Charles Millet

Root cause analysis of generator failures

The failure rate in one of Hydro-Quebecs power plants is about three times higher than in the rest of the fleet. Over the years a total of 10 failures occurred for the six units of this plant. Spare bars were put in after every failure and the units returned to service. Apart from a few incidents, the failures were almost exclusively on bottom bars. Failed bars were retrieved and cross-section slices were obtained from them for inspection. The same was done with one un-failed bar removed from service and with a spare bar. Microscopic observations revealed differences between top and bottom bars with regard to curvature of the copper strand radii, the quality of the conductor stacking, the thickness of the groundwall insulation, the type of transposition filler material and the void content. The paper analyses the differences and their impact on the electrical field calculation in the groundwall insulation and the relative contribution of each factor to the high failure rate.

Case study - high ozone concentration in hydro generators

Ozone emission in hydro generators is a sign of surface partial discharge (PD) in the stator windings. Surface PD activity can take different forms: slot PD, corona at the junction of the slot semiconductive coating with the voltage grading coating outside the slot, PD between adjacent coils in the end-winding, PD between adjacent connections and surface tracking. An ozone concentration above the daily exposure limit, for which there is a health hazard for workers, was found at several locations in a 6-hydro generator plant. The ozone level in the vicinity of the generators was in the same range for all of them. This case study presents how this problem was identified and addressed.

Comparative AC and DC voltage endurance tests performed on Roebel bars

Measurement of the charging and discharging current of stator winding insulation under DC high voltage is one of the tests used in the condition assessment of rotating machines. Since the high voltage may be applied for a relatively long period of time during the charging of the winding, its value can be a matter of concern. No aging of the insulation under test should occur while the condition assessment is performed. In order to be able to compare the aging effect of AC and DC high voltage on the groundwall insulation of a stator winding, the so-called Voltage Endurance Test (VET) was performed on 12 spare 13.8-kV Roebel bars. Half the test specimens were subjected to a voltage of 35 kVac; for this test, a ratio of 1.7 between direct voltage and power frequency voltage (rms) was used. The other half were then tested at a voltage of 59.5 kVdc. This paper presents the results of this comparative VET between AC and DC voltages performed on epoxy-mica based insulation of hydro-generator Roebel bars.

Evaluation of the change in winding pattern of a large hydro generator

Uprating and refurbishment of large hydro generators presents opportunities for increase in power output, efficiency and life time of the machine. Several parts of the machine can be changed depending on the need and predicted performance outcome. For the generator unit in this paper, refurbishment of the stator winding is to be done while maintaining the same stator core design, therefore a new winding pattern is proposed for the machine. This paper numerically compares the proposed new winding pattern and the old winding pattern using flux density distribution and radial electromagnetic forces in the air gap. Emphasis is given to the low frequency harmonics with the potential to excite the natural frequency of the core, causing noise and vibration. The calculated total harmonic distortion (THD) of the new winding pattern is higher than the old winding pattern by 3%. This was reflected in the increased damper bar and core losses in the machine with the new winding pattern.

Analysis of turn insulation degradation under voltage surge stress

The turn insulation of multiturn coils is often involved in machine failures. Until now, there has been no type test for detecting the long-term performance of turn insulation. The purpose of this study is to establish an aging test that will sufficiently stress the turn insulation, in order to evaluate and compare the degradation of both turn insulation types: coils with dedicated turn insulation and coils without dedicated turn insulation. Two aging tests were performed on new 13.8 kV coils with dedicated turn insulation. A group of four coils was submitted to 200 surges at 5.8 pu while another group of four coils was submitted to a 2,000 surges at the same voltage. Afterwards, both groups of coils were submitted to an impulse breakdown voltage IBDV test and compared to IBDV on new coils. On coils without dedicated turn insulation, a group of three coils was submitted to 200 surges at 5.8 pu while another group of three coils was submitted to 200 surges at 4.0 pu. Afterwards, both groups were submitted to IBDV test and compared to IBDV on new coils. No sign of induced aging was observed in the turn insulation system under repetitive surges. However, it was observed that IBDV can reveal defects or differences within a same group of coils. To compare coils with and without dedicated turn insulation, dimensions and number of turns need to be the same.

