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.
Figure 6: The main – tie – tie – main 13.2-kilovolt service entrance switchgear has automatic transfer controls. Courtesy: CDM Smith[/caption]
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).