Cost-Effective Online Partial Discharge Measurements for Electrical Machines: Preventing Insulation Failure

Transcription

1 F E A T U R E A R T I C L E Cost-Effective Online Partial Discharge Measurements for Electrical Machines: Preventing Insulation Failure Key words: online PD monitoring, diagnosis, insulation assessment, motor, generator Introduction Medium-voltage (11-kV) motors or generators are key pieces of power plant equipment. Insulation failure of a motor or generator can result in severe production losses. Partial discharge (PD) monitoring provides early warning so that maintenance can be undertaken to prevent in-service insulation failure. Figure 1 shows the PD levels in three phases of a generator prior to failure of its insulation. The large difference (more than a factor of 40) between the PD levels of phases in good and poor condition and the increasing levels of PD activity with time strongly suggest the presence of faults in the Red and Yellow phases, which are likely to cause failure in the near future. The results of on-site PD measurements may be used as the basis for preventative maintenance, repair, and planned replacement. The demand for near-continuous operation of key equipment, and constraints on preventative maintenance due to limited capital and human resources, mean that PD measurements for insulation assessment must be cost-effective, conveniently implemented, and widely applicable. Research has shown that 56% of failed high-voltage rotating machines show insulation damage [1]. Cost-effective detection of defects in a small percentage of a large population of assets, using PD techniques, demands that the type of PD sensor used and the ease of its attachment to the equipment under test be carefully considered. The portable online PD measuring system described in this article was developed in New Zealand and has been in use there over the last 15 years. It features noninvasive clamp-on PD sensors, which are easily and rapidly attached [2] [6] and have been used for PD measurements on various types of high-voltage equipment. In this article several on-site applications of the system to generators and motors are described and discussed. The article also shows how the clamp-on PD sensors may be used under various site conditions to obtain informative and reliable online test results. The advantages of this portable system, compared with conventional setups, are highlighted. Yafei Zhou and Yang Qin AP EnerTec Ltd., 9A Delph St., Christchurch, New Zealand Colin Leach PanPac Forest Products Ltd., 1161 SH2 Wairoa Rd., Napier, New Zealand A portable online PD measuring system using noninvasive clamp-on PD sensors is described, and some results obtained using the system are presented. The sensors allow online PD tests to be conducted rapidly and cost effectively. The Online PD Measuring System The portable online PD measurement system consists of inductive clamp-on PD sensors and a portable digital storage oscilloscope. Development of the clamp-on PD sensors started in 1994 [7]. They have been used extensively on-site since then, many incremental improvements of the design having been made along the way. They can be temporarily or permanently attached at several positions on an operating machine, avoiding the need for expensive shutdown or disconnection before testing. They function as high-frequency current transducers, with flat response in the range of 200 khz to 100 MHz. A specially September/October Vol. 26, No /07/$25/ 2010IEEE 23

