Spotting electrical problems with IR thermography
There is perhaps no industry that requires zero room for error than nuclear power generation. Exelon Nuclear, part of Exelon Corp., one of the nation’s largest electric utilities, uses infrared thermography as a key component — along with other technologies such as vibration and lubrication analysis — in its extremely successful predictive maintenance program. Thermography makes it possible to measure the temperature of each part of an entire assembly — including parts that are inaccessible or at high voltage — and do it rapidly and without touching anything. Temperature anomalies often reveal otherwise-undetectable problems.
Once a problem is identified and diagnosed, a comprehensive approach eliminates the possibility of recurrence and improves future maintenance efficiency. The following examples are taken from the component health database, with thermographic data gathered with a FLIR ThermaCAM PM 695.
Chiller control panel hot spot
During routine thermography, a potentially serious hot spot was identified on the main power supply, “B” phase connection in the control panel of a large commercial chiller. The delta-T between phases trended up to 57 F (Fig. 1A), which turned it yellow or in “alert” range in the component health-tracking program. We assumed that the temperature was higher given the insulating wrap installed on the connection. Since the hot spot was on the supply side, the breaker may not have tripped to prevent failure.
An analysis of the bolted connection indicated that there was RTV-type sealant between the areas of contact, causing a high resistance connection and the subsequent heating. Repair procedures were adjusted to prevent a recurrence. Figure 1B shows the result.
Hot spot in distribution panel
During routine thermography, a 244 F hot spot was identified on a lug to the busbar for multiple control power breakers inside a critical panel. The delta-T from similar connections was around 165 F, making this a red component requiring immediate action. As shown in Fig. 2A, some covers on the front of the panel had to be removed to see the connection more clearly (Fig. 2B).
The bolt that holds the lug to the copper bus bar, and carries most of the current, appeared to be visibly loose. This was detected about one month prior to a refueling outage. Making the repair while online would have been risky, so after assessing the risk we decided to complete the repair during the upcoming outage.
This connection would not fail at or below measured temperatures of 600 F, so we did a thermal test every day until the scheduled outage. Fortunately, the temperature remained stable until repairs were completed. Electrical maintenance technicians removed the degraded hardware, cleaned the connection and installed a new bolt on the lug assembly.
Minor hot spot?
In many cases, the control instrumentation of large circuit breakers is just as important as the breaker itself. A terminal board connection in a control panel for a switchgear breaker is a good example of a problem that on the surface, appeared to be minor — and indeed had a low risk of failure — but whose possible consequences were high. During an annual inspection, we found a hot spot on the terminal board connection for a circuit that provides amp indication in the panel and plant control room. The connection was 5.5 F hotter than other terminal board connections, most likely because of a loose terminal screw or inadequate lug crimp connection. Failure of this connection would cause a loss of amp indication and could affect the trip function of the breaker. The trip function of the breaker not only protects the load but also the bus.
Electricians investigated and found the terminal screw loose approximately one-quarter turn. Temperature on the connection returned to normal after corrective maintenance was completed.
|Mike Ralph is program coordinator for the LaSalle (IL) Station Vibration, Thermography and Lubrication Program for Exelon Nuclear. He is a Level II thermographer, Level II vibration analyst and Level II lubrication analyst, and has been using these technologies for nearly 18 years. For more information, go to www.goinfrared.com/electricalproblems .|