Abstract 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 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 Partial discharge is one of the most sensitive indicators of generator insulation deterioration. However, PD measurements on in service generators are very difficult because of the large interference. Noise of various origins can enter a generator from the HV terminals and ground, which significantly effect the accuracy and reliability of PD measurements. During the last ten years, the author and his research team have intensively investigated several PD detection techniques. Based on the outcomes of research, a new PD detector GDD-3 has been developed that uses advanced digital signal processing techniques and multiple noise gating channels for noise discrimination and blocking. The noise detected by noise sensors is used to block a small window in the PD measurement circuit. With a specially designed high frequency sensor installed at the neutral of a generator, partial discharges of an inservice generator can be better identified and the Noise from the thyristor excitor insulation condition continuously monitored. Also, the test results are recorded and analysed by computer software. After analysis, the discharge magnitude and various statistical distributions can be displayed on the screen or printed out. The main results can also be saved in a computer database so that further analysis can be done to determine the trend of insulation deterioration. The detector includes a main unit, a personal computer, two PD sensing units of different frequency bands and various noise sensors. 2. NOISE ON IN-SERVICE GENERATORS An in-service generator can experience noise of various origins. The most important noise sources are from the exciter, auxiliary equipment such as transformers, motors and corona discharges of HV busbars. The main entries of noise to a generator are the HV terminals and the rotor winding, as shown in Fig.1. Stator winding Noise from the HV busbars DC alternator Noise coupled through the 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 TECHNIQUE Noise suppression can be realised in a gating circuit, as shown in Fig.2. The PD measurement circuit in the detector is inserted with a fast analogue or digital switching circuit S. The circuit is controlled by a triggering circuit that is activated whenever a noise is detected. The switch is kept open for a certain period, eg 1-50µs, depending on the behaviour of the noise and its oscillating nature. The PD measurement circuit is then temporarily blocked and no noise can enter the detecting circuit. After the noise passes, the switch is reclosed and is ready to measure the subsequent PDs. For noise from corona discharges and PD from outside of the generator, it usually last for 1-5µs for high frequency band measurements (up to 10MHz) and 10-20µs for lower frequency band measurements (up to 500kHz). In order to block noise from thyristor excitors, the blocking window should be 20-100µs long. PD + Noise Fig. 2 Noise gating circuit Noise triggering circuit Block diagram of the noise gating system PD The noise blocking can also be illustrated in the diagram of Fig.3 in which the noise pulses are blocked in several windows marked leaving clean PDs detected by the system. For the detection of air-born noise and those entering the HV terminals, a RF antenna and Rogowski coils, or coupling plate sensors are used for different generators. Noise entering the generator HV terminals are detected by a directional sensor. All noise detected will generate gating signals. With a certain width of windows closed to the noise, most interference are removed from the measuring unit giving more reliable measurement results of PDs inside generator windings. 4. GDD-3 PARTIAL DISCHARGE DETECTOR Based on the techniques above explained, a new partial discharge detector GDD-3 has been developed for insulation condition monitoring of generators. This detector uses advanced digital signal processing techniques and multiple noise gating channels for noise discrimination and blocking. With a specially designed dual CT sensor or capacitive coupler installed at the neutral of a generator, partial discharges of an in-service generator can be continuously monitored. Test results are recorded and analysed by the computer software. After analysis, the discharge magnitude and various statistical distributions can be displayed on the screen or printed out. The main results can also be saved in a computer database so that further analysis can be done to determine the trend of insulation deterioration. The measurement circuit connection is shown in Fig.4. The detector has the following key features: Easy to use: The signals are detected, analysed and calculated by the computer-based measurement system. There is no need for expert explanation of test results. User-friendly interface: 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 discrimination: