Simultaneous Measurement of Partial Discharge Using TEV, IEC60270 and UHF Techniques Alistair J. Reid School of Engineering and Built Environment Glasgow Caledonian University Glasgow, UK
[email protected]
Abstract—This paper explores a means of quantifying the relationship between transient earth voltage (TEV), ultra-highfrequency (UHF) and IEC60270 partial discharge (PD) measurements on a pulse-by-pulse basis for well-defined, laboratory-based discharge sources. Since each technique responds differently to the same PD event, there is no theoretical relationship between the measured quantities; TEV and UHF techniques respond to the rate of change of charge movement, while IEC60270 responds to its integral. Empirical measurement is therefore necessary. Discharge pulses for each system were captured simultaneously using a 3 GHz, 20 GS/s digital sampling oscilloscope. PD amplitudes recorded by the respective systems were analysed and their relationships evaluated. Although these relationships are complex and bear characteristics particular to the PD geometry, the order-of-magnitude variation in amplitude between the sources allows approximate linear interpolation of the relationships when pulses are plotted on a logarithmic scale. The TEV/IEC correlation approximated to 1 μV/pC. The TEV/UHF correlation approximated to 0.05 mV/mV (TEV/UHF). Using TEV data measured at various points at an on-line medium voltage substation, approximate corresponding pC levels have been estimated based on the above relationships. The implications for PD severity judgment are discussed. Keywords-Partial discharges; Condition monitoring; UHF; TEV; Switchgear.
I.
INTRODUCTION
Partial discharge (PD) measurement is a well established condition monitoring technique used to detect incipient failures within electrical insulation systems. A number of detection techniques have been applied over the years to measure the many effects of PD. PD measurement according to IEC standard 60270 [1] is a well established technique, in which PD is quantified in terms of ‘apparent charge’. The authors have previously carried out investigations examining the correlation between IEC measurement and the increasingly used UHF technique [2–4]. Recently, the transient earth voltage phenomenon has been more widely exploited for condition monitoring and asset management of MV switchgear [5–10]. Transient earth voltage sensors make use of the skin effect to measure electromagnetic radiation due to internal partial discharge. This is an attractive sensing option since measurements are inherently safe and can be made without any physical intrusion or modification to the switchgear.
Martin D. Judd and Graeme Duncan Institute for Energy and Environment University of Strathclyde Glasgow, UK
[email protected]
Quantifying the transient earth voltage output in terms of partial discharge severity can be challenging since there is no widely accepted standard. It is the purpose of this study to investigate the relationship between apparent charge, UHF energy and Transient Earth Voltage. The benefits of TEV measurement derive from the ability to install sensors nonintrusively on in-service equipment. Since PD magnitude is conventionally well understood in terms of apparent charge, it is desirable to quantify the IEC/TEV relationship for a better understanding of PD severity in MV switchgear. For completeness, UHF data was measured simultaneously along with TEV and IEC. II.
EXPERIMENTAL PROCEDURE
Figs. 1 and 2 show the laboratory-based setup, in which an enclosed metal chamber was used, together with PD test cells with well-defined topologies and discharge characteristics. A proprietary TEV sensor was used. A Lemke LDS-6 IEC60270 system was used to quantify apparent charge. The system was calibrated when connected to each respective test object by injecting current pulses of known charge, as specified in the IEC60270 standard. The hardware-integrated output pulse from the Lemke system, the peak of which is proportional to the apparent charge, was connected directly to a 3 GHz Lecroy 7300 Oscilloscope (top trace, Fig. 3) along with the TEV and UHF signals. A UHF monopole antenna was installed internally (Fig. 2). The output voltage from the UHF sensor was also connected directly to the oscilloscope. Although the test chamber is shown with the front face removed in Fig. 2, it was bolted in place for the duration of each test. The TEV sensor was placed on top of the test chamber and its output connected to the oscilloscope. Fig. 3 shows three traces captured simultaneously. All three traces arise from the same PD event. Since PD signal propagation to the UHF and TEV sensors is essentially at the speed of light, both signals arrive around the same time. The measured IEC60270 trace arrives approximately 500 ns later due to circuit delay and other propagation effects in the measurement system. Four PD test cells were used, representing four common PD source topologies: 1. A floating electrode in SF6 2. A point-plane configuration in SF6
Integrated IEC apparent charge output
PD test chamber
300 mV
TEV 2 μs
Interlocked test cage
Figure 3. Oscilloscope screenshot showing simultaneously captured IEC60270, UHF and TEV traces from a single PD event.
