Our Response Use of Resistive Temperature Detectors as Partial Discharge Sensors in

Rotating Equipment

Claude Kane

Alexander Golubev Electrical Diagnostic Innovations, Inc

Plymouth, MN

Igor Blokhintsev

Cal Patterson Eaton Corporation - Predictive Diagnostics group

Minnetonka, MN and Burlington, ON

Abstract: Much debate and misinformation has been published in various technical journals and in the marketplace on the use of Resistive Temperature Detector (RTDs) as a sensor of partial discharges (PD) in motors and generators. The authors have more experience in this field of technology than any other company in the world. With hundreds of applications working in operating facilities and thousands of data records, this technology is well proven. This paper addresses the concerns in using RTDs as a PD sensor through sound technical analysis and case studies. Specific response to the paper titled Investigations Into the Use of Temperature Detectors as Stator Winding Partial Discharge Detectors presented at the 2006 IEEE International Symposium on Electrical Insulation is made.

I. INTRODUCTION

Four new case studies are presented to once again substantiate the use of this technology. One case study compares signals from an RTD that is installed in the same slot as a Stator Slot Coupler (SSC). A point by point response to the paper referenced in the abstract is not made, however, the conclusions of the 2006 paper are analyzed, and appropriate responses given. In general, points made in the 2006 paper do not reflect our experience and do not recognize the main issues in making PD measurements in the high frequency ranges. The main issue is signal attenuation and having limited sensors, only a very small portion of the winding can be monitored. Each individual sensor has a limited coverage area and provides limited information as to the health of stator insulation. Use of more sensors distributed throughout the insulation will provide more information and the ability to provide a better assessment of the health of the insulation. Thousands of data records and dozens of calibrations show that RTDs are a valuable and inexpensive, ready-to-go technology for the measurement of PD in rotating equipment. Based on both scientific evidence and actual field experience this technology can not be rejected.

II. RESPONSE TO CONCLUSIONS OF THE 2006 PAPER

Table I reviews the conclusions from the 2006 paper and our response to each point made.

TABLE I RESPONSES TO THE CONCLUSIONS OF 2006 PAPER

2006 Paper Conclusion Experiments have been conducted to determine the ability of RTDs in stator windings to detect PD. Tests were done on operating and off-line stators from motors, hydro generators and a turbine generator, as well as more controlled simulated coils in slots. It is clear that in most circumstances PD can be detected by the RTDs. However, rather than the RTD itself detecting the PD, it is the lead (or wires) from the RTD that detect the PD signal. Response This is a well known fact and has never been disputed. Our experience shows that both the RTD and the wiring will detect the PD signals. What difference does it make what picks up the PD? The RTD or the wire? A PD signal is a PD signal. Antennae are used by many companies to detect PD in large power transformers, switchgear and other electrical equipment. If one does not understand the physics of sensing does it mean that there is no sensing? In many cases the leads in a motor or generator act as an antenna. For example, when a pulse was injected into phase B neutral, the highest pulse was detected on the Phase C output lead with opposite polarity. 2006 Paper Conclusion Since the length of the lead wire, its proximity to high voltage stator coils, as well as whether the lead is shielded or not are all usually unknown it is very difficult to interpret the severity of any PD from the magnitude of the detected signals. Response RTDs with spiral armor at times can attenuate the PD signals. This again is not a new revelation and has been presented in technical papers. Experience shows that in about 10-15% of the cases on older large generators and rarely on motors, the RTDs have this spiral armor and in 2-3% of those machines the spiral armor attenuates the signals as the writers state. It looks like the authors have a sample of One and therefore draw a conclusion that this is true for All

Machines. One can usually assess if the RTD wiring has this spiral armor during the installation of the RTD Module/Sensor), therefore it is a known and not an unknown, unlike what the authors state. The responders have extensive experience in the use of RTDs and have calibrated many systems to verify results. Thousands of data records are available showing significant PD found deeper in windings detected by the RTDs and not by coupling capacitors installed at the line terminals. Also, the health of the insulation should not be based on magnitude alone. Pulse count, partial discharge intensity and phase resolved data should be considered during evaluation. The same can be said for traditional coupling capacitors. Without calibration, one has no idea about true magnitude. 2006 Paper Conclusion This is verified by the on-line and off-line PD tests on 8 stators where the magnitude of the detected signals from the RTDs had no correlation to either the detected PD using conventional sensors, nor the known condition of the insulation. Response This is exactly what the responders have been saying for nearly ten years. The main reasons there is little to no correlation between the PD signals on the RTDs and the 80 pF coupling capacitors are twofold: 1. The 80 pF coupling capacitors are not detecting the PD that the

RTDs are since this PD is deeper in the winding. This demonstrates that these coupling capacitors see very little of the winding.

