Selecting a standby generator for waste water treatment and water recycling plants. Although most water treatment plants have mains power supply, standby generators are widely used to provide backup power during failure of normal power.
When choosing a standby generator, electrical engineers and plant managers should always assess the details of each piece of equipment and the load that will be connected to the generator. In particular, they should consider the required starting load and harmonic content.
Required starting load
Choose a standby generator with a capacity (rated at 0.8PF by international standards) that is greater than the total load in both KW (real power) and KVA (apparent power).
Unlike normal mains power, a standby generator cannot support any overload above the standby power even for a few seconds. All motors draw from five to eight times more current when starting directly. Therefore, the capacity of the standby generator should be able to accommodate the starting overload.
To reduce the amount of overload there are a number of indirect starting methods that can be implemented including a star-delta starter, auto transformer and a soft starter. Variable Speed Drive (VSD) also helps to minimise the starting load demand as does changing the motor speed.
Starting each load in sequence with only a few seconds in between each load step reduces the generators capacity. In addition, it also allows turbocharged engines to gradually take up the full load. Turbocharged engines only accommodate about 60-70% of the rated capacity in one step. Some manufacturers claim that a 100% step load is possible, however the excess speed drop during the step load makes this feature ineffective.
Mechanical engineers and plant managers should also be aware that some motors, such as those attached to conveyor belts and vibration screens, will start on load. Motors attached to these types of applications require considerably more power when compared to the same motor driving a pump, blower or air compressor.
Calculating the required load of the standby generator can be done manually. However reputable generator manufacturers such as Kohler and SDMO provide sophisticated software to help simplify such calculations.
Harmonic content due to non-linear load such as VSD and UPS
When selecting a standby generator, mechanical engineers and plant managers should also consider harmonic content. High harmonic content causes the motor to overheat and lighting flashing. It also affects the normal operation of the automatic voltage regulator (AVR) of the generator, often causing it to fail. The harmonic voltage should be kept below 10% but most AVRs are able accommodate up to 15%.
Although generators with a low subtransient reactance (X''d), such as 12%, will help reduce the harmonic voltage, those lower than a 10% subtransient reactance are usually oversized and uneconomic.
All VSD and UPS manufacturers offer power supply harmonic filters. Some also provide active harmonic treatment to guarantee that the power supply harmonic is less than 5%. This is always more economic than oversizing the generator alone.
For more information on standby generators contact Compressed Air and Power Systems (CAPS Australia) on 1300 858 763 or email [email protected]