Comparing generator sizing software
Generator sizing software can help engineers optimize emergency system designs for various load types.
Mission critical facilities, such as hospitals, data centers, financial institutions, and other data processing facilities, are required to protect against possible failures of normal electric utility power sources and provide a reliable alternate supply of power. Engine-driven generators are the workhorses that fulfill this need for emergency and standby power. With critical operations—and many times, human life—relying on these systems, it is important for the engineer to design an emergency system that is sized correctly for the various types of loads. If a generator system is undersized, it will cause voltage fluctuations and damage to the generator or equipment it serves, or worse, lose the load entirely. If it is oversized, it may develop maintenance problems and other issues such as wet stacking.
The most common method for sizing a generator system is total connected load. However, there are various other factors, such as altitude, temperature, fuel type, and application, that influence generator sizing. In today’s mission critical facilities, there are also number of different types of loads, such as fire pumps, chillers, and UPS systems connected to an emergency/standby power system, that can impact the generator size. Each of these load types has a different characteristic during start-up and run time, and reacts differently under various electrical anomalies. Some load types, such as fire pumps, have specific voltage drop (less than 15%) or frequency dip requirements that must be met during start-up. These factors must be considered and analyzed when sizing a generator.
A detailed analysis of the load and sizing parameters to manually size a generator can be a daunting task. However, sophisticated software is available to assist the designer in sizing and selecting the correct engine. Generator sizing software programs in today’s marketplace are intricately developed to allow for a precise selection that is specifically catered to each application.
To take an in-depth look at their characteristics, this article presents a hypothetical project using four popular sizing software programs. The case study includes predefined load parameters and includes tips and techniques to optimize the generator size.
For this example, consider a facility that requires a standby diesel genset to support various loads (see Figure 1). For these loads, assume that the system can tolerate a maximum of 35% voltage dip and 10% frequency dip. The loads that the genset must support are listed in Table 1.
Cummins Power Suite 5.0 GenSize
Cummins Power Suite 5.0 GenSize is a software program developed by Cummins Power Generation, Inc. According to developers, the goal of the program was to have a tool that is easy to use, yet very flexible. The program allows users to input specific parameters, such as voltage drop of motor loads, which allows for maximum flexibility. The program also includes default parameters that help ensure the designer does not undersize a generator. In addition, on each page, it includes page-specific help tools that include tips and direct links for contacting the local distributor. The software is available as an online database that allows the developers to provide current performance data, which is updated regularly to ensure the user is not using obsolete data. Cummins Power Suite GenSize includes an online tool that allows collaborative work in which projects can be shared on the online dashboard among designers or application engineers. The data are not visible to any other user unless the project is specifically shared on the collaboration site. The program is not available in a downloadable version, but projects that have been initiated in earlier versions of the downloadable version can be uploaded to the site.
The example project load data was entered into the program based on a number of available presets (see Table 2). The loads were then assigned to the quantity of steps applicable to the project. After the data are entered and the steps are assigned, the software first selects the smallest genset that meets the performance requirements. For the example project, the software recommended a single 1,500 kW/1,875 kVA genset. In addition to this selection, the program provided alternate selections of generators that may meet the needs and highlights parameters that won’t meet the entered requirements. The program highlights the parameters in two different colors: red if it won’t meet the requirements, and yellow if there are some cautionary items the designer should consider. This feature helps the user understand exactly what is driving the size of the engine and provides guidance on some design changes that could be made to reduce the system size. On the results page, the specification sheets are directly linked to the system-selected engine as well as building information modeling (BIM) results. There are several print features available directly from the results page as well.
Caterpillar Electric Power SpecSizer
SpecSizer is a software program developed by Caterpillar, Inc. It is currently available as a downloadable program from the Caterpillar website, and the software resides on each individual computer. According to developers, the system will be available as an online program by the end of 2013. The program allows users to enter specific parameters, such as percentage of intermittent motor starts, which allows the designer to account for real-world operating conditions when working with systems that have a large quantity of motors. Other features expected in the online version include a specific chiller load parameter, which will help system designers model chiller loads more accurately than the currently available ac load feature.
The load data from the example project was entered into the program based on the available predetermined loads (see Table 1). As the loads are added, the software displays an up-to-date report of the connected load on the load scenario screen. The loads are assigned a step when the load is entered. After the data are entered and the loads are assigned, the program generates a selection based on the smallest generator that will meet the performance requirements.
For the example project, the software indicated that “no standard SINGLE genset is large enough under the specified site conditions. Sizing will continue by paralleling the minimum number of generator sets necessary.” The software recommended four 550 kW/687.5 kVA paralleled units. The initial results were unexpected because Caterpillar offers larger size engines. After further discussion with the software expert, it was discovered that SpecSizer offers three different input parameters for the genset duty. If emergency standby is selected—as opposed to standby or standby/ESP—the program limits the selection to a very narrow rating category that isn’t applicable here. The software expert indicated that this will not even be an option in the next revision of the software.
After the correct engine duty was selected, the software recommended one 2,000 kW/2,500 kVA genset. Under the technical section of the software, the program generates project sizing reports and provides a transient performance report showing the true characteristics to be expected per step. The software also provides links where the user can download the spec sheets or go directly to the website. BIM models are currently not available directly through links from the software. However, drawings and other application data are provided.
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