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PARTIAL DISCHARGE MEASUREMENTS ON GENERATORS USING A NOISE GATING

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					              PARTIAL DISCHARGE MEASUREMENTS ON GENERATORS
                         USING A NOISE GATING SYSTEM
                                                     Q. SU
                           Department of Electrical & Computer Systems Engineering
                                   Monash University, Clayton VIC 3168
                                      Email: qi.su@eng.monash.edu.au

       Abstract
       Partial discharge level is one of the most important indicators of generator insulation condition.
       However, PD measurements often fail because of the interference problem. A new PD detector
       GDD-3 has been jointly developed by Monash University and Insultest Australia. This detector
       uses advanced digital signal processing techniques and multiple noise gating channels for noise
       discrimination and blocking. With aid of a specially designed high frequency sensor installed at
       the neutral of a generator and several directional noise detectors, partial discharges of an in-service
       generator can be better identified and the insulation condition continuously monitored.

1.   INTRODUCTION                                           insulation condition continuously monitored. Also,
                                                            the test results are recorded and analysed by computer
Partial discharge is one of the most sensitive              software. After analysis, the discharge magnitude and
indicators of generator insulation deterioration.           various statistical distributions can be displayed on
However, PD measurements on in service generators           the screen or printed out. The main results can also be
are very difficult because of the large interference.       saved in a computer database so that further analysis
Noise of various origins can enter a generator from         can be done to determine the trend of insulation
the HV terminals and ground, which significantly            deterioration. The detector includes a main unit, a
effect the accuracy and reliability of PD                   personal computer, two PD sensing units of different
measurements. During the last ten years, the author         frequency bands and various noise sensors.
and his research team have intensively investigated
several PD detection techniques. Based on the               2.    NOISE ON IN-SERVICE GENERATORS
outcomes of research, a new PD detector GDD-3 has
been developed that uses advanced digital signal            An in-service generator can experience noise of
processing techniques and multiple noise gating             various origins. The most important noise sources are
channels for noise discrimination and blocking. The         from the exciter, auxiliary equipment such as
noise detected by noise sensors is used to block a          transformers, motors and corona discharges of HV
small window in the PD measurement circuit. With a          busbars. The main entries of noise to a generator are
specially designed high frequency sensor installed at       the HV terminals and the rotor winding, as shown in
the neutral of a generator, partial discharges of an in-    Fig.1.
service generator can be better identified and the
                       Noise from the
                       thyristor excitor
                                                                                         Noise from the
                                                      Stator winding                     HV busbars

                                               Noise coupled through the
                  DC alternator                rotor winding
                                                       Rotor winding




