A load bank is a self-contained unit which is used to apply controlled load on a power system. Ideally, load banking is a system which is used to determine the maximum backup power performance of a system. This technique should be performed regularly to make sure that the performance system is running efficiently.
Load banking can be used to:
• Verify system operation
• Commissioning and maintenance of a system
• Certify system capacity
Why perform a load bank test?
This test is carried out to ensure that the portable back-up generator is running to its full potential to make sure that it an be able to provide enough power when needed. Since emergency generators are sized so that they can be able to accommodate full start-up load of the facility they are supposed to supply with power, most emergency generators are sized above the normal operating load if they don’t have load management system. This serves as a guarantee that the generator will be able to produce enough energy for the whole facility.
Most generators are sized to about 30% or more of kW rating of a facility. This is important to regulate the surge of the building, as well as the equipment, start up load. However, this challenges diesel powered generators because it can develop conditions such as wet stacking and carbon build up (in piston rings, turbo chargers, injector nozzles, combustion chambers and exhaust piping).
What is wet stacking?
Wet stacking is the unburnt fuel which usually accumulates in diesel exhaust. Usually, wet stacking can be detected after warm up as a black seepage around the exhaust connections or as a continuous black exhaust. To avoid wet stacking, exhaust gas must be maintained at a temperature of about 275⁰ Fahrenheit or else it would lead to exhaust pollution. Load banking plays an important role in the conservation of the environment because it minimizes exhaust pollution when carried out properly and regularly.
For a generator load bank to operate at peak efficiency, its engine must be able to provide the right fuel-air ratio to be able to maintain the right temperature which would help in complete burning of fuel. Wet stacking is very common when diesel engines operate for a long period with little or no load testing procedures. However, due to exhaust pollution and wet stacking problem, EPA has set requirements which have resulted in diesel engine design improvements to minimize wet stacking.
Additionally, the use of electronic controls has increased diesel engine performance and thereby reducing the amount of unburnt fuel from these engines.
Load bank testing for standby applications should per performed annually for a minimum of about two hours to make sure that the system is working optimally.
If you are commissioning an emergency power system, you are supposed to have a load bank testing to help you:
• Ensure proper installation
• Ensure adequate cooling to ambient temperatures
• Determine whether there is sufficient fuel delivery
• Have proper load transfer
Load banking process
It is important to be logical when carrying out load banking because the load bank emits heat. This heat can destroy plants, trees or even discolor the paint of objects found too close or even on the path of exhaust air produced. Also, to facilitate cooling, the load bank should be in a place
b) You are supposed to disable the emergency switch gear to avoid accidental transfer to a facility load.
c) You should confirm that the cable size to be used can be able to carry the full Amp load between the transfer switch and generator breaker depending on the rating given by the manufacturer to avoid overloading the cable.
d) The cables used should carry only the rated load with additional cable length leading to an increase in cable size.
e) To avoid inconveniencies, the facility manager should be notified of the start time and the estimated completion time depending on how long load bank testing will take.
f) For safety, all cables should have color codes before installation to ease identification. Additionally, during load bank testing, you should ascertain that all cables used have proper insulation, correct phasing, and secure termination.
g) After using the master switch to ‘block’ the load, you should start your generator and then allow it to run for 5-10 minutes to warm up.
h) Later, you should apply load stepwise allowing about 15-30 minutes between the steps.
i) While being very careful and ready to remove the load in case you detect problems, monitor the engine closely for leaks, temperature, and oil pressure.
j) You are then required to block load in each step while still recording the ability of the generator to pick up the load in one step by switching the master switch. After blocking the load, you are required to measure the exhaust temperature and then compare it with the recommendations given by the manufacturer for both the engine and exhaust filter if they are installed.
k) Finally, give the generator a 30 minute cool down period without a load.
Before testing the generator, you should review the worst case scenarios with your client since there could be potential risks of failure during testing. Most generators have not been tested to their full capacity since they were last tested after their manufacture and hence load bank testing could result in them breaking down during testing. Ideally, the older the generator, the higher the risk of breaking down. However, the results given from load banking shows the level of neglect or under use of the generator and hence it is important to carry out the test.
The power supply to the facility should be disconnected from the generator while testing to avoid overloading the generator. Additionally, the staff should be notified in advance to prepare in case of a blackout, and the facility runs life support machines which need to run all through. Therefore, backup generators should be stand by to address the situation. Also, testing should be scheduled to the most favorable time of the day, week or month when the situation could be managed easily.
While planning for load bank testing, you should also consider local Environmental Protection Regulations to avoid heavy fines for failing to follow these laws.
Methods of load bank testing
Resistive and reactive testing are the two methods which are used in load bank testing.
• The resistive method is the most common method of the two, and it entails measuring kW and not kVA at the rated power factor. The method is very effective in testing the cooling system, exhaust system and fuel delivery system of engines.• Reactive testing is generator specific, and it is carried out in the factory to test Observations
• If the exhaust is smoke-free during testing, the engine should be repaired. A lot of smoke is caused by rings which have not been set properly, bad fuel or a faulty fuel pump
• If your generator’s temperatures are not within the recommended range, your cooling system, thermostat or water pump could be faulty and needs fixing
• If generator oil falls below the recommended range, the oil pump may need some fixing because it could be having a problem.
Alternatively, the low pressure could indicate an excessive engine wear meaning that the engine should be overhauled.
In documentation, a digital recorder should be connected to the chart frequency, voltage and amperage during one-second sampling. The information recorded should then be downloaded to a performance report. Also, the engine oil pressure and temperature of the nameplate rating of the generator should be documented at 25, 50, 75 and 100 percentages.
Load bank testing should be considered as a necessity to get optimum performance from your generator. If you are a maintenance provider, you should recommend load bank testing to your client since it is essential to know how their generators are performing.
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