the suitability of UHF PD monitoring techniques developed on .... captured using a transient digitiser (1 GHz bandwidth). The ... appeared to be loss-free. In some ...
Paper presented at: 10th International Symposium on High Voltage Engineering, August 1997, Montréal
UHF Partial Discharge Monitoring for 132 kV GIS M. D. Judd
B. F. Hampton
W. L. Brown
Centre for Electrical Power Engineering, University of Strathclyde, Glasgow, UK
Diagnostic Monitoring Systems Limited, 204 George Street, Glasgow, UK
China Light and Power Company Limited, Kowloon, Hong Kong
Abstract
GIS chamber
A theoretical and experimental study into the propagation of partial discharge (PD) excited UHF signals in a 132 kV gas insulated substation (GIS) is described. The aim is to assess the suitability of UHF PD monitoring techniques developed on 400 kV GIS for application to substations operating at lower voltages. The PD signal energy is found at higher frequencies due to the effects of the smaller chambers and the pressure window on which the UHF coupler is mounted. The monitoring system frequency response must therefore be matched to the 132 kV GIS and the use of an active coupler is recommended.
90mm
R120mm
R19mm pressure window
Fig. 1
UHF coupler
Part of the 132 kV GIS showing the pressure window and a UHF coupler.
The propagation of UHF signals in 132 kV GIS was investigated using both waveguide theory and experimental measurements in full scale test rigs. The study and its results are described below.
1. Introduction A considerable body of research now exists showing the UHF method of partial discharge (PD) detection in 400 kV gas insulated substations (GIS) to be very sensitive [1,2]. GIS designed for lower operating voltages have smaller chambers, increasing the cut-off frequencies of higher-order electromagnetic modes. The UHF signal will be affected because most of its energy propagates in these modes. If UHF signal levels are still adequate for PD detection, it may be considered economically viable to monitor substations operating at lower voltages. This is particularly the case when external UHF couplers can be fitted without interrupting substation operation.
2. Theory The window structure and the dimensions of the 132 kV GIS are shown in Fig. 1. The internal radius of the GIS is about half that of a 400 kV design. As a result, the cut-off wavelengths of the higher-order modes in which the PD signals propagate [3] are decreased by a similar factor. Table 1 lists some of the cut-off frequencies. The attenuation caused by Ohmic losses can be calculated using waveguide theory [4]; results for several modes are shown in Fig. 2 (for aluminium conductors). While the attenuation is slightly higher than in 400 kV GIS [3], it is not sufficient to cause a significant reduction in UHF signal power over distances in the region of 10 m. Dielectric materials used for barriers also have a low loss at frequencies up to several GHz [5].
China Light and Power Company Limited have at Black Point a 132 kV GIS fitted with glass pressure windows that provide an electrical aperture in the GIS chamber. Initial measurements using a coupler designed to fit these windows revealed lower than expected UHF signal levels when a PD test cell was placed in an open chamber. A study was therefore instigated to determine whether there are any fundamental obstacles to using the UHF technique in 132 kV GIS by:
Table 1
1. Establishing typical values for the attenuation of UHF signals caused by various GIS components. 2. Identifying any monitoring system requirements that differ from those used in 400 kV GIS.
mode TE11 TE21 TM01
1
Cut-off frequencies in the 132 kV GIS. fc ( MHz ) 696.3 1211.3 1431.3
mode TM11 TE31 TE12
fc ( MHz ) 1627.1 1670.2 1986.4
0.08
0.06 0.04
0.06
0.02 0
0.04
0.02 0.04
0.02
0.06 0 0
500
1000
1500 frequency ( MHz )
2000
2500
0.06
3000
0.04 0.02
Fig. 2
Theoretical mode attenuation as a function of frequency.
