By Michael Stuart, Fluke
Imagine your typical substation: filled with transformers, above ground, easy to monitor and inspect as needed. Now imagine an underground vault: filled with water, bad air, rats, poisonous snakes and transformers, confined dark spaces under city streets–but spaces that need to be monitored and, occasionally, inspected and maintained.
Oil-cooled transformers like these can be quickly scanned for potential failure points.
Electrical vault component failures can have severe consequences ranging from power supply interruptions to damaged equipment, fire, explosion or arc flash. For these reasons, regular maintenance is critical.
Where possible, most utilities now remotely monitor the equipment in underground electrical vaults. There are several reasons for this trend, but most of them boil down to the fact that underground electrical vaults are dangerous places for humans.
Justifying Secondary Monitoring
Of course, no system is foolproof, and there must be a way to check up on the sensors that are monitoring the vault equipment. This is especially important given the role vaults play in the power distribution business.
Imagine, for example, the consequences if an oil-filled transformer in a city-based vault exploded. In addition to causing a power-outage, the incident could require digging up streets and disrupting foot traffic, automobile traffic or both. The whole scenario spells a public relations nightmare for the utility and safety concerns for its employees and the public. So, many utilities back up the primary remote monitoring system with a secondary onsite inspection.
Handheld thermal imagers, also known as infrared (IR) imagers or IR cameras, are powerful tools for this secondary inspection. Thermal imagers create electronic color-palette representations of the surface temperatures of electrical components and other objects, a process called thermography. And, since abnormally hot (as well as abnormally cool) operating temperatures may signal the degradation of an electrical device or component, imagers can provide valuable confirmation of what the remote sensors are reporting.
The regular, perhaps annual, use of a thermal imager to confirm what automatic, remote monitors are reporting about vault equipment is a way of “calibrating” the remote sensors to ensure that they are not missing deteriorating equipment on its way to failure. In other words, such tests will help guard against “false positives.”
Utility field teams also take thermal imagers onsite when a remote sensor signals a problem. Thermal data will either support or refute the remote sensor’s finding and assist in further pinpointing the cause of the alarm.
Getting Started with Thermal Imaging
Getting started safely means using extreme caution. The first step involves reviewing the vault’s history. If a thermal image inspection has already occurred within the vault’s history, reviewing those saved thermal images will help the next inspection crew prepare for what to expect–and what to inspect–this time.
Before a crew enters a vault, it needs to perform a preliminary thermal scan of equipment inside from the outside. It needs to determine if any thermal reading signals a short-term failure or impending disaster. Such conditions require an immediate response.
At 94 F, one of the terminals on this 1320 V — 480 V main transformer …
Thermographers and support personnel preparing to enter an electrical vault must also check for flooding and, if it exists, arrange to have the water pumped from the vault. People standing in water, even in rubber boots, performing electrical inspections is a frightening prospect.
… is running about 20 F hotter than it should.
Lighting can be a problem in vaults. The thermography team needs to ensure that there is sufficient lighting to safely descend the ladder into the vault and perform the tasks assigned. If there is insufficient lighting, and the vault history indicates that, then auxiliary lighting must be ordered to properly illuminate the vault and access to it.
Regarding air quality, a recent vault history may describe potential problems, but even if the remote-monitoring installation team encountered no problems, the air in a vault must be tested with an air quality meter before any personnel enter the vault. An indication that explosive or toxic gasses are present mandates that the vault be flushed with fresh air. Then, it should be monitored again before the thermography team enters. And, depending upon the hazard–the nature of the gas, its concentration, etc.–it may be necessary to constantly monitor the vault’s atmosphere whenever it is occupied.
Lastly, before the team physically enters the vault, team members should use the thermal imager to survey from a distance the floor, corners and other places where dangerous reptiles and rats could be hiding. Worries of a bite aside, the greater danger to the inspector is being startled within a confined space and leaping back into something electrified.
And, when they do enter the vault, the team should always leave someone at the entrance with instructions not to enter the vault under any circumstances. This person’s job is to keep unauthorized people a safe distance from the vault entrance and, in the event of an emergency, to call for help and avoid becoming a victim, too.
In short, electrical vaults are dangerous places, and any condition that could impede work or lead to an injury (or worse) must be corrected before personnel enter. After all, the hazards electrical vaults present to people entering them is a major factor in why remote monitoring is so popular.
Monitoring the Equipment
If an alarm condition brought an inspection team to a vault, they will want to pay special attention to the equipment in question. However, having gone through the trouble to enter the vault, most teams will also conduct a general thermal inspection of the entire system: transformers, surge protection and fuse banks, and various types of connections.
