Preventing Load Bank Overheating
Facilities use load banks to place electrical loads on power systems to ensure that those systems will operate reliably when needed. If load banks overheat, however, they can present their own reliability issues. This article summarizes causes of overheating and describes how to avoid them.
Most load banks apply load by converting electricity from a power source to heat, then ejecting this heat from the unit. In doing so, a 1000-kilowatt 480-volt load bank must continuously absorb more than 1200 amps per phase and will produce 3.4 million British Thermal Units of heat per hour. This heat must continually be removed from the load bank to avoid overheating.
Heat removal is typically accomplished using an electric-powered blower to produce airflow that ejects hot exhaust from the load bank enclosure. Figure 1 presents a schematic showing how fan-driven air flows through and past load elements to remove the heat. Figure 2 illustrates how this looks in an actual load bank.
To operate properly, a load bank must push enough cool air through the unit to remove the heat it generates. To operate at its rated capacity, the intake air temperature cannot exceed the manufacturer’s specified maximum value. Likewise, the designed airflow rate must be maintained to remove this heat. Anything that interferes with these conditions can cause a load bank to run hotter.
Reasons to Avoid Overheating
While load banks and their components are designed and tested to withstand the temperatures that result from their normal operation, they must be operated and maintained as designed to ensure that overheating does not occur.
One reason to avoid overheating is to prevent short-term or long-term thermal damage to materials in a load bank. Materials and components can deteriorate when temperatures exceed their normal operating ranges. For example, wire insulation and components such as terminal blocks could deform or melt, increasing the risk for electrical fault. In extreme instances, overheating could elevate the risk of fire.
Even if overheating does not cause immediate damage or failure, excess operating temperatures could present long-term stresses to materials and components. For instance, repeated overheating and cooling increases the range of temperatures components pass through during every operating cycle. This can affect the properties of some materials and result in stresses due to expansion and contraction that are greater than the unit was designed to withstand, leading to decreased performance or eventual failure.
Another reason to avoid overheating is the risk this condition could pose to people. Overheated equipment could present a risk to people who touch or work with load banks as well as a fire safety risk if extreme temperatures develop.
Common Causes of Overheating
Limitation of Air Flow
Anything that limits the flow of air through an operating load bank will increase the temperatures in the device and its exhaust. Two common causes include:
Blockage: Load bank intakes and exhausts are covered by screened panels to prevent inadvertent access and avoid objects from being entrained into the equipment. If an object even partially blocks the intake or the exhaust, then airflow will be reduced together with the unit’s ability to dissipate heat.
Inadequate Clearance: If a load bank’s intake or exhaust is located too close to another object, such as an adjacent wall or nearby permanent or temporary equipment, airflow through the unit may be restricted, decreasing its capacity to remove heat from the load elements.
An underperforming blower: If the power to or speed of a fan is inadvertently reduced, the airflow rate could be insufficient to remove all of the heat generated by the load elements. A reduced fan speed could result from a number of causes, including faulty sensors, faulty controls, or a fan motor that develops inadequate power because of wear or a mechanical or electrical issue.
Faulty Overtemperature Sensor: Overtemperature sensors detect excess heating and are used to shut down a unit when out-of-range temperatures occur. Depending on their design, a dirty or faulty sensor could allow a load bank to run at temperatures beyond its designed operating range. Notably, an overtemperature sensor should never be bypassed to enable a load bank to operate – such issues should be evaluated and corrected by a qualified technician.
High ambient air temperature: Manufacturers specify maximum ambient temperatures in which their load banks can operate. If a load bank operates in a hot climate where temperatures could exceed operating limits, a different or larger load bank may provide better service.
High altitude: Because air is less dense at higher altitudes, it absorbs less heat per volume as it flows past load elements, reducing cooling.
Nearby hot equipment: It is important to evaluate all sources of heat near a load bank. If exhaust heat is entrained into a load bank from a nearby device such as a generator or HVAC equipment, it could contribute to reduced efficiency or overheating.
Entrainment of load bank exhaust: High internal operating temperatures can result if any portion of a load bank’s own exhaust is inadvertently recirculated to its intake.
High solar load: In hot environments with high ambient temperatures, heat from intense sunshine can further increase load bank operating temperatures.
Following the appropriate specification, installation, and maintenance practices will mitigate many causes of load bank overheating.
- At high altitudes or where high ambient temperatures can occur, select and derate according to the manufacturer’s guidance
- Where intense sunshine can present a high solar load, consider:
o Locating load banks where they will be shaded by another structure during the hottest times of day
o Providing shade by installing a manufacturer’s shade accessory, when available
o Provide an overhead structure to shade a horizontal discharge load bank
- Consider the location of nearby equipment and sources of heat to ensure that hot air will not be entrained into a load bank
- Ensure that a load bank’s hot exhaust will not be recirculated to its intake or the intake of another unit.
- Install load banks according to the minimum clearances specified by manufacturers (Figure 3).
Regular Inspection and Maintenance
- Keep the area surrounding a load bank free from objects that could impede the flow of intake and exhaust air
- Perform regular inspections to verify that intake and exhaust screens and internal components are free from objects that could limit airflow
- Engage a qualified service provider to inspect, clean, and maintain each load bank according to the scope and frequency recommended by its manufacturer. These services should evaluate the proper function of safety features such as overtemperature and airflow sensors and verify the load bank and its component operate properly.
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