2. The authors of the paper used two types of sensors that have very poor frequency response to PD signals when the tests were performed. A 10 turn HFCT used by the authors is 2.5 to 3 times less sensitive than used in the responders RTD-PD sensors. Even direct connecting to the RTD with a scope probe has better sensitivity. This is still not as good as the sensor the responders normally use for measuring PD from RTDs. Therefore the claims being made about sensitivity are invalid.

The responders have performed dozens of calibration tests over the last ten years on a variety of machines. There is an excellent knowledge base established as far as RTD sensitivity and effectiveness is concerned. The same is true for 80 pF coupling capacitors. One can not start comparing sensitivities of different sensors without understanding the calibration of all sensors. The sensitivity of a coupling capacitor will vary between one that is directly coupled to the bus and one that has a two or three foot lead length. Some professionals feel calibration is not necessary since trending is the key component in PD diagnostics. If that is truly the case, then sensitivity plays no part. 2006 Paper Conclusion These results also reveal other difficulties with interpreting PD results using RTD sensors. Since it is the lead that detects the PD, and since in most motors the RTD leads take a circuitous path from the slot around the stator the lead is likely to detect PD from all three phases. Thus one does not normally see just two clumps of pulses per cycle (often indicative of PD), but 6 clumps that often merge into a mass of pulses across the AC cycle, which makes separating noise from actual stator PD very difficult even for an expert. In addition, the actual position of the pulses with respect to the AC phase position is always unknown, and the pulse polarity information is lost when combined RTD/HFCT sensors are used.

Response Utilizing advanced signal processing techniques, the data is sorted and presented showing the common two clumps. It seems the authors have no knowledge in using such techniques. Thousands of data records from RTDs are available and many have been presented in technical papers that show the common two clumps of data. As a matter fact the authors failed to reference or acknowledge any of the professional technical papers that have been presented on this very specific topic. The actual position of the pulses is not lost. A voltage reference is always used in order to determine phase position. Also, a set of coupling capacitors are ALWAYS installed at the line terminals. The use of both types of sensors is complementary and not mutually exclusive. 2006 Paper Conclusion The conclusion of these investigations is that while PD may be detected by RTD leads, even an expert with decades of experience will have a great deal of difficulty making useful, accurate conclusions on the condition of the stator winding insulation. Response Interpretation of data from the RTDs is no different than data from traditional coupling capacitors. Magnitudes, pulse counts, phase relationships, polarity and trend are all items used in the diagnostics. There is more data to look at since there are more sensors, but more information as to the overall health of the machine is also provided.

III. MAIN ISSUE OF FIELD MEASUREMENTS SIGNAL ATTENUATION

It is well known in that PD signals have significant attenuation especially in inductive equipment such as motors and generators. Due to the attenuation of PD pulses while propagating from the place of origin, all PD sensors in the high frequency range have a limited zone of sensitivity. This feature allows the technology to localize a source of PD activity with certain accuracy and take preventive measures such as locally repair the insulation, replace coils or bars and otherwise save healthy stators from failure.

Type of Sensor It is well accepted that sensors installed at the line terminals of a machine can not cover the entire insulation system. The type of sensor and location of sensors are very important. Fig. 1 shows PD pulses obtained from a 13.8 kV motor that has three 80 pF coupling capacitors and three radio frequency current transformers (RFCTs) installed on the surge capacitor ground circuit. Traces 1, 2 and 3 are the 80 pF coupling capacitors, while traces 4, 5 and 6 are from the RFCTs. The location of the defect in the machine is known and is close to the main termination box. It can be easily seen the RFCTs are far more sensitive (ten times) to the defect than the coupling capacitors. This substantiates that the coupling capacitors see very little of the winding. Due to the frequency response of the RFCTs, they can see deeper into the winding.

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1) CCA: 2 Volt 500 ns 2) CCB: 2 Volt 500 ns 3) CCC: 2 Volt 500 ns 4) SCA: 2 Volt 500 ns 5) SCB: 2 Volt 500 ns 6) SCC: 2 Volt 500 ns

Figure 1 - Comparison of RFCTs sensors on Surge Capacitors ground (traces 4,5 and 6) and 80pF coupling capacitors (traces1, 2, 3) on the same 13.8 kV motor

Use of Multiple Sensors One can expect to have higher magnitudes on the sensor closest to the origin of the PD pulse. The responses of six RTDs to a PD pulse originating in the vicinity of RTD#5 in a 13.8 kV motor are shown in Fig. 2. As can be observed, RTD5 is the closest RTD to the defect since higher magnitudes are present.