                                                           Noise from the
                                                           ground

Fig.1 Various noises can enter the stator winding making it difficult to measure PDs on in-service generators
3.   NOISE SUPPRESSION USING A GATING                      for insulation condition monitoring of generators.
     TECHNIQUE                                             This detector uses advanced digital signal processing
                                                           techniques and multiple noise gating channels for
Noise suppression can be realised in a gating circuit,     noise discrimination and blocking. With a specially
as shown in Fig.2. The PD measurement circuit in the       designed dual CT sensor or capacitive coupler
detector is inserted with a fast analogue or digital       installed at the neutral of a generator, partial
switching circuit S. The circuit is controlled by a        discharges of an in-service generator can be
triggering circuit that is activated whenever a noise is   continuously monitored. Test results are recorded and
detected. The switch is kept open for a certain period,    analysed by the computer software. After analysis, the
                                                           discharge magnitude and various statistical
eg 1-50µs, depending on the behaviour of the noise
                                                           distributions can be displayed on the screen or printed
and its oscillating nature. The PD measurement circuit
                                                           out. The main results can also be saved in a computer
is then temporarily blocked and no noise can enter the     database so that further analysis can be done to
detecting circuit. After the noise passes, the switch is   determine the trend of insulation deterioration. The
reclosed and is ready to measure the subsequent PDs.       measurement circuit connection is shown in Fig.4.
For noise from corona discharges and PD from
outside of the generator, it usually last for 1-5µs for    The detector has the following key features:
high frequency band measurements (up to 10MHz)
and 10-20µs for lower frequency band measurements          • Easy to use:
(up to 500kHz). In order to block noise from thyristor        The signals are detected, analysed and calculated
excitors, the blocking window should be 20-100µs              by the computer-based measurement system.
long.                                                         There is no need for expert explanation of test
                          Noise gating                        results.
                             circuit
                                                           • User-friendly interface:
 PD + Noise                                      PD           The window-based computer software has all
                                                              functions in pull-down manual or push button
                                                              format. On-line instructions and help are also
                                                              available.
                        Noise triggering                   • Noise discrimination:
                            circuit
                                                              Using advanced sensors, noise gating channels
                                                              and digital signal processing techniques, most
                                                              interference such as the thyristor switching pulses
Fig. 2   Block diagram of the noise gating system             and noise from the HV terminals can be identified
                                                              and blocked. The HF and LF components of each
The noise blocking can also be illustrated in the             PD are analysed. From comparison between their
diagram of Fig.3 in which the noise pulses are                peak voltages and time delays, discharges from
blocked in several windows marked leaving clean               different positions can be grouped and displayed
PDs detected by the system.                                   in different forms for its identification.
For the detection of air-born noise and those entering     • Portability:
the HV terminals, a RF antenna and Rogowski coils,
                                                              The detector is light in weight and very portable.
or coupling plate sensors are used for different
                                                              The high frequency sensors are installed at the
generators. Noise entering the generator HV terminals
                                                              neutral of the generator and connected to a
are detected by a directional sensor. All noise detected
will generate gating signals. With a certain width of         terminal panel on the generator neutral cubicle.
windows closed to the noise, most interference are            The computer-based detector, including the GDD-
removed from the measuring unit giving more reliable          3 main unit and a notebook computer, can be
measurement results of PDs inside generator                   easily moved around and connected to the panel
windings.                                                     outlets for PD measurements.
                                                           • Database software:
4. GDD-3 PARTIAL DISCHARGE DETECTOR
                                                              The analysis results can be stored in a
Based on the techniques above explained, a new                comprehensive database for future analysis. The
partial discharge detector GDD-3 has been developed           trend of test results for a particular generator can
be also determined for better assessment of its         insulation condition.

                                                                                   Signals + noise




                               Noise detected by antenna                           Noise gate inputs




                          Noise detected by directional sensor


                                                                                Gate operating voltage




                                                                                       Signals output to
                                                                                        A/D converter




 Fig.3 Noise is blocked by blocking windows leaving PD signals to be measured by an A/D converter.
        Exciter                       Generator

                                                                                      HV bus
                                                                                      noise sensor
                                           HF CTs and pre-amp

Noise sensor and
 pre-amplifier
                                                    Noise gating                      Noise gating
                                                      input 1                         inputs 2


                                                                   GDD-3H



                            Replace the HF CTs
                            when using the HV                      GDD-3H main unit         PC for control and data
                            capacitor and sensing                                                 processing
                            impedance box.



                   Fig. 4     Measurement circuit connection of GDD-3 partial discharge detector




   Fig.5     A photo of GDD-3 including the main unit, HV capacitor coupler and a notebook computer
5.   PD MEASUREMENT RESULTS ON IN-                         other generators, motors and DC exciters are
     SERVICE GENERATORS                                    eliminated. After the generator is put into service, PDs
                                                           inside the stator winding can then be reliably
On-line PD measurements were carried out on several        measured. Fig.6 shows some typical test results from
generators using GDD-3 partial discharge detector.         an 80MW hydro-generator. It can be seen that the DC
The test results are encouraging. In most cases, GDD-      exciter noise was completely removed from the
3 can be adjusted to block all noise from outside of a     measurement results after the gating is activated by
de-energised generator. This means that noise from         the noise sensor.