Voltage supply Lemke LDS-6 Oscilloscope
Figure 1. Experimental setup. From left to right: 3 GHz oscilloscope capturing TEV, IEC and UHF pulses simultaneously, Lemke LDS-6 ‘apparent charge’ measurement system (calibration device can be see on top of the unit), Voltage supply. The test envlodure is located inside the interlocked HV test cage.
HV supply
TEV sensor
UHF
6 mV
Although SF6 and oil are not common insulation media in the equipment of interest in this study, these test cells were used to provide a wide dynamic range in PD levels, allowing greater confidence in the reported correlations.
UHF monopole
Due to the large variation in output voltage levels it was necessary to apply attenuation or amplification to the respective measurements for some test configurations. For example, due to high amplitude discharges produced by the floating electrode, it was necessary to apply a 10 dB attenuator to the UHF channel. Since TEV measurements have comparatively low sensitivity, a 25 dB amplifier was needed when measuring discharge from the point-plane and free particle test cells. All measurements were correlated in post-processing using specially developed MATLAB code to produce the graphs shown in the following section. The software reads the peak of the measured waveform (approximately 6000 waveforms for each test configuration) and correlates this into a 3-column array of data. At this point any attenuation or amplification was corrected through scaling to arrive at the actual output voltage for each sensor. The relationship between output voltage and pC level for the IEC measurements was also calibrated for each configuration and factored into the calculation. The frequency response of the TEV sensor was measured using a pulsed GTEM cell [11]. The sensor has a mean effective height between 0.1 and 0.2 mm in the 200 MHz - 2 GHz range. This is considered low in terms of UHF sensitivity (normally > 6 mm mean effective height). The TEV sensor is likely to have higher sensitivity at frequencies below the calibration system’s lower limit of 200 MHz. III.
PD test cell
150 mV
3. A free metallic particle in SF6 4. A free metallic particle in oil (L10B)
Coupling capacitor
Figure 2. Experimental setup. PD test cell is located within the aluminium test chamber and connected to voltage supply through an HV bushing at the top. TEV sensor is located on top of the chamber and a UHF monopole sensor is mounted internally. The front plate of the chamber was re-fitted during testing.
RESULTS
A. Laboratory Measurements Figs. 4 and 5 show the TEV/IEC and TEV/UHF relationships respectively for all four test cells. TEV/IEC results for the individual defects follow a non-linear trend, characteristic to each PD source. However, when all data is plotted on the same graph (Fig. 4) the approximate gradient of the best-fit straight line is 1μV/pC. Since both UHF and TEV techniques respond to the rate of change of charge, they exhibit a more linear relationship, as shown in Fig. 5, with the best-fit straight line voltage ratio approximating to 0.05 mV/mV (TEV/UHF). This correlation has been included for completeness. Although the UHF measurement appears more sensitive by a factor of almost 20, it must be noted that the monopole sensor was mounted internally, while the TEV sensor was mounted on the outside of the chamber. Clearly a trade off exists between ease of installation and sensitivity for the two sensor technologies.
Floating electrode in SF6
Free particle in SF6
Free particle in oil
Point-plane in SF6
Figure 4. Correlation between apparent charge and TEV output voltage for four partial discharge sources
Floating electrode in SF6 Free particle in SF6
Figure 6. Results of on-site TEV measurement for PD location at varoius points on a MV substation. The approximate location of the fault is evidenced by high measured dBmV levels around the Bus Section.
