Test items the standards require
The genset tests defined in ISO 8528-6 and GB/T 2820 mainly comprise: continuous running and temperature rise at rated power; steady-state voltage and frequency regulation; transient voltage dip, frequency deviation and recovery time on load acceptance and rejection; overload capability, typically 110% for one hour; and load-sharing accuracy between paralleled sets.
Every one of them needs a controllable, steppable and repeatable load source. Using real consumers for acceptance testing is neither controllable nor safe, which is why load banks are the industry norm.
Avoiding low-load damage
Standby sets running below about 30% of rated load accumulate unburnt fuel in the exhaust — wet stacking — which shows up as black smoke, lost power and rising maintenance cost. A periodic full-load exercise with a load bank raises cylinder temperatures and clears the deposits; it is standard maintenance practice.
Choosing the equipment
Factory test bays generally use fixed high-power banks with the accompanying test software. Site acceptance and maintenance work suits mobile or containerised units that travel by trailer with their own control cabinet and cable drums. Medium-voltage sets from 3.3 kV to 13.8 kV need a purpose-built HV load bank; stepping down through a transformer to a low-voltage bank masks the transient behaviour you are trying to measure.