2008 International Conference on Condition Monitoring and Diagnosis, Beijing, China, April 21-24, 2008
Experiences in On-site Partial Discharge Measurements and Prospects for PD Monitoring Dr. K. Rethmeier1* , Dr. M. Krüger1, Dr. A. Kraetge1, Dr. R. Plath2, Dr. W. Koltunowicz2, 1
A. Obralic3, Prof. W. Kalkner3 Omicron electronics, Klaus, Austria 2Omicron electronics, Berlin, Germany 3Technische Universität Berlin, Germany *E-mail :
[email protected]
critical network points. Abstract--With four case studies (dry-type transformer, power transformer, GIS, generator) this paper describes the principle of synchronous multi-channel PD measurements and its benefits for PD data evaluation. With respect to monitoring the tracing of a charge value as QIEC over the measuring time is not sufficient to assess the PD activity of an HV apparatus. With state-of-the-art PD evaluation techniques as 3PARD and 3CFRD a simple and fast (all calculations during the PD measurements are carried out in real-time) tool is available to separate PD from noise. Separated PD sources can easily be back-transformed to phaseresolved patterns or pulse sequence patterns, for instance, to be evaluated automatically by computerized PD expert systems or by experienced human experts.
A. 3-Phase-Amplitude-Relation-Diagram (3PARD) The synchronous acquisition of PD data for all three phases of an HV apparatus allows a pulse-per-pulse amplitude comparison.
Index Terms--Partial Discharge, PD, offline, online, monitoring, diagnosis, noise suppression, pulse-waveform analysis, synchronous acquisition, multi-channel acquisition
I. INTRODUCTION measurement of Partial Discharges (PD) is a THwEorldwide accepted tool for quality control of high voltage (HV) apparatus [1]. PD signals are very often superposed by noise pulses, a fact that makes a PD data analysis more difficult for human experts and for software expert systems, respectively. With the ongoing development of permanently installed PD monitoring systems the PD data analysis needs to become more effective to be done automatically. Using synchronous multi-channel PD acquisition it is possible to gain de-noised PD data from separated PD sources in order to make PD measurements more reliable. II. PD DATA EVALUATION PD measurements are often performed under noisy conditions. The PD signal is superposed by stochastic noise pulses or even multiple PD sources, which will lead to a complex phase resolved PD pattern that is not easy to analyze. Conventional frequency filters are not able to eliminate these pulse-shaped disturbances. PD experts and atomized computer expert systems will have difficulties with the superposition of multiple PD faults and noise. Some well-known evaluation techniques as pulse-sequence-analyses will even fail with noncorrelated PD pulses to be compared. The PD source separation must be the first step to de-noise PD data. In the future this will become even more important with the rising numbers of automated PD monitoring systems installed at 978-1-4244-1622-6/08/$25.00 ©2008 IEEE
Fig. 1. Creating 3PARD from PD voltage signals
The amplitude relations of acquired pulse triples are constant for different PD sources and for different noise sources due to their unique propagation path from PD fault location to PD decoupling location. So the first step of a PD location is a PD source separation. The creation of the diagram as well as the back-transformation of 3PARD clusters can be done during the PD measurement in real-time.
The 3PARD source separation has been in practical use now for seven years with reliable results as described in several scientific publications [2,3,4]. Finally it found its way into university textbooks for students' education [5]. It has to be pointed out that synchronous PD data acquisition is an absolute must for 3PARD data evaluation. Due to their sequential measuring procedure three-channel multiplex systems are only able to deliver uncorrelated PD pulses instead of pulse triples.
More detailed information about a PD measuring device suitable for synchronous PD data acquisition is given in [6]. B. 3-Center-Frequency-Relation-Diagram (3CFRD) The 3PARD method requires PD pulse triples. Three PD amplitudes are compared and plotted into a single diagram to form separable clusters. However, in principle this type of diagram can be created by any synchronously gained pulse triples. Even for a single phase or a single PD decoupling position pulse
triples can be gained by using three different PD filter settings, for instance. So the signal output of three filters with different center frequencies or bandwidths allows a pulse-waveform-analysis. This refers to the fact that, due to the discharge physics, different PD types or noise pulses have different but characteristic energy spectrums. A separation of different PD sources and different noise sources is possible by focusing on single 3CFRD clusters. A real-time back-transformation will result in clear and de-noised phase resolved patterns.
