Generators and transfer switches for mission critical facilities
In some larger installations where a single generator is not sufficient or multiple gensets are required to provide system resilience, paralleling switchgear (PSG) provides a scalable and intelligent automation option. Typical PSG systems provide the programming and logic to synchronize the generators and optionally operate a transfer pair of circuit breakers. Some PSG designs include a utility feeder input on the load bus. The utility input circuit breaker must be opened and closed simultaneously with the main generator input circuit breaker such that only one input is closed onto the load bus. In this sense, this equipment could operate like an ATS. The PSG can be programmed to operate in manual open transition, manual closed transition, automatic open transition, and automatic closed transition.
Many configurations can be devised to meet the client’s expectations for system performance. Figure 3 represents a simple generator paralleling configuration with minimal additional distribution costs. Keep in mind that the functionality of this system will be limited to the synchronizing of generators only. A few generator manufacturers can provide synchronizing equipment without the need for sophisticated PSG programmable logic control (PLC) hardware. The ATSs perform the remaining sequencing and operate with little difference from a traditional one-generator configuration.
In Figure 4, the PSG has taken on a larger role in the sequence of operations. It includes a programmable logic controller (PLC) and automatic breaker control of sources and loads. There is a transfer pair that maintains the currently available source to all downstream feeder breakers, though some AHJs reject this technique, so engineers must work closely with the AHJ. During a loss of utility, all breakers open and the generator main and priority 1 emergency feeder breakers close. The individual generator breakers remain open until the generator is synchronized. Simultaneously, all generators are provided a start signal and the first one to stabilize closes to the generator bus. This must occur within 10 seconds (NEC 700.12). As additional generators synchronize with the first, they close to the generator bus. Additional feeder breakers may then close until all feeder breakers are closed or the maximum capacity of the generator system is reached.
Dynamic real-time load demand management functionality can be provided to add and subtract generators from the system as loading dictates. For example, if the peak demand in the system meets a specific programmed setting indicating all four generators are not required, the excess generator may go into a programmed shutdown sequence initiated by the demand management system. This configuration will normally provide a robust facility backup system because of aggregation of generator capacity. It also can accommodate large motor starting currents and nonlinear loads.
On-site fuel storage
The generator run time is largely dependent on the amount of fuel storage. The most common fuel source for a standby generator system is diesel. Natural gas and hybrid generators also are available; this article will focus on diesel.
The first priority for determining the amount of on-site fuel needed to be stored is to study the load applications. If loads are limited to backup for life safety to meet code minimum criteria, the fuel supply must provide a minimum of 2 hours of run time (NEC 700.12(B)(2)). However, if additional standby loads are served or the client indicates a need to provide additional backup time, the fuel storage must be sized appropriately to address the requirement. The amount of fuel stored on-site must be considered; diesel fuel does not remain very stable for long and will foul if untreated and/or stored for an extended period of time (usually more than 3 to 6 months). There are methods for conditioning the fuel, but these methods add complication to the system. To determine the amount of fuel required for the desired run time, obtain product data from a few manufacturers that meet the system specifications. A good rule of thumb is 7 gal/hr/100 kW of generator nameplate at full load. Again, fuel consumption should be reviewed with the specific manufacturer for the project’s generator.
The manufacturer data will most often include the amount of fuel consumption at 50%, 75%, and 100% loading. When determining the amount of fuel storage, the prudent design approach is to use the value for 100% loading. Even if the system is not fully loaded initially, this provides a safety factor to ensure the system run time does not fall below client expectations or code minimum requirements in the event additional loads are incorporated in the future.
There are multiple methods for on-site fuel storage. For exterior generators, a sub-base (or belly) tank is usually the least complicated. It should be sized to provide the amount of fuel required to operate the individual generator through the specified number of hours. In this sense, it is a self-contained fuel system and is a part of the packaged generator system. If a centralized fuel farm is to be provided, then additional components and design will be required.
The fuel storage and supply must be designed by a qualified professional. Each generator will require a day tank in the form of a small fuel storage tank located on or in close proximity to the generator to provide the initial fuel for startup. Day tanks are typically sized to store 15 min to 1 hr of fuel, based on the generator fuel consumption rate. The day tanks will have fuel piping from the bulk storage tank(s) with additional return piping from the generators back to the bulk storage tanks for fuel recirculation.
Recirculation back to the bulk storage tank is generally advised as opposed to being returned to the small day tank. The temperature of the unused fuel is elevated after passing through the engine and, if returned directly to the small day tank, may cause engine trip on fuel over temperature. Returning the unused fuel to the bulk storage reduces this risk because the fuel is cooled when mixed with the higher volume of fuel in the bulk storage tank. Day tanks add capacity to the system in addition to the bulk storage tank, which in most cases allows for a reduction of the bulk storage tank capacity equal to the aggregate total of day tank capacity.
Depending on the type of facility and operational expectations/requirements, the level of resilience built into the system varies. Cost typically is a concern, so if no resilience is expected, then a single generator is likely sufficient. If the client expects a higher level of redundancy, then a sophisticated PSG system can provide the flexibility and logic required for implementation of a highly resilient system.
There are a variety of approaches between a minimalistic single unit and a highly redundant PSG system. As shown in Figure 6, a 12.47 kV system where the estimated peak demand of priorities 1, 2, and 3 on the generator system is 16 MW. The breakdown of demand for each system and feeder is detailed in Table 1. Note the generator system includes some normal feeders (or low-priority optional standby), but the system capacity only accounts for the anticipated priority 1, 2, and 3 loads. The additional normal feeders are included to provide access to the generator for additional non-essential loads as system capacity permit. For lower priority feeders, this approach takes advantage of the diversity in the system loading rather than increasing capacity based on the peak requirements of all connected feeders. When the peak demand exceeds the available capacity of the generator system, the PSG real-time demand management functionality will shed the nonessential loads based on the established priorities.
There are a total of nine 2 MW/2.5 MVA generators connected between three generator buses. This provides a capacity that ensures the top three priorities always have access to generator backup, even upon the loss of one generator. However, it is not uncommon for a generator system to be lightly loaded as diversity of loads based on time of day or seasonal variations for demand is a factor that creates capacity for feeders to additional normal loads. Large PSG systems are, for the most part, not typical installations. However, in our experience with high-profile mega-resorts where the client has a budget that does not support complete property backup and has high expectations to maintain a seamless guest experience whenever possible, PSG systems have proven to be a viable, cost-effective approach.
The robustness of this system is twofold. It is technically an N+1 configuration for the three highest priorities through the use of load pickup and shedding. It also provides the client with the opportunity to maximize the system usage during off-peak times of the year and supply other loads. An additional benefit of the PSG system is flexibility for planned outages or for outages with extended duration as the owner can decide which loads, beyond code minimum, are desired based on the situation.
Determine which system—emergency, legally required standby, or optional standby—is required by code, and carefully select and specify the product to best suit the facility’s needs.
Robert R. Jones Jr. is an electrical project engineer at JBA Consulting Engineers with more than 10 years of design experience. Jones has experience in market sectors including hospitality, commercial, medical, and government projects. He specializes in medium- and low-voltage distribution systems, emergency/standby power systems, renewable energy design and implementation, circuit analysis calculations, and equipment space planning.
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