Before starting the inspection, let the vault air-out. The tightly confined space will likely either have a higher-than-ambient air temperature, or, in reverse, moisture in the air. Water causes evaporative cooling of hot components. Hotter-than-usual ambient air masks unusual hot spots. Letting fresh air circulate in the vault before conducting a thermal inspections will reduce the chance of problems being masked by heating or cooling effects.
Transformers are usually the central component of an inspection. They are typically cooled by oil and operate at an average of 65 C. Problems with transformers often manifest as overheating or hot spots, making thermal imaging a good tool for finding problems in external connections and cooling components as well as the transformers themselves.
NFPA Standard 70B, Recommended Practice for Electrical Equipment Maintenance, Chapter 9: “Power and Distribution Transformers” is a good reference for general transformer inspection guidelines. The essential external components on an oil-cooled transformer that the thermography team should inspect are:
- High- and low-voltage bushing connections. Overheating in a connection indicates high resistance and that the connection is loose or dirty. Also, a comparison of phases will reveal unbalance and overloading.
- Cooling tubes. On oil-cooled transformers, cooling tubes will normally appear warm. If one or more tubes are comparatively cool, oil flow is being restricted and the root cause of the problem needs to be determined.
- Cooling fans/pumps. Inspecting fans and pumps while they are running will pinpoint problems. A normally operating fan or pump will be warm. A fan or pump with failing bearings will be hot. A fan or pump that is not functioning at all will be cold. (Note: In vaults, sump pumps play a particularly important role in maintaining the operating environment and should be regularly inspected, even if it means partially draining out the water around them.)
For thermography to be effective in pinpointing an internal transformer problem, the malfunction must generate enough heat to be detectable on the outside. Oil-filled transformers may experience internal problems with the following:
- Internal bushing connections. Connections will be much hotter than surface temperatures read by an imager. In a load-break elbow (a shielded and insulated termination that connects underground cables to transformers), internal temperatures can often be six to 10 times greater than what an imager sees on the surface. Even small changes in this surface temperature are often indicative of a more serious problem internally.
- Tap changers. Tap changers are devices for regulating transformer output voltage to required levels. An external tap changer compartment should be no warmer than the body of the transformer. Since not all taps will be connected at the time of an inspection, thermal inspection results may not be conclusive.
Finally, it is important to note that like an electric motor, a transformer has a minimum operating temperature that represents the maximum allowable rise in temperature above ambient, where the specified ambient is typically 40 C (104 F). It is generally accepted that a 10 C (18 F) rise above its maximum rated operating temperature will reduce a transformer’s life by 50 percent.
Line connectors typically operate at the same temperature as the line or slightly cooler, but not much above the temperature of the ambient air. When connectors produce abnormal resistance they will heat up compared to normal connectors. The same statements are true of ground connectors.
Surge protection varies significantly from vault to vault in design and type, but is usually some version of a high-resistance path connected to ground. Because there is no current flow, surge protection normally operates at ambient air temperature.
The breakdown of a surge device is often the result of moisture intrusion and electrical vaults are typically quite moist. The result is persistent but small current leakage to ground, even when a surge is not present. This condition causes the surge arrestor to heat to only a few Fahrenheit degrees above ambient, but to do so continuously. This is a condition that while seemingly insignificant may signify imminent failure. Failure, especially if the device is ceramic, can be catastrophic and pose a threat to any nearby personnel. A thermography team ought not to ignore even slightly overheated surge arrestors.
One common type of arrestor used to protect circuits against excessive transient voltages (lightning in particular) is the varistor. A varistor, when triggered, pushes the current away from the sensitive components. The most common type of varistor is the metal oxide varistor (MOV).
In normal operations, a MOV may operate a few Fahrenheit degrees above ambient because MOVs constantly leak current to ground. Field reports suggest that a MOV operating 5 F or greater above ambient requires further monitoring or testing by a technology other than thermography.
Other equipment, as well as equipment already discussed, falls under NETA (InterNational Electrical Testing Association) guidelines that suggest when the temperature difference (à¢Ë†â€ T) between similar components under similar loading exceeds 15 C (27 F), immediate repairs should be performed. NETA also recommends the same action when the à¢Ë†â€ T between a component and ambient air exceeds 40 C (72 F). If remote monitoring does not correctly report these thresholds, catastrophic failure could occur. Thermographic backup can avert that outcome.
Much of the information referenced and cited above, including directions for thermal vault inspection, is from “Best Practices for Using Infrared Thermography for Condition Monitoring of Utility Substation Assets” by John Snell. ©2008 The Snell Group. Used by the author with express, unique permission.
Michael Stuart is a Level II thermographer and a thermography specialist for Fluke Corp.