Light touch: Predictive maintenance for electrical systems: Part 1

Build the right combination of predictive tools and technologies to proactively monitor your electrical systems.

By Sheila Kennedy, contributing editor

The potentially catastrophic consequences of electrical system failures and hazards must be avoided at all costs. Regular predictive maintenance (PdM) on electrical systems lets maintenance teams detect and correct problems before they can shut down equipment or production lines or cause a safety incident.

Whether conducted in-house or outsourced to reliability specialists, PdM inspections – followed up with timely, precise repairs – can protect against electrical accidents and save companies millions of dollars by reducing unscheduled downtime, lowering equipment maintenance costs, and extending the useful life of machinery.

Is your electrical PdM program up to snuff? Here, check out some of the newest electrical PdM tools and applications as well as popular testing options and practical PdM recommendations from industry pros.

Effective tools and technologies

Wide-ranging options exist for online/energized and offline testing of electrical systems. Tom Bishop, senior technical support specialist at the Electrical Apparatus Service Association (EASA), notes that there's a wide scope of available inspection types with these common motor tests:

Offline tests for motors

  • Insulation resistance tests
  • Polarization index tests
  • Motor electrical circuit parameters evaluation (resistance, inductance and capacitance)
  • Rotor influence tests to check for open rotor bars

Online tests for motors

  • Measuring line voltages and currents
  • Motor current signature analysis
  • Vibration analysis for electrically induced frequencies
  • Thermography
  • Ultrasonic inspection

Technology has come a long way in the past 30 years, remarks John Bernet, mechanical application and product specialist at Fluke Industrial Group. “There are now easy-to-use tools available for operators to screen components of electrical systems for potential problems," Bernet says. "Technicians can then troubleshoot the problem to find the specific fault and recommend a correction and then use the same screening tools to verify that the fault has been corrected and quickly get the machine back into service.”

The emergence of the internet of things (IoT) is extending the value of PdM. “By collecting data through connected maintenance technologies, personnel can better evaluate the asset’s utilization, environment, lifecycle and performance,” says Emanuel Kourounis, services business development manager at Schneider Electric. “This data also enables a more condition-based maintenance approach to help companies allocate limited resources in managing their op-ex and cap-ex budgets along with maximizing their general maintenance activities.”

Thermal imaging

One of the most effective PdM technologies for operating electrical systems is infrared (IR) thermography, says R. James Seffrin, director of the Infraspection Institute. “A thermal imager converts the normally invisible infrared radiation emitted by an object into a monochrome or multicolor image that is representative of the thermal patterns across the surface of the imaged object." Many thermal imagers also can provide temperatures of imaged objects, he notes.

Infrared inspections are conducted while the electrical system is energized and under load. Typical defects that may be found include loose/deteriorated connections, overloaded circuits, unbalanced loads, and defective equipment, Seffrin says.

Predictive IR technologies can be employed to mitigate the potential for human error in monitoring and maintaining assets, notes Schneider Electric’s Kourounis. “Asset-specific algorithms, thresholds, and rules can be used to detect thermal abnormalities in the performance of the monitored assets, which can then be further analyzed by industry experts to validate the findings and develop a list of recommended next steps,” he says.

Infrared technology reduces troubleshooting time, saves money, and can be safer – especially when a user is put in a potential arc-flash situation, adds Jim Huekels, business development manager at FLIR Systems. “With thermal imaging technology becoming more affordable, facilities are able to have multiple cameras or ‘eyes’ in the hands of their maintenance personnel," he says. "If you go back five years, users had to spend thousands of dollars on a thermal camera.”

Ultrasonic inspection

Ultrasonic inspection, another nondestructive test (NDT) technique, detects and isolates high-frequency sounds that are otherwise inaudible to the human ear. It is used to differentiate electrical discharge noises from normal sound patterns.

Electrical inspection with ultrasound is being driven by three main factors: safety, electrical maintenance standards, and insurance company interest, says Adrian Messer, manager of U.S. operations at UE Systems.

“Safety is improved because we are able to inspect the energized electrical equipment without having to open it up," Messer says. "Electrical standards, such as NFPA 70B and CSA Z463, specifically mention the use of ultrasound to detect corona, tracking, and arcing from energized electrical equipment." He adds: "And, insurance companies are writing into procedures that before opening anything up to do visual and infrared inspections, ultrasound should first be used to detect any fault that may be producing high-frequency sound.”

It's best practice to use both infrared and ultrasound technologies together, as they detect different failure modes and together provide significant safety benefits, suggests Doug Waetjen, vice president of global operations at UE Systems. “Infrared detects resistance-based problems, like overheating fuses and insulation breakdown, while ultrasound detects ionization-based problems like tracking on loose and faulty connections in switchgear, tracking on transformer windings as well as cracked insulators, and destructive corona in substations,” he notes.

Another effective combination is ultrasonic inspection and vibration monitoring. EASA’s Bishop said he has seen these methods used together to detect failing bearings – a situation that could have escalated into a rubbing fault between stator and rotor that would have damaged the stator windings.

Infrared and ultrasound windows

Infrared and ultrasound windows from companies like Exiscan let inspectors look at and listen to energized switchgear without being forced to open enclosed cabinets, says UE Systems’ Waetjen. “By listening to what’s going on internally and only opening the door when no evidence of tracking or arcing sounds are present, the chances of an arc flash incident are greatly reduced.”

Motor current signature analysis

Motor current signature analysis records and analyzes motor current readings to diagnose faults. This test is able to identify rotor cage failures that result in reduced output torque, increased stator current, and increased motor temperature, says EASA’s Bishop.

Insulation resistance testing

Insulation degradation can be detected with insulation resistance testing. This test is temperature-sensitive, and the readings must be corrected to a common base temperature. Bishop has seen insulation resistance tests identify motors with ground insulation weakness caused by moisture contamination that could have resulted in winding failure to ground if the motor had been started.

Partial discharge testing

Partial discharge (PD) monitoring looks for pockets of electrical breakdown within the insulation of electrical equipment. “In the past, PD monitoring was primarily used on higher-voltage machines rated over five kilovolts, but with the prevalence of low-voltage motors powered by variable frequency drives (VFDs), the use of PD monitoring is increasing,” remarks Bishop.

PD technology is applicable to voltages higher than 2400 volts AC and can be applied to a wide variety of assets within an electrical distribution system, such as switchgear, some cable types, some bus duct applications, large rotating equipment, and power transformers, says Schneider Electric’s Kourounis. “Continuous online monitoring is the ideal methodology for capturing activity and facilitating trending based upon available data,” he says.

Wireless temperature monitoring

Wireless temperature monitoring can be applied to everything from dry-type transformers to cable connections. Thermal monitoring combined with load profiling has proved to be a reliable means of monitoring equipment performance, says Kourounis.

Oil analysis

Oil quality and gases-in-oil analyses are additional PdM testing options. “Transformer oil should be monitored for its physical characteristics compared to the ASTM standards, which might include color, specific gravity, dielectric strength, moisture content, surface tension, contents of acids, carbon or furans,” says Kourounis. Gases-in-oil analysis, as Kourounis explains it, measures the existing percentages of specific combustible gases, such as hydrogen, oxygen, methane, ethane, ethylene and acetylene, and indicates combinations and ratios that indicate potential problems.

Click here to read "Light touch: Predictive maintenance for electrical safety: Part 2"

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