Offshore Wind Turbines Power Electronics Design and Reliability Research F. P. McCluskey CALCE/Dept. Of Mechanical Engineering University of Maryland, College Park, MD (301) 405-0279 mcclupa@umd.edu 1
Power electronics are an integral part of offshore wind turbine energy production University of Maryland research focuses on the power electronics in the nacelle where variable frequency converters modify the AC electric energy created by the wind-driven generators to high voltage AC (50 or 60 Hz) or high voltage DC for transmission to shore and distribution on the grid.. 2
The Marine Environment The marine environment refers to all the physical, chemical, and biological stressors that would typically be present and acting on wind turbine electronics in an offshore application. The primary concern and the main focus of this study is the effects of the presence of salt and moisture content, in the form either of sea water, sea air, or salt spray on corrosion of the electronics. This corrosion can take many forms and occur at many sites in wind turbine electronics. Some of this concern is mitigated by the practice of normalizing the turbine internal atmospheric environment by removing salt from all air inflows and providing a positive pressure within the turbine itself. The mitigating effect of lower salt and moisture levels remaining in the air around the electronics after normalization will be addressed. There are, however, other physical and electrical stresses related to location offshore that can cause failures as well. Locating wind turbines offshore exposes them to harsh weather conditions such as: highly variable temperatures powerful storms and lightning strikes. These can cause failures resulting from solder fatigue due to temperature cycling or due to the shock or vibration caused by high winds or waves during storms. Application life cycle profiles are being generated based on the marine environmental and operational conditions and extremes. 3
Corrosion in Wind Turbines 4
Corrosion Avoidance and Mitigation 5
Estimation of the overall system Power Electronic System PoF Reliability Assessment PoF mechanism identification Parts arranged in different configurations e.g., series, parallel Failure mechanism 1 mechanism 1 Part 1 Overall system Sub-system 1 Sub-system 2 Sub-system n Part 2 Failure Failure mechanism 222 Part n Failure mechanism n n (R f )d (r f )d r i i i i i i i f da dn A K m Nf = 0.5 ( g/2 f ) c 6
Steps to Addressing Corrosion Failures Design information has been collected for power electronics used in offshore wind turbines. Currently identifying models and mitigation approaches for key failure mechanisms in power electronics in marine environments, such as the following: Electrochemical Migration on Power/Gate Driver Boards Silver Metal Migration Conductive Filament Formation in Power Boards Corrosion of the Direct Bond Copper (DBC) Substrate Corrosion of Copper Leads 7
Electrochemical Migration Electrochemical migration (ECM) is the growth of conductive metal filaments on or in a printed circuit board (PCB) under the influence of a DC voltage bias [IPC-TR-476A]. Solder alloy + - DC voltage source Plating Anode (Cu): electrodissolution Ion transport Cathode (Cu): electrodeposition Substrate Necessary Conditions for ECM Electrical carriers (such as ions). A medium, usually water, to dissolve the ionic materials and sustain them in their mobile ionic state. Electrical potential between the electrodes to establish an ionic current in the liquid medium. Stages of ECM Path formation Electrodissolution Ionic transport Electrodeposition Filament growth 8
Dendritic Growth Solder Solder Mask (green area) Exposed Substrate Dendritic growth occurs on the top surface of the printed wiring boards between adjacent isolated conductors. Electrical bias and surface contamination from flux residue or process residue can contribute to the growth. Surface damage may also play a role. 9
Silver Migration on Lead-Free PCB During THB Testing Sn-3.5Ag Solder on Polyimide Substrate with Immersion Sn Plating 250 μm EDS mapping revealed that silver migrated between the two electrodes. Sn-3.5Ag was the only source of silver in this sample. 10
Conductive Filament Formation (CFF) Conductive filament formation (CFF), also referred to as metallic electromigration or conductive anodic filament (CAF), is an electrochemical process which involves the transport (usually ionic) of a metal across a nonmetallic medium under the influence of an applied electric field. CFF can cause current leakage, intermittent electrical shorts and dielectric breakdown between conductors in printed wiring boards. Anode (Positive) Cathode (Negative) 150 µm 11
Plated Through Hole Plated Through Hole Typical CFF Paths A. Plated-through hole (PTH) to PTH B. Trace to trace (or plane) C. Trace/Plane to PTH 12
Formation of Conductive Filaments Oxidation Sites PTH Epoxy Resin Glass Fiber PTH Cu Cu Cu Cu ++ Cu ++ Cu ++ Cu Cu Cu ++ Cu Delamination Epoxy Resin Reduction Sites Water Monolayers 13
Time to Failure (hrs) Model for Assessing Time-to-Failure for CFF 1600 1200 1000 SM/Pb-Sb/No PC No SM/No PC Experimental data (300 VDC) Experimental data (800 VDC) 300 VDC 800 600 400 T f a f V (1000L m eff ) ( M M n t ) 200 0 800 VDC 0 10 20 30 40 Spacing (mils) T f = time to failure (hrs) a = filament formation acceleration factor f = multilayer correction factor L eff = effective length between conductors (inches) V = Voltage (Volts dc) n = geometry; acceleration factor m = voltage acceleration factor M = Moisture absorbed M t = Threshold moisture content 14
DBC Corrosion Galvanic corrosion between dissimilar metals in contact, such as Al wires on DBC substrates in power modules 15
Copper Lead Failure Initial optical inspection of lead cross-section New module - no vibration or thermal cycling Microcracks visible on edges of lead pads 1 mm 1 mm 16
Other Potential Corrosion Failures Pitting corrosion or crevice corrosion, where moisture and chlorides are present but there is a localized reduction in the oxygen level leading these sites to become anodic and initiating corrosion on substrates or PWB traces; Erosion-corrosion, where any salt or sand particulates from the marine environment can cause wear of metal, or of the native protective oxide films or protective coatings on the metal, for example, on PWB traces, leading to corrosion. Marine biological Corrosion in which microbiological organisms can deposit and exclude oxygen, creating locally corrosive conditions. Furthermore, deposits of these microbiological organisms can short traces together and increase resistance. 17
Microbiologically Influenced Corrosion (MIC) Initiation and acceleration of corrosion due to the interaction between microbial activity and construction materials Applications Offshore construction, piping, ship ballast tanks, sprinkler systems, water infrastructure Example of MIC: Accelerated Low Water Corrosion 18
Conclusion We are developing PoF models for failure of offshore wind turbine electronics in marine environments We are developing methods to integrate these failure models into techniques for system-wide reliability assessments, based on: Fault tree analysis Probabilistic physics of failure Bayesian approaches 19