C. 3-Phase-Time-Relatio- Diagram (3PTRD) A third method using the 3PARD principle is the evaluation of the pulse time delays of acquired PD triples. Similar to the well-known TDR method for PD fault location on HV cables every pulse source has a characteristic fingerprint of time differences due to the pulse propagation to the three locations of PD decoupling. Figure 2 shows two typical PD pulse triples caused by different PD sources.
Fig. 3. Temporarily installed PD monitoring device
Fig. 2. Typical time differences for PD fault location Fig. 4. Excerpt of PD trend curve
III. CASE STUDIES The evaluation of synchronously gained PD data is neither limited to online or offline PD measurements nor to a certain type of HV apparatus [7,8,9]. In the following several case studies for PD measurements are shown to demonstrate the principle, the versatility and the benefits of multi-channel PD data acquisition. A. GIS Substation A PD monitoring device for synchronous 4-channel data acquisition [10] was temporarily installed at the cable/GIS interface of a 110 kV substation as to be seen in figure 3. To observe the PD activity of the GIS cable termination and the power transformer connected by a short length of a 110 kV XLPE cable a charge trend recording was performed for three phases. Figure 4 shows a one hour timeframe of the acquired PD data of all three phases. It can be seen that the recorded PD charge level is altering rapidly in time. A look on four of the automatically generated sets of phase resolved patterns (see fig. 5, channel 4 for PD decoupling at neutral or star point not connected) explains these unstable curves. Beside the PD activity (black marks in channel 2 of all sets of PD histograms) a lot of noise and pulse disturbances were recorded and flew in the finally stored QIEC value. In this case the
Fig. 5. Timed PD histogram acquisition
simple PD trend recording does not represent the PD activity of the HV apparatus. Due to the synchronicity of all four PD acquisition channels of this monitoring device a real-time 3PARD clustering can be performed to focus on PD activity correlated to a single phase. B. Power Transformer After repair works a 3 phased synchronous PD measurement on a 30 MVA 115 kV/11.3 kV transformer (see fig. 6) was performed to ensure the quality of the undertaken maintenance work. The PDs were decoupled at the measuring taps of the transformers' bushings.
Figure 7 shows the recorded PRPD pattern of phase L1. The pattern shows three different discharge structures. Statistically distributed noise pulses not related to the test voltage frequency generated a band at the bottom of the diagram (base noise level of 5 pC). Two phase-stable clusters due to the power generator (voltage source for the induced voltage test) are noticeable at a discharge level of ca. 20 pC. As a third structure the partial discharge pattern can bee seen with a discharge magnitude of about 100 pC.
capacitor, measuring impedance, MPD 600 PD acquisition unit and battery for potential-free power supply.
Fig. 6. PD test on transformer, 30 MVA 115 kV Fig. 8. 3PARD and PRPD pattern separated by discharge type
Fig. 7. PRPD pattern of phase L1
By the use of 3PARD all this structures can be separated and real-time re-calculated into cleaned-up PRPD patterns, as shown in figure 8. With 3PARD clustering a pulse source separation could be done by comparing the amplitudes of PD and its cross-talk to the neighbor phases. In this way the PD pattern could be clearly separated from noise and other non-PD pulses. As PD activity is located very close to the axes of the 3PARD diagram this information can be used to specify an additional filter option to be used for automated data evaluation at online PD-monitoring.
C. Dry-Type Transformer In a heavily disturbed industrial environment of a transformer manufacturer's workshop a 500 kVA 240 V/10.5 kV dry-type transformer was tested according to IEC 60076-11 with 1.8 times of nominal voltage and 1.3 times of nominal voltage, respectively. Figure 9 shows the HV set up with transformer, coupling
Fig. 9. PD measuring set up
Figure 10 shows the recorded test voltage (red curve scaled in kV) and the referring PD level (green curve scaled in nC)versus time. At 18.9 kV test voltage level heavy pulse activity (multiple PD sources and noise pulses) with more than 1000 pC could be recorded as to be seen in the trend curve (fig. 10). After lowering the test voltage to 14 kV and finally 13 kV a PD level of ca. 260 pC was still present. For a deeper investigation figure 11 shows a summary of the performed real-time 3CFRD analysis with measuring frequencies of 1.4MHz (unit 1.1), 2.0MHz (unit 1.2) and 2.4MHz (unit 1.3).
