By Ross Kirschbaum
Within water and wastewater treatment plants (WWTPs) are critical power electrical systems, such as generators that provide backup power during grid outages. But, how do engineers correctly size generators for water applications, while meeting all applicable codes and standards? Also, how do they meet other criteria such as minimizing noise and emissions? These factors all need careful consideration when specifying a generator to ensure reliable and sustainable operation over the long term.
To answer those questions, we first need to understand different types of loads. Water facilities have dynamic electrical loads that impact generator sizing. The single largest load and starting method drives the minimum electrical requirements for motor starting and corresponding alternator selection for the standby generator. Generator manufacturers offer multiple alternators on a single model to provide an oversized alternator when electrical loads have high motor starting requirements.
Sizing considerations
Overall minimum or maximum load requirements can impact the longevity of the equipment. Diesel engines running at low loads (less than 35% of the nameplate rating) for extended periods may experience performance issues when not maintained properly.
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Failure to perform routine maintenance can cause excessive moisture in the oil, leading to scoring or glazing of the cylinder walls and excessive soot and unburnt fuel in the overhead assembly and exhaust piping, which is known as “wet stacking.”
Minimally loading gas generators can also be an issue.
Another crucial factor to consider is the maximum average load factor. This is different from a generator’s electrical power factor. The load factor is the average percentage of loading applied to a generator. The average load factor is defined in ISO 8528-1.
Diesel versus gas
Another factor that may impact the overall size of the generator equipment is the desired power quality performance or acceptable voltage and frequency dips and time to recover. Highly inductive or capacitive loads may present a significant challenge to a generator compared to an “infinite” utility source.
There is a difference in transient performance between diesel and gaseous generators, and ISO 8528-5 defines three class type standards (G1/G2/G3) that generator manufacturers use for each fuel type. Water applications often demand more stringent performance, which stems from large motor starting requirements.
Stationary and mobile emissions
A transition is taking place in some areas, where less utility-scale generation is being produced and there are increasing power demands on the infrastructure. Utility companies are now incentivizing end-users to install equipment that can operate at any time to counter demand or potentially even return surplus power to the grid.
Larger water facilities with substantial megawatt (MW) installation bases are considering moving from emergency standby (Tier 2 or Tier 3) to Tier 4 certified engine generators required for operation in non-emergency applications. Any mobile (non-stationary) generators, commonly seen at treatment plants and pump stations, must also adhere to the latest Canadian emissions standards.
Incentives are offered by utilities for these peak shaving, curtailment, or demand response programs. Utility companies are even providing the equipment themselves on behalf of an end-user in instances for flexibility to control equipment operation times and reduce grid power demand for improved stability.
Understanding Tier 4 applications requires additional exhaust after-treatment components driving complexity with diesel oxidizing catalysts, selective catalytic reduction, and/or diesel particulate filters.
Optimizing load step management
The starting sequence of loads or load set management is important in balancing the starting and running loads and sizing the generator properly. As referenced earlier, the single largest load and starting method will drive the required starting kilovolt-amps to select the correct alternator to use for a specific generator model.
Therefore, we must ask specific questions to understand the application and sequence of steps. For example, how many loads are considered emergency or life safety loads? Are line-input filters required to reduce line harmonics? How is the starting sequence initiated (e.g., SCADA System, ATS, VFD Master Control, or other)? These questions, and others, can help optimize load management as required.
Motor starting methods and alternator selection
The starting method for a motor is also a significant factor in ensuring an appropriately sized generator. A reduced start method like a soft-start, VFD, or wye-delta may be more cost-effective than up-sizing a generator to handle the large instantaneous load in a worst-case across-the-line start (ALS). In many cases with an ALS, any high horsepower motor will require such an oversized alternator, or it will not work with the generator model sized for the running loads.
A reduced voltage starting method decreases the size of equipment. Input filtering or a higher quality VFDs (18-pulse vs. 6-pulse) can mitigate overall harmonics on the alternator,
reducing equipment size. Typically, ensuring less than 10 – 12% total harmonic distortion (THD) ensures the VFD will stay engaged, but design engineers should reference and specify required power quality characteristics to avoid the VFD disengaging.
Harmonics impact from inductive and capacitive loads
Non-linear loads can drive larger alternators within the system to manage harmonic distortion of the waveform. Non-linear loads create harmonics (i.e., current harmonics). Depending on its characteristics, an alternator simply enhances or mitigates the voltage distortion (i.e., voltage harmonics) at its output terminals.
Several alternator features can be specified to mitigate harmonics. For instance, an alternator with lower sub-transient reactance will produce less voltage harmonics. Meanwhile, Class H insulation offers better “cushion” than Class F insulation against overheating, and a lower alternator temperature rise reduces the chances of overheating.
Future considerations
For water applications, current sizing needs are often compared to potential future state scenarios. Increased power demands are usually a direct result of projected population growth and the need for larger facilities. The focus is to understand optimal-sized equipment requirements and standardize generator models in multiple scenarios to allow modular plug-and-play for future equipment to address growth. This provides for commonality of parts, identical service requirements, and familiarity with the same footprints.
There are many benefits when connecting multiple generators to act as a larger power source, including redundancy, efficiency, different kW size generators, diesel and gas combinations, and value and scalability. However, a few items to be aware of are space constraints for the multiple generator systems, more complex operations & maintenance, and additional complexity in the overall control system.
Considerations must be made for low versus medium voltage generation on larger projects. When generating at 5kV or 15kV class, it must be decided whether to have a disconnect at the generator or in a switchboard panel. This can be a fused disconnect or a medium voltage breaker; however, both are large gear sections and drive custom packaging inside the generator enclosure.
Medium voltage automatic transfer switch (ATS) requirements can increase packaging footprint. Manufacturer assistance early in the design phase can address physical size, footprint, and power delivery voltage variances, clearly outlining low or medium voltage advantages and disadvantages. Medium voltage applications require a decision about where to locate and wire a fused disconnect (when desired) and a potential neutral grounding resistor (NGR), as well as determining if it should be arc-resistant and required withstand ratings.
Additionally, the human-machine interface (HMI) monitors and protects the engine and alternator. A controller with UL 6200 is the latest standard for generator controls and may allow for removing the line circuit breaker on the generator, protecting against overcurrent conditions. It can also act as an energy reduction maintenance mode switching device, as now required by the latest code standards.
Finally, a breaker-free generator with a UL 6200 controller can facilitate short-circuit selective coordination.
Ross Kirschbaum is with Rehlko. Email: ross.kirschbaum@rehlko.com
This article appeared in the April 2025 edition of Environmental Science & Engineering Magazine. Read the full issue below.
