at frequencies offset from the input signal by the Raman scattering were analyzed [6,7]. Mechanical stress, as an indicator of cable movement or laying damage, can also be measured by optical TDR. In this case, for example, the backscattered signal of the Brillioun scattering was analyzed [8, 91.
* Contributions by the US Government, not subject to copyright in the United States.
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Water infiltration, as an indicator damage to the cable jacket, can be measured by special fibers that change their index of reflection through water absorption, leading to a strong backscatter signal in the wet region [lo]. However, presently there is no direct method to measure PD. While temperature, mechanical stress and water infiltration need to be measured as a hnction of cable length, it is generally accepted to limit PD measurement to cable accessories for the following reason: The cables themselves are manufactured under controlled conditions and are strictly tested, so most of the failures occur in accessories such as cable joints or terminations that are mounted under usually less favorable construction-site conditions. In addition, problems may arise in cable joints because of tangential field components at dielectric interfaces.
At the monitoring station, the optical signal is converted back into an electrical signal for further analysis. Such a system needs no electrical power at the location of the sensor, can be made very reliable, and can be buried underground. Thus, it is an attractive choice for PD monitoring. We tested the concept by injecting artificially generated partial-discharge pulses into 15 kV and 126 kV cables with the setup shown in Fig. 2. In our experiment, we used a Michelson interferometric, electrooptic modulator fabricated by attaching phase modulators with reflecting facets to two legs of a 3 dB fiber-optic coupler. Due to different path lengths in each leg, the modulator can be tuned to quadrature by changing the laser wavelength. This is achieved by a feedback-control circuit that adjusts the laser's temperature and current. The system is described in [ 121 and is characterized by: Base wavelength: 1319 nm
MEASUREMENT SYSTEM The basic concept of the measurement system is shown in Fig. 1. Possible improvements upon it are analyzed in the discussion section of this paper. A directional coupling sensor [I 13 having one side terminated into 50 Q couples energy from the PD pulse directly to an electrooptic modulator. The resulting modulated optical signal can be transmitted over large distances (km) due to the low loss of single-mode fibers.
XLPE insulator
directional coupling sensor'"
cable joint (schematically shown)
V,: 1.8 V. V, is the voltage required to shift the optical phase difference between two legs of the interferometer by n radians. 0
0
Excess attenuation: 16 dB. The excess attenuation is the loss in the optical system not accounted for by the 3 dB splitting ratio in the modulator. Laseroutput: 4mW.
cable sheath f.''
outer s k i c o n layer 50 Ohm termination
PD-signal coax
Laser diode
1
+-
modulator
I
Photo diode
optical fiber
I
Figure 1 :Basic concept for a directly modulating partial discharge measurement system.
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126 kV XLPE cable
directional coupling sensor
-I
0
5
10 Time [ns]
15
20
5
10 Time [ns]
15
20
50 Oscilloscope 1.5 GHz / 8 Gsample
40
2 Y
30
0
Figure 2: Experimental setup. The 126 kV cable was approx. 1.5 m long. Details of the optical setup can be found in [12]. . Partial discharge pulses in cross-linked polyethylene (XLPE) cable typically have a risetime of less than 1 ns and pulse widths at half maximum of only 1 or 2 ns. The output from the pulse generator used to emulate such an impulse and the integrated charge across a 50 R oscilloscope impedance are shown in Fig. 3. These pulses were injected between the inner conductor and the lead sheath of the XLPE cable. The pulse parameters are typical for partial discharges within XLPE cable, although slower impulses exist and may lead to reduced sensitivity of the measurement system. Pulse characteristics The 3 dB bandwidth of our measurement system was 30lcHz to 300MHz, while the spectrum of the above pulse rolls off 3 dB at 420 MHz, as is shown in Fig. 4 . The transfer impedance of the directional coupling sensor [l 1J that we used is shown in Fig. 5 and is defined as
z,
=
vs 0 -
I where I is the partial discharge current in the high voltage cable and V5,,is the voltage across a 50 R load on the coupler’s output port.
P 20 10
0 0
Figure 3: Output pulse shape (across a 50 f2 load) and the integrated charge
-a
0
2z
2. -5
.+
8 3 -10
a
2
,? -15 2 c
-20 Frequency [MHz]
Figure 4: FFT of the pulse shown in Fig. 3.
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The signal recorded after passing through the photonic link, 17 dB of amplification, and the 300 MHz low pass filter is shown in Fig. 7. A sensitivity of better than 10 pC is indicated for our setup.
...... 100 Frequency [MHz]
IO
1000
Figure 5: Transfer impedance in the forward direction of the directional coupling sensor attached to the 126 kV cable. The theoretical limit as shown in Fig. 5 is 50 R.This is reached if the full PD current is transferred from the high-voltage cable into the 50 SZ load of the coupler. Our sensor was approximately 15 dl3 below the theoretical limit, which is still quite sensitive for a partial discharge sensor.
-100
-50
0
50
100 150
Time [ns] Figure I: Output signal recorded for a "40 pC" pulse into the 126 kV cable.
DISCUSSION Signals measured
The signals shown in Fig. 6 illustrate the directivity and sensitivity of the sensor. The directivity is expressed as the ratio between the signals recorded across the forward and reverse output ports of the coupler, and was 1:3.5. The output voltage reaches 380 mV,. The pulse reflected from the open-ended cable reaches the sensor 15 ns later and can be identified by having a larger reverse signal than the forward signal. Due to some additional reflections between the pulse source and the high-
voltage cables, the waveform of the forward pulse in the 15 - 20 ns region is somewhat distorted.