Free particle in oil
Re-arranging Eqn. 1, the TEV output voltage can be defined, in this case, as: Vout [mV] = 10(dBmV/20) x 1
(2)
Point-plane in SF6
Figure 5. Correlation between UHF output voltage and TEV output voltage for four partial discharge sources.
Using the previously measured approximate relationship between TEV and IEC of 1μV/pC (or 1x10-3 mV/pC), and defining this constant as A, an approximation for the apparent charge levels in the substation may be computed as follows (using units of mV and pC): q [pC] = 10(dBmV/20) x A-1
B. On-Site Measurements In November 2009, a site survey was conducted at a medium voltage substation having a suspected defect after previous PD monitoring was carried out approximately a year beforehand. TEV measurements were taken to establish whether PD was still present and to determine if the condition had worsened. The measurement device used expresses readings in dBmV. The definition of a dBmV is: dBmV = 20 log10 (Vout / Vref)
(1)
where Vout is the TEV sensor output voltage in mV. By definition, dBmV uses a reference voltage of Vref = 1 mV. A plan of the substation along with sensor positions and TEV dBmV levels at these positions is depicted in Fig. 6.
(3)
= 10(dBmV/20) x 103 From the above equation, an approximate dBmV level of 15 dB measured on switchgear units 1-6 corresponds to an apparent charge level of around 5,600 pC. Using the same approximation, 40 dB measured close to the source of the discharge at the bus section corresponds to an apparent charge of around 100,000 pC. IV.
DISCUSSION
It must be noted that the reported relationships are only approximate. Since the TEV and IEC techniques respond differently to the same PD event it is clear on examination of the results that the relationship is not always linear. However, the assumption of an approximate linear relationship is helpful when considering a wide range of measurements. In establishing these relationships, the idealised laboratory test setup was necessary to minimise any attenuation and
dispersion effects of each measurement system. These effects are evident on examination of the on-site measurements. For example, the TEV dBmV level is greatly attenuated by the shielding effects of the enclosures as the distance from the PD source increases. The TEV mV level closest to the PD site should therefore be used when approximating the corresponding apparent charge. Conventional IEC measurement techniques only approximate the PD quantity – hence ‘apparent’ charge. The use of TEV or UHF measurements to judge PD severity is equally valid, but since more experience has been gained in the application of conventional techniques, the apparent charge level has become the internationally accepted quantity for the assessment of PD magnitude and severity. It is hoped that this initial study will go some way in assisting in the diagnosis of PD severity based on measurements on MV switchgear using the more convenient TEV method. V.
CONCLUSIONS
Partial discharge has been measured simultaneously on a pulse-by-pulse basis using conventional IEC60270, UHF and TEV methods in a laboratory-based test chamber using a number of well-defined PD sources. Since the TEV and IEC techniques respond differently to the same PD event, the measured relationship between the two quantities is not only dependent on the charge but on the dynamics of charge transfer. This gives rise to characteristic patterns when the pulse magnitudes for a particular PD source are plotted against each other on the same graph. Although these correlations vary between PD sources, an approximately linear relationship can be estimated when all measured PD data is considered. A more linear relationship exists between TEV and UHF since both techniques respond to the dynamics of charge motion. The gradient of the best-fit straight line in each case was 1μV/pC (TEV/IEC) and 0.05 mV/mV (TEV/UHF). Based on these correlations, TEV data was examined for on-site measurements on a MV switchgear enclosure. The PD source was pinpointed to the Bus Section and the apparent charge quantity approximated to 100,000 pC in the region of the defect based on the measured TEV dBmV levels and the reported TEV/IEC correlation.
ACKNOWLEDGMENT This work was funded through the EPSRC Supergen V, UK Energy Infrastructure (AMPerES) grant in collaboration with UK electricity network operators working under Ofgem's Innovation Funding Incentive scheme; - full details on www.supergen-amperes.org REFERENCES [1]
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