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Power Design Pro™: Load Sequencing (Groups vs. Steps)

Power Design Pro™: Load Sequencing (Groups vs. Steps)

1/20/2015

This is the fifth article in our series about Power Design Pro™ software. It highlights the options users can make to load characteristics to better manage loads entered into the program. 

There is often a misunderstanding by users that all loads, or large groupings of loads within a building or application, start at the same time (concurrent starting). This typically does not happen in practice, for example:

  • motors have control circuits which must be re-sequenced,
  • cooling compressors must bleed off head pressures,
  • computers must be restarted,
  • UPS’s wait for voltage stability before coming back on-line, etc. 

Typically, when a generator transfer switch closes into an application, there is an initial minimum load step followed by the loads in the application naturally sequencing back on. If an application has two significant loads that inadvertently start concurrently, it is often easier and more cost-effective to add a delay or interlock with a control relay, than to size the generator for the concurrent starting of two significant motor loads.

Limitations of Competitive Sizing Programs
Most generator sizing programs have a contrived method of entering loads into factiously structured load steps. This method assumes that all the loads are running and that they start concurrently. Both are generally false assumptions in the real world. Loads cycle on and off constantly, and rarely start concurrently.  As a result, when using typical sizing programs, you are forced to separate the loads into an arbitrary load sequence to avoid the oversized generator that will result from the concurrent starting assumption.

Power Design Pro™ supports both traditional concurrent starting (step #1, step #2… step #20) and the flexibility to enter loads into load steps that assume non-concurrent starting (group #1 to group #4).

Power Design Pro™ (Steps vs. Groups)
When loads are entered into a non-concurrent starting sequence (group #1, group #2…), natural sequencing is assumed. Each load in this load grouping will start at a different time. In Power Design Pro™, the sequences’ starting kW (skW) is the largest skW in the sequence. The sequences’ starting KVA (skVA) is the largest skVA value in the sequence. 

When loads are entered into a traditional, concurrent starting sequence (step #1, step #2...), simultaneous starting is assumed. The sequences’ skW is the sum of all skW values in the sequence. The sequences’ skVA is the sum of all the skVA values in the sequence. 

Cyclic Sequences
Both groups and steps assume a load factor of 100% (meaning all the loads are always on) with the exception of the cyclic sequences. These sequences (cyclic #1 and cyclic #2) assume that the largest load in the sequence is on and applies the user selected load factor (set on the project page) to the remainder of the loads in the sequence.  This allows the users to enter all the loads into the program but not have them all running.

By assuming that the largest load is always on ensures that the generator has enough capacity to support the starting and running of the largest load in the load sequence. This load sequence assumes non-concurrent starting just like the group sequences.

Power Design Pro™ is a powerful program with many load management features assuring you obtain a properly-sized generator versus an oversized generator. Understanding how to best use these features ensures that you are getting the maximum benefits that the program can offer.

Click here more information on Power Design Pro™.


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