demonstrates that a simple PD charge level tracing will not show the PD activity but only disturbances not related to the test object. D. Hydro Generator
After maintenance works on a 12 MVA 16.8 kV hydro generator ( see fig. 12) a single phase offline PD measurement was performed to compare the results with previous PD measurements on this machine.
Fig. 10. Test voltage level and PD charge versus time
Fig. 12. Hydro generator to be tested
The results showed typical phase resolved PD patterns for rotating machines and are not to be discussed in this paper. To demonstrate the power of the 3CFRD algorithm three PD acquisition units were connected to one phase (see fig. 13).
Fig. 11. Synchronous PD data evaluation
From the original PD data, as to be recorded with any kind of commercial PD measuring instrument, no significant result can be obtained from the phase-resolved PD pattern due to heavy noise (s. row "All Data" in fig. 11). By performing a pulse-waveformanalysis the 3CFRD visualization shows different separable clusters due to differences in pulse shape of different pulse types. A real-time back-transformation allows viewing the cleared PRPD patterns. Cluster 1 is representing statistical noise; cluster 2 is showing a single burst of pulses (probably resulting from a switching operation close to the HV set up). Cluster 3 finally shows the transformer's PD activity, formerly hidden behind other pulses. A PD fault with ca. 200 pC charge level could be confirmed with this evaluation method. By concentrating only on cluster 3 the PD extinction voltage (PDEV) could be recognized at 13.5 kV. As a consequence for PD monitoring this example
Fig. 13. Measuring set up, 3 MPDs measuring with different frequency Settings
A pulse calibrator was connected to the HV bus bar to simulate PD activity from the generator winding. Figure 13 shows the 3CFRD date evaluation. The 3CFRD showed three separable clusters representing three pulse sources with different energy spectrums. The realtime back-transformation delivered the correlated PRPD patterns. The calibrator pulses, represented by cluster 3, could be made visible even with present noise of higher charge levels. IV. SUMMARY AND PROSPECTS FOR PD MONITORING PD measurements are performed to evaluate the insulation condition of all types of HV apparatus. These measurements are possible under offline and online conditions, respectively. Important and critical components will be more and more
permanently monitored in the future. Beside PD monitoring
Omicron electronics: "MPD600 - Product brief and specification" Schaper, Kalkner, Plath.: "Synchronous multi-terminal on-site PD measurements on power transformers", 14th International Symposium on High Voltage Engineering, Bejing/P.R.China, 2005 [8] Obralic, Kalkner et al.: “Verbesserte Zustandsbewertung durch neue Auswerteverfahren bei der Synchronen Mehrstellen-TE-Messung an Hochspannungsmaschinen,” ETG conference on Diagnosic, Kassel, Germany, 2006 [9] Weissenberg, Farid, Plath, Rethmeier, Kalkner: "On-Site PD Detection at Cross-Bonding Links of HV Cables", CIGRE Session 2004 -Paris, France, 2004 [10] Omicron electronics: "MPD402- Product brief and specification" [11] Plath: "System Concept for Partial Discharge Monitoring on HV/EHV Cable Systems", CMD – „International Conference on Condition Monitoring and Diagnosis”, Changwon, Korea, 2006 [12] Neumann et al.: "The Impact of Insulation Monitoring and Diagnostics on Reliability and Exploitation of Service Life", CIGRE Session 2006 Paris, France, 2006
Fig. 14. Synchronous PD data evaluation
hardware this will require sophisticated concepts to handle the measured data [11,12]. With the exponential increase of computer calculation power several de-noising methods are applicable in real-time to gain relevant PD data from a disturbed set of PD data. As a robust and available tool PD instruments with synchronous multi-channel PD data acquisition deliver reliable results due to advanced PD data evaluation techniques. Their variable system concept allows offline measurements and permanent PD monitoring for all kinds of applications.
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