0 0.02 0.04
A theoretical model for the excitation of UHF signals by PD [3] allows the UHF electric field at a remote coupler to be calculated from a knowledge of the PD current and its position in the GIS. To determine whether the UHF signals in 132 kV GIS are inherently smaller than those in 400 kV GIS, identical PD sources were modelled in both types so that the UHF fields could be compared. Inner and outer conductor diameters of 0.1 m and 0.5 m were used for the 400 kV GIS. A real PD current pulse was sampled and used as the excitation source in the simulation. The 0.5 pC current pulse was measured on a 25 mm needle PD source. The UHF electric field was calculated at a distance of 5 m from the PD source in the absence of signal reflections and the results for both GIS sizes are shown in Figs. 3 and 4. Note that these results do not include the effects of the coupler and PD measurement system frequency responses.
0.06 0
Fig. 3
10
20
30
40
50 time ( ns )
60
70
80
90
100
UHF electric fields at the GIS chamber wall, 5 m from a 0.5 pC PD source. (a) 400 kV GIS, (b) 132 kV GIS.
0.08 0.06 0.04 0.02 0 0.08 0.06 0.04 0.02 0 0
Fig. 4
500
1000
1500 frequency ( MHz )
2000
2500
3000
FFT of the electric field. (a) 400 kV, (b) 132 kV GIS.
UHF signals excited within the test rigs were detected using a second 25 mm probe which has a known response to an electric field [3]. Holes along the top and sides of the test chambers allowed for mounting of the excitation and field probes. The signal from the field probe was fed through a pre-amplifier (25 dB gain, 1.7 GHz bandwidth) and captured using a transient digitiser (1 GHz bandwidth). The maximum record length that can be captured using the digitiser is 20 ns so in each configuration the first 100 ns of UHF signal was assembled from five records captured with the trigger delay incremented in steps of 20 ns.
Although the higher cut-off frequencies in 132 kV GIS restrict the propagation of low frequency components of the signal, the excitation in the smaller GIS is greater (for a given defect size and PD current) because the defect occupies a larger percentage of the gap between the conductors. This effect appears to dominate and the simulation results consistently showed a greater UHF electric field amplitude in the 132 kV GIS, although at higher frequencies. These results indicate that there is no fundamental obstacle to the use of UHF monitoring in 132 kV GIS provided the coupler and monitoring system have a suitable frequency response.
Comparing UHF signal amplitudes proves difficult because the signal envelope can vary considerably. A better method of comparison involves calculating the average UHF signal power PUHF over a representative measurement
3. Experiment Various full scale components were fabricated in aluminium to the dimensions of the 132 kV GIS so that the UHF signal attenuation caused by a range of discontinuities could be investigated. A typical measurement arrangement is shown in Fig. 5. A stable source of excitation was necessary to allow the UHF signal powers measured at various locations to be compared. This was accomplished using an avalanche transistor circuit to generate a 1.4 pC current pulse on a 25 mm probe inserted through the wall of the GIS chamber. Peak amplitude of the pulse was 5.7 mA and its width at half-amplitude was 300 ps.
pulse circuit
input probe to simulate PD
trig input
PC
Top view of GIS test chamber
UHF field probe
1.4m
4.5m
Pre-amplifier
Digitiser
Fig. 5
2
Measurement equipment and a typical GIS test chamber.
30 20
(1)
10
(2)
0 10 20
(3)
(4)
30
0.15
Scale:
0.1
Fig. 7
1m
Four configurations of the 132 kV GIS test chambers.
