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FEATURE TEST & MEASUREMENT


Maintenance inspection windows on transformers


Transformers feed voltages everywhere, so when a transformer fails in commercial or industrial environments, it significantly disrupts facility operations


W


hen commercial or industrial entities perform a criticality assessment of


their infrastructure assets, main transformers with power ratings of 500+ KVA usually rank high in risk priority number tables. With high replacement costs, long lead times (typically 12+ weeks), high in/out swap costs, and implausible onsite repair, transformers can be an electrical distribution system’s weak link. Many transformers are oil-filled, increasing flammability and environmental concerns, and a complete failure easily garners tens of thousands of dollars in costs. Fortunately, many condition based


maintenance (CBM) technologies can catch early signs of transformer failure. Infrared inspection can detect loose connections, weak crimps, and cable creepage from thermal cycling, while contact ultrasound locates loose windings. Ultraviolet (UV) cameras can confirm a corona event’s location. Visual inspection reveals airborne contaminants and water and pest ingress. Most of these CBM techniques require


that equipment be energised and operating under normal load conditions to provide useful quantitative data. Of course, this raises safety concerns. If inspection requires opening transformer doors or covers, arc flash or electrocution risk elevates. Workers must wear appropriate personal protective equipment (PPE) – high risk can prevent inspection and data collection altogether. Fortunately, safer data collection methods can be substituted via a safety- by-design approach and electrical maintenance safety devices (EMSD). Maintenance inspections solutions, like


those from Flir, can be installed on virtually any transformer and allow users to perform visual, infrared, and UV inspections with a single device. Large format IRW-XPx rectangular windows allow a single unit to be used for LV connections and another for HV connections. Equipment remains guarded during window cover manipulation and technicians stay behind restricted approach boundaries. Oil sampling ports can also be brought


outside of the transformer cable compartment, with several vendors offering retrofit kits for safe sampling as


30 JULY/AUGUST 2019 | ELECTRICAL ENGINEERING


Visual • Dust, water and pest ingress • Water stains from previously standing water • Observable partial discharge (if detected by other CBM techniques) • Fan operation • Oil leakage • Soiled bushings


Airborne ultrasound • Decibel values at pre-determined test points • Analysis of waveform (time and frequency domain analysis) to determine PD nature


Structure borne ultrasound • Decibel values at defined test points • Waveform analysis for fault type determination


well as providing an optional external pressure gauge and nitrogen insert. Contact ultrasound and PD (TEV) detection are performed on the equipment’s external skin in a closed condition, requiring no special EMSD. The optimum frequency of different


inspection techniques is a function of asset criticality. Following a cross- functional failure modes and effects analysis (FMEA) approach, each facility should classify their assets based on lead time, mean time to repair, repair/replacement and downtime costs, and potential safety and environmental impacts. Assets are then classified as critical to facility operation, important to facility operation, or as support assets with limited impact to the facility. Often, a ‘normal’ operation baseline


can be set after the transformer goes into service. Asset health assessments are only


possible via regularly collecting data and comparing trends. The key parameters collected for each inspection type are:


Infrared • Booted connection temperature • Crimped connection temperature • Bolted connection temperature • Oil tank temperature scan (for hot and cold spots indicating problems) • Load tap changer tank temperature differential to main tank


Pests and their predators can be attracted to warm, dry spaces inside transformers


TEV (partial discharge) • Decibel values at pre-determined test points • Phase-resolved partial discharge plots (comparative analysis for trends)


Oil sampling • Oil pressure (gauge reading) • Oil temperature (gauge reading) • Oil quality (acidity, moisture content, dielectric properties) dissolved gas levels (ppm) including atmospheric gases, carbon oxides, hydrocarbons and hydrogen


Using EMSD, such as maintenance inspection windows and external oil sampling ports, can eliminate energised open panel work and make CBM data collection safer. Then, a single technician, without cumbersome arc flash PPE, can perform inspections. With increased inspection frequency, potential problems are detected earlier and prevented. Not only does this ensure compliance with NFPA 70E guidelines, but it also makes economic sense to monitor and protect critical transformer assets. Experience has shown transformer protection by fuses alone is inadequate to prevent fires during a short circuit – detecting early warning signs with CBM techniques is key.


Flir Systems flir.co.uk 


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