We have demonstrated a measurement system which confirms the principles of a PD-based monitor. Remaining challenges in the system design are: 0 the need for polarization maintaining fibers the sensitivity (V,) of the modulators, the number of modulators needed, the number of fibers needed. The system shown in Fig. 8 would overcome these challenges,. But a number of improvements are needed for Fig. 8 to be realistic. cable joints
cable joints
3
, 1 Port A of the directiona1,couplingsensor
. . . . . . . . .
3
'&$.signal combiner
............ modulator ................... analysis
..............................
.opfical' ~ ............................. e ~ .
....... ..I
Figure 8: Fiber optical monitoring system concept with reduced number offibers and modulators.
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The number of modulators can be reduced by combining signals from neighboring cable joints. The cable joint could be identified by assigning different PD-signal frequency ranges to each phase. As long as the modulators are not chained, the necessity for polarization maintaining fibers could be eliminated by using non-polarized sources such as superradiant, light emitting diodes. To chain modulators, it is important that they are polarization independent, as otherwise very long polarization maintaining fiber 'would be needed. As such modulators are under development, this seems achievable [ 131 . The sensitivity of the system is determined by the laser power, the losses and, most of all, the Vx,. Presently modulators have VT, in the range of a few volts. But recent developments of polymer modulators show significantly reduced values of less than 0.8 V [ 141.
CONCLUSIONS The proposed system allows measurement of partial discharges in XLPE cable without the need for electrical power at the site of the sensor. Additional advantages are high reliability and small space requirements. The principle of its operation has been verified experimentally.
REFERENCES R.Ross, 'Dealing with Interface Problems in Polymer Cable Terminations', DEIS, JulyIAug. 1999, V01.15, No.4, pp.5-9 C.G.Henningsen, K.Polster, B.A.Fruth, D.W.Gross, 'Experience with an On-line monitoring system for 400 kV XLPE Cables', IEEE Power Eng. Soc. Trans. and Distr. Conf. 1996, pp.1099-1102 J.C.Santos, M.C. Taplamacioglu, K.Hidaka, 'Pockels high-voltage measurement system', Symp. High Volt., Aug. 1999, pp.53-57 Vol.1 Laurensse, I.J.,Koreman, C.G.A.,Rutgers, W.R.,Van Der Wey, A.H., 'Applications for optical current and voltage sensor', J. SenSors and Actuators, Vo1.17, No.1-2, 1989, 181-186 R.Minkner, J.Schmid, 'A new flexible fiber optic current-measuring -system for AC-and DCcurrent in high voltage systems', Int. Symp. High Volt., Aug. 1999, pp 29-32, vol. 1 T.Kawai, et.al., 'Cablefault location method using fiber optic distributed temperature sensor', Fujikura Tech. Rev., Jan 1996, pp.52-60 M.Angenend, E.Zinburg, B.Harjes, M.Kirchner, ' Temperature monitoring of XLPE-insulated power cables by means of optical fibers', Elektrizitaetswirtschaft, Vo1.92, No. 10, May 1993, pp.598-602 T.Nishimoto et.al, 'New type of 66 kV XLPE submarine cable with mechanical-damagedetection-sensor by optical fiber', Trans. Inst.
Int.
E.E.of Japan, Part B, Vol.ll2-B, No.10, Oct. 1992, pp.921-926. Check also work by A.H.Wey, B.J.Grotenhuis and A.Kerstens 'Fibre-optical technology in medium-voltage XLPE cables in the Netherlands' to be published. The Application of Optical Sensors for Temperature, Mechanical Stress and Moisture in Energy Cables in The Netherlands, OFS, 2000, Venice, Italy, October 2000. Xiaoyi Bao, David J. Webb, David A. Jackson, 'Recent progress in disributed fiber optic sensors based on Brillouin scattering,' in Distributed and Multiplexed Fiber Optic Sensors, V, Proc. SPIE Vol. 2507, 1995, pp. 175-185 M.Minami, et.al, 'Insulation monitoring system for XLPE cable containing water sensor and optical fiber', Showa Electric- Wire and Cable Review, V01.40, No.2, pp.50-54, 1990, . Check also work by A.H.Wey,- -B.J.Grotenhuis and AKerstens 'Fibre-optical technology in medium-voltage XLPE cables in the Netherlands' to be published. D.Pommerenke, T.Streh1, R.Heinrich, W.Kalkner, F.Schmidt, W.Weissenberg, 'Discrimination between internal PD and other pulses using directional coupling sensors on HV Cable Systems', IEEE Trans. Diel and Elec. Insul. Vol.6, No.6, Dec.1999, pp. 814-824 K.D.Masterson, D.R.Novotny, K.H.Cavcey, 'Standard antennas designed with electrooptic modulators and optical-Jiber linkage, ' in Intense Microwave Pulses IV, H.E. Brandt, ed, Proc. SPIE, Vol. 2834, 1996, pp.188-196 A:Donval, E.Toussaere, R.Hierle, J.Zyss, 'Polarization insensitive electro-optic polymer modulator', J. Applied Physics, Vol. 87, No.7, April 2000, pp.3256-3562 YShi, C.Zhang, H.Zhang, J.Bechte1, L.Dalton, B.Robinson, W.Steier, 'Low (Sub-1- Volt) halfivave voltage polymeric electro-optic modulators achieved by controlling chromophore shape', Science, Vol. 288, No. 5463, April 2000, pp. 119-122