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1) RTD1: 200 mVolt 100 ns 2) RTD2: 200 mVolt 100 ns 3) RTD3: 200 mVolt 100 ns 4) RTD4: 200 mVolt 100 ns 5) RTD5: 200 mVolt 100 ns 6) RTD6: 200 mVolt 100 ns

Figure 2 - Response of six RTDs to a PD pulse originating close to RTD 5.

Sorting out Cross Coupled Signals Utilizing proprietary algorithms and advanced signal processing techniques, one can sort out cross-coupled signals and only show pulses from the sensor that is most sensitive to each individual pulse. The authors of the 2006 paper apparently do not understand the use of these advanced technologies and therefore are confused about the 6 clumps of data versus the expected 2 clumps. Fig. 3 shows phase resolved PD distributions for different sensors that have different phase positions and patterns. One

can see PD distributions from six RTD sensors and two 80 pF coupling capacitors. This demonstrates that each sensor is detecting a different PD event.

Figure 3 - Different patterns are shown on the RTDs and coupling capacitors. Most of the PD activity is phase to ground.

Crosstalk can be significant and depends on multiple factors, including winding design, sensor location and the origin of a PD pulse. In some cases, cross-coupling can be significant, and mislead one about the location of the PD site. This is valid for all types of sensors including coupling capacitors. In one case a signal was injected to the neutral of B phase of a small generator stator, and one should expect to see highest magnitude on B phase line lead. However, the highest magnitude was detected on phase C and pulse polarity was opposite to the injected pulse. Calibration performed by injecting signals all over a winding can provide valuable information on sensitivity and crosstalk. Fig. 4 shows the response of three RTD sensors to a signal injected near the RTD in slot #2 (Trace 1) at turbine end. RTD#6 (Trace 2) located +4 slots from RTD#2, while RTD#52 (Trace 3) is located -3 slots at exciter end. The signals from RTD#2 are five times higher than RTD#6 and 15 times higher than RTD#52 located on other end.

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3 >1) RTD2: 10 mVolt 100 ns 2) RTD6: 10 mVolt 100 ns 3) RTD52: 10 mVolt 100 ns

Figure 4 - Signals from RTD#2 are five times higher than RTD#6 and 15 times higher than from RTD#52.

The differences are the combined effect of attenuation along the stator coils, along RTD wires and crosstalk in the stator and between RTD wires. This simple example shows how PD sensors are sensitive to a localized area.

Use of Multiple Sensors and How Many Sensors are Needed To cover a complete stator on a large turbine or hydro generator with equal high sensitivity one would require sensors in every slot on both ends of the stator. Since this is not realistic it is recommend that 12 RTDs located in the most electrically stressed parts of the winding, and that are evenly distributed on the turbine and exciter end of the stator winding be used. It can be argued that it is a difficult task to choose the proper RTD sensor configuration, but reliability of PD measurements must be taken very seriously and it is strongly felt that optimal sensor location is the most important factor of the RTD technology. Use of multiple sensors will provide the user more data to review, but with a significant increase in reliability in determining the health of an insulation system. It is similar to a physician making a conclusion on a patients health based on temperature only compared to a physician using full lab tests and a MRI.

CASE STUDIES

Case 1- Hydro Generator Fig. 5 shows two snapshots from RTDs and 80 pF coupling capacitors on a hydro generator where one can see low or no response from the capacitors, while having high magnitudes on RTD sensors and vice-versa. The stator had two sets of capacitors along the ring bus and 12 RTDs evenly distributed along the stator.

PD pulse in vicinity of RTD#4 no response from CCs

Phase A to phase B discharge detected by CCA2 and CCB2

Figure 5 - Independent registration of two PD events by different sensors.

Fig. 6 shows multiple and different PD patterns detected by the RTDs and 80 pF capacitors. Even if the capacitors connected to the same ring bus 90 degrees apart, have different PD patterns detected by those sensors, which confirm that the sensors are local and picking up different partial discharges. Attenuation of a calibration pulse along a quarter of the ring bus is ~0.4 and one can expect only ~15% of the PD signal coming from the most distant point on the ring bus to the sensor.

Figure 6 - Multiple PD patterns detected in the stator of a hydro generator showing complexity of pulse propagation and attenuation.