                                                                        Noise from DC
                                                                            exciter


       nC




                                                         (a)



                                                                       Noise from DC
                                                                       exciter is removed



       nC




                                                     (b)

                     Fig. 6 Typical PD measurement results on an 80MW hydro-generator
                          (a) without and (b) with blocking of noise from the DC exciter
6.   CONCLUSIONS                                          [4] A.Wilson, "Stator winding testing using partial
                                                              discharge techniques", IEEE Conference,
Noise identification and suppression are essential in         Chicago, 1987.
PD measurements on in-service generators. Since the
main noise sources are from the DC exciter and from       [5] Q. Su and R.E. James, "Examination of Partial
outside the HV terminals, the special noise sensors for       Discharge Propagation in Hydro-Generator
the detection of air-borne noise and the noise entering       Stator Windings Using Digital Signal Processing
the HV terminals are developed. In conjunction with           Techniques," Proceedings of the 26th
the GDD-3 detector that has 2-4 noise gating                  Universities Power Engineering Conference,
channels, noise from the main sources are blocked in          Brighton, U.K.. 18-20 September, 1991
the gating circuits leading to more reliable
measurements of PDs inside a generator. This noise
                                                          [6] Q. Su, "Techniques for insulation condition
gating technique can be extended to the measurement
                                                              monitoring of electrical plant", AUPEC'95,
of PDs in any HV equipment such as transformers and
                                                              UWA, Perth, 27 -29 Sept 1995, pp.206-211.
power cables, whose noise origins can be identified.
The new PD detector GDD-3 has the advantages of
                                                          [7] Q.Su, R.E.James, T.Blackburn, B.Phung,
noise gating and PD grouping, which can be very
useful for on-line condition monitoring of HV                 R.Tychsen and J.Simpson, "Development of a
equipment. The main advantages of GDD-3 are                   Computer-Based Measurement System for the
                                                              Location of Partial Discharges in Hydro-
• Easy to install. There is no need to overhaul the           Generator Stator Windings," Proceedings of
  generator.                                                  Australian Universities Power and Control
• Advanced noise discrimination techniques                    Engineering Conference, Melbourne, 3-4 Oct.
  including directional sensing.                              1991.
• The whole winding is monitored.
• Continuous on-line monitoring of all generators in      [8] W. Hutter and R. Schuler, "Experience with new
  a station.                                                  PD diagnostic and monitoring systems for
• Comprehensive database and analysis software.               rotating electrical machines", 7th International
• Cheap and easy to maintain                                  Symposium on High Voltage Engineering,
                                                              Dresden, 26-30 Aug. 1991, paper75.01, pp157-
It is recommended that reliable on-line PD detectors          160.
and alarm systems should be installed on large
generators.                                               [9] Q. Su and R.C. Tychsen, "Generator insulation
                                                              condition assessment by partial discharge
6.   REFERENCES                                               measurements", IPEC'95, 27 Feb - 1 March
                                                              1995, Singapore.
[1] J. W. L. Simpson, R. C. Tychsen, Q. Su, T. R
    Blackburn And R. E. James, " Evaluation of            [10] Q. Su, C. Chang and R. Tychsen, “Travelling
    Partial Discharge Detection Techniques on                  wave propagation of partial discharges along
    Hydro Generators in the Australian Snowy                   generator stator windings”, International
    Mountains Scheme TUMUT 1 Case Study",                      Conference on Properties and Application of
    IEEE Trans. on Energy Conversion, Vol. 10,                 Dielectric Materials, 25-30 May 1997, Seoul,
    No. 1, 1995, pp 18-22.                                     Korea, pp. 1132-1135.

[2] H.Huttner,    H.Koglek,   E.Schopper     and
    S.Wenger, "Some aspects on diagnosis methods
    and operational monitoring for large A.C.
    generators", Proc. CIGRE, Paper 11-01, Paris,
    1986.

[3] R.E.James, Q.Su, T.Phung, S.Foong and
    R.Tychsen, "Location of Partial Discharges On
    an 80MW/12.5kV Hydro-Generator With the
    Aid of Digital Filtering Techniques", the
    Proceedings of Electrical Engineers, Australia,
    No.4, Volume, Dec. 1990.

				
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