Bringing anomalies to light with IR inspection

Load imbalance

Ideally, three-phase electrical systems should have similar operating load conditions on each phase for what is known as a “balanced load.” While a perfect load distribution is not always achievable, load differences should not vary greatly between phases in any given system. In commercial office buildings, hospitals, and other places where lighting and receptacle loads make up a large portion of load, unevenly distributed loads can occur. Conversely, in systems where motors make up a large portion of load devices, loads should be evenly distributed between phases with little variation.

Uneven loads cause conductors to heat unevenly and are readily detectable with an infrared camera. As with all load-generated heat, the entire conductor warms evenly, but the one or more conductors will have either warmer or cooler temperatures (figures 18 and 19).

Figures 18 and 19. The left image is a three-phase circuit breaker with the B-phase circuit under higher load than the C-phase circuit that is under higher load than the A-phase circuit. An ammeter was used to confirm the uneven load conditions. The right image shows elevated heating on the middle and left windings on a dry-type transformer as compared to the right winding. An ammeter confirmed the load imbalance condition observed with the infrared camera.

Harmonic problems

Perhaps one of the least understood of the thermal problems is harmonics. Destructive harmonics are a distortion in the electrical power waveform caused by non-linear loads. Common load devices that cause harmonic problems include:

• solid-state electronics
• office equipment
• high-intensity discharge lamps (HID Lamps)
• electronic lighting ballasts
• variable frequency drives (VFDs)
• UPS systems.

Common symptoms of harmonics are overheated transform windings and neutral wires (Figure 20). When inspecting hot transformer windings, it is important to verify load conditions using a true RMS ammeter. Load conditions should be close to balanced and running within specified current ratings; if not, the problem could be due to a load imbalance or overload condition.

When hot neutral wires area observed, harmonic problems are confirmed and higher load is found on the main neutral conductor than on any of the other hot legs (Figure 21). Another testing method to confirm harmonic issues is using waveform analyzers.

Figures 20 and 21. The right image shows hot windings on a dry-type transformer under balanced loads. When uneven winding temperatures are encountered during an inspection, it’s important to check load conditions to determine the cause of the thermal exception. The right image shows a hot neutral wire under elevated load. Typically, the neutral leg will have a higher load than the hot legs when harmonic problems are present.

Harmonic problems are not as common in newer electrical systems as they are in older systems that have installed electronics, UPSs, VFDs, and other non-linear loads. More recent systems have oversized neutral wire mains and, if necessary, harmonic filtering devices installed to counteract the effects of harmonics (Figure 22).

Figure 22. More recent systems have oversized neutral wire mains and, if necessary, harmonic filtering devices installed to counteract the effects of harmonics. An enhanced neutral system (left) is common in a 120/208 V breaker panel. A typical harmonic filter system (center and right) can be installed in-line with the main electrical trunk.

Inductive heating