2 Figure 1. Partial discharge levels in the phases of a generator prior to failure of its insulation. selected ferrite core is used to enhance the sensor sensitivity (exceeding 6 V/A). Similar sensors including Rogowski coils (air core), yoke-coils, or high-frequency current transducers have been used for online PD measurements [8], [9]. The digital oscilloscope is a Tektronix TDS3000, selected on the basis of sampling rates, display, weight and dimensions, ease and flexibility of use, and cost. Importantly, it may be battery operated. Partial discharge tests normally start with phase-resolved PD measurements to determine whether PD is present in each phase. The PD patterns are then recorded. Such tests can be readily carried out using some of the sophisticated functions available on the oscilloscope and provide detailed information on PD levels and phase and frequency of occurrence. They usually allow identification of the type of PD and provide some measure of its severity. Observation of continuous displays of real-time, phase-resolved PD measurements is an essential PD measuring process. It confirms the repeatability of the PD signatures, thus helping to ensure that reliable PD measurements are recorded, and it indicates simultaneously whether it is necessary to filter out noise. Figure 2(a) shows phase-resolved PD measurements on three phases of a generator. Positive pulses dominate each phase, suggesting that slot discharge is occurring. Figure 2(b) shows PDs from a generator and six large evenly distributed (in time) pulses due to the brushes of the generator excitation system. Brush noise is often encountered during online PD measurement. These two signal sources are easily distinguished by viewing the continuous real-time measurement display, and thus the risk of incorrect diagnosis is reduced. Channel 1 shows much higher PD frequencies than the other two channels, suggesting that there are more PD sites, or erosion due to PD is faster, in the relevant phase. The oscilloscope sampling rate (>1 GHz) enables the detailed shape of a PD pulse of 10 ns duration to be recorded and displayed. Thus a PD site in a cable can be located within 0.1 m using the PD incident and reflection pulses. Alternatively, the PD wave front and width can be used for the same purpose and to differentiate between PD and noise. Figure 3(a) shows a PD pulse of 20 ns duration originating at one end of a cable 35 m in length. Its width after reflection from the other end of the cable Figure 2. (a) Phase-resolved partial discharge measurements from the three phases of a generator (channels 1, 3, and 4). The ac phase reference (channel 2) is the phase 1 current measured by a clamp-on ac transducer. (b) Partial discharges from a generator and six large regularly occurring pulses from the brushes of the generator excitation system. The ac phase reference (channel 2) is the phase 4 current measured by a clamp-on ac transducer. In (a) and (b), the vertical scales of channels 1, 3, and 4 are 50.0 mv/division. The common time base is 2 ms/ division. and return to its point of origin (after about 420 ns) is approximately 40 ns. (High-frequency components of the pulse are rapidly attenuated during transit.) Figure 3(b) shows a 500-ns pulse of low-frequency interference noise, originating at the far end of a cable connected to a generator. Its width and shape differ considerably from those of a PD pulse. It can be eliminated by appropriately selecting the frequency band of the filter attached to the PD sensor. 24 IEEE Electrical Insulation Magazine

3 Figure 3. (a) Propagation and reflection of a PD pulse initially of 20 ns duration. The vertical scale of channel 1 is 50.0 mv/division, and the time base is 100 ns/division. (b) A noise pulse with 500-ns pulse width. The vertical scale is 10.0 mv/division, and the time base is 2 μs/division. PD Sensor Positions on a Generator or Motor Installing conventional PD sensors on the high-voltage terminations of operating motors or generators can present considerable difficulty, e.g., the performance of the insulation in a relatively small termination box may be unsatisfactory under lightning impulse or switching overvoltages. Sensor installation may be prohibitively costly and is not practicable for many fully insulated terminations. The portable online PD measuring system with clamp-on sensors is a much more cost-effective solution. Figures 4(a) and 4(b) show a practical, noninvasive method for attaching clamp-on PD sensors for online PD measurements on motors or generators. In Figure 4(a) the cable connected to the high-voltage termination of the generator is used as a PD coupling capacitor. If the cable screen earth leads are brought outside the cable termination box, the clamp-on PD sensor (like a PD measuring impedance) can be attached to the earth lead while the machine is in operation. Alternatively, a ring-type PD sensor can be preinstalled on the screen earth lead inside the termination box. Figure 4(b) shows three preinstalled PD sensors with their signal cables placed in a plastic box. If the cable termination of a generator or motor is accessible, the clamp-on PD sensor can be temporarily attached to the earth end of the cable termination, as shown in Figure 5. In that case the PD sensor behaves similarly to a conventional PD measuring impedance connected at the high-voltage end, which often has a higher signal-to-noise ratio than that at the earth end. The polarity and wave shape of PD generated at the motor differ considerably from those of corona discharge generated at the far-end termination. Partial discharge from the motor is not attenuated in the cable, but any corona discharge or noise from the far end of the cable is significantly attenuated on arrival at the motor end. An additional benefit of attaching the sensor to the earth end of the cable termination is that the terminations can then be examined visually or by using an infrared camera. Such examination sometimes reveals insulation or connection problems that might not have been discovered if the terminations had been covered in the termination box. For a large generator, a bus bar, rather than cables, is installed at the termination box. In that case the earth lead for the bus covering can be used for online PD measurement. Alternative positions for noninvasive attachment of the sensors are (a) to the secondary wires of the bus current transformer (CT) in the vicinity of the high-voltage terminations of the generator, (b) to the earth leads of a lightning protection capacitor or arrester, and (c) to the screen earth of a short cable connecting the generator to the voltage transformer in the termination box. A typical application to a large thermal turbine generator is described in the following. The 600-MW 20-kV generator with fully enclosed inaccessible buses had been operating 24 hours a day for almost a year. Conventional online PD measurement would have necessitated taking the generator off-line while the sensors were being fitted, which was contrary to the operator s wishes. Following inspection of the installation drawing of the generator, the CTs on the bus of each phase of the generator were selected for PD coupling, as shown in Figure 6. The CTs were close to the high-voltage winding, and the stray capacitance between CT winding and bus provided good PD coupling. A clamp-on PD sensor was attached to all secondary wires of the CTs while the generator was in operation, and the insulation assessment was completed in an hour. The online PD measurements for each phase of the generator indicated the state of the insulation of each phase, and the points at which the insulation should be visually examined during the next overhaul. The frequency of PD measurements on a generator should be based on the insulation condition indicated by the PD test September/October Vol. 26, No. 5 25