Comparative AC VET on new bars and on bars previously subjected to DC VET

A previous experiment was performed to compare the aging effect of AC and DC high-voltage on the groundwall insulation of 13.8 kV Roebel bars [1]. For this purpose, Voltage Endurance Test (VET) was performed on twelve spare bars. Half of the test specimens were submitted to a voltage of 35 kVac, and failures occurred on all bars in less than 150 hours. For this experiment, a ratio of 1.7 between direct voltage and power frequency voltage (RMS) was used. The other half of the test specimens were then tested at a voltage of 59.5 kVdc. Further in this experiment, since none of the bars subjected to the DC VET had failed during a test period of more than 2000 hours, it was decided to perform an AC voltage endurance test on these bars. This paper presents the test results of a comparative AC voltage endurance test performed on new bars and on bars previously subjected to a long term DC VET.

Investigation of rotor hotspot temperature using Fiber Bragg Gratings

Outward migration of the turn insulations of several poles was observed on every generators in one of Hydro-Québecs power plant and was attributed to thermomechanical stresses. The concern for these rotors is more related to consequences of such degradation and to decide what should be the next course of action considering that the units have been operating for just over thirty years out of 50 years of expected life. If shorted turns start to occur and cause magnetic, thermal or mechanical problems, this may lead to unplanned outage. A prudent alternative would be to re-insulate the poles from class-B to class-F insulation. However, since the actual hotspot temperature is not known and could be above the 155°C of maximum for class-F insulation, the problem could manifest itself again. Therefore, determination of the hotspot temperature was the primary goal of this investigation. Fiber Bragg Gratings (FBG) were used in conjunction with an optical rotating joint to carried out measurements of four of the poles of one unit in the plant. The results of this investigation are presented, highlighting the benefits of carrying localized on-line rotor temperature measurements.

Effect of long-term aging test under DC voltage on Roebel bars

Measurement of the charging and discharging current of stator winding insulation under DC high voltage is one of the tests used in assessing the condition of rotating machines. Since the DC high voltage may be applied for a relatively long period of time during the charging of the winding, the value of the applied voltage can be a matter of concern, and no aging of the insulation under test should occur while the condition assessment is performed. Previous experimentation compared the aging effect of AC and DC high voltage on the groundwall insulation of 13.8-kV Roebel bars. A Voltage Endurance Test (VET) was performed on 12 spare bars for this purpose. Half of the test specimens were subjected to a voltage of 35 kVac, and failures occurred on all bars in less than 150 h. A ratio of 1.7 was used between the direct voltage and power frequency voltage (rms) for these tests. The other half of the specimens were then tested at a voltage of 59.5 kVdc and no failure occurred after more than 2000 h. It was later decided to perform an AC voltage endurance test on bars previously tested under DC voltage, to try to evaluate the effect of DC VET on the integrity of the insulation. Another aspect of the aging process that was evaluated during these experiments was the effect of corona partial discharge activity at the junction of semi-conducting (S/C) and stress-grading coatings. Another six bars of the same production were modified in order to generate sustained corona PD activity to try to establish the associated risk. This paper presents the results of comparative VETs between AC and DC voltages performed on the epoxy-mica based insulation of hydro-generator Roebel bars, as well as results of AC VET performed on six bars previously subjected to a long-term DC VET and the effect of corona activity on the integrity of the insulation.

EIC volunteer profile

Charles Millet of Hydro- Quebec has one of the most important jobs for the 2009 EIC this June. He is the Local Arrangements Chair. The Local Arrangements chair gets caught for a whole variety of preparations involved in organizing a conference. These duties include involvement in hotel accommodations, audio visual resources, arrangements for banquets, rooms for meetings, registration desk set-up, coffee breaks, etc.