A systematic approach for inspecting transformers
November 27, 2019 | By Kavita Sabharwal-Chomiuk
Everyone knows Thomas Alva Edison invented the light bulb, but who invented the transformer, perhaps the most important electrical machine ever developed? The ZBD model alternating-current (A/C) transformer was invented in 1885 at the Ganz Works in Budapest, Hungary, by three engineers: Karoly Zipernowsky, Otto Blathy and Miksa Deri. (‘ZBD’ represents the initials of their last names.)
Today, transformers are everywhere, feeding various voltages into homes and businesses. And when one of them fails in a commercial or industrial environment, that failure can disrupt facility operations significantly.
When commercial or industrial entities assess their infrastructure assets, main transformers with power ratings of 500 kVA and above usually rank very high among risk priorities. With their high cost of replacement, long lead times of typically 12 weeks or more, high swap-in/swap-out costs and many modes of failure impossible to repair on-site, they can become a weak link for reliability within an electrical distribution system.
While a system that has been designed for redundancy—such as main-tie-main configurations, with each transformer loaded at less than 50% of nameplate capacity—can alleviate the impact of a failure, that is not the only consideration. Large amounts of fault energy are available, making some failure modes potentially hazardous to on-site personnel, and many transformers are oil-filled, so flammability and environmental impacts are additional considerations.
It is much better to come up with ways to detect problems and prevent failures in the first place, rather than wait until they occur before rectifying them. A complete transformer failure can easily add up to tens of thousands of dollars in repair and downtime costs.
Fortunately, many types of condition-based maintenance (CBM) technologies can be deployed to catch the early warning signs of failure for transformers.
Infrared (IR) inspection can detect loose connections, weak crimps and cable creepage due to thermal cycling. The key parameters to collect include:
- temperature at booted connections.
- temperature at crimped connections.
- temperature at bolted connections.
- oil tank temperature scan, for hot and cold spots indicating possible problems.
- load tap changer tank temperature (differential to main tank).
Contact ultrasound—also known as structure-borne ultrasound—can detect loose windings and other mechanical issues. This type of inspections checks decibel values at defined test points and conducts waveform analysis for fault type determination.
Airborne ultrasound can detect arcing, tracking and corona, all of which emit high-frequency signals in the ultrasound spectrum above 20 kHz. Similar to the structure-borne approach, the inspection checks decibel values at predetermined test points and analyzes waveforms based on time and frequency to determine the nature of partial discharge (PD).
Ultraviolet (UV) cameras can help confirm the exact location of a corona event, while a visual inspection is useful for detecting dust, water or pest ingress, corrosion, signs of PD, oil leakage, soiled bushings, fan operation issues and stains from previously standing water.
For oil-filled transformers, periodic analyses can detect degradation, leaks and excessive acidity (due to insulation breakdown). Dissolved gas analysis of oil can further detect signs of thermal faults and PD activity inside the transformer chamber. Oil pressure and temperature can be checked by reading gauges. Sampling is important for checking oil quality, including moisture content, dielectric properties and dissolved levels of atmospheric gases, oxides of carbon, hydrocarbons and hydrogen.
Finally, transient earth voltage (TEV) detection—addressing another form of PD event—can find hidden defects inside a transformer’s insulating components. Like ultrasound, it checks decibel levels at predetermined test points. It also plots phase-resolved PDs for comparative analysis.
Accounting for safety
Most of these CBM techniques require the equipment to be energized and operating under normal load conditions to provide useful quantitative data. This creates some safety issues that must be accounted for, especially under the scrutiny of new guidelines in the CE Code and the 2018 edition of NFPA 70E, Standard for Electrical Safety in the Workplace.
If any inspection tasks require opening the doors or covers of the transformer, then there is an elevated risk of an arc flash or electrocution to the personnel involved. With this in mind, workers must be qualified and wear an appropriate level of personal protective equipment (PPE) for the arc flash incident energy available.
At the transformer, this arc flash risk can be significant and a barrier to performing inspection and data collection tasks altogether. Furthermore, the ‘hierarchy of control’ mandates other alternatives to open-panel work must be deployed if possible and practical, including the substitution of non-hazardous for hazardous tasks.
Fortunately, there are practical solutions for virtually all of these types of inspection issues, which substitute safer methods of data collection through a ‘safety-by-design’ approach and the use of electrical maintenance safety devices (EMSDs).
Maintenance inspection systems can be installed on virtually any transformer to allow users to perform visual, IR and UV inspections. A single unit can be used for low-voltage (LV) connections and another for high-voltage (HV) connections.
When the large, rectangular inspection window’s cover is being manipulated, the equipment stays in a closed and guarded condition and technicians do not violate the restricted approach boundary. So, they do not need to wear any special PPE, as there is no increased likelihood of an arc flash occurrence.
Oil sampling ports can also be brought outside of the transformer cable compartment. Some vendors offer retrofit kits that permit safe sampling and provide an optional external gauge and nitrogen insert to relieve vacuum pressure.
(Contact ultrasound, PD and TEV detections are performed on the external ‘skin’ of the equipment in a closed condition, so no special EMSD is normally required in these situations.)
The optimal frequency of different inspection techniques depends on the importance of the assets in question.
Following a failure modes and effects analysis (FMEA) approach by a cross-functional team, each facility should attempt to classify its assets based on replacement cost, lead time, average repair cost, mean time to repair (MTTR), the potential safety and environmental impacts of failure and cost of downtime.
Assets can then be classified, using agreed-upon point system, as (a) critical to the operation of the facility, (b) important to the operation of the facility or (c) supportive but with limited impact to the facility. See Table 1 for inspection frequency examples for different CBM technologies, based on the importance of different assets.
To enable accurate assessments of asset ‘health,’ data must be collected at regular intervals, so long-term trends can be compared. For many of the measured parameters, a baseline can be set for ‘normal’ operation shortly after the transformer goes into service.
The EMSD advantage
In these ways, the use of EMSDs—such as maintenance inspection windows and external oil sampling ports—with transformers can take the danger out of CBM data collection tasks, by eliminating the need to work on energized open panels. With the related safety risks eliminated, inspections become feasible for a single technician, with no cumbersome arc flash PPE. This means data can be collected much more efficiently.
Then, with increased frequency of inspections, problems that could lead to unexpected failures of transformers can be detected earlier and preventative intervention can be initiated. Not only does this ensure regulatory compliance, but it also makes economic sense for the monitoring and protection of important electrical assets.
Experience has shown protecting transformers with fuses alone is not adequate to prevent fires in the event of a short circuit. Instead, the warning signs and possible causes of a short circuit should be detected early on, through CBM techniques.
This article originally appeared in the October 2019 issue of Electrical Business Magazine. With files from FLIR.
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