4 Figure 4. (a) Partial discharge (PD) sensor position and the equivalent circuit of the PD measurement. (b) Three PD sensors installed at the cable screen earth leads for online PD measurement. results. Annual PD tests should be adequate for most machines not showing severe PD. Visual examination would be required if the PD level exceeds 1,000 mv and is increasing with time. Online PD Measurements for Motors Motors are key pieces of equipment for large pulp and paper mills. In one local enterprise, online PD tests of the insulation of aged motors, cables, and switchboards were carried out in Seventeen 11-kV motors and a generator were connected to the switchboards, each through a 30- to 50-m cable. Clamp-on sensors were attached to the three core feeder cable terminations in the termination box on the back of switchboard, as shown in Figure 7. During the PD tests, the terminations of some feeder cables connected to motors were selected for online PD measurements. The PD levels in the motors were usually significantly higher than those in the switchboards and cables. Three motors showed severe PD at a factor of 5 to 10 higher than that in a sound phase. Figure 8 shows a phase-resolved PD measurement on one phase of one of the three motors. The PD level was extremely high (2.52 V) in both positive and negative cycles. 26 IEEE Electrical Insulation Magazine

5 Figure 7. Clamp-on partial discharge (PD) sensors attached to the cable terminations at the rear of the switchboard. The top left inset shows how the clamp-on PD sensors are attached. The red cable is the motor feeder cable. Figure 5. Two clamp-on partial discharge (PD) sensors and a clamp-on ac current transducer attached to the cable termination of an 11-kV motor. The yellow channel on the digital storage oscilloscope shows PD from all three phases, and the magenta channel shows PD from only one phase. In these three motors, white powder was found at the slot ends of the conductor bars, as shown in Figure 9(a). Presumably the powder deposit was due to PD erosion. When the conductor bars were removed from the stator, it was clear that the insulation had been severely eroded by the PD, as illustrated in Figure 9(b). The semiconducting layer of the most affected winding bar had been completely eroded. The erosion could loosen the winding bar in the slot, thereby accelerating the erosion. After the three motors had been rewound, tests showed that the PD levels had been reduced to approximately 3% of the earlier levels, as shown in Figure 10. Clearly the PD tests provided essential information that prompted location and repair of the damaged insulation, thus preventing in-service failure. Figure 6. A clamp-on partial discharge sensor attached to the current transformer secondary wires of one of three phases. Figure 8. Phase-resolved partial discharge (PD) measurement showing severe PD (maximum intensity 2.52 V) from one phase of an 11-kV motor. The vertical scale of channel 3 is 500 mv/ division, and the time base is 2 ms/division. September/October Vol. 26, No. 5 27