Case Study 2 - Comparing RTD and SSC Measurements When making an attempt to compare PD sensors one must understand that sensors connected to different parts of an object, most probably, will detect different PD activity and simple correlation of the response of capacitors to RTD sensors is just wrong. Fig. 7 shows the response of a Stator Slot Coupler (SSC) as well as a RTD that is located in the same slot directly under the SSC on a 20 kV, 300 MW turbine generator. The SSC is installed above the top bar (under the wedge) and the RTD is between the top and bottom bars. Measurements show that both types of sensors have almost identical sensitivity. Fig. 7 shows a snapshot of PD pulses detected simultaneously on SSC72 and RTD72

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1) RTD72: 50 mVolt 100 ns 2) SC72: 50 mVolt 100 ns

Figure 7 - Almost identical response of SSC and RTD sensors to PD pulse.

Fig. 8 shows the PD patterns look almost identical for the three sets of SSCs and RTDs. Minor differences can be explained by the symmetrical position of RTDs between stator bars and the asymmetrical position of SSC. The PD from the bottom bar will be shielded by the top bar and one can expect lower sensitivity on the SSC to PD pulses originating in the bottom bar.

Figure 8 - PD patterns by SSC and RTD sensors located next to each other.

Contrary to the conclusion of the authors of the 2006 paper that RTDs having a spiral armor have low sensitivity, this case study shows the RTD has a slightly higher sensitivity than the SSC. RTDs in this case have a spiral metal armor as can be seen in Fig. 9.

Figure 9 - RTD termination compartment, one can see bundles of RTD wires in spiral armor coming from the stator.

Case Study 3- Swapping of Line and Neutral Leads This study shows the progression of aging insulation on a machine where the line and neutral leads were interchanged. Fig. 10 shows both the similarities and differences of trend from different sensors. The capacitors and RTDs each show different trends illustrating a very interesting case of fast aging of old insulation by electrical stress. The effect of CO2 cleaning of the end winding area is seen on the capacitors.

Figure 10 - Fast aging of electrical insulation in different parts of the stator after swapping line and neutral leads.

Case Study 4 Capacitors and RTDs are Complimentary It is strongly promoted and recommended that both capacitors and RTDs be used to provide a reliable assessment of stator insulation health. Using both types of sensors is complementary and not meant to be mutually exclusive. The capacitors will be more sensitive to PD occurring near the line termination area while the RTDs will be more sensitive to PD occurring deeper in the winding. This provides an opportunity for much more reliable diagnostics. Fig. 11 and Fig. 12 show the Partial Discharge Intensity (PDI) distributions and phase resolved data for two distinctive cases with PD activity mostly detected only by one type of sensor. Fig. 11 shows a case where the detected PD is deeper in the windings and there are very low signals at the coupling capacitors. Conversely, Fig. 12 shows a case where the PD is near the line terminals and the capacitors detect the majority of the PD and the RTDs show fairly low PD activity. In this case the defects are closer to the line terminal area.

Figure 11 - Low PD activity in line termination area are sensed by the capacitors and high PD activity occurring deeper in winding are sensed by the RTDs.

Figure 12 - Significant PD activity in line termination area sensed by the capacitors and low PD activity occurring deeper in the winding sensed by the RTDs.

IV. CONCLUSIONS

Responses to the conclusions of the paper titled Investigations Into the Use of Temperature Detectors as Stator Winding Partial Discharge Detectors presented at the 2006 IEEE International Symposium on Electrical Insulation were presented. Four new case studies also were discussed showing that PD information obtained from multiple sensors (RTDs located throughout the winding) is very valuable. Most

of the conclusions from this paper are a response to the misleading statements that reflect a lack of understanding the measurement of PD in the higher frequency ranges. It is time to stop debating the value of utilizing existing RTDs as PD sensors. With minimum effort from electrical societies, manufacturers and repair facilities, one can have an excellent, natural technology for estimation of stator insulation conditions on-line, verifying quality of repairs, saving costs on maintenance, extending insulation life and preventing critical failures.

REFERENCES [1] S.R. Campbell, G.C. Stone Investigations into the Use of Temperature

Detectors as Stator Winding Partial Discharge Detectors, Conference Record of the 2006 IEEE International Symposium on Electrical Insulation, Toronto, ON, pp 369-375.

[2] IEEE Std 1434-2000 Guide to Measurement of Partial Discharges in Rotating Machinery

[3] Z. Berler, I. Blokhintsev, A. Golubev, G. Paoletti, A. Romashkov, RTD as a Valuable Tool in Partial Discharge Measurements on Rotating Machines, 67th Annual International Conference of Doble Clients, March 27-31, 2000, Boston, MA

[4] C. Kane, A. Golubev, I. Blokhintsev, Further Experience in the Use of Existing RTDs in Windings of Motors and Generators for the Measurement of Partial Discharges, Conference Record of the 2004 IEEE International Symposium on Electrical Insulation, Indianapolis, IN, pp 434-439.

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