0.05 0
Fig. 6
0
10
20
30
40
50 time ( ns )
60
70
80
90
• Gaps in the inner conductor had little effect on the UHF signals (see Fig. 8). At a T-junction, the power measured in the side arm increased when the inner conductor leading into it was completely removed. • Flat PVC discs (εr ≈ 3.5) that were mounted in the chamber to represent an insulating gas barrier increased the power measured in the compartment containing the source while decreasing the power in the compartment beyond the disc. This effect becomes more significant as the disc thickness increases (see Fig. 9). At 36 mm thick, the signal power measured beyond this barrier is about 3 dB below the level in the source chamber. • Signal power measured in a side compartment (such as a corner or T-junction) at 90° to the chamber containing the source was in the range 5-9 dB below that in the main chamber. The termination of the side compartment had a significant effect on the signal power, with lower attenuation occurring if a metal closing plate was used rather than a less reflective material such as a PVC disc. • An interesting result was obtained for the propagation of UHF signals through a larger compartment (diameter 360 mm based on the dimensions of a circuit breaker). No additional attenuation relative to a uniform 240 mm diameter was observed when the source and coupler were either side of the large compartment. However, when either the source or coupler was in the large compartment, a decrease of 6 dB in signal power was recorded.
100
Calculating the UHF signal power. (a) Initial part of UHF signal into 50Ω. (b) Cumulative energy delivered to the 50 Ω load, amounting to 0.102 pJ in 97 ns.
period. This is particularly useful because PUHF can be directly related to the detection threshold of a PD monitoring system. Fig. 6 illustrates the signal processing used to determine PUHF. In this example, the energy delivered to the load during the first 97 ns after the arrival of the UHF signal at the coupler is 0.102 pJ. The average power at the digitiser input is: 0.102 pJ 97 ns
=
1.05 µW
=
- 29.8 dBm
(1)
The gain of the pre-amplifier (including losses in the associated cables) was 24 dB ± 1 dB in the range 700→1500 MHz where most of the UHF signal energy recorded by the digitiser is concentrated. Subtracting this gain gives an average power of PUHF = -53.8 dBm for the PD generated signal at the coupler output. Fig. 7 shows four configurations of the GIS components that were investigated. Within each of these, further modifications were made such as introducing gaps in the inner conductor or adding dielectric discs to represent barriers. Pulses at a repetition rate of 10 kHz were injected into the chamber and the resulting UHF signals were recorded at various positions. Each measurement was processed to determine an average signal power over the first 95 - 100 ns. These UHF power levels were used to obtain typical values for the attenuation through various signal paths. Power variations of less than ± 1 dB were regarded as statistically insignificant. The results of these experiments were as follows:
50 52 54 56 58 0
Fig. 8
• A uniform straight GIS section of up to 4.5 m in length appeared to be loss-free. In some instances slightly more power was recorded at positions further from the source than at a distance of 1 m. This is because during 100 ns the signal can traverse the length of the chamber more than six times in both directions. The electric field at the UHF coupler is due to the superposition of all these reflected signals and variations with position can occur which are not unlike the effects of standing waves.
20
40
60
80 100 120 140 inner conductor gap ( mm )
160
180
200
The effect of a gap in the inner conductor between PD source and coupler on the UHF signal power. 50 52 54 56 58 0
Fig. 9
3
0.5
1
1.5 2 2.5 3 No. of PVC disks ( each 9mm thick )
3.5
4
Effect on the UHF signal power of inserting plastic discs between PD source and coupler to simulate a gas barrier.
400
4. Window coupler
300 200
The coupler consists of a sensor mounted against a pressure window on the GIS (Fig. 1). The cut-off frequency of the dominant TE11 mode [4] in the window tube (∅90 mm) is 1950 MHz. Most of the PD signal energy is at frequencies below cut-off and is therefore subject significant attenuation. Fig. 10 shows the theoretical attenuation as a function of frequency (calculated according to [6]). The window structure acts as a high-pass filter and therefore has a considerable effect on the coupler sensitivity and its frequency response. For example, at 1 GHz the insertion loss of the window tube is 300 dB m-1, so a recess 50 mm deep will cause an attenuation of about 15 dB. The effects of the window structure are investigated further in [7].
100 0 0
Fig. 10
Attenuation of the TE11 mode in the window tube.