Using advanced sensors, noise gating channels and digital signal processing techniques, most interference such as the thyristor switching pulses and noise from the HV terminals can be identified and blocked. The HF and LF components of each PD are analysed. From comparison between their peak voltages and time delays, discharges from different positions can be grouped and displayed in different forms for its identification. Portability: The detector is light in weight and very portable. The high frequency sensors are installed at the neutral of the generator and connected to a terminal panel on the generator neutral cubicle. The computer-based detector, including the GDD- 3 main unit and a notebook computer, can be easily moved around and connected to the panel outlets for PD measurements. Database software: The analysis results can be stored in a comprehensive database for future analysis. The 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 Noise sensor and pre-amplifier HF CTs and pre-amp Noise gating input 1 GDD-3H HV bus noise sensor Noise gating inputs 2 Replace the HF CTs when using the HV capacitor and sensing impedance box. GDD-3H main unit PC for control and data processing 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- SERVICE GENERATORS On-line PD measurements were carried out on several generators using GDD-3 partial discharge detector. The test results are encouraging. In most cases, GDD- 3 can be adjusted to block all noise from outside of a de-energised generator. This means that noise from other generators, motors and DC exciters are eliminated. After the generator is put into service, PDs inside the stator winding can then be reliably measured. Fig.6 shows some typical test results from an 80MW hydro-generator. It can be seen that the DC exciter noise was completely removed from the measurement results after the gating is activated by 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 Noise identification and suppression are essential in PD measurements on in-service generators. Since the main noise sources are from the DC exciter and from outside the HV terminals, the special noise sensors for the detection of air-borne noise and the noise entering the HV terminals are developed. In conjunction with the GDD-3 detector that has 2-4 noise gating channels, noise from the main sources are blocked in the gating circuits leading to more reliable measurements of PDs inside a generator. This noise gating technique can be extended to the measurement of PDs in any HV equipment such as transformers and power cables, whose noise origins can be identified. The new PD detector GDD-3 has the advantages of noise gating and PD grouping, which can be very useful for on-line condition monitoring of HV equipment. The main advantages of GDD-3 are Easy to install. There is no need to overhaul the generator. Advanced noise discrimination techniques including directional sensing. The whole winding is monitored. Continuous on-line monitoring of all generators in a station. Comprehensive database and analysis software. Cheap and easy to maintain It is recommended that reliable on-line PD detectors and alarm systems should be installed on large generators. 6. REFERENCES [1] J. W. L. Simpson, R. C. Tychsen, Q. Su, T. R Blackburn And R. E. James, " Evaluation of Partial Discharge Detection Techniques on Hydro Generators in the Australian Snowy Mountains Scheme TUMUT 1 Case Study", IEEE Trans. on Energy Conversion, Vol. 10, No. 1, 1995, pp 18-22. [4] A.Wilson, "Stator winding testing using partial discharge techniques", IEEE Conference, Chicago, 1987. [5] Q. Su and R.E. James, "Examination of Partial Discharge Propagation in Hydro-Generator Stator Windings Using Digital Signal Processing Techniques," Proceedings of the 26th Universities Power Engineering Conference, Brighton, U.K.. 18-20 September, 1991 [6] Q. Su, "Techniques for insulation condition monitoring of electrical plant", AUPEC'95, UWA, Perth, 27-29 Sept 1995, pp.206-211. [7] Q.Su, R.E.James, T.Blackburn, B.Phung, R.Tychsen and J.Simpson, "Development of a Computer-Based Measurement System for the Location of Partial Discharges in Hydro- Generator Stator Windings," Proceedings of Australian Universities Power and Control Engineering Conference, Melbourne, 3-4 Oct. 1991. [8] W. Hutter and R. Schuler, "Experience with new PD diagnostic and monitoring systems for rotating electrical machines", 7th International Symposium on High Voltage Engineering, Dresden, 26-30 Aug. 1991, paper75.01, pp157-160. [9] Q. Su and R.C. Tychsen, "Generator insulation condition assessment by partial discharge measurements", IPEC'95, 27 Feb - 1 March 1995, Singapore. [10] Q. Su, C. Chang and R. Tychsen, Travelling wave propagation of partial discharges along generator stator windings, International Conference on Properties and Application of Dielectric Materials, 25-30 May 1997, Seoul, 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.