6 Figure 10. Phase-resolved partial discharge measurement (channel 1 max 83.6 mv) of one phase of an 11-kV motor after it had been rewound. The vertical scale of channel 1 is 50 mv/division, and the time base is 2 ms/division. Figure 9. (a) White powder at the slot end of the conductor bars of a motor, due to severe partial discharge erosion. (b) Winding bars of a motor with severely eroded semiconducting layers. Conclusions On-site applications of the portable online PD measurement system for generators and motors have demonstrated that it is a noninvasive, fast, and cost-effective form of insulation assessment and monitoring. The sophisticated functions of the oscilloscope allow various PD quantities to be continuously displayed in real time, thus providing essential information for preventative or condition-based maintenance, repairs, and planned replacement. In this way the service lifetime of aged assets is extended, the probability of insulation failure is reduced, and the reliability of power plants and systems is enhanced. References [1] R. Brütsch, M. Tari, K. Fröhlich, T. Weiers, and R. Vogelsang, Insulation failure mechanisms of power generators, IEEE Electr. Insul. Mag., vol. 24, no. 4, pp , July/Aug [2] Y. Zhou, P. Chappell, and Y. Qin, Cost-effective on-line partial discharge measurement for cables, IEEE Electr. Insul. Mag., vol. 22, no. 2, pp , Mar./Apr [3] Y. Zhou, K. Gardner, and Y. Qin, A portable partial discharge measuring device for cost-effective insulation assessment, J. Elect. Electron. Eng., Australia, vol. 22, no. 2, pp , [4] Y. Zhou, A. Gardiner, G. Mathieson, and Y. Qin, A portable partial discharge measuring system for insulation condition monitoring, in Proceeding of the International Symposium on Electrical Insulating Materials, 1998, pp [5] Y. Zhou, A. Gardiner, G. Mathieson, and Y. Qin, Partial discharge measurements on the winding bars from a failed machine, in Proceedings of the IEEE Electrical Insulation Conference, 1997, pp [6] Y. Zhou, A. Gardiner, G. Mathieson, and Y. Qin, New methods of partial discharge measurement for the assessment and monitoring of insulation in large machines, in Proceedings of the IEEE Electrical Insulation Conference, 1997, pp [7] Y. Zhou, G. I. Dix, and P. W. Quaife, Insulation condition monitoring and testing for large electrical machines, in Proceedings of the IEEE International Symposium on Electrical Insulation, 1996, pp [8] M. Muhr, T. Strehl, E. Gulski, K. Feser, E. Gockenbach, W. Hauschild Highvolt, and E. Lemke, Sensors and sensing used for non-conventional PD detection, D1-102 CIGRE [9] L. Renforth, R. Mackinlay, M. Seltzer-Grant, and R. Shuttleworth, Online partial discharge (PD) spot testing and monitoring of high voltage cable sealing ends, B1-205 CIGRE Yafei Zhou obtained his PhD degree from The University of Queensland, Australia, in He spent two years with the Queensland Electricity Commission (now Powerlink) Australia, where he developed infrared and ultrasonic techniques for the detection of faulty insulators in insulator strings. In 1994 he joined Industrial Research Ltd. (IRL) in New Zealand, where he initiated research and development on new online partial discharge measuring techniques for insulation assessment and monitoring. These have been applied to numerous pieces of HV equipment and in substations. He was a principal research scientist and engineer, project manager, and HV 28 IEEE Electrical Insulation Magazine

7 lab manager at IRL. He is an International Accreditation New Zealand-approved signatory for high-voltage and heavy-current tests. He managed projects developing a live-line joint resistance measuring device, new aluminum connectors, and intelligent V/I sensors for HV cable state indication. Working in New Zealand over the last 15 years, he has carried out insulation failure studies for power companies, acted as consultant to manufacturers of high-voltage insulation testing equipment, and provided expert evidence for court cases and insurance companies. Since 2006 he has provided technical services through AP EnerTec Ltd. E- mail: yafei.zhou@gmail.com Yang Qin joined the Electrotec group of Industrial Research Ltd. New Zealand as a research engineer in 1996 and became a contract research engineer in She has many years of R&D experience in partial discharge online measuring techniques and on-site insulation assessment of HV equipment and substations. She has also conducted many high-voltage and heavy-current laboratory tests for various power companies and high-voltage equipment manufacturers. nzy.q@163.com Colin Leach is an electrical engineer with PanPac Forest Products Ltd., Napier, New Zealand. Custom-wound on small and large spools Based in Rhode Island, Conneaut Industries is a global supplier of specialty fibers for electrical insulation. Conneaut offers the world s highest quality poly-glass insulating yarns for magnet wire. Precision-wound with proprietary technology, Conneaut packages are defect-free, consistent and uniform. Plastic spools are available in metered lengths, up to 60 ends. Founded in 1930, Conneaut is known for technological support, on-time shipments and excellent customer support. Call John Santos, President, with inquiries YARN (9276) Fax: ISO 9001: info@conneaut.com Incorporated 1930 Shipping: 89 Hopkins Hill Road, West Greenwich, RI Mail: P.O. Box 1425, Coventry, RI September/October Vol. 26, No. 5 29