Load-bank testing ensures performance, reliability
Load-bank tests assure facility managers that their generators will perform well at 100% of their nameplate rating. The absence of regular load-bank testing dramatically increases the potential risk of generator failure during an emergency. Load-bank testing should be a regular part of a generator maintenance program to avoid mechanical failure caused by extreme stress on the engine and other components when the generator is used in a power outage (see Figure 1). It also helps maintain vital engine system parameters such as oil pressure and coolant temperature while preventing wet stacking and carbon buildup.
Wet stacking occurs when unburned fuel accumulates in the exhaust system due to running the generator at less than full load and therefore not producing enough heat to burn off the excess fuel. This accumulation can reduce engine lifecycle and efficiency by fouling the fuel injectors, engine valves, and exhaust system. If the correct mixture of fuel and air is not maintained, the engine will not operate at its highest level and will deliver less power.
As with wet stacking, carbon buildup typically occurs when the generator’s engine runs at less than full load, producing insufficient heat to remove the buildup. If carbon buildup continues unchecked, over time the piston rings can seize, which could allow the superheated fuel/air mixture to sneak past the rings and scorch the sides of the pistons (see Figure 2).
A load bank applies a stable controllable electrical load on a generator. It can be permanent or portable, can accommodate ac or dc, and can range from a few kW to multiple MW. Most load-bank units have an internal fan that cools the resistive elements and discharges the high temperature air away from the unit. It is very important to properly position the load-bank unit away from combustible material and to ensure that it is operated in a well-ventilated environment to keep it—and the generator—from overheating.
The two types of load-bank testing methods are resistive and reactive. Resistive is the most common field testing method because it encompasses the full kW rating of the generator and the engine systems, such as the cooling, exhaust, and fuel delivery system. Reactive testing is typically performed at the genset factory to test kVA ratings.
Load-bank testing schedule
Generators must be able to go from a cold start to a full load in a matter of seconds. Just as the human body needs to be stretched and warmed up before a 100-yard sprint, the generator must be exercised at 30% load for at least 30 min monthly. Testing at this level and frequency keeps engine parts lubricated, prevents oxidation of electrical contacts, consumes fuel before it deteriorates, and enables reliable engine starting.
An annual load-bank test should be performed at 100% of the generator’s kW capacity for a minimum of 1 hr, or loaded to the minimum engine exhaust temperature recommended by the engine manufacturer. The load-bank test should be performed by stepping up the load at certain time intervals (see Table 1).
One of the goals of load-bank testing is to discover problems in a controlled situation rather than during an actual power failure. The following list suggests tips for discovering problems safely:
Load-bank testing should be incorporated into a regular generator maintenance schedule to ensure peak performance and maximum reliability. Testing allows the operator to determine if the genset can run at its full kW capacity (without the risk of having to use the facility’s electrical distribution system and its load). Regularly-scheduled load-bank testing removes unused fuel in the exhaust and carbon buildup in the engine that can damage the genset and compromise its ability to produce emergency power when it’s needed most.
Ballas is the sales team leader for DynaTech, a generator sales and service company in Lebanon, Pa. He has experience creating and selling preventive maintenance agreements for air compressor and emergency generator systems.