References [1] J S Pearson, O Farish, B F Hampton, M D Judd, D Templeton, B M Pryor and I M Welch, "Partial discharge diagnostics for gas insulated substations", IEEE Trans. Dielectrics and Electrical Insulation, Vol. 2, No. 5, pp. 893-905, October 1995 [2] R Kurrer, K Klunzinger, K Feser, N de Kock and D Sologuren, "Sensitivity of the UHF-method for defects in GIS with regard to on-line partial discharge detection", Conf. Record of IEEE Int. Symp. on Electrical Insulation (Montreal), Vol. 1, pp. 95-98, 1996
Effective height specification H e ≥ 10 mm over a total
bandwidth of at least 250 MHz. 1250 → 1500 MHz
2000
China Light and Power Company Limited is currently considering the application of UHF PD monitoring techniques to the Black Point 132 kV GIS, making use of window couplers with built-in pre-amplification.
Proposed window coupler sensitivity specification.
700 → 1250 MHz
1500
The tube which houses the window forms a high-pass filter with a cut-off frequency that is inversely proportional to its diameter. When such pressure windows are specified for UHF monitoring, the diameter should be made as large as possible within the constraints of pressure vessel design. To overcome the attenuation of the window structure an internal pre-amplifier can be used in the window coupler.
Simulations including the effects of the monitoring system bandwidth showed that a first-order low-pass response with a 3 dB cut-off at 1 GHz reduces the available PUHF by 6 dB from its theoretical maximum. Extending the bandwidth to 1.5 GHz causes a 3 dB improvement. To extend the bandwidth beyond 1.5 GHz was considered uneconomical in view of the increased cost of the signal processing components and the relatively small further gains in PUHF (< 3 dB) that can be achieved.
Frequency range
1000 frequency ( MHz )
conclusion is supported by calculations of the skin effect losses which, even if they were increased tenfold by surface roughness, would account for less than 1 dB in 10 m. The attenuation along the GIS is caused by reflections at discontinuities which tend to confine the UHF energy within chambers close to the PD source. The attenuation introduced by various obstacles is therefore partly dependent on adjacent components. However, some typical UHF attenuation levels for certain components have been measured and these can be used to estimate the total signal attenuation between a PD source and the nearest coupler.
The attenuation caused by the window mount must be overcome using additional gain. Locating a pre-amplifier within the coupler housing which is bolted to the GIS window ensures a good level of immunity to external interference. To achieve a PD detection sensitivity comparable with that of 400 kV systems, the coupler specification given in Table 2 was proposed. This sensitivity (specified in terms of an effective height He [7]) should be measured with the coupler mounted on a test plate which replicates the pressure window structure.
Table 2
500
H e ≥ 10 mm
[3] M D Judd, O Farish and B F Hampton, "The excitation of UHF signals by partial discharges in GIS", IEEE Trans. Dielectrics and Electrical Insulation, Vol. 3, No. 2, pp. 213-228, April 1996
5. Summary and conclusions
[4] N Marcuvitz, Waveguide Handbook, Peter Peregrinus Ltd, 1986
This study has shown that the reduced dimensions of 132 kV GIS do not impede UHF detection of PD. However, the bandwidth of a monitoring system should extend to around 1.5 GHz because of the higher cut-off frequencies compared to 400 kV GIS. Experimental results indicate that a uniform 132 kV GIS chamber of up to at least 5 m in length can be considered loss-free when it has no discontinuities such as barriers, corners or T-junctions. This
[5] S Pack, A Diessner and J Gorablenkow, "PD signal propagation in GIS considering frequencies up to GHz", Proc. 8th ISH (Yokohama), Vol. 3, pp. 93-96, 1993 [6] J D Kraus and K R Carver, Electromagnetics. McGraw-Hill, 2nd Ed., 1981 [7] M D Judd, O Farish and P Coventry, "UHF Couplers for GIS Sensitivity and Specification", Proc. 10th ISH (Montreal), 1997
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