Comparing generator sizing software

12/12/2013


Generac Power Design Pro

Power Design Pro is a software program developed by Generac Power Systems, Inc. The software can be downloaded from the Generac website to an individual computer. After the registration process, program operation begins with the project setup. This software includes very thorough data preset options, such as NEC application, enclosure types, fuel tank options, and run time. A user settings tab offers overall parameter data input for future project use if desired. The software also offers downloadable material from the start-up page including specifications, spec sheets, installation instructions, and an engine emissions certificate for the recommended genset. 

After the project setup is complete, the user adds loads from the predetermined load menu. This software offers the largest selection of load options compared to the other programs (see Table 2). After the load data are entered, the loads are assigned to either a group (non-concurrent load) or a step (concurrent load). The load summary page is continually updated with the recommended generators as loads and steps are added. 

A unique feature of Power Design Pro is the ability to manually select the generator. Instead of using an autoselect function, the designer may input a specific kW rating and configuration. For the example project, after the loads were entered under the autoselect function, the program recommended six 500 kW/625 kVA paralleled generators. When switched to the manual function, and 600 kW units were selected (the maximum kW rating offered by Generac), the program recommended three 600 kW/750 kVA units. Although it wasn’t recommended by the autoselect function, alternate quantities and sizes of generators were allowed to be selected that still met the project parameters. If a system that will not meet the parameters is selected, the entries are highlighted in red indicating that the manual selection would not meet the project parameters. This is a very flexible option for the program, allowing the engineer to determine the quantity and size of the generators for each specific application. Although the program does not appear to automatically select the lowest quantity and kW rating that can be used, the flexibility is a benefit. 

Figure 2: This one-line diagram of the example facility represents selected strategies for optimizing emergency system designs for various load types. Courtesy: ccrdAfter the generator system has been selected, the mechanical design tab assists with gas and exhaust piping layout and paralleled generator configuration. The analysis tab provides direct access to several reports including a project summary and transient and harmonic analyses. 

Kohler QuickSize

QuickSize is a software program developed by Kohler Power Systems. It is an easy-to-use, Web-based application that requires a login and password from a local distributor. The login information is the same for each member of a company for easy file sharing. All projects developed using this software are automatically available to the Kohler representative and can be viewed if additional help is desired. Specification sheets and other PDF drawings are available through the site. However, CAD drawings and BIM models must be requested through a representative. After running through the sizing software, a custom specification is available (as a Word document) for the project based on the parameters and genset selected. The specification can be used as a whole, or copied and pasted into another specification format to suit the user’s needs. Similar to the other programs, QuickSize uses preset pull-down buttons for entering parameters, which can be revised during any step of data entry. All of the information is shown on one screen, and all of the data can be changed at any time. 

First, the load steps are entered, and then specific loads are added. The loads can be moved up or down between the steps at any time, and all information is easily editable throughout the process. After all data are entered and load steps are assigned, a click of the search result button yields a list of applicable gensets, listed by model number, alternator, and engine with the smallest recommended size at the top. Engines that will not meet the requirements are indicated in red at the bottom with an explanation for the program’s rejection. For the example project, the software recommended a single 1,500 kW/1,875 kVA generator. From here, a direct link to a summary report of all the inputs and other parameters used is available, and specifications may be created. 

Kohler indicated it will continue to develop its software program to provide the necessary tools and options to enhance the user’s experience by making the program comprehensive and intuitive. 

Strategies for optimizing system selection

After initial project data are entered into the system, there are several tips and tricks for reducing the required system size. A partial list of tips includes:

  • Provide higher quality rectifiers on equipment to reduce harmonics.
  • Include VFDs or reduced voltage starters on motor loads to allow soft starting and minimize the starting impact.
  • Allow for additional load steps on the project by adding automatic transfer switches, programmable logic controllers, or time delays.
  • Load the system with the largest motor loads first.
  • Disable capacitors on emergency loads.
  • Include a UPS upstream of medical imaging or other sensitive loads to reduce their voltage and frequency dip requirements. 

Applying some of these changes in the design can have a huge impact on the generator size. These items may also be higher in cost, but if the results reduce a generator size or even the quantity of generators required for the project, it may provide significant financial benefits to the project as a whole. In the example project, a few of these elements were applied to the loads as shown in Figure 2 with the results shown in Figure 3. 

Results analysis

Figure 3: This graph shows the results of applying the selected optimization strategies shown in Figure 2. • Cummins Power Suite 5.0 GenSize reduced the required generator from 1,500 kW to 1,000 kW—a reduction of 33% and two full engine sizes. • CaterpilAs illustrated in the analysis, each software program yields different results. Each manufacturer has a unique way of modeling the load parameters with built-in baseline defaults. The only defaults changed for the example project are those called for in the example project description. 

In addition, each program models the operating functions of the gensets differently. For example, when calculating the sustained voltage dip for the project parameters, some programs use an average voltage dip over several cycles while others use an instantaneous voltage dip. Also, some manufacturers use their own alternators while others purchase outside alternators. Therefore, the internal modeling differs because of the inherent properties in the equipment. 

The most important factor in using generator sizing software is to know as much as possible about the loads and remember that sometimes it can be more of an art than a science. The more exact the data input—as opposed to using the system default parameters—the more accurate the required generator size will be. Using a generator sizing program properly will provide the designer with a higher level of confidence for specifying a system that is large enough to meet the project needs, but not oversized. However, even with sophisticated software solutions, the designer must use his or her engineering judgment in selecting the proper genset size.


Danna Jensen is associate principal at ccrd in Dallas. With 13 years of electrical engineering experience, most of her work consists of designing electrical distribution systems for hospitals; however, she also designs electrical systems for office and retail facilities. Jensen was a 2009 Consulting-Specifying Engineer 40 Under 40 winner and is a member of the Consulting-Specifying Engineer Editorial Advisory Board. 

Abby Lipperman has 8 years of electrical engineering experience with ccrd in Dallas. Her primary project experience is in the health care industry, but she is also involved in many different project types including data centers, technology labs, museums, schools, banks, and commercial and office spaces. Lipperman was also a Consulting-Specifying Engineer 40 Under 40 winner in 2009.


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