appendix

Load Step
Considerations
Step Sequence
Guidelines
Recommendations
and Reports
APPENDIX A
Rev. Jan 2011
Application Manual – Liquid Cooled Generator Sets
To enter multiple loads into the step, click on the loads folder and all loads are listed on
the right side of the screen. Using the Shift or Ctrl key and the mouse, select the desired
loads, click on any of the selected loads on the right, and drag to the step. All of the
selected loads should appear in the step.
Use the tool bar to add one or more additional steps, as desired. You can view the loads
and steps using View on the menu to find out either which steps individual loads were
placed in or to get a summary of all the loads in each step.
For many applications, the generator set will be sized to be able to pick up all of the loads
in a single step. For some applications it is advantageous to start the loads with the
larger starting surge requirements first, then after those loads are running, to start the
rest of the loads in different steps. The starting sequence of loads might also be
determined by codes in which the emergency loads must come on first, then the standby
equipment, then the optional loads.
Starting step sequencing of generator sets may be accomplished with transfer switches
using transfer time delays, load sequencer, or other controller, such as a PLC. You may
use this application to tell your distributor how many starting steps your application
requires. Remember, even though there is a controlled initial loading sequence, there
may be uncontrolled load stopping and starting of certain loads and you may wish to
check surge loading under those conditions.
Single Step Simultaneous Starting: One commonly used approach is to assume that all
connected loads will be started in a single step, regardless of the number of transfer
switches used. This assumption will result in the most conservative (largest) generator
set selection. Use a single step load unless something will be added, such as multiple
transfer switches with staggered time delays or a step load sequencer.
Single Step with a Diversity Factor: This is similar to simultaneous starting in a single
step, except that an estimated diversity factor, of perhaps 80%, is applied to reduce the
SkVA and SkW totals to account for whatever automatic starting controls may be
provided with the load equipment.
Multiple Step Sequence: Sequenced starting of loads (where possible) will often permit
the selection of a smaller generator set. GenSize assumes that adequate time is allowed
between load steps for the generator set voltage and frequency to stabilize, typically 5–10
seconds.
Consider the following when controls or delays are provided to step sequence the loads
onto the generator set:
• Start the largest motor first.
• When starting motors that use electronic drives (VFD or VSD) the largest motor first
rule may not apply. Using electronic drives for starting and running motors allows the
designer to better control the actual load applied to the generator set by controlling
the maximum current load, rate of load application, etc. The thing to remember about
these loads is that they are more sensitive to voltage variation than motors that are
started ”across–the–line.”
• Load the UPS last. UPS equipment is typically frequency sensitive, especially to the
rate of change of frequency. A pre–loaded generator set will be more stable in
accepting UPS load.
• For each step, the SkW required is the total of the RkW of the previous step(s) plus
the SkW for that step.
The following is intended to help you understand the GenSize recommendation for a
generator set and available reports that can be printed. Figure A–3 illustrates the default
screen on which GenSize makes its recommendation for the single Cummins Power
Generation generator set model that most closely matches the current project
A–15