Abstract
Early failures in bearings of wind turbine drivetrains have increased after introduction of power electronic switches, which leads to shaft voltages and bearing currents. In presence of voltage, a rupture of bearing insulation could occur due to several plausible electro-physical mechanisms viz., asperities, electric breakdowns, particles, etc. The flow of high amperage current through the bearing during a breakdown mechanism could lead to early failures. Our aim is to understand the electrical behaviour of a bearing and elaborate by an equivalent electric circuit model, emphasizing on particle-initiated breakdowns. In presence of a shaft voltage, the particles form a path of low resistance through the bearing and results in flow of shaft or bearing currents, which could cause pre-mature failure of the bearing. Particles such as Arizona Test Dust (ATD), carbon black, aluminium powder and fine iron powder were mixed in lubricant at particle concentrations ranging between 7.5 mg/L and 150 mg/L. The breakdown characteristics of electrical insulation of the bearing during a given test is quantified as time of conduction, which is expressed as a percentage of the time the bearing is in the conducting state during a test. An investigation of time of conduction for different lubricant samples was conducted along with studying the effects of start and stops of the rotating shaft. The electrical conductive nature of the particle played no role in breakdown of bearing voltage. At a fixed concentration of 150 mg/L, the insulation breakdown events were highest in lubricant with ATD, followed by iron powder, aluminium power and carbon black particles. The time of conduction increases up to 24 times for the same test lubricant, as the particle concentration was increased from 7.5 mg/L to 150 mg/L. The current activity reduced to almost half in the test after stopping the shaft rotation. The resistance of bearing during insulation breakdown events is highest for aluminium powder, followed by fine iron powder, carbon black and ATD.
Highlights
If the bearing voltage exceeds a certain threshold value, a path of low resistance is formed between the bearing raceways and could lead to a ow of high amperage bearing currents. e source of voltage in this case is caused by charging of stray capacitance within the machine due to the common mode voltage source and causes Journal of Renewable Energy
To establish a further understanding of electrical properties of a miniature bearing when exposed to a sha voltage, an electric circuit model is proposed. e parameters associated with the two proposed electrical states are discussed and in this paper, the breakdown characteristics due to particles of di erent type, size, and concentrations is presented
It is concluded from these measurements that the current activity is highest for Arizona Test Dust (ATD), followed by iron powder, aluminium powder and carbon black particles. e current threshold limit of 150 mg/L for cleanliness levels in gear box oils in wind turbines is recommended to be lowered to 30 mg/L as the time of conduction is reduced by one order of magnitude
Summary
Failure analyses of electrical machines have revealed that prematurely failed bearings have been the leading cause for downtimes in wind power generation industry [1, 2], and petroleum and chemical industries [3,4,5]. e high failure rate in bearings result in large downtimes, which increases costs and reduces feasibility of energy generated from wind turbines. us, higher reliability of bearings is important, and their pre-mature failures should be reduced. ese pre-mature failures have already been linked to the presence of additional electrical stress in the form of stray bearing currents and voltage across the bearing [6]. e stray bearing currents are commonly classi ed as: (i) Capacitive bearing currents [7,8,9,10,11,12], (ii) Capacitive discharge currents popularly knowns as Electrostatic Discharge Machining (EDM) type of bearing currents [7, 13,14,15,16,17], (iii) High Frequency (HF) circulating type of bearing currents [7, 11, 12, 18,19,20], (iv) HF ground currents [7, 21], and (v) Ohmic currents [7, 22].At speeds higher than few 100’s of rpm, the bearing rolls over a thin lm of lubricant that bears the load of operation. Failure analyses of electrical machines have revealed that prematurely failed bearings have been the leading cause for downtimes in wind power generation industry [1, 2], and petroleum and chemical industries [3,4,5]. E high failure rate in bearings result in large downtimes, which increases costs and reduces feasibility of energy generated from wind turbines. Ese pre-mature failures have already been linked to the presence of additional electrical stress in the form of stray bearing currents and voltage across the bearing [6]. Since the lubricating lm formed around the rolling element is electrically insulating in nature, the bearing could withstand certain voltage across its inner and outer raceways; commonly referred to as bearing voltage. If the bearing voltage exceeds a certain threshold value, a path of low resistance is formed between the bearing raceways and could lead to a ow of high amperage bearing currents. e source of voltage in this case is caused by charging of stray capacitance within the machine due to the common mode voltage source and causes
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