Case study: EPSS retrofit at water treatment plant
A water treatment plant required an electrical distribution system upgrade
CDM Smith was contracted in 2017 to design the electrical distribution system upgrade at the City of New Bedford’s Quittacas Water Treatment Plant in Freetown, Mass. The project included the replacement of an existing 2,400-volt switchgear which was used to manually select between two separate utility services and a 2,250-kilowatt diesel-driven generator.
During preliminary design, the stakeholders decided to change the site distribution voltage to 13.2 kilovolts, which is the same voltage obtained from the utility. This case study discusses the design decisions and process to best meet the client’s needs and budget, while also providing a reliable system for this essential drinking water infrastructure for the City of New Bedford.
The first design discussions with the client considered whether the upgrade should replace the existing generator. Because this facility is a water treatment plant, the emergency power supply system was designed in accordance with NFPA 70: National Electrical Code (2017), NEC Article 701, Legally Required Standby.
The treatment plant already had two separate electrical utility services that were confirmed by the utility to be from separate substations, so the team investigated using the additional service as the standby power source. NEC Article 701.12(F) permits a separate electrical service to be used as a standby power source for a legally required standby system.
After further investigation, it was determined that although the utility services come from completely separate substations, the services share common utility poles coming into the plant. These common poles were deemed as a common failure point and the use of Article 701.12(F) and the separate utility service as the standby power source was not fully met.
Due to the proximity of the generator to a drinking water supply source, a directive from the Massachusetts Department of Environmental Protection recommended using a gaseous fuel in lieu of diesel fuel. The design team investigated using a gaseous fuel supply because of this directive. Article 701 of the NEC requires on-site fuel supply except where acceptable to the authority having jurisdiction and as permitted by Chapter 5.1.1(3) of NFPA 110: Standard for Emergency and Standby Power Systems. The use of liquid propane (LP) would meet the requirement for on-site fuel supply, but would have required multiple generators paralleled together to meet the generator set size needs for the essential loads and the existing site would not be able to accommodate the multiple units.
Ultimately, the use of a natural gas driven generator was determined to be acceptable in accordance with NEC 701.12(D)(3) due to the low probability of simultaneous loss of the two electric utility services and the natural gas service.
The final step in the design process before implementation was to determine the size of the generator. The existing generator size of 2,250 kilowatts was oversized as a result of improvements in the water treatment plant processes and increased efficiencies of electrically driven equipment that was replaced because the generator’s installation. The design team sized the generator based on an analysis of the utility bills and by developing a sizing calculation on current operation.
The soft starting and stopping of large raw water and finish water pumps, now controlled by variable frequency drives, was also considered, resulting in a 13.2-kilovolt natural gas generator rated at 1,200 kilowatts. Coordination with major generator equipment vendors was critical on this project to determine the physical size of the generator set including the alternator and remote radiator. The finished product was a natural gas engine generator set footprint similar to a 2,500-kilowatt diesel engine (see Figure 4).
Although most decisions discussed above rely heavily on NEC Article 701, NFPA 110 was also critical in the design of the distribution system design for this upgrade. A main (utility A) – tie – (generator bus) – tie – main (utility B) 13.2-kilovolt switchgear with automatic transfer controls was used as the main distribution system for this water treatment plant. A generator bus was separated by two tie breakers and the generator will only be called to start when both utility sources lose power.
Chapter 6.1.6 of NFPA 110 permits the use of electrically interlocked medium-voltage circuit breakers for source transfer control. The source selection automatic transfer controls graphical user interface is displayed on the front of the 13.2-kilovolt switchgear (see Figure 5).