PARTIAL DISCHARGE MEASUREMENTS ON GENERATORS USING A NOISE GATING SYSTEM Q. SU Department of Electrical & Computer Systems Engineering Monash University, Clayton VIC 3168 Email: email@example.com 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  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  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  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  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  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.  Q. Su and R.C. Tychsen, "Generator insulation condition assessment by partial discharge 6. REFERENCES measurements", IPEC'95, 27 Feb - 1 March 1995, Singapore.  J. W. L. Simpson, R. C. Tychsen, Q. Su, T. R Blackburn And R. E. James, " Evaluation of  Q. Su